Metallomacromolecules containing cobalt sandwich complexes: Synthesis and functional materials properties

Accepted Manuscript Review Metallomacromolecules Containing Cobalt Sandwich Complexes: Synthesis and Functional Materials Properties Li Zhao, Xiong Liu, Li Zhang, Guirong Qiu, Didier Astruc, Haibin Gu PII: DOI: Reference:

S0010-8545(16)30539-2 http://dx.doi.org/10.1016/j.ccr.2017.02.009 CCR 112396

To appear in:

Coordination Chemistry Reviews

Received Date: Revised Date: Accepted Date:

27 December 2016 10 February 2017 10 February 2017

Please cite this article as: L. Zhao, X. Liu, L. Zhang, G. Qiu, D. Astruc, H. Gu, Metallomacromolecules Containing Cobalt Sandwich Complexes: Synthesis and Functional Materials Properties, Coordination Chemistry Reviews (2017), doi: http://dx.doi.org/10.1016/j.ccr.2017.02.009

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Metallomacromolecules Containing Cobalt Sandwich Complexes: Synthesis and Functional Materials Properties Li Zhao,a Xiong Liu,a Li Zhang,a Guirong Qiu,a Didier Astrucb,*, Haibin Gu,a,* a

Key Laboratory of Leather Chemistry and Engineering of Ministry of Education, Sichuan

University, Chengdu 610065, P. R. China. E-mail: [email protected] b

ISM, UMR CNRS No 5255, Univ. Bordeaux, 33405 Talence Cedex, France. E-mail:

[email protected] ABSTRACT Metallomacromolecules are attracting considerable attention as a class of macromolecular materials with unique properties and applications. Although ferrocene (Fc)-containing macromolecules have occupied the dominant position for several decades in this area and have led to a variety of architectures and applications, the class of cobalt-sandwich-containing macromolecules has recently undergone a booming development. Especially, polymers and dendrimers containing the 18-e cationic cobalticenium (Cc) unit isoelectronic to Fc have been developed during the last few years after overcoming Cc functionalization problems and following the burst of living polymers and dendrimers. Subsequently, remarkable applications have been disclosed including materials physical properties, healthcare and engineering. Meanwhile several other metallopolymers with Co sandwiches containing cyclobutadiene or carborane ligands have also emerged. This review summarizes the most recent progress in the synthesis of metallomacromolecules containing cobalt sandwich complexes and their functional materials properties. Polymerization techniques have included radical polymerization, reversible addition fragmentation chain transfer (RAFT), ring-opening metathesis polymerization (ROMP), condensation polymerization, ring-opening polymerization (ROP), and atom transfer radical polymerization (ATRP), and postpolymerization has also been used. The applications include the formation of nanostructured materials, magnetic materials, redox recognition and sensing, lithographic patterning, antimicrobial materials, stimuli-responsive materials, catalysis and electrochemical devices. Keywords: Metallopolymer; cobalt sandwich, cobalticenium, dendrimer, cyclobutadienecobalt, cobaltacarborane

Contents 1. Introduction 2. Synthesis 2.1. Polymers containing cobalt-sandwich complexes in the side chain 2.1.1. Cobalticenium-containing polymers 2.1.1.1 Post-polymerization 2.1.1.2 Radical polymerization 2.1.1.3 RAFT 2.1.1.4 ROMP 2.1.2. CpCoCb-containing polymers 2.2. Polymers containing cobalt-sandwich complexes in the main chain

1

2.2.1. Cobalticenium-containing polymers 2.2.1.1 Condensation polymerization 2.2.1.2 ROP 2.2.1.3 ATRP 2.2.2. CpCoCb-containing polymers 2.2.2.1 Alkyne metathesis 2.2.2.2 Condensation polymerization 2.2.2.3 Thermal rearrangement 2.2.2.4 Electrochemical polymerization 2.2.3. Cobaltacarborane-containing polymers 2.3. Dendrimers containing cobalt-sandwich complexes 2.3.1. Cobalticenium-containing dendrimers 2.3.2. CbCoCp-containing dendrimers 2.3.3. Cobaltacarborane-containing dendrimers 3. Functional Materials 3.1. Nanostructured materials 3.2. Magnetic materials 3.3. Molecular recognition and sensing 3.4. Lithographic patterning 3.5. Antimicrobial materials 3.6. Stimuli-responsive materials 3.7. Catalysis 3.8. Electrochemical devices 4. Conclusion References

1. Introduction Metal-containing polymers have been known for a long time but have recently encountered considerable development. This enhanced interest is due to the introduction of modern methods of living polymerization and the appearance of a large spectrum of physical and chemical properties in materials science [1-25]. Ferrocene (Fc) has long been a standard in metal-containing polymers because of the rich chemistry and redox and electrochromic properties of Fc-containing materials [1,4,5,26-29]. However, the area of metallocenecontaining macromolecules [30-33] has recently largely expanded owing to the progress in particular in cobalticenium (Cc) functionalization. Thus Cc-containing macromolecules now form a large family with potential applications related to the cationic charge of Cc unit providing aqueous solubilization and new redox properties. This family extends to neutral cobalt-sandwich-containing macromolecules that are derived from cationic ones either by reversible reduction of Cc-containing macromolecules or replacement of a cyclopentadienyl (Cp) ligand by a cyclobutadienyl (Cb) one in [CoCp(η4-C4R4)] units or a carborane ligand. In these metallopolymers, the redox properties of the cobalt center allow interconversion between the neutral and cationic forms, which leads to possible switch of the materials properties. Among these cobalt-sandwich-containing macromolecules, metallodendrimers occupy a special place with specific properties such as the near-identity of redox potential of cobalt sandwich systems located at their tether termini [2,18,34,35]. This review will cover both the syntheses and materials applications of polymer and dendrimer families containing cobalt-sandwich complexes such as Cc, [(η5-cyclopentadienyl)(η4-cyclobutadiene)cobalt] (CpCoCb) and cobaltacarborane, this field being largely dominated by Cc-containing 2

macromolecules.

2. Synthesis 2.1. Polymers containing cobalt-sandwich complexes in the side chain 2.1.1. Cobaltocenium-containing polymers Polymers containing Cc units in the side chain have been synthesized by post-polymerization, radical polymerization, reversible-addition-fragmentation chain transfer (RAFT) and ringopening metathesis polymerization (ROMP) [19,31,32,36]. Being isoelectronic with Fc, the Cc cation satisfies the 18-valence electron rule, and it is stable and even robust. Cc salts are readily generated by removing an electron from the neutral 19-electron (19-e) complex cobaltocene [37]. It is not possible to conduct direct electrophilic substitution on the Cp rings of cobaltocene because of its easy oxidation to Cc. In 1950s, cobaltocene [38] was synthesized shortly after the preparation of Fc, and in 1970 Sheats and Rausch first developed the synthesis of substituted Cc salts [39]. Cyclopentadiene (CpH), methylcyclopentadiene (MeCpH) and cobalt bromide (CoBr2) were used as starting materials, and pyrrolidine was applied as a strong base and good solvent (Scheme 1). The mixture of Cc, methylcobalticenium, and 1,l'-dimethylcobalticenium salts was first synthesized in 20-30% yield by using a modified method for the preparation of metallocenes. Then, basic aqueous potassium permanganate (KMnO4) was applied to oxidize the methyl groups of the obtained mixture to carboxyls, whereas Cc was removed as a precipitate by adding excess sodium hexafluorophosphate (NaPF6) to the basic solution. After the addition of hydrochloric acid, a mixture of the mono- and dicarboxylic cobalticenium hexafluorophosphate was precipitated. Due to their different solubilities in acetone, the monoand dicarboxylic derivatives were separated using hot acetone (Scheme 1), and the overall yield of carboxycobalticenium hexafluorophosphate 1 was only 6.6% [39].

Scheme 1 Synthesis of complex 1. Reproduced with permission from Ref. [40]. Copyright 2010 American Chemical Society. In order to improve the yield of 1, the procedure of Sheats and Rausch was modified by conducting exhaustive extraction and recrystallization techniques to eliminate any remaining 1,1'-dicarboxycobalticenium [40]. Almost 100% pure 1 was obtained, which increased the overall yield to 14-20%. Bildstein et al. also enhanced the yield of the desired compound 1 by rationalizing the reaction conditions, changing the stoichiometry, or adopting other starting compounds. Indeed cyclopentadienyl lithium (CpLi)/methylcyclopentadienyl lithium (MeCpLi) were used instead of CpH/MeCpH/pyrrolidine, and an 11% yield was reached [4143]. The targeted compound 1 was best prepared, however, by an oxidative cleavage of the alkyne group of ethynylcobalticenium hexafluorophosphate 2 (vide infra) by KMnO4 (Scheme 2). The overall yield of 1 from the starting cobalticenium hexafluorophosphate 3

(CcPF6) reaches 80.6% [41]. This is up to now the most convenient and practical protocol to prepare 1 [41,44,45]. H

Si Co+ PF6-

(CH 3)3SiC

CLi/THF

99%

K2CO3/CH3OH

Co H

(C6H5)3C+PF6 CH2Cl2/n-hexane

(C6H5) 3C+PF6CH2Cl 2/n-hexane 87%

COOH Co+ PF6-

1

NaF/CH 3CN, KMnO4/CH3CN, H2O/HPF 6 85%

Si

Co+ PF6-

Co

H

96%

NaF/CH 3CN, H2O 87%

45%

Co+ PF6-

2

Scheme 2 Modified synthesis of 1 and 2. Reprinted with permission from Ref. [41]. Copyright 2014 American Chemical Society. The synthesis of 2, an important synthon for the preparation of Cc-containing polymers by click chemistry or hydroamination, was first reported in 1990 by Schwarzhans and coworkers [43]. It involves nucleophilic attack of suitable alkynyl carbanions and subsequent hydride abstraction. These authors reported a two-step method in which lithium ethynylide in tetramethylethylenediamine (TMEDA) was added to CcPF6 yielding (η5cyclopentadienyl)[η4-(exo-5-trimethylsilylethynyl)-1,3-cyclopentadiene]cobalt(I) followed by addition of tritylium hexafluorophosphate for endo hydride abstraction. The overall yield was 30% [43]. Recently a three-step synthetic pathway was reported affording the target compound 2 [41,45]. Bildstein and coworkers [41] further improved the yield of 2 by changing the sequence of deprotection and hydride abstraction (Scheme 2). As in the case of the synthesis of 2, (η5-cyclopentadienyl)[η4-(exo-5-trimethylsilylethynyl)-1,3cyclopentadiene]cobalt(I) was oxidized to trimethylsilylethynylcobalticenium hexafluorophosphate by a chemoselective endo-hydride abstraction with triphenylcarbenium hexafluorophosphate. Then, deprotection of the trimethylsilyl group was conducted using sodium fluoride. This new three-step synthetic route provided a 75% yield from CcPF6 [41]. 2.1.1.1. Post-polymerization Post-polymerization is a feasible and versatile technique to prepare side-chain Cc-containing polymers [40,46-48]. The Casado group reported the first example of polysiloxane copolymers 3 containing pendant Cc units via condensation reactions (Scheme 3a) [46]. The condensation reaction of cobalticenium acyl chloride (CcCOCl) with the random copolymer of poly(aminopropylmethylsiloxane)-co-poly(dimethylsiloxane) was conducted at room temperature (r. t.) in chloroform (CHCl3) in the presence of triethylamine (Et3N). The copolymer 3 is obtained as a yellow gum, which hardens and solidifies on standing. Its structure and compositions were characterized by 1H, 13C, and 29Si NMR spectroscopy and by elemental analysis (EA). Especially, 1H NMR analysis confirmed that all the pendant amine groups in the siloxane chains have been modified by Cc moieties, and the degree of Cc functionalization was estimated to be 3-5%.

4

Scheme 3 Syntheses of side-chain Cc-containing polymers by various post-polymerization methods [40,46-48]. Tang et al. [40] integrated the Cc unit into the side-chain of a diblock polyacrylate copolymer (Scheme 3b). The side-chain Cc-containing block copolymers 4 were synthesized through the esterification reaction of CcCOCl and hydroxyl groups in poly(tert-butyl acrylate)-blockpoly(2-hydroxyethyl acrylate) (PtBA-b-PHEA-Br) synthesized by atom-transfer radical polymerization (ATRP). However, this route resulted in only 70% esterification yield of hydroxyl groups, which is probably due to the steric hindrance of the bulky Cc moieties [40]. Recently, Astruc et al. [47] reported the synthesis of a water-soluble triazolylcobalticeniumcontaining polymer 5 using a three-step route including a click reaction between the polymer precursor and 2 (Scheme 3c). The monomer of p-chloromethylstyrene was firstly polymerized at 80 oC using azobisisobutyronitrile (AIBN) to provide p-polychloromethylstyrene that was then used to prepare p-polyazidomethylstyrene by the azidation reaction with sodium azide (NaN3). The targeted triazolylcobalticinium polyelectrolyte 5 was synthesized by Cu Icatalyzed azide-alkyne cycloaddition (CuAAC) click reaction between ppolyazidomethylstyrene and 2 at 50 oC for 12 hours in the solvent mixture of tetrahydrofuran (THF), dimethyl formamide (DMF) and water. All of the azido groups were transformed into triazole groups, which was confirmed by the disappearance of characteristic infrared peak at 5

2094 cm-1 assigned to the azido groups [47]. So, the post-polymerization functionalization by click chemistry is a highly efficient method to prepare Cc-containing polymers. The same group further prepared Cc-containing polymers 6 according to the mild uncatalyzed hydroamination of 2 [48]. The starting compound, 5-norbornene-2-carboxaldehyde (endo + exo), was polymerized with using the efficient Grubbs' 3rd generation olefin metathesis catalyst (noted [Ru], Scheme 3d). The polynorbornene obtained with aldehyde groups in the side chain then reacted with excess butylamine to provide the imine polymer. The Schiff base CH=N bonds of this polymer were then reduced to C-NH bonds by sodium borohydride (NaBH4) to form the corresponding secondary amine-substituted polymer. A slight excess of 2 was used in reaction with the secondary amine polymer at 35 oC in the absence of any catalyst in a mixture of dichloromethane (CH2Cl2) and acetonitrile (MeCN) for 16 hours to provide the Cc-enamine polymer 6 with the yield of 97% [48].

Scheme 4 Synthesis of permethylcobalticenium(Cp*2Co)-functionalized polysulfone 9. Reproduced with permission from Ref. [49]. Copyright 2015 Nature Publishing Group. Yan et al. [49] reported covalent linking of the [Cp*2Co]+ salt to polysulfone used to prepare polymer hydroxide-exchange membranes (HEMs) (Scheme 4). The PF6--paired [Cp*2Co]+ salt was used as the starting compound. It was first brominated to introduce a single bromo group in a Cp* ligand group yielding [(η5-C5Me4CH2Br)CoCp*][PF6-]. Then, the hexafluorophosphate counter anion was exchanged on an anion exchange resin balanced with hydrophilic chloride anion to produce brominated [(η5-C5Me4CH2Br)CoCp*][Cl-], 7. In parallel, the chloromethylated polysulfone was aminated with excess hexamethylenediamine (HMDA) to give the HMDA-aminated polysulfone 8. Finally, the desired functionalized polysulfone 9 was synthesized by reaction of 7 with 8 followed by anion exchange from Cl− to hydroxyl ion using potassium hydroxide [49]. 2.1.1.2. Radical polymerization A Cc-containing vinyl monomer 10 was prepared by reaction between CcCOCl and 2hydroxyethyl acrylate, and the radical polymerization with AIBN was used to synthesize the corresponding side-chain Cc-containing polymer 11 with the PF6- anion, a water-soluble hydrophilic polyelectrolyte [50]. The PF6- anion was replaced by tetraphenylborate (BPh4) anion through an effective ion-exchange process (Scheme 5).

6

Scheme 5 Synthesis of side-chain Cc-containing polymers by radical polymerization [50]. Reproduced with permission from Ref. [50]. Copyright 2010 American Chemical Society. This method yielded a hydrophobic BPh4--paired polyelectrolyte 12 that was soluble only in strong polar solvents such as DMF and dimethyl sulfoxide (DMSO). Thus the solubility of these polymers was adjusted by ion exchange. Free radical polymerization prepared only acrylic polymers with low molecular weight, however, and the polymerization process was not well controlled. 2.1.1.3. RAFT The controlled RAFT technique [51,52] that involves living polymers was adopted to prepare well-defined side-chain Cc-containing homopolymers and block copolymers [45,53]. A new Cc-containing methacrylate monomer 13 was prepared and polymerized in MeCN at 90 oC via RAFT. Polymerization kinetic studies confirm the linear relationship between reaction time and monomer conversion, which means that the RAFT of monomer 13 follows the controlled/living character. The cationic character of the obtained metallopolymers 14 rendered very difficult characterization of their molecular weights by gel permeation chromatography (GPC) technique. Consequently the ester bonds in the side chain were hydrolyzed to remove the cationic Cc units. The neutral polymer was then checked by GPC, and a suitable curve was observed with PDI (polydispersity index) = 1.25 and Mn = 9800 g/mol, which further showed the control obtained with the RAFT technique. Three different Cc-containing diblock copolymers 15-17 were successfully synthesized by chain extension of the Cc-containing homopolymer 14 to the second monomers including a Fc-containing monomer (Scheme 6).

7

Scheme 6 RAFT synthesis of side-chain Cc-containing homopolymers and diblock copolymers [45,53]. Kinetic studies showed that all the chain extensions followed a well-controlled/living process, too. Thus, RAFT is a powerful tool to prepare various Cc-containing polymers. Recently, click chemistry and the RAFT technique were adopted to develop a more facile route to sidechain Cc-containing polymers (Scheme 6). Using 2 and 2-azidoethyl methacrylate as starting materials, a new Cc-containing methacrylate monomer 18 was prepared by CuAAC click reaction, then polymerized to provide side-chain Cc-containing homopolymers 19. This RAFT also followed the controlled/living process [45]. 2.1.1.4. ROMP The living ROMP technique [54-59] is also a powerful tool to prepare side-chain Cccontaining homopolymers and block copolymers. A Cc-containing norbornene monomer 20 with PF6- anion was synthesized by the esterification reaction between CcCOCl and N-[3hydroxylpropyl]-cis-5-norbornene -exo-2,3-dicarboximide (Scheme 7) [60].

8

Scheme 7 ROMP synthesis of side-chain Cc-containing homopolymers and diblock copolymers [60,61]. The counter anion in 20 was exchanged for BPh4- (21) or Cl- anions (22). Using the catalyst [Ru] [56,57], it took only 10 minutes in open air at r. t. to finish the ROMP of these Cc monomers 20-22 with high yields (~100%), and the polymerizations followed a living and controlled manner. The obtained polymers 23-25 showed monodispersity with PDI < 1.2, and their molecular weights reached 167,000 g mol-1. These Cc-containing homopolymers with ruthenium (Ru)-end were not quenched but used as macromolecular initiators for further introduction of the second polymer block. Several Cc-containing block copolymers 26-29 with different counter anions and organic blocks were introduced via the one-pot two-step sequential ROMP. All the chain extensions displayed living kinetics, and block copolymers had full end-group retention [60,61]. Cc-containing polymers with different counter ions show different solubility characteristics. This results in the self-assemblies of these copolymers in selected solvents (vide infra) [61]. Co+ PF61) n-BuNH2 THF, r. t., 16h

CHO 2) NaBH4o/PhCOOH THF, 0 C, 30min

H N

2 CH3CN 35oC, 16h

n

Ph 1)

N

H

[Ru] N

CH2Cl2, r. t., 30min

H

2) EVE

H

H Co+ PF6-

Co+ PF6-

30

31

Scheme 8 ROMP synthesis of Cc-containing polymers from Cc-enamine monomer. Reproduced with permission from Ref. [48]. Copyright 2014 American Chemical Society. The Cc-containing monomer 30 (Scheme 8) was prepared by Astruc et al. [48] using the reaction of 2 with a norbornene-functionalized secondary amine under mild conditions in the absence of catalyst. It took 30 minutes to finish the ROMP of the cationic monomer 30 at r. t. in CH2Cl2 in the presence of the metathesis catalyst [Ru]. The yield of polymers 31 reached up to 99%, and the polymers 31 obtained were characterized by 1H, 13C, and DOSY NMR, IR, UV-vis., matrix-assisted laser desorption-ionization time-of-flight mass spectrometry 9

(MALDI-TOF MS), cyclic voltammetry (CV), and EA. Especially, the numbers of Cc redox units in the polymers were determined by end-group analysis by 1H NMR, DOSY NMR and CV, respectively, and the results obtained were similar and close to the feed mole ratio of monomer and catalyst. This shows that ROMP is a powerful and controlled tool to obtain Cccontaining polymers with well-defined size [48]. Recently, this group developed a new amidocobalticenium-containing norbornene monomer 32 by amidation between CcCOCl and N-(2-aminoethyl)-cis-5-norbornene-exo-2,3dicarboximide [62]. 32 is different from the monomer 20 reported earlier [60], and it contains an amido group used to connect Cc and the norbornene structures. Kinetic study by 1 H NMR confirmed that 30 minutes were sufficient to complete the ROMP of the monomer 32 in the presence of the metathesis catalyst [Ru] in DMF. The slower rate is perhaps caused by the lower solubility of 32 than that of 20. The structure of the corresponding Cc-containing polymers 33 was confirmed, and especially the molecular weights of the polymers were characterized by several techniques such as MALDI-TOF MS, end-group analysis and BardAnson’s electrochemical method[62]. Further, the living ROMP was used to synthesize the first diblock polyelectrolytic copolymers containing cationic Cc and the electron-reservoir complex of [Fe(η5‑C5H5)(η6‑C6Me6)][PF6] [63,64]. In view of the faster polymerization rate, the ROMP of 32 was first carried out in dry DMF using the catalyst [Ru], and the second block was then synthesized by chain extension of the produced polymer 33 with Ru-end to the second cationic organoiron monomer 34. It took 30 minutes to finish the synthesis of the first Cc block, whereas overnight stirring was required for 100% conversion of the monomer 34. The structure, molecular weight and redox properties of the diblock metallopolymer polyelectrolyte 35 were characterized and analyzed using various methods such as 1H, 13C NMR, IR, UV-vis., MALDI-TOF MS and CV[64] .

10

nRu

Ph O N O HN

O N O

Co+ PF6-

HN

DMF r. t., 30min

O

Fe

Co+ PF6- Fe+ PF6-

35 nRu

O

[Ru] O

CH2Cl2, r. t. 15 min

HN Fe

36

37

1.

HN

O

CH2Cl2, r. t. 10 min

Fe

40

Ru

n

HN

Ph

m

36 in CH2Cl2

O N O O N O HN

n

2 hrs, r. t.

O

36 in CH2Cl2 15 min, r. t.

HN

C O HN

CO

Co+ PF6-

CO

HN Co+ PF6- C O Fe

42 Ru

n

m

O N O O N O O HN

O C O

Fe

41

Ph

p

m

O N O O N O O N O

Fe

O CO

Co+ PF6-

38

nRu

HN

O

Fe

O N O [Ru]

NH O

2. EVE

O

O N O

O

32 in DMF, r. t., 60min

32 in DMF 20 min, r. t. Ph

m

O N O O N O

Ph

39

n

Ph

O N O

Fe

Fe

O

2. EVE

O N O

HN CO

NH

NH O

33

O

m

O N O O N O

in DMF, r. t., overnight

O

Ph

O

n

Ph PF6-

+

Co+PF6-

32

HN

N H

N

34

[Ru] O

O

O

1.

O

HN CO CO

Fe

Fe

43

32 in DMF Overnight r. t.

Ph

n

p

m

O N O O N O O N O O HN

O

HN CO CO

Fe

HN C O Co+ PF6-

Fe

44

Scheme 9 Synthesis of amidocobalticenium-containing homopolymers and block copolymers by ROMP [62,64,65]. A diblock Fc/Cc-containing copolymer 38 was synthesized with the aid of the catalyst [Ru] by chain extension of a Fc-containing homopolymer 37 with Ru-end to the amidocobalticenium monomer 32 via one-pot two-step sequential ROMP [62]. It took 15 minutes to complete the ROMP of the Fc monomer 36 in CH2Cl2, then the obtained polymer 37 with Ru-end was used as a macromolecular initiator for the ROMP of 32 in dry DMF/CH2Cl2. In situ 1H NMR analysis of the reaction mixture confirmed that 60 minutes were needed to finish the fully polymerization for the second Cc block. The intensity of the first Fc block and the successful formation of the second Cc block were fully confirmed by various measurements including 1 H, 13C NMR, IR, UV-vis. spectroscopy, MALDI-TOF MS and CV. Especially, the numbers of metallocenyl units in each block, determined by the Bard-Anson’s electrochemical method, showed high consistency with the theoretical numbers, which further showed that the ROMP technique is a valuable synthetic tool for the synthesis of well-defined metallopolymers [62].

11

Using the very efficient catalyst [Ru], two triblock copolymers were successfully prepared by ROMP technique with Fc, pentamethylferrocene (Fc*), and Cc moieties in the side chains of a polynorbornene backbone [65]. Two of the six possible synthetic routes were successfully conducted for successive ROMP reactions of the three metalloblocks. The ROMP reaction of the neutral Fc* monomer 39 was first conducted in dry CH2Cl2, and the obtained homopolymer 40 with Ru-end was then used as a macromolecular initiator to initiate the following ROMP of different metallomonomers. In one route, the diblock copolymer 41 containing the pendant neutral Fc* and cationic Cc moieties was constructed by chain extension of 40 with Ru-end to the Cc monomer 32 via one-pot, two-step sequential ROMP, and then the obtained diblock copolymer 41 with Ru-end was adopted as a new macromolecular initiator to initiate the ROMP of Fc monomer 36 to prepare the triblock copolymer 42 containing successively pendant neutral Fc*, cationic Cc and neutral Fc units. Following another synthetic route, a similar but different triblock copolymer was synthesized with successively pendant neutral Fc*, neutral Fc and cationic Cc units using the same three metallomonomers. The ROMP of Fc monomer 36 initiated by 40 with Ru-end led to the diblock copolymer 43 with pendant Fc* and Fc units, then the targeted triblock copolymer 44 was synthesized by ROMP of 32 with the aid of the diblock copolymer 43 with Ru-end as a macromolecular initiator. The success of all the syntheses was monitored and confirmed by kinetic studies using in situ 1H NMR analysis. The structures of all the triblock copolymers and corresponding intermediate diblock copolymers were characterized by 1 H, 13C NMR, IR and UV-vis. spectroscopies, and the numbers of each block were estimated by end-group analysis, MALDI-TOF MS and Bard-Anson’s electrochemical method. The results further confirmed the living and controlled characteristics for ROMP synthesis of the triblock copolymers 42 and 44 [65].

Scheme 10 Synthesis of Cc-containing brush polymers by a combination of ROMP and RAFT. Reprinted with permission from Ref. [66]. Copyright 2013 American Chemical Society. A Cc-containing brush polymer using both ROMP and RAFT techniques (Scheme 10) was also synthesized [66]. A new norbornene monomer 45 containing a chain transfer agent was first obtained, then polymerized using the catalyst [Ru]. Using the obtained polymer 46 as a macromolecular chain transfer agent, the Cc-containing methacrylate monomer 13 was polymerized by the RAFT technique, which produced the targeted brush polymers 47. Atomic force microscope (AFM) images (Fig. 1) indicated that these Cc-containing brush polymers 47 exhibited an elongated wormlike morphology in MeCN. When the small PF6- counter anion is partly exchanged by the bulky BPh4- anion by addition of NaBPh4 into the brush polymer solution, a conformational transition and change is observed. For example, when the amount of BPh4- anion is increased to 25 mol %, spherical nanoparticles (Fig. 1C) instead of wormlike morphology are found. The morphology change of the Cc-based polymers resulted from the quantitative counter anion exchange could find various applications including layerby-layer assembly and ion-triggered drug delivery [66]. 12

Fig. 1. AFM height images of Cc-containing brush polymers 47 in MeCN at different stages of anion exchange of PF6- by BPh4-. [PF6-] : [BPh4-] = 1.0 : 0 (A), 1.0 : 0.1 (B), 1.0 : 0.25 (C) and 1.0 : 1.0 (D). Image size: 800 nm × 800 nm. Reprinted with permission from Ref. [66]. Copyright 2013 American Chemical Society.

Scheme 11 Synthesis of side-chain [(η5-cyclopentadienyl)(η4-cyclopentadiene)cobalt] (CpCoCp#)-containing polymers by ROMP [67]. Reprinted with permission from Ref. [67]. Copyright 2014 Wiley. The ROMP technique was successfully used to prepare side-chain CpCoCp #-containing polymers (Scheme 11) [67]. The neutral cobalt(I) complex CpCoCp # [41,68], obtained by reduction of a Cc salt by NaBH4, has an electronic structure resembling that of CpCoCb, and also is isolobal [69] to Fc and Cc cation. Because of its relative open structure and neutral nature, it has specific and noteworthy properties. For example, it can be easily functionalized through nucleophilic addition to a Cc salt. The CuAAC click reaction between an alkynecontaining CpCoCp# complex 48 and 2-azido ethanol resulted in the hydroxyl-terminated CpCoCp # intermediate 49 that was then used to synthesize a CpCoCp #-containing monomer 50 by esterification with norbornene carboxylic acid (Scheme 11). ROMP of 50 was conducted by using the catalyst [Ru] in CH2Cl2 at r. t. to synthesize the side-chain CpCoCp#containing polymer 51. According to kinetic studies conducted by 1H NMR and GPC analysis, a linear semi-logarithmic plot was observed, which demonstrated the controlled and living character of the ROMP process. It took 10 minutes to finish the ROMP of 50, and the final polymer 51 that was synthesized with 98% yield exhibited excellent monodispersity with a PDI of 1.05. Unlike Cc-containing polyelectrolytes, 51 was readily dissolved in common organic solvents including CH2Cl2, CHCl3 and THF, but it showed moderate stability. It is only stable in air for several days, and the nitrogen (N2) atmosphere is required for its long time storage [67].

13

2.1.2. CpCoCb-containing polymers The 18-electron mixed cobalt(I) sandwich complex CpCoCb is as Fc also highly air and moisture stable and soluble in common organic solvents. Many reports focused on the incorporation of this functional unit in the side chain of polymer backbones [70]. Ragogna’s group [71-74] has established a synthetic platform to prepare side-chain CpCoCb-containing polyacrylates. Unlike Fc, the electrophilic substitution on the Cp ring of CpCoCb is difficult. To prepare substituted Cp derivatives, an alternative route was normally followed using sodium cyclopentadienide containing various substituents as starting compounds that are then transferred into the targeted derivatives. A CpCoCb derivative 52 containing hydroxyl group was synthesized by reaction between lithium (1-carboxypropan-3-ol) cyclopentadiene and (C4Me4)Co(CO)2I, then with acryloyl and methacryloyl chloride, respectively yielding the final CpCo(C4Me4)-containing monomers 53 and 54 (Scheme 12). Ragogna et al. [71] polymerized these monomers by free radical polymerization with AIBN. Over a period of 3 d at 60 oC in deuterated benzene, only 65% conversion for acrylate monomer 53 was achieved according to in situ 1H NMR analysis. The polymer 55 obtained exhibited a low molecular weight (Mn = 10.6 × 103 g mol-1), due to the high percentage of AIBN (9 mol%) required, whereas no polymerization was observed with the methacrylate monomer 54 under the same conditions. This mediocre reactivity of the methacrylate moiety was taken into account by its low solubility and the chain transfer processes of the CpCo(C4Me4) unit [71].

Scheme 12 Synthesis of side-chain CpCo(C4Me4)-containing polymers by free-radical polymerization. Reproduced with permission from Ref. [71]. Copyright 2011 Royal Society of Chemistry.

Scheme 13 Synthesis of side-chain CpCo(C4Ph4)-containing polymers by free radical polymerization and ATRP. Reproduced with permission from Ref. [71]. Copyright 2011 Royal Society of Chemistry. Ragogna’s group [71] also reported the synthesis of the CpCo(C4Ph4)-containing acrylate monomer and its free radical polymerization and ATRP (Scheme 13). The starting methyl ester derivative of CpCo(C4Ph4) 56 was first hydrolyzed with lithium iodide in 2,4,6-collidine under refluxing condition, and the acid derivative 57 obtained was then transformed into the acid chloride 58 using oxalyl chloride and catalytic amount of DMF. Finally, the halide

14

displacement with hydroxyethylacrylate (HEA) yielded the targeted monomer 59. The polymerization of 59 was carried out via ATRP and conventional free-radical methods, and the polymers 60 obtained exhibited the same general structure, but had different numbers of units. (Scheme 13) The ATRP method resulted in 60% yield of the polymer with low molecular weight (Mn = 3.6 × 103 g mol-1) and good monodispersity (PDI = 1.15) determined by size exclusion chromatography (SEC). The free radical method that used less initiator revealed higher molecular weights up to 30.5 × 103 g mol-1 and higher PDI values of 2.0. This seminal work of Ragogna’s group [71] opened a door to the incorporation of the CpCoCb organometallic species into a poly(acrylate) architecture.

Scheme 14 Synthesis of side-chain CpCo(C4Ph4)-containing polymers by RAFT polymerization. Reproduced with permission from Ref. [72]. Copyright 2014 American Chemical Society. A new CpCo(C4Ph4)-containing acrylate monomer 61 was prepared according to an established synthetic protocol and polymerized using the RAFT polymerization technique with 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid as the RAFT agent and AIBN as the initiator in benzene at 80 °C (Scheme 14). Various RAFT conditions including solvent, temperature, RAFT agent, concentration, and monomer/RAFT agent/initiator ratio were optimized to synthesize well-defined polymers [72,73]. Monomer conversion of 90% was observed for the polymerization of 61 under the optimized conditions; however, the homopolymer 62 produced exhibited a 10.8 kDa molecular weight by GPC analysis which is lower than the estimated value of 35 kDa. Moreover, its high PDI of 1.3 indicated that a wellcontrolled polymerization reaction did not take place. The steric hindrance of the CpCo(C4Ph4)-containing monomer 61 resulted in chain transfer and termination reactions instead of controlled chain growth, leading to short polymers with high PDIs.[72] In order to overcome this obstacle, a small monomer, methyl acrylate (MA) was adopted as a comonomer to be copolymerized with 61. When the feed mole ratio of MA to 61 was 3 : 1, as expected, the obtained random copolymer 63 indicated well-controlled and high molecular weight with low PDI of 1.1. Kinetics studies by 1H NMR spectra analysis confirmed the controlled polymerization of 61, and the ratio of PMA and poly-CpCo(C4Ph4) blocks in the 15

copolymer 63 was close to the monomer feed ratio of 3 : 1. Then, 63 was used as a macroRAFT agent for polymerization of styrene in order to prepare diblock copolymers 64 with high molecular weight, and similarly data of 1H NMR spectra and GPC also pointed to a controlled polymerization [72]. Furthermore, a polydimethylsiloxane (PDMS) functionalized macro-RAFT agent 65 was utilized to copolymerize monomer 61 with MA under the same conditions [73]. This RAFT polymerization produced a new diblock copolymer 66 containing PDMS and CpCo(C4Ph4) blocks. So the RAFT polymerization technique is an effective “living/controlled” polymerization to produce CpCo(C4Ph4)-containing homo- and diblock copolymers [72,73]. A block copolymer containing the neutral CpCo(C4Ph4) block and a cationic polyelectrolyte block was synthesized using the “living/controlled” RAFT polymerization of 61 and a phosphonium-containing styrene monomer (Scheme 15) [74]. The end group of a RAFT agent and anion of phosphonium monomer were labeled by fluorine atoms, resulting in accurate end group analysis using 19F{1H} NMR spectroscopy for the prepared polymer 67.

Scheme 15 Synthesis, salt metathesis and AuNPs of side-chain CpCo(C4Ph4)-containing polymers by RAFT. Reproduced with permission from Ref. [74]. Copyright 2015 American Chemical Society. According to the plotting change of monomer concentration over reaction time, a pseudofirst-order plot was observed, which indicated the characteristics of a controlled polymerization of 61. Two copolymers 68 with different block lengths of polyelectrolyte were prepared and characterized. A heterobimetallic block copolymer 69 with distinct gold and cobalt sections was prepared by a typical salt metathesis reaction of triflate anion in polyelectrolyte block with a gold anion (AuCl4-). The anion exchange was accomplished upon one-hour stirring of a CH2Cl2 solution of copolymer 68 with an aqueous HAuCl4 solution. The copolymer 69 obtained shows distinctive physical properties; especially its solubility properties are obviously different from that of the initial copolymer 68. Gold nanoparticles 70 were obtained by reducing the AuCl4- anion with NaBH4 in the polyelectrolyte block of the heterobimetallic block copolymer 69 [74]. Recently, Gallei’s group [75] reported the synthesis of side-chain CpCo(C4Ph4)-containing polymethacrylates by thermal, free radical polymerization and ATRP techniques, respectively.

16

Using sodium cyclopentadienylide and γ-butyrolactone as starting materials, the CpCo(C4Ph4) hydroxyl derivative with hydroxyl group was synthesized according to a convenient singlestep route. Then its reaction with methacrylic acid using the Steglich esterification protocol led to the synthesis of the new CpCo(C4Ph4)-containing methacrylate monomer 71 (Scheme 16). Thermal polymerization of 71 was conducted at 100 oC under vacuum, and the prepared homopolymer 72 exhibited a quite high molecular weight up to 192 kg mol-1 (Mn) and polydispersity (PDI = 3.99) [75].

Scheme 16 Synthesis of side-chain CpCo(C4Ph4)-containing polymethacrylates by thermal, free radical polymerization and ATRP [75].

Free radical polymerization of 71 was conducted in toluene at 60 °C for 24 h using AIBN as the initiator. According to the NMR analysis, the polymer obtained showed a similar molecular structure to 72 produced by thermal polymerization. Only a small polymer (Mn = 8900 g mol-1 via SEC) with PDI of 2.09 was observed, however. Additionally, the copolymer 73 was also successfully prepared by radical copolymerization of methyl methacrylate and 71. In order to obtained well-defined polymers, the living and controlled ATRP technique [76-78] was used to polymerize 71 at 80 °C with 2-bromoisobutyric tert-butyl ester (tBbib) as the ATRP initiator and [CuI(PMDETA)Br] as the catalyst. Consequently, a good monodispersity was observed for the prepared homopolymers 74, which confirmed that ATRP was a reliable method [75]. A polymethacrylate 75 was used as a macro-initiator of 71 for the preparation of block copolymers 76 under similar conditions. The size, structure and composition of 76 were analyzed by 1H NMR spectroscopy, SEC and differential scanning calorimetry (DSC). In summary, ATRP afforded CpCo(C4Ph4)-containing block copolymers with low polydispersity values (PDI ≤ 1.15), and their compositions was well controlled [75].

2.2. Polymers containing cobalt-sandwich complexes in the main chain 2.2.1. Cobalticenium-containing polymers The first main-chain Cc-containing polymers are a series of polymeric oligomethylene Cc salts prepared by Ito and Kenjo [79,80] in 1968 as a new type of thermostable anion17

exchanger (Scheme 17). Cyclopentadienyl sodium and α,ω-dibromoalkanes were used as starting compounds to synthesize α,ω-dicyclopentadienyl alkanes that then reacted with sodium metal to prepare their disodium salts. Then, hexamminecobalt(II) chloride was added resulting in the neutral poly(methylene-cobaltocene) 77, while the corresponding poly(methylene-cobalticenium) salts 78 were obtained by treating polycobaltocene 77 with dilute HCl. However, the obtained oligomers 78 were not well characterized by many techniques except for IR and UV spectroscopy [79,80].

Scheme 17 Synthesis of main-chain Cc-containing oligomers [79,80]. 2.2.1.1. Condensation polymerization Sheats et al. [81-84] pioneered the synthesis of main-chain Cc-containing polymers using condensation polymerization techniques. The first polyesters 81 containing Cc units in the backbone were synthesized by Pittman and Sheats et al. by condensation polymerization of 1,l’-bis(chlorocarbonyl) cobalticenium hexafluorophosphate 79 with diols 80 such as 1,4butanediol and 1,4-bis(hydroxymethyl)benzene (Scheme 18) [81,82]. O Cl

Cl O Co+ PF6-

HO R1 OH

80

O

150 mm CH3CN, N2 0oC/r.

O

t. to reflux

O R1 O Co+ PF6-

81a: R1 = (CH2)4 81b: R1 = H2C

CH2

n

81

79 O Cl

Cl O Co+ PF6-

O H3N (CH2)3 Si O Si (CH2)3 NH2

O R2 O Co+ PF 6-

HO R3 OH

85 84

N2, r. t., overnight

82

79

R2

Et 3N, CHCl3

HO (H3C)3C

Si (CH2)3 N H

N (CH2)3 Si O H PF6n

83

C(CH3)3 OH

PbO 170 - 175 oC, N2, 3h

O Co+

O

O O R3 O Co+ PF6-

84a: R2 = OC2H5

n

86

84b: R2 = OH

85a: R3 = (CH2)10 85b: R3 = H2C

CH2

Scheme 18 Synthesis of main-chain Cc-containing polymers by condensation polymerization of Cc salts [46, 81-84]. The polymerization, conducted in MeCN at reduced pressure, resulted in dimers, trimers and low-molecular-weight polyesters 81 with 4-20 repeat units, according to intrinsic viscosity measurements. Polymers 81 were briefly characterized by IR spectroscopy and EA. The Casado group also reported the synthesis of siloxane-based main-chain Cc-containing polymer 83 according to the condensation polymerization of 79 with diamine-containing siloxane 82 [46]. The polycondensation was conducted in CHCl3 solution at r. t. using Et3N, and the resulting polymer 83 contained a backbone consisting in alternating dimethylsiloxane segments and Cc units bonded through amide linkages. 83 showed high insolubility in common organic solvents, except for partial solubility in DMSO. Its assigned structure was confirmed by 1H, 13C, and 29Si NMR spectroscopy and EA [46]. Pittman and Sheats et al. [83,84] prepared main-chain Cc-containing polyesters 86 by melt phase polytransesterification of 1,l’-bis(carbethoxy) cobalticenium hexafluorophosphate 84 and diols 85. These melt polymerizations were conducted using equimolar amounts of Cc

18

salts 84 and diols 85 in the presence of PbO and di-tert-butylhydroquinone as catalysts in order to prevent dehydration reactions. The ionic polyesters 86 obtained contained both DMF soluble and insoluble fractions, and exhibited fairly low molecular weights (Mn = 3000-10000 by GPC) [83].

Scheme 19 Synthesis of main-chain Cc-containing copolymers by interfacial technique [82,84]. The interfacial technique was adopted by Carraher and Sheats [82] to produce heterobimetallic copolymers 89 containing titanium and Cc units in the main chain (Scheme 19). The aqueous solution of the disodium Cc salt 87 was added into the nitrobenzene solution of dicyclopentadienyltitanium dihexafluorophosphate 88, and highly stirred (23500 rpm) at 30 o C for 5-180 seconds to produce the desired polymer 89. The oligomer obtained was insoluble in common solvents such as DMF, DMSO, THF and toluene, and slightly soluble in aqueous phenol, 2-chloroethanol and H2SO4. The same group reported the synthesis of oligomeric Cc-containing dialkylstannanediyl polyesters by interfacial technique (Scheme 16) [84]. The polymers 91 were prepared by condensation of alkali metal salt of 87 with dialkyl (or diaryl) tin dihalides 90 in CCl4/water. 2.2.1.2. ROP

Scheme 20 Synthesis of main-chain Cc-containing homopolymers and block copolymers through ring-opening polymerization (ROP) [85,86]. OTf -= trifluoromethanesulfonate (triflate anion) Following his seminal discovery and development of the ROP of ferrocenophanes [1], Manners’ group [5,12,87] extended the ROP method to a number of other metallocenophanes. In particular, this group reported the ROP of 19-electron dicarba[2]cobaltocenophanes 92 yielding main-chain Cc-containing homopolymers and block copolymers (Scheme 20) [85,86]. The anionic polymerization of 92 in THF, using Li[tBuC5H4] as an initiator, in the presence or absence of light yielded only oligocobalticenium 93a (DPn ≤ 5) and 93b (DPn ≤ 9), respectively, while its thermal ROP gave high-molecular-weight, water-soluble

19

polycobalticenium polyelectrolytes 94. Cobaltocenophane 92 was sealed in a glass tube, heated at 140 °C for 1 h, cooled and stirred overnight in a 50: 50 methanol/water mixture saturated with NH4NO3 in air. The Cc-containing polymer 94 was obtained in 47% yield and characterized by 1H, 13C NMR and UV-vis. spectra, and its redox activity was assessed by CV showing a chemically reversible but electrochemically irreversible reduction wave at -0.98 V vs. SCE. The molecular weight of 94 was estimated through dynamic light scattering (DLS) experiments as ∼55 000 (Mw) for sodium polystyrene sulfonate and a DPw of 198 [85]. Manners’ group reported the template-induced chiral structure of the water-soluble poly(cobalticeniumethylene) 94 using chiral DNA as an anionic template [88]. The aqueous solutions of 94 and DNA were combined at different mass ratios, and a fast complexation of 94 and DNA was observed, presumably as a result of the strong electrostatic interactions (Fig. 2) between the Cc cations and the anionic phosphodiester groups of the DNA backbone. At a 1 : 1 mass ratio of DNA and 94, a spherical nanoparticles with diameters of 15-40 nm was observed by cryogenic transmission electron microscopy (cryo-TEM). The induced chirality of 94 in these complexes was confirmed by circular dichroism (CD). The main chain of polymer 94 probably presents helical conformation and is embedded in the major or minor groove along the DNA helices (Fig. 2). The induction of chirality was presumably facilitated by the location of the positive charges and the structural flexibility of 94 [88].

Fig. 2. A speculated helical structure of the DNA/94 complex and the electrostatic interaction between the Cc cation and the anionic phosphodiester group of the DNA backbone. Reprinted with permission from Ref. [88]. Copyright 2013 Royal Society of Chemistry. Manners’ group [86] successfully synthesized new heterobimetallic block copolymers 97 containing Fc and Cc units in the main chain by living photocontrolled ROP, a process that had not previously been demonstrated for 92. The synthesis of 97 was accomplished by sequential photocontrolled ROP of 18-electron sila[1]ferrocenophane 95 and 19-electron cobaltocenophane 92 followed by oxidation of the 19-electron cobalt centers. Using sodium cyclopentadienide as initiator, the synthesis of the living polyferrocenylsilane (PFS) block 96 was firstly conducted in THF at 5 oC, then, the 19-electron cobaltocenophane 92 was combined with the obtained chains and irradiated at 20 oC for 24 h. The polymers obtained were quenched by adding methanol and oxidized by air in the presence of ammonium triflate ([NH4][OTf]) to provide the final copolymers 97. The copolymer structure was confirmed by 1 H, 13C, 29Si and 19F NMR and electronic spectra. GPC was used to determine the Mn of the first PFS block, and its polymerization degree was also calculated, then the molecular weights of 97 were estimated by comparing the relative integrations of the -CH2CH2- signal from the Cc block and the -SiMe2- signal from the PFS block in the 1H NMR spectra. The oxidation wave of Fc and the reduction waves of Cc cation in the copolymers were separated by around 1.5 V in CV curves obtained in a 9:1 mixture of THF and water [86]. 2.2.1.3. ATRP Cc-labeled polymers in which the Cc moiety was located either at the end or in the middle of 20

the main chain were synthesized using ATRP by Tang’s group [89]. Specifically, two Ccbased α-haloesters, monosubstituted and 1,1’-disubstituted bromoisobutyrate cobalticenium carboxylate 98 and 99, were synthesized and used as ATRP initiators, respectively, to prepare Cc-labeled polymers 100 and 101 from three different classes of monomers including styrene (St), tert-butyl acrylate (tBA), and methyl methacrylate (MMA) with the aid of N,N,N’,N’’,N’’- pentamethyldiethylenetriamine (PMDETA) and Cu(I)Br [89] (Scheme 21). O Co

+

O PF 6-

Br

O O

Monomer

98

O

O Co+ PF 6-

Br 100a: R1 = H, R2 =

n

R2

O

100b: R1 = H, R2 = -COOC(CH3)3 100c: R1 = CH3, R2 = -COOCH3

100 O O Co PF 6-

O Br

R1

O Cu(I)Br, PMDETA

+

O

O O

Br

O O

Cu(I)Br, PMDETA

R1

O Co + PF 6-

O

R2

Monomer

Br

99

R1

O

O n

O O

O

Br

n

O

R2

101a: R1 = H, R2 =

101

101b: R1 = H, R2 = -COOC(CH3)3 101c: R1 = CH3, R2 = -COOCH3

Scheme 21 ATRP synthesis of polymers containing Cc unit at the end or in the middle of the main chain. Reproduced with permission from Ref. [89]. Copyright 2012 American Chemical Society. Kinetic studies of all monomer polymerization through in situ 1H NMR analysis confirmed that most polymerization systems followed a living and control kinetics, whereas polymerization of MMA exhibited significant termination with the aid of a mono-initiator. It is not very clear at this stage what induced the significant termination reactions. The GPC traces of all of the polymers were determined and showed different PDI values. The PDIs of 101a and 101c were ∼1.20, whereas a higher PDI of 1.45 was observed for 101b. The PDI of 100b is as low as 1.18, and 100a had a higher PDI of 1.35, whereas the polymer 100c exhibited an asymmetrical GPC trace with a rough baseline, indicating that the polymerization was not controlled. Furthermore, the characteristic UV-vis. absorption of the Cc unit was observed for all the Cc-ended and -centered polymers. CV showed irreversible redox waves for all of these polymers. These main-chain Cc-containing polymers are expected to find applications such as sensing and imaging [89]. Metzler-Nolte’s group synthesized a series of Cc-centered or ended peptide bioconjugates 102-113 using solid phase techniques (Scheme 22) [90-94] .

21

H-Phe-OMe

H O N OMe O CH2Ph BPh4

Co+

TBTU, NEt3, DMF

102 H O N N O CH H 3 BPh4

O OH Co PF6+

H-Ala-Phe-OMe TBTU, NEt3, DMF

Co+

H-Phe-OMe

Co+

TBTU, NEt3, DMF

O OH Co+ PF6 OH O

H-Ala-Phe-OMe TBTU, NEt3, DMF

PF6- Co+

106

H N S-H2N-CH(CH3)-Ph

105

Ph OMe

103

1

PF6- Co+

TBTU, NEt3, DMF

O

H O N OMe O CH2Ph O CH2Ph OMe N H O Ph H O OMe N N O CH3H O O CH3H O N OMe N H O Ph H N Ph

Ph

O CH3 BPh4-

S-H2N-CH(CH3)-Ph

O CH3 O CH3

PF6- Co+

TBTU, NEt3, DMF

104

N H

107

H N

O

H O N N O + Co PF 6

N H

H2N

H2N

H2N O

N H O

O

O N H O

H O N

OH

HN

HN

NH2

COOH

108 H2N

H2N

H2N

OH O

S

HN

NH

H2N

H2N

H N

H O N N H O + Co PF 6

Ph

O

OH

O

109 H O N N O + Co PF 6 HN HN

H N

O N H

O N H O

O

H N

O N H O

H O N

OH

HN HN

NH

H2N

HN

H N

NH2

S

H N

O N H

OCo+ TFA

NH2 NH

O N H O

O

H O N

NH2

NH HN

COOH O

110

OH

O

H N Co+ PFO -

H N

O N H NH NH2

HN

O N H O

O

6

HN

111

HN

HN

H N

NH HN

NH2

O

H N

NH2

Co+ PFO-

NH2 NH H N

O N H

HN

O

NH2 NH

O N H O

H O N

NH2

6

NH HN

NH2

HN

HN

HN

NH2

113

112

Scheme 22 Cc-centered or ended peptide bioconjugates [90-94]. Initially, the direct reaction of Cc mono- or di-carboxylic acid with various peptides gave a series of Cc organometallic β-turn mimetics 102-107 used to investigate the influence of a positive charge on the structure and stability of peptide turn structures that were stabilized by hydrogen bonds. The structures of these peptide bioconjugates were characterized by EA, electrochemistry, MS, IR and NMR spectroscopies, and single-crystal X-ray diffraction (102 and 104). The positive charge of the Cc unit exhibited minor influence of hydrogen bonds in peptide turn structures [90]. Metzler-Nolte and his coworkers presented the first example of directed nuclear delivery of Cc cation by conjugation to the SV-40 T antigen nuclear 22

localization signal (NLS) with a peptide containing the primary sequence of H-Pro-Lys-LysLys-Arg-Lys-Val-OH [91]. The bioconjugate was synthesized using standard solid-phase peptide synthesis (SPPS) on Wang resin and Fmoc (Fmoc = 9-fluorenylmethyloxycarbonyl) strategy. An additional lysine residue was introduced at the N-terminus. After peptide assembly, the N-terminal Fmoc protecting group was cleaved and the Cc carboxylic acid was coupled using 2-(1H-benzotriazole-1-yl)-1,3,3-tetramethyluronium tetrafluoroborate (TBTU). Cleavage from the resin was achieved by concentrated trifluoroacetic acid (TFA) with concurrent deprotection of all amino acid side chains. The targetted product 108 was purified by preparative HPLC and fully characterized by MALDI-TOF MS, 1H NMR and square-wave voltammetry. The Cc-peptide bioconjugate showed enhanced cellular pptake and directed nuclear delivery [91]. Moreover, the bioconjugates 111-113 exhibited moderate inhibitory effect against Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus [92].

2.2.2. CpCoCb-containing polymers 2.2.2.1. Alkyne metathesis The Bunz group designed bis-ethynyl CbCoCp-containing complexes to serve as starting blocks for coupling using various organic fragments as linkers to build remarkable exotic organocobalt-containing architectures. First a series of homo- and co-polymers containing CpCoCb complexes in the main chain were synthesized by Bunz and coworkers [95-104] using Stille or Heck-type coupling reactions. (Scheme 23)

Scheme 23 Synthesis of main-chain CpCoCb-containing polymers by alkyne metathesis [95104].

23

The liquid crystalline rigid rod polymer 115 was firstly prepared by heating the monomer, 2,4-diethynyl-1,3-bis(tri-methylsily1)-substituted cobalt 114 to 122 oC in TMEDA with equimolar copper(I) chloride under O2 [95]. The stable polymer 115 obtained is well-soluble in common solvents such as CH2Cl2 and CHCl3 and characterized by NMR, IR and UV-vis. spectroscopies, DSC, thermogravimetry (TG) and powder X-ray crystallography, and its molecular weight was determined by GPC to be 6.2 × 103 (PDI = 3.7) corresponding to 16 monomer units. The oligomers (n = 2-9) of monomer 114 were synthesized and purified by column chromatography and preparative HPLC in moderate yields [99]. A bathochromic shift was observed in the UV-vis. absorption spectrum with a concomitant increase of peak intensity when the polymerization degree was increased. The UV-vis. spectra of higher oligomers (> 7) are almost superimposable with that of polymer 115, which shows that the effective conjugation length seems to be reached. The nature of the visible optical transitions in these oligomers and polymer was determined by electronic absorption, Raman and resonance Raman spectroscopies [105,106]. Poly(p-phenylene ethynylene)s (PPEs) copolymers 118 [96,98,104] containing cobaltoarenocene units were prepared from the reaction of diiodobenzenes 117 with the ethynyl-substituted cobalt complexes 116 under the modified Heck-Cassar-SonoshigaraHagihara conditions developed by Alami and Linstrumelle [107]. The copolymers were obtained in 74-86% yields and showed average degrees of polymerization of 12-58 depending on the types of substituents on the cyclobutadiene and benzene rings. These copolymers are yellow-brownish film forming substances that are stable to atmospheric conditions for prolonged periods of time and exhibit thermotropic liquid crystal behavior. Similar reaction conditions were adopted for the reactions of 114 and 119, yielding copolymers 120 containing thiophene and cyclobutadienecobalt complexes, and the polymers obtained formed smectic lyotropic liquid crystalline phases [97,98]. The acyclic diyne metathesis (ADIMET) polymerization of monomers 121 also led to PPE copolymers 122 containing CpCoCb moieties, and the organometallic polymers obtained showed a degree of polymerization (Pn) of 20-60 [101]. These polymers exhibited nematic, lyotropic liquid crystalline phases as well as chiroptical properties from which aggregation in poor solvents and in the solid state was concluded. The copolymerization of the dodecylcontaining monomer 121 and a dipropynylbenzene 123 was conducted to furnish a series of organometallic PPE copolymers 124 in which CpCoCb units are intercalated into the phenyleneethynylene chains, and this incorporation led to an efficient quenching of the PPE fluorescence in solution and in the solid state [100]. A series of remarkable cyclic organometallic dehydroannulenes with fused CpCoCb units were reported by Bunz et al. [108,109]. The desired cyclic oligomers were prepared by Hay or Vögtle-type coupling using the corresponding 1,2-diethynylated cobalt sandwich complexes as starting compounds (Scheme 24).

24

R R

126a: n =1, R = TMS, trans 126b: n =1, R = TMS, cis 126c: n =1, R = H, trans

Co TMS TMS

Co

Cu(OAc)2 MeCN, 80 oC

Co

127a-d: n =2, R = TMS

R R

125

Co

Cu(OAc)2 MeCN 24 oC, 24 h

TMS TMS

n

126-128 TMS TMS

128: n =3, R = TMS

Co R R

Co n

129

130 TIPS TIPS 132a: n =1, trans 132b: n =1, cis

Co TIPS

Co

TIPS

133a-d: n =2

Cu(OAc)2 MeCN, 80 oC

Co TIPS

Co

131

TIPS

TIPS TIPS n

132, 133 Scheme 24 Synthesis of cyclic CpCoCb-containing oligomers [108,109]. Diyne 125 was subjected to Hay coupling (boiling TMEDA, 1 h, introduction of O2) and led to 63% yield of cyclic oligomers 126-128 according to mass spectroscopic analysis, while the Vögtle-type reaction (Cu(OAc)2, boiling MeCN) of 125 resulted in a crude 87% yield of cycles 126-128. The cyclotrimers 126, cyclotetramers 127 and cyclopentamers 128 were isolated using repeated column chromatography combined with preparative GPC and HPLC. The structures of these oligomers were confirmed using EA, MS, 1H, 13C NMR, IR, and UVvis. spectroscopies, and X-ray single-crystal diffraction. The extended monomer 129 prepared from monomer 125 according to a two-step reaction was also polymerized using Cu(OAc)2 in MeCN, but the resulting product was the linear polymer 130, and no cycle was detected. Similarly, subjecting monomer 131 to the conditions of Vögtle-type coupling using Cu(OAc)2 at 80 oC in MeCN led to a mixture of the trimers 132 and tetramers 133 in a combined yield of 76% [108,109]. Other spectacular polycyclic hydrocarbons containing cobaltoarenocene with various remarkable topological structures were also synthesized by Bunz and coworkers [110-117] from bis-ethynylCbCoCp (Scheme 25).

25

Scheme 25 Synthesis of cyclic CpCoCb-containing oligomers with various topological structures [110-117] . Starting from the diethynylcyclobutadiene complex 125, its Pd-catalyzed coupling with 1,2diiodobenzene 134 produced a dimeric dehydrobenzannulene 135 containing two cobaltoarenocene units, while the coupling with 136 under Pd catalysis led to a new precursor 137 which was then used to prepare a triangle-type cycle 138 following the deprotection and Vögtle coupling procedures. Starting from suitably designed precursors, a series of organometallic cyclynes containing triangle-type (139 and 140), expanded bicyclo[1.1.0]butane 141, tricyclo[2.1.0.0]pentane 142 and butterfly (143-146) topology were synthesized according to similar coupling conditions [110-117]. 2.2.2.2. Condensation polymerization Ni(0)-mediated dehalogenative polycondensation was used by Endo et al. [118-120] to synthesize several liquid crystalline organocobalt poly(arylene)s 147 and 148 containing 1,3or 1,2- type CpCoCb moieties in the main chain (Scheme 26). The structures of the polymers 147 and 148 were confirmed using 1H and 13C NMR and IR spectra. These polymers show π-

26

conjugated structures, good solubility in common organic solvents such as CH2Cl2, CHCl3 and THF and were estimated to have DPn (degree of polymerization) > 15 (GPC, Mn ≈ 22,000 by the polystyrene standard). RO

RO

Co X

Co

Ni(0)

X

X =Cl, Br, OTf R = C12H25, C14H29, C16H33 n

OR

OR

147 OR

OR

RO

RO Co

Co

Ni(0)

R = C6H13, C12H25, C13H27, C14H29, C15H31, C16H33

Cl

Cl

n

148 R1

R1 Y = O, R2 = (CH2)k; Co

HO

OH

X R2

Y = OOC, R2 = (CH2) k;

Co

X

Y R2

Y

,

Y = OOC, R2 =

, n

R1 R1 = H, OC6H13-n,

R1

149

C10H21-n, C14H29-n R1

R1 Co

R1

R1 X R2

Y = O, R2 = (CH2)k; Y = OOC, R2 = (CH2) k;

Co

X

Y = OOC, R2 = HO

Y R2

Y

OH R1 = H, OC 6H13-n,

, ,

n

150

C10H21-n, C14H29-n C14H29O

C14H29O Co Br

Br

Pd(OAc) 2 Tri(o-tolyl)phosphine

Co

(C4H9)3N, DMF n

OC 14H29

OC14H29

151 OC 14H29

C14H29O

Pd(OAc) 2 Tri(o-tolyl)phosphine

Co

OC 14H29

C14H29O Co

(C4H9)3N, DMF

Br

Br

152

n

Scheme 26 Condensation synthesis of polymers with CpCoCb moieties in the main chain [118-123].

27

Similarly, the synthesis of polyethers and polyesters 149 and 150 [121,122] with CpCoCb moieties in the main chain and flexible alkoxy groups in the side chains was achieved by interfacial polycondensations of the regioisomer 1,3- and 1,2- (η4tetraarylcyclobutadiene)cobalt-containing bis-phenols with aliphatic dichlorohydrocarbons and aromatic (or aliphatic) diacyl dichlorides, respectively. The 1,3-type diols offered rigid rod p-terphenyl-like structural units, whereas the 1,2- type isomers led to 900 kinks in polymeric backbones. The structures and properties of the polymers were characterized using 1 H NMR and IR spectroscopies, TG/DTA, DSC and GPC. DPn was found to range from 10 to >50, which corresponds to 40-250 aromatic rings along the polymeric backbones. Most of the polymers exhibited good solubility in THF, DMF, DMSO, CHCl3, benzene and 1,4dioxane, and the solubilities of the all-aromatic random-coil polyesters were higher than that of their rigid rod isomers presumably due to the entropy factor. Glass transition temperatures of the polymers showed good correlation with the content of aliphatic moieties and rigidities of main chains, but were insensitive to the kinks in the backbones. The introduction of rigid rod 1,3-type organocobalt units promoted crystalline or liquid-crystalline phases in the corresponding polymers 149, while the polymers 150 containing the bent 1,2-type organocobalt moieties remained amorphous [121,122]. Heck reaction of p-divinylbenzene with 1,3- or 1,2- type CpCoCb-containing monomers bearing two aryl bromide units, respectively, yielded two poly(arylene-vinylene)s 151 and 152 with organocobalt moieties in the main chain. Both polymers are yellow and soluble in CH2Cl2, CHCl3 and THF. Molecular weights were evaluated as 4000 or so (Mn, GPC on the basis of polystyrene) for the prepared organocobalt poly(arylene-vinylene)s. The UV-vis. spectra confirmed that both polymers 151 and 152 exhibited thermotropic liquid crystallinity in the range of r. t. to ca. 80 °C according to DSC and optical measurement using crossed polarizers [123]. 2.2.2.3. Thermal rearrangement The thermal rearrangement reaction of a series of organocobalt polymers 153 with (η5cyclopentadienyl)cobaltacyclopentadiene moieties in the main chain was conducted by Endo et al. [124-127] to prepare thermally stable polymers 154 bearing CpCoCb units in the main chain. (Scheme 27).

28

Scheme 27 Synthesis of polymers with CpCoCb moieties in the main chain by thermal rearrangement of cobaltacyclopentadiene-containing polymers [124-132]. The starting cobaltacyclopentadiene-containing polymers 153 were synthesized by oxidative coupling (i.e., oxidative ring closure) of diynes with (η5-cyclopentadienyl) bis(tripheny1phosphine)cobalt complex. The used diynes contain flexible aliphatic, electrondonating, or electron-withdrawing groups between the acetylene moieties or as lateral groups, which result in the diversity of the corresponding organocobalt polymers 153. A typical rearrangement reaction was performed in a sealed tube at 110 oC in THF for 1 h (or in toluene for 2 h) without any added reagent, from which the quantitative conversion of 153 to 154 was observed. The structures of 154 were confirmed using 1H, 13C NMR and 31P NMR, IR and UV-vis. spectroscopies. These polymers are soluble in benzene, chlorobenzene, THF, CHCl3, diethyl ether (Et2O) and DMF, and their Mn values were estimated as 5300-33 200 by GPC on the basis of standard polystyrene samples. According to the data by TG/DTA, the polymers 154 showed higher stability, and no weight loss was observed below 400 oC [124-127]. 2-Pyridone-, pyridine- and thiophene-containing copolymers 155, 156 and 158 with CpCoCb moieties in the main chain were also synthesized by the reaction of cobaltacyclopentadienecontaining polymers 153 with various isocyanates, nitrile and sulfur, respectively along with rearrangement of (η5-cyclopentadienyl)cobaltacyclopentadiene moieties in the main chain. For example, 2-pyridone-containing polymers 155 were obtained in 60-99% yields by heating polymers 153 and various isocyanates in THF at 120 oC for 6 h in a sealed tube, and their structures were confirmed using IR, 1H, 13C and 31P NMR spectroscopies. The composition of the 2-pyridone and cyclobutadienecobalt moieties in 155 was controlled by changing the feed ratio of polymers 153 and isocyanates [128-131]. Evidences for formation of the CpCoCb units in the main chain of some polymers 153 were obtained from their detailed structural elucidation and from model experiments. This means that some copolymers 157 bearing both cobaltacyclopentadiene and cyclobutadienecobalt 29

units in the main were formed during the oxidative coupling reaction of diynes with the (η5cyclopentadienyl) bis(tripheny1phosphine)cobalt complex. The structure and properties of the linkage and lateral groups of diynes affects the mole ratio of the two organocobalt moieties in the final polymers. For instance, a diyne having electron-withdrawing groups between the acetylenes and lateral aliphatic moieties (i.e., 1, 4-bis(1-oxo-2-undecynyl)benzene) resulted in 100% of the cobaltacyclopentadiene moieties in the main chain [130]. Following the reaction mechanism of a cobaltacyclopentadiene derivative and t-butyl isocyanide that effectively yields a (η5-cyclopentadienyl)( η4-iminocyclopentadiene)cobalt derivative, several copolymers 159 containing (η5-cyclopentadienyl)(η4iminocyclopentadiene)cobalt and CpCoCb moieties were synthesized quantitatively from the organocobalt copolymers 157 as starting compounds, and then were converted into copolymers 160 containing both Cc and CpCoCb units. Since the intermediate polymers 159 are relatively unstable in air and would convert into an insoluble material, the next alkylation of polymers 159 was conducted in situ to prepare air-stable polymers 160. The alkylation step does not proceed quantitatively, which may be caused by the insufficient solubility of the ionic polymers 160. The structures of the polymers 159 and 160 were confirmed by 1H and 13 C NMR, UV-vis. and IR spectra, and the redox behavior and thermal properties of the obtained polymers 160 were examined by CV and TGA [132]. 2.2.2.4. Electrochemical polymerization Swager et al. [16,133] reported the synthesis of a series of CpCoCb-containing thiophenes and their electrochemical polymerization (Scheme 28).

30

O

O

O

TMS Co

S S

S

S

S S

TMS

TMS Co

S

polymerizatiom

O

O

Electrochemical

S

S S TMS

O

161 Co

S

n

O

O

162 R1 O

R1 O

TMS

S

S

S

Co

S

S

S

S

n

TMS

S

164

163

S

n

O R1

S

165

S n

R1 = CH3, C12H25

O R1

Co

S

S

S S

n

S

S

TMS Co

S

S S

Co

166

S

S

4

TMS

Co

S

TMS

S

S S

S

167

S

S

S

TMSCo

S

S

S

n

R

S

R

S

S S

S

S

S

n

Co S

R

S

168

S R

S

169

Scheme 28 Main-chain CpCoCb-containing polymers prepared by electrochemical polymerization [133]. A typical electrochemical synthesis of the polymers such as 162 was achieved by anodic electrochemical polymerization of complex 161 using repeated CV scans that deposited an insoluble polymer of unknown molecular weight onto the working electrode. These polymers 162-168 were characterized using CV, in situ conductivity and UV-vis. spectroelectrochemistry. The CpCoCb complex remained intact when the oxidation potential of the thiophene fragment was below the CoII/I redox couple. Side reactions as shown in polymer 169 occurred during the polymerization when the oxidation potential of the thiophene fragment was above that of the CoII/I redox couple, however [133]. 2.2.3. Cobaltacarborane-containing polymers Grimes et al. [134-136] reported the spectacular step-by-step synthesis of linear multidecker cobalt sandwiches oligomers by stacking reactions which have a potential application as electroactive materials. Scheme 29 represents a typical synthesis route for the construction of tridecker, pentadecker and hexadecker cobalt sandwiches.

31

H

H

Me B B B Et

Co

2-

n-C4H 9Li

Et

Me B B B Et

2) O2

Et

Co Et Me Me2N B B B Co

NMe2

Et

172

H

B B B Et Co Et Me n-C4H9Li B B B Et THF Co Et

173

2B B B Et Co Et Me B B B Et Co Et

174

Co

Et

Co Et Me B B B

Et

Et

171

Et B B Co Me Et Et B B B M B B B Et B

1) CoCl2 Et2C2B 4H42-

Et

Co

THF

170

H

B B B B

Co

2-

M2+, O2 M = Co, Pt

B B B Et Co Et Me B B B Et Co Et

M2+,

O2 M = Co, Ni, Pt X = H, Me X H B B B

Et

Et

Co

Et

Co

Et B B X M B B B Et Co Et Me B B B Et Co Et Et B

174

176

175

Scheme 29 Synthesis of linear cobaltacarborane-containing oligomers by stacking reactions [134-136]. A cobalt sandwich complex 170 containing planar C2B3 ring was used as the starting compound, and its apex BH units were removed with the aid of n-C4H9Li to yield the Cp*Co(2,3-Et2C2B3H2-5-Me)2- anion 171. Then, the C2B4-capped triple-decker 172 was synthesized by the reaction between 171 and 2,3-Et2C2B4H42- dianions with cobaltous ion followed by workup in air on silica columns. Compound 172 was characterized by NMR or ESR data, UV-vis., MS and EA as a C2B4-capped triple-decker sandwich complex. Decapitation of 172 with wet TMEDA yielded the triple-decker complex 173 having open C2B3 end rings that are amenable to further stacking. For example, deprotonation of 173 provided the anionic triple-decker 174. Three-way reactions of 174 with the double-decker anions Cp*Co(Et2C2B3H2-5-X)2- (X = H or Me), and M2+ ion (M = Co, Ni using Pt) in cold THF, with workup of the products in air, afforded the Co4 or heterometallic Co 3Ni/Co 3Ni pentadecker sandwiches 175. Hexadecker sandwiches 176 were obtained by the treatment of 174 with Co 2+ or Pt2+ ions. These multidecker products were obtained as black, moderately air-stable solids, and their structures were confirmed using ESR or NMR, UV-vis., MS, EA, and for some complexes X-ray crystal structure determination [134-136]. The stacking reaction was also used by Grimes et al. to stepwise synthesize staircase cobaltacarborane oligomers (Scheme 30) [137-139].

32

H H B B B

B B B B Et

Co

Et

Et

Et

1) (Me3Si)2NLi, THF 2) CH3SO2Cl

Double-decapitation Co Et Et B B B B

Co

NMe2

Me2N

Co Et Et B B B H

177

Bridge-deprotonation

H

179

H Cl B B B Et

Co Et Et B B B Cl M Cl B B B Et

Co

Co

H Cl B B B H

1) Bridge -deprotonation 2) MCl2 M = Co, Ni

Et

Et

Et

Co Et Et B B B Cl M Cl B B B Et

Co

5 Et B B B Cl H H

Co Et Et B B B Cl H H

178 H

Et Co

Cl B B B Et Co Et

H

Co

Et

Et

1) Bridge -deprotonation 2) MCl2 M = Co, Ni

Et

H Cl B B B H

MCl2 M = Co, Ni

Co

Et

Co Et Et B B B Cl M Cl B B B Et

Co

Et

3 Co Et Et B B B Cl H H

Co Et Et B B B Cl H H

181 182 180 Scheme 30 Synthesis of staircase cobaltacarborane oligomers by stacking reactions [137-139].

The starting bimetallic linked sandwich complex 177 was produced by reaction of the 1,4bis(tetramethylcyclopentadienyl)phenylene dianion [Me4C5-C6H4-C5Me4]2- with CoCl2 and the carborane anion Et2C2B4H5-. Double decapitation of 177 with TMEDA proceeded easily, quantitatively giving the desired product 178 containing reactive carborane C2B3 end rings. 178 is an effective synthon for the stepwise construction of air-stable multi-sandwich staircase oligomers. Specifically, the monovalent anionic dicobalt complex 179 was produced by bridge deprotonation of the end ring of 178 with equimolar (Me3Si)2NLi followed by treatment with methanesulfonyl chloride. Then, the coordination of two reactive C2B3 end rings in two complexes 179 to a central transition metal (NiIV, Co IV, Co IIIH) formed multilevel staircase-like species 180 having a central tetradecker sandwich linked to double-decker end units. The latter groups have exposed C2B3 rings on which the same reaction sequence was repeated to generate progressively larger oligomers such as three-step 181 and five-step 182 that contain 11 and 17 metal atoms, respectively. All the prepared oligomers were isolated via chromatography on silica and characterized via lH and 1lB NMR, UV-vis., fast atom bombardment mass spectrometry (FAB MS) and X-ray diffraction. Furthermore, the CV curves of staircase oligomers are composites of those found for homologous double-decker and tetradecker monomeric sandwich complexes [139]. A seven-step sequence was depicted by Grimes et al. as follows. The iodo derivative Cp*Co(2,3-Et2C2B4H3-7-I) 183 was converted into a B(5,7)-dialkynyl species 184 that was then dimerized and subsequently cyclized to give a tetrametallic species 185 containing four identical cobaltacarborane clusters and featuring a planar octagonal (tetratruncated square) {C16B8} macrocycle (Scheme 31) [140,141].

33

X B B B Et B Et Co

SiMe3 ClZn-C C-SiMe 3 Pd0, THF

B B B Et B Et Co

H

SiMe3 B B B I Et B Et Co

N-iodosuccinimide

B B B I Et B Et Co

Bu4NF

X = Br, I

183 Et Et

Pd0, THF ClZn-C C-SiMe 3

Co B B B B

Co Et B B B Et B

Et Et

Co B B B B

Et Et

H

Cu(Ac) 2, CuCl

Co B B B B

H SiMe3 B B B Et B I2, Et3N/py Et Co Pd0/CuI

Bu4NF

MeCN, py

B B B Et B Et Co

B B B Et B Et Co

B B B Et B Co Et

B B B Et B Et Co

H

SiMe3

SiMe3

184 C 6H 3I3 Pd(PPh 3)2Cl2/CuI Et3N/THF

185 Co Et B B B Et B

H

Bu4NF

H Et Et B B Co BB

Co Et B B B Et B

Me3Si

SiMe3 Et Et B B Co BB

B BB B Co Et Et H

B BB B Co Et Et SiMe3

187

186

Cu(Ac) 2, CuCl

Et Et Co B BB B

MeCN, py

Et Et Co BBB B

Et B Et B Co BB

BB Co BB Et Et

B Co B Et

B Et B B B B Co Et Et

188 Scheme 31 Synthesis of cyclic cobaltacarborane oligomers [140,141].

The cobaltacarborane-based macrocycle 185, obtained as a red crystalline air-stable solid, has an octogonal macrocyclic geometry in solution confirmed by its extremely simple 11B, 1H, and 13C NMR spectra. An X-ray crystallographic study showed that the 24-atom ring formed by the four {-C≡C-C≡C-} and four carborane {-B-B-} linkages is essentially planar. Two separate one-electron reductions followed by a single two-electron reduction were observed in the CV curve of 185, indicating significant intramolecular electronic communication between

34

the four cobalt centers. A hexanuclear three-dimensional cobaltacarborane-containing macrocycle was also systematically constructed from monomeric precursor complexes [141]. The key intermediate 186 was synthesized in 83% yield from the B(5,7)-dialkynyl species 184 through Pd-catalyzed cross-coupling with 1,3,5-triiodobenzene. The desilylation of 186 yielded, tris(metallacarboranylalkynyl)benzene ‘pinwheel’ complex 187, and a three-way diethynyl linkage of two complexes 187 reacted with MeCN, pyridine, copper(II) acetate and copper(I) chloride to give the target metallomacrocycle 188 as an air-stable red-orange crystalline solid. The electronic interactions and metal-metal communication in 188 was investigated via CV, controlled potential coulometry, and UV-vis. spectroelectrochemistry [142].

2.3. Dendrimers containing cobalt-sandwich complexes 2.3.1. Cobalticenium-containing dendrimers The first Cc-containing dendrimers were reported in 1997 by Astruc’s group [143-145]. These dendrimers are benzene-centered metallodendrimers 189-191 containing respectively 3, 9 and 18 amidocobalticenium units at the periphery (Scheme 32).

35

Scheme 32 Synthesis of benzene-centered polycationic amidocobalticenium dendrimers [143145]. A nonaallyl arene core was first synthesized by CpFe+-mediated nonaallylation of mesitylene under ambient conditions [144,145] followed by visible-light photo-decomplexation using a simple 100 W desk lamp [145-147]. Then, its reaction with acrylonitrile in dioxane in the

36

presence of KOH resulted in the formation of the nitrile-containing dendrimer that was converted into the nona-amine dendrimer by the treatment with BH3 in THF. Finally, the targeted Cc-containing dendrimer 190 was prepared by condensation reaction of the amine dendrimers with CcCOCl in MeCN at r. t. in the presence of Et3N. The weakly MeCN– soluble dendrimer 191 that contained 18 Cc units at its periphery was synthesized using the same route [145]. Casado’s group synthesized poly(propyleneimine) dendrimers 192-195 functionalized with 4, 8, 16 and 32 peripheral Cc subunits, respectively (Scheme 33) [148-153]. The amidation reaction between CcCOCl and poly(propyleneimine) dendrimers having 4, 8, 16 and 32 amino groups, respectively, gave the desired dendrimers 192-195 as air-stable, yellow (192 and 193) or green solids (194 and 195) [148-150].

Scheme 33 Poly(propyleneimine) dendrimers having peripheral Cc moieties [148-150]. The electrochemical behavior of these metallodendrimers and their binding interactions with β-cyclodextrin (β-CD) were investigated by cyclic and normal pulse voltammetry. All the Cc moieties are reduced around a single potential value. The one-electron reduction of each of the Cc centers in dendrimers 192-194 triggers strong binding interactions with freely diffusing β-CD hosts, which resulted in the formation of multisite β-CD-dendrimer supramolecular complexes. The films derived from dendrimers 192-195 were used to modify Pt and glassy carbon electrodes that showed a well-defined and persistent electrochemical response [150]. The synthesis and redox properties of the metallodendrimers 196-199 containing both Fc and Cc units were reported by the Casado’s and Abruna’s groups [151-153] (Scheme 34).

37

O Cl Co+ PF6O Cl

Fe

PF6Co+

O DAB-dendr-(NH 2)x

NH

CH2Cl2/CH3CN Et3N, r. t.

n

O N H

Fe x-n

196: x = 4 198: x = 16 197: x = 8 199: x = 32

Fe Co+ Co+ HO N O Co+ H N Fe

O H N

Co+

O H N

Co+

Fe

NHO

Fe NH O Co+

NHO N

NH

N

N

O

N N

N

Fe NH O

N

N

N

O

N

N

NH O

N N

N

N H

N

N

N HO

N

N

N

N H O

N N

N N N

O

N

HN Co+

Fe

Co+

n PF6-

O

N

H N

Fe

NH

N

O HN Fe

NH

O

NH

O NH

N

O HN O

Fe Co+

O NH

O O NH

Fe

Fe

Co+

N

N

OHN O HN Fe Co

O HN

O HN

HN O

HN O O HN O

+

Co+

HN

Co+

N OH

Fe

Co+

Co+

Fe

Fe Co+

Fe

199

Scheme 34 Synthesis of mixed Fc/Cc dendrimers 196-199 and the structure of 199 [151]. The heteromultimetallic dendrimers 196-199 were produced as air-stable orange-brown shiny solids by the condensation reactions of poly(propyleneimine) dendrimers DAB-dendr-(NH2)x (x = 4, 8, 16, 32) with an equimolar mixture of CcCOCl and chlorocarbonylferrocene (FcCOCl) (Scheme 34). The formation of a single defined product was not obtained, however, and several components with various peripheral Fc/Cc ratios were obtained. The ratios of Cc and Fc units were estimated by 1H NMR and total X-ray fluorescence (TXRF), namely, 3:1 for 196, 6.5:1.5 for 197, 13:3 for 198 and 19:13 for 199. In CV experiments with 196-199, the reversible one-electron oxidation peak of Fc units was observed at about +0.60 V, and the single reduction wave near -0.70 V corresponded to the Cc moieties. Kaifer’s group [154-156] reported two series of dendrimers containing a single Cc unit covalently attached to the apical position of Newkome dendrons (Scheme 35).

38

Scheme 35 Dendrimers containing a single Cc unit covalently attached to the apical position of Newkome dendrons [154-156]. Cc-containing dendrimers 200-202 containing respectively 3, 9, and 27 tert-butyl ester in their peripheries were synthesized by reaction of complex 1 with the corresponding dendritic amine building blocks using 1-[bis(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-b]pyridinium 3-oxide hexafluorophosphate (HATU) and 1,8-bis(dimethylamino)naphthalene (proton sponge). The dendrimers obtained were hydrolyzed in 96% HCOOH to afford the corresponding water-soluble dendrimers as PF6- salts with carboxylic acid groups on their surfaces. The expected one-electron reversible reduction of the Cc center was observed in the CV curves of all the dendrimers, and the heterogeneous electron transfer rate decreased with generation in two dendrimer series. The host-guest binding interactions between the hydrophilic Cc-containing dendrimers and the cucurbit[7]uril (CB7) were investigated and confirmed by the association equilibrium constant (K) of 5.7 ± 0.6 × 109 M-1 [154]. Giant pentamethylcobalticenium(Cc*)-terminated dendrimers were synthesized by Astruc et al. [157] with Cc* termini for the zero, fifth, sixth, and seventh generations up to a theoretical number of 3 9 tethers (seventh generation, G7). The bulk constraint at the periphery of shorttethered dendrimers was overcome by a tether-lengthening strategy for the introduction of the bulky cobalt-sandwich moieties (Scheme 36).

39

p-chlorotoluene

Cl

FE

mesitylene

Fe

AlCl 3

FE

FE = [n5-CpFe][PF6]

AlCl 3

5

FE = [n -CpFe][PF6]

Si O

Co+ PF6-

(ii) hv, MeCN, PPh 3

(i) CH2CHCH 2Br

203

KOH, DME, 20o C

EtOH, K2CO 3 reflux, 24h

PF6Co+

-

EtO

OH

PF6 Co+

FE CH2CHCH 2Br t-BuOH, THF, -40oC

G0-9-allyl (ii) NaI, butanone (i) HSiMe2CH 2Cl Kartstedt, cat. 80 oC, 16h ether, 25oC, 24h

I Si

HO

I Si

I

PF6- Co+

Si

I Si

phenoltriallyl dendron

Si I

I Si

Si

Si

Si I

I

PF6-

Co+ O Si

K2CO 3, DMF 80 oC, 24h

Si O

Si Si Si O O O

O

G0-9-CH2I

Si O

O Si O Si

O Si O Si

DMF, 80 oC, 48h

Si

I

O Si

Si O

203, K2CO 3

PF6Co+

Si O

Si O

Co+ PF6-

O Si

Co+ PF6

-

O Si

O Si

Co+ PF6

Co+ PF6-

Si O

204

O

O

Si

O Si

G5-2187-CH2I Si

Si

O Si

O

G6-6561-CH2I

Si Si

O

Si

O G7-19683-CH2I

O

203, K2CO 3 DMF, 80 oC, 48h

203, K2CO3 DMF, 80 oC, 48h

203, K2CO 3 DMF, 80 oC, 48h

G5-2187-Co*PF6

205 G6-6561-Co*PF6

206 G7-19683-Co*PF6

207

G1-27-allyl

Scheme 36 Synthesis of Cc*-containing dendrimers [157]. The Cc*-containing complex 203 bearing a long alkyl chain with phenol termini was prepared, in which the robust pentamethylcyclopentadienyl ligand was used to stabilize the cobalt(III) species and improve the solubility of the corresponding metallodendrimers. The dendritic core was synthesized by the known allylation and decomplexation procedures [158], and giant iodomethyl dendrimers were constructed until generation 7 by using the iterative hydrosilylation and Williamson reactions [159]. The Williamson coupling reaction of 203 with the iodomethyl-terminated dendrimers gave the corresponding Cc*-terminated dendrimers of generations 0, 5, 6 and 7. These metallodendrimers 204-207 were characterized using 1 H, 13C, and 29Si NMR, MALDI-TOF MS (for 204, G0), EA, UV-vis. spectroscopy, DLS, AFM and CV. UV-vis. spectroscopy showed that these metallodendrimers contained a number of redox groups that were relatively close to the theoretical numbers until G6 with numbers of defects increasing as the generation number increased. Especially, the G7 metallodendrimer 207 had a large number of defects, with only 14 000 ±1000 Cc* termini, instead of the theoretical number of 19 683. The full chemical and electrochemical reversibility was observed by CV for the Cc* centers. These metallodendrimers were expected to find applications as molecular batteries because of their behaviors as fast electron-transfer connectors [160]. The CuAAC click chemistry of 2 was used by Astruc’s group [161] to construct three

40

generations (G0, G1, and G2) of metallodendrimers 208-210 containing 9, 27, and 81 Cc termini, respectively. The G0, G1, and G2 azido-terminated dendrimers (Scheme 37) were synthesized by reactions between sodium azide and the corresponding chloromethyl(dimethyl)silyl dendrimers.

41

Scheme 37 Click synthesis of metallodendrimers bearing 9, 27, and 81 triazolylcobalticinium

42

termini, respectively [161]. The synthesis of dendrimers 208-210 were conducted at 60 oC in a mixture of MeCN/H2O (for 208) or THF/MeCN/H2O (for 209 and 210) using CuSO4/sodium ascorbate between 2 and three generations G0, G1, and G2 of dendrimers dend-N3 containing 9, 27, 81 azido termini respectively. The dendrimer sizes were characterized by DOSY NMR, MALDI-TOF MS, and AFM measurements. CV experiments of 208-210 in MeCN and DMF confirmed the solvent-dependent reversibility of the CoIII/II wave (18e/19e) and generation dependent reversibility of the CoII/I (19e/20e) wave in DMF [161]. In order to improve the robustness of these triazolylcobalticinium dendrimers in various experimental conditions, Astruc et al. recently investigated the synthesis of azido-terminated dendrimers 211 without ether linkages. A new series of Cc-terminated dendrimers 212 were constructed by combining hydrosilylation reactions with first trichlorosilane and then chloromethyl(dimethyl)silane, followed by metallation reactions using click chemistry (Scheme 38) [162]. Cl

Cl

Si

Si Si

Cl Cl Si

Cl

Si

Cl

Si

(i) HSiCl 3, Karstedt cat. Et2 O, r. t.,17h

HSi(CH 3)2CH2 Cl Karstedt cat.

Si

Cl Si

Si

Si

Si

Si

Si

Cl

Si

Si

Si

Cl Si

Si

Si

Si Si

Co

PF6 Co+ PF6

-

Co

PF6- Co+ N

PF6- Co+

N

+

N N N

Co

N

PF6

Si

Si

Si

Si

Si Si

Si

N NN N N Co+ N Co+

PF6-

Si

Si

Si

Si

Si Si

Si

N N NN N N

N

Si N N

N

Co+ PF6-

Co PF6-

+

Si N N

+

Co PF6-

NN N Co+ PF6-

NN N Co+ PF6-

N

N NN

NN N

Si

Si

N

Si

6

-

N3 N3 Si

Si

Co+ PF6-

N3 Si

Si

Si

Si

N3

N3 Si

Si

N3

Si

Si

N3

Si

N3

Si

Si

Si Si

Si

N3

Si

Si

Si

Si N3

N3

Si Si

N3

Si

Si Si

N3

N3

N3

N3 N3

Si

Si

N3

N3

Si

Si

Si Co+PF

Co+ PF6-

N3 Si Si

Si

N3

2

Si

Si

N3

CuSO 4 Na ascorbate MeCN/THF/H2O N + 3 Co PF r. t., 17h 6

Co+ PF6

N3

N3 N3

NN PF Co+ 6 N Si NN N Si Co+ PF6 N Si N N

N

Si

Cl

NaN3 DMF, 60oC,18h

Si

Si

Si

Cl

Cl

PF6Co+

Si

Si

N N N

Co PF6-

Si

Si

N

+

+

Si

Si Si

Si Cl

PF6-

N PF6N N Co+ N N N NN PF6Si Co+ N Si

Si Si

Si

Si NN

N NN PF6-

N NN

N N N

Cl Co+

N N N

N N N NN N

+

-

Co

Co+

Co+

PF6-

Cl

Si

Cl PF6-

Cl

Si

Si Cl

PF6-

Cl

Si

Si

Cl

PF6+

Cl

Si

Si

Si

Si

Si

Cl

Si

Si

Cl

Si

G0-9-allyl

Cl Si

Si

Et2O, r. t.,18h

(ii) CH2=CHCH 2 MgBr Et2 O, r. t.,40h

Cl

Si

Si Si

Cl

Si

Cl

Si

Si

N3

211

212

Scheme 38 Click synthesis of 27-Cc dendrimer 212 from the nona-allyl core [ 162]. The uncatalyzed hydroamination of 2 discovered by Astruc’s group [163,164] opened a new avenue for the construction of Cc-containing dendrimers. This mild “green” reaction with primary and secondary amines in the absence of a catalyst and an additional solvent

43

quantitatively yield trans-enamines of Cc without formation of any byproduct. The dendrimers terminated with primary or secondary amino groups react with 2 to give corresponding Cc dendrimers. The hydroamination of 2 with a poly(amido amine) (PAMAM) dendrimers containing eight terminal amino groups in MeCN/CH2Cl2 (1:1) produced the alltrans-enamine-Cc dendrimer 213, whereas that with arene-centered dendrimers terminated with secondary amine groups under similar conditions yielded metallodendrimers 214-216 containing 9, 27, and 81 all-trans-enamine-Cc termini, respectively [164] (Schemes 39 and 40).

Scheme 39 Synthesis of all-trans-enamine-Cc dendrimer by reaction of 2 with the firstgeneration PAMAM dendrimer [164].

44

45

6 -

PF

Co

PF6-

N

+

n-Bu

O

n-Bu

Si

O Si

Si Si

O Si

O

Si

O

Si

Si

O

O

Si O

Si

Si

O

n-Bu

O

N n-Bu N

PF6-

n-Bu N

Si

Si

O

O

n-Bu

N

Co+

n-Bu

PF6+

Co Co+ PF6-

Co+

n-Bu

Co+ PF6-

Co+ PF6-

Co+

Co+

N

N n-Bu N n-Bu

Co+

n-Bu N

O

O

N

n-Bu N

O

Si Si

Co+ PF6-

n-Bu N

O

PF6-

Si

Si n-Bu

O

O

O

O n-Bu

n-Bu

Si

Si Si

Co+ PF6

O N

Si

+

Co+

N

Si

O

N

n-Bu

Si

O Si

PF6-

O Si O

O Si

N

Co+

N

O

N n-Bu

Co+

Si

O Si

PF6-

O

Si

Si

Si

O Si

Si

Si

O

Co+

PF6-

n-Bu N

Si

+ PF6- Co

PF6-

O

O

O

N n-Bu

Co

N n-Bu N

N

Si

PF6-

PF6-

n-Bu O

N n-Bu

N n-Bu

Co+

N

n-Bu

n-Bu N

O PF6-

Co+

Co+

n-Bu

Co+

Co

+

PF6Co

PF6-

Co+

+

Co+

PF6-

PF6-

PF6-

PF6-

PF6-

Co+ PF6-

215

46

PF6-

PF6-

Scheme 40 Synthesis of metallodendrimers containing 9, 27, and 81 all-trans-enamine-Cc termini, respectively [164]. The hydroamination reaction with dendrimers bearing nine ferrocenylmethylamino groups yielded the bimetallic dendrimers 217 containing both Fc and Cc termini [164]. These bimetallic dendrimers are the first ones in which the ferrocenyl and Cc groups are precisely distributed, as confirmed in particular by 1 H (including DOSY) and multinucleus NMR and CV in DMF (Scheme 41).

47

Scheme 41 Synthesis of bimetallic dendrimers containing Cc and octamethylferrocenyl units [164]. A water-soluble supramolecular Cc dendrimer 218 was prepared by binding cationic Cc to a polyanionic dendrimer by means of contact ion-pairing interactions [165] (Scheme 42).

48

-

O OC

CO

O

-

OO C

CO OCO O-

CO

O-

OO C

C - OO

Scheme 42 Synthesis of the water-soluble Cc dendrimer [165]. The Williamson reaction of the G1 polyiodomethylsilyl dendrimer with methyl 4hydroxybenzoate yielded the polybenzoate-terminated dendrimer that was then solubilized in water upon addition of a stoichiometric amount of NaOH to give a polyanionic dendrimer [166]. This sodium polybenzoate dendrimer reacted with cobalticenium chloride according to a metathetic ion exchange among the two reacting ion pairs to form a polybenzoate Ccterminated dendrimer 218. The expected structure of 218 was confirmed by comparing their 1 H and 13C NMR and IR spectra, and the hydrodynamic radius from DOSY NMR spectra with that of the starting sodium polybenzoate dendrimer [165]. 2.3.2. CpCoCb-containing dendrimers Bunz et al. [167] reported the synthesis and characterization of new CpCoCb-cored organometallic polyphenylene dendrimers containing 24 or 44 phenyl rings (Scheme 43). The convergent CpCo(CO)2-mediated dimerization of di- or tetraethynyltolanes and the following divergent core extension using tetraphenylcyclopentadienone resulted in the air and water stable dendrimers 219-221. The dendrimers exhibited oxidation potentials ranging from 0.8 to

49

0.83 V, and the dendrimer with higher steric hindrance showed higher oxidation potential.

Scheme 43 Synthesis of CpCoCb-centered dendrimers [167]. 2.3.3. Cobaltacarborane-containing dendrimers A review on boron clusters-based metallodendrimers was recently published [168]. A series of benzene-centered and cobaltacarborane-ended branched oligomers were synthesized by Grimes et al. (Scheme 44) [142,169-173]. The reaction of Li2[Cp*Co(2,3-Et2C2B3H3)] 222 with one-third molar equivalent of 1,3,5-tris(diiodoboryl)benzene in toluene yielded the tricobaltacarborane sandwich complexes 223 as moderately air-stable yellow solid. Then, the thermal displacement of cyclooctatriene from [(η6-C8H10)Fe(Et2C2B4H4)] complex by the central benzene of 223 provided the heterotetranuclear dendritic complex 224. The trigonally symmetric structures of dendrimers 223 and 224 were shown by various spectroscopies and X-ray crystallography. The CoC2B4-type cobaltacarborane-containing dendrimer 226 was constructed by reaction of 1,3,5-triiodobenzene with the B(5)-ethynyl closo-cobaltacarborane 225 in THF [169].

50

BI 2

Et Et

2B B B Et Co Et

B B B B Fe

Co B B B B

Et Et

BI 2

I2B

Toluene, 0 oC

222

Et Et BB Co BB

B B B B Co Et Et

180 oC

B B B BB B Co Fe BB Et Et

Et Et Et Et Co BBB B

B B B Et B Co Et

223

224

Et Et B B B Et B Et Co

C 6H3I3 H Pd(PPh 3)2Cl2/CuI

Co B B B B

Et3N/THF B B B B Et Co Et

225

H

Et Et

Et Et

Et Et

Co B B H HB

Co B Co B B

C6H 3I3 Pd(PPh 3)2Cl 2/CuI Et 3N/THF

227

Co

226

TMEDA/H2O

H B B Et BH Et Co

BB B B

1) NaH

H B B Co HB Et Et

H B BH B Et Et Co

2) (Cp*CoCl)2

Co B B B Et Et Co

B B Co B Et Co Et

229

228

Scheme 44 Synthesis of benzene-centered cobaltacarborane-containing dendrimers [142,169173]. The decapitation of 225 in aqueous TMEDA yielded the nido-cobaltacarborane 227 that reacted with 1,3,5-triiodobenzene under palladium catalysis to afford the CoC2B3-type cobaltacarborane-containing pinwheel complex 228. Bridge deprotonation of the three open C2B3 faces in 228 with NaH followed by capping with (Cp*CoCl)2 yielded the dark-red airstable dendritic tris(triple-decker sandwich) complex 229 in 69% yield [170,171]. The trigonal symmetrical benzene-centered dendritic structures 226 and 229 were confirmed using multinuclear NMR, IR, MS and X-ray diffraction studies. CV data demonstrate that small but measurable metal-metal communication between the cobalt centers is observed in the CoC2B4-containing cobaltacarborane dendrimers 226, whereas enhanced and considerable electronic communication occurs in oxidized and reduced species of the C2B3-bridged tripledecker systems 229. The apically substituted complex Cp*CoIII(Et2C2B4H3)-7-C≡CH 230 was used to construct cobaltacarborane-containing dendrimers with various branching types [141,171,173]. Reaction of 230 with n-butyllithium and zinc chloride and finally with 1,3,5-triiodobenzene afforded the benzene-centered CoC2B4-type cobaltacarborane-containing dendrimer 231, an isomer of 226 (Scheme 45).

51

Co Et B B B Et B Co2(CO)6 Et Et B B Co B B

B BB B Co Et Et

Co Et B B B Et B OC CO C OC Co Co CO C OC CO OC CO OCCO OC Co Co CO Et C C C B BB Et BCC B Co B o CO Co Co BB OC CO OCOC CO Et Et

231

232

I

I

I Et3N/THF

H

B B B Et B Et Co

I

I

I

I

Pd(PPh3)2Cl2/CuI Et3N/THF

230

Co Et B B B Et B

I

I

Pd(PPh3)2Cl2/CuI

Co Et B B B Et B

Et Et B B Co B B

B BB B Co Et Et

Et Et B B Co BB

B BB B Co Et Et

233

Scheme 45 Synthesis of the large cobaltacarborane-containing dendrimers [141,171,173]. The treatment of 231 with dicobalt octacarbonyl at r. t. in CH2Cl2 further yielded the redbrown nonacobalt complex 232 containing three cobaltacarborane-ended branches. A larger cobaltacarborane-ended dendrimer 233 was generated from the reaction of 230 with one-sixth molar equivalent of 1,3,5-tris(m-diiodophenyl)benzene. These dendrimers were characterized by 11B, 1H, and 13C NMR, UV-vis. and IR spectroscopies combined with EA [141,171,173]. Grimes et al. reported the first poly(propyleneimine) dendrimers containing cobaltacarborane units at the periphery [174]. Considering the relatively small and metallocene-like steric requirements, well-established redox properties and synthetic tailorability, the 6- and 7-vertex CoC2Bn (n = 3, 4) cobaltacarborane clusters were attached to two poly(propyleneimine) dendrimers with 16 and 32 amido-ended branches, respectively. The starting CpCo(2,3Et2C2B4H4) 234 was successively converted to its Cp-substituted carboxylic acid 235 and acyl derivative 236 [174] (Scheme 46).

52

Scheme 46 Synthesis of the dendrimer containing 32 cobaltacarborane units [174]. Then, the decapitation of the acyl chloride 236 in ethyl acetate and water quantitatively produced nido-[η5-C5H4C(O)Cl]Co(2,3-Et2C2B3H5) 237 as a red oil. Its reaction with the polyamine diaminobutane-dend(NH2)16 (DAB-16) in CH2Cl2 in the presence of Et3N yielded the targeted 16-cobaltacarborane dendrimer DAB-dend-[NHC-(O)-C5H4Co(2,3-Et2C2B3H5)]16 as an air-stable yellow-orange solid. The metallodendrimer 238 containing 32 cobaltacarborane-terminated branches was generated under similar condition from the fourthgeneration dendrimer diaminobutane-dend(NH2)32 (DAB-32). The structures of these two dendrimers were confirmed by 1H, 11B and 13C NMR, IR, UV-vis. and MALDI TOF MS. Their CV curves in THF solution showed a single reduction process exhibiting chemical reversibility, and no electronic interaction was observed among the cobaltacarborane subunits [174].

53

3. Functional materials Metal-containing polymers [1-20] have attracted attention for a long time for their functions and materials properties, and the great ease of introduction of Fc units into polymers has largely contributed to this interest. Thus the engineering of Fc-containing polymers towards nanomaterials applications has been largely developed in particular by Manners’ group [1,5,11,13,15] and others [6,10,16,18,20] for more than two decades. Applications that have evolved include memory and light-emitting devices, solar cells, photonic crystal display, nanolithography, controlled release, sensors, and catalysis. Cobalt-sandwich-containing polymers are relevant from the same strategy with the difference that in the Cc-containing macromolecules the most common redox form is cationic, whereas it is neutral for Fc. There is a continuum of concept between the iron- and cobalt-sandwich-containing materials, however, because both of them benefit from a stable redox couple, complementarity essentially residing in the very large difference of redox potential regions of these redox couples. Applications of the cobalt-sandwich-containing macromolecules have therefore been more recently developed than those of the Fc analogues. They involve strategies of counteranion exchange, polyelectrolytes, formation of vesicles, nanotubes, micelles, magnetic materials obtained by pyrolysis, lithographic patterning via microprinting, antibiotic binding and health care in general, stimuli responsive materials, electrochemical devices, sensing and catalysis. 3.1. Nanostructured materials

Fig. 3. TEM images under bright-field (a) and dark-field (b) modes for the self-assembled nanotubes of side-chain Cc-containing block copolymer 4 in acetone/CHCl3. The inset is a proposed illustration of the self-assembled nanotubes. Scale bar: 1000 nm. Reprinted with permission from Ref. [40]. Copyright 2010 American Chemical Society. The block copolymers 4 self-assemble into vesicles and nanotubes according to the adopted solvent system [19,40]. An acetone/CHCl3 solvent system results in the formation of nanotubular structures with 12 ± 2 nm of wall thickness and 26 ± 3 nm of cavity width (Fig. 3). The walls of nanotubes were supposedly constructed from CHCl3-insoluble electron-rich Cc-containing domains, their center being located at the cavity of the nanotubes. It is possible to change the solubility of polycationic Cc-containing polymers by exchanging their counter anions. For example, the Cl--containing polymer 25 is water-soluble, whereas the BPh4--containing polymer 24 has very poor solubility in water, and the solubility of the PF6--containing polymer 23 in water is intermediate between the two others. Thus, copolymers containing these different blocks showed amphiphilic property and selfassembled into micelles in certain solvent (Scheme 3). Block copolymer 26 containing Cl- and BPh4- anions self-assembled into spherical micelles with size of about 110 ± 20 nm and are stable in water (Fig. 4A-C).

54

Scheme 47 Self-assembly of Cc-containing block copolymers 26 and its use as precursor for the template synthesis of inorganic nanoparticles. Reproduced with permission from Ref. [61]. Copyright 2012 Wiley.

Fig. 4. TEM/AFM pictures and particle-size distribution of 26 forming nano-micelles. (A) TEM images, (B) particle-size distribution and (C) AFM height image of micelles of 26 in aqueous solution. (D) AFM height image of UV/ozone and pyrolysis-treated micelles of 26. Reproduced with permission from Ref. [61]. Copyright 2012 Wiley. The micelles obtained should exhibit a characteristic core-shell structure in which the core is constituted by the hydrophobic BPh4- blocks, while the shell is formed by the hydrophilic Clblocks. It was not possible to distinguish between the core and the shell, however, because of the presence of cobalt (III) in both blocks. These self-assembled block copolymer micelles were further used as templates to prepare inorganic oxidized cobalt nanoparticles (Fig. 4D) via UV/ozonolysis and thermal pyrolysis successively to remove their organic components. Results of X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) showed that the main contents of these nanoparticles are Co 3O4 and Co2PO4F, and their anti-ferromagnetic properties were also confirmed by magnetic tests [53]. Also, spherical nano-micelles were observed from the self-assembly of the Fc/Cc-containing block copolymer 17 in both DMF/THF and DMF/MeCN solvent systems (Fig. 5).

Fig. 5. TEM pictures of nano-micelles self-assembled by copolymer 17 in (A) DMF/MeCN (v/v: 1:2) and (B) DMF/THF (v/v: 1:2), and their particle size distribution by DLS in (C) 55

DMF/THF and (D) DMF/MeCN. Reprinted with permission from Ref. [53]. Copyright 2012 American Chemical Society. The size of micelles ranged in 45-150 nm. DMF is a good solvent for both blocks, whereas MeCN is a selective solvent for Cc block, and THF can only dissolve Fc block. In DMF/MeCN, separated nano-micelles were observed, whereas in DMF/THF they tended to aggregate. It was believed that, in DMF/MeCN the shells of micelles were formed from the Cc-containing block and their cores contained Fc-containing block, whereas in DMF/THF the locations of these two blocks were opposite. Obviously, the different core-shell component is the reason for aggregation or separation of micelles. The solution self-assembly behavior of the main-chain Cc-containing block copolymer 97a was studied, and spherical nano-micelles (radius ca. 8 nm) were observed in the solution of 97a in a methanol/water mixture[86]. The seeded self-assembly of 97a was studied by adding 97a in THF to seed poly(ferrocenyldimethylsilane)-block-poly(2-vinylpyridine) (PFS34-bP2VP272) micelles in methanol. Fig. 6 shows the TEM images of the cylindrical micelles obtained. Obviously, 97a grew in three dimensions around the PFS34-b-P2VP272 seed micelle, burying it within the hierarchical micelle structures [86].

Fig. 6. TEM pictures (a, b) of cylindrical micelles resulting from the addition of 97a to PFS34b-P2VP272 seed micelles and their idealized graphical representation (c). Scale bars correspond to 200 nm. The PFS core is drawn in orange, the Cc-containing block ([PCE][OTf]) corona is drawn in red, and the P2VP corona is drawn in blue. Reprinted with permission from Ref. [86]. Copyright 2011 Wiley. Nanostructures were formed by the solution- and solid-state self-assembly of side-chain CpCo(C4Ph4)-containing copolymers [72-74]. The solid-state self-assembly of 66, in which the prepared bulk film was solvent annealed with THF vapor, thermally annealed at 150 oC under reduced pressure and then quickly cooled by immersion in liquid N2, resulted in the long-ranged order of hexagonal morphology (Fig. 7).

56

Fig. 7. TEM images of solid-state phase-separated nanostructures (left) and nano-micelles (right) from the solution self-assembly of side-chain CpCo(C4Ph4)-containing copolymer 66. Reproduced with permission from Ref. [73]. Copyright 2015 Wiley. The dark hexagonal domains are PolyCpCoCb-r-PMA and appear as dark circles within the light grey polydimethylsiloxane (PDMS) matrix background. Also, spherical nano-micelles with about 20 nm diameter sizes were obtained by injecting THF solution of 66 into n-hexane, a selective solvent for the PDMS block. These nano-micelles contain a metallic core and are stabilized by a PDMS corona. The presence of silicon and cobalt in the self-assembled nanostructures was confirmed by energy-dispersive X-ray spectroscopy (EDX) analysis. The polyelectrolyte block copolymer 68 containing the side-chain CpCo(C4Ph4) was also selfassembled into different nanostructures in solution and in the solid state, respectively [74]. A THF solution of 68 was injected into methanol, a selective solvent for its polyelectrolyte block, which generated nano-micelles with metallopolymer cores and polyelectrolyte coronas. TEM imaging (Fig. 8A) visualized the obtained spherical nano-micelles with 25 ± 5 nm diameters, and the presence of cobalt, phosphorus, and fluorine was confirmed by EDX technique. The salt metathesis reaction of 68 resulted in the gold functionalized block copolymer 69 that self-assembled into heterobimetallic nano-micelles with a gold containing core and cobalt containing corona by injecting a CH2Cl2 solution of 69 into benzene. Spherical micelles with a diameter of 40 ± 5 nm were obtained, and the presence of cobalt, gold, and phosphorus was identified on the basis of EDX analysis (Fig. 8B). The AuCl4--containing cores of the produced nano-micelles were then reduced by sodium borohydride to form gold nanoparticles (AuNPs). The UV-vis. spectrum showed a characteristic plasmon band at 525 nm, and the TEM imaging exhibited a size of 10 ± 5 nm for the AuNPs obtained (Fig. 8C) [74].

Fig. 8. TEM images of nanostructures formed by self-assembly of block copolymers 68 and 69. Nano-micelles were solution-state self-assembled by 68 (A) and 69 (B). AuNPs (C) are produced by reduction of the nano-micelles of 69. The solid-state assembled nanostructures of 68 are stained with RuO4 (D) and HAuCl4 (E). Reproduced with permission from Ref. [74]. Copyright 2015 American Chemical Society. A significant size decrease was observed upon comparison of the original spherical micelles and the AuNPs obtained that exhibited a similar size distribution despite being produced from micelles with different sizes. The possible reason is that the reduced AuNPs contained a rigid core, while the original micelles had a soft solvent expanded core. Similar sizes of the AuNPs were due to the close packed crystalline core, regardless of the core making block length. Phase-separation behavior of block copolymer 68 exhibited hexagonally packed cylinders of 57

metallopolymer in the sea of polyelectrolyte. To clarify the discrimination and assignment of phase-separated domains, RuO4 was used to stain the cobalt-containing metallopolymer domain. As expected, TEM analysis (Fig. 8D) showed the presence of dark regions of hexagonally packed cylinders corresponding to the metallopolymer block in a comparatively less dark background assigned to polyelectrolyte region. Moreover, to further confirm domain assignments, salt metathesis with AuCl4- anion in solid-state was used for selectively staining the phosphonium-based polyelectrolyte block. As a complementary pattern, the bright spherical regions observed by TEM (Fig. 8E) in a hexagonal arrangement were assigned to the metallopolymer regions left intact during the staining process, whereas the polyelectrolyte domains underwent anion exchange and appeared as a dark background region. A phase-transfer ion-exchange method was reported to prepare cationic Cc-containing polymers with diverse counter anions including F-, Cl-, Br-, I-, NO3-, and OAc- using tetrabutylammonium salts [175]. Various cobalt-containing bulk materials, nanoparticles and 1D nanowires were prepared by pyrolyzing these polyelectrolytes with different counter anions under air or reduced H2/N2 atmosphere (Fig. 9).

Fig. 9. Diagrammatic preparation of versatile cobalt-based bulk materials, nanoparticles and 1D nanowires by using cationic Cc-containing homopolymers, block copolymers, and polymer brushes as precursors. Reprinted with permission from Ref. [175]. Copyright 2013 Wiley. The prepared cobalt materials contained cobalt metal, cobalt monoxide (CoO), cobalt phosphide (Co 2P), cobalt-iron (CoFe) hybrid, and cobalt ferrite (CoFe2O4). The controlled molecular architecture, self-assembled morphology, type of counter anion and pyrolysis conditions played key role in the formation of these cobalt-containing materials. For example, pure and high quality Co 2P was obtained by the pyrolysis of PF6--paired Cc-containing methacrylate homopolymer under H2/N2 atmosphere at 800 oC, while the treatment of corresponding I--paired polymers under the same conditions resulted in the formation of cobalt metal, and CoO was obtained through UV/ozonolysis and further pyrolysis in air at 800 o C. Copolymers containing a hydrophilic NO3--paired Cc block and a hydrophobic poly-tert-butyl acrylate (PtBA) block self-assembled into nanoscale micelles in aqueous solution. These nano-micelles were converted into CoO nanoparticles (by pyrolysis in air at 800 oC) and elemental cobalt nanoparticles (by pyrolysis under H2/N2 atmosphere). The heterobimetallic diblock copolymer containing I--paired Cc and Fc units in the side chain self-assembled into nano-micelles with Fc in the core and Cc in the shell in aqueous solution. CoFe2O4 nanoparticles were formed by pyrolysis of these micelles under air, whereas FeCo hybrid was 58

obtained by pyrolysis under a reductive atmosphere. Worm-like Cc-containing polymer brushes 47 were also used as precursors for the formation of 1D cobalt-based nanowires. 1D cobalt phosphide nanowires were produced by pyrolysis of PF6--paired brushes under H2/N2 atmosphere, whereas for I--paired brushes, nanowires of elemental cobalt metal were obtained by pyrolysis under reduced atmosphere, and nanowires of CoO were prepared by pyrolysis under air [175].

3.2. Magnetic materials Manners’ group pioneered ten years ago the area of magnetic cobalt nanomaterials formed from metallopolymers containing organometallic sandwich species [176]. Indeed, pyrolysis at high temperatures of these molecular materials leads to the decomposition of the cobaltcontaining molecules in the polymers yielding such magnetic cobalt-containing inorganic materials. Related papers were also published some years later by others [177-179]. Various nanostructured iron-cobalt/carbon and iron-cobalt phosphide/carbon magnetic materials were prepared by pyrolysis of heterobimetallic diblock copolymers 17 side-chain containing both Fc and Cc units [180]. The facile thermal pyrolysis was conducted at 800 oC in reductive atmosphere for 8 h, and resulted in core/shell metal/carbon nanoparticles in which metal nanoparticles were embedded in amorphous carbon films or encapsulated in well-graphitized carbon nanotubes (Fig. 10).

Fig. 10. Diagrammatic relationship between the Fc/Cc block ratios of heterobimetallic diblock copolymers 17 and their magnetic properties. Reprinted with permission from Ref. [180]. Copyright 2014 American Chemical Society. Upon utilizing the improvement of the Fc block, a transition of these inorganic materials from metal (iron-cobalt) phosphide to (iron-cobalt) alloy was observed, and the values of their magnetic saturation were increased. Magnetization of these inorganic nanomaterials was tuned by adjusting the heterobimetallic diblock copolymer compositions of 17 taking into account the close correlation between the saturated magnetization and the weight percentage of phosphorus and cobalt in the pyrolyzed materials. Applications including new oxygen reduction catalysts and magnetic bioimaging devices are expected for these controlled magnetic materials given their high tolerance toward harsh conditions [180]. The pyrolysis of thermally annealed 66, bearing CpCo(C4Ph4) units in the side chain, under nitrogen atmosphere for 4 h at 800 oC resulted in the formation of a cobalt-containing magnetic nanomaterial showing attraction to a permanent magnet (Fig. 11). TEM analysis

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showed nanoparticles with the size of 7±2 nm, and the presence of cobalt and silicon in the nanostructures was also confirmed by EDX analysis.

Fig. 11. TEM image of the pyrolyzed 66 and its attraction to a permanent magnet. Reproduced with permission from Ref. [73]. Copyright 2015 Wiley.

Cobalt-phosphide nanoparticles with magnetic properties were produced upon pyrolyzing annealed CpCo(C4Ph4)-containing block copolymers 68 and 74 under similar conditions [74,75]. 3.3. Molecular recognition and sensing Molecular recognition and sensing are of importance in many fields, ranging from environmental monitoring, biology and industrial applications to clinical diagnostics [161]. The Cc group allows redox sensing and recognition, because it electrostatically binds anions. This binding is switched by redox change to Co(II) upon adjusting the electrode potential to that corresponding to the reduction of Co(III) to Co(II). This binding capacity is enhanced by the presence of a Cc substituent such as an amido or 1,2,3-triazolyl group in the case of oxoanions that form H bonds with them. Thus in the presence of an anion to analyze, the redox potential Co(III/II) is modified, but this modification is only small or modest with a Cc monomer. This potential change, however, becomes larger in Cc-terminated dendrimers, and all the larger as the dendrimer generation increases. Metallodendrimers have a well-defined topology whereby the anions are trapped at the inner periphery in the dendritic exoreceptor, this effect being favored by higher generations. The first recognition of anions such as H2PO4HSO4- and Cl- using this strategy with Cc-terminated dendrimers was reported 20 years ago by Astruc’s group [146], followed by others [181-183]. The method has also been applied by Casado’s group to polymerized pyrrole-Cc receptor systems [183,184]. Recently, click Ccterminated dendrimers have shown related redox recognition properties for the recognition of n-Bu4N+H2PO4- [161] (Fig. 12).

Fig. 12. Recognition of H2PO4- anion by triazolylcobaticenium dendrimer 210. CVs of compound 210: (a) in DMF, [n-Bu 4N][PF6] 0.1M; (b) splitting of the reduction CV wave upon addition of n-Bu4N+H2PO4-; (c) complete disappearance of the first wave upon addition of nBu 4N+H2PO4-. (d) Binding mode between 210 and H2PO4- anion. Reprinted with permission from Ref. [161]. Copyright 2013 American Chemical Society. 60

Splitting the CoIII/II CV wave is observed at the beginning of the addition of n-Bu 4N+H2PO4into the electrochemical cell containing 210, and the resulting new reduction wave is located at a potential 0.15 V more negative than the original one. Further addition of n-Bu 4N+H2PO4results in the complete disappearance of the first CoIII/II CV wave when the equivalent point is reached. The stronger electrostatic binding of the anionic guest than that of the initial counter anion partly masks the positive charge, rendering the positively charged Cc moiety more difficult to reduce to its neutral 19-electron form [161]. Amperometric enzyme electrodes are subjected to an important analytical area that has been addressed by Casado’s group [184]. Amperometric enzyme electrodes were prepared by electrostatically immobilizing glucose oxidase (GOx) onto carbon and platinum electrodes modified with dendrimers containing both Fc and Cc units. They were used to monitor glucose under anaerobic and aerobic conditions (Fig. 13).

Fig. 13. Glucose calibration plots of the mixed Fc/Cc dendrimers 196-199/enzyme sensors (Pt electrode area 0.07 cm2, Γ = 7×10 -10 mol ferrocene cm-2). Steady-state currents were measured at +0.55V (vs. SCE) in phosphate buffer (pH 7.0). Inset: typical amperometric response of the dendrimer 198/enzyme modified electrode to consecutive additions of glucose aliquots in the electrolyte [Reproduced by courtesy of Prof. C.-M. Casado]. The presence of the Cc units in the nanosystem was expected to prevent the loss of GOx resulting from the reduction of the ferricenium groups, thus improving the long-term stability of the sensors. These surface-confined heterometallic dendrimers 196-199 work in a double way. The Fc units successfully act as mediators in enzymatic processes under anaerobic conditions, while the Cc moieties serve as electrocatalysts in the reduction of oxygen in solution. This makes possible for the determination of the oxygen variation with high sensitivity due to the enzymatic reaction. The sensor behavior was affected by the dendrimer generation, and indeed the glucose calibration curves show that the sensitivity of the sensors containing various dendrimer generations as mediators increased in the order 196 < 197 < 198 < 199 (Fig. 13) [184]. 3.4. Lithographic patterning The side-chain copolymer 66 containing CpCo(C4Ph4) was used as ink material for soft lithographic patterning via microcontact printing (µCP). A 0.5 wt % toluene solution of 66 was transferred onto the PDMS stamp, and the pattern was printed on a freshly cleaned silicon wafer substrate by gentle pressure. Long-ranged patterns including holes, lines, and pillars were successfully produced on silicon wafers. Such structures were believed to be excellent candidates to make patterned ceramics by means of pyrolysis of these well-defined domains

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[73] (Fig. 14).

Fig. 14. SEM images of holes (left), lines (middle), and pillars (right) using 66 as ink material for µCP. Reproduced with permission from Ref. [73]. Copyright 2015 Wiley.

3.5. Antimicrobial materials Halide anion (Cl-, Br-, and I-)-paired side-chain Cc-containing polymers 14 and their bioconjugates with conventional β-lactam antibiotics were found to show high inhibiting efficacy against several stains of methicillin-resistant Staphylococcus aureus (MRSA) including community-associated MRSA (CA-MRSA, ATCC 1717), HA-MRSA (ATCC 29213), and MRSA-252 (ATCC 1720) [185]. The excretion of β-lactamase that hydrolyze βlactam antibiotics is a major defense mechanism employed by stains of MRSA (Fig. 15a, b). Various β-lactam antibiotics (Fig. 15e) were protected by Cc polymers via the formation of unique ion-pairs between their carboxylate anions and cationic Cc moieties. Carboxylate anions in β-lactam antibiotics readily underwent counter anion exchange with halide counter anions in Cc polymers, resulting in the formation of antibiotics-metallopolymer conjugates with 1:1 pairing between antibiotics and Cc units. (Fig. 15d) These bioconjugates perform anion exchange with negatively charged cell walls or carboxylate anions in extracellular solution, leading to the release of unbroken antibiotics. (Fig. 15c, f) Thus, the β-lactam antibiotics were protected from β-lactamase hydrolysis and effectively exhibited their inhibitory effect against MRSA. Compared with metallopolymers alone and antibiotics alone, their corresponding bioconjugates exhibited the largest inhibition zones against different strains of MRSA, especially for HA-MRSA (Fig. 15g). This point was also confirmed by confocal scanning laser microscopy (CSLM) and SEM images of the tested HA-MRSA cells (Figure 6h). The water-soluble halide anion (Cl-, Br- and I-)-paired cationic Cc-containing polymers 14 themselves also exhibited excellent inhibition against different MRSA cells by selectively disrupting their cell membranes when their concentrations were increased to about 5 µM. These metallopolymers showed negligible hemolytic effects on red blood cells and minimal in vitro and in vivo toxicity.

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Fig. 15. Bioconjugates between Cc-containing polymers 14 and β-lactam antibiotics and their inhibitory effects against MRSA. (a) MRSA cells. (b) Typical β-lactamase hydrolysis of βlactam antibiotics. (c) Proposed interactions between bioconjugates, β-lactamase and cell wall. (d) Formation of ion-pairs between β-lactam antibiotics and cationic Cc-containing polymers. (e) Molecular structures of four used β-lactam antibiotics. (f) Antibiotic release from antibiotic-metallopolymer ion-pairs via lipoteichoic acid or β-lactamases. (g) Size of inhibition zones of antibiotics alone, Cl--paired metallopolymers and their conjugates against HA-MRSA, CA-MRSA, and MRSA-252. (h) CSLM images (left) and corresponding SEM images (right) of HA-MRSA cells incubated respectively in the presence of control solution, penicillin-G, Cl--paired Cc polymers and penicillin-G-metallopolymer bioconjugate. Scale bars in CSLM images, 50 µm; scale bars in SEM images, 1 µm. Reproduced with permission from Ref. [185]. Copyright 2014 American Chemical Society. Antibacterial metallopolymer hydrogels (Fig. 16) were also prepared by Cc-containing copolymers [186]. A monomer, 2-cobalticenium amidoethyl methacrylate hexafluorophosphate (CoAEMAPF6) was synthesized, and subsequently copolymerized with poly(ethylene glycol) dimethacrylate (PEGDMA) as a crosslinker to produce cationic Cccontaining brownish organogels (PCoPF6-Gel) (Fig. 16a). Then, the phase-transfer ion exchange of PF6- counter anion in CoAEMAPF6 by Cl- anion was conducted in MeCN to produce light yellow hydrogels PCoCl-Gel. Because of the ability of Cc units to bind βlactam antibiotics, the obtained hydrogels showed the ability to remove antibiotics from aqueous solution. Moreover, these hydrogels exhibited inhibitory effects against Gramnegative Escherichia coli (90% inhibition, Fig.16b), Gram-positive Staphylococcus aureus (80% inhibition, Fig. 16c), and HA-MRSA (80% inhibition, Fig. 16d). This work might open up new avenues for Cc-containing polymers in the field of healthcare and environmental treatment [186].

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Fig. 16. Anion-paired (Cl-, PF6- and antibiotics) Cc-containing organogels and hydrogels, and their antibacterial activities. (a) Preparation process of three gels. Inhibition of PCoCl-Gel hydrogels against (b) gram-negative Escherichia coli (c) gram-positive Staphylococcus aureus and (d) HA-MRSA under different concentrations by standard solution micro-broth measurement. Reproduced with permission from Ref. [186]. Copyright 2015 Nature Publishing Group.

3.6. Stimuli-responsive materials Electroactive polyelectrolyte multilayers (PEMs) were fabricated as nanoscale thin films (Fig. 17) using the layer-by-layer (LbL) assembly technique [187] utilizing the ionic interactions between cationic Cl--paired Cc-containing polymer 25 and anionic poly(styrenesulfonate) (PSS) [188]. The mass and thickness of the PEM were adjusted by the salt concentration. When a potential of -0.6 V or lower was applied to the PEM, the cationic Cc units were rapidly reduced to neutral cobaltocene, and when the electrostatic forces responsible for holding the PEM together disappeared the PEM film disintegrated. The disassembly process is highly controllable, rapid and efficient characteristics depend on the time of electric stimuli, and complete decomposition of the film from the substrate is observed within a few minutes. The PEM was used as a trigger to control the release of non-responsive molecules (drugs and antibodies, for example) in an overlaying layer into the solution upon application of the electric potential [188].

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Fig. 17. Schematic illustration of the fabrication and electric stimulus-triggered disassembly of Cc-containing multilayer film. Reprinted with permission from Ref. [188]. Copyright 2014 Royal Society of Chemistry. 3.7. Catalysis Polymers 51 containing cyclopentadienyl-cobalt(I)-1,3-cyclopentadiene were reported as a heterogeneous macromolecular catalysts for ATRP of methyl methacrylate (MMA) and styrene in the presence of 2-ethyl bromoisobutyrate (EBiB) as an initiator in toluene [67]. The “living” characteristic of the polymerization (Fig. 18a, b) was confirmed by the linear semilogarithmic plot, and the polymerization rate was comparable to those of molecular catalysts reported in the literature. In the proposed mechanism (Fig. 18c) the halide and Co(I) unit undergo a redox process involving atom transfer during the activation and deactivation steps. The cobalt(I) polymer that is insoluble in toluene is easily removed from the polymerization system by filtration, due to the heterogeneity of the polymerization. Thus, the obtained PMMA and polystyrene were not contaminated with the color of the catalyst and are much whiter than the corresponding light-brown polymer obtained with the aid of cobalt(I) monomer as a molecular catalyst (Fig. 18d) [189,190].

Fig. 18. CpCoCp #-containing polymer 51 as a heterogeneous macromolecular catalyst for ATRP of MMA. (a) Semilogarithmic plot of polymerization of MMA) (b) Plots of molecular weight (Mn, GPC, left) and PDI (right) vs. monomer conversion (1H NMR) for MMA. (c) Possible ATRP mechanism upon using cobalt(I) polymer as catalyst. (d) Optical images showing the homogeneous and heterogeneous process catalyzed by the cobalt(I)-containing monomer (left) and polymer 51 (right). Reprinted with permission from Ref. [67]. Copyright

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2014 Wiley.

3.8. Electrochemical devices

Fig. 19. HEMs prepared by Cp*2Co-containing polysulfone 9. (a) Photograph (2” x 2”, 100 µm thick) of membrane made by 9. (b) TGA and derivative thermal gravimetric (DTG) curves. (c) Ion exchange capacity (IEC) change of membrane vs. time during the alkaline stability test. Reproduced with permission from Ref. [49]. Copyright 2015 Nature Publishing Group. Cp*2Co-containing polysulfone 9 was used to prepare polymer HEMs (Fig. 19a) that are attractive electrolytes for electrochemical energy conversion devices such as fuel cells, electrolyzers and solar hydrogen generators [49]. [Cp*2Co]+ salts presents superior stabilities and basicities, and they are ultra-stable redox moieties for polymer HEMs. The prepared films are similar to normal cation-based HEMs in terms of hydroxide conductivity, water uptake in deionized water at r. t. as well as IEC-normalized hydroxide conductivity, suggesting similar cation basicity and hydroxide conduction efficiency. As expected, the membranes exhibited enhanced (short-term) thermal stability and improved (long-term) alkaline stability. The onset of decomposition temperature of 305 oC (Fig. 19b) is the highest among all the reported HEMs. Heating at 80 oC in 1 M KOH for 2,000 hours led to only 27% IEC loss, whereas at 100 oC the loss was about 50%, mainly caused by the polysulfone backbone scission. Thus the Cp*2Co cation may be useful in designing more durable HEM electrochemical devices.

4. Conclusion The class of cobalt sandwich-containing polymers is a recent and very valuable complement to that of the Fc-containing polymers. Progress in the functionalization of Cc complexes and the increasing need for metal-containing nanomaterials are responsible for this late development. Impressive materials properties have been disclosed involving supramolecular aspects (nanowires, vesicles, micelles), magnetism, biological relevance, electrochemical applications, lithographic patterning and catalysis. Probably the greatest advantage of this cobalt family of metal-containing materials is the positive charge of the robust 18-electron Cc sandwich allowing solubilization of the nanomaterials in water and pairing flexibility with a variety of anions including those of biomedical importance. Materials properties considerably 66

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Co

Co Co Co

Co Co Co

n

n

Co Co Co

Co

H

H B

Co =

Co+

Co

Et

B

Co

B Et

74

Highlights: Macromolecules containing Co sandwich complexes and materials properties are reviewed Synthesis and application of polymers and dendrimers containing Co sandwich complexes Macromolecules containing Cp2Co+ or Co sandwiches with a C4 or metallocarborane ring

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