Titanium complexes based on pyridine containing dialcohols: Effect of a ligand

    Titanium complexes based on pyridine containing dialcohols: Effect of a ligand Ekaterina A. Kuchuk, Badma N. Mankaev, Kirill V. Zaitsev, Yuri F. Oprunenko, Andrei V. Churakov, Galina S. Zaitseva, Sergey S. Karlov PII: DOI: Reference:

S1387-7003(16)30053-3 doi: 10.1016/j.inoche.2016.02.023 INOCHE 6249

To appear in:

Inorganic Chemistry Communications

Received date: Revised date: Accepted date:

9 January 2016 22 February 2016 27 February 2016

Please cite this article as: Ekaterina A. Kuchuk, Badma N. Mankaev, Kirill V. Zaitsev, Yuri F. Oprunenko, Andrei V. Churakov, Galina S. Zaitseva, Sergey S. Karlov, Titanium complexes based on pyridine containing dialcohols: Effect of a ligand, Inorganic Chemistry Communications (2016), doi: 10.1016/j.inoche.2016.02.023

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ACCEPTED MANUSCRIPT Titanium complexes based on pyridine containing dialcohols: effect of a ligand Ekaterina A. Kuchuka, Badma N. Mankaeva, Kirill V. Zaitseva,*, Yuri F. Oprunenkoa,

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Andrei V. Churakovb,c, Galina S. Zaitsevaa and Sergey S. Karlova

Chemistry Department, M.V. Lomonosov Moscow State University, B-234 Leninskie Gory,

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119991 Moscow, Russia

N.S. Kurnakov Institute of General and Inorganic Chemistry, Russian Acad. Sci., Leninskii pr.,

31, 119991 Moscow, Russia

Department of Chemistry, Tomsk State University, Lenina prosp. 36, Tomsk 634050, Russia

*

Corresponding

Author:

Phone:

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+7(495)939-3887;

fax:

+7(495)939-0067;

e-mail:

Abstract:

Substituted

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[email protected] (Dr. K.V. Zaitsev)

2,6-bis(hydroxyalkyl)pyridines,

H2L,

1-3

(2,6-

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Py(CH2(X)OH)(CH2(Y)OH), X= Y= cyclo-C6H10, 1; X= Y= 1-Ad, 2; X= CPh2, Y= CH2CPh2,

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3), were used as ligands for the synthesis of titanium(IV) complexes, (i-PrO)2Ti(L), 1a-3a. On the contrary, application of related dialcohol based on 2,2’-bipyridine, H2L’, 5 (2,2’-bipy-6,6’-

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(CH2Ph2OH)2), resulted in titanyl complex, (L’)Ti=O, 6. The molecular structure of 6 was investigated by X-ray analysis. Compounds 1a and 3a were tested as initiators in ring-opening polymerization of L-lactide and ε-caprolactone showing moderate activity.

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Keywords: polydentate ligand, pyridine dialcohol, titanium, complex, titanyl, ring-opening polymerization

Nowadays titanium complexes based on polydentate ligands, such as amino alсohols, find wide application in chemistry, for example, as precursors for ceramic materials using sol-gel [1] or MOCVD techniques [2], catalysts for organic reactions [3], cytotoxic compounds [4], in synthesis of oligomeric complexes [5] or initiators in polymerization of lactones [6] or polymerization of alkenes [7]. Therefore, an investigation of these compounds may be regarded as an actual scientific subject. General approach for the synthesis of titanium(IV) oxo-alkoxidees consists in partial hydrolysis followed by dehydration and dealkoxylation of intermediate Ti alkoxides [8]. However, this method does not allow to predict the structure of oxo-alkoxide product. It was found for secondary Ti alkoxides that sometimes the oxo-fragment may be formed in water-free 1

ACCEPTED MANUSCRIPT conditions at high temperature evolving ethers [9], chloroalkanes or ketones [10], alkene [11]. In this case, the product structure strongly depends on ligand structure. The mechanism of this transformation has not been fully understood yet, but the initial skepticism about oxo-fragment

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formation under stringently anhydrous conditions has gradually been dispelled.

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Pyridine containing dialcohols were used earlier as ligands for the synthesis of titanium complexes [12] and stabilized germylenes and stannylenes [13]. In this work three pyridine

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dialcohols with voluminous substituents, 1 [14], 2 [15], and 3 [16] were used for the synthesis of

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titanium(IV) complexes (Scheme 1).

Scheme 1. Structures of ligands 1-3.

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Our goal was to investigate how switching from pyridine to 2,2’-bipyridine fragment will

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influence the structure and reactivity of titanium complexes. From structural viewpoint the additional nitrogen donor atom can lead to more effective stabilization of complexes in the

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monomeric form which is essential for ring-opening polymerization (ROP) catalysis in accordance with the «single-site catalyst» concept. For the synthesis of 5 we used a method which is similar for the synthesis of known compounds 1 and 2. The method somewhat differed from that used for the synthesis of 4 and 5 by Kellogg and co-workers [17]. Our approach

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includes two-step synthesis with isolation of intermediate product 4 and the application of nBuLi as lithiating agent instead of LDA (Scheme 2). Isolation of monosubstituted product 4 is useful for subsequent modifications.

Scheme 2. The synthesis of ligand 5. Compounds 4 and 5 were isolated as white solids in moderate yields. The structure of tetradentate ligand 5 in the solid state was investigated by X-ray analysis. This molecule exists in 2

ACCEPTED MANUSCRIPT transoid conformation with strong intramolecular hydrogen bonds (Fig. 1; Supporting

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Information, Table S1).

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Fig. 1. Molecular structure of ligand 5. Selected bond length (Å): d(O···N)= 2.738(2) Å.

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Transesterification reaction [3a, 12, 18] was carried out between corresponding ligands 13 and Ti(O-i-Pr)4 in toluene at room temperature to synthesize complexes 1a-3a (Scheme 3).

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These compounds were obtained in analytically pure form without admixtures of the corresponding bis-ligated species due to more rigid structure of the ligands. Compounds 1a-3a

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were obtained in a good yield as air- and moisture sensitive compounds.

Scheme 3. The synthesis of titanium complexes 1a-3a.

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ACCEPTED MANUSCRIPT The structures of complexes 1a-3a were established on the basis of NMR spectroscopy. The 1H and

13

C NMR spectra of these compounds contain one set of ligand signals which is

typical for such compounds; the protons of CH2 groups appear as singlets. This situation is common for monomeric structures with trigonal bipyramidal Ti atom with C2v (1a, 2a) or Cs (3a)

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molecular symmetry structure (or in fast exchange between different possible geometries). Only

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in the case of 3a the methyl groups are diastereotopic due to nonsymmetrical ligand nature (two

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signals of Me groups in 13C NMR).

In contrast to the synthesis of 1a-3a, the reaction between of Ti(O-i-Pr)4 and bipyridine ligand 5 under similar conditions (room temperature, toluene or dichloromethane) resulted in a

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mixture of several compounds (according to NMR), including the expected di-isopropoxy complex 5a and another bipyridine containing species (approximate ratio 2:1). The attempts to

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recrystallize this mixture in absolutely anhydrous conditions from toluene resulted in decreasing the quantity of 5a. The final product in this reaction is titanyl derivative 6 (Scheme 4). The formation of complex 5a can be clearly observed, but the attempts to obtain it in analytically

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pure form were unsuccessful. Furthermore, compound 6 may be obtained in moderate yield

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under prolonged reflux of the reaction mixture.

Scheme 4. Synthesis of titanium complex 6.

The formation of compound 6 may be explained by thermal decomposition of intermediate product 5a which is caused by the ligand structure. The appearance of additional oxygen atom under anhydrous conditions in titanium complexes based on polydentate ligands [1] has been observed earlier [19]. In the case of complex 6 the elimination of propene was detected by gas chromatography - mass spectrometry which is similar to known reaction [11]. According to the literature, titanyls complexes are not sufficiently studied [20]. In the precedent works Ti=O fragment is also stabilized by organic ligands of porphyrine or phtalocyanine types, though there are some examples of Ti=O fragments in the presence of cyclopentadienyl organic framework [21]. Synthesis of titanyl complexes based on 4

ACCEPTED MANUSCRIPT phtalocyanines can be carried out by the reaction of ligands with TiCl3, TiCl4, Ti(O-n-Bu)4 or Ti(O-i-Pr)4 in high boiling solvents in the presence of urea [22] or without solvent [23]. The following scheme of the formation of 6 was proposed. The formation of derivative

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5a is followed by its decomposition into two molecules of propene and dihydroxytitanium

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complex, then the last complex decomposes yielding a molecule of water and complex complex 6 (Scheme 4). The molecular structure of complex 6 was established by X-ray analysis (Fig. 2;

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Supporting Information, Table S1).

Fig. 2. Molecular structure of 6. Solvated CH2Cl2 molecules and hydrogen atoms are omitted.

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Selected bond lengths (Å) and angles (°): Ti(1)-O(3) 1.6691(15), Ti(1)-O(2) 1.8565(15), Ti(1)O(1) 1.8551(15), Ti(1)-N(2) 2.2073(17), Ti(1)-N(1) 2.2088(16); O(3)-Ti(1)-O(1) 112.17(7), O(3)-Ti(1)-O(2) 112.09(8), O(2)-Ti(1)-N(2) 84.04(6), O(1)-Ti(1)-N(1) 84.16(6), N(2)-Ti(1)N(1) 72.42(6), O(1)-Ti(1)-O(2) 97.42(7). The average bond lengths Ti-O (1.856(2) Å) and Ti←N (2.208(2) Å) are very close to the values found in the related Ti complex, [2,6-Py(CPh2O)2]2Ti (1.890(1) and 2.161(2) Å, respectively) [12]. At the same time, Ti=O distance is very close to that in phthalocyanine derivative, [(Me4C12H6N)4N4]Ti=O (1.669(2) vs. 1.619(8) Å) [22]. Coordination polyhedron of titanium atom is a square pyramid (torsion factor (τ) [24] is 0.03) with two nitrogen atoms and two oxygen atoms of the ligand at its base and oxygen atom О(3) at the vertex. In this sense the structure is similar to phthalocyanine titanyls mentioned above. Two pyridine rings are almost coplanar; furthermore, titanium, nitrogen atoms and carbon atoms C(12), C(22) are also in the 5

ACCEPTED MANUSCRIPT plane. The chelate six-membered cycle N-C-C-C-O-Ti adopts bath conformation with Ti and C(12)/C(22) (CH2) atoms as flaps. To the best of our knowledge, compound 6 is the first example of molecular titanium

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alkoxide with double Ti=O bond based on tetradentate chelate ligand (compare with nonchelate

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(NC5H4-4-NC4H8)2(2,6-i-PrC6H3O)2Ti=O [25] or bidentate (t-BuNC(Ph)NBu-t)2(Py-O)Ti=O [26]). Usually, the formation of Ti-O-Ti fragment is observed. The possibility for the formation

groups and high electron donating properties).

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of such bond in 6 is explained by the ligand structure (geometrically rigid, sterically voluminous

Compound 6 is more stable than complexes 1a-3a, since it can be kept for some time in

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the air without decomposition. Also, it is readily soluble in polar organic solvents (CH2Cl2, THF)

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and has poor solubility in toluene or hexane.

The structure of 6 in the solution corresponds to the structure established in the solid state. The presence of tight Ti←N coordination bond results in diastereotopic protons of Ph and

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CH2 groups, which proves the monomeric structure with Cs symmetry.

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Thus, there is a correlation between the structure of the ligand and the type of the complex. In the case of pyridine containing dialcohols titanium complexes, containing two 13

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isopropoxy groups (according to the NMR 1H and

C spectroscopy) are formed. When

incorporating additional nitrogen donor atom and changing the ligand to bipyridine dialcohol titanyl complex is formed, which is the first example of mononuclear titanyl structure based on

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organic ligand with ONNO surrounding. There are some examples when changes in the ligand’s structure led to the formation of different types of titanium complexes [27]. Complexes 1a and 3a were tested as initiators in the polymerization of ε-caprolactone (CL) and L-LA, respectively (Table 1). In the case of 1a the ROP of CL started only at elevated temperatures. Complex 3a revealed moderate activity in this process, giving polylactide with low polydispersity (1.1). Practically full conversion was observed within 2 h. There is a good agreement between NMR and GPC data. All these data indicate highly controlled character of polymerization, especially for L-lactide. Table 1. Ring-opening polymerization of lactones using 1a and 3a as initiators. initiator

monomer

t, [h]

[M]/[cаt]

conversion,

Mn(GPC),

Mn(NMR),

Mn(theor),

Ne

Mw/Mn

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CLf

19

300:1

85

7190

11400

29166

1.4

4.0

CLg

18

600:1

60

10100

10260

41150

1.8

4.1

LAh

0.5

50:1

80

2540

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[g/mol]c

2480

5820

1.1

2.3

97

3182

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3a

[g/mol]b

7483

1.1

2.4

1.5 a

3100

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1a

[%]a

Obtained from 1H NMR spectroscopy for crude reaction mixture; bCalculated according to the equation Mn= 0.58×Mn(GPC) for

polylactide and Mn= 0.56×Mn(GPC) for polycaprolactone; cCalculated using 1H NMR spectra: Mn= I(CH)PLLAMw(LA) + Mw(iPrOH) or Mn= I(OCH2)PCLMw(CL) + Mw(i-PrOH);

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Mn(theor)= Mw(LA)[LA]o/[cat](conversion) + Mw(i-PrOH) or

Mn(theor)= Mw(CL)[Cl]o/[cat](conversion) + Mw(i-PrOH); eNumber of polymer chains per catalyst molecule, calculated as o

C; hPolymerization of L-lactide (LA) in bulk at 100°C.

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N=Mn(theor)/Mn(exp); fPolymerization of ε-caprolactone (CL) in bulk, 100 oC; gPolymerization of ε-caprolactone in bulk, 130

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The catalytic activity of the complexes obtained in ROP of cyclic esters is similar to the related titanium derivatives based on alkanolamines [6a, 28], and somewhat lower than for Ti complexes based on chelating phenols [6b, 29]. The molecular masses of polylactide are in good

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correlation with the calculated ones in assumption that each initiator molecule, 3a, initiates the

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growth of two polymer chains (i.e. the polymerization process begins with the O-i-Pr groups). In CL polymerization, each catalyst molecule, 1a, initiates the growth of four polymer chains per

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catalyst molecule. In this case, alkoxide groups of the ligand are also involved in the growth of polymer chains (that is evident by NMR of the polymers [6a, 16]), but significant chain transfer

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side reactions are observed what is evident from comparison of calculated and found Mn values. So, in this work it was established that ligand structure determines the Ti complex structure. Application of dialcohol based on 2,2’-bipyridine results in titanyl complex. New titanium complexes based on 2,6-bis(hydroxyalkyl)pyridines may be used as active initiators of ROP of cyclic lactones. Further research concerning ligand design and polymerization activity of Ti complexes is in progress.

Acknowledgments X-ray diffraction studies were performed at the Centre of Shared Equipment of IGIC RAS. We thank Russian Fund for Basic Research (12-03-00206) for financial support. This work in part was supported by President Grant for Young Russian Scientists (MD-3634.2012.3) and by M. V. Lomonosov Moscow State University Program of Development. 7

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The structure of the ligand determines the nature of the titanium complex formed. Substituted give

typical

complexes.

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2,6-bis(hydroxyalkyl)pyridines

On

contrary,

(2,2’-bipy-6,6’-

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(CH2Ph2OH)2) results in complex with Ti=O bond.

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights

-O,N,N,O-type ligand, 2,2’-bipy-6,6’-(CH2Ph2OH)2, gives titanyl complex

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-rare example of Ti complex with Ti=O bond is obtained

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-titanium complexes based on substituted 2,6-bis(hydroxyalkyl)pyridines are obtained

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-titanium complexes are active initiators of ROP of lactones

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