Synthesis and intramolecular electronic interactions of hexaarylbenzene bearing redox-active Cp*(dppe)Fe-C≡C- termini

Accepted Manuscript Synthesis and intramolecular electronic interactions of hexaarylbenzene bearing redox-active Cp*(dppe)Fe-C≡C- termini Yuya Tanaka, Munetaka Akita PII:

S0022-328X(18)30758-7

DOI:

10.1016/j.jorganchem.2018.10.002

Reference:

JOM 20587

To appear in:

Journal of Organometallic Chemistry

Received Date: 30 August 2018 Revised Date:

1 October 2018

Accepted Date: 4 October 2018

Please cite this article as: Y. Tanaka, M. Akita, Synthesis and intramolecular electronic interactions of hexaarylbenzene bearing redox-active Cp*(dppe)Fe-C≡C- termini, Journal of Organometallic Chemistry (2018), doi: https://doi.org/10.1016/j.jorganchem.2018.10.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Abstract Hexaphenyl- (1) and hexathienyl-benzene (2) complexes with the six redox-active Cp*(dppe)Fe-C≡C- units are synthesized and their properties in terms of molecular Both complexes show stepwise multi-redox processes and

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junction are investigated.

IVCT bands in the NIR region upon progressive oxidation from the neutral to hexacationic state, as a result of intramolecular electron transfer processes via troidal conjugation through the peripheral aromatic rings. Judging from the KC and Vab values

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obtained by deconvolution analysis, electronic interaction for thienylene derivative 2 turns out to be stronger than that for the phenylene complex 1. DFT analysis of model complexes support superior electron transfer properties for the thienyl derivatives.

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Keywords: molecular junction, metal acetylide, hexaarylbenzene.

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Graphical abstract

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Revised (JORGANCHEM_2018_316)

Synthesis and Intramolecular Electronic Interactions of Hexaarylbenzene

Yuya Tanaka,* Munetaka Akita,*

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Bearing Redox-Active Cp*(dppe)Fe-C≡C- Termini

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Laboratory for Chemistry and Life Science, Institute of Innovative Research

Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan *E-mails: [email protected] [email protected].

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FAX: +(81)-44-924-5230.

Dedicated to Prof. Armando J.L. Pombeiro on the occasion of his 70th birthday.

Introduction

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Compounds bearing branched structures with redox-active peripherals, including metallodendrimers, have played important roles in materials sciences such as molecular electronics, sensing, and multielectron redox catalysis.[1] In particular, those having

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electronic interactions between the remote redox-active units are attractive not only due to being electron reservoir but also due to showing interesting multi-step redox processes via electron/hole transfer between the peripheral redox sites, leading to

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promising components for single molecule devices.[2] To date, in contrast to 1D electron transport systems, i.e. molecular wires [3,4], only limited examples of 2D and 3D multidimensional redox-active systems have been reported.[5] Hexaarylbenzene (HAB) is one of the promising scaffolds to construct

multi-dimensional molecular wiring due to its unique structure with six peripheral aromatic rings (Figure 1).[6-8] Electrons and holes may be transported through ccumulative π-π interactions, between the peripheral aromatic rings, so-called “toroidal conjugation”.[6a]

The orthogonal arrangement of the peripheral aromatic rings with

respect to the central benzene core and the facile synthesis allow us to construct relatively large architectures with better processability compared to those based on 1

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planar polyaromatic hydrocarbon systems. Despite its fascinating structural motif, poor electronic interactions have been found for the systems combined with redox-active metal termini.[8] Systematic studies on derivatization of the hexaarylbenzene peripherals is needed for further improvement of molecular junction systems. Herein, disclose

synthesis

and

properties

of

hexa(ethynylphenyl)-

(1)

and

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we

hexa(ethynylthienyl)-benzene complexes (2) with the highly redox active Cp*Fe(dppe)

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fragments (Figure 1).

Figure 1. Structures of hexaarylbenzene junctions 1 and 2 bearing the redox-active

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Cp*Fe(dppe) metal termini.

Results and Discussion Synthesis and Characterization: Hexaphenylbenzene complex 1 was prepared by reaction of the corresponding terminal alkyne and Cp*Fe(dppe)Cl in the presence of a PF6 salt to give vinylidene intermediates, which were further deprotonated by t-BuOK (Scheme 1). In contrast, hexathienylbenzene complex 2 was synthesized by Co2(CO)8-catalyzed cyclotrimerization of the diiron-substituted dithienylacetylene precursor 3 [9] (Scheme 1). The complexes were fully characterized by 1H NMR,

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NMR, IR, and MALDI- or ESI-TOF-MS spectroscopy. Single but slightly broad Cp* (1H-NMR) and dppe signals (31P-NMR) observed for 1 and 2 reveal highly symmetrical 2

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structures in solution. Complex 2 shows two sets of broad thiophene signals presumably due to hindered rotation of the thiophene rings.[10] IR vibrations for the C≡C parts in 1 and 2 are located at 2053 and 2032 cm–1, respectively. The value of 1 is similar to those of Cp*Fe(dppe)C≡CC6H5 (2053 cm–1),[11] while the value of 2 is slightly shifted to

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lower energies compared to that of Cp*Fe(dppe)C≡CC5H2S (2040 cm–1).[12] Complex 4, a dinuclear analogue of complex 1 prepared in a manner similar to 1, was characterized crystallographically (Figure 2).[13] A propeller-like conformation of the peripheral phenyl rings is noted for complex 4, and the dihedral angles between the

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central and the peripheral benzene rings range from 60˚ to 71˚, which are similar to those observed for related organic counterparts.[14] No significant distortion was observed for the organic skeleton, indicating that bulky Cp*(dppe)Fe-C≡C- units do not

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strongly affect the structure of the central organic skeleton. Distances between the ipso carbon atoms of the peripheral benzene rings, through which electronic communication may take place, are around 2.90 Å and within the range of van der Waals contacts.[14]

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The through-space Fe-Fe distance is found to be 10.90 Å.

Scheme 1. Synthesis of 1 and 2.

3

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P Fe

Fe

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P

P

Fe

Fe

4

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P

Figure 2. The molecular structure and an ORTEP drawing of 4.

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Electrochemistry: Electrochemical measurements were performed to investigate the electronic interaction between the iron centers. Hexanuclear complexes 1 and 2 show quite broad CV waves in the range from −500 to −900 mV (vs. FeCp2+/FeCp2 couple), which can be assigned as FeII/FeIII redox couples. Since these broad waves are the sum of the six electron redox processes, differential pulse voltammetry (DPV) measurements were performed to resolve the complicated redox events (Figures 3a,b; bottom). DPV

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charts of complexes 1 and 2 contain broad unsymmetrical waves which are deconvoluted into a couple of redox waves. Good fitting curves were obtained for complex 1 when the redox events are assumed to occur with successive 1e−, 1e−, 1e−, and 3e− oxidation processes (Figure 3a). Similarly, DPV curve fitting analysis for 2

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revealed occurrence of successive 1e−, 2e−, and 3e− processes (Figure 3b, c). It is notable that the last three oxidation processes occurred at once. Taking symmetry and

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electrostatic repulsion into consideration, in the tricatinic species shown in Figure 3d, the three cationic centers are located at the meta positions with each other to avoid electrostatic repulsion. KC values, which show thermodynamic stability of mixed valence state, were calculated for each redox process on the basis of the differences of the electrochemical potentials (∆E), and the results are summarized in Table 1, although absolute values of the estimated ∆E and KC should contain some errors. The fourth, fifth, and sixth oxidation processes cause generation of the cationic centers at the positions between the adjacent two cationic centers. Because the electrostatic interactions caused by the last three oxidation processes may be significant and similar when compared with the first three oxidation processes, which generate the cationic centers at the meta4

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or para-positions, the last three processes should occur at the same potential. In other words, tricationic species are relatively stable compared to the tetra-, penta-, and

Figure 3.

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hexa-cationic species of the other oxidation states.

CV (top) and DPV (bottom) traces of (a) 1 and (b) 2. Observed curves (bold lines),

deconvoluted curves (plane lines), and sum of deconvoluted curve (dotted lines); [complex]

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~1.0 mM in CH2Cl2, electrolyte: [Bu4N][PF6]= 0.1 M, scan speed: 100 mV s–1 (CV) and 20 mV s–1 (DPV). (c) A proposed oxidation sequence and (d) a proposed oxidation state for tricationic

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species.

Table 1.

complex

1 2

Electrochemical data and determined KC values.a ∆E1

∆E2

∆E3

∆E4

∆E5

(KC1)/

(KC2)/

(KC3)/

(KC4)/

(KC5)/

mV

mV

mV

mV

mV

–727 (E1; 1e–), –676 (E2; 1e–),

51

45

84

~0

~0

–632 (E3; 1e–), –548 (E4; 3e–)

(7)

(6)

(26)

(≥4)

(≥4)

–690 (E1; 1e–), –591 (E2 &

99

~0

113

~0

~0

E1/2 v.s. Fc/Fc+ (mV)b

5

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E3; 2e–), –478 (E4; 3e–) a

(47)

(≥4)

(82)

(≥4)

(≥4)

KCn = exp(F/RT×∆En), where ∆En = E1/2n+1- E1/2n. b E1/2 = (Epc + Epa)/2, where Epc and

Epa were determined by oxidative and reductive DPV measurements. Spectroscopic study of neutral and cationic species: The efficiency of electron

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transfer processes can be evaluated by the electronic coupling (Vab), which can be determined on the basis of intervalence charge transfer (IVCT) bands observed for the mixed valence species in the NIR region. Judging from the electrochemical data with the very small KC values, isolation of each mixed valence species should be difficult.

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Thus, cationic species of [1]n+ and [2]n+ was generated in situ by mixing the neutral and hexacationic complexes with varying the [1] : [1]6+ ratio. We, therefore, prepared hexacationic species [1]6+ and [2]6+ by chemical oxidation of the neutral complexes with

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5.9 eq. of [FeCp2][PF6]. The hexacationic complexes [1]6+ and [2]6+ show the ν(C≡C) vibrations at 1988 and 1950 cm–1 and at 1952 cm–1, respectively. UV-vis absorption spectra of the neutral species 1 and 2 show bands with the peak maxima at 381 and 431 nm, respectively, which are attributed to metal to ligand charge transfer bands (Figures 4a,c).[15] In addition, shoulder bands extending to ~700

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nm are also observed for both complexes. The hexacationic complex [1]6+ shows a new absorption band at 703 nm, which could be attributed to a ligand to metal charge transfer band. An absorption band in the NIR region is observed around 1847 nm (Figure 4b). This NIR band could be ascribed to the ligand field (LF) transition usually

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observed for Fe(III) species. Complex [2]6+ shows a LMCT and a LF band at 792 and 1769 nm, respectively. UV-vis-NIR spectra of the mixed-valence species generated in situ by mixing the neutral and hexacationic complexes with varying the [1] : [1]6+ ratio

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were measured. The sum of the concentrations of the two complexes was kept constant at 2.0 × 10–5 M. For complex 2, UV-vis-NIR spectra of the mixtures with various mixing ratios are shown in Figures 4c,d. Upon increasing the content of [2]6+, the broad bands in the NIR region emerged, and reached the maximum at the mixing ratio of 2 : [2]6+ = 1 : 1.

Since further addition of [2]6+ induces disappearance of the bands, the

new NIR bands should include the IVCT transition bands. To estimate the electronic coupling Vab for the tricationic species, we followed the procedure reported by Lambert.[6c] While the Marcus-Hush theory is frequently used to determine Vab for dinuclear mixed-valence system, for the multinuclear

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mixed-valance systems, the generalized Mulliken-Hush treatment is applied.[16] In principle, the IVCT bands should result from three transitions; transitions from the neutral metal fragments located at the ortho, meta, and para positions with respect to the oxidized iron fragment, to the oxidized iron fragment. To estimate those electronic

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couplings for tricationic species, one has to solve a 20 x 20 secular determinant (see the supporting information for details), which is too complicate to be solved. Therefore, for simplicity, the following assumptions were adopted: (1) Vab(ortho) values of the tricationic species [1]3+ and [2]3+ are significantly larger than those of Vab(para) and

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Vab(meta) because of the shortest distance between the redox-active metal centers located in the ortho-positions compared to those in the meta- and para-positions. and, thus, Vab ~ Vab(ortho), i.e. Vab(para) and Vab(meta) are negligible,[6c, 17] and (2) IVCT

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transitions arise from single electron transfer processes. Taking these assumptions into consideration, solving the 7 x 7 secular determinant leads to 6 degenerated excited states (see the supporting information for details). Therefore, the Vab value obtained by the Marcus-Hush treatment has to be divided by the symmetry factor (√6). In addition, we also consider contribution of the trications in solution ([1]3+: 72 %, [2]3+: 82%) to estimate Vab.[19] Because the IVCT and LF transition bands were ovelapped,

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deconvolution analysis of the NIR bands of a 1 : 1 mixture of the neutral and hexacationic species was performed (Figure 5, Table 2). For the benzene derivatives, deconvolution analysis of the NIR bands afforded three Gaussian bands, and band A located at 6100 cm–1 is ascribed to the IVCT band. On the basis of the band, Vab value

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of [1]3+ is determined to be 0.014 eV. The thienyl complex [2]/[2]6+ shows the three Gaussian bands and the dominant curve (band A) with the peak maximum at 5800 cm–1

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with a half height width of 3800 cm–1 is assigned as an IVCT band. The band around 5627 cm–1 (band B) is assigned as a LF transition band. The half height width of the band A is close to that (3660 cm–1) calculated as a class II compound according to the Robin-Day classification. On the basis of this curve, the Vab value was determined to be 0.052 eV. The analysis and method used for the estimation of Vab values contain non-negligible assumption, simplification and experimental errors, making the exact determination of Vab values difficult. Nevertheless, it is clearly demonstrated that the electronic interactions for complex 2 are stronger than those for complex 1.

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Figure 4. UV-vis-NIR spectra of mixtures of (a and b) 1 and [1]6+[PF6]6, and (c and d) those of

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2 and [2]6+[PF6]6 in CH2Cl2. [total complex] = 2.0 x 10–5 M.

Figure 5. Deconvoluted NIR spectra of mixtures of (a) 1 and [1]6+[PF6]6 (1 : 1) and (b) 2 and [2]6+[PF6]6 (1 : 1) in CH2Cl2. [total complex] = 2.0 x 10–5 M.

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Table 2.

Deconvoluted NIR spectral data and calculated Vab values.

εmax /

νmax /

ν1/2exp /

ν1/2calc /

assigned

M-1cm-1

cm-1

cm-1

cm-1

A

IVCT

500

6100

3800

B

LF

250

5400

1150

C

IVCT

100

9500

3800

A

IVCT

6600

5800

3800

B

LF

460

5627

1127

C

IVCT

5350

10492

3800

transition

23+

Vab / eV (cm-1)

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13+

band

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complex

3660

0.014 (115)

0.052 (422)

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Vab = (2.55 × 10-4)(νmax×εmax×ν1/2exp)1/2(r×√6×X)–1 (eV), where r values were 10.9 Å for 1 (from the crystal structure of 4) and 9.26 Å for 2 (from a DFT optimized

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structure of 5’), and X is contribution of the tricationic species. DFT calculation: In order to get insight into the enhanced electronic interactions for the thienyl derivative 2, DFT calculations were performed for the simplified ortho-substituted diiron HAB derivatives 4’ (phenylene) and 5’ (thienylene), in which

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Cp* and dppe ligands on the iron fragments were replaced with the Cp and dmpe ligands, respectively (Figure 6). For the optimized structures, the iron-iron distance of 5’ LUMOs of 4’ and 5’ are nearly

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(9.26 Å) is somewhat shorter than that of 4’ (9.95 Å).

degenerated and mainly localized on the Ar4C6 parts. HOMO and HOMO–1 of 4’ are closely located in energy and are extended to the two iron centers. The orbitals are unequally distributed over the two Fe-C≡C-C6H4 moieties. By contrast, HOMO and HOMO–1 of 5’ are almost fully and equally delocalized over the two ethynylenethienylene peripherals (Figure 6). Notably, for HOMO–1 of 5’, a clear bonding interaction between the ipso carbon atoms of the Fe-C≡C-C4H2S parts is found. The HOMO and HOMO–1 orbital energies of 5’ are higher than those of 4’. In addition, when the orbital composition of HOMO and HOMO–1 is compared, contribution of the metal fragments for 5’ (HOMO: 37%, HOMO–1: 41%) is smaller than that for 4’

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(HOMO: 48%, HOMO–1: 53%). Therefore, these differences are derived from the electron-rich nature of the thiophene part, which is the origin of efficient through-space interactions. It should be noted that the contribution of the central aromatic rings is very small for both 4’ and 5’, and that metal-metal communication in 5’ being stronger than

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that in 4’ is not due to stronger through-bond interactions but due to the stronger toroidal through-space interactions.

LUMO

HOMO

HOMO–1

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LUMO+1

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4'

–0.456 eV

–0.447 eV

C6H 4 C2 Fe' C6Ph 4

6

1 1 86

10

1 2 71

18 20 41 5

3

C6H 4 C2 Fe'

5

0

12

1

5

16 21 43

1

–0.811 eV C4SH2 C2 Fe' C6(C4H 2S)4

10 1 1 76

C4SH2 C2 Fe'

10 1

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1

2

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5'

–4.193 eV

–0.815 eV

4

7

–4.085 eV

–4.362 eV 4 10 4

–4.183 eV

5

0 1 88

17 11 18 8

14 12 20 7

5

0

17 11 19

14 12 21

1

Figure 6. Frontier orbitals of 4’ (upper) and 5’ (lower). Numbers indicate atomic Mulliken percentages of each fragment. Calculations were performed with B3LYP/LanL2DZ (for Fe) and

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6-31G(d) (for the others) levels of theory.

Fe’ denotes FeCp(dmpe).

Conclusion

Novel design and synthesis of hexapolar hexaphenyl- (1) and hexathienyl-benzene complexes (2) with the six redox active Cp*Fe(dppe) fragments are reported.

The

complex 2 shows the more prominent IVCT bands than those of the phenyl derivative 1, suggesting that electronic communication of hexaarylbenzene can be reasonably modulated by changing the peripheral aromatic rings. Theoretical calculations suggest that interaction through the central benzene rings for both of the benzene and thienyl derivatives are negligible and the stronger electronic interactions for the thienyl 10

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derivatives occur through the enhanced troidal conjugations. To the best of our knowledge, this is the first report on a hexanuclear HAB metal complex with electronic interactions showing IVCT bands. This result provides a new opportunity for the use of

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hexaarylbenzene cores as molecular junctions and programmed coordination systems. Experiment

General Methods. All manipulations were carried out under N2 atmosphere by using standard Schlenk tube techniques. THF, ether, hexane (Na−K alloy), CH2Cl2 (P2O5), and and stored under N2. 1H and 1

spectrometer ( H, 300 MHz; ( H and

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P NMR spectra were recorded on a Bruker AC-300

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P, 121.5 MHz). Chemical shifts (downfield from TMS

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C) and H3PO4 ( P)) and coupling constants are reported in ppm and in Hz,

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1

31

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ROH (Mg(OR)2; R = Me, Et) were treated with the appropriate drying agents, distilled,

respectively. Solvents for NMR measurements containing 0.5% TMS were dried over molecular sieves, degassed, distilled under reduced pressure, and stored under N2. IR and UV−vis−near-IR spectra were obtained on a JASCO FT/IR 5300 spectrometer and a JASCO V-570 spectrometer, respectively. Mass spectra were recorded on ThermoQuest Finnigan LCQ Duo (ESI), micrOTOF II (ESI-TOF), Bruker

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UltrafleXtreme (MALDI-TOF) and JEOL JMS-700 mass spectrometers (FD/FAB), respectively. For cationic complexes, M stands for the molecular weight for the cationic parts. Electrochemical measurements were made with a BAS 100B/W analyzer.

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Crystals of 4 suitable for X-ray crystallography was obtained by recrystallization from toluene-pentane, and crystallographic data for 4 has been deposited at The Cambridge

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Crystallographic Data Centre (CCDC No. 1863732). [13] Preparation of (Cp*(dppe)Fe-C≡ ≡C-C6H4)6C6 (1) : A mixture of (H-C≡C-C6H4)6C6 [20] (55.4 mg, 0.816 mmol), Cp*Fe(dppe)Cl[21] (408.0 mg, 0.653 mmol), KPF6 (120.2 mg, 0.653 mmol) dissolved in MeOH (30 mL) and THF (6 mL) was stirred at room temperature for 17 hours under N2 atmosphere. The volatiles were removed under reduced pressure, the residue was washed with diethyl ether (5 ml × 3), and then dried under vacuum. To the obtained mixture dissolved in THF (10 mL) was added tert-BuOK (88.0 mg, 0.784 mmol) in one portion, and the mixture turned into red after being stirred at room temperature for 30 min. The volatiles were removed under reduced pressure and the residue was extracted with toluene, and filtered through a 11

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Celite® plug. The filtrate was dried under vacuum to afford red solid, which was washed with diethyl ether (10 mL × 2) to give complex 1 (267.8 mg, 0.0636 mmol, 78 % yield). analysis: found (calcd. for C270H106Fe2P12 (1)): C, 77.03 (76.55), H, 6.18 (6.31), N 0.00 (0.00); 1H-NMR (300 MHz, C6D6): δH 6.60-7.45, 7.80-8.10 (m, dppe-Ph and C6H4, (121.5 MHz, C6D6): δP 100.8 (s, dppe), IR (KBr) 2049 cm +

(m/z) 4210 (M +H).

–1

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P-NMR

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144H), 1.70-2.00, 2.50-2.75 (m, dppe-CH2, 24H), 1.54 (s, η5-C5Me5, 90H),

(νC≡C); MALDI-TOF-MS

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(Cp*(dppe)Fe-C≡C-(2,5-C4H2S)-C≡C-(2,5-C4H2S)-C≡C-Fe(dppe)Cp*) (3)[9] : The titled complex was reported previously, but we have synthesized 3 in a manner different from

the

report.

To

a

mixture

of

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TMS-C≡C-(2,5-C4H2S)-C≡C-(2,5-C4H2S)-C≡C-TMS[22] (206.7 mg, 0.540 mmol) dissolved in MeOH (20 mL) and THF (8 mL) was added 0.5 M KOH aq. (17 mL) and the reaction mixture was stirred for 20 min at r.t. After the reaction completion was confirmed by TLC, the mixture was quenched by H2O. The organic layer was extracted with pentane, dried over Na2SO4 and evaporated to give an yellow oil, which was used to the next reaction without further purification. To a mixture of the obtained oil and

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Cp*Fe(dppe)Cl (674.9 mg, 1.08 mmol), KPF6 (198.8 mg, 1.08 mmol) was added MeOH (40 mL) and THF (8 mL), and the mixture was stirred at r.t. for 16 hours. The volatiles were removed under reduced pressure, and the obtained residue in THF (20

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mL) was treated with tert-BuOK (152.0 mg, 1.36 mmol). The reaction mixture was stirred for 1 hour. The volatiles were removed under reduced pressure and the residue was extracted with dichloromethane and filtered through a Celite® plug. The filtrate was

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concentrated and hexane was added to give precipitates, which were collected, washed with hexane, and dried under vacuum to give 3 as a red solid (578.0 mg, 0.409 mmol, 76% yield). analysis: found (calcd. for C88.2H86.4Cl4.4Fe2P4S2 (3·(CH2Cl2)2.2)): C, 65.94 (66.12), H, 6.16 (5.44), N 0.00 (0.00); 1H-NMR (300 MHz, C6D6): δH 1.47 (30H, s, C5Me5), 1.50-1.90, 2.47-2.63 (8H, m, C2H4), 6.49 (2H, brs, C4SH2), 6.80-8.15 (44H, m, C6H5);

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P-NMR (300 MHz, C6D6): δP 100.1; IR(KBr) 2026, 2171 cm–1 (νC≡C);

HR-ESI-TOF-MS (m/z) Found (Calcd. for C256H246Fe6P12S6 [M]+): 1414.3502 (1414.3508). Preparation (Cp*(dppe)Fe-C≡ ≡C-C4H2S)6C6 (2) : A mixture of 3 (148 mg, 0.105 12

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mmol), and Co2(CO)8 (10.7 mg, 0.0314 mmol) dissolved in dioxane (10 mL) was refluxed for 7 hours under N2 atmosphere. After the completion of the reaction was checked by IR spectroscopy, the volatiles were removed under reduced pressure and the residue was extracted by dichloromethane. The extract was passed through a Celite®

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plug, and the filtrate was concentrated and the product was precipitated by addition of hexane. The precipitate was washed with hexane and dried under vacuum to give a brown solid 2 (124.4 mg, 0.0293 mmol, 84% yield). 1H-NMR (300 MHz, C6D6): δH 1.54 (90H, s, C5Me5), 1.40-2.00, 2.43-2.85 (24H, m, C2H4), 6.52, 6.59 (6H x 2, brs, 2032 cm

–1

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P-NMR (300 MHz, C6D6): δP 100.7; IR(KBr)

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C4H2S), 6.80-8.15 (126H, m, C6H5) ;

(νC≡C); MALDI-TOF-MS (m/z): 4247 (M++H); HR-ESI-TOF-MS Found

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(Calcd. for C256H246Fe6P12S6 [M]+): 4245.0633 (4245.0599). o-(Cp*(dppe)Fe-C≡C-C6H4)2C6(C6H5)4 (4)

Complex 4 was prepared by metalation of o-(H-C≡C-C6H4)2C6Ph4, which was obtained by desilylation of o-(TMS-C≡C-C6H4)2C6Ph4. o-(TMS-C≡C-C6H4)2C6Ph4:

To

a

mixture

of

1,2-di(4-bromophenyl)-3,4,5,6-tetraphenylbenzene [23] (143.5 mg, 0.207 mmol),

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Pd(PPh3)2Cl2 (14.5 mg, 0.0207 mmol), CuI (7.9 mg, 0.414 mmol), and PPh3 (10.8 mg, 0.0414 mmol) in a glass autoclave purged with N2 was added degassed toluene (15 mL) and NEt3 (30 mL) and the mixture was heated at 100˚C for 12 hours in the closed system. The mixture was allowed to be cooled to room temperature and filtered through

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a short Celite® plug, and the filtrate was evaporated to dryness. The residue was subjected to silica gel column chromatography (CH2Cl2 : hexane = 1 : 3) to give the title

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compound as a white powder (136.6 mg, 0.188 mmol, 91% yield). 1H-NMR (300 MHz, CDCl3): δH 6.98 (d, J = 7.3 Hz, C6H4, 4H), 6.65-6.90 (m, aromatic, 24H), 0.19 (s, Si-Me, 18H), FT-IR (KBr / cm–1) 2164 (νC≡C). o-(H-C≡C-C6H4)2C6Ph4: To a mixture of o-(TMS-C≡C-C6H4)2C6Ph4 (136.6 mg, 0.188 mmol) dissolved in THF (5 mL) and MeOH (5 mL) was added NaOH (104.3 mg, 2.67 mmol) in one portion under N2 atmosphere, and the solution was stirred for 4 hours at room temperature. The reaction was quenched by H2O and the organic layer was extracted with diethyl ether. The combined organic layer was dried over Na2SO4. The filtrate was evaporated and the residue was subjected to silica gel column chromatography (CH2Cl2 : hexane = 1 : 3) to give the title compound as a white solid 13

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(105.3 mg, 0.180 mmol, 96％ yield). 1H-NMR (300 MHz, CDCl3): δH 7.01 (d, J = 8.0 Hz, C6H4, 4H), 6.73-6.95 (m. aromatic, 24H), 2.94 (2H, s, C≡C-H), FT-IR (KBr / cm–1) 2105 (νC≡C), 3286 (ν≡C-H). 4: A mixture of o-(H-C≡C-C6H4)2C6(C6H5)4 (105.1 mg, 0.180 mmol), Cp*Fe(dppe)Cl

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(225.4 mg, 0.361 mmol), KPF6 (66.4 mg, 0.361 mmol) dissolved in a mixture of MeOH (20 mL) and THF (4 mL) was stirred at room temperature overnight. The volatiles were removed under reduced pressure and the residue was washed with diethyl ether to give a brown powder. To the residue was added THF (10 mL) and then t-BuOK (56.1 mg,

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0.501) in one portion. The resultant mixture turned into red after being stirred at room temperature for 30 min. The volatiles were removed under reduced pressure, and the residue was extracted with diethyl ether and passed through a short almina pad to give

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crude 4. An analytically pure sample was obtained by crystallization by slow diffusion of pentane into a toluene solution of 4 (94.5 mg, 0.0537 mmol, 30% yield). analysis: found (calcd. for C118H106Fe2P4 (4)): C, 80.13 (80.54), H, 5.87 (6.07), N 0.00 (0.00); 1

H-NMR (300 MHz, C6D6): δH: 6.65-7.40, 7.80-8.10 (m, aromatic, 68H), 1.60-2.00,

2.35-2.75 (m, 16H, dppe-CH2), 1.51(s, 30H, η5-C5Me5), 31P-NMR (121.5 MHz, C6D6):

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δP 100.8; IR (KBr) 2052 cm–1 (νC≡C); FD-MS (m/z) 1759 (M+).

General Procedure for Preparation of Hexacationic Complexes To a THF solution of 1 or 2 cooled at –78˚C was added 5.90 eq. of [FeCp2][PF6] and the

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mixture was stirred at –78˚C for 2 hours. Then, the mixture was allowed to be warmed to r.t. and stirred for an additional 1 hour. To the reaction mixture was added pentane to precipitate the product, which was collected, washed with pentane and dried under

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vacuum to give hexacationic complexes. The purity of cationic complexes was confirmed by CV measurements. [(Cp*(dppe)Fe-C≡ ≡C-C6H4)6C6](PF6)6 (16+) A green powder (92% yield) ; IR(KBr) 1988, 1950 cm–1 (νC≡C), 840 cm–1 (νPF6); ESI-MS (m/z): 701 (M6+); UV-vis-NIR: λmax / nm (10–3 × ε / M–1cm–1) (in CH2Cl2) 321 (sh, 81.8), 412 (sh, 17.0), 600 (8.3), 703 (15.6), 1847 (0.6). [(Cp*(dppe)Fe-C≡ ≡C-C4H2S)6C6](PF6)6 (26+) A green powder (99%) ; IR(KBr) 1952 cm–1 (νC≡C), 840 cm-1 (νPF6); ESI-MS (m/z): 708 (M6+) ; UV-vis-NIR: λmax / nm (10–3 × ε / M-1cm-1) (in CH2Cl2) 267 (132), 380 (sh, 46.0), 476 (sh, 32.6), 792 (32.1), 1769 (1.6). 14

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Calculation of the Equilibrated Concentrations of Hexanuclear Complexes of Different Oxidation States.[6d] The redox equilibria and the equilibrium constant Kn (n = 1-5) in solutions of a mixture of neutral complex 1 and hexacationic complex 16+

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can be presented as follows: (1)

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(2) The equilibrium concentrations of [1]n+ are related to the set of equations as noted above.

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(3) On the basis of eq. (2) (with the equilibrium constants calculated by eq. (3), and taking into account the mass balance, ∑ [1n+]= [1]0 + [1]6+0, and the charge balance, ∑ n[1n+]= 6 × [1]6+0 (n = 1−6), the equilibrium concentrations of all the components were

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numerically calculated with the Mathematica program (Table 3).

Table 3. Numerically calculated ratios of oxidized species. 1/%

Total

12+

13+

14+

15+

16+

2

2+

22+

23+

24+

25+

26+

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charge

1+

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1 a

2/%

0

100

0

0

0

0

0

0

100

0

0

0

0

0

0

1

23

56

20

1

0

0

0

12

77

11

0

0

0

0

2

1

21

54

23

0

0

0

0

25

50

25

0

0

0

3

0

0

14

71

14

1

0

0

0

9

82

9

0

0

4

0

0

1

26

48

22

3

0

0

0

27

49

22

3

5

0

0

0

2

22

49

27

0

0

0

2

22

49

27

15

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6 a

0

0

0

0

0

0

100

0

0

0

0

0

0

100

Total charge represent ([complex]6+0×6)/([complex]0+[complex]6+0).

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Theoretical Calculations

Density functional theory (DFT) calculations were carried out using the Gaussian 09D and 16A3 package [24] for the simplified structures 4’ and 5’. The iron atoms were described with a LanL2DZ basis set of valence double-ζ quality including the

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relativistic effective core potential of Hay and Wadt.[25] The 6-31G(d) split-valence basis set was used for the other atoms. No symmetrical constraint was applied. Geometry optimization was confirmed by frequency analysis, in which no imaginary

Supporting Information

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vibration mode was found for both of the optimized structures of 4’ and 5’.

IR spectral data of [1]n+ and [2]n+(n = 0-6), and the Cartesian coordinates for 4’ and 5’ are included in Supporting information.

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Aknowledgement

This work was supported by JSPS KAKENHI Grant Number 18K05139 and research grants from the Murata Science Foundation and the Kato Foundation for Promotion of Science. The computations were performed by using a computer in the Research Center

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for Computational Science, Institute of Molecular Science, Okazaki.

[1]

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References and Notes

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(1968) 1277–1287. [15] K. Costuas, F. Paul, L. Toupet, J.-F. Halet, C. Lapinte, Organometallics 23 (2004) 2053−2068. [16] A. Heckmann, C. Lambert, Angew. Chem. Int. Ed. 51 (2012) 326-392. rings

(Vab(para)

>

Vab(ortho)

>>

Vab(meta))

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[17] On the contrary to the trend of Vab with respect to the connection of the phenyl for

the

series

of

{Cp*(dppe)FeC≡C}2C6H4,[18] it is demonstrated that Vab(ortho) is larger than Vab(para) for the hexaarylbenzene derivatives.[9b]

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Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, 19

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J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, and D. J. Fox, Gaussian, Inc., Wallingford CT, 2009; (b) Gaussian 16, Revision B.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A. Petersson,

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H. Nakatsuji, X. Li, M. Caricato, A. V. Marenich, J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci, H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov, J. L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini, F. Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V. G. Zakrzewski, J. Gao, N.

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Rega, G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, K. Throssell, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. J. Bearpark, J. J.

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Heyd, E. N. Brothers, K. N. Kudin, V. N. Staroverov, T. A. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. P. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, J. M. Millam, M. Klene, C. Adamo, R. Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma, O. Farkas, J. B. Foresman, and D. J. Fox, Gaussian, Inc., Wallingford CT, 2016.

[25] (a) P. J. Hay, W. R. Wadt, J. Chem. Phys. 82 (1985) 270–283; (b) Hay, P. J.; Wadt,

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W. R. J. Chem. Phys. 82 (1985) 299–310; (c) Wadt, W. R.; Hay, P. J. J. Chem.

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Phys. 82 (1985) 284–298.

20

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Highlights 1) Synthesis of hexaphenyl- and hexathienyl-benzencomplexes bearing the six peripheral redox-active Cp*(dppe)Fe-C≡C- termini

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analyzed by electrochemistry and analysis of IVCT bands

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2) Communication between the peripheral iron centers through toroidal conjugation as