Spectroscopic Study of δ Electron Transfer between Two Covalently

Article pubs.acs.org/JPCC

Spectroscopic Study of δ Electron Transfer between Two Covalently Bonded Dimolybdenum Units via a Conjugated Bridge: Adequate Complex Models to Test the Existing Theories for Electronic Coupling Chun Y. Liu,*,†,‡ Xuan Xiao,‡ Miao Meng,† Yu Zhang,‡ and Mei Juan Han‡ †

Department of Chemistry, Jinan University, 601 Huang-Pu Avenue West, Guangzhou 510632, China Department of Chemistry, Tongji University, Shanghai 200092, China

‡

S Supporting Information *

ABSTRACT: Three symmetrical and one unsymmetrical dimolybdenum dimers, namely, [Mo2(DAniF)3]2(E2CC6H4CE2) (DAniF = N,N′-di(p-anisyl)formamidinate and E = O or S), are structurally and electronically closely related. The mixed-valence cation radicals display well-defined metal to ligand (ML), ligand to metal (LM), and metal to metal (MM) charge transfer absorption bands. Successive thiolations of the complexes result in steady increases of the electronic coupling between the two [Mo2] units. The electronic coupling matrix elements (Hab) calculated from the Hush model fall in the range of 600− 900 cm−1, which are remarkably consistent with the results from the CNS superexchange formalism. Spectroscopic analyses suggest that the intramolecular electron transfer occurs by electron-hopping and hole-hopping in concert. The rate constants (ket) are estimated in the range of 1011−1012 s−1 for the symmetrical analogues and 107 s−1 for the unsymmetrical species. The ultrafast electron transfer in such a weakly coupled system (Hab < 1000 cm−1) is attributed to the d(δ)−p(π) conjugation between the dimetal centers and the bridge.

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as the Creutz−Taube ion, is the first designed D−B−A metal complex model.11,12 With this complex as the landmark, numerous binuclear d 5−6 metal complexes have been synthesized and studied in terms of intervalence transition.13 Pure organic MV compounds have been actively involved in the research of this field since the 1990s.14,15 These works in turn prompted the refinement and development of ET theories. While the Hush model has long been applied to the analysis of IVCT bands for a wide range of electron donor−acceptor systems, an alternate spectral method for quantitatively determining the electronic coupling element has been sought. The so-called “CNS” model was introduced by Creutz, Newton, and Sutin based upon a superexchange formalism.16 According to the CNS theory, when direct coupling between two bridged metal sites M and M′ is neglected, effective metal to metal coupling is dominated by mixing metal to bridging ligand charge transfer (MLCT) and bridging ligand to metal charge transfer (LMCT) states. For a bridged binuclear system M−BL(bridging ligand)−M′, the electronic coupling parameter (denoted by HMM′ to distinguish from Hab of Hush theory) may be calculated from eq 2.

INTRODUCTION For decades, great efforts have been made in understanding and controlling electron transfer (ET) reactions that are ubiquitous in chemical, physical, and biological systems. Today, it remains one of the broadest and most active research areas due to its underlying significance for multidisciplinary research interests such as the elucidation of biochemical process,1,2 the development of molecular electronics,3,4 and the conversion of solar energy.5,6 Exploration in this realm has been relying on the development of synthetic model compounds and construction of theoretical frameworks, thus generating a chemistry-rich interplaying field for experimental and theoretical scientists. The classical model for the fastest ET involves charge transfer from an electron-donor site (D) to an electronacceptor site (A) across the bridge (B). In late 1960s, Hush provided an equation (eq 1) for evaluating the electronic coupling effect between metal-containing donor and acceptor,7,8 which successfully correlates the intervalence charge transfer (IVCT) bands with the thermal ET energy barrier (ΔG*) from the Marcus theory.9,10 Hab = 2.06 × 10−2

(Δν1/2 ̃ εmax E IT)1/2 rab

(1)

Received: June 25, 2013 Revised: August 16, 2013 Published: August 29, 2013

In experimental study, mixed-valence (MV) coordination compound [(NH3)5Ru(pyrazine)Ru(NH3)5]5+, better known © 2013 American Chemical Society

19859

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The Journal of Physical Chemistry C HMM ′ =

HMLHM ′ L H H + LM LM ′ 2ΔEML 2ΔE LM

Article

Scheme 1 (2)

Here, HML and HM′L and HLM and HLM′ are the metal to ligand and ligand to metal coupling elements, respectively, for the two sites M−BL and BL−M′ at the geometries associated with the corresponding metal oxidation level. According to eq 2, the metal to metal coupling element HMM′ is composed of two components, which account for the contributions from MLCT and LMCT. ΔEML and ΔELM represent the effective energy gaps for the corresponding charge transfer, which can be derived from the MLCT, LMCT, and IVCT data from the spectra through eq 3 and 4, if applicable.16,17 ⎛ 1 1 1 ⎞ = 0.5 × ⎜ + ⎟ ΔEML EML ⎠ ⎝ EML − E IT

(3)

⎛ 1 1 1 ⎞ = 0.5 × ⎜ + ⎟ ΔE LM E LM ⎠ ⎝ E LM − E IT

(4)

Testing of the latest theoretical model has been carried out in binuclear ruthenium16,18−20 and organic systems.21 However, significant differences were found in results from the two distinct models. Therefore, understanding the discrepancy between the Hush and CNS methods and further verifying the latest theory require desirable D−B−A complex models. In this report, dimolybdenum “dimers of dimer” have been investigated as complex models to test the existing ET theories through quantitative determination of the electronic coupling matrix elements (Hab or HMM′). Spectral method based on semiclassical theories has been employed to study the ET reaction kinetics. Importantly, a dimetal unit has a well-defined electronic configuration, e.g., σ2π4δ2 for a quadruply bonded Mo2 and W2 unit,22 of which the d orbital degeneracy is diminished. One-electron redox of the dimetal sites and singleelectron transfer across the bridge are explicitly due to behaviors of the δ electrons. Therefore, the interferences from other electrons to the metal associated CT occurrences are precluded. These electronic features make the Mo2−Mo2 system significantly different from other experimetal models, such as Ru−Ru,23 Ru3−Ru3,24,25 Fc−Fc,26 and pure organic systems.14,15,27 In this context, a dimetal unit as the electronic donor or acceptor will facilitate the spectroscopic analysis and conseqently reduce the inaccuracy of the results.

As shown in Figure 1, while all the neutral molecules show a strong metal to ligand charge transfer (MLCT) band, the cations display a well-defined intervalence transition (IT) band (or MMCT). Furthermore, all the thiolated MV speices present an additional band at the low energy side of the MLCT band. In formal sense, the singly occupied δ obital of the oxidized [Mo2] unit has the energy lower than that of the neutral [Mo2] unit. In other words, the acceptor site has a small ligand−metal orbital energy gap. Therefore, this band arises from the ligand to metal charge transfer (LMCT) occurring at the oxidized site. More importantly, the energy, intensity and shape of these charge transfer bands vary largely due to the subtle differences of the bridging lignds. As presented in Table 1, in the series, increasing the sulfur contents generally lowers the charge transfer energy and increases the absorption intensity. The MMCT bands for the terephthalate and dithioterephthalate analogues are essentially Gaussian-shaped. On the contrary, the fully thiolated complex [S2−S2]+ presents an unsymmetrical intervalence transition band with high intensity (Figure 1 and Table 1). It is significant that the MMCT band for the unsymmetrical species [O2−S2]+ has exceptionally high energy (Table 1). This is due to the free energy change for electron transfer from the donor to the acceptor. Determination of the Electronic Coupling Matrix Elements. In weak electronic coupling and high temperature limit, the intervalence transition energy (EIT) is a sum of free energy change (ΔG0) and vibrational reorganization energy (λ).29

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RESULTS AND DISCUSSTION The series under investigation includes three symmetrical and one unsymmetrical complexes, [Mo 2 (DAniF) 3 ] 2 (μO 2 CC 6 H 4 CO 2 ), [Mo 2 (DAniF) 3 ] 2 (μ-OSCC 6 H 4 CSO), [Mo2(DAniF)3]2(μ-S2CC6H4CS2), and [Mo2(DAniF)3]2(μO2CC6H4CS2), hereafter abbreviated as [O2−O2], [OS−OS], [S2−S2], and [O2−S2], respectively (Scheme 1). They were synthesized by assembling two dimolybdenum units [Mo2(DAniF)3]+ (DAniF = N,N′-di-p-anisylformamidinate) with either a terephthalate or a thiolated terephthalate bridging ligand. The enhanced metal−metal interaction for [S2−S2] was reported in the prelimilary communiction.28 Compounds of this series possess the same molecular scaffold and isoelectronic valence shell and thus share a common intramolecular ET pathway over similar ET distances. Single-electron oxidation of the four compounds yielded the corresponding mixed-valence cation radicals, [O2−O2]+, [OS−OS]+, [S2−S2]+, and [O2− S2]+ as confirmed by EPR spectra.

E IT = ΔG 0 + λ

(5)

For the symmetrical complexes (ΔG = 0), λ = EIT. It is interesting to note that with EIT = 4200 cm−1 for [O2−O2]+, increasing the sulfur content of the complex lowers the EIT value by 800 cm−1 (see Table 1). The unsymmetrical cation [O2−S2]+, however, has a EIT value of 6560 cm−1, much higher than those for the symmetrical anlogues. The increased EIT accounts for the internal energy difference ΔG°. In eq 1 or the Mulliken−Hush expression, rab is the ET distance, which is usually determined from the geometrical separation between the two metal centers. Four compounds in this series have very similar [Mo2]···[Mo2] separation, ca. 11− 12 Å. Using these geometrical distances (rab), as the oxygen 0

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generally underestimated because of the inappropriate estimation of the ET distance. For instance, [(NH3)5Ru)]2(4,4′-bpy) has a Hab value of 900 cm−1 based on the effective ET distance (r′ab) of 4.6 Å as determined by the dipole moment change in electroabsorption (Stark effect).33 In a [Mo2] unit, the δ electrons are delocalized over the dimetal coordination shell via d(δ)−p(π) conjugation.34,35 The redox potential is very sensitive to the equatorial ligands.36 Thus, the complex unit as a whole rather than the Mo24+/5+ cation functions as the electron donor or acceptor. In the current system, the same dimetal building block was used for construction of the D−B−A molecules. Therefore, the differences in the electron donating (or accepting) ability of the donor (or acceptor) for the analogues are due to the versatility of chelating groups from the bridging ligands. From this point of view, the real bridge that connects the electron donor and acceptor is the central moiety −CC6H4C−; hence, the effective ET distance (r′ab) is reasonably estimated to be 5.8 Å from the geometrical length of this group. Since r′ab ≈ 1/2rab, the Hab values are enlarged by a factor of 2 (Table 1). For instance, with r′ab = 5.8 Å, [S2−S2]+ has a Hab value of 864 cm−1, close to 900 cm−1 for [(NH3)5Ru)]2(4,4′-bpy).33 In application of the CNS methods, the electronic coupling elements for metal to ligand (HML) and ligand to metal (HLM) are calculated from eq 1 using the centroid to centroid distances (rML) between the metal unit and the phenyl group (Table 2), as suggested by CNS.16 Prior to this work, only one metal−ligand charge transfer state, either MLCT16 or LMCT,19,20 was considered in the calculation of HMM′, which is likely due to the unavailability of full charge transfer data. For example, for electron-hopping pathway, by neglecting the LMCT, the metal to metal coupling elements were determined only by the first term of eq 2. Provided with the unambiguous assignments to the MLCT, LMCT, and MMCT bands, the HMM′ values for the [Mo2−Mo2]+ complexes were calculated using the original CNS forms. The data are shown in Table 2. By doing so, the contributions to HMM′ from MLCT and LMCT are both taken into account. For the symmetrical complexes, the calculated effective energy gaps, ΔEML and ΔELM, are close to but less than the band energies, EML and ELM from for the MLCT and LMCT, respectively, and the variation tendencies are parallel with the experimental data (Table 2). Large deviations between experimental and computational results were found for the unsymmetrical [O2−S2]+, which might have something to do with the intrinsic potential difference between the donor and acceptor. Remarkably, our results show that the magnitudes of HMM′ from the CNS equations are in excellent agreement with the Hab values derived from the Hush model, and Hab/HMM′ is around 1. For [S2−S2]+, the two spectral methods gave exactly the same value, ca. 864 cm−1 (Tables 1 and 2). For [O2−O2]+ and [OS−OS]+, the small deviations of ±40 cm−1 are essentially in the range of experimental error. Such a consistency has not been obtained in other systems since the CNS model was originally proposed.16,18−21 For the unsymmetrical analogue [O2−S2]+, for which Hush model is not applicable, the CNS equations yielded a HMM′ value of 711 cm−1, reasonably smaller than 764 cm−1 for the symmetric isomer. The results indicate that these two different theoretical treatments are in best accordance with each other as full consideration to the CT occurrences is made in calculation of HMM′. Besides, application of effective electron transfer distance (r′ab) for the Mulliken−Hush equation is equally important.

Figure 1. Comparison of the Vis−Near-IR−Mid-IR spectra between the neutral and the MV complexes.

atoms on the bridging ligand are substituted stepwise by sulfur atoms, the calculated Hab values increase from about 300 up to 400 cm−1 (see Table 1). The magnitudes of Hab are comparable with those for [(NH3)5Ru]2[1,4-(NC)C6H4(CN)] (320 cm−1)30 and [(NH3)5Ru)]2(4,4′-bpy) (390 cm−1),16 which have similar metal to metal distances. However, as pointed out in the literature,31,32 the Hab values derived from eq 1 were 19861

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Table 1. Electronic Coupling Matrix Elements (Hab) Calculated from Hush Model compd [O2−O2] [OS−OS]+ [S2−S2]+ [O2−S2]+ +

EIT (cm−1)

εIT (M−1 cm−1)

exptl Δν̃1/2 (cm−1)

calcd Δν̃1/2 (cm−1)

Hab(1) (cm−1)a

Hab(2) (cm−1)b

4240 3440 2640 6560

1470 3690 12660 2270

4410 3290 1770 4130

3190 2820 2470 3890

304 360 410 NA

589 727 864 NA

Values based on the metal to metal separation (rab = Mo2···Mo2, 11−12 Å) determined from the X-ray structures. bValues based on the geometrical length of the central moiety −CC6H4C− (r′ab = 5.8 Å).

a

Table 2. Metal to Ligand and Ligand to Metal Electronic Coupling Parameters Determined from the MLCTa and LMCTb Bands in the Spectra, Respectively, and Comparison of HMM′ with Hab compd

rMLc (Å)

EML (cm−1)

εML (M−1 cm−1)

HML (cm−1)

ΔEML (cm−1)

ELM (cm−1)

εLM (M−1 cm−1)

HLMc (cm−1)

ΔELM (cm−1)

HMM′ (cm−1)

Hab(2)/HMM′

[O2−O2] [OS−OS] [S2−S2] [O2−S2]

5.6 5.8 6.1 5.9

20600 16040 13850 15920

15230 25870 39960 22500

4480 4300 4190 3820

18230 14110 12390 11790

0 12330 10630 12970

0 5445 17500 2780

0 1500 1690 1260

0 10330 9120 8580

551 764 864 711

1.07 0.95 1.00 NA

a

Calculations based on the spectroscopic data of the neutral compounds. bCalculations based on the spectroscopic data of the mixed-valence complexes. cAssuming rLM′ = rML.

hopping. However, in the case of [Mo2 −Mo 2 ] + , the intervalence transition is realized through MLCT and LMCT, which occur simultaneously. By orbital description, the MLCT band is caused by electron transition from δ (Mo2) to empty ϕπ*−BL of the bridge, while the LMCT is due to the electron transition from the filled ϕπ−BL to the singly occupied δ orbital or hole hopping from the δ orbital to the bridge. From this point of view, it is believed that the electron transfer from the donor to the acceptor is accomplished by electron hopping and hole hopping pathways, as described in Scheme 2. On this basis, our results provide a straightforward interpretation of the effective two-state model for the CNS superexchange formalism. Kinetics of the Intramolecular Transfer Reaction. In the semiclassical treatment,7−10 by neglecting the nuclear tunneling effects, the ET rate is calcualted by eq 6,39 in which the electronic factor (κ) is generally assumed to be unity. Since the electronic frequency νel is in the order of 1014 s−1, larger than 2νn (1013 s−1),39 an averaged nuclear frequency (νn = 5 × 1012 s−1) was then adopted for determination of the rate constant (ket).13 The free energy of activation (ΔG*) can be derived from the total reorganization energy λ and the electronic coupling element (HMM′) according to eq 7.40

Mechanism for the Intramolecular Electron Transfer. In the CNS model (eq 2), the second term signifies the contribution of LMCT to HMM′, which varies depending on the nature of the bridging ligand. This is manifested in the present Mo2−Mo2 system. Clearly, the sulfur donor atoms on the bridging ligands are capable of enhancing the MLCT, as reported elsewhere,37,38 and improving the LMCT as well. The fully thiolated [S2−S2]+ exhibits a low energy LMCT band, which gives rise to a large value of HLM and a small value of ΔELM (Table 2). In contrast, in the spectrum of [O2−O2]+, the CT band is slightly blue-shifted, with increased intensity relative to the MLCT band of the neutral complex (Figure 1A). This unusual phenomenon can be rationalized by the presence of a high energy LMCT band that is overlapped with the MLCT band. Nevertheless, the contribution to HMM′ from the unresolved LMCT band was included in the calculation of HML from the MLCT band. In superexchange framework, reactions of electron transfer from donor to acceptor crossing the bridge are generally treated with one of the two pathways, either electron-hopping or holeScheme 2. Schematic Illustration of Superexchange Mechanism for the Electron Transfer from the Donor to the Acceptor via the Bridgea

ket = κνnexp( −ΔG*/kBT )

ΔG2* =

(6)

(λ − 2HMM ′)2 4λ

(7)

The calculated ET rate constants (ket) for the symmetrical series fall in the range of 1011−1012 s−1 (Table 3). The Table 3. Calculated Activation Energies and Reaction Rate Constants for the Intramolecular Electron Transfera ΔG*adia(cm−1)

complex [O2−O2] [OS−OS]+ [S2−S2]+ [O2−S2]+ +

(A) Electron hopping from the δ orbital of the donor to the bridging π orbital; meanwhile, hole hopping from the δ orbital of the acceptor to the high-lying filled bridging π orbital. The MLCT and LMCT correspond to the electron and hole hopping, respectively. (B) A possible intermediate state of electron transfer at which the odd electrons on the ligand are paired in the bonding orbitals through relaxation. (C) Completion of electron transfer. a

a

19862

forward reverse

581 266 79 2430 364

ket(adia) (s−1) 3.0 1.4 3.4 4.1 8.6

× × × × ×

1011 1012 1012 107 1011

Results calculated from electronic coupling elements HMM′. dx.doi.org/10.1021/jp406261w | J. Phys. Chem. C 2013, 117, 19859−19865

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Given ΔG° 2226 cm−1, the activation barrier for the forward reaction (ΔG*f ) is determined to be 2482 cm−1, and 256 cm−1 for the reverse reaction (ΔGr*) calculated from eq 8.39

magnitude of ket increases expectedly as a result of stepwise increase of the sulfur contents. A ket value in the order of 1011 s−1 is found for [O2−O2]+ and 1012 s−1 for [S2−S2]+. These ET rates are comparable with those observed in the Ru3−Ru3 systems,24,40 and those for organic D−B−A systems having conjugated bridges.27 However, in our study, the ultra fast intramolecular electron transfer is observed in the system where the donor and acceptor sites are weakly coupled (2Hab ≪ λ).41 The exceptionally fast ET must be attributed to the d(δ)−p(π) conjugation between the donor/acceptor and the bridge. Therefore, it is evident that in addition to electronic coupling and ET distance, orbital overlap is one of the major factors that govern the intramolecular electron transfer. Furthermore, this dimolybdenum intervalence system exemplifies that introduction of sulfur atoms along the electron transfer axis flattens the adiabatic energy surfaces and accelerates the electron transfer, as shown in Figure 2.

λ⎛ ΔG 0 ⎞ ΔG* = ⎜1 + ⎟ 4⎝ λ ⎠ ΔG*(adia) =

2

(8)

(ΔG°)2 λ ΔG° + + − Hab 4 2 4(λ − 2Hab) +

Hab 2 (λ + ΔG°)

(9) 42

A more complicated equation (eq 9) with HMM′ involved can be used to calculate ΔG° and ΔG* for the adiabatic ET reaction, which give ΔG*f = 2430 cm−1 and ΔG*r = 364 cm−1. The small differences of the ΔG* values for the diabatic and adiabatic processes prove the adiabatic nature of the ET reaction in [O2−S2]+. The ET rate constant (ket) is then calculated to be 4.1 × 107 s−1 and 8.6 × 1011 s−1 for the forward and reverse reactions, respectively. These results indicate that the two energy minima are remaining in the adiabatic state as shown in Figure 2.

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CONCLUSIONS In summary, we have studied a series of strucurally and electronically closely related Mo2−Mo2 complexes regarding the electronic coupling and the intramolecular electron transfer mechanism and kinetics. It is found that succesive substitution of the oxygen atoms on the terephthalate bridging ligand by sulfur atoms resulted in a constant increase of the electronic coupling in the range of 550 to 900 cm−1. The Hab values calculated from the Mulliken−Hush expression are consistent with the metal to metal coupling elements HMM′ derived from the CNS superexchange formalism. Applying the structurally determined effective ET distances (r′ab) and adopting the original CNS equations lead to the best agreement between Hab and HMM′. On this basis, the conformity of the two distinct spectral methods is proven. These results suggest that the electronic coupling in the Mo2−Mo2 system is invoked by orbital interactions between the intervening molecular fragments, which strongly support the CNS effective two-state model. Spectroscopic analyses show that in the dimetal systems, electron-hopping and hole-hopping work in concert for the electron transfer from donor to acceptor. It is unusual that although the mixed-valence systems under investigation are weakly coupled, ultrafast adiabatic electron transfer is predicted. This is attributed to the d(δ)−p(π) conjugation between the dimetal center and the bridging ligand. The study of electron exchange kinetics on the symmetrical and unsymmetrical MV complexes verifies that, by using Marcus−Hush theories, intramolecular ET rate can be accurately calculated through analyses of the spectroscopically determined adiabatic energy surfaces.

Figure 2. Schematic energy-coordinate diagram of the adiabatic energy surfaces for the [Mo2−Mo2]+ series, showing the variations of the optical excitation energy (EIT), electronic coupling strength (2HMM′ at the intersection), and free energy (ΔG°) as a result of atomic alternation of O/S on the bridging ligands.

For the unsymmetrical mixed-valence complex [O2−S2]+, there exists an intrinsic potential difference between the donor and acceptor. The carboxylate associated [Mo2] unit serves as the acceptor, while the thiolated [Mo2] unit is the donor. The intramolecular electron transfer from donor to acceptor is an uphill process (ΔG° > 0). The magnitude of free energy change (ΔG°) equals numerically the potential differnece between the two redox sites. A direct way to determine the ΔG° value is to find the difference in potential (ΔE1/2) between the two redox sites. Our approach was to exploit two reference compounds that structurally and electronically resemble [Mo2] units, which presumably have the half-wave potentials similar to that for the corresponding [Mo2] unit in the unsymmetrical complex. For this purpose, we have prepared two dimolybdenum monomers Mo 2 (DAniF) 3 (O 2 CC 6 H 5 ) and Mo 2 (DAniF) 3 (S 2 CC 6 H 5 ), which correspond to the carboxylate and dithiocarboxylate [Mo2] sites, respectively. The potentials of oxidation Mo24+ → Mo25+ are measured to be 375 mV for the former and 651 mV for the latter. Therefore, the potential difference of 276 mV, or 2226 cm−1, is taken as the internal potential difference (ΔG°) for [O2−S2]+.

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EXPERIMENTAL SECTION Methods. Metal complexes were synthesized using standard Schlenk line. The neutral dimolybdenum dimers for the study were characterized by 1H NMR spectra. Dichloromethane solutions of the corresponding mixed-valence complexes were prepared by chemical oxidation using one equiv of ferrocenium hexafluorophosphate prior to the magnetic and spectroscopic 19863

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NMR δ (ppm in CDCl3): 8.46 (s, 1H, −NCHN−), 8.32 (m, 2H, aromatic C−H), 8.29 (s, 2H, −NCHN−), 7.35 (d, 3H, aromatic C−H), 6.62 (d, 8H, aromatic C−H), 6.56 (d, 8H, aromatic C−H), 6.40 (d, 4H, aromatic C−H), 6.10 (d, 4H, aromatic −H), 3.72 (s, 12H, −OCH3), 3.65 (s, 6H, −OCH3). Half-wave potential ΔE1/2 (V, vs Ag/AgCl): 0.651.

measurements. The UV−vis−Near-IR spectra were measured on a Shimadzu UV-3600 and Mid-IR spectra on a Nicolet 6700 spectrophotometer. The four cation radicals, [O2−O2]+, [OS− OS]+ [S2−S2]+, and [O2−S2]+ were characterized by EPR spectra. A g value of 1.942−1.947 for all the complexes indicates that the odd electron resides on a metal-based orbital. Cyclic voltammograms (CVs) were performed using a CH Instruments Model-CHI660D electrochemical analyzer in 0.10 M nBu4NPF6 solution in CH2Cl2 with Pt working and auxiliary electrodes, Ag/AgCl reference electrode, and a scan rate of 100 mV/s. All potentials are referenced to the Ag/AgCl electrode General Procedure for Preparation of the Bridged Dimolybdenum Compounds. One equiv of the bridging ligand and two equiv of the dimolybdenum precursor [Mo2(DAniF)3(O2CCH3)] were mixed in THF. To the mixture, two equiv of sodium methoxide in methanol was added. After stirring at room temperature for 2 h, the solvents were evaporated under reduced pressure. The residue was dissolved in dichloromethane and filtrated through a Celitepacked column. To the filtrate, ethanol was added to generate solid product, which was collected and dried under vacuum. The yield ranges from 70 to 80%. Preparation of Tetraethylammonium Dithiobenzoate. About 30 mL of methanol was added to a 100 mL flask containing sodium methoxide (1.080 g, 20.00 mmol) and sulfur (0.640 g, 20.00 mmol). The mixture was refluxed under N2 for 3 h, and then, benzyl chloride (1.266 g, 10.00 mmol) was added. The reaction was allowed to reflux for additional 15 h, generating a purple solution and some precipitates. After filtration, the residue was washed with about 5 mL of methanol. To the combined filtrate was added slowly dilute hydrochloric acid to generate dithiobenzoic acid. The organic acid was extracted with 30 mL of chloroform. With stirring, 1.5 mL of aqueous solution of tetraethylammonium hydroxide (25% v/v) was added to the chloroform solution, yielding purple solid product. From this reaction, 2.46 g (81%) of product was obtained. 1H NMR δ (ppm in acetone-d6): 8.33 (d, 2H, aromatic C−H), 8.07 (d, 2H, aromatic C−H), 7.86 (d, 2H, aromatic C−H), 7.44 (d, 4H, aromatic C−H), 1.86−1.57 (m, 20H, aromatic −CH2−). UV−vis, λmax nm (ε, M−1 cm−1): 365 (2800). Preparation of Mo2(DAniF)3(μ-O2CC6H5). In a 100 mL flask, Mo2(DAniF)3(O2CCH3) (0.508 g, 0.50 mmol) and benzoic acid (0.061 g, 0.50 mmol) were mixed in 30 mL of THF, to which a solution of sodium methoxide (0.054 g, 1.00 mmol in 10 mL of methanol) was transferred through a cannula. After stirring at room temperature for 4 h, the solvents were removed under vacuum. The residue was washed with ethanol (3 × 20 mL), producing a yellow solid. Yield: 0.43 g (80%). 1H NMR δ (ppm in CDCl3): 8.47 (s, 1H, −NCHN−), 8.37 (s, 2H, −NCHN−), 8.31 (m, 2H, aromatic C−H), 7.46 (d, 3H, aromatic C−H), 6.64 (d, 8H, aromatic C−H), 6.56 (d, 8H, aromatic C−H), 6.45 (d, 4H, aromatic C−H), 6.24 (d, 4H, aromatic −H), 3.72 (s, 12H, −OCH3), 3.67 (s, 6H, −OCH3). Half-wave potential ΔE1/2 (V, vs Ag/AgCl): 0.375. Preparation of Mo2(DAniF)3(μ-S2CC6H5). To a mixture of Mo2(DAniF)3(O2CCH3) (0.508 g, 0.50 mmol) and tetraethylammonium dithiobenzoate (0.156 g, 0.55 mmol) in 30 mL of THF was added a solution of sodium methoxide, 0.027 g (0.50 mmol), in 10 mL of methanol. The resulted blue solution was stirred at room temperature for 3 h. The solvents were removed under reduced pressure. The residue was washed with ethanol (3 × 20 mL), giving blue solid product. Yield: 0.45 g (81%). 1H

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ASSOCIATED CONTENT

S Supporting Information *

UV−vis−near-IR and mid-IR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(C.Y.L.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation of China (No. 20871093 and 90922010), Tongji University, Jinan University, and Sun Yat-Sen University for financial support.

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REFERENCES

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