Electronic Coupling and Electron Transfer between Two

Article pubs.acs.org/JPCC

Electronic Coupling and Electron Transfer between Two Dimolybdenum Units Spaced by a Biphenylene Group Xuan Xiao,† Miao Meng,‡ Hao Lei,‡ and Chun Y. Liu*,†,‡ †

Department of Chemistry, Tongji University, 1239 Siping Road, Shanghai 200092, China Department of Chemistry, Jinan University, 601 Huang-Pu Avenue West, Guangzhou 510632, China

‡

S Supporting Information *

ABSTRACT: Three symmetrical dimolybdenum dimers bridged by 4,4′-biphenyldicarboxylate and the partially and fully thiolated derivatives have been synthesized and studied with respect to electronic coupling and intramolecular electron transfer. As generally denoted by [Mo2]−(ph)2−[Mo2], the complexes are differentiated by the [Mo 2 ] units but have a biphenylene spacer in common, where [Mo 2 ] = [Mo2(DAniF)3(EE′C)] with auxiliary ligands DAniF (N,N′-di(p-anisyl)formamidinate) and donor atoms E and E′ (O or S). The radical cations {[Mo2]−(ph)2−[Mo2]}+, prepared by one-electron oxidation of the corresponding neutral precursor, exhibit a characteristic intervalence (IV) charge transfer absorbance in the near-IR spectra. The electronic coupling matrix elements (Hab) calculated from the Mulliken−Hush expression vary in the range of 245−415 cm−1 depending on the number of sulfur atoms in the [Mo2] units. These parameters are also calculated by CNS superexchange formalism, in which only the electron-hopping pathway is taken into account because of the lack of ligand to metal charge transfer absorptions in the spectra. The results show remarkable alignment between the two different methods. Thus, the mixed-valence complexes are assigned to weakly coupled Class II in terms of Robin−Day’s classification. Under the Marcus−Hush theoretical framework, the adiabatic electron transfer rate constants (ket) are optically determined in the range of 109 − 1011 s−1. The fastest electron transfer is observed in the fully thiolated species. In comparison with the reported (ph)1 series, a relatively small attenuation factor, ca. β(Hab) 0.17, is also estimated for the tetrathiolated system. Therefore, the introduction of sulfur atoms along the charge transfer axis efficiently enhances the electronic coupling and facilitates the electron transfer between the two dimetal centers.

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according to the Robin−Day’s scheme,8 Class I, II, and III depending on the degree of electron delocalization over the bridge. With the Creutz−Taube complex {[(NH3)5Ru]2(pz)}+ as the prototype of MV complexes,9,10 innumerous d5−6 metal compounds with various metal centers (Ru, Fe, and Os),11,12 bridging ligands,13 and auxiliary ligands14 have been investigated. Quantitative expression of electronic coupling effect is given by the electronic coupling matrix element (H), which is a physical parameter derived by harmonic oscillator model or quantum mechanics treatment on a D−B−A system.15,16 Given the H parameter, one can determine the class of a given MV compound in Robin−Day’s definition and estimate the ET rate according to the Marcus−Hush theories.16−18 There are two approaches, Mulliken−Hush model15,16 and the CNS formalism,19 to derive the H parameter from the vibronic spectra. While the former is based on the classical two-state model, the latter, proposed by Creutz, Newton, and Sutin, stems from the McConnell superexchange principle.20 However, with hundreds of synthetic D−B−A systems, there are very limited cases

INTRODUCTION Studies of electronic coupling (EC) and intramolecular electron transfer (ET) have significance in the fundamentals of chemistry, and hence, has been a long-standing topic of research.1−3 Much work has also been directed to biological systems for the elucidation of important naturally occurring redox processes.4 Recently, the research in this field has been sparked by the application potentials of the developed synthetic methods and theories in molecular electronic devices, such as molecular wires,5 switches,6 and rectifiers.7 Binuclear D−B−A mixed-valence (MV) complexes, in which transition metal coordination units serve as the electron donor (D) and acceptor (A) with an organic species as the bridge (B), have been the experimental models for developing theories in electronic coupling and intramolecular electron transfer. The advantage of employing coordination compounds for this research purpose is that a metal complex unit is electronically addressable by means of electrochemical, optical, and magnetic techniques. Thus, the assessments of charge distribution and monitoring of electron (or energy) transfer can be made by the excited state dynamics and energetics. For symmetrical MV systems, the electron donor and acceptor have the same structural entity but different formal oxidation states. Compounds of this category are classified into three classes © 2014 American Chemical Society

Received: March 3, 2014 Revised: March 30, 2014 Published: April 2, 2014 8308

dx.doi.org/10.1021/jp502163a | J. Phys. Chem. C 2014, 118, 8308−8315

The Journal of Physical Chemistry C

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coupling effect and intramolecular electron transfer kinetics were investigated in comparison with the results obtained for (ph)1 series. On this basis, the impacts of two major factors, coordination shell and bridge length, on the donor−acceptor interaction and electron transfer are discussed.

where the reaction kinetics is well understood. Major challenges still exist on the interplaying region for theoretical21 and experimental22 research practices. Therefore, interpreting and modeling the experimental results under the existing theoretical frameworks is still the research focus in the field.23 By exploiting their multiple advantages in synthetic, analytical, structural, and electronic aspects, covalently bonded dimetal units M2 (M = Mo, W, and Ru)24−26 have been used to construct so-called “dimer of dimers” for the research in this regard. Through analyzing the optical behaviors by the semiclassical two-state model, we have recently studied the ET reaction kinetics in a series of dimolybdenum dimers with terephthalate and its thiolated derivatives as the bridging ligands (Scheme 1, n = 1).27,28 We tend to formulate these

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RESULTS AND DISCUSSION Magnetic Properties for the Mixed-Valence Complexes. The three neutral dimolybdenum pairs [O2−(ph)2− O2], [OS−(ph)2−OS], and [S2−(ph)2−S2] were synthesized in good yield by assembling two dimetal building blocks [Mo2(DAniF)3]+ with the bridging ligands,29 4,4′-biphenyldicarboxylate, 4,4′-biphenyldithiocarboxylate, and 4,4′-biphenyltetrathiocarboxylate, respectively. 1H NMR characterizations show that the compounds have the molecular skeletons similar to the reported (ph)1 series (Scheme 1),28 although X-ray structural determinations have not been performed. The products for measurements and analyses have a purity of 95% or higher. The mixed-valence complexes, [O2−(ph)2−O2]+, [OS−(ph)2−OS]+, and [S2−(ph)2−S2]+, were prepared by one-electron oxidation of the corresponding neutral compound with one equiv of ferrocenium hexafluorophosphate (Cp2FePF6). These radical cations were characterized by Xband electron paramagnetic resonance (EPR) spectra. In the EPR spectra, each complex exhibits one main signal with some weak hyperfine structures as shown in Figure 1. The EPR signal is attributed to molecules containing only the 96Mo (I = 0) isotope, while the hyperfine structure is due to molecules with 95, 97 Mo (I = 5/2) isotope (natural abundance of approximately 25%). The EPR peaks center at g 1.944 for [O2−(ph)2−O2]+, 1.945 for [OS−(ph)2−OS]+, and 1.946 for [S2−(ph)2−S2]+. The g values being significantly smaller than 2.003 for an organic radical indicate that the odd electron resides essentially in a metal-based orbital, the δ orbital. The g values for this series are comparable with those for complexes in (ph)1 series, ca. 1.942 for [O2−(ph)1−O2]+, 1.945 for [OS−(ph)1−OS]+, and 1.947 for [S2−(ph)1−S2]+.28 Notably, for the two series, the g values increase as the number of sulfur atoms in the [Mo2] units increases. This variation tendency is consistent with the results in electrochemical cyclic voltammograms and electronic and vibronic spectra, implying that, somehow, the g value is correlated with the extent of electron delocalization. Spectroscopic Properties of the Complexes. All the neutral complexes display an intense absorption band in the electronic spectra (Figure 2). Theoretical work on similar systems has confirmed that this band is attributed to electronic transition from the δ orbital of the Mo2 unit (HOMO) to the empty π* orbital of the bridging ligand (LUMO). Thus, this band is assigned to metal to ligand charge transfer (MLCT).24 For this biphenyl series, the ML transition occurs at higher energy than that for (ph)1 series. For example, [O2−(ph)2− O2] has the MLCT band energy (EML) 21 012 cm−1, higher than that of [O2−(ph)1−O2] (EML, 20 600 cm−1). Large blue shifts of the MLCT band are found for the thiolated analogues. For instance, for complexes [S2−(ph)n−S2], the ML transition energy (EML) is increased by 1800 cm−1 as the Mo2···Mo2 separation increases.28 On the other hand, the MLCT energy is steadily lowered as the oxygen atoms of the bridging ligand are stepwise substituted by sulfur atoms; however, the variation is relatively small. For example, in (ph)1 series, replacements of the first and second two oxygen atoms lower the MLCT energy by 4560 and 2190 cm−1, respectively, while the corresponding energy changes are 3690 and 1670 cm−1 for (ph)2 series.

Scheme 1

complexes as [Mo2]−(ph)1−[Mo2], where [Mo2] represents the coordinatively saturated dimolybdenum unit [Mo2(DAniF)3(EE′C)] (E, E′ = O or S) with three N,N′di(p-anisyl)formamidinate (DAniF) auxiliary ligands and a three-atom chelating group (EE′C) from the bridging ligand. Thus, the mixed-valence complexes {[Mo2]−(ph)1−[Mo2]}+ have slightly different [Mo2] units as the electron donor and acceptor but a common phenylene group as the bridge. The MV complex series present a transition from weak (Class II) to moderately strong (Class II−III) coupling interactions.28 Given the same auxiliary ligand and similar Mo2···Mo2 separations, the adiabatic ET rate is dependent on the chelating group (EE′C) which controls the potential energies of the donor and acceptor. In this report, three dimolybdenum pairs spaced by a biphenyl group, [Mo2 (DAniF)3]2(μ-O2CC 6H4C6H 4CO2), [Mo2(DAniF)3]2(μ-SOCC6H4C6H4COS), and [Mo2(DAniF)3]2(μ-S2CC6H4C6H4CS2), abbreviated as [O2− (ph)2−O2], [OS−(ph)2−OS], and [S2−(ph)2−S2], respectively, were synthesized by assembling two dimetal units with 4,4′-biphenyldicarboxylate and its thiolated derivatives. These compounds correspond to those in the (ph)1 series by having the same [Mo2] units as shown in Scheme 1 (n = 2). Inserting an additional phenyl group between two dimolybdenum units increases the metal−metal separations to 15−16 Å. The mixedvalence species, [O2−(ph)2−O2]+, [OS−(ph)2−OS]+, and [S2−(ph)2−S2]+, were prepared by one electron oxidation of the corresponding neutral molecules. These radical cations, which are all in the weakly coupled Class II regime, show a characteristic intervalence charge transfer band in the near-IR region. Applying the Marcus−Hush theories, the electronic 8309



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Figure 2. Vis−near-IR spectra for the neutral compounds and the MV complexes: (A) for [O2−(ph)2−O2] and [O2−(ph)2−O2]+, (B) for [OS−(ph)2−OS], and [OS−(ph)2−OS]+, and (C) for [S2−(ph)2− S2], and [S2−(ph)2−S2]+. In the insets, amplified spectra are presented to show the intervalence bands.

interesting to note that in (ph)2 series, the intervalence transition energy is decreased by 1700 cm−1 for every two sulfur atoms introduced, while in (ph)1 series, the variation of MMCT energy is exactly 800 cm−1.27 These results show that for compounds having similar metal−metal separations (rMM), the intervalence transition energy is strictly dependent upon the variable donor atoms in the dimetal coordination shell. Thus, it is reasonable that the complex unit [Mo2] is considered to be the electronic donor (or acceptor). It is also important to note that the ligand to metal charge transfer (LMCT) absorptions present in the [Mo2]−(ph)1−[Mo2] systems are not observed in the current systems. A plausible explanation for this is that the energies of the filled ligand π orbitals are significantly lowered by the extended conjugation of the bridge, which increases the LM transition gap. In terms of the extent of electron delocalization, the broadness of MMCT absorption band for MV complex can be measured by comparing the experimental bandwidth (Δν1/2) with the calculated value (Δνo1/2 = (2310EIT)1/2). According to the equation derived from the Hush model, Γ = 1 − Δν1/2/(2310EIT)1/2, a broad IV band gives a small Γ value (< 0.5), meaning that the complex is weakly coupled or at the Class II regime. In this study, the IV bands are very broad and negative Γ values are found for the three complexes: −0.96 for [O2−(ph)2−O2]+, −0.63 for [OS−(ph)2−OS]+, and −0.56 for [S2−(ph)2−S2]+. Accordingly, the MV cations should be assigned to valence trapped Class II. As expected, the Γ value increases in order as the sulfur atoms are stepwise introduced in the [Mo2] units (Table 1). Previous study has shown that [S2−

Figure 1. X-band EPR spectra of the radical cations generated by single oxidation of the neutral compounds. Samples were measured in situ in CH2Cl2 solutions at 110 K.

As shown in the spectra (Figure 2), the mixed-valence complexes present a MLCT band at similar wavelength with lower intensity when compared to the corresponding neutral precursor, except for the dicarboxylate bridged complex. The MLCT energy for the cation [O2−(ph)2−O2]+ (EML, 21 505 cm−1) is slightly higher than that for the corresponding neutral [O2−(ph)2−O2] (EML, 21 012 cm−1). Similar results were observed for the [O2−(ph)1−O2] system where the oxidized species displays a slightly blue-shifted MLCT band.27 The most important spectroscopic feature for the MV complexes is the weak and broad absorption band appearing in the near-IR region, which should be assigned to intervalence (IV) or metal to metal (MM) charger transfer (Figure 2). For {[Mo2]− (ph)2−[Mo2]}+, the intervalence transition energies (EIT) are high, but the intensities (εIT) are low relative to those for the {[Mo2]−(ph)1−[Mo2]}+ complexes (Table 1).27 These results indicate that the electronic interaction between the two [Mo2] units in this series is generally weak. Two factors, the long donor−acceptor separation (ca. 15−16 Å) and the torsion angle (ca. 30−50°) between the two phenyl rings, are likely responsible for the weak metal−metal interactions.27 The former eliminates the donor−acceptor Coulomb repulsion, while the latter diminishes the orbital interactions. It is 8310



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Table 1. Electronic Coupling Parameters Calculated from Hush Model, along with the Data for [Mo2]−(ph)1−[Mo2]+ for Comparisona compd [O2−(ph)1−O2] [OS−(ph)1−OS]+ [S2−(ph)1−S2]+ [O2−(ph)2−O2]+ [OS−(ph)2−OS]+ [S2−(ph)2−S2]+ +

r′abb (Å)

EIT (cm−1)

εIT (M−1 cm−1)

exptl Δν1/2 (cm−1)

cacld Δν1/2 (cm−1)

Γ

Hab (cm−1)

5.8 5.8 5.8 10 10 10

4240 3440 2640 8300 6536 4826

1470 3690 12 660 198 715 1610

4410 3290 1770 8578 6338 5231

3190 2820 2470 4379 3886 3339

−0.17 −0.14 0.30 −0.96 −0.63 −0.56

589 727 864 245 354 415

a Data cited from ref 27. br′ab is the geometrical length of the spacing moieties “−CC6H4C−” for [Mo2]−(ph)1−[Mo2] and “−CC6H4C6H4C−” for [Mo2]−(ph)2−[Mo2].

Table 2. Electronic Coupling Parameters Calculated from CNS Model in Comparison with the Data Derived from the Mulliken−Hush Expression compd {[O2−(ph)2−O2]} {[OS−(ph)2−OS]}+ {[S2−(ph)2−S2]}+ +

rML (Å)

EML (cm−1)

εML (M−1 cm−1)

EIT (cm−1)

HML (cm−1)

ΔEML (cm−1)

HMM′ (cm−1)

Hab/HMM′

7.76 8 8.3

21 012 17 319 15 647

9272 21 840 37 350

8300 6536 4826

2573 2797 2986

15 840 13 291 12 794

209 294 348

1.2 1.2 1.2

(ph)1−S2]+ (Γ = 0.30) is on the Class II−III borderline, while [O2−(ph)1−O2]+ (Γ, −0.17) and [OS−(ph)1−OS]+ (Γ, −0.14) are typical Class II complexes. Therefore, by changing the donor atoms and bridge length, the donor−acceptor interactions in the {[Mo2]−(ph)n−[Mo2]}+ series vary from weakly to strongly coupled until reaching the Class II−III borderline. Determination of the Electronic Coupling Matrix Elements. Determining the effective electron transfer distance is critical to accurately estimating the electronic coupling parameter from the classical Mulliken−Hush equation (eq 1).15,16 Hab = 2.06 × 10−2

from 5.8 to 10 Å, the magnitudes of Hab are lowered by a factor of 2 compared to the analogues having the same [Mo2] units. These Hab parameters are also comparable with the data obtained in other MV systems with similar metal−metal separations (rMM). For example, in the diferrocenylpolyene system, Hab = 430 cm−1 is found for the Fc-6-Fc species with rMM of 16.26 Å.34 In pure organic donor−acceptor system with polyphenylene bridges, the biphenyl (13 Å) and triphenyl (17 Å) species have the Hab values of 430 and 217 cm−1, respectively.35 Based on the McConnell’s superexchange theory,20 Creutz, Newton, and Sutin proposed an alternative approach to calculate the electronic coupling matrix element (HMM′, to distinguish from Hab).19 By the CNS formalism, the calculation of HMM′ involves three equations (eqs 2, 3, and 4).

(Δv1/2εmax E IT)1/2 rab

(1)

Adopting metal to metal distance for rab would lead to an underestimated Hab value because the actual electron transfer distance (r′ab) is significantly shorter.30,31 When applicable, r′ab can be obtained from the dipole moment change in Stark effect.32,33 For the covalently bonded dimetal systems, we seek for experimental approach to estimate r′ab for determination of Hab. As is well known, the δ electrons on the donor (or acceptor) site are delocalized over the coordination shell through d(δ)−p(π) conjugation, which apparently shortens the electron transfer distance. In calculation of Hab for (ph)1 systems, to lower the errors caused by mistaken use of the geometrical distance, we utilized the length of the spacer (−CC6H4C−) (ca. 5.8 Å) as the effective electron transfer distance r′ab. Calculations have yielded the electronic coupling parameters that conform with the optical properties and electron transfer kinetics.27 By the same principle, for the current systems, the length of the central moiety “− CC6H4C6H4C−” (ca. 10 Å), rather than the Mo2···Mo2 separation (∼16 Å), is taken as the effective electron transfer distance (r′ab). Given the spectroscopic data, calculations from eq 1 yielded the Hab parameters for the three complexes as presented in Table 1, along with the data for [Mo2]−(ph)1− [Mo2]+ for comparison. As shown in Table 1, the derived Hab values (245−415 cm−1) for {[Mo2]−(ph)2−[Mo2]}+ are significantly smaller than those for the phenyl series as predicted from the increased metal− metal separation. It is worthwhile to note that as rab′ increases

HMM ′ =

HMLHM ′ L H H + LM LM ′ 2ΔEML 2ΔE LM

(2)

⎛ 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)

From eq 2, electronic coupling constant HMM′ is composed of two components that account for the contributions from metal to ligand (the first term) and ligand to metal charge transfer (the second term). These two terms correspond to the electron-hopping pathway and hole-hopping pathway, in the case where the two pathways act synergistically in the course of electron transfer. Equations 3 and 4 are given to calculate the effective energy gaps for metal to ligand (ΔEML) and ligand to metal (ΔEML) charge transfer from the optical data. By employing the Mulliken−Hush expression (eq 1), the metal− ligand electronic coupling constant (HML) is derived from the MLCT absorption of the neutral precursor, while the ligand− metal coupling parameter (HLM) is calculated from the LMCT band of the mixed-valence complex.19 However, not all the intramolecular electron transfer processes involve both pathways. Single pathway dominated systems, either by electronhopping,19,36 or hole-hopping,37 have been reported for several 8311



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systems and the CNS equations are usually simplified accordingly. In the {[Mo2]−(ph)1−[Mo2]}+ series, MLCT and LMCT absorption bands are both resolved in the spectra. The application of the complete form of eq 2 yielded the coupling constants HMM′ consistent with the data obtained from the Mulliken−Hush expression. In the absence of the LMCT absorption band in (ph)2 system, calculations of HMM′ are carried out based on the MLCT and MMCT data. In calculation of HML from eq 1, rML is the distance between the centroid of the Mo−Mo bond and that of the biphenyl spacer.19,27 In addition, it is assumed that the metal to ligand coupling elements at the reduced and oxidized sites are equal, e.g., HM′L = HML.19 The calculated results are presented in Table 2. Importantly, the two different theoretical models give the electronic coupling matrix elements that agree with each other. The variation tendencies for the two data sets are parallel with a Hab:HMM′ ratio of 1.2, while in other systems, large deviations between Hab and HMM′ were found.35,38,39 Undoubtedly, the consistency in this study results from reasonable estimates of the effective electron transfer distances (r′ab). Therefore, it is suggested that for conjugated systems, the alternative method to determine r′ab, when electroabsorption spectroscopy is not available, is to measure the distance between the coordination shells of the two metal centers. On the other hand, it is noticed that for (ph)2 series, the HMM′ values are generally smaller than the Hab values. Presumably, this is because the contribution from LMCT is not taken into account in HMM′ calculation. Interestingly, better alignment between Hab and HMM′ is achieved for the dicarboxylate bridged complexes. As noticed, complexes {[O2−(ph)n−O2]}+ (n = 1 and 2) exhibit slightly blue-shifted and broad MLCT bands (Figure 2), which are attributed to the overlap of the MLCT band with the LMCT band. Thus, in the calculations of HMM′ from the spectral parameters, the contribution from hole-hopping has been taken into consideration. Nevertheless, provided with the satisfactory conformity for the electronic coupling matrix elements derived from the Hush and CNS models, the latest superexchange approach, or CNS formalism, is revalidated in the dimetal systems. Kinetics of the Intramolecular Electron Transfer Reactions. For weakly coupled systems (Class II), the kinetics of electron transfer reaction are discussed according to the semiclassical theories (eqs 5 and 6)32,40

ΔG* =

(λ − 2Hab)2 4λ

ket = A exp( −ΔG*/kBT )

Table 3. Calculated Activation Energies (ΔG*) and ET Rate Constants (ket),a along with the Data for the (ph)1 System for Comparisonb ΔG* (cm−1)

complex [O2−(ph)2−O2] [OS−(ph)2−OS]+ [S2−(ph)2−S2]+ [O2−(ph)1−O2]+ [OS−(ph)1−OS]+ [S2−(ph)1−S2]+ +

a

1838 1299 827 581 266 79

ket (s−1) 7.0 9.5 9.2 3.0 1.4 3.4

× × × × × ×

108 109 1010 1011 1012 1012

Results calculated from Hab. bData from ref 27.

organic D−B−A system with a biphenyl bridge, the rate constant ket measured by ESR is about 3 × 109 s−1.35 As shown in Table 3, the δ electron transfer in {[O2−(ph)2−O2]}+ is relatively slow as expected, and the fastest ET process is found for [S2−(ph)2−S2]+. Significantly, incorporating every two sulfur atoms on the bridging ligand raises ket by an order of magnitude. Compared with the (ph)1 analogues, lengthening the bridge lowers the rate constant by 1−2 orders of magnitude depending on the sulfur content in [Mo2]. Therefore, it is evident that for this [Mo2]−(ph)n−[Mo2] system, the electronic coupling and electron transfer are controlled by the [Mo2] coordination shell as well as the bridge. Distance Dependence of Hab and ket. Following the preceding discussion, it is clear that the electronic coupling effect (Hab) and electron transfer rate (ket) decay with the electron transfer distance (rab). According to the widely accepted decay laws, Hab and ket are exponentially related to rab by an attenuation factor (β) that reflects the intrinsic electronic characteristics of the bridge Hab = H0 exp( −βrab)

(7)

ket = k 0 exp( −βrab)

(8)

where H0 is the electronic coupling at contact distance and k0 is a kinetic prefactor. From eqs 7 and 8, a linear relationship in plot of ln(Hab) or ln(ket) versus rab is obtained and the attenuation factor β is then determined from the slope. Unfortunately, for the [Mo2]−(ph)n−[Mo2] systems, there is not enough data for statistically satisfactory results. Hence, for systems differing in [Mo2] units, the β factors are estimated from the Hab or ket values for the two complexes (n = 1 and 2) (Table 4). Consistent with the treatments in calculations of Table 4. Estimated Attenuation Factors (β) for Systems with Various Donor Atoms on the Bridging Ligands

(5) (6)

bridging ligand

where ΔG* is the activation energy of the ET reaction and A is the prefactor of the Arrhenius equation. For symmetrical Class II systems, the reorganization energy λ equals the vertical excitation energy EIT. In the first-order adiabatic system (or Class II), the vibrational motion (1012 − 1013 s−1) is slower than the electronic motion (≫1014 s−1). The prefactor A is then determined by an averaged nuclear vibration frequency factor, that is, νn = 5 × 1012 s−1. Calculations indicate that the three long-range weakly coupled systems have the adiabatic electron transfer rate constants (ket) falling in the range of 109 − 1011 s−1. The calculated kinetic parameters, along with those for the (ph)1 systems, are shown in Table 3. These results are compatible with mixed-valence systems having similar donor− acceptor separations. For example, it is reported that in an

[O2C−(ph)n− CO2] [OSC− (ph)n−COS]2− [S2C− (ph)n−CS2]2− 2−

β(Hab)

β(ket)

0.21 0.17 0.17

1.44 1.19 0.86

Hab, here, rab = 5.8 Å for (ph)1 series and 10 Å for (ph)2 series. The attenuation factors of 0.17−0.21 Å−1for Hab decay fall in a normal range for a π-bridged conjugated system, but are significantly smaller than those obtained for saturated hydrocarbon bridges41 (β = 0.6−1.2 Å−1). Attenuation factors β(Hab) = 0.04−0.2 Å−1 were reported for dinuclear metal complexes with oligoene bridges.42 Smaller attenuation factors were found for the oligoynes,43 e.g., β = 0.04−0.17 Å−1. It is important to note that, in this study, the attenuation factors vary depending 8312



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on the presence of sulfur atoms in the [Mo2] units. Smaller β (Hab) values for the thiolated systems demonstrate that the sulfur donor atom is capable of enhancing the electronic coupling interaction. The attenuations of electron transfer rate are estimated with β = 0.86−1.44 Å−1 (Table 4). In comparison with the data in literature, the β(ket) values are considerably larger. For example, β = 0.32−0.66 Å−1 was reported for conjugated oligophenylenes.44 Dinuclear MV complexes with a polyphenyl spacer showed an attenuation factor of 0.4 Å−1 for the intramolecular electron transfer.45 The higher β(ket) from our results might be a reflection of the nature of the systems. In light of the superexchange theory, electron-hopping and hole-hopping may contribute in concert to facilitate the electron transfer process. Accordingly, the attenuation factor (β) can be divided into two components, βHT and βET, that is, β = βHT + βET, as proposed by Closs and co-workers.46 In the previous section, we have seen that there is significant difference in electron transfer mechanism for complexes with different spacers or lengths. In the {[Mo2]−(ph)1−[Mo2]}+ cations, the electron transfer takes place by simultaneous electron-hopping and hole-hopping pathways,27 while in (ph)2 systems, only the electron-hopping pathway takes effect as indicated by the absence of the LMCT absorption bands. Therefore, lengthening the metal−metal separation by inserting an additional phenyl group not only increases the electron transfer distance, but also affects the twopathway mechanism. These two effects would jointly lower the electron transfer rate in the {[Mo2]−(ph)2−[Mo2]}+ complexes. On the other hand, as discussed earlier, the ultrafast electron transfer occurring in the (ph) 1 analogues is attributable, at least in part, to the d(δ)−p(π) conjugation within a [Mo2] site, which is extended to the other [Mo2] site through the conjugated phenylene group. However, in (ph)2 systems, the extension of the conjugation through the bridge is partially inhibited by the torsion angle between the two neighboring phenyl rings, consequently, diminishing the electron transfer process.44

biphenylene group (ca. 10 Å), the electronic matrix elements (Hab) calculated from the Mulliken−Hush expression fall in the range of 245−415 cm−1. The absence of the LMCT in the spectra for the MV cations implies that electron-hopping dominates the donor−acceptor electron transfer process. This is different from the phenylene bridged analogues, for which electron-hopping and hole-hopping work in concert for the intramolecular electron transfer. Remarkably, the electronic coupling parameters calculated from the CNS superexchange formalism are in good agreement with the Mulliken−Hush Hab values. Under the Marcus−Hush theoretical framework, the electron transfer rate constants (ket) are determined in the range of 109 − 1011 s−1, less than those for the corresponding (ph)1 systems by 1 order of magnitude. The attenuations of Hab and ket vary depending on the number of sulfur atoms in the [Mo2] units. Smaller attenuation factors are found for the complexes with S-containing electron donor and acceptor. Therefore, it is confirmed that introducing sulfur atoms on the charge transfer axis efficiently enhances the electronic coupling and facilitates the electron transfer.

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EXPERIMENTAL SECTION Materials and Methods. All reactions and manipulations were performed under a nitrogen atmosphere, using either a nitrogen-filled glovebox or standard Schlenk line techniques. Solvents were freshly distilled over appropriate drying agents under nitrogen. The auxiliary ligand HDAniF47 and the paddlewheel precursor Mo2(DAniF)3(O2CCH3)48 were prepared as reported. Physical Measurements. Elemental analyses were determined using an Elementar Vario EL elemental analyzer. 1H NMR spectra were recorded on a Bruker Avance III 500 spectrometer using CDCl3 or DMSO-d6 as the solvent. UV− vis−NIR spectra were measured on a Shimadzu UV-3600 UV− vis−NIR spectrophotometer in CH2Cl2 solutions using IR quartz cell with light path length of 2 mm. EPR spectra were recorded on a Bruker A300-10-12 electron paramagnetic resonance spectrometer. Measurements for the mixed-valence complexes were carried out in situ after one-electron oxidation of the corresponding neutral compounds using 1 equiv of ferrocenium hexafluorophosphate (Cp2FePF6). Cyclic voltammograms (CVs) and differential pulse voltammograms (DPVs) were collected in CH2Cl2 on a CH Instruments CHI660D electrochemical analyzer equipped with Pt working and auxiliary electrodes and a Ag/AgCl reference electrode and using a scan rate of 100 mV s−1 and 0.10 M nBu4NPF6 as the electrolyte. Preparation of Tetraethylammonium 4,4′-Biphenyltetrathiodicarboxylate. Sulfur (1.28 g, 0.04 mol) and sodium methoxide (2.16 g, 0.04 mol) were mixed in 60 mL of methanol. The mixture was then refluxed for 3 h. 4,4′bis(chloromethyl) biphenyl (2.51 g, 0.01 mol) was added in portions over a period of 1 h. The reaction was allowed to reflux for additional 15 h, generating a red solution with some solids suspended. After the solution was cooled at room temperature, dilute hydrochloric acid was added dropwise. The resultant 4,4′-biphenyltetrathiocarboxylic acid was extracted using 30 mL of dichloromethane. With stirring, the extract was mixed with aqueous solution of tetraethylammonium hydroxide (25%, 12 mL), yielding red precipitates. The dark red product was collected by filtration and dried in vacuo overnight. Yield: 4.02 g (78%). 1H NMR δ (ppm in CDCl3): 7.64 (d, 4H, aromatic C−H), 7.40 (d, 4H, aromatic C−H), 2.40 (q, 16H,

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CONCLUSIONS To further explore the electronic properties and optical behaviors of Mo2-containing D−B−A complexes, three novel symmetrical dimolybdenum pairs having 4,4′-biphenyldicarboxylate and their partially and fully thiolated derivatives as the bridging ligands have been synthesized. In our study in electronic coupling and intramolecular electron transfer in these dimers of dimers, the dimetal complex unit with the coordinatively saturated ligand sphere, i.e. [Mo2], is considered to be the electron donor (or acceptor) and the phenyl spacer between the two [Mo2] units as the functional bridge. Therefore, this series can be expressed by a general formula [Mo2]−(ph)2−[Mo2], where [Mo2] = [Mo2(DAniF)3(EE′C)] (E, E′ = O or S) and the three complexes differing by dimetal units are abbreviated as [O2−(ph)2−O2], [OS−(ph)2−OS], and [S2−(ph)2−S2]. From their optical behaviors, the three MV complexes [O2−(ph)2−O2]+, [OS−(ph)2−OS]+, and [S2−(ph)2−S2]+ generated by one-electron oxidation are assigned to the weakly coupled Class II system. The current [Mo2]−(ph)2−[Mo2] systems present well-defined MLCT bands and broader, higher energy MMCT bands compared with (ph)1 series. However, different from the previously reported [Mo2]−(ph)1−[Mo2] systems, the LMCT absorption bands are not observed in the spectra. Given the effective electron transfer distances estimated from the length of the 8313



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8.41 (d, 4H, aromatic C−H), 8.30 (s, 4H, −NCHN−), 7.66 (d, 4H, aromatic C−H), 6.64 (d, 16H, aromatic C−H), 6.57 (d, 16H, aromatic C−H), 6.41 (d, 8H, aromatic C−H), 6.11 (d, 8H, aromatic C−H), 3.72 (s, 24H, −OCH3), 3.66 (s, 12H, −OCH3). UV−vis, λmax nm (ε, M−1 cm−1): 639 (37350). Anal. Calcd for C104H98Mo4N12O12S4: C, 56.27; H, 4.45; N, 7.57; S, 5.78. Found: C, 56.28; H, 4.15; N, 7.46; S, 5.92.

−CH2−), 1.02 (t, 24H, −CH3). Anal. Calcd for C30H48N2S4: C, 63.77; H, 8.56; N, 4.96; S, 22.71. Found: C, 63.33; H, 8.24; N, 4.68; S, 22.88. Preparation of 4,4′-Biphenyldithiodicarboxylic Acid. In a 100-mL flask, thionyl chloride (30 mL, 0.41 mol) was mixed with 4,4′-biphenyldicarboxylic acid (2.24 g, 0.01 mol) and two drops of DMF. The mixture was then refluxed for 24 h. After removal of the solvents by a rotary evaporator, the yielded 4,4′-biphenyldicarbonyl dichloride was mixed with thioacetamide (2.25 g, 0.03 mol) in 30 mL of THF. The solution was allowed to stir for 3 h at room temperature. Dilute hydrochloric acid (∼ 1 M, 50 mL) was added slowly to produce 4,4′biphenyldithiodicarboxylic acid. The product was extracted using 30 mL of diethyl ether. Yield: 2.41 g (88%). 1H NMR δ (ppm in DMSO-d6): 8.20 (d, 4H, aromatic C−H), 7.79 (d, 4H, aromatic C−H). Anal. Calcd for C14H10O2S2: C, 61.29; H, 3.67; S, 23.38. Found: C, 61.18; H, 3.85; S, 23.16. Preparation of [Mo2(DAniF)3]2(μ-O2CC6H4C6H4CO2). A 100-mL flask was charged with Mo2(DAniF)3(OOCCH3) (0.51 g, 0.50 mmol) and tetrabutylammonium 4,4′-biphenyldicarboxylate (0.09 g, 0.25 mmol), to which 30 mL of dichloromethane was added. The mixture was allowed to stir at room temperature for 6 h, developing a red solution. After beng filtered through a Celite-packed frit, the solvent was evaporated under reduced pressure. The solid product was washed with ethanol (20 mL × 3) and dried under vacuum. Yield: 0.39 g (72%). 1H NMR δ (ppm in CDCl3): 8.49 (s, 2H, −NCHN−), 8.38 (s, 4H, −NCHN−), 8.37 (d, 4H, aromatic C−H), 7.72 (d, 4H, aromatic C−H), 6.65 (d, 8H, aromatic C−H), 6.59 (d, 8H, aromatic C−H), 6.49 (d, 16H, aromatic C−H), 6.17 (d, 16H, aromatic C−H), 3.73 (s, 12H, −OCH3), 3.70 (s, 24H, −OCH3). UV−vis, λmax nm (ε, M−1 cm−1): 476 (9270). Anal. Calcd for C104H98Mo4N12O16: C, 57.94; H, 4.58; N, 7.80. Found: C, 57.68; H, 4.45; N, 7.76. Preparation of [Mo2(DAniF)3]2(μ-OSCC6H4C6H4COS). A methanol solution of sodium methoxide (0.027 g, 0.50 mmol) in 5 mL of methanol, was added slowly to a solution prepared by dissolving Mo2(DAniF)3(OOCCH3) (0.51 g, 0.50 mmol) and 4,4′-biphenyldithiodicarboxylic acid (0.07 g, 0.25 mmol) in 30 mL of THF. Upon mixing, the color of the solution turned to purple immediately. After stirring for 3 h, the solvents were removed under vacuum, yielding a purple solid. The dark color product was washed with ethanol (20 mL × 3) and collected by filtration. Yield: 0.48 g (88%). 1H NMR δ (ppm in CDCl3): 8.49 (s, 2H, −NCHN−), 8.35 (s, 4H, −NCHN−), 8.33 (d, 4H, aromatic C−H), 7.68 (d, 4H, aromatic C−H), 6.65 (d, 16H, aromatic C−H), 6.61 (d, 8H, aromatic C−H), 6.53 (d, 8H, aromatic C−H), 6.45 (d, 8H, aromatic C−H), 6.21 (d, 4H, aromatic C−H), 6.12 (d, 4H, aromatic C−H), 3.73 (s, 12H, −OCH3), 3.72 (s, 12H, −OCH3), 3.67 (s, 6H, −OCH3), 3.66 (s, 6H, −OCH3). UV−vis, λmax nm (ε, M−1 cm−1): 577 (21 840). Anal. Calcd for C104H98Mo4N12O14S2: C, 57.09; H, 4.51; N, 7.68; S, 2.93. Found: C, 57.18; H, 4.55; N, 7.72; S, 3.16. Preparation of [Mo2(DAniF)3]2(μ-S2CC6H4C6H4CS2). Mo2(DAniF)3(OOCCH3) (0.51 g, 0.50 mmol) and tetraethylammonium 4,4′-biphenyltetrathiodicarboxylate (0.10 g, 0.25 mmol) were placed in a 100-mL flask, to which 30 mL of THF was added. After stirring for 30 min, the solution changed to blue. The mixture was stirred at room temperature for 3 h. After that, the solvent was removed under reduced pressure. The solid was washed with ethanol (20 mL × 3). The blue product was collected and dried under vacuum. Yield: 0.48 g (86%). 1H NMR δ (ppm in CDCl3): 8.48 (s, 2H, −NCHN−),

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR spectra and computational details. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*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 (20871093,90922010, 21371074, and 21301070), Tongji University, Jinan University, and Sun Yat-Sen University for financial support.

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REFERENCES

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