Optical Determination of Electron Transfer Dynamics and Kinetics for

Article pubs.acs.org/JPCC

Optical Determination of Electron Transfer Dynamics and Kinetics for Asymmetrical [Mo2]−ph−[Mo2] Systems Wei Yong Yu, Miao Meng, Hao Lei, Xue Dan He, and Chun Y. Liu* Department of Chemistry, Jinan University, 601 Huang-Pu Avenue West, Guangzhou 510632, China S Supporting Information *

ABSTRACT: We report the study, in terms of electronic coupling (EC) and electron transfer (ET), on three asymmetrical Mo 2 dimers [Mo2(DAniF)3]2[μ-(NH)OCC6H4CO2] ([NO−ph−OO]) (DAniF = N,N′di(p-anisyl)formamidinate), [Mo2(DAniF)3]2[μ-(NH)2CC6H4CO2] ([NN− ph−OO]), and [Mo2(DAniF)3]2[μ-(NH)2CC6H4C(NH)O] ([NN−ph− NO]), which are closely related to the three symmetrical analogues [OO− ph−OO], [NO−ph−NO], and [NN−ph−NN] reported earlier. The mixedvalence (MV) complexes [NO−ph−OO]+, [NN−ph−OO]+, and [NN−ph− NO]+ exhibit metal to ligand and ligand to metal charge transfer bands, along with an intervalence (IV) transition absorption in the Near-IR region. The free energy change (ΔG°) for ET is determined by comparing the redox potential splitting (ΔE1/2) and IV transition energy (EIT) with the data for the symmetrical species. The reorganization energy (λ) is estimated from the Hush model (Δν1/2 = [16ln(2)λRT]1/2). Significantly, electrochemical and optical analyses verify EIT = ΔG° + λ, the core energetic relationship underlying the semiclassical theories. With the electronic coupling parameters calculated from the method suggested by Creutz, Newton and Sutin (HMM′ = ∼ 500 cm−1), the adiabatic ET rate constants ket (f) are determined to be ∼1010 s−1 for [NO−ph−OO]+ and [NN−ph−NO]+, smaller than ket (r) for the backward reaction and ket for the symmetrical analogues by 1 order of magnitude, and ∼109 s−1 for [NN−ph−OO]+. This work illustrates that the redox asymmetry in D−B− A systems controls the ET rate and direction.

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INTRODUCTION Mixed-valence (MV) compounds consisting of three assembled units serving as electron donor (D), bridge (B) and acceptor (A) have been valuable models to theoretically and experimentally elucidate the processes of electron transfer (ET).1−4 Starting with Creutz−Taube complexes {[Ru(NH3)5](pyrazine)[Ru(NH3)5]}5+,5,6 innumerous D−B−A assemblies, typically Ru−Ru,7−9 Fc−Fc,10−12 Ru3−Ru313,14 metal complex and pure organic systems15,16 have been intensively investigated. Most of the studies deal with symmetrical systems in which the donor and acceptor are structurally identical. In comparison, much less work has been done on asymmetrical D−B−A systems, which is partially due to the synthetic obstacles. In fact, with distinguishable donor and acceptor, asymmetrical systems greatly enrich the chemistry in the fields of charge transfer and mixed-valency. For example, in asymmetrical Ru3−Ru3 systems, Kubiak and coworkers observed mixed-valence isomers in dynamic equilibrium on the picoseconds time scale.17−19 Semiclassical theories predict that the free energy change (ΔG°) controls the ET direction and rates. Verifying the existing theories demands elegantly designed and synthesized D−B−A systems with a nonzero ΔG°.20−22 In asymmetric cases, the D−B−A system becomes electronically nondegenerate before and after electron transfer. The ET reaction is favored or disfavored depending upon the free © XXXX American Chemical Society

energy change (ΔG°) or driving force (−ΔG°). Thus, the magnitude of ΔG° is one of the major factors that govern the ET reactions. Through the vibronic two-state model, Marcus− Hush theories have elucidated the kinetics, dynamics and pathways of ET reactions in asymmetrical systems,1,2,23 as described by Figure 1 for the case of ΔG° > 0. In semiclassical theories, by assuming that the reactant and product at the equilibrium of the energy surfaces have identical force constants, the energy required for optical (diabatic) ET is the sum of reorganization energy (λ) and the free energy change (ΔG°),1,3,4 E IT = ΔG° + λ

(1)

This energetic relationship regulates the dynamics and kinetic of the diabatic and adiabatic ET processes. However, it is hardly proven by experiments because the reorganization energy (λ) of the system, or Franck−Condon barrier (EFC) for electron transfer, is not measurable. For adiabatic electron transfer, donor−acceptor electronic coupling is another major factor that controls the electron transfer. Quantitatively, the coupling parameter Hab is equal to half of the splitting between the upper and lower energy surfaces (Figure 1).24 Increasing electronic Received: April 13, 2016 Revised: May 21, 2016

A

DOI: 10.1021/acs.jpcc.6b03792 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

(NH)O] ([NN−ph−NO]. The three compounds are differentiated from each other by atomic alternation of N/O on the bridging ligand, but have similar molecular structures as characterized by 1H NMR and single-crystal X-ray diffraction, as shown in Figure 2. The corresponding mixed−valence

Figure 1. Marcus−Hush potential energy surfaces for asymmetrical IV systems (ΔG° > 0), the diabatic surfaces and adiabatic surfaces are presented by dashed and solid lines, respectively.

coupling lowers the activation energies for the forward (ΔGf*) and backward (ΔGr*) reactions, consequently, accelerating thermal electron transfer in both directions. It is remarkable that based on the semiclassical theories, the ET kinetics and dynamics in MV systems may be determined by optical analyses.25−27 However, reports on full optical determination of ET rates for D−B−A systems are limited,28,29 especially in asymmetrical systems. Troubles in practical application of the theories and methodologies include assignments of the intervalence bands, and completeness and reliability of the energetic data. On the other hand, asymmetrical D−B−A molecules have been actively involved in the research aiming at development of molecular electronics.30,31 In 1974, Aviram and Ratner proposed that when embeded in an electric circuit, an asymmetric D−B−A molecule could function as a molecular rectifier, exhibiting unilateral conductivity.32 It is even more interesting that such a rectifying behavior was observed with molecules whose asymmetry is caused by single atom difference between the two termini. For instance, with thiophene (C4S) and thiazole (C3NS) diode molecules, it was observed that alternation of N/C on the building blocks changes the bias current direction.33 In recent work by Tao and Yu, the rectifying behavior observed on dipyrimidinyldiphenyl was interpreted in terms of localization of the LUMO.34 Therefore, the unilateral conductivity of asymmetrical molecules in an electric circuit is somehow similar to oriented direction of the intramolecular electron transfer in D−B−A systems with ΔG ≠ 0. As is known, for structurally asymmetrical molecules there must be an internal energy difference between the two termini; however, the correlation of redox asymmetry of the molecule with its diode behavior is not well understood. To gain insight into these issues, asymmetrical metal complex D−B−A assemblies are of particular interest, for which the donor− acceptor potential difference is controllable and measurable. In this report, we studied the electronic coupling and electron transfer between two bridged Mo2 units in three asymmetrical dimolybdenum dimers, [Mo2(DAniF)3]2[μ(NH)OCC6H4CO2] ([NO−ph−OO]) (DAniF = N,N′-di(panisyl)formamidinate), [Mo2(DAniF)3]2[μ-(NH)2CC6H4CO2] ([NN−ph−OO]), and [Mo2(DAniF)3]2[μ-(NH)2CC6H4C-

Figure 2. Schematic representation of the molecular scaffold for the three closely related asymmetrical [Mo2]−ph−[Mo2] complexes under investigation.

radical cations, [NO−ph−OO]+, [NN−ph−OO]+, and [NN− ph−NO]+, were obtained by chemical one-electron oxidation of the neutral compounds. All of the neutral complexes show a metal to ligand charge transfer (MLCT) absorption band, while the MV species exhibit an additional electronic absorption in the visible region for ligand to metal charge transfer (LMCT), plus an intervalence (IV) or metal to metal (MM) charge transfer band in the near-IR region. The optical behaviors of these complexes are explicitly attributed to the transferring δ electrons on the quadruply bonded Mo2 centers.35 Importantly, these complexes present electronic properties quite different from those for the closely related symmetrical analogues, [OO−ph−OO],36 [NO−ph−NO], and [NN−ph−NN]37,38 because of the subtle redox asymmetry. Provided with these structurally and electronically well-defined asymmetrical systems, we were able to examine the energetic relationship in the mixed-valence systems by means of electrochemistry and spectroscopy and investigate the electron transfer dynamics and kinetics for both optical and thermal electron transfer within the established theoretical framework.

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RESULTS AND DISCUSSION Syntheses and Structures. Synthetically, an asymmetric D−B−A system can be created by using different chemical building blocks for the D and A entities or employing an asymmetrical bridging ligand. For complex systems, to introduce redox asymmetry, one can change either the metal nuclearity or the supporting ligands. In this work, we obtained the asymmetric [Mo2]−bridge−[Mo2] systems by assembling building blocks [Mo2(DAniF)3]+ with an asymmetric bridging ligand −EOCC6H4C(NH)E− (E = NH or O). By successive variation of the donor atoms (or groups) E (= NH and O), three asymmetric complexes in a series, [NO−ph−OO], [NN−ph−OO], and [NN−ph−NO] (vide supra), were produced. The advantage of this method is that the target molecules can be synthesized by a convergent one-pot reaction, B


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similar Mo2···Mo2 distances, ca. 11.3 Å. Complex [NO−ph− OO] has a small dihedral angle between the central phenyl group and the adjacent Mo2 chelating rings, ca. 13.03°, in comparison with [NN−ph−OO] (27.24°) and [NN−ph−NO] (29.36°). In the series, the torsion angle increases as more NH groups are introduced to the bridging ligand, which is likely due to the steric repulsion between the NH and the phenylene groups. Nevertheless, the relative small torsion angles indicate that all three complexes adopt quasi-coplanar geometry in solution. The subtle differences in structure for the series guarantee similar force constants of nuclear vibrations. Electrochemical Properties and Redox Asymmetries. The cyclic voltammograms (CVs) for [NO−ph−OO], [NN− ph−OO], and [NN−ph−NO] are shown in Figure 4 and the electrochemical parameters are listed in Table 1. Each of these asymmetrical compounds presents two electrochemically reversible redox waves, corresponding to consecutive oneelectron oxidations of the two dimetal centers from Mo24+ to Mo25+. The potential separations (ΔE1/2) between the two [Mo2] units are significantly larger than those for the closely related symmetrical analogues, although they differ only by one or two redox inactive ligating atom(s) (Table 1). For asymmetrical D−B−A complexes, the potential separation ΔE1/2 is attributed to the electronic interaction and the internal potential difference between D and A, namely, Eip. The magnitude of Eip directly measures the extent of redox asymmetry of the molecule. The larger ΔE1/2 values for [NO−ph−OO] (150 mV), [NN−ph−OO] (230 mV) and [NN−ph−NO] (145 mV), relative to those for [OO−ph− OO] (100 mV),39 [NO−ph−NO] (96 mV), and [NN−ph− NN] (80 mV),37 are indicative of the redox asymmetry, which is introduced by the subtle structural asymmetry. The largest ΔE1/2 value for [NN−ph−OO] suggests that electronically, it is the most asymmetrical species. Our earlier work showed that compared to O atoms, N chelating atoms of the Mo 2 coordination shell raise the δ electron energy.37 Accordingly, for these dimers of dimers, the single oxidation occurs on the [Mo2] unit with more N ligating atoms. Therefore, for the MV D−B−A complex, this cationic [Mo2]+ unit serves as the electron acceptor, and the neutral [Mo2] unit, which has more O chelating atoms, is the electron donor. As indicated by the comproportionation constants (KC) (Table 1), the redox asymmetry also increases the thermodynamic stability of the mixed-valence species. In previous work, we demonstrated that the internal potential difference (Eip) in an asymmetrical donor−acceptor system may be determined from the redox potentials for two Mo2 reference compounds that are structurally and electronically related to the donor and acceptor sites, respectively,36 as shown by eq 2.

the same procedure for preparation of the symmetric analogues.37,38 The three asymmetrical complexes can be distinguished in solution by 1H NMR though the molecular structures are quite similar. In the 1H NMR spectra, the amidate (NH) proton resonances are observed at 9.10 for [NN−ph−NO] and 9.14 ppm for [NO−ph−OO], and the amidinate (NH) proton signals appear at high field, i.e., 8.35 ppm for [NN−ph−NO] and 8.38 ppm for [NN−ph−OO]. All the three compounds in the series crystallized in the monoclinic space group P21/c with the molecules residing in a special position (Z = 2). The crystallographic data and collection parameters are presented in Table S1 and selected bond distances and angles shown in Table S2 (see Supporting Information). For symmetry reason, the crystal structures of these molecules are intrinsically disordered. Routine treatments were carried out by separating the Mo2 coordination shell into two parts. Refinements of the “two halves” of the [Mo2] unit with a fixed FVAR (free variable of 0.5) gave satisfactory results. The molecule as a whole is generated by combining two asymmetrical parts (Figure 3). The three compounds have very

E ip = E1/2(donor) − E1/2(acceptor)

(2)

In this study, three Mo2 monomers, Mo2(DAniF)3[(NH)2CC6H5] ([NN−ph]), Mo2(DAniF)3[(NH)OCC6H5] ([NO−ph]) and Mo2(DAniF)3(O2CC6H5) ([OO−ph]) were prepared, which serve as the reference compounds for the donor or acceptor in the Mo2 dimers. The half-wave potentials were measured from the reversible redox couples (Figure 4). For these Mo2 monomers, the variation trend of E1/2 is consistent with N > O in basicity, as indicated by the E1/2 values, 195 mV for [NN−ph], 280 mV for [NO−ph], and 335 mV for [OO−ph]. Therefore, [NO−ph] is the reference

Figure 3. X-ray crystal structures for [NO−ph−OO] (A), [NN−ph− OO] (B) and [NN−ph−NO] (C). The atoms are differentiated by color codes: Mo (gray green), O (red), N (blue), and C (gray). C


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Figure 4. Cyclic voltammograms (CVs) for the dimers of dimers (left) and the associated monomers (right).

(Figure 5). This absorption band can be assigned unambiguously to the metal (δ) to bridging ligand (π*) charge transfer (MLCT).36,41,42 For the series, the MLCT energy is lowered as the nitrogen atoms of the bridging ligands are stepwise substituted by oxygen atoms (Table 2), showing similar variation trend as reported for the symmetric analogues.37,38 The MLCT band energies are significantly higher than those for the analogues with carboxylate and thiocarboxylate bridging ligands.36 The mixed-valence compounds were produced by chemical one-electron oxidation using one equiv of ferrocenium hexafluorophosphate (Cp2FePF6) and characterized by electron paramagnetic resonance spectroscopy (EPR) (see Supporting Information). The g values of ∼1.94 indicate that for the radical cations, the odd electron resides on a δ orbital of the Mo2 center. In comparison with those of the neutral precursors (Figure 5), the electronic spectra for the MV complexes are broadened, and the band maxima are blue-shifted and the intensity lowered. Gaussian deconvolution of this broad absorbance gives two overlapping bands (Figure 5). In previous study, we have seen that the MLCT absorption band for a MV complex is similar, in energy and shape, to the spectra for the corresponding neutral compound, but the intensity is significantly lower.43 Different from the neutral precursor, the MV complex may exhibit an isolated LMCT band. However, for weak coupling complexes, including the symmetrical species with N/O chelating atoms, only one absorption band was observed because the LMCT band is high in energy and overlapped with the MLCT band.37,43−45 Therefore, for these asymmetrical MV complexes, the electronic spectra are attributed to the MLCT and LMCT absorptions without ambiguity. The lower energy band is assigned to the MLCT absorption because the band maximum corresponds to that for its neutral precursor. As shown in Figure 5, the mixed-valence compounds display a typical metal to metal charge transfer (MMCT) or intervalence transition (IV) absorption band in the Near-IR region. The spectroscopic data are presented in Table 3. For Mo2 D−B−A systems as such, the intervalence absorption records the energy required for optical transition of an δ electron from the neutral Mo2 unit (donor) to the singly oxidized one (acceptor), namely, EIT. The IV absorptions appear at high energy with low intensity in comparison with the spectra for the closely related symmetrical analogues.37,38 For example, for [NO−ph−OO]+, the IV band maximum appears at 5330 cm−1 with a molar extinction coefficient (εIT) of 940

Table 1. Redox Potentials for the Mo2 Dimers and the Associated Monomers, and the Comproportionation Constants for the Dimers, along with the Data for the Symmetric Complexesa compound [NO−ph− OO] [NN−ph− OO] [NN−ph− NO] [OO−ph− OO] [NO−ph− NO] [NN−ph− NN] [NN−ph] [NO−ph] [OO−ph]

E1/2(1) (mV)

E1/2(2) (mV)

ΔE1/2 (mV)

KC (cm−1)

Eip (mV)

ΔE1/2(net) (mV)

286

436

150

−1210

55

95

170

400

230

−1854

140

90

170

315

145

−1170

85

60

225

325

100

−807

0

100

302

398

96

−774

0

96

201

281

80

−645

0

80

195 280 335

For the dimers of dimers with ΔE1/2 < 160 mV, the ΔE1/2 values were obtained from the Richardson and Taube methods.40 According to ref 40, the E1/2 values were calculated from E1/2(2) = Ec + (ΔE1/2 + Epul)/ 2 and E1/2(1) = E1/2(2) − ΔE1/2. Ec = the center potential of the DPV trace. Epul =50 mV. a

compound for the donor of [NN−ph−NO]+, but for the acceptor of [NO−ph−OO]+. Given these E1/2 values, the internal potential differences Eip are calculated to be 55 mV (444 cm−1) for [NO−ph−OO], 140 mV (1130 cm−1) for [NN−ph−OO], and 85 mV (686 cm−1) for [NN−ph−NO]. For these asymmetrical D−B−A systems, subtracting Eip from ΔE1/2 gives a “net” potential displacement, ΔE1/2(net). The two major factors affecting the magnitude of ΔE1/2(net) are electrostatic interaction and electron delocalization. Thus, this parameter can be used to evaluate the relative coupling strength for complexes having similar Mo2···Mo2 separations. As shown in Table 1, the asymmetrical compounds have the ΔE1/2(net) values smaller than the ΔE1/2 values for the related symmetrical analogues, meaning that the metal to metal interaction in the asymmetrical complexes is weakened. Similar results were obtained for the dithiolated isomers [SO−ph−SO] and [OO− ph−SS].36 Spectroscopic Properties of the Neutral and MixedValence Complexes. Each of the neutral [Mo2]−ph−[Mo2] complexes presents an intense absorption in the visible region D


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Table 3. Electron Transfer Dynamic Parameters Determined from Different Methods compd

Eip (cm−1)

Δ(ΔE1/2) (cm−1)

ΔEIT (cm−1)

ΔG°(av) (cm−1)

λ (cm−1)

[NO−ph−OO]+ [NN−ph−OO]+ [NN−ph−NO]+

444 1129 686

436 1210 524

680 1300 560

520 1213 590

4800 5242 4975

M−1cm−1, while for [OO−ph−OO]+, EIT = 4240 cm−1 and εIT = 1470 M−1cm−1.42 Replacing a single N atom in [NN−ph− NN]+ with an O atom generates [NN−ph−NO]+ and increases the transition energy EIT from 4980 to 5540 cm−1 (Table 3). The highest band energy for [NN−ph−OO]+ (6280 cm−1) is consistent with its largest redox asymmetry (Eip = 140 mV). The blue shifts and low intensities of the IV bands indicate that electronic coupling in the asymmetrical complexes is generally weakened. Therefore, the optical behaviors of the MV complexes conform well to their electrochemical properties. Determination of ΔG° for Electron Transfer. Nonzero free energy change, i.e., ΔG° ≠ 0, is an important thermodynamic character for ET reaction occurring in asymmetrical systems. In the current study, the donor [Mo2] unit has more positive reduction potential as compared to that of the acceptor. With ΔG° > 0, the donor−acceptor electron transfer is an energetically uphill process, as shown in Figure 1. The internal potential difference between the donor and acceptor, Eip, may be viewed as the enthalpy change for the ET reactions. It can be taken as the free energy change (ΔG°) for the reaction when the entropy change (ΔS) is negligible.20,46 Similar treatments have been reported in metal complex and organic systems.47−49 By assuming ΔG° = Eip, the ET kinetics for asymmetrical [OO−ph−SS]+ was studied recently.43 For the three complex systems, the Eip values in wavenumbers (cm−1) are relisted in Table 3 (vide supra). According to the semiclassical theories, ΔG° is correlated to the vertical transition energy (EIT) and the reorganization energy (λ) by EIT = ΔG° + λ.2,46 From this expression, driving force (−ΔG°) may be derived directly from the optical data if reorganization energy (λ) is available. However, this approach is difficult to achieve because the determination of reorganization energy is practically infeasible. There are two terms consisting of the total reorganization energy (λ), nuclear (λv) and solvent (λs) reorganization energies, that is, λ = λs + λv. While λs can be quantitatively evaluated according to dielectric continuum theory established by Marcus50−52 and Hush,53,54 experimental determination of nuclear vibrational reorganization energy is impossible. Earlier work suggested that ΔG° values for asymmetrical D−B−A complexes may be estimated simply from the differences in redox potential splitting (ΔE1/2) or IVCT energies (EIT) between the asymmetrical and the

Figure 5. Vis−near−mid-IR spectra of the mixed-valence complexes (darker lines), (A) for [NO−ph−OO]+, (B) for [NN−ph−OO]+, and (C) for [NN−ph−NO] + , in comparison with that for the corresponding neutral precursor (lighter lines). In the insets, amplified spectra are presented to show the intervalence absorption bands. The dashed lines are the deconvoluted LMCT and MLCT bands for the MV complexes.

Table 2. Electronic Coupling Parameters Calculated from the CNS Model in Comparison with Those for Symmetric Systems

a

compd

EML (cm−1)

εML (M−1 cm−1)

Δv1/2(ML) (cm−1)

ELM (cm−1)

εLM (M−1 cm−1)

Δν1/2(LM) (cm−1)

EIT (cm−1)

εIT (M−1 cm−1)

Δν1/2(IT) (cm−1)

[NO−ph−OO]+ [NN−ph−OO]+ [NN−ph−NO]+ [OO−ph−OO]+ a [NO−ph−NO]+ b [NN−ph−NN]+ b

19780 20000 20400 20600 20410 21500

8060 8050 10830 15230 21720 16270

4500 5100 4400 5070 3780 4610

22350 22500 22900

7500 7500 7780

3700 3260 3400

5330 6280 5540 4240 4650 4980

940 850 1010 1470 1170 520

3330 3480 3390 4410 5240 5840

Data cited from ref 43. bData calculated by CNS model from the neutral spectra in ref 37. E


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The Journal of Physical Chemistry C closely related asymmetrical analogue,55 as described by eq 3 and 4, respectively. ΔG° = Δ(ΔE1/2) = ΔE1/2(asym) − ΔE1/2(sym)

(3)

ΔG° = ΔE IT = E IT(asym) − E IT(sym)

(4)

bandwidths.58 Therefore, the three systems are differentiated by having different energy parameters EIT, ΔG° and λ, which are determined independently (Tables 2 and 3). Significantly, summation of free energy change, ΔG°(ave), and reorganization energy λ equals essentially the IV transition energy EIT. For example, for [NO−ph−OO]+, EIT = 5330 cm−1 ≈ ΔG° + λ = 520 + 4800 = 5320 cm−1. From these data, EIT = ΔG° + λ, the core energetic relationship in semiclassical theories, is proven directly. Furthermore, as the system alternates, there exists a correlation of parameter differences, δEIT ≈ δΔG° + δλ. For example, for system variation from [NO−ph−OO]+ to [NN− ph−OO]+, δEIT = 950 cm−1 and δΔG° = 774 cm−1 where ΔG° = Δ(ΔE1/2); from [NN−ph−OO]+ to [NN−ph−NO]+, the variations of EIT and ΔG° are 740 cm−1 and 686 cm−1, respectively. Here, the quantity δEIT/δΔG° of 1.1−1.2 indicates that while the change of reorganization energy is minimized, manipulation of the redox asymmetry would cause an equivalent variation of the intervalence transition energy.20 The potential energy surfaces (Figure 6), drawn based on the two-state model24 and the experimental data, illustrate the quantitative correlation between these parameters.

However, since for a given asymmetrical system, there are two possible symmetrical analogues, the following question is how to choose an appropriate symmetrical system for comparison. For Ru−Os systems, the comparison was made with Ru− Ru.55,56 Alternatively, the averaged ΔE1/2 value from the two related symmetrical species may be used as the reference point for estimating ΔG° for the asymmetrical complexes.57 In this study, we chose the symmetric reference compound for a given asymmetrical dimer so that they have similar potentials (E1/2(1)) for the first redox couple. In other words, the two MV complexes in a pair have an identical [Mo2] unit as the electron acceptor but different donor. For example, for [NO−ph−OO]+, we consider [NO−ph−NO]+ to be the reference compound, because for both, the first one-electron oxidation occurs at the same [Mo2] unit, or the “[NO]” terminal. Linking “[NO]” with the “[OO]” through a phenylene group enlarges the potential separation between the two redox sites, with respect to ΔE1/2 for [NO−ph−NO]+. For the same reasons, [NN−ph−NN]+ is the reference compound for [NN−ph−OO]+ and [NN−ph−NO]+. Using the ΔE1/2 data in Table 1, the Δ(ΔE1/2) values for the asymmetrical species are calculated from eq 3 (Table 3). The ΔG° value determined by this way agrees generally with the internal potential difference Eip. However, there are two factors that cause the deviations between Eip and Δ(ΔE1/2), electron delocalization in the dimer and different entropy effects for the monomers and dimers. In calculation of ΔG° from EIT (eq 4), the same symmetrical reference compounds were employed. The results are given in Table 3 in comparison with the data obtained from different methods. For [NN−ph−OO]+ and [NN−ph−NO]+, ΔEIT ≈ Δ(ΔE1/2), confirming the relationship between ΔEIT and Δ(ΔE1/2) proposed by Meyer.55 For [NO−ph−OO]+, however, notable deviation is found between ΔEIT (680 cm−1) and Δ(ΔE1/2) (436 cm−1). To lower the systematic deviations, averaged ΔG° values were obtained (Table 3). According to Hush, for weakly coupled systems, the halfheight bandwidth of an IV band (Δv1/2) is a function of reorganization energy (λ) eq 5.58,59 Thus, at the high temperature limit, λ can be calculated directly from eq 6, where Δν1/2 is the measured bandwidth.29 Δv1/2 = [16(ln(2)λkBT ]1/2

(5)

λ = (Δν1/2)2 /2310

(6)

Figure 6. Potential energy surfaces for the redox asymmetric Mo2 dimers. For these systems, the free energy change (ΔG°) is manipulated by atomic alternation of N/O on the bridging ligand, while the reorganization energy λ remains nearly constant. The energy parameters (EIT, ΔG°, and λ) are labeled based on the mostly asymmetrical system [NN−ph−OO]+ (green). The differences in IV transition energy (δEIT) and free energy change (δΔG°) between [NN−ph−OO]+ (blue) and [NN−ph−NO]+ (red) are shown in the diagram.

The calculated λ values are listed in Table 3. It is remarkable that the three complex systems present similar λ values, ca. 5000 ± 300 cm−1, which is ascribable to the structural similarity of the systems. On the other hand, these data, although the differences are small, show that the reorganization energy changes with the redox asymmetry. Corresponding to their similar ΔG° values, [NO−ph−OO]+ and [NN−ph−NO]+ have similar λ values for the ET reactions, while appreciably larger λ is found for [NN−ph−OO]+. It should be addressed that this method of deriving λ is applicable only for valence trapped Class II systems, for which there exists reasonable consistency between the observed and calculated IV

Electronic Coupling Matrix Elements and Electron Transfer Mechanism. The Mulliken−Hush expression eq 7 is widely used to calculate the electronic coupling matrix element Hab for mixed-valence D−B−A systems.1,58 A distinct approach to derive the electronic coupling parameter was introduced by Creutz, Newton, and Sutin based upon the McConnell F


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The Journal of Physical Chemistry C Table 4. Electronic Coupling Matrix Elements HMM′, in Comparison with Hab Derived from the Hush Modela compd +

[NO−ph−OO] [NN−ph−OO]+ [NN−ph−NO]+

HML (cm−1)

ΔEML (cm−1)

HLM (cm−1)

ΔELM (cm−1)

HMM′ (cm−1)

Hab (cm−1)

3010 3220 3500

16700 16280 17200

2800 2630 2760

19320 18850 19750

470 500 540

460 480 490

a

For calculation of HML and HLM from eq 7, rML = rLM = 5.8 Å, estimated from the centroid to centroid distance between the Mo2 unit and the phenylene spacer.

Figure 7. Schematic illustration of superexchange mechanism for donor−acceptor electron transfer via the bridge in the asymmetrical [Mo2]−ph− [Mo2] systems.

superexchange mechanism, as a so-called “CNS” formalism.60,61 The CNS model adopts spectral data for the electronic ML and LM transitions and vibronic IV transitions through three eqs eq 8−10. Owing to the well-defined electronic configuration and unambiguous spectroscopic assignments, the CNS alternative is considered to be more suitable for calculation of the electronic HMM′ (to distinguish from Hab derived from the Hush model). Hab = 2.06 × 10−2 HMM ′ =

found for [NN−ph−NO]+ because there are more N chelating atoms on the charge transfer platform. The HMM′ values are also compatible with the Hab data for asymmetrical Ru−Os complexes, for example, 406 cm−1 for [Os(bpy)2(bpt)Ru(tpy)Cl] (bpy =2,2′-bipyridyl; Hbpt =3,5-bis(pyridin-2-yl)1,2,4triazole and tpy =2,2′,6′,2″-terpyridine)62 and 598 cm−1 for [(bpy)2Ru(bpt)Os(bpy)2]3+.63 The coupling parameters HMM′ are confirmed by the data Hab calculated from Hush model (Table 4). It is recognized that calculation of coupling parameters from eq 7 underestimates the electronic coupling strength when rab is the distance separating the localized (diabatic) donor and acceptor centroids.64−66 In calculation of Hab, the effective electron transfer distance (r′ab) is estimated from the size of the phenylene spacer “−CC6H4C−” in considering that the δ electrons are fully delocalized within the [Mo2] unit via d(δ)−p(π) conjugation.36,38,43 For all the complex systems, the results from the two different approaches show excellent consistency with HMM′/Hab ≈ 1. This is remarkable because major discrepancies between HMM′ and Hab were reported in other donor−acceptor systems.67−70 Furthermore, in most of the MV systems, only one kind of absorption in the spectra, either MLCT60,68 or LMCT69,70 band, was observed or assigned unambiguously; therefore, only one term in eq 8 was involved in calculation of HMM′. Here, both ML and LM contributions to the MM coupling are accounted for, exemplifying the application of the CNS formalism. The appearance of both MLCT and LMCT bands in the electronic spectra illustrates that the electron coupling and electron transfer between the two [Mo2] units are dominated by the superexchange mechanism involving simultaneous electron-hopping and hole-hopping.71−73 According to CNS, the magnitudes of HML and HLM reflect quantitatively the contributions of ML and LM interactions to the MM coupling. This weakly coupled N/O series has HLM values of 2600−2800 cm−1, less than HML by about 200−300 cm−1. However, for the O/S series,43 much smaller HLM values (ca. 1500−1700 cm−1) were found, although the HML values (ca. 4200−4300 cm−1) are generally large. Evidently, in the asymmetrical systems the ligand to metal coupling plays a more important role than in symmetrical systems. For systems as such, the hole-hopping is also the major pathway and relaxation of the intermediate state becomes a prominent process. Accordingly, the superexchange mechanism for the Mo2−Mo2

(Δν1/2εmax EIT )1/2 rab

HMLHM ′ L H H + LM LM ′ 2ΔEML 2ΔE LM

(7)

(8)

⎛ 1 1 1 ⎞ = 0.5 × ⎜ + ⎟ ΔEML EML ⎠ ⎝ EML − E IT

(9)

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

(10)

According to eq 8, when direct electronic coupling between the two bridged redox sites is neglected, effective metal to metal couplings are contributed by the metal−ligand coupling interactions HML and HLM. In addition, the metal to ligand coupling for the two sites M−BL and M′−BL, HML and HM′L, are assumed to be equal, and so are HLM and HLM′, which are calculated from eq 7 using the corresponding spectroscopic data. In eq 8−10, ΔEML and ΔELM represent the effective energy gaps for ML and LM transitions, respectively. Calculations from the spectroscopic data in Table 2 yielded the CNS parameters as listed in Table 4. Prior to this work, it was noted that in the CNS formalism the calculated metal−ligand transition gaps, ΔEML and ΔELM, are related to MLCT and LMCT band energies (EML and ELM), respectively.43 In these asymmetrical systems, this correlation exists as well. The MLCT and LMCT energies in the spectra are determined in the range of 20000−23000 cm−1, larger than ΔEML and ΔELM by about 3000 cm−1. For each of the systems, it is found that HML > HLM, and ΔEML < ΔELM, consistent with the experimental results (EML < ELM). The calculated HMM′ values, 470 cm−1 for [NO−ph−OO]+ and 500 cm−1 for [NN− ph−OO]+, are reasonably smaller than the data for the symmetrical analogues. Relatively large HMM′ (540 cm−1) is G


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Table 5. Calculated Kinetic Parameters for the Adiabatic and Diabatic ET Reactions in the [Mo2]−ph−[Mo2] Systems adiabatic asymmetric

ΔG*(r)

ket ( f)

ket (r)

ΔG*( f)

1050 1470 1070

530 260 480

3.1× 1010 4.1 × 109 2.8 × 1010

1470 1990 1480

ΔG*

3.8 × 1011 1.4 × 1012 4.9 × 1011 ket (s−1)

580 650 760

3.0 × 1011 2.1 × 1011 1.3 × 1011

[NO−ph−OO]+ [NN−ph−OO]+ [NN−ph−NO]+ symmetric +a

[OO−ph−OO] [NO−ph−NO]+ b [NN−ph−NN]+ b a

diabatic

ΔG*( f)

ΔG*(r) 950 780 890 ΔG* 1060 1240 1160

ket ( f) 4.1× 109 3.3 × 108 3.9 × 109

ket (r) 1.0 × 1010 1.2 × 1011 6.8 × 1010 ket (s−1) 3.0 × 1010 1.2 × 1010 1.8 × 1010

Cited from ref 43. bCited from ref 38.

reaction (ΔG*(r)). The activation energy for adiabatic ET reaction is generally lower than that for the diabatic reaction because of the electronic coupling. Systems [NO−ph−OO]+ and [NN−ph−NO]+ have similar activation energies, corresponding to their similar extent of redox asymmetry and reorganization energies. The ΔG*( f) values (∼1000 cm−1) for thermal electron transfer are appreciably less than, but close to λ/4 (∼1250 cm−1), which reflects the nature of weak coupling of the systems. For [NN−ph−OO]+, a relative large ΔG*(f) is found for both optical and thermal electron transfer. It is also interesting to note that the ET barrier (260 cm−1) for the backward reaction is substantially lower than that for the symmetrical systems (ΔG*) (Table 5). Obviously, these results are ascribable to its large redox asymmetry and electron localization. Importantly, for these systems, the energy barriers for optical electron transfer are higher than those for the thermal process by ∼500 cm−1. This difference is about the magnitude of HMM′, thus, proving that the separations between the lower and upper potential energy surfaces in the transition state is about 2HMM′ (Figure 6), as predicted by the two-state model. For [NO−ph−OO]+ and [NN−ph−NO]+, the rate constants ket( f) (∼1010 s−1) are less than ket(r) by 1 order of magnitude. These rates are comparable with the data for other systems with similar electron transfer distances. For example, asymmetrical Ru3−pz−Ru3 systems have a ket value of 1011 s−1 for the uphill ET process, determined from IR line shape for carbonyl stretch frequency ν(CO),17 while the ET reaction for the downhill reactions is faster by 1 order of magnitude.18 The Kubiak group also showed that the IR band ν(CO) analyses gave rate constants significantly larger than the results from IV band analyses.14 Thus, for the [Mo2]−ph−[Mo2] and Ru3− pz−Ru3 systems, the rate constants determined from semiclassical theories fall in the same range. With the largest internal potential difference (Eip), the least ket ( f) (4.1 × 109 s−1) and the largest ket (r) (1.4 × 1012 s−1) are found for [NN−ph− OO]+. For this complex, the backward ET reaction is even faster than the process in [NO−ph−NO]+ (ket = 2.1 × 1011 s−1) due to the large driving force. Consistent results were obtained for the asymmetrical O/S analogue. For [OO−ph− SS]+, which has a redox asymmetry of 2226 cm−1, the forward and backward ET reactions have rate constants of 4.1 × 107 s−1 and 8.6 × 1011 s−1, respectively.43 Interestingly, the amidate supported [Mo2] unit in [NN−ph−NO]+ functions as the electron donor, while the same [Mo2] unit in [NO−ph−OO]+ is the acceptor, although they have similar rate constants. In other words, in the two systems the electron transfer directions are reversed, with respect to the same terminal. Significantly, our results demonstrate that alternation of N/O on the bridging ligand modulate the electron transfer rates and control

systems can be described by Figure 7. In addition, in contrast to the strongly coupled [OS−ph−OS]+ and [SS−ph−SS]+,43 the asymmetrical complexes show a high energy LM transition in the spectra. Again, this observation can be rationalized by the redox asymmetry. For instance, for [NN−ph−OO]+, the odd electron is localized in the amidinate bridged Mo2 center (acceptor), which is higher in energy than the carboxylate supported Mo 2 center (donor) because of the redox asymmetry. Consequently, on the acceptor site, the energy gap for electron-hopping from the bridging ligand (π) to the δ orbital of the Mo2 center (L → M) is larger than that for the metal (δ) to ligand (π*) charge transfer. Kinetics of the Intramolecular Electron Transfer Reactions. Similar reorganization energies are expected for the asymmetrical and the associated symmetrical systems. In comparison with EIT (= λ) for the related symmetrical systems, [NO−ph−OO]+ and [NN−ph−OO]+ have the λ values larger than those for the symmetrical analogues by several hundred wavenumbers (Table 2). Since the differences are fairly small, and almost within the range of experimental errors, similar λ values were obtained for [NN−ph−NO]+ (4975 cm−1) and [NN−ph−NN]+ (4980 cm−1). These results prove the reliability of the calculated λ values for the asymmetrical systems. With 2HMM′ ≪ λ, all the species should be considered to be weakly coupled Class II in terms of Robin−Day’s classification of mixed-valence compounds.36,74 For systems of Class II, electron transfer may proceed via optical and thermal pathways, corresponding to the diabatic and adiabatic processes, respectively. As shown in Figure 6, the ET process must overcome an activation energy (ΔG*). The activation energies ΔG*(adia) and ΔG*(dia) can be calculated by eq 11,52 and eq 12,24 respectively. The electron transfer rate constant (ket) is derived from eq 13, for which the electronic factor (κ) is assumed to be unity and an averaged nuclear frequency, νn = 5 × 1012 s−1, is adopted.3,23 ΔG*(dia) =

ΔG*(adia) =

(λ + ΔG°)2 4λ

(11)

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

2 Hab (λ + ΔG°)

ket = κνnexp( −ΔG*/kBT )

(12) (13)

The calculated ΔG* and ket are summarized in Table 5. For these systems, the free energy of activation for the forward reaction (ΔG*(f)) is about twice as large as that of the reverse H


Article


appropriate drying agents and collected for further use under nitrogen atmosphere. Benzamidine hydrochloride, benzamide, 4−carbamoylbenzoic acid and 4-amidinobenzamide hydrochloride were obtained from commercial sources. 4-amidinobenzoic acid,75 HDAniF,76 Mo2(DAniF)3(O2CCH3),77 and Mo2(DAniF)3(μ-O2CC6H5)43 were synthesized according to literature procedures. Physical Measurements. Electronic spectra were measured on a Shimadzu UV-3600 UV−vis-NIR spectrophotometer in CH2Cl2 solutions. 1H NMR spectra were recorded on a Bruker Avance III 300 spectrometer. Cyclic voltammograms (CVs) and differential pulse voltammograms (DPVs) were performed using a CH Instruments Model-CHI660D electrochemical analyzer in 0.10 M CH2Cl2 solution of nBu4NPF6 with Pt working and auxiliary electrodes, Ag/AgCl reference electrode, and a scan rate of 100 mVs−1. Elemental analyses were determined using an Elementar Vario EL elemental analyzer. EPR spectra were measured using a Bruker A300− 10−12 electron paramagnetic resonance spectrometer. Measurements for the mixed-valence complexes were carried out in situ after single electron oxidation of the corresponding neutral compounds by FcPF6. X-ray Crystal Structure Determinations. Single-crystal data for [NN−ph−OO] were collected on an Agilent Gemini S Ultra diffractometer with Cu Kα radiation (λ = 1.54178 Å) at 150(2) K and single-crystal data for [NO−ph−OO] and [NN−ph−NO] were collected on an Agilent Xcalibur Nova diffractometer with Cu Kα radiation (λ = 1.54178 Å) at 173(2) K. The empirical absorption corrections were applied using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm.78All the structures were solved using direct methods, which yielded the positions of all non-hydrogen atoms. Hydrogen atoms were placed in calculated positions in the final structure refinement. Structure determination and refinement were carried out using SHELXS-97 and SHELXL97 programs, respectively.79 All non-hydrogen atoms were refined with anisotropic displacement parameters. Mo2(DAniF)3[(NH)2CC6H5]. Mo2(DAniF)3(O2CCH3) (0.203 g, 0.20 mmol) and benzamidine hydrochloride (0.0313 g, 0.20 mmol) were mixed in THF (30 mL) in a 100 mL flask, to which a solution of sodium ethoxide (0.0272 g, 0.40 mmol) in ethanol (10 mL) was transferred through a cannula. After addition, the mixture was stirred at room temperature for 3 h. All volatiles were removed under vacuum, affording a yellow microcrystalline solid. The product was washed with ethanol (20 mL × 3) and collected by filtration. Yield: 0.150 g (70%). 1 H NMR δ (ppm in CDCl3): 8.44 (d, 1H, −NCHN−), 8.17 (s, 2H, −NCHN−), 8.26 (s, 2H, C(NH)NH), 7.70 (d, 2H, aromatic C−H), 7.41 (t, 2H, aromatic C−H), 7.34 (d, 1H, aromatic C−H), 6.60 (d, 8H, aromatic C−H), 6.45 (d, 4H, aromatic C−H), 6.38 (d, 8H, aromatic C−H), 6.26 (d, 4H, aromatic C−H), 3.71 (m, 18H, − OCH3). Half-wave potential E1/2 (V vs Ag/AgCl): 0.195. Mo2(DAniF)3[N(H)OCC6H5]. A 100 mL flame-dried flask was charged with Mo2(DAniF)3(O2CCH3) (0.203 g, 0.20 mmol), benzamide (0.0242 g, 0.20 mmol) and THF (30 mL), to which a solution of sodium ethoxide (0.0136 g, 0.20 mmol) in ethanol (10 mL) was added. The produced yellow solution was stirred at room temperature for 3 h. All volatiles were removed under vacuum, and the residue was washed with ethanol (20 mL × 3), giving yellow solid product. Yield: 0.156 g (72%). 1H NMR δ (ppm in CDCl3): 9.03 (s, 1H, C(O)NH), 8.46 (s, 1H, −NCHN−), 8.34 (s, 2H, − NCHN−), 8.01 (t, 2H, aromatic

the intramolecular ET direction, although N and O atoms are close in atomic radii and electronic properties. The donor− acceptor electron transfer in asymmetrical D−B−A molecules resembles somehow the conductive behavior of molecular diodes.

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CONCLUSION Assembling two [Mo2] building blocks with asymmetrical bridging ligands [EOCC6H4CE(NH)]2− (E = O, NH) generated three Mo2 dimers, namely, [NO−ph−OO], [NN− ph−OO], and [NN−ph−NO], which share a common molecular scaffold with the symmetrical analogues [NO−ph− NO], [OO−ph−OO], and [NN−ph−NN] reported previously. These newly synthesized complexes are slightly redox asymmetrical because of the similarity in atomic radii and electronic properties between N and O atoms. The redox asymmetry, defined by the internal potential difference Eip in the dimer, was determined by the redox potentials of two independent Mo2 reference compounds that correspond to the donor and acceptor. The largest Eip (140 mV) was found for [NN−ph−OO], in accordance with the predication from the molecular structure. The MV complexes [NO−ph−OO]+, [NN−ph−OO]+, and [NN−ph−NO]+ were prepared by single electron oxidation using one equiv of ferrocenium hexafluorophosphate (Cp2FePF6). These asymmetrical donor− acceptor systems exhibit larger redox potential splitting (ΔE1/2) and higher IV transition energy (EIT) than the symmetrical species. In comparison with ΔE1/2 and EIT for the closely related symmetrical analogue, the free energy change ΔG° for the ET reaction was estimated. From the Hush model, Δν1/2 = [16ln(2)λRT]1/2, optical analyses gave the reorganization energies (λ) of ∼5000 cm−1. Remarkably, in the three systems, the important energetic relationship EIT = ΔG° + λ is verified, which underlies the semiclassical theories. As the system alternates by manipulation of the redox asymmetry, correlated variations of parameters ΔG°, EIT, and λ are found. The explicit assignments of the MLCT and LMCT absorptions and IV band allow application of CNS superexchange formalism for determination of the electronic coupling matrix element HMM′. For the three asymmetrical systems, the magnitudes of HMM′ (∼500 cm−1), which are in good agreement with the results from the Hush model, are slightly smaller than those for the symmetrical analogues. Both optical and thermal ET dynamics and kinetics have been evaluated within the Marcus−Hush theoretic framework. For [NO−ph−OO]+, and [NN−ph−NO] +, the rate constants for the forward ET reaction ket ( f) were determined in the order of 1010 s−1, smaller than those for the backward reaction ket(r) by 1 order of magnitude. Smaller ket(f) (4.1 × 109 s−1) but larger ket(r) (1.4 × 1012 s−1) values are found for [NN−ph−OO]+ due to the larger redox asymmetry. The optical behaviors for these Mo2 donor−acceptor systems confirm that the intramolecular electron transfer proceeds via superexchange mechanism by which electron-hopping and hole-hopping occur simultaneously. The experimental results and theoretical analyses illustrate that the donor−acceptor redox asymmetry controls the direction and rate of electron transfer.

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EXPERIMENTAL SECTION Materials and Methods. All manipulations were performed in a nitrogen-filled glovebox or by using standard Schlenk-line techniques. All solvents were freshly distilled over I


Article


(ppm in CDCl3): 9.10 (s, 1H, C(O)NH), 8.49 (d, 2H, −NCHN−), 8.35 (s, 2H, C(NH)2), 8.27 (s, 4H, −NCHN−), 8.03 (d, 2H, aromatic C−H), 7.79 (d, 2H, aromatic C−H), 6.63 (m, 16H, aromatic C−H), 6.54 (s, 1H, aromatic C−H), 6.48 (m, 8H, aromatic C−H), 6.44 (s, 1H, aromatic C−H), 6.38 (m, 12H, aromatic C−H), 6.34 (s, 1H, aromatic C−H), 6.29 (m, 8H, aromatic C−H), 6.22 (s, 1H, aromatic C−H), 3.74 (d, 24H, −OCH3), 3.72 (s, 3H, −OCH3), 3.70 (s, 6H, −OCH3), 3.68 (s, 3H, −OCH3). UV−vis, λmax nm (ε, M−1 cm−1): 475 (1.76 × 104). Anal. Calcd for C98H97Mo4N15O13: C, 56.67; H, 4.67; N, 10.12; Found: C, 56.54; H, 4.61; N, 10.27.

C−H), 7.44 (t, 2H, aromatic C−H), 7.38 (t, 1H, aromatic C− H), 6.62 (d, 12H, aromatic C−H), 6.49 (d, 2H, aromatic C− H), 6.41 (d, 2H, aromatic C−H), 6.38 (d, 4H, aromatic C−H), 6.31 (d, 2H, aromatic C−H), 6.22 (d, 2H, aromatic C−H), 3.72 (d, 12H, −OCH3), 3.69 (s, 3H, −OCH3), 3.65 (s, 3H, −OCH3). Half-wave potential E1/2 (V vs Ag/AgCl): 0.280. [Mo2(DAniF)3]2[μ-O2CC6H4CO(NH)]. A 100 mL flask was charged with Mo2(DAniF)3(O2CCH3) (0.305 g, 0.30 mmol) and sodium ethoxide (0.0408 g, 0.60 mmol), to which 30 mL of THF and 10 mL of ethanol were added. After the mixture was stirred for 1 h, a solution of 4-carbamoylbenzoic acid (0.025 g, 0.15 mmol) in 30 mL of methanol was added slowly with stirring, producing a red solution. The mixture was stirred at room temperature for another 5 h. All volatiles were removed under vacuum and the residue was extracted with 25 mL of CH2Cl2 and filtered through a Celite-packed funnel. The solvent was evaporated under reduced pressure. The resulting red product was washed with ethanol (30 mL × 3) and collected by filtration. Yield: 0.20 g (64%). 1H NMR δ (ppm in CDCl3): 9.14 (s, 1H, C(O)NH), 8.52 (d, 2H, − NCHN−), 8.39 (m, d, 4H, − NCHN−), 8.36 (d, 2H, aromatic C−H), 8.06 (d, 2H, aromatic C−H), 6.66 (m, 18H, aromatic C−H), 6.61 (m, 8H, aromatic C−H), 8.51 (m, 8H, aromatic C−H), 6.41(m, 7H, aromatic C−H), 6.27 (m, 7H, aromatic C−H), 3.75 (d, 24H, − OCH3), 3.72 (s, 3H, − OCH3), 3.70 (s, 6H, − OCH3), 3.68 (s, 3H, − OCH3). UV−vis, λmax nm (ε, M−1 cm−1): 484 (1.38 × 104). Anal. Calcd for C98H95Mo4N13O15: C, 56.62; H, 4.57; N, 8.76; Found: C, 56.45; H, 4.64; N, 8.46. [Mo 2 (DAniF) 3 ] 2 [μ-O 2 CC 6 H 4 C(NH) 2 ]. A solution of Mo2(DAniF)3(O2CCH3) (0.305 g, 0.30 mmol) in 30 mL of THF was mixed with a solution of sodium methoxide (0.0324 g, 0.60 mmol) in 10 mL of methanol. After the mixture was stirred for 1 h, it was added to another flask charged with 4amidinobenzoic acid (0.030 g, 0.15 mmol) through a cannula. The solution turned to red color immediately upon mixing and was stirred for another 3 h. All volatiles were removed under vacuum and the residue was extracted using 25 mL of CH2Cl2 and filtered through a Celite-packed funnel. The filtrate was concentrated in vacuo to ca. 20% of its original volume. Ethanol (30 mL) was added to afford red precipitates. The red solid was washed with ethanol (20 mL × 2) and collected by filtration. Yield: 0.220 g (71%). 1H NMR δ (ppm in CDCl3): 8.51 (d, 2H, −NCHN−), 8.38 (s, 2H, C(NH)NH), 8.34 (d, 2H, aromatic H), 8.26 (s, 4H, −NCHN−), 7.78 (d, 2H, aromatic C−H), 6.62 (m, 24H, aromatic C−H), 6.50 (d, 8H, aromatic C−H), 6.38 (d, 8H, aromatic C−H), 6.27 (d, 8H, aromatic C− H), 3.74 (d, 24H, −OCH3), 3.70 (d, 12H, −OCH3). UV−vis, λmax nm (ε, M−1 cm−1): 480 (1.37 × 104). Anal. Calcd for C98H96Mo4N14O14: C, 56.66; H, 4.66; N, 9.44. Found: C, 56.39; H, 4.60; N, 9.57. [Mo2(DAniF)3]2[μ-(NH)OCC6H4C(NH)2]. A methanol solution (5 mL) of sodium methoxide (0.0243 g, 0.45 mmol) was added slowly to a solution of Mo2(DAniF)3(O2CCH3) (0.305 g, 0.30 mmol) in THF (30 mL). After the mixture was stirred for 1 h, a solution of benzamidine hydrochloride (0.030 g, 0.15 mmol) in 5 mL of methanol was added slowly. The resulting red solution was stirred at room temperature for another 3 h. All volatiles were removed under reduced pressure. The residue was extracted with 25 mL of CH2Cl2 and filtered through a Celite-packed funnel. The filtrate was concentrated to ca. 5 mL in vacuo, and ethanol (30 mL) was added to afford red precipitates. The red solid was washed with ethanol (20 mL × 2) and collected by filtration. Yield: 0.23 g (74%). 1H NMR δ

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b03792. 1 H NMR spectra, EPR spectra, crystallographic data, and selected bond distances and angles (PDF) Cif data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*(C.Y.L.) E-mail: [email protected]. Phone: +86-02085222191. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation of China (No.21371074, 90922010, 21301070), and Jinan University for financial support.

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REFERENCES

(1) Hush, N. S. Qualitative Evidence for Intervalence-Transfer Absorption in Inorganic Systems in Solution and in Solid State. Prog. Inorg. Chem. 1967, 8, 391−444. (2) Hush, N. S. Distance Dependence of Electron Transfer Rates. Coord. Chem. Rev. 1985, 64, 135−157. (3) Creutz, C. Mixed Valence Complexes of d5-d6 Metal Centers. Prog. Inorg. Chem. 1983, 30, 1−73. (4) Crutchley, R. J. Intervanlence Charge Transfer and Electron Exchange Studies of Dinuclear Ruthenium Complexes. Adv. Inorg. Chem. 1994, 41, 273−325. (5) Creutz, C.; Taube, H. A Direct Approach to Measuring the Franck-Condon Barrier to Electron Transfer between Metal Ions. J. Am. Chem. Soc. 1969, 91, 3988−3989. (6) Creutz, C.; Taube, H. Binuclear Complexes of Ruthenium Ammines. J. Am. Chem. Soc. 1973, 95, 1086−1094. (7) Parthey, M.; Gluyas, J. B. G.; Fox, M. A.; Low, P. J.; Kaupp, M. Mixed-Valence Ruthenium Complexes Rotating through a Conformational Robin−Day Continuum. Chem. - Eur. J. 2014, 20, 6895−6908. (8) Kaim, W.; Lahiri, G. K. Unconventional Mixed-Valent Complexes of Ruthenium and Osmium. Angew. Chem., Int. Ed. 2007, 46, 1778− 1796. (9) Yao, C.-J.; Zhong, Y.-W.; Yao, J. Charge Delocalization in a Cyclometalated Bisruthenium Complex Bridged by a Noninnocent 1,2,4,5-Tetra(2-pyridyl)benzene Ligand. J. Am. Chem. Soc. 2011, 133, 15697−15706. (10) Venkatasubbaiah, K.; Doshi, A.; Nowik, I.; Herber, R. H.; Rheingold, A. L.; Jäkle, F. Examination of the Mixed-Valence State of the Doubly Boron-Bridged Diferrocene Cation [(FeCp) 2 {C10H6(BPh)2}]+. Chem. - Eur. J. 2008, 14, 444−458. (11) Hadt, R. G.; Nemykin, V. N. Exploring the Ground and Excited State Potential Energy Landscapes of the Mixed-Valence Biferrocenium Complex. Inorg. Chem. 2009, 48, 3982−3992. J


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of a Single Molecular Diode with Controlled Orientation. Nat. Chem. 2009, 1, 635−641. (35) Cotton, F. A.; Murillo, C. A.; Walton, R. A. Multiple Bonds between Metal Atoms, 3rd ed.; Springer: New York, 2005. (36) Xiao, X.; Liu, C. Y.; He, Q.; Han, M. J.; Meng, M.; Lei, H.; Lu, X. Control of the Charge Distribution and Modulation of the Class II−III Transition in Weakly Coupled Mo2−Mo2 Systems. Inorg. Chem. 2013, 52, 12624−12633. (37) Shu, Y.; Lei, H.; Tan, Y. N.; Meng, M.; Zhang, X. C.; Liu, C. Y. Tuning the Electronic Coupling in Mo2−Mo2 Systems by Variation of the Coordinating Atoms of the Bridging Ligands. Dalton Trans. 2014, 43, 14756−14765. (38) Lei, H.; Xiao, X.; Meng, M.; Cheng, T.; Shu, Y.; Tan, Y. N.; Liu, C. Y. Dimolybdenum Dimers Spaced by Phenylene Groups: The Experimental Models for Study of Electronic Coupling. Inorg. Chim. Acta 2015, 424, 63−67. (39) Cotton, F. A.; Donahue, J. P.; Murillo, C. A. Polyunsaturated Dicarboxylate Tethers Connecting Dimolybdenum Redox and Chromophoric Centers: Syntheses, Structures, and Electrochemistry. J. Am. Chem. Soc. 2003, 125, 5436−5450. (40) Richardson, D. E.; Taube, H. Determination of E2°-E1° in Multistep Charge Transfer by Stationary-Electrode Pulse and Cyclic Voltammetry: Application to Binuclear Ruthenium Ammines. Inorg. Chem. 1981, 20, 1278−1285. (41) Cotton, F. A.; Donahue, J. P.; Murillo, C. A.; Pérez, L. M. Polyunsaturated Dicarboxylate Tethers Connecting Dimolybdenum Redox and Chromophoric Centers: Absorption Spectra and Electronic Structures. J. Am. Chem. Soc. 2003, 125, 5486−5492. (42) Chisholm, M. H.; Feil, F.; Hadad, C. M.; Patmore, N. J. Electronically Coupled MM Quadruply-Bonded Complexes (M = Mo or W) Employing Functionalized Terephthalate Bridges: Toward Molecular Rheostats and Switches. J. Am. Chem. Soc. 2005, 127, 18150−18158. (43) Liu, C. Y.; Xiao, X.; Meng, M.; Zhang, Y.; Han, M. J. 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. J. Phys. Chem. C 2013, 117, 19859−19865. (44) Zhang, H. L.; Zhu, G. Y.; Wang, G.; Meng, M.; Liu, C. Y.; Lei, H. Electronic Coupling in [Mo2]−Bridge−[Mo2] Systems with Twisted Bridges. Inorg. Chem. 2015, 54, 11314−11322. (45) Kang, M. T.; Meng, M.; Tan, Y. N.; Cheng, T.; Liu, C. Y. Tuning the Electronic Coupling and Electron Transfer in Mo2 Donor−Acceptor Systems by Variation of the Bridge Conformation. Chem. - Eur. J. 2016, 22, 3115−3126. (46) Marcus, R. A.; Sutin, N. The Relation Between the Barriers for Thermal and Optical Electron Transfer Reactions in Solution. Comments Inorg. Chem. 1986, 5, 119−133. (47) Ceccon, A.; Santi, S.; Orian, L.; Bisello, A. Electronic Communication in Heterobinuclear Organometallic Complexes through Unsaturated Hydrocarbon Bridges. Coord. Chem. Rev. 2004, 248, 683−724. (48) Santi, S.; Orian, L.; Durante, C.; Bisello, A.; Benetollo, F.; Crociani, L.; Ganis, P.; Ceccon, A. Tuning the Electronic Communication in Heterobimetallic Mixed-Valence Ions of (1Ferrocenyl)- and (2-Ferrocenyl)indenyl Rhodium Isomers. Chem. Eur. J. 2007, 13, 1955−1968. (49) Paul, F.; Meyer, W. E.; Toupet, L.; Jiao, H.; Gladysz, J. A.; Lapinte, C. A. A “Conjugal” Consanguineous Family of ButadiynediylDerived Complexes: Synthesis and Electronic Ground States of Neutral, Radical Cationic, and Dicationic Iron/Rhenium C4 Species. J. Am. Chem. Soc. 2000, 122, 9405−9414. (50) Marcus, R. A. On the Theory of Oxidation-Reduction Reactions Involving Electron Transfer. J. Chem. Phys. 1956, 24, 966−978. (51) Marcus, R. A. On the Theory of Oxidation-Reduction Reactions Involving Electron Transfer. V. Comparison and Properities of Electrochemical and Chemical Rate Constants. J. Phys. Chem. 1963, 67, 853−857.

(12) Li, Y.; Josowicz, M.; Tolbert, L. M. Diferrocenyl Molecular Wires. The Role of Heteroatom Linkers. J. Am. Chem. Soc. 2010, 132, 10374−10382. (13) Ito, T.; Hamaguchi, T.; Nagino, H.; Yamaguchi, T.; Washington, J.; Kubiak, C. P. Effects of Rapid Intramolecular Electron Transfer on Vibrational Spectra. Science 1997, 277, 660−663. (14) Kubiak, C. P. Inorganic Electron Transfer: Sharpening a Fuzzy Border in Mixed Valency and Extending Mixed Valency across Supramolecular Systems. Inorg. Chem. 2013, 52, 5663−5676. (15) Hankache, J.; Wenger, O. S. Organic Mixed Valence. Chem. Rev. 2011, 111, 5138−5178. (16) Heckmann, A.; Lambert, C. Organic Mixed-Valence Compounds: A Playground for Electrons and Holes. Angew. Chem., Int. Ed. 2012, 51, 326−392. (17) Salsman, J. C.; Kubiak, C. P.; Ito, T. Mixed Valence Isomers. J. Am. Chem. Soc. 2005, 127, 2382−2383. (18) Salsman, J. C.; Ronco, S.; Londergan, C. H.; Kubiak, C. P. Tuning the Electronic Communication and Rates of Intramolecular Electron Transfer of Dimers of Trinuclear Ruthenium Clusters: Bridging and Ancillary Ligand Effects. Inorg. Chem. 2006, 45, 547− 554. (19) Ito, T.; Imai, N.; Yamaguchi, T.; Hamaguchi, T.; Londergan, C. H.; Kubiak, C. P. Observation and Dynamics of “Charge-Transfer Isomers. Angew. Chem., Int. Ed. 2004, 43, 1376−1381. (20) Chang, J. P.; Fung, E. Y.; Curtis, J. C. Evidence for Specific Solvent-Solute Interactions as a Major Contributor to the FranckCondon Energy in Intervalence-Transfer Absorptions of Ruthenium Ammine Complexes. Inorg. Chem. 1986, 25, 4233−4241. (21) Salaymeh, F.; Berhane, S.; Yusof, R.; de la Rosa, R.; Fung, E. Y.; Matamoros, R.; Lau, K. W.; Zheng, Q.; Kober, E. M.; Curtis, J. C. Electronic Coupling in Mixed-Valence Binuclear Ruthenium Ammine Complexes As Probed by an Electrochemical Method and an Extension of Mulliken’s Theory of Donor-Acceptor Interactions. Inorg. Chem. 1993, 32, 3895−3908. (22) Heckmann, A.; Lambert, C. Neutral Organic Mixed-Valence Compounds: Synthesis and All-Optical Evaluation of ElectronTransfer Parameters. J. Am. Chem. Soc. 2007, 129, 5515−5527. (23) Sutin, N. Theory of Electron Transfer Reactions: Insights and Hindsights. Prog. Inorg. Chem. 1983, 30, 441−483. (24) Brunschwig, B. S.; Sutin, N. Energy Surfaces, Reorganization Energies, and Coupling Elements in Electron Transfer. Coord. Chem. Rev. 1999, 187, 233−254. (25) Moneo, Á .; Telo, J. P. Electron Transfer within 1,3Dinitrobenzene Radical Anions: Electron Hopping or Superexchange? J. Phys. Org. Chem. 2012, 25, 1139−1143. (26) Rocha, R. C.; Toma, H. E. Mixed-Valence Properties of Ruthenium-Polypyridine Dimers Bridged by Imidazolate and Triazolate Ligands. J. Coord. Chem. 2004, 57, 303−312. (27) Semenov, S. N.; Blacque, O.; Fox, T.; Venkatesan, K.; Berke, H. Electronic Communication in Dinuclear C4-Bridged Tungsten Complexes. J. Am. Chem. Soc. 2010, 132, 3115−3127. (28) Lambert, C.; Nö ll, G. The Class II/III Transition in Triarylamine Redox Systems. J. Am. Chem. Soc. 1999, 121, 8434−8442. (29) Demadis, K. D.; Hartshorn, C. M.; Meyer, T. J. The Localizedto-Delocalized Transition in Mixed-Valence Chemistry. Chem. Rev. 2001, 101, 2655−2685. (30) Metzger, R. M. Unimolecular Electrical Rectifiers. Chem. Rev. 2003, 103, 3803−3834. (31) Zhao, J.; Yu, C.; Wang, N.; Liu, H. Molecular Rectification Based on Asymmetrical Molecule-Electrode Contact. J. Phys. Chem. C 2010, 114, 4135−4141. (32) Aviram, A.; Ratner, M. A. Molecular Rectifiers. Chem. Phys. Lett. 1974, 29, 277−283. (33) Oleynik, I. I.; Kozhushner, M. A.; Posvyanskii, V. S.; Yu, L. Rectification Mechanism in Diblock Oligomer Molecular Diodes. Phys. Rev. Lett. 2006, 96, 096803. (34) Díez-Pérez, I.; Hihath, J.; Lee, Y.; Yu, L.; Adamska, L.; Kozhushner, M. A.; Oleynik, I. I.; Tao, N. J. Rectification and Stability K


Article

The Journal of Physical Chemistry C (52) Marcus, R. A.; Sutin, N. Electron Transfers in Chemistry and Biology. Biochim. Biophys. Acta, Rev. Bioenerg. 1985, 811, 265−322. (53) Hush, N. S. Adiabatic Rate Processes at Electrodes. I. EnergyCharge Relationships. J. Chem. Phys. 1958, 28, 962−972. (54) Hush, N. S. Adiabatic Theory of Outer Sphere ElectronTransfer Reactions in Solution. Trans. Faraday Soc. 1961, 57, 557− 580. (55) Goldsby, K. A.; Meyer, T. J. Intervalence Transfer in MixedMetal Dimers of Ruthenium and Osmium: Relationship between Redox Asymmetry and Intervalence-Transfer Absorption Band Energy. Inorg. Chem. 1984, 23, 3002−3010. (56) D’Alessandro, D. M.; Davies, M. S.; Keene, F. R. Multisite Effects on Intervalence Charge Transfer in a Clusterlike Trinuclear Assembly Containing Ruthenium and Osmium. Inorg. Chem. 2006, 45, 1656−1666. (57) Shao, J. Y.; Zhong, Y. W. Tuning the Electronic Coupling in Cyclometalated Diruthenium Complexes through Substituent Effects: A Correlation between the Experimental and Calculated Results. Chem. - Eur. J. 2014, 20, 8702−8713. (58) Hush, N. S. Homogeneous and Heterogeneous Optical and Thermal Electron Transfer. Electrochim. Acta 1968, 13, 1005−1023. (59) Brunschwig, B. S.; Creutz, C.; Sutin, N. Optical Transitions of Symmetrical Mixed-Valence Systems in the Class II−III Transition Regime. Chem. Soc. Rev. 2002, 31, 168−184. (60) Creutz, C.; Newton, M. D.; Sutin, N. Metal-Ligand and MetalMetal Coupling Elements. J. Photochem. Photobiol., A 1994, 82, 47−59. (61) D’Alessandro, D. M.; Keene, F. R. Current Trends and Future Challenges in the Experimental, Theoretical and Computational Analysis of Intervalence Charge Transfer (IVCT) Transitions. Chem. Soc. Rev. 2006, 35, 424−440. (62) Halpin, Y.; Dini, D.; Younis Ahmed, H. M.; Cassidy, L.; Browne, W. R.; Vos, J. G. Excited State Localization and Internuclear Interactions in Asymmetric Ruthenium(II) and Osmium(II) bpy/tpy Based Dinuclear Compounds. Inorg. Chem. 2010, 49, 2799−2807. (63) Hage, R.; Haasnoot, J. G.; Nieuwenhuis, H. A.; Reedijk, J.; de Ridder, D. J. A.; Vos, J. G. Synthesis, X-ray Structure, and Spectroscopic and Electrochemical Properties of Novel Heteronuclear Ruthenium-Osmium Complexes with an Asymmetric Triazolate Bridge. J. Am. Chem. Soc. 1990, 112, 9245−9251. (64) D’Alessandro, D. M.; Dinolfo, P. H.; Hupp, J. T.; Junk, P. C.; Keene, F. R. The Effective Electron-Transfer Distance in Dinuclear Ruthenium Complexes Containing the Asymmetrical Bridging Ligand 3,5-Bis(2-pyridyl)-1,2,4-Triazolate. Eur. J. Inorg. Chem. 2006, 4, 772− 783. (65) Nelsen, S. F.; Newton, M. D. Estimation of Electron Transfer Distances from AM1 Calculations. J. Phys. Chem. A 2000, 104, 10023− 10031. (66) Karki, L.; Lu, H. P.; Hupp, J. T. Electroabsorption Studies of Intervalence Charge Transfer in (NC)5FeCNOs(NH3)5−:Experimental Assessment of Charge-Transfer Distance, Solvent Reorganization, and Electronic Coupling Parameters. J. Phys. Chem. 1996, 100, 15637−15639. (67) Rosokha, S. V.; Sun, D.-L.; Kochi, J. K. Conformation, Distance, and Connectivity Effects on Intramolecular Electron Transfer between Phenylene-Bridged Aromatic Redox Centers. J. Phys. Chem. A 2002, 106, 2283−2292. (68) Distefano, A. J.; Wishart, J. F.; Isied, S. S. Convergence of Spectroscopic and Kinetic Electron Transfer Parameters for MixedValence Binuclear Dipyridylamide Ruthenium Ammine Complexes. Coord. Chem. Rev. 2005, 249, 507−516. (69) Evans, C. E. B.; Naklicki, M. L.; Rezvani, A. R.; White, C. A.; Kondratiev, V. V.; Crutchley, R. J. An Investigation of Superexchange in Dinuclear Mixed-Valence Ruthenium Complexes. J. Am. Chem. Soc. 1998, 120, 13096−13103. (70) Rezvani, A. R.; Bensimon, C.; Cromp, B.; Reber, C.; Greedan, J. E.; Kondratiev, V. V.; Crutchley, R. J. Inner Coordination Sphere Control of Metal-Metal Superexchange in Ruthenium Dimers. Inorg. Chem. 1997, 36, 3322−3329.

(71) McConnell, H. M. Intramolecular Charge Transfer in Aromatic Free Radicals. J. Chem. Phys. 1961, 35, 508−515. (72) Newton, M. D. Quantum Chemical Probes of Electron-Transfer Kinetics: The Nature of Donor-Acceptor Interactions. Chem. Rev. 1991, 91, 767−792. (73) Mosher, P. J.; Yap, G. P. A.; Crutchley, R. J. A Donor-AcceptorDonor Bridging Ligand in a Class III Mixed-Valence Complex. Inorg. Chem. 2001, 40, 1189−1195. (74) Robin, M. B.; Day, P. Mixed valence Chemistry-A Survey and Classification. Adv. Inorg. Chem. Radiochem. 1968, 10, 247−422. (75) Miyashita, M.; Hanai, Y.; Awane, H.; Yoshikawa, Y. T.; Miyagawa, H. Improving Peptide Fragmentation by N-Terminal Derivatization with High Proton Affinity. Rapid Commun. Mass Spectrom. 2011, 25, 1130−1140. (76) Lin, C.; Protasiewicz, J. D.; Smith, E. T.; Ren, T. Linear Free Energy Relationships in Dinuclear Compounds. 2. Inductive Redox Tuning via Remote Substituents in Quadruply Bonded Dimolybdenum Compounds. Inorg. Chem. 1996, 35, 6422−6428. (77) Cotton, F. A.; Liu, C. Y.; Murillo, C. A.; Villagrán, D.; Wang, X. Modifying Electronic Communication in Dimolybdenum Units by Linkage Isomers of Bridged Oxamidate Dianions. J. Am. Chem. Soc. 2003, 125, 13564−13575. (78) CrysAlis RED, Version 1.171.31.7; Oxford Diffraction Ltd.: Abington, U.K., 2006. (79) Sheldrick, G. M. SHELXTL, Version 6.12; Bruker Analytical Xray Systems, Inc.: Madison, WI, 2000.

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