Electronic Coupling between Two Covalently Bonded Dimolybdenum

May 31, 2016 - Synopsis. Three Mo2 dimers bridged by a 2,6-naphthalene group are synthesized and studied. The electronic coupling parameters Hab, ...
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Electronic Coupling between Two Covalently Bonded Dimolybdenum Units Bridged by a Naphthalene Group Guang Yuan Zhu,† Miao Meng,† Ying Ning Tan, Xuan Xiao, and Chun Y. Liu* Department of Chemistry, Jinan University, 601 Huang-Pu Avenue West, Guangzhou 510632, China S Supporting Information *

ABSTRACT: Using 2,6-naphthalenedicarboxylate and its thiolated derivatives as bridging ligands, three Mo2 dimers of the type [Mo2(DAniF)3](E2CC10H6CE2)[Mo2(DAniF)3] (DAniF = N,N′-dip-anisylformamidinate; E = O, S) have been synthesized and characterized by X-ray diffraction. These compounds can be generally formulated as [Mo2]−naph−[Mo2], where the complex unit [Mo2] ([Mo2(DAniF)3(μ-E2C)]) functions as an electron donor (acceptor) and the naphthalene (naph) group is the bridge. The mixed-valence (MV) complexes, generated by one-electron oxidation of the neutral precursors, display weak, very broad intervalence charge-transfer absorption bands in the near-to-mid-IR regions. The electronic coupling matrix elements for the MV complexes, Hab = 390−570 cm−1, are calculated from the Mulliken−Hush equation, which fall between those for the phenyl (ph) and biphenyl (biph) analogues reported previously. The three series consisting of three complexes with the same [Mo2] units exhibit exponential decay of Hab as the bridge changes from ph to biph via naph, with decay factors of 0.21−0.17 Å−1. Therefore, it is evidenced that while the extent of the bridge conjugacy varies, the electronic coupling between the two [Mo2] units is dominated by the Mo2···Mo2 separation. The absorption band energies for metal-to-ligand charge transfer are in the middle of those for the ph and biph analogues, which is consistent with variation of the HOMO−LUMO energy gaps for the complex series. These results indicate that the interplay of the bridge length and conjugacy is to affect the enegy for charge transfer crossing the intervening moiety, in accordance with a superechange mechanism.

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INTRODUCTION

favorable for the construction of molecular electron devices with “single-electron” conductance. Synthetically, [M2]−bridge−[M2] complexes can be obtained by assembling two dimetal building blocks with a dicarboxylate bridging ligand or its nitrogen and sulfur derivatives.1a,9−11 In these systems, the δ orbital energy accounts for the electron-donating (accepting) ability of the donor (acceptor), which is controlled by the coordination sphere of the M2 center. Therefore, the coordinatively saturated [M2] unit, rather than the M2 cations, should be considered to be the electron donor or acceptor. For instance, terephthalatebridged compound [Mo 2 (DAniF) 3 ](O 2 CC 6 H 4 CO 2 )[Mo2(DAniF)3], where auxiliary ligand DAniF = N,N′-di-panisylformamidinate, can be reformulated as [Mo2(DAniF)3(O2C)](C6H4)[(CO2)Mo2(DAniF)3]. In the MV system, the neutral [Mo2(DAniF)3(O2C)] unit functions as the electron donor and the cationic [Mo2(DAniF)3(O2C)]+ unit is the acceptor, and the phenylene group (−C6H4−) serves as the bridge. By assembling [Mo2(DAniF)3]+ with tetrephthalate and biphenyldicarboxylate as well as their thiolated derivatives, two three-membered [Mo2]−bridge−[Mo2] series have been

Diverse compounds in the form of [M2]−bridge−[M2] (M = Mo, W), corresponding to D (donor)−B (bridge)−A (acceptor) assembly, have been synthesized and studied in terms of electronic coupling (EC) and electron transfer (ET).1−3 These dimers of dimers are significantly different from the intensively studied d5 and d6 bimetallic D−B−A complexes in electronic configurations. One of most distinct features for the mixedvalence (MV) systems is that the transferring electron resides on the δ orbitals of the M2 centers before and after the occurrence of the electronic event, and ET leads to the breakage of the δ bond on one site and simultaneous formation of a δ bond on the other site.4 Therefore, the δ electron energy determines the diabatic and adiabatic states of the ET system. Vertical transition of the ground state generates a well-defined intervalence charge-transfer (IVCT) absorption band in the near-to-mid-IR regions.3,5 The optical behaviors correspond to the classes in Robin−Day’s scheme for MV compounds,6 that is, Classes I and III for weakly and strongly coupled systems, respectively, and Class II for intermediate systems. Given these electronic and spectroscopic properties, [M2]−bridge−[M2] complexes become desirable experimental models for the studies of EC and ET within the established theoretical framework.7,8 In addition, the complex moieties as such are © 2016 American Chemical Society

Received: April 24, 2016 Published: May 31, 2016 6315

DOI: 10.1021/acs.inorgchem.6b01021 Inorg. Chem. 2016, 55, 6315−6322

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Inorganic Chemistry

Figure 1. Molecular scaffold for complexes under investigation. The dimolybdenum building block [Mo2] = [Mo2(DAniF)3]+.

the dimer of dimers. However, in the preparation of [O2−(2,6naph)−O2], a similar reaction did not give the expected compound; instead, a stoichiometric reaction of Mo2(DAniF)3(O2CCH3) with 2,6-naphthalenedicarboxylic acid resulted in the product in a satisfactory yield. 1H NMR characterization showed that the complexes had a purity of >90%, suitable for electrochemical and spectroscopic measurements. For the three Mo2 dimers, single crystals suitable for crystallographic analyses were obtained by the slow diffusion of ethanol into a dichloromethane solution of the compound. These compounds all crystallized in a triclinic P1̅ space group, with the molecules residing in a special position (Z = 1) in the unit cell. The crystallographic data and structural parameters are given in the Supporting Information (SI). As shown in Figure 2, the three molecules have the same structural skeleton. The bond distances and angles are comparable with those for the phenylene and biphenylene analogues (see the SI). The Mo−Mo bond distances are about 2.10 Å, as seen in other quadruply bonded Mo2 compounds,18,19 and increase in order as the chelating groups of the bridging ligand change from −CO2 to −COS to −CS2. The Mo2···Mo2 separations (ca. 13− 14 Å) are longer than those for the phenyl analogues12 but shorter than those for the biphenyl analogues by about 2 Å. The naphthalene group deviates from the five-membered Mo2 chelating rings with varying torsion angles. Relatively large torsion angles, ca. 23°, are found for [S2−(2,6-naph)−S2], similar to that for [S2−ph−S2].12 It is worthwhile to note that the C(4)−C(5) bonds that connect the chelating ring and the aromatic spacer are significantly shorter than a C−C single bond (ca. 1.51 Å). For example, in [OS−(2,6-naph)−OS] and [S2−(2,6-naph)−S2], this bond is about 1.47 Å in length, which is shorter than those for the ph-spaced series (1.49 Å).12 The shortened C(4)−C(5) bonds are indicative of d(δ)−p(π) conjugation between the Mo2 center and the bridging ligand. Electrochemical Studies. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were employed to study the electrochemical properties of the Mo2 dimers. In the CV diagrams (Figure 3), the two redox events occurring on the dimetal centers are not resolved. The electrochemical behavior for this series is quite different from that for related phenyl analogues, which exhibit two isolated redox couples,12 and that

developed and studied recently.12−14 Each of the two series, namely, [Mo2]−ph−[Mo2] and [Mo2]−biph−[Mo2], has three complexes differing in the electron donor (acceptor) by variation of the chelating group on the bridging ligands from −CO2 to −COS to −CS2. For convenience of discussion, the complexes in the two series are denoted as [O2−ph−O2], [OS−ph−OS], and [S2−ph−S2] and as [O2−biph−O2], [OS−biph−OS], and [S2−biph−S2]. For the three pairs with different [Mo2] units, distance dependences of EC and ET were studied and the attenuation factors for the EC parameters Hab were estimated in the range of 0.17−0.21 Å−1.14 In the present work, when the bridging ligand is changed to 2,6-naphthalenedicarboxylate and its thiolated derivatives, the third series consisting of [O2−(2,6-naph)−O2], [OS−(2,6naph)−OS], and [S2−(2,6-naph)−S2] is produced. The molecular skeletons are shown in Figure 1. Variation of the bridging ligands modifies the Mo2···Mo2 separations and the extent of conjugation of the bridge, which are the two major factors affecting EC and ET. Having a 2,6-naphthalene group as the bridge for this series, the Mo2···Mo2 distances fall between those for the phenylene and biphenylene analogues, and the d(δ)−p(π) conjugation along the charge-transfer platform is improved, relative to the ph- and biph-bridged systems.12−14 Therefore, structural modifications allow us to evaluate the impact of the interplay between the two effects on EC and verify the distance dependence of Hab for [Mo2]−bridge− [Mo2] systems.

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RESULTS AND DISCUSSION Syntheses and Structures. Following literature methods, 2,6-naphthalenedithiodicarboxylic acid15 and 2,6-naphthalenetetrathiodicarboxylic acid16,17 are prepared. The three neutral dimolybdenum pairs [O 2 −(2,6-naph)−O 2 ], [OS−(2,6naph)−OS], and [S2−(2,6-naph)−S2] were synthesized by assembling two dimetal building blocks [Mo2(DAniF)3]+ with the associated bridging ligand. Utilization of the mixed-ligand dimolybdenum compound Mo2(DAniF)3(O2CCH3) is the key for convergent assembly with the bridging ligand. In the generalized practical procedure,12 Mo2(DAniF)3(O2CCH3) reacts with 1 equiv of sodium alkoxide first, giving [Mo2(DAniF)3(OEt) (EtOH)], for example. This intermediate is highly reactive and directs the assembling reaction to form 6316

DOI: 10.1021/acs.inorgchem.6b01021 Inorg. Chem. 2016, 55, 6315−6322

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Inorganic Chemistry

in ΔE1/2 are found for the tetrathiolated analogues, i.e., 108 mV for [S2−(2,6-naph)−S2] versus 195 mV for [S2−ph−S2] (Table 1). These results indicate that of the three series the electronic communication in the naph-bridged complexes is moderately strong. For the series, the ΔE1/2 values increase as the oxygen atoms on the bridging ligands are replaced stepwise by sulfur atoms, similar to the observation in the phenyl series.12 Given the ΔE1/2 values, the comproportionation constants (KC) for the equilibrium established between the neutral [Mo2−Mo2], the doubly oxidized [Mo2−Mo2]2+, and the MV species [Mo2−Mo2]+ can be derived by the expression KC = exp(ΔE1/2/25.69).20 The free-energy change (ΔGC) for the equilibrium, which measures the thermodynamic stability of the MV species in solution, is calculated from ΔGC = −RT ln(KC) (Table 1). Spectroscopic Properties and Electronic Structures. The three complexes exhibit symmetrical absorption in the electronic spectra, with the band energy decreasing but the intensity increasing as the bridging ligand is stepwise thiolated (Figure S5). According to previous studies,11,21 this band stems from the electronic transition from the δ orbital of the Mo2 center to the π* orbital of the bridging ligand, thus being assigned to metal-to-ligand charge-transfer (MLCT) absorbance. The MLCT absorption energies (EML) are provided in Table S1 for the three series for comparison. In general, the EML values fall between those for the related ph- and biphbridged analogues. For example, EML = 14850 cm−1 for [S2− (2,6-naph)−S2] is higher than that for [S2−ph−S2] (EML = 13850 cm−1) but lower than that for [S2−biph−S2] (EML = 15647 cm−1). However, similar MLCT energies are found for the dicarboxylate derivatives [O2−ph−O2] (20600 cm−1) and [O2−(2,6-naph)−O2] (20010 cm−1) (Table S1). To further investigate the electronic structures of the compounds, density functional theory (DFT) calculations were performed on the calculation models derived from the molecular structures by replacement of the anisyl groups with hydrogen atoms. The frontier molecular orbitals (MOs) and the relative energies are shown in Figure 4. The calculated bond distances and the spectroscopic data for the three models are in general agreement with the experimental data (see the SI), which verifies the theoretical results. As shown in Figure 4, the highest occupied molecular orbital (HOMO) and HOMO−1 result mainly from the combination of the two δ orbitals in a pattern of δ+δ and δ−δ, respectively, thus belonging to the metal (δ) orbitals in nature. It is noted that, for the naphbridged complexes, the patterns of mixing of the δ orbitals are different from those in the ph-spaced analogues where HOMO and HOMO−1 are generated from the out-of-phase (δ−δ) and in-phase (δ+δ) combinations, respectively.12 This is because the node numbers of the conjugated bridge are increased by introducing one more fused aromatic ring on the bridge. A similar situation is seen for the Mo2 dimer with a 2,6azulenedicarboxylate bridge.22 Therefore, the calculation results confirm the d(δ)−p(π) conjugation along the charge-transfer platform. In the series, the extent of involvement of the bridge π orbitals varies depending on the composition of the [Mo2] units. The contributions of the π orbitals to the metal-based MOs increase as more sulfur atoms are introduced to the [Mo2] units (Figure 4). For each of the complexes, the HOMO−lowest unoccupied molecular orbital (LUMO) energy gap (ΔEH−L) is compatible with the measured MLCT band energy in the spectrum (see the SI). Excellent consistency between the calculated ΔEH−L

Figure 2. X-ray crystal structures for [O2−(2,6-naph)−O2] (top), [OS−(2,6-naph)−OS] (middle), and [S2−(2,6-naph)−S2] (below). Displacement ellipsoids are drawn at the 30% probability level. Hydrogen atoms are omitted for clarity.

Figure 3. Cyclic voltammograms for complexes [O2−(2,6-naph)− O2], [OS−(2,6-naph)−OS], and [S2−(2,6-naph)−S2] in CH2Cl2 solutions.

for the biphenyl analogues, which show a quasi-two-electron redox wave.14 The potential separations (ΔE1/2) for the two successive one-electron oxidations are estimated by Richardson−Taube’s method.20 The electrochemical parameters are given in Table 1, along with those for the phenyl analogues for comparison. The ΔE1/2 values are significantly smaller than that for the corresponding phenylene-spaced one. Large differences 6317

DOI: 10.1021/acs.inorgchem.6b01021 Inorg. Chem. 2016, 55, 6315−6322

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Inorganic Chemistry Table 1. Electrochemical Measurements and the Parameters for the Comproportionation Equilibriuma compound

E1/2(1) (mV)

E1/2(2) (mV)

ΔE1/2 (mV)

KC

ΔGC (cm−1)

[O2−(2,6-naph)−O2] [OS−(2,6-naph)−OS] [S2−(2,6-naph)−S2] [O2−ph−O2]b [OS−ph−OS]b [S2−ph−S2]b

283 443 591 335 468 502

351 531 699 426 584 697

68 88 108 91 116 195

14 31 67 35 91 1980

−544 −708 −874 −733 −935 −1572

For complexes [Mo2]−naph−[Mo2], the ΔE1/2 values were obtained from the Richardson and Taube methods (ref 20). According to ref 20, the E1/2 values were calculated from E1/2(2) = Ec + (ΔE1/2 + Epul)/2 and E1/2(1) = E1/2(2) − E1/2. Ec = the center potential of the DPV trace. Epul = 50 mV. bData cited from ref 12. a

Figure 4. Illustrations of the 0.04 contour surface and energy of the frontier MOs for the computational models [Mo2(N2H2CH)3]2(μE2CC10H6CE2) [E = O (red), S (yellow)] corresponding to compounds [Mo2(DAniF)3]2(μ-E2CC10H6CE2).

value (19917 cm−1) and the spectral MLCT energy (20010 cm−1) is obtained for [O2−(2,6-naph)−O2]. Therefore, the MLCT energy is a measure of the HOMO−LUMO gap. For the series, the variation trends of ΔEH−L and EML are parallel to each other. The ΔEH−L value for a naphthalene-spaced complex is larger than that for the phenylene counterpart.12 Large increases of ΔEH−L, as the bridge changes from ph to naph, are found for the thiolated complexes, consistent with variations of the MLCT energies in the spectra and the electrochemical results. Both experimental and computational results show that [O2−(2,6-naph)−O2] and [O2−ph−O2] have relatively small differences in the MLCT energy. These results imply that increasing the size of the conjugated bridge effectively enhances EC for weakly coupled [O2−(2,6-naph)−O2]. This can be rationalized by the similar atomic radii of the chelating atoms (O) and the naphthalene spacer (C) of the bridging ligand, which give rise to better orbital interaction. This hypothesis is supported by the calculation results. Substitution of the naph for ph in the dicarboxylate-bridged complexes lowers the LUMO energy from −1.19 to −1.43 eV, while for the thiolated complexes, similar LUMO energies were found for the two series (Figure 4).12 HOMO and HOMO−1 are nondegenerated because of mediation of the bridging ligand. Therefore, the HOMO− HOMO−1 energy gap (ΔEH−H−1) also reflects the strength of the electronic interaction between the two [Mo2] units. Strong metal−bridge−ligand interaction increases the splitting in energy between HOMO and HOMO−1. Indeed, the ΔEH−H−1 values in the series increase from 0.1 to 0.15 to 0.2

eV as EC increases (Table S4). In comparison with the phbridged analogues, the naphthalene complexes have relatively small ΔEH−H−1 values, consistent with the predictions from the ΔEH−L values and electrochemical results. Importantly, the consistency between the experimental observation and computational result indicate that, in these complex systems, the interplay between the bridge length and conjugacy affects the EC through control of the energy gap between the donor and bridge, which is in accordance with the superexchange formalism.23,24 Optical Properties of the MV Complexes. The MV complexes [O2−(2,6-naph)−O2]+, [OS−(2,6-naph)−OS]+, and [S2−(2,6-naph)−S2]+ were prepared by one-electron oxidation of the corresponding neutral compound with 1 equiv of ferrocenium hexafluorophosphate (Cp2FePF6). These radical cations were characterized by X-band electron paramagnetic resonance (EPR) spectra. In the EPR spectra, each complex exhibits one main signal with some weak hyperfine structures (Figure S6). The EPR signal is attributed to molecules containing the 96Mo (I = 0) isotope, while the hyperfine structure is due to molecules with the 95,97Mo (I = 5 /2) isotope (natural abundance of approximately 25%).25 The EPR peaks center at g = 1.941 for [O2−(2,6-naph)−O2]+, 1.940 for [OS−(2,6-naph)−OS]+, and 1.942 for [S2−(2,6naph)−S2]+, similar to the g values for other Class II MV complexes of this catagory.12,14,26 The EPR spectra for Mo2 dimers with carboxylate supporting ligands also show similar g values, for example, 1.942 for perfluoroterephthalate-bridged complexes, but significantly different hyperfine structures.27 6318

DOI: 10.1021/acs.inorgchem.6b01021 Inorg. Chem. 2016, 55, 6315−6322

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Inorganic Chemistry The small g values, compared to 2.003 for an organic radical, indicate that the odd electron resides essentially on the δ orbital. As shown in Figure 5, the MV complexes present MLCT absorption with a maximum energy similar to that for the

The most important spectroscopic feature for the MV complexes is the weak and extremely broad absorption band in the near-to-mid-IR regions (Figure 5), which should be assigned to IVCT or metal-to-metal charger-transfer (MMCT). Similar to the ph- and biph-bridged cases, the MMCT energy of this series is steadily lowered but the intensity increased, as shown in Table 2. For example, the fully Table 2. Spectroscopic Data of the IVCT Bands for the [O2− (2,6-naph)−O2]+, [OS−(2,6-naph)−OS]+, and [S2−(2,6naph)−S2]+ and Electronic Coupling Parameters, in Comparison with the Data for the [Mo2]−ph−[Mo2]a and [Mo2]−biph−[Mo2]b Series

a

compound

EIT (cm−1)

εIT (M−1 cm−1)

exptl Δν1/2 (cm−1)

Hab (cm−1)

[O2−(2,6-naph)−O2]+ [OS−(2,6-naph)−OS]+ [S2−(2,6-naph)−S2]+ [O2−ph−O2]+ [OS−ph−OS]+ [S2−ph−S2]+ [O2−biph−O2]+ [OS−biph−OS]+ [S2−biph−S2]+

6030 4900 3333 4240 3440 2640 8300 6536 4826

496 1080 2700 1470 3690 12660 198 715 1610

7620 6330 5331 4410 3290 1770 8578 6338 5231

390 470 570 589 727 864 245 354 415

Data cited from ref 12. bData cited from ref 14.

thiolated complex [S2−(2,6-naph)−S2]+ exhibits a IVCT band at 3333 cm−1 (εIT = 2700 M−1 cm−1), but for [O2−(2,6naph)−O2]+, this band has much higher energy (EIT = 6030 cm−1) and lower intensity (εIT = 496 M−1 cm−1). It is important to note that, for the three series, the IVCT band energy (EIT) increases and the intensity (εIT) decreases in order with the bridge changing from ph to naph to biph.12,14 EC Matrix Elements Hab. The EC matrix elements (Hab) are calculated from the Mulliken−Hush expression:29 Figure 5. Vis−near-IR (NIR) spectra for the neutral compounds (light line) and MV complexes (dark line): (A) for [O2−(2,6-naph)−O2] and [O2−(2,6-naph)−O2]+; (B) for [OS−(2,6-naph)−OS], and [OS−(2,6-naph)−OS]+; (C) for [S2−(2,6-naph)−S2] and [S2−(2,6naph)−S2]+. In the insets, amplified spectra are presented to show the intervalence bands. Gaussian simulation of the IVCT bands is based on the high-energy profile because the absorptions in the low-energy region are covered by the raised baseline.

Hab =

2.06 × 10−2 (εITΔν1/2E IT)1/2 rab

(1)

where Δν1/2 is the IVCT bandwidth at half-height. With application of eq 1, the effective ET distance (rab) is the key for accurate determination of Hab. As is known, for complex D−B− A systems, the coordination sphere of the metal centers shortens the ET distance.30,31 In [Mo2]−bridge−[Mo2], the δ electrons are delocalized over the coordination shell of the Mo2 centers through d(δ)−p(π) conjugation, which decreases the D−A distance.32,33 Therefore, in previous works, the lengths of the spacer, ca. 5.8 Å of “−CC6H4C−”12 and 10 Å of “− CC6H4C6H4C−”,14 rather than the geometrical Mo2···Mo2 separations are considered to be the effective ET distances for the ph and biph systems, respectively. Accordingly, for the present system, rab is estimated to be 8 Å from the size of the naphthalene group (−CC 10 H 6 C−). The calculated H ab parameters are presented in Table 2, along with the data for the related systems. The magnitudes of Hab increase from 390 cm−1 ([O2−(2,6-naph)−O2]+) to 470 cm−1 ([OS−(2,6naph)−OS]+) to 570 cm−1 ([S2−(2,6-naph)−S2]+), with a variation trend consistent with those observed in the other two series.12,14 For the three analogues having the same [Mo2] unit but differing bridges, the naph-spaced complexes have the Hab parameters falling between those for their ph and biph counterparts (Table 2),12−14 as expected.

corresponding neutral precursor, but with significantly low intensity. The high-energy δ → δ* transition absorbance is manifested upon one-electron oxidation, as seen for [OS−(2,6naph)−OS]+ and [S2−(2,6-naph)−S2]+. For [O2−(2,6naph)−O2]+, this band is overlapped with the MLCT band, which shifts the maximum ∼2 nm toward high energy (Figure 5). Substantial spectral differences between the MV complexes and the precursors include the ligand-to-metal charge-transfer (LMCT) absorptions, which do not appear in the spectra of the neutral compounds. More specifically, this absorption is due to charge transfer from the occupied bridging π orbital to the singly occupied δ orbital on the oxidized [Mo2] unit.13 For this series, the LMCT band as a shoulder of the MLCT band has very low intensity (Figure 5). From the superexchange formalism,23,24 the MLCT and LMCT spectral data account for the contributions of ET and hole transfer on EC and ET, respectively. Low-energy, intense LMCT absorption bands are observed in a strong coupling system.13,28 6319

DOI: 10.1021/acs.inorgchem.6b01021 Inorg. Chem. 2016, 55, 6315−6322

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Inorganic Chemistry

cordingly, the three molecules in a series share a common naph bridge but have different [Mo2] units as the electron donor (acceptor), of which the electron-donating (accepting) ability varies depending on the composition of the [Mo2] units. In comparison with the phenylene-bridged analogues reported previously, the D−A distances (rab = 8 Å) are lengthened, which weakens the metal−metal interaction, and the conjugated system enlarged, which supposedly enhances the EC. Qualitatively, the electronic interaction of this series, as judged by electrochemical and spectroscopic analyses, is weaker than that for the ph one (rab = 5.8 Å) but stronger than that for the biph one (rab = 10 Å). Optical analyses of the MV complexes, based on the Hush formalism, gave the EC matrix elements Hab = 390−570 cm−1, which fall between the data for the ph and biph series. For the three series having the same [Mo2] units and different bridges, the electron coupling parameters decay exponentially with the D−A separation with the decay factor γ = 0.21−0.17 Å−1. The largest γ value is found for the dicarboxylate-bridged series. These results confirm that introducing sulfur atoms on the charge-transfer platform enhances the EC. Relative to the phenylene-bridged series, the positive effect of increasing the bridge conjugacy on the EC does not complement the negative one, resulting from increasing charge-transfer distance.

The influence of the naph bridge on EC was examined by the exponential decay law (eq 2)31,34

Hab = Hab° exp( −γR )

(2) 14

in comparison with the ph and biph systems. In eq 2, Hab and Hab° are the EC parameters at distance R (= rab) and van der Waals contact, respectively, and γ is the empirical decay factor. For convenience of discussion, the three groups of compounds are denoted as [O2−B−O2], [OS−B−OS], and [S2−B−S2], each of which has the same [Mo2] unit (or donor and acceptor) but varying bridges. Application of the data in Table 2 to eq 2 demostrates that, for each group, ln(Hab) is linearly related to R with a correlation coefficient of >0.99, as shown in Figure 6.

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Figure 6. Distance dependence plot of ln(Hab) versus R (Å). The data (Table 2) were fitted to three linear equations for [O2−B−O2], [OS− B−OS], and [S2−B−S2], ln(Hab) = ln(H0) −γR, with correlation coefficients (R2) of >0.99.

EXPERIMENTAL SECTION

Materials and Methods. All manipulations were performed in a nitrogen-filled glovebox or by using standard Schlenk-line techniques. All solvents were purified using a Vacuum Atmospheres (VAC) solvent purification system or freshly distilled over appropriate drying agents under nitrogen. HDAniF18 and Mo2(DAniF)3(O2CCH3)19 were synthesized according to literature methods. 2,6-Naphthalenedithiodicarboxylic acid15 and 2,6-naphthalenetetrathiodicarboxylic acid16,17 were prepared by following published procedures. X-ray Structure Determinations. Single-crystal data for [O2− (2,6-naph)−O2]·2CH2Cl2 were collected on an Agilent Xcalibur Nova diffractometer with Cu Kα radiation (λ = 1.54178 Å) at 100 K, and single-crystal data for [OS−(2,6-naph)−OS]·2CH2Cl2 and [S2−(2,6naph)−S2]·2C2H5OH were collected on an Agilent Gemini S Ultra diffractometer with Cu Kα radiation (λ = 1.54178 Å) at 173 K. For them, empirical absorption corrections were applied using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm.40 All of 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 the SHELXS-97 and SHELXL-97 programs, respectively.41 The solvent molecules were disordered in multiple orientations, which were refined isotropically. All non-hydrogen atoms were refined with anistropic displacement parameters. Physical Measurements. Elemental analyses were determined using an Elementar Vario EL elemental analyzer. UV−vis and NIR spectra were measured on a Shimadzu UV-3600 UV−vis−NIR spectrophotometer. The mid-IR spectra were recorded on a Thermo Electron Corporation Nicolet 6700 spectrophotometer. Both near-IR and IR spectra were measured in CH2Cl2 solutions using IR quartz cells with a light path length of 2 mm. CV was performed using a CH Instruments model CHI 660D electrochemical analyzer in a 0.10 M n Bu4NPF6 solution in CH2Cl2 with platinum working and auxiliary electrodes, a Ag/AgCl reference electrode, and a scan rate of 100 mV s−1. All potentials were referenced to the Ag/AgCl electrode. 1H NMR spectra were recorded on a Bruker 300 spectrometer. EPR spectra were measured using a Bruker A300-10-12 EPR spectrometer. Measurements for the MV complexes were carried out in situ after single-electron oxidation of the corresponding neutral compounds. Computational Details. All DFT calculations were performed with the hybrid O3LYP42 functional implemented in the Gaussian 09

Then, the decay factors γ are determined to be 0.21 Å−1 ([O2− B−O2]), 0.17 Å−1 ([OS−B−OS]), and 0.17 Å−1 ([S2−B−S2]) from the slopes. When the Mo2···Mo2 separations are considered to be the ET distatnce, these γ values remain almost the same. The decay factors vary depending on the nature of the donor (or acceptor). Smaller γ values (0.17 Å−1) are found for the thiolated systems, confirming that the involvement of sulfur atoms enhances the EC. Notably, in previous studies on the ph and biph series,14 the γ values were estimated from the two data sets. Here, the data for the naph complexes fit satisfactorily the respective linear equations, giving the same decay factors (γ). The implication of these results is that, with a naph bridge, the charge-transfer distance is the major factor that affects the EC. In other words, the larger conjugated bridge (naph) does not efficiently complement the Hab decay with increasing distance. The decay factors herein are comparable with those for inorganic−organic hybridized systems (0.19 Å−1),35 diruthenim dimers bridged by polyyndiylene (0.12−0.15 Å−1)36 and polyphenylene (0.14 Å−1)37 groups. Significantly smaller decay factors (0.07−0.10 Å−1) were found for some bimetallic D−B−A systems with conjugated bridges.31,38 In general, conjugated bridges offer much smaller decay factors than saturated hydrocarbon bridges (0.6−1.2 Å−1).39

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CONCLUDING REMARKS We have synthesized and structurally characterized three novel Mo2 dimers with the generated formula [Mo2(DAniF)3]2(μE2CC10H6CE2) (E = O, S), for which the bridging ligands are 2,6-naphthalenedicarboxylate and its thioated derivatives. Corresponding to a donor−bridge−acceptor assembly, these compounds may be reformulated as [(DAniF)3Mo2(E2C)](C10H6)[(CE2)Mo2(DAniF)3)] or [Mo2]−naph−[Mo2]. Ac6320

DOI: 10.1021/acs.inorgchem.6b01021 Inorg. Chem. 2016, 55, 6315−6322

Inorganic Chemistry

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package (revision A0.2).43 The model complexes were fully optimized. The standard 6-31G*basis set was used for hydrogen, carbon, and nitrogen atoms and the aug-CC-pvDZ basis set for sulfur and oxygen atoms of the bridging ligands. The SDD basis set, together with the SDD pseudopotential, was used for the heavy-metal molybdenum atoms. Time-dependent DFT calculations were carried out to obtain 60 excitations for all of the model compounds. By replacement of the p-anisyl groups on [Mo2(DAniF)3]+ with hydrogen atoms, the employed calculation models have [Mo2(NHCHNH)3]+ units as building blocks. This simplification has been successfully used in the Mo2 analogues.2a,b,12,21 Preparation of [O2−(2,6-naph)−O2]. Complex Mo2(DAniF)3(O2CCH3) (0.25 mmol) and 2,6-naphthalenedicarboxylic acid (0.125 mmol) were mixed and dissolved in 30 mL of tetrahydrofuran (THF). The solution was stirred at room temperature for 5 h. The solvent was evaporated under reduced pressure. The residue was washed with ethanol (3 × 15 mL). The product was collected by filtration and dried under vacuum. Yield of [O2−(2,6naph)−O2]: 0.18 g (68%). 1H NMR (CDCl3): δ 8.49 (s, 2H, −NCHN−), 8.40 (s, 4H,−NCHN−), 8.80 (s, 2H, aromatic H), 8.41 (d, 2H, aromatic H), 8.01 (d, 2H, aromatic H), 6.65 (d, 16H, aromatic H), 6.58 (d, 16H, aromatic H), 6.46 (d, 8H, aromatic H), 6.26 (d, 8H, aromatic H), 3.73 (s, 24H, −OCH3), 3.68 (s, 12H, −OCH3). UV−vis [λmax, nm (ε, M−1 cm−1)]: 460 (5.4 × 103), 618 (2.6 × 104). Anal. Calcd for C102H96Mo4N12O16: C, 57.52; H, 4.51; N, 7.89. Found: C, 56.98; H, 4.562; N, 7.93. General Procedure for the Preparation of [OS−(2,6-naph)− OS] and [S2−(2,6-naph)−S2]. A solution of sodium methoxide (0.50 mmol) in 10 mL of methanol was transferred to a solution of Mo2(DAniF)3(O2CCH3) (0.25 mmol) and 2,6-dithionaphthalenedicarboxylic acid (or 2,6-tetrathionaphthalenedicarboxylic acid; 0.125 mmol) in 30 mL of THF. The mixture was stirred at room temperature for 5 h. The solvent was evaporated under reduced pressure. The residue was dissolved using 15 mL of CH2Cl2 and filtered off through a Celite-packed funnel. The filtrate was evaporated under reduced pressure. The residue was washed with ethanol (3 × 15 mL) and collected by filtration. The product was dried under vacuum. [OS−(2,6-naph)−OS]. Yield: 0.13 g (61%). 1H NMR (CDCl3): δ 8.50 (s, 2H, −NCHN−), 8.35 (s, 4H, −NCHN−), 8.70 (s, 2H, aromatic H), 8.39 (d, 2H, aromatic H), 7.89 (d, 2H, aromatic H), 6.64 (d, 16H, aromatic H), 6.58 (d, 16H, aromatic H), 6.42 (d, 8H, aromatic H), 6.10 (d, 8H, aromatic H), 3.72 (s, 24H, −OCH3), 3.66 (s, 12H, −OCH3). UV−vis [λmax, nm (ε, M−1 cm−1)]: 460 (5.4 × 103), 618 (2.6 × 104). Anal. Calcd for C102H96Mo4N12O14S2: C, 56.65; H, 4.44; N, 7.77. Found: C, 56.88; H, 4.62; N, 7.852. [S2−(2,6-naph)−S2]. Yield: 0.13 g (61%). 1H NMR (CDCl3): δ 8.47 (s, 2H, −NCHN−), 8.33 (s, 4H, −NCHN−), 8.75 (s, 2H, aromatic H), 8.53 (d, 2H, aromatic H), 7.87 (d, 2H, aromatic H), 6.65 (d, 16H, aromatic H), 6.61 (d, 8H, aromatic H), 6.53 (d, 8H, aromatic H), 6.45 (d, 8H, aromatic H), 6.24 (d, 4H, aromatic H), 6.18 (d, 4H, aromatic H), 3.73 (s, 12H, −OCH3), 3.72 (s, 12H, −OCH3), 3.68 (s, 6H, −OCH3), 3.66 (s, 6H, −OCH3). UV−vis [λmax, nm (ε, M−1 cm−1)]: 460 (5.4 × 103), 618 (2.6 × 104). Anal. Calcd for C102H96Mo4N12O12S4: C, 55.82; H, 4.38; N, 7.66. Anal. Calcd for C102H96Mo4N12O12S4: C, 55.82; H, 4.38; N, 7.66. Found: C, 56.08; H, 4.52; N, 7.82.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.Y.L.). Author Contributions †

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the National Science Foundation of China (Grants 21371074 and 90922010) and Jinan University for financial support.

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REFERENCES

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01021. Syntheses, 1H NMR data, DPV figures, electronic spectra, EPR figures, and computational data (PDF) Crystallographic information in CIF format (CIF) 6321

DOI: 10.1021/acs.inorgchem.6b01021 Inorg. Chem. 2016, 55, 6315−6322

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DOI: 10.1021/acs.inorgchem.6b01021 Inorg. Chem. 2016, 55, 6315−6322