Mapping Bridge Conformational Effects on Electronic Coupling in Mo2

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

pubs.acs.org/IC

Mapping Bridge Conformational Effects on Electronic Coupling in Mo2−Mo2 Mixed-Valence Systems Huo Wen Chen,† Suman Mallick,† Shan Feng Zou, Miao Meng, and Chun Y. Liu* Department of Chemistry, Jinan University, 601 Huang-Pu Avenue West, Guangzhou 510632, China S Supporting Information *

ABSTRACT: The large bridging ligand 9,10-anthracenedicarboxylate and its thiolated derivatives have been employed to assemble two dimolybdenum complex units and develop three Mo 2 dimers, [Mo 2 (DAniF) 3 ] 2 (μ-9,10-O 2 CC 14 H 8 CO 2 ), [Mo2(DAniF)3]2(μ-9,10-OSCC14H8COS), and [Mo2(DAniF)3]2(μ-9,10-S2CC14H8CS2) (DAniF = N,N′-di(panisyl)formamidinate), for the study of conformation dependence of the electronic coupling between the two Mo2 centers. These compounds feature a large deviation of the central anthracene ring from the plane defined by the Mo−Mo bond vectors, with the torsion angles (ϕ = 50−76°) increasing as the chelating atoms of the bridging ligand vary from O to S. Consequently, the corresponding mixed-valence complexes do not exhibit characteristic intervalence charge transfer absorptions in the near-IR spectra, in contrast to the phenylene and naphthalene analogues, from which these systems are assigned to the Class I in Robin-Day’s scheme. Together with the phenylene and naphthalene series, the nine total mixed-valence complexes in three series complete a transition from the electronically uncoupled Class I to the strongly coupled Class II−III borderline via moderately coupled Class II and permit a systematic mapping of the bridge conformation effects on electronic coupling. Density functional theory calculations show that the HOMO−LUMO energy gap, corresponding to the metal (δ) to ligand (π*) transition energy, and the magnitude of HOMO−HOMO−1 splitting in energy are linearly related to cos2 ϕ. Therefore, our experimental and theoretical results concur to indicate that the coupling strength decreases in the order of the bridging units: phenylene > naphthalene > anthracene, which verifies the through-bond superexchange mechanism for electronic coupling and electron transfer.

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INTRODUCTION The understanding obtained of electronic coupling (EC) and electron transfer (ET) through a bridging molecule (or group) in the donor (D)−bridge (B)−acceptor (A) triad is of fundamental importance in chemistry,1 biology,2 and molecular electronics,3 an emerging field that integrates chemistry, physics, and material science. The bridge controls the EC effects and the ET kinetics through its molecular topology and electronic configuration as well as the interplay between them. Experimental study has demonstrated that a π conjugated system is favored for electron transfer within the molecule and for electric conductance in the molecular junction, which conforms to the through-bond (TB) superexchange formalism.4 This is exemplified by the study of D−B−A mixed-valence (MV) systems with an unsaturated 1,4-diethynylarene spacer, in which the arene moiety varies from phenyl, naphthalene, and anthracene.5−9 Indeed, it is found that replacement of phenylene by a larger aromatic unit, i.e., naphthalene or anthracene, enhances significantly the electronic coupling for both inorganic5,6 and purely organic7,8 MV compounds. Consistent results also have been obtained in molecular junctions.9 Study on the bridged bishydrazine radical reveals that an anthracene linker is able to enhance the electronic © XXXX American Chemical Society

coupling, with respect to phenylene and naphthalene, consequently, lowering the ET barrier for the system.10 However, Lambert et al.11 reported that in tetraanisylarylenediamine MV systems, the opposite trend holds, that is, that the EC is found to be less effective through the anthracene spacer than phenylene and naphthalene bridges. These results suggest that the electronic coupling is controlled by the energetic balance between conjugation and conformation of the bridge. In order to achieve intellectual control of the electron migration along the molecule, detailed knowledge on how the structural and electronic factors interplay is required. In this context, much work has been done, by taking experimental and theoretical approaches,12 to address the bridge conformation effects on EC and ET.13 For instance, in the system with two porphyrin units bridged by a 4,4′-biphenylene group, the dihedral angle (ϕ) between the phenyl rings is controlled by substituents at the 2,2′-position, and the minimum of throughbond ET rate was observed at an angle of 45°,14 which follows roughly a cos2(2ϕ) function. In the study by Benniston et al.,15 a tethered strap across the 2,2′-position of the biphenyl bridge Received: April 17, 2018

A

DOI: 10.1021/acs.inorgchem.8b01056 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

[OS−(9,10-anth)−OS], and [SS−(9,10-anth)−SS] respectively (Scheme 1). They share a common anthracene bridge

is incorporated into the binuclear ruthenium(II)−bis(2,2′:6′,2″-terpyridine) complexes, which permits a free change of bridge conformation without altering its electronic property. In this case, the maximal coupling was found at 0° and a minimum at orthogonal orientation, i.e., at 90°. However, the coupling effect does not vanish at 90° because of thermal fluctuations about the mean angle. Quantitatively, a linear correlation between the electronic coupling matrix element (Hab) and cos2 ϕ is observed,

Scheme 1. Molecular Scaffold of the [Mo2]−(9,10-anth)− [Mo2] Complexesa

Hab = H0 + H1 cos2 ϕ

where H0 is the inherent coupling element for the coplanar geometry and H1 represents the angle-dependent coupling parameter.15a Similarly, in molecular junctions, it is found that electric conductance (or resistance) is linearly related to cos2 ϕ.16 Mixed-valence complexes with two bridged multiply bonded Mo2 units possess distinct molecular topology and well-defined electronic configurations, σ2π4δ2 for the Mo2 donor and σ2π4δ1 for the acceptor. In such a D−B−A system, the δ electrons, which are discriminated from the other d electrons on the Mo2 unit by orbital symmetry and energy, are the migrating electrons, which depart from and arrive at the δ orbitals but in different Mo2 centers. These unique electronic features give rise to characteristic metal (δ) to bridging ligand (π*) (ML) and bridging ligand (π) to metal (δ) (LM) electronic transition bands and more importantly a single metal to metal (MM) vibronic transition band or intervalence charge transfer (IVCT) absorption in the near-IR region. Structurally, equatorial linkage of the two [Mo2] units across the bridge arranges the two Mo− Mo bonds in a parallel setting, which allows d(δ)−p(π) orbital interactions between the dimetal centers and the bridging ligand. These structural and electronic features make the Mo2 D−B−A complexes desirable experimental models for evaluation of the bridge-mediated electronic coupling within the established theoretical framework.17 Our previous works18,19 have shown that sulfur-containing chelating groups of the bridging ligand are more efficient for electronic communication between the two [Mo2] termini compared to those with oxygen atoms. However, the coupling strength is also dependent on the dihedral angles between the Mo2 chelate ring and aromatic bridge,20,21 which has been clearly shown by the phenylene18 and naphthalene20 bridged series with distinct optical behaviors. While in the [Mo2]−bridge−[Mo2] model systems, the [Mo2] unit and the bridge affect significantly the donor−acceptor electronic coupling, determining which factor dominants and understanding how these two factors interplay to control the electronic coupling are of great interest. This can be done by mapping the electronic coupling in a large group of subtly differentiated individuals with the variables concerning the structural, electronic, spectroscopic, and energetic aspects. In this report, a large anthracenyl bridge, which imposes severe steric hindrance to the adjacent units, is introduced to the [Mo2]−bridge−[Mo2] complex system to gate the electronic coupling and electron transfer through conformation control. The topologic gradient of the bridge is manipulated by O/S alteration of the chelating group of the bridging ligand, a strategy that has been utilized in the naphthalene bridged series. Therefore, the three complexes in a series involved in this study are [Mo2(DAniF)3]2(μ-9,10-O2CC14H8CO2), [Mo2(DAniF)3]2(μ-9,10-OSCC14H8COS), and [Mo2(DAniF)3]2(μ-9,10-S2CC14H8CS2) (DAniF = N,N′-di(panisyl)formamidinate), denoted as [OO−(9,10-anth)−OO],

The torsion angle ϕ depends on the choice of the chelating E atoms, O or S. a

−(9,10-C14H8)− and thus, have similar Mo2···Mo2 distances, but differ in the chelating atoms E (O, S) which also modifies the electron donating (or accepting) ability of the [Mo2] units. Importantly, in these compounds the bridge moiety is highly twisted from the charge transfer platform, efficiently gating the TB coupling and electron transfer as shown by the electrochemical and spectroscopic properties. Together with the phenylene and naphthalene bridged series, the nine total Mo2 dimers allow us to map the conformational effects on electronic coupling.

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RESULTS AND DISCUSSION Molecular Design and Synthesis. It has been shown that in a mixed-valent [Mo2]−bridge−[Mo2] system, a conjugated bridge enables electron transfer occurring between the [Mo2] units via a π platform. This is because the δ orbital of a Mo2 unit is symmetrically related to the π orbitals of the bridging ligand, forming the so-called d(δ)−p(π) conjugation.18 This d(δ)−p(π) interaction between the Mo2 center and bridging ligand establishes a π-conjugated charge transfer platform only when the coplanar requirement for the bridge is satisfied. Thus, such a system allows explicitly the bridge conformation to control the electron coupling through orbital interactions. Conformational control on the electronic coupling and electron transfer in the [Mo2]−bridge−[Mo2] system has been illustrated in previous work with phenylene (ph) and 1,4naphthalene (1,4-naph) bridges.20 We have shown that increasing the bridge size enlarges the torsion angles and eventually lowers the electronic coupling, despite the increased π system as in the case of the naphthalene bridge. Moreover, in prior studies, by stepwise substitution of S atoms for the O atoms of dicarboxylate bridging ligand, the two series of [Mo2]−bridge−[Mo2] complexes with ph and 1,4-naph bridges were developed, each of which has three members differing in the [Mo2] units. In order to systematically evaluate the conformational effects on the electronic coupling, in this work, we chose a 9,10-anthracene (9,10-anth) group as the bridge in hoping to further increase the torsion angle and lower the electronic coupling. This can be achieved by assembling two dimolybdenum building blocks [Mo2(DAniF)3]+ with anthraB


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Inorganic Chemistry

Scheme 2. Compounds in Series [Mo2]−ph−[Mo2] (A), [Mo2]−(1,4-naph)−[Mo2] (B), and [Mo2]−(9,10-anth)−[Mo2] (C), or [OO−bridge−OO] (I), [OS−bridge−OS] (II), and [SS−bridge−SS] (III), where [Mo2] = [Mo2(DAniF)3(EEC)]+ (E = O or S)

Scheme 3. Synthetic Routes for the Anthracenyl Bridging Ligandsa

a (i) n-BuLi/diethyl ether, r.t., 30 min; (ii) CS2, 4 days; (iii) (a) CO2, 1 h; (b) HCl; (iv) SOCl2/DMF, reflux, 4 h; (v) (a) CH3CSNH2/THF, r.t., 4 h; (b) NaOH, HCl.

[Mo2(DAniF)3(OSC)], or [Mo2(DAniF)3(S2C)]. There are two effects resulting from introducing S atoms to the [Mo2] unit, enhancing the electronic coupling and enlarging the torsion angles, which affect the electronic coupling in an opposite way. For the three series, the torsion angles are expected to increase with increasing S contents in the [Mo2] units. With these compounds available, three different series with the same [Mo2] units but varying bridges can be constructed, that is, [OO−bridge−OO] (I), [OS−bridge− OS] (II), and [SS−bridge−SS] (III), as shown in Scheme 2. In each of these series, the torsion angles increase as the bridge size increases. Collectively, the availability of the nine species with continuously varied structures and electronic properties allow us to systematically evaluate the interplay of various factors on the electronic coupling.

cenedicarboxylate and its thiolaged derivatives, generally formulated as −E2C−(9,10-anth)−CE2− (E = O, S). Then, three Mo2 dimers [Mo2(DAniF)3]2(μ-9,10-O2CC14H8CO2), [Mo2(DAniF)3]2(μ-9,10-OSCC14H8COS), and [Mo2(DAniF)3]2(μ-9,10-S2CC14H8CS2), abbreviated as [OO− (9,10-anth)−OO], [OS−(9,10-anth)−OS], and [SS−(9,10anth)−SS], respectively, were synthesized by following the procedures developed in this group. With addition of the 9,10-anth bridged series, this study involves three series of [Mo2]−bridge−[Mo2] complexes (Scheme 2), namely, [Mo2]−ph−[Mo2] (A), [Mo2]−(1,4naph)−[Mo2] (B), and [Mo2]−(9,10-anth)−[Mo2] (C). Each of the three series is composed of three members, of which the [Mo2] complex units vary with increasing the number of S atoms , that is, [ Mo 2 ] = [ Mo 2 ( D A n i F) 3 (O 2 C ) ] , C


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Inorganic Chemistry The synthetic routes for the anthracenyl bridging ligands are summarized in Scheme 3. 9,10-Anthracenedicarboxylic acid (4) and the dilithium salt of 9,10-anthracenetetrathiodicarboxylic acid (3) were synthesized from commercially available 9,10dibromoanthracene (1). For preparation of 9,10-anthracenedithiodicarboxylic acid (6), a similar method that was used to synthesize dithioterephthalic acid was adopted.22 The three dimolybdenum dimers [(Mo2(DAniF)3)]2(μ-E2CC14H8CE2)] (E = O or S) were obtained by convergent reaction18 of the mixed-ligand complex [Mo2(DAniF)3(O2CCH3)] with the corresponding bridging ligand in the presence of sodium alkoxide. Molecular Structures. All three compounds crystallized in the triclinic space group P1̅ with Z = 1. The X-ray crystallographic parameters are presented in Table S1, and the selected bond lengths and dihedral angles ϕ between the anthracene ring and the adjacent Mo2-chelate ring planes are listed in Table 1. As shown by the crystal structures (Figure 1), Table 1. Selected Bond Lengths (Å) and Torsion Angles (deg)a of [OO−(9,10-anth)−OO], [OS−(9,10-anth)−OS], and [SS−(9,10-anth)−SS]

Mo(1)−Mo(2) Mo(1)−O(1) Mo(2)−O(2) Mo(1)−S(1) Mo(2)−S(2) C(4)−C(8) C(4)−S(1) C(4)−S(2) C(4)−O(1) C(4)−O(2) Mo2···Mo2 ϕ(1) ϕ(2)

[OO−(9,10anth)−OO]

[OS−(9,10anth)−OS]

[SS−(9,10anth)−SS]

2.0922(7) 2.121(4) 2.152(4)

2.1033(7) 2.190(5)

2.1130(6)

1.498(9)

1.266(8) 1.269(8) 11.256 50.3 48.6

2.4566(19) 1.486(8) 1.681(7) 1.320(8) 11.662 70.4 70.3

2.4405(14) 2.4594(14) 1.470(7) 1.687(6) 1.697(6)

12.139 76.7 70.6

a

ϕ(1) refers to O(1)/S(1)−C(4)−C(8)−C(9), and ϕ(2) to O(2)/ S(2)−C(4)−C(8)−C(7).

Figure 1. X-ray crystal structures of the dimolybdenum dimers [OO− (9,10-anth)−OO] (top), [OS−(9,10-anth)−OS] (middle), and [SS−(9,10-anth)−SS] (bottom), drawn at the 40% ellipsoid probability level. All hydrogen atoms have been omitted for clarity.

these compounds share the same molecular scaffold with two parallel [Mo2] units spaced by an anthracene group. The dimeric structures are also present in solution, as confirmed by the 1H NMR spectra (Figures S1−S3). Typically, the 1H NMR spectra show two sets of resonances in a ratio of 1:2 for protons of horizontally and vertically arranged auxiliary DAniF ligands, respectively. In the spectra, the aromatic protons on the anthracene moiety give also two sets of resonances at 8.50 and 7.30 ppm. The Mo−Mo bond lengths, ca. 2.1 Å, are in accordance with a quadruple bond;23 however, it increases slightly as the O chelating atoms of the bridge are replaced by S atoms, for example, 2.0922(7) Å for [OO−(9,10-anth)−OO] and 2.1130(6) Å for [SS−(9,10-anth)−SS]. In the crystal structure of [OS−(9,10-anth)−OS], the O and S atoms of the bridging ligand are arranged in trans position. As expected, replacement of S atoms for the O atoms in the chelating groups also modifies the [Mo2]···[Mo2] separations. From the shortest distance of 11.256 Å for [OO−(9,10-anth)−OO] to the longest (12.139 Å) for [SS−(9,10-anth)−SS], there is a length variation of less 1 Å. Importantly, in the three compounds, the anthracene group is highly twisted from the adjacent dimetal chelate ring planes, and the torsion angle ϕ increases by

stepwise introducing S atoms to replace the dicarboxylate chelating groups (Table 1). Thus the largest torsion angle (ϕ = 76°) is found for [SS−(9,10-anth)−SS] and the smallest ϕ (50°) for [OO−(9,10-anth)−OO], consistent with previously reported phenylene and naphthalene series.18,20 On the other hand, in the series [OO−bridge−OO] (I), the torsion angles are tuned by the bridge. [OO−(9,10-anth)−OO] presents the largest torsion angle (ϕ = 50°), compared to the less sterically hindered naphthalene analogue [OO−(1,4-naph)−OO] (ϕ = 24°) and phenylene analogue [OO−ph−OO] (ϕ = 9°). The other two series, [OS−bridge−OS] (II) and [SS−bridge−SS] (III), show the same variation trend of torsion angles.20 These structural differences in the three series indicate that the bulky bridge and large S chelating atoms exert severe steric repulsion between the chelating group and aromatic unit of the bridging ligand, which alters the bridge conformation. Electrochemical Studies. Electrochemical measurements on the neutral compounds in CH2Cl2 solution were carried out for general evaluation of the electronic coupling effect between two Mo2 redox sites. The cyclic voltammograms (CVs) and differential pulse voltammograms (DPVs) are shown in Figure 2, and the electrochemical parameters are listed in Table 2, D


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Inorganic Chemistry

clearly conformation dependence of electronic communication between the two bridged Mo2 centers. As is known, in a symmetrical D−B−A system, electrostatic (through space) and electron resonant (through bond) are two major interactions involved in donor−acceptor coupling.25,26 The former is correlated to the donor−acceptor distance, while the latter is determined by the orbital interaction, which is responsible for electron delocalization. Interestingly, the ΔE1/2 of 74 mV for [OO−(9,10-anth)−OO] is close to that for the 1,4-cyclohexylenedicarboxylate bridged analogue (69 mV),27 which has similar Mo2···Mo2 separations, although for the former a large π conjugated bridge is involved. For the cyclohexylene bridged analogue, however, the TB coupling is completely disrupted, and the weak coupling (small ΔE1/2) is likely due to the electrostatic interaction between the metal centers. This comparison indicates that the large torsion angle (50°) in [OO−(9,10-anth)−OO] has a pronounced effect in lowering the electronic coupling. These results manifest that a conjugated bridge with coplanar orientation is necessary for effective through bond interaction or resonant electronic coupling. Given the similar Mo2···Mo2 separations, the cyclohexylene analogue is suitable as a reference to quantify the electron resonant contribution (ΔEr) to ΔE1/2 for the present series of complexes. Indeed, subtraction of the ΔE1/2 value of 69 mV from the measured ΔE1/2 yields the resonant contribution (ΔEr) for each of the complexes. As shown in Table 2, of the three series, great increasing of ΔEr is found for the ph bridged analogues with the chelating atoms of the bridging ligands changing from oxygen (O) to sulfur (S), for which the ΔE is dominated by the resonant effect (ΔEr). For the anthracene-bridged complexes, however, the ΔEr values and the variation of ΔEr are much smaller because of the large torsion angles. Clearly, the large torsion angles of the anthracenyl complexes destroy, to a large extent, the d(δ)− p(π) conjugation across the bridge, and consequently, disables the TB electronic coupling, which conforms well to the superexchange formalism. Electronic Spectroscopy and DFT Calculations. All the three compounds under investigation display an intense absorption band in the visible region (Figure 3), which is assigned to the metal to bridging ligand charge transfer (MLCT) according to earlier work.18,19 The spectroscopic data are listed in Table 3, along with those for the phenylene and naphthalene series for comparison. For [Mo2]−(9,10-anth)− [Mo2] complexes, the MLCT energy is steadily lowered as the

Figure 2. Differential pulse voltammograms (DPVs, top) and cyclic voltammograms (CVs, bottom) for complexes [OO−(9,10-anth)− OO] (black), [OS−(9,10-anth)−OS] (red), and [SS−(9,10-anth)− SS] (green).

Table 2. Electrochemical Parametersa compound

E1/2 (1)

E1/2 (2)

ΔE1/2

ΔEr

ref

[OO−(9,10-anth)−OO] [OS−(9,10-anth)−OS] [SS−(9,10-anth)−SS] [OO−(1,4-naph)−OO] [OS−(1,4-naph)−OS] [SS−(1,4-naph)−SS] [OO−ph−OO] [OS−ph−OS] [SS−ph−SS]

320 578 600 292 470 606 335 468 502

394 660 690 388 583 713 426 584 697

74 82 90 96 113 107 100 116 195

5 13 21 31 48 42 35 51 130

this work this work this work 20 20 20 18 18 18

a

All electrochemical potentials are given in millivolts.

along with those for phenylene and naphthalene bridged analogues for comparison. For these Mo2 dimers, two redox events are expected due to removal of one δ electron from each of the two [Mo2] sites upon electrochemical oxidation. In the given potential range, E1/2 (1) and E1/2 (2) correspond to the [Mo 2 5+ −Mo 2 4+ ]/[Mo 2 4+ −Mo 2 4+ ] and [Mo 2 5+ −Mo 2 5+ ]/ [Mo25+−Mo24+] redox couples, respectively. Thus, the redox potential separation ΔE1/2 effectively indicates the relative strength of electronic communication between two dimetal centers. Note that this electrochemical criterion is applicable under the condition that the counterparts in comparison have similar distances between the two redox sites. This condition is met by the nine species of the three series (Scheme 2). The three anth bridged complexes are generally weakly coupled, as shown by the unresolved redox waves in the CVs (Figure 2). The ΔE1/2 was estimated by Richardson−Taube’s method24 to be 74 mV for [OO−(9,10-anth)−OO], 82 mV for [OS− (9,10-anth)−OS], and 90 mV for [SS−(9,10-anth)−SS]. Compared to the related phenylene and naphthalene bridged analogues, all three compounds show much smaller ΔE1/2 values (Table 2), although the anth bridge has a larger π conjugated system. For example, the magnitude of ΔE1/2 for [SS−(9,10-anth)−SS] is about half of the value for [SS−ph− SS] (195 mV). These two complexes share a common [Mo2] unit as the donor (acceptor); thus, the difference in ΔE1/2 should be attributed to the large conformational variation between them, that is, 76° ([SS−(9,10-anth)−SS]) versus 23° ([SS−ph−SS]). As indicated in Table 2, for the three series with different bridges (A, B, and C), the ph bridged analogue has the largest ΔE1/2, while the corresponding anth derivative has the smallest ΔE1/2. Therefore, for all the compounds listed, the ΔE1/2 values vary as a function of the torsion angle, showing

Figure 3. Electronic absorption spectra of [OO−(9,10-anth)−OO] (black), [OS−(9,10-anth)−OS] (red), and [SS−(9,10-anth)−SS] (green). E


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Table 3. DFT Calculated Torsion Angles and Orbital Energy Gaps for [Mo2]−(9,10-anth)−[Mo2], [Mo2]−(1,4-naph)−[Mo2], and [Mo2]−ph−[Mo2] Complexes in Comparison with the Experimental Dataa torsion angle (deg) compound [OO−(9,10-anth)−OO] [OS−(9,10-anth)−OS] [SS−(9,10-anth)−SS] [OO−(1,4-naph)−OO] [OS−(1,4-naph)−OS] [SS−(1,4-naph)−SS] [OO−ph−OO] [OS−ph−OS] [SS−ph−SS] a

expt 50 70 76 26 60 65 9 10 23

calcd 55 73 90 21 35 58 0 0 9

MLCT (expt, neutral) ΔEH−L (eV/cm−1) 2.28/18390 2.13/17180 2.37/19115 2.25/18148 2.10/16938 2.30/18550 2.24/18067 1.99/16050 1.89/15244

ΔEH−H−1 (eV) 0.08 0.10 0.02 0.15 0.16 0.10 0.19 0.29 0.37

MLCT (expt, MV)

EML (nm/cm−1)

εML (M−1 cm−1)

EML (nm/cm−1)

εML (M−1 cm−1)

505/19801 510/19607 545/18348 493/20284 560/17857 575/17391 492/20325 618/16181 715/13986

× × × × × × × × ×

447/22371 502/19920 546/18315 434/23041 552/18115 573/17452 479/20877 652/15337 721/13869

7.70× 103 7.80 × 103 1.84 × 104 1.45 × 104 1.42 × 104 2.32 × 104 1.70 × 104 1.77 × 104 2.75 × 104

4.60 7.20 1.93 1.36 1.66 2.20 1.52 2.60 4.00

3

10 103 104 104 104 104 104 104 104

For complexes [Mo2]−ph−[Mo2] and ([Mo2]−(1,4-naph)−[Mo2], data are cited from refs 18 and 20 respectively.

Figure 4. Frontier molecular orbitals (isodensity value ±0.04) for the models of [Mo2−(9,10-anth)−Mo2] complexes, showing the relative orbital energies and the HOMO−LUMO energy gaps.

(ORCA) and the same basis set; the overall results are consistent with the previous study.18,20 The calculated torsion angles for the models are in good agreement with those in the crystal structures, except for [OS−(1,4-naph)−OS] and [SS− (9,10-anth)−SS], which show a relatively large deviation due to the crystal lattice distortion. The optimized structure of [SS−(9,10-anth)−SS] shows that the anth bridge is orthogonal to the Mo2 chelating ring, similar to the results in Chisholm’s work,29 which indicates significant structural fluctuation in solution. As shown in Figure 4, the HOMO and HOMO−1 are obtained from the combination of δ orbitals in out-of-phase (δ − δ) and in-phase (δ + δ) modes, respectively, while the LUMO results from the bridge π* orbital. The orbital interaction modes of the dimetal centers with the bridging ligand and the constructions of these frontier MOs are similar to those found in the ph and naph bridged series.18,20 As shown in Table 3, calculations show that the LUMO energy is significantly impacted by the [Mo2] units, although the electron density is concentrated on the aromatic ring. In this [Mo 2 ]−(9,10-anth)−[Mo 2] series, the LUMO for dicarboxylate bridged complex ([OO−(9,10-anth)−OO]) is much higher than the other two (Figure 4). Similar results are

oxygen atoms of the bridging ligand are stepwise substituted by sulfur atoms; however, the variation is relatively small compared to other series (Table 3). Compound [OO−(9,10anth)−OO] exhibits a broad absorption band at 505 nm, close in energy to [OO−(1,4-naph)−OO] (493 nm) and [OO− ph−OO] (492 nm). Compared to the ph and naph bridged analogues, for the two thiolated complexes [OS−(9,10-anth)− OS] and [SS−(9,10-anth)−SS], the band maxima shift to the high energies, i.e., 510 and 545 nm, respectively, but the intensities are lowered (Table 3). These results are in contrast to the general expectations that with a large π conjugated system as the bridge, the MLCT absorptions are decreased in energy and increased in intensity.28 To understand the electronic properties of these compounds and the conformation dependence of electronic coupling, density functional theory (DFT) calculations were performed on the simplified models derived by replacing the p-anisyl groups in the DAniF ligands with hydrogen atoms. The calculated results are presented in Table 3, and the frontier molecular orbitals are shown in Figure 4. For comparison, in this study, the ph and naph bridged series of complexes also have been investigated theoretically using the same program F


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Inorganic Chemistry

Table 4. Symmetry and Energy (eV) of DFT Calculated Frontier Molecular Orbitals for [Mo2]−(9,10-anth)−[Mo2] with [Mo2]−(1,4-naph)−[Mo2] and [Mo2]−ph−[Mo2] complexes [Mo2]−(9,10-anth)−[Mo2]

a

[Mo2]−(1,4-naph)−[Mo2]

[Mo2]−ph−[Mo2]

MOs

[OO]a (D2, ϕ = 55°)

[OS] (C2, ϕ = 73°)

[SS] (D2h, ϕ = 90°)

[OO] (C2, ϕ = 21°)

[OS] (C1, ϕ = 35°)

[SS] (C2, ϕ = 58°)

[OO] (D2h, ϕ = 0°)

[OS] (C2h, ϕ = 0°)

[SS] (D2, ϕ = 9°)

LUMO HOMO HOMO−1

B1 (−1.81) B2 (−4.09) B1 (−4.17)

A (−2.11) B (−4.24) A (−4.34)

B2u (−2.06) B2g (−4.43) B1u (−4.45)

A (−1.75) B (−4.00) A (−4.15)

A (−2.11) A (−4.21) A (−4.37)

A (−2.04) B (−4.34) A (−4.44)

B1u (−1.77) B2g (−4.01) B1u (−4.20)

Au (−2.12) Bg (−4.11) Au (−4.40)

B1 (−2.36) B2 (−4.25) B1 (−4.62)

[EE] = [Mo2] = [Mo2(DAniF)3(EEC)]+ (E = O or S).

Figure 5. (A−E) HOMO−LUMO (ΔEH−L) and HOMO−HOMO−1 (ΔEH−H−1) band gaps for the [OO−bridge−OO], [OS−bridge−OS], and [SS−bridge−SS] series are plotted against cos2 ϕ, where ϕ is the torsion angle obtained from the X-ray structures.

[OS−(9,10-anth)−OS]. In this series, the highest ΔEH−L (2.37 eV) for [SS−(9,10-anth)−SS] indicates a relatively weak TB interaction. Similar results are obtained for the [Mo2]−(1,4naph)−[Mo2] series (Table 3). However, disagreement is found on the variation trend of ΔEH−L between calculated and measured data. For the series B and C, the observed metal to ligand transition energies decrease as the torsion angles increase with the chelating group changing from −CO2 to −COS to −CS2. However, for both, as shown in Table 3, the partially thiolated analogues have the smallest ΔEH−L, instead of the fully thiolated species. This inconsistency is introduced mainly by the low Mo2-based orbital energy in calculation; the increased ΔEH−L for [SS−(1,4-naph)−SS] and [SS−(9,10anth)−SS] predicts the conformational effects of electronic coupling. Experimentally, small variations of the transition energies are observed in comparison with the ph bridged series. For example, for [Mo2]−(9,10-anth)−[Mo2], the difference in MLCT energy between [OO−(9,10-anth)−OO] and [SS− (9,10-anth)−SS] is only 40 nm, while for the [Mo2]−ph− [Mo2] series, the transition energy decreases by 223 nm (Table 3) with varying the bridging ligand. These results indicate that in the series, the increased torsion angle, by inducing S atoms to the charge transfer platform, cannot fully offset the enhancement of electronic coupling, likely due to the fluctuation of the molecule in solution. For the fully thiolated series (III), [SS−(9,10-anth)−SS] has the smallest HOMO and HOMO−1 splitting (ΔEH−H−1 = 0.02 eV) due to the large torsion angle, implying a very weak δ−δ

seen for the [Mo2]−ph−[Mo2] and [Mo2]−(1,4-naph)−[Mo2] series as well (Table 4). This is because the LUMO results from mixing the π* orbital of bridging ligand with metal (δ + δ); the higher energy of dicarboxylate bridging ligand raises the energy level of the LUMO.18 In view of the compositions of these frontier MOs (Figure 4), LUMO, HOMO, and HOMO−1, we treat reasonably these dimers of dimers as an assembly of [Mo2] units and bridge, thus, namely, [Mo2]−bridge−[Mo2]. By this formula, the chelating group of the bridging ligand is integrated into the [Mo2] unit as part of the donor (acceptor), while the central moiety of the bridging ligand is taken as the bridge. Calculations show that the chelating group affects the energies of the metal-based HOMOs through its chemical composition and the energy of the ligand-based LUMO through the torsion angle. Therefore, the HOMO−LUMO energy gap (ΔEH−L) accounts for metal−ligand interactions by taking the various factors into account. As a result of the metal−ligand interactions, the degeneracy between HOMO and HOMO−1 is removed; thus, the orbital splitting in energy (ΔEH−H−1) reflects the strength of the metal−metal interaction. A small HOMO−LUMO gap invokes strong metal−metal interaction, creating a large splitting. Therefore, practically, the magnitude of ΔEH−H−1 can be used to predict the relative strength of electronic coupling. For each of the complexes, the ΔEH−L value is comparable with the MLCT energy in the spectra (Table 3). Better numerical consistency is found for [OO−(9,10-anth)−OO] and [SS−(9,10-anth)−SS], but a relatively large deviation for G


Article

Inorganic Chemistry orbital interaction, in contrast to the largest ΔEH−H−1 value for [SS−ph−SS] (0.37 eV). The other two series, [OO−bridge− OO] (I) and [OS−bridge−OS] (II), show the same variation trend, as indicated in Table 3. These results are consistent with the electrochemical results, showing clearly that the metal− metal electronic coupling is denominated by the bridge conformation, i.e., the torsion angle. As shown in Table 3, the conformation dependence of electronic coupling is also reflected by the complex series with naph and anth bridges, i.e., [Mo2]−(1,4-naph)−[Mo2] and [Mo2]−(9,10-anth)−[Mo2], in which the HOMO−HOMO−1 splitting decreases as the torsion angles increase. In contrast, for the phenylene bridged three complexes, the largest ΔEH−H−1 (0.37 eV) is found for [SS−ph−SS] because the deviation of the bridge from the Mo−Mo bond vectors is small (Table 3), probably in the range of structural fluctuation in solution. In this case, the Scontaining [Mo2] units enhance greatly the electronic coupling. Theoretical study predicts that the electron transmission is proportional to cos2ϕ considering the contribution of tunneling transport through the conjugated π orbitals,30 which has been proved by experimental observations in molecular junction.12a,c,d,h,16 In this study, for the three series with varying bridges, it is found that ΔEH−L decreases and ΔEH−H−1 increases as the bridges change from the ph to 1,4-naph to 9,10-anth, which tunes continuously the torsion angle (ϕ) (Table 3). In Figure 5, for these three series (I, II, and III), the ΔEH−L and ΔEH−H−1 values are plotted against cos2 ϕ, showing a remarkable linear relationship between the calculated energy gaps and cos2 ϕ. From these plots, the conformation dependence of electronic coupling is represented typically by the series [SS−bridge−SS] which exhibits a large variation of the torsion angle (Figure 5). The presented energy-geometry correlations illustrate that the interplay of structural and electronic factors affects the electronic coupling through energetic control to the system. The HOMO−LUMO energy gaps are found to be linearly related to cos2 ϕ in the bridged diruthenium MV system12b and in the molecular junction.12a These results are in accordance with the functional relationship of cos2 ϕ on electronic coupling parameter Hab observed in the literature.15a Optical Properties, Electronic Coupling and System Transition of the Mixed-valence Complexes. The MV complexes [OO−(9,10-anth)−OO]+, [OS−(9,10-anth)− OS]+, and [SS−(9,10-anth)−SS]+ were prepared by singleelectron oxidation of the corresponding neutral compounds using 1 equiv of ferrocenium hexafluorophosphate (Cp2FePF6) in CH2Cl2 solution. In the X-band EPR spectra, each complex cation displays a symmetrical isotropic peak with some weak hyperfine structures (Figure S4). The g values fall in the range of 1.940−1.950, similar to those for the phenylene and naphthalene bridged MV analogues,18,20 which indicates that the odd electron is localized essentially on the δ orbital of the Mo2 acceptor. All the MV compounds exhibit an absorption band in the visible region of the spectra, with the transition energy similar to the MLCT band of its neutral precursor (Table 3 and Figure S5), except for [OO−(9,10-anth)−OO]+, which has this band slightly blue-shifted. Thus, this high energy absorption can be assigned unambiguously to the electronic transition from the Mo2 center (δ) to the bridging ligand (π*).18 In the series, the dicarboxylate bridged species has the highest energy MLCT band, and the tetrathiolated analogue has the lowest energy MLCT band. Interestingly, these results are consistent with the

calculated the HOMO−LUMO energy gaps for the neutral complexes (Table 3), which proves the application of the Koopmans’ theorem31 once again in the [Mo2]−bridge−[Mo2] system.19 It is worthwhile to note that compared to the phenylene and naphthalene bridged series, the [Mo2]−(9,10anth)−[Mo2]+ series shows much lower MLCT absorption intensities (Figure 6). For the three series differing in bridge,

Figure 6. Vis/near-mid-IR absorption spectra of the MV complexes [OO−(9,10-anth)−OO]+ (A), [OS−(9,10-anth)−OS]+ (B), and [SS−(9,10-anth)−SS]+ (C) in comparison with those for the corresponding phenylene and naphthalene analogues.

the most intense MLCT absorbances are found for the phenylene analogues, which have the smallest torsion angles in the series (Figure 6). A bridging ligand (π) to metal (δ) charge transfer (LMCT) band may be observed in the [Mo2]− ph−[Mo2]+ system, depending on the coupling strength. It is found that the LMCT bands decrease in energy but increases in intensity as the electronic coupling increase.18,32,33 Unlike the phenylene bridged analogues and the other Mo2 dimers in Robin−Day’s Class II,18,20,34 as shown in Figure 6, the three complexes with an anthracene bridge lack the LMCT band, consistent with their extremely weak coupling between the two H


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Inorganic Chemistry

the donor−acceptor EC through the bridge,35 which is the case for the anthracene analogues. Therefore, this study verifies the superexchange formalism through the nine complexes with a torsion angle gradient but nearly constant donor−acceptor separations, which maps exclusively the conformation dependence of electronic coupling. Figure 7 correlates the electronic coupling strength with bridge conformation for the studied complexes, in terms of Hab

Mo2 centers. In comparison with the other series, the most striking optical feature for this series is the lack of the intervalence charge transfer (IVCT) band, which is an indication of MV compounds in Class I. The strength of electronic coupling in mixed-valence systems can be evaluated quantitatively by the electronic coupling matrix element Hab from the Hush model17a,d (eq 1) based on the spectral features of the IVCT band. Hab =

2.06 × 10−2 (εITΔν1/2E IT)1/2 rab

(1)

In the Mo2 D−B−A systems, distinguished optical behaviors have been observed for weakly and strongly coupled systems corresponding to Class II and III, respectively. The variation of the IVCT parameters (energy EIT, intensity εIT and bandwidth Δν1/2), as functions of the coupling strength, is well represented by the [Mo2]−ph−[Mo2] series.18,20,33 The most strongly coupled species [SS−ph−SS]+ exhibits a intense, “halfcut” IVCT band in the mid-IR region (EIT = 2640 cm−1), while a high energy, low intensity, and symmetric IVCT band is observed for [OO−ph−OO]+ (Figure 6). We have also seen that after crossing the Class II−III borderline, the system in the Class III regime displays a high energy “IVCT” band with more electronic features.19,33 Remarkably, in the anthracene bridged system, the three MV complexes are lack of the IVCT bands, which is attributed explicitly due to the molecular topology. The electronic coupling is lowered to the minimum by the bridge conformation, practically giving Hab = 0 according to eq 1; thus, these MV complexes should be assigned to Class I. On the other hand, Creutz, Newton, and Sutin proposed, based on the McConnell’s superexchange theory,25,26 a method to estimate the electronic coupling matrix element (HMM′) for mixed-valence systems with metal complex units as the donor and acceptor.35 According to the CNS formalism (eq 2), the HMM′ parameter is determined by the coupling parameters (HML, HM′L and HLM, HLM′) between the metal centers (M and M′) and the bridging ligand as well as the energy gaps for metal to ligand (ΔEML) and ligand to metal (ΔELM) charge transfer. HMM′ =

HMLHM L H H ′ + LM LM ′ 2ΔEML 2ΔE LM

Figure 7. Coupling constants (Hab) plotted against the torsion angle (deg). For each of the three series, the Hab value decreases as the torsion angle increases. Data for cmplexes in [Mo2]−ph−[Mo2]+ and [Mo2]−(1,4-naph)−[Mo2]+ series refer to refs 18 and 20 respectively.

and ϕ, respectively. It should be noted first that these series have the minimum coupling for the anthrancene bridged complexes, as judged by the absence of the IVCT band, which have the largest torsion angles for the series but in varying degrees. This indicates that to eliminate the metal to metal coupling the bridge may not necessarily be orthogonal with the charge transfer platform. From these correlations, the torsion angle for removing the coupling effect depends on how strongly coupled the system is. For a very strongly coupled system, for example, [SS−ph−SS]+, the torsion angle should be increased to 76° to reduce the coupling to the minimum, while the weak coupling in the dicarboxylate bridged series can be removed by a relatively small torsion angle (ca. 50°). For the three series distinguished by the [Mo2] units, i.e., I, II, and III, the Hab values decrease as the bridge size increases, which enlarge the torsion angles. In each series, as shown in Figure 7, the phenylene bridged analogue has the smallest ϕ and thus, the largest Hab, while the anthrancene derivatives exhibit the negligible coupling effect. For the three phenylene bridged complexes, the electronic coupling is dominated by the nature of [Mo2] units, and the Hab values increase as more S atoms are integrated.18 This is because the S atoms exert a substantial effect of enhancing the electronic coupling, while a small steric effect cannot offset the positive effect. This situation is changed when a naphthalene bridge employed, in which the bridge conformation plays a critical role in controlling the coupling strength.20 Combining the S-containing [Mo2] units with the naphthalene bridge yields large torsion angles for [SS−(1,4naph)−SS]+, which cancels effectively the enhancement of coupling from the [Mo2] units with a −CSS chelating group. This is why similar Hab values are obtained for [OO−(1,4naph)−OO]+ (258 cm−1) and [SS−(1,4-naph)−SS]+ (268

(2)

In eq 2, the first term represents to the contribution to HMM′ from the metal to ligand interaction, and the second term arises from the ligand to metal interaction. These two terms account for the electron- and hole-hopping pathways, respectively, in the case where the two coupling pathways are in operation simultaneously. Importantly, the physical parameters in these two terms are derived from the ML and LM absorption bands,35,36 respectively. Figure 6 shows that the ML, LM, and IVCT absorptions vary corresponding to the molecular topologies. The large torsion angles lower the bandwidth and intensity, consequently reducing the metal−ligand orbital interactions (HML, HM′L and HLM, HLM′) according to eq 1. Moreover, the effective energy gaps for charge transfer, ΔEML and ΔELM, correspond to the measured MLCT and LMCT energies, respectively. For the current systems, as indicated by the DFT calculations, the energy gaps increase with increasing the torsion angles (Figure 5), which decreases the coupling HMM′ (eq 1). In a strongly coupled system, for example, [SS− ph−SS]+, optical analysis on the low energy MLCT and LMCT bands yielded large metal−ligand coupling parameters,37 while the high energy MLCT and LMCT absorptions would suppress I


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cm−1).20 Here the observed torsion angle dependences of electronic coupling for the mixed-valence systems are consistent with the variations of energy gaps between the frontier MOs in DFT calculations for the neutral compounds (Figure 5). Finally, it is worth noting that these MV complexes are distributed in the three different classes in the Robin−Day’s classification.34 While the anthrancene bridged systems belong to noninteracting Class I and [SS−ph−SS]+ exhibits characteristic spectral features for systems on the Class II−III borderline, the rest are in the weakly coupled Class II regime. A system transition from noninteracting Class I to strongly coupled Class II−III borderline via moderately coupled [OS−ph−OS]+ is therefore achieved through the complexes with the same Mo2 center and similar donor−acceptor separations. In these [Mo2]−bridge−[Mo2] complexes, the bridge conformation becomes the dominant factor that affects the strength of electronic coupling and the extent of electron delocalization, while the [Mo2] donor (acceptor) and the size of the π bridge are varied. Therefore, this study shows that in a mixed-valence system, control of the charge distribution can be realized by manipulating the molecular topology.

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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 appropriate drying agents and collected for further use under a nitrogen atmosphere. HDAniF38 and Mo2(DAniF)3(O2CCH3)39 were synthesized according to published methods, while 9,10-anthracenedicarboxylic acid40 was synthesized by a modification of the literature method. Physical Measurements. UV−vis−NIR spectra were measured in CH2Cl2 solutions using IR quartz cells with a light path length of 2 mm on a Shimadzu UV-3600 UV−vis−NIR spectrophotometer. Cyclic voltammograms (CVs) were performed using a CH Instruments model CHI660D electrochemical analyzer in a 0.10 M CH2Cl2 solution of nBu4NPF6 with Pt working and auxiliary electrodes, an Ag/ AgCl reference electrode, and a scan rate of 100 mV/s−1. All potentials are referenced to the Ag/AgCl electrode. 1H NMR spectra were recorded on a Bruker Avance 300 spectrometer. 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. X-ray Structure Determinations. Single-crystal data for [OO− (9,10-anth)−OO]·6CH2Cl2, [OS−(9,10-anth)−OS]·6CH2Cl2, and [SS−(9,10-anth)−SS]·5CH2Cl2 were collected on an Agilent Xcalibur Nova diffractometer with Cu−Kα radiation (λ = 1.54178 Å) at 100 and 150 K. The empirical absorption corrections were applied using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm.41 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-2014 and SHELXL-2014 programs, respectively.42 The solvent molecules are disordered in multiple orientations, which were refined isotropically. All non-hydrogen atoms were refined with anisotropic displacement parameters. Computational Details. The ORCA 2.9.1 software packages43 were used for all DFT computations assuming an S = 0 spin state. The geometry of the model complexes was optimized in the gas phase, employing the Becke−Perdew (BP86) functional 44 and RI/J approximation45 without imposing any symmetry constraints. Geometry optimizations for the complexes were converged with the def2-SV(P) basis set46 and def2-SVP/J auxiliary basis set47 for C and H atoms, def2-TZVP(−f) basis set48 and def2-TZVP/J auxiliary basis set43 for S, N, and O atoms and def2-TZVPP basis set44 and def2TZVPP/J auxiliary basis set43 for Mo atoms including the ZORA approximation.49 Tight optimization and tight self-consistent field convergence were employed along with a dense integration grid (ORCA Grid 5) for all geometry optimization calculations. Singlepoint calculations on the optimized geometries were performed using the B3LYP functional50 and TZVP basis set51,46 for all atoms, together with the COSMO methodology52 (using ε = 9.08 for CH2Cl2 solvent). Isosurface plots of molecular orbitals were generated using the gOpenMol 3.00 program53 with isodensity values of 0.04. Preparation of 9,10-Anthracenedicarboxylic Acid (4). A suspension of 9,10-dibromoanthracene (2 g, 5.95 mmol) in anhydrous diethyl ether (20 mL) was cooled in an ice bath, to which with stirring 9.76 mL of n-butyl lithium (1.6 M solution in hexane) was added. After being stirred for 30 min at ambient temperature, an orange precipitate formed. At low temperature (∼0 °C), carbon dioxide bubbled through the mixture for 1 h, and then 20 mL of water was added. The aqueous phase was separated and washed with diethyl ether (3 × 20 mL). The aqueous solution was acidified with diluted sulfuric acid, yielding yellow precipitate. The solid material was filtered off and dried under a vacuum for overnight. Yield: 1.2 g (76%). 1H NMR δ (ppm in DMSO-d6): 8.07 (dd, 4H, anthracenyl −CH), 7.70 (dd, 4H, anthracenyl −CH). Preparation of 9,10-Anthracenedithiodicarboxylic Acid (6). In a 100 mL flask, thionyl chloride (14.6 mL, 75 mmol) was mixed with 9,10-anthracenedicarboxlic acid (1 g, 3.75 mmol) and two drops of DMF. The mixture was heated to reflux for 4 h. After removal of the

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CONCLUSION By assembling two quadruply bonded Mo2 building blocks with 9,10-anthracenedicarboxylate and its thiolated derivatives, three complexes of the type [Mo2]−(9,10-anth)−[Mo2], denoted as [OO−(9,10-anth)−OO], [OS−(9,10-anth)−OS], and [SS− (9,10-anth)−SS], have been synthesized. As shown by the crystal structures, these complexes are closely related to the phenylene and naphthalene analogues by having the same [Mo2] units and similar Mo2···Mo2 distances, but differently, larger torsion angles (ϕ = 50−76°) between the Mo2 chelating ring and the bridge ring planes. The nine compounds in three series allow us to systematically evaluate the impacts of bridge conformation on the through-bond electronic coupling in the case where the extent of through-space interaction is essentially fixed, if there is any. Each of the present compounds shows a potential separations (ΔE1/2) for the two dimetal redox centers smaller than those for the phenylene and naphthalene bridged analogues but close to that of the 1,4-cyclohexylenedicarboxylate bridged analogue, although a larger π conjugated bridge is involved. Notably, the MV complexes, generated by oneelectron oxidation using ferrocenium hexafluorophosphate, are lack of the characteristic vibronic IVCT absorptions in the nearIR region spectra, in contrast to the phenylene and naphthalene series. Therefore, this series can be assigned to noninteracting Class I, for which the electronic coupling is eliminated explicitly by the large torsion angle. The nine compounds in three series are distributed in Class I, Class II, and Class II−III due to the differences in molecular topology. DFT calculations on the neutral models show that the HOMO and LUMO are derived largely from the Mo2 (δ) and bridging ligand (π*), respectively, and the HOMO−LUMO energy gap (ΔEH−L) correspond to the metal to ligand transition energy. For all these compounds, the ΔEH−L increases, but the HOMO−HOMO−1 splitting (ΔEH−H−1) decreases as the torsion angle increases, showing a remarkable linear relationship between ΔEH−L, and ΔEH−H−1 and cos2 ϕ. This work verifies experimentally and theoretically the through-bond superexchange formalism for electronic coupling and electron transfer. J


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solvents in a rotary evaporator, the residue was washed with hexane and then dried under a vacuum. This crude material of 5 was used for the preparation of 6 without further purification. Crude product 5 was mixed with thioacetamide (0.704 g, 9.37 mmol) in 20 mL of THF; the mixture was stirred at room temperature for 4 h. Dilute sodium hydroxide solution (10%, 50 mL) was added, and the mixture was stirred for additional 30 min. The solvents were removed under reduced pressure. To the residue, dilute hydrochloric acid (1 M, 50 mL) was added. The mixture was extracted with diethyl ether (3 × 20 mL). The collected organic layer was dried over MgSO4. Removal of solvents under reduced pressure gives the yellow solid product. Yield: 0.82 g (73%). 1H NMR δ (ppm in CDCl3): 8.20 (dd, 4H, anthracenyl −CH), 7.64 (dd, 4H, anthracenyl −CH). Preparation of Dilithium Salt of 9,10-Anthracenetetrathiodicarboxylic Acid (3). In a 200 mL flask, 9,10-dibromoanthracene (2 g, 5.95 mmol) was suspended in mixed solvents of hexane (50 mL) and diethyl ether (20 mL). At low temperature (−20 °C), 20.4 mL of n-butyl lithium (1.6 M solution in hexane) was added dropwise. The mixture was stirred for 30 min at ambient temperature. After the reaction was further cooled to −70 °C, an excess amount of CS2 (5 mL) was added slowly over 20 min; then, the mixture was allowed to warm up to room temperature and stirred for 3 days. The resultant dark orange precipitate was filtered off and washed with hexane several times, and dried under a vacuum. Yield: 0.70 g (34%). 1H NMR δ (ppm in DMSO-d6): 8.50 (dd, 4H, anthracenyl −CH), 7.45 (dd, 4H, anthracenyl −CH). General Procedure for the Preparation of [OO−(9,10-anth)− OO], [OS−(9,10-anth)−OS], and [SS−(9,10-anth)−SS]. A solution of sodium ethoxide (0.017 g, 0.25 mmol) in ethanol (10 mL) was added to a solution of Mo2(DAniF)3(O2CCH3) (0.254 g, 0.25 mmol) in 30 mL of THF. After being stirred for 30 min, the mixture was transferred into a flask containing 9,10-anthracenedicarboxylic acid (0.029 g, 0.13 mmol) for [OO−(9,10-anth)−OO] or 9,10anthracenedithiodicarboxylic acid (0.038 g, 0.13 mmol) for [OS− (9,10-anth)−OS] or dilithium salt of 9,10-anthracenetetrathiodicarboxylic acid (0.044 g, 0.13 mmol) for [SS−(9,10-anth)−SS] in 10 mL of THF. The solution was stirred at room temperature for another 5 h. Then the solvents were evaporated under reduced pressure. The residue was dissolved in CH2Cl2 (15 mL), and the solution was filtered through a Celite-packed funnel. The filtrate was evaporated under reduced pressure, and the residue was washed with ethanol (3 × 15 mL). The product was collected by filtration and dried under a vacuum. Single crystals were grown over about 1 week after layering ethanol onto a dichloromethane or THF solution of the compound. [OO−(9,10-anth)−OO]. Dark red powder (0.219 g, 80% yield). 1H NMR δ (ppm in CDCl3): 8.67 (s, 4H, −NCHN−), 8.55 (s, 2H, −NCHN−), 8.50 (d, 4H, anthracenyl −CH), 7.26 (d, 4H, anthracenyl −CH), 6.63 (d, 32H, aromatic −CH), 6.48 (d, 8H, aromatic −CH), 6.30 (d, 8H, aromatic −CH), 3.69 (s, 24H, −OCH3), 3.67 (s, 12H, −OCH3). UV−vis, λmax nm (ε, M−1 cm−1): 505 (4600). Anal. Calcd. (%) for C106H98N12O16Mo4: C, 58.41; H, 4.53; N, 7.71. Found: C, 58.32; H, 4.59; N, 7.65. [OS−(9,10-anth)−OS]. Purple powder (0.205 g, 74% yield). 1H NMR δ (ppm in CDCl3): 8.60 (s, 4H, −NCHN−), 8.55 (s, 2H, −NCHN−), 8.43 (d, 4H, anthracenyl −CH), 7.29 (d, 4H, anthracenyl −CH), 6.63 (d, 24H, aromatic −CH), 6.54 (d, 8H, aromatic −CH), 6.43 (d, 8H, aromatic −CH), 6.19 (d, 8H, aromatic −CH), 3.69 (d, 18H, −OCH3), 3.64 (d, 18H, −OCH3). UV−vis, λmax nm (ε, M−1 cm−1): 510 (7200). Anal. Calcd. (%) for C106H98N12O14S2Mo4: C, 57.56; H, 4.47; N, 7.60. Found: C, 57.65; H, 4.54; N, 7.71. [SS−(9,10-anth)−SS]. Blue powder (0.151 g, 54% yield). 1H NMR δ (ppm in CDCl3): 8.59 (s, 2H, −NCHN−), 8.50 (d, 4H, anthracenyl −CH), 8.43 (s, 4H, −NCHN−), 7.49 (d, 4H, anthracenyl −CH), 6.64 (d, 32H, aromatic −CH), 6.48 (d, 8H, aromatic −CH), 6.15 (d, 8H, aromatic −CH), 3.69 (d, 24H, −OCH3), 3.67 (s, 12H, −OCH3). UV−vis, λmax nm (ε, M−1 cm−1): 545 (19300). Anal. Calcd. (%) for C106H98N12O12S4Mo4: C, 56.73; H, 4.40; N, 7.49. Found: C, 56.82; H, 4.49; N, 7.58.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01056. 1 H NMR spectra, EPR spectra, and DFT computed Cartesian coordinates for neutral model compounds (PDF) Accession Codes

CCDC 1837099−1837101 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-020-85222191. E-mail: [email protected]. ORCID

Chun Y. Liu: 0000-0001-6908-9929 Author Contributions †

H.W.C. and S.M. equally contributed to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are very thankful to the National Natural Science Foundation of China (No. 21371074), Jinan University, and Fundamental Research Funds for the Central Universities for the financial support.

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REFERENCES

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Mapping Bridge Conformational Effects on Electronic Coupling in Mo2

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