Aromaticity-Driven Molecular Structural Variation and Electronic

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Cite This: Inorg. Chem. 2017, 56, 14888−14899

Aromaticity-Driven Molecular Structural Variation and Electronic Configuration Alternation: An Example of Cyclic π Conjugation Involving a Mo−Mo δ Bond Suman Mallick,† Ye Lu,‡ Ming Hui Luo,† Miao Meng,† Ying Ning Tan,† Chun Y. Liu,*,†,‡ and Jing-Lin Zuo*,§ †

Department of Chemistry, Jinan University, 601 Huang-Pu Avenue West, Guangzhou-510632, China Department of Chemistry, Tongji University, Shanghai-200092, China § State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing-210023, China ‡

S Supporting Information *

ABSTRACT: We have synthesized and characterized the mixedligand dimolybdenum paddlewheel complex Na[(DAniF)3Mo2(C3S5)] (Na[1]; DAniF = N,N′-di-p-anisylformamidinate, dmit = 1,3-dithiole-2-thione-4,5-dithiolate), which has a six-membered chelating [Mo2S2C2] ring created by equatorial coordination of the dmit (C3S5) ligand to the Mo2 unit. Oneelectron oxidation of Na[1] using Cp2FePF6 yields the neutral complex [(DAniF)3Mo2(C3S5)] ([1]), and removal of two electrons from Na[1] using AgBPh4 gives [(DAniF)3Mo2(C3S5)]BPh4 ([1]BPh4). In the crystal structures, [1]− and [1] present dihedral angles of 118.9 and 142.3° between the plane defined by the Mo−Mo bond vector and the dmit ligand, respectively, while DFT calculations show that in [1]+ the Mo−Mo bond and the dmit ligand are coplanar. Complex [1] is paramagnetic with a g value of 1.961 in the EPR spectrum and has a Mo−Mo bond distance of 2.133(1) Å, increased from 2.0963(9) Å for [1]−. Consistently, a broad absorption band is observed for [1] in the near-IR region, which arises from charge transfer from the dmit ligand to the cationic Mo25+ centers. Interestingly, complex [1]+ has an aromatic [Mo2S2C2] core, as evidenced by a large diamagnetic anisotropy, in addition to the coplanarity of the core structure, which shifts downfield the 1H NMR signal of the horizontal methine proton (ArN−(CH)−NAr) but upfield those of the vertical protons, relative to the methine proton resonances for the precursor ([1]−). The magnetic anisotropy (Δχ = χ⊥ − χ∥) for the [Mo2S2C2] ring in [1]+ is −105.5 ppm cgs, calculated from the McConnell equation, which is about 2-fold larger than that for benzene. The aromaticity of the [Mo2S2C2] ring is supported by theoretical studies, including single-point calculations and gauge-including atomic orbital (GIAO) NMR spectroscopic calculations at the density functional theory (DFT) level. DFT calculations also show that the [Mo2S2C2] core in [1]+ possesses a set of three highest occupied and three lowest unoccupied molecular orbitals in π character, corresponding to those of benzene in symmetry, and six π electrons that conform to the Hückel 4n + 2 rule for aromaticity. Therefore, this study shows that an aromatic [Mo2S2C2] core is formed by coupling the δ orbital of the Mo≣Mo bond with the π orbital of the CC bond through the bridging atoms (S), thus validating the equivalency in bonding functionality between δ and π orbitals.

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INTRODUCTION Benzene, the well-known aromatic prototype, was first discovered by Faraday in 1825,1 and its structure and bonding were described by Kekulé in 1865.2 Ever since, “aromaticity” has been one of the most fascinating fields in chemistry and there has been intensive research aiming at the preparation of new aromatic compounds for an insightful understanding of aromaticity.3 By introduction of noncarbon elements to the aromatic hydrocarbon frameworks, a new area of research, i.e., “heteroaromaticity”, has been developed, which provides a fruitful interplay between theory and experiment.4 Heterobenzene is produced by formal replacement of a CH unit in © 2017 American Chemical Society

benzene with isoelectronic main-group elements. In addition to pyridine, an ordinary chemical in the laboratory, phospha-,5 sila-,6 germa-,7 gallata-,8 arsa-,9 stiba-, 10 bisma-,11 and aluminabenzenes12 have been synthesized to study their aromaticity. Integrating an isolobal transition-metal fragment [MLn] with an organic C5 moiety has generated various aromatic complexes, the so-called “metallabenzenes”,13 such as osma-,14 irida-,15 ruthena-,16 platina-,17 nickela-,18 and rhenabenzenes.19 Recently, metal analogues of higher arenes, Received: August 18, 2017 Published: December 5, 2017 14888

DOI: 10.1021/acs.inorgchem.7b02133 Inorg. Chem. 2017, 56, 14888−14899

Article

Inorganic Chemistry

Scheme 1. Bonding Analogy between the δ Orbital of a Mo≣Mo Quadruple Bond and the π Orbital of a CC Double Bond

osmanaphthalene,20 iridanaphthalene,21 and iridaanthracene,22 have been synthesized and studied in terms of aromaticity. Sixmembered aromatic systems with two metal centers, e.g., 1,3dimetallabenzenes,23 of Nb and Ta have also been reported. With additional heteroatom(s) involved (O, N, or S) in the benzene-like complexes, the so-called “heterometallabenzenes”,13a,24 i.e., metallapyridines,25 metallapyryliums,26 and metallathiabenzenes,27 have been synthesized. Although theoretically transition-metal aromatics are thermodynamically stable, some of them, such as ferrobenzene,28 chromobenzene,29 molybdenabenzene,30tungstabenzene,31 ruthenaphenanthrene,32 and rhenaphenanthrene33 have been identified as reactive intermediates, and other metallabenzenes undergo cycloadditions with dienophiles and rearrangements to form cyclopentadienyl complexes.34 For two decades, study on “metallaaromatics” has attracted growing interest. In this field, both theoretical and synthetic chemists have made every effort to answer questions such as whether or not the aromaticity is retained after substitution of a metal complex fragment for a CH unit in the organic aromatics4,35 and, if so, what is the physical origin of the aromaticity. It is generally accepted that aromatic compounds are cyclic, planar, and fully conjugated systems which possess a delocalized π-electron cloud. Likewise, most metallabenzenes exhibit the properties of aromatic systems, such as ring planarity, bond-length equalization, and relatively deshielded resonance of the nearby protons in the NMR spectrum.4,24,35 The main difference between transition metallaaromatics and regular aromatic hydrocarbons is the involvement of metal d orbitals in the π-bonding interactions. In metallabenzenes, a CH unit is isolobally substituted by a transition-metal fragment so that such types of metallacycles conserve the electronic delocalization, consequently exhibiting aromatic properties, as proposed by Hoffmann.36 In light of this, most of the metallaaromatics have been characterized with 6 or 10 π electrons, depending on the degree of participation of the metal d orbitals in the cyclic π conjugation.37,13,24 However, Jia et al.35a have considered 8 π electrons for the aromaticity of metallabenzenes, in opposition to the Hückel 4n + 2 π-electron rule. To support this idea, it is suggested that metallabenzenes exhibit “Möbius aromaticity” arising from incorporation of dπ orbitals of transition-metal atoms in the π-orbital system.38 Calculations of nucleus-independent chemical shift (NICS)39 and magnetic susceptibility anisotropy (Δχ) are theoretical approaches to evaluate the degree of aromaticity for a given system; unfortunately, these methods are practically inapplicable to metallaaromatics due to the local magnetic anisotropy of the transition-metal center and the associated auxiliary ligands. Recently, an isomerization method has been introduced to measure the aromaticity of a metallabenzene ring by determination of the aromatic stabilization energy.35b,37b Nevertheless, metallabenzenes present appreciable aromatic character but the degree of aromaticity may vary depending on the electronic configuration of the metal center.24,35 Unlike the mononuclear metal fragment [MLn] in which the d orbitals are degenerate, a quadruply bonded M2 unit has a nondegenerate electronic configuration: that is, σ2π4δ2. δ and δ* are the HOMO and LUMO of the dimetal unit, respectively. These two orbitals resemble in symmetry the π and π* orbitals of a CC double bond, respectively, as shown in Scheme 1. The symmetry compatibility allows the δ orbital to participate in the π interaction between a M2 unit and a CC double bond, forming a dδ−pπ conjugation. This dδ−pπ bonding

analogy inspired us to construct a unique heterometallabenzene by incorporating a dimolybdenum (Mo2) fragment into a cyclic π system to examine its “δ aromaticity”. Recently we reported several chalcogen-bridged dimyolybdenum complexes, [Mo2(DAniF)3]2(μ-E)2 (DAniF = N,N′-di-p-anisylformamidinate and E = O, S, Se),40 with a central six-membered core of [Mo2E2Mo2]. These complexes have a benzene-like electronic configuration for the cyclic π system with participation of the δ orbitals and exhibits typical aromatic behaviors. For the series, the aromaticity increases from the O to S to Se analogue, consistent with the aromaticity sequence of a chalcogencontaining five-membered aromatic family: i.e., furan < thiophene < selenophene.40a Remarkably, these [Mo2E2Mo2] (E = O, S, Se) “chalcogendimolybdenumbenzenes” show an aromaticity of higher degree relative to the benzene, as evidenced by the exceptionally large deshielding and shielding shifts of the 1H NMR signals of the remote DAniF methine protons in and out of the ring plane, respectively, and the dramatically large diamagnetic anisotropy (Δχ) calculated from the McConnell equation (Δσ = Δχ[(1 − 3 cos2 θ)/3R3N]). A cyclically delocalized six-membered aromatic [Mo2C4] ring was reported recently by Tsai et al., which was obtained by [2 + 2 + 2] cycloaddition of quintuply bonded Mo2 amidinate with terminal alkynes.41 As is well-known, multiple-sulfur 1,2-dithiolene ligands are electron rich and their metal complexes have been extensively studied as materials for molecular conductors and superconductors.42 In the present work, a quadruply bonded dimolybdenum unit [Mo2(DAniF)3]+ was integrated with a 1,3-dithiole-2-thione-4,5-dithiolate (dmit) ligand, giving the complex Na[(DAniF)3Mo2(C3S5)] (Na[1]), in which two sp3 S atoms bridge a Mo≣Mo bond and a CC bond, forming a sixmembered [Mo2S2C2] chelating ring. One-electron oxidation yielded [(DAniF)3Mo2(C3S5)] ([1]), and double oxidations using AgBPh4 removed one electron from the Mo2 center and another from the dmit ligand, producing the complex [(DAniF)3Mo2(C3S5)]BPh4 ([1]BPh4). Compounds [1]− and [1] have similar molecular structures and electronic configurations. Remarkably, complex [1]+ is diamagnetic and possesses an aromatic [Mo2S2C2] ring system. The diamagnetic anisotropy of the [Mo2S2C2] core is manifested by the downfield and upfield shifts of 1H NMR signals of the DAniF methine protons positioned in (H∥) and out (H⊥) of the ring plane, respectively. These results are consistent with the magnetic shielding tensors (σ) of the methine protons, calculated using the gauge-including atomic orbital (GIAO) method. From the McConnell equation, a diamagnetic 14889


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Inorganic Chemistry Scheme 2. Preparation of Complexes Na[1], [1], and [1]BPh4

anisotropy (Δχ) of −105.5 ppm cgs is determined for the [Mo2S2C2] system, which is about 2-fold larger than that for benzene (−62.9 ppm cgs). Density functional theory (DFT) calculations show that the [Mo2S2C2] core in [1]+ has a coplanar structure and the hybridizing state of the S atoms changes from sp3 to sp2 upon removal of two electrons from the core. Theoretical work reveals that there exists a set of six πMOs for the cyclic π-conjugated system, corresponding to those for benzene in symmetry and relative energy level. The residing of six π electrons on the three bonding MOs satisfies the Hückel 4n + 2 rule.

control of the reaction stoichiometry is important to obtain pure oxidized compounds in good yield. After a routine workup, the dark oxidized products [1] and [1]BPh4 were collected in good yield (80%). Single crystals of Na[1] and [1] suitable for crystallographic analysis were obtained by diffusion of ethanol into the THF solution of Na[1] and diffusion of hexane into the dichloromethane solution of [1], respectively. Unfortunately, we were unable to grow single crystals of the doubly oxidized compound for X-ray diffraction to determine the structure of [1]+. Efforts made in crystal growth included using different techniques (vapor and solvent diffusion and slow solvent evaporation), different counteranions (BPh4−, PF6−, and CF3SO3−) and varying solvent combinations. Crystals of Na[1] and [1] belong to the monoclinic P21/n space group with the molecules residing in a special position (Z = 4 (Na[1]) and Z = 8 ([1])) in the unit cell. The molecular structures of these compounds are shown in Figure 1, and the selected bond parameters are summarized in Table S1 in the Supporting Information. The structure of Na[1] shows that the Mo2 unit is equatorially coordinated by three formamidinate (DAniF) ligands and one dmit ligand. The Na+ ion is captured by the complex through

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RESULTS AND DISCUSSION Syntheses, Characterization, and Molecular Structures. The mixed-ligand complex Na[(DAniF)3Mo2(C3S5)] (Na[1]) was prepared readily by substitution of 1,3-dithiole-2thione-4,5-dithiolate (dmit) for the acetate in the precursor Mo2(DAniF)3(O2CCH3) (Scheme 2). In synthesis, the sodium salt of dmit (Na2C3S5), freshly prepared by treating the benzoyl ester of dmit (C3S5(COPh)2) with sodium methoxide, was mixed with the precursor complex in THF at room temperature and, subsequently, the sodium salt of complex [1]− was isolated as an orange solid in high yield. One-electron oxidation of Na[1] with 1 equiv of ferrocenium hexafluorophosphate (Cp2FePF6) produced the paramagnetic complex [(DAniF)3Mo2(C3S5)] ([1]). Complex [1] displays a pronounced peak with a g value of 1.961 in the electron paramagnetic resonance spectrum (EPR) (Figure S1 in the Supporting Information), indicating that the odd electron resides in a metal-based orbital, the δ orbital, and thus, the Mo−Mo bond order is lowered to 3.5. Using 2 equiv of the strong oxidizing reagent silver tetraphenylborate (AgBPh4 ), diamagnetic [(DAniF)3Mo2(C3S5)](BPh4) ([1]BPh4) was isolated from the oxidation reaction (Scheme 2). The composition and structure of [1]BPh4 have been characterized by 1H NMR spectra, which show an integration ratio of 2:1 for the methine protons from the vertically and horizontally positioned DAniF ligands, respectively. The anionic ([1]−) and cationic ([1]+) complexes are both diamagnetic and share the same chemical composition and molecular skeleton. The difference in 1H NMR spectra, specifically the chemical shifts for the methine protons, indicates the notable difference in complex structure between them. For the preparation of [1] and [1]BPh4, precise

Figure 1. X-ray crystal structures for [1]− in Na[1] (A) and neutral complex [1] (B) drawn at the 40% ellipsoid probability level. Hydrogen atoms and solvent molecules are omitted for clarity. The full structure of Na[1] is presented in Figure S2 in the Supporting Information. 14890


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Inorganic Chemistry

for Mo2(DAniF)443 because one of DAniF ligands is replaced by the electron-rich dmit ligand. Therefore, the higher potential redox couple (0.65 V) should be attributed to the dmit ligand. This assignment is supported by the predominant contribution of the dmit ligand on the HOMO of compound [1] (vide infra). By assembling with the Mo2 unit, the dmit ligand displays a reversible redox process in the CV diagram (Figure 2), which is likely due to the feasible charge transfer between the two redox sites. Complex [1] presents an electrochemical behavior similar to that of [1]− (Figure 2). To verify the electrochemical reversibility of the redox processes, CV measurements with variable scan rates were carried out for [1]− and [1]. For [1]−, the CVs at different scan rates (10−200 mV/s; Figure S3 in the Supporting Information) show that the potentials for the two redox waves remain essentially unchanged, verifying the reversibility of the redox processes. However, for [1], with slow scanning (10 to 100 mV/s), the CVs exhibit two redox processes with potentials at 0.10 and 0.65 V, but fast scanning (100 to 200 mV/s) shifts the second wave toward a higher potential (∼0.7 V) and increases the current density (Figure S4 in the Supporting Information). Therefore, for this species, a more complicated redox processes may be involved and this different electrochemical behavior is reflective of its electronic and structural nature as intermediate between [1]− and [1]+. The slow scan permits an adjustment between structures of [1] and [1]− and, thus, shows a behavior of Na[1]. It appears that, with a fast scan, multiple electrons (likely from the dmit ligand) are involved in the high-potential (∼0.7 V) redox process. Interestingly, although two-electron oxidation of [1]− generates [1]+ as a diradical, the chemically oxidized complex [1]+ is diamagnetic and shows completely different electrochemical behavior. Compound [1]+ exhibits two redox waves at E1/2(1) = 0.10 V and E1/2(2) = 0.30 V within the scanning range from −0.2 to +0.6 V (Figure 2), which are scan rate independent (Figure S5 in the Supporting Information). The first redox wave (0.10 V) has a potential similar to that for [1]− and, therefore, is due to the one-electron oxidation of the electrochemically reduced species of [1]+, or [1′]−. However, the second wave appears at much lower potential in comparison with that for [1]−. More importantly, the current density for this redox wave is exceptionally high. The implication of these results is 2-fold: first, the closed-shell complex [1]+ obtained from chemical oxidation is thermodynamically more stable than the diradical form of the doubly oxidized species, and second, [1]+ has an electronic structure completely different from those of its precursors [1]− and [1]. Electrochemical measurements with different scan rates (Figure S5) show same wave profile of the voltammograms; therefore, these redox processes are considered to be quasi-reversible and there is no isomerization from [1]+ to [1], at least under electrochemical conditions. It is observed that the oxidation current density of [1]+ at E1/2(2) is 3 times larger than the reduction current density at E1/2(1) as shown in Figure 2 and Figure S5 in the Supporting Information. Thus, we assign the redox wave at 0.3 V to a three-electron oxidation of the oneelectron-reduced species of [1]+, or [1′], generated at E1/2(1). The redox processes occurring in the given potential range can be described as

Na−S interactions and solvated with three THF molecules (Figure S2 in the Supporting Information). The Mo−Mo distance of 2.0963(9) Å is typical for a dimolybdenum quadruple bond with chelating ligands.43 Coordination of the dmit ligand to the dimetal unit creates the six-membered chelating ring [Mo2S2C2], in which the dmit moiety is not coplanar with the Mo−Mo bond vector. The [Mo2S2C2] core in [1]− has a large dihedral angle between the S1−Mo1−Mo2− S2 and S1−C5−C4−S2 planes of 61.1°. The Mo1−S1−C5 and Mo2−S2−C4 bond angles are 102.9(1) and 103.5(2)°, respectively, suggesting that the bridging S atoms possess an sp3-hybridizing character. In contrast to the anionic precursor [1]−, the complex [(DAniF)3Mo2(C3S5)] ([1]) is a neutral molecule. The Mo− Mo distance is increased to 2.1326(12) Å ([1]) from 2.0963(9) Å ([1]−), which confirms that one electron is removed from the Mo2 center upon the oxidative reaction with 1 equiv of Cp2FePF6. Consistently, the Mo−N (DAniF) bonds are shortened from an average of 2.147 Å ([1]−) to 2.133 Å ([1]). It is interesting to note that removal of one electron from the Mo2 unit significantly alters the molecular topology, specifically affecting the [Mo2S2C2] ring structure (Table S1 in the Supporting Information). In [1], the dihedral angle between the S1−Mo1−Mo2−S2 and S1−C5−C4−S2 planes is reduced to 37.7°; the Mo−S−C bond angle is increased to ∼113°, while the Mo−S bonds (2.446 Å) are significantly shortened. In [1], the [Mo2S2C2] ring tends to be flat in comparison with that of [1]−, but the chairlike structure still remains, as shown in Figure 1. The structural parameters indicate that the bridging S atoms have a dominant sp2hybridizing feature. Electrochemical Studies. The three compounds Na[1], [1] and [1]BPh4 are all electroactive, and the cyclic voltammograms (CVs) were recorded in THF solutions in the potential range of 0.1−0.7 V (vs Ag/AgCl), as shown in Figure 2. Compound [1]− displays two quasi-reversible redox

Figure 2. Cyclic voltammograms of Na[1] (black), [1] (blue), and [1]BPh4 (red) in THF solution recorded at a scan rate of 100 mV s−1. The CVs at variable scan rates are shown in the Supporting Information.

processes at E1/2(1) = 0.11 V and E1/2(2) = 0.65 V. Oxidation of DAniF-supported Mo2 complexes usually occurs at ∼0.4 V.43 The redox-noninnocent property of the dmit ligand has been well documented, and it generally exhibits an irreversible anodic peak around +0.69 V.44 Therefore, the low-potential oneelectron oxidation should be assigned to the Mo24+ → Mo25+ process, corresponding to the configuration change σ2π4δ2 → σ2π4δ1. This potential (0.11 V) is significantly lower than that

+2e

+e

+e

−2e

−e

−e

[1]3 + XoooY [1]+ ⇄ [1′] ⇄ [1′] 14891


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Inorganic Chemistry Here [1′] and [1′]− are the isoelectronic analogues of [1] and [1]−, respectively; on the basis of the distinct electrochemical behaviors the isoelectronic species of each pair are not identical and interconvertible electronically and structurally. The relatively low oxidation potential for [1]+ (0.3 V) illustrates the low stability of this complex in solution. Spectroscopic Analyses. Figure 3 shows the absorption spectra recorded in the UV−vis−NIR region for the three

Information), which characterize the diamagnetism and closed-shell electronic configuration for these compounds. For [1]+, the diamagnetic behaviors may be understood from the strong electronic and antiferromagnetic coupling between the two redox sites with one odd electron residing on each of them.40 However, the distinct electrochemical properties of [1]− and [1]+ indicate that these two diamagnetic species are not convertible under both electrochemical and chemical conditions. In other words, complexes [1]− and [1]+ must have different molecular and electronic structures. While the electrochemical and spectroscopic measurements imply a distinct electronic configuration for the doubly oxidized species, the complex core of [1]+ must have a different molecular geometrical topology, as indicated by the different 1H NMR spectrum (Figure S8). For this complex system, only the DAniF ligands present 1H NMR signals that convey important information on the molecular topology. Geometrically, there are two types of methine (ArN−CH−NAr) protons from the vertical and horizontal DAniF ligands, denoted as H⊥ and H∥, respectively. In the spectrum, the H⊥ and H∥ signals appear as two singlets with a 2:1 integration ratio, thus acting as useful structural probes.40 In a study of electronic coupling between two bridged [Mo2] units in the Mo2 dimers, we found that the downfield shifts of the H∥ NMR peaks can be used as an effective criterion for assessment of the extent of electron delocalization: the larger the 1H NMR shift, the greater the delocalization.47 For the nonoxidized species [1]−, the chemical shifts (δ) for the H⊥ and H∥ atoms are 8.90 and 8.18 ppm, respectively, with a δ displacement of ∼0.7 ppm. These downfield signals have normally been attributed to the magnetic anisotropy imposed by the metal−metal multiple bond.48 Interestingly, upon oxidation, the H⊥ signal is shifted upfield (δ 8.58 ppm), while the H∥ atoms exhibit a downfield shift (δ 8.47 ppm), reducing the separation to 0.1 ppm (Figure 4). Remarkably,

Figure 3. UV−vis−NIR spectra of Na[1] (black), [1] (red), and [1]BPh4 (blue) measured in THF solution.

complexes Na[1], [1], and [1]BPh4 at different oxidation levels. In the spectra, compounds [1]− and [1] exhibit a mediumintensity peak at ∼440 nm, which can be assigned to the δ(Mo2) to π*(ligand) transitions (MLCT) according to prior studies.45 The similarity in electronic spectra for [1]− and [1] implies that they have similar electronic structures. In contrast, for [1]+, this absorption is largely diminished (Figure 3), indicating that this species has an electronic structure significantly different from those for [1]− and [1]. Interestingly, compound [1] exhibits a broad absorption band in the near-IR region (1096 nm). From the band shape and low transition energy, this absorption bears vibronic features and arises from distant charge transfer. In our previous study,46 a similar absorption band was observed for a dimolybdenum (Mo25+) complex with a pendant triarylamine ligand. Experimental and theoretical analyses illustrated that the low-energy band arises from charge transfer across a significant distance from the noninnocent amine ligand to the oxidized Mo2 center and is therefore denoted as a vibronic LMCT band. Similarly, in this study, charge transfer from the dmit ligand to the Mo25+ center occurs in [1]. Furthermore, this LMCT absorption is solvent dependent, and the transition energy increases in polar solvents (Figure S6 in the Supporting Information). For example, the LMCT band has a absorption maximum at 9862 cm−1 in acetone, higher in energy than that in dichloromethane (7987 cm−1) (Table S2 in the Supporting Information). It is worthwhile to note that this type of vibronic LMCT band has the same physical origin as that for an intervalence charge transfer (IVCT) band in mixed-valence compounds but differs somehow from the high-energy MLCT band. This optical behavior conforms well to the electrochemical results, suggesting that two-electron oxidation removes one electron from each of the two redox sites. Diamagnetic Anisotropy of the [Mo2S2C2] ring in [1]+. Both [1]− and [1]+ are EPR silent and exhibit well-resolved 1H NMR spectra (Figures S7 and S8 in the Supporting

Figure 4. 1H NMR spectra in DMSO-d6 showing the upfield and downfield chemical shifts of H⊥ and H∥ methine protons, respectively, upon oxidation of compound [1]− (red) to [1]+ (black).

here, the shifts of the NMR signals for H⊥ and H∥ toward opposite directions correspond to the deshielding and shielding effects induced by an aromatic system, respectively. Therefore, the displacements of the NMR signals for the methine protons, relative to those for [1]−, are indicative of the aromaticity of the [Mo2S2C2] ring system in [1]+. As is well-known, for aromaticity to be invoked in a ring system, all of the ring14892


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Inorganic Chemistry forming atoms must be coplanar. The crystal structures of [1]− and [1] have shown that removal of one electron from the ring system alters the structural geometry toward better coplanarity of the [Mo2S2C2] core. From the shifts of the 1H NMR for the concerned protons, it is assumed that the structural condition for aromaticity is satisfied. Therefore, the [Mo2S2C2] ring system in [1]+ becomes an analogue of benzene in terms of aromaticity. By taking advantage of the H⊥ and H∥ protons, we can further investigate the aromaticity of this “heterometallabenzene” system by NMR spectral analysis on the basis of the ringcurrent model.40 This is unique because there are few elegantly designed aromatic systems49 which have two types of protons located in and above the ring plane, corresponding to different zones of the aromaticity-induced diamagnetic anisotropy. Hence, in [1]+, the two H⊥ atoms are located at the shielding zone (+) of the magnetic anisotropy of the [Mo2S2C2] ring, which shifts the NMR signal upfield (smaller δ value). In contrast, the H∥ atom lies in the deshielding zone (−) of the anisotropic magnetic field; consequently, the H∥ signal moves downfield (larger δ value). A similar situation was observed for an aromatic [Mo2S2Mo2] ring constructed by two Mo−Mo δ bonds bridged by two sp2 S atoms.40 It is noted that the remote phenyl protons on the in-plane DAniF ligand also experience appreciable deshielding of the [Mo2S2C2] ring, with the chemical shift changing from δ 6.32 ppm ([1]−) to 6.42 ppm ([1]+), while the NMR resonances for the perpendicular DAniF ligands remain unshifted. It is worthwhile to note that the H⊥ and H∥ protons are far from the center of the [Mo2S2C2] ring, in comparison with the benzene H atoms from the ring center. This means that the aromaticity-induced magnetic field in [1]+ is much larger than that for benzene. For a cyclic π-conjugated ring system, the diamagnetic anisotropy (Δχ) can be calculated according to the McConnell equation50 (eq 1) derived on the basis of the ring current model. Δσ = Δχ[(1 − 3 cos2 θ )/3R3N ]

value is about 2-fold larger than that for benzene (−62.9 ppm cgs) and reasonably smaller than those for higher benzenoid ring systems such as naphthalene (−130.3 ppm cgs) and azulene (−144.0 ppm cgs).52 The deshielding effect experienced by the in-plane proton (H∥) in proximity to the [Mo2S2C2] core confirms the aromaticity of this ring system, meanwhile verifying the ring current model. Computational Studies. The gauge-including atomic orbital (GIAO) method at the DFT level was employed to compute the NMR magnetic shielding tensor (σ) for the H⊥ and H∥ atoms in both of the two diamagnetic species [1]− and [1]+. The calculated magnetic shielding tensors are given in Table 1, along with experimental chemical shifts. The calculated Table 1. Summary of Experimental 1H NMR Spectroscopic Chemical Shifts (δ, ppm), along with the Calculated Magnetic Shielding Tensor (σ, ppm) of Methine Hydrogen Atoms Ha for Compounds [1]− and [1]+ δ(exptl) (ppm)

[1]−

[1]+

Δδ([1]−→[1]+)

8.18 8.90

0.29 −0.32 −Δσ([1]−→[1]+) 0.27 −0.47

δ(H∥) δ(H⊥) σ(calcd) (ppm)

[1]− b

8.47 8.58 [1]+ b

σ(H∥) σ(H⊥)

22.80 (8.29) 21.87 (9.22)

22.53 (8.56) 22.34 (8.75)

a H∥ and H⊥ refer to the methine protons of DAniF ligands which are essentially parallel and perpendicular to the [Mo2S2C2] ring plane, respectively. bData in parentheses indicate the calculated chemical shifts (δ) with respect to the reference TMS, for which the magnetic shielding tensor (σ) is 31.09 ppm.

upfield shift (−0.47 ppm) for the H⊥ atoms and downfield shift (0.27 ppm) for the H∥ atom are in excellent agreement with the observed chemical shift changes: −0.47 ppm for H⊥ and 0.29 ppm for H∥. These results support the presence of a large anisotropic magnetic field induced by the aromatic π system [Mo2S2C2]. Thus, the experimental and theoretical results provide substantial support for the conventional ring-current model. To obtain better insight into the ground-state geometries, electronic structures, and nature of frontier molecular orbitals (FMOs) for this series of compounds, density functional theory (DFT) calculations were performed by assuming an S = 0 or 1/ 2 spin state. The structural geometry used for ground-state optimization was based on the crystal structure parameters of the complex anion [1]−. The optimized geometries of the complexes are shown in Figure 5, and the important metrical parameters are given in Table 2. The calculated bond parameters of [1]− and [1] are in good agreement with those found by X-ray crystallography, and a slight discrepancy arises likely due to crystal lattice distortion existing in the real molecules. On this basis, in the present case where the crystal structure of [1]+ is not available, the calculated molecular geometry for [1]+ is reliable and was used for the structural analysis. As shown in Figure 5, upon transformation of complex [1]− to [1] and [1]+, the calculated dihedral angle between two planes defined by the Mo2S2 and S2C2 moieties, respectively, changes from 54.2° to 33.1 and 5.8°, and the chairlike geometry of the [Mo2S2C2] ring in [1]− changes eventually to a planar geometry for [1]+. For a quick visualization of the coplanarity of the [Mo2S2C2] ring, ∠φ (∠φ = 180° − dihedral angle) may be defined as shown in Figure 5. For [1]+, ∠φ = 174.2°, close

(1)

Here, Δσ (shielding tensor σ) accounts for the magnetic shielding effect, which equals the negative value of the observed change of chemical shift (−Δδ) in the NMR spectrum. In eq 1, Δχ is the magnetic anisotropy (ppm, cgs) defined by Δχ = χ⊥ − χ∥, where χ⊥ and χ∥ represent the out-of-plane and in-plane components of magnetic susceptibility, respectively, R is the distance (Å) from the center of the aromatic ring plane to the proton concerned, θ is the angle between the vector R and the axis perpendicular on the aromatic ring center, and N is Avogadro’s number. For [1]+, the distance of the in-plane methine proton (H∥) from the centroid of the [Mo2S2C2] ring (R) is 5.86 Å and θ = 90°, resulting from the optimized structure of [1]+ in DFT calculations (vide infra). The change in chemical shift (Δδ or −Δσ) for H∥ is 0.29 ppm, in comparison to the H ∥ resonance for the nonoxidized nonaromatic compound [1]−. Because the aromatic species [1]+ and its reference complex [1]− share a common molecular framework with similar structural parameters, we assume that the local magnetic effects arising from the Mo−Mo multiple bond are essentially canceled out51 and the downfield shift of the H∥ signal is predominantly due to the six-center−sixelectron (6c−6e) π conjugation within the [Mo2S2C2] core. Thus, from the McConnell equation, the magnitude of diamagnetic anisotropy Δχ induced by the [Mo2S2C2] core is determined to be −105.5 ppm cgs. It is remarkable that this 14893


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Figure 5. DFT optimized molecular structures of [1]−, [1], and [1]+ (H, white; C, gray; N, blue; O, red; S, yellow; Mo, magenta).

Table 2. DFT Calculated Bond Distances (Å) and Angles (deg) of [1]−, [1], and [1]+, in Comparison with X-ray Crystal Parameters [1]−

[1]

bond param

X-ray

DFT

X-ray

DFT

[1]+ DFT

Mo1−Mo2 Mo1−S1 Mo2−S2 S1−C5 S2−C4 C4−C5 Mo1−S1−C5 Mo2−S2−C4 S1−Mo1−Mo2 S2−Mo2−Mo1 ∠φ

2.0963(9) 2.5418(18) 2.5382(18) 1.750(6) 1.764(6) 1.341(7) 102.92(19) 103.5(2) 103.75(4) 105.84(4) 118.9

2.112 2.519 2.519 1.755 1.755 1.383 107.46 107.46 105.41 105.30 125.8

2.1326(12) 2.445(3) 2.446(3) 1.727(9) 1.724(9) 1.373(11) 112.1(3) 113.0(3) 106.94(7) 106.87(7) 142.3

2.130 2.468 2.468 1.733 1.733 1.398 115.21 115.06 107.36 107.48 146.9

2.153 2.410 2.414 1.707 1.706 1.418 122.44 122.03 107.36 107.78 174.2

in [1]− to 3.03 for the [Mo2]5+ unit in [1], consistent with removal of one electron from the δ(Mo2) orbital. A further decrease in Mo−Mo bond order to 2.86 in [1]+ is presumably due to electronic delocalization within the aromatic [Mo2S2C2] core. The Mo−S and C−S bond orders increase (Table 3) from [1] to [1]+, consistent with the formation of the cyclic π system. The overall changes in the Mo−N bond orders are very small, indicating that in the transformation from [1]− to [1]+ via [1] significant alternations of the electronic structures take place in the six-membered [Mo2S2C2] ring system. This is quite interesting because, in such a paddlewheel structure of Mo2, the δ orbitals interact equally with the π orbitals from the four equatorial bridging ligands. Each of the coordinated formamidinate ligands is able to provide four π electrons; thus, each of the five-membered chelating rings can be a 5c−6e π system with d(δ)−p(π) interaction. However, the calculated bond order indices indicate that the δ electrons are involved in the cyclic π conjugation of [Mo2S2C2] to meet the 6c−6e requirement for an aromatic system. During the transformation from [1]− to [1]+, the sp3 hybridization state of the bridging S atoms is converted to sp2 so that the coplanarity of all the ringforming atoms within the [Mo2S2C2] core is realized, as described by Scheme 3. Therefore, it is confirmed that, from [1]− to [1]+, modification of the electronic structure and variation of the molecular structure are both invoked by the [Mo2S2C2] aromaticity resulting from the removal of two electrons from complex [1]−. Single-point calculations at the DFT level were carried out on the optimized molecular geometries of the complexes, and the selected frontier molecular orbitals (FMOs) are shown in Figure 6. In compound [1]−, the highest occupied molecular orbital (HOMO) is mainly constructed by the δ(Mo2) orbital with some admixture of π orbitals of bridging S atoms, while

to 180°. Accompanying this change, the bond distances for the [Mo2S2C2] ring are altered. The calculated Mo−Mo distance increases from 2.112 Å ([1]−) to 2.153 Å ([1]+), and the C−C bond increases from 1.383 Å ([1]−) to 1.418 Å ([1]+), indicating that the electron density is lowered on these bonds (Table 2). On the other hand, Mo−S and C−S bonds in [1]+ are significantly shortened: for example, 2.519 Å ([1]−) versus 2.410 Å ([1]+) for the Mo−S bond and 1.755 Å ([1]−) versus 1.707 Å ([1]+) for the C−S bond. Obviously, the shortening of these bonds is due to the increase in the π-bonding character. Furthermore, the difference between Mo−Mo and Mo−S distances or Δd = d(Mo−S) − d(Mo−Mo) is 0.40 Å for [1]−, which decreases to 0.26 Å for [1]+. These results show the bond equalization for the heteroatomic [Mo2S2C2] ring in [1]+, which is driven by aromaticity. Mayer bond order indices for the selected bonds, obtained from DFT analyses, are given in Table 3. Upon oxidation, the Mo−Mo bond order decreases from 3.22 for the [Mo2]4+ unit Table 3. Mayer Bond Orders from DFT Analyses of the Selected Bonds for Compounds [1]−, [1], and [1]+, in Comparison with the Deviation of Bond Distances (Δ) Induced by Oxidation of [1]− to [1]+ Mayer bond order bond type

[1]−

[1]

[1]+

Δ([1]−→[1]+)

Mo−Mo C−C Mo−S C−S Mo−N⊥(DAniF) Mo−N∥(DAniF)

3.22 1.52 0.66 1.05 0.60 0.56

3.03 1.42 0.75 1.18 0.61 0.63

2.86 1.30 0.90 1.27 0.63 0.67

0.041 0.035 −0.109 −0.049 −0.016 −0.036 14894


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Scheme 3. Transformation of the [Mo2S2C2] Core from Nonaromatic to Aromatic upon Stepwise Removal of Two Electrons

Figure 6. Selected molecular orbitals (isodensity value 0.04) of [1]−, [1], [1]+, and benzene along with their energies (eV) obtained from singlepoint calculations at the DFT level.

HOMO-1 is contributed by the π orbital of the dmit ligand. The lowest unoccupied molecular orbital (LUMO) and LUMO +1 are based on the π* orbital of the dmit ligand and δ* orbital of the Mo2 unit, respectively. A similar bonding situation is seen for the neutral complex [1] with an almost identical HOMO− LUMO energy gap of 3.0 eV. The similar electronic structures for [1]− and [1] predicted by theoretical computations are consistent with the electrochemical and spectroscopic results. Importantly, two-electron oxidation of [1]− to [1]+ changes the

composition of FMOs and lowers the MO energies significantly, as shown in Figure 6, which is obviously due to the formation of the cyclic π conjugation within [Mo2S2C2]. For [1]+, the three highest occupied and three lowest unoccupied π-MOs in a set resemble correspondingly the πMOs of benzene in symmetry (Figure 6). For instance, the well-recognized “doughnut-shaped” HOMO-1 of benzene is clearly similar to the HOMO-2 of [1]+, produced by the combination of dδ(Mo2) and pπ(S,C) orbitals. Likewise, the 14895


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Inorganic Chemistry HOMO-1 and HOMO of [1]+ correspond to the two degenerate HOMOs of benzene. Note that these two MOs have similar orbital energies, although they are nondegenerate. The three HOMOs comprise a 6c−6e dδ−pπ π conjugation within the six-membered [Mo2S2C2] ring, formally involving one electron from the δ(Mo2) orbital plus three electrons in two pπ(S) orbitals plus two electrons in pπ(CC) orbitals, and thus behave as the archetype of the Hückel aromatic system, benzene. In this study, the calculated ΔEH−L values are in excellent agreement with the experimental MLCT band energies (EMLCT) in the absorption spectra (Figure S9 in the Supporting Information). Different from the other two species [1]− and [1], in the heterometallabenzene [1]+ the HOMO to LUMO excitation involves a significant amount of d−d character (Laporte forbidden), mixing with some metal to ligand charge transfer character; as a consequence, the intensity of this band is comparatively lowered, as shown in the spectrum (Figure 3).

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 Na[1]· 3THF·0.5C2H5OH and [1]·0.75CH2Cl2 were collected on a Bruker SMART diffractometer with Mo Kα radiation (λ = 0.71073 Å) at 173 K. The empirical absorption corrections were applied using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm.56 The crystallographic data and refinement parameters are given in Table S3 in the Supporting Information. The structures were solved using direct methods, which yielded the positions of all nonhydrogen 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 SHELXL2014 programs, respectively.57 All non-hydrogen atoms were refined with anisotropic displacement parameters. Computational Details. The ORCA 2.9.1 and 4.0 software packages were used for DFT computations.58 The geometry of the complexes was fully optimized in the gas phase, employing the Becke− Perdew (BP86) functional59 and RI/J approximation60 without imposing any symmetry constraints. Geometry optimizations for the complexes were converged with the def2-SV(P) basis set61 and def2SVP/J auxiliary basis set62 for C and H atoms, def2-TZVP(−f) basis set63 and def2-TZVP/J auxiliary basis set58 for S, N, and O atoms and def2-TZVPP basis set59 and def2-TZVPP/J auxiliary basis set58 for Mo atoms including the ZORA approximation.64 Tight optimization and tight self-consistent field convergence were employed along with a dense integration grid (ORCA Grid 5) for all geometry optimization calculations. Single-point calculations on the optimized geometries were performed with the B3LYP functional65 and def2-TZVP(−f) basis set for all atoms, except Mo atoms, for which the def2-TZVPP basis set was used including the ZORA approximation. The gaugeincluding atomic orbital (GIAO) method66 was used to compute magnetic shielding tensors, employing the B3LYP functional and RI/ JK approximation67 along with the def2-TZVP basis set68 for all atoms. Isosurface plots of molecular orbitals were generated using the UCSF Chimera package69 using isodensity values of 0.04. Preparation of Na[(DAniF)3Mo2(C3S5)] (Na[1]). A THF solution (10 mL) of the benzoyl ester of dmit (80 mg, 0.196 mmol) was mixed with sodium methoxide (22 mg, 0.393 mmol) solution in methanol (5 mL) and stirred for 10 min. To this was added a solution of Mo2(DAniF)3(O2CCH3) (200 mg, 0.196 mmol) in THF (20 mL). After the mixture was stirred for 3 h at room temperature, the solvent was evaporated under reduced pressure. The orange residue was thoroughly washed with toluene (3 × 20 mL) followed by hexane (2 × 20 mL) and collected by filtration. The product was dried under vacuum. Yield: 0.21 g (90%). 1H NMR δ (ppm in DMSO-d6): 8.90 (s, 2H, −NCHN−), 8.18 (s, 1H, −NCHN−), 6.69 (q, 16H, aromatic C− H), 6.27 (d, 4H, aromatic C−H), 5.93 (d, 4H, aromatic C−H), 3.66 (s, 12H, −OCH3), 3.53 (s, 6H, −OCH3). UV−vis, λmax, nm (ε, M−1 cm−1): 441 (6830). Anal. Calcd for C48H45N6O6S5Mo2Na: C, 48.98; H, 3.85; N, 7.14. Found: C, 48.79; H, 3.92; N, 7.25. Preparation of [(DAniF)3Mo2(C3S5)] ([1]). An orange solution of Na[(DAniF)3Mo2(C3S5)] (100 mg, 0.085 mmol) in 15 mL of THF was transferred to a flask containing ferrocenium hexafluorophosphate (28 mg, 0.085 mmol) via cannula. A dark brown color appeared immediately, and the mixture was stirred at room temperature for 2 h. The solvent was evaporated under vacuum, leaving a brown residue, which was washed with diethyl ether (2 × 20 mL). The remaining solid was extracted with dichloromethane (10 mL) and layered with hexane (25 mL). A dark microcrystalline solid was obtained after 1 week. Yield: 0.08 g (81%). UV−vis, λmax, nm (ε, M−1 cm−1): 1096 (820), 443 (6580), 377 (5110). Anal. Calcd for C48H45N6O6S5Mo2: C, 49.95; H, 3.93; N, 7.28. Found: C, 49.81; H, 3.86; N, 7.32. Preparation of [(DAniF)3Mo2(C3S5)]BPh4 ([1]BPh4). The orange solid of Na[(DAniF)3Mo2(C3S5)] (100 mg, 0.085 mmol) was dissolved in 15 mL of THF. A solution of AgBPh4 (72 mg, 0.17 mmol) in THF (5 mL) was then added slowly. A dark brown solution was obtained immediately. After the solution was stirred for 2 h at

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CONCLUSIONS In summary, a Mo≣Mo quadruple bond is integrated with a 1,3-dithiole-2-thione-4,5-dithiolate (dmit) ligand through equatorial coordination, developing a complex ([1]−) with a sixmembered heteroatomic [Mo2S2C2] ring in which the Mo2 unit and the CC bond are bridged by two sp3-hybridized S atoms. Successive one-electron oxidations of the complex remove one electron from the dimetal center and another from the dmit moiety, producing [1] and [1]+, respectively. Interestingly, the two-electron-oxidized complex [1]+ is diamagnetic and has an aromatic [Mo2S2C2] core. The aromaticity is confirmed by the displacement of 1H NMR signals of the methine protons of DAniF ligands positioned differently with respect to the [Mo2S2C2] plane due to the diamagnetic anisotropy. The magnetic shielding tensors (σ) calculated from the gaugeincluding atomic orbital (GIAO) NMR spectroscopic method are in excellent agreement with the observed chemical shifts of the related protons. The induced diamagnetic anisotropy (Δχ), estimated from the McConnell equation, is −105.5 ppm cgs, 2fold larger than that for benzene, consistent with the abnormal 1 H NMR chemical shifts for the methane protons. Remarkably, DFT calculations give rise to six π molecular orbitals, respectively, resembling those of benzene in symmetry, relative energy level, and electron occupation. These results indicate that the δ bond in an M≣M quadruple bond is an equivalent of the π bond in organic compounds in bonding functionality. This understanding should be beneficial in the design of metal−organic hybridized functional molecules and materials.

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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. HDAniF,53 Mo2(DAniF)3(O2CCH3),54 and the benzoyl ester of 1,3-dithiole-2thione-4,5-dithiolate55 were synthesized according to published methods. Physical Measurements. UV−vis−NIR spectra were measured in THF 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 THF 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 14896


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Inorganic Chemistry room temperature, filtration was performed with the aid of Celite. Addition of hexane to the filtrate resulted in precipitation of a brown powder in quantitative yield, which was collected by filtration. The product was dried under vacuum. Yield: 0.1 g (80%). 1H NMR δ (ppm in DMSO-d6): 8.58 (s, 2H, −NCHN−), 8.47 (s, 1H, −NCHN−), 7.36−6.77 (m, 20H, aromatic C−H from BPh4), 6.69 (q, 16H, aromatic C−H), 6.46 (d, 4H, aromatic C−H), 6.32 (d, 4H, aromatic C−H), 3.71 (s, 12H, −OCH3), 3.62 (s, 6H, −OCH3). UV−vis, λmax, nm (ε, M−1 cm−1): 442 (1040). Anal. Calcd for C72H65N6O6S5Mo2B: C, 58.69; H, 4.45; N, 5.70. Found: C, 58.80; H, 4.54; N, 5.62.

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Tuning Their Kinetics by Substituents. Organometallics 2014, 33, 2336−2340. (5) Märkl, G. 2,4,6-Triphenylphosphabenzen. Angew. Chem., Int. Ed. Engl. 1966, 5, 846−847. (6) Wakita, K.; Tokitoh, N.; Okazaki, R.; Nagase, S. Synthesis and Properties of an Overcrowded Silabenzene Stable at Ambient Temperature. Angew. Chem., Int. Ed. 2000, 39, 634−636. (7) Nakata, N.; Takeda, N.; Tokitoh, N. Synthesis and Properties of the First Stable Germabenzene. J. Am. Chem. Soc. 2002, 124, 6914− 6920. (8) Ashe, A. J. Aromatic Gallium Heterocycles: Synthesis of the First Gallatabenzene. Angew. Chem., Int. Ed. Engl. 1995, 34, 1357−1359. (9) Ashe, A. J. Phosphabenzene and Arsabenzene. J. Am. Chem. Soc. 1971, 93, 3293−3295. (10) Ashe, A. J. Stibabenzene. J. Am. Chem. Soc. 1971, 93, 6690− 6691. (11) Ashe, A. J.; Diephouse, T. R.; El-Sheikh, M. Y. Stabilization of Stibabenzene and Bismabenzene by 4-Alkyl Substituents. J. Am. Chem. Soc. 1982, 104, 5693−5699. (12) Nakamura, T.; Suzuki, K.; Yamashita, M. An Anionic Aluminabenzene Bearing Aromatic and Ambiphilic Contributions. J. Am. Chem. Soc. 2014, 136, 9276−9279. (13) (a) Bleeke, J. R. Metallabenzenes. Chem. Rev. 2001, 101, 1205− 1227. (b) Wright, L. J. Metallabenzenes and metallabenzenoids. Dalton Trans. 2006, 1821−1827. (14) Elliott, G. P.; Roper, W. R.; Waters, J. M. Metallacyclohexatrienes or ‘metallabenzenes.’ Synthesis of osmabenzene derivatives and X-ray crystal structure of [Os(CSCHCHCHCH)(CO)(PPh3)2]. J. Chem. Soc., Chem. Commun. 1982, 811−813. (15) (a) Bleeke, J. R.; Xie, Y. F.; Peng, W. J.; Chiang, M. Metallabenzene: Synthesis, Structure, and Spectroscopy of a l-Irida3,5-dimethylbenzeneComplex. J. Am. Chem. Soc. 1989, 111, 4118− 4120. (b) Bleeke, J. R. Aromatic Iridacycles. Acc. Chem. Res. 2007, 40, 1035−1047. (c) Bleeke, J. R.; Behm, R.; Xie, Y.; Chiang, M. Y.; Robinson, K. D.; Beatty, A. M. Synthesis, Structure, Spectroscopy, and Reactivity of a Metallabenzene. Organometallics 1997, 16, 606−623. (16) (a) Wu, L.; Feng, L.; Zhang, H.; Liu, Q.; He, X.; Yang, F.; Xia, H. Synthesis and Characterization of a Novel Dialdehyde and Cyclic Anhydride. J. Org. Chem. 2008, 73, 2883−2885. (b) Zhang, H.; Feng, L.; Gong, L.; Wu, L.; He, G.; Wen, T. B.; Yang, F.; Xia, H. Synthesis and Characterization of Stable Ruthenabenzenes Starting from HC CCH(OH)CCH. Organometallics 2007, 26, 2705−2713. (c) Zhang, H.; Xia, H.; He, G.; Wen, T. B.; Gong, L.; Jia, G. Synthesis and Characterization of Stable Ruthenabenzenes. Angew. Chem., Int. Ed. 2006, 45, 2920−2923. (d) Clark, G. R.; O’Neale, T. R.; Roper, W. R.; Tonei, D. M.; Wright, L. J. Stable Cationic and Neutral Ruthenabenzenes. Organometallics 2009, 28, 567−572. (e) Lin, R.; Zhao, J.; Chen, H.; Zhang, H.; Xia, H. Interconversion of Metallabenzenes and Cyclic η2-Allene Coordinated Complexes. Chem. - Asian J. 2012, 7, 1915−1924. (f) Lin, R.; Zhang, H.; Li, S.; Wang, J.; Xia, H. New Highly Stable Metallabenzenes via Nucleophilic Aromatic Substitution Reaction. Chem. - Eur. J. 2011, 17, 4223−4231. (17) Landorf, C. W.; Jacob, V.; Weakley, T. J. R.; Haley, M. M. Rational Synthesis of Platinabenzenes. Organometallics 2004, 23, 1174−1176. (18) Bertling, U.; Englert, U.; Salzer, A. From Triple-Decker to Metallabenzene: A New Generation of Sandwich Complexes. Angew. Chem., Int. Ed. Engl. 1994, 33, 1003−1004. (19) (a) Poon, K. C.; Liu, L.; Guo, T.; Li, J.; Sung, H. H. Y.; Williams, I. D.; Lin, Z.; Jia, G. Synthesis and Characterization of Rhenabenzenes. Angew. Chem., Int. Ed. 2010, 49, 2759−2762. (b) Lin, R.; Lee, K. H.; Poon, K. C.; Sung, H. H. Y.; Williams, I. D.; Lin, Z.; Jia, G. Synthesis of Rhenabenzenes from the Reactions of Rhenacyclobutadienes with Ethoxyethyne. Chem. - Eur. J. 2014, 20, 14885−14899. (20) Liu, B.; Xie, H.; Wang, H.; Wu, L.; Zhao, Q.; Chen, J.; Wen, T. B.; Cao, Z.; Xia, H. Selective Synthesis of Osmanaphthalene and Osmanaphthalyne by Intramolecular C−H Activation. Angew. Chem., Int. Ed. 2009, 48, 5461−5464.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02133. 1 H NMR and EPR spectra and DFT calculation data (PDF) Cartesian coordinates for the calculated structures (XYZ) Accession Codes

CCDC 1569091−1569092 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 Authors

*C.Y.L: tel, +86-020-85222191; e-mail, [email protected]. *J.-L.Z.: tel, +86-025-83593893; e-mail, [email protected]. ORCID

Chun Y. Liu: 0000-0001-6908-9929 Jing-Lin Zuo: 0000-0003-1219-8926 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are greatly thankful to the National Natural Science Foundation of China (No. 21371074, 90922010, and 21631006), Jinan University, and Fundamental Research Funds for the Central Universities for financial support.

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REFERENCES

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Inorganic Chemistry

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