Aromaticity-Driven Molecular Structural Variation and Electronic Configuration Alternation: An Example of Cyclic π Conjugation Involving a Mo–Mo δ Bond

We have synthesized and characterized the mixed-ligand 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. One-electron 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.