Aromaticity-Driven Molecular Structural Variation and Electronic Configuration Alternation: An Example of Cyclic π...

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.