Effect of Pyramidalization of the M₂(SR)₂ Center: The Case of (C₅H₅)₂Ni₂(SR)₂

The effects of simple thiolates vs chelating dithiolates on M-M bonding, redox potentials, and synthetic outcomes have been probed experimentally and computationally. Nickelocene (Cp₂Ni) has long been known to react with simple thiols to give diamagnetic Cp₂Ni₂(SR)₂ with planar Ni₂S₂ cores and long Ni- - -Ni distances. Ethane- and propanedithiol (edtH₂ and pdtH₂, respectively) instead give di-, tri-, and pentanickel complexes, with nonplanar Ni₂S₂ cores. The 36e Cp₂Ni₂(pdt) (1^pdt) adopts a symmetrical butterfly Ni₂S₂ structure. Variable-temperature NMR spectra indicate that 1^pdt possesses a thermally accessible triplet state (ΔG = 2.65(5) kcal/mol) in equilibrium with a diamagnetic ground state. DFT calculations indicate that the singlet-triplet gap is highly sensitive to the nonplanarity of the Ni₂S₂ core. The calculations further reveal that only the high-spin form of 1^pdt features Ni-Ni bonding, which is unprecedented. Cp₂Ni₃(pdt)₂ (2^pdt), which derives from 1^pdt, crystallized as cis and trans isomers, both with a central Ni(pdt)₂^2- unit that is S,S-chelated to two CpNi⁺ centers. Reaction of Cp₂Ni with 1,2-ethanedithiol (H₂edt) and 1,2-benzenedithiol (bdtH₂) exclusively gave the trinickel species 2^edt and 2^bdt, which are structurally analogous to cis-2^pdt. Solutions of 2^edt are unstable, depositing crystals of Cp₂Ni₅(edt)₄ (3^edt). Cyclic voltammetric studies show that the Ni₂ species oxidize readily to give the mixed-valence cations [1^pdt]⁺ and [1^edt]⁺. Crystallographic and EPR analyses indicate that these cations are delocalized mixed-valence Ni(II)-Ni(III) species. Oxidation of Cp₂Ni₂(SEt)₂, which features a planar Ni₂S₂ core, afforded a mixed-valence cation, showing the pyramidal Ni₂S₂ core observed in [1^pdt]^0/+ and [1^edt]^0/+. Although not obtained from the Cp₂Ni/H₂edt reaction, the neutral complex 1^edt was obtained by reduction of [1^edt]⁺. Variable-temperature NMR measurements and DFT calculations indicate that the triplet is further stabilized in this highly pyramidalized species.