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) (1pdt) adopts a symmetrical butterfly Ni2S2 structure. Variable-temperature NMR spectra indicate that 1pdt 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 1pdt features Ni-Ni bonding, which is unprecedented. Cp2Ni3(pdt)2 (2pdt), which derives from 1pdt, crystallized as cis and trans isomers, both with a central Ni(pdt)22- 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 2edt and 2bdt, which are structurally analogous to cis-2pdt. Solutions of 2edt are unstable, depositing crystals of Cp2Ni5(edt)4 (3edt). Cyclic voltammetric studies show that the Ni2 species oxidize readily to give the mixed-valence cations [1pdt]+ and [1edt]+. 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 [1pdt]0/+ and [1edt]0/+. Although not obtained from the Cp2Ni/H2edt reaction, the neutral complex 1edt was obtained by reduction of [1edt]+. Variable-temperature NMR measurements and DFT calculations indicate that the triplet is further stabilized in this highly pyramidalized species.