Lewis Acid Enhanced Axial Ligation of [Mo2]4+ Complexes

We report here the syntheses, X-ray crystal structures, electrochemistry, and density functional theory (DFT) single-point calculations of three new complexes: tetrakis­(monothiosuccinimidato)­dimolybdenum­(II) [Mo2(SNO5)4, 1a], tetrakis­(6-thioxo-2-piperidinonato)­dimolybdenum­(II) [Mo2(SNO6)4, 1b], and chlorotetrakis­(monothiosuccinimidato)­pyridinelithiumdimolybdenum­(II) [pyLiMo2(SNO5)4Cl, 2-py]. X-ray crystallography shows unusually short axial Mo2–Cl bond lengths in 2-py, 2.6533(6) Å, and dimeric 2-dim, 2.644(1) Å, which we propose result from an increased Lewis acidity of the Mo2 unit in the presence of the proximal Li+ ion. When 2-py is dissolved in MeCN, the lithium reversibly dissociates, forming an equilibrium mixture of (MeCNLiMo2(SNO5)4Cl) (2-MeCN) and [Li­(MeCN)4]+[Mo2(SNO5)4Cl]− (3). Cyclic voltammetry was used to determine the equilibrium lithium binding constant (room temperature, Keq = 95 ± 1). From analysis of the temperature dependence of the equilibrium constant, thermodynamic parameters for the formation of 2-MeCN from 3 (ΔH° = −6.96 ± 0.93 kJ mol–1 and ΔS° = 13.9 ± 3.5 J mol–1 K–1) were extracted. DFT calculations indicate that Li+ affects the Mo–Cl bond length through polarization of metal–metal bonding/antibonding molecular orbitals when lithium and chloride are added to the dimolybdenum core.