Lawrence Berkeley National Laboratory

Reduced dithiolene ligands are bound to high valent Mo centers in the active site of the oxotransferase family of enzymes. Related model complexes have been studied with great insight by Prof. Holm and his colleagues. This study focuses on the other limit of dithiolene chemistry: an investigation of the 2-electron oxidized dithiolene bound to low-valent late transition metal (TM) ions (Zn^II, Cu^I, and Cu^II). The bonding descriptions of the oxidized dithiolene [N,N-dimethyl piperazine 2,3-dithione (Me₂Dt⁰)] complexes are probed using S K-edge X-ray absorption spectroscopy (XAS) and the results are correlated to density functional theory (DFT) calculations. These experimentally supported calculations are then extended to explain the different geometric structures of the three complexes. The Zn^II(Me₂Dt⁰)₂ complex has only ligand-ligand repulsion so it is stabilized at the D_2d symmetry limit. The Cu^I(Me₂Dt⁰)₂ complex has additional weak backbonding thus distorts somewhat from D_2d toward D_2h symmetry. The Cu^II(Me₂Dt⁰)₂ complex has a strong σ donor bond that leads to both a large Jahn-Teller stabilization to D_2h and an additional covalent contribution to the geometry. The combined strong stabilization results in the square planar, D_2h structure. This study quantifies the competition between the ligand-ligand repulsion and the change in electronic structures in determining the final geometric structures of the oxidized dithiolene complexes, and provides quantitative insights into the Jahn-Teller stabilization energy and its origin.