Syntheses and properties of dinuclear group 6 metal complexes with the zwitterionic sulfur donor ligand bis(N, N -diethylamino) carbeniumdithiocarboxylate

A class of dinuclear group 6 metal complexes [{M⁰(CO) ₅}₂(μ-S,S′-EtL)] (M = Cr, Mo, W) with the asymmetrically bridged zwitterionic sulfur donor bis(N,N-diethylamino) carbeniumdithiocarboxylate (EtL) was prepared by photoreaction of [M ⁰(CO)₆] with EtL in hexane-CH₂Cl₂ for Cr and Mo or THF for W. The same structure in a syn-anti coordination mode was revealed by X-ray analyses for chromium(0) and tungsten(0) complexes, 1 and 3, respectively, and speculated from various analytical data for the molybdenum(0) complex 2. The pertinent criteria for the coordination mode are found to be the wavenumber of the asymmetric stretching vibration of the -CS₂ moiety in the solid state and the ligand-based reduction potential in solution. Complexes 1 and 3 showed quasi-reversible reduction waves, which are assigned to a two-step, one-electron reduction process derived from the bridging EtL. In a series of [Cr(CO)₅L] complexes (L = monodentate ligand), the ¹³C NMR chemical shifts of the trans-to-L carbonyl group showed the order of increasing σ-donor/π-acceptor ratio of the ligands. The sulfur donor ligands, including EtL and thione, are positioned in the middle of this range. All complexes exhibited negative solvatochromism: the wavelength of the absorption maximum shifted to the blue side in the range 500-600 nm with an increase in the Reichardt solvent polarity parameters E_T(30), except for protic solvents. Quantum chemical calculations by time-dependent density functional theory-polarized continuum model were employed for understanding the excited states and solvatochromic properties of complex 3. The calculated vertical excitation energies in solution are consistent with the experimental data, suggesting that the transition is a metal-to-ligand charge-transfer transition. In addition, UV-vis, NMR, and cyclic voltammetry data showed that complex 3 dissociates into two mononuclear species in polar solvents: [W(CO)₅(EtL)] and [W(CO)₅(solvent)].