Syntheses and Properties of Dinuclear Group 6 Metal
Complexes with the Zwitterionic Sulfur Donor Ligand Bis(<i>N</i>,<i>N</i>‑diethylamino)carbeniumdithiocarboxylate
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Abstract
A class
of dinuclear group 6 metal complexes [{M<sup>0</sup>(CO)<sub>5</sub>}<sub>2</sub>(μ-<i>S</i>,<i>S′</i>-EtL)] (M = Cr, Mo, W) with the asymmetrically bridged zwitterionic
sulfur donor bis(<i>N,N</i>-diethylamino)carbeniumdithiocarboxylate
(EtL) was prepared by photoreaction of [M<sup>0</sup>(CO)<sub>6</sub>] with EtL in hexane–CH<sub>2</sub>Cl<sub>2</sub> for Cr and
Mo or THF for W. The same structure in a <i>syn</i>–<i>anti</i> coordination mode was revealed by X-ray analyses for
chromium(0) <i>and</i> tungsten(0) complexes, <b>1</b> and <b>3</b>, respectively, and speculated from various analytical
data for the molybdenum(0) complex <b>2</b>. The pertinent criteria
for the coordination mode are found to be the wavenumber of the asymmetric
stretching vibration of the −CS<sub>2</sub> moiety in the solid
state and the ligand-based reduction potential in solution. Complexes <b>1</b> and <b>3</b> 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)<sub>5</sub>L] complexes
(L = monodentate ligand), the <sup>13</sup>C NMR chemical shifts of
the <i>trans</i>-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 <i>E</i><sub>T</sub>(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 <b>3</b>. 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 <b>3</b> dissociates into two mononuclear species
in polar solvents: [W(CO)<sub>5</sub>(EtL)] and [W(CO)<sub>5</sub>(solvent)]
