Coordination Chemistry of the Parent Dithiocarbamate H₂NCS₂^–: Organometallic Chemistry and Tris-Chelates of Group 9 Metals

Abstract

Two tris-chelate complexes of cobalt and rhodium and two complexes of Ru(II) of dithiocarbamate, [S 2 CNH 2 ] − , were synthesized. The complexes were spectroscopically characterized by IR, NMR, UV−vis, and MS and structurally characterized by X-ray diffraction. The structural features of the rhodium complex were compared to those of other tris-chelate Rh(III) dithiocarbamate complexes and are characterized by a change in the ground-state geometry in comparison to expected octahedral tris-chelate complexes. This was confirmed both experimentally by X-ray diffraction and theoretically using DFT calculations. The inversion barriers of Rh(Bz 2 dtc) 3 , Ir(Bz 2 dtc) 3 , and Rh(Et 2 dtc) 3 were determined using VT-NMR in DMSO. These barriers were found to be surprisingly low for heavy group 9 elements of d 6 tris-chelate complexes: values of 16.7, 17.1, and 16.4 kcal/mol were calculated, respectively. By comparing structural features, we are able to determine that the activation barrier for the inversion of stereochemistry of Rh(H 2 dtc) 3 must have a similarly low value. A modified version of the Bailar twist involving an intermediate with C 3h geometry was proposed as the mechanism of inversion.

Three-Center M–H–B Bonds Are Strong Field Interactions. Synthesis and Characterization of M(CH2NMe2BH3)3 Complexes of Titanium, Chromium, and Cobalt

“…Octahedral cobalt­(III) compounds almost always have large activation free energies for the Δ/Λ racemization process; in fact, most are stereochemically rigid on the NMR time scale (and even on the chemical time scale). – To our knowledge, the only exceptions to this generalization are cobalt tris­(tropolonate) and tris­(dithiocarbamate) complexes, in which the ligands potentially are electronically non-innocent; these complexes have activation free energies Δ G ⧧ of between 14 and 15 kcal mol –1 at either 5 or 25 °C. ,, As shown from the variable temperature NMR studies above, BDAM complex 4 has by far the smallest Δ/Λ racemization barrier (Δ G ⧧ = 10.1 kcal mol –1 at 25 °C) seen for any octahedral cobalt­(III) complex.…”