Electron paramagnetic resonance and electrochemical study of the oxidation chemistry of mononuclear and binuclear chromium carbonyl thiolates

Article abstract of DOI:10.1021/ja00171a018

The oxidation chemistry of RSCr (CO)₅^- thiolate anions, 1(R)^-, and of their binuclear analogues RS[Cr(CO)₅]₂^-, 2(R)^-, has been studied by cyclic voltammetry, EPR, and low-temperature IR. Rapid potential scan cyclic voltammetry shows that the radicals 1(R)^. have lifetimes of less than 50 μs and decay by first-order kinetics. Chemical studies are consistent with fragmentation of 1(R)^. into an alkylthiyl radical RS^. and a Cr(CO)₅ fragment. The slower reactions following the initial irreversible oxidation have been elucidated by cyclic voltammetry. The major product of chemical and electrochemical oxidation of 1(R)^- is the monodentate disulfide complex (RS-SR)Cr(CO)₅ formed by complexation of Cr(CO)₅ fragments to the organic disulfide resulting from dimerization of RS^. radicals. The electrochemical oxidation of 2(R)^- proceeds in two chemically reversible steps to the corresponding 2(R)^. radicals and 2(R)⁺ sulfonium cations. The intensely colored 2(R)^. radicals have limited thermal stability and were characterized by EPR, low-temperature IR, and extended Hu?ckel MO calculations. The unpaired electron resides to a large extent in a p-type orbital on the trigonal sulfur, which is μ-conjugated to d orbitals of the neighboring Cr atoms. This orbital is part of a three-center pseudoallylic μ system occupied by five electrons, totally analogous to that with four-electron occupancy put forward previously for binuclear phosphinidine complexes RP[ML_n]₂, e.g., RP[Cr(CO)₅]₂, which is isoelectronic with 2(R)⁺. The X-ray structures of t-BuSCr(CO)₅^- and t-BuS[Cr(CO)₅]₂^- are also presented.