ELECTRON-PARAMAGNETIC RESONANCE AND ELECTROCHEMICAL STUDY OF THE OXIDATION CHEMISTRY OF MONONUCLEAR AND BINUCLEAR CHROMIUM CARBONYL THIOLATES
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abstract
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 s 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 Hckel MO calculations. The unpaired electron resides to a large extent in a p-type orbital on the trigonal sulfur, which is -conjugated to d orbitals of the neighboring Cr atoms. This orbital is part of a three-center pseudoallylic system occupied by five electrons, totally analogous to that with four-electron occupancy put forward previously for binuclear phosphinidene complexes RP[MLn]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. 1990, American Chemical Society. All rights reserved.
