Mechanistic studies on reactivities of organometallic macrocyclic complexes of chromium and cobalt

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Date
1990
Authors
Shi, Shu
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James Espenson
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Chemistry
Abstract

Reaction pathways leading to the formation and cleavage of a transition metal-carbon bond at various oxidation states of the metal occupy a central position in understanding many enzymatic reactions and designing catalysts;Part I focuses on the homolysis vs. heterolysis of a C-Cr(III) bond. Rate constants of homolysis and heterolysis of the C-Cr(III) bonds along with corresponding [delta]H[superscript]\ddagger and [delta]S[superscript]\ddagger were measured by ESR and stopped flow techniques. Factors favoring one pathway over another to cleave a C-Cr(III) bond are discussed;Part II describes a unique chain reaction and a S[subscript] E2 reaction between I[subscript]2 and RCrL[superscript]2+. The molecular structure of RCrL[superscript]2+, determined by x-ray diffraction for the first time, provides clearly a structural basis for the reactivity discussions;Part III concerns the oxidation of organochromium(III) complexes by dihalide and pseudo-dihalide radical anions generated by pulse radiolysis. The reaction mechanism and reactivity trend are discussed;Part IV concentrates on the oxidation mechanism of RCrL(H[subscript]2O)[superscript]2+ and the fate of RCr(H[subscript]2O)[superscript]3+ as well as the corresponding reduction potentials. An outer-sphere electron transfer mechanism was established. A facile C-Cr(IV) bond hemolyses was discovered along with its negligible heterolysis. Reduction potential of E[subscript]1/2(RCr[superscript] IV/RCr[superscript] III) and the relevant self exchange rate constant k[subscript]11 were estimated by a kinetic method;In Part V the study is extended to organocobalt complexes with attention turned to reduction induced cleavages of a transition metal-carbon bond. Electrochemical methods in conjunction with ESR, NMR, and GC-MASS are used to reveal evidence of novel reactions--reduction induced hydrogen atom transfer and reduction induced alkyl group migration. Part V also concerns the kinetics of reductions of the organocobalt compounds by a nickel(I) complex;In Part VI, the crystallization of ((CH[subscript]3)[subscript]4N) (Co(dmgBF[subscript]2)[subscript]2py) and its molecular structure determined by X-ray diffraction are described. The Co[superscript] I-N[subscript] eq distances are unusually short (1.839 A), even shorter than the corresponding bond (1.878 A) in the cobalt(II) analogue. Reasons of this unusual Co-N bond shortening is discussed along with the electronic structure of the d[superscript]8 cobalt(I) anion.

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Mon Jan 01 00:00:00 UTC 1990