Reversible Sequence of Intramolecular Associative and Dissociative Electron-Transfer Reactions in Hydrotris(Pyrazolylborate) Complexes of Rhodium

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• 作者：William E. Geiger ; Nicole Camire Ohrenberg ; Brett Yeomans ; Neil G. Connelly ; and David J. H. Emslie
• 刊名：Journal of the American Chemical Society
• 出版年：2003
• 出版时间：July 16, 2003
• 年：2003
• 卷：125
• 期：28
• 页码：8680 - 8688
• 全文大小：164K
• 年卷期：v.125,no.28(July 16, 2003)
• ISSN：1520-5126

文摘

The one-electron chemically reversible oxidation of four neutral [RhLL'(chars/kappa.gif" BORDER=0 >²-Tp^Me2)]complexes{Tp^Me2 = hydrotris(3,5-dimethylpyrazolyl)borate}, which leads to chars/kappa.gif" BORDER=0 >³-Tp^Me2 bonding in the correspondingmonocations, has been studied by cyclic voltammetry (CV) and other electrochemical methods. The CVbehavior of [Rh(CO){P(OPh)₃}Tp^Me2] (1) and [Rh(CO)(PPh₃)Tp^Me2] (2) is quasi-nernstian at slow CV scanrates, with heterogeneous charge-transfer rates, k_s, of 0.025 cm s^-1 and 0.015 cm s^-1 (at 273 K),respectively. By contrast, [Rh(CO)(PCy₃)Tp^Me2] (3, Cy = cyclohexyl) and [Rh(PPh₃)₂Tp^Me2] (4) displayelectrochemically irreversible CV curves that arise from rate-limiting slow electron-transfer reactions. Boththe oxidation of 3 (or 4) and the rereduction of 3⁺ (or 4⁺) have two-step (EC-type) mechanisms in whichthe electron transfer (e.t.) process is followed by a separate structural change, leading to an overall squarescheme with irreversible charge-transfer kinetics. Homogeneous redox catalysis was used to determinethe E_1/2 value of the oxidation of 3 to an intermediate 3C⁺ which is postulated to have a pseudo-squarepyramidal structure. Digital simulations gave k_s = 9 × 10^-3 cm s^-1 for the heterogeneous charge-transferrate of 3/3C⁺. The close-to-nernstian CV behavior of 1 is ascribed to the fact that, unlike the stericallyconstrained derivatives 3 and 4, the third pyrazolyl ring in 1 is already in a configuration which favorsformation of the Rh-N(2) bond in 1⁺. The overall redox mechanism for this series of compounds involvesan associative oxidative e.t. reaction followed by a dissociative reductive e.t. process.