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

The one-electron chemically reversible oxidation of four neutral [RhLL'(kappa(2)-Tp(Me2))]complexes [Tp(Me2) = hydrotris(3,5-dimethylpyrazolyl)borate], which leads to kappa(3)-Tp(Me2) bonding in the corresponding monocations, has been studied by cyclic voltammetry (CV) and other electrochemical methods. The CV behavior of [Rh(CO)[P(OPh)(3)]Tp(Me2)] (1) and [Rh(CO)(PPh(3))Tp(Me2)] (2) is quasi-nernstian at slow CV scan rates, 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)(PCy(3))Tp(Me2)] (3, Cy = cyclohexyl) and [Rh(PPh(3))(2)Tp(Me2)] (4) display electrochemically irreversible CV curves that arise from rate-limiting slow electron-transfer reactions. Both the oxidation of 3 (or 4) and the rereduction of 3(+) (or 4(+)) have two-step (EC-type) mechanisms in which the electron transfer (e.t.) process is followed by a separate structural change, leading to an overall square scheme with irreversible charge-transfer kinetics. Homogeneous redox catalysis was used to determine the E(1/2) value of the oxidation of 3 to an intermediate 3C(+) which is postulated to have a pseudo-square pyramidal structure. Digital simulations gave k(s) = 9 x 10(-3) cm s(-1) for the heterogeneous charge-transfer rate of 3/3C(+). The close-to-nernstian CV behavior of 1 is ascribed to the fact that, unlike the sterically constrained derivatives 3 and 4, the third pyrazolyl ring in 1 is already in a configuration which favors formation of the Rh-N(2) bond in 1(+). The overall redox mechanism for this series of compounds involves an associative oxidative e.t. reaction followed by a dissociative reductive e.t. process.