Infrared spectroelectrochemical and electrochemical kinetics studies of the reaction of nickel cluster radicals [Ni3(μ2-dppm)3(μ3-L)(μ3-I)]• (L = CNR, R = CH3, i-C3H7, C6H11, CH2C6H5, t-C4H9, 2,6-Me2C6H3; L = CO) with carbon dioxide

The trinuclear nickel clusters [Ni-3(mu(2)-dppm)(3)(mu(3)-L)(mu(3)-I)][PF6] (L = CNR, R = CH3 (1), i-C3H7 (2), C6H11 (3), CH2C6H5 (4), t-C4H9 (5), 2,6-Me2C6H3 (6); L = CO (7), dppm = bis(diphenylphosphino)methane) undergo single electron reduction over a relatively narrow range of E-1/2(+/0) (-1.08 to -1.18 V versus SCE in acetonitrile) to form neutral radicals, [Ni-3(mu(2)-dppm)(3)(mu(3)-L)(mu(3)-I)](.). Specular reflectance IR spectroelectrochemical measurements were used to characterize these species and their reactions with CO2. Studies in the absence of CO2 show that the capping isocyanide or carbonyl ligand remains triply bridging (mu(3),eta(1)) upon single electron reduction. Electrochemical kinetics studies indicate that the rates of reaction with CO2 depend to first order on [cluster] and to first order on [CO2]. The rate constants for the rate limiting step in the reduction of CO2 by the clusters, k(CO2) (M(-1)s(-1)), are 1.6 +/- 0.3 (1), 1.4 +/- 0.3 (2), 0.5 +/- 0.1 (3), 0.2 +/- 0.05 (4), 0.0 +/- 0.05 (5), 0.0 +/- 0.05 (6), and 0.1 +/- 0.1 (7). Thus, the relative rates of reaction of the alkyl or aryl substituted isocyanide or carbonyl capped clusters with CO2 follow the order: CNCH3 (1) similar to-CN(i-C3H7) (2) > CNC6H11 (3) > CNCH2C6H5 (4) > CO (7) > CN(t-C4H9) (5) similar to CN(2,6-Me2C6H3) (6). On the basis of these kinetic and spectroscopic studies, a mechanism for the catalytic reduction of CO2 involving CO2 activation on the isocyanide-capped face of the trinuclear nickel clusters is proposed. (C) 1998 Elsevier Science S.A.