Infrared spectroelectrochemical and electrochemical kinetics studies of the reaction of nickel cluster radicals [Ni 3( μ2-dppm) 3( μ3-L) ( μ3I)] •(L = CNR, R = CH 3, i-C 3H 7, C 6H 11, CH 2C 6H 5, t-C 4H 9, 2,6-Me 2C 6H 3; L = CO) with carbon dioxide

Abstract

The trinuclear nickel clusters [Ni 3( μ 2-dppm) 3( μ 3-L) ( μ 3-I)][PF 6] (L = CNR, R = CH 3 ( 1), i-C 3H 7 ( 2), C 6H 11 ( 3), CH 2C 6H 5 ( 4), 1-C 4H 9 ( 5), 2,6-Me 2C 6H 3 ( 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( μ 2-dppm) 3 ( μ 3-L) ( μ 3-I)] •. Specular reflectance IR spectroelectrochemical measurements were used to characterize these species and their reactions with CO 2. Studies in the absence of CO 2 show that the capping isocyanide or carbonyl ligand remains triply bridging ( μ 3, η 1) upon single electron reduction. Electrochemical kinetics studies indicate that the rates of reaction with CO 2 depend to first order on [cluster] and to first order on [CO 2]. The rate constants for the rate limiting step in the reduction of CO 2 by the clusters, k CO 2 (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 CO 2 follow the order: CNCH 3 ( 1) ∼ CN(i-C 3H 7) ( 2) > CNC 6H 11 ( 3) > CNCH 2C 6H 5 ( 4) > CO ( 7) > CN ( t-C 4H 9) ( 5) ∼ CN(2,6-Me 2C 6H 3) ( 6). On the basis of these kinetic and spectroscopic studies, a mechanism for the catalytic reduction of CO 2 involving CO 2 activation on the isocyanide-capped face of the trinuclear nickel clusters is proposed.