Fast and conventional voltammetry of organometallic complexes and electrocatalysts

A fast potentiostat equipped with on-line iR compensation has been constructed, tested, and used to characterized electroactive organometallic transient intermediates. This potentiostat was modeled after the one published by Christian Amatore and coworkers at the Ecole Normale Superieure in Paris, France. The fast potentiostat has been used to probe two electrocatalysts involved in the reduction of CO$\sb2$. The electrochemistry of the nickel trimer complex, (Ni$\sb3(\mu\sb3$-$\eta\sp1$-CNMe)($\mu\sb3$-$\eta\sp1$-I)(dppm)$\sb3$) (I), (1) is known to be an electrocatalyst for the reduction of CO$\sb2$. Reduction of CO$\sb2$ with the neutral form of 1 occurs at E$\sb{1/2}$ = $-$1.09V vs Ag/AgCl (about 1.07V more favorable than direct electrochemical reduction of CO$\sb2$ itself). Previously, steady-state voltammetry measured the overall rate constant for CO$\sb2$ reduction of 0.011M$\sp{-1}$s$\sp{-1}$ Several electrochemical products had been detected by conventional cyclic voltammetry as evidenced by their oxidation waves present during electrocatalysis of CO$\sb2$. Fast cyclic voltammetry (CV) has been used to intercept the electron transfer to CO$\sb2$ by the reduced form of 1. The chemical rate constant prior to regeneration of the electrocatalyst, has been estimated at 1.2 $\times$ 10$\sp3$ M$\sp{-1}$s$\sp{-1}$. The electrochemistry of the copper dimer (Cu$\sb2(\mu\sb2$-PPh$\sb2$bipy)$\sb2$(MeCN)$\sb2$) (PF$\sb6\rbrack \sb2$ (2) has been previously characterized by two reversible one-electron reductions at E$\sb{1/2}$(2+/+) = $-$1.38V and E$\sb{1/2}$(+/0)= $-$1.58V vs SCE. The neutral state of 2 reacts with CO$\sb2$ in the electrocatalytic reduction of CO$\sb2$ by 2. Steady-state chronoamperometry by coworkers has estimated the limiting chemical rate constant, k(CO$\sb2$), of the doubly reduced form of 2 with CO$\sb2$ to be 0.7 M$\sp{-1}$s$\sp{-1}$ in acetonitrile. Fast CV estimates the rate of adduct formation of CO$\sb2$ with the doubly reduce state of 2 to be 1.3 $\times$ 10$\sp5$ M$\sp{-1}$s$\sp{-1}$. The previous chronoamperometric data appears to represent a later reaction step involving CO$\sb2$ in the electrocatalytic reduction of CO$\sb2$. Other electroactive organometallic complexes of interest have involved metal-metal dimers of the type M$\sb2$(CNMe)$\sb6\sp{2+}$ (M = Pd, Pt). Photolysis of these dimers leads to homolytic cleavage of the M-M bond. Previous studies have shown that these radicals recombine at nearly diffusion-controlled rate. It is also known these transient organometallic radicals are more potent oxidants and reductants than their parent dimers. The electrochemical behavior of organometallic radicals, M(CNMe)$\sb3\sp{+\circ}$ (M = Pd and Pt), were examined by photo-modulated voltammetry. The radicals are more powerful oxidants than their parent dimers by about 1V. Hence, metallic films were prepared upon glassy and n-Si ($\Omega$ cm, $\langle 100\rangle$) electrodes. The metallic component within the films was verified by X-ray photoelectron spectroscopy (XPS).