Photochemical and electrochemical reduction of carbon dioxide by nickel clusters

David Alan Morgenstern, Purdue University

Abstract

A triangular nickel cluster, (Ni$\sb3(\mu\sb3$-I)$\sb2(\mu\sb2$-dppm)$\sb3$), 1, (dppm = bis(diphenylphosphino) methane) was prepared in high yield by the conproportionation of Ni(COD)$\sb2$ (COD = 1,5-cyclooctadiene) and nickel iodide in the presence of dppm. 1 is stable to strong reducing agents, but can be cleanly oxidized to a monocation, 1$\sp+$, which has essentially the same structure. Both 1 and 1$\sp+$ were characterized by X-ray diffraction. 1 can be photooxidized in the presence of a variety of acceptors at irradiation wavelengths shorter than 400 nm. Photolysis in methylene chloride leads to the reduction of solvent to chloride ion and chloromethyl radical, which was trapped with CO$\sb2$. Irradiation of a THF solution of 1 saturated with carbon dioxide causes photoinduced electron transfer to occur from 1 to CO$\sb2$, forming the carbon dioxide radical anion, CO$\sb2\sp{\cdot-}$ which disproportionates to carbon monoxide and carbonate. The intermediacy of CO$\sb2\sp{\cdot-}$ was confirmed by trapping the radical with a hydrogen donor (toluene) to produce formate, or with cyclohexene. In the latter case, the result is the double carboxylation of cyclohexene to form cis and trans 1,2-cyclohexanedicarboxylic acid which was confirmed by GC/MS after esterification of the photoproduct. Clusters related to 1 can be formed by displacement of a face-bridging iodide with CO, NO, or isocyanides. The structure of the CO derivative was determined by crystallography. All of these derivatives have highly similar electrochemical and spectroscopic properties reflecting the similarity of their physical and electronic structures. Reaction of two equivalents of 1 with one equivalent of 1,6-diisocyanohexane or 1,4-phenylene diisocyanide produces a linked pair of isocyanide-capped clusters which were characterized by IR and PDMS and FAB mass spectroscopy. The hexane-linked "dimer of trimers" proved to be a selective electrocatalyst for the reduction of CO$\sb2$ to oxalate. The same linkage was used to bind nickel clusters to gold and platinum surfaces.

Degree

Ph.D.

Advisors

Kubiak, Purdue University.

Subject Area

Chemistry|Chemistry|Molecules

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