Synthetic and Mechanistic Studies of Organo-Cobalt Clusters

Abstract

<p> Dicobalt octacarbonyl reacts with a wide variety of molecules containing trichloromethyl functionalities to yield carbynyl tricobalt nonacarbonyl clusters of the general formula R-CCo 3 (CO) 9 . While these clusters have been shown to undergo many reactions, mechanistic studies on these systems are sparse. In particular, their rather facile decarbonylation processes are not well understood. </p> <p> Complexes of the type Ar-CO-CCo 3 (CO)9 readily lose CO to produce the corresponding Ar-CCo 3 (C0) 9 clusters but the origin of the carbon monoxide extruded was not known . Speculation had focussed on two possibilities: firstly, direct elimination of the ketonic carbonyl - perhaps via radical intermediates - and, secondly, via initial loss of a cobalt carbonyl ligand (to produce a 16-electron cobalt vertex) with subsequent migration of the original ketonic group onto the coordinatively unsaturated cobalt center. These two mechanistic possibilities are differentiable by labelling either the cobalt carbonyl sites or the ketonic position with carbon-13 and then examining the decarbonylated product to locate the isotopically enriched positions by 13c NMR spectroscopy. It is shown that the carbon monoxide initially eliminated is a cobalt carbonyl and the ketonic CO migrates from its apical position onto a cobalt atom. </p> <p> A second project involves the synthesis and characterization of a cobalt cluster derived from the reaction of dicobalt octacarbonyl with the insecticide DDT . This cluster contains the bis(4-chlorophenyl)methylcarbynyl capping group which is so bulky that it has the potential to stop the rotation of the carbynyl ligand and also prevent carbonyl exchange on the metal triangle. The DDT-tricobalt cluster was characterized by X-ray crystallography. The compound crystallizes in the space group P21/n: the monoclinic cell has dimensions a = 13.083 A, b = 14.222 A, c = 14.165 A, B = 95.871 degrees and Z = 4. The molecule adopts almost Cs symmetry except that the phenyl rings are twisted slightly so as to destroy the potential mirror plane. At low temperature, the cobalt carbonyl ligands are non-equivalent on the NMR time-scale and the possible causes for this behaviour are discussed. </p>