Reductive activation of carbon dioxide by anionic group 6b alkyl metal carbonyl complexes
Description
The reductive activation of carbon dioxide by group 6b metal alkyl and aryl carbonylates to produce metallocarboxylates is demonstrated. The reaction was established to be first order in metal complex and first order in carbon dioxide. One of the metal alkyl anions, cis-CH(,3)W(CO)(,4)PMe(,3)('-), which had not been reported previously was characterized by X-ray crystallography, as were some of the carboxylate products. The activation parameters for the reaction of cis-CH(,3)W(CO)(,4)P(OMe)(,3)('-) with carbon dioxide were determined and found to suggest a concerted I(,a) mechanism, wherein the (pi)-acid character of CO(,2) promotes binding to the nucleophilic metal center. Consistent with this proposal, significant rate enhancements were observed as the metal alkyl complex was made more basic, either by substitution of CO with phosphine or phosphite ligands or by replacement of Cr with W Rates of labilization of CO cis to X in XM(CO)(,5)('-) (M = Cr, Mo, W) (X = O(,2)CCH(,3), OCHO, SH, Cl, Br, I) were measured. Cis-labilization ability decreases in the following order: O(,2)CCH(,3)('-) (TURN) OCHO('-) (TURN) SH('-) > Cl('-) > Br('-) > I('-) The stereoselectivity of the dissociative process involving the reaction of trans-Cr(CO)(,4)(PPh(,3)) with ('13)C-labeled CO to afford Cr(CO)(,5)PPh(,3) was examined. The data presented are consistent with a completely stereoselective process where the incoming ('13)CO ligand occupies a site cis to the PPh(,3) ligand