CARBON-DIOXIDE INSERTION PROCESSES INVOLVING METAL-CARBON BONDS - SOLID-STATE AND SOLUTION STRUCTURE OF [NA(18-CROWN-6)][W(CO)5CH3]
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Carbon dioxide insertion reactions into the CH3-W bonds of cis-CH3W(CO)4L- derivatives (L = CO or phosphorus donor ligands) to afford the corresponding acetato complexes have provided mechanistic insight concerning this C-C bond-forming process. When L = PMe3, the insertion reaction was found to occur about 250 times faster than when L = CO. Relevant to these insertion reactions, the structure of [Na(18-crown-6)(THF)2][CH3W(CO)5] has been established by X-ray crystallography, and a comparison is made with the phosphine-substituted derivative. The compound crystallizes in the monoclinic space group C2/c with cell dimensions a = 12.962 (2) ,b~ 12.607 (5) , c = 20.22 (1) , = 91,76 (2),V = 3302.2 3, and Dcalcd= 1.55 g cm-3for Z = 4. The structure consists of an array of the two discrete ionic units at normal van der Waals distances, where the Na+is octacoordinated and the tungsten metal is in an octahedral environment. The W-CH3bond distance at 2.313 (17) is almost identical with that previously noted in the isoelectronic neutral rhenium analogue CH3Re(CO)5and is longer than that found in cis-CH3W(CO)4PMe3-. Although Na(18-crown-6)+is complexed by two tetrahydrofuran molecules in the solid-state structure, these ligands are necessarily labile in THF solution. This is manifested in the observation that Na(18-crown-6)(THF)2+behaves in much the same manner as Na(THF)n+in enhancing the rate of carbon dioxide insertion into the W-CH3bond, a phenomenon ascribed to a neutralization of negative charge buildup on the incipient acetate ligand by the Na+ion. 1987, American Chemical Society. All rights reserved.