CATION-ANION INTERACTION IN THE [NA-KRYPTOFIX-221][W(CO)5O2CH] DERIVATIVE AND ITS RELEVANCE IN CARBON-DIOXIDE REDUCTION PROCESSES
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The structure of [Na-kryptofix-221] [W(CO)5O2CH] has been determined by X-ray crystallography and shown to exhibit a novel strong contact ion pairing between the uncoordinated oxygen of the formate ligand with the cryptand-encapsulated sodium cation. The compound crystallizes in the triclinic space group Pi with cell dimensions a = 10.221 (2) , b = 10.845 (2) , c = 12.779 (2) , a = 87.93 (1), = 89.47 (1), and = 82.99 (1). The most significant structural aspect is that the distance between the formate's distal oxygen atom and sodium (2.388 (9) A) is the shortest NaO bond distance in the cation's coordination sphere. Although this ion pairing persists for uncomplexed sodium cations in tetrahydrofuran solution, the cryptand-encapsulated species exists as a solvent-separated or free-ion pair in solution. Similar solution behavior is observed for the acetatotungsten pentacarbonyl anionic derivative. In addition, the CH3CO2W(CO)5- anion, in the presence of 18-crown-6 complexed Na+, displays a solution structure approximating that of the anion in the existence of the uncomplexed Na+ cation. Consistent with these solution structures, the rate of reaction of carbon dioxide with CH3W(CO)5- to provide CH3CO2W(CO)5- is enhanced by a factor of 10 in the presence of Na+ or Na(18-crown-6)+ over that carried out with the non-interacting PPN+ (bis(triphenylphosphine)iminium cation) or Na-kryptofix-221+. These rate accelerations are attributed to a cation stabilization of the incipient carboxylate during the carbon dioxide insertion process. 1985, American Chemical Society. All rights reserved.
