Homogeneous Carbon Monoxide Reduction – Catalyst Design Strategies

A new approach to the activation of carbon monoxide toward homogeneous reduction by hydrogen is presented. Attack at transition metal bound carbon monoxide by nucleophiles generates anionic carbene complexes; hydrogenation of such complexes should give rise to products derived from reduced carbon monoxide. Model stoichiometric reactions using lithium dimethylamide as the activating nucleophile and various binary transition metal carbonyls have been found to give rise to varying amounts of methanol, dimethylformamide (DMF), and trimethylamine, products derived from reduction of carbon monoxide (as confirmed by carbon-13 labeling), under exceptionally mild conditions [35 psi H2, 130 °C, tetrahydrofuran or hexamethylphosphoric triamide (HMPA) solvent]. In the Cr(CO)6/LiNMe2 system in HMPA, trimethylamine appears to be formed by hydrogenation of the primary product DMF by a chromium carbonyl hydride anion. In support of this, the pentacarbonyl chromium hydride anion, (CO)5CrH-, has been found to quantitatively reduce N,N-dialkylamides to the corresponding amines under the above hydrogenation conditions. Nucleophiles weaker than LiNMe2 do not give rise to the desired activation of carbon monoxide for homogeneous reduction. Attempts to drive unfavorable equilibria through the use of bifunctional nucleophiles are reported. Intramolecular delivery of weaker nucleophiles has also been investigated in a series of functionalized arene chromium tricarbonyl complexes; no sign of the desired nucleophilic attack at carbon monoxide is obtained, though interesting reductive degradations and couplings of arenes are observed. The electrochemical oxidation of a series of arene chromium and arene tungsten tricarbonyl complexes has been examined. Studies regarding the oxidatively-promoted decomposition of the chromium complexes are reported. Extension of these studies to the tungsten analogs has led to the observation of metal coordination sphere expansion upon one-electron oxidation, an observation of fundamental significance with regards to recent reports of dramatic reaction rate enhancements in odd-electron organometallic systems. The relationship of coordination sphere expansion to these and other problems of current mechanistic organometallic chemical interest, including the oxidative instability of arene chromium tricarbonyl complexes, is discussed.