Multi-electron reduction of CO2 via Ru&z.sbnd;CO2, &z.sbnd;C(O)OH, &z.sbnd;CO, &z.sbnd;CHO, and &z.sbnd;CH2OH species

A series of [Ru(bpy)2(CO)L]n+ (L=CO2, C(O)OH, CO, CHO, CH2OH, CH3, and C(O)CH3; n=0, 1, 2) were prepared and their molecular structures determined by X-ray analyses. These complexes are reasonable models of reaction intermediates in the multi-electron reduction of CO2 catalyzed by metal complexes, since reductive cleavage of the Ru&z.sbnd;L bonds of the complexes in protic media affords HCOOH, CO, HCHO, CH3OH, and CH4 as two-, four-, six- and eight-electron reduction products of CO2. Thermodynamically, the free energy required in the reduction of CO2 progressively decreases with an increase of the number of electrons participating in the reduction of CO2. The Ru&z.sbnd;L bond character of the series of [Ru(bpy)2(CO)L]n+ was assessed by the ν(Ru&z.sbnd;L) bands and the Ru&z.sbnd;L bond distances from the viewpoint of elucidation of a correlation between free energy changes in the multi-electron reduction of CO2 catalyzed by metal complexes and the metal&z.sbnd;carbon bond strength of each intermediate. The ruthenium&z.sbnd;carbon bond distance of [Ru(bpy)2(CO)L]n+ largely depends on the hybrid orbital of the carbon atom bonded to ruthenium and lengthens in the order Ru&z.sbnd;Csp<Ru&z.sbnd;Csp2<Ru&z.sbnd;Csp3. An unusual shift of the ν(Ru&z.sbnd;L) bands to higher wavenumber with decrease of the Ru&z.sbnd;L bond distances was discussed in terms of σ- and π-character of the ruthenium&z.sbnd;carbon bonds. [Copyright &y& Elsevier]