Diversity and design of metal-based carbon monoxide-releasing molecules (CO-RMs) in aqueous systems: revealing the essential trends

The CO-releasing ability of a diverse library of primary metal carbonyl complexes has been assessed using a deoxymyoglobin-carbonmonoxymyglobin assay. A wide spectrum of rates for the CO-release process was observed in aqueous systems. For octahedral d(6) complexes, the rate was found to decrease in the sequence FeI2(CO)(4) > [NEt4][V(CO)(6)] > MnBr(CO)(5) > Cr(CO)(6) implying that CO-release is not controlled by the metal-carbon bond strengths. Within the series, [NEt4][MX(CO)(5)] (M = Cr, Mo, W; X = Cl, Br, I), the rate of CO-release was found to decrease down the group (Cr > Mo > W), whilst within the chromium series a similar trend was observed for the halide (Cl > Br > I). The d(4) complexes [NEt4][MI3(CO)(4)] (M = Mo, W) exhibit faster release than their d(6) congeners. A mechanistic investigation into the [NEt4][MX(CO)(5)] series revealed the intermediacy of [{M(CO)(5)}(2)(mu-X)](-) in the CO-release process and that the hydrolysis of the M-X bond, rather than the intrinsic strength of M-CO bonds, controls the rate of CO-release in aqueous systems.