Group 6 Carbon Monoxide-Releasing Metal Complexes with Biologically-Compatible Leaving Groups

A series of carbon monoxide-releasing molecules (CO-RMs) based on the M(CO)5 framework (M = Cr, Mo, W) is reported. The metal carbonyl anions [MCl(CO)5]− are shown to be highly versatile precursors to Group 6 pentacarbonyl complexes containing amino-ester groups, namely, [M(CO)5(NH2CH{R}CO2R′)]. The structures of five of the complexes, including an enantiomeric pair based on (R) and (S)-alanine, were determined by single crystal X-ray diffraction. These species exhibit rapid CO-release, as shown by a myoglobin-based assay. The rate of release is affected by the nature of both the metal and the amino-ester employed. A mechanistic study shows that a common intermediate is formed corresponding to loss of the amino-ester from the metal. In addition, a further series of potential CO-RMs have been prepared based on Fischer-type carbenes complexes, which contain either amino esters or amino acids. The amino esters and amino acids are introduced into the coordination sphere of the metal by a nucleophilic substitution reaction at the carbene carbon atom. The Michael addition of NH2CH2CO2Et across the triple bond in [Cr(CO)5(C{OMe}CCPh)] affords crystallographically characterized [Cr(CO)5(C{OMe}-(Z)CHC{Ph}NHCH2CO2Et)]. The rate of CO-release from the carbene complexes depends primarily on the specific heteroatom connected to the carbene center. Rapid CO-release is observed in the case of sulfur- and methoxy-stabilized carbenes whereas in the case of amino-substituted carbenes, release is far more sluggish. This may be correlated with the electrophilic character at the carbene carbon atom.