title: Group 6 carbon monoxide-releasing metal complexes with biologically-compatible leaving groups.
abstract: 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)(NH(2)CH{R}CO(2)R')]. 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 NH(2)CH(2)CO(2)Et across the triple bond in [Cr(CO)(5)(═C{OMe}-C≡CPh)] affords crystallographically characterized [Cr(CO)(5)(═C{OMe}-(Z)-CH═C{Ph}NHCH(2)CO(2)Et)]. 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.
authors.affiliation: Department of Chemistry, University of York, Heslington, York YO10 5DD, UK.
authors.forename: Wei-Qiang
authors.initials: WQ
authors.lastname: Zhang
authors.forename: Adrian C
authors.initials: AC
authors.lastname: Whitwood
authors.forename: Ian J S
authors.initials: IJ
authors.lastname: Fairlamb
authors.forename: Jason M
authors.initials: JM
authors.lastname: Lynam
mesh_terms: D002248:Carbon Monoxide / Q000737:chemistry*; D002857:Chromium / Q000737:chemistry*; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D015394:Molecular Structure; D008982:Molybdenum / Q000737:chemistry*; D009211:Myoglobin / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*; D013237:Stereoisomerism; D014414:Tungsten / Q000737:chemistry*
chemical_list: D009211:Myoglobin; D009942:Organometallic Compounds; D002857:Chromium; D002248:Carbon Monoxide; D008982:Molybdenum; D014414:Tungsten