title: Proton walk in the aqueous platinum complex [TpPtMeCO] via a sticky sigma-methane ligand.
abstract: Both experimental and theoretical evidence suggest that the proton exchange between water and the methyl group in [TpPt(CO)CH(3)] (1, Tp=hydridotripyrazolylborate) involves the formation and deprotonation of a "sticky" sigma-methane ligand. The efficiency of this nontrivial process has been attributed to the spatial orientation of functional groups that operate in concert to activate a water molecule and then achieve a multistep proton walk from water to an uncoordinated pyrazolyl nitrogen atom, to the methyl ligand, and then back to the nitrogen atom and water. The overall proton-exchange process has been proposed to involve an initial attack of water at the CO ligand in 1 with concerted deprotonation by the uncoordinated pyrazolyl nitrogen atom. The pyrazolium proton is then transferred to the Pt--CH(3) bond, leading to a sigma-methane intermediate. Subsequent rotation and deprotonation of the sigma-methane ligand, followed by reformation of 1 and water, result in scrambling of the methyl protons with the hydrogen atoms of water. An alternative two-step process that involves oxidative addition and reductive elimination has also been considered. The two competing mechanistic routes from 1 into [D(3)]-1, as well as the conversion of 1 into [TpPt(CH(3))H(2)] (2), have been examined by density functional theory (DFT) using a variety of exchange-correlation methods, primarily PW6B95, which was recently shown to be highly accurate for evaluating reactions of late-transition-metal complexes. The key role played by the free pyrazolyl nitrogen atom, acting as a proton carrier that abstracts a proton from water and transfers the proton to the Pt--CH(3) bond, is reminiscent of the dual functionality of histidine in the catalytic triad of natural serine proteases.
authors.affiliation: Department of Molecular Biology and The Skaggs Institute for Chemical Biology, The Scripps Research Institute, 10550 N. Torrey Pines Rd., La Jolla, CA 92037, USA.
authors.forename: H Christine
authors.initials: HC
authors.lastname: Lo
authors.forename: Mark A
authors.initials: MA
authors.lastname: Iron
authors.forename: Jan M L
authors.initials: JM
authors.lastname: Martin
authors.forename: Ehud
authors.initials: E
authors.lastname: Keinan
mesh_terms: D001881:Borates / Q000737:chemistry*; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D008697:Methane / Q000031:analogs & derivatives*; D008956:Models, Chemical; D009584:Nitrogen / Q000737:chemistry; D009942:Organometallic Compounds / Q000737:chemistry*; D010984:Platinum / Q000737:chemistry*; D011522:Protons; D011758:Pyrroles / Q000737:chemistry*; D021241:Spectrometry, Mass, Electrospray Ionization; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
chemical_list: D001881:Borates; D008024:Ligands; D009942:Organometallic Compounds; D011522:Protons; D011758:Pyrroles; C419464:hydridotrispyrazolylborate-(CO)2Mo(pyridinyl); D014867:Water; D010984:Platinum; D009584:Nitrogen; D008697:Methane