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Proton Walk in the Aqueous Platinum Complex [TpPtMeCO] via a Sticky σ‐Methane Ligand
Author(s) -
Lo H. Christine,
Iron Mark A.,
Martin Jan M. L.,
Keinan Ehud
Publication year - 2007
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200600709
Subject(s) - deprotonation , chemistry , ligand (biochemistry) , proton , density functional theory , hydrogen bond , molecule , photochemistry , computational chemistry , organic chemistry , ion , biochemistry , physics , receptor , quantum mechanics
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” σ‐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 PtCH 3 bond, leading to a σ‐methane intermediate. Subsequent rotation and deprotonation of the σ‐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 PtCH 3 bond, is reminiscent of the dual functionality of histidine in the catalytic triad of natural serine proteases.