Exploring the mechanism of aqueous C-H activation by Pt(II) through model chemistry: Evidence for the intermediacy of alkylhydridoplatinum(IV) and alkane sigma-adducts

The protonolysis mechanisms of several alkylplatinum(II) complexes [(meda)PtMeCl (2) (tmeda = N,N,N',N'-tetramethylethylenediamine), (tmeda)Pt(CH(2)Ph)Cl (5), (tmeda)PtMe(2) (11), and trans-(PEt(3))(2)Pt(CH3)Cl (15)] in CD2Cl2 and CD3OD have been investigated. These reactions model the microscopic reverse of C-H activation by aqueous Pt(II). Each of the four systems (2 in CD3OD, 5 in CD2Cl2, 11 in CD3OD, and 15 in CD3OD) exhibits different behavior in the protonolysis reaction as observed by low-temperature H-1 NMR spectroscopy. Protonolysis of 2 in methanol-d(4) proceeds with no observable intermediates. Reversible reaction between 5 and HCl in CD2Cl2 at -78 degrees C produces (tmeda)Pt(CH(2)Ph)(H)Cl-2 (6), which undergoes reductive elimination of toluene at higher temperatures. Treatment of 11 with HCl in methanol at -78 degrees C generates (tmeda)PtMe(2)(H)Cl (12), which incorporates deuterium from solvent (CD3OD) into the methyl groups prior to reductive elimination of methane. Finally, 15 reacts with H+ in methanol to liberate methane with no intermediates observed. However, hydrogen/deuterium exchange takes place between the solvent (CD3OD) and Pt-Me prior to methane loss. Each of these reactions was evaluated further to determine the kinetics of the reaction, activation parameters, and isotope effects. Based on the results, a common mechanistic sequence is proposed to operate in all the reactions: (1) chloride- or solvent-mediated protonation of Pt(II) to generate an alkylhydridoplatinum(IV) intermediate, (2) dissociation of solvent or chloride to generate a cationic, five-coordinate platinum(IV) species, (3) reductive C-H bond formation producing a platinum(II) alkane sigma-complex, and (4) loss of alkane either through an associative or dissociative substitution pathway. The characteristics of each system differ due to changes in the relative stabilities of the intermediates and/or transition states upon varying the solvent or alkylplatinum species.