Mechanistic insight into protonolysis and cis-trans isomerization of benzylplatinum(II) complexes assisted by weak ligand-to-metal interactions. A combined kinetic and DFT study

Low-temperature NMR measurements showed that protonolysis and deuterolysis by H(D)X acids on metaand para-substituted dibenzylplatinum(II) complexes cis-[Pt- (CH2Ar)2(PEt3)2] (Ar = C6H4Y-; Y = 4-Me, 1a; 3-Me, 1b; H, 1c; 4-F, 1d; 3-F, 1e; 4-Cl, 1f; 3-Cl, 1g; 3-CF3, 1h) in CD3OD leads directly to the formation of trans-[Pt(CH2Ar)(PEt3)2- (CD3OD)]X (4a-4h) and toluene derivatives. The reaction obeys the rate law kobsd = kH[Hþ]. For CH2Ar = CH2C6H5 -, kH = 176 ( 3 M-1 s-1 and kD = 185 ( 5 M-1 s-1 at 298.2 K, ΔHq = 46 ( 1 kJ mol-1 and ΔSq = -47 ( 1 J K-1 mol-1. In contrast, in acetonitrile-d3, three subsequent stages can be distinguished, at different temperature ranges: (i) instantaneous formation of new benzylhydridoplatinum(IV) complexes cis-[Pt(CH2Ar)2(H)(CD3CN)(PEt3)2]X (2a-2h, at 230 K), (ii) reductive elimination of 2a-2h to yield cis-[Pt(CH2Ar)(CD3CN)(PEt3)2]X (3a-3h) and toluene derivatives (in the range 230-255 K), and finally (iii) spontaneous isomerization of the cis cationic solvento species to the corresponding trans isomers (4a-4h, in the range 260-280 K). All compoundswere detected and fully characterized through their 1H and 31P{1H} NMR spectra. Kinetics monitored by 1H and 31P{1H} NMR and isotopic scrambling experiments on cis-[Pt(CH2Ar)2(H)(CD3CN)(PEt3)2]X gave some insight onto the mechanism of reductive elimination of 2a-2h. Systematic kinetics of isomerization of 3a-3h were followed in the temperature range 285-320 K by stopped-flow techniques. The process goes, as expected, through the relatively slow dissociative loss of the weakly bonded solvent molecule and interconversion of two geometrically distinct T-shaped three-coordinate intermediates. The dissociation energy depends upon the solvent-coordinating ability.DFT optimization reveals that along the energy profile the “cis-like” [Pt(CH2Ar)(PMe3)2]þ intermediate is strongly stabilized by a Pt 3 3 3 η2-C1- Cipso bond between the unsaturatedmetal and benzyl carbons. The value of the ensuing stabilization energy was estimated by computational data to be greater than that found for similar β-agostic Pt 3 3 3 η2-CH interactions with alkyl groups containing β-hydrogens. An observed consequence of the strong stabilization of “cis”-[Pt(η2-CH2Ar)(PMe3)2]þ is the remarkable acceleration of the rate of isomerization, greater than that produced by the so-called “β-hydrogen kinetic effect”. Kinetic and DFT data concur to indicate that electron donation by substituents on the benzyl ring leads to further stabilization of the “cis”-[Pt(η2-CH2Ar)(PMe3)2]þ cationic species.