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 meta- and para-substituted dibenzylplatinum(II) complexes cis-Pt(CH(2)Ar)(2)(PEt(3))(2)] (Ar = C(6)H(4)Y(-); Y = 4-Me, 1a; 3-Me, 1b; H, 1c; 4-F, 1d; 3-F, 1e; 4-Cl, 1f; 3-Cl, 1g; 3-CF(3), 1h) in CD(3)OD leads directly to the formation of trans-Pt(CH(2)Ar)(PEt(3))(2)(CD(3)OD)]X (4a-4h) and toluene derivatives. The reaction obeys the rate law k(obsd) = k(H)H(+)]. For CH(2)Ar = CH(2)C(6)H(5)(-), k(H) = 176 ± 3 M(-1) s(-1) and k(D) = 185 ± 5 M(-1) s(-1) at 298.2 K, ΔH(double dagger) = 46 ± 1 kJ mol(-1) and ΔS(double dagger) = -47 ± 1 J K(-1) mol(-1). In contrast, in acetonitrile-d(3), three subsequent stages can be distinguished, at different temperature ranges: (i) instantaneous formation of new benzylhydridoplatinum(IV) complexes cis-Pt(CH(2)Ar)(2)(H)(CD(3)CN)(PEt(3))(2)]X (2a-2h, at 230 K), (ii) reductive elimination of 2a-2h to yield cis-Pt(CH(2)Ar)(CD(3)CN)(PEt(3))(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 compounds were detected and fully characterized through their (1)H and (31)P{(1)H} NMR spectra. Kinetics monitored by (1)H and (31)P{(1)H} NMR and isotopic scrambling experiments on cis-Pt(CH(2)Ar)(2)(H)(CD(3)CN)(PEt(3))(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(CH(2)Ar)(PMe(3))(2)](+) intermediate is strongly stabilized by a Pt···η(2)-C1-C(ipso) bond between the unsaturated metal 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···η(2)-CH interactions with alkyl groups containing β-hydrogens. An observed consequence of the strong stabilization of "cis"-Pt(η(2)-CH(2)Ar)(PMe(3))(2)](+) 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(η(2)-CH(2)Ar)(PMe(3))(2)](+) cationic species.