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(CH2Ar)(2)(PEl(3))(2)] (Ar = C6H4Y-; Y = 4-Me, 1a; 3-Me, 1b; H, 1c; 4-F, Id; 3-F, le; 4-Cl, if; 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 k(obsd) = k(H)[H+]. For CH2Ar = CH2C6H5-, k(H) = 176 +/- 3 M-1 s(-1) and k(D) = 185 +/- 5 M-1 s(-1) at 298.2 K, Delta H-double dagger = 46 +/- 1 kJ mol(-1) and Delta S-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(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 compounds were detected and fully characterized through their H-1 and P-31{H-1} NMR spectra. Kinetics monitored by H-1 and P-31{11-1) 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)](+) intermediate is strongly stabilized by a Pt center dot center dot center dot eta(2)-Cl-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 beta-agostic Pt center dot center dot center dot eta(2)-CH interactions with alkyl groups containing beta-hydrogens. An observed consequence of the strong stabilization of "cis"-[Pt(eta(2)-CH2Ar)(PMe3)(2)](+) is the remarkable acceleration of the rate of isomerization, greater than that produced by the so-called "beta-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(eta(2)-CH2Ar)(PMe3)(2)](+) cationic species.