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(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[H+]. For CH2Ar = CH2C6H5−, kH = 176 ± 3 M−1 s−1 and kD = 185 ± 5 M−1 s−1 at 298.2 K, ΔH⧧ = 46 ± 1 kJ mol−1 and ΔS⧧ = −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 compounds were 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]+ intermediate is strongly stabilized by a Pt···η2-C1−Cipso 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···η2-CH interactions with alkyl groups containing β-hydrogens. An observed consequence of the strong stabilization of “cis”-[Pt(η2-CH2Ar)(PMe3)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(η2-CH2Ar)(PMe3)2]+ cationic species.