A dissociative route to C-H bond activation: Ligand cyclometalation in cis-dimethyl[(sulfinyl-kappa S)bis [methane]][tris(2-methylphenyl)phosphine]platinum(2+)

The dialkyl compound cis-dimethyl[(sulfinyl-kappa S)bis[methane]][tris(2-methylphenyl)phosphine]platinum(2+) (cis-[Pt(Me)(2)(dmso)(P(o-tol)(3)]; 1) has been isolated from the reaction of cis-dimethylbis[(sulfinyl-kappa S)bis[methane]]platinum(2+) (cis-[ Pt(Me)(2)(dmso)(2)]) with tris(2-methylphenyl)phosphane (P(o-tol)(3)). Restricted rotation around the P-C-ipso bonds of the phosphane ligand generates two different conformers, 1a and 1b, in rapid exchange in non-polar solvents at low temperature. Strong through-space contacts between the ortho-Me substituent groups on the ligand and the cis-Me groups in the coordination plane were determined, which proved useful for identifying the atropisomers formed. At room temperature, H-1-NMR spectra of 1 maintain a 'static' pattern upon onset of easy and rapid ortho-platination, leading to [[2-[bis(2-methylphenyl)-phosphino-kappa P]phenyl]methyl-kappa C]methyl[(sulfinyl-kappa S)bis[methane]]platinum(2+) (2), a new C,P-cyclometalated compound of platinum(II), with liberation of methane. The process has been studied by H-1- and P-31{H-1}-NMR in CDCl3, and kinetics experiments were performed by conventional spectrophotometric techniques. The first-order rate constants k(c) decrease with the addition of dimethyl sulfoxide until the process is blocked by the presence of a sufficient excess of sulfoxide. This behavior reveals a mechanism initiated by ligand dissociation and formation of a three-coordinate species. The value of the rate constant for dimethyl sulfoxide dissociation k(1) has been measured independently over a wide temperature range by both H-1-NMR ligand exchange (isotopic labeling experiments) and ligand substitution (stopped-flow pyridine for dimethyl sulfoxide substitution). The rates of the two processes are in reasonable agreement at the same temperature, and a single Eyring plot can be constructed with the two sets of kinetics data. However, the value of the derived dissociation constant at 308 K (k(1) = 6.5 +/- 0.3 s(-1)) is at least two orders of magnitude higher than that of cyclometalation (k(c) = 0.0098 +/- 0.0009 s(-1) at 308 K). Clearly, the dissociation step is not rate-determining for cyclometalation. A multistep mechanism consistent with mass-law retardation is derived, which involves a pre-equilibrium that controls the concentration of an unsaturated three-coordinate, 14-electron T-shaped cis- [PtMe2{P(o-tol)(3)}] intermediate. Cyclometalation is initiated in this latter by an agostic interaction with the sigma(C-H) orbital of a methyl group. Oxidative addition of the C-H bond follows, yielding a cyclometalated-hydrido 16-electron Pt(IV) five-coordinate intermediate. Finally, reductive elimination and re-entry of dimethyl sulfoxide with liberation of methane should yield the cyclometalated species 2.