852456-80-5Relevant academic research and scientific papers
A dissociative route to C-H bond activation: Ligand cyclometalation in cis-dimethyl[(sulfinyl-κS)bis[methane]][tris(2-methylphenyl)-phosphine] platinum(2+)
Romeo, Raffaello,Plutino, Maria Rosaria,Romeo, Andrea
, p. 507 - 522 (2005)
The dialkyl compound cis-dimethyl[(sulfinyl-κS)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-κS)bis[methane]]platinum(2+) (cis-[Pt(Me) 2(dmso)2]) with tris(2-methylphenyl)phosphane (P(o-tol)3). Restricted rotation around the P-Cipso 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, 1H-NMR spectra of 1 maintain a 'static' pattern upon onset of easy and rapid orrto-platination, leading to [[2-[bis(2- methylphenyl)-phosphino-κP]phenyl]methyl-κC]methyl[(sulfinyl- κS)bis[methane]]platinum(2+) (2), a new C,P-cyclometalated compound of platinum(II), with liberation of methane. The process has been studied by 1H- and 31P{1H}-NMR in CDCl3, and kinetics experiments were performed by conventional spectrophotometric techniques. The first-order rate constants kc 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 k1 has been measured independently over a wide temperature range by both 1H-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 (k1 = 6.5 ± 0.3 s -1) is at least two orders of magnitude higher than that of cyclometalation (kc = 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 o(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.
