Reactivity of Platinum−Oxygen Bonds:  Kinetic and Mechanistic Studies of the Carbonylation of Platinum Aryloxide Complexes and the Formation of (Aryloxy)carbonyls

The reactions of [Pt(triphos)(Cl)][Cl] (1) {triphos = bis[2-(diphenylphosphino)ethyl]phenylphosphine} with NaOC6H4-p-R, in the presence of NaPF6, yields the aryloxy complexes [Pt(triphos)(OC6H4-p-R)][PF6] (R = OMe (2a), Me (2b), H (2c), F (2d), Cl (2e)). Upon reaction of 2ae with carbon monoxide at pressures from 10 to 134 psi in acetonitrile the (aryloxy)carbonyl complexes [Pt(triphos)(C(O)OC6H4-p-R)][PF6] (3ae) were obtained. The molecular structure of [Pt(triphos)(C(O)OC6H4-p-Me)][PF6] (3b) was determined by X-ray diffraction. Complex 3b crystallized in the monoclinic space group P21/n (no. 14) with a = 10.797(1) Å , b = 19.927(3) Å, c = 19.113(2) Å, β = 98.07(1)°, V = 4071(2) Å3, and Z = 4. The structure was solved and refined to R = 0.035 and Rw = 0.040 for 3958 reflections with I > 3σ(I). The kinetics of the carbonylation of 2ae to form 3ae were studied by 31P{1H} NMR. Rates of carbonylation exhibit a first order dependence on [CO], but are independent of the concentration of free aryloxide in solution. Rates of aryloxide ligand exchange were also found to be significantly faster than rates of carbonylation. The rates of carbonylation depend on the para-substituent of the aryloxy ligand and follow the order F (2d) > Me (2b) > OMe (2a). These observations are interpreted in terms of a carbonylation mechanism that proceeds via a migratory insertion pathway, rather than by nucleophilic attack at coordinated carbon monoxide by free or dissociated aryloxide.