KINETICS AND MECHANISM OF BIARYL REDUCTIVE ELIMINATION FROM ELECTRON-POOR [(C6F5)2PCH2CH2P(C6F5)2]PT(AR)2 COMPLEXES

The syntheses and elimination properties of electron-deficient (fluoroaryl)-and (fluoroalkyl)phosphine complexes of platinum are described. Treatment of (cod)PtCl2 With (C6F5)2PCH2CH2P(C6F5)2 (dfppe) Or (C2F5)2PCH2CH2P(C2F5)2 (dfepe) at elevated temperatures gives the dichlorides (dfepe)PtCl2 (1) and (dfppe)PtCl2 (2) in high yield. The diaryls (dfppe)Pt(Ph)2 (3) and (dfppe)Pt(p-tol)2 (4) and the dialkyls (dfppe)Pt(Me)2 (5) and (dfepe)Pt(Me)2 (6) are prepared by metathesis of 1 or 2 with the appropriate Grignard reagent. Thermolysis of the diphenyl complex 3 cleanly yields biphenyl as the sole observed organic elimination product. No organic products derived from ancillary ligand degradation are observed. The fate of the released metal fragment is solvent-dependent, with the bis chelate (dfppe)2Pt (7) being the major product in noncoordinating aromatic solvents. The crystal structure of 7 has been determined. Crossover labeling studies for 3 and the regiospecific formation of 4,4'-dimethylbiphenyl from the thermolysis of 4 confirm that these elimination reactions proceed via an intramolecular 1,1'-coupling process. Elimination kinetics for 3 and 4 are conveniently followed by H-1 NMR spectroscopy. Decomposition rates follow first-order behavior in all solvents examined and are independent of added phosphine concentration. The rate of biphenyl reductive elimination from 3 is found to be inversely proportional to solvent polarity, varying by almost a factor of 10 between benzene (epsilon = 2.28, k = [2.44 (5)] x 10(-4) s-1) and DMSO (epsilon = 45.0, k = [2.50 (2)] X 10(-5) s-1) media at 100-degrees-C. Entropies of activation obtained in benzene (DELTA-S(double dagger) = -1.5 +/- 1 eu) and DMSO (DELTA-S(double dagger) = -1.7 +/- 2 eu) are essentially equivalent and suggest that there is no significant solvent coordination along the elimination reaction coordinate. The mechanistic interpretation of these results and comparisons with prior group 10 elimination studies are discussed.