Terminal platinum(II) phosphido complexes: Synthesis, structure, and thermochemistry

A series of terminal Pt(II) phosphido complexes Pt(dppe)(Me)(PRR') (R = H; R' = Mes* (1), R' = Mes (2), R' = Ph (3), R' = Cy (4); R = R' = Mes (5); R = R' = Ph (6); R = R' = Cy (7); R = R' = Et (8); R = Ph, R' = i-Bu (9)) has been prepared by proton transfer from the appropriate phosphine to the methoxide ligand of Pt(dppe)(Me)(OMe) (10) (dppe = Ph2PCH2-CH2PPh2; Mes* = 2,4,6-(t-Bu)(3)C6H2; Mes = 2,4,6-Me3C6H2; Cy = cyclo-C6H11). Complexes 1 and 2 were also made by deprotonation of the cations [Pt(dppe)(Me)(PH2Ar)][BF4] (Ar = Mes* (13); Ar = Mes (14)). For comparison to 1, the arylthiolate and aryloxide complexes Pt(dppe)(Me)(EMes*) (E = S (11); E = O (12)) were also prepared from 10. NMR studies of the proton-transfer equilibria between Pt(dppe)(Me)(X), Pt(dppe)(Me)(Y), and the acids HY and HX (see Bryndza, H. E.; Fong, L. K.; Paciello, R. A.; Tam, W.; Bercaw, J. E. J. Am. Chem. Sec. 1987, 109, 1444-1456 and Bryndza, H. E.; Domaille, P. J.; Tam, W.; Fong, L. K.; Paciello, R. A.; Bercaw, J. E. Polyhedron 1988, 7, 1441-1452) provide an approximate partial ranking of Pt-P bond strengths in this series: Pt-PHPh > Pt-PHMes > Pt-PHMes*; Pt-PPh2 > Pt-PMes(2). Complementary solution calorimetry investigations probe the role of entropic effects on the equilibria. Both steric and electronic factors appear to be important in controlling relative Pt-P bond strengths. The Pt-S bonds in 11 and Pt(dppe)(Me)(SPh) are stronger than the analogous Pt-P bonds in 1 and 3. Complexes 1 and 5.THF were structurally characterized by X-ray crystallography.