INPLANE OLEFIN COORDINATION AND UNUSUALLY SMALL SUBSTITUENT DEPENDENCY OF STABILITY IN (ETA-3-ALLYL)(PENTAFLUOROPHENYL)(OLEFIN)-PLATINUM(II) COMPLEXES

The crystal structure of the styrene complex Pt(eta-3-CH2CMeCH2)(CH2=CHC6H5)(C6F5) (2a) was determined. Crystal data: C18H15F5Pt, fw = 521.40, triclinic, space group PBAR1, a = 6.292 (2) angstrom, b = 11.874 (4) angstrom, c = 12.736 (4), alpha = 117.39 (2)-degrees, beta = 82.07 (3)-degrees, gamma = 97.50 (4)-degrees, V = 834.3 (5) angstrom-3, Z = 2, D(c) = 2.075 g cm-3, R = 0.074 for 3410 reflections (\F(o)\ > 3-sigma-(\F(o)\)). The structure thus determined revealed the C = C bond of styrene oriented parallel to the coordination plane (the angle between the C = C axis and the coordination plane is 10.2-degrees). When dissolved in CDCl3, 2a takes two isomeric forms. The H-1 NMR NOE experiments indicated that the configuration of the major isomer in solution is similar to that in the solid state. The minor isomer corresponds to a diastereomer of the major isomer, where both isomers contain the C = C bond lying almost in the coordination plane. The measurements of the epimerization rate of 2a in the absence and presence of free styrene suggested that the interconversion between the two isomers is an intramolecular process without invoking Pt-styrene bond dissociation. Substituted styrene complexes Pt(eta-3-CH2CMeCH2)(CH2=CHC6H4Y)(C6F5) (2b-f) (Y = m-NO2, p-Cl, p-Me, p-OMe, o-Me) were generated by ligand exchange between 2a and appropriate olefin. Relative olefin coordination ability was determined by H-1 NMR analysis of an equilibrium mixture containing 2a, a substituted styrene complex, 2b-f, and the appropriate olefins to show the stability trend unusually weakly dependent on the electronic property of the substituent Y (Hammett rho = -0.38). The nature of the Pt-olefin bond in 2 has been discussed in terms of these stability and structural trends.