PLATINUM(II) DIMETHYL COMPLEXES OF CHELATING POLYPHOSPHINE LIGANDS - P-31 NMR INVESTIGATION OF THEIR DYNAMIC BEHAVIOR

A series of Pt(CH3)2(poiyphosphine) complexes was prepared and studied by P[H] NMR spectroscopy. The di-phosphine ligands [Ph2PCH2CH2P(Ph)neo-C10H10H19, (CH3)2PCH2CH2PPh2, and Ph2PCH2CH,CH2P(Ph)CH2CH2-CH=CH2}, as well as all of the triphosphine ligands {CH3C(CH2PPh2)3, PhP(CH2CH2CH2PPh2)2, PhP(CH2-CH2CH2PCy2)2. PhP(CH2CH2PR2)2 (R = Ph, Me), and Ph2PCH2CH2CH2P(Ph)CH2CH>Ph2} and a tetraphosphine {P(CH2CH2PPh2)3 = PP3} function as bidentate ligands at low temperature. For the complexes containing the triphosphine ligands, one terminal phosphine group remains unbonded, while the Pt(CH3)2(PP3) complex has two terminal groups unbonded at low temperatures in solution and in the solid state. Some of the tri-and tetradentate ligands undergo rapid exchange of the terminal phosphorus atoms. On the basis of line-shape analyses of the p NMR spectra, which show large negative ΔS‡ values, an associative mechanism involving five-coordinate platinum(ll) is proposed to explain the exchange process. Additional support for an associative mechanism is deduced from comparisons between ligands of differing steric requirements and che-late-bite angles. A five-member chelate ring containing two phosphine groups is thermodynamically preferable to a six-member ring in these platinum(ll) compounds, and the cis phosphorus-phosphorus coupling constants vary in a regular and predictable manner for five-and six-member chelate rings. The changes in the phosphorus-phosphorus coupling constants are factored into “through-the-backbone” and “through-the-metal” components. © 1979, American Chemical Society. All rights reserved.