Monodentate sigma-n and bidentate sigma-n,sigma-n' coordination of 1,1-bis((N-p-tolylimino)diphenylphosphoranyl)ethane, CHCH3(PPh(2)=NC6H4-4-CH3)(2), to platinum(II)

The new ligand, 1,1-bis((N-p-tolylimino)diphenylphosphoranyl)ethane (1,1-BIPE), 1, has been synthesized by means of a Staudinger reaction of 1,1-bis(diphenylphosphino)ethane (1,1-dppe) with 2 equiv of p-tolylazide. Bridge-splitting reactions of Pt2Cl4(PR(3))(2) with 1 readily afforded sigma-N monodentate complexes, [PtCl2(PR(3)){1,1-BIPE-sigma N}] (2a, PR(3) = PEt(3); 2b, PR(3) = PMe(2)Ph). Conversion of 2 into the six-membered platinacycle [PtCl(PR(3)){1,1-BIPE-sigma N,sigma N '}](+)[X](-) (3) (X = Cl, PtCl3(PR(3)), BF4) took place after prolonged stirring, its reaction rate being strongly dependent on the type of phosphine (>5 days for 2a in the presence of NaBF4, 1 h for 2b) and the metal-to-ligand ratio. The compounds 1, 2, and 3 have been fully characterized by H-1, P-31{H-1}, and C-13{H-1} NMR and IR spectroscopy, elemental analysis, or FAB mass spectroscopy. The molecular structures of CHCH3- (PPh(2)=NC6H4-4-CH3)(2) (1) and [PtCl(PMe(2)Ph){(N(pTol)=PPh(2))(2)CHCH3}](+)[Cl](-) (3b) have been determined by X-ray crystallography. Crystal data for 1: space group P2(1)/c with a = 8.9591(5) Angstrom, b = 19.1961(12) Angstrom, c = 21.9740(9) Angstrom, beta = 105.069(4)degrees, V = 3649.1(3) Angstrom(3), and Z = 4. The structure refinement converged to R = 0.080 and R(w) = 0.109. Crystal data for 3b: monoclinic, space group P2(1)/c with a 12.4021(7) Angstrom, b = 16.9705(11) Angstrom, c = 23.760(2) Angstrom, beta = 109.544(5)degrees, V = 4712.7(5) Angstrom(3), and Z = 4. The structure refinement converged to R1 = 0.057, wR2 = 0.122. Variable temperature NMR spectroscopy has revealed that complexes 3 exclusively adopt a twisted boat conformation with the methyl group in equatorial position at low temperature, in agreement with the solid state structure of 3b as determined by X-ray crystallography. Boat-to-boat inversion is assumed to take place at temperatures above 293 K. Furthermore, for 3, hindered rotation of one of the p-tolyl substituents on nitrogen has been established at low temperatures.