Synthesis, molecular structures, and fluxional behavior of dppm-bridged complexes of platinum(II) with linear gold(I), trigonal silver(I), or tetrahedral mercury(II) centers

The complexes [PtR(dppm-P,P)(dppm-P)]PF6 (R = Me, Et, Ph) react with [AuCl(SMe(2))] to generate the dppm-bridged platinum-gold complexes trans-[Pt,ClR(mu-dppm)(2)Au]PF6 (1). Addition of [PtR(dppm-P,P)(dppm-P)]Cl to a solution of silver(I) acetate, followed by acetyl chloride, yields the neutral platinum-silver species trans-[PtClR(mu-dppm)(2)AgCl] (2). Similarly, addition of [PtR(dppm-P,P)(dppm-P)]Cl to a solution of mercury(II) acetate, followed by acetyl chloride, yields the neutral platinum-mercury species trans-[PtClR(mu-dppm)(2)HgCl2] (3). Each of the complexes has been characterized by H-1 and P-31{H-1} NMR spectroscopy and by elemental analysis. The crystal structure of 1 as its methyl derivative 1a (CHCl3 solvate) has been determined by X-ray diffraction. The compound exhibits approximate square-planar geometry at platinum and linear geometry at gold. The platinum-gold internuclear distance is 3.008(1) Angstrom. The methyl complex 2a crystallizes as its CHCl3 solvate. The geometry at platinum is distorted square planar, and the Pt ... Ag distance is 3.049(1) Angstrom. The geometry at silver is approximately trigonal planar. The pyramidal distortion taken together with a long Ag-Cl bond (2.610(4) Angstrom) is discussed in the context of C-H ... Cl hydrogen bonding and a possible incipient Pt ... Ag interaction. The structure of 3a was also determined by X-ray diffraction. The geometry at platinum is distorted square planar, and the Pt ... Hg distance is 3.302(1) Angstrom. The geometry at mercury can be described as pyramidally distorted tetrahedral. However, taken together with an unusually long axial Hg-Cl distance (2.715(4) Angstrom), this distortion may be viewed in terms of an incipient S(N)2 reaction at the Hg center that invokes a weak Pt ... Hg interaction. The complexes have static structures at ambient temperature, as evidenced by the nonequivalence of the methylene hydrogens of the dppm ligands, but they exhibit fluxional behavior at higher temperatures. Free energies of activation in the range 13-19 kcal/mol have been determined, being higher for the Pt-Au species, and a mechanism involving reversible dissociation of chloride is proposed to account for the observed behavior.