Synthesis and Structures of Platinum A-Frame Complexes

Reactions of [PtCl2(cod)] with the appropriate Grignard reagents produce [PtR2(cod)] (R
= C6H5, C6H4CH3-4, C6H4CH3-2, CH2C6H5), which, on treatment with 1 mol equiv of HCl,
yield the corresponding chloroplatinum complexes [PtClR(cod)]. The 2-tolyl compounds exhibit
hindered rotation about the Pt−C bonds at ambient temperature, the barrier to rotation
being greater in [PtCl(C6H4CH3-2)(cod)] than in the ditolyl derivative. The chloroplatinum
compounds react with 1 mol equiv of dppm to give [PtClR(dppm)], which are in equilibrium
with the A-frame complexes [Pt2R2(μ-Cl)(μ-dppm)2]Cl. The extent of dimerization depends
on the nature of R, but in each case the A-frame complex could be obtained quantitatively
by treatment of the solution with NH4PF6 or TlPF6. The structures of [Pt2R2(μ-Cl)(μ-dppm)2]PF6 (R = CH2C6H5, C6H4CH3-4) were determined by X-ray crystallography. In the benzyl
derivative, one of the ortho hydrogens on each phenyl (benzyl) ring points towards the
centroid of a dppm phenyl ring, and this may account for the low-frequncy signal associated
with the ortho hydrogens in solution. The chloride-bridged A-frames could be converted to
the corresponding hydride-bridged derivatives, [Pt2R2(μ-H)(μ-dppm)2]PF6 (R = C6H5, C6H4CH3-4, CH2C6H5), by treatment with NaBH4.