Substitution of chloride in [PtCl2(dppe)] by the chiral anionic ligand [Mo(CO)5(PPhH)]-: Synthesis and X-ray structure of [Pt(μ-PPhH)2{Mo(CO)5}2(dppe)] and a 31P{1H} NMR analysis of its isomeric products and fluxionality

Article abstract of DOI:10.1016/0277-5387(95)00321-5

Reaction of the chiral anionic ligand [Mo(CO)₅(PPhH)]^- with [PtCl₂(L-L)] affords the neutral trimetallic monophosphido bridged complexes [Pt(μ-PPhH)₂{Mo-(CO)₅}₂(L-L)] (L-L = dppe, Ph₂PCH₂CH₂PPh₂, 2; dpae, Ph₂AsCH₂CH₂AsPh₂, 3; dppe′, cis-Ph₂PCH=CHPPh₂, 4). Compounds 2-4 are the first examples of heterometallic complexes that contain two chiral primary phosphido bridges existing as pairs of diastereoisomers. [Pt(μ-PPhH)₂{Mo(CO)₅}₂(dppe)] (2) has been characterized spectroscopically and by a single-crystal X-ray analysis. The molecular structure of 2 can be considered either as Mo₂Pt phosphido-bridged trimer or a square-planar Pt^II complex bonded to one chelating dppe and two anionic phosphine ligands. The latter PtP₄ description successfully accounts for the line broadening observed in the ³¹P{¹H} NMR spectra of 2-4, which can be understood in terms of the existence of several interconverting rotameric forms arising from restricted rotation about the Pt-phosphido bond. The Pt - P - Mo angles of 122.1(1) and 123.5(1)° are some of the largest ever to be reported for phosphido-bridged heterometallics, reflecting the long Pt - Mo separations [4.349(2) and 4.320(2) A] and the electronic and structural flexibility of the phosphido bridge. The PtMo compound [Pt(μ-PPhH)₂{Mo-(CO)4}(dppe)] (5), comprised of one neutral and one dianionic chelate ligand, was prepared from [Li]₂[Mo(CO)₅(PPhH)₂] and [PtCl₂(dppe)] and was shown to contain a conformationally rigid PtP₄ structure with characteristically sharp ³¹P resonances. The ³¹P{¹H} NMR spectrum of 5 is rich with information analysing as two overlapping AA′XX′ spin systems together with their AA′XX′M (M = ¹⁹⁵Pt) counterparts. The values of ²J(P - P) and ¹J(¹⁹⁵Pt - P) are significantly lower for PPhH{Mo(CO)₅}^- than dppe, a difference that cannot be accounted for solely on the basis of bond length variations. Meso-[Pt(μ-PPhH)₂{Mo(CO)₅}₂(dppe)] (2) is thermodynamically unstable, undergoing a rapid intramolecular elimination of [Mo(CO)₆] to generate [Pt(μ-PPhH)₂{Mo(CO)₄}(dppe)] (5), while its racemic diastereoisomer, that which was observed in the crystal, decomposes to unidentified phosphorus-containing products.