Synthesis and characterization of cationic dinuclear complexes of platinum with bridging hydrides; crystal structures of [Pt2(µ-H)2{But2P(CH2)3PBut2}2][BF4]2 and [Pt2(µ-H)2{(C6H11)2P(CH2)3P(C6H11)2}2][BF4]2

Abstract

A series of diplatinum dications [Pt₂(µ-H)₂(L–L)₂][BF₄]₂[L–L =(C₆H₁₁)₂P(CH₂)_nP(C₆H₁₁)₂, Bu^t₂P(CH₂)_nPBu^t₂, n= 2 or 3] with two chelating diphosphine and two bridging hydride ligands has been prepared by the elimination of ethene from the agostic alkyl complexes [PtEt(L–L)]⁺ or alkene–hydride complexes [PtH(C₂H₄)(L–L)]⁺, or by the reaction of the dihydride complexes [PtH₂(L–L)] with an excess of HBF₄·OMe₂. The complexes have been characterized by multinuclear (¹H, ³¹P and ¹⁹⁵Pt) NMR spectroscopy and for [Pt₂(µ-H)₂{Bu^t₂P(CH₂)₃PBu^t₂}₂][BF₄]₂ and [Pt₂(µ-H)₂{(C₆H₁₁)₂P(CH₂)₃P(C₆H₁₁)₂}₂][BF₄]₂ by single-crystal X-ray crystallography. The latter complex has a structure in which the platinum and phosphorus atoms are coplanar whereas in the former the co-ordination planes of the platinum atoms are twisted with respect to each other by 36.6°. The twisting in [Pt₂(µ-H)₂{Bu^t₂P(CH₂)₃PBu^t₂}₂][BF₄]₂ is ascribed to the steric pressure of the large diphosphine which destabilises the planar geometry. Significantly this complex is fluxional on the NMR time-scale at 290 K whereas the others are static and there is a shift in colour from yellow to red for the strained complex. The dinuclear species are useful synthetic precursors of the [PtH(L–L)]⁺ fragment, particularly when L–L is large. Thus [Pt₂(µ-H)₂{Bu^t₂P(CH₂)₃PBu^t₂}₂]²⁺, which has the most sterically demanding diphosphine, reacts with alkenes [e.g. ethene (reversibly) or norbornene] to form mononuclear alkyl complexes with a three-centre, two-electron (agostic) bond.