trans-[Pt(BCat′)Me(PCy3)2]: An Experimental Case Study of Reductive Elimination Processes in Pt-Boryls through Associative Mechanisms

A stable trans-(alkyl) (boryl) platinum complex trans-[Pt(BCat') Me(PCy3)(2)] (Cat'= Cat-4-tBu; Cy= cyclohexyl=C6H11) was synthesised by salt metathesis reaction of trans-[Pt( BCat')Br(PCy3)(2)] with LiMe and was fully characterised. Investigation of the reactivity of the title compound showed complete reductive elimination of Cat'BMe at 80 degrees C within four weeks. This process may be accelerated by the addition of a variety of alkynes, thereby leading to the formation of the corresponding h 2-alkyne platinum complexes, of which [Pt(eta(2)-MeCCMe)-(PCy3)(2)] was characterised by X-ray crystallography. Conversion of the trans-configured title compound to a cis derivative remained unsuccessful due to an instantaneous reductive elimination process during the reaction with chelating phosphines. Treatment of trans-[Pt(BCat') Me(PCy3)(2)] with Cat(2)B(2) led to the formation of CatBMe and Cat'BMe. In the course of further investigations into this reaction, indications for two indistinguishable reaction mechanisms were found: 1) associative formation of a six-coordinate platinum centre prior to reductive elimination and 2) s-bond metathesis of B-B and C-Pt bonds. Mechanism 1 provides a straightforward explanation for the formation of both methylboranes. Scrambling of diboranes(4) Cat(2)B(2) and Cat'B-2(2) in the presence of [Pt(PCy3)(2)], fully reductive elimination of CatBMe or Cat'BMe from trans-[Pt(BCat') Me(PCy3)(2)] in the presence of sub-stoichiometric amounts of Cat(2)B(2), and evidence for the reversibility of the oxidative addition of Cat(2)B(2) to [Pt(PCy3)(2)] all support mechanism 2, which consists of sequential equilibria reactions. Furthermore, the solid-state molecular structure of cis-[Pt(BCat)(2)(PCy3)(2)] and cis-[Pt(BCat')(2)(PCy3)(2)] were investigated. The remarkably short B-B separations in both bis(boryl) complexes suggest that the two boryl ligands in each case are more loosely bound to the PtII centre than in related bis(boryl) species.