trans-[Pt(BCat')Me(PCy3)2]: an experimental case study of reductive elimination processes in Pt-Boryls through associative mechanisms |
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| Authors: | Braunschweig Holger Bertermann Rüdiger Brenner Peter Burzler Michael Dewhurst Rian D Radacki Krzysztof Seeler Fabian |
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| Institution: | Institut für Anorganische Chemie, Julius‐Maximilians‐Universit?t Würzburg, Am Hubland, 97074 Würzburg (Germany), Fax: (+49)?931‐31‐84623 |
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| Abstract: | 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 °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 η2‐alkyne platinum complexes, of which Pt(η2‐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 Cat2B2 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) σ‐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) Cat2B2 and Cat′2B2 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 Cat2B2, and evidence for the reversibility of the oxidative addition of Cat2B2 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. |
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| Keywords: | boron platinum reaction mechanisms reduction transition metals |
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