Backbone Modified Small Bite-Angle Diphosphines: Synthesis, Structure, Fluxionality and Regioselective Thermally-Induced Transformations of Ru3(CO)10{µ-Ph2PCH(Me)PPh2}

Abstract

The synthesis of Ru3(CO)10{µ-Ph2PCH(Me)PPh2} (1) has been achieved from the radical-catalysed reaction of Ru3(CO)12 with 1,1′-bis(diphenylphosphino)ethane and the fluxionality, protonation and regioselective thermally-induced on-metal transformations of the small bite-angle diphosphine have been studied. Cluster 1 is fluxional in solution and variable temperature 13C{1H} NMR spectroscopy shows that the six carbonyls on the phosphine-bound metal centers interconvert rapidly on the NMR timescale. Protonation of 1 is facile at room temperature and affords the cationic-hydride [Ru3(CO)10{µ-Ph2PCH(Me)PPh2}(μ-H)][BF4] (1H +) which is fluxional, the hydride migrating between bridged and non-bridged ruthenium–ruthenium vectors, location across an unbridged metal–metal bond being thermodynamically favoured. Thermolysis of 1 in heptane affords moderate amounts of the expected benzene-CO elimination product, Ru3(CO)8(µ-CO){µ3-PhPCH(Me)PPh(C6H4)} (2), along with smaller amounts of Ru3(CO)10{μ-PhP(CHMe)(C6H4)PPh} (3) containing a novel doubly-bridged diphosphine ligand. Hydrogenation of 1 in refluxing cyclohexane affords the hydride cluster Ru3(CO)9{μ3-PhPCH(Me)PPh2}(μ-H) (4), the same species also being obtained when 2 was treated with hydrogen under similar conditions. All thermally-induced transformations are regioselective, with only a single isomer being generated. In light of the observed regioselectivity a mechanism is proposed for the formation of 2 from 1 which results from an intermediate in which the methyl-group is held over the triruthenium framework. Cluster Ru3(CO)10{µ-Ph2PCH(Me)PPh2} has been synthesized from the radical-catalysed reaction between Ru3(CO)12 and 1,1′-bis(diphenylphosphino)ethane and the fluxionality, protonation and regioselective thermally-induced on-metal transformations of the small bite-angle diphosphine have been investigated.