Backbone Modified Small Bite-Angle Diphosphines: Synthesis, Structure and Regioselective Thermal Rearrangements of Os3(CO)10{μ-Ph2PCH(Me)PPh2}

Addition of 1,1-bis(diphenylphosphino)ethane, Ph2PCH(Me)PPh2, to Os3(CO)12 in benzene at 60 °C in the presence of Me3NO affords Os3(CO)10{μ-Ph2PCH(Me)PPh2} (1) in almost quantitative yield. Thermolysis of 1 in boiling toluene gives the expected electronically-unsaturated 46-electron triosmium cluster Os3(CO)8{μ3-Ph2PCH(Me)P(Ph)C6H4}(μ-H) (2) as the major product formed from the regioselective activation of a phenyl ring lying anti to the methyl group in 1. A second minor product is Os3(CO)8{μ3-Ph2PC(Me)P(Ph)C6H4}(μ-H)2 (3) resulting from the activation of methine and phenyl protons. This complex contains an unusual semi-bridging tertiary phosphine ligand characterized by short [2.398(1) A] and long [2.872(1) A] osmium–phosphorus interactions which structurally resembles an intermediate during phosphine migration across a metal–metal edge. The extent of the bonding in this potential three-center, two-electron interaction was examined by electronic structure calculations. The Wiberg bond index for the short Os(1)–P(1) bond is as expected but the long Os(3)–P(1) interaction is nearly an order of magnitude smaller, which suggests that there is no significant interaction between the Os(3) and P(1) centers. Prolonged heating of 2 in boiling toluene leads to the formation of Os3(CO)7(η6-C6H5CH3){μ3-PhPCH(Me)P(Ph)C6H4} (4), while in octane Os3(CO)9{μ3-PhPCH(Me)P(Ph)C6H4} (5) is generated. Both contain the 6-electron μ3-PhPCH(Me)P(Ph)C6H4 ligand formed as a result of carbon–phosphorus bond cleavage, while in 4 toluene has displaced three carbonyls. The molecular structures of 1–4 have been determined by single crystal X-ray diffraction analysis.