Eleven-vertex polyhedral monocarbaplatinaborane chemistry. Structure and fluxionality of [8,8-(PMe2Ph)2-nido-8,7-PtCB9H11]

Abstract

Reaction between [PtCl₂(PMe₂Ph)₂] and [arachno-6-CB₉H₁₄]^– in CH₂Cl₂ solution at room temperature yields yellow air-stable eleven-vertex [8,8-(PMe₂Ph)₂-nido-8,7-PtCB₉H₁₁]1(46–64%) together with small quantities of ten-vertex [9,9-(PMe₂Ph)₂-arachno-9,6-PtCB₈H₁₂]2(0–3%). Crystals of compound 1 are monoclinic, space group P2₁/n, with a= 1186.8(1), b= 1554.1(2), c= 1344.1(2) pm, β= 95.02(1)° and Z= 4. The structure refined to R,R′= 0.0297, 0.0310 using 3938 data with I > 3σ(I). The cluster structure is of the nido eleven-vertex type, with the platinum and carbon atoms occupying adjacent positions on the open face. There is some geometrical closure from pure nido, so that the gross cluster structure tends towards intermediacy between those of [7,7-(PMe₂Ph)₂-nido-7-PtB₁₀H₁₂]4 and of the closo-structured species [1,1-(PMe₂Ph)₂-1,2,3-PtC₂B₈H₁₀]3. The NMR properties of 1 are reported and assessed for intermediacy between those of 3 and 4, and the analytical utility of NMR spectroscopy in assessing the metal-to-cluster bonding is developed by comparison with non-metallated species. The metal-to-carborane bonding sphere of compound 1 is fluxional, with ΔG^‡₃₁₃ca. 62 kJ mol^–1. The fluxional process involves an interconversion of enantiomers with the platinum centre and the bridging hydrogen atom migrating around the open face of the {CB₉H₁₀} cage while the {(PMe₂Ph)₂}exo polyhedral ligand sphere undergoes a half-twist.