The Conformations of 13-Vertex ML₂C₂B₁₀ Metallacarboranes:  Experimental and Computational Studies

The docosahedral metallacarboranes 4,4-(PMe₂Ph)₂-4,1,6-closo-PtC₂B₁₀H₁₂, 4,4-(PMe₂Ph)₂-4,1,10-closo-PtC₂B₁₀H₁₂, and [N(PPh₃)₂][4,4-cod-4,1,10-closo-RhC₂B₁₀H₁₂] were prepared by reduction/metalation of either 1,2-closo-C₂B₁₀H₁₂ or 1,12-closo-C₂B₁₀H₁₂. All three species were fully characterized, with a particular point of interest of the latter being the conformation of the {ML₂} fragment relative to the carborane ligand face. Comparison with conformations previously established for six other ML₂C₂B₁₀ species of varying heteroatom patterns (4,1,2-MC₂B₁₀, 4,1,6-MC₂B₁₀, 4,1,10-MC₂B₁₀, and 4,1,12-MC₂B₁₀) reveals clear preferences. In all cases a qualitative understanding of these was afforded by simple MO arguments applied to the model heteroarene complexes [(PH₃)₂PtC₂B₄H₆]^2- and [(PH₃)₂PtCB₅H₆]^3-. Moreover, DFT calculations on [(PH₃)₂PtC₂B₄H₆]^2- in its various isomeric forms approximately reproduced the observed conformations in the 4,1,2-, 4,1,6-, and 4,1,10-MC₂B₁₀ species, although analogous calculations on [(PH₃)₂PtCB₅H₆]^3- did not reproduce the conformation observed in the 4,1,12-MC₂B₁₀ metallacarborane. DFT calculations on (PH₃)₂PtC₂B₁₀H₁₂ yielded good agreement with experimental conformations in all four isomeric cases. Apparent discrepancies between observed and computed Pt−C distances were probed by further refinement of the 4,1,2- model to 1,2-(CH₂)₃-4,4-(PMe₃)₂-4,1,2-closo-PtC₂B₁₀H₁₀. This still has a more distorted structure than measured experimentally for 1,2-(CH₂)₃-4,4-(PMe₂Ph)₂-4,1,2-closo-PtC₂B₁₀H₁₀, but the structural differences lie on a very shallow potential energy surface. For the model compound a henicosahedral transition state was located 8.3 kcal mol^-1 above the ground-state structure, consistent with the fluxionality of 1,2-(CH₂)₃-4,4-(PMe₂Ph)₂-4,1,2-closo-PtC₂B₁₀H₁₀ in solution.