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.