The Conformations of 13-Vertex ML_2C_2B_(10) Metallacarboranes: Experimental and Computational Studies

摘要

The docosahedral metallacarboranes 4,4-(PMe_2Ph)_2-4,1,6-closo-PtC_2B_(10)H_(12), 4,4-(PMe_2Ph)_2-4,1,10-closo-PtC_2B_(10)H_(12), and [N(PPh_3)_2][4,4-cod-4,1,10-closo-RhC_2B_(10)H_(12)] were prepared by reduction/ metalation of either 1,2-closo-C_2B_(10)H_(12) or 1,12-closo-C_2B_(10)H_(12). All three species were fully characterized, with a particular point of interest of the latter being the conformation of the {ML_2} fragment relative to the carborane ligand face. Comparison with conformations previously established for six other ML_2C_2B_(10) species of varying heteroatom patterns (4,1,2-MC_2B_(10), 4,1,6-MC_2B_(10), 4,1,10-MC_2B_(10), and 4,1,12-MC_2B_(10)) reveals clear preferences. In all cases a qualitative understanding of these was afforded by simple MO arguments applied to the model heteroarene complexes [(PH_3)_2PtC_2B_4H_6]~(2-) and [(PH_3)_2PtCB_5H_6]~(3-). Moreover, DFT calculations on [(PH_3)_2PtC_2B_4H_6]~(2-) in its various isomeric forms approximately reproduced the observed conformations in the 4,1,2-, 4,1,6-, and 4,1,10-MC_2B_(10) species, although analogous calculations on [(PH_3)_2PtCB_5H_6]~(3-) did not reproduce the conformation observed in the 4,1,12-MC_2B_(10) metallacarborane. DFT calculations on (PH_3)_2PtC_2B_(10)H_(12) 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-(CH_2)_3-4,4-(PMe_3)_2-4,1,2-closo-PtC_2B_(10)H_(10). This still has a more distorted structure than measured experimentally for 1,2-(CH_2)_3-4,4-(PMe_2Ph)2-4,1,2-closo-PtC_2B_(10)H_(10), 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-(CH_2)_3-4,4-(PMe_2Ph)_2-4,1,2-closo-PtC_2B_(10)H_(10) in solution.