RESUMO
The docosahedral metallacarboranes 4,4-(PMe(2)Ph)2-4,1,6-closo-PtC(2)B(10)H(12), 4,4-(PMe(2)Ph)2-4,1,10-closo-PtC(2)B(10)H(12), and [N(PPh(3))2][4,4-cod-4,1,10-closo-RhC(2)B(10)H(12)] were prepared by reduction/metalation of either 1,2-closo-C(2)B(10)H(12) or 1,12-closo-C(2)B(10)H(12). 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 ML(2)C(2)B(10) species of varying heteroatom patterns (4,1,2-MC(2)B(10), 4,1,6-MC(2)B(10), 4,1,10-MC(2)B(10), and 4,1,12-MC(2)B(10)) reveals clear preferences. In all cases a qualitative understanding of these was afforded by simple MO arguments applied to the model heteroarene complexes [(PH3)2PtC(2)B(4)H(6)]2- and [(PH3)2PtCB(5)H(6)]3-. Moreover, DFT calculations on [(PH3)2PtC(2)B(4)H(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(2)B(10) species, although analogous calculations on [(PH3)2PtCB(5)H(6)]3- did not reproduce the conformation observed in the 4,1,12-MC(2)B(10) metallacarborane. DFT calculations on (PH3)2PtC(2)B(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-(CH2)3-4,4-(PMe3)2-4,1,2-closo-PtC(2)B(10)H(10). This still has a more distorted structure than measured experimentally for 1,2-(CH2)3-4,4-(PMe(2)Ph)2-4,1,2-closo-PtC(2)B(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-(CH2)3-4,4-(PMe(2)Ph)2-4,1,2-closo-PtC(2)B(10)H(10) in solution.
RESUMO
Reduction of the tethered carborane 1,2-(CH2)3-1,2-closo-C2B10H10 followed by treatment with CoCl2/NaCp, [(p-cymene)RuCl2]2(p-cymene=C6H4MeiPr-1,4), (PMe2Ph)2PtCl2 or (dppe)NiCl2(dppe=Ph2PCH2CH2PPh2) affords reasonable yields of the new 13-vertex metallacarboranes 1,2-(CH2)3-4-Cp-4,1,2-closo-CoC2B10H10 (1), 1,2-(CH2)3-4-(p-cymene)-4,1,2-closo-RuC2B10H10 (2), 1,2-(CH2)3-4,4-(PMe2Ph)2-4,1,2-closo-PtC2B10H10 (3) and 1,2-(CH2)3-4,4-(dppe)-4,1,2-closo-NiC2B10H10 (4), respectively. All compounds were characterised spectroscopically and crystallographically. The cobalt and ruthenium species 1 and 2 have Cs symmetry in both solution and the solid state, having henicosahedral cage structures featuring a trapezoidal C1C2B9B5 face. The platinum and nickel compounds 3 and 4 have asymmetric docosahedral cage structures in the crystal (the more so for 4 than for 3) although both appear, by 11B and 31P NMR spectroscopy, to have Cs symmetry in solution. Low-temperature experiments on the more soluble platinacarborane could not freeze out the diamond-trapezium-diamond fluctional process that we assume is operating in solution, and we therefore conclude that this process has a relatively low activation barrier, probably <35 kJ mol-1.
