title: The conformations of 13-vertex ML2C2B10 metallacarboranes: experimental and computational studies.
abstract: 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.
authors.affiliation: School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, U.K.
authors.forename: Kelly J
authors.initials: KJ
authors.lastname: Dalby
authors.forename: David
authors.initials: D
authors.lastname: Ellis
authors.forename: Stefan
authors.initials: S
authors.lastname: Erhardt
authors.forename: Ruaraidh D
authors.initials: RD
authors.lastname: McIntosh
authors.forename: Stuart A
authors.initials: SA
authors.lastname: Macgregor
authors.forename: Karen
authors.initials: K
authors.lastname: Rae
authors.forename: Georgina M
authors.initials: GM
authors.lastname: Rosair
authors.forename: Volker
authors.initials: V
authors.lastname: Settels
authors.forename: Alan J
authors.initials: AJ
authors.lastname: Welch
authors.forename: Bruce E
authors.initials: BE
authors.lastname: Hodson
authors.forename: Thomas D
authors.initials: TD
authors.lastname: McGrath
authors.forename: F Gordon A
authors.initials: FG
authors.lastname: Stone