Synthetic, structural and computational studies on adducts of the 4,1,2-SnC2B10 supraicosahedral stannacarborane

The stannacarborane 1,2-mu-(CH(2))(3)-4,1,2-closo-SnC(2)B(10)H(10) (1) and its adducts with 2,2'-bipyridine (bipy), 1,10-phenanthroline (o-phen) and 4,4'-diphenyl-2,2'-bipyridine (Ph(2)bipy), 1,2-mu-(CH(2))(3)-4-(bipy)-4,1,2-closo-SnC(2)B(10)H(10) (2), 1,2-mu-(CH(2))(3)-4-(o-phen)-4,1,2-closo-SnC(2)B(10)H(10) (3) and 1,2-mu-(CH(2))(3)-4-(Ph(2)bipy)-4,1,2-closo-SnC(2)B(10)H(10) (4), respectively, together with the analogous compound 1,2-mu-{C(6)H(4)(CH(2))(2)}-4-(bipy)-4,1,2-closo-SnC(2)B(10)H(10) (5) have been prepared and characterised. In solution at ambient temperature, compounds 1-5 all display NMR spectra which are interpreted in terms of (time-averaged) C(s) molecular symmetry, but whilst (effectively) C(s) symmetry is retained in the structures of 2-5 in the crystal (i.e. henicosahedral cage structures are observed), 1 has a (C(1)-symmetric) docosahedral structure. A method for quantifying the "percentage docosahedral character" of 13-vertex 1,2-C(2) heteroboranes is described, based on the angles around the C1C2B9B5 quadrilateral. The structures of "carbons adjacent" 1-5 all reveal less slipping of the Sn atom (or {SnL(2)} fragment) across the C(2)B(4) carborane face than has previously been observed in analogous "carbons apart" 4,1,6-closo-SnC(2)B(10) species, a surprising result in the context of previous studies of slipping in icosahedral platinacarboranes. A computational study of "carbons adjacent" and "carbons apart" icosahedral and supraicosahedral platinacarboranes has revealed that the origin of this observation is steric control of the slipping distortion in both "carbons apart" species and in the "carbons adjacent" 13-vertex species, with orbital interactions proving dominant only in the case of the "carbons adjacent" icosahedral compound.