Macropolyhedral boron-containing cluster chemistry. The reversible disassembly and reassembly of the hexagonal pyramidal {B7} feature in the [S2B18H19]- anion

The S(2)B(18)H(19)](-) anion 1, from syn-B(18)H(22) 2 with NaH and elemental sulfur, has an unusual arachno-type eleven-vertex {SB(10)} subcluster that has an open hexagonal pyramidal {B(7)} structural feature. This is conjoined, with two boron atoms in common, to a second {SB(10)} subcluster of conventional nido eleven-vertex geometry. Protonation of 1 forms neutral S(2)B(18)H(20)] 4. Subsequent deprotonation of 4 yields the fluxional S(2)B(18)H(19)](-) anion 5, which is isomeric with 1. Neutral 4and anion 5 do not have the {B(7)} hexagonal pyramidal feature. Neutral 4 consists of conventional nido eleven-vertex {SB(10)} and arachno ten-vertex {SB(9)} subclusters conjoined with a single spiro boron atom in common. Anion 5 is closely related to 4, but with an additional inter-boron intercluster link. Anion 5 spontaneously reverts to anion 1 over a few hours at room temperature, remarkable in that the open {B(7)} hexagonal pyramid is regenerated. DFT B3LYP/6-31G* calculations suggest definitive structures for 4 and 5 that are substantiated by agreement between observed NMR delta((11)B) values and boron nuclear shielding as calculated by the GIAO approach on the DFT-calculated structures. Extension of this approach additionally defines transition states and intermediates for the fluxionality of 5, and also for the reassembly of the starting anion 1, together with its {B(7)} feature, from fluxional 5. The fluxionality of 5 involves the inter-subcluster transfer of a {BH} unit. The reassembly of 1 from 5 involves a DSD rearrangement and two successive hydrogen-atom hops. Confidence in the application of this method to these large macropolyhedral assemblies is afforded in the first instance by good agreement between delta((11)B)(OBS) and delta((11)B)(CALC) for the structurally characterised original anion 1, the only species amongst these to be crystallographically established.