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

Abstract

The [S₂B₁₈H₁₉]⁻ anion 1, from syn-B₁₈H₂₂2 with NaH and elemental sulfur, has an unusual arachno-type eleven-vertex {SB₁₀} subcluster that has an open hexagonal pyramidal {B₇} structural feature. This is conjoined, with two boron atoms in common, to a second {SB₁₀} subcluster of conventional nido eleven-vertex geometry. Protonation of 1 forms neutral [S₂B₁₈H₂₀] 4. Subsequent deprotonation of 4 yields the fluxional [S₂B₁₈H₁₉]⁻ anion 5, which is isomeric with 1. Neutral 4 and anion 5 do not have the {B₇} hexagonal pyramidal feature. Neutral 4 consists of conventional nido eleven-vertex {SB₁₀} and arachno ten-vertex {SB₉} subclusters conjoined with a single spiroboron 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 {B₇} hexagonal pyramid is regenerated. DFT B3LYP/6-31G* calculations suggest definitive structures for 4 and 5 that are substantiated by agreement between observed NMR δ(¹¹B) 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 {B₇} 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 δ(¹¹B)_OBS and δ(¹¹B)_CALC for the structurally characterised original anion 1, the only species amongst these to be crystallographically established.