Nine-vertex metallaborane chemistry. Preparation and characterisation of [1,1,1-(PMe3)2H-isocloso-IrB8H7-8-X], where X = H or Cl

The heating of solutions of [(CO)(PMe3)2H-arachno-IrB8H11Cl] 1a or [(CO)(PMe3)2-nido-IrB8H10Cl] 2 in xylene results in the iridanonaborane cluster compound [1,1,1-(PMe3)2H-isocloso-IrB8H7-8-Cl] 3a in yields of up to 42%. Significantly less stable is the non-chlorinated analogue [1,1,1-(PMe3)2H-isocloso-IrB8H8] 3b, prepared via solutions of [(CO)(PMe3)2H-arachno-IrB8H12] 1b and excess nido-B10H14 in refluxing xylene. The structures of the {IrB8} units of 3a and 3b, characterised by multielement NMR spectroscopy and by single-crystal X-ray diffraction analyses, both approximate closely to an idealised C2v symmetry with the {IrH(PMe3)2} moiety capping the six-membered open faces of arachno/nido-shaped {B8} units. This structural geometry differs significantly from the classical nine-vertex closo D3h tricapped trigonal prismatic (ttp) structure of the [closo-B9H9]2− anion. NMR spectroscopy shows that both 3a and 3b are fluxional with regard to the orientation of the {Ir(H(PMe3)} ligand set relative to the {IrB8} unit. Density functional theory (DFT) calculations have been carried out to help rationalise and extend the conclusions from the experimental observations. The calculations suggest that the isocloso cluster structure for chloro-substituted 3a may be both the kinetically and the thermodynamically favoured one, whereas for unsubstituted 3b a ttp structure would be thermodynamically the more stable and so the observed isocloso structure may well be the favoured kinetic product. However, the differences in these relative stabilities are marginal, and both compounds are unstable in solution relative to a general decomposition, 3b much more so than 3a, so there may be little significance in the relative stabilities of the ttp structures: the calculations suggest that the energy barrier to an isocloso-to-ttp conversion would preclude a decomposition pathway for 3a or 3b via a ttp structure at room temperature.