[(Cp2M)2B9H11] (M = Zr or Hf): early transition metal ‘guarded’ heptaborane with strong covalent and electrostatic bonding

Abstract

Among the series of stable closo-borate dianions, [B_nH_n]²⁻, the X-ray crystallographic structure of [B₇H₇]²⁻ was determined only in 2011. To explore its chemistry and stability, we have isolated and structurally characterized two new transition metal complexes of the heptaborane, [(Cp₂M)₂B₉H₁₁] (Cp = η⁵-C₅H₅; M = Zr or Hf). The structures of [(Cp₂M)₂B₉H₁₁] contain a pentagonal bipyramidal B₇ core, coordinated by two {Cp₂M} and two {BH₂} units equatorially. Structural and spectroscopic characterizations and DFT calculations show that [(Cp₂M)₂B₉H₁₁] complexes are substantially more stable than the parent dianion, in either [B₇H₇]²⁻ or (ⁿBu₄N)₂[B₇H₇]. Our theoretical study and chemical bonding analyses reveal that the surprising stability of the two new heptaborane metal complexes is due to multi-center covalent bonds related to the two exo-{Cp₂M} units, as well as electrostatic interactions between the {Cp₂M} units and the B₇ core. The facile syntheses of the heptaborane metal-complexes will allow further exploration of their chemistry.