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

Abstract

Among the series of stable <i>closo</i>-borate dianions, [B _<i>n</i> H _<i>n</i> ]^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 ( ^<i>n</i> Bu₄N)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 <i>exo</i>-{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.