Carbon-Rich Metallacarboranes. 15. Novel Metal-Promoted Cluster Fusion Reactions

In contrast to our recently reported syntheses² of Co-M-Co tetradecker sandwich complexes (M = Co, Ni, Ru) via reactions of double-decker cobaltacarborane anions Cp*Co(Et₂C₂B₃H₃X)⁻ (1⁻, X = H, Me, Cl, Br, acetyl) with metal halides, reactions of the anions having X = H, Me, Et, or Cl with FeCl₂ did not produce the intended Co-Fe-Co tetradeckers. Instead, oxidative fusion of the CoC₂B₃ units occurred, generating tetracarbon metallacarboranes Cp*₂Co2Et₄C₄B₆H₄X₂ (3a–d), which have been characterized via NMR, IR, and mass spectroscopy, supported by X-ray crystallographic analyses of the parent (3a) and dichloro (3d) clusters. These compounds have an open Co₂C₄B₆ 12-vertex cage structure that is a fragment of a 16-vertex T_d closo polyhedron, a geometry that was previously unknown in cluster chemistry and (in these species) is not consistent with the widely applied skeletal-electron counting rules. Moreover, it differs from the Co₂C₄B₆ cage structures of several known cobaltacarboranes.⁴ An attempt to prepare a diiodo Co-Ni-Co tetradecker complex from 1⁻ (X = I) with NiBr₂ gave instead the dimer [Cp*Co(Et₂C₂B₃H₃)]₂ (6), which was shown by X-ray crystallography to have an edge-fused structure in which the two C₂B₃ rings are mutually tilted. A centrosymmetric isomer of this dimer (8) having coplanar C₂B₃ rings with the Cp*Co units on opposite sides of the ring plane was isolated from a reaction of 1⁻ (X = H) with CoCl₂ and Cp₂Co and characterized by X-ray diffraction. Also isolated from this reaction were a dimer of a different type (joined via a single B—B bond), [Cp*Co(Et₂C₂B₃H4)]₂ (9), together with previously reported^2a Co-Co triple-decker and Co-Co-Co tetradecker products. The implications of the formation and structures of the fused and partially fused products found in this work are discussed in terms of multidecker stacking mechanisms and skeletal electron-counting theory.