Organotransition-Metal Metallacarboranes. 42. Synthesis and Cluster Fusion of Iron-Centered Tetradecker Sandwiches

Recent work in our laboratory has shown that metal-stacking reactions of Cp*Co(Et₂ C₂ B₃ H₃ x)⁻ anions (X = alkyl, acyl, halogen) with metal halides generate air-stable tetradecker sandwiches of the type [Cp*Co-(Et₂ C₂ B₃ H₂ X)]₂ M in which M is Co, CoH, Ni, Ru, Rh, or Ir. However, attempts to prepare analogous Co-Fe-Co tetradeckers via reactions with FeCh have given only CO₂ C₄ B₆ fused clusters or uncharacterizable products. It has been assumed that Fe-centered tetradeckers are formed in such reactions but undergo oxidative fusion during the usual workup on silica in air. We have confirmed this hypothesis via the synthesis and isolation of iron-centered [Cp*Co(Et₂ C₂ B₃ H2X)]₂ FeH sandwiches (1, X = Cl; 2, X = Me) while avoiding contact with silica in air and demonstrating that both species undergo air oxidation to form respectively the fused products Cp*2⁻ Co₂ (Et₄ C₄ B₆ H₄ X₂) (5, X = Cl; 6, X = Me). Different cage structures are exhibited by 5, a previously prepared and structurally characterized compound, and 6, whose geometry was established in this work by X-ray crystallography. The structural relationships between these cages and those of other known CO₂ C₄ B₆ clusters are examined from mechanistic and skeletal electron-counting viewpoints. Crystal data for 6: Space group P2₁ /c; a = 15.737(5) k,b= 13.575(4) Å, c = 17.212(5) Å, β = 111.13(2)°; Z = 4; R = 0.058 for 3340 independent reflections having I > 3σ(I).