Deltahedral ferratricarbaboranes: analogues of ferrocene

Abstract

The neutral ferratricarbaboranes (η⁵-RC₃B₈H₁₀)Fe(η⁵-C₅H₅) (R = aryl), analogues of ferrocene, have recently (2013) been synthesized by Štíbr and co-workers and structurally characterized by X-ray crystallography. We have now used density functional theory to study the structures of the complete series of related ferratricarbaboranes CpFeC₃B_n−4H_n−1 (n = 8 to 12). The lowest energy structures are found to have the maximum number of carbon atoms at degree 4 rather than at degree 5 vertices and avoid adjacent carbon atoms, i.e., C–C edges. This can lead to structures deviating from the most spherical closo deltahedra. For the 8-vertex CpFeC₃B₄H₇ system, hexagonal bipyramidal structures are found to be of comparable energy to the closo bisdisphenoidal structures. For the 9-vertex CpFeC₃B₅H₈ system the unique closo tricapped trigonal prismatic structure having carbon atoms at the three non-adjacent degree 4 vertices lies ∼16 kcal mol⁻¹ below the next lowest energy structure. The 10-vertex CpFeC₃B₆H₉ system has the most complicated potential energy surface of the CpFeC₃B_n−4H_n−1 structures with nine structures within 13 kcal mol⁻¹ of the global minimum. Six of these nine structures are based on the closo bicapped square antiprism, which has only two degree 4 vertices for carbon atoms. The remaining three low-energy CpFeC₃B₆H₉ structures are derived from the isocloso 10-vertex deltahedron with the iron atom at the unique degree 6 vertex and all three carbon atoms at degree 4 vertices. The low-energy 11-vertex CpFeC₃B₇H₁₀ structures are based on the 11-vertex closo/isocloso deltahedron with the unique structure having the iron atom at the degree 6 vertex, carbon atoms at the two degree 4 vertices, and no C–C edge being the lowest energy structure. The lowest energy 12-vertex CpFeC₃B₈H₁₁ structures are all based on the regular icosahedron with the three lowest energy structures having no C–C edges. The two lowest energy CpFeC₃B₈H₁₁ structures correspond to those recently found experimentally in the (η⁵-RC₃B₈H₁₀)Fe(η⁵-C₅H₅) (R = aryl) systems.