Deltahedral ferratricarbaboranes: analogues of ferrocene

The neutral ferratricarbaboranes (η(5)-RC3B8H10)Fe(η(5)-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-4)H(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-4)H(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 (η(5)-RC3B8H10)Fe(η(5)-C5H5) (R = aryl) systems.