Nonsphericity in diferratetracarbaboranes having 2n + 2 Wadean skeletal electrons: deviations from closo deltahedral geometries and high-energy kinetically stable isomers

Abstract

The diferratetracarbaboranes Cp₂Fe₂C₄B_n−6H_n−2 (n = 10 to 14; Cp = η⁵-C₅H₅) as well as the experimentally known C-tetramethyl derivatives Cp₂Fe₂C₄Me₄B₈H₈ have been studied by density functional theory methods. For the Cp₂Fe₂C₄Me₄B₈H₈ system, the three structurally characterized isomers produced under relatively mild conditions having an “open” tetragonal or pentagonal face correspond to the lowest energy structures not based on the 14-vertex closo deltahedron, namely the bicapped hexagonal antiprism. These structures provide examples of kinetically favored but thermodynamically disfavored high-energy metallacarborane structures. Thus the lowest energy such structure lies ∼22 kcal mol⁻¹ above the global minimum, namely a C_2vcloso structure with no C–C deltahedral edges. This latter global minimum 14-vertex closo structure is found experimentally to be the ultimate pyrolysis product in the Cp₂Fe₂C₄Me₄B₈H₈ system at 300 °C. The lowest energy structures for the smaller 11 to 13 vertex Cp₂Fe₂C₄B_n−6H_n−2 systems are the corresponding most spherical closo deltahedra as expected by the Wade–Mingos rules for these 2n + 2 skeletal electron systems. However, for the 11- and 12-vertex systems, less spherical deltahedral structures providing additional degree 6 vertices for the iron atoms and degree 4 vertices for the carbon atoms become energetically competitive. For the 10-vertex Cp₂Fe₂C₄B₄H₈ system a relatively non-spherical deltahedral structure with four degree 4 vertices for the carbon atoms and two degree 6 vertices for the iron atoms is energetically preferred by a substantial margin. Thus such a structure lies ∼23 kcal mol⁻¹ in energy below the isomeric 10-vertex closo bicapped tetragonal antiprism structure expected from the Wade–Mingos rules for this 2n + 2 skeletal electron system.