Novel non-spherical deltahedra in trirhenaborane structures

Abstract

The geometries and energetics of the trirhenaboranes Cp₃Re₃B_n−3H_n−3 (Cp = η⁵-C₅H₅; n = 5 to 12) have been investigated using density functional theory for comparison with the experimentally known oblatocloso dirhenaboranes Cp*₂Re₂B_n−2H_n−2 (Cp* = η⁵-Me₅C₅; n = 8 to 12). The low-energy Cp₃Re₃B_n−3H_n−3 (7 ≤ n ≤ 12) structures are found to be Re₃B_n−3 deltahedra with internally bonded Re₃ triangles. The rhenium atoms are generally located at degree 6 to 8 vertices representing sites of low local curvature and the boron atoms at degree 3 to 5 vertices representing sites of high local curvature. The Re–Re bonds in the Re₃ triangles of such clusters typically range from 2.6 to 2.7 Å if they are located on or near the deltahedral surface and from 2.8 to 3.0 Å if they go through the interior of the deltahedron away from the surface. Such highly non-spherical structures, typically having little symmetry, are related to the oblatocloso structures of the dirhenaboranes. A low-energy, more nearly spherical 12-vertex Cp₃Re₃B₉H₉ structure, essentially degenerate with the global minimum, has an Re–Re–Re chain embedded in a deltahedron having degree 5 and 6 rhenium vertices and degree 4 and 5 boron vertices. Low-energy structures for the 5-vertex Cp₃Re₃B₂H₂ system are derived from a trigonal bipyramid. Similarly, low-energy 6-vertex Cp₃Re₃B₃H₃ structures have central Re₃B₃ bicapped tetrahedra.