Deviations from the Most Spherical Deltahedra in Rhenatricarbaboranes Having 2n + 2 Wadean Skeletal Electrons

Abstract

Density functional theory studies on the rhenatricarbaboranes CBHRe(CO) (n = 7-12) show that the lowest energy polyhedra for n-vertex metallaboranes having 2n + 2 skeletal electrons and sufficiently dissimilar vertex atoms can deviate from the most spherical closo deltahedra predicted by application of the Wade-Mingos rules. Furthermore, the lowest energy structures of these rhenatricarbaboranes are found to avoid C-C edges and have carbon atoms located at degree 4 rather than degree 5 vertices. The lowest energy structures for the 7-vertex CBHRe(CO) system all have a central CBRe closo deltahedron, namely the pentagonal bipyramid with the rhenium atom at a degree 5 axial vertex and all three carbon atoms at degree 4 equatorial vertices. However, the lowest energy structure for the 8-vertex CBHRe(CO) is not the most spherical closo 8-vertex deltahedron, namely the bisdisphenoid, but instead a central CBRe hexagonal bipyramid with mutually nonadjacent degree 4 vertices for the carbon atoms. Similarly, the lowest energy 10-vertex CBHRe(CO) structures are derived from isocloso deltahedra having three degree 4 vertices for all three carbon atoms rather than from the most spherical 10-vertex closo deltahedron, namely the bicapped square antiprism with only two degree 4 vertices. However, for the 9-vertex CBHRe(CO) system, the most spherical closo deltahedron, namely the tricapped trigonal prism, has three mutually nonadjacent degree 4 vertices, which is ideal for the three carbon atoms and thus is the preferred deltahedron. The preferred deltahedron for the 11-vertex CBHRe(CO) remains the most spherical closo deltahedron despite having only two degree 4 vertices for the carbon atoms. Furthermore, the six lowest energy 12-vertex CBHRe(CO) structures are all based on the regular icosahedron generally favored in polyhedral borane chemistry despite its complete lack of degree 4 vertices for the carbon atoms.

Neutral Rhenadicarbaboranes with Re(CO)2(NO) Vertices: A Theoretical Study of Building Blocks for Rhenacarborane-Based Drug Delivery Agents

“…Structure B8C2Re-4, lying 10.7 kcal/mol in energy above B8C2Re-1, also has the rhenium atom located at the degree 6 vertex, one carbon atom located at a degree 4 vertex, and the other carbon atom located at a degree 5 vertex different from the degree 5 carbon vertex in B8C2Re-2. The clear energetic preference of the 10-vertex rhenadicarbaborane C 2 B 7 H 9 Re(CO) 2 (NO) for the closo bicapped tetragonal antiprism structures ( Figure 1) contrasts with the previously discovered [23] energetic preference of the 10-vertex rhenatricarbaborane C 3 B 6 H 9 Re(CO) 3 for isocloso deltahedral structures (Figure 2). This difference may relate to the number of carbon vertices in the central deltahedron.…”

“…B7C2Re-1 (Cs) 0.0 kcal/mol The clear energetic preference of the 10-vertex rhenadicarbaborane C2B7H9Re(CO)2(NO) for the closo bicapped tetragonal antiprism structures ( Figure 1) contrasts with the previously discovered [23] energetic preference of the 10-vertex rhenatricarbaborane C3B6H9Re(CO)3 for isocloso deltahedral structures ( Figure 2). This difference may relate to the number of carbon vertices in the central deltahedron.…”

“…The closo deltahedra for the 8-through 12-vertex systems have only degree 4 and 5 vertices except for the 11-vertex system, which necessarily has one degree 6 vertex. However, density functional theory studies [23] show that the asymmetry in the C 3 B n−4 Re polyhedra leads to deviation from the most spherical deltahedra to give lowest energy structures for the 8-and 10-vertex systems with two degree 6 vertices. We now report similar density theory functional studies on the neutral (C 2 B n−3 H n−1 )Re(CO) 2 (NO) systems (n = 9 to 12), which are the basis for the rhenacarborane drug delivery systems.…”