Cyclopentadienylcobalt azaboranes violating the Wade–Mingos rules: a degree 3 vertex for the nitrogen atom

Abstract

The experimentally realized chemistry of polyhedral azaboranes includes the very stable cobalt derivative CpCoNHB₉H₉, which has been synthesized and structurally characterized by X-ray crystallography. The structures and energetics of the complete series of cobaltaazaboranes CpCoNHB_n−2H_n−2 have now been studied by density functional theory. Low-energy structures are found for the 8- and 9-vertex systems based on non-spherical deltahedra providing a degree 3 vertex for the nitrogen atom and thus violating expectations from the Wade–Mingos rules. Thus the lowest energy CpCoNHB₆H₆ structure is an antipodally bicapped octahedron with the nitrogen atom at a degree 3 vertex rather than the most spherical bisdisphenoid. For the 9-vertex CpCoNHB₇H₇ system the lowest energy structures are based on the most spherical tricapped trigonal prism. However, isomeric CpCoNHB₇H₇ structures at ∼13 kcal mol⁻¹ in energy above these structures are found with a central capped bisdisphenoid having the nitrogen atom at the degree 3 capping vertex. In contrast to the 8- and 9-vertex systems, the low-energy CpCoNHB_n−2H_n−2 structures for the 10 to 12 vertex systems are based on the most spherical deltahedra. For the 10- and 11-vertex systems all of the low-energy structures have the nitrogen atom at a degree 4 vertex. The predicted Co–N and Co–B distances in the lowest energy CpCoNHB₉H₉ structure are very close to the experimental values.