Intra- and inter-molecular carboranyl C–H⋯N hydrogen bonds in pyridyl-containing ortho-carboranes

Four C-substituted derivatives of ortho-carborane, 1-R-1,2-C2B10H11, where R = 2′-pyridyl (1), 2′-picolyl (2), 5′-bromo-2′-pyridyl (3) or 3′-pyridyl (4) have been prepared using adaptations of standard procedures, and structurally characterised by single crystal X-ray diffraction studies in an exploration of C–H⋯N hydrogen bonding effects involving their carborane CH units. Calculations at the MP2/6-31G* level of theory were used to assess the strength of the hydrogen bonding detected, and calculated NMR shifts at the GIAO-B3LYP/6-311G* level were compared with measured C–H shifts to show that intramolecular C–H⋯N hydrogen bonding persisted in solution in the case of compound 1. The value of IR C–H stretching frequencies for probing hydrogen bonding in these systems was also studied. An unsuccessful attempt to convert compound 3 into a macrocyclic species (C2B10H10C5H3N)3 in which three ortho-carborane units are linked through 2,5-disubstituted pyridine rings is also described.