Description of an unusual hydrogen bond between carborane and a phenyl group

Boranes and carboranes form non-classical H-bonds with the p-face of an arene provided entropic factors are excluded, for example via a suitably designed template. The self-assembled Ir-dithiolene phosphine complexes of the type [Cp*Ir(PR3)S2C2B10H10] (R = C6H4X, X = H, F, OMe) provide such a template. Steric crowding causes that one of the B-H bonds of the carborane is positioned above the phenyl plane of R to form a non-classical B-H center dot center dot center dot pi hydrogen bond. For the gas phase, this is predicted by quantum chemical calculations of the local B-H center dot center dot center dot pi stretching force constants. The solid-state structures of six [Cp*Ir(PR3)S2C2B10H10] complexes synthesized are in close agreement with the quantum chemical predictions thus suggesting the existence of B-H center dot center dot center dot pi hydrogen bonds. The H-1[B-11] NMR titration experiments reveal that non-covalent B-H center dot center dot center dot pi bonds also exists in solution. Calculations show that the bond strength order of the B-H center dot center dot center dot pi H-bond determined with the help of the local stretching force constant is 0.35, comparable to what is found for the H-bond in the water dimer. However, the B-H center dot center dot center dot pi H-bond is electrostatic in nature as confirmed by the topological analysis of the electron density rho(r). The role of this H-bond in carborane supramolecular chemistry is elucidated and discussed. (C) 2018 Published by Elsevier B.V.