Structural, spectroscopic, electrochemical and computational studies of C,C′-diaryl-ortho-carboranes, 1-(4-XC6H4)-2-Ph-1,2-C2B10H10 (X = H, F, OMe, NMe2, NH2, OH and O-)

The influence of aryl ring substituents X (F, OMe, NMe2, NH2, OH and O-) on the physical and electronic structure of the ortho-carborane cage in a series of C,C'-diaryl-ortho-carboranes, 1-(4-XC6H4)-2-Ph-1,2-C2B10H10 has been investigated by crystallographic, spectroscopic [nuclear magnetic resonance (NMR), UV-vis], electrochemical and computational methods. The cage C1-C2 bond lengths in this carborane series show small variations with the electron-donating strength of the substituent X, but there is no evidence of a fully evolved quinoid form within the aryl substituents in the ground state. In the B-11 and C-13 NMR spectra, the 'antipodal' shift at B12, and the C1 shift correlates with the Hammett sigma(p) value of the substituent X. The UV-visible absorption spectra of the cluster compounds show marked differences when compared with the spectra of the analogous substituted benzenes. These spectroscopic differences are attributed to variation in contributions from the cage orbitals to the unoccupied/virtual orbitals involved in the transitions responsible for the observed absorption bands. Electrochemical studies (cyclic and square-wave voltammetry) carried out on the diarylcarborane series reveal that one-electron reduction takes place at the cage in every case with the voltage required for reduction of the cage influenced by the electron-donating strength of the substituent X, affording a series of carborane radicals with formal [2n+3] electron counts.