Direct Observation of Bis(dicarbollyl)nickel Conformers in Solution by Fluorescence Spectroscopy: An Approach to Redox-Controlled Metallacarborane Molecular Motors

As a continuation of work on metallacarborane-based molecular motors, the structures of substituted bis­(dicarbollyl)­nickel complexes in NiIII and NiIV oxidation states were investigated in solution by fluorescence spectroscopy. Symmetrically positioned cage-linked pyrene molecules served as fluorescent probes to enable the observation of mixed meso-trans/dl-gauche (pyrene monomer fluorescence) and dl-cis/dl-gauche (intramolecular pyrene excimer fluorescence with residual monomer fluorescence) cage conformations of the nickelacarboranes in the NiIII and NiIV oxidation states, respectively. The absence of energetically disfavored conformers in solutiondl-cis in the case of nickel­(III) complexes and meso-trans in the case of nickel­(IV)was demonstrated based on spectroscopic data and conformer energy calculations in solution. The conformational persistence observed in solution indicates that bis­(dicarbollyl)­nickel complexes may provide attractive templates for building electrically driven and/or photodriven molecular motors.