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.