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

Abstract

As a continuation of work on metallacarborane-based molecular motors, the structures of substituted bis(dicarbollyl)nickel complexes in Ni(III) and Ni(IV) 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 Ni(III) and Ni(IV) 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.

Ferrocene and Transition Metal Bis(Dicarbollides) as Platform for Design of Rotatory Molecular Switches

“…Recently this concept was realized via attachment of pendant pyrene molecules as fluorescent probes ( Figure 23 ). As expected, an intramolecular pyrene excimer emission was observed for the nickel(IV) complex, while the corresponding nickel(III) complex revealed only monomer fluorescence [ 64 ].…”

“…The calculated energy difference between the most stable transoid and the neighboring gauche rotamers in the nickel(III) complex was found to be negligible low (~0.1 kcal/mol) therefore the nickel(III) complex exists in solution as a mixture of transoid and gauche conformers, whereas the nickel(IV) complex due to higher difference in energies of the most stable cisoid and the neighboring gauche rotamers (~1.2 kcal/mol) in solution exists mainly as the cisoid conformer [ 64 ].…”

“… Design and switching process in pyrene-substituted nickel bis(dicarbollides). Adapted with permission from [ 64 ]. Copyright (2014) American Chemical Society.…”

Methylsulfanyl‐Stabilized Rotamers of Cobalt Bis(dicarbollide)

“…The interconversion of the transoid and cisoid geometries through the controlled change of the nickel oxidation state provides the basis for the controlled rotation of the dicarbollide ligands and makes the nickel bis(dicarbollide) moiety a promising module for the design of rotation molecular switches. [5,6] However, the nickel bis(dicarbollide) system is not very stable in the presence of some Lewis bases such as 2,2′-bipyridine, [7] 1,10-phenanthroline, [8] and triphenylphosphine [9] and affords the corresponding nikelacarboranes 3,3′-L 2 -3,1,2-NiC 2 B 9 H 11 . Therefore, an exciting challenge is the design of molecular switches based on the extremely stable cobalt bis(dicarbollide) anion {[3,3′-Co(1,2-C 2 B 9 H 11 ) 2 ] -}.…”

“…For example, the transoid conformation with two pairs of carbon vertices reflected through a center of symmetry is preferred for nickel(III) bis(dicarbollide), whereas the cisoid conformation (rotation angle 36°) is preferred for nickel(IV) bis(dicarbollide). The interconversion of the transoid and cisoid geometries through the controlled change of the nickel oxidation state provides the basis for the controlled rotation of the dicarbollide ligands and makes the nickel bis(dicarbollide) moiety a promising module for the design of rotation molecular switches , . However, the nickel bis(dicarbollide) system is not very stable in the presence of some Lewis bases such as 2,2′‐bipyridine, 1,10‐phenanthroline, and triphenylphosphine and affords the corresponding nikelacarboranes 3,3′‐L 2 ‐3,1,2‐NiC 2 B 9 H 11 .…”