posted on 2018-12-18, 00:00authored byMartina Cirulli, Amanpreet Kaur, James E. M. Lewis, Zhihui Zhang, Jonathan A. Kitchen, Stephen M. Goldup, Maxie M. Roessler
Early
work by Sauvage revealed that mechanical bonding alters the
stability and redox properties of their original catenane metal complexes.
However, despite the importance of controlling metal ion properties
for a range of applications, these effects have received relatively
little attention since. Here we present a series of tri-, tetra-,
and pentadentate rotaxane-based ligands and a detailed study of their
metal binding behavior and, where possible, compare their redox and
electronic properties with their noninterlocked counterparts. The
rotaxane ligands form complexes with most of the metal ions investigated,
and X-ray diffraction revealed that in some cases the mechanical bond
enforces unusual coordination numbers and distorted arrangements as
a result of the exclusion of exogenous ligands driven by the sterically
crowded binding sites. In contrast, only the noninterlocked equivalent
of the pentadentate rotaxane CuII complex could be formed
selectively, and this exhibited compromised redox stability compared
to its interlocked counterpart. Frozen-solution EPR data demonstrate
the formation of an interesting biomimetic state for the tetradentate
CuII rotaxane, as well as the formation of stable NiI species and the unusual coexistence of high- and low-spin
CoII in the pentadentate framework. Our results demonstrate
that readily available mechanically chelating rotaxanes give rise
to complexes the noninterlocked equivalent of which are inaccessible,
and that the mechanical bond augments the redox behavior of the bound
metal ion in a manner analogous to the carefully tuned amino acid
framework in metalloproteins.