Entasis through Hook-and-Loop Fastening in a Glycoligand with Cumulative Weak Forces Stabilizing Cu^I

The idea of a possible
control of metal ion properties by constraining
the coordination sphere geometry was introduced by Vallee and Williams
with the concept of entasis, which is frequently postulated to be
at stake in metallobiomolecules. However, the interactions controlling
the geometry at metal centers remain often elusive. In this study,
the coordination properties toward copper ionsCuII or CuIof a geometrically constrained glycoligand
centered on a sugar scaffold were compared with those of an analogous
ligand built on an unconstrained alkyl chain. The sugar-centered ligand
was shown to be more preorganized for CuII coordination
than its open-chain analogue, with an unusual additional stabilization
of the CuI redox state. This preference for CuI was suggested to arise from geometric constraints favoring an optimized
folding of the glycoligand minimizing steric repulsions. In other
words, the CuI d10 species is stabilized by
valence shell electron pair repulsion (VSEPR). This idea was rationalized
by a theoretical noncovalent interactions (NCI) analysis. The cumulative
effects of weak forces were shown to create an efficient buckle as
in a hook-and-loop fastener, and fine structural features within the
glycoligand reduce repulsive interactions for the CuI state.
This study emphasizes that monosaccharide platforms are appropriate
ligand backbones for a delicate geometric control at the metal center,
with a network of weak interactions within the ligand. This structuration
availing in glycoligands makes them attractive for metallic entasis.