Aromatic ring-ring or ð,ð-stacking interactions are intraand/
or intermolecular forces, which play relevant roles in
molecular recognition processes, supramolecular chemistry,
and biological systems. In this broad context, an interesting
account for ð,ð-stacking interactions in metal complexes
having aromatic N-donor ligands has been recently reported
by Ch. Janiak1. In this connection, structural studies on
Cu(II)-dicarboxylato complexes having suitable N-heterocyclic
auxiliary ligands have proved the contribution of both
hydrogen bonding and aromatic ring-ring stacking interactions
to the crystal building. Structural comparisons on
copper(II)-malonato complexes where the referred stackings
are2 or are not3 presented reveal the influence of the
ð-acceptor character of the N-heterocyclic ligand (such as
2,2¢-bipyridine2a (2,2¢-bipy) or 1,10-phenanthroline2c,d (phen)
and closely related ones2b,e) on the copper(II)-O(carboxyl)
bond lengths. These findings strongly suggest an active
electronic delocalization within the metal-N-heterocyclic
chelate ring in such a way that it could exhibit some degree
of “metalloaromaticity”. This is a classic concept recently
reviewed and improved by H. Masui.4 The increasing interest
in this subject arises from its usefulness as a unifying
approach for the reactivity, magnetic and/or spectral properties,
and structure of so-called “metalloaromatic compounds”.
Resonance energy stabilization, spectral and/or magnetic
measurements, and various molecular structural parameters
are considered the most suitable criteria for assessing the
degree of aromaticity in appropriate compounds.4
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