Self-assembled Transition Metal Coordination Frameworks of ...

g g _ g Introduction
1.2.2. Metallosupramolecular squares
Cyclic tetranuclear metal complexes with ~90° angles at the corners
(molecular squares or molecular boxes) are of great interest [25]. The complexes
having an ‘array of metal centers at the vertices of a (approximate) square lattice’ is
the reason behind the name. ‘Polytopic’ ligands, with well defined and appropriately
separated coordination compartments, in principle have a better chance of control over
the outcome of a self-assembly process to produce a cluster with a predefined
nuclearity. Here, the ligands have two potential coordination pockets (ditopic) and
involve monatomic bridging groups. Thermodynamically favored grid complexes are
formed by self-assembly process in high yield as suggested by so far obtained results
[13], with homoleptic and non-homoleptic examples.
Based on rapid chemical exchanges among starting materials, intermediates
(e. g. oligomers and polymers) and final ensembles during the coordinative assembling
processes the composition of the final products depends primarily on the
thermodynamic parameters of the possible products and intermediates. Such exchange
provides an efficient mechanism for error correction, which may result in the
conversion of thermodynamically unfavorable intermediates into a single final
product. However, thermodynamic control only affords the formation of a single
product if this product has a sufficient thermodynamic advantage over the other
possible species. For many metallosupramolecular systems, two or more species are in
equilibrium because no clear thermodynamic preference for one species is given.
From the viewpoint of thermodynamics, enthalpy favors the formation of squares,
which have less conformational strain (90° corner) than triangles (60° corner), while
entropy favors the formation of triangles, which are assembled from fewer
components than squares. As a consequence, both the triangular and square species
may co-exist in solution. However, the reversibility of metal—ligand coordination
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