Self-assembled Transition Metal Coordination Frameworks of ...

Chapter I g g _
exhibit considerable thermodynamic stability. To achieve this, the binding constant
(and the related Gibbs free energy) for the respective monotopic model building
blocks should be high. A large EM is desired for the square macrocycle to avoid
transformation into metallosupramolecular polymers even at high concentrations.
Finally, other cyclic species should exhibit much smaller EM values than that of the
square to provide a clear thermodynamic advantage for square macrocycles. To
accomplish these goals rigid building blocks are required whose structural
predisposition affords square macrocycles that are free of strain.
Molecular design and chemical templates will help provide the species and
processes that create order. It is as it has always been that the enemy is the second law
of thermodynamics. The job of coordination chemists is, within the systems that are
their focus, to confound the second law. That is the essence of molecular organization,
and, within limits specified in the concept of a coordination entity, the broad mission
of the complete coordination chemistry [5]. It may appear obvious that the self­
assembly of geometrically shaped polygons and polyhedra must involve ligands
which are somewhat confomiationally inflexible. However square complexes, which
.mploy linkers that are generally considered flexible or semiflexible are known.
Molecular squares involving these ligands are typically stabilized by (i)
conformational restraints inherent to the motif itself, (ii) steric or repulsive
interactions to minimize crowding, (iii) attractive it-interactions, or (iv) the presence
of bridging atomsl groups on the sides of squares [1 l]. For example, the first molecular
square of a thiocarbohydrazone [28] is stabilized by bridging thiocarbohydrazide S­
atoms on each side; as this molecule contains a large central cavity surrounded by a
lattice-like arrangement of ligands, it can also be considered a grid [ll]. So by using
rigid building blocks of suitably substituted thiocarbohydrazones and taking
advantage of octahedral geometries of metal centers, molecular square grids [28-31]
can be readily achieved.
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