Self-assembled Transition Metal Coordination Frameworks of ...

Chapter I _ _ ______g__H _ g
involved, (ii) the bonds must be kinetically labile so as to allow self-correction and
(iii) the desired assembly must be thermodynamically more favorable than any
competing species. Several studies have examined the role of thermodynamic factors
in the self-assembly of metallocyclic compounds [ll]. These have generally
concluded that cyclic structures are preferred over linear ones for enthalpic reasons,
while small cycles are favored over large cycles (at low concentrations) for entropic
reasons. In coordination chemistry, the enthalpic driving forces in a self-assembly
reaction invariably dominate the entropic ones because of the large enthalpy of
coordinate bond fomiation.
The size of large polymetallic clusters is difficult to control, and these systems
are invariably obtained serendipitously through self-assembly reactions. Controlling
the nuclearity for clusters of large numbers of metals is very difficult, and requires an
ingenious approach to the design of a single ligand. A more practical approach is
based on the propensity of metal ions, given the right coordination environment, to
self-assemble into a cluster. This occurs readily with simple bidentate ligands, which
do not fully satisfy the coordination requirements of a single metal, and the vacant
coordination sites are filled by spare donor fragments from a neighboring subunit as
the overall cluster forms in a self-assembly process [23]. However, a significant
amount of control can be exerted over the formation of relatively low nuclearity
molecular clusters (