Self-assembled Transition Metal Coordination Frameworks of ...

Chapter-4 __ U g __ ,7
thermodynamic stability. To achieve this, the binding constant (and the related Gibbs
free energy) for the respective monotopic model building blocks should be high and
also the square macrocycles should be free of strain [18]. Jahn Teller distortions of
octahedral geometry, rigidity of carbohydrazones and thiocarbohydrazones, etc may
also be behind the reluctance of Cu(ll) towards self-assembly.
Maubaraki er al. [ll] have reported the synthesis of binuclear Cu(Il)
compounds of bis(pyridine-2-aldehyde) thiocarbohydrazone in acid medium by
stirring and their selective elimination of N, N, N coordinated copper in appropriate
ethanolic mineral acids to form mononuclear complexes. In our study, the dibasic
ligands are found to be deprotonated under neutral conditions itself, and resulted in
metal complexes with ligand to metal ratio 1:2, coordinating first Cu(II) through
thiolate sulfur (or enolate oxygen for carbohydrazones), azomethine N and pyridyl or
quinolyl N. The binding atoms to second metal involve the azomethine N and pyridyl
or quinolyl N of the remaining half of ligands: usually, the imine nitrogen of the first
half of ligand acts as the third coordinating atom. The second copper can be
coordinated by the NNS/ NNO mode also to form symmetric dicopper complex
through sulfur/ oxygen bridging. However, for the present complexes the spectroscopic
data are more consistent with asymmetric dicopper complexes, as would be expected
primarily. Unfortunately, we could not get any X-ray quality single crystals of any of
the copper complexes for confirming the exact coordination. The coordination mode
towards Cu(II) is flexible, and thus, the final assemblies are less predictable. In the
majority of cases the NNS/NNO and NNN coordination modes of
(thio)carbohydrazones are seen, and the only one crystal study of a Cu(II)
carbohydrazone [17] agree with NNO and NNN coordination.
The complexes prepared were either green or dark brown in color. All the
complexes were found to be soluble in DMF and DMSO, but only partially soluble in
other organic solvents such as CHCI3, ethanol, methanol etc. The variable temperature
magnetic susceptibility measurements of all complexes show antiferromagnetic
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