Cobalt(III)
complexes with three unsymmetrical bidentate ligands
containing a noncoordinating N–H bond and a phenolate-O donor as hydrogen-bond donor and acceptor, respectively,
were prepared and characterized. 1H NMR spectroscopy indicated
that all the tris-chelate Co(III) complexes prepared favor the mer configuration in solution. [Co(Hthp)3] and
[Co(Himn)3] also possess the mer configuration
in the crystals (Hthp– = 2-(1,4,5,6-tetrahydropyrimidin-2-yl)phenolate,
Himn– = 2-(2-imidazolinyl)phenolate). On the other
hand, [Co(Himl)3] takes the fac configuration
in the crystal (Himl– = 2-(2-imidazolyl)phenolate).
These Co(III) complexes showed three types of characteristic supramolecular
structures: ladder, distorted hexagonal sheet, and honeycomb sheet
structure, constructed by intermolecular hydrogen bonds. Heating [Co(Himn)3] and [Co(Himl)3] in methanol selectively afforded
precipitates of the fac isomer due to the low solubility
of the hydrogen-bonded supramolecular structures. This mer to fac isomerization upon crystallization in methanol
is presumably induced by the formation of highly ordered hydrogen-bond
networks via the methanol molecule. The fac isomers
remained intact in dimethyl sulfoxide (DMSO) for longer than a week
at room temperature. Upon heating, however, fac to mer geometrical isomerization of both fac-[Co(Himn)3] and fac-[Co(Himl)3] was observed in DMSO. Thus, mer/fac interconversion was achieved by heating in two different solvents,
due to the formation of a supramolecular assembly of hydrogen-bond
networks.
Quenching of quantum tunneling of magnetization was observed in a tetrahedral cobalt(ii) complex with 1-D chain hydrogen-bond networks by partially substituting the CoII ion with the ZnII ion, up to 33%.
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