Self-assembled Transition Metal Coordination Frameworks of ...

C0pper(II) complexes
The spectrum of an uncoupled Cu(II) species is expected to show clearly four
well resolved hyperfine lines, due to the interaction of the electron spin with the
copper nuclear spin (°5Cu, I=3/2) corresponding to M, = -3/2, - 1/2, 1/2, 3/2 transitions
(AM, = il), need not show well resolved lines if another Cu(II) species is present,
eventhough there are no interactions taking place. The spectrum of such a sample is
expected to show a resultant combination depending on the g values and their
intensities of each species.
hi the dissolved frozen state at 77 K in a suitable solvent, usually, the
intermolecular interactions are reduced, and the EPR spectrum normally is expected to
exhibit appreciable hyperfine interactions for a Cu(H) center. The hyperfine splitting,
A i arising from the nuclear magnetic moment of the Cu(H) at g L is usually very
small so that this feature of the spectrum often shows no splitting. On the other hand,
A" the nuclear hyperfine splitting at g" is usually much larger and the four features at
g" are often resolved. It is the magnitude of AH and g" which are dependent, among
other things, on the nature of the ligands of Cu(H), and these values can often be used
to assign structure. It has been suggested that the smaller value of A" arises from a
distortion of a copper site away from planarity [35]. The hyperfine spacing are more
or less twice than that for dimers indicating the complex under investigation behaves
like localized electrons [36] in each copper(H) centers.
The spectra of all the compounds, except 13 and 16, exhibit some common
features as evidenced by the nature of spectra. The spectra of 13 and 16 are broad but
not isotropic in nature and don’t give much information, but may be consistent with
antiferromagnetic interaction between Cu(H) centers, as it is expected. This can be
attributed to concentration difference of solutions taken for recording the spectra. The
experimental and simulated best fits of EPR spectra of all complexes are given in
Figs. 4.22 to 4.34. All of these spectra are found axial in nature and g“ > g L > 2.0023
155