centre invariant, a change in spin-state for cobalt(II) brings about
an anodic potential shift of ~0.57 V [0.48 + 0.09] in its CoIII–
CoII redox potential value. In our opinion, this assessment carries
a notable consequence in predicting E1/2 value of the CoIII–CoII
redox process, as a change in the spin-state.
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It is worth mentioning that while discussing the trends in redox
potentials, one should also take into account the significance of
solvent effect on redox thermodynamics.34 However, comparison
of the CoIII–CoII redox potential values of closely similar com-
plexes and prediction of the redox potential value for ‘hypothetic’
low-spin complex [CoII(L9)] as done in this work is in order, given
the fact that for [CuII(L10)] the CuII–CuI redox potential values are
within 0.05 V in the two solvents CH2Cl2 and DMSO.9g
5f
Conclusions
In summary, a new hexadentate pyridine amide ligand having
a –OCH2CH2O– spacer between the two pyridine carboxamido
functionalities has been synthesized and its bivalent cobalt, nickel
and copper complexes and also trivalent cobalt complex have
been synthesized. The coordination environment around the
metal(II) centre in each case has been authenticated by single-
crystal X-ray crystallography. Given the literature reports on
structurally characterized low-spin iron(II) (S = 0), cobalt(II)
(S = 1/2), nickel(II) (S = 1) and copper(II) (S = 1/2) complexes
of the deprotonated form of pyridine/pyrazine amide ligands
with –SCH2CH2S– and –SCH2CH2CH2S– spacers the present
investigation has given us a unique opportunity to pinpoint the
effect of ligand donor atom type (O vs. S) on (i) the spin-state
of cobalt(II), (ii) on coordination geometry about the metal(II)
centre and associated structural characteristics and (iii) on the
MIII–MII redox potential values. Our efforts are now to synthesize
and structurally characterize and to determine the FeIII–FeII redox
potential value of a high-spin iron(II) complex of L9(2-) [FeII(L9)].
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Acknowledgements
This work was supported by the Department of Science &
Technology, Government of India. RM sincerely thanks DST
for a J. C. Bose national fellowship. We sincerely acknowledge
the preliminary work done by Pradip Kumar Das. We deeply
appreciate the experimental assistance provided by Amit Rajput
and Debanjan Dhar in reproducing the synthesis of 2 and critical
examination of its 1H NMR spectra. We also acknowledge Suman
K. Barman for his help with simulation of the EPR spectra of the
solid sample of 4. We also thank Nanda Dulal Paul of Department
of Inorganic Chemistry, Indian association for the Cultivation
of Science, Kolkata for recording and simulating the glass EPR
spectrum of 4. Comments of the reviewer’s were very helpful at
the revision stage.
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