Self-assembled Transition Metal Coordination Frameworks of ...

5.3.4. Magnetochemistry
Manganese(lI) complexes
The temperature dependence of molar magnetic susceptibility X“, for all the
five compounds were carried out in the temperature range 5-325 K. All compounds
show common features. The effective magnetic moments Ilgff at room temperature of
tetranuclear complexes are found to be low, compared to that expected for a complex
having four independent Mn(II) ions {(Z4Si(Si+l));= 11.83 pg}, and also
decreases regularly with decreasing temperature suggesting antiferromagnetic
interactions between Mn(II) ions. The room temperature effective magnetic moment
[J-eff of dinuclear compound 20, 2.95 pg, is very low compared to 8.37 pg expected for a
compound with two noninteracting Mn(II) centers. The temperature dependence curve
of llcff shows a regular decrease on cooling and reaches a minimum of 1.13 pg at 5 K,
clearly indicating strong antiferromagnetism similar to other Mn(II) complexes.
The room temperature effective magnetic moment pgg of sulfur-bridged
square 19 is found to be 4.59 pg. On cooling pgg decreases gradually and reaches a
minimum of 2.20 pg at l0 K and shows a slight increase, 2.27 pg, at 5 K. The variable
temperature magnetic behaviour of 19 is indicative of strong antiferromagnetic
coupling between Mn(II) ions. The sharp increase in the X,“ values at lower
temperature is attributed to the presence of monomeric impurity. The carbohydrazone
Mn(II) complexes 21, 22 and 23 show cormnon features. The room temperature pggof
4.08 pg shown by complex 22 is low compared to that of the sulfur bridged compound
19. The llgff values show a regular decrease upon decreasing the temperature and reach
a minimum of 0.53 pg at 5 K. Similarly, the compounds 21 and 23 show magnetic
moments of 4.24 and 4.18 pg at room temperature and undergo a regular decrease on
cooling to reach a minimum value of 0.52 pg at 5 K for both complexes. The thermal
dependence curve of all these Mn(II) square complexes implies a dominant strong
antiferromagnetic interactions between Mn(II) centers. The results are as expected for
the proposed structures of these complexes.
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