Self-assembled Transition Metal Coordination Frameworks of ...
Self-assembled Transition Metal Coordination Frameworks of ...
Self-assembled Transition Metal Coordination Frameworks of ...
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3.3.4. Magnetochemistry <strong>of</strong> the molecular square grids<br />
Ni(ll) complexes<br />
For all the four square grid compounds the temperature dependence <strong>of</strong> molar<br />
magnetic susceptibility X,“ was investigated over the temperature range 5-325 K. All<br />
compounds show common features. The effective magnetic moments pgfi at room<br />
temperature <strong>of</strong> 2 and 3 are found to be low compared to that expected for a complex<br />
having four independent Ni(II) ions (5.66 pg) and also decreases with decreasing<br />
temperature suggesting antiferromagnetic interactions between the Ni(II) ions. The<br />
temperature dependence curves <strong>of</strong> 1 and 4 are similar to that <strong>of</strong> 2 and 3, but with<br />
slightly higher |J.gff values at room temperature. These features indicate a dominant<br />
antiferromagnetic interaction in these compounds.<br />
The room temperature effective magnetic moment pm <strong>of</strong> sulfur-bridged<br />
squares are found to be 6.05 pg for 1 and 4.01 pg for 2. As the temperature is lowered<br />
pfff <strong>of</strong> both 1 and 2 decreases gradually and reaches a minimum <strong>of</strong> 0.51 pg for 2 and<br />
0.45 pg for 1 at 5 K. The variable temperature magnetic behaviour <strong>of</strong> both compounds<br />
are indicative <strong>of</strong> antiferromagnetic coupling between the Ni(II) ions and this is greater<br />
in 2. The room temperature Ilgff <strong>of</strong> 4.50 pg shown by complex 3 is low compared to its<br />
sulfur bridged counterpart compound 1. The thermal dependence curve <strong>of</strong> 3 implies<br />
strong antiferromagnetic interactions between Ni(II) centers. The pgg values show a<br />
regular decrease upon decreasing the temperature and reach a minimum value <strong>of</strong> 0.66<br />
pg at 5 K. The antiferromagnetic coupling in complex 4 is found to be low compared<br />
to that <strong>of</strong> 3, as evidenced by the room temperature magnetic moment <strong>of</strong> 5.80 pg. On<br />
cooling, pggdecreases and reaches a minimum value <strong>of</strong> 0.83 pg at 5 K.<br />
To fit and interpret the magnetic susceptibility data <strong>of</strong> complexes, first it is<br />
necessary to find all possible magnetic pathways in the complicated but regular<br />
structures. In a general case a square (D4,,) or distorted square (D24) [2 >< 2] grid can be<br />
described by an exchange coupling scheme involving three exchange integrals (J1, J2<br />
and .13) according to the appropriate Heisenberg exchange Hamiltonian [51], eqn (1).<br />
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