Probing the magnetic and magnetothermal properties of M(II)-Ln(III) complexes (where M(II) = Ni or Zn; Ln(III) = La or Pr or Gd)

We establish the coordination potential of the Schiff base ligand (2-methoxy-6-[(E)-2'-hydroxymethyl-phenyliminomethyl]-phenolate (H2L)) via the isolation of various M(II)-Ln(III) complexes (where M(II) = Ni or Zn and Ln(III) = La or Pr or Gd). Single crystals of these five complexes were isolated and their solid state structures were determined by single crystal X-ray diffraction. Structural determination revealed molecular formulae of [NiGd(HL)(2)(NO3)(3)] (1), [NiPr(HL)(2)(NO3)(3)] (2) and [Ni2La(HL)(4)(NO3)](NO3)(2) (3), [Zn2Gd(HL)(4)(NO3)](NO3)(2) (4), and [Zn2Pr(HL)(4)(NO3)](NO3)(2) (5). Complexes 1 and 2 were found to be neutral heterometallic dinuclear compounds, whereas 3-5 were found to be linear heterometallic tri-nuclear cationic complexes. Direct current (dc) magnetic susceptibility and magnetization measurements conclusively revealed that complexes 1 and 4 possess a spin ground state of S = 9/2 and 7/2 respectively. Empirically calculated Delta T-chi M derived from the variable temperature susceptibility data for all complexes undoubtedly indicates that the Ni(II) ion is coupled ferromagnetically with the Gd(III) ion, and anti-ferromagnetically with the Pr(III) ion in 1 and 2 respectively. The extent of the exchange interaction for 1 was estimated by fitting the magnetic susceptibility data using the parameters (g = 2.028, S = 9/2, J = 1.31 cm(-1) and zJ = +0.007), supporting the phenomenon observed in an empirical approach. Similarly using a HDVV Hamiltonian, the magnetic data of 3 and 4 were fitted, yielding parameters g = 2.177, D = 3.133 cm(-1), J = -0.978 cm(-1), (for 3) and g = 1.985, D = 0.508 cm(-1) (for 4). The maximum change in magnetic entropy (-Delta S-m) estimated from the isothermal magnetization data for 1 was found to be 5.7 J kg(-1) K-1 (Delta B = 7 Tesla) at 7.0 K, which is larger than the -Delta S-m value extracted from 4 of 3.5 J kg(-1) K-1 (Delta B = 7 Tesla) at 15.8 K, revealing the importance of the exchange interaction in increasing the overall ground state of a molecule for better MCE efficiency.