Importance of Orbital Complementarity in Spin Coupling through Two Different Bridging Groups. Synthesis, Crystal Structure, Magnetic Properties and Magneto-Structural Correlations in a Dicopper(II) Complex of Endogenous Alkoxo Bridging Ligand with Exogenous Pyrazolate

[Qi2(L3)(3,5-prz)] (L3 = 1,3-bis(2-hydroxy-4-methoxybenzylideneamino)-propan-2-ol) (3) was synthesized and its crystal structure determined. The compound consists of discrete binu­ clear units, in which copper atoms are linked by the alkoxide oxygen atom of the ligand and the pyrazolate nitrogen atoms. Variable-temperature magnetic susceptibility measurements for a powdered sample of the complex were carried out in the temperature range 4.4 308 K and anal­ ysed to obtain values of the parameter J in the exchange Hamiltonian H = —2 JSi -S2. Recently, the dicopper(II) complexes [Cu2(L’)(3,5-prz)] (L1 = l,3-bis(2-hydroxy-l-napthylideneamino)propan-2-ol) (1) and [Cu2(L2)(3,5-prz)], (L2 = l,3-bis(2-hydroxy-5-chlorosalicylideneamino)propan-2-ol) (2) were reported. These compounds show antiferromagnetic behaviour (—2 J: 444 cm-1 for 1, 164 cm-1 for 2, and 472 cm-1 for 3). The strength of the super-exchange interaction (—2 J) of 2 is much less than that of 1 and 3, a result which is difficult to explain in terms of structural factors on the basis of widely accepted criteria. The differences in the mag­ netic behaviour have been rationalized in terms of the bridging ligand orbital complementary / countercomplementary concept.