Coordination dependence of magnetic properties within a family of related [FeII2] complexes of a triazine-based ligand

The coordination chemistry of the polydentate ligand 2,4,6tris(dipyridin-2-ylamino)-1,3,5-triazine (dpyatriz) with Fell has been explored, leading through variation of the counterion and the solvent system to the preparation of three different dinuclear complexes: [Fe-2(dpyatriz)(2)(H2O)(2)(CH3CN)(2)]-(ClO4)(4) (1), [Fe-2(dpyatriz)(2)(H2O)2(CH3OH)(2)](BF4)(4) (2) and [Fe-2(dpyatriz)(2)Cl-2](CF3SO3)(2) (3). The X-ray structure of these compounds has revealed that besides the difference in the noncoordinated anion, complex 1 differs from complex 2 only in the nature of the terminal ligands. Bulk magnetisation studies and Mossbauer spectroscopy have shown that such a subtle difference produces a change to the crystal field on the metal atoms, originating an important disparity of the magnetic behaviour. Thus, complex 1 experiences a partial spin crossover centred at approximately 265 K, whereas complex 2 shows two uncoupled high-spin Fe-II centres over most of the studied temperature range, experiencing the effect of very weak antiferromagnetic coupling and/or single-ion zero-field splitting at low temperature. By contrast, complex 3, in which the triazine ring of dpyatriz is coordinated for the first time, displays (very uncommon) ferromagnetic coupling between both Fell ions within the molecule, leading to an S = 4 spin ground state. A fit of the experimental data led to a value of the coupling constant of J = +0.23 cm(-1) (using H = -2JS(1)S(2) as the convention for the exchange Hamiltonian) and provided an estimation of D = 0.63 cm(-1) for the ground-state axial zero-field splitting parameter. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006).