Dinuclear Complexes with a Triple N1,N2-Triazole Bridge That Exhibit Partial Spin Crossover and Weak Antiferromagnetic Interactions



The reaction of 4-phenylimino-1,2,4-triazole (1) with FeII, CoII, NiII and CuII thiocyanate produces a series of analogous dinuclear compounds of formula [M2(1)5(NCS)4] (25) as demonstrated by single-crystal X-ray diffraction studies of the FeII (2) and CoII (3) analogues. The magnetic properties of [Fe2(1)5(NCS)4]·xMeOH (x = 3.5–5) reveal a partial and gradual spin crossover (SCO) centred at TSCO = 115 K. This is confirmed by its crystal structure solved at 100, 150 and 250 K, which exhibits a gradual decrease of the Fe–N bond lengths with temperature. However, the bulk hydrated form of 2 that is generated upon exposure to air of crystals is a high-spin compound that exhibits weak antiferromagnetic interaction. The exchange coupling among the FeII S = 2 ions within the dinuclear neutral complex was evaluated as J/kB = –1.33(3) K by using the Heisenberg Hamiltonian H = –2JS1·S2. Similarly, the magnetic properties of the NiII (4) and CuII (5) analogues are dominated by moderate and weak antiferromagnetic interactions evaluated as J/kB = –13.9(3) and –0.30(5) K, respectively. The presence of strong spin–orbit coupling of the individual CoII ions impeded the evaluation of the likely antiferromagnetic interaction that leads to a singlet ground state in 3. The reported structures of 2 and 3 are new additions to a very scarce family of dinuclear complexes bearing a unique triple N1,N2-triazole bridge. Owing to its relevance in the peculiar properties of 1D triazole-based SCO materials, which are widely studied for their various potential applications, a structural analysis of this triple N1,N2-triazole bridge in reported structures of FeII and CoII trinuclear and 1D compounds is provided.