Synthesis, structures and magnetic properties of polynuclear mixed-valence Mn^IIMn^III complexes containing 3-(2-phenol)-5-(pyridin-2- yl)-1,2,4-triazole ligand

This paper reports the synthesis, crystal structures, and magnetic properties of a series of mixed-valence polynuclear manganese complexes bridged by a dianionic 1,2,4-triazole-based ligand L^2- (H₂L = 3-(2-phenol)-5-(pyridin-2-yl)-1,2,4-triazole). Complexes 1 to 3 show a similar binuclear structure, with the central core of [Mn^IIMn ^III(L)₂] and some other small molecules also coordinated to the two Mn centers. The two [Mn^IIMn^III(L)₂] subunits can be further connected by different dicarboxylic acids to construct non-charged tetranuclear complexes 4 to 7. In these complexes, each deprotonated dianionic ligand L^2- acts as a μ₂-bridged ligand to coordinate two Mn centers via the 1,2,4-triazole ring. Bond valence sum (BVS) calculations and Mn surrounding bond lengths indicate that the Mn center coordinated to the pyridine N atoms in the +2 oxidation state, while another Mn center coordinated to the phenolic O atoms is in the +3 oxidation state. The Mn^III centre shows Jahn-Teller elongation along one of the axes. Magnetic studies show the presence of ferromagnetic Mn^III-Mn ^II coupling in the binuclear (1-3) and tetranuclear (4-7) complexes with the magnetic coupling constant (J) ranging from 0.58 to 1.17 cm ^-1, based on the Hamiltonian H = -2JS₁·S ₂ (S₁ = 5/2 and S₂ = 2). In all cases both z′J′ (intermolecular interactions) and D parameter (Zero Field Splitting of the Mn (III) ion) has been introduced to improve the corresponding fit. The relationship of the coupling interaction between the Mn ^II/Mn^III and their electronic structures is discussed.