Magnetic exchange switching in a series of niobium(IV)-based molecular magnets: chemical design vs. mechanical stimulus

A series of isostructural magnetic coordination polymers {[MII(pyrazole)4]2[NbIV(CN)8].4H2O}n (M = Mn, Fe, Co, Ni for 1, 2, 3 and 4, respectively) show long-range magnetic ordering below 24, 8, 6 and 13 K due to the strong magnetic coupling between M and Nb. The exchange coupling changes from -6.8 for 1, through -3.1 for 2, +3.5 for 3 to +8.1 cm-1 for 4. This affects the nature of the long-range magnetic ordering in the 1-4 series: ferrimagnetic in 1 and 2 with antiparallael alignment of the Mn-Nb and Fe-Nb magnetic moments to ferromagnetic in 3 and 4 with parallel alignment. The proposed XAS/XMCD experiments performed at the Nb L2,3 edges with the application of sum rules are the only reliable way to unequivocally determine the magnetic structure of the four compounds. Moreover, compound 3 demonstrates switching of the type of long-range magnetic ordering under pressure. This will also be investigated by collecting XAS/XMCD spectra at the Nb L2,3 edge under high pressure.