Nature of Magnetic Interactions in 3D {[MII(pyrazole)4]2[NbIV(CN)8]•4H2O}n (M = Mn, Fe, Co, Ni) Molecular Magnets

The self-assembly of [Nb-IV(CN)(8)](4-) with different 3d metal centers in an aqueous solution and an excess of pyrazole resulted in the formation of four 3D isostructural compounds {[M-II(pyrazole)(4)](2)[Nb-IV(CN)8]center dot 4H(2)O}(n), where M-II= Mn, Fe, Co, and Ni for 1-4, respectively. All four assemblies crystallize in the same l4(1)/a space group and show identical cyanido-bridged structures decorated with pyrazole molecules coordinated to M-II centers. All four compounds show also long-range magnetic ordering below 24, 8, 6, and 13 K, respectively. A thorough analysis of the structural and magnetic data utilizing the molecular field model has allowed for an estimation of the values of coupling constants J(M-Nb) attributed to the one type of M-II-NC-Nb-IV linkage existing in 1-4. The J(M-Nb) values increase monotonically from -6.8 for 1 through -3.1 for 2 and +3.5 for 3, to +8.1 cm(-1) for 4 and are strongly correlated with the number of unpaired electrons on the M-II metal center. Average orbital contributions to the total exchange coupling constants J(M-Nb) have also been identified and calculated: antiferromagnetic JAF = 21.6 cm-1 originating from the d(xy), d(xz) and d(yz) orbitals of M-II and ferromagnetic J(F) = +15.4 cm(-1) originating from de and d(x2-y2) orbitals of Mil. Antiferromagnetic interaction is successively weakened in the 1-4 row with each additional electron on the t20 level, which results in a change of the sign of J(M-Nb) and the nature of long-range magnetic ordering from ferrimagnetic in 1 and 2 to ferromagnetic in 3 and 4.