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

• Published in: Inorganic chemistry 2010-08, Vol.49 (16), p.7565-7576
• Main Authors: Pinkowicz, Dawid, Pełka, Robert, Drath, Olga, Nitek, Wojciech, Bałanda, Maria, Majcher, Anna Małgorzata, Poneti, Giordano, Sieklucka, Barbara
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic100937h

The self-assembly of [NbIV(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 {[MII(pyrazole)4]2[NbIV(CN)8]·4H2O} n , where MII = Mn, Fe, Co, and Ni for 1−4, respectively. All four assemblies crystallize in the same I41/a space group and show identical cyanido-bridged structures decorated with pyrazole molecules coordinated to MII 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 MII−NC−NbIV 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 MII metal center. Average orbital contributions to the total exchange coupling constants J M−Nb have also been identified and calculated: antiferromagnetic J AF = −21.6 cm−1 originating from the d xy , d xz , and d yz orbitals of MII and ferromagnetic J F = +15.4 cm−1 originating from d z 2 and d x 2−y 2 orbitals of MII. Antiferromagnetic interaction is successively weakened in the 1−4 row with each additional electron on the t2g 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.