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The self-assembly of [NbIV(CN)8]4- with different 3d metal cenfers in an aqueous solution and an escess 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, 6, 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 JM-Nb attributed to the one type of MII-NC-NbIV linkage existing in 1-4. The JM-Nb values increase monotonically from -6.6 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 JM-Nb have also been identified and calculated: antiferromagnetic JAF = -21.6 cm-1 originating from the dxy, dxz, and dyz orbitals of MII and ferromagnetic JF = +15.4 cm-1 originating from dz2 and dx2-y2 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 JM-Nb and the nature of long-range magnetic ordering from ferrimagnetic in 1 and 2 to ferromagnetic in 3 and 4. [ABSTRACT FROM AUTHOR]

Graphical abstract A combination of dinuclear Fe O Fe and tetranuclear Co CN Fe O Fe NC Co units with strong intramolecular antiferromagnetic coupling. Abstract The synthesis, characterization, structure and magnetic properties of a hexametallic mixed-metal iron(III)–cobalt(III) compound are described. The compound has been characterized by standard spectroscopic and analytical methods to determine its composition. Single-crystal X-ray diffraction has shown that the asymmetric unit consists of discrete dinuclear dicationic units of {(µ-oxido-µ-[sulfato- O1 , O2 ])bis[tris(2-pyridyl-methyl)amineiron(III)]}2+, herein designated as Fe12, and two half-dinuclear dianionic units of {(µ-oxido)bis[(µ-cyanido- κN -pentacyanidocobaltato(III))(tris(2-pyridyl-methyl)-amine)iron(III)]}2− units, herein designated as Fe33a and Fe44b, generating the overall composition Co 2 Fe 4 O 2 (CN) 12 (tpa) 4 , 1 , where tpa is tris(2-pyridyl-methyl)amine. X-ray structural results and thermogravimetric and differential scanning calorimetry measurements also indicate that the compound, depending upon its history, may contain up to nine interstitial waters of hydration, herein designated as 1 (H 2 O) 9. In the dinuclear dicationic unit, the bridging Fe O Fe bond in Fe12 is bent and there is also an Fe O S O Fe sulfato-based bridge with an angle of 132.8°. In contrast, in the two dinuclear dianionic units, Fe33a and Fe44b, the Fe O Fe bond angle is crystallographically constrained to be 180°. The Co CN Fe bonds are almost co-linear, with Co C N angles of 176° and C N Fe angles of 169°. In each species the tpa ligand is tripodal tetradentate with the tertiary amine trans to the sulfato ligand or to the cyanide ligand in the dianions or the bridged oxido ligand in the dications. Bond lengths and angles are all in the typical range for Fe(III) and Co(III) compounds. The magnetic behavior of 1 (H 2 O) 9 , obtained upon cooling from 300 to 2 K, reveals a strong antiferromagnetic interaction between the Fe(III) ions in each dinuclear unit. Attempts to discriminate between the two Fe(III) dinuclear units in 1 (H 2 O) 9 have in all cases led to two very different Heisenberg isotropic exchange coupling constants, namely J = −220(2) and −716(32) cm−1 for 1 (H 2 O) 9 ; i.e. one of the dinuclear units, probably the Fe12 unit, is so strongly antiferromagnetically coupled that it is close to diamagnetic between 2 and ca. 250 K and has a Heisenberg S = 0 ground state. [ABSTRACT FROM AUTHOR]

A new dinuclear Fe(II) complex with the general formula [Fe 2 (1,4-tpbd)(dca) 4 (H 2 O) 2 ] ( 1 ) (in which 1,4-tpbd = N,N,N′,N′ -tetrakis(2-pyridylmethyl)benzene-1,4-diamine; dca = dicyanamide, [N(CN) 2 ] − ), has been prepared under hydrothermal conditions and fully characterized by elemental analysis, infrared spectroscopy, X-ray diffraction, and magnetic measurements. The compound crystallizes in the triclinic system and P -1 space group with Z = 1 and the following unit cell parameters: a = 8.165(5) Å, b = 10.236(5) Å, c = 13.105(5) Å, α = 81.421(5)°, β = 78.810(5)°, γ = 72.941(5)°, and V = 1022.2(9) Å 3 . The Fe(II) ions are six-coordinated, in distorted octahedral fashion, by three N atoms of tpbd ligand, one water oxygen atom and two N atoms of two dicyanamide anions. The intra-dinuclear Fe⋯Fe distance via the bridged μ 2 -tpbd ligand is 8.293(8) Å. In the molecular arrangement, the combination of O H⋯N and C H⋯N hydrogen bonds links the title complex to form a three-dimensional supramolecular structure. Magnetic susceptibility measurements (2–300 K) show that 1 presents weak antiferromagnetic exchange between the two nearest Fe 2+ ions with the coupling constant J = −1.05 cm −1 . [ABSTRACT FROM AUTHOR]