Two tetranuclear Ni complexes, namely [Ni(L)(CHOH)(HO)]·CHOH (1) and (PrNH)[Ni(L)(CHCOO)] (2, PrN = tripropylamine), were synthesized from a tridentate Schiff base ligand HL (2-[(E)-(2-hydroxybenzylidene)amino]phenol) and Ni(CHCOO)·4HO, using different solvents and their ratios (CHOH and/or CHCl). The prepared Ni complexes are of different structural types, involving an NiO cubane-like core (1) and NiO defective dicubane-like core (2), with all the Ni atoms hexacoordinated. The complexes were characterized by elemental analysis, FT-IR spectroscopy, variable temperature and field magnetic measurements, and single crystal X-ray analysis. The DFT and CASSCF/NEVPT2 theoretical calculations were utilized to reveal information about the isotropic exchange parameters (J) and single-ion zero-field splitting parameters (D, E). The variable temperature magnetic data suggested the competition of the antiferromagnetic and ferromagnetic intracluster interactions in compound 1, which is in contrast to compound 2, where all intracluster interactions are ferromagnetic resulting in the ground spin state S = 4 with an easy-axis type of anisotropy quantified by the axial zero-field splitting parameter D = -0.81 cm. This resulted in the observation of a field-induced slow-relaxation of magnetization (U = 3.3-6.7 K), which means that the complex 2 represents the first Ni single-molecule magnet with the defective dicubane-like topology.

The synthesis, and the structural and magnetic properties, of the following new iron(III) Schiff base complexes with the {O',N,O″}-chelating ligand H2L (2-hydroxyphenylsalicylaldimine) are reported: K[FeL2]·H2O (1), (Pr3NH)[FeL2]·2CH3OH (2), [FeL(bpyO2) (CH3OH)][FeL2]·CH3OH (3), [Fe2L3(CH3OH)]·2CH3OH·H2O (4), and [{Fe2L2}(μ-OH)2{FeL(bpyO2)}2][BPh4]2·2H2O (5), where Pr3NH(+) represents the tripropylammonium cation and bpyO2 stands for 2,2'-bipyridine-N-dioxide. A thorough density functional theory (DFT) study of magnetic interactions (the isotropic exchange) at the B3LYP/def-TZVP level of theory was employed, and calculations have revealed superexchange pathways through intramolecular/intermolecular noncovalent contacts (π-π stacking, C-H···O and O-H···O hydrogen bonds, diamagnetic metal cations) and/or covalent bonds ((μ-O(Ph), μ-OH) or bis(μ-O(Ph)) bridging modes), which helped us to postulate trustworthy spin Hamiltonians for magnetic analysis of experimental data. Within the reported family of compounds 1-5, the mediators of the antiferromagnetic exchange can be sorted by their increasing strength as follows: π-π stacking (J(DFT) = -0.022 cm(-1)/J(mag) = -0.025(4) cm(-1) in 2) < C-H···O contacts and π-π stacking (J(DFT) = -0.19 cm(-1)/J(mag) = -0.347(9)cm(-1) in 1) < O-H···O hydrogen bonds (J(DFT) = -0.53 cm(-1)/J(mag) = -0.41(1) cm(-1) in 3) < bis(μ-O(Ph)) bridge (J(DFT) = -13.8 cm(-1)/J(mag) = -12.3(9) cm(-1) in 4) < (μ-O(Ph), μ-OH) bridge (J(DFT) = -18.0 cm(-1)/J(mag) = -17.1(2) cm(-1) in 5), where J(DFT) and J(mag) are the isotropic exchange parameters derived from DFT calculations, and analysis of the experimental magnetic data, respectively. The good agreement between theoretically calculated and experimentally derived isotropic exchange parameters suggests that this procedure is applicable also for other chemical and structural systems to interpret magnetic data properly.

Two tetranuclear Ni4 and Co4 complexes were prepared and characterized. Their magnetic properties were thoroughly studied and it was revealed that the Co4 compound behaves as a zero-field single-molecule magnet.