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Reliable prediction of the ground-state spin and magnetic coupling constants in transition-metal complexes is a well-known challenge for density functional theory (DFT). One popular strategy for addressing this long-standing issue involves the modification of the fraction of Fock exchange in a hybrid functional. Here we explore the viability of this approach using three polynuclear metal-organic complexes based on a Ni4O4 cubane motif, having different ground state spin values (S = 0, 2, 4) owing to the use of different ligands. We systematically search for an optimum fraction of Fock exchange, across various global, range-separated, and double hybrid functionals. We find that for all functionals tested, at best there only exists a very narrow range of Fock exchange fractions which results in a correct prediction of the ground-state spin for all three complexes. The useful range is functional dependent, but general trends can be identified. Typically, at least two similar systems must be used in order to determine both an upper and lower limit of the optimal range. This is likely owing to conflicting demands of minimizing delocalization errors, which typically requires a higher percentage of Fock exchange, and addressing static correlation, which typically requires a lower one. Furthermore, we find that within the optimal range of Fock exchange, the sign and relative magnitude of Ni–Ni magnetic coupling constants are reasonably well reproduced, but there is still room for quantitative improvement in the prediction. Thus, the prediction of spin state and magnetic coupling in polynuclear complexes remains an ongoing challenge for DFT. [ABSTRACT FROM AUTHOR]

A numerically exact spin-Hamiltonian approach has been proposed for the frustrated Cr8Ni molecule. The non-uniform exchange couplings parameters, improving the fit of the experimental magnetic susceptibility data, have been obtained, using a genetic algorithm search procedure. The energy intervals between the lowest multiplets, relevant for envisaged transitions observable in the INS spectra, have been determined and the critical fields corresponding to the first-level crossing have been estimated in agreement with experiment.

A new family of [3 + 3] hexanuclear 3d–4f complexes [(μ3-CO3){CoIILnIIIL(μ3-OH)(OH2)}3]-(ClO4)·mC2H5OH·nH2O (1–5) [Ln = La (1), Gd (2), Tb (3), Dy (4), and Ho (5)] have been prepared in moderate to high yields (62–78%) following a self-assembly reaction between the ligand 6,6′,6′′-(nitrilotris(methylene))tris-(2-methoxy-4-methylphenol) (H3L), Co(OAc)2·4H2O and the lanthanide ion precursors in the mandatory presence of tetrabutylammonium hydroxide. During the reaction, atmospheric carbon dioxide is fixed in the product molecule as a bridging carbonato ligand which connects all the three lanthanide centers of this molecular assembly through a rare η2:η2:η2–μ3 mode of bridging as revealed from X-ray crystallography. The metal centers in all these compounds, except the GdIII analogue (2), are coupled in antiferromagnetic manner while the nature of coupling in the CoII3GdIII3 complex is ferromagnetic. DFT calculations revealed that this ferromagnetic interaction occurs most likely by the CoII–GdIII superexchange, mediated via the bridging oxygen atoms. Only the CoII–DyIII compound (4) displayed a slow relaxation of the magnetization at a very low temperature as established by AC susceptibility measurements. The data provides an estimation of the activation energy U/kB = 9.2 K and the relaxation time constant τ0 = 1.0 × 10−7 s. [ABSTRACT FROM AUTHOR]

Abstract: The magnetic properties of the bimetallic compound {(CuL1)[Co(NCS)4]}, where L1 = N-rac-5,12-dimethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene have been numerically analysed on the basis of the anisotropic Heisenberg spin-chain model. In the model the single-ion anisotropy of the Co(II) ions is taken into account and the interchain interactions are neglected. The thermodynamic properties are calculated using the DMRG technique adapted to the molecular-based chains for the first time which is reliable in the entire temperature region. The high accuracy results of our simulations have been successfully fitted to the corresponding experimental susceptibility and magnetization data measured for the powder sample and the model parameters (the exchange coupling J, the absolute value of the anisotropy constant D, the anisotropic g factors) have been estimated. The quantum model parameters are consistent with the previous findings, although the ferromagnetic interactions between Cu(II) and Co(II) ions through the thiocyanate bridge are somewhat weaker (J/kB =1.0±0.1) than found before. [Copyright &y& Elsevier]

A new family of [3 + 3] hexanuclear 3d-4f complexes [(μ 3 -CO 3 ){Co II Ln III L(μ 3 -OH)(OH 2 )} 3 ]-(ClO 4 )·mC 2 H 5 OH·nH 2 O (1-5) [Ln = La (1), Gd (2), Tb (3), Dy (4), and Ho (5)] have been prepared in moderate to high yields (62-78%) following a self-assembly reaction between the ligand 6,6',6''-(nitrilotris(methylene))tris-(2-methoxy-4-methylphenol) (H 3 L), Co(OAc) 2 ·4H 2 O and the lanthanide ion precursors in the mandatory presence of tetrabutylammonium hydroxide. During the reaction, atmospheric carbon dioxide is fixed in the product molecule as a bridging carbonato ligand which connects all the three lanthanide centers of this molecular assembly through a rare η 2 :η 2 :η 2 -μ 3 mode of bridging as revealed from X-ray crystallography. The metal centers in all these compounds, except the Gd III analogue (2), are coupled in antiferromagnetic manner while the nature of coupling in the CoGd complex is ferromagnetic. DFT calculations revealed that this ferromagnetic interaction occurs most likely by the Co II -Gd III superexchange, mediated via the bridging oxygen atoms. Only the Co II -Dy III compound (4) displayed a slow relaxation of the magnetization at a very low temperature as established by AC susceptibility measurements. The data provides an estimation of the activation energy U/k B = 9.2 K and the relaxation time constant τ 0 = 1.0 × 10 -7 s.