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Magnetic Interactions in Hexanuclear Tantalum Halide Cluster Compounds with a Diamond Related Framework

The occurrence of intercluster magnetic exchange interactions in selected solid-state tantalum cluster compounds has been evidenced by means of magnetic susceptibility measurements and broken symmetry DFT calculations. The compounds are based on the paramagnetic [Ta6X12(H2O)6]3+ (X = Cl or/and Br) halides with edge-bridged Ta6 octahedral clusters. They all show a similar structural arrangement of cluster units with a diamond related framework. Magnetic susceptibility measurements are in accordance with the magnetic model of Heisenberg antiferromagnet (S = 1/2) in a diamond lattice, concerning only the interactions between nearest neighbours. The [5, 4] Padé approximant of the high temperature series expansion for susceptibility was found to be an accurate theoretical model for these systems in the temperature region above 0.7 J/k. The experimental results point out that the interaction between cluster units is enhanced by replacing chlorine inner-ligands for bromine ones. The interaction between two [Ta6Cl12(H2O)6]3+ cluster units in [Me4N][Ta6Cl12(H2O)6]Br4 is characterized by a J value of –92 K (–64 cm−1), J for the interaction between two [Ta6Br7.7(1)Cl4.3(1)(H2O)6]3+ clusters in [Et4N][Ta6Br7.7(1)Cl4.3(1)(H2O)6]Br4•4H2O is estimated to be –177 K (–123 cm−1) and J for the interaction between two [Ta6Br12(H2O)6]3+ clusters in [Et4N][Ta6Br12(H2O)6]Br4•4H2O is estimated to be –223 K (–155 cm−1). Broken symmetry DFT calculations corroborate this result. From the X-ray crystallographic data for [Me4N][Ta6Cl12(H2O)6]Br4 and [Et4N][Ta6Br12(H2O)6]Br4• 4H2O, the energy of the BS and T spin states of dimmers [Ta6X12(H2O)6]26+ (X = Cl, Br) has been computed. In both cases the BS spin state is more stable than the T state ; (EBS – ET’) is equal to –0.018 and –0.058 eV for Cl and Br dimmers, respectively. Within a strong delocalized limit (overlap Sab = 1), this corresponds to the theoretical J values of –145 and –468 cm−1, respectively. Considering the influence of the inner-ligand, the same trend as in calculated J values is observed for the data obtained from the measurements. This suggests that the magnetic exchange interaction between clusters can be explained by an overlap between singly occupied orbitals (a2u symmetry) localized on neighbouring cluster units. This overlap is enhanced for clusters with bromine inner-ligands because bromine atoms possess more diffused valence orbitals than chlorine ones.

cluster compounds; density functional calculations; magnetic properties; tantalum; X-ray diffraction

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