High-Temperature Experimental and Theoretical Study of Magnetic Interactions in Diamond and Pseudo-Diamond Frameworks Built up from Hexanuclear Tantalum Clusters

Magnetic interactions in solid-state tantalum cluster compounds have been evidenced by using magnetic susceptibility measurements and corroborated by broken-symmetry DFT calculations. The three selected compounds are based on [Ta6X12(H2O)(6)](3+) (X= Cl or/and Br) units with edge-bridged Ta-6 octahedral clusters. Although two of them crystallise in the tetragonal space group I4(1)/a, all compounds exhibit a similar arrangement of paramagnetic clusters related to the diamond structural framework (Fd (3) over barm space group). Magnetic parameters were fitted by using the [5,4] Pade approximant of high-temperature series expansion of susceptibility for the Heisenberg model (S= 1/2) in the diamond framework, assuming only nearest-neighbouring interactions. Such a model appropriately describes magnetic-susceptibility measurements at temperatures T > 0.7 |J|/ k. The magnetic interaction parameter J between two [Ta6Cl12(H2O)(6)](3+) clusters is estimated to be -64.28(7) cm(-1); it has been enhanced by replacing several chlorine inner ligands with bromine ones (J= -123(3) cm(-1) for two [Ta6Br7.7(1) Cl-4.3(1) (H2O)(6)](3+) clusters) and is strongest between two bromine [Ta6Br12(H2O)(6)](3+) clusters with a value of -155(1) cm(-1). Broken-symmetry DFT calculations within spin-dimer analysis confirmed this trend. Those interactions can be explained on the basis of the overlap between singly occupied a(2u) orbitals localised on neighbouring clusters.