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 [Ta 6 X 12 (H 2 O) 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 I 4 1 / a , all compounds exhibit a similar arrangement of paramagnetic clusters related to the diamond structural framework ( Fd $\bar 3$ m space group). Magnetic parameters were fitted by using the [5,4] Padé 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 [Ta 6 Cl 12 (H 2 O) 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 [Ta 6 Br 7.7(1) Cl 4.3(1) (H 2 O) 6 ] 3+ clusters) and is strongest between two bromine [Ta 6 Br 12 (H 2 O) 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.