Crystal structure, magnetic properties and theoretical study of a bithiazolebis(oxamate)-containing [Ni II3] helicate

Abstract

A new metallosupramolecular compound of formula Na₆Ni₃(dabtzox)₃·10H₂O (1) was prepared by the metal-mediated self-assembly of the potentially tris(chelating) N,N′-2,2′-(4,4′-bithiazole)bis(oxamate) (dabtzox) ligand and its structure was determined by single-crystal X-ray diffraction. The structure of 1 comprises linear centrosymmetric helicate-type [Ni^II₃(dabtzox)₃]⁶⁻ complex anions which are interconnected through hydrated Na^I cations. The two crystallographically independent nickel(II) ions [Ni(1) and Ni(2)] are six-coordinate in distorted octahedral geometries with Ni(1)N₃O₃ and Ni(2)N₆ chromophores. The value of the intra-helicate Ni(1)⋯Ni(2) separation is 4.082(2) Å. The investigation of the static (dc) magnetic properties of 1 in the temperature range 1.9–300 K reveals the occurrence of weak intramolecular ferro- [J₁ = +0.6 cm⁻¹ for Ni(1)⋯Ni(2)] and antiferromagnetic interactions [J₂ = −0.9 cm⁻¹ for Ni(1)⋯Ni(1)ⁱ] together with a moderate zero-field splitting (|D| = 2.3 cm⁻¹), the Hamiltonian being defined as DFT- and CASSCF/NEVPT-type theoretical calculations were carried out to substantiate these parameters and illustrate the occurrence of the strict orthogonality between the d_z² [Ni(1)] and d_(x²−y²) [Ni(2)] magnetic orbitals and the spin polarisation mechanism [Ni(1) and Ni(1)ⁱ] accounting for the unusual intramolecular ferro- and antiferromagnetic interactions, respectively. Dynamic (ac) magnetic susceptibility measurements in the temperature range 2.0–10.0 K showed the lack of any out-of-phase signal for 1, unlike what is observed for the previously reported triple helicate of formula K₆Co₃(dabtzox)₃·8H₂O·MeOH (2) which exhibits intramolecular antiferromagnetic coupling and field-induced slow magnetic relaxation. A detailed magneto-structural comparison between 1 and 2 is carried out to elucidate their different magnetic behaviours.