Ferro- vs. antiferromagnetic exchange between two Ni(ii) ions in a series of Schiff base heterometallic complexes: what makes the difference?

Abstract

Three new Ni^II/Zn^II heterometallics, [NiZnL′₂(OMe)Cl]₂ (1), [NiZnL′′(Dea)Cl]₂·2DMF (2) and [Ni₂(H₃L′′′)₂(o-Van)(MeOH)₂]Cl·[ZnCl₂(H₄L′′′)(MeOH)]·2MeOH (3), containing three-dentate Schiff bases as well as methanol or diethanolamine (H₂Dea) or o-vanillin (o-VanH), all deprotonated, as bridging ligands were synthesized and structurally characterized. The Schiff base ligands were produced in situ from o-VanH and CH₃NH₂ (HL′), or NH₂OH (HL”), or 2-amino-2-hydroxymethyl-propane-1,3-diol (H₄L′′′); a zerovalent metal (Ni and Zn in 1, Zn only in 2 and 3) was employed as a source of metal ions. The first two complexes are dimers with a Ni₂Zn₂O₆ central core, while the third compound is a novel heterometallic cocrystal salt solvate built of a neutral zwitterionic Zn^II Schiff base complex and of ionic salt containing dinuclear Ni^II complex cations. The crystal structures contain either centrosymmetric (1 and 2) or non-symmetric di-nickel fragment (3) with Ni⋯Ni distances in the range 3.146–3.33 Å. The exchange coupling is antiferromagnetic for 1, J = 7.7 cm⁻¹, and ferromagnetic for 2, J = −6.5 cm⁻¹ (using the exchange Hamiltonian in a form Ĥ = JŜ₁Ŝ₂). The exchange interactions in 1 and 2 are comparable to the zero-field splitting (ZFS). High-field EPR revealed moderate magnetic anisotropy of opposite signs: D = 2.27 cm⁻¹, E = 0.243 cm⁻¹ (1) and D = −4.491 cm⁻¹, E = −0.684 cm⁻¹ (2). Compound 3 stands alone with very weak ferromagnetism (J = −0.6 cm⁻¹) and much stronger magnetic anisotropy with D = −11.398 cm⁻¹ and E = −1.151 cm⁻¹. Attempts to calculate theoretically the exchange coupling (using the DFT “broken symmetry” method) and ZFS parameters (with the ab initio CASSCF method) were successful in predicting the trends of J and D among the three complexes, while the quantitative results were less good for 1 and 3.