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

Three new Ni^II/Zn^IIheterometallics, [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) oro-vanillin (o-VanH), all deprotonated, as bridging ligands were synthesized and structurally characterized. The Schiff base ligands were producedin situfromo-VanH and CH₃NH₂(HL′), or NH₂OH (HL”), or 2-amino-2-hydroxymethyl-propane-1,3-diol (H₄L′′′); a zerovalent metal (Ni and Zn in1, Zn only in2and3) was employed as a source of metal ions. The first two complexes are dimers with a Ni₂Zn₂O₆central core, while the third compound is a novel heterometallic cocrystal salt solvate built of a neutral zwitterionic Zn^IISchiff base complex and of ionic salt containing dinuclear Ni^IIcomplex cations. The crystal structures contain either centrosymmetric (1and2) or non-symmetric di-nickel fragment (3) with Ni-Ni distances in the range 3.146-3.33 Å. The exchange coupling is antiferromagnetic for1,J= 7.7 cm⁻¹, and ferromagnetic for2,J= −6.5 cm⁻¹(using the exchange Hamiltonian in a formĤ=JŜ₁Ŝ₂). The exchange interactions in1and2are 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) andD= −4.491 cm⁻¹,E= −0.684 cm⁻¹(2). Compound3stands alone with very weak ferromagnetism (J= −0.6 cm⁻¹) and much stronger magnetic anisotropy withD= −11.398 cm⁻¹andE= −1.151 cm⁻¹. Attempts to calculate theoretically the exchange coupling (using the DFT “broken symmetry” method) and ZFS parameters (with theab initioCASSCF method) were successful in predicting the trends ofJandDamong the three complexes, while the quantitative results were less good for1and3.