Dependence of magnetic coupling on ligands at the axial positions of Ni^II in phenoxido bridged dimers: experimental observations and DFT studies

Abstract

A nickel(ii) complex [Ni₂L^R₂(CH₃CN)₄](ClO₄)₂·2CH₃CN (1) has been synthesized by using a tridentate reduced Schiff base ligand, HL^R, 2-[(3-methylamino-propylamino)-methyl]-4-nitrophenol and Ni(ClO₄)₂·6H₂O. Stoichiometrically controlled addition of NH₄SCN to the acetonitrile-water solution of 1 produced two other complexes, [Ni₂L^R₂(μ_1,1-NCS)(CH₃CN)₂](ClO₄)·CH₃CN (2) and [Ni₂L^R₂(NCS)₂ (CH₃CN)₂] (3) by replacing one and two coordinated CH₃CN by NCS^-, respectively. The dinuclear Ni^II complexes 1 and 3 are structurally very similar containing a diphenoxido bridge between the Ni^II ions. Complex 2 also consists of a dinuclear entity, but a μ_1,1-isothiocyanato bridge is present in addition to the diphenoxido bridge. The magnetic measurements indicate that antiferromagnetic interactions (J = -33.85 cm^-1) for 1, (J = -23.43 cm^-1) and 3 and ferromagnetic interaction (J = 5.01 cm^-1) for 2 are mediated between the intra-dimer Ni^II ions. The nature of coupling (ferromagnetic for 2 and antiferromagnetic for 1 and 3) is understandable from the phenoxido bridging angles (91.27° (av. angle) for 2, 102.41° for 1 and 102.42° for 3) but it cannot explain the considerable difference of J values between 1 and 3. Other important factors such as NiO bond distances, the out-of-plane shift of phenyl rings and the hinge distortions are also of no use for a possible explanation. With the help of computational evidence, we have identified for the first time that magnetic coupling could be dramatically modified by the non-bridging axially coordinated ligands: antiferromagnetic coupling becomes considerably stronger for a neutral ligand in comparison to the negatively charged ligand. Moreover, for a negatively charged axially coordinated ligand, antiferromagnetic coupling decreases with increase in crystal field strength.