Rational design of ferromagnetic coupled diphenoxocarboxylate triply bridged dinuclear nickel(ii) complexes: orbital countercomplementarity of the bridging ligands

Abstract

Four new dinuclear complexes [Ni(μ-L)(μ-OAc)Ni(H₂O)₂]NO₃ (1), Ni(μ-L)(μ-OBz)Ni(H₂O)(MeOH)]NO₃·3MeOH (2), [Ni(μ-L)(μ-9-An)Ni(H₂O)(MeOH)]NO₃ (3) and [Ni(μ-L)(μ-OAc)Ni(H₂O)(N₃)]·CH₃OH (4) have been prepared from the compartmental ligand N,N′,N′′-trimethyl-N,N′′-bis(2-hydroxy-3-methoxy-5-methylbenzyl)diethylenetriamine (H₂L) and different carboxylate ligands (OBz = benzoate and 9-An = 9-anthracenecarboxylate), and then magnetically and structurally characterized. The X-ray structures of these complexes reveal that the Ni²⁺ ions are bridged by two phenoxo groups belonging to the L²⁻ ligand and one syn–syn carboxylate group, giving rise to diphenoxocarboxylate triply bridged dinuclear nickel(II) complexes. The syn–syn carboxylate bridging group folds the structure with hinge angles (β) in the range 32–36°. The folding of the structure takes place with a concomitant decrease of the O–Ni–O bridging angle (θ) and an increase of the shift carbon atom directly linked to the phenoxo oxygen atom with respect to the Ni–Ni–O plane in the bridging region (τ angle). All the complexes exhibit ferromagnetic interactions between the slightly distorted octahedral Ni²⁺ ions. A comparative analysis of the magneto-structural data for 1–4 and other diphenoxocarboxylate bridged complexes clearly show that not only the θ angle, but also the τ and β angles play a significant role in determining the sign and magnitude of the magnetic exchange coupling in these complexes. The ferromagnetic behaviour observed for 1–4 has been justified on the basis of their relatively low θ and high β and τ values, all of them decreasing the antiferromagnetic contribution in complexes 1–4, as well as the countercomplementarity effects between the diphenoxo and carboxylate bridging ligands. DFT theoretical calculations unequivocally demonstrate that the countercomplementarity mechanism is operative in compounds 1–4.