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

Four new dinuclear complexes [Ni(mu-L)(mu-OAc)Ni(H2O)(2)]NO3 (1), Ni(mu-L)(mu-OBz)Ni(H2O)(MeOH)]NO3 center dot 3MeOH (2), [Ni(mu-L)(mu-9-An)Ni(H2O)(MeOH)]NO3 (3) and [Ni(mu-L)(mu-OAc)Ni(H2O)(N-3)]center dot 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 (beta) in the range 32-36 degrees. The folding of the structure takes place with a concomitant decrease of the O-Ni-O bridging angle (theta) 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 (tau 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 theta angle, but also the tau and beta angles play a significant role in determining the sign and magnitude o the magnetic exchange coupling in these complexes. The ferromagnetic behaviour observed for 1-4 has been justified on the basis of their relatively low theta and high beta and tau 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.