Facile strategies for the synthesis and crystallization of linear trinuclear nickel (II)-Schiff base complexes with carboxylate bridges: tuning of coordination geometry and magnetic properties

Abstract

Three new linear trinuclear nickel (II) complexes, [Ni₃(salpen)₂(OAc)₂(H₂O) ₂]•4H₂O (1) (OAc = acetate, CH₃COO⁻), [Ni₂(salpen)₂(OBz)₂] (2) (OBz = benzoate, PhCOO⁻) and [Ni₃(salpen) ₂(OCn)₂(CH₃CN) ₂] (4) (OCn = cinnamate, PhCH CHCOO⁻), H₂salpen = tetradentate ligand, N,N′-bis(salicylidene)-1,3-pentanediamine have been synthesized and characterized structurally and magnetically. The choice of solvent for growing single crystal was made by inspecting the morphology of the initially obtained solids with the help of SEM study. The magnetic properties of a closely related complex, [Ni₃(salpen) ₂(OPh)₂(EtOH)] (3) (OPh = phenyl acetate, PhCH₂COO⁻) whose structure and solution properties have been reported recently, has also been studied here. The structural analyses reveal that both phenoxo and carboxylate bridging are present in all the complexes and the three Ni (II) atoms remain in linear disposition. Although the Schiff base ligand and the syn–syn bridging bidentate mode of the carboxylate group remain the same in complexes 1–4, the change of alkyl/aryl group of the carboxylates brings about systematic variations between six- and five-coordination in the geometry of the terminal Ni (II) centres of the trinuclear units. The steric demand as well as hydrophobic nature of the alkyl/aryl group of the carboxylate is found to play a crucial role in the tuning of the geometry. Variable-temperature (2–300 K) magnetic susceptibility measurements show that complexes 1–4 are antiferromagnetically coupled (J = −3.2(1), −4.6(1), −3.2(1) and −2.8(1) cm⁻¹ in 1–4, respectively). Calculations of the zero-field splitting parameter indicate that the values of D for complexes 1–4 are in the high range (D = +9.1(2), +14.2(2), +9.8(2) and +8.6(1) cm⁻¹ for 1–4, respectively). The highest D value of +14.2(2) and +9.8(2) cm⁻¹ for complexes 2 and 3, respectively, are consistent with the pentacoordinated geometry of the two terminal nickel (II) ions in 2 and one terminal nickel (II) ion in 3.