Templated synthesis of Ni(ii) complexes of unsymmetrical Schiff base ligands derived from 1,3-diamino-2-propanol: structural diversity and magnetic properties

Abstract

An attempt at the synthesis of complexes of unsymmetrical Schiff base ligands derived from 1,3-diamino-2-propanol, o-hydroxyacetophenone and salicylaldehyde or o-vanillin by a Ni(II) templated method resulted in the isolation of mononuclear [NiL¹] (1) and tetranuclear [Ni₄L²₂(μ₃-OCH₃)₂(H₂O)₂]·10CH₃OH (2) where H₃L¹ = N-(α-methylsalicylidene)–N′-(salicylidene)–1,3-diamino-2-propanol, and H₃L² = N-(3-methoxysalicylidene)–N′-(α-methylsalicylidene)–1,3-diamino-2-propanol. Complex 1 on reaction with Ni(NO₃)₂ yielded a hexa-nuclear complex, [Ni₂Ni₄L¹₂L³₂(NO₃)₂]·23CH₃OH (3). Ligand H₂L³ was formed in situ by partial hydrolysis of H₃L¹ during the synthesis of 3. Single crystal X-ray diffraction studies revealed a square planar geometry of Ni(II) in complex 1 and a hexa-coordinated distorted octahedral geometry of all four Ni(II) centres, forming a defective dicubane core in complex 2. In hexa-nuclear complex 3, two square planar Ni(II) centres are connected to a distorted cubane core, formed by four octahedral Ni(II). Temperature dependent molar magnetic susceptibility measurements illustrated overall ferromagnetic coupling among the Ni(II) in complex 2 with exchange coupling parameters, J₁ = 10.9(2) cm⁻¹, J₂ = −3.1(1) cm⁻¹, J₃ = 13.8(8) cm⁻¹ and g = 2.200(1) and predominating anti-ferromagnetic coupling interactions among the four paramagnetic Ni(II) in the cubane core of complex 3 with exchange coupling parameters, J₁ = −1.99(1) cm⁻¹, J₂ = 2.00(1) cm⁻¹, J₃ = −1.93(1) cm⁻¹ and g = 2.359(1). Cyclic voltammetry measurements showed quasi-reversible and irreversible Ni(II)/Ni(I) couples for complexes 1 and 2, respectively, whereas complex 3 showed one irreversible and one quasi reversible reduction process for the octahedral and square planar Ni(II) centres, respectively.