Mono-, Tetra- and octanuclear transition metal complexes of in situ generated schiff baseligands containing up to 12 coordinating atoms: syntheses, structures and magnetism

Abstract

By the reaction of an alkoxyl containing tetraamine, 1,3-bis[(2-aminoethyl)amino]-2-propanol, with various aldehydes, O-vanillin and 2,6-diformyl-4-cresol, in the presence of various metal cations, four novel complexes were synthesized and structurally characterized. Complexes [CoHL¹](ClO₄)·2MeOH (1) and [NiH₂L²](ClO₄)₂·0.5MeOH (2) have mononuclear structures, with the linear di-Schiff base ligand (HL¹)²⁻ and mono-Schiff base ligand H₂L² generated from in situ reactions under various pH conditions. In complexes [Ni₄L³(SCN)₃(MeOH)]Cl·2.5MeOH (3), and [Cu₈(H₂L³)₂(OH)₄(ClO₄)](SCN)₆(ClO₄)·8H₂O (4), an interesting 32-membered dodecadentate macrocyclic ligand H₄L³ comprising four Schiff base CN double bonds was generated by a “2+2” type condensation reaction between the tetraamine and the dialdehyde. Complex 3 has a tetranuclear Ni₄ coordination structure with all 12 coordinating atoms of (L³)⁴⁻ utilized for coordination, and the alkoxyl groups are deprotonated and coordinated in a bridging mode. Complex 4 exhibits an octanuclear Cu₈ coordination structure, with a ClO₄⁻ bridging two tetranuclear [Cu₄H₂L³(OH)₂]⁴⁺ moieties through four weak axial Cu–O coordination bonds. In complex 4, only 10 coordinating atoms of (L³)⁴⁻ are utilized for coordination, with the two alkoxyl groups left neutral and noncoordinated. It is noteworthy that H₄L³ has two long saturated chains in the molecule. Therefore, it has good flexibility and tends to have a nonplanar structure. Variable temperature magnetic data for 3 and 4 indicated the existence of antiferromagnetic interactions, which were discussed based on the structural results.