Controlled assemblies of hepta- and trideca-CoII clusters by a rational derivation of salicylalde schiff bases: microwave-assisted synthesis, crystal structures, ESI-MS solution analysis and magnetic properties

Abstract

Hydrothermal reaction of Co(ClO₄)₂·6H₂O with 2-ethoxy-6-(iminomethyl)phenol (Heimp) or 3-ethoxysalicylaldoxime (H₂eosa) leads to the formation of two completely different Co^II complexes [Co₇(eimp)₆(OMe)₆]·2ClO₄ (1) and [Co₁₃(eosao)₈(Heosao)₆(ClO₄)₄]·H₂O (2). Compound 1 is a disc-like heptanuclear Co^II cluster with all the eimp⁻ ligands displaying the η¹: η²: η¹: μ₂ binding mode. Compound 2 is a butterfly-like nano-tridecanuclear Co^II cluster and the oximation of iminomethyl group results in its complicated binding modes. Four of the six Heosao⁻ ligands adopt the η¹: η²: η¹: μ₂ mode and the other two take the η¹: η²: η¹: η¹: μ₃ fashion, while eight eosao²⁻ ligands are in the η²: η¹: η²: μ₄, η¹: η³: η¹: η²: μ₅ manner. Microwave heating during the synthetic process will dramatically improve the reaction rate and yield of 1, which however, is not valid for the formation of 2. Electrospray ionization mass spectrometry (ESI-MS) solution analysis was performed, which reveals a ligand substitution reaction of the core bridge for 1. For 2, the result of ESI-MS indicates that the Co₁₃ cluster still exists in solution, and maintains a conformation similar to that observed in the crystal structure. Magnetic studies show that 1 exhibits a dominated ferromagnetic coupling between the Co^II ions below 30 K, while 2 is a typical antiferromagnetic system.