Strategic synthesis of [Cu2], [Cu4] and [Cu5] complexes: inhibition and triggering of ligand arm hydrolysis and self-aggregation by chosen ancillary bridges

Abstract

The Schiff base ligand HL1 ({2,6-bis(allylimino)methyl}-4-methylphenol) having no coordinating donor arm has been examined for its reaction medium and ancillary bridge dependent reactivity for a hierarchical family of Cu^II complexes. The ligand showed a unique reactivity pattern toward Cu^II in solution. The bridging nature of in situ generated HO⁻ ions in the absence and presence of externally added carboxylates (RCOO⁻; R = CF₃, C₆H₅ and CH₃) has been utilized to produce complexes {[Cu₂(μ-L2)₂(H₂O)]₂[Cu₂(μ-L2)₂(H₂O)₂](ClO₄)₆} (1) (HL2 = 3-{(allylimino)methyl}-2-hydroxy-5-methylbenzaldehyde), [Cu₄(μ₄-O)(μ-L1)₂(μ_1,3-O₂CCF₃)₄] (2), [Cu₄(μ₄-O)(μ-L1)₂(μ_1,3-O₂CC₆H₅)₄]·H₂O (3), and [Cu₅(μ₃-OH)₂(μ-L1)₂(μ_1,3-OAc)₂(OAc)₂(H₂O)₄][Cu₅(μ₃-OH)₂(μ-L1)₂(μ_1,3-OAc)₂(OAc)₃(H₂O)](ClO₄)₃·2C₂H₅OH (4). The absence of carboxylate anions did not yield HO⁻ ions in situ and triggered single ligand arm hydrolysis. The formation of tetra- and pentanuclear aggregates as well as ligand hydrolyzed dinuclear products has been rationalized to identify the possible roles of carboxylate anions in solution. Detailed characterization of the complexes in the solid state and in solution has been carried out using spectroscopic measurements, X-ray crystallography, variable temperature magnetic measurements and functional behavior. In MeOH solutions at 298 K, the complexes 1–4 showed catalytic oxidation of 3,5-di-tert-butyl catechol (3,5-DTBCH₂) saturated with O₂ from the air.