Surprising difference between two closely similar O(phenoxo)2O(ether)2 compartments as hosts for an aquated proton and a novel type of host–guest system

With the aim of checking whether the O(phenoxo)2O(methoxy)2 compartment of 3-methoxysalicylaldehyde-diamine ligands can interact with an aquated proton, the work in this investigation presents the syntheses, characterization, crystal structures and diffuse reflectance spectra of the products obtained from the reactions of three mononuclear copper(II) compounds derived from three H2LOMe ligands (H2LOMe–en, H2LOMe–op and H2LOMe–hex) with perchloric acid in DMF, MeOH and MeOH–MeCN, where ‘en’, ‘op’ and ‘hex’ in the superscripts of the ligand symbols indicate that the diamine functionality comes from ethylenediamine, o-phenylenediamine and trans-1,2-diaminocyclohexane, respectively. From these reactions, the following six compounds have been isolated: [{CuIILOMe−en}(Me2NH2)](ClO4) (1), [{CuIILOMe−op}(Me2NH2)](ClO4) (2), [{CuIILOMe–hex}(Me2NH2)](ClO4) (3), [{CuIILOMe–en}2(enH2)](ClO4)2 (4, where enH2 means diprotonated ethylenediamine), [{CuIILOMe−op}2(opdaH2)](ClO4)2·2H2O (5, where opdaH2 means diprotonated o-phenylenediamine) and [{CuIILOMe−hex}(MeOH2)](ClO4) (6). The Me2NH2+ moiety in 1–3, NH3+ sites in 4 and 5 and MeOH2+ moiety in 6 interact with the O4 compartment by forming bifurcated hydrogen bonds. Interestingly, compound 6 is an unprecedented type of host–guest system. The individual units in 1–6 are self-assembled by non-covalent interactions to generate one, two and three-dimensional self-assemblies. A spectroscopic correlation has been enlightened.

Treatment of copper(II) compounds of 3-methoxysalicylaldehyde-diamine ligands with acid does not result in the interaction of an aquated proton with the O4 compartment, but hydrolyses DMF (solvent) or the diimine moiety of the ligands, or even attacks MeOH (solvent). This phenomenon is remarkably different from that of 3-ethoxysalicylaldehyde-diamine ligands, where an aquated proton is trapped in similar reactions.Figure optionsDownload as PowerPoint slide