Metal complex analogues of crown ethers as the preorganized motif to stabilize aquated proton in solid state

Abstract

The work in this report presents the syntheses, characterization and crystal structures of [{Cu^IIL^op}₂(H₅O₂)](ClO₄) (1), [{Cu^IIL^hex}₂(H₅O₂)](ClO₄) (2) and [{Cu^IIL^en}₂(H₅O₂)](ClO₄) (3) derived from three 3-ethoxysalicylaldehyde-diamine Schiff base compartmental ligands H₂L^op, H₂L^hex and H₂L^en, in which the diimine moieties come from ortho-phenylenediamine, trans-1,2-diaminocyclohexane and ethylenediamine, respectively. Compounds 1 and 2 crystallize in triclinic crystal system having P space group, while the crystal system and space group of compound 3 is monoclinic P2₁/c. In the [{Cu^IIL^op/hex/en}₂(H₅O₂)]⁺ cation in 1–3, the two OH₂ sites of the aquated proton, H₅O₂⁺, interact with two O(phenoxo)₂O(ethoxy)₂ compartments of the two [Cu^IIL^op/hex/en] moieties by forming bifurcated hydrogen bonds and thus [{Cu^IIL^op/hex/en}₂(H₅O₂)]⁺ may be considered as a supramolecular dimer of two mononuclear moieties, self-assembled by H₅O₂⁺ as the tecton. On the basis of the extent of the difference in the two O–H distances involving the central hydrogen atom, H₅O₂⁺ moieties in 1–3 may be considered as H₂O⁺–H⋯OH₂ (in 1) or as the resonance hybrid of H₂O⁺–H⋯OH₂ and [H₂O⋯H⋯OH₂]⁺ (in 2 and 3). A unique aspect in the composition of 1–3 in terms of the interaction of metal complexes with an aquated proton has been discussed.