Piperazine linked salicylaldoxime and salicylaldimine-based dicopper(ii) receptors for anions

Abstract

The syntheses and single crystal X-ray analyses of five strapped salicylaldoxime/salicylaldimine based dicopper(II) receptors utilising a new piperazine linker are described. The complexes 1–4 form 2 + 2 metallocycles and the molecular structures of all four complexes possess a small internal cavity with the utilisation of a short piperazine linker. The molecular structures of complexes [Cu₂(L⁴ − H)(L⁴ − 2H) ⊂ DMF]BF₄·DMF, 1 and [Cu₂(L⁴ − H)₂Br]Br·1.25DMSO·H₂O·MeOH, 2 show that intramolecular H-bonding interactions due to the presence of –OH (oxime moiety) groups lead to a Pacman-like cleft arrangement of the two metal coordinating subunits in the metallo-macrocycle. The geometrical constraints brought about by this constrained cleft make the receptor coordinate strongly to a bromide anion involving both metal centres as evidenced by 2 whereas in 1 the larger tetrafluroborate anion is excluded. Absence of the oxime moiety around the metal coordination site of the ligand as demonstrated in the complexes [Cu₂(L⁵)₂BF₄](BF₄)₃, 3 and [Cu₂(L⁵)₂Br]Br₃·2MeOH, 4 resulted in less constrained dicopper(II) helicate forms. For these complexes no anion size discrimination was observed. The addition of pyridine solvent to a slightly modified piperazine-linked ligand produces an expanded 3 + 3 tube-like tricopper complex [Cu₃(L^4a − H)₃Py₃](BF₄)₂·(MeOH)₃·PF₆·(H₂O)₃, 5, with two coordinated pyridine molecules occupying the newly formed cavity.