Aryl-Linked Salicylaldoxime-Based Copper(II) Helicates and “Boxes”: Synthesis, X‑ray Analysis, and Anion Influence on Complex Structure Ajay Pal Singh Pannu, James R. Stevens, and Paul G. Plieger* Institute of Fundamental Sciences, Massey University, Private Bag 11 222, Palmerston North, New Zealand * S Supporting Information ABSTRACT: The synthesis and spectroscopic analysis of both “metal-only” and anion encapsulated salicylaldoxime-based complexes utilizing a new 1,3- xylyl strap are described. X-ray crystallographic analysis reveals that the aromatic spacer restricts the confirmation flexibility of the resulting complexes leading to dicopper(II) double helicate and dicopper(II) 2 + 2 “box” structural forms. The choice of the structural motif is influenced by the anion present, with the copper(II) nitrate-containing complex [NO 3 ⊂(Cu 2 L 3 2 )](NO 3 ) 3 , 4, adopting a double helicate form, whereas the analogous copper(II) bromide complexes [2Br⊂(Cu 2 L 3 2 )](Br) 2 , 5, and [2Br⊂(Cu 2 L 3 2 )](BF 4 ) 2 , 6, both adopt 2 + 2 “box” structural configurations. Spectroscopic analysis has shown an enhancement in the binding strength of ClO 4 - over the anions SO 4 2- and NO 3 - . The enhanced rigidity caused by the use of the 1,3-xylyl spacer in this series of complexes has favored the formation of the “double loaded” dibromide complex. ■ INTRODUCTION Over the last few decades, the field of selective anion recognition has attracted much attention and has developed rapidly as a subdiscipline of supramolecular chemistry. The initiative taken by Park and Simmons to describe how simple bicyclic diaza katapinanda could encapsulate halide ions 1 marked the birth of “anion coordination chemistry”. This work inspired many efforts into the research of ammonium and polyammonium-based anion receptors around that time. 2 Hydrogen bonding and/or electrostatic interactions coupled with topological complementarity between protonated amines and the anion guests govern the binding in these systems. A number of researchers are now involved in this fast evolving field, which includes a wide variety of different types of amine- or ammonium-based receptors. 3 Among such systems, the synthesis of ditopic receptors designed for simultaneous binding of both cations and their attendant anions is a further challenging task due to the specific requirements needed to be met by both the metal coordinating site and the anion binding site/pocket. 4 Nevertheless, such innovative and well-designed receptors have been reported on a regular basis. 5 In the past few years, we have been actively designing new polyammonium-based ditopic receptors and exploring the use of dicopper(II) helical complexes of these new ligands as anion binding receptors. 6,7 Our design consists of utilizing salicylaldi- mine 6 or salicyalaldoxime 7 functionality for metal coordination; these groups are then in turn linked together via straps of alkyl (L 1 in Scheme 1) or aromatic (L 2 in Scheme 1) tertiary amines. Coordination of these ligands to metal salts results in the formation of neutral dimetallic helicates with metal(II) centers coordinated to phenoxide oxygen and oxime nitrogen atoms. In the absence of a suitably alkaline metal salt, this process results in the protonation of the tertiary amine sites within the newly formed cage complex, thus enabling the receptor to form both hydrogen bonds, electrostatic interactions, and (in some cases) metal covalent bonds with encapsulated anions. 6,7 In the course of this work, we have observed that the uptake of an anion within the helical cage leads to a major contraction of the complex bringing the two metal centers closer to one another. 7a Further studies among these complexes have shown selective uptake of sulfate over dihydrogen phosphate in aqueous media, Received: April 9, 2013 Scheme 1. General Form of the Ligands Used To Make the Dicopper Helicates Article pubs.acs.org/IC © XXXX American Chemical Society A dx.doi.org/10.1021/ic400829d | Inorg. Chem. XXXX, XXX, XXX-XXX