Chiral heterocyclic ligands. XIV. Control of directionality in silver coordination polymers through ligand design

Abstract

[DO NOT ALTER/DELETE THIS TEXT]Four C 2 -symmetric homochiral ligands derived from (+)-camphor are shown to produce linear chain and ladder silver coordination polymers wherein the directionality and polarisation can be controlled by subtle differences in ligand design.There has been much recent interest in the study of linear 1-D silver coordination polymers involving bridging N-heterocyclic ligands. 1 Coordination polymers 2 are now recognized as offering considerable potential as functional materials, 3 such as in the area of non-linear optics (NLO). 4 An important requirement for these properties is that the material be non-centrosymmetric and we believe that the simplest approach to the synthesis of noncentrosymmetric coordination polymers is to employ chiral bridging ligands. 5 Furthermore, it has been stated that such effects can be amplified by the additional polarisation associated with directionality in the chain. 6 We have previously shown 7 that the homochiral C 2 -symmetric ligand 1 (scheme 1), consisting of a central pyrazine ring with two fused bornane groups, readily forms a linear 1-D silver coordination polymer. However, the symmetry of 1 is such that the resulting polymer, although chiral, is non-directional (Figure 1(a)). This is because the C 2 -axis within the ligand is perpendicular to the plane of the central pyrazine ring, making the two nitrogen donors equivalent. As a consequence this coordination polymer is identical when viewed from either end of the chain. We now report the use of an isomeric chiral ligand 2 for the construction of a directional silver coordination polymer and the extension of this design strategy to a new class of tetradentate chiral ligand, viz 3 and 4, for the rational construction of chiral ladder polymers.Ligand 2 has previously been prepared, 8 in very low yield, from 3-aminocamphor but has not been used as a ligand in coordination chemistry. We prepared it from (1R)-(+)-camphor by conversion to camphorquinone and subsequent reaction with ammonium acetate and hexamethylenetetramine in acetic acid. This gave a 9:1 mixture of 2 and 1, which were separated by column chromatography ‡. In order to confirm the structure of this isomer an X-ray structure determination of the free ligand was carried out (Figure 2). It crystallizes in the tetragonal space group P4 1 2 1 2 and lies on a C 2 -axis that passes through the two nitrogen atoms ¶. This ligand differs from 1 in that the two nitrogen donors are now very different; N4 has a much less hindered environment than N1, which is shielded by two adjacent bridgehead methyl groups. Canterbury, Christchurch 8020, New Zealand. Fax:++64-3-3642110; Tel: ++64-3-3642432' E-mail: peter.steel@canterbury.ac.nz This journal is

catena-Poly[[silver(I)-μ-4-aminopyridine] perchlorate]: a 1-D staircase coordination polymer

“…For metallosupramolecular assemblies derived from bridging heterocyclic ligands, see: Steel (2005). For the use of silver(I) for the self-assembly of both discrete and polymeric aggregates with diverse molecular architectures, see: Fitchett & Steel (2006); O'Keefe & Steel (2007). For a review of the use of pyrazine and analogues as bridging ligands for silver(I)based assemblies, see: Steel & Fitchett (2008).…”

“…For some time we have been involved in the study of metallosupramolecular assemblies derived from bridging heterocyclic ligands (Steel, 2005). In recent years we have focused on the use of silver(I) for the self-assembly of both discrete and polymeric aggregates with diverse molecular architectures (Fitchett & Steel, 2006;O′Keefe & Steel, 2007).…”

Bis(2-acetylpyridine-κ2N,O)silver(I) tetrafluoridoborate: a complex with silver in a seesaw coordination geometry

“…For other silver complexes with the same ligand, see: Bowmaker et al (2005); Drew et al (2005); Di Nicola et al (2010). For examples of our previous work on silver complexes, see: Steel (2005); Fitchett & Steel (2006); O'Keefe & Steel (2007); Steel & Fitchett (2008); Golder et al (2010). For details of the coordination geometry of four-coordinate silver, see: Young & Hanton (2008).…”

(5R)-5,9,9-Trimethyl-5,6,7,8-tetrahydro-5,8-methanoquinoxaline: a chiral Mills–Nixon pyrazine

“…We have long been interested in the synthesis and study of chiral heterocyclic ligands derived from the readily available monoterpene (+)-camphor (Steel, 1983;Steel, 2005). More recently, we have reported the preparations of a number of chiral pyrazines derived from (+)-camphor that contain one or two bornane units fused to the sides of a pyrazine ring (Fitchett & Steel, 2000;Fitchett & Steel, 2006a;Fitchett & Steel, 2006b;Fitchett & Steel, 2006c). The title ligand (1) is one such compound that we have shown to form both discrete (Fitchett & Steel, 2006a) and polymeric (Fitchett & Steel, 2006b) metallosupramolecular assemblies.…”