Hierarchical importance of coordination and hydrogen bonds in the formation of homochiral 2D coordination polymers and 2D supramolecular assemblies

Abstract

In exploring the chemistry of reduced Schiff base derivatives of amino acids with Cu(II) ions, a series of homochiral two-dimensional (2D) coordination polymers (CPs) with a unique loop-like structure comprised of five Cu(II) centers, {[Cu₂(Hsersal)₂(H₂O)]·2.5H₂O}_n, (1), {[Cu₂(Hser-5OMe-sal)₂(H₂O)]·DMF}_n (2), [Cu₂(Hser-5NO₂-sal)₂(H₂O)]_n (3), {[Cu₂(Hser-5Cl-sal)₂(H₂O)]·2H₂O}_n (4), {[Cu₂(Hser-3Cl-sal)₂(H₂O)]·3H₂O}_n (5) and {[Cu₂(Hser-o-Van)₂(H₂O)]·3H₂O}_n (6) [where H₃sersal = N-(2-hydroxybenzyl)-serine, H₃ser-5OMe-sal = N-(2-hydroxy-5-methoxybenzyl)-serine, H₃ser-5NO₂-sal = N-(2-hydroxy-5-nitrobenzyl)-serine, H₃ser-5-Cl-sal = N-(2-hydroxy-5-chlorobenzyl)-serine, H₃ser-3-Cl-sal = N-(2-hydroxy-3-chlorobenzyl)-serine, H₃ser-o-van = N-(2-hydroxy-3-methoxybenzyl)-serine], have been isolated in good yields from the reaction of a methanolic solution of CuSO₄·5H₂O and potassium salt of the respective ligands (in a 1 : 1 ratio) either at room temperature or under reflux. In these CPs, the two Cu(II) centers have different coordination environments with one coordinated to a water molecule. Using a bifunctional linker, such as 4,4′-bipyridine, four of these 2D CPs are converted in methanol under reflux to the corresponding 2D supramolecular coordination complexes (SCCs) constructed through very strong hydrogen bonding interactions, [Cu₂(4,4′-bpy)(Hsersal)₂]·2H₂O (7), [Cu₂(4,4′-bpy)(Hser-5-OMe-sal)₂]·6H₂O (8), [Cu₂(4,4′-bpy)(Hser-5-NO₂-sal)₂]·H₂O (9) and [Cu₂(4,4′-bpy)(Hser-5-Cl-sal)₂]·4H₂O·DMF (10). This chemical conversion of a CP to an SCC is unknown in the literature and indicates the hierarchical importance of coordination and hydrogen bonds in their formation. The complexes are structurally characterized by elemental analysis, UV-Vis spectroscopy, circular dichroism, IR and Raman spectroscopy, ESI mass spectrometry, single crystal and powder X-ray diffraction, polarimetry and thermogravimetric analysis. A magneto-structural correlation for the change from 1 to 7 is established through variable temperature magnetic susceptibility measurements (2–390 K) indicating strong antiferromagnetic coupling (2J = −278 cm⁻¹) in 1 and no interaction in 7 between the Cu(II) centers. As an example, water adsorption studies of 1 and 7 were carried out to demonstrate the porous nature of the SCCs compared to the CPs.