Amino-acid based coordination polymers

Abstract

Five new transition metal–amino acid coordination polymers, including two copper compounds 1D-{Cu(d-α-PhGly)2} (1), 1D-{Cu(l-Trp)2} (2) and three cadmium compounds 2D-{Cd(d-Leu)2} (3) 2D-{Cd(d-Phe)2} (4) and 2D-{[Cd(d-Phe)(4,4′-bipy)H2O][NO3]} (5) are structurally characterized (PhGly=phenylglycinato, Trp=tryptophanato, Leu=leucinato, Phe=phenylalaninato in enantiopure form, 4,4′-bipy=4,4′-bipyridine). Single crystals were obtained by slow diffusion of the metal salt and the ligand in a water–methanol mixture. The enantiopure amino acetate ligands led to homochiral crystallization of 1–5 in the non-centrosymmetric space group P21. Characterization by single-crystal X-ray diffraction, infrared and Raman spectroscopy showed that the copper compounds 1 and 2 form twofold helices around the 21 screw axis and the cadmium compounds 3–5 yield two-dimensional coordinative layers also with twofold helices around the 21 axes for 3 and 4. The Cu-amino-acetate/carboxlyate helices are connected by N–H···O hydrogen bonds and very long Cu⋯O–carboxylate contacts to supramolecular 2D layers. Crystal packings of 2D layer structures in 1–4 show a separation of the hydrophobic alkyl groups from the hydrophilic M-amino-carboxylate region. Charge-assisted Cu(+)⋯(−)O interactions and N–H···(−)O hydrogen-bonding interactions in the hydrophilic region are the driving force of “hydrophobic exterior layer” formation with a hydrophilic interior exposing the hydrophobic alkyl groups to the exterior. Stacking of the layers occurs through weak van der Waals interactions between the alkyl groups. This correlates with the crystal morphology, with formation of thin crystal plates along the stacking direction.In 5 the Cd-μ-amino carboxylate-κN,O:O′ units with a pentagonal–bipyramidal Cd atom form strands, and these strands are then joined through the rigid 4,4′-bipy linkers to a coordinative 2D layer. The nitrate anions connect these layers with charge-assisted (aqua)O–H···(−)O(nitrate) and (amino)N–H···(−)O(nitrate) hydrogen bonds to a supramolecular 3D network.