Structural Evidence for an Unusual Conformation and Weak Interligand Interactions in Two Copper Chelates with (o-Nitrophenyl)ethylenediaminediacetic Acid

Two mixed-ligand copper(II) complexes, [Cu(L-1)(H2O)]center dot H2O (1) and [Cu(L-1)(bipy)center dot 2H(2)O (2), containing the title N-substituted iminodiacetate (IDA) derivative (L-1(2-)) as primary ligand with and without 2,2'-bipyridine as auxiliary ligand, have been synthesised. Both have been fully characterised by means of IR spectroscopy, mass spectrometry, elemental analysis and conductimetry, as well as X-ray crystallography. The dominant structural feature of these complexes is the presence of a flexible coordinating arm on the IDA skeleton which controls the conformation of the resulting complexes. The Cu-II centre in complex 1 exhibits a distorted square-pyramidal [4+1] coordination in which the apical site is unexpectedly occupied by the N(2) atom of the coordinating arm instead of a water molecule. The IDA skeleton adopts a tridentate mer-O2N arrangement in 1, whereas in 2 it exhibits an unusual fac-O2N(apical) configuration, which is observed for the first time for a tridentate IDA unit bearing a flexible arm containing a coordinating atom. The o-nitro aromatic group in the flexible arm gives rise to different inter- and/or intramolecular stacking interactions. Classic intermolecular "nitrophenyl-nitrophenyl" pi-pi stacking interactions are observed between pairs of complex molecules in crystals of 1, whereas intramolecular "nitrophenyl-pyridyl" interactions alternate with unprecedented interligand pi-pi nitrophenyl-metal chelate ring" interactions in crystals of 2. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)