Synthesis, complexation and cyclisation reactions of a new acyclic diamide: observation of intramolecular ligand exchange in a structurally characterised nickel(II) complex

A new acyclic diamide ligand, H(2)L1, has been prepared and characterised. H(2)L1, 2,6-bis(1-propanecarboxamido-3-amino)pyr is prepared from 2,6-dimethylpyridinedicarboxylate and excess I,3-diaminopropane. Nickel(II) complexes derived from H(2)L1 [Ni(L1+H)]X, are orange, square planar and diamagnetic (X=ClO4-, CF3SO3- or NO3-). A single-crystal X-ray analysis of [Ni(L1+H)]CLO4 confirmed that both of the amide groups are deprotonated and are bound to the nickel ion with a trans-amide configuration and that the remaining coordination sites are occupied by the pyridine nitrogen atom and one of the amine "arms". The second amine "arm" is protonated and participates in an array of hydrogen bonds in the solid state. In solution intramolecular exchange of the "arms" is observed on the NMR timescale. Three macrocycles have been prepared from this acyclic ligand. Reaction of H(2)L1 with 2,6-diacetylpyridine or 2,6-diformylpyridine, in the presence of a barium(II) template ion, results in two new unsymmetrical, amide-containing and potentially dinucleating macrocycles as the complexes [Ba(H(2)L2)](ClO4)(2) and [Ba (H(2)L3)] (ClO4)(2), respectively. In contrast to the template cyclisations, a symmetrical metal-free tetraamide macrocycle, H(2)L4, is produced from the direct cyclisation of H(2)L1 with 2,6-dimethylpyridinedicarboxylate. (C) 1999 Elsevier Science Ltd. All rights reserved.