Internal hydrogen bonding and amide co-ordination in zinc(ii) complexes of a tripodal N4 ligand: structural, spectroscopic and reactivity studies

Abstract

The tripodal N4 ligand N,N-bis(2-pyridylmethyl)-N-(6-pivaloylamido-2-pyridylmethyl)amine (bppapa) presents an N–H group for hydrogen bonding to an adjacent metal-bound ligand, and a carbonyl group for metal co-ordination. These binding features are key in metallopeptidase catalysis, which is an area of considerable current interest. The X-ray crystal structure and ¹H NMR studies of bppapa show an intramolecular C–H⋯OC interaction involving the pivaloylamido unit that determines the orientation of the amide N–H and CO groups relative to the N4 metal binding site. The reaction of [Zn(NCCH₃)₄](PF₆)₂ with bppapa affords [(bppapa)Zn](PF₆)₂1. The X-ray crystal structure of 1·0.5CH₃OH shows a zinc(II) ion in a trigonal-bipyrimidal environment in which the bridgehead nitrogen atom of the ligand and the carbonyl oxygen of the pivaloylamido group co-ordinate axially. ¹H and ¹³C NMR and IR spectra show that this structure is retained in acetonitrile solution. The reaction of ZnCl₂ with bppapa in acetonitrile affords the salt [(bppapa)Zn(Cl)](Cl) 2, which in methanol undergoes anion metathesis with NaBPh₄ (1 equiv.) to form [(bppapa)Zn(Cl)](BPh₄) 2′and NaCl. The X-ray crystal structure of 2′·CH₃CN shows that the chloride ion occupies one of the axial co-ordination sites of the trigonal-bipyramidal co-ordination geometry of the zinc(II) center. In addition, this structure reveals internal N–H⋯Cl–Zn, C–H⋯Cl–Zn and C–H⋯OC hydrogen bonding. Remarkably, all these interactions are retained in solution and are clearly reflected in the ¹H NMR spectra, which we prove can be used as a powerful diagnostic tool for determining the solution structures of these and related metal complexes. IR spectroscopy was used to determine the strength of the N–H⋯Cl hydrogen bond, which was estimated to be at least 10.3 ± 0.6 kJ mol⁻¹ in acetonitrile solution and 14.9 ± 0.6 kJ mol⁻¹ in the solid state. The [(bppapa)Zn(Cl)]⁺ cation is very stable to substitution of the chloride ion by water, which may be an indication of the stabilising effect exerted by internal hydrogen bonding.