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

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...O=C interaction involving the pivaloylamido unit that determines the orientation of the amide N-H and C=O 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)2ʹ and NaCl. The X-ray crystal structure of 2ʹ·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...O=C 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 (plus-minus) 0.6 kJ mol-1 in acetonitrile solution and 14.9 (plus-minus) 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.