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 H-1 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)(2 1). The X-ray crystal structure of 1.0.5CH(3)OH 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. H-1 and C-13 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...O=C hydrogen bonding. Remarkably, all these interactions are retained in solution and are clearly reflected in the H-1 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.