Juan C. Mareque Rivas

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⋯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.

Structures and reactivity of synthetic zinc(II) complexes resembling the active sites and reaction intermediates of aminopeptidases

Herein we report the first crystallographic characterization and hydrolysis of a synthetic zinc(II) complex that resembles the active site and reaction intermediates proposed for aminopeptidases.

Structural characterization of a dizinc(II) complex with bridging eta(2)-phosphate diesters and internal N-H center dot center dot center dot O-P hydrogen bonding

The first structurally characterized dizinc(II) complex with bridging η2-phosphate diesters, in this case dibenzyl phosphate, and internal N–H⋯O–P H-bonding are reported.