Internal hydrogen bonding and amide co-ordination in zinc(ii) complexes of a tripodal N4 ligand: structural, spectroscopic and reactivity studies
Mareque Rivas, Juan C.; Torres Martín de Rosales, Rafael; Parsons, Simon; Mareque Rivas Juan C.; School of Chemistry, The University of Edinburgh; Torres MartÃn de Rosales Rafael; School of Chemistry, The University of Edinburgh; Parsons Simon; School of Chemistry, The University of Edinburgh
Журнал:
Dalton Transactions
Дата:
2003
Аннотация:
The tripodal N4 ligand N,N-bis(2-pyridylmethyl)-N-(6-pivaloylamido-2-pyridylmethyl)amine (bppapa) presents an Nâ 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 <sup>1</sup>H NMR studies of bppapa show an intramolecular Câ Hâ ¯O.dbd;C interaction involving the pivaloylamido unit that determines the orientation of the amide Nâ H and C.dbd;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 1·0.5CH3OH 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. <sup>1</sup>H and <sup>13</sup>C 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) 2â ²and 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 Nâ Hâ ¯Clâ Zn, Câ Hâ ¯Clâ Zn and Câ Hâ ¯O.dbd;C hydrogen bonding. Remarkably, all these interactions are retained in solution and are clearly reflected in the <sup>1</sup>H 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 Nâ Hâ ¯Cl hydrogen bond, which was estimated to be at least 10.3 ± 0.6 kJ mol<sup>â 1</sup> in acetonitrile solution and 14.9 ± 0.6 kJ mol<sup>â 1</sup> in the solid state. The [(bppapa)Zn(Cl)]<sup>+</sup> 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.
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