Issue 17, 2003

Internal hydrogen bonding in tetrahedral and trigonal bipyramidal zinc(ii) complexes of pyridine-based ligands

Abstract

Polydendate ligands (6-R1-2-pyridylmethyl)-R2 (R1 = NHCOtBu, R2 = bis-(2-pyridylmethyl)amine L1; R1 = NH2, R2 = bis-(2-pyridylmethyl)amine L2; R1 = NHCOtBu, R2 = N(CH2CH2)2NMe L3; R1 = NH2, R2 = N(CH2CH2)2NMe L4; R1 = NHCOtBu, R2 = N(CH2CH2)2O L5; R1 = NH2, R2 = N(CH2CH2)2O L6) were prepared as part of an effort to rationally design ligands that induce internal hydrogen bonding to other metal-bound ligands to be used as active site models of metallohydrolases and oxygenases. L1, L3 and L5 were prepared by alkylation of the appropriate amine (bis-(2-pyridylmethyl)amine, N-methylpiperazine or morpholine) with 2-(pivaloylamido)-6-(bromomethyl)pyridine. L2, L4 and L6 were prepared by acid hydrolysis of L1, L3 and L5, respectively. L1,2 were metallated with ZnCl2 to give [(L1)Zn(Cl)](Cl) 1′ and [(L2)Zn(Cl)](Cl) 2′ salts, which after a metathesis reaction with NaBPh4 in MeOH, afford [(L1)Zn(Cl)](BPh4) 1 and [(L2)Zn(Cl)](BPh4) 2. The reaction of L3–6 with ZnCl2, however, affords the neutral complexes [(L3)Zn(Cl)2] 3, [(L4)Zn(Cl)2] 4, [(L5)Zn(Cl)2] 5 and [(L6)Zn(Cl)2] 6. X-Ray crystallographic studies of 1, 2 and 4–6 revealed that these complexes adopt trigonal bipyramidal (N4Cl) and tetrahedral (N2Cl2) geometries, respectively, with ‘internal’ N–H⋯Cl–Zn hydrogen bonding. 1H NMR, IR and X-ray crystallographic studies indicated that internal N–H⋯Cl–Zn hydrogen bonding in 4–6 is of similar strength and weaker than in the trigonal bipyramidal complexes 1 and 2. The chemical shift of the amine and amide NH proton associated with the internal N–H⋯Cl–Zn hydrogen bond is shifted downfield by 2.2–2.5 ppm in 1, 2 and by 1.1–1.2 ppm in 3–6 relative to in the corresponding ligand L1–6. Thus, in the 1–6 series, the magnitude of the chemical shift changes experienced by the hydrogen bonded N–H can be correlated with the hydrogen bond energies determined by IR and 1H NMR variable temperature coalescence studies, and with the hydrogen bond geometries revealed by X-ray crystallography.

Graphical abstract: Internal hydrogen bonding in tetrahedral and trigonal bipyramidal zinc(ii) complexes of pyridine-based ligands

Supplementary files

Article information

Article type
Paper
Submitted
15 May 2003
Accepted
08 Jul 2003
First published
29 Jul 2003

Dalton Trans., 2003, 3339-3349

Internal hydrogen bonding in tetrahedral and trigonal bipyramidal zinc(II) complexes of pyridine-based ligands

J. C. M. Rivas, E. Salvagni, R. T. M. de Rosales and S. Parsons, Dalton Trans., 2003, 3339 DOI: 10.1039/B305476B

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