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

Abstract

Polydendate ligands (6-R¹-2-pyridylmethyl)-R² (R¹ = NHCO^tBu, R² = bis-(2-pyridylmethyl)amine L¹; R¹ = NH₂, R² = bis-(2-pyridylmethyl)amine L²; R¹ = NHCO^tBu, R² = N(CH₂CH₂)₂NMe L³; R¹ = NH₂, R² = N(CH₂CH₂)₂NMe L⁴; R¹ = NHCO^tBu, 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. L^1,2 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 L^3–6 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 L^1–6. 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.