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

Polydendate ligands (6-R-1-2-pyridylmethyl)-R-2 (R-1 = (NHCOBu)-Bu-t, R-2 = bis-(2-pyridylmethyl)amine L-1; R-1 = NH2, R-2 = bis-(2-pyridylmethyl)amine L-2; R-1 = (NHCOBu)-Bu-t, R-2 = N(CH2CH2),NMe L-3; R-1 = NH2, R-2 = N(CH2CH2)(2)NMe V; R-1 = (NHCOBu)-Bu-t, R-2 = N(CH2CH2)(2)O L-5; R-1 = NH2, R-2 = N(CH2CH2)(2)O L-6) 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. L-1, L-3 and L-5 were prepared by alkylation of the appropriate amine (bis-(2-pyridylmethyl)amine, N-methylpiperazine or morpholine) with 2-(pivaloylamido)-6-(bromomethyl)pyridine. L-2, L-4 and L-6 were prepared by acid hydrolysis of L-1, L-3 and L-5, respectively L-1,L-2 were metallated with ZnCl2 to give [(L-1)Zn(Cl)](Cl) 1' and [(L-2)Zn(Cl)](Cl) 2' salts, which after a metathesis reaction with NaBPh4 in MeOH, afford [(L-1)Zn(Cl)](Bph(4)) 1 and [(L-2)Zn(Cl)](BPh4) 2. The reaction of L3-6 with ZnCl2, however, affords the neutral complexes [(L-3)Zn(Cl)(2)] 3, [(L-4)Zn(Cl)(2)] 4, [(L-5)Zn(Cl)(2)] 5 and [(L-6)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. H-1 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 H-1 NMR variable temperature coalescence studies, and with the hydrogen bond geometries revealed by X-ray crystallography.