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

Polydendate ligands (6-R1-2-pyridylmethyl)-R2(R1= NHCOtBu, R2= bis-(2-pyridylmethyl)amine L^1; R^1= NH2, R^2= bis-(2pyridylmethyl)amine L2; R1= NHCOtBu, R^2= N(CH2CH2)2NMe L3; R1= NH2, R^2= N(CH2CH2)2NMe L^4; R^1= NHCOtBu, R^2= N (CH2CH2)2O L^5; R^1= NH2, R^2= N(CH2CH2)2O 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. 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)1ʹ and [(L2)Zn(Cl)](Cl)2ʹsalts, 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, [(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 ʹinternalʹ 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.