CONFORMATIONAL STUDIES OF CHELATED ETHYLENEDIAMINES BY NUCLEAR MAGNETIC RESONANCE - PARAMAGNETIC NICKEL(II) COMPLEXES OF N-ALKYLETHYLENEDIAMINES

Large proton magnetic resonance contact shifts were observed for ethylenediamines coordinated with nickel(ll) ion in aqueous solution. The observed spectra are highly sensitive to the conformational states of the ligand molecule and considerable structural information can be obtained by investigating the effects of alkyl substitution and of temperature on the spectra. The chelated ligand was found to exist in various gauche forms, with fast conformational exchange between corresponding k and k' forms and with slow exchange between configurations that require N'N bond rupture for interconversion. The alkyl substituents are shown to prefer the pseudo-equatorial position in the ring; the N-methyl group, for example, was found to be 0.43 kcal/mole more stable thermodynamically at this position than at the pseudo-axial position for the tetraaquo-N,N'-dimethylethylenediaminenickel(II) ion. © 1969, American Chemical Society. All rights reserved.