Conformational studies of chelated ethylenediamines by nuclear magnetic resonance. Paramagnetic nickel(II) complexes of N-alkylethylenediamines

Abstract

Large proton magnetic resonance contact shifts were observed for ethylenediamines coordinated with nickel(l1) 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 Ni-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(l1) ion.CONSIDERABLE EVIDENCE, based on X-ray and infrared studies in the solid-state, has now been accumulated ( I , 2) to establish convincingly that the ethylenediamine molecule in coordination complexes exists as a puckered five-membered ring and in a gauche configuration; much earlier it had been tacitly assumed that a planar cis configuration was predominant. Conformational energy differences estimated on the basis of a theoretical model proposed by Corey and Bailar (3) also suggest the gauche form to be the preferred configuration. Several attempts to acquire experimental proof of this preference by the nuclear magnetic resonance (NMR) technique (1,(4)(5)(6) have been reported. However, in the most thoroughly studied example, tris-ethylenediaminecobalt(II1) ion, the methylene protons do not exhibit well refined NMR splitting, which would theoretically be an AzBz system in gauche configuration and in the absence of additional splitting from N, Co, and NH protons. A broad, highly overlapping pattern was observed in HzO, D20, DzSOd, and in trifluoroacetic acid, even over a range of temperatures. This had been rationalized as arising either from too small differences in the chemical shifts between the magnetically different protons or too great mobility of the ring in the gauche form.In this report, conformational studies were carried out, for a number of reasons, on various N-alkylethylenediamines coordinated with nickel(I1) ion in D20. Because of the strong magnetic moment of the unpaired electron spins of the para-

Structure of the Metal Site in Rhus vernicifera Stellacyanin: A Paramagnetic NMR Study on Its Co(II) Derivative

“…63 Signals A and B are assigned as the -CH 2 protons of Cys87. 38,46 These shifts are expected to follow a general equation of this type: 54,67,68,69 where A and B are constants, θ i is the M-S-C -H i dihedral angle (i ) 1, 2), and F s is the spin density localized in the coordinating sulfur atom. 44,64,65 These shifts are mainly due to a contact contribution, 40,54 so that it may be inferred that the electron delocalization onto the cysteine moiety is large.…”

Proton NMR studies of the cobalt(II)-metallothionein system

“…Rabbit liver MT2 was isolated from rabbits injected subcutaneously 15 times in 2-3-day intervals with 1 mg of Cd2+/kg of body weight in the form of the chloride salt. 12 The protein was purified as previously reported.13 Each preparation was characterized by amino acid analysis (Durrum 500) and by metal analysis using atomic absorption spectroscopy (Instrumentation Laboratory, IL 157). The apo form of the protein was prepared by gel filtration of the native form at low pH.…”

Complexation of nickel(II) by ethylenediamine investigated by means of electrospray ionization mass spectrometry

“…Ethylenediamine as the most simple of polyamines behaves as a bidentate ligand that can form two coordinative bonds with a metal atom through the lone pair electrons on both nitrogens [14]. The coordination of ethylenediamine to a metal ion leads to a five-membered chelate ring with many of the stereochemical characteristics of cyclopentane [15,16]. Transition-metal cations generally show higher affinities to ethylenediamine in comparison with alkaline earth and alkali metal cations [17].…”