Speciation 98: Abstracts

Experimental estimation of "true" denticity of polydentate organic ligands in aqueous solutions is a serious problem of Bio-inorganic and Co-ordination Chemistry. It gains a key importance if "potential" denticity of ligand L exceeds co-ordination capacity of particular cation M and a competition within different donor groups of ligand for metal bonding takes place. The more complicated but common cases involve competition of several polydentate ligands for cation's co-ordination sphere in bis-complexes ML₂ or in ML'L complexes. Both situation require reliable donor group speciation (differentiation of mono-, bidentate and "free" groups) for an advanced understanding of consequences associated with chelated metal application in biology and Medicine.

For most of diamagnetic cations (e.g. Be(II), Al(III), V(V), Mo(V), W(VI), Ge(IV), Sn(IV), Zr(IV), Tl(III), La(III), Y(III), Lu(III), Pt(II,IV), Pd(II) the desirable donor group speciation could be settled by means of ¹³C of ³¹P NMR spectroscopy due to a selective paramagnetic line broadening of a target complex ML (substrate) by paramagnetic impurity of relaxant: [Gd cdta]^- (H₄cdta - 1,2 diaminocyclohexane-N,N,N'N'-tetraethanoic acid). The proposed approach is based on a sufficiently different ability of mono-, bidentate and "free" donor groups of substrate to form unstable bonds with Gadolinium within short-living binuclear complexes: {[Gdcdta]MLN. The co-ordinated carboxylate group of substrate is less capable to form additional bonds with chelated paramagnetic cation then the "free" one. Then the corresponding ¹³NMR bands of unco-ordinated COO-groups are remarkably broadened while those of co-ordinated undergo a very poor broadening.

A special attention should be attracted to speciation of phosphonic groups. Both "free" and monodentate donor groups of substrate seem to have comparable ability to interact with [Gd cdta]^- Paramagnetic ³¹P NMR linebroadening for both types of groups is therefore large but reveals no reliable differences. At the same time bidentate phosphonic groups demonstrate exclusively low linebroadening. These could be clearly identified in presence of others.

The main restriction of the proposed method is associated with a requirement of low rate for both intermolecular exchange of ligands L (e.g. between two complexes ML), and for intramolecular exchange of "free" and co-ordinted donor groups within a single complex ML. Protonation of donor groups in ML is found to be also not desirable in speciation experiments.

The validity of the proposed method is demonstrated for 20 complexes formed by NTA, EDTA, DTPA and by some organophosphonates (e.g. nitrilotrimethylenphosphonic and 1-hydroxyethane-1,1-bisphosphonic acid) with both known and unknown X-Ray structures.

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