Report No xxxx - Instytut Fizyki JÄ drowej PAN

15 N NMR COORDINATION SHIFTS IN STABLE AND LABILE
d-ELECTRON METAL COMPLEXES WITH AZINES
Leszek Pazderski, Iwona Łakomska, Edward Szłyk
Faculty of Chemistry, Nicholas Copernicus University, Toruń, Poland
Jerzy Sitkowski, Lech Kozerski
Institute of Organic Chemistry, Polish Academy of Sciences, Warsaw, Poland
National Institute for Public Health, Warsaw, Poland
The 15 N NMR coordination shift, i.e. the difference between the chemical shift of the
same nitrogen atom within the complex and ligand molecules (∆ coord = δ compl - δ lig ), is a useful
tool indicating the metallation site in many N-donors. For azines, possessing the pyridine-type
nitrogens, the coordination shifts induced by binding of diamagnetic transition metal ions are
usually negative (shielding effect being observed). Their absolute values seem to depend
primarily on the stability/lability of the formed metal-nitrogen bondings. ∆ coord can reach 70-
100 ppm for stable Pd(II), Pt(II), Pt(IV) complexes and does not exceed 10 ppm for labile
Zn(II) species.
The stability of the coordination compounds in commonly used NMR solvents can be
easily tested by 1 H NMR measurements of their equimolar mixtures with the ligands. In the
case of platinides two distinct sets of 1 H signals appear, the deshielding of protons being
usually observed upon metal coordination. For zinc compounds an average spectrum is
detected, with the chemical shifts intermediate between those characteristic for the pure
complex and the free ligand. Hence one can assume that in the solution the changes of δ
values can be regarded rather as the result of temporary Zn-N interactions than the real
bonding formation (although the latter is present in the solid phase).
The studied examples concern Pd(II), Pt(II), Pt(IV) and Zn(II) chloride complexes with such
azines as: pyridine, pyridazine, purine and 1,2,4-triazolo-[1,5a]-pyrimidine.
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