Application of INEPT 109Ag and 15N NMR Spectroscopy for the study of metal-ligand interactions of silver analogues of copper(I) model compounds

Abstract

Because of the presence of copper (in its reduced state) at active sites in protein it has become very ▪ important to study copper(I) model complexes by spectroscopic techniques. We now report that if copper(I) in model complexes can be substituted by silver(I) with retention of the structural features then 107Ag or 109Ag NMR spectroscopy (natural abundances 50% I = 1 2 ) using the recently developed polarization transfer sequence INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) [1] provides an excellent tool for studying the metal IB-ligand interaction 1H NMR studies show that the analogous copper(I) and silver(I) complexes of potentially quadridentate N 4 ( R)( S)-1,2-(6-R-pyridine-2-CHN) 2-cyclohexane (R = H or Me) as well as the N 2S 2 donor ligand ( R)( S)-1,2-(5-R-thiophene-2-CHN) 2-cyclohexane (R = H or Me) have similar structures (confirmed by X-ray studies [2]). However, study of the direct coordination sphere and copper(I)-ligand interactions in these model complexes by Cu NMR is hampered by the large quadropole moments of both 63Cu and 65Cu (natural abudances 70 and 30% respectively, I = 3 2 ). We have measured directly 109Ag NMR (INEPT) spectra with large enhancements in signal to noise and enormous experimental time saving (a factor 400–500) as compared to the conventional methods (see Fig. 1) [3]. the influence of the nature of the hetero-atoms coordinating the metal centre is directly reflected in the chemical shift differences of the 109Ag resonances. It is shown that for silver(I) coordination complexes information about the ligand-to-metal interaction can be obtained by using not only direct 109Ag NMR, but also INEPT 15N NMR spectroscopy. In particular the 15N NMR (INEPT) spectra of the silver(I) complexes [M 2(N 4) 2] 2+ 2O 3SCF − 3 show that the structure of these complexes as found in the solid by X-ray methods, in which each metal ion has a distorted tetrahedral coordination geometry of four N-atoms, is fully retained in solution. From different 1J( 15N 107,109Ag) coupling constants the relative bond strengths of the various AgN interactions can be deduced. Where Cu I can be substituted by Ag I either in coordination (model) complexes or biological systems (bovine superoxide dismutase [4]) INEPT 109Ag NMR spectroscopy has potential as a novel technique for the study of the coordinating properties of the metal centres.