Silver(I) and copper(I) bis(pyridine-2-carbaldehyde-imine) triflate complexes studied in solution by 1H, 1H-{109Ag}, INEPT 15N and INEPT 109Ag NMR

Abstract

The reactions of the neutral pyridine-imine, 6-R-py-2-CH=N-R' (NN'), donor ligands with [M(O{3}SCF{3})] [M = Ag(I) or Cu(I)] yield ionic complexes, consisting of a [M(NN'){2}]}+{ cation and a O{3}SCF{3}}-{ anion. }1{H NMR studies of the complexes which contain the prochiral substituent (R') i-Pr or the chiral (S)-CHMePh show that in the slow exchange limit the metal IB centres have tetrahedral geometries as a result of chelate bonding of the pyridine-imine ligands. The metal IB centres have either @D or @L configurations. The complexes with R' is (S)-CHMePh exist in solution in two diastereomeric forms, i.e. @D(S)(S) and @L(S)(S), the most abundant of which (according to }1{H NMR spectra) is the @L(S)(S) diastereomer. The (diastereomeric) selectivity is higher for the silver(I) complexes (@D(S)(S)/@D(S)(S) for R = H > 99/1; R = Me, 95/5) than for the corresponding copper(I) complexes (@L(S)(S)@D(S)(S) for R = H, 85/15; R = Me, 75/25). An INEPT}1{}0{}9{ Ag NMR study, which was performed for the silver(I) complex with R is Me and R' is (S)-CHMePh shows a large chemical shift difference (24 ppm) between the signals arising from the Ag(I) centres in the two diastereoisomers. Temperature-dependent }1{H NMR experiments of the complexes with R' is i-Pr show that fluxional processes occur. In the case of the silver(I) complex with R is Me and R' is i-Pr it was established by }1{H and INEPT }1{}5{N NMR that both an intramolecular process involving inversion of configuration (@D @? @L) at the metal centre and a slower intermolecular ligand exchange process are taking place. It appeared that the rates of these processes depend on the bulkiness of the 6-pyridine as well as the imine substituents, R and R'. Moreover, the copper(I) complexes are more stable than the corresponding silver(I) complexes. Possible mechanisms for the observed exchange processes are discussed.