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 3SCF 3)] [M = Ag(I) or Cu(I)] yield ionic complexes, consisting of a [M(NN′) 2] + cation and a O 3SCF 3 - anion. 1H 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 Δ or Λ configurations. The complexes with R′ is (S)-CHMePh exist in solution in two diastereomeric forms, i.e. Δ(S)(S) and Λ(S)(S), the most abundant of which (according to 1H NMR spectra) is the Λ(S)(S) diastereomer. The (diastereomeric) selectivity is higher for the silver(I) complexes (Δ(S)(S)/Δ(S)(S) for R = H > 99/1; R = Me, 95/5) than for the corresponding copper(I) complexes (Λ(S)(S)Δ(S)(S) for R = H, 85/15; R = Me, 75/25). An INEPT 109 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 1H 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 1H and INEPT 15N NMR that both an intramolecular process involving inversion of configuration (Δ ⇄ Λ) 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.