The Impact of Methyl Substituents on the Ag–^–O–N⁺ Coordination Bond Denticity of Pyridine <i>N</i>-Oxide Oxygen

The coordination bond (CB) denticities of the N-oxide oxygen in three methyl-substituted pyridine N-oxides, 2-methyl (1), 4-methyl (2), and 2,6-dimethyl (3), have been investigated across 52 crystal structures comprising 29 different silver(I) complexes formed with seven different Ag(I) salts (AgX, X = TFA, OTf, ClO4, NO3, BF4, PF6, and SbF6). These complexes were crystallized in methanol under three PyNO:silver(I) equivalent ratios (1:1, 1:2, and 2:1). Ligands 1 and 2, each featuring a single methyl group at the ortho or para position relative to the N–O group, exhibit monodentate, bidentate, and tridentate CBs. Ligand 2 even forms tetradentate Ag···O contacts that are shorter than the sum of the van der Waals radii for silver and oxygen (3.24 Å). In contrast, ligand 3, with two ortho-methyl groups adjacent to the N–O group, consistently adopts a bidentate CB mode across all silver(I) salts and PyNO:silver(I) ratios tested, due to steric hindrance that limits access to the N-oxide oxygen. Density functional theory studies were used to study the hybridization characteristics of the N-oxide oxygen and the corresponding Ag–O(N-oxide) interaction energies.