Heavier alkali metal complexes of 2-phenylamidopyridine: An X-ray crystallographic and theoretical study of a structurally diverse series of crown ether adducts

Abstract

Complexes of the anion of the secondary amine 2-phenylaminopyridine (LH) with the heavier alkali metals Na–Cs have been prepared in the presence of various macrocyclic polyether crowns [12-crown-4 (12C4), 15-crown-5 (15C5), and 18-crown-6 (18C6)], which coordinate to the metal ions in all cases. Depending on the combination of alkali metal and crown, the products include separated ion pairs [(crown)₂M]⁺L⁻ (12C4/Na, 15C5/K, 15C5/Rb, 15C5/Cs) and contact-ion-pair neutral molecules [(crown)ML] (15C5/Na, 18C6/Na, 18C6/K, 18C6/Rb) in which L⁻ acts as a bidentate ligand. [{(12C4)KL}₂] is a dimer in which the amido and pyridine N atoms of two ligands bridge the metal ions, while [{(18C6)KL₂K}_∞] is a chain polymer with crown O and pyridyl N atoms acting as bridges in corner-sharing KOKN four-membered rings and may be regarded as a potassium potassate complex. [{(18C6)Cs₂L₂}_∞] is also polymeric, with a basic arrangement like that of [{(12C4)KL}₂], but with each 18C6 ligand μ-κ⁶:κ⁶ to two metal centres, generating the polymer. Although most of the [(crown)₂M]⁺ sandwich cations have essentially parallel crown ligands, [(12C4)₂Rb]⁺ is markedly bent, both in the complex incorporating THF as an additional ligand and in the THF-free complex, where two of these cations form a centrosymmetric dimer through two bridging oxygen atoms; DFT calculations indicate that the bending is inherent, thus enabling the coordination by an extra oxygen atom rather than being a consequence of this coordination. Attempts to isolate the caesium 12C4 derivative were unsuccessful. The compounds have been characterized by NMR spectroscopy, CHN microanalysis and, in most cases, X-ray crystallography.