N-Alkyl pyrrolidone ether podands as versatile alkali metal ion chelants

Abstract

This work explores the coordination chemistry of a bis(pyrrolidone) ether ligand. Pyrrolidones are commercially important functional groups because of the high polarity and hence high hydrophilicity and surface affinity. An array of alkali metal ion complexes of a podand bearing two pendant pyrrolidone functionalities, namely 1-{2-[2-(2-oxo-pyrrolid-1-yl)-ethoxy]-ethyl}-pyrrolid-2-one (1) are reported. Reaction of this ligand with sodium hexafluorophosphate gives two discrete species of formulae [Na(1)₂]PF₆ (3) and [Na₃(H₂O)₂(μ-1)₂](PF₆)₃ (4), and a coordination polymer {[Na₃(μ₃-1)₃(μ₂-1)](PF₆)₃}_n (5). The same reaction in methanol gives a 1 : 1 complex, namely [Na₂(μ-1)₂(MeOH)₂](PF₆)₂ (6). Use of tetraphenyl borate as a less coordinating counter ion gives [Na₂(1)₂(H₂O)₄](BPh₄)₂ (7) and [Na₂(1)₄](BPh₄)₂ (8). Two potassium complexes have also been isolated, a monomer [K(1)₂]PF₆ (9) and a cyclic tetramer [K₄(μ₄-H₂O)₂(μ-1)₄](PF₆)₄ (10). The structures illustrate the highly polar nature of the amide carbonyl moiety within bis(pyrrolidone) ethers with longer interactions to the ether oxygen atom. The zinc complex is also reported and {[ZnCl₂(μ-1)]}_n (11) exhibits bonding only to the carbonyl moieties. The ether oxygen atom is not necessary for Na⁺ complexation as exemplified by the structure of the sodium complex of the analogue 1,3-bis(pyrrolid-2-on-1-yl)butane (2). Reaction of compound 1 with lithium salts results in isolation of the protonated ligand.