Metallocene bis(aza-crown ether) ligands and related compounds. Their syntheses, co-ordination chemistry, and electrochemical properties

Abstract

The syntheses of new ferrocene and ruthenocene bis(aza-crown ethers)(11)–(14) containing amide bond linkages between the metallocene redox centre and aza-15-crown-5 (1,4,7,10-tetraoxa-13-azacyclopentadecane) and aza-18-crown-6 (1,4,7,10,13-pentaoxa-16-azacyclo-octadecane) are described. Variable-temperature ¹³C n.m.r. studies on these compounds reveal a common intramolecular dynamic process involving rotation about the N–CO bond whose energy barrier to rotation is cation (Na⁺ or Li⁺) dependent. The sodium cation forms 1 : 2 stoicheiometric complexes with the ferrocene bis(aza-crown ethers), whereas potassium produces a 1 : 1 intramolecular sandwich complex with the former and a 1 : 2 complex with the latter. Electrochemical investigations reveal that binding of Na⁺ and K⁺ guest cations at the respective aza-crown ether co-ordinating sites results in shifts of the ferrocene oxidation wave to more positive potentials. Lithium produces analogous electrochemical effects with these two compounds, however ¹³C n.m.r. results suggest this cation exclusively co-ordinates to the respective amide carbonyl oxygen donor atoms. This rationalisation is supported by ‘model’ ferrocene bis(tertiary amide) derivatives which exhibit redox-responsive behaviour to Li⁺ while being electrochemically insensitive to Na⁺ and K⁺.