Neutral Molecule Receptor Systems Using Ferrocene's “Atomic Ball Bearing” Character as the Flexible Element

Abstract

Fourteen novel ferrocene derivatives have been designed to serve as receptors for low molecular weight diamines. The compounds that have been prepared and fully characterized possess two ferrocenedicarboxylic acid residues bridged by amide formation in their respective 1‘-positions by 4,4‘-benzidinyl (15), 3,3‘-dimethoxy-4,4‘-benzidinyl (16), 2,7-fluorenyl (17), 3-methoxy-2,7-fluorenyl (18), 4-N-piperazinoanilinyl (19), N,N‘-4,4‘-bipiperdinyl (20), and 4,13-diaza-18-crown-6 (21). In two cases, ferrocenecarboxylic acid was bridged by spacers attached using 1-methylene groups. The bridges in these cases were 4,13-diaza-18-crown-6 (22) and 1,5-diaminoanthraquinone (24). In a single case, ferrocenecarboxylic acid was bridged by 1,5-dicarbonylnaphthalene (25). In one additional case, the bridge was created by formation of an imine followed by hydrogenation, but both compounds (26, 27) proved to be relatively unstable. Attempts to increase solubility afforded the N-ethylated derivative 28 of 15 and the derivative 29 of 27 having carboxamides in the 1‘-positions. A solid state structure of the diethyl ester of 20 confirms the design criteria. Complexation constants were determined in THF-d8 or CDCl3 for combinations of receptors 18, 19, and 20 with 4-aminopyridine, N,N,N‘,N‘-tetramethylethylenediamine, N,N,N‘,N‘-tetramethylpropylenediamine, DABCO, 3-propyladenine, and benzimidazole and were in the range 102−104. The anticipated complexation mechanism for 20 with N,N,N‘,N‘-tetramethylpropylenediamine was confirmed by observation of an NOE between host and guest.