Chemical and Electrochemical Formation of Pseudorotaxanes Composed of Alkyl(ferrocenylmethyl)ammmonium and Dibenzo[24]crown-8

Abstract

Protonation of p-xylylaminomethylferrocene (1) and n-hexylaminomethylferrocene (2) by HCl and NH(4)PF(6) forms the ferrocenylmethyl(alkyl)ammonium salt. Inclusion of the compounds by dibenzo[24]crown-8 (DB24C8) produces [2]pseudorotaxanes, [(DB24C8)(1-H)](+)(PF(6)) and [(DB24C8)(2-H)](+)(PF(6)), respectively. X-ray diffraction of the former product indicates an interlocked structure composed of the axis and the macrocyclic molecule. Intermolecular N-H...O and C-H...O interactions and stacking of the aromatic planes are observed. [(DB24C8)(1-H)](+)(PF(6)), in the solid state, is characterized by IR spectroscopy and elemental analyses. A similar reaction of 1,1'-bis(p-xylylaminomethyl)ferrocene (3) forms a mixture of [2] and [3]pseudorotaxanes, [(DB24C8)(3-H(2))](2+)(PF(6))(2) and [(DB24C8)(2)(3-H(2))](2+)(PF(6))(2). The latter product having two DB24C8 molecules is isolated and characterized by X-ray crystallography. Formation of these pseudorotaxanes in a CD(3)CN solution is evidenced by (1)H NMR and mass spectrometry. Electrochemical oxidation of 1-3 at 0.4 V (vs Ag(+)/Ag) in the presence of TEMPOH (1-hydroxy-2,2,6,6-tetramethylpiperidine) and DB24C8 affords the corresponding pseudorotaxanes. The ESR spectrum of the reaction mixture indicates the formation of a TEMPO radical in high yield. Details of the conversion of the dialkylamino group of the ligand to the dialkylammonium group are investigated by using a flow electrolysis method linked to spectroscopic measurements. The proposed mechanism for the reaction involves the ferrocenium species, formed by initial oxidation, which undergoes electron transfer from nitrogen to the Fe(III) center, producing a cation radical at the nitrogen. Transfer of hydrogen from TEMPOH to the cation radical and inclusion of the resulting dialkylammonium species by DB24C8 yields the pseudorotaxanes.