Crystal and molecular structure of protonated (N-propyl)-aminomethyl ferrocene, a proton-sensitive redox-responsive fragment

In order to rule out any possible 'chelate' iron-proton interaction which could be responsible for the dramatic changes observed in the Fc(+)/Fc redox potential on proton addition in a series of N-substituted aminomethyl ferrocenes (4a-d), we report the results of electrochemical titration experiments with strong acids on the free amines in protic and aprotic solvents and the X-ray diffraction structure of (ferrocenylmethyl) propylammonium p-toluenesulfonate, [4aH][OTs]. Crystals of [4aH][OTs] are orthorhombic, space group P2(1)2(1)2(1), with a = 11.919(2), b = 21.313(3), c = 8.246(2) Angstrom, Z = 4. The conformation of the FcCH(2)NH(2)C(3)H(7)(+) cation, which prevents any intramolecular iron-proton interaction, seems imposed by the intermolecular N-H ... O hydrogen bonds that the amino H atoms form with oxygen atoms from p-toluenesulfonate anions. (C) 1998 Elsevier Science S.A.