We extended our research on N-benzamidothiourea based anion receptors to N-ferrocamidothioureas and synthesized N-ferrocamido-N'-(substituted phenyl)thioureas (3a～3e, substituent X＝p-OCH₃, p-CH₃, H, m-Br, and m-CF₃). In 3a～3e an electrochemically active ferrocene moiety was employed as an aromatic counterpart of phenyl and —NH— was a spacer linking ferrocene signal reporter and the thiourea binding-site. Cyclic voltammetric ex-periments carried out in acetonitrile indicated that the half-wave potentials of 3a～3e of 0.31 V vs. Ag/AgNO₃ were independent of the substituent at N'-phenyl ring. It was found that the ¹H NMR chemical shifts of the aromatic protons at ferrocene moi-ety and the amido —NH proton in 3 did not, whereas those of the N'-phenyl aromatic pro-tons and the thioureido —NH protons did vary when the N'-phenyl substituent was changed. The azino N—N bond was hence concluded to be twisted that blocked the electronic communication between the ferrocene moiety and thiourea anion-binding site. Yet we experimentally observed that the oxidation poten-tials of 3a～3e shifted by ca. －200 mV in the presence of anions such as CH₃CO₂^－ and F^－ in acetonitrile and the chemical shifts of the ferrocene aromatic protons shifted to high field upon anion binding to the thiourea moiety in 3a～3e. A conformation change in the N—N bond was hence assumed for 3a～3e upon anion binding. Absorption titrations indicated that anion binding to 3a～3e resulted in a new absorption band at ca. 315 nm with a spectral red-shift of 5820 cm^-1 irrespective of the substituent. The spectral red-shift, anion affinity of 10⁶ mol^－1•L orders of magnitude for CH₃CO₂^－ and F^－, and substituent effect on the anion affinity of 3a～3e were found much higher than those of the corresponding traditional N,N'-diphenylthiourea coun-terparts (1). It was concluded that anion binding induced conformational change in 3a～3e led to enhanced anion affinity and substituent effect on the anion affinity, information indicative of allosteric character of the receptors and instructive for designing N-arylamido-N'-arylthiourea based anion receptors. Results reported here further supported the advantages of the “—NH—” spacer in N-benzamidothiourea-based anion receptors.

Key words: anion receptor, ferrocene, thiourea, allosterism, —NH— spacer, cyclic voltammetry