Ferrocenyl derivatives of dithizone : photochromic properties and potential as electrochemical sensors
New ferrocenyl derivatives of dithizone were successfully prepared and characterised by various analytical techniques. In the first part of this project interactions between photochromic and redox properties were investigated for the new ferrocenylmercury dithizonate complexes (N,S-dithizonato )(ferrocenyl)mercury(II) (11) and (ferrocene-l, 1'- diyl)bis[(N,S-dithizonato)mercury(II)] (12). UV-Vis spectroscopy showed that these complexes exhibited similar photochromic behaviour to related compounds known in the literature, i.e., a change in colour from orange to blue upon irradiation of organic solutions with visible light. The molar extinction coefficient of compound (12) was almost double that of compound (11), indicating that there are two chromophores absorbing in compound (12) compared to one in compound (11). Cyclic voltammetry on acetonitrile solutions of these photochromic compounds showed no noticeable differences in the voltammograms for the activated blue form compared to the normal orange form when irradiated with sunlight. This suggests that the interactions between the redoxactive group and the changes in the conformation of the photochromic group were not significant enough to make any noticeable changes to the voltammetric behaviour. In the second part of this project the electrochemical recognition of various metal ions by the new ligands S-ferrocenyldithizone (13) and S-ferrocenylmethyldithizone (14) was investigated. In both compounds the transducer is the ferrocenyl substituent and the binding site is the formazan group. These receptors showed a reversible redox wave in acetonitrile assigned to the ferrocene/ferrocenium couple: for receptor (13), E1I2 = 203 mV and for receptor (14), E1I2 = 136 mV relative to ferrocene as a standard reference. Addition of Cu2 + ions to ligand solutions in acetonitrile showed contrasting behaviour: ligand (13) produced a new wave with shift in E1I2 from 203 to 387 mV (M1I2 = 184 mY). For receptor (14), titration with Cu2 + ions resulted in a continuous shift of the EI/2 of the original receptor from 136 mV up to a maximum of 180 mV (MlI2= 44 mY). It was discovered that the formazan binding site of the receptor was likely oxidised by Cu2 + ions to form a tetrazolium salt, and this was proven from the product obtained during the attempted synthesis of a receptor(13)-Cu2 + complex: the product obtained was detennined crystallographically to be 5-ferrocenylthio-2,3-diphenyltetrazolium perchlorate. On the other hand, upon addition of PhHg+ to a solution of receptor (13) a new wave with M1/2 = 177 m V was observed resulting from the formation of a receptormetal complex, while addition of this organomercury ion to a solution of receptor (14) resulted in the continuous shift of the original receptor wave up to a maximum of M1/2 = 32 mY. No binding was observed when receptor (13) was titrated wi1h Ni2 + ions; when receptor (14) was titrated with this metal ion a gradual shift of the ferrocenyl wave up to a maximum of M,1/2 = 39 mV was obtained. Large potential shifts were observed when the receptor (13) was titrated with various metal ions compared to small potential shifts which were observed during titration experiments with receptor (14). The methylene link present in receptor (14) increased the distance between the binding site and the transducer, decreasing the interaction between the binding events and the transducer. The electronic communication between the transducer and the metal ions during titration of receptor (13) with metal ions is most likely a combination of through-space and throughbond interaction, while through-space communication seems likely for receptor (14). Synthesis of several receptor-metal complexes was attempted in order to elucidate the coordination mode of the ligand with the metal ion; however, no solid compound was successfully isolated. In addition, the data obtained from cyclic voltammetry did not allow confinnation of reliable receptor : metal ratios, thus unambiguous coordination modes for the receptor-metal complexes could not be specified. The possible application in bio-electrochemistry of the new compound (13) in its oxidized form as the tetrazolium salt (5-ferrocenylthio-2,3-diphenyltetrazolium perchlorate) has also been recognized and preliminary experiments were conducted.
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