Solvent, N-alkyl substituent and coordination effects on the electrochemistry of 2,11-diaza-5,8-dithia[12](1,1')ferrocenophanes

The synthesis of N, N-diethyi-2,11-diaza-5,8-dithia[12](1,1')ferrocenophane (6), cyclic voltammetric studies of diazadithiaferrocenophanes in ten organic solvents and the electrochemical characteristics of four diazadithiaferrocenophane complexes with palladium(II) are reported with the objective of better understanding the factors which govern Fe-S bonding in such compounds. As compared with the N-decyl analog 7 (E_1/2(Fe^III/II = 727 mV vs. SHE), the half-wave reduction potential of 6 (388 mV) in acetonitrile solution (25°C, 0.1 M N(^uBu)₄ClO₄) is displaced by 0.34 V to more negative potential. However, the E_1/2 values for 6 and 7 are essentially identical (average of 0.39 ± 0.02 V) in butyronitrile, benzonitrile, dichloromethane, N,N-dimethylformamide, acetone and methanol. On this basis it is concluded that conformations which permit Fe^II-S bonding in 7 are supported by hydrophobic attractions between the N-decyl substituents. The perturbation of Fe-S bonding by coordination of S and/or N atoms to Pd^II was examined by preparing [7]PdCl₂, [6]PdCl₂. 2HCl, [6]Pd(CA). 2H₂O and [7]Pd(CA), where CA^2- = 2,5-dioxo-3,6-dichloro-1,4-benzoquinone (chloranilate dianion). Electrochemical and ¹³C NMR findings suggest that both ferrocenophane sulfur atoms ligate Pd^II in the former three complexes, while [7]Pd(CA) prefers ligation through both N and S donor atoms, leaving one thioether S to interact with the Fe^II center.