Electrochemistry of 1,2,5-thiadiazole 1,1-dioxide derivatives with 3,4 substituents presenting separated, connected, and fused pi-systems

The voltammetric electroreduction properties of 1,2,5-thiadiazole 1,1-dioxi de derivatives with different 3,4 substituents have been investigated in ap rotic (mainly acetonitrile) solvent solution. The 3,4 substituents, selecte d according to the characteristics of their pi-system, were phenanthro-9,10 (connected pi-system) and acenaphtho-1,2 (fused a-system). New measurement s are also presented for 3,4-diphenyl (separated pi-systems). The basic pro perties of the substrates were investigated through voltammetry in acidic a cetonitrile solutions. The substrates were more easily electroreduced (betw een -0.6 and -1.0 V vs. Ag/Ag+, acetonitrile solvent) than the parent (non- 1,1-dioxide) heterocycles (which are electroreduced at ca. -2.5 V). They we re also more easily electroreduced (by ca. 0.4 V) than the corresponding di ketones (benzil, 9,10-phenanthrenequinone, and acenaphthenequinone). The in fluence on voltammetric behavior of the electron-withdrawing properties of the 1,2,5-thiadiazole 1,1-dioxide ring and that of the structure of the mol ecules was analyzed in light of previous structural studies by X-ray crysta llography and theoretical chemistry calculations. Bulk electrolysis experim ents in acidic acetonitrile solutions of the phenanthro[9,10-c] derivative produced its previously unreported dihydrogenated derivative (thiadiazoline ), which oxidizes easily. Infrared spectroscopy data suggested that the str ucture of this compound differs from that of 3,4-diphenyl-1,2,5-thiadiazoli ne 1,1-dioxide.