THE SYNTHESIS AND PROPERTIES OF COBALT CAGE COMPLEXES WITH N3S3 DONORSETS

An understanding of how variations in the cage ligands modify the redo x behaviour of cobalt(II)/(III) couples has led to rational syntheses of a series N3S3 donor cobalt cage complexes that have redox potential s and electron self-exchange rates appropriate for their use as electr on carriers in systems devised for the photoreduction of water. Diazot ization of [Co(NH3-capten)] Cl4 (NH3-capten = 13,16-trithia-6,10,19-tr iazabicyclo[6.6.6]icosane) in nitric acid resulted in a mixture of fiv e complexes: [Co(NO2-capten)]3+, [Co(Cl-Capten)]3+ and [Co(HO-Capten)] 3+ (8-nitro-, 8-chloro- and hia-6,10,19-triazabicyclo[6.6.6]icosanecob alt(III) respectively), in which the cage framework remained intact, t ogether with two complexes with a contracted cap, [Co(ClCH2-abcapten)] 3+ and [Co(HOCH2-abcapten)]3+, (8-chloromethyl- and bicyclo[6.6.5]nona decanecobalt(III),respectively). Reductive elimination occurred with [ Co(Cl-capten)]3+ in the presence of Zn or Ni/Al alloy to give the pare nt cage complex, [Co(capten)]3+ a-6,10,19-triazabicyclo[6.6.6]icosanec obalt(III)). When [Co(ten)]3+ ''-ethylidynetris(3-thiabutan-1-amine)co balt(III)) and an aqeuous solution of diethyl malonate and formaldehyd e were reacted under basic conditions, the amide cage complex, [Co(EtO OC-oxocapten-H)]2+, triazabicyclo[6.6.6]icosan-7-onato(1-)cobalt(III)) was obtained. Hydrolysis of the ester group in base yielded the carbo xylate derivative [Co(OOC-oxocapten - H)]+ riazabicyclo[6.6.6]icosan-7 -onato(1-)cobalt(III)). The complexes were characterized by microanaly ses, H-1 and C-13 n.m.r. spectroscopy, and electrochemistry. The value s of the cobalt(III)/(II) redox potentials change with the nature of t he apical substituents in a similar manner to that observed for the an alogous hexaamine cage complexes, but they are all more positive, and the cobalt(ii) complexes are low spin. The N3S3 donor set stabilizes t he lower oxidation state and the low-spin electronic configuration. Al l of these cage complexes are effective at quenching the lowest lying triplet excited state of the [Ru(bpy)3]2+ CoMpleX, with rate constants typically c. 10(9) dm3 mol-1 S-1. [Co(EtOOC-oxocapten - H)]2+ quenche s [Ru(bpy)3]2+ very efficiently and has a suitable redox potential fo r hydrogen production, but it is only moderately efficient as an elect ron-transfer agent in the photoreduction of water. It is apparent that the high molar absorption coefficients of these cage complexes in the visible region, the too positive redox potentials and competing energ y transfer and/or back electron-transfer inhibit their ability to be u sed as effective electron-transfer agents in these reactions at pH c. 5. However, the molecules are inherently interesting and stable redox reagents which undergo rapid one-electron reactions.