The Synthesis and Properties of Cobalt Cage Complexes With N₃S₃ Donor Sets

Abstract

An understanding of how variations in the cage ligands modify the redox behaviour of cobalt(II)/(III) couples has led to rational syntheses of a series N₃S₃ donor cobalt cage complexes that have redox potentials and electron self-exchange rates appropriate for their use as electron carriers in systems devised for the photoreduction of water. Diazotization of [Co(NH₃-capten)]Cl₄ (NH₃-capten = 8-ammonio-1-methyl-3,13,16-trithia-6,10,19-triazabicyclo[6.6.6] icosane ) in nitric acid resulted in a mixture of five complexes: [Co(NO₂-capten)]³⁺, [Co( Cl-capten )]³⁺ and [Co(HO- capten )]³⁺ (8-nitro-, 8-chloro- and 8-hydroxy-1-methyl-3,13,16-trithia-6,10,19-triazabicyclo[6.6.6] icosanecobalt (III) respectively), in which the cage framework remained intact, together with two complexes with a contracted cap, [Co(ClCH₂-abcapten)]³⁺ and [Co(HOCH₂-abcapten)]³⁺, (8-chloromethyl- and 8-hydroxymethyl-1-methyl-3,13,16-trithia-6,10,19-triazabicyclo[6.6.5] nonadecanecobalt (III), respectively). Reductive elimination occurred with [Co( Cl-capten )]³⁺ in the presence of Zn or Ni/Al alloy to give the parent cage complex, [Co( capten )]³⁺ (1-methyl-3,13,16-trithia-6,10,19-triazabicyclo[6.6.6] icosanecobalt (III)). When [Co(ten)]³⁺(4,4′,4″-ethylidynetris(3-thiabutan-1-amine)cobalt(III)) and an aqeuous solution of diethyl malonate and formaldehyde were reacted under basic conditions, the amide cage complex, [Co( EtOOC-oxocapten-H )]²⁺, (8-ethoxycarbonyl-1-methyl-3,13,16-trithia-6,10,19-triazabicyclo [6.6.6]icosan-7-onato(1-)cobalt(III)) was obtained. Hydrolysis of the ester group in base yielded the carboxylate derivative [Co(OOC- oxocapten - H)]⁺(8-carboxylate-1-methyl-3,13,16-trithia-6,10,19-triazabicyclo[6.6.6]icosan-7-onato(1-)cobalt(III)). The complexes were characterized by microanalyses, ¹H and ¹³C n.m.r. spectroscopy, and electrochemistry. The values of the cobalt(III)/(II) redox potentials change with the nature of the apical substituents in a similar manner to that observed for the analogous hexaamine cage complexes, but they are all more positive, and the cobalt(II) complexes are low spin. The N₃S₃ donor set stabilizes the lower oxidation state and the low-spin electronic configuration. All 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. 10⁹ dm³ mol-¹s-¹. [Co( EtOOC-oxocapten-H )]²⁺ quenches [ Ru*( bpy )₃]²⁺ very efficiently and has a suitable redox potential for hydrogen production, but it is only moderately efficient as an electron-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 energy transfer and/or back electron-transfer inhibit their ability to be used 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.