EFFICIENT AND SELECTIVE ELECTRON MEDIATION OF COBALT COMPLEXES WITH CYCLAM AND RELATED MACROCYCLES IN THE P-TERPHENYL-CATALYZED PHOTOREDUCTION OF CO2

Cobalt(III) complexes of cyclam (cyclam (L1) = 1,4,8,11-tetraazacyclotetradecane) (Co(III)L1) or related 14-membered tetraazamacrocycles (L2-L8) modiate electron transfer in the photoreduction of CO2 with p-terphenyl (OPP-3) as a photocatalyst and tertiary amines as sacrificial electron donors in methanolic acetonitrile. Tertiary amines (e.g., triethylamine (TEA)) used as electron donors play an important role in the electron mediation of Co(III)L1 through coordination, and the mediation of the amine-coordinatod Co(III)L1 suppresses the degradative and competitive photo-Birch reduction of OPP-3 and enhances the activity of OPP-3, leading to efficient and selective formation of both carbon monoxide (CO) and formate (HCO2-) without producing much H-2. The degradation of OPP-3 is mostly suppressed in the presence of beta-hydroxylated tertiary amines such as triethanolamine (TEOA) and tri-2-propanolamine (TIPOA), leading to much more efficient and selective production of CO and HCO2-. The total quantum yield of CO and HCO2- is 0.25 at 313 nm in the presence of TEOA. Preferential electron transfer from the photoformed radical anion of OPP-3 (OPP-3.-) to the TEA-coordinated Co(III)L1, [Co(III)L1(TEA)2]3+, is confirmed by the quenching of OPP-3.- by [Co(III)L1(TEA)2]3+ with a diffusion-controlled rate (k(s) = 1.1 x 10(10) M-1 s-1). Successive reduction of [Co(II)L1(TEA)]2+ by OPP-3.- results in the formation of [Co(I)L1]+. [Co(I)L1]+ can react with CO2 to give [Co(I)L1(CO2)]+ or react with a proton to give a d6 hydride [Co(III)L1(H-)(TEA)]2+. The extensive charge transfer from metal to bound CO2 and the coordination of tertiary amines may lead to the formation of d6 complexes like [Co(III)L1(CO22-)(TEA)]+, which may react with an electron from OPP-3.- or Co(I) species to form CO, OH-, and Co(II) species such as [Co(II)L1(TEA)]2+. As for the mechanism for the formation of HCO2-, the insertion of CO2 into intermediary hydride complexes such as [Co(III)L1(H-)(TEA)]2+ derived from [Co(I)L1]+ and H+ is proposed. The structural and electrochemical properties of cobalt complexes of the 14-membered tetraazamacrocycles investigated (L2-L8) are also discussed in view of the distribution of the reduction products of CO, HCO2-, and H-2.