Efficient and selective electron mediation of cobalt complexes with cyclam and related macrocycles in the p-terphenyl-catalyzed photoreduction of carbon dioxide

Abstract

Cobalt(III) complexes of cyclam (cyclam (L[sub 1]) = 1,4,8,11-tetraazacyclotetradecane) (Co[sup III]L[sub 1]) or related 14-membered tetraazamacrocycles (L[sub 2]-L[sub 8]) mediate electron transfer in the photoreduction of CO[sub 2] 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[sup III]L[sub 1] through coordination, and the mediation of the amine-coordinated Co[sup III]L[sub 1] 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 (HCO[sub 2][sup [minus]]) without producing much H[sub 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 HCO[sub 2][sup [minus]]. As for the mechanism for the formation of HCO[sub 2][sup [minus]], the insertion of CO[sub 2] into intermediary hydride complexes such as [Co[sup III]L[sub 1](H[sup [minus]])(TEA)][sup 2+] derived from [Co[sup I]L[sub 1]][sup +] and H[sup +] is proposed. The structural and electrochemical properties of cobalt complexes of the 14-membered tetraazamacrocycles investigated (L[sub 2]-L[submore » 8]) are also discussed in view of the distribution of the reduction products of CO, HCO[sub 2][sup [minus]], and H[sub 2]. 45 refs., 14 figs., 3 tabs.« less