Electrocatalytic reduction and fixation of carbon dioxide by some transition metal complexes

Since the excessive production of CO{dollar}sb2{dollar} is a major environmental problem, reduction of CO{dollar}sb2{dollar} to energetically rich raw materials would help to reduce atmospheric pollution and provide an alternative source of fuels and chemicals. Although much attention has been paid to the development of metal catalysts for CO{dollar}sb2{dollar} reduction, relatively little attention was paid to the importance of the presence of Bronsted acids in the system. In this study the effect of weak Bronsted acid on two metal catalysts, namely (Re(bpy)(CO){dollar}sb3{dollar}py) {dollar}sp+{dollar} (bpy = 2,2{dollar}spprime{dollar}-bipyridine, py = pyridine) and (Fe(dophen)(1-MeIm){dollar}sb2rbracksp+{dollar} (dophen = 2,9-bis(2{dollar}spprime{dollar}-hydroxyphenyl)-1,10-phenanthroline dianion, 1-MeIm = 1-methylimidazole) were investigated. Addition of weak Bronsted acids such as trifluoroethanol, phenol, methanol or water can significantly enhance the rate of the catalytic process and improve the lifetime of the catalyst. For (Re(bpy)(CO){dollar}sb3{dollar}py) {dollar}sp+{dollar}, CO was produced as the only product with faradaic efficiency close to 100%. Mechanistic studies suggest that CO was produced by successive protonation of the coordinated CO{dollar}sb2{dollar} by the Bronsted acids followed by cleavage of one C-O bond and subsequent release of CO from the metal center. When (Fe(dophen)(1-MeIm){dollar}sb2rbracksp+{dollar} was employed as catalyst, a mixture of carbon monoxide, formate and oxalate were produced. We proposed that early cleavage of the Fe-C bond before the first and second protonation steps will lead to the formation of CO{dollar}sb2sp-{dollar} (which will subsequently dimerize to yield oxalate) and formate respectively.; Electroreductive carboxylation is an alternative method of using CO{dollar}sb2{dollar} molecule as a Cl building block in organic synthesis. In this study. the electrochemical carboxylation of arylmethyl chlorides (RCI) such as 4-(trifluoromethyl)benzyl, 4-methoxybenzyl and diphenylmethyl chloride with (Co(dobpy)) (dobpy = 6,6{dollar}spprime{dollar}-bis(2{dollar}spprime{dollar}-hydroxyphenyl)-2,2{dollar} spprime{dollar}-bipyridine dianion) and related complexes as catalysts were investigated. A mixture of carboxylic acids and hydroxycarbons were formed and the selectivity pattern depends on both the catalyst and electrolysis potential. It is believed that the electrogenerated (Co{dollar}rmsp{lcub}I{rcub}Lrbracksp-{dollar} reacts with RCl to give an organometallic complex, (Co{dollar}rmsp{lcub}II{rcub}{dollar}LR). The one-electron reduced complex (Co{dollar}rmsp{lcub}II{rcub}LRrbracksp-{dollar} is unstable and its decomposition in the presence of CO{dollar}sb2{dollar} is the key step in the electrocatalytic process. The decomposition involves the Co-C bond cleavage which can be either homolytic or heterolytic. When (Co(TPP)) (TPP = meso-tetraphenylporphine dianion) was employed as the electrocatalyst, the highly conjugated electron-withdrawing porphyrin ring favours hydrocarbon formation. Although the activities of (Co(dobpy)) and (Co(salen)) (salen N,N{dollar}spprime{dollar}-bis(salicylidene)ethane-1,2-diamine dianion) are similar, the higher stability of the former complex can be attributed to the absence of imino bond in the dobpy ligand.