- Mechanistic and kinetic studies of cobalt macrocycles in a photochemical CO2 reduction system: Evidence of Co-CO2 adducts as intermediates
-
Cobalt macrocycles mediate electron transfer in the photoreduction of CO2 with p-terphenyl as a photosensitizer and a tertiary amine as a sacrificial electron donor in a 5:1 acetonitrile/methanol mixture. The mechanism and kinetics of this system have been studied by continuous and flash photolysis techniques. Transient spectra provide evidence for the sequential formation of the p-terphenyl radical anion, the CoIL+ complex, the [CoIL-CO2]+ complex, and the [S-CoIIIL-(CO22-)]+ complex (L = HMD = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene; S = solvent) in the catalytic system. The electron-transfer rate constant for the reaction of p-terphenyl radical anion with CoIIL2+ is 1.1 × 1010 M-1 s-1 and probably diffusion controlled because of the large driving force (~+1.1 V). Flash photolysis studies yield a rate constant 1.7 × 108 M-1 s-1 and an equilibrium constant 1.1 × 104 M-1 for the binding of CO2 to CoIL+ in the catalytic system. These are consistent with those previously obtained by conventional methods in acetonitrile. Studies of catalytic systems with varying cobalt macrocycles highlight some of the factors controlling the kinetics of the photoreduction of CO2. Steric hindrance and reduction potentials are important factors in the catalytic activity for photochemical CO2 reduction.
- Ogata, Tomoyuki,Yanagida, Shozo,Brunschwig, Bruce S.,Fujita, Etsuko
-
p. 6708 - 6716
(2007/10/02)
-
- Collision-induced Dissociations of Carboxylate Negative Ions from 2-Ethylbutanoic, 2-Methylpropanoic, and Pivalic Acids. An Isotopic Labelling Study
-
Deprotonation of Et2CHCO2H yields Et2CHCO21-.On collisional activation this ion forms CO2-., CH2=CH-, and MeCH=CH-.In addition, elimination of H. and Et. yield Et(R)C=CO2-. (R=Et and H, respectively).The elimination of Et. is not a simple cleavage but occurs by loss of H. from a methyl group followed by loss of ethene.The carboxylate ion also rearranges to ; this species decomposes to OH-, , and also eliminates the elements of C3H8 and CH4.All fragmentations have been studied using 2H and 13C labelling: for example it is proposed that loss of CH4 from occurs by a six-centre stepwise process in which the first step (formation of an incipient methyl anion) is rate determining.The collisional activation mass spectra of Et2CHCO2-, Me2CHCO2-, and Me3CCO2- are different, all showing characteristic decompositions.For example, all three ions eliminate methane; the mechanism is different in each case.
- Stringer, Michael B.,Bowie, John H.,Eichinger, Peter C. H.,Currie, Graeme J.
-
p. 385 - 390
(2007/10/02)
-