The Thermodynamics and Kinetics of CO2 and H+ Binding to Ni(cyclam)+ in Aqueous Solution

Abstract

Pulse radiolysis and laser flash photolysis have been used to generate and characterize Ni(cyclam)+, where cyclam is 1,4,8,11-tetraazacyclotetradecane, in aqueous solution. Reduction of Ni(cyclam)2+ was carried out at ambient temperature (22 ± 2 °C) by solvated electrons, eaq", hydrogen atoms, H", and carbon dioxide radical anions, C02-. Reduction by eaq~o ccurs with a rate constant of (4.1 ± 0.2) x 1010 M_1 s_1 (7 < 2 x -4 M). Reduction by C02*_ and H' proceeds via an inner-sphere mechanism to yield [Ni(cyclam)(C02)]+ and [Ni(cyclam)(H)]2+, with rate constants of (6.7 ± 0.2) x 109 and (5 ± 2) x 109 M_l s_1, respectively. Decomposition of the adducts, to yield Ni(cyclam)+ and C02 or H+, proceeds with rate constants of (2.0 ± 0.2) x 106 and (5.3 ± 0.7) x 105 s_l, respectively.Carbon dioxide and proton dissociation constants for [Ni(cyclam)(C02)]+ and [Ni(cyclam)(H)]2+ were found to be (6.2 ± 0.3) x 10-2 and (1.6 ± 0.4) x 10-2 M, respectively. The rate constants for the addition of C02 and H+ to Ni(cyclam)+ were calculated to be (3.2 ± 0.4) x 107 and (3 ± 1) x 107 M-1 s_1, respectively. The selective reduction of C02 by Ni(cyclam)+ in the presence of protons at pH 4 is rationalized.

Diffusion-Controlled Charge Transfer from Excited Ru(bpy)32+ into Nanosized TiO2 Colloids Stabilized with EDTA

“…k q ) πr′ND/250 (17) k q ) πr′ 2 Nk et /250 (18) log k q ) log(πr′ 2 N/250) + log k et 0 + 12.2R -RpH…”

Application of Pulse Radiolysis to Mechanistic Investigations of Catalysis Relevant to Artificial Photosynthesis

“…Inset:Dependenceo ft he absorbance at l = 380 nm (A 380 )o nt he concentration of dissolved CO 2 .Reproducedw ith permission from Ref. [39].Copyright 1995 American Chemical Society. ChemSusChem 2017, 10,4359 -4373 www.chemsuschem.org Herein, we have shown how PR has been applied in mechanistic investigations of redox catalysis related to AP.F or example, PR has been used to obtain kineticand mechanistic knowledge of water oxidation by both colloidal metal oxides and molecular catalysts.…”

“…+ is an active catalystf or CO 2 reduction in water, Kelly et al [39,44] + spectrum( l max = 380 nm) exhibitedasmalll inear decrease with [CO 2 ], which indicated the presenceo fa ne quilibrium, as shown in Equation (31). Assuming that the CO 2 adduct does not have any significant absorption in the l = 300-700 nm region, the K CO 2 for Equation (31) was estimated to be 16 m À1 (see Ta ble 3).…”

Reactivity of Inorganic Radicals in Aqueous Solution

“…82 This logic was supported by an analysis of the low rate constant for the reaction of Fe 3 þ (aq). In the case of the actinyl ions, MO 2 2 þ (M ¼ U, Pu, Np, and Am), the rate constants range from 4.5 Â 10 7 to 1.6 Â 10 9 M À1 s À1 and increase uniformly as the metal ion becomes a stronger oxidant.…”

“…An example of H-atom addition to metal complexes is provided by the reaction of [Ni(cyclam)] 2 þ and a Co (II) tetraazamacrocycle. 82,125 The hydroxyl radical reacts with many metal centers through an apparent substitution mechanism:…”