Figure 119.

Schematic employed to experimentally determine reaction kinetics [i.e., the 1:1 primary interaction of O₂ and CO with various [(ligand)Cu^I] complexes (ligand = ^dL)], using transient absorption spectroscopy and the “flash-and trap” approach.^940,1002 Experiments are conducted in the presence of precisely determined concentrations of O₂ and CO (as mixtures) within the temperature range of interest and on nanosecond and longer time scales. As depicted here, initial laser photolysis of ^DLCu^I-CO leads to CO photoejection, giving solvated ^dLCu^I in a solution of both CO and O₂. The initial “fast” process (k_fast) involves the competitive binding of both CO and O₂ with ^DLCu^I, either regenerating ^DLCu^I-CO or forming ^DLCu^II-O₂^•−; thus, k_fast is a combination of two rate constants, k_O2 and k_co. The latter is independently determined from transient absorption experiments with ^DLCu^I–CO with only CO_(g) present (i.e., in the absence of dioxygen). A complementary determination of the key parameters comes from observations that binding of carbon monoxide is thermodynamically favored compared to dioxygen, as is well-known for heme proteins and is also the case in copper-dioxygen proteins and complexes, K_co ≫ K_o2 (where K is the binding constant). Thus, CO subsequently displaces coordinated O₂ from ^DLCu^II-O₂^•− to reform the initial ^DLCu^I–CO species (k_slow; millisecond time scale), according to the equation for 1/k_slow shown in the graphic, as originally deduced by Antonini and Burnori¹²⁵⁵ in studies of heme protein O₂-carriers. Adapted from ref 1002. Copyright 2010 American Chemical Society.