Ab Initio Characterization of the Isomerism between the μ-η²:η²-Peroxo- and Bis(μ-oxo)dicopper Cores

Abstract

The interconversion of model compounds {[(NH3)3Cu]2(μ-η2:η2-O2)}2+ (1) and {[(NH3)3Cu]2(μ-O)2}2+ (2) has been examined using multireference second-order perturbation theory with an 8-electron/8-orbital active space. At this level of theory, 1 and 2 are separated by only 0.3 kcal/mol, and the barrier to isomerization is predicted to be very low based on single-point energy calculations for intermediate structures. The flat nature of the potential energy surface along the interconversion coordinate derives from a balancing of Coulomb forces and nondynamic electron correlation. The latter effect depends critically on the significant energy change experienced by the 13au virtual orbital on passing from one isomer to the other. In addition, solvation electrostatics favor 2 over 1.

The restricted active space followed by second-order perturbation theory method: Theory and application to the study of CuO2 and Cu2O2 systems

“…In both of these cases, the resulting compounds with a variety of ligands can have singlet ground states that exhibit substantial biradical character because of the spin separation associated either with two d 9 Cu͑II͒ ions or one such ion and a superoxide radical anion. [40][41][42][43][44][45][46][47][48][49][50][51][52] Wave function theories restricted to a single determinant are poorly suited to the description of such species since singlet biradicals are intrinsically two determinantal. 53,54 Moreover, even when oxidation states more likely to be characterized as closed shell in nature are considered, e.g., ͓LCu͑III͔͒ 2 ͓O͑2−͔͒ 2 , computational studies have found that large contributions from dynamical correlation effects influence relative isomer energetics.…”

“…53,54 Moreover, even when oxidation states more likely to be characterized as closed shell in nature are considered, e.g., ͓LCu͑III͔͒ 2 ͓O͑2−͔͒ 2 , computational studies have found that large contributions from dynamical correlation effects influence relative isomer energetics. [40][41][42][47][48][49]51,52 In this section, we apply the RASPT2 method to two problems previously studied in considerable detail using a wide range of theoretical models ͑see Fig. 2͒.…”

Density matrix renormalization group calculations on relative energies of transition metal complexes and clusters

“…Two main motifs of metal-containing cores containing two copper ions were synthetically characterized, the bis͑-oxo͒ ͓Cu 2 ͑-O͒ 2 ͔ 2+ ͑1͒ and the peroxo ͓Cu 2 ͑-2 : 2 -O 2 ͔͒ 2+ ͑2͒ cores. 58 Cramer et al 59 examined the interconversion of the model compounds…”

Theoretical Models on the Cu2O2 Torture Track: Mechanistic Implications for Oxytyrosinase and Small-Molecule Analogues

“…6 However, Tolman and co-workers, 11,13,17,20,21 and later others, [22][23][24][25][26] have demonstrated that in select instances the relative energies of isomeric µ-η 2 :η 2 peroxo and bis(µ-oxo) complexes can be sufficiently close to one another, and the barrier to their interconversion sufficiently low, that the two species may be in rapid equilibrium with one another; theoretical studies have rationalized many of the electronic structural details governing this phenomenon. [27][28][29][30][31][32][33][34] This observation potentially complicates mechanistic analysis in model systems and, by extension, oxytyrosinase, because the kinetics for systems characterized by a rapid preequilibrium can be indistinguishable from systems involving only a single reactant. 35 Thus, even in the absence of observing a spectral signature for a bis(µ-oxo) intermediate, because it is present at, say, part per million concentration, one cannot a priori discount the possibility that this same isomer is so much more reactive than the µ-η 2 :η 2 peroxo that product derives predominantly from prior isomerization and subsequent reaction.…”