Ligand Macrocycle Structural Effects on Copper−Dioxygen Reactivity

Abstract

With the goal of understanding how the nature of the tridentate macrocyclic supporting ligand influences the relative stability of isomeric mu-eta 2:eta 2-peroxo- and bis(mu-oxo)dicopper complexes, a comparative study was undertaken of the O2 reactivity of Cu(I) compounds supported by the 10- and 12-membered macrocycles, 1,4,7-R3-1,4,7-triazacyclodecane (R3TACD; R = Me, Bn, iPr) and 1,5,9-triisopropyl-1,5,9-triazacyclododecane (iPr3TACDD). While the 3-coordinate complex [(iPr3TACDD)Cu]SbF6 was unreactive with O2, oxygenation of [(R3TACD)Cu(CH3CN)]X (R = Me or Bn; X = ClO4- or SbF6-) at -80 degrees C yielded bis(mu-oxo) species [(R3TACD)2Cu2(mu O)2]X2 as revealed by UV-vis and resonance Raman spectroscopy. Interestingly, unlike the previously reported system supported by 1,4,7-triisopropyl-1,4,7-triazacyclononane (iPr3TACN), which yielded interconverting mixtures of peroxo and bis(mu-oxo) compounds (Cahoy, J.; Holland, P. L.; Tolman, W. B. Inorg. Chem. 1999, 38, 2161), low-temperature oxygenation of [(iPr3TACD)Cu(CH3CN)]SbF6 in a variety of solvents cleanly yielded a mu-eta 2:eta 2-peroxo product, with no trace of the bis(mu-oxo) isomer. The peroxo complex was characterized by UV-vis and resonance Raman spectroscopy, as well as an X-ray crystal structure (albeit of marginal quality due to disorder problems). Intramolecular attack at the alpha C-H bonds of the substituents was indicated as the primary decomposition pathway of the oxygenated compounds through examination of the decay kinetics and the reaction products, which included bis(mu-hydroxo)- and mu-carbonato-dicopper complexes that were characterized by X-ray diffraction. A rationale for the varying results of the oxygenation reactions was provided by analysis of (a) the X-ray crystal structures and electrochemical behavior of the Cu(I) precursors and (b) the results of theoretical calculations of the complete oxygenated complexes, including all ligand atoms, using combined quantum chemical/molecular mechanics (integrated molecular orbital molecular mechanics, IMOMM) methods. The size of the ligand substituents was shown to be a key factor in controlling the relative stabilities of the peroxo and bis(mu-oxo) forms, and the nature of this influence was shown by both theory and experiment to depend on the ligand macrocycle ring size.

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.…”

Spin Crossover in a Family of Iron(II) Complexes with Hexadentate ligands: Ligand Strain as a Factor Determining the Transition Temperature

“…For example, significant chelate-ring-size effects have been found in the copper-dioxygen chemistry, [14,15] catechol dioxygenase reactivity of ironA C H T U N G T R E N N U N G (III) compounds, [16] electrochemical properties of manganeseA C H T U N G T R E N N U N G (III) and (IV) complexes, [17] spectroscopic properties of nickel(II) [18] and cobaltA C H T U N G T R E N N U N G (III) compounds. [19][20][21] Some attempts to consider the effect of chelate cycles size on the spin state of iron(II) have been done in the literature.…”

The catalytic cycle of tyrosinase: peroxide attack on the phenolate ring followed by O-O bond cleavage

“…The mechanism for O-O activation by copper dimer complexes related to tyrosinase has been the subject of several previous theoretical studies [20,21,22,23,24,25,26]. Most of these studies used ammonia as ligands rather than imidazoles.…”