Ligand Macrocycle Structural Effects on Copper−Dioxygen Reactivity

With the goal of understanding how the nature of the tridentate macrocyclic supporting ligand influences the relative stability of isomeric μ-η<sup>2</sup>:η<sup>2</sup>-peroxo- and bis(μ-oxo)dicopper complexes, a comparative study was undertaken of the O<sub>2</sub> reactivity of Cu(I) compounds supported by the 10- and 12-membered macrocycles, 1,4,7-R<sub>3</sub>-1,4,7-triazacyclodecane (R<sub>3</sub>TACD; R = Me, Bn, iPr) and 1,5,9-triisopropyl-1,5,9-triazacyclododecane (iPr<sub>3</sub>TACDD). While the 3-coordinate complex [(iPr<sub>3</sub>TACDD)Cu]SbF<sub>6</sub> was unreactive with O<sub>2</sub>, oxygenation of [(R<sub>3</sub>TACD)Cu(CH<sub>3</sub>CN)]X (R = Me or Bn; X = ClO<sub>4</sub><sup>-</sup> or SbF<sub>6</sub><sup>-</sup>) at −80 °C yielded bis(μ-oxo) species [(R<sub>3</sub>TACD)<sub>2</sub>Cu<sub>2</sub>(μ-O)<sub>2</sub>]X<sub>2</sub> 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 (iPr<sub>3</sub>TACN), which yielded interconverting mixtures of peroxo and bis(μ-oxo) compounds (Cahoy, J.; Holland, P. L.; Tolman, W. B. <i>Inorg. </i><i>Chem.</i> <b>1999</b>, <i>38</i>, 2161), low-temperature oxygenation of [(iPr<sub>3</sub>TACD)Cu(CH<sub>3</sub>CN)]SbF<sub>6</sub> in a variety of solvents cleanly yielded a μ-η<sup>2</sup>:η<sup>2</sup>-peroxo product, with no trace of the bis(μ-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 α 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(μ-hydroxo)− and μ-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(μ-oxo) forms, and the nature of this influence was shown by both theory and experiment to depend on the ligand macrocycle ring size.