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We have synthesized and characterized bis(mu-oxo)dicopper(III) dimers 1b-4b (Os) based on a core family of peralkylated trans-(1R,2R)-cyclohexanediamine (CD) ligands, self-assembled from the corresponding LCu(MeCN)CF3SO3 species 1a-4a and O2 at 193 K in aprotic media; additional Os based on peralkylated ethylenediamine and tridentate polyazacyclononane ligands were synthesized analogously for comparative purposes (5b-7b and 8b-9b, respectively). Trigonal-planar LCu(MeCN)1+ species are proposed as the active O precursors. The 3-coordinate Cu(I) complexes (L(TE))Cu(MeCN)CF3SO3 (4a) and (L(TB))Cu(MeCN)CF3SO3 (10a) were structurally characterized; the apparent O2-inertness of 10a correlates with the steric demands of its four benzyl substituents. The rate of O formation, a multistep process that likely proceeds via associative formation of a 1:1 LCu(O2)1+ intermediate, exhibits significant dependence upon ligand sterics and solvent: oxygenation of 4a-the slowest-reacting O precursor of the CD series-is first-order with respect to 4a and proceeds at least 300 times faster in tetrahydrofuran than in CH2Cl2. The EPR, UV-vis, and resonance Raman spectra of 1b-9b are all characteristic of the diamagnetic bis(mu-oxo)dicopper(III) core. The intense ligand-to-metal charge transfer absorption maxima of CD-based Os are red-shifted proportionally with increasing peripheral ligand bulk, an effect ascribed to a slight distortion of the Cu2O2 rhomb. The well-ordered crystal structure of (L(ME))2Cu2(mu-O)2(CF3SO3)2.4CH2Cl2 (3b. 4CH2Cl2) features the most metrically compact Cu2O22+ core among structurally characterized Os (av Cu-O 1.802(7) A; Cu...Cu 2.744(1) A) and exemplifies the minimal square-planar ligation environment necessary for stabilization of Cu(III). The reported Os are mild oxidants with moderate reactivity toward coordinating substrates, readily oxidizing thiols, certain activated alkoxides, and electron-rich phenols in a net 2e-, 2H+ process. In the absence of substrates, 1b-9b undergo thermally induced autolysis with concomitant degradation of the polyamine ligands. Ligand product distribution and primary kinetic isotope effects (kobsH/kobsD approximately 8, 1b/d24-1b, 293 K) support a unimolecular mechanism involving rate-determining C-H bond cleavage at accessible ligand N-alkyl substituents. Decomposition half-lives span almost 3 orders of magnitude at 293 K, ranging from approximately 2 s for 4b to almost 30 min for d(24)-1b, the most thermally robust dicationic O yet reported. Dealkylation is highly selective where ligand rigidity constrains accessibility; in 3b, the ethyl groups are attacked preferentially. The observed relative thermal stabilities and dealkylation selectivities of 1b-9b are correlated with NC(alpha)-H bond dissociation energies, statistical factors, ligand backbone rigidity, and ligand denticity/axial donor strength. Among the peralkylated amines surveyed, bidentate ligands with oxidatively robust NC(alpha)-H bonds provide optimal stabilization for Os. Fortuitously, the least sterically demanding N-alkyl substituent (methyl) gives rise to the most thermally stable and most physically accessible O core, retaining the potential for exogenous substrate reactivity.
Cole et al. (Wed,) studied this question.