Bis(μ-oxo)dicopper(III) Complexes of a Homologous Series of Simple Peralkylated 1,2-Diamines: Steric Modulation of Structure, Stability, and Reactivity

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Abstract
We have synthesized and characterized bis(μ-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 seriesis 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(μ-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(μ-O)2](CF3SO3)2·4CH2Cl2 ([3b· 4CH2Cl2]) features the most metrically compact [Cu2O2]2+ core among structurally characterized Os (av Cu−O 1.802(7) Å; Cu···Cu 2.744(1) Å) 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 ≈ 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 ∼2 s for 4b to almost 30 min for d24-1b, 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 NCα−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 NCα−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.