Bis(μ-oxo)dicopper(III) complexes of a homologous series of simple peralkylated 1,2-diamines: Steric modulation of structure, stability, and reactivity

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)]CF₃SO ₃ (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 CH ₂Cl₂. 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)₂Cu ₂(μ-O)₂](CF₃SO₃) ₂·4CH₂Cl₂ ([3b-4CH₂Cl ₂]) features the most metrically compact [Cu₂O ₂]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 (k_obs^H/k _obs^D ≈ 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 NC^α-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 ^α-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.