A new hybrid permethylated-amine-guanidine ligand based on a 1,3-propanediamine backbone ( 2 L) and its Cu-O 2 chemistry is reported. [( 2 L)Cu I (MeCN)] 1+ complex readily oxygenates at low temperatures in polar aprotic solvents to form a bis(μ-oxo)dicopper(III) (O) species (2b), similar to the parent bis-guanidine ligand complex (1b) and permethylated-diamine ligand complex (3b). UVvis and X-ray absorption spectroscopy experiments confirm this assignment of 2b as an O species, and full formation of the 2:1 Cu-O 2 complex is demonstrated by an optical titration with ferrocenemonocarboxylic acid (FcCOOH). The UV-vis spectra of 1b and 2b with guanidine ligation show low-intensity visible features assigned as guanidine π → Cu 2 O 2 core transitions by time-dependent density functional theory (TD-DFT) calculations. Comparison of the reactivity among the three related complexes (1b-3b) with phenolate at 195 K is particularly insightful as only 2b hydroxylates 2,4-di-tert-butylphenolate to yield 3,5-di-tert-butylcatecholate (>95% yield) with the oxygen atom derived from O 2 , reminiscent of tyrosinase reactivity. 1b is unreactive, while 3b yields the C-C radical-coupled bis-phenol product. Attenuated outer-sphere oxidative strength of the O complexes and increased phenolate accessibility to the Cu 2 O 2 core are attributes that correlate with phenolate hydroxylation reactivity observed in 2b. The comparative low-temperature reactivity of 1b-3b with FcCOOH (O-H BDE 71 kcal mol −1 ) to form the two-electron, two-proton reduced bis(μ-hydroxo) dicopper(II,II) complex is quantitative and presumably precedes through two sequential protoncoupled electron transfer (PCET) steps. Optical titrations along with DFT calculations support that the reduced complexes formed in the first step are more powerful oxidants than the parent O complexes. These mechanistic insights aid in understanding the phenol to bis-phenol reactivity exhibited by 2b and 3b.

A new catalyst (see structure) hydroxylates phenols with O2 via a stable side‐on peroxide complex, which is similar to the active site of tyrosinase in terms of the ligand environment and its spectroscopic properties. The catalytic oxidation of phenols to quinones proceeds at room temperature in the presence of NEt3 and even non‐native substrates can be oxidized catalytically. The reaction mechanism is analogous to that of the enzyme‐catalyzed reaction.