Phenolate Hydroxylation in a Bis(μ-oxo)dicopper(III) Complex: Lessons from the Guanidine/Amine Series
2009-01-28T00:00:00Z (GMT) by
A new hybrid permethylated-amine-guanidine ligand based on a 1,3-propanediamine backbone (<sup><b>2</b></sup><b>L</b>) and its Cu−O<sub>2</sub> chemistry is reported. [(<sup><b>2</b></sup><b>L</b>)Cu<sup>I</sup>(MeCN)]<sup>1+</sup> complex readily oxygenates at low temperatures in polar aprotic solvents to form a bis(μ-oxo)dicopper(III) (<b>O</b>) species (<b>2b</b>), similar to the parent bis-guanidine ligand complex (<b>1b</b>) and permethylated-diamine ligand complex (<b>3b</b>). UV−vis and X-ray absorption spectroscopy experiments confirm this assignment of <b>2b</b> as an <b>O</b> species, and full formation of the 2:1 Cu−O<sub>2</sub> complex is demonstrated by an optical titration with ferrocene-monocarboxylic acid (FcCOOH). The UV−vis spectra of <b>1b</b> and <b>2b</b> with guanidine ligation show low-intensity visible features assigned as guanidine π → Cu<sub>2</sub>O<sub>2</sub> core transitions by time-dependent density functional theory (TD-DFT) calculations. Comparison of the reactivity among the three related complexes (<b>1b</b>−<b>3b</b>) with <i>phenolate</i> at 195 K is particularly insightful as only <b>2b</b> hydroxylates 2,4-di-<i>tert</i>-butylphenolate to yield 3,5-di-<i>tert</i>-butylcatecholate (>95% yield) with the oxygen atom derived from O<sub>2</sub>, reminiscent of tyrosinase reactivity. <b>1b</b> is unreactive, while <b>3b</b> yields the C−C radical-coupled bis-phenol product. Attenuated outer-sphere oxidative strength of the <b>O</b> complexes and increased phenolate accessibility to the Cu<sub>2</sub>O<sub>2</sub> core are attributes that correlate with phenolate hydroxylation reactivity observed in <b>2b</b>. The comparative low-temperature reactivity of <b>1b</b>−<b>3b</b> with FcCOOH (O−H BDE 71 kcal mol<sup>−1</sup>) to form the two-electron, two-proton reduced bis(μ-hydroxo)dicopper(II,II) complex is quantitative and presumably precedes through two sequential proton-coupled 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 <b>O</b> complexes. These mechanistic insights aid in understanding the phenol to bis-phenol reactivity exhibited by <b>2b</b> and <b>3b</b>.