10<sup>6</sup>-fold faster C-H bond hydroxylation by a Co<sup>III,IV</sup><sub>2</sub>(µ-O)<sub>2</sub> complex [via a Co<sup>III</sup><sub>2</sub>(µ-O)(µ-OH) intermediate] versus its Fe<sup>III</sup>Fe<sup>IV</sup> analog.
basic_science · Level V
Where this comes from
- Record sourced from PubMed, PMID 38085777.
- Also identified by DOI 10.1073/pnas.2307950120 and PMC identifier 10743362.
- Licence recorded as CC BY-NC-ND.
- Because redistribution is not established, this page shows the abstract only. Follow the links below for the full text.
Abstract
The hydroxylation of C-H bonds can be carried out by the high-valent Co<sup>III,IV</sup><sub>2</sub>(µ-O)<sub>2</sub> complex <b>2a</b> supported by the tetradentate tris(2-pyridylmethyl)amine ligand via a Co<sup>III</sup><sub>2</sub>(µ-O)(µ-OH) intermediate (<b>3a</b>). Complex <b>3a</b> can be independently generated either by H-atom transfer (HAT) in the reaction of <b>2a</b> with phenols as the H-atom donor or protonation of its conjugate base, the Co<sup>III</sup><sub>2</sub>(µ-O)<sub>2</sub> complex <b>1a</b>. Resonance Raman spectra of these three complexes reveal oxygen-isotope-sensitive vibrations at 560 to 590 cm<sup>-1</sup> associated with the symmetric Co-O-Co stretching mode of the Co<sub>2</sub>O<sub>2</sub> diamond core. Together with a Co•••Co distance of 2.78(2) Å previously identified for <b>1a</b> and <b>2a</b> by Extended X-ray Absorption Fine Structure (EXAFS) analysis, these results provide solid evidence for their "diamond core" structural assignments. The independent generation of <b>3a</b> allows us to investigate HAT reactions of <b>2a</b> with phenols in detail, measure the redox potential and p<i>K</i><sub>a</sub> of the system, and calculate the O-H bond strength (<i>D</i><sub>O-H</sub>) of <b>3a</b> to shed light on the C-H bond activation reactivity of <b>2a</b>. Complex <b>3a</b> is found to be able to transfer its hydroxyl ligand onto the trityl radical to form the hydroxylated product, representing a direct experimental observation of such a reaction by a dinuclear cobalt complex. Surprisingly, reactivity comparisons reveal <b>2a</b> to be 10<sup>6</sup>-fold more reactive in oxidizing hydrocarbon C-H bonds than corresponding Fe<sup>III,IV</sup><sub>2</sub>(µ-O)<sub>2</sub> and Mn<sup>III,IV</sup><sub>2</sub>(µ-O)<sub>2</sub> analogs, an unexpected outcome that raises the prospects for using Co<sup>III,IV</sup><sub>2</sub>(µ-O)<sub>2</sub> species to oxidize alkane C-H bonds.