DUGi

Rapid hydrogen and oxygen atom transfer by a high-valent nickel-oxygen species

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Terminal high-valent metal-oxygen species are key reaction intermediates in thecatalytic cycle of both enzymes (e.g., oxygenases) and synthetic oxidation catalysts. Whiletremendous efforts have been directed towards the characterization of the biologically relevantterminal manganese-oxygen and iron-oxygen species, the corresponding analogues based onlate-transition metals such as cobalt, nickel or copper are relatively scarce. This is in part relatedto the “Oxo Wall” concept, which predicts that late transition elements cannot support aterminal oxido ligand in a tetragonal environment. Here, the nickel(II) complex (1) of thetetradentate macrocyclic ligand bearing a 2,6-pyridinedicarboxamidate unit is shown to be aneffective catalyst in the chlorination and oxidation of C-H bonds with sodium hypochlorite asterminal oxidant in the presence of acetic acid (AcOH). Insight into the active speciesresponsible for the observed reactivity was gained through the study of the reaction of 1 withClO- at low temperature by UV/Vis absorption, resonance Raman, EPR, ESI-MS, and XASanalyses. DFT calculations aided the assignment of the trapped chromophoric species (3) as anickel-hypochlorite species. Despite the fact that the formal oxidation state of the nickel in 3 is+4, experimental and computational analysis indicate that 3 is best formulated as a NiIII complexwith one unpaired electron delocalized in the ligands surrounding the metal center. Mostremarkably, 3 reacts rapidly with a range of substrates including those with strong aliphatic C-Hbonds, indicating the direct involvement of 3 in the oxidation/chlorination reactions observed inthe 1/ClO-/AcOH catalytic system

Financial support for this work was provided by the European Commission (FP7-PEOPLE-2011-CIG-303522 to A.C.). The MINECO of Spain is acknowledged for a Ramón y Cajalcontract to A.C. and for CTQ2013-43012-P to A.C. F. A.-P. thanks Universitat de Girona for apredoctoral grant. W.R.B. acknowledges the European Research Council (ERC-2011-StG-279549) and the Ministry of Education, Culture and Science (Gravity program 024.001.035). Xrayabsorption data was collected on beamline BM25-Spline at the European SynchrotronRadiation Facility (ESRF), Grenoble, France

American Chemical Society (ACS)