Terminal high-valent metal-oxygen species are key reaction intermediates in the catalytic cycle of both enzymes (e.g., oxygenases) and synthetic oxidation catalysts. While tremendous efforts have been directed towards the characterization of the biologically relevant terminal manganese-oxygen and iron-oxygen species, the corresponding analogues based on late-transition metals such as cobalt, nickel or copper are relatively scarce. This is in part related to the “Oxo Wall” concept, which predicts that late transition elements cannot support a terminal oxido ligand in a tetragonal environment. Here, the nickel(II) complex (1) of the tetradentate macrocyclic ligand bearing a 2,6-pyridinedicarboxamidate unit is shown to be an effective catalyst in the chlorination and oxidation of C-H bonds with sodium hypochlorite as terminal oxidant in the presence of acetic acid (AcOH). Insight into the active species responsible for the observed reactivity was gained through the study of the reaction of 1 with ClO- at low temperature by UV/Vis absorption, resonance Raman, EPR, ESI-MS, and XAS analyses. DFT calculations aided the assignment of the trapped chromophoric species (3) as a nickel-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 complex with one unpaired electron delocalized in the ligands surrounding the metal center. Most remarkably, 3 reacts rapidly with a range of substrates including those with strong aliphatic C-H bonds, indicating the direct involvement of 3 in the oxidation/chlorination reactions observed in the 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 Cajal contract to A.C. and for CTQ2013-43012-P to A.C. F. A.-P. thanks Universitat de Girona for a predoctoral 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). Xray absorption data was collected on beamline BM25-Spline at the European Synchrotron Radiation Facility (ESRF), Grenoble, France