Na-Promoted\nBimetallic\nHydroxide Nanoparticles for\nAerobic C–H Activation: Catalyst Design Principles and Insights\ninto Reaction Mechanism

A precious metal-free\nbimetallic Fe<sub><i>x</i></sub>Mn<sub>1–<i>x</i></sub>(OH)<sub><i>y</i></sub> hydroxide catalyst\nwas developed that is capable of catalyzing\naerobic C–H oxidation reactions at low temperatures, without\nthe need for an initiator, relying sustainably on molecular oxygen.\nThrough a systematic synthetic effort, we scanned a wide nanoparticle\nsynthesis parameter space to lay out a detailed set of catalyst design\nprinciples unraveling how the Fe/Mn cation ratio, NaOH(aq) concentration\nused in the synthesis, catalyst washing procedures, extent of residual\nNa<sup>+</sup> promoters on the catalyst surface, reaction temperature,\nand catalyst loading influence catalytic C–H activation performance\nas a function of the electronic, surface chemical, and crystal structure\nof Fe<sub><i>x</i></sub>Mn<sub>1–<i>x</i></sub>(OH)<sub><i>y</i></sub> bimetallic hydroxide nanostructures.\nOur comprehensive XRD, XPS, BET, ICP-MS, <sup>1</sup>H NMR, and XANES\nstructural/product characterization results as well as mechanistic\nkinetic isotope effect (KIE) studies provided the following valuable\ninsights into the molecular level origins of the catalytic performance\nof the bimetallic Fe<sub><i>x</i></sub>Mn<sub>1–<i>x</i></sub>(OH)<sub><i>y</i></sub> hydroxide nanostructures:\n(i) catalytic reactivity is due to the coexistence and synergistic\noperation of Fe<sup>3+</sup> and Mn<sup>3+</sup> cationic sites (with\nminor contributions from Fe<sup>2+</sup> and Mn<sup>2+</sup> sites)\non the catalyst surface, where in the absence of one of these synergistic\nsites (i.e., in the presence of monometallic hydroxides), catalytic\nactivity almost entirely vanishes, (ii) residual Na<sup>+</sup> species\non the catalyst surface act as efficient electronic promoters by increasing\nthe electron density on the Fe<sup>3+</sup> and Mn<sup>3+</sup> cationic\nsites, which in turn, presumably enhance the electrophilic adsorption\nof organic reactants and strengthen the interaction between molecular\noxygen and the catalyst surface, (iii) in the fluorene oxidation reaction\nthe step dictating the reaction rate likely involved the breaking\nof a C–H bond (<i>k</i><sub>H</sub><i>/k</i><sub>D</sub> = 2.4), (iv) reactivity patterns of a variety of alkylarene\nsubstrates indicate that the C–H bond cleavage follows a stepwise\nPT-ET (proton transfer-electron transfer) pathway.