Nucleophilic versus Electrophilic Activation of Hydrogen Peroxide over Zr-Based Metal–Organic Frameworks

Abstract

Zr-based metal–organic frameworks (Zr-MOF) UiO-66 and UiO-67 catalyze thioether oxidation in nonprotic solvents with unprecedentedly high selectivity toward corresponding sulfones (96–99% at ca. 50% sulfide conversion with only 1 equiv of H2O2). The reaction mechanism has been investigated using test substrates, kinetic, adsorption, isotopic (18O) labeling, and spectroscopic tools. The following facts point out a nucleophilic character of the peroxo species responsible for the superior formation of sulfones: (1) nucleophilic parameter XNu = 0.92 in the oxidation of thianthrene 5-oxide and its decrease upon addition of acid; (2) sulfone to sulfoxide ratio of 24 in the competitive oxidation of methyl phenyl sulfoxide and p-Br-methyl phenyl sulfide; (3) significantly lower initial rates of methyl phenyl sulfide oxidation relative to methyl phenyl sulfoxide (k S/k SO = 0.05); and (4) positive slope ρ = +0.42 of the Hammett plot for competitive oxidation of p-substituted aryl methyl sulfoxides. Nucleophilic activation of H2O2 on Zr-MOF is also manifested by their capability of catalyzing epoxidation of electron-deficient CC bonds in α,β-unsaturated ketones accompanied by oxidation of acetonitrile solvent. Kinetic modeling on methyl phenyl sulfoxide oxidation coupled with adsorption studies supports a mechanism that involves the interaction of H2O2 with Zr sites with the formation of a nucleophilic oxidizing species and release of water followed by oxygen atom transfer from the nucleophilic oxidant to sulfoxide that competes with water for Zr sites. The nucleophilic peroxo species coexists with an electrophilic one, ZrOOH, capable of oxygen atom transfer to nucleophilic sulfides. The predominance of nucleophilic activation of H2O2 over electrophilic one is, most likely, ensured by the presence of weak basic sites in Zr-MOFs identified by FTIR spectroscopy of adsorbed CDCl3 and quantified by adsorption of isobutyric acid.

Catalytic Performance of Zr‐Based Metal–Organic Frameworks Zr‐abtc and MIP‐200 in Selective Oxidations with H2O2

“…The unprecedentedly high selectivity toward sulfones observed at low conversions of thioethers implies that nucleophilic oxidizing species predominate over electrophilic ones in Zr‐MOFs. The capability of UiO‐66 and UiO‐67 to epoxidize electron‐deficient C=C bonds in α,β‐unsaturated compounds also supported this conclusion [7b] …”

“…Finally, we tried to analyze a possible role of defects in the Zr‐MOF structures in the H 2 O 2 ‐based catalytic oxidations. Previously, two groups concluded that defects play a significant role in the thioether oxidation over UiO‐66 [6c, 7b] . TGA measurements showed that the concentration of defects corresponding to the total amount of terminal Zr(OH/H 2 O) groups decreased in the order Zr‐abtc > MIP‐200 > UiO‐67 > UiO‐66 (Table S2, Figures S8–S12 in the Supporting Information).…”

“…The catalytic performance of Zr‐abtc and MIP‐200 was also evaluated for the oxidation of methyl phenyl sulfide (MPS) and methyl phenyl sulfoxide (MPSO), organic substrates that possess nucleophilic and electrophilic character, respectively (Table 5 and Table S4 in the Supporting Information). In the MPS oxidation with 1 equivalent of H 2 O 2 , both Zr‐abtc and MIP‐200 behaved similar to UiO‐66, revealing high oxidant utilization efficiency, and produced sulfone as the main oxidation product, indicating predomination of nucleophilic activation of H 2 O 2 [7b] . In contrast, SOC MOFs with metals other than Zr revealed much less activity, and H 2 O 2 efficiency, and produced sulfoxide as the principle product (Table 5), which is typical of electrophilic oxidation.…”

“…Zr‐MOFs constructed from the highly robust Zr 6 O n (OH) m clusters and various carboxylate linkers are one of the most attractive classes of MOFs owing to their outstanding thermal, hydrothermal, chemical, and mechanical stability as well as a tunable amount of defects [5] . In particular, the most studied representative of Zr‐MOFs, Zr IV terephthalate UiO‐66, has been employed as a leaching‐tolerant and recyclable catalyst for a range of H 2 O 2 ‐based oxidative transformations, [6, 7] the majority of which were oxidations of S compounds [6a–c, 7b] …”

“…It was demonstrated that protons facilitate heterolytic activation of the oxidant and prevent its unproductive degradation on Zr‐MOFs, thereby leading to selective oxygenation of the C=C bond without oxidation of allylic C−H bonds via a homolytic mechanism. More recently, we investigated in detail the mechanism of organic sulfides and sulfoxides oxidation with aqueous H 2 O 2 over UiO‐66 and UiO‐67 by using product, kinetic, adsorption, and spectroscopic tools [7b] . All the results strongly suggested that H 2 O 2 is activated on a Zr IV site to give a hydroperoxo zirconium species, “ZrOOH”, which possesses electrophilic properties, but an adjacent basic oxygen atom implements deprotonation of “ZrOOH”, favoring its transformation into a nucleophilic oxidant, “ZrOO − ”.…”