Formation, Reactivity, and Catalytic Behavior of a Keggin Polyoxometalate/Bipyridine Hybrid in the Epoxidation of Cyclooctene with H₂O₂

Abstract

The present study further explores the behavior of polyoxometalate-based hybrid compounds as catalysts for liquidphase cyclooctene epoxidation with H 2 O 2 . Precisely, it unveils the nature of the relevant active species derived from the hybrid based on Keggin polyoxometalate (POM) and bipyridines (bpy) of formula (2,2′-Hbpy) 3 [PW 12 O 40 ] (1). Whereas (i) it is generally accepted that the catalytic oxidation of organic substrates by H 2 O 2 involving Keggin HPAs proceeds via an oxygen transfer route from a peroxo intermediate and (ii) the catalytically active peroxo species is commonly postulated to be the polyperoxotungstate {PO 4 [W(O)(O 2 ) 2 ] 4 } 3− complex (PW 4 ), we show that the studied epoxidation reaction seems to be more sophisticated than commonly reported. During the catalytic epoxidation, 1 underwent a partial transformation into two oxidized species, 2 and 3. Compound 3 corresponding to 2,2′-bipyridinium oxodiperoxotungstate of formula [WO(O 2 ) 2 (2,2′-bpy)] was shown to be the main species responsible for the selective epoxidation of cyclooctene since 2 (in which the POM is associated with a protonated mono-Noxide derivative of 2,2′-bpy of formula (2,2′-HbpyO) 3 [PW 12 O 40 ]) exhibited no activity. The structures of 1, 2, and 3 were solved by single-crystal X-ray diffraction and were independently synthesized. The speciation of 1 was monitored under catalytic conditions by 1 H and 1 H DOSY NMR spectroscopies, where the formation in situ of 2 and 3 was revealed. A reaction mechanism is proposed that highlights the pivotal, yet often underestimated, role of H 2 O 2 in the reached catalytic performances. The active species responsible for the oxygen transfer to cyclooctene is a hydroperoxide intermediate species that is formed by the interaction between the anionic structure of the catalyst and H 2 O 2 . The latter operates as a "conservative agent" whose presence in the catalytic system is required to prevent the catalysts from deactivating irreversibly.

Homogeneous Tungsten Catalysis for Controllable Selective Oxidation of Anilines via the W(O)(η2-O2)2 Intermediate

“…In contrast, treating W-1 with H 2 O 2 at 60 °C changed the solution’s color from colorless to light yellow, indicating the simultaneous formation of the peroxo-tungsten complex W(O)(η 2 -O 2 ) 2 ( W-2 ) (Figure a). Single-crystal X-ray diffraction (SC-XRD) analysis of this complex showed that W-2 has a W(VI) center coordinated to two peroxo groups (O–O), one terminal oxo group (WO), and a N , N ′-chelated 4,4′-dimethoxy-2,2′-bipyridine ligand (Figure d). − The WOO geometry is similar to the crystallographically characterized structure of naphthalene dioxygenase, where dioxygen binds side-on close to the mononuclear metal at the active site (1.75 A° resolution, O–O distance, 1.45 A°) . The structurally determined O–O bond lengths of 1.47(1) and 1.51(1) A° in our complex are indicative of peroxo character of the OO moieties .…”

“…Subsequently, Liu, Engle, and their co-workers showed that a specific amino-directing group could guide a tungsten catalyst to achieve selective carbonylative functionalization of terminal alkenes at a predetermined internal position . In the context of high-valent tungsten, its use in petroleum upgrading processes and catalytic transformations, involving tungsten (+6) in a constant oxidation state for alkene , and alkyne metathesis, polymerization reactions, and epoxidation reactions, − is particularly notable (Figure a). Despite significant advances, expanding the scope of applications for W(VI) beyond these areas and achieving controllable selectivity for challenging transformations not only represent meaningful chemical innovation but also enhance its organometallic characteristics. , …”

“…To make this tactic synthetically viable, certain preconditions should be satisfied: ( i ) the catalytic systems should be capable of navigating the oxidation network; ( ii ) they should selectively stop at a specific oxidation level; and ( iii ) they should tolerate other reactive functional groups embedded within the substrate. The focal point of our research program encompasses the study of molecular tungsten catalysis , and alkene functionalization. , Specifically, on the basis of previous literature studies − and our group’s explorations, , we have noted that ( i ) the W catalyst has the ability to adopt multiple coordination geometries and ( ii ) the center of W features high Lewis acidity compared to that of later transition metals, especially for high-valent tungsten. We envisaged that the unique configuration geometries and Lewis acidic properties of W(VI) complexes would be particularly beneficial in molecular (e.g., H 2 O 2 and O 2 ) activation.…”

“…NMR spectra were obtained on 400 MHz spectrometers. Chemical shifts are referenced to tetramethylsilane (TMS) utilizing residual 1 H or13 C signals of the deuterated solvents as internal standards. All synthetic reactions were performed in a glovebox under a dry nitrogen atmosphere, unless otherwise noted.…”