The kinetics and mechanism of oxidation of reduced phosphovanadomolybdates by molecular oxygen: theory and experiment in concert

Abstract

The reactivity of the H 5 PV 2 Mo 10 O 40 polyoxometalate and its analogues as an electron transfer and electron transfer-oxygen transfer oxidant has been extensively studied in the past and has been shown to be useful in many transformations. One of the hallmarks of this oxidant is the possibility of its re-oxidation with molecular oxygen, thus enabling aerobic catalytic cycles. Although the re-oxidation reaction was known, the kinetics and mechanism of this reaction have not been studied in any detail.Experimentally, we show that both the one-and two-electron reduced polyoxometalate are reactive with O 2 , the two-electron one more so. The reactions are first-order in the polyoxometalate and O 2 .Solvents also have a considerable effect, protic solvents being preferred over aprotic ones.

Oxygen atom transfer with organofunctionalized polyoxovanadium clusters: O-atom vacancy formation with tertiary phosphanes and deoxygenation of styrene oxide

“…We have demonstrated an alternative route for O-atom vacancy formation at the surface of alkoxy-bridged POV clusters via OAT to a series of tertiary phosphanes (PR 3 = PMe 3 , PMe 2 Ph, PMePh 2 , PPh 3 ). While the reactivity of polyoxometalates with phosphane substrates has been reported previously,29,30,53 evidence suggesting an important role of surface-bound water ligands has created ambiguity as to whether oxygen-atom vacancies are truly formed during the course of the reaction 55,78. In contrast, this work offers clear indication of OAT between cluster and substrate, resulting in the formation of mono-vacant POV-alkoxide clusters with a datively coordinated phosphine oxide ligand bound to the oxygen deficient moiety.…”

“…The lengthened V1–O1 bond and similarity in PO bond distances between free and bound OPMe 3 collectively provide support for a purely dative interaction between a reduced, vanadium( iii ) centre and a phosphine oxide ligand. These bond metrics refute the alternative product of one electron transfer, namely the ion-paired species, [Me 3 P˙ + ···V 6 O 7 (OCH 3 ) 12 ] 21,30,31. Further support for complete OAT can be garnered from examination of V1–O c bond distances.…”

“…Theoretical investigations, supported by spectroscopic analysis, have revealed that OAT occurs via an initial one-electron reduction of the polyoxoanion by the substrate, resulting in the formation of a reactive ion pair. In a subsequent mechanistic step requiring water, the oxidation of said substrate occurs 16,21,30–33. Density functional theory supports the transient formation of a coordinatively unsaturated vanadium ion at the surface of the cluster that rapidly reacts with a terminal oxidant to restore the vacant site.…”

Conductivity via Thermally Induced Gap States in a Polyoxometalate Thin Layer

“…On the basis of IR data of catalytic studies, it has often been argued that reduction of POMs, especially polyoxomolybdates, is accompanied by the elimination of one or several bridging oxygens . In this respect, it may also be noted that unsaturated sites, namely, defect structures, when one or more metal–oxygen bonds around the same metal atom are broken while the latter remains connected to the defect site, are commonly considered as reactive intermediates in the Mars–van Krevelen-type oxidation reactions catalyzed by POMs, especially H 5 PV 2 Mo 10 O 40 . , The existence of oxygen vacancies has been advanced to explain the observation of band gaps in SiO 2 /APTES/H 3 PW 12 O 40 /IPA stacks . However, although the presence of oxygen vacancies is well-established in molybdenum and tungsten oxides, where they are associated with the appearance of new features below the Fermi level, , as yet no oxygen-defect POM has been reported.…”

O2 Activation with a Sterically Encumbered, Oxygen-Deficient Polyoxovanadate-Alkoxide Cluster

“…Indeed, these clusters have demonstrated catalytic activity toward the aerobic oxidation of a variety of substrates invoked in heterogeneous catalytic systems. − Extensive kinetic and theoretical studies have established that O 2 reduction in these systems typically proceeds through outer-sphere electron transfer mechanisms. − This is credited to the fact that the oxide-terminated surfaces of these metal oxide assemblies inhibit direct interaction of O 2 with reduced transition metal centers within the cluster. A notable exception, however, is the Keggin-type, phosphovanadomolybdate cluster, H 5 PV 2 Mo 10 O 40 , where the coordinatively flexible vanadium centers are purported to serve as the location of inner-sphere reactivity with O 2 . − …”