In Situ Assembly, De‐Metalation and Induced Repair of a Copper‐Polyoxovanadate Oxidation Catalyst

Abstract

Polyoxometalates (POMs) offer a rich dynamic solution chemistry so that in principle, the assembly and repair of POM oxidation catalysts in situ, i. e. under catalytic conditions, is possible. Here, we report the oxidative activity of a molecular copper vanadium oxide cluster [(Me 2 NH 2 )Cu(MeCN)V 12 O 32 Cl] 2À : the catalyst self-assembles by reaction of Cu II and [(Me 2 NH 2 ) 2 V 12 O 32 Cl] 3À in solution and selectively oxidizes secondary alcohols (here: 1-phenylethanol) to the corresponding ketones in near quantitative yield. The catalyst can be deactivated in solution by removal of the Cu II site from the catalyst. Subsequent catalyst re-assembly is achieved by addition of Cu II ions, leading to the recovery of the catalytic activity. The system might serve as molecular model to study repair mechanism in POM catalysts with relevance for "green" oxidation processes.Catalyst deactivation is a significant technological problem with an estimated global economic impact in the tens of billions of dollars per year. [1] For homogeneous catalysts, decomposition is most often due to chemical or thermal degradation. [2] Therefore, exploring means of catalyst reactivation or repair is a current challenge in catalyst research. Ideally, this process should work under operating conditions to avoid the time-consuming removal of the catalyst from the reactor. [1] Therefore, molecular catalysts which feature dynamic assembly in solution would be ideal candidates to study model deactivation and repair scenarios. [3] Prime candidates for this approach are molecular metal oxides, so-called polyoxometalates (POMs). [4] POMs are formed in situ by spontaneous self-assembly of small oxometalate precursors and feature wide chemical tunability [4,5] so that specific catalytic properties can be introduced on the molecular level. [6,7] These features have led to a range of POM-based catalytic processes [8][9][10][11] with a particular focus on industrially relevant oxidation reactions [12][13][14][15] including alcohol oxidations, [16,17] epoxidations [18,19] and CÀH-activations. [11,20] Further advantages of POM catalysts are their high thermal [21] and oxidative [19] stability together with their dynamic structure which allows the cluster to undergo self-assembly (and possibly self-repair) reactions [22] under catalytic conditions. [23][24][25] While exploring the incorporation of reactive metal sites into vanadium oxide clusters, [7,21,[26][27][28][29][30][31] we have recently discovered a new class of metal-functionalized polyoxovanadates: programmed reactivity tuning was achieved by selective incorporation of catalytic transition metal centers into the framework of a dodecavanadate cluster, see Figure 1. [32,33] Briefly, the vanadate precursor (Me 2 NH 2 ) 2 [V 12 O 32 Cl] 3À (= {V 12 }) features two Me 2 NH 2 + -blocked metal binding sites; the Me 2 NH 2 + placeholder cations can be selectively replaced by a catalytic metal center of choice. Using our published procedure, {V 12 } was reacted with Cu II to give the homogeneous ...

Aerobic Oxidation Catalysis by a Molecular Barium Vanadium Oxide

“…To enable a targeted metal‐functionalization of vanadate clusters, we developed a chloride‐templated vanadate “host” cluster (Me 2 NH 2 ) 2 [V 12 O 32 Cl] 3− (= {V 12 } ) featuring two Me 2 NH 2 + ‐blocked metal binding sites . Incorporation of one or two reactive 3d metal “guests” is possible, leading to metal‐functionalized vanadates as multi‐electron transfer sites, for example, in battery electrodes or as catalysts for C−H‐activation and alcohol oxidation . Based on these studies, we became interested in expanding this “host–guest” assembly strategy by developing a vanadate “host” capable of binding larger metals, for example, large alkali or alkali earth cations.…”

Reply to Comment on Stabilization of Low‐Valent Iron(I) in a High‐Valent Vanadium(V) Oxide Cluster

“…Nevertheless,t he absence of as traightforward Fe III to Fe II reduction step in the potential window studied, as proposed by Dr. Sproules,w ould require explanation, since analogous species {TMV 12 },containing divalent transition metals, TM = Cu 2+ , [3] Co 2+ [2] and Zn 2+ [2] have been synthesized, thus demonstrating that divalent metal cation in the dodecavanadate shell is possible. Nevertheless,t he absence of as traightforward Fe III to Fe II reduction step in the potential window studied, as proposed by Dr. Sproules,w ould require explanation, since analogous species {TMV 12 },containing divalent transition metals, TM = Cu 2+ , [3] Co 2+ [2] and Zn 2+ [2] have been synthesized, thus demonstrating that divalent metal cation in the dodecavanadate shell is possible.…”

Beyond Charge Balance: Counter‐Cations in Polyoxometalate Chemistry

“…In addition, the presence of dimethyl ammonium cations during cluster assembly is critical to block the vacant metal binding sites. Subsequent exchange of these placeholders with metal cations enables controlled functionalization of the cluster architecture …”