We report a rare example of oxygen atom transfer (OAT) from a polyoxometalate cluster to a series of tertiary phosphanes. Addition of PR 3 (PR 3 = PMe 3 , PMe 2 Ph, PMePh 2 , PPh 3 ) to a neutral methoxide-bridged polyoxovanadate-alkoxide (POV-alkoxide) cluster, [V 6 O 7 (OMe) 12 ] 0 , results in isolation of a reduced structure with phosphine oxide datively coordinated to a site-differentiated V III ion. A positive correlation between the steric and electronic properties of the phosphane and the reaction rate was observed. Further investigation of the steric influence of the alkoxy-bridged clusters on OAT was probed through the use of POV clusters with bridging alkoxide ligands of varying chain length ([V 6 O 7 (OR′) 12 ]; R′ = Et, n Pr). These investigations expose that steric hinderance of the vanadyl moieties has significant influence on the rate of OAT. Finally, we report the reactivity of the reduced POV-alkoxide clusters with styrene oxide, resulting in the deoxygenation of the substrate to generate styrene. This result is the first example of epoxide deoxygenation using homometallic polyoxometalate clusters, demonstrating the potential for mono-vacant Lindqvist clusters to catalyze the removal of oxygen atoms from organic substrates.
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Reductive silylation of polyoxovanadate surfaces using Mashima's reagent
Here, we present the first example of reductive silylation for oxygen defect formation at the surface of a polyoxometalate. Upon addition of 1,4-bis(trimethylsilyl)dihydropyrazine (Pyz(SiMe 3 ) 2 ) to [V 6 O 7 (OMe) 12 ] 1− , quantitative formation of the oxygen-deficient vanadium oxide assembly, [V 6 O 6 (OMe) 12 ] 1− was observed. Substoichiometric reactions of Pyz(SiMe 3 ) 2 with the parent cluster revealed the mechanism of defect formation; addition of 0.5 equiv. of Pyz(SiMe 3 ) 2 to [V 6 O 7 (OMe) 12 ] 1− results in isolation of [V 6 O 6 (OSiMe 3 )(OMe) 12 ] 1− . This reactivity was extended to reduced and oxidized forms of the cluster, [V 6 O 7 (OMe) 12 ] n ( n = 2-, 0), revealing the consequences of modifying the oxidation states of remote transition metal ions on the stability of the siloxide functional group, and thus the extent of reactivity of the cluster surface with Pyz(SiMe 3 ) 2 . The work offers a new understanding of the mechanisms of surface activation of reducible metal oxides via reductive silylation, and reveals new chemical routes for the formation of oxygen atom vacancies in polyoxometalate ions.
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- Award ID(s):
- 1653195
- PAR ID:
- 10337036
- Date Published:
- Journal Name:
- Inorganic Chemistry Frontiers
- Volume:
- 8
- Issue:
- 20
- ISSN:
- 2052-1553
- Page Range / eLocation ID:
- 4507 to 4516
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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We report the synthesis and characterisation of a series of siloxide-functionalised polyoxovanadate–alkoxide (POV–alkoxide) clusters, [V 6 O 6 (OSiMe 3 )(OMe) 12 ] n ( n = 1−, 2−), that serve as molecular models for proton and hydrogen-atom uptake in vanadium dioxide, respectively. Installation of a siloxide moiety on the surface of the Lindqvist core was accomplished via addition of trimethylsilyl trifluoromethylsulfonate to the fully-oxygenated cluster [V 6 O 7 (OMe) 12 ] 2− . Characterisation of [V 6 O 6 (OSiMe 3 )(OMe) 12 ] 1− by X-ray photoelectron spectroscopy reveals that the incorporation of the siloxide group does not result in charge separation within the hexavanadate assembly, an observation that contrasts directly with the behavior of clusters bearing substitutional dopants. The reduced assembly, [V 6 O 6 (OSiMe 3 )(OMe) 12 ] 2− , provides an isoelectronic model for H-doped VO 2 , with a vanadium( iii ) ion embedded within the cluster core. Notably, structural analysis of [V 6 O 6 (OSiMe 3 )(OMe) 12 ] 2− reveals bond perturbations at the siloxide-functionalised vanadium centre that resemble those invoked upon H-atom uptake in VO 2 through ab initio calculations. Our results offer atomically precise insight into the local structural and electronic consequences of the installation of hydrogen-atom-like dopants in VO 2 , and challenge current perspectives of the operative mechanism of electron–proton co-doping in these materials.more » « less
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