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 ¼ PMe 3 , PMe 2 Ph, PMePh 2 , PPh 3 ) to a neutral methoxidebridged 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 0 ) 12 ]; R 0 ¼ 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. † Electronic supplementary information (ESI) available: 1 H NMR, ESI-MS (+ve), UV-vis, and IR spectroscopic data for all complexes, crystallographic parameters and BVS calculations of complexes 2-OPMe 3 , 2-OPMe 2 Ph, 2-OPMePh 2 , 2-OPPh 3 , and 4-OPPh 3 , attempted synthesis of 4-OPMePh 2 , 4-OPPh 3 , and propoxide derivatives, 1 H NMR analysis of styrene oxide OAT reactions with 2-OPMe 3 and 4-OPMe 3 . For ESI and crystallographic data (CCDC1914250-1914254) in CIF or other electronic format see Scheme 1 Previously reported synthesis of the mono-vacant neutral cluster, 2-MeCN, via addition of VMes 3 (THF) addition (left) 35 and synthesis of [V 6 O 6 (OMe) 12 OPMe 3 ] (2-OPMe 3 ) via OAT to PMe 3 reported in this work (right). 8036 | Chem. Sci., 2019, 10, 8035-8045 This journal is Scheme 2 Synthesis of complexes 2-OPMe 2 Ph, 2-OPMePh 2 , and 2-OPPh 3 . 8038 | Chem. Sci., 2019, 10, 8035-8045 This journal is Scheme 3 Synthesis of complexes 4-OPMe 3 and 4-OPMe 2 Ph. This journal is Scheme 5 Stoichiometric oxygen atom transfer facilitated by an oxygen-atom vacancy on a POV-alkoxide cluster. 8040 | Chem. Sci., 2019, 10, 8035-8045 This journal is
We report the synthesis and characterization of a monochloride-functionalized polyoxovanadate-alkoxide (POV-alkoxide) cluster, which can serve as a molecular model for halogen-doped vanadium oxide (VO2) materials that have recently attracted great interest as advanced materials for energy-saving smart window applications. Chloride-substituted variants of the Lindqvist vanadium-oxide cluster were obtained via two distinct chemical pathways: (1) direct halogenation of the isovalent parent POV-alkoxide architecture, [V6O7(OC2H5)12]−2 with AlCl3 and (2) coordination of a chloride ion to a coordinatively unsaturated vanadium center within a cluster that bears a single oxygen-atom vacancy, [V6O6(OC2H5)12]0. Notably, our direct halogenation constitutes the first example of selective, single-site halide doping of homometallic metal oxide clusters. The chloride-containing compound, [V6O6Cl(OC2H5)12]−1, was characterized by 1H NMR spectroscopy and X-ray crystallography. The electronic structure of the chloride-functionalized POV-alkoxide cluster was established by infrared, electronic absorption, and X-ray photoelectron spectroscopy and revealed formation of a site-differentiated VIII ion upon halogenation. Cyclic voltammetry was employed to assess the electrochemical response of halide doping. A comparison of the Cl-VO2 model to the fully oxygenated cluster, [V6O7(OC2H5)12]−2, provides molecular-level insights into a new proposed mechanism by which halogenation increases the carrier density in solid VO2, namely, through prompting charge separation within the material.
The rational control of the electrochemical properties of polyoxovanadate-alkoxide clusters is dependento n understanding the influenceo fv arious synthetic modifications on the overall redox processes of these systems. In this work, the electronic consequences of ligand substitution at the heteroion in ah eterometal-functionalized clusterwas examined. The redoxp roperties of [V 5 O 6 (OCH 3) 12 FeCl] (1-[V 5 FeCl])a nd [V 5 O 6 (OCH 3) 12 Fe]X (2-[V 5 Fe]X;X = ClO 4 ,O Tf) were compared in order to assess the effects of changing the coordination environmenta roundt he iron center on the electrochemical properties of the cluster.C oordination of a chloride anion to iron leads to an anodic shift in redox events. Theoretical modelling of the electronics tructure of these heterometal-functionalizedc lusters reveals that differences in the redox profiles of 1-[V 5 FeCl] and 2-[V 5 Fe]X arise from changes in the number of ligands surroundingt he iron center (e.g.,6-coordinate vs. 5-coordinate). Specifically,b inding of the chloride to the sixth coordination site appears to change the orbitali nteraction between the iron and the delocalized electronic structure of the mixed-valent polyoxovanadate core. Tuning the heterometal coordination environment can therefore be used to modulate the redox properties of the whole cluster.
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