NMR, Voltammetric, and Photoelectrochemical Studies on the Dark and Light-Catalyzed Reactions of α-[S₂Mo₁₈O₆₂]^4- with Aryl- and Alkylphosphines

Abstract

The polyoxometalate, α-[S2Mo18O62]4- is reduced by phosphines in the mixed-solvent medium CH3CN/H2O (95/5 v/v). Monitoring of reaction pathways by voltammetric and NMR (17O, 31P) techniques shows that the identity of the final products is determined by the basicity of the phosphine. Thus reaction with aryl- (Ph3P, Ph2PCH2PPh2, and Ph2PCH2CH2PPh2) and alkyl- (Et3P and nBu3P) phosphines leads to two- and one-electron reduced polyoxometalate products, respectively, for example: [S2Mo18O62]4- + Ph3P + H2O → [HS2Mo18O62]5- + Ph3PO + H+; 2[S2Mo18O62]4- + 3nBu3P + H2O → 2[S2Mo18O62]5- + nBu3PO + 2nBu3PH+. For each reaction, the primary electron-transfer step is believed to be [S2Mo18O62]4- + R3P → [S2Mo18O62]5- + R3P•+with R3P•+ then reacting with water to generate R3PO and protons. If R3P is relatively basic (R = nBu, Et), a 1:2 mixture of R3PO and R3PH+ is formed due to the protonation of R3P. However, if R3P (R = Ph) is a weak base, H+ preferentially initiates the following disproportionation reaction: 2[S2Mo18O62]5- + H+ ⇄ [HS2Mo18O62]5- + [S2Mo18O62]4-. Reactions in the presence of light lead to significant photocatalysis. A quantitative photoelectrochemical channel electrode study demonstrates that oxidation of Ph3P by [S2Mo18O62]4- is accelerated substantially by irradiation in the 300−400 nm wavelength range, where absorption bands of both [S2Mo18O62]4- and [S2Mo18O62]5- are present.

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.…”

Reduction of the Phosphododecamolybdate Ion by Phosphonium Ylides and Phosphanes

“…Neither does the product correspond to [HPMo 12 O 40 ] 4Ϫ (HII) as described by Mattes et al Definitive evidence for the true nature of the product is provided by electrochemistry which shows that the behaviour of the product is identical to that of (nBu 4 (9)]. [18] [PMo 12 O 40 ] 3Ϫ ϩ R 3 P Ǟ [PMo 12 O 40 ] 4Ϫ ϩ R 3 P •ϩ (7) The net reaction can then be written as Equation (10) …”

Spectroscopic and electrochemical examination of the dark and photo-catalysed redox reactions that occur at the interface between solid α-[Hex4N]4[S2Mo18O62], solid triphenylphosphine and water

“…The k obs data in Table 2 show that UV/visible irradiation accelerates the overall rate of reaction by an order of magnitude, similar to the observation made in the solution phase reaction. 17 Further, the rate of reaction is reduced by three orders of magnitude in the absence of atmospheric water (Table 2). 3a).…”

“…The anions [H x S 2 Mo 18 O 62 ] (6 Ϫ x)Ϫ derive from disproportionation of the initial 1e Ϫ -reduced product [S 2 Mo 18 O 62 ] 5Ϫ in the presence of protons derived from water. 10,11, 17 The magnitude of the oxidative limiting current (I lim ) measured by steady-state voltammetry at a microdisk electrode allows determination of the concentration of the product [H x S 2 Mo 18 O 62 ] (6 Ϫ x)Ϫ from the simple relationship ( 2) 24 where n is the number of electrons…”

“…The reactivity is quantitatively compared with that observed in the solution phase in order to establish whether the same products and analogous mechanistic behaviour is observed when a redox reaction occurs in a different phase. 17 Applic-ations of this new electrochemical approach may assist the development of photovoltaic devices based upon solid state mixtures of polyoxometalates and a range of electron donors.…”