Studies on electron transfer reactions of Keggin-type mixed addenda heteropolytungstovanadophosphates with NADH

Catalytic System for Aerobic Oxidation That Simultaneously Functions as Its Own Redox Buffer

The control of the solution electrochemical potential as well as pH impacts products in redox reactions, but the former gets far less attention. Redox buffers facilitate the maintenance of potentials and have been noted in diverse cases, but they have not been a component of catalytic systems. We report a catalytic system that contains its own built-in redox buffer. Two highly synergistic components (a) the tetrabutylammonium salt of hexavanadopolymolybdate TBA₄H₅[PMo₆V₆O₄₀] (PV₆Mo₆) and (b) Cu(ClO₄)₂ in acetonitrile catalyze the aerobic oxidative deodorization of thiols by conversion to the corresponding nonodorous disulfides at 23 °C (each catalyst alone is far less active). For example, the reaction of 2-mercaptoethanol with ambient air gives a turnover number (TON) = 3 × 10² in less than one hour with a turnover frequency (TOF) of 6 × 10^-2 s^-1 with respect to PV₆Mo₆. Multiple electrochemical, spectroscopic, and other methods establish that (1) PV₆Mo₆, a multistep and multielectron redox buffering catalyst, controls the speciation and the ratio of Cu(II)/Cu(I) complexes and thus keeps the solution potential in different narrow ranges by involving multiple POM redox couples and simultaneously functions as an oxidation catalyst that receives electrons from the substrate; (2) Cu catalyzes two processes simultaneously, oxidation of the RSH by PV₆Mo₆ and reoxidation of reduced PV₆Mo₆ by O₂; and (3) the analogous polytungstate-based system, TBA₄H₅[PW₆V₆O₄₀] (PV₆W₆), has nearly identical cyclic voltammograms (CV) as PV₆Mo₆ but has almost no catalytic activity: it does not exhibit self-redox buffering.

Synthesis and Characterisation of Monolacunary Keggin Monovanado-deca-tungstophosphate and Its Complexes with Transition Metal Cations

Five new complexes with metal cations (Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, and Cu²⁺) of monolacunary Keggin monovanado-deca-tungstophosphate, K₈[PVW₁₀O₃₉]·15H₂O, have been synthesised. The molar ratio of the combination between metal cations and K₈[PVW₁₀O₃₉]·15H₂O has been established to be 1:1, and its general molecular formulas were found to be: K_n[MPVW₁₀O₃₉(H₂O)]·xH₂O (n = 5 for M = Fe³⁺ and n = 6 for M = Mn²⁺, Co²⁺, Ni²⁺, and Cu²⁺). Optimal conditions for complexes' synthesis (pH, temperature, and reaction time) have been determined. The characterisation of K₈[PVW₁₀O₃₉]·15H₂O and of its compounds K_n[MPVW₁₀O₃₉(H₂O)]·xH₂O have been performed using AAS, TG-DTA-DTG, UV-VIS, IR, Raman, and powder XRD methods. In UV spectra, two maximums of absorption were obtained, at 200 and 250 nm, characteristic of Keggin polycondensate compounds. The coordination of cations Ni²⁺, Co²⁺, and Cu²⁺ through oxygen atoms from K₈[PVW₁₀O₃₉]·15H₂O in an octahedron system has been reflected with VIS spectroscopy. All these methods have proved the compositions and structures of K₈[PVW₁₀O₃₉]·15H₂O and K_n[MPVW₁₀O₃₉(H₂O)]·xH₂O, their similarity with other vanadotungstophosphates, and their achievements in the Keggin class. Additionally, all analysis methods have shown an increase in the degree of structural symmetry and the thermal stability of a polyoxoanion complex after attaching metal cations compared to the monolacunary, K₈[PVW₁₀O₃₉]·15H₂O.

A lacunary tungstomolybdophosphate as an electronic pendulum: The "blue" electron under examination

The photoreduction of a Keggin type lacunary tungstomolybdophosphate, α-(Bu₄N)₄[H₃PW₉Mo₂O₃₉], in acetonitrile, led to the formation of a monoreduced lacunary heteropoly anion, or a one electron reduced "heteropoly blue" species, whereby the added "blue" electron was captured by the molybdenum atoms. The magnetic properties and behavior of the "blue" electron were studied by a modified Evans nuclear magnetic resonance method (small downshift of the ³¹P signal) and variable-temperature electron paramagnetic resonance (g = 1.936 for Mo^V). The intermolecular exchange of the "blue" electron was limited by a geometrical factor, which requires the contact between Mo caps to exchange it between the heteropoly couple. The intramolecular exchange of the "blue" electron between Mo atoms was rather fast (5.3 × 10⁹ s^-1), with a rate of more than six orders of magnitude larger than the intermolecular exchange rate. Density functional theory was used to determine the most prevalent protonation sites in the mixed lacunary isomers with the aim of studying the intramolecular electron transfer pathway in the isolated [H₄PW₉Mo₂O₃₉]^4- species. The singly occupied molecular orbital (SOMO) is essentially localized in one of the two nonequivalent molybdenum sites. The kinetics of the intramolecular electron exchange equilibrium Mo^V + Mo^VI → Mo^VI + Mo^V between the two molybdenum atoms bridged by an oxygen atom was found to be fast in agreement with the experimental result. The transition state is of mixed-valence type, with the SOMO delocalized over the Mo-O-Mo group. Spectroscopic parameters were found to be in fair agreement with experimental results.