Anna Proust

Structural, Physicochemical, and Reactivity Properties of an All-Inorganic, Highly Active Tetraruthenium Homogeneous Catalyst for Water Oxidation

Several key properties of the water oxidation catalyst Rb(8)K(2)[{Ru(IV)(4)O(4)(OH)(2)(H(2)O)(4)}(gamma-SiW(10)O(36))(2)] and its mechanism of water oxidation are given. The one-electron oxidized analogue [{Ru(V)Ru(IV)(3)O(6)(OH(2))(4)}(gamma-SiW(10)O(36))(2)](11-) has been prepared and thoroughly characterized. The voltammetric rest potentials, X-ray structures, elemental analysis, magnetism, and requirement of an oxidant (O(2)) indicate these two complexes contain [Ru(IV)(4)O(6)] and [Ru(V)Ru(IV)(3)O(6)] cores, respectively. Voltammetry and potentiometric titrations establish the potentials of several couples of the catalyst in aqueous solution, and a speciation diagram (versus electrochemical potential) is calculated. The potentials depend on the nature and concentration of counterions. The catalyst exhibits four reversible couples spanning only ca. 0.5 V in the H(2)O/O(2) potential region, keys to efficient water oxidation at low overpotential and consistent with DFT calculations showing very small energy differences between all adjacent frontier orbitals. The voltammetric potentials of the catalyst are evenly spaced (a Coulomb staircase), more consistent with bulk-like properties than molecular ones. Catalysis of water oxidation by [Ru(bpy)(3)](3+) has been examined in detail. There is a hyperbolic dependence of O(2) yield on catalyst concentration in accord with competing water and ligand (bpy) oxidations. O(2) yields, turnover numbers, and extensive kinetics data reveal several features and lead to a mechanism involving rapid oxidation of the catalyst in four one-electron steps followed by rate-limiting H(2)O oxidation/O(2) evolution. Six spectroscopic, scattering, and chemical experiments indicate that the catalyst is stable in solution and under catalytic turnover conditions. However, it decomposes slowly in acidic aqueous solutions (pH < 1.5).

Cs(9)[(gamma-PW(10)O(36))(2)Ru(4)O(5)(OH)(H(2)O)(4)], a new all-inorganic, soluble catalyst for the efficient visible-light-driven oxidation of water.

The tetraruthenium-substituted polyoxometalate Cs(9)[(gamma-PW(10)O(36))(2)Ru(4)O(5)(OH)(H(2)O)(4)] was synthesized and structurally, spectroscopically and electrochemically characterized; it was shown to be a catalyst for visible-light-induced water oxidation.

Phenylimido derivatives of [Mo6O19]2−: syntheses, X-ray structures, vibrational, electrochemical, 95Mo and 14N NMR studies

Abstract Both [Mo 6 O 18 (NPh)] 2− and [Mo 6 O 17 (NPh) 2 ] 2− species are formed by reaction of (n-Bu 4 N) 2 [Mo 6 O 19 ] with either 1 or 2 equiv. of Ph 3 PNPh in pyridine or acetonitrile and have been isolated as mixtures. Single-crystal X-ray diffraction analyses have shown that [Mo 6 O 18 (NPh)] 2− , [Mo 6 O 17 (NPh) 2 ] 2− and [Mo 6 O 19 ] 2− can be found together in the same crystal of average composition (n-Bu 4 N) 2 [Mo 6 O 19− x (NPh) x ]. The structural parameters of the averaged anions have been determined for x =1.16 (sample 1 ) and x =0.92 (sample 2 ). Crystal data for 1 : a =12.684(4), b =22.750(4), c =19.483(3) A, β=103.04(2)°, space group P 2 1 / c , Z =4, R =0.048 and R w =0.051 for 3846 reflections with I ⩾3σ( I ). Crystal data for 2 : a = 12.678(2), b =22.645(3), c =19.462(4) A, β=104.16(2)°, space group P 2 1 / c , Z =4, R =0.058 and R w =0.061 for 5243 reflections with I ⩾3σ( I ). The products have been studied in acetonitrile solution by 95 Mo and 14 N NMR spectroscopy, Raman spectrophotometry and electrochemistry. Each of these techniques has confirmed the presence of the mono- and bis-imido derivatives together with the parent species, and has allowed both the characteristics of the individual components and the composition of the solution to be determined; the results obtained by the different methods are in reasonable agreement.

Discrete Covalent Organic–Inorganic Hybrids: Terpyridine Functionalized Polyoxometalates Obtained by a Modular Strategy and Their Metal Complexation

The rational design and synthesis of organic-inorganic hybrids as functional molecular materials relies on both the careful conception of building-blocks and the strategy for their assembly. Three families of trialkoxo polyoxometalates (Lindqvist 2, Anderson 3, Dawson 4) grafted with remote terpyridine coordination sites have been synthesized to extend the available building-blocks. These new units can be combined with metal complexes that play a role as (i) chromophores toward charge-separated systems in light-harvesting devices and (ii) coordination motifs for metal-directed self-assembly toward multifunctional molecular hybrid materials. The X-ray crystal structures of polyoxometalate-terpyridine hybrids indicate distances of 21 Å and 19 Å between the two terpyridyl coordination sites in 2 and 3, respectively, with angles between the coordination vectors of 180° and 177.4°, respectively. Lindqvist 2 displays a reduction at -0.52 V vs SCE while Anderson 3 exhibits one reversible oxidation attributed to Mn(III)/Mn(IV) (+0.75 V vs SCE) and a broad wave at -1.28 V vs SCE assigned to the Mn(III)/Mn(II) reduction. Dawson 4 displays several processes on a wide range of potentials (+0.5 to -2.0 V vs SCE) centered on V(V), W(VI) and the organic ligand in order of decreasing potentials. The grafted terpyridine ligands in Anderson 3 and Dawson 4 were successfully coordinated to {PdCl}(+) and {RuCl(3)} moieties, respectively. The polyoxometalates and transition metal complexes retain their intrinsic properties in the final assemblies.

Elaboration of Covalently Linked Polyoxometalates with Ruthenium and Pyrene Chromophores and Characteriation of Their Photophysical Properties

Keggin and Dawson-type polyoxometalates (POMs) decorated by organometallic [cyclometalated ruthenium(II) polypyridine complex] or organic (pyrene) chromophores were prepared by postfunctionalization of hybrid disilylated POM platforms. The connection is made in a very efficient and modular way via Sonogashira coupling reactions, which provide a rigid linkage between the POM and the photoactive centers. Electronic properties have been inferred from electrochemical and photophysical studies and reflect poor electronic interactions between both partners. The presence of the POM leads to luminescence quenching of the chromophores, which was attributed to an intramolecular electron transfer from the chromophore to the POM. The rate of this process is much faster in the POM-pyrene than in the POM-Ru system. It depends on the driving force dictated by the redox potentials of both partners but also in the case of the POM-Ru system on the presence of the metallacycle, which acts as a molecular insulator and delays the intramolecular electron transfer. In the POM-Ru system, a comparative study of the luminescence quenching showed that the electron transfer is still more important in the covalently bonded hybrids than in systems where the POM and the ruthenium complexes are assembled via electrostatic interactions.

Merging Organometallic Chemistry with Polyoxometalate Chemistry

A series of polyoxomolybdate-incorporated organometallic complexes has been obtained by reaction of [MBr(CO)5] or solvated M(CO)3+ ions (M = Mn or Br) with (nBu4N)2[Mo2O7] in methanol, sometimes in the presence of triols of the type RC(CH2OH)3 (R = Me or CH2OH). Their molecular structures are related to those of previously described polyoxoalkoxomolybdates through the formal replacement of fac-MoO2(OR)+ units by topologically equivalent fac-M(CO)3+ units. Representative pairs of structurally related clusters include [Mo2O6(OMe)4-Re(CO)32]2- and [Mo4O10(OMe)6]2-, [Mo2O4MeC(CH2O)32Mn(CO)3]- and [Mo3O6(OMe)MeC(CH2O)32]-, [Mo2O4HOCH2C(CH2O)32Mn(CO)32] and [Mo4O8(OEt)2MeC(CH2O)32], [Mo6O16(OMe)2MeC(CH2O)32-Mn(CO)32]2- and [Mo8O20(OMe)4-MeC(CH2O)32]2-. Although the frameworks of the majority of derivatives are based on tetranuclear units which display the common rhomb-like structure, the alternative cubane-type arrangement is observed in [Mo2O5(OMe)5M(CO)32]-.

Charge photo-accumulation and photocatalytic hydrogen evolution under visible light at an iridium(III)-photosensitized polyoxotungstate

Steady-state irradiation under visible light of a covalent Ir(III)-photosensitized polyoxotungstate is reported. In the presence of a sacrificial electron donor, the photolysis leads to the very efficient photoreduction of the polyoxometalate. Successive formation of the one-electron and two-electron reduced species, which are unambiguously identified by comparison with spectroelectrochemical measurements, is observed with a significantly faster rate reaction for the formation of the one-electron reduced species. The kinetics of the photoreduction, which are correlated to the reduction potentials of the polyoxometalate (POM), can be finely tuned by the presence of an acid. Indeed light-driven formation of the two-electron reduced POM is considerably facilitated in the presence of acetic acid. The system is also able to perform photocatalytic hydrogen production under visible light without significant loss of performance over more than 1 week of continuous photolysis and displays higher photocatalytic efficiency than the related multi-component system, outlining the decisive effect of the covalent bonding between the POM and the photosensitizer. This functional and modular system constitutes a promising step for the development of charge photoaccumulation devices and subsequent photoelectrocatalysts for artificial photosynthesis.

Hybrid Polyoxometalates: Keggin and Dawson Silyl Derivatives as Versatile Platforms

A new series of polyoxometalate-based hybrids has been synthesized. These covalently linked organic-inorganic materials represent valuable elementary building blocks ready for postfunctionalization, using classical organic reactions and couplings. This approach is exemplified by the grafting of an organic chromophore via a Sonogashira coupling.

Second-Order Nonlinear Optical Properties of Polyoxometalate Salts of a Chiral Stilbazolium Derivative

The synthesis of nonlinear optical (NLO) active salts with stilbazolium derivatives and polyoxometalate (POM) counterions has been investigated. With known nonchiral stilbazolium derivatives, such as MOMS(+), compounds with centrosymmetric structures have been isolated, like for instance the centrosymmetric salt (MOMS)(4)[Mo(8)O(26)] (1), synthesized under hydrothermal conditions. A new chiral derivative of the known DAMS(+) molecules, named here CHIDAMS(+), has therefore been synthesized in order to force the crystallization of the hybrid ionic salts in noncentrosymmetric space groups. The CHIDAMS(+) cation has been crystallized under two polymorphic PF(6)(-) salts, (CHIDAMS)PF(6) (2a and 2b), and its reactivity with various POMs has been investigated. The ionic salt (CHIDAMS)(2)[Mo(5)O(13)(OEt)(4)(NO){Na(H(2)O)(0.5)(DMF)(0.5)}] (4) crystallizes in the noncentrosymmetric P2(1) group, but the push-pull axis of the CHIDAMS(+) cations adopts a quasi-antiparallel alignment. The ionic salt (CHIDAMS)(3)[PW(12)O(40)].2DMF (5) associating three CHIDAMS(+) cations and a PW(12)O(40)(3-) Keggin anion crystallizes also in the P2(1) space group, but the disposition of the cations in the solid state is far more favorable. Diffuse reflectance experiments have evidenced a charge transfer between the organic and inorganic components in 5, and Kurtz-Perry experiments show that this salt exhibits a second harmonic generation efficiency more than 10 times higher than those of the PF(6)(-) salts 2a and 2b, the hybrid salt 4, and all of the other NLO active POM molecular materials reported in the literature.

Organosilyl/‐germyl Polyoxotungstate Hybrids for Covalent Grafting onto Silicon Surfaces: Towards Molecular Memories

Organosilyl/-germyl polyoxotungstate hybrids [PW(9)O(34)(tBuSiO)(3)Ge(CH(2))(2)CO(2)H](3-) (1a), [PW(9)O(34)(tBuSiO)(3)Ge(CH(2))(2)CONHCH(2)C[triple bond]CH](3-) (2 a), [PW(11)O(39)Ge(CH(2))(2)CO(2)H](4-) (3a), and [PW(11)O(39)Ge(CH(2))(2)CONHCH(2)C[triple bond]CH](4-) (4a) have been prepared as tetrabutylammonium salts and characterized in solution by multinuclear NMR spectroscopy. The crystal structure of (NBu(4))(3)1a.H(2)O has been determined and the electrochemical behavior of 1a and 2a has been investigated by cyclic voltammetry. Covalent grafting of 2a onto an n-type silicon wafer has been achieved and the electrochemical behavior of the grafted clusters has been investigated. This represents the first example of covalent grafting of Keggin-type clusters onto a Si surface and a step towards the realization of POM-based multilevel memory devices.