Serena Berardi

Photocatalytic water oxidation: tuning light-induced electron transfer by molecular Co4O4 cores.

Isostructural cubane-shaped catalysts [Co(III)(4)(μ-O)(4)(μ-CH(3)COO)(4)(p-NC(5)H(4)X)(4)], 1-X (X = H, Me, t-Bu, OMe, Br, COOMe, CN), enable water oxidation under dark and illuminated conditions, where the primary step of photoinduced electron transfer obeys to Hammett linear free energy relationship behavior. Ligand design and catalyst optimization are instrumental for sustained O(2) productivity with quantum efficiency up to 80% at λ > 400 nm, thus opening a new perspective for in vitro molecular photosynthesis.

Photocatalytic Water Oxidation by a Mixed‐Valent MnIII3MnIVO3 Manganese Oxo Core that Mimics the Natural Oxygen‐Evolving Center

The functional core of oxygenic photosynthesis is in charge of catalytic water oxidation by a multi-redox Mn(III)/Mn(IV) manifold that evolves through five electronic states (S(i), where i=0-4). The synthetic model system of this catalytic cycle and of its S0→S4 intermediates is the expected turning point for artificial photosynthesis. The tetramanganese-substituted tungstosilicate [Mn(III)3Mn(IV)O3(CH3COO)3(A-α-SiW9O34)](6-)(Mn4POM) offers an unprecedented mimicry of the natural system in its reduced S0 state; it features a hybrid organic-inorganic coordination sphere and is anchored on a polyoxotungstate. Evidence for its photosynthetic properties when combined with [Ru(bpy)3](2+) and S2O8(2-) is obtained by nanosecond laser flash photolysis; its S0→S1 transition within milliseconds and multiple-hole-accumulating properties were studied. Photocatalytic oxygen evolution is achieved in a buffered medium (pH 5) with a quantum efficiency of 1.7%.

Is [Co4(H2O)2(α-PW9O34)2]10− a genuine molecular catalyst in photochemical water oxidation? Answers from time-resolved hole scavenging experiments

Water oxidation catalysts: evolution of [Co(4)(H(2)O)(2)(α-PW(9)O(34))(2)](10-) to catalytically active species is assessed by laser flash photolysis in sacrificial photocatalytic cycles with Ru(bpy)(3)(2+) as a photosensitizer.

Photoinduced Water Oxidation by a Tetraruthenium Polyoxometalate Catalyst: Ion-pairing and Primary Processes with Ru(bpy)32+ Photosensitizer

The tetraruthenium polyoxometalate [Ru(4)(μ-O)(4)(μ-OH)(2)(H(2)O)(4)(γ-SiW(10)O(36))(2)](10-) (1) behaves as a very efficient water oxidation catalyst in photocatalytic cycles using Ru(bpy)(3)(2+) as sensitizer and persulfate as sacrificial oxidant. Two interrelated issues relevant to this behavior have been examined in detail: (i) the effects of ion pairing between the polyanionic catalyst and the cationic Ru(bpy)(3)(2+) sensitizer, and (ii) the kinetics of hole transfer from the oxidized sensitizer to the catalyst. Complementary charge interactions in aqueous solution leads to an efficient static quenching of the Ru(bpy)(3)(2+) excited state. The quenching takes place in ion-paired species with an average 1:Ru(bpy)(3)(2+) stoichiometry of 1:4. It occurs by very fast (ca. 2 ps) electron transfer from the excited photosensitizer to the catalyst followed by fast (15-150 ps) charge recombination (reversible oxidative quenching mechanism). This process competes appreciably with the primary photoreaction of the excited sensitizer with the sacrificial oxidant, even in high ionic strength media. The Ru(bpy)(3)(3+) generated by photoreaction of the excited sensitizer with the sacrificial oxidant undergoes primary bimolecular hole scavenging by 1 at a remarkably high rate (3.6 ± 0.1 × 10(9) M(-1) s(-1)), emphasizing the kinetic advantages of this molecular species over, e.g., colloidal oxide particles as water oxidation catalysts. The kinetics of the subsequent steps and final oxygen evolution process involved in the full photocatalytic cycle are not known in detail. An indirect indication that all these processes are relatively fast, however, is provided by the flash photolysis experiments, where a single molecule of 1 is shown to undergo, in 40 ms, ca. 45 turnovers in Ru(bpy)(3)(3+) reduction. With the assumption that one molecule of oxygen released after four hole-scavenging events, this translates into a very high average turnover frequency (280 s(-1)) for oxygen production.

Polyoxometalate-Based N-Heterocyclic Carbene (NHC) Complexes for Palladium-Mediated C-C Coupling and Chloroaryl Dehalogenation Catalysis

Molecular hybrids are of particular interest in catalysis, due to the interplay of joint organic–inorganic domains with very diverse functional environments. The hybrid strategy is expected to allow for fine-tuning of the stereo-electronic properties of the catalyst through a tailored choice of the interacting organic–inorganic moieties. Such a hybrid upgrade of the catalytic system can be readily accessed, through the covalent functionalization of molecular polyoxometalates (POMs). Indeed, covalent POM hybrids are characterized by discrete, nanosized, multi-metal oxides as polyanionic scaffolds, which allow for anchoring on-surface organic pendants, including chiral residues. The hybrid structures display a remarkable stability, and resist harsh catalytic conditions, enabling a multi-turnover performance and sequential recycling in ionic-liquid media exposed to MW irradiation. Interestingly, the hybrid modification of the POM framework has been successfully used to expand the coordination potential by introduction of tailored organic domains. In this respect, polydentate salen-type or thiol/phosphineterminated ligands have been immobilized on the POM surface to bind Mn, Pd or Rh ions, or to implement the stabilization of Pd colloids. We present herein the synthesis, characterization, and catalytic applications of new POM-appended N-heterocyclic carbene (NHC) palladium complexes and their remarkable performance in catalyzing C C crosscoupling and aromatic dehalogenation reactions. To this end, imidazolium moieties have been successfully grafted on the defect site of the divacant Keggin polyanion [gSiW10O36] 8 . These organic domains are useful precursors of N-heterocyclic (NHC) carbenes, which are known to provide strong M NHC bonds, thus imparting high thermal robustness and stability to organometallic intermediates in multi-turnover palladium catalysis. In addition to the specifically designed Pd binding site, such POM-based hybrids display an extended polyanionic surface. This setup is expected to contribute sterically and by virtue of electrostatic repulsions to prevent agglomeration of incipient Pd, which generally deteriorates the turnover efficiency of the system. The molecular nature of such composite structures is a further point of interest, offering a single-site model for extended materials. Specifically, the molecularly defined structure of the hybrid allows for selective tailoring of the active site as well as for detecting subtle changes by solution characterization techniques, including heteronuclear NMR spectroscopy and ESI-MS. The synthetic route to Keggin-type hybrids involves the reaction of organosilane reagents with the nucleophilic oxygen atoms that border a defect site on the POM surface. Grafting strategies have been optimized in acetonitrile in which the presence of excess nBu4NBr promotes the solubilization of the POM by counterion metathesis (Scheme 1). Under these conditions, decoration of the lacunary POM is known to yield hybrids with two surface-anchored organosilyl (RSi-) groups, each one linked to two oxygen atoms of two edge-shared WO6 octahedra. [4,10] A first attempt focused on a divergent approach, including the covalent attachment of the triethoxysilyl-functionalized 4,5-dihydro-imidazolium bromide (1) to the divacant decatungstosilicate. The hybrid POM (2) was isolated and displayed spectroscopic data (H, C, Si and W NMR, FT-IR) that are in agreement with the expected bis-functionalized structure. However, palladation of 2 met with little success (Scheme 1). Neither direct metalation using Pd ACHTUNGTRENNUNG(OAc)2 nor transmetalation using Ag2O resulted in the [a] Dr. S. Berardi, Dr. M. Carraro, Dr. A. Sartorel, Prof. G. Scorrano, Dr. M. Bonchio ITM-CNR and Department of Chemical Sciences University of Padova, via Marzolo 1, 35131 Padova (Italy) Fax: (+39)0498275239 E-mail : marcella.bonchio@unipd.it [b] Dr. M. Iglesias, Prof. M. Albrecht Department of Chemistry, University of Fribourg Current address: School of Chemistry and Chemical Biology University College Dublin, Belfield, Dublin 4 (Ireland) Fax: (+353)17162501 E-mail : martin.albrecht@ucd.ie Supporting information for this article is available

Porous versus Compact Nanosized Fe(III)-Based Water Oxidation Catalyst for Photoanodes Functionalization

Integrated absorber/electrocatalyst schemes are increasingly adopted in the design of photoelectrodes for photoelectrochemical cells because they can take advantage of separately optimized components. Such schemes also lead to the emergence of novel challenges, among which parasitic light absorption and the nature of the absorber/catalyst junction features prominently. By taking advantage of the versatility of pulsed-laser deposition technique, we fabricated a porous iron(III) oxide nanoparticle-assembled coating that is both transparent to visible light and active as an electrocatalyst for water oxidation. Compared to a compact morphology, the porous catalyst used to functionalize crystalline hematite photoanodes exhibits a superior photoresponse, resulting in a drastic lowering of the photocurrent overpotential (about 200 mV) and a concomitant 5-fold increase in photocurrents at 1.23 V versus reversible hydrogen electrode. Photoelectrochemical impedance spectroscopy indicated a large increase in trapped surface hole capacitance coupled with a decreased charge transfer resistance, consistent with the possible formation of an adaptive junction between the absorber and the porous nanostructured catalyst. The observed effect is among the most prominent reported for the coupling of an electrocatalyst with a thin layer absorber.

Oxygenation by Ruthenium Monosubstituted Polyoxotungstates in Aqueous Solution: Experimental and Computational Dissection of a Ru(III)–Ru(V) Catalytic Cycle

Molecular polyoxometalates with one embedded ruthenium center, with general formula [Ru(II/III)(DMSO)XW11O39](n-) (X = P, Si; n = 4-6), are readily synthesized in gram scale under microwave irradiation by a flash hydrothermal protocol. These nanodimensional and polyanionic complexes enable aerobic oxygenation in water. Catalytic oxygen transfer to dimethylsulfoxide (DMSO) yielding the corresponding sulfone (DMSO2 ) has been investigated with a combined kinetic, spectroscopic and computational approach addressing: (i ) the Ru(III) catalyst resting state; (ii ) the bimolecular event dictating its transformation in the rate-determining step; (iii ) its aerobic evolution to a high-valent ruthenium oxene species; (iv ) the terminal fate to diamagnetic dimers. This pathway is reminiscent of natural heme systems and of bioinspired artificial porphyrins. The in silico characterization of a key bis-Ru(IV)-μ-peroxo-POM dimeric intermediate has been accessed by density functional theory. This observation indicates a new landmark for tracing POM-based manifolds for multiredox oxygen reduction/activation, where metal-centered oxygenated species play a pivotal role.

Photo-induced water oxidation: New photocatalytic processes and materials

New progress towards artificial photosynthetic methods and solar fuels will depend on the discovery of highly robust multi-electron catalysts and materials enabling light-activated water splitting with high quantum efficiency and low overpotential, thus mimicking the natural process.