Template-dependent photochemical reactivity of molecular metal oxides.

A combined experimental and theoretical study shows that the photooxidative activity of two isostructural metal oxide clusters depends on their internal templates. To this end, two halide-templated bismuth vanadium oxide clusters [X(Bi(dmso)3 )2 V12 O33 ](-) (X=Cl(-) , Br(-) ) are reported and fully characterized. The two clusters show similar absorption features and illustrate that bismuth incorporation results in increased visible-light absorption. Significantly higher photooxidative activity is observed for the bromide-templated cluster compared with the chloride-templated one. Detailed photophysical assays and complementary DFT calculations suggest that the more efficient triplet excited state formation in the Br(-) -containing cluster is the decisive step in the photocatalysis and is due to the heavy-atom effect of the bromide. This concept can therefore open new pathways towards the optimization of photocatalytic activity in metal oxide clusters.

Multiply bonded metal(II) acetate (rhodium, ruthenium, and molybdenum) complexes with the trans-1,2-bis(N-methylimidazol-2-yl)ethylene ligand.

The synthesis and structural characterization of new coordination polymers with the N,N-donor ligand trans-1,2-bis(N-methylimidazol-2-yl)ethylene (trans-bie) are reported. It was found that the acetate-bridged paddlewheel metal(II) complexes [M2(O2CCH3)4(trans-bie)]n with M = Rh, Ru, Mo, and Cr are linked by the trans-bie ligand to give a one-dimensional alternating chain. The metal-metal multiple bonds were analyzed with density functional theory and CASSCF/CASPT2 calculations (bond orders: Rh, 0.8; Ru, 1.7; Mo, 3.3).

Photocatalytic reactivity tuning by heterometal and addenda metal variation in Lindqvist polyoxometalates.

A systematic study into the effects of metal substitution on the visible-light photocatalytic activity of prototype metal oxide cluster anions is presented. Four isostructural Lindqvist clusters [V(x)M(6-x)O19]((2+x)-) (M = W, Mo, x = 1, 2) with photooxidative activity in the visible range are reported. It is shown that the photooxidative performance correlates with the number of vanadium atoms in the cluster. Further, two divergent reaction mechanisms are observed depending on the type of addenda metal (i.e. Mo or W) used. When comparing the reactivity under aerated vs. de-aerated conditions, it was found that molybdate-based clusters show significantly increased reaction rates in the absence of oxygen; in contrast, marginally reduced reaction rates were observed for the tungstate-based species under de-aerated conditions. Wavelength-dependent quantum efficiency studies provide insight into the visible-light reactivity of all four species. Radical scavenging experiments suggest that the photocatalysis proceeds via formation of hydroxyl radicals. Cluster recycling studies demonstrate the robust nature of the homogeneous photocatalysts.

Visible light photooxidative performance of a high-nuclearity molecular bismuth vanadium oxide cluster.

The visible light photooxidative performance of a new high-nuclearity molecular bismuth vanadium oxide cluster, H3[{Bi(dmso)3}4V13O40], is reported. Photocatalytic activity studies show faster reaction kinetics under anaerobic conditions, suggesting an oxygen-dependent quenching of the photoexcited cluster species. Further mechanistic analysis shows that the reaction proceeds via the intermediate formation of hydroxyl radicals which act as oxidant. Trapping experiments using ethanol as a hydroxyl radical scavenger show significantly decreased photocatalytic substrate oxidation in the presence of EtOH. Photocatalytic performance analyses using monochromatic visible light irradiation show that the quantum efficiency Φ for indigo photooxidation is strongly dependent on the irradiation wavelength, with higher quantum efficiencies being observed at shorter wavelengths (Φ395nm ca. 15%). Recycling tests show that the compound can be employed as homogeneous photooxidation catalyst multiple times without loss of catalytic activity. High turnover numbers (TON ca. 1200) and turnover frequencies up to TOF ca. 3.44 min(-1) are observed, illustrating the practical applicability of the cluster species.

Chemical and photochemical functionality of the first molecular bismuth vanadium oxide.

Anionic metal oxide clusters, so-called polyoxometalates, can be developed as molecular model compounds to mimic the chemical and photochemical reactivity of solid-state metal oxides on the molecular level. Inspired by the well-known visible-light photocatalyst BiVO(4), the first molecular bismuth vanadium oxide has been synthesized to investigate the chemical and photochemical similarities between the solid-state and molecular compounds. The cluster H(3)[(Bi(dmso)(3))(4)V(13)O(40)]·ca. 4 DMSO was obtained from simple precursors in almost quantitative yield. Structural analysis showed that the cluster shell is based on the unusual all-vanadium ε-Keggin framework [ε-V(12)O(40)](15-), which is stabilized by coordination of four Bi(III) centers. The acidic character of the three cluster protons was demonstrated by titration studies. The cluster shows promising photocatalytic properties in visible-light photooxidation reactions and has high activity (turnover number >1200), high quantum yield (Φ=7.6 %), and good recyclability, which make it a promising first example of a new class of heterometallic polyoxometalates.

Metal substitution in a Lindqvist polyoxometalate leads to improved photocatalytic performance.

The photochemical properties of homo- and heterometallic molybdate-based Lindqvist polyoxometalate clusters are investigated in a comparative study and it is shown that vanadium substitution can be used as a facile synthetic tool to optimize the visible light absorption and photocatalytic activity of the cluster. The mono-vanadium substituted unit, [VMo(5)O(19)](3-) shows light absorption up to 480 nm whereas the light absorption of the molybdate analogue [Mo(6)O(19)](2-) is mainly in the UV region below 400 nm. The electronic absorption properties of both clusters are further investigated using TD-DFT calculations which show that vanadium incorporation leads to the formation of low-energy O → V LMCT transitions. In comparative photochemical dye decomposition test reactions under UV and Vis irradiation, a higher reactivity is observed for [VMo(5)O(19)](3-) together with turnover numbers of more than 1600. In addition it is shown that under anaerobic conditions, the photoreaction proceeds faster than in the presence of oxygen, suggesting that oxygen acts as a quencher in one of the photoredox steps.