Catalytic Reactions over Halide Cluster Complexes of Group 5–7 Metals

Catalytic ammonia synthesis on HY-zeolite-supported angstrom-size molybdenum cluster

The development of new catalysts with high N2 activation ability is an effective approach for low-temperature ammonia synthesis. Herein, we report a novel angstrom-size molybdenum metal cluster catalyst for efficient ammonia synthesis. This catalyst is prepared by the impregnation of a molybdenum halide cluster complex with an octahedral Mo6 metal core on HY zeolite, followed by the removal of all the halide ligands by activation with hydrogen. In this activation, the size of the Mo6 cluster (ca. 7 Å) is almost retained. The resulting angstrom-size cluster shows catalytic activity for ammonia synthesis from N2 and H2, and the reaction proceeds continuously even at 200 °C under 5.0 MPa. DFT calculations suggest that N[triple bond, length as m-dash]N bond cleavage is promoted by the cooperation of the multiple molybdenum sites.

Octahedral Tantalum Bromide Clusters as Catalysts for Light-Driven Hydrogen Evolution

The development of an efficient hydrogen generation strategy from aqueous protons using sunlight is a current challenge aimed at the production of low-cost, easily accessible, renewable molecular hydrogen. For achieving this goal, non-noble metal containing and highly active catalysts for the hydrogen evolution reaction (HER) are desirable. Octahedral tantalum halide clusters {Ta6(μ-X)12}2+ (X = halogen) represent an emerging class of such HER photocatalysts. In this work, the photocatalytic properties of octahedral aqua tantalum bromide clusters toward HER and in acid and homogeneous aqueous conditions were investigated. The [{Ta6Bri12}Bra2(H2O)a4]·4H2O (i = inner ligand; a = apical ligand) compound is revealed to be an efficient precatalyst in acid (HBr) conditions and with methanol as the sacrificial agent. A response surface methodology (RSM) study was applied for the optimization of the HER conditions, considering the concentrations of both additives (methanol and HBr) as independent variables. An optimal H2 production of 11 mmol·g-1 (TON = 25) was achieved, which displays exceptional catalytic properties compared to regular Ta-based materials. The aqua tantalum bromide clusters assist in the photocatalytic hydrogen generation in agreement with energy-conversion schemes, and plausible active catalytic species and a reaction mechanism were proposed from computational and experimental perspectives.

Nanostructured Hybrids Based on Tantalum Bromide Octahedral Clusters and Graphene Oxide for Photocatalytic Hydrogen Evolution

The generation of hydrogen (H₂) using sunlight has become an essential energy alternative for decarbonization. The need for functional nanohybrid materials based on photo- and electroactive materials and accessible raw materials is high in the field of solar fuels. To reach this goal, single-step synthesis of {Ta₆Brⁱ₁₂}@GO (GO = graphene oxide) nanohybrids was developed by immobilization of [{Ta₆Brⁱ₁₂}Br^a₂(H₂O)^a₄]·4H₂O (i = inner and a = apical positions of the Ta₆ octahedron) on GO nanosheets by taking the advantage of the easy ligand exchange of the apical cluster ligands with the oxygen functionalities of GO. The nanohybrids were characterized by spectroscopic, analytical, and morphological techniques. The hybrid formation enhances the yield of photocatalytic H₂ from water with respect to their precursors and this is without the presence of precious metals. This enhancement is attributed to the optimal cluster loading onto the GO support and the crucial role of GO in the electron transfer from Ta₆ clusters into GO sheets, thus suppressing the charge recombination. In view of the simplicity and versatility of the designed photocatalytic system, octahedral tantalum clusters are promising candidates to develop new and environmentally friendly photocatalysts for H₂ evolution.

Revisiting properties of edge-bridged bromide tantalum clusters in the solid-state, in solution and vice versa: an intertwined experimental and modelling approach

Edge-bridged halide tantalum clusters based on the {Ta₆Br₁₂}⁴⁺ core have been the topic of many physicostructural investigations both in solution and in the solid-state. Despite a large number of studies, the fundamental correlations between compositions, local symmetry, electronic structures of [{Ta₆Brⁱ₁₂}L^a₆]^m+/n- cluster units (L = Br or H₂O, in solution and in the solid-state), redox states, and vibrational and absorption properties are still not well established. Using K₄[{Ta₆Brⁱ₁₂}Br^a₆] as a starting precursor (i: inner and a: apical), we have investigated the behavior of the [{Ta₆Brⁱ₁₂}Br^a₆]^4- cluster unit in terms of oxidation properties and chemical modifications both in solution (water and organic solvent) and after recrystallization. A wide range of experimental techniques in combination with quantum chemical simulations afford new data that allow the puzzling behavior of the cluster units in response to changes in their environment to be revealed. Apical ligands undergo changes like modifications of interatomic distances to complete substitutions in solution that modify noticeably the cluster physical properties. Changes in the oxidation state of the cluster units also occur, which modify significantly their physical properties, including optical properties, which can thus be used as fingerprints. A subtle balance exists between the number of substituted apical ligands and the cluster oxidation state. This study provides new information about the exact nature of the species formed during the transition from the solid-state to solutions and vice versa. This shows new perspectives on optimization protocols for the design of Ta₆ cluster-based materials.

Proton motion inside [(DMF)2H]2[W6Cl14]: structural, Raman and luminescence studies

Protonation of DMF by (H₇O₃)₂[W₆Cl₁₄] results in the appearance of strongly proton coupled [(DMF)₂H]⁺ dimers. Such units are captured as the cationic part of [(DMF)₂H]₂[W₆Cl₁₄] (1). The proton behavior in such dimers was studied for the first time with single crystal X-ray diffraction (XRD) and ¹H MAS NMR, Raman and photoluminescence (PL) spectroscopic techniques. The experimental data reveal the presence of two types of [(DMF)₂H]⁺ dimers in 1 (cisoidal and transoidal, with respect to the mutual orientations of their C-O groups) which differ in terms of the degree to which they interact with the cluster anions as the temperature decreases. At room temperature all the OO distances in the [(DMF)₂H]⁺ dimers are very short (2.375 Å) and almost equal. ¹H MAS NMR spectra show a resonance line at 18.7 ppm which is very close to that observed in sodium hydrogen maleate with a strong hydrogen bond belonging to a single-well potential of proton motion. The temperature decrease leads to the differentiation of [(DMF)₂H]⁺ dimers due to the elongation of the OO distance in one pair while keeping a practically constant OO distance in the second pair. The analysis of the cation-anion interactions reveals a strong difference between these two types of dimers which results from the shifting of one DMF molecule toward a terminal Cl^- ligand of the cluster anion. The DFT calculations were used to show the difference in OH⁺O stretches for two different dimers. Moreover, we have found the PL of such dimeric units in the solid state. The temperature screening of the PL behavior demonstrates two types of luminescent centers at low temperatures which coalesce at 298 K. The proton motion in the hydrogen bond was studied using Raman spectroscopy, which was beneficial to monitor the complex behavior over a very broad temperature range from 5 to 298 K. According to the Raman data, we are dealing with a symmetric double-well potential for the hydrogen bond at room temperature, which becomes a broad single well potential below 110 K for the [(DMF)₂H]⁺ cation with a longer OO distance (the cisoidal isomer) and below 60 K for the [(DMF)₂H]⁺ cation with a shorter OO distance (the transoidal isomer).

Enhanced Photocatalytic Activity and Stability in Hydrogen Evolution of Mo6 Iodide Clusters Supported on Graphene Oxide

Catalytic properties of the cluster compound (TBA)₂[Mo₆Iⁱ₈(O₂CCH₃)^a₆] (TBA = tetrabutylammonium) and a new hybrid material (TBA)₂Mo₆Iⁱ₈@GO (GO = graphene oxide) in water photoreduction into molecular hydrogen were investigated. New hybrid material (TBA)₂Mo₆Iⁱ₈@GO was prepared by coordinative immobilization of the (TBA)₂[Mo₆Iⁱ₈(O₂CCH₃)^a₆] onto GO sheets and characterized by spectroscopic, analytical, and morphological techniques. Liquid and, for the first time, gas phase conditions were chosen for catalytic experiments under UV–Vis irradiation. In liquid water, optimal H₂ production yields were obtained after using (TBA)₂[Mo₆Iⁱ₈(O₂CCH₃)^a₆] and (TBA)₂Mo₆Iⁱ₈@GO) catalysts after 5 h of irradiation of liquid water. Despite these remarkable catalytic performances, “liquid-phase” catalytic systems have serious drawbacks: the cluster anion evolves to less active cluster species with partial hydrolytic decomposition, and the nanocomposite completely decays in the process. Vapor water photoreduction showed lower catalytic performance but offers more advantages in terms of cluster stability, even after longer radiation exposure times and recyclability of both catalysts. The turnover frequency (TOF) of (TBA)₂Mo₆Iⁱ₈@GO is three times higher than that of the microcrystalline (TBA)₂[Mo₆Iⁱ₈(O₂CCH₃)^a₆], in agreement with the better accessibility of catalytic cluster sites for water molecules in the gas phase. This bodes well for the possibility of creating {Mo₆I₈}⁴⁺-based materials as catalysts in hydrogen production technology from water vapor.

In Situ Generation of Active Molybdenum Octahedral Clusters for Photocatalytic Hydrogen Production from Water

The photocatalytic hydrogen evolution reaction (HER) from water under homogeneous and heterogeneous conditions is explored for the {Mo6 Br(i) 8 }(4+) cluster core based unit starting from (TBA)2 [Mo6 Br(i) 8 F(a) 6 ] (TBA=tetra-n-butylammonium; "i" and "a" refer to the face-capping inner and terminal apical ligand, respectively). The catalytic activity of {Mo6 Br(i) 8 }(4+) is enhanced by the in situ generation of [Mo6 Br(i) 8 F(a) 5 (OH)(a) ](2-) , [Mo6 Br(i) 8 F(a) 3 (OH)(a) 3 ](2-) , and [Mo6 Br(i) 8 (OH)(a) 6 ](2-) , which are identified by ESIMS, luminescence, and NMR techniques. Full substitution of F(-) by OH(-) leads to the formation of (H3 O)2 [Mo6 Br(i) 8 (OH)(a) 6 ]⋅10 H2 O; its structure was determined by single-crystal XRD. The immobilization of the active {Mo6 Br(i) 8 }(4+) onto graphene oxide (GO) surfaces enhances its stability under catalytic conditions. The catalytic activity of the resulting (TBA)2 Mo6 Br(i) 8 @GO material is improved with respect to GO, but is reduced compared to the activity under homogeneous conditions because of changes in the GO semiconducting properties as well as lower activity and/or accessibility of the anchored cluster.