High-Density Formation of Metal/Oxide Interfacial Catalytic Active Sites through Hybrid Clustering

Reactivity of Organoiridium Tungsten Oxide Clusters with Transition Metal Aquo Cations

Organometallic-polyoxometalate (POM) complexes form a unique class of molecular organometallic oxides characterized by the dynamic behavior of the organometallic cations. Herein, we investigated the reactivity of Cp*Ir-octatungstate clusters (where Cp* represents pentamethylcyclopentadienyl, C5Me5-) with Werner-type transition-metal aquo cations. The addition of Ag+, Co2+, Ni2+, and M3+ (M = Cr, Fe, or In) cations to the aqueous solution of Cp*Ir-octatungstate clusters resulted in the formation of [{Ag(OH2)2}2{Cp*Ir(OH2)}2{Cp*IrW3O12(OH)}2(WO2)2] (1), Co1.5K0.8Na0.2[{trans-Co(OH2)2}{Cp*IrW3O12(OH)}2(WO2)1.3{cis-Co(OH2)2}0.7] (2-Co), Ni0.2K1.4Na0.2[{Ni(OH2)4}2{Cp*IrW3O12(OH)}2(WO2)1.1{cis-Ni(OH2)2}0.9] (2-Ni), and [{M(OH2)4}2{Cp*IrW3O12(OH)}2{cis-M(OH2)2}2](NO3)2 (M = Cr, 3-Cr; Fe, 3-Fe; or In, 3-In), respectively. All clusters share the same cubane-type {Cp*IrW3O12(OH)}5- building block, representing the first examples of organoiridium-POMs functionalized by transition-metal aquo cations. These compounds are insoluble in water, facilitating the evaluation of their heterogeneous water-oxidation properties. Notably, 2-Co generates the highest catalytic water oxidation current. This work provides a new synthetic method to introduce metal-aquo complexes on an organometallic oxide cluster, producing multimetallic molecules that model the catalytic sites of complex oxides.

High-Density Formation of Ir/MoOx Interface through Hybrid Clustering for Chemoselective Nitrostyrene Hydrogenation

To form high-density metal/oxide interfacial active sites, we developed a catalyst preparation method based on hybrid clustering. An iridium-molybdenum (Ir-Mo) hybrid clustering catalyst was prepared by using the hybrid cluster [(IrCp*)4Mo4O16] (Cp* = η5-C5Me5) as the precursor. The Ir-Mo hybrid clustering catalyst selectively reduced the nitro group in the hydrogenation of 4-nitrostyrene, whereas the coimpregnated Ir-Mo catalyst reduced both the nitro and vinyl groups nonselectively. The hybrid clustering catalyst also exhibited high selectivity, even at a high Ir loading (5 wt %), in contrast to Ir/MoO3, which exhibited high selectivity only at low Ir loadings (<0.3 wt %). In situ X-ray absorption spectroscopy analysis revealed that oxygen vacancies were formed at the Ir/MoOx interface in the presence of H2. We concluded that a high-density Ir/MoOx interface contributes to the preferential adsorption of nitro groups on vacant sites, promoting the selective hydrogenation of nitro groups.

Molar-Ratio-Dependent Coordination Assembly of Organoiridium(III)-Octatungstate Complexes in Aqueous Solution

We scrutinized the speciation of Cp*Ir-containing tungsten oxide clusters (Cp* is pentamethylcyclopentadienyl anion) in aqueous mixtures of [(Cp*IrCl)₂(μ-Cl)₂] and Na₂WO₄ in varying molar ratios. ¹H nuclear magnetic resonance (NMR) spectroscopy revealed the formation of three distinct Cp*Ir-polyoxotungstate species in the reaction solution, and they were isolated as Na₄[(Cp*Ir)₂(μ-OH)₃]₂[(Cp*Ir)₂H₂W₈O₃₀] (1), [(Cp*Ir)₂(μ-OH)₃]₂[(Cp*Ir)₂{Cp*Ir(OH₂)}₂H₂W₈O₃₀] (2), and [(Cp*Ir)₂{Cp*Ir(OH₂)}₂{Cp*Ir(OH₂)₂}₂H₂W₈O₃₀](NO₃)₂ (3) from the mixtures in which iridium concentration is less than, equal to, and more than the tungsten concentration, respectively. These results show the octatungstate [H₂W₈O₃₀]^10- anion is the major polyoxotungstate species in the presence of {Cp*Ir}²⁺ cations, and it has high nucleophilicity enough to bind up to six {Cp*Ir}²⁺ cations on its surfaces producing a cationic Cp*Ir-octatungstate complex. The octatungstate anion was also generated from the reaction of [(Cp*IrCl)₂(μ-Cl)₂] and methylammonium paratungstate-B, (CH₃NH₃)₁₀[H₂W₁₂O₄₂], and was isolated as a methylamine-coordinated complex (CH₃NH₃)₂[(Cp*Ir)₂{Cp*Ir(NH₂CH₃)}₂H₂W₈O₃₀] (4), indicating {Cp*Ir}²⁺ cations function as a structure-directing agent that converts tungsten species into octatungstate anions in aqueous solution. In addition, the coordination environment of {Cp*Ir}²⁺ can be further modified by coordination with pyridine forming [{Cp*Ir(NC₅H₅)}₂(μ-OH)₂][(Cp*Ir)₂{Cp*Ir(NC₅H₅)}₂H₂W₈O₃₀] (5).