Reinvestigation of a Ru2-Incorporated Polyoxometalate Dioxygenase Precatalyst, “[WZnRu2III(H2O)(OH)(ZnW9O34)2]11−”: Evidence For Marginal, ≤0.2 Equivalents of Ru Incorporation Plus Faster Catalysis by Physical Mixtures of [RuII(DMSO)4Cl2] and the Parent Polyoxometalate [WZn3(H2O)2(ZnW9O34)2]12−

Pathways towards true catalysts: computational modelling and structural transformations of Zn-polyoxotungstates

Current catalysis undergoes a paradigm shift from molecular and heterogeneous realms towards new dynamic catalyst concepts. This calls for innovative strategies to understand the essential catalytic motifs and true catalysts emerging from oxidative transformation processes. Polyoxometalate (POM) clusters offer an inexhaustible reservoir for new noble metal-free catalysts and excellent model systems whose structure-activity relationships and mechanisms remain to be explored. Here, we first introduce a new {Zn_nNa_6-n(B-α-SbW₉O₃₃)₂} (n = 3-6) catalyst family with remarkable tuning options of the Zn-based core structure and high activity in H₂O₂-assisted catalytic alcohol oxidation as a representative reaction. Next, high level solution-based computational modelling of the intermediates and transition states was carried out for [Zn₆Cl₆(SbW₉O₃₃)₂]^12- as a representative well-defined case. The results indicate a radical-based oxidation process with the involvement of tungsten and adjacent zinc metal centers. The {Zn_nNa_6-n(B-α-SbW₉O₃₃)₂} series indeed efficiently catalyses alcohol oxidation via peroxotungstate intermediates, in agreement with strong spectroscopic support and other experimental evidence for the radical mechanism. Finally, the high performance of [Zn₆Cl₆(SbW₉O₃₃)₂]^12- was traced back to its transformation into a highly active and robust disordered Zn/W-POM catalyst. The atomic short-range structure of this resting pre-catalyst was elucidated by RMC modelling of the experimental W-L₃ and Zn-K edge EXAFS spectra and supported with further analytical methods. We demonstrate that computational identification of the reactive sites combined with the analytical tracking of their dynamic transformations provides essential input to expedite cluster-based molecular catalyst design.

A Novel Ruthenium-Decorating Polyoxomolybdate Cs₃Na₆H[MoVI14RuIV₂O50(OH)₂]·24H₂O: An Active Heterogeneous Oxidation Catalyst for Alcohols

The first example of wholly inorganic ruthenium-containing polyoxomolybdate Cs₃Na₆H[MoVI14RuIV₂O50(OH)₂]·24H₂O (1) was isolated and systematically characterized by element analysis, infrared spectroscopy (IR), thermogravimetric analyses (TGA), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX) and single-crystal X-ray diffraction. Compound 1 is composed of an unprecedented {Mo14}-type isopolymolybdate with a di-ruthenium core precisely encapsulated in its center, exhibiting a three-tiered ladder-like structure. The title compound can act as an efficient heterogeneous catalyst in the transformation of 1-phenylethanol to acetophenone. This catalyst is also capable of being recycled and reused for at least ten cycles with its activity being retained under the optimal conditions.

Mechanistic Investigation into Olefin Epoxidation with H2O2 Catalyzed by Aqua-Coordinated Sandwich-Type Polyoxometalates: Role of the Noble Metal and Active Oxygen Position

Aqua-coordinated sandwich-type polyoxometalates (POMs), {[WZnTM2(H2O)2](ZnW9O34)2} n- (TM=RhIII, PdII, and PtII), catalyze olefin epoxidation with hydrogen peroxide and have been well established, and they present an advance toward the utilization of olefins. To elucidate the epoxidation mechanism, we systematically performed density functional calculations. The reaction proceeds through a two-step mechanism: activation of H2O2 and oxygen transfer. The aqua-coordinated complexes show two distinct H2O2 activation pathways: "two-step" and "concerted". The concerted processes are more facile and proceed with similar and rate-determining energy barriers at the Rh-, Pd-, and Pt-containing transition states, which agrees well with the experimental results. Next, the resulting TM-OH-(μ-OOH) intermediate transfers an O atom to olefin to form an epoxide. The higher reactivity of the Rh-containing POM is attributed to more interactions between the Rh and hydroperoxo unit. We also calculated all active oxygen positions to locate the most favorable pathway. The higher reactivity of the two-metal-bonded oxygen position is predominantly ascribed to its lower stereoscopic hindrance. Furthermore, the presence of one and two explicit water solvent molecules significantly reduces the energy barriers, making these sandwich POMs very efficient for the olefin epoxidation with H2O2.