ABSTRACT
To overcome the bottleneck of water splitting, the exploration of efficient, selective, and stable water oxidation catalysts (WOCs) is crucial. We report an all-inorganic, oxidatively and hydrolytically stable WOC based on a polyoxometalate [(A-α-SiW9 O34)2Co8(OH)6(H2O)2(CO3)3](16-) (Co8 POM). As a cobalt(II)-based cubane water oxidation catalyst, Co8POM embeds double Co(II)4O3 cores. The self-assembled catalyst is similar to the oxygen evolving complex (OEC) of photosystemâ II (PSâ II). Using [Ru(bpy)3](2+) as a photosensitizer and persulfate as a sacrificial electron acceptor, Co8POM exhibits excellent water oxidation activity with a turnover number (TON) of 1436, currently the highest among bioinspired catalysts with a cubical core, and a high initial turnover frequency (TOF). Investigation by several spectroscopy, spectrometry, and other techniques confirm that Co8POM is a stable and efficient catalyst for visible light-driven water oxidation. The results offer a useful insight into the design of water oxidation catalysts.
Subject(s)
Cobalt/chemistry , Coordination Complexes/chemistry , Oxides/chemistry , Tungsten Compounds/chemistry , Biomimetic Materials , Catalysis , Cobalt/radiation effects , Coordination Complexes/radiation effects , Light , Organometallic Compounds/chemistry , Organometallic Compounds/radiation effects , Oxidation-Reduction , Photochemical Processes , Photosensitizing Agents/chemistry , Photosensitizing Agents/radiation effects , Tungsten Compounds/radiation effects , Water/chemistryABSTRACT
Two MOF-like but all-inorganic polyoxometalate-based networks, [Na7X2W18Sn9Cl5O68·(H2O)m]n (1, X = Si, m = 35; 2, X = Ge, m = 41), and the molecular tetramer Na6[{Na(µ-OH2)(OH2)2}6{Sn6(B-SbW9O33)2}2]·50H2O (3) have been prepared and characterized by X-ray diffraction and spectroscopic methods. All three compounds exhibit unique structural features, and networks 1 and 2 incorporate the highest nuclearity of Sn(II)-containing POMs to date. Tetramer 3 comprises bridging Sn(II) ions with [B-SbW9O33](9-) units and exhibits two highly unusual features, a long-range Sb···Sb interaction and an intramolecular charge-transfer transition involving donation of the lone-pair electron density on both Sb(III) and Sn(II) to the POM. The electronic structure and excited-state dynamics have been studied by transient spectroscopy, spectroelectrochemistry, DFT calculations, and resonance Raman spectroscopy. The synergistic effect of two types of stereoactive lone-pairs on Sb(III) and Sn(II) is critical for the charge-transfer absorption feature in the visible.
Subject(s)
Antimony/chemistry , Models, Chemical , Tin Compounds/chemistry , Tungsten Compounds/chemistry , Crystallography, X-Ray , Electrochemistry , Electron Transport , Spectrum Analysis, Raman , Tungsten Compounds/chemical synthesisABSTRACT
Several polyoxometalate-supported metal carbonyl complexes, K7Na3P2W23O80{Re(CO)3}2·38H2O (1), (C3H10N)8Na2P2W23O80{Re(CO)3}2·10H2O (1a) and (C3H10N)6KNa3P2W23O80{Mn(CO)3}2·7H2O (2), have been prepared from the dimerization of the monovacant Keggin [α-PW11O39](7-) with metal carbonyl complexes [M(CO)3](+) (M = Re, Mn) in acidic aqueous solutions. The resulting "twisted-sandwich" architectures are chiral, but their crystalline solids are racemic. A detailed investigation involving syntheses, crystal structures, and electrochemistry is presented.
ABSTRACT
A polyoxometalate-supported trirhenium carbonyl cluster, mimicking metal oxide supported interfacial dyadic structures, has been synthesized and characterized. Multiple techniques, including computational and transient absorption spectroscopy, have been applied to characterize the charge-transfer dynamics occurring at the interfaces of this "double cluster". The stepwise kinetics of charge separation and recombination has been thoroughly investigated.
ABSTRACT
A new series of complexes containing two electron donating groups, {M(CO)(3)}(+) ions, M = Re or Mn, on one polytungstate electron acceptor group have been prepared and characterized. These complexes containing two electron donating groups, {M(CO)(3)}(+) ions, M = Re or Mn, on one polytungstate electron acceptor group have been prepared and characterized. These two-component polyoxometalate (POM) compounds have been made by reaction of solvated {M(CO)(3)}(+) ions (M = Re or Mn) with [X(2)W(22)O(74)(OH)(2)](12-) (X = Sb or Bi) POM multidentate ligands in aqueous solution. These syntheses reveal that the fac-{WO(OH)(2)}(2+) groups in the terminal positions of these two POM ligands are easily replaced by the topologically equivalent units fac-{M(CO)(3)}(+). Four compounds, [X(2)W(20)O(70){M(CO)(3)}(2)](12-) (1a: X = Sb, M = Re; 1b: X = Bi, M = Re; 2a: X = Sb, M = Mn; 2b: X = Bi, M = Mn) have been isolated and characterized of X-ray crystallography, spectroscopic, and computational methods. The charge transfer dynamics, investigated by femtosecond transient absorption (TA) spectroscopy of 1a and 1b combined with the density functional theory (DFT) calculations indicate that both complexes exhibit metal-to-polyoxometalate charge-transfer (MPCT) from the Re centers to the POM ligands, while MPCT from the Mn centers to the POM ligands in 2a and 2b leads to decomposition of starting compounds. The studies suggest a general synthetic route to a potentially very large class of POM-based hybrid compounds.
ABSTRACT
Di- and tricobalt silicotungstate complexes, K(5)Na(4)H(4)[{Na(3)(µ-OH(2))(2)Co(2)(µ-OH)(4)} (Si(2)W(18)O(66))]·37H(2)O (1) and K(6)Na(3)[Na(H(2)O){Co(H(2)O)(3)}(2){Co(H(2)O)(2)}(Si(2)W(18)O(66))]·22H(2)O (2), have been synthesized through reaction of cobalt chloride and [A-α-SiW(9)O(34)](10-) in acidic buffer solution. They have been characterized by X-ray crystallography, elemental analysis, cyclic voltammetry, infrared, and UV-vis spectroscopy. In 1, two cobalt atoms as well as three sodium atoms are incorporated in the central pocket of the [Si(2)W(18)O(66)](16-) polyanion. In 2, one cobalt atom and one sodium atom are incorporated in the pocket of [Si(2)W(18)O(66)](16-); two other cobalt atoms in this complex protrude outside the pocket and connect with WO(6) units of other [Si(2)W(18)O(66)](16-) polyanions to form a one-dimensional polymeric structure. The crucial parameters in the synthesis of these two compounds are discussed, and their stability in different buffer solutions is studied. The decomposition of 1 or 2 in heated potassium acetate buffer (pH 4.8, 1 M) yields K(11)[{Co(2)(H(2)O)(8)}K(Si(2)W(18)O(66))]·17H(2)O (3) based on spectroscopic studies and an X-ray crystal structure.
ABSTRACT
A new pentanickel silicotungstate complex, K(10)H(2)[Ni(5)(OH)(6)(OH(2))(3)(Si(2)W(18)O(66))]·34H(2)O (KH-), has been synthesized and characterized by X-ray crystallography and several other methods. Dynamic light scattering, kinetics and other experiments confirm that in the presence of [Ru(bpy)(3)](2+) (the photosensitizer for light-driven water oxidations) and [Ru(bpy)(3)](3+) (the oxidant in the dark water oxidations) exists in an equilibrium between solution (soluble) and a [Ru(bpy)(3)](n+)- complex (minimally soluble) form. This new pentanickel polyoxometalate catalyzes efficient water oxidation in both the dark and on irradiation with 455 nm LED light with 1.0 mM [Ru(bpy)(3)](2+) photosensitizer and 5.0 mM Na(2)S(2)O(8), sacrificial electron acceptor. Four lines of evidence indicate that in this solution [symbol:see text] Ru(bpy)(3)](n+)- complex equilibrium remains molecular and does not decompose to nickel hydroxide particles.
ABSTRACT
In the last five years and currently, research on solar fuels has been intense and no sub-area in this field has been more active than the development of water oxidation catalysts (WOCs). In this timeframe, a new class of molecular water oxidation catalysts based on polyoxometalates have been reported that combine the advantages of homogeneous and heterogeneous catalysts. This review addresses central issues in green energy generation, the challenges in water oxidation catalyst development, and the possible uses of polyoxometalates in green energy science.
ABSTRACT
A dodecazinc silicotungstate K(20)Na(2)[Zn(6)(OH)(7)(H(2)O)(Si(2)W(18)O(66))](2)·34H(2)O (1) has been synthesized and characterized by X-ray crystallography, elemental analysis, infrared, UV-vis spectroscopy, cyclic voltammetry, acid-base titration, and DFT calculations. The twelve zinc atoms between the two [Si(2)W(18)O(66)](16-) frameworks make this complex more stable hydrolytically than the heteropolytungstate ligands, [Si(2)W(18)O(66)](16-), themselves. The structurally unique central Zn(12) core is formed by the fusion of two [Zn(6)(OH)(7)(H(2)O)](5+) units through two edge-sharing Zn6 atoms. DFT B3LYP calculations give HOMO-LUMO and (HOMO - 1)-LUMO energy gaps of â¼3.65 and 3.91 eV, respectively, as compared to the band gap in ZnO of 3.35 eV.
Subject(s)
Silicon/chemistry , Tungsten Compounds/chemistry , Zinc/chemistry , Coordination Complexes/chemical synthesis , Coordination Complexes/chemistry , Crystallography, X-Ray , Electrochemical Techniques , Ligands , Molecular Conformation , Oxidation-ReductionABSTRACT
Terminal oxo complexes of the late transition metals Pt, Pd, and Au have been reported by us in Science and Journal of the American Chemical Society. Despite thoroughness in characterizing these complexes (multiple independent structural methods and up to 17 analytical methods in one case), we have continued to study these structures. Initial work on these systems was motivated by structural data from X-ray crystallography and neutron diffraction and (17)O and (31)P NMR signatures which all indicated differences from all previously published compounds. With significant new data, we now revisit these studies. New X-ray crystal structures of previously reported complexes K(14)[P(2)W(19)O(69)(OH(2))] and "K(10)Na(3)[Pd(IV)(O)(OH)WO(OH(2))(PW(9)O(34))(2)]" and a closer examination of these structures are provided. Also presented are the (17)O NMR spectrum of an (17)O-enriched sample of [PW(11)O(39)](7-) and a careful combined (31)P NMR-titration study of the previously reported "K(7)H(2)[Au(O)(OH(2))P(2)W(20)O(70)(OH(2))(2)]." These and considerable other data collectively indicate that previously assigned terminal Pt-oxo and Au-oxo complexes are in fact cocrystals of the all-tungsten structural analogues with noble metal cations, while the Pd-oxo complex is a disordered Pd(II)-substituted polyoxometalate. The neutron diffraction data have been re-analyzed, and new refinements are fully consistent with the all-tungsten formulations of the Pt-oxo and Au-oxo polyoxometalate species.
ABSTRACT
A new polyoxometalate of earth adundant elements [{Co(4)(µ-OH)(H(2)O)(3)}(Si(2)W(19)O(70))](11-) has been synthesized, characterized and shown to be a water oxidation catalyst. The initial catalytic complex is unstable and slowly undergoes hydrolysis. The hydrolysis products have been isolated and characterized, and their catalytic water oxidation activity is assessed.
ABSTRACT
[P(4)W(35)O(124){Re(CO)(3)}(2)](16-) (1), a Wells-Dawson [α(2)-P(2)W(17)O(61)](10-) polyoxometalate (POM)-supported [Re(CO)(3)](+) complex containing covalent W(VI)-O-Re(I) bonds has been synthesized and characterized by several methods, including X-ray crystallography. This complex shows a high visible absorptivity (ε(470 nm) = 4000 M(-1) cm(-1) in water) due to the formation of a Re(I)-to-POM charge transfer (MPCT) band. The complex was investigated by computational modeling and transient absorption measurements in the visible and mid-IR regions. Optical excitation of the MPCT transition results in instantaneous (<50 fs) electron transfer from the Re(I) center to the POM ligand.