Solution and Solid State Structural Chemistry of Th(IV) and U(IV) 4-Hydroxybenzoates.

Organic ligands with carboxylate functionalities have been shown to affect the solubility, speciation, and overall chemical behavior of tetravalent metal ions. While many reports have focused on actinide complexation by relatively simple monocarboxylates such as amino acids, in this work we examined Th(IV) and U(IV) complexation by 4-hydroxybenzoic acid in water with the aim of understanding the impact that the organic backbone has on the solution and solid state structural chemistry of thorium(IV) and uranium(IV) complexes. Two compounds of the general formula [An6O4(OH)4(H2O)6(4-HB)12]· nH2O [An = Th (Th-1) and U (U-1); 4-HB = 4-hydroxybenzoate] were synthesized via room-temperature reactions of AnCl4 and 4-hydroxybenzoic acid in water. Solid state structures were determined by single-crystal X-ray diffraction, and the compounds were further characterized by Raman, infrared, and optical spectroscopies and thermogravimetry. The magnetism of U-1 was also examined. The structures of the Th and U compounds are isomorphous and are built from ligand-decorated oxo/hydroxo-bridged hexanuclear units. The relationship between the building units observed in the solid state structure of U-1 and those that exist in solution prior to crystallization as well as upon dissolution of U-1 in nonaqueous solvents was investigated using small-angle X-ray scattering, ultraviolet-visible optical spectroscopy, and dynamic light scattering. The evolution of U solution speciation as a function of reaction time and temperature was examined. Such effects as well as the impact of the ligand on the formation and evolution of hexanuclear U(IV) clusters to UO2 nanoparticles compared to prior reported monocarboxylate ligand systems are discussed. Unlike prior reported syntheses of Th and U(IV) hexamers where the pH was adjusted to ∼2 and 3, respectively, to drive hydrolysis, hexamer formation with the HB ligand appears to be promoted only by the ligand.

Cobalt-to-vanadium charge transfer in polyoxometalate water oxidation catalysts revealed by 2p3d resonant inelastic X-ray scattering.

Two isostructural cobalt containing polyoxometalate water oxidation catalysts, [Co4(H2O)2(α-PW9O34)2]10- (Co4P2) and [Co4(H2O)2(α-VW9O34)2]10- (Co4V2), exhibit large differences in their catalytic performance. The substitution of phosphorus centers in Co4P2 with redox-active vanadium centers in Co4V2 leads to electronic structure modifications. Evidence for the significance of the vanadium centers to catalysis, predicted by theory, was found from soft X-ray absorption (XAS) and resonant inelastic X-ray scattering (RIXS). The XAS and RIXS spectra determine the electronic structure of the cobalt and vanadium sites in the pre-reaction state of both Co4V2 and Co4P2. High-energy resolution RIXS results reveal that Co4V2 possesses a smaller ligand field within the tetra-cobalt core and a cobalt-to-vanadium charge transfer band. The differences in electronic structures offer insights into the enhanced catalysis of Co4V2.

Stabilization of Polyoxometalate Water Oxidation Catalysts on Hematite by Atomic Layer Deposition.

Fast and earth-abundant-element polyoxometalates (POMs) have been heavily studied recently as water oxidation catalysts (WOCs) in homogeneous solution. However, POM WOCs can be quite unstable when supported on electrode or photoelectrode surfaces under applied potential. This article reports for the first time that a nanoscale oxide coating (Al2O3) applied by the atomic layer deposition (ALD) aids immobilization and greatly stabilizes this now large family of molecular WOCs when on electrode surfaces. In this study, [{RuIV4(OH)2(H2O)4}(γ-SiW10O34)2]10- (Ru4Si2) is supported on hematite photoelectrodes and then protected by ALD Al2O3; this ternary system was characterized before and after photoelectrocatalytic water oxidation by Fourier transform infrared, X-ray photoelectron spectroscopy, energy-dispersive X-ray, and voltammetry. All these studies indicate that Ru4Si2 remains intact with Al2O3 ALD protection, but not without. The thickness of the Al2O3 layer significantly affects the catalytic performance of the system: a 4 nm thick Al2O3 layer provides optimal performance with nearly 100% faradaic efficiency for oxygen generation under visible-light illumination. Al2O3 layers thicker than 6.5 nm appear to completely bury the Ru4Si2 catalyst, removing all of the catalytic activity, whereas thinner layers are insufficient to maintain a long-term attachment of the catalytic POM.

Distorted Tetrahedral CoII in K5H[CoW12O40]·xH2O Probed by 2p3d Resonant Inelastic X-ray Scattering.

The Co 2p3/2 X-ray absorption spectroscopy and high-energy-resolution (∼0.09 eV fwhm) 2p3d resonant inelastic X-ray scattering (RIXS) spectra of the single-cobalt-centered polyoxometalate K5H[CoW12O40]·xH2O were measured. The low-energy dd transition features at 0.55 eV, unmeasurable with ultraviolet-visible (UV/vis) spectroscopy, were experimentally revealed in 2p3d RIXS spectra. RIXS simulations based on ligand-field multiplet theory were performed to assess the potential cobalt tetragonal symmetry distortion, which is described with the ligand-field parameters 10Dq (-0.54 eV), Ds (-0.08 eV), and Dt (0.005 eV). Because 2p3d RIXS probes not only the optical spin-allowed transitions but also the spin-forbidden transitions, we show that the current 2p3d RIXS simulation enables a series of dd feature assignments with higher accuracy than those from previous optical data. Furthermore, by wave-function decomposition analyses, we demonstrate the more realistic and detailed origins of a few lowest dd transitions using both one-electron-orbital and term-symbol descriptions.

Transition Metal Substitution Effects on Metal-to-Polyoxometalate Charge Transfer.

A series of hetero-bimetallic transition metal-substituted polyoxometalates (TMSPs) were synthesized based on the Co(II)-centered ligand [Co(II)W11O39](10-). The eight complex series, [Co(II)(M(x)OHy)W11O39]((12-x-y)-) (M(x)OHy = V(IV)O, Cr(III)(OH2), Mn(II)(OH2), Fe(III)(OH2), Co(II)(OH2), Ni(II)(OH2), Cu(II)(OH2), Zn(II)(OH2)), of which six are reported for the first time, was synthesized starting from [Co(III)W11O39](9-) and studied using spectroscopic, electrochemical, and computational techniques to evaluate the influence of substituted transition metals on the photodynamics of the metal-to-polyoxometalate charge transfer (MPCT) transition. The bimetallic complexes all show higher visible light absorption than the plenary [Co(II)W12O40](6-) and demonstrate the same MPCT transition as the plenary complex, but they have shorter excited-state lifetimes (sub-300 ps in aqueous media). The decreased lifetimes are rationalized on the basis of nonradiative relaxation due to coordinating aqua ligands, increased interaction with cations due to increased negative charge, and the energy gap law, with the strongest single factor appearing to be the charge on the anion. The most promising results are from the Cr- and Fe-substituted systems, which retain excited-state lifetimes at least 50% of that of [Co(II)W12O40](6-) while more than tripling the absorbance at 400 nm.

All-inorganic networks and tetramer based on tin(II)-containing polyoxometalates: tuning structural and spectral properties with lone-pairs.

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.

Extending metal-to-polyoxometalate charge transfer lifetimes: the effect of heterometal location.

In an effort to develop robust molecular sensitizers for solar fuel production, the electronic structure and photodynamics of transition-metal-substituted polyoxometalates (POMs), a novel class of compound in this context, was examined. Experimental and computational techniques including femtosecond (fs) transient absorption spectroscopy have been used to study the cobalt-containing Keggin POMs, [Co(II) W12 O40 ](6-) (1 a), [Co(III) W12 O40 ](5-) (2 a), [SiCo(II) (H2 O)W11 O39 ](6-) (3 a), and [SiCo(III) (H2 O)W11 O39 ](5-) (4 a), finding the longest lived charge transfer excited state so far observed in a POM and elucidating the electronic structures and excited-state dynamics of these compounds at an unprecedented level. All species exhibit a bi-exponential decay in which early dynamic processes with time constants in the fs domain yield longer lived excited states which decay with time constants in the ps to ns domain. The initially formed states of 1 a and 3 a are considered to result from metal-to-polyoxometalate charge transfer (MPCT) from Co(II) to W, while the longer-lived excited state of 1 a is tentatively assigned to a localized intermediate MPCT state. The excited state formed by the tetrahedral cobalt(II) centered heteropolyanion (1 a) is far longer-lived (τ=420 ps in H2 O; τ=1700 ps in MeCN) than that of 3 a (τ=1.3 ps), in which the single Co(II) atom is located in a pseudo-octahedral addendum site. Short-lived states are observed for the two Co(III) -containing heteropolyanions 2 a (τ=4.4 ps) and 4 a (τ=6.3 ps) and assigned solely to O→Co(III) charge transfer. The dramatically extended lifetime for 1 a versus 3 a is ascribed to a structural change permitted by the coordinatively flexible central site, weak orbital overlap of the central Co with the polytungstate framework, and putative transient valence trapping of the excited electron on a single W atom, a phenomenon not noted previously in POMs.

Complexation of metal carbonyl units with [α-PW11O39]7- and [α2-P2W17O61]10-: insights into the chiral "twisted-sandwich" dimeric structures.

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.

Di- and tri-cobalt silicotungstates: synthesis, characterization, and stability studies.

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.

A nickel containing polyoxometalate water oxidation catalyst.

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.

Synthesis of panchromatic Ru(II) thienyl-dipyrrin complexes and evaluation of their light-harvesting capacity.

Ru(II) complexes with 5-(3-thienyl)-4,6-dipyrrin (3-TDP), containing 2,2'-bipyridine (bpy) or 4,4'-bis(methoxycarbonyl)-2,2'-bipyridine (dcmb) as coligands, have been prepared and extensively characterized. Crystal structure determination of [Ru(bpy)(2)(3-TDP)]PF(6) (1a) and [Ru(bpy)(3-TDP)(2)] (2) reveals that the 3-thienyl substituent is rotated with respect to the plane of the dipyrrinato moiety. These complexes, as well as [Ru(dcmb)(2)(3-TDP)]PF(6) (1b), act as panchromatic light absorbers in the visible range, with two strong absorption bands observable in each case. A comparison to known Ru(II) complexes and quantum-chemical calculations at the density functional theory (DFT) level indicate that the lower-energy band is due to metal-to-ligand charge transfer (MLCT) excitation, although the frontier occupied metal-based molecular orbitals (MOs) contain significant contributions from the 3-TDP moiety. The higher energy band is assigned to the π-π* transition of the 3-TDP ligand. Each complex exhibits an easily accessible one-electron oxidation. According to DFT calculations and spectroelectrochemical experiments, the first oxidation takes place at the Ru(II) center in 1a, but is shifted to the 3-TDP ligand in 1b. An analysis of MO energy diagrams suggests that complex 1b has potential to be used for light harvesting in the dye-sensitized (Grätzel) solar cell.