A Well-defined Magnesium Complex of C706.

Controlling and understanding charge state and metal coordination in carbon nanomaterials is crucial to harnessing their unique properties. Here we describe the synthesis of the well-defined fulleride complex [{(Mesnacnac)Mg}6C70], 2, (Mesnacnac) = HC(MeCNMes)2, Mes = 2,4,6-Me3C6H2, from the reaction of the ß-diketiminate magnesium(I) complex [{(Mesnacnac)Mg}2] with C70 in aromatic solvents. The molecular structure of complex 2 was determined, providing the first high-quality structural study of a complex with the C706- ion. In combination with solution state NMR spectroscopic and DFT computational studies, the changes in geometry and charge distribution in the various atom and bond types of the fulleride unit were investigated. Additionally, the influence of the (Mesnacnac)Mg+ cations on the global and local fulleride coordination environment was examined.

Exploring the Water Oxidation Catalytic Activity of a Mn-Based Magnetic Metal-Organic Framework: The Role of Proton Conductivity and Oxygen Evolution Reaction Overpotential.

The present work evaluates the water oxidation catalytic activity of a Mn-based metal-organic framework (MOF), which we envisioned to reduce the oxygen evolution reaction (OER) overpotential because of its high electrical conductivity, facilitated by solvent-encapsulated structural features. The presence of Mn centers induces interesting magnetic features in the MOF, which exhibits impressive cryogenic magnetic refrigeration with a ΔSM value of 29.94 J kg-1 K-1 for a field change of ΔH = 5T at 2.3 K. To the best of our knowledge, the ΔSM value of the current system ranked the highest position among the published examples. The crystal structure aligns perfectly with the thematic expectations and features as many as ten metal-coordinated water molecules, forming an extensive web of a hydrogen-bonded network while facing toward the porous channel filled with another set of much-anticipated entrapped lattice water molecules. Such structural features are expected to manifest high proton conductivity, and detailed investigation indeed yields the best value for the system at 1.57 × 10-4 S/cm at 95% humidity and 85 °C. In order to evaluate the thematic notion of a one-to-one relationship between OER and improved electrical conductivity, extensive electrocatalytic water splitting (WS) investigations were carried out. The final results show highly encouraging WS ability of the Mn-MOF, showing the electrocatalytic surface area value of the active species as 0.0686 F/g with a turnover frequency value of 0.043 [(mol. O2) (mol. Mn-MOF)-1 s-1]. Another fascinating aspect of the current communication is the excellent synergy observed between the experimental WS outcomes and the corresponding theoretical data calculated using density functional theory (DFT). Consequently, a plausible mechanism of the overall OER and the role of the Mn-MOF as a water oxidation catalyst, along with the importance of water molecules, have also been derived from the theoretical calculations using DFT.

Predicting the Solubility of Inorganic Ion Pairs in Water.

Polyoxometalates (POMs), ranging in size from 1 to 10's of nanometers, resemble building blocks of inorganic materials. Elucidating their complex solubility behavior with alkali-counterions can inform natural and synthetic aqueous processes. In the study of POMs ([Nb24 O72 H9 ]15- , Nb24 ) we discovered an unusual solubility trend (termed anomalous solubility) of alkali-POMs, in which Nb24 is most soluble with the smallest (Li+ ) and largest (Rb/Cs+ ) alkalis, and least soluble with Na/K+ . Via computation, we define a descriptor (σ-profile) and use an artificial neural network (ANN) to predict all three described alkali-anion solubility trends: amphoteric, normal (Li+ >Na+ >K+ >Rb+ >Cs+ ), and anomalous (Cs+ >Rb+ >K+ >Na+ >Li+ ). Testing predicted amphoteric solubility affirmed the accuracy of the descriptor, provided solution-phase snapshots of alkali-POM interactions, yielded a new POM formulated [Ti6 Nb14 O54 ]14- , and provides guidelines to exploit alkali-POM interactions for new POMs discovery.

The Smallest Polyoxotungstate Retained by TRIS-Stabilization.

A polycondensation reaction of the orthotungstate anion WO42-, buffered at pH 7.5 in a TRIS-HCl (0.15 M) solution, results in the first example of a discrete polyoxotungstate anion, with just two W ions stabilized with TRIS ligands. It was isolated and characterized as Na2[WVI2O6(C4O3NH10)2]·6H2O by single-crystal and powder X-ray diffraction, FT-IR spectroscopy, thermogravimetrical analysis (TGA), and elemental analysis in solid state and by electro-spray ionization mass spectrometry (ESI-MS), 13C, and 183W NMR, as well as Raman spectroscopy in solution. This synthesis demonstrates the crucial and new role of the added tris-alkoxy ligand in the development of a new hybrid TRIS-isopolytungstate with the lowest known nuclearity (so far) and the terminal oxygens substituted with two nitrogen atoms arising from amines of the TRIS ligands.

Computational exploration of heterometal substitution into the decaniobate framework, [Nb10O28]6.

The factors governing the substitution of group 4B-12B metals into the decaniobate framework are explored using density functional theory in order to ascertain whether (1) recently isolated [MNb9O28]x- clusters are kinetic or thermodynamic products, (2) density functional theory is a sufficient level of theory to accurately predict substitution patterns in polyoxometalates where ion pairing and other effects may operate, and (3) it can be used to guide future synthetic efforts. Computations using restricted, unrestricted and open-shell density functional theory at PBE0/def2-tzvp were found to correctly predict substitution patterns in known clusters, and were subsequently used to calculate the relative energies of a large series of [MNb9O28]x- clusters, to reveal trends and suggest potential synthetic approaches. OPBE/def2-tzvp correctly predicted favoured spin states of known substituted decametalates.

Energetics of paramagnetic oxide clusters: the Fe(iii) oxyhydroxy Keggin ion.

The energetics of the different spin states of the five Baker-Figgis isomers of the iron(iii) Keggin ion, [Fe(O4)(Fe(OH)2(OH2))12]7+, have been investigated using density functional theory in order to demonstrate how the energy landscape of medium-to-large discrete paramagnetic transition metal oxide clusters with large numbers of antiferromagnetically coupled centres can be resolved. Antiferromagnetic coupling causes the energies to span a surprisingly large range of 30 kcal mol-1, as determined by calculating the energies of all 664 unique spin configurations based on determination of the antiferromagnetic coupling constants by density functional theory. A program which simplifies the resolution of the energetics of this type of system is also provided.

Direct Single- and Double-Side Triol-Functionalization of the Mixed Type Anderson Polyoxotungstate [Cr(OH)3W6O21]6.

Since the first successful triol-functionalization of the Anderson polyoxometalates, the six protons of their central octahedron X(OH)6 (X-heteroatom, p- or d-element) have been considered as a prerequisite for their functionalization with tripodal alcohols, and therefore, the functionalization of Anderson structures from the unprotonated sides have never been reported. Here, we describe the triol-functionalization of [Cr(OH)3W6O21]6- leading to the single-side grafted anions [Cr(OCH2)3CRW6O21]6- (CrW6-tris-R, R = -C2H5, -NH2, -CH2OH) and the unprecedented double-side functionalized anion [Cr((OCH2)3CC2H5)2W6O18]3- (CrW6-(tris-C2H5)2), despite the lack of protons in the parent anion in the solid state. CrW6-(tris-C2H5)2 demonstrates the first example of double-side functionalized Anderson POT with the partially one-side protonated corresponding parent anion. The new heteropolytungstates were characterized by single-crystal X-ray diffraction, elemental analysis, Fourier-transform infrared spectroscopy, thermal gravimetric analysis, cyclic voltammetry, and electrospray ionization mass spectrometry. Density functional theory calculations were performed to investigate and compare the stability among the different isomers of the parent anion [Cr(OH)3W6O21]6-.

Solution, Solid-State, and Computational Analysis of Agostic Interactions in a Coherent Set of Low-Coordinate Rhodium(III) and Iridium(III) Complexes.

A homologous family of low-coordinate complexes of the formulation trans-[M(2,2'-biphenyl)(PR3 )2 ][BArF4 ] (M=Rh, Ir; R=Ph, Cy, iPr, iBu) has been prepared and extensively structurally characterised. Enabled through a comprehensive set of solution phase (VT 1 H and 31 P NMR spectroscopy) and solid-state (single crystal X-ray diffraction) data, and analysis in silico (DFT-based NBO and QTAIM analysis), the structural features of the constituent agostic interactions have been systematically interrogated. The combined data substantiates the adoption of stronger agostic interactions for the IrIII compared to RhIII complexes and, with respect to the phosphine ligands, in the order PiBu3 >PCy3 >PiPr3 >PPh3 . In addition to these structure-property relationships, the effect of crystal packing on the agostic interactions was investigated in the tricyclohexylphosphine complexes. Compression of the associated cations, through inclusion of a more bulky solvent molecule (1,2-difluorobenzene vs. CH2 Cl2 ) in the lattice or collection of data at very low temperature (25 vs. 150 K), lead to small but statistically significant shortening of the M-H-C distances.

Heavier Group 13 Metal(I) Heterocycles Stabilized by Sterically Demanding Diiminophosphinates: A Structurally Characterized Monomer-Dimer Pair For Gallium.

We have synthesized and characterized the monomeric diiminophosphinate-stabilized Group 13 metal(I) complexes [Dip LE:], Dip L=Ph2 P(NDip)2 , Dip=2,6-iPr2 C6 H3 ; E=Ga (1), In (2) and Tl (3). In addition, we structurally characterized the dimeric complex [(Dip LGa)2 ], 12 . Similar synthetic attempts using Mes L=Ph2 P(NMes)2 , Mes=2,4,6-Me3 C6 H2 afforded product mixtures from which the mixed oxidation state species [(Mes L)3 Ga4 I3 ] 4 was isolated. [Dip LGa:] 1 is converted with dry air to the gallium(III) oxide species [(Dip LGaO)2 ] 5. Density Functional Theory studies on [Dip LE:] and [(Dip LE)2 ], E=Al-Tl, shed light on the bonding in these compounds and show that the newly formed E-E bonding interactions can be described as weak single σ-bond with no significant π-bonding contribution for E=Al, Ga. A large contribution to the dimer binding enthalpies results from London dispersion forces.

Non-Aqueous Microwave-Assisted Syntheses of Deca- and Hexa-Molybdovanadates.

We report a new approach for the synthesis of heterohexa- and heterodecametalates via the use of non-aqueous, microwave-assisted reaction conditions. The two novel molybdovanadates have been isolated and characterized in the solid and solution states using single-crystal X-ray diffraction, FT-IR, UV/Vis, multinuclear NMR spectroscopy, and ESI-MS. The relative stabilities of the possible structural isomers were probed using dispersion-corrected DFT calculations for both polyoxometalate systems.

Solid-State 27Al NMR Spectroscopy of the γ-Al13 Keggin Containing Al Coordinated by a Terminal Hydroxyl Ligand.

We report solid-state 27Al NMR spectroscopic results for the sulfate salt of the γ-Al13 Keggin cluster, γ-[AlO4Al12(OH)25(OH2)11][SO4]3·[H2O]14, that provide a spectroscopic signature for partial hydrolysis of this Keggin-type cluster. In 27Al multiple-quantum magic-angle spinning NMR spectra, all 13 Al positions of the cluster are at least partially resolved and assigned with the aid of density functional theory (DFT) calculations of the 27Al electric field gradients. The isotropic chemical shift of the single tetrahedral site, 75.7 ppm, is nearly identical to that reported for solutions from which the cluster crystallizes. Reflecting broadly similar coordination environments, the octahedral Al show mostly small variations in isotropic chemical shift (+7 to +11 ppm) and quadrupolar coupling constant (CQ; 6-7.5 MHz), except for one resonance that exhibits a much smaller CQ and another site with a larger value. DFT calculations show that deprotonation of a terminal water ligand, to form an η-OH group, causes a large reduction in the 27Al CQ, allowing assignment of a distinct, narrow peak for octahedral Al to this hydroxyl-terminated site. This result suggests a relationship between octahedral 27Al NMR line width and hydrolysis for solids prepared from Keggin-type clusters.

Predicting (17)O NMR chemical shifts of polyoxometalates using density functional theory.

We have investigated the computation of (17)O NMR chemical shifts of a wide range of polyoxometalates using density functional theory. The effects of basis sets and exchange-correlation functionals are explored, and whereas pure DFT functionals generally predict the chemical shifts of terminal oxygen sites quite well, hybrid functionals are required for the prediction of accurate chemical shifts in conjunction with linear regression. By using PBE0/def2-tzvp//PBE0/cc-pvtz(H-Ar), lanl2dz(K-) we have computed the chemical shifts of 37 polyoxometalates, corresponding to 209 (17)O NMR signals. We also show that at this level of theory the protonation-induced pH dependence of the chemical shift of the triprotic hexaniobate Lindqvist anion, [HxNb6O19]((8-x)), can be reproduced, which suggests that hypotheses regarding loci of protonation can be confidently tested.

Application of the tris(acetylacetonato)iron(III)/(II) redox couple in p-type dye-sensitized solar cells.

An electrolyte based on the tris(acetylacetonato)iron(III)/(II) redox couple ([Fe(acac)3](0/1-)) was developed for p-type dye-sensitized solar cells (DSSCs). Introduction of a NiO blocking layer on the working electrode and the use of chenodeoxycholic acid in the electrolyte enhanced device performance by improving the photocurrent. Devices containing [Fe(acac)3](0/1-) and a perylene-thiophene-triphenylamine sensitizer (PMI-6T-TPA) have the highest reported short-circuit current (J(SC)=7.65 mA cm(-2)), and energy conversion efficiency (2.51%) for p-type DSSCs coupled with a fill factor of 0.51 and an open-circuit voltage V(OC)=645 mV. Measurement of the kinetics of dye regeneration by the redox mediator revealed that the process is diffusion limited as the dye-regeneration rate constant (1.7×10(8) M(-1) s(-1)) is very close to the maximum theoretical rate constant of 3.3×10(8) M(-1) s(-1). Consequently, a very high dye-regeneration yield (>99%) could be calculated for these devices.

Introducing manganese complexes as redox mediators for dye-sensitized solar cells.

The abundance and low toxicity of manganese have led us to explore the application of manganese complexes as redox mediators for dye sensitized solar cells (DSCs), a promising solar energy conversion technology which mimics some of the key processes in photosynthesis during its operation. In this paper, we report the development of a DSC electrolyte based on the tris(acetylacetonato)manganese(iii)/(iv), [Mn(acac)3](0/1+), redox couple. PEDOT-coated FTO glass was used as a counter electrode instead of the conventionally used platinum. The influence of a number of device parameters on the DSC performance was studied, including the concentration of the reduced and oxidized mediator species, the concentration of specific additives (4-tert-butylpyridine, lithium tetrafluoroborate, and chenodeoxycholic acid) and the thickness of the TiO2 working electrode. These studies were carried out with a new donor-π-acceptor sensitizer K4. Maximum energy conversion efficiencies of 3.8% at simulated one Sun irradiation (AM 1.5 G; 1000 W m(-2)) with an open circuit voltage (VOC) of 765 mV, a short-circuit current (JSC) of 7.8 mA cm(-2) and a fill factor (FF) of 0.72 were obtained. Application of the commercially available MK2 and N719 sensitizers resulted in an energy conversion efficiency of 4.4% with a VOC of 733 mV and a JSC of 8.6 mA cm(-2) for MK2 and a VOC of 771 mV and a JSC of 7.9 mA cm(-2) for N719. Both dyes exhibit higher incident photon to current conversion efficiencies (IPCEs) than K4.

A high-pressure NMR probe for aqueous geochemistry.

A non-magnetic piston-cylinder pressure cell is presented for solution-state NMR spectroscopy at geochemical pressures. The probe has been calibrated up to 20 kbar using in situ ruby fluorescence and allows for the measurement of pressure dependencies of a wide variety of NMR-active nuclei with as little as 10 µL of sample in a microcoil. Initial (11)B NMR spectroscopy of the H3BO3-catechol equilibria reveals a large pressure-driven exchange rate and a negative pressure-dependent activation volume, reflecting increased solvation and electrostriction upon boron-catecholate formation. The inexpensive probe design doubles the current pressure range available for solution NMR spectroscopy and is particularly important to advance the field of aqueous geochemistry.

A soluble phosphorus-centered Keggin polyoxoniobate with bicapping vanadyl groups.

A water-soluble tetramethylammonium (TMA) salt of a novel Keggin-type polyoxoniobate has been isolated as TMA9[PV2Nb12O42]·19H2O (1). This species contains a central phosphorus site and two capping vanadyl sites. Previously only a single example of a phosphorus-containing polyoxoniobate, [(PO2)3PNb9O34](15-), was known, which is a lacunary Keggin ion decorated with three PO2 units. However, that cluster was isolated as an insoluble structure consisting of chains linked by sodium counterions. In contrast, the [PV2Nb12O42](9-) cluster in 1 is stable over a wide pH range, as evident by (31)P and (51)V NMR, UV/Vis spectroscopy, and ESI-MS spectrometry. The ease of substitution of phosphate into the central tetrahedral position suggests that other oxoanions can be similarly substituted, promising a richer set of structures in this class.

Dynamics of a nanometer-sized uranyl cluster in solution.

A class of uranyl peroxide clusters was discovered before as nanometer-sized ions that form spontaneously in aqueous solutions. The uranyl(VI) cluster investigated here is approximately 2 nm in diameter, contains 24 uranyl moieties, and 12 pyrophosphate units. NMR spectroscopy shows that the ion has two distinct forms that interconvert in milliseconds to seconds depending on the temperature and the size of the counterions. P blue, O red, U yellow.

Rates of water exchange on the [Fe4(OH)2(hpdta)2(H2O)4]0 molecule and its implications for geochemistry.

The ammonium salt of [Fe(4)O(OH)(hpdta)(2)(H(2)O)(4)](-) is soluble and makes a monospecific solution of [Fe(4)(OH)(2)(hpdta)(2)(H(2)O)(4)](0)(aq) in acidic solutions (hpdta = 2-hydroxypropane-1,3-diamino-N,N,N',N'-tetraacetate). This tetramer is a diprotic acid with pK(a)(1) estimated at 5.7 ± 0.2 and pK(a)(2) = 8.8(5) ± 0.2. In the pH region below pK(a)(1), the molecule is stable in solution and (17)O NMR line widths can be interpreted using the Swift-Connick equations to acquire rates of ligand substitution at the four isolated bound water sites. Averaging five measurements at pH < 5, where contribution from the less-reactive conjugate base are minimal, we estimate: k(ex)(298) = 8.1 (±2.6) × 10(5) s(-1), ΔH(++) = 46 (±4.6) kJ mol(-1), ΔS(++) = 22 (±18) J mol(-1) K(-1), and ΔV(++) = +1.85 (±0.2) cm(3) mol(-1) for waters bound to the fully protonated, neutral molecule. Regressing the experimental rate coefficients versus 1/[H(+)] to account for the small pH variation in rate yields a similar value of k(ex)(298) = 8.3 (±0.8) × 10(5) s(-1). These rates are ∼10(4) times faster than those of the [Fe(OH(2))(6)](3+) ion (k(ex)(298) = 1.6 × 10(2) s(-1)) but are about an order of magnitude slower than other studied aminocarboxylate complexes, although these complexes have seven-coordinated Fe(III), not six as in the [Fe(4)(OH)(2)(hpdta)(2)(H(2)O)(4)](0)(aq) molecule. As pH approaches pK(a1), the rates decrease and a compensatory relation is evident between the experimental ΔH(++) and ΔS(++) values. Such variation cannot be caused by enthalpy from the deprotonation reaction and is not well understood. A correlation between bond lengths and the logarithm of k(ex)(298) is geochemically important because it could be used to estimate rate coefficients for geochemical materials for which only DFT calculations are possible. This molecule is the only neutral, oxo-bridged Fe(III) multimer for which rate data are available.

Molecular Parameters Promoting High Relaxivity in Cluster-Nanocarrier Magnetic Resonance Imaging Contrast Agents.

We have investigated the mechanism of relaxivity for two magnetic resonance imaging contrast agents that both employ a cluster-nanocarrier design. The first system termed Mn8Fe4-coPS comprises the cluster Mn8Fe4O12(L)16(H2O)4 or Mn8Fe4 (1) (L = carboxylate) co-polymerized with polystyrene to form ∼75 nm nanobeads. The second system termed Mn3Bpy-PAm used the cluster Mn3(O2CCH3)6(Bpy)2 or Mn3Bpy (2) where Bpy = 2,2'-bipyridine, entrapped in ∼180 nm polyacrylamide nanobeads. Here, we investigate the rate of water exchange of the two clusters, and corresponding cluster-nanocarriers, in order to elucidate the mechanism of relaxivity in the cluster-nanocarrier. Swift-Connick analysis of O-17 NMR was used to determine the water exchange rates of the clusters and cluster-nanocarriers. We found distinct differences in the water exchange rate between Mn8Fe4 and Mn8Fe4-coPS, and we utilized these differences to elucidate the nanobead structure. Using the transverse relaxivity from O-17 NMR line widths, we were able to determine the hydration state of the Mn3Bpy (2) cluster as well as Mn3Bpy-PAm. Using these hydration states in the Swift-Connick analysis of O-17 NMR, we found the water exchange rate to be extremely close in value for the cluster Mn3Bpy and cluster-nanocarrier Mn3Bpy-PAm.

Electronic structure description of a [Co(III)3Co(IV)O4] cluster: a model for the paramagnetic intermediate in cobalt-catalyzed water oxidation.

Multifrequency electron paramagnetic resonace (EPR) spectroscopy and electronic structure calculations were performed on [Co(4)O(4)(C(5)H(5)N)(4)(CH(3)CO(2))(4)](+) (1(+)), a cobalt tetramer with total electron spin S = 1/2 and formal cobalt oxidation states III, III, III, and IV. The cuboidal arrangement of its cobalt and oxygen atoms is similar to that of proposed structures for the molecular cobaltate clusters of the cobalt-phosphate (Co-Pi) water-oxidizing catalyst. The Davies electron-nuclear double resonance (ENDOR) spectrum is well-modeled using a single class of hyperfine-coupled (59)Co nuclei with a modestly strong interaction (principal elements of the hyperfine tensor are equal to [-20(±2), 77(±1), -5(±15)] MHz). Mims (1)H ENDOR spectra of 1(+) with selectively deuterated pyridine ligands confirm that the amount of unpaired spin on the cobalt-bonding partner is significantly reduced from unity. Multifrequency (14)N ESEEM spectra (acquired at 9.5 and 34.0 GHz) indicate that four nearly equivalent nitrogen nuclei are coupled to the electron spin. Cumulatively, our EPR spectroscopic findings indicate that the unpaired spin is delocalized almost equally across the eight core atoms, a finding corroborated by results from DFT calculations. Each octahedrally coordinated cobalt ion is forced into a low-spin electron configuration by the anionic oxo and carboxylato ligands, and a fractional electron hole is localized on each metal center in a Co 3d(xz,yz)-based molecular orbital for this essentially [Co(+3.125)(4)O(4)] system. Comparing the EPR spectrum of 1(+) with that of the catalyst film allows us to draw conclusions about the electronic structure of this water-oxidation catalyst.