Computational Modeling of Di-Transition-Metal-Substituted γ-Keggin Polyoxometalate Anions. Structural Refinement of the Protonated Divacant Lacunary Silicodecatungstate

Catalytic Synthesis of Silanols by Hydroxylation of Hydrosilanes: From Chemoselectivity to Enantioselectivity

As a crucial class of functional molecules in organosilicon chemistry, silanols are found valuable applications in the fields of modern science and will be a potentially powerful framework for biologically active compounds or functional materials. It has witnessed an increasing demand for non-natural organosilanols, as well as the progress in the synthesis of these structural features. From the classic preparative methods to the catalytic selective oxidation of hydrosilanes, electrochemical hydrolysis of hydrosilanes, and then the construction of the most challenging silicon-stereogenic silanols. This review summarized the progress in the catalyzed synthesis of silanols via hydroxylation of hydrosilanes in the last decade, with a particular emphasis on the latest elegant developments in the desymmetrization strategy for the enantioselective synthesis of silicon-stereogenic silanols from dihydrosilanes.

Recent advancements in understanding the self-assembly of macroions in solution via molecular modeling

Macroions fill the gap between simple ions and colloids in size but display a completely different self-assembly behavior in solution.

A large molecular cluster with high proton release capacity

We present a single molecular polyoxometalate cluster (K₄₁[(P₂W₁₂Nb₆O₆₂)₆{Mn₃(OH)₃(H₂O)₆}₄{Mn₃Na(H₂O)₁₆}]·26H₂O) with controllable release of a large number of protons (∼40 per molecule) in its aqueous solution upon addition of a base. The deprotonation/protonation process is reversible with the clusters remaining intact. This molecule can also absorb up to 11 protons per cluster when an acid, HCl, was added to its original aqueous solution. To the best of our knowledge, such large proton absorption/release capacity along with excellent stability is unprecedented.

DFT study of α-Keggin, lacunary Keggin, and ironII-VI substituted Keggin polyoxometalates: the effect of oxidation state and axial ligand on geometry, electronic structures and oxygen transfer

Herein, the geometry, electronic structure, Fe-ligand bonding nature and simulated IR spectrum of α-Keggin, lacunary Keggin, iron(ii/iii)-substituted and the important oxidized high-valent iron derivatives of Keggin type polyoxometalates have been studied using the density functional theory (DFT/OPTX-PBE) method and natural bond orbital (NBO) analysis. The effects of different Fe oxidation states (ii-vi) and H2O/OH-/O2- ligand interactions have been addressed concerning their geometry and electronic structures. It has been revealed that the d-atomic orbitals of Fe and 2p orbitals of polyoxometalate's oxygen-atoms contribute in ligand binding. Compared with other high valent species, the considered polyoxometalate system of [PW11O39(FeVO)]4-, possesses a high reactivity for oxygen transfer.

Hydrogen peroxide activation by fluorophilic polyoxotungstates for fast and selective oxygen transfer catalysis

Polyoxometalate (POM)-based polyanionic fluorosurfactants self-assemble in hexafluoroisopropanol (HFIP) and activate hydrogen peroxide (H₂O₂), yielding efficient epoxidation of terminal alkenes.

Structure, properties and reactivity of polyoxometalates: a theoretical perspective

In the thematic review dedicated to polyoxometalate (POM) chemistry published in Chemical Reviews in 1998, no contribution was devoted to theory. This is not surprising because computational modelling of molecular metal-oxide clusters was in its infancy at that time. Nowadays, the situation has completely changed and modern computational methods have been successfully applied to study the structure, electronic properties, spectroscopy and reactivity of POM clusters. Indeed, the progress achieved during the past decade has been spectacular and herein we critically review the most important papers to provide the reader with an almost complete perspective of the field.

Divacant polyoxotungstates: reactivity of the gamma-decatungstates [γ-XW10O36]8-(X = Si, Ge)

The dilacunary, Keggin-based gamma-decatungstate ions [γ-XW(10)O(36)](8-) (X = Si, Ge) {XW(10)} exhibit an exciting and versatile solution chemistry, which is probably unmatched by any other lacunary polytungstate. The reactivity of {XW(10)} in the presence, and even absence, of electrophiles, includes loss/gain of tungsten, isomerization, and dimerization. Ever since the syntheses and structures of {XW(10)} were reported, many research groups around the world have investigated the reactivity of these polyanions towards nucleophiles (mostly d-block metal ions) and different products with various shape, size and composition were obtained. Here we provide an overview of the state-of-the-art in this subarea of polyoxometalate chemistry, with a focus on synthetic and structural aspects.

Tetrairon and Hexairon Hydroxo/Acetato Clusters Stabilized by Multiple Polyoxometalate Scaffolds. Structures, Magnetic Properties, and Chemistry of a Dimer and a Trimer

Investigation of the catalytically relevant gamma-diiron(III) Keggin complexes in aqueous acetate buffer leads to a dimeric C(2v)-symmetric polyanion, [{Fe(OH)(OAc)}(4)(gamma-SiW(10)O(36))(2)](12-) (3) and a trimeric C(2)-symmetric polyanion, [{Fe(6)(OH)(9)(H(2)O)(2)(OAc)(2)}(gamma-SiW(10)O(36))(3)](17-) (4). Polyanion 3 incorporates a hydroxo/acetato-bridged tetrairon(III) core, while 4 incorporates a trigonal prismatic hydroxo/acetato-bridged hexairon(III) core. The monomeric building unit of 3 and 4, {gamma-SiW(10)Fe(2)}, adopts the "out-of-pocket" structural motif (with two corner-sharing FeO(6) coordination polyhedra no longer connected to the internal SiO(4) tetrahedron of the Keggin unit) also observed in the {gamma-SiW(10)Fe(2)}(-)type structures isolated from nonbuffered aqueous solutions. Following hydrolysis, 3 is converted to 4 as confirmed by (29)Si NMR. Magnetic measurements establish that in both 3 and 4 all exchange interactions are antiferromagnetic.

Density functional theory and ab initio study of electronic and electrochemistry properties of the tetranuclear sandwich complex [FeIII4(H2O)2(PW9O34)2]6-

Quantum chemistry calculations have been performed to unravel the electronic and electrochemical properties of a FeIII-sandwich polyoxometalate. Using a combination of methods, it is shown that in these clusters the first reduction occurs in the so-called external Fe, which is bonded to a water ligand. Calculations also show that the electron reductions are coupled with protonation processes, in full agreement with existing experimental results.

Structural and electronic properties of hetero-transition-metal Keggin anions: a DFT Study of alpha/beta-[XW12O40]n- (X = CrVI, VV, TiIV, FeIII, CoIII, NiIII, CoII, and ZnII) relative stability

Density functional theory calculations have been carried out to investigate the electronic structures and the alpha/beta relative stability of Keggin-typed [XW(12)O(40)]n- anions with transition metal as heteroatom X (X = Cr(VI), V(V), Ti(IV), Fe(III), Co(III), Ni(III), Co(II) and Zn(II)). Nice agreement in geometries between computation and experiment has been obtained, and the higher stability of the alpha isomer over the beta one has been confirmed. Structural parameter analysis reveals that the {M(3)O(13)} triads in both alpha and beta isomers contract considerably with the increase of the negative anionic charge, while the overall size of both isomers shrinks only slightly. Fragment molecular orbital analysis shows that except alpha/beta-[TiW(12)O(40)]4-, the electronic structures of Keggin anions can be described by the insertion of the e and/or t2 orbital of XO4n- into the frontier orbitals of W(12)O(36) cage, and this leads to the specific redox property, which is different from that of the Keggin anions with main-group elements as heteroatoms. Energy decomposition analysis shows that the enhanced intrinsic stability of the alpha isomer in Td arrangement of W(12)O(36) shell and the larger deformation of the alpha over the beta isomer are two dominating factors and contribute oppositely to the alpha/beta relative stability.

Metallopyrrole-Capped Carbon Nanocones

We design nickel-doped and nitrogen-doped carbon nanocones with various amounts of buckling that feature square-planar, (approximate) tetrahedral, and octahedral coordination. The optimized geometries and electronic structures of these novel metallocarbon complexes are calculated by using the B3LYP (Gaussian03) and GGA-BLYP (ADF) exchange-correlation functionals. We analyze buckling and stability of the nanocones, and discuss their potential for additional functionalization at the metallic site.

The True Nature of the Di-iron(III) γ-Keggin Structure in Water: Catalytic Aerobic Oxidation and Chemistry of an Unsymmetrical Trimer

The complex [gamma(1,2)-SiW(10){Fe(OH(2))}(2)O(38)](6)(-) (1) has been reported to catalyze the much sought reductant-free selective O(2)-based epoxidation of alkenes (Nishiyama, Y.; Nakagawa, Y.; Mizuno, N. Angew. Chem. Int. Ed. 2001, 40, 3639-3641) in chlorocarbon-acetonitrile solution. The challenge of reproducing catalysis by 1 led us to examine this chemistry in detail. In H(2)O, a desirable solvent for catalysis, 1, does not exist in the proposed organic-medium form in which the two iron atoms are in the binding pocket defined by the equatorial oxygens and, importantly, by two oxygens bound to the central Si heteroatom. Instead, 1 in H(2)O initially forms an unusual trimer [{Fe(2)(OH)(3)(H(2)O)(2)}(3)(gamma-SiW(10)O(36))(3)](15)(-) (2). The X-ray structure of 2 shows that the Fe-O(Si) bonds are cleaved and new bonds (mu-hydroxo bridges) form between these Fe centers and those of the neighboring [gamma(1,2)-SiW(10)Fe(2)] units. Structural, physical, and computational evidence indicate that if the bonds between the d-electron center, M (Fe in the case of 1 and 2), and the terminal ligands on M are stronger than the M-O(x)() bonds, then the out-of-pocket form is more stable and is the one observed. Significantly, 2 in H(2)O forms an intermediate that catalyzes the effective aerobic oxidation of sulfur compounds (mercaptoethanol is oxidized to the corresponding disulfide by O(2) at ambient pressure and temperature). All experimental findings are consistent with dissociation of a gamma-SiW(10) Keggin unit from the trimer, 2, to form the catalytically active species.

Insights into the Mechanism of Selective Olefin Epoxidation Catalyzed by [γ-(SiO4)W10O32H4]4-. A Computational Study

A mechanism for the H2O2-based epoxidation of olefins catalyzed by the lacunary polyoxometalate (POM) [gamma-(SiO4)W10O32H4]4- (1) has been investigated at the DFT level. In this study, for the first time a "hydroperoxy" mechanism for this important process has been proposed. It is divided into two steps and investigated using the whole lacunary compound as a model. In the first step, a hydroperoxy (W-OOH) species and a water molecule are generated. The formation of this nonradical oxidant (W-OOH), consistent with the experimental suggestions, occurs with a barrier of 4.4 (7.2) kcal/mol (the number without parenthesis includes solvent effects in benzene, while the one with parenthesis is in the gas phase). In the second step, the O-O bond of the W-OOH species is cleaved, and an epoxide is formed. This step has a barrier of 38.7 (40.0) kcal/mol. It was found that the presence of one and two (CH3)4N+ countercations significantly reduces the rate-limiting barrier by 7.6 (8.3) and 11.9 (12.6) kcal/mol, respectively, and makes this lacunary POM a very efficient catalyst for epoxidation of olefins by hydrogen peroxide. It was demonstrated that the lacunary polyoxometalate basically acts as a mononuclear W(VI) complex in activating the oxidant, a conceptually noteworthy finding.

Experimental and Theoretical Study of the Regiospecific Coordination of RuII and OsII Fragments on the Lacunary Polyoxometalate [α-PW11O39]7-

New Ru(II) and Os(II) derivatives of the monovacant [alpha-PW(11)O(39)](7-) anion ([PW(11)O(39){M(DMSO)(3)(H(2)O)}](5-) (M = Ru (1), Os (2)) and [PW(11)O(39){Os(eta(6)-p-cym)(H(2)O)}](5-) (3)) have been synthesized and characterized. The binding mode of the d(6)-{M(II)L(3)(H(2)O)}(2+) moieties in these compounds is similar to that in the previously described [PW(11)O(39){Ru(eta(6)-p-cym)(H(2)O)}](5-) (4) complex: bidentate, on two nonequivalent oxygen atoms of the lacuna, leading to a loss of the C(s) symmetry of the parent anion, which thus plays the role of a prochiral bidentate ligand. The density functional theory (DFT) (B3PW91) computation of the lowest unoccupied molecular orbitals of the {ML(3)(H(2)O)}(2+) (M = Os, Ru; L(3) = fac-(DMSO)(3), eta(6)-C(6)H(6)) fragments reveals the similarities between their electrophilic properties. The origin of the regioselectivity of the grafting was investigated through a DFT (B3PW91) analysis of (i) the highest occupied molecular orbital of [alpha-PW(11)O(39)](7-) and (ii) the relative energies of the different potential regioisomers obtained by a bidentate grafting of the {ML(3)(H(2)O)}(2+) moiety onto the lacuna of [alpha-PW(11)O(39)](7-). The role of the water ligand in the stabilization of this peculiar structure was studied.

Synthesis and Structure of Dilacunary Decatungstogermanate, [γ-GeW10O36]8-

The dilacunary decatungstogermanate [gamma-GeW10O36]8- (1) has been synthesized and structurally characterized in solution and in the solid state. Reaction of germanium dioxide with sodium tungstate in aqueous acidic medium results in the formation of [beta2-GeW11O39]8- (2), which is then used as a precursor for the synthesis of 1. The (183)W spectrum of 2 shows the expected 11 peaks of equal intensity, whereas that of 1 exhibits the expected three peaks with relative intensities 2:2:1. Polyanion 1 represents a novel lacunary polyoxometalate, giving rise to a multitude of derivatives by reaction with transition metals, lanthanides, and other electrophiles.

Density Functional Study of the Roles of Chemical Composition of Di-Transition-Metal-Substituted γ-Keggin Polyoxometalate Anions

The roles of chemical composition (X, M and M(FW)) of di-transition-metal-substituted gamma-Keggin polytungstates and polymolybdates, [(X(n)(+)O(4))M(2)(OH)(2)(M(FW))(10)O(32)]((8-n)-), on the geometry, electronic structure, and magnetic properties of these species have been investigated at the density functional level. It was shown that the change of the heteroatom X via Al(III)-Si(IV)-P(V)-S(VI) slightly stabilizes the broken-symmetry (BS) state over the high-spin (HS) state, increases the antiferromagnetic coupling constant, J, of these species, and lowers the energies of their highest-occupied molecular orbitals (HOMOs) and lowest-unoccupied molecular orbitals (LUMOs). The change of the redox-active center M from Mn to Fe slightly increases the M-(XO(4)) interaction, J-coupling constant, and energy gap between the HS and BS states. Meanwhile, the LUMOs are stabilized, indicating the stronger oxidant character of [(X(n)(+)O(4))M(2)(OH)(2)W(10)O(32)]((8-n)-) for M = Fe than Mn. It was shown that the change of addenda atom M(FW) from W to Mo makes (a) the geometry of Keggin "cage" slightly smaller, (b) the interaction of redox-active centers (Fe) with the central XO(4)-unit slightly stronger, and (c) the J-coupling constant, as well as the energy gap DeltaE(BS-HS), slightly larger.

The Role of the Central Atom in Structure and Reactivity of Polyoxometalates with Adjacent d-Electron Metal Sites. Computational and Experimental Studies of γ-[(Xn+O4)RuIII2(OH)2(MFM)10O32](8-n)- for MFM = Mo and W, and X = AlIII, SiIV, PV, and SVI

The role of the central atom X in the structure and reactivity of di-Ru-substituted gamma-Keggin polyoxometalates (POMs), gamma-[(Xn+O4)RuIII2(OH)2(MFM)10O32](8-n)-), where MFM = Mo and W, and X = AlIII, SiIV, PV, and SVI., was computationally investigated. It was shown that for both MFM = Mo and W the nature of X is crucial in determining the lower lying electronic states of the polyoxoanions, which in turn likely significantly impacts their reactivity. For the electropositive X = AlIII, the ground state is a low-spin state, while for the more electronegative X = SVI the ground state is a high-spin state. In other words, the heteroatom X can be an "internal switch" for defining the ground electronic states of the gamma-M2-Keggin POMs. The obtained trends, in general, are less pronounced for MFM = Mo than for W. On the basis of the comparison of the calculated energy gaps between low-spin and high-spin states of polytungstates and polymolybdates, we predict that the gamma-M2-Keggin polytungstates could be more reactive than their polymolybdate analogues. For purposes of experimental verification the computationally predicted and evaluated polytungstate gamma-[(SiO4)RuIII2(OH)2(OH2)2W10O32]4- was prepared and characterized.

Asymmetric terminal ligation on substituted sites in a disorder-free Keggin anion, [β-SiFe2W10O36(OH)2(H2O)Cl]5–

A new monomeric diiron beta-Keggin derivative, [(CH3)2NH2]5[beta-SiFe2W10O36(OH)2(H2O)Cl].7H2O (1) with non-adjacent substituted sites has been obtained in good yield by reaction of Fe(III) cations with [gamma-SiW10O36]8- in aqueous solution. The use of hydrogen bonding counter-cations produced a disorder-free polyanion. This situation facilitates unequivocal identification of the terminal ligands on Fe atoms and interpretation of the magnetic properties, which are also addressed by DFT calculations. Electrochemical studies establish that 1 can be step-wise reduced by two electrons and then reversibly oxidized.