Polyoxometalates in solution: Molecular dynamics simulations on the alpha-PW12O40(3-) Keggin anion in aqueous media

The Keggin anion, PW12O40(3-), is one of the most representative polyoxometalates (POMs). In recent years increasing theoretical work focused on this family of compounds has explained or even predicted some of their properties using quantum mechanics methods. In this report we applied for the first time molecular dynamics (MD) to the title compound to analyze its interactions with water. We used three atomic charge definitions (Mulliken, ChelpG, and formal charges) to carry out MD simulations. The results show that the terminal oxygens of the cluster are invariably most effectively solvated by water because of their prominent position within the framework. On the other hand, bridging oxygens, which are confined in more internal positions, concentrate a smaller portion of the whole solvation. We investigated the hydrogen bonds existing between water and the cluster, confirming that the terminal positions form more contacts with H2O than any other site of the cluster. In the end, the lifetime of such contacts is longer with bridging oxygens, presumably due to their higher atomic charge.     

Construction of Heterometallic Cubanes

Incorporation of M(CO)(3) fragments by trinuclear Ti complexes [{Ti(3)Cp(μ(3)-CR)}(μ-O)(3)] and [{Ti(3)Cp(μ(3)-N)}(μ-NH)(3)] (Cp*=eta(5)-C(5)Me(5)) leads to the formation of an unprecedented class of heterometallic clusters with cubane structure [e.g., Eq. (a)]. Density functional calculations on these complexes indicate the existence of electron delocalization in the Ti(3)M cores (M=Cr, Mo, W).     

Electronic and magnetic properties of alpha-Keggin anions: A DFT study of [XM12O40](n-), (M = W, Mo; X = Al(III), Si(IV), P(V), Fe(III), Co(II), Co(III)) and [SiM11VO40](m- (M = Mo and W).

Calculations based on density functional theory (DFT) have been carried out to investigate the electronic and magnetic properties of the alpha-Keggin anions mentioned in the title. The atomic populations and the distribution of the electron density computed for the studied clusters support the hypothesis that an oxidized Keggin anion is an XO(4)(n-) clathrate inside a neutral M(12)O(36) cage. The energy gap between the band of occupied orbitals, formally delocalized over the oxo ligands, and the unoccupied d-metal orbitals, delocalized over the addenda, has been found to be independent of the central ion. However, substitution of a W or a Mo by V modifies the relative energy of the LUMO and then induces important changes in the redox properties of the cluster. In agreement with the most recent X-ray determination of [Co(III)W(12)O(40)](5-) and with the simplicity of the (183)W NMR and (17)O NMR spectra observed for this anion the calculations suggest that [Co(III)W(12)O(40)](5-) has a slightly distorted T(d ) geometry. For the parent cluster [CoW(12)O(40)](6-) the quadruplet corresponding to the anion encapsulating a Co(II) was found to be approximately 1 eV more stable than the species formed by a Co(III) and 1 e delocalized over the sphere of tungstens. The one-electron reduction of [Co(II)W(12)O(40)](6-) and [Fe(III)W(12)O(40)](5-) leads to the formation of the 1 e blue species [Co(II)W(12)O(40)](7-) and [Fe(III)W(12)O(40)](6-). The blue-iron cluster is considerably antiferromagnetic, and in full agreement with this behavior the low-spin state computed via a Broken Symmetry approach is 196 cm(-1) lower than the high-spin solution. In contrast, the cobalt blue anion has a low ferromagnetic coupling with an S-T energy gap of +20 cm(-1). This blue species is more stable than the alternative reduction product [Co(I)W(12)O(40)](7-) by more than 0.7 eV.     

Magnetochemical Complexity of Hexa‐ and Heptanuclear Wheel Complexes of Late‐3d Ions Supported by N,O‐Donor Pyridyl–Methanolate Ligands

The scaffold geometries, stability and magnetic features of the (pyridine‐2‐yl)methanolate (L) supported wheel‐shaped transition‐metal complexes with compositions [M₆L₁₂] ( 1 ), [Na?(ML₂)₆]⁺ ( 2 ), and [M′?(ML₂)₆]²⁺ ( 3 ), in which M=Co^II, Ni^II, Cu^II, and Zn^II were investigated with density functional theory (DFT). The goals of this study are manifold: 1) To advance understanding of the magnetism in the synthesized compounds [Na?(ML₂)₆]⁺ and [M′?(ML₂)₆]²⁺ that were described in Angew. Chem. Int. Ed.­ 2010 , 49, 4443 ( I ‐{Na?Ni₆}, I ‐{Ni′?Ni₆}) and Dalton Trans.­ 2011 , 40, 10526 ( II ‐{Na?Co₆}, II ‐{Co′?Co₆}); 2) To disclose how the structural, electronic, and magnetic characteristics of 1 , 2 , and 3 change upon varying M^II from d⁷ (Co²⁺) to d¹⁰ (Zn²⁺); 3) To estimate the influence of the Na⁺ and M′²⁺ ions (X^Q+) occupying the central voids of 2 and 3 on the external and internal magnetic coupling interactions in these spin structures; 4) To assess the relative structural and electrochemical stabilities of 1 , 2 , and 3 . In particular, we focus here on the net spin polarization, the determination of the strength and the sign of the exchange coupling energies, the rationalization of the nature of the magnetic coupling, and the ground‐state structures of 1 , 2 , and 3 . Our study combines the broken symmetry DFT approach and the model Hamiltonian methodology implemented in the computational framework CONDON 2.0 for the modeling of molecular spin structures, to interpret magnetic susceptibility measurements of I ‐{Na?Ni₆} and I ‐{Ni′?Ni₆}. We illustrate that whereas the structures, stability and magnetism of 1 , 2 , and 3 are indeed influenced by the nature of 3d transition‐metals in the {M₆} rims, the X^Q+ ions in the inner cavities of 2 and 3 impact these properties to an even larger degree. As exemplified by I ‐{Ni′?Ni₆}, such heptanuclear complexes exhibit ground‐state multiplets that cannot be described by simplistic model of spin‐up and spin‐down metal centers. Furthermore, we assess how future low‐temperature susceptibility measurements at high magnetic fields can augment the investigation of compound 3 with M=Co, Ni.     

Chemical reactivity of D3h C78 (metallo)fullerene: regioselectivity changes induced by Sc3N encapsulation

We report here for the first time a full comparison of the exohedral reactivity of a given fullerene and its parent trinitride template endohedral metallofullerene. In particular, we study the thermodynamics and kinetics for the Diels-Alder [4 + 2] cycloaddition between 1,3-butadiene and free D3h'-C78 fullerene and between butadiene and the corresponding endohedral D3h-Sc3N@C78 derivative. The reaction is studied for all nonequivalent bonds, in both the free and the endohedral fullerenes, at the BP86/TZP//BP86/DZP level. The change in exohedral reactivity and regioselectivity when a metal cluster is encapsulated inside the cage is profound. Consequently, the Diels-Alder reaction over the free fullerene and the endohedral derivative leads to totally different cycloadducts. This is caused by the metal nitride situated inside the fullerene cage that reduces the reactivity of the free fullerene and favors the reaction over different bonds.     

Hydration of hydrogentungstate anions at different ph conditions: a car-parrinello molecular dynamics study

Standard density functional theory calculations with a continuous model of solvation as well as Car-Parrinello molecular dynamics simulations with explicit solvent molecules are carried out to analyze the effect of the pH of the solution on the coordination sphere of the W (VI) ion. Both methodologies agree in predicting an expansion of the coordination sphere of the W (VI) ion upon a decrease in the pH. Continuous solvation models, however, are unable to predict as stable some structural isomers of a hydrated hydrogentungstate anion and tungstic acid.     

Large fullerenes stabilized by encapsulation of metallic clusters

Structures are proposed for six endohedral metallofullerenes with large carbon cages (from C(92) to C(100)) on the basis of sizeable (LUMO-4)-(LUMO-3) gap and the formal transfer of six electrons to the cages.     

Towards the computational modelling of polyoxoanions on metal surfaces: IR spectrum characterisation of [SiW12O40](4-) on Ag(111)

The adsorption of a polyoxometalate (POM) on a metal surface has been studied by means of a periodic plane wave DFT based method for the first time. In particular, we have analysed the most favourable adsorption sites of anion [SiW(12)O(40)](4-) on the Ag(111) surface and computed the infrared spectrum. Despite the intrinsic complexity of the system we have been able to fairly reproduce the observed IR spectrum and unambiguously discern the signal corresponding to W-O-Ag symmetric stretching, which appears at around 800 cm(-1).     

Polyoxometalates paneling through {Mo2O2S2} coordination: cation-directed conformations and chemistry of a supramolecular hexameric scaffold

The chemical system based on the [Mo(2)O(2)S(2)(OH(2))(6)](2+) aqua cation (noted L) and the trivacant [AsW(9)O(33)](9-) polyoxometalate (noted POM) has been investigated. Depending upon the ionic strength and the nature of the alkali cations, these complementary components assemble to yield three different architectures derived as hexamer (1), tetramer (2), and dimer (3). This series of clusters displays the same stoichiometry {POM(6)L(9)}(36-), {POM(4)L(6)}(24-), and {POM(2)L(3)}(12-) for 1, 2, and 3, respectively, and their conditions of formation differ mainly by the nature and the concentration of the alkali cation (from Li to Cs). Structural characterizations of 1 reveal a large hexameric supramolecular scaffold (about 25 ? in diameter), which encloses a large internal hole (about 200 ?(3)) filled by water molecules and alkali cations (Na(+) or K(+)). The hexameric scaffold 1 exhibits a rare flexibility property evidenced in the solid state by two distinct conformations, either eclipsed (1a) or staggered-off (1b). Both conformations appear clearly separated by a large twist angle (~40°) and depend mainly on the composition of the internal hole. Structure of anion 2 shows a tetrahedral arrangement where the four POM units and the six connecting {Mo(2)O(2)S(2)} linkers are located at the corners and at the edges, respectively. The structure of anion 3 corresponds to the simplest arrangement, described as a dimeric association of two POM units linked by three {Mo(2)S(2)O(2)} pillars. Stability of the hexameric scaffold has been investigated in solution by (183)W and (39)K NMR and by UV-vis, showing that stability of 1 depends strongly on the proportion of potassium ions, which interfere through host-guest exchange. Density functional methodology (DFT) has been applied to compute the geometries and energies of dimer (3), tetramer (2) and hexamer (1) based on {AsW(9)O(33)} (POM) and {Mo(2)O(2)S(2)} (L) units. Calculations tend to show that internal cations act as "glue" to maintain the POM units connected through the conformationally inward-directed {Mo(2)O(2)S(2)} linkers.     

Lewis Base Behavior of Bridging Nitrido Ligands of Titanium Polynuclear Complexes

The Lewis base behavior of μ₃‐nitrido ligands of the polynuclear titanium complexes [{Ti(η⁵‐C₅Me₅)(μ‐NH)}₃(μ₃‐N)] ( 1 ) and [{Ti(η⁵‐C₅Me₅)}₄(μ₃‐N)₄] ( 2 ) to MX Lewis acids has been observed for the first time. Complex 1 entraps one equivalent of copper(I ) halide or copper(I ) trifluoromethanesulfonate through the basal NH imido groups to give cube‐type adducts [XCu{(μ₃‐NH)₃Ti₃(η⁵‐C₅Me₅)₃(μ₃‐N)}] (X=Cl ( 3 ), Br ( 4 ), I ( 5 ), OSO₂CF₃ ( 6 )). However, the treatment of 1 with an excess (≥2 equiv) of copper reagents afforded complexes [XCu{(μ₃‐NH)₃Ti₃(η⁵‐C₅Me₅)₃(μ₄‐N)(CuX)}] (X=Cl ( 7 ), Br ( 8 ), I ( 9 ), OSO₂CF₃ ( 10 )) by incorporation of an additional CuX fragment at the μ₃‐N nitrido apical group. Similarly, the tetranuclear cube‐type nitrido derivative 2 is capable of incorporating one, two, or up to three CuX units at the μ₃‐N ligands to give complexes [{Ti(η⁵‐C₅Me₅)}₄(μ₃‐N)_4?n{(μ₄‐N)CuX}_n] (X=Br ( 11 ), n=1; X=Cl ( 12 ), n=2; X=OSO₂CF₃ ( 13 ), n=3). Compound 2 also reacts with silver(I ) trifluoromethanesulfonate (≥1 equiv) to give the adduct [{Ti(η⁵‐C₅Me₅)}₄(μ₃‐N)₃{(μ₄‐N)AgOSO₂CF₃}] ( 14 ). X‐ray crystal structure determinations have been performed for complexes 8 – 13 . Density functional theory calculations have been carried out to understand the nature and strength of the interactions of [{Ti(η⁵‐C₅H₅)(μ‐NH)}₃(μ₃‐N)] ( 1′ ) and [{Ti(η⁵‐C₅H₅)}₄(μ₃‐N)₄] ( 2′ ) model complexes with copper and silver MX fragments. Although coordination through the three basal NH imido groups is thermodynamically preferred in the case of 1′ , in both complexes the μ₃‐nitrido groups act as two‐electron donor Lewis bases to the appropriate Lewis acids.