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Counterions are deemed “spectators” in aqueous solutions of cationic or anionic molecular metal-oxo clusters. While pH and concentration drive aqueous metal speciation as a first approximation, the important effect of counterions is usually overlooked and never considered in standard Pourbaix databases. Alkali counterions for polyoxometalate (POM) clusters control solubility with distinct periodic trends, but evidence for alkali control over speciation is ambiguous. Here we show that a simple Nb-POM, [Nb10O28]6– ({Nb10}), converts to oligomers of (HxNb24O72)(24–x)– ({Nb24}) upon adding only alkali chloride salts, even in buffered neutral solutions. Raman and X-ray scattering reveal that the rate of {Nb10} to {Nb24} conversion increases with alkali cation radius and cation concentration. Cation-bridged oligomers of {Nb24}y (y = 2,4) are defined by comparing experimental to computed small-angle X-ray scattering spectra. Computational studies and mass spectrometry indicate that the alkalis open the compact {...

We report experimental and theoretical evidence of restrained axial rotation for heteroleptic L2·Ir(I)·1,5-cyclooctadiene (COD) complexes included in the aromatic cavity of Rebek's self-folding octaamide cavitand. At 298 K, the axial spinning motion of the included organometallic guests was slow on the (1)H NMR time scale and produced a proton spectrum for the bound host indicative of C2 symmetry. Signals corresponding to aromatic protons of the bound host coalesced at 323 K, indicating that the spinning process of the included guest became fast on the (1)H NMR time scale and that the complex approached C4 symmetry. Surprisingly, lowering the temperature of the solution to 193 K induced an additional splitting of the proton signals observed at room temperature for both the bound host and the included guest. We propose the emergence of a new element of chirality in the complexes, which was associated with a slow interconversion, on the (1)H NMR time scale between the two chiral twisted-boat conformers of the chelated COD included in the already chiral cavity of the container. This leads to the inclusion complexes existing in solution as pairs of two racemic diastereomers. We estimated that the racemization barrier for the two cyclochiral conformers of the Ir(I) chelated COD was 5 kcal mol(-1) higher as an included organometallic complex than as free in solution. Furthermore, we performed a van't Hoff plot and determined that the inclusion of the organometallic complex in the cavitand was endothermic and exclusively driven by entropy (ΔH = 5.9 kcal mol(-1) and ΔS = 33.9 cal mol(-1) K(-1)).

The 30 cationic {(Mo2O4)-O-V(acetate)}(+) units linking 12 negatively charged pentagonal "ligands," {(Mo-VI)-(Mo5O21)-O-VI(H2O)(6)}(6-) of the porous metal-oxide capsule, [{(Mo6O21)-O-VI(H2O)(6)}(12){(Mo2O4)-O-V(acetate)}(30)](42-) provide active sites for catalytic transformations of organic "guests". This is demonstrated using a well-behaved model reaction, the fully reversible cleavage and formation of methyl tert-butyl ether (MTBE) under mild conditions in water. Five independent lines of evidence demonstrate that reactions of the MTBE guests occur in the ca. 6 x 10(3) angstrom(3) interior of the spherical capsule. The Mo atoms of the {(Mo2O4)-O-V(acetate)}(+) linkers-spanning an ca. 3-nm truncated icosahedron-are sterically accessible to substrate, and controlled removal of their internally bound acetate ligands generates catalytically active {(Mo2O4)-O-V(H2O)(2)}(2+) units with labile water ligands, and Lewis- and Bronsted-acid properties. The activity of these units is demonstrating by kinetic data that reveal a first-order dependence of MTBE cleavage rates on the number of acetate-free {(Mo2O4)-O-V(H2O)(2)}(2+) linkers. DFT calculations point to a pathway involving both Mo(V) centers, and the intermediacy of isobutene in both forward and reverse reactions. A plausible catalytic cycle satisfying microscopic reversibility is supported by activation parameters for MTBE cleavage, deuterium and oxygen-18 labeling studies, and by reactions of deliberately added isobutene and of a water-soluble isobutene analog. More generally, pore-restricted encapsulation, ligand-regulated access to multiple structurally integral metal-centers, and options for modifying the microenvironment within this new type of nanoreactor, suggest numerous additional transformations of organic substrates by this and related molybdenum-oxide based capsules.

Uranyl-peroxide nanoclusters display different topologies based on square, pentagonal and hexagonal building blocks. Computed complexation energies of different cations (Li(+), Na(+), K(+), Rb(+), and Cs(+)) with [UO(2)(O(2))(H(2)O)](n) (n = 4, 5, and 6) macrocycles suggest a strong cation templating effect. The inherent bent structure of a U-O(2)-U model dimer is demonstrated and justified through the analysis of its electronic structure, as well as of the inherent curvature of the four-, five-, and six-uranyl macrocyles. The curvature is enhaced by cation coordination, which is suggested to be the driving force for the self-assembly of the nanocapsules.

The Preyssler anion, of general formula [Xn+P5W30O110](15-n)-, is the smallest polyoxometalate (POM) with an internal cavity allowing cation exchange with the solution. The Preyssler anion features a rich chemistry evidenced by its ability to accept electrons at low potentials, to selectively capture various metal cations, and to undergo acid−base reactions. A deep understanding of these topics is herein provided by means of DFT calculations on the title series of compounds. We evaluate the energetics of the release/encapsulation process for several Xn+ cations and identify the effect of the encapsulated ion on the properties of the Preyssler anion. We revisit the relationship between the internal cation charge and the electrochemical behavior of the POM. A linear dependence between the first one-electron reduction energies and the encapsulated Xn+ charge is found, with a slope of 48 mV per unit charge. The protonation also shifts the reduction potential to more positive values, but the effect is much lar...

The origin of regioselectivity in rhodium diphosphine catalyzed hydroformilation was investigated by means of hybrid QM/MM calculations using the IMOMM method. The roles of the diphosphine bite angle and of the nonbonding interactions were analyzed in detail by considering rhodium systems containing xantphos-type ligands, for which a correlation between the natural bite angle and regioselectivity has been recently reported. From the pentacoordinated equatorial--equatorial HRh(CO)(alkene)(diphosphine) key intermediate, eight possible reaction paths were defined and characterized through their respective transition states (TS). We succeeded in reproducing the experimentally observed trends for the studied diphosphines. By performing additional calculations on model systems, in which the steric effects induced by the phenyl substituents of xantphos ligands were canceled, we were able to separate, identify, and evaluate the different contributions to regioselectivity. These additional calculations showed that regioselectivity is governed by the nonbonding interactions between the diphenylphosphino substituents and the substrate, whereas the effects directly associated to the bite angle, what we call orbital effects, seem to have a smaller influence.

Calculations based on density functional theory (DFT) have been carried out to investigate the electronic and magnetic properties of the α-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 XO4n- clathrate inside a neutral M12O36 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 [CoIIIW12O40]5- and with the simplicity of the 183W NMR and 17O NMR spectra observed for this anion the calculations suggest that [CoIIIW12O40]5- has a slightly distorted Td geometry. For the parent clust...

A receptor for fullerenes featuring three exTTF units linked to a CTV scaffold is described. The exTTF-CTV host forms remarkably stable complexes with both C(60) (log K(a) = 5.3 +/- 0.2) and C(70) (log K(a) = 6.3 +/- 0.6). Light-induced ESR spectra demonstrate that intracomplex PET processes take place in solution.

The electronic effect in the rhodium diphosphine catalyzed hydroformylation was investigated. A series of electronically modified thixantphos ligands was synthesized, and their effects on coordination chemistry and catalytic performance were studied. Phosphine basicity was varied by using p-(CH3)2N, p-CH3O, p-H, p-F, p-Cl, or p-CF3 substituents on the diphenylphosphine moieties. X-ray crystal structure determinations of the complexes (thixantphos)Rh(CO)H(PPh3) and (p-CH3O-thixantphos)Rh(CO)H(PPh3) were obtained. The solutions structures of the (diphosphine)Rh(CO)H(PPh3) and (diphosphine)Rh(CO)2H complexes were studied by IR and NMR spectroscopy. IR and 1H NMR spectroscopy showed that the (diphosphine)Rh(CO)2H complexes consist of dynamic equilibria of diequatorial (ee) and equatorial−apical (ea) isomers. The equilibrium compositions proved to be dependent on phosphine basicity; the ee:ea isomer ratio shifts gradually from almost one for the p-(CH3)2N-substituted ligand to more than nine for the p-CF3-subs...

The Preyssler anion, of general formula [Xn+P5W30O110](15-n)-, is the smallest polyoxometalate (POM) with an internal cavity allowing cation exchange with the solution. The Preyssler anion features a rich chemistry evidenced by its ability to accept electrons at low potentials, to selectively capture various metal cations, and to undergo acid-base reactions. A deep understanding of these topics is herein provided by means of DFT calculations on the title series of compounds. We evaluate the energetics of the release/encapsulation process for several Xn+ cations and identify the effect of the encapsulated ion on the properties of the Preyssler anion. We revisit the relationship between the internal cation charge and the electrochemical behavior of the POM. A linear dependence between the first one-electron reduction energies and the encapsulated Xn+ charge is found, with a slope of 48 mV per unit charge. The protonation also shifts the reduction potential to more positive values, but the effect is much larger. In connection to this, the last proton's pKa = 2 for the Na+ derivative was estimated to be in reasonable agreement with experiment. The electronic structure of lanthanide derivatives is more complex than conventional POM structures. The reduction energy for the CeIV-Preyssler + 1e- --CeIII-Preyssler process was computed to be more exothermic than that of very oxidant species such as S2Mo18O624-.

Keggin heteropolyanions [XM(12)O(40)](n-) have various isomeric structures, alpha and beta being the most common. Conventionally, the alpha structure appears to be the most stable, but calculations carried out at the DFT level for X = P(V), Si(IV), Al(III), As(V), Ge(IV), and Ga(III) and M = W(VI) and Mo(VI) show that this stability depends on several factors, particularly on the nature of the heteroatom (X) and the total charge of the cluster. In this paper, we apply the clathrate model to the Keggin molecule to carry out a fragment-interaction study to elucidate when and why the traditional relative stability of various isomers can be inverted. The fully oxidized anions that have inverted the traditional stability trend in this series are [AlW(12)O(40)](5-) and [GaW(12)O(40)](5-), both of which contain a third-group heteroatom and an overall charge of -5. beta-isomers are always more easily reduced than alpha-isomers. This experimental observation suggests that reduction favors the stability of beta-isomers and one of the most important results of this study is that the alpha/beta inversion is achieved in most cases after the second reduction. The alpha- and beta-isomers may have different properties because the energy of the LUMO, a symmetry-adapted d(xy)-metal orbital, is different.