Gated and Differently Functionalized (New) Porous Capsules Direct Encapsulates' Structures: Higher and Lower Density Water

Either more or less neighbors favored? By deliberate choice of the internal ligands of the porous nanocapsules [{(Mo)Mo₅}₁₂{Mo₂(ligand)}₃₀]^n?, the respective cavities can be differently sized/functionalized. This allows one to trap, in two different corresponding capsules, 100 water molecules arranged in shells that exhibit more and less dense packing, respectively—the latter type in a capsule with smaller internal ligands; the picture shows the related 100 O atoms at the vertices of the four polyhedra, while two dodecahedra (shown in pale green and red) have only 10 H₂O molecules instead of 20.

     

Ab initio study of the singlet-triplet splitting in reduced polyoxometalates

A valence bond analysis of the wave function of doubly reduced polyoxometales is presented, using the M₆O₁₉ Lindqvist structure as test case. By a unitary transformation of the delocalised valence orbitals to localised metal centred orbitals, the multiconfigurational wave function is mapped onto a valence bond function with three different types of configurations: the two electrons are on the same site, on neighbouring sites, or on next-nearest neighbour sites. The inspection of the relative weights of these configurations for triplet and singlet state shows that the triplet-coupled electrons are confined to a smaller volume, and hence have a higher energy than the singlet-coupled electrons. This is in line with the experimental observation that the doubly reduced polyoxometalates show non-mangetic behaviour.     

Ammonia activation by μ3-alkylidyne fragments supported on a titanium molecular oxide model

Ammonolysis of the μ(3)-alkylidyne derivatives [{Ti(η(5)-C(5)Me(5))(μ-O)}(3)(μ(3)-CR)] [R = H (1), Me (2)] produces a trinuclear oxonitride species, [{Ti(η(5)-C(5)Me(5))(μ-O)}(3)(μ(3)-N)] (3), via methane or ethane elimination, respectively. During the course of the reaction, the intermediates amido μ-alkylidene [{Ti(η(5)-C(5)Me(5))(μ-O)}(3)(μ-CHR)(NH(2))] [(R = H (4), Me (5)] and μ-imido ethyl species [{Ti(η(5)-C(5)Me(5))(μ-O)}(3)(μ-NH)Et] (6) were characterized and/or isolated. This achievement constitutes an example of characterization of the three steps of successive activation of N-H bonds in ammonia within the same transition-metal molecular system. The N-H σ-bond activation of ammonia by the μ(3)-alkylidyne titanium species has been theoretically investigated by DFT method on [{Ti(η(5)-C(5)H(5))(μ-O)}(3)(μ(3)-CH)] model complex. The calculations complement the characterization of the intermediates, showing the multiple bond character of the terminal amido and the bridging nature of imido ligand. They also indicate that the sequential ammonia N-H bonds activation process goes successively downhill in energy and occurs via direct hydron transfer to the alkylidyne group on organometallic oxides 1 and 2. The mechanism can be divided into three stages: (i) coordination of ammonia to a titanium center, in a trans disposition with respect to the alkylidyne group, and then the isomerization to adopt the cis arrangement, allowing the direct hydron migration to the μ(3)-alkylidyne group to yield the amido μ-alkylidene complexes 4 and 5, (ii) hydron migration from the amido moiety to the alkylidene group, and finally (iii) hydron migration from the μ-imido complex to the alkyl group to afford the oxo μ(3)-nitrido titanium complex 3 with alkane elimination.     

The shape of the Sc2(μ2-S) unit trapped in C82: crystallographic, computational, and electrochemical studies of the isomers, Sc2(μ2-S)@C(s)(6)-C82 and Sc2(μ2-S)@C(3v)(8)-C82

Single-crystal X-ray diffraction studies of Sc(2)(μ(2)-S)@C(s)(6)-C(82)·Ni(II)(OEP)·2C(6)H(6) and Sc(2)(μ(2)-S)@C(3v)(8)-C(82)·Ni(II)(OEP)·2C(6)H(6) reveal that both contain fully ordered fullerene cages. The crystallographic data for Sc(2)(μ(2)-S)@C(s)(6)-C(82)·Ni(II)(OEP)·2C(6)H(6) show two remarkable features: the presence of two slightly different cage sites and a fully ordered molecule Sc(2)(μ(2)-S)@C(s)(6)-C(82) in one of these sites. The Sc-S-Sc angles in Sc(2)(μ(2)-S)@C(s)(6)-C(82) (113.84(3)°) and Sc(2)(μ(2)-S)@C(3v)(8)-C(82) differ (97.34(13)°). This is the first case where the nature and structure of the fullerene cage isomer exerts a demonstrable effect on the geometry of the cluster contained within. Computational studies have shown that, among the nine isomers that follow the isolated pentagon rule for C(82), the cage stability changes markedly between 0 and 250 K, but the C(s)(6)-C(82) cage is preferred at temperatures ≥250 °C when using the energies obtained with the free encapsulated model (FEM). However, the C(3v)(8)-C(82) cage is preferred at temperatures ≥250 °C using the energies obtained by rigid rotor-harmonic oscillator (RRHO) approximation. These results corroborate the fact that both cages are observed and likely to trap the Sc(2)(μ(2)-S) cluster, whereas earlier FEM and RRHO calculations predicted only the C(s)(6)-C(82) cage is likely to trap the Sc(2)(μ(2)-O) cluster. We also compare the recently published electrochemistry of the sulfide-containing Sc(2)(μ(2)-S)@C(s)(6)-C(82) to that of corresponding oxide-containing Sc(2)(μ(2)-O)@C(s)(6)-C(82).     

Product formation in the Prato reaction on Sc3N@D(5h)-C80: preference for [5,6]-bonds, and not pyracylenic bonds

We report here reaction energies and NMR data for the N-tritylpyrrolidino mono-adducts of Sc(3)N@D(5h)-C(80). The pyracylene adduct that was previously suggested to be important for the Prato reaction is ruled out both from energetic and NMR points of view.     

Redox properties of polyoxometalates: new insights on the anion charge effect

In this paper we study the electronic structure of Lindqvist, Keggin, Dawson and Preyssler polyoxometalates (POMs) at the DFT level, particularly their LUMOs and reduction energies. Our aim was to revisit the previously reported evidence that a linear relationship exists between reduction potentials and molecular charges in Keggin anions. In this line of thought, we calculated one simple structural parameter-volume of the clusters-so that the corresponding volume charge density, rho(v), could be estimated. Contrary to what we expected, the connection between rho(v) and the experimental reduction potentials is not evident since q/V itself does not justify the scale of oxidizing powers. Complementary calculations were performed using the clathrate model, anion@W(m)O(3m), analyzing separately the effects of the size of the neutral cages and the molecular charge, q, upon the redox properties. The parameter m emulates the size (volume) of the clusters, only approximately, but with the benefit that it is easily accounted for. A linear relationship was found between the difference in LUMO energies for the neutral and charged clusters and the q/m ratio.     

(99)Tc and re incorporated into metal oxide polyoxometalates: oxidation state stability elucidated by electrochemistry and theory

The radioactive element technetium-99 ((99)Tc, half-life = 2.1 × 10(5) years, β(-) of 253 keV), is a major byproduct of (235)U fission in the nuclear fuel cycle. (99)Tc is also found in radioactive waste tanks and in the environment at National Lab sites and fuel reprocessing centers. Separation and storage of the long-lived (99)Tc in an appropriate and stable waste-form is an important issue that needs to be addressed. Considering metal oxide solid-state materials as potential storage matrixes for Tc, we are examining the redox speciation of Tc on the molecular level using polyoxometalates (POMs) as models. In this study we investigate the electrochemistry of Tc complexes of the monovacant Wells-Dawson isomers, α(1)-P(2)W(17)O(61)(10-) (α1) and α(2)-P(2)W(17)O(61)(10-) (α2) to identify features of metal oxide materials that can stabilize the immobile Tc(IV) oxidation state accessed from the synthesized Tc(V)O species and to interrogate other possible oxidation states available to Tc within these materials. The experimental results are consistent with density functional theory (DFT) calculations. Electrochemistry of K(7-n)H(n)[Tc(V)O(α(1)-P(2)W(17)O(61))] (Tc(V)O-α1), K(7-n)H(n)[Tc(V)O(α(2)-P(2)W(17)O(61))] (Tc(V)O-α2) and their rhenium analogues as a function of pH show that the Tc-containing derivatives are always more readily reduced than their Re analogues. Both Tc and Re are reduced more readily in the lacunary α1 site as compared to the α2 site. The DFT calculations elucidate that the highest oxidation state attainable for Re is VII while, under the same electrochemistry conditions, the highest oxidation state for Tc is VI. The M(V)→ M(IV) reduction processes for Tc(V)O-α1 are not pH dependent or only slightly pH dependent suggesting that protonation does not accompany reduction of this species unlike the M(V)O-α2 (M = (99)Tc, Re) and Re(V)O-α1 where M(V/IV) reduction process must occur hand in hand with protonation of the terminal M═O to make the π*(M═O) orbitals accessible to the addition of electrons. This result is consistent with previous extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) data that reveal that the Tc(V) is "pulled" into the -α1 framework and that may facilitate the reduction of Tc(V)O-α1 and stabilize lower Tc oxidation states. This study highlights the inequivalency of the two sites, and their impact on the chemical properties of the Tc substituted in these positions.