Activation of carbon dioxide by a terminal uranium-nitrogen bond in the gas-phase: a demonstration of the principle of microscopic reversibility.

Activation of CO2 is demonstrated by its spontaneous dissociative reaction with the gas-phase anion complex NUOCl2(-), which can be considered as NUO(+) coordinated by two chloride anion ligands. This reaction was previously predicted by density functional theory to occur exothermically, without barriers above the reactant energy. The present results demonstrate the validity of the prediction of microscopic reversibility, and provide a rare case of spontaneous dissociative addition of CO2 to a gas-phase complex. The activation of CO2 by NUOCl2(-) proceeds by conversion of a U[triple bond, length as m-dash]N bond to a U[double bond, length as m-dash]O bond and creation of an isocyanate ligand to yield the complex UO2(NCO)Cl2(-), in which uranyl, UO2(2+), is coordinated by one isocyanate and two chloride anion ligands. This activation of CO2 by a uranium(vi) nitride complex is distinctive from previous reports of oxidative insertion of CO2 into lower oxidation state U(iii) or U(iv) solid complexes, during which both C-O bonds remain intact. This unusual observation of spontaneous addition and activation of CO2 by NUOCl2(-) is a result of the high oxophilicity of uranium. If the computed Gibbs free energy of the reaction pathway, rather than the energy, is considered, there are barriers above the reactant asymptotes such that the observed reaction should not proceed under thermal conditions. This result provides a demonstration that energy rather than Gibbs free energy determines reactivity under low-pressure bimolecular conditions.

Gas-phase reactions of molecular oxygen with uranyl(V) anionic complexes-synthesis and characterization of new superoxides of uranyl(VI).

Gas-phase complexes of uranyl(V) ligated to anions X(-) (X = F, Cl, Br, I, OH, NO3, ClO4, HCO2, CH3CO2, CF3CO2, CH3COS, NCS, N3), [UO2X2](-), were produced by electrospray ionization and reacted with O2 in a quadrupole ion trap mass spectrometer to form uranyl(VI) anionic complexes, [UO2X2(O2)](-), comprising a superoxo ligand. The comparative rates for the oxidation reactions were measured, ranging from relatively fast [UO2(OH)2](-) to slow [UO2I2](-). The reaction rates of [UO2X2](-) ions containing polyatomic ligands were significantly faster than those containing the monatomic halogens, which can be attributed to the greater number of vibrational degrees of freedom in the polyatomic ligands to dissipate the energy of the initial O2-association complexes. The effect of the basicity of the X(-) ligands was also apparent in the relative rates for O2 addition, with a general correlation between increasing ligand basicity and O2-addition efficiency for polyatomic ligands. Collision-induced dissociation of the superoxo complexes showed in all cases loss of O2 to form the [UO2X2](-) anions, indicating weaker binding of the O2(-) ligand compared to the X(-) ligands. Density functional theory computations of the structures and energetics of selected species are in accord with the experimental observations.

Synthesis and hydrolysis of gas-phase lanthanide and actinide oxide nitrate complexes: a correspondence to trivalent metal ion redox potentials and ionization energies.

Several lanthanide and actinide tetranitrate ions, M(III)(NO3)4(-), were produced by electrospray ionization and subjected to collision induced dissociation in quadrupole ion trap mass spectrometers. The nature of the MO(NO3)3(-) products that result from NO2 elimination was evaluated by measuring the relative hydrolysis rates under thermalized conditions. Based on the experimental results it is inferred that the hydrolysis rates relate to the intrinsic stability of the M(IV) oxidation states, which correlate with both the solution IV/III reduction potentials and the fourth ionization energies. Density functional theory computations of the energetics of hydrolysis and atoms-in-molecules bonding analysis of representative oxide and hydroxide nitrates substantiate the interpretations. The results allow differentiation between those MO(NO3)3(-) that comprise an O(2-) ligand with oxidation to M(IV) and those that comprise a radical O(-) ligand with retention of the M(III) oxidation state. In the particular cases of MO(NO3)3(-) for M = Pr, Nd and Tb it is proposed that the oxidation states are intermediate between M(III) and M(IV).

Gas-phase uranyl, neptunyl, and plutonyl: hydration and oxidation studied by experiment and theory.

The following monopositive actinyl ions were produced by electrospray ionization of aqueous solutions of An(VI)O(2)(ClO(4))(2) (An = U, Np, Pu): U(V)O(2)(+), Np(V)O(2)(+), Pu(V)O(2)(+), U(VI)O(2)(OH)(+), and Pu(VI)O(2)(OH)(+); abundances of the actinyl ions reflect the relative stabilities of the An(VI) and An(V) oxidation states. Gas-phase reactions with water in an ion trap revealed that water addition terminates at AnO(2)(+)·(H(2)O)(4) (An = U, Np, Pu) and AnO(2)(OH)(+)·(H(2)O)(3) (An = U, Pu), each with four equatorial ligands. These terminal hydrates evidently correspond to the maximum inner-sphere water coordination in the gas phase, as substantiated by density functional theory (DFT) computations of the hydrate structures and energetics. Measured hydration rates for the AnO(2)(OH)(+) were substantially faster than for the AnO(2)(+), reflecting additional vibrational degrees of freedom in the hydroxide ions for stabilization of hot adducts. Dioxygen addition resulted in UO(2)(+)(O(2))(H(2)O)(n) (n = 2, 3), whereas O(2) addition was not observed for NpO(2)(+) or PuO(2)(+) hydrates. DFT suggests that two-electron three-centered bonds form between UO(2)(+) and O(2), but not between NpO(2)(+) and O(2). As formation of the UO(2)(+)-O(2) bonds formally corresponds to the oxidation of U(V) to U(VI), the absence of this bonding with NpO(2)(+) can be considered a manifestation of the lower relative stability of Np(VI).

Gas-phase lanthanide chloride clusters: relationships among ESI abundances and DFT structures and energetics.

Anionic lanthanide chloride clusters, Ln(n)Cl(3n+1)(-), were produced by electrospray ionization (ESI) of LnCl(3) in isopropanol, where Ln = La-Lu (except Pm); the clusters were characterized using a quadrupole ion trap mass spectrometer. High-abundance "magic number" clusters were apparent at n = 4 for the early Ln (La-Sm), and at n = 5 for the late Ln (Dy-Lu). Density functional theory computations of La(n)Cl(3n+1)(-) and Lu(n)Cl(3n+1)(-) clusters (n = 1-6) indicate that the clusters with n = 4-6 are rings with a central chlorine atom. Computed structures show six-coordinate Ln in distorted octahedral sites in "magic number" La(4)Cl(13)(-) and Lu(5)Cl(16)(-), which have particularly large dissociation energies. For lanthanum, larger anionic chloride clusters with multiple charges of down to -5 were observed; their fragmentation by collision-induced dissociation in the ion trap revealed La(4)Cl(13)(-) as a common product. Gas-phase hydrolysis to Ln(n)Cl(3n+1-y)(OH)(y)(-) (y = 1, 2) was prevalent for the late lanthanides, but only for small clusters, n = 2 or 3; larger clusters were evidently resistant to gas-phase hydrolysis. ESI of selected LnBr(3) and LnI(3) resulted in Ln(n)X(3n+1)(-) clusters (X = Br, I)--in contrast to Ln(n)Cl(3n+1)(-) clusters, the only observed (minor) high-abundance clusters were La(4)Br(13)(-) and Ce(4)Br(13)(-).

Gas-phase reactions of the bare Th2+ and U2+ ions with small alkanes, CH4, C2H6, and C3H8: experimental and theoretical study of elementary organoactinide chemistry.

The gas-phase reactions of two dipositive actinide ions, Th(2+) and U(2+), with CH(4), C(2)H(6), and C(3)H(8) were studied by both experiment and theory. Fourier transform ion cyclotron resonance mass spectrometry was employed to study the bimolecular ion-molecule reactions; the potential energy profiles (PEPs) for the reactions, both observed and nonobserved, were computed by density functional theory (DFT). The experiments revealed that Th(2+) reacts with all three alkanes, including CH(4) to produce ThCH(2)(2+), whereas U(2+) reacts with C(2)H(6) and C(3)H(8), with different product distributions than for Th(2+). The comparative reactivities of Th(2+) and U(2+) toward CH(4) are well explained by the computed PEPs. The PEPs for the reactions with C(2)H(6) effectively rationalize the observed reaction products, ThC(2)H(2)(2+) and UC(2)H(4)(2+). For C(3)H(8) several reaction products were experimentally observed; these and additional potential reaction pathways were computed. The DFT results for the reactions with C(3)H(8) are consistent with the observed reactions and the different products observed for Th(2+) and U(2+); however, several exothermic products which emerge from energetically favorable PEPs were not experimentally observed. The comparison between experiment and theory reveals that DFT can effectively exclude unfavorable reaction pathways, due to energetic barriers and/or endothermic products, and can predict energetic differences in similar reaction pathways for different ions. However, and not surprisingly, a simple evaluation of the PEP features is insufficient to reliably exclude energetically favorable pathways. The computed PEPs, which all proceed by insertion, were used to evaluate the relationship between the energetics of the bare Th(2+) and U(2+) ions and the energies for C-H and C-C activation. It was found that the computed energetics for insertion are entirely consistent with the empirical model which relates insertion efficiency to the energy needed to promote the An(2+) ion from its ground state to a prepared divalent state with two non-5f valence electrons (6d(2)) suitable for bond formation in C-An(2+)-H and C-An(2+)-C activated intermediates.

On the Nature of the CP Bond in Phosphaalkynes.

In this work, we report results of calculations based on the density functional theory (B3LYP/6-311+G(2d,2p)) of different species containing a terminal cyaphide bond. The chosen species range from small molecules and anions (C⋮P(-), HC⋮P, tBuC⋮P, [(CF3)3BC⋮P)](-)) to large transition-metal containing complexes ([(dppe)2Ru(H)(C⋮P)], trans-[Pt(PMe3)2(Cl)(C⋮P)], trans-[Pt(PMe3)2(Cl)(CP)Pt(PMe3)2]). A comparative analysis of the description of the C⋮P bond obtained by different methodologies is presented. Topological analyses of the electron density in the framework of the theory of atoms in molecules (AIM) and of the electron localization function (ELF) are complemented with the results obtained by natural bond orbital analysis (NBO).