The new binuclear chromium Pacman complex [Cr₂(L)] of the Schiff base pyrrole macrocycle H₄L has been synthesized and structurally characterized. Addition of isocyanide, C≡NR (R = xylyl, ^tBu), or triphenylphosphine oxide donors to [Cr₂(L)] gives contrasting chemistry with the formation of the new coordination compounds [Cr₂(μ-CNR)(L)], in which the isocyanides bridge the two Cr(II) centers, and [Cr₂(OPPh₃)₂(L)], a Cr(II) phosphine oxide adduct with the ligands exogenous to the cleft.

The oxo- and catecholate-bridged U^IV/U^IV Pacman complex [{(py)U^IVOU^IV(μ-O₂C₆H₄)(py)}(L^A)] A (L^A = a macrocyclic "Pacman" ligand; anthracenylene hinge between N₄-donor pockets, ethyl substituents on meso-carbon atom of each N₄-donor pocket) featuring a bent U^IV-O-U^IV oxo bridge readily reacts with small molecule substrates to undergo either oxo-atom functionalisation or substitution. Complex A reacts with H₂O or MeOH to afford [{(py)U^IV(μ-OH)₂U^IV(μ-O₂C₆H₄)(py)}(L^A)] (1) and [{(py)U^IV(μ-OH)(μ-OMe)U^IV(μ-O₂C₆H₄)(py)}(L^A)] (2), respectively, in which the bridging oxo ligand in A is substituted for two bridging hydroxo ligands or one bridging hydroxo and one bridging methoxy ligand, respectively. Alternatively, A reacts with either 0.5 equiv. of S₈ or 4 equiv. of Se to provide [{(py)U^IV(μ-η²:η²-E₂)U^IV(μ-O₂C₆H₄)(py)}(L^A)] (E = S (3), Se (4)) respectively, in which the [E₂]^2- ion bridges the two U^IV centres. To the best of our knowledge, complex A is the first example of either a d- or f-block bimetallic μ-oxo complex that activates elemental chalcogens. Complex A also reacts with XeF₂ or 2 equiv. of Me₃SiCl to provide [{(py)U^IV(μ-X)₂U^IV(μ-O₂C₆H₄)(py)}(L^A)] (X = F (5), Cl (6)), in which the oxo ligand has been substituted for two bridging halido ligands. Reacting A with either XeF₂ or Me₃SiCl in the presence of O(Bcat)₂ at room temperature forms [{(py)U^IV(μ-X)(μ-OBcat)U^IV(μ-O₂C₆H₄)(py)}(L^A)] (X = F (5A), Cl (6A)), which upon heating to 80 °C is converted to 5 and 6, respectively. In order to probe the importance of the bent U^IV-O-U^IV motif in A on the observed reactivity, the bis(boroxido)-U^IV/U^IV complex, [{(py)(pinBO)U^IVOU^IV(OBpin)(py)}(L^A)] (B), featuring a linear U^IV-O-U^IV bond angle was treated with H₂O and Me₃SiCl. Complex B reacts with two equiv. of either H₂O or Me₃SiCl to provide [{(py)HOU^IVOU^IVOH(py)}(L^A)] (7) and [{(py)ClU^IVOU^IVCl(py)}(L^A)] (8), respectively, in which reactions occur preferentially at the boroxido ligands, with the μ-oxo ligand unchanged. The formal U^IV oxidation state is retained in all of the products 1-8, and selective reactions at the bridging oxo ligand in A is facilitated by: (1) its highly nucleophilic character which is a result of a non-linear U^IV-O-U^IV bond angle causing an increase in U-O bond covalency and localisation of the lone pairs of electrons on the μ-oxo group, and (2) the presence of the bridging catecholate ligand, which destabilises a linear oxo-bridging geometry and stabilises the resulting products.

Fens and wet meadows are important mountain wetland types, but influences onassemblage structure of associated invertebrates are poorly understood compared with other aspects of the ecology of these habitats. We sought to determine the relative contributions of terrestrial and aquatic invertebrates to diversity and abundance in these wetlands, the extent to which terrestrial and aquatic invertebrate assemblages differ with wetland type, and to what degree the aquatic assemblages vary as a function of slow sheet flow. We compared assemblages in fens and wet meadows, with and without flow, at 80 backcountry sites dispersed across the 6200 km2 landscape of Yosemite, Sequoia,and Kings Canyon National Parks in the Sierra Nevada mountains of California,U.S.A., using standard aquatic and terrestrial sweep netting. Cicadellid leafhoppers, aphids, and thomisid crab spiders were the most abundant terrestrial taxa. Cicadellids,Lepidoptera, anthomyiid, muscid, chloropid, and ephydrid flies, and thomisids were more abundant in fens than in wet meadows. Only mirid leaf bugs were significantly more abundant in wet meadows than fens. Sphaeriid clams and chironomid midges dominated aquatic assemblages both with and without flow. Chloroperlid stoneflies, mites, clams, and flatworms were all more abundant in flow, and Hemiptera and mosquitos were significantly more abundant in quiescent water. Mosquitos were more abundant in wet meadows, but there were few other population differences as a functionof wetland type. Terrestrial diversity was 1.1 to 2.0 times that of aquatic diversity,depending on metric and habitat. Fens had greater terrestrial abundance, richness,evenness, and diversity than wet meadows; there were fewer differences as a function of wetland type for aquatic fauna. Presence or absence of slow sheet flow had more effect on these aquatic assemblages than did wetland type. Cluster analyses, ordination, and multi-response permutation procedures were generally consistent with the univariate results. Vegetation-based wetland classifications should be extrapolated to faunal assemblages with caution, particularly for aquatic invertebrates.

Crystals of a Cu complex of the macrocyclic Schiff-base calixpyrrole or 'Pacman' ligand, Cu₂(L), do not contain any solvent-accessible void space at ambient pressure, but adsorb neon at 4.67 GPa, forming Cu₂(L)·3.5Ne.

We report the coordination chemistry of the tripodal tris[2-amido(2-pyridyl)ethyl]amine ligand, L, with thorium(iv) and uranium(iv). Using a salt-metathesis strategy from the potassium salt of this ligand, K3L, new actinide complexes were isolated, namely the dimeric thorium complex [ThCl(L)]₂ (1) and the monomeric uranium complex UI(THF)(L) (2); under different crystallisation conditions, the dimeric uranium complex is also isolated, [UI(L)]₂ (2-dimer). With the aim of studying electronic phenomena such as magnetic exchange between two actinide ions, we have synthesised the first examples of dinuclear, quinoid-bridged actinide complexes from dianionic 2,5-bis[2,6-(diisopropyl)anilide]-1,4-benzoquinone (QDipp) and 2,5-bis[2-(methoxy)anilide]-1,4-benzoquinone (QOMe) ligands. The resulting complexes are [Th(L)]₂Q^Dipp (3), [Th(THF)(L)]₂Q^OMe (5) and [U(L)]₂Q^OMe (6). The targeted [U(L)]₂Q^Dipp complex (4) could not be isolated. All isolated complexes have been characterised by spectroscopic methods and X-ray crystallography. The uranium(iv) complexes 2-dimer and 6 have been studied by SQUID magnetometry but indicate that there is negligible magnetic exchange between the two uranium(iv) ions. The reduced form of 6, [K(18-c-6)][6-] is unstable and highly sensitive, but X-ray crystallography indicates that it is a novel U^IVU^IV complex bridged by a quinoid-radical.

The uranyl(vi) 'Pacman' complex [(UO₂)(py)(H₂L)] A (L = polypyrrolic Schiff-base macrocycle) is reduced by Cp₂Ti(η²-Me₃SiC[triple bond, length as m-dash]CSiMe₃) and [Cp₂TiCl]₂ to oxo-titanated uranyl(v) complexes [(py)(Cp₂Ti^IIIOUO)(py)(H₂L)] 1 and [(ClCp₂Ti^IVOUO)(py)(H₂L)] 2. Combination of Zr^II and Zr^IV synthons with A yields the first Zr^IV-uranyl(v) complex, [(ClCp₂ZrOUO)(py)(H₂L)] 3. Similarly, combinations of Ae⁰ and Ae^II synthons (Ae = alkaline earth) afford the mono-oxo metalated uranyl(v) complexes [(py)₂(ClMgOUO)(py)(H₂L)] 4, [(py)₂(thf)₂(ICaOUO)(py) (H₂L)] 5; the zinc complexes [(py)₂(XZnOUO)(py)(H₂L)] (X = Cl 6, I 7) are formed in a similar manner. In contrast, the direct reactions of Rb or Cs metal with A generate the first mono-rubidiated and mono-caesiated uranyl(v) complexes; monomeric [(py)₃(RbOUO)(py)(H₂L)] 8 and hexameric [(MOUO)(py)(H₂L)]₆ (M = Rb 8b or Cs 9). In these uranyl(v) complexes, the pyrrole N-H atoms show strengthened hydrogen-bonding interactions with the endo-oxos, classified computationally as moderate-strength hydrogen bonds. Computational DFT MO (density functional theory molecular orbital) and EDA (energy decomposition analysis), uranium M₄ edge HR-XANES (High Energy Resolution X-ray Absorption Near Edge Structure) and 3d4f RIXS (Resonant Inelastic X-ray Scattering) have been used (the latter two for the first time for uranyl(v) in 7 (ZnI)) to compare the covalent character in the U^V-O and O-M bonds and show the 5f orbitals in uranyl(vi) complex A are unexpectedly more delocalised than in the uranyl(v) 7 (ZnI) complex. The O_exo-Zn bonds have a larger covalent contribution compared to the Mg-O_exo/Ca-O_exo bonds, and more covalency is found in the U-O_exo bond in 7 (ZnI), in agreement with the calculations.

The reduction of U^VI uranyl halides or amides with simple Ln^II or U^III salts forms highly symmetric, linear, oxo-bridged trinuclear U^V /Ln^III /U^V , Ln^III /U^IV /Ln^III , and U^IV /U^IV /U^IV complexes or linear Ln^III /U^V polymers depending on the stoichiometry and solvent. The reactions can be tuned to give the products of one- or two-electron uranyl reduction. The reactivity and magnetism of these compounds are discussed in the context of using a series of strongly oxo-coupled homo- and heterometallic poly(f-block) chains to better understand fundamental electronic structure in the f-block.

The uranyl(vi) complex UO₂Cl(L) of the redox-active, acyclic diimino-dipyrrin anion, L^- is reported and its reaction with inner- and outer-sphere reductants studied. Voltammetric, EPR-spectroscopic and X-ray crystallographic studies show that chemical reduction by the outer-sphere reagent CoCp₂ initially reduces the ligand to a dipyrrin radical, and imply that a second equivalent of CoCp₂ reduces the U(vi) centre to form U(v). Cyclic voltammetry indicates that further outer-sphere reduction to form the putative U(iv) trianion only occurs at strongly cathodic potentials. The initial reduction of the dipyrrin ligand is supported by emission spectra, X-ray crystallography, and DFT; the latter also shows that these outer-sphere reactions are exergonic and proceed through sequential, one-electron steps. Reduction by the inner-sphere reductant [TiCp₂Cl]₂ is also likely to result in ligand reduction in the first instance but, in contrast to the outer-sphere case, reduction of the uranium centre becomes much more favoured, allowing the formation of a crystallographically characterised, doubly-titanated U(iv) complex. In the case of inner-sphere reduction only, ligand-to-metal electron-transfer is thermodynamically driven by coordination of Lewis-acidic Ti(iv) to the uranyl oxo, and is energetically preferable over the disproportionation of U(v). Overall, the involvement of the redox-active dipyrrin ligand in the reduction chemistry of UO₂Cl(L) is inherent to both inner- and outer-sphere reduction mechanisms, providing a new route to accessing a variety of U(vi), U(v), and U(iv) complexes.

The first use of a dinuclear U^III/U^III complex in the activation of small molecules is reported. The octadentate Schiff-base pyrrole, anthracene-hinged 'Pacman' ligand L^A combines two strongly reducing U^III centres and three borohydride ligands in [M(THF)₄][{U(BH₄)}₂(μ-BH₄)(L^A)(THF)₂] 1-M, (M = Li, Na, K). The two borohydride ligands bound to uranium outside the macrocyclic cleft are readily substituted by aryloxide ligands, resulting in a single, weakly-bound, encapsulated endo group 1 metal borohydride bridging the two U^III centres in [{U(OAr)}₂(μ-MBH₄)(L^A)(THF)₂] 2-M (OAr = OC₆H₂^t Bu₃-2,4,6, M = Na, K). X-ray crystallographic analysis shows that, for 2-K, in addition to the endo-BH₄ ligand the potassium counter-cation is also incorporated into the cleft through η⁵-interactions with the pyrrolides instead of extraneous donor solvent. As such, 2-K has a significantly higher solubility in non-polar solvents and a wider U-U separation compared to the 'ate' complex 1. The cooperative reducing capability of the two U^III centres now enforced by the large and relatively flexible macrocycle is compared for the two complexes, recognising that the borohydrides can provide additional reducing capability, and that the aryloxide-capped 2-K is constrained to reactions within the cleft. The reaction between 1-Na and S₈ affords an insoluble, presumably polymeric paramagnetic complex with bridging uranium sulfides, while that with CS₂ results in oxidation of each U^III to the notably high U^V oxidation state, forming the unusual trithiocarbonate (CS₃)^2- as a ligand in [{U(CS₃)}₂(μ-κ²:κ²-CS₃)(L^A)] (4). The reaction between 2-K and S₈ results in quantitative substitution of the endo-KBH₄ by a bridging persulfido (S₂)^2- group and oxidation of each U^III to U^IV, yielding [{U(OAr)}₂(μ-κ²:κ²-S₂)(L^A)] (5). The reaction of 2-K with CS₂ affords a thermally unstable adduct which is tentatively assigned as containing a carbon disulfido (CS₂)^2- ligand bridging the two U centres (6a), but only the mono-bridged sulfido (S)^2- complex [{U(OAr)}₂(μ-S)(L^A)] (6) is isolated. The persulfido complex (5) can also be synthesised from the mono-bridged sulfido complex (6) by the addition of another equivalent of sulfur.

The uranyl(vi) Pacman complex [(UO2)(py)(H2L)] A (L = polypyrrolic Schiff-base macrocycle) is reduced by Cp2Ti(η2-Me3SiC[triple bond, length as m-dash]CSiMe3) and [Cp2TiCl]2 to oxo-titanated uranyl(v) complexes [(py)(Cp2TiIIIOUO)(py)(H2L)] 1 and [(ClCp2TiIVOUO)(py)(H2L)] 2. Combination of ZrII and ZrIV synthons with A yields the first ZrIV-uranyl(v) complex, [(ClCp2ZrOUO)(py)(H2L)] 3. Similarly, combinations of Ae0 and AeII synthons (Ae = alkaline earth) afford the mono-oxo metalated uranyl(v) complexes [(py)2(ClMgOUO)(py)(H2L)] 4, [(py)2(thf)2(ICaOUO)(py) (H2L)] 5; the zinc complexes [(py)2(XZnOUO)(py)(H2L)] (X = Cl 6, I 7) are formed in a similar manner. In contrast, the direct reactions of Rb or Cs metal with A generate the first mono-rubidiated and mono-caesiated uranyl(v) complexes; monomeric [(py)3(RbOUO)(py)(H2L)] 8 and hexameric [(MOUO)(py)(H2L)]6 (M = Rb 8b or Cs 9). In these uranyl(v) complexes, the pyrrole N-H atoms show strengthened hydrogen-bonding interactions with the endo-oxos, classified computationally as moderate-strength hydrogen bonds. Computational DFT MO (density functional theory molecular orbital) and EDA (energy decomposition analysis), uranium M4 edge HR-XANES (High Energy Resolution X-ray Absorption Near Edge Structure) and 3d4f RIXS (Resonant Inelastic X-ray Scattering) have been used (the latter two for the first time for uranyl(v) in 7 (ZnI)) to compare the covalent character in the UV-O and O-M bonds and show the 5f orbitals in uranyl(vi) complex A are unexpectedly more delocalised than in the uranyl(v) 7 (ZnI) complex. The Oexo-Zn bonds have a larger covalent contribution compared to the Mg-Oexo/Ca-Oexo bonds, and more covalency is found in the U-Oexo bond in 7 (ZnI), in agreement with the calculations.