Nora Planas

Cooperative insertion of CO2 in diamine-appended metal-organic frameworks

The process of carbon capture and sequestration has been proposed as a method of mitigating the build-up of greenhouse gases in the atmosphere. If implemented, the cost of electricity generated by a fossil fuel-burning power plant would rise substantially, owing to the expense of removing CO2 from the effluent stream. There is therefore an urgent need for more efficient gas separation technologies, such as those potentially offered by advanced solid adsorbents. Here we show that diamine-appended metal-organic frameworks can behave as ‘phase-change’ adsorbents, with unusual step-shaped CO2 adsorption isotherms that shift markedly with temperature. Results from spectroscopic, diffraction and computational studies show that the origin of the sharp adsorption step is an unprecedented cooperative process in which, above a metal-dependent threshold pressure, CO2 molecules insert into metal-amine bonds, inducing a reorganization of the amines into well-ordered chains of ammonium carbamate. As a consequence, large CO2 separation capacities can be achieved with small temperature swings, and regeneration energies appreciably lower than achievable with state-of-the-art aqueous amine solutions become feasible. The results provide a mechanistic framework for designing highly efficient adsorbents for removing CO2 from various gas mixtures, and yield insights into the conservation of Mg2+ within the ribulose-1,5-bisphosphate carboxylase/oxygenase family of enzymes.

Oxidation of ethane to ethanol by N2O in a metal–organic framework with coordinatively unsaturated iron(II) sites

Enzymatic haem and non-haem high-valent iron-oxo species are known to activate strong C-H bonds, yet duplicating this reactivity in a synthetic system remains a formidable challenge. Although instability of the terminal iron-oxo moiety is perhaps the foremost obstacle, steric and electronic factors also limit the activity of previously reported mononuclear iron(IV)-oxo compounds. In particular, although natures non-haem iron(IV)-oxo compounds possess high-spin S = 2 ground states, this electronic configuration has proved difficult to achieve in a molecular species. These challenges may be mitigated within metal-organic frameworks that feature site-isolated iron centres in a constrained, weak-field ligand environment. Here, we show that the metal-organic framework Fe2(dobdc) (dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate) and its magnesium-diluted analogue, Fe0.1Mg1.9(dobdc), are able to activate the C-H bonds of ethane and convert it into ethanol and acetaldehyde using nitrous oxide as the terminal oxidant. Electronic structure calculations indicate that the active oxidant is likely to be a high-spin S = 2 iron(IV)-oxo species.

Defining the Proton Topology of the Zr6-Based Metal-Organic Framework NU-1000.

Metal-organic frameworks (MOFs) constructed from Zr6-based nodes have recently received considerable attention given their exceptional thermal, chemical, and mechanical stability. Because of this, the structural diversity of Zr6-based MOFs has expanded considerably and in turn given rise to difficulty in their precise characterization. In particular it has been difficult to assign where protons (needed for charge balance) reside on some Zr6-based nodes. Elucidating the precise proton topologies in Zr6-based MOFs will have wide ranging implications in defining their chemical reactivity, acid/base characteristics, conductivity, and chemical catalysis. Here we have used a combined quantum mechanical and experimental approach to elucidate the precise proton topology of the Zr6-based framework NU-1000. Our data indicate that a mixed node topology, [Zr6(μ3-O)4(μ3-OH)4(OH)4 (OH2)4](8+), is preferred and simultaneously rule out five alternative node topologies.

Are Zr6-based MOFs water stable? Linker hydrolysis vs. capillary-force-driven channel collapse

Metal-organic frameworks (MOFs) built up from Zr6-based nodes and multi-topic carboxylate linkers have attracted attention due to their favourable thermal and chemical stability. However, the hydrolytic stability of some of these Zr6-based MOFs has recently been questioned. Herein we demonstrate that two Zr6-based frameworks, namely UiO-67 and NU-1000, are stable towards linker hydrolysis in H2O, but collapse during activation from H2O. Importantly, this framework collapse can be overcome by utilizing solvent-exchange to solvents exhibiting lower capillary forces such as acetone.

Catalytic Silylation of Dinitrogen with a Dicobalt Complex

A dicobalt complex catalyzes N2 silylation with Me3SiCl and KC8 under 1 atm N2 at ambient temperature. Tris(trimethylsilyl)amine is formed with an initial turnover rate of 1 N(TMS)3/min, ultimately reaching a turnover number of ∼200. The dicobalt species features a metal-metal interaction, which we postulate is important to its function. Although N2 functionalization occurs at a single cobalt site, the second cobalt center modifies the electronics at the active site. Density functional calculations reveal that the Co-Co interaction evolves during the catalytic cycle: weakening upon N2 binding, breaking with silylation of the metal-bound N2 and reforming with expulsion of [N2(SiMe3)3](-).

Experimental and quantum chemical characterization of the water oxidation cycle catalysed by [RuII(damp)(bpy)(H2O)]2+

The water-oxidation catalytic activity of [RuII(damp)(bpy)(H2O)]2+ has been determined from manometric and mass spectroscopy studies. Mechanistic details of the catalytic cycle have been studied both experimentally and using DFT and CASSCF/CASPT2 calculations. Characterisation of this Ru(II) complex and more highly oxidized catalytic intermediates has been accomplished through UV-vis and XAS spectroscopy, as well as through electrochemical techniques. Comparison of XAS spectra with CASSCF/CASPT2 calculations provides insight into the electronic structures of the more highly oxidized species, especially the degree to which oxidation occurs over both atoms of the Ru–O fragment. 18O-labelling experiments indicate that the O–O bond formation step proceeds via a water nucleophilic attack mechanism, and a detailed DFT analysis of the catalytic cycle predicts that step to be rate-determining and to take place for a formal Ru(V)O species. A number of alternative higher energy pathways have also been characterised in order to provide a more complete vision of the whole system.

Unusual structure, bonding and properties in a californium borate

The participation of the valence orbitals of actinides in bonding has been debated for decades. Recent experimental and computational investigations demonstrated the involvement of 6p, 6d and/or 5f orbitals in bonding. However, structural and spectroscopic data, as well as theory, indicate a decrease in covalency across the actinide series, and the evidence points to highly ionic, lanthanide-like bonding for late actinides. Here we show that chemical differentiation between californium and lanthanides can be achieved by using ligands that are both highly polarizable and substantially rearrange on complexation. A ligand that suits both of these desired properties is polyborate. We demonstrate that the 5f, 6d and 7p orbitals are all involved in bonding in a Cf(III) borate, and that large crystal-field effects are present. Synthetic, structural and spectroscopic data are complemented by quantum mechanical calculations to support these observations.

Multiple metal-metal bonds in iron-chromium complexes

Multiple metal–metal bonds are uncommon in heterobimetallic complexes. Rare examples of heterobimetallics with short metal–metal bonds include [TiRh(OCMe2CH2PPh2)3], [2] [CoZr(MesNPiPr2)3(thf)], [3] and [CrMo(O2CCH3)4]. [4] We report the first isolable examples of metal–metal multiple bonds between two different first-row transition metals, namely iron and chromium. We have conducted spectroscopic and theoretical investigations of two Fe!Cr coordination complexes to understand the nature of these unprecedented metal–metal bonds. We previously reported a dinucleating, double-decker ligand that was designed to enable the synthesis of heterobimetallic complexes in a modular manner. To prepare the iron–chromium complexes herein, the ligand was first deprotonated and metalated with CrCl3. The resulting monometallic species, [Cr(N(o-(iPr2PCH2N)C6H4)3)], which was confirmed by combustion analysis, acts as a metalloligand in a subsequent metalation (Scheme 1). For example, reaction of [Cr(N(o-(iPr2PCH2N)C6H4)3)] with FeBr2 and two equiv KC8 resulted in a color change from dark brown to green brown within minutes. The product, [FeCr(N(o(iPr2PCH2N)C6H4)3)] (1), is paramagnetic, and its proposed structure has been confirmed by X-ray crystallography. The redox properties of 1 were examined by electrochemical methods. The cyclic voltammogram (CV) of 1 (Supporting Information, Figure S3) reveals a rich redox profile, including: 1) a reversible reduction at !2.33 V; 2) a reversible oxidation at !1.32 V; and 3) a second, quasi-reversible oxidation at !0.62 V (versus FeCp2/FeCp2, 0.1m N(nBu)4PF6 in THF, 10 mVs!1). In contrast, the redox profile of the related iron–alane adduct, [FeAl(N(o-(iPr2PCH2N)C6H4)3)], is much more limited. It is characterized by a single reversible event, a reduction at !2.08 V (vs FeCp2/FeCp2). Reduction of 1 with 1 equiv KC8 generates a red-brown solution of paramagnetic [K(solv)n][1] (2). Alternatively, 2 can be directly prepared by mixing [Cr(N(o(iPr2PCH2N)C6H4)3)] with FeBr2 and three equiv KC8. Encapsulation of the potassium ion with either [18]crown-6 or crypt222 enabled the isolation of crystalline [K([18]crown-6)][1] (2a) or [K(crypt-222)][1] (2b), respectively. X-ray diffraction studies were conducted on single crystals of 1 and 2b (Figure 1; Supporting Information, Table S1). The Scheme 1. Synthesis of iron–chromium complexes.

Mechanism of Oxidation of Ethane to Ethanol at Iron(IV)–Oxo Sites in Magnesium-Diluted Fe2(dobdc)

The catalytic properties of the metal-organic framework Fe2(dobdc), containing open Fe(II) sites, include hydroxylation of phenol by pure Fe2(dobdc) and hydroxylation of ethane by its magnesium-diluted analogue, Fe0.1Mg1.9(dobdc). In earlier work, the latter reaction was proposed to occur through a redox mechanism involving the generation of an iron(IV)-oxo species, which is an intermediate that is also observed or postulated (depending on the case) in some heme and nonheme enzymes and their model complexes. In the present work, we present a detailed mechanism by which the catalytic material, Fe0.1Mg1.9(dobdc), activates the strong C-H bonds of ethane. Kohn-Sham density functional and multireference wave function calculations have been performed to characterize the electronic structure of key species. We show that the catalytic nonheme-Fe hydroxylation of the strong C-H bond of ethane proceeds by a quintet single-state σ-attack pathway after the formation of highly reactive iron-oxo intermediate. The mechanistic pathway involves three key transition states, with the highest activation barrier for the transfer of oxygen from N2O to the Fe(II) center. The uncatalyzed reaction, where nitrous oxide directly oxidizes ethane to ethanol is found to have an activation barrier of 280 kJ/mol, in contrast to 82 kJ/mol for the slowest step in the iron(IV)-oxo catalytic mechanism. The energetics of the C-H bond activation steps of ethane and methane are also compared. Dehydrogenation and dissociation pathways that can compete with the formation of ethanol were shown to involve higher barriers than the hydroxylation pathway.

Electronic structure of oxidized complexes derived from cis-[Ru II(bpy) 2(H 2O) 2] 2+ and its photoisomerization mechanism

The geometry and electronic structure of cis-[Ru(II)(bpy)(2)(H(2)O)(2)](2+) and its higher oxidation state species up formally to Ru(VI) have been studied by means of UV-vis, EPR, XAS, and DFT and CASSCF/CASPT2 calculations. DFT calculations of the molecular structures of these species show that, as the oxidation state increases, the Ru-O bond distance decreases, indicating increased degrees of Ru-O multiple bonding. In addition, the O-Ru-O valence bond angle increases as the oxidation state increases. EPR spectroscopy and quantum chemical calculations indicate that low-spin configurations are favored for all oxidation states. Thus, cis-[Ru(IV)(bpy)(2)(OH)(2)](2+) (d(4)) has a singlet ground state and is EPR-silent at low temperatures, while cis-[Ru(V)(bpy)(2)(O)(OH)](2+) (d(3)) has a doublet ground state. XAS spectroscopy of higher oxidation state species and DFT calculations further illuminate the electronic structures of these complexes, particularly with respect to the covalent character of the O-Ru-O fragment. In addition, the photochemical isomerization of cis-[Ru(II)(bpy)(2)(H(2)O)(2)](2+) to its trans-[Ru(II)(bpy)(2)(H(2)O)(2)](2+) isomer has been fully characterized through quantum chemical calculations. The excited-state process is predicted to involve decoordination of one aqua ligand, which leads to a coordinatively unsaturated complex that undergoes structural rearrangement followed by recoordination of water to yield the trans isomer.