Ligand and Metal Effects on the CO-Release Reactivity of Metal Acireductone and Flavonolate Complexes

The research reported herein involves synthetic metal complexes of relevance to dioxygenase enzymes (Ni(II)-containing acireductone dioxygenase (Ni(II)-ARD) and quercentinase (2,4-flavonol dioxygenase) that promote oxidative carbon-carbon bond cleavage and CO release. The experiments focus on the elucidation of structure-reactivity relationships and evaluation of the conditions under which CO is generated. It had been proposed that hydrogen bond donors in the secondary environment of the active site metal center in Ni(II)-ARD influence the coordination of the acireductone substrate on the nickel center. To evaluate this proposal, we investigated the Ni(II) coordination chemistry of an acireductone-type enolate anion using a supporting chelate ligand having two internal hydrogen bond-donors. The resulting complex exhibited differences in terms of the organic product distribution in a CO release reaction resulting from oxidative C-C bond cleavage of the enolate ligand relative to this reported for the iv hydrophobic, 6-Ph2TPA-supported (6-Ph2TPA = N,N-bis((6-phenyl-2-pyridyl)methyl)- N-((2-pyridyl)methyl)amine) analogue. In another study, we found that changes in the supporting chelate ligand or metal center influenced the coordination chemistry of the acireductone-type enolate anion. This chemistry highlighted the propensity of the enolate to undergo isomerization without CO release in the presence of water. Rigorously excluding water enabled the isolation of a Ni(II) enolate complex of the 6-PhTPA ligand and examination of its oxidative CO release chemistry, as well as the spectroscopic characterization of the first Co(II) complex of an acireductone-type enolate. To elucidate factors influencing the CO-release reactivity of metal-flavonoid complexes, some of which have relevance to quercentinase enzymes, we synthesized and characterized the first series of structurally-related metal-flavonolate complexes [(6- Ph2TPA)M(3-Hfl)]X (M = Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II), Hg(II); X = OTfor ClO4 - ). Exposure of these complexes to UV light initiated photooxidative carboncarbon bond cleavage and CO release in a metal-dependent manner, with closed-shell d 10 metals giving rise to the highest rate of CO release. These studies suggest that metal flavonolate species may be useful as a new type of photo-induced CO release molecules (CORMs). Such species are of current interest for possible therapeutic applications

Hydrogen storage in liquid hydrogen carriers: recent activities and new trends

Efficient storage of hydrogen is one of the biggest challenges towards a potential hydrogen economy. Hydrogen storage in liquid carriers is an attractive alternative to compression or liquefaction at low temperatures. Liquid carriers can be stored cost-effectively and transportation and distribution can be integrated into existing infrastructures. The development of efficient liquid carriers is part of the work of the International Energy Agency Task 40: Hydrogen-Based Energy Storage. Here, we report the state-of-the-art for ammonia and closed CO2-cycle methanol-based storage options as well for liquid organic hydrogen carriers

Evaluation of Hydrogen Gettering Rates Correlated to Surface Composition and Texture of Nickel-Plated Zircaloy Getters of Different Heat Treatment Procedures

Coatings of metal specimens are known to have an impact on hydrogen gettering (hydrogen absorption). The coating can have one or more functions, such as enhancing gettering, preventing gettering and/or preventing oxidation of the metal substrate. It is known that contaminants and surface texture can impact hydrogen gettering/absorption performance, but has not previously been thoroughly explored. This study evaluated the role of different post-plating heat treatments of nickel-plated zircaloy-4 getters (NPGs) and the role of the heat treatments on gettering rates, surface composition and texture. Nickel plating is applied to prevent oxidation of the Zircaloy-4 surface and also enhances gettering. The nickel plating must be heat treated before desirable gettering can occur. Our NPG getters with historically known satisfying performance were pre-heat treated in air followed by activation heat treatment in a vacuum at a higher temperature. In this study, we were interested in finding out if both heat treatment steps were necessary to obtain a desirable gettering performance, or if one step could be omitted. XPS analysis showed that if the nickel surface is not heat treated before bonding the nickel to the zirconium in the activation step, there will be carbon contaminants on the surface, which significantly reduces gettering. We studied the texture of Zircaloy-4 using SEM/EBSD to compare NPGs with both heat treatment steps with NPGs that had no post-plating heat treatment to learn if the degree of cold work could be impacted by the heat treatment steps. We did not observe any differences in texture between them. We measured gettering rates of both pretreated and activated NPGs and NPGs that had been activated without first being pre-heat treated. We found that the NPGs without the first post-plating heating step had up to a seven times slower gettering rate and obtained higher plateau pressures due to the contaminated surface. Thus, the pre-heat treatment in air before activation is necessary to avoid slower gettering rates and higher plateau pressures

Experimentally Quantifying Small-Molecule Bond Activation Using Valence-to-Core X‑ray Emission Spectroscopy

This work establishes the ability of valence-to-core X-ray emission spectroscopy (XES) to serve as a direct probe of N₂ bond activation. A systematic series of iron-N₂ complexes has been experimentally investigated and the energy of a valence-to-core XES peak was correlated with N–N bond length and stretching frequency. Computations demonstrate that, in a simple one-electron picture, this peak arises from the N₂ 2s2s σ* orbital, which becomes less antibonding as the N–N bond is weakened and broken. Changes as small as 0.02 Å in the N–N bond length may be distinguished using this approach. The results thus establish valence-to-core XES as an effective probe of small molecule activation, which should have broad applicability in transition-metal mediated catalysis

CCDC 832449: Experimental Crystal Structure Determination

Related Article: K.Grubel, B.J.Laughlin, T.R.Maltais, R.C.Smith, A.M.Arif, L.M.Berreau|2011|Chem.Commun.|47|10431|doi:10.1039/c1cc13961d,An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures

CCDC 832451: Experimental Crystal Structure Determination

Related Article: K.Grubel, B.J.Laughlin, T.R.Maltais, R.C.Smith, A.M.Arif, L.M.Berreau|2011|Chem.Commun.|47|10431|doi:10.1039/c1cc13961d,An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures

Alkali Metal Control over N–N Cleavage in Iron Complexes

Though N₂ cleavage on K-promoted Fe surfaces is important in the large-scale Haber–Bosch process, there is still ambiguity about the number of Fe atoms involved during the N–N cleaving step and the interactions responsible for the promoting ability of K. This work explores a molecular Fe system for N₂ reduction, particularly focusing on the differences in the results obtained using different alkali metals as reductants (Na, K, Rb, Cs). The products of these reactions feature new types of Fe–N₂ and Fe-nitride cores. Surprisingly, adding more equivalents of reductant to the system gives a product in which the N–N bond is not cleaved, indicating that the reducing power is not the most important factor that determines the extent of N₂ activation. On the other hand, the results suggest that the size of the alkali metal cation can control the number of Fe atoms that can approach N₂, which in turn controls the ability to achieve N₂ cleavage. The accumulated results indicate that cleaving the triple N–N bond to nitrides is facilitated by simultaneous approach of least three low-valent Fe atoms to a single molecule of N₂

CCDC 832450: Experimental Crystal Structure Determination

Related Article: K.Grubel, B.J.Laughlin, T.R.Maltais, R.C.Smith, A.M.Arif, L.M.Berreau|2011|Chem.Commun.|47|10431|doi:10.1039/c1cc13961d,An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures

CCDC 832452: Experimental Crystal Structure Determination

Related Article: K.Grubel, B.J.Laughlin, T.R.Maltais, R.C.Smith, A.M.Arif, L.M.Berreau|2011|Chem.Commun.|47|10431|doi:10.1039/c1cc13961d,An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures