338 research outputs found

    Double protonation of a cis-Bipyridoallenophane detected via Chiral-Sensing Switch: the role of Ion Pairs

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    We prove that the confinement of the conformational space of pyridoallenophanes leads to intense chiroptical responses. Unlike the cyclic dimer [142], single-conformation [141]pyridoallenophanes isomerize under thermal and photochemical conditions. Yet, less-strained [141]-bipyridoallenophanes are stable and are prepared successfully. They, unexpectedly, undergo double protonation as a result of cooperative ion-pairing and hydrogen bonding. The complex formation forces a single configuration of the axis connecting both pyridyl rings recognized by a diagnostic circular dichroism (CD) signal at 330 nm.Agencia Estatal de Investigación | Ref. CTQ2017-84354-PAgencia Estatal de Investigación | Ref. CTQ2017-85919-RXunta de Galicia | Ref. IN607C 2016/03Xunta de Galicia | Ref. ED431C 2017/70Xunta de Galicia | Ref. Ref. ED431G/0

    Synthesis of a Dicyclopenta[a, e]pentalene by [6 + 2]-Cycloaddition of 1,3-Di-tert-butyl-5-vinylidenecyclopentadiene and Consecutive 8ŌÄ-Electrocyclic Reaction

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    The first non-benzenoid, linearly annelated, tetracyclic system with [4n + 2]-perimeter and [4n]-partial structures was obtained by dimerization of the allene derivative 1 via [6 + 2]-cycloaddition. Subsequent 8ŌÄ-electrocyclization yielded a tetraquinane, which could be dehydrogenated to the title compound 2

    Electrochemical Hydrogen Gas Production and Carbon Dioxide Sequestration via \u3ci\u3eIn Situ\u3c/i\u3e Formation of Nickel (II) Tris-(Pyridinethiolate) Derivatives

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    In the research presented, we use electrochemical techniques, such as cyclic voltammetry and controlled potential electrolysis, to study in situ formation of hydrogen evolution catalysts which also reduce carbon dioxide. Using pyridinethiol-based ligands and a nickel (II) precursor, both homo- and heteroleptic complexes were investigated as catalysts to produce alternative fuels while mitigating greenhouse gas emissions. Existing synthetic procedures to obtain these Ni(II) catalysts lead to low yields of the complexes and difficulties in crystallizing samples for further analysis, limiting sample size and restricting the number of studies. While current research is heavily focused on photochemically-driven experiments, given that solar radiation is highly inconsistent across the world, a major shift must be made to understand electrocatalysts and how they yield pertinent information about solar fuel production. Cyclic voltammetry was used to investigate production of hydrogen gas from the in situ formation of nickel(II) tris-(pyridinethiolate), 3P+Ni. Comparing a sample of the isolated versus the in situ catalyst, the voltammograms suggested that the complex not only self-assembled in solution but also performed proton reduction. Four separate homoleptic catalysts with electron donating and electron withdrawing groups were employed to explore how ligand modifications could influence the reduction potential needed to produce hydrogen gas. Although 3P+Ni did not have ligand substituents, it afforded the most positive reduction potential (-1.41 V v. SCE) and concurrently a low rate-constant (376 mM-1·s-1) for hydrogen production. 3(3-F)+Ni, in situ nickel(II) tris (3 (trifluoromethyl)pyridine-2-thiolate), exhibited the most negative reduction potential (-1.68 V v. SCE) and the highest rate constant (951 mM-1·s-1) for proton reduction. Using these findings, heteroleptic catalysts were proposed to develop catalysts with tunable reduction potentials. By using a 1:2:1 stoichiometric ratio (metal precursor, major ligand, and minor ligand), heteroleptic catalytic solutions were developed in situ and tested for hydrogen production. Reduction potentials for all the heteroleptic pairs were found to be more positive than their ligand modified homoleptic parent complex, i.e., heteroleptic 2P+1(3 F)+Ni (-1.53 V v. SCE) and 2(3-F)+1P+Ni (-1.50 V v. SCE) have lower overpotentials when compared to 3(3-F)+Ni (-1.68 V v. SCE). When comparing voltammograms for the heteroleptic pairs, similarities in the traces suggested that one hydrogen evolution electrocatalyst is formed for both ratio pairs. Despite the differences in the stochiometric ligand ratios, the electrochemical studies and reduction potentials are consistent with this theory. With respect to the rate constants, most of the catalytic solutions were also found to work better than the ligand modified parent analogues; therefore, the studies imply that in situ heteroleptic catalysts offer improvements without rigorous synthetic techniques. Expanding on the versatility of these in situ nickel(II) catalysts, preliminary experiments using controlled potential electrolysis and quantitative gas analysis of the headspace tested carbon dioxide reduction. Exploring homoleptic 3P+Ni and 3(3-F)+Ni, both electrocatalysts were found to produce a maximum partial pressure change of ~14-15% CH4, ~15-16% HCOOH, and ~21% CO. However, they were found to produce the highest pressure changes from H2 evolution (~49%) which identified 3P+Ni and 3(3-F)+Ni as non-selective for CO2 reduction over proton reduction catalysts. Three heteroleptic catalysts, namely 2P+1(3-F)+Ni, 2(3 M)+1P+Ni, and 2P+1(3-M)+Ni, were investigated similarly and where found to show a significant improvement in the pressure changes related to HCOOH (+ ~4-5%). Although these in situ heteroleptic catalysts were also deemed better proton reduction catalysts, they afforded selectivity for HCOOH formation that was not expected. Through this research, the benefits of in situ studies in yielding measurable hydrogen gas production through electrochemical studies is shown, as well as interesting results that showcase the further development of these Ni(II) catalysts for carbon dioxide reduction and formic acid formation

    Group 14 metallole dianions as ő∑5^5-coordinating ligands

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    As heavier aromatic analogs of the cyclopentadienides, group 14 dianionic metalloles exhibit more versatile reactivity and coordination modes due to the additional lone pair at the heteroatom. Compared to the well-established chemistry of monoanionic cyclopentadienide ligands, the coordination chemistry with those dianionic ligands remains underexplored. This perspective provides an overview of literature-known examples of group 14 metallole dianions (silole, germole, stannole and plumbole) adopting ő∑5^5-coordinating modes. The diverse coordination modes and reactivity exhibited by these compounds highlight their potential as intriguing ligands in organometallic chemistry

    Steuerung der elektronischen Struktur von redoxaktiven Guanidin-Verbindungen: Einfluss einer sekundären Koordinationssphäre sowie der Homokonjugation

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    Die vorliegende Dissertation besch√§ftigt sich mit zwei neuen Wegen, die elektronische Struktur von √úbergangsmetallkomplexen mit redoxaktiven Guanidin-Liganden (GFA) zu steuern: durch die Einf√ľhrung einer sekund√§ren Koordinationssph√§re sowie durch Homokonjugation. Die Kombination dieser Ligandenklasse als starke Elektronendonoren mit redoxaktiven √úbergangsmetallen f√ľhrt zur M√∂glichkeit intramolekularer Elektronen√ľbertragungen von den Liganden auf die Metallatome. Besonders in CuII-Komplexen wurden solche Prozesse bereits intensiv untersucht und es konnten mehrere Einflussfaktoren wie die Elektronendonorst√§rke des Liganden, die Art der Coliganden, sowie der L√∂sungsmittelpolarit√§t und der Temperatur gefunden werden. Der Fokus lag stets auf der prim√§ren Koordinationssph√§re der Liganden mit dem Metallatom. Durch den Einbau einer [18]Krone-6-Funktion als R√ľckgrat eines redoxaktiven Bisguanidin-Liganden gelang in dieser Arbeit die Einf√ľhrung einer sekund√§ren Koordinationssph√§re. In einem CuCl2-Komplex konnte ein valenztautomeres Gleichgewicht beobachtet werden. Erstmalig konnte gezeigt werden, dass die Einlagerung von Kalium- und Barium-Ionen in den Kronenether zu einer drastischen √Ąnderung der elektronischen Struktur f√ľhrt und weit √ľber eine typische anodische Verschiebung des Redoxpotentials des Liganden hinausgeht. Die Koordination der Ionen initiiert einen intramolekularen Elektronen-transfer vom Metallatom auf den Liganden. Auch in einfach oxidierten CoII-Komplexen wurde der Einfluss auf deren elektronische Struktur untersucht, wobei unabh√§ngig von der Metall-Einlagerung in den Kronenether ein L√∂sungsmitteleinfluss beobachtet werden konnte. Dar√ľber hinaus konnte dieses neue Ligandensystem auf Dibenzo-Kronenether erweitert werden. So gelang die Synthese neuer Tetraguanidine, in denen zwei o-Bisguanidinobenzol-Einheiten √ľber eine sekund√§re Koordinationssph√§re verkn√ľpft sind. Besonders interessant ist hierbei die Vergr√∂√üerung des Kronenethers zu [24]Krone-8, wobei quantenchemische Untersuchungen zeigen, dass die elektronische Struktur von Metallkomplexen durch die Koordination eines linearen Molek√ľls durch den Makrozyklus gesteuert werden kann. Weitere experimentelle Untersuchungen k√∂nnten den GFA den Weg in die supramolekulare Chemie ebnen. Des Weiteren wurde in dieser Arbeit der Einfluss der Homokonjugation in GFA-Verbindungen untersucht. So wurden erstmalig Hexaguanidino-Triptycen-Derivate synthetisiert, in denen drei o-Bisguanidinobenzol-Einheiten durch diese Art der Konjugation in Wechselwirkung stehen. Das Cyclovoltammogramm zeigt drei reversible Zwei-Elektronen-Oxidationen, was im Einklang mit der Abh√§ngigkeit dieser Einheiten aufgrund der elektronischen Kopplung durch die Homokonjugation steht. Zweifache Oxidation f√ľhrt zu paramagnetischen Salzen, welche intensive ŌÄ ŌÄ*-√úberg√§nge mit Charge-Transfer-Charakter im sichtbaren-NIR-Spektralbereich aufweisen. Detaillierte magnetometrische Messungen (SQUID, ESR) neuer CoII-Komplexe zeigen einen elektronischen high-spin Grundzustand mit neun ungepaarten Elektronen. Partielle Oxidation der Komplexe f√ľhrt zu √§hnlichen ligandenzentrierten elektronischen √úberg√§ngen wie bei den freien Liganden. Im Gegensatz dazu konnten diese Beobachtungen mit bereits bekannten Hexaguanidino-Triphenylen-Verbindungen nicht gemacht werden. Dadurch konnte gezeigt werden, dass die elektronische Kopplung durch die Homokonjugation einen entscheidenden Einfluss auf die optischen Eigenschaften und somit die elektronische Struktur solcher Verbindungen hat

    How Metal Nuclearity Impacts Electrocatalytic H‚āā Production in Thiocarbohydrazone-Based Complexes

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    Thiocarbohydrazone-based catalysts feature ligands that are potentially electrochemically active. From the synthesis point of view, these ligands can be easily tailored, opening multiple strategies for optimization, such as using different substituent groups or metal substitution. In this work, we show the possibility of a new strategy, involving the nuclearity of the system, meaning the number of metal centers. We report the synthesis and characterization of a trinuclear nickel-thiocarbohydrazone complex displaying an improved turnover rate compared with its mononuclear counterpart. We use DFT calculations to show that the mechanism involved is metal-centered, unlike the metal-assisted ligand-centered mechanism found in the mononuclear complex. Finally, we show that two possible mechanisms can be assigned to this catalyst, both involving an initial double reduction of the system

    Activities of International Joint Usage/Research Center

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    This Annual Report covers from 1 January to 31 December 202

    CuO Nanoparticles Solubility as Influenced by Soil Pore Water, Native Microorganisms, and Wheat Rhizosphere Chemistry in a Sand Matrix

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    Nanoparticles (NPs) are particles less than 100 nm (~4 millionths of an inch) in a direction. NPs, due to their small size, are used in a variety of products, such as silver (Ag) NPs as an antimicrobial in clothes. Copper Oxide (CuO) NPs are used in electronics as semiconductors and other fields as antimicrobials and purposefully or accidentally end up in the environment. Copper (Cu) is a necessary nutrient for plants, but at higher amounts is toxic to plants and beneficial soil microbes. In order to understand how the CuO NPs interacts with plants, wheat seedlings were grown in sand for 10 days. The sand was watered with water extracted from three soils differing in properties relevant to CuO NP dissolution. The SPEs were either sterilized, to provide information on the wheat releases (exudates) into the sand as it grows, or contained native microbes, so that the wheat with microbes could mimic a natural environment. The sand was amended with a low and high dose of CuO NPs to provide two different environments for the wheat to react in. The higher dose of CuO caused the wheat to release greater amounts of exudates. In the presence of microbes, the amounts of exudates drastically decreased, yet the amount of Cu complexed did not drastically change; instead, soil components such as fulvic acid complexed with the Cu. The amount of Cu associated with the roots depended on the amount of NPs added and resulted in root mass with wheat that had less organic matter in the SPEs, but the wheat, regardless of the conditions it was grown in, was able to regulate the amount of Cu taken into the shoots
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