I20-Scanning-X-ray spectroscopy (XAS/XES)
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Yue
Pang
,
Nils
Nöthling
,
Markus
Leutzsch
,
Liqun
Kang
,
Eckhard
Bill
,
Maurice
Van Gastel
,
Edward
Reijerse
,
Richard
Goddard
,
Lucas
Wagner
,
Daniel
Santalucia
,
Serena
Debeer
,
Frank
Neese
,
Josep
Cornella
Diamond Proposal Number(s):
[30449]
Abstract: Large Spin-Orbit Coupling (SOC) is an intrinsic property of the heavy-elements that directly affects the electronic structures of the compounds. Herein we report the synthesis and characterization of a mono-coordinate bismuthinidene featuring a rigid and bulky ligand. All magnetic measurements (SQUID, NMR) point to a diamagnetic compound. However, multiconfigurational quantum chemical calculations predict the ground state of the compound to be dominated (76%) by a spin-triplet. The apparent diamagnetism is explained by an extremely large SOC induced positive zero-field-splitting of more than 4500 cm−1 that leaves the MS = 0 magnetic sublevel thermally isolated in the electronic ground state.
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May 2023
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I19-Small Molecule Single Crystal Diffraction
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Abstract: The reactions of a terminal aluminium imide with a range of oxygen-containing substrates have been probed with a view to developing its use as a novel main group transfer agent for the [NR]2- fragment. We demonstrate transfer of the imide moiety to [N2], [CO] and [Ph(H)C] units driven thermodynamically by Al-O bond formation. N2O reacts rapidly to generate the organoazide DippN3 (Dipp = 2,6- iPr2C6H3), while CO2 (under dilute reaction conditions) yields the corresponding isocyanate, DippNCO. Mechanistic studies, using both experimental and quantum chemical techniques, identify a carbamate complex K2[(NON)Al{κ2-(N,O)-N(Dipp)CO2}]2 (formed via [2+2] cycloaddition) as an intermediate in the formation of DippNCO, and also in an alternative reaction leading to the generation of the amino-dicarboxylate complex K2[(NON)Al{κ2(O,O')-(O2C)2N(Dipp)}] (via the take-up of a second equivalent of CO2). In the case of benzaldehyde, a similar [2+2] cycloaddition process generates the metallacyclic hemi-aminal complex, Kn[(NON)Al{κ2-(N,O)-(N(Dipp)C(Ph)(H)O}]n. Extrusion of the imine, PhC(H)NDipp, via cyclo-reversion is disfavoured thermally, due to the high energy of the putative aluminium oxide co-product, K2[(NON)Al(O)]2. However, addition of CO2 allows the imine to be released, driven by the formation of the thermodynamically more stable aluminium carbonate co-product, K2[(NON)Al(κ2-(O,O')-CO3)]2.
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Jan 2023
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I19-Small Molecule Single Crystal Diffraction
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Diamond Proposal Number(s):
[15059]
Abstract: Four bis [2-{pyrazol-1-yl}-6-{pyrazol-3-yl}pyridine] ligands have been synthesized, with butane-1,4-diyl ( L 1 ), pyrid-2,6-diyl ( L 2 ), benzene-1,2-dimethylenyl ( L 3 ) and propane-1,3-diyl ( L 4 ) linkers between the tridentate metal-binding domains. L 1 and L 2 form [Fe 2 ( μ - L ) 2 ]X 4 (X ‒ = BF 4 ‒ or ClO 4 ‒ ) helicate complexes when treated with the appropriate iron(II) precursor. Solvate crystals of [Fe 2 ( μ - L 1 ) 2 ][BF 4 ] 4 exhibit three different helicate conformations, which differ in the torsions of their butanediyl linker groups. The solvates exhibit gradual thermal spin-crossover, with examples of stepwise switching and partial spin-crossover to a low temperature mixed-spin form. Salts of [Fe 2 ( μ - L 2 ) 2 ] 4+ are high-spin, which reflects their highly twisted iron coordination geometry. The composition and dynamics of assembly structures formed by iron(II) with L 1 - L 3 vary with the ligand linker group, by mass spectrometry and 1 H NMR. Gas phase DFT calculations imply the butanediyl linker conformation in [Fe 2 ( μ - L 1 ) 2 ] 4+ influences its spin state properties, but show anomalies attributed to intramolecular electrostatic repulsion between the iron atoms.
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Nov 2022
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I19-Small Molecule Single Crystal Diffraction
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Diamond Proposal Number(s):
[16139]
Abstract: Covalency involving the 5f orbitals is regularly invoked to explain the reactivity, structure and spectroscopic properties of the actinides, but the ionic versus covalent nature of metal-ligand bonding in actinide complexes remains controversial. The tetrakis 2,6-di-tert-butylphenoxide complexes of Th, U and Np form an isostructural series of crystal structures containing approximately tetrahedral MO4 cores. We show that up to 3 GPa the Th and U crystal structures show negative linear compressibility as the OMO angles distort. At 3 GPa the angles snap back to their original values, reverting to a tetrahedral geometry with an abrupt shortening of the M-O distances by up to 0.1 Å. The Np complex shows similar but smaller effects, transforming above 2.4 GPa. Electronic structure calculations associate the M-O bond shortening with a change in covalency resulting from increased contributions to the M-O bonding by the metal 6d and 5f orbitals, the combination promoting MO4 flexibility at little cost in energy.
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Oct 2022
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B18-Core EXAFS
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Diamond Proposal Number(s):
[11226]
Open Access
Abstract: Over 50 million people in South Asia are exposed to groundwater contaminated with carcinogenic arsenic(III). Photocatalyst-adsorbent composite materials are popularly developed for removing arsenic in a single-step water treatment. Here, As(III) is oxidised to As(V), which is subsequently removed via adsorption. We previously developed a component additive surface complexation model (CA-SCM) to predict the speciation of arsenic adsorbed onto TiO 2/Fe2O3 under different environmental conditions, using surface complexes taken from studies of single-phase minerals. In this work, we critically evaluate this approach, using experimental observations of the surface structures of arsenic adsorbed onto TiO 2/Fe2O3. Extended X-ray absorption fine structure spectroscopy (EXAFS) indicates significant As(III) surface precipitation, and the possible formation of tridentate 3C complexes. EXAFS was unable to identify As binding modes for TiO 2 and Fe2O3 surface complexes simultaneously, highlighting the challenge of analysing composite surfaces. FTIR and zeta potential analysis indicate that As(III)-Fe2O3 surface complexes are protonated at neutral pH, whilst As(III)-TiO 2, As(V)-Fe2O3 and As(V)-TiO 2 surface complexes are negatively charged. Our study confirms the speciation predicted by CA-SCM, particularly As(III) surface precipitation, but also introduces the possibility of tridentate As(III) at acidic pH. This study highlights how experiment and modelling can be combined to assess surface complexation on composite surfaces.
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Sep 2022
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I19-Small Molecule Single Crystal Diffraction
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Diamond Proposal Number(s):
[16139]
Open Access
Abstract: A range of reasons has been suggested for why many low-coordinate complexes across the periodic table exhibit a geometry that is bent, rather a higher symmetry that would best separate the ligands. The dominating reason or reasons are still debated. Here we show that two pyramidal UX3 molecules, in which X is a bulky anionic ligand, show opposite behaviour upon pressurisation in the solid state. UN″3 (UN3, N″ = N(SiMe3)2) increases in pyramidalization between ambient pressure and 4.08 GPa, while U(SAr)3 (US3, SAr = S-C6H2-tBu3−2,4,6) undergoes pressure-induced planarization. This capacity for planarization enables the use of X-ray structural and computational analyses to explore the four hypotheses normally put forward for this pyramidalization. The pyramidality of UN3, which increases with pressure, is favoured by increased dipole and reduction in molecular volume, the two factors outweighing the slight increase in metal-ligand agostic interactions that would be formed if it was planar. The ambient pressure pyramidal geometry of US3 is favoured by the induced dipole moment and agostic bond formation but these are weaker drivers than in UN3; the pressure-induced planarization of US3 is promoted by the lower molecular volume of US3 when it is planar compared to when it is pyramidal.
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Jul 2022
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I19-Small Molecule Single Crystal Diffraction
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Diamond Proposal Number(s):
[25315]
Open Access
Abstract: The reactivity of the Ir(I) PONOP pincer complex [Ir(iPr-PONOP)(η2-propene)][BArF4], 6, [iPr-PONOP = 2,6-(iPr2PO)2C6H3N, ArF = 3,5-(CF3)2C6H3] was studied in solution and the solid state, both experimentally, using molecular density functional theory (DFT) and periodic-DFT computational methods, as well as in situ single-crystal to single-crystal (SC-SC) techniques. Complex 6 is synthesized in solution from sequential addition of H2 and propene, and then the application of vacuum, to [Ir(iPr-PONOP)(η2-COD)][BArF4], 1, a reaction manifold that proceeds via the Ir(III) dihydrogen/dihydride complex [Ir(iPr-PONOP)(H2)H2][BArF4], 2, and the Ir(III) dihydride propene complex [Ir(iPr-PONOP)(η2-propene)H2][BArF4], 7, respectively. In solution (CD2Cl2) 6 undergoes rapid reaction with H2 to form dihydride 7 and then a slow (3 d) onward reaction to give dihydrogen/dihydride 2 and propane. DFT calculations on the molecular cation in solution support this slow, but productive, reaction, with a calculated barrier to rate-limiting propene migratory insertion of 24.8 kcal/mol. In the solid state single-crystals of 6 also form complex 7 on addition of H2 in an SC-SC reaction, but unlike in solution the onward reaction (i.e., insertion) does not occur, as confirmed by labeling studies using D2. The solid-state structure of 7 reveals that, on addition of H2 to 6, the PONOP ligand moves by 90° within a cavity of [BArF4]− anions rather than the alkene moving. Periodic DFT calculations support the higher barrier to insertion in the solid state (ΔG‡ = 26.0 kcal/mol), demonstrating that the single-crystal environment gates onward reactivity compared to solution. H2 addition to 6 to form 7 is reversible in both solution and the solid state, but in the latter crystallinity is lost. A rare example of a sigma amine-borane pincer complex, [Ir(iPr-PONOP)H2(η1-H3B·NMe3)][BArF4], 5, is also reported as part of these studies.
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Jul 2022
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I19-Small Molecule Single Crystal Diffraction
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Deepak
Asthana
,
Dean
Thomas
,
Selena J.
Lockyer
,
Adam
Brookfield
,
Grigore A.
Timco
,
Iñigo J.
Vitorica-Yrezabal
,
George F. S.
Whitehead
,
Eric J. L.
Mcinnes
,
David
Collison
,
David A.
Leigh
,
Richard E. P.
Winpenny
Open Access
Abstract: Polymer beads have been used as the core of magnetic particles for around twenty years. Here we report studies to attach polymetallic complexes to polymer beads for the first time, producing beads of around 115 microns diameter that are attached to 1014 hybrid inorganic-organic [2]rotaxanes. The bead is then formally a [1014] rotaxane. The number of complexes attached is counted by EPR spectroscopy after including TEMPO radicals within the thread of the hybrid [2]rotaxanes.
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Jun 2022
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I19-Small Molecule Single Crystal Diffraction
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Diamond Proposal Number(s):
[22240]
Open Access
Abstract: Four novel homo- and heterometallic sodium and/or aluminium complexes based on the TrenSal ligand, [LH3], have been synthesised and fully characterised, including by single-crystal X-ray diffraction experiments. While [LAl] was completely inactive towards rac-lactide ring-opening polymerisation, incorporating sodium to form heterometallic [LNaAlMe] changes the aluminium geometry from octahedral to tetrahedral, leading to good catalytic activity in the presence of co-initiator BnOH (kobs=3.19×10−2 min−1; room temperature, toluene solvent) and good polymerisation control. Under identical conditions, homometallic sodium complexes showed higher activities in rac-lactide polymerisation than [LNaAlMe], with [LNa3] being extremely active (kobs=1.21 min−1) but displaying unusual second-order monomer dependency and poor polymerisation control. 1H NMR spectroscopic studies suggest that polymerisation with [LNaAlMe] or [LH2Na]/BnOH follows an activated monomer mechanism, whereas [LNa3] operates via simultaneous coordination-insertion and activated monomer mechanisms. Overall, heterometallic [LNaAlMe] provides a balance of good activity and control compared to the homometallic analogues.
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Apr 2022
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B18-Core EXAFS
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Fabio
Del Bello
,
Maura
Pellei
,
Luca
Bagnarelli
,
Carlo
Santini
,
Gianfabio
Giorgioni
,
Alessandro
Piergentili
,
Wilma
Quaglia
,
Chiara
Battocchio
,
Giovanna
Iucci
,
Irene
Schiesaro
,
Carlo
Meneghini
,
Iole
Venditti
,
Nitya
Ramanan
,
Michele
De Franco
,
Paolo
Sgarbossa
,
Cristina
Marzano
,
Valentina
Gandin
Diamond Proposal Number(s):
[25674]
Open Access
Abstract: Bis(pyrazol-1-yl)- and bis(3,5-dimethylpyrazol-1-yl)-acetates were conjugated with the 2-hydroxyethylester and 2-aminoethylamide derivatives of the antineoplastic drug lonidamine to prepare Cu(I) and Cu(II) complexes that might act through synergistic mechanisms of action due to the presence of lonidamine and copper in the same chemical entity. Synchrotron radiation-based complementary techniques [X-ray photorlectron spectroscopy and near-edge X-ray absorption fine structure (NEXAFS)] were used to characterize the electronic and molecular structures of the complexes and the local structure around the copper ion (XAFS) in selected complexes. All complexes showed significant antitumor activity, proving to be more effective than the reference drug cisplatin in a panel of human tumor cell lines, and were able to overcome oxaliplatin and multidrug resistance. Noticeably, these Cu complexes appeared much more effective than cisplatin against 3D spheroids of pancreatic PSN-1 cancer cells; among these, PPh3-containing Cu(I) complex 15 appeared to be the most promising derivative. Mechanistic studies revealed that 15 induced cancer cell death by means of an apoptosis-alternative cell death.
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Mar 2022
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