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Abstract: Single-component molecular conductors form an important class of materials showing exotic quantum phenomena, owing to the range of behavior they exhibit under physical stimuli. We report the effect of high pressure on the electrical properties and crystal structure of the single-component crystal [Ni(dddt)2] (where dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate). The system is isoelectronic and isostructural with [Pd(dddt)2], which is the first example of a single-component molecular crystal that exhibits nodal line semimetallic behavior under high pressure. Systematic high pressure four-probe electrical resistivity measurements were performed up to 21.6 GPa, using a Diamond Anvil Cell (DAC), and high pressure single crystal synchrotron X-ray diffraction was performed up to 11.2 GPa. We found that [Ni(dddt)2] initially exhibits a decrease of resistivity upon increasing pressure but, unlike [Pd(dddt)2], it shows pressure-independent semiconductivity above 9.5 GPa. This correlates with decreasing changes in the unit cell parameters and intermolecular interactions, most notably the π-π stacking distance within chains of [Ni(dddt)2] molecules. Using first-principles density functional theory (DFT) calculations, based on the experimentally-determined crystal structures, we confirm that the band gap decreases with increasing pressure. Thus, we have been able to rationalize the electrical behavior of [Ni(dddt)2] in the pressure-dependent regime, and suggest possible explanations for its pressure-independent behavior at higher pressures.

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Abstract: [FeFe]-hydrogenases contain strongly electronically coupled diiron [2Fe]H and tetrairon [Fe4–S4]H clusters, and thus much recent effort has focused on the chemistry of diiron-dithiolate biomimics with appended redox-active ligands. Here we report on the synthesis and electrocatalytic activity of Fe2(CO)4(μ-edt)(κ2-bpcd) (2) in which the electron-acceptor 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) acts as a surrogate of the [Fe4–S4]H sub-cluster. The complex is prepared in low yield but has been fully characterised, including a crystallographic study which shows that the diphosphine adopts a basal-apical coordination geometry in the solid state. Cyclic voltammetry shows that 2 undergoes four reduction events with DFT studies confirming that the first reduction is localised on the low-lying π* system of the diphosphine ligand. The addition of the second electron furnishes a triplet dianion that exhibits spin density distributed over the diphosphine and diiron subunits. Protonation at the Fe–Fe bond of the triplet dianion furnishes the corresponding bridging hydride as the thermodynamically favoured species that contains a reduced bpcd ligand. Complex 2 functions as a catalyst for proton-reduction at its second reduction potential, in contrast to the related 2,3-bis(diphenylphosphino)maleic anhydride (bma) complex, Fe2(CO)4(μ-pdt)(κ2-bma) (1), which shows similar electrochemical behaviour but is not catalytically active. The difference in chemical behaviour is attributed to greater stability of the 4-cyclopenten-1,3-dione platform in 2 as compared to the maleic anhydride ring of the bma ligand in 1 following the uptake of the second electron. Thus protonation of the Fe–Fe bond in the 22− affords a species which is stable enough to undergo a further reduction–protonation event, unlike the bma ligand whose maleic anhydride ring undergoes deleterious C–O bond scission upon protonation or reaction with adventitious moisture. DFT studies, however, suggest that electron-transfer from the diphosphine to the diiron centre is not significant, probably due to their poor redox levelling. Thus, while the diphosphine is readily reduced, the added electron is apparently not utilised in proton-reduction and hence cannot truly be considered as an [Fe4–S4]H surrogate.

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Abstract: Solid/gas Single–Crystal to Single–Crystal (SC–SC) hydrogenation of appropriate diene precursors forms the corresponding σ–alkane complexes [Rh(Cy2P(CH2)nPCy2)(L)][BArF4] (n = 3, 4) and [RhH(Cy2P(CH2)2(CH)(CH2)2PCy2)(L)][BArF4] (n = 5, L = norbornane, NBA; cyclooctane, COA). Their struc-tures, as determined by single–crystal X-ray diffraction, have cations exhibiting Rh···H–C σ–interactions which are modulated by both the chelating ligand and the identity of the alkane, while all sit in an octahedral anion–microenvironment. These range from chelating η2,η2 Rh···H–C (e.g. [Rh(Cy2P(CH2)nPCy2)(η2η2–NBA)][BArF4], n = 3 and 4), through to more weakly bound η1 Rh··H–C in which C–H activation of the chelate backbone has also occurred (e.g. [RhH(Cy2P(CH2)2(CH)(CH2)2PCy2)(η1–COA)][BArF4]) and ultimately to systems where the alkane is not ligated with the metal center, but sits encapsulated in the supporting anion microenvironment – [Rh(Cy2P(CH2)3PCy2)][COA⊂BArF4] – in which the metal center instead forms two intramolecular agostic η1 Rh···H–C interactions with the phosphine cyclohexyl groups. CH2Cl2 adducts formed by displacement of the η1–alkanes in solution (n = 5; L = NBA, COA), [RhH(Cy2P(CH2)2(CH)(CH2)2PCy2)(η1–ClCH2Cl)][BArF4], are characterized crystallographically. Analyses via periodic DFT, QTAIM, NBO and NCI calculations, alongside variable temperature solid–state NMR spectroscopy, provide snapshots marking the onset of Rh σ-alkane interactions along a C···H activation trajectory. These are negligible in [Rh(Cy2P(CH2)3PCy2)][COA⊂BArF4]; in [RhH(Cy2P(CH2)2(CH)(CH2)2PCy2)(η1–COA)][BArF4] σC–H→Rh σ-donation is supported by Rh→σ*C-H ‘pregostic’ donation; and in [Rh(Cy2P(CH2)nPCy2)(η2η2-NBA)][BArF4] (n = 2-4) σ-donation dominates, supported by classical Rh(dπ)→σ*C-H π-back donation. Dispersive interactions with the [BArF4]- anions and Cy substituents further stabilize the alkanes within the binding pocket.

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Abstract: We present a detailed kinetic study of photo-induced solid state linkage isomerism in the compound [Pd(Bu4dien)NO2]BPh4 (Bu4dien = N,N,N’,N’-tetrabutyldiethylenetriamine) using in-situ photocrystallographic techniques. We explore the key variables that influence the photoconversion and develop a detailed kinetic model for the excitation and decay processes and the temperature dependence of the conversion rates. We show that by varying the temperature the lifetime of the excited state can be varied over orders of magnitude, making these systems ideal test cases for the development of new time-resolved X-ray diffraction methods. The kinetic model is used to build a numerical-simulation tool, which we use to explore the practicalities of pump-probe single-crystal diffraction experiments with minute and second time-resolution.

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Abstract: Herein, we describe the development of a novel dual air-bearing fixed-χ diffractometer for beamline I19 at Diamond Light Source. The diffractometer is designed to facilitate the rapid data collections possible with a Dectris Pilatus 2M pixel-array photon-counting detector, while allowing remote operation in conjunction with a robotic sample changer. The sphere-of-confusion is made as small as practicably possible, through the use of air-bearings for both the ω and φ axes. The design and construction of the new instrument is described in detail and an accompanying paper by Johnson et al. (also in this issue) will provide a user perspective of its operation.