The instrument suite of the European Spallation Source

An overview is provided of the 15 neutron beam instruments making up the initial instrument suite of the European Spallation Source (ESS), and being made available to the neutron user community. The ESS neutron source consists of a high-power accelerator and target station, providing a unique long-pulse time structure of slow neutrons. The design considerations behind the time structure, moderator geometry and instrument layout are presented. The 15-instrument suite consists of two small-angle instruments, two reflectometers, an imaging beamline, two single-crystal diffractometers; one for macromolecular crystallography and one for magnetism, two powder diffractometers, and an engineering diffractometer, as well as an array of five inelastic instruments comprising two chopper spectrometers, an inverse-geometry single-crystal excitations spectrometer, an instrument for vibrational spectroscopy and a high-resolution backscattering spectrometer. The conceptual design, performance and scientific drivers of each of these instruments are described. All of the instruments are designed to provide breakthrough new scientific capability, not currently available at existing facilities, building on the inherent strengths of the ESS long-pulse neutron source of high flux, flexible resolution and large bandwidth. Each of them is predicted to provide world-leading performance at an accelerator power of 2 MW. This technical capability translates into a very broad range of scientific capabilities. The composition of the instrument suite has been chosen to maximise the breadth and depth of the scientific impact o

Iron(II) complexes of 2,6-bis(imidazo[1,2‑a]pyridin-2-yl)pyridine and related ligands with annelated distal heterocyclic donors

Following a published synthesis of 2,6-bis(imidazo[1,2-a]pyridin-2-yl)pyridine (L1), treatment of α,α′-dibromo-2,6-diacetylpyridine with 2 equiv. 2-aminopyrimidine or 2-aminoquinoline in refluxing acetonitrile respectively gives 2,6-bis(imidazo[1,2-a]pyrimidin-2-yl)pyridine (L2) and 2,6-bis(imidazo[1,2-a]quinolin-2-yl)pyridine (L3). Solvated crystals of [Fe(L1)2][BF4]2 (1[BF4]2) and [Fe(L2)2][BF4]2 (2[BF4]2) are mostly high-spin, although one solvate of 1[BF4]2 undergoes thermal spin-crossover on cooling. The iron coordination geometry is consistently distorted in crystals of 2[BF4]2 which may reflect the influence of intramolecular, inter-ligand N⋯π interactions on the molecular conformation. Only 1[thin space (1/6-em)]:[thin space (1/6-em)]1 Fe[thin space (1/6-em)]:[thin space (1/6-em)]L3 complexes were observed in solution, or isolated in the solid state; a crystal structure of [FeBr(py)2L3]Br·0.5H2O (py = pyridine) is presented. A solvate crystal structure of high-spin [Fe(L4)2][BF4]2 (L4 = 2,6-di{quinolin-2-yl}pyridine; 4[BF4]2) is also described, which exhibits a highly distorted six-coordinate geometry with a helical ligand conformation. The iron(II) complexes are high-spin in solution at room temperature, but 1[BF4]2 and 2[BF4]2 undergo thermal spin-crossover equilibria on cooling. All the compounds exhibit a ligand-based emission in solution at room temperature. Gas phase DFT calculations mostly reproduce the spin state properties of the complexes, but show small anomalies attributed to intramolecular, inter-ligand dispersion interactions in the sterically crowded molecules

A Survey of the Angular Distortion Landscape in the Coordination Geometries of High-Spin Iron(II) 2,6-Bis(pyrazolyl)pyridine Complexes

Reaction of 2,4,6-trifluoropyridine with sodium 3,4-dimethoxybenzenethiolate and 2 equiv of sodium pyrazolate in tetrahydrofuran at room temperature affords 4-(3,4-dimethoxyphenylsulfanyl)-2,6-di(pyrazol-1-yl)pyridine (L), in 30% yield. The iron(II) complexes [FeL2][BF4]2 (1a) and [FeL2][ClO4]2 (1b) are high-spin with a highly distorted six-coordinate geometry. This structural deviation from ideal D2d symmetry is common in high-spin [Fe(bpp)2]2+ (bpp = di{pyrazol-1-yl}pyridine) derivatives, which are important in spin-crossover materials research. The magnitude of the distortion in 1a and 1b is the largest yet discovered for a mononuclear complex. Gas-phase DFT calculations at the ω-B97X-D/6-311G** level of theory identified four minimum or local minimum structural pathways across the distortion landscape, all of which are observed experimentally in different complexes. Small distortions from D2d symmetry are energetically favorable in complexes with electron-donating ligand substituents, including sulfanyl groups, which also have smaller energy penalties associated with the lowest energy distortion pathway. Natural population analysis showed that these differences reflect greater changes to the Fe–N{pyridyl} σ-bonding as the distortion proceeds, in the presence of more electron-rich pyridyl donors. The results imply that [Fe(bpp)2]2+ derivatives with electron-donating pyridyl substituents are more likely to undergo cooperative spin transitions in the solid state. The high-spin salt [Fe(bpp)2][CF3SO3]2, which also has a strong angular distortion, is also briefly described and included in the analysis

Activating a High-Spin Iron(II) Complex to Thermal Spin-Crossover with an Inert Non-Isomorphous Molecular Dopant

[Fe(bpp)2][ClO4]2 (bpp = 2,6-bis{pyrazol-1-yl}pyridine; monoclinic, C2/c) is high-spin between 5-300 K, and crystallises with a highly distorted molecular geometry that lies along the octahedral−trigonal prismatic distortion pathway. In contrast, [Ni(bpp)2][ClO4]2 (monoclinic, P21) adopts a more regular, near-octahedral coordination geometry. Gas phase DFT minimisations (ω-B97X-D/6-311G**) of [M(bpp)2]2+ complexes show the energy penalty associated with that coordination geometry distortion runs as M2+ = Fe2+ (HS) ≈ Mn2+ (HS) < Zn2+ ≈ Co2+ (HS) ≲ Cu2+ << Ni2+ << Ru2+ (LS; HS = high-spin, LS = low-spin). Slowly crystallised solid solutions [FexNi1−x(bpp)2][ClO4]2 with x = 0.53 (1a) and 0.74 (2a) adopt the P21 lattice, while x = 0.87 (3a) and 0.94 (4a) are mixed-phase materials with the high-spin C2/c phase as the major component. These materials exhibit thermal spin-transitions at T½ = 250 ±1 K which occurs gradually in 1a, and abruptly and with narrow thermal hysteresis in 2a-4a. The transition proceeds to 100 % completeness in 1a and 2a; that is, the 26 % Ni doping in 2a is enough to convert high-spin [Fe(bpp)2][ClO4]2 into a cooperative, fully SCO-active material. These results were confirmed crystallographically for 1a and 2a, which revealed similarities and differences between these materials and the previously published [FexNi1−x(bpp)2][BF4]2 series. Rapidly precipitated powders with the same compositions (1b-4b) mostly resemble 1a-4a, except that 2b is a mixed-phase material; 2b-4b also contain a fraction of amorphous solid in addition to the two crystal phases. The largest iron fraction that can be accommodated by the P21 phase in this system is 0.7 ±0.1

A study of the thermal and light induced spin transition in FeL2 (BF4)(2) and FeL2 (ClO4)(2) L=2,6-di(3-methylpyrazol-1-yl)pyrazine

The spin crossover compounds [FeL2](BF4)(2), L = 2,6-di(3-methylpyrazol-1-yl) pyrazine and [FeL2](ClO4)(2) have very unusual two stage spin transitions which are initially steep and then become more gradual. A detailed variable temperature single crystal X-ray diffraction study has shown that the course of the spin transition is controlled by an order - disorder transition in the counter anions. The high and low spin states both crystallise in the tetragonal space group I (4) over bar, the structures of the high and low spin states are presented at 290 and 30 K, respectively. The title compounds are shown to undergo LIESST ( Light Induced Excited Spin State Trapping) under irradiation with either red or green laser light with wavelengths of 632.8 and 532.06 nm, respectively, at 30 K. The cell parameters for the tetragonal photo-induced metastable high spin state at this temperature are a = 9.169(6), c = 17.77(1) Angstrom for [FeL2](ClO4)(2) with an increase in unit cell volume of 21 Angstrom(3), and a = 9.11(1), c = 17.75(2) Angstrom and an increase in volume of 42.8 Angstrom(3) for [FeL2](BF4)(2)

An X-ray powder diffraction study of the spin-crossover transition and structure of bis (2,6-dipyrazol-1-ylpyrazine)iron(II) perchlorate

The crystal structure of the iron(II) spin-crossover compound [Fe(C10H8N6)(2)](ClO4)(2) in the high-spin state has been solved from powder X-ray diffraction data using the DASH program and refined using Rietveld refinement. The thermal spin transition has been monitored by following the change in unit-cell parameters with temperature. The title compound has been found to undergo a crystallographic phase change, involving a doubling of the crystallographic a axis, on undergoing the spin transition

Photomagnetic properties of iron(II) spin crossover complexes of 2,6-dipyrazolylpyridine and 2,6-dipyrazolylpyrazine ligands.

The photomagnetic properties of the following iron(II) complexes have been investigated: [Fe(L1)2][BF4]2 (1), [Fe(L2)2][BF4]2 (2), [Fe(L2)2][ClO4]2 (3), [Fe(L3)2][BF4]2 (4), [Fe(L3)2][ClO4]2 (5) and [Fe(L4)2][ClO4]2 (6) (L1 = 2,6-di{pyrazol-1-yl}pyridine; L2 = 2,6-di{pyrazol-1-yl}pyrazine; L3 = 2,6-di{pyrazol-1-yl}-4-{hydroxymethyl}pyridine; and L4 = 2,6-di{4-methylpyrazol-1-yl}pyridine). Compounds 1–6 display a complete thermal spin transition centred between 200–300 K, and undergo the light-induced excited spin state trapping (LIESST) effect at low temperatures. The T(LIESST) relaxation temperature of the photoinduced high-spin state for each compound has been determined. The presence of sigmoidal kinetics in the HS → LS relaxation process, and the observation of LITH hysteresis loops under constant irradiation, demonstrate the cooperative nature of the spin transitions undergone by these materials. All the compounds in this study follow a previously proposed linear relation between T(LIESST) and their thermal spin-transition temperatures T1/2: T(LIESST) = T0 − 0.3T1/2. T0 for these compounds is identical to that found previously for another family of iron(II) complexes of a related tridentate ligand, the first time such a comparison has been made. Crystallographic characterisation of the high- and low-spin forms of 5 and 6, the light-induced high-spin state of 5, and the low-spin complex [Fe(L4)2][BF4]2 (7), are described

The effect of inert dopant ions on spin-crossover materials is not simply controlled by chemical pressure

[Fe(bpp)2][BF4]2 (bpp = 2,6-bis{pyrazol-1-yl}pyridine) undergoes abrupt thermal spin-crossover (SCO) at 261 K with a small 2–3 K thermal hysteresis. Different compositions of doped materials [FezZn1−z(bpp)2][BF4]2 and [FezRu1−z(bpp)2][BF4]2 (0 < z < 1) show similar broadening of the SCO transition with increased doping, but differ in their effect on the transition temperature. Doping with zinc strongly lowers T½, which is consistent with previous work. In contrast, doping with ruthenium increases T½ to a smaller degree, which cannot be explained by the chemical pressure arguments that are conventionally applied to doped SCO materials. Mechanoelastic simulations imply that different dopants exert opposite effects on the lattice elastic interactions in the material during the SCO transition. Consistent with that, the materials show a complicated dependence of the crystallographic lattice parameters and thermal expansion properties on the iron spin state, for different dopant ions. These changes correlate with small perturbations to the molecular structure of high-spin [Fe(bpp)2]2+, in the presence of dopants with different geometric preferences and conformational rigidities. We conclude the effect of isomorphous dopants on T½ reflects how the dopant influences the coordination geometry of the iron centres, as well as the chemical pressure exerted by the dopant ion size