High-pressure single-crystal diffraction at the Australian Synchrotron.

A new high-pressure single-crystal diffraction setup has been designed and implemented at the Australian Synchrotron for collecting molecular and protein crystal structures. The setup incorporates a modified micro-Merrill-Bassett cell and holder designed specifically to fit onto the horizontal air-bearing goniometer, allowing high-pressure diffraction measurements to be collected with little to no modification of the beamline setup compared with ambient data collections. Compression data for the amino acid, L-threonine, and the protein, hen egg-white lysozyme, were collected, showcasing the capabilities of the setup.

Guest-Induced Multistep to Single-Step Spin-Crossover Switching in a 2-D Hofmann-Like Framework with an Amide-Appended Ligand.

A detailed study of the two-dimensional (2-D) Hofmann-like framework [Fe(furpy)2Pd(CN)4]·nG (furpy: N-(pyridin-4-yl)furan-2-carboxamide, G = H2O,EtOH (A·H2O,Et), and H2O (A·H2O)) is presented, including the structural and spin-crossover (SCO) implications of subtle guest modification. This 2-D framework is characterized by undulating Hofmann layers and an array of interlayer spacing environments─this is a strategic approach that we achieve by the inclusion of a ligand with multiple host-host and host-guest interaction sites. Variable-temperature magnetic susceptibility studies reveal an asymmetric multistep SCO for A·H2O,Et and an abrupt single-step SCO for A·H2O with an upshift in transition temperature of ∼75 K. Single-crystal analyses show a primitive orthorhombic symmetry for A·H2O,Et characterized by a unique FeII center─the multistep SCO character is attributed to local ligand orientation. Counterintuitively, A·H2O shows a triclinic symmetry with two inequivalent FeII centers that undergo a cooperative single-step high-spin (HS)-to-low-spin (LS) transition. We conduct detailed structure-function analyses to understand how the guest ethanol influences the delicate balance between framework communication and, therefore, the local structure and spin-state transition mechanism.

Regulation of Multistep Spin Crossover Across Multiple Stimuli in a 2-D Framework Material.

We investigate the effects of a broad array of external stimuli on the structural, spin-crossover (SCO) properties and nature of the elastic interaction within the two-dimensional Hofmann framework material [Fe(cintrz)2Pd(CN)4]·guest (cintrz = N-cinnamalidene 4-amino-1,2,4-triazole; A·guest; guest = 3H2O, 2H2O, and Ø). This framework exhibits a delicate balance between ferro- and antiferro-elastic interaction characters; we show that manipulation of the pore contents across guests = 3H2O, 2H2O, and Ø can be exploited to regulate this balance. In A·3H2O, the dominant antiferroelastic interaction character between neighboring FeII sites sees the low-temperature persistence of the mixed spin-state species {HS-LS} for {Fe1-Fe2} (HS = high spin, LS = low spin). Elastic interaction strain is responsible for stabilizing the {HS-LS} state and can be overcome by three mechanisms: (1) partial (2H2O) or complete (Ø) guest removal, (2) irradiation via the reverse light-induced excited spin-state trapping (LIESST) effect (λ = 830 nm), and (3) the application of external hydrostatic pressure. Combining experimental data with elastic models presents a clear interpretation that while guest molecules cause a negative chemical pressure, they also have consequences for the elastic interactions between metals beyond the simple chemical pressure picture typically proposed.

Elastically flexible molecular crystals.

The discovery of molecular single crystals that display interesting elastic behaviour has generated excitement regarding their potential applications as it has upended the common perception of crystals as brittle objects. In order to design new functional materials based on molecular crystals, a comprehensive understanding of how these materials respond to deformation on a molecular-level is required. An introduction to the underlying mechanical theory and how it may be applied to single crystals is provided, along with a comprehensive discussion on how these mechanical properties can be characterised. While this field has already presented a large number of elastically flexible crystals, there is a lack of detailed mechanical characterisation data and some contention regarding the atomic-scale mechanism of elasticity. Due to the discrepancies and contradictions between theories proposed in the literature, it is not yet understood why some crystals are elastic while others shatter under applied force. To dispel ambiguity and guide future research, a set of criteria are proposed to define an elastically flexible crystal, so that these materials may find applications among future technologies.

Guest-Dependent Isomer Convergence of a Permanently Fluxional Coordination Cage.

A fluxional bis-monodentate ligand, based on the archetypal shape-shifting molecule bullvalene, self-assembles with M2+ (M=Pd2+ or Pt2+ ) to produce a highly complex ensemble of permanently fluxional coordination cages. Metal-mediated self-assembly selects for an M2 L4 architecture while maintaining shape-shifting ligand complexity. A second level of simplification is achieved with guest-exchange; the binding of halides within the M2 L4 cage mixture results in a convergence to a cage species with all four ligands present as the "B isomer". Within this confine, the reaction graph of the bullvalene is greatly restricted, but gives rise to a mixture of 38 possible diastereoisomers in rapid exchange. X-ray crystallography reveals a preference for an achiral form consisting of both ligand enantiomers. Through a combination of NMR spectroscopy and DFT calculations, we elucidate the restricted isomerisation pathway of the permanently fluxional M2 L4 assembly.

Visible-Light-Responsive Self-Assembled Complexes: Improved Photoswitching Properties by Metal Ion Coordination.

A photoswitchable ligand based on azobenzene is self-assembled with palladium(II) ions to form a [Pd2 (E-L)4 ]4+ cage. Irradiation with 470 nm light results in the near-quantitative switching to a monomeric species [Pd(Z-L)2 ]2+ , which can be reversed by irradiation with 405 nm light, or heat. The photoswitching selectivity towards the metastable isomer is significantly improved upon self-assembly, and the thermal half-life is extended from 40 days to 850 days, a promising approach for tuning photoswitching properties.

Side-Chain Interactions in d/l Peptide Nanotubes: Studies by Crystallography, NMR Spectroscopy and Molecular Dynamics.

Our understanding of the factors affecting the stability of cyclic d/l peptide (CP) nanotubes remains underdeveloped. In this work, we investigate the impact of side chain alignment, hydrophobicity and charge on CP nanotube stability through X-ray crystallography, NMR spectroscopy and molecular dynamics (MD) simulations. We characterise the distinct CP-CP alignments that can form and identify stable and unstable dimers by MD simulation. We measure H-bond half-lives of synthesised CPs by 1 H-D exchange experiments and find good correlation with predicted CP-CP stabilities. We find that hydrophobic amino acids improve CP dimer stability but experimentally reduce solubility. Charged amino acids either increase or decrease CP dimer stability depending on the relative orientation and composition of charged groups. X-ray crystal structures are solved for two CPs, revealing non-tubular folded conformations. Ultimately, this work will assist the educated design of stable tubular structures for potential applications in biomedicine.

[U(H2O)2]{[(UO2)10O10(OH)2][(UO4)(H2O)2]}: A Mixed-Valence Uranium Oxide Hydrate Framework.

A uranium oxide hydrate framework, [U(H2O)2]{[(UO2)10O10(OH)2][(UO4)(H2O)2]} (UOF1), was synthesized hydrothermally using schoepite as a uranium precursor. The crystal strucutre of UOF1 was revealed with synchrotron single-crystal X-ray diffraction and confirmed with transmission electron miscroscopy. The typical uranyl oxide hydroxide layers similar to those in ß-U3O8 are further connected via double-pentagonal-bipyramidal uranium polyhedra to form a three-dimensional (3D) framework structure with tetravalent uranium species inside the channels. The presence of mixed-valence uranium was investigated with a combination of X-ray absorption near-edge structure and diffuse reflectance spectroscopy. Apart from the major hexavalent uranium, evidence for tetravalent uranium was also found, consistent with the bond valence sum calculations. The successful preparation of UOF1 as the first pure uranium oxide hydrate framework sheds light on the structural understanding of the alteration of UO2+x as either a mineral or spent nuclear fuel.

Guest Removal and External Pressure Variation Induce Spin Crossover in Halogen-Functionalized 2-D Hofmann Frameworks.

The effect of halogen functionalization on the spin crossover (SCO) properties of a family of 2-D Hofmann framework materials, [FeIIPd(CN)4(thioX)2]·2H2O (X = Cl and Br; thioCl = (E)-1-(5-chlorothiophen-2-yl)-N-(4H-1,2,4-triazol-4-yl)methanimine) and thioBr = (E)-1-(5-bromothiophen-2-yl)-N-(4H-1,2,4-triazol-4-yl)methanimine)), is reported. Inclusion of both the chloro- and bromo-functionalized ligands into the Hofmann-type frameworks (1Cl·2H2O and 2Br·2H2O) results in a blocking of spin-state transitions due to internal chemical pressure effects derived by the collective steric bulk of the halogen atoms and guest molecules. Cooperative one-step SCO transitions are revealed by either guest removal or the application of external physical pressure. Notably, removal of solvent water reveals a robust framework scaffold with only marginal variation between the solvated and desolvated structures (as investigated by powder and single crystal X-ray diffraction). Yet, one-step complete SCO transitions are revealed in 1Cl and 2Br with a transition temperature shift between the analogues due to various steric, structural, and electronic considerations. SCO can also be induced in the solvated species, 1Cl·2H2O and 2Br·2H2O, with the application of physical pressure, revealing a complete one-step SCO transition above 0.62 GPa (as investigated by magnetic susceptibility and single crystal X-ray diffraction measurements).

Diastereoselective Control of Tetraphenylethene Reactivity by Metal Template Self-Assembly.

The reaction of 4,4',4'',4'''-(ethene-1,1,2,2-tetrayl)tetraaniline with 2-pyridinecarboxaldehyde and iron(II) chloride resulted, after aqueous workup, in the diastereoselective formation of an [Fe2 L3 ]4+ triple-stranded helicate structure, irrespective of the stoichiometry employed. The helicate structure was characterized in solution by multinuclear NMR spectroscopy, and in the solid state by single-crystal X-ray crystallography. The reaction of iron(II) tetrafluoroborate or iron(II) bistriflimide with the tetraaniline and 2-pyridinecarboxaldehyde allowed the formation of an [Fe8 L6 ]16+ cube when the appropriate stoichiometry was used, but these structures were unstable with respect to hydrolysis. The pendant amine groups on the helicate can be functionalized by reaction with acid chlorides or anhydrides, and the resulting functionalized tetraphenylethene (TPE) units were isolated by the reaction of the helicate with tris(2-aminoethyl)amine. The emission properties of the TPE units were studied in THF/water mixtures, and they were found by dynamic light scattering to self-assemble into large (av. diameter 250 nm) structures.

Proximity Enforced Agostic Interactions Involving Closed-Shell Coinage Metal Ions.

A proximity enforcing diarylsilane ligand is reported, which gives rise to unusual Si-H···M interactions with the d10 metal ions Cu+ and Ag+ upon complexation. These interactions are studied in detail both experimentally and computationally and can be classified to be weakly agostic in nature for the Si-H···Cu interaction. The Si-H···Ag interaction has more signatures of an electrostatic contact.

Predictable Substituent Control of CoIII/II Redox Potential and Spin Crossover in Bis(dipyridylpyrrolide)cobalt Complexes.

A family of five easily prepared tridentate monoanionic 2,5-dipyridyl-3-(R1)-4-(R2)-pyrrolide anions (dppR1,R2)-, varying in the nature of the R1 and R2 substituents [R1, R2 = CN, Ph; CO2Et, CO2Et; CO2Me, 4-Py; CO2Me, Me; Me, Me], has been used to generate the analogous family of neutral [CoII(dppR1,R2)2] complexes, two of which are structurally characterized at both 100 and 298 K. Both the oxidation and spin states of these complexes can be switched in response to appropriate external stimuli. All complexes, except [CoII(dppMe,Me)2], exhibit gradual spin crossover (SCO) in the solid state, and SCO activity is observed for three complexes in CDCl3 solution. The cobalt(II) centers in the low spin (LS) complexes are Jahn-Teller tetragonally compressed along the pyrrolide-Co-pyrrolide axis. The complexes in their high spin (HS) states are more distorted than in the LS states, as is also usually the case for SCO active iron(II) complexes. The reversible CoIII/II redox potentials are predictably tuned by choice of substituents R1 and R2, from -0.95 (Me,Me) to -0.45 (CN,Ph) V vs Fc+/Fc, with a linear correlation observed between E1/2(CoIII/II) and the Swain-Lupton parameters of the pyrrolide substituents.

Molecular Switches for any pH: A Systematic Study of the Versatile Coordination Behaviour of Cyclam Scorpionands.

Molecular switches have many potential applications in nanoscience and biomedicine. Transition metal complexes that can be switched from an inert, unreactive state to a catalytically active one by a simple change in conditions (e.g. pH shift) or by binding to a specific biomolecular target-so-called target-activated metal complexes (TAMCs)-hold particular allure as a means of harnessing the potent but at times indiscriminate reactivity of metal-based drugs. Towards this goal, we have prepared a series of ten structurally related ligands, each of which bears a different pendant side-arm functional group appended to a common macrocyclic core, along with copper(II) and nickel(II) complexes of these cyclam-based "molecular scorpionands". X-ray crystal structures reveal a variety of binding modes between pendant side-arm and metal centre that depend on the constituent donor atoms. To investigate the switchability of side-arm coordination in solution, spectrophotometric pH titrations were carried out for all 20 metal complexes. The majority of the complexes undergo spectroscopic changes that are consistent with a switch in pendant coordination state at a specific pH. This ligand series represents a comprehensive model platform from which to build pH-switchable metal complexes for applications in nanoscience and biomedicine.

Thorium(IV) and Uranium(IV) Complexes with Cucurbit[5]uril.

Tetravalent thorium and uranium complexes with cucurbit[5]uril (Q[5]) were investigated with eight new complexes being synthesized and structurally characterized. [Th(Q[5])(OH)(H2O)2]6·18NO3· nH2O (1) has a hexagonal nanowheel structure with each of the six Th4+ ions being cap-coordinated by a Q[5] and monodentate-coordinated to the nearby Q[5]. [Th(Q[5])(HCOO)(H2O)4][Th(NO3)5(H2O)2]2[Th(NO3)3(HCOO)(H2O)2]0.5·NO3· nH2O (2) has a heteroleptic mononuclear structure with a Th4+ ion cap-coordinated on one side of the Q[5] portal and monodentate-coordinated to a formate anion inside the Q[5] cavity. [KTh1.5(Q[5])Cl(NO3)3][Th(NO3)5(H2O)2]·2NO3·2.5H2O (3) has a heterometallic structure with both Th4+ and K+ ions each occupying one side of the two Q[5] portals forming a capsule. [CsTh(Q[5])Cl(NO3)2(H2O)3]·2NO3· nH2O (4) has a heterometallic 1D polymeric structure with both Th4+ and Cs+ ions each occupying one side of the two Q[5] portals, forming monomers which are linked together by sharing two water molecules and one carbonyl oxygen atom between Th4+ and Cs+ ions. [Th(Q[5])Cl(H2O)][CdCl3][CdCl4]·0.5HCl·4H2O (5), [Th(Q[5])Cl(H2O)][Ru2OCl9(H2O)]·0.5HCl·9.5H2O (6), [Th(Q[5])Cl(H2O)][IrCl6]1.5·3H2O (7), and [U(Q[5])Cl(H2O)][ZnCl3(H2O)][(ZnCl4)]·8H2O (8) have similar 1D polymeric structures with Th4+/U4+ ions cap-coordinated on one side of a Q[5] and bidentate coordinated to the nearby Q[5]. The transition metal chlorides act as anions for charge compensation as well as structure inducers via cation-anion interactions forming various anion patterns around the 1D polymers. Actinide contraction has been observed in the early actinide series.

"114"-Type Nitrides LnAl(Si4-x Alx )N7 Oδ with Unusual [AlN6 ] Octahedral Coordination.

Aluminum-nitrogen six-fold octahedral coordination, [AlN6 ], is unusual and has only been seen in the high-pressure rocksalt-type aluminum nitride or some complex compounds. Herein we report novel nitrides LnAl(Si4-x Alx )N7 Oδ (Ln=La, Sm), the first inorganic compounds with [AlN6 ] coordination prepared via non-high-pressure synthesis. Structure refinements of neutron powder diffraction and single-crystal X-ray diffraction data show that these compounds crystallize in the hexagonal Swedenborgite structure type with P63 mc symmetry where Ln and Al atoms locate in anticuboctahedral and octahedral interstitials, respectively, between the triangular and Kagomé layers of [SiN4 ] tetrahedra. Solid-state NMR data of high-purity La-114 powders confirm the unusual [AlN6 ] coordination. These compounds are the first examples of the "33-114" sub-type in the "114" family. The additional site for over-stoichiometric oxygen in the structure of 114-type compounds was also identified.

Chiral Ruthenium(II) Complexes as Supramolecular Building Blocks for Heterometallic Self-Assembly.

A series of enantiopure ruthenium(II) polypyridyl complexes are reported that feature pendant pyridyl groups suitable for building larger self-assembled structures. The complexes are characterized in detail in solution using NMR spectroscopy, cyclic voltammetry, and photophysical methods and in the solid state using single-crystal X-ray crystallography. The complexes are luminescent, displaying long excited-state lifetimes that are quenched when the pendant pyridyl groups are protonated. Reaction with cadmium(II) ions results in the formation of a mixed-metal one-dimensional coordination polymer, which was characterized by single-crystal X-ray crystallography.

Hysteretic Four-Step Spin Crossover within a Three-Dimensional Porous Hofmann-like Material.

Materials that display multiple stepped spin crossover (SCO) transitions with accompanying hysteresis present the opportunity for ternary, quaternary, and quinary electronic switching and data storage but are rare in existence. Herein, we present the first report of a four-step hysteretic SCO framework. Single-crystal structure analysis of a porous 3D Hofmann-like material showed long-range ordering of spin states: HS, HS0.67 LS0.33 , HS0.5 LS0.5 , HS0.33 LS0.67 , and LS. These detailed structural studies provide insight into how multistep SCO materials can be rationally designed through control of host-host and host-guest interactions.

MX1: a bending-magnet crystallography beamline serving both chemical and macromolecular crystallography communities at the Australian Synchrotron.

MX1 is a bending-magnet crystallography beamline at the 3 GeV Australian Synchrotron. The beamline delivers hard X-rays in the energy range from 8 to 18 keV to a focal spot at the sample position of 120 µm FWHM. The beamline endstation and ancillary equipment facilitate local and remote access for both chemical and biological macromolecular crystallography. Here, the design of the beamline and endstation are discussed. The beamline has enjoyed a full user program for the last seven years and scientific highlights from the user program are also presented.

Selective gas adsorption in a pair of robust isostructural MOFs differing in framework charge and anion loading.

Activation of the secondary assembly instructions in the mononuclear pyrazine imide complexes [Co(III)(dpzca)2](BF4) or [Co(II)(dpzca)2] and [Ni(II)(dpzca)2] has facilitated the construction of two robust nanoporous three-dimensional coordination polymers, [Co(III)(dpzca)2Ag](BF4)2·2(H2O) [1·2(H2O)] and [Ni(II)(dpzca)2Ag]BF4·0.5(acetone) [2·0.5(acetone)]. Despite the difference in charge distribution and anion loading, the framework structures of 1·2(H2O) and 2·0.5(acetone) are isostructural. One dimensional channels along the b-axis permeate the structures and contain the tetrafluoroborate counterions (the Co(III)-based MOF has twice as many BF4(-) anions as the Ni(II)-based MOF) and guest solvent molecules. These anions are not readily exchanged whereas the solvent molecules can be reversibly removed and replaced. The H2, N2, CO2, CH4, H2O, CH3OH, and CH3CN sorption behaviors of the evacuated frameworks 1 and 2 at 298 K have been studied, and modeled, and both show very high selectivity for CO2 over N2. The increased anion loading in the channels of Co(III)-based MOF 1 relative to Ni(II)-based MOF 2 results in increased selectivity for CO2 over N2 but a decrease in the sorption kinetics and storage capacity of the framework.

Perturbation of spin crossover behavior by covalent post-synthetic modification of a porous metal-organic framework.

Covalent post-synthetic modification is a versatile method for gaining high-level synthetic control over functionality within porous metal-organic frameworks and for generating new materials not accessible through one-step framework syntheses. Here we apply this topotactic synthetic approach to a porous spin crossover framework and show through detailed comparison of the structures and properties of the as-synthesised and covalently modified phases that the modification reaction proceeds quantitatively by a thermally activated single-crystal-to-single-crystal transformation to yield a material with lowered spin-switching temperature, decreased lattice cooperativity, and altered color. Structure-function relationships to emerge from this comparison show that the approach provides a new route for tuning spin crossover through control over both outer-sphere and steric interactions.