Lattice oxygen self-spillover on reducible oxide supported metal cluster: the water–gas shift reaction on Cu/CeO2 catalyst

In this work we have tackled one of the most challenging problems in nanocatalysis namely understanding the role of reducible oxide supports in metal catalyzed reactions. As a prototypical example, the very well-studied water gas shift reaction catalyzed by CeO₂ supported Cu nanoclusters is chosen to probe how the reducible oxide support modifies the catalyst structures, catalytically active sites and even the reaction mechanisms. By employing density functional theory calculations in conjunction with a genetic algorithm and ab initio molecular dynamics simulations, we have identified an unprecedented spillover of the surface lattice oxygen from the ceria support to the Cu cluster, which is rarely considered previously but may widely exist in oxide supported metal catalysts under realistic conditions. The oxygen spillover causes a highly energetic preference of the monolayered configuration of the supported Cu nanocluster, compared to multilayered configurations. Due to the strong metal–oxide interaction, after the O spillover the monolayered cluster is highly oxidized by transferring electrons to the Ce 4f orbitals. The water–gas-shift reaction is further found to more favorably take place on the supported copper monolayer than the copper-ceria periphery, where the on-site oxygen and the adjacent oxidized Cu sites account for the catalytically active sites, synergistically facilitating the water dissociation and the carboxyl formation. The present work provides mechanistic insights into the strong metal–support interaction and its role in catalytic reactions, which may pave a way towards the rational design of metal–oxide catalysts with promising stability, dispersion and catalytic activity.

The lattice oxygen on the reducible CeO₂ support could self-spillover to surface of Cu cluster, generating the on-site oxygen to promote the catalytic water–gas shift reaction.     

Engineering plasticization resistant gas separation membranes using metal–organic nanocapsules

Membrane technologies hold great potential for industrial gas separation. Nevertheless, plasticization, a common phenomenon that is responsible for the loss of gas pair selectivity and the decrease of membrane lifespan, is one of the top challenges withholding the deployment of advanced membrane materials in realistic applications. Here, we report a highly generalizable approach, that utilizes PgC₅Cu, a copper metal–organic nanocapsule (MONC) containing 24 open metal sites (OMSs) as a multi-dentate node to coordinatively crosslink polymers. By adding merely 1–3 wt% of PgC₅Cu, a wide range of carbonyl group-containing polymers can be effectively crosslinked. Through rigorous dissolution tests, molecular dynamic simulations, and in situ FT-IR spectroscopy, we qualitatively and quantitatively unveiled the coordinative binding nature at the polymer–MONC interface. As a result, we produced a series of composite membranes showing near complete plasticization resistance to CO₂, C₂H₄, and C₂H₆ under high pressure with no loss of mechanical and gas transport properties.

Ultra-small metal–organic nanocapsules (MONCs) with open metal sites (OMSs) are used as multi-dentate nodes to form coordinative crosslinking networks with polymers.     

Au3-to-Ag3 coordinate-covalent bonding and other supramolecular interactions with covalent bonding strength

An efficient strategy for designing charge-transfer complexes using coinage metal cyclic trinuclear complexes (CTCs) is described herein. Due to opposite quadrupolar electrostatic contributions from metal ions and ligand substituents, [Au(μ-Pz-(i-C₃H₇)₂)]₃·[Ag(μ-Tz-(n-C₃F₇)₂)]₃ (Pz = pyrazolate, Tz = triazolate) has been obtained and its structure verified by single crystal X-ray diffraction – representing the 1^st crystallographically-verified stacked adduct of monovalent coinage metal CTCs. Abundant supramolecular interactions with aggregate covalent bonding strength arise from a combination of M–M′ (Au → Ag), metal–π, π–π interactions and hydrogen bonding in this charge-transfer complex, according to density functional theory analyses, yielding a computed binding energy of 66 kcal mol⁻¹ between the two trimer moieties – a large value for intermolecular interactions between adjacent d¹⁰ centres (nearly doubling the value for a recently-claimed Au(i) → Cu(i) polar-covalent bond: Proc. Natl. Acad. Sci. U.S.A., 2017, 114, E5042) – which becomes 87 kcal mol⁻¹ with benzene stacking. Surprisingly, DFT analysis suggests that: (a) some other literature precedents should have attained a stacked product akin to the one herein, with similar or even higher binding energy; and (b) a high overall intertrimer bonding energy by inferior electrostatic assistance, underscoring genuine orbital overlap between M and M′ frontier molecular orbitals in such polar-covalent M–M′ bonds in this family of molecules. The Au → Ag bonding is reminiscent of classical Werner-type coordinate-covalent bonds such as H₃N: → Ag in [Ag(NH₃)₂]⁺, as demonstrated herein quantitatively. Solid-state and molecular modeling illustrate electron flow from the π-basic gold trimer to the π-acidic silver trimer with augmented contributions from ligand-to-ligand’ (LL′CT) and metal-to-ligand (MLCT) charge transfer.

A stacked Ag₃–Au₃ bonded (66 kcal mol⁻¹) complex obtained crystallographically exhibits charge-transfer characteristics arising from multiple cooperative supramolecular interactions.     

“Bottled” spiro-doubly aromatic trinuclear [Pd2Ru]+ complexes

Following an ongoing interest in the study of transition metal complexes with exotic bonding networks, we report herein the synthesis of a family of heterobimetallic triangular clusters involving Ru and Pd atoms. These are the first examples of trinuclear complexes combining these nuclei. Structural and bonding analyses revealed both analogies and unexpected differences for these [Pd₂Ru]⁺ complexes compared to their parent [Pd₃]⁺ peers. Noticeably, participation of the Ru atom in the π-aromaticity of the coordinated benzene ring makes the synthesized compound the second reported example of ‘bottled’ double aromaticity. This can also be referred to as spiroaromaticity due to the participation of Ru in two aromatic systems at a time. Moreover, the [Pd₂Ru]⁺ kernel exhibits unprecedented orbital overlap of Ru d_z² AO and two Pd d_xy or d_x²−y² AOs. The present findings reveal the possibility of synthesizing stable clusters with delocalized metal–metal bonding from the combination of non-adjacent elements of the periodic table which has not been reported previously.

Synthesis of a triangular [Pd₂Ru]⁺ complex with delocalized metal–metal bonding between non-adjacent elements of the periodic table, double aromaticity and overlap of d-AOs with different angular momentum.     

Ir6In32S21, a polar,metal-rich semiconducting subchalcogenide

Subchalcogenides are uncommon, and their chemical bonding results from an interplay between metal–metal and metal–chalcogenide interactions. Herein, we present Ir₆In₃₂S₂₁, a novel semiconducting subchalcogenide compound that crystallizes in a new structure type in the polar P31m space group, with unit cell parameters a = 13.9378(12) Å, c = 8.2316(8) Å, α = β = 90°, γ = 120°. The compound has a large band gap of 1.48(2) eV, and photoemission and Kelvin probe measurements corroborate this semiconducting behavior with a valence band maximum (VBM) of −4.95(5) eV, conduction band minimum of −3.47(5) eV, and a photoresponse shift of the Fermi level by ∼0.2 eV in the presence of white light. X-ray absorption spectroscopy shows absorption edges for In and Ir do not indicate clear oxidation states, suggesting that the numerous coordination environments of Ir₆In₃₂S₂₁ make such assignments ambiguous. Electronic structure calculations confirm the semiconducting character with a nearly direct band gap, and electron localization function (ELF) analysis suggests that the origin of the gap is the result of electron transfer from the In atoms to the S 3p and Ir 5d orbitals. DFT calculations indicate that the average hole effective masses near the VBM (1.19m_e) are substantially smaller than the average electron masses near the CBM (2.51m_e), an unusual feature for most semiconductors. The crystal and electronic structure of Ir₆In₃₂S₂₁, along with spectroscopic data, suggest that it is neither a true intermetallic nor a classical semiconductor, but somewhere in between those two extremes.

Subchalcogenides are uncommon, and their chemical bonding results from an interplay between metal–metal and metal–chalcogenide interactions.     

Backbonding contributions to small molecule chemisorption in a metal–organic framework with open copper(i) centers

Metal–organic frameworks are promising materials for applications such as gas capture, separation, and storage, due to their ability to selectively adsorb small molecules. The metal–organic framework Cu^I-MFU-4l, which contains coordinatively unsaturated copper(i) centers, can engage in backbonding interactions with various small molecule guests, motivating the design of frameworks that engage in backbonding and other electronic interactions for highly efficient and selective adsorption. Here, we examine several gases expected to bind to the open copper(i) sites in Cu^I-MFU-4l via different electronic interactions, including σ-donation, π-backbonding, and formal electron transfer. We show that in situ Cu L-edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy can elucidate π-backbonding by directly probing excitations to unoccupied backbonding orbitals with Cu d-character, even for gases that participate in other dominant interactions, such as ligand-to-metal σ-donation. First-principles calculations based on density functional theory and time-dependent density functional theory additionally reveal the backbonding molecular orbitals associated with these spectroscopic transitions. The energies of the transitions correlate with the energy levels of the isolated small molecule adsorbates, and the transition intensities are proportional to the binding energies of the guest molecules within Cu^I-MFU-4l. By elucidating the molecular and electronic structure origins of backbonding interactions between electron rich metal centers in metal–organic frameworks and small molecule guests, it is possible to develop guidelines for further molecular-level design of solid-state adsorbents for energy-efficient separations of relevance to industry.

In situ near edge X-ray absorption fine structure spectroscopy directly probes unoccupied states associated with backbonding interactions between the open metal site in a metal–organic framework and various small molecule guests.     

Efficient and organic host–guest room-temperature phosphorescence: tunable triplet–singlet crossing and theoretical calculations for molecular packing

Organic host–guest doped materials exhibiting the room temperature phosphorescence (RTP) phenomenon have attracted considerable attention. However, it is still challenging to investigate their corresponding luminescence mechanism, because for host–guest systems, it is very difficult to obtain single crystals compared to single-component or co-crystal component materials. Herein, we developed a series of organic doped materials with triphenylamine (TPA) as the host and TPA derivatives with different electron-donating groups as guests. The doped materials showed strong fluorescence, thermally activated delayed fluorescence (τ: 39–47 ms), and efficient room temperature phosphorescence (Φ_phos: 7.3–9.1%; τ: 170–262 ms). The intensity ratio between the delayed fluorescence and phosphorescence was tuned by the guest species and concentration. Molecular dynamics simulations were used to simulate the molecular conformation of guest molecules in the host matrix and the interaction between the host and guest molecules. Therefore, the photophysical properties were calculated using the QM/MM model. This work provides a new concept for the study of molecular packing of guest molecules in the host matrix.

Molecular dynamics simulations were used to simulate the molecular conformation and interaction between hosts and guests. This work provides a new concept for the study of molecular packing for the investigation of the luminescence mechanism.     

Crystallographic characterization of the metal–organic framework Fe2(bdp)3 upon reductive cation insertion

Precisely locating extra-framework cations in anionic metal–organic framework compounds remains a long-standing, yet crucial, challenge for elucidating structure–performance relationships in functional materials. Single-crystal X-ray diffraction is one of the most powerful approaches for this task, but single crystals of frameworks often degrade when subjected to post-synthetic metalation or reduction. Here, we demonstrate the growth of sizable single crystals of the robust metal–organic framework Fe₂(bdp)₃ (bdp²⁻ = benzene-1,4-dipyrazolate) and employ single-crystal-to-single-crystal chemical reductions to access the solvated framework materials A₂Fe₂(bdp)₃·yTHF (A = Li⁺, Na⁺, K⁺). X-ray diffraction analysis of the sodium and potassium congeners reveals that the cations are located near the center of the triangular framework channels and are stabilized by weak cation–π interactions with the framework ligands. Freeze-drying with benzene enables isolation of activated single crystals of Na_0.5Fe₂(bdp)₃ and Li₂Fe₂(bdp)₃ and the first structural characterization of activated metal–organic frameworks wherein extra-framework alkali metal cations are also structurally located. Comparison of the solvated and activated sodium-containing structures reveals that the cation positions differ in the two materials, likely due to cation migration that occurs upon solvent removal to maximize stabilizing cation–π interactions. Hydrogen adsorption data indicate that these cation–framework interactions are sufficient to diminish the effective cationic charge, leading to little or no enhancement in gas uptake relative to Fe₂(bdp)₃. In contrast, Mg_0.85Fe₂(bdp)₃ exhibits enhanced H₂ affinity and capacity over the non-reduced parent material. This observation shows that increasing the charge density of the pore-residing cation serves to compensate for charge dampening effects resulting from cation–framework interactions and thereby promotes stronger cation–H₂ interactions.

Single-crystal X-ray diffraction reveals structural influences on gas adsorption properties in anionic metal–organic frameworks.     

Metal–ligand–Lewis acid multi-cooperative catalysis: a step forward in the Conia-ene reaction

An original multi-cooperative catalytic approach was developed by combining metal–ligand cooperation and Lewis acid activation. The [(SCS)Pd]₂ complex featuring a non-innocent indenediide-based ligand was found to be a very efficient and versatile catalyst for the Conia-ene reaction, when associated with Mg(OTf)₂. The reaction operates at low catalytic loadings under mild conditions with HFIP as a co-solvent. It works with a variety of substrates, including those bearing internal alkynes. It displays complete 5-exo vs. 6-endo regio-selectivity. In addition, except for the highly congested ^tBu-substituent, the reaction occurs with high Z vs. E stereo-selectivity, making it synthetically useful and complementary to known catalysts.

An original multi-cooperative catalytic approach was developed by combining metal–ligand cooperation and Lewis acid activation.     

Ligand-controlled and nanoconfinement-boosted luminescence employing Pt(ii) and Pd(ii) complexes: from color-tunable aggregation-enhanced dual emitters towards self-referenced oxygen reporters

In this work, we describe the synthesis, structural and photophysical characterization of four novel Pd(ii) and Pt(ii) complexes bearing tetradentate luminophoric ligands with high photoluminescence quantum yields (Φ_L) and long excited state lifetimes (τ) at room temperature, where the results were interpreted by means of DFT calculations. Incorporation of fluorine atoms into the tetradentate ligand favors aggregation and thereby, a shortened average distance between the metal centers, which provides accessibility to metal–metal-to-ligand charge-transfer (³MMLCT) excimers acting as red-shifted energy traps if compared with the monomeric entities. This supramolecular approach provides an elegant way to enable room-temperature phosphorescence from Pd(ii) complexes, which are otherwise quenched by a thermal population of dissociative states due to a lower ligand field splitting. Encapsulation of these complexes in 100 nm-sized aminated polystyrene nanoparticles enables concentration-controlled aggregation-enhanced dual emission. This phenomenon facilitates the tunability of the absorption and emission colors while providing a rigidified environment supporting an enhanced Φ_L up to about 80% and extended τ exceeding 100 μs. Additionally, these nanoarrays constitute rare examples for self-referenced oxygen reporters, since the phosphorescence of the aggregates is insensitive to external influences, whereas the monomeric species drop in luminescence lifetime and intensity with increasing triplet molecular dioxygen concentrations (diffusion-controlled quenching).

Pt(ii) and Pd(ii) complexes with unprecedented photophysical properties were developed. Encapsulation in nanoparticles boosted their performance while rendering them as self-referenced oxygen sensors.     

A highly emissive AIE-active luminophore exhibiting deep-red to near-infrared piezochromism and high-quality lasing

Further development of high-efficiency and low-cost organic fluorescent materials is intrinsically hampered by the energy gap law and spin statistics, especially in the near-infrared (NIR) region. Here we design a novel building block with aggregation-induced emission (AIE) activity for realizing highly efficient luminophores covering the deep-red and NIR region, which originates from an increase in the orbital overlap and electron-withdrawing ability. An organic donor–acceptor molecule (BPMT) with the building block is prepared and can readily form J-type molecular columns with multiple C–H⋯N/O interactions. Notably, such synthesized materials can emit fluorescence centered at 701 nm with extremely high photoluminescence quantum yields (PLQYs) of 48.7%. Experimental and theoretical investigations reveal that the formation of the hybridized local and charge-transfer (HLCT) state and substantial C–H⋯N/O interactions contribute to a fast radiative decay rate and a slow nonradiative decay rate, respectively, resulting in high PLQYs in the solid state covering the NIR range. Remarkably, such BPMT crystals, as a first example, reveal strong-penetrability piezochromism along with a distinct PL change from the deep-red (λ_max = 704 nm) to NIR (λ_max = 821 nm) region. Moreover, such typical AIE-active luminophores are demonstrated to be a good candidate as a lasing medium. Together with epoxy resin by a self-assembly method, a microlaser is successfully illustrated with a lasing wavelength of 735.2 nm at a threshold of 22.3 kW cm⁻². These results provide a promising approach to extend the contents of deep-red/NIR luminophores and open a new avenue to enable applications ranging from chemical sensing to lasing.

A HLCT-type luminophore is prepared with bright deep-red fluorescence, showing high-performance piezochromism and lasing.     

Pressure-and temperature induced phase transitions,piezochromism, NLC behaviour and pressure controlled Jahn–Teller switching in a Cu-based framework

In situ single-crystal diffraction and spectroscopic techniques have been used to study a previously unreported Cu-framework bis[1-(4-pyridyl)butane-1,3-dione]copper(ii) (CuPyr-I). CuPyr-I was found to exhibit high-pressure and low-temperature phase transitions, piezochromism, negative linear compressibility, and a pressure induced Jahn–Teller switch, where the switching pressure was hydrostatic media dependent.

In situ high-pressure single-crystal diffraction and spectroscopic techniques have been used to study a previously unreported Cu-framework bis[1-(4-pyridyl)butane-1,3-dione]copper(ii) (CuPyr-I).

High-pressure crystallographic experiments over the last 25 years or so have proved to be a unique tool in probing the mechanical properties of the organic solid state,¹ metal-complexes, and 2D/3D coordination compounds.² In particular, high-pressure techniques have been used to study an array of mechanical and chemical properties of crystals, such as changes in electrical and thermal conductivity,³ pressure-induced melting,⁴ solubility,⁵ amorphisation,⁶ post-synthetic modification,⁷ and chemical reactions such as polymerisation,⁸ cycloaddition⁹ and nanoparticle formation.¹⁰ Previous high-pressure experiments on porous metal-organic framework (MOF) materials have shown that on loading a diamond anvil cell (DAC) with a single-crystal or polycrystalline powder, the hydrostatic medium that surrounds the sample (to ensure hydrostatic conditions) can be forced inside the pores on increasing pressure, causing the pore and sample volume to increase with applied pressure.¹¹ This technique has also been used to determine the position of CH₄ and CO₂ molecules inside the small pores of a Sc-based MOF at room temperature using a laboratory X-ray diffractometer, and has proved useful in experimentally determining the maximum size of guest molecules that can penetrate into a pore.¹²On direct compression of more dense frameworks, negative linear compressibility (NLC) has also been observed, which results in an expansion of one or more of the unit cell dimensions with an overall contraction in volume. Such changes in the compressibility behaviour of metal-containing framework materials is usually as a result of common structural motifs which rotate or bend in order to accommodate increases in length along particular crystallographic directions.¹³ Changes in coordination environment can also be induced at pressure, as metal–ligand bonds are more susceptible to compression than covalent bonds.^2a In previous high pressure studies on metal complexes or coordination compounds, in which the metal ion has an asymmetric octahedral environment caused by Jahn–Teller (JT) distortions for example (such as those observed in Cu²⁺ and Mn³⁺ complexes), the application of pressure can result in compression of the JT axis, and can even be switched to lie along another bonding direction within the octahedron.¹⁴ Such distortions often result in piezochromism, often observed within a single crystal.¹⁵Here, we present a high-pressure crystallographic study on a novel and unreported Cu-framework bis[1-(4-pyridyl)butane-1,3-dione]copper(ii) (hereafter referred to as CuPyr-I). On application of pressure, CuPyr-I is highly unusual in that it demonstrates several of these phenomena within the same framework, including a single-crystal to single-crystal phase transition, a switching of the JT axis that depends on the hydrostatic medium used to compress the crystal, piezochromism and NLC behaviour. To date, we are unaware of any other material which exhibits all of these phenomena, with the first ever reported hydrostatic media ‘tuneable’ JT-switching.Under ambient temperature and pressure CuPyr-I crystallises in the rhombohedral space group R$\bar{3}$ (a/b = 26.5936(31) Å, c = 7.7475(9) Å). Each Cu-centre is coordinated to four 1-(4-pyridyl)butane-1,3-dione linkers, two of these ligands are bound through the dione O-atoms, with the final two bonding through the N-atom of the pyridine ring to form a 3D polymer. The crystal structure of CuPyr-I is composed of an interpenetration of these 3D polymers to form one-dimensional porous channels (∼2 Å in diameter) that run along the c-axis direction (Fig. 1).Open in a separate windowFig. 1Ball and stick model showing the coordination environment around the Cu²⁺ ion in CuPyr-I, and 3D-pore structure as viewed along the c-axis direction. The yellow sphere represents the available pore-space. Colour scheme is red: oxygen, blue: nitrogen, black: carbon, white: hydrogen and cyan: copper. The Cu²⁺ octahedron is illustrated in green.On increasing pressure from 0.07 GPa to 1.56 GPa using Fluorinert FC-70 (a mixture of large perfluorinated hydrocarbons) as a hydrostatic medium, compression of the framework occurs, resulting in a 9.89% reduction in volume, while the a/b-axes and c-axis are reduced by 4.46% and 1.25% respectively (Fig. 2 (blue triangles) and Table S1^†).Open in a separate windowFig. 2 a/b and c-axes as a function of pressure in a hydrostatic medium of FC-70 (blue triangles) and MeOH (red/black circles). The vertical line indicates the transition from CuPyr-I (red circles) to CuPyr-II (black circles) above 2.15 GPa. Errors in cell-lengths are smaller than the symbols plotted.On increasing pressure to 1.84 GPa, the framework became amorphous, though this is unsurprising as the hydrostatic limit for FC-70 is ∼2 GPa, and compression of frameworks in non-hydrostatic conditions usually results in amorphisation.¹⁶ On increasing pressure to 1.56 GPa, the three-symmetry independent Cu–O/N bond lengths to the ligand were monitored (Fig. 3 and Table S5^†). Under ambient pressure conditions, the two Cu–N1 pyridine bonds are longer than the four Cu–O1/O2 dione bonds, typical for an elongated JT distorted Cu²⁺ complex. However, on increasing pressure the direction of the JT axis gradually changed from Cu–N1 to the Cu–O1 bond (the dione oxygen in the 3-position), becoming equidistant at ∼0.57 GPa. By 1.56 GPa, the lengths of the Cu–N1 and Cu–O1 bonds had steadily reduced and increased by 12.3% and 8.9%, respectively. Throughout this the Cu–O2 bond remained essentially unchanged.Open in a separate windowFig. 3Cu–O1 (orange), Cu–N1 (blue) and Cu–O2 (green) bond lengths on increasing pressure in both FC-70 (triangles and dashed lines) and MeOH (circles and solid lines).Pressure induced JT switching has been observed in other systems, including a Mn₁₂ single-molecule magnet cluster that re-orientates the JT axis on one of the Mn centres at 2.5 GPa.^14a A similar transition was also observed in [CuF₂(H₂O)₂(pyz)] (pyz = pyrazine) and Rb₂CuCl₄(H₂O)₂,¹⁵ where the JT axis was reoriented from the Cu–N bond to the perpendicular Cu–O bond, though this occurs during a crystallographic phase transition at 1.8 GPa.¹⁸ Here, in CuPyr-I, no phase transition takes place, and unusually the JT switching appears to occur progressively on increasing pressure with no phase transition.^14bUsing methanol (MeOH) as the hydrostatic medium, CuPyr-I was compressed in two separate experiments, from 0.52 GPa to 5.28 GPa using synchrotron radiation, and from 0.34 GPa to 2.95 GPa using a laboratory X-ray diffractometer. On increasing pressure to 2.15 GPa, the a/b and c-axes compressed by 6.22% and 0.39% respectively (Fig. 2, Tables S2 and S3^†). On increasing pressure from 2.15 GPa to 2.78 GPa, CuPyr-I underwent a single-crystal to single-crystal isosymmetric phase transition to a previously unobserved phase (hereafter referred to as CuPyr-II).The transition to CuPyr-II resulted in a doubling of the a/b-axes, whilst the c-axis remained essentially unchanged. On increasing the pressure further, the a/b-axes continued to be compressed, whilst the c-axis increased in length, exhibiting negative linear compressibility (NLC) until the sample became amorphous at 5.28 GPa. The diffraction data were of poor quality after the phase transition, and only the connectivity of the CuPyr-II phase could be determined at 3.34 GPa. Above 3.34 GPa, only unit cell dimensions could be extracted. The occurrence of positive linear compressibility (PLC) followed by NLC is unusual in a framework material, and we could find only a few examples in the literature where this occurs.¹⁹During the NLC, the c-axis expanded by 1.46%, to give a compressibility of K_NLC = −5.3 (0.8) TPa⁻¹ (Δp = 2.23–4.90 GPa). K_NLC is calculated using the relationship K = −1/l(∂l/∂p)_T, where l is the length of the axis and (∂l/∂p)_T is the length change in pressure at constant temperature.²⁰ The value of K_NLC here is rather small compared to the massive NLC behaviour observed in the low pressure phase of Ag₃[Co(CN)₆]⁹ (K_NLC = −76(9) TPa⁻¹, Δp = 0–0.19 GPa) or the flexible MOF MIL-53(Al) (K_NLC = −28 TPa⁻¹, Δp = 0–3 GPa) for example,^17b and is much more comparable to the dense Zn formate MOF [NH₄][Zn(HCOO)₃] (−1.8(8) TPa⁻¹ (Δp = 0–0.94 GPa)).²¹ Because of the quality of the data, the exact nature, or reason for the NLC in CuPyr-II is unknown, although we aim to investigate this in the future.On increasing pressure using MeOH, the JT axis was again supressed on compression, with the Cu–N1 bond reducing in length by 0.288 Å (12%) between 0.34 and 2.15 GPa, while the Cu–O1 bond length increased by 0.216 Å (11%). The pressure at which Cu–N1 and Cu–O1 became equidistant was 1.28 GPa, measuring 2.140(5) Å and 2.131(6) Å respectively (Fig. 3 and Table S6^†). Across the entire pressure range, little to no compression or expansion was observed in the Cu–O2 bond in the 1-position of the dione in CuPyr-I, the same trend observed when compressed in FC-70. The JT switching pressure in MeOH however was 0.71 GPa higher than observed by direct compression in FC-70 (0.57 GPa). This, to our knowledge, is the first time that pressure induced JT switching has been observed to be hydrostatic media dependent.Changes to the Cu–N and Cu–O bond lengths were supported by high-pressure Raman spectroscopy of CuPyr-I, using MeOH as the hydrostatic medium (Fig. S10^†). Gradual growth of a shoulder on a band at ∼700 cm⁻¹ during compression is tentatively assigned to the Cu²⁺ coordination environment shifting from elongated to compressed JT distorted geometry. The shouldered peak becomes split above 2 GPa, after which the isosymmetric phase transition occurs.The gradual JT switch is thought to be principally responsible for reversible piezochromism in single crystals of CuPyr-I, which change in colour from green to dark red under applied pressure (Fig. 4b, S1 and S2^†). UV-visible spectroscopy confirms a bariometric blue-shift in the absorption peak at ∼700 nm assigned to d–d electronic transitions, and a red-shift of the tentatively assigned ligand-to-metal charge-transfer (LMCT) edge around 450 nm during the elongated to compressed switch (Fig. 4a and S8^†), accounting for this colour change. The red-shift is observed during compression in both Fluorinert® FC-70 and MeOH hydrostatic media, with a slightly suppressed shift measured in the latter due to filling of the framework pores (Table S2^†). Geometric switching at the metal centre leads to electronic stabilisation of the Cu²⁺ ion, as electrons transfer from higher energy d_x²−y² (Cu–O) orbitals to the lower energy d_z² (Cu–N) state (Fig. S7^†), evidenced by the blue-shift of the d–d intraconfigurational band as the d_z² (Cu–N) is progressively mixing with d_x²−y² (Cu–O) increasing its energy with respect to the lower energy d_xy and d_xz,yz levels becoming the highest energy level at the nearly compressed rhombic geometry. On the other hand, the redshift in the hesitantly assigned O²⁻ to Cu²⁺ LMCT band below 450 nm is ascribed to increase of the Cu–O bond distance and a likely bandwidth broadening with pressure both yielding a pressure redshift of the absorption band gap edge (Fig. 4a).Open in a separate windowFig. 4(a) UV-visible spectroscopy of CuPyr-I during compression in Fluorinert® FC-70 showing a gradual BLUE-shift in the d–d intraconfigurational band (∼700 nm) and a gradual red-shift of the absorption band assigned to LMCT (∼450 nm) with increasing pressure. (b) Gradual pressure-induced Jahn–Teller switch of the Cu²⁺ octahedral coordination environment in CuPyr-1 from tetragonal elongated (left, green) to rhombic compressed (right, red), causing piezochromism. Atom colouring follows previous figures.Compression of the coordination bonds was not the only distortion to take place in CuPyr-I, with the Cu-octahedra also twisting with respect to the 1-(4-pyridyl)butane-1,3-dione linkers on increasing pressure. Twisting of the Cu-octahedra in CuPyr-I with respect to the dione section of the linker could be quantified by measuring both the ∠N1Cu1O2C4 and the ∠N1Cu1O1C2 torsion angles from the X-ray data, which in MeOH gradually decrease and increase by 12.2° and 7.3°, respectively, to 2.15 GPa (Table S8^†). In FC-70, ∠N1Cu1O2C4 and ∠N1Cu1O1C2 decrease and increase by 5.4° and 2.8°, respectively, to 1.56 GPa. On increasing pressure to 1.57 GPa in a hydrostatic medium of MeOH, a difference of ∼5° for both angles was observed compared to FC-70 at 1.56 GPa. Twisting about the octahedra allows compression of the channels to take place in a ‘screw’ like fashion and has been observed in other porous materials with channel structures.²² The overall effect is to reduce the pore volume, and decrease the size of the channels (Tables S2 and S3^†). Using MeOH as a hydrostatic medium therefore appears to reduce this effect by decreasing the compressibility of the framework.It was not possible to determine the pressure dependence in other longer-chain alcohols, including ethanol (EtOH) and isopropanol (IPA), due to cracking of the crystal upon loading into the diamond anvil cell (Fig. S1^†). We believe this is a result of these longer chain alcohols acting as reducing agents, as indicated by the loss in colour of the crystals.To ascertain the origin of the hydrostatic media-induced change in the JT switching pressure and unit cell compressibility, the pore size and content were monitored as a function of pressure. A dried crystal of CuPyr-I was collected at ambient pressure and temperature in order to compare to the high-pressure data and is included in the ESI.^† The pore volume and electron density were estimated and modelled respectively using the SQUEEZE algorithm within PLATON (Tables S1–S3^†).²³ CuPyr-I under ambient pressure conditions has three symmetry equivalent channels per unit cell with a total volume of ∼1152 ų containing diethyl ether (2.5 wt%) trapped in the pores during the synthesis of the framework, confirmed by TGA analysis (Fig. S6^†).On surrounding the crystal with FC-70, direct compression of the framework occurred. The pore content remained almost constant during compression up to 0.88 GPa, inferring no change in the pore contents. On increasing pressure further to 1.56 GPa, an increase in the calculated electron density was observed (23%), though the data here were of depreciating quality and less reliable. During compression of CuPyr-I in MeOH to 0.52 GPa, the pore volume and electron density in the channels increased by 4.5% and 54%, respectively, reflecting ingress of MeOH into the pores. The electron density in the channels continued to increase to a maximum of 0.466e⁻ A⁻³ by 0.96 GPa, although the pore volume began to decrease at this pressure. The uptake of MeOH into the pores therefore results in the marked decrease in compressibility, as noted above.Previous high-pressure experiments on porous MOFs have resulted in similar behaviour on application of pressure, with the uptake of the media significantly decreasing the compressibility of the framework.²⁴ However, using different hydrostatic media to control the JT switch in any material is, to the best of our knowledge, previously unreported. On increasing pressure above 0.96 GPa, the electron density in the pores decreases, and coincides with a steady reduction in volume of the unit cell. Both an initial increase and then subsequent decrease in uptake of hydrostatic media is common in high-pressure studies of MOFs, and has been seen several times, for example in HKUST-1 (ref. 24c) and MOF-5.^24a The ingress of MeOH into the pores on initially increasing pressure to 0.52 GPa is also reflected in a twisting of the octahedra, in-particular the ∠N1Cu1O2C4 angle decreases by 5.8° in MeOH, whereas on compression in FC-70, little to no change is observed in the ∠N1Cu1O2C4 angle to 1.56 GPa. These angles represent a twisting of the dione backbone, which we speculate must interact with the MeOH molecules which penetrate into the framework.Upon compression in n-pentane, the lightest alkane that is a liquid at ambient temperature, we see different behaviour to that in MeOH or FC-70. Poor data quality permitted only the extraction of unit cell parameters but from this it can be seen that CuPyr-I has undergone the transition to CuPyr-II by 0.77 GPa. This is a significantly lower pressure than is required to induce the phase transition in MeOH (ca. 2.15 GPa). We speculate that this difference in pressure is caused by the n-pentane entering the channels at a lower pressure than MeOH due to the hydrophobic nature of the channels. This can be overcome by MeOH but not until substantially higher pressures, as seen in other MOFs that contain hydrophobic pores.²⁵On undergoing the transition to CuPyr-II at 2.78 GPa the unit cell volume quadruples, resulting in three symmetry independent channels (12 per unit cell), with the % pore volume continuing to decrease (Table S4^†). Additionally, the reflections become much broader, significantly depreciating the data quality. Nevertheless, changes in metal–ligand bond lengths and general packing features can be extracted. In particular, the transition to CuPyr-II results in two independent Cu-centres, with six independent Cu–N/O bond distances per Cu. Each exhibits a continuation of the trend seen in CuPyr-I, with the Cu–O bonds (equivalent to the Cu–O1 bond in CuPyr-I) remaining longer than the JT suppressed Cu–N bonds. However, the transition to CuPyr-II results in both an increase and decrease in three of the four Cu–N and Cu–O bonds respectively, compared to CuPyr-I at 2.15 GPa (Table S6^†). The net result is a framework which contains a Cu-centre where the coordination bonds are more equidistant, while the JT axis becomes much more prominent in the other Cu-centre, with the Cu–O dione bond continuing to increase in length. The data for CuPyr-II depreciates rapidly after the phase transition, and more work would be required to study the effect of the anisotropic compression of the JT axis in CuPyr-II on increasing pressure further.It is difficult to determine the mechanism behind the NLC behaviour observed upon compression of CuPyr-II because the phase transition results in a significant reduction in data quality. Further work will be carried out computationally in order to elucidate the structural mechanism that gives rise to the PLC followed by NLC. However, we propose this effect is inherent to this framework and the ingress of MeOH molecules into the channels allows the retention of crystallinity to allow this behaviour to be observed crystallographically.In order to determine whether the JT switch could be induced by decreasing temperature and remove any effect the ingress of hydrostatic media has into the pores on the JT switch, variable temperature X-ray diffraction measurements were undertaken on a powder and single-crystal sample. On cooling below 175 K and 150 K in a powder and single-crystal sample respectively, a phase transition was observed, however, this was to a completely different triclinic phase, hereafter referred to as CuPyr-III. The transition here appears to occur when the disordered diethyl ether becomes ordered in the pores, confirmed by determination of the structure by single-crystal X-ray diffraction, where the diethylether could be modelled inside the pore-channel (see ESI Sections 7 & 8 for details^†).In conclusion, we have presented a compression study on the newly synthesised Cu-based porous framework bis[1-(4-pyridyl)butane-1,3-dione]copper(ii), referred to as CuPyr, compressed in FC-70 to 1.56 GPa and MeOH to 4.90 GPa. In both FC-70 and MeOH hydrostatic media, the JT axis, which extends along the Cu–N pyridyl bond, steadily compresses and then switches to lie along one of the Cu–O dione bonds. Compression in MeOH results in ingress of the medium into the framework pores, which increases the JT switching pressure to 1.47 GPa, compared with 0.64 GPa during compression in Fluorinert® FC-70. Interaction of stored MeOH with the host framework prompts twisting of the ligand backbone, which is not observed in the absence of adsorbed guest. Suppression of the JT axis is accompanied by a piezochromic colour change in the single crystals from green to dark red, as confirmed by crystallographic and spectroscopic measurements. Increasing the applied pressure to at least 2.15 GPa causes the framework to undergo an isosymmetric phase transition to a previously unobserved phase, characterised by a doubling of the a/b axes. Between 2.15 GPa and 4.90 GPa, NLC behaviour is observed.This is to the best of our knowledge the first time a phase transition, NLC, piezochromic and pressure induced JT switching behaviour have been observed within the same material. We have also reported for the first time a pressure induced JT axis switch which is hydrostatic media dependent. In further analysis of this system, we intend to study the magnetic properties under ambient and high pressure.     

Modulation of the superficial electronic structure via metal–support interaction for H2 evolution over Pd catalysts

Electronic interactions can radically enhance the performance of supported metal catalysts and are critical for fundamentally understanding the nature of catalysts. However, at the microscopic level, the details of such interactions tuning the electronic properties of the sites on the metal particle''s surface and metal–support interface remain obscure. Herein, we found polarized electronic metal–support interaction (pEMSI) in oxide-supported Pd nanoparticles (NPs) describing the enhanced accumulation of electrons at the surface of NPs (superficial Pd^δ−) with positive Pd atoms distributed on the interface (interfacial Pd^δ+). More superficial Pd^δ− species mean stronger pEMSI resulting from the synergistic effect of moderate Pd–oxide interaction, high structural fluxionality and electron transport activity of Pd NPs. The surface Pd^δ− species are responsible for improved catalytic performance for H₂ evolution from metal hydrides and formates. These extensive insights into the nature of supported-metal NPs may open new avenues for regulating a metal particle''s electronic structure precisely and exploiting high-performance catalysts.

A new type of electronic effect, polarized metal-support interaction (pEMSI), in oxide-supported Pd nanoparticles describing the enhanced accumulation of electrons at the superficial surface is responsible for improved catalytic H₂ evolution.     

Understanding the geometric diversity of inorganic and hybrid frameworks through structural coarse-graining

Much of our understanding of complex structures is based on simplification: for example, metal–organic frameworks are often discussed in the context of “nodes” and “linkers”, allowing for a qualitative comparison with simpler inorganic structures. Here we show how such an understanding can be obtained in a systematic and quantitative framework, combining atom-density based similarity (kernel) functions and unsupervised machine learning with the long-standing idea of “coarse-graining” atomic structure. We demonstrate how the latter enables a comparison of vastly different chemical systems, and we use it to create a unified, two-dimensional structure map of experimentally known tetrahedral AB₂ networks – including clathrate hydrates, zeolitic imidazolate frameworks (ZIFs), and diverse inorganic phases. The structural relationships that emerge can then be linked to microscopic properties of interest, which we exemplify for structural heterogeneity and tetrahedral density.

A coarse-graining approach enables structural comparisons across vastly different chemical spaces, from inorganic polymorphs to hybrid framework materials.     

Ultrafast and long-time excited state kinetics of an NIR-emissive vanadium(iii) complex II. Elucidating triplet-to-singlet excited-state dynamics

We report the non-adiabatic dynamics of V^IIICl₃(ddpd), a complex based on the Earth-abundant first-row transition metal vanadium with a d² electronic configuration which is able to emit phosphorescence in solution in the near-infrared spectral region. Trajectory surface-hopping dynamics based on linear vibronic coupling potentials obtained with CASSCF provide molecular-level insights into the intersystem crossing from triplet to singlet metal-centered states. While the majority of the singlet population undergoes back-intersystem crossing to the triplet manifold, 1–2% remains stable during the 10 ps simulation time, enabling the phosphorescence described in Dorn et al. Chem. Sci., 2021, DOI: 10.1039/D1SC02137K. Competing with intersystem crossing, two different relaxation channels via internal conversion through the triplet manifold occur. The nuclear motion that drives the dynamics through the different electronic states corresponds mainly to the increase of all metal–ligand bond distances as well as the decrease of the angles of trans-coordinated ligand atoms. Both motions lead to a decrease in the ligand-field splitting, which stabilizes the interconfigurational excited states populated during the dynamics. Analysis of the electronic character of the states reveals that increasing and stabilizing the singlet population, which in turn can result in enhanced phosphorescence, could be accomplished by further increasing the ligand-field strength.

The ultrafast triplet-to-singlet mechanism, responsible for the photoluminescence of the open-shell V^IIICl₃(ddpd) complex – based on Earth-abundant vanadium – is unraveled using non-adiabatic dynamics in full dimensionality.     

Direct observation of the evolving metal–support interaction of individual cobalt nanoparticles at the titania and silica interface

Understanding the metal–support interaction (MSI) is crucial to comprehend how the catalyst support affects performance and whether this interaction can be exploited in order to design new catalysts with enhanced properties. Spatially resolved soft X-ray absorption spectroscopy (XAS) in combination with Atomic Force Microscopy (AFM) and Scanning Helium Ion-Milling Microscopy (SHIM) has been applied to visualise and characterise the behaviour of individual cobalt nanoparticles (CoNPs) supported on two-dimensional substrates (SiO_xSi(100) (x < 2) and rutile TiO₂(110)) after undergoing reduction–oxidation–reduction (ROR). The behaviour of the Co species is observed to be strongly dependent on the type of support. For SiO_xSi a weaker MSI between Co and the support allows a complete reduction of CoNPs although they migrate and agglomerate. In contrast, a stronger MSI of CoNPs on TiO₂ leads to only a partial reduction under H₂ at 773 K (as observed from Co L₃-edge XAS data) due to enhanced TiO₂ binding of surface-exposed cobalt. SHIM data revealed that the interaction of the CoNPs is so strong on TiO₂, that they are seen to spread at and below the surface and even to migrate up to ∼40 nm away. These results allow us to better understand deactivation phenomena and additionally demonstrate a new understanding concerning the nature of the MSI for Co/TiO₂ and suggest that there is scope for careful control of the post-synthetic thermal treatment for the tuning of this interaction and ultimately the catalytic performance.

Understanding the metal–support interaction (MSI) is crucial to comprehend how the catalyst support affects performance and whether this interaction can be exploited in order to design new catalysts with enhanced properties.     

How to not build a cage: endohedral functionalization of polyoxometalate-based metal–organic polyhedra

Introducing functionalities into the interior of metal–organic cage complexes can confer properties and utilities (e.g. catalysis, separation, drug delivery, and guest recognition) that are distinct from those of unfunctionalized cages. Endohedral functionalization of such cage molecules, for decades, has largely relied on modifying their organic linkers to covalently append targeted functional groups to the interior surface. We herein introduce an effective coordination method to bring in functionalities at the metal sites instead, for a set of polyhedral cages where the nodes are in situ formed polyoxovanadate clusters, [V^IV₆O₆(OCH₃)₉(μ₆-SO₄)(COO)₃]²⁻. Replacing the central sulfates of these hexavanadate clusters with more strongly coordinating phosphonate groups allows the installation of functionalities within the cage cavities. Organophosphonates with phenyl, biphenyl, and terphenyl tails were examined for internalization. Depending on the size/shape of the cavities, small phosphonates can fit into the molecular containers whereas larger ones inhibit or transform the framework architecture, whereby the first non-cage complex was isolated from a reaction that otherwise would lead to entropically favored regular polyhedra cages. The results highlight the complex and dynamic nature of the self-assembly process involving polyoxometalates and the scope of molecular variety accessible by the introduction of endo functional groups.

Installation of oversized functions within a metal–organic cage may “burst” or even transform the molecular cage itself.     

Linking metal–organic cages pairwise as a design approach for assembling multivariate crystalline materials

Using metal–organic cages (MOCs) as preformed supermolecular building-blocks (SBBs) is a powerful strategy to design functional metal–organic frameworks (MOFs) with control over the pore architecture and connectivity. However, introducing chemical complexity into the network via this route is limited as most methodologies focus on only one type of MOC as the building-block. Herein we present the pairwise linking of MOCs as a design approach to introduce defined chemical complexity into porous materials. Our methodology exploits preferential Rh-aniline coordination and stoichiometric control to rationally link Cu₄L₄ and Rh₄L₄ MOCs into chemically complex, yet extremely well-defined crystalline solids. This strategy is expected to open up significant new possibilities to design bespoke multi-functional materials with atomistic control over the location and ordering of chemical functionalities.

A new strategy to design atomically precise multivariate metal–organic frameworks is presented. This is achieved by linking two preformed metal–organic cages via a precisely tuned Rh–aniline interaction.     

Impact of sphingosine and acetylsphingosines on the aggregation and toxicity of metal-free and metal-treated amyloid-β

Pathophysiological shifts in the cerebral levels of sphingolipids in Alzheimer''s disease (AD) patients suggest a link between sphingolipid metabolism and the disease pathology. Sphingosine (SP), a structural backbone of sphingolipids, is an amphiphilic molecule that is able to undergo aggregation into micelles and micellar aggregates. Considering its structural properties and cellular localization, we hypothesized that SP potentially interacts with amyloid-β (Aβ) and metal ions that are found as pathological components in AD-affected brains, with manifesting its reactivity towards metal-free Aβ and metal-bound Aβ (metal–Aβ). Herein, we report, for the first time, that SP is capable of interacting with both Aβ and metal ions and consequently affects the aggregation of metal-free Aβ and metal–Aβ. Moreover, incubation of SP with Aβ in the absence and presence of metal ions results in the aggravation of toxicity induced by metal-free Aβ and metal–Aβ in living cells. As the simplest acyl derivatives of SP, N-acetylsphingosine and 3-O-acetylsphingosine also influence metal-free Aβ and metal–Aβ aggregation to different degrees, compared to SP. Such slight structural modifications of SP neutralize its ability to exacerbate the cytotoxicity triggered by metal-free Aβ and metal–Aβ. Notably, the reactivity of SP and the acetylsphingosines towards metal-free Aβ and metal–Aβ is determined to be dependent on their formation of micelles and micellar aggregates. Our overall studies demonstrate that SP and its derivatives could directly interact with pathological factors in AD and modify their pathogenic properties at concentrations below and above critical aggregation concentrations.

The reactivity of sphingosine and acetylsphingosines towards both metal-free and metal-treated amyloid-β is demonstrated showing a correlation of their micellization properties.     

Single-step synthesis and interface tuning of core–shell metal–organic framework nanoparticles

Control over the spatial distribution of components in metal–organic frameworks has potential to unlock improved performance and new behaviour in separations, sensing and catalysis. We report an unprecedented single-step synthesis of multi-component metal–organic framework (MOF) nanoparticles based on the canonical ZIF-8 (Zn) system and its Cd analogue, which form with a core–shell structure whose internal interface can be systematically tuned. We use scanning transmission electron microscopy, X-ray energy dispersive spectroscopy and a new composition gradient model to fit high-resolution X-ray diffraction data to show how core–shell composition and interface characteristics are intricately controlled by synthesis temperature and reaction composition. Particle formation is investigated by in situ X-ray diffraction, which reveals that the spatial distribution of components evolves with time and is determined by the interplay of phase stability, crystallisation kinetics and diffusion. This work opens up new possibilities for the control and characterisation of functionality, component distribution and interfaces in MOF-based materials.

Core–shell metal–organic framework nanoparticles have been synthesised in which the internal interface and distribution of components is found to be highly tunable using simple variations in reaction conditions.