Metal-free tandem carbene N–H insertions and C–C bond cleavages

A metal-free C–H [5 + 1] annulation reaction of 2-arylanilines with diazo compounds has been achieved, giving rise to two types of prevalent phenanthridines via highly selective C–C cleavage. Compared to the simple N–H insertion manipulation of diazo, this method elegantly accomplishes a tandem N–H insertion/S_EAr/C–C cleavage/aromatization reaction, and the synthetic utility of this new transformation is exemplified by the succinct syntheses of trisphaeridine and bicolorine alkaloids.

A metal-free C–H [5 + 1] annulation reaction of 2-arylanilines with diazo compounds has been achieved, giving rise to two types of prevalent phenanthridines via highly selective C–C cleavage.     

Spontaneous S–Si bonding of alkanethiols to Si(111)–H: towards Si–molecule–Si circuits

We report the synthesis of covalently linked self-assembled monolayers (SAMs) on silicon surfaces, using mild conditions, in a way that is compatible with silicon-electronics fabrication technologies. In molecular electronics, SAMs of functional molecules tethered to gold via sulfur linkages dominate, but these devices are not robust in design and not amenable to scalable manufacture. Whereas covalent bonding to silicon has long been recognized as an attractive alternative, only formation processes involving high temperature and/or pressure, strong chemicals, or irradiation are known. To make molecular devices on silicon under mild conditions with properties reminiscent of Au–S ones, we exploit the susceptibility of thiols to oxidation by dissolved O₂, initiating free-radical polymerization mechanisms without causing oxidative damage to the surface. Without thiols present, dissolved O₂ would normally oxidize the silicon and hence reaction conditions such as these have been strenuously avoided in the past. The surface coverage on Si(111)–H is measured to be very high, 75% of a full monolayer, with density-functional theory calculations used to profile spontaneous reaction mechanisms. The impact of the Si–S chemistry in single-molecule electronics is demonstrated using STM-junction approaches by forming Si–hexanedithiol–Si junctions. Si–S contacts result in single-molecule wires that are mechanically stable, with an average lifetime at room temperature of 2.7 s, which is five folds higher than that reported for conventional molecular junctions formed between gold electrodes. The enhanced “ON” lifetime of this single-molecule circuit enables previously inaccessible electrical measurements on single molecules.

Spontaneously formed Si–S bonds enable monolayer and single-molecule Si–molecule–Si circuits.     

Electron-enriched thione enables strong Pb–S interaction for stabilizing high quality CsPbI3 perovskite films with low-temperature processing

Cesium lead iodide (CsPbI₃) perovskite is a promising photovoltaic material with a suitable bandgap and high thermal stability. However, it involves complicated phase transitions, and black-phase CsPbI₃ is mostly formed and stabilized at high temperatures (200–360 °C), making its practical application challenging. Here, for the first time, we have demonstrated a feasible route for growing high quality black-phase CsPbI₃ thin films under mild conditions by using a neutral molecular additive of 4(1H)-pyridinethione (4-PT). The resulting CsPbI₃ thin films are morphologically uniform and phase stable under ambient conditions, consisting of micron-sized grains with oriented crystal stacking. With a range of characterization experiments on intermolecular interactions, the electron-enriched thione group in 4-PT is distinguished to be critical to enabling a strong Pb–S interaction, which not only influences the crystallization paths, but also stabilizes the black-phase CsPbI₃via crystal surface functionalization. The 4-PT based CsPbI₃ achieves 13.88% power conversion efficiency in a p–i–n structured device architecture, and encapsulated devices can retain over 85% of their initial efficiencies after 20 days of storage in an ambient environment, which are the best results among fully low-temperature processed CsPbI₃ photovoltaics.

A neutral molecular additive of 4(1H)-pyridinethione (4-PT) is used for growing high quality black-phase CsPbI₃ thin films at low temperatures.     

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.     

C4-arylation and domino C4-arylation/3,2-carbonyl migration of indoles by tuning Pd catalytic modes: Pd(i)–Pd(ii) catalysis vs. Pd(ii) catalysis

Efficient C4-arylation and domino C4-arylation/3,2-carbonyl migration of indoles have been developed. The former route enables C4-arylation in a highly efficient and mild manner and the latter route provides an alternative straightforward protocol for synthesis of C2/C4 disubstituted indoles. The mechanism studies imply that the different reaction pathways were tuned by the distinct acid additives, which led to either the Pd(i)–Pd(ii) pathway or Pd(ii) catalysis.

C4-arylation via Pd(i)–Pd(ii) catalysis and domino C4-arylation/3,2-carbonyl migration of indoles via Pd(ii) catalysis tuning by acids have been developed.     

Site-selective functionalization of remote aliphatic C–H bonds via C–H metallation

Directing group assistance provided a paradigm for controlling site-selectivity in transition metal-catalyzed C–H functionalization reactions. However, the kinetically and thermodynamically favored formation of 5-membered metallacycles has greatly hampered the selective activation of remote C(sp³)–H bonds via larger-membered metallacycles. Recent development to achieve remote C(sp³)–H functionalization via the C–H metallation process largely relies on employing specific substrates without accessible proximal C–H bonds. Encouragingly, recent advances in this field have enabled the selective functionalization of remote aliphatic C–H bonds in the presence of equally accessible proximal ones by taking advantage of the switch of the regiodetermining step, ring strain of metallacycles, multiple non-covalent interactions, and favourable reductive elimination from larger-membered metallacycles. In this review, we summarize these advancements according to the strategies used, hoping to facilitate further efforts to achieve site- and even enantioselective functionalization of remote C(sp³)–H bonds.

Recent advances in site-selective functionalization of remote aliphatic C–H bonds in organometallic pathways are summarized.     

Palladium-catalyzed synthesis of β-hydroxy compounds via a strained 6,4-palladacycle from directed C–H activation of anilines and C–O insertion of epoxides

A palladium-catalyzed C–H activation of acetylated anilines (acetanilides, 1,1-dimethyl-3-phenylurea, 1-phenylpyrrolidin-2-one, and 1-(indolin-1-yl)ethan-1-one) with epoxides using O-coordinating directing groups was accomplished. This C–H alkylation reaction proceeds via formation of a previously unknown 6,4-palladacycle intermediate and provides rapid access to regioselectively functionalized β-hydroxy products. Notably, this catalytic system is applicable for the gram scale mono-functionalization of acetanilide in good yields. The palladium-catalyzed coupling reaction of the ortho-C(sp²) atom of O-coordinating directing groups with a C(sp³) carbon of chiral epoxides offers diverse substrate scope in good to excellent yields. In addition, further transformations of the synthesized compound led to biologically important heterocycles. Density functional theory reveals that the 6,4-palladacycle leveraged in this work is significantly more strained (>10 kcal mol⁻¹) than the literature known 5,4 palladacycles.

The combined experimental and computational study on palladium-catalyzed regioselective C–H functionalization of O-coordinating directing groups with epoxides is described.     

Zinc catalysed electrophilic C–H borylation of heteroarenes

Cationic zinc Lewis acids catalyse the C–H borylation of heteroarenes using pinacol borane (HBPin) or catechol borane (HBCat). An electrophile derived from [IDippZnEt][B(C₆F₅)₄] (IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) combined with N,N-dimethyl-p-toluidine (DMT) proved the most active in terms of C–H borylation scope and yield. Using this combination weakly activated heteroarenes, such as thiophene, were amenable to catalytic C–H borylation using HBCat. Competition reactions show these IDipp–zinc cations are highly oxophilic but less hydridophilic (relative to B(C₆F₅)₃), and that borylation proceeds via activation of the hydroborane (and not the heteroarene) by a zinc electrophile. Based on DFT calculations this activation is proposed to proceed by coordination of a hydroborane oxygen to the zinc centre to generate a boron electrophile that effects C–H borylation. Thus, Lewis acid binding to oxygen sites of hydroboranes represents an under-developed route to access reactive borenium-type electrophiles for C–H borylation.

Cationic zinc Lewis acids catalyse the C–H borylation of heteroarenes using pinacol borane (HBPin) or catechol borane (HBCat).     

Bound oxygen-atom transfer endows peroxidase-mimic M–N–C with high substrate selectivity

Advances in nanoscience have stimulated the wide exploration of nanozymes as alternatives to enzymes. Nonetheless, nanozymes often catalyze multiple reactions and are not specialized to a specific substrate, restricting their broad application. Here, we report that the substrate selectivity of the peroxidase-mimic M–N–C can be significantly altered via forming bound intermediates with variable interactions with substrates according to the type of metal. Taking two essential reactions in chemical sensing as an example, Fe–N–C and Co–N–C showed opposite catalytic selectivity for the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) and 3-aminophthalhydrazide (luminol), respectively, by factors of up to 200-fold. It was revealed that specific transition metal-N coordination was the origin of the selective activation of H₂O₂ forming critically bound oxygen intermediates (M O) for oxygen-atom transfer and the consequent oxidization of substrates. Notably, owing to the embedded ligands in the rigid graphitic framework, surprisingly, the selectivity of M–N–C was even superior to that of commonly used horseradish peroxidase (HRP).

Learning principles from biology, this work highlights the great potential of biomimetic bound-intermediates in endow nanozymes with high reaction selectivity towards industrial reactions previously not accessible to biology.     

Rhodium-catalysed tetradehydro-Diels–Alder reactions of enediynes via a rhodium-stabilized cyclic allene

Efficient methods for the synthesis of fused-aromatic rings is a critical endeavour in the creation of new pharmaceuticals and materials. A direct method for preparing these systems is the tetradehydro-Diels–Alder reaction, however this is limited by the need for harsh reaction conditions. A potential, but underdeveloped, route to these systems is via transition metal-catalysed cycloaromatisation of ene-diynes. Herein, tethered unconjugated enediynes have been shown to undergo a facile room-temperature Rh^I-catalysed intramolecular tetradehydro-Diels–Alder reaction to produce highly substituted isobenzofurans, isoindolines and an indane. Furthermore, experimental and computational studies suggest a novel mechanism involving an unprecedented and complex Rh^I/Rh^III/Rh^I/Rh^III redox cycle involving the formation of an unusual strained 7-membered rhodacyclic allene intermediate and a Rh^III-stabilized 6-membered ring allene complex.

Room temperature Rh-catalysed tetradehydro-Diels–Alder reaction via an unusual Rh-stabilised allene.     

Pd-catalyzed cross-electrophile Coupling/C–H alkylation reaction enabled by a mediator generated via C(sp3)–H activation

Transition-metal-catalyzed cross-electrophile C(sp²)–(sp³) coupling and C–H alkylation reactions represent two efficient methods for the incorporation of an alkyl group into aromatic rings. Herein, we report a Pd-catalyzed cascade cross-electrophile coupling and C–H alkylation reaction of 2-iodo-alkoxylarenes with alkyl chlorides. Methoxy and benzyloxy groups, which are ubiquitous functional groups and common protecting groups, were utilized as crucial mediators via primary or secondary C(sp³)–H activation. The reaction provides an innovative and convenient access for the synthesis of alkylated phenol derivatives, which are widely found in bioactive compounds and organic functional materials.

A cascade Pd-catalyzed cross-electrophile coupling and C–H alkylation reaction of 2-iodo-alkoxylarenes with alkyl chlorides has been developed by using an ortho-methoxy or benzyloxy group as a mediator via C(sp³)–H activation.     

Electrochemical C–N coupling with perovskite hybrids toward efficient urea synthesis

Electrocatalytic C–N coupling reaction by co-activation of both N₂ and CO₂ molecules under ambient conditions to synthesize valuable urea opens a new avenue for sustainable development, while the actual catalytic activity is limited by poor adsorption and coupling capability of gas molecules on the catalyst surface. Herein, theoretical calculation predicts that the well-developed built-in electric field in perovskite hetero-structured BiFeO₃/BiVO₄ hybrids can accelerate the local charge redistribution and thus promote the targeted adsorption and activation of inert N₂ and CO₂ molecules on the generated local electrophilic and nucleophilic regions. Thus, a BiFeO₃/BiVO₄ heterojunction is designed and synthesized, which delivers a urea yield rate of 4.94 mmol h⁻¹ g⁻¹ with a faradaic efficiency of 17.18% at −0.4 V vs. RHE in 0.1 M KHCO₃, outperforming the highest values reported as far. The comprehensive analysis further confirms that the local charge redistribution in the heterojunction effectively suppresses CO poisoning and the formation of the endothermic *NNH intermediate, which thus guarantees the exothermic coupling of *N N* intermediates with the generated CO via C–N coupling reactions to form the urea precursor *NCON* intermediate. This work opens a new avenue for effective electrocatalytic C–N coupling under ambient conditions.

The local charge redistribution in BiFeO₃/BiVO₄ hybrids promotes the targeted adsorption and activation of inert gas molecules and guarantees the exothermic coupling of *N N* with generated CO via C–N coupling reactions to form *NCON* precursor.     

Accelerating the insertion reactions of (NHC)Cu–H via remote ligand functionalization

Most ligand designs for reactions catalyzed by (NHC)Cu–H (NHC = N-heterocyclic carbene ligand) have focused on introducing steric bulk near the Cu center. Here, we evaluate the effect of remote ligand modification in a series of [(NHC)CuH]₂ in which the para substituent (R) on the N-aryl groups of the NHC is Me, Et, ^tBu, OMe or Cl. Although the R group is distant (6 bonds away) from the reactive Cu center, the complexes have different spectroscopic signatures. Kinetics studies of the insertion of ketone, aldimine, alkyne, and unactivated α-olefin substrates reveal that Cu–H complexes with bulky or electron-rich R groups undergo faster substrate insertion. The predominant cause of this phenomenon is destabilization of the [(NHC)CuH]₂ dimer relative to the (NHC)Cu–H monomer, resulting in faster formation of Cu–H monomer. These findings indicate that remote functionalization of NHCs is a compelling strategy for accelerating the rate of substrate insertion with Cu–H species.

Remote modification of an N-heterocyclic carbene ligand with bulky or electron-rich groups in [(NHC)Cu(μ-H)]₂ increases the rate of substrate insertion, which kinetics studies suggest arises from changes in the Cu–H monomer–dimer equilibrium.     

Understanding the unique reactivity patterns of nickel/JoSPOphos manifold in the nickel-catalyzed enantioselective C–H cyclization of imidazoles

The 3d transition metal-catalyzed enantioselective C–H functionalization provides a sustainable strategy for the construction of chiral molecules. A better understanding of the catalytic nature of the reactions and the factors controlling the enantioselectivity is important for rational design of more efficient systems. Herein, the mechanisms of Ni-catalyzed enantioselective C–H cyclization of imidazoles are investigated by density functional theory (DFT) calculations. Both the π-allyl nickel(ii)-promoted σ-complex-assisted metathesis (σ-CAM) and the nickel(0)-catalyzed oxidative addition (OA) mechanisms are disfavored. In addition to the typically proposed ligand-to-ligand hydrogen transfer (LLHT) mechanism, the reaction can also proceed via an unconventional σ-CAM mechanism that involves hydrogen transfer from the JoSPOphos ligand to the alkene through P–H oxidative addition/migratory insertion, C(sp²)–H activation via σ-CAM, and C–C reductive elimination. Importantly, computational results based on this new mechanism can indeed reproduce the experimentally observed enantioselectivities. Further, the catalytic activity of the π-allyl nickel(ii) complex can be rationalized by the regeneration of the active nickel(0) catalyst via a stepwise hydrogen transfer, which was confirmed by experimental studies. The calculations reveal several significant roles of the secondary phosphine oxide (SPO) unit in JoSPOphos during the reaction. The improved mechanistic understanding will enable design of novel enantioselective C–H transformations.

Several unique reactivity patterns of the Ni/JoSPOphos manifold, including facile hydrogen transfer via the two-step oxidative addition/migratory insertion and C(sp²)–H activation via an unconventional σ-CAM mechanism, were disclosed in this work.     

trans-Hydroboration–oxidation products in Δ5-steroids via a hydroboration-retro-hydroboration mechanism

Herein, we report for the first time a “trans-hydroboration–oxidation product” isolated and characterized under traditional hydroboration–oxidation conditions using cholesterol and diosgenin as substrates. These substrates are excellent starting materials because of the rigidity and different structural environments around the double bond. Further investigations based on experimental evidence, in conjunction with theoretical studies, indicate that the formation of this trans-species occurs via a retro-hydroboration of the major product to generate the corresponding Δ⁶-structure and the subsequent hydroboration by the β-face. Besides, the corresponding Markovnikov type products have been isolated in synthetically useful yields. The behavior of the reaction under a range of temperatures is also investigated.

A trans-product is isolated and characterized under traditional hydroboration–oxidation conditions using Δ⁵-steroids as substrates. Experimental and theoretical studies indicate that the trans-species occurs via a retro-hydroboration mechanism.     

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.     

Facilitating the reduction of V–O bonds on VOx/ZrO2 catalysts for non-oxidative propane dehydrogenation

Supported vanadium oxide is a promising catalyst in propane dehydrogenation due to its competitive performance and low cost. Nevertheless, it remains a grand challenge to understand the structure–performance correlation due to the structural complexity of VO_x-based catalysts in a reduced state. This paper describes the structure and catalytic properties of the VO_x/ZrO₂ catalyst. When using ZrO₂ as the support, the catalyst shows six times higher turnover frequency (TOF) than using commercial γ-Al₂O₃. Combining H₂-temperature programmed reduction, in situ Raman spectroscopy, X-ray photoelectron spectroscopy and theoretical studies, we find that the interaction between VO_x and ZrO₂ can facilitate the reduction of V–O bonds, including V O, V–O–V and V–O–Zr. The promoting effect could be attributed to the formation of low coordinated V species in VO_x/ZrO₂ which is more active in C–H activation. Our work provides a new insight into understanding the structure–performance correlation in VO_x-based catalysts for non-oxidative propane dehydrogenation.

Low coordinated VO_x species on ZrO₂ with more reduced V–O bonds leads to improved catalytic activity for propane dehydrogenation.     

Spontaneous emergence of membrane-forming protoamphiphiles from a lipid–amino acid mixture under wet–dry cycles

Dynamic interplay between peptide synthesis and membrane assembly would have been crucial for the emergence of protocells on the prebiotic Earth. However, the effect of membrane-forming amphiphiles on peptide synthesis, under prebiotically plausible conditions, remains relatively unexplored. Here we discern the effect of a phospholipid on peptide synthesis using a non-activated amino acid, under wet–dry cycles. We report two competing processes simultaneously forming peptides and N-acyl amino acids (NAAs) in a single-pot reaction from a common set of reactants. NAA synthesis occurs via an ester–amide exchange, which is the first demonstration of this phenomenon in a lipid–amino acid system. Furthermore, NAAs self-assemble into vesicles at acidic pH, signifying their ability to form protocellular membranes under acidic geothermal conditions. Our work highlights the importance of exploring the co-evolutionary interactions between membrane assembly and peptide synthesis, having implications for the emergence of hitherto uncharacterized compounds of unknown prebiotic relevance.

Synthesis of lipoamino acids via ester–amide exchange under prebiotically plausible wet-dry cycling conditions that results in vesicles at acidic pH.     

Charge stabilization via electron exchange: excited charge separation in symmetric,central triphenylamine derived,dimethylaminophenyl–tetracyanobutadiene donor–acceptor conjugates

Photoinduced charge separation in donor–acceptor conjugates plays a pivotal role in technology breakthroughs, especially in the areas of efficient conversion of solar energy into electrical energy and fuels. Extending the lifetime of the charge separated species is a necessity for their practical utilization, and this is often achieved by following the mechanism of natural photosynthesis where the process of electron/hole migration occurs distantly separating the radical ion pairs. Here, we hypothesize and demonstrate a new mechanism to stabilize the charge separated states via the process of electron exchange among the different acceptor entities in multimodular donor–acceptor conjugates. For this, star-shaped, central triphenylamine derived, dimethylamine–tetracyanobutadiene conjugates have been newly designed and characterized. Electron exchange was witnessed upon electroreduction in conjugates having multiple numbers of electron acceptors. Using ultrafast spectroscopy, the occurrence of excited state charge separation, and the effect of electron exchange in prolonging the lifetime of charge separated states in the conjugates having multiple acceptors have been successfully demonstrated. This work constitutes the first example of stabilizing charge-separated states via the process of electron exchange.

The significance of electron exchange in stabilizing the charge-separated state is revealed in multi-modular donor–acceptor conjugates.