Multi-Variable Multi-Metric Optimization of Self-Assembled Photocatalytic CO2 Reduction Performance Using Machine Learning Algorithms.

The sunlight-driven reduction of CO2 into fuels and platform chemicals is a promising approach to enable a circular economy. However, established optimization approaches are poorly suited to multivariable multimetric photocatalytic systems because they aim to optimize one performance metric while sacrificing the others and thereby limit overall system performance. Herein, we address this multimetric challenge by defining a metric for holistic system performance that takes multiple figures of merit into account, and employ a machine learning algorithm to efficiently guide our experiments through the large parameter matrix to make holistic optimization accessible for human experimentalists. As a test platform, we employ a five-component system that self-assembles into photocatalytic micelles for CO2-to-CO reduction, which we experimentally optimized to simultaneously improve yield, quantum yield, turnover number, and frequency while maintaining high selectivity. Leveraging the data set with machine learning algorithms allows quantification of each parameter's effect on overall system performance. The buffer concentration is unexpectedly revealed as the dominating parameter for optimal photocatalytic activity, and is nearly four times more important than the catalyst concentration. The expanded use and standardization of this methodology to define and optimize holistic performance will accelerate progress in different areas of catalysis by providing unprecedented insights into performance bottlenecks, enhancing comparability, and taking results beyond comparison of subjective figures of merit.

Electrostatic [FeFe]-hydrogenase-carbon nitride assemblies for efficient solar hydrogen production.

The assembly of semiconductors as light absorbers and enzymes as redox catalysts offers a promising approach for sustainable chemical synthesis driven by light. However, achieving the rational design of such semi-artificial systems requires a comprehensive understanding of the abiotic-biotic interface, which poses significant challenges. In this study, we demonstrate an electrostatic interaction strategy to interface negatively charged cyanamide modified graphitic carbon nitride (NCNCNX) with an [FeFe]-hydrogenase possessing a positive surface charge around the distal FeS cluster responsible for electron uptake into the enzyme. The strong electrostatic attraction enables efficient solar hydrogen (H2) production via direct interfacial electron transfer (DET), achieving a turnover frequency (TOF) of 18 669 h-1 (4 h) and a turnover number (TON) of 198 125 (24 h). Interfacial characterizations, including quartz crystal microbalance (QCM), photoelectrochemical impedance spectroscopy (PEIS), intensity-modulated photovoltage spectroscopy (IMVS), and transient photocurrent spectroscopy (TPC) have been conducted on the semi-artificial carbon nitride-enzyme system to provide a comprehensive understanding for the future development of photocatalytic hybrid assemblies.

Photocatalytic CO2 Reduction Using Homogeneous Carbon Dots with a Molecular Cobalt Catalyst.

A simple and precious-metal free photosystem for the reduction of aqueous CO2 to syngas (CO and H2) is reported consisting of carbon dots (CDs) as the sole light harvester together with a molecular cobalt bis(terpyridine) CO2 reduction co-catalyst. This homogeneous photocatalytic system operates in the presence of a sacrificial electron donor (triethanolamine) in DMSO/H2O solution at ambient temperature. The photocatalytic system exhibits an activity of 7.7 ± 0.2 mmolsyngas gCDs -1 (3.6 ± 0.2 mmolCO gCDs -1 and 4.1 ± 0.1 mmolH2 gCDs -1) after 24 hours of full solar spectrum irradiation (AM 1.5G). Spectroscopic and electrochemical characterization supports that this photocatalytic performance is attributed to a favorable association between CDs and the molecular cobalt catalyst, which results in improved interfacial photoelectron transfer and catalytic mechanism. This work provides a scalable and inexpensive platform for the development of CO2 photoreduction systems using CDs.

Predictable electronic tuning of FeII and RuII complexes via choice of azine: correlation of ligand pKa with Epa(MIII/II) of complex.

Five new mononuclear ruthenium(II) tris-ligated complexes have been synthesised, varying through the choice of azine in the family of 3-azinyl-4-(4-methylphenyl)-5-phenyl-4H-1,2,4-triazole ligands (Lazine): [Ru(Lpyridine)](PF6)2 (1), [Ru(Lpyridazine)](PF6)2 (2), [Ru(L4-pyrimidine)](PF6)2 (3), [Ru(Lpyrazine)](PF6)2 (4), [Ru(L2-pyrimidine)](PF6)2 (5). Three of them, 1·2MeCN·Et2O, 3·2MeCN·Et2O and 4·2MeCN, have been structurally characterised, confirming the presence of the meridional isomer, as was previously reported for the FeII analogues. Cyclic voltammetry studies, in dry CH3CN vs. Ag/0.01 M AgNO3, show that all five RuII complexes undergo a reversible RuIII/RuII process, with the midpoint potential (Em) increasing from 0.87 to 1.18 V as the azine is changed: pyridine < pyridazine < 2-pyrimidine < 4-pyrimidine < pyrazine. A strong inverse linear correlation (R2 = 0.98) is found between the RuIII/RuII redox potential and the calculated HOMO orbital energies, which is consistent with the expectation that it is easier to oxidise (lower Em) a metal ion with a higher HOMO orbital energy. The same trend was reported earlier for the family of analogous FeII complexes, albeit at lower values of Em in all cases. In addition, the ionisation potentials of the RuII complexes, as well as those of the other group 8 analogues (FeII and OsII), showed a linear relationship with Epa. As the MIII/II redox potentials of a family of complexes has been previously reported to correlate with ligand pKa values, a computational protocol to calculate, in silico, the pKa of the Lazine family of ligands was developed. A strong linear relationship was found between the readily calculated pKa of the Lazine ligand and the Epa of the MII complex, for all three families of complexes (R2 = 0.98).

Connecting Biological and Synthetic Approaches for Electrocatalytic CO2 Reduction.

Electrocatalytic CO2 reduction has developed into a broad field, spanning fundamental studies of enzymatic 'model' catalysts to synthetic molecular catalysts and heterogeneous gas diffusion electrodes producing commercially relevant quantities of product. This diversification has resulted in apparent differences and a disconnect between seemingly related approaches when using different types of catalysts. Enzymes possess discrete and well understood active sites that can perform reactions with high selectivity and activities at their thermodynamic limit. Synthetic small molecule catalysts can be designed with desired active site composition but do not yet display enzyme-like performance. These properties of the biological and small molecule catalysts contrast with heterogeneous materials, which can contain multiple, often poorly understood active sites with distinct reactivity and therefore introducing significant complexity in understanding their activities. As these systems are being better understood and the continuously improving performance of their heterogeneous active sites closes the gap with enzymatic activity, this performance difference between heterogeneous and enzymatic systems begins to close. This convergence removes the barriers between using different types of catalysts and future challenges can be addressed without multiple efforts as a unified picture for the biological-synthetic catalyst spectrum emerges.

Valorisation of lignocellulose and low concentration CO2 using a fractionation-photocatalysis-electrolysis process.

The simultaneous upcycling of all components in lignocellulosic biomass and the greenhouse gas CO2 presents an attractive opportunity to synthesise sustainable and valuable chemicals. However, this approach is challenging to realise due to the difficulty of implementing a solution process to convert a robust and complex solid (lignocellulose) together with a barely soluble and stable gas (CO2). Herein, we present the complete oxidative valorisation of lignocellulose coupled to the reduction of low concentration CO2 through a three-stage fractionation-photocatalysis-electrolysis process. Lignocellulose from white birch wood was first pre-treated using an acidic solution to generate predominantly cellulosic- and lignin-based fractions. The solid cellulosic-based fraction was solubilised using cellulase (a cellulose depolymerising enzyme), followed by photocatalytic oxidation to formate with concomitant reduction of CO2 to syngas (a gas mixture of CO and H2) using a phosphonate-containing cobalt(ii) bis(terpyridine) catalyst immobilised onto TiO2 nanoparticles. Photocatalysis generated 27.9 ± 2.0 µmolCO gTiO2-1 (TONCO = 2.8 ± 0.2; 16% CO selectivity) and 147.7 ± 12.0 µmolformate gTiO2-1 after 24 h solar light irradiation under 20 vol% CO2 in N2. The soluble lignin-based fraction was oxidised in an electrolyser to the value-added chemicals vanillin (0.62 g kglignin-1) and syringaldehyde (1.65 g kglignin-1) at the anode, while diluted CO2 (20 vol%) was converted to CO (20.5 ± 0.2 µmolCO cm-2 in 4 h) at a Co(ii) porphyrin catalyst modified cathode (TONCO = 707 ± 7; 78% CO selectivity) at an applied voltage of -3 V. We thus demonstrate the complete valorisation of solid and a gaseous waste stream in a liquid phase process by combining fractioning, photo- and electrocatalysis using molecular hybrid nanomaterials assembled from earth abundant elements.

Self-Assembled Liposomes Enhance Electron Transfer for Efficient Photocatalytic CO2 Reduction.

Light-driven conversion of CO2 to chemicals provides a sustainable alternative to fossil fuels, but homogeneous systems are typically limited by cross reactivity between different redox half reactions and inefficient charge separation. Herein, we present the bioinspired development of amphiphilic photosensitizer and catalyst pairs that self-assemble in lipid membranes to overcome some of these limitations and enable photocatalytic CO2 reduction in liposomes using precious metal-free catalysts. Using sodium ascorbate as a sacrificial electron source, a membrane-anchored alkylated cobalt porphyrin demonstrates higher catalytic CO production (1456 vs 312 turnovers) and selectivity (77 vs 11%) compared to its water-soluble nonalkylated counterpart. Time-resolved and steady-state spectroscopy revealed that self-assembly facilitates this performance enhancement by enabling a charge-separation state lifetime increase of up to two orders of magnitude in the dye while allowing for a ninefold faster electron transfer to the catalyst. Spectroelectrochemistry and density functional theory calculations of the alkylated Co porphyrin catalyst support a four-electron-charging mechanism that activates the catalyst prior to catalysis, together with key catalytic intermediates. Our molecular liposome system therefore benefits from membrane immobilization and provides a versatile and efficient platform for photocatalysis.

Shorter Alkyl Chains Enhance Molecular Diffusion and Electron Transfer Kinetics between Photosensitisers and Catalysts in CO2 -Reducing Photocatalytic Liposomes.

Covalent functionalisation with alkyl tails is a common method for supporting molecular catalysts and photosensitisers onto lipid bilayers, but the influence of the alkyl chain length on the photocatalytic performances of the resulting liposomes is not well understood. In this work, we first prepared a series of rhenium-based CO2 -reduction catalysts [Re(4,4'-(Cn H2n+1 )2 -bpy)(CO)3 Cl] (ReCn ; 4,4'-(Cn H2n+1 )2 -bpy=4,4'-dialkyl-2,2'-bipyridine) and ruthenium-based photosensitisers [Ru(bpy)2 (4,4'-(Cn H2n+1 )2 -bpy)](PF6 )2 (RuCn ) with different alkyl chain lengths (n=0, 9, 12, 15, 17, and 19). We then prepared a series of PEGylated DPPC liposomes containing RuCn and ReCn , hereafter noted Cn , to perform photocatalytic CO2 reduction in the presence of sodium ascorbate. The photocatalytic performance of the Cn liposomes was found to depend on the alkyl tail length, as the turnover number for CO (TON) was inversely correlated to the alkyl chain length, with a more than fivefold higher CO production (TON=14.5) for the C9 liposomes, compared to C19 (TON=2.8). Based on immobilisation efficiency quantification, diffusion kinetics, and time-resolved spectroscopy, we identified the main reason for this trend: two types of membrane-bound RuCn species can be found in the membrane, either deeply buried in the bilayer and diffusing slowly, or less buried with much faster diffusion kinetics. Our data suggest that the higher photocatalytic performance of the C9 system is due to the higher fraction of the more mobile and less buried molecular species, which leads to enhanced electron transfer kinetics between RuC9 and ReC9 .

Roadmap towards solar fuel synthesis at the water interface of liposome membranes.

Artificial photosynthesis has experienced rapid developments aimed at producing photocatalytic systems for the synthesis of chemical energy carriers. Conceptual advances of solar fuel systems have been inspired by improved understanding of natural photosynthesis and its key operational principles: (a) light harvesting, (b) charge separation, (c) directional proton and electron transport between reaction centres and across membranes, (d) water oxidation and (e) proton or CO2 reduction catalysis. Recently, there has been a surge of bio-inspired photosynthetic assemblies that use liposomes as nanocompartments to confine reaction spaces and enable vectorial charge transport across membranes. This approach, already investigated in the 1980s, offers in principle a promising platform for solar fuel synthesis. However, the fundamental principles governing the supramolecular assemblies of lipids and photoactive surfactant-like molecules in membranes, are intricate, and mastering membrane-supported photochemistry requires thorough understanding of the science behind liposomes. In this review, we provide an overview of approaches and considerations to construct a (semi)artificial liposome for solar fuel production. Key features to consider for the use of liposomes in solar fuel synthesis are highlighted, including the understanding of the orientation and binding of different components along the membrane, the controlled electron transport between the reaction centres, and the generation of proton gradients as driving force. Together with a list of experimental techniques for the characterisation of photoactive liposomes, this article provides the reader with a roadmap towards photocatalytic fuel production at the interface of lipid membranes and aqueous media.

Electroactive Metal Complexes Covalently Attached to Conductive PEDOT Films: A Spectroelectrochemical Study.

The successful covalent attachment, via copper(I)-catalyzed azide alkyne cycloaddition (CuAAC), of alkyne-functionalized nickel(II) and copper(II) macrocyclic complexes onto azide (N3)-functionalized poly(3,4-ethylenedioxythiophene) (PEDOT) films on ITO-coated glass electrodes is reported. To investigate the surface attachment of the selected metal complexes, which are analogues of the cobalt-based complex previously reported to be a molecular catalyst for hydrogen evolution, first, three different PEDOT films were formed by electropolymerization of pure PEDOT or pure N3-PEDOT, and last, 1:2N3-PEDOT:PEDOT were formed by co-polymerizing a 1:4 mixture of N3-EDOT:EDOT monomers. The successful surface immobilization of the complexes on the latter two azide-functionalized films, by CuAAC, was confirmed by X-ray photoelectron spectroscopy (XPS) and electrochemistry as well as by UV-vis-NIR and resonance Raman spectroelectrochemistry. The ratio between the N3 groups, and hence, the number of surface-attached metal complexes after CuAAC functionalization, in pristine N3-PEDOT versus 1:2N3-PEDOT:PEDOT is expected to be 3:1 and seen to be 2.86:1 with a calculated surface coverage of 3.28 ± 1.04 and 1.15 ± 0.09 nmol/cm2, respectively. The conversion, to the metal complex attached films, was lower for the N3-PEDOT films (Ni 74%, Cu 76%) than for the copolymer 1:2N3-PEDOT:PEDOT films (Ni 83%, Cu 91%) due to the former being more sterically congested. The Raman and UV-vis-NIR results were simulated using density functional theory (DFT) and time-dependent DFT (TD-DFT), respectively, and showed good agreement with the experimental data. Importantly, the spectroelectrochemical behavior of both anchored metal complexes is analogous to that of the free metal complexes in solution. This proves that PEDOT films are promising conducting scaffolds for the covalent immobilization of metal complexes, as the existing electrochromic features of the complexes are preserved on immobilization, which is important for applications in electrocatalytic proton and carbon dioxide reduction, optoelectronics, and sensing.

Qualitative Guest Sensing via Iron(II) Triazole Complexes.

The pyridazine-pyridine triazole-based Rat ligand, Lpydzpy [4-(4-methylphenyl)-3-(3-pyridazinyl)-5-(2-pyridinyl)-1,2,4-triazole], is potentially ditopic. Nevertheless, Lpydzpy is shown herein to exclusively form mononuclear iron(II) complexes, [FeII(Lpydzpy)2(NCE)2]·solvent, in the presence of coordinating NCE anions (E = S or Se). Specifically, a new family of 10 mononuclear complexes, in which Lpydzpy binds in a monotopic bidentate manner, has been made: two solvent-free complexes, [FeII(Lpydzpy)2(NCS)2] (1) and [FeII(Lpydzpy)2(NCSe)2] (2); six solvatomorphs, 1·4CH3CN, 2·4CH3CN, 1·2.25CH3CN, 2·3CH3CN, 2·tetrahydrofuran, and 2·CHCl3; and a pair of desolvated polymorphs, 1' and 2'. Seven of them are spin crossover-active, the exceptions being 1, 2, and 2'. This is confirmed by single-crystal X-ray diffraction (XRD) for 1, 2, 1·4CH3CN, and 2·4CH3CN and is consistent with variable-temperature optical microscopy observations on single crystals of 1·4CH3CN and 2·4CH3CN and on samples of 1' and 2'. Powder XRD, thermogravimetric analysis, and solid-state magnetometry reveal that desolvated 1' and 2' are capable of absorbing and desorbing a range of volatile guests: CH3CN in both cases and also tetrahydrofuran and CHCl3 in the case of 2'.

Predictable Electronic Tuning By Choice of Azine Substituent in Five Iron(II) Triazoles: Redox Properties and DFT Calculations.

Five new mononuclear iron(II) tris-ligand complexes, and four solvatomorphs, have been made from the azine-substituted 1,2,4-triazole ligands (Lazine ): [FeII (Lpyridazine )3 ](BF4 )2 (1), [FeII (Lpyrazine )3 ](BF4 )2 (2), [FeII (Lpyridine )3 ](BF4 )2 (3), [FeII (L2pyrimidine )3 ](BF4 )2 (4), and [FeII (L4pyrimidine )3 ](BF4 )2 (5). Single-crystal XRD and solid-state magnetometry reveal that all of them are low-spin (LS) iron(II), except for solvatomorph 5⋅4 H2 O. Evans method NMR studies in CD2 Cl2 , (CD3 )2 CO and CD3 CN show that all are LS in these solvents, except 5 in CD2 Cl2 (consistent with L4pyrimidine imposing the weakest field). Cyclic voltammetry in CH3 CN vs. Ag/0.01 m AgNO3 reveals an, at best quasi-reversible, FeIII/II redox process, with Epa increasing from 0.69 to 0.99 V as the azine changes: pyridine< pyridazine<2-pyrimidine<4-pyrimidine< pyrazine. The observed Epa values correlate linearly with the DFT calculated HOMO energies for the LS complexes.

Solvent Polarity Predictably Tunes Spin Crossover T1/2 in Isomeric Iron(II) Pyrimidine Triazoles.

Two isomeric pyrimidine-based Rdpt-type triazole ligands were made: 4-(4-methylphenyl)-3-(2-pyrimidyl)-5-phenyl-4 H-1,2,4-triazole (L2pyrimidine) and 4-(4-methylphenyl)-3-(4-pyrimidyl)-5-phenyl-4 H-1,2,4-triazole (L4pyrimidine). When reacted with [FeII(pyridine)4(NCE)2], where E = S, Se, or BH3, two families of mononuclear iron(II) complexes are obtained, including six solvatomorphs, giving a total of 12 compounds: [FeII(L2pyrimidine)2(NCS)2] (1), [FeII(L2pyrimidine)2(NCSe)2] (2), 2·1.5H2O, [FeII(L2pyrimidine)2(NCBH3)2]·2CHCl3 (3·2CHCl3), 3 and 3·2H2O, [FeII(L4pyrimidine)2(NCS)2] (4), 4·H2O, [FeII(L4pyrimidine)2(NCSe)2] (5), 5·2CH3OH, 5·1.5H2O, and [FeII(L4pyrimidine)2(NCBH3)2]·2.5H2O (6·2.5H2O). Single-crystal X-ray diffraction reveals that the N6-coordinated iron(II) centers in 1, 2, 3·2CHCl3, 4, 5, and 5·2CH3OH have two bidentate triazole ligands equatorially bound and two axial NCE co-ligands trans-coordinated. All structures are high spin (HS) at 100 K, except 3·2CHCl3, which is low spin (LS). Solid-state magnetic measurements show that only 3·2CHCl3 ( T1/2 above 400 K) and 5·1.5H2O ( T1/2 = 110 K) undergo spin crossover (SCO); the others remain HS at 300-50 K. When 3·2CHCl3 is heated at 400 K it desorbs CHCl3 becoming 3, which remains HS at 400-50 K. UV-Vis studies in CH2Cl2, CHCl3, (CH3)2CO, CH3CN, and CH3NO2 solutions for the BH3 analogues 3 and 6 led to a 6:1 ratio of L npyrimidine/Fe(II) being employed for the solution studies. These revealed SCO activity in all five solvents, with T1/2 values for the 2-pyrimidine complex (247-396 K) that were consistently higher than for the 4-pyrimidine complex (216-367 K), regardless of solvent choice, consistent with the 2-pyrimidine ring providing a stronger ligand field than the 4-pyrimidine ring. Strong correlations of solvent polarity index with the T1/2 values in those solvents are observed for each complex, enabling predictable T1/2 tuning by up to 150 K. While this correlation is tantalizing, here it may also be reflecting solvent-dependent speciation-so future tests of this concept should employ more stable complexes. Differences between solid-state (ligand field; crystal packing; solvent content) and solution (ligand field; solvation; speciation) effects on SCO are highlighted.

Solid Versus Solution Spin Crossover and the Importance of the        Fe-N≡C(X) Angle.

A new family of mononuclear [FeII(Rdpt)2(NCE)2] complexes (E = S, Se, or BH3) is formed by 1:2 reaction of [FeII(pyridine)4(NCE)2] with the monotopic pyridyl triazole ligand 4-(4-methylphenyl)-3-(2-pyridinyl)-5-phenyl-4H-1,2,4-triazole (tolpyph). The three complexes are obtained as six different solvatomorphs: [FeII(tolpyph)2(NCS)2]·H2O (1·H2O), 1·1.5CH3OH·0.5H2O, [FeII(tolpyph)2(NCSe)2] (2), 2·1.5H2O, [FeII(tolpyph)2(NCBH3)2] (3), and 3·H2O. Single-crystal X-ray diffraction reveals that 1·1.5CH3OH·0.5H2O and 2 are high-spin (HS) at 100 K, while 3 is low-spin (LS) at 100 K and HS at 373 K. Compound 3 is the first structurally characterized example of an [FeII(Rdpt)2(NCE)2]-type complex with NCBH3 co-ligand: the crystal packing is dominated by aromatic stacking interactions. Solid-state magnetic measurements show that 1·H2O and 2·1.5H2O remain HS down to 50 K, whereas 3·H2O undergoes spin crossover (SCO) with a T1/2 of 309 K, slightly above room temperature. A literature survey of analogous trans-[FeII(Rdpt)2(NCX)2]-type complexes (53 distinct crystal structures) shows that for the complexes that are SCO active in the solid state the Fe-N≡C(X) angle is usually close to straight, 162-178°, whereas it is usually lower, 142-159°, for the complexes that remain HS. UV-vis studies in CHCl3 solution show that in each case the use of a 6:1 ratio of tolpyph/Fe(II) is required to ensure the iron(II) is present in solution as [FeII(tolpyph)2(NCE)2]. Interestingly, using this ratio, all three compounds are SCO-active in CDCl3 solution-in dramatic contrast to the solid-state findings. Specifically, while compounds 1 and 2 are not SCO-active in the solid state (they remain HS), they undergo gradual SCO in CDCl3 solution, with T1/2 values of 290 and 310 K, respectively. In CDCl3 solution, compound 3 has a T1/2 value of 288 K, which is 21 K lower than in the solid state. These results highlight the differences between solid state (ligand field; crystal packing) and solution (ligand field; solvation) effects on SCO, with the latter studies revealing room-temperature SCO for all three of these complexes.

A Simple Method of Predicting Spin State in Solution.

A simple method, using density functional theory (DFT), of predicting spin-state in advance of synthesis is reported. Specifically, an excellent correlation is observed between the switching temperatures (T1/2) in CDCl3 solution of five spin-crossover (SCO)-active [FeII(Lazine)2(NCBH3)2] complexes and the DFT-calculated (and observed) 15N NMR chemical shift (δNA) of the five different azine-substituted 1,2,4-triazole ligands employed, Lazine = 4-(4-methylphenyl)-3-phenyl-5-(azine)-1,2,4-triazole, where azine = pyridine, pyridazine, 4-pyrimidine, pyrazine, and 2-pyrimidine. To test the generality of this finding, DFT was also employed to readily predict the δNA values for a family of 16 literature ligands, known as bppX,Y [X,Y-substituted 2,6-(pyrazol-1-yl)pyridines], which have produced 16 SCO-active [FeII(bppX,Y)2](Z)2 complexes (Z = BF4 or in one case PF6) in (CD3)2CO solution: again an excellent correlation was found between the computed δNA and the observed T1/2. These correlations represent a key advance in the field, as they allow a simple DFT calculation on a modified ligand to be used to reliably predict, before synthesis of the ligand or complex, the T1/2 that would result from that modification. Achieving such easily predictable tuning of T1/2, and hence of spin-state, is a significant step forward in the field of SCO and also has big implications in many other fields in which spin-state is key, including catalysis, metallo-enzyme modeling studies, and host-guest chemistry.

Non-Porous Iron(II)-Based Sensor: Crystallographic Insights into a Cycle of Colorful Guest-Induced Topotactic Transformations.

Materials capable of sensing volatile guests at room temperature by an easily monitored set of outputs are of great appeal for development as chemical sensors of small volatile organics and toxic gases. Herein the dinuclear iron(II) complex, [FeII2 (L)2 (CH3 CN)4 ](BF4 )4 ⋅2 CH3 CN (1) [L=4-(4-methylphenyl)-3-(3-pyridazinyl)-5-pyridyl-4H-1,2,4-triazole], is shown to undergo reversible single-crystal-to-single-crystal (SCSC) transformations upon exposure to vapors of different guests: 1 (MeCN)⇌2 (EtOH)→3 (H2 O)⇌1 (MeCN). Whilst 1 and 2 remain dimetallic, SCSC to 3 involves conversion to a 1D polymeric chain (due to a change in L bridging mode), which, remarkably, can undergo SCSC de-polymerization, reforming dimetallic 1. Additionally, SC-XRD studies of two ordered transient forms, 1TF3 and 2TF3, confirm that guest exchange occurs by diffusion of the new guests into the non-porous lattices as the old guests leave. These reversible SCSC events also induce color and magnetic responses. Indeed dark red 1 is spin crossover active (T1/2 ↓ 356 K; T1/2 ↑ 369 K), whilst orange 2 and yellow 3 remain high spin.

Three-way crystal-to-crystal reversible transformation and controlled spin switching by a nonporous molecular material.

Porous materials capable of hosting external molecules are paramount in basic and applied research. Nonporous materials able to incorporate molecules via internal lattice reorganization are however extremely rare since their structural integrity usually does not resist the guest exchange processes. The novel heteroleptic low-spin Fe(II) complex [Fe(bpp)(H2L)](ClO4)2·1.5C3H6O (1; bpp = 2,6-bis(pyrazol-3-yl)pyridine, H2L = 2,6-bis(5-(2-methoxyphenyl)pyrazol-3-yl)pyridine) crystallizes as a compact discrete, nonporous material hosting solvate molecules of acetone. The system is able to extrude one-third of these molecules to lead to [Fe(bpp)(H2L)](ClO4)2·C3H6O (2), switching to the high-spin state while experiencing a profound crystallographic change. Compound 2 can be reversed to the original material upon reabsorption of acetone. Single crystal X-ray diffraction experiments on the latter system (1') and on 2 show that these are reversible single-crystal-to-single-crystal (SCSC) transformations. Likewise, complex 2 can replace acetone by MeOH and H2O to form [Fe(bpp)(H2L)](ClO4)2·1.25MeOH·0.5H2O (3) through a SCSC process that also implies a switch to the spin state. The 3→1 transformation through acetone reabsorption is also demonstrated. Besides the spin switching at room temperature, this series of SCSC transformations causes macroscopic changes in color that can be followed by the naked eye. The reversible exchanges of chemicals are therefore easily sensed at the temperature at which these occur, contrary to what is the case for most of the few existing nonporous spin-based sensors, which feature a large temperature gap between the process monitored and the mechanism of detection.