Halide Perovskites Breathe Too: The Iodide-Iodine Equilibrium and Self-Doping in Cs2SnI6.

The response of an oxide crystal to the atmosphere can be personified as breathing-a dynamic equilibrium between O2 gas and O2- anions in the solid. We characterize the analogous defect reaction in an iodide double-perovskite semiconductor, Cs2SnI6. Here, I2 gas is released from the crystal at room temperature, forming iodine vacancies. The iodine vacancy defect is a shallow electron donor and is therefore ionized at room temperature; thus, the loss of I2 is accompanied by spontaneous n-type self-doping. Conversely, at high I2 pressures, I2 gas is resorbed by the perovskite, consuming excess electrons as I2 is converted to 2I-. Halide mobility and irreversible halide loss or exchange reactions have been studied extensively in halide perovskites. However, the reversible exchange equilibrium between iodide and iodine [2I-(s) ↔ I2(g) + 2e-] described here has often been overlooked in prior studies, though it is likely general to halide perovskites and operative near room temperature, even in the dark. An analysis of the 2I-(s)/I2(g) equilibrium thermodynamics and related transport kinetics in single crystals of Cs2SnI6 therefore provides insight toward achieving stable composition and electronic properties in the large family of iodide perovskite semiconductors.

Tailored Design of a Water-Based Nanoreactor Technology for Producing Processable Sub-40 Nm 3D COF Nanoparticles at Atmospheric Conditions.

Covalent organic frameworks (COFs) are crystalline materials with intrinsic porosity that offer a wide range of potential applications spanning diverse fields. Yet, the main goal in the COF research area is to achieve the most stable thermodynamic product while simultaneously targeting the desired size and structure crucial for enabling specific functions. While significant progress is made in the synthesis and processing of 2D COFs, the development of processable 3D COF nanocrystals remains challenging. Here, a water-based nanoreactor technology for producing processable sub-40 nm 3D COF nanoparticles at ambient conditions is presented. Significantly, this technology not only improves the processability of the synthesized 3D COF, but also unveils exciting possibilities for their utilization in previously unexplored domains, such as nano/microrobotics and biomedicine, which are limited by larger crystallites.

Mosaic CuI -CuII -InIII 2D Perovskites: Pressure-Dependence of the Intervalence Charge Transfer and a Mechanochemical Alloying Method.

The perovskite (BA)4 [CuII (CuI InIII )0.5 ]Cl8 (1BA ; BA+ =butylammonium) allows us to study the high-pressure structural, optical, and transport properties of a mixed-valence 2D perovskite. Compressing 1BA reduces the onset energy of CuI/II intervalence charge transfer from 1.2 eV at ambient pressure to 0.2 eV at 21 GPa. The electronic conductivity of 1BA increases by 4 orders of magnitude upon compression to 20 GPa, when the activation energy for conduction decreases to 0.16 eV. In contrast, CuII perovskites achieve similar conductivity at ≈50 GPa. The solution-state synthesis of these perovskites is complicated, with more undesirable side products likely from the precursor mixtures containing three different metal ions. To circumvent this problem, we demonstrate an efficient mechanochemical synthesis to expand this family of halide perovskites with complex composition by simply pulverizing together powders of 2D CuII single perovskites and CuI InIII double perovskites.

Dielectric behavior of curcuminoid polymorphs on different substrates by direct soft vacuum deposition.

Our work examines the structural-electronic correlation of a new curcuminoid, AlkCCMoid, as a dielectric material on different substrates. For this purpose, we show a homemade sublimation method that allows the direct deposition of molecules on any type of matrix. The electronic properties of AlkCCMoid have been evaluated by measurements on single crystals, microcrystalline powder, and sublimated samples, respectively. GIWAXS studies on surfaces and XRD studies on powder have revealed the existence of polymorphs and the effect that substrates have on curcuminoid organization. We describe the dielectric nature of our system and identify how different polymorphs can affect electronic parameters such as permittivity, all corroborated by DFT calculations.

Quasi-One-Dimensional Metallicity in Compressed CsSnI3.

Low-dimensional metal halides exhibit strong structural and electronic anisotropies, making them candidates for accessing unusual electronic properties. Here, we demonstrate pressure-induced quasi-one-dimensional (quasi-1D) metallicity in δ-CsSnI3. With the application of pressure up to 40 GPa, the initially insulating δ-CsSnI3 transforms to a metallic state. Synchrotron X-ray diffraction and Raman spectroscopy indicate that the starting 1D chain structure of edge-sharing Sn-I octahedra in δ-CsSnI3 is maintained in the high-pressure metallic phase while the SnI6 octahedral chains are distorted. Our experiments combined with first-principles density functional theory calculations reveal that pressure induces Sn-Sn hybridization and enhances Sn-I coupling within the chain, leading to band gap closure and formation of conductive SnI6 distorted octahedral chains. In contrast, the interchain I...I interactions remain minimal, resulting in a highly anisotropic electronic structure and quasi-1D metallicity. Our study offers a high-pressure approach for achieving diverse electronic platforms in the broad family of low-dimensional metal halides.

Charge Reservoirs in an Expanded Halide Perovskite Analog: Enhancing High-Pressure Conductivity through Redox-Active Molecules.

As halide perovskites and their derivatives are being developed for numerous optoelectronic applications, controlling their electronic doping remains a fundamental challenge. Herein, we describe a novel strategy of using redox-active organic molecules as stoichiometric electron acceptors. The cavities in the new expanded perovskite analogs (dmpz)[Sn2 X6 ], (X=Br- (1Br) and I- (1I)) are occupied by dmpz2+ (N,N'-dimethylpyrazinium), with the LUMOs lying ca. 1 eV above the valence band maximum (VBM). Compressing the metal-halide framework drives up the VBM in 1I relative to the dmpz LUMO. The electronic conductivity increases by a factor of 105 with pressure, reaching 50(17) S cm-1 at 60 GPa, exceeding the high-pressure conductivities of most halide perovskites. This conductivity enhancement is attributed to an increased hole density created by dmpz2+ reduction. This work elevates the role of organic cations in 3D metal-halides, from templating the structure to serving as charge reservoirs for tuning the carrier concentration.

Water oxidation electrocatalysis using ruthenium coordination oligomers adsorbed on multiwalled carbon nanotubes.

Photoelectrochemical cells that utilize water as a source of electrons are one of the most attractive solutions for the replacement of fossil fuels by clean and sustainable solar fuels. To achieve this, heterogeneous water oxidation catalysis needs to be mastered and properly understood. The search continues for a catalyst that is stable at the surface of electro(photo)anodes and can efficiently perform this reaction at the desired neutral pH. Here, we show how oligomeric Ru complexes can be anchored on the surfaces of graphitic materials through CH-π interactions between the auxiliary ligands bonded to Ru and the hexagonal rings of the graphitic surfaces, providing control of their molecular coverage. These hybrid molecular materials behave as molecular electroanodes that catalyse water oxidation to dioxygen at pH 7 with high current densities. This strategy for the anchoring of molecular catalysts on graphitic surfaces can potentially be extended to other transition metals and other catalytic reactions.

Second Coordination Sphere Effects in an Evolved Ru Complex Based on Highly Adaptable Ligand Results in Rapid Water Oxidation Catalysis.

A new Ru complex containing the deprotonated 2,2':6',2''-terpyridine-6,6''-diphosphonic acid (H4tPa) and pyridine (py) of general formula [RuII(H3tPa-κ-N3O)(py)2]+, 2+, has been prepared and thoroughly characterized by means of spectroscopic and electrochemical techniques, X-ray diffraction analysis, and density functional theory (DFT) calculations. Complex 2+ presents a dynamic behavior in the solution that involves the synchronous coordination and the decoordination of the dangling phosphonic groups of the tPa4- ligand. However, at oxidation state IV, complex 2+ becomes seven coordinated with the two phosphonic groups now bonded to the metal center. Further, at this oxidation state at neutral and basic pH, the Ru complex undergoes the coordination of an exogenous OH- group from the solvent that leads to an intramolecular aromatic O atom insertion into the CH bond of one of the pyridyl groups, forming the corresponding phenoxo-phosphonate Ru complex [RuIII(tPaO-κ-N2OPOC)(py)2]2-, 42-, where tPaO5- is the 3-(hydroxo-[2,2':6',2''-terpyridine]-6,6''-diyl)bis(phosphonate) ligand. This new in situ generated Ru complex, 42-, has been isolated and spectroscopically and electrochemically characterized. In addition, a crystal structure has been also obtained using single-crystal X-ray diffraction techniques. Complex 42- turns out to be an exceptional water oxidation catalyst achieving record maximum turnover frequencies (TOFmax) on the order of 16 000 s-1. A mechanistic analysis complemented with DFT calculations has also been carried out, showing the critical role of intramolecular second coordination sphere effects exerted by the phosphonate groups in lowering the activation energy at the rate-determining step.

Three-Dimensional Phthalocyanine Metal-Catecholates for High Electrochemical Carbon Dioxide Reduction.

The synthesis of a new anionic 3D metal-catecholate framework, termed MOF-1992, is achieved by linking tetratopic cobalt phthalocyanin-2,3,9,10,16,17,23,24-octaol linkers with Fe3(-C2O2-)6(OH2)2 trimers into an extended framework of roc topology. MOF-1992 exhibits sterically accessible Co active sites together with charge transfer properties. Cathodes based on MOF-1992 and carbon black (CB) display a high coverage of electroactive sites (270 nmol cm-2) and a high current density (-16.5 mA cm-2; overpotential, -0.52 V) for the CO2 to CO reduction reaction in water (faradaic efficiency, 80%). Over the 6 h experiment, MOF-1992/CB cathodes reach turnover numbers of 5800 with turnover frequencies of 0.20 s-1 per active site.

Reticular Synthesis of Multinary Covalent Organic Frameworks.

Hexagonal hexaminophenyl benzene, tetragonal tetrakis(4-aminophenyl) ethane, and trigonal 1,3,5-tris(p-formylphenyl)benzene were all joined together by imine linkages to yield a 2D porous covalent organic framework with unprecedented tth topology, termed COF-346. Unlike the 5 simple existing 2D topologies reported in COFs, COF-346 has 3 kinds of vertices and 2 kinds of edges and is constructed with linkers of 3 kinds of connectivity, and thus represents a higher degree of complexity in COF structures. The success in crystallizing COF-346 was based on precisely chosen geometry and metrics of the linkers and error correction offered by dynamic imine formation. We also report two additional related COFs: a crystalline, porous COF, termed COF-360 with a rare kgd topology, as well as the first crystalline, porous COF with defected tth topology, termed COF-340.

Seven Coordinated Molecular Ruthenium-Water Oxidation Catalysts: A Coordination Chemistry Journey.

Molecular water oxidation catalysis is a field that has experienced an impressive development over the past decade mainly fueled by the promise of generation of a sustainable carbon neutral fuel society, based on water splitting. Most of these advancements have been possible thanks to the detailed understanding of the reactions and intermediates involved in the catalytic cycles. Today's best molecular water oxidation catalysts reach turnover frequencies that are orders of magnitude higher than that of the natural oxygen evolving center in photosystem II. These catalysts are based on Ru complexes where at some stage, the first coordination sphere of the metal center becomes seven coordinated. The key for this achievement is largely based on the use of adaptative ligands that adjust their coordination mode depending on the structural and electronic demands of the metal center at different oxidation states accessed within the catalytic cycle. This Review covers the latest and most significant developments on Ru complexes that behave as powerful water oxidation catalysts and where at some stage the Ru metal attains coordination number 7. Further it provides a comprehensive and rational understanding of the different structural and electronic factors that govern the behavior of these catalysts.

Electrochemically Driven Water Oxidation by a Highly Active Ruthenium-Based Catalyst.

The highly active ruthenium-based water oxidation catalyst [RuX (mcbp)(OHn )(py)2 ] [mcbp2- =2,6-bis(1-methyl-4-(carboxylate)benzimidazol-2-yl)pyridine; n=2, 1, and 0 for X=II, III, and IV, respectively], can be generated in a mixture of RuIII and RuIV states from either [RuII (mcbp)(py)2 ] or [RuIII (Hmcbp)(py)2 ]2+ precursors. The precursor complexes are isolated and characterized by single-crystal X-ray analysis, NMR, UV/Vis, EPR, and FTIR spectroscopy, ESI-HRMS, and elemental analysis, and their redox properties are studied in detail by electrochemical and spectroscopic methods. Unlike the parent catalyst [Ru(tda) (py)2 ] (tda2- =[2,2':6',2''-terpyridine]-6,6''-dicarboxylate), for which full transformation into the catalytically active species [RuIV (tda)(O)(py)2 ] could not be carried out, stoichiometric generation of the catalytically active Ru-aqua complex [RuX (mcbp)(OHn )(py)2 ] from the RuII precursor was achieved under mild conditions (pH 7.0) and short reaction times. The redox properties of the catalyst were studied and its activity for electrocatalytic water oxidation was evaluated, reaching a maximum turnover frequency (TOFmax ) of around 40 000 s-1 at pH 9.0 (from foot-of-the-wave analysis), which is comparable to the activity of the state-of-the-art catalyst [RuIV (tda)(O)(py)2 ].

Bioinspired Metal-Organic Framework Catalysts for Selective Methane Oxidation to Methanol.

Particulate methane monooxygenase (pMMO) is an enzyme that oxidizes methane to methanol with high activity and selectivity. Limited success has been achieved in incorporating biologically relevant ligands for the formation of such active site in a synthetic system. Here, we report the design and synthesis of metal-organic framework (MOF) catalysts inspired by pMMO for selective methane oxidation to methanol. By judicious selection of a framework with appropriate topology and chemical functionality, MOF-808 was used to postsynthetically install ligands bearing imidazole units for subsequent metalation with Cu(I) in the presence of dioxygen. The catalysts show high selectivity for methane oxidation to methanol under isothermal conditions at 150 °C. Combined spectroscopies and density functional theory calculations suggest bis(µ-oxo) dicopper species as probable active site of the catalysts.

Behavior of Ru-bda Water-Oxidation Catalysts in Low Oxidation States.

The Ru complex [RuII (bda-κ-N2 O2 )(N-NH2 )2 ] (1; bda2- =2,2'-bipyridine-6,6'-dicarboxylate, N-NH2 =4-(pyridin-4-yl)aniline) was used as a synthetic intermediate to prepare new RuII and RuIII bda complexes that contain NO+ , MeCN, or H2 O ligands. In acidic solution complex 1 reacts with an excess of NO+ (generated in situ from sodium nitrite) to form a new Ru complex in which the aryl amine ligand N-NH2 is transformed into a diazonium salt [N-N2+ =4-(pyridin-4-yl)benzenediazonium)] together with the formation of a new Ru(NO) moiety in the equatorial zone, to generate [RuII (bda-κ-N2 O)(NO)(N-N2 )2 ]3+ (23+ ). Here the bda2- ligand binds in a κ-N2 O tridentate manner with a dangling carboxylate group. Similarly, complex 1 can also react with a coordinating solvent, such as MeCN, at room temperature to give [RuII (bda-κ-N2 O)(MeCN)(N-NH2 )2 ] (3). In acidic aqueous solutions, a related reaction occurs in which solvent water coordinates to the Ru center to form {[RuII {bda-κ-(NO)3 }(H2 O)(N-NH3 )2 ](H2 O)n }2+ (42+ ) and is strongly hydrogen-bonded with additional water molecules in the second coordination sphere. Furthermore, under acidic conditions the aniline ligands are also protonated to form the corresponding anilinium cationic ligands N-NH3+ . Additionally, the one-electron oxidized complex {[RuIII {bda-κ-(NO)3.5 }(H2 O)(N-NH3 )2 ](H2 O)n }3+ (53+ ) was characterized, in which the fractional value in the κ notation indicates the presence of an additional contact to the pseudo-octahedral geometry of the Ru center. The coordination modes of the complexes were studied in the solid state and in solution through single-crystal XRD, X-ray absorption spectroscopy, variable-temperature NMR spectroscopy, and DFT calculations. While κ-N2 O is the main coordination mode for 23+ and 3, an equilibrium that involves isomers with κ-N2 O and κ-NO2 coordination modes and neighboring hydrogen-bonded water molecules is observed for 42+ and 53+ .

Synthesis, Structure, and Redox Properties of a trans-Diaqua Ru Complex That Reaches Seven-Coordination at High Oxidation States.

In this work we have prepared and characterized two Ru complexes that contain the pentadenatate tda2- ligand (tda2- = [2,2':6',2″-terpyridine]-6,6″-dicarboxylate) that occupies the equatorial positions and two monodentate ligands aqua and/or dmso that occupy the axial positons: [trans-RuIII(tda-κ-N3O)(OH2ax)2]+, 3III(OH2)2+, and [RuII(tda-κ-N3O)(dmso)(OH2ax)], 4II. The latter is a useful synthetic intermediate for the preparation of Ru-tda complexes with different axial ligands. The two complexes have been characterized in the solid state by single-crystal XRD and by elemental analysis. In solution, complex 4II has been characterized by NMR spectroscopy as well as the one-electron reduction of complex 3III(OH2)2+. The electrochemical properties of 3III(OH2)2+ and 4II have been assessed by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Complex 3III(OH2)2+ shows the presence of four redox waves that are assigned to the VI/V, V/IV, IV/III, and III/II redox couples. The variation of the redox potentials is analyzed as a function of pH and is graphically presented as a Pourbaix diagram. Finally, the redox potentials displayed by both 3III(OH2)2+ and 4II are compared to related complexes previously reported in the literature and rationalized on the basis of the electron donating or withdrawing capacity of the auxiliary ligands as well as with regard to their ability to undergo seven-coordination at high oxidation states.

Photoelectrochemical Behavior of a Molecular Ru-Based Water-Oxidation Catalyst Bound to TiO2-Protected Si Photoanodes.

A hybrid photoanode based on a molecular water oxidation precatalyst was prepared from TiO2-protected n- or p+-Si coated with multiwalled carbon nanotubes (CNT) and the ruthenium-based water oxidation precatalyst [RuIV(tda)(py-pyr)2(O)], 1(O) (tda2- is [2,2':6',2″-terpyridine]-6,6″-dicarboxylato and py-pir is 4-(pyren-1-yl)-N-(pyridin-4-ylmethyl)butanamide). The Ru complex was immobilized by π-π stacking onto CNTs that had been deposited by drop casting onto Si electrodes coated with 60 nm of amorphous TiO2 and 20 nm of a layer of sputtered C. At pH = 7 with 3 Sun illumination, the n-Si/TiO2/C/CNT/[1+1(O)] electrodes exhibited current densities of 1 mA cm-2 at 1.07 V vs NHE. The current density was maintained for >200 min at a constant potential while intermittently collecting voltammograms that indicated that over half of the Ru was still in molecular form after O2 evolution.

How to make an efficient and robust molecular catalyst for water oxidation.

Energy has been a central subject for human development from Homo erectus to date. The massive use of fossil fuels during the last 50 years has generated a large CO2 concentration in the atmosphere that has led to the so-called global warming. It is very urgent to come up with C-neutral energy schemes to be able to preserve Planet Earth for future generations to come and still preserve our modern societies' life style. One of the potential solutions is water splitting with sunlight (hν-WS) that is also associated with "artificial photosynthesis", since its working mode consists of light capture followed by water oxidation and proton reduction processes. The hydrogen fuel generated in this way is named as "solar fuel". For this set of reactions, the catalytic oxidation of water to dioxygen is one of the crucial processes that need to be understood and mastered in order to build up potential devices based on hν-WS. This tutorial describes the different important aspects that need to be considered to come up with efficient and oxidatively robust molecular water oxidation catalysts (Mol-WOCs). It is based on our own previous work and completed with essential contributions from other active groups in the field. We mainly aim at describing how the ligands can influence the properties of the Mol-WOCs and showing a few key examples that overall provide a complete view of today's understanding in this field.

A Million Turnover Molecular Anode for Catalytic Water Oxidation.

Molecular ruthenium-based water oxidation catalyst precursors of general formula [Ru(tda)(Li )2 ] (tda2- is [2,2':6',2''-terpyridine]-6,6''-dicarboxylato; L1 =4-(pyren-1-yl)-N-(pyridin-4-ylmethyl)butanamide, 1 b; L2 =4-(pyren-1-yl)pyridine), 1 c), have been prepared and thoroughly characterized. Both complexes contain a pyrene group allowing ready and efficiently anchoring via π interactions on multi-walled carbon nanotubes (MWCNT). These hybrid solid state materials are exceptionally stable molecular water-oxidation anodes capable of carrying out more than a million turnover numbers (TNs) at pH 7 with an Eapp =1.45 V vs. NHE without any sign of degradation. XAS spectroscopy analysis before, during, and after catalysis together with electrochemical techniques allow their unprecedented oxidative ruggedness to be monitored and verified.

Foot of the Wave Analysis for Mechanistic Elucidation and Benchmarking Applications in Molecular Water Oxidation Catalysis.

The description of the foot of the wave analysis (FOWA) applied to the electrocatalytic oxidation of water to dioxygen is reported for cases where the rate determining step is first order and second order with regard to catalyst concentration, coinciding mechanistically with the so-called water nucleophilic attack (WNA) and the interaction of two M-O units (I2M, where M represents the metal center of the catalyst), respectively. The newly adapted equations are applied to a range of relevant molecular catalysts, both in homogeneous and heterogeneous phase, and the kinetic parameters are determined, including apparent rate constants and turnover frequencies. In this respect, the application of FOWA at different catalyst concentrations allows elucidation of the reaction mechanism that operates in each case. In addition, catalytic Tafel plots are used for assessing the performance of several molecular water oxidation catalysts (WOCs) as a function of overpotential under analogous conditions, and thus can be used for benchmarking purposes. This analysis was carried out earlier for oxide-based WOCs; however, this is the first report using molecular WOCs.

Intramolecular Proton Transfer Boosts Water Oxidation Catalyzed by a Ru Complex.

We introduce a new family of complexes with the general formula [Ru(n)(tda)(py)2](m+) (n = 2, m = 0, 1; n = 3, m = 1, 2(+); n = 4, m = 2, 3(2+)), with tda(2-) being [2,2':6',2″-terpyridine]-6,6″-dicarboxylate, including complex [Ru(IV)(OH)(tda-κ-N(3)O)(py)2](+), 4H(+), which we find to be an impressive water oxidation catalyst, formed by hydroxo coordination to 3(2+) under basic conditions. The complexes are synthesized, isolated, and thoroughly characterized by analytical, spectroscopic (UV-vis, nuclear magnetic resonance, electron paramagnetic resonance), computational, and electrochemical techniques (cyclic voltammetry, differential pulse voltammetry, coulometry), including solid-state monocrystal X-ray diffraction analysis. In oxidation state IV, the Ru center is seven-coordinated and diamagnetic, whereas in oxidation state II, the complex has an unbonded dangling carboxylate and is six-coordinated while still diamagnetic. With oxidation state III, the coordination number is halfway between the coordination of oxidation states II and IV. Species generated in situ have also been characterized by spectroscopic, computational, and electrochemical techniques, together with the related species derived from a different degree of protonation and oxidation states. 4H(+) can be generated potentiometrically, or voltammetrically, from 3(2+), and both coexist in solution. While complex 3(2+) is not catalytically active, the catalytic performance of complex 4H(+) is characterized by the foot of the wave analysis, giving an impressive turnover frequency record of 8000 s(-1) at pH 7.0 and 50 000 s(-1) at pH 10.0. Density functional theory calculations provide a complete description of the water oxidation catalytic cycle of 4H(+), manifesting the key functional role of the dangling carboxylate in lowering the activation free energies that lead to O-O bond formation.