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Several metalloenzymes, including [FeFe]-hydrogenase, employ cofactors wherein multiple metal atoms work together with surrounding ligands that mediate heterolytic and concerted proton-electron transfer (CPET) bond activation steps. Herein, we report a new dinucleating PNNP expanded pincer ligand, which can bind two low-valent iron atoms in close proximity to enable metal-metal cooperativity (MMC). In addition, reversible partial dearomatization of the ligand's naphthyridine core enables both heterolytic metal-ligand cooperativity (MLC) and chemical non-innocence through CPET steps. Thermochemical and computational studies show how a change in ligand binding mode can lower the bond dissociation free energy of ligand C(sp3)-H bonds by ∼25 kcal mol−1. H-atom abstraction enabled trapping of an unstable intermediate, which undergoes facile loss of two carbonyl ligands to form an unusual paramagnetic (S = ) complex containing a mixed-valent iron(0)-iron(i) core bound within a partially dearomatized PNNP ligand. Finally, cyclic voltammetry experiments showed that these diiron complexes show catalytic activity for the electrochemical hydrogen evolution reaction. This work presents the first example of a ligand system that enables MMC, heterolytic MLC and chemical non-innocence, thereby providing important insights and opportunities for the development of bimetallic systems that exploit these features to enable new (catalytic) reactivity.

Low‐nuclearity copper hydrides are rare and few well‐defined dicopper hydrides have been reported. Herein, we describe the first example of a structurally characterized anionic dicopper monohydride complex. This complex does not display typical reactivity associated with low‐nuclearity copper hydrides, such as alcoholysis or insertion reactions. Instead, its stoichiometric and catalytic reactivity is akin to that of copper hydride clusters. The distinct reactivity is ascribed to the robust dinuclear core that is bound tightly within the dinucleating ligand scaffold.

We present the synthesis and coordination chemistry of a bulky, tripodal N,N,O ligand, ImPh2NNO tBu (L), designed to model the 2‐His‐1‐carboxylate facial triad (2H1C) by means of two imidazole groups and an anionic 2,4‐di‐tert‐butyl‐subtituted phenolate. Reacting K‐L with MCl2 (M = Fe, Zn) affords the isostructural, tetrahedral non‐heme complexes [Fe(L)(Cl)] (1) and [Zn(L)(Cl)] (2) in high yield. The tridentate N,N,O ligand coordination observed in their X‐ray crystal structures remains intact and well‐defined in MeCN and CH2Cl2 solution. Reacting 2 with NaSPh affords a tetrahedral zinc thiolate complex, [Zn(L)(SPh)] (4), that is relevant to isopenicillin N synthase (IPNS) biomimicry. Cyclic voltammetry studies demonstrate the ligand's redox non‐innocence, where phenolate oxidation is the first electrochemical response observed in K‐L, 2 and 4. However, the first electrochemical oxidation in 1 is iron‐centred, the assignment of which is supported by DFT calculations. Overall, ImPh2NNO tBu provides access to well‐defined mononuclear, monoligated, N,N,O‐bound metal complexes, enabling more accurate structural modelling of the 2H1C to be achieved., How N,N,O can you go? A new bulky, tripodal N,N,O phenolate ligand provides access to well‐defined mononuclear, monoligated non‐heme iron and zinc complexes that accurately model the 2‐His‐1‐carboxylate facial triad. These complexes are ideal synthons from which to create a range of biomimetic N,N,O‐bound complexes featuring biorelevant cofactors.

Herein, we report the use of structured hydroxyapatite composite (SHCs) as highly efficient and recyclable solid base catalysts for various condensation reactions. Catalyst performance as function of catalyst loading, reaction time and reaction temperature were studied in the solventless self-aldol condensation reaction of butyraldehyde to 2-ethylhexenal under mild reaction conditions. SHC catalysts were found to outperform benchmark solid base catalysts such as MgO, TiO2, calcium carbonate and hydroxyapatites. Characterization of the synthesized SHC catalysts by a range of surface analysis, spectroscopic and electron microscopy techniques, showed that a moderate acid/base ratio and high BET surface area to be key to their high efficiency. Furthermore, recycling experiments showed the catalyst to be stable over multiple runs. Moreover, the most active SHC catalyst was investigated in other prototypical condensation reactions such as the Knoevenagel condensation, Claisen-Schmidt condensation and Henry reaction, again showing excellent performance. These results highlight the versatility of these SHC materials and their potential for industrial employment as solid base catalysts.

Volatile fatty acids (VFA) produced by fermentation of organic-rich wastewater streams can, after efficient recovery from the dilute fermentation broth, serve as a circular source of carbon and be catalytically upgraded into various valuable platform molecules. Waste-derived VFA, that is, a mixture of acetic, propionic, and butyric acids, can thus be converted into mixed ketones, which in turn are valuable intermediates for light aromatics synthesis. Here, an integrated process is presented for the recovery and in-line catalytic conversion of VFA extracted from a fermentation broth by adsorption on a nonfunctionalized resin adsorbent. Gas-phase ketonization of the VFA was studied with and without co-fed water, which is inevitably coextracted from the broth, over TiO2 anatase catalysts to assess catalyst performance, including stability as a function of time on stream. While VFA conversion over bare TiO2 at 375 °C proceeded at 90% conversion with 100% selectivity to ketones, the presence of water in the feed resulted in an activity drop to 40%. Catalyst stability toward water could be greatly improved by dispersing the titania on a hydrophobic carbon support. The carbon-supported catalyst showed superior performance in the presence of excess water, providing a quantitative yield toward ketones at 400 °C. The approach thus allows coupling of VFA recovery from a fermentation broth with successful catalytic upgrading to mixed ketones, thus providing a novel route for the production of value-added products from waste streams.

Surface‐mounted metal–organic frameworks (SURMOFs) show promising behavior for a manifold of applications. As MOF thin films are often unsuitable for conventional characterization techniques, understanding their advantageous properties over their bulk counterparts presents a great analytical challenge. In this work, we demonstrate that MOFs can be grown on calcium fluoride (CaF2) windows after proper functionalization. As CaF2 is optically (in the IR and UV/Vis range of the spectrum) transparent, this makes it possible to study SURMOFs using conventional spectroscopic tools typically used during catalysis or gas sorption. Hence, we have measured HKUST‐1 during the adsorption of CO and NO. We show that no copper oxide impurities are observed and also confirm that SURMOFs grown by a layer‐by‐layer (LbL) approach possess Cu+ species in paddlewheel confirmation, but 1.9 times less than in bulk HKUST‐1. The developed methodology paves the way for studying the interaction of any adsorbed gases with thin films, not limited to MOFs, low temperatures, or these specific probe molecules, pushing the boundaries of our current understanding of functional porous materials., Calcium fluoride windows were functionalized to facilitate the growth of surface‐mounted metal–organic frameworks (SURMOF). The resulting HKUST‐1 SURMOF was measured by in situ IR spectroscopy during CO and NO adsorption. This work showcases the practicality for utilizing such calcium fluoride windows for in‐depth in situ studies of various functional porous materials.

Iron-containing metalloenzymes that contain the 2-His-1-Carboxylate facial triad at their active site are well known for their ability to activate molecular oxygen and catalyse a broad range of oxidative transformations. Many of these reactions are synthetically challenging, and developing small molecular iron-based catalysts that can achieve similar reactivity and selectivity remains a long-standing goal in homogeneous catalysis. This review focuses on the development of bioinspired facial N,N,O ligands that model the 2-His-1-Carboxylate facial triad to a greater degree of structural accuracy than many of the polydentate N-donor ligands commonly used in this field. By developing robust, well-defined N,N,O facial ligands, an increased understanding could be gained of the factors governing enzymatic reactivity and selectivity.

The modes of deactivation -and the extent to which their properties can be restored- of two catalyst bodies used in cascade for bio-oil synthesis have been studied. These catalysts include a solid acid granulate (namely ZrO2/desilicated zeolite ZSM-5/attapulgite clay) employed in ex-situ catalytic fast pyrolysis of biomass, and a base extrudate (K-exchanged zeolite USY/attapulgite clay) for the subsequent bio-oil upgrading. Post-mortem analyses of both catalyst bodies with Raman spectroscopy and confocal fluorescence microscopy revealed the presence of highly poly-aromatic coke distributed in an egg-shell manner. Deactivation due to coke adsorption onto acid sites affected the zeolite ZSM-5-based catalyst, while for the base catalyst it is structural integrity loss, resulting from KOH-mediated zeolite framework collapse, the main deactivating factor. A hydrothermal regeneration process reversed the detrimental effects of coke in the acid catalyst, largely recovering catalyst acidity (∼80%) and textural properties (∼90%), but worsened the structural damage suffered by the base catalyst.

The maritime sector accounts for almost 3% of global greenhouse gas emissions and is under increasing pressure to decarbonise rapidly. Renewable fuels represent a promising pathway for decarbonisation, but their high costs hinder adoption. Carbon Capture and Storage (CCS) can further augment marine fuel decarbonisation but adds to the cost. This work presents a harmonised cost comparison of four promising renewable carbon fuels (methanol, dimethyl ether (DME), liquefied natural gas (LNG) and bio-oil) produced either from routes utilising biomass (biofuels, including CCS) or CO2 (e-fuels). The differing technology status of the fuel production routes has been accounted for using the RAND Corporation method to estimate the cost of pioneer plants. Additionally, the impact of different levels of carbon taxation (15 or 140 €/t CO2) on the economic viability of the alternative fuels has been examined. None of the renewable fuels were found to be close to the incumbent fuels without carbon taxation, which needs to be considerable to adequately bridge the cost gap. Methanol and DME produced using point CO2 capture are the lowest cost choices if full scale sale of the oxygen by-product is considered. The biofuel routes remain at a premium to the existing fuels, while the direct air capture (DAC)-based fuels are the most expensive among the options studied, besides requiring completely renewable electricity for their carbon footprint to not exceed that of fossil fuels. Renewable LNG has a particularly high cost gap, bringing its status as a potential bridging fuel into doubt.

The direct enantioselective hydroxylation of benzylic C–H bonds to form chiral benzylic alcohols represents a challenging transformation. Herein, we report on the exploration of new biologically inspired manganese and iron complexes bearing highly electron-rich aminopyridine ligands containing 4-pyrrolidinopyridine moieties ((S,S)-1, (R,R)-1, 2 and 5) in combination with chiral bis-pyrrolidine and N,N-cyclohexanediamine backbones in enantioselective oxidation catalysis with aqueous H2O2. The current manganese complexes outperform the analogous manganese complexes containing 4-dimethylaminopyridine moieties (3 and 4) in benzylic oxidation reactions in terms of alcohol yield while keeping similar ee values (~ 60% ee), which is attributed to the higher basicity of the 4-pyrrolidinopyridine group. A detailed investigation of different carboxylic acid additives in enantioselective benzylic oxidation provides new insights into how to rationally enhance enantioselectivities by means of proper tuning of the environment around the catalytic active site, and has resulted in the selection of Boc-L-tert-leucine as the preferred additive. Using these optimized conditions, manganese complex 2 was shown to be effective in the enantioselective benzylic oxidation of a series of arylalkane substrates with up to 50% alcohol yield and 62% product ee. A final set of experiments also highlights the use of the new 4-pyrrolidinopyridine-based complexes in the asymmetric epoxidation of olefins (up to 98% epoxide yield and >99% ee).

Nature provides much inspiration for the design of multistep conversion processes, with numerous reactions running simultaneously and without interference in cells, for example. A key challenge in mimicking nature's strategies is to compartmentalize incompatible reagents and catalysts, for example, for tandem catalysis. Here, we present a new strategy for antagonistic catalyst compartmentalization. The synthesis of bifunctional Janus catalyst particles carrying acid and base groups on the particle's opposite patches is reported as is their application as acid-base catalysts in oil/water emulsions. The synthesis strategy involved the use of monodisperse, hydrophobic and amine-functionalized silica particles (SiO2 -NH2 -OSi(CH3 )3 ) to prepare an oil-in-water Pickering emulsion (PE) with molten paraffin wax. After solidification, the exposed patch of the silica particles was selectively etched and refunctionalized with acid groups to yield acid-base Janus particles (Janus A-B). These materials were successfully applied in biphasic Pickering interfacial catalysis for the tandem dehydration-Knoevenagel condensation of fructose to 5-(hydroxymethyl)furfural-2-diethylmalonate (5-HMF-DEM) in a water/4-propylguaiacol PE. The results demonstrate the advantage of rapid extraction of 5-hydroxymethylfurfural (5-HMF), a prominent platform molecule prone to side product formation in acidic media. A simple strategy to tune the acid/base balance using PE with both Janus A-B and monofunctional SiO2 -NH2 -OSi(CH3 )3 base catalysts proved effective for antagonistic tandem catalysis.

Invited for this month's cover is the group of Bert M. Weckhuysen at the Utrecht University (The Netherlands). The cover picture shows an attenuated Total Reflection Infrared spectroscopy (ATR-IR) probe monitoring a chemical reaction in the liquid phase, particularly water, at demanding conditions. The light penetrates few microns deep in proximity of the ATR-IR probe and thus any IR-active entity in that range is probed. This research systematically addresses the challenges associated with acquiring operando ATR-IR spectra for liquid/aqueous phase reactions, which is essential for better understanding of chemical reactions in the liquid phase. Read the full text of their Full Paper at 10.1002/cmtd.202100041.

Activation of a Si–H bond is commonly a critical step in catalytic hydrosilylation reactions. Herein, we investigate the cooperative reactivity of Ni(0) centers bearing a side-bound imine ligand toward silanes. Such complexes activate a Si–H bond of diphenylsilane, resulting in formal hydrosilylation of the imine backbone, which acts as a hydride acceptor. The resulting hydrosilazane motif engages either in coordination to nickel via the Si–H bond, forming an 18-electron η2-Si–H complex, or oxidative addition to Ni to form 16-electron Ni(II) silyl/hydride complexes. DFT calculations suggest a cooperative activation of the silane via ligand-to-ligand hydride transfer. In addition, the silicon fragment readily exchanges with external hydrosilanes, showing that the Si–N bond can be reversibly cleaved under mild conditions.

We report the synthesis and characterization of a series of cationic, neutral and anionic dicopper(I) complexes featuring a µ-mesityl ligand and a naphthyridine-derived PNNP expanded pincer ligand. Structural characterization showed that the protonation state of the dinucleating ligand has a pronounced effect on the bending and tilting of the µ-mesityl ligand. DFT calculations indicate that the varying orientations of the µ-mesityl ligand are inherent due to changes in electronic structure rather than crystal packing effects. NBO analysis reveals how the interactions that contribute to the 3-center 2-electron bond between the µ-mesityl ligand and the dicopper core change for the various degrees of observed bending and tilting.

The transition towards a circular economy requires closing the carbon loop, e.g. by the development of new synthesis routes to valuable intermediates and products from organic-rich waste streams. Volatile fatty acids (VFA) can be fermentatively produced from wastewater and serve as circular platform chemicals. We show that these VFA can be catalytically upgraded to light aromatics (i.e., benzene, toluene, ethylbenzene and xylenes, BTEX) via a tandem catalytic reaction involving TiO2-catalyzed ketonization and zeolite ZSM-5 catalyzed aromatization. Including this intermediate ketonization step is demonstrated to be much more efficient than direct VFA aromatization, as direct acid conversion mainly gave rise to short-chain olefins by decarboxylation and low BTEX yields of 1%. A one-reactor, tandem catalytic conversion instead significantly improved the yield to 45% when zeolite Ga/ZSM-5 was used. Furthermore, the effect of VFA-derived ketone composition, a process parameter set by the fermentation process, on aromatics production efficiency and product distribution was found to be very pronounced for zeolite Ga/ZSM-5, but not for non-promoted zeolite HZSM-5. This suggests a different reaction mechanism to dominate on zeolite Ga/ZSM-5, involving dehydration on the Brønsted acid sites and cyclization/aromatization on the Ga sites. Finally, water, expected to be present in the feed during VFA upgrading, caused the activity of zeolite Ga/ZSM-5 to drop reversibly, but also led to lower coke buildup. Analysis of the spent catalyst with solid-state 27Al nuclear magnetic resonance spectroscopy and temperature-programmed reduction with H2 showed that the catalyst structure remained intact, also with water present in the feed. Together, the results demonstrate that catalytic ketonization/aromatization is an attractive circular approach for converting waste-derived carboxylic acids into renewable aromatics.

Production of volatile fatty acids (VFAs) by fermentation is a potential sustainable alternative for conventional petrochemical routes to VFAs. Due to the low VFA content of fermentation broths, robust and economical separation technology has to be devised to recover the VFA. Liquid-liquid extraction of VFAs with the phosphonium phosphinate ionic liquid (IL) [P 666,14 ][Phos] allows good VFA extractability. For an extraction process using [P 666,14 ][Phos] to be green, it is essential to efficiently regenerate the solvent and recover the VFA. To obtain insight into the (strong) intermolecular interactions between [P 666,14 ][Phos] and acetic acid, selected as a model VFA, 1 H NMR, 31 P NMR, FT-IR and isothermal titration calorimetry (ITC) were applied. The observations were used to interpret operations to recover acetic acid from the IL, which included evaporation at elevated temperature under vacuum, possibly assisted by nitrogen stripping, in situ esterification and back-extraction with volatile bases. Through evaporative regeneration with nitrogen stripping, HAc could be removed, but only down to an HAc/IL molar ratio of 1. The remaining molar equivalent of HAc-IL interacts tightly with the IL by partial proton transfer and strong hydrogen bonding interactions with the phosphinate anion. Back-extraction of HAc with trimethylamine (TMA) and subsequent decomposition of the HAc-TMA complexes allowed for successful IL regeneration. This process uses ten times less amine (TMA) than conventional amine-based extraction processes (e.g. tri-n-octyl amine), and provides a sustainable process route to obtain pure carboxylic acids from highly diluted aqueous solutions without generating large streams of byproducts. Further valorization via in-line vaporization/catalytic ketonization or via in-line thermal decomposition and ketonization of the TMA-HAc salt was also demonstrated, showing the potential of the VFAs as a green platform for bio-based chemicals.

Background The aromatic compounds vanillin and vanillic acid are important fragrances used in the food, beverage, cosmetic and pharmaceutical industries. Currently, most aromatic compounds used in products are chemically synthesized, while only a small percentage is extracted from natural sources. The metabolism of vanillin and vanillic acid has been studied for decades in microorganisms and many studies have been conducted that showed that both can be produced from ferulic acid using bacteria. In contrast, the degradation of vanillin and vanillic acid by fungi is poorly studied and no genes involved in this metabolic pathway have been identified. In this study, we aimed to clarify this metabolic pathway in Aspergillus niger and identify the genes involved. Results Using whole-genome transcriptome data, four genes involved in vanillin and vanillic acid metabolism were identified. These include vanillin dehydrogenase (vdhA), vanillic acid hydroxylase (vhyA), and two genes encoding novel enzymes, which function as methoxyhydroquinone 1,2-dioxygenase (mhdA) and 4-oxo-monomethyl adipate esterase (omeA). Deletion of these genes in A. niger confirmed their role in aromatic metabolism and the enzymatic activities of these enzymes were verified. In addition, we demonstrated that mhdA and vhyA deletion mutants can be used as fungal cell factories for the accumulation of vanillic acid and methoxyhydroquinone from guaiacyl lignin units and related aromatic compounds. Conclusions This study provides new insights into the fungal aromatic metabolic pathways involved in the degradation of guaiacyl units and related aromatic compounds. The identification of the involved genes unlocks new potential for engineering aromatic compound-producing fungal cell factories.

The oxidation of aromatic substrates to phenols with H2O2 as a benign oxidant remains an ongoing challenge in synthetic chemistry. Herein, we successfully achieved to catalyze aromatic C−H bond oxidations using a series of biologically inspired manganese catalysts in fluorinated alcohol solvents. While introduction of bulky substituents into the ligand structure of the catalyst favors aromatic C−H oxidations in alkylbenzenes, oxidation occurs at the benzylic position with ligands bearing electron‐rich substituents. Therefore, the nature of the ligand is key in controlling the chemoselectivity of these Mn‐catalyzed C−H oxidations. We show that introduction of bulky groups into the ligand prevents catalyst inhibition through phenolate‐binding, consequently providing higher catalytic turnover numbers for phenol formation. Furthermore, employing halogenated carboxylic acids in the presence of bulky catalysts provides enhanced catalytic activities, which can be attributed to their low pKa values that reduces catalyst inhibition by phenolate protonation as well as to their electron‐withdrawing character that makes the manganese oxo species a more electrophilic oxidant. Moreover, to the best of our knowledge, the new system can accomplish the oxidation of alkylbenzenes with the highest yields so far reported for homogeneous arene hydroxylation catalysts. Overall our data provide a proof‐of‐concept of how Mn(II)/H2O2/RCO2H oxidation systems are easily tunable by means of the solvent, carboxylic acid additive, and steric demand of the ligand. The chemo‐ and site‐selectivity patterns of the current system, a negligible KIE, the observation of an NIH‐shift, and the effectiveness of using tBuOOH as oxidant overall suggest that hydroxylation of aromatic C−H bonds proceeds through a metal‐based mechanism, with no significant involvement of hydroxyl radicals, and via an arene oxide intermediate The European Commission is acknowledged for financial support through the NoNoMeCat project (675020- MSCA-ITN-2015-ETN). We also thank Utrecht University. Support by the Spanish Ministry of Science (PGC2018-101737- B-I00 to M.C. and PhD grant to M.B. BES-2016-076349), and Generalitat de Catalunya (ICREA Academia Award to M.C. and 2017 SGR 00264) is acknowledged