Twinned versus linked organometallics - bimetallic "half-baguette" pentalenide complexes of Rh(I)

The application of Mg[Ph4Pn] and Li·K[Ph4Pn] in transmetalation reactions to a range of Rh(I) precursors led to the formation of "half-baguette" anti-[RhI(L)n]2[μ:η5:η5Ph4Pn] (L = 1,5-cyclooctadiene, norbornadiene, ethylene; n = 1, 2) and syn-[RhI(CO)2]2[μ:η5:η5Ph4Pn] complexes as well as the related iridium complex anti-[IrI(COD)]2[μ:η5:η5Ph4Pn]. With CO exclusive syn metalation was obtained even when using mono-nuclear Rh(I) precursors, indicating an electronic preference for syn metalation. DFT analysis showed this to be the result of π overlap between the adjacent M(CO)2 units which overcompensates for dz2 repulsion of the metals, an effect which can be overridden by steric clash of the auxiliary ligands to yield anti-configuration as seen in the larger olefin complexes. syn-[RhI(CO)2]2[μ:η5:η5Ph4Pn] is a rare example of a twinned organometallic where the two metals are held flexibly in close proximity, but the two d8 Rh(I) centres did not show signs of M-M bonding interactions or exhibit Lewis basic behaviour as in some related mono-nuclear Cp complexes due to the acceptor properties of the ligands. The ligand substitution chemistry of syn-[RhI(CO)2]2[μ:η5:η5Ph4Pn] was investigated with a series of electronically and sterically diverse donor ligands (P(OPh)3, P(OMe)3, PPh3, PMe3, dppe) yielding new mono- and bis-substituted complexes, with E-syn-[RhI(CO)(P{OR})3]2[μ:η5:η5Ph4Pn] (R = Me, Ph) characterised by XRD.

Water co-catalysis in aerobic olefin epoxidation mediated by ruthenium oxo complexes

We report the development of a versatile Ru-porphyrin catalyst system which performs the aerobic epoxidation of aromatic and aliphatic (internal) alkenes under mild conditions, with product yields of up to 95% and turnover numbers (TON) up to 300. Water is shown to play a crucial role in the reaction, significantly increasing catalyst efficiency and substrate scope. Detailed mechanistic investigations employing both computational studies and a range of experimental techniques revealed that water activates the RuVI di-oxo complex for alkene epoxidation via hydrogen bonding, stabilises the RuIV mono-oxo intermediate, and is involved in the regeneration of the RuVI di-oxo complex leading to oxygen atom exchange. Distinct kinetics are obtained in the presence of water, and side reactions involved in catalyst deactivation have been identified.

Synthesis, Structure, and Reactivity of Magnesium Pentalenides

The first magnesium pentalenide complexes have been synthesized via deprotonative metalation of 1,3,4,6-tetraphenyldihydropentalene (Ph4PnH2) with magnesium alkyls. Both the nature of the metalating agent and the reaction solvent influenced the structure of the resulting complexes, and an equilibrium between Mg[Ph4Pn] and [nBuMg]2[Ph4Pn] was found to exist and investigated by NMR, XRD, and UV-vis spectroscopic techniques. Studies on the reactivity of Mg[Ph4Pn] with water, methyl iodide, and trimethylsilylchloride revealed that the [Ph4Pn]2- unit undergoes electrophilic addition at 1,5-positions instead of 1,4-positions known for the unsubstituted pentalenide, Pn2-, highlighting the electronic influence of the four aryl substituents on the pentalenide core. The ratio of syn/anti addition was found to be dependent on the size of the incoming electrophile, with methylation yielding a 60:40 mixture, while silylation yielded exclusively the anti-isomer.

Paramagnetic Relaxation Agents for Enhancing Temporal Resolution and Sensitivity in Multinuclear FlowNMR Spectroscopy

Sensitivity in FlowNMR spectroscopy for reaction monitoring often suffers from low levels of pre-magnetisation due to limited residence times of the sample in the magnetic field. While this in-flow effect is tolerable for high sensitivity nuclei such as 1 H and 19 F, it significantly reduces the signal-to-noise ratio in 31 P and 13 C spectra, making FlowNMR impractical for low sensititvity nuclei at low concentrations. Paramagnetic relaxation agents (PRAs), which enhance polarisation and spin-lattice relaxation, could eliminate the adverse in-flow effect and improve the signal-to-noise ratio. Herein, [Co(acac)3 ], [Mn(acac)3 ], [Fe(acac)3 ], [Cr(acac)3 ], [Ni(acac)2 ]3, [Gd(tmhd)3 ] and [Cr(tmhd)3 ] are investigated for their effectiveness in improving signal intensity per unit time in FlowNMR applications under the additional constraint of chemical inertness towards catalytically active transition metal complexes. High-spin Cr(III) acetylacetonates emerged as the most effective compounds, successfully reducing 31 P T1 values four- to five-fold at PRA concentrations as low as 10 mM without causing adverse line broadening. Whereas [Cr(acac)3 ] showed signs of chemical reactivity with a mixture of triphenylphosphine, triphenylphosphine oxide and triphenylphosphate over the course of several hours at 80° C, the bulkier [Cr(tmhd)3 ] was stable and equally effective as a PRA under these conditions. Compatibility with a range of representative transition metal complexes often used in homogeneous catalysis has been investigated, and application of [Cr(tmhd)3 ] in significantly improving 1 H and 31 P{1 H} FlowNMR data quality in a Rh-catalysed hydroformylation reaction has been demonstrated. With the PRA added, 13 C relaxation times were reduced more than six-fold, allowing quantitative reaction monitoring of substrate consumption and product formation by 13 C{1 H} FlowNMR spectroscopy at natural abundance.

Connect Four: Tetraarylated Dihydropentalenes and Triarylated Monocyclic Pentafulvenes from Cyclopentadienes and Enones

In search of novel pentalenide ligands for use in organometallic chemistry and homogeneous catalysis, we report the scope of a straightforward base-promoted Michael annulation of cyclopentadienes with α,β-unsaturated ketones that allows the introduction of symmetrical as well as unsymmetrical aryl and alkyl substitution patterns including electron-donating as well as electron-withdrawing substituents. More than 16 examples of various isomers of 1,3,4,6-tetraarylated dihydropentalenes have been synthesized in isolated yields of up to 78%, representing a substantial expansion of the range of dihydropentalene scaffolds known to date. Double bond isomerization between the two pentacyclic rings in 1,2-dihydropentalenes with electronically different substituents occurred depending on the polarization of the molecule. The melting points of the air-stable dihydropentalenes decrease, and their solubilities in organic solvents improve with increasing substitution and decreasing symmetry of the molecule. A competitive pseudo-retro-aldol pathway produces 1,3,6-triarylated monocyclic pentafulvenes as side products in yields of 9-68%, which can be cleanly isolated (8 new examples) and used for other synthetic purposes, including separate cyclization to other dihydropentalenes.

Sustainable Synthesis of Dimethyl- and Diethyl Carbonate from CO2 in Batch and Continuous Flow-Lessons from Thermodynamics and the Importance of Catalyst Stability

Equilibrium conversions for the direct condensation of MeOH and EtOH with CO₂ to give dimethyl- and diethyl carbonate, respectively, have been calculated over a range of experimentally relevant conditions. The validity of these calculations has been verified in both batch and continuous flow experiments over a heterogeneous CeO₂ catalyst. Operating under optimized conditions of 140 °C and 200 bar CO₂, record productivities of 235 mmol/L·h DMC and 241 mmol/L·h DEC have been achieved using neat alcohol dissolved in a continuous flow of supercritical CO₂. Using our thermodynamic model, we show that to achieve maximum product yield, both dialkyl carbonates and water should be continuously removed from the reactor instead of the conventionally used strategy of removing water alone, which is much less efficient. Catalyst stability rather than activity emerges as the prime limiting factor and should thus become the focus of future catalyst development.

Mapping Catalyst Activation, Turnover Speciation and Deactivation in Rh/PPh3-catalysed Olefin Hydroformylation

We report new insights into the fate of the precious metal during hydroformylation catalysis of 1-hexene with Rh/PPh3 complexes using multi-nuclear operando FlowNMR spectroscopy. By applying selectively excited 1H and...

Fast and Accurate Diffusion NMR Acquisition in Continuous Flow

FlowNMR spectroscopy has become a popular and powerful technique for online reaction monitoring. DOSY NMR is an established technique for obtaining information about diffusion rates and molecular size on static...

Convenient and accurate insight into solution-phase equilibria from FlowNMR titrations

Solution phase titrations are made easy by multi-nuclear FlowNMR spectroscopy with automated, continuous titre addition to give accurate insights into Brønsted acid/base, hydrogen bonding, Lewis acid/base and metal/ligand binding equilibria under native conditions.

Does the Configuration at the Metal Matter in Noyori-Ikariya Type Asymmetric Transfer Hydrogenation Catalysts?

Noyori–Ikariya type [(arene)RuCl(TsDPEN)] (TsDPEN, sulfonated diphenyl ethylenediamine) complexes are widely used C=O and C=N reduction catalysts that produce chiral alcohols and amines via a key ruthenium–hydride intermediate that determines the stereochemistry of the product. Whereas many details about the interactions of the pro-chiral substrate with the hydride complex and the nature of the hydrogen transfer from the latter to the former have been investigated over the past 25 years, the role of the stereochemical configuration at the stereogenic ruthenium center in the catalysis has not been elucidated so far. Using operando FlowNMR spectroscopy and nuclear Overhauser effect spectroscopy, we show the existence of two diastereomeric hydride complexes under reaction conditions, assign their absolute configurations in solution, and monitor their interconversion during transfer hydrogenation catalysis. Configurational analysis and multifunctional density functional theory (DFT) calculations show the λ-(R,R)S^Ru configured [(mesitylene)RuH(TsDPEN)] complex to be both thermodynamically and kinetically favored over its λ-(R,R)R^Ru isomer with the opposite configuration at the metal. Computational analysis of both diastereomeric catalytic manifolds show the major λ-(R,R)S^Ru configured [(mesitylene)RuH(TsDPEN)] complex to dominate asymmetric ketone reduction catalysis with the minor λ-(R,R)R^Ru [(mesitylene)RuH(TsDPEN)] stereoisomer being both less active and less enantioselective. These findings also hold true for a tethered catalyst derivative with a propyl linker between the arene and TsDPEN ligands and thus show enantioselective transfer hydrogenation catalysis with Noyori–Ikariya complexes to proceed via a lock-and-key mechanism.

Multi-nuclear, high-pressure, operando FlowNMR spectroscopic study of Rh/PPh3 - catalysed hydroformylation of 1-hexene

The hydroformylation of 1-hexene with 12 bar of 1 : 1 H2/CO in the presence of the catalytic system [Rh(acac)(CO)2]/PPh3 was successfully studied by real-time multinuclear high-resolution FlowNMR spectroscopy at 50 °C. Quantitative reaction progress curves that yield rates as well as chemo- and regioselectivities have been obtained with varying P/Rh loadings. Dissolved H2 can be monitored in solution to ensure true operando conditions without gas limitation. 31P{1H} and selective excitation 1H pulse sequences have been periodically interleaved with 1H FlowNMR measurements to detect Rh-phosphine intermediates during the catalysis. Stopped-flow experiments in combination with diffusion measurements and 2D heteronuclear correlation experiments showed the known tris-phosphine complex [RhH(CO)(PPh3)3] to generate rapidly exchanging isomers of the bis-phosphine complex [Rh(CO)2(PPh3)2] under CO pressure that directly enter the catalytic cycle. A new mono-phosphine acyl complex has been identified as an in-cycle reaction intermediate.

Engineering aspects of FlowNMR spectroscopy setups for online analysis of solution-phase processes

In this article we review some fundamental engineering concepts and evaluate components and materials required to assemble and operate safe and effective FlowNMR setups that reliably generate meaningful results.

Selective Catalytic Synthesis of 1,2- and 8,9-Cyclic Limonene Carbonates as Versatile Building Blocks for Novel Hydroxyurethanes

The selective catalytic synthesis of limonene-derived monofunctional cyclic carbonates and their subsequent functionalisation via thiol-ene addition and amine ring-opening is reported. A phosphotungstate polyoxometalate catalyst used for limonene epoxidation in the 1,2-position is shown to also be active in cyclic carbonate synthesis, allowing a two-step, one-pot synthesis without intermittent epoxide isolation. When used in conjunction with a classical halide catalyst, the polyoxometalate increased the rate of carbonation in a synergistic double-activation of both substrates. The cis isomer is shown to be responsible for incomplete conversion and by-product formation in commercial mixtures of 1,2-limomene oxide. Carbonation of 8,9-limonene epoxide furnished the 8,9-limonene carbonate for the first time. Both cyclic carbonates underwent thiol-ene addition reactions to yield linked di-monocarbonates, which can be used in linear non-isocyanate polyurethanes synthesis, as shown by their facile ring-opening with N-hexylamine. Thus, the selective catalytic route to monofunctional limonene carbonates gives straightforward access to monomers for novel bio-based polymers.

Online tracing of molecular weight evolution during radical polymerization via high-resolution FlowNMR spectroscopy

High-resolution FlowNMR was coupled to a continuous flow reactor to monitor polymer molecular weight evolution online by diffusion ordered NMR spectroscopy.

Graphite-protected CsPbBr3 perovskite photoanodes functionalised with water oxidation catalyst for oxygen evolution in water

Metal-halide perovskites have been widely investigated in the photovoltaic sector due to their promising optoelectronic properties and inexpensive fabrication techniques based on solution processing. Here we report the development of inorganic CsPbBr3-based photoanodes for direct photoelectrochemical oxygen evolution from aqueous electrolytes. We use a commercial thermal graphite sheet and a mesoporous carbon scaffold to encapsulate CsPbBr3 as an inexpensive and efficient protection strategy. We achieve a record stability of 30 h in aqueous electrolyte under constant simulated solar illumination, with currents above 2 mA cm-2 at 1.23 VRHE. We further demonstrate the versatility of our approach by grafting a molecular Ir-based water oxidation catalyst on the electrolyte-facing surface of the sealing graphite sheet, which cathodically shifts the onset potential of the composite photoanode due to accelerated charge transfer. These results suggest an efficient route to develop stable halide perovskite based electrodes for photoelectrochemical solar fuel generation.

Evidence for tetranuclear bis-μ-oxo cubane species in molecular iridium-based water oxidation catalysts from XAS analysis

Latest EXAFS results suggest a ‘dimer-of-dimers’ as the dominant resting state of Ir-pyalk WOCs in aqueous solution.

Insight into catalyst speciation and hydrogen co-evolution during enantioselective formic acid-driven transfer hydrogenation with bifunctional ruthenium complexes from multi-technique operando reaction monitoring

Operando spectroscopy shows a transition from dehydrogenation to hydrogen transfer during the reaction, and allows measuring optimal conditions for maximum rate and efficiency.

Synthesis of organometallic pentalenide complexes

While a number of reports have established the unique structures and electronic properties of mono- and dinuclear pentalenide complexes of s, p, d and f block elements, access to these intriguing compounds is restricted by synthetic challenges. Here we review various strategies for the synthesis, functionalisation and (trans)metalation of pentalenide complexes from a practical point of view, pointing out promising avenues for future research that may allow wider access to novel pentalenide complexes for application in many different areas.

Electrochemical and Kinetic Insights into Molecular Water Oxidation Catalysts Derived from Cp*Ir(pyridine-alkoxide) Complexes

We report the solution-phase electrochemistry of seven half-sandwich iridium(III) complexes with varying pyridine-alkoxide ligands to quantify electronic ligand effects that translate to their activity in catalytic water oxidation. Our results unify some previously reported electrochemical data of Cp*Ir complexes by showing how the solution speciation determines the electrochemical response: cationic complexes show over 1 V higher redox potentials that their neutral forms in a distinct demonstration of charge accumulation effects relevant to water oxidation. Building on previous work that analysed the activation behaviour of our pyalk-ligated Cp*Ir complexes 1-7, we assess their catalytic oxygen evolution activity with sodium periodate (NaIO4) and ceric ammonium nitrate (CAN) in water and aqueous tBuOH solution. Mechanistic studies including H/D kinetic isotope effects and reaction progress kinetic analysis (RPKA) of oxygen evolution point to a dimer-monomer equilibrium of the IrIV resting state preceding a proton-coupled electron transfer (PCET) in the turnover-limiting step (TLS). Finally, true electrochemically driven water oxidation is demonstrated for all catalysts, revealing surprising trends in activity that do not correlate with those obtained using chemical oxidants.

Online monitoring of a photocatalytic reaction by real-time high resolution FlowNMR spectroscopy

We demonstrate how FlowNMR spectroscopy can readily be applied to investigate photochemical reactions that require sustained input of light and air to yield mechanistic insight under realistic conditions. The Eosin Y mediated photo-oxidation of N-allylbenzylamine is shown to produce imines as primary reaction products from which undesired aldehydes form after longer reaction times. Facile variation of reaction conditions during the reaction in flow allows for probe experiments that give information about the mode of action of the photocatalyst.

Mixed-linker approach in designing porous zirconium-based metal–organic frameworks with high hydrogen storage capacity

New zirconium based metal–organic framework (UBMOF-31) synthesised using mixed-linker strategy showing permanent porosity, excellent hydrogen uptake, and high selectivity for adsorption of CO₂ over N₂.

Practical aspects of real-time reaction monitoring using multi-nuclear high resolution FlowNMR spectroscopy

FlowNMR spectroscopy is an excellent technique for non-invasive real-time reaction monitoring under relevant conditions that avoids many of the limitations that bedevil other reaction monitoring techniques.

Unlocking the potential of supported liquid phase catalysts with supercritical fluids: low temperature continuous flow catalysis with integrated product separation

Solution-phase catalysis using molecular transition metal complexes is an extremely powerful tool for chemical synthesis and a key technology for sustainable manufacturing. However, as the reaction complexity and thermal sensitivity of the catalytic system increase, engineering challenges associated with product separation and catalyst recovery can override the value of the product. This persistent downstream issue often renders industrial exploitation of homogeneous catalysis uneconomical despite impressive batch performance of the catalyst. In this regard, continuous-flow systems that allow steady-state homogeneous turnover in a stationary liquid phase while at the same time effecting integrated product separation at mild process temperatures represent a particularly attractive scenario. While continuous-flow processing is a standard procedure for large volume manufacturing, capitalizing on its potential in the realm of the molecular complexity of organic synthesis is still an emerging area that requires innovative solutions. Here we highlight some recent developments which have succeeded in realizing such systems by the combination of near- and supercritical fluids with homogeneous catalysts in supported liquid phases. The cases discussed exemplify how all three levels of continuous-flow homogeneous catalysis (catalyst system, separation strategy, process scheme) must be matched to locate viable process conditions.

Probing the Viability of Oxo-Coupling Pathways in Iridium-Catalyzed Oxygen Evolution

A series of Cp*IrIII dimers have been synthesized to elucidate the mechanistic viability of radical oxo-coupling pathways in iridium-catalyzed O2 evolution. The oxidative stability of the precursors toward nanoparticle formation and their oxygen evolution activity have been investigated and compared to suitable monomeric analogues. We found that precursors bearing monodentate NHC ligands degraded to form nanoparticles (NPs), and accordingly their O2 evolution rates were not significantly influenced by their nuclearity or distance between the two metals in the dimeric precursors. A doubly chelating bis-pyridine-pyrazolide ligand provided an oxidation-resistant ligand framework that allowed a more meaningful comparison of catalytic performance of dimers with their corresponding monomers. With sodium periodate (NaIO4) as the oxidant, the dimers provided significantly lower O2 evolution rates per [Ir] than the monomer, suggesting a negative interaction instead of cooperativity in the catalytic cycle. Electrochemical analysis of the dimers further substantiates the notion that no radical oxyl-coupling pathways are accessible. We thus conclude that the alternative path, nucleophilic attack of water on high-valent Ir-oxo species, may be the preferred mechanistic pathway of water oxidation with these catalysts, and bimolecular oxo-coupling is not a valid mechanistic alternative as in the related ruthenium chemistry, at least in the present system.