Highlights

Manipulating electron transfer – the influence of substituents on novel copper guanidine quinolinyl complexes

Type 1 copper proteins, which are also called blue copper proteins, are responsible in nature for electron transfer processes in all living organisms. The fast electron transfer is made possible by the entatic state in which the type-1 copper proteins are found. In previous work of the Herres-Pawlis group, it was shown that copper guanidine quinoline complexes are highly suitable as model complexes for the entatic state of the electron transfer of type 1 copper proteins. This suitability is caused by their special structural properties. The copper complexes exhibit a highly distorted structure, which is strongly determined by the donor properties of the guanidine quinoline ligand. As a result, the associated copper(I) and copper(II) complexes of the redox pairs show a high structural resemblance. The special electron transfer properties of the copper-guanidine-quinoline complexes are described by the electron self-exchange rate.

In this publication, guanidine quinoline ligands with different substituents were synthesized to gain a deeper understanding of the electron transfer properties and how to affect and enhance them. Therefore, the copper complexes were characterized and analyzed in detail using a variety of methods. The influences of the substituents on the structures of the copper complexes were investigated experimentally using XRD and XAS measurements as well as DFT calculations. In addition, the influences of the substituents on the donor properties of the ligands in the complexes were determined by NBO calculations. The electrochemical properties were characterized by cyclic voltammetry and related to the influences of the substituents on the complex structures and donor properties of the ligands. Using Markus theory, the electron self-exchange rates of the complexes were determined performing stopped-flow UV/Vis measurements. In this work, different substituents were revealed to be suitable for manipulating and enhancing the electron transfer properties of the complexes significantly for different reasons. The results were correlated with the influences of the substituents by DFT-calculations of the reorganization energies.

The article Manipulating electron transfer – the influence of substituents on novel copper guanidine quinolinyl complexesis available on the webpage of the publisher.

Image reproduced by permission of Sonja Herres-Pawlis and The Royal Society of Chemistry from Chem. Sci. 2022, 13, 8445-8446., DOI: 10.1039/D2SC90149H; Link zum Back cover.

Guanidine carboxy zinc complexes for the chemical recycling of renewable polyesters

Plastics are ubiquitous in our lives. However, facing the increasing environmental pollution caused by petroleum-based, non-degradable plastics is one of the greatest global challenges of the near future. Bioplastics, such as the bio-based and fully biodegradable polyester polylactide (PLA), can make a decisive contribution to solving this problem. Unfortunately, in the current linear plastics economy, PLA can be a danger to the environment, since the toxic catalyst stannous octanoate, currently used for industrial production, is released from the polymer, when degraded and can accumulate in nature. Therefore, extensive research has been made by the Herres-Pawlis group to develop non-toxic, robust and highly active catalysts for the ring-opening polymerization of lactide to produce PLA. Various iron and zinc guanidine catalysts have already been developed that are significantly more active than the toxic tin catalyst using industrially preferred melt conditions. In addition to the development of non-toxic catalysts for production, efficient recycling of PLA after its use in desirable, to exploit its full potential in a circular plastics economy. Due to the reactive ester bond in the polymer chain, PLA is particularly well suited for chemical recycling methods, such as alcoholysis.

In the current work, we show that three robust guanidine-carboxy-zinc complexes, previously developed in our group and suitable for lactide polymerization, are also catalytically active in the alcoholysis of PLA. Herein, all tested catalysts transform PLA to methyl lactate in a transesterification reaction using mild reaction conditions (60 °C in THF). Methyl lactate can be cyclized to the monomer lactide to recycle PLA, used as a solvent or as a starting material for the synthesis of new compounds. Electron density donating substituents on the ligand increased the catalytic activity, whereas electron density withdrawing substituents decrease the catalytic activity. The most active catalyst [ZnCl2(TMG5NMe2asme)] has additionally been tested towards applicability in industry. Both the scalability of the reaction and the recycling of the catalyst itself are fulfilled. Furthermore, the conversion of other alcohols, the selective recycling of PLA from plastic mixtures and blends, and the recycling of the bioplastic polycaprolactone can be realised using the catalyst. Using solvent free conditions at 150 °C, accelerated alcoholysis of PLA, reaching full conversion after only 1 h. which further demonstrates the industrial relevance of the catalyst. Therefore, guanidine carboxy Zn catalysts can be a vital tool in the implementation of a sustainable circular bioplastics economy.

The article Guanidine Carboxy Zinc Complexes for the Chemical Recycling of Renewable Polyesters is available on the webpage of the publisher.

Master of Chaos and Order: Opposite Microstructures of PCL-co-PGA-co-PLA Accessible by a Single Catalyst

Iron guanidine complexes have shown exceptional properties as polymerization catalysts in the past years: They have been the first robust, biocompatible catalysts having a higher polymerization activity than the industrially used, but toxic tin octoate and open whole new synthesis routes to block copolymers by polymerizing via ATRP and ROP simultaneously.

In this publication, the way of success is continued by the presentation of a single iron guanidine catalysts being one of the rare examples capable to produce totally different microstructures in copolymers of lactide, glycolide and ε-caprolactone. The catalyst is active under immortal conditions, stable at temperatures as high as 180 °C and polymerizes all three monomers in a controlled manner. By sequential addition of ε-caprolactone followed by glycolide and lactide highly defined block copolymers are accessible with narrow molar mass distributions. Using a bifunctional co-initiator, up to the pentablock copolymer PLA-b-PGA-b-PCL-b-PGA-b-PLA can be synthesized. Random copolymers on the other hand are obtained by polymerizing a monomer mixture and increasing the reaction temperature to 180 °C. Due to the high temperature, transesterifcations are enhanced and scramble the polymer chain to a random copolymer. Mechanistic investigations revealed ε-caprolactone as the driving force for the synthesis of random copolymers since its active chain end rather performs a transesterification with the existing polymer chain than incorporating an available ε-caprolactone monomer.

These findings significantly contribute to the understanding of copolymerization systems and therefore pave the way for the synthesis of bioplastics with tailor-made properties replacing traditional plastics currently causing the environmental crisis.

The article Master of Chaos and Order: Opposite Microstructures of PCL-coPGA-co-PLA Accessible by a Single Catalyst is available on the webpage of the publisher.

Active in Sleep: Iron Guanidine Catalyst Performs ROP on Dormant Side of ATRP

In the past 100 years the variety of plastics has brought countless innovations. A treasure yet to be fully understood lies in the copolymerization of different monomers to copolymers with adjustable properties. Typically, only monomers containing the same functionality enabling the polymerization via the same mechanism are combined. However, not all desired properties can be accessed by a single monomer class. For monomers with different functionalities, work-intensive multiple step procedures with distinct catalysts are often necessary.

In this publication we show that the copolymerization of lactide via a ring-opening polymerization (ROP) and styrene via an atom transfer radical polymerization (ATRP) can be performed in a one-pot synthesis. The presented iron guanidine catalyst is highly active for lactide and establishes the ATRP equilibrium promoting the controlled polymerization of styrene. The catalyst is not only active for both mechanisms but performs them simultaneously leading to block copolymers if a suited bifunctional initiator is used. Mechanistic investigations suggest that the ROP of lactide is promoted by the iron(II) species on the dormant side of the ATRP equilibrium which proceeds without interfering with the controlled radical polymerization of styrene at the active side. The way both mechanisms work hand-in-hand shows the great possibilities yet to be discovered simplifying copolymerization procedures leading to materials with tailor-made properties.

The article Active in Sleep: Iron Guanidine Catalyst Performs ROP on Dormant Side of ATRP is available on the webpage of the publisher.

A new generation of terminal copper nitrenes and their application in aromatic C–H amination reactions

Nitrenes are a highly reactive nitrogen species with six valence electrons. These can be stabilized by copper(I) bis(pyrazolyl)methane complexes at low temperatures and be applied for the direct C–H amination and the aziridination of styrene derivatives.

In this publication, three novel copper nitrene complexes with different third N donor moieties were synthesized. They were characterized with different methods, for example UV/Vis spectroscopy and ultra-high resolution cryo ESI mass spectrometry. Furthermore, the structure of the copper nitrene complexes was studied by DFT and DLPNO(T) calculations. The copper nitrene complexes were applied for the C–H amination of challenging substrates like benzene or cyclohexane. In addition, the aziridination of styrene derivatives was achieved in good yields. Moreover, the mechanism of the aziridination of Styrene was investigated experimentally and by DFT calculation.

The article A new generation of terminal copper nitrenes and their application in aromatic C–H amination reactions is available on the webpage of the publisher.

Image reproduced by permission of Sonja Herres-Pawlis and The Royal Society of Chemistry from Dalton Trans., 2021, 50, 6444-6462, DOI: 10.1039/D1DT00832C; link to the Cover.

Next Generation of Zinc Bisguanidine Polymerization Catalysts towards Highly Crystalline, Biodegradable Polyesters

The Herres-Pawlis group showed repeatedly the potential of biocompatible catalysts for the production of bioplastics by ring-opening polymerization, which are an alternative use to the toxic industrially used tin(II) 2-ethylhexanoate. The focus of the group is the investigation of robust alternatives that can be used under industrial conditions. The iron(II) hybrid guanidine complex published in this group in 2019 is the first robust catalyst that surpasses the industrial catalyst in its activity in the ring-opening polymerization of lactide.

With the zinc-bisguanidine complex shown in the new publication it is possible to obtain an even higher activity under industrial conditions in the ring-opening polymerization of cyclic esters. Not only lactide as monomer was investigated, but also the cyclic ester e-caprolactone. The high activity of the published zinc complex is mainly reflected in the lactide polymerization. Here the catalyst shows a reaction rate that is about ten times faster than the industrial tin complex. Additionally, this high catalyst activity enables polymerization in solution, which is particularly advantageous for specific applications, e.g. in the medical field. Thus, this zinc bisguanidine complex is not only the most active catalyst at present, but also the first guanidine complex which can polymerize lactide in solution. Additional material investigations of the produced polymers showed that zinc-based polylactide has a significantly higher crystallinity than the same material produced with tin. This is an important property for e.g. medical implants.

The authors thank the Bioeconomy Science Center (BioSC) for generous funding within the project R2HPBio.

The article Next Generation of Zinc Bisguanidine Polymerization Catalysts towards Highly Crystalline, Biodegradable Polyesters is available on the website of the publisher.

Exceptional Substrate Diversity in Oxygenation Reactions Catalyzed by a Bis(μ‐oxo) Copper Complex

Activation and transfer of molecular oxygen is crucial for oxidation processes, because dioxygen is cheap and readily available. Tyrosinase model systems serve as tool to investigate the reaction mechanisms. Hybrid guanidine-stabilized copper(I) complexes are able to bind dioxygen in a bis(µ-oxo) dicopper(III) complex and to transfer it to phenolic substrates. We succeeded in characterizing the reactive bis(µ-oxo) species spectroscopically at low temperatures and evaluating its activity in catalytic oxygenation reactions of mono- and bicyclic aromatic alcohols. For the first time, this phenolase activity was successfully applied on more complex substrate classes. The selective C-H activation of these substrates paves the way to future developments on tyrosinase model systems on the route to atom-economic oxygenation reactions.

The article Exceptional Substrate Diversity in Oxygenation Reactions Catalyzed by a Bis(μ-oxo) Copper Complex is available on the website of the publisher.

Enhanced catalytic activity of copper complexes in microgels for aerobic oxidation of benzyl alcohols

Microgels are porous and flexible macromolecular networks, which are able to react to external stimuli like temperature, ionic strength or pH by their swelling behavior. Thus, it is not possible to classify microgels as one of the common colloidal groups (flexible macromolecules, surfactants or rigid colloids). Instead, they unite the characteristics of all those groups. Therefore, microgel networks are promising and adaptable systems for various applications, for example biosensors or tissue engineering.

In this publication the application of microgels in catalysis is shown by incorporating copper bis(pyrazolyl)methane complexes on specified positions within the microgels, generating functionalized nanoreactors. Similar to enzymes, these nanoreactors provide a tailor-made environment for the catalytically active centers increasing the stability of the copper complexes and hence their activity in catalysis. The microgels were analyzed by means of dynamic light scattering (DLS), scanning electron microscopy (SEM) and inductively coupled plasma mass spectrometry (ICP-MS). To determine the enhancement in catalytic activity the functionalized microgels were used as catalysts in the oxidation of various benzyl alcohols to the corresponding aldehydes as a test reaction and were compared to the molecular copper bis(pyrazolyl)methane complex and the copper salt.

The article Enhanced catalytic activity of copper complexes in microgels for aerobic oxidation of benzyl alcohols is available on the webpage of the publisher.

Image reproduced by permission of Sonja Herres-Pawlis and The Royal Society of Chemistry from Chem. Commun., 2020, 56, 5601-5604, DOI: 10.1039/D0CC02433C; link to the Cover.

Undiscovered Potential: Ge Catalysts for Lactide Polymerization

Polylactide (PLA) is a high potential bioplastic that can replace oil‐based plastics in a number of applications. To date, in spite of its known toxicity, a tin catalyst is used on industrial scale which should be replaced by a benign catalyst in the long run. Germanium is known to be unharmful while having similar properties as tin. Only few germylene catalysts are known so far and none has shown the potential for industrial application.

In this paper, Ge complexes in combination with zinc and copper are presented, which show amazingly high polymerization activities for lactide in bulk at 150 °C. In these complexes, germylenes with a 2‐Et2NCH2‐4,6‐tBu2‐C6H2 group act as ancillary ligands for Zn or Cu. The complexes were characterized using single-crystal X-ray diffraction (XRD), and the bond situations were studied using density functional theory (DFT) calculations. By systematical variation of the complex structure, structure–property relationships are found regarding the polymerization activity. Even in the presence of zinc and copper, germanium acts as the active site for polymerizing probably through the coordination–insertion mechanism to high molar mass polymers.

The article Undiscovered Potential: Ge Catalysts for Lactide Polymerization is available on the webpage of the publisher.

New kids in lactide polymerization: highly active and robust iron guanidine complexes as superior catalysts

The development of novel and sustainable catalysts for the ring-opening polymerization of lactide is one of the research fields of the Herres-Pawlis group. In order to obtain an alternative to the currently applied tin(II)octanoate, the applicability of the developed catalysts in industrial conditions is of special interest.

In contrast to the so far mainly investigated zinc(II)-complexes, this publication deals with iron(II)-complexes. Three novel and promising iron(II)-complexes where synthesized from iron(II)chloride and a N,N-donor-hybrid guanidine and two N,O-donor-hybrid guanidine ligands, respectively. The X-ray structures of these complexes were determined. Moreover, the complexes where applied in the bulk polymerization of technical-grade rac-lactide. The polymerizations were monitored with in-situ Raman spectroscopy.

The observed activity of the three complexes proved that the catalysts are robust and tolerate the impurities present in technical-grade lactide. However, the two investigated iron(II)-N,O-donor complexes showed a significantly higher activity than the iron(II)-N,N-complex. The two complexes exhibit an even higher activity than the fastest known robust zinc(II)-catalysts.

In addition, the rate constant of propagation k p was determined for the two N,O-donor complexes: One of the complexes shows an activity similar to tin(II)-octanoate, the other complex surpasses the activity of the tin catalyst by one order of magnitude. With molar masses above 90.000 g mol-1, the produced polymers are suited for industrial processing.

In conclusion, the presented novel iron(II)-hybrid guanidine catalysts provide promising results and represent a decisive advance towards the tin-free lactide polymerization.

The article New kids in lactide polymerization: highly active and robust iron guanidine complexes as superior catalysts is available on the webpage of the publisher.

Next Generation of Guanidine Quinoline Copper Complexes for Highly Controlled ATRP: Influence of Backbone Substitution on Redox Chemistry and Solubility

One aim of the Herres-Pawlis group is the development of new homogeneous catalysts based on guanidinoquinoline copper complexes, which are suitable for the atom transfer radical polymerisation (ATRP). The first generation of ATRP catalysts (with an unsubstituted ligand) exhibited limited solubility in the chosen monomer (styrene) and thus limited polymerisation control (see Chem. Eur J. 2016). Since guanidinoquinolines represent a promising class of ligands, they should be improved by targeted substitution. Consequently, a strategy for the synthesis of substituted guanidinoquinolines was developed. The strategy allows the synthesis of various guanidinoquinolines in high yields. The second generation of ligands possesses an alkylated quinoline backbone. n-Butylation of the backbone leads to a dramatic increase of the catalysts solubility. The higher solubility of the catalysts is furthermore affiliated with an increased polymerisation control. Consequently, polymer samples, produced with second-generation catalysts, exhibit polydispersities (PDs) of 1.1, which are associated with an extremely narrow molar mass distribution. Additionally, the influence of the substituent on the electronic properties of the ligands were comprehensively studied with various methods. As expected, alkylated TMG ligands reveal a higher activity in ATRP as the unsubstituted ligands. The knowledge of a new strategy for the synthesis of substituted guanidinoquinoline ligands allows future development of tailor-made complexes for homogeneous catalysis. The whole article is available on the webpage of the publisher.

Terminal Copper Nitrenes and their Application in Catalytic C–H Aminations

The discovery of terminal copper nitrenes led us to find a long-missing link in the family of copper nitrenes. They have been predicted by theory and are pursued since decades. Their remarkable reactivity in terms of direct C–H-aminations of unsaturated C–H-compounds is direcetly linked to the existence of terminal copper nitrenes. The term terminal simply describes the occurrence of an end-on bound nitrene fragment to the copper centre. To date, only a few copper nitrenes are reported, but neither a terminal species or monometallic species was found. Our low-temperature stabilised nitrenes are based on bis(pyrazolyl)methane ligands and have been directly observed by Cryo-ESI mass spectrometry and UV/Vis spectroscopy. Their reactivity in stoichiometric reactions was followed by UV/Vis spectroscopy. In the end, a catalytic protocol was discovered, leading to the C–H-Amination of cyclohexane and toluene, compounds with very strong C–H bonds. This research will stimulate further efforts for the atom-efficient synthesis of organic compounds and helps to better understand the optimisation of crucial components in C–H insertions. The article Designed to react: Terminal Copper Nitrenes and their Application in Catalytic C-H Aminations is available on the webpage of the publisher.

Transferring the entatic state principle into copper photochemistry

So-called pre-distorted states accelerate photochemical reactions too

What enables electrons to be transferred swiftly, for example during photosynthesis? An interdisciplinary team of researchers has worked out the details of how important bioinorganic electron transfer systems operate. Using a combination of very different, time-resolved measurement methods at DESY's X-ray source PETRA III and other facilities, the scientists were able to show that so-called pre-distorted states can speed up photochemical reactions or make them possible in the first place. The group headed by Sonja Herres-Pawlis from the RWTH Aachen University (RWTH) Michael Rübhausen from the University of Hamburg and Wolfgang Zinth from Munich’s Ludwig Maximilian University, is presenting its findings in the journal Nature Chemistry.

Entatic state model complexes optimize the energies of starting and final configuration to enable fast reaction rates (illustrated by the hilly ground). The authors demonstrate that the entatic state principle can be used to tune the photochemistry of copper complexes.

The scientists had studied the pre-distorted, “entatic” state using a model system. An entatic state is the term used by chemists to refer to the configuration of a molecule in which the normal arrangement of the atoms is modified by external binding partners such that the energy threshold for the desired reaction is lowered, resulting in a higher speed of reaction. One example of this is the metalloprotein plastocyanin, which has a copper atom at its centre and is responsible for important steps in the transfer of electrons during photosynthesis. Depending on its oxidation state, the copper atom either prefers a planar configuration, in which all the surrounding atoms are arranged in the same plane (planar geometry), or a tetrahedral arrangement of the neighbouring ligands. However the binding partner in the protein forces the copper atom to adopt a sort of intermediate arrangement. This highly distorted tetrahedron allows a very rapid shift between the two oxidation states of the copper atom.

“Pre-distorted states like this play an important role in many biochemical processes,” explains Rübhausen, who works at the Centre for Free-Electron Laser Science (CFEL) in Hamburg, a cooperation between DESY, the University of Hamburg and the Max Planck Society. “The principle of the entatic state helps the electron transfer reactions that occur everywhere in nature and also in human beings, for example when we breathe or a plant photosynthesises,” adds Herres-Pawlis.

Biologically relevant, pre-distorted states always involve a metal atom. The scientists examined a model system consisting of a copper complex with specially tailored molecules bound to it, so-called ligands. Using a wide range of observation methods as well as theoretical calculations, the scientists showed that the ligands used did indeed put the copper complex into a pre-distorted (entatic) state and were then able to observe the details of the reaction that occurred when light was absorbed.

The combination of time-dependent UV, infra-red, X-ray and visual fluorescence spectroscopy produces a detailed picture of the dynamics of the structural changes on a timescale of pico- to nanoseconds (trillionths to billionths of a second). “We are now able for the first time to understand how pre-distorted states favour charge transfer,” explains Rübhausen. “Also, our studies demonstrate that pre-distorted states are important for photochemical reactions, in other words for certain biochemical processes which are triggered by light,” explains Herres-Pawlis.

The study shows in detail how the process proceeds: from the initial state (copper in an oxidation state of +1) an electron is transferred from the copper to one of the ligands, by optical excitation. Within femtoseconds (trillionths of a second) the excited state created decays into another, still excited state, known as the S1 state. In this configuration, the geometry is slightly relaxed.

Shortly afterwards, the electron undergoes a change in spin. The spin of an electron is comparable to the direction in which a top rotates. Although one of the electrons has so far remained on the ligand, this electron and its corresponding partner on the copper were spin-coupled. The spin of the electron on the ligand now reverses, and this very rapid transition to the so-called triplet state, within just about two picoseconds, removes the spin coupling. This T1 state exists for 120 picoseconds and drops back into the original state again after once again reversing its spin. All the time constants are distinctly shorter compared with other copper complexes. “A complete understanding of all the processes taking place has only become possible through the unique combination of different methods of study,” emphasises Zinth.

The detailed analysis of the reaction principle not only improves our understanding of natural processes. It can also help to customise new bioinorganic complexes that imitate nature but whose range of reactions extend beyond those of natural molecules. These complexes could also accelerate or make possible chemical reactions associated with electron transfers in other areas, too.

Scientists from the University of Hamburg, RWTH Aachen University, the Ludwig Maximilian University in Munich, DESY, the University of Paderborn, the European research facility ELI Beamlines, the Institute of Physics of the Czech Academy of Sciences, the University of Uppsala, the Chalmers University of Technology in Göteborg, European XFEL and the Danish Technical University were all involved in the research. The study received grants from Deutsche Forschungsgemeinschaft as part of the dislocated research group FOR1405 (Dynamics of Electron Transfer Processes within Transition Metal Sites in Biological and Bioinorganic Systems) and the SFB749 (Dynamics and Intermediates of Molecular Transformations) and the cluster of excellence CIPSM.

The article Transferring the entatic state principle into copper photochemistry is available on the webpage of the publisher.

Highly Active N,O Zinc Guanidine Catalysts for the Ring-Opening Polymerization of Lactide

The Herres-Pawlis group develops new catalysts for the polymerisation of lactide to polylactide. New guanidine ligands with N,O-donors are synthesised and the desired zinc complexes are characterised and tested as catalyst for the solvent-free ring-opening polymerisation of racemic lactide at 150°C. All complexes showed high activity and the fastest complex affords colorless polylactide within 90 minutes with high molar masses (M w=69 100, polydispersity=1.4). Different monomer-to-initiator ratios were studied to identify the rate constant kP . The polymerisation of the most active catalyst was monitored by in situ Raman spectroscopy. To obtain insights into the mechanism, a end group analysis was performed. All four complexes show robustness against impurities in the lactide and an excellent catalytic activity. These zinc guanidine complexes with N,O-donors represent the fastest robust lactide catalysts to date, opening new avenues to a sustainable ring-opening polymerisation catalyst family for industrial use. Here you can go directly to this publication.

On the Way to a Trisanionic {Cu3O2} Core for Oxidase Catalysis: Evidence of an Asymmetric Trinuclear Precursor Stabilized by Perfluoropinacolate Ligands

In a cooperation with the working group Doerrer from Boston, USA, we study the oxygenation of copper(I) complexes with perfluoropinacolate. At low temperatures, the oxygenation results in a trisanionic bis(µ3-oxo) trinuclear copper (II,II,III) species. This species were characterised with UV/Vis spectroscopy, cryospray-ionization mass spectrometry, and X-band EPR spectroscopy. The trimeric {Cu3O2} species was furthermore studied with stopped-flow spectroscopy and the kinetics of this system were hereby elucidated. Density function theory calculations provide isights in the electronic structures. During the reaction, an asymmetric {Cu3O2} species is detectable which structure is similar to multicopper oxidases. The final species is a symmetric trinuclear species which shows catalytic activity: It oxidises para-hydroquinone to benzoquinone. Here you can go directly to this publication.

The Federation de Recherche FERMaT "Fluids, Energy, Reactors, Materials and Transfer" from Toulouse, France, invited the members of the priority program 1740 "The Influence of Local Transport Processes on Chemical Reactions in Bubble Flows" for the symposium Non-Invasive Measuring Tools and Numerical Methods for the Investigation of Non-Reactive and Reactive Gas-Liquid Flows. On this symposium researchers from Germany and France presented the latest results of their research and discussed about yield and selectivity of chemical reactions with mass transport. The most important results of this symposium were published within an special issue of Chemical Engineering & Technology. Here you can go directly to this special issue.

Record Broken: A Copper Peroxide Complex with Enhanced Stability and Faster Hydroxylation Catalysis

For decades, functional tyrosinase models have been sought for. Our model complexes use N-donor ligands which mimic the histidine ligands in the actice site of the enzyme. Bis(pyrazolyl)methanes with pyridinyl or imidazolyl are excellent ligands for tyrosinase model complexes and herein we substited the pyridinyl donor. The new ligand HC(3-tBuPz)2(4-CO2MePy) stabilises a room temperature stable μ-η22-peroxide dicopper(II) species upon oxygenation and show high catalytic activity. This species hydroxylates 8-hydroxyquinoline in high yields (TONs of up to 20) and much faster than all other model systems (max. conversion within 7.5 min). The stoichiometric reaction behaviour with para-substituted sodium phenolates constrains saturation kinetics and a electrophilic aromatic substitution mechanism. Density functional theory calculations show the influence of the substitution of the pyridinyl donor: the carboxymethyl group adjusts the basicity and nucleophilicity without additional steric demand. This substitution opens up new pathways in reactivity tuning. Here you can go directly to this publication.

Implications of Guanidine Substitution on Copper Complexes as Entatic-State Models

In this publication, we study copper complexes with the guanidine-quinoline ligand DMEGqu. The bis(chelate) copper(I) and copper(II) complexes show a geometry between tetrahedral and square-planar coordination. These structures model the entatic state and have been investigated in solid state by single-crystal X-ray diffraction and in the solid state and in solution by X-ray absorption spectroscopy. The DMEG unit of DMEGqu reveals a smaller steric encumbrance than the tetramethylguanidine (TMG) counterpart. So in DMEGqu complexes, we observe slightly larger structural changes upon oxidation than those for TMGqu complexes. DFT analyses show a good aggreement to the experiment (structur and optical features) with the functional B3LYP or TPSSh with the basis set def2-TZVP and empirical dispersion correction with Becke-Johnson damping. An orbital analysis explains the electronic structure of the complexes and their charge-transfer behaviour. Here you can go directly to this publication.

Decay kinetics of sensitive bioinorganic species in a SuperFocus mixer at ambient conditions

In this key publication of the Schwerpunktprogramm 1740, we determine the intrinsic kinetics (formation and decay) of the oxygenation reaction of a copper(I) complex with a guanidine ligand with a stopped-flow UV/Vis setup and with SuperFocus mixer. By means of this setup, we were able for the first time to detect the formation and decay of a thermally very sensitive bis(μ-oxo)dicopper species at ambient temperature. Comparing these data to results from a stopped-flow setup we could confirm the performance of the SuperFocus mixer setup. Here you can go directly to this publication.

Image reproduced by permission of Sonja Herres-Pawlis and The Royal Society of Chemistry from Reaction Chemistry & Engineering 2016, 1, 485-493, DOI: 10.1039/C6RE00119J, link to the Cover.

Donor-driven conformational flexibility in a real-life catalytic dicopper(II) peroxo complex

In this study we reported on a comprehensive theoretical analysis of the conformers of a real-life P species [Cu2O2{HC(3-tBuPz)2(Py)}2]2+ which displays catalytic reactivity. By second-order perturbation theory we elucidated the donor competition between pyrazolyl and pyridinyl moieties and found that pyrazolyl units are the stronger donors in bis(pyrazolyl)pyridinylmethane copper complexes. Geometry optimisations and TD-DFT calculations proved to be robust in the prediction of the experimental data: the XAS distances and both charge-transfer bands are well reproduced. Four stable conformers were found which are energetically rather close. We calculated for each conformer the energies of closed-shell and open-shell P state as well as the isomeric O state. Here, the basis set 6-31g(d) fails in the O-P energetics and predict in some cases randomly the O state with too large energetic favourisation. For a correct description of the energetics, empirical dispersion correction with Becke-Johnson damping, a PCM solvent model and a 3z basis set are required. The CDA analyses gave insights into the electronic structure of the real-life P species. The classic P MOs can be relocated but they are mixing with each other or with ligand orbitals. Hence, the CDA is a great method to parse the crucial interactions. In summary, we find that the N donor interactions to the core are extremely stabilising and that in the pzpz conformer, the equatorial interactions are more stabilising than the axial ones. We relate the extraordinary catalytic activity of the [Cu2O2{HC(3-tBuPz)2(Py)}2]2+ system to the subtle interplay of the different donor moieties which allow a flexible re-orientation of the dioxygen as well as the substrate approach. Both features are crucial requirements for efficient hydroxylation catalysis. Here you can go directly to this paper.

Image reproduced by permission of Sonja Herres-Pawlis and The Royal Society of Chemistry from Phys. Chem. Chem. Phys., 2016, 18, 6430-6440, DOI: 10.1039/C5CP05009J link to the cover.

Efficient Biomimetic Hydroxylation Catalysis with a Bis(pyrazolyl)imidazolylmethane Copper Peroxide Complex

We have reported that fine-tuning of biomimetic ligands leads to enhanced catalytic activity of a catalytic tyrosinase model. We have provided syntheses and structural characterisation of a new bis(pyrazolyl)imidazolylmethane ligand and its copper(II) halide complexes. The reaction of dioxygen with its copper(I)hexafluoridoantimonate complex gives a peroxo species that has been characterised by UV/Vis and XAS spectroscopy as well as cryo-UHR-ESI mass spectrometry. The classical CT absorptions are strongly blueshifted and have been analysed by TD-DFT and NTO analysis. They show the special role of the third donor function. The new peroxo species displays stoichiometric and catalytic hydroxylation activity. The catalytic activity towards 8-hydroxyquinoline is more rapid than that observed
for the parent pyridinyl–peroxo species. Analysis of the substrate-binding kinetics quantitatively proves that the electrondeficient substrate p-carbomethoxyphenolate is hydroxylated approximately six times faster. This possibility of efficiently tuning catalytic activity by donor exchange opens up new directions for industrial applications of this mild, selective and atom-economic hydroxylation chemistry. Here you can go directly to this paper.

The Cu2O2 torture track for a real-life system: [Cu2(btmgp)2O2]2+ oxo and peroxo species in density functional calculations

The structural, vibrational, electronic, and optical properties of [Cu2(btmgp)2(µ-O)2] 2+ and [Cu2(btmgp)2($\eta$ 2: $\eta$ 2-O2)]2+ have been calculated within DFT using semilocal as well as hybrid density functionals in conjunction with both localized and plane wave basis sets. The geometrical data derived within the various theoretical approaches largely agree with each and are close to the experimental data available. This does also hold for the general shape of the energy landscape that describes the oxo–peroxo transition. Details, and in particular, the position of the global energy minimum of the PES, however, are strongly dependent on the XC functional and, in cases of small basis sets, depend additionally on the type of basis function. Notably, we find that hybrid-DFT calculations tend to favor the P species over O. If one additionally allows for spin relaxation – which leads to a favorable BS solution for P-this trend is enhanced. This behavior is not observed for local XC functionals, however. The inclusion of dispersion correction, conversely, leads to a small bias toward the O core. The CDA analysis for the O state reveals the strong stabilization by the guanidine donors and highlights the role of the guanidine as $\sigma$- and $\pi$-donor. The structural modification accompanying the O-P transition leads to pronounced changes of the excitation energies. In particular, the interchange of the O LUMO and P LUMO13 at the TS geometry leads to strongly dispersive excited-state PESs that might be used for optically induced isomerization. The geometry changes along the O-P transition give rise to significant changes of the optical absorption features as well as modify the vibrational fingerprints of the Cu complex. Here you can go directly to this paper.

Geometrical and optical benchmarking of copper(II) guanidine-quinoline complexes: Insights from TD-DFT and many-body perturbation theory (Part II)

In summary, our results indicate a strong influence of the exchange and correlation functional on the structural properties of copper(II) guanidine–quinoline complexes. In addition, calculations performed with a plane-wave basis show that the influence of the basis set on the structural details of the molecular complexes studied here is of similar magnitude as the one due to various XC functionals. The structural benchmarking indicates that B3LYP/def2-TZVP and TPSSh/def2-TZVP offer both reasonable structural descriptions (predictions within the error bars of the experimental structures) but that the addition of dispersion with Becke–Johnson damping offers best structural agreement for extremely low computational costs. With def2-TZVP and Becke–Johnson damping, the functionals B3LYP and TPSSh give the accuracy of crystallographically determined structures (error: 0.012 A, 0.5% of bond lengths). Concerning the optical response calculations, it is found from IPA and DSCF calculations that adiabatic linear-response TD-DFT is a viable approach to copper(II) charge-transfer systems while the Bethe–Salpeter approach, at least using a static kernel with a simplified screening, may give rise to spurious excitations. In any event, a realistic description of the molecular optical response within MBPT requires the inclusion of the resonant–nonresonant coupling terms in the exciton Hamiltonian as well as selfconsistency effects in the Greens functions. The TD-DFT optical benchmarking demonstrates that B3LYP/def2-TZVP calculations model well the experimental data (error <0.1 eV) with little influence of the description of dispersion interaction and that the Becke–Johnson damping does not change this picture. Here you can go directly to this paper.

Insights into the influence of dispersion correction in the theoretical treatment of guanidine-quinoline copper(I) complexes

In summary, we could show that the correct description of copper bis(chelate) complexes requires modern dispersion correction using BJ damping. The triple-zeta basis set def2-TZVP of the Ahlrichs series is balanced and converged for the structural description. The best structural description is obtained with the TPSSh functional but B3LYP is very suited as well. Especially when application for the calculation of optical transitions (and the resulting structures) as well as Raman spectra prediction is targeted, B3LYP seems to be the best choice. Cutting of ligand substituents leads to distortions which limit the predictive ability of such calculations. We recommend the calculation of “full” chemical systems with inclusion of dispersion correction using BJ damping. In the further analysis of the regarded copper bis(chelate) complexes, we found that the theoretical description of optical and Raman spectra is not much affected by the dispersion although charge transfer excitations come into play. Hence, we can derive the result that the correct structural description with dispersion serves as crucial basis for subsequent calculation steps and very much attention has to be paid here as all errors propagate from this step. Here you can go directly to this paper.

Hiking on the potential energy surface of a functional tyrosinase model – implications of singlet, broken-symmetry and triplet description

In summary, we herein present the first full PES analysis of a P/O core equilibrium for the three important states. Moreover, this complete analysis has been accomplished for a real-life catalytic tyrosinase model. We could show that the P core lies in a deep global minimum with additional broken-symmetry stabilisation. With regard to the simple optical excitation by visible light, the low-lying triplet P state appears as a viable alternative for induction of biological hydroxylation activity. Here you can go directly to this paper.

Image reproduced by permission of Sonja Herres-Pawlis and The Royal Society of Chemistry from Chem. Commun., 2014, 50, 403-405, DOI: 10.1039/C3CC46893C link to the cover.