Special Issue "N‐Heterocyclic Carbenes and Their Complexes in Catalysis"

A special issue of Catalysts (ISSN 2073-4344).

Deadline for manuscript submissions: 30 November 2019.

Special Issue Editors

Guest Editor
Dr. Fady Nahra Website E-Mail
VITO (Flemish Institute for Technological Research), Separation and Conversion Technology, Boeretang 200, B-2400 Mol, Belgium
Interests: N-Heterocyclic carbenes; Multicatalysis; Fluorination; Organic Chemistry; Organometallic chemistry
Guest Editor
Dr. David J. Nelson Website E-Mail
WestCHEM Department of Pure and Applied Chemistry, University of Strathclyde, UK
Interests: N-Heterocyclic carbenes; Nickel catalysis; C-H activation; Physical (in)organic chemistry; Organometallic chemistry

Special Issue Information

Dear Colleagues,

Over the last 30 years, N-Heterocyclic carbenes (NHCs) have had a profound impact on catalysis and on organometallic chemistry in general. Their widely tuneable electronic and steric features have contributed significantly to their recognition as both: 1) an important class of ligands in organometallic chemistry, as demonstrated by the numerous applications ranging from homogeneous catalysis to material and medicinal sciences; and 2) as excellent nucleophilic organocatalysts. This exponential growth in the preparation and application of NHCs has elevated this class of compounds to the forefront of the modern chemical era, making them prevalent in academia and industry. The structural diversity of this exciting class of compounds is still being explored and exploited. Their synthesis and applications are more carefully being investigated in terms of sustainability, user-friendliness and recyclability. New designs are continuously being deployed as organocatalysts or as ligands for p-, d-, and f-block metal complexes that are active catalysts in a number of transformations.

This Special Issue highlights some of the recent work in the design, characterisation, and application of NHC ligands. Submissions are welcome in the form of original research papers or short reviews that reflect the state of the art of this research area.

Dr. Fady Nahra
Dr. David J. Nelson
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Catalysts is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • N-heterocyclic carbenes
  • Metal Catalysis
  • Organocatalysis
  • Coordination Chemistry
  • Ligand Design
  • Sustainability

Published Papers

This special issue is now open for submission, see below for planned papers.

Planned Papers

The below list represents only planned manuscripts. Some of these manuscripts have not been received by the Editorial Office yet. Papers submitted to MDPI journals are subject to peer-review.

Title: Recent advances in Computational Analysis in NHC catalyzed reactions: Scope and Limitations
Author: Rajat Maji*
Affiliation: Physical Organic Chemistry, Texas A & M University, USA
Abstract: Our understanding of NHC catalysed reactions have evolved considerably over the last decade, and these reactions now constitute a key feature in many stereoselective transformations. However, there are still gaps in our understanding the physical underpinnings of these ligand and catalysts and means by which they can be modulated, impeding our ability to fully harness their power in organic synthesis. Computational chemistry has made enormous strides in explaining how conformations, reactivity and stereoselectivity can be impacted by subtle changes in NHC catalysts. Ultimately, more effective catalysts using virtual screening, new reactions and ligands can be developed by harnessing the insights available from theory. The success of computational chemistry in explaining the mode of action of NHC catalysts and the underlying reason of stereo-induction can serve as a model for the cooperation of theory and experiment. By combining key insights provided by computational chemistry with experimental investigations, tremendous advances can be made in the design of more effective catalysts. Our hope is that this tutorial review will lay the foundation for such collaborative endeavours.

Title: Strong solvent effects on catalytic transfer hydrogenation of ketones with [Ir(cod)(NHC)(PR3)] precatalysts in 2-propanol-water mixtures
Authors: Krisztina Orosza, Gábor Pappa, Ágnes Kathóa, Ferenc Joóa,b, Henrietta Horváthb*
Affiliation:aUniversity of Debrecen, Department of Physical Chemistry, P.O.Box 400, Debrecen, H-4002 Hungary
bMTA-DE Redox and Homogeneous Catalytic Reaction Mechanisms Research Group, P.O.Box 400, Debrecen, H-4002 Hungary
Abstract: The zwitterionic complexes [Ir(cod)(NHC)(mtppms)] and Na2[Ir(cod)(NHC)(mtppts)] (NHC=emim or bmim; mtppms-Na and mtppts-Na3 = sodium salts of mono- and trisulfonated triphenylphosphine, respectively) were found effective precatalysts for transfer hydrogenation of aromatic and aliphatic ketones in basic 2-propanol-water mixtures with initial turnover frequencies up to 390 h-1 at 80 ºC, and their catalytic performances were compared to those of  [IrCl(emim)(cod)], [IrCl(bmim)(cod)], and [Ir(emim)(cod)(PPh3)]Cl. The reaction rates of the transfer hydrogenation of acetophenone and benzophenone showed strong dependence on the water concentration of the solvent indicating preferential solvation of the substrate and/or the catalytically active metal complexes.

Title: Catalytic Asymmetric Transformations Using Chiral N-Heterocyclic Carbene Coinage Metal (Cu, Ag and Au) Complexes
Author: Michael G. Coleman
Affiliation: Ph.D. Associate Professor of Chemistry, Rochester Institute of Technology
Abstract: N-heterocyclic carbenes (NHCs) are a class of extremely versatile ligands for nearly every metal on the periodic table. NHC ligands have unusually high σ-donor basicity and their equilibrium favors the intact NHC-metal complex. Consequently, the coordination chemistry of NHC-metal complexes make them excellent candidates for catalytic asymmetric transformations. In the last decade, several excellent reviews and book chapters have been reported in the field of chiral NHC-transition metal asymmetric catalysis. Much like the field of asymmetric catalysis, these reports are largely dominated by the coordination chemistry of chiral NHC-precious metals (metal = Pd, Ru, Rh, Ir, Pr). Given the recent developments in field of chiral NHC-coinage metal (metal = Cu, Ag, Au) catalytic asymmetric transformations, a focused review is warranted to highlight their high activity and stereoselectivity towards the synthesis of optically pure compounds.

Title: Photochemistry of Iron Carbene Complexes” (prel. topic)
Authors: Pavel Chábera, Nils W. Rosemann, Kenneth Wärnmark, Jens Uhlig, Arkady Yartsev, Villy Sundström, Petter Persson
Affiliation: 1 Division of Chemical Physics, Department of Chemistry, Lund University, Box 118, SE-22100 Lund, Sweden
2 Center for Analysis and Synthesis, Department of Chemistry, Lund University, Box 118, SE-22100 Lund, Sweden
3 Division of Theoretical Chemistry, Department of Chemistry, Lund University, Box 118, SE-22100 Lund, Sweden
Abstract: Transition metal complexes as sensitizers and catalysts for solar cells, solar fuel and lighting applications have traditionally often been based on rare and expensive transition metals like ruthenium, osmium, rhenium and Iridium. For large scale implementation of such complexes in these technologies cheaper and more abundant metals are needed. Zinc, copper and cobalt are examples of metals that have been explored with some success. Iron, the fourth most common element in Earth’s crust, would be an obvious candidate, but until very recently extensive scientific efforts have not been rewarded with much progress. The reason being the low ligand-field strength of iron which results in femtosecond-fast deactivation of the photo-functional charge transfer states by low-lying optically dark metal-centered (MC) states. Various strategies have been explored to remedy this unwanted situation, but until very recently no photochemically useful or luminescent Fe-complex had been produced. Six years ago our group published the first Fe-NHC complex with a 3MLCT lifetime approx. hundred times longer (9 ps) than at that time typical polypyridyl-Fe complexes. This first break-through triggered vigorous activity that now has led to iron-based complexes with both nanosecond-long and luminescent charge transfer states. Here we will review this development with a focus on spectroscopic and photochemical properties and how they can be rationalized with the help of quantum chemistry calculations. Following an introduction and description of our strategy to use NHC ligands to achieve photofunctional iron complexes we start with a discussion of the excited state landscape of Fe-NHCs and how quantum chemistry methods can guide the design of complexes by suggesting ligand properties that would lead to a particular state ordering and geometry. A comparison of the excited state structures of FeII- and FeIII-NHCs leads to insight into state characteristics leading to long-lived and luminescent CT states. Next we will summarize results on excited state dynamics of FeII-NHCs, both for molecules in solution and attached to a nanostructured metal oxide surface emulating the photoanode of a dye sensitized solar cell. We will show how various aspects of the dynamics and excited state properties can be probed with the help of ultrafast spectroscopy covering wavelengths from the far infrared (THz) to the X-rays. This will be followed by a discussion of the paradigm-shifting FeIII-NHCs developed by our group, their excited state dynamics and photoelectrochemical processes. In an Outlook and Conclusion we will try to identify photophysical limitations of Fe-NHC complexes and propose how they can be overcome. And finally, we will discuss emerging opportunities towards photovoltaic, photoelectrochemical and photocatalytic processes.

Title: Design and Synthesis of Iron N-heterocyclic Carbene complexes as photosensitisers and photocatalysts
Authors: Simon Kaufhold, Yizhu Liu, Reiner Lomoth and Kenneth Wärnmark
Affiliation: Department of Chemistry and Lund University, Sweden
AbstractN-heterocyclic carbene (NHC) complexes of iron have attracted considerable attention in recent years in the field of photochemistry. These complexes are promising candidates as successors for widely used ruthenium and iridium polypyridine complexes as photosensitisers in dye-sensitised solar cells and in (photo)catalysis. Recently, the progress in overcoming fundamental difficulties for the use of iron complexes in photoapplications has been enormous. This is due to manipulate the electronic states of iron by using cleverly designed NHC ligands that enable the exploitation of the photoproperties of the resulting complexes. In this review we want to map out the crucial design principles and synthesis strategies, showcase state of the art iron compounds based on N-heterocyclic carbenes and give inspiration for new generations of photoactive iron NHC complexes. A detailed analysis of the photophysical properties iron NHC complexes is presented in the complementary review by Persson and Sundström et al. in this special issue of Catalysts.

Title: Chiral N-heterocyclic carbene gold complexes: Synthesis and applications in catalysis
Authors: Szymon Musioł and Michał Michalak
Affiliation: Institute of Organic Chemistry Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
Abstract:N-Heterocyclic carbenes have found many applications in modern metal catalysis, as well as organocatalysis, due to the formation of stable metal complexes. Among myriad of N-heterocyclic carbene metal complexes, gold complexes have gained a lot of attention due to their unique propensity for the activation of carbon-carbon multiple bonds, allowing many useful transformations of alkynes, allenes, and alkenes, not accessible by other metal complexes. The present review summarizes synthetic efforts for the preparation of chiral N-heterocyclic gold(I) complexes exhibiting C2 and C1 symmetry, as well as their applications in enantioselective catalysis. Finally, the emerging area of rare gold(III) complexes and their preliminary usage in asymmetric catalysis is also presented.

Title: Intermolecular alkyne hydroalkoxylation reaction catalyzed by dinuclear gold(I) complexes with bidentate N-heterocyclic carbene ligands: a case study.
Authors: Marco Baron, Elena Marcheggiani, Andrea Biffis, Cristina Tubaro
Affiliation: Dipartimento di Scienze Chimiche, Università degli Studi di Padova, via Marzolo 1, 35131 Padova (Italy)
Abstract: By coupling their Lewis acid character and affinity to carbon-carbon multiple bonds, gold(I) complexes have proven to be extremely good activators of alkenes, alkynes and allenes towards hydrofunctionalization reactions. In the playground of ancillary ligands able to stabilize gold(I) centers in a catalytic environment, N-heterocyclic carbenes certainly hold a privileged position, allowing the achievement of outstanding catalytic performances. In this study, the catalytic activity of a family of dinuclear gold(I) complexes with bridging bidentate N-heterocyclic carbene ligand in intermolecular alkyne hydroalkoxylation reactions has been analyzed. The influence of the ligand structure on the catalytic performance of the different complexes has been investigated by using model experiments aimed at rationalizing the possible presence of cooperative effects. The reaction scope has been extended to different alkynes and alcohols.

Title: The natural product Lepidiline A as an N-heterocyclic carbene ligand precursor in complexes of the type [Ir(cod)(NHC)PR3)]X: synthesis, characterisation and application in hydrogen isotope exchange catalysis
Authors: William J. Kerr,* David M. Lindsay and Marc Reid
Affiliation: WestCHEM, University of Strathclyde, Department of Pure and Applied Chemistry, Glasgow, G1 1XL, Scotland (UK)
Abstract: A range of iridium(I) complexes of type [Ir(cod)(NHC)PR3)]X are reported, where the N-heterocyclic carbine is derived from the naturally-occurring imidaozlium salt Lepidiline A (1,3-Dibenzyl-4,5-dimethylimidazolium chloride). A range of complexes were prepared, with a number of phosphine ligands and counter-ions, and various steric and electronic parameters were evaluated. The activity of these [Ir(cod)(NHC)PR3)]X complexes in hydrogen isotope exchange reactions was then studied, and compared to established iridium(I) complexes.

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