Editor's Page Cite This: Organometallics 2018, 37, 273−274
Pushing ElectronsWhich Carbene Ligand for Which Application?
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and where are the electrons? Do electrophilic Schrock carbenes exist? A multitude of questions are waiting to be answered by a comprehensive tutorial (DOI: 10.1021/acs.organomet. 7b00720). Dominik Munz is an excellent choice for the assembly of such a tutorial review, because his academic education allowed him to obtain insights from several varying perspectives on carbene chemistry. During his graduate studies at TU Dresden, he studied the application of late transition metal NHC complexes for catalysis and the molecular modeling of the electronic properties of NHC complexes. In the group of Thomas Strassner in Dresden, he studied applications of NHC and mesoionic carbene complexes as light-emitting or -harvesting molecules. After a 1 year stint with Brent Gunnoe at the University of Virginia in the field of C−H bond activation, he continued with carbene research as a postdoctoral fellow in Guy Bertrand’s lab at UC San Diego. There, he worked on the synthesis of novel carbenes with small singlet−triplet gaps, functionalized CAAC ligands, and stable carbene radicals, as well as p-block carbene analogs. Currently, his research interests include the synthesis of carbene compounds of the electropositive metals as well as the application of ancillary carbene ligands for the stabilization of reactive transition metal intermediates. I believe that this tutorial gives a concise and comprehensible introduction on the electronic structure of carbene ligands and helps to unveil how electronic effects translate into chemical behavior. The conceptual analysis of a variety of carbene ligands and their coordination chemistry can serve as an expedient guide to understand design principles and choice of specific carbene ligand for a particular application. Thus, I hope that chemists will be inspired to hop into the adventurous playground of carbene ligands, “push electrons”, and to discover the vast worlds beyond conventional NHCs and classic Fischer or Schrock carbene ligands.
he isolation and scrutiny of metal carbene complexes is one of the core disciplines of organometallic chemistry and hence lies at the heart of Organometallics. Research has focused for many years on the development of carbon atom transfer reactivity after Fischer’s discovery of heteroatomstabilized nucleophilic carbene complexes.1 These efforts culminated in the development of Tebbe’s reagent2 for methylene transfer as well as the application of carbene catalysts for cyclopropanation,3 C−H insertion,4 and the Nobel Prize winning olefin metathesis reaction.5 The field started to pick up even more speed with the isolation of the first stable acyclic free carbene, reported by Bertrand in the 1980s.6 Synthesis of Arduengo’s crystalline Nheterocyclic carbene (NHC) in 1991, also known as “bottleable carbene”, eventually demonstrated that persistent carbenes are not merely laboratory curiosities.7 Following these seminal discoveries, heterocyclic carbene ligands were introduced as ancillary ligands for catalysis, and the number of reports on NHC metal complexes skyrocketed. The success story of NHC ligands was, and still is, strongly connected with the names of Herrmann and Nolan.8 Subsequently, further applications emerged in the fields of light/energy conversion and drug design.9 Computational chemistry proved essential for the development of new carbenes and offered tools to decipher their electronic structures, causing carbene stability and reactivity trends.11 After the early days of transition metal− NHC chemistry, it was again Bertrand’s group who was pushing the field, and within a few years, they isolated a remarkable series of stable acyclic and cyclic free carbenes.10 These “unconventional” carbenes and derivatives showed a huge diversity of electronic properties, which was unprecedented for conventional NHCs. Currently, this new class of carbenes finds increasing application in organometallic coordination chemistry. Outstanding examples include the stabilization of radicals and low-valent complexes by cyclic alkyl amino carbene (CAAC) ligands or the isolation of metal complexes with nucleophilic carbene ligands derived from carbodiphosphoranes and carbodicarbenes. In fact, “classic” Schrock-type carbenes also are experiencing a remarkable renaissance. For instance, heterogeneous Schrock carbenes were shown to catalyze the metathesis of alkanes,12 and smart ligand design13 allowed for the development of stereoselective olefin metathesis catalysts. Historically, traditional NHC ligands were believed to be pure σ-donors.14 Accordingly, differences to Fischer- and especially Schrock-type carbenes were overemphasized, and they were classified as a new type of ligand. However, spotting the true differences between carbene ligands is very difficult, complex, and proved to be a challenging task.15 Can diarylcarbenes be considered Schrock carbenes? Is a mesoionic carbene really a carbene or rather a heteroaryl-substituent? Is a particular carbene ligand a surprisingly strong π-acceptor (or is this behavior not surprising at all)? Is it a carbene, carbenoid, or carbocation? Is it a Schrock carbene, alkylidene, or nucleophilic carbene complex? How important are electrostatic considerations? What is the oxidation state of the coordinating metal © 2018 American Chemical Society
Karsten Meyer*
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Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 1, 91058 Erlangen, Germany
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Karsten Meyer: 0000-0002-7844-2998 Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS.
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REFERENCES
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DOI: 10.1021/acs.organomet.8b00014 Organometallics 2018, 37, 273−274
Organometallics
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DOI: 10.1021/acs.organomet.8b00014 Organometallics 2018, 37, 273−274
