Editorial Cite This: Chem. Rev. 2018, 118, 9455−9456
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Introduction: Carbene Chemistry
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properties and reactivity patterns to MICs coordinated to transition metals. Complexes of MICs and related less heteroatom-stabilized N-heterocyclic carbenes are discussed in the review. Over the last years, the coordination chemistry of poly-NHC ligands has developed rapidly. The use of such poly-NHC ligands for the preparation of discrete metallosupramolecular assemblies is covered by a review by Y.-F. Han and co-workers. Specific features such as self-sorting of poly-NHC ligands during the synthesis of metallosupramolecular assemblies and postsynthetic modifications on such assemblies are also covered by the review. Recently, complexes bearing protic NHC ligands (pNHCs), i.e. NHCs featuring hydrogen atoms at the N-wingtips, have received increased attention. Complexes of such carbenes possess an N−H molecular recognition unit in close proximity to the metal center. The N−H group can be deprotonated to yield the complex with a Lewis-basic ring-nitrogen atom and a Lewis-acidic metal center. Such complexes are capable of cooperative substrate activation and have been reviewed by S. Kuwata and F. E. Hahn. Electron-rich NHCs have been used for the stabilization of various reactive intermediates both in transition metal chemistry but also for p-element compounds. A review by S. Inoue and co-workers on NHCs in low-coordinate main group element compounds summarizes recent developments in this area. It covers NHC adducts of groups 1, 2, and 13−17. Immediately after the first NHCs were prepared, their potential as spectator ligands for the preparation of catalytically active metal complexes has been established. A review by F. Li and co-workers deals with reusable NHC complex catalysts. The review summarizes recycling strategies for organometallic catalysts in general and discusses in detail advantages and disadvantages of various recycling strategies for specific reactions. High oxidation state 3d metal complexes with NHC ligation have been reviewed by L. Deng and co-workers. The review covers the preparation, properties, and selected catalytic applications for high oxidation state 3d metal NHC complexes. Finally, “smart” NHCs, i.e. NHC ligands that are capable to adapt their properties to the specific requirements of individual catalytic transformations, have been reviewed by E. Peris. The review covers photoswitchable, redox-switchable, and chemoactive NHCs. This thematic issue would not have been realized without the support of the editorial staff of Chemical Reviews and particularly to Dr. Michele Soleilhavoup, whom the guest editor sincerely thanks. Writing a comprehensive review is challenging, so thanks are also due to all participating authors. Given the still expanding interest in NHCs and their metal complexes, it is hoped that this thematic issue will provide a useful overview for the interested reader and will be thoughtprovoking for those who enter the field.
arely since the introduction of the cyclopentadienyl derivatives has a new type of ligand captured the imagination of chemists as was the case with the Nheterocyclic carbenes (NHCs) and other divalent carbon species. While numerous tetravalent carbon compounds in which all four valence electrons of carbon are engaged in bonding are known, those considered to have only two electrons involved in bonding while two nonbonding electrons remain at the carbon atom are rare. These divalent carbon derivatives comprise carbon monoxide, isocyanides, and carbenes. While carbon monoxide and stable isocyanides have been known for more than a century, carbenes have initially only been observed when stabilized in metal complexes.1−3 However, as early as 1960 Wanzlick proposed the synthesis of an imidazolidin-2-ylidene by α-elimination of chloroform from an imidazoline derivative but could only isolate the dimeric enetetramine.4 Pioneering work by Bertrand and co-workers in 1988 provided the stable [bis(diisopropylamino)phosphino](trimethylsilyl)carbene, stabilized by phosphorus and silicon substituents adjacent to the carbene center.5 Three years later, the isolation and characterization of 1,3-bis(adamantyl)imidazol-2-ylidene by Arduengo and co-workers provided the first example for a free and stable “bottle-able” N-heterocyclic carbene.6 The report by Arduengo et al. from 1991 initiated intensive research on these compounds and their metal complexes which now has lasted unabated for more than 25 years. Over this period, various types of cyclic diaminocarbenes derived from smaller or larger diaminoheterocycles than the originally used imidazoline derivatives have been prepared. In addition, different heteroatoms have been introduced into heterocyclic carbenes. From transient laboratory curiosities, carbenes have developed into a diverse and important class of carbon(II) donor ligands. They have found multiple applications as sterically unique spectator ligands for the preparation of catalytically active metal complexes. Organometallic metallodrugs have been developed using NHC ligands and poly-NHC ligands have yielded various discrete metallosupramolecular assemblies. This thematic issue provides reviews on selected topics of the still rapidly developing carbene chemistry. Next to some senior researchers, early career scientists, who will define carbene chemistry in the future, have been invited to contribute. The electronic properties of NHCs are reviewed by H. V. Huynh by discussing the most common and popular methods for the experimental determination of their donor-strength. Each individual method and the underlying principles of detection it uses are highlighted in terms of strength and weakness. The author places particular emphasis on the use of 13 C NMR spectroscopy and compiles data for various NHCs for comparison. Mesoionic carbenes (MICs), a subclass of the family of Nheterocyclic carbenes, form the topic of the review by M. Albrecht and co-workers. The reduced heteroatom stabilization of the carbenic carbon atom imparts specific donor © 2018 American Chemical Society
Special Issue: Carbene Chemistry Published: October 10, 2018 9455
DOI: 10.1021/acs.chemrev.8b00495 Chem. Rev. 2018, 118, 9455−9456
Chemical Reviews
Editorial
F. Ekkehardt Hahn* Westfälische Wilhelms-Universität Münster
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
F. Ekkehardt Hahn: 0000-0002-2807-7232 Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS. Biography
F. Ekkehardt Hahn was born in Jena, Germany in 1955. He studied chemistry at the Technische Universität Berlin and the University of Oklahoma, receiving his Dr. rer. nat. degree from the Technical University Berlin in 1985. The same year he moved to UC Berkeley as a postdoctoral associate in the group of K. N. Raymond. After returning to the Technical University Berlin in 1988, he completed the habilitation in 1990. In 1992 he moved to the Free University Berlin as a Professor of Inorganic Chemistry before accepting the position as Chair in Inorganic Chemistry at the University of Münster in 1998. His research focuses on the chemistry of divalent group 14 compounds with special emphasis on the coordination chemistry of isocyanides and (protic) N-heterocyclic carbenes. Since 2004 he acts as Permanent Secretary of the International Conference of Organometallic Chemistry (ICOMC).
REFERENCES (1) Wanzlick, H.-W.; Schönherr, H.-J. Direct Synthesis of a Mercury Salt-Carbene Complex. Angew. Chem., Int. Ed. Engl. 1968, 7, 141−142. (2) Ö fele, K. 1,3-Dimethyl-4-imidazolinyliden-(2)-pentacarbonylchrom ein neuer Ü bergangsmetall−Carben-Komplex. J. Organomet. Chem. 1968, 12, P42−P43. (3) Cardin, D. J.; Cetinkaya, B.; Lappert, M. F.; Manojlović-Muir, L.; Muir, K. W. An Electron-rich Olefin as a Source of Co-ordinated Carbene; Synthesis of trans-PtCl2[C(NPhCH2)2]PEt3. Chem. Commun. 1971, 400−401. (4) Wanzlick, H.-W.; Schikora, E. Ein neuer Zugang zur CarbenChemie. Angew. Chem. 1960, 72, 494. (5) Igau, A.; Grutzmacher, H.; Baceiredo, A.; Bertrand, G. Analogous α,α’-Bis-Carbenoid Triply Bonded Species: Synthesis of a Stable λ3-Phosphinocarbene−λ5-Phosphaacetylene. J. Am. Chem. Soc. 1988, 110, 6463−6466. (6) Arduengo, A. J., III; Harlow, R. L.; Kline, M. A Stable Crystalline Carbene. J. Am. Chem. Soc. 1991, 113, 361−363.
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DOI: 10.1021/acs.chemrev.8b00495 Chem. Rev. 2018, 118, 9455−9456
