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Carbon Ligands: From Fundamental Aspects to Applications
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Carbon Ligands: From Fundamental Aspects to Applications

A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Inorganic Chemistry".

Deadline for manuscript submissions: closed (30 September 2020) | Viewed by 53064

Printed Edition Available!
A printed edition of this Special Issue is available here.

Special Issue Editor

Dr. Yves Canac

E-Mail Website
Guest Editor
Laboratoire de Chimie de Coordination du CNRS (LCC), UPR–8241, 205, route de Narbonne, 31077 Toulouse, CEDEX 04, France
Interests: My research activities focus on main group chemistry, coordination chemistry, and homogeneous catalysis. Special attention is given on the design, the coordination, and the catalytic properties of electron-rich neutral carbon ligands (carbenes, phosphonium ylides) and electron-poor cationic phosphorus ligands (cyclopropenio- and imidazoliophosphines)

Special Issue Information

Dear Colleagues,

Homogeneous catalysis owes its tremendous development to the advent of a wide range of ligands with well-defined electronic and steric properties, which can finely control the reactivity of organometallic complexes. The ligands involved in catalysis have been centered for a long time on elements of group 15, and it is only more recently that carbon representatives have proved to be valuable alternatives with the emergence of the famous cyclic diaminocarbenes (NHCs). Beyond their activity, the predominance of N- and P-based ligands can be explained by their high stability as opposed to the carbon ligands that have long been thought to be unstable. The isolation of the first stable carbenes proved to be a trigger in the minds of chemists, leading in a short time to remarkable advances in the field of homogeneous catalysis. From there, considerable efforts have been undertaken and carbene ligands have become unavoidable surpassing even their illustrious predecessors in some transformations. Like carbenic species (NHCs, aNHCs (MICS), CAACs, BACs, non-NHCs, etc.), onium (P+, N+, S+) ylides and related (bis-ylides, carbones, carbodiphosphoranes, carbodicarbenes, NHOs, etc.) which are charge-neutral in their free state and act as strong σ-donor ligands, have experienced a revival of interest, thus, confirming their potential as Lewis bases in main group chemistry, coordination chemistry, and in homogeneous catalysis.

The aim of this Special Issue is to provide a contemporary overview of the advances in carbon ligand chemistry. From fundamental aspects to applications, all contributions involving carbon ligands, where the coordinating carbon atom is either of C-sp2 type as in carbenic derivatives, or of C-sp3 type as in ylidic species, are thus welcome. More generally, all carbon ligands regardless of the nature of the coordinating carbon center will also be of major interest for this special issue.

Dr. Yves Canac
Guest Editor

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Keywords

  • Carbon ligand
  • Carbene
  • Ylide
  • Carbon–metal bond
  • Coordination mode
  • Organometallic complex
  • Main group chemistry
  • Coordination chemistry
  • Homogeneous catalysis

Published Papers (16 papers)

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Editorial

Jump to: Research, Review

3 pages, 198 KiB  
Editorial
Carbon Ligands: From Fundamental Aspects to Applications
by Yves Canac
Molecules 2021, 26(8), 2132; https://doi.org/10.3390/molecules26082132 - 08 Apr 2021
Cited by 3 | Viewed by 1361
Abstract
Ligand design is at the forefront of many advances in various areas of chemistry such as organometallic chemistry, functional materials, and homogeneous catalysis [...] Full article
(This article belongs to the Special Issue Carbon Ligands: From Fundamental Aspects to Applications)

Research

Jump to: Editorial, Review

9 pages, 1584 KiB  
Article
Complexes of Dichlorogermylene with Phosphine/Sulfoxide-Supported Carbone as Ligand
by Ugo Authesserre, Sophie Hameury, Aymeric Dajnak, Nathalie Saffon-Merceron, Antoine Baceiredo, David Madec and Eddy Maerten
Molecules 2021, 26(7), 2005; https://doi.org/10.3390/molecules26072005 - 01 Apr 2021
Cited by 2 | Viewed by 2056
Abstract
Due to their remarkable electronic features, recent years have witnessed the emergence of carbones L2C, which consist in two donating L ligands coordinating a central carbon atom bearing two lone pairs. In this context, the phosphine/sulfoxide-supported carbone 4 exhibits a strong [...] Read more.
Due to their remarkable electronic features, recent years have witnessed the emergence of carbones L2C, which consist in two donating L ligands coordinating a central carbon atom bearing two lone pairs. In this context, the phosphine/sulfoxide-supported carbone 4 exhibits a strong nucleophilic character, and here, we describe its ability to coordinate dichlorogermylene. Two original stable coordination complexes were obtained and fully characterized in solution and in the solid state by NMR spectroscopy and X-ray diffraction analysis, respectively. At 60 °C, in the presence of 4, the Ge(II)-complex 5 undergoes a slow isomerization that transforms the bis-ylide ligand into an yldiide. Full article
(This article belongs to the Special Issue Carbon Ligands: From Fundamental Aspects to Applications)
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12 pages, 1996 KiB  
Article
The Fascinating Flexibility and Coordination Modes of a Pentamethylene Connected Macrocyclic CNC Pincer Ligand
by Ronja Jordan and Doris Kunz
Molecules 2021, 26(6), 1669; https://doi.org/10.3390/molecules26061669 - 17 Mar 2021
Cited by 7 | Viewed by 2269
Abstract
The coordination chemistry of an electron-rich macrocyclic CNC pincer-ligand consisting of two pentamethylene tethered N-heterocyclic carbene moieties on a carbazole backbone (bimcaC5) is investigated by mainly NMR spectroscopy and X-ray crystal structure analysis. A bridging coordination mode is found for the [...] Read more.
The coordination chemistry of an electron-rich macrocyclic CNC pincer-ligand consisting of two pentamethylene tethered N-heterocyclic carbene moieties on a carbazole backbone (bimcaC5) is investigated by mainly NMR spectroscopy and X-ray crystal structure analysis. A bridging coordination mode is found for the lithium complex. With the larger and softer potassium ion, the ligand adopts a facial coordination mode and a polymeric structure by intermolecular potassium nitrogen interactions. The facial coordination is also confirmed at a Cp*Ru fragment, while C-H activation under dehydrogenation at the alkyl chain is observed upon reaction with [Ru(PPh3)3Cl2]. In contrast, Pd(OAc)2 reacts under C-H activation at the central carbon atom of the pentamethylene tether to an alkyl-pincer macrocycle. Full article
(This article belongs to the Special Issue Carbon Ligands: From Fundamental Aspects to Applications)
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23 pages, 4098 KiB  
Article
Synthesis and Characterization of Ion Pairs between Alkaline Metal Ions and Anionic Anti-Aromatic and Aromatic Hydrocarbons with π-Conjugated Central Seven- and Eight-Membered Rings
by Jan Bloch, Stefan Kradolfer, Thomas L. Gianetti, Detlev Ostendorf, Subal Dey, Victor Mougel and Hansjörg Grützmacher
Molecules 2020, 25(20), 4742; https://doi.org/10.3390/molecules25204742 - 15 Oct 2020
Cited by 6 | Viewed by 3456
Abstract
The synthesis, isolation and full characterization of ion pairs between alkaline metal ions (Li+, Na+, K+) and mono-anions and dianions obtained from 5H-dibenzo[a,d]cycloheptenyl (C15H11 = trop) is reported. According [...] Read more.
The synthesis, isolation and full characterization of ion pairs between alkaline metal ions (Li+, Na+, K+) and mono-anions and dianions obtained from 5H-dibenzo[a,d]cycloheptenyl (C15H11 = trop) is reported. According to Nuclear Magnetic Resonance (NMR) spectroscopy, single crystal X-ray analysis and Density Functional Theory (DFT) calculations, the trop‒ and trop2−• anions show anti-aromatic properties which are dependent on the counter cation M+ and solvent molecules serving as co-ligands. For comparison, the disodium and dipotassium salt of the dianion of dibenzo[a,e]cyclooctatetraene (C16H12 = dbcot) were prepared, which show classical aromatic character. A d8-Rh(I) complex of trop− was prepared and the structure shows a distortion of the C15H11 ligand into a conjugated 10π -benzo pentadienide unit—to which the Rh(I) center is coordinated—and an aromatic 6π electron benzo group which is non-coordinated. Electron transfer reactions between neutral and anionic trop and dbcot species show that the anti-aromatic compounds obtained from trop are significantly stronger reductants. Full article
(This article belongs to the Special Issue Carbon Ligands: From Fundamental Aspects to Applications)
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21 pages, 3092 KiB  
Article
Unsaturated and Benzannulated N-Heterocyclic Carbene Complexes of Titanium and Hafnium: Impact on Catalysts Structure and Performance in Copolymerization of Cyclohexene Oxide with CO2
by Lakshmi Suresh, Ralte Lalrempuia, Jonas B. Ekeli, Francis Gillis-D’Hamers, Karl W. Törnroos, Vidar R. Jensen and Erwan Le Roux
Molecules 2020, 25(19), 4364; https://doi.org/10.3390/molecules25194364 - 23 Sep 2020
Cited by 8 | Viewed by 3171
Abstract
Tridentate, bis-phenolate N-heterocyclic carbenes (NHCs) are among the ligands giving the most selective and active group 4-based catalysts for the copolymerization of cyclohexene oxide (CHO) with CO2. In particular, ligands based on imidazolidin-2-ylidene (saturated NHC) moieties have given catalysts which [...] Read more.
Tridentate, bis-phenolate N-heterocyclic carbenes (NHCs) are among the ligands giving the most selective and active group 4-based catalysts for the copolymerization of cyclohexene oxide (CHO) with CO2. In particular, ligands based on imidazolidin-2-ylidene (saturated NHC) moieties have given catalysts which exclusively form polycarbonate in moderate-to-high yields even under low CO2 pressure and at low copolymerization temperatures. Here, to evaluate the influence of the NHC moiety on the molecular structure of the catalyst and its performance in copolymerization, we extend this chemistry by synthesizing and characterizing titanium complexes bearing tridentate bis-phenolate imidazol-2-ylidene (unsaturated NHC) and benzimidazol-2-ylidene (benzannulated NHC) ligands. The electronic properties of the ligands and the nature of their bonds to titanium are studied using density functional theory (DFT) and natural bond orbital (NBO) analysis. The metal–NHC bond distances and bond strengths are governed by ligand-to-metal σ- and π-donation, whereas back-donation directly from the metal to the NHC ligand seems to be less important. The NHC π-acceptor orbitals are still involved in bonding, as they interact with THF and isopropoxide oxygen lone-pair donor orbitals. The new complexes are, when combined with [PPN]Cl co-catalyst, selective in polycarbonate formation. The highest activity, albeit lower than that of the previously reported Ti catalysts based on saturated NHC, was obtained with the benzannulated NHC-Ti catalyst. Attempts to synthesize unsaturated and benzannulated NHC analogues based on Hf invariably led, as in earlier work with Zr, to a mixture of products that include zwitterionic and homoleptic complexes. However, the benzannulated NHC-Hf complexes were obtained as the major products, allowing for isolation. Although these complexes selectively form polycarbonate, their catalytic performance is inferior to that of analogues based on saturated NHC. Full article
(This article belongs to the Special Issue Carbon Ligands: From Fundamental Aspects to Applications)
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15 pages, 3305 KiB  
Article
Cu(I) Complexes of Multidentate N,C,N- and P,C,P-Carbodiphosphorane Ligands and Their Photoluminescence
by Marius Klein, Nemrud Demirel, Alexander Schinabeck, Hartmut Yersin and Jörg Sundermeyer
Molecules 2020, 25(17), 3990; https://doi.org/10.3390/molecules25173990 - 01 Sep 2020
Cited by 9 | Viewed by 3435
Abstract
A series of dinuclear copper(I) N,C,N- and P,C,P-carbodiphosphorane (CDP) complexes using multidentate ligands CDP(Py)2 (1) and (CDP(CH2PPh2)2 (13) have been isolated and characterized. Detailed structural information was gained by single-crystal XRD analyses [...] Read more.
A series of dinuclear copper(I) N,C,N- and P,C,P-carbodiphosphorane (CDP) complexes using multidentate ligands CDP(Py)2 (1) and (CDP(CH2PPh2)2 (13) have been isolated and characterized. Detailed structural information was gained by single-crystal XRD analyses of nine representative examples. The common structural motive is the central double ylidic carbon atom with its characteristic two lone pairs involved in the binding of two geminal L-Cu(I) fragments at Cu–Cu distances in the range 2.55–2.67 Å. In order to enhance conformational rigidity within the characteristic Cu–C–Cu triangle, two types of chelating side arms were symmetrically attached to each phosphorus atom: two 2-pyridyl functions in ligand CDP(Py)2 (1) and its dinuclear copper complexes 2–9 and 11, as well as two diphenylphosphinomethylene functions in ligand CDP(CH2PPh2)2 (13) and its di- and mononuclear complexes 14–18. Neutral complexes were typically obtained via the reaction of 1 with Cu(I) species CuCl, CuI, and CuSPh or via the salt elimination reaction of [(CuCl)2(CDP(Py)2] (2) with sodium carbazolate. Cationic Cu(I) complexes were prepared upon treating 1 with two equivalents of [Cu(NCMe)4]PF6, followed by the addition of either two equivalents of an aryl phosphine (PPh3, P(C6H4OMe)3) or one equivalent of bisphosphine ligands bis[(2-diphenylphosphino)phenyl] ether (DPEPhos), 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (XantPhos), or 1,1′-bis(diphenyl-phosphino) ferrocene (dppf). For the first time, carbodiphosphorane CDP(CH2PPh2)2 (13) could be isolated upon treating its precursor [CH(dppm)2]Cl (12) with NaNH2 in liquid NH3. A protonated and a deprotonated derivative of ligand 13 were prepared, and their coordination was compared to neutral CDP ligand 13. NMR analysis and DFT calculations reveal that the most stable tautomer of 13 does not show a CDP (or carbone) structure in its uncoordinated base form. For most of the prepared complexes, photoluminescence upon irradiation with UV light at room temperature was observed. Quantum yields (ΦPL) were determined to be 36% for dicationic [(CuPPh3)2(CDP(Py)2)](PF6)2 (4) and 60% for neutral [(CuSPh)2(CDP(CH2PPh2)2] (16). Full article
(This article belongs to the Special Issue Carbon Ligands: From Fundamental Aspects to Applications)
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Graphical abstract

10 pages, 1963 KiB  
Article
Mercury(II) Complexes of Anionic N-Heterocyclic Carbene Ligands: Steric Effects of the Backbone Substituent
by Chandrakanta Dash, Animesh Das and H. V. Rasika Dias
Molecules 2020, 25(16), 3741; https://doi.org/10.3390/molecules25163741 - 16 Aug 2020
Cited by 4 | Viewed by 3206
Abstract
Mercury(II) complexes (Me-maloNHCDipp)HgCl (1b), (t-Bu-maloNHCDipp)HgCl (2b) and (t-Bu-maloNHCDipp)HgMe (2c) supported by anionic N-heterocyclic carbenes have been obtained in good yields from the reaction of the potassium [...] Read more.
Mercury(II) complexes (Me-maloNHCDipp)HgCl (1b), (t-Bu-maloNHCDipp)HgCl (2b) and (t-Bu-maloNHCDipp)HgMe (2c) supported by anionic N-heterocyclic carbenes have been obtained in good yields from the reaction of the potassium salt of N-heterocyclic carbene ligand precursors and mercury(II) salts, HgCl2 and MeHgI. These molecules have been characterized by 1H-NMR, 13C-NMR and IR spectroscopy and elemental analysis. X-ray crystal structures of 1b and 2b are also presented. Interestingly, complex 1b is polymeric {(Me-maloNHCDipp)HgCl}n in the solid state, as a result of inter-molecular Hg-O contacts, and features rare three coordinate mercury sites with a T-shaped arrangement, whereas the (t-Bu-maloNHCDipp)HgCl (2b) is monomeric and has a linear, two-coordinate mercury center. The formation of T-shaped structure and the aggregation of complex 1b is attributable to the reduced steric demand of the N-heterocyclic carbene ligand backbone substituent. Full article
(This article belongs to the Special Issue Carbon Ligands: From Fundamental Aspects to Applications)
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15 pages, 2174 KiB  
Article
Syntheses and Reactivity of New Zwitterionic Imidazolium Trihydridoborate and Triphenylborate Species
by Maura Pellei, Riccardo Vallesi, Luca Bagnarelli, H. V. Rasika Dias and Carlo Santini
Molecules 2020, 25(14), 3184; https://doi.org/10.3390/molecules25143184 - 13 Jul 2020
Cited by 1 | Viewed by 3104
Abstract
In this study, four new N-(alkyl/aryl)imidazolium-borates were prepared, and their deprotonation reactions were investigated. Addition of BH3•THF to N-benzylimidazoles and N-mesitylimidazoles leads to imidazolium-trihydridoborate adducts. Ammonium tetraphenylborate reacts with benzyl- or mesityl-imidazoles with the loss of one of [...] Read more.
In this study, four new N-(alkyl/aryl)imidazolium-borates were prepared, and their deprotonation reactions were investigated. Addition of BH3•THF to N-benzylimidazoles and N-mesitylimidazoles leads to imidazolium-trihydridoborate adducts. Ammonium tetraphenylborate reacts with benzyl- or mesityl-imidazoles with the loss of one of the phenyl groups yielding the corresponding imidazolium-triphenylborates. Their authenticity was confirmed by CHN analysis, 1H-NMR, 13C-NMR, 11B-NMR, FT-IR spectroscopy, and electrospray ionization mass spectrometry (ESI-MS). 3-Benzyl-imidazolium-1-yl)trihydridoborate, (HImBn)BH3, and (3-mesityl-imidazolium-1-yl)trihydridoborate, (HImMes)BH3, were also characterized by X-ray crystallography. The reactivity of these new compounds as carbene precursors in an effort to obtain borate-NHC complexes was investigated and a new carbene-borate adduct (which dimerizes) was obtained via a microwave-assisted procedure. Full article
(This article belongs to the Special Issue Carbon Ligands: From Fundamental Aspects to Applications)
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10 pages, 2001 KiB  
Article
N-Heterocyclic Carbene Platinum(IV) as Metallodrug Candidates: Synthesis and 195Pt NMR Chemical Shift Trend
by Mathilde Bouché, Bruno Vincent, Thierry Achard and Stéphane Bellemin-Laponnaz
Molecules 2020, 25(14), 3148; https://doi.org/10.3390/molecules25143148 - 09 Jul 2020
Cited by 4 | Viewed by 2700
Abstract
A series of octahedral platinum(IV) complexes functionalized with both N-heterocyclic carbene (NHC) ligands were synthesized according to a straightforward procedure and characterized. The coordination sphere around the metal was varied, investigating the influence of the substituted NHC and the amine ligand in [...] Read more.
A series of octahedral platinum(IV) complexes functionalized with both N-heterocyclic carbene (NHC) ligands were synthesized according to a straightforward procedure and characterized. The coordination sphere around the metal was varied, investigating the influence of the substituted NHC and the amine ligand in trans position to the NHC. The influence of those structural variations on the chemical shift of the platinum center were evaluated by 195Pt NMR. This spectroscopy provided more insights on the impact of the structural changes on the electronic density at the platinum center. Investigation of the in vitro cytotoxicities of representative complexes were carried on three cancer cell lines and showed IC50 values down to the low micromolar range that compare favorably with the benchmark cisplatin or their platinum(II) counterparts bearing NHC ligands. Full article
(This article belongs to the Special Issue Carbon Ligands: From Fundamental Aspects to Applications)
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Figure 1

20 pages, 527 KiB  
Article
Hybrid Gold(I) NHC-Artemether Complexes to Target Falciparum Malaria Parasites
by Manel Ouji, Guillaume Barnoin, Álvaro Fernández Álvarez, Jean-Michel Augereau, Catherine Hemmert, Françoise Benoit-Vical and Heinz Gornitzka
Molecules 2020, 25(12), 2817; https://doi.org/10.3390/molecules25122817 - 18 Jun 2020
Cited by 11 | Viewed by 2270
Abstract
The emergence of Plasmodium falciparum parasites, responsible for malaria disease, resistant to antiplasmodial drugs including the artemisinins, represents a major threat to public health. Therefore, the development of new antimalarial drugs or combinations is urgently required. In this context, several hybrid molecules combining [...] Read more.
The emergence of Plasmodium falciparum parasites, responsible for malaria disease, resistant to antiplasmodial drugs including the artemisinins, represents a major threat to public health. Therefore, the development of new antimalarial drugs or combinations is urgently required. In this context, several hybrid molecules combining a dihydroartemisinin derivative and gold(I) N-heterocyclic carbene (NHC) complexes have been synthesized based on the different modes of action of the two compounds. The antiplasmodial activity of these molecules was assessed in vitro as well as their cytotoxicity against mammalian cells. All the hybrid molecules tested showed efficacy against P. falciparum, in a nanomolar range for the most active, associated with a low cytotoxicity. However, cross-resistance between artemisinin and these hybrid molecules was evidenced. These results underline a fear about the risk of cross-resistance between artemisinins and new antimalarial drugs based on an endoperoxide part. This study thus raises concerns about the use of such molecules in future therapeutic malaria policies. Full article
(This article belongs to the Special Issue Carbon Ligands: From Fundamental Aspects to Applications)
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20 pages, 3327 KiB  
Article
The Influence of Various N-Heterocyclic Carbene Ligands on Activity of Nitro-Activated Olefin Metathesis Catalysts
by Michał Pieczykolan, Justyna Czaban-Jóźwiak, Maura Malinska, Krzysztof Woźniak, Reto Dorta, Anna Rybicka, Anna Kajetanowicz and Karol Grela
Molecules 2020, 25(10), 2282; https://doi.org/10.3390/molecules25102282 - 12 May 2020
Cited by 7 | Viewed by 3359
Abstract
A set of nitro-activated ruthenium-based Hoveyda-Grubbs type olefin metathesis catalysts bearing sterically modified N-hetero-cyclic carbene (NHC) ligands have been obtained, characterised and studied in a set of model metathesis reactions. It was found that catalysts bearing standard SIMes and SIPr ligands ( [...] Read more.
A set of nitro-activated ruthenium-based Hoveyda-Grubbs type olefin metathesis catalysts bearing sterically modified N-hetero-cyclic carbene (NHC) ligands have been obtained, characterised and studied in a set of model metathesis reactions. It was found that catalysts bearing standard SIMes and SIPr ligands (4a and 4b) gave the best results in metathesis of substrates with more accessible C–C double bonds. At the same time, catalysts bearing engineered naphthyl-substituted NHC ligands (4d–e) exhibited high activity towards formation of tetrasubstituted C–C double bonds, the reaction which was traditionally Achilles’ heel of the nitro-activated Hoveyda–Grubbs catalyst. Full article
(This article belongs to the Special Issue Carbon Ligands: From Fundamental Aspects to Applications)
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Graphical abstract

14 pages, 2397 KiB  
Article
A Self-Assembling NHC-Pd-Loaded Calixarene as a Potent Catalyst for the Suzuki-Miyaura Cross-Coupling Reaction in Water
by Arnaud Peramo, Ibrahim Abdellah, Shannon Pecnard, Julie Mougin, Cyril Martini, Patrick Couvreur, Vincent Huc and Didier Desmaële
Molecules 2020, 25(6), 1459; https://doi.org/10.3390/molecules25061459 - 24 Mar 2020
Cited by 12 | Viewed by 4359
Abstract
Nanoformulated calix[8]arenes functionalized with N-heterocyclic carbene (NHC)-palladium complexes were found to be efficient nano-reactors for Suzuki-Miyaura cross-coupling reactions of water soluble iodo- and bromoaryl compounds with cyclic triol arylborates at low temperature in water without any organic co-solvent. Combined with an improved [...] Read more.
Nanoformulated calix[8]arenes functionalized with N-heterocyclic carbene (NHC)-palladium complexes were found to be efficient nano-reactors for Suzuki-Miyaura cross-coupling reactions of water soluble iodo- and bromoaryl compounds with cyclic triol arylborates at low temperature in water without any organic co-solvent. Combined with an improved one-step synthesis of triol arylborates from boronic acid, this remarkably efficient new tool provided a variety of 4′-arylated phenylalanines and tyrosines in good yields at low catalyst loading with a wide functional group tolerance. Full article
(This article belongs to the Special Issue Carbon Ligands: From Fundamental Aspects to Applications)
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12 pages, 2874 KiB  
Article
Towards the Preparation of Stable Cyclic Amino(ylide)Carbenes
by Henning Steinert, Christopher Schwarz, Alexander Kroll and Viktoria H. Gessner
Molecules 2020, 25(4), 796; https://doi.org/10.3390/molecules25040796 - 12 Feb 2020
Cited by 7 | Viewed by 4009
Abstract
Cyclic amino(ylide)carbenes (CAYCs) are the ylide-substituted analogues of N-heterocyclic Carbenes (NHCs). Due to the stronger π donation of the ylide compared to an amino moiety they are stronger donors and thus are desirable ligands for catalysis. However, no stable CAYC has been [...] Read more.
Cyclic amino(ylide)carbenes (CAYCs) are the ylide-substituted analogues of N-heterocyclic Carbenes (NHCs). Due to the stronger π donation of the ylide compared to an amino moiety they are stronger donors and thus are desirable ligands for catalysis. However, no stable CAYC has been reported until today. Here, we describe experimental and computational studies on the synthesis and stability of CAYCs based on pyrroles with trialkyl onium groups. Attempts to isolate two CAYCs with trialkyl phosphonium and sulfonium ylides resulted in the deprotonation of the alkyl groups instead of the formation of the desired CAYCs. In case of the PCy3-substituted system, the corresponding ylide was isolated, while deprotonation of the SMe2-functionalized compound led to the formation of ethene and the thioether. Detailed computational studies on various trialkyl onium groups showed that both the α- and β-deprotonated compounds were energetically favored over the free carbene. The most stable candidates were revealed to be α-hydrogen-free adamantyl-substituted onium groups, for which β-deprotonation is less favorable at the bridgehead position. Overall, the calculations showed that the isolation of CAYCs should be possible, but careful design is required to exclude decomposition pathways such as deprotonations at the onium group. Full article
(This article belongs to the Special Issue Carbon Ligands: From Fundamental Aspects to Applications)
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Review

Jump to: Editorial, Research

48 pages, 13226 KiB  
Review
Carbones and Carbon Atom as Ligands in Transition Metal Complexes
by Lili Zhao, Chaoqun Chai, Wolfgang Petz and Gernot Frenking
Molecules 2020, 25(21), 4943; https://doi.org/10.3390/molecules25214943 - 26 Oct 2020
Cited by 41 | Viewed by 5403
Abstract
This review summarizes experimental and theoretical studies of transition metal complexes with two types of novel metal-carbon bonds. One type features complexes with carbones CL2 as ligands, where the carbon(0) atom has two electron lone pairs which engage in double (σ and [...] Read more.
This review summarizes experimental and theoretical studies of transition metal complexes with two types of novel metal-carbon bonds. One type features complexes with carbones CL2 as ligands, where the carbon(0) atom has two electron lone pairs which engage in double (σ and π) donation to the metal atom [M]⇇CL2. The second part of this review reports complexes which have a neutral carbon atom C as ligand. Carbido complexes with naked carbon atoms may be considered as endpoint of the series [M]-CR3 → [M]-CR2 → [M]-CR → [M]-C. This review includes some work on uranium and cerium complexes, but it does not present a complete coverage of actinide and lanthanide complexes with carbone or carbide ligands. Full article
(This article belongs to the Special Issue Carbon Ligands: From Fundamental Aspects to Applications)
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45 pages, 17735 KiB  
Review
Selenonium Ylides: Syntheses, Structural Aspects, and Synthetic Applications
by Józef Drabowicz, Aneta Rzewnicka and Remigiusz Żurawiński
Molecules 2020, 25(10), 2420; https://doi.org/10.3390/molecules25102420 - 22 May 2020
Cited by 9 | Viewed by 2927
Abstract
The goals of this mini review constitute (a) a presentation of the synthetic protocols applied to the preparation of achiral and non-racemic selenonium ylides; (b) discussion of their basic structural features, including their optical activity; and (c) a description of their synthetic applications [...] Read more.
The goals of this mini review constitute (a) a presentation of the synthetic protocols applied to the preparation of achiral and non-racemic selenonium ylides; (b) discussion of their basic structural features, including their optical activity; and (c) a description of their synthetic applications in general synthetic methodology and in asymmetric synthesis. Full article
(This article belongs to the Special Issue Carbon Ligands: From Fundamental Aspects to Applications)
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19 pages, 3278 KiB  
Review
NHC Core Pincer Ligands Exhibiting Two Anionic Coordinating Extremities
by Rachid Taakili and Yves Canac
Molecules 2020, 25(9), 2231; https://doi.org/10.3390/molecules25092231 - 09 May 2020
Cited by 27 | Viewed by 4538
Abstract
The chemistry of NHC core pincer ligands of LX2 type bearing two pending arms, identical or not, whose coordinating center is anionic in nature, is here reviewed. In this family, the negative charge of the coordinating atoms can be brought either by [...] Read more.
The chemistry of NHC core pincer ligands of LX2 type bearing two pending arms, identical or not, whose coordinating center is anionic in nature, is here reviewed. In this family, the negative charge of the coordinating atoms can be brought either by a carbon atom via a phosphonium ylide (R3P+–CR2−) or by a heteroatom through amide (R2N−), oxide (RO−), or thio(seleno)oxide (RS−, RSe−) donor functionalities. Through selected examples, the synthetic methods, coordination properties, and applications of such tridentate systems are described. Particular emphasis is placed on the role of the donor ends in the chemical behavior of these species. Full article
(This article belongs to the Special Issue Carbon Ligands: From Fundamental Aspects to Applications)
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