The chemistry of heterocyclic carbenes has experienced a rapid development over the last years. In addition to the imidazolin-2-ylidenes, a large number of cyclic diaminocarbenes with different ring sizes have been described. Aside from diaminocarbenes, P-heterocyclic carbenes, and derivatives with only one, or even no heteroatom within the carbene ring are known. New methods for the synthesis of complexes with N-heterocyclic carbene ligands such as the oxidative addition or the metal atom template controlled cyclization of β-functionalized isocyanides have been developed recently. This review summarizes the new developments regarding the synthesis of N-heterocyclic carbenes and their metal complexes.

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Organocatalytic Reactions Enabled by N-Heterocyclic Carbenes.

Lead halide perovskite materials have attracted significant attention in the context of photovoltaics and other optoelectronic applications, and recently, research efforts have been directed to nanostructured lead halide perovskites. Collodial nanocrystals (NCs) of cesium lead halides (CsPbX3, X = Cl, Br, I) exhibit bright photoluminescence, with emission tunable over the entire visible spectral region. However, previous studies on CsPbX3 NCs did not address key aspects of their chemistry and photophysics such as surface chemistry and quantitative light absorption. Here, we elaborate on the synthesis of CsPbBr3 NCs and their surface chemistry. In addition, the intrinsic absorption coefficient was determined experimentally by combining elemental analysis with accurate optical absorption measurements. 1H solution nuclear magnetic resonance spectroscopy was used to characterize sample purity, elucidate the surface chemistry, and evaluate the influence of purification methods on the surface composition. We fi...

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Highly Dynamic Ligand Binding and Light Absorption Coefficient of Cesium Lead Bromide Perovskite Nanocrystals

Quantification and variation of characteristic properties of different ligand classes is an exciting and rewarding research field. N-Heterocyclic carbenes (NHCs) are of special interest since their electron richness and structure provide a unique class of ligands and organocatalysts. Consequently, they have found widespread application as ligands in transition-metal catalysis and organometallic chemistry, and as organocatalysts in their own right. Herein we provide an overview on physicochemical data (electronics, sterics, bond strength) of NHCs that are essential for the design, application, and mechanistic understanding of NHCs in catalysis.

The Measure of All Rings—N‐Heterocyclic Carbenes

Facile Derivatization of a “Chemo-active” NHC Incorporating an Enolate Backbone and Relevant Tuning of Its Electronic Properties

Imidazol-2-ylidene-4-olate: an anionic N-heterocyclic carbene pre-programmed for further derivatization.

The synthesis of a new tetrapyridyl ligand, bis[di-1,1-(2-pyridyl)ethyl]amine (BDPEA), is described. Complexation of this ligand with manganese(II), iron(III) or copper(II) chlorides afforded mononuclear complexes: Mn(BDPEA)Cl2 (1) [Fe (BDPEA)Cl2]Cl (2) and [Cu(BDPEA)Cl]Cl (3). In all cases, BDPEA is coordinated to the metal center by three pyridine nitrogen atoms and the secondary amine. The geometrical environments around the metals in Mn(BDPEA)Cl2 and [Fe(BDPEA)Cl2]Cl are best described as distorted octahedrals and in [Cu (BDPEA)Cl]Cl as a slightly distorted square pyramid. The DNA cleavage activities of manganese(II), iron (III) or copper(II) complexes of both BDPEA and another tetrapyridyl ligand, bis[di(2-pyridyl) methyl]amine (BDPMA), in the presence of an oxidant (H2O2) or a reducing agent (ascorbate) with air, are reported. The iron(III) complexes exhibited significantly enhanced efficiencies, compared to copper(II) complexes. [Fe(BDPEA)Cl2]Cl is found to be the most active DNA cleaver, in agreement with a better stability of BDPEA in oxidizing conditions.

Preparation, characterization and crystal structures of manganese(II), iron(III) and copper(II) complexes of the bi.

We report the synthesis of Au(I) and Au(III) complexes, involving alcohol functionalized bis(N-heterocyclic carbene) ligands. Two short reaction pathways lead to the diimidazolium precursors, namely 1,1′-(2,6-pyridinediyl)bis[3-(2-hydroxyethyl)-1H-imidazol-3-ium]diiodide (2), 3,3′(methanediyl)bis[1-(2-hydroxy-2-methylpropyl)-1H-imidazol-3-ium]dibromide (4) and 3,3′-(1,3-propanediyl)bis[1-(2-hydroxy-2-methylpropyl)-1H-imidazol-3-ium]bis(4-methylbenzenesulfonate) (6), in which the two azolium rings are bridged by a rigid pyridine unit or an aliphatic chain (C1 or C3). The Au(I) complexes [AuI(Lpy)]22+[PF6−]2 (11) and [AuI(LC1)]22+[PF6−]2 (13) were obtained by direct metallation of the salts 2 and 4, respectively, in the presence of sodium acetate with Au(SMe2)Cl, followed by an anionic metathesis in the presence of KPF6. The trimethylene compound [AuI(LC3)]22+[PF6−]2 (15) was prepared by transmetallation between the related precursor [AgI(LC3)]22+[PF6−]2 (10) and Au(SMe2)Cl. The Au(III) complexes, [AuIII(Lpy)Br2]22+[PF6−]2 (16), [AuIII(LC1)Br2]22+[PF6−]2 (17) and [AuIII(LC3)Br2]22+[PF6−]2 (18) were generated by oxidation of the corresponding Au(I) species with an excess of elemental bromine. Complexes 11, [AuI(LC1)]22+[Br−]2 (12) and 15 have been characterized by single-crystal X-ray diffraction analyses. The electrochemical and luminescence properties of the Au(I) and Au(III) compounds have been studied.

Dinuclear gold(I) and gold(III) complexes of bridging functionalized bis(N-heterocyclic carbene) ligands: synthesis, structural, spectroscopic and electrochemical characterizations

A tetraphosphabenzene analog of the postulated anti-tricyclohexylene, a singlet biradical valence isomer of benzene, has been isolated. The tricyclic derivative features one-electron phosphorus-phosphorus bonds, which result from the pi*-pi* interaction between two diphosphirenyl radicals. Such one-electron bonds may play a wider role in phosphorus chemistry.

https://science.sciencemag.org/content/sci/279/5359/2080.full.pdf

Heinz Gornitzka

Papers

Facile Derivatization of a “Chemo-active” NHC Incorporating an Enolate Backbone and Relevant Tuning of Its Electronic Properties

Imidazol-2-ylidene-4-olate: an anionic N-heterocyclic carbene pre-programmed for further derivatization.

Preparation, characterization and crystal structures of manganese(II), iron(III) and copper(II) complexes of the bi.

Dinuclear gold(I) and gold(III) complexes of bridging functionalized bis(N-heterocyclic carbene) ligands: synthesis, structural, spectroscopic and electrochemical characterizations

Isolation of a Benzene Valence Isomer with One-Electron Phosphorus-Phosphorus Bonds