N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry

Abstract

N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and lowcoordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element−metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.

MeCAAC=N−: A Cyclic (Alkyl)(Amino)Carbene Imino Ligand

“…[32] The boron centers in both compounds remaini np lanar environments and the C-N-B angles in compound 4 (164.20 (12)8) (4) )i ss imilart oo ther known dibromodiboranes such as bis(dimethylamino)dibromodiborane, (Me 2 N)B 2 Br 2 (1.682(16) ) [33] and the bis(pyrrolyl)dibromodiborane, (C 4 H 4 N) 2 B 2 Br 2 (1.696 (4) ). [35] Due to their electronic similarity to cyclopentadienyl ligands, Group 4c omplexes have been the dominant focusl ikely due to their use as olefin polymerization catalysts. [35] Due to their electronic similarity to cyclopentadienyl ligands, Group 4c omplexes have been the dominant focusl ikely due to their use as olefin polymerization catalysts.…”

Low‐Valent Group 14 Phosphinidenide Complexes [({SIDipp}P)2M] Exhibit P–M pπ–pπ Interaction (M=Ge, Sn, Pb)

“…It was possible to separate (SIDippP) 2 M and SIMes (which are soluble in pentane) after changing the solvent to pentane and collection of the insoluble residue ([(SI-DippP)MX] 2 , (SIMes)MX 2 and KX). After dissolving this residue in toluene and subsequent separation of the insoluble KX through centrifugation, the isolation of [(SIDippP)SnCl] 2 (6) and [(SIDippP)PbBr] 2 (7) was possible in moderate yields. However, the isolation of the germanium compound was not successful because in this reaction the obtained compounds are (SI-DippP) 2 Ge and (SIMes)GeCl 2 .…”

Stable Mesoionic N‐Heterocyclic Olefins (mNHOs)

“…Forschungsartikel 5834 www.angewandte.de backbonding.Importantly,inline with the basicity trends,the IR frequencies follow the trend n av (NHO) > n av (8) % n av (9) > n av (10). Note, 10 (TEP:2023 cm À1 )isanexceptional strong donor exceeding common ligand classes such as phosphines, NHCs,NHOs,a nd even MICs and aNHOs.…”

“…[4] So far NHOs and their derivatives,w hich can be considered as deoxy-Breslow type intermediates, [5] have found broad application in several fields such as transition metal and organo catalysis, [6] main-group [7] and coordination chemistry, [8] or polymer chemistry. [9,10] Besides Arduengotype N-heterocyclic carbenes, [11] abnormal N-heterocyclic carbenes (aNHC), [12] and mesoionic carbenes (MICs), 1H-1,2,3-triazol-5-ylidenes [13] have been shown to be isolable, stable and bottable carbenes with strong s-donor properties exceeding traditional NHCs. [14] While formal methylene extension of phosphines and Nheterocyclic carbenes leads to methylene phosphoranes (I/II) and NHOs (III/IV)r espectively,w ew ere curious if formal methylene extension of mesoionic and abnormal carbenes would result in stable compounds (Scheme 1).…”