Luong Phong Ho

The new diaminoacetylene (DAA) dimorpholinoacetylene (3) was prepared from 1,1‐dimorpholinoethene (1) by bromination to form the dibromoketene aminal 2, which upon lithiation afforded 3 through a Fritsch‐Buttenberg‐Wiechell rearrangement. Heating 3 at elevated temperatures resulted in a complete conversion into the dimer 1,1,2,4‐tetramorpholino‐1‐buten‐3‐yne (4), which was used for the synthesis of four‐membered cyclic bent allene (CBA) transition‐metal complexes of the type [(CBA)ML_n] (5‐7; ML_n=AuCl, RhCl(COD), RhCl(CO)₂; CBA=1,3,4,4‐tetramorpholino‐1,2‐cyclobutadiene; COD=1,5‐cyclooctadiene). The reaction of 3 with tetraethylammonium bromide gave 1,2,3,4‐tetramorpholinocyclobutenylium bromide (8), which reacted with bromine to form 1,2,3,4‐tetra(morpholino)cyclobutenediylium bis(tribromide) (9). Compound 9 represents the first fully characterized compound containing a tetraaminocyclobutadiene dication and displays a nearly planar C₄N₄ core as shown by X‐ray diffraction analysis. Detailed quantum chemical calculations were performed to assess the aromaticity of tetraaminocyclubutadiene dications by employing the Nucleus Independent Chemical Shift (NICS) method and current density analysis.

In this Concept article we provide a brief overview of the design and preparation of N‐heterocyclic carbenes carrying weakly coordinating anions (WCA‐NHCs). The anionic charge in these ligand systems is located on an exocyclic group, for example, B(C₆F₅)₃, tethered to the backbone of the imidazole ring, thus resembling a weakly coordinating moiety. With the general guiding principle behind the application of WCA‐NHCs being the conversion of otherwise cationic NHC complexes into their overall neutral congeners, numerous transition metal as well as main group element complexes were isolated during the last decade, which are summarized herein.

Chalcogen‐pnictogen dihalide complexes which bear an anionic N‐heterocyclic carbene ligand with a tethered weakly coordinating borate moiety (WCA‐IDipp, WCA=B(C₆F₅)₃, IDipp=1,3‐bis(2,6‐diisopropyphenyl)imidazolin‐2‐ylidene) were isolated and fully characterized. The metathesis reaction between the lithium salts [{(WCA‐IDipp)E}Li(toluene)] (E=S, Se) and pnictogen trihalides E′X₃ (E=P, As, Sb; X=Cl, Br) afforded the complexes of the general type (WCA‐IDipp)E−E′X₂ (E=S, Se; E=P, As, Sb; X=Cl, Br). In contrast, lithium chloride elimination was not observed with BiCl₃ under the same reaction conditions and instead, the lithium complexes [{(WCA‐IDipp)E−BiCl₃}Li(thf)₄] (E=S, Se) were obtained. However, the corresponding bismuth dichloride derivatives (WCA‐IDipp)E−BiCl₂ were accessible from the reaction of (WCA‐IDipp)ESiMe₃ with BiCl₃ upon Me₃SiCl elimination.

The lithium salts of anionic N‐heterocyclic thiones and selones [{(WCA‐IDipp)E}Li(toluene)] (1: E=S; 2: E=Se; WCA=B(C₆F₅)₃, IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene), which contain a weakly coordinating anionic (WCA) borate moiety in the imidazole backbone were reacted with Me₃SiCl, to furnish the silylated adducts (WCA‐IDipp)ESiMe₃ (3: E=S; 4: E=Se). The reaction of the latter with [(η⁵‐C₅Me₅)MCl₂]₂ (M=Rh, Ir) afforded the rhodium(III) and iridium(III) half‐sandwich complexes [{(WCA‐IDipp)E}MCl(η⁵‐C₅Me₅)] (5–8). The direct reaction of the lithium salts 1 and 2 with a half or a full equivalent of [M(COD)Cl]₂ (M=Rh, Ir) afforded the monometallic complexes [{(WCA‐IDipp)E}M(COD)] (9–12) or the bimetallic complexes [μ₂‐{(WCA‐IDipp)E}M₂(COD)₂(μ₂‐Cl)] (13–16), respectively. The bonding situation in these complexes has been investigated by means of density functional theory (DFT) calculations, revealing thiolate or selenolate ligand character with negligible metal‐chalcogen π‐interaction.

Anionic N-heterocyclic carbenes with weakly coordinating borate, aluminate, and gallate moieties of the type [(F₅C₆)₃E-NHC]⁻ (E = B, Al, Ga) were isolated as lithium salts by the lithiation of 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (IMes) or 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene (IDipp) followed by the addition of E(C₆F₅)₃ (E = B, Al, Ga). Treatment with elemental selenium afforded the lithium salts of the corresponding anionic selenourea derivatives [{(F₅C₆)₃E-NHC}Se]⁻ (NHC = IMes, E = B; NHC = IDipp, E = B, Al, Ga), which were examined, among other things, by means of ⁷⁷Se{¹H} NMR spectroscopy to assess the π-accepting properties of the WCA-NHC ligands in comparison with their neutral NHC congeners.

Anionic N-heterocyclic carbenes have been employed for the isolation of the first dicarbene–dibismuth complex; the resulting dibismuthene features a trans -bent geometry with a Bi–Bi double bond and short intramolecular Bi–C _ipso contacts.