Dimorpholinoacetylene and Its Use for the Synthesis of Tetraaminocyclobutadiene Species.

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)MLn ] (5-7; MLn =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 C4 N4 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.

N-Heterocyclic Carbenes Carrying Weakly Coordinating Anions.

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(C6 F5 )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.

Rhodium and Iridium Complexes of Anionic Thione and Selone Ligands Derived from Anionic N-Heterocyclic Carbenes.

The lithium salts of anionic N-heterocyclic thiones and selones [{(WCA-IDipp)E}Li(toluene)] (1: E=S; 2: E=Se; WCA=B(C6 F5 )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 Me3 SiCl, to furnish the silylated adducts (WCA-IDipp)ESiMe3 (3: E=S; 4: E=Se). The reaction of the latter with [(η5 -C5 Me5 )MCl2 ]2 (M=Rh, Ir) afforded the rhodium(III) and iridium(III) half-sandwich complexes [{(WCA-IDipp)E}MCl(η5 -C5 Me5 )] (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.

Determination of the π-Accepting Properties of Borate-, Aluminate-, and Gallate-Functionalized N-Heterocyclic Carbenes by 77Se NMR Spectroscopy.

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.

Stabilization of a bismuth-bismuth double bond by anionic N-heterocyclic carbenes.

The bismuth dichloride complex (WCA-IDipp)BiCl2, which bears an anionic N-heterocyclic carbene ligand with a weakly coordinating borate moiety (WCA-IDipp, WCA = B(C6F5)3, IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene), was prepared by salt metathesis reaction between BiCl3 and the lithium salt (WCA-IDipp)Li·toluene. Subsequent two-electron reduction with 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine afforded the dibismuthene (WCA-IDipp)2Bi2, which displays a bismuth-bismuth double bond.

Chalcogen complexes of anionic N-heterocyclic carbenes.

Several group 16 adducts of the type [(WCA-IDipp)E]Li(solv.) that bear an anionic N-heterocyclic carbene ligand with a weakly coordinating borate moiety (WCA-IDipp, WCA = B(C6F5)3, IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene, E = O, S, Se, Te) were prepared. This was achieved by deprotonation of the corresponding ketone (IDipp)O or thione (IDipp)S with n-BuLi and subsequent reaction with B(C6F5)3 or by direct reaction of [WCA-IDipp]Li(toluene) with elemental Se or Te. The oxygen, sulfur and selenium adducts can be protonated to give derivatives (WCA-IDipp)EH (E = O, S, Se), whereas the oxidation of the sulfur and selenium adducts yielded neutral disulfide and diselenide species (WCA-IDipp)2E2 (E = S, Se).

Group 13 Element Trihalide Complexes of Anionic N-Heterocyclic Carbenes.

A series of group 13 complexes of the general type [{(WCA-IDipp)EX3 }Li(solv)] (E=B, Al, Ga, In; X=Cl, Br) that bear an anionic N-heterocyclic carbene ligand with a weakly coordinating borate moiety (WCA-IDipp, WCA=B(C6 F5 )3 and IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) were prepared by the reaction of the respective group 13 trihalides (EX3 ) with the lithium salt [(WCA-IDipp)Li ⋅ toluene]. The molecular structures of the BBr3 , AlCl3 , AlBr3 , GaCl3 and InCl3 adducts were established by X-ray diffraction analyses, revealing the formation of coordination polymers linked by halide-lithium interactions, except for the indium derivative, which consists of isolated [Li(THF)4 ]+ and [(WCA-IDipp)InCl3 ]- ions in the solid state.

Heteroleptic diphosphenes and arsaphosphenes bearing neutral and anionic N-heterocyclic carbenes.

The reaction of the phosphorus and arsenic dihalides (WCA-IDipp)ECl2 (E = P, As) that bear an anionic N-heterocyclic carbene ligand with a weakly coordinating borate moiety (WCA = B(C6F5)3, IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with (IDipp)PSiMe3 afforded the monochlorides (WCA-IDipp)E(Cl)P(IDipp), which can be transformed into the cationic dipnictenes [(WCA-IDipp)EP(IDipp)][GaCl4] or neutral radicals [(WCA-IDipp)EP(IDipp)] by treatment with GaCl3 or reduction with 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine.

London Dispersion Interactions in Pnictogen Cations [ECl2 ]+ and [E=E]2+ (E=P, As, Sb) Supported by Anionic N-Heterocyclic Carbenes.

Several pnictogen dihalide complexes of the type (WCA-IDipp)EX2 (E=P, As, Sb; X=Cl, Br) that bear an anionic N-heterocyclic carbene ligand with a weakly coordinating borate moiety (WCA-IDipp, WCA=B(C6 F5 )3 , IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) were prepared by salt metathesis reactions between the respective pnictogen trihalides EX3 and the lithium salt (WCA-IDipp)Li⋅toluene. Two-electron reduction of the dihalides (WCA-IDipp)EX2 with 1,3-bis(trimethylsilyl)-1,4-dihydropyrazine or elemental magnesium afforded the dipnictenes (WCA-IDipp)2 E2 , which display typical element-element double bonds as observed in diaryldiphosphenes, -arsenes and -stibenes. To provide an insight into the factors contributing to the structural stability of the pnictogen dihalide and dipnictene compounds, quantum chemical calculations were performed at the domain-based local pair natural orbital coupled-cluster (DLPNO-CCSD(T)) level. A local energy decomposition (LED) analysis of the interaction between the carbene and the pnictogen dihalide or dipnictene moiety demonstrates that London dispersion is an essential factor for the stabilization of these compounds.

Ethylene-Bridged Oligo-BODIPYs: Access to Intramolecular J-Aggregates and Superfluorophores.

A versatile and rapid access to various chain lengths of ethylene-bridged BODIPY motifs was discovered. Corresponding oligomers comprising up to eight monomeric units were studied with respect to their microstructures by photophysical, X-ray crystallographic, and computational means. The investigation of three different dipyrrin cores revealed a crucial dependence on the substitution pattern of the core, whereas the nature of the meso-periphery is less critical. The impact of substituent effects on the conformational space was investigated by Monte Carlo simulations and a set of DFT methods (B3LYP, PBEh-3c, TPSS/PWPB95), including dispersion effects. Cryptopyrrole-derived oligo-BODIPYs are characterized by a tight intramolecular arrangement triggering a dominant J-type excitonic coupling with red-shifts up to 45 nm, exceptionally small line widths of the absorption and emission event (up to 286 cm-1), outstandingly high attenuation coefficients (up to 1 042 000 M-1 cm-1), and quantum yields of up to unity.