Triptycene‐Based Chiral and meso‐N‐Heterocyclic Carbene Ligands and Metal Complexes

Based on 1‐amino‐4‐hydroxy‐triptycene, new saturated and unsaturated triptycene‐NHC (N‐heterocyclic carbene) ligands were synthesized from glyoxal‐derived diimines. The respective carbenes were converted into metal complexes [(NHC)MX] (M=Cu, Ag, Au; X=Cl, Br) and [(NHC)MCl(cod)] (M=Rh, Ir; cod=1,5‐cyclooctadiene) in good yields. The new azolium salts and metal complexes suffer from limited solubility in common organic solvents. Consequently, the introduction of solubilizing groups (such as 2‐ethylhexyl or 1‐hexyl by O‐alkylation) is essential to render the complexes soluble. The triptycene unit infers special steric properties onto the metal complexes that enable the steric shielding of selected areas close to the metal center. Next, chiral and meso‐triptycene based N‐heterocyclic carbene ligands were prepared. The key step in the synthesis of the chiral ligand is the Buchwald–Hartwig amination of 1‐bromo‐4‐butoxy‐triptycene with (1S,2S)‐1,2‐diphenyl‐1,2‐diaminoethane, followed by cyclization to the azolinium salt with HC(OEt)₃. The analogous reaction with meso‐1,2‐diphenyl‐1,2‐diaminoethane provides the respective meso‐azolinium salt. Both the chiral and meso‐azolinium salts were converted into metal complexes including [(NHC)AuCl], [(NHC)RhCl(cod)], [(NHC)IrCl(cod)], and [(NHC)PdCl(allyl)]. An in situ prepared chiral copper complex was tested in the enantioselective borylation of α,β‐unsaturated esters and found to give an excellent enantiomeric ratio (er close to 90:10).     

Porphyrins Fused to N‐Heterocyclic Carbenes (NHCs): Modulation of the Electronic and Catalytic Properties of NHCs by the Central Metal of the Porphyrin

We report herein a detailed study of the use of porphyrins fused to imidazolium salts as precursors of N‐heterocyclic carbene ligands 1 M . Rhodium(I) complexes 6 M – 9 M were prepared by using 1 M ligands with different metal cations in the inner core of the porphyrin (M=Ni^II, Zn^II, Mn^III, Al^III, 2H). The electronic properties of the corresponding N‐heterocyclic carbene ligands were investigated by monitoring the spectroscopic changes occurring in the cod and CO ancillary ligands of [( 1 M )Rh(cod)Cl] and [( 1 M )Rh(CO)₂Cl] complexes (cod=1,5‐cyclooctadiene). Porphyrin–NHC ligands 1 M with a trivalent metal cation such as Mn^III and Al^III are overall poorer electron donors than porphyrin–NHC ligands with no metal cation or incorporating a divalent metal cation such as Ni^II and Zn^II. Imidazolium salts 3 M (M=Ni, Zn, Mn, 2H) have also been used as NHC precursors to catalyze the ring‐opening polymerization of L ‐lactide. The results clearly show that the inner metal of the porphyrin has an important effect on the reactivity of the outer carbene.     

Symmetric and dissymmetric N‐heterocyclic carbene rhodium(I) complexes: a comparative study of their catalytic activities in transfer hydrogenation reaction

Six new [RhBr(NHC)(cod)] (NHC = N‐heterocyclic carbene; cod = 1,5‐cyclooctadiene) type rhodium complexes ( 4–6 ) have been prepared by the reaction of [Rh(μ‐OMe)(cod)]₂ with a series of corresponding imidazoli(in)ium bromides ( 1–3 ) bearing mesityl (Mes) or 2,4,6‐trimethylbenzyl (CH₂Mes) substituents at N¹ and N³ positions. They have been fully characterized by ¹ H, ¹³ C and heteronuclear multiple quantum correlation NMR analyses, elemental analysis and mass spectroscopy. Complexes of type [(NHC)RhBr(CO)₂] (NHC = imidazol‐2‐ylidene) ( 7b–9b ) were also synthesized to compare σ‐donor/π‐acceptor strength of NHC ligands. Transfer hydrogenation (TH) reaction of acetophenone has been comparatively studied by using complexes 4–6 as catalysts. The symmetrically CH₂Mes‐substituted rhodium complex bearing a saturated NHC ligand ( 5a ) showed the highest catalytic activity for TH reaction. Copyright © 2012 John Wiley & Sons, Ltd.     

Iridium(I) Hydroxides: Powerful Synthons for Bond Activation

A family of iridium(I) hydroxides of the form [Ir(cod)(NHC)(OH)] (cod=1,5‐cyclooctadiene, NHC=N‐heterocyclic carbene) is reported. Single‐crystal X‐ray analyses and computational methods were used to explore the structural characteristics and steric properties of these new complexes. The model complex [Ir(cod)(IiPr)(OH)] (IiPr=1,3‐(diisopropyl)imidazol‐2‐ylidene) undergoes reaction with a wide variety of substrates including boronic acids and silicon compounds. In addition, O? H, N? H and C? H bond activation was achieved with alcohols, carboxylic acids, amines and various sp‐, sp²‐ and sp³‐hybridised carbon centres, giving access to a wide range of new Ir^I complexes. These studies have allowed us to explore the exciting reactivity of this motif, revealing a versatile and useful synthon capable of activating important chemical bonds under mild (typically room temperature) conditions. No additives were required and, in the case of X? H bond activation, water was the only waste product, rendering this an atom efficient procedure for bond activation. This system has great potential for the construction of new catalytic cycles for organic synthesis and small‐molecule activation.     

Synthesis,characterization and catalytic activity in Suzuki–Miyaura coupling of palladacycle complexes with n‐butyl‐substituted N‐heterocyclic carbene ligands

A series of monomeric palladacycle complexes bearing n‐butyl‐substituted N‐heterocyclic carbenes, namely [Pd(NHC)X(dmba)] (dmba: dimethylbenzylamine and [Pd(NHC)X(ppy)]; NHC: 1‐n‐butyl‐3‐substituted benzylimidazol‐2‐ylidene; ppy: 2‐phenylpyridine), were prepared either by transmetallation from the corresponding silver carbene complexes or by the reaction of the corresponding acetate‐bridged palladacycle dimer with N‐heterocyclic carbene ligands in high yields. The palladium(II) complexes were characterized using elemental analyses, APCI‐MS, ¹H NMR and ¹³C NMR spectroscopies. These complexes are efficient in the Suzuki–Miyaura coupling reaction between phenylboronic acid and aryl bromides.     

Synthesis of a Metallo‐Iminosilane via a Silanone–Metal π‐Complex

Facile oxygenation of the acyclic amido‐chlorosilylene bis(N‐heterocyclic carbene) Ni⁰ complex [{N(Dipp)(SiMe₃)ClSi:→Ni(NHC)₂] ( 1 ; Dipp=2,6‐ⁱPr₂C₆H₄; N‐heterocyclic carbene=C[(ⁱPr)NC(Me)]₂) with N₂O furnishes the first Si‐metalated iminosilane, [DippN=Si(OSiMe₃)Ni(Cl)(NHC)₂] ( 3 ), in a rearrangement cascade. Markedly, the formation of 3 proceeds via the silanone (Si=O)–Ni π‐complex 2 as the initial product, which was predicted by DFT calculations and observed spectroscopically. The Si=O and Si=N moieties in 2 and 3 , respectively, show remarkable hydroboration reactivity towards H−B bonds of boranes, in the former case corroborating the proposed formation of a (Si=O)–Ni π‐complex at low temperature.     

Metal Complexes of a Boron‐Dipyrromethene (BODIPY)‐Tagged N‐Heterocyclic Carbene (NHC) as Luminescent Carbon Monoxide Chemodosimeters

Several metal complexes with a boron dipyrromethene (BODIPY)‐functionalized N‐heterocyclic carbene (NHC) ligand 4 were synthesized. The fluorescence in [( 4 )(SIMes)RuCl₂(ind)] complex is quenched (Φ=0.003), it is weak in [( 4 )PdI₂(Clpy)] (Φ=0.033), and strong in [( 4 )AuI] (Φ=0.70). The BODIPY‐tagged complexes can experience pronounced changes in the brightness of the fluorophore upon ligand‐exchange and ligand‐dissociation reactions. Complexes [( 4 )MX(1,5‐cyclooctadiene)] (M=Rh, Ir; X=Cl, I; Φ=0.008–0.016) are converted into strongly fluorescent complexes [( 4 )MX(CO)₂] (Φ=0.53–0.70) upon reaction with carbon monoxide. The unquenching of the Rh and Ir complexes appears to be a consequence of the decreased electron density at Rh or Ir in the carbonyl complexes. In contrast, the substitution of an iodo ligand in [( 4 )AuI] by an electron‐rich thiolate decreases the brightness of the BODIPY fluorophore, rendering the BODIPY as a highly sensitive probe for changes in the coordination sphere of the transition metal.     

Electronic Tuning of a Carbene Center via Remote Chemical Induction,and Relevant Effects in Catalysis

The present report develops the idea that an N‐heterocyclic carbene incorporating a remote anionic functionality—here, a malonate group—as a backbone component of its heterocyclic framework, can be “post‐functionalized” directly from its transition‐metal complexes, upon simple addition of a variety of electrophiles interacting directly with the malonate group in the outer coordination sphere. From a palette of selected electrophilic reagents, it was thus possible to modulate the electronic donor properties of the carbene center over a rather broad range. Both the zwitterionic complex [Rh{malo‐NHC}(cod)] and the cationic derivatives [Rh{malo‐NHC^E}(cod)]⁺ (where “malo‐NHC^E” represents the ligand modified by a selected electrophile “E”) were used as pre‐catalysts in two types of catalytic reactions, namely, the polymerization of phenylacetylene and the hydroboration of styrene. The results indicate that, in both cases, the zwitterionic species is by far the best catalyst, whereas a decrease in the ligand donicity induced by the added electrophile results in a concomitant reduction of catalytic activity. Apparent deviations to such a trend in the case of the hydroboration of styrene were rationalized in terms of an interaction between the reactive catecholborane substrate and the remote functionality of the N‐heterocyclic carbene leading to an in situ modification of the nature of the active species. These observations serve as a useful basis to define the scope and limitations of the present conceptual approach in catalysis.     

NHC/Nickel(II)‐Catalyzed [3+2] Cross‐Dimerization of Unactivated Olefins and Methylenecyclopropanes

Cross‐dimerization of a methylenecyclopropane ( 1 ) and an unactivated alkene ( 2 ) with typical hydroalkenylation reactivity was observed for the first time by using a [NHC‐Ni(allyl)]BAr^F catalyst (NHC=N‐heterocyclic carbene). Results show that the C?C cleavage of 1 did not involve a Ni⁰ oxidative addition, which was crucial in former systems. Thus the method reported here emerges as a complementary method for attaining highly chemo‐ and regioselective synthesis of methylenecyclopentanes ( 3 ) with broad scope. An efficient NHC/Ni^II‐catalyzed rearrangement of 1 leads to the convergent synthesis of 3 in the presence of 2 .     

Ligand‐Centred Reactivity of Bis(picolyl)amine Iridium: Sequential Deprotonation,Oxidation and Oxygenation of a “Non‐Innocent” Ligand

Treatment of [Ir(bpa)(cod)]⁺ complex [ 1 ]⁺ with a strong base (e.g., tBuO^?) led to unexpected double deprotonation to form the anionic [Ir(bpa?2H)(cod)]^? species [ 3 ]^?, via the mono‐deprotonated neutral amido complex [Ir(bpa?H)(cod)] as an isolable intermediate. A certain degree of aromaticity of the obtained metal–chelate ring may explain the favourable double deprotonation. The rhodium analogue [ 4 ]^? was prepared in situ. The new species [M(bpa?2H)(cod)]^? (M=Rh, Ir) are best described as two‐electron reduced analogues of the cationic imine complexes [M^I(cod)(Py‐CH₂‐N?CH‐Py)]⁺. One‐electron oxidation of [ 3 ]^? and [ 4 ]^? produced the ligand radical complexes [ 3 ]^. and [ 4 ]^.. Oxygenation of [ 3 ]^? with O₂ gave the neutral carboxamido complex [Ir(cod)(py‐CH₂‐N‐CO‐py)] via the ligand radical complex [ 3 ]^. as a detectable intermediate.     

A Comparison of the Stability and Reactivity of Diamido‐ and Diaminocarbene Copper Alkoxide and Hydride Complexes

The mononuclear N‐heterocyclic carbene (NHC) copper alkoxide complexes [(6‐NHC)CuOtBu] (6‐NHC=6‐MesDAC ( 1 ), 6‐Mes ( 2 )) have been prepared by addition of the free carbenes to the tetrameric tert‐butoxide precursor [Cu(OtBu)]₄, or by protonolysis of [(6‐NHC)CuMes] (6‐NHC=6‐MesDAC ( 3 ), 6‐Mes ( 4 )) with tBuOH. In contrast to the relatively stable diaminocarbene complex 2 , the diamidocarbene derivative 1 proved susceptible to both thermal and hydrolytic ring‐opening reactions, the latter affording [(6‐MesDAC)Cu(OC(O)CMe₂C(O)N(H)Mes)(CNMes)] ( 6 ). The intermediacy of [(6‐MesDAC)Cu(OH)] in this reaction was supported by the generation of Cu₂O as an additional product. Attempts to generate an isolable copper hydride complex of the type [(6‐MesDAC)CuH] by reaction of 1 with Et₃SiH resulted instead in migratory insertion to generate [(6‐MesDAC‐H)Cu(P(p‐tolyl)₃)] ( 9 ) upon trapping by P(p‐tolyl)₃. Migratory insertion was also observed during attempts to prepare [(6‐Mes)CuH], with [(6‐Mes‐H)Cu(6‐Mes)] ( 10 ) isolated, following a reaction that was significantly slower than in the 6‐MesDAC case. The longer lifetime of [(6‐Mes)CuH] allowed it to be trapped stoichiometrically by alkyne, and also employed in the catalytic semi‐reduction of alkynes and hydrosilylation of ketones.     

Enantiopure Cycloiridiated Complexes Bearing a Pentahelicenic N‐Heterocyclic Carbene and Displaying Long‐Lived Circularly Polarized Phosphorescence

A fused π‐helical N‐heterocyclic carbene (NHC) system was prepared and examined through its diastereoisomerically pure cycloiridiated complexes. The latter display light‐green phosphorescence with unusually long lifetimes and circular polarization that depends on both the helical NHC P /M stereochemistry and the iridium Δ/Λ stereochemistry. These unprecedented features are attributed to extended π conjugation within the helical carbenic ligand and efficient helicene‐NHC–Ir interaction.     

Facile Syntheses of N‐Heterocyclic Carbene Precursors through I2‐ or NIS‐Promoted Amidiniumation of N‐Alkenyl Formamidines

We have developed I₂‐ or N‐iodosuccinimide (NIS)‐mediated amidiniumation of N‐alkenyl formamidines for the syntheses of cyclic formamidinium salts, some of which could be directly used as N‐heterocyclic carbene (NHC) precursors. Treatment of iodine‐containing formamidinium salts with Al₂O₃ led to the formation of cyclic formamidinium salts with an unsaturated backbone. A rhodium(I) complex ligated by a representative NHC was prepared by the reaction of [Rh(cod)Cl]₂ (cod=1,5‐cyclooctadiene) with the free carbene obtained in situ from deprotonation of the corresponding formamidinium salts. The NHCs prepared in situ can also react with S₈ to afford the corresponding thiones.     

Silylene–Nickel Promoted Cleavage of B−O Bonds: From Catechol Borane to the Hydroborylene Ligand

The first 16 valence electron [bis(NHC)](silylene)Ni⁰ complex 1 , [(^TMSL)ClSi:→Ni(NHC)₂], bearing the acyclic amido‐chlorosilylene (^TMSL)ClSi: (^TMSL=N(SiMe₃)Dipp; Dipp=2,6‐Prⁱ₂C₆H₄) and two NHC ligands (N‐heterocyclic carbene=:C[(Prⁱ)NC(Me)]₂) was synthesized in high yield and structurally characterized. Compound 1 is capable of facile dihydrogen activation under ambient conditions to give the corresponding HSi‐NiH complex 2 . Most notably, 1 reacts with catechol borane to afford the unprecedented hydroborylene‐coordinated (chloro)(silyl)nickel(II) complex 3 , {[cat(^TMSL)Si](Cl)Ni←:BH(NHC)₂}, via the cleavage of two B−O bonds and simultaneous formation of two Si−O bonds. The mechanism for the formation of 3 was rationalized by means of DFT calculations, which highlight the powerful synergistic effects of the Si:→Ni moiety in the breaking of incredibly strong B−O bonds.     

Expanded‐Ring N‐Heterocyclic Carbenes Efficiently Stabilize Gold(I) Cations,Leading to High Activity in π‐Acid‐Catalyzed Cyclizations

A series of six‐ and seven‐membered expanded‐ring N‐heterocyclic carbene (er‐NHC) gold(I) complexes has been synthesized using different synthetic approaches. Complexes with weakly coordinating anions [(er‐NHC)AuX] (X^?=BF₄^?, NTf₂^?, OTf^?) were generated in solution. According to their ¹³C NMR spectra, the ionic character of the complexes increases in the order X^?=Cl^?<NTf₂^?<OTf^?<BF₄^?. Additional factors for stabilization of the cationic complexes are expansion of the NHC ring and the attachment of bulky substituents at the nitrogen atoms. These er‐NHCs are bulkier ligands and stronger electron donors than conventional NHCs as well as phosphines and sulfides and provide more stabilization of [(L)Au⁺] cations. A comparative study has been carried out of the catalytic activities of five‐, six‐, and seven‐membered carbene complexes [(NHC)AuX], [(Ph₃P)AuX], [(Me₂S)AuX], and inorganic compounds of gold in model reactions of indole and benzofuran synthesis. It was found that increased ionic character of the complexes was correlated with increased catalytic activity in the cyclization reactions. As a result, we developed an unprecedentedly active monoligand cationic [(THD‐Dipp)Au]BF₄ (1,3‐bis(2,6‐diisopropylphenyl)‐3,4,5,6‐tetrahydrodiazepin‐2‐ylidene gold(I) tetrafluoroborate) catalyst bearing seven‐membered‐ring carbene and bulky Dipp substituents. Quantitative yields of cyclized products were attained in several minutes at room temperature at 1 mol % catalyst loadings. The experimental observations were rationalized and fully supported by DFT calculations.     

Stepwise synthesis of a hydrido, N-heterocyclic dicarbene iridium(III) pincer complex featuring mixed NHC/abnormal NHC ligands

We describe a stepwise synthesis of the hydrido, N-heterocyclic dicarbene iridium(III) pincer complex [Ir(H)I(C(NHC)CC(aNHC))(NCMe)] (3) which features a combination of normal and abnormal NHC ligands. The reaction of the bis(imidazolium) diiodide [(CH(imid)CHCH(imid))]I(2) (1) with [Ir(μ-Cl)(cod)](2) afforded first the mono-NHC Ir(I) complex [IrI(cod)(CH(imid)CHC(NHC))]I (2), which was then reacted with 2 equiv. of Cs(2)CO(3) in acetonitrile at 60 °C for 40 h to yield 3. These observations support our previously proposed mechanism for the formation of hydrido, N-heterocyclic dicarbene iridium(III) pincer complexes from the reaction of bis(imidazolium) salts with weak bases involving a mono-NHC Ir(I) intermediate. We describe the reactivity of the mono-NHC Ir(I) complex 2 under various conditions. By changing the reaction solvent from MeCN to toluene, we observed the cleavage of the imidazol-2-ylidene ring and the formation of an iminoformamide-containing mono-NHC Ir(I) complex [IrI(cod){[NHCH=CHN(Ad)CHO]CHC(NHC)}] (4). Complex 4 was also prepared in high yield from the reaction of 2 with strong bases (potassium tert-butoxide or potassium hexamethyldisilazane), via the initial formation of the complex [IrI(cod)(CH(NHC)CHC(NHC))] (5), which contains a coordinated NHC moiety and a free carbene arm, followed by subsequent hydrolysis of the latter. The bis(imidazolium) salt 1 can be deprotonated by strong bases to form the bis(carbene) ligand C(NHC)CHC(NHC) (6), which readily reacts with [Ir(μ-Cl)(cod)](2) to give the dinuclear complex [{IrI(cod)}(2)(μ-C(NHC)CHC(NHC))] (7), in which the N-heterocyclic bis(carbene) ligand bridges the two metals through the carbene carbon atoms.     

[(NHC)NiIIH]‐Catalyzed Cross‐Hydroalkenylation of Cyclopropenes with Alkynes: Cyclopentadiene Synthesis by [(NHC)NiII]‐Assisted C−C Rearrangement

A cross‐hydroalkenylation/rearrangement cascade (HARC), using a cyclopropene and alkyne as substrate pairs, was achieved for the first time by using new [(NHC)Ni(allyl)]BAr^F catalysts (NHC=N‐heterocyclic carbenes). By controlling the (NHC)Ni^IIH relative insertion reactivity with cyclopropene and alkyne, a broad scope of cyclopentadienes was obtained with highly selectively. The structural features of the new (NHC)Ni^II catalyst were important for the success of the reaction. The mild reaction conditions employed may serve as an entry for exploring (NHC)Ni^II‐assisted vinylcyclopropane rearrangement reactivity.     

CH–π and CF–π Interactions Lead to Structural Changes of N‐Heterocyclic Carbene Palladium Complexes

The role of CH–π and CF–π interactions in determining the structure of N‐heterocyclic carbene (NHC) palladium complexes were studied using ¹H NMR spectroscopy, X‐ray crystallography, and DFT calculations. The CH–π interactions led to the formation of the cis‐anti isomers in 1‐aryl‐3‐isopropylimidazol‐2‐ylidene‐based [(NHC)₂PdX₂] complexes, while CF–π interactions led to the exclusive formation of the cis‐syn isomer of diiodobis(3‐isopropyl‐1‐pentafluorophenylimidazol‐2‐ylidene) palladium(II).     

Copper,Silver and Sodium Salt‐Mediated Quaternization by Arylation: Syntheses of N‐Heterocyclic Carbene Precursors and 6‐H‐Phenanthridine Derivatives

We have developed a Cu^II‐, Ag^I‐, and NaOTf‐mediated intramolecular quaternization by arylation reactions to synthesize a variety of N‐heterocyclic carbene (NHC) precursors with a benzene‐fused backbone. The methodology also provides a convenient alternative route for the synthesis of 6‐H‐phenanthridine derivatives. A novel silver–NHC complex was prepared by treatment of Ag₂O with the free carbene, which was in situ prepared from the deprotonation of a representative quinazolinonium salt.     

Iron‐ and Cobalt‐Catalyzed Synthesis of Carbene Phosphinidenes

In the presence of stoichiometric or catalytic amounts of [M{N(SiMe₃)₂}₂] (M=Fe, Co), N‐heterocyclic carbenes (NHCs) react with primary phosphines to give a series of carbene phosphinidenes of the type (NHC)?PAr. The formation of (IMe₄)?PMes (Mes=mesityl) is also catalyzed by the phosphinidene‐bridged complex [(IMe₄)₂Fe(μ‐PMes)]₂, which provides evidence for metal‐catalyzed phosphinidene transfer.