NHC Adducts Research Articles

Phosphaarsenes - Combining Phospha- and Arsa-Wittig-Reagents.

Dipnictenes of the type RPn=PnR (Pn=P, As, Sb, Bi) can be viewed as dimers of the corresponding pnictinidenes R-Pn. Phosphanylidene- and arsanylidenephosphoranes (R-Pn(PMe3); Pn=P, As) have been shown to be versatile synthetic surrogates for the delivery of pnictinidene fragments. We now report that thermal treatment of 1 : 1 mixtures of R-P(PMe3) and R'-As(PMe3) gives access to arsaphosphenes of the type RP=AsR'. Three examples are presented and the properties and reactivity of Mes*P=AsDipTer (1) (Mes*=2,4,6-tBu3-C6H2; DipTer=2,6-(2,6-iPr2C6H3)2-C6H3) were studied in detail. Solid state 31P NMR spectroscopy revealed a large 31P NMR chemical shift anisotropy with a span of ca. 920 ppm for 1 while computational methods were employed to investigate this pronounced magnetic deshielding of the P atom in 1. In the presence of the carbene IMe4 (IMe4=:C(MeNCMe)2) 1 is shown to be split into the corresponding NHC adducts Mes*P(IMe4) and DipTerAs(IMe4), which is additionally shown for diarsenes.

Low-Coordinate Magnesium Sulfide and Selenide Complexes.

The reactions of [{(iPrDipNacNac)Mg}2] 1 (iPrDipnacnac = HC(iPrCNDip)2) with Ph3P═O at 100 °C afforded the phosphinate complex [(iPrDipNacNac)Mg(OPPh3)(OPPh2)] 3. Reactions of 1 with Ph3P═E (E = S, Se) proceeded rapidly at room temperature to low-coordinate chalcogenide complexes [{(iPrDipNacNac)Mg}2(μ-S)] 4 and [{(iPrDipNacNac)Mg}2(μ-Se)] 5, respectively. Similarly, reactions of RNHC═S ((MeCNR)2C═S with R = Me, Et, or iPr) with 1 afforded NHC adducts of magnesium sulfide complexes, [{(iPrDipNacNac)Mg(RNHC)}(μ-S){Mg(iPrDipNacNac)}] 6, that could alternatively be obtained by adding the appropriate RNHC to sulfide complex 4. Complex 4 reacted with 1-adamantylazide (AdN3) to give [{(iPrDipNacNac)Mg}2(μ-SN3Ad)] 7 and can form various simple donor adducts in solution, of which [(iPrDipNacNac)Mg(OAd)}2(μ-S)] 8a (OAd = 2-adamantanone) was structurally characterized. The nature of the ionic Mg-E-Mg unit is described by solution and solid-state studies of the complexes and by DFT computational investigations.

Heteroleptic Aluminum Allyls as Initiators for Lactide Polymerization

Organoaluminum complexes with the formula [(NHC)AlCl3-nA′n] (NHC = IMes (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) or IDipp (IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene); A′ = [1,3-(SiMe3)2C3H3]−; n = 0–2) were synthesized either through solution or mechanochemical methods. Although an NHC adduct with [AlA′3] did not form, [(IDipp)AlCl2A′] and [(IMes)AlClA′2] were obtained and X-ray crystallography confirmed their construction around Al in a distorted tetrahedral environment. The [(NHC)AlCl3-nA′n] complexes, [AlA′3], and [Al(OiPr)3] (as a reference) were examined for their ability to polymerize l-lactide via ring-opening polymerization to produce polylactide. All of them did so with varying degrees of effectiveness. Of the organoaluminum species, the [(NHC)AlCl3] complexes were very weak initiators, and [AlA′3] the most effective, producing polymer molecular weights up to 49 kDa (Mw), with the mixed chloro/allyl complexes in between. All initiators were slower than [Al(OiPr)3].

Direct Formation and Reactivity of a Bromo- and Amido-Substituted Cyclotrisilene

The reaction of {N(tBu)Trip}SiBr3 (1, Trip = 2,4,6-iPr3C6H2) with 1.5 equiv of activated magnesium (Mg*) directly provides the amido-substituted cyclotrisilene 2 with one bromine substituent in 29% yield. In 2, negative hyperconjugation from the Si═Si π-bond into antibonding σ*-orbitals of the Si1–N1 and Si1–Br1 bonds occurs. This results in remarkably more shielded silicon atoms of the Si═Si bond compared to aryl- and silyl-substituted cyclotrisilenes. The reaction with the N-heterocyclic carbene NHCMe4 does not result in ring-opening but instead yields the NHC adduct 3. Nevertheless, cyclotrisilene 2 reacts with ethylene and dimethylacetylene in [2 + 2] cycloadditions to result in bicyclic saturated and unsaturated housanes 4 and 5, respectively.

Open samarocene and ytterbocenes and their adducts with

Reaction of the open samarocene [(η5-pdl′)2Sm(thf)2] (1-Sm; pdl′ = 2,4-tBu2C5H5) towards IMes (= 1,3-dimesitylimidazolin-2-ylidene), ItBu (= 1,3-di-tert-butylimidazolin-2-ylidene) and IiPr2Me2 (= 1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene) forms the corresponding NHC adducts, [(pdl′)2Sm(IMes)] (2-Sm), [(pdl′)2Sm(ItBu)] (3-Sm) and [(pdl′)2Sm(IiPr2Me2)] (4-Sm), respectively in good to excellent yields. Diverging reactivity patterns emerge when we attempted to prepare the related adducts using the open ytterbocene [(pdl′)2Yb(thf)] (1-Yb). Attributed to the smaller ionic radius of YbII no adduct is observed for IMes. However, for the smaller ItBu ligand iso-butene elimination occurs to yield the imidazole YbII adduct (3′-Yb). For the slightly less sterically encumbered IiPr2Me2 two products were isolated, albeit in low yield: [(pdl′)2Yb(IiPr2Me2)] (4-Yb) and [{(pdl′)Yb(IiPr2Me2)(μ-S)}2] (5-Yb). The surprising formation of the μ-sulfido bridged species 5-Yb suggested that this product might result from trace impurities of imidazoline-2-thione originating from the synthesis of IiPr2Me2. To verify this hypothesis the first imidazoline-2-thione adducts in organolanthanide chemistry were prepared by the reaction of 1-Sm and 1-Yb towards S=IMe2Me2 and S=IiPr2Me2, respectively. All of these adducts were structurally authenticated. However, the imidazoline-2-thione adducts slowly degrade in solution with the Yb derivatives being less stable. Analyses of the organic degradation products suggest that reduction of imidazoline-2-thione to imidazoline-2-ylidene can indeed be accomplished by the lanthanide metal (providing one electron) as well as the (pdl′)− ligand (also delivering one electron).

Bis‐NHC Aluminium and Gallium Dihydride Cations [(NHC)2EH2]+ (E = Al, Ga)

The NHC alane and gallane adducts (NHC)·AlH2I (NHC = Me2ImMe 7, iPr2Im 8, iPr2ImMe 9) and (NHC)·GaH2I (NHC = Me2ImMe 10, iPr2ImMe 11, Dipp2Im 12; R2Im = 1,3‐di‐organyl‐imidazolin‐2‐ylidene; Dipp = 2,6‐diisopropylphenyl; iPr = isopropyl; Me2ImMe = 1,3,4,5‐tetra‐methyl‐imidazolin‐2‐ylidene) were prepared either by the simple yet efficient reaction of the NHC adduct (NHC)·AlH3 with elemental iodine or by the treatment of (NHC)·GaH3 with an excess of methyl iodide at room temperature. The reaction of one equivalent of the group 13 NHC complexes with an additional equivalent of the corresponding NHC afforded cationic aluminium and gallium hydrides [(NHC)2·AlH2]+I− (NHC = Me2ImMe 13, iPr2Im 14, iPr2ImMe 15) and [(NHC)2·GaH2]+I− (NHC = Me2ImMe 16, iPr2ImMe 17) and the normal and abnormal NHC coordinated compound [(Dipp2Im)·GaH2(aDipp2Im)]+I− 18. Compounds 7–18 were isolated and characterized by means of elemental analysis, IR and multinuclear NMR spectroscopy and by X‐ray diffraction of the compounds 7, 9, 10, 15, 16 and 18.

Specialized Olefin Metathesis Catalysts Featuring Unsymmetrical N-Heterocyclic Carbene Ligands Bearing N-(Fluoren-9-yl) Arm

Beneficial structural motifs from two known state-of-the-art olefin metathesis catalysts types, bearing unsymmetrical N-heterocyclic carbenes (uNHCs), were combined into a new hybridized design thereby translating the complementary beneficial reactivity demonstrated by their ‘parent’ complexes to the new N-fluorene derived olefin metathesis catalysts. Two chelating 2-iso-propoxy-benzylidene (Hoveyda-type) and two 3-phenyl-1H-inden-1-ylidene (indenylidene-type) complexes were successfully prepared by in situ generation of either the N′-mesityl (Mes) or N′-diisopropylphenyl (Dipp) derived uNHCs taking advantage of the thermal decomposition of the corresponding 2-(penta-fluorophenyl)-imidazolidines (NHC adducts). The new fluorene-derived catalysts mediate challenging olefin metathesis processes, such as α-olefin self-metathesis, with high selectivity and conversion.

Benzyl Borane NHC Adducts: Beyond B-C Bond Scission.

Benzyl-substituted boronates and borates are widely employed as mild sources in radical or anionic transfer reactions of benzyl entities. In this process the B-C bond to the benzyl moiety is essentially ruptured. In contrast, reactions with retention of the B-C bond are poorly investigated although several other reactive sites in benzyl-boron systems are clearly inherent. In this respect, the novel reactivity of the representative borane adduct IiPr-BH2 Bn [IiPr=:C{N(iPr)CH}2 , Bn=CH2 C6 H5 ] is demonstrated. Dihalogenation of the BH2 entity is observed with BCl3 and BBr3 , whereas BI3 either affords IiPr-BHI2 or proceeds with borylation of the aromatic phenyl ring to give a hydride-bridged bisborylated species. The photochemical mono- and dihalogenation of the benzylic CH2 group was demonstrated with elemental bromine Br2 . The brominated product IiPr-BBr2 -CHBr-C6 H5 was borylated at the benzylic carbon atom in an umpolung event with BI3 to afford the zwitterion IiPr-BI-CH(BI3 )-C6 H5 .

Unsymmetrically substituted benzimidazolium based Silver(I)-N-heterocyclic carbene complexes: Synthesis, characterization and in vitro anticancer study against human breast cancer and colon cancer

The promising biomedical applications of silver complexes stimulated the researchers to test these compounds against cancer. The present research work was designed to achieve this goal. In this work, a series of 5-methyl benzimidazole based N-Heterocyclic carbene ligands and respective silver(I) complexes were synthesized and tested on cancer cell lines to assess their anticancer activity. Unsymmetrically substituted benzimidazole was found unique in its reactivity and generation of a single product during NHC ligand formation was only possible after two successive alkylations with same alkyl halide. The corresponding Ag(I)-NHC adducts were obtained by in situ deprotonation of the NHC ligands. Synthesized compounds were characterized by various physcio-chemical and spectroscopic methods. Single crystal X-ray diffraction study of complex 7 revealed its mononuclear structure. Preliminary in vitro anticancer study of azolium salts and respective Ag(I)-NHC complexes against human breast cancer (MDA-MB-231), colon cancer (HCT-116) and normal endothelial cells (EA.hy926) cells revealed that all the compounds are more cytotoxic to cancer cells than normal cells and the complexes are relatively more potent compared to the corresponding NHC ligands. It was found that increased chain length and presence of methyl substituent on benzimidazole ring enhance the biopotency of Ag(I)-NHC complexes. The synthesized compounds were further studied for pro-apoptotic mechanism of action via Rhodamine 123 test. The tested compounds were found to induce apoptosis via extrinsic mitochondrial pathway.

The Quest for Stable Silaaldehydes: Synthesis and Reactivity of a Masked Silacarbonyl

The first donor-acceptor complex of a silaaldehyde, with the general formula (NHC)(Ar)Si(H)OGaCl3 (NHC=N-heterocyclic carbene), was synthesized using the reaction of silyliumylidene-NHC complex [(NHC)2 (Ar)Si]Cl with water in the presence of GaCl3 . Conversion of this complex to the corresponding silacarboxylate dimer [(NHC)(Ar)SiO2 GaCl2 ]2 , free silaacetal ArSi(H)(OR)2 , silaacyl chloride (NHC)(Ar)Si(Cl)OGaCl3 , and phosphasilene-NHC adduct (NHC)(Ar)Si(H)PTMS unveil its true potential as a synthon in silacarbonyl chemistry.

Recent Developments on N-Heterocyclic Carbene Supported Zinc Complexes: Synthesis and Use in Catalysis

The present contribution reviews the synthesis, reactivity, and use in catalysis of NHC–Zn complexes reported since 2013. NHC-stabilized Zn(II) species typically display enhanced stability relative to common organozinc species (such as Zn dialkyls), a feature of interest for the mediation of various chemical processes and the stabilization of reactive Zn-based species. Their use in catalysis is essentially dominated by reduction reactions of various unsaturated small molecules (including CO2), thus primarily involving Zn–H and Zn–alkyl derivatives as catalysts. Simple NHC adducts of Zn(II) dihalides also appear as effective catalysts for the reduction amination of CO2 and borylation of alkyl/aryl halides. Stable and well-defined Zn alkoxides have also been prepared and behave as effective catalysts in the polymerization of cyclic esters/carbonates for the production of well-defined biodegradable materials. Overall, the attractive features of NHC-based Zn(II) species include ready access, a reasonable stability/reactivity balance, and steric/electronic tunability (through the NHC source), which should promote their further development.1 Introduction2 NHC-Supported Zinc Alkyl/Aryl Species2.1 Synthesis2.2 Reactivity and Use in Catalysis3 NHC-Supported Zinc Hydride Species3.1 Synthesis3.2 Reactivity and Use in Catalysis4 NHC-Supported Zinc Amido/Alkoxide Species4.1 Synthesis4.2 Use in Catalysis5 NHC-Supported Zinc Dihalide Species5.1 Synthesis5.2 Use in Catalysis6 Other NHC-Stabilized Zn Species7 Conclusion

Sterically Bulky NHC Adducts of GaMe3 and InMe3 for H2 Activation and Lactide Polymerization

The sterically bulky Ga(III) and In(III) (IPr*)MMe3 adducts (1 and 2) and (SItBu)MMe3 adducts (3 and 4) (M = Ga, In; IPr* = 1,3-bis{2,6-bis(diphenylmethyl)-4-methylphenyl}-1,3-dihydro- imidazol-2-ylidene; SItBu = 1,3-bis(1,1-dimethylethyl)-imidazolidin-2-ylidene) were prepared and structurally characterized, allowing an estimation of the steric hindrance of such Lewis pairs (yields in 1–4: 92%, 90%, 73%, and 42%, respectively). While the IPr* adducts 1 and 2 are robust species, the more severely congested SItBu adducts 3 and 4 are more reactive and exhibit a limited stability in solution. Adduct (SItBu)GaMe3 (3) reacts quickly with H2 at room temperature to afford the corresponding aminal product, 1,3-di-tert-butylimidazolidine (5), along with free GaMe3. Such Frustrated Lewis Pair (FLP) reactivity constitutes the first instance of a H2 activation involving a simple trialkyl GaR3 species. Adduct 3 also mediates the ring-opening polymerization (ROP) of rac-lactide at room temperature to afford cyclic polylactide (PLA).

Reactivity enhancement of a diphosphene by reversible N-heterocyclic carbene coordination.

Diphosphene TerMesP = PTerMes (1; TerMes = 2,6-Mes2C6H3; Mes = 2,4,6-Me3C6H2) and NHCMe42 (NHCMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene) exist in an equilibrium mixture with the NHCMe4 -coordinated diphosphene 3. While uncoordinated 1 is inert to hydrolysis, the NHC adduct 3 readily undergoes hydrolysis to afford a phosphino-substituted phosphine oxide with the liberation of NHCMe4 . On this basis, conditions suitable for the catalytic use of NHCMe4 were identified. Similarly, while the hydrogenation of free diphosphene 1 with H3N·BH3 is very slow, 3 reacts instantaneously with H3N·BH3 at room temperature to afford a dihydrodiphosphane.

N-Heterocyclic Carbene-Carbodiimide ("NHC-CDI") Adduct or Zwitterionic-Type Neutral Amidinate-Supported Magnesium(II) and Zinc(II) Complexes.

A series of structurally characterized magnesium and zinc complexes of the form L4-tBuPh-M{N(SiMe3)2}2 [M = Mg (1) and Zn (2); L4-tBuPh = 1,3-diethyl-4,5-dimethylimidazolium-2-{N,N'-bis(4-tert-butylphenyl)amidinate}], L4-iPrPh-M{N(SiMe3)2}2 [M = Mg (3) and Zn (4); L4-iPrPh = 1,3-diethyl-4,5-dimethylimidazolium-2-{N,N'-bis(4-isopropylphenyl)amidinate}], and L4-iPrPh-ZnEt2 (5) bearing a zwitterionic-type neutral amidinate or N-heterocyclic carbene-carbodiimide ("NHC-CDI") adduct and monoanionic amido or alkyl ligands have been reported. The synthesis of compounds 1-5 was achieved by the direct addition of a "NHC-CDI" adduct to a corresponding metal bis(amide) or dialkyl reagent. All compounds 1-5 exist as monomers in the solid state. In all cases, the metal (magnesium or zinc) centers adopt a distorted four-coordinate tetrahedral geometry bonded to one N,N'-chelated neutral zwitterionic ligand and two monoanionic amido or alkyl moieties. In contrast, sterically bulky zwitterionic amidinate 1,3-diethyl-4,5-dimethylimidazolium-2-{N,N'-bis(2,6-diisopropylphenyl)amidinate} (LDipp) upon treatment with lithium bis[(trimethylsilyl)amide], Li{N(SiMe3)2}, affords the NHC-lithium complex MeIEt-[Li{N(SiMe3)2}]2 (6), in which one molecule of NHC (MeIEt = 1,3-diethyl-4,5-dimethylimidazol-2-ylidene) coordinates to one of the two lithium centers. In a similar way, the reaction between LDipp and Mg{N(SiMe3)2}2 allowed the formation of a NHC adduct of metal bis(amide), MeIEt-Mg{N(SiMe3)2}2 (7), instead of a zwitterionic adduct of metal bis(amide). Alternatively, the synthesis of both compounds 6 and 7 was achieved by the direct addition of 1 equiv of NHC, i.e., MeIEt to Li{N(SiMe3)2} (2.0 equiv) and Mg{N(SiMe3)2}2 (1.0 equiv) in benzene-d6, respectively. All compounds (1-7) were characterized by multinuclear {1H, 13C, and 29Si (for 1-4, 6, and 7) and 7Li (for compound 6)} magnetic resonance spectroscopy, mass spectrometry, elemental analysis, and single-crystal X-ray structural analysis. In addition, preliminary reactivity studies of zwitterion-supported metal complexes have been investigated. Furthermore, density functional theory calculations have been carried out to obtain the energetics of zwitterion-supported lithium and magnesium complexes.

Mode of Activation of Cobalt(II) Amides for Catalytic Hydrosilylation of Alkenes with Tertiary Silanes.

Cobalt(II) complexes capable of catalyzing alkene hydrosilylation in the absence of external activators are rarely known, and their activation mode has remained poorly understood. We present here that cobalt(II) amide complexes, [Co(N(SiMe3)2)2] and its NHC adducts [(NHC)Co(N(SiMe3)2)2] (NHC = N-heterocyclic carbene), are effective catalysts for the hydrosilylation of alkenes with tertiary silanes. Mechanistic studies revealed that cobalt(II) amides can react with hydrosilane to form cobalt(I) species, silylamide, and hydrogen, which serves as the entry to the genuine catalytically active species, presumably cobalt(I) species, for the cobalt-catalyzed hydrosilylation reaction.

NHC adducts of tantalum amidohalides: the first example of NHC abnormally coordinated to an early transition metal.

The reaction of Ta(NMe2)5 with NHC·HBF4 (NHC = IMes and SIMes) leads to new carbene adducts of the mixed TaV amidofluoride, [(NHC)TaF3(NMe2)2]. On the contrary, the reaction of Ta(NMe2)5 with IMes·HCl gives a complex mixture, where [(aIMes)TaCl2(NMe2)3] is identified as one of the products. This is the first example of abnormal NHC coordination to an early transition metal.

NHC Adducts of Disilylated Germylenes and Stannylenes and Their Coordination Chemistry with Group 11 Metals

Syntheses of NHC adducts of bis[tris(trimethylsilyl)silyl]germylene and ‐stannylene and some structurally related cyclic disilylated stannylenes are reported. Reactions of a cyclic stannylene NHC adduct with gold and silver cyanide led to cationic metal complexes with two cyanostannyl ligands. The Ag complex decomposed further to a cyanostannide. Reactions of bis[tris(trimethylsilyl)silyl]germylene and ‐stannylene NHC adducts with gold and silver cyanide displayed more unusual coordination chemistry with one silyl group of the ylene unit being transferred to the group 11 metal.

N‐Heterocyclic Carbenes and Parent Cations: Acidity, Nucleophilicity, Stability, and Hydrogen Bonding—Electrochemical Study and Ab Initio Calculations

AbstractN‐Heterocyclic carbenes (NHCs) are widely used as organocatalysts. Their reactivity (and instability) is related to their basicity and nucleophilicity, which, in turn, are linked to their scaffold. NHCs can be generated by chemical deprotonation or electrochemical reduction of the parent azolium cations, NHCH+s. Cyclic voltammetry enabled the reduction potential of the NHCH+s to be determined; the reduction potential is related to the acidity of the NHCH+s and the oxidation potential of the NHCs, which is related to the nucleophilicity of the NHCs. It was thus possible to order different NHCH+s and NHCs by their acidity and nucleophilicity, respectively. A study on the stability of NHCs was also performed in the absence and in the presence of acetic acid to assess the possibility of the coexistence of NHC and an acid in the same solution, opening the possibility of co‐catalysis. Finally, ab initio calculations confirmed the presence, in DMF, of hydrogen‐bonded NHCH+–NHC adducts, which could influence catalyst activity.

Green synthesis of mono- and di-selenium-N-heterocyclic carbene adducts: Characterizations, crystal structures and pro-apoptotic activities against human colorectal cancer

From mono- and bis-benzimidazolium salts, three new Se–NHC adducts were synthesized using water as a solvent. Single crystals analyses indicated that crystal structures of Se–NHC adducts are stabilized by face-to-face and edge-to-face π–π interactions in the absences of any potential hydrogen bonds donor and acceptor. Benzimidazolium salts and respective Se–NHC adducts were preliminarily screened against human colorectal cell line (HCT 116) where the benzyl substituted benzimidazolium salt and respective Se–NHC adduct showed the far best results among the test compounds. The selected compounds were further studied in detail for pro-apoptotic mechanism of action (Hoest 33342 stain and Rhodamine 123 tests). The tested compounds were found to induce apoptosis via mitochondria pathway.

Normal-to-Abnormal NHC Rearrangement of Al(III) , Ga(III) , and In(III) Trialkyl Complexes: Scope, Mechanism, Reactivity Studies, and H2 Activation.

The present contribution reports experimental and theoretical mechanistic investigations on a normal-to-abnormal (C2-to-C4-bonded) NHC rearrangement processes occurring with bulky group 13 metal NHC adducts, including the scope of such a reactivity for Al compounds. The sterically congested adducts (nItBu)MMe3 (nItBu=1,3-di-tert-butylimidazol-2-ylidene; M=Al, Ga, In; 1 a-c) readily rearrange to quantitatively afford the corresponding C4-bonded complexes (aItBu)MMe3 (4 a-c), a reaction that may be promoted by THF. Thorough experimental data and DFT calculations were performed on the nNHC-to-aNHC process converting the Al-nNHC (1 a) to its aNHC analogue 4 a. A nItBu/aItBu isomerization is proposed to account for the formation of the thermodynamic product 4 a through reaction of transient aItBu with THF-AlMe3 . The reaction of benzophenone with (nItBu)AlMe3 afforded the zwitterionic species (aItBu)(CPh2 -O-AlMe3 ) (6), reflecting the unusual reactivity that such bulky adducts may display. Interestingly, the nItBu/Al(iBu)3 Lewis pair behaves like a frustrated Lewis pair (FLP) since it readily reacts with H2 under mild conditions. This may open the way to future reactivity developments involving commonly used trialkylaluminum precursors.