N-heterocyclic Imine Research Articles

Copper Imidazolin-imine Coordination Compounds as Precursors for a Cu/Al Complex.

The reactions of [(CF3SO3Cu)2(C6H6)] with the sterically hindered imidazolin-2-imine ligands DippImTMS (1,3-Bis(2,6-diisopropylphenyl)-2-(trimethylsilylimino)imidazoline) or DippImH (1,3-bis(2,6-diisopropylphenyl) imidazolin-2-imine) lead to the formation of the linear copper(I) complexes [Cu(DippImTMS)(OTf)] (1) and [Cu(DippImH)2][OTf] (2), respectively. The triflate counteranion in 2 can be easily exchanged to the weakly coordinating [BArF] giving [Cu(DippImH)2][BArF] (3) (BArF = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate). Substitution of the N-heterocyclic imine (NHI) ligand in 3 by AlCp* (Cp* = pentamethylcyclopentadienyl) gives the tetrahedral [Cu(AlCp*)4][BArF] (5). The reaction between lithiated imidazolin-2-iminate DippImLi and CuCl results in the triangular cluster [Cu3(DippIm)2Cl] (4). All products have been fully characterized by 1H- and 13C NMR, mass spectrometry, as well as SC-XRD.

Electron-rich pyridines with para-N-heterocyclic imine substituents: ligand properties and coordination to CO2, SO2, BCl3 and PdII complexes.

Electron-rich pyridines with π donor groups at the para position play an important role as nucleophiles in organocatalysis, but their ligand properties and utilization in coordination chemistry have received little attention. Herein, we report the synthesis of two electron-rich pyridines 1 and 2 bearing N-heterocyclic imine groups at the para position and explore their coordination chemistry. Experimental and computational methods were used to assess the donor ability of the new pyridines showing that they are stronger donors than aminopyridines and guanidinyl pyridines, and that the nature of the N-heterocyclic backbone has a strong influence on the pyridine donor strength. Coordination compounds with Lewis acids including the CO2, SO2, BCl3 and PdII ions were synthesized and characterized. Despite the ambident character of the new pyridines, coordination preferentially occurs at the pyridine-N atom. Methyl transfer experiments reveal that 1 and 2 can act as demethylation reagents.

Reactivity of NHI-Stabilized Heavier Tetrylenes towards CO2 and N2 O.

A heteroleptic amino(imino)stannylene (TMS2 N)(It BuN)Sn: (TMS=trimethylsilyl, It Bu=C[(N-t Bu)CH]2 ) as well as two homoleptic NHI-stabilized tetrylenes, (It BuN)2 E: (NHI=N-heterocyclic imine, E=Ge, Sn) are presented. VT-NMR investigations of (It BuN)2 Sn: (2) reveal an equilibrium between the monomeric stannylene at room temperature and the dimeric form at -80 °C as well as in the solid state. Upon reaction of the homoleptic tetrylenes with CO2 , both compounds insert two equivalents of CO2 , however differing bonding modes can be observed. (It BuN)2 Sn: (2) inserts one equivalent of CO2 into each Sn-N bond, giving carbamato groups coordinated κ2 O,O' to the metal center. With (It BuN)2 Ge: (3), the Ge-N bonds stay intact upon activation, being bridged by one molecule of CO2 respectively, forming 4-membered rings. Furthermore, the reactivity of 2 towards N2 O was investigated, resulting in partial oxidation to form stannylene dimer [((It BuN)3 SnO)(It BuN)Sn:]2 (6).

A Guanidine-Supported π-Complex of Germanium Amenable to Intramolecular C-C Cleavage in Arene and Ge Atom Transfer.

The germylone dimNHCGe (dimNHC = diimino N-heterocyclic carbene) reacts with azides N3R (R = SiMe3 or p-tolyl) to furnish the first examples of germanium π-complexes, i.e. guanidine-ligated compounds (dimNHI-SiMe3)Ge (NHI= N-heterocyclic imine, R= SiMe3) and (dimNHI-Tol)Ge (R= p-tolyl). DFT calculations suggest that these species are formed by a Staudinger type replacement of dinitrogen in the azide by a nucleophilic germylone, leading to a transient carbene adduct of iminogermylidene. Heating a solution of compound (dimNHI-SiMe3)Ge to 70 °C results in extrusion of the iminogermylidene that further aggregates to produce the known [Me3SiNGe]4 tetramer, whereas the imidazolylidene fragment transforms into an unusual heptatriene species that can be considered as a product of carbene insertion into the C-C bond of a pendant Ar substituent at the imidazolylidene nitrogen of the dimNHC. Reaction of (dimNHI-SiMe3)Ge with tetrachloro-o-benzoquinone results in the net transfer of a germanium atom and formation of the free diimino-guanidine ligand. This ligand also forms when (dimNHI-SiMe3)Ge is treated with azide N3(p-Tol), with the germanium product being [(p-Tol)NGe]n.

Room Temperature Intermolecular Dearomatization of Arenes by an Acyclic Iminosilylene.

A novel nontransient acyclic iminosilylene (1), bearing a bulky super silyl group (-SitBu3) and N-heterocyclic imine ligand with a methylated backbone, was prepared and isolated. The methylated backbone is the feature of 1 that distinguishes it from the previously reported nonisolable iminosilylenes, as it prevents the intramolecular silylene center insertion into an aromatic C-C bond of an aryl substituent. Instead, 1 exhibits an intermolecular Büchner-ring-expansion-type reactivity; the silylene is capable of dearomatization of benzene and its derivatives, giving the corresponding silicon analogs of cycloheptatrienes, i.e. silepins, featuring seven-membered SiC6 rings with nearly planar geometry. The ring expansion reactions of 1 with benzene and 1,4-bis(trifluoromethyl)benzene are reversible. Similar reactions of 1 with N-heteroarenes (pyridine and DMAP) proceed more rapidly and irreversibly forming the corresponding azasilepins, also with nearly planar seven-membered SiNC5 rings. DFT calculations reveal an ambiphilic nature of 1 that allows the intermolecular aromatic C-C bond insertion to occur. Additional computational studies, which elucidate the inherent reactivity of 1, the role of the substituent effect, and reaction mechanisms behind the ring expansion transformations, are presented.

Terminal methylene phosphonium ions: precursors for transient monosubstituted phosphinocarbenes.

Isolable singlet carbenes are among the most important tools in chemistry, but generally require the interaction of two substituents with the electron deficient carbon atom. We herein report a synthetic approach to monosubstituted phosphinocarbenes via deprotonation of hitherto unknown diprotic terminal methylene phosphonium ions. Two methylene phosphonium salts bearing bulky N-heterocyclic imine substituents at the phosphorus atom were isolated and fully characterized. Deprotonation studies indicate the formation of transient monosubstituted carbenes that undergo intermolecular cycloadditions or intramolecular Buchner ring expansion to afford a cycloheptatriene derivative. The reaction mechanism of the latter transformation was elucidated using DFT calculations, which reveal the ambiphilic nature of the phosphinocarbene enabling the insertion into the aromatic C-C bond. Additional computational studies on the role of substituent effects are presented.

Umpolung in a Pair of Cobalt(III) Terminal Imido/Imidyl Complexes.

Reaction of the CoI complex [(TIMMNmes)CoI](PF6) (1) (TIMMNmes= tris‐[2‐(3‐mesityl‐imidazolin‐2‐ylidene)‐methyl]amine) with mesityl azide yields the CoIII imide [(TIMMNmes)CoIII(NMes)](PF6) (2). Oxidation of 2 with [FeCp2](PF6) provides access to a rare CoIII imidyl [(TIMMNmes)Co(NMes)](PF6)2 (3). Single‐crystal X‐ray diffractometry and EPR spectroscopy confirm the molecular structure of 3 and its S=1/2 ground state. ENDOR, X‐ray absorption spectroscopy and computational analyses indicate a ligand‐based oxidation; thus, an imidyl‐radical electronic structure for 3. Migratory insertion of one ancillary NHC to the imido ligand in 2 gives the CoI N‐heterocyclic imine (4) within 12 h. Conversely, it takes merely 0.5 h for 3 to transform to the CoII congener (5). The migratory insertion in 2 occurs via a nucleophilic attack of the imido ligand at the NHC to give 4, whereas in 3, a nucleophilic attack of the NHC at the electrophilic imidyl ligand yields 5. The reactivity shunt upon oxidation of 2 to 3 confirms an umpolung of the imido ligand.

Modification of bidentate bis(N-heterocyclic imine) ligands for low-valent main group complexes

The synthesis of modified neutral bis-NHI (NHI is N-heterocyclic imine) ligands and their application for the stabilization of tetryliumylidenes are reported. The ligands’ scaffolding consists of either saturated or methylated imidazoline backbones, and the bridge alternated from flexible ethylene to more rigid o-phenylene. Transmetalation reactivity of the cationic SnII compounds was tested towards LiAlH4 and IDipp→SiCl2 [IDipp is 1,3-bis(2,6-diisopropyl- phenyl)imidazol-2-ylidene] affording the respective aluminium and silicon complexes.

Aluminium alkyl complexes supported by imino-phosphanamide ligand as precursors for catalytic guanylation reactions of carbodiimides.

Herein, we report the synthesis, characterisation, and application of three aluminium alkyl complexes, [κ2-{NHIRP(Ph)NDipp}AlMe2] (R = Dipp (2a), Mes (2b); tBu (2c), Dipp = 2,6-diisopropylphenyl, Mes = mesityl, and tBu = tert-butyl), supported by unsymmetrical imino-phosphanamide [NHIRP(Ph)NDipp]− [R = Dipp (1a), Mes (1b), tBu (1c)] ligands as molecular precursors for the catalytic synthesis of guanidines using carbodiimides and primary amines. All the imino-phosphanamide ligands 1a, 1b and 1c were prepared in good yield from the corresponding N-heterocyclic imine (NHI) with phenylchloro-2,6-diisopropylphenylphosphanamine, PhP(Cl)NHDipp. The aluminium alkyl complexes 2a, 2b and 2c were obtained in good yield upon completion of the reaction between trimethyl aluminium and the protic ligands 1a, 1b and 1c in a 1 : 1 molar ratio in toluene via the elimination of methane, respectively. The molecular structures of the protic ligands 1b and 1c and the aluminium complexes 2a, 2b and 2c were established via single-crystal X-ray diffraction analysis. Complexes 2a, 2b and 2c were tested as pre-catalysts for the hydroamination/guanylation reaction of carbodiimides with aryl amines to afford guanidines at ambient temperature. All the aluminium complexes exhibited a high conversion with 1.5 mol% catalyst loading and broad substrate scope with a wide functional group tolerance during the guanylation reaction. We also proposed the most plausible mechanism, involving the formation of catalytically active three-coordinate Al species as the active pre-catalyst.

Crystalline phosphino(silyl)carbenes that readily form transition metal complexes.

Phosphino(silyl)carbenes are known for their ineptitude to form transition metal complexes. We describe the synthesis of phosphino(silyl)carbenes bearing N-heterocyclic imine groups and show that these isolable, crystalline carbenes readily form stable copper(I) and gold(I) complexes. The solid-state structures of the free carbenes and their transition metal complexes are reported.

Fluoride-Mediated Nucleophilic Aromatic Amination of Chloro-1H-1,2,3-triazolium Salts.

We report a fluoride-mediated nucleophilic aromatic amination of chloro-1H-1,2,3-triazolium salts with aliphatic amines. The reaction proceeded under mild reaction conditions to provide amino-1,2,3-triazolium salts with various functional groups, which can be utilized for further transformations. Moreover, it was found that an amino-1,2,3-triazolium salt was transformed via deprotonation into the N-heterocyclic imine (NHI), which exhibited the excellent catalytic activity for the cyanosilylation of acetophenone with trimethylsilyl cyanide.

Alkali metal complexes of an unsymmetrical imino–phosphanamidinate (N-P-N) ligand

The syntheses and characterization of alkali metal complexes of an unsymmetrical monoanionic imidazolin-2-imino–phosphanamidinate ligand, {NHIDippP(Ph)N(Dipp)}− {L}− [Dipp = 2,6-diisopropylphenyl] is reported. The protio-ligand [NHIDippP(Ph)NH(Dipp)] (LH) was obtained by the reaction of N-heterocyclic imine (NHI) and [PhP(Cl)NHDipp] in the presence of Et3N in toluene at room temperature. The lithium complex [{NHIDippP(Ph)N(Dipp)}Li(OEt2)] (1) (Et = ethyl) was synthesized by the reaction of the protio ligand LH and one equivalent LiCH2SiMe3 in diethyl ether at room temperature. The reaction of sodium or potassium hexamethyldisilazide [MN(SiMe3)2] (M = Na or K) with LH in 1:1 molar ratio yielded corresponding alkali metal complexes [{NHIDippP(Ph)N(Dipp)}Na(THF)] (2) and [{NHIDippP(Ph)N(Dipp)}K(THF)2] (3) respectively in good yield. The molecular structures of the protio-ligand LH and alkali metal complexes 1 and 3 were established by single-crystal X-ray diffraction analysis. In the solid-state structures of 1 and 3, the lithium and potassium ions are κ2 bonded to the monoanionic ligand moiety through the imine nitrogen and phosphanamidinate nitrogen atoms.

An Unsymmetric Imino-Phosphanamidinate Ligand and its Y(III) Complex: Synthesis, Characterization, and Catalytic Hydroboration of Carbonyl Compounds.

An imino-phosphanamide ligand, [NHIiPr2Me2P(Ph)NH-2,6-iPr2C6H3] (LH), containing two different N-substituents was prepared by the direct reaction of the lithium salt of N-heterocyclic imine (NHI) with phenylchloro-2,6-diisopropylphenyl phosphanamine, PhP(Cl)NH-2,6-iPr2-C6H3. Reaction of LH with Y(N(SiMe3)2)3 afforded the heteroleptic complex, [{L}Y(N(SiMe3)2)2] (1), by elimination of HN(SiMe3)2. Compound 1 was characterized by multinuclear NMR and X-ray crystallography. In the complex, the Y(III) center was found to be tetracoordinate in a distorted tetrahedral geometry. The ligand, imino-phosphanamidinate, [L]-, functions in a chelating manner, and its coordination to Y(III) results in a distorted 4-membered YPN2 ring. As a proof of principle of its activity, 1 was used as a precatalyst for the hydroboration of various aldehydes and ketones using HBpin as the hydrogen source. The hydroboration reaction was rapid and clean even with low catalyst loadings (0.01-0.1 mol %). In addition, a very good functional group tolerance was observed in these reactions.

A Proton-Responsive Annulated Mesoionic Carbene (MIC) Scaffold on Ir Complex for Proton/Hydride Shuttle: An Experimental and Computational Investigation on Reductive Amination of Aldehyde

A Cp*Ir(III) complex (1) bearing a proton-responsive hydroxy unit on an annulated imidazo[1,2-a][1,8]naphthyridine based mesoionic carbene scaffold was synthesized by two different synthetic routes. The molecular structure of 1 revealed an anionic lactam form of the ligand. The acid–base equilibrium between the lactam-lactim tautomers on the ligand scaffold was examined by 1H NMR and UV–vis spectra. The pKa of the appendage −OH group in the lactim form of 1 was estimated to assess the proton transfer property of the catalyst. The catalytic efficacy of 1 for reductive amination of aldehyde was evaluated by utilizing three different hydrogen sources: molecular H2, iPrOH/KOtBu combination, and HCOOH/Et3N (5:2) azeotropic mixture. The HCOOH/Et3N (5:2) azeotropic mixture protocol was found to be the best among the three different hydrogenation methods. Catalyst 1 hydrogenates imines chemoselectively over carbonyls under the reaction conditions. A range of aldehydes was reductively aminated to the corresponding secondary amines using the HCOOH/Et3N (5:2) azeotropic mixture. Further, catalyst 1 showed high efficiency for the reduction of a wide variety of N-heterocyclic imine derivatives. The lactam-lactim tautomerization of the ligand system is proposed for direct hydrogenation, whereas only the lactam form operates in the strongly basic medium (iPrOH/KOtBu). Under HCOOH/Et3N (5:2) conditions, the lactam scaffold is not protonated; rather, an outer-sphere hydride transfer from formate to the Ir is proposed, which is supported by 1H NMR and DFT calculations. Finally, ligand-promoted hydride transfer from metal-hydride to the protonated imine affords the corresponding amine. A close agreement between the experimentally estimated and computed thermodynamic/kinetic parameters gives credence to the metal-ligand cooperative mechanism for the imine hydrogenation reaction using the HCOOH/Et3N (5:2) azeotropic mixture.

Isolation and Reactivity of Chlorotetryliumylidenes Using a Bidentate Bis(N-heterocyclic imine) Ligand

By use of the ethylene-bridged bidentate bis-NHI (NHI = N-heterocyclic imine) ligand 1 toward the complexation of ECl2·(dioxane) (E = Ge, Sn) the synthesis of the first bis-NHI-stabilized chloroger...

Isolation, characterization and reactivity of three-coordinate phosphorus dications isoelectronic to alanes and silylium cations.

Three-coordinate main group Lewis acids are exceedingly important reagents in chemical synthesis. In contrast to the well-established chemistries of neutral group 13 and cationic group 14 species, isoelectronic group 15 element dications are unknown. In this work, we use stabilizing N-heterocyclic imine substituents to isolate and characterize phosphorandiylium dications ([R3P]2+) and show that the electrophilicity at the phosphorus atoms is controlled by the π-electron-donating ability of these subtituents. Structural, spectroscopic and theoretical results reveal that the phosphorus dications adopt a perfectly trigonal-planar geometry with the electron-deficient phosphorus centres being well separated from the borate anions. The reactivity of the dications reveal their exceptional Lewis acidity at phosphorus; the adjacent nitrogen atoms, however, are weakly basic, resulting in transformations such as chloride ion abstraction from Me3SiCl and the selective monodefluorination of trifluoromethyl groups.

Isolation of an N-Heterocyclic Carbene Complex of a Borasilene.

Borasilenes, that is complexes which contain a boron-silicon double bond, have scarcely been isolated to date. In pursuit of such species, (Me3 Si)3 SiB(Cl)NHI (2, NHI=bulky N-heterocyclic imine) was prepared and treated with KOtBu to achieve formal extrusion of ClSiMe3 . The formation of an elusive borasilene (3int ) is postulated and it was verified by isolation of the N-heterocyclic carbene adduct (Me3 Si)2 SiB(I )NHI (4, I =1,3,4,5-tetramethyl-imidazolin-2-ylidene). X-ray crystallographic study and theoretical calculations on 4 diagnosed a boron-silicon double bond with marked zwitterionic character. The negative charge resides at the Si atom which marks the apex of a trigonal pyramid. Structural comparison of 4 with boron cation congeners (5+ , 6+ ) suggests that the positive charge is mainly located at the trigonal planar-coordinated B center. The conversion of 4 with pinacolborane (HBpin, 2 equiv) resulted in cleavage of the double bond to produce (Me3 Si)2 Si(Bpin)2 and (NHI)BH2 (I ).

NHI- and NHC-Supported Al(III) Hydrides for Amine–Borane Dehydrocoupling Catalysis

The catalytic dehydrocoupling of amine–boranes has recently received a great deal of attention due to its potential in hydrogen storage applications. The use of aluminum catalysts for this transformation would provide an additional cost-effective and sustainable approach towards the hydrogen economy. Herein, we report the use of both N-heterocyclic imine (NHI)- and carbene (NHC)-supported Al(III) hydrides and their role in the catalytic dehydrocoupling of Me2NHBH3. Differences in the σ-donating ability of the ligand class resulted in a more stable catalyst for NHI-Al(III) hydrides, whereas a deactivation pathway was found in the case of NHC-Al(III) hydrides.

Successive Protonation of an N-Heterocyclic Imine Derived Carbonyl: Superelectrophilic Dication Versus Masked Acylium Ion.

Carbonyl cations are among the most commonly invoked reactive intermediates in organic synthesis. While Olah pioneered superacids to provide a "stable ion" environment for their study in situ, isolated examples are rare. Here, we disclose successive protonation of an N-heterocyclic imine (NHI) derived carbonyl compound (IDippN)2 CO, 2, to the monocation [(IDippN)(IDippNH) CO]+ , [3]+ , and the doubly protonated dication [(IDippNH)2 CO]2+ , [4]2+ . [3]+ represents a rare example of an N-protonated carbonyl cation and [4]2+ the first example of a superelectrophilic carbonyl dication. All three compounds have been characterized by X-ray crystallography and IR spectroscopy, revealing stepwise strengthening of the C=O bond on protonation. The unique stability of these systems is attributed to the enhanced basicity and steric profile provided by the NHI substituents. In addition, we report the related singly NHI-stabilized cation [IDippNCO]+ , [5]+ . Crystallographic and DFT analyses provide insight into the interaction between the carbonyl fragment and the NHI, which reveals that the [CNCO]+ unit (isoelectronic to CCCO) can be described as an acylium cation "masked" as a cumulene.

Successive Protonation of an N‐Heterocyclic Imine Derived Carbonyl: Superelectrophilic Dication Versus Masked Acylium Ion

AbstractCarbonyl cations are among the most commonly invoked reactive intermediates in organic synthesis. While Olah pioneered superacids to provide a “stable ion” environment for their study in situ, isolated examples are rare. Here, we disclose successive protonation of an N‐heterocyclic imine (NHI) derived carbonyl compound (IDippN)2CO, 2, to the monocation [(IDippN)(IDippNH) CO]+, [3]+, and the doubly protonated dication [(IDippNH)2CO]2+, [4]2+. [3]+ represents a rare example of an N‐protonated carbonyl cation and [4]2+ the first example of a superelectrophilic carbonyl dication. All three compounds have been characterized by X‐ray crystallography and IR spectroscopy, revealing stepwise strengthening of the C=O bond on protonation. The unique stability of these systems is attributed to the enhanced basicity and steric profile provided by the NHI substituents. In addition, we report the related singly NHI‐stabilized cation [IDippNCO]+, [5]+. Crystallographic and DFT analyses provide insight into the interaction between the carbonyl fragment and the NHI, which reveals that the [CNCO]+ unit (isoelectronic to CCCO) can be described as an acylium cation “masked” as a cumulene.