Carbene Adducts Research Articles

PH3 as a Phosphorus Source for Phosphinidene–Carbene Adducts and Phosphinidene–Transition Metal Complexes

The parent phosphinidene-carbene adduct NHC=PH is a versatile synthon in main group chemistry and can act as an electronically flexible ligand for transition metals. Previously, it could only be synthesized with sterically demanding N-aryl substituents. This paper describes simple methods for the synthesis of NHC=PH adducts with varying steric demand from elemental phosphorus or easily accessible phosphorus sources. Furthermore, the reactivity of NHCs towards PH3 was investigated. It was shown how a NHC inserts into the P-H bond of PH3, which opens up a route to NHC=PH adducts from an imidazolium salt and PH3. The adducts were used in the simple syntheses of bis(phosphinidene) mercury(<small>II</small>) and group 6 pentacarbonyl parent phosphinidene complexes. Their electronic and structural properties were investigated to elucidate the influence of the NHC on the phosphinidene and identify possible applications.

Theoretical study of the mechanism for the sequential N-O and N-N bond cleavage within N2O adducts of N-heterocyclic carbenes by a vanadium(iii) complex.

A theoretical study into the reactions of the N2O adducts of N-heterocyclic carbenes (NHCs) and a V((III)) complex was carried out using DFT calculations. Unlike most transition metal reactions with N2O that simply release N2 following O-atom transfer onto the metal centre, this NHC-based system traps the entire N2O molecule and then cleaves both the N-O and N-N bond in two consecutive reactions. The NHC presence increases the reactivity of N2O by altering the distribution of electron density away from the O-atom towards the two N-atoms. This electronic redistribution enables V-N binding interactions to form a reactive N,O-donor intermediate species. Our results show that bond breaking with concomitant ligand migration occurs via a concerted process for both the N-O and N-N cleavage reactions.

Carbene insertion into a P-H bond: parent phosphinidene-carbene adducts from PH3 and bis(phosphinidene)mercury complexes.

PH3 reacts with the in situ generated N-heterocyclic carbene DippNHC* (DippNHC* = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) to give the phosphanyl-imidazolidine [(Dipp)NHC*-H]-[PH2]. Upon treatment with an ortho-quinone, [(Dipp)NHC*-H]-[PH2] is dehydrogenated to give the parent phosphinidene-carbene adduct (Dipp)NHC*[double bond, length as m-dash]PH. Alternative routes to [(Dipp)NHC*-H]-[PH2] and (Dipp)NHC*[double bond, length as m-dash]PH employ NaPH2 and (TMS)3P7 (TMS = trimethylsilyl), respectively, as phosphorus sources. The adduct (Dipp)NHC*[double bond, length as m-dash]PH and the related adduct (Dipp)NHC[double bond, length as m-dash]PH ((Dipp)NHC = bis(2,6-diisopropylphenyl)imidazol-2-ylidene) possessing an unsaturated NHC backbone both react with HgCl2 to give the bis(carbene-phosphinidenyl) complexes [((Dipp)NHC*[double bond, length as m-dash]P)2Hg] and [((Dipp)NHC[double bond, length as m-dash]P)2Hg].

N-Heterocyclic carbene adducts to [Cp′FeI]2: synthesis and molecular and electronic structure

Adducts [Cp′FeI(NHC)] exhibit a highly anisotropic magnetic Ms = ±2 ground state resulting in unusual large spin–lattice (Orbach) relaxation barriers observed by zero-field 57Fe Mössbauer spectroscopy.

Reactions of Li/Cl Phosphinidenoid Complexes with 1,3,4,5‐Tetramethylimidazol‐2‐ylidene: A New Route to N‐Heterocyclic Carbene Adducts of Terminal Phosphin­idene Complexes and an Unprecedented Transformation of an Oxaphosphirane Complex

AbstractThe reaction of Li/Cl phosphinidenoid tungsten(0) complex 2 [R = CH(SiMe3)2] with 1,3,4,5‐tetramethylimidazol‐2‐ylidene (3) yielded the zwitterionic phosphaalkene complex 4. A comparative study on chloro‐ (5) or dichloro(organo)phosphane complexes 1 [R = CH(SiMe3)2] and NHC 3 revealed the formation of 4 at ambient temperature. First evidence for the formation of phosphinidenoid complex 7, having a C‐Cl imidazolium cation, and its reaction with benzaldehyde to give oxaphosphirane complex 9 are presented. Furthermore, the first example of a formal exchange at the phosphorus center of an oxaphosphirane complex to give zwitterion 4 is demonstrated.

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.

Copper(I) Complexes of Zwitterionic Imidazolium-2-Amidinates, a Promising Class of Electroneutral, Amidinate-Type Ligands.

The first complexes containing imidazolium-2-amidinates as ligands (betaine-type adducts of imidazolium-based carbenes and carbodiimides, NHC-CDI) are reported. Interaction of the sterically hindered betaines ICyCDI(DiPP) and IMeCDI(DiPP) [both bearing 2,6-diisopropylphenyl (DiPP) substituents on the terminal N atoms] with Cu(I) acetate affords mononuclear, electroneutral complexes 1a and 1b, which contain NHC-CDI and acetate ligands terminally bound to linear Cu(I) centers. In contrast, the less encumbered ligand ICyCDI(p-Tol), with p-tolyl substituents on the nitrogen donor atoms, affords a dicationic trigonal paddlewheel complex, [Cu2(μ-ICyCDI(p-Tol))3](2+)[OAc(-)]2 (2-OAc). The nuclear magnetic resonance (NMR) resonances of this compound are broad and indicate that in solution the acetate anion and the betaine ligands compete for binding the Cu atom. Replacing the external acetate with the less coordinating tetraphenylborate anion provides the corresponding derivative 2-BPh4 that, in contrast with 2-OAc, gives rise to sharp and well-defined NMR spectra. The short Cu-Cu distance in the binuclear dication [Cu2(μ-ICyCDI(p-Tol))3](2+) observed in the X-ray structures of 2-BPh4 and 2-OAc, ca. 2.42 Å, points to a relatively strong "cuprophilic" interaction. Attempts to force the bridging coordination mode of IMeCDI(DiPP) displacing the acetate anion with BPh4(-) led to the isolation of the cationic mononuclear derivative [Cu(IMeCDI(DiPP))2](+)[BPh4](-) (3b) that contains two terminally bound betaine ligands. Compound 3b readily decomposes upon being heated, cleanly affording the bis-carbene complex [Cu(IMe)2](+)[BPh4(-)] (4) and releasing the corresponding carbodiimide (C(═N-DiPP)2).

N–N bond cleavage in a nitrous oxide–NHC adduct promoted by a PNP pincer cobalt(I) complex

The reactivity of a PNP cobalt(I) complex towards the nitrous oxide N-heterocyclic carbene adduct (NHC–N2O) was explored. Several products were isolated including the cobalt nitrosyl complex (HPNP)Co(NO) (1) and the NHC-derived imine NHCNH (2). This study provides a rare example of metal-mediated rupture of the N–N bond in a nitrous oxide derivative, a route that could be used to generate late-metal nitrosyls.

ChemInform Abstract: Structurally Simple Complexes of CO2

AbstractReview: 169 refs.

Lithium Complexes of Neutral Bis-NHC Ligands

The employment of lithium hexamethyldisilazide for the deprotonation of methylene-bridged bis(imidazolium) salts led to the formation of lithium carbene adducts. Depending on the crystallization method and the substituents of the ligands, monomeric, dimeric, or polymeric solid-state structures were obtained. These lithium carbene complexes represent the first examples of lithium complexes bearing neutral bis(N-heterocyclic carbene) ligands.

Dimeric platinum-stannylene complexes by twofold ligand transfer from an NHC adduct to an organotin(II) hydride.

Upon ligand transfer of hydrides and NHC a dimeric formal Pt(I)–Sn(I) (2) complex was obtained from a 1:1 reaction of Pt(cod)2 (cod = 1,5-cyclooctadiene) and the NHC adduct to a bulky terphenyltin(II) hydride Ar*SnH(NHC) (1) [NHC = 1,3,4,5-tetramethylimidazol-2-ylidene, Ar* = 2,6-(2,4,6-triisopropylphenyl)C6H3]. The bridging stannylene Sn atoms each reveal an individual coordinative interaction with the platinum atoms and the hydrides with ‘non-classic’ Pt–H–Sn bonds.

Zwitterionic carbene adducts and their carbene isomers

N-heterocyclic carbenes (NHC) and abnormal NHCs (aNHC) form the stable adducts,1and2, with X (CH2, SiH2, NH, PH, O, S), which are excellent nucleophiles at X.

Remarkably stable chelating bis-N-heterocyclic carbene adducts of phosphorus(i) cations.

A convenient one-pot synthesis of chelating bis-N-heterocyclic carbene-ligated P(I) salts is described. The solid state structures of these remarkably stable phosphamethine cyanine dyes with various N-alkyl groups and counter-anions are reported, and initial reactivity results are discussed.

Two alternative approaches to access mixed hydride-amido zinc complexes: synthetic, structural and solution implications.

Using bis(amide) Zn(HMDS)2 (HMDS = 1,1,1,3,3,3-hexamethyldisilazide) as a precursor, this study explores the synthesis of N-heterocyclic carbene stabilized mixed amido-hydride zinc complexes using two alternative hydride sources, namely dimethylamine borane (DMAB) and phenylsilane PhSiH3. Hydride-rich zinc cluster Zn4(HMDS)2H6·2IPr () (IPr = 1,3-bis(2,6-di-isopropylphenyl)imidazol-2-ylidene), which can be envisaged as a co-complex of IPr·ZnH2 and (HMDS)ZnH, is obtained when DMAB is employed, with the concomitant formation of heteroleptic bis(amido)borane [HB(NMe2)(HMDS)] and H2 evolution. NMR studies in d8-THF show that although the bulky carbene IPr does not bind to the zinc bis(amide), its presence in the reaction media is required in order to stabilise . Reactions using the slightly less sterically demanding NHC IXy (IXy = 1,3-bis-(2,6-dimethylphenyl)imidazol-2-ylidene) led to the isolation and structural elucidation of the carbene adduct Zn(HMDS)2·IXy (). Contrastingly, mixtures of equimolar amounts of PhSiH3 and the zinc bis(amide) (60 °C, 3 h, hexane) afforded monomeric heteroleptic hydride (HMDS)ZnH·IPr (). NMR studies, including DOSY experiments, revealed that while the integrity of is retained in polar d8-THF solutions, in lower polarity C6D6 it displays a much more complex solution behaviour, being in equilibrium with the homoleptic species ZnH2·IPr, Zn(HMDS)2 and IPr.

C- and N-Adducts of N-alkenyl substituted Arduengo carbene and N-alkyl substituted imidazole with PF5: synthesis and structural investigation

Thermolytic decomposition of 3,3’-(ethan-1,2-diyl)bis(1-methyl-1H-imidazolium) bis-(hexafluorophosphate) (1) under reduced pressure results in a 1: 1 mixture of pentafluo-ro(1-ethenyl-3-methyl-2,3-dihydro-1H-imidazole-2-ylidene-κC 2)phosphorous (2) and penta- fluoro(3-methyl-1H-imidazole-κN 3)phosphorous (3) in an almost quantitate yield. Products 2 and 3 were isolated as individual compounds by fractional recrystallization from hot methanol. Direct observation of a 1: 1 co-crystallite of 2-pentafluorophosphanyl-1-ethenyl-3-methyl-1H-imidazolium hexafluorophosphate and adduct 2 (compound 5) as a minor coproduct of this reaction reveals a complicated path of the thermolysis. Crystal and molecular structures of compounds 2, 3, and 5 were established by X-ray diffraction analysis. Compound 2 presents the first example of an N-alkenyl substituted Arduengo carbene adduct with a main Group element Lewis acid. Compound 3 presents the second known organic base N-adduct with PF5.

Expanded ring N-heterocyclic carbene adducts of group 15 element trichlorides: synthesis and reduction studies.

Reactions of the expanded ring N-heterocyclic carbene, 6-Dip (:C{N(Dip)CH2}2CH2, Dip = 2,6-diisopropylphenyl), with group 15 element trichlorides have yielded the monomeric complexes, [(6-Dip)ECl3] (E = P, As or Sb), two examples of which (E = P and Sb) have been crystallographically characterised. Reduction of [(6-Dip)PCl3] with KC8 yielded the unusual tetraphosphorus dicationic complex, [(6-Dip)2(μ-P4)]Cl2, the X-ray crystal structure of which shows it to be an ion-separate salt. The compound can also be prepared from the direct reaction of excess 6-Dip with PCl3. Treatment of the cyclic amidinium salt, [6-MesH]Br (6-MesH = [HC{N(Mes)CH2}2CH2](+), Mes = mesityl) with KC8, leads to reductive coupling of the heterocycle and formation of the hindered bis(hexahydropyrimidine), (6-MesH)2. An X-ray crystallographic analysis of (6-MesH)2 shows the compound to have a long central C-C bond, while an electrochemical analysis reveals it to undergo an irreversible two-electron oxidation in dichloromethane solutions.

Zincocene and dizincocene N-heterocyclic carbene complexes and catalytic hydrogenation of imines and ketones.

The N-heterocyclic carbene (NHC) adducts Zn(Cp(R))(2) (NHC)] (Cp(R) =C(5)HMe(4), C(5)H(4) SiMe(3); NHC=ItBu, IDipp (Dipp=2,6-diisopropylphenyl), IMes (Mes=mesityl), SIMes) were prepared and shown to be active catalysts for the hydrogenation of imines, whereas decamethylzincocene [ZnCp*(2)] is highly active for the hydrogenation of ketones in the presence of noncoordinating NHCs. The abnormal carbene complex [Zn(OCHPh(2))(2) (aItBu)](2) was formed from spontaneous rearrangement of the ItBu ligand during incomplete hydrogenation of benzophenone. Two isolated Zn(I) adducts [Zn(2)Cp*(2) (NHC)] (NHC=ItBu, SIMes) are presented and characterized as weak adducts on the basis of (13)C NMR spectroscopic and X-ray diffraction experiments. A mechanistic proposal for the reduction of [ZnCp*(2)] with H(2) to give [Zn(2)Cp*(2)] is discussed.

ChemInform Abstract: Carbene Adduct of Cyclopalladated Ferrocenylimine‐Assisted Synthesis of Aminopyridine Derivatives by the Amination of Chloropyridines with Primary and Secondary Amines.

AbstractThis presents a catalyst for the Buchwald‐Hartwig amination of chloropyridines.

Zwitterionic borane adducts of N-heterocyclic carbenes from mesomeric betaines of uracil

We prepared a series of imidazolium-substituted uracil-anions which are members of the class of cross-conjugated heterocyclic mesomeric betaines. They are in tautomeric equilibrium with their N-heterocyclic carbenes, uracil-6-yl-imidazol-2-ylidenes. These carbenes can be trapped by reaction with sulfur, selenium, as well as by triethylborane and triphenylborane, respectively. The latter trapping reaction yielded the first representatives of a new heterocyclic zwitterionic ring system, imidazo[2',1':3,4][1,4,2]diazaborolo[1,5-c]pyrimidinium-10-ide. Results of two single crystal X-ray structure analyses are presented.

The first bismuth–NHC complexes

The synthesis, isolation and crystallographic characterization of the first N-heterocyclic carbene adducts of bismuth is reported, by direct reaction of the Dipp2NHC (Dipp = 2,6-diisopropylphenyl) or (i)Pr2(Me2)NHC with BiCl3. This represents the last non-radioactive element from groups 13-17 for which an NHC-element fragment remained unreported.