Carbene Donor Research Articles

Solvent-dependent switch of ligand donor ability and catalytic activity of ruthenium(II) complexes containing pyridinylidene amide (PYA) N-heterocyclic carbene hybrid ligands.

Chelating ligands incorporating both N-[1-alkylpyridin-4(1H)-ylidene]amide (PYA) and N-heterocyclic carbene (NHC) donor sites were prepared and used for the synthesis of ruthenium(II) complexes. Cyclic voltammetry, NMR, and UV-vis spectroscopy of the complexes indicate a solvent-dependent contribution of the limiting resonance structures associated with the ligand in solution. The neutral pyridylidene imine structure is more pronounced in apolar solvents (CH2Cl2), while the mesoionic pyridinium amide form is predominant in polar solvents (MeOH, DMSO). The distinct electronic properties of these hybrid PYA-NHC ligands in different solvents have a direct influence on the catalytic activity of the ruthenium center, e.g., in the dehydrogenation of benzyl alcohol to benzaldehyde. The activity in different solvents qualitatively correlates with the solvent permittivity.

Accessing zinc monohydride cations through coordinative interactions.

We present isolable examples of formal zinc hydride cations supported by N-heterocyclic carbene (NHC) donors, and investigate the dual electrophilic and nucleophilic (hydridic) character of the encapsulated [ZnH](+) units by computational methods and preliminary hydrosilylation catalysis.

ChemInform Abstract: Beyond Click Chemistry — Supramolecular Interactions of 1,2,3‐Triazoles

AbstractReview: 583 refs.

Physicochemical Analysis of Ruthenium(II) Sensitizers of 1,2,3-Triazole-Derived Mesoionic Carbene and Cyclometalating Ligands

A series of heteroleptic bis(tridentate) ruthenium(II) complexes bearing ligands featuring 1,2,3-triazolide and 1,2,3-triazolylidene units are presented. The synthesis of the C^N^N-coordinated ruthenium(II) triazolide complex is achieved by direct C-H activation, which is enabled by the use of a 1,5-disubstituted triazole. By postcomplexation alkylation, the ruthenium(II) 1,2,3-triazolide complex can be converted to the corresponding 1,2,3-triazolylidene complex. Additionally, a ruthenium(II) complex featuring a C^N^C-coordinating bis(1,2,3-triazolylidene)pyridine ligand is prepared via transmetalation from a silver(I) triazolylidene precursor. The electronic consequences of the carbanion and mesoionic carbene donors are studied both experimentally and computationally. The presented complexes exhibit a broad absorption in the visible region as well as long lifetimes of the charge-separated excited state suggesting their application in photoredox catalysis and photovoltaics. Testing of the dyes in a conventional dye-sensitized solar cell (DSSC) generates, however, only modest power conversion efficiencies (PCEs).

Beyond click chemistry – supramolecular interactions of 1,2,3-triazoles

The research on 1,2,3-triazoles has been lively and ever-growing since its stimulation by the advent of click chemistry. The attractiveness of 1H-1,2,3-triazoles and their derivatives originates from their unique combination of facile accessibility via click chemistry and truly diverse supramolecular interactions, which enabled myriads of applications in supramolecular and coordination chemistry. The nitrogen-rich triazole features a highly polarized carbon atom allowing the complexation of anions by hydrogen and halogen bonding or, in the case of the triazolium salts, via charge-assisted hydrogen and halogen bonds. On the other hand, the triazole offers several N-coordination modes including coordination via anionic and cationic nitrogen donors of triazolate and triazolium ions, respectively. After CH-deprotonation of the triazole and the triazolium, powerful carbanionic and mesoionic carbene donors, respectively, are available. The latter coordination mode even features non-innocent ligand behavior. Moreover, these supramolecular interactions can be combined, e.g., in ion-pair recognition, preorganization by intramolecular hydrogen bond donation and acceptance, and in bimetallic complexes. Ultimately, by clicking two building blocks into place, the triazole emerges as a most versatile functional unit allowing very successful applications, e.g., in anion recognition, catalysis, and photochemistry, thus going far beyond the original purpose of click chemistry. It is the intention of this review to provide a detailed analysis of the various supramolecular interactions of triazoles in comparison to established functional units, which may serve as guidelines for further applications.

A Tripodal Ligand Constructed from Mesoionic Carbene Donors

A tripodal ligand constructed solely from mesoionic carbene donors is reported. The donor strength of this ligand is lower than most imidazol-2-ylidene-based tris(carbene)borate ligands, as measured by IR spectroscopy of {NiNO}(10) and {Mn(CO)3}(+) derivatives. The attenuated donor strength is proposed to be due to the collective electron-withdrawing effect of the ligand's aryl substituents.

Carbene-Bound Borane and Silane Adducts: A Comprehensive DFT Study on Their Stability and Propensity for Hydride-Mediated Ring Expansion

The stability of a variety of borane (BH3 and BH2NHMe) and silane (SiHnPh4–n, n = 0–4) adducts with diamino (NHC) and aminoalkyl (CAAC) carbenes has been carefully examined using M06-2X/cc-pVDZ computations, including natural bond orbital and atoms-in-molecules analyses. Moreover a potential mechanism for the hydride-mediated ring expansion of the carbene donors is reported. While the NHC adducts can undergo thermally induced ring-expansion chemistry, the CAAC adducts show increased stability due to a large energetic barrier for the insertion of boron (or silicon) atoms into the CAAC heterocycle. A series of substituted NHCs were investigated to further explore the roles of both electronic and steric effects on adduct stabilities and on their propensities for undergoing ring-expansion transformations.

Heterobimetallic Carbene Complexes by a Single-Step Site-Selective Metalation of a Tricarbene Ligand

The unsymmetrical tris(imidazolium) salt H3-1(Br)3, featuring a 1,2,4-substitution pattern of the central phenyl ring, after triple imidazolium C2 deprotonation reacts in a one-pot reaction with Pd(OAc)2 and [M(Cp*)(Cl)2]2 (M = Rh(III), Ir(III)) to yield heterobimetallic complexes [3] (M = Rh) and [4] (M = Ir), in which the Pd(II) ion is chelated by two ortho N-heterocyclic carbene (NHC) donors while the third NHC donor coordinates to the M(III) center, which orthometalates the central phenyl ring.

Simple, efficient and reusable Pd–NHC catalysts for hydroamination

A series of chelating NHC–palladium complexes with different alkane-bridges of the type Pd[NHC–(CH2)n–NHC]X2 (X = Br or Cl, n = 2–4) were synthesized, where NHC is a triazolyl-N-heterocyclic carbene donor ligand. The bromide complexes with n = 2 and 3 were characterized by X-ray crystallography. The effects of the length of the bridge and halide ligand on the catalytic reactivity in the hydroamination reaction were investigated. The best catalytic performance was observed with the propylene-bridged Pd–NHC bromide complex 2. The hydroamination of phenylacetylene or 4-methylphenylacetylene with various substituted anilines catalyzed by 2 yielded only Markovnikov addition products in good yield. Interestingly, this homogenous catalyst can be reused three times without significant loss in activity.

A dimer of bis(N-heterocyclic carbene)rhodium(I) centres spanned by a dibenzo-18-crown-6 bridge from synchrotron radiation

The compound (μ-3,3′,3′′,3′′′-{[2,5,8,15,18,21-hexa­oxatricyclo­[20.4.0.09,14]hexa­cosa-1(22),9,11,13,23,25-hexa­ene-11,12,24,25-tetra­yl]tetra­kis­(methyl­ene)}tetra­kis­(1-methyl-1H-imidazol-2-yl))bis­[(η4-cyclo­octa-1,4-diene)rhodium(I)] bis­(hexa­fluoridophosphate) acetonitrile sesquisolvate dihydrate, [Rh2(C8H12)2(C40H42N8O6)](PF6)2·1.5CH3CN·2H2O, crystallized from acetonitrile under an atmosphere of diethyl ether. In the crystal structure, the complex cation exhibits two square-planar RhI centres, each bound by a cyclo­octa­diene (COD) ligand and by two adjacent imidazolyl­idene N-heterocyclic carbene (NHC) donors from the same phen­oxy ring of the {[dibenzo-18-crown-6-11,12,24,25-tetra­yl]tetra­kis­(methyl­ene)}tetra­kis­(1-methyl-1H-imidazol-2-yl) (L) ligand. The dibenzo-crown ether bridge of L spans the Rh centres and forms hydrogen bonds with water mol­ecules. One water mol­ecule with half occupancy bridges adjacent macrocycles in the lattice. Another water with full occupancy forms weak hydrogen bonds to the crown ether O atoms and is, in turn, part hydrogen bonded by a lattice water with half occupancy. The latter water is within hydrogen-bonding distance of a fourth water also with partial occupancy. The result of these inter­actions is the formation of a layer in the ab plane. Two PF6 − ions, one of which is twofold disordered, and one ordered and one twofold disordered (with 0.5 occupancy) lattice acetonitrile mol­ecules complete the crystal structure.

Carbene-Stabilized Phosphenium Oxides and Sulfides

Carbene→chalcogenophosphenium adducts, which correspond to an intermolecular stabilization mode of the so far elusive, free oxo- and thiooxophosphenium species [R(2)P(+) = X] (X = O, S) by imidazolylidene (NHC) and diaminocyclopropenylidene (BAC) donors, have been isolated and fully characterized. The dative character of the R(2)C:→P(+)(X)Ph(2) bond was confirmed experimentally by nucleophilic displacement of the carbene donor with a chloride ion and by an exchange reaction of the NHC ligand of the NHC:→P(+)(O)Ph(2) adduct with an independently prepared BAC ligand, thereby giving the BAC:→P(+)(O)Ph(2) adduct. This dative character was further characterized by the DFT-calculated preference of carbene→chalcogenophosphenium systems for a heterolytic dissociation mode over a homolytic one.

Preparation of Stable Low‐Oxidation‐State Group 14 Element Amidohydrides and Hydride‐Mediated Ring‐Expansion Chemistry of N‐Heterocyclic Carbenes

Various low oxidation state (+2) group 14 element amidohydride adducts, IPr⋅EH(BH(3))NHDipp (E=Si or Ge; IPr=[(HCNDipp)(2)C:], Dipp=2,6-iPr(2)C(6)H(3)), were synthesized. Thermolysis of the reported adducts was investigated as a potential route to Si- and Ge-based clusters; however, unexpected transmetallation chemistry occurred to yield the carbene-borane adduct, IPr⋅BH(2)NHDipp. When a solution of IPr⋅BH(2)NHDipp in toluene was heated to 100 °C, a rare C-N bond-activation/ring-expansion reaction involving the bound N-heterocyclic carbene donor (IPr) transpired.

Iridium(III) Cp* Complexes for the Efficient Hydroamination of Internal Alkynes

A series of iridium(III) and rhodium(III) 1,2,3,4,5-pentamethylcyclopentadienyl (Cp*) complexes of the type [M(Cp*)(LL)Cl]BPh4 were synthesized, where LL is a chelating diimine or mixed pyrazolyl-phosphine or pyrazolyl-N-heterocyclic carbene donor ligand. The majority of these complexes were characterized by X-ray crystallography, allowing for a detailed comparison of their structural properties. These complexes were shown to yield active hydroamination catalysts upon in situ abstraction of the chloride coligand using AgBF4. The activity of these catalysts for the intramolecular hydroamination of alkyne-amines to yield indolyl and pyrolyl heterocycles was investigated. It was found that chelating ligand groups containing a strongly coordinating N-heterocyclic carbene donor led to the most effective catalysis and that an increase in the steric bulk of the ligand was detrimental to the catalyst efficiency.

Stepwise Intramolecular Photoisomerization of NHC-Chelate Dimesitylboron Compounds with C–C Bond Formation and C–H Bond Insertion

C,C-chelate dimesitylboron (BMes(2)) compounds containing an N-heterocyclic carbene (NHC) donor have been obtained. Single-crystal X-ray diffraction analyses established that the boron atom in these compounds is bound by four carbon atoms in a distorted tetrahedral geometry. Compared to previously reported N,C-chelate dimesitylboron compounds, the new C,C-chelate boron compounds have a much larger HOMO-LUMO energy gap (>3.60 eV). They do, however, respond to UV irradiation (300 nm) in the same manner as N,C-chelate BMes(2) compounds do, undergoing photoisomerization and converting to an intensely colored (yellow or orange) isomer A quantitatively, with a high quantum efficiency (0.60-0.75). NMR and single-crystal X-ray diffraction analyses established that the structure of A is similar to the dark isomers obtained from N,C-chelate BMes(2) compounds. However, unlike the N,C-chelate dark isomers that have the tendency to thermally reverse back to the light colored isomers, the isomers A of the C,C-chelate BMes(2) are thermally stable and no reverse isomerization was observed even when heated to 80 °C (or 110 °C) for hours. The most unusual finding is that isomers A undergo further photoisomerization when irradiated at 350 nm, forming a new colorless species B nearly quantitatively. NMR and single-crystal X-ray diffraction analyses established the structure of isomer B, which may be considered as an intramolecular C-H insertion product via a borylene intermediate. Mechanistic aspects of this unusual two-step photoisomerization process have been examined by DFT computational studies.

β-Elimination-Immune PCcarbeneP Iridium Complexes via Double C–H Activation: Ligand–Metal Cooperation in Hydrogen Activation

The synthesis of two members of a new PCP pincer ligand family and their complexation with iridium has been developed. Double C–H activation results in a ligand anchored by a carbene donor; hydrogen reversibly adds to the (PCcarbeneP)IrCl complexes 3R (R = iPr, tBu). Aryl, amido, and phenoxy derivatives are available from the chlorides via salt metathesis. These compounds add H2 and eliminate H–X; the iridium compound is trapped as a PCalkylP iridium polyhydride.

Synthesis of N-heterocyclic carbene rhenium(i) carbonyl complexes

Reaction of aminophosphinimine [RHN(CH(2))(2)N[double bond, length as m-dash]PPh(3)] (R = H, Et) with Re(2)(CO)(10) provided the NH-functionalized carbene rhenium complex [Re(2)(CNHCH(2)CH(2)NR)(CO)(9)] (3a, R = H, 3b, R = Et). Treatment of 3 with Br(2) provided the mono nuclear [Re(CNHCH(2)CH(2)NR)(CO)(4)Br] (1, R = H, 2, R = Et). However, NH-functionalized carbene complexes 1-3 did not undergo N-alkylation with alkyl halides to yield the N-substituted NHC complexes. The direct ligand substitution of [Re(CO)(5)Br] with a carbene donor was employed to prepare [Re(IMes(2))(CO)(4)Br] (6a, IMes(2) = 1,3-di-mesitylimidazol-2-ylidene; 6b, IMes(2) = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene). Analyses of spectroscopic and crystal data of 6a and 6b show similar corresponding data among these complexes, suggesting the saturated and unsaturated NHCs have similar bonding with Re(I) metal centers. Reduction of 6a and 6b with LiEt(3)BH yielded the corresponding hydrido complexes 7a-b [ReH(CO)(4)(IMes(2))], but not 1 and 2. Ligand substitution of 1, 6a and 6b toward 2,2'-bipyridine (bipy) was investigated. Crystal structures of 1, 3a-b, 6a-b and 7b were determined for characterization and comparison.

Interaction of Carbene and Olefin Donors with [Cl2PN]3: Exploration of a Reductive Pathway toward (PN)3

The iminophosphine-phosphazene [P(III)-P(V)] heterocyclic adduct [IPr·PN(PCl(2)N)(2)] was prepared via reduction of the cyclic phosphazene [Cl(2)PN](3) in the presence of the carbene donor IPr {IPr = [(HCNDipp)(2)C:], where Dipp = 2,6-(i)Pr(2)C(6)H(3)}. By contrast, the treatment of [Cl(2)PN](3) with the N-heterocyclic olefin IPr═CH(2) yielded the olefin-grafted phosphazene ring [(IPr═CH)P(Cl)N(PCl(2)N)(2)].

Distinct Reactivity of Mono- and Bis-NHC Silver Complexes: Carbene Donors versus Carbene–Halide Exchange Reagents

The synthesis of N-heterocyclic carbene (NHC) complexes of silver is an established technique for their use as carbene transfer reagents. While it is known that both mono-NHC Ag(I) and bis-NHC Ag(I) complexes are accessible, different reactivities of these species have not been explored synthetically. Whereas the commonly used mono-NHC Ag(I) complexes act as NHC donors only, we found that a bis-NHC Ag(I) complex was capable of transferring one carbene under halide abstraction. This is exemplified in the transformation of P–Cl compounds into the respective imidazolium–phosphines by a bis-NHC Ag(I) complex but not by its corresponding mono-NHC Ag(I) complex. When the bis-NHC Ag(I) complex was allowed to react with [M(cod)Cl]2 (M = Rh, Ir), both NHCs were transferred under chloride abstraction to give [M(cod)(NHC)2]+ exclusively, a reaction which is reported to stop at [M(cod)Cl(NHC)] even with an excess of mono-NHC Ag(I) complex in the presence of AgPF6.

Synthesis and characterization of κ-2- bis- N-heterocyclic carbene rhodium(I) catalysts: Application in enantioselective arylboronic acid addition to cyclohex-2-enones

This manuscript describes the synthesis and characterization of new derivatives of di- N-heterocyclic carbene (diNHC) ligands derived from trans-9,10-dihydro-9,10-ethanoanthracene-11,12-diylmethanediyl (DEAM) and trans-9,10-dihydro-9,10-ethanoanthracene-11,12-diyl (DEA). Synthesized as the diazolium salts, the new ligands are employed, in conjunction with known derivatives, to examine specific properties than influence the chiral induction in rhodium-catalyzed arylboronic acid addition to cyclohex-2-enones. Three properties of the diNHC ligands are modified and include: 1) the size and composition of the N-heterocyclic substituent (Me, i Pr, Bn, CHPh 2, o-MeBn, and R-CHMePh), 2) the type of N-heterocycle (benzimidazole versus imidazole), and 3) the size of the chiral pocket (DEAM versus DEA). Results from the catalytic studies indicate the DEAM ligand, which contains an extra CH 2 group compared to DEA, is too flexible to induce enantiomeric excess. The DEA derivatives containing an imidazole or benzimidazole provide distinctly different results despite relatively small differences between the heterocyclic carbene donors. An unexpected source of chiral induction is rationalized using results from catalytic data (% e.e.) and complemented by X-ray structural data and DFT calculations.

Modelling fundamental arene–borane contacts: spontaneous formation of a dibromoborenium cation driven by interaction between a borane Lewis acid and an arene π system

Reaction of 2,6-dimesityl pyridine (L(py)) with BBr(3) leads to the spontaneous formation of the trigonal dibromoborenium cation [L(py)·BBr(2)](+)via bromide ejection. Systematic structural and computational studies, and the reactivity displayed by a closely related N-heterocyclic carbene (NHC) donor, reveal the role played by arene-borane interactions in this chemistry. [L(py)·BBr(2)](+) features a structurally characterized (albeit weak) electrostatic interaction between the borane Lewis acid and flanking arene π systems.