Remote N-heterocyclic Carbene Research Articles

Iridium Complexes with Bidentate Pyridinylidene/N-Amidate Ligands for Transfer Hydrogenation Catalysis.

A series of iridium pentamethylcyclopentadienyl (Cp*) complexes, [Cp*Ir(κ2-RLp/m)Cl], that contain the strongly coordinating bidentate ligands RLp/m were synthesized. The donor groups of the bidentate ligands were an N-amidate and either a para-pyridinylidene remote N-heterocyclic carbene (RLp) or a meta-pyridinylidene remote N-heterocyclic carbene (RLm). For each type of bidentate ligand, a set of iridium complexes was synthesized, which differed only according to the substituents (R) on the phenyl ring associated with the amidate group. The iridium complexes were all fully characterized and molecular structures were obtained by single-crystal X-ray diffraction studies for representative examples. The complexes were found to be good precatalysts in iso-propanol for the transfer hydrogenation of benzaldehyde to give benzyl alcohol. The catalytic activity correlated with the Hammett σm/p parameters of the phenyl ring substituents, with more electron-donating substituents leading to increased catalytic activity. In all cases, the meta-pyridinylidene complexes, [Cp*Ir(κ2-RLm)Cl], performed better than the corresponding para analogues, [Cp*Ir(κ2-RLp)Cl].

Metal-Ligand Proton Tautomerism, Electron Transfer, and C(sp3)-H Activation by a 4-Pyridinyl-Pincer Iridium Hydride Complex.

The para-N-pyridyl-based PCP pincer proligand 3,5-bis(di-tert-butylphosphinomethyl)-2,6-dimethylpyridine (pN-tBuPCP-H) was synthesized and metalated to give the iridium complex (pN-tBuPCP)IrHCl (2-H). In marked contrast with its phenyl-based congeners, e.g., (tBuPCP)IrHCl and derivatives, 2-H is highly air-sensitive and reacts with oxidants such as ferrocenium, trityl cation, and benzoquinone. These oxidations ultimately lead to intramolecular activation of a phosphino-t-butyl C(sp3)-H bond and cyclometalation. Considering the greater electronegativity of N than C, 2-H is expected to be less easily oxidized than simple PCP derivatives; cyclic voltammetry and DFT calculations support this expectation. However, 2-H is calculated to undergo metal-ligand-proton tautomerism (MLPT) to give an N-protonated complex that can be described with resonance forms representing a zwitterionic complex (with a negative charge on Ir) and a p-N-pyridylidene (a remote N-heterocyclic carbene) Ir(I) complex. One-electron oxidation of this tautomer is calculated to be dramatically more favorable than direct oxidation of 2-H (ΔΔG° = -31.3 kcal/mol). The resulting Ir(II) oxidation product is easily deprotonated to give metalloradical 2• which is observed by NMR spectroscopy. 2• can be further oxidized to give cationic Ir(III) complex, 2+, which can oxidatively add a phosphino-t-butyl C-H bond and undergo deprotonation to give the observed cyclometalated product. DFT calculations indicate that less sterically hindered analogues of 2+ would preferentially undergo intermolecular addition of C(sp3)-H bonds, for example, of n-alkanes. The resulting iridium alkyl complexes could undergo facile β-H elimination to afford olefin, thereby completing a catalytic cycle for alkane dehydrogenation driven by one-electron oxidation and deprotonation, enabled by MLPT.

Mesoionic and N-Heterocyclic Carbenes Coordinated N+ Center: Experimental and Computational Analysis.

N-Heterocyclic carbenes, carbocyclic carbenes, remote N-heterocyclic carbenes and N-heterocyclic silylenes are known to form L→N+ coordination bonds. However, mesoionic carbenes (MICs) are not reported to form coordination bonds with cationic nitrogen. Herein, synthesis and quantum chemical studies were performed on 1,2,3-triazol-5-ylidene stabilized N+ center. Six compounds with MIC→N+ ←NHC were synthesized. Density functional theory calculations and energy decomposition analysis were carried out to explore the bonding situation between MIC and N+ center. The C→N+ bond lengths were in the range of 1.295-1.342 Å and bond dissociation energies were <400 kcal/mol. Natural bond orbital analysis supported the presence of excess electron density (>3 electrons) at the N+ center. The computational and X-ray diffraction analysis results confirmed the presence of divalent NI character of center nitrogen and MIC→N+ ←NHC coordination interactions.

Investigation of fused remote N-heterocyclic silylenes (frNHSis), at DFT.

We compared and contrasted the ΔΕs-t, band gap (ΔΕHOMO-LUMO), aromaticity, charge distribution, and reactivity of singlet (s) and triplet (t) benzopyridine-4-ylidene as the fused remote N-heterocyclic carbene (frNHC) and frNHSis with different fused aromatic rings, at (U)B3LYP/AUG-cc-pVTZ and (U)M06-2X/AUG-cc-pVTZ levels of theory. In this investigation, we found (1) all s and t divalent states appear as minimum structures, for having no negative force constant. Nonetheless, only singlets present more thermodynamic stability than their triplet analogous; (2) the trend of ΔΕs-t in kcal/mol is ortho-pyrrole (52.94) > ortho-furan (51.84) > ortho-thiophene (50.38) > para-furan (49.36) > para-pyrrole (49.00) > para-phosphole (48.67) ≥ para-thiophene (48.64) > benzene (44.33) > ortho-phosphole frNHSi (27.50), while ΔΕs-t of frNHC is 15.65kcal/mol; (3) apart from phosphole frNHSis, the order of ΔΕs-t in a "ortho position or zigzag array" about 1.8-4.0kcal/mol is more than that of in a "para position or chair array"; (4) the highest ΔΕHOMO-LUMO is demonstrated by ortho-pyrrole frNHSi (95.65kcal/mol) while the lowest ΔΕHOMO-LUMO is verified by the reference frNHC (63.44kcal/mol); (5) in contradiction of frNHC, all singlet frNHSis reveal higher band gap and lower global reactivity than their triplet congeners; (6) charge distribution along with MEP maps indicate differentially electronic cloud in middle of rings frNHSis vs. frNHC; (7) we anticipate higher nucleophilicity and lower electrophilicity of triplet frNHSis than singlet congeners, will make them worthy of synthetic surveys.

Substituent effects on the stability of cyclic - unsaturated remote N-heterocyclic Hammick carbenes using density functional theory

We investigated the structure, stability and chemical properties of novel singlet (s) and triplet (t) Hammick carbenes with different fused aromatic rings, at (U)B3LYP/6‐311+G*, (U)M06-2X/6‐311+G*, (U)B3LYP/AUG-cc-pVTZ and (U)M06-2X/AUG-cc-pVTZ levels of theory. It is observed that: (1) All singlet and triplet structures appear as minima on their energy surfaces and each singlet carbene displays more thermodynamic stability than its corresponding triplet state; (2) Regarding to the relationship among carbenic center and substituted heteroatom, the order of thermodynamic stability based on singlet-triplet energy differences (ΔΕs-t) for fused rings is ortho-pyrrole > furan > thiophene > phosphole in a “W” arrangement; (3) The highest kinetic stability based on band gap (ΔΕHOMO-LUMO) is demonstrated by substituted ortho-pyrrole carbene in a "W" arrangement; (4) Regardless of how orchestrated, the order of stabilization for fused rings is pyrrole > furan > thiophene > phosphole; (5) The ortho-phosphole ring in a “W” arrangement destabilizes carbene more than benzene ring.

A DFT quest for effects of fused rings on the stability of remote N-heterocyclic carbenes

Assuming aromaticity (cyclic continuous conjugation, planarity, and obeying the Huckel 4n + 2 rule), effects of one and two fused six-membered heterocyclic rings are investigated on the energy lowering (stabilization) of 22 novel singlet (s) and triplet (t) carbenes, at B3LYP/AUG-cc-pVTZ and M06-2X/AUG-cc-pVTZ. Results display that (1) exclusive of triplet pyridine-4-ylidene, and s and t states, other species appear as ground state, so every s Hammick carbene exhibits more stability than its corresponding t state; (2) the highest stability is demonstrated by unsubstituted pyridine-4-ylidene as reference carbene, and the lowest stability is shown by carbene situated between two nitrogen heteroatoms of two fused rings, in a “W” arrangement; (3) regarding the relationship between carbenic center (CC) and substituted heteroatom, the order of stabilization for fused rings is meta > para > ortho; (4) regardless of how organized, fusion of one six-membered ring, in a given arrangement, has more stabilizing effect than two six-membered rings; (5) contrary to our expectation, t Hammick carbenes show higher band gap (ΔEHOMO-LUMO) than their corresponding s species; (6) based on the NICS (nuclear independent chemical shift) results, the least stable carbene has the most aromaticity in its pyridine ring; and (7) according to proposed homomolecular isodesmotic reactions, all s states are stabilized via π-donor/σ-acceptor substitution more than the t states.

Divalent NI Compounds: Identifying new Carbocyclic Carbenes to Design Nitreones using Quantum Chemical Methods.

Nitreones are compounds with oxidation state 1 at the nitrogen, these compounds carry formal positive charge as well as two lone pairs of electrons at nitrogen center. These compounds are also known as divalent NI compounds and can be represented with the general formula L → N+ ← L, where L is an electron donating ligand. In the recent past, several divalent NI compounds have been reported with L = N-heterocyclic carbene (NHC), remote N-heterocyclic carbene (rNHC), carbocyclic carbene (CCC) and diaminocarbene. Recently, our group reported that a novel six-membered CCC (cyclohexa-2,5-diene-4-[diaminomethynyl]-1-ylidene) can stabilize N+ center in nitreones. As an independent carbene, this species is very unstable. In this work, modulation of this CCC using (a) annulation, (b) heterocyclic ring modification, (c) substitutions adjacent to the carbenic carbon, (d) exocyclic double bond insertion and (e) ring contraction, has been reported. These modulations and quantum chemical analyses helped in the identification of five new six-membered CCCs which carry improved donation and stability properties. Further, these CCCs were employed in the design of new divalent NI compounds (nitreones) which carry coordination bonds between ligands and N+ center. The molecular and electronic structure properties, and the donor→acceptor coordination interactions present in the resultant low oxidation state divalent NI compounds have been explored.

Asymmetric catalysis with a chiral-at-osmium complex.

The first example of a chiral osmium catalyst is reported in which the overall chirality originates exclusively from a stereogenic metal center (metal-centered chirality) with all coordinating ligands being achiral. The non-C2-symmetric chiral-at-metal complex contains two cyclometalated 7-methyl-1,7-phenanthrolinium heterocycles which can be described as two chelating pyridylidene remote N-heterocyclic carbene (rNHC) ligands. The octahedral coordination sphere is completed with one CO and one acetonitrile ligand. A monodentate chiral oxazoline ligand is used as a chiral auxiliary ligand to obtain enantiomerically pure chiral-at-osmium complexes (>99 : 1 e.r.). Finally, it is demonstrated that the developed chiral-at-osmium complex is suitable for ring-closing enantioselective C(sp3)-H aminations, including the first example of catalytic enantioselective cyclizations of azidoformates to chiral 2-oxazolidinones.

Non-C2-Symmetric Chiral-at-Ruthenium Catalyst for Highly Efficient Enantioselective Intramolecular C(sp3)-H Amidation.

A new class of chiral ruthenium catalysts is introduced in which ruthenium is cyclometalated by two 7-methyl-1,7-phenanthrolinium heterocycles, resulting in chelating pyridylidene remote N-heterocyclic carbene ligands (rNHCs). The overall chirality results from a stereogenic metal center featuring either a Λ or Δ absolute configuration. This work features the importance of the relative metal-centered stereochemistry. Only the non-C2-symmetric chiral-at-ruthenium complexes display unprecedented catalytic activity for the intramolecular C(sp3)-H amidation of 1,4,2-dioxazol-5-ones to provide chiral γ-lactams with up to 99:1 er and catalyst loadings down to 0.005 mol % (up to 11 200 TON), while the C2-symmetric diastereomer favors an undesired Curtius-type rearrangement. DFT calculations elucidate the origins of the superior C-H amidation reactivity displayed by the non-C2-symmetric catalysts compared to related C2-symmetric counterparts.

Exploring the reactivity of carbene supported diboraanthracene towards dihydrogen activation

Quantum chemical calculations have been carried out on some N-heterocyclic carbene (NHC) stabilized boraanthracenes to investigate their possibility to act as H2 activators. Different coordination modes such as normal, abnormal and remote NHC are considered. Moreover, a 1,3,2,5-diazadiborinine molecule, which is experimentally known to activate H2 has also been considered for comparison. All the studied systems have a lower HOMO–LUMO gap than this molecule, an important factor for rendering higher reactivity. Quasiclassical trajectory for the reaction between H2 and these molecules indicates a single dynamically concerted step. Electronic structure analysis reveals synergism between donation and back donation in the activation process. The effect of substituents has also been studied which reveals that electron withdrawing substituents increase the activation barrier while electron donating substituents decrease it. The position of the substituents is also very crucial so far as the reactivity is concerned. With remote NHC stabilized boraanthracenes, it may be possible to achieve a low kinetic barrier and thermodynamic reversibility for activation of H2.

Can Remote N-Heterocyclic Carbenes Be Used for Designing Efficient Blue Triplet Emitters? An Answer from Quantum Chemical Investigation

For the first time, we have tried to show the effect of remote carbene ligands on the emission and quantum efficiency of Ir-based triplet emitters useable for organic light-emitting diodes. It has already been shown that N-heterocyclic carbenes (NHCs) are comparatively stronger than mesoionic carbene (MICs) and hence provide a blue emissive color, but the latter ones are superior in terms of quantum efficiency. Similar arguments prevail for remote NHCs (rNHCs), which are supposed to be even less stronger than MICs. Therefore, any triplet emitter designed with rNHC might experience a bathochromic shift and might not therefore achieve the blue emission. However, we demonstrate that the efficiency of rNHC-based triplet emitters is much higher than that of NHCs and MICs. Hence, our target is toward designing efficient blue emitters having rNHCs. We have used both pyrazolium-pyridine- and phenyl-pyridine-based rNHCs, and we find that the combination of NHC as a cyclometallating ligand and phenyl-pyridine-based...

Is the term “Carbene” justified for remote N-heterocyclic carbenes (r-NHCs) and abnormal N-heterocyclic carbenes (aNHCs/MICs)?

The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of typical N-heterocyclic carbenes NHCs, r-NHCs, a-NHCs and MICs have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept and visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction. Prior to that both structures and 13C chemical shifts were calculated and in case of isolated carbenes the computed δ(13C)/ppm values compared (as a quality criterion for obtained structures) with the experimental ones. The TSNMRS values of the studied carbenes, which are in mesomeric equilibrium with zwitterionic (ylide/betaine/mesoionic) resonance contributors, are employed to qualify and quantify the present electronic structure and if the term carbene is still justified to denote the compounds studied. The results, thus obtained from spatial magnetic properties (TSNMRS), are compared with the geometry of the compounds, the corresponding WIBERG's bond index values, and the 13C chemical shifts especially of the carbene electron-deficient centre.

Can Remote N-Heterocyclic Carbenes Coordinate with Main Group Elements? Synthesis, Structure, and Quantum Chemical Analysis of N+ -Centered Complexes.

Remote N-heterocyclic carbenes (rNHCs), such as N-methyl-4-pyridylidene, are known to form coordination complexes with TMs. Herein, it is established that rNHCs can also coordinate to the N+ centre. Synthesis of some novel divalent NI complexes with the general formula (rNHC)→N+ ←(NHC) and (rNHC)→N+ ←(rNHC) was achieved, and X-ray diffraction studies supported the coordination bond character between the rNHCs and the N+ centre. Quantum chemical analysis established the presence of divalent NI character at the central nitrogen in these systems.

ChemInform Abstract: Recent Advances in Neutral and Anionic N‐Heterocyclic Carbene‐Betaine Interconversions. Synthesis, Characterization, and Applications

AbstractReview: 147 refs.

Reaching for two new stable ambiphilic quinoline-derivedN-heterocyclic carbenes at DFT level

A detailed investigation on the thermodynamic and kinetic stability of four carbenic tautomers of quinoline 1, including quinoline-2-ylidene 2, quinoline-3-ylidene 3, quinoline-4-ylidene 4, and 3,4-dihydroquinoline-4-ylidene 5, reveals that singlet planar six-membered ring N-heterocyclic carbenes (NHCs) 2 and 4 have less stability than Arduengo type NHC but seems to have enough conceivably for reaching at B3LYP/aug-cc-pVTZ//B3LYP/6–31+G* and B3LYP/6–311++G**//B3LYP/6–31+G* levels. All these six-membered NHCs are extremely ambiphilic with the more nucleophilic and electrophilic characters compared to the Arduengo type one. The aromaticity of singlet 2 and 4 is a significant contributor to their stability which is confirmed through their Nucleus-independent chemical shift(1)zz values. Finally, among 2–5, the normal NHC 2 is thermodynamically preferred but the remote NHC 4 is kinetically proffered over the other isomeric carbenes. The effects of different N- or C-substituted NHCs of 2 are studied using appropriate isodesmic reactions. The trimethylsilyl substituent exhibits slightly larger carbene stabilization in quinoline-derived NHCs than the pyridine analogue. © 2014 Wiley Periodicals, Inc.

Tuning the electronic and ligand properties of remote carbenes: a theoretical study.

The effect of annulation and carbonylation on the electronic and ligating properties of remote N-heterocyclic carbenes (rNHCs) has been studied quantum-chemically. The thermodynamic stability of these complexes has been assessed on the basis of their hydrogenation and stabilization energies, while HOMO-LUMO gaps are used to measure the kinetic stabilities. Annulated/carbonylated rNHCs are found to be weaker σ donors but better π acceptors compared with the parent rNHCs. The reactivity of these rNHCs has been studied by evaluating their nucleophilicity and electrophilicity indices. The nucleophilicity values are in good agreement with the σ basicities of all of the rNHCs. The (31)P NMR chemical shifts of the corresponding rNHC-phosphinidene adducts have been calculated and found to correlate well with the π acidities of these rNHCs.

Proton-induced generation of remote N-heterocyclic carbene-Ru complexes.

The proton-induced Ru-C bond variation, which was previously found to be relevant in the water oxidation, has been investigated by using cyclometalated ruthenium complexes with three phenanthroline (phen) isomers. The designed complexes, [Ru(bpy)2 (1,5-phen)](+) ([2](+)), [Ru(bpy)2 (1,6-phen)](+) ([3](+)), and [Ru(bpy)2 (1,7-phen)](+) ([4](+)) were newly synthesized and their structural and electronic properties were analyzed by various spectroscopy and theoretical protocols. Protonation of [4](+) triggered profound electronic structural change to form remote N-heterocyclic carbene (rNHC), whereas protonation of [2](+) and [3](+) did not affect their structures. It was found that changes in the electronic structure of phen beyond classical resonance forms control the rNHC behavior. The present study provides new insights into the ligand design of related ruthenium catalysts.

Changes in ligating abilities of the singlet and triplet states of normal, abnormal and remote N-heterocyclic carbenes depending on their aromaticities

Quantum chemical calculations at B3LYP/aug-cc-pVTZ level about singlet N-heterocyclic carbene (NHC) ligands, imidazol-2-ylidene, imidazol-4-ylidene, pyrazol-3-ylidene and pyrazol-4-ylidene, and their protonated analogues show that they are considerably aromatic except for pyrazol-3-ylidene. This result is experimentally verified by approximately five thousand NHC transition metal complexes retrieved from the Cambridge Structural Database (CSD). CSD search discloses that NHCs can participate in π-stacking interactions, albeit scarce. Geometry-based HOMA and electronic aromaticity index FLU rather than NICS provide a satisfactory description of the bonding situations in NHC ligands. Singlet state of the normal NHC has electron-deficient aromaticity as compared to those of the abnormal and remote NHCs. Depending on the transition from the singlet to triplet state, NHCs become electron-deficient ligands except for remote NHC. Computational studies regarding electronic nature of free NHC ligands show that the π-electronic population of the formally vacant pπ orbital on the carbene atoms in abnormal and remote NHC is occurred as a result of the aromaticity of NHCs, not as a result of the direct electron donation from LP-orbitals of N atoms to carbene atom according to putative push-pull effect used in understanding the electronic stabilization of normal NHC. Increase in the aromaticity raises σ-donating ability of both imidazol- and pyrazol-based NHC ligands. Free abnormal and remote NHC ligands have higher σ-donation ability than normal NHC ligands. The lack of σ-donating ability of normal NHC is compensated by its relatively high π-accepting ability, whereas π-back donation abilities of abnormal and remote NHCs are prohibited by their almost fully occupied π-orbitals. Aromaticities of the triplet NHC ligands are higher than that of the lowest-lying triplet state of benzene. Increase in the aromaticity of NHC ligands decreases van der Waals shortening in TM-NHC bonds mainly due to diminishing dative character of these bonds.

Recent advances in neutral and anionic N-heterocyclic carbene – betaine interconversions. Synthesis, characterization, and applications

Some mesoionic compounds, i.e. five-membered representatives of the class of conjugated mesomeric betaines (CMB), are in equilibrium with their tautomeric normal N-heterocyclic carbenes (nNHC). In addition, anionic N-heterocyclic carbenes, generated by deprotonation of mesoionic compounds, have been described. The first examples of conversions of crossconjugated mesomeric betaines (CCMB), 6-oxopyrimidinium-4-olates, into normal Nheterocyclic carbenes have been reported as well. CCMB such as imidazolium-4-carboxylate and pyrazolium-4-carboxylate can decarboxylate to form abnormal (aNHC) or remote N-heterocyclic carbenes (rNHC). Most conversions of betaines into N-heterocyclic carbenes start from pseudocross-conjugated mesomeric betaines (PCCMB) which can be regarded as heterocumulene adducts of nNHC. Thus, decarboxylations of imidazolium-2-carboxylates, 1,2,4-triazole-3carboxylates, pyrazolium-3-carboxylates or indazolium-3-carboxylates yield N-heterocyclic carbenes which have been used in catalysis, complex chemistry, heterocyclic synthesis, and organocatalysis.

Mesomeric Betaines and N-Heterocyclic Carbenes of Pyrazole and Indazole

The chemistry of the N-heterocyclic carbenes pyrazol-3-ylidene and indazol-3-ylidene, and of the remote N-heterocyclic carbene pyrazol-4-ylidene, is reviewed. Heterocumulene adducts of pyrazol- and indazol-3-ylidene are pseudo-cross-conjugated heterocyclic mesomeric betaines, from which the corresponding N-heterocyclic carbenes can be generated by thermal cleavage of the carbene–heterocumulene bond. Cross-conjugated heterocyclic mesomeric betaines such as pyrazolium-4-carboxylates are formal 1:1 adducts of the heterocumulene carbon dioxide with the remote N-heterocyclic carbene pyrazol-4-ylidene. These relationships give rise to an interesting interplay between the two compound classes, N-heterocyclic carbenes and heterocyclic mesomeric betaines. 1 Introduction 2 Classifications 2.1 Mesomeric Betaines of Pyrazole and Indazole 2.2 N-Heterocyclic Carbenes and Relatives of Pyrazole and Indazole 3 Conjugated Mesomeric Betaines of Pyrazole and Indazole 4 Ylides of Pyrazole 5 Cross-Conjugated Mesomeric Betaines and Remote N-Heterocyclic Carbenes of Pyrazole (Pyrazol-4-ylidenes) 6 Pseudo-Cross-Conjugated Mesomeric Betaines and N-Heterocyclic Carbenes of Pyrazole (Pyrazol-3-ylidenes) 7 Pseudo-Cross-Conjugated Mesomeric Betaines and N-Heterocyclic Carbenes of Indazole (Indazol-3-ylidenes) 8 Conclusions