Denitrogenative Ring-Contraction of Pyridines to a Cyclopentadienyl Skeleton at a Dititanium Hydride Framework

Selective removal of the nitrogen atom from an aromatic N-heterocycle, such as pyridine, is of significant interest and importance, yet it remains highly challenging. Here, we report an unprecedented denitrogenative ring-contraction reaction of pyridines at a dititanium hydride framework, yielding cyclopentadienyl and nitride species under mild conditions. The reaction of pyridine with a dititanium tetrahydride complex (1) bearing rigid acridane-based PNP-pincer ligands at room temperature produced a cyclopentadienyl/nitride complex (2), in which the two Ti atoms are bridged by a nitride atom and one Ti atom is bonded to a cyclopentadienyl group formed by pyridine denitrogenation and ring-contraction. The reactions of 2-, 3-, and 4-methylpyridines with 1 under similar conditions yielded the same product (3), a methylcyclopentadienyl-ligated analog of 2. When 2,4- or 3,5-dimethylpyridine reacted with 1 at 60 °C, the 1,3-dimethylcyclopentadienyl-ligated analog (5) formed almost quantitatively. The mechanistic details have been elucidated by isolation of key intermediates and density functional theory calculations. It was revealed that the reaction proceeded via coordination of the N atom of a pyridine unit to a Ti atom in 1 followed by H2 release, C═N reduction, two C-N bond cleavage (ring-opening and denitrogenation), and C-C coupling (ring closing). The involvement of C-H activation in an isopropyl group of a PNP ligand at the later stages of the reaction significantly contributed to the stabilization of the denitrogenative ring-contraction product. This work not only provides unprecedented mechanistic insights into denitrogenation of aromatic N-heterocycles but also represents a novel example of skeletal editing of aromatic N-heterocycles.

Elucidation of the Pyridine Ring-Opening Mechanism of 2,2'-Bipyridine or 1,10-Phenanthroline Ligands at Re(I) Carbonyl Complexes

The cleavage of the C-N bonds of aromatic heterocycles, such as pyridines or quinolines, is a crucial step in the hydrodenitrogenation (HDN) industrial processes of fuels in order to minimize the emission of nitrogen oxides into the atmosphere. Due to the harsh conditions under which these reactions take place (high temperature and H2 pressure), the mechanism by which they occur is only partially understood, and any study at the molecular level that reveals new mechanistic possibilities in this area is of great interest. Herein, we unravel the pyridine ring-opening mechanism of 2,2'-bipyridine (bipy) and 1,10-phenanthroline (phen) ligands coordinated to the cis-{Re(CO)2(N-RIm)(PMe3)} (N-RIm= N-alkylimidazole) fragment under mild conditions. Computational calculations show that deprotonation of the pyridine ring, once dearomatized, is crucial to induce ring contraction, triggering extrusion of the nitrogen atom from the ring and cleavage of the C-N bond. It is noteworthy that different products (regioisomers) are obtained depending on whether the ligand used is bipy or phen due to the additional rigidity and stability conferred by the central ring of the phen ligand, an issue also addressed and clarified computationally. Strong support for the proposed mechanism is provided by the characterization and isolation, including three single-crystal X-ray diffraction structures, of several of the proposed reaction intermediates.

Recent Advances in Homogeneous Catalysts for the Asymmetric Hydrogenation of Heteroarenes

The asymmetric hydrogenation of heteroarenes has recently emerged as an effective strategy for the direct access to enantioenriched, saturated heterocycles. Although several homogeneous catalyst systems have been extensively developed for the hydrogenation of heteroarenes with high levels of chemo- and stereoselectivity, the development of mild conditions that allow for efficient and stereoselective hydrogenation of a broad range of substrates remains a challenge. This Perspective highlights recent advances in homogeneous catalysis of heteroarene hydrogenation as inspiration for the further development of asymmetric hydrogenation catalysts, and addresses underdeveloped areas and limitations of the current technology.

Hydrodenitrogenation of pyridines and quinolines at a multinuclear titanium hydride framework

Investigation of the hydrodenitrogenation (HDN) of aromatic N-heterocycles such as pyridines and quinolines at the molecular level is of fundamental interest and practical importance, as this transformation is essential in the industrial petroleum refining on solid catalysts. Here, we report the HDN of pyridines and quinolines by a molecular trinuclear titanium polyhydride complex. Experimental and computational studies reveal that the denitrogenation of a pyridine or quinoline ring is easier than the ring-opening reaction at the trinuclear titanium hydride framework, which is in sharp contrast with what has been reported previously. Hydrolysis of the pyridine-derived nitrogen-free hydrocarbon skeleton at the titanium framework with H2O leads to recyclization to afford cyclopentadiene with the generation of ammonia, while treatment with HCl gives the corresponding linear hydrocarbon products and ammonium chloride. This work has provides insights into the mechanistic aspects of the hydrodenitrogenation of an aromatic N-heterocycle at the molecular level.

Room-Temperature Ring-Opening of Quinoline, Isoquinoline, and Pyridine with Low-Valent Titanium

The complex (PNP)Ti═CH^tBu(CH₂^tBu) (PNP = N[2-PⁱPr₂-4-methylphenyl]₂^-) dehydrogenates cyclohexane to cyclohexene by forming a transient low-valent titanium-alkyl species, [(PNP)Ti(CH₂^tBu)], which reacts with 2 equiv of quinoline (Q) at room temperature to form H₃C^tBu and a Ti(IV) species where the less hindered C₂═N₁ bond of Q is ruptured and coupled to another equivalent of Q. The product isolated from this reaction is an imide with a tethered cycloamide group, (PNP)Ti═N[C₁₈H₁₃N] (1). Under photolytic conditions, intramolecular C-H bond activation across the imide moiety in 1 occurs to form 2, and thermolysis reverses this process. The reaction of 2 equiv of isoquinoline (Iq) with intermediate [(PNP)Ti(CH₂^tBu)] results in regioselective cleavage of the C₁═N₂ and C₁-H bonds, which eventually couple to form complex 3, a constitutional isomer of 1. Akin to 1, the transient [(PNP)Ti(CH₂^tBu)] complex can ring-open and couple two pyridine molecules, to produce a close analogue of 1, complex (PNP)Ti═N[C₁₀H₉N] (4). Multinuclear and multidimensional NMR spectra confirm structures for complexes 1-4, whereas solid-state structural analysis reveals the structures of 2, 3, and 4. DFT calculations suggest an unprecedented mechanism for ring-opening of Q where the reactive intermediate in the low-spin manifold crosses over to the high-spin surface to access a low-energy transition state but returns to the low-spin surface immediately. This double spin-crossover constitutes a rare example of a two-state reactivity, which is key for enabling the reaction at room temperature. The regioselective behavior of Iq ring-opening is found to be due to electronic effects, where the aromatic resonance of the bicycle is maintained during the key C-C coupling event.

DFT study of dihydrogen addition to molybdenum π-heteroaromatic complexes: a prerequisite step for the catalytic hydrodenitrogenation process

The mechanism proceeds via consecutive steps of oxidative H₂ addition, hydrogen metal-to-ligand transfer, accompanied by a change of metal–ligand hapticity.

[Ir(P-OP)]-catalyzed asymmetric hydrogenation of diversely substituted C═N-containing heterocycles

Iridium(I) complexes of enantiomerically pure phosphine-phosphite ligands ([Ir(Cl)(cod)(P-OP)]) efficiently catalyze the enantioselective hydrogenation of diverse C═N-containing heterocyclic compounds (benzoxazines, benzoxazinones, benzothiazinones, and quinoxalinones; 25 examples, up to 99% ee). A substrate-to-catalyst ratio as high as 2000:1 was reached.

Density Functional Theory Study of Pyrrole Adsorption on Mo(110)

The objective of the present study is to identify possible adsorption configurations of pyrrole on Mo(110) using density functional theory (DFT) calculations. Several adsorption configurations were studied including pyrrole and pyrrolyl adsorption as parallel, perpendicular, and tilted adsorption modes relative to the Mo(110) surface plane. Based on the DFT calculations, pyrrole is suggested to adsorb in a parallel mode with respect to the Mo(110) surface through its pi-orbital as mu3,eta(5)-Pyr-0 degrees with an adsorption energy of -28.7 to -31.5 kcal mol(-1). The possibility of a coexisting mode where pyrrole adsorbs on the surface with a slightly tilted molecular plane as mu3,eta(4)(N,C2,C3,C4)-Pyr-5 degrees is also likely to occur, particularly at higher pyrrole coverages. The slightly tilted configuration is suggested to arise from the lateral interactions of adsorbed pyrrole on Mo(110), and not the result of a phase transformation on the surface since this requires a relatively high activation energy as indicated by additional linear synchronous transit (LST)/quadratic synchronous transit (QST) calculations. Both adsorption geometries bond to three surface Mo atoms, and Mo(110) did not promote hydrogen abstraction. Pyrrolyl adsorption on Mo(110) is energetically possible, but unlikely to occur because gas-phase hydrogen has not been previously experimentally observed as a pyrrole decomposition product on Mo(110).

Synthesis and reactivity of iridium complexes with pyridine and piperidine ligands: models for hydrodenitrogenation

The complexes [Ir(H)2(eta1-N-L)2(PPh3)2]PF6, L = py (1), iQ (2) and pip (3) (py = pyridine, iQ = isoquinoline, pip = piperidine) have been synthesized in high yields by hydrogenation of [Ir(cod)(PPh3)2]PF6 in the presence of the appropriate nitrogen compound. When hydrogen is bubbled through 1,2-dichloroethane solutions of 1 or 2, two new species were formed in each case by C-Cl bond activation of the solvent, Ir(H)2Cl(eta1-N-L)(PPh3)2 (L = py, 4; iQ, 5) and IrH(Cl)2(eta1-N-L)(PPh3)2 (L = py, 6; iQ, 7). Reaction of 3 with py or iQ yielded complexes 1 and 2, respectively, while under a slow stream of carbon monoxide the complex [Ir(H)2(eta1-N-pip)(CO)(PPh3)2]PF6 (8) was produced. Complex 3 also reacts with halide and 4-bromothiophenolate anions leading to the corresponding neutral species Ir(H)2(X)(eta1-N-pip)(PPh3)2, X = Cl (9), I (10) and 4-BrC6H4S (11), or with [MoS4]2- to yield the hetero-bimetallic complex [Ir(H)(PPh3)2(mu-S)2MoS2]- (13). All the new complexes were characterized by analytical and spectroscopic methods. The X-ray structures of , 2 and 8 consist of distorted octahedra with a mutually cis disposition of the two hydrides and mutually trans phosphines. Complexes 1, 2 and 3 and their derivatives are of interest as models for the chemisorption step in hydrodenitrogenation reactions on solid catalysts.

The reactivity of Mo(PMe3)(6) towards heterocyclic nitrogen compounds: transformations relevant to hydrodenitrogenation

The reactions of Mo(PMe3)6 towards a variety of five- and six-membered heterocyclic nitrogen compounds (namely, pyrrole, indole, carbazole, pyridine, quinoline, and acridine) have been studied to provide structural models for the coordination of these heterocycles to the molybdenum centers of hydrodenitrogenation catalysts. Pyrrole reacts with Mo(PMe3)6 to yield the eta5-pyrrolyl derivative (eta5-pyr)Mo(PMe3)3H, while indole gives sequentially (eta1-indolyl)Mo(PMe3)4H, (eta5-indolyl)Mo(PMe3)3H, and (eta6-indolyl)Mo(PMe3)3H, with the latter representing the first example of a structurally characterized complex with an eta6-indolyl ligand. Likewise, carbazole reacts with Mo(PMe3)6 to give (eta6-carbazolyl)Mo(PMe3)3H with an eta6-carbazolyl ligand. The reactions of Mo(PMe3)6 with six-membered heterocyclic nitrogen compounds display interesting differences in the nature of the products. Thus, Mo(PMe3)6 reacts with pyridine to give an eta2-pyridyl derivative [eta2-(C5H4N)]Mo(PMe3)4H as a result of alpha-C-H bond cleavage, whereas quinoline and acridine give products of the type (eta6-ArH)Mo(PMe3)3 in which both ligands coordinate in an eta6-manner. For the reaction with quinoline, products with both carbocyclic and heterocyclic coordination modes are observed, namely [eta6-(C6)-quinoline]Mo(PMe3)3 and [eta6-(C5N)-quinoline]Mo(PMe3)3, whereas only carbocyclic coordination is observed for acridine.

Early transition metal complexes containing 1,2,4-triazolato and tetrazolato ligands: synthesis, structure, and molecular orbital studies

Several early transition metal complexes bearing 1,2,4-triazolato and tetrazolato ligands have been prepared by reaction of the pyrazolato complexes Ti(tBu(2)pz)(4-x)Cl(x) (tBu(2)pz = 3,5-di-tert-butylpyrazolato; x = 1, 2) and M(tBu(2)pz)(5-x)Cl(x) (M = Nb, Ta: x = 2, 3) with the sodium or potassium salts derived from 1,2,4-triazoles and tetrazoles. The X-ray structure analysis of Ti(tBu(2)pz)(2)(Me(2)C(2)N(3))(2) shows eta(2)-coordination of the 1,2,4-triazolato ligands, while in Ti(tBu(2)pz)(3)(C(2)H(2)N(3)) and Nb(tBu(2)pz)(3)(Me(2)C(2)N(3))(2) the analogous groups are joined in a eta(1)-fashion in the solid-state structure. Solution NMR studies at different temperatures suggest transition states involving eta(2)-1,2,4-triazolato ligands for the complexes containing eta(1)-1,2,4-triazolato ligands in the solid state. X-ray crystal structures of analogous tetrazolato complexes Ti(tBu(2)pz)(3)(PhCN(4)) and Nb(tBu(2)pz)(3)(PhCN(4))(2) show eta(1)-coordination of the 2-nitrogen atoms of the tetrazolato ligands. Molecular orbital calculations have been carried out on several model titanium complexes and provide detailed insight into the bonding between early transition metal centers and 1,2,4-triazolato and tetrazolato ligands. The eta(2)-coordination mode of 1,2,4-triazolato and tetrazolato ligands is predicted to be more stable than the eta(1)-coordination mode by 13.8-5.2 kcal/mol.