Interligand C–C Coupling between α‑Methyl N‑Heterocycles and bipy or phen at Rhenium Tricarbonyl Complexes

Intramolecular C–C coupling between N-bonded 1,2-dimethylimidazole, 2-methyloxazoline, or 2-methylpyridine and either 2,2′-bipyridine (bipy) or 1,10-phenanthroline (phen) ligands results from α-methyl group deprotonation in the coordination sphere of Re­(CO)₃ fragments. The nucleophilic CH₂ group generated by the deprotonation attacks the 6 (bipy) or 2 (phen) positions of the diimines, dearomatizing the involved pyridine ring and generating new asymmetric, <i>fac</i>-capping tridentate ligands

Intramolecular Nucleophilic Addition to the 2 Position of Coordinated 2,2′-Bipyridine by a Deprotonated Dimethyl Sulfide Ligand

Deprotonation of the dimethyl sulfide ligand in [Re­(bipy)­(CO)₃(SMe₂)]­[OTf] (<b>1</b>) by KN­(SiMe₃)₂ afforded a mixture of two diastereomers (<b>2M</b> and <b>2m</b>) in which a C–C bond has been formed between the S-bonded CH₂ group and the 2 position of 2,2′-bipyridine. The solid-state structure of the more stable 2,6-^<i>i</i>Pr-BIAN analogue could be determined by X-ray diffraction

Hydroxylamine Diffusion Can Enhance N₂O Emissions in Nitrifying Biofilms: A Modeling Study

Wastewater treatment plants can be significant sources of nitrous oxide (N₂O), a potent greenhouse gas. However, little is known about N₂O emissions from biofilm processes. We adapted an existing suspended-growth mathematical model to explore N₂O emissions from nitrifying biofilms. The model included N₂O formation by ammonia-oxidizing bacteria (AOB) via the hydroxylamine and the nitrifier denitrification pathways. Our model suggested that N₂O emissions from nitrifying biofilms could be significantly greater than from suspended growth systems under similar conditions. The main cause was the formation and diffusion of hydroxylamine, an AOB nitrification intermediate, from the aerobic to the anoxic regions of the biofilm. In the anoxic regions, hydroxylamine oxidation by AOB provided reducing equivalents used solely for nitrite reduction to N₂O, since there was no competition with oxygen. For a continuous system, very high and very low dissolved oxygen (DO) concentrations resulted in lower emissions, while intermediate values led to higher emissions. Higher bulk ammonia concentrations and greater biofilm thicknesses increased emissions. The model effectively predicted N₂O emissions from an actual pilot-scale granular sludge reactor for sidestream nitritation, but significantly underestimated the emissions when the NH₂OH diffusion coefficient was assumed to be minimal. This numerical study suggests an unexpected and important role of hydroxylamine in N₂O emission in biofilms

Re-Mediated C–C Coupling of Pyridines and Imidazoles

Rhenium tricarbonyl complexes with three <i>N</i>-heterocyclic ligands (<i>N</i>-alkylimidazoles or pyridines) undergo deprotonation with KN­(SiMe₃)₂ and then oxidation with AgOTf to afford complexes with pyridylimidazole or bipyridine bidentate ligands resulting from deprotonation, C–C coupling and rearomatization

Re-Mediated C–C Coupling of Pyridines and Imidazoles

Rhenium tricarbonyl complexes with three <i>N</i>-heterocyclic ligands (<i>N</i>-alkylimidazoles or pyridines) undergo deprotonation with KN­(SiMe₃)₂ and then oxidation with AgOTf to afford complexes with pyridylimidazole or bipyridine bidentate ligands resulting from deprotonation, C–C coupling and rearomatization

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

Influence of the N–N Coligand: C–C Coupling Instead of Formation of Imidazol-2-yl Complexes at {Mo(η³‑allyl)(CO)₂} Fragments. Theoretical and Experimental Studies

New <i>N</i>-methylimidazole (N-MeIm) complexes of the {Mo­(η³-allyl)­(CO)₂(N–N)} fragment have been prepared, in which the N,N-bidentate chelate ligand is a 2-pyridylimine. The addition of a strong base to the new compounds deprotonates the central CH group of the imidazole ligand and subsequently forms the C–C coupling product that results from the nucleophilic attack to the imine C atom. This reactivity contrasts with that previously found for the analogous 2,2′-bipyridine compounds [Mo­(η³-allyl)­(CO)₂­(bipy)­(N-RIm)]­OTf [N-RIm = N-MeIm, <i>N</i>-mesitylimidazole (N-MesIm, Mes= 2,4,6-trimethylphenyl); OTf = trifluoromethanesulfonate) which afforded imidazol-2-yl complexes upon deprotonation. Density Functional Theory (DFT) computations uncover that the reactivity of the imine C atom along with its ability to delocalize electron density are responsible for the new reactivity pattern found for the kind of molybdenum complexes reported herein

Insights on the Reactivity of Terminal Phosphanido Metal Complexes toward Activated Alkynes from Theoretical Computations

Herein we present a theoretical study on the reaction of [Re­(PPh₂) (CO)₃(phen)] (phen = 1,10-phenanthroline) and [Re­(PPh₂) (CO)₃(bipy)] (bipy = 2,2′-bipyridine) toward methyl propiolate. In agreement with experimental results for the phen ligand, the coupling of the substituted acetylenic carbon with the nonsubstituted <i>ortho</i> carbon of the phen ligand is the preferred route from both kinetic and thermodynamic viewpoints with a Gibbs energy barrier of 18.8 kcal/mol and an exoergicity of 11.1 kcal/mol. There are other two routes, the insertion of the acetylenic fragment into the P–Re bond and the coupling between the substituted acetylenic carbon and a carbonyl ligand in <i>cis</i> disposition, which are kinetically less favorable than the preferred route (by 2.8 and 1.9 kcal/mol, respectively). Compared with phen, the bipy ligand shows less electrophilic character and also less π electron delocalization due to the absence of the fused ring between the two pyridine rings. As a consequence, the route involving the coupling with a carbonyl ligand starts to be kinetically competitive, whereas the product of the attack to bipy is still the most stable and would be the one mainly obtained after spending enough time to reach thermal equilibrium