Ashfaq A. Bengali

Further development in the area of medicinal chemistry requires facile and atom-economical C-N bond formation from readily accessible precursors using recyclable and reusable catalysts with low process toxicity. In this work, direct N-alkylation of amines with alcohols is performed with a series of Ir-phosphine-functionalized metal-organic framework (MOF) heterogeneous catalysts. The grafted monophosphine-Ir complexes were studied comprehensively to illustrate the ligand-dependent reactivity. The afforded MOF catalysts exhibited high reactivity and selectivity toward N-alkylamine product formation, especially UiO-66-PPh₂-Ir, which showed 90% conversion after recycling with no catalyst residue remaining in the product after the reaction. Furthermore, analyses of the active catalyst, mechanistic studies, control experiments, and H₂ adsorption tests are consistent with the conclusion that immobilization of the iridium complex on the MOF support enables the formation of the iridium-monophosphine complex and enhances its stability during the reaction. To illustrate the potential of the catalyst for application in medicinal chemistry, two pharmaceutical precursors were synthesized with up to 99% conversion and selectivity.

The predictive ability of density functional theory is fundamental to its usefulness in chemical applications. Recent work has compared solution-phase enthalpies of activation for metal-ligand bond dissociation to enthalpies of reaction for bond dissociation, and the present work continues those comparisons for 43 density functional methods. The results for ligand dissociation enthalpies of 30 metal-ligand complexes tested in this work reveal significant inadequacies of some functionals as well as challenges from the dispersion corrections to some functionals. The analysis presented here demonstrates the excellent performance of a recent density functional, M11plus, which contains nonlocal rung-3.5 correlation. We also find a good agreement between theory and experiment for some functionals without empirical dispersion corrections such as M06, r²SCAN, M06-L, and revM11, as well as good performance for some functionals with added dispersion corrections such as ωB97X-D (which always has a correction) and BLYP, B3LYP, CAM-B3LYP, and PBE0 when the optional dispersion corrections are added.

Metal-organic frameworks (MOFs) are typically assembled by bridging metal centers with organic linkers for various applications, including providing robust support for heterogeneous catalysts for CO₂ reduction. In this study, we have demonstrated the solubilization of a MOF tethered to a CO₂-reducing electrocatalyst and studied its fundamental electrochemistry in THF solvent using infrared spectroelectrochemistry (IR-SEC). The fundamental electrochemical properties of this immobilized catalyst were compared to that of its homogeneous counterpart. This approach provides a foundation for future experimental studies to bridge the gap between homogeneous and heterogeneous electrocatalysis.

Manganese tricarbonyl diimine complexes bearing pyridine and imidazole ligands have been prepared as electrocatalysts for proton reduction using acetic acid as the proton source. The electron-donor ability of the diimine ligand is found to play an important role in determining the efficiency of the electrocatalysts with [MnBr(pybz)(CO)₃] (pybz = 2-(2-pyridyl)benzimidazole) exhibiting the lowest overpotential (0.28 V) toward proton reduction. The [Mn(pybz)(CO)₃(MeCN)]⁺ cationic complex prepared via debromination of [MnBr(pybz)(CO)₃] by a silver salt has also been shown to catalyze proton reduction upon its electrochemical reduction. A neutral complex [Mn(pyridine-benzimidazolate)(CO)₃(MeCN)], which can be synthesized by reacting [MnBr(pybz)(CO)₃] with a strong base, has been detected using IR-SEC (infrared spectroelectrochemistry) as an intermediate species in the catalytic process. Using [MnBr(pybz)(CO)₃] as the model electrocatalyst, we have carried out density functional calculations to propose a proton reduction mechanism consistent with our experimental observations.

This paper describes the catalytic activity of air stable and easy to handle manganese complexes towards the hydrosilylation of aldehydes. These catalysts incorporate a bulky diazabutadiene ligand and exhibit good functional group tolerance and chemoselectivity in the hydrosilylation of aldehydes, utilizing primary silanes as the reducing agent. The reactions proceed with turnover frequencies approaching 150 h⁻¹ in some instances, similar to those observed for other manganese-based catalysts. The conversion of aromatic aldehydes to the corresponding alcohols was found to be more efficient than that for the analogous aliphatic systems.

Transition metal catalysts immobilized on the surface of MOFs, are being utilized for an ever increasing number of reactions ranging from couplings to olefin oligomerization. While these reactions are usually performed in solution, unlike their homo-geneous counterparts, the insolubility of the MOF systems makes it difficult to obtain detailed mechanistic information by in situ spectroscopic analysis in solution. In this report, we present a synthetic method to solubilize these systems by grafting oligomers on the surface of the MOF particles, making it possible to characterize these species by transmission IR spectros-copy. The fundamental photochemistry of these catalysts was also studied and compared to that of their homogeneous counterparts. This work establishes a proof of principle for in solution monitoring of heterogeneous catalysts.

Oxidative cleavage of aromatic alkenes and oxidation of alkynes have been catalyzed by a simple Fe(II) complex [Fe(CH₃CN)₆][BF₄]₂ using H₂O₂ as an oxidant at room temperature. A free radical mechanism has been proposed involving the peroxy (OOH) radical to account for our experimental findings.

The photochemistry of two Mn(bpy)(CO)₃X complexes (X = PhCC^–, Br^–) has been studied in the coordinating solvents THF (terahydrofuran) and MeCN (acetonitrile) employing time-resolved infrared spectroscopy. The two complexes are found to exhibit strikingly different photoreactivities and solvent dependencies. In MeCN, photolysis of 1-(CO)(Br) [1 = Mn(bpy)(CO)₂] affords the ionic complex [1-(MeCN)₂]Br as a final product. In contrast, photolysis of 1-(CO)(CCPh) in MeCN results in facial to meridional isomerization of the parent complex. When THF is used as solvent, photolysis results in facial to meridional isomerization in both complexes, though the isomerization rate is larger for X = Br^–. Pronounced differences are also observed in the photosubstitution chemistry of the two complexes where both the rate of MeCN exchange from 1-(MeCN)(X) by THFA (tetrahydrofurfurylamine) and the nature of the intermediates generated in the reaction are dependent upon X. DFT calculations are used to support analysis of some of the experiments.