Palladium hetero‐di(N‐heterocyclic carbene) complexes and their catalytic activities in direct C–H arylation of heteroarenes

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Palladium hetero-di(N-heterocyclic carbene) complexes and their catalytic activities in direct C–H arylation of heteroarenes

Palladium hetero-dicarbene complexes have been prepared and tested for their catalytic activities in the direct C-H arylation of thiophenes with aryl halides. Complexes containing more distinct NHC units give rise to superior catalysts due to an amplified “stereoelectronic asymmetry.” A preliminary photophysical study of some selected thiophene reaction products has been conducted as well. The ability to individually change each NHC donor in such dicarbene ligands allows for a better fine-tuning of complex properties in search for superior catalysts.


A library of neutral and cationic palladium complexes of cis-chelating hetero-dicarbene ligands have been prepared. These ligands contain two different NHC donors allowing for a wider degree of variation, and the impact of the distinct NHC units has been compared using various spectroscopic means. In addition, the catalytic activities of these complexes in the direct C-H arylation of thiophenes with aryl halides were studied leading to the finding that the neutral dibromido complexes of the type [PdBr2(diNHC)] generally outperformed their cationic [Pd (NCMe)2(diNHC)](OTf)2 counterparts. More importantly, complexes containing more distinct NHC units give rise to superior catalysts due to an amplified “stereoelectronic asymmetry” within the complex. A preliminary photophysical study of some selected thiophene reaction products has been conducted as well. The ability to individually change each NHC donor in such dicarbene ligands allows for a better fine-tuning of complex properties in search for superior catalysts.

Variation in electrophilicity on electronic excitation

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Variation in electrophilicity on electronic excitation

The validity of the minimum electrophilicity principle is tested in the course of molecular electronic excitation.


Abstract

The rationality of the minimum electrophilicity principle (MElP) as a companion of minimum polarizability principle and maximum hardness principle is studied for simple diatomic, triatomic, and tetratomic molecules. The applicability is further justified considering organic molecules (e.g., pyrene and acridine yellow) are known for their photophysical properties and accordingly their excited state properties. Single excitation CI (CIS) and time-dependent density functional theory (TDDFT) are employed to study the excited state reactivity. Two types of excitations, namely vertical and adiabatic, are considered. Processes involving conservation and change in spin multiplicity are included during excitation. The general trend is that the molecules are less electrophilic in the ground state than those in the corresponding excited states. It is found that adiabatic excitation validates the principle even for the triplet ground state molecules undergoing an excitation where spin multiplicity gets altered. The TDDFT method explains the validity of the MElP augmenting the CIS method. This study echoed the MElP during molecular electronic excitation.

Reusable water‐soluble homogeneous catalyst in aqueous‐phase transfer hydrogenation of N‐heteroarenes with formic acid: Uracil‐based bifunctional Ir‐NHC catalyst is the key

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Reusable water-soluble homogeneous catalyst in aqueous-phase transfer hydrogenation of N-heteroarenes with formic acid: Uracil-based bifunctional Ir-NHC catalyst is the key

A water-soluble Ir-NHC complex is demonstrated as an efficient, durable, reusable, and sustainable alternative catalyst for transfer-hydrogenation of quinolines and related N-heteroarenes with (buffered) formic acid.


Abstract

Transfer hydrogenation of N-heteroarenes was successfully achieved using a water-soluble half-sandwich Cp*Ir-based catalyst containing a uracil-based bifunctional abnormal NHC ligand, using HCOOH/HCOONa buffer solution as the hydrogen source. Reduction of N-heteroarenes was shown to be highly pH-dependent, and an acidic pH = 3.0 was found to be suitable for the best activity. The catalyst showed excellent functional group compatibility and high turnover number (up to 10,400), with catalyst loadings as low as 0.005 mol%. Finally, we demonstrated the catalyst's efficacy and applicability toward reusable and repetitive transfer hydrogenation using liquid/liquid extraction methodology, which underscored the significance of sustainable usage of noble metal catalysts in such transformations.