Bis(N-Heterocyclic Carbene) Manganese(I) Complexes in Catalytic N-Formylation/N-Methylation of Amines Using Carbon Dioxide and Phenylsilane

A series of six Mn(I) complexes with general formula [MnBr(bisNHC)(CO)3 ], having a bidentate bis(N-heterocyclic carbene) ligand (bisNHC), has been developed by varying the bridging group between the NHC donors, the nitrogen wingtip substituents and the heterocyclic ring. The synthesis of the complexes has been accomplished by in situ transmetalation of the bisNHC from the corresponding silver(I) complexes. Removal of the bromide anion affords the corresponding solvento complexes [Mn(bisNHC)(CO)3 (CH3 CN)](BF4 ). The influence of the bisNHC structure on its electron donor ability has been evaluated by FTIR and 13 C NMR spectroscopy, both in the neutral and cationic complexes. Finally, the isolated Mn(I)-bisNHC complexes have been employed as homogeneous catalysts in the reductive N-formylation and N-methylation of amines with CO2 as C1 source and phenylsilane as reducing agent, showing a high selectivity for the N-methylated product. Preliminary mechanistic investigations suggest that, in the adopted reaction conditions, the formylated product can be formed via different reaction pathways, either metal-catalyzed or not, while the methylation reaction requires the use of the Mn(I) catalyst.

Mononuclear and dinuclear gold(i) complexes with a caffeine-based di(N-heterocyclic carbene) ligand: synthesis, reactivity and structural DFT analysis

Two gold(i) complexes with caffeine-based di(N-heterocyclic carbene) ligands were synthesised and fully characterised.

Multiscale modeling of reaction rates: application to archetypal SN2 nucleophilic substitutions

We propose an approach to the evaluation of kinetic rates of elementary chemical reactions within Kramers' theory based on the definition of the reaction coordinate as a linear combination of natural, pseudo Z-matrix, internal coordinates of the system. The element of novelty is the possibility to evaluate the friction along the reaction coordinate, within a hydrodynamic framework developed recently [J. Campeggio et al., J. Comput. Chem. 2019, 40, 679-705]. This, in turn, allows to keep into account barrier recrossing, i.e. the transmission coefficient that is employed in correcting transition state theory evaluations. To test the capabilities and the flaws of the approach we use as case studies two archetypal SN2 reactions. First, we consider to the standard substitution of chloride ion to bromomethane. The rate constant at 295.15 K is evaluated to k/c⊖ = 2.7 × 10-6 s-1 (with c⊖ = 1 M), which compares well to the experimental value of 3.3 × 10-6 s-1 [R. H. Bathgate and E. A. Melwyn-Hughes, J. Chem. Soc 1959, 2642-2648]. Then, the method is applied to the SN2 reaction of methylthiolate to dimethyl disulfide in water. In biology, such an interconversion of thiols and disulfides is an important metabolic topic still not entirely rationalized. The predicted rate constant is k/c⊖ = 7.7 × 103 s-1. No experimental data is available for such a reaction, but it is in accord with the fact that the alkyl thiolates to dialkyl disulfides substitutions in water have been found to be fast reactions [S. M. Bachrach, J. M. Hayes, T. Dao and J. L. Mynar, Theor. Chem. Acc. 2002, 107, 266-271].

New homoleptic gold carbene complexes via Ag–Au transmetalation: synthesis and application of [Au(diNHC)2]3+ cations as 1H-NMR and UV-vis halide sensors

Novel homoleptic gold(iii) complexes with bidentate N-heterocyclic carbene ligands have been successfully synthesised by transmetalation reaction and studied as anion sensors in solution by means of ¹H NMR and UV-vis titration experiments.

Structural and Luminescent Properties of Homoleptic Silver(I), Gold(I), and Palladium(II) Complexes with nNHC-tzNHC Heteroditopic Carbene Ligands

Novel silver(I), gold(I), and palladium(II) complexes were synthesized with bidentate heteroditopic carbene ligands that combine an imidazol-2-ylidene (nNHC) with a 1,2,3-triazol-5-ylidene (tzNHC) connected by a propylene bridge. The silver(I) and gold(I) complexes were dinuclear species, [M₂(nNHC-tzNHC)₂](PF₆)₂ (M = Ag or Au), with the two bidentate ligands bridging the metal centers, whereas in the palladium(II) complex [Pd(nNHC-tzNHC)₂](PF₆)₂, the two ligands were chelated on the same metal center. Because of the presence of two different carbene units, isomers were observed for the gold(I) and palladium(II) complexes. The molecular structures of the head-to-tail isomer for gold(I) complexes, with a twisted or folded-syn conformation of the bridge between the carbene units, were determined by X-ray diffraction analysis. The study was completed with a systematic structural investigation through density functional theory (DFT) calculations. For palladium(II) species, the head-to-head form was structurally characterized. The dinuclear gold(I) complexes were emissive in the solid state in the blue region (PLQY up to 8%); time-dependent density functional theory (abbreviated as TD-DFT) calculations disclosed that the absorption bands have metal-to-ligand-charge-transfer character and evidenced that the emission occurs from the T₁ level (phosphorescence).

Coordination chemistry of gold with N-phosphine oxide-substituted imidazolylidenes (POxIms)

Diverse gold complexes are found to be accessible with the title ligands, which exhibit peculiar structural features and promising catalytic performances.