ABSTRACT
It has been experimentally observed that water-ice-embedded polycyclic aromatic hydrocarbons (PAHs) form radical cations when exposed to vacuum UV irradiation, whereas ammonia-embedded PAHs lead to the formation of radical anions. In this study, we explain this phenomenon by investigating the fundamental electronic differences between water and ammonia, the implications of these differences on the PAH-water and PAH-ammonia interaction, and the possible ionization pathways in these complexes using density functional theory (DFT) computations. In the framework of the Kohn-Sham molecular orbital (MO) theory, we show that the ionic state of the PAH photoproducts results from the degree of occupied-occupied MO mixing between the PAHs and the matrix molecules. When interacting with the PAH, the lone pair-type highest occupied molecular orbital (HOMO) of water has poor orbital overlap and is too low in energy to mix with the filled π-orbitals of the PAH. As the lone-pair HOMO of ammonia is significantly higher in energy and has better overlap with filled π-orbitals of the PAH, the subsequent Pauli repulsion leads to mixed MOs with both PAH and ammonia character. By time-dependent DFT calculations, we demonstrate that the formation of mixed PAH-ammonia MOs opens alternative charge-transfer excitation pathways as now electronic density from ammonia can be transferred to unoccupied PAH levels, yielding anionic PAHs. As this pathway is much less available for water-embedded PAHs, charge transfer mainly occurs from localized PAH MOs to mixed PAH-water virtual levels, leading to cationic PAHs.
ABSTRACT
A series of NHC-gold(i) (NHC = N-heterocyclic carbene) complexes has been studied by DFT calculations, enabling comparison of electronic and NMR behaviour with related protonated and free NHC molecules. Based on calculations, the NMR resonances of the carbenic C2 carbon atom in [Au(NHC)(Cl)] and [NHC(H)][Cl] exhibit increased shielding when compared to the free N-heterocyclic carbenes by an average of 46.6 ± 2.2 and 73.7 ± 4.3 ppm, respectively. A similar trend is observed when analysing the paramagnetic term of the magnetic shielding tensor. Although gold(i) and proton are considered isolobal fragments, imidazolium compounds lack π-backdonation due to the energetic unavailability of d-orbitals in H+. We propose that NHC-gold(i) complexes exhibit important π-backdonation irrespective of the relative amount of σ-donation between the NHC and gold(i)-X (X = anionic ligand) moieties in Au-NHC complexes. Interestingly, a correlation exists between the calculated shielding for gold (197Au) and the π-donation and π-backdonation contributions. We describe that this correlation also exists when analysing the σ-backdonation term, a property generally ignored yet representing a significant energetic contribution to the stability of the C2-Au bond.
ABSTRACT
New [Au(IPr)(CHR2)] complexes have been synthesised through protonolysis reactions of [Au(IPr)(OH)] with moderately acidic substrates, CH2R2. An array of spectroscopic (IR and NMR), structural (X-ray), electronic (DFT) and experimental (reactivity) parameters was collected to quantify the variation in stereoelectronic properties of these new and previously reported [Au(IPr)(CHR2)] complexes. Variation of the R substituents on the carbanion ligands (CHR2-) was found to have a crucial impact on parameters characterising the resulting gold complexes. A regression analysis of both experimental and modelled parameters, guided by network analysis techniques, produced linear models that supported trends within the [Au(IPr)(CHR2)] complexes.
ABSTRACT
We report the use of cationic gold complexes [Au(NHC)(CH3CN)][BF4] and [{Au(NHC)}2(µ-OH)][BF4] (NHC = N-heterocyclic carbene) as highly active catalysts in the solvent-free hydroalkoxylation of internal alkynes using primary and secondary alcohols. Using this simple protocol, a broad range of (Z)-vinyl ethers were obtained in excellent yields and high stereoselectivities. The methodology allows for the use of catalyst loadings as low as 200 ppm for the addition of primary alcohols to internal alkynes (TON = 35 000, TOF = 2188 h-1).
