László Könczöl

Significant Cation Effects in Carbon Dioxide–Ionic Liquid Systems

Carbon dioxide–ionic liquid systems are of great current interest, and significant efforts have been made lately to understand the intermolecular interactions in these systems. In general, all the experimental and theoretical studies have concluded so far that the main solute–solvent interaction takes effect through the anion, and the cation has no, or only a secondary role in solvation. In this theoretical approach it is shown that this view is unfounded, and evidence is provided that, similarly to the benzene–CO2 system, dispersion interactions are present between the solute and the cation. Therefore, this defines a novel site for tailoring solvents to tune CO2 solubility.

Pyridyl-Functionalised 3H-1,2,3,4-Triazaphospholes: Synthesis, Coordination Chemistry and Photophysical Properties of Low-Coordinate Phosphorus Compounds

Novel conjugated, pyridyl-functionalised triazaphospholes with either tBu or SiMe3 substituents at the 5-position of the N3 PC heterocycle have been prepared by a [3+2] cycloaddition reaction and compared with structurally related, triazole-based systems. Photoexcitation of the 2-pyridyl-substituted triazaphosphole gives rise to a significant fluorescence emission with a quantum yield of up to 12 %. In contrast, the all-nitrogen triazole analogue shows no emission at all. DFT calculations indicate that the 2-pyridyl substituted systems have a more rigid and planar structure than their 3- and 4-pyridyl isomers. Time-dependent (TD) DFT calculations show that only the 2-pyridyl-substituted triazaphosphole exhibits similar planar geometry, with matching conformational arrangements in the lowest energy excited state and the ground state; this helps to explain the enhanced emission intensity. The chelating P,N-hybrid ligand forms a Re(I) complex of the type [(N^N)Re(CO)3 Br] through the coordination of nitrogen atom N(2) to the metal centre rather than through the phosphorus donor. Both structural and spectroscopic data indicate substantial π-accepting character of the triazaphosphole, which is again in contrast to that of the all-nitrogen-containing triazoles. The synthesis and photophysical properties of a new class of phosphorus-containing extended π systems are described.

Substituent effect on the aromaticity of the silolide anion

The effect of the substituents on the planarity and aromaticity of the silolide anion was studied computationally. It was revealed that π-electron acceptor groups (e.g., silyl or trimethylsilyl) at the α position of the ring reduce substantially the inversion barrier about the central silicon increasing the aromaticity according to isomeric stabilization energy (ISE) and NICS values. In the planar and highly aromatic silolide anions, the mesomeric structures with the largest weight exhibit a Si=C double bond and a negative charge which is located at the α or β carbons based on NRT calculations. 2,5-disilylsilacyclopentadienides coordinated by naked Li+ have planar minima, however, further coordination by THF molecules (as in a solution) reduces somewhat the flattening of the silicon pyramid. NMR calculations were carried out to understand the connection between the 29Si chemical shift and the aromaticity of the ring. It was revealed that the commonly accepted charge transfer–chemical shift relationship is strongly influenced by the substituents and the counter cation. THF complexation of the Li counter cation has a small influence on the NMR shift.

Π-Rich σ2P-Ligands: Unusual Coordination Behavior of 1H-1,3-Benzazaphospholes Toward Late Transition Metals

GRAPHICAL ABSTRACT Abstract Coordination chemical studies of 1H-1,3-benzazaphospholes, π-excess aromatic σ2P-ligands, demonstrate similar ligand properties to those of phosphabenzenes toward MVI(CO)n but marked differences toward nonzerovalent transition metals. Benzazaphosphole d8-metal halide complexes are more strongly destabilized and undergo consecutive reactions, whereas benzazaphosphole coinage metal halide and HgCl2 complexes generally display μ2P- and/or bent η1P-coordination with π-P donor bond contribution, contrasting with the preference of phosphinine CuX complexes for η1P-coordination within the ring plane.

Specific Photochemical Dehydrocoupling of N-Heterocyclic Phosphanes and Their Use in the Photocatalytic Generation of Dihydrogen

N-Heterocyclic phosphanes react under UV irradiation in a highly selective dehydrocoupling reaction to diphosphanes and H2. Computational studies suggest that the product formation is initiated by the formation of dimeric molecular associates whose electronic excitation yields H2 and a diphosphane. Combining the dehydrocoupling of sterically demanding phosphanes with Mg-reduction of the formed diphosphanes allows constructing a reaction cycle for the photocatalytic reductive generation of H2 from Et3NH(+).

Di(phosphavinyl) ethers (2,4-diphospha-3-oxapentadienes)

Hydrolytic cleavage of the P-chlorophosphaalkenes (RMe2Si)2C=PCl (R = Me: 1a; R = iPr: 1b) in the presence of triethylamine leads to di(phosphavinyl) ethers (2,4-diphospha-3-oxapentadienes) [(RMe2Si)2C=P]2O (2a, 2b) as main products, accompanied by alkylphosphinic acids (RMe2Si)2(H)CP(H)(O)OH (3a, 3b). The hydrolysis of (PhMe2Si)2C=PCl (1c) proceeds less selectively. Reactions with metal oxides under aprotic conditions provide 2a [impure, from 1a with (nBu3Sn)2O] and 2b [from iodophosphaalkene (iPrMe2Si)2C=PI with Ag2O] as oils. 1H, 13C, 29Si and 31P NMR spectra, however, allow unambiguous characterisation of 2a and 2b. Formation mechanisms, structure, and C=P-O π stabilisation of the oxabisphosphaalkene [(H3Si)2C=P]2O (2ʹ) were studied with DFT methods. The double [2+4] cycloaddition reaction of 2a with two equivalents of cyclopentadiene leads to the phosphinous anhydride 7 as a mixture of diastereomers whereas the addition of two equivalents of tetrachloro-o-benzoquinone proceeds in a diastereoselective fashion. An X-ray crystal structure determination of the resulting oxo-bridged bis(2-phospha-2,5-dioxa-3,4-benzophospholene) derivative 8 revealed the presence of a racemic mixture of (R,R)- and (S,S)-configurated molecules. The solid state structure of a by-product, bisylphosphonic tetrachlorocatechol monoester (Me3Si)2CH-P(=O)(OH)-o-OC6Cl4OH 9, was also determined crystallographically. Graphical Abstract Di(phosphavinyl) Ethers (2,4-Diphospha-3-oxapentadienes)

Asymmetric hydrogenation of prochiral and kinetic resolution of chiral cyclohexanone derivatives with Pd catalysts

This paper focuses on the investigation of the role of catalytic and molecular factors determining the optical yield in the enantioselective hydrogenation of C=C double bonds in prochiral unsaturated ketones and in the kinetic resolution of racemates of chiral cyclic ketones with proline mediated reductive alkylation. The hydrogenation of two unsaturated methyl cyclohexenones, and the kinetic resolution of two saturated methyl cyclohexanones with proline were studied with different Pd catalysts. The aim was to find out the effects of catalyst modifiers and chiral auxiliary, beside that of the structure and the degree of substitution of the substrates on the stereochemical outcome and yield of these reactions.

CnH2nCl+ ion formation in electron impact MS conditions: A theoretical study

The mass spectral fragmentation of different 1-chloroalkanes (of the 1-chlorohexane-1-chlorooctadecane series) has been investigated, quantifying the relative abundance of the fragment ions. The base peak is dominantly at m/z 91, 93 in each investigated case, although with the increasing chain length, its contribution to the total ion current exhibits some reduction. Among the possible fragmentation products, the five-membered chloronium containing ring is the most stable as measured by an isodesmic reaction, although the six-and seven-membered rings exhibit only slightly reduced stability. The most stable structure of the 1-chlorohexane radical cation has a hydrogen bonded structure with the involvement of chlorine and the HC(δ), pre-forming the five-membered cationic ring. Accordingly, among the reactions leading to alkyl (or H) radical and a chloronium containing ring, this transition structure has the lowest energy, providing explanation for the experimental observations.