László Nyulászi

Phosphole‐Containing π‐Conjugated Systems: From Model Molecules to Polymer Films on Electrodes

Two series of 2,5-dipyridyl- and 2,5-dithienylphosphole derivatives containing sigma3- or sigma4-P atoms were prepared, and their optical (UV/Vis absorption, fluorescence spectra) and electrochemical properties were systematically evaluated. These physical properties depend mainly on the natures of the 2,5-substituents and of the phosphorus moiety, and they revealed that these compounds contain extended pi-conjugated systems. Structure-property relationships were established on the basis of these experimental data and ab initio calculations on the parent molecules. The limited aromatic character and low-lying LUMO of the phosphole ring appear to be crucial for achieving a highly delocalised pi system. Electrooxidation of 2,5-dithienylphosphole derivatives affords electroactive films with low optical band gaps. As observed for the corresponding monomers, the optical and electrochemical properties of the polymers can be varied over a wide range by modifying the nature of the phosphorus moiety.

Carbenes in ionic liquids

The chemistry of 1,3-dialkylimidazolium-based ionic liquids (ILs) can easily be linked to that of N-heterocyclic carbenes (NHCs) in the presence of sufficiently basic counteranions. B3LYP/6-31+G*, B3LYP/aug-cc-pVTZ and MP2/6-311+G** studies show that increasing the basicity of the anionic component the relative stability of the ion pair and that of the hydrogen bonded complex of the corresponding free acid and NHC itself can be shifted toward the formation of NHC. In the case of the acetate anion, the ion pair and the NHC-acetic acid complex have similar stability. Photoelectron spectroscopic studies show that the vapor of EMIM-acetate is dominated by the NHC-acetic acid complex. The mass spectrum of the same compound shows the presence of both acetic acid and 1-ethyl-3-methylimidazolium-2-ylidene, in agreement with the low pressure during the MS experiment, which facilitates dissociation. The possibility of systematic and simple variation of the NHC content of the ILs facilitates the extension of carbene chemistry in ionic liquids.

Hydrolysis of Imidazole-2-ylidenes

The direct reaction of an imidazole-2-ylidene in a predominantly aqueous environment [about 0.1 M solution in a H(2)O (>60%)/THF solvent system] was investigated for the first time. The reaction yielded a stable solution of the corresponding imidazolium-hydroxide of pH 13, which is in agreement with results from an ab initio molecular dynamics simulation. In contrast, hydrolysis of the carbene in a mainly aprotic environment (>80% THF) gives a hydrogen-bridged carbene-water complex which could be detected by NMR and IR spectroscopies for the first time. This complex converts slowly to two isomeric ring opened products and is at higher water concentration in dynamic equilibrium with the imidazolium hydroxide. A computational mechanistic study of the carbene hydrolysis with a gradually increasing number of water molecules revealed that the imidazolium-hydroxide structure can only be optimized with three or more water molecules as reactants, and with the increasing number of water molecules its stability is increasing with respect to the carbene-water complex. In agreement with the experimental results, these findings point out that solvent stabilization and basicity of the hydroxide ion plays a crucial role in the reaction. With increasing number of water molecules the barriers connecting the reaction intermediates are getting smaller, and the ring opened hydrolysis products can be derived from imidazolium-hydroxide type intermediates. Computational studies on the hydrolysis of a nonaromatic imidazolidine-2-ylidene analogue clearly indicated the analogous ring-opened product to be by 10-12 kcal/mol more stable than the appropriate ion pair and the carbene-water complex, in agreement with the known aromatic stabilization of imidazol-2-ylidenes. Accordingly, these molecules hydrolyze with exclusive formation of the ring-opened product.

Dibenzophosphapentaphenes: Exploiting P Chemistry for Gap Fine-Tuning and Coordination-Driven Assembly of Planar Polycyclic Aromatic Hydrocarbons

A synthetic route to planar P-modified polycylic aromatic hydrocarbons (PAHs) is described. The presence of a reactive σ(3),λ(3)-P moiety within the sp(2)-carbon scaffold allows the preparation of a new family of PAHs displaying tunable optical and redox properties. Their frontier molecular orbitals (MOs) are derived from the corresponding phosphole MOs and show extended conjugation with the entire π framework. The coordination ability of the P center allows the coordination-driven assembly of two molecular PAHs onto a Au(I) ion.

An organocatalytic ionic liquid

The carbene concentration in 1-ethyl-3-methylimidazolium-acetate ionic liquid is sufficiently high to act as a catalyst in benzoin condensation, hydroacylation and also in oxidation of an alcohol by using CO(2) and air. This observation reveals the potential of ionic liquid organocatalysts, uniting the beneficial properties of these two families of compounds.

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.

About the aromaticity of five-membered heterocycles

Abstract The aromaticity of five-membered heterocycles with one and two heteroatoms (O, S, NH, N and P) has been investigated, using energetic and structural criteria. It has been shown that to obtain a correct order for the aromaticity, a nonhomodesmic reaction (SEH) is sufficient, provided it compares the energy of the ring to that of the conjugated building blocks in the ring. Contrary to general expectations, superhomodesmic reactions seem less reliable owing to unpredictable steric interactions in the conjugated reference molecules. The stabilization shows a considerable increase if calculated at the MP2 versus HF level of theory in all reactions. The aromatic stabilization in the ring is mainly determined by the “first heteroatom” which provides two electrons for the π-system (O, S or NH). The effect of the second heteroatom is much smaller. In agreement with earlier observations, N as a second heteroatom increases the stabilization in the SEH reaction. In the case of P, stabilization has again been observed, but only at the MP2 level. An aromaticity index has been defined based on the double bond characters of the peripheral bonds in the ring. According to this geometry aromaticity index ( I ), there is no apparent increase in aromaticity for the phosphorus compounds; thus, I can be related to the decrease in ring strain, attributable to the small bonding angle about phosphorus.

The First Delocalized Phosphole Containing a Planar Tricoordinate Phosphorus Atom: 1-[Bis(trimethylsilyl)methyl]-3,5-bis(trimethylsilyl)-1,2,4-triphosphole

The sterically demanding groups on the tricoordinate phosphorus atom, the π-electron acceptors substituted on the ring, and the dicoordinate phosphorus atoms within the ring are the most significant factors contributing to the planarity and aromaticity of the 1,2,4-triphosphole ring in 1. The Bird aromaticity index for 1 shows that it has the most pronounced aromatic character of all known phospholes.

An abnormal N-heterocyclic carbene-carbon dioxide adduct from imidazolium acetate ionic liquids: the importance of basicity.

In the reaction of 1-ethyl-3-methylimidazolium acetate [C2C1Im][OAc] ionic liquid with carbon dioxide at 125 °C and 10 MPa, not only the known N-heterocyclic carbene (NHC)-CO2 adduct I, but also isomeric aNHC-CO2 adducts II and III were obtained. The abnormal NHC-CO2 adducts are stabilized by the presence of the polarizing basic acetate anion, according to static DFT calculations and ab initio molecular dynamics studies. A further possible reaction pathway is facilitated by the high basicity of the system, deprotonating the initially formed NHC-CO2 adduct I, which can then be converted in the presence of the excess of CO2 to the more stable 2-deprotonated anionic abnormal NHC-CO2 adduct via the anionic imidazolium-2,4-dicarboxylate according to DFT calculations on model compounds. This suggests a generalizable pathway to abnormal NHC complex formation.

Aromatic Compounds with Planar Tricoordinate Phosphorus

Abstract Tricoordinate phosphorus can be planarized, and used as an excellent building block of aromatic compounds. The pyramidality can be reduced by (i) σ donor and π acceptor groups; (ii) steric strain, which increases the bonding angle about phosphorus; (iii) bulky substituent groups; and (iv) delocalization or aromatic stabilization effects. The aromaticity of π systems with planar tricoordinate phosphorus is largest among those rings containing nitrogen or chalcogen heteroatoms. For the flat systems the aromatic stabilization is just compensated by the energy cost of the planarization at phosphorus. Due to this counterbalancing effect a very rich chemistry can be expected for these compounds.