Dajana Barišić

Mechanism of Mechanochemical C–H Bond Activation in an Azobenzene Substrate by Pd(II) Catalysts

Mechanism of C-H bond activation by various PdII catalysts under milling conditions has been studied by in situ Raman spectroscopy. Common PdII precursors, that is PdCl2 , [Pd(OAc)2 ]3 , PdCl2 (MeCN)2 and [Pd(MeCN)4 ][BF4 ]2 , have been employed for the activation of one or two C-H bonds in an unsymmetrical azobenzene substrate. The C-H activation was achieved by all used PdII precursors and their reactivity increases in the order [Pd(OAc)2 ]3 <PdCl2 (MeCN)2 <PdCl2 <[Pd(MeCN)4 ][BF4 ]2 . In situ Raman monitoring in combination with stepwise ex situ NMR, IR and PXRD experiments has provided direct probing of the reaction mechanism and kinetics, and revealed how liquids of different acid-base properties and proticity as well as selected solids used as additives modify precursors or intermediates and their reactivity. Reaction intermediates that were isolated and structurally characterized agree with the observed species during reaction. In situ Raman spectroscopy has also enabled the derivation of reaction profiles suggesting an electrophilic process which proceeds via a coordination complex (adduct) undergoing deprotonation by a bound or an external base depending on the used PdII precursor. Slow step of the first palladation for two chloride precursors and [Pd(MeCN)4 ][BF4 ]2 is the C-H bond cleavage whereas palladation using [Pd(OAc)2 ]3 depends primarily on breaking of its trimeric structure by the azobenzene substrate and/or liquid additives.

Acid-base properties of phosphoric and acetic acid in aprotic organic solvents – A complete thermodynamic characterisation

Knowledge regarding the acid-base behaviour in non-aqueous media has remained relatively scarce in spite of its importance for many aspects of chemistry. The research presented in this work fills some of particularly important gaps in the corresponding thermodynamic data. We report on a detailed study of acid-base properties of dihydrogen phosphate and acetate in aprotic organic solvents (acetonitrile, dimethyl sulfoxide, and dimethylformamide). It was found that several processes, i.e. protonation, homoassociation, and dimerisation play important roles in defining the basicity of these widely important anions. In the case of dihydrogen phosphate, formation of higher homoassociates (two anions, one acid molecule and vice versa) was detected, whereas acetate formed only simple homoassociates of 1:1 stoichiometry. The dimerisation of dihydrogen phosphate and acetic acid were confirmed to be important processes as well. The thermodynamics of the above mentioned reactions was characterised in detail by means of various experimental methods: ITC, spectrophotometry, NMR-spectroscopy, and conductometry. Reliable equilibrium constants and other thermodynamic reaction functions were determined. The obtained results were discussed in terms of hydrogen bonding potential of the anions and their conjugated acids, as well as solvent properties, i.e. their ability to solvate the species involved in the studied processes.

Dehydroacetic acid derivatives bearing amide and urea moieties as effective anion receptors

Derivatives of dehydroacetic acid comprising amide or urea subunits have been synthesized and their anion-binding properties investigated. Among a series of halides and oxyanions, the studied compounds selectively bind acetate and dihydrogen phosphate in acetonitrile and dimethyl sulfoxide. The corresponding complexation processes were characterized by means of 1 H NMR titrations, which revealed a 1:1 complex stoichiometry in most cases, with the exception of dihydrogen phosphate, which formed 2:1 (anion/ligand) complexes in acetonitrile. The complex stability constants were determined and are discussed with respect to the structural properties of the receptors, the hydrogen-bond-forming potential of the anions, and the characteristics of the solvents used. Based on the spectroscopic data and results of Monte Carlo simulations, the amide or urea groups were affirmed as the primary binding sites in all cases. The results of the computational methods indicate that an array of both inter- and intramolecular hydrogen bonds can form in the studied systems, and these were shown to play an important role in defining the overall stability of the complexes. Solubility measurements were carried out in both solvents and the thermodynamics of transfer from acetonitrile to dimethyl sulfoxide were characterized on a quantitative level. This has afforded a detailed insight into the impact of the medium on the complexation reactions.