G. V. Markin

Zero Kinetic Energy Spectroscopy: Mass-Analyzed Threshold Ionization Spectra of Chromium Sandwich Complexes with Alkylbenzenes, (η6-RPh)2Cr (R = Me, Et, i-Pr, t-Bu)

For over 25 years zero kinetic energy (ZEKE) spectroscopy has yielded a rich foundation of high-resolution results of molecular ions. This was based on the discovery in the late 60s of long-lived ion states throughout the ionization continuum of molecular ions. Here, an example is chosen from another fundamental system pioneered at this university. The mass-analyzed threshold ionization (MATI) spectra of jet-cooled chromium bisarene complexes (η(6)-RPh)(2)Cr (R = Me (1), Et (2), i-Pr (3), and t-Bu (4)) have been measured and interpreted on the basis of DFT calculations. The MATI spectra of complexes 1 and 2 appear to reveal features arising from ionizations of the isomers formed by the rotation of one arene ring relative to the other. The 1 and 2 MATI spectra show two intense peaks corresponding to the 0(0)(0) ionizations with inverse intensity ratios. As indicated by the DFT calculations, the intensity ratio change on going from 1 to 2 results from different isomers contributing to each MATI peak. The ionization energies corresponding to the 0(0)(0) peaks are 42746 ± 5 and 42809 ± 5 cm(-1) for compound 1 and 42379 ± 5 and 42463 ± 5 cm(-1) for complex 2. The 1 and 2 spectra show also the weaker features representing transitions to the vibrationally excited cationic levels, the signals of individual rotamers being detected and assigned on the basis of calculated vibrational frequencies. The MATI spectra of compounds 3 and 4 reveal only one strong peak because of close ionization potentials of the isomers contributing to the MATI signal. The 3 and 4 ionization energies are 42104 ± 5 and 41917 ± 5 cm(-1), respectively. The precise values of ionization energies obtained from the MATI spectra reveal a nonlinear dependence of the IE on the number of Me groups in the alkyl substituents of (η(6)-RPh)(2)Cr. This can be explained by an increase in the molecular zero point energies on methylation of the substituents.

The thermodynamic properties of bis(η6-ethoxybenzene)chromium fulleride from T → 0 to 340 K

The temperature dependence of the heat capacity of crystalline bis-(η6-ethoxybenzene)chromium fulleride [(η6-(EtOPh))2Cr]·+[C60]·− was studied for the first time by adiabatic vacuum calorimetry over the temperature range 6–340 K with errors of ±0.2%. The temperature dependence of the EPR signal parameters of bis-(η6-ethoxybenzene)chromium fulleride was studied for the first time from 120 to 340 K. A reversible endothermic transformation was observed between 160 and 250 K during heating; it was caused by the dissociation of the [(C60)2]2− dimer and the formation of the [(η6-(EtOPh))2Cr]·+[C60]·− fulleride; its standard thermodynamic characteristics were estimated and analyzed. The experimental data were used to calculate the standard thermodynamic functions, including the heat capacity, enthalpy, entropy, and Gibbs function of the fulleride dimer from T → 0 to 160 K and the [(η6-(EtOPh))2Cr]·+[C60]·− monomeric complex over the temperature range 250–340 K. The standard thermodynamic properties of the fulleride studied, fullerides studied earlier, and fullerite C60 were compared.

Thermodynamic Characteristics of Bis(η 6 ethylbenzene)chromium Fulleride

The temperature dependence of heat capacity of crystalline bis(η6-ethylbenzene)chromium fulleride [(η6-EtPh)2Cr]·+ [C60]·− was studied in adiabatic vacuum calorimeter in the range of 6.7 to 344 K with an error of ±0.3%. Dependence of the parameters of EPR signal of bis(η6-ethylbenzene)chromium fulleride on temperature was studied by electron paramagnetic resonance (EPR) in the range of 120 to 290 K. In the range of 204 to 246 K, upon heating, reversible endothermic transformation was recorded, which is caused by the dissociation of dimer (C60−)2 and formation of fulleride [(η6-EtPh)2Cr]·+ [C60]·−; its standard thermodynamic parameters were estimated and analyzed. Standard thermodynamic functions were calculated by the experimental data obtained: heat capacity, enthalpy, entropy, and Gibbs function of fulleride dimmer in the range of T → 0 to 204 K and monomer complex [(η6-EtPh)2Cr]·+ [C60]·− in the range of 246 to 344 K. Standard thermodynamic properties of fulleride under study, fullerides studied earlier, and fullerite C60 were compared.

Synthesis of 3′ H -cyclopropa[1,9](C 60 - I h )[5,6]fullerenes from 2′-aryl-1′,1′-dimethyl-2′,5′-dihydro-1′ H -pyrrolo[3′,4′:1,9]-(C 60 - I h )[5,6]fulleren-1-ium iodides

New 1′,1′-dimethyl-2′,5′-dihydro-1′H-pyrrolo[3′,4′:1,9](C60-Ih)[5,6]fulleren-1-ium iodides containing different aryl groups in position 2 of the pyrrolidine ring have been synthesized. Their reactions with bis-(toluene)chromine afforded the corresponding 3′-aryl-3′H-cyclopropa[1,9](C60-Ih)[5,6]fullerenes.

DFT-Supported Threshold Ionization Study of Chromium Biphenyl Complexes: Unveiling the Mechanisms of Substituent Influence on Redox Properties of Sandwich Compounds

High-resolution mass-analyzed threshold ionization (MATI) spectra of (η6 -Ph2 )2 Cr and (η6 -Ph2 )(η6 -PhMe)Cr demonstrate that the Ph groups work as electron donors, decreasing the ionization energy of the gas-phase bisarene complexes. In contrast to electrochemical data, a close similarity of the Ph and Me group effects on the oxidation of free sandwich molecules has been revealed. However, DFT calculations testify for the opposite shifts of the electron density caused by the Me and Ph substituents in the neutral complexes, the latter behaving as an electron-accepting fragment. On the contrary, in the bisarene cations, the Ph group becomes a stronger donor than methyl. This change provides the similar substituent effects observed with the MATI experiment. On the other hand, the well-documented opposite influence of the Me and Ph fragments on the redox potential of the (η6 -arene)2 Cr+/0 couple in solution appears to be a result of solvation effects but not intramolecular interactions as shown for the first time in this work.

Synthesis and properties of ion-radical salts based on bis(arene)chromium complexes and fullerene derivatives

A reaction of bis(arene)chromium complexes with [60]fullerene derivatives leads to ionradical bis(arene)chromium [60]fullerides. Bis(1,2,4,5-tetramethylbenzene)chromium [70]fulleride was synthesized similarly. The compounds obtained were characterized by spectroscopic methods, their thermal decomposition was studied and thermal stability of their dimers was evaluated.

Crystalline complexes of fullerene with anisole derivatives

For the first time, bis(anisole)chromium fulleride (PhOMe)2Cr+·[C60]−· and a crystalline complex of fullerene with ortho-butoxyanisole have been obtained. The temperature dependence of the parameters of the EPR spectrum of bis(anisole)chromium fulleride (PhOMe)2Cr+·[C60]−· has been studied. The molecular structure of the complex of fullerene with ortho-butoxyanisole has been established.