Density functional theory studies of molecules that undergo single two electron redox reactions accompanied by structural changes.

摘要

Three different molecules, (i) Fe2(CO)6(mu 2-P(CH3)2)2 = 1-CH3; (ii) Cyclooctatetraene = COT and (iii) Ru(eta 6-(C6(CH3)6)22+ = Ru(Ar*)22+ that have been observed to undergo single two-electron redox reactions accompanied by structural change were investigated using Density Functional Theory (DFT) combined with the COnductor like Solvation MOdel (COSMO). The multielectron redox reaction, observed as a single step in the cyclic voltammogramm (CV), arises from the thermodynamic instability of the 1e-reduced intermediate with respect to disproportionation. The structures and energies of all three oxidation states involved in the redox reaction are calculated for each system in gas phase and solution to evaluate the energetics of the disproportionation reaction. Gas phase calculations poorly model the experimental redox behavior, and the inclusion of solvation is necessary to correctly predict the energetics of the disproportionation reaction.; In the study of 1-CH3, the experimentally reported iron-iron bond cleavage of the dinuclear complex is reproduced correctly and a disproportionation reaction enthalpy of -0.23 eV is calculated in acetonitrile. A study of ion pairing effects and the distribution of the added electrons over the molecules as well as the charge distribution as a function of alkali metal counter cation (Li, Na+, K+) were evaluated using the Hirshfeld charge analysis scheme. The role of the carbonyl ligand in dissipating the excess charge and the effect of substituting the methyl group of the bridging phosphido-ligand by CF3 are also addressed.; COT displays counterion-dependent redox behavior. In the presence of alkali-metal cations, a single 2e-wave is observed in the CV, whereas two separated 1e-waves are observed in the absence coordinating counterions. Upon 2e-reduction, the tub-shaped neutral COT ring flattens and forms a planar aromatic system. DFT/COSMO calculations reproduce the counterion-dependent disprortionation reaction. An energetically favorable disproportionation reaction is only predicted for the ion paired system. A novel formal energy partitioning scheme is developed and used to analyze the effects of ion pairing on the relative energies of the different redox species.; The third type of electrochemically induced structural change is displayed by Ru(Ar*)22+. The formation of an energetically unfavorable 20-electron species is avoided by a eta 6 → eta4 hapticity change. The larger eta 4 bond energy is identified as the main cause of the energy profile distortion leading to an energetically favorable disproportionation reaction. The electronic structure of Ru(Ar)22+ (Ar = eta6-C6H6), a model for Ru(Ar*)22+, and its reduced forms are examined in detail and compared to the first row transition metal analogue Fe(Ar) 22+. The isoelectronic group 9 analogues Rh/Co-CpAr (Cp = eta5-C5H5) are also investigated. Vibrational frequencies of the model systems are used to deduce the zero-point-energy and entropy corrections to calculate absolute redox potentials and a good correlation with the experimentally measure redox potentials is obtained for the metallocene series.