Synthesis and Characterization of Stable Iron Pentacarbonyl Radical Cation Salts

The open-shell iron pentacarbonyl cation [Fe(CO)5 ].+ was isolated by deelectronation, i.e., the single-electron oxidation of the parent neutral Fe(CO)5 using [phenazineF ].+ as the [Al(ORF )4 ]- and [F-{Al(ORF )3 }2 ]- salt (RF =C(CF3 )3 ; phenazineF =perfluoro-5,10-bis(perfluorophenyl)-5,10-dihydrophenazine). [Fe(CO)5 ].+ [Al(ORF )4 ]- was fully characterized (scXRD analysis, IR, NMR, EPR, 57 Fe spectroscopy, CV and SQUID magnetization study) and, apart from being a compound of fundamental interest, may serve as a precursor for low-valent iron coordination chemistry.

Measurements and Utilization of Consistent Gibbs Energies of Transfer of Single Ions: Towards a Unified Redox Potential Scale for All Solvents

Utilizing the "ideal" ionic liquid salt bridge to measure Gibbs energies of transfer of silver ions between the solvents water, acetonitrile, propylene carbonate and dimethylformamide results in a consistent data set with a precision of 0.6 kJ mol-1 over 87 measurements in 10 half-cells. This forms the basis for a coherent experimental thermodynamic framework of ion solvation chemistry. In addition, we define the solvent independent pe abs H 2 O - and the E abs H 2 O values that account for the electronating potential of any redox system similar to the pH abs H 2 O value of a medium that accounts for its protonating potential. This E abs H 2 O scale is thermodynamically well-defined enabling a straightforward comparison of the redox potentials (reducities) of all media with respect to the aqueous redox potential scale, hence unifying all conventional solvents' redox potential scales. Thus, using the Gibbs energy of transfer of the silver ion published herein, one can convert and unify all hitherto published redox potentials measured, for example, against ferrocene, to the E abs H 2 O scale.

The Inverted Philosopher’s Stone: how to turn silver to a base metal

Metals often are classified as “noble” or “base”—characterizing their reduction potential as one of the most important chemical properties. We show that metals are only as noble as allowed by their environment, i.e. this is a relative term, and the “frame of reference” simply is the solvent in which the redox system is present. We prove that silver is a prime example for a noble metal that forfeits its noble character in the simple ionic liquid HMIM Br (1-hexyl-3-methylimidazolium bromide) as an example for such a solvent.

Synthesis and Application of a Perfluorinated Ammoniumyl Radical Cation as a Very Strong Deelectronator

The perfluorinated dihydrophenazine derivative (perfluoro-5,10-bis(perfluorophenyl)-5,10-dihydrophenazine) ("phenazine^F ") can be easily transformed to a stable and weighable radical cation salt by deelectronation (i.e. oxidation) with Ag[Al(OR^F )₄ ]/ Br₂ mixtures (R^F =C(CF₃ )₃ ). As an innocent deelectronator it has a strong and fully reversible half-wave potential versus Fc⁺ /Fc in the coordinating solvent MeCN (E°'=1.21 V), but also in almost non-coordinating oDFB (=1,2-F₂ C₆ H₄ ; E°'=1.29 V). It allows for the deelectronation of [Fe^III Cp*₂ ]⁺ to [Fe^IV (CO)Cp*₂ ]²⁺ and [Fe^IV (CN-^t Bu)Cp*₂ ]²⁺ in common laboratory solvents and is compatible with good σ-donor ligands, such as L=trispyrazolylmethane, to generate novel [M(L)_x ]ⁿ⁺ complex salts from the respective elemental metals.

Proton Thermodynamics in a Protic Ionic Liquid, Ethylammonium Nitrate

In order to investigate the proton solvation state in protic ionic liquids (PILs), ten acid dissociation enthalpies and entropies of eight compounds were determined in ethylammonium nitrate (EAN). Regardless of the nature of the compound, 24 kJ mol^-1 larger enthalpy and 65 J mol^-1  K^-1 larger entropy than those in water, respectively, were observed. These values were reasonably explained by the differences in the proton solvation structure in EAN and water. Namely, protons in EAN exist as HNO₃ , having a higher reaction energy than that of H₃ O⁺ in water, undergo entropic stabilization as a result of the less-structured solvation. As such, the entropic effect of the proton solvation structure on the acid-base property is possibly applicable to all PILs. In addition, based on these proton thermodynamics, enthalpy and entropy windows were proposed as a novel perspective for the characterization of solvents. Use of this concept enabled the visualization of similarities and differences between EAN and water.

Generalization of Acid-Base Diagrams Based on the Unified pH-Scale

So far, pH‐lgc_i diagrams have been exclusively used for the theoretical description of acid‐base equilibria in aqueous solutions. Here, this approach is extended to include acid‐base equilibria in nonaqueous non‐hydrogen‐bond‐donor (non‐HBD) solvents following the acid‐base theory of Brønsted and Lowry, and using a unified pH scale (pH_abs scale). This way, it is possible to estimate the approximate concentrations of all the species involved in acid‐base equilibria in a solution at any solution acidity pH(HS) , and vice versa. The diagrams are excellent to illustrate the differences in behavior of species involved in the solution equilibria in different solvents, for example, an aqueous solution of acetic acid is an acidic solution; however, the same concentration of acetic acid in DMSO has a pHabsH2O value of about 10.6, that is, it is a basic solution with reference to the aqueous pH scale.

Predicting Mole-Fraction-Dependent Dissociation for Weak Acids

We demonstrate for formic and acetic acid dissolved in water as examples that the binary quantum cluster equilibrium (bQCE) approach can predict acid strengths over the whole range of acid concentrations. The acid strength increases in a complex rather than a simple way with increasing mole fraction of the acid from 0 to 0.7, reflecting the complex interplay between the dissociated ions or conjugate bases available as compared to the acid and water molecules. Furthermore, our calculated ion concentrations meet the experimental maximum of the conductivity with excellent agreement for acetic acid and satisfactorily for the formic acid/water mixture. As only a limited number of simple quantum-chemical calculations are required for the prediction, bQCE is clearly a valuable approach to access these quantities also in non-aqueous solutions. It is a highly valuable asset for predicting ionization processes in highly concentrated solutions, which are relevant for biological and chemical systems, as well as technological processes.

Enhancement of Thermodynamic Gas-Phase Acidity and Basicity of Water by Means of Secondary Interactions

A series of A⋅water, B⋅water complexes (A=acid, B=base) are studied at the G4 level of theory to show that water acidity or basicity can be modulated by non-covalent interactions. Protic and non-protic acids interacting with water form hydrogen bonds or other kinds of non-covalent interactions, respectively, that may dramatically change the acidity of water up to almost 360 kJ ⋅ mol^-1 in terms of enthalpy. Similarly, hydrogen bonds responsible for the interaction between typical small nitrogen-containing Lewis bases and water can enhance the proton affinity of water by almost 300 kJ ⋅ mol^-1 . Our results reveal that these large enhancements are linearly related with the binding energy of the charged complexes, and are determined by the Lewis acid-base properties of the molecule involved in the interaction, allowing a quite precise modulation of the corresponding acid-base properties of water.