Piotr J. Leszczyński

Separation of the Energetic and Geometric Contributions to Aromaticity. 2. Analysis of the Aromatic Character of Benzene Rings in Their Various Topological Environments in the Benzenoid Hydrocarbons. Crystal and Molecular Structure of Coronene

Statistical analysis of the aromatic character and its geometric and energetic contributions of 167 benzene rings embedded in various topological environments in 26 benzenoid hydrocarbons leads to the following conclusions:  the aromatic character of benzene rings with three or fewer fused rings is due mostly to geometric contributions, whereas in other cases energetic contribution is decisive. Aromaticity indices for individual rings (local aromaticity) depend strongly on the kind of topological environment. Terminal rings always exhibit a strong aromatic character, whereas those fused to many rings are often weakly aromatic. The study is based on precisely solved X-ray or neutron crystal structure determination retrieved from Cambridge Structural Database supplemented by our own precise determination of coronene.

KAgF3, K2AgF4 and K3Ag2F7: important steps towards a layered antiferromagnetic fluoroargentate(II),

Crystal structure and magnetic properties of K2AgF4, related to recently studied Cs2AgF4, have been scrutinized. It crystallizes orthorhombic (Cmca No.64) with a = 6.182(3) A, b = 12.632(5) A, c = 6.436(3) A (Z = 4, V = 502.6(7) A3). K2AgF4 exhibits slightly puckered [AgF2] sheets and a compressed octahedral coordination of Ag(II) and it is not isostructural to related Cs2AgF4. Violet–coloured K2AgF4 orders ferromagnetically below 26 K. The DFT calculations reproduce semiconducting properties and ferromagnetism of K2AgF4 at the LSDA + U level but only if substantial values of Mott–Hubbard on-site electron–electron repulsion energies for Ag and F are used in calculations. We have also succeeded to solve the crystal structure of a brown KAgF3 (1D antiferromagnet below 64 K; GdFeO3–type, PnmaNo. 62, a = 6.2689(2) A, b = 8.3015(2) A, c = 6.1844(2) A, Z = 4, V = 321.84(2) A3) and to prepare K3Ag2F7, a novel KAgF3/K2AgF4 intergrowth phase and a member of the Ruddlsden–Popper KnAgFn+2 series (n = 1.5). Dark brown K3Ag2F7 crystallizes orthorhombic (K3Cu2Cl7-type, CccaNo. 68, setting 2) with a = 20.8119(14) A, b = 6.3402(4) A, c = 6.2134(4) A (Z = 4, V = 819.87(9) A3).

Redetermination of crystal structure of Ag(II)SO4 and its high-pressure behavior up to 30 GPa

Here we redetermine the crystal structure of Ag(II)SO4, an unusual d9 system, at 1 atm from powder X-ray data and we report hydrostatic pressure X-ray diffraction experiments on Ag(II)SO4 inside the diamond anvil cell. AgSO4 crystallizes in the monoclinic C2/c cell, with a = 12.8476(2) A, b = 13.6690(4) A, c = 9.36678(19) A, β = 47.5653(13)°, and V = 1214.04(5) A3 (Z = 16). AgSO4 exhibits bulk modulus, B0, of 36.9 GPa, and undergoes sluggish decomposition at ∼23 GPa yielding a high-pressure phase of Ag2S2O7 (K2S2O7-type), with the substrate and product coexisting at 30 GPa. Theoretical calculations within Density Functional Theory for the C2/c cell nicely reproduce the observed trend for lattice constants as well as the B0 values of AgSO4, and suggest that the rigidity of the infinite [Ag(SO4)] chains as well as the Jahn–Teller effect for the Ag(II) cation persist even at 30 GPa.

Facile formation of thermodynamically unstable novel borohydride materials by a wet chemistry route.

A novel wet synthetic method utilizing weakly coordinating anions that yields LiCl-free Zn-based materials for hydrogen storage has recently been reported. Here we show that this method may also be applied for the synthesis of the pure yttrium derivatives, M[Y(BH4)4] (M = K, Rb, Cs). Moreover, it can be extended to the preparation of previously unknown thermodynamically unstable derivatives, Li[Y(BH4)4] and Na[Y(BH4)4]. Importantly, these two H-rich phases cannot be accessed by standard dry (mechanochemical) or solid/gas synthetic methods due to the thermodynamic obstacles. Here we describe their crystal structures and selected important physicochemical properties.

Complete Series of Alkali-Metal M(BH3NH2BH2NH2BH3) Hydrogen-Storage Salts Accessed via Metathesis in Organic Solvents

We report a new efficient way of synthesizing high-purity hydrogen-rich M(BH3NH2BH2NH2BH3) salts (M = Li, Na, K, Rb, Cs). The solvent-mediated metathetic synthesis applied here uses precursors containing bulky organic cations and weakly coordinating anions. The applicability of this method permits the entire series of alkali-metal M(BH3NH2BH2NH2BH3) salts (M = Li, Na, K, Rb, Cs) to be obtained, thus enabling their comparative analysis in terms of crystal structures and hydrogen-storage properties. A novel polymorphic form of Verkades base (C18H39N4PH)(BH3NH2BH2NH2BH3) precursor was also characterized structurally. For all compounds, we present a comprehensive structural, spectroscopic, and thermogravimetric characterization (PXRD, NMR, FTIR, Raman, and TGA/DSC/MS).

Solvent effect on kinetics of the chloride ion cleavage from anion radicals of 4-chlorobenzophenone

Abstract Formal potentials (expressed on the ferrocene scale) for the formation of 4-chlorobenzophenone anion radicals E RX/RX − o as well as for the Cl /Cl − couple increase with the solvent acceptor number but the second dependence is at least twice as strong. However, the rate constant for CCl bond cleavage in these radicals decreases with solvent acidity, indicating that solvation of the leaving anion does not determine the kinetics of cleavage. It is shown that solvent effects on the intrinsic rate constant are dominant and can be described by the Pekar factor, as proposed in the Saveant model.

Unusual Thermal Decomposition of AgIISO4 Yielding AgI2S2O7: Bending Hammond’s Rule

“It could be said that among the coinage metals (Cu, Ag, Au), only silver is normal . (...) gold is anomalous due to large relativistic effects. Copper is anomalous as it has a nodeless and therefore very compact d shell, with strong electron–electron repulsion (...)”. The “normality” of elemental silver, is reflected in the prevalence of its monovalent oxidation state, +1 (as expected for a Group 11 element), which also has severe consequences for its second oxidation state—it renders Ag an extremely powerful oxidizer. The standard redox potential for the Ag/Ag redox pair equals +1.98 V versus NHE and it is surpassed only by those of FC/F , F2/2F , ClC/Cl and OHC/OH . Thus Ag is stabilized in connections with fluoride ligands, whereas its oxo and aza compounds are rare and are quite unstable thermodynamically and thermally. Here we investigate in detail the thermal decomposition of recently synthesized Ag sulfate, AgSO4. [4] The activation energy for decomposition turns out to be substantial ( 127 kJmol ) rendering this compound metastable at ambient (p, T) conditions. We show, based on literature about thermal decomposition for over 50 different sulfates and oxo-sulfates (see the Supporting Information), that solely AgSO4 s low-temperature thermal decomposition is associated with the reduction of a metal cation (“reductive decomposition”). We consider the reaction pathway for decomposition and we point out structural links between AgSO4 and crystalline product of its decomposition, AgSO3.5 Ag2S2O7. Ag sulfate was reported to decompose thermally in a single step while yielding Ag disulfate and releasing O2 [Eq. (1)]:

Rates of the halide ion cleavage from halo-9,10-diphenylanthracene anion radicals in DMF

The rate constants k for the cleavage of the carbon-halogen bond in anion radicals of 1- and 2-chloro-9,10-diphenylanthracene, 9,10-dichloroanthracene and 2-bromo-9,10-diphenylanthracene have been determined and compared with the literature data for other anthracenyl halides. There is no significant correlation between log k and the formal potentials of the anion radical formation. However, linear relationships of the experimental RT In k vs. the relative thermodynamic contribution to the activation barrier as well as the relative intrinsic activation energy (calculated on the basis of the recent Saveant model and using the ionic component of the bond enthalpy estimated from the absolute electronegativity and hardness) have been found.

Hammett reaction constants for the electroreduction of substituted chloro- and bromobenzenes: Effect of solvent dynamics on the two-electron irreversible reductive cleavage

Abstract The half-wave potentials, corresponding to irreversible two-electron transfers, were determined for the electroreduction of substituted chloro- and bromobenzenes in a number of organic solvents. In each solvent they obey the Hammett equation, and the reaction constants ϱ, found for each series are solvent dependent. Since the first electron transfer is the rate-determining step for the overall process and the irreversible half-wave potentials depend on the heterogeneous rate constants, it is shown that the ϱ values obtained are linear functions of the logarithms of the solvent longitudinal relaxation times. This is the first example indicating the role of solvent relaxation dynamics in irreversible electrode processes accompanied by bond cleavage. Differences in the Hammett behaviour obtained and that described by us previously for the reduction of substituted iodobenzenes are discussed on the basis of a mechanism of electrode processes.

Dynamic solvent effects on the rates of homogeneous and heterogeneous electron transfer to chlorobenzene

A voltammetric method and homogeneous redox catalysis have been used to determine rate constants for the electrode and solution electron transfer to chlorobenzene and its formal potential in four aprotic solvents. It has been shown that both reactions are adiabatic and are controlled by the overdamped solvent relaxation.