The Chemical Bond: When Atom Size Instead of Electronegativity Difference Determines Trend in Bond Strength

We have quantum chemically analyzed element−element bonds of archetypal H_nX−YH_n molecules (X, Y=C, N, O, F, Si, P, S, Cl, Br, I), using density functional theory. One purpose is to obtain a set of consistent homolytic bond dissociation energies (BDE) for establishing accurate trends across the periodic table. The main objective is to elucidate the underlying physical factors behind these chemical bonding trends. On one hand, we confirm that, along a period (e. g., from C−C to C−F), bonds strengthen because the electronegativity difference across the bond increases. But, down a period, our findings constitute a paradigm shift. From C−F to C−I, for example, bonds do become weaker, however, not because of the decreasing electronegativity difference. Instead, we show that the effective atom size (via steric Pauli repulsion) is the causal factor behind bond weakening in this series, and behind the weakening in orbital interactions at the equilibrium distance. We discuss the actual bonding mechanism and the importance of analyzing this mechanism as a function of the bond distance.     

A Pressure-Induced Inverse Order–Disorder Transition in Double Perovskites

Given the consensus that pressure improves cation ordering in most of known materials, a discovery of pressure-induced disordering could require recognition of an order–disorder transition in solid-state physics/chemistry and geophysics. Double perovskites Y₂CoIrO₆ and Y₂CoRuO₆ polymorphs synthesized at 0, 6, and 15 GPa show B-site ordering, partial ordering, and disordering, respectively, accompanied by lattice compression and crystal structure alteration from monoclinic to orthorhombic symmetry. Correspondingly, the long-range ferrimagnetic ordering in the B-site ordered samples are gradually overwhelmed by B-site disorder. Theoretical calculations suggest that unusual unit-cell compressions under external pressures unexpectedly stabilize the disordered phases of Y₂CoIrO₆ and Y₂CoRuO₆.     

PIII/PV=O Catalyzed Cascade Synthesis of N‐Functionalized Azaheterocycles

An organocatalytic method for the modular synthesis of diverse N‐aryl and N‐alkyl azaheterocycles (indoles, oxindoles, benzimidazoles, and quinoxalinediones) is reported. The method employs a small‐ring organophosphorus‐based catalyst (1,2,2,3,4,4‐hexamethylphosphetane P‐oxide) and a hydrosilane reductant to drive the conversion of ortho‐functionalized nitroarenes into azaheterocycles through sequential intermolecular reductive C?N cross coupling with boronic acids, followed by intramolecular cyclization. This method enables the rapid construction of azaheterocycles from readily available building blocks, including a regiospecific approach to N‐substituted benzimidazoles and quinoxalinediones.     

A Pressure‐Induced Inverse Order–Disorder Transition in Double Perovskites

Given the consensus that pressure improves cation ordering in most of known materials, a discovery of pressure‐induced disordering could require recognition of an order–disorder transition in solid‐state physics/chemistry and geophysics. Double perovskites Y₂CoIrO₆ and Y₂CoRuO₆ polymorphs synthesized at 0, 6, and 15 GPa show B‐site ordering, partial ordering, and disordering, respectively, accompanied by lattice compression and crystal structure alteration from monoclinic to orthorhombic symmetry. Correspondingly, the long‐range ferrimagnetic ordering in the B‐site ordered samples are gradually overwhelmed by B‐site disorder. Theoretical calculations suggest that unusual unit‐cell compressions under external pressures unexpectedly stabilize the disordered phases of Y₂CoIrO₆ and Y₂CoRuO₆.     

Friction force microscopy of alkylphosphonic acid and carboxylic acids adsorbed on the native oxide of aluminum

Monolayers of alkylphosphonic acids (APA) and alkylcarboxylic acids (ACA) on magnetron-sputtered aluminum films have been investigated by friction force microscopy (FFM), contact angle measurement, and polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS). Clear evidence has been provided from PM-IRRAS that friction coefficients, determined from FFM data, may be correlated directly with variations in adsorbate molecular structure. The friction coefficient increased with the length of the adsorbate molecule, but reached a limiting value when the alkyl chain of the adsorbate contained eight carbons in the case of APA or 12 carbons in the case of ACA. For a given alkyl chain length, APA monolayers yielded coefficients of friction that were similar to those of monolayers of alkylthiols of the same length, but smaller than those of ACA. These data indicate that APA monolayers are better ordered than ACA monolayers. These inferences were supported by PM-IRRAS data, which enabled the density of gauche defects to be estimated and correlated with variations in the coefficient of friction.     

Boron Subphthalocyanines and Silicon Phthalocyanines for Use as Active Materials in Organic Photovoltaics

Organic photovoltaics (OPVs) have experienced continued interest over the last 25 years as a viable technology for the generation of power. Phthalocyanines are among the oldest commercial dyes and have been utilized in some of the earliest examples of OPVs. In recent years, the use of boron subphthalocyanines (BsubPcs) and silicon phthalocyanines (SiPcs) has attracted a flurry of interest with some examples of fullerene‐free devices reaching power conversion efficiencies >8 %. Unlike other more common divalent phthalocyanines such as copper or zinc, BsubPcs and SiPcs contain additional axial groups that can easily be functionalized without significantly affecting the optoelectronic properties of the macrocycle. This handle facilitates our ability to tune the solid‐state arrangement and other physical characteristics such as solubility ultimately giving us the ability to improve the thin film processing and final device performance. This review covers recent studies on the development of BsubPcs and SiPcs for use as active materials in organic photovoltaics.     

Regioselective,Asymmetric Formal Hydroamination of Unactivated Internal Alkenes

We report the regioselective and enantioselective formal hydroamination of unsymmetrical internal alkenes catalyzed by a copper catalyst ligated by DTBM‐SEGPHOS. The regioselectivity of the reaction is controlled by the electronic effects of ether, ester, and sulfonamide groups in the homoallylic position. The observed selectivity underscores the influence of inductive effects of remote substituents on the selectivity of catalytic processes occurring at hydrocarbyl groups, and the method provides direct access to various 1,3‐aminoalcohol derivatives with high enantioselectivity.