Hexahalodiborate Dianions: A New Family of Binary Boron Halides

The electron‐precise binary boron subhalide species [B₂X₆]^2? X=F, Br, I) were synthesized and their structures confirmed by X‐ray crystallography. The existence of the previously claimed [B₂Cl₆]^2?, which had been questioned, was also confirmed by X‐ray crystallography. The dianions are isoelectronic to hexahaloethanes, are subhalide analogues of the well‐known tetrahaloborate anions (BX₄^?), and are rare examples of molecular electron‐precise binary boron species beyond B₂X₄, BX₃, and [BX₄]^?.     

Expanded Porphyrin Contraction: From [22]Triphyrin(6.6.0) to [22]Triphyrin(6.5.0)

An expanded triphyrin containing a bipyrrole moiety and annulene links, namely tetraphenyl-[22]triphyrin(6.5.0), 2 , has been synthesized. The synthesis proceeded by a postsynthetic transformation of tetraphenyl-[22]triphyrin(6.6.0), 1 , in a metal-free unexpected and unprecedented ring contraction during column chromatography on alumina. The observed transformation, located at the hydrocarbon chain linking the pyrrole units, formally corresponds to a subtraction of one carbon atom from an annulene circuit. In contrast to the flexible substrate 1 , the product 2 is conformationally rigid, and capable of chloride anion binding in its protonated form.     

Gold(III)‐Mediated Contraction of Benzene to Cyclopentadiene: From p‐Benziporphyrin to Gold(III) True Tetraarylcarbaporphyrin

The reaction of p‐benziporphyrin, sodium tetrachloroaurate(III) dihydrate, and potassium carbonate in dichloromethane yielded gold(III) 5,10,15,20‐tetraaryl‐21‐carbaporphyrin owing to the contraction of p‐phenylene to cyclopentadiene. This molecule is the very first representative of a true 5,10,15,20‐tetraaryl‐21‐carbaporphyrin complex where four trigonal donor atoms are involved in equatorial coordination. The contraction adds an unprecedented route to numerous organic transformations of aromatic compounds catalyzed by simple gold(III) compounds. p‐Benziporphyrin provided the unique environment to alter the fundamental reactivity of the benzene unit facilitating its contraction to cyclopentadiene.     

硼原子的键合形式对硼酸盐晶体非线性光学行为的影响

在几种具有典型复杂结构的硼酸盐晶体中 ,从硼原子的键合形式出发讨论了不同键合情况下的B—O键对硼酸盐晶体的非线性光学行为的具体影响。结果表明 ,晶体中B—O键的键合形式是制约硼酸盐晶体非线性光学行为的重要因素。     

A 1,1‐Carboboration Route to Bora‐Nazarov Systems

Hydroboration of the conjugated enynes 1 a and 1 b with Piers’ borane [HB(C₆F₅)₂] gave the respective dienylboranes trans‐ 2 c and trans‐ 2 d . Their photolysis resulted in the formation of the dihydroborole products 3 c and 3 d . Both were converted to their pyridine adducts 5 c and 5 d , respectively. Compounds 3 c and 5 c,d were characterized by X‐ray diffraction. The reaction of the bis(enynyl)boranes 6 a and 6 b with B(C₆F₅)₃ resulted in the formation of the dihydroboroles 7 a and 7 b , respectively. This reaction is thought to proceed by 1,1‐carboboration of one of the enynyl substituents at boron to generate the dienylborane intermediates 8 a / 8 b , followed by thermally induced bora‐Nazarov ring‐closure and subsequent stabilizing 1,2‐pentafluorophenyl group migration from boron to carbon. Compound 7 a was characterized by X‐ray diffraction and solid‐state ¹¹B NMR spectroscopy.     

A Rhodium‐Mediated Contraction of Benzene to Cyclopentadiene: Transformations of Rhodium(III) m‐Benziporphyrin

The contraction of benzene is one of an exclusive group of reactions where the cleavage of aromatic structure is of fundamental importance. Rhodium(III) m‐benziporphyrin undergoes an unprecedented transformation of the built‐in m‐phenylene in which a perimeter carbon atom is extruded to form rhodium(III) 21‐carbaporphyrin, stabilizing the formyl‐unit‐substituted rhodacyclopropane motif.     

Metastable Dianions and Dications

A theoretical study of metastable dianions and dications has been carried out at the CCSD(T)//MP2 level. MX₃²⁻ and LX₄²⁻ (M=Li and Na, L=Be and Mg, X=F and Cl) have been considered as dianions, M₃X²⁺ (M=Li and Na, X=F and Cl), YH₃²⁺ and ZH₄²⁺ (Y=F and Cl and Z=O, S) as dications. Minima structures are found in all cases, but they are less stable than the corresponding dissociated pair of mono-ions. The dissociation profile of the molecules in two mono-ions has been explored showing in all cases a maximum that prevent their spontaneous dissociation. The strength and nature of the chemical bond in the dianions and dications have been analyzed with the QTAIM, NBO and LMOEDA method and compared to the corresponding monoanions and monocations.     

A unified orbital model of delocalised and localised currents in monocycles, from annulenes to azabora-heterocycles

Why are some (4n+2)π systems aromatic, and some not? The ipsocentric approach to the calculation of the current density induced in a molecule by an external magnetic field predicts a four‐electron diatropic (aromatic) ring current for (4n+2)π carbocycles and a two‐electron paratropic (antiaromatic) current for (4n)π carbocycles. With the inclusion of an electronegativity parameter, an ipsocentric frontier‐orbital model also predicts the transition from delocalised currents in carbocycles to nitrogen‐localised currents in alternating azabora‐heterocycles, which rationalises the differences in (magnetic) aromaticity between these isoelectronic π‐conjugated systems. Ab initio valence‐bond calculations confirm the localisation predicted by the naïve model, and coupled‐Hartree–Fock calculations give current‐density maps that exhibit the predicted delocalised‐to‐localised/carbocycle–heterocycle transition.