CAAC‐Based Thiele and Schlenk Hydrocarbons

Diradicals have been of tremendous interest for over a century ever since the first reports of p‐ and m‐phenylene‐bridged diphenylmethylradicals in 1904 by Thiele and 1915 by Schlenk. Reported here are the first examples of cyclic(alkyl)(amino)carbene (CAAC) analogues of Thiele's hydrocarbon, a Kekulé diradical, and Schlenk's hydrocarbon, a non‐Kekulé diradical, without using CAAC as a precursor. The CAAC analogue of Thiele's hydrocarbon has a singlet ground state, whereas the CAAC analogue of Schlenk's hydrocarbon contains two unpaired electrons. The latter forms a dimer, by an intermolecular double head‐to‐tail dimerization. This straightforward synthetic methodology is modular and can be extended for the generation of redox‐active organic compounds.     

meta‐ and para‐Phenylenediamine‐Fused Porphyrin Dimers: Synthesis and Magnetic Interactions of Their Dication Diradicals

meta‐ and para‐Phenylenediamine‐fused nickel(II) porphyrin dimers were synthesized by S_NAr reaction of meso,β,β‐trichloro nickel(II) porphyrin with meta‐ and para‐phenylenediamines and subsequent Pd‐catalyzed intramolecular C?H arylation. Their tetrachlorinated dication diradicals are very stable, allowing SQUID magnetometry and revealing clear open‐shell characters for both meta and para isomers with ferro‐ and anti‐ferromagnetic interactions, respectively. The nitrogen analogue of Thiele's hydrocarbon usually displays predominant closed‐shell nature but its hidden diradical characters increase either in a twisted conformation or upon insertion of an additional phenylene spacer. The observed distinct diradical nature of the para‐congener indicates that diradical properties can be enhanced also by efficient spin delocalization.     

Bis-Olefin Based Crystalline Schlenk Hydrocarbon Diradicals with a Triplet Ground State

A modular approach for the synthesis of isolable crystalline Schlenk hydrocarbon diradicals from m-phenylene bridged electron-rich bis-triazaalkenes as synthons is reported. EPR spectroscopy confirms their diradical nature and triplet electronic structure by revealing a half-field signal. A computational analysis confirms the triplet state to be the ground state. As a proof-of-principle for the modular methodology, the 4,6-dimethyl-m-phenylene was further utilized as a coupling unit between two alkene motifs. The steric conjunction of the 4,6-dimethyl groups substantially twists the substituents at the nonbonding electron bearing centers relative to the central coupling m-phenylene motif. As a result, the spin delocalization is decreased and the exchange coupling between the two unpaired spins, hence, significantly reduced. Notably, 108 years after Schlenk's m-phenylene-bis(diphenylmethyl) synthesis as a diradical, for the first time we were able to isolate its derivative with the same spacer, i.e. m-phenylene, between two radical centers in a crystalline form.     

Nitrogen Analogues of Thiele’s Hydrocarbon

A series of bis[N,N‐di‐(4‐methoxylphenyl)amino]arene dications 1 ²⁺– 3 ²⁺ have been synthesized and characterized. Their electronic structures were investigated by various experiments assisted by theoretical calculations. It was found that they are singlets in the ground state and that their diradical character is dependent on the bridging moiety. 3 ²⁺ has a smaller singlet–triplet energy gap and its excited triplet state is thermally readily accessible. The work provides a nitrogen analogue of Thiele’s hydrocarbon with considerable diradical character.     

Color centers in poly(arylenesulfophthalides) and the ground state of Müller's hydrocarbon molecule

Published data on the properties of Müller's hydrocarbon are analyzed. The total energies of several hydrocarbon biradicals withp-phenylene bridges, including Thiele's, Chichibabin's, and Müller's hydrocarbons in the singlet and triplet states were calculated by the AM1 and PM3 semiempirical quantum-chemical methods. Contrary to popular opinion, our calculations revealed that the Müller's hydrocarbon molecule has a triplet rather than singlet ground state. The results obtained make it possible to explain the fact that quinoid color centers do not form in the course of reduction of poly(terphenylsulfophthalide). The calculated parameters of electronic spectra for singlet states of some related biradicals are reported.     

Disclosing Cyclic(Alkyl)(Amino)Carbenes as One-Electron Reductants: Synthesis of Acyclic(Amino)(Aryl)Carbene-Based Kekulé Diradicaloids

Herein, we disclose cyclic(alkyl)(amino)carbenes (CAACs) to be one-electron reductants under the formation of a transient radical cation as indicated by EPR spectroscopy. The disclosed CAAC reducing reactivity was used to synthesize acyclic(amino)(aryl)carbene-based Thiele and Chichibabin hydrocarbons, a new class of Kekulé diradicaloids. The results demonstrate CAACs to be potent organic reductants. Notably, the acyclic(amino)(aryl)carbene-based Chichibabin's hydrocarbon shows an appreciable population of the triplet state at room temperature, as evidenced by both variable-temperature NMR and EPR spectroscopy.     

Stable Oxindolyl‐Based Analogues of Chichibabin's and Müller's Hydrocarbons

Chichibabin's and Müller's hydrocarbons are classical open‐shell singlet diradicaloids but they are highly reactive. Herein we report the successful synthesis of their respective stable analogues, OxR‐2 and OxR‐3 , based on the newly developed oxindolyl radical. X‐ray crystallographic analysis on OxR‐2 reveals a planar quinoidal backbone similar to Chichibabin's hydrocarbon, in accordance with its small diradical character (y₀=11.1 %) and large singlet–triplet gap (ΔE_S‐T=−10.8 kcal mol⁻¹). Variable‐temperature NMR studies on OxR‐2 disclose a slow cis/trans isomerization process in solution through a diradical transition state, with a moderate energy barrier (ΔG^≠_298K=15–16 kcal mol⁻¹). OxR‐3 exhibits a much larger diradical character (y₀=80.6 %) and a smaller singlet–triplet gap (ΔE_S‐T=−3.5 kcal mol⁻¹), and thus can be easily populated to paramagnetic triplet diradical. Our studies provide a new type of stable carbon‐centered monoradical and diradicaloid.     

Tuning Diradical Properties of Boron-Containing π-Systems by Structural Isomerism

Tuning diradical character is an important topic for organic diradicaloids. Herein, we report the precise borylation enabling structural isomerism as an effective strategy to modulate diradical character and thereby properties of organic diradicaloids. We synthesized a new B-containing polycyclic hydrocarbon that has the indeno[1,2-b]fluorene π-skeleton with the β-carbons bonding to two boron atoms. Detailed theoretical and experimental results show that this bonding pattern leads to its distinctive electronic structures and properties in comparison to that of its isomeric molecule. This molecule has the efficient conjugation between boron atoms and π-skeleton, resulting in downshifted LUMO and HOMO levels. Moreover, it exhibits smaller diradical character and thereby inhibited diradical properties, such as significantly blue-shifted light absorption, larger energy bandgap and weak para-magnetic resonance. Notably, this B-containing polycyclic hydrocarbon possesses much stronger Lewis acidity and its Lewis acid-base adducts display enhanced diradical character, demonstrating the positive effects of Lewis coordination on modulating diradical performance.     

A Stable Trimethylenemethane Triplet Diradical Based on a Trimeric Porphyrin Fused π‐System

Trimethylenemethane (TMM) diradical is the simplest non‐Kekulé non‐disjoint molecule with the triplet ground state (ΔE_ST=+16.1 kcal mol^?1) and is extremely reactive. It is a challenge to design and synthesize a stable TMM diradical with key properties, such as actual aliphatic TMM diradical centers and the triplet ground state with a large positive ΔE_ST value, since such species provide detailed information on the electronic structure of TMM diradical. Herein we report a TMM derivative, in which the TMM segment is fused with three Ni^II meso‐triarylporphyrins, that satisfies the above criteria. The diradical shows delocalized spin density on the propeller‐like porphyrin π‐network and the triplet ground state owing to the strong ferromagnetic interaction. Despite the apparent TMM structure, the diradical can be handled under ambient conditions and can be stored for months in the solid state, thus allowing its X‐ray diffraction structural analysis.     

Acyclic diaminocarbene-based Thiele,Chichibabin, and Müller hydrocarbons

Thiele, Chichibabin and Müller hydrocarbons are considered as classical Kekulé diradicaloids. Herein we report the synthesis and characterization of acyclic diaminocarbene (ADC)-based Thiele, Chichibabin, and Müller hydrocarbons. The calculated singlet–triplet energy gaps are ΔE_S–T = −27.96, −3.70, −0.37 kcal mol⁻¹, respectively, and gradually decrease with the increasing length of the π-conjugated spacer (p-phenylene vs. p,p′-biphenylene vs. p,p′′-terphenylene) between the two ADC-scaffolds. In agreement with the calculations, we also experimentally observed the enhancement of paramagnetic diradical character as a function of the length of the π-conjugated spacer. ADC-based Thiele''s hydrocarbon is EPR silent and exhibits very well resolved NMR spectra, whereas ADC-based Müller''s hydrocarbon displays EPR signals and featureless NMR spectra at room temperature. The spacer also has a strong influence on the UV-Vis-NIR spectra of these compounds. Considering that our methodology is modular, these results provide a convenient platform for the synthesis of an electronically modified new class of carbon-centered Kekulé diradicaloids.

We report the synthesis of acyclic diaminocarbene (ADC)-scaffold based Thiele, Chichibabin, and Müller hydrocarbons. Studies support that the singlet-triplet energy gap depends on the π-conjugated spacer between the ADC scaffolds.     

Oxidative Addition at a Carbene Center: Synthesis of an Iminoboryl–CAAC Adduct

The reaction of a cyclic (alkyl)(amino)carbene (CAAC) with dichloro‐ and dibromobis(trimethylsilyl)aminoborane results in the formation of haloiminoborane–CAAC adducts. When the iodo analogue is used, an oxidative addition at the carbene center affords a cationic iminoboryl–CAAC adduct, featuring a boron–nitrogen triple bond. Similar salts are also obtained by halide abstraction from the chloro‐ and bromoiminoborane–CAAC adducts. The reactivity of all of these compounds towards CO₂ is discussed.     

SYNTHESE D'ACIDES ω-AMINO-ω-CARBOXY-ALKYLPHOSPHONIQUES

Abstract

La synthèse des acides ω-amino ω-carboxyalkylphosphoniques 1, a été réexaminée. Pour n = 4, 5 et 6, une synthèse générale, efficace et économique en 3 étapes est proposée: les acétamido (ω-bromoalkyl) malonates 12 sont préparés par transfert de phase à partir de l'acétamidomalonate de diéthyle et des dibromoalcanes commerciaux, puis condensés quantitativement sur le phosphite de triéthyle par une réaction d'Arbuzov. Une hydrolyse acide et une purification sur résine Dowex fournissent les aminoacides 1 (n = 4, 5, 6) avec un rendement global de l'ordre de 70%. L'acide amino-2 carboxy-2 éthylphosphonique 1 (n = 2) isolé de façon analogue avec un rendement de 70% est préparé par réaction de l'acétamidomalonate de diéthyle avec un halogéno-2 éthylphosphonate dans les conditions du transfert de phase liquide-solide.

ω-Amino ω-carboxyalkylphosphonic acids 1 synthesis is reviewed. For n = 4, 5, 6 a general, efficient and inexpensive synthesis is described; acetamido(ω-bromoalkyl)malonates 12 are prepared from commercial dibromoalkanes and acetamidomalonate using the phase transfer catalysis process, then condensed with triethylphosphite through an Arbuzov reaction. An acid hydrolysis followed by purification on Dowex gives aminophosphonic acids 1 (n=4, 5, 6) with a 70% overall yield. The lower acid 1 (n = 2) is obtained with an identical overall yield from acetamidomalonate and 2-haloethylphosphonate using also liquid-solid phase transfer catalysis.     

Singlet�Ctriplet energy gap for trimethylenemethane, oxyallyl diradical, and related species: single- and multireference computational results

We have investigated the through-bond exchange interactions in three non-Kekulé hydrocarbon diradicals on the basis of single- and multireference coupled cluster and related broken-symmetry (BS) methods. The singlet?Ctriplet energy gap (S-T gap) and diradical characters for these species are evaluated. It is found that the spin contamination involved in the BS solutions is non-negligible and the approximate spin-projection method greatly improves the usual BS solutions. As for Mukherjee??s state-specific multireference coupled cluster (MkMRCC) computations, the size-consistent correction with the UHF localized natural orbitals (ULO) is useful to obtain the qualitatively correct 2J values.     

Rational Design of an Air-Stable,High-Spin Diradical with Diazapyrene

Stable carbon-based polyradicals exhibiting strong spin-spin coupling and slow depolarization processes are particularly attractive functional materials. A new molecular motif synthesized by a convenient method that allows the integration of stable, high-spin radicals to (hetero)aromatic polycycles has been developed, as illustrated by a non-Kekulé diradical showing a triplet ground state with long persistency (τ_1/2≈31 h) in air. Compared to the widely used 1,3-phenylene, the newly designed (diaza)pyrene-4,10-diyl moiety is for the first time demonstrated to confer ferromagnetic (FM) spin coupling, allowing delocalized non-disjoint SOMOs. With the X-ray crystallography unambiguously proving the diradical structure, the triplet ground state was thoroughly characterized. A large ΔE_S-T of 1.1 kcal/mol, proving the strong FM coupling effect, was revealed consistently by superconducting quantum interference device (SQUID) measurements and variable-temperature electron paramagnetic resonance (EPR) spectroscopy, while the zero-field splitting and triplet nutation characters were examined by continuous-wave and pulsed EPR spectroscopy. A millisecond spin-lattice relaxation time was also detected. The current study not only offers a new molecular motif enabling FM coupling between carbon-based spins, but more importantly presents a general method for installing stable polyradicals into functional π-systems.     

A Peri‐tetracene Diradicaloid: Synthesis and Properties

Peri‐acenes are good model compounds for zigzag graphene nanoribbons, but their synthesis is extremely challenging owing to their intrinsic open‐shell diradical character. Now, the successful synthesis and isolation of a stable peri‐tetracene derivative PT‐2ClPh is reported; four 2,6‐dichlorophenyl groups are attached onto the most reactive sites along the zigzag edges. The structure was confirmed by X‐ray crystallographic analysis and its electronic properties were systematically investigated by both experiments and theoretical calculations. It exhibits an open‐shell singlet ground state with a moderate diradical character (y₀=51.5 % by calculation) and a small singlet–triplet gap (ΔE_S‐T=?2.5 kcal mol^?1 by SQUID measurement). It displays global aromatic character, which is different from the smaller‐size bisanthene analogue BA‐CF3 .     

Facile Cleavage of the P=P Double Bond in Vinyl‐Substituted Diphosphenes

The reactions of the cyclic alkyl amino carbene (CAAC) 1 with phosphaalkynes generate the kinetically unstable CAAC‐derived phosphirenes 4 and 5 , which undergo rearrangement/dimerization reactions to give the vinyl‐substituted diphosphenes 2 , 3 , and 6 . The P=P double bond scission of 2 or 3 is unprecedentedly effected by S₈, [AuCl(tht)], or MeOTf at room temperature, which affords a dithiophosphorane 7 , a phosphepine Au complex 8 , or phosphepinium cations 9 and 10 , respectively. The cationic species feature little homoaromaticity while representing the first examples of the phosphorus‐containing analogue of the tropylium ion.     

Disjoint and coextensive amminium radical cations: a general problem in making amminium radical cation based high-spin polymers

A two-electron oxidation of N,N,N′,N′-tetrakis(2-methoxyphenyl)-1,3-diaminobenzene 12 gives the diradical dication 12²⁺, which is an aza analogue of the Schlenk hydrocarbon and, like the Schlenk hydrocarbon, has a triplet ground state. However, an attempt to produce pentuplet tetraradical tetracations by extending the same ferromagnetic spin-coupling motif in a linear or a cyclic fashion was unsuccessful. Two-electron oxidations to give disjoint diradical dications (in which the charges and spins are spatially separated) are relatively easy but it proved impossible to remove the third and fourth electrons. This would require generation of coextensive radical ions in which the charges and spin distributions overlap. The results obtained with these model oligomers illustrate what is a general problem in the creation of high-spin polymers in which the spin-bearing centres are amminium radical cations. Strong ferromagnetic spin-coupling depends on the formation of coextensive rather than disjoint radical cations but the formation of coextensive radical cations with strong ferromagnetic coupling involves a large additional coulombic penalty.     

A Highly Stable Organic Luminescent Diradical

It is very challenging to obtain stable room-temperature luminescent open-shell singlet diradicals. Herein we report the first stable Müller's hydrocarbon TTM-PhTTM with luminescent properties. Variable-temperature electron paramagnetic resonance spectroscopy measurements and theoretical calculations show that TTM-PhTTM has an open-shell singlet ground state with a diradical character of 90 %. Because of a small singlet-triplet energy gap, the open-shell singlet ground state can be thermally excited to a triplet state. TTM-PhTTM shows room-temperature deep-red emission in various solutions. Unusually high stability of TTM-PhTTM was also observed owing to effective steric hindrance and spin delocalization. Our results are beneficial to the rational design and discovery of more stable luminescent diradical materials.     

Anselme Payen (1795–1871), Learned Manufacturer of Chemical Products

Abstract

Between 1835 and 1871, Anselme Payen (1795-1871) taught industrial chemistry at both the Ecole centrale des arts et manufactures and the Conservatoire des arts et métiers. In addition, he held several administrative functions in such institutions as the Société d'encouragement pour l'industrie nationale and the Société centrale d'agriculture. Prior to 1835, however, he owned a chemical manufactory situated in the plain of Grenelle. This paper is about Payen's career as a chemical manufacturer (1815–1835) and his various innovation strategies. The themes discussed are the lead chamber process of sulfuric acid production, artificial borax as a substitute for refined Tibetan tincal, the decolourising power of boneblack, and the “export” of the knacker's trade to the countryside. Different forms of applied science can be seen at work here: analytical chemistry for chemical bookkeeping, laboratory-scale analysis of various manufacturing conditions, theoretical explanation, and the reconstruction of traditional practices. These various ways of applying science to manufacturing problems make Payen stand out as what Chaptal had called a fabricant éclairé.     

Mechanical and Chemical Explanations in Du Clos' Chemistry

Abstract

Samuel Cottereau Du Clos (1598–1685) appears as the first French chemist to combine in chemistry (for him, the science of substances, the physics of qualities) demonstrations using the laws of motion with demonstrations using the qualities of chemical principles. In this way, he brought to bear two different and complementary orders of explanation. According to Du Clos, the mechanical considerations represent a first approach, a stage towards the knowledge of “the truth of things” (la vérité des choses) in natural philosophy. He set out his chemistry at the Académie royale des sciences de Paris, especially through his criticism of Boyle's Certain Physiological Essays in 1668–1669.