Isolation of a Neutral Boron‐Containing Radical Stabilized by a Cyclic (Alkyl)(Amino)Carbene

Utilizing a cyclic (alkyl)(amino)carbene (CAAC) as a ligand, neutral CAAC‐stabilized radicals containing a boryl functionality could be prepared by reduction of the corresponding haloborane adducts. The radical species with a duryl substituent was fully characterized by single‐crystal X‐ray structural analysis, EPR spectroscopy, and DFT calculations. Compared to known neutral boryl radicals, the isolated radical species showed larger spin density on the boron atom. Furthermore, the compound that was isolated is extraordinarily stable to high temperatures under inert conditions, both in solution and in the solid state. Electrochemical investigations of the radical suggest the possibility to generate a stable formal boryl anion species.     

Radical Complexes of Nickel(II)/Copper(II) and Redox Non-innocent MB-DIPY Ligands: Unusual Stability and Strong Near-Infrared Absorption at λmax ∼1300 nm

The preparation of radicals with intense and redox-switchable absorption beyond 1000 nm is a long-standing challenge in the chemistry of functional dyes. Here we report the preparation of a series of unprecedented stable neutral nickel(II) and copper(II) complexes of “Manitoba dipyrromethenes” (MB-DIPYs) in which the organic chromophore is present in the radical-anion state. The new stable radicals have an intense absorption at λ_max∼1300 nm and can be either oxidized to regular [M^II(MB-DIPY)]⁺ (M=Cu or Ni) or reduced to [M^II(MB-DIPY)]⁻ compounds. The radical nature of the stable [M^II(MB-DIPY)] complexes was confirmed by EPR spectroscopy with additional insight into their electronic structure obtained by UV-Vis spectroscopy, electro- and spectroelectrochemistry, magnetic measurements, and X-ray crystallography. The electronic structures and spectroscopic properties of the radical-based chromophores were also probed by density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. These nickel(II) and copper(II) complexes represent the first stable radical compounds with a MB-DIPY ligand.     

Electronic Delocalization in Two and Three Dimensions: Differential Aggregation in Indium “Metalloid” Clusters

Reduction of indium boryl precursors to give two‐ and three‐dimensional M−M bonded networks is influenced by the choice of supporting ligand. While the unprecedented nanoscale cluster [In₆₈(boryl)₁₂]⁻ (with an In₁₂@In₄₄@In₁₂(boryl)₁₂ concentric structure), can be isolated from the potassium reduction of a bis(boryl)indium(III) chloride precursor, analogous reduction of the corresponding (benzamidinate)In^IIIBr(boryl) system gives a near‐planar (and weakly aromatic) tetranuclear [In₄(boryl)₄]²⁻ system.     

An Olefinic 1,2‐Boryl‐Migration Enabled by Radical Addition: Construction of gem‐Bis(boryl)alkanes

A series of in situ formed alkenyl diboronate complexes from alkenyl Grignard reagents (commercially available or prepared from alkenyl bromides and Mg) with B₂Pin₂ (bis(pinacolato)diboron) react with diverse alkyl halides by a Ru photocatalyst to give various gem‐bis(boryl)alkanes. Alkyl radicals add efficiently to the alkenyl diboronate complexes, and the adduct radical anions undergo radical‐polar crossover, specifically, a 1,2‐boryl‐anion shift from boron to the α‐carbon sp² center. This transformation shows good functional‐group compatibility and can serve as a powerful synthetic tool for late‐stage functionalization in complex compounds. Measurements of the quantum yield reveal that a radical‐chain mechanism is operative in which the alkenyl diboronates acts as reductive quencher for the excited state of the photocatalyst.     

Synergistic Effects of Lewis Bases and Substituents on the Electronic Structure and Reactivity of Boryl Radicals

Boryl radicals have the potential for the development of new molecular entities and for application in new radical reactions. However, the effects of the substituents and coordinating Lewis bases on the reactivity of boryl radicals are not fully understood. By using first‐principles methods, we investigated the spin‐density distribution and reactivity of a series of boryl radicals with various substituents and Lewis bases. The substituents, along with the Lewis bases, only affect the radical reactivity when an unpaired electron is in the boron p_z orbital, that is, for three‐coordinate radicals. We found evidence of synergistic effects between the substituents and the Lewis bases that can substantially broaden the tunability of the reactivity of the boryl radicals. Among Lewis bases, pyridine and imidazol‐2‐ylidene show a similar capacity for stabilization by delocalizing the spin density. Electron‐donating substituents, such as nitrogen, more efficiently stabilize boryl radicals than oxygen and carbon atoms. The reactivity of a boryl radical is always boron based, irrespective of the spin density on boron.     

Radical CpNi(dithiolene) and CpNi(diselenolene) complexes: Synthetic routes and molecular properties

The formally Ni(III) d⁷ radical organometallic complexes formulated as [CpNi(dithiolene)] can be prepared by different routes involving different CpNi sources such as the Ni^(I) [CpNi(CO)]₂, the Ni^(II) [Cp₂Ni] or [CpNi(cod)]⁺ or the Ni^(III) [Cp₂Ni]⁺ complexes. As dithiolene precursors, the naked dithiolate, the mono- as well as bis-(dithiolene) metal complexes were investigated. The highest yields are generally associated with an appropriate redox match, that is a CpNi^(II) precursor with a formally Ni^(IV) [Ni(dithiolene)₂]⁰ complex, or a CpNi^(III) precursor with a formally Ni^(III) [Ni(dithiolene)₂]^? complex. The structural, electrochemical and spectroscopic (UV–vis–NIR, EPR) properties of more than twenty complexes are described and compared, with the help of DFT calculations. They all exhibit a small optical gap with a low-energy absorption band in the Near Infra-Red region, between 700 and 1000 nm. The smaller electrochemical and optical gap found in the [CpNi(dmit)] and [CpNi(dddt)] complexes is correlated with an extensive delocalisation of the spin density in these complexes, while the other members of the series are characterized with a larger and sizeable spin density on the cyclopentadienyl ring.     

On the Synthesis,Characterization and Reactivity of N‐Heteroaryl–Boryl Radicals,a New Radical Class Based on Five‐Membered Ring Ligands

The synthesis and physical characterization of a new class of N‐heterocycle–boryl radicals is presented, based on five membered ring ligands with a N(sp²) complexation site. These pyrazole–boranes and pyrazaboles exhibit a low bond dissociation energy (BDE; B?H) and accordingly excellent hydrogen transfer properties. Most importantly, a high modulation of the BDE(B?H) by the fine tuning of the N‐heterocyclic ligand was obtained in this series and could be correlated with the spin density on the boron atom of the corresponding radical. The reactivity of the latter for small molecule chemistry has been studied through the determination of several reaction rate constants corresponding to addition to alkenes and alkynes, addition to O₂, oxidation by iodonium salts and halogen abstraction from alkyl halides. Two selected applications of N‐heterocycle–boryl radicals are also proposed herein, for radical polymerization and for radical dehalogenation reactions.     

New Radical Borylation Pathways for Organoboron Synthesis Enabled by Photoredox Catalysis

Radical borylation using N‐heterocyclic carbene (NHC)‐BH₃ complexes as boryl radical precursors has emerged as an important synthetic tool for organoboron assembly. However, the majority of reported methods are limited to reaction modes involving carbo‐ and/or hydroboration of specific alkenes and alkynes. Moreover, the generation of NHC‐boryl radicals relies principally on hydrogen atom abstraction with the aid of radical initiators. A distinct radical generation method is reported, as well as the reaction pathways of NHC‐boryl radicals enabled by photoredox catalysis. NHC‐boryl radicals are generated via a single‐electron oxidation and subsequently undergo cross‐coupling with the in‐situ‐generated radical anions to yield gem‐difluoroallylboronates. A photoredox‐catalyzed radical arylboration reaction of alkenes was achieved using cyanoarenes as arylating components from which elaborated organoborons were accessed. Mechanistic studies verified the oxidative formation of NHC‐boryl radicals through a single‐electron‐transfer pathway.     

Anti-oxidant and pro-oxidant reactivities of copper(II), manganese(II) and iron(III) 3,5-di-<Emphasis Type="Italic">i</Emphasis>-propylsalicylate chelates during peroxidation of alkylbenzenes

Bioactive copper(II), iron(III), and manganese(II) 3,5-di-i-propylsalicylate (3,5-DIPS) chelates were investigated in order to determine their ability to inhibit the free radical initiated chain reactions leading to the peroxidation of isopropylbenzene (i-PrPh) and ethylbenzene (EtPh). Quantitative kinetic studies of these chelates established the following order of anti-oxidant reactivities: manganese(II)-(3,5-DIPS)₂>iron(III)(3,5-DIPS)₃>copper(II)₂(3,5-DIPS)₄> > 3,5-DIPS acid. The mechanism of anti-oxidant reactivity of these three chelates is established as being due, in part, to their chain-breaking capacity resulting from the chemical reduction of the generated peroxyl radical to yield alkybenzenelhydroperoxides via reaction of the 3,5-DIPS ligand with the peroxyl radical. In the case of manganese(II)3,5-di-i-propylsalicylate, the central metalloelement also interacts with the peroxyl radical. The manganese(II)-(3,5-DIPS)₂ and copper(II)₂(3,5-DIPS)₄ chelates were also found to exhibit alkylhydroperoxide pro-oxidative reactivity leading to the formation of the alkylbenzeneperoxyl radical. In addition, the manganese(II) atom underwent oxidation to manganese(III) with the formation of the alkylbenzenehydroperoxide or superoxide with air oxygen oxidation. Amyl acetate and dipropylamine (n-Pr₂NH) were added to the reaction mixture to model the biochemical presence of ester or amine cellular components. Addition of amyl acetate to the reaction mixture increased the anti-oxidant reactivity of manganese(II)-(3,5-DIPS)₂ while decreasing its pro-oxidant reactivity. The weaker anti-oxidant reactivites of iron(III)(3,5-DIPS)₃ and copper(II)₂(3,5-DIPS)₄ were less affected by the addition of amyl acetate and the pro-oxidant reactivity of copper(II)₂(3,5-DIPS)₄ was not changed by the addition of amyl acetate, while the pro-oxidant property of iron(III)(3,5-DIPS)₃ was eliminated. In contrast to 2,6-di-t-butyl-4-methylphenol, butylated hydroxy toluene (BHT), anti-oxidant reactivities of copper(II), iron(III), and manganese(II) 3,5-DIPS chelates were dramatically enhanced by the addition of n-Pr₂NH to the reaction mixture. It is concluded that all three metalloelement chelates react with and remove alkylbenzeneperoxyl radicals and the hydroperoxyl radical. The manganese(II)-(3,5-DIPS)₂ and copper(II)₂(3,5-DIPS)₄ chelates may also be useful in removing hydroperoxides in vivo. These reactivities, in addition to their established superoxide dismutase (SOD)-mimetic and catalase-mimetic reactivities, are suggested to possibly permit anti-oxidant and pro-oxidant reactivities in aqueous and organic cellular compartments.     

Persistent Borafluorene Radicals

N‐Heterocyclic carbene (NHC)‐ and cyclic (alkyl)(amino)carbene (CAAC)‐stabilized borafluorene radicals have been isolated and characterized by elemental analysis, single‐crystal X‐ray diffraction, UV/Vis absorption, cyclic voltammetry (CV), electron paramagnetic resonance (EPR) spectroscopy, and theoretical studies. Both the CAAC–borafluorene radical ( 2 ) and the NHC–borafluorene radical ( 4 ) have a considerable amount of spin density localized on the boron atoms (0.322 for 2 and 0.369 for 4 ). In compound 2 , the unpaired electron is also partly delocalized over the CAAC ligand ^carbeneC and N atoms. However, the unpaired electron in compound 4 mainly resides throughout the borafluorene π‐system, with significantly less delocalization over the NHC ligand. These results highlight the Lewis base dependent electrostructural tuning of materials‐relevant radicals. Notably, this is the first report of crystalline borafluorene radicals, and these species exhibit remarkable solid‐state and solution stability.     

(o-Phenylenediamino)borylstannanes: Efficient Reagents for Borylation of Various Alkyl Radical Precursors

(o-Phenylenediamino)borylstannanes were newly synthesized to achieve radical boryl substitutions of a variety of alkyl radical precursors. Dehalogenative, deaminative, decharcogenative, and decarboxylative borylations proceeded in the presence of a radical initiator to give the corresponding organic boron compounds. Radical clock experiments and computational studies have provided insights into the mechanism of the homolytic substitution (S_H2) of the borylstannanes with alkyl radical intermediates. DFT calculation disclosed that the phenylenediamino structure lowered the LUMO level including the vacant p-orbital on the boron atom to enhance the reactivity to alkyl radicals in S_H2. Moreover, C(sp³)-H borylation of THF was accomplished using the triplet state of xanthone.     

Identification of radical species derived from allergenic 15-hydroperoxyabietic acid and insights into the behaviour of cyclic tertiary allylic hydroperoxides in Fe(II)/Fe(III) systems

To understand the skin sensitization mechanism of 15-hydroperoxyabietic acid, the major allergen in colophony, we first examined the formation of potential reactive radicals derived from its reaction with light, heat and TPP-Fe³⁺. Trapping with 1,1,3,3-tetramethylisoindolin-2-yloxyl nitroxide confirmed the formation of carbon-centred radicals derived from allyloxyl/allylperoxyl radicals as a consequence of the hydroperoxide scission. Particular interest was further given to the reactivity with Fe(II)/Fe(III) due to the biological importance of haem containing enzymes. Using a monocyclic 15-hydroperoxyabietic acid-like compound as a model of allergenic allylic hydroperoxides, we evidenced, by the ESR spin-trapping technique, the competition between carbon and oxygen-centred radicals formed in the presence of Fe(II)/Fe(III) in organic/aqueous media. We complemented the study by showing the possibility of formation, via a radical mechanism induced by ferric chloride, of an adduct between the allylic hydroperoxide and N-acetyl-cysteine ethyl ester. The results gave new knowledge on the possible generation of highly reactive radicals that could lead to the formation of antigenic structures.     

The interaction of carbon-centered radicals with copper(I) and copper(II) complexes*

Abstract

Although alkylcopper(I) reagents are widespread, compounds containing alkyl ligands on Cu^II or Cu^III are much less common. Such complexes, however, are generated as transient species when carbon-center radicals add to Cu^I or Cu^II complexes, respectively, and appear to be involved in several copper-catalyzed organic transformations. A few organocopper(II) and organocopper(III) complexes were found sufficiently robust to allow isolation and full characterization. This article reviews the reactivity of carbon-centered radicals with Cu^I and Cu^II ions, both in aqueous and non-aqueous environments, with focus on the importance of the resulting organocopper species on atom transfer radical polymerization and on copper-catalyzed radical termination.     

Stable Mixed‐Valent Radicals from Platinum(II) Complexes of a Bis(dioxolene) Ligand

Three diplatinum(II) complexes [{PtL}₂(μ‐thea)] (H₄thea=2,3,6,7‐tetrahydroxy‐9,10‐dimethyl‐9,10‐dihydro‐9,10‐ethanoanthracene) have been prepared, with diphosphine or bipyridyl “L” co‐ligands. One‐electron oxidation of these complexes gave radical cations containing a mixed‐valent [thea]^3? ligand with discrete catecholate and semiquinonate centers separated by quaternary methylene spacers. The electronic character of these radicals is near the Robin–Day class II/III border determined by UV/Vis/NIR and EPR spectroscopies. Crystal‐structure determinations and a DFT calculation imply that oxidation of the thea^4? ligand may lead to an increased through‐space interaction between the dioxolene π systems.     

L3C3P3: Tricarbontriphosphide Tricyclic Radicals and Cations Stabilized by Cyclic (alkyl)(amino)carbenes

Alkynes usually oligomerize to give rings with a conjugated π‐electron system. In contrast, phosphaalkynes, R?C≡P, frequently give compounds with polycyclic structures, which are thermodynamically more stable than the corresponding π‐conjugated isomers. The syntheses of the first C₃P₃ tricyclic compounds are reported with either radical or cationic ground states stabilized by cyclic (alkyl)(amino)carbenes (CAACs). These compounds may be considered as examples of tricarbontriphosphide coordinated by carbenes and are likely formed via trimerization of the corresponding mono‐radicals CAAC‐CP^.. The mechanism for the formation of these tricarbontriphosphide radicals has been rationalized by a combination of experiments and DFT calculations.     

Pulse and steady‐state radiolysis studies of some homonuclear copper(II) and heteronuclear copper(II)‐molybdenum(VI) peroxo compounds containing a simple peptide molecule

The reactions of e_aq⁻, OH·, CO₂⁻·, and N₃· radicals with some novel homo nuclear and hetero nuclear peroxo peptide complexes viz: copper peroxo glycylglycine, [Cu(O₂)(H₂L)₂]; molybdenum oxoperoxo glycylglycine, [Mo(O)(O₂)₂(H₂L)₂]; Cu,Mo oxoperoxo glycylglycine [CuMo(O)(O₂)(L)₂] and Cu,Mo, oxo glycylglycine, [CuMo(O)₂(L)₂] (H₂L=glycyl glycine) in aqueous solutions were investigated by pulse radiolysis. Three types of reactions were observed: (1) reduction of Cu(II) to Cu(I) by e_aq⁻ and CO₂⁻·, (2) oxidation of Cu(II) to Cu(III) by N₃⁻·, and (3) formation of a radical on H abstraction from the ligand by OH· radical. Rate constants were reported for formation and decomposition of all intermediates. In case of one electron reduced complexes of hetero atoms, CuMo(O)₂L₂ and CuMo(O)(O₂)L₂, prepared via the reduction of the corresponding complexes by e_aq⁻, formation of a new dimer radical anion complex was observed. It is also noteworthy to mention the possibility of the effect of peroxo ligand on intermediate steps during the formation of Cu clusters. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 159–168, 1999     

Electrochemical activation of acyl ligands in organometals. ESR studies of acetyl- and benzoyliron(III) complexes

The anodic oxidation of two series of acyliron(II) complexes, the neutral CpFe(CO)₂COR(I) and the anionic CpFe(CO)(CN)COR^? (II) where R = CH₃ and C₆H₅, are examined in acetonitrile solutions. The cyclic voltammograms of II are reversible, whereas those of I are irreversible even at sweep rates as high as 10 V s^?1. The neutral CpFe^III (CO)(CN)COCH₃ radical is sufficiently stable at 20°C to examine its ESR spectrum, as well as the kinetics and mechanism of thermal decomposition to afford acetone in high yields. However the cation-radical CpFe^III(CO)₂COCH⁺₃ from I is too reactive to observe directly, and it undergoes rapid solvolytic substitution in the presence of ethanol to afford ethyl acetate. The distinction between neutral and cationic acyliron(III) radicals is discussed in the context of acyl activation of organometals by electrochemical methods.     

Crystalline Cyclic (Alkyl)(amino)carbene‐tetrafluoropyridyl Radical

A stable cyclic (alkyl)(amino)carbene (CAAC) 1 inserts into the para‐CF bond of pentafluoropyridine, and after fluoride abstraction, the iminium‐pyridyl adduct [ 3 ]⁺ was isolated. A cyclic voltammetry study shows a reversible three‐state redox system involving [ 3 ]⁺, [ 3 ] ^? , and [ 3 ] ^? . The CAAC‐pyridyl radical [ 3 ] ^? , obtained by reduction of [ 3 ]⁺ with magnesium, has been spectroscopically and crystallographically characterized. In contrast to the lack of π communication between the CAAC and the pyridine units in cation [ 3 ]⁺, the unpaired electron of [ 3 ] ^? is delocalized over an extended π system involving both heterocycles.     

Magnetic interactions in a new heterospin complex involving nitroxide radical and oxamido-bridged copper(II)

A new binuclear copper(II) complex [Cu₂(oxpn)(IM2py)₂](ClO₄)₂, containing four spin carriers with pyridyl-substituted nitroxide radicals has been synthesized and characterized structurally and magnetically (oxpn?=?N,N′-bis(3-aminopropyl)oxamido, IM2py?=?2-(2′-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl). The structure of the complex consists of centrosymmetric trans oxamido-bridged copper(II) binuclear units and nitroxide radicals. The coordination geometry around each copper atom is distorted square pyramidal and the apical position is occupied by a nitrogen atom of the imidazoline ring of a radical ligand. Magnetic analysis indicates that the complex exhibits strong antiferromagnetic coupling between copper(II) ions through the oxamido bridge and a ferromagnetic interaction between copper(II) ions and radical ligands. The magnetic behaviour is discussed with reference to the crystal structure.     

Divergent Synthesis of Chelating Aziridines and Cyclic(Alkyl)(Amino)Carbenes (CAACs) from Pyridyl-Tethered Robust Azomethine Ylides

Azomethine ylides are typically in situ generated synthons for making N-heterocycles through cycloaddition reactions. But an offbeat aspect about them is the isomeric nature of aldiminium-based azomethine ylides and (alkyl/aryl)(amino)carbenes, interconvertible by a formal 1,3-H⁺ transfer. Herein, two thermally robust azomethine ylides with a N-appended picolyl sidearm are isolated, which cyclize to ^pyaziridines at 80 °C but unprecedentedly result ^N−picoCAAC-CuCl (CAAC=cyclic(alkyl)(amino)carbene) complexes when heated with CuCl at merely 60 °C. The pendant N_py, as revealed by computational analysis, plays a crucial role in this unusual 1,3-H⁺ shift using a deprotonation-protonation sequence, as well as in placing the CuCl at the carbenic site in tandem. The softer nature of Cu^(I) is also critical. Chelating CAACs are rare and one with a N-tethered additional donor is priorly unknown. Both ^N-picoCAAC and ^pyaziridine are bidentate chelators giving highly active cationic Rh^(I) catalysts for hydrosilylating unactivated olefins by Et₃SiH. Notably, the ^pyaziridine-Rh^(I) is superior than the ^N-picoCAAC-Rh^(I) catalyst.