Tricoordinate Nontrigonal Pnictogen‐Centered Radical Anions: Isolation,Characterization, and Reactivity

The search for main‐group element‐based radicals is one of the main research topics in contemporary chemistry because of their fascinating chemical and physical properties. The Group 15 element‐centered radicals mainly feature a V‐shaped two coordinate structure, with a couple of radical cations featuring trigonal tricoordinated geometry. Now, nontrigonal compounds R₃E (E=P, As, Sb) were successfully synthesized by introducing a new rigid tris‐amide ligand. The selective one‐electron reduction of R₃E afforded the first stable tricoordinate pnictogen‐centered radical anion salts; the pnictogen atoms retain planar T‐shaped structures. EPR spectroscopy and calculations reveal that the spin density mainly resides at the p orbitals of the pnictogen atoms, which is perpendicular to the N₃E planes.     

Synthesis of H‐shaped A3BA3 copolymer by methyl‐2‐nitrosopropane induced single electron transfer nitroxide radical coupling

Amphiphilic H‐shaped [poly(ethylene oxide)]₃‐polystyrene‐[poly(ethylene oxide)]₃(PEO₃‐PS‐PEO₃) copolymer was synthesized by 2‐methyl‐2‐nitrosopropane (MNP) induced single electron transfer nitroxide radical coupling (SETNRC) using PEO₃‐(PS‐Br) as a single precursor. First, the A₃B star‐shaped precursor PEO₃‐(PS‐Br) was synthesized by atom transfer radical polymerization (ATRP) using three‐arm star‐shaped PEO₃‐Br as macro‐initiator. Then, in the presence of Cu(I)Br/Me₆TREN, the bromide group at PS end was sequentially transferred into carbon‐centered radical by single electron transfer and then nitroxide radical by reacting with MNP in mixed solvents of dimethyl sulfoxide (DMSO)/tetrahydrofuran (THF), and in situ generated nitroxide radical could again capture another carbon‐centered radical by fast SETNRC to form target PEO₃‐PS‐PEO₃ copolymer. The MNP induced SETNRC could reach to a high efficiency of 90% within 60 min. After the product PEO₃‐PS‐PEO₃ was cleaved by ascorbic acid, the SEC results showed that there was about 30% fraction of product formed by single electron transfer radical coupling (SETRC) between carbon‐centered radicals. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Synthesis,Crystal Structure,and Magnetic Property of New Trispin Ln(III)‐Nitronyl Nitroxide Complexes

Four Ln(III) complexes based on a new nitronyl nitroxide radical have been synthesized and structurally characterized: {Ln(hfac)₃[NITPh(MeO)₂]₂} (Ln = Eu( 1 ), Gd( 2 ), Tb( 3 ), Dy( 4 ); NITPh(MeO)₂ = 2‐(3′,4′‐dimethoxyphenyl)‐4,4,5,5‐tetramethylimidazoline‐1‐oxyl‐3‐oxide; hfac = hexafluoroacetylacetonate). The single‐crystal X‐ray diffraction analysis shows that these complexes have similar mononuclear trispin structures, in which central Ln(III) ion is eight‐coordinated by two O‐atoms from two nitroxide groups and six O‐atoms from three hfac anions. The variable temperature magnetic susceptibility study reveals that there exist ferromagnetic interactions between Gd(III) and the radicals, and antiferromagnetic interactions between two radicals (J_Gd‐Rad = 3.40 cm^?1, J_Rad‐Rad = ?9.99 cm^?1) in complex 2 . Meanwhile, antiferromagnetic interactions are estimated between Eu(III) (or Dy(III)) and radicals in complexes 1 and 4 , and ferromagnetic interaction between Tb(III) and radicals in complex 3 , respectively.     

Cyclic Group 15 Radical Cations

Singlet cyclo‐1,3‐dipnicta‐2,4‐diazane‐1,3‐diyls of the type [E(μ‐NTer)₂E] ( 2 , E=P, As, Ter=2,6‐dimesitylphenyl) can undergo a one‐electron‐oxidation utilizing silver salts of weakly coordinating anions such as [AgL_n][B(C₆F₅)₄] (L=donor solvents) to afford the novel cyclic radical cations, [E(μ‐NTer)₂E]^+. ( 3 ^+.). When smaller and more basic anions were employed in the reaction, the anions were found to form covalent bonds to the radical centers yielding dipnictadiazanes, [FP(μ‐NTer)₂PF] ( 5 ) and [(CF₃CO₂)P(μ‐NTer)₂P(CF₃CO₂)] ( 6 ). A two‐electron oxidation process, resulting in the formation of dications of the type [E(μ‐NTer)₂E]²⁺, could not be observed. Computational and EPR data revealed that the spin density is almost completely localized at the two heavier pnictogen centers E of the former 1,3‐dipnictadiazane‐1,3‐diyls. The bonding situation in the radical cations features a rare example of a transannular one‐electron π bond without having a σ bond.     

Fluorinated Sulfilimino Iminiums: Efficient and Versatile Sources of Perfluoroalkyl Radicals under Photoredox Catalysis

Reported herein is the use of S‐perfluoroalkyl sulfilimino iminiums as a new source of R_F radicals under visible‐light photoredox catalysis (R_F=CF₃, C₄F₉, CF₂Br, CFCl₂). These shelf‐stable perfluoroalkyl reagents, readily prepared on gram scale from the corresponding sulfoxide using a one‐pot procedure, allow the efficient photoredox‐induced oxyperfluoroalkylation of various alkenes using fac‐Ir(ppy)₃ as the photocatalyst. Importantly, spin‐trapping/electron paramagnetic resonance experiments were carried out to characterize all the radical intermediates involved in this radical/cationic process.     

Addition of sulfur radicals to fullerenes

The addition of oxygen‐centered radicals to fullerenes has been intensively studied due to their role in cell protection against against hydrogen peroxide induced oxidative damage. However, the analogous reaction of sulfur‐centered radicals has been largely overlooked. Herein, we investigate the addition of S‐centered radicals to C₅₀, C₆₀, C₇₀, and C₁₀₀ fullerenes by means of DFT calculations. The radicals assayed were: S, SH, SCH₃, SCH₂CH₃, SC₆H₅, SCH₂C₆H₅, and the open‐disulfide SCH₂CH₂CH₂CH₂S. Sulfur, the most reactive species, prefers to be attached to a 66‐bond of C₆₀ with a binding energy (E_bind) of 2.4 eV. For the SR radicals the electronic binding energies to C₆₀ are 0.77, 0.74, 0.58, 0.67, and 0.35 eV for SH, SCH₃, SCH₂CH₃, SCH₂C₆H₅, and SC₆H₅, respectively. The reactivity of C₆₀ toward SR radicals can be increased by lithium doping. For Li@C₆₀, the E_bind is increased by 0.65 eV with respect to C₆₀, but only by 0.33 eV for the exohedral doping. Fullerenes act like free radical sponges. Indeed, the C₆₀‐SR E_bind can be duplicated if two radicals are added in ortho or para positions. The enhanced reactivity because of multiple additions is mostly a local effect, although the addition of one radical makes the whole cage more reactive. Therefore, as observed for hydroxylated fullerenes, they should protect cells from oxidative damage. However, the thiolated fullerenes have one advantage, they can be easily attached to gold nanoparticles. For the addition on pentagon junctions smaller fullerenes like C₅₀ are more reactive than C₆₀. Interestingly, C₇₀ is as reactive as C₆₀, even for the addition on the equatorial belt. For larger fullerenes like C₁₀₀, reactivity decreases for the carbon atoms belonging to hexagon junctions. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Syntheses,Structures, and Properties of Two New Tri‐spin Lanthanide‐Nitronyl Nitroxide (LnIII = GdIII and HoIII) Complexes

Abstract. Two radical–Ln^III–radical complexes, [Ln(hfac)₃(NITPh‐Ph)₂] [Ln = Gd ( 1 ) and Ho ( 2 ), hfac = hexafluoroacetylacetonate; and NITPh‐Ph = 4′‐biphenyl‐4, 4, 5, 5‐tetramethylimidazoline‐1‐oxyl‐3‐oxide] were synthesized and characterized by X‐ray diffraction, elemental analysis, magnetic measurements, as well as IR and UV/Vis spectroscopy. X‐ray crystal structure analysis revealed that the structures of both complexes are isomorphous, the central Ln^III ions are coordinated by six oxygen atoms from three hfac ligand molecules and two oxygen atoms from nitronyl radicals. The temperature dependencies of the magnetic susceptibilities were studied. They showed that in the Gd^III complex, ferromagnetic interactions between Gd^III and the radicals and antiferromagnetic interactions between the radicals coexist in this system (with J_Rad–Gd = 0.1 cm^–1, J_Rad–Rad = –0.309 cm^–1).     

Intramolecular hydrogen bond in the hydroxycyclohexadienyl peroxy radicals

The hydroxycyclohexadienyl peroxy radicals (HO? C₆H₆? O₂) produced from the reaction of OH‐benzene adduct with O₂ were studied with density functional theory (DFT) calculations to determine their characteristics. The optimized geometries, vibrational frequencies, and total energies of 2‐hydroxycyclohexadienyl peroxy radical II_s and 4‐hydroxycyclohexadienyl peroxy radical III_s were calculated at the following theoretical levels, B3LYP/6‐31G(d), B3LYP/6‐311G(d,p), and B3LYP/6‐311+G(d,p). Both were shown to contain a red‐shifted intramolecular hydrogen bond (O? H … O? H bond). According to atoms‐in‐molecules (AIM) analysis, the intramolecular hydrogen bond in the 2‐hydroxycyclohexadienyl peroxy radical II_s is stronger than that one in 4‐hydroxycyclohexadienyl peroxy radical III_s, and the former is the most stable conformation among its isomers. Generally speaking, hydrogen bonding in these radicals plays an important role to make them more stable. Based on natural bond orbital (NBO) analysis, the stabilization energy between orbitals is the main factor to produce red‐shifted intramolecular hydrogen bond within these peroxy radicals. The hyperconjugative interactions can promote the transfer of some electron density to the O? H antibonding orbital, while the increased electron density in the O? H antibonding orbital leads to the elongation of the O? H bond and the red shift of the O? H stretching frequency. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Stereochemical Selection in Phosphoranyl Radical Formation Using Ionizing Radiation

Abstract

Recent electron spin resonance (ESR) experiments on phosphorus-centered radicals generated by ionizing radiation demonstrate that stereochemical aspects act strongly on the rate of radical formation and can be decisive in the selection between the possible resulting radical structures. This phenomenon was first established in a single crystal ESR study on radiogenic electron-capture phosphorus-centered radicals of the racemic and meso stereoisomers of 1.2-dimethyl-1,2-diphenyldiphosphine disulfide (1). The radiation process of the racemic form involves the formation of a symmetric species with a threeelectron bond in an overall low yield. The meso isomer, on the other hand, yields exclusively asymmetric radical configurations in which the unpaired electmn resides on one of the two phosphorus nuclei. The high intensity of the ESR spectra for the meso compound indicate a more efficient electron-capture process. A similar pronounced difference in radiosensitivity was observed for the R_p (1 and S_p (2) isomers of (4S,5R)-2-chloro-3,4-dimethyl-5-phenyl-1,3,2-Oxazaphospholidine 2-sulfide. Upon X irradiation, 1 readily results in an electron-capture phosphorus centered radical, whereas the concurrent process in 2 is almost completely absent. Since the geometric parameters of the atoms directly linked to phosphorus are very much alike for 1 and 2 il can be concluded that the efficiency of electron-capture at phosphorus strongly depends on the relative configuration of the distant chiral centers at C₄ and C₅.     

A third polymorph of 4‐(2,6‐difluorophenyl)‐1,2,3,5‐dithiadiazolyl

The crystal structure of a third polymorphic form of the known 4‐(2,6‐difluorophenyl)‐1,2,3,5‐dithiadiazolyl radical, C₇H₃F₂N₂S₂, is reported. This new polymorph represents a unique crystal‐packing motif never before observed for 1,2,3,5‐dithiadiazolyl (DTDA) radicals. In the two known polymorphic forms of the title compound, all of the molecules form cis‐cofacial dimers, such that two molecules are π‐stacked with like atoms one on top of the other, a common arrangement for DTDA species. By contrast, the third polymorph, reported herein, contains two crystallographically unique molecules organized such that only 50% are dimerized, while the other 50% remain monomeric radicals. The dimerized molecules are arranged in the trans‐antarafacial mode. This less common dimer motif for DTDA species is characterized by π–π interactions between the S atoms [S...S = 3.208 (1) Å at 110 K], such that the two molecules of the dimer are related by a centre of inversion. The most remarkable aspect of this third polymorph is that the DTDA dimers are co‐packed with monomers. The monomeric radicals are arranged in one‐dimensional chains directed by close lateral intermolecular contacts between the two S atoms of one DTDA heterocycle and an N atom of a neighbouring coplanar DTDA heterocycle [S...N = 2.857 (2) and 3.147 (2) Å at 110 K].     

Visible‐Light‐Induced ortho‐Selective Migration on Pyridyl Ring: Trifluoromethylative Pyridylation of Unactivated Alkenes

The photocatalyzed ortho‐selective migration on a pyridyl ring has been achieved for the site‐selective trifluoromethylative pyridylation of unactivated alkenes. The overall process is initiated by the selective addition of a CF₃ radical to the alkene to provide a nucleophilic alkyl radical intermediate, which enables an intramolecular endo addition exclusively to the ortho‐position of the pyridinium salt. Both secondary and tertiary alkyl radicals are well‐suited for addition to the C2‐position of pyridinium salts to ultimately provide synthetically valuable C2‐fluoroalkyl functionalized pyridines. Moreover, the method was successfully applied to the reaction with P‐centered radicals. The utility of this transformation was further demonstrated by the late‐stage functionalization of complex bioactive molecules.     

Redox‐Active‐Ligand‐Mediated Formation of an Acyclic Trinuclear Ruthenium Complex with Bridging Nitrido Ligands

Coordination of a redox‐active pyridine aminophenol ligand to Ru^II followed by aerobic oxidation generates two diamagnetic Ru^III species [ 1 a (cis) and 1 b (trans)] with ligand‐centered radicals. The reaction of 1 a / 1 b with excess NaN₃ under inert atmosphere resulted in the formation of a rare bis(nitrido)‐bridged trinuclear ruthenium complex with two nonlinear asymmetrical Ru‐N‐Ru fragments. The spontaneous reduction of the ligand centered radical in the parent 1 a / 1 b supports the oxidation of a nitride (N^3?) to half an equivalent of N₂. The trinuclear omplex is reactive toward TEMPO‐H, tin hydrides, thiols, and dihydrogen.     

Stereochemistry of the Methyl Group in (R)‐3‐Methylitaconate Derived by Rearrangement of 2‐Methylideneglutarate Catalysed by a Coenzyme B12‐Dependent Mutase

2‐Methylideneglutarate mutase is an adenosylcobalamin (coenzyme B₁₂)‐dependent enzyme that catalyses the equilibration of 2‐methylideneglutarate with (R)‐3‐methylitaconate. This reaction is believed to occur via protein‐bound free radicals derived from substrate and product. The stereochemistry of the formation of the methyl group of 3‐methylitaconate has been probed using a `chiral methyl group'. The methyl group in 3‐([²H₁,³H]methyl)itaconate derived from either (R)‐ or (S)‐2‐methylidene[3‐²H₁,3‐³H₁]glutarate was a 50 : 50 mixture of (R)‐ and (S)‐forms. It is concluded that the barrier to rotation about the C−C bond between the methylene radical centre and adjacent C‐atom in the product‐related radical [^.CH₂CH(⁻O₂CC=CH₂)CO₂⁻] is relatively low, and that the interaction of the radical with cob(II)alamin is minimal. Hence, cob(II)alamin is a spectator of the molecular rearrangement of the substrate radical to product radical.     

A theoretical study of the addition of silyl radicals to olefinic monomers

In this paper, the high reactivity of silyl macroradicals toward double bonds of olefinic compounds has been explained by means of quantum‐mechanical calculations through their frontier orbital characteristics. In this way, the main orbital interaction corresponds to the overlapping between the SOMO of the disilyl radical and the LUMO of the olefin. In order to obtain more accurate results of differential reactivity, an orbitalic SOMO‐HOMO interaction should be included in addition to the main SOMO‐LUMO one. Also, we theoretically studied the regioselectivity of the addition of silyl radicals to double bonds obtaining similar results as for carbon centered radicals where the reaction takes place on the less hindered carbon of the olefin. Regarding to the geometrical and electronic parameters, it has been shown that carbon radicals have a sp² geometry and a negative charge on the radical center whilst silyl radicals have a sp³ goemetry and a positive charge. Both factors contribute to the enhanced reactivity of silyl radicals with respect to carbon ones.     

Sulfamate Esters Guide Selective Radical‐Mediated Chlorination of Aliphatic C−H Bonds

Masked alcohols are particularly appealing as directing groups because of the ubiquity of hydroxy groups in organic small molecules. Herein, we disclose a general strategy for aliphatic γ‐C(sp³)?H functionalization guided by a masked alcohol. Specifically, we determine that sulfamate ester derived nitrogen‐centered radicals mediate 1,6‐hydrogen‐atom transfer (HAT) processes to guide γ‐C(sp³)?H chlorination. This reaction proceeds through a light‐initiated radical chain‐propagation process and is capable of installing chlorine atoms at primary, secondary, and tertiary centers.     

Influence of the nature of ligands in iridium complexes with C60 fullerene and ·P(O)(OPri)2, ·CMe3, and ·CCl3 radicals on regioselectivity of addition and stability of spin-adducts formed: an EPR study

The addition of ^·P(O)(OPrⁱ)₂ (R¹), ^·CMe₃ (R²), and ^·CCl₃ (R³) radicals to metallofullerenes (η²-C₆₀)IrH(CO)(CNBu^t)₂(o-HCB₁₀H₉CCH₂PPh₂-B,P) (1), (η²-C₆₀)IrH(CO)(DIOP) (DIOP is (4R,5R)-(+)-4,5-bis(diphenylphosphinomethyl)-2,2-dimethyl-1,3-dioxolane, 2), and (η²-C₆₀)IrH(CO)(PPh₃)₂ (3) was studied by EPR spectroscopy. A stability study of spin adducts (SAs) of R¹ radicals with complexes 1 and 2 showed that when the reactions are initiated by illumination with 366-nm light, the EPR spectra exhibit only signals of those isomers that are formed upon attack of the R¹ radicals on the carbon atoms of the cis-1 and cis-2 bonds (i.e., carbon atoms of the fullerene hemisphere to which the metallofragment is attached). Investigations of the reactions of R² and R³ radicals with complexes 1–3 initiated with 366-nm light made it possible to detect (i) regioisomers formed by adding these radicals to carbon atoms of the cis-n bonds and (ii) SAs formed by adding the radicals to carbon atoms of other bonds in complexes 1–3. The hyperfine structure of the EPR spectrum essentially depends on the spatial structure of substituents at the metal atom and allows individual regioisomers of not only phosphoryl radicals, but also carbon-centered radicals R² and R³ with metallofullerenes 1–3 to be identified. The rate constants for addition of R² and R³ radicals to complexes 2 and 3 were determined. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1302–1309, July, 2007.     

Two new compounds, β‐ScTe and Y3Au2, and a reassessment of Y2Au

Two new compounds, β‐ScTe (scandium telluride) and Y₃Au₂ (triyttrium digold), have been synthesized by high‐temperature solid‐state techniques and their crystal structures, along with that of Y₂Au (diyttrium gold), have been refined by single‐crystal X‐ray diffraction methods. β‐ScTe is a superstructure of ScTe (NiAs‐type), featuring double hexagonal close‐packed layers of Te atoms with the octahedral cavities filled by Sc atoms. Y₃Au₂ displays a U₃Si₂‐type structure and is built from Au₂‐centered bitrigonal prisms and centered cubes of Y atoms. The structure of Y₂Au is better described as an inverse PbCl₂‐type structure rather than a Co₂Si‐type.     

Computational studies on the reactions of 3‐butenyl and 3‐butenylperoxy radicals

The reactions of 3‐butenyl (^?CH₂CH₂CH?CH₂) radicals—unimolecular decomposition, isomerization, as well as reaction with O₂—and the subsequent unimolecular rearrangement reactions of the 3‐butenylperoxy radicals have been investigated and are compared to the analogous reactions of butyl (^?CH₂CH₂CH₂CH₃) and butylperoxy radicals using transition‐state theory based on the quantum chemical calculations at the CBS‐QB3 level. For alkyl‐analogue processes, the reactions of 3‐butenyl and 3‐butenylperoxy radicals can be well characterized by the decreased and increased bond dissociation energies at the allylic and vinylic sites, respectively. The intramolecular addition reactions of the radical center atoms to the double bonds were found to be important non‐alkyl‐analogue reactions of 3‐butenyl and 3‐butenylperoxy radicals. As a consequence, the thermal decomposition of 3‐butenyl radicals was found to be slower than that of butyl radicals by one order of magnitude at temperature near 1000 K. Intramolecular addition reactions are suggested to be the predominant unimolecular rearrangement processes of 3‐butenylperoxy radicals over the entire temperature range investigated (500–1200 K). The intramolecular addition reactions of the alkenyl peroxy radicals, which have not been included in combustion kinetic models, and their implications for the autoignition of alkenes are discussed. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 273–288, 2010     

Synthesis,Crystal Structure Determination and Fe/Rh Site Preference in the New Ternary Boride FeRh6B3

Single crystals of FeRh₆B₃ were synthesized by arc‐melting the elements in a water‐cooled copper crucible under an argon atmosphere. The new silver‐like compound, structurally characterized by single‐crystal X‐ray analysis, crystallizes in the hexagonal Th₇Fe₃ structure type (space group P6₃mc, no. 186, Z = 2; a = 7.4135(6) Å, c = 4.7168(6) Å, R₁ = 0.040, wR₂ = 0.083 for all 376 reflections and 21 parameters). The structure consists of layers of boron‐centered trigonal prisms of rhodium/iron atoms and one‐dimensional strings of face‐sharing octahedral Rh₆ clusters.