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The Staudinger reaction provides chemists with a valuable tool for the reduction of azides, which are notoriously unstable and can decompose explosively. By providing a controlled method for the conversion of azides to amines, the reaction opened up new avenues for the synthesis of various amine‐containing compounds that are widely used in natural products, pharmaceuticals and polymers. The Staudinger reaction begins with the nucleophilic attack of a trivalent phosphine (usually triphenylphosphine), leading to the formation of a triazenide intermediate, a highly reactive species. Here we report how a divalent phosphorus‐centered biradicaloid reacts with covalent azides and show that it is possible to capture and fully characterize the transient intermediate. The experimental data is supported by quantum chemical calculations of the reaction paths and in terms of thermodynamics and chemical bonding. [ABSTRACT FROM AUTHOR]

This study investigates the formation of partial sigma (σ) covalent bonds in experimentally synthesizable biradicals formed from hydrogenated and fluorinated C8, C20, and C60 cage structures, by assessing their stability, geometry, and bonding character in singlet and triplet states using restricted B3LYP‐D3/6–31+G(d,p) theory, natural bond orbital (NBO) analysis, and complete active space self‐consistent field (CASSCF) method. The results show that these partial σCC bonds have Wiberg bond orders of 0.38 to 0.48 and bond lengths ranging from 2.62 Å to 5.93 Å. Cage size influences the characteristics of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), with electrons favoring more antibonding orbitals in smaller cages where electrons reside more on the exterior of the cage and favoring bonding orbitals in larger ones where electrons are more in the interior. Fluorination enhances electron density on bonding orbitals. The analysis further clarified that the differentiation between antibonding and bonding features of HOMOs and LUMOs extends beyond merely electron transfer from s‐ to p‐atomic orbitals, also noting possible interactions of the same symmetry repel. The study also introduces hyperconjugation from α‐position CH bonds as a factor in stabilizing partial σ‐bond formation. The results also caution against the use of broken symmetry methodology in unrestricted SCF wavefunctions for biradicals, such as those in this study as it may cause large spin contamination and thus errors in the calculated electronic properties results. [ABSTRACT FROM AUTHOR]

Phosphorus‐centered disbiradicals, in which the radical sites exist as individual spin doublets with weak spin‐spin interaction have not been known so far. Starting from monoradicals of the type [⋅P(μ‐NTer)2P−R], we have now succeeded in linking two such monoradical phosphorus centers by appropriate choice of a linker. To this end, biradical [⋅P(μ‐NTer)2P⋅] (1) was treated with 1,6‐dibromohexane, affording the brominated species {Br[P(μ‐NTer)]2}2C6H12 (3). Subsequent reduction with KC8 led to the formation of the disbiradical {⋅[P(μ‐NTer)]2}2C6H12 (4) featuring a large distance between the radical phosphorus sites in the solid state and formally the highest biradical character observed in a P‐centered biradical so far, approaching 100 %. EPR spectroscopy revealed a three‐line signal in solution with a considerably larger exchange interaction than would be expected from the molecular structure of the single crystal. Quantum chemical calculations revealed a highly dynamic conformational space; thus, the two radical sites can approach each other with a much smaller distance in solution. Further reduction of 4 resulted in the formation of a potassium salt featuring the first structurally characterized P‐centered distonic radical anion (5−). Moreover, 4 could be used in small molecule activation. [ABSTRACT FROM AUTHOR]

Reactions of a masked N‐heterocyclic phosphinidene (mNHP, 1), a four‐membered heterocycle with a divalent P atom, with various isonitriles (tBu−NC, Dmp−NC) and carbonyls (CO, Fe(CO)5, and benzalacetone) have been studied. The isonitriles (tBu−NC and Dmp−NC) insert into a P−N bond of the four‐membered ring of 1, leading to the formation of a five‐membered heterocycle (2tBu, 2Dmp), indicating carbene‐like reactivity of the isonitriles. In exactly this way, CO, which is isolobal to isonitriles, reacts to form a five‐membered ring (3) with an exocyclic C=O double bond. If, on the other hand, Fe(CO)5 is used as a CO source, a CO molecule is inserted analogously, however, the remaining Fe(CO)4 fragment is bound via a phosphorus‐iron dative bond to the five‐membered heterocycle. When mNHP 1 reacts with the α,β‐unsaturated ketone benzalacetone, phosphinidene reactivity is observed for 1. This means that the four‐membered ring opens and the P atom reacts with the benzalacetone in a cyclization reaction to form a 1,2‐oxaphospha‐cyclopentene. Theoretical studies of the five‐membered heterocycles 2 and 3 carrying eight π electrons show that zwitterionic Lewis representations dominate in the resonance scheme and only a very low biradical character (<5 %) is found. [ABSTRACT FROM AUTHOR]

Ground triplet 4,6-bis(trifluoromethyl)-1,3-phenylene bis(tert-butyl nitroxide) (TF2PBN) reacted with [Y(hfac)3(H2O)2] (hfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dionate), affording a doubly hydrogen-bonded adduct [Y(hfac)3(H2O)2(TF2PBN)]. The biradical was recovered from the adduct through recrystallization. Crystallographic analysis indicates that the torsion angles (|θ| ≤ 90°) between the benzene ring and nitroxide groups were 74.9 and 84.8° in the adduct, which are larger than those of the starting material TF2PBN. Steric congestion due to o-trifluoromethyl groups gives rise to the reduction of π-conjugation. Two hydrogen bonds enhance this deformation. Susceptometry of the adduct indicates a ground singlet with 2J/kB = −128(2) K, where 2J corresponds to the singlet–triplet gap. The observed magneto-structure relation is qualitatively consistent with Rajca's pioneering work. A density functional theory calculation at the UB3LYP/6-311+G(2d,p) level using the atomic coordinates determined provided a result of 2J/kB = −162.3 K for the adduct, whilst the corresponding calculation on intact TF2PBN provided +87.2 K. After a comparison among a few known compounds, the 2J vs. |θ| plot shows a negative slope with a critical torsion of 65(3)°. The ferro- and antiferromagnetic coupling contributions are balanced in TF2PBN, being responsible for ground-state interconversion by means of small structural perturbation like hydrogen bonds. [ABSTRACT FROM AUTHOR]

Photoinitiated radical chemistry has proven to be useful for breaking covalent bonds within many biomolecules in the gas phase. Herein, we demonstrate that radical chemistry is useful for bond synthesis in the gas phase. Single peptides containing two cysteine residues capped with propylmercaptan (PM) often form disulfide bonds following ultraviolet excitation at 266 nm and loss of both PM groups. Similarly, noncovalently bound peptide pairs where each peptide contains a single cysteine residue can be induced to form disulfide bonds. Comparison with disulfide bound species sampled directly from solution yields identical collisional activation spectra, suggesting that native disulfide bonds have been recapitulated in the gas phase syntheses. Another approach utilizing radical chemistry for covalent bond synthesis involves creation of a reactive diradical that can first abstract hydrogen from a target peptide, creating a new radical site, and then recombine the second radical with the new radical to form a covalent bond. This chemistry is illustrated with 2-(hydroxymethyl-3,5-diiodobenzoate)-18-crown-6 ether, which attaches noncovalently to protonated primary amines in peptides and proteins. Following photoactivation and crosslinking, the site of noncovalent adduct attachment can frequently be determined. The ramifications of these observations on peptide structure and noncovalent attachment of 18-crown-6-based molecules is discussed.

Photoinitiated radical chemistry has proven to be useful for breaking covalent bonds within many biomolecules in the gas phase. Herein, we demonstrate that radical chemistry is useful for bond synthesis in the gas phase. Single peptides containing two cysteine residues capped with propylmercaptan (PM) often form disulfide bonds following ultraviolet excitation at 266 nm and loss of both PM groups. Similarly, noncovalently bound peptide pairs where each peptide contains a single cysteine residue can be induced to form disulfide bonds. Comparison with disulfide bound species sampled directly from solution yields identical collisional activation spectra, suggesting that native disulfide bonds have been recapitulated in the gas phase syntheses. Another approach utilizing radical chemistry for covalent bond synthesis involves creation of a reactive diradical that can first abstract hydrogen from a target peptide, creating a new radical site, and then recombine the second radical with the new radical to form a covalent bond. This chemistry is illustrated with 2-(hydroxymethyl-3,5-diiodobenzoate)-18-crown-6 ether, which attaches noncovalently to protonated primary amines in peptides and proteins. Following photoactivation and crosslinking, the site of noncovalent adduct attachment can frequently be determined. The ramifications of these observations on peptide structure and noncovalent attachment of 18-crown-6-based molecules is discussed.

The redox behaviors of macrocyclic molecules with an entirely π-conjugated system are of interest due to their unique optical, electronic, and magnetic properties. In this study, defect-free cyclic P3HT with a degree of polymerization (DPn) from 14 to 43 was synthesized based on our previously established method, and its unique redox behaviors arising from the cyclic topology were investigated. Cyclic voltammetry (CV) showed that the HOMO level of cyclic P3HT decreases from –4.86 eV (14 mer) to –4.89 eV (43 mer), in contrast to the linear counterparts increasing from –4.94 eV (14 mer) to –4.91 eV (43 mer). During the CV measurement, linear P3HT suffered from electro-oxidation at the chain ends, while cyclic P3HT was stable. ESR and UV–Vis–NIR spectroscopy suggested that cyclic P3HT has stronger dicationic properties due to the interactions between the polarons. On the other hand, linear P3HT showed characteristics of polaron pairs with multiple isolated polarons. Moreover, the dicationic properties of cyclic P3HT were more pronounced for the smaller macrocycles. [ABSTRACT FROM AUTHOR]

In spite of achievement of a lot of Ln‐radical SMMs, how to improve magnetic behavior of Ln‐radical system remains challenging. Here, two series of Ln‐radical complexes have successfully been built using an imino nitroxide biradical, namely, [Ln2(hfac)6(ImPhPyobis)2] (LnIII=Gd 1, Tb 2, Dy 3) and [Ln2Cu2(hfac)10(ImPhPyobis)2] (LnIII=Gd 4, Dy 5; hfac=hexafluoroacetylacetonate and ImPhPyobis=5‐(4‐oxypyridinium‐1‐yl)‐1,3‐bis(1'‐oxyl‐4',4',5',5'‐tetramethyl‐4,5‐hydro‐1H‐imidazol‐2‐yl)benzene). For these biradical‐metal complexes, two imino nitroxide biradicals bind two Ln(III) ions via their oxygen atoms coming from 4‐oxypyridinium units to produce a binuclear {Ln2O2} unit. Those imino nitroxide groups are free for complexes 1–3, however one of imino nitroxide groups of the biradical is ligated to the copper(II) ion for complexes 4 and 5. The distinct magnetic relaxation behaviors are observed for two Dy derivatives, as revealed by ac magnetic studies: complex 3 presents one magnetic process with the effective energy barrier(Ueff) of 74.0 K while complex 5 exhibits dual relaxation processes with Ueff values for the fast‐ and slow‐relaxation being 20.2 K and 30.9 K, respectively, which implies that the second coordination sphere of Dy ion plays a critical role for magnetic relaxation. [ABSTRACT FROM AUTHOR]

Artemisinin is the most successful antimalarial drug against malaria caused by Plasmodium falciparum. Despite its tremendous success and popularity in malaria therapeutics, the molecular mechanism of artemisinin's activity is still elusive. The activation of artemisinin, i.e., cleavage of the endoperoxide bond at the infected cell that generates radical intermediates and the subsequent chemical rearrangements, plays a key role in the antimalarial activities. In this work, adopting state‐of‐the‐art computational techniques based on the spin‐constrained density functional theory (CDFT) along with ab initio thermodynamics, we have investigated the activation of artemisinin by two different pathways, homolytic and heterolytic cleavage. The homolytic scission of the endoperoxide bond is further followed by subsequent chemical reactions that propagate via biradical intermediates. Here we report that the free energy of activation of artemisinin associated with the homolytic cleavage is less (21.77 kcal/mol) than the heterolytic cleavage (23.64 kcal/mol). The nonreductive homolytic cleavage of the endoperoxide bond is thermodynamically slightly favorable. Thus, the latter could occur in parallel with the heterolytic activation of the artemisinin. [ABSTRACT FROM AUTHOR]

The photochromic molecules showing wavelength-selective or light intensity-dependent photoresponse are receiving increased attention in recent years. Although a photoswitch with a single chromophore can control the ON and OFF states of a function, that consisting of multi-chromophores would be useful for the specific control in complex systems. Herein, we designed stepwise two-photon induced photochromic molecules (PABI–PIC and PABI–PIC2) consisting of two different photochromic units (PABI and PIC). One-photon absorption reaction in the UV light region of PABI–PIC generates the short-lived transient biradical (BR) that absorbs an additional photon in the visible and UV light region in a stepwise manner to produce the two-photon photochemical product, the quinoidal species (Quinoid). The photochromic properties of these transient species are completely different in color and fading speed. In addition, PABI–PIC also shows the excitation wavelength-dependent photochromism because the excited states of the PABI and PIC units are electronically orthogonal. Therefore, the stepwise photochromic properties of PABI–PIC are easily controlled depending on the excitation light intensity and wavelength. These molecular designs are important for the development of advanced photoresponsive materials. [ABSTRACT FROM AUTHOR]

The present minireview describing the mono- and biradical structures of heterocyclic radicals (verdazyls) in the role of ligands and types of their metal complexes is based on an analysis of scientific literature published since 2000 to 2021. In the majority of cases, monoverdazyl radicals in the composition of the complexes are described as N,N-bidentate or N,N,N-tridentate redox-active ligands. The verdazyl-containing biradical systems are considered. Examples of other verdazyl compounds with metals are presented. [ABSTRACT FROM AUTHOR]

Dichalcogenolenes are archetypal redox non‐innocent ligands with numerous applications. Herein, a diselenolene ligand with fundamentally different electronic properties is described. A mesoionic diselenolene was prepared by selenation of a C2‐protected imidazolium salt. This ligand is diamagnetic, which is in contrast to the paramagnetic nature of standard dichalcogenolene monoanions. The new ligand is also redox‐active, as demonstrated by isolation of a stable diselenolene radical dianion. The unique electronic properties of the new ligand give rise to unusual coordination chemistry. Thus, preparation of a hexacoordinate aluminum tris(diselenolene) complex and a Lewis acidic aluminate complex with two ligand‐centered unpaired electrons was achieved. [ABSTRACT FROM AUTHOR]

Spiro-substituted nitroxyl biradicals are widely used as reagents for dynamic nuclear polarization (DNP), which is especially important for biopolymer research. The main criterion for their applicability as polarizing agents is the value of the spin–spin exchange interaction parameter (J), which can vary considerably when different couplers are employed that link the radical moieties. This paper describes a study on biradicals, with a ferrocene-1,1′-diyl-substituted 1,3-diazetidine-2,4-diimine coupler, that have never been used before as DNP agents. We observed a substantial difference in the temperature dependence between Electron Paramagnetic Resonance (EPR) spectra of biradicals carrying either methyl or spirocyclohexane substituents and explain the difference using Density Functional Theory (DFT) calculation results. It was shown that the replacement of methyl groups by spirocycles near the N-O group leads to an increase in the contribution of conformers having J ≈ 0. The DNP gain observed for the biradicals with methyl substituents is three times higher than that for the spiro-substituted nitroxyl biradicals and is inversely proportional to the contribution of biradicals manifesting the negligible exchange interaction. The effects of nucleophiles and substituents in the nitroxide biradicals on the ring-opening reaction of 1,3-diazetidine and the influence of the ring opening on the exchange interaction were also investigated. It was found that in contrast to the methyl-substituted nitroxide biradical (where we observed the ring-opening reaction upon the addition of amines), the ring opening does not occur in the spiro-substituted biradical owing to a steric barrier created by the bulky cyclohexyl substituents. [ABSTRACT FROM AUTHOR]

Summary: Heterocyclobutane-1,3-diyls bridge the gap between organic and inorganic chemistry and are known for the majority of group 13–15 elements. The kinetically stabilized compounds have been employed in the activation of small molecules, akin to conventional transition metal complexes, and are thus cost-efficient alternatives for future processes. Here, we present the synthesis and characterization of a particularly unusual example of a heterocyclobutane-1,3-diyl, [RC(μ-NR′)2P], that blurs the line between biradical, zwitterion, and phosphinidene. While the reaction with AuCl and an NHC (N-heterocyclic carbene) shows resemblance to biradicals, the reaction with an alkyne to form a phosphirene is characteristic for phosphinidenes. Quantum chemical investigations allow for the classification of the amidinate-based heterocycle [RC(μ-NR′)2P] next to comparable literature examples.

Among various polycyclic aromatic hydrocarbons, C6−C5−C6−C7−C6 fused pentacyclic aromatic hydrocarbons have the unique potential to adopt quinonoid, zwitterion, singlet, or triplet biradical electronic configurations. Two such hybrid structures between pentacene and azulene were synthesized and their ground state electronic configurations were deduced from the reactivity patterns they exhibit respectively. Compound 6, where the radicaloid carbons are linked through a para‐phenylene, forms a head‐to‐head dimer like a singlet biradical. In contrast, isomer 7, where the para‐linkage was switched to meta, reacts readily with oxygen which resembles the reactivity of a triplet state. The oxidized intermediate(s) then undergoes rearrangement to furnish the C6−C5−C6−C6−C6 ring contraction product 13. Cation 14, the protonated form of 7, was synthesized, which implies 7 also reacts like a zwitterion. It was revealed the oxidative rearrangement takes place even with mesityl dibenzotropylium cation despite its perceived aromaticity. DFT calculations confirm the most stable forms of 6 and 7 are singlet and triplet diradical, which is consistent with the observed reactivity of respective molecules. [ABSTRACT FROM AUTHOR]

Visible-light sensitized photoswitches have been paid particular attention in the fields of life sciences and materials science because long-wavelength light reduces photodegradation, transmits deep inside of matters, and achieves the selective excitation in condensed systems. Among various photoswitch molecules, the phenoxyl-imidazolyl radical complex (PIC) is a recently developed thermally reversible photochromic molecule whose thermal back reaction can be tuned from tens of nanoseconds to tens of seconds by rational design of the molecular structure. While the wide range of tunability of the switching speed of PIC opened up various potential applications, no photosensitivity to visible light limits its applications. In this study, we synthesized a visible-light sensitized PIC derivative conjugated with a benzil unit. Femtosecond transient absorption spectroscopy revealed that the benzil unit acts as a singlet photosensitizer for PIC by the Dexter-type energy transfer. Visible-light sensitized photochromic reactions of PIC are important for expanding the versatility of potential applications to life sciences and materials science.

Volatile organic compounds (VOCs) react with atmospheric OH, NO3, and O3 to form compounds having lower volatility, contributing to secondary organic aerosols (SOAs). Consequently, characterizing the reaction intermediate for this reaction is essential for modeling the formation of SOAs. Herein, we proposed a mechanism for the ozonolysis of p‐cymene and calculated the energies of the intermediates generated during each step by density functional theory (DFT). According to our calculation, high‐energy biradicals were produced in the ozonolysis of p‐cymene, which plays an important role in the formation of SOAs, thereby affecting the climate and air quality. [ABSTRACT FROM AUTHOR]

The one‐electron reduction of a cyclic (alkyl)(amino)carbene (CAAC)‐stabilized arylborylene carbonyl complex yields a dimeric borylketyl radical anion, resulting from an intramolecular aryl migration to the CO carbon atom. Computational analyses support the existence of a [(CAAC)B(CO)Ar].− radical anion intermediate. Further reduction leads to a highly nucleophilic dianionic (boraneylidene)methanolate. [ABSTRACT FROM AUTHOR]

We have investigated the cannibalistic self‐trapping reaction of an ortho‐benzyne derivative generated from 1,11‐bis(p‐tolyl)undeca‐1,3,8,10‐tetrayne in an HDDA reaction. Without adding any specific trapping agent, the highly reactive benzyne is trapped by another bisdiyne molecule in at least three different modes. We have isolated and characterized the resulting products and performed high‐level calculations concerning the reaction mechanism. During the cannibalistic self‐trapping process, either a C≡C triple bond or an sp–sp3 C−C single bond is cleaved. Up to seven rings and nine C−C bonds are formed starting from two 1,11‐bis(p‐tolyl)undeca‐1,3,8,10‐tetrayne molecules. Our experiments and calculations provide considerable insight into the variety of reaction pathways which the ortho‐benzyne derivative, generated from a bisdiyne, can take when reacting with another bisdiyne molecule. [ABSTRACT FROM AUTHOR]

Presented here is a comprehensive study of highly oxidized multiple‐decker complexes composed of TbIII and CdII ions and two to five phthalocyaninato ligands, which are stabilized by electron‐donating n‐butoxy groups. From X‐ray structural analyses, all the complexes become axially compressed upon ligand oxidation, resulting in bowl‐shaped distortions of the ligands. In addition, unusual coexistence of square antiprism and square prism geometries around metal ions was observed in +4e charged species. From paramagnetic 1H NMR studies on the resulting series of triple, quadruple and quintuple‐decker complexes, ligand oxidation leads to a decrease in the magnetic anisotropy, as predicted from theoretical calculations. Unusual paramagnetic shifts were observed in the spectra of the +2e charged quadruple and quintuple‐decker complexes, indicating that those two species are actually unexpected triplet biradicals. Magnetic measurements revealed that the series of complexes show single‐molecule magnet properties, which are controlled by the multi‐step redox induced structural changes. [ABSTRACT FROM AUTHOR]

The reaction of biradical [P(μ‐NTer)]2 (1, Ter = 2,6‐bis(2,4,6‐trimethylphenyl)phenyl) towards different alkenes (R = 2,3‐dimethyl–butadiene, 2,5‐dimethyl‐2,4‐hexadiene, 1,7‐octadiene, 1,4‐cyclohexadiene) and alkynes (R = 1,4‐diphenyl‐1,3‐butadiyne) was studied experimentally. Although these olefins can react in different ways, only [2+2] cycloaddition products (1R) were observed. The reaction with 2,3‐dimethylbutadiene also led to the [2+2] product (1dmb). Thermal treatment of 1dmb above 140 °C resulted in the recovery of biradical 1 upon homolytic bond cleavage of the two P–C bonds and the release of 2,3‐dimethylbutadiene. In contrast to this reaction, all other [2+2] additions products (1R, R = 1,7‐octadiene, 1,4‐cyclohexadiene, 1,4‐diphenyl‐1,3‐butadiyne) began to decompose at temperatures between 200 °C and 300 °C. Only unidentified products were obtained but no temperature‐controlled equilibrium reactions were observed. Computations were carried out to shed light into the formal [2+2] as well as the possible [4+2] addition reaction. [ABSTRACT FROM AUTHOR]

The reaction of biradical [P(μ‐NTer)]2 (1, Ter = 2,6‐bis(2,4,6‐trimethylphenyl)phenyl) towards different alkenes (R = 2,3‐dimethyl–butadiene, 2,5‐dimethyl‐2,4‐hexadiene, 1,7‐octadiene, 1,4‐cyclohexadiene) and alkynes (R = 1,4‐diphenyl‐1,3‐butadiyne) was studied experimentally. Although these olefins can react in different ways, only [2+2] cycloaddition products (1R) were observed. The reaction with 2,3‐dimethylbutadiene also led to the [2+2] product (1dmb). Thermal treatment of 1dmb above 140 °C resulted in the recovery of biradical 1 upon homolytic bond cleavage of the two P–C bonds and the release of 2,3‐dimethylbutadiene. In contrast to this reaction, all other [2+2] additions products (1R, R = 1,7‐octadiene, 1,4‐cyclohexadiene, 1,4‐diphenyl‐1,3‐butadiyne) began to decompose at temperatures between 200 °C and 300 °C. Only unidentified products were obtained but no temperature‐controlled equilibrium reactions were observed. Computations were carried out to shed light into the formal [2+2] as well as the possible [4+2] addition reaction. [ABSTRACT FROM AUTHOR]

The main advances in the synthesis and studies of the properties of new stable heterocyclic verdazyl radicals and verdazyl radical coordination chemistry are reviewed. The promising applications of these radicals are also discussed. [ABSTRACT FROM AUTHOR]

Four new pincer-type bisnitroxides, o -C 6 H 4 {CH 2 O-(p -C 6 H 4) n -N(O•)-Bu t } 2 (abbreviated as p Bi and p Ter for n = 2 and 3, respectively; p Ph for n = 1 (known)), and o -C 6 H 4 {CH 2 O-(p -C 6 H 4) n -1 -(m -C 6 H 4)–N(O•)-Bu t } 2 ( m Bi and m Ter for n = 2 and 3, respectively) were prepared, where o -xylylenedioxy was chosen as a hinge and phenylene, biphenylene, and terphenylene were selected as an arm. The crystallographic analysis clarified the intramolecular proximity of the two radical groups was observed for m Bi, whereas p Bi and m Ter possess an open V-shape. In toluene solution ESR, five hyperfine splitting lines were recorded for every compound at room temperature or higher. The antiferromagnetic exchange interaction is estimated as 2 J / k B = −0.05 to −0.0002 K from ESR simulation. The orders of the exchange interactions are as follows: Ph > Bi > Ter regarding the arm lengths and m > p regarding the positional isomers. The temperature at which alternating linewidths appeared was found to be a convenient temperature scale in this study. [Display omitted] • Bisnitroxide compounds were synthesized where the radical groups can come closer. • Crystal structure analysis clarified a U-shaped molecular structure. • The solution ESR spectra were satisfactorily reproduced with simulation. • DFT geometry optimization supported the approach of two nitroxide group. [ABSTRACT FROM AUTHOR]

A novel nitronyl nitroxide biradical-based MnII 2 DyIII 2 compound [Dy 2 Mn 2 (hfac) 10 (NITPhPybis) 2 ] has been designed and assembled successfully, representing the first example of nitronyl nitroxide-MnII-LnIII compounds and expanding the realm of radical-metal based magnetic materials. DC magnetic susceptibility studies reveal the existence of ferromagnetic Dy-nitronyl nitroxide interaction and strong antiferromagnetic interaction between direct coordinated NO group and Mn(II) ion. The rational design of a polynuclear 2 p -3 d -4 f cluster made of MnII ions, DyIII ions and functionalized nitronyl nitroxide biradicals is presented. The cluster represents the first the nitronyl nitroxide-MnII-LnIII complex. Image 1 • The first nitronyl nitroxide biradical-based MnII-DyIII heterometallic cluster has been successfully obtained. • Compound possesses a centrosymmetric annular heterometallic tetranuclear structure. • DC magnetic susceptibility studies reveal the ferromagnetic interaction between the Dy(III) ion and mono-radical. [ABSTRACT FROM AUTHOR]

A novel Dynamic Nuclear Polarization (DNP) NMR polarizing agent ToSMTSL-PTE representing a phospholipid with a biradical TOTAPOL tethered to the polar head group has been synthesized, characterized, and employed to enhance solid-state Nuclear Magnetic Resonance (SSNMR) signal of a lipid-reconstituted integral membrane protein proteorhodopsin (PR). A matrix-free PR formulation for DNP improved the absolute sensitivity of NMR signal by a factor of ca. 4 compared to a conventional preparation with TOTAPOL dispersed in a glassy glycerol/water matrix. DNP enhancements measured at 400 MHz/263 GHz and 600 MHz/395 GHz showed a strong field dependence but remained moderate at both fields, and comparable to those obtained for PR covalently modified with ToSMTSL. Additional continuous wave (CW) X-band electron paramagnetic resonance (EPR) experiments with ToSMTSL-PTE in solutions and in lipid bilayers revealed that an unfavorable conformational change of the linker connecting mononitroxides could be one of the reasons for moderate DNP enhancements. Further, differential scanning calorimetry (DSC) and CW EPR experiments indicated an inhomogeneous distribution and/or a possibility of a partial aggregation of ToSMTSL-PTE in DMPC:DMPA bilayers when the concentration of the polarizing agent was increased to 20 mol% to maximize the DNP enhancement. Thus, conformational changes and an inhomogeneous distribution of the lipid-based biradicals in lipid bilayers emerged as important factors to consider for further development of this matrix-free approach for DNP of membrane proteins. Image 1 [ABSTRACT FROM AUTHOR]

• Highly selective synthesis of disulfide bonds in the gas phase. • Covalent bond formation to identify sites of noncovalent crown ether binding. • Both biradical and diradical species can be used for bond synthesis. Photoinitiated radical chemistry has proven to be useful for breaking covalent bonds within many biomolecules in the gas phase. Herein, we demonstrate that radical chemistry is useful for bond synthesis in the gas phase. Single peptides containing two cysteine residues capped with propylmercaptan (PM) often form disulfide bonds following ultraviolet excitation at 266 nm and loss of both PM groups. Similarly, noncovalently bound peptide pairs where each peptide contains a single cysteine residue can be induced to form disulfide bonds. Comparison with disulfide bound species sampled directly from solution yields identical collisional activation spectra, suggesting that native disulfide bonds have been recapitulated in the gas phase syntheses. Another approach utilizing radical chemistry for covalent bond synthesis involves creation of a reactive diradical that can first abstract hydrogen from a target peptide, creating a new radical site, and then recombine the second radical with the new radical to form a covalent bond. This chemistry is illustrated with 2-(hydroxymethyl-3,5-diiodobenzoate)-18-crown-6 ether, which attaches noncovalently to protonated primary amines in peptides and proteins. Following photoactivation and crosslinking, the site of noncovalent adduct attachment can frequently be determined. The ramifications of these observations on peptide structure and noncovalent attachment of 18-crown-6-based molecules is discussed. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • [2 + 2], [3 + 2], and [4 + 2] Photocycloadditions selectively proceeded. • Intramolecular π-π interaction in exciplexes governed the reaction modes. • Di-π-methane rearrangement of [4 + 2] photocycloadducts gave angular triquinanes. Abstract Photoreactions of 1-cyanonaphthalene derivatives containing 5-arylpent-4-enyl groups at C-2 or C-4 of the naphthalene ring were investigated. Photoreactions of C-2 derivatives in benzene or CH 3 CN generated intramolecular [2 + 2] and [3 + 2] photocycloadducts. Substances bearing electron-rich and electron-poor phenyl groups produced respective [2 + 2] and [3 + 2] photocycloadducts preferentially. In contrast, photoreactions of C-4 linked 1-cyanonaphthalenes produced intramolecular [4 + 2] photocycloadducts as main products along with stereoisomers of tricyclic compounds as minor products. The formation of [4 + 2] photocycloadducts was promoted by using benzophenone or Michler's ketone triplet photosensitization, and quenched by O 2 or ferrocene. Intramolecular singlet exciplexes, which serve as important intermediates in the mechanistic pathways for these reactions, governed the site-selectivities in photoreactions of both of the 2- and 4-(5-arylpent-4-enyl)-1-cyanonaphthalenes. In addition, [4 + 2] photocycloadducts were observed to undergo di-π-methane rearrangement under irradiation conditions to produce angular triquinane derivatives. [ABSTRACT FROM AUTHOR]

Superoxide radical represents one of the most biologically relevant reactive oxygen species involved in numerous physiological and pathophysiological processes. Superoxide measurement through the decay of an electron paramagnetic resonance (EPR) signal of a triarylmethyl (TAM) radical possesses the advantage of a high selectivity and relatively high rate constant of TAM reaction with the superoxide. Hereby we report a straightforward synthesis and characterization of a TAM-TAM biradical showing a high reactivity with superoxide (second-order rate constant, (6.7 ± 0.2) × 103 M−1 s−1) enabling the measurement of superoxide radical by following the increase of a sharp EPR signal associated with the formation of a TAM-quinone-methide monoradical product. [ABSTRACT FROM AUTHOR]

Spin trapping coupled with electron paramagnetic resonance spectroscopy is the most direct method for detection of very low concentrations of free radicals, and it has been intensively used to detect superoxide or alkylperoxyl radicals in biological systems, using cyclic nitrones as spin traps. The half-life time of the ensuing spin adducts depends dramatically on the spin trap structure; however, their mechanism of decay has never been definitely established. We investigated at the MRMP2/CASSCF (MultiReference second-order Møller-Plesset perturbation theory/Complete Active Set Self Consistent Field) level of theory the mechanism of decay of methylperoxyl and tert-butylperoxyl spin adducts formed with various cyclic nitrones. We showed that no transition state can be located for the O─O homolytic cleavage, which yields an intermediate biradical with the following sequence •O─N─C─O•. Then, homolytic cleavage of the N─C bond yields a nitrosoaldehyde, through an early transition state with a very low activation energy. For each nitrone used as spin trap, electrospray ionization mass spectrometry analysis of the spin trapping mixture allowed to detect the presence of the expected nitrosoaldehyde. We generated tert-butylperoxyl spin adducts in toluene, and we found a good correlation between their half-life time and the bond dissociation energy of their peroxidic O─O bond. The theoretical method was then extended to hydroperoxyl spin adducts in water and provided promising results. [ABSTRACT FROM AUTHOR]

We designed bisnitroxide compounds where the radical sites are located close to each other in a molecule. Two new pincer-type bisnitroxide compounds have been synthesized, involving xanthene-4,5-diyl as a spacer and tert -butyl phenyl nitroxides as arms. From the X-ray crystal structure analysis, the shortest intramolecular interatomic N⋯O and O⋯O distances respectively are 5.074(6) and 5.258(6) Å for the m , m ′-derivative and 3.624(3) and 3.771(3) Å for the p , p ′-derivative. The N⋯O distance in the latter satisfies the empirical criterion for possible dimerization/degradation reaction accompanied by dia-/paramagnetic transition. However, the magnetic study clarified paramagnetic behavior in all the temperature range. According to a singlet-triplet model, antiferromagnetic couplings were characterized with 2 J / k B = −7.71(2) and −8.83(4) K for the m , m ′ and p , p ′-derivatives, respectively. The present result suggests that a more flexible spacer is required for realization of possible dia-/paramagnetic transition. [ABSTRACT FROM AUTHOR]

Adiabatic photochemical reactions of cyclopropanes possessing a benzophenone moiety ( trans- and cis- 5), which generate a luminescent excited biradical 1 6••*, were studied. Various photochemical and spectroscopic techniques were used for this purpose. Excitation (350 nm, n, π* band) of 5 at 77 K affords the lowest excited state 1 5* that undergoes intersystem crossing to form 3 5*, which deactivates through phosphorescence. In contrast, 295-nm excitation ( π, π* band) of 5 affords the higher electronically excited state 1 5**, which undergoes adiabatic C-C bond cleavage reaction to form the excited biradical 1 6••* that luminesces at 630 nm. The results suggest that 5 follows 2 photoreaction modes depending on the excitation wavelength, a finding that disobeys Kasha's rule. [ABSTRACT FROM AUTHOR]

A novel nickel(II) complex of a tridentate iminonitroxyl biradical, [Ni( bisimpy )MeOH(ReO 4 ) 2 ], ( 1 ) (bisimpy = 2,6-bis(4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-2-yl-1-oxyl)pyridine) was structurally and magnetically characterized. Due to an effective separation of the molecules in solid the magnetic behavior of 1 differs considerably from that of a related complex studied earlier, the spin carriers in the former having been strongly ferromagnetically coupled even at room temperature. Dynamic behavior of the paramagnetic cation [Ni( bisimpy )(CD 3 OD) 3 ] in CD 3 OD solution was studied by 1 H NMR. A temperature dependence of the spectrum of 1 has been analyzed using a dynamic NMR approach. It was shown that an interconversion of two conformers is liable for a fast dynamic process occurring in solution of 1 at elevated temperatures. An estimated value of ΔG ‡ (298 K) for this process was found to be of 59 ± 3 kJ/mol. It was confirmed that a considerable extension of the measurable rate constants range compared to that for diamagnetic compounds is a result of the paramagnetic chemical shifts in the Ni complex. Application of paramagnetic d -metal complexes as probes for studying of the free-energy changes in chemical exchange processes, as well as the advantages of this method compared to diamagnetic species are considered. The investigated complex represents an example of the new type of thermometric NMR sensors being to our knowledge is the most temperature sensitive reagent among the d -metal paramagnetic coordination compounds. [ABSTRACT FROM AUTHOR]

Relativistic quantum chemistry investigations are carried out to tackle the puzzling oxidation state problem in a series of MO trioxide anions of all d- and f-block elements with five valence electrons. We have shown here that while the oxidation states of V, Nb, Ta, Db, Pa are, as usual, all +V with divalent oxygen O(-II) in MO anions, the lanthanide elements Pr and Gd cannot adopt such high +V oxidation state in similar trioxide anions. Instead, lanthanide element Gd retains its usual +III oxidation state, while Pr retains a +IV oxidation state, thus forcing oxygen into a non-innocent ligand with an uncommon monovalent radical (O) of oxidation state -I. A unique Pr - (O) biradical with highly delocalized unpairing electron density on Pr(IV) and three O atoms is found to be responsible for stabilizing the monovalent-oxygen species in PrO ion, while GdO ion is in fact an OGd(O ) complex with Gd(III). These results show that a naïve assignment of oxidation state of a chemical element without electronic structure analysis can lead to erroneous conclusions. [ABSTRACT FROM AUTHOR]

The scope of Bergman cyclization is expanded computationally by exploring the cyclization in inorganic B, N substituted derivative. This substitution has introduced polarity into the transition state, which resulted in dramatic lowering of the activation barrier. Natural charge distribution throughout the reaction profile has ascertained this hypothesis. Single B and N atom substitution at 1 and 6 terminal positions lowers the activation barrier by almost half of the original value which becomes even lower in the complete B, N analogue. The parent Bergman and single B, N substituted products were characterized by significant biradical character while the complete B, N substituted analogue was characterized by significant zwitterionic character. Reduction in electron delocalization is also observed in the complete B, N substituted analogue., (© 2022 Wiley-VCH GmbH.)

A novel tetrabenzo[a,f,j,o]perylene, namely 'bistetracene' in which two tetracenes are connected side by side with two bonds, was synthesized and characterized. An optical energy gap of about 1.56 eV is derived from the UV/Vis absorption spectrum, showing the low optical gap feature of such zigzag-edged polycyclic aromatic hydrocarbons (PAHs). Theoretical calculations and physical property investigations manifest that such a PAH possesses a prominent biradical character in the ground state, which is the first example among bistetracene derivatives. However, the bistetracene can easily undergo oxidation to tetrabenzo[a,f,j,o]perylene-9,19-dione (diketone) under ambient conditions. Thereby, such zigzag-edged PAH provides insight into understanding the edge state of other expanded homologues, such as peri-tetracenes or peri-pentacenes. [ABSTRACT FROM AUTHOR]

Magic Angle Spinning (MAS) combined with Dynamic Nuclear Polarization (DNP) has been proven in recent years to be a very powerful method for increasing solid-state NMR signals. Since the advent of biradicals such as TOTAPOL to increase the nuclear polarization new classes of radicals, with larger molecular weight and/or different spin properties have been developed. These have led to unprecedented signal gain, with varying results for different experimental parameters, in particular the microwave irradiation strength, the static field, and the spinning frequency. Recently it has been demonstrated that sample spinning imposes DNP enhancement processes that differ from the active DNP mechanism in static samples as upon sample spinning the DNP enhancements are the results of energy level anticrossings occurring periodically during each rotor cycle. In this work we present experimental results with regards to the MAS frequency dependence of the DNP enhancement profiles of four nitroxide-based radicals at two different sets of temperature, 110 and 160 K. In fact, different magnitudes of reduction in enhancement are observed with increasing spinning frequency. Our simulation code for calculating MAS–DNP powder enhancements of small model spin systems has been improved to extend our studies of the influence of the interaction and relaxation parameters on powder enhancements. To achieve a better understanding we simulated the spin dynamics of a single three-spin system { e a - e b - n } during its steady state rotor periods and used the Landau–Zener formula to characterize the influence of the different anti-crossings on the polarizations of the system and their necessary action for reaching steady state conditions together with spin relaxation processes. Based on these model calculations we demonstrate that the maximum steady state nuclear polarization cannot become larger than the maximum polarization difference between the two electrons during the steady state rotor cycle. This study also shows the complexity of the MAS–DNP process and therefore the necessity to rely on numerical simulations for understanding parametric dependencies of the enhancements. Finally an extension of the spin system up to five spins allowed us to probe the first steps of the transfer of polarization from the nuclei coupled to the electrons to further away nuclei, demonstrating a decrease in the spin-diffusion barrier under MAS conditions. [ABSTRACT FROM AUTHOR]

A dinitroxide biradical (dCdO) has the peculiar tendency to crystallize in non-centrosymmetric polar space groups either as a stable dichloromethane solvate or as a pure phase. The molecules arrange with no compensation of their dipolar moment resulting in net macroscopic polar order. [ABSTRACT FROM AUTHOR]

Pseudo-diabatic electronic states buried in Hartree–Fock (HF) wavefunctions are applied to describe bond dissociation processes. The limited variational freedom of HF wavefunctions turns to their own advantage to approximately describe the “diabatic” ionic-pair state or the diabatic biradical state. As an application of the pseudo-diabatic states, CH-bond dissociation processes of normal alkanes and their branched isomers are investigated. Reasonable relationships between the diabatic crossing energies and the number of alkyl substituents on a radical center are obtained in accordance with hyperconjugation theory. [ABSTRACT FROM AUTHOR]

Several indazolo[3,2- b ]quinazolines were synthesized in moderate to good yields by exposing 2-(2-nitrophenyl)-1,2,3,4-tetrahydroquinazolines to UV light (306 nm) in acetonitrile. The scope, limitation, and possible mechanism of this light-mediated reaction as well as the redox-switching properties of the target compounds were explored. Reduction of the colored indazolo[3,2- b ]quinazoline with sodium borohydride resulted in a distinct change to colorless and a sharp increase in fluorescence intensity. The reduced product can be swiftly reverted to the original form by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone oxidation. The reversible redox-switching between the indazolo[3,2- b ]quinazoline and its reduced product utilizing chemical reduction and oxidation as two external stimuli with dual output properties; that is, color change and emission variation was reported. [ABSTRACT FROM AUTHOR]