Isolation and X-ray Crystal Structures of Triarylphosphine Radical Cations

An Isolable Triarylphosphine Radical Cation Electronically Stabilized by Through-Space Radical Delocalization

Synthesis and characterization of a thermally stable triarylphosphine radical cation, [P(8-Br-C10H6)3][BArF24] ([1][BArF24], BArF24 = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate), enabled by stabilization through peri-bromo-substituted naphthalenes, are described. Unlike previously reported phosphine radical cations that rely on sterically bulky substituents for stabilization, our approach leverages electronic stabilization via "through-space" radical delocalization. Single-crystal X-ray diffraction of [1][BArF24] reveals a tricapped tetrahedral geometry, resulting from the spatial proximity of the three bromine atoms to the phosphorus center, differentiated from the trigonal planar geometry observed in the previously reported triarylphosphine radical cations with sterically bulky substituents. EPR spectroscopy shows an isotropic signal with hyperfine couplings to both the phosphorus and the three bromine atoms, indicating spin delocalization over these four atoms and consequent formation of a four-center, seven-electron (4c-7e) bond. DFT computational studies further support the through-space radical delocalization mechanism, revealing that the HOMO of 1 exhibits antibonding character between the phosphorus center and the three adjacent Br atoms, distinct from common triarylphosphines.

Mechanistic studies on single-electron transfer in frustrated Lewis pairs and its application to main-group chemistry

Advances in the field of frustrated Lewis pair (FLP) chemistry have led to the discovery of radical pairs, obtained by a single-electron transfer (SET) from the Lewis base to the Lewis acid. Radical pairs are intriguing for their potential to enable cooperative activation of challenging substrates (e.g., CH4, N2) in a homolytic fashion, as well as the exploration of novel radical reactions. In this review, we will cover the two known mechanisms of SET in FLPs-thermal and photoinduced-along with methods (i.e., CV, DFT, UV-vis) to predict the mechanism and to characterise the involved electron donors and acceptors. Furthermore, the available techniques (i.e., EPR, UV-vis, transient absorption spectroscopy) for studying the corresponding radical pairs will be discussed. Initially, two model systems (PMes3/CPh3+ and PMes3/B(C6F5)3) will be reviewed to highlight the difference between a thermal and a photoinduced SET mechanism. Additionally, three cases are analysed to provide further tools and insights into characterizing electron donors and acceptors, and the associated radical pairs. Firstly, a thermal SET process between LiHMDS and [TEMPO][BF4] is discussed. Next, the influence of Lewis acid complexation on the electron acceptor will be highlighted to facilitate a SET between (pBrPh)3N and TCNQ. Finally, an analysis of sulfonium salts as electron acceptors will demonstrate how to manage systems with rapidly decomposing radical species. This framework equips the reader with an expanded array of tools for both predicting and characterizing SET events within FLP chemistry, thereby enabling its extension and application to the broader domain of main-group (photo)redox chemistry.

Sequential radical and cationic reactivity at separated sites within one molecule in solution

Distonic radical cations (DRCs) with spatially separated charge and radical sites are expected to show both radical and cationic reactivity at different sites within one molecule. However, such "dual" reactivity has rarely been observed in the condensed phase. Herein we report the isolation of crystalline 1λ2,3λ2-1-phosphonia-3-phosphinyl-cyclohex-4-enes 2a,b˙+, which can be considered delocalized DRCs and were completely characterized by crystallographic, spectroscopic, and computational methods. These DRCs contain a radical and cationic site with seven and six valence electrons, respectively, which are both stabilized via conjugation, yet remain spatially separated. They exhibit reactivity that differs from that of conventional radical cations (CRCs); specifically they show sequential radical and cationic reactivity at separated sites within one molecule in solution.

Photoredox-Catalyzed Selective α-Scission of PR3-OH Radicals to Access Hydroalkylation of Alkenes

Photoinduced generation of phosphoranyl radicals offers a versatile strategy to access a variety of synthetically valuable radicals. A long-standing challenge remains in the regulation of phosphoranyl radical to undergo α-scission pathway, although the β-scission mode has been intensively studied. We herein developed an unprecedented protocol for selective α-scission of the P(OH)R3 radical intermediate under photocatalytic conditions. This efficient P-C bond cleavage via α-scission of the P(OH)R3 radicals has been successfully utilized in the alkylation/fluoroalkylation of alkenes.

Homolytic C-H Bond Activation by Phosphine-Quinone-Based Radical Ion Pairs

Herein, we present the formation of transient radical ion pairs (RIPs) by single-electron transfer (SET) in phosphine-quinone systems and explore their potential for the activation of C-H bonds. PMes3 (Mes=2,4,6-Me3 C6 H2 ) reacts with DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) with formation of the P-O bonded zwitterionic adduct Mes3 P-DDQ (1), while the reaction with the sterically more crowded PTip3 (Tip=2,4,6-iPr3 C6 H2 ) afforded C-H bond activation product Tip2 P(H)(2-[CMe2 (DDQ)]-4,6-iPr2 -C6 H2 ) (2). UV/Vis and EPR spectroscopic studies showed that the latter reaction proceeds via initial SET, forming RIP [PTip3 ]⋅+ [DDQ]⋅- , and subsequent homolytic C-H bond activation, which was supported by DFT calculations. The isolation of analogous products, Tip2 P(H)(2-[CMe2 {TCQ-B(C6 F5 )3 }]-4,6-iPr2 -C6 H2 ) (4, TCQ=tetrachloro-1,4-benzoquinone) and Tip2 P(H)(2-[CMe2 {oQtBu -B(C6 F5 )3 }]-4,6-iPr2 -C6 H2 ) (8, oQtBu =3,5-di-tert-butyl-1,2-benzoquinone), from reactions of PTip3 with Lewis-acid activated quinones, TCQ-B(C6 F5 )3 and oQtBu -B(C6 F5 )3 , respectively, further supports the proposed radical mechanism. As such, this study presents key mechanistic insights into the homolytic C-H bond activation by the synergistic action of radical ion pairs.

Syntheses and Structures of 5-Membered Heterocycles Featuring 1,2-Diphospha-1,3-Butadiene and Its Radical Anion

LGa(P2 OC)cAAC 2 features a 1,2-diphospha-1,3-butadiene unit with a delocalized π-type HOMO and a π*-type LUMO according to DFT calculations. [LGa(P2 OC)cAAC][K(DB-18-c-6)] 3[K(DB-18-c-6] containing the 1,2-diphospha-1,3-butadiene radical anion 3⋅- was isolated from the reaction of 2 with KC8 and dibenzo-18-crown-6. 3 reacted with [Fc][B(C6 F5 )4 ] (Fc=ferrocenium) to 2 and with TEMPO to [L-H Ga(P2 OC)cAAC][K(DB-18-c-6)] 4[K(DB-18-c-6] containing the 1,2-diphospha-1,3-butadiene anion 4- . The solid state structures of 2, 3K(DB-18-c-6], and 4[K(DB-18-c-6] were determined by single crystal X-ray diffraction (sc-XRD).

Transformation of phosphorus species during phosphoric acid-assisted pyrolysis of lignocellulose

Phosphoric acid-assisted pyrolysis (PAAP) is a pyrolysis technique with potential for the engineered and environmental application. Nevertheless, the volatilisation, immobilisation, and dissolution of phosphorus (P) species have been neglected during PAAP of lignocellulose. Therefore, we compared the transformation of P species with direct-pyrolysis and PAAP system, using multiple techniques including gas chromatography tandem mass spectrometry (GCMS) and ³¹P nuclear magnetic resonance (NMR). It was also investigated that the properties of pyrogenic and modified carbons obtained from lignocellulose pyrolysis at 200-650 °C. As the temperature increased, volatile P species evolved into gas-phase during PAAP, inhibiting the formation of the macromolecular volatile components. Compared with pyrogenic carbons, modified carbons with more aromatic structures experienced a higher degree of dehydration and cyclisation via catalytic crosslinking reaction. PAAP system facilitated more generation of persistent free radical (PFR) below 500 °C and the attenuation of PFR signals was observed at 500-650 °C, which may be associated with the sequestration and elimination of P species between carbon matrix. Notably, three configurations of C₃PO, CPO, and COP were the major combinations of P and C elements on modified carbons. Increased gaseous P and decreased soluble P were observed with elevated temperatures in PAAP system. The species proportion of immobilised P clearly demonstrated the transformation of partial P species from inorganic to organic through pyrolysis. The immobilised P could serve as a potential sustained-release source participating in P biogeochemical cycles. These findings are fundamental for the technical design of lignocellulose pyrolysis.

Revisiting the Reaction of IPr with Tritylium: An Alternative Mechanistic Pathway

Despite a recent proposal on the mechanism of a single-electron transfer (SET) process between tritylium and 2,6-bis(diisopropylphenyl)imidazol-2-ylidene (IPr) based on evidence of transient IPr radical cation intermediate ([IPr]⋅⁺ ) formation, such oxidation is still contentious because of the high oxidation potential of N-heterocyclic carbenes. Our experimental analysis indicates that the appearance of deep purple color, previously considered to be from transient [IPr]⋅⁺ , originates from a zwitterionic intermediate (3 a), not a radical cation. Here, we propose an alternative mechanism for the reaction involving tritylium and IPr. This mechanism is noteworthy for explaining how [NHC-H]⁺ can be generated without the formation of transient [NHC]⋅⁺ , which has been frequently proposed as an intermediate for the reaction between NHC and oxidants. These results also show that a transient strong single-electron donor (3 a) could be generated by the alternative mechanism for oxidants using NHCs, which is a more feasible explanation for the reactivity of NHCs with oxidants.

Manipulating D-A interaction to achieve stable photoinduced organic radicals in triphenylphosphine crystals

New strategies for the design and synthesis of stable organic radicals without additives are highly desirable. Herein, we design a series of donor-acceptor structured triarylphosphines and disclose the fast color change triggered by UV-irradiation in the crystalline state. Photoinduced organic radicals are undoubtedly verified and proved to be the reason for the color change by time-dependent and quantitative electron paramagnetic resonance analysis, X-ray crystallographic analysis, and theoretical calculations. It is revealed that the intrinsic symmetry breaking of peripheral architecture helps to form continuous molecular chains by donor-acceptor counterpart pairing. Intermolecular electron-transfer occurs among molecular chains and results in radical ion pairs upon photoirradiation.

Modulating the frontier orbitals of L(X)Ga-substituted diphosphenes [L(X)GaP]2 (X = Cl, Br) and their facile oxidation to radical cations

Modulating the electronic structures of main group element compounds is crucial to control their chemical reactivity. Herein we report on the synthesis, frontier orbital modulation, and one-electron oxidation of two L(X)Ga-substituted diphosphenes [L(X)GaP]2 (X = Cl 2a, Br 2b; L = HC[C(Me)N(Ar)]2, Ar = 2,6-i-Pr2C6H3). Photolysis of L(Cl)GaPCO 1 gave [L(Cl)GaP]22a, which reacted with Me3SiBr with halide exchange to [L(Br)GaP]22b. Reactions with MeNHC (MeNHC = 1,3,4,5-tetramethylimidazol-2-ylidene) gave the corresponding carbene-coordinated complexes L(X)GaPP(MeNHC)Ga(X)L (X = Cl 3a, Br 3b). DFT calculations revealed that the carbene coordination modulates the frontier orbitals (i.e. HOMO/LUMO) of diphosphenes 2a and 2b, thereby affecting the reactivity of 3a and 3b. In marked contrast to diphosphenes 2a and 2b, the cyclic voltammograms (CVs) of the carbene-coordinated complexes each show one reversible redox event at E 1/2 = -0.65 V (3a) and -0.36 V (3b), indicating their one-electron oxidation to the corresponding radical cations as was confirmed by reactions of 3a and 3b with the [FeCp2][B(C6F5)4], yielding the radical cations [L(X)GaPP(MeNHC)Ga(X)L]B(C6F5)4 (X = Cl 4a, Br 4b). The unpaired spin in 4a (79%) and 4b (80%) is mainly located at the carbene-uncoordinated phosphorus atoms as was revealed by DFT calculations and furthermore experimentally proven in reactions with n Bu3SnH, yielding the diphosphane cations [L(X)GaPHP(MeNHC)Ga(X)L]B(C6F5)4 (X = Cl 5a, Br 5b). Compounds 2-5 were fully characterized by NMR and IR spectroscopy as well as by single crystal X-ray diffraction (sc-XRD), and compounds 4a and 4b were further studied by EPR spectroscopy, while their bonding nature was investigated by DFT calculations.

Recent advances in stable main group element radicals: preparation and characterization

Radical species are significant in modern chemistry. Their unique chemical bonding and novel physicochemical properties play significant roles not only in fundamental chemistry, but also in materials science. Main group element radicals are usually transient due to their high reactivity. Highly stable radicals are often stabilized by π-delocalization, sterically demanding ligands, carbenes and weakly coordinating anions in recent years. This review presents the recent advances in the synthesis, characterization, reactivity and physical properties of isolable main group element radicals.

Strategies to Generate Nitrogen-centered Radicals That May Rely on Photoredox Catalysis: Development in Reaction Methodology and Applications in Organic Synthesis

For more than 70 years, nitrogen-centered radicals have been recognized as potent synthetic intermediates. This review is a survey designed for use by chemists engaged in target-oriented synthesis. This review summarizes the recent paradigm shift in access to and application of N-centered radicals enabled by visible-light photocatalysis. This shift broadens and streamlines approaches to many small molecules because visible-light photocatalysis conditions are mild. Explicit attention is paid to innovative advances in N-X bonds as radical precursors, where X = Cl, N, S, O, and H. For clarity, key mechanistic data is noted, where available. Synthetic applications and limitations are summarized to illuminate the tremendous utility of photocatalytically generated nitrogen-centered radicals.

Phosphorus radicals and radical ions

Synthesis and characterization of isolable radicals of main-group elements have been a long-pursued quest. Although there has been considerable progress in this area, particularly in isolating carbon- radicals, the isolation...

Sterically Crowded Tris(2-(trimethylsilyl)phenyl)phosphine - Is it Still a Ligand?

Tris(2-(trimethylsilyl)phenyl)phosphine, P( o -TMSC 6 H 4 ) 3 , was synthesised and characterised in solution and in the solid state. The large steric bulk prevents most reactions of the phosphorus donor and makes the compound air stable both in the solid state as well as in solution. This shielded phosphine can still undergo three reactions, namely protonation, oxidation to the phosphine oxide under harsh conditions and complexation to Au I , thus forming a complex with linear coordination. Unexpectedly, complexation was unsuccessful with a range of other metal cations. Neither Pd II , Pt II , Zn II nor Hg II reacted and even the remaining coinage metal cations Cu I and Ag I could not be coordinated. Both the parent molecule as well as the reaction products were structurally characterised by single crystal X-ray di raction, and the conformational change of geometry required to accommodate the additional atoms was analysed in detail. Apart from chemical oxidation with H 2 O 2 , P( o -TMSC 6 H 4 ) 3 displays reversible electrochemical oxidation with a potential not unlike the one of sterically unencumbered phosphines for which the oxidation is usually not reversible. P( o -TMSC 6 H 4 ) 3 can thus be considered a model compound for the investigation of the electronic properties of sterically unencumbered phosphines.

One- and Two-Electron Transfer Oxidation of 1,4-Disilabenzene with Formation of Stable Radical Cations and Dications

Electron‐transferable oxidants such as B(C₆F₅)₃/nBuLi, B(C₆F₅)₃/LiB(C₆F₅)₄, B(C₆F₅)₃/LiHBEt₃, Al(C₆F₅)₃/(o‐RC₆H₄)AlH₂ (R=N(CMe₂CH₂)₂CH₂), B(C₆F₅)₃/AlEt₃, Al(C₆F₅)₃, Al(C₆F₅)₃/nBuLi, Al(C₆F₅)₃/AlMe₃, (CuC₆F₅)₄, and Ag₂SO₄, respectively were employed for reactions with (L)₂Si₂C₄(SiMe₃)₂(C₂SiMe₃)₂ (L=PhC(NtBu)₂, 1). The stable radical cation [1]^+. was formed and paired with the anions [nBuB(C₆F₅)₃]⁻ (in 2), [B(C₆F₅)₄]⁻ (in 3), [HB(C₆F₅)₃]⁻ (in 4), [EtB(C₆F₅)₃]⁻ (in 5), {[(C₆F₅)₃Al]₂(μ‐F)]⁻ (in 6), [nBuAl(C₆F₅)₃]⁻ (in 7), and [Cu(C₆F₅)₂]⁻ (in 8), respectively. The stable dication [1]²⁺ was also generated with the anions [EtB(C₆F₅)₃]⁻ (9) and [MeAl(C₆F₅)₃]⁻ (10), respectively. In addition, the neutral compound [(L)₂Si₂C₄(SiMe₃)₂(C₂SiMe₃)₂][μ‐O₂S(O)₂] (11) was obtained. Compounds 2–11 are characterized by UV‐vis absorption spectroscopy, X‐ray crystallography, and elemental analysis. Compounds 2–8 are analyzed by EPR spectroscopy and compounds 9–11 by NMR spectroscopy. The structure features are discussed on the central Si₂C₄‐rings of 1, [1]^+., [1]²⁺, and 11, respectively.

Highly Efficient Production of Heteroarene Phosphonates by Dichromatic Photoredox Catalysis

A new strategy to achieve efficient aerobic phosphorylation of five-membered heteraroenes with excellent yields using dichromatic photoredox catalysis in a gel-based nanoreactor is described here. The procedure involves visible aerobic irradiation (cold white LEDs) of a mixture containing the heteroarene halide, trisubstituted phospite, N,N-diisopropylethylamine (DIPEA) as sacrificial agent, and catalytic amounts of 9,10-dicyanoanthracene (DCA) in the presence of an adequate gelator, which permits a faster process than at the homogeneous phase. The methodology, which operates by a consecutive photoinduced electron transfer (ConPET) mechanism, has been successfully applied to the straightforward and clean synthesis of a number of different heteroarene (furan, thiophene, selenophene, pyrrole, oxazole, or thioxazole) phosphonates, extending to the late-stage phosphonylation of the anticoagulant rivaroxaban. Strategically, employment of cold white light is critical since it provides both selective wavelengths for exciting first DCA (blue region) and subsequently its corresponding radical anion DCA^•- (green region). The resultant strongly reducing excited agent DCA^•-* is capable of even activate five-membered heteroarene halides (Br, Cl) with high reduction potentials (∼-2.7 V) to effect the C(sp²)-P bond formation. Spectroscopic and thermodynamic studies have supported the proposed reaction mechanism. Interestingly, the rate of product formation has been clearly enhanced in gel media because reactants can be presumably localized not only in the solvent pools but also through to the fibers of the viscoelastic gel network. This has been confirmed by field-emission scanning electron microscopy images where a marked densification of the network has been observed, modifying its fibrillary morphology. Finally, rheological measurements have shown the resistance of the gel network to the incorporation of the reactants and the formation of the desired products.

Persistent 2c-3e σ-bonded heteronuclear radical cations centered on S/Se and P/As atoms

The two-center three-electron (2c-3e) bonded species are important in chemical and biological science. Reported isolable 2c-3e σ-bonded species are usually constructed in homoatomic radicals. The one-electron oxidation of main-group heteronuclear species Nap(SPh)(P(Mes)2) (1), Nap(SePh)(P(Mes)2) (2), Nap(SPh)(As(Mes)2) (3) and Nap(SePh)(As(Mes)2) (4) produced persistent radical cations 1˙+-4˙+ in solution. Large couplings of heteroatoms in EPR spectra of 1˙+-4˙+, shorter bond distances and bigger Wiberg bond orders of Ch-Pn in 1˙+-4˙+ than those in 1-4 in DFT calculations indicate large amounts of spin densities over heteroatoms and the formation of 2c-3e σ-bonds between chalcogen and pnicogen atoms. This work provides evidence of 2c-3e σ-bonds constructed between main-group heteronuclears and rare examples of radical cations involving three-electron σ-bonds between S/Se and P/As atoms.

Reversible Stereoisomerization of 1,3-Diphosphetane Frameworks Revealed by a Single-Electron Redox Approach

The discovery of pyramidal inversion has continued to impact modern organic and organometallic chemistry. Sequential alkylation reactions of an N-heterocyclic carbene (NHC) ligated dicarbondiphosphide 1 with RI (R = Me, Et, or iBu) and ZnMe₂ give rise to the highly stereoselective synthesis of cis-1,3-diphosphetanes 3. cis-3 is conformationally favorable at room temperature, whereas inversion to trans-3 is observed at 110 °C. One-electron oxidation of cis-3 with Fc⁺(BAr^F) (Fc = [Fe(C₅H₅)₂]; BAr^F = [B(3,5-(CF₃)₂C₆H₃)₄)]^-) leads to the stereoselective formation of trans-1,3-diphosphetane radical cation salts 3^•+(BAr^F), which can be reversibly transformed to cis-3 upon one-electron reduction. Salts 3^•+(BAr^F) represent the first examples of 1,3-diphosphetane radical cations. These results provide a potential application of planar four-membered heterocycle-based building blocks for electrically fueled molecular switches.

Site-Selective Csp3-Csp/Csp3-Csp2 Cross-Coupling Reactions Using Frustrated Lewis Pairs

The donor-acceptor ability of frustrated Lewis pairs (FLPs) has led to widespread applications in organic synthesis. Single electron transfer from a donor Lewis base to an acceptor Lewis acid can generate a frustrated radical pair (FRP) depending on the substrate and energy required (thermal or photochemical) to promote an FLP into an FRP system. Herein, we report the Csp3-Csp cross-coupling reaction of aryl esters with terminal alkynes using the B(C6F5)3/Mes3P FLP. Significantly, when the 1-ethynyl-4-vinylbenzene substrate was employed, the exclusive formation of Csp3-Csp cross-coupled products was observed. However, when 1-ethynyl-2-vinylbenzene was employed, solvent-dependent site-selective Csp3-Csp or Csp3-Csp2 cross-coupling resulted. The nature of these reaction pathways and their selectivity has been investigated by extensive electron paramagnetic resonance (EPR) studies, kinetic studies, and density functional theory (DFT) calculations both to elucidate the mechanism of these coupling reactions and to explain the solvent-dependent site selectivity.

Probing the Charge Transfer in a Frustrated Lewis Pair by Resonance Raman Spectroscopy and DFT Calculations

A classical Lewis adduct derives from a covalent bond between a Lewis acid and a base. When the adduct formation is precluded by means of steric hindrance the association of the respective acid-base molecular system is defined as a frustrated Lewis pair (FLP). In this work, the archetypal FLP Mes₃ P/B(C₆ F₅ )₃ was characterized for the first time by resonance Raman spectroscopy, and the results were supported by density functional theory (DFT) calculations. The charge transfer nature of the lowest energy electronic transition, from phosphine to borane, was confirmed by the selective enhancement of the Raman bands associated to the FLP chromophore at resonance condition. Herein, we demonstrate the use of resonance Raman spectroscopy as a distinguished technique to probe the weak interaction involved in FLP chemistry.

Alkene homologation via visible light promoted hydrophosphination using triphenylphosphonium triflate

A hydrophosphination reaction of alkenes with triphenylphosphonium triflate under photocatalytic conditions is described. The reaction is promoted by naphthalene-fused N-acylbenzimidazole and is believed to proceed through intermediate formation of a phosphinyl radical cation. The resulting phosphonium salts are directly involved in the Wittig reaction leading to homologated alkenes.

A Room-Temperature Stable Distonic Radical Cation

Distonic radical cations (DRCs) with spatially separated charge and radical sites have, so far, largely been observed by gas-phase mass spectrometry and/or matrix isolation spectroscopy work. Herein, we disclose the isolation of a crystalline dicarbondiphosphide-based β-distonic radical cation salt 3^.+ (BARF) (BARF=[B(3,5-(CF₃ )₂ C₆ H₃ )₄ )]^- ) stable at room temperature and formed by a one-electron-oxidation-induced intramolecular skeletal rearrangement reaction. Such a species has been validated by electron paramagnetic resonance (EPR) spectroscopy, single-crystal X-ray diffraction, UV/Vis spectroscopy and density functional theory (DFT) calculations. Compound 3^.+ (BARF) exhibits a large majority of spin density at a two-coordinate phosphorus atom (0.74 a.u.) and a cationic charge located predominantly at the four-coordinate phosphorus atom (1.53 a.u.), which are separated by one carbon atom. This species represents an isolable entity of a phosphorus radical cation that is the closest to a genuine phosphorus DRC to date.

Trialkylphosphines Having a Bulky Phosphacyclopentane Backbone: Structural and Redox Properties Depending on the Exocyclic Alkyl Groups and EPR Observation of a Persistent Trialkylphosphine Radical Cation

Bulky phosphines and their redox properties have received increased attention in the view of useful auxiliary ligands for transition metal catalysts and Lewis-base components of frustrated Lewis pairs for chemical transformations. Herein we report the synthesis, structure, and properties of a series of trialkylphosphines 2R (R = methyl, ethyl, isopropyl, tert-butyl, 1-adamantyl) that possess the bulky 2,2,5,5-tetrakis(trimethylsilyl)-1-phosphacyclopentane as a structural backbone. Among these phosphines, 2Ad, which contains an adamantyl moiety, has a very large buried volume (%V_bur) for a trialkylphosphine (62.0) and shows a quasi-reversible oxidative wave at a lower oxidation potential (-0.12 V in CH₂Cl₂, vs ferrocene/ferrocenium couple) by cyclic voltammetry. The reaction of 2Ad with AgPF₆ afforded a cationic silver aquo complex [Ag(2Ad)(H₂O)]⁺[PF₆]^-, whereas the reaction with NOSbF₆ gave a persistent phosphine radical cation [2Ad]^•+. Based on the EPR spectra and DFT studies, the spin and positive charge of [2Ad]^•+ are localized on the phosphorus atom.

Phosphoranyl Radical Fragmentation Reactions Driven by Photoredox Catalysis

Photocatalytic generation of phosphoranyl radicals is fast emerging as an essential method for the generation of diverse and valuable radicals, typically via deoxygenation or desulfurization processes. This Perspective is a comprehensive evaluation of all studies using phosphoranyl radicals as tunable mediators in photoredox catalysis, highlighting how two distinct methods for phosphoranyl radical formation (radical addition and nucleophilic addition) can be used to generate versatile radical intermediates with diverse reactivity profiles.

Ligand Effects on the Electronic Structure of Heteroleptic Antimony-Centered Radicals

We report on the structures of three unprecedented heteroleptic Sb-centered radicals [L(Cl)Ga](R)Sb. (2-R, R=B[N(Dip)CH]2 2-B, 2,6-Mes2 C6 H3 2-C, N(SiMe3 )Dip 2-N) stabilized by one electropositive metal fragment [L(Cl)Ga] (L=HC[C(Me)N(Dip)]2 , Dip=2,6-i-Pr2 C6 H3 ) and one bulky B- (2-B), C- (2-C), or N-based (2-N) substituent. Compounds 2-R are predominantly metal-centered radicals. Their electronic properties are largely influenced by the electronic nature of the ligands R, and significant delocalization of unpaired-spin density onto the ligands was observed in 2-B and 2-N. Cyclic voltammetry (CV) studies showed that 2-B undergoes a quasi-reversible one-electron reduction, which was confirmed by the synthesis of [K([2.2.2]crypt)][L(Cl)GaSbB[N(Dip)CH]2 ] ([K([2.2.2]crypt)][2-B]) containing the stibanyl anion [2-B]- , which was shown to possess significant Sb-B multiple-bonding character.

Isolation of singlet carbene derived 2-phospha-1,3-butadienes and their sequential one-electron oxidation to radical cations and dications

A synthetic strategy for the 2-phospha-1,3-butadiene derivatives [{(IPr)C(Ph)}P(cAACMe)] (3a) and [{(IPr)C(Ph)}P(cAACCy)] (3b) (IPr = C{(NDipp)CH}2, Dipp = 2,6-iPr2C6H3; cAACMe = C{(NDipp)CMe2CH2CMe2}; cAACCy = C{(NDipp)CMe2CH2C(Cy)}, Cy = cyclohexyl) containing a C[double bond, length as m-dash]C-P[double bond, length as m-dash]C framework has been established. Compounds 3a and 3b have a remarkably small HOMO-LUMO energy gap (3a: 5.09; 3b: 5.05 eV) with a very high-lying HOMO (-4.95 eV for each). Consequently, 3a and 3b readily undergo one-electron oxidation with the mild oxidizing agent GaCl3 to afford radical cations [{(IPr)C(Ph)}P(cAACR)]GaCl4 (R = Me 4a, Cy 4b) as crystalline solids. The main UV-vis absorption band for 4a and 4b is red-shifted with respect to that of 3a and 3b, which is associated with the SOMO related transitions. The EPR spectra of compounds 4a and 4b each exhibit a doublet due to coupling of the unpaired electron with the 31P nucleus. Further one-electron removal from the radical cations 4a and 4b is also feasible with GaCl3, affording the dications [{(IPr)C(Ph)}P(cAACR)](GaCl4)2 (R = Me 5a, Cy 5b) as yellow crystals. The molecular structures of compounds 3-5 have been determined by X-ray diffraction and analyzed by DFT calculations.

Stabilization and isolation of radical cation and dication salts of a tetrathiafulvalene derivative functionalized with amino groups

The radical cation and dication salts of a tetrathiafulvalene derivative functionalized with amino groups have been stabilized and isolated by chemical oxidation. Comprehensive research on their structure–property relationship was fully performed.

Isolable cyclic radical cations of heavy main-group elements

The first stable radical cations bearing both heavy group 14 and 15 elements have been isolated and fully characterized.

Isolation of singlet carbene derived 2-arsa-1,3-butadiene radical cations and dications

2-Arsa-1,3-butadienes (L)As(cAACR) (L = PhC[double bond, length as m-dash]C{(NDipp)CH}2, Dipp = 2,6-iPr2C6H3; cAACR = C{(NDipp)CMe2CH2C(R)}, R = Me22a, R = cyclohexyl (Cy) 2b) and the corresponding radical cations [(L)As(cAACR)]GaCl4 (R = Me23a, Cy 3b) and dications [(L)As(cAACR)](GaCl4)2 (R = Me 4a, Cy 4b) featuring a C[double bond, length as m-dash]C-As[double bond, length as m-dash]C π-conjugated framework are reported.

Radicals derived from Lewis acid/base pairs

While conventional approaches to stabilizing main group radicals have involved the use of Lewis acids or bases, this tutorial review focuses on new avenues to main group radicals derived from combinations of donor and acceptor molecules.

Diphosphene radical cations and dications with a π-conjugated C2P2C2-framework

The synthesis and characterization of the crystalline diphosphene radical cations [{(NHC)C(Ph)}P]2(GaCl4) (NHC = IPr = C{(NDipp)CH}23, SIPr = C{(NDipp)CH2}24; Dipp = 2,6-iPr2C6H3) and dications [{(NHC)C(Ph)}P]2(GaCl4)2 (NHC = IPr 5, SIPr 6) featuring a π-conjugated C2P2C2-framework has been reported.

Single-Electron Transfer Reactions in Frustrated and Conventional Silylium Ion/Phosphane Lewis Pairs

Silylium ions undergo a single-electron reduction with phosphanes, leading to transient silyl radicals and the corresponding stable phosphoniumyl radical cations. As supported by DFT calculations, phosphanes with electron-rich 2,6-disubstituted aryl groups are sufficiently strong reductants to facilitate this single-electron transfer (SET). Frustration as found in kinetically stabilized triarylsilylium ion/phosphane Lewis pairs is not essential, and silylphosphonium ions, which are generated by conventional Lewis adduct formation of solvent-stabilized trialkylsilylium ions and phosphanes, engage in the same radical mechanism. The trityl cation, a Lewis acid with a higher electron affinity, even oxidizes trialkylphosphanes, such as tBu₃ P, which does not react with either B(C₆ F₅ )₃ or silylium ions.