An Isolable and Monomeric Phosphorus Radical That Is Resonance-Stabilized by the Vanadium(IV/V) Redox Couple

Expanding the Landscape of Phosphorous-Based Open Shell Species: Stable Mono-, Di-, and Trianionic Radicals Based on a Contorted Triphosphaalkene

The stepwise reduction of the highly contorted truxene-based triphosphaalkene 1 using KC8 led to the isolation of mono-, di-, and tri-anionic species. The solid-state molecular structures of mono- and diradical anionic species were elucidated by single crystal X-ray diffractions, revealing elongated P-C bonds and a pronounced "indene" aromatization compared to the parent system. All three radical species displayed distinct Electron Paramagnetic Resonance (EPR) spectra, providing compelling evidence for the open-shell electronic configuration of both the diradical and triradical species-an observation unprecedented in any previously reported phosphorous-based anionic polyradicals. Mulliken spin density calculations revealed a dominant localization of radical spin on a single phosphorous atom in the monoanion. In the dianion, spin localization is observed on two phosphorous atoms (~34 % each), with a minor contribution from the third phosphorous (0.13 %), while the trianion demonstrates a uniform distribution of spin density (~30 %) across each phosphorous atom.

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.

A quest for stable phosphonyl radicals: limitations and possibilities of carbocyclic backbones and bulky substituents

Although phosphonyl radicals play an important role as transient species in many chemical transformations, such as photoinitiated polymerisation reactions, permanently stable phosphonyl radicals are yet to be discovered. In this computational study, we aim at a conceptual understanding of the electronic effects influencing the stabilities of phosphonyl radicals through computing radical stabilisation energies (RSEs) for a large set of phosphonyl radicals with carbocyclic backbones. The studied radicals exhibit ring sizes varying from 3- to 7-membered with full saturation or different grades of unsaturation adjacent to the P-centre in an endo or exocyclic fashion. To gain deeper insight into the stabilisation effects and delocalisation, the geometrical aspects, electronic structures, and spin distributions of the radicals were scrutinised. The five-membered, fully unsaturated ring (phospholyl oxide), which has a planar structure, offers the most substantial electronic stabilisation. By embedding this ring into a more extended π-system, the possibility of gaining further stabilisation was also explored. To screen the effect of steric congestion on the stabilities of previously selected radicals toward dimerisation, a large number of bulky substituents with different sizes and shapes were systematically investigated. Our results outline that stable phosphonyl radicals seem accessible, provided that the electronic stabilisation effects are supplemented by well-designed bulky substituents.

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).

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.

Stability of Carbocyclic Phosphinyl Radicals: Effect of Ring Size, Delocalization, and Sterics

In this computational study, we report on the stability of cyclic phosphinyl radicals with an aim for a systematical assessment of stabilization effects. The radical stabilization energies (RSEs) were calculated using isodesmic reactions for a large number of carbocyclic radicals possessing different ring sizes and grades of unsaturation. In general, the RSE values range from −1.2 to −14.0 kcal·mol^–1, and they show practically no correlation with the spin populations at the P-centers. The RSE values correlate with the reaction Gibbs free energies calculated for the dimerization of the studied simple radicals. Therefore, the more easily accessible RSE values offer a cost-effective estimation of global stability in a straightforward manner. To explore the effect of unsaturation on the RSE values, delocalization energies were determined using appropriate isodesmic reactions. Introducing unsaturations beside the P-center into the backbone of the rings leads to an additive increase in the magnitude of the delocalization energy (∼10, 20, and 30 kcal·mol^–1, respectively, for radicals with one, two, and three C=C bonds in the conjugation). Parallelly, the spin populations at the P-centers also dwindle gradually by ∼0.1 e in the same order, indicating that the lone electron delocalizes over the π-system. Radicals containing exocyclic C=C π-bonds were also investigated, and all of these radicals have rather similar stabilities independently of the ring size, outlining the primary importance of the two exocyclic π-bonds in the conjugation. Among the radicals involved in our study, those with the best electronic stabilization are the unsaturated three-, five-, six-, and seven-membered rings containing the maximum number of conjugated vinyl fragments. The largest delocalization energy of 31.5 kcal·mol^–1 and the lowest obtained spin population of 0.665 e were found for the fully unsaturated seven-membered radical (phosphepin derivative). Importantly, the electronic stabilization effects alone are insufficient for stabilizing the radicals in monomeric forms epitomized by the exothermic dimerization energies (−40 to −58 kcal·mol^–1). Therefore, it is essential to apply sterically demanding bulky substituents on the α-C-atoms. Tweaking the steric congestion enabled us to propose radicals that are expected to be stable against dimerization and, consequently, may be realistic target species for synthetic investigations. The effects contributing to the stability of radicals having sterically encumbered substituents have also been explored.

To systematically evaluate the stabilization effects, the radical stabilization energies of various carbocyclic phosphinyl radicals having saturated backbones or unsaturation(s) in either endocyclic or exocyclic manner have been determined and analyzed. As the electronic stabilization is alone insufficient to hamper the possible dimerization of these species, the effect of several sterically demanding substituents has been explored for the congeners with best electronic stabilizations, thus enabling us to propose synthetically accessible candidates in the future.

Radical Reactivity of the Biradical [⋅P(μ-NTer)2 P⋅] and Isolation of a Persistent Phosphorus-Cantered Monoradical [⋅P(μ-NTer)2 P-Et]

The activation of C-Br bonds in various bromoalkanes by the biradical [⋅P(μ-NTer)2 P⋅] (1) (Ter=2,6-bis-(2,4,6-trimethylphenyl)-phenyl) is reported, yielding trans-addition products of the type [Br-P(μ-NTer)2 P-R] (2), so-called 1,3-substituted cyclo-1,3-diphospha-2,4-diazanes. This addition reaction, which represents a new easy approach to asymmetrically substituted cyclo-1,3-diphospha-2,4-diazanes, was investigated mechanistically by different spectroscopic methods (NMR, EPR, IR, Raman); the results suggested a stepwise radical reaction mechanism, as evidenced by the in-situ detection of the phosphorus-centered monoradical [⋅P(μ-NTer)2 P-R].< To provide further evidence for the radical mechanism, [⋅P(μ-NTer)2 P-Et] (3Et⋅) was synthesized directly by reduction of the bromoethane addition product [Br-P(μ-NTer)2 P-Et] (2 a) with magnesium, resulting in the formation of the persistent phosphorus-centered monoradical [⋅P(μ-NTer)2 P-Et], which could be isolated and fully characterized, including single-crystal X-ray diffraction. Comparison of the EPR spectrum of the radical intermediate in the addition reaction with that of the synthesized new [⋅P(μ-NTer)2 P-Et] radical clearly proves the existence of radicals over the course of the reaction of biradical [⋅P(μ-NTer)2 P⋅] (1) with bromoethane. Extensive DFT and coupled cluster calculations corroborate the experimental data for a radical mechanism in the reaction of biradical [⋅P(μ-NTer)2 P⋅] with EtBr. In the field of hetero-cyclobutane-1,3-diyls, the demonstration of a stepwise radical reaction represents a new aspect and closes the gap between P-centered biradicals and P-centered monoradicals in terms of radical reactivity.

On metal coordination of neutral open-shell P-ligands focusing on phosphanoxyls, their electron residence and reactivity

This feature article highlights the discovery and development of phosphanoxyl complex chemistry starting from (neutral) low-coordinate phosphorus radicals and the quest of metal ligation effects. We describe synthesis and reactions of precursors, namely 2,2,6,6-tetramethylpiperidinoxyl (TEMPO) substituted phosphane tungsten(0) complexes. Trapping reactions of transient phosphanoxyl complexes, formed via thermal homolytic N-O bond cleavage, as well as their use in radical polymerisations are illustrated, thus revealing an interesting reactivity dichotomy. DFT calculations provide insight into thermal stabilities of precursors and the resulting spin density distributions (SDDs) in these reactive intermediates. Systematic studies on the dependance of the electron delocalisation in phosphanoxyl complexes have been performed examining different substitution pattern at phosphorus and different co-ligand combinations at the tungsten(0) center. Preliminary results on Mn and Fe complexes are reported.

1,3,2-Diheterophospholane complexes: access to new tuneable precursors of phosphanoxyl complexes and P-functional polymers

Synthesis of a testbed of P-H functional diheterophospholane complexes (3 and 6a,b) with no or little steric bulk at the α-position was achieved using [NEt₄][WH(CO)₅] as a combined reductant and complexation reagent. Reaction with TEMPO leads to P-OTEMP substituted tungsten complexes (4 and 7a,b) possessing different thermostabilities towards N-O bond cleavage. The transient phosphanoxyl complexes obtained were used for the polymerisation of styrene and acrylonitrile. DFT calculations were performed on the formation of various open-shell complexes and Loewdin spin density distributions.

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...

Heteroatom (N, P, As, Sb, Bi) Effects on the Resonance-Stabilized 2-, 3-, and 4-Picolyl Radicals

Important recent experimental studies have allowed the isomer-selective identification of the 2-, 3-, and 4-picolyl radicals. The picolyl radicals and their valence isoelectronic P, As, Sb, and Bi congeners are investigated here. For the three observed parent radicals, the theoretical ionization potentials agree with experiment to within 0.02 eV. Two rules are proposed for predicting vertical ionization potentials (E_VIE) and relative energies. The E_VIE values for these radicals will be higher when large percentages of the SOMO orbitals are distributed on the atoms with greater electronegativities. The cations of these systems were also studied along with the closed-shell methylpyridines and their P, As, Sb, and Bi analogs. The energies for the cationic species will lie lower when high percentages of π natural localized molecular orbitals occur on the more electronegative atoms. The structures of the 2- and 4-isomers strongly depend upon the heteroatoms, with the C-C linkages adopting a single-double alternating bond manner when the heteroatoms become heavier. The 3-isomers adopt roughly equal C-C bond distances with small changes from N to Bi.

Redox-Controlled Reactivity at Boron: Parallels to Frustrated Lewis/Radical Pair Chemistry

We report the synthesis of new Lewis-acidic boranes tethered to redox-active vanadium centers, (Ph₂N)₃V(μ-N)B(C₆F₅)₂ (1a) and (N(CH₂CH₂N(C₆F₅))₃)V(μ-N)B(C₆F₅)₂ (1b). Redox control of the V^IV/V couple resulted in switchable borane versus "hidden" boron radical reactivity, mimicking frustrated Lewis versus frustrated radical pair (FLP/FRP) chemistry, respectively. Whereas heterolytic FLP-type addition reactions were observed with the V^V complex (1b) in the presence of a bulky phosphine, homolytic peroxide, or Sn-hydride bond cleavage reactions were observed with the V^IV complex, [CoCp₂^*][(N(CH₂CH₂N(C₆F₅))₃)V(μ-N)B(C₆F₅)₂] (3b), indicative of boron radical anion character. The extent of radical character was probed by spectroscopic and computational means. Together, these results demonstrate that control of the V^IV/V oxidation states allows these compounds to access reactivity observed in both FLP and FRP chemistry.

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.

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.

Crystalline Divinyldiarsene Radical Cations and Dications

The divinyldiarsene radical cations [{(NHC)C(Ph)}As]2 (GaCl4 ) (NHC=IPr: C{(NDipp)CH}2 3; SIPr: C{(NDipp)CH2 }2 4; Dipp=2,6-iPr2 C6 H3 ) and dications [{(NHC)C(Ph)}As]2 (GaCl4 )2 (NHC=IPr 5; SIPr 6) are readily accessible as crystalline solids on sequential one-electron oxidation of the corresponding divinyldiarsenes [{(NHC)C(Ph)}As]2 (NHC=IPr 1; SIPr 2) with GaCl3 . Compounds 3-6 have been characterized by X-ray diffraction, cyclic voltammetry, EPR/NMR spectroscopy, and UV/vis absorption spectroscopy as well as DFT calculations. The sequential removal of one electron from the HOMO, that is mainly the As-As π-bond, of 1 and 2 leads to successive elongation of the As=As bond and contraction of the C-As bonds from 1/2→3/4→5/6. The UV/vis spectrum of 3 and 4 each exhibits a strong absorption in the visible region associated with SOMO-related transitions. The EPR spectrum of 3 and 4 each shows a broadened septet owing to coupling of the unpaired electron with two 75 As (I=3/2) nuclei.

Interconversion of Phosphinyl Radical and Phosphinidene Complexes by Proton Coupled Electron Transfer

The isolable complex [Os(PHMes*)H(PNP)] (Mes*=2,4,6-t Bu3 C6 H3 ; PNP=N{CHCHPt Bu2 }2 ) exhibits high phosphinyl radical character. This compound offers access to the phosphinidene complex [Os(PMes*)H(PNP)] by P-H proton coupled electron transfer (PCET). The P-H bond dissociation energy (BDE) was determined by isothermal titration calorimetry and supporting DFT computations. The phosphinidene product exhibits electrophilic reactivity as demonstrated by intramolecular C-H activation.

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.

Probing Hydrogen Atom Transfer at a Phosphorus(V) Oxide Bond Using a "Bulky Hydrogen Atom" Surrogate: Analogies to PCET

Recent computational studies suggest that the phosphate support in the commercial vanadium phosphate oxide (VPO) catalyst may play a critical role in initiating butane C-H bond activation through a mechanism termed reduction-coupled oxo activation (ROA) similar to proton-coupled electron transfer (PCET); however, no experimental evidence exists to support this mechanism. Herein, we present molecular model compounds, (Ph₂N)₃V═N-P(O)Ar₂ (Ar = C₆F₅ (2a), Ph (2b)), which are reactive to both weak H atom donors and a Me₃Si^• (a "bulky hydrogen atom" surrogate) donor, 1,4-bis(trimethylsilyl)pyrazine. While the former reaction led to product decomposition, the latter resulted in the isolation of the reduced, silylated complexes (Ph₂N)₃V-N═P(OSiMe₃)Ar₂ (3a/b). Detailed analyses of possible reaction pathways, involving the isolation and full characterization of potential stepwise square-scheme intermediates, as well as the determination of minimum experimentally and computationally derived thermochemical values, are described. We find that stepwise electron transfer (ET) + silylium transfer (ST) or concerted EST mechanisms are most likely. This study provides the first experimental evidence supporting a ROA mechanism and may inform future studies in homogeneous or heterogeneous C-H activation chemistry, as well as open up a possible new avenue for main group/transition metal cooperative redox reactivity.

A Mono-, Di-, and Trivanadocene Phosphorus Oxide Series: Synthesis, Magnetism, and Chemical/Electrochemical Properties

In this Article, we outline the synthesis of B(C₆F₅)₃-coordinated mono-, di-, and trivanadocene phosphorus oxide complexes, Cp₂VOP(OB(C₆F₅)₃)Ph₂ (2), (Cp₂VO)₂P(OB(C₆F₅)₃)Ph (3), and (Cp₂VO)₃P(OB(C₆F₅)₃) (4), respectively (Cp = η⁵-cyclopentadienyl). The complexes were synthesized from the known reagents, Cp₂VF and Ph₂P(O)OSiMe₃ (for 2) or PhP(O)(OSiMe₃)₂ (for 3) or (Me₃SiO)₃PO (for 4), via Me₃SiF elimination and in the presence of B(C₆F₅)₃. The multimetallic complexes (3 and 4) could not be synthesized without the capping B(C₆F₅)₃ Lewis acid, whereas the uncapped version of 2, Cp₂VOP(O)Ph₂ (1), has previously been reported by us. Spectroscopic and crystallographic analyses of 2-4 support an increasingly Lewis basic P═O bond upon substitution of -Ph for -OVCp₂ fragments (2-4). The increased metal nuclearity also results in increasingly reducing complexes as evidenced by cyclic voltammetry (CV). Magnetic measurements (SQUID) further revealed high-spin complexes with negligible magnetic exchange between V centers. Chemical oxidation of 2 with 0.5 equiv of [Ag][B(C₆F₅)₄] resulted in a ligand rearrangement reaction producing the V^IV product, Cp₂V(OP(OB(C₆F₅)₃)Ph₂)₂ (7). In contrast, the oxidation of 4 with the trityl salt, [Ph₃C][B(C₆F₅)₄], resulted in the isolation of a mixed-valent V^III/V^IV dimetallic species, (Cp₂VO₂P(OB(C₆F₅)₃)OVCp₂ (9). Both oxidations likely produce the [Cp₂V][B(C₆F₅)₄] byproduct and evidence for its formation is presented. The synthesis and characterization of the mono- and dimetallic species, Cp₂VOP(OBPh₃)Ph₂ (8) and (Cp₂VO)₂P(OB(C₆F₅)₃)OSiMe₃ (5), is also reported.

From stable Sb- and Bi-centered radicals to a compound with a Ga=Sb double bond

Neutral stibinyl and bismuthinyl radicals are typically short-lived, reactive species. Here we show the synthesis and solid-state structures of two stable stibinyl [L(Cl)Ga]₂Sb· 1 and bismuthinyl radicals [L(I)Ga]₂Bi· 4, which are stabilized by electropositive metal centers. Their description as predominantly metal-centered radicals is consistent with the results of NMR, EPR, SQUID, and DFT studies. The Lewis-acidic character of the Ga ligands allow for significant electron delocalization of the Sb- and Bi- unpaired radical onto the ligand. Single-electron reduction of [L(Cl)Ga]₂Sb· gave LGaSbGa(Cl)L 5, the first compound containing a Ga=Sb double bond. The π-bonding contribution is estimated to 9.56 kcal mol⁻¹ by NMR spectroscopy. The bonding situation and electronic structure is analyzed by quantum mechanical computations, revealing significant π backdonation from the Sb to the Ga atom. The formation of 5 illustrates the high-synthetic potential of 1 for the formation of new compounds with unusual electronic structures.

Radicals of heavy main-group elements represent important intermediates in chemical synthesis, yet few have been isolated. Here the authors stabilize neutral stibinyl and bismuthinyl radicals using gallium-based ligands, and reduce the former to afford a Ga=Sb double bond-containing complex.

Syntheses, structures and theoretical calculations of stable triarylarsine radical cations

Two stable triarylarsine radical cation salts have been synthesized and fully characterized by X-ray crystallography and EPR spectroscopy.

Group 15 biradicals: synthesis and reactivity of cyclobutane-1,3-diyl and cyclopentane-1,3-diyl analogues

Synthesis, structure and reactivity of cyclobutane-1,3-diyl and cyclopentane-1,3-diyl analogues are discussed along with their application as molecular switches or reagents to activate or trap small molecules with single or multiple bonds.

α-Radical Phosphines: Synthesis, Structure, and Reactivity

A series of phosphines featuring a persistent radical were synthesized in two steps by condensation of dialkyl-/diarylchlorophosphines with stable cyclic (alkyl)(amino)carbenes (cAACs) followed by one-electron reduction of the corresponding cationic intermediates. Structural, spectroscopic, and computational data indicate that the spin density in these phosphines is mainly localized on the original carbene carbon from the cAAC fragment; thus, it remains in the α-position with respect to the central phosphorus atom. The potential of these α-radical phosphines to serve as spin-labeled ligands is demonstrated through the preparation of several Au^I derivatives, which were also structurally characterized by single-crystal X-ray diffraction.

Isolation and Characterization of Radical Anions Derived from a Boryl-Substituted Diphosphene

Radical anions of a diphosphene with two boryl substituents were isolated and characterized by single-crystal X-ray diffraction, electron spin resonance (ESR), and UV/Vis absorption spectroscopy as well as DFT calculations. Structural analysis of the radical anions revealed an elongation of the P=P bond and a contraction of the B-P bonds relative to the neutral diphosphene. The UV/Vis spectra of these radical anions showed a strong absorption in the visible region, which was assigned to SOMO-related transitions on the basis of DFT calculations. The ESR spectra revealed that the hyperfine coupling constant with the phosphorus nuclei is the smallest that has been reported thus far. The results of the DFT calculations furthermore suggest that this should be attributed to a soaking of electron spin to the vacant p orbitals of the boryl substituents.

Oxidative formation of phosphinyl radicals from a trigonal pyramidal terminal phosphide Rh(i) complex, with an unusually long Rh–P bond

A coordinatively saturated triolefinic rhodium(i) complex, bearing a terminal pyramidal phosphido ligand, generates phosphinyl radical species under oxidative conditions.

Phosphinidene Reactivity of a Transient Vanadium P≡N Complex

Toward the preparation of a coordination complex of the heterodiatomic molecule PN, P≡N-V(N[^tBu]Ar)₃ (1, Ar = 3,5-Me₂C₆H₃), we report the use of ClPA (A = C₁₄H₁₀, anthracene) as a formal source of phosphorus(I) in its reaction with Na[NV(N[^tBu]Ar)₃] (Na[4]) to yield trimeric cyclo-triphosphane [PNV(N[^tBu]Ar)₃]₃ (3) with a core composed exclusively of phosphorus and nitrogen. In the presence of NapS₂ (peri-1,8-naphthalene disulfide), NapS₂P-NV(N[^tBu]Ar)₃ (6) is instead generated in 80% yield, suggesting trapping of transient 1. Upon mild heating, 3 readily fragments into dimeric [PNV(N[^tBu]Ar)₃]₂ (2), while in the presence of bis(trimethylsilyl)acetylene or cis-4-octene, the respective phosphirene (Ar[^tBu]N)₃VN-PC₂(SiMe₃)₂ (7) or phosphirane (Ar[^tBu]N)₃VN-P(C₈H₁₆) (8) compounds are generated. Kinetic data were found to be consistent with unimolecular decay of 3, and [2+1]-cycloaddition with radical clocks ruled out a triplet intermediate, consistent with intermediate 1 reacting as a singlet phosphinidene. In addition, both 7 and 8 were shown to reversibly exchange cis-4-octene and bis(trimethylsilyl)acetylene, serving as formal sources of 1, a reactivity manifold traditionally reserved for transition metals.