Isolation of crystalline carbene-stabilized P2-radical cations and P2-dications

N-Heterocyclic Carbene-Derived Carbon Disulfide Radical Ligands for Palladium Diradicals

N-heterocyclic carbenes (NHCs) are recognized for their ability to stabilize various main group radicals; however, NHC-derived, sulfur-based radicals remain rare. In this study, we successfully synthesized and characterized a series of palladium diradical complexes that featured new sulfur-based radical ligands from NHC-carbon disulfide adducts. Spectroscopic and computational characterizations of the palladium complexes confirmed the open-shell singlet ground state, which resulted from the antiferromagnetic coupling of two unpaired electrons on each ligand. Proton nuclear magnetic resonance relaxometry was used to experimentally confirm the presence of these unpaired electrons. Moreover, the redox behavior of the complexes was localized on the ligand center, confirming the redox activity of the ligands. The discovery of this sulfur-based, redox-active radical ligand underscores the versatility and significance of NHC-derived radicals, thereby expanding the repertoire of radical ligands and opening new avenues for advanced material and catalytic systems.

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.

A neutral diphosphene radical: synthesis, electronic structure and white phosphorus activation

(ClImDipp)P-P(Dipp)˙ (1˙), a rare example of a neutral, mixed substituted diphosphene radical, has been prepared by reduction of (ClImDipp)PP(Dipp)[OTf] (2[OTf]) and (ClImDipp)PP(Cl)(Dipp) (3) with cobaltocene (CoCp2) (ClImDipp = 4,5-dichloro-1,3-bis(1,3-diisopropylphenyl)-imidazol-2-yl, Dipp = 2,6-diisopropylphenyl). Radical compound 1˙ readily activates white phosphorus (P4), resulting in the formation of an intriguing octaphosphane butterfly compound P8(ClImDipp)2(Dipp)2 (4).

A Digermanium(III) 1,2-Dication Stabilized by Amidinate and cAAC-Phosphinidenide Ligands

A digermanium(III) 1,2-dication comprises two cationic centers located at two interconnected Ge atoms. The strong Coulombic repulsion between two positively charged germanium cations hinders their bond formation. Balancing these two oppositions was achieved by using amidinate and cyclic (alkyl)amino carbene (cAAC)-phosphinidenide ligands, where an amidinato cAAC-phosphinidenidogermylene complex, [LGeP(cAACMe)] (2, where L = PhC(NtBu)2, cAACMe = :C{C(Me)2CH2C(Me)2NAr}, and Ar = 2,6-iPr2C6H3), underwent one-electron oxidation with a bis(phosphinidene) radical cation, [(cAACMe)P]2•+, to form a digermanium(III) 1,2-dication, [LGeP(cAACMe)]22+, in compound 4.

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.

Synthesis of Carbene-Stabilized PNPN Fragments and Their Carbene-Dependent Redox Properties

Herein, we report the synthesis of carbene-stabilized 1,3-diaza-2,4-diphosphabutenes CAACMePNPNCAACMe 4CAAC (CAACMe = 1-[2,6-bis(isopropyl)phenyl]-3,3,5,5-tetramethyl-2-pyrrolidinylidene) and IPrPNPNIPr 4NHC (IPr = 1,3-Bis(2,6-diisopropylphenyl)-imidazol-2-ylidene). The bonding in both systems is defined by a delocalized polar covalent π-system, with 4NHC exhibiting increased conjugation relative to 4CAAC. The nature of the stabilizing carbene also influences the redox properties of the compound, with 4CAAC undergoing potassium-mediated reduction to the closed-shell P-P bonded dimer K252, which upon treatment with Kryptofix-2,2,2 converts to the transient radical anion [Kcrypt][5], the formal one-electron reduction product of 4CAAC. In contrast, 4NHC undergoes reversible one-electron oxidation to the stable radical cation [6NHC][SbF6]. Computational and spectroscopic analyses of both radical species are suggestive of unevenly delocalized spin, with the bulk of the spin density residing on phosphorus in both cases.

Bicyclic (alkyl)(amino)carbene (BICAAC)-supported phosphinidenes

Herein, the synthesis and characterization of bicyclic (alkyl)(amino)carbene (BICAAC)-stabilized phosphinidenes (1-4) are reported. Compounds 1-3 were obtained by reacting trihalophosphine [PX3, X = Cl (1), Br (2), I (3)] with BICAAC in THF. A BICAAC-stabilized bis-phosphinidene (4) was obtained from the reduction of compound 2. All four compounds were characterized by X-ray crystallography and heteronuclear NMR spectroscopy. Theoretical calculations indicated the predominant C(carbene)P double bond characteristic in compounds 1-4.

A silylene-stabilized ditin(0) complex and its conversion to methylditin cation and distannavinylidene

Due to their intrinsic high reactivity, isolation of tin(0) complexes remains challenging. Herein, we report the synthesis of a silylene-stabilized ditin(0) complex (2) by reduction of a silylene-supported dibromostannylene (1) with 1 equivalent of magnesium (I) dimer in toluene. The structure of 2 was established by single crystal X-ray diffraction analysis. Density Functional Theory calculations revealed that complex 2 bears a Sn=Sn double bond and one lone pair of electrons on each of the Sn(0) atoms. Remarkably, complex 2 is readily methylated to give a mixed-valent methylditin cation (4), which undergoes topomerization in solution though a reversible 1,2-Me migration along a Sn=Sn bond. Computational studies showed that the three-coordinate Sn atom in 4 is the dominant electrophilic center, and allows for facile reaction with KHBBus3 furnishing an unprecedented N-heterocyclic silylenes-stabilized distannavinylidene (5). The synthesis of 2, 4 and 5 demonstrates the exceptional ability of N-heterocyclic silylenes to stabilize low valent tin complexes.

Synthesis and Reactivity of Hetero-Pnictogen Diazonium Analogs Stabilized by Transition Metal Units

The reactivity of the mixed dipnictogen complexes [{CpMo(CO)2 }2 (μ,η2 : 2 -PE)] (E=P, As, Sb) towards different group 14 electrophiles is reported. The resulting library of cationic compounds [{CpMo(CO)2 }2 (μ,η2 : 2 -EPR)]+ (R=Mes (2,4,6-C6 H2 Me3 ), CH3 , CPh3 , SnMe3 ) represents formally inorganic diazonium homologs which are stabilized by transition metal units. Modifying the steric and electronic properties of the electrophile drastically impacts the respective P-R bond lengths and is accompanied by increasing (SnMe3 >CPh3 >CH3 ) dynamic behavior in solution. In contrast to the well-studied organic analogs, the prepared compounds are stable at room temperature. The subsequent reaction of the model substrate [{CpMo(CO)2 }2 (μ,η2 : 2 -P2 Me)][OTf] ([OTf]- =[CF3 SO3 ]- ) with different N-heterocyclic carbenes (NHCs) leads to an addition at the unsubstituted P atom which is also predicted by computational methods. NMR spectroscopy confirms the formation of two isomers sync/gauche-[{CpMo(CO)2 }(μ,η2 : 1 -P(NHC)PMe){CpMo(CO)2 }][OTf]. X-ray crystallographic characterization and additional DFT calculations shed light on the spatial arrangement as well as on the possible formation pathways of the isomers.

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).

Tetrakis(N-heterocyclic Carbene)-Diboron(0): Double Single-Electron-Transfer Reactivity

The use of 1,3,4,5-tetramethylimidazol-2-ylidene (IMe) to coordinate with diatomic B₂ species afforded a tetrakis(N-heterocyclic carbene)-diboron(0) [(IMe)₂B-B(IMe)₂] (2). The singly bonded B₂ moiety therein possesses a valence electronic configuration 1σ_g²1π_u²1π_g*² with four vacant molecular orbitals (1σ_u*, 2σ_g, 1π_u', 1π_g'*) coordinated with IMe. Its unprecedented electronic structure is analogous to the energetically unfavorable planar hydrazine with a D_2h symmetry. The two highly reactive π_g* antibonding electrons enable double single-electron-transfer (SET) reactivity in small-molecule activation. Compound 2 underwent a double SET reduction with CO₂ to form two carbon dioxide radical anions CO₂^•-, which then reduced pyridine to yield a carboxylated pyridine reductive coupling dianion [O₂CNC₅(H)₅-C₅(H)₅NCO₂]^2- and converted compound 2 to the tetrakis(N-heterocyclic carbene)-diborene dication [(IMe)₂B═B(IMe)₂]²⁺ (3²⁺). This is a remarkable transition-metal-free SET reduction of CO₂ without ultraviolet/visible (UV/vis) light conditions.

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.

Counterintuitive Chemistry: Carbene Stabilization of Zero-Oxidation State Main Group Species

Carbenes have evolved from transient laboratory curiosities to a robust, diverse, and surprisingly impactful ligand class. A variety of different carbenes have significantly contributed to the development of low-oxidation state main group chemistry. This Perspective focuses upon advances in the chemistry of carbene complexes containing main group element cores in the formal oxidation state of zero, including their diverse synthetic strategies, unusual bonding and structural motifs, and utility in transition metal coordination chemistry and activation of small molecules.

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.

Cage-size effects on the encapsulation of P2 by fullerenes

The classic pnictogen dichotomy stands for the great contrast between triply bonding very stable N₂ molecules and its heavier congeners, which appear as dimers or oligomers. A banner example involves phosphorus as it occurs in nature as P₄ instead of P₂ , given its weak π-bonds or strong σ-bonds. The P₂ synthetic value has brought Lewis bases and metal coordination stabilization strategies. Herein, we discuss the unrealized encapsulation alternative using the well-known fullerenes' capability to form endohedral and stabilize otherwise unstable molecules. We chose the most stable fullerene structures from C_n (n = 50, 60, 70, 80) and experimentally relevant from C_n (n = 90 and 100) to computationally study the thermodynamics and the geometrical consequences of encapsulating P₂ inside the fullerene cages. Given the size differences between P₂ and P₄ , we show that the fullerenes C₇₀ -C₁₀₀ are suitable cages to side exclude P₄ and host only one molecule of P₂ with an intact triple bond. The thermodynamic analysis indicates that the process is favorable, overcoming the dimerization energy. Additionally, we have evaluated the host-guest interaction to explain the origins of their stability using energy decomposition analysis.

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.

An Isolable 2,5‐Disila‐3,4‐Diphosphapyrrole and a Conjugated Si=P−Si=P−Si=N Chain Through Degradation of White Phosphorus with a N,N ‐Bis(Silylenyl)Aniline

White phosphorus (P₄) undergoes degradation to P₂ moieties if exposed to the new N,N‐bis(silylenyl)aniline PhNSi₂1 (Si=Si[N(tBu)]₂CPh), furnishing the first isolable 2,5‐disila‐3,4‐diphosphapyrrole 2 and the two novel functionalized Si=P doubly bonded compounds 3 and 4. The pathways for the transformation of the non‐aromatic 2,5‐disila‐3,4‐diphosphapyrrole PhNSi₂P₂2 into 3 and 4 could be uncovered. It became evident that 2 reacts readily with both reactants P₄ and 1 to afford either the polycyclic Si=P‐containing product [PhNSi₂P₂]₂P₂3 or the unprecedented conjugated Si=P−Si=P−Si=NPh chain‐containing compound 4, depending on the employed molar ratio of 1 and P₄ as well as the reaction conditions. Compounds 3 and 4 can be converted into each other by reactions with 1 and P₄, respectively. All new compounds 1–4 were unequivocally characterized including by single‐crystal X‐ray diffraction analysis. In addition, the electronic structures of 2–4 were established by Density Functional Theory (DFT) calculations.

Carbene-Anchored Boryl- and Stibanyl-Phosphaalkenes as Precursors for Bis-Phosphaalkenyl Dichlorogermane and Mixed-Valence AgI/AgII Phosphinidenide

Cyclic alkyl(amino) carbene (cAAC)-anchored boryl- and stibanyl-phosphaalkenes with general formula cAAC = P-ER₂ [E = B, R = (NⁱPr₂)₂ (3a-c); E = Sb, R = 2,4,6-triisopropylphenyl (5a-b)] have been synthesized and utilized as precursors for the bis-phosphaalkenyl dichlorogermane [(cAAC = P)₂GeCl₂] (6) and the first molecular example of a neutral polymeric mixed-valence Ag^I/Ag^II phosphinidenide complex [(cAACP)₂Ag₄^IAg^IICl₄]_n (7). All compounds have been characterized by single-crystal X-ray diffraction and further investigated by nuclear magnetic resonance (NMR), mass spectrometric analysis, and UV-vis/fluorescence measurements. The paramagnetic complex 7 has been characterized by ESR spectroscopy. Cyclic voltammetry studies of compounds 3/5 have suggested possible one-electron quasi-reversible reductions, indicating their redox noninnocent behavior in solution. Quantum chemical studies revealed the electron-sharing nature of the P-B and P-Sb σ bonds in compounds 3 and 5, and the polar C_cAAC = P bonds in compounds 3, 5, and 6 prevailing their phosphaalkene structures over phosphinidenes.

Single-Electron Oxidation of Carbene-Coordinated Pnictinidenes-Entry into Heteroleptic Radical Cations and Metalloid Clusters

Stable heavy main group element radicals are challenging synthetic targets. Although several strategies have been developed to stabilize such odd-electron species, the number of heavier pnictogen-centered radicals is limited. We report on a series of two-coordinated pnictogen-centered radical cations [(^MecAAC)EGa(Cl)L][B(C₆F₅)₄] (^MecAAC = [H₂C(CMe₂)₂NDipp]C; Dipp = 2,6-i-Pr₂C₆H₃; E = As 1, Sb 2, Bi 3; L = HC[C(Me)NDipp]₂) synthesized by one-electron oxidation of L(Cl)Ga-substituted pnictinidenes (^MecAAC)EGa(Cl)L (E = As I, Sb II, Bi III). 1-3 were characterized by electron paramagnetic resonance (EPR) spectroscopy and single crystal X-ray diffraction (sc-XRD) (1, 2), while quantum chemical calculations support their description as carbene-coordinated pnictogen-centered radical cations. The low thermal stability of 3 enables access to metalloid bismuth clusters as shown by formation of [{LGa(Cl)}₃Bi₆][B(C₆F₅)₄] (4).

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.

Recent advances in the chemistry of the phosphaethynolate and arsaethynolate anions

Over the past decade, the reactivity of 2-phosphaethynolate (OCP^-), a heavier analogue of the cyanate anion, has been the subject of momentous interest in the field of modern organometallic chemistry. It is used as a precursor to novel phosphorus-containing heterocycles and as a ligand in decarbonylative processes, serving as a synthetic equivalent of a phosphinidene derivative. This perspective aims to describe advances in the reactivities of phosphaethynolate and arsaethynolate anions (OCE^-; E = P, As) with main-group element, transition metal, and f-block metal scaffolds. Further, the unique structures and bonding properties are discussed based on spectroscopic and theoretical studies.

Base-stabilized formally zero-valent mono and diatomic molecular main-group compounds

Various compounds are known for transition metals in their formal zero-oxidation state, while similar compounds of main-group elements are recently realized and limited to only a few examples. Lewis-base-stabilized mono and diatomic molecular species (B₂, C, C₂, Si, Si₂, Ge, Ge₂, Sn, P₂, As₂, Sb₂) represent groundbreaking examples of main-group compounds with formally zero-oxidation state. In recent years, the isolation of low-valent main-group compounds has attracted increasing attention of both experimental and theoretical chemists. This is not only due to their fascinating electronic structures and exceptional reactivities, but also their use as valuable precursors for the synthesis of exotic yet important chemical species. This has led to a better understanding of the intricate balance of the donor-acceptor properties of the ligand(s) used to stabilize elements in a formally zero-oxidation state. Owing to the unusual oxidation state of the central element, many compounds containing formally zero-valent elements can efficiently activate otherwise inert small molecules. This review describes the synthesis, characterization, and reactivity of reported mono and diatomic formal zero-oxidation state main-group compounds. This review also emphasizes the comparative description of systems where different ligands are used to stabilize an element in its formal zero-oxidation state.

Fluorescent organo-antimony compounds as precursors for syntheses of redox-active trimeric and dimeric alkali metal antimonides: an insight into electron transfer reduction processes

(Tip)₂SbCl (1, Tip = 2,4,6-triisopropylphenyl) has been utilized as a precursor for the synthesis of the distibane (Tip)₄Sb₂ (4) via one-electron reduction using KC₈. The two-electron reduction of 1 and 4 afforded the novel trinuclear antimonide cluster [K₃((Tip)₂Sb)₃(THF)₅] (6). Changing the reducing agent from KC₈ to a different alkali metal resulted in the solid-state isolation of corresponding stable dimeric alkali metal antimonides with the general formula [M₂((Tip)₂Sb)₂(THF)_p-x(tol)_x] (M = Li (14), Na (15), Cs (16)). In this report, different aspects of the various reducing agents [K metal, KC₈, and [K₂(Naph)₂(THF)]] used have been studied, correlating the experimental observations with previous reports. Additional reactivity studies involving 1 and AgNTf₂ (Tf = trifluoromethanesulfonyl) afforded the corresponding antimony cation (Tip)₂Sb⁺NTf₂^- (19). The Lewis acidic character of 19 has been unambiguously proved via treatment with Lewis bases to produce the corresponding adducts 20 and 21. Interestingly, the precursors 1 and 4 have been observed to be highly luminescent, emitting green light under short-wavelength UV radiation. All the reported compounds have been characterized via NMR, UV-vis, mass spectrometry, and single-crystal X-ray diffraction analysis. Cyclic voltammetry (CV) studies of 1 in THF showed possible two electron reduction, suggesting the in situ generation of the corresponding radical-anion intermediate 1˙- and its subsequent conversion to the monomeric intermediate (Tip)₂Sb^- (5) upon further reduction. 5 undergoes oligomerization in the solid state to produce 6. The existence of 1˙- was proved using electron paramagnetic resonance (EPR) spectroscopy in solution. CV studies of 6 suggested its potential application as a reducing agent, which was further proved via the conversion of Tip-PCl₂ to trimeric (Tip)₃P₃ (17), and cAACP-Cl (cAAC = cyclic alkyl(amino)carbene) to (cAAC)₂P₂ (18) and 4, utilizing 6 as a stoichiometric reducing agent.

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...

Synthesis, structure and bonding nature of heavy dipnictene radical anions

Cyclic voltammetry (CV) studies of two L(X)Ga-substituted dipnictenes [L(R2N)GaE]2 (E = Sb, R = Me 1; E = Bi; R = Et 2; L = HC[C(Me)NDipp]2; Dipp = 2,6-i-Pr2C6H3) showed reversible reduction events. Single electron reduction of 1 and 2 with KC8 in DME in the presence of benzo-18-crown-6 (B-18-C-6) gave the corresponding dipnictenyl radical anions (DME)[K(B-18-C-6)][L(R2N)GaE]2 (E = Sb, R = Me 3; E = Bi, R = Et 4). Radical anions 3 and 4 were characterized by EPR, UV-vis and single crystal X-ray diffraction, while quantum chemical calculations gave deeper insight into the nature of the chemical bonding.

Phosphine-catalysed reductive coupling of dihalophosphanes

Classically tetraaryl diphosphanes have been synthesized through Wurtz-type reductive coupling of halophosphanes R₂PX or more recently, through the dehydrocoupling of phosphines R₂PH. Catalytic variants of the dehydrocoupling reaction have been reported, but are limited to R₂PH compounds. Using PEt₃ as a catalyst, we now show that TipPBr₂ (Tip = 2,4,6-iPr₃C₆H₂) is selectively coupled to give the dibromodiphosphane (TipPBr)₂ (1), a compound not accessible using classic Mg reduction. Surprisingly, when using DipPBr₂ (Dip = 2,6-iPr₃C₆H₃) in the PEt₃ catalysed reductive coupling the diphosphene (PDip)₂ (2) with a PP double was formed selectively. In benzene solutions (PDip)₂ has a half life time of ca. 28 days and can be utilized with NHCs to access NHC-phosphinidene adducts. To show that this protocol is more widely applicable, we show that Ph₂PCl and Mes₂PX (X = Cl, Br) are efficiently coupled using 10 mol% of PEt₃ to give (Ph₂P)₂ and (Mes₂P)₂, respectively. Control experiments show that [BrPEt₃]Br is a potential oxidation product in the catalytic cycle, which can be debrominated by Zn dust as a sacrificial reductant.

Highly Stable 1,2-Dicarbonyl Radical Cations Derived from N-Heterocyclic Carbenes

Stable organic radicals have been of great academic interest not only in the context of fundamental understanding of reactive intermediates but also because of their numerous applications as functional materials. Apart from the early examples of triphenylmethyl and TEMPO derivatives, reports on air- and water-stable organic radicals are scarce, and their development remains a challenge. Herein, we present the design and synthesis of a novel organic radical based on a 1,2-dicarbonyl scaffold supported by N-heterocyclic carbenes (NHCs). The presented radical cations exhibit remarkable stability toward various harsh conditions, such as the presence of reactive chemicals (reductants, oxidants, strong acids, and bases) or high temperatures, by far exceeding the stability of triphenylmethyl and TEMPO radicals. In addition, physiological conditions including aqueous buffer and blood serum are tolerated. The steric and electronic stabilization provided by the two NHC moieties enabled the successful design of the highly stable radical.

Indirect Access to Carbene Adducts of Bismuth- and Antimony-Substituted Phosphaketene and Their Unusual Thermal Transformation to Dipnictines and [(NHC)2OCP][OCP]

The synthesis and thermal redox chemistry of the first antimony (Sb)– and bismuth (Bi)–phosphaketene adducts are described. When diphenylpnictogen chloride [Ph₂PnCl (Pn = Sb or Bi)] is reacted with sodium 2-phosphaethynolate [Na[OCP]·(dioxane)_x], tetraphenyldipnictogen (Ph₂Pn–PnPh₂) compounds are produced, and an insoluble precipitate forms from solution. In contrast, when the N-heterocyclic carbene adduct (NHC)–PnPh₂Cl is combined with [Na[OCP]·(dioxane)_x], Sb– and Bi–phosphaketene complexes are isolated. Thus, NHC serves as an essential mediator for the reaction. Immediately after the formation of an intermediary pnictogen–phosphaketene NHC adduct [NHC–PnPh₂(PCO)], the NHC ligand transfers from the Pn center to the phosphaketene carbon atom, forming NHC–C(O)P-PnPh₂ [Pn = Sb (3) or Bi (4)]. In the solid state, 3 and 4 are dimeric with short intermolecular Pn–Pn interactions. When compounds 3 and 4 are heated in THF at 90 and 70 °C, respectively, the pnictogen center Pn^III is thermally reduced to Pn^II to form tetraphenyldipnictines (Ph₂Pn–PnPh₂) and an unusual bis-carbene-supported OCP salt, [(NHC)₂OCP][OCP] (5). The formation of compound 5 and Ph₂Pn–PnPh₂ from 3 or 4 is unique in comparison to the known thermal reactivity for group 14 carbene–phosphaketene complexes, further highlighting the diverse reactivity of [OCP]⁻ with main-group elements. All new compounds have been fully characterized by single-crystal X-ray diffraction, multinuclear NMR spectroscopy (¹H, ¹³C, and ³¹P), infrared spectroscopy, and elemental analysis (1, 2, and 5). The electronic structure of 5 and the mechanism of formation were investigated using density functional theory (DFT).

An N-heterocyclic carbene (NHC) was used to support the otherwise unstable Ph₂Sb—P=C=O and Ph₂Bi—P=C=O moieties. Exploration of the thermal chemistry of these NHC−phosphaketene adducts reveals the formation of the salt [NHC₂OCP][OCP]. This present work demonstrates the thermal chemistry of the 2-phospaethynolate anion with heavier pnictogens (Sb and Bi).

Metalloradical Cations and Dications Based on Divinyldiphosphene and Divinyldiarsene Ligands

Metalloradicals are key species in synthesis, catalysis, and bioinorganic chemistry. Herein, two iron radical cation complexes (3‐E)GaCl₄ [(3‐E)^.+ = [{(IPr)C(Ph)E}₂Fe(CO)₃]^.+, E = P or As; IPr = C{(NDipp)CH}₂, Dipp = 2,6‐iPr₂C₆H₃] are reported as crystalline solids. Treatment of the divinyldipnictenes {(IPr)C(Ph)E}₂ (1‐E) with Fe₂(CO)₉ affords [{(IPr)C(Ph)E}₂Fe(CO)₃] (2‐E), in which 1‐E binds to the Fe atom in an allylic (η³‐EEC_vinyl) fashion and functions as a 4e donor ligand. Complexes 2‐E undergo 1e oxidation with GaCl₃ to yield (3‐E)GaCl₄. Spin density analysis revealed that the unpaired electron in (3‐E)^.+ is mainly located on the Fe (52–64 %) and vinylic C (30–36 %) atoms. Further 1e oxidation of (3‐E)GaCl₄ leads to unprecedented η³‐EEC_vinyl to η³‐EC_vinylC_Ph coordination shuttling to form the dications (4‐E)(GaCl₄)₂.

Reactivity of P4 butterfly complexes towards NHCs - generation of a metal-bridged P2 dumbbell complex

The reactivity of the P₄ butterfly complexes [{Cp'''Fe(CO)₂}₂(μ,η^1:1-P₄)] (A, Cp''' = C₅H₂^tBu₃) and [{Cp*Cr(CO)₃}₂(μ,η^1:1-P₄)] (B, Cp* = C₅(CH₃)₅) towards the N-heterocyclic carbene IMe (1,3,4,5-tetramethyl-imidazol-2-ylidene) is reported. The reaction of A affords [P(IMe)₂][Fe(CO)₂Cp'''] (1) or [P(IMe)₂][{Cp'''Fe}₂(μ,η^3:3-P₃)] (2), the latter possessing a P₃-allylic moiety. In contrast, the reaction of B yields [P(IMe)₂][Cr(CO)₃Cp*] (3) and [{Cp*Cr(CO)₂}(η²-P₂IMe₂)][Cr(CO)₃Cp*] (4), featuring a novel metal-bridged P₂ dumbbell.

N-Heterocyclic carbene-carbodiimide (NHC-CDI) betaine adducts: synthesis, characterization, properties, and applications

N-Heterocyclic carbenes (NHCs) are an important class of reactive organic molecules used as ligands, organocatalysts, and σ-donors in a variety of electroneutral ylide or betaine adducts with main-group compounds. An emerging class of betaine adducts made from the reaction of NHCs with carbodiimides (CDIs) form zwitterionic amidinate-like structures with tunable properties based on the highly modular NHC and CDI scaffolds. The adduct stability is controlled by the substituents on the CDI nitrogens, while the NHC substituents greatly affect the configuration of the adduct in the solid state. This Perspective is intended as a primer to these adducts, touching on their history, synthesis, characterization, and general properties. Despite the infancy of the field, NHC-CDI adducts have been applied as amidinate-type ligands for transition metals and nanoparticles, as junctions in zwitterionic polymers, and to stabilize distonic radical cations. These applications and potential future directions are discussed.

Stabilization of a bismuth–bismuth double bond by anionic N-heterocyclic carbenes

Anionic N-heterocyclic carbenes have been employed for the isolation of the first dicarbene–dibismuth complex; the resulting dibismuthene features a trans-bent geometry with a Bi–Bi double bond and short intramolecular Bi–C_ipso contacts.

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.

Solid-State Isolation of Cyclic Alkyl(Amino) Carbene (cAAC)-Supported Structurally Diverse Alkali Metal-Phosphinidenides

Cyclic alkyl(amino) carbene (cAAC)-supported, structurally diverse alkali metal-phosphinidenides 2-5 of general formula ((cAAC)P-M)_n (THF)_x [2: M=K, n=2, x=4; 3: M=K, n=6, x=2; 4: M=K, n=4, x=4; 5: M=Na, n=3, x=1] have been synthesized by the reduction of cAAC-stabilized chloro-phosphinidene cAAC=P-Cl (1) utilizing metallic K or KC₈ and Na-naphthalenide as reducing agents. Complexes 2-5 have been structurally characterized in solid state by NMR studies and single crystal X-ray diffraction. The proposed mechanism for the electron transfer process has been well-supported by cyclic voltammetry (CV) studies and Density Functional Theory (DFT) calculations. The solid state oligomerization process has been observed to be largely dependent on the ionic radii of alkali metal ions, steric bulk of cAAC ligands and solvation/de-solvation/recombination of the dimeric unit [(cAAC)P-M(THF)_x ]₂ .

NHCs as Neutral Donors towards Polyphosphorus Complexes

The first adducts of NHCs (=N‐heterocyclic carbenes) with aromatic polyphosphorus complexes are reported. The reactions of [Cp*Fe(η⁵‐P₅)] (1) (Cp*=pentamethyl‐cyclopentadienyl) with IMe (=1,3,4,5‐tetramethylimidazolin‐2‐ylidene), IMes (=1,3‐bis(2,4,6‐trimethylphenyl)‐imidazolin‐2‐ylidene) and IDipp (=1,3‐bis(2,6‐diisopropylphenyl)‐imidazolin‐2‐ylidene) led to the corresponding neutral adducts which can be isolated in the solid state. However, in solution, they quickly undergo a dissociative equilibrium between the adduct and 1 including the corresponding NHC. The equilibrium is influenced by the bulkiness of the NHC. [Cp′′Ta(CO)₂(η⁴‐P₄)] (Cp′′=1,3‐di‐tert‐butylcyclopentadienyl) reacts with IMe under P atom abstraction to give an unprecedented cyclo‐P₃‐containing anionic tantalum complex. DFT calculations shed light onto the energetics of the reaction pathways.