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.

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).

Stannyl phosphaketene as a synthon for phosphorus analogues of β-lactams

The reaction of the stannyl phosphaketene (Nacnac)SnPCO 1 (Nacnac = CH{(CMe)(2,6-iPr2C6H3N)}2) with B(C6F5)3 produced the 1,4-addition product of (Nacnac)SnPCO(B(C6F5)3). However, the corresponding reactions in the presence of dimethyl maleate, diisopropyl fumarate or diethyl-but-2-ynedioate gave [2+2] addition yielding four-membered phosphacycles, ((Nacnac)Sn(MeO2C))CHPC(OB(C6F5)3)CH(CO2Me), [(C6F5)3B)PC(OSn)C(CO2Me)CH(CO2Me)]2, (Nacnac)Sn(iPrO2C)CC(OAl(C6F5)3)P[CH(CO2iPr)CH2(CO2iPr)]CH(CO2iPr), and (Nacnac)SnP (EtO2CCC(CO2Et))CO(B(C6F5)3), respectively. In contrast, the corresponding reaction of phenylacetylene gave the FLP-addition product (Nacnac)SnOC(P)C(Ph)CH(B(C6F5)3). Collectively, this reactivity demonstrates that the stannyl phosphaketene 1 can act as a synthon for P-analogues of β-lactam derivatives.

Metal-Mediated Oligomerization Reactions of the Cyaphide Anion

The cyaphide anion, CP- , is shown to undergo three distinct oligomerization reactions in the coordination sphere of metals. Reductive coupling of Au(IDipp)(CP) (IDipp=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) by Sm(Cp*)2 (OEt2 ) (Cp*=1,2,3,4,5-pentamethylcyclopentadienyl), was found to afford a tetra-metallic complex containing a 2,3-diphosphabutadiene-1,1,4,4-tetraide fragment. By contrast, non-reductive dimerization of Ni(SIDipp)(Cp)(CP) (SIDipp=1,3-bis(2,6-diisopropylphenyl)-imidazolidin-2-ylidene; Cp=cyclopentadienyl), gives rise to an asymmetric bimetallic complex containing a 1,3-diphosphacyclobutadiene-2,4-diide moiety. Spontaneous trimerization of Sc(Cp*)2 (CP) results in the formation of a trimetallic complex containing a 1,3,5-triphosphabenzene-2,4,6-triide fragment. These transformations show that while cyaphido transition metal complexes can be readily accessed using metathesis reactions, many such species are unstable to further oligomerization processes.

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.

Recent Advances in the Domain of Cyclic (Alkyl)(Amino) Carbenes

Isolation of cyclic (alkyl) amino carbenes (cAACs) in 2005 has been a major achievement in the field of stable carbenes due to their better electronic properties. cAACs and bicyclic(alkyl)(amino)carbene (BicAAC) in essence are the most electrophilic as well as nucleophilic carbenes are known till date. Due to their excellent electronic properties in terms of nucleophilic and electrophilic character, cAACs have been utilized in different areas of chemistry, including stabilization of low valent main group and transition metal species, activation of small molecules, and catalysis. The applications of cAACs in catalysis have opened up new avenues of research in the field of cAAC chemistry. This review summarizes the major results of cAAC chemistry published until August 2021.

A Primary Acyl Phosphine Stabilized by a Phosphonium Ylide

Primary acyl-phosphines are scarce in the literature. Here we show that the reaction of Ph₃ GePCO with the ylide Ph₃ PCH₂ proceeds to give the species Ph₃ PCHC(O)PH(GePh₃ ) 1. Deprotonation of 1 with Na[N(SiMe₃ )₂ ] generates the salt [Na(THF)₂ ][Ph₃ PCHC(O)P(GePh₃ )] 2 which provides subsequent access to the bis-germanylated acylphosphine, Ph₃ PCHC(O)P(GePh₃ )₂ 3. Alternatively, treatment of 1 with HCl in dioxane affords the primary acylphosphine Ph₃ PCHC(O)PH₂ 4. Compound 4 is a rare example of an air stable primary acyl-phosphines and the first devoid of a stabilizing heteroatom adjacent to the carbonyl fragment.

Analysis of Non-innocence of Phosphaquinodimethane Ligands when Charge and Aromaticity Come into Play

Several phosphaquinodimethanes and their M(CO)5 complexes (M=Cr, Mo, W) and model derivatives have been theoretically investigated regarding the quest of non-innocence. Computed structural and electronic properties of the P-Me/NH2 substituted phosphaquinodimethanes and tungsten complexes revealed an interesting non-innocent ligand behaviour for the radical anion complexes with distonic ion character and a strong rearomatization of the middle phenyl ring. The latter was further probed taking also geometric aromaticity (HOMA) and quinoid distortion parameters (HOMQc) into account, as well as NICS(1). Furthermore, the effect of the P-substitution was investigated for real (or plausible) complexes and their free ligands focusing on the resulting aromaticity at the middle phenyl ring and vertical one-electron redox processes. The best picture of ligand engagement in redox changes was provided by representing NICS(1) values versus HOMA and the new geometric distortion parameter HOMQc8.

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.

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.

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.

Facile addition of E–H bonds to a dicarbondiphosphide

Charge transfer at P atoms in an N-heterocyclic carbene stabilized 6π-electron aromatic dicarbondiphosphide 1 has been observed upon interaction with a variety of small molecule substrates that feature a polar E–H bond (E = C, N, and O).

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.

Saturated N-Heterocyclic Carbene Based Thiele's Hydrocarbon with a Tetrafluorophenylene Linker

The synthesis of a SIPr [1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene] derived Kekulé diradicaloid with a tetrafluorophenylene spacer (3) has been described. Two synthetic routes have been reported to access 3. The cleavage of C-F bond of C₆ F₆ by SIPr in the presence of BF₃ led to double C-F activated compound with two tetrafluoro borate counter anions (2), which upon reduction by lithium metal afforded 3. Alternatively, 3 can be directly accessed in one step by reacting SIPr with C₆ F₆ in presence of Mg metal. Compounds 2 and 3 were well characterized spectroscopically and by single-crystal X-ray diffraction studies. Experimental and computational studies support the cumulenic closed-shell singlet state of 3 with a singlet-triplet energy gap (ΔE_S-T ) of 23.7 kcal mol^-1 .

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.

Base induced isomerisation of a phosphaethynolato-borane: mechanistic insights into boryl migration and decarbonylation to afford a triplet phosphinidene

We report on the (tert-butyl)isocyanide-catalysed isomersation of a phosphaethynolato-borane, [B]OCP ([B] = N,N′-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2-diazaboryl), to its linkage isomer, a phosphaketenyl-borane, [B]PCO. Mechanistic insight into this unusual isomerisation was gained through a series of stoichiometric reactions of [B]OCP with isocyanides and theoretical calculations at the Density Functional Theory (DFT) level. [B]PCO decarbonylates under photolytic conditions to afford a novel boryl-substituted diphosphene, [B]P Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> P[B]. This reaction proceeds via a transient triplet phosphinidene which we have been able to observe spectroscopically by Electron Paramagnetic Resonance (EPR) spectroscopy.

We report on the (tert-butyl)isocyanide-catalysed isomersation of a phosphaethynolato-borane, [B]OCP, to its linkage isomer, a phosphaketenyl-borane, [B]PCO.

Bent Phosphaallenes With "Hidden" Lone Pairs as Ligands

Phosphaheteroallenes R-P=C=L, with L = N-heterocyclic carbenes (NHCs), can be viewed to a certain extent as phosphaisonitriles stabilized with NHCs, R-P=C:←L. The suitability of these molecules as ligands for coinage-metal ions was investigated and coordination through the central carbon center was observed in most cases. A combination of experiments, spectroscopic methods, and DFT calculations indicates the presence of a hidden electron pair at the carbon center of R-P=C:←L. Remarkably, this lone pair also inserts intramolecularly in C-H bonds showing the carbene-type reactivity which is expected for phosphaisonitriles.

Stable cyclic (alkyl)(amino)carbene (cAAC) radicals with main group substituents

Recent attempts to isolate cyclic (alkyl)(amino)carbene stabilized radicals of p-block elements have been described here.

Isolation and characterization of stable radicals has been a long-pursued quest. While there has been some progress in this field particularly with respect to carbon, radicals involving heavier p-block elements are still considerably sparse. In this review we describe our recent successful efforts on the isolation of stable p-block element radicals particularly those involving aluminum, silicon, and phosphorus.

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.

Metastable phosphorus neutral monoradical: a key intermediate in the bicyclic cage formation

A key intermediate in the formation of a bicyclic cage formed between a biradical (LCP)₂ (L = carbene) and an unsaturated substrate via a [2 + 2] cycloaddition reaction has been isolated and fully characterized including by X-ray diffraction analysis.

The Chemistry of the 2-Phosphaethynolate Anion

In all likelihood the first synthesis of the phosphaethynolate anion, PCO^- , was performed in 1894 when NaPH₂ was reacted with CO in an attempt to make Na(CP) accompanied by elimination of water. This reaction was repeated 117 years later when it was discovered that Na(OCP) and H₂ are the products of this remarkable transformation. Li(OCP) was synthesized and fully characterized in 1992 but this salt proved to be too unstable to allow for a detailed investigation of its chemistry. It was not until the heavier analogues of this lithium salt were isolated, Na(OCP) and K(OCP) (both of which are remarkably stable and can be even dissolved in water), that the chemistry of this new functional group could be explored. Here we review the chemistry of the 2-phosphaethynolate anion, a heavier phosphorus-containing analogue of the cyanate anion, and describe the wide breadth of chemical transformations for which it has been thus far employed. Its use as a ligand, in decarbonylative and deoxygenative processes, and as a building block for novel heterocycles is described. In the mere twenty-six years since Becker first reported the isolation of this remarkable anion, it has become a fascinating reagent for the synthesis of a vast library of, often unprecedented, molecules and compounds.