Reactivity enhancement of a diphosphene by reversible N-heterocyclic carbene coordination

Phosphaarsenes - Combining Phospha- and Arsa-Wittig-Reagents

Dipnictenes of the type RPn=PnR (Pn=P, As, Sb, Bi) can be viewed as dimers of the corresponding pnictinidenes R-Pn. Phosphanylidene- and arsanylidenephosphoranes (R-Pn(PMe3); Pn=P, As) have been shown to be versatile synthetic surrogates for the delivery of pnictinidene fragments. We now report that thermal treatment of 1 : 1 mixtures of R-P(PMe3) and R'-As(PMe3) gives access to arsaphosphenes of the type RP=AsR'. Three examples are presented and the properties and reactivity of Mes*P=AsDipTer (1) (Mes*=2,4,6-tBu3-C6H2; DipTer=2,6-(2,6-iPr2C6H3)2-C6H3) were studied in detail. Solid state 31P NMR spectroscopy revealed a large 31P NMR chemical shift anisotropy with a span of ca. 920 ppm for 1 while computational methods were employed to investigate this pronounced magnetic deshielding of the P atom in 1. In the presence of the carbene IMe4 (IMe4=:C(MeNCMe)2) 1 is shown to be split into the corresponding NHC adducts Mes*P(IMe4) and DipTerAs(IMe4), which is additionally shown for diarsenes.

Selective monooxygenation of diphosphenes with molecular oxygen

The O2 splitting mediated by the bulky Rind-based diphosphenes resulted in the clean formation of the mixed-valent diphosphorus compounds, diphosphene oxides, with P2O moieties. Their structural features and electronic properties have been clearly characterized by experimental and theoretical methods.

Triphosphiranes as phosphinidene-transfer agents - synthesis of regular and chelating NHC phosphinidene adducts

In this contribution we describe the general use of aryl-substituted triphosphiranes (Ar3P3; Ar = Mes, Dip, Tip) as phosphinidene transfer reagents towards N-heterocyclic carbenes (NHCs) to give a library of twelve N-heterocyclic carbene phosphinidene adducts of the type ArPNHC (NHCPs), in which the NHCs have varying steric profiles, allowing a systematic evaluation of their structural and NMR-spectroscopic properties. In the next series of experiments we utilized 1,3- and 1,4-phenylene bridged bis-NHCs to access a new class of chelating bis(NHCP)s, of which three derivatives could be structurally characterized. The 1,4-phenylene derivatives were shown to be susceptible to P-CNHC bond cleavage when irradiated with an LED (396 nm), providing a rare example of phosphinidene release from NHCPs. The coordination chemistry of 1,3-phenylene bridged bis(NHCP)s towards GeCl2(dioxane) and GaI3 was investigated and revealed the formation of ion-separated cationic complexes, with significant charge transfer from the ligand to the metal center according to NBO analyses.

Synthesis and Ligand Properties of Chelating Bis(N-heterocyclic carbene)-Stabilized Bis(phosphinidenes)

NHC-phosphinidene (NHCP) adducts are an emerging class of ligands with proven binding ability for main group and transition metal elements. They possess electron-rich P atoms with two lone pairs (LPs) of electrons, making them interesting platforms for the formation of multimetallic complexes. We describe herein a modular, high-yielding synthesis of bis(NHCP)s, starting from alkylidene-bridged bis(NHC)s ((IMe)2CnH2n; n = 1,3) and triphosphirane (PDip)3 (Dip = 2,6-iPr2C6H3) as phosphinidene transfer reagent. The coordination chemistry of [{DipP(IMe)}2CH2], 1, was studied in detail, and complexes [1·FeBr2] and [1·Rh(cod)]Cl were prepared, showing that the ligand has a flexible bite angle. The dicarbonyl complex [1·Rh(CO)2]Cl, with an average value for the CO stretching frequency of 2029 cm-1, indicates a strongly donating ligand when compared to related complexes. The binding ability of the remaining two phosphorus LPs was demonstrated with AuCl(SMe2), giving the heterotrimetallic complex [1·(AuCl)2·Rh(cod)]Cl. Moreover, [1·Rh(cod)]X (X- = Cl, B(3,5-(CF3)2-C6H3)4) was tested in the catalytic hydrogenation of methyl-Z-α-acetamidocinnamate (MAC) and dimethyl itaconate (ItMe2), revealing that the chloride complex was inactive, while the BArF complex demonstrated moderate activity. Additionally, [1·Rh(cod)]Cl was shown to be moderately air- and moisture-stable, slowly decomposing to the corresponding NHC-stabilized bis-dioxophosphorane, which was independently synthesized by treating the free ligand with dry O2.

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.

A PH-functionalized dicationic bis(imidazolio)diphosphine

Reaction of the iodide salt of a secondary imidazolio-iodophosphine [(L)PHI]I (L⁺ = 1,3-diarylimidazolium-yl) with an imidazolio-phosphide (L)PH in the presence of GaI₃ afforded the isolable salt of a dicationic, bis(imidazolio)-substituted dihydro-diphosphine [(L)₂P₂H₂][GaI₄]₂. Non-preparative formation of the cationic diphosphines was also observed upon spontaneous "dehalo-coupling" of [(L)PHI]⁺, or in reactions of [(L)PHI]I and (L)PH in the absence of GaI₃. Further reaction of [(L)₂P₂H₂]²⁺ with (L)PH produced an iodide salt of a known (bis)imidazolio-diphosphide monocation [(L)₂P₂H]⁺. The identity of cationic diphosphines and diphosphides was established by single-crystal X-ray diffraction studies. NMR spectroscopy revealed that dications [(L)₂P₂H₂]²⁺ exist as a mixture of meso- and rac-diastereomers in solution. Computational studies confirmed the stereochemical assignment of the isomers observed, and gave insight into the bonding situation of the diphosphine dications.

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.

Metathesis of P=C Bonds Catalyzed by N-Heterocyclic Carbenes

The catalytic metathesis of C=C bonds is a textbook reaction that has no parallel in the widely studied area of multiple bonds involving heavier p-block elements. A high-yielding P=C bond metathesis of phosphaalkenes (ArP=CPh₂ , Ar=Mes, o-Tol, Ph) has been discovered that is catalyzed by N-heterocyclic carbenes (NHC=Me₂ IMe, Me₂ Iⁱ Pr). The products are cyclic oligomers formally derived from ArP=PAr [i. e. cyclo-(ArP)_n ; n=3, 4, 5, 6] and Ph₂ C=CPh₂ . Preliminary mechanistic studies of this remarkable transformation have established NHC=PAr (Ar=Mes, o-Tol, Ph) as key phosphinidene transfer agents. In addition, novel cyclic intermediates, such as, cyclo-(ArP)₂ CPh₂ and cyclo-(ArP)₄ CPh₂ have also been observed. This work represents a rare application of non-metal-based catalysts for transformations involving main-group elements.

Synthesis and reactivity of NHC-coordinated phosphinidene oxide

Here we report the synthesis of an N-heterocyclic carbene (NHC)-stabilised phosphinidene oxide by the controlled oxygenation of a phosphinidene under ambient conditions. This compound can be further oxygenated to a phosphinidene dioxide. The stoichiometric reduction of a phosphinidene oxide with KC₈ resembles the pinacol coupling reaction-the reduction of a carbonyl compound. We also looked at the stoichiometric oxidation of NHC-coordinated phosphinidene, phosphinidene oxide and phosphinidene dioxide with [NO][SbF₆].

Reactivity of NHC/diphosphene-coordinated Au(I)-hydride

We report the reactivity of isolable Au(i)-hydride stabilized by an NHC-coordinated diphosphene towards substrates containing C-C and N-N multiple bonds (NHC = N-heterocyclcic carbene). Reactions with dimethyl acetylenedicarboxylate and azobenzene lead to a trans-addition of the Au(i)-H across the C-C triple bond and the N-N double bond, respectively. In contrast, the reaction with ethyl diazoacetate affords a gold(i)-hydrazonide as the 1,1-addition product to the terminal nitrogen atom. With phenyl acetylene, the corresponding Au(i)-alkynyl complex is obtained under the elimination of dihydrogen. Strikingly, diphosphene-containing Au(i)-hydride is more reactive - affording different products in some cases - than a related NHC-stabilized Au(i)-hydride without the mediating diphosphene moiety.

Boosting Low-Valent Aluminum(I) Reactivity with a Potassium Reagent

The reagent RK [R=CH(SiMe3 )2 or N(SiMe3 )2 ] was expected to react with the low-valent (DIPP BDI)Al (DIPP BDI=HC[C(Me)N(DIPP)]2 , DIPP=2,6-iPr-phenyl) to give [(DIPP BDI)AlR]- K+ . However, deprotonation of the Me group in the ligand backbone was observed and [H2 C=C(N-DIPP)-C(H)=C(Me)-N-DIPP]Al- K+ (1) crystallized as a bright-yellow product (73 %). Like most anionic AlI complexes, 1 forms a dimer in which formally negatively charged Al centers are bridged by K+ ions, showing strong K+ ⋅⋅⋅DIPP interactions. The rather short Al-K bonds [3.499(1)-3.588(1) Å] indicate tight bonding of the dimer. According to DOSY NMR analysis, 1 is dimeric in C6 H6 and monomeric in THF, but slowly reacts with both solvents. In reaction with C6 H6 , two C-H bond activations are observed and a product with a para-phenylene moiety was exclusively isolated. DFT calculations confirm that the Al center in 1 is more reactive than that in (DIPP BDI)Al. Calculations show that both AlI and K+ work in concert and determines the reactivity of 1.

Influence of N-heterocyclic carbenes (NHCs) on the hydrolysis of a diphosphene

We report the influence of N-heterocyclic carbenes (NHCs) on the hydrolysis of a diphosphene TerP[double bond, length as m-dash]PTer (1; Ter = 2,6-Mes₂C₆H₃; Mes = 2,4,6-Me₃C₆H₂), a phosphorus-analogue of an alkene. The diphosphene 1 itself is completely inert towards water. However, NHCs have been found to activate 1 towards ready hydrolysis. While sterically less-encumbered NHCs react with 1 affording NHC-adducts which are in equilibrium with 1 in solution, sterically encumbered NHCs do not bind to 1 at all. Interestingly, in both of these situations hydrolysis of the P[double bond, length as m-dash]P motif proceeds efficiently. At low temperatures, sterically less-encumbered NHCs are catalytic while the sterically encumbered NHCs play a catalytic role at room temperature. To gain insight on this striking influence of NHCs on the hydrolysis of diphosphene detailed low-temperature ³¹P-NMR studies along with theoretical calculations have been carried out. In addition, systematic hydrolysis studies of all the NHCs used in this study have also been performed.

Proton transfer vs. oligophosphine formation by P–C/P–H σ-bond metathesis: decoding the competing Brønsted and Lewis type reactivities of imidazolio-phosphines

Cationic imidazolio-phosphines show two-sided reactivity towards bases, undergoing either Brønsted-type proton transfer to imidazolio-phosphides or autocatalytic Lewis acid/base reaction cascades to yield P-free imidazolium ions and oligophosphines.

NHC-Coordinated Diphosphene-Stabilized Gold(I) Hydride and Its Reversible Conversion to Gold(I) Formate with CO2

An NHC-coordinated diphosphene is employed as ligand for the synthesis of a hydrocarbon-soluble monomeric AuI hydride, which readily adds CO2 at room temperature yielding the corresponding AuI formate. The reversible reaction can be expedited by the addition of NHC, which induces β-hydride shift and the removal of CO2 from equilibrium through the formation of an NHC-CO2 adduct. The AuI formate is alternatively formed by dehydrogenative coupling of the AuI hydride with formic acid (HCO2 H), thus in total establishing a reaction sequence for the AuI hydride mediated dehydrogenation of HCO2 H as chemical hydrogen storage material.

Crystalline Divinyldiarsenes and Cleavage of the As=As Bond

The first divinyldiarsenes [{(NHC)C(Ph)}As]₂ (NHC=IPr 3 a, SIPr 3 b; IPr=C{(NAr)CH}₂ ; SIPr=C{(NAr)CH₂ }₂ ; Ar=2,6-iPr₂ C₆ H₃ ) are reported. Compounds 3 a and 3 b were prepared by the reduction of corresponding chlorides {(NHC)C(Ph)}AsCl₂ (NHC=IPr 2 a, SIPr 2 b) with Mg. Calculations revealed a small HOMO-LUMO energy gap of 3.86 (3 a) and 4.24 eV (3 b). Treatment of 3 a with (Me₂ S)AuCl led to the cleavage of the As=As bond to restore 2 a, which is expected to proceed via the diarsane [{(IPr)C(Ph)}AsCl]₂ (4). Remarkably, 4 as well as 2 a can be selectively accessed on treatment of 3 a with an appropriate amount of C₂ Cl₆ . Moreover, 3 a readily reacts with PhEEPh (E=Se or Te) at room temperature to give {(IPr)C(Ph)}As(EPh)₂ (E=Se 5 a; Te 5 b), revealing the cleavage of As=As and E-E bonds and the formation of As-E bonds. Such highly selective stepwise oxidation (3 a→4→2 a) and bond metathesis (3 a→5 a,b) reactions are unprecedented in main-group chemistry.

Structural Diversity in Supramolecular Organization of Anionic Phosphate Monoesters: Role of Cations

Syntheses and structures of anionic arylphosphate monoesters [ArOP(O)₂(OH)]^- (Ar = 2,6-CHPh₂-4-R-C₆H₂; R = Me/Et/iPr/tBu) with different counter cations are reported. The counter cations were varied systematically: imidazolium cation, 2-methyl imidazolium cation, N-methyl imidazolium cation, N,N'-alkyl substituted imidazolium cation, 1,4-diazabicyclo[2.2.2]octan-1-ium cation, 4,4'-bipyridinium dication, and magnesium(II) dication. The objective was to examine if the supramolecular structure of anionic arylphosphate monoesters could be modulated by varying the cation. It was found that an eight-membered P₂O₄H₂-hydrogen-bonded dimeric motif involving intermolecular H-bonding between the [P(O)(OH)] unit of the anionic phosphate monoester along with the counter cation is formed with 2-methyl imidazolium cation, N-methyl imidazolium cation, N,N'-alkyl substituted imidazolium cation, 1,4-diazabicyclo[2.2.2]octan-1-ium cation, and magnesium(II) dication; both discrete and polymeric H-bonded structures are observed. In the case of imidazolium cations and 1,4-diazabicyclo[2.2.2]octan-1-ium cation, the formation of one-dimensional polymers (single lane/double lane) was observed. On the other hand, two types of phosphate motifs, intermolecular H-bonded dimer and an open-form, were observed in the case of 4,4'-bipyridinium dication.

Facile Cleavage of the P=P Double Bond in Vinyl-Substituted Diphosphenes

The reactions of the cyclic alkyl amino carbene (CAAC) 1 with phosphaalkynes generate the kinetically unstable CAAC-derived phosphirenes 4 and 5, which undergo rearrangement/dimerization reactions to give the vinyl-substituted diphosphenes 2, 3, and 6. The P=P double bond scission of 2 or 3 is unprecedentedly effected by S₈ , [AuCl(tht)], or MeOTf at room temperature, which affords a dithiophosphorane 7, a phosphepine Au complex 8, or phosphepinium cations 9 and 10, respectively. The cationic species feature little homoaromaticity while representing the first examples of the phosphorus-containing analogue of the tropylium ion.

A Room-Temperature-Stable Phosphanorcaradiene

A room-temperature-stable crystalline phosphanorcaradiene 2 has been synthesized via a C-C bond forming strategy induced by the demetalation of a phosphepine-Au complex 1 using a Lewis base (Ph₃P, IDipp or CyNC). Compound 2 undergoes photolysis to afford a 2 H-phosphirene 4. Ru(PPh₃)₃Cl₂ is shown to catalyze the equilibration of 2 and 4 in solution. In addition, the transformation of 2 to 4 can be achieved by a two-step strategy, namely cyclopropenylidene-induced ring opening to give cyclopropenylidene-stabilized vinylphosphinidene 7 followed by elimination of cyclopropenylidene by an electrophile including TMSOTf and Me₂SBH₃.