Versatile Reagent Ph3As(OTf)2: One-Pot Synthesis of [P7(AsPh3)3][OTf]3from PCl3

Reactivity of Tetrel-Functionalized Heptaphosphane Clusters toward Azides

In this work, the reactivity of tetrel-functionalized phosphorus clusters toward organoazides is probed. Clusters (Me3Si)3P7 (1) and (Me3Ge)3P7 (2) were reacted with benzyl azide, phenyl azide, and 4-bromophenyl azide, and it was found that the [RN] (R = benzyl, phenyl, and 4-bromophenyl) unit from the azide inserted into the phosphorus-tetrel bonds on the cluster, accompanied by N2 elimination. Through control of the azide stoichiometry, the mono-, bis-, and tris-inserted products could be observed, consistent with these insertions proceeding in a stepwise manner. The bonding between the amine moieties and clusters was further investigated by computational chemistry, and the findings were consistent with the phosphorus cluster having undergone a formal oxidation. These insertion reactions are a convenient means of accessing Zintl clusters functionalized with exo-nitrogen-bonded moieties, which, to the best of our knowledge, were previously unknown.

Recent Breakthroughs in P4 Chemistry: Towards Practical, Direct Transformations into P1 Compounds

For several decades, academic researchers have been intensively studying the chemistry of white phosphorus (P₄ ) in the hope of developing direct methods for its transformation into useful P-containing products. This would bypass the hazardous, multistep procedures currently relied on by industry. However, while academically interesting P₄ activation reactions have become well established, their elaboration into useful, general synthetic procedures has remained out of reach. Very recently, however, a series of independent reports has begun to change this state of affairs. Each shows how relatively simple and practical synthetic methods can be used to access academically or industrially relevant P₁ compounds from P₄ directly, in "one pot" or even in a catalytic fashion. These reports mark a step change in the field of P₄ chemistry, and suggest its possible transition from an area of largely academic interest to one with the promise of true synthetic relevance.

Carbene chemistry of arsenic, antimony, and bismuth: origin, evolution and future prospects

The discovery of the first isolable N-heterocyclic carbene in 1991 ushered in a new era in coordination chemistry. The remarkable bonding properties of carbenes have led to their rapid proliferation as auxiliary ligands for a wide range of transition metals and main group elements. In the case of group 15, while carbene-stabilized nitrogen and phosphorus compounds are extensively studied, the scope of research has shrunk significantly from arsenic to bismuth. This is essentially attributed to the decrease in stability of the C-E bond upon descending the group. Even so, modulating the carbene backbone or introducing alternative synthetic strategies not only alleviates the stability issues but also offers promising results in terms of the bonding and reactivities of these compounds. The purpose of the present perspective is to provide a comprehensive overview of the origins and development of carbene chemistry of arsenic, antimony, and bismuth, as well as to highlight the future prospects of this field.

Direct conversion of white phosphorus to versatile phosphorus transfer reagents via oxidative onioation

The main feedstock for the value-added phosphorus chemicals used in industry and research is white phosphorus (P₄), from which the key intermediate for forming P(III) compounds is PCl₃. Owing to its high reactivity, syntheses based on PCl₃ are often accompanied by product mixtures and laborious work-up procedures, so an alternative process to form a viable P(III) transfer reagent is desirable. Our concept of oxidative onioation, where white phosphorus is selectively converted into triflate salts of versatile P₁ transfer reagents such as [P(L_N)₃][OTf]₃ (L_N is a cationic, N-based substituent; that is, 4-dimethylaminopyridinio), provides a convenient alternative for the implementation of P-O, P-N and P-C bonds while circumventing the use of PCl₃. We use p-block element compounds of type R_nE (for example, Ph₃As or PhI) to access weak adducts between nitrogen Lewis bases L_N and the corresponding dications [R_nEL_N]²⁺. The proposed equilibrium between [R_nEL_N]²⁺ + L_N and [R_nE(L_N)₂]²⁺ allows for the complete oxidative onioation of all six P-P bonds in P₄ to yield highly reactive and versatile trications [P(L_N)₃]³⁺.

Drastic Enhancement of Photosensitized Energy Transfer Efficiency of a Eu(III) Complex Driven by Arsenic

In this study, we focused on arsenic as a new potential motif for the ligand design of high-efficiency, luminous lanthanide complexes. A Eu³⁺ complex bearing triphenylarsine oxide had a photosensitized energy-transfer efficiency 7.9 times higher than that of a Eu³⁺ complex bearing triphenylphosphine oxide. This is mainly due to the heavy-atom effect of arsenic, which was supported by evaluating the photoluminescence spectra of their corresponding Gd³⁺ complexes.

Bicyclic and tricyclic phosphanes with p-block substituents

This review summarises the experimental and structural knowledge on polycyclic phosphanes, with a focus on bicyclic and tricyclic phosphanes, as they have not only been the most studied in the last 25 years, but also show the greatest diversity in terms of constitutional isomerism and structural motifs. Moreover, only polycyclic phosphanes that have p-block substituents at all free valences are discussed.

Isolation and Structural Determination of a Hexacoordinated Antimony(V) Dication

The hexacoordinated antimony(V) dication [(ppy)₃ Sb]²⁺ ([1]²⁺ ; ppy=2-(2-pyridyl)phenyl), stabilized by three intramolecular donor-acceptor interactions, has been isolated as its hexachloroantimonate salt [1][SbCl₆ ]₂ , prepared by the oxidative addition of chlorine to the neutral stibine [(ppy)₃ Sb] (1), followed by the abstraction of chloride. Air-stable [1][SbCl₆ ]₂ exhibits remarkable thermal stability and the three ppy ligands on the antimony atom are shown to be magnetically inequivalent in the ¹ H and ¹³ C NMR spectra. A hexacoordinated, meridional octahedral bonding geometry has been determined for [1][SbCl₆ ]₂ by X-ray crystallographic analysis. Theoretical calculations were performed to investigate why the meridional form was generated preferentially over the facial form. In addition, the dynamics of the ppy ligands were investigated by variable-temperature ¹ H NMR spectroscopy. The potential to generate dications by using a single-electron-transfer reagent has also been investigated. The dication [1]²⁺ is the first [12-Sb-6]²⁺ chemical species to have been structurally determined.

Toward N,P-Doped π-Extended PAHs: A One-Pot Synthesis to Diannulated 1,4,2-Diazaphospholium Triflate Salts

A novel method for the one-pot synthesis of diannulated 1,4,2-diazaphospholium triflate salts by a Me₃SiOTf-mediated self-condensation of dichlorophosphaneyl aza-(poly)cyclic aromatic hydrocarbons (aza-(P)AHs; namely, pyridine, quinoline, phenanthridine, and benzo[d]thiazole) is reported. The diannulated 1,4,2-diazaphospholium triflate salts are characterized by multinuclear NMR spectroscopy, X-ray analysis, as well as their calculated NICS values, underlining their aromatic character. Quantum mechanical calculations shed light on the intermolecular reaction mechanism. Follow up chemistry, such as the halogenation reaction with XeF₂ or SO₂Cl₂ with the dipyridinium derivative selectively yields the respective dihalo-σ⁴,λ⁵- and tetrahalo-σ⁵,λ⁶-diazaphospholium triflate salts. The dihalo-σ⁴,λ⁵-diazaphospholium triflate salt serves well as a surrogate for the introduction of the cationic 2-(1,2'-bipyridin)-1-iumyl ligand (1,2'-bipyl), the monocationic structural isomer of the prototypical 2,2'-bipyridine ligand (2,2'-bipy).

Formation of an imidazoliumyl-substituted [(LC)4P4]4+ tetracation and transition metal mediated fragmentation and insertion reaction (LC = NHC)

Tetracationic cyclo-tetraphosphane [(L_C)₄P₄]⁴⁺ as triflate salt (3[OTf]₄) (L_C = 4,5-dimethyl-1,3-diisopropyl-imidazol-2-yl) is obtained in high yield from the reduction of [L_CPCl₂]⁺ (4[OTf]) with 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (6) and represents the first salt of the cationic cyclo-phosphane series with the general formula [L _n P _n ] ⁿ⁺. Theoretical calculations reveal the electrophilic nature of the P atoms within the P₄-ring due to the influence of the imidazoliumyl-substituents. Further reduction of 3[OTf]₄ with 6 affords the unexpected formation of the notricyclane P₇-type cation [(L_C)₃P₇]³⁺ (9[OTf]₃). Selective transition metal mediated [2 + 2]-fragmentation of 3 ⁴⁺ is achieved when 3[OTf]₄ is reacted with Fe₂(CO)₉, Pd(PPh₃)₄ and Pt(PPh₃)₄ leading to the formation of the dicationic diphosphene complexes [(η²-L_CP[double bond, length as m-dash]PL_C)Fe(CO)₄]²⁺ (12[OTf]₂) and [(η²-L_CP[double bond, length as m-dash]PL_C)M(PPh₃)₂]²⁺ (13[OTf]₂ for M = Pd; 14[OTf]₂ for M = Pt). In contrast, the reaction of 3[OTf]₄ with an excess of AuCl(tht) gives rise to the formation of the five-membered ring complex [((L_C)₄P₄)AuCl₂]³⁺ (15[OTf]₃), where the Au(i) atom reductively inserts into a P-P bond of 3 ⁴⁺.

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry

N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and low-coordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element-metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.

Tris(benzoimidazol)amine (L) complexes of pnictogen(iii) and pnictogen(v) cations and assessment of the [LP]3+/[LPF2]3+ redox couple

A series of cationic complexes involving a pnictogen(iii) (Pn = P, As, Sb) centre and the tetradentate ligand tris((1-ethyl-benzoimidazol-2-yl)methyl)amine (BIMEt3) have been synthesized and comprehensively characterized. Oxidation of [P(BIMEt3)]3+ with XeF2 provides access to [PF2(BIMEt3)]3+ representing the first structurally characterized example of a phosphorus(v) centred trication.

Selective Derivatization of a Hexaphosphane from Functionalization of White Phosphorus

The reaction of LGa (L = Dipp(4-(Dipp-imino)pent-2-en-2-yl)amide; Dipp: 2,6-diisopropylphenyl) and white phosphorus was revisited. A plethora of unprecedented polyphosphanes in addition to the known monoinserted product LGaP₄ (1) are observed. An optimized synthesis of the hitherto unknown hexaphosphane (LGa)₂P₆ (3) is presented, and its subsequent selective derivatization with Brønsted acids, MeOTf, Ph₂ECl (E = P, As), and NaOCP provides access to a wealth of functionalized hexa- and heptaphosphanes.

A Tetracyclic Octaphosphane by Successive Addition, Inversion, and Condensation Reactions

An example of an octaphosphane of type R₂ P₈ (R=(DDP)Ga) was isolated by treatment of cage compound (DDP)GaP₄ (2, DDP=(2,6-diisopropylphenyl)(4-((2,6-diisopropylphenyl)imino)pent-2-en-2-yl)amide) with (C₆ F₅ )₂ PBr. The initially formed endo-exo butterfly shaped pentaphosphane 7 rapidly rearranges to the more stable exo-exo isomer 8, which undergoes dimerization to decaphosphane 11. Compound 11 unexpectedly eliminates tetraaryldiphosphane 13 to give tetracyclo[3.3.0.0^2,7 .0^4,6 ]octaphosphane [(DDP)GaBr]₂ P₈ (12). The reaction steps were confirmed by crystal structure analysis of the key intermediates and supported by kinetic studies using NMR techniques.

Carbodiphosphorane mediated synthesis of a triflyloxyphosphonium dication and its reactivity towards nucleophiles

A carbodiphosphorane ((Ph₃P)₂C) mediated synthesis of the first triflyloxyphosphonium dication (12+) bearing two electrophilic sites is presented. Depending on the nucleophile, 12+ reacts selectively either at the sulfur atom of the triflyloxy moiety or at the directly attached phosphorus atom. In substitution reactions at the phosphorus atom the triflyloxy moiety serves as a leaving group and enables the synthesis of rare examples of pseudo-halophosphonium dications.

Reduction of [Cp*Sb]4 with Subvalent Main-Group Metal Reductants: Syntheses and Structures of [(L1 Mg)4 (Sb4 )] and [(L2 Ga)2 (Sb4 )] Containing Edge-Missing Sb4 Units

[Cp*Sb]₄ (Cp*=C₅ Me₅ ) reacts with [L¹ Mg]₂ and L² Ga with formation of [(L¹ Mg)₄ (μ₄ ,η^1:2:2:2 -Sb₄ )] (L¹ =iPr₂ NC[N(2,6-iPr₂ C₆ H₃ )]₂ , 1) and [(L² Ga)₂ (μ,η^2:2 -Sb₄ )] (L² =HC[C(Me)N(2,6-iPr₂ C₆ H₃ )]₂ , 2). The cleavage of the Sb-Sb and Sb-C bonds in [Cp*Sb]₄ are the crucial steps in both reactions. The formation of 1 occurred by elimination of the Cp* anion and formation of Cp*MgL¹ , while 2 was formed by reductive elimination of Cp*₂ and oxidative addition of L² Ga to the Sb₄ unit. 1 and 2 were characterized by heteronuclear NMR spectroscopy and single-crystal X-ray diffraction, and their bonding situation was studied by quantum chemical calculations.

Tris(2-pyridyl)phosphine as a versatile ligand for pnictogen acceptors

We report cationic complexes of arsenic and antimony with the tris(2-pyridyl)phosphine ligand. Chloride ion abstraction from the main group halide using TMSOTf in presence of the ligand gives [P(Pyr)₃Pn][OTf]₃, in which the trication adopts a Janus Head type complex with a C_3v symmetric cage structure and two apical lone pairs.

α-Cationic Arsines: Synthesis, Structure, Reactivity, and Applications

A series of structurally differentiated cationic arsines containing imidazolium, cyclopropenium, formamidinium, and pyridinium substituents have been synthesized through short and scalable routes. Evaluation of the donor properties of these compounds by IR spectroscopy and DFT calculations reveals similar σ-electron-releasing abilities for all of them; however, their π-acceptor properties are strongly influenced by the nature of the positively charged group. We describe the coordination chemistry of the newly prepared α-cationic arsines toward different metal centers and their reactivity in the presence of strong oxidants to afford cationic As(V) species. Their unique electronic properties have been exploited in Pt(II) catalysis to develop a new catalyst with remarkable activity in the cycloisomerization of enynes to trisubstituted cyclopropanes. To the best of our knowledge, this is the first report on the use of α-cationic arsine ligands in catalysis.

Reactive p-block cations stabilized by weakly coordinating anions

The chemistry of the p-block elements is a huge playground for fundamental and applied work. With their bonding from electron deficient to hypercoordinate and formally hypervalent, the p-block elements represent an area to find terra incognita. Often, the formation of cations that contain p-block elements as central ingredient is desired, for example to make a compound more Lewis acidic for an application or simply to prove an idea. This review has collected the reactive p-block cations (rPBC) with a comprehensive focus on those that have been published since the year 2000, but including the milestones and key citations of earlier work. We include an overview on the weakly coordinating anions (WCAs) used to stabilize the rPBC and give an overview to WCA selection, ionization strategies for rPBC-formation and finally list the rPBC ordered in their respective group from 13 to 18. However, typical, often more organic ion classes that constitute for example ionic liquids (imidazolium, ammonium, etc.) were omitted, as were those that do not fulfill the - naturally subjective -"reactive"-criterion of the rPBC. As a rule, we only included rPBC with crystal structure and only rarely refer to important cations published without crystal structure. This collection is intended for those who are simply interested what has been done or what is possible, as well as those who seek advice on preparative issues, up to people having a certain application in mind, where the knowledge on the existence of a rPBC that might play a role as an intermediate or active center may be useful.

Formation of the spirocyclic, Si-centered cage cations [ClP(NSiMe3)2Si(NSiMe3)2P5](+) and [P5(NSiMe3)2Si(NSiMe3)2P5](2)

On account of our interest in P4 activation by phosphenium ion insertion into P-P bonds we have developed synthetic routes to bicyclic N-P-Si-heterocycle 7 and probed its reactivity towards GaCl3 and P4. A GaCl3-induced rearrangement of 7 leads to the in situ formation of spirocyclic, Si-centered phosphenium ions. Their insertion into P-P bonds of one or two P4 tetrahedra yields polyphosphorus cages [ClP(NSiMe3)2Si(NSiMe3)2P5](+) (19(+)) and [P5(NSiMe3)2Si(NSiMe3)2P5](2+) (13(2+)).

Bipyridine complexes of E3+ (E = P, As, Sb, Bi): strong Lewis acids, sources of E(OTf)3 and synthons for EI and EV cations

Triflate salts of trications [(bipy)₂E]³⁺ ([6E][OTf]₃) and [(tbbipy)₂E]³⁺ ([6′E][OTf]₃) (bipy = 2,2′-bipyridine, tbbipy = 4,4′-di-^tbutyl-2,2′-bipyridine; E = P, As, Sb, Bi) have been synthesized and comprehensively characterized.

Triflate salts of trications [(bipy)₂E]³⁺ ([6E][OTf]₃) and [(tbbipy)₂E]³⁺ ([6′E][OTf]₃) (bipy = 2,2′-bipyridine, tbbipy = 4,4′-di-^tbutyl-2,2′-bipyridine; E = P, As, Sb, Bi) have been synthesized and comprehensively characterized. The unique molecular and electronic structures of this new class of complexes involving pnictogen Lewis acids has been assessed in the solid, solution and gas phases to reveal systematic variations in metric parameters, ligand lability and charge concentration. While the Lewis acidity of E³⁺ has the trend E = Bi < Sb < As < P as determined by gas-phase calculations and ¹H NMR spectroscopy, the Lewis acidity of [6E]³⁺ has the trend E = P < As < Sb < Bi according to gas-phase calculations. Derivatives of [6′E][OTf]₃ (E = P, As) are latent sources of E(OTf)₃ as demonstrated by their reactions with dmap, which give the corresponding derivatives of [(dmap)₃E][OTf]₃. The highly oxidizing nature of P(OTf)₃ and As(OTf)₃ is evidenced in reactions of [6′E][OTf]₃ (E = P, As) with phosphines, which give E^I-containing monocations [(R₃P)₂E]¹⁺ and oxidatively coupled dications [R₃PPR₃]²⁺, illustrating new P–P and P–As bond forming strategies. Cations [6′E]³⁺ (E = P, As) are C–H bond activating agents that dehydrogenate 1,4-cyclohexadiene, with higher activity observed for E = P. Combinations of [6′E]³⁺ and ^tBu₃P activate H₂ and D₂ under mild conditions, evidencing frustrated Lewis pair activity. Oxidation of [6′P][OTf]₃ with SO₂Cl₂ gives [(tbbipy)₂PCl₂][OTf]₃, containing a P^V-trication, but there is no evidence of the analogous reaction with [6′As][OTf]₃. The observations highlight new directions in the chemistry of highly charged cations and reveal a rich reactivity for p-block triflates E(OTf)₃, which can be accessed through derivatives of [6E][OTf]₃ and [6′E][OTf]₃.

Establishing the Coordination Chemistry of Antimony(V) Cations: Systematic Assessment of Ph4 Sb(OTf) and Ph3 Sb(OTf)2 as Lewis Acceptors

The coordination chemistry of the stiboranes Ph4 Sb(OTf) (1 a, OTf = OSO2 CF3 ) and Ph3 Sb(OTf)2 (3) with Lewis bases has been investigated. The significant steric encumbrance of the Sb center in 1 a precludes interaction with most ligands, but the relatively low steric demands of 4-methylpyridine-N-oxide (OPyrMe) and OPMe3 enabled the characterization of [Ph4 Sb(OPyrMe)][OTf] (2 a) and [Ph4 Sb(OPMe3 )][OTf] (2 b), rare examples of structurally characterized complexes of stibonium acceptors. In contrast, 3 was found to engage a variety of Lewis bases, forming stable isolable complexes of the form [Ph3 Sb(donor)2 ][OTf]2 [donor=OPMe3 (6 a), OPCy3 (6 b, Cy=cyclohexyl), OPPh3 (6 c), OPyrMe (6 d)], [Ph3 Sb(dmap)2 (OTf)][OTf] (6 e, dmap=4-(dimethylamino)pyridine) and [Ph3 Sb(donor)(OTf)][OTf] [donor=1,10-phenanthroline (7 a) or 2,2'-bipy (7 b, bipy=bipyridine)]. These compounds exhibit significant structural diversity in the solid-state, and undergo ligand exchange reactions in line with their assignment as coordination complexes. Compound 3 did not form stable complexes with phosphine donors, with reactions instead leading to redox processes yielding SbPh3 and products of phosphine oxidation.