N-heterocyclic carbene induced reductive coupling of phosphorus tribromide. Isolation of a bromine bridged P-P bond and its subsequent reactivity

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.

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.

Boron-based octahedral dication experimentally detected: DFT surface confirms its availability

Borane and heteroborane clusters have been known as neutral or anionic species. In contrast to them, several ten-vertex monocationic nido and closo dicarbaborane-based systems have recently emerged from the reaction of the parent bicapped-square antiprismatic dicarbaboranes with N-heterocyclic carbenes followed by the protonization of the corresponding nido intermediates. The expansion of these efforts has afforded the very first closo-dicationic octahedral phosphahexaborane along with new closo-monocationic pnictogenahexaboranes of the same shapes. All are the products of the one-pot procedure that consists in the reaction of the same carbenes with the parent closo-1,2-Pn₂B₄Br₄ (Pn = As, P). Whereas in the case of phosphorus such a monocation appears to be a mixture of stable intermediates, and arsenahexaboranyl monocation has occurred as the final product, all of them without using any subsequent reaction. The well-established DFT/ZORA/NMR approach has unambiguously confirmed the existence of these species in solution, and computed electrostatic potentials have revealed the delocalization of the positive charge in these monocations and in the very first dication, namely within the octahedral shapes in both cases.

Halogenation and Nucleophilic Quenching: Two Routes to E−X Bond Formation in Cobalt Triple‐Decker Complexes (E=As, P; X=F, Cl, Br, I)

The oxidation of [(Cp’’’Co)₂(μ,η² : η²‐E₂)₂] (E=As (1), P (2); Cp’’’=1,2,4‐tri(tert‐butyl)cyclopentadienyl) with halogens or halogen sources (I₂, PBr₅, PCl₅) was investigated. For the arsenic derivative, the ionic compounds [(Cp’’’Co)₂(μ,η⁴ : η⁴−As₄X)][Y] (X=I, Y=[As₆I₈]_0.5 (3 a), Y=[Co₂Cl_6‐nI_n]_0.5 (n=0, 2, 4; 3 b); X=Br, Y=[Co₂Br₆]_0.5 (4); X=Cl, Y=[Co₂Cl₆]_0.5 (5)) were isolated. The oxidation of the phosphorus analogue 2 with bromine and chlorine sources yielded the ionic complexes [(Cp’’’Co)₂(μ‐PBr₂)₂(μ‐Br)][Co₂Br₆]_0.5 (6 a), [(Cp’’’Co)₂(μ‐PCl₂)₂(μ‐Cl)][Co₂Cl₆]_0.5 (6 b) and the neutral species [(Cp’’’Co)₂(μ‐PCl₂)(μ‐PCl)(μ,η¹ : η¹‐P₂Cl₃] (7), respectively. As an alternative approach, quenching of the dications [(Cp’’’Co)₂(μ,η⁴ : η⁴‐E₄)][TEF]₂ (TEF=[Al{OC(CF₃)₃}₄]⁻, E=As (8), P (9)) with KI yielded [(Cp’’’Co)₂(μ,η⁴ : η⁴‐As₄I)][I] (10), representing the homologue of 3, and the neutral complex [(Cp’’’Co)(Cp’’’CoI₂)(μ,η⁴ : η¹‐P₄)] (11), respectively. The use of [(CH₃)₄N]F instead of KI leads to the formation of [(Cp’’’Co)₂(μ‐PF₂)(μ,η² : η¹ : η¹‐P₃F₂)] (12) and 2, thereby revealing synthetic access to polyphosphorus compounds bearing P−F groups and avoiding the use of very strong fluorinating reagents, such as XeF₂, that are difficult to control.

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.

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.

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).

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 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.

Heteroleptic diphosphenes and arsaphosphenes bearing neutral and anionic N-heterocyclic carbenes

A modular approach afforded heteroleptic arsaphosphene species supported by neutral and anionic N-heterocyclic carbenes.

Taming a silyldiium cation and its reactivity towards sodium phosphaethynolate

A dicationic bis(NHC)-stabilised silyldiium complex, [bis(NHC)-SiPh₂]²⁺ (7²⁺) (bis(NHC) = [CH₂(NC₃H₂NDipp)₂], Dipp = 2,6-ⁱPr₂C₆H₃), was synthesised for the first time. It reacts with sodium phosphaethynolate (NaOCP) as a source of monoanionic phosphorus to give the P-insertion product [bis(NHC)-PSiPh₂]⁺ (8⁺). The latter comprises a seven-membered heterocycle containing a Si-P moiety which can easily be desilylated when exposed to dichlorophosphanes as exemplified by the synthesis of the diphosphanide cations [bis(NHC)-PPCy]⁺ (9⁺) and [bis(NHC)-PPPh]⁺ (10⁺).

Isolable N-heterocyclic carbene adducts of the elusive diiodophosphine

Reactions of PH-substituted phosphinidene-imidazolylidenes with I₂ afford isolable NHC-adducts of the transient diiodophosphine PHI₂. The products, which show a surprising structural diversity, are according to DFT studies best formulated as charge-transfer complexes of onio-substituted cationic phosphines with I^- and are capable of reaction under formal transfer of an NHC-P⁺ unit.

NHCs in Main Group Chemistry

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC-main group element adducts, and might be useful for the broad research community.

Recent Advances in Imidazoliumyl-Substituted Phosphorus Compounds

This review aims to highlight the recent developments in the chemistry of selected imidazoliumyl-substituted phosphorus compounds. The synthetic approaches for their preparation with phosphorus in various oxidation states and coordination environments are discussed. Their intriguing properties and versatile chemistry strongly depends on the bonding motif at the P atoms, which is given special focus.

N-Heterocyclic carbene adducts of the heavier group 15 tribromides. Normal to abnormal isomerism and bromide ion abstraction

The reactivity of the heavier group 15 tribromides, SbBr₃ and BiBr₃, towards 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr) is described.

A modular approach to carbene-stabilized diphosphorus species

A modular approach has been developed for the synthesis of a series of neutral, cationic and dicationic heteroleptic dicarbene–diphosphorus species.