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.

Synthesis of an Indazole/Indazolium Phosphine Ligand Scaffold and Its Application in Gold(I) Catalysis

Advances in ligand development have allowed for the fine-tuning of gold catalysis. To contribute to this field, we designed an indazole phosphine ligand scaffold that allows facile introduction of cationic charge through methylation. With minimal changes to the structure upon methylation, we could assess the importance of the electronic effects of the insertion of a positive charge on the catalytic activity of the resulting gold(I) complex. Using the benchmark reactions of propargyl amide cyclization and enyne cyclization with and without hexafluoroisopropanol (HFIP), we observed marked differences in the catalytic activities of the neutral and cationic gold species.

1,2,3-Triazole based ligands with phosphine and pyridine functionalities: synthesis, PdII and PtII chemistry and catalytic studies

This manuscript describes the syntheses of pyridine appended triazole-based mono- and bisphosphines, [o-Ph₂P(C₆H₄){1,2,3-N₃C(Py)C(H)}] (2), [o-Br(C₆H₄){1,2,3-N₃C(Py)C(PPh₂)}] (3), [C₆H₅{1,2,3-N₃C(Py)C(PPh₂)}] (4), [Ph₂P(C₆H₄){1,2,3-N₃C(Py)C(PPh₂)}] (5) and [3-Ph₂P-2-{1,2,3-N₃C(Ph)C(PPh₂)}C₅H₃N] (6), their palladium and platinum chemistry and catalytic applications. These ligands upon treatment with [M(COD)Cl₂] (M = Pd or Pt) yielded complexes with different coordination modes, depending on the reaction conditions. Both κ²-P,N and κ²-P,P coordination modes were observed in many of the complexes indicating the ambidentate nature of these ligands. Monophosphine 2 in the presence of a base afforded rare fused-5,6-membered PCN pincer complexes [MCl{o-Ph₂P(C₆H₄){1,2,3-N₃C(Py)C(H)}}-κ³-P,C,N] (7, M = Pd; 8, M = Pt), whereas the reactions of 4 with [M(COD)Cl₂] (M = Pd, Pt) produced κ²-P,N chelate complexes [MCl₂{C₆H₅{1,2,3-N₃C(Py)C(PPh₂)}-κ²-P,N}] (9, M = Pd; 10, M = Pt). Similar reactions of 5 and 6 resulted in κ²-P,P chelate complexes [MCl₂{{3-Ph₂P-2-{1,2,3-N₃C(Ph)C(PPh₂)}C₅H₃N}-κ²-P,P}] (11, M = Pd; 12, M = Pt) and [MCl₂{3-Ph₂P-2-{1,2,3-N₃C(Ph)C(PPh₂)}C₅H₃N}-κ²-P,P}] (13, M = Pd; 14, M = Pt), respectively. The palladium(II) complexes have shown excellent catalytic activity in the α-alkylation reaction of acetophenone derivatives.

N-Heterocyclic Carbene-Phosphinidenide Complexes as Hydroboration Catalysts

The reactions of the N-heterocyclic carbene-phosphinidene adducts (NHC)PSiMe₃ and (NHC)PH with the dinuclear ruthenium and osmium complexes [(η⁶-p-cymene)MCl₂]₂ (M = Ru, Os) afforded the half-sandwich complexes [(η⁶-p-cymene){(NHC)P}MCl] and [(η⁶-p-cymene){(NHC)PH}MCl₂] with two- and three-legged piano-stool geometries, respectively (NHC = IDipp, IMes; IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene; IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene). The complexes were initially tested as precatalysts for the hydroboration of benzonitrile, and the most active species, the ruthenium complex [(η⁶-p-cymene){(IMes)P}RuCl], was further used for the efficient hydroboration of a wide range (ca. 50 substrates) of nitriles, carboxylic esters, and carboxamides in neat pinacolborane (HBpin) under comparatively mild reaction conditions (60-80 °C, 3-5 mol % catalyst loading). Preliminary mechanistic and kinetic studies are reported, and stoichiometric reactions with HBpin indicate the initial formation of the monohydride complex [(η⁶-p-cymene){(IMes)P}RuH] as the putative catalytically active species.

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.

Isolation and reactivity of a gold(I) hydroxytrifluoroborate complex stabilized by anion-π+ interactions

A 9,9-dimethylxanthene-based ligand substituted at the 4- and 5-positions by a phosphine and a xanthylium unit, respectively, has been prepared and converted into an AuCl complex, the structure of which reveals an intramolecular Au-Cl⋯π⁺ interaction. This new ligand platform was also found to support the formation of an unprecedented hydroxytrifluoroborate derivative featuring a "hard/soft" mismatched Au-μ(OH)-BF₃ motif. Despite its surprising stability, this gold hydroxytrifluoroborate complex is a remarkably potent carbophilic catalyst which readily activates alkynes, without activator.

Direct Access to Palladium(II) Complexes Based on Anionic C,C,C-Phosphonium Ylide Core Pincer Ligand

The reaction of readily available imidazolium-phosphonium salt [MesIm(CH₂)₃PPh₃](OTf)₂ with PdCl₂ in the presence of an excess of Cs₂CO₃ afforded selectively in one step the cationic Pd(II) complex [(C,C,C)Pd(NCMe)](OTf) exhibiting an LX₂-type NHC-ylide-aryl C,C,C-pincer ligand via formal triple C-H bond activation. The replacement of labile MeCN in the latter by CNtBu and CO fragments allowed to estimate the overall electronic properties of this phosphonium ylide core pincer scaffold incorporating three different carbon-based donor ends by IR spectroscopy, cyclic voltammetry, and molecular orbital analysis, revealing its significantly higher electron-rich character compared to the structurally close NHC core pincer system with two phosphonium ylide extremities. The pincer complex [(C,C,C)Pd(CO)](OTf) represents a rare example of Pd(II) carbonyl species stable at room temperature and characterized by X-ray diffraction analysis. The treatment of isostructural cationic complexes [(C,C,C)Pd(NCMe)](OTf) and [(C,C,C)Pd(CO)](OTf) with (allyl)MgBr and nBuLi led to the formation of zwitterionic phosphonium organopalladates [(C,C,C)PdBr] and [(C,C,C)Pd(COnBu)], respectively.

Ligand-enforced intimacy between a gold cation and a carbenium ion: impact on stability and reactivity

Controlling the reactivity of transition metal complexes by positioning non-innocent functionalities around the catalytic pocket is a concept that has led to significant advances in catalysis. Here we describe our efforts toward the synthesis of dicationic phosphine gold complexes of general formula [(o-Ph₂P(C₆H₄)Carb)Au(tht)]²⁺ decorated by a carbenium moiety (Carb) positioned in the immediate vicinity of the gold center. While the most acidic examples of such compounds have limited stability, the dicationic complexes with Carb⁺ = 9-N-methylacridinium and Carb⁺ = [C(Ar^N)₂]⁺ (Ar^N = p-(C₆H₄)NMe₂) are active as catalysts for the cycloisomerization of N-propargyl-4-fluorobenzamide, a substrate chosen to benchmark reactivity. The dicationic complex [(o-Ph₂P(C₆H₄)C(Ar^N)₂)Au(tht)]²⁺, which also promotes hydroarylation and enyne cyclization reactions, displays a higher catalytic activity than its acridinium analog, indicating that the electrophilic reactivity of these complexes scales with the Lewis acidity of the carbenium moiety. These results support the role of the carbenium unit as a non-innocent functionality which can readily enhance the activity of the adjacent metal center. Finally, we also describe our efforts toward the generation and isolation of free γ-cationic phosphines of general formula [(o-Ph₂P(C₆H₄)Carb)]⁺. While cyclization into phosphonium species is observed for Carb⁺ = [C(Ar^N)₂]⁺, [C(Ph)(Ar^N)]⁺, and 9-xanthylium, [(o-Ph₂P(C₆H₄)-9-N-methylacridinium)]⁺ can be isolated as an air stable, biphilic derivative with uncompromised Lewis acidic and basic properties.

This work describes the synthesis of carbenium-based, γ-cationic phosphines and their coordination to Au(i) cations , leading to carbophilic catalysts whose activity is enhanced by the ligand-enforced convergence of the positively charged moieties.

Stretching the P-C Bond. Variations on Carbenes and Phosphanes

The stability and the structure of adducts formed between four substituted phosphanes (PX3, X:H, F, Cl, and NMe2) and 11 different carbenes have been investigated by DFT calculations. In most cases, the structure of the adducts depends strongly on the stability of the carbene itself, exhibiting a linear correlation with the increasing dissociation energy of the adduct. Carbenes of low stability form phosphorus ylides (F), which can be described as phosphane → carbene adducts supported with some back-bonding. The most stable carbenes, which have high energy lone pair, do not form stable F-type structures but carbene → phosphane adducts (E-type structure), utilizing the low-lying lowest unoccupied molecular orbital (LUMO) of the phosphane (with electronegative substituents), benefiting also from the carbene-pnictogen interaction. Especially noteworthy is the case of PCl3, which has an extremely low energy LUMO in its T-shaped form. Although this PCl3 structure is a transition state of rather high energy, the large stabilization energy of the complex makes this carbene-phosphane adduct stable. Most interestingly, in case of carbenes with medium stability both F- and E-type structures could be optimized, giving rise to bond-stretch isomerism. Likewise, for phosphorus ylides (F), the stability of the adducts G formed from carbenes with hypovalent phosphorus (PX-phosphinidene) is in a linear relationship with the stabilization of the carbene. Adducts of carbenes with hypervalent phosphorus (PX5) are the most stable when X is electronegative, and the carbene is highly nucleophilic.

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.

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.

Efficient and Recyclable RuCl3 ⋅ 3H2O Catalyst Modified with Ionic Diphosphine for the Alkoxycarbonylation of Aryl Halides

A series of ionic (mono-/di-)phosphines (L2, L4, and L6) with structural similarity and their corresponding neutral counterparts (L1, L3, and L5) were applied to modulate the catalytic performance of RuCl3 ⋅ 3H2O. With the involvement of the ionic diphosphine (L4), in which the two phosphino-fragments were linked by butylene group, RuCl3 ⋅ 3H2O with advantages of low cost, robustness, and good availability was found to be an efficient and recyclable catalyst for the alkoxycarbonylation of aryl halides. The L4-based RuCl3 ⋅ 3H2O system corresponded to the best conversion of PhI (96 %) along with 99 % selectivity to the target product of methyl benzoate as well as the good generality to alkoxycarbonylation of different aryl halides (ArX, X=I and Br) with alcohols MeOH, EtOH, i-PrOH and n-BuOH. The electronic and steric effects of the applied phosphines, which were analyzed by the 31P NMR for 1 J 31 P- 77 Se 1 J measurement and single-crystal X-ray diffraction, were carefully co-related to the performance RuCl3 ⋅ 3H2O catalyst. In addition, the L4-based RuCl3 ⋅ 3H2O system could be recycled successfully for at least eight runs in the ionic liquid [Bmim]PF6.

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.

Carbeniophosphines versus Phosphoniocarbenes: The Role of the Positive Charge

The chemistry of carbeniophosphines and phosphoniocarbenes, which have general structures derived formally from the three-component "carbene/phosphine/positive charge" association, is presented. These two complementary classes of carbon-phosphorus-based ligands, defined by the presence of an inverted cationic coordinating structure (C+ ∼P: vs. P+ ∼C:) have the common purpose of positioning a positive charge in the vicinity of the metal center. Through selected examples, the synthetic methods, coordination properties, and general reactivity of both cationic species is described. Particular emphasis is placed on the influence of the positive charge on the respective chemical behavior of the two classes of compound.

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.

Au(i)-Catalyzed hydroarylation of alkenes with N,N-dialkylanilines: a dual gold catalysis concept

In the present work, we report the efficiency of the commercially available [(PPh₃)AuCl] complex in combination with a set of potassium salts to catalyze the hydroarylation of a broad variety of alkenes using N,N-dialkylanilines, leading to full conversion and complete selectivity toward the Markovnikov products.

α-Dicationic Chelating Phosphines: Synthesis and Application to the Hydroarylation of Dienes

A series of new P^P-chelating ligands constituted by a dicationic -[P(H₂Im)₂]⁺² unit (H₂Im = 1,3-dimethyl-4,5-dihydroimidazol-2-ylidene) and a -PPh₂ group connected through structurally different backbones have been synthesized. Evaluation of their reactivity toward different metal centers provides evidence that the dicationic fragment, otherwise reluctant to coordinate metals, readily participates in the formation of chelates when embedded into such a scaffold. Moreover, it significantly enhances the Lewis acidity of the metals to which it coordinates. This property has been used to develop a Rh catalyst that efficiently triggers the hydroarylation of dienes with electron-rich aromatic molecules. Kinetic studies and deuterium-labeling experiments, as well as density functional theory calculations, were performed in order to rationalize these findings.

Amidine functionalized phosphines: tuneable ligands for transition metals

A diastereomeric, cationic phosphine ligand [α,β-CgPAmMe]⁺ composed of a chiral amidinium substituent and a racemic phosphacycle has been prepared and compared to its neutral parent α,β-CgPAm.

A convenient access to N-phosphonio-substituted NHC metal complexes [M = Ag(i), Rh(i), Pd(ii)]

NHC Pd(ii) complexes featuring a propyl bridge were ionized by the introduction of a phosphonium moiety, which may coordinate to the metal center or remain pendant.

Synthesis, Structure, and Applications of α-Cationic Phosphines

In α-cationic phosphines, at least one of the three substituents on phosphorus corresponds to a cationic (normally, but not always heteroaromatic) group, which is attached without any spacer to the phosphorus atom by a relatively inert P-C bond. This unique architecture confers to the resulting ligand strong acceptor properties, which frequently surpass those of traditional acceptor ligands such as phosphites or polyfluorinated phosphines. In addition, the fine-tuning of the stereoelectronic properties of α-cationic phosphines is also possible by judicious selection of the number and nature of the cationic groups. The opportunities offered in catalysis by α-cationic ligands arise from this ability to deplete electron density from the metals they coordinate. Thus, if in a hypothetical catalytic cycle the step that determines the rate is facilitated by an increase of the Lewis acidity at the metal center, then an acceleration of the whole process is expected by their use as ancillary ligands. Interestingly, this situation is found more frequently than one might think; many common elementary steps involved in catalytic cycles, such as reductive eliminations, coordination of substrates to metals, or attack of nucleophiles to coordinated substrates, belong to this category and are often fostered by electron poor metal centers. In this regard, our group has observed remarkable ligand acceleration effects by the employment of α-cationic phosphines in Au(I)- and Pt(II)-promoted hydroarylation and cycloisomerization reactions. These results seem to be general in π-acid catalysis when the nucleophile used is not especially electron rich because then their attack to the activated alkene or alkyne is normally rate determining. On the other hand, the use of cationic phosphines also presents drawbacks that limit their range of application. As a general rule, the reduced σ-donation from the phosphine is not compensated by the increased π-back-donation from the metal making the resulting phosphorus-metal bond weaker, and the corresponding catalysts more prone to decomposition. This can be critical when di- or tricationic ancillary ligands are used. In addition, the positively charged groups occasionally participate in undesired side reactions, with either the metal or the substrate, which are not present when their neutral congeners are used. Stimulated by both the fundamental questions regarding bonding and their valuable applications in catalysis, the chemistry of α-cationic phosphines has experienced an enormous growth during the last years. This Account describes our group's efforts and those of others to understand their coordination behavior, study their reactivity, and further develop their range of applications in catalysis.

Bis(cyclopropenium)phosphines: Synthesis, Reactivity, and Applications

A straightforward route for the preparation of a set of bis(cyclopropenium)-substituted phosphines is reported. Due to their dicationic nature, these ligands depict an excellent π-acceptor character. The effect of the ligand substituent pattern on the catalytic activity of the metal complexes thereof derived is also studied. Whereas sterically demanding biaryl groups directly attached to the phosphorus atom seem to facilitate elementary steps such as the product release from the catalyst, long chain dialkylamino groups on the cyclopropenium units maximize the catalysts solubility and, thus, allow the use of typical apolar solvents such as toluene. Importantly, all new ligands prepared can be easily handled in air. Finally, the impact of the newly prepared dicationic phosphines in hydroarylation reactions is demonstrated. In particular, their use in the synthesis of several naphtho[1,2-b]furanes and naturally occurring naphthalene derivatives such as Calanquinone C is reported.

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.

Immobilization of a rhodium catalyst using a diphosphine-functionalized ionic liquid in RTIL for the efficient and recyclable biphasic hydroformylation of 1-octene

A highly efficient and stable Rh–P catalytic system in the RTIL of [PEmim]BF₄ was developed for the biphasic hydroformylation of 1-octene by using the diphosphine-functionalized ionic liquid (FIL) of 2. While 2-Rh(acac)(CO)₂ was immobilized in [PEmim]BF₄ (solvent), a typical biphasic catalysis was fulfilled with advantages of facile separation and recycling ability – 9 runs without any loss of activity. It was found that not only the acquired π-acceptor character of 2, but also the synergetic role of the piperidyl group in [PEmim]BF₄ as an N-containing donor, cooperatively contributed to the efficient hydroformylation due to the facilitated formation and stability of the Rh-H active species (ν 2045 cm⁻¹). This was supported by the in situ high-pressure FT-IR spectral analysis.

Cationic 5-phosphonio-substituted N-heterocyclic carbenes

Cationic NHCs featuring a phosphonium moiety in the 5-position are prepared from the reaction of imidazolium salts with a tert. phosphane. They are used as ligands for the preparation of transition metal complexes, dehydrogenation reactions of prim. and sec. phosphanes and the preparation of a N-heterocyclic olefin (NHO).

Synthesis and reactivity of α-cationic phosphines: the effect of imidazolinium and amidinium substituents

Mono- and dicationic phosphines have been synthesized through the reaction of chloroimidazolinium or chloroamidinium salts with secondary or primary phosphines respectively. The resulting ligands, which depict a significantly reduced donor ability compared with their neutral analogues, have been used to design Pt(II) and Au(I) complexes that effectively catalyse the hydroarylation of alkynes.

Bis[(dialkylamino)cyclopropenimine]-Stabilized P(III) - and P(V) -Centered Dications

The treatment of bis[(dialkylamino)cyclopropenimines] with dihalophosphines in the presence of trimethylsilyl trifluoromethanesulfonate (TMSOTf) to form diimine-stabilized P(III) -centered dications is reported. The structures of the new compounds were determined by using X-ray diffraction analysis and their donor abilities as ligands evaluated through electrochemical methods. Despite the two positive charges that they bear, these compounds depict intermediate behavior between that of phosphines and phosphites. The coordination of the [L2 PR](2+) moiety to Au(I) and Ag(I) is also reported. Even more surprisingly, these phosphorus centers can be oxidized to the corresponding P(V) dications in the presence of strong oxidants such as peroxides or XeF2 .