On the P-Coordinating Limit of NHC-Phosphenium Cations toward RhICenters

Carbon-Phosphorus Ligands with Extreme Donating Character

Carbeniophosphines [R2 C+ -PR2 ] and phosphonium ylides [R3 P+ -CR2 - ] are two complementary classes of carbon-phosphorus based ligands defined by their unique donor properties. Indeed, while carbeniophosphines are electron-poor P-ligands due to the positioning of a positive charge near the coordinating P-atom, phosphonium ylides are electron-rich C-ligands due to the presence of a negatively charged coordinating C-atom. Based on this knowledge, this account summarizes our recent contribution on these two classes of carbon-phosphorus ligands, and in particular the strategies developed to lower the donor character of carbeniophosphines and enhance that of phosphonium ylides. This led us to design, at both extremities of the donating scale, extremely electron-poor P-ligands exemplified by imidazoliophosphonites [R2 C+ -P(OR)2 ] and dicarbeniophosphines [(R2 C+ )2 -PR], and extremely electron-rich C-ligands illustrated by pincer architectures exhibiting several phosphonium ylide donor extremities. In the context of carbon-phosphorus analogy, the closely related cases of ligands where the C-atom of a NHC ligand is in close proximity of two positive charges, and that of a phosphonium ylide coordinated through its P-atom are also discussed. An overview of the synthetic methods, coordinating properties, general reactivity and electronic structure of all these C,P-based species is presented here.

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.

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.

Filling a Niche in "Ligand Space" with Bulky, Electron-Poor Phosphorus(III) Alkoxides

The chemistry of phosphorus(III) ligands, which are of key importance in coordination chemistry, organometallic chemistry and catalysis, is dominated by relatively electron-rich species. Many of the electron-poor P^III ligands that are readily available have relatively small steric profiles. As such, there is a significant gap in "ligand space" where more sterically bulky, electron-poor P^III ligands are needed. This contribution discusses the coordination chemistry, steric and electronic properties of P^III ligands bearing highly fluorinated alkoxide groups of the general form PR_n (OR^F )_3-n , where R=Ph, R^F =C(H)(CF₃ )₂ and C(CF₃ )₃ ; n=1-3. These ligands are simple to synthesize and a range of experimental and theoretical methods suggest that their steric and electronic properties can be "tuned" by modification of their substituents, making them excellent candidates for large, electron-poor ligands.

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.

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.

α-Cationic phosphines: synthesis and applications

In coordination chemistry, typical ancillary ligands are anionic or neutral species. Cationic ones are exceptions and, when used, the positively charged groups are normally attached to the periphery and not close to the donating atom. However, this concept article highlights a series of recent experimental, as well as theoretical results, suggesting that the utility in catalysis of cationic phosphines with no spacer between the phosphorus atom and the positively charged group(s) has been largely overlooked. In fact, a growing number of studies indicate that, because of their specific architecture, these cationic ligands depict excellent π-acceptor character that can exceed that of phosphites or polyfluorinated phosphines. This property has been used to increase the Lewis acidity of the metals they coordinate. Specifically, new extreme π-acid catalysts, mainly based on Pt(II) and Au(I) , have been recently prepared and their superior performance demonstrated along several mechanistically distinct transformations. In this concept article the current state of the art is critically assessed and possible future directions of the topic discussed.

Nickel(II) complexes of the new pincer-type unsymmetrical ligands PIMCOP, PIMIOCOP, and NHCCOP: versatile binding motifs

Chelation/nickellation of an unsymmetrical meta-phenylene-based ligand featuring phosphinite and imidazolophosphine functionalities gives the corresponding pincer complex. N-Methylation of the latter generates a new complex featuring the ternary moiety NHC→Ph(2)P(+)→Ni, which can be converted subsequently into the binary moiety NHC→Ni by extrusion of Ph(2)P(+), thus allowing a sequential synthesis of pincer complexes displaying varying degrees of dipolar character.