Part of the renaissance in main group chemistry has been a result of the focus on reactivity. This has led to the development of applications in stoichiometric reactivity and catalysis. In this tutorial review, we focus attention on the role of phosphorus-based Lewis acids in such advances. While early literature recognizes the role of P(iii) and P(v) electrophiles in coordination chemistry, it has generally been more recent studies that have focused on applications of this Lewis acidity. Applications of these novel P-based Lewis acids in stoichiometric reactivity, Lewis acid catalysis and frustrated Lewis pair (FLP) reactivity are reviewed. These advances demonstrate that P-based Lewis acids are a powerful tool for further developments in metal-free catalysis.

A highly Lewis acidic diphosphonium dication [(C10H6)(Ph2P)2](2+) (1), in combination with a Lewis basic phosphine, acts as a purely phosphorus-based frustrated Lewis pair (FLP) and abstracts hydride from [HB(C6F5)3](-) and Et3SiH demonstrating the remarkable hydridophilicity of 1. The P-based FLP is also shown to activate H2 and C-H bonds.

Four perfluoroalkyl cobalt(III) fluoride complexes have been synthesized and characterized by elemental analysis, multinuclear NMR spectroscopy, X-ray crystallography, and powder X-ray diffraction. The remarkable cobalt fluoride (19)F NMR chemical shifts (-716 to -759 ppm) were studied computationally, and the contributing paramagnetic and diamagnetic factors were extracted. Additionally, the complexes were shown to be active in the catalytic fluorination of p-toluoyl chloride. Furthermore, two examples of cobalt(III) bis(perfluoroalkyl)complexes were synthesized and their reactivity studied. Interestingly, abstraction of a fluoride ion from these complexes led to selective formation of cobalt difluorocarbene complexes derived from the trifluoromethyl ligand. These electrophilic difluorocarbenes were shown to undergo insertion into the remaining perfluoroalkyl fragment, demonstrating the elongation of a perfluoroalkyl chain arising from a difluorocarbene insertion on a cobalt metal center. The reactions of both the fluoride and bis(perfluoroalkyl) complexes provide insight into the potential catalytic applications of these model systems to form small fluorinated molecules as well as fluoropolymers.

Using commercially available 2-pyridyldiphenylphosphine (o-NC5H4)PPh2, a family of electrophilic phosphonium cations [(o-NC5H4)PFPh2](+) (2) and dications [(o-MeNC5H4)PRPh2](2+) (R = F (4); Me (5)) were prepared. The Lewis acidity of these pyridinium-phosphonium dications was probed in Friedel-Crafts dimerization, hydrodefluorination, hydrosilylation, dehydrocoupling and hydrodeoxygenation reactions. The influence of the counterion on the catalytic activity of the electrophilic phosphonium cations is also discussed.

The activation and functionalization of CÀFb onds has garnered significant attention in the scientificc ommunity as as trategy to mitigate toxicitya nd environmental concerns, as well as provide new pathways to agro-and pharmaceuticalc hemicals and materials. Although severalt ransition-metal-based systemsh ave been developed for this transformation,t he use of main-group compoundsr emains less explored. In recent years, several strategies for CÀF bond activation have focused on the use of phosphorusbased reagents. In this Minireview,a no verview of strategies is provided that exploits P V andP III -based Lewis acids as well as P III Lewis bases in frustrated Lewis pair (FLP) protocols for hydrodefluorination, CÀCc ouplings andC ÀFd erivatizations.

Two strategies were used to prepare dicationic phosphonium cations. The first method consists of the reaction of 1-chlorocyclopropenium salts with phosphines to obtain cyclopropenium-substituted phosphonium salts 10a-f[BF4]. Anion exchange was performed to access the corresponding [B(CF)] analogues 10a-f[B(C6F5)4], which showed much higher solubility in organic solvents. In addition, we developed a synthesis of dicationic phosphonium salts containing 2-, 3-, or 4-methylpyridinium substituents 11a-c[TfO], which were converted as well to their [B(CF)] analogues 11a-c[B(C6F5)4]. Finally, the phenoxy-substituted phosphonium salt 12[B(C6F5)4] was also prepared. All salts demonstrated remarkable stability in air as compared with their fluorinated analogues. The Lewis acidity of these salts was evaluated by means of theoretical calculations and finally, they were shown to be effective in initiating the Mukaiyama-aldol reaction.

A new class of electrophilic phosphonium cations (EPCs) containing a -CF group attached to the phosphorus(v) center is readily accessible in high yields, via a scalable process. These species are stable to air, water, alcohol and strong Brønsted acid, even at raised temperatures. Thus, P-CF EPCs are more robust than previously reported EPCs containing P-X moieties (X = F, Cl, OR), and despite their reduced Lewis acidity they function as Lewis acid catalysts without requiring anhydrous reaction conditions.

The water/base intolerance of the previously reported electrophilic phosphonium cations has been overcome by replacing the labile electron-withdrawing groups generally attached to phosphorus (e.g. –F, –OAr, –CF3) with methyl groups.