α‐Cationic Phosphines: Synthesis and Applications

Abstract

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.

Cationic 5-phosphonio-substituted N-heterocyclic carbenes

“…[20][21][22] The quantitative oxidation of 10a,b[OTf ] via the slow addition of a solution of XeF 2 in CH 2 Cl 2 affords the difluorophosphoranes 15a,b[OTf ] (isolated yield >80%; Scheme 4). This effect might be attributed to a slightly increased electrophilic character of the Cl1 atom caused by the high group electronegativity of dication 14b 2+ (vide infra).…”

Charge Effects in PCP Pincer Complexes of NiII bearing Phosphinite and Imidazol(i)ophosphine Coordinating Jaws: From Synthesis to Catalysis through Bonding Analysis

“…As the central carbyl moiety of ECE pincer ligands is strongly s-electron donating, particulare fforts have been devoted to the study of "balanced" pincers with electron-withdrawing "coordinating jaws", like phosph(in)ite moieties.T he objective is to design new,u nsymmetrical pincerl igands by judicious combinations of charge-neutral donor moieties, represented by E, and positivelyc harged jaws such as imidazoliophosphine moieties, also regarded as acceptor-donor adducts of phospheniums with N-heterocyclic carbenes (NHCs), [(NHC)R 2 P:] + .O ur interests in the coordination chemistry of bi- [4] or tridentate [5] ligandsf eaturing imidazoliophosphine units, [6] and in the organonickelc hemistry of pincer complexes (Figure 1; e.g.,N CN, [7] PCP, [8] POCOP, [9] POCN [10] ) [11] led us to collaborate on the development of af amily of unsymmetricalE CE'-type pincer ligands incorporating[ (NHC)R 2 P:] + and related coordination moieties.…”

Carbeniophosphines versus Phosphoniocarbenes: The Role of the Positive Charge

“…The most general method used to prepare carbeniophosphines consists of the condensation of secondary diaryl or dialkyl phosphines with chloro‐substituted onium cations (Scheme ) ,. This strategy gives direct access in good yields to imidazolinio‐ 1 , pyridinio‐ 2 , cyclopropenio‐ 3 , and formamidiniophosphines 4 , respectively.…”

“…Although the previous method developed by Alcarazo et al. has considerably broadened the range of carbeniophosphines,, imidazoliophosphines 8 , generally obtained by phosphinylation of N‐heterocyclic carbene (NHC) species, historically represent the first members of the family (Scheme ) . Kuhn et al.…”

“…As mentioned in the literature,, experimental ν CO values may present inconsistencies, especially in trans ‐[RhCl(CO)L 2 ] complexes of sterically hindered cationic ligands . However, it should be noted that the measurement of the oxidation potential by cyclic voltammetry, which is an alternative experimental method for evaluating the electronic properties of ligands and does not depend on steric factors, has led to the same relative ranking of α‐cationic phosphines ,…”