Experimental and Theoretical Predictors for Redox Potentials of Bispyridinylidene Electron Donors.

Bispyridinylidenes are neutral organic molecules capable of two-electron oxidation at a range of redox potentials that are widely tunable by choice of substituent, making them attractive as homogeneous organic reductants and active materials in redox flow batteries. In an effort to readily predict the redox potentials of this important class of compounds, we have developed correlations between the experimental redox potentials and both experimental and theoretical predictors. On the experimental side, we show that multinuclear NMR chemical shifts of related pyridinium ions correlate well with the redox potentials of bispyridinylidenes, with R and standard errors (S) reaching 0.

Enhancing catalytic activity of pyridines -iminophosphorano substituents.

Four pyridines decorated with π-donating iminophosphorano substituents (RPN-) in the 4-position were assessed as acylation catalysts. These catalysts display high sensitivity to the groups at phosphorus, with activities that are well correlated to the corresponding Hammett-type substituent constants (σ), and can achieve superior activity over the most active dialkylamino-substituted pyridines. Iminophosphorano-substituted pyridines represent an easily accessible, tunable, and highly active class of nucleophilic organocatalysts.

Bis(Iminophosphorano)-Substituted Pyridinium Ions and their Corresponding Bispyridinylidene Organic Electron Donors.

Optimized synthetic procedures for pyridinium ions featuring iminophosphorano (-N=PR ; R=Ph, Cy) π-donor substituents in the 2- and 4- positions are described. Crystallographic and theoretical studies reveal that the strongly donating substituents severely polarize the π-electrons of the pyridyl ring at the expense of aromaticity. Moreover, the pyridinium ions are readily deprotonated to generate powerful bispyridinylidene (BPY) organic electron donors.

Iminophosphorano-Substituted Bispyridinylidenes: Redox Potentials and Substituent Constants from Tolman Electronic Parameters.

Bispyridinylidenes (BPYs) have emerged as an important class of neutral organic electron donors, with redox potentials that vary widely with choice of substituent. Methods to predict the effect of substitution on the redox potential are therefore highly desirable. Here we show that the redox potential of BPYs featuring iminophosphorano substituents (R P=N-), which represent the most reducing class of BPYs, can be predicted based on the well-known Tolman electronic parameter (TEP) for the respective phosphine fragment (R P).

A strong organic electron donor incorporating highly π-donating triphenylphosphonium ylidyl substituents.

The π-electron donor strength of a triphenylphosphonium ylidyl group (PhP[double bond, length as m-dash]CH-) was explored through its substitution onto a bispyridinylidene (BPY) scaffold. Electrochemical studies revealed that the new triphenylphosphonium ylidyl-substituted BPY is the most reducing di-substituted derivative reported to date (E = -1.55 V vs.

Pushing the limits of neutral organic electron donors: a tetra(iminophosphorano)-substituted bispyridinylidene.

A new ground-state organic electron donor has been prepared that features four strongly π-donating iminophosphorano substituents on a bispyridinylidene skeleton. Cyclic voltammetry reveals a record redox potential of -1.70 V vs.

Powerful Bispyridinylidene Organic Reducing Agents with Iminophosphorano π-Donor Substituents.

Four members of a new family of powerful bispyridinylidene organic reducing agents have been prepared, which exploit iminophosphorano (-N=PR3; R = Ph, Cy) π-donor substituents. Electrochemical studies show exceptionally high oxidation potentials, ranging from 1.30 to 1.

Diverse reactivity of the cyclo-diphosphinophosphonium cation [(PtBu)₃Me]⁺: parallels with epoxides and new catena-phosphorus frameworks.

The cyclo-diphosphinophosphonium salt [(PtBu)3Me][OTf] (2) has been shown to be highly reactive toward Lewis bases, exhibiting diverse reactivity with phosphines, 4-(dimethylamino)pyridine (dmap) and chlorophosphines, providing approaches to new open-chain and cyclic catena-phosphorus frameworks. Reaction of 2 with R3P (R = Me or nPr) or dmap led to the ring-opened adducts [R3P-PtBu-PtBu-P(Me)tBu][OTf] (R = Me (4a), nPr (4b)) and [(dmap)-PtBu-PtBu-P(Me)tBu][OTf] (6), respectively. The complicated (31)P{(1)H} NMR spectra of the three compounds were simulated, evidencing the presence of two diastereomeric forms of 4a, and single diastereomers of 4b and 6.

Exocyclic delocalization at the expense of aromaticity in 3,5-bis(pi-donor) substituted pyrazolium ions and corresponding cyclic bent allenes.

Small ring allenes are usually highly strained and highly reactive species, and for a long time considered only as transient intermediates. The recent isolation of a five membered heterocyclic allene 1f has raised questions and debate regarding the factors responsible for its stability. Since 1f has been derived by deprotonation of a pyrazolium ion 2f, it has been suggested that the stability of 1f comes from its aromatic character.

2,3-Diphosphino-1,4-diphosphonium ions.

Salts of the first crystallographically characterized chlorophosphinophosphonium ions have been prepared, and their reaction with Ph3P results in reductive coupling of the chlorophosphine centers to give the first acyclic 2,3-diphosphino-1,4-diphosphonium ions, representing a key framework in the development of catena-phosphorus chemistry. These new salts of general formula [R3P-PR'-PR'-PR3][OTf]2 are also obtained in a one-pot diastereoselective reaction of a dichlorophosphine, a tertiary phosphine, and trimethylsilyltrifluoromethanesulfonate. The structural and spectroscopic features of the new dications complement those of the known diphosphonium and 2-phosphino-1,3-diphosphonium dications.

catena-phosphorus cations.

Recent advances towards a systematic development of catena-phosphorus cations are reviewed. The cations represented in this new and developing chapter in fundamental phosphorus chemistry complement the series of neutral and anionic polyphosphorus compounds.

Cyclotetraphosphinophosphonium ions: synthesis, structures, and pseudorotation.

The first derivatives of catenated cyclotetraphosphinophosphonium cations, [(PhP)4PPhMe]+ (8a), [(MeP)4PMe2]+ (8b), [(CyP)4PPh2]+ (8d), [(CyP)4PMe2]+ (8e), [(PhP)4PPh2]+ (8f), [(PhP)4PMe2]+ (8g), are synthesized as trifluoromethanesulfonate (triflate, OSO2CF3-) salts through the reaction of cyclopentaphosphines (PhP)5 (4a) or (MeP)5 (4b) with methyl triflate (MeOTf) or by a net phosphenium ion [PR2+, R = Ph, Me; from R2PCl and trimethylsilyltriflate (Me3SiOTf)] insertion into the P-P bond of either cyclotetraphosphine (CyP)4 (3c) or cyclopentaphosphines (PhP)5 (4a) or (MeP)5 (4b). Although more conveniently prepared from 4a, compound 8a[OTf] can also be formed from (PhP)4 (3a) and MeOTf, and derivatives 8f[OTf] and 8g[OTf] are also accessible through reactions of 3a and R2PCl/Me3SiOTf with R = Ph or Me, respectively. A tetrachlorogallate salt of [(PhP)4PPhtBu]+ (8c) has been synthesized by alkylation of 4a with tBuCl/GaCl3.

Monocyclic di- and triphosphinophosphonium cations: new foundational frameworks for catena-phosphorus chemistry.

The synthesis and characterization for trifluoromethanesulfonate (triflate) salts of the first definitive examples of cyclotriphosphinophosphonium and cyclodiphosphinophosphonium cations are described, representing new prototypical frameworks in the rational and systematic development of catena-phosphorus chemistry. Addition of methyl triflate (MeOTf) or triflic acid (HOTf) to cyclotetraphosphines (tBuP)4 (1a) or (CyP)4 (1b) gives [(tBuP)3PtBuMe][OTf] (2a[OTf]), [(CyP)3PCyMe][OTf] (2b[OTf]), [(tBuP)3PtBuH][OTf] (3a[OTf]), and [(CyP)3PCyH][OTf] (3b[OTf]), respectively. Cyclotriphosphine (tBuP)3 (4a) reacts with HOTF or Me2PCl/Me3SiOTf to give the ring expanded cations 3a[OTf] and [(tBuP)3PMe2][OTf] (5[OTf]), respectively, but reactions with MeOTf and HCl give cyclic diphosphinophosphonium cation [(tBuP)2PtBuMe][OTf] (6a[OTf]) and ring-opened triphosphine HtBuP-PtBu-PtBuCl (7), respectively.

Acyclic catena-diphosphinodiphosphonium dications [R3P-PR'-PR'-PR3]2+ or bisphosphine-diphosphenium complexes [R3PPR'-PR'PR3]2+: synthesis by reductive P-P coupling of [R3P-PR'Cl]+ and phosphine ligand exchange.

The acyclic tetraphosphorus dication [Ph3P-PPh-PPh-PPh3]2+ has been formed by the reductive coupling of [Ph3P-PPhCl]+, providing a new synthetic method for the systematic development of catena-phosphorus cations. Ligand exchange (Ph3P for Me3P) gives [Me3P-PPh-PPh-PMe3]2+, implicating these dications as bisphosphine-diphosphenium complexes.

The synthesis, characterisation and electronic structure of N-heterocyclic carbene adducts of P(I) cations.

Two different syntheses of N-heterocyclic carbene complexes of P(I) cations are presented; the structural features of these phosphamethine cyanine salts, in conjunction with the results of computational investigations, provide insight into the nature of the bonding of these heavy allene analogues.

Ylidene-->iminophosphine coordination complexes and reversible dissociation of dichlorophosphetidines.

Chloro-, bromo-, iodo-, and trifluoromethylsulfonyloxy-(2,4,6-tri-tert-butylphenylimino)phosphines (MesNPX; X = Cl, Br, I, OTf) react quantitatively with 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (Im) to give Lewis acid-base complexes with the general formula MesNP(Im)X. The dichlorophosphetidine (DippNPCl)(2) (Dipp = 2,6-diisopropylphenyl) represents a formal cyclodimer of an iminophosphine and reacts with Im to give a similar complex. The process represents a ligand induced dissociation of the phosphetidine framework and is reversed by the introduction of an appropriate Lewis acid.