A zwitterionic triphosphenium compound as a tunable multifunctional donor

Template Synthesis of NPN' Pincer-type Ligands at Titanium Using an Ambiphilic Phosphide Scaffold

Ti-imido complex [TiCl(N^tBu)(BIPP)] [1; BIPP = bis(iminophosphoranyl)phosphide ligand] reacts with terminal alkynes R-C≡CH (R = phenyl, isopropenyl, cyclopropyl, and 2-pyridyl) via P-P bond cleavage of the BIPP ligand. The resulting complexes [TiCl(NPN')(NPhPPh₂)] (2a-d) contain a pincer-type NPN' phosphide ligand that incorporates the terminal alkyne and the imido ligand from complex 1. Complexes 2a-d feature two chiral centers (Ti and P) with interdependent absolute configurations; thus, they are formed stereoselectively. Complex 2a (R = phenyl) undergoes chloride abstraction with [Et₃SiHSiEt₃][B(C₆F₅)₄], yielding [Ti(NPN')(NPhPPh₂)][B(C₆F₅)₄] (3). Complex 3 is a moderately active and stereoselective initiator for the ring-opening polymerization of rac-lactide. Complex 3 activates the C═O bond of 4-iodobenzaldehyde to give complex 4 as a single diastereomer despite the presence of three chiral centers. Complex 3 undergoes transmetallation with SbCl₃, yielding [Sb(NPN')][B(C₆F₅)₄] (5) and [TiCl₃(NPhPPh₂)] (6) selectively. The bonding situation in 3 and 5 was analyzed using Bader's atoms in molecules and the electron localization function, showing that the nitrogen atoms of the NPN' ligand are electronically similar, and that the metal-phosphide interaction is more polar in the case of titanium.

Recent Developments in the Chemistry of Pn(I) (Pn=N, P, As, Sb, Bi) Cations

The Group 15 Pn(I) cations (Pn=N, P, As, Sb and Bi), which are isoelectronic with the donor-stabilized carbones, have emerged recently. Despite the presence of two lone pair of electrons, the Pn(I) cations are weakly nucleophilic due to their inherent positive charge. Strongly electron-donating supporting ligands including zwitterionic forms have been used to enhance their Lewis basicity. Furthermore, the chelating effect of cyclic ligand systems proved effective in increasing their nucleophilicity. The strategies involved in successfully isolating the fleeting Sb(I) and Bi(I) cations as the recent most achievements in this field have been discussed. The syntheses, structure, bonding situations and reactivity of the Pn(I) cations are discussed. An outlook on the periodic trends and future applications of these electronically unique electron-rich cationic moieties have been provided.

Reappraising Schmidpeter's bis(iminophosphoranyl)phosphides: coordination to transition metals and bonding analysis

The synthesis and characterization of a range of bis(iminophosphoranyl)phosphide (BIPP) group 4 and coinage metals complexes is reported. BIPP ligands bind group 4 metals in a pseudo fac-fashion, and the central phosphorus atom enables the formation of d0-d10 heterobimetallic complexes. Various DFT computational tools (including AIM, ELF and NCI) show that the phosphorus-metal interaction is either electrostatic (Ti) or dative (Au, Cu). A bridged homobimetallic Cu-Cu complex was also prepared and its spectroscopic properties were investigated. The theoretical analysis of the P-P bond in BIPP complexes reveals that (i) BIPP are closely related to ambiphilic triphosphenium (TP) cations; (ii) the P-P bonds are normal covalent (i.e. not dative) in both BIPP and TP.

Triphosphenium salts: air-stable precursors for phosphorus(I) chemistry

The chemistry of low-coordinate phosphorus-containing species is an area of intense interest in modern main group chemistry. While typical routes for accessing such species include pyrophoric phosphorus-centered precursors or harsh reducing agents, triphosphenium cations represent a more convenient and safer alternative. This Perspective summarizes the use of air- and moisture-stable triphosphenium salts of [dppeP]+ as a source of P+ ions for the generation of a variety of new and/or useful low-coordinate phosphorus-containing species. These range from phosphorus-rich oligomers to phosphamethine cyanine dyes. Special emphasis is placed on the electronic structure of the newly generated species as well as their subsequent reactivity.

Diphosphoniodiphosphene Formation by Transition Metal Insertion into a Triphosphenium Zwitterion

Treatment of two equivalents of the triphosphenium zwitterion L with sources of Ni⁰ and Pd⁰ form the mononuclear η² -diphosphoniodiphosphene complexes 1 and 2. The reaction between L and [FeCp(CO)₂ ]₂ results in the binuclear μ-η¹ :η¹ -diphosphoniodiphosphene iron complex 3, which features an alternative bonding motif of the diphosphoniodiphosphene unit. The formation of these species has been confirmed by spectroscopic methods and single-crystal X-ray diffraction analysis, and their electronic structures have been elucidated using computational methods.

Synthesis of Heteroleptic Phosphorus(I) Cations by P+ Transfer

Reported are general synthetic approaches for the syntheses of asymmetrically substituted phosphorus(I) cations by P⁺ transfer from [dppeP]⁺ (dppe = 1,2-bis(diphenylphosphino)ethane). The first method grants access to acyclic derivatives and is accomplished by the sequential substitution of dppe using first a sterically encumbered ligand which cannot form a stable homoleptic complex, followed by a second equivalent of a less sterically demanding ligand. The second method grants access to cyclic derivatives and utilizes asymmetric hybrid phosphine/N-heterocyclic carbene ligands. Interplay between the different ligand types and their stoichiometries relative to those of [dppeP]⁺ also allows for the isolation of symmetrical derivatives with pendant phosphines.

Synthesis of Heavy Dicyanamide Homologues from Air-Stable Precursors

A convenient synthesis of dicyanophosphide and dicyanoarsenide anions is reported. These heavy homologues of the long-known and fundamentally important dicyanamide anion were formed through the nucleophilic displacement of bis(diphenylphosphino)ethane (dppe) from the pnictogen⁺ transfer agents [dppePn][BPh₄ ] (Pn=P, As) by exposure to cyanide salts. The protocol requires three synthetic steps from commercially available materials and the [dppePn][BPh₄ ] salts are remarkably temperature, air, and moisture stable. All products have been fully characterized by spectroscopic methods and by single-crystal X-ray diffraction, and the electronic structures of the DCPn anions have been assessed computationally.

Accessing multimetallic complexes with a phosphorus(i) zwitterion

We present the synthesis of a zwitterionic triphosphenium molecule, ^tBu(C₅H₂)(PPh₂)₂P^I (L), which can act as a single- or multidentate ligand with group 6, 7, 8 and 9 metal carbonyl complexes. Group 6, [M(CO)₅L] complexes are formed under photolytic conditions, where the metal is bound at the P(i) center. In the case of Mo(CO)₆, the bimetallic complex [M(CO)₅LMo(CO)₃] is generated, which features bonds to both the phosphorus(i) center and the cyclopentadienyl moiety of the molecule. Interestingly, group 7 and 9 metal carbonyl dimers generate bimetallic complexes in the form [M₂(CO)_nL], where both metal centers are bound at the phosphorus(i) center. A detailed analysis of these molecules is provided, including X-ray diffraction, multinuclear NMR, infrared spectroscopy and computational investigations.

Synthesis of bis(trithio)phosphines by oxidative transfer of phosphorus(i)

We report the oxidative transfer of the phosphorus(i) centre in [P^Idppe][Br] to tetrathiocin and disulfide ligands. Characterization of the resulting bis(trithio)phosphines and mechanistic insight into the formation of these products is provided.

Preparation and Reactivity of a Triphosphenium Bromide Salt: A Convenient and Stable Source of Phosphorus(I)

We present herein the optimized synthesis of a triphosphenium bromide salt. Apart from being a versatile metathesis reagent, this unusually stable low-valent-phosphorus-containing compound acts as a useful P⁺ transfer agent. Unlike traditional methods employed to access low-coordinate phosphorus species which usually require pyrophoric phosphorus-containing precursors (white phosphorus, Tris(trimethylsilyl)phosphine, etc.), or harsh reducing agents (alkali metals, potassium graphite, etc.), the current approach does not involve pyrophoric or explosive reagents and can be done on large scales (>20 g) in excellent yields by undergraduates with basic air-free synthetic training. The bromide counter ion is readily exchanged with other anions such as tetraphenyl borate (described herein) using typical salt metathesis reagents to obtain materials with desired properties and reactivities. The versatility of this P⁺ transfer approach is exemplified by the reactions of these triphosphenium precursors with an N-heterocyclic carbene and an anionic bisphosphine, each of which readily displace the neutral bisphosphine to give an NHC-stabilized phosphorus(I) cation and a phosphorus(I) containing zwitterion, respectively.