Allenylidene Phosphonium Ion: An Isoelectronic Phosphorus Analogue of [3]Cumulene.

Herein we disclose the synthesis, characterization and bonding analysis of a crystalline allenylidene phosphonium ion ([R2P=C=C=CAr2]+) [2]+ (R = 1,3-diisopropylphenylimidazolin-2-ylidenamino, CAr2 = 9-fluorenylidene), an isolobal analogue of [3]cumulenes. The cation was prepared via methoxide ion abstraction of an alkynyl phosphine 1. Electron rich N-heterocylic imine substituents and a pi-conjugated fluorenyl scaffold together stabilize the resulting three-coordinate phosphonium center of [2]+.

Light-Driven, Reversible Spatiotemporal Control of Dynamic Covalent Polymers.

Dynamic covalent polymer networks exhibit a cross-linked structure like conventional thermosets and elastomers, although their topology can be reorganized through externally triggered bond exchange reactions. This characteristic enables a unique combination of repairability, recyclability and dimensional stability, crucial for a sustainable industrial economy. Herein the application of a photoswitchable nitrogen superbase is reported for the spatially resolved and reversible control over dynamic bond exchange within a thiol-ene photopolymer.

Phosphonium SF Salts Derived from Sulfur Hexafluoride as Deoxyfluorination Reagents.

Sulfur based deoxyfluorination reagents are usually derived from the corrosive gas SF. Herein, we report the synthesis and properties of an easily accessible phosphonium salt [(tmg)PF]SF (1) which was obtained from the reaction of sulfur hexafluoride (SF) with tris(tetramethylguanidinyl)phosphine. The performance of this crystalline SF salt as a reagent in deoxyfluorination reactions was investigated together with a second SF salt [(R)PF]SF (2) containing bulky substituents (R=1,3-di-tert-butylimidazolidin-2-ylidenamino).

Electron-rich pyridines with -N-heterocyclic imine substituents: ligand properties and coordination to CO, SO, BCl and Pd complexes.

Electron-rich pyridines with π donor groups at the position play an important role as nucleophiles in organocatalysis, but their ligand properties and utilization in coordination chemistry have received little attention. Herein, we report the synthesis of two electron-rich pyridines 1 and 2 bearing N-heterocyclic imine groups at the position and explore their coordination chemistry. Experimental and computational methods were used to assess the donor ability of the new pyridines showing that they are stronger donors than aminopyridines and guanidinyl pyridines, and that the nature of the N-heterocyclic backbone has a strong influence on the pyridine donor strength.

Photoswitchable electron-rich phosphines: using light to modulate the electron-donating ability of phosphines.

The synthesis and properties of photoswitchable electron-rich phosphines containing N-heterocyclic imines equipped with a photochromic dithienylethene unit are reported. Heteronuclear NMR spectroscopy and UV/vis studies reveal that the imine substituents undergo reversible electrocyclic ring-closing and ring-opening reactions upon exposure to UV and visible light, respectively. The photoisomerization alters the electron-donating ability of the phosphines by up to ΔTEP = 8 cm.

Terminal methylene phosphonium ions: precursors for transient monosubstituted phosphinocarbenes.

Isolable singlet carbenes are among the most important tools in chemistry, but generally require the interaction of two substituents with the electron deficient carbon atom. We herein report a synthetic approach to monosubstituted phosphinocarbenes deprotonation of hitherto unknown diprotic terminal methylene phosphonium ions. Two methylene phosphonium salts bearing bulky -heterocyclic imine substituents at the phosphorus atom were isolated and fully characterized.

Facile One-Step Access to Pyrrole-Based 1,4-Diphosphabarrelenes.

1,4-Diphosphabarrelenes are bicyclic diphosphines relevant, for example, for the generation of polymetallic coordination compounds. However, current synthetic protocols either suffer from low yields or require multiple reaction steps. Herein, we report the one-step synthesis of pyrrole-based 1,4-diphosphabarrelenes that are obtained in very good yields from the reaction of 1,2,5-trimethylpyrrole with 1,2-bis(dichlorophosphino)ethane or 1,2-bis(dichlorophosphino)benzene.

Synthesis and Reactivity of a Cyclooctatetraene-Like Polyphosphorus Ligand Complex [Cyclo-P ].

The thermolysis of Cp'''Ta(CO) with white phosphorus (P ) gives access to [{Cp'''Ta} (μ,η -P )] (A), representing the first complex containing a cyclooctatetraene-like (COT) cyclo-P ligand. While ring sizes of n >6 have remained elusive for cyclo-P structural motifs, the choice of the transition metal, co-ligand and reaction conditions allowed the isolation of A. Reactivity investigations reveal its versatile coordination behaviour as well as its redox properties.

Crystalline phosphino(silyl)carbenes that readily form transition metal complexes.

Phosphino(silyl)carbenes are known for their ineptitude to form transition metal complexes. We describe the synthesis of phosphino(silyl)carbenes bearing N-heterocyclic imine groups and show that these isolable, crystalline carbenes readily form stable copper(I) and gold(I) complexes. The solid-state structures of the free carbenes and their transition metal complexes are reported.

Synthesis of N-Heterocyclic Carbenes and Their Complexes by Chloronium Ion Abstraction from 2-Chloroazolium Salts Using Electron-Rich Phosphines.

N-Heterocyclic carbenes (NHCs) are commonly prepared by deprotonation of azolium salts using strong anionic bases. This reaction is often unselective, yielding alkali metal NHC complexes or dimerized NHCs. Alternatively, free NHCs are obtained by the dechlorination of 2-chloroazolium salts using electron-rich phosphines.

Tris(tetramethylguanidinyl)phosphine: The Simplest Non-ionic Phosphorus Superbase and Strongly Donating Phosphine Ligand.

We report the synthesis and properties of the much sought-after tris(1,1,3,3-tetramethylguanidinyl) phosphine P(tmg) , a crystalline, superbasic phosphine accessible through a short and scalable procedure from the cheap and commercially available bulk chemicals 1,1,3,3-tetramethylguanidine, tris(dimethylamino)-phosphine and phosphorus trichloride. The new phosphine exhibits exceptional electron donor properties and readily forms transition metal complexes with gold(I), palladium(II) and rhodium(I) precursors. The formation of zwitterionic Lewis base adducts with carbon dioxide and sulfur dioxide was explored.

Photoswitchable Nitrogen Superbases: Using Light for Reversible Carbon Dioxide Capture.

Using light as an external stimulus to alter the reactivity of Lewis bases is an intriguing tool for controlling chemical reactions. Reversible photoreactions associated with pronounced reactivity changes are particularly valuable in this regard. We herein report the first photoswitchable nitrogen superbases based on guanidines equipped with a photochromic dithienylethene unit.

Thiophosphonium-Alkyne Cycloaddition Reactions: A Heavy Congener of the Carbonyl-Alkyne Metathesis.

While the metathesis reaction between alkynes and thiocarbonyl compounds has been thoroughly studied, the reactivity of alkynes with isoelectronic main group RE=S compounds is rarely reported and unknown for [RP=S] analogues. We show that thiophosphonium ions, which are the isoelectronic phosphorus congeners to thiocarbonyl compounds, undergo [2 + 2]-cycloaddition reactions with different alkynes to generate 1,2-thiaphosphete ions. The four-membered ring species are in an equilibrium state with the corresponding P=C-C=S heterodiene structure and thus undergo hetero-Diels-Alder reactions with acetonitrile.

Multistimuli-Responsive [3]Dioxaphosphaferrocenophanes with Orthogonal Switches.

Novel multistimuli-responsive phosphine ligands comprising a redox-active [3]dioxaphosphaferrocenophane backbone and a P-bound imidazolin-2-ylidenamino entity that allows switching by protonation are reported. Investigation of the corresponding metal complexes and their redox behaviour are reported and show the sensitivity of the system towards protonation and metal coordination. The experimental findings are supported by DFT calculations.

Lewis base-free thiophosphonium ion: a cationic sulfur atom transfer reagent.

Phosphorus(v) sulfide and Lawesson's reagent are commonly used thionating reagents which are considered to operate after dissociation into highly reactive dithiophosphorane fragments. We report the synthesis and properties of a monomeric thiophosphonium ion [R2P[double bond, length as m-dash]S]+. The highly electrophilic species reacts with carbonyls in oxo-for-sulfido exchange reactions at room temperature and undergoes phosphorus-chalcogen bond metathesis reactions with phosphine chalcogenides.

Oxophosphonium-Alkyne Cycloaddition Reactions: Reversible Formation of 1,2-Oxaphosphetes and Six-membered Phosphorus Heterocycles.

While the metathesis reaction between alkynes and carbonyl compounds is an important tool in organic synthesis, the reactivity of alkynes with isoelectronic main-group RE═O compounds is unexplored. Herein, we show that oxophosphonium ions, which are the isoelectronic phosphorus congeners to carbonyl compounds, undergo [2 + 2] cycloaddition reactions with different alkynes to generate 1,2-oxaphosphete ions, which were isolated and structurally characterized. The strained phosphorus-oxygen heterocycles open to the corresponding heterodiene structure at elevated temperature, which was used to generate six-membered phosphorus heterocycles via hetero Diels-Alder reactions.

A General Pathway to Heterobimetallic Triple-Decker Complexes.

A systematic study on the reactivity of the triple-decker complex [(Cp'''Co) (μ,η :η -C H )] (A) (Cp'''=1,2,4-tritertbutyl-cyclopentadienyl) towards sandwich complexes containing cyclo-P , cyclo-P , and cyclo-P ligands under mild conditions is presented. The heterobimetallic triple-decker sandwich complexes [(Cp*Fe)(Cp'''Co)(μ,η :η -P )] (1) and [(Cp'''Co)(Cp'''Ni)(μ,η :η -P )] (3) (Cp*=1,2,3,4,5-pentamethylcyclopentadienyl) were synthesized and fully characterized. In solution, these complexes exhibit a unique fluxional behavior, which was investigated by variable temperature NMR spectroscopy.

Isolation, characterization and reactivity of three-coordinate phosphorus dications isoelectronic to alanes and silylium cations.

Three-coordinate main group Lewis acids are exceedingly important reagents in chemical synthesis. In contrast to the well-established chemistries of neutral group 13 and cationic group 14 species, isoelectronic group 15 element dications are unknown. In this work, we use stabilizing N-heterocyclic imine substituents to isolate and characterize phosphorandiylium dications ([RP]) and show that the electrophilicity at the phosphorus atoms is controlled by the π-electron-donating ability of these subtituents.

Switching the Electron-Donating Ability of Phosphines through Proton-Responsive Imidazolin-2-ylidenamino Substituents.

Stimuli-responsive ancillary ligands are valuable tools to control the activity and selectivity of transition-metal catalysts. The synthesis and characterization of a series of metal complexes containing phosphines with proton-responsive imidazolin-2-ylidenamino substituents are reported. Determination of the ligand-donor properties revealed that protonation of each substituent increases the Tolman electronic parameter (TEP) of the phosphine by 22 cm , hence allowing for switching of the electron-donor power of phosphine 2 within an unprecedented range (ΔTEP=43.

Crystalline, room-temperature stable phosphine-SO adducts: generation of sulfur monoxide from sulfur dioxide.

The particularly basic phosphines 1a-c readily form isolable, zwitterionic Lewis base adducts with SO that were fully characterized including by X-ray diffraction studies. Computational and reactivity studies show that these adducts readily release SO at room temperature driven by the formation of the corresponding phosphine oxides.

Lewis Base Free Oxophosphonium Ions: Tunable, Trigonal-Planar Lewis Acids.

Oxophosphonium ions (R P=O) are fascinating chemical intermediates related to the well-known acylium cations (RC=O) , and comprise a tricoordinate phosphorus(V) center with a phosphorus-oxygen double bond. Here, we report the synthesis of two oxophosphonium ions stabilized by bulky imidazolin-2-imine and imidazolin-2-olefin substituents attached to phosphorus. The novel species were characterized by NMR spectroscopy and single-crystal X-ray diffraction analysis, and the bonding situation was probed by DFT calculations.

Nucleophilic Activation of Sulfur Hexafluoride: Metal-Free, Selective Degradation by Phosphines.

The development of new methods for the chemical activation of the extremely inert greenhouse gas sulfur hexafluoride (SF ) not only is of current environmental interest, but also offers new opportunities for applications of SF as a reagent in organic synthesis. We herein report the first nucleophilic activation of SF by Lewis bases, namely by phosphines, which results either in its complete degradation to phosphine sulfides and difluorophosphoranes or in the selective conversion of SF into a bench-stable, crystalline salt containing the SF anion. Quantum chemical calculations reveal a nucleophilic substitution mechanism (S 2) for the initial fluorine abstraction from SF by the phosphine.

Stable Bromiranium Ions with Weakly-Coordinating Counterions as Efficient Electrophilic Brominating Agents.

Electrophilic halogenating agents are an important class of reagents in chemical synthesis. Herein, we show that sterically demanding bromiranium ions with weakly coordinating counterions are highly reactive electrophilic brominating agents. Despite their high reactivity these reagents are stable, in one case even under ambient conditions and can be applied in electrophilic halogenations of alkenes as well as heteroatoms.