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-1.

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 via deprotonation of hitherto unknown diprotic terminal methylene phosphonium ions. Two methylene phosphonium salts bearing bulky N-heterocyclic imine substituents at the phosphorus atom were isolated and fully characterized. Deprotonation studies indicate the formation of transient monosubstituted carbenes that undergo intermolecular cycloadditions or intramolecular Buchner ring expansion to afford a cycloheptatriene derivative. The reaction mechanism of the latter transformation was elucidated using DFT calculations, which reveal the ambiphilic nature of the phosphinocarbene enabling the insertion into the aromatic C-C bond. Additional computational studies on the role of substituent effects are presented.

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. The new caged diphosphines are strong donor ligands and act as bridging ligand in nickel(0), rhodium(I), iridium(I) and copper(I) coordination compounds.

Synthesis and Reactivity of a Cyclooctatetraene-Like Polyphosphorus Ligand Complex [Cyclo-P8 ]

The thermolysis of Cp'''Ta(CO)₄ with white phosphorus (P₄ ) gives access to [{Cp'''Ta}₂ (μ,η^{2 : 2 : 2 : 2 : 1 : 1} -P₈ )] (A), representing the first complex containing a cyclooctatetraene-like (COT) cyclo-P₈ ligand. While ring sizes of n >6 have remained elusive for cyclo-P_n 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. Oxidation leads to dimerization to afford [{Cp'''Ta}₄ (μ₄ ,η^{2 : 2 : 2 : 2 : 2 : 2 : 2 : 2 : 1 : 1 : 1 : 1} -P₁₆ )][TEF]₂ (4, TEF=[Al(OC{CF₃ }₃ )₄ ]^- ). Reduction, however, leads to the fission of one P-P bond in A followed by rapid dimerization to form [K@[2.2.2]cryptand]₂ [{Cp'''Ta}₄ (μ₄ ,η^{2 : 2 : 2 : 2 : 2 : 2 : 2 : 2 : 1 : 1 : 1 : 1} -P₁₆ )] (5), which features an unprecedented chain-type P₁₆ ligand. Lastly, A serves as a P₂ synthon, via ring contraction to the triple-decker complex [{Cp'''Ta}₂ (μ,η^6 : 6 -P₆ )] (B).

Oxidative Fluorination of Selenium and Tellurium Compounds using a Thermally Stable Phosphonium SF5 - Salt Accessible from SF6

Fluorinated group 16 moieties are attractive building blocks in synthetic chemistry but only few synthetic methods are available to prepare them. Herein, we report a new oxidative fluorination reagent capable of stabilizing reactive fluorinated anions. It consists of an SF₅ ^- anion and a chemically inert phosphonium cation and is exceptionally thermally stable. Accordingly, it was used to generate the SeF₅ ^- and TeF₅ ^- anions from the elemental chalcogens and to prepare the unknown tetrafluoro(phenyl)-λ⁵ -selenate PhSeF₄ ^- and -tellurate PhTeF₄ ^- from the corresponding diphenyl dichalcogenides. In addition, we show that further derivatization of [PhTeF₄ ]^- by oxidation to trans-PhTeF₄ O^- and subsequent alkylation gives access to a new class of trans-(alkoxy)(phenyl)tetrafluoro-λ⁶ -tellanes (trans-PhTeF₄ OR), thus providing an approach to introduce the functional group into organic molecules.

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. PPh₃, PCy₃, and PtBu₃ are unsuitable for Cl⁺ abstraction, while the sterically encumbered tris(1,3‐tert‐butylimidazolidin‐2‐ylidenamino)phosphine 1 selectively removes Cl⁺ from 2‐chloroazolium salts. Since bulky 1 does not bind to metal complexes, it was used for the preparation of NHC complexes via in situ Cl⁺ abstraction from 2‐chloroazolium salts. The dechlorination was employed for the site‐selective monometallation with Ir^I, Ir^III, Rh^I, Rh^III, and Ru^II of a bis‐NHC precursor composed of a 2‐chlorobenzimidazolium and a 2‐chlorobenzimidazole group, followed by the preparation of the heterobimetallic Ir^III/Pd^II complex [18](BF₄)₂ by a dechlorination/oxidative addition reaction sequence.

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...

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)₃, 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. In addition, the complete series of phosphine chalcogenides was prepared from the reaction of P(tmg)₃ with N₂O and the elemental chalcogens.

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. Protonation and oxidation events are applied in Rh‐catalysed hydrosilylations and demonstrate a remarkable influence on reactivity and/or selectivity.

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. The resulting N-heterocyclic imines (NHIs) undergo reversible, near quantitative electrocyclic isomerization upon successive exposure to UV and visible irradiation, as demonstrated over multiple cycles. Switching between the ring-opened and ring-closed states is accompanied by substantial pK_a shifts of the NHIs by up to 8.7 units. Since only the ring-closed isomers are sufficiently basic to activate CO₂ via the formation of zwitterionic Lewis base adducts, cycling between the two isomeric states enables the light-controlled capture and release of CO₂ .

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 R₂E=S compounds is rarely reported and unknown for [R₂P=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. Heteroatom and substituent effects on the energy profile of the 1,2-thiaphosphete formation were elucidated by means of quantum chemical methods.

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. Heteroatom and substituent effects on the energy profile of the 1,2-thiaphosphete formation were elucidated by means of quantum chemical methods.

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 R₂E═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. Insights into the influence of the substituents at the phosphorus center on the energy profile of the oxygen atom transfer reaction were obtained by quantum-chemical calculations.

A General Pathway to Heterobimetallic Triple-Decker Complexes

A systematic study on the reactivity of the triple‐decker complex [(Cp’’’Co)₂(μ,η⁴:η⁴‐C₇H₈)] (A) (Cp’’’=1,2,4‐tritertbutyl‐cyclopentadienyl) towards sandwich complexes containing cyclo‐P₃, cyclo‐P₄, and cyclo‐P₅ ligands under mild conditions is presented. The heterobimetallic triple‐decker sandwich complexes [(Cp*Fe)(Cp’’’Co)(μ,η⁵:η⁴‐P₅)] (1) and [(Cp’’’Co)(Cp’’’Ni)(μ,η³:η³‐P₃)] (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. The dynamic processes can be blocked by coordination to {W(CO)₅} fragments, leading to the complexes [(Cp*Fe)(Cp’’’Co)(μ₃,η⁵:η⁴:η¹‐P₅){W(CO)₅}] (2 a), [(Cp*Fe)(Cp’’’Co)(μ₄,η⁵:η⁴:η¹:η¹‐P₅){(W(CO)₅)₂}] (2 b), and [(Cp’’’Co)(Cp’’’Ni)(μ₃,η³:η²:η¹‐P₃){W(CO)₅}] (4), respectively. The thermolysis of 3 leads to the tetrahedrane complex [(Cp’’’Ni)₂(μ,η²:η²‐P₂)] (5). All compounds were fully characterized using single‐crystal X‐ray structure analysis, NMR spectroscopy, mass spectrometry, and elemental analysis.

Pyridinylidenaminophosphines: Facile Access to Highly Electron-Rich Phosphines

Electron-rich tertiary phosphines are valuable species in chemical synthesis. However, their broad application as ligands in catalysis and reagents in stoichiometric reactions is often limited by their costly synthesis. Herein, we report the synthesis and properties of a series of phosphines with 1-alkylpyridin-4-ylidenamino and 1-alkylpyridin-2-ylidenamino substituents that are accessible in a very short and scalable route starting from commercially available aminopyridines and chlorophosphines. The determination of the Tolman electronic parameter (TEP) value reveals that the electron donor ability can be tuned by the substituent pattern at the aminopyridine backbone and it can exceed that of common alkylphosphines and N-heterocyclic carbenes. The potential of the new phosphines as strong nucleophiles in phosphine-mediated transformations is demonstrated by the formation of Lewis base adducts with CO2 and CS2 . In addition, the coordination chemistry of the new phosphines towards CuI , AuI , and PdII metal centers has been explored, and a convenient procedure to introduce the most basic phosphine into metal complexes starting from air-stable phosphonium salt is described.

Cooperative activation of azides by an Al/N-based active Lewis pair – unexpected insertion of nitrogen atoms into C–Si bonds and formation of AlCN3 heterocycles

The active Lewis pairs (ALPs) 2,6-Me2H8C5N–C(H) = C(SiMe3)–AlR2 (1a: R =  t Bu, 1b, R =  i Bu) have strained AlC2N heterocycles and relatively weak Al–N bonds. They react readily with a series of organic azides R′N3 [R′ = Ph, CH2C6H4(4- t Bu), t Bu, SiMe3, CH2Ph] by cleavage of the heterocycles and addition of the azides with their α-N atoms to the Al atom. The Al–N interactions result in an activation of the azide groups which insert into the C–Si bonds of the vinyl groups with their terminal γ-N atoms. Compounds with approximately planar five-membered AlCN3 heterocycles and intact N3 groups are formed in highly selective reactions.

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^-1 , hence allowing for switching of the electron-donor power of phosphine 2 within an unprecedented range (ΔTEP=43.4 cm^-1 ).

Crystalline, room-temperature stable phosphine–SO2 adducts: generation of sulfur monoxide from sulfur dioxide

Particularly basic phosphines form isolable Lewis base adducts with SO₂ that readily release SO at room temperature driven by the formation of the corresponding phosphine oxides.

Quantifying the efficiency of CO2 capture by Lewis pairs

A microfluidic strategy is used to assess the relative efficiency and thermodynamic parameters of CO₂ binding by three Lewis acid/base combinations.

A microfluidic strategy has been used for the time- and labour-efficient evaluation of the relative efficiency and thermodynamic parameters of CO₂ binding by three Lewis acid/base combinations, where efficiency is based on the amount of CO₂ taken up per binding unit in solution. Neither tBu₃P nor B(C₆F₅)₃ were independently effective at CO₂ capture, and the combination of the imidazolin-2-ylidenamino-substituted phosphine (NIiPr)₃P and B(C₆F₅)₃ was equally ineffective. Nonetheless, an archetypal frustrated Lewis pair (FLP) comprised of tBu₃P and B(C₆F₅)₃ was shown to bind CO₂ more efficiently than either the FLP derived from tetramethylpiperidine (TMP) and B(C₆F₅)₃ or the highly basic phosphine (NIiPr)₃P. Moreover, the proposed microfluidic platform was used to elucidate the thermodynamic parameters for these reactions.

The Cobalt cyclo-P4 Sandwich Complex and Its Role in the Formation of Polyphosphorus Compounds

A synthetic approach to the sandwich complex [Cp′′′Co(η⁴‐P₄)] (2) containing a cyclo‐P₄ ligand as an end‐deck was developed. Complex 2 is the missing homologue in the series of first‐row cyclo‐P_n sandwich complexes, and shows a unique tendency to dimerize in solution to form two isomeric P₈ complexes [(Cp′′′Co)₂(μ,η⁴:η²:η¹‐P₈)] (3 and 4). Reactivity studies indicate that 2 and 3 react with further [Cp′′′Co] fragments to give [(Cp′′′Co)₂(μ,η²:η²‐P₂)₂] (5) and [(Cp′′′Co)₃P₈] (6), respectively. Furthermore, complexes 2, 3, and 4 thermally decompose forming 5, 6, and the P₁₂ complex [(Cp′′′Co)₃P₁₂] (7). DFT calculations on the P₄ activation process suggest a η³‐P₄ Co complex as the key intermediate in the synthesis of 2 as well as in the formation of larger polyphosphorus complexes via a unique oligomerization pathway.

cyclo-P4 Building Blocks: Achieving Non-Classical Fullerene Topology and Beyond

The cyclo‐P₄ complexes [Cp^RTa(CO)₂(η⁴‐P₄)] (Cp^R: Cp′′=1,3‐C₅H₃tBu₂, Cp′′′=1,2,4‐C₅H₂tBu₃) turned out to be predestined for the formation of hollow spherical supramolecules with non‐classical fullerene‐like topology. The resulting assemblies constructed with CuX (X=Cl, Br) showed a highly symmetric 32‐vertex core of solely four‐ and six‐membered rings. In some supramolecules, the inner cavity was occupied by an additional CuX unit. On the other hand, using CuI, two different supramolecules with either peanut‐ or pear‐like shapes and outer diameters in the range of 2–2.5 nm were isolated. Furthermore, the spherical supramolecules containing Cp′′′ ligands at tantalum are soluble in CH₂Cl₂. NMR spectroscopic investigations in solution revealed the formation of isomeric supramolecules owing to the steric hindrance caused by the third tBu group on the Cp′′′ ligand. In addition, a 2D coordination polymer was obtained and structurally characterized.

Reversible Carbon Dioxide Binding by Simple Lewis Base Adducts with Electron-Rich Phosphines

For the efficient utilization of carbon dioxide as feedstock in chemical synthesis, low-energy-barrier CO2 activation is a valuable tool. We report a metal-free approach to reversible CO2 binding under mild conditions based on simple Lewis base adducts with electron-rich phosphines. Variable-temperature NMR studies and DFT calculations reveal almost thermoneutral CO2 binding with low-energy barriers or stable CO2 adduct formation depending on the phosphines donor ability. The most basic phosphine forms an air-stable CO2 adduct that was used as phosphine transfer agent, providing a convenient access to transition-metal complexes with highly electron-rich phosphine ligands relevant to catalysis.

Redox and Coordination Behavior of the Hexaphosphabenzene Ligand in [(Cp*Mo)2 (μ,η(6) :η(6) -P6 )] Towards the "Naked" Cations Cu(+) , Ag(+) , and Tl(.)

Although the cyclo-P₆ complex [(Cp*Mo)₂(μ,η⁶:η⁶-P₆)] (1) was reported 30 years ago, little is known about its chemistry. Herein, we report a high-yielding synthesis of 1, the complex 2, which contains an unprecedented cyclo-P₁₀ ligand, and the reactivity of 1 towards the “naked” cations Cu⁺, Ag⁺, and Tl⁺. Besides the formation of the single oxidation products 3 a,b which have a bisallylic distorted cyclo-P₆ middle deck, the [M(1)₂]⁺ complexes are described which show distorted square-planar (M=Cu(4 a), Ag(4 b)) or distorted tetrahedral coordinated (M=Cu(5)) M⁺ cations. The choice of solvent enabled control over the reaction outcome for Cu⁺, as proved by powder XRD and supported by DFT calculations. The reaction with Tl⁺ affords a layered two-dimensional coordination network in the solid state.