Series Of Phosphines Research Articles

Phosphine Oxide‐Containing Gold(III) Complexes with Tunable Emission Color and Thermally Enhanced Luminescence Behavior

AbstractA series of phosphine oxide‐containing gold(III) complexes with tunable emission colors spanning from sky‐blue to near‐infrared region is reported. This is accomplished by the switching of the excited state characters from intraligand to ligand‐to‐ligand charge transfer through the replacement of the auxiliary ligand from aryl to nitrogen‐based ligands. In addition to high photoluminescence quantum yields in both solution and solid‐state thin films, these complexes exhibit large radiative decay rate constants of the order of 106 s−1, much larger than those commonly found for other gold(III) complexes. The origin of such enhanced performance is believed to be arising from the occurrence of both thermally activated delayed fluorescence and thermally stimulated delayed phosphorescence processes within the emitters. This is probed by ns‐ and fs‐transient absorption spectroscopy, time‐resolved, and temperature‐dependent emission spectroscopy. In particular, the direct observation of the upconversion processes and the determination of the activation barriers are achieved in the variable‐temperature fs‐transient absorption spectroscopic studies. Solution‐processed organic light‐emitting devices with satisfactory external quantum efficiencies of up to 15.2% are achieved, which could be ascribed to the presence of thermally activated delayed fluorescence and/or thermally stimulated delayed phosphorescence processes.

Facile Assembly of Modular-Type Phosphines for Tackling Modern Arylation Processes.

ConspectusThis Account presents an overview of a promising collection of phosphine ligands simply made from the modular Fischer indolization process and their applications in modern arylation processes. Using one easily accessible 2-arylindole scaffold, three major phosphino-moiety-positioned ligand series can be readily generated. We have attempted to explore challenging electrophilic and nucleophilic partners for the coupling reaction using the modular ligand tool. For the electrophilic partner study, CM-phos-type ligands, where the phosphino group is located at the 2-arene position of 2-arylindole, allow the successful cross-coupling of aryl mesylates. The CM-phos ligand forms a palladacycle before entering the cross-coupling catalytic cycle. For the nucleophilic partner investigation, the indole C3-positioned phosphines show the first accomplishment of Pd-catalyzed organotitanium nucleophile arylation. Indeed, the aryl-titanium nucleophile undergoes cross-coupling more efficiently than does the organoboron coupling partner in particular cases. Moreover, in the indole C3-positioned phosphine series, the -PPh2-containing ligands perform better in the highly sterically hindered cross-coupling of aryl chlorides than do ligands containing the -PCy2 moiety. The catalyst loading can even be reduced to 0.2 mol % Pd for tetra-ortho-substituted biaryl synthesis. This finding offers a new perspective on the next-generation design of phosphine ligands in which the sterically bulky and electron-rich -PR2 group (R = alkyl) may not be necessary for the cross-coupling of aryl chlorides. In general, we hypothesize that a good balance of steric and electronic properties for entertaining the oxidative addition and reductive elimination steps is crucial to the success of the reaction. For the steric factor, the highly sterically congested -PR2 group normally favors the reductive elimination, yet we conjecture that this sterically bulky group would serve as an obstacle for the incoming aryl halides. For the electronic factor, the electron rich -PR2 group is believed to support the oxidative cleavage of the C(Ar)-Cl bond by donating more electron density to the corresponding σ* orbital. Nevertheless, the high electron richness of the -PR2 group may disfavor the reductive elimination electronically. Overall, an appropriate balance of both electron density and steric bulkiness is suggested to allow the sterically hindered cross-coupling to proceed smoothly. We have found that the -PPh2-containing ligand is a good starting point for this investigation. The formation of aromatic carbon-carbon (C-C) and carbon-heteroatom (C-X) bonds from aryl chlorides was successfully realized using our proprietary phosphines.In addition to the indole-core-bearing ligand skeleton, we also explored the relevant imidazolyl and carbazolyl phosphines for their unique applications. Interestingly, the carbazolyl ligand, having more flexible C-N axial chirality, displays particular interchangeable Pd-N and Pd-arene coordination, which facilitates both oxidative addition and reductive elimination processes. Moreover, this C-N axially chiral ligand allows the successful asymmetric Suzuki-Miyaura coupling for attaining the most sterically hindered tetra-ortho-substituted biaryls with excellent enantioselectivity. The rationale behind these scientifically interesting findings is presented in detail.

Phosphine-Functionalized Syndiotactic Polystyrenes: Synthesis and Application to Immobilization of Transition Metal Nanoparticle Catalysts.

The precise control of monomer sequence and stereochemistry in copolymerization is of much interest and importance for the synthesis of high performance materials, but studies toward this goal have met with only limited success to date. The coordination polymerization of diphenylphosphinostyrene (p-StPPh2 and o-StPPh2 ) and its copolymerization with styrene (St) by (C5 Me4 SiMe3 )Sc(CH2 C6 H4 NMe2 -o)2 have been achieved for the first time to afford a new series of phosphine functionalized syndiotactic polystyrene. By the design of the polymer structure, the copolymer of o-StPPh2 and St (poly(o-StPPh2 -alt-St)-b-sPS) containing o-StPPh2 and St atactic alternating copolymer block and syndiotactic polystyrene block (sPS) showed excellent thermal stability and chemical resistance. The simple combination of the triphenylphosphine and syndiotactic polystyrene realized the stable immobilization of metal nanoparticles to afford highly robust metal@poly(o-StPPh2 -alt-St)-b-sPS nanocatalysts at high temperature and various atmospheres. The Cu@poly(o-StPPh2 -alt-St)-b-sPS catalyst can serve as a highly efficient heterogeneous catalyst for the synthesis of quinoline derivatives by acceptorless dehydrogenative coupling of o-aminobenzylalcohol with ketones.

Synthesis of a Sterically Encumbered Pincer Au(III)−OH Complex

AbstractWe report the synthesis and crystallographic characterization of a novel Au(III)−OH complex featuring a N^N^N‐pincer ligand. Reactivity studies towards oxygen atom transfer (OAT), a type of reactivity previously reported for a Au(III)−OH complex, indicate that this complex provides both a sterically encumbered Au atom and a sterically poorly accessible OH group leading to no reactivity with a series of phosphines. The steric encumbrance sets this example apart from the known examples of Au(III)−OH (pincer) complexes, which commonly feature planar ligands that provide little control over steric accessibility of the Au and O atoms in these complexes. Implications for the mechanism of OAT from Au−OH complexes are briefly discussed.

Chiral P,Olefin Ligands with Rotamers for Palladium-Catalyzed Asymmetric Allylic Substitution Reactions

We synthesized a series of phosphine–olefin-type chiral aminophosphines, and we confirmed that these each exists as two rotamers at the C(aryl)–N(amine) bond. We also investigated the ability of these aminophosphines to act as chiral ligands for Pd-catalyzed asymmetric allylic substitution reactions, such as the alkylation of allylic acetates with malonates or indoles, and we found they gave high enantio­selectivities (up to 98% ee).

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.

Balancing Bulkiness in Gold(I) Phosphino-triazole Catalysis.

The syntheses of a series of 1‐phenyl‐5‐phosphino 1,2,3‐triazoles are disclosed, within which, the phosphorus atom (at the 5‐position of a triazole) is appended by one, two or three triazole motifs, and the valency of the phosphorus(III) atom is completed by two, one or zero ancillary (phenyl or cyclohexyl) groups respectively. This series of phosphines was compared with tricyclohexylphosphine and triphenylphosphine to study the effect of increasing the number of triazoles appended to the central phosphorus atom from zero to three triazoles. Gold(I) chloride complexes of the synthesised ligands were prepared and analysed by techniques including single‐crystal X‐ray diffraction structure determination. Gold(I) complexes were also prepared from 1‐(2,6‐dimethoxy)‐phenyl‐5‐dicyclohexyl‐phosphino 1,2,3‐triazole and 1‐(2,6‐dimethoxy)‐phenyl‐5‐diphenyl‐phosphino 1,2,3‐triazole ligands. The crystal structures thus obtained were examined using the SambVca (2.0) web tool and percentage buried volumes determined. The effectiveness of these gold(I) chloride complexes to serve as precatalysts for alkyne hydration were assessed. Furthermore, the regioselectivity of hydration of but‐1‐yne‐1,4‐diyldibenzene was probed.

Phosphine interactions with high oxidation state metals

Phosphines and related ligands (PE3) are some of the most versatile and well-studied in the chemist’s arsenal. The vast majority of studies on these ligands have focused on their interactions with low valent transition metals. From these investigations, it is now widely accepted that there are two major electronic interactions between the PE3 ligand and a low valent metal: a σ-donor bond between the metal and phosphorus and a π-acceptor interaction, where the phosphorus accepts electron density from the metal. The π-acceptor interaction is a form of negative hyperconjugation, which becomes more significant for electron-withdrawing E groups, such as those in phosphites, P(OR)3. In this study, the bonding between a high valent transition metal, chromium(VI), and a variety of different PE3 ligands was interrogated. The Lewis acidity of the NCr(NiPr2)2+ fragment was quantified using the Gutmann method, and the acceptor number was found to be 100, similar to highly Lewis acidic SbCl5. Using calculations (DFT, Natural Resonance Theory) it was found that rarely discussed resonance forms were important for some phosphines on d0 metal centers. These alternative resonance forms included negative hyperconjugative forms that appear when aryl or heteroatom substituents are placed on the phosphorus atom, leading to increased electron density at the Lewis acidic metal center. Attempts to explore the consequences of the computational results experimentally with the [NCr(NiPr2)2PE3]+X− system were undertaken, however, due to experimental difficulties with ion pairing and solvent effects, experimental quantification of donor ability across a diverse series of phosphines was not possible. However, trialkylphosphines were fit to a simple 2-parameter model, which showed that both steric and electronic effects were extremely important in the system. While we could not reliably quantify the observation, it was noted that aryl and heteroatom substituted PE3, P(OEt)3 and PMePh2, for example, have faster amide rotation at lower temperatures than many of the trialkylphosphines. Despite experimental complications, these studies suggest that bonding to high valent metals could be more complex than simple lone pair donation, and it appears that several resonance forms may be of importance when d0 metals bond to aryl- and heteroatom substituted phosphines.

31 P NMR Evidence for Peroxide Intermediates in Lipid Emulsion Photooxidations: Phosphine Substituent Effects in Trapping.

Intralipid is a lipid emulsion used in photodynamic therapy (PDT) for its light scattering and tissue-simulating properties. The purpose of this study is to determine whether or not Intralipid undergoes photooxidation, and we have carried out an Intralipid peroxide trapping study using a series of phosphines [2'-dicyclohexylphosphino-2,6-dimethoxy-1,1'-biphenyl-3-sulfonate, 3-(diphenylphosphino)benzenesulfonate, triphenylphosphine-3,3',3''-trisulfonate and triphenylphosphine]. Our new findings are as follows: (1) An oxygen atom is transferred from Intralipid peroxide to the phosphine traps in the dark, after the photooxidation of Intralipid. 3-(Diphenylphosphino)benzenesulfonate is the most suitable trap in the series owing to a balance of nucleophilicity and water solubility. (2) Phosphine trapping and monitoring by 31 P NMR are effective in quantifying the peroxides in H2 O. An advantage of the technique is that peroxides are detected in H2 O; deuterated NMR solvents are not required. (3) The percent yield of the peroxides increased linearly with the increase in fluence from 45 to 180 J cm-2 based on our trapping experiments. (4) The photooxidation yields quantitated by the phosphines and 31 P NMR are supported by the direct 1 H NMR detection using deuterated NMR solvents. These data provide the first steps in the development of Intralipid peroxide quantitation after PDT using phosphine trapping and 31 P NMR spectroscopy.

α‐Radical Phosphines: Synthesis, Structure, and Reactivity

AbstractA series of phosphines featuring a persistent radical were synthesized in two steps by condensation of dialkyl‐/diarylchlorophosphines with stable cyclic (alkyl)(amino)carbenes (cAACs) followed by one‐electron reduction of the corresponding cationic intermediates. Structural, spectroscopic, and computational data indicate that the spin density in these phosphines is mainly localized on the original carbene carbon from the cAAC fragment; thus, it remains in the α‐position with respect to the central phosphorus atom. The potential of these α‐radical phosphines to serve as spin‐labeled ligands is demonstrated through the preparation of several AuI derivatives, which were also structurally characterized by single‐crystal X‐ray diffraction.

α-Radical Phosphines: Synthesis, Structure, and Reactivity.

A series of phosphines featuring a persistent radical were synthesized in two steps by condensation of dialkyl-/diarylchlorophosphines with stable cyclic (alkyl)(amino)carbenes (cAACs) followed by one-electron reduction of the corresponding cationic intermediates. Structural, spectroscopic, and computational data indicate that the spin density in these phosphines is mainly localized on the original carbene carbon from the cAAC fragment; thus, it remains in the α-position with respect to the central phosphorus atom. The potential of these α-radical phosphines to serve as spin-labeled ligands is demonstrated through the preparation of several AuI derivatives, which were also structurally characterized by single-crystal X-ray diffraction.

Understanding the Role of Dispersion in Frustrated Lewis Pairs and Classical Lewis Adducts: A Domain-Based Local Pair Natural Orbital Coupled Cluster Study.

The interaction of Lewis acids and bases in both classical Lewis adducts and frustrated Lewis pairs (FLPs) is investigated to elucidate the role that London dispersion plays in different situations. The analysis comprises 14 different adducts between tris(pentafluorophenyl)borane and a series of phosphines, carbenes, and amines with various substituents, differing in both steric and electronic properties. The domain-based local pair natural orbital coupled-cluster (DLPNO-CCSD(T)) method is used in conjunction with the recently introduced local energy decomposition (LED) analysis to obtain state-of-the-art dissociation energies and, at the same time, a clear-cut definition of the London dispersion component of the interaction, with the ultimate goal of aiding in the development of designing principles for acid/base pairs with well-defined bonding features and reactivity. In agreement with previous DFT investigations, it is found that the London dispersion dominates the interaction energy in FLPs, and is also remarkably strong in Lewis adducts. In these latter systems, its magnitude can be easily modulated by modifying the polarizability of the substituents on the basic center, which is consistent with the recently introduced concept of dispersion energy donors. By counteracting the destabilizing energy contribution associated with the deformation of the monomers, the London dispersion drives the stability of many Lewis adducts.

Electrophilic phosphonium cations (EPCs) with perchlorinated-aryl substituents: towards air-stable phosphorus-based Lewis acid catalysts.

A series of phosphines incorporating (C6Cl5) substituents, Ph2P(C6Cl5) 1, PhP(C6Cl5)22, P(C6Cl5)33 and (C6F5)P(C6Cl5)24 were prepared. In the case of 1, 2 and 4, these were converted to the corresponding aryl-difluorophosphoranes 5-7via reaction with XeF2, whereas reaction of 3 with XeF2 afforded only an inseparable mixture of products. The compounds 5-7 were converted to the fluorophosphonium cations 8-10, whereas the reaction of 3 with Selectfluor afforded (C6Cl5)2POF and (C6Cl5)2. The fluorophosphonium salts showed evidence of improved air stability as well as Lewis acid catalytic activity in hydrodefluorination, hydrosilylation, deoxygenation and dehydrocoupling chemistry.

Structural, spectroscopic and computational examination of the dative interaction in constrained phosphine-stibines and phosphine-stiboranes.

A series of phosphine-stibine and phosphine-stiborane peri-substituted acenaphthenes containing all permutations of pentavalent groups -SbCln Ph4-n (5-9), as well as trivalent groups -SbCl2 , -Sb(R)Cl, and -SbPh2 (2-4, R=Ph, Mes), were synthesised and fully characterised by single crystal diffraction and multinuclear NMR spectroscopy. In addition, the bonding in these species was studied by DFT computational methods. The P-Sb dative interactions in both series range from strongly bonding to non-bonding as the Lewis acidity of the Sb acceptor is decreased. In the pentavalent antimony series, a significant change in the P-Sb distance is observed between -SbClPh3 and -SbCl2 Ph2 derivatives 6 and 7, respectively, consistent with a change from a bonding to a non-bonding interaction in response to relatively small modification in Lewis acidity of the acceptor. In the Sb(III) series, two geometric forms are observed. The P-Sb bond length in the SbCl2 derivative 2 is as expected for a normal (rather than a dative) bond. Rather unexpectedly, the phosphine-stiborane complexes 5-9 represent the first examples of the σ(4) P→σ(6) Sb structural motif.

The Multiple Roles of Imidazolium Ionic Liquids in Transition‐Metal Catalysis: The Palladium‐Catalyzed Telomerization of 1,3‐Butadiene with Acetic Acid

AbstractThe telomerization of 1,3‐butadiene with acetic acid catalyzed by palladium(II) acetate associated with a series of phosphines in 3‐(2‐methoxyethyl)‐1‐methylimidazolium acetate was used to investigate the role of the ionic liquid. The ionic liquid plays multiple roles in this reaction as it acts as the solvent, stabilizer, ligand, and cocatalyst. The reaction performed in the presence of Dan2phos, a trifluoromethylated sulfonated triarylphosphine, at 100 °C for 24 h gave a turnover number of 14 600 with 89 % selectivity to telomers at 75 % 1,3‐butadiene conversion and complete acetic acid conversion.

Electronic effects of triarylphosphines in metal-free hydrogen activation: a kinetic and computational study

The frustrated Lewis pair-mediated reversible hydrogen activation is studied as a function of the electron-donor quality of a series of phosphines. The increasing acidity of the generated phosphonium species leads to a stepwise lowering of the temperature for the highly reversible H2-activation and permits concrete classification for the first time. The influence of the acid strength on the metal-free hydrogenation of selected olefins is investigated by kinetic experiments and quantum chemical calculations. Detailed information for the rate-determining steps fully support our mechanistic model of a protonation step prior to hydride transfer. The rate of hydrogenation is strongly dependent on the electronic nature of the phosphine and of the acidity of the corresponding phosphonium cation. A careful balance of these two factors provides highly efficient metal-free hydrogenation catalysts. The provided findings are used to revise the reactivity of Lewis bases in the hydrogenation of imines, one of the most recognized applications of FLPs.

Unexpected Acidity Enhancement Triggered by AlH3Association to Phosphines

The complexes formed by the interaction between a series of phosphines R-PH(2) (R = H, CH(3), c-C(3)H(5), C(6)H(5)) and AlH(3) have been investigated through the use of high-level G4 ab initio calculations. These very stable complexes behave as much stronger acids than the isolated phosphines. This dramatic acidity enhancement, which can be as high as 174 kJ mol(-1), results from a much greater stabilization of the anionic deprotonated species with respect to the neutral one, upon AlH(3) association. This effect depends quantitatively on the nature of the substituent R and is smaller for R = C(6)H(5) because of the conjugation of the P lone pair with the aromatic system. More unexpectedly, however, the phosphine-alane complexes, RPH(2):AlH(3), are more acidic than the corresponding phosphine-borane RPH(2):BH(3) analogues. This unexpected result is due to the enhanced stability of the anionic deprotonated species for complexes involving AlH(3), because the delocalization of the newly created P lone pair with the P-Al bonding density is more favorable when the Lewis acid is aluminum trihydride than when it is borane.

Natural Bond Orbital Rationalizations of NMR Observations for Metal‐Ligand Bonding (II): Rehybridization of Phosphorus Arising from Coordination of Methyl‐Phenyl‐Phosphines

AbstractCarbonyls' 2π orbital populations, [2π], in W(CO)5L {L = PPh3, PPh2Me, PPhMe2} have been determined by NMR spin‐lattice relaxation techniques. Experimental values of axial [2π], compared with those reported for PMe3 and P(OMe)3, reveal that PMe3 is a slightly better π‐acid than PPh3. Through space interactions between carbonyl and phenyl groups are insignificant since values of [2π] do not vary significantly in the series of phosphines, going from PMep3 to PPh3. Natural bond orbital (NBO) studies indicate that π‐accepting capabilities for these phosphines are primarily governed by the nature of P‐C anti‐bonding, σ*P‐C. Compared with PPh3, the better π‐accepting σ*P‐C, as well as the better s‐donating lone‐pair LP(P), in PMe3 can both be explained by the higher extents of rehybridization of the coordinated phosphorus atom. Based on this rehybridization argument, the NBO predicted order of increasing π‐acidic strengths PPh3 < PPh2Me < PPhMe2 < PMe3, which cannot be clearly distinguished by NMR experiments, is ascribed to the same NBO trend of σ‐donating capabilities in a synergistic manner. Effects of coordination on P‐Y (Y = C, O, F) bonding strengths in phosphines (or phosphites) are depending on two conflicting effects: rehybridization of LP(P) and the hyperconjugative‐like dπ → σ*P‐Y back‐donation.

Quantum-chemical calculations of NMR chemical shifts of organic molecules: II. Influence of medium, relativistic effects, and vibrational corrections on phosphorus magnetic shielding constants in the simplest phosphines and phosphine chalcogenides

The influence of solvent nature, relativistic effects, and vibrational corrections on the accuracy of calculation of 31P chemical shifts of the simplest phosphines, phosphine oxides, phosphine sulfides, and phosphine selenides was studied. Consideration of the above factors at the stage of both geometry optimization and calculation of magnetic shielding constants was found to appreciably improve the accuracy of calculation of 31P NMR chemical shifts in the series of phosphines and phosphine chalcogenides.

The Ever‐Surprising Chemistry of Boron: Enhanced Acidity of Phosphine⋅Boranes

The acidity-enhancing effect of BH(3) in gas-phase phosphineboranes compared to the corresponding free phosphines is enormous, between 13 and 18 orders of magnitude in terms of ionization constants. Thus, the enhancement of the acidity of protic acids by Lewis acids usually observed in solution is also observed in the gas phase. For example, the gas-phase acidities (GA) of MePH(2) and MePH(2)BH(3) differ by about 118 kJ mol(-1) (see picture).The gas-phase acidity of a series of phosphines and their corresponding phosphineborane derivatives was measured by FT-ICR techniques. BH(3) attachment leads to a substantial increase of the intrinsic acidity of the system (from 80 to 110 kJ mol(-1)). This acidity-enhancing effect of BH(3) is enormous, between 13 and 18 orders of magnitude in terms of ionization constants. This indicates that the enhancement of the acidity of protic acids by Lewis acids usually observed in solution also occurs in the gas phase. High-level DFT calculations reveal that this acidity enhancement is essentially due to stronger stabilization of the anion with respect to the neutral species on BH(3) association, due to a stronger electron donor ability of P in the anion and better dispersion of the negative charge in the system when the BH(3) group is present. Our study also shows that deprotonation of ClCH(2)PH(2) and ClCH(2)PH(2)BH(3) is followed by chloride departure. For the latter compound deprotonation at the BH(3) group is found to be more favorable than PH(2) deprotonation, and the subsequent loss of Cl(-) is kinetically favored with respect to loss of Cl(-) in a typical S(N)2 process. Hence, ClCH(2)PH(2)BH(3) is the only phosphineborane adduct included in this study which behaves as a boron acid rather than as a phosphorus acid.