The chemistry of phosphines in constrained, well-defined microenvironments

Exploring Supramolecular Frustrated Lewis Pairs

Frustrated Lewis pairs (FLPs) have rapidly become one of the key metal-free catalysts for a variety of chemical transformations. Embedding these catalysts within a supramolecular assembly can offer improvements to factors such as recyclability and selectivity. In this review we discuss advances in this area, covering key supramolecular assemblies such as metal organic frameworks (MOFs), covalent organic frameworks (COFs), polymers and macrocycles.

Singlet Carbenes Are Stereoinductive Main Group Ambiphiles

Stereogenic units are a critical source of molecular complexity, but their stereoselective formation via main group ambiphiles─which are suitable for derivatizing a wide scope of functionalities─is largely unexplored. Herein, using chiral cyclic (alkyl)(amino)carbenes (ChiCAACs), we study stereoinduction during the oxidative addition of E-H σ-bonds (E = C, N, O, Si, P). Through computational modeling, the relationship between stereochemical outcome and mechanism is elucidated, providing insight into when and why ChiCAACs exhibit excellent stereoselectivities. Altogether, these results demonstrate the potential for chiral main group ambiphiles to generate stereogenic units in a highly controlled manner opening avenues for applying "metal-like" reactivity in metal-free asymmetric syntheses.

MOF-Supported Diphosphine Ligands for Iridium-Catalyzed C-H Borylation of Arenes

Postsynthetic methods have been used to immobilize a carboxylate-functionalized diphosphine ligand, N,N-bis[(diphenylphosphino)methyl]glycinate (dppmg), in different metal-organic framework (MOF) supports. H(dppmg) reacts quantitatively with Zn-OH groups in MFU-4l-OH (1) to provide solid-state ligands (1-H-x) with controllable diphosphine loadings. Postsynthetic metalation with [Ir(OMe)(cod)]2 (cod = 1,5-cyclooctadiene) generates heterogeneous precatalysts (1-Ir-x) that show excellent activity toward C-H borylation of arenes. This activity is dependent on both the catalyst site density and initial concentration of the borylating reagent. Homogeneous catalysts supported by analogous diphosphine ligands exhibit low catalytic activity, demonstrating the beneficial role of catalyst site isolation. Immobilization of dppmg-Ir catalysts at the Zr-based nodes of MOF-808 (2-P-Ir) and NU-1000 (3-P-Ir) results in materials with relatively poor catalytic activity toward C-H borylation of toluene, revealing the importance of the MOF support in catalyst design.

Supramolecular Chemistry in Metal-Organic Framework Materials

Far from being simply rigid, benign architectures, metal-organic frameworks (MOFs) exhibit diverse interactions with their interior environment. From developing crystal sponges to studying reactions in framework materials, the role of both supramolecular chemistry and framework structure is evident. We explore the role of supramolecular chemistry in determining framework…guest interactions and attempts to understand the dynamic behavior in MOFs, including attempts to control pore behavior through the incorporation of mechanically-interlocked molecules. Appreciating and understanding the role of supramolecular interactions and dynamic behavior in metal-organic frameworks emerge as important directions for the field.

Robust phosphine-based covalent-organic framework palladium catalysts for the highly efficient carbonylation coupling reaction

The first vinyl-linked PPh3-based covalent-organic frameworks (Pd@TMBen-PPh3 and Pd@TMBen-4F-PPh3) were facilely constructed via direct three-component Knoevenagel condensation, which could serve as outstanding heterogeneous catalysts for the Pd-catalyzed carbonylation coupling reaction owing to the synergistic effect of the flexible PPh3-ligated Pd complexes and the rigid framework.

An Isolable Triarylphosphine Radical Cation Electronically Stabilized by Through-Space Radical Delocalization

Synthesis and characterization of a thermally stable triarylphosphine radical cation, [P(8-Br-C10H6)3][BArF24] ([1][BArF24], BArF24 = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate), enabled by stabilization through peri-bromo-substituted naphthalenes, are described. Unlike previously reported phosphine radical cations that rely on sterically bulky substituents for stabilization, our approach leverages electronic stabilization via "through-space" radical delocalization. Single-crystal X-ray diffraction of [1][BArF24] reveals a tricapped tetrahedral geometry, resulting from the spatial proximity of the three bromine atoms to the phosphorus center, differentiated from the trigonal planar geometry observed in the previously reported triarylphosphine radical cations with sterically bulky substituents. EPR spectroscopy shows an isotropic signal with hyperfine couplings to both the phosphorus and the three bromine atoms, indicating spin delocalization over these four atoms and consequent formation of a four-center, seven-electron (4c-7e) bond. DFT computational studies further support the through-space radical delocalization mechanism, revealing that the HOMO of 1 exhibits antibonding character between the phosphorus center and the three adjacent Br atoms, distinct from common triarylphosphines.

Strain-Releasing Ring-Opening Diphosphinations for the Synthesis of Diphosphine Ligands with Cyclic Backbones

Diphosphine ligands based on cyclobutane, bicyclo[3.1.1]heptane, and bicyclo[4.1.1]octane were synthesized from the corresponding highly strained, small, cyclic organic molecules, i.e., bicyclo[1.1.0]butane, [3.1.1]propellane, and [4.1.1]propellane, employing a ring-opening diphosphination. Under photocatalytic conditions, the three-component reaction of a diarylphosphine oxide, one of the aforementioned strained molecules, and a diarylchlorophosphine results in the smooth formation of the corresponding diphosphines in high yield. The obtained diphosphines can be expected to find applications in functional molecules due to their unique structural characteristics, which likely impart specific properties on their associated metal complexes and coordination polymers (e.g., a zigzag-type structure). The feasibility of the initial radical addition can be estimated using density-functional-theory calculations using the artificial force induced reaction (AFIR) method. This study focuses on the importance of integrating experimental and computational methods for the design and synthesis of new diphosphination reactions that involve strained, small, cyclic organic molecules.

Installation of Copper(I) and Silver(I) Sites into TREN-Based Porous Organic Cages via Postsynthetic Metalation

Porous organic cages (POCs) and metal-organic polyhedra (MOPs) function as zero-dimensional porous materials, able to mimic many functions of insoluble framework materials while offering processability advantages. A popular approach to access tailored metal-based motifs in extended network materials is postsynthetic metalation, which allows metal installation to be decoupled from framework assembly. Surprisingly, this approach has only sparingly been reported for molecular porous materials. In this report, we demonstrate postsynthetic metalation of tetrahedral [4 + 4] POCs assembled from tris(2-aminoethyl)amine (TREN) and 1,3,5-tris(4-formylphenyl)benzene. The trigonally symmetric TREN motif is a common chelator in coordination chemistry and, in the POCs explored herein, readily binds copper(I) and silver(I) to form cationic cages bearing discrete mononuclear coordination fragments. Metalation retains cage porosity, allowing us to compare the sorption properties of the parent organic and metalated cages. Interestingly, introduction of copper(I) facilitates activated oxygen chemisorption, demonstrating how targeted metalation can be exploited to tune the sorption characteristics of porous molecular materials.

Crowding for Confinement: Reversible Isomerization of First-Generation Donor-Acceptor Stenhouse Adduct Derivatives in Water Modulated by Thermoresponsive Dendritic Macromolecules

Mimicking nature, the reversible isomerization of hydrophobic dyes in aqueous solutions is appealing for bio-applications. Here, we report on the reversible isomerization of first-generation solvatochromic donor-acceptor Stenhouse adducts (DASAs) in water within dendritic matrices, realized either through the dendronization of DASAs or the incorporation of DASA pendants into dendronized copolymers. These dendritic macromolecules contain three-fold dendritic oligoethylene glycols (OEGs), which afford the macromolecules water-solubility and unprecedented thermoresponsive behavior. The thermoresponsive behavior of both dendronized DASAs and dendronized copolymers is dominated by the peripherals of dendritic OEGs. However, the hydrophilicity of the acceptor from DASA moieties also play a role in mediating their thermal phase transitions, and more importantly, tailor the hydrophobic interactions between dendritic OEGs and DASA moieties. Intriguingly, dendritic topologies contribute confinement to encapsulate the DASA moieties through crowding effects, and cooperative interactions from the crowded dendritic OEGs modulate the DASA moieties with different isomerization in aqueous media. The thermally induced collapse of dendritic OEGs, accompanied by the aggregation of dendritic macromolecules, leads to the formation of hydrophobic domains, which exert enhanced crowding effects to efficiently encapsulate the DASA moieties. Compared to the low molar mass of dendronized DASAs, thermally collapsed dendronized copolymers can efficiently retard the hydration of DASA pendants through cooperation between neighboring dendritic OEGs and afford the DASA pendants with better confined microenvironments to mediate their isomerization recovery by up to 90% from a cyclic charged (hydrophilic) state into a noncharged (hydrophobic) linear state in water. This dendritic confinement exhibits excellent fatigue resistance after several cycles of alternating photo-irradiation and thermal annealing at elevated temperatures.

Continuous-Flow Catalysis Using Phosphine-Metal Complexes on Porous Polymers: Designing Ligands, Pores, and Reactors

Continuous-flow syntheses using immobilized catalysts can offer efficient chemical processes with easy separation and purification. Porous polymers have gained significant interests for their applications to catalytic systems in the field of organic chemistry. The porous polymers are recognized for their large surface area, high chemical stability, facile modulation of surface chemistry, and cost-effectiveness. It is crucial to immobilize transition-metal catalysts due to their difficult separation and high toxicity. Supported phosphine ligands represent a noteworthy system for the effective immobilization of metal catalysts and modulation of catalytic properties. Researchers have been actively pursuing strategies involving phosphine-metal complexes supported on porous polymers, aiming for high activities, durabilities, selectivities, and applicability to continuous-flow systems. This review provides a concise overview of phosphine-metal complexes supported on porous polymers for continuous-flow catalytic reactions. Polymer catalysts are categorized based on pore sizes, including micro-, meso-, and macroporous polymers. The characteristics of these porous polymers are explored concerning their efficiency in immobilized catalysis and continuous-flow systems.

A phosphine-free molecularly-defined Ni(II) complex in catalytic hydrogenation of CO2

The development of base metal catalysts capable of CO2 hydrogenation is a challenge and a necessity to progress from the scarce noble metal catalysts. In this regard, we report herein the first non-phosphine-based Ni complex, supported by a "carbazolato-bis-NHC" pincer ligand framework, for efficient catalytic hydrogenation of CO2 to formate. A tailored combination of the Ni complex as a catalyst, DBU as a base, and Zn(OAc)2 as an additive offered enhanced activity leading to a TON up to 5476 and an excellent yield up to 92% for the formate product from a reaction on ∼27 mmol scale.

Recent progress in transition metal complexes featuring silylene as ligands

Silylenes, divalent silicon(II) compounds, once considered highly reactive and transient species, are now widely employed as stable synthons in main-group and coordination chemistry for myriad applications. The synthesis of stable silylenes represents a major breakthrough, which led to extensive exploration of silylenes in stabilizing low-valent main-group elements and as versatile ligands in coordination chemistry and catalysis. In recent years, the exploration of transition metal complexes stabilized with silylene ligands has captivated significant research attention. This is due to their robust σ-donor characteristics and capacity to stabilize transition metals in low valent states. It has also been demonstrated that the transition metal complexes of silylenes are effective catalysts for hydroboration, hydrosilylation, hydrogenation, hydrogen isotope exchange reactions, and small molecule activation chemistry. This review article focuses on the recent progress in the synthesis and catalytic application of transition metal complexes of silylenes.

Phosphine-incorporated Metal-Organic Framework for Palladium Catalyzed Heck Coupling Reaction

As an emerging material with the potential to combine the high efficiency of homogeneous catalysts and high stability and recyclability of heterogeneous catalysts, metal-organic frameworks (MOFs) have been viewed as one of the candidates to produce catalysts of the next generation. Herein, we heterogenized the highly active mono(phosphine)-Pd complex on surface of UiO-66 MOF, as a catalyst for Suzuki and Heck cross coupling reactions. The successful immobilization of these Pd-monophosphine complexes on MOF surface to form UiO-66-PPh2-Pd was characterized and confirmed via comprehensive set of analytical methods. UiO-66-PPh2-Pd showed high activity and selectivity for both Suzuki and Heck Cross Coupling Reactions. This strategy enabled facile access to mono(phosphine) complexes which are challenging to design and require multistep synthesis in homogeneous systems, paving the way for future MOF catalysts applications by similar systems.

CYTOP® 366: A Tertiary Phosphine Inaccessible by Most Traditional Hydrophosphination Methods

Homogenous catalysis is an essential tool within the commercial manufacture of bulk and fine chemicals. Within this, phosphine ligands, such as tricyclohexylphosphine, otherwise known as CYTOP® 366, are a crucial component. When designing a pathway to your ligand of choice, some key considerations include safety, yield and quality, but at commercial volumes we must also balance cost and consider the technologies readily available. Herein, we report the synthetic route that was chosen to manufacture tricyclohexylphosphine at commercial scale. We also consider, with the use of computational calculations, why traditional hydrophosphination methods failed, where the selected pathway succeeded.

Ligand-Free Iron-Catalyzed Construction of C-P Bonds via Phosphorylation of Alcohols: Synthesis of Phosphine Oxides and Phosphine Compounds

An efficient method for the construction of C-P(V) and C-P(III) bonds via the iron-catalyzed phosphorylation of alcohols under ligand-free conditions is disclosed. This strategy represents a straightforward process to prepare a series of phosphine oxides and phosphine compounds in good to excellent yields from the readily available alcohols and P-H compounds. A plausible mechanism is also proposed. We anticipate that this mode of transforming simple alcohols would apply in chemical synthesis widely.

Enantioselective copper-catalyzed B-H bond insertion reaction of α-diazo phosphonates to access chiral α-boryl phosphonates

Chiral phosphorus-containing compounds find applications across various fields, including asymmetric catalysis, medicinal chemistry, and materials science. Despite the abundance of reported highly enantioselective methods for synthesizing various chiral phosphorus compounds, the enantioselective synthesis of α-boryl phosphorus compounds still remains an unknown territory. Here, we report a method for the construction of chiral α-boryl phosphates by asymmetric B-H insertion reaction using α-diazo phosphates as carbene precursors, cheap and readily available copper salt as the catalyst and chiral oxazoline as the ligand. This method can directly afford a series of stable α-boryl phosphates with a yield up to 97% and an enantioselectivity up to 98% ee. The operating procedure of this method is straightforward, offering a broad substrate applicability, remarkable tolerance towards various functional groups, and gentle reaction conditions.

A Simple Manganese(I) Catalyst for the Efficient and Selective Hydrophosphination of Olefins with PH3, Primary, and Secondary Phosphanes

A tridentate ligand L with a P,NH,N donor motif was synthesized in few steps from commercially available precursors. Upon reaction with [MnBr(CO)5], an octahedral 18-electron complex [Mn(CO)3(L)]Br (1) is obtained in which L adopts a facial arrangement. After deprotonation of the NH group in the cationic complex unit, a neutral Mn(I) amido complex [Mn(CO)2(L-H)] (2) is formed under loss of CO. Rearrangement of L-H leads to a trigonal bipyramidal structure in which the P and N donor centers are in trans position. Further deprotonation of 2 results in a dep-blue anionic complex fragment [Mn(CO)2(L-2H)]- (3). DFT calculations and a QTAIM analysis show that the amido complex 2 contains a Mn-N bond with partial double bond character and 3 an aromatic MnN2C2 ring. The anion [Mn(CO)2(L-2H)]- reacts with Ph2PH to give a phosphido complex, which serves as phosphide transfer reagent to activated olefins. But the catalytic activity is low. However, the neutral amido complex 2 is an excellent catalyst and with loadings as low as 0.04 mol %, turn over frequencies of >40'000 h-1 can be achieved. Furthermore, secondary and primary alkyl phosphines as well as PH3 can be added in a catalytic hydrophosphination reaction to a wide range of activated olefins such as α,β-unsaturated aldehydes, ketones, esters, and nitriles. But also, vinyl pyridine and some styrene derivatives are converted into the corresponding phosphanes.

Bridging Homogeneous and Heterogeneous Catalysis: Phosphine-Functionalized Metal-Organic Frameworks

Phosphine-functionalized metal-organic frameworks (P-MOFs) as an emerging class of coordination polymers, have provided novel opportunities for the development of heterogeneous catalysts. Yet, compared with the ubiquitous phosphine systems in homogeneous catalysis, heterogenization of phosphines in MOFs is still at its early stage. In this Minireview, we summarize the synthetic strategies, characterization and catalytic reactions based on the P-MOFs reported in literature. In particular, various catalytic reactions are discussed in detail in terms of phosphine ligand structure-function relationship, including the potential obstacles for future development. Finally, we discuss the possible solutions, including new types of reactions and techniques as the perspectives for the development of P-MOF catalysts, highlighting the opportunities and challenges.

Cavity-Shape-Dependent Divergent Chemical Reaction inside Aqueous Pd6L4 Cages

Chemical reactions inside the confined pockets of enzyme-mimicking hosts, such as cages and macrocycles, have been an emerging field of interest over the past decade. Although many such reactions are known, the use of such cages toward the divergent synthesis of nonisomeric products has not been well explored. Divergent synthesis is a technique of forming two or more distinct products from the same reagents by changing the catalyst or reaction conditions. Changing the shape of the cage can also change the nature and magnitude of the host-guest interactions. Thus, is it possible for such changes to cause differences in the reaction pathways leading to formation of nonisomeric products? Herein, we report a divergent chemical transformation of anthrone [anthracen-9(10H)-one] inside different water-soluble M6L4 cages. When anthrone was encapsulated inside a newly synthesized M6L4 octahedral cage 1, it dimerized to form dianthrone [9,9'-bianthracen-10,10'(9H,9'H)-dione]. In contrast, when the same chemical reaction was performed inside a M6L4 double-square shaped cage 2, it was oxidized to form anthraquinone [anthracene-9,10-dione]. Similar results were obtained with a different set of isomeric aqueous Pd6 cages 3a (octahedral cage) and 3b (double-square cage), indicating the dependence of the shape of cavity on the divergent synthesis. The present report demonstrates a unique example of different outcomes/results of a reaction depending on the shape of the molecular container, which was driven by the host-guest interactions and the preorganization of the substrates.

An Inorganic Click Reaction for the Synthesis of Interlocked Molecules

We describe the use of the cyaphide-azide 1,3-dipolar cycloaddition reaction for the synthesis of a new class of inorganic rotaxanes containing gold(I) triazaphosphole stoppers. Electron-deficient bis-azides, which thread perethylated pillar[5]arene in aromatic solvents, readily react with two equivalents of Au(IDipp)(CP) (IDipp=1,3-bis-(2,6-diisopropylphenyl)-imidazol-2-ylidene) to afford interlocked molecules via an inorganic click reaction. These transformations proceed in good yields (ca. 65 %) and in the absence of a catalyst. The resulting organometallic rotaxanes are air- and moisture-stable and can be purified by column chromatography under aerobic conditions. The targeted rotaxanes were characterized by multi-element nuclear magnetic resonance (NMR) spectroscopy, mass-spectrometry, and single-crystal X-ray diffraction.

Catalytic Asymmetric P-H Insertion Reactions

Albeit notable endeavors in enantioselective carbene insertion into X-H bonds (X = C, O, N, S, Si, B), the catalytic asymmetric P-H insertion reactions still stand for a long-lasting challenge. By merging transition-metal catalysis with organocatalysis, we achieve a scalable enantioselective P-H insertion transformation between diazo pyrazoleamides and H-phosphine oxides that upon subsequent reduction delivers a wide variety of optically active β-hydroxyl phosphine oxides in good yields with high enantioselectivity. The achiral copper catalyst fosters the carbenoid insertion into the P-H bond, while the chiral cinchona alkaloid-derived organocatalyst controls the subsequent enantioselective outcome. Density functional theory (DFT) calculations further reveal that the copper catalyst chelates to the organocatalyst, enhances its acidity, and accordingly promotes the enantioselective proton transfer. Our work showcases the potential of combining transition-metal catalysis with organocatalysis to realize elusive asymmetric reactions.

Synthesis of a rhodium(III) dinitrogen complex using a calix[4]arene-based diphosphine ligand

The synthesis and characterisation of the rhodium(III) dinitrogen complex [Rh(2,2'-biphenyl)(CxP₂)(N₂)]⁺ are described, where CxP₂ is a trans-spanning calix[4]arene-based diphosphine and the dinitrogen ligand is projected into the cavity of the macrocycle.

An all-aqueous and phosphine-free integrated amine-assisted CO2 capture and catalytic conversion to formic acid

A phosphine-free Ir(III)-NHC-based efficient catalytic system is developed for integrated CO₂ capture with tetramethylguanidine as a capturing agent and conversion to formate with H₂ gas, conducting both the steps in water, affording product yield up to 85% and TON up to 19 171 in just 12 h. In the segment of "integrated CO₂-capture and conversion to formate", this system represents not only the first phosphine-free module, but also one of the few best known homogeneous catalysts.

A cavity-shaped cis-chelating P,N ligand for highly selective nickel-catalysed ethylene dimerisation

The presence of a permethylated α-cyclodextrin (α-CD) cavity in a chelating P,N ligand promotes exclusive formation of 1 : 1 ligand/metal complexes. In MX₂ complexes, one of the two halido ligands is forced to reside inside the CD hollow while the second one is pointing outside. Unlike its cavity-free analogue, a Ni(II) complex of the CD ligand is a highly selective precatalyst for ethylene dimerisation (96% C₄ selectivity with up to 95% of 1-butene within the C₄ fraction).

Synthesis and Structure of a Ferrocenylsilane-Bridged Bisphosphine

A bisferrocenylsilane-bridged bisphosphine, i.e., a bisphosphine bridged by bis(1’-dicyclohexylphosphino-1-ferrocenyl)dimethoxysilane, was synthesized and structurally characterized. Its redox behavior was examined by cyclic voltammetry and differential pulse voltammetry, which revealed two-step oxidation processes.

Diametric calix[6]arene-based phosphine gold(I) cavitands

We report the synthesis and characterization, in low polarity solvents, of a novel class of diametric phosphine gold(I) cavitands characterized by a 1,2,3-alternate geometry. Preliminary catalytic studies were performed on a model cycloisomerization of 1,6-enynes as a function of the relative orientation of the bonded gold(I) nuclei with respect to the macrocyclic cavity.