Synthesis and coordination chemistry of macrocyclic phosphine ligands

A Coppoborylene Stabilized by Multicenter Covalent Bonding and Its Amphoteric Reactivity to CO

A cationic copper-stabilized coppoborylene was prepared and structurally characterized via infrared photodissociation spectroscopy and density functional theory calculations. This structure exemplifies a new class of borylenes stabilized by three-center-two-electron metal-boron-metal covalent bonding interaction, displaying exceptional σ-acidity and unparalleled π-donor capability for CO activation that outperforms all of the known transition metal cations and is comparable or even superior to the documented base-trapped borylenes. Its neutral form represents a monovalent boron compound with a strongly reactive amphoteric boron center built on transition-metal-boron bonds, which inspires the design and synthesis of new members of the borylene family.

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.

Thiacalix[4]arene-Protected Silver Nanoclusters Encapsulating Different Two-Electron Superatom Oligomers

The subvalent silver kernel represents the nascent state of silver cluster formation, yet the growth mechanism has long been elusive. Herein, two silver nanoclusters (Ag30 and Ag34) coprotected by TC4A4- (H4TC4A = p-tert-butylthiacalix[4]arene) and TBPMT- (TBPMTH = 4-tert-butylbenzenemethanethiol) containing 6e and 4e silver kernels are synthesized and characterized. The trimer of the 2e superatom Ag14 kernel in Ag30 is built from a central Ag6 octahedron sandwiched by two orthogonally oriented Ag5 trigonal bipyramids through sharing vertexes, whereas a double-octahedral Ag10 kernel in Ag34 is a dimer of 2e superatoms. They manifest disparate polyhedron fusion growth patterns at the beginning of the silver cluster formation. Their excellent solution stabilities are contributed by the multisite and multidentate coordination fashion of TC4A4- and the special valence electron structures. This work demonstrates the precise control of silver kernel growth by the solvent strategy and lays a foundation for silver nanocluster application in photothermal conversion.

Amidophosphine Boranes as Hydroboration Reagents for Nitriles, Alkynes, and Carboxylic Acids

We report here the hydroboration of nitriles, alkynes, and carboxylic acids using amidophosphine boranes {(BH3)(PPh2)-NC(CH3)3}, {(BH3)2(PPh)2N(CH2)C6H5}, and {(BH3)2(PPh2)2N-(BH3)CH2C6H4N} as reducing agents. These compounds were synthesized to replace more commonly used borane reagents. Solid amidophosphine boranes, which were synthesized with ease, demonstrated excellent reactivity and functional group tolerance toward a wide variety of nitriles, alkynes, and carboxylic acids, affording the corresponding ammonium salts, alkenes, and alcohols in good yield.

First Example of Cage P4N4-Macrocycle Copper Complexes with Intracavity Location of Unusual Cu2I Fragments

In this study, 28-membered macrocyclic 1,5(1,5)-di(1,5-diaza-3,7-diphosphacyclooctana)-2,4,6,8(1,4)-tetrabenzenacyclooctaphane were synthesized by condensation of pyridinephosphine, paraformaldehyde, and primary diamines (bis(4-aminophenyl)methane or -sulfide. The first representatives of binuclear copper(I) complexes of P,N-containing cyclophanes with two 1,5-diaza-3,7-diphosphacyclooctane rings incorporated into a macrocyclic core and intracavity location of unusual, developed angle Cu₂I moiety were obtained. The structure of one complex was established by X-ray diffraction analysis. The complexation led to a slight distortion of the cyclophane conformations.

The Labile Nature of Air Stable Ni(II)/Ni(0)-phosphine/Olefin Catalysts/Intermediates: EDA-NOCV Analysis

Metal ions-based inorganic-organic hybrid composites are often reported acting as good to excellent catalysts with various substrate scopes under milder reaction conditions. The active catalyst of a catalytic cycle is sometimes proposed to be a short-lived reactive intermediate species. A three coordinate (L-Me)Ni(II) intermediate species [L-Me = O 2 N donor dianionic ligand] can bind with short-lived carbene-ester ligands to produce four coordinate Ni(II) species which can act as carbene transfer intermediates under suitable reaction conditions for C-H functionalization or cyclopropanation reactions. The dissociation of phosphine (PPh 3 ) from the Ni(II) centre of (L-Me)Ni(II)(PPh 3 ) ( 1a ) and binding of short lived carbene esters (:CR 1 -CO 2 R 2 ; R 1 = H, Ph; R 2 = aliphatic group; 2-4 and other carbenes; 5-10 ) to Ni(II) rationalize the phenomenon in solution. Air stable Ni(0)-olefin complexes/intermediates ( 12-18 ) have recently been shown to mediate a variety of organic transformations. This analysis will further help organic/organometallic chemists to rationalize the design and synthesis of future catalysts for organic transformation. EDA-NOCV calculations have been performed to shed light on the stability and bonding of those species. Additionally, our analysis provides a proper reason why the analogous (L-Me)Pd-PPh 3 complex ( 1b ) does not dissociate in solution and hence, a similar catalytic product has not been isolated from identical reaction conditions. The stability and the labile nature of Ni(II/0) complexes has been investigated by state-of-the-art EDA-NOCV analyses.

Nickel-catalysed diversification of phosphine ligands by formal substitution at phosphorus

We report a diversification strategy that enables the direct substituent exchange of tertiary phosphines. Alkylated phosphonium salts, prepared by standard alkylation of phosphines, are selectively dearylated in a nickel-catalysed process to access alkylphosphine products via a formal substitution at the phosphorus center. The reaction can be used to introduce a wide range of alkyl substituents into both mono- and bisphosphines. We also show that the alkylation and dearylation steps can be conducted in a one-pot sequence, enabling accelerated access to derivatives of the parent ligand. The phosphine products of the reaction are converted in situ to air-stable borane adducts for isolation, and versatile derivatisation reactions of these adducts are demonstrated.

Phosphine substituents can be exchanged by standard alkylation of a phosphine and a subsequent dearylation of the resulting phosphonium salt. A wide variety of alkyl groups can be introduced into both mono- and bidentate ligands using this method.

Molecular Catalysts with Diphosphine Ligands Containing Pendant Amines

Pendant amines play an invaluable role in chemical reactivity, especially for molecular catalysts based on earth-abundant metals. As inspired by [FeFe]-hydrogenases, which contain a pendant amine positioned for cooperative bifunctionality, synthetic catalysts have been developed to emulate this multifunctionality through incorporation of a pendant amine in the second coordination sphere. Cyclic diphosphine ligands containing two amines serve as the basis for a class of catalysts that have been extensively studied and used to demonstrate the impact of a pendant base. These 1,5-diaza-3,7-diphosphacyclooctanes, now often referred to as "P₂N₂" ligands, have profound effects on the reactivity of many catalysts. The resulting [Ni(P^R₂N^R'₂)₂]²⁺ complexes are electrocatalysts for both the oxidation and production of H₂. Achieving the optimal benefit of the pendant amine requires that it has suitable basicity and is properly positioned relative to the metal center. In addition to the catalytic efficacy demonstrated with [Ni(P^R₂N^R'₂)₂]²⁺ complexes for the oxidation and production of H₂, catalysts with diphosphine ligands containing pendant amines have also been demonstrated for several metals for many different reactions, both in solution and immobilized on surfaces. The impact of pendant amines in catalyst design continues to expand.

The chemistry of phosphines in constrained, well-defined microenvironments

Developments in the confinement of phosphines within micro- or nano-environments are explored. Phosphines are ubiquitous across metal coordination chemistry and underpin some of the most famous homogeneous transition metal catalysts. Constraining phosphines within confined environments influences not only their behaviour but also that of their metal complexes. Notable examples include the use of metal-organic frameworks (MOFs) or metal-organic cages (MOCs) to support phosphines which demonstrate how the microenvironment within such constructs leads to reactivity modification. The development of phosphine confinement is explored and parallels are drawn with related constrained macrocyclic systems and mechanically interlocked molecules. The review concludes by identifying areas that remain a challenge and those that will provide new avenues for research.

The Coordination Chemistry of the N-Donor-Substituted Phosphazanes

Phosph(III)azanes, featuring the heterocyclobutane P₂ N₂ ring, have now been established as building blocks in main-group coordination and supramolecular compounds. Previous studies have largely involved their use as neutral P-donor ligands or as anionic N-donor ligands, derived from deprotonation of amido-phosphazanes [RNHP(μ-NR)]₂ . The use of neutral amido-phosphazanes themselves as chelating, H-bond donors in anion receptors has also been an area of recent interest because of the ease by which the proton acidity and anion binding constants can be modulated, by the incorporation of electron-withdrawing exo- and endo-cyclic groups (R) and by the coordination of transition metals to the ring P atoms. We observed recently that the effect of P,N-chelation of metal atoms to the P atoms of cis-[(2-py)NHP(μ-N^t Bu)]₂ (2-py=2-pyridyl) not only pre-organises the N-H functionality for optimum H-bonding to anions but also results in a large increase in anion binding constants, well above those for traditional organic receptors like squaramides and ureas. Here, we report a broader investigation of ligand chemistry of [(2-py)NHP(μ-^t NBu)]₂ (and of the new quinolyl derivative [(8-Qu)NHP(μ-N^t Bu)]₂ (8-Qu=8-quinolyl). The additional N-donor functionality of the heterocyclic substituents and its position has a marked effect on the anion and metal coordination chemistry of both species, leading to novel structural behaviour and reactivity compared to unfunctionalized counterparts.

Aza-crown compounds synthesised by the self-condensation of 2-amino-benzyl alcohol over a pincer ruthenium catalyst and applied in the transfer hydrogenation of ketones

A well-defined PNN-Ru catalyst was revisited to self-condense 2-aminobenzyl alcohol in forming a series of novel aza-crown compounds [aza-12-crown-3 (1), aza-16-crown-4 (2) and aza-20-crown-5 (3)]. All aza-crown compounds are separated and determined by NMR, IR, and ESI-MS spectroscopy as well as X-ray crystallography, indicating the saddle structure of 1 and the twisted 1,3-alternate conformation structure of 3. These aza-crown compounds have been explored to study ferric initiation of transfer hydrogenation (TH) of ketones into their corresponding secondary alcohols in the presence of 2-propanol with a basic t-BuOK solution, achieving a high conversion (up to 95%) by a ferric complex with 2 in a low loading (0.05 mol%).

Thermoregulated Ionic Liquid-Stabilizing Ru/CoO Nanocomposites for Catalytic Hydrogenation

Catalytic hydrogenations represent fundamental processes and allow for atom-efficient and clean functional group transformations for the production of chemical intermediates and fine chemicals in chemical industry. Herein, the Ru/CoO nanocomposites have been constructed and applied as nanocatalysts for the hydrogenation of phenols and furfurals into the corresponding cyclohexanols and tetrahydrofurfuryl alcohols, respectively. The functionalized ionic liquid acted not only as a ligand for stabilizing the Ru/CoO nanocatalyst but also as a thermoregulated agent. The as-obtained nanocatalyst showed superior activity, and it could be conveniently recovered via the thermoregulating phase separation. In six recycle experiments, the catalysts maintained excellent performance. It was observed that the catalytic performance highly hinged on the molar ratio of Ru to Co in the nanocatalyst. The catalyst characterization was carried out by high-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), X-ray photoelectron spectroscopy, X-ray diffraction, high-resolution mass spectrometry, Fourier transform infrared, nuclear magnetic resonance, and UV-vis. Especially, the characterization by HRTEM and HAADF-STEM images of the nanocatalyst demonstrated that Ru(0) and Co(II) species were distributed uniformly and the Ru and Co(II) species were close to each other. However, Co(0) was generated and an electronic transfer from Co to Ru species could occur under the hydrogenation conditions. The ¹³C NMR characterization indicated further that Co(II) sites were mainly responsible for phenol adsorption. Meanwhile, the adjacent electron-rich Ru(0) sites can promote H₂ dissociation and favor for the sequential hydrogenation.

Chiral [16]-ane P4N2 macrocycles: stereoselective synthesis and unexpected intermolecular exchange of endocyclic fragments

Chiral 1,9-diaza-3,7,11,15-tetraphosphacyclohexadecanes were stereoselectively synthesized as pure RPSPSPRP isomers via a one-pot Mannich-type condensation reaction of 1,3-bis(arylphosphino)propane, formaldehyde and optically active, as well as racemic, primary amines. An unprecedented intermolecular exchange of endocyclic amino fragments of 1,9-diaza-3,7,11,15-tetraphosphacyclohexadecanes was observed. The lability of the P-CH2-N fragment in macrocyclic aminomethylphosphines is the reason of the stereoisomerization, formation of products with medium-sized cycles as well as the exchange of endocyclic amino fragments. The mechanism of these transformations involving the formation of a methylenephosphonium intermediate and further intra- and intermolecular nucleophilic substitution is discussed.

The first representatives of tetranuclear gold(i) complexes of P,N-containing cyclophanes

The first representatives of the tetranuclear gold(i) complexes of P,N-containing cyclophanes with two 1,5-diaza-3,7-diphosphacyclooctane rings incorporated into the macrocyclic core have been obtained. The complexation leads to a change in ligand conformations so that the diazadiphosphacyclooctane fragments of the complexes adopt twist-chair conformations, and two of the four gold(i) ions are located over and under the partially collapsed macrocyclic cavity. The complexes demonstrate moderate solid-state green emission.

Catalytic Silylation of N2 and Synthesis of NH3 and N2H4 by Net Hydrogen Atom Transfer Reactions Using a Chromium P4 Macrocycle

We report the first discrete molecular Cr-based catalysts for the reduction of N₂. This study is focused on the reactivity of the Cr-N₂ complex, trans-[Cr(N₂)₂(P^Ph₄N^Bn₄)] (P₄Cr(N₂)₂), bearing a 16-membered tetraphosphine macrocycle. The architecture of the [16]-P^Ph₄N^Bn₄ ligand is critical to preserve the structural integrity of the catalyst. P₄Cr(N₂)₂ was found to mediate the reduction of N₂ at room temperature and 1 atm pressure by three complementary reaction pathways: (1) Cr-catalyzed reduction of N₂ to N(SiMe₃)₃ by Na and Me₃SiCl, affording up to 34 equiv N(SiMe₃)₃; (2) stoichiometric reduction of N₂ by protons and electrons (for example, the reaction of cobaltocene and collidinium triflate at room temperature afforded 1.9 equiv of NH₃, or at -78 °C afforded a mixture of NH₃ and N₂H₄); and (3) the first example of NH₃ formation from the reaction of a terminally bound N₂ ligand with a traditional H atom source, TEMPOH (2,2,6,6-tetramethylpiperidine-1-ol). We found that trans-[Cr(¹⁵N₂)₂(P^Ph₄N^Bn₄)] reacts with excess TEMPOH to afford 1.4 equiv of ¹⁵NH₃. Isotopic labeling studies using TEMPOD afforded ND₃ as the product of N₂ reduction, confirming that the H atoms are provided by TEMPOH.

Covalent self-assembly of the specific RSSR isomer of 14-membered tetrakisphosphine

The first representative of the specific RSSR isomer of 14-membered tetrakisphosphine has been obtained instead of the RRRR/SSSS isomer predicted according to the empirical rule formulated recently. The geometry of the obtained 14-P₄N₂ is preorganized for the dicopper complex formation with the unique coordination mode in the row of P₄N₂ corands.

Aminophosphine Oxides: A Platform for Diversified Functions

This review summarizes significant contributions reported on aminophosphine oxides (AmPOs), specifically those containing at least one amino group present as amino substituents on α- and β-carbons including direct P-N bond containing molecules. AmPOs have additional 'N' site(s), including highly basic 'P=O' groups, and these features make favor smooth and unexpected behavior. The most striking manifestations of flexibility of AmPOs are that they are exciting ligand systems for the coordination chemistry of actinides, and their involvement in catalytic organic reactions including enantioselective opening of meso-epoxides, addition of silyl enol ethers, allylation with allyltributylstannane, etc. The diverse properties of the AmPOs and their metal complexes demonstrate both the scope and complexity of these systems, depending on the basicity of phosphoryl group, and nature of the substituents on the pentavalent tetracoordinate phosphorus atom and metal. Two components key to understanding the challenges of actinide separations are detailed here, namely, previously described separation methods, and recent investigations into the fundamental coordination chemistry of actinides. Both are aimed at probing the critical features necessary for improved selectivity of separations. This review leads to the conclusion that, although many AmPOs have already been discovered and developed over the past century, many opportunities nevertheless exist for further developments towards new extraction processes and new catalytic materials by fine tuning the electronic and steric properties of substituents on the central phosphorus atom.

A family of cis-macrocyclic diphosphines: modular, stereoselective synthesis and application in catalytic CO2/ethylene coupling

The stereoselective synthesis of a family of cis-macrocyclic diphosphines was achieved in only three steps from white phosphorus and commercial materials. These new ligands showed activity in the nickel-catalyzed coupling of CO₂ and ethylene.

A family of cis-macrocyclic diphosphines was prepared in just three steps from white phosphorus and commercial materials using a modular synthetic approach. Alkylation of bicyclic diphosphane 3,4,8,9-tetramethyl-1,6-diphosphabicyclo(4.4.0)deca-3,8-diene, or P₂(dmb)₂, produced phosphino-phosphonium salts [R-P₂(dmb)₂]X, where R is methyl, benzyl and isobutyl, in yields of 90–96%. Treatment of these salts with organolithium or Grignard reagents yielded symmetric and unsymmetric macrocyclic diphosphines of the form cis-1-R-6-R′-3,4,8,9-tetramethyl-2,5,7,10-tetrahydro-1,6-DiPhospheCine, or R,R′-DPC, in which R′ is methyl, cyclohexyl, phenyl or mesityl, in yields of 46–94%. Alternatively, symmetric diphosphine Cy₂-DPC was synthesized in 74% yield from the dichlorodiphosphine Cl₂P₂(dmb)₂. As a first application, these cis-macrocyclic diphosphines were used as ligands in the nickel-catalyzed synthesis of acrylate from CO₂ and ethylene, for which they showed promising catalytic activity.

Synthesis of tetraphosphine macrocycles using copper(i) templates

The synthesis of phosphine macrocycles is a relatively underdeveloped area and no standard synthetic routes have emerged. Accordingly, two general synthetic routes to tetradentate phosphine macrocycles were investigated. Both routes use Cu(i) ions as template ions because, unlike other metals such as Pd(ii) and Pt(ii), the Cu(i) ions can be removed from the macrocyclic complex without degrading the macrocycle ligand. The first route involves the coupling of two bidentate secondary phosphines bonded to Cu(i) using 1,3-dibromopropane or 1,4-dibromobutane. Using this route, tetradentate phosphine macrocycles with either -(CH2)3OCH3 or Ph groups bonded to the P atoms were synthesized. Macrocycle phosphines containing the -(CH2)3OCH3 groups were investigated for their potential water-solubility, but experiments showed these phosphines were not water soluble. The second synthetic route involved the alkylation of an open-chain, mixed tertiary-secondary, tetradentate phosphine coordinated to Cu(i). Following formation of the macrocyclic ligand, the Cu(i) template was removed by reaction with aqueous KCN to yield the free macrocyclic phosphine. This route was demonstrated for the preparation of the macrocyclic phosphine ligand 1,5,9,13-tetraphenyl-1,5,9,13-tetraphosphacycloheptadecane. Following demetallation, this macrocyclic ligand was coordinated to Fe(ii) and Co(ii) to form the corresponding macrocyclic phosphine complexes.

Distinction between coordination and phosphine ligand oxidation: interactions of di- and triphosphines with Pn3+ (Pn = P, As, Sb, Bi)

Hard and soft acceptors react differently towards phosphine donors, defining a range of phosphine coordination chemistry outcomes for p-block elements.

Synthesis and unique reversible splitting of 14-membered cyclic aminomethylphosphines on to 7-membered heterocycles

A novel type of 14-membered cyclic polyphosphine, namely 1,8-diaza-3,6,10,13-tetraphosphacyclotetradecanes 2a–4ahas been synthesized by the condensation of 1,2-bis(phenylphosphino)ethane, formaldehyde and alkylamines (isopropylamine, ethylamine and cyclohexylamine) as a RRRR/SSSS-stereoisomer. The structure of macrocycle 2a was investigated by NMR-spectroscopy and X-ray crystal structure analysis. The unique reversible processes of macrocycles 2a–4a splitting onto the corresponding rac- (2b–4b) and meso- (2c–4c) stereoisomers of 1-aza-3,6-diphosphacycloheptanes were discovered.

Phenylphosphinacalix[3]trifuran: synthesis, coordination and application in the Suzuki–Miyaura cross-coupling reaction in water

Phenylphosphinacalix[3]trifuran and supramolecular assemblies with palladium have been synthesized. Phenylphosphinacalix[3]trifuran affords turnover numbers as high as 3.05 × 10⁷ in Suzuki–Miyaura cross-coupling reactions in pure water.

Alternating stereoselective self-assembly of SSSS/RRRR or RSSR isomers of tetrakisphosphines in the row of 14-, 16-, 18- and 20-membered macrocycles

Covalent self-assembly of novel 18- and 20-membered P,N-corands as single stereoisomer is described.