Ligand Scaffold Research Articles

Valence tautomerism, non-innocence, and emergent magnetic phenomena in lanthanide-organic tessellations.

Coordination networks based on lanthanide ions entangle collective magnetic phenomena, otherwise only observed in inorganic 4f materials, and the tunable spatial and electronic structure engineering intrinsic to coordination chemistry. In this review, we discuss the use of 2D-structure-directing linear {LnII/IIII2} nodes to direct the formation of polymeric coordination networks. The equatorial coordination plasticity of {LnII/IIII2} results in broad structural diversity, including previously unobtainable tessellations containing motifs observed in quasicrystalline tilings. The new phases host also magnetic frustration, which is at the origin of enhanced magnetic refrigeration potential. Finally, careful redox matching of Ln node and frontier orbitals of the ligand scaffold has culminated in the discovery of quantitative valence tautomeric conversion in a molecule-based Ln material, opening up new avenues for combining exotic magnetic phenomena with an encoded switch.

Metal complexes with selenium-based ligands in organic synthesis

Designing ligands is a crucial and significant element in homogeneous catalysis. Over the past two decades, a range of compounds with selenium substitutions have been employed as scaffolds for ligands in the development of transition metal catalytic systems. Air and moisture stability, good solubility in different solvents, stability toward oxidation and excellent electron-donating properties of selenium atom are some of the key features of organoselenium compounds that make them suitable candidates for application as ligands. The metal complexes of selenium ligands are widely acknowledged for their ability to catalyze diverse organic reactions, including but not limited to the Sonogashira coupling reaction, Heck coupling reaction, Suzuki-Miyaura coupling reaction, imidazole arylation, transfer hydrogenation reaction, oxidation of alcohols, and ortho-arylation of phenols. In certain instances, there has been notable observation of exceptionally high catalytic efficiency. Additionally, catalysis often enables a broad substrate range in numerous scenarios, as it is tolerant towards different functional groups. This review highlights the key synthetic methods employed in the design of recent instances of organoselenium compounds as ligands for synthesizing transition metal complexes. Additionally, it explores their utilization in various preparative organic transformations.<br> The bibliography includes 133 references.

Nickel(II) Complexes Bearing Electron Donating OR/OH Groups on a Tetradentate Ligand Scaffold for Photocatalytic Carbon Dioxide Reduction

Nickel(II) Complexes Bearing Electron Donating OR/OH Groups on a Tetradentate Ligand Scaffold for Photocatalytic Carbon Dioxide Reduction

Heteroaryl Glutarimides and Dihydrouracils as Cereblon Ligand Scaffolds for Molecular Glue Degrader Discovery

Heteroaryl Glutarimides and Dihydrouracils as Cereblon Ligand Scaffolds for Molecular Glue Degrader Discovery

Nucleophilic Fluoride Anion Delivery from Triazacyclononane-Supported Molecular Ca-F Complexes.

The major source of fluoride, namely calcium difluoride (CaF2), is derived from the mineral fluorspar. Recent advances in the activation of CaF2 via mechanochemical methods have inspired investigation of the fundamental properties of Ca-F bonds in molecular complexes. However, the paucity of well-defined molecular Ca-F-containing complexes undermines systematic understanding of the factors governing nucleophilic fluoride delivery. Here we report the use of a multidentate bis(phenoxide) ligand based on a 1,4,7-triazacyclononane (TACN) scaffold for the formation of well-defined Ca-F complexes and demonstrate their capabilities in fluoride delivery. Key to this synthesis is a desymmetrization step carried out on the TACN ligand within the Ca coordination sphere. A series of stable anionic dinuclear calcium fluoride complexes has then been accessed and characterized by NMR spectroscopy (critically by 19F NMR) and X-ray crystallography. Nucleophilic fluoride delivery can be used to generate C-F, Si-F and S-F bonds, with a notable advance over amide-derived ligand scaffolds being the reduced extent of side reactions derived from competing attack on the electrophile by the less nucleophilic O-donor anionic ligand set.

From Virtual Screens to Cellular Target Engagement: New Small Molecule Ligands for the Immune Checkpoint LAG-3.

Herein, we performed a virtual screening study to discover new scaffolds for small molecule-based ligands of the immune checkpoint lymphocyte-activation gene 3 (LAG-3). Molecular dynamics (MD) simulations using the LAG-3 structure revealed two putative binding sites for small molecules: the antibody interface and the lipophilic canyon. A 3D pharmacophore screening resulted in the identification of potential ligands for these binding sites and afforded a library of 25 compounds. We then evaluated the screening hits for LAG-3 binding via microscale thermophoresis (MST) and surface plasmon resonance (SPR). Our biophysical screening identified two binders with K D values in the low micromolar range, compounds 3 (antibody interface) and 25 (lipophilic canyon). Furthermore, we investigated the ability of LAG-3 hits to engage LAG-3 on a cellular level using a cellular thermal shift assay (CETSA). In summary, compound 3 shows potential as a lead but is not yet a development candidate.

Stabilization of the Compressed Planar Benzene Dianion in Inverse-Sandwich Rare Earth Metal Complexes.

For the first time, the capture of a planar antiaromatic benzene dianion in between two trivalent rare earth (RE) metal cations, each stabilized by two guanidinate ligands, is reported. The synthesized inverse-sandwich complexes [{(Me3Si)2NC(NiPr)2}2RE]2(µ-ƞ6:ƞ6-C6H6), (RE = Y (1), Dy (2), and Er (3)) feature a remarkably planar benzene dianion, previously not encountered for any metal ion prone to low or absent covalency. The -2 charge localization at the benzene ligand was deduced from the results obtained by single-crystal X-ray diffraction analyses, spectroscopy, magnetometry, and Density Functional Theory (DFT) calculations. In the 1H NMR spectrum of the diamagnetic Y complex 1, the equivalent proton resonance of the bridging benzene dianion ligand is drastically shifted to higher field in comparison to free benzene. This and the calculated highly positive Nucleus-Independent Chemical Shift (NICS) values are attributed to the antiaromatic character of the benzene dianion ligand. The crucial role of the ancillary guanidinate ligand scaffold in stabilizing the planar benzene dianion conformation was also elucidated by DFT calculations. Remarkably, the planarity of the benzene dianion originates from the stabilization of the π-type orbitals of the d-manifold and compression through its strong electrostatic interaction with the two REIII sites.

Stabilization of the Compressed Planar Benzene Dianion in Inverse‐Sandwich Rare Earth Metal Complexes

For the first time, the capture of a planar antiaromatic benzene dianion in between two trivalent rare earth (RE) metal cations, each stabilized by two guanidinate ligands, is reported. The synthesized inverse‐sandwich complexes [{(Me3Si)2NC(NiPr)2}2RE]2(µ–ƞ6:ƞ6–C6H6), (RE = Y (1), Dy (2), and Er (3)) feature a remarkably planar benzene dianion, previously not encountered for any metal ion prone to low or absent covalency. The ‐2 charge localization at the benzene ligand was deduced from the results obtained by single‐crystal X‐ray diffraction analyses, spectroscopy, magnetometry, and Density Functional Theory (DFT) calculations. In the 1H NMR spectrum of the diamagnetic Y complex 1, the equivalent proton resonance of the bridging benzene dianion ligand is drastically shifted to higher field in comparison to free benzene. This and the calculated highly positive Nucleus‐Independent Chemical Shift (NICS) values are attributed to the antiaromatic character of the benzene dianion ligand. The crucial role of the ancillary guanidinate ligand scaffold in stabilizing the planar benzene dianion conformation was also elucidated by DFT calculations. Remarkably, the planarity of the benzene dianion originates from the stabilization of the π‐type orbitals of the d‐manifold and compression through its strong electrostatic interaction with the two REIII sites.

C-H amination chemistry mediated by trinuclear Cu(I) sites supported by a ligand scaffold featuring an arene platform and tetramethylguanidinyl residues.

Tripodal ligands that can encapsulate single or multiple metal sites in C3-symmetric geometric configurations constitute valuable targets for novel catalysts. Of particular interest in ligand development are efforts toward incorporating apical elements that exhibit little if any electron donicity, to enhance the electrophilic nature of a trans positioned active oxidant (e.g., metal-oxo, -nitrene). The tripodal ligand TMG3trphen-Arene has been synthesized, featuring an arene platform 1,3,5-substituted with phenylene arms possessing tetramethylguanidinyl (TMG) residues. Compound [(TMG3trphen-Arene)Cu3(μ-Cl)3] has been subsequently synthesized by extracting a Cu3(μ-Cl)3 cluster from anhydrous CuCl and shown to encapsulate a crown-shaped Cu3(μ-Cl)3 fragment, supported by Cu-NTMG bonds and modest Cu3⋯arene long-range contacts. Energy decomposition analysis (EDA) indicates that electrostatic contributions to the total interaction energy far exceed those due to orbital interactions. The latter involve orbital pairings largely associated with the NTMG stabilization of the Cu3(μ-Cl)3 cluster. The independent gradient model based on the Hirshfeld partition (IGMH) corroborates that contacts between the arene platform and the Cu3 triangle are noncovalent in nature. Catalyst [(TMG3trphen-Arene)Cu3(μ-Cl)3] enables amination of sec-benzylic and tert-C-H bonds of a panel of substrates by pre-synthesized PhINTces in solvent matrices that incorporate small amounts of HFIP. The involvement of an electrophilic aminating agent is evidenced by the better yields obtained for electron-rich benzylic sites and is further supported by Hammett analysis that reveals the development of a small positive charge during C-H bond activation. A rather modest KIE effect (2.1) is obtained from intramolecular H(D) competition in the amination of ethylbenzene, at the borderline of reported values for concerted and stepwise C-H amination systems. DFT analysis of the putative copper-nitrene oxidant indicates that the nitrene N atom is bridging between two copper sites in closely spaced triplet (ground state) and broken-symmetry singlet electronic configurations.

Electrocatalytic Conversion of CO2 to Formic Acid: A Journey from 3d-Transition Metal-Based Molecular Catalyst Design to Electrolyzer Assembly.

ConspectusElectrochemical CO2 reduction to obtain formate or formic acid is receiving significant attention as a method to combat the global warming crisis. Significant efforts have been devoted to the advancement of CO2 reduction techniques over the past few decades. This Account provides a unified discussion on various electrochemical methodologies for CO2 to formate conversion, with a particular focus on recent advancements in utilizing 3d-transition-metal-based molecular catalysts. This Account primarily focuses on understanding molecular functions and mechanisms under homogeneous conditions, which is essential for assessing the optimized reaction conditions for molecular catalysts. The unique architectural features of the formate dehydrogenase (FDH) enzyme provide insight into the key role of the surrounding protein scaffold in modulating the active site dynamics for stabilizing the key metal-bound CO2 intermediate. Additionally, the protein moiety also triggers a facile proton relay around the active site to drive electrocatalytic CO2 reduction forward. The fine-tuning of FDH machinery also ensures that the electrocatalytic CO2 reduction leads to the production of formic acid as the major yield without any other carbonaceous products, while limiting the competitive hydrogen evolution reaction. These lessons from the enzymes are key in designing biomimetic molecular catalysts, primarily based on multidentate ligand scaffolds containing peripheral proton relays. The subtle modifications of the ligand framework ensure the favored production of formic acid following electrocatalytic CO2 reduction in the solution phase. Next, the molecular catalysts are required to be mounted on robust electroactive surfaces to develop their corresponding heterogeneous versions. The surface-immobilization provides an edge to the molecular electrocatalysts as their reactivity can be scaled up with improved durability for long-term electrocatalysis. Despite challenges in developing high-performance, selective catalysts for the CO2 to formic acid transformation, significant progress is being made with the tactical use of graphene and carbon nanotube-based materials. To date, the majority of the research activity stops here, as the development of an operational CO2 to formic acid converting electrolyzer prototype still remains in its infancy. To elaborate on the potential future steps, this Account covers the design, scaling parameters, and existing challenges of assembling large-scale electrolyzers. A short glimpse at the utilization of electrolyzers for industrial-scale CO2 reduction is also provided here. The proper evaluation of the surface-immobilized electrocatalysts assembled in an electrolyzer is a key step for gauging their potential for practical viability. Here, the key electrochemical parameters and their expected values for industrial-scale electrolyzers have been discussed. Finally, the techno-economic aspects of the electrolyzer setup are summarized, completing the journey from tactical design of molecular catalysts to their appropriate application in a commercially viable electrolyzer setup for CO2 to formate electroreduction. Thus, this Account portrays the complete story of the evolution of a molecular catalyst to its sustainable application in CO2 utilization.

Isolation and Crystallographic Characterization of an Octavalent Co2O2 Diamond Core.

High-valent cobalt oxides play a pivotal role in alternative energy technology as catalysts for water splitting and as cathodes in lithium-ion batteries. Despite this importance, the properties governing the stability of high-valent cobalt oxides and specifically possible oxygen evolution pathways are not clear. One root of this limited understanding is the scarcity of high-valent Co(IV)-containing model complexes; there are no reports of stable, well-defined complexes with multiple Co(IV) centers. Here, an oxidatively robust fluorinated ligand scaffold enables the isolation and crystallographic characterization of a Co(IV)2-bis-μ-oxo complex. This complex is remarkably stable, in stark contrast with previously reported Co(IV)2 species that are highly reactive, which demonstrates that oxy-Co(IV)2 species are not necessarily unstable with respect to oxygen evolution. This example underscores a new design strategy for highly oxidizing transition-metal fragments and provides detailed data on a previously inaccessible chemical unit of relevance to O-O bond formation and oxygen evolution.

Asymmetric N-oxidation catalyzed by bisguanidinium dinuclear oxodiperoxomolybdosulfate

N-oxides play a pivotal role in natural products and emerging drug design, while also serving as valuable ligand scaffolds in organometallic chemistry. Among heteroatom oxidations, the conversion of amines to N-oxides is a critical and challenging facet. We present here a highly enantioselective N-oxidation methodology for both cyclic and acyclic amines. The method employs an ion-pair catalyst comprising a chiral bisguanidinium [BG]2+ cation and an achiral oxodiperoxomolybdosulfate anion [(µ-SO4)2Mo2O2(µ-O2)2(O2)2]2-. Notably, the bisguanidinium cation undergoes modification through silyl group incorporation and is elucidated by X-ray crystallography. Our findings underscore the crucial role of the side chain in the determination of the chiral pocket size, allowing for the oxidation of diverse tertiary amines with enantioselectivities. Comprehensive mechanistic investigations are conducted to explain the catalytic system’s efficacy in achieving dynamic kinetic resolution (DKR) with high efficiency.

Unsymmetric Chiral Ligands for Large Metallo‐Macrocycles: Selectivity of Orientational Self‐Sorting

AbstractNature uses various chiral and unsymmetric building blocks to form substantial and complex supramolecular assemblies. In contrast, the majority of organic ligands used in metallosupramolecular chemistry are symmetric and achiral. Here we extend the group of unsymmetric chiral bile acids used as a scaffold for organic bispyridyl ligands by employing chenodeoxycholic acid (CDCA), an epimer of the previously used ursodeoxycholic acid (UDCA). The epimerism, flexibility, and bulkiness of the ligands leads to large structural differences in coordination products upon reaction with Pd(NO3)2. The UDCA‐bispyridyl ligand self‐assembles quantitatively into a single crown‐like Pd3L6 complex, whereas the CDCA ligand provides a mixture of coordination complexes of general formula PdnL2n, i.e., Pd2L4, Pd3L6, Pd4L8, Pd5L10, and even Pd6L12 containing an impressive 120 chiral centers. The coordination products were studied by a combination of analytical methods, with ion‐mobility mass spectrometry (IM‐MS) providing valuable details on their structure and allowed an effective separation of m/z 1461 to individual signals according to the arrival time distribution, thereby revealing four different ions of [Pd3L6(NO3)3]3+, [Pd4L8(NO3)4]4+, [Pd5L10(NO3)5]5+, and [Pd6L12(NO3)6]6+. The structures of all the complexes were modelled using DFT calculations. Finally, the challenges and conclusions in determining the specific structural identity of these unsymmetric species are discussed.

Twisted and Disconnected Chains: Flexible Linear Tetracuprous Arrays and a Decanuclear CuI Cluster as Blue- and Green/Yellow-Light Emitters.

Defined arrays of transition metal ions embedded in tailored polydentate ligand scaffolds allow for a systematic design of their physical properties. Such molecular strings of closed-shell transition metal centers are particularly interesting for Group 11 metal ions in the oxidation state +1 if they undergo metallophilic d10···d10 contact interactions since these clusters are oftentimes efficient photoluminescence (PL) emitters. Copper is particularly attractive as a sustainable earth-abundant coinage metal source and because of the ability of several CuI complexes to serve as powerful thermally activated delayed fluorescence (TADF) emitters in molecular/organic light-emitting devices (OLEDs). Our combined synthetic, crystallographic, photophysical, and computational study describes a straight tetracuprous array possessing a centrally disconnected CuI2···CuI2 chain and a continuous helically bent CuI4 complex. This molecular helix undergoes a facile rearrangement in diethyl ether solution, yielding an unprecedented nanosized CuI10 cluster (2.9 × 2.0 nm) upon crystallization. All three clusters show either bright blue phosphorescence, TADF, or green/yellow multiband phosphorescence with quantum yields between 6.5 and 67%, which is persistent under hydrostatic pressure up to 30 kbar. Temperature-dependent PL investigations in combination with time-dependent density-functional theory (TD-DFT) calculations and void space analyses of the crystal packings complement a comprehensive correlation between the molecular structures and photoluminescence properties.

Self-Illuminating Nanoagonist Simultaneously Induces Dual Cell Death Pathways via Death Receptor Clustering for Cancer Therapy.

Inducing death receptor 5 (DR5) clustering holds particular promise in tumor-specific therapeutics because it could trigger an apoptotic cascade in cancerous cells. Herein, we present a tumor microenvironment H2O2-responsive self-illuminating nanoagonist, which could induce dual tumor cell death pathways through enhancing DR5 clustering. By conjugating DR5 ligand peptides onto the surfaces of self-illuminating nanoparticles with cross-linking capacity, this strategy not only provides scaffolds for ligands to bind receptors but also cross-links them through photo-cross-linking. This strategy allows for efficient activation of DR5 downstream signaling, initiating the extrinsic apoptosis pathway and immunogenic cell death of tumor cells, and contributes to improved tumor-specific immune responses, resulting in enhanced antitumor efficacy and minimized systemic adverse effects.

Enhanced Intersystem Crossing, Yet Still Fluorescence Upon Introduction of Intermediate Charge-Transfer States in Hemicaged [Zn(bpy)3]2.

Photoactive zinc(II) complexes typically undergo fluorescence from the singlet excited state as the dominant radiative pathway, as the operative spin-orbit coupling is usually very small and phosphorescence from the triplet state is strongly forbidden. Although dicationic zinc(II) tris(bipyridine) strictly follows this scheme with fluorescence at λem = 326 nm, constructing the ligand sphere as a hemicage was reported to lead to quantitative intersystem crossing (ISC) and subsequent fast phosphorescence with λem = 485 and a short radiative lifetime of ca. 1 μs. Surprised by this finding, we reinvestigated [Zn(bpy)3]2+ and its hemicage derivative in great detail, including variable temperature and time-resolved photophysical measurements in solution and solid state as well as high-level theoretical calculations to resolve their excited state behavior. Our investigations suggest that both compounds undergo fluorescence at room temperature with significantly different radiative rate constants of kr = 2 × 108 and 1.2 × 106 s-1, respectively, and only weak phosphorescence on the millisecond time scale at low temperatures. The major difference is the occurrence of additional charge-transfer states within the ligand scaffold of the hemicage, which accelerate the ISC to the 3LC(bpy) state from 350 s down to 82 ns and reduce the fluorescence rate constant.

Unsymmetric Chiral Ligands for Large Metallo-Macrocycles: Selectivity of Orientational Self-Sorting.

Nature uses various chiral and unsymmetric building blocks to form substantial and complex supramolecular assemblies. In contrast, the majority of organic ligands used in metallosupramolecular chemistry are symmetric and achiral. Here we extend the group of unsymmetric chiral bile acids used as a scaffold for organic bispyridyl ligands by employing chenodeoxycholic acid (CDCA), an epimer of the previously used ursodeoxycholic acid (UDCA). The epimerism, flexibility, and bulkiness of the ligands leads to large structural differences in coordination products upon reaction with Pd(NO3)2. The UDCA-bispyridyl ligand self-assembles quantitatively into a single crown-like Pd3L6 complex, whereas the CDCA ligand provides a mixture of coordination complexes of general formula PdnL2n, i.e., Pd2L4, Pd3L6, Pd4L8, Pd5L10, and even Pd6L12 containing an impressive 120 chiral centers. The coordination products were studied by a combination of analytical methods, with ion-mobility mass spectrometry (IM-MS) providing valuable details on their structure and allowed an effective separation of m/z 1461 to individual signals according to the arrival time distribution, thereby revealing four different ions of [Pd3L6(NO3)3]3+, [Pd4L8(NO3)4]4+, [Pd5L10(NO3)5]5+, and [Pd6L12(NO3)6]6+. The structures of all the complexes were modelled using DFT calculations. Finally, the challenges and conclusions in determining the specific structural identity of these unsymmetric species are discussed.

Influence of Donor-Acceptor Interactions on MLCT Hole Reconfiguration in {Ru(bpy)} Chromophores.

In MLCT chromophores, internal conversion (IC) in the form of hole reconfiguration pathways (HR) is a major source of dissipation of the absorbed photon energy. Therefore, it is desirable to minimize their impact in energy conversion schemes by slowing them down. According to previous findings on {Ru(bpy)} chromophores, donor-acceptor interactions between the Ru ion and the ligand scaffold might allow to control HR/IC rates. Here, a series of [Ru(tpm)(bpy)(R-py)]2+ chromophores, where tpm is tris(1-pyrazolyl)methane, bpy is 2,2'-bipyridine and R-py is a 4-substituted pyridine, were prepared and fully characterized employing electrochemistry, spectroelectrochemistry, steady-state absorption/emission spectroscopy and electronic structure computations based on DFT/TD-DFT. Their excited-state decay was monitored using nanosecond and femtosecond transient absorption spectroscopy. HR/IC lifetimes as slow as 568 ps were obtained in DMSO at room temperature, twice as slow as in the reference species [Ru(tpm)(bpy)(NCS)]+.

Linear, Electron-Rich Erbium Single-Molecule Magnet with Dibenzocyclooctatetraene Ligands.

Judicious design of ligand scaffolds to highly anisotropic lanthanide ions led to substantial advances in molecular spintronics and single-molecule magnetism. Erbium-based single-molecule magnets (SMMs) are rare, which is attributed to the prolate-shaped ErIII ion requiring an equatorial ligand field for enhancing its single-ion magnetic anisotropy. Here, we present an electron-rich mononuclear Er SMM, [K(crypt-222)][Er(dbCOT)2], 1 (where dbCOT = dibenzocyclooctatetraene), that was obtained from a salt metathesis reaction of ErCl3 and K2dbCOT. The dipotassium salt, K2dbCOT, was generated through a two-electron reduction of the bare dbCOT0 ligand employing potassium graphite and was crystallized from DME to give the new solvated complex, [K(DME)]2[dbCOT]n, 2. 1 was analyzed through crystallography, electrochemistry, spectroscopy, magnetometry, and CASSCF calculations. The structure of 1 consists of an anionic metallocene complex featuring a linear (180.0°) geometry with an ErIII ion sandwiched between dianionic dbCOT ligands and an outer-sphere K+ ion encapsulated in 2.2.2-cryptand. Two pronounced redox events at negative potentials allude to the formation of a trianionic erbocene complex, [Er(dbCOT)2]3-, on the electrochemical time scale. 1 shows slow magnetic relaxation with an effective spin-reversal barrier of Ueff = 114(2) cm-1, which is close in magnitude to the calculated energies of the first and second excited states of 96.9 and 109.13 cm-1, respectively. 1 exhibits waist-constricted hysteresis loops below 4 K and constitutes the first example of an erbocene-SMM bearing fused aromatic rings to the central COT ligand. Notably, 1 comprises the largest COT scaffold implemented in erbocene SMMs, yielding the most electron-rich homoleptic erbium metallocene SMM.

Bis(bicyclo[1.1.1]pentyl)chlorophosphine as a Precursor for the Preparation of Bis(bicyclo[1.1.1]pentyl)phosphines.

Dialkylchlorophosphines are among the most versatile building blocks for tertiary phosphine ligands, but their synthesis relies on the nucleophilic substitution of PCl3, leaving substituents that require P-H precursors largely inaccessible. The primary phosphine reagent iPr2NPH2·BH3 can serve as a doubly protected PH2Cl proxy, enabling the synthesis of bis(bicyclo[1.1.1]pentyl)chlorophosphine (Bcp2PCl) for the first time. Bcp2PCl serves as a general reagent for the preparation of a family of bis(bicyclo[1.1.1]pentyl) alkyl- and arylphosphines, including new members of privileged phosphine ligand scaffolds.