Highly active alkyne metathesis catalysts operating under open air condition

Triarylmethanol Derivatives with Ultralong Organic Room-Temperature Phosphorescence

Triphenylmethyl-based compounds such as rhodamines and fluoresceine representing an old and well-known class of triphenylmethane dyes, are widely used in fluorescent labeling of bioimaging. Inspired by ultralong room temperature phosphorescence of triphenylphosphine derivatives, herein we report a methoxy substituted triarylmethanol ((4-methoxyphenyl)diphenylmethanol, LJW-1) exhibits ultralong room temperature phosphorescence (RTP) under ambient condition with afterglows of about 7 seconds. Its multiple C-H ⋅ ⋅ ⋅ π intermolecular interactions, C-H ⋅ ⋅ ⋅ O intermolecular interactions, hydrogen bond and π-π interactions are beneficial for forming rigid environment in the aggregated state which is evidently an important factor in the appearance of excellent RTP. Different substituents on triarylmethanol result in compounds with different lifetimes varying from 7 μs to 818 ms. The substituent effects on the phosphorescence of triarylmethanol derivatives provide an efficient method for the design of organic RTP materials and may be enlightening the phosphorescence research of triarylmethanol derivatives.

Exploring efficient and air-stable d2 Re(v) alkylidyne catalysts: toward room temperature alkyne metathesis

Transition metal-catalyzed alkyne metathesis has become a useful tool in synthetic chemistry. Well-defined alkyne metathesis catalysts comprise alkylidyne complexes of tungsten, molybdenum and rhenium. Non-d0 Re(v) alkylidyne catalysts exhibit advantages such as remarkable tolerance to air and moisture as well as excellent functional group compatibility. However, the known Re(v) alkylidynes with a pyridine leaving ligand require harsh conditions for activation, resulting in lower catalytic efficiency compared to d0 Mo(vi) and W(vi) alkylidynes. Herein, we report the first non-d0 alkylidyne complex capable of mediating alkyne metathesis at room temperature, namely, the Re(v) aqua alkylidyne complex Re([triple bond, length as m-dash]CCH2Ph)( Ph PO)2(H2O) (14). The aqua complex readily dissociates a water ligand in solution, confirmed by ligand substitution reactions with other σ-donor ligands. The aqua complex can be readily prepared on a large scale, and is stable to air and moisture in the solid state and compatible with a variety of functional groups. The versatile ability of the catalyst has been demonstrated through examples of alkyne cross-metathesis (ACM), ring-closing alkyne metathesis (RCAM), and acyclic diyne metathesis macrocyclization (ADIMAC) reactions. All in all, this work presents a solution for an efficient and air-stable alkyne metathesis catalytic system based on d2 Re(v)-alkylidynes.

Rhenium Alkyne Catalysis: Sterics Control the Reactivity

Metathesis reactions, including alkane, alkene, and alkyne metatheses, have their origins in the fundamental understanding of chemical reactions and the development of specialized catalysts. These reactions stand as transformative pillars in organic chemistry, providing efficient rearrangement of carbon-carbon bonds and enabling synthetic access to diverse and complex compounds. Their impact spans industries such as petrochemicals, pharmaceuticals, and materials science. In this work, we present a detailed mechanistic study of the Re(V) catalyzed alkyne metathesis through density functional theory calculations. Our findings are in agreement with the experimental evidence from Jia and co-workers and unveil critical factors governing catalyst performance. Our work not only enhances our understanding of alkyne metathesis but also contributes to the broader landscape of catalytic processes, facilitating the design of more efficient and selective transformations in organic synthesis.

Vanadium Alkylidyne Initiated Cyclic Polymer Synthesis: The Importance of a Deprotiovanadacyclobutadiene Moiety

Reported is the catalytic cyclic polymer synthesis by a 3d transition metal complex: a V(V) alkylidyne, [(dBDI)V≡CtBu(OEt2)] (1-OEt2), supported by the deprotonated β-diketiminate dBDI2- (dBDI2- = ArNC(CH3)CHC(CH2)NAr, Ar = 2,6-iPr2C6H3). Complex 1-OEt2 is a precatalyst for the polymerization of phenylacetylene (PhCCH) to give cyclic poly(phenylacetylene) (c-PPA), whereas its precursor, complex [(BDI)V≡CtBu(OTf)] (2-OTf; BDI- = [ArNC(CH3)]2CH, Ar = 2,6-iPr2C6H3, OTf = OSO2CF3), and the zwitterion [((C6F5)3B-dBDI)V≡CtBu(OEt2)] (3-OEt2) exhibit low catalytic activity despite having a neopentylidyne ligand. Cyclic polymer topologies were verified by size-exclusion chromatography (SEC) and intrinsic viscosity studies. A component of the mechanism of the cyclic polymerization reaction was probed by isolation and full characterization of 4- and 6-membered metallacycles as model intermediates. Metallacyclobutadiene (MCBD) and deprotiometallacyclobutadiene (dMCBD) complexes (dBDI)V[C(tBu)C(H)C(tBu)] (4-tBu) and (BDI)V[C(tBu)CC(Mes)] (5-Mes), respectively, were synthesized upon reaction with bulkier alkynes, tBu- (tBuCCH) and Mes-acetylene (MesCCH), with 1-OEt2. Furthermore, the reaction of the conjugate acid of 1-OEt2, [(BDI)V≡CtBu(OTf)] (2-OTf), with the conjugated base of phenylacetylene, lithium phenylacetylide (LiCCPh), yields the doubly deprotio-metallacycle complex, [Li(THF)4]{(BDI)V[C(Ph)CC(tBu)CC(Ph)]} (6). Protonation of the doubly deprotio-metallacycle complex 6 yields 6-H+, a catalytically active species toward the polymerization of PhCCH, for which the polymers were also confirmed to be cyclic by SEC studies. Computational mechanistic studies complement the experimental observations and provide insight into the mechanism of cyclic polymer growth. The noninnocence of the supporting dBDI2- ligand and its role in proton shuttling to generate deprotiometallacyclobutadiene (dMCBD) complexes that proposedly culminate in the formation of catalytically active V(III) species are also discussed. This work demonstrates how a dMCBD moiety can react with terminal alkynes to form cyclic polyalkynes.

Aromatic Hydrocarbon Based and Space Interactions Induced Color-tunable Single-component Organic Phosphorescence

Construction of new system and exploration of new approach are of great importance for the improvement of their photophysical properties to meet the growing various uses of phosphorescent materials. Triphenylmethane (TPM), composed only of carbon and hydrogen, exhibits excellent color tunable phosphorescence in air, with ultralong lifetime (836 ms), and wide color-tunable range (from cyan to green, then to yellow and finally to orange, 525 nm-616 nm). Through careful comparison with the single crystal diffraction structure of tetraphenylmethane (TTPM) and theoretical calculation analysis, we believe that various clusters formed through space interactions are crucial for color-tunable phosphorescence.

From the Glovebox to the Benchtop: Air-Stable High Performance Molybdenum Alkylidyne Catalysts for Alkyne Metathesis

Molybdenum alkylidynes endowed with tripodal silanolate ligands belong to the most active and selective catalysts for alkyne metathesis known to date. This paper describes a new generation that is distinguished by an unprecedented level of stability and practicality without sacrificing the chemical virtues of their predecessors. Specifically, pyridine adducts of type 16 are easy to make on gram scale, can be routinely weighed and handled in air, and stay intact for many months outside the glovebox. When dissolved in toluene, however, spontaneous dissociation of the stabilizing pyridine ligand releases an active species of excellent performance and functional group tolerance. Specifically, a host of polar and apolar groups, various protic sites, and numerous basic functionalities proved compatible. The catalysts are characterized by crystallographic and spectroscopic means, including 95Mo NMR; their activity and stability are benchmarked in detail, and the enabling properties are illustrated by advanced applications to natural product synthesis. For the favorable overall application profile and ease of handling, complexes of this new series are expected to replace earlier catalyst generations and help encourage a more regular use of alkyne metathesis in general.

Color-Tunable Ultralong Organic Phosphorescence: Commercially Available Triphenylmethylamine for UV-Light Response and Anticounterfeiting

Due to the unclear mechanism and lack of effective design for color-tunable ultralong organic phosphorescence (UOP) in a single-component molecule, the development of new types of single-component UOP materials with color-tunable property remains challenging. Herein, commercially available triphenylmethylamine-based single-component phosphors featuring color-tunablity and ultralong lifetime (0.56 s) are reported. The changed afterglow colors from cyan to orange were observed after different wavelengths of UV excitation. Crystal structure and calculation studies show that multiple emission centers in the aggregated states may be responsible for the color-tunablity. In addition, visual probing of UV light (from 260 to 370 nm) and colorful anti-counterfeiting were conducted. More importantly, UV light ranging from 350 to 370 nm could be detected with the minimal interval of 2 nm. The findings provide a new type of single-component color-tunable UOP materials and shed new light on mechanism and design for such materials.

Classifying and Understanding the Reactivities of Mo-Based Alkyne Metathesis Catalysts from 95Mo NMR Chemical Shift Descriptors

The most active alkyne metathesis catalysts rely on well-defined Mo alkylidynes, X₃Mo≡CR (X = OR), in particular the recently developed canopy catalyst family bearing silanolate ligand sets. Recent efforts to understand catalyst reactivity patterns have shown that NMR chemical shifts are powerful descriptors, though previous studies have mostly focused on ligand-based NMR descriptors. Here, we show in the context of alkyne metathesis that ⁹⁵Mo chemical shift tensors encode detailed information on the electronic structure of these catalysts. Analysis by first-principles calculations of ⁹⁵Mo chemical shift tensors extracted from solid-state ⁹⁵Mo NMR spectra shows a direct link of chemical shift values with the energies of the HOMO and LUMO, two molecular orbitals involved in the key [2 + 2]-cycloaddition step, thus linking ⁹⁵Mo chemical shifts to reactivity. In particular, the ⁹⁵Mo chemical shifts are driven by ligand electronegativity (σ-donation) and electron delocalization through Mo-O π interactions, thus explaining the reactivity patterns of the silanolate canopy catalysts. These results further motivate exploration of transition metal NMR signatures and their relationships to electronic structure and reactivity.

Organometallic Chemistry of Transition Metal Alkylidyne Complexes Centered at Metathesis Reactions

Transition metals form a variety of alkylidyne complexes with either a d⁰ metal center (high-valent) or a non-d⁰ metal center (low-valent). One of the most interesting properties of alkylidyne complexes is that they can undergo or mediate metathesis reactions. The most well-studied metathesis reactions are alkyne metathesis involving high-valent alkylidynes. High-valent alkylidynes can also undergo metathesis reactions with heterotriple bonded species such as N≡CR, P≡CR, and N≡NR⁺. Metathesis reactions involving low-valent alkylidynes are less known. Highly efficient alkyne metathesis catalysts have been developed based on Mo(VI) and W(VI) alkylidynes. Catalytic cross-metathesis of nitriles with alkynes has also been achieved with M(VI) (M = W, Mo) alkylidyne or nitrido complexes. The metathesis activity of alkylidyne complexes is sensitively dependent on metals, supporting ligands and substituents of alkylidynes. Beyond metathesis, metal alkylidynes can also promote other reactions including alkyne polymerization. The remaining shortcomings and opportunities in the field are assessed.

Tandem Imine Formation and Alkyne Metathesis Enabled by Catalyst Choice

Three-rung molecular ladder 8 was prepared in one pot via tandem imine condensation and alkyne metathesis. Catalyst VI is demonstrated to successfully engender the metathesis of imine-bearing substrate 7, while catalyst III does not. The susceptibility of catalyst VI to deactivation by hydrolysis and ligand exchange is demonstrated. Assembly and disassembly of ladder 8 in one pot were demonstrated in the presence and absence of a Lewis acid catalyst.

Imparting Functionality and Enhanced Surface Area to a 2D Electrically Conductive MOF via Macrocyclic Linker

The development of 2D electrically conductive metal-organic frameworks (EC-MOFs) has significantly expanded the scope of MOFs' applications into energy storage, electrocatalysis, and sensors. Despite growing interest in EC-MOFs, they often show low surface area and lack functionality due to the limited ligand motifs available. Herein we present a new EC-MOF using 2,3,8,9,14,15-hexahydroxyltribenzocyclyne (HHTC) linker and Cu nodes, featuring a large surface area. The MOF exhibits an electrical conductivity up to 3.02 × 10^-3 S/cm and a surface area up to 1196 m²/g, unprecedentedly high for 2D EC-MOFs. We also demonstrate the utilization of alkyne functionality in the framework by postsynthetically hosting heterometal ions (e.g., Ni²⁺, Co²⁺). Additionally, we investigated particle size tunability, facilitating the study of size-property relationships. We believe that these results not only contribute to expanding the library of EC-MOFs but shed light on the new opportunities to explore electronic applications.

Alkyne Metathesis with d2 Re(V) Alkylidyne Complexes Supported by Phosphino-Phenolates: Ligand Effect on Catalytic Activity and Applications in Ring-Closing Alkyne Metathesis

A family of d² Re(V) alkylidyne complexes bearing two decorated phosphino-phenolates (POs) and a labile pyridine ligand were prepared that can efficiently promote alkyne metathesis reactions in toluene. The relative activity of these complexes varies with the PO ligands. Complexes with an electron-rich metal center have a higher activity. Ligand exchange experiments suggest that the pyridine ligands of the Re(V) alkylidynes with more electron-donating PO ligands are more labile and are more easily released to generate catalytically active species. However, complexes with electron-withdrawing PO ligands are more air-stable than those with electron-donating PO ligands. These Re(V) alkylidyne catalysts can promote the homometathesis of functionalized internal alkyl- and aryl-alkynes, as well as ring-closing alkyne metathesis (RCAM) of methyl-capped diynes, forming macrocycles with a ring size ≥12 efficiently for concentrations ≤5 mM. These reactions represent the first examples of RCAM mediated by non-d⁰ alkylidyne complexes. The Re(V) alkylidyne catalysts tolerate a wide range of functional groups including ethers, esters, ketones, aldehydes, alcohols, phenols, amines, amides, and heterocycles. Moreover, the catalytic RCAM reactions promoted by robust Re(V) alkylidyne catalysts could also proceed normally in wet toluene.

Canopy Catalysts for Alkyne Metathesis: Investigations into a Bimolecular Decomposition Pathway and the Stability of the Podand Cap

Molybdenum alkylidyne complexes with a trisilanolate podand ligand framework ("canopy catalysts") are the arguably most selective catalysts for alkyne metathesis known to date. Among them, complex 1 a endowed with a fence of lateral methyl substituents on the silicon linkers is the most reactive, although fairly high loadings are required in certain applications. It is now shown that this catalyst decomposes readily via a bimolecular pathway that engages the Mo≡CR entities in a stoichiometric triple-bond metathesis event to furnish RC≡CR and the corresponding dinuclear complex, 8, with a Mo≡Mo core. In addition to the regular analytical techniques, 95 Mo NMR was used to confirm this unusual outcome. This rapid degradation mechanism is largely avoided by increasing the size of the peripheral substituents on silicon, without unduly compromising the activity of the resulting complexes. When chemically challenged, however, canopy catalysts can open the apparently somewhat strained tripodal ligand cages; this reorganization leads to the formation of cyclo-tetrameric arrays composed of four metal alkylidyne units linked together via one silanol arm of the ligand backbone. The analogous tungsten alkylidyne complex 6, endowed with a tripodal tris-alkoxide (rather than siloxide) ligand framework, is even more susceptible to such a controlled and reversible cyclo-oligomerization. The structures of the resulting giant macrocyclic ensembles were established by single-crystal X-ray diffraction.

The Ascent of Alkyne Metathesis to Strategy-Level Status

For numerous enabling features and strategic virtues, contemporary alkyne metathesis is increasingly recognized as a formidable synthetic tool. Central to this development was the remarkable evolution of the catalysts during the past decades. Molybdenum alkylidynes carrying (tripodal) silanolate ligands currently set the standards; their functional group compatibility is exceptional, even though they comprise an early transition metal in its highest oxidation state. Their performance is manifested in case studies in the realm of dynamic covalent chemistry, advanced applications to solid-phase synthesis, a revival of transannular reactions, and the assembly of complex target molecules at sites, which one may not intuitively trace back to an acetylenic ancestor. In parallel with these innovations in material science and organic synthesis, new insights into the mode of action of the most advanced catalysts were gained by computational means and the use of unconventional analytical tools such as ⁹⁵Mo and ¹⁸³W NMR spectroscopy. The remaining shortcomings, gaps, and desiderata in the field are also critically assessed.

Impact of Ligands and Metals on the Formation of Metallacyclic Intermediates and a Nontraditional Mechanism for Group VI Alkyne Metathesis Catalysts

The intermediacy of metallacyclobutadienes as part of a [2 + 2]/retro-[2 + 2] cycloaddition-based mechanism is a well-established paradigm in alkyne metathesis with alternative species viewed as off-cycle decomposition products that interfere with efficient product formation. Recent work has shown that the exclusive intermediate isolated from a siloxide podand-supported molybdenum-based catalyst was not the expected metallacyclobutadiene but instead a dynamic metallatetrahedrane. Despite their paucity in the chemical literature, theoretical work has shown these species to be thermodynamically more stable as well as having modest barriers for cycloaddition. Consequentially, we report the synthesis of a library of group VI alkylidynes as well as the roles metal identity, ligand flexibility, secondary coordination sphere, and substrate identity all have on isolable intermediates. Furthermore, we report the disparities in catalyst competency as a function of ligand sterics and metal choice. Dispersion-corrected DFT calculations are used to shed light on the mechanism and role of ligand and metal on the intermediacy of metallacyclobutadiene and metallatetrahedrane as well as their implications to alkyne metathesis.

By-design molecular architectures via alkyne metathesis

Shape-persistent purely organic molecular architectures have attracted tremendous research interest in the past few decades. Dynamic Covalent Chemistry (DCvC), which deals with reversible covalent bond formation reactions, has emerged as an efficient synthetic approach for constructing these well-defined molecular architectures. Among various dynamic linkages, the formation of ethynylene linkages through dynamic alkyne metathesis is of particular interest due to their high chemical stability, linearity, and rigidity. In this review, we focus on the synthetic strategies of discrete molecular architectures (e.g., macrocycles, molecular cages) containing ethynylene linkages using alkyne metathesis as the key step, and their applications. We will introduce the history and challenges in the synthesis of those architectures via alkyne metathesis, the development of alkyne metathesis catalysts, the reported novel macrocycle structures, molecular cage structures, and their applications. In the end, we offer an outlook of this field and remaining challenges.

The recent synthesis of novel shape-persistent 2D and 3D molecular architectures via alkyne metathesis is reviewed and the critical role of catalysts is also highlighted.

Truxene-based covalent organic polyhedrons constructed through alkyne metathesis

Dynamic alkyne metathesis has successfully been employed toward the synthesis of a truxene-based shape-persistent covalent organic polyhedron (COP) with high binding affinity for fullerenes.