Electron-rich Ligands Research Articles

Photoinduced Reductive C-O Couplings from Unsymmetrical Bis-Cyclometalated Pt(IV) Dicarboxylato Complexes.

Unsymmetrical bis-cyclometalated dicarboxylato complexes (OC-6-32)-[Pt(tpy)2(O2CR)2] [tpy = cyclometalated 2-(p-tolyl)pyridine, R = t-Bu (1), Me (2), Ph (3), CF3 (4)], are obtained from the reaction of cis-[Pt(tpy)2] with the appropriate PhI(O2CR)2 reagent. Treatment of complexes of this type with different carboxylates (R'CO2-) results in the formation of mixed-carboxylato derivatives, namely (OC-6-43)-[Pt(tpy)2(O2CMe)(O2CR')] [R' = t-Bu (5), CF3 (6), Ph (7)], (OC-6-34)-[Pt(tpy)2(O2CCF3)(O2CR')] [R' = t-Bu (8), Me (9), Ph (10)], and (OC-6-34)-[Pt(tpy)2(O2C-t-Bu)(O2CMe)] (11). Irradiation of 1-3 and 5-11 with UV light (365 nm) in MeCN gives 5-methyl-2-(2-pyridyl)phenyl pivalate (12), 5-methyl-2-(2-pyridyl)phenyl acetate (13) or 5-methyl-2-(2-pyridyl)phenyl benzoate (14) as the major photoproduct from most complexes, resulting from a reductive C-O coupling between a tpy ligand and a carboxylato ligand. Cyclometalation of 12-14 at the ensuing Pt(II) species to produce cis-[Pt(tpy)(tpyO2CR/R')], reduction to cis-[Pt(tpy)2] and isomerization to (OC-6-33)-[Pt(tpy)2(O2CR/R')2] are identified as secondary processes in most cases. In contrast, complex 4 exclusively photoisomerizes to (OC-6-33)-[Pt(tpy)2(O2CCF3)2] (4'). The C-O couplings are favored for the most electron-rich carboxylato ligands and occur predominantly from the carboxylato trans to N. Consistent with this, a computational study reveals that the lowest singlet and triplet LMCT excited states result from electronic transitions to a dσ* orbital distributed along the N-Pt-O axis, which would trigger the observed processes.

Ancillary Ligand Steric Effects and Cyclometalating Ligand Substituents Control Excited-State Decay Kinetics in Red-Phosphorescent Platinum Complexes.

Achieving high-efficiency red phosphorescence remains a significant challenge, especially in cyclometalated platinum complexes where radiative rates are inherently slower than their iridium counterparts. In this work, six red-emitting cyclometalated platinum complexes of the formula Pt(C∧N)[(Ar)acNac] (C∧N is the cyclometalating ligand, and (Ar)acNac is an aryl-substituted β-ketoiminate ancillary ligand) were synthesized and characterized. Two C∧N ligands were employed, 1-phenylisoquinoline (piq) and its cyano-substituted analogue 1-phenylisoquinoline-4-carbonitrile (piqCN), which both result in red phosphorescence in cyclometalated platinum complexes. These were paired with three (Ar)acNac ligands that are sterically differentiated via the N-aryl group, which is phenyl in the unsubstituted analogue (Ph)acNac and 2,6-dimethylphenyl or 2,6-diisopropylphenyl in the sterically encumbered analogues. An in-depth photophysical analysis of all compounds was performed and compared to the related compounds with the acetylacetonate (acac) ancillary ligand. While quantum yields are modest in the unsubstituted (Ph)acNac complexes, steric bulk on the β-ketoiminate has a pronounced effect on the excited-state dynamics and can lead to photoluminescence quantum yields of more than 0.50 in both solution and transparent polymer films, with the photoluminescence λmax ∼ 630 nm. We show that both steric effects on the electron-rich β-ketoiminate ancillary ligands and the cyano substituent on the cyclometalating ligand play a role in achieving high-efficiency phosphorescence in the red region.

Coupling of Aryl Chlorides with Lithium Nucleophiles Enabled by Molecularly Defined Alkynyllithium Palladium Catalysts.

Palladium-catalyzed cross-couplings of aryl chlorides usually call for bulky, electron-rich ligands such as phosphines or heterocyclic carbenes. We have now found that similarly powerful cross-coupling catalysts are obtained by the reaction of palladium salts with alkynyllithium reagents. The species initially formed in this process was characterized as a dilithium tetraalkinyl palladate complex. It catalyzes the coupling of aryl chlorides with the lithium salts of various terminal alkynes to give alkynyl arenes. The isolated Li-alkynyl-Pd complex also efficiently promotes the reaction of aryl, and allyl chlorides with (hetero)aryl-, alkyl-, and allyllithium compounds as well as lithium amides. None of these reactions proceeded in the presence of palladium salts alone. The preparative utility of this approach was demonstrated by the synthesis of 49 molecules, including pharmaceutically relevant compounds.

Coordination environment modulation to optimize d-orbit arrangement of Mn-based MOF electrocatalyst for lithium-oxygen battery

Nowadays lithium-oxygen (Li-O2) batteries with metal-organic frameworks (MOFs) based oxygen electrodes have suffered from the sluggish kinetics and irreversible behavior of Li2O2 formation/decomposition, which originates from weak orbit coupling with oxygen species caused by narrow orbit arrangement of metal sites in MOFs. Modulation of coordination environment of metal sites has been regarded as the vital strategy to tune 3d orbit arrangement and electron distribution of metal sites, which is favorable for altering electron transfer pathway and oxygen electrode reaction kinetics. Herein, electron-rich ferrocene acid (FcA), as grafted ligand, is introduced on Mn sites of Mn-MOF-74 (named as Mn-MOF-74-FcA) for facilitating the oxygen electrode reactions in Li-O2 batteries. In details, due to the strong electron delocalization on electron-rich FcA ligand, the resulted metal-ligand interaction between FcA and Mn sites distorts [MnOx] coordination sphere, which induces 3d orbits rearrangement of Mn sites from (dx2-y2/dxy, dz2, dxz/dyz) to (dx2-y2, dz2, dxz, dyz, dxy) and electron transfer from electron-rich FcA to Mn sites. And the strong orbit overlapping between LiO2 intermediate and widened Mn d-band ensures the electron exchange with each other and thus reduces the energy barrier of oxygen electrode reactions. Moreover, electron transfer from FcA ligand to Mn sites also leads to the decreased energy gap between valence band (VB) and conduction band (CB). As expected, Li-O2 battery with Mn-MOF-74-FcA exhibits low overpotential of 0.82 V and long lifespan of 276 cycles at the current density of 200 mA g−1.

C–heteroatom coupling with electron-rich aryls enabled by nickel catalysis and light

Nickel photoredox catalysis has resulted in a rich development of transition-metal-catalysed transformations for carbon–heteroatom bond formation. By harnessing light energy, the transition metal can attain oxidation states that are difficult to achieve through thermal chemistry in a catalytic manifold. For example, nickel photoredox reactions have been reported for both the synthesis of anilines and aryl ethers from aryl(pseudo)halides. However, oxidative addition to simple nickel systems is often sluggish in the absence of special, electron-rich ligands, leading to catalyst decomposition. Electron-rich aryl electrophiles therefore currently fall outside the scope of many transformations in the field. Here we provide a conceptual solution to this problem and demonstrate nickel-catalysed C–heteroatom bond-forming reactions of arylthianthrenium salts, including amination, oxygenation, sulfuration and halogenation. Because the redox properties of arylthianthrenium salts are primarily dictated by the thianthrenium, oxidative addition of highly electron-rich aryl donors can be unlocked using simple NiCl2 under light irradiation to form the desired C‒heteroatom bonds.

Nickel-Based Catalysts for the Selective Monoarylation of Dichloropyridines: Ligand Effects and Mechanistic Insights.

This report describes a detailed study of Ni phosphine catalysts for the Suzuki-Miyaura coupling of dichloropyridines with halogen-containing (hetero)aryl boronic acids. With most phosphine ligands these transformations afford mixtures of mono- and diarylated cross-coupling products as well as competing oligomerization of the boronic acid. However, a ligand screen revealed that PPh2Me and PPh3 afford high yield and selectivity for monoarylation over diarylation as well as minimal competing oligomerization of the boronic acid. Several key observations were made regarding the selectivity of these reactions, including: (1) phosphine ligands that afford high selectivity for monoarylation fall within a narrow range of Tolman cone angles (between 136° and 157°); (2) more electron-rich trialkylphosphines afford predominantly diarylated products, while less-electron rich di- and triarylphosphines favor monoarylation; (3) diarylation proceeds via intramolecular oxidative addition; and (4) the solvent (MeCN) plays a crucial role in achieving high monoarylation selectivity. Experimental and DFT studies suggest that all these data can be explained based on the reactivity of a key intermediate: a Ni0-π complex of the monoarylated product. With larger, more electron-rich trialkylphosphine ligands, this π complex undergoes intramolecular oxidative addition faster than ligand substitution by the MeCN solvent, leading to selective diarylation. In contrast, with relatively small di- and triarylphosphine ligands, associative ligand substitution by MeCN is competitive with oxidative addition, resulting in selective formation of monoarylated products. The generality of this method is demonstrated with a variety of dichloropyridines and chloro-substituted aryl boronic acids. Furthermore, the optimal ligand (PPh2Me) and solvent (MeCN) are leveraged to achieve the Ni-catalyzed monoarylation of a broader set of dichloroarene substrates.

Ligand doping engineering induced robust internal electric field in MOFs/BiVO4 photoanode for water splitting

A robust internal electric field (IEF) within heterojunctions is essential for enhancing the separation and transfer of photogenerated charge carriers during photoelectrochemical (PEC) water splitting. In this contribution, a ligand doping engineering approach is innovatively proposed to design 3,5-Th2-TzH = 3,5-di(thiophen-2-yl)-1H-1,2,4-triazole (CuMTZ)/BiVO4 photoanodes, in which thiophene-containing electron-rich ligands (3,5-Th2-TzH) strengthen the interactions between CuMTZ and BiVO4. This results in the formation of a robust IEF, which serves as a strong driving force for efficient charge carrier transfer and separation, leading to an enhanced photovoltage. Furthermore, the uniform CuMTZ nanolayers act as a cocatalyst that not only increases the number of active reaction sites but also boosts photogenerated holes injection into the electrolyte to participate directly in the water oxidation process. Consequently, CuMTZ/BiVO4 photoanode exhibits a significant 4.26-fold improvement in photocurrent for water oxidation compared to BiVO4. This work highlights the significance of ligand doping engineering in the design of metal-organic framework (MOF)/semiconductor photoanodes for enhancing PEC efficiency.

In silico modelling of chelate stabilized tetrylene derivatives.

The steric and electronic effects of specific ligands can play crucial roles in stabilizing unsaturated tetrylene species. In this work, hybrid density functional theory (DFT) methods, quantum theory of atoms in molecules (QTAIM) investigations and natural bond orbital (NBO) calculations are employed to evaluate the stabilization of low-valent E(ii) centers (E = Si, Ge, Sn, Pb) through the chelating effect generated by an electron-rich ligand containing the P[double bond, length as m-dash]C-P[double bond, length as m-dash]X moiety (X = O or S). Based on several types of analyses, such as the bond dissociation energy (BDE) or the interplay between attractive (i.e., charge-transfer) and repulsive (i.e., Pauli-exchange) effects, we highlight that the stabilization energy induced by chelation is up to ca. 70 kcal mol-1 for silylenes, yet slightly decreases within the heavier analogues. Moreover, it is emphasized that chelate-stabilized silylenes can form highly stable hybrid metal-metalloid complexes with transition metals (e.g., gold). Due to push-pull effects occurring in the X→Si(ii)→Au fragment, the Si(ii)→Au bonding is significantly stronger than the X→Au, P(sp2)→Au or π(C[double bond, length as m-dash]P)→Au donor-acceptor bonds, which are potentially formed by the electron-rich P[double bond, length as m-dash]C-P[double bond, length as m-dash]X unit with the AuCl fragment. These findings are supported by energy decomposition analysis (EDA) calculations.

Photoinduced copper-catalyzed asymmetric cyanoalkylalkynylation of alkenes, terminal alkynes, and oximes.

The asymmetric dicarbofunctionalization of alkenes via a radical relay process can provide routes to diverse hydrocarbon derivatives. Three-component carboalkynylation, limited to particular alkyl halides and using readily available cycloketone oxime esters as redox-active precursors, is restricted by the available pool of suitable chiral ligands for broadening the redox potential window of copper complexes and simultaneously creating the enantiocontrol environment. Herein, we report a new hybrid tridentate ligand bearing a guanidine-amide-pyridine unit for photoinduced copper-catalyzed cyanoalkylalkynylation of alkenes. Leveraging the copper catalyst's redox capability is achieved via merging the electron-rich ligand with a readily organized configuration and enhanced absorption in the visible light range, which also facilitates the enantioselectivity. The generality of the catalyst system is exemplified by the efficacy across a number of alkenes, terminal alkynes and cycloketone oxime esters, working smoothly to give alkyne-bearing nitriles with good yields and excellent enantioselectivity. A mechanistic study reveals that the chiral copper catalyst meets the requirements of possessing sufficient reduction ability, good light absorption properties, and appropriate steric hindrance.

Kumada-Tamao-Corriu Type Reaction of Aromatic Bromo- and Iodoamines with Grignard Reagents.

The first example of the Kumada-Tamao-Corriu type reaction of unprotected bromoanilines with Grignard reagents is described. The method uses a palladium source and a newly designed Buchwald-type ligand as the catalytic system. Secondary and tertiary bromo- and iodoamines were also successfully coupled to alkyl Grignard reagents. The products of the competitive β-hydride elimination reaction were successfully reduced using a highly efficient electron-deficient phosphine ligand (BPhos). Mechanistic considerations allowed us to establish that the less electron-rich phosphine ligands stabilize the transition state much better than the electron-rich ones; hence, they increase the reaction yield and reduce the amount of β-hydride elimination products. The developed method proved to be tolerant of many functional groups and can be applied to many different aromatic bromo- and iodoamines. Multigram synthesis of p-toluidine from 4-bromoaniline was achieved with a palladium catalyst loading of only 0.03 mol%.

Nickel-Catalyzed Selective C(sp2 )-F Bond Alkylation of Industrially Relevant Hydrofluoroolefin HFO-1234yf.

The selective transition-metal catalyzed C-F bond functionalization of inexpensive industrial fluorochemicals represents one of the most attractive approaches to valuable fluorinated compounds. However, the selective C(sp2 )-F bond carbofunctionalization of refrigerant hydrofluoroolefins (HFOs) remains challenging. Here, we report a nickel-catalyzed selective C(sp2 )-F bond alkylation of HFO-1234yf with alkylzinc reagents. The resulting 2-trifluoromethylalkenes can serve as a versatile synthon for diversified transformations, including the anti-Markovnikov type hydroalkylation and the synthesis of bioactive molecule analogues. Mechanistic studies reveal that lithium salt is essential to promote the oxidative addition of Ni0 (Ln ) to the C-F bond; the less electron-rich N-based ligands, such as bipyridine and pyridine-oxazoline, feature comparable or even higher oxidative addition rates than the electron-rich phosphine ligands; the strong σ-donating phosphine ligands, such as PMe3 , are detrimental to transmetallation, but the less electron-rich and bulky N-based ligands, such as pyridine-oxazoline, facilitate transmetallation and reductive elimination to form the final product.

Tracking an FeV (O) Intermediate for Water Oxidation in Water.

High-valent iron-oxo species are appealing for conducting O-O bond formation for water oxidation reactions. However, their high reactivity poses a great challenge to the dissection of their chemical transformations. Herein, we introduce an electron-rich and oxidation-resistant ligand, 2-[(2,2'-bipyridin)-6-yl]propan-2-ol to stabilize such fleeting intermediates. Advanced spectroscopies and electrochemical studies demonstrate a high-valent FeV (O) species formation in water. Combining kinetic and oxygen isotope labelling experiments and organic reactions indicates that the FeV (O) species is responsible for O-O bond formation via water nucleophilic attack under the real catalytic water oxidation conditions.

Towards Green Synthesis of Fluorescent Metal Nanoclusters.

In the modern development of nanoscience and nanotechnology, metal nanoclusters have emerged as a foremost category of nanomaterials exhibiting remarkable biocompatibility and photo-stability having dramatically distinctive optical, electronic, and chemical properties. This review focuses on synthesizing fluorescent metal nanoclusters in a greener way to make them suitable for biological imaging and drug delivery application. The green methodology is the desired route for sustainable chemical production and should be utilized for any form of chemical synthesis including nanomaterials. It aims to eliminate harmful waste, uses non-toxic solvents, and employs energy-efficient processes for the synthesis. This article provides an overview of conventional synthesis methods, including stabilizing nanoclusters by small organic molecules in organic solvents. Then we focus on the improvement of properties, applications of green synthesized metal nanoclusters, challenges involved, and further advancement required in the direction of green synthesis of MNCs. There are plenty of problems for scientists to solve to make nanoclusters suitable for bio-applications, chemical sensing, and catalysis synthesized by green methods. Using bio-compatible and electron-rich ligands, understanding ligand-metal interfacial interactions, employing more energy-efficient processes, and utilizing bio-inspired templates for synthesis are some immediate problems worth solving in this field that requires continued efforts and interdisciplinary knowledge and collaboration.

Ligand-Dictated Regiodivergent Allylic Functionalizations via Palladium-Catalyzed Remote Substitution.

Different from classical allylic substitutions requiring a vicinal leaving group, an olefin bearing a remote leaving group is scarcely viewed as a potential allylation substrate. Here we describe feasible protocols to achieve the regiodivergent allylic C-H functionalizations via palladium-catalyzed remote substitution, which provides a novel strategy for seldomly studied migratory Tsuji-Trost reaction. With the dictation of a suitable ligand, a processinvolved 4,3-hydrofunctionalization of the generated conjugated diene intermediate via metal walking is observed in generally >20:1 regioselectivity. Unexpectedly, related 1,4-hydrofunctionalization pathway is tuned to work as a major route with a newly synthesized electron-rich bisphosphine ligand, which breakouts the conventional viewpoint on the potential regioselectivity of hydrofunctionalizations of linear internal conjugated dienes via ƞ3-substitution. A series of deuterium experiments and kinetic studies provide preliminary insight into the potential catalytic cycle.

Enantioselective Nickel-Catalyzed Reductive anti-Arylative Annulation of Alkyne-Tethered Malononitriles to Construct Quaternary Stereocenters

A nickel-catalyzed reductive desymmetrizing annulation of alkyne-tethered malononitriles and (hetero)aryl iodides is reported for the access of cyclohexenones containing an α-all-carbon quaternary stereocenter. The use of a nickel catalyst derived from an electron-rich phosphinooxazoline ligand combined with iron powder as a reductant is crucial to the success of this transformation.

Stereospecific Acylative Suzuki-Miyaura Cross-Coupling: General Access to Optically Active α-Aryl Carbonyl Compounds.

A novel strategy for the stereospecific Pd-catalyzed acylative cross-coupling of enantiomerically enriched alkylboron compounds has been developed. The protocol features an extremely high level of enantiospecificity to allow facile access to synthetically challenging and valuable chiral ketones and carboxylic acid derivatives. The use of a sterically encumbered and electron-rich phosphine ligand proved to be crucial for the success of the reaction. Furthermore, on the basis of experimental and computational studies, a unique mechanism for the transmetalation, assisted by the noncovalent interactions of the C(sp3)-based organoboron reagent, has been identified.

The modulation effect of auxiliary ligands on photochromic properties of 3D naphthalene diimide coordination polymers.

Two novel naphthalene diimide (NDI) coordination polymers (CPs), [Cd(NicNDI)(4,4'-SBC)] (1) and [Cd(NicNDI)(2,2'-BPC)] (2) (NicNDI = (3-pyridylacylamino)-1,4,5,8-naphthalene diimide, 4,4'-SBC = 4,4'-stilbene dicarboxylic acid, 2,2'-BPC = 2,2'-biphenyl dicarboxylic acid), were designed and prepared by the combination of electron-deficient NicNDI and electron-rich aromatic carboxylic acid ligands in the presence of cadmium ions. The usage of aromatic carboxylic acid ligands with different conjugation degrees, sizes, shapes and charge densities leads to the generation of distinct interpenetrated three-dimensional (3D) frameworks. Interestingly, photochromism of 1 and weak photoactivity of 2 should be attributed to the introduction of different auxiliary ligands and consequently the formation of distinct interfacial contacts of electron donors (EDs)/electron acceptors (EAs) (dπ-π = 3.427 Å, infinite -ED-EA-ED-EA- for 1vs. dπ-π = 3.634 Å, discrete ED-EA-ED for 2), suggesting a subtle modulating effect of auxiliary ligands on interfacial contacts, photoinduced intermolecular electron transfer (PIET) and photoresponsive behaviors.

Ruthenium(v) terminal arylimido corroles: isolation, spectroscopic characterization and reactivity.

Terminal Ru(v)-imido species are thought to be as reactive to group transfer reactions as their Ru(v)-oxo homologues, but are less studied. With the electron-rich corrole ligand, relatively stable and isolable Ru(v)-arylimido complexes [Ru(tBu-Cor)(NAr)] (H3(tBu-Cor) = 5,15-diphenyl-10-(p-tert-butylphenyl)corrole, Ar = 2,4,6-Me3C6H2 (Mes), 2,6-(iPr)2C6H3 (Dipp), 2,4,6-(iPr)3C6H2 (Tipp), and 3,5-(CF3)2C6H3 (BTF)) can be prepared from [Ru(tBu-Cor)]2 under strongly reducing conditions. This type of Ru(v)-monoarylimido corrole complex with S = ½ was characterized by high-resolution ESI mass spectrometry, X-band EPR, resonance Raman spectroscopy, magnetic susceptibility, and elemental analysis, together with computational studies. Under heating/light irradiation (xenon lamp) conditions, the complexes [Ru(tBu-Cor)(NAr)] (Ar = Mes, BTF) could undergo aziridination of styrenes and amination of benzylic C(sp3)-H bonds with up to 90% product yields.

Electronic effects of amine-imine nickel and palladium catalysts on ethylene (co)polymerization

A series of bis-(aryl)-amine-imine nickel and palladium complexes with electron-donating (OMe), H, and -withdrawing (F and Br) groups at the para-aryl position were synthesized and characterized to probe the ligand electronic effects on homo- and copolymerization of ethylene. The influences of electronic perturbations on complex synthesis, polymerization property, and polymer microstructure were investigated systematically. The electronic effects of para-substituents were verified by Hammett constants and topographic steric maps of nickel and palladium complexes. Electron-rich ligands had stronger complexation ability to the nickel and palladium metal, thereby facilitating the synthesis of nickel and palladium complexes. Ethylene polymerizations at different temperatures showed that electron-deficient amine-imine nickel and palladium catalysts were more active than electron-rich analogues at low temperatures while the reverse trend was observed at high temperatures. The electronic effects of para-substituents affected polyethylene molecular weight, but had little influence on the branching density of highly branched polyethylenes. Copolymerization of ethylene and methyl acrylate (MA) using palladium catalysts demonstrated that the electron-donating group promoted copolymerization performance, and was favorable for direct insertion of MA to produce main-chain MA unit. Electronic effects of amine-imine nickel and palladium catalysts on ethylene polymerization were comprehensively summarized to electronic perturbations on the metal electrophilicity and the thermal tolerance of catalyst.

Synthesis and characterization of Ni(0) complexes supported by an unsymmetric C,N ligand

Over the past decade, N-heterocyclic carbene (NHC), bipyridine (bpy), and pyridine oxazoline (PyOx) ligands have been increasingly applied in nickel catalysis, where the combination of a low-oxidation-state 3d metal and an electron-rich ligand can enable otherwise challenging bond activations. Herein we report the synthesis and characterization of two Ni(0) complexes supported by an alternative, bidentate, C,N ligand, hIMesPy (“half-IMes-pyridine” or 1-(2,4,6-trimethylphenyl)-3-(2-pyridinyl)-imidazol-2-ylidene). The unsymmetric ligand combines the strong σ-donating properties of an NHC with the π-accepting properties of pyridine to afford homoleptic [Ni(hIMesPy)2] and heteroleptic [Ni(cod)(hIMesPy)] complexes. Characterization through the combination of single-crystal X-ray diffraction analysis and NMR spectroscopy, as well as reactivity studies demonstrating reversible interconversion between the two species, provide a strong basis for future applications to overcome reactivity challenges.