Tridentate Pincer Ligand Research Articles

Iron Complexes of 4,5-Bis(diorganophosphinomethyl)acridine Ligands.

The search for an iron analog of the established ruthenium-based catalysts containing methylene-extended 4,5-bis(diorganophosphinomethyl)acridine ligands, [FeHCl(CO)(LR)], resulted in the discovery of a bidentate coordination mode of these usually tridentate pincer ligands toward iron. The acridines nitrogen atom does not coordinate to iron, leading to the formation of iron diphos-type complexes with unusually large cis bite angles of up to 124° as well as trans bite angles around 155°. The iron-containing complexes [FeCl2(κ2-LR)] (R = iPr, Ph), [FeX2(κ2-LCy)] (X = Cl, Br) and [Fe(CO)3(κ2-LR)] (R = iPr, Cy) have been isolated in crystalline form and characterized by spectroscopic methods and mass spectrometry. Their structures were verified unambiguously through X-ray diffraction. The stability of the iron(II) complexes decreased in the order Cy > Ph > iPr and Cl > Br > I, although all iron(II) complexes were found to be relatively stable enough for short-term handling in air in the solid state. Notably, no iron(0) complex of the phenyl derivative could be isolated. The iron(0) complex [Fe(CO)3(κ2-LCy)] was found to be significantly more stable toward hydrolysis and oxygen compared to [Fe(CO)3(κ2-LiPr)] and can be stored in air for months without significant decomposition in the solid state, while [Fe(CO)3(κ2-LiPr)] decomposes in air within seconds. The decomposition products [FeI2(κ2-O2LCy)], [{Fe(CO)3(κ2-HLR)}2] (R = iPr, Cy) and [FeCl2(CO)2(κ1-LCy)(κ1-OLCy)] were identified and characterized crystallographically. The iron(0) complex [Fe(CO)3(κ2-LCy)] is oxidized by [Fe(Cp)2](BPh4) to give the paramagnetic, low-spin iron(I) cation [Fe(CO)3(κ2-LCy)]+. The electron paramagnetic resonance spectrum of the highly sensitive cation as well as density functional theory calculations suggest a partial delocalization of the unpaired electron over the three carbonyl ligands and the acridines aromatic ring system. The catalytic activity and photophysical properties of the complexes have been preliminarily investigated.

LNL-Carbazole Pincer Ligand: More than the Sum of Its Parts.

The utility of carbazole in photo-, electro-, and medicinal applications has ensured its widespread use also as the backbone in tridentate pincer ligands. In this review, the aim is to identify and illustrate the key features of the LNL-carbazolide binding to transition metal centers (with L = flanking donor moieties, e.g., C, N, P, and O-groups) in a systematic bottom-up progression to illustrate the marked benefits attainable from (i) the rigid aromatic carbazole scaffold (modulable in both the 1,8- and 3,6-positions), (ii) the significant electronic effect of central carbazole-amido binding to a metal, and the tunable sterics and electronics of both the (iii) flanking donor L-moieties and (iv) the wingtip R-groups on the L-donors, with their corresponding influence on metal coordination geometry, d-electron configuration, and resultant reactivity. Systematic implementation of the ligand design strategies not in isolation, but in a combinatorial approach, is showcased to demonstrate the potential for functional molecules that are not only modulable but also adaptable for wide-ranging applications (e.g., stereoselective (photo)catalysis, challenging small molecule activation, SET and redox applications, and even applications in chemotherapeutics) as an indication of future research efforts anticipated to stem from this versatile pincer assembly, not only for the transition metals but also for s-, p-, and f-block elements.

Synthesis and characterization of cobalt (II) pincer complexes and their application as dyes in dye-sensitized solar cells

A series of eight cobalt(II) pincer complexes were prepared by using NNN tridentate and NN bidentate pincer ligand systems. All the ligands and complexes were completely characterized using various spectroscopic techniques and analytical methods including single crystal X-ray analysis. Optimization of the molecular structures of all the compounds was done by B3LYP computational method and the HOMO-LUMO energy gaps of the compounds were calculated. The optical band-gaps of the cobalt complexes were determined by using electronic spectral data and the corresponding electrochemical gaps were obtained from cyclic voltammetry studies. All the cobalt (II) complexes were used as dyes in the dye-sensitized solar cell and the photovoltaic parameters of the constructed solar cells were investigated. Complex C7, comprising of bis(benzimidazole) system with an ethylene backbone exhibited highest power conversion efficiency among the tested compounds.

The tritopic phosphanylthiolato ligand P(2-SHC6H4)2(2,4,6-iPr3C6H2) enables formation of a thiolato-bridged dinuclear nickel(II) complex featuring a Ni2S2 butterfly core

The bulky tritopic phosphanylthiolato pincer ligand (Tipp)P(2-SHC6H4)2 (Tipp = 2,4,6-iPr3C6H2) (TippPS2H2) reacts with nickel dichloride to give the dinuclear nickel(II) complex [Ni{(Tipp)P(µ-2-SC6H4)2-κ3S,S',P}]2 (1). The X-ray structure analysis of 1 shows a dinuclear complex consisting of two nickel(II) phosphanyldithiolato units formed with the dianionic tridentate pincer ligand, [Ni(TippPS2)], which dimerise via thiolato bridges involving one of the two sulfur atoms of each pincer ligand. The resulting four-membered central Ni2S2 ring has a butterfly conformation. The molecule is located on a crystallographic twofold axis and displays two chiral phosphorus centres. Only the rac isomer is formed diastereoselectively; both enantiomers of 1 (R,R and S,S) are present in the unit cell.

Realization of Long Operational Lifetimes in Vacuum-Deposited Organic Light-Emitting Devices Based on para-Substituted Pyridine Carbazolylgold(III) C^C^N Complexes.

A new series of robust C^C^N carbazolylgold(III) complexes is designed and synthesized through the introduction of inert and sterically bulky oligophenyl substituents on the pyridyl moiety of the cyclometalating ligand. High photoluminescence quantum yields of up to 96% are recorded with these complexes doped in solid-state thin films, and short excited-state lifetimes of 0.3 μs or less in the solid state at room temperature are found. Promising electroluminescence (EL) performances are shown by the vacuum-deposited organic light-emitting devices (OLEDs) based on this series of gold(III) complexes. High external quantum efficiencies of up to 19.5% with efficiency roll-offs of down to 10% at a practical luminance brightness level of 1000 cd m-2 are achieved. More importantly, record-long operational lifetimes (LT50) of up to 470,700 h at 100 cd m-2 are realized, which is currently the highest value among all classes of gold(III) complexes with tridentate pincer ligands. Particularly, by introducing a sterically bulky terphenyl moiety on the reactive site of the pyridine ring, the LT50 value is shown to attain ∼7 times longer half-lifetime than that based on the unsubstituted complex. These unprecedented EL performances and the simple synthetic route in a mercury-free fashion make them promising emitting materials for practical OLEDs toward commercialization.

Unsymmetrical ONO-type pincer complexes of palladium(II) containing amino acid barbiturate derivatives: Synthesis, characterization, and catalytic applications in Suzuki–Miyaura cross-coupling reaction

New two tridentate pincer ligands were prepared by the reaction between the selected amino acids and 1,3-dimethyl barbituric acid. The corresponding Pd(II) complexes were synthesized from the treatment of palladium(II) acetate with the prepared ligands in presence of an ancillary ligand such as pyridine, triphenylphosphine, or 1-methylimidazole. All ligands and Pd(II) complexes were characterized by standard spectroscopic and analytical techniques. The crystal and molecular structures of two complexes were determined by single-crystal X-ray diffraction revealing a slightly distorted square-planar geometry around the metal center to which the ligand bonded in an ONO-tridentate fashion. The catalytic activities of Pd(II) complexes were evaluated in the Suzuki-Miyaura coupling reaction of phenylboronic acid with the different aryl halides. They showed good activity for 4-substituted aryl bromide with phenylboronic acid.

Ruthenium-Catalyzed Dehydrogenative Functionalization of Alcohols to Pyrroles: A Comparison between Metal-Ligand Cooperative and Non-cooperative Approaches.

Herein, we report the synthesis and characterization of two ruthenium-based pincer-type catalysts, [1]X (X = Cl, PF6) and 2, containing two different tridentate pincer ligands, 2-pyrazolyl-(1,10-phenanthroline) (L1) and 2-arylazo-(1,10-phenanthroline) (L2a/2b, L2a = 2-(phenyldiazenyl)-1,10-phenanthroline; L2b = 2-((4-chlorophenyl)diazenyl)-1,10-phenanthroline), and their application in the synthesis of substituted pyrroles via dehydrogenative alcohol functionalization reactions. In catalyst [1]X (X = Cl, PF6), the tridentate scaffold 2-pyrazolyl-(1,10-phenanthroline) (L1) is apparently redox innocent, and all the redox events occur at the metal center, and the coordinated ligands remain as spectators. In contrast, in catalysts 2a and 2b, the coordinated azo-aromatic scaffolds are highly redox-active and known to participate actively during the dehydrogenation of alcohols. A comparison between the catalytic activities of these two catalysts was made, starting from the simple dehydrogenation of alcohols to further dehydrogenative functionalization of alcohols to various substituted pyrroles to understand the advantages/disadvantages of the metal-ligand cooperative approach. Various substituted pyrroles were prepared via dehydrogenative coupling of secondary alcohols and amino alcohols, and the N-substituted pyrroles were synthesized via dehydrogenative coupling of aromatic amines with cis-2-butene-1,4-diol and 2-butyne-1,4-diol, respectively. Several control reactions and spectroscopic experiments were performed to characterize the catalysts and establish the reaction mechanism.

Mechanisms of Electrochemical N2 Splitting by a Molybdenum Pincer Complex.

Molybdenum complexes supported by tridentate pincer ligands are exceptional catalysts for dinitrogen fixation using chemical reductants, but little is known about their prospects for electrochemical reduction of dinitrogen. The viability of electrochemical N2 binding and splitting by a molybdenum(III) pincer complex, (pyPNP)MoBr3 (pyPNP = 2,6-bis(tBu2PCH2)-C5H3N)), is established in this work, providing a foundation for a detailed mechanistic study of electrode-driven formation of the nitride complex (pyPNP)Mo(N)Br. Electrochemical kinetic analysis, optical and vibrational spectroelectrochemical monitoring, and computational studies point to two concurrent reaction pathways: In the reaction-diffusion layer near the electrode surface, the molybdenum(III) precursor is reduced by 2e- and generates a bimetallic molybdenum(I) Mo2(μ-N2) species capable of N-N bond scission; and in the bulk solution away from the electrode surface, over-reduced molybdenum(0) species undergo chemical redox reactions via comproportionation to generate the same bimetallic molybdenum(I) species capable of N2 cleavage. The comproportionation reactions reveal the surprising intermediacy of dimolybdenum(0) complex trans,trans-[(pyPNP)Mo(N2)2](μ-N2) in N2 splitting pathways. The same "over-reduced" molybdenum(0) species was also found to cleave N2 upon addition of lutidinium, an acid frequently used in catalytic reduction of dinitrogen.

Cyclometallated complexes as catalysts for C-H activation and functionalization.

The development of novel catalysts for C-H activation reactions with increased reactivity and improved selectivities has been attracting significant interest over the last two decades. More recently, promising results have been developed using tridentate pincer ligands, which form a stable C-M bond. Furthermore, based on mechanistic studies, the unique catalytic role of some metallacyclic intermediate species has been revealed. These experimental observations have subsequently translated into the rational design of advanced C-H activation catalysts in both Ru- and Ir-based systems. Recent breakthroughs in the field of C-H activation catalysed by metallacyclic intermediates are thus discussed.

Green and chemo selective amine methylation using methanol by an organometallic ruthenium complex

Herein a green and convenient catalytic N-methylation of aniline and n-hexylamine using methanol as a dual methylation agent and solvent has been investigated. A new ruthenium carbonyl complex was synthesized and applied as a homogeneous catalyst in methylation reaction. The solid-state structure of the complex was determined by X-ray crystallographic analysis which indicate xantphos ligand bonded to ruthenium (II) as a tridentate pincer ligand by two P donor and one O atom. The catalytic system showed excellent conversion and selectivity toward N-methylaniline, and N,N-hexyldimethylamine at 140°C.

Heteroleptic Palladium(II) Complexes of Thiazolinyl‐picolinamide Derived N∩N∩N Pincer Ligand: An Efficient Catalyst for Acylative Suzuki Coupling Reactions

AbstractSynthesis, reactivity, characterization, and catalytic activities of a series of palladium(II) complexes containing the thiazolinyl‐picolinamide derived pincer ligand is reported. Treatment of 2‐aminobenzonitrile with cysteamine and subsequent treatment with 2‐picolinic acid afforded the thiazolinyl‐picolinamide derived tridentate pincer ligand N∩N∩N (2). Treatment of 2 either with (COD)PdCl2 or PdCl2 in 1 : 1 ratio afforded the palladium(II) complex [(κ3‐N∩N∩N)PdCl] (3). A reaction between Pd(OAc)2 and 2 in equimolar ratio in dichloromethane gave [(κ3‐N∩N∩N)Pd(η1‐OAc)] (4) which on further treatment with triflic acid in acetonitrile produced the [(κ3‐N∩N∩N)Pd(N≡C−CH3)]OTf (5). The complex 5 was further treated with auxiliary ligands to produce the cationic heteroleptic palladium complexes, [(κ3‐N∩N∩N)Pd(NC5H5)]OTf (6), [(κ3‐N∩N∩N)Pd(NC5H4CN)]OTf (7), and [(κ3‐N∩N∩N)Pd(NC5H4NMe2)]OTf (8), respectively. The newly synthesized compounds were characterized by 1H and 13C{1H} NMR spectroscopic techniques and the molecular structures of 2–4, 6 and 8 were established. The catalytic activity of palladium(II) complexes (3–8) were evaluated for the acylative Suzuki coupling reaction between aryl carboxylic acids and arylboronic acids to yield unsymmetrically substituted biaryl ketones. The effect of reaction conditions viz., solvent, base, temperature, catalyst loading were evaluated, and complex 4 was found to be an excellent catalyst with a maximum TOF of 19.8 min−1.

Effects of a Tridentate Pincer Ligand on Parahydrogen Induced Polarization.

The role of ligands in rhodium- and iridium-catalyzed Parahydrogen Induced Polarization (PHIP) and SABRE (signal amplification by reversible exchange) chemistry has been studied in the benchmark systems, [Rh(diene)(diphos)]+ and [Ir(NHC)(sub)3 (H)2 ]+ , and shown to have a great impact on the degree of hyperpolarization observed. Here, we examine the role of the flanking moieties in the electron-rich monoanionic bis(carbene) aryl pincer ligand, Ar CCC (Ar=Dipp, 2,6-diisopropyl or Mes, 2,4,6-trimethylphenyl) on the cobalt-catalyzed PHIP and PHIP-IE (PHIP via Insertion and Elimination) chemistry that we have previously reported. The mesityl groups were exchanged for diisopropylphenyl groups to generate the (Dipp CCC)Co(N2 ) catalyst, which resulted in faster hydrogenation and up to 390-fold 1 H signal enhancements, larger than that of the (Mes CCC)Co-py (py=pyridine) catalyst. Additionally, the synthesis of the (Dipp CCC)Rh(N2 ) complex is reported and applied towards the hydrogenation of ethyl acrylate with parahydrogen to generate modest signal enhancements of both 1 H and 13 C nuclei. Lastly, the generation of two (Mes CCC)Ir complexes is presented and applied towards SABRE and PHIP-IE chemistry to only yield small 1 H signal enhancements of the partially hydrogenated product (PHIP) with no SABRE hyperpolarization.

Oxidative Addition of Aryl and Alkyl Halides to a Reduced Iron Pincer Complex.

The two-electron oxidative addition of aryl and alkyl halides to a reduced iron dinitrogen complex with a strong-field tridentate pincer ligand has been demonstrated. Addition of iodobenzene or bromobenzene to (3,5-Me2MesCNC)Fe(N2)2 (3,5-Me2MesCNC = 2,6-(2,4,6-Me-C6H2-imidazol-2-ylidene)2-3,5-Me2-pyridine) resulted in rapid oxidative addition and formation of the diamagnetic, octahedral Fe(II) products (3,5-Me2MesCNC)Fe(Ph)(N2)(X), where X = I or Br. Competition experiments established the relative rate of oxidative addition of aryl halides as I > Br > Cl. A linear free energy of relative reaction rates of electronically differentiated aryl bromides (ρ = 1.5) was consistent with a concerted-type pathway. The oxidative addition of alkyl halides such as methyl-, isobutyl-, or neopentyl halides was also rapid at room temperature, but substrates with more accessible β-hydrogen positions (e.g., 1-bromobutane) underwent subsequent β-hydride elimination. Cyclization of an alkyl halide containing a radical clock and epimerization of neohexyl iodide-d2 upon oxidative addition to (3,5-Me2MesCNC)Fe(N2)2 are consistent with radical intermediates during C(sp3)-X bond cleavage. Importantly, while C(sp2)-X and C(sp3)-X oxidative addition produces net two-electron chemistry, the preferred pathway for obtaining the products is concerted and stepwise, respectively.

Metal-Ion Influence on Ligand-Centered Hydrogen-Atom Transfer.

The ligand-centered hydrogen-atom-transfer (HAT) reactivity was examined for a family of group 10 metal complexes containing a tridentate pincer ligand derived from bis(2-mercapto-p-tolyl)amine, [SNS]H3. Six new metal complexes of palladium and platinum were synthesized with the [SNS] ligand platform in different redox and protonation states to complete the group 10 series previously reported with nickel. The HAT reactivity was examined for this family of nickel, palladium, and platinum complexes to determine the impact of a metal ion on the ligand-centered reactivity. Thermodynamic measurements revealed that N-H bond dissociation free energies increased by approximately 10 kcal mol-1 along the series Ni < Pd < Pt driven by changes to both the redox potential and pKa of the ligand. Kinetic analyses for all three metal complexes suggest that the barrier to the HAT reactivity is primarily entropic rather than enthalpic for this system.

Biaryl Group 4 Metal Complexes as Non‐Metallocene Catalysts for Polyethylene with Long Chain Branching

A series of biaryl Group 4 complexes with a bidentate and a tridentate pincer ligand have been synthesized and characterized. The complexes have been applied as metallocene analogues for the controlled polymerization of ethylene and the copolymerization of ethylene and 1‐hexene, with a particular focus on the control of the degree of long chain branching in these polymers.

Electronic and molecular characterization of an air-stable Cr(II) complex containing azo-anion-radicals

A new mononuclear homoleptic chromium complex has been synthesized by reaction of one equiv. of Cr(CO)6 with two equiv. of the azoaromatic pincer ligand namely 2,6-bis[(4-methylphenyl)azo]pyridine [LMe] in n-octane under reflux condition. The complex has been fully characterized by ESI-MS, 1H-NMR, FT-IR and UV/Vis spectroscopy. The X-ray structure of the complex reveals that the Cr-metal is coordinated to the central pyridine N-atom and two azo N-atoms of each tridentate pincer ligand in an octahedral environment. The 1H-NMR spectrum of the complex is indicative of diamagnetic ground state. The elongation of N-N bond lengths [d(N-N)av = 1.3275 Å] in the complex is consistent with the presence of azo-anion-radical character in the ligand. Since 2,6-bis(phenylazo)pyridine and its 4-Me-derivative are redox non-innocence in nature and therefore can coordinate to the chromium metal center as neutral (L0) or as mono-anionic mono-radical (L·)1− or as di-anionic di-radical (L··)2- or even as tri-anionic mono-radical (L·)3− resulting an ambiguity on the true oxidation state of the metal center. Thus the present work nicely elaborates the importance of suitably designed bis-azopyridine containing pincer ligand in accessing an air-stable Cr(II) complex starting with low-valent metal carbonyl precursor via electron transfer from the metal center to the highly π-acidic ligand leading to stable azo-anion-radicals. The stability of azo-anion-radical bound Cr(II) complex has been investigated by DFT calculations.

Transition metal pincer complex based self-healable, stretchable and transparent triboelecctric nanogenerator

As a promising energy converter, triboelectric nanogenerator (TENG) demonstrates unparalleled advantages. However, its performance is extremely affected by wear, tear and damage of the functional materials during triboelectrification. In this work, by incorporating tridentate pincer ligand moieties into a linear polydimethylsiloxane (PDMS) chain, the Zn-Pdca-PDMS elastomer for triboelectric layer was synthesized with excellent viscoelastic behaviors, super-stretchability (>5200%), self-healing ability and transparency. As a newly developed strategy, the strength of metal-ligand complex could be tuned efficiently by changing molar ratio of metal ions and ligands and three kinds of anions (CF3SO3−, BF4−, ClO4−) to adapt the desirable material performances. Subsequently, a stretchable, self-healable and transparent Zn-Pdca-PDMS based TENG (ZPP-TENG) was fabricated. With the molar ratio of Zn2+ to Pdca-PDMS ligand of 2:1, it exhibits the best output performance of 145 V, 6.7 nC cm−2 and 0.74 μA cm−2, respectively. The self-healable ZPP-TENG was employed as energy harvester to light up LEDs and charge a capacitor to driven electronic watch effectively. Even after several cutting-healing cycles, the output performance of the ZPP-TENG still maintain consistent. Finally, it was demonstrate to be employed a self-powered sensor for monitoring finger movement and pulse wave with excellent repeatability and stability.

Preparation of SNS Cobalt(II) Pincer Model Complexes of Liver Alcohol Dehydrogenase.

Chemical model complexes are prepared to represent the active site of an enzyme. In this protocol, a family of tridentate pincer ligand precursors (each possessing two sulfur and one nitrogen donor atom functionalities (SNS) and based on bis-imidazole or bis-triazole compounds) are metallated with CoCl2·6H2O to afford tridentate SNS pincer cobalt(II) complexes. Preparation of the cobalt(II) model complexes for liver alcohol dehydrogenase is facile. Based on a quick color change upon adding the CoCl2·6H2O to acetonitrile solution that contains the ligand precursor, the complex forms rapidly. Formation of the metal complex is complete after allowing the solution to reflux overnight. These cobalt(II) complexes serve as models for the zinc active site in liver alcohol dehydrogenase (LADH). The complexes are characterized using single crystal X-ray diffraction, electrospray mass spectrometry, ultra-violet visible spectroscopy, and elemental analysis. To accurately determine the structure of the complex, its single crystal structure must be determined. Single crystals of the complexes that are suitable for X-ray diffraction are then grown via slow vapor diffusion of diethyl ether into an acetonitrile solution that contains the cobalt(II) complex. For high quality crystals, recrystallization typically takes place over a 1 week period, or longer. The method can be applied to the preparation of other model coordination complexes and can be used in undergraduate teaching laboratories. Finally, it is believed that others may find this recrystallization method to obtain single crystals beneficial to their research.

Push-pull ligands and the oxidation of monomethylplatinum(II) complexes with oxygen or hydrogen peroxide

A rare example of the oxidation of a monomethylplatinum(II) complex with dioxygen is reported, aided by an appended phenol group. The ligands X-2-HO-C6H3CH2N(CH2-2-C5H4N)2 (L1, X = H; L2, X = 5-Cl; L3, X = 5-NO2; L4, X = 4-Me) react with [PtClMe(SMe2)2] to give the corresponding complexes [PtMe(κ3-N,N’,N’’-L)]Cl in which L acts as a tridentate pincer ligand, binding through all three nitrogen donors with the phenol group not coordinated. When L = L1 – L3, the complexes react rapidly with hydrogen peroxide and slowly with dioxygen from air to give the corresponding platinum(IV) complexes [Pt(OH)Me(κ4-N,N’,N”,O-L-H)]Cl, in which the deprotonated phenol group is coordinated trans to the hydroxo ligand. The structures of three of these complexes are determined and all are shown to self-assemble with water molecules to form hydrogen-bonded supramolecular polymers. The mechanisms of the oxidation reactions are discussed.

Copper(I) Complexes of Anionic Tridentate CNC Pincer Ligands

New monoanionic CNC pincer ligands, [N{SiMe2CH2(RIm)}2]– (R = tBu, iPr, Ph) featuring three different N‐heterocyclic carbenes and a disilylamido moiety is reported. Treatment of the lithium salt of [N{SiMe2CH2(RIm)}2]– with CuIOTf afforded the corresponding copper complexes [N{SiMe2CH2(RIm)}2]Cu in 41–56 % yield. X‐ray crystal structures of [N{SiMe2CH2(RIm)}2]Cu show that they are monomeric and feature three‐coordinate, pseudo T‐shaped copper(I) sites. The X‐ray crystal structure of one of the precursor lithium complexes, [N{SiMe2CH2(tBuIm)}2]Li is also presented.