NNN Pincer Ligand Research Articles

Ruthenium Complexes with NNN-Pincer Ligands for N-Methylation of Amines Using Methanol.

A series of ruthenium complexes (Ru1-Ru4) bearing new NNN-pincer ligands were synthesized in 58-78% yields. All of the complexes are air and moisture stable and were characterized by IR, NMR, and high-resolution mass spectra (HRMS). In addition, the structures of Ru1-Ru3 were confirmed by X-ray crystallographic analysis. These Ru(II) complexes exhibited high catalytic efficiency and broad functional group tolerance in the N-methylation reaction of amines using CH3OH as both the C1 source and solvent. Experimental results indicated that the electronic effect of the substituents on the ligands considerably affects the catalytic reactivity of the complexes in which Ru3 bearing an electron-donating OMe group showed the highest activity. Deuterium labeling and control experiments suggested that the dehydrogenation of methanol to generate ruthenium hydride species was the rate-determining step in the reaction. Furthermore, this protocol also provided a ready approach to versatile trideuterated N-methylamines under mild conditions using CD3OD as a deuterated methylating agent.

Ruthenium NNN‐Based Pincer Complexes with Metal Ligand Cooperation as Catalysts for N‐Methylation of Anilines and Nitroarenes with Methanol as a C1 Source

AbstractA novel phosphine‐free ruthenium pincer complex based on an NNN pincer ligand has been prepared and fully characterized. The complex was subsequently employed as an efficient catalyst for the N‐methylation of amines and the direct N‐methylation of nitroarenes using methanol as a C1 source under mild reaction conditions following the borrowing‐hydrogen approach. Both of the catalytic transformations were performed with only catalytic amounts of base under closed air conditions without using any other additives.

Catalytic Application of Copper Complex Prepared by a NNN Pincer Ligand Supported on Silica Nanoparticles Using first Generation of Polyamidoamine Dendrimers

Catalytic Application of Copper Complex Prepared by a NNN Pincer Ligand Supported on Silica Nanoparticles Using first Generation of Polyamidoamine Dendrimers

Transfer semi-hydrogenation of terminal alkynes with a well-defined iron complex.

The synthesis and characterization of a bis-iron(II) complex was accomplished upon treatment of a phosphine free NNN-pincer ligand (L) with FeCl2·4H2O under ambient conditions. The deep greenish colored iron(II) complex (Fe-1) was characterized by a single-crystal X-ray diffraction study along with IR spectroscopy, UV-Vis spectroscopy, mass spectrometry, and elemental analysis. The Fe-1 complex was tested for the transfer semi-hydrogenation of terminal alkynes to the corresponding alkenes through the dehydrogenation of dimethyl amine-borane. This procedure enables the conversion of various structurally different terminal alkynes to alkenes under mild conditions. Control experiments were performed to shed light on the possible intermediates generated during the present protocol.

Machine Learning-Enabled Predictions of Condensed Fukui Functions and Designing of Metal Pincer Complexes for Catalytic Hydrogenation of CO2.

This research showcases the machine learning (ML)-enabled homogeneous catalyst discovery to be employed in carbon dioxide hydrogenation. To achieve the desired turnover frequency (TOF), the electrophilicity of the central metal atom is a crucial factor in transition metal pincer complexes. The condensed Fukui function is a direct measure of the catalytic performance of these pincer complexes. Herein, we demonstrate that machine learning is a convenient and effiecient method to calculate condensed Fukui functions of the central metal atom. The electrophilicity values of 202 pincer complexes were calculated by using density functional theory (DFT) to train the ML model. The test data of the experimentally established pincer complexes show a direct linkage between calculated electrophilicity and experimental TOF. Further, this data was used to develop an ML protocol to screen 2,84,062 catalyst complexes to get the electrophilicity values of the Mn, Fe, Co, and Ni transition metals encompassing various permutation combinations of PNP, PNN, NNN, and PCP pincer ligands. These findings validate the efficacy of machine learning in the rapid screening of metal pincer catalysts based on condensed Fukui functions.

Reactivity of a Strictly T-Shaped Phosphine Ligated by an Acridane Derived NNN Pincer Ligand.

The steric tuning of a tridentate acridane-derived NNN pincer ligand allows for the isolation of a strictly T-shaped phosphine that exhibits ambiphilic reactivity. Well-defined phosphorus-centered reactivity towards nucleophiles and electrophiles is reported, contrasting with prior reports on this class of compounds. Reactions towards oxidants are also described. The latter result in the two-electron oxidation of the phosphorus atom from +III to +V and are accompanied by a strong geometric distortion of the NNN pincer ligand. By contrast, cooperative activation of E-H (HCl, HBcat, HOMe) bonds proceeds with retention of the phosphorus redox state. When using H2 O as a substrate, the reaction results in the full disassembly of H2 O to its constituent atoms, highlighting the potential of this platform for small molecule activation reactions.

Catalytic reduction of nitroarenes and degradation of dyes at room temperature by an efficient NNN pincer palladium catalyst based on the magnetic amino-triazole-modified chitosan

In this study, a magnetic chitosan biosupport was synthesized and functionalized with a NNN pincer ligand to facilitate palladium immobilization. FT-IR, FE-SEM, EDX, XRD, VSM, ICP, BET, and TGA analyses were used to characterize the synthesized nanocomposite (Fe3O4@Triazole-CS@NNN-Pd). Then, its catalytic activity in the reduction of a wide variety of nitroaromatic compounds at room temperature was investigated using sodium borohydride as a reducing agent, as well as the degradation of three hazardous dyes, including Methylene Blue (MB), Methyl Orange (MO), and Rhodamine B (RhB). This technique is feasible due to its ease of purification, environmental friendliness, and high yield of product generation in a relatively short response time. The nanocatalyst is easily separated, recovered, and reused for numerous cycles without significantly degrading catalytic activity.

An unusual mixed-valence cobalt dimer as a catalyst for the anti-Markovnikov hydrophosphination of alkynes

A Co dimer with a 3e-reduced NNN pincer ligand [(PMe3)2CoII(L13−)CoI(PMe3)3] is an efficient catalyst for the hydrophosphination reaction of both internal and terminal alkynes including α-hydroxy-functionalized alkyne substrates with excellent stereo- and regioselectivity.

Synthesis and characterization of trigonal bipyramidal FeIII complexes and their solution behavior

A series of air-stable trigonal bipyramidal FeIII complexes supported by a redox non-innocent NNN pincer ligand, CztBu(PyrR)2− (R = iPr, Me, or H), were synthesized, fully characterized, and utilized for the investigation of the interaction between acetone and the FeIII center. The magnetic moments determined from the paramagnetic 1H NMR spectra in conjunction with EPR and Mössbauer spectroscopy indicate the presence of a high-spin ferric center. Cyclic voltammetry studies feature two quasi-reversible events corresponding to a metal-centered FeIII/II reduction around −0.40 V (vs. Fc) and a ligand-centered CztBu(PyrR)2/CztBu(PyrR)2+ oxidation at potentials near +0.70 V (vs. Fc). UV–Visible spectroscopy in CH2Cl2 showcases ligand–metal charge transfer (LMCT) bands, as well as coordination of acetone to CztBu(PyrH)2FeCl2. In situ IR spectroscopy and solution conductivity (̺κ) measurements of CztBu(PyrR)2FeCl2 with varied equivalents of acetone reveal that acetone is weakly associated with the iron center.

Iron(II) Complexes Featuring a Redox-Active Dihydrazonopyrrole Ligand.

Nature uses control of the secondary coordination sphere to facilitate an astounding variety of transformations. Similarly, synthetic chemists have found metal-ligand cooperativity to be a powerful strategy for designing complexes that can mediate challenging reactivity. In particular, this strategy has been used to facilitate two electron reactions with first row transition metals that more typically engage in one electron redox processes. While NNN pincer ligands feature prominently in this area, examples which can potentially engage in both proton and electron transfer are less common. Dihydrazonopyrrole (DHP) ligands have been isolated in a variety of redox and protonation states when complexed to Ni. However, the redox-state of this ligand scaffold is less obvious when complexed to metal centers with more accessible redox couples. Here, we synthesize a new series of Fe-DHP complexes in two distinct oxidation states. Detailed characterization supports that the redox-chemistry in this set is still primarily ligand based. Finally, these complexes exist as 5-coordinate species with an open coordination site offering the possibility of enhanced reactivity.

Reactivity of a T-shaped cobalt(I) pincer-complex.

The reactivity of a paramagnetic T-shaped cobalt(i) complex, [(iPrboxmi)Co], stabilised by a monoanionic bis(oxazolinylmethylidene)-isoindolate (boxmi) NNN pincer ligand is described. The exposure to carbon monoxide as an additional neutral ligand resulted in the square-planar species [(iPrboxmi)Co(CO)], accompanied by a change in the electronic spin state from S = 1 to S = 0. In contrast, upon treatment with trimethylphosphine the formation of the distorted tetrahedral complex [(iPrboxmi)Co(PMe3)] was observed (S = 1). Reacting [(iPrboxmi)Co] with iodine (I2), organic peroxides (tBu2O2, (SiMe3)2O2) and diphenyldisulphide (Ph2S2) yielded the tetracoordinated complexes [(iPrboxmi)CoI], [(iPrboxmi)Co(OtBu)], [(iPrboxmi)Co(OSiMe3)] and [(iPrboxmi)Co(SPh)], respectively, demonstrating the capability of the boxmi-supported cobalt(i) complex to homolytically cleave bonds and thus its distinct one-electron reactivity. Furthermore, a square-planar cobalt(ii) alkynyl complex [(iPrboxmi)Co(CCArF)] was identified as the main product in the reaction between [(iPrboxmi)Co] and a terminal alkyne, 4-fluoro-1-ethynylbenzene. Putting such species in the context of the previously investigated hydroboration catalysis, its stoichiometric reaction with pinacolborane revealed its potential conversion into a cobalt(ii) hydride complex, thus confirming its original attribution as off-cycle species.

Electrochemical behaviors of a pincer-type NNN-Fe complex and catalytic H2 evolution activity.

We describe electrochemical reactivity of a pincer-type [NNN-Fe(tBuNC)3](ClO4)2 complex. Upon electron reduction, the Fe(i) species experienced disproportionation to Fe(0) and Fe(ii). An electron-reduced Fe center dissociated a tBuNC ligand to make an open coordination site, where a proton could be transferred. The low-spin Fe center, assisted by isocyanide and a pyridine-based NNN-pincer ligand, catalyzed efficiently the proton reduction reaction. Also, a Lewis basic amine site in the side 'arm' of the NNN-pincer ligand lowered the free energy for the protonation of an Fe center during the proton reduction process. DFT calculations provided insight into a plausible catalytic pathway.

Synthesis and characterization of group 13 dichloride (M = Ga, In), dimethyl (M = Al) and cationic methyl aluminum complexes supported by monoanionic NNN-pincer ligands

A series of monoanionic NNN-pincer ligands effectively stabilize five-coordinate gallium and indium dichloride complexes, as well as neutral dimethyl aluminum species, and organometallic cations thereof.

Magnetically coupled iron azide chains

Three well-defined iron(II) compounds CztBu(PyriPr)2FeCl (1), CztBu(PyriPr)2FeCl(THF) (2), and CztBu(PyriPr)2FeN3 (3), supported by the NNN pincer ligand CztBu(PyriPr)2− were synthesized and characterized.Compound 3 features a one-dimensional chain structure built by CztBu(PyriPr)2Fe units and bridged by end-to-end azido ligands. This chain material exhibits magnetic hysteresis at low temperatures and non-superimposable zero-field cooled and field cooled magnetization. These properties are consistent with single-chain magnet behavior, likely arising from superexchange coupling of Fe(II) centers via the azide bridges.

Coordination diversity in transition metal complexes with 4-aminoantipyrine tethered bis(imino)pyridine ligand: structures, superoxide dismutase and anticancer properties

In this article, we report Mn(II), Fe(II) and Cu(I) complexes (BIP-Mn, BIP-Fe and BIP-Cu) bearing a non-innocent, NNN pincer ligand, 4-aminoantipyrine tethered 2,6-bis(iminoantipyrine)pyridine (BIP) and structural diversity. The complexes were characterized by different spectroscopic and single-crystal X-ray analysis. Superoxide dismutase (SOD) activity has been evaluated for BIP and complexes BIP-Mn, BIP-Fe and BIP-Cu. It was observed that the seven-coordinate BIP-Mn complex showed preeminent SOD activity on comparing to BIP-Fe and BIP-Cu complexes. Besides, preliminary in vitro anticancer properties of BIP-Mn, BIP-Fe and BIP-Cu have been studied. Again, BIP-Mn complex showed stronger anticancer activity compared to ligand, BIP-Fe and BIP-Cu complexes.

Reaction Pathways and Redox States in α‐Selective Cobalt‐Catalyzed Hydroborations of Alkynes

AbstractCobalt(II) alkyl complexes supported by a monoanionic NNN pincer ligand are pre‐catalysts for the regioselective hydroboration of terminal alkynes, yielding the Markovnikov products with α:β‐(E) ratios of up to 97:3. A cobalt(II) hydride and a cobalt(II) vinyl complex appear to determine the main reaction pathway. In a background reaction the highly reactive hydrido species specifically converts to a coordinatively unsaturated cobalt(I) complex which was found to re‐enter the main catalytic cycle.

Reaction Pathways and Redox States in α-Selective Cobalt-Catalyzed Hydroborations of Alkynes.

Cobalt(II) alkyl complexes supported by a monoanionic NNN pincer ligand are pre‐catalysts for the regioselective hydroboration of terminal alkynes, yielding the Markovnikov products with α:β‐(E) ratios of up to 97:3. A cobalt(II) hydride and a cobalt(II) vinyl complex appear to determine the main reaction pathway. In a background reaction the highly reactive hydrido species specifically converts to a coordinatively unsaturated cobalt(I) complex which was found to re‐enter the main catalytic cycle.

Synthesis of Molybdenum Pincer Complexes and Their Application in the Catalytic Hydrogenation of Nitriles

AbstractA series of molybdenum(0), (I) and (II) complexes ligated by different PNP and NNN pincer ligands were synthesized and structurally characterized. Along with previously described Mo−PNP complexes Mo‐1 and Mo‐2, all prepared compounds were tested in the catalytic hydrogenation of aromatic nitriles to primary amines. Among the applied catalysts, Mo‐1 is particularly well suited for the hydrogenation of electron‐rich benzonitriles. Additionally, two aliphatic nitriles were transformed into the desired products in 80 and 86 %, respectively. Moreover, catalytic intermediate Mo‐1a was isolated and its role in the catalytic cycle was subsequently demonstrated.

Hydrophosphination of Activated Alkenes by a Cobalt(I) Pincer Complex

AbstractHerein we report the synthesis of three heteroleptic first‐row transition metal(II) complexes containing carbazolido NNN pincer ligands and conversion to the corresponding metal(I)‐carbonyl complexes via a reductive carbonylation route. These complexes are precatalysts for the hydrophosphination of activated alkenes, affording a cobalt‐catalysed hydrophosphination process that solely and selectively yields the β addition (anti‐Markovnikov) product. The scope of this transformation has been investigated using a variety of activated alkenes. Isolation and characterisation of substrate‐coordinated intermediates reveal available coordination sites, which provide insight into the proposed catalytic cycle.magnified image

Squeezing Bi: PNP and P2N3 pincer complexes of bismuth.

We report the first application of a rigid P2N3 pincer ligand in p-block chemistry by preparing its bismuth complex. We also report the first example of bismuth complexes featuring a flexible PNP pincer ligand, which shows phase-dependent structural dynamics. Highly electrophilic, albeit thermally unstable, Bi(iii) complexes of the PNP ligand were also prepared.