NNN Ligand Research Articles

Cu(II) Coordination Polymers for the Selective Oxidation of Biomass-Derived Veratryl Alcohol in Green Solvents: A Sustainable Catalytic Approach.

Four air-stable one-dimensional copper(II) coordination polymers (CP1-CP4) with azide linkers were synthesized using tridentate NNS and NNN ligands. Single-crystal X-ray diffraction (XRD) analysis confirmed the molecular structures of CP1, CP3, and CP4. In the presence of TEMPO, all four coordination polymers demonstrated effective catalytic activity for the selective aerobic oxidation of veratryl alcohol, a biomass model compound, under base-free conditions. CP4 exhibited the best catalytic efficiency. Oxidations were conducted at ambient temperature (40 °C) utilizing air as a sustainable oxidant. Selective oxidation of veratryl alcohol to veratraldehyde was also conducted in the presence of a catalytic amount of base (5 mol %), and enhanced reactivity was observed. The green solvents, acetone, and water, were used to maximize sustainability. The optimized reaction conditions were applied to broaden the substrate scope of various lignin model alcohols and substituted benzylic alcohols with wide electronic variability. CP4 exhibited high recyclability, consistently providing quantitative yields even after ten consecutive runs. The catalytic protocol demonstrated sustainability and environmental compatibility, as evidenced by a low E-factor (4.29) and a high Eco-scale score (90). Based on experimental evidence and theoretical calculations, a plausible catalytic cycle was proposed. Finally, the sustainability credentials of the different optimized reaction protocols were evaluated using the CHEM21 green metrics toolkit.

Cobalt‐Catalyzed Difunctionalization of Styrenes via Ligand Relay Catalysis†

Comprehensive SummaryHere, we report a cobalt‐catalyzed sequential dehydrogenative Heck silylation/hydroamination of styrenes with hydrosilane and diazo compound to access 1‐amino‐2‐silyl compounds with excellent regioselectivity. This difunctionalization reaction could undergo smoothly using 1 mol% catalyst loading with good functional group tolerance. Not only di‐ and tri‐substituted hydrosilanes, but also alkoxysilane is suitable, which does explore the scope of the family of 1‐amino‐2‐silyl compounds. The ligand relay phenomenon between neutral tridentate NNN ligand and anionic NNN ligand is observed for the first time via absorption spectral analysis in this one‐pot, two‐step transformations. The primary mechanism has been proposed based on the control experiments.

Efficient additive-free formic acid dehydrogenation with a NNN-ruthenium complex.

A new ruthenium complex containing a pyridylidene amine-based NNN ligand was developed as a catalyst precursor for formic acid dehydrogenation, which, as a rare example, does not require basic additives to display high activity (TOF ∼10 000 h-1). Conveniently, the complex is air-stable, but sensitive to light. Mechanistic investigations using UV-vis and NMR spectroscopic monitoring correlated with gas evolution profiles indicate rapid and reversible protonation of the central nitrogen of the NNN ligand as key step of catalyst activation, followed by an associative step for formic acid dehydrogenation.

Synthesis, structure elucidation and dft study of a new thiazole–pyridine anchored nnn donor and it's cobalt(II) complex: In-vitro antitumor activity against U937 cancer cells, dna binding property and molecular docking study

The reaction between 4-(2-bromoacetyl)benzonitrile and 2-benzoylpyridine thiosemicarbazone produces a tridentate NNN ligand, 4-(2-(2-(phenyl(pyridine-2-yl)methylene)hydrazinyl)thiazole-4-yl)benzonitrile (ppytbH). The ligand constructs an octahedral complex with composition [Co(ppytbH)2](ClO4)2.3(H2O) (1) when reacted with cobalt(II) perchlorate salt in acetonitrile methanol (1:1) solution. The structures of ppytbH and 1 have been established by single X-ray crystallography, spectroscopic (1H NMR, IR, UV-Visible and fluorescence), thermal and electrochemical methods. The chemical reactivity and HOMO–LUMO energy of the compounds have been calculated using Density Functional Theory (DFT). The compounds ppytbH and 1 exhibit potential anticancer activity against U937 human monocytic cells and IC50 values are found 12.76 ± 0.75 and 12.83 ± 1.37 µM, respectively. The fluorescence and molecular docking study interpret the intercalative DNA binding mode with the titled molecules.

Photocatalytic degradation of dyes by mononuclear copper(II) complexes from bis-(2-pyridylmethyl)amine NNN-derivative ligands

Photocatalytic degradation of organic pollutant dyes under ultraviolet radiation has emerged as an efficient wastewater treatment. This work describes the application of four mononuclear copper(II) complexes coordinated to NNN ligands: bis-(2-pyridylmethyl)amine (BMPA), N-methylpropanoate-N,N-bis-(2-pyridylmethyl)amine (MPBMPA), N-propanoate-N,N-bis-(2-pyridylmethyl)amine (PBMPA) and N-propanamide-N,N-bis-(2-pyridylmethyl)amine (PABMPA); in the photocatalytic degradation of different dyes: methyl orange (MO), methylene blue (MB), crystal violet (CV), Congo red (CR) and Rhodamine B (RhB). The reactions were carried out under a UV lamp of 250 W, where 100% of degradation was achieved in 90 min for all complexes using hydrogen peroxide as oxidant. Kinetic experiments were carried out to investigate the photodegradation of the dyes under a UV lamp of 24 W. The reactions followed a zero-order model in relation to the dye, showing that its concentration did not play a significant role in the photocatalysis. The reaction order in relation to hydrogen peroxide varied from 0 to 0.8, from low to high concentrations of oxidant. The light intensity and the intrinsic catalytic activity of the complexes are the most important features for the dye photodegradation pathway.

Tridentate NNN Ligand Associating Amidoquinoline and Iminophosphorane: Synthesis and Coordination to Pd and Ni Centers

The synthesis of a tridentate NNN ligand incorporating an amidoquinoline and an iminophosphorane (LRH; R = Ph, Cy) was carried out. Coordination to PdII and NiII precursors gave, as expected, square-planar complexes of the general formula [LRMCl]. The most surprising result came from the reaction of LPhH with [Ni(COD)2], which gave a NiII phenyl complex bearing a tridentate amidoquinoline-aminophosphine ligand. This rearrangement would result from a proton transfer assisted by the Ni0 on stabilization by a phenyl substituent at the phosphorus, as suggested by DFT calculations and complementary experiments. This nickel phenyl complex formed the corresponding benzoyl complex under a CO atmosphere.

Ru(II)‐PBTNNXN complex bearing functional 2‐(pyridin‐2‐yl)benzo[d]thiazole ligand catalyzed α‐alkylation of nitriles with alcohols

Six tridentate NNN ligand precursors derived from 2‐(pyridin‐2‐yl)benzo[d]thiazole(PBT) with different linkers, PBTNNXN (X = NH, NMe, O, S) (1a–1f), have been successfully prepared. The electronic properties of PBTNNXN ligands are well tunable by differing linkers between PBT skeleton and the pyridine ring, and/or by introducing electron‐donating/withdrawing groups on the pyridine ring (R = OMe or F). The ligand precursors and representative complexes Ru (PBTNNNHN)Cl2(PPh3) (2a), Ru (PBTNNNMeN)Cl2(PPh3) (2b), and Ru (PBTNNSN)Cl2(PPh3) (2f) have been characterized by NMR spectroscopy, high‐resolution mass spectroscopy, and Fourier transform infrared (FT‐IR). The molecular structures of 1f, 2a, and 2f have been determined by X‐ray diffraction study. The results indicate that PBTNNNHN ligand in the complex presented coplanar with two five‐membered chelating rings. It should be noted that 2a featuring a NH group exhibits superior performance compared to those with other linkers (such as NMe, O, or S). A variety of heterocyclic and aromatic nitriles with aromatic and aliphatic alcohols have been explored in α‐alkylation for good to excellent yields. Based on kinetic experiments and mechanistic studies, a proposed mechanism was put forward. Ru‐H species and benzaldehyde, which was oxidized from benzyl alcohol, were detected in the catalytic cycle.

Degradation of organic dyes by water soluble iron(III) mononuclear complexes from bis-(2-pyridylmethyl)amine NNN-derivative ligands

This work describes the degradation of the dyes methyl orange (MO), methylene blue (MB), crystal violet (CV), Congo red (CR) and Rhodamine B (RhB), catalyzed by four water soluble mononuclear iron(III) complexes, in presence of hydrogen peroxide. The NNN ligands are bis-(2-pyridylmethyl)amine (BMPA) and three -derivatives: N-methylpropanoate-N,N-bis-(2-pyridylmethyl)amine (MPBMPA), N-propanoate-N,N-bis-(2-pyridylmethyl)amine (PBMPA), N-propanamide-N,N-bis-(2-pyridylmethyl)amine (PABMPA). More than 85% of degradation was achieved after 180 min for all dyes. The complex [Fe(MPBMPA)Cl3] was the most active. Kinetic experiments indicated a two-step reaction, where the intermediate Fe(III)-OOH is the catalytic species in the first step (k1), followed by the hydroxyl radical degradation in the second step (k2). The degradation percentage was well correlated with k2, being the degradation of the dye by the hydroxyl radical the most important step.

An H-Substituted Rhodium Silylene.

Divergent reactivity of organometallic rhodium(I) complexes, which led to the isolation of neutral rhodium silylenes, is described. Addition of PhRSiH2 (R=H, Ph) to the rhodium cyclooctene complex (iPr NNN)Rh(COE) (1-COE; iPr NNN=2,5-[iPr2 P=N(4-iPrC6 H4 )]2 N(C6 H2 )- , COE=cyclooctene) resulted in the oxidative addition of an Si-H bond, providing rhodium(III) silyl hydride complexes (iPr NNN)Rh(H)SiHRPh (R=H, 2-SiH2 Ph; Ph, 2-SiHPh2 ). When the carbonyl complex (iPr NNN)Rh(CO) (1-CO) was treated with hydrosilanes, base-stabilized rhodium(I) silylenes κ2 -N,N-(iPr NNN)(CO)Rh=SiRPh (R=H, 3-SiHPh; Ph, 3-SiPh2 ) were isolated and characterized using multinuclear NMR spectroscopy and X-ray crystallography. Both silylene species feature short Rh-Si bonds [2.262(1) Å, 3-SiHPh; 2.2702(7) Å, 3-SiPh2 ] that agree well with the DFT-computed structures. The overall reaction led to a change in the iPr NNN ligand bonding mode (κ3 →κ2 ) and loss of H2 from PhSiRH2 , as corroborated by deuterium labelling experiments.

Synthesis, characterization and catalytic activity of novel ruthenium complexes bearing NNN click based ligands.

Novel air stable ruthenium(ii) complexes bearing tridentate ligands bis((1-benzyl-1H-1,2,3-triazol-4-yl)methyl)amine (L1), 1-(1-benzyl-1H-1,2,3-triazol-4-yl)-N-(pyridin-2-ylmethyl)methanamine (L2) or 2-(4-phenyl-1H-1,2,3-triazol-1-yl)-N-(pyridin-2-ylmethyl)ethan-1-amine (L3) were synthesised. The nitrogen based ligands were easily prepared by virtue of click chemistry using cheap and commercially available reagents. The ruthenium complexes were obtained by heating the Ru(PPh3)3Cl2 precursor and the tridentate NNN ligand in toluene under reflux for 2 hours, achieving yields of 82-87%. These complexes were fully characterized by means of NMR, FT-IR and high resolution ESI spectroscopy. The crystal structure of one of the complexes was determined. These complexes showed excellent activity and selectivity in the hydrogenation of ketones and aldehydes. DFT calculations show that complex 3 may react through an outer-sphere catalytic cycle rather than via an inner-sphere mechanism.

Nickel(II) complexes with tripodal NNN ligands as homogenous and supported catalysts for ethylene oligomerization

Four new coordination compounds of nickel (II) with derivatives of N,N-bis(pyrazol-1-ylmethyl)propylamine were synthesized; their composition and structure were confirmed with IR-spectroscopy and elemental analysis. The structures of products 13 and 15 were unambiguously established in an X-ray diffraction study. Compounds 13 and 15 crystallize in the orthorhombic space groups Pna21 and P212121 correspondingly and represent a monomeric octahedral nickel complexes, that are typical for tridentate scorpion-type ligands. New method for immobilization of nickel complexes with derivatives of N,N-bis(pyrazol-1-ylmethyl)propylamine on silica gel modified with aminopropyl groups was proposed. The EXAFS/XANES analysis indicated that Ni atom in the supported complexes adopt almost octahedral geometry, being partly surrounded by nitrogen atoms from organic ligand and partly grafted to silica surface through silanol groups, with Br− in outer coordination sphere. Both the original and the supported complexes, when activated with Et2AlCl or Et3Al2Cl3, catalyze ethylene oligomerization with the predominant formation of butene isomers. Generally, the immobilized complexes show higher activity and better selectivity towards 1-butene formation.

Synthesis and Reactivity of Metal–Ligand Cooperative Bifunctional Ruthenium Hydride Complexes: Active Catalysts for β-Alkylation of Secondary Alcohols with Primary Alcohols

Three unsymmetrical NNN ligands with a 2-hydroxypyridyl fragment were used to react with RuHCl(PPh3)3(CO), affording the three bidentate ruthenium hydride complexes [(R1-C5H3N-CH2-C5H3N-C5H3N-R2)Ru...

Photophysical Properties of Pt(II) Polypyridines with Five- versus Six-Membered Chelate Rings: Trade-Offs in Angle Strain.

This report describes the synthesis and characterization of a series of eight [Pt(NNN)X]+ complexes where the tridentate NNN ligand is (2,2'-bipyrid-6-yl)(pyrid-2-yl)sulfide (btp) or methyl(2,2'-bipyrid-6-yl)(pyrid-2-yl)amine (bmap) and X is OMe, Cl, phenylethynyl (C2Ph), or cyclohexylethynyl (C2Cy). The expectation was that inserting a heteroatom into the backbone of 2,2':6',2″-terpyridine (trpy) would expand the overall intraligand bite angle, introduce ILCT character into the excited states, and improve the photophysical properties. Crystal structures of [Pt(bmap)C2Ph]+ and [Pt(btp)Cl]+ reveal that atom insertion into the trpy backbone successfully expands the bite angle of the ligand by 8-10°. However, the impact on the photophysics is minimal. Indeed, of the eight systems investigated, only the [Pt(bmap)C2Ph]+ and [Pt(btp)C2Ph]+ complexes display appreciable emission in fluid solution, and they exhibit shorter emission lifetimes than [Pt(trpy)C2Ph]+. One reason is that the bond angle preferences of platinum and the inserted heteroatom induce the six-membered rings to deviate from planarity and adopt a boat-like conformation, impairing charge delocalization within the ligand. In addition, angle strain induces the donor atoms about platinum to assume a pseudotetrahedral arrangement, which offsets any benefit due to the increase in overall bite angle by promoting deactivation via d-d excited states. The results reveal that, in order to improve the luminescence of a [Pt(NNN)X]+ system, one must take care to avoid trading one kind of angle strain for another.

NNN Pincer Ru(II)-Complex-Catalyzed α-Alkylation of Ketones with Alcohols.

A series of novel ruthenium(II) complexes supported by a symmetrical NNN ligand were prepared and fully characterized. These complexes exhibited good performance in transfer hydrogenation to form new C-C bonds using alcohols as the alkylating agents, generating water as the only byproduct. A broad range of substrates, including (hetero)aryl- or alkyl-ketones and alcohols, were well tolerated under the optimized conditions. Notably, α-substituted methylene ketones were also investigated, which afforded α-branched steric hindrance products. A potential application of α-alkylation of methylene acetone to synthesize donepezil was demonstrated, which provided the desired product in 83% yield. Finally, this catalytic system could be applied to a one-pot double alkylation procedure with sequential addition of two different alcohols. The current protocol is featured with several characteristics, including a broad substrate scope, low catalyst (0.50 mol %) loadings, and environmental benignity.

Cobalt‐Catalyzed Regioselective Hydroboration of Terminal Alkenes

Cobalt(II) catalysts based on flexible PNP or NNN ligands were explored for the regioselective hydroboration of alkenes. A known CoII–PNP pincer complex was found to efficiently catalyze alkene hydroboration with excellent anti‐Markovnikov selectivity, whereas a newly synthesized dinuclear CoII–NNN complex was found to catalyze the hydroboration of a range of aromatic terminal alkenes with good Markovnikov selectivities (up to 98:2, b/l). This represents a rare example of Markovnikov selectivity in the hydroboration of alkenes using an inexpensive, flexible‐ligand‐supported dinuclear cobalt catalyst.

Ambient-Stable Bis-Azoaromatic-Centered Diradical [(L•)M(L•)] Complexes of Rh(III): Synthesis, Structure, Redox, and Spin–Spin Interaction

Bis-azoaromatic electron traps, viz. 2-(2-pyridylazo)azoarene 1, have been synthesized by colligating electron-deficient pyridine and azoarene moieties, and they act as apposite proradical templates for the formation of stable open-shell diradical complexes [(1•-)RhIII(1•-)]+ ([2]+), starting from the low-valent electron reservoir [RhI]. The less stable monoradical [RhIII(1•-)Cl2(PPh3)3] (3) has also been isolated as a minor product. These π-radical complexes are multiredox systems, and the electron transfer processes occur exclusively within the pincer-type NNN ligand backbone 1. Molecular and electronic structures of the diradicals and monoradicals have been ascertained with the aid of X-ray diffraction, electrochemical, spectroelectrochemical, and spectral (electronic, IR, NMR, and EPR) studies. In the diradicals [2]+, the orthogonal disposition of two ligand π orbitals linked via a closed-shell metal center (t26) impedes significant coupling between the radicals. Indeed, the observed magnetic moment of [2a]+ lies near ∼2.3 μB over the temperature range 50-300 K. A very weak antiferromagnetic (AF) intramolecular spin-spin interaction between two ligand π arrays in [(1•-)RhIII(1•-)]+ have been found experimentally (J ≈ -5 cm-1), and this is further substantiated by density functional theory (DFT) calculations at the (U)B3LYP/6-31G(d,p) level.

Methane C-H Activation via 3d Metal Methoxide Complexes with Potentially Redox-Noninnocent Pincer Ligands: A Density Functional Theory Study.

This paper reports a density functional theory study of 3d transition-metal methoxide complexes with potentially redox-noninnocent pincer supporting ligands for methane C-H bond activation to form methanol (LnM-OMe + CH4 → LnM-Me + CH3OH). The three types of tridentate pincer ligands [terpyridine (NNN), bis(2-pyridyl)phenyl-C,N,N' (NCN), and 2,6-bis(2-phenyl)pyridine-N,C,C' (CNC)] and different first-row transition metals (M = Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) are used to elucidate the reaction mechanism as well as the effect of the metal identity on the thermodynamics and kinetics of a methane activation reaction. Spin-density analysis indicates that some of these systems, the NNN and NCN ligands, have redox-noninnocent character. A four-centered, kite-shaped transition state, σ-bond metathesis, or oxidative hydrogen migration has been found for methane activation for the complexes studied. Calculations suggest that the d electron count is a more significant factor than the metal formal charge in controlling the thermodynamics and kinetics of C-H activation and late 3d metal methoxides, with high d counts preferred. Notably, early-to-middle metals tend toward oxidative hydrogen migration and late metals undergo a pathway that is more akin to σ-bond metathesis, suggesting that metal methoxide complexes that favor σ-bond metathesis pathways for methane activation will yield lower barriers for C-H activation.

NHTs Effect on the Enantioselectivity of Ru(II) Complex Catalysts Bearing a Chiral Bis(NHTs)-Substituted Imidazolyl-Oxazolinyl-Pyridine Ligand for Asymmetric Transfer Hydrogenation of Ketones

Pincer-type ruthenium(II)-NNN complex catalysts bearing a chiral bis(NHTs)-substituted imidazolyl-oxazolinyl-pyridine ligand were synthesized and structurally characterized by NMR, IR, elemental analysis, and X-ray single-crystal crystallographic determinations. The two NHTs groups substituted on the imidazolyl moiety of the chiral NNN ligand exhibited a remarkable effect on the enantioselectivity of the Ru(II)-NNN complexes for the asymmetric transfer hydrogenation (ATH) of ketones. The Ru(II)-NNN complex bearing a chiral (NHTs)2-substituted imidazolyl-(isopropyl)oxazolinyl-pyridine ligand exhibited excellent catalytic activity, reaching an enantioselectivity up to 99.9% ee for the target alcohol products.

PH value-dependent electronic absorption and Ru(III/II) potential of bis-tridentate pincer ruthenium complexes

Two bis-tridentate pincer complexes [Ru(H2bip)(Mebib)](PF6)2 (1(PF6)2) and [Ru(H2bip)(H2bib)](PF6)2 (2(PF6)2) were prepared and studied, where H2bip is 2,6-bis(benzimidazol-2-yl)pyridine, Mebib is the 2-deprotonated 1,3-bis(N-methylbenzimidazol-2-yl)benzene, and H2bib is 2-deprotonated 1,3-bis(benzimidazol-2-yl)benzene. These complexes were isolated as cyclometalated Ru(III) salts with a [(NCN)(NNN)] coordination due to a low Ru(III/II) redox potential. Complex 1(PF6)2 has two N−H protons on the NNN ligand side, while complex 2(PF6)2 has four N−H protons. Absorption spectral analysis showed that two deprotonation steps were distinguished for 1(PF6)2 with pKa1 and pKa2 of 3.46 and 5.88, respectively. In comparison, four deprotonation steps were observed for 2(PF6)2 with four pKa values of 3.45, 5.95, 8.95, and 10.98, respectively. When the solution was changed from acidic to basic, the Ru(III/II) redox wave of 1(PF6)2 and 2(PF6)2 shifted from +0.20 V vs Ag/AgCl to a less positive region by 540 and 850 mV, respectively. These studies suggest that the electronic absorption and electrochemical properties of these complexes are very sensitive to pH effect.

Synthesis, Characterization, and Reversible Multielectron Redox Properties of a Biradical Yttrium Complex Containing Bis(2‐isopropylaminophenyl)amide

The redox‐active ligand bis(2‐isopropylaminophenyl)amide (NNN) was used to prepare ionic yttrium complex [(NNNsq·)2Y]–[Li(thf)4]+ (2; sq = semiquinonate form). A solid‐state magnetic measurement on 2 (µeff = 2.40 µB) indicated the presence of two unpaired electrons on two separate NNN ligands. The paramagnetic compound 2 was characterized by EPR spectroscopy in a dilute frozen solution of thf, which suggested there was no intramolecular radical–radical interaction. Electrochemistry of 2 featured a quasireversible two‐electron reduction process at –1.193 V attributed to the 22–/2 redox couple, and two successive one‐electron oxidation processes at –0.406 V and –0.156 V attributed to the reversible 2/2+ redox couple and quasireversible 2+/22+ redox couple, respectively.