Amide Ligands Research Articles

On the Single-Molecule Magnetic Behavior of Linear Iron(I) Arylsilylamides.

The rational design of 3d-metal-based single-molecule magnets (SMM) requires a fundamental understanding of their intrinsic electronic and structural properties and how they translate into experimentally observable features. Here, we determined the magnetic properties of the linear iron(I) silylamides K{crypt}[FeL2] and [KFeL2] (L = -N(Dipp)SiMe3; crypt = 4,7,13,16,21,24-Hexaoxa-1,10-diazabicyclo[8.8.8]hexacosan). For the former, slow-relaxation of the magnetization with a spin reversal barrier of Ueff = 152 cm-1 as well as a closed-waist magnetic hysteresis and magnetic blocking below 2.5 K are observed. For the more linear [KFeL2], in which the potassium cation is encapsulated by the aryl substituents of the amide ligands, the relaxation barrier and the blocking temperature increase to Ueff = 184 cm-1 and TB = 4.5 K, respectively. The increase is rationalized by a more pronounced axial anisotropy in [KFeL2] determined by dc-SQUID magnetometry. The effective relaxation barrier of [KFeL2] is in agreement with the energy spacing between the ground and first-excited magnetic states, as obtained by field-dependent IR-spectroscopy (178 cm-1), magnetic measurements (208 cm-1), as well as theoretical analysis (212 cm-1). In comparison with the literature, the results show that magnetic coercivity in linear iron(I) silylamides is driven by the degree of linearity in conjunction with steric encumbrance, whereas the ligand symmetry is a marginal factor.

Pincer Platinum(II) Hydrides: High Stability Imparted by Donor-Flexible Pyridylidene Amide Ligands and Evidence for Adduct Formation before Protonation

Donor-flexible ligands are an emerging class of noninnocent ligands. Their ability to adapt their donating strength toward a metal center has had numerous catalytic advantages yet has never been utilized to stabilize and isolate intermediate complexes within these processes. We demonstrate through the use of a pincer ligand containing two donor-flexible pyridylidene amide (PYA) arms in coordination with platinum(II) that this ligand adaptability revealed remarkably stable hydride and formate complexes. These are typically fleeting catalytic intermediates within formic acid dehydrogenation and CO2 hydrogenation catalytic cycles. The PYA platinum hydride complexes are indefinitely stable in air, while formate complexes show no sign of β-hydrogen elimination. This robustness allowed us to investigate hydride protonation as a seemingly simple reaction, though in-depth kinetic analysis reveals a pre-equilibrium step prior to platinum hydride protonation. This initial step has been attributed to adduct formation and is slower than the protonation, and therefore a relevant aspect when designing catalytic cycles for hydrogen release and its microscopic reverse, viz., hydrogen uptake.

Amide-Ligand-Promoted Silver-Catalyzed C-H Fluorination via Radical/Polar Crossover.

We describe an efficient method for benzylic C-H fluorination via sequential hydrogen-atom transfer (HAT) and oxidative radical-polar crossover utilizing the Ag(I)/Selectfluor system. Amide ligands, such as benzamide and sulfonamide, substantially facilitate the processes leading to a carbocation intermediate, which subsequently reacts with nucleophilic fluorinating reagent to form a C-F bond. This protocol is applicable to the fluorination of all 1°, 2°, and 3° C-H bonds as well as to late-stage C-H fluorination of bioactive molecules.

Metal-regulated d10 coordination polymers constructed from bis(pyridyl)-bis(amide) ligands with different spacers as high-efficiency fluorescence sensors for identifying chlortetracycline and furaltadone

Three new Cd(ii)/Zn(ii) coordination polymers derived from two bis(pyridyl)-bis(amide) ligands with different spacers and trimellitic acid (H3MTC) were designed and synthesized, which can effectively detect chlortetracycline and furaltadone.

Complexation of cobalt (II) with new Schiff base and amide ligands

Complexation of cobalt (II) with new Schiff base and amide ligands

Production and characterization of chitosan of crustacean shells

After cellulose, chitin is the most prevalent biopolymer. The purpose of this work was to produce and characterize chitosan from expended crustacean shells (crab, prawn, and shrimp), primarily from Lagos State in Nigeria. Three main processes—deproteination, demineralization, and deacetylation—were used to manufacture their chitosan. The produced chitosan samples were characterized using Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), and X-ray Fluorescence (XRF) analyses, respectively, for functional components (ligands), surface shape, mineral constituent, and chemical constituent. The crab, shrimp, and prawn shell chitosan absorption altitudes ranged from 3360 to 893, 3775 to 570, and 3761 to 707 cm−1, respectively, in FTIR wavelengths; while both shrimp and prawn shell chitosan showed a clear and small filament lucent shape, the SEM pictures of crab shell chitosan showed a non-permeable, loosen skin semblance made up of protruding openings, tiny threadlike, and mineral form. Additionally, the chitosan found in shrimp shells is permeable and comprises granular layers. The crustaceans' related XRD results showed two tiny altitudes at 20°, 35°, 19.50° and 29.52°, 25.79° and 32.02°. Shrimp and prawn chitosan were both found to contain CaO, with larger proportions of 81.60 and 84.02%, respectively, according to XRF analysis. In general, all of the chitosan samples had amine, amide, carbonyl, carboxylic acid, hydrogen, and hydroxyl ligands (among others), which gave them better metal comity and band capability. As a result, the prepared chitosan of crab, prawn, and shrimp portrayed appropriate attributes for the adsorption techniques.

Kinetic and mechanistic investigations of dioxygen reduction by a molecular Cu(II) catalyst bearing a pentadentate amidate ligand.

A pentadentate Cu(II) complex, [CuII(dpaq)](ClO4) (1), featuring a redox active ligand, H-dpaq (H-dpaq = 2-[bis(pyridine-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate), catalyses four-electron reduction of dioxygen by decamethylferrocene (Fc*) in the presence of trifluoroacetic acid (CF3COOH) in acetone at 298 K. No catalytic oxygen reduction was observed in the presence of stronger Brønsted acids than CF3COOH, such as perchloric acid (HClO4) or trifluoromethanesulphonic acid (HOTf). In contrast, facile catalytic reduction of O2 occurs by Fc* with 1 and HClO4 or HOTf in dimethylformamide (DMF). The use of CF3COOH as the proton source in DMF results in the suppression of O2 reduction under otherwise identical reaction conditions. While the O2 reduction reactions in DMF are linearly dependent on the pKa of Brønsted acids, the acid dependence on catalytic O2-reduction reactivity by 1 in acetone showed complete reversal. Cyclic voltammetry studies using p-chloranil as the probe substrates in the presence of acids in the solvents reveal that the strengths of the protonic acids increase significantly in acetone compared to that in DMF. The amidate-N in [CuII(dpaq)](ClO4) (1) undergoes protonation in the presence of HClO4 or HOTf in DMF to form [CuII(H-dpaq)]2+ (1-H+), but not in the presence of CF3COOH. Enhanced acid strength of CF3COOH in acetone, however, effectively protonates 1 and triggers O2 reduction. Protonation of 1 with HClO4 or HOTf in acetone results in the change of its coordination environment, and this protonated species does not trigger O2 reduction. Detailed kinetic studies indicate that 1-H+ undergoes reduction by two-electrons and the reduced species binds O2 to form a Cu(II)-superoxo intermediate. This is followed by a rate-determining proton-coupled electron-transfer (PCET) reduction to generate the Cu(II)-hydroperoxo intermediate. While catalytic O2 reduction in acetone occurs predominantly via a 4e-/4H+ pathway, product selectivity (H2O vs. H2O2) in DMF depends upon the concentration of the reductant (Fc*). While dioxygen reduction to H2O2 is favoured at low [Fc*], mechanistic studies suggest that O2 reduction with high [Fc*] proceeds via a [2e- + 2e-] mechanism, where the released H2O2 during catalysis is further reduced to water.

Efficient β-alkylation of secondary alcohols to α-substituted ketones catalyzed by functionalized Ir complexes via borrowing hydrogen in water

Highly efficient and selective alkylation via borrowing hydrogen in water was achieved with a functional Ir complex bearing an amidate ligand. The significant cooperation effect of the ligand on the high efficiency and selectivity was demonstrated.

A Cobalt(III)−Hydroxo Complex Bearing a Pentadentate Amidate Ligand as an Electrocatalyst for Water Oxidation

AbstractA Co(III)−hydroxo complex, [CoIII(dpaq)OH]ClO4 (1‐OH) bearing a pentadentate ligand, H‐dpaq, (H‐dpaq=(2‐[bis(pyridin‐2‐ylmethyl)]amino‐N‐quinolin‐8‐yl‐acetamidate]) catalyses water oxidation in mildly alkaline medium (pH 8.0) at a potential of 1.4 VNHE with an average Turn‐Over‐Frequency (TOFmax) of 2.8×104 s−1 and faradaic efficiency of 88 %. Post‐electrolysis characterization of the electrode rules out the formation of any heterogeneous electroactive species. Electrochemical results and theoretical calculations confirm the occurrence of both metal and ligand centered PCET processes during anodic scanning. The resulting formally Co(V)−oxo/oxyl intermediate undergoes water nucleophilic attack to install the O−O bond. The role of axial ligand in water oxidation by Co(III)−dpaq system has been examined by comparing the reactivity of the Co‐hydroxide complex (1‐OH) with that of its chloride‐ligated counterpart, [CoIII(dpaq)Cl]Cl (1‐Cl). The results confirm the ability of the Co‐dpaq complexes to bind water/or water derived ligands over chloride or non‐aqueous solvents. The interplay of ligand redox non‐innocence and σ‐donating ability of the N5‐carboxamido ligand helps to store oxidizing equivalents and triggers O−O bond formation.

Two 2D Polyoxometalate‐Based Metal‐Organic Complexes Constructed from a New Flexible N‐Donor Ligand with High Capacitance Performances and Low Detection Limits of Cr(VI)

AbstractBy designing and using a new flexible bis(pyrimidine)‐bis(amide) ligand H2L [H2L=N,N′‐bis(4‐pyrimidinecarboxamido)‐1,3‐propane], two new polyoxometalate (POM)‐based metal‐organic complexes (MOCs), H3[Cu2(L)2(PMo12O40)] (1) and [Cu2(H2L)2(β‐Mo8O26)] (2), were synthesized under solvothermal and hydrothermal conditions, respectively. In complexes 1 and 2, metal‐organic units and POM anions are linked together to form two distinct 2D structures. The [PMo12O40]3− (PMo12) anions were used as μ4‐bridging ligands in complex 1 and linked the 1D [Cu(L)]n metal‐organic chains to form a 2D layered structure. The [β‐Mo8O26]4− (Mo8) anions adopted two diverse coordination modes in complex 2 and connected the 1D [Cu(H2L)]n2n+ metal‐organic chains to generate a 2D grid structure. Complexes 1–2 can serve as electrode materials of supercapacitor and show large specific capacitances, up to 1065 and 956 F g−1 at current density of 2 A g−1, respectively, which surpass the parent POM and most of the previous reported POM‐based electrode materials, thus demonstrating the important role of introducing metal‐organic units in improving capacitive performances. Besides, the electrocatalytic redox activities of complexes 1–2 were also studied, both of them can be used as electrochemical sensors to detect Cr(VI) ions. They possess high sensitivity of 0.537 and 0.455 μA μM−1 and low detection limits of 0.16 and 0.33 μM, which are below the maximum content of Cr(VI) in groundwater required by the WHO.

Nitrogen Reduction to Ammonia by a Phosphorus-Nitrogen PN 3 P-Mo(V) Nitride Complex: Significant Enhancement via Ligand Postmodification

Nitrogen Reduction to Ammonia by a Phosphorus-Nitrogen PN ³ P-Mo(V) Nitride Complex: Significant Enhancement via Ligand Postmodification

Syntheses of Group 5 Amide Amidinates and Their Reactions with Water: Different Reactivities of Nb(V) and Ta(V) Complexes.

Chemistries of Nb(V) and Ta(V) compounds are essentially identical as a result of lanthanide contraction. Hydrolysis of M(NMe2)5 (M = Nb, Ta), for example, yields [M(μ3-O)(NMe2)3]4 (M = Nb, 1; Ta, 2) reported earlier. The similar reactivities of Nb(V) and Ta(V) compounds make it challenging, for example, to separate the two metals from their minerals. We have found that the reactions of H2O with amide amidinates M(NMe2)4[MeC(NiPr)2] (M = Nb, 3; Ta, 4) show that the niobium and tantalum analogues take different principal paths. For the Nb(V) complex 3, the amidinate and one amide ligand are liberated upon treatment with water, yielding [Nb(μ3-O)(NMe2)3]4 (1). For the Ta(V) complex 4, the amide ligands are released in the reaction with H2O, leaving the amidinate ligand intact. [Ta(μ3-O)(NMe2)3]4 (2), the analogue of 1, was not observed as a product in the reaction of 4 with H2O. To our knowledge, this is the first example of the formation of two different complexes that maintain the (V) oxidation state in both metals. The new complexes M(NMe2)4[MeC(NiPr)2] (M = Nb, 3; Ta, 4) have been prepared by the aminolysis of M(NMe2)5 (M = Nb, Ta) with iPrN(H)C(Me)=NiPr (5). The hydrolysis of 3 and 4 has been investigated by DFT electronic structure calculations. The first step in each hydrolysis reaction involves the formation of a hydrogen-bonded complex that facilitates a proton transfer to the amidinate ligand in 3 and protonation of an axial dimethylamide ligand in 4. Both proton transfers furnish an intermediate metal-hydroxide species. The atomic charges in 3 and 4 have been computed by Natural Population Analysis (NPA), and these data are discussed relative to which of the ancillary ligands is protonated initially in the hydrolysis sequence. Ligand exchanges in 3 and 4 as well as the exchange in iPrN(H)C(Me)=NiPr (5) were probed by EXSY NMR spectroscopy, giving rate constants of the exchanges: 0.430(13) s-1 (3), 0.033(6) s-1 (4), and 2.23(7) s-1 (5), showing that the rate of the Nb complex Nb(NMe2)4[MeC(NiPr)2] (3) is 13 times faster than that of its Ta analogue 4.

Targeting Ultrasmall Gold Nanoparticles with cRGD Peptide Increases the Uptake and Efficacy of Cytotoxic Payload.

Cyclic arginyl-glycyl-aspartic acid peptide (cRGD) peptides show a high affinity towards αVβ3 integrin, a receptor overexpressed in many cancers. We aimed to combine the versatility of ultrasmall gold nanoparticles (usGNP) with the target selectivity of cRGD peptide for the directed delivery of a cytotoxic payload in a novel design. usGNPs were synthesized with a modified Brust-Schiffrin method and functionalized via amide coupling and ligand exchange and their uptake, intracellular trafficking, and toxicity were characterized. Our cRGD functionalized usGNPs demonstrated increased cellular uptake by αVβ3 integrin expressing cells, are internalized via clathrin-dependent endocytosis, accumulated in the lysosomes, and when loaded with mertansine led to increased cytotoxicity. Targeting via cRGD functionalization provides a mechanism to improve the efficacy, tolerability, and retention of therapeutic GNPs.

Study on crystallographic structure and antiproliferative effect of mixed-ligand strontium(II) complex and N, Nˊ–bis(2-hydroxy-5-methylphenyl) pyridine-2,6-dicarboxamide ligand

The novel amide ligand, N,Nˊ–bis(2-hydroxy-5-methylphenyl) pyridine-2,6-dicarboxamide (L) was synthesized through the condensation reaction of pyridine-2,6-dicarbonylchloride with 2-amino-4-methylphenol (1:2). The new mixed-ligand complex [Sr(pydcH)2(H2O)3][Sr(pydcH2)(pydcH)(pydc)]2.[Sr(pydcH2)2(H2O)3].2H2O (C) (where pydc is pyridine-2,6-dicarboxylate) was prepared from the reaction of L with Sr(NO3)2·4H2O metal salt. The structures were characterized by single-crystal X-ray diffraction. A study on the antiproliferative effect of compounds was done by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide) assay against two cancer cell lines including A375 (a human melanoma) and HT29 (a human colon adenocarcinoma), and also a normal HFF (a human foreskin fibroblast) cell line. A potent and selective cytotoxicity effect was exhibited for the complex toward A375 (IC50= 2.42 μM, MAE= 73.91%) cells. Further evaluations confirmed that ROS (Reactive oxygen species) level has increased by 307.7% in A375 cells following treatment with the complex compared to the control. Also, the decrease of MMP (Mitochondrial membrane potential) in the mentioned cell line was significant. Finally, the apoptotic cell death and the cell cycle arrest in the S-phase were recognized in A375 cells treated with the complex.

Development of a Library of Extremely Bulky Amide Ligands, and their use for the Stabilization of Germanium(I) and Germanium(II) Compounds

AbstractEleven new extremely bulky aryl/silyl substituted secondary amines, L1–11H, have been prepared. These incorporate a combination of one of the bulky aryl groups, C6H2{C(H)Ph2}2R‐2,6,4; R=Me (Ar*), Pri (Ar†); C6H2{C(H)(C6H3But2‐3,5)2}2Me‐2,6,4) (Ar#); or C6H2{C(H)(2‐naphthyl)2}2Me‐2,6,4 (Ar¥), and one of the silyl fragments, SiMe3, SiCy3 (Cy=cyclohexyl), SiPri3, Si(p‐C6H4But)3, or Si(OBor)3 (OBor=[(1S)‐endo]‐(−)‐borneoxy). Amines including the latter silyl group represent the first enantiopure examples of this ligand class. All amines are readily deprotonated with n‐butyllithium or KH. Reactions of five alkali metal amide complexes with GeCl2⋅dioxane have given monomeric amido‐germanium(II) chloride complexes, LxGeCl (x=1–4, 7). Treatment of two of these with KOBut afforded monomeric amido‐germanium(II) tert‐butoxide species, LxGeOBut (x=1 or 3). Reductions of two amido‐germanium(II) chloride systems with a dimagnesium(I) compound afforded amido‐digermyne species, LxGeGeLx (x=3 or 4), the X‐ray crystal structures of which show them to possess very short Ge−Ge bonds, with considerable implied multiple bond character.

π-Extended Ligands in Two-Coordinate Coinage Metal Complexes.

Two-coordinate carbene-MI-amide (cMa, MI = Cu, Ag, Au) complexes have emerged as highly efficient luminescent materials for use in a variety of photonic applications due to their extremely fast radiative rates through thermally activated delayed fluorescence (TADF) from an interligand charge transfer (ICT) process. A series of cMa derivatives was prepared to examine the variables that affect the radiative rate, with the goal of understanding the parameters that control the radiative TADF process in these materials. We find that blue-emissive complexes with high photoluminescence efficiencies (ΦPL > 0.95) and fast radiative rates (kr = 4 × 106 s-1) can be achieved by selectively extending the π-system of the carbene and amide ligands. Of note is the role played by the increased separation between the hole and electron in the ICT excited state. Analysis of temperature-dependent luminescence data and theoretical calculations indicate that the hole-electron separation exerts a primary effect on the energy gap between the lowest-energy singlet and triplet states (ΔEST) while keeping the radiative rate for the singlet state relatively unchanged. This interpretation provides guidelines for the design of new cMa derivatives with even faster radiative rates in addition to those with slower radiative rates and thus extended excited state lifetimes.

Synthesis and structural isomerism of sterically hindered isobornyl amide tethered N-heterocyclic carbene complexes

Enantiomerically pure chelating NHC-isobornylamide 1 was prepared and reacted with palladium and platinum salts. This gave monodentate complexes with carbonate (Pd12CO3, 4), and chelating or monodentate complexes with acetylacetonate M1acac (M = Pd (5a), Pt (5b)) and M1(acac)2 (M = Pd (7a), Pt (7b)). This is in contrast to the reactions of the achiral NHC-cyclohexylamide derivative 2, which gave only chelating complexes (M2acac (M = Pd (6a), Pt (6b)). The reaction of 1 and 2 with Pd(allyl)Cl gave exclusively monodenate complexes 8 (Pd1(allyl)Cl) and 9 (Pd2(allyl)Cl). Several of the complexes were characterized structurally in the solid-state using X-ray crystallography, which showed that, when chelated, significant steric bulk of the chiral isobornyl group was directed towards the metal atom. This was also demonstrated in the formation of monodentate 7a and 7b, which showed E/Z isomerism of the amide bond. We were able to identify the major isomer as the Z-anti, with the minor isomer E-anti forming due to the hydrophobic nature of the isobornyl group and weak hydrogen bonding between the amide and acetylacetonate ligands.

Solid-State End-On to Side-On Isomerization of (N═N)2- in {[(R2N)3Nd]2N2}2- (R = SiMe3) Connects In Situ LnIII(NR2)3/K and Isolated [LnII(NR2)3]1- Dinitrogen Reduction.

Examination of the reduction chemistry of Nd(NR2)3 (R = SiMe3) under N2 has provided connections between the in situ Ln(III)-based LnIII(NR2)3/K reductions of N2 that form side-on bound neutral (N=N)2- complexes, [(R2N)2(THF)Ln]2[μ-η2:η2-N2], and the Ln(II)-based [LnII(NR2)3]1- reductions by Sc, Gd, and Tb that form end-on bound (N=N)2- complexes, {[(R2N)3Ln]2[μ-η1:η1-N2]}2-, which are dianions. The reduction of Nd(NR2)3 by KC8 under dinitrogen in Et2O in the presence of 18-crown-6 (18-c-6) forms dark yellow solutions of [K2(18-c-6)3]{[(R2N)3Nd]2N2} at low temperatures that become green as they warm up to -35 °C in a glovebox freezer. Green crystals obtained from the solution turn yellow-brown when cooled below -100 °C, and the yellow-brown compound has an end-on Nd2(μ-η1:η1-N2) structure. The yellow-brown crystals isomerize in the solid state on the diffractometer upon warming, and at -25 °C, the crystals are green and have a side-on Nd2(μ-η2:η2-N2) structure. Collection of X-ray diffraction data at 10 °C intervals from -50 to -90 °C revealed that the isomerization occurs at temperatures below -100 °C. In the presence of tetrahydrofuran (THF), the dianionic {[(R2N)3Nd]2N2}2- system can lose an amide ligand to provide the monoanionic [(R2N)3NdIII(μ-η2:η2-N2)NdIII(NR2)2(THF)]1-, characterized by X-ray crystallography. These data suggest a connection between the in situ Ln(III)/K reductions and Ln(II) reductions that depends on solvent, temperature, the presence of a chelate, and the specific rare-earth metal.

Pincer and Macrocyclic Pyridylidene Amide (PYA) AuIII Complexes.

Gold-based homogeneous catalysis is dominated by redox neutral AuI systems. Redox-active gold-based catalysts are less common, principally because of redox cycles between AuI and AuIII being hampered by unfavorable potentials. We report gold(III) complexes containing pincer-based, donor-flexible pyridylidene amide (PYA) ligands to address these issues. These complexes act as electron reservoirs through two limiting resonance structures consisting of either soft, imine coordination sites or harder, zwitterionic amide donors. We further alter the donor properties by using the ortho-, meta-, and para-pyridylidene amide variants of the PYA pincer arms. These bis-PYA pincer ligands exhibited a high contribution of amide coordination in the solid-state of the gold(III) complexes; however, the solution data suggests a high contribution from the neutral L-type resonance forms. This L-type contribution, primarily shown through cyclic voltammetry studies, prevents reversible gold(III) reduction and also disfavors abstraction of the ancillary chloride ligand. Furthermore, a novel macrocyclic-PYA ligand is introduced, which shows secondary metal-ligand interactions.

Ligand-Promoted Fluorinated Olefination of Isatins at the C5 Position via a Palladium Catalyst.

A palladium-catalyzed nondirected fluorinated olefination was developed. The oxalyl amide ligand greatly improved the yield of the reaction. A wide variety of isatin derivatives were well tolerated and yielded the corresponding products in moderate to good yields. Various fluorinated olefins were also compatible. The application and synthesis of bioactive compounds such as a Metisazone derivative highlight the synthetic value of this approach.