Ruthenium(II) carbonyl complexes containing ‘pincer like’ ONS donor Schiff base and triphenylphosphine as catalyst for selective oxidation of alcohols at room temperature

Nickel Pincer Complexes Catalyzed Sustainable Synthesis of 3,4-Dihydro-2H-1,2,4-benzothiadiazine-1,1-dioxides via Acceptorless Dehydrogenative Coupling of Primary Alcohols

We report the atom-economic and sustainable synthesis of biologically important 3,4-dihydro-2H-1,2,4-benzothiadiazine-1,1-dioxide (DHBD) derivatives from readily available aromatic primary alcohols and 2-aminobenzenesulfonamide catalyzed by nickel(II)-N∧N∧S pincer-type complexes. The synthesized nickel complexes have been well-studied by elemental and spectroscopic (FT-IR, NMR, and HRMS) analyses. The solid-state molecular structure of complex 2 has been authenticated by a single-crystal X-ray diffraction study. Furthermore, a series of 3,4-dihydro-2H-1,2,4-benzothiadiazine-1,1-dioxide derivatives have been synthesized (24 examples) utilizing a 3 mol % Ni(II) catalyst through acceptorless dehydrogenative coupling of benzyl alcohols with benzenesulfonamide. Gratifyingly, the catalytic protocol is highly selective with the yield up to 93% and produces eco-friendly water/hydrogen gas as byproducts. The control experiments and plausible mechanistic investigations indicate that the coupling of the in situ generated aldehyde with benzenesulfonamide leads to the desired product. In addition, a large-scale synthesis of one of the thiadiazine derivatives unveils the synthetic usefulness of the current methodology.

Mechanistic investigation of the aerobic oxidation of 2-pyridylacetate coordinated to a Ru(II) polypyridyl complex

A new ruthenium polypyridyl complex, [Ru(bpy)₂(acpy)]⁺ (acpy = 2-pyridylacetate, bpy = 2,2'-bipyridine), was synthesized and fully characterized. Distinct from the previously reported analog, [Ru(bpy)₂(pic)]⁺ (pic = 2-pyridylcarboxylate), the new complex is unstable under aerobic conditions and undergoes oxidation to yield the corresponding α-keto-2-pyridyl-acetate (acpyoxi) coordinated to the Ru^II center. The reaction is one of the few examples of C-H activation at mild conditions using O₂ as the primary oxidant and can provide mechanistic insights with important implications for catalysis. Theoretical and experimental investigations of this aerobic oxidative transformation indicate that it takes place in two steps, first producing the α-hydroxo-2-pyridyl-acetate analog and then the final product. The observed rate constant for the first oxidation was in the order of 10^-2 h^-1. The reaction is hindered in the presence of coordinating solvents indicating the role of the metal center in the process. Theoretical calculations at the M06-L level of theory were performed for multiple reaction pathways in order to gain insights into the most probable mechanism. Our results indicate that O₂ binding to [Ru(bpy)₂(acpy)]⁺ is favored by the relative instability of the six-ring chelate formed by the acpy ligand and the resulting Ru^III-OO˙^- superoxo is stabilized by the carboxylate group in the coordination sphere. C-H activation by this species involves high activation free energies (ΔG^‡ = 41.1 kcal mol^-1), thus the formation of a diruthenium μ-peroxo intermediate, [(Ru^III(bpy)₂(O-acpy))₂O₂]²⁺via interaction of a second [Ru(bpy)₂(acpy)]⁺ was examined as an alternative pathway. The dimer yields two Ru^IVO centers with a low ΔG^‡ of 2.3 kcal mol^-1. The resulting Ru^IVO species can activate C-H bonds in acpy (ΔG^‡ = 23.1 kcal mol^-1) to produce the coordinated α-hydroxo-2-pyridylacetate. Further oxidation of this intermediate leads to the α-keto-2-pyridyl-acetate product. The findings provide new insights into the mechanism of C-H activation catalyzed by transition-metal complexes using O₂ as the sole oxygen source.

Synthesis, structural characterization and catalytic activity of chlororuthenium(II) complexes with substituted Schiff base/phosphine ancillary ligands

Treatment of [(η 6-p-cymene)RuCl2]2 with one equivalent of chlorodiphenylphosphine in tetrahydrofuran at reflux afforded a neutral complex [(η 6-p-cymene)RuCl2(κ 1-P-PPh2OH)] (1). Similarly, the reaction of [Ru(bpy)2Cl2·2H2O] (bpy = 2,2′-bipyridine) and chlorodiphenylphosphine in methanol gave a cationic complex [Ru(bpy)2Cl(κ 1-P-PPh2OCH3)](PF6) (2), while treatment of [RuCl2(PPh3)3] with [2-(C5H4N)CH=N(CH2)2N(CH3)2] (L1) in tetrahydrofuran at room temperature afforded a ruthenium(II) complex [Ru(PPh3)Cl2(κ 3-N,N,N-L1)] (3). Interaction of the chloro-bridged complex [Ru(CO)2Cl2] n with one equivalent of [Ph2P(o-C6H4)CH=N(CH2)2N(CH3)2] (L2) led to the isolation of [Ru(CO)Cl2(κ 3-P,N,N-L2)] (4). The molecular structures of the ruthenium(II) complexes 1–4 have been determined by single-crystal X-ray crystallography. The properties of the ruthenium(II) complex 4 as a hydrogenation catalyst for acetophenone were also tested.

Study of the dual functional behaviour of 1,2-bis(4-bromobenzamide)benzene by synchronous fluorescence spectroscopy

This study presents the dual functionality of 1,2-bis(4-bromobenzamide)benzene, where depending on the stoichiometric conditions, temperature and reaction time, the compound may act as a PdCl₂ ligand or as one of the reactants of the Suzuki coupling reaction.

Synthesis of a new series of Ni(ii), Cu(ii), Co(ii) and Pd(ii) complexes with an ONS donor Schiff base: crystal structure, DFT study and catalytic investigation of palladium and nickel complexes towards deacylative sulfenylation of active methylenes and regioselective 3-sulfenylation of indoles via thiouronium salt formation

A series of Ni(ii), Cu(ii), Co(ii), and Pd(ii) complexes have been synthesized with a chelating Schiff base ligand coordinated to a metal center with ONS donor atoms. The ligand and complexes are characterized by elemental analysis and spectroscopic techniques like FT-IR, 1H-NMR, and UV-Visible spectroscopy. The single crystal structure of the Pd(ii) complex is obtained by X-ray diffraction analysis and exhibits slightly distorted square planar geometry. The structure is optimized by DFT, TD-DFT calculation to elaborate the electronic structure and NBO for the charge distribution analysis of the Pd(ii) complex. The synthesized Pd(ii) and Ni(ii) complexes as catalysts have been investigated in the C-S cross-coupling of indoles and active methylenes. The metal propelled regioselective transformation afforded 3-sulfenylated indoles while β-diketones favored deacylated monosulfenyl ketones in an excellent yield via thiouronium salt formation. The Pd(ii) complex displays slightly better reactivity whereas the Ni(ii) complex is cost-efficient. The method is fast, easy to handle and cost effective in terms of high reactivity of catalysts, use of non-toxic solvents, and cheaper aryl halides and thiourea replace conventional sulfur sources, providing a practical access to organic transformations.

Complexes of (η5-Cp*)Ir(iii) with 1-benzyl-3-phenylthio/selenomethyl-1,3-dihydrobenzoimidazole-2-thione/selenone: catalyst for oxidation and 1,2-substituted benzimidazole synthesis

The treatment of 1-benzyl-3-phenylthio/selenomethyl-1,3-dihydrobenzoimidazole-2-thione/selenone [L1-L4] with [(η⁵-Cp*)IrCl(μ-Cl)]₂ at 25 °C followed by NH₄PF₆ results in [(η⁵-Cp*)Ir(L)Cl][PF₆] (1-4 for L = L1 to L4), authenticated with high-resolution mass spectrometry (HR-MS) and multi-nuclei nuclear magnetic resonance (NMR) imaging (¹H, ¹³C{¹H} and ⁷⁷Se{¹H}). The structures of 1-4, established with single-crystal X-ray diffraction, reveal a "piano-stool" geometry around the Ir. The Ir-thio/selenoether (Ir-S/Ir-Se) bond distances (Å) are 2.347(18)-2.355(4)/2.4663(12)-2.4663(13) and Ir-thione/selenone (Ir-S/Ir-Se) distances are 2.4146(19)-2.417(2)/2.5141(16)-2.5159(12). The reaction of 1,2-phenylenediamine with benzylic alcohols and furfuryl alcohol under mild and ambient conditions, catalyzed efficiently with complexes 1-4, generates bisimine in situ. Cyclization and rearrangement via 1,3-hydride shift triggered by its electrophilic activation with Ir(iii) species finally results in 1,2-disubstituted benzimidazole. The yield of the heterocycles in this one-pot synthesis is excellent to good. The aldehydes generated in situ by aerial oxidation of alcohols in the presence of 1-4 as catalysts are precursors to the bisimine as the protocols of this heterocycle synthesis carried out in the absence of 1,2-phenylenediamine give them in excellent-to-good yield. The oxidation of alcohols by hydrogen transfer to acetone was catalyzed efficiently with complexes 1-4 and resulted in aldehyde/ketone in excellent-to-good yield. Each catalytic process is marginally more efficient with 1 than its counterparts.

Synthesis of an oligomer ruthenium complex and its catalysis in the oxidation of alcohols

The catalyst showed high efficiency in the oxidation of alcohols to ketones or acids and can be recycled several times.

Efficient preparation of multimetallic ONO-based Schiff base complexes of nickel(ii) and copper(ii)

Spectroscopy, structures, electrochemical behavior and magnetic properties of a series of readily prepared [(R-ONO)M(μ-4,4′-bipy)M(R-ONO)] complexes (R = anisyl or ferrocenyl; M = Ni, Cu) have been explored.

Experimental and theoretical spectroscopic study and structural determination of nickel(II) tridentate Schiff base complexes

Some new complexes of [NiL(PR3)] (where L=(E)-1-[(2-amino-5-nitrophenyl)iminio-methyl]naphthalene-2-olate (L(1)), (E)-1-[(2-hydroxiphenyl)iminio-methyl]naphthalene-2-olate (L(2)), R=Bu and Ph) containing tridentate ONN and ONO Schiff bases were synthesized and characterized by IR, UV-Vis, (1)H-NMR spectroscopy and elemental analysis. The geometry of [NiL(1)(PBu3)] and [NiL(2)(PBu3)] complexes were determined by X-ray crystallography. It was indicated that the complexes have a square planar structure and four coordinates in the solid state. Theoretical calculations were also performed to optimize the structures of the ligands and complexes in the gas phase and ethanol solvent, separately to confirm the structures proposed by X-ray crystallography. In addition, UV-Visible and IR spectra of the complexes were calculated and compared with the corresponding experimental spectra to complete the experimental structural identification.

A review on versatile applications of transition metal complexes incorporating Schiff bases

Schiff bases and their complexes are versatile compounds synthesized from the condensation of an amino compound with carbonyl compounds and widely used for industrial purposes and also exhibit a broad range of biological activities including antifungal, antibacterial, antimalarial, antiproliferative, anti-inflammatory, antiviral, and antipyretic properties. Many Schiff base complexes show excellent catalytic activity in various reactions and in the presence of moisture. Over the past few years, there have been many reports on their applications in homogeneous and heterogeneous catalysis. The high thermal and moisture stabilities of many Schiff base complexes were useful attributes for their application as catalysts in reactions involving at high temperatures. The activity is usually increased by complexation therefore to understand the properties of both ligands and metal can lead to the synthesis of highly active compounds. The influence of certain metals on the biological activity of these compounds and their intrinsic chemical interest as multidentate ligands has prompted a considerable increase in the study of their coordination behaviour. Development of a new chemotherapeutic Schiff bases and their metal complexes is now attracting the attention of medicinal chemists. This review compiles examples of the most promising applied Schiff bases and their complexes in different areas.

Binuclear ruthenium(III) bis(thiosemicarbazone) complexes: synthesis, spectral, electrochemical studies and catalytic oxidation of alcohol

A new series of binuclear ruthenium(III) thiosemicarbazone complexes of general formula [(EPh3)2(X)2Ru-L-Ru(X)2(EPh3)2] (where E=P or As; X=Cl or Br; L=NS chelating bis(thiosemicarbazone ligands) has been synthesized and characterized by analytical and spectral (FT-IR, UV-Vis and EPR). IR spectra show that the thiosemicarbazones behave as monoanionic bidentate ligands coordinating through the azomethine nitrogen and thiolate sulphur. The electronic spectra of the complexes indicate that the presence of d-d and intense LMCT transitions in the visible region. The complexes are paramagnetic (low spin d(5)) in nature and all the complexes show rhombic distortion around the ruthenium ion with three different 'g' values (gx≠gy≠gz) at 77K. All the complexes are redox active and exhibit an irreversible metal centered redox processes (Ru(III)-Ru(III)/Ru(IV)-Ru(IV); Ru(III)-Ru(III)/Ru(II)-Ru(II)) within the potential range of 0.38-0.86V and -0.39 to -0.66 V respectively, versus Ag/AgCl. Further, the catalytic efficiency of one of the complexes [Ru2Cl2(AsPh3)4(L1)] (4) has been investigated in the case of oxidation of primary and secondary alcohols into their corresponding aldehydes and ketones in the presence of N-methylmorpholine-N-oxide(NMO) as co-oxidant. The formation of high valent Ru(V)O species is proposed as catalytic intermediate for the catalytic cycle.

Synthesis, characterization and structural determination of some nickel(II) complexes containing imido Schiff bases and substituted phosphine ligands

Some new tridentate ONN Schiff base complexes of [NiL(PR3)] (where L=Salicylidene2-amino4-nitrobenzene (L(1)), 5-BrSalicylidene2-amino4-nitrobenzene (L(2)), 5-NO2Salicylidene2-amino4-nitrobenzene (L(3)), 5-MeOSalicylidene2-amino4-nitrobenzene (L(4)) and 3-MeOSalicylidene2-amino4-nitrobenzene (L(5)), R=Bu and Ph (with L(1))) were synthesised and characterized by IR, UV-Vis, (1)H NMR spectroscopy and elemental analysis. The geometry of [NiL(1)(PPh3)] was determined by X-ray crystallography. It indicated that the complex had a planar structure and four coordinates in the solid state. The thermogravimetry (TG) and differential thermoanalysis (DTA) of the synthesized complexes were carried out in the range of 20-600°C, leading to the decomposition of L(1)-L(3) type in three stages and of L(4)-L(5) and [NiL(1)(PPh3)] type in four stages. Thermal decomposition of the complexes was closely the dependent upon the nature of the Schiff base ligands and proceeded via the first order kinetics.