Biomimetic catalytic activity and structural diversity of cobalt complexes with N3O-donor Schiff base ligand

New synthetic approaches for the construction of 2-aminophenoxazinone architectures

Elaborated molecular architectures, specifically those containing a 2-aminophenoxazinone scaffold, belong to one of the most ubiquitous and prominent classes of heterocyclic frameworks, going from natural products to biologically active pharmaceutical molecules and from agrochemicals to functional materials and polymers. Therefore, efficient synthetic strategies for the assembly of 2-aminophenoxazinone frameworks are always in demand and have gained attention in academic and industrial communities. Methodologies that involve cascade reactions generally catalyzed by transition metal complexes, such as iron, cobalt, manganese, copper, and zinc complexes, have stood out as a representative approach. Over the past few decades, a great deal of versatile, atom-economic, and straightforward protocols have been reported for the generation of value-added 2-aminophenoxazinone frameworks in a sustainable, powerful, and applicable manner. The state-of-the-art methodologies toward the construction of 2-aminophenoxazinone skeletons are summarized in this review, which could be divided into four categories: (1) construction of 2-aminophenoxazinone compounds catalyzed by transition metal complexes; (2) construction of 2-aminophenoxazinone compounds catalyzed by biosynthetic enzymes; (3) synthetic process routes of 2-aminophenoxazinone compounds; and (4) construction of 2-aminophenoxazinone compounds via other innovative methods.

Exploring structural, magnetic, and catalytic diversity in tetranuclear {Cu4O4} cubane cores and a dinuclear complex derived from closely related ligand systems

The coordination compounds featuring a {Cu4O4} core, typically bridged by hydroxo or alkoxo groups, are particularly intriguing due to their notable magnetic properties and catalytic activity. In this study, we explored the synthesis and characterization of four new Schiff base ligands and their subsequent complexation with CuII salts, which resulted in the formation of three tetranuclear complexes: [Cu4(L1)4]·2H2O (1), [Cu4(L2)2(HL2)2](Cl)(NO3)·5H2O (2), and [Cu4(L3)4] (3), as well as one dinuclear complex: [Cu2(L4)2] (4). These tetranuclear complexes all feature a {Cu4O4} core, but with differing coordination environments around the CuII centers. For instance, complex 1 exhibits μ3-phenoxido bridges and a distorted octahedral geometry, while complex 3 features μ3-alkoxido bridges and a square pyramidal geometry. Complex 2 displays an open cubane core with mixed μ2-phenoxido and μ3-alkoxido bridges, with both square planar (CuNO4) and octahedral (CuNO5) geometries. In addition, dinuclear complex 4 was synthesized, featuring square planar CuII centers linked by μ2-alkoxido bridges. The magnetic studies revealed that 1 and 2 exhibit strong antiferromagnetic coupling, which is attributed to their larger Cu-O-Cu bond angles, while complex 3 demonstrates moderate ferromagnetic behavior, associated with smaller bond angles. Literature reports indicate that {Cu4O4} cubane cores generally show ferromagnetic interactions at bond angles near 104°, but in complex 3, we observed moderate ferromagnetic interactions with a Cu-O-Cu bridging angle of 108.41(9)°, making it one of the highest bond angles observed for ferromagnetic interactions in a {Cu4O4} cubane-like system. The planar dinuclear complex 4 exhibits extremely strong antiferromagnetic coupling, which is attributed to the ideal Cu-O-Cu bridging angles and the planar Cu-O-O-Cu core. Additionally, EPR measurements of complex 3 at 4 K reveal a well-isolated S = 2 ground state, separated by a gap of 65.8 cm-1 from the three closest degenerate spin levels (S = 0, 1, and 1). Finally, all four complexes were used as catalysts for the aerobic oxidation of 2-aminophenol, and the mechanism of the oxidation process was elucidated by EPR spectroscopy.

Supramolecular Dimeric MnIII Complexes: Synthesis, Structure, Magnetic Properties, and Catalytic Oxidation Studies

In this study, a tetradentate Schiff-base ligand (H2L), synthesized by the condensation of ethylenediamine with 2-hydroxy-3-methoxy-5-methylbenzaldehyde, was reacted with either manganese salts or manganese salts in the presence of various pseudohalides in methanol. This reaction resulted in the formation of five mononuclear MnIII complexes: [Mn(L)(H2O)2](NO3)·1/2H2O·1/2CH3OH (1), [Mn(L)(H2O)2](ClO4)·H2O (2), [Mn(L)(N3)(H2O)]·1/3H2O (3), [Mn(L)(NCS)(H2O)] (4), and [Mn(L)(H2O)2](dca) (5) (where dca is dicyanamide ion). X-ray crystallography revealed that the MnIII centers adopt a hexa-coordinate pseudo-octahedral geometry, where the equatorial plane is constructed with phenoxo oxygen and imine nitrogen atoms from the Schiff base ligand, while the axial positions are occupied by water molecules or a combination of water and pseudohalides. Supramolecular interactions, primarily π-π stacking and hydrogen bonding, contribute to the formation of pseudodimeric structures in the solid state. Magnetic susceptibility measurements indicated antiferromagnetic coupling within quasi-dimers, primarily through hydrogen bonds. Catalytic studies showed that the complexes effectively catalyze the aerobic oxidation of substrates such as 2-aminophenol and 3,5-di-tert-butylcatechol to yield 2-aminophenoxazin-3-one and 3,5-di-tert-butylquinone, respectively. They also catalyze the oxidation of styrene to its corresponding oxirane, demonstrating their versatile catalytic proficiency. Mechanistic insights, supported by ESI mass spectrometry and EPR studies, suggest that catalysis involves the formation of a complex-substrate aggregate, followed by an intramolecular electron transfer.

Chemical fixation of atmospheric CO2 in tricopper(II)-carbonato complexes with tetradentate N-donor ligands: reactive intermediates, probable mechanisms, and catalytic and magneto-structural studies

In the present era, the fixation of atmospheric CO2 is of significant importance and plays a crucial role in maintaining the balance of carbon and energy flow within ecosystems. Generally, CO2 fixation is carried out by autotrophic organisms; however, the scientific community has paid substantial attention to execute this process in laboratory. In this report, we synthesized two carbonato-bridged trinuclear copper(II) complexes, [Cu3(L1)3(μ3-CO3)](ClO4)3 (1) and [Cu3(L2)3(μ3-CO3)](ClO4)3 (2) via atmospheric fixation of CO2 starting with Cu(ClO4)2·6H2O and easily accessible pyridine/pyrazine-based N4 donor Schiff base ligands L1 and L2, respectively. Under very similar reaction conditions, the ligand framework embedded with the phenolate moiety (HL3) fails to do so because of the reduction of the Lewis acidity of the metal center, inhibiting the formation of a reactive hydroxide bound copper(II) species, which is required for the fixation of atmospheric CO2. X-ray crystal structures display that carbonate-oxygen atoms bridge three copper(II) centers in μ3syn-anti disposition in 1 and 2, whereas [Cu(HL3)(ClO4)] (3) is a mononuclear complex. Interestingly, we also isolated an important intermediate of atmospheric CO2 fixation and structurally characterized it as an anti-anti μ2 carbonato-bridged dinuclear copper(II) complex, [Cu2(L2)2(μ2-CO3)](ClO4)2·MeOH (2-I), providing an in-depth understanding of CO2 fixation in these systems. Variable temperature magnetic susceptibility measurement suggests ferromagnetic interactions between the metal centers in both 1 and 2, and the results have been further supported by DFT calculations. The catalytic efficiency of our synthesized complexes 1-3 was checked by means of catechol oxidase and phenoxazinone synthase-like activities. While complexes 1 and 2 showed oxidase-like activity for aerobic oxidation of o-aminophenol and 3,5-di-tert-butylcatechol, complex 3 was found to be feebly active. ESI mass spectrometry revealed that the oxidation reaction proceeds through the formation of complex-substrate intermediations and was further substantiated by DFT calculations. Moreover, active catalysts 1 and 2 were effectively utilized for the base-free oxidation of benzylic alcohols in the presence of air as a green and sustainable oxidant and catalytic amount of TEMPO in acetonitrile. Various substituted benzylic alcohols smoothly converted to their corresponding aldehydes under very mild conditions and ambient temperature. The present catalytic protocol showcases its environmental sustainability by producing minimal waste.

Structure-directing role of CH⋯X (X = C, N, S, Cl) interactions in three ionic cobalt complexes: X-ray investigation and DFT study using QTAIM Vr predictor to eliminate the effect of pure Coulombic forces

Three cobalt complexes, namely [CoIII(HL1)2(N3)2]ClO4 (1), [CoIII(L2)(HL2)(N3)]ClO4·1.5H2O (2), and [CoIII(L3)(HL3)(NCS)]2 [CoIICl2(NCS)2] (3), where HL1 = 2-(3-(dimethylamino)propyliminomethyl)-6-methoxyphenol, HL2 = 2-(2-(dimethylamino)ethyliminomethyl)-4,6-dichlorophenol, and HL3 = 2-(2-(dimethylamino)ethyliminomethyl)-6-methoxyphenol, as potential tridentate N2O-donor Schiff base ligands, were synthesized and characterized using elemental analysis, IR and UV-vis spectroscopy, and single-crystal X-ray diffraction studies. All three were found to be monomeric ionic complexes. Complex 1 crystallizes in the orthorhombic space group Pbcn, whereas both complexes 2 and 3 crystallize in triclinic space groups, P1̄. Further, 1 and 2 are cationic complexes of octahedral cobalt(iii) with perchlorate anions, whereas complex 3 contains a cationic part of octahedral cobalt(iii) and an anionic part of tetrahedral cobalt(ii). Hydrogen-bonding interactions involving aromatic and aliphatic CH bonds as H-bond donors and the pseudo-halide co-ligands as H-bond acceptors were established, which are important aspects governing the X-ray packing. These interactions were analyzed theoretically using the quantum theory of atoms in molecules (QTAIM) and non-covalent interaction plot (NCI plot) analyses. Moreover, energy decomposition analysis (EDA) was performed to analyze the stabilization of the complexes in terms of the electrostatic, dispersion, and correlation forces.

A facile biomimetic catalytic activity through hydrogen atom abstraction by the secondary coordination sphere in manganese(III) complexes

This paper describes the synthesis and structural characterization of four new manganese(iii) complexes (1-4) derived from N3O donor Schiff base ligands and their biomimetic catalytic activities related to catechol oxidase and phenoxazinone synthase. X-ray crystallography reveals that the Schiff bases coordinate the metal centre in a tridentate fashion, leaving the pendant tertiary amine nitrogen atom either protonated or free to balance the charge of the system, and these pendant triamines participate in strong hydrogen bonding interactions in the solid state. The hydrogen bonding ability of the pendant triamines at the second coordination sphere plays a crucial role in the substrate recognition and the stability of the complex-substrate intermediates. The effect of substitution at the phenolate ring towards the redox potential of the metal centre and the catalytic activity of these complexes has been observed. Detailed kinetic studies further disclose the deuterium kinetic isotope effect in which the transfer of the proton along the hydrogen bond from the substrates to the pendant triamine group at the secondary coordination sphere occurs at the key step in the catalytic reaction. The present reactivity nicely resembles the biochemical reactivities in the natural system in which a concerted electron and proton transfer to different species is usually observed. Remarkably, although some sort of influence of the secondary coordination sphere on catalytic activity has been reported mimicking the function of these metalloenzymes, such a direct participation of the secondary coordination sphere, particularly in modelling phenoxazinone synthase, has not been observed to date.

The first report of a tetra-azide bound mononuclear cobalt(iii) complex and its comparative biomimetic catalytic activity with tri-azide bound cobalt(iii) compounds

Three new azide-bound cobalt(iii) complexes derived from three different triamines with extensive hydrogen bonded supramolecular chain structures and the role of their structural factors in oxidative coupling of o-aminophenols have been reported.

Synthesis, structure and phenoxazinone synthase-like activity of three unprecedented alternating CoII–CoIII 1D chains

Three new one-dimensional Co^II/Co^III chains have been synthesized using N-salicylidene-l-alanine/N-salicylidene-l-phenylalanine and 2,2′-bipyridine/1,10-phenanthroline. All three complexes exhibit phenoxazinone synthase-like activity.