Metal Complexes with N-Donor Ligands: Second Edition

A wide variety of complexes containing N-donor ligands from ammonia, amines, Schiff bases or N-heterocycles have been prepared since the first complex compound of the family, which was an ammonia cobalt complex, was discovered [1]. Their enormously variable structural features and importance in chemistry, biochemistry, material science, and engineering have kept them at the center of emerging chemical research and developments [2,3,4,5,6,7,8,9,10,11,12,13,14,15]. The chemistry of coordination compounds containing N-base ligands has high variability depending on the chemical and stereochemical aspects of the central metal atoms and other factors such as the basicity, number, and arrangement of N-containing donor ligands in the ligand structure.

The properties and reactivity of the complex compounds are strongly dependent on the nature of the central ion and ligand, the coordination modes and sites, geometry and distortions, and the presence/absence of co-ligands. One of the main factors is the presence or absence of secondary intramolecular or intermolecular interactions, especially between the ligands and anionic components [3,5,12,16]. A lot of unexpected reactions, such as solid-phase redox reactions or hydrolysis reactions in aqueous solutions, may be attributed to these interactions. The solid-phase redox reactions produce nanosized mixed oxides for catalysts in various technologically important reactions [4,13,14]. The solution-phase reactions provide opportunities to prepare hardly available compounds such as ammonium salts of oxidizing anions [15].

The development of the chemistry of complexes with N-donor ligands has remained unbroken for 200 years due to the diversity of the chemical and structural properties of this kind of compound family. Intensive studies on the field of complexes with N-donor ligands are promoted by their various application fields and the catalytic properties of their decomposition/reaction products.

The six articles that form this Special Issue of Inorganics present data on the latest advances in the chemistry of N-donor ligand complexes, including ammonia, 8-arylnaphthylamines, substituted pyrazoles, benzimidazoles, Schiff bases, and pyridine dicarboxylates.

The review paper by Prof. Haiyang Gao discusses late transition metal complex catalysts derived from 8-aryxlnaphthylamines. These complexes are active catalysts of the olefin polymerization with low oxophilicity. This review focuses on Ni, Pd, and Fe-containing catalysts derived from 8-arylnaphthylamines, surveying their synthesis, structural features, and catalytic applications in olefin polymerization/co-polymerization reactions. The relationship between catalyst (ligand) structure and the catalytic performance of compounds is analyzed, highlighting how these ligand systems can influence the polymerization activity of complexes and molecular weight and polymer branching of products [17].

The Ni(II) and Cu(II) complexes of a novel Schiff base ligand ((2E,2′E)-N,N′-(2-hydroxypropane-1,3-diyl)bis[(2-hydroxyimino)propanamide]) and the coordination modes in the solid state and in aqueous medium were studied by a research group led by Prof. Craig Grimmer [18]. To study the effect of polymethylene spacers, detailed potentiometric titration studies were performed, various complex species were identified, and their stability constants were determined. The anionic pseudo-macrocyclic complexes’ dimerization was observed due to the presence of the centrosymmetric hydrogen bonding between the hydroxy group of the propane spacer and the oximato oxygen of the opposing unit, or due to the back-to-back π-stacking of the planar complexes. The ESR measurements indicated coupling of Cu-Cu paramagnetic centers in the dimers [18].

Structural and antimicrobial studies on benzimidazole and methylated benzimidazole complexes of copper methacrylate were performed by Mihaela Badea and colleagues [19]. Five benzimidazole, 2-methyl- and 5,6-dimethylbenzimidazole complexes of copper(II) methacrylate, including two dimerized complexes, were synthesized, and their structure and antimicrobial activities were studied. The mononuclear complexes exhibit a distorted octahedral geometry, whereas the binuclear ones show a paddlewheel square pyramidal structure. The methacrylate ions act as a chelate or a bridge, and all benzimidazole derivatives showed only unidentate coordination. The supramolecular networks are built by both intermolecular π–π stacking and hydrogen bond interactions. The antimicrobial assays showed inhibition of planktonic strain proliferation and adherence to inert substrates. Some complexes exhibit remarkable activity against Candida albicans and inhibit the microbial adhesion of the clinical Escherichia coli strain [19].

The preparation, structural characterization, and thermal decomposition of [(carbonato)tetraamminecobalt(III)] permanganate and its monohydrate were described by László Kótai and his group [20]. The controlled thermal decomposition is a convenient reaction route for preparing a phase-pure Co_1.5Mn_1.5O₄ spinel with 1:1 Co:Mn stoichiometry and a MnCo₂O₄-type spinel structure. The hydrated complex loses its water at 100 °C and decomposes under refluxing toluene at 110 °C into amorphous Co-Mn-oxide, ammonia, nitrogen oxides, water, and ammonium nitrate. These intermediates produced spinel oxides even at 300 and 400 °C, either with or without washing out of ammonium nitrate content. The particle size of the manganese cobalt spinel products was found to be as low as 4.0 and 5.7 nm between 300 and 500 °C, respectively, and thus these products were deemed to be potential precursors for Fischer–Tropsch catalysts [20].

The supramolecular arrangement of a multicomponent Ni(II) and a mononuclear Zn(II) compound involving pyridine dicarboxylates with ethylenediamine and 3,5-dimethylpyrazole as co-ligands, respectively, was studied experimentally and theoretically by Prof. Manjit K. Bhattacharyya and coworkers [21]. The Ni-complex was built up of five discrete complex moieties and unfolded dual encapsulation of the cationic moieties in the supramolecular host cavities. This structural feature was stabilized by anionic π, π-stacking, and various (N–H⋯O, C–H⋯O, and O–H⋯O) hydrogen bonding interactions, which were analyzed with the use of density functional theory, a non-covalent interaction plot index, molecular electrostatic potential surfaces, and quantum theory of atoms in molecule methods. The theoretical studies displayed that π-stacking or H bonds greatly tune the directionality of the Ni-complex, although the non-directional electrostatic forces are strongly dominant. The zinc complex showed an unconventional C–H⋯π(chelate ring) interaction. The energy contributions of the weak but significant non-covalent interactions were determined computationally [21].

Two isomers of a novel Ag(I) complex with a neutral pyrazole (ethyl-5-amino-1-methyl-1H-pyrazole-4-carboxylate) ligand (L) were isolated and studied by Prof. Berta Barta Holló [22]. The reaction of silver perchlorate with the pyrazole-type ligand in a molar ratio of 1:1 resulted in a bis(ligand) complex [AgL₂]ClO₄, whereas in the presence of 4-formylbenzonitrile, a monoperiodic polymer {[AgL₂]ClO₄}_n was formed. The ligand is coordinated through the basic nitrogen atom of the pyrazole ring in both forms of the complex. The monomer complex has a linear coordination environment for the silver cation, whereas the polymer has a square-planar coordination achieved with the involvement of two oxygen atoms from bridging perchlorate anions. The thermal properties of both isomers were found to be almost identical. The isomer that had a reverse-oriented pyrazole-type ligand exhibited significant antioxidant activity [22].

The papers included in this Special Issue provide new and important contributions to the modern areas of inorganic chemistry of complexes with N-donor ligands. The comprehensive review offers a broad overview of the discussed compound types.

I would like to express my sincere gratitude to all the authors who contributed to this Special Issue for sharing their high-quality, cutting-edge research results. I also thank all the reviewers for their rigorous efforts in ensuring the papers were of high quality. Additionally, I sincerely appreciate the professional support and assistance provided by the editorial team of Inorganics during the organization and publication of this Special Issue.