Introduction
Nitrogen-donor ligand is a ligand that provides nitrogen, in which nitrogen is both electron donor atom and good coordination group. With development of the scientific research, various nitrogen-donor ligands have been developed. Nowadays, nitrogen-donor ligands are some of the most successful, versatile, and commonly used ligand classes, the synthesis and application of nitrogen-donor ligands as well as its complexes have become an attractive research area. These substances have a variety of structures and properties, and have been widely used in industrial catalysis, life science, material science and others.
Classifications
According to different classification methods, nitrogen-donor ligands can be divided into chiral nitrogen-donor ligands and achiral nitrogen-donor ligands, as well as monodentate, bidentate, tridentate and polydentate nitrogen-donor ligands. The most common and easy understanding classification way is on the basis of their structure. The details are as follows:
- Amines ligands: The general structure of amine ligand is a nitrogen atom with a lone pair of electrons. Most primary amines are good ligands and react with metal ions to yield coordination complexes. The extended arm of amines like pyrrole, azole and phenolate also are good ligands, which can aid metals used in homogenous catalysis.
- Imines and related ligands: An imine ligand refers to a ligand having a C=N structure in its structure, which are common ligands in coordination chemistry. The related ligands like oxime, hydrazone and thiosemicarbazone are also good candidates in coordination chemistry, the complexes produced by them have better stability and have broad application prospects such as catalysis and materials science.
Fig.1 The general structure of an imine
- Amido and amidate related ligands: In this class, the ligands include amido, amidate ligands, pyridinium amide, pyridinium amidate and pyrrolidone may form complexes with metals, those metal complexes in which there is negative charge on the nitrogen in M-N (M refers to metal).
- Imido and nitrosyl ligands: Both imido and nitrosyl are the group of nitrogen donor ligands usually with no pendant arms bound to the metal center. The reaction chemistry of nitrido and imido ligands is intriguing since these ligands behave as nucleophiles, electrophiles, or both. The nitrido and imido complexes have excellent reactivity as N-atom and NR group transfer agents.
Fig.2 Structure of a representative imido complex (py = pyridine, CMe3 = tert-butyl)
- Azole ligands: In this class, the ligands constitute at least five membered ring with one donor atom (one nitrogen called pyrrole) and with two donor atoms may be similar (two nitrogens together called pyrazolyl or separated by a carbon called imidazole, benzimidazole) or different (one nitrogen with other atom like oxygen called oxazole or sulphur called thiazole) and finally with three donor atoms (three nitrogens together called triazole). These above ligands are varied and have broad application.
Applications
Due to the wide variety and special properties of nitrogen-donor ligands, they are widely used in the fields of catalysis, medicine, separation science and others.
- Catalysis: Variety of nitrogen-donor ligands combine with a lot of metals provide new dimensions in catalytic homogeneous reactions from 2011 to update. For examples, amines complexes of Zn, Ru and Ni were used as catalysts for oxidation of epoxides and carbonyl containing organic compounds. Imines complexes with Co, Ni and Pd were used for oligomerization and polymerization of olefins, Cr and Mn imines complexes were used for carbon dioxide addition in styrene and asymmetric epoxidation respectively. Azoles complexes with various metals are used for polymerization, suzuki coupling and for hydrogen aminiation reactions. Substituted porphyrrins complexes are used as catalyst for oxygen evolving reaction and for oxidation of organic compounds[1].
- Medicine: Nitrogen-donor ligands combine with some metals show admirable anticancer activity that can be ascribed to the electronic interactions between the metal centre and the π-electrons in rings. For instance, Zn(II) complexes containing ligands of N as donor atoms generally exert good therapeutic effect, lower toxicity and offer few side effects, which have been proposed as antitumor agent. Thereinto, the ligands of bis-pyrazole, bipyridine, and phenanthroline derivatives are often be used in the development of anticancer agent. Furthermore, pyrazole, bipyridine, and phenanthroline derivatives possess good anticancer activities by themselves[2].
- Separation science: Nitrogen-donor ligands like triazine ligands stand out as the preeminent family of ligands for use in an actinide/lanthanide separation process and satisfy many of the criteria, which can be well used in the removal of the most long-lived radiotoxic elements from used nuclear fuel, minor actinides. This is foreseen as an essential step toward increasing the public acceptance of nuclear energy as a key component of a low-carbon energy future[3].
Fig.3 The schematic diagram of process of removal of the radiotoxic elements by nitrogen-donor ligands
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References
- Zafar M.N.; et al. Development of Some Important nitrogen donor ligands for transition metal homogeneous catalysis[J]. Russian Journal of Coordination Chemistry. 2016, 42, 225−251.
- Marina P.; et al. Zinc complexes with nitrogen donor ligands as anticancer agents[J]. Molecules. 2020, 25, 15814.
- Hudson M.J.; et al. Use of soft heterocyclic N donor ligands to separate actinides and lanthanides[J]. Inorganic Chemistry. 2013, 52(7):3414-3428.
