The oxygen-donor ligands refer to the ligands like alcohol, phenolic, ketone, aldehyde that contains oxygen-containing groups such as carbonyl group, hydroxyl group and ether bond in their structure. The oxygen atoms in carbonyl group can break the double bond and then combine metal ion to form coordination bond under certain conditions, and hydroxyl group can remove proton to attract metal cation to form stable structure under certain conditions, thus oxygen-donor ligands can form a variety of coordination compounds. Different coordination compounds have different physicochemical properties, making them widely used in many fields, which has attracted plenty of attention.
Oxygen-donor ligands are generally divided into chiral oxygen-donor ligands and achiral oxygen-donor ligands. In addition, according to their structures and functional groups, the most widely used oxygen-donor ligands are as follows.
- Diketone ligands: Diketone ligands refer to a ketone ligand that has a β-diketone structure (1,3-diketone structure) in its structure, which are among the most widely used ligands in co-ordination chemistry and has been used in co-ordination chemistry for more than 100 years. The diketone compounds can exist in solution as well as in solid as keto and enol tautomers, usually, diketone ligands coordinate in the form of a two-tooth chelation with metal ions to form a stable six membered chelating ring because the enolic hydrogen is labile.
Fig.1 The coordination mode of diketone ligands with the metal ions
- BINOL ligands: BINOL is an acronym for binaphthol, which is an organic compound that often used as a ligand for transition-metal catalysed asymmetric synthesis such as asymmetric cyanophosphorylation, cyanosilylation, cyanobenzylation, and cyanoformylation of aldehydes, enantioselective C-alkylation of Schiff bases, and asymmetric Michael addition. Nowadays, BINOL ligands and their extensive derivatives have generated particular interest because their versatile backbone can be modified, thereby affecting the reaction environment by influencing the properties of the metal center.
Fig.2 The general structural formula of BINOL ligands
- Crown ligands: Crown ethers are cyclic chemical compounds that consist of a ring containing several ether groups. Crown ethers as ligand can strongly bind certain cations, forming complexes. In crown ligands, the oxygen atoms are well situated to coordinate with a cation located at the interior of the ring, whereas the exterior of the ring is hydrophobic. The resulting cations often form salts that are soluble in nonpolar solvents, and for this reason crown ethers are useful in phase transfer catalysis.
Fig.3 Some representative crown ligands
- Others: BIPHEN and TADDOL are acronym for 3,3'-di-tert-butyl-5,5',6,6' -tetramethylbiphenyl-2,20-diol and α,α,α',α'-tetraaryl-2,2-disubstituted1,3- dioxolane-4,5-dimethanol, respectively. They and their derivatives are the other two more common oxygen-donor ligands.
Oxygen-donor ligands can form complexes with various metals such as transition metals, alkaline earth metals and rare earth metals, which are widely used in catalysis, luminescence and medical fields.
- Catalysis: Complexes by synthesis of oxygen-donor ligands are extensively applied in homogeneous and asymmetric catalysis. The homogeneous catalysis involve the organic transformations such as oxidation, deoxygenation, carbon-carbon and carbon-heteroatom bond forming reactions. Complexes containing BINOL ligands are mainly used in asymmetric catalysis involved two kinds of reaction, carbon-carbon bond-forming reactions (Mannich-type reaction, Diels−Alder reaction, Friedel−Crafts reaction, Michael addition reaction, cyanation of aldehydes, allylation of aldehydes, olefin metathesis, aldol reaction and others) and asymmetric heteroatom-transfer reactions (ring opening of epoxides, 1,3-dipolar cycloaddition reactions, Pudovik reaction, Baeyer−Villiger reaction, oxidation of sulfides to sulfoxides and so on).
- Luminescence: Fascinating features of lanthanide ions have encouraged the exploitation of their coordination complexes in distinguished area as in OLEDs, and luminescent sensors, unfortunately, they suffer in weak luminescence intensity. However, diketone ligands give lanthanide complexes with more enhanced luminescence characteristics such as noticeable emission properties and efficient energy transfer. Because diketone ligands not only substituted the solvent molecules in coordination sphere but also indirectly sensitize metal ion through transfer of energy to emissive level of respective Ln3+ ion from triplet state of ligand, thus enhancing the features of complexes in the luminescence intensity.
Fig.4 Luminescence properties of some coordination complexes
- Medicine: Metal based drugs like cisplatin have gained popularity against a wide range of cancer cell. At present, the oxygen-donor ligands can combine with copper and platinum form complexes, which are used in the development of anticancer drugs. For example, copper(II) complexes was synthesized by the mixed ligand of oxygen and nitrogen donor ligands, in which, the ligands strongly bind and cleave DNA and could exhibit pronounced anticancer activity and regulate apoptosis.
Fig.5 Schematic of anticancer drug of coordination complexes based on oxygen-donor
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- Devender S.; et al. Synthesis and investigation of enhanced luminescence of Ln(III)-complexes containing fluorinated β-diketone and oxygen donor ancillary ligands for efficient advanced displays[J]. Journal of Luminescence. 2020, 223, 117255.
- Surbhi J.; et al. Natesan S. Bio-affinity of copper(II) complexes with nitrogen and oxygen donor ligands: Synthesis, structural studies and in vitro DNA and HSA interaction of copper(II) complexes[J].Journal of Photochemistry & Photobiology, B: Biology. 2017, 174, 35-43.