BLD Insights
Salen Ligands and Co-Salen Complexes
14 February 2022
2,2'-((1E,1'E)-(((1S,2S)-Cyclohexane-1,2-diyl)bis(azanylylidene))bis(methanylylidene))bis(4-(tert-butyl)phenol)
2,2'-((1E,1'E)-(((1R,2R)-Cyclohexane-1,2-diyl)bis(azanylylidene))bis(methanylylidene))bis(4-(tert-butyl)phenol)
6,6'-((1E,1'E)-(((1S,2S)-Cyclohexane-1,2-diyl)bis(azanylylidene))bis(methanylylidene))bis(4-(tert-butyl)-2-methylphenol)
[[2,2'-[(1S,2S)-1,2-Cyclohexanediylbis[(nitrilo-κN)methylidyne]]bis[4-(1,1-dimethylethyl)phenolato-κO]](2-)](nitrato-κO)cobalt
[[2,2'-[(1R,2R)-1,2-Cyclohexanediylbis[(nitrilo-κN)methylidyne]]bis[4-(1,1-dimethylethyl)phenolato-κO]](2-)](nitrato-κO)cobalt
[[2,2'-[(1S,2S)-1,2-Cyclohexanediylbis[(nitrilo-κN)methylidyne]]bis[4-(1,1-dimethylethyl)-6-methyl-phenolato-κO]](2-)](nitrato-κO)cobalt
[[2,2'-[(1R,2R)-1,2-Cyclohexanediylbis[(nitrilo-κN)methylidyne]]bis[4-(1,1-dimethylethyl)-6-methyl-phenolato-κO]](2-)](nitrato-κO)cobalt
[[2,2'-[(1S,2S)-1,2-Cyclohexanediylbis[(nitrilo-κN)methylidyne]]bis[4-bis(1,1-dimethylethyl)phenolato-κO]](2-)][tetrafluoroborato(1-)-κF]cobalt
Salen ligands are prepared from primary diamines and salicylaldehydes. They are chelating ligands and could form stable metal-Salen complexes with various metal ions. The complexes play an important role in coordination chemistry and homogeneous catalysis. It has a wide range of application prospects in chiral catalysis, biomedicine and material science because of its excellent properties: (1) The synthesis of Salen ligands is convenient and the ligands can be recovered and reused after the reaction; (2) It has strong coordination ability and can form complexes with various metal ions; (3) It is suitable for a wide range of reactions, such as asymmetric catalytic oxidation of olefins, addition reaction, polymerization reaction, cyclopropanation reaction, Diels-Alder reaction, (photocatalytic) redox reaction, electrocatalytic reaction and kinetic resolution of epoxide hydrolysis. 1-7 (4) Metal-Salen complexes exhibit molecular magnetism, anti-tumor activity and optical properties in different research programs. 8-13
Development of Salen Ligands
ased on the research of porphyrin complexes, Jacobsen and Katsuki designed a similar ligand: Mn(III)-Salen complex in 1990-1991. The complex was used as a catalyst in asymmetric catalytic reactions (Jacobsen-Katsuki Epoxidation, Fig 1).14, 15
Fig 1: Jacobsen-Katsuki Epoxidation 16
Due to the four electron-donor sites (N/O atoms) in Salen ligands, the Metal-Salen complexes usually have a very high formation constant and can use different conformations (such as square planes, tetrahedron, square pyramid, and octahedron) to stabilize the spatial structures and control the catalytic performance of metal ions. In previous studies, various metal ions (including main-group metals and transition metals) have been used in the synthesis of Metal-Salen Complexes to generate a variety of complex catalysts for asymmetric catalysis, (photocatalytic) redox catalysis and other reactions.1, 17-21(Fig 2)
Fig 2: Metal-Salen complex catalysts2
Metal-Salen complexes with higher stereoselectivity and conversion have been continuously developed depending on the adjustment of (1) the connecting chain of diamine & (2) asymmetric substitution of salicylaldehyde derivatives.
Hotspot in Research & Application: Co(II/III)-Salen complex
Currently, catalyst systems mostly rely on precious metals such as ruthenium, rhodium, iridium, and platinum. However, the applications of these precious metal catalyst systems in industrial production are limited by their high toxicity, lack of reserves, and high price. Therefore, the development of catalytic systems based on cheap metals such as manganese, iron, cobalt, and nickel has become a research hotspot in recent years. The environmentally friendly and inexpensive transition metal cobalt has attracted extensive attention due to its high catalytic activity and stereoselectivity. Hence, Co-Salen complexes are constantly being developed and widely used. They are usually used in oxidation, polymerization and asymmetric catalysis. 13, 18, 22-26 (Fig 3A-C)
Fig 3A: Oxidations catalyzed by Co-Salen complexes
Fig 3B: Polymerizations catalyzed by Co-Salen complexes
Fig 3C: asymmetric catalysis & other applications of Metal-Salen complexes
The research projects on Co-Salen complexes in biosensors, material applications (MOF/magnetic materials), tumor treatment and other fields are also being carried out accordingly. (Fig 4) 13, 22, 27-30
Fig 4: Applications of Co-Salen complexes
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