A square planar metal–salen complex. The M denotes the metal atom; R and R′ denote precursor ingredients.
The metal-free salen compound (H2salen or salenH2) has two phenolichydroxyl groups. The salen ligand is usually its conjugate base (salen2−), resulting from the loss of protons from those hydroxyl groups. The metal atom usually makes four coordination bonds to the oxygen and nitrogen atoms.
Preparation of complexes
The salen anion forms complexes with most transition metals. These complexes are usually prepared by the reaction of H2salen ("proligand") with metal precursors containing built-in bases, such as alkoxides, metal amides, or metal acetate. The proligand may also be treated with a metal halide, with or without an added base. Lastly, the proligand may be deprotonated by a nonnucleophilic base, such as sodium hydride, before treatment with the metal halide. For example, Jacobsen's catalyst is prepared from the salen ligand precursor with manganese acetate.[4]
Other metal–salen complexes may have additional ligands above the salen nitrogen–oxygen plane. Complexes with one extra ligand, such as VO(salen), may have a square pyramidal molecular geometry. Complexes with two extra ligands, such as Co(salen)Cl(py), may have octahedral geometry. Usually the MN2O2 core is relatively planar, even though the ethylene backbone is skewed and the overall salen ligand takes a twisted C2symmetry. Examples exist where ancillary ligands force the N2O2 donors out of planarity.[6] No evidence indicates that salen is a redox-noninnocent ligand.
Reactions
Structure of Co(salen)(CH2CHMe2)(4-picoline), a mimic of the organocobalt center in vitamin B12.[7]
Enzyme mimics
Tsumaki described the first metal–salen complexes in 1938. He found that the cobalt(II) complex Co(salen) reversibly binds O2, which led to intensive research on cobalt complexes of salen and related ligands for their capacity for oxygen storage and transport, looking for potential synthetic oxygen carriers.[1] Cobalt salen complexes also replicate certain aspects of vitamin B12.
The presence of bulky groups adjacent to the phenoxide group can give complexes with enhanced catalytic activity. These substituents suppress formation of dimers. For these reasons, salen ligands derived from 3,5-di-tert-butylsalicylaldehyde have received particular scrutiny.
"Salen-type" metal complexes are formed with ligands with similar chelating groups, such as acacen, salph, and salqu. Salqu copper complexes have been investigated as oxidation catalysts.[12]
Complexes with salan ligands
Complexes with the similar salan or salalen ligands, with one or two saturated nitrogen–aryl bonds (amines rather than imines) tend to be less rigid and more electron-rich at the metal center than the corresponding salen complexes.[13][14]
Further reading
Hazra, S.; Mohanta, S. (2019). "Metal–tin derivatives of compartmental Schiff Bases: Synthesis, structure and application". Coordination Chemistry Reviews. 395:1-24. https://doi.org/10.1016/j.ccr.2019.05.013
McGarrigle, Eoghan M.; Gilheany, Declan G. (2005). "Chromium− and Manganese−salen Promoted Epoxidation of Alkenes". Chemical Reviews. 105 (5): 1563–1602. doi:10.1021/cr0306945. PMID15884784.
Bandini, Marco; Cozzi, Pier Giorgio; Umani-Ronchi, Achille (2002). "[Cr(Salen)] as a 'bridge' between asymmetric catalysis, Lewis acids and redox processes". Chemical Communications (9): 919–927. doi:10.1039/b109945k. PMID12123051.
^Baleizão, Carlos; Garcia, Hermenegildo (2006). "Chiral Salen Complexes: An Overview to Recoverable and Reusable Homogeneous and Heterogeneous Catalysts". Chemical Reviews. 106 (9): 3987–4043. doi:10.1021/cr050973n. PMID16967927.
^Decortes, Antonello; Castilla, Ana M.; Kleij, Arjan W. (2010). "Salen-Complex-Mediated Formation of Cyclic Carbonates by Cycloaddition of CO2 to Epoxides". Angewandte Chemie International Edition. 49 (51): 9822–9837. doi:10.1002/anie.201002087. PMID20957709.
^Coggon, P.; McPhail, A. T.; Mabbs, F. E.; Richards, A.; Thornley, A. S. (1970). "Preparation, Magnetic, and Electronic Spectral Properties of Some Chromium(III)–NN′-Ethylenebis(salicylideneiminato) Complexes: Crystal and Molecular Structure of N,N′-Ethylenebis(salicylideneiminato)diaquochromium(III) Chloride". J. Chem. Soc. A: 3296–3303. doi:10.1039/j19700003296.
^Lauffer, Randall B.; Heistand, Robert H.; Que, Lawrence (1983). "Dioxygenase models. Crystal Structures of the 2,4-Pentanedionato, Phenanthrenesemiquinone, and Catecholato Complexes of N,N′-Ethylenebis(salicylideneaminato)iron(III)". Inorganic Chemistry. 22: 50–55. doi:10.1021/ic00143a013.
^Huilan, Chen; Deyan, Han; Tian, Li; Hong, Yan; Wenxia, Tang; Jian; Peiju; Chenggang (1996). "Synthesis and Crystal Structure of Organocobalt(III) Complexes with Secondary Alkyls or Bulky Schiff Base Equatorial Ligands". Inorganic Chemistry. 35 (6): 1502–1508. doi:10.1021/ic940516h. PMID11666365.
^Makoto Tokunaga; Jay F. Larrow; Fumitoshi Kakiuchi; Eric N. Jacobsen (1997). "Asymmetric Catalysis with Water: Efficient Kinetic Resolution of Terminal Epoxides by Means of Catalytic Hydrolysis". Science. 277 (5328): 936–938. doi:10.1126/science.277.5328.936. PMID9252321. S2CID23745844.
^D. J. Darensbourg (2007). "Making Plastics from Carbon Dioxide: Salen Metal Complexes as Catalysts for the Production of Polycarbonates from Epoxides and CO2". Chemical Reviews. 107 (6): 2388–2410. doi:10.1021/cr068363q. PMID17447821.
^Wu, Xianghong; Gorden, A. V. E. (2009). "2-Quinoxalinol Salen Copper Complexes for Oxidation of Aryl Methylenes". Eur. J. Org. Chem.2009 (4): 503–509. doi:10.1002/ejoc.200800928.
^Atwood, David A.; Remington, Michael P.; Rutherford, Drew (1996). "Use of the Salan Ligands to Form Bimetallic Aluminum Complexes". Organometallics. 15 (22): 4763. doi:10.1021/om960505r.
^Berkessel, Albrecht; Brandenburg, Marc; Leitterstorf, Eva; Frey, Julia; Lex, Johann; Schäfer, Mathias (2007). "A Practical and Versatile Access to Dihydrosalen (Salalen) Ligands: Highly Enantioselective Titanium. In Situ Catalysts for Asymmetric Epoxidation with Aqueous Hydrogen Peroxide". Adv. Synth. Catal.349 (14–15): 2385. doi:10.1002/adsc.200700221.