Metal salen complexes

Salen are a class of organic compounds used as ligands in coordination chemistry. This class of compounds are named after the simplest example, N,N'-bis(salicylidene)ethylenediamine, more commonly referred to as salen. The ligands consist of phenol and Schiff base (aryl-imine groups.

Preparation of the ligands

Salen ligands are prepared by the condensation of a salicylaldehyde with an amine. For example, the preparation of salen:^[1]

The reactants are in principle in equilibrium with the product, and water may be removed via an added drying agent or by azeotropic distillation.^[2] In practice, the reaction of the salicylaldehyde with the amine in alcoholic solvent usually goes to completion.

Preparation of complexes

A square planar M(salen) complex.

Free salen ligand is often written as SalenH₂ to emphasize its diprotic character. Its conjugate base forms complexes with most transition metals. These complexes are usually prepared by the reaction of the diprotic pro-ligand with metal precursors containing built-in bases, such as alkoxides, metal amides, or metal acetate. The pro-ligand may also be treated with a metal halide, with or without an added base. Lastly, the pro-ligand may be deprotonated by a non-nucleophilic base, e.g. sodium hydride, before treatment with the metal halide. Jacobsen's catalyst is prepared from the salen ligand precursor with manganese acetate:^[3]

Structures

In many cases, a square pyramidal complex with composition M(salen)L or an octahedral coordination sphere with stoichiometry M(salen)L₂ is formed. Illustrative examples include VO(salen) and Co(salen)Cl(py). With d⁸ metal ions, low-spin square planar complexes form, such as Ni(salen).

Reactions

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.^[4] Cobalt salen complexes also replicate certain aspects of vitamin B12.

Homogeneous catalysis

The manganese-containing catalyst is used for the asymmetric epoxidation of olefins. In the hydrolytic kinetic resolution technique, racemic mixture of epoxides may be separated by selectively hydrolyzing one enantiomer, catalyzed by the analogous cobalt(III) complex.^[5] In subsequent work, chromium(III) and cobalt(III) salen complexes were found to be good catalysts for preparing poly(carbonates) from carbon dioxide and epoxides.^[6]

Ligand design

Unsubstituted salen complexes are poorly soluble in organic solvent. The presence of bulky groups near the coordination site is generally desirable, as it enhances catalytic activity and prevents dimerization. Salen ligands derived from 3,5-di-tert-butylsalicylaldehyde are popular because they fulfill both criteria; such complexes tend to be soluble even in non-polar solvents like pentane.

Chirality may be introduced into the ligand either via the diamine backbone, via the phenyl ring, or both.^[2] For example, condensation of the C₂-symmetric trans-1,2-diaminocyclohexane with 3,5-di-tert-butylsalicylaldehdye gives a ligand that forms complexes with Cr, Mn, Co, Al, which have proven useful for asymmetric transformations. For an example, see the Jacobsen epoxidation.^[3]

Salen-type ligands

Numerous types of salen are derived from salicylaldehyde or the diamine.

• The ligand acacen (parent: H₂acacen) is derived by condensation of acetylacetone and ethylenediamine.
• The ligand abbreviated "Salph" is derived from the condensation of 1,2-phenylenediamine and salicyaldehyde.
• The ligand "Salqu," derived by condensation of salicylaldehyde and 2-quinoxalinol, is an anionic tetradendate ligand, reminiscent of other macrocyclic ligands. Salqu copper complexes have been investigated as oxidation catalysts.^[7]

Salpn, prepared from 1,2-diaminopropane, is a typical metal deactivator additive in fuels.^[8]

Salen-like ligands and complexes

The similar salan ligands^[9] are saturated at the nitrogen; these compounds are amines rather than imines. Salalens are intermediate between salan and salens: one side of the compound bears an imine, while the other side bears a free amine.^[10] These complexes tend to be less rigid and more electron rich at the metal center than the corresponding salen complexes. Salan pro-ligands may be synthesized by reduction of salen pro-ligands. They may also be synthesized by the alkylation of an appropriate amine with a phenolic alkyl halide. Like the name suggests, half-salen compounds are not symmetrical. They are prepared from a salicylaldehyde and a monoamine.

References

1. Harvey Diehl; Clifford C. Hach (1950). "Bis(N,N'-Disalicylalethylenediamine)-μ-Aquodicobalt(II)". Inorg. Synth. Inorganic Syntheses. 3: 196–201. doi:10.1002/9780470132340.ch53. ISBN 978-0-470-13234-0.
2. Cozzi, Pier Giorgio (2004). "Metal-Salen Schiff base complexes in catalysis: Practical aspects". Chem. Soc. Rev. 33 (7): 410–21. doi:10.1039/B307853C. PMID 15354222.
3. Larrow, J. F.; Jacobsen, E. N. (2004). "(R,R)-N,N'-Bis(3,5-Di-tert-Butylsalicylidene)-1,2-Cyclohexanediamino Manganese(III) Chloride, A Highly Enantioselective Epoxidation Catalyst". Org. Synth.; Coll. Vol., 10, p. 96
4. Tsumaki, T. (1938). "Nebenvalenzringverbindungen. IV. Über einige innerkomplexe Kobaltsalze der Oxyaldimine". Bull. Chem. Soc. Jap. (in German). 13 (2): 252–260. doi:10.1246/bcsj.13.252.
5. 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. PMID 9252321.
6. 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. PMID 17447821.
7. Wu, Xianghong, Gorden, A. V. E., (2009). "2-Quinoxalinol Salen Copper Complexes for Oxidation of Aryl Methylenes". Eur. J. Org. Chem. 4 (4): 503–509. doi:10.1002/ejoc.200800928.
8. Dabelstein, W.; Reglitzky A.; Schutze A.; Reders, K. (2005), "Automotive Fuels", Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH, doi:10.1002/14356007.a16_719.pub2
9. 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.
10. Berkessel, Albrecht; Brandenburg, Marc; Leitterstorf, Eva; Frey, Julia; Lex, Johann; Schäfer, Mathias (2007). "A Practical and Versatile Access to Dihydrosalen (Salalen) Ligands: Highly Enantioselective TitaniumIn Situ Catalysts for Asymmetric Epoxidation with Aqueous Hydrogen Peroxide". Adv. Synth. Catal. 349 (14–15): 2385. doi:10.1002/adsc.200700221.