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Metal salen complex

May 04, 2024

A metal salen complex is a coordination compound between a metal cation and a ligand derived from N,N′-bis(salicylidene)ethylenediamine, commonly called salen. The classical example is salcomine, the complex with divalent cobalt Co2+, usually denoted as Co(salen).[1] These complexes are widely investigated as catalysts and enzyme mimics.[2][3]

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 phenolic hydroxyl 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 edit

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]

Structures edit

 
Structure of [Cr(salen)(H2O)2]+.[5]

Salen complexes with d8 metal ions, such as Ni(salen), typically have a low-spin square planar molecular geometry in the coordination sphere.

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 C2 symmetry. Examples exist where ancillary ligands force the N2O2 donors out of planarity.[6] No evidence indicates that salen is a redox-noninnocent ligand.

Reactions edit

 
Structure of Co(salen)(CH2CHMe2)(4-picoline), a mimic of the organocobalt center in vitamin B12.[7]

Enzyme mimics edit

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.

Homogeneous catalysis edit

The manganese-containing salen complex catalyzes the asymmetric epoxidation of alkenes. In the hydrolytic kinetic resolution technique, a racemic mixture of epoxides may be separated by selectively hydrolyzing one enantiomer, catalyzed by the analogous cobalt(III) complex.[8] In subsequent work, chromium(III) and cobalt(III) salen complexes catalyze the reaction of carbon dioxide and epoxides to give polycarbonates.[9]

Related complexes edit

Substituted salen complexes edit

 
Salpn, a substituted salen ligand

Complexes of salen per se are poorly soluble in organic solvents. Substitution of the organic framework increases the solubility of the complex. An example is the salpn ligand, derived from 1,2-diaminopropane instead of ethylenediamine, which is used as a metal deactivating additive in motor oils and motor fuel.[10]

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.

Chirality may be introduced into the ligand either via the diamine backbone, via the phenyl ring, or both.[11] 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, which is catalyzed by a chiral manganese-salen complex:[4]

 

Complexes with salen-type ligands edit

"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 edit

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 edit

  • 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. PMID 15884784.
  • 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. PMID 12123051.

References edit

  1. ^ a b Tsumaki, T. (1938). "Nebenvalenzringverbindungen. IV. Über einige innerkomplexe Kobaltsalze der Oxyaldimine". Bulletin of the Chemical Society of Japan (in German). 13 (2): 252–260. doi:10.1246/bcsj.13.252.
  2. ^ 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. PMID 16967927.
  3. ^ 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. PMID 20957709.
  4. ^ a b 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". Organic Syntheses; Collected Volumes, vol. 10, p. 96.
  5. ^ 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.
  6. ^ 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.
  7. ^ 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. PMID 11666365.
  8. ^ 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. S2CID 23745844.
  9. ^ 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.
  10. ^ Dabelstein, W.; Reglitzky A.; Schutze A.; Reders, K. "Automotive Fuels". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a16_719.pub2. ISBN 978-3527306732.
  11. ^ 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.
  12. ^ 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.
  13. ^ 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.
  14. ^ 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.

May 04, 2024
metal, salen, complex, been, suggested, that, this, article, merged, with, salen, ligand, discuss, proposed, since, december, 2023, metal, salen, complex, coordination, compound, between, metal, cation, ligand, derived, from, salicylidene, ethylenediamine, com. It has been suggested that this article be merged with Salen ligand Discuss Proposed since December 2023 A metal salen complex is a coordination compound between a metal cation and a ligand derived from N N bis salicylidene ethylenediamine commonly called salen The classical example is salcomine the complex with divalent cobalt Co2 usually denoted as Co salen 1 These complexes are widely investigated as catalysts and enzyme mimics 2 3 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 phenolic hydroxyl 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 Contents 1 Preparation of complexes 1 1 Structures 2 Reactions 2 1 Enzyme mimics 2 2 Homogeneous catalysis 3 Related complexes 3 1 Substituted salen complexes 3 2 Complexes with salen type ligands 3 3 Complexes with salan ligands 4 Further reading 5 ReferencesPreparation of complexes editThe 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 Structures edit nbsp Structure of Cr salen H2O 2 5 Salen complexes with d8 metal ions such as Ni salen typically have a low spin square planar molecular geometry in the coordination sphere 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 C2 symmetry Examples exist where ancillary ligands force the N2O2 donors out of planarity 6 No evidence indicates that salen is a redox noninnocent ligand Reactions edit nbsp Structure of Co salen CH2CHMe2 4 picoline a mimic of the organocobalt center in vitamin B12 7 Enzyme mimics edit 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 Homogeneous catalysis edit The manganese containing salen complex catalyzes the asymmetric epoxidation of alkenes In the hydrolytic kinetic resolution technique a racemic mixture of epoxides may be separated by selectively hydrolyzing one enantiomer catalyzed by the analogous cobalt III complex 8 In subsequent work chromium III and cobalt III salen complexes catalyze the reaction of carbon dioxide and epoxides to give polycarbonates 9 Related complexes editSubstituted salen complexes edit nbsp Salpn a substituted salen ligand Complexes of salen per se are poorly soluble in organic solvents Substitution of the organic framework increases the solubility of the complex An example is the salpn ligand derived from 1 2 diaminopropane instead of ethylenediamine which is used as a metal deactivating additive in motor oils and motor fuel 10 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 Chirality may be introduced into the ligand either via the diamine backbone via the phenyl ring or both 11 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 which is catalyzed by a chiral manganese salen complex 4 nbsp Complexes with salen type ligands edit 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 edit 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 editHazra 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 PMID 15884784 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 PMID 12123051 References edit a b Tsumaki T 1938 Nebenvalenzringverbindungen IV Uber einige innerkomplexe Kobaltsalze der Oxyaldimine Bulletin of the Chemical Society of Japan in German 13 2 252 260 doi 10 1246 bcsj 13 252 Baleizao 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 PMID 16967927 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 PMID 20957709 a b 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 Organic Syntheses Collected Volumes vol 10 p 96 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 PMID 11666365 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 S2CID 23745844 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 Dabelstein W Reglitzky A Schutze A Reders K Automotive Fuels Ullmann s Encyclopedia of Industrial Chemistry Weinheim Wiley VCH doi 10 1002 14356007 a16 719 pub2 ISBN 978 3527306732 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 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 Schafer 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 Retrieved from https en wikipedia org w index php title Metal salen complex amp oldid 1199207093, wikipedia, wiki, book, books, library,

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