1 / 15

Imperial College London

Imperial College London. 3I3 Advanced Organometallics. Lectures 1 - 4. Dr . Ed Marshall, M220, RCS 1 e.marshall@imperial.ac.uk Additional materials available on: www.ch.ic.ac.uk/marshall/3I3.html Lecture notes also available on Blackboard. 3I3 Slide 1. Imperial College London.

twyla
Télécharger la présentation

Imperial College London

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Imperial College London 3I3 Advanced Organometallics Lectures 1 - 4 Dr. Ed Marshall, M220, RCS 1 e.marshall@imperial.ac.uk Additional materials available on: www.ch.ic.ac.uk/marshall/3I3.html Lecture notes also available on Blackboard 3I3 Slide 1

  2. Imperial College London A question for you What properties do you think are desirable for a catalyst? • Cheap, robust and long-lived • Low toxicity • Lewis acidic metal centre – electronic unsaturation • At least one vacant coordination site – coordinative unsaturation • Variable oxidation states? • Flexible metal-based frontier orbitals (energy, direction) L Large ligands (L) are often used to give coordinative (and electronic) unsaturation. If L bonds to M using a flexible mixture of orbitals, then M may also use a mixture of orbitals to bind to a substrate. X M 3I3-2

  3. Imperial College London The next four lectures Alkene and polyene ligands Bonding, synthesis & reactivity Alkene polymerisation Metal-carbon multiple bonds Olefin metathesis 3I3-3

  4. Imperial College London Learning objectives By the end of lecture 4, you should be able... Use simple MO theory to explain how a carbon-carbon p-cloud bonds to a metal. To list methods used to synthesise metal complexes of alkenes and polyenes, and metal-carbon multiple bonds. To describe typical reactions of these complexes. To appreciate how polyene ligands may respond to the electronic needs of a metal, and how such a property is useful for catalysis. To describe how cyclopentadienyl-based catalysts can be used to polymerise alkenes. To outline the most important applications of olefin metathesis. 3I3-4

  5. Imperial College London Assumed Knowledge Assumed knowledge • In order to get the most out of this course, it is worth making sure that you understand the following concepts… • Crystal field theory versus molecular orbital theory • LX ligand classifications • How to count electrons and the 18 electron rule • Metal-alkene bonding 3I3-5

  6. Imperial College London 3A Advanced Organometallics 3I1 Advanced Organometallics Section 1: Metal-alkene complexes 3I3-6

  7. Imperial College London The Dewar-Chatt-Duncanson Model of Metal-Alkene Bonding Dewar-Chatt-Duncanson model for metal-alkene bonding s-component: C-C p → empty metal orbital p-component: occupied metal d → emptyC-C p* Note the similarity to CO ligands... s-component: donation of C lone pair p-component: backbonding into CO p* 3I3-7

  8. Imperial College London Best Described as Metal-Alkenes or Metallacyclopropanes? Metal-alkenes versus metallacyclopropanes C-C bond distance in ethene= 1.34 Å H atoms no longer planar with the C-C bond C-C = 1.37 Å C-C = 1.43 Å C-C = 1.49 Å C-C = 1.62 Å 3I3-8

  9. [Pt(C2H4)Cl3]2- versus [Pt(C2Cl4)(PPh3)2]

  10. Imperial College London The Concept Of Umpolung - Reversal Of Polarity The impact of metal coordination and backbonding on reactivity 1. Free alkenes undergo electrophilic additions, but coordinated alkene ligands are susceptible to nucleophilic attack No backbonding: “metal-alkene" sp2 carbons d+ With backbonding: “metallacyclopropane" sp3 carbons 2. Backbondingreduces d+ charge and reduces reactivity to nucleophiles Backbonding occurs to the p* antibonding orbital, therefore reducing the C-C bond order Why sp3? 3I3-10

  11. Imperial College London Appendix: Synthesis & Reactivity of PolyeneLIgands Synthesis of metal-alkene complexes Two common methods: 1. Addition to electron poor metal centres / displacement of other L-ligands 16e- 18e- 2. Reduction of a metal complex in the presence of the neutral -ene ligand Oxidation state: N Oxidation state: N-2 3I3-11

  12. Imperial College London Synthesis Of Metal-Alkene Complexes Synthesis of metal-alkene complexes: examples • 1 (a) Addition to 16 electron species: • e.g. [Ir(CO)Cl(PPh3)2] + C60 18 e- 16 e- [Ir(CO)Cl(PPh3)2C60] 1 (b) Displacement of other L-type ligands: e.g. (h5-C5H5)2Zr(PMe3)2+ C2H4 18 e- 18 e- (h5-C5H5)2Zr(C2H4)(PMe3) 3I3-12

  13. Imperial College London Synthesis Of Metal-Alkene Complexes Synthesis of metal-alkene complexes 2. Reduction of a metal in the presence of an alkene e.g. RhCl3+ CH3CH2OH + CH3CHO Rh(III) Rh(I) [(nbd)Rh(m-Cl)]2 nbd = norbornadiene C2H4 e.g. (h5-C5H5)2TiCl2 + 2Na Ti(IV) Ti(II) (h5-C5H5)2Ti(C2H4) 3I3-13

  14. Imperial College London Reactivity of metal-alkene complexes Alkene ligands are often susceptible to nucleophilic attack 3I3-14

  15. Imperial College London Summary of section 1 Catalysis at a metal centre often requires a responsive metal (and therefore a responsive ligand set) Binding an alkene to a metal often increases its susceptibility to nucleophilic attack Most useful ligands are often those that can use different MOs to bind to a metal Binding any organic fragment to a metal may activate it towards chemical modification 3I3-15

More Related