Imperial College London

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