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Imperial College London 3I3 Advanced Organometallics Lectures 1 - 4 Dr. Ed Marshall, M220, RCS 1 [email protected] Additional materials available on: www.ch.ic.ac.uk/marshall/3I3.html Lecture notes also available on Blackboard 3I3 Slide 1 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) Large ligands (L) are often used to give coordinative (and electronic) unsaturation. IfXL bonds to M using a flexible L then M M may also use a mixture of orbitals mixture of orbitals, to bind to a substrate. 3I3-2 Imperial College London The next four lectures Alkene and polyene ligands Bonding, synthesis & reactivity Alkene polymerisation Metal-carbon multiple bonds Olefin metathesis 3I3-3 Learning objectives Imperial College London By the end of lecture 4, you should be able... 1. Use simple MO theory to explain how a carbon-carbon p-cloud bonds to a metal. 2. To list methods used to synthesise metal complexes of alkenes and polyenes, and metal-carbon multiple bonds. 3. To describe typical reactions of these complexes. 4. To appreciate how polyene ligands may respond to the electronic needs of a metal, and how such a property is useful for catalysis. 5. To describe how cyclopentadienyl-based catalysts can be used to polymerise alkenes. 6. To outline the most important applications of olefin metathesis. 3I3-4 Assumed knowledge Knowledge Imperial College London 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 Imperial College London 3A Advanced 3I1 Advanced Organometallics Organometallics Section 1: Metal-alkene complexes 3I3-6 The Dewar-Chatt-Duncanson Dewar-Chatt-Duncanson model Model for metal-alkene of Metal-Alkene bonding Bonding s-component: C-C p → empty metal orbital Imperial College London p-component: occupied metal d → empty C-C p* Note the similarity to CO ligands... s-component: donation of C lone pair p-component: backbonding into CO p* 3I3-7 Best Describedversus Metal-alkenes as Metal-Alkenes metallacyclopropanes or Metallacyclopropanes? Imperial College London 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 = 1.49 Å C-C = 1.43 Å C-C = 1.62 Å 3I3-8 [Pt(C2H4)Cl3]2- « Chem3D Embed » versus [Pt(C2Cl4)(PPh3)2] « Chem3D Embed » The impact ConceptofOfmetal Umpolung coordination - Reversal andOf backbonding Polarity on reactivity 1. Free alkenes undergo electrophilic additions, but coordinated alkene ligands are susceptible to nucleophilic attack 2. Backbonding reduces d+ charge and reduces reactivity to nucleophiles Why sp3? d+ Imperial College London No backbonding: “metal-alkene" sp2 carbons With backbonding: “metallacyclopropane" sp3 carbons Backbonding occurs to the p* antibonding orbital, therefore reducing the C-C bond order 3I3-10 Imperial College London Appendix:of Synthesis Synthesis metal-alkene & Reactivity complexes of Polyene LIgands 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 Imperial College London Synthesis of Of metal-alkene Metal-Alkenecomplexes: 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 Imperial College London Synthesis of Of metal-alkene Metal-Alkenecomplexes 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 e.g. (h5-C5H5)2TiCl2 + 2Na C2H4 Ti(IV) Ti(II) (h5-C5H5)2Ti(C2H4) 3I3-13 Reactivity of metal-alkene complexes Imperial College London Alkene ligands are often susceptible to nucleophilic attack 3I3-14 Imperial College London Summary of section 1 1. Catalysis at a metal centre often requires a responsive metal (and therefore a responsive ligand set) Most useful ligands are often those that can use different MOs to bind to a metal 2. Binding an alkene to a metal often increases its susceptibility to nucleophilic attack Binding any organic fragment to a metal may activate it towards chemical modification 3I3-15