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Imperial College London Dr. Ed Marshall, M220, RCS 1 Additional materials available on: Lecture.

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Presentation on theme: "Imperial College London Dr. Ed Marshall, M220, RCS 1 Additional materials available on: Lecture."— Presentation transcript:

1 Imperial College London Dr. Ed Marshall, M220, RCS 1 Additional materials available on: Lecture notes also available on Blackboard 3I3 Slide 1 3I3 Advanced Organometallics Lectures 1 - 4

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 L M M X X 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. 3I3-2

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

4 Imperial College London Learning objectives 1.Use simple MO theory to explain how a carbon-carbon  -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. By the end of lecture 4, you should be able... 3I3-4

5 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 Assumed Knowledge Imperial College London Assumed knowledge 3I3-5

6 Section 1: Metal-alkene complexes Section 1: Metal-alkene complexes Imperial College London 3A Advanced Organometallics3I1 Advanced Organometallics 3I3-6

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

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

9 [Pt(C 2 H 4 )Cl 3 ] 2- versus [Pt(C 2 Cl 4 )(PPh 3 ) 2 ]

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

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

12 Synthesis Of Metal-Alkene Complexes Imperial College London 1 (a) Addition to 16 electron species: e.g. [Ir(CO)Cl(PPh 3 ) 2 ] + C 60 [Ir(CO)Cl(PPh 3 ) 2 C 60 ] 16 e - 18 e - 1 (b) Displacement of other L-type ligands: e.g. (  5 -C 5 H 5 ) 2 Zr(PMe 3 ) 2 + C 2 H 4 (  5 -C 5 H 5 ) 2 Zr(C 2 H 4 )(PMe 3 ) 18 e - Synthesis of metal-alkene complexes: examples 3I3-12

13 2. Reduction of a metal in the presence of an alkene nbd = norbornadiene Rh(III) Synthesis Of Metal-Alkene Complexes Imperial College London e.g. (  5 -C 5 H 5 ) 2 TiCl 2 + 2Na C2H4C2H4 Ti(IV) Ti(II) (  5 -C 5 H 5 ) 2 Ti(C 2 H 4 ) Synthesis of metal-alkene complexes e.g. RhCl 3 + CH 3 CH 2 OH + CH 3 CHO Rh(I) [(nbd)Rh(  -Cl)] 2 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 3I Catalysis at a metal centre often requires a responsive metal (and therefore a responsive ligand set) 2.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


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