1 / 37

Understanding Transition Metal Organometallics

Explore the unique properties, catalytic reactions, and synthesis methods of transition metal organometallic compounds. Learn about the different groups, oxidation states, and reactivities of these compounds, as well as strategies to prevent b-hydrogen elimination and reactions with various ligands.

elba
Download Presentation

Understanding Transition Metal Organometallics

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. Transition-metal Organometallics Peter H.M. Budzelaar

  2. Transition metals are never on time Transition-metal Organometallics

  3. Early Transition MetalsGroups 3,4 • Strongly electrophilic and oxophilic • Few redox reactions (exception: Ti) • Nearly always < 18e • Polar and very reactive M-C bonds(to alkyl and aryl) • Few d-electrons: • preference for "hard" s-donors (N/O/F) • weak complexation of p-acceptors (olefins, phosphines) • Typical catalysis: Polymerisation Transition-metal Organometallics

  4. "Middle" Transition MetalsGroups 5-7 • Many accessible oxidation states • Mostly 18e • Ligands strongly bound • Strong, not very reactive M-C bonds • Preference for s-donor/p-acceptor combinations (CO!) • Typical catalysis: Alkene and alkyne metathesis Transition-metal Organometallics

  5. Late Transition MetalsGroups 8-10 (and 11) • Many accessible oxidation states • Mostly 18e or 16e16e common for square-planar complexes • Easy ligand association/dissociation • Weak, not very reactive M-C bonds • Even weaker, reactive M-O/M-N bonds • Preference for s-donor/weak p-acceptor ligands (phosphines) • Typical catalysis: Hydroformylation Transition-metal Organometallics

  6. Front- and Back-benchers Transition-metal Organometallics

  7. Going down... 1st row: • often unpaired electrons • different spin states (HS/LS) accessible • "highest possible" oxidation states not very stable • MnO4- is a strong oxidant 2nd/3rd row: • nearly always "closed shell" • virtually same atomic radii (except Y/La) • highest oxidation states fairly stable • ReO4- is hardly oxidizing • 2nd row often more reactive than 3rd Transition-metal Organometallics

  8. M-H and M-C s-bonds Transition-metal Organometallics

  9. Synthesis of metal alkyls • Metathesis • Electrophilic attack on metal • Insertion Transition-metal Organometallics

  10. Synthesis of metal alkyls • Oxidative addition • often starts with electrophilic attack Transition-metal Organometallics

  11. Decomposition of metal alkyls Dominant: b-hydrogen elimination Alternatives: • homolysis • a/g/d-eliminations • reductive elimination (especially with H or another alkyl) • ligand metallation Transition-metal Organometallics

  12. How to prevent b-hydrogen elimination ? • No b-hydrogen CH3, CH2CMe3, CH2SiMe3, CH2Ph • No empty site cis to alkyl • Product of elimination unstable Transition-metal Organometallics

  13. How to prevent b-hydrogen elimination ? • Planar transition state inaccessible even for 5-membered metallacycles b-elimination is difficult !(basis of selective ethene trimerization) Transition-metal Organometallics

  14. Reactions of metal alkyls • Insertion, of both polar and non-polar C=X bonds: • olefins, acetylenes, allenes, dienes • (ketones etc) • CO, isocyanides • Reductive elimination Transition-metal Organometallics

  15. Synthesis of metal hydrides • Metathesis • b-elimination • Protonation / oxidative addition Transition-metal Organometallics

  16. Structure of WH6(PiPr2Ph)3 Transition-metal Organometallics

  17. Synthesis of metal hydrides • Hydrogenolysis Transition-metal Organometallics

  18. Reactivity of metal hydrides • "Hydride is the smallest alkyl" • Can react as H+ or H- • HCo(CO)4 nearly as acidic as H2SO4 • Cp2ScH gives H2 with acids, alcohols, ... • Insertion reactions • CO insertion rare (endothermic!) Transition-metal Organometallics

  19. Metal aryls Usually much more stable than alkyls Synthesis: • Metathesis • Oxidative addition Reaction/decomposition: • Reductive elimination Transition-metal Organometallics

  20. The other ligands... Common ligands for transition metals:p ligands, CO, phosphines General characteristic: s-donating, p-accepting, "soft" Transition-metal Organometallics

  21. p ligands, CO, phosphines s-donor character:phosphines > alkenes, CO p-acceptor character:CO > alkenes > phosphines Depends strongly on the substituents on P, C=C ! Transition-metal Organometallics

  22. Donor and acceptor orbitals s-donor p-acceptor alkene CO phosphine p p* p* LP s* LP Transition-metal Organometallics

  23. p ligands, CO, phosphines • CO is one of the best p-acceptors (p-acids) • isocyanides are stronger donors, weaker acceptors • PMe3 very weak p-acceptor, good s-donor • PF3 nearly as strongly p-acidic as CO • C2H4 weak p-acceptor • C2(CN)4 very strong p-acceptor • even forms stable radical anions Transition-metal Organometallics

  24. Synthesis of CO and p-ligand complexes Stable, neutral ligands:generate empty site(s) in presence of free ligand 1.13 1.37 1.417 1.410 2.141 2.223 1.841 1.913 1.157 1.141 Transition-metal Organometallics

  25. Synthesis of CO and p-ligand complexes Variation:reductive synthesis Transition-metal Organometallics

  26. Synthesis of CO and p-ligand complexes Anionic ligands:introduce via metathesis (Cp-), substitutionor oxidative addition (allyl) Transition-metal Organometallics

  27. Synthesis of CO and p-ligand complexes Transition-metal Organometallics

  28. Modification of p ligandsby H+/H- addition/abstraction Transition-metal Organometallics

  29. Reactivity of p-ligand complexes Ligand activated for nucleophilic attack both internal and external Transition-metal Organometallics

  30. Reactivity of p-ligand complexes Change of hapticity Transition-metal Organometallics

  31. More than 18 e ? 1.47 1.51 1.40 2.33 2.45 1.22 1.15 1.99 2.97 1.95 2.30-2.32 2.46 1.40 2.35 2.32 1.41 1.41-1.44 1.40 Transition-metal Organometallics

  32. s complexes A s bond as 2-electron donor for a metal. • H2 complexes (non-classical hydrides) • C-H bonds Usually intramolecular Sometimes intermolecular In "matrix" Characterized by IR Transition-metal Organometallics

  33. s complex ? 2.10 1.92 Transition-metal Organometallics

  34. s complex ? Transition-metal Organometallics

  35. s complexes • C-Si bonds 2.63 3.22 103° Transition-metal Organometallics

  36. s complexes • Si-H bonds • etc: B-H, Sn-Cl, P-H, .. ?? Transition-metal Organometallics

  37. s complexes A s complex is an (arrested) intermediate for oxidative addition: Stable s complexes are formed when the metal is notp-basic enough to enable completion of the addition. Transition-metal Organometallics

More Related