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- 1. Organometallic Chemistry between organic and inorganic Peter H.M. Budzelaar
- 2. Organometallic Chemistry2 Course Roadmap • A real example: the Monsanto Acetic Acid process • Introduction: what is organometallic chemistry, and why should you care? • Electron counting: the basis for understanding structure and reactivity • An overview of Main-group and Transition metal chemistry • Main group metal chemistry and "Umpolung" • Intermezzo: characterization of organometallic compounds • Transition metal chemistry: overview of common ligands • Ligand substitution • Insertion and elimination • Oxidative insertion and reductive elimination • More exotic steps • Your own presentations • Applications in catalysis
- 3. Organometallic Chemistry3 Example: Acetic Acid synthesis Acetic acid is an important industrial chemical. The traditional synthesis uses bio-oxidation of ethanol obtained via fermentation: C6H12O6 → 2 C2H5OH + 2 CO2 C2H5OH + O2 → CH3COOH + H2O This is not a clean and efficient process! Industrial acetic acid synthesis: CH3OH + CO → CH3COOH Catalyzed by a rhodium complex.
- 4. Organometallic Chemistry4 Acetic Acid synthesis Moderately complex catalytic cycle: H2O HI CH3OH CH3I CH3COOH CH3COI Rh(CO)2I2 - MeRh(CO)2I3 - MeCORh(CO)I3 - MeCORh(CO)2I3 - CO
- 5. Organometallic Chemistry5 Acetic Acid synthesis This cycle is known in considerable detail: To understand it, you need to be familiar with electron counting and common reaction types CO Rh(CO) I2 2 - MeRh(CO) I2 3 - MeCORh(CO) I2 3 - MeCORh(CO)I3 - CH I3 CH COI3 CH COOH3 H O2 HI CH OH3 RhI 16 e RhIII 18 e RhIII 16 e RhIII 18 e oxidative addition insertion reductive elimination ligand binding
- 6. Organometallic Chemistry6 What is organometallic chemistry ? Strictly speaking, the chemistry of compounds containing at least one metal-carbon bond. Metal hydrides are often included, H being considered as the "smallest organic group" (as in propyl, ethyl, methyl, hydride). Metal-carbon bonds are often formed temporarily or potentially, so in practice many compounds are included that do not actually contain metal-carbon bonds. Phosphine, NO CO etc…metal-nitrogen, metal-oxygen, metal-halogen, and even metal-hydrogen bonds all play a role (Ph3P)3RhCl H2 O OMe Li O OMe Li
- 7. Transition metal organometallic compounds & Catalysis Metal-carbon bond: a few from many? Which one is organometallic? Ni(CO)4 or NaCN ? French Chemist L. C. Cadet 1760 As2Me4 dicacodyl Re R3P CO PR3 CH3 CO Mo P R2 R2 P Cl C C Mo Cl R2 P P R2 C C O O OO
- 8. Organometallic Chemistry8 Why should you care ? Organometallic chemistry is the basis of homogeneous catalysis, which is the method of choice for clean and efficient synthesis of fine chemicals, pharmaceuticals and many larger-scale chemicals. Many plastics (polythene, polypropene, butadiene rubber, ...) and detergents are made via organometallic catalysis. Organometallic chemistry is also the basis for understanding important steps in heterogeneous catalysis reactions such as olefin hydrogenation and CO oxidation. Organometallic compounds are used on a large scale as precursors for generation of semiconductors (AlN, GaAs, etc). Silicone rubbers are one of the few classes of organometallic compounds used as "final products".
- 9. Organometallic Chemistry9 Course Objectives By the end of this course, you should be able to: • Make an educated guess about stability and reactivity of a given compound, based on (a.o.) electron counting rules • Propose reasonable mechanisms, based on "standard" organometallic reaction steps, for many metal-catalyzed reactions • Use steric and electronic arguments to predict how changes in reactants, metal or ligands affect the outcome of reactions • Read a current research literature paper, understand and explain its content and significance
- 10. Organometallic Chemistry10 Useful Background Knowledge Organometallic chemistry senior course because it builds on: • Organic chemistry: reaction mechanisms, primarily nucleophilic and electrophilic attack • Inorganic Chemistry: electronegativity; electron counting and stability; properties of (transition) metals • Physical chemistry: orbitals and MO theory; free energy, enthalpy and entropy You will (now and then) need this background to understand the material or make assignments etc
- 11. Historical Perspective Organometallic chemistry timeline • 1760 Louis Claude Cadet de Gassicourt investigates inks based on Cobalt salts and isolates Cacodyl (CH3)2As—As(CH3)2from cobalt mineral containing arsenic • 1827 Zeise's salt is the first platinum / olefin complex Na[PtCl3C2H4] • 1848 Edward Frankland discovers diethylzinc, Zn(Et)2, first metal alkyl • 1863 Charles Friedel and James Crafts prepare organochlorosilanes • 1890 Ludwig Mond discovers Nickel carbonyl • 1899 Introduction of Grignard reaction, organomagnesium halides • 1900 Paul Sabatier works on hydrogenation organic compounds with metal catalysts. • 1909 Paul Ehrlich introduces Salvarsan for the treatment of syphilis, an early arsenic based organometallic compound • 1912 Nobel Prize Victor Grignard and Paul Sabatier Organometallic Chemistry11
- 12. • 1917. Schenk, lithium alkyls • 1930 Henry Gilman works on lithium cuprates, see Gilman reagent • 1951 Walter Hieber was awarded the Alfred Stock prize for his work with metal carbonyl chemistry. Fe(CO)4H2 first transition metal hydride • 1951 Ferrocene is discovered, Pauson and Miller • 1955, Fisher, bis arene metal complex • 1963 Nobel prize for Karl Ziegler and Giulio Natta on Ziegler-Natta catalyst • 1965 Discovery of cyclobutadieneiron tricarbonyl • 1968 Heck reaction • 1973 Nobel prize Geoffrey Wilkinson and Ernst Otto Fischer on sandwich compounds Cr(CO)4(CR), first carbyne complex • 1981 Nobel prize Roald Hoffmann and Kenichi Fukui for creation of the Woodward-Hoffman Rules Organometallic Chemistry12
- 13. • 1983. Bergman, Graham, C-H bond activation • 1985, Green, Brookhart, agostic metal-hydrogen interaction • 2001 Nobel prize W. S. Knowles, R. Noyori and Karl Barry Sharpless for asymmetric hydrogenation • 2005 Nobel prize Yves Chauvin, Robert Grubbs, and Richard Schrock on metal-catalyzed alkene metathesis • 2010 Nobel prize Richard F. Heck, Ei-ichi Negishi, Akira Suzuki for palladium catalyzed cross coupling reactions Organometallic Chemistry13
- 14. Hapticity , ηx - “eta-x” • word used to describe the bonding mode of a ligand to a metal center • ηx values for carbon ligands where the x value is odd usually indicate anionic carbon ligands (e.g., η5 -Cp, η1 -CH3, η 1 -allyl or η3 -allyl, η1 -CH=CH2) • The # of electrons donated (ionic method of electron counting) by the ligand is usually equal to x + 1 • Even ηx values usually indicate neutral carbon π-system ligands (e.g., η6 - C6H6, η2 -CH2=CH2, η4 -butadiene, η4 cyclooctadiene) • The # of electrons donated by the ligand in the even (neutral) case is usually just equal to x. Organometallic Chemistry14
- 16. μx “mu-x” • is the nomenclature used to indicate the presence of a bridging ligand between two or more metal centers • The x refers to the number of metal centers being bridged by the ligand • Usually most authors omit x = 2 and just use μ to indicate that the ligand is bridging the simplest case of two metals Organometallic Chemistry16
- 17. Ordering • There is no set method of naming or ordering the listing of metal and ligands in a metal/ligand complex that most authors follow • There are some qualitative rules that most authors seem to use in American Chemical Society (ACS) publications • 1) in formulas with Cp (cyclopentadienyl) ligands, the Cp usually comes first, followed by the metal center: Cp2TiCl2 • 2) other anionic multi-electron donating ligands are also often listed in front of the metal, e.g., trispyrazolylborate anion (Tp) Organometallic Chemistry17
- 18. • 3) in formulas with hydride ligands, the hydride is sometimes listed first. Rules # 1 & 2, however, take precedence over this rule: HRh(CO)(PPh3)2 and Cp2TiH2 • 4) bridging ligands are usually placed next to the metals in question, then followed by the other ligands (note that rules 1 & 2 take precedence): Co2(μ-CO)2(CO)6 , Rh2(μ-Cl)2(CO)4 , Cp2Fe2(μ- CO)2(CO)2 • 5) anionic ligands are often listed before neutral ligands: • RhCl(PPh3)3, CpRuCl(=CHCO2Et)(PPh3) (neutral carbene ligand), PtIMe2(C≡CR)(bipy). Organometallic Chemistry18
- 19. • Problem: Sketch structures for the following: • a) CpRuCl(=CHCO2Et)(PPh3) • b) Co2(μ-CO)2(CO)6 (Co-Co bond, several possible structures) • c) trans-HRh(CO)(PPh3)2 [Rh(+1) = d8 ] • d) Ir2(μ-Cl)2(CO)4 [Ir(+1) = d8 ] • e) Cp2TiCl2 Organometallic Chemistry19
- 21. Common organometallic ligands M H M C M C C M M H H M H X M M M PR3 M CO M CNR M CS M NO M N2 M M M M M M C M C
- 22. 18 Electron "Rule“ , Effective Atomic Number rule (EAN) • Organic compounds, of course, follow the 8 electron rule: there can only be a maximum of 8 valence electrons around a carbon center • The vast majority of stable diamagnetic organometallic compounds have 16 or 18 valence electrons due to the presence of the five d orbitals which can hold 10 more electrons relative to C, O, N, etc. • Electron counting is the process of determining the number of valence electrons about a metal center in a given transition metal complex • Complexes with 18 e- counts are referred to as saturated, because there are no empty low-lying orbitals to which another incoming ligand can coordinate. Complexes with counts lower than 18e- are called unsaturated and can electronically bind additional ligands Organometallic Chemistry22
- 23. : To determine the electron count for a metal complex • 1) Determine the oxidation state of the transition metal center(s) and the metal centers resulting d-electron count. To do this one must: a) note any overall charge on the metal complex b) know the charges of the ligands bound to the metal center (ionic ligand method) c) know the number of electrons being donated to the metal center from each ligand (ionic ligand method) • 2) Add up the electron counts for the metal center and ligands Organometallic Chemistry23
- 24. Ligands, Bonding Types, Charges, and Electron Donor Numbers Organometallic Chemistry24
- 30. Two methods of electron counting Organometallic Chemistry30 Ionic Method of electron-counting 95% of inorganic/organometallic chemists use the ionic method The ionic method assigns formal charges to the metal and ligands in order to keep the ligands with an even # of electrons and (usually) a filled valence shell the neutral method Considers everything to be neutral Synthetically, the ionic method makes more sense
- 31. The two methods compared: some examples N.B. like oxidation state assignments, electron counting is a formalism and does not necessarily reflect the distribution of electrons in the molecule – useful though Some ligands donate the same number of electrons Number of d-electrons and donation of the other ligands can differ Now we will look at practical examples on the black board
- 33. Metal –Metal bonded examples Organometallic Chemistry33
- 35. Total valence electron counts in d-block organometallic clusters • Each low oxidation state metal cluster cage possesses a characteristic number of valence electrons (ve) • Any organometallic complex with a triangular M3 framework requires 48 valence electrons, for example: Organometallic Chemistry35
- 38. Exercise Organometallic Chemistry38 Limitations of total valence counting schemes For example rhodium carbonyl chemistry [Rh5CO15 ]- It possesses 76 valence electrons and yet has a trigonal bipyramidal Rh5-core, for which 72 electrons are usual
- 41. Biphase/multiple phaseBiphase/multiple phase Activity (of metal content)Activity (of metal content) selectivityselectivity Conditions of reactionConditions of reaction Life-time of catalystLife-time of catalyst Sensitivity of deactivationSensitivity of deactivation Problems due to diffusionProblems due to diffusion Recycling of catalystRecycling of catalyst Steric and electronic propertiesSteric and electronic properties MechanismMechanism Mono phaseMono phase highhigh variablevariable highhigh variablevariable mildmild harshharsh variablevariable longlong lowlow highhigh nonenone usually difficultusually difficult easily changedeasily changed realistic models do existrealistic models do exist difficult to solvedifficult to solve can easily be donecan easily be done no variation possibleno variation possible not obviousnot obvious Homogenous versus Heterogenous CatalysisHomogenous versus Heterogenous Catalysis
Notes de l'éditeur
- Katalyse ist eine Schlüssel- und Querschnittstechnologie für das neue Jahrhundert. Die verschiedenen Katalysedisziplinen strahlen in nahezu alle Bereiche unserer Gesellschaft hinein.