Presentation on theme: "Organometallic Chemistry"— Presentation transcript:
1Organometallic Chemistry JHU CourseProf. Kenneth D. Karlin Spring, 2009Kenneth D. KarlinDepartment of Chemistry, Johns Hopkins University
2p. 1Organometallic Chemistry Prof. Kenneth D. Karlin Spring, 2009 Class Meetings: TTh, 12:00 – 1:15 pmTextbook – The Organometallic Chemistry of the Transition Metals” 4th Ed., R. H. Crabtree Course Construction: Homeworks, Midterm Exams (1 or 2), Oral PresentationsRough Syllabus Most or all of these topics• Introduction, History of the field• Transition Metals, d-electrons• Bonding, 18 e– Rule (EAN Rule)• Ligand Types / Complexes• Types of CompoundsM-carbonyls, M-alkyls/hydridesM-olefins/arenesM-carbenes (alkylidenes alkylidynes)Other• Reaction TypesOxidative AdditionReductive eliminationInsertion – EliminationNucleophilic/electrophilic Rxs.• Catalysis – ProcessesWacker oxidationMonsanto acetic acid synthesisHydroformylationPolymerization- Olefin metathesisWater gas-shift reactionFischer-Tropsch reaction
5Reaction Examples - continued • Wacker OxidationC2H4 (ethylene) + ½ O2 –––> CH3CH(O) (acetaldehyde)Pd catalyst, Cu (co-catalyst)• Monsanto Acetic Acid SynthesisCH3OH (methanol) + CO –––> CH3C(O)OH (acetic acid) (Rh catalyst)• Ziegler-Natta catalysts – Stereoregular polymerization of 1-alkenes (a-olefins)1963 Nobel PrizeCatalyst: Ti compounds and organometalllic Al compound (e.g., (C2H5)3Al )• Olefin metathesis – variety of metal complexes2005 Nobel Prize – Yves Chauvin, Robert H. Grubbs, Richard R. Schrockn CH2=CHR –––> –[CH2-CHR]n–
6Organo-transition Metal Chemistry History-Timeline p. 5Organo-transition Metal Chemistry History-TimelineMain-group OrganometallicsCacodyl – tetramethyldiarsine,from Co-mineral with arsenic1899 –> 1912 Nobel Prize: Grignard reagents (RMgX)1827 – “Zeise’s salt” - K+ [(C2H4)PtCl3]–n-Butyl-lithiumSynthesis: PtCl4 + PtCl2 in EtOH, reflux, add KCl Bonding- Dewar-Chatt-Duncanson model
7Organo-transition Metal Chemistry History-Timeline (cont.) p. 6Organo-transition Metal Chemistry History-Timeline (cont.)st metal-carbonyl, [PtCl2(CO)2]1890 – L. Mond, (impure) Ni + xs CO –––> Ni(CO)4 (highly toxic)1900 – M catalysts; organic hydrogenation (---> food industry, margerine)1930 – Lithium cuprates, Gilman regent, formally R2Cu–Li+1951 – Ferrocene discovered Sandwich structure proposedFerrocene was first prepared unintentionally. Pauson and Kealy, cyclopentadieny-MgBr and FeCl3 (goal was to prepare fulvalene) But, they obtained a light orange powder of "remarkable stability.”, later accorded to the aromatic character of Cp– groups. The sandwich compound structure was described later; this led to new metallocenes chemistry (1973 Nobel prize, Wilkinson & Fischer). The Fe atom is assigned to the +2 oxidation state (Mössbauer spectroscopy).The bonding nature in (Cp)2Fe allows the Cp rings to freely rotate, as observed by NMR spectroscopy and Scanning Tunneling Microscopy > Fluxional behavior. (Note: Fe-C bond distances are 2.04 Å).Asides:Oxidation statesUS vs. UK18-electrons ?(Cp)2FeCp = cyclopentadienyl anion)(h5-C5H5)2Fe(pentahapto)Solid-statestructure
8Organo-transition Metal Chemistry History-Timeline (cont.) p. 7Organo-transition Metal Chemistry History-Timeline (cont.)Cotton and Wilkinson (of the Text) discover organometallic-complex fluxional behavior (stereochemical non-rigidity)The capability of a molecule to undergo fast and reversible intramolecular isomerization, the energy barrier to which is lower than that allowing for the preparative isolation of the individual isomers at room temperature. It is conventional to assign to the stereochemically non-rigid systems those compounds whose molecules rearrange rapidly enough to influence NMR line shapes at temperatures within the practical range (from –100 °C to +200 °C ) of experimentation. The energy barriers to thus defined rearrangements fall into the range of 5-20 kcal/mol (21-85 kJ/mol).Aside:Oxidation State18-electron Rule
9Fluxional behavior; stereochemical non-rigidity (cont.) p. 8Fluxional behavior; stereochemical non-rigidity (cont.)Butadiene iron-tricarbonylXray- 2 CO’s equiv, one diff., If retained in solution, expect, 2:1 for 13-C NMR. But, see only 1 peak at RT. Cooling causes a change to the 2:1 ratio expected.Two possible explanations:Dissociation and re-association or (2) rotation ofthe Fe(CO)3 moiety so that CO’s become equiv.Former seems not right, because for example additionof PPh3 does NOT result in substitution to give (diene)M(CO)2PPh3.Note: You can substitute PPh3 for CO, but that requireseither high T or hv. So, the equivalency of the CO groupsis due to rotation without bond rupture, pseudorotation.13C-NMR spectraCO region, only
10p. 9Berry PseudorotationPseudorotation: Ligands 2 and 3 move from axial to equatorial positions in the trigonal bipyramid whilst ligands 4 and 5 move from equatorial to axial positions. Ligand 1 does not move and acts as a pivot. At the midway point (transition state) ligands 2,3,4,5 are equivalent, forming the base of a square pyramid. The motion is equivalent to a 90° rotation about the M-L1 axis. Molecular examples could be PF5 or Fe(CO)5.
11p. 10The Berry mechanism, or Berry pseudorotation mechanism, is a type of vibration causing molecules of certain geometries to isomerize by exchanging the two axial ligands for two of the equatorial ones. It is the most widely accepted mechanism for pseudorotation. It most commonly occurs in trigonal bipyramidal molecules, such as PF5, though it can also occur in molecules with a square pyramidal geometry.The process of pseudorotation occurs when the two axial ligands close like a pair of scissors pushing their way in between two of the equatorial groups which scissor out to accommodate them. This forms a square based pyramid where the base is the four interchanging ligands and the tip is the pivot ligand, which has not moved. The two originally equatorial ligands then open out until they are 180 degrees apart, becoming axial groups perpendicular to where the axial groups were before the pseudorotation.