Organometallic Chemistry Organometallic Chemistry

Kenneth D. KarlinDepartment of Chemistry, Johns Hopkins University

karlin@jhu.edu http://www.jhu.edu/~chem/karlin/

JHU Course 030.442Prof. Kenneth D. Karlin

Spring, 2009

JHU Course 030.442Prof. Kenneth D. Karlin

Spring, 2009

Organometallic Chemistry030.442 Prof. Kenneth D. Karlin Spring, 2009

Class Meetings: TTh, 12:00 – 1:15 pm

Textbook – The Organometallic Chemistry of the Transition Metals”4th Ed., R. H. Crabtree

Course Construction: Homeworks, Midterm Exams (1 or 2), Oral Presentations

Rough 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 Compounds

M-carbonyls, M-alkyls/hydrides

M-olefins/arenes

M-carbenes (alkylidenes alkylidynes)

Other

• Reaction Types Oxidative Addition

Reductive elimination Insertion – Elimination Nucleophilic/electrophilic Rxs.

• Catalysis – Processes Wacker oxidation Monsanto acetic acid synthesis

Hydroformylation Polymerization- Olefin metathesis

Water gas-shift reaction Fischer-Tropsch reaction

p. 1

p. 2

Reaction Examplesp. 3

• Oxidative Addition

Reductive Elimination

• Carbonyl Migratory Insertion

• Reaction of Coordinated Ligands

CH3Mn(CO)5 CH3CMn(CO)5

OCO

Vaska’s complex

(Iron pentacarbonyl) (CO)4Fe– + :OH– ––––> (CO)4Fe

––––––> (CO)4Fe–H + CO2

O

O

HC O

Reaction Examples - continuedp. 4

• Wacker Oxidation C2H4 (ethylene) + ½ O2 –––> CH3CH(O) (acetaldehyde)

Pd catalyst, Cu (co-catalyst)

• Monsanto Acetic Acid Synthesis CH3OH (methanol) + CO –––> CH3C(O)OH (acetic acid) (Rh catalyst)

• Ziegler-Natta catalysts – Stereoregular polymerization of 1-alkenes (-olefins)

1963 Nobel Prize

Catalyst: Ti compounds and organometalllic Al compound (e.g., (C2H5)3Al )

• Olefin metathesis – variety of metal complexes

2005 Nobel Prize – Yves Chauvin, Robert H. Grubbs, Richard R. Schrock

n CH2=CHR –––> –[CH2-CHR]n–

Organo-transition Metal Chemistry History-Timelinep. 5

• Main-group Organometallics

1760 - Cacodyl – tetramethyldiarsine, from Co-mineral with arsenic

1899 –> 1912 Nobel Prize: Grignard reagents (RMgX)

• 1827 – “Zeise’s salt” - K+ [(C2H4)PtCl3]–

n-Butyl-lithium

Synthesis: PtCl4 + PtCl2 in EtOH, reflux, add KCl Bonding- Dewar-Chatt-Duncanson model

Organo-transition Metal Chemistry History-Timeline (cont.)p. 6

1863 - 1st 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. 1952 -- Sandwich structure proposed

Ferrocene 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).

(Cp)2Fe

Cp = cyclopentadienyl anion)

(h5-C5H5)2Fe

(pentahapto) Solid-statestructure

1955 - 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).

Organo-transition Metal Chemistry History-Timeline (cont.)

Aside:Oxidation State18-electron Rule

p. 7

Fluxional behavior; stereochemical non-rigidity (cont.)

Butadiene iron-tricarbonyl

Xray- 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: (1)Dissociation and re-association or (2) rotation of the Fe(CO)3 moiety so that CO’s become equiv.

Former seems not right, because for example addition of 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 groups is due to rotation without bond rupture, pseudorotation.

13C-NMR spectraCO region, only

p. 8

Pseudorotation: 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.

Berry Pseudorotationp. 9

The 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.

p. 10