Transcript OMC-Karlin-Spring-`09-Lecture-Set

Organometallic Chemistry
JHU Course 030.442
Prof. Kenneth D. Karlin
Spring, 2009
Kenneth D. Karlin
Department of Chemistry, Johns Hopkins University
[email protected]
http://www.jhu.edu/~chem/karlin/
Organometallic Chemistry
p. 1
030.442
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
• Reaction Types
Oxidative Addition
• Transition Metals, d-electrons
Reductive elimination
–
• Bonding, 18 e Rule (EAN Rule)
Insertion – Elimination
Nucleophilic/electrophilic Rxs.
• Ligand Types / Complexes
• Types of Compounds
• Catalysis – Processes
Wacker oxidation
M-carbonyls, M-alkyls/hydrides
Monsanto acetic acid synthesis
M-olefins/arenes
Hydroformylation
M-carbenes (alkylidenes alkylidynes) Polymerization- Olefin metathesis
Water gas-shift reaction
Other
Fischer-Tropsch reaction
p. 2
p. 3
Reaction Examples
• Oxidative Addition
Reductive Elimination
Vaska’s complex
• Carbonyl Migratory Insertion
CH3Mn(CO)5
CO
O
CH3CMn(CO)5
• Reaction of Coordinated Ligands
O
(Iron pentacarbonyl)
(CO)4Fe–C O + :OH– ––––> (CO)4Fe
––––––>
(CO)4Fe–H
+
CO2
O
H
Reaction Examples - continued
p. 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 (a-olefins)
1963 Nobel Prize
n CH2=CHR
–––>
–[CH2-CHR]n–
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
p. 5
Organo-transition Metal Chemistry History-Timeline
• Main-group Organometallics
1760 - Cacodyl – tetramethyldiarsine,
from Co-mineral with arsenic
1899 –> 1912 Nobel Prize: Grignard reagents (RMgX)
n-Butyl-lithium
• 1827 – “Zeise’s salt” - K+ [(C2H4)PtCl3]–
Synthesis: PtCl4 + PtCl2 in EtOH, reflux, add KCl
Bonding- Dewar-Chatt-Duncanson model
p. 6
Organo-transition Metal Chemistry History-Timeline (cont.)
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
(Cp)2Fe
Cp = cyclopentadienyl anion)
(h5-C5H5)2Fe
(pentahapto)
Solid-state
structure
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).
p. 7
Organo-transition Metal Chemistry History-Timeline (cont.)
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).
Aside:
Oxidation State
18-electron Rule
p. 8
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 requires
either high T or hv. So, the equivalency of the CO groups
is due to rotation without bond rupture, pseudorotation.
13C-NMR
spectra
CO region, only
p. 9
Berry Pseudorotation
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.
p. 10
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.