Download Organometallic Reactions and Catalysis

Organometallic Reactions and
Catalysis
Chapter 14
Gain or Loss of Ligands
• CO dissociation
– In many cases to add another ligand.
– Dissociative and associative mechanisms
– More complicated reactions.
• Dissociation of phosphine (steric effects)
– cis-Mo(CO)4L2 + CO  Mo(CO)5L + L
– Figure 14-1 and Table 14-1 (Article)
– Rate dependence on cone angle and other factors.
• Reaction follows the first-order rate law.
Oxidative Addition (OA)
• Increases the coordination number and the
oxidation state of the metal.
• OA reactions of square-planar d8
complexes.
– trans-Ir(CO)Cl(PEt3)2 (Figure 14-3)
• Changes in CN and oxidation state
• Reactions may occur between ligands due
to close proximity.
Reductive Elimination (RE,
reverse of OA)
• Decrease in coordination number and
oxidation state of the
metal.
OA
(-C5H5)2TaH + H2  (5-C5H5)TaH3
RE
• RE reaction rates are also affected by ligand
bulk. How? (Table 14-2)
Nucleophlic Displacement
(attack by a Lewis base)
• A strong nucleophile would be a ligand with strong
electron-donating character.
• Organometallic complexes can behave as nucleophiles
in displacement reactions (especially negativelycharged complexes).
[Co(CO)4]- + RX  RCo(CO)4 + XRCo(CO)4 + CO  R(C=O)Co(CO)4 (acyl complex)
R(C=O)Co(CO)4 +R’OH  R(C=O)OR’ + HCo(CO)4
(generates the ester from an alcohol).
Modification of Ligands
• Insertion – a molecular fragment appears to
insert itself into a metal-ligand bond.
– Many reaction mechanism can be complicated.
– 1,1-insertion (both bonds are made to the same
atom).
• illustrate
– 1,2-insertion (bonds to the inserted molecule
are made to adjacent atoms in that molecule).
• illustrate
Insertion of Ligands
• How is CO inserted in the complex shown
previously (1,1-insertion)?
– Work through this and understand.
• 1,2-insertions
Hydride Elimination
• Transfer of a hydrogen atom from a ligand
to a metal.
–  elimination is the most common type.
•  position on the alkyl ligand.
• Stability
– Alkyl complexes that lack  hydrogens are
more stable.
– Coordinatively saturated complexes containing
alkyl ligands are also more stable.
Abstraction
• Removal of a substituent from a ligand in
which the coordination number of the metal
does not change (can be removed by an
acid).
Organometallic Catalysts
(hydroformylation)
• Converting terminal alkenes into other organic products.
– (oxo process) H and HCO are formally added across a double
bond.
• Show reaction
• Largest-scale industrial process that is homogeneous
• Mechanism was suggested by Heck and Breslow in
1961.
– Examine each step in the cycle and characterize the reaction
according to type.
• 18-,16-electron cycling is common.
Comments on the
Hydroformylation Mechanism
• CO pressure has to be controlled carefully.
Why?
• Rate-determining step is the insertion of the
olefin (alkene).
• Main purpose of the reaction is to produce
butanal from propene.
CH3CH=CH2CH3CH2CH2CHO
Union Carbide Hydroformylation
Process
• Contain Rh and bulky phosphine groups.
How will this affect the mechanism?
– (Ph3P)3Rh(CO)H
• In many cases, the linear/branched ration
needs to high.
• The catalyst is also water-soluble.
Hydrogenation of Alkenes
• Wilkinson’s catalyst
– Show reaction (alkenes and alkynes)
• Show mechanism and discuss
– Step 9 is slow, the sequence 123 is favored.
– The rate determining step is insertion, 4.
• The catalyst hydrogenizes terminal and internal
olefins.
• Examine Table 14-3.
Hydrogenation Catalyst
• Selective hydrogenation can be observed if the
ligand contains multiple double bonds.
• Another hydrogenation catalyst, (PPh3)2Rh(CO)H,
is very selective toward hydrogenation of only
terminal olefins.
• Asymmetric hydrogenation
– If the catalyst, [L2RHS2]+, bears an optically active
diphosphane, prochiral unsaturated molecules can be
hydrogenated to chiral products (enantiomeric
selectivity).
• L-Dopa (treatment of Parkinson’s disease).
Alkene Metathesis
• Demonstrate
– Propene and 1-butene (what are the 4 new products that
may form from methathesis?)
• Ring-opening metathesis (ROM)
• Chauvin mechanism is most widely accepted.
– Involved a carbene complex
– The carbene reacts with an alkene to form a
metallocyclobutane intermediate. The intermediate can
either revert to reactants or form new products.
– Schrock metathesis catalysts are most effective and the
most studied (available commercially).
• Ring-closing methathesis (page 545)
Heterogeneous Catalysis
• Used much more
extensively in industry
than homogeneous
catalysts.
– Robust at high
temperatures.
– Easy to separate out the
catalyst.
Composition of Heterogeneous
Catalysts
• Uniform – bulk of the
high-surface area serves
as the catalyst.
– ZSM-5 (zeolite)
• Multiphase – highsurface-area material
serves as a support for
the active catalyst.
– Pt/Re on alumina
Surface Ligands
• In many cases, the nature of the surface ligand is
inferred by comparison of IR spectra with those of
organometallic or inorganic complex.
– Terminal and bridging CO.
Surface-Sensitive Techniqus
• Temperature-programmed desorption (mass
spectroscopy).
• Photoelectron spectroscopy (XPS and Auger)
• Low-Energy Electron Diffraction (LEED)
• Scanning Tunneling and Atomic Force
Microscopies.
• Vibrational Techniques (RAIRS and HREELS).
• Many othes.
Catalytic Steps
• Many parallels can be drawn in comparison
to organometallic mechanisms studied
previously.
• Chemisorption and physisorption
– Similar to interactions present in complexes
with low oxidation states.
– Physisorption and chemisorption.
Catalytic Steps
• Similar to homogeneous
catalysis, there is also a
balance between strong
enough adsorption for the
reaction to occur and weak
enough desorption that the
species can be removed
for further reactions.
– HCOOH  CO + H2O
• (on a metal surface)
Diversity of Sites
• Real surfaces possess a
large diversity of surface
types. Each surface type
may have a different
reactivity and/or produce
different products.
– Lower selectivity.
– Most reactive sites.
Examples of Heterogeneous
Catalysts
• Hydrogenation of alkenes on
metal surfaces.
– H2 is dissociatively
chemisorbed
– Ethylene is associated
– Hydrogen adds to produce an
alkyl species
– Another hydrogen atom
coordinates and ethane leaves.
– Actual species produced
Ziegler-Natta Polymerization
• TiCl4 + Al(C2H5)3
– A titanium alkyl complex is produce.
– Ethylene or propylene associates and inserts
into the titanium-carbon bond.
– The 1,2-insertion continues.
– Mechanism has proved difficult to understand.
In Miessler and Tarr
Fundamental Studies of
Hydrocarbons on Platinum Surfaces
• The techniques used.
– Reflection-absorption infrared spectroscopy.
– Auger electron spectroscopy
– Temperature-programmed desorption/reaction
spectroscopy.
– Others as needed.
Reflection-Absorption Infrared
Spectroscopy (RAIRS)
• The dynamic dipole moment must have a component
normal to the surface to be visible.
• The intensity of the vibration signature reveals orientation
information.
• Position of the signature indicates identity of species on the
surface.
_
q(t)
+
qcos
+ 
_
=
+
_
+
_
+
_
_
+
qsin
Absorbance
A Typical RAIRS Spectrum
c
b
a
1300 1350
1400 1450 1500 1550 1600 2400 2500 2600 2700 2800 2900 3000 3100
Frequency (cm-1)
The Labeling Study
0.0006
0.00005
Absorbance
Absorbance
CD2HCD2(CH2)2CD2CD2H
n-C8H18
x 15
CH3CH2(CD2)2CH2CH3
x4
CD3(CH2)6CD3
1440
CH3(CH2)4CH3
2400
2500
2600
2700
2750
2800
Frequency (cm-1)
2700
2800
-1
Frequency (cm )
2900
3000
2850
2900
2950
Cyclic C8 Systems on Pt(111)
.
.
250-325 K
cyclooctene
cyclooctenediyl
C8H12
C8H14
5
37
532
K
1,3-cyclooctadiene
C8H12
325
-375
K
.
250-300 K
1,5-cyclooctadiene
C8H12
. or
cyclooctadienediyl
C8H10
325-375 K
cyclooctatetraene
1,5-dihydropentalene
K
325-375
K
00
3
522
cyclooctatetraene
C8H8
175-225 K
bicyclo[3.3.0]2-octene
C8H12
1,5-dihydropentalene
C8H8
Pentalene
C8H6
Stable to ~500 K,
decomposes to yield
predominantly surfacebound carbon,
hydrogen desorbs.
1,4-Cyclohexadiene on Pt(111)
0.0003
Absorbance
100 K
Hdistal
Hdistal
150
Hproximal
200
f
250
300
350
2400
2500
Hproximal
2600
2700
2800
2900
-1
Wavenumber (cm )
3000
3100