Kinetics Database Workshop Solution Phase Organometallic Reactions

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Kinetics Database WorkshopSolution Phase
Organometallic Reactions

• Donald J. Darensbourg
• Department of Chemistry
• Texas AM University
• College Station, TX 77843
• djdarens_at_mail.chem.tamu.edu

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Ligand Substitution via Dissociative Pathway
ML5A B ? ML5B A
For example, cis-Mo(CO)4PPh3NHC5H10 CO ?
Mo(CO)5PPh3 NHC5H10
A
B
Steady-state approximation on intermediate, ML5
(specifically, W(CO)4PPh3).
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(No Transcript)
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cis-Mo(CO)4PPh3NHC5H10 CO ? Mo(CO)5PPh3
NHC5H10
?H? bond dissociation energy
?H? 108 kJ/mol
Energy
Reaction coordinate
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Other Considerations
- The bimolecular rate constant, k2, for the
reaction of the intermediate Mo(CO)4PPh3 (16
electron species) with CO in perfluorohydrocarbon
solvent is expected to be 109M-1-sec-1
(diffusion controlled) vs 106M-1-sec-1 in
hydrocarbon solvent.

• flash photolysis studies (rate)
• time-resolved infrared studies in ?CO
• region (structure)

• solid-state (matrix isolation)
• solution (TRIR)

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contd
In general with better nucleophiles than CO as
incoming ligands (B), e.g., trialkylphosphine,
there is a concurrent substitution pathway which
is dependent on the PR3.
kobsd k1 k2 PR3
Reaction carried out in absence of added
leaving group (A) with increasing excesses of
entering group (B).

• k2 term cannot be ascribed to an associative
  process (exceeds 18e- requirement) and is
  attributed to be interchange process. Id or Ia
  (decided on basis of ?H?, ?S?, and ?V?)
• Any report of rate constants must contain
  solvent information. (purity of PR3 important
  to eliminate effects of R3PO)

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Changes of Reaction Order
Many other Inorganic/Organometallic reactions
have a change of reaction order with reagent
concentration.
A B ? D , via a transient species C
B gtgt A

• low B (but still gtgt A), kobsd proportioned
  to B
• high B, kobsd independent of B

kobsd
B
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Mechanistic Ambiguity
Several circumstances where this rate behavior is
observed
rapid preequilibrium step k1, k-1 gtgt k2
Hence, C will be in equilibrium with A B
throughout the reaction. Reactions are
typically run under pseudo first order conditions
i.e., B gtgt A
In this instance, C is kinetically competent.
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contd
Alternatively, A B can react directly to give
D, but are also in a rapid dead end equilibrium
with C. That is, C is not kinetically competent.
, same as before except k3 replaces k2K1
Once the rapid equilibrium is established, the
steady-state kinetics are identical for the two
different processes.
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An Example Involving Electron-Transfer Process
Cis-Ru(NH3)4Cl2 Cr2 ? Ru(NH3)4(H2O)Cl
CrCl2
A
B
D
The effect of Cr2 on kobsd is depicted below
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contd
Hence, reaction could be taking place via an
Inner Sphere mechanism, i.e, where C represents
the chloride bridged intermediate
K1 4.65 ? 102M-1 _at_ 6ºC k2 1.54 ? 102sec-1
first-order rate constant
or, reaction occurs via an outer-sphere process
(A B ? D) with a k3 7.14 x 104M-1-sec-1
and a K1 4.65 x 102M-1 for the dead end
equilibrium.
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Current Industrial Process forPolycarbonate
Production
Bottenbruch, L., Engineering Thermoplastics
Polycarbonates, Polyacetals, Polyesters,
Cellulose Esters Hanser Pub. New York 1996, p.
112.
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CO2 and Epoxide Coupling

• Elimination of hazardous starting materials.

• Elimination of methylene chloride solvents.

• Utilization of CO2 as a feedstock.

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CO2 / Epoxide Copolymerization Process
TOFa
Inoue (1969) Heterogenous catalyst lt 1 h-1
Soga (1981) Zinc dicarboxylates 1 h-1
Darensbourg (1995) Discrete zinc phenoxide complexes 10 h-1
Kruper (1995) Chromium porphyrins 100 h-1
Beckman (1997) ZnO/fluorinate carboxylic acid 10 h-1
Coates (1998) ?-diiminates zinc carboxylates and alkoxides up to 2300 h-1 (generally lt 800 h-1)
Holmes (2000) Chromium fluorinated porphyrins 78 h-1
Darensbourg (2002) Chromium salen complexes Up to 500 h-1
a moles of epoxide consumed/mole of catalyst-hour a moles of epoxide consumed/mole of catalyst-hour a moles of epoxide consumed/mole of catalyst-hour
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super-critical
liquid
solid
pressure p
critical point
Tc 31.0C
pc 73.75 bar
gas
temperature T
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Reaction Conditions

• Expansion of the epoxide solvent (reactant) by
  the application of gaseous (subcritical) CO2
  pressure
• Catalyst is soluble in this phase.
• in situ infrared probe of this more dense phase,
  typically
• 40 to 80ºC
• 35 to 55 bars

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ReactIRTM In-SituInfrared Technology
Si-based Crystal
Attenuated Total Reflectance (ATR)
spectroscopy Infrared light penetrates only a
few microns into the reaction mixture
Schematic of In-Situ Probe
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In Situ Infrared Spectroscopy
1750cm-1
1810cm-1
1802cm-1
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Delineated Mechanism of Copolymerization
Step 1 Initiation
Step 2 Chain propagation
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Summary of Reactivity
Based on 4 hour reactions 1 mol CHO consumed/mol
Cr 2 mol CHO consumed/mol Cr/hour 5 methoxy
derivative