Kinetics Database Workshop

Solution Phase Organometallic Reactions

Donald J. Darensbourg

Department of ChemistryTexas A&M UniversityCollege Station, TX 77843djdarens@mail.chem.tamu.edu

ML5A + B ML5B + A

Ligand Substitution Ligand Substitution viavia Dissociative Pathway Dissociative Pathway

ML5A {ML5} + Ak1

k -1

ML5A{ML5} + Bk2

For example, cis-Mo(CO)4PPh3[NHC5H10] + CO Mo(CO)5PPh3 + NHC5H10

AB

Steady-state approximation on intermediate, ML5 (specifically, W(CO)4PPh3).

d[ML5]dt

= k1[ML5A] - k-1[ML5][A] - k2[ML5][B] = 0

d[ML5A]dt

=k1k2[ML5A][B]k-1[A] + k2[B] = kobsd[ML5A]

kobsd =k1k2[B]

k-1[A] + k2[B] , reduces to k1 where k2[B] >> k-1[A]

Alternatively, kobsd

1=

k1

1 +k1k2

k-1 [A][B]

Plots of kobsd

1vs [A]

[B]k1 and (competition ratio)

k2

k-1

0

10,000

20,000

30,000

40,000

50,000

0.0 2.0 4.0 6.0 8.0 10.0 12.0

kobsd

1

[HNC5H10][CO]

k2

k-1 = 2.74

k1 = 7.40 x 10-4sec-1 @ 30ºC

cis-Mo(CO)4PPh3[NHC5H10] + CO Mo(CO)5PPh3 + NHC5H10

Reaction coordinate

En

erg

y

ΔH≠ = 108 kJ/mol

PPh3

PPh3

CO

PPh3

NHC5H10

ΔH≠ ≈ bond dissociation energy

Other ConsiderationsOther 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.

Mo(CO)5PPh3 Mo(CO)4PPh3 + COh

• flash photolysis studies (rate)

• time-resolved infrared studies in CO

region (structure)

PPh3

C4V

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

– – cont’d –cont’d –

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

PR3

k1 (dissociative)

k2

dependent on B

* 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 ΔH≠, ΔS≠, and ΔV≠)

- Any report of rate constants must contain solvent information. (purity of PR3 important to eliminate effects of R3PO)

kobsd = k1 + k2 [PR3]

Changes of Reaction OrderChanges 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] >> [A]

-dAdt = kobsd [A] =

a [A][B]1 + b [B]

- low [B] (but still >> [A]), kobsd proportioned to [B]

- high [B], kobsd independent of [B]

kobsd kobsd

1

B 1[B]

Mechanistic AmbiguityMechanistic Ambiguity

Several circumstances where this rate behavior is observed

k1

k -1

A + B C rapid preequilibrium step k1, k-1 >> k2

k2DC Hence, C will be in equilibrium with A + B

throughout the reaction. [Reactions are typically run under pseudo first order conditions i.e., [B] >> [A]]

d[D]dt

= kobsd ([A] + [C]) = k2[C] = k2K1[A][B]

kobsd =k2K1[A][B][A] + [C]

k2K1[B]1 + K1[B]=

In this instance, C is kinetically competent.

–– cont’d cont’d ––

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.

Once the rapid equilibrium is established, the steady-state kinetics are identical for the two different processes.

k1

k -1

k3

A + B C

DA + B

, K = k-1

k1

d[D]dt

= kobsd ([A] + [C]) = k3[A][B]

kobsd = k3[B]1 + K1[B]

, same as before except k3 replaces k2K1

An Example Involving Electron-Transfer ProcessAn Example Involving Electron-Transfer Process

Cis-Ru(NH3)4Cl2+ + Cr+2 Ru(NH3)4(H2O)Cl+ + CrCl+2

A B D

The effect of [Cr+2] on kobsd is depicted below:

–– cont’d cont’d ––

Hence, reaction could be taking place via an Inner Sphere mechanism, i.e, where C represents the chloride bridged intermediate

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.

A + B {RuIII-Cl-CrII}K1

atom transfer {RuII + CrIIICl

k2

C

intercept =

slope =

1k2

1k2K1

K1 = 4.65 102M-1 @ 6ºCk2 = 1.54 102sec-1

first-order rate constant

intercept =

slope =

k3

1k3

K1

Current Industrial Process forCurrent Industrial Process forPolycarbonate ProductionPolycarbonate Production

Bottenbruch, L., Engineering Thermoplastics: Polycarbonates, Polyacetals, Polyesters, Cellulose Esters; Hanser Pub.; New York; 1996, p. 112.

Cl Cl

O

CH3H3C

HOOH

aq. NaOH

CH2Cl2

CH3H3C

OO C

O

n

COCO22 and Epoxide Coupling and Epoxide Coupling

• Elimination of hazardous starting materials.

• Elimination of methylene chloride solvents.

• Utilization of CO2 as a feedstock.

n

O

+ CO2

catalyst O C

O

O

COCO22 / Epoxide Copolymerization Process / Epoxide Copolymerization ProcessO

+ CO2 catalystO O

On

TOFa

Inoue (1969) Heterogenous catalyst < 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 alkoxidesup to 2300 h-1

(generally < 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

pres

sure

p

temperature T

solid

gas

critical point

liquid

super-critical

Tc = 31.0°Cpc = 73.75 bar

Reaction ConditionsReaction 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

ReactIRTM In-SituInfrared Technology

Schematic of In-Situ Probe

Reaction Mixture

ZnSe Focusing Material

Si-basedCrystal

• Attenuated Total Reflectance (ATR) spectroscopy

• Infrared light penetrates only a fewmicrons into the reaction mixture

In Situ Infrared Spectroscopy

17

50

cm

-1

18

02

cm

-1

18

10

cm

-1

O O

On

O O

O

Delineated Mechanism of CopolymerizationDelineated Mechanism of Copolymerization

Cr

Nu

L

E Cr

Nu

E

L

Cr

Nu

L

O

CrO

L

Nu

CrNu LOCrNu

CrO

L

CO

O

R

CO2

First Order Second Order

CO2 Insertion

Active SpeciesR=Cyclohexyl

E = EpoxideL = Cocatalyst

Step 1:Initiation

Cr

O

L

C

O

O

R

OCr

O

L

O

C

O

O

Polymer

O O

On

Step 2:Chain propagation

N N

O O

Cr

tBu

tButBu

tBuX

Based on 4 hour reactions 1 mol CHO consumed/mol Cr 2 mol CHO consumed/mol Cr/hour

* 5 methoxy derivative

0

0.1

0.2

0.3

0.4

0.5

0 50 100 150 200

Time(minutes)

Ab

sorb

ance

PPNCl

PPNN3

3eq PCy3

3eq PPh3

Summary of Reactivity

Cocatalyst Initiator(X) TON1 TOF2

PPNN3 N3 1293.0 323.3

PPNCl N3 1021.6 255.4

PCy3 N3 391.3 97.8

PPh3 N3 284.2 71.1

PPNBr* N3 1976.0 494.0

N-methylimidazole N3 189.2 47.3PPNCl Cl 748.0 187.0

PPNOAc Cl 592.0 148.0PPNI Cl 421.0 105.3