A Quantitave View of MetalOlefin Bonds: A Step Beyond the DewarChattDuncanson Description

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A Quantitave View of Metal-Olefin Bonds A Step
Beyond the Dewar-Chatt-Duncanson Description 

• David L. Cedeno
• Illinois State University
• Normal, IL 61790

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Why do we need to know more about metal-olefin
bonding interactions?

• Metal-olefin complexes are involved in many
  catalytic systems polymerization, hydrogenation,
  epoxidation, etc.
• Metal-olefin complexes are involved in biological
  systems ageing of plants, flowers, fruits.
• Lots of processes involve breaking and forming
  metal-olefin bonds.
• We want to understand how olefins bind to metals
• why do some olefins bind better than others to a
  particular metal?
• why do some metals bind better than others to a
  given olefin?
• How can we control metal-olefin interactions?

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Classical description of the metal-olefin
interaction The Dewar-Chatt-Duncanson Model
s bond

• Based on Frontier Molecular Orbital Theory
  (Dewar, Bull. Chem. Soc. Fr., 1951, 18, C71-79
  Chatt and Duncanson, J. Chem. Soc., 1953, 2939)
• Bond results from two interactions
• Sigma Olefin HOMO (p) donates electron density
  to the metal LUMO (a dsp hybrid)
• Pi (or back bonding) Metal HOMO (d-character)
  donates electron density to the olefin LUMO (p)

p bond
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Classical description of the metal-olefin
interaction The Dewar-Chatt-Duncanson Model

• DCD is commonly used to qualitatively explain
• Geometrical changes in the olefin CC bond
  stretches due to back bonding.
• Olefin rotation around the metal-olefin bond axis
• Extent of the metal-olefin interaction, which can
  be measured as a bond strength. Model is very
  limited in this aspect.

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The Dewar-Chatt-Duncanson Model and metal-olefin
bond strengths?

• Qualitative nature of model prevents any complete
  rationalization of metal-olefin bond strengths
• .
• For example, one may qualitatively predict that
  the more electron withdrawing an olefin is, the
  stronger the metal-olefin bond is. Then, for the
  series M(CO)5(C2X4) metal-olefin bond energies
  may be arranged in the order
• C2F4 gt C2Cl4 gt C2H4
• Cedeno and coworkers have shown that
  reorganizational and steric effects may be large
  enough to alter the order.

Cedeno and Weitz, J. Am. Chem. Soc. 2001, 123,
12857 Schlappi and Cedeno, J. Phys. Chem. A,
2003, 107, 8763
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DCD and experimental bond strenghts

• Cedeno and Weitz (J. Am. Chem. Soc., 2001, 123,
  12857)
• For the series Cr(CO)5(X2CCX2), X F, Cl, H
• The most electron withdrawing olefin does not
  necessarily form the strongest metal-olefin bond

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Research Strategy

• Since DCD does not take into account all the
  factors involved in the interaction, we propose a
  quantitative extension to DCD.
• Gather more experimental data Measurement of
  Bond Enthalpies
• Bond energies reflect the strength of the
  interaction
• (L)nM-olefin heat ? (L)nM olefin
• Use Quantum Mechanical Calculations to account
  for all factors in the interaction Bond Energy
  Decomposition

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Current Studies

• Metal-Cycloolefin complexes How does ring
  strain affects the metal-olefin bond energy?
• Metal-haloolefins complexes Why is that
  metal-olefin bond energies do not increase with
  an increase in the number of electron withdrawing
  atoms around the CC bond?

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Experimental Techniques Measurement of Bond
Dissociation Enthalpies (BDE) in Solution

• Time Resolved Photoacoustic Calorimetry

Conservation of energy analysis DHtotal DH1
DH2 are obtained
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Photoacoustic calorimetry
f is the fraction of laser energy that is not
used in the reaction. Using a suitable
calorimetric reference (f 1, Frxn 0) K is
eliminated and f is obtained for any sample. For
the reaction
Acoustic detector
MLn(CO) olefin ? MLn(olefin) CO
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Experimental Techniques Measurement of Bond
Dissociation Enthalpies (BDE) in Solution

• Time Resolved Photoacoustic Calorimetry

Signal Deconvolution (Sound Analysis V 1.50D,
Quantum Northwest Inc.)
Thermodynamic Parameters f1 and f2 Kinetic
Parameter k2
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Bond Energetics from PAC Summary
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Quantum Mechanical Studies Bond Energy
Decomposition Analysis
Method Density Functional Theory (DFT) Geometry
Jaguar, PWP91 and BP86, LACV3P Decompositiona
ADF, BP86 and VWNP91, STO-TZ
a. Ziegler and Rauk, Inorg. Chem., 1979, 18, 1755
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Current Studies

• Metal-Cycloolefin complexes How does ring
  strain affects the metal-olefin bond energy?
• Klassen et al., in Bonding Energetics of
  Organometallic Compounds, Marks, T. J., Ed., ACS
  Symp. Ser. 428, ACS, Washington D.C., 1990, pp.
  195.
• Regarding the differences in Cr-olefin (olefin
  hexene, cis-cyclooctene, and trans-cyclooctene)
  bond energies in Cr(CO)5(olefin) complexes
• Perhaps this stronger bonding interaction
  (Cr-ciscycloctene vs. Cr-hexene) can be
  attributed to the 6 kcal/mol ring strain found in
  cis-cyclooctenethe metal-olefin bond in
  (Cr(CO)5(transcyclooctene)) is more than 5
  kcal/mol stronger than that in (Cr(CO)5(ciscyclooc
  tene)). Coordination of the olefin relieves a
  substantial portion of this strain resulting in a
  greater bond strength

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Studies on M(CO)6(Cyclo-olefins), M Cr, Mo, W
Assignment based on Pope and Wrighton, Inorg.
Chem., 1985, 24, 2792 Schultz and Krav-Ami, J.
Chem. Soc, Dalton Trans, 1999, 115
W(CO)5(cyclopentene) is stable in solution (lasts
for hours). Its slow decay suggests that W-olefin
bond energy is above 20 kcal/mol.
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DFT Calculated Results W(CO)5(Cycloolefin)
Geometries
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Photoacoustic Calorimetry Results
M(CO)5(Cycloolefin), M Cr, Mo, W Cycloolefin
Cyclopentene, Cyclohexene
Cedeno et al., manuscript in preparation
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Experimental and DFT Calculated Results Bond
Energy Trends
M Cr
M Mo
M W
Cedeno and Sniatynsky, manuscript in preparation
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DFT Calculated Results Bond Energy Trends
Thus, the M-Olefin bond energy trend follows the
ring strain energy trend, but Why do
cyclopropene and cyclobutene actually bond weaker
than anticipated from ring strain energy?
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Electronic interactions and Ring Strain Relief
Data for M W
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Quantum Mechanical Studies Bond Energy
Decomposition Analysis
Method Density Functional Theory (DFT) Geometry
Jaguar, PWP91 and BP86, LACV3P Decompositiona
ADF, BP86 and VWNP91, STO-TZ
a. Ziegler and Rauk, Inorg. Chem., 1979, 18, 1755
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Bond Energy Decomposition Results (M Cr)
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Electronic Interactions and Strain Relief
Data shown is for M Mo
Cyclopropene
Cyclobutene
Cyclopentene
Cyclohexene
Cycloheptene
cis-Cyclooctene
trans-Cyclooctene
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The Energetic Cost of Olefin Reorganization
Data shown is for M Mo
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Electronic interactions and Ring Strain Relief
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Conclusions

• Metal-cycloolefins bond strengths correlate well
  with the trend in ring strain energy
• Ring strain is alleviated by the reorganization
  of the olefin as it binds
• Both elongation of the CC bond and torsion
  around the CC bond relieve strain.
• Extent of the electronic interaction is the
  dominant factor. The larger the rehybridization,
  the more strain is relieved
• However, olefin reorganization is energetically
  costly, thus reducing the overall BDE.
• Reorganizational energy affects cyclopropane the
  most as it goes from a planar to highly pyramidal
  structure (0o to 26o).
• Interestingly, trans-cyclooctene suffers little
  reorganization since it is already highly
  pyramidal when not bonded, its pyramidalization
  angle just changes from 21o to 25o

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Current Studies

• Metal-haloolefins complexes
• Why is that metal-olefin bond energies do not
  increase with an increase in the number of
  electron withdrawing atoms around the CC bond?

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DCD and experimental bond strengths

• Tolman, C. A. (J. Am. Chem. Soc., 1974, 96,
  2780)
• Regarding to a series of (olefin)bis(tri-o-tolyl
  phosphite)nickel complexes
• It is commonly believed that fluoro-olefins
  form more stable metal-olefin bonds than do
  hydrocarbonsWe were extremely surprised to find
  that none of the fluoro olefins examined were as
  good as C2H4 in coordinating to Ni(0)

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Nickel-Olefin Bond Energies as a function of the
number of halogens around CC (Schlappi and
Cedeno, J. Phys. Chem. A, 2003, 107, 8763)
Tolmans complexes Ni(P(O-otolyl)32(C2FnH4-n),
n 0-4 This Study Ni(CO)(PH3)2(C2XnH4-n), n
0-4, X F, Cl
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Nickel-Olefin Bond Energies as a function of the
number of halogens around CC
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Nickel-Olefin Bond Energies as a function of the
number of halogens around CC
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Nickel-Olefin Bond Energies as a function of the
number of halogens around CC
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Conclusions

• As predicted by the Dewar-Chatt-Duncanson model
  orbital interactions increase as the
  electronegativity of the olefinic substituent
  increases but the Metal-Olefin bond strength will
  depend on the extent of the steric interactions
  and deformations of both the olefin and the metal
  fragment.
• The deformation energy in the metal fragment is
  mainly a consequence of the metal fragment-ligand
  steric interactions
• The bulkier the ligand the larger the
  deformation.
• The more sterically restricted is the metal the
  larger is the metal fragment deformation
• The olefin deforms as a consequence of both
  steric interactions and the change in
  hybridization in the olefinic carbons
• The more electron withdrawing are the
  substituent is, the larger the elongation of the
  C-C bond.
• Increase in rehybridization increase
  reorganizational energy at expenses of the
  overall bond strength

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Acknowledgements

• Illinois State University - Students
• Darin Schlappi, Richard Sniatynsky
• Tom Walczak, Cole Hexel, Kenneth Kite
• Julio Martinez, Casey Huftington, Delano Robinson
• Professor Eric Weitz at Northwestern University
• Funding
• ACS-PRF (39604-GB3)
• Illinois State University Office of the Provost,
  College of Arts and Sciences, Department of
  Chemistry
• ACS-Project SEED
• Strem Chemicals