Why are we interested on metalolefin bonding interactions

1
Laser-based Methods Applied to the Study of
Metal-Olefin Interactions and the Photophysics of
Sensitizers
David L. Cedeno, Ph.D. Department of
Chemistry Illinois State University Box
4160 Normal, Illinois, 61790-4160 Ph
1-309-438-5595 E-mail dcedeno_at_ilstu.edu
2
Research Focus
General Use spectroscopic and computational
tools to study thermodynamic and kinetic aspects
of different systems

• Energetics and kinetics in Organometallic
  Chemistry. Relevance Agriculture Rational
  design of anti-ripening compounds
• 2. Effects of molecular structure on yields of
  triplet state of oxygen photosensitizers.
• Relevance Clinical Rational design of
  efficient photodynamic therapy compounds and
  fluorescent diagnostic probes.

3
Why are we interested on metal-olefin bonding
interactions?

• Lots of processes involve breaking and forming
  metal-olefin bonds.
• Metal-olefin complexes are involved in many
  catalytic systems polymerization, hydrogenation,
  epoxidation, etc.

• Metal-olefin complexes are involved in biological
  systems growth of plants, senescence of flowers,
  ripening of fruits.
• 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?

4
Ethylene as a plant hormone

• Ethylene binds to protein receptors in plants to
  signal developmental processes as seed
  germination, plant growth, fruit ripening, flower
  abscission and senescence.
• Ethylene signaling involves a family of
  sensor/response regulator proteins (ETR, EIN,
  AIN, etc. The receptor is a negative regulator of
  a protein kinase.

Ciardi and Klee, Annals Bot., 2001, 88, 813 Chang
and Stadler, BioEssays, 2001, 23, 619
5
Anti-ripening control Blocking ethylene action

• It has been proposed that ethylene action at the
  receptor site requires two steps
• Competitive antagonists block the receptor by
  bonding to it for a longer time that ethylene
  does, thus preventing the activation of the
  receptor for signaling.

Burg and Burg, Science, 1965, 148, 1190 Sisler
and Serek, Bot. Bull. Acad. Sin. 1999, 40, 1
6
Anti-ripening control Blocking ethylene action

• Known competitive antagonists include

• An understanding of the metal-olefin interaction
  is important in designing anti-ripening
  compounds.
• Why are cyclic olefins so special? Ring strain
  affects olefin-receptor interaction.
• Are there any other effects? Is it possible to
  control the strength of the metal-olefin bond?

7
Towards a quantitaive description of metal-olefin
interactions Extending the Dewar-Chatt-Duncanson
Model

• Qualitative nature of model prevents any complete
  rationalization of metal-olefin bond strengths,
  because it 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

• Dewar, Bull. Chem. Soc. Fr., 1951, 18, C71-79
  Chatt and Duncanson, J. Chem. Soc., 1953, 2939)

8
Experimental Techniques Time Resolved Laser
Photoacoustic Calorimetry
laser
Acoustic detector
f 1 for reference compound
9
Studies on M(CO)6(Cycloolefins), M Cr, Mo, W

• Metal-cycloolefins bond strengths correlate well
  but not exactly with the trend in ring strain
  energy

t
t
10
Electronic interactions, ring strain relief and
Reorganizational effects A molecular paradox
Ring strain is relieved by the reorganization of
the olefin as it binds, however, olefin
reorganization is energetically costly, thus
reducing the overall BDE.
11
Photodynamic Therapy (PDT)
The ideal photosensitizer

• Strong absorption in the red or near infrared
  (gt630 nm) region of the spectrum.
• High quantum yield of triplet state to obtain
  large concentrations of the activated drug.
• High reactivity of the triplet with ground state
  oxygen to obtain measured high yields of active
  oxygen
• High affinity for diseased tissue against healthy
  one, to avoid the risk of photodestruction of
  healthy tissue
• Rapid metabolic rates so it can be excreted from
  the body
• Low toxicity in the dark
• Simple formulation
• Facile synthesis and modification of the
  structure.

12
Mechanisms of oxygen photosensitization
13
Research Goal To establish correlations between
the yield of triplet sensitizer and active oxygen
and the molecular structure of the
photosensitizer. Correlations will lead to a
rational design of sensitizers with optimized
yields of triplet sensitizer and singlet oxygen
Absorption Spectra
Novel Photosensitizers (Prof. T.D. Lash)
Emission Spectra Fluorescence yield Triplet
energy
1O2 Emission
Triplet yield (PAC)
Time
1O2 yield (Traps, TRF)
Computational Methods
14
Experimental Techniques Time Resolved VIS-NIR
Emission Spectroscopy
15
Extension of Conjugation, Distortion of Planarity
and Photophysical Properties
16
Extension of Conjugation, Distortion of Planarity
and Photophysical Properties
TAAP
TAP
TPP
17
Research is a TEAM effort
Collaborators Tim Lash, Marge Jones, Pilar
Mejia (ISU) Eric Weitz (Northwestern
University) Graduate Students Richard
Sniatynsky, Ken Kite, Darin Schlappi Undergraduat
e Students Hal Steiner, Jakoah Brgoch Past
Cole Hexel, Paul Brackemeyer, Joel Eagles, Jeremy
Woods,Tom Walczack, Ken Kite, Darin
Schlappi. High School Students Casey
Huftington, Delano Robinson, Julio Martinez.
Funding Petroleum Research Fund American
Chemical Society. Project SEED American
Chemical Society. Illinois State University
Office of the Provost, College of Arts and
Sciences, Department of Chemistry.