|
|
|
Computational
Subgroup The computational subgroup has
three main interests: ab-initio modeling of
reaction mechanisms at catalytically active metal centers; use of
localized bond concepts to understand molecular shapes and binding energies,
especially around transition metal centers; and advancement of Natural Bond
Orbital methods, especially as
relates to transition metals. In
the past, our group has used ab initio methods to develop the VALBOND force
field method for transition metal-ligand interactions.
Our most current projects are a potential energy surface for the binding
of O2 and C2H4 substrates to a model Pd
catalyst in solution, a survey of bond enthalpies for various metal-ligand
bonding types, and developing a Natural Bond Orbital–based initial guess
method for electronic structure calculations. Potential
Energy Surfaces for Pd-X Binding (X = O2, C2H4) Using hybrid density
functional theory, we have computed both ground and excited state potential
surfaces for the binding of O2 and C2H4 to a
small model Pd catalyst. We have
used Natural Bond Orbital analysis to interpret the electronic structure,
leading us to a detailed explanation of the binding mechanism.
Our findings are consistent with the
results of published experimental studies on a similar system.
CASSCF studies are underway to confirm the overall picture and to obtain
more accurate energies at key points along the potential surfaces. Survey
of Bond Enthalpies Using a variety of ab initio
methods, we have calculated gas-phase binding energies for a selection of
ligands to compounds of all of the transition metals in groups 3-11.
Ligands are grouped by the type of bond according to the VALBOND
classification, and the secondary effect of other ligands on binding energies
are explored for some metal centers. Through
an understanding of periodic trends and dependence of bond enthalpies on steric
and electronic factors, we hope to establish a knowledge base from which many
other metal-ligand bond enthalpies can easily be estimated. Natural
Bond Orbital Method Development Our main interest at this time is the use of Natural Bond Orbitals to generate ground and excited state initial guesses for ab initio electronic structure calculations. We have also recently been involved in improving the Natural Resonance Theory description of delocalized transition metal-ligand bonding patterns.
The development of new pharmaceuticals is increasingly dependent on
enantiomerically pure compounds. Generating
enantiomerically pure chemicals from racemic starting materials can be
accomplished with enantioselective transition metal catalysts.
The primary focus of our research is to design enzyme-like phosphines
containing properly positioned functional groups that can interact with the
chiral environment of an active catalyst. It
is difficult to predict which ligand design will be most beneficial for a
catalyst system, therefore it is advantageous to be able to easily synthesize a
large array of structurally and electronically diverse phosphines and to be able
to screen this ligands rapidly for a range of different catalytic
transformations. The
diazaphospholane ligand structure that we have developed (see figure), allows us
to easily access a large range of structures. We have applied chiral diazaphospholanes to one catalytic
reaction (allylic alkylation) and have achieved the highest enantioselectivity
reported to date. We are currently
exploring other reactions such as hydrogenation, hydroformylation, and allylic
amination and we are also working on new types of diazaphospholanes.
The polymerization subgroup uses a
variety of techniques to perform detailed kinetic studies on homogenous Zeigler-Natta
olefin polymerization catalysts in order to answer fundamental mechanistic
questions about these catalysts. Currently,
we are using low-temperature in-situ Nuclear Magnetic Resonance (NMR)
spectroscopy to observe the polymeryl zirconocene species and determine rates of
initiation, propagation, and termination. Our
current catalyst system, rac-(C2H4(1-indenyl)2)Zr(CH3)2,
can be reacted with B(C6F5)3 to produce species
1, which was characterized via 1H NMR spectroscopy at –40°
C. Compound 1 can be reacted
with 1-hexene to produce the polymeryl species 2 and by careful control
of temperature and concentration, the rate law for this reaction is derived.
Compound 2 is useful in that it can be reacted with propene or
ethylene (species 3 and 4, respectively) to determine rate
constants for these reactions. One
current project involves reacting species 2 with a variety of olefins
displaying different electronic and steric properties in order to determine
structure-activity relationships. Other
projects include examining different catalysts with different activators and
studying the effects of a more polar solvent on the catalyst system.
|
