The Jordan Group
Jordan Group Research
Research in the Jordan group is focused on synthetic
and mechanistic organometallic chemistry. The central theme of this
program is the interplay between the structures and reactivity of
organometallic compounds, especially in systems that are relevant to
catalysis. We design and study novel organometallic complexes for use
as practical catalysts, synthetic reagents, or probes of fundamental
mechanistic issues in catalysis. We use a wide array of synthetic,
mechanistic, spectroscopic and computational tools for these studies,
including advanced anaerobic synthesis techniques, NMR spectroscopy,
ESI-mass spectrometry, X-ray crystallography, and molecular modeling.
Ph.D. graduates and postdocs from the group go on to leading positions
in industry, academia and national labs.
Our current efforts are focused on five major
topics: (i) fundamental studies of d0 metal olefin
complexes, (ii) insertion polymerization of polar olefins to
functionalized polyolefins, (iii) stereoselective catalysis using
chiral metallocenes, (iv) scorpionate-based post-metallocene catalysts,
and (v) the design of multidentate Lewis acids for catalytic
applications. We are also interested in main group chemistry. These
projects are summarized below.
d0 Metal Olefin Complexes
Catalysts derived from Cp2ZrX2
and other "metallocenes" exhibit high activity for the insertion
polymerization of olefins. Metallocenes are single-site catalysts and
produce polyolefins with narrow molecular weight and composition
distributions. We have shown that the active species in Cp2ZrX2-based
catalysts are d0 Cp2ZrR+ cations,
which are generated from Cp2ZrX2 precursors by
alkylation and R-/X- abstraction reactions. Chain
growth occurs by coordination of monomer to the active species and
migratory insertion (Scheme 1). Chain transfer typically occurs by β-H
elimination.
The Cp2Zr(R)(olefin)+ and Cp2Zr(H)(olefin)+
intermediates are transient species that have never been directly
observed! These d0 species are challenging to study because the
metal-olefin binding is weak, due to the absence of d-π* backbonding,
and the insertion barriers are very low. We have designed model d0
metal olefin complexes (Scheme 2) that cannot undergo insertion,
and we are probing their structures, bonding and dynamics. These
studies are providing new insights to important issues in
polymerization catalysis, including how the monomer is activated upon
coordination, catalyst structure/performance relationships, insertion
regiochemistry, tacticity control, and monomer reactivity trends in
copolymerization. Our current objective in this area is to design Cp2Zr(R)(olefin)+
species that are stable enough to be observed, and reactive enough to
undergo insertion. We will use these systems to learn how the Cp2Zr
structure influences the thermodynamics of monomer binding and the
insertion barrier.
The Polar Monomer Problem
It is possible to polymerize simple hydrocarbon
olefins such as ethylene, α-olefins, dienes, styrenes, and norbornenes
to a broad range of polymers with exquisite control by insertion
chemistry using appropriately tailored catalysts. However, known
catalysts have essentially no tolerance for polar functional groups on
the monomer, which significantly limits the scope of insertion
polymerization. The development of catalysts that can incorporate functionalized olefins in insertion
polymerization would enable the synthesis of new polymers that are not
accessible by current methods. "Polar" CH2=CHX monomers with
functional groups directly bonded to the olefin unit, such as vinyl
halides, vinyl esters, vinyl ethers, alkyl acrylates, and acrylonitrile
are of particular interest. A major goal of our current research is to
develop catalysts that can handle such polar monomers. Long term
polymer targets include stereoregular PVC and linear ethylene/vinyl
chloride copolymers.
We are studying the reactions of single-site
catalysts with CH2=CHX monomers to probe monomer
coordination modes, monomer insertion rates, insertion
regioselectivity, and the properties of metal alkyls that contain
functional groups at the alpha and beta positions of the alkyl chain.
We are using the results of these basic studies to develop new concepts
for the design of functional-group-tolerant catalysts. For example, we
have investigated the reactions of Zr, Ti, Fe, Co, Ni and Pd
single-site catalysts with vinyl chloride (Scheme 3). Active species in
these systems readily coordinate and insert VC, and ultimately form
olefins and metal chloride species by net 1,2 insertion and
beta-chloride elimination. This process terminates chain growth.
However, deuterium labeling experiments show that several mechanisms
are operative for these reactions and provide leads to how a catalyst
that does not undergo this termination process might be developed.
Stereoselective Catalysis using Chiral Metallocenes
Chiral group 4 ansa-metallocenes, in which the two
cyclopentadienyl ligands are linked, are important stereoselective
olefin polymerization catalysts, and catalyze stereoselective olefin
and imine hydrogenation, Diels-Alder reactions, olefin
cyclopolymerizations, and other processes. We are investigating the
synthesis of ansa-metallocenes and the application of these compounds
in asymmetric catalysis. We recently developed a powerful synthesis of
enantiopure metallocenes that exploits the conformational properties of
a chiral chelating bisamide ligand to control stereoselectivity (Scheme
4).
We are now using this approach to prepare libraries of enantiopure
metallocenes that we will use in diverse asymmetric catalytic
applications. For example, some years ago we showed that Cp2ZrR+
species activate and functionalize C-H bonds of pyridine substrates
(Scheme 5). We are exploring new versions of these reactions for
application in asymmetric heterocycle synthesis.
Scorpionate-Based Post-Metallocene Catalysts
Our fundamental studies of catalytic mechanisms and
cationic metal alkyls have provided a detailed understanding of the
structural and electronic factors that are necessary for polymerization
activity in metallocene catalysts. We are using these principles to
design new "post-metallocene" catalysts with novel properties. One
current effort focuses on catalysts that contain tris-pyrazolyl-borate
(Tp), i.e. "scorpionate" ligands.
Scorpionate ligands are anionic, 6-electron donors
like Cp-, but are hard N-donors instead of soft C-donors.
Scorpionates provide new opportunities for tuning the electronic and
steric properties of catalysts. We have shown that MAO activation of
group 4 TpMCl3 complexes that contain bulky scorpionate
ligands generates highly active ethylene polymerization catalysts,
which display interesting properties, including the production of ultra
high molecular weight polyethylene and high 1-hexene incorporation in
ethylene/hexene copolymerization. Our working hypothesis is that the
active species in these catalysts are TpMR2+
cations. We are exploring the chemistry of Tp*Zr(CH2Ph)2+
(Tp* = HB(3,5-Me2pz)3) to probe this hypothesis.
The reaction of Tp*Zr(CH2Ph)3
with [Ph3C][B(C6F5)4] at
–60 °C yields the Tp*Zr(CH2Ph)2+
cation, which is shown in Scheme 6. Tp*Zr(CH2Ph)2+
undergoes an unusual rearrangement to a bis(pyrazolyl)borate complex
{(PhCH2)(H)B(-Me2pz)2}Zr(2-Me2pz)(CH2Ph)
at 0 °C! Both cations are highly active for ethylene polymerization
and both display pseudo-living behavior at low temperature. We also
found that both cations undergo multiple insertions of the model
substrate 2-butyne, but in the latter case a spectacular rearrangement
occurs to generate a Cp* species! These studies show that the TpMCl3/MAO
catalysts are much more complex than previously appreciated. We are
currently exploring the basic reaction chemistry of these fascinating
systems and their utility as "living catalysts" for the synthesis of
novel polymers.
Multidentate Lewis Acids
A critical component of metallocenes and other
catalysts that contain highly electrophilic cationic active species is
a weakly-coordinating, non-reactive anion that charge-balances but does
not deactivate the active cation. Only a few suitable anions are known,
notable examples being fluorinated aryl borates such as B(C6F5)4-,
halogenated closo polyhedral carboranes such as HCB11Me5Br6-,
and the poorly characterized anions formed in MAO-activated catalysts.
We are investigating a new strategy for generating stable anions which
is based on encapsulation of simple anions in tailored multidentate
Lewis acid hosts (Scheme 7).
Catalytic Chemistry of Metal Carborane Complexes
The nido carborane cluster C2B9H112-
coordinates to metals in an eta-6 fashion by formal donation of six
electrons, and thus is a dianionic analogue of the commonly used
monoanionic Cp- ligand. The use of C2B9H112- in place of Cp-
enables the construction of unique complexes with new metal/charge
combinations. For example, group 4 metal (η5-C2B9H11)(Cp)MR
complexes are neutral analogues of Cp2MR+ olefin
polymerization catalysts. We have studied the chemistry of (η5-C2B9H11)(η5-C5R5)MR
species to probe how the metal charge influences the olefin insertion
and beta-H elimination reactivity, and hence chain growth and chain
transfer reactivity in polymerization. We have also discovered that
group 4 metal carboranes exhibit unique reactivity. For example, (C2B9H11)(C5Me5)HfR
species are incredibly active catalysts for the selective dimerization
of terminal alkynes, and operate by a "self-correcting" mechanism in
which a catalyst "error" that would normally lead to a side product
triggers a cascade of reactions that modify the catalyst structure and
enhance selectivity (Scheme 8).
Super Electrophilic Main Group Complexes
Simple aluminum halides and alkyls such as AlX3
and AlR3 are strong Lewis acids and good alkylating agents,
and are widely used in catalysis and synthesis. Cationic low coordinate
Al species are interesting because the increased electrophilicity
resulting from the cationic charge may enhance substrate coordination
and activation. We have developed methods for the generation of
cationic Al, Ga and In alkyls and are investigating their chemistry. We
have prepared many interesting species including the first 3-coordinate
cationic Al alkyls, the first Al chlorocarbon complexes, and unique
dinuclear dicationic Al species (Scheme 9). We are exploring the
reactivity of these unusual species.
