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| UNC-CH Collaborators |
| Joseph DeSimone |
| Ed Samulski |
| Joe Templeton |
| Brookhart Group Research Projects | |
| Mechanistic Investigations of the (a-Diimine)Ni(II)- and Pd(II)-Catalyzed Polymerization of Olefins | |
In 1995, our group reported a new class of catalysts
based on the group VIII metals nickel and palladium which are capable
of polymerizing ethylene to high molecular weight, branched polyethylene.
The mechanism of propagation involves insertion of ethylene into a
metal-alkyl bond and subsequent isomerization of the product via b-H
elimination and reinsertion with opposite regiochemistry. We
have independently synthesized a number of polymerization intermediates,
including ethyl, propyl, and butyl agostic cations and their corresponding
ethylene complexes. Low temperature NMR studies of these
species allow us to observe and quantify equilibria, insertion,
and isomerization processes for this catalyst system. One
example of a recent experiment showing that agostic tertiary alkyl
complexes are quite stable is shown below:
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| Neutral Nickel-Catalyzed Olefin Polymerization | |
The
goal of this project is to develop neutral nickel catalysts capable
of homopolymerizing ethylene and alpha olefins and of copolymerizing
ethylene and alpha olefins with functionalized monomers such as
acrylates and vinyl acetates. Developmentof efficient transition
metal catalysts for such copolymerizations is a major unsolved
problem and its solution would result in numerous practical applications.
We are also interested in the mechanistic aspects of neutral nickel-catalyzed
homo- and copolymerization as well.
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| Living Polymerization with Pd a-diimine Catalysts | |
The Pd a-diimine catalysts have been shown
to polymerize ethylene, a-olefins and
functionalized monomers. The goals of this project have included
the development living polymerization conditions such that polymer
uniformity in molecular weight and molecular weight distribution
could be controlled. Living polymerization of ethylene and a-olefins
has been explored and achieved under the conditions shown in the
figure below. Currently, we are working towards block copolymerizations
of ethylene with monomers which yield a more crystalline material
to create thermoplastic elastomers as well as the further use of
functional monomers with ethylene polymerization.
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| Ni(II)-Catalyzed Internal Olefin Polymerization for the Synthesis of New Polymeric Microstructures | |
(a-Diimine)Ni(II)
catalysts of the general structure shown below have demonstrated
high polymerization activity as well as the production of branched
ethylene homopolymers, "chain-straightened" poly(a-olefins)
and cyclic olefin polymers. Currently, these systems are being
investigated for the polymerization of acyclic internal olefins
such as trans-2-butene. These homopolymers posess a unique microstructure
similar to that of polypropylene with a methyl branch on every
third backbone carbon. Studies of ligand structure effects on polymer
properties are in progress.
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| Variation of Ligand Electronics in Highly Active Iron and Cobalt Olefin Polymerization Catalysts | |
Research focus is on the synthesis of pyridinebisimine-based complexes of
iron and cobalt with variation of substituents on the imine carbon
(R), and their effects on the polymerization. Studies to determine
the mechanism of activation and polymerization by the iron and
cobalt catalysts are also underway. The synthesis and characterization
of model iron and cobalt alkyl complexes is currently being explored.
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| Mechanistic Studies of Nickel(II) and Palladium(II) Catalysts for the Copolymerization of Ethylene and Carbon Monoxide | |
Investigations of the mechanistic aspects of olefin / CO copolymerization catalyzed
by bisphosphine nickel(II) and palladium(II) catalysts have been
underway in our group for some time. The current project in this
area focuses on cationic nickel(II) complexes of the general formula
[(P-P)NiCH3(OEt2)][BAr'4] (P-P = bisphosphine). Model studies have
elucidated several four- and five-coordinate intermediates relevant
to the alternating copolymerization. In addition, in situ reaction
monitoring techniques have elucidated aspects of these copolymerizations
such as the catalyst resting state and catalyst decomposition pathways.
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| Development of New Cationic Nickel Olefin Polymerization Catalysts | |
| Research focus is on the synthesis, characterization, and evaluation of a new series of olefin polymerization catalysts based on cationic nickel systems. Polymerization of a variety of olefins as well as copolymerization with polar monomers is being explored. In addition, mechanistic studies to determine the catalyst resting state, initiation, propagation, and decomposition pathways are underway. | |
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| Supported Late Transition Metal Catalysts for Slurry and Gas-Phase Polymerization of Ethylene | |
| Pd and Ni diimine catalysts for olefin polymerization give unique control of the microstructure of obtained polymers. To be able to employ these late transition metal complexes in slurry or gas-phase polymerization they have to be supported on different carrier materials. Investigations of the activity of these supported catalysts and properties of the resulting polymers are currently in progress. | |
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| New Catalytic Transformations Based on C-H Bond Activation by Rh Complexes | |
While
extensive synthetic methodology has been developed based on oxidative
additions of C-X bonds (X = halogen, heteroatom), reactions based
on transition metal-catalyzed C-H bond activations are rare. In
our group, a number of C-H bond activations catalyzed by Cp*Rh(olefin)2 complexes
have been developed, namely o-alkylation of aromatic ketones, hydrovinylation
of aldehydes, isomerization of alkyl aldehydes, and transfer hydrogenation
of silyl enol ethers. The main focus is the synthesis of modified
CpRh(olefin)2 complexes with electron withdrawing or bulky substituents
on the Cp ring, use of these complexes to achieve milder reaction
conditions for known transformations, and the development of new
reactions based on these complexes.
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| Catalytic Transfer Hydrogenation | |
Complexes
of the type [C5Me5Rh(olefin)2]
are capable of converting vinylalkoxysilanes to silyl enolates via
catalytic transfer hydrogenation. A series of vinylalkoxysilanes containing
various functionalities and steric bulk have been prepared and used
as substrates for catalytic transfer hydrogenation. The rates and selectivities
of conversion to the resulting silyl enolates are dependent upon the
metal, temperature, and the functionality of the alkoxy substituent,
using [C5Me5M(CH2=CHSiMe3)2]
(M = Co, Rh) as precatalysts. We are continuing to introduce new substrates
in an effort to broaden the scope of these reactions. To probe the
mechanistic details of C-H activation and subsequent hydrogen transfer,
intramolecular H/D exchange experiments using deuterated vinyl alkoxysilanes
have been performed. We aim to develop new catalysts based our mechanistic
findings to optimize catalyst turnover and selectivity.
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| Catalytic C-H Activation with Late Transition Metal Complexes | |
Our group has found cobalt(I) and rhodium(I) complexes mentioned previously to catalyze various C-H activation processes, e.g. the hydroacylation of olefins, the intramolecular transferhydrogenation, the Murai reaction, and aldehyde isomerization. Nevertheless, the catalysts suffer from degradation due to the formation of catalytically inactive carbonyl species. In order to circumvent these pathways, studies are conducted to prepare isoelectronic d8 complexes with ruthenium(0) and palladium(II) as the metal center. Complexes 1, 2, and 3 are currently targeted as potential catalysts for the above mentioned C-H activation processes.
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| Bond Activation Reactions Using [Cp*(PMe3)Rh(R)]+ (R=H, Me, Ph) | |
Cationic rhodium(III) complexes of the form [Cp*(PMe3)Rh(R)]+ (R = H, Me, Ph) have been used to study C-H, C-C, and Si-H activation reactions using a variety of organic substrates. The primary research focus is on understanding the mechanism under which these processes operate and the general viability of the rhodium complexes. Sample reactions are shown below:
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Last Updated:
January 17, 2007
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