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Refreshing Catalytic
Studies

12 July
2001

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A
new way of looking at catalytic reaction mechanisms that could
revolutionise our understanding of countless industrially
important processes and ultimately lead to new and improved
versions has been developed by a team at the University of
Nottingham, UK, writes David
Bradley.

Martyn
Poliakoff, Mike
George and their colleagues are perhaps best known for
their pioneering work in the area of supercritical fluids but
underpinning their research in this field is the basic need to
develop more efficient and effective catalytic reactions. One
area in which they have been particularly active is in
studying homogeneously catalysed reactions involving gases, in
particular, hydrogen. Such reactions, which occur between
volatile transition-metal complexes and gases, are a recurrent
feature of organometallic chemistry. However, looking closely
at the way in which the catalyst assists the conversion of
reactants into products has until now relied on several rather
specialised spectroscopic approaches, simply because the
gas-phase intermediates in these reactions are far too
short-lived at room temperature to catch in action without
them.

Low-temperature matrix isolation has been the key to
studying catalytic reactions so far. By slowing the reaction
to a rate at which the intermediates are essentially suspended
for longer between reactant and product, conventional
spectroscopic methods can be used to get a close look at their
structures. For instance, an organometallic might be frozen in
a noble gas or hydrocarbon glass between 10 and 77 K. The
main drawback is that any thermal steps in the reaction cannot
then be seen because of the low temperatures involved.

“The intermediate is caught in
a freeze frame — like a dancer under a strobe
light.”

Ultraviolet (UV) flash photolysis offered an alternative
view that could be seen at room temperature using
time-resolved infrared (IR) spectroscopy. The UV flash
essentially blasts apart the intermediate on the femto- to
milli-second timescale so that it is caught in a freeze frame
— like a dancer under a strobe light. The technique is
powerful not least because it allows reactions to be studied
under more realistic conditions than in matrix isolation but
still some aspects of a catalytic mechanism remain opaque to
the flash.

The polymer matrix
immobiliser

In their latest work appearing in J Am Chem Soc, the
Nottingham team reveal how they have discovered that a polymer
matrix can be used to immobilise a reaction in limbo. With the
intermediates suspended in this manner, Fourier transform IR
(FTIR) spectroscopy and coupled gas chromatography–mass
spectrometry (GC–MS) can be used to look at even the most
complicated catalytic mechanism. They have obtained 'a
surprisingly high level of control' over otherwise thermally
unstable intermediates. The team has applied the method to two
catalytic reactions — the hydrogenation of dimethyl fumarate
(DF) and of norbornadiene (NBD) to demonstrate proof of
principle.

The team used a polyethylene (PE) matrix to freeze out the
intermediates in a similar manner to standard matrix
isolation, but at high pressure inside a unique cell. The
major difference, however, was that they could then warm the
matrix without it vaporising to track how any intermediates
trapped inside the PE change as the temperature rises.

The team applied the technique to the hydrogenation of DF
using [Fe(CO)4(η2-DF)] as the catalyst. They first
irradiated the catalyst in the PE matrix at 150 K. This
led to the formation of an intermediate complex in which the
DF is coordinated to the iron, revealed by FTIR spectroscopy.
Now, when the matrix is warmed to 260 K under hydrogen
gas, H2 is tacked on to this
intermediate, a step which they again could see clearly using
FTIR spectroscopy. The final stage involved further heating
which hydrogenates the coordinated DF forming dimethyl
succinate (DS) as the product as identified by GC–MS at room
temperature.

Similarly, UV photolysis of [(NBD)M(CO)4] (where the metal is either Cr or
Mo) in the PE matrix under hydrogen gas yields the
hydrogenated product norbornene (NBN) and nortricyclene (NTC),
and a trace of norbornane (NBA). The team explains that the
product ratios are close to those observed in a more
conventional hydrocarbon solution. However, for the molybdenum
catalyst the team found they could change the outcome
considerably if they removed free NBD. NBA then becomes the
main product. Further studies with hydrogen and deuterium gas
exchange, using the specially designed pressure cell, then
allowed them to see how this change in product profile might
arise. Such isotopic labelling experiments, the team points
out, are highly revealing but nigh on impossible in standard
experiments because there is no means of controlling the
various gases without releasing the pressures.

“A change of perspective can
generate something new.”

Considering that this reaction has been studied for more
than 30 years it is intriguing to find that a change of
perspective can generate something new that might lead to a
clearer understanding of its complex reaction mechanism. The
clue lies in the previously recognised non-classical
hydrogen-complexed intermediates in which the hydrogen
molecule is coordinated to the metal 'through' its bond rather
than with direct M–H atom connectivity. The difference between
the outcome of the molybdenum and the chromium-catalysed
reaction results from the difference in how each of those
metals favour bonding in this non-classical way to the
hydrogen molecule and the impact this has on the overall
structure of the intermediate formed.

The team believe there are still several questions about
these particular reactions that remain to be answered. The
origin of the doubly hydrogenated NBA, for instance, remains a
minor mystery. The researchers also point out that while they
could detect and identify many intermediates with their new
technique they never actually observed directly the crucial
reaction step — the transfer of hydrogen from the metal to the
organic substrate. This may become transparent to further
studies in time, but the demonstration of the power of the new
technique is beyond doubt. Homogeneous catalysis has never
before been so see-through.
Reference: Gavin I. Gavin I. Childs, Andrew
I. Cooper, Trevor F. Nolan, Michael J. Carrott, Michael W.
George & Martyn Poliakoff. A new approach to studying
the mechanism of catalytic reactions: an investigation into
the photocatalytic hydrogenation of norbornadiene and
dimethylfumarate using polyethylene matrices at low
temperature and high pressure. J Am Chem Soc 2001,
123(28):6857–6866.

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