High-Energy Processes in Organometallic Chemistry

341 Pages·1987·5.289 MB·English

by  Kenneth S. Suslick (Eds.)

333
ACS S Y M P O S I UM SERIES
High-Energy Processes
in Organometallic Chemistry
Kenneth S. Suslick, EDITOR
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0 University of Illinois at Urbana-Champaign
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bk- Developed from a symposium sponsored by
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02 the Division of Inorganic Chemistry
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0. at the 192nd Meeting
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American Chemical Society, Washington, DC 1987
In High-Energy Processes in Organometallic Chemistry; Suslick, K.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
Library of Congress Cataloging-in-Publication Data
High-energy processes in organometallic chemistry.
(ACS symposium series, ISSN 0097-6156; 333)
"Developed from a symposium sponsored by the
Division of Inorganic Chemistry at the 192nd Meeting
of the American Chemical Society, Anaheim,
California, September 7-12, 1986."
Includes bibliographies and indexes.
1
0 1. Organometallic chemistry—Congresses.
0
w 2. Radiation chemistry—Congresses.
33.f I. Suslick, Kenneth Sanders. II. American Chemical
3
0 Society. Division of Inorganic Chemistry. III. American
87- Chemical Society. Meeting (192nd: 1986: Anaheim,
19 Calif.) IV. Series.
k-
b QD410.H54 1987 547'.07 86-32245
21/ ISBN 0-8412-1018-7
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In High-Energy Processes in Organometallic Chemistry; Suslick, K.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
ACS Symposium Series
M. Joan Comstock, Series Editor
1987 Advisory Board
Harvey W. Blanch Vincent D. McGinniss
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Chemistry Research Environmental Health
In High-Energy Processes in Organometallic Chemistry; Suslick, K.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
Foreword
SYMPOSIUM SERIES
The ACS was founded in 1974 to provide a
medium for publishing symposia quickly in book form. The
ADVANCES
format of the Series parallels that of the continuing
IN CHEMISTRY SERIES
except that, in order to save time, the
1 papers are not typeset but are reproduced as they are submitted
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w0 by the authors in camera-ready form. Papers are reviewed under
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0 Advisory Board and are selected to maintain the integrity of the
7-
8 symposia; however, verbatim reproductions of previously pub
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k- lished papers are not accepted. Both reviews and reports of
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In High-Energy Processes in Organometallic Chemistry; Suslick, K.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
Preface
IN THIS VOLUME,
an attempt is made to draw together a wide range of
disciplines and applications that involve high-energy processes in orga
nometallic chemistry. The reader will find here state-of-the-art descriptions
of methodologies that, at first glance, are dramatically disparate. There is,
1
0
0 however, an underlying commonality that is sometimes conspicuous and
pr
3. other times subtle. The cross-fertilization of these approaches should benefit
3
03 them all.
7-
8 The opening chapter gives some historical background to the investi
9
1
k- gation of high-energy processes. The discussions in the subsequent chapters
b
1/ proceed from current studies in the gas phase, to examination of liquid-
2
0
1 phase techniques, and finally to applications in the solid state. The final
0.
doi: 1 ochf athptee rfise lpdr.o vide a perspective on current and future industrial applications
87 | The symposium upon which this book is based was truly international
9
6, 1 with respect to both speakers and audience. One measure of the multidis-
y 2 ciplinary aspects of the topic was its cross-listing in three ACS divisions
uar other than the sponsoring division. The symposium also allowed a thorough
ebr intermingling of academic and industrial researchers: More than a quarter
F
e: of this volume's contributions are from the latter.
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D Funding for the conference came from the ACS Division of Inorganic
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atio Chemistry; the Petroleum Research Fund, administered by the ACS; and the
blic Alfa Products Division of Morton Thiokol, Inc. Portions of travel expenses
Pu were borne by AT&T Bell Laboratories; BBC-Brown, Boveri & Co.; Exxon

Research and Engineering; and Imperial Chemical Industries, Ltd. The
conference would not have occurred without the generous support of these
organizations. Finally, I wish to thank all of the contributing authors, whose
cooperation and timeliness have been greatly appreciated.
KENNETH S. SUSLICK
University of Illinois at Urbana-Champaign
Urbana, Illinois 61801
November 21, 1986
In High-Energy Processes in Organometallic Chemistry; Suslick, K.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
Chapter 1
High-Energy Processes in Organometallic Chemistry
in Perspective
Peter L. Timms
School of Chemistry, University of Bristol, Bristol, BS8 1TS, United Kingdom
1
0
0
h
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3. The relationship between "conventional" and "high energy"
3
3 processes in organometallic chemistry is defined. The
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7- growth of metal atom chemistry is surveyed as a model for
8
9 the impact of a "high energy process" on the field of
1
k- organometallic chemistry.
b
1/
2
0
1
0. Chemistry can be viewed as a balance between thermodynamic and
1
oi: kinetic factors which dictate the course of chemical reactions and the
d stability of compounds. Chemists seeking to achieve particular goals,
7 | manipulate these factors using chemical or physical means. The papers
8
19 in this symposium on "High Energy Processes in Organometallic
6, Chemistry" describe recent attempts to apply mainly physical means to
2
y get around the thermodynamic and kinetic constraints of conventional
uar organometallic chemistry.
br
e
F
e: Definitions
at
D
n The term "high energy process" needs explanation. Its meaning is seen
atio most easily if we first state that "conventional processes" in
c organometallic chemistry are reactions which occur at a useful rate at
ubli some temperature within the range of about -100° to +250 °C and
P
which do not involve the injection of other forms of energy or of
very shortlived reagents such as atoms or radicals. Then "high energy
processes" are broadly those in which the energy input is not thermal,
e.g. the absorption of light, or is non-equilibrium thermal, e.g. the
effect of ultrasonic cavitation, or in which unstable gaseous species,
produced by an external, energetic process, are added as reactants.
Our definition of a "conventional process" covers an enormous
range of organometallic chemistry. Many organometallic chemists will
want to include photochemical methods as part of their armoury of
"conventional" techniques. This can be taken as a measure of the
success of photochemical methods in the field. Indeed, it is the
ambition of many of us who have worked on the development of
"high energy processes" that our process should be accepted within the
core of basic synthetic techniques of organometallic chemists.
0097-6156/87/0333-0001 $06.00/0
© 1987 American Chemical Society
In High-Energy Processes in Organometallic Chemistry; Suslick, K.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
2 HIGH-ENERGY PROCESSES IN ORGANOMETALLIC CHEMISTRY
One of the first uses of a high energy process in organometallic
chemistry was the work by Paneth (JJ and then Rice (2) on the
reaction of metal mirrors with free alkyl radicals. The alkyl radicals
were formed by thermal decomposition of a zinc or lead alkyl in a
flowing gas stream and the radicals then reacted with a film or finely
divided form of another metal downstream to give a new
organometallic compound. Only a little earlier, the work of Kraus and
Callis (3) had established the very important synthesis of tetraethyllead
from NaPb and chloroethane. This is properly classified as a
"conventional" organometallic reaction and put alongside the many other
reactions where alkali metals or organic derivatives of alkali metals
are used as reducing or coupling reagents. Nevertheless, the work of
Mishima (4) showed that the NaPb/EtCl reaction probably occurs by a
free radical mechanism. This means that there is a very fine
distinction between the "conventional" synthesis of tetraethyllead via in
1
0 situ generation of ethyl radicals and their reaction with lead and a
0
h "high energy" synthesis of tetraethyllead from ethyl radicals and lead
c
3. in a flowing gas stream. The distinctions get more blurred if the
3
03 NaPb alloy is regarded as an activated metal and compared with the
7- activated metals made chemically by Rieke (£), the use of which is
8
19 often grouped with "high energy processes". A related case of the
bk- narrow but definite distinction between "conventional" and "high
1/ energy" processes, come in the search for dimethylsilene some 35 years
2
10 later than Paneth's work. The intermediacy of dimethylsilene in a
10. number of reactions in the condensed phase at temperatures up 250°C
oi: had been proposed for years (6), but the first convincing proof came
87 | d [frCoHm3 0(CthHe3 )Swi]o rkt o roef leasAe twdeimll ethaynlsdil eneW eiyne nbseitrug ati n 1301°9-6260,0 °C (usIi)n.g
9 2 2
1 Shortly before this, Skell and Goldstein (8) had reported the generation
6, of dimethylsilene in a "high energy process" by reacting
2
y dichlorodimethylsilane with potassium vapour in a helium stream at
ar
u 260°C, and allowing the resulting dimethyl- silene to react with
ebr trapping agents downstream. There are often less mechanistic
F
e: ambiguities in the use of free gaseous species as reagents than in the
at in situ generation of the same species in the condensed phase.
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n
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ati Energy Considerations,
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Pu As already implied, "high energy processes" can be divided into two

groups. First, those in which energy is applied in situ, to reactants to
excite molecules to high translational, vibrational or electronic states so
that the excited molecules can undergo immediate intra- or
inter-molecular reactions. Such processes include photochemistry,
radiolysis, shock wave studies, sonication and some discharge reactions.
Second, those in which high energy species such as free atoms,
radicals, carbene-like molecules or ions are formed in the gas phase by
a chemical, thermal, photo- chemical or discharge process, and then
reacted with other molecules in the gas or condensed phase. Before
we consider any of these in detail, it is instructive to see what useful
amounts of energy they involve compared with the energies of
"conventional" processes.
Bonds between many neutral ligands and transition metal atoms in
low oxidation states, have mean bond disruption enthalpies in the
range 80-200 kJ mol-1 (9). Bonds to charged ligands, e.g.
In High-Energy Processes in Organometallic Chemistry; Suslick, K.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
1. TIM MS High-Energy Processes in Perspective 3
cyclopentadiene, are generally stronger. These bond energies are strong
enough to allow many "conventional" chemical synthetic approaches to
be successful but they are often weak enough to permit reactions of
organometallics to occur at moderate temperatures by dissociative or
associative mechanisms. Hence the success of "conventional"
organometallic chemistry.
The in situ "high energy processes" vary in the form of energy
they provide. For example, the use of ultrasonics (sonochemistry) gives
short-lived hot spots in liquids with transient, non-equilibrium
temperatures of thousands of degrees (10). giving local translational and
vibrational excitation to molecules, with the possibility of bond
breaking followed by thermalization and secondary reactions.
Photolysis with light of around 300 nm wavelength, provides electronic
excitation energy of around 400 kJ mol"1 sufficient to bring about
ligand dissociations, rearrangements, reductive elimination and many
1
0 other processes (11). If 7-radiolysis is used, the absorbed energy per
0
ch quanta is vastly greater than bond energies but one of the useful
33. secondary processes is electron capture and the formation of ionic
3
0 species (12).
7-
8 Gaseous transition metal atoms can be taken as representative of
9
k-1 the class of endothermic species which are formed in one place and
b then reacted to form organometallic compounds in another place. The
1/
2 atoms have heats of formation of about 300-800 kJ mol"1.
0
0.1 Nevertheless, their real value in chemical synthesis depends as much on
1 kinetic as thermodynamic factors. A reaction (where M is
oi:
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87 | M(gas) + nL(gas) > MLn(solid) (i)
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1
6, a transition metal atom and L is a ligand molecule) will be
y 2 exothomeric if there is a positive M-L bond dissociation energy; this
ar result is independent of the heat of formation of M(gas). Comparing
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br reaction (i) with another reaction (ii) starting with the solid metal, (i)
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F will be more exothermic than (ii) by an amount
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D M(solid) + nL(gas) > ML(solid) (ii)
n n
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ati proportional to the heat of atomisation of the metal. However,
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bli attempts to achieve direct reactions between ligands and solid transition
u
P metals often failed because the kinetics and sometimes the

thermodynamics are unfavourable. It is more meaningful to compare
reaction (i) with the common reductive process (iii) (where M is an
electropositive metal and X an halogen). The greater
MX + nL + mM' > ML + mMX' (iii)
m
stability of mM'x compared to MX means that the reaction will be
m
exothermic even if the product ML is quite endothermic. So,
n
thermodynamically, reductive routes can achieve almost as much as can
be achieved using the reaction of metal atoms and ligands. In
practise, atom-ligand reactions have such low activation energies that
they will occur at lower temperatures than most reductive reactions,
enhancing the chances of isolating unstable products, and there are less
opportunities for unwanted side reactions in metal atom than in
reductive reactions. So for these reasons, atom reactions are more
successful than reductive reactions in some cases.
In High-Energy Processes in Organometallic Chemistry; Suslick, K.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
4 HIGH-ENERGY PROCESSES IN ORGANOMETALLIC CHEMISTRY
The Development of High Energy Processes in Organometallic Chemistry
The Period 1930-60. Following the work of Paneth (J_), there was a
long period of active work on free radicals, mostly of importance in
organic chemistry but with some relevance to organometallic chemistry.
The radicals were generated by thermal, photochemical or discharge
processes on organic or organometallic compounds and detection was
often by reaction with metal mirrors (13-14). In the early 1950's, the
matrix isolation method was developed to study radicals (15). and this
spectroscopic technique has proved of great value subsquently to metal
atom chemistry. Also in this decade, there was a resurgence of
interest in discharge reactions and organosilicon compounds were made
this way (16-17) (chemistry in arcs and discharges has a long history
and in 1907 Svedberg (1_8) was making metal colloids by striking arcs
between metal electrodes under liquid methane~it is interesting to
1
0 speculate on what organometallic compounds he formed at the same
0
ch time!).
3.
3 The rapid growth of the scope of organometallic chemistry during
3
0 the 1950's meant that by the early 1960's the situation was ripe for
7-
8 the application of "high energy processes". Here we will trace only
9
k-1 one of these processes in detail, the use of free metal atoms in
b synthesis, but with the expectation that there are lessons to be learned
1/
2 from this of relevance to the role of other "high energy processes".
0
1
0.
oi: 1 Work on Free Transition Metal Atoms
d
7 | The Key Developments. The work initiated by Skell and his
98 co-workers in the early 1960's on carbene chemistry, had a profound
1
6, influence on experimental techniques. He was the first to use high
2 temperatures to generate unstable gaseous species under vacuum inside
y
ar a liquid nitrogen cooled reaction vessel. The condensation of radicals
u
br and related species at liquid nitrogen temperatures was not new
Fe (19-20). However, the containment of a very hot radical generator
e: within a liquid nitrogen cooled reactor and the attainment of a
at
D line-of-sight path from the generator to the cooled walls was new.
on His results on reactions of carbon vapour, generated from a carbon
ati arc, with organic compounds on the liquid nitrogen cooled walls of the
c
bli surrounding vacuum vessel (20. paved the way for the subsequent
Pu development of metal atom chemistry.

Between 1965 and 1969, there were rapid developments in the use
of other high temperature species, particularly the silicon dihalides
(22-23). boron monofluoride (24), boron atoms (25), silicon atoms (26),
and alkali metal atoms (27-28). in reactions at liquid nitrogen
temperatures. Clearly this experimental method had to be applied to
transition metal atoms and this happend in 1968-69 (29). All that was
required was to harness the simple and long established methods for
vacuum evaporation of metals used in thin film studies (30), to make
metal atoms to condense with organic compounds at low temperatures.
As is often apparent in retrospect, these experimental methods could
have merged anytime in the previous five years.
The demonstration that stable and unstable organometallic
compounds could be made in gram quantities from transition metal
In High-Energy Processes in Organometallic Chemistry; Suslick, K.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
1. TIMMS High-Energy Processes in Perspective 5
atoms sparked off enormous interest. The resulting chemistry has been
extensively reviewed (31-35) and need not be repeated. More
appropriate is to highlight some points in the development of the
technique which have relevance to its acceptance by organometallic
chemists.
The first reactions with transition metal atoms were carried out
in the quite large and sophisticated stainless steel apparatus built
originally for work on electron bombardment evaporation of boron (25).
However, the immediate next generation of apparatuses were smaller
glass reactors. This simplification was prompted in two ways; the
adaption by Skell of his glass carbon-arc reactors for metal atom
synthesis (36), and the desire here at Bristol to create more apparatuses
quickly and cheaply to put the method into the hands of students
(37). The appearance of simple apparatus which could be assembled
for $1000 or less, greatly stimulated the field.
1
00 Spectroscopists also saw the potential of reacting ligands with
ch transition metal under matrix isolation conditions. Photolysis of metal
33. carbonyls in organic (38) or inert gas matrices (39) had already been
3
0 done, but atoms offered the possibility of step-wise addition of ligands.
7-
8 DeKock (40), Turner (41), and Moskovits and Ozin (42) made early
9
1 contributions, but the work of the last two became dominant (32). By
k-
b 1972, there were the two distinct branches in transition metal atom
1/
2 chemistry, the preparative and matrix spectroscopic studies.
0
1
0.
1 The Years of Rapid Growth. In the period, 1972-75, many people
oi: entered the field of atom synthesis, Klabunde (43), von Gustorf (44),
d
7 | Green (45), and Lagowski (46) to mention a few. The result was that
98 a very large number of new organometallic compounds was synthesised
1
6, sufficient to convince even the most conventional organometallic
2 chemists that this approach had something to offer.
y
ar Most of the work initially was with the more volatile transition
u
br metals, i.e. the first row metals plus palladium, silver and gold,
e
F because these were easy to evaporate in reasonable quantities in simple
e: apparatus. However, efforts to use the less volatile metals of the
at
D second and third rows gained momentum. Skell used sublimation of
on resistively heated wires of molybdenum and tungsten to make the
ati remarkably stable [Mo(r4-CH)] and [W(7}4-CH)] (47). Green
blic attempted to use electron ? bo4m6b3ardment evapora4tio6n3 but initially there
u
P were difficulties because of electron reflection off the target metal at

ground potential, the electrons then destroying the organometallic
products. By 1973, he could evaporate titanium (45) but electron
damage obstructed work with less volatile metals. The problem was
solved by putting the target metal at a high positive potential (48-49)
and from that time on electron bombardment evaporation was a
growing force in the field.
Also in this period, the technique of reacting metal atoms with
ligands in cold solutions was developed (50) and this gave the
possibility of reacting metal atoms with involatile ligands which was
later exploited (51-52). An instructive failure was the largely abortive
attempts by von Gustorf (53) to use a 200 W continuous wave YAG
laser for metal evaporation. Reflection of the laser light from the
metal surface meant that heat transfer was poor and evaporation much
less than was hoped. The much more recent spectroscopic scale work
with laser evaporation (54-55) is successful because of the higher
energy input per laser pulse.
In High-Energy Processes in Organometallic Chemistry; Suslick, K.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.