Applications of Polyhedral Oligomeric Silsesquioxanes

449 Pages·2011·18.92 MB·English

by  Elda Markovic

Applications of Polyhedral Oligomeric
Silsesquioxanes
ADVANCES IN SILICON SCIENCE
VOLUME3
SeriesEditor:
JANISMATISONS
SchoolofChemistry,PhysicsandEarthSciences,FlindersUniversity,SouthAustralia.
AdvancesinSiliconScienceisabookserieswhichpresentsreviewsofthepresent
and future trends in silicon science and will benefit those in chemistry, physics,
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NanomaterialsGroup,
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Volume3
ApplicationsofPolyhedralOligomericSilsesquioxanes
VolumeEditor
ClaireHartmann-Thompson
MichiganMolecularInstitute
Midland,MI
USA
Forothertitlespublishedinthisseries,gotohttp://www.springer.com/series/7926
Claire Hartmann-Thompson
Editor
Applications of Polyhedral
Oligomeric Silsesquioxanes
123
Editor
ClaireHartmann-Thompson
MichiganMolecularInstitute
1910W.SaintAndrewsRd.
MidlandMichigan48640
USA
[email protected]
Chapter6wascreatedwithinthecapacityofanUSgovernmentalemploymentandthereforeisinthe
publicdomain.
ISBN978-90-481-3786-2 e-ISBN978-90-481-3787-9
DOI10.1007/978-90-481-3787-9
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Foreword: The Re-Birth of Polyhedral
Oligosilsesquioxane Chemistry
Frank J. Feher
Fundamental research is a wonderful example of chaos. Each researcher makes
individual choices about which fi elds to pursue, which experiments to do and how
to do them, how to interpret their data and the data of others, where to seek funding,
who to consult or collaborate with, and where to present or publish their work. The
end results from fundamental research are deterministic because they are ultimately
governed by universal Laws of Nature, but the path to discovery is impossible to
predict and the order in which Nature’s secrets are unlocked depends greatly on
seemingly trivial decisions. In many ways, fundamental research seems analogous
to the situation described by Lorenz [1], where the fl apping of a butterfl y’s wings in
Brazil can set off a tornado in Texas.
When I was asked to write the foreword to this book, I was struck by the
enormous progress made with polyhedral oligosilsesquioxanes (POS) over the past
two decades. Numerous researchers have made important contributions to the fi eld,
and it is truly remarkable what has been accomplished using these versatile and now
ubiquitous organosilicon compounds. To a newcomer, the contents of this book will
probably create an impression that the current state-of-the-art was preordained to
evolve over the past 25 years. Maybe it was, but as one of the earlier researchers
responsible for the renaissance of POS chemistry, I have often wondered where the
fi eld would be now if some of the early butterfl ies had not fl apped their wings at just
the right moment. From my vantage point, POS chemistry as we know it today is
the result of many unexpected twists and turns that uniquely shaped how the fi eld
developed. I am grateful to the editor of this book (Claire Hartmann-Thompson)
for the opportunity to share my personal perspective about the early days of this
renaissance and some of reasons for the success enjoyed by its participants.
The discovery of polyhedral oligosilsesquioxanes (POS) was inevitable
v
vi Foreword
because many [RSiO ] frameworks form rapidly via hydrolytic condensation of
3/2 2n
RSiX, and spontaneously crystallize in pure form from common organic solvents.
3
In fact, silsesquioxanes were undoubtedly synthesized (by accident) shortly after
the fi rst trifunctional organosilanes (i.e., RSiX) were prepared, and there are
3
reports consistent with silsesquioxane formation back as far as the 1870s [2]. Not
surprisingly, much of the early work in the fi eld was done by researchers at the
major silicone producers. Scott of General Electric (GE) described the formation
of MeSiO in 1946 [3]. Although his analytical data was only good enough to
8 8 12
establish that the molecule was an oligomer with the formula [MeSiO ] , he
3/2 2n
correctly realized that the molecule had a highly symmetric structure because of
its poor solubility and tendency to sublime without melting. Several years later,
Barry and Gilkey (Dow Corning) [4,5] convincingly established that the molecule
indeed possessed a cube-octameric structure, and that a variety of crystalline POS
frameworks with the formulae RSiO, RSiO , and R Si O could be prepared
6 6 9 8 8 12 12 12 18
from readily available organyltrichlorosilanes (RSiCl with R = Me, Et, Pr, Bu, Cy,
3
Ph).
The early pioneers in POS chemistry faced truly formidable challenges because
most of the analytical tools we now take for granted were not yet available. Quite
often, the only data available for making structural assignments were from elemental
analysis (including silicon) and molecular weight measurements in solution. This
was generally suffi cient to identify pure compounds with the formula [RSiO ] ,
3/2 2n
but it was a major obstacle to making timely progress in the broader fi eld. This
situation began to change by the 1960s, when IR spectroscopy became widely
available. While it is likely that many POS researchers during this time felt blessed
to have IR spectroscopy available to characterize their products and mixtures, it was
still extremely diffi cult to make unambiguous structural assignments for many POS
compounds, especially compounds with low symmetry, mixed organyl groups, or
multiple stereoisomeric possibilities.
By the 1970s, analytical methods for characterizing organic compounds
were advancing rapidly, a sizable body of early knowledge about POS was being
accumulated, and efforts to identify commercially viable applications were
underway. At the same time, related studies of silicones were moving forward
with dramatic success; in comparison, development of POS and POS applications
was much less fruitful. Despite three decades of work, the availability of tank-car
quantities of several trifunctional silanes (e.g., PhSiCl, MeSiCl and vinyl-SiCl),
3 3 3
and the ability to make some POS compounds in practically quantitative yield, no
one had discovered a large-scale commercial application for POS. Time seemed
to run out because continuation of this work was diffi cult to justify during the
economic climate of the late 1970s and early 1980s. So as the early pioneers retired
or moved on to other fi elds, the fi rst generation of POS research came to an end.
Completely oblivious to anything related to POS, I started graduate school at
the University of Rochester in 1980 to pursue a Ph.D. in Chemistry. Although I
initially planned to specialize in natural product (i.e., drug) synthesis, I eventually
teamed up with a starting assistant professor (William D. Jones) to explore
Foreword vii
whether homogeneous (i.e., soluble) organometallic complexes could be used to
catalytically activate C-H bonds for functionalization by small organic molecules
(e.g., CO, CO, CH). In the course of my studies, I became familiar with a related
2 2 4
body of work, seeking to understand and mimic the chemistry of metal surfaces
using soluble model compounds containing small metal clusters (e.g., Os(CO) ).
3 12
These clusters exhibited a rich reaction chemistry with hydrocarbons, and provided
valuable insights into possible mechanisms for the reactions of hydrocarbons on
metal surfaces. However, none of these cluster models truly behaved like the metal
surfaces they were intended to model. This sparked my interest in developing
soluble clusters that could better mimic the chemistry of catalyst surfaces.
Towards the end of my graduate career, a fellow graduate student (Prudence
Bradley) from another research group introduced me to POS through a presentation
she made at a late-night group meeting. It was Prudence’s turn to give a review of
any “special topic” of her choosing, and by chance she saw a recent review article
by Voronkov [6] and decided to focus on [RSiO ] polyhedra for her presentation.
3/2 2n
Because the people in attendance were transition-metal chemists with little interest
in the chemistry of organosilicon clusters, the presentation did not generate much
enthusiasm. Most attendees simply drank more beer than usual and revisited
familiar topics of discussion. At one point near the end of our meeting, the group
began a familiar debate about the value of using small molecular clusters to model
surface chemistry. This time the debate took an interesting turn as I stared at a hand-
drawn structure of RSiO on an adjacent chalkboard. Rather than questioning the
8 8 12
value of cluster models for surfaces, it occurred to me that these models might work
well for cases where the surface could be reasonably approximated as an insulating
solid with a localized electronic structure. I then suggested that molecules similar
to POS might be great models for silica surfaces, if methods could be developed
for synthesizing structurally well-defi ned silsesquioxane frameworks containing
reactive Si-OH groups. One of my mentors (Richard Eisenberg) reacted favorably
to the idea, but quickly asserted that I would “never be able to make useful quantities
of these frameworks.” As our meeting adjourned, I took that as a challenge and
promptly headed off to the library in search of a silsesquioxane framework that
might be suitable as a cluster model for silica, or as a ligand in cluster models for
silica-supported catalysts. By the morning, I had found what I was looking for, was
reasonably convinced that the idea could be made to work, and was well on my way
to creating an independent research program for this purpose.
Upon reading Voronkov’s review [6] and diving into the early silsesquioxane
literature, it became apparent that a wide variety of structurally well-defi ned
silsesquioxane frameworks could be obtained in synthetically useful quantities,
even though synthetic methods, product yields, procedures for separation and
purifi cation, characterization data, structural assignments, and mechanistic
explanations for product formation often left a lot to be desired. Frameworks
with reactive Si-OH groups were much less common and generally less well
characterized than fully condensed [RSiO ] frameworks, but they still seemed
3/2 2n
to be accessible. John F. Brown Jr.’s work at GE appeared to offer the best starting
viii Foreword
point, and his work with cyclohexyl-substituted silsesquioxanes [7] provided
the most compelling evidence for the formation of structurally well-defi ned,
incompletely-condensed silsesquioxane frameworks. Trisilanol 1, in particular,
seemed to be an ideal candidate for modeling both the chemistry of silica and the
chemistry of monometallic silica-supported catalysts. This eventually proved to be
the case and provided the foundation for many of my research group’s contributions
to the re-birth of POS chemistry.
Starting in 1985 with trisilanol 1 and a goal to develop realistic solution-state
cluster models for silica and silica-supported catalysts, my group at the University
of California, Irvine (UCI) began a journey that ultimately produced a large number
of new silsesquioxane frameworks and metal-containing frameworks. Some of these
frameworks indeed appeared to be good models for silica surface sites [8, 9], as
well as excellent ligands for catalytically active cluster models for silica-supported
catalysts [10]. At the same time, an increasing number of researchers were starting
to discover practical routes to POS frameworks with synthetically useful groups
attached to Si (e.g., H, CHCHCHZ with Z = Cl, SH and NH). The pool of known
2 2 2 2
POS frameworks expanded rapidly over a very short period of time as general and
highly effi cient methodology was developed for synthetically manipulating both the
Si/O framework and the pendant groups attached to silicon.
Efforts to develop practical applications for discrete POS frameworks gained a
major champion in the early 1990’s when Joseph D. Lichtenhan initiated a research
program at Edwards Air Force Base (California, USA) to use POS-containing
polymers as precursors to hybrid inorganic/organic materials. Joe was a former UCI
student who learned quickly how to scale up the synthetic methods developed in
my laboratory. His team quickly identifi ed a number of promising applications for
POS-containing polymers in military applications, and along the way they became
so skilled at making POS that they were able to supply samples to Aldrich Chemical
Company for sale to other researchers who were interested in working with POS.
Eventually, Joe and several colleagues left Edwards Air Force in the late 1990s to
form the company now known as Hybrid Plastics, Inc., which is recognized as the
Foreword ix
commercial leader for POS production and applications.
Much of the new generation of researchers’ success with POS was a direct
consequence of advances in routine analytical instrumentation for characterizing
complex organic molecules, especially multi-nuclear NMR spectrometers, mass
spectrometers, and single-crystal X-ray diffraction systems. The fi rst generation
of POS researchers simply did not have the necessary tools for the quick and
unambiguous assignment of molecular structures, or for the analyses of complex
mixtures of products. This was highlighted to me during a visit to GE around 2000,
where I had the pleasure of meeting John F. Brown Jr., who was about to start his
sixth decade with the company. Although he had not worked with silsesquioxanes
since the mid-1960’s, he still remembered an impressive amount of information
about his earlier work and was enthusiastic about discussing the topic. At one
point during our discussion, John volunteered a few seemingly minor details that
undoubtedly played an important role in his successes. In particular, John told me
that his practice of allowing POS compounds to crystallize slowly from resinous
mixtures of products over days, weeks, months or sometimes even years stemmed
from his graduate research with sugars, which can be notoriously slow to crystallize
from syrupy mixtures. Without this patient approach to synthesis, which was a
unique characteristic of his early work with silsesquioxanes, it might have been very
diffi cult to create conditions conducive to the formation of trisilanol 1. Isolation of
pure 1 from other polycondensation products might also have been very diffi cult
if a more expedient synthetic approach had been pursued. Upon hearing this,
and realizing that my entry into POS chemistry was largely motivated by John’s
successful synthesis of trisilanol 1, I could not resist wondering whether a butterfl y
had been fl apping its wings in Schenectady, New York.
Better analytical tools were only partly responsible for the re-birth of
POS chemistry and applications. Two other important reasons for the successes
observed after 1985 were an increased focus on mechanistic studies involving POS
frameworks, and the deliberate use of POS as precursors to more complex Si/O
and Si/O/M frameworks (as well as materials containing these frameworks). In
addition to attracting broad interest, expertise, and generous funding from outside
the traditional organosilicon community, this emphasis shifted the focus of POS
research away from identifi cation and isolation of compounds that spontaneously
formed during hydrolytic condensation of RSiX, and towards the rational design
3
and synthesis of specifi c compounds for specifi c purposes. This approach eventually
paid big and unexpected dividends when studies aimed at developing methods for
selective cleavage of Si-O-Si linkages in POS frameworks led to the discovery that
the RSiO(OH) structure shown for trisilanol 1 is actually the thermodynamic
7 7 9 3
product when many different trifunctional organosilanes are subjected to hydrolytic
condensation in the presence of alkali metal ions. This discovery opened the door to
a broad range of options for producing new POS derivatives, and ultimately allowed
Hybrid Plastics, Inc. to develop practical methods for large-scale manufacturing of
many potentially useful materials. Between the compounds derived from trisilanols
such as 1, and other compounds derived directly or indirectly from hydrolytic