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Part IV Summary and Evaluation
PART IV SUMMARY AND EVALUATION
607
Metal clusters constitute an extensive new class of compounds, offering new modes of bonding of ligands to metal and subtle new patterns of reactivity.
Most of the literature of metal cluster chemistry still has to do
with synthesis and structure.
The predictive basis of thermochemistry is
still weakly developed, and the understanding of reactivity has only just begun to develop rapidly. A number of assessments of the state of metal cluster chemistry have emphasized the potential value of the clusters as catalysts.
Catalysis has
been a major driving force for the advancement of organometallic chemistry in general, and the major technological successes represented by processes such as alkene hydroformylation and methanol carbonylation loom as monuments justifying research in this field. The forecasts of exciting catalytic routes related to the new bonding and reactivity patterns offered by metal clusters have not been borne out. The clusters are fragile.
A rapid and selective cycle of organometallic
catalysis requires a complex and delicate dynamic balance; the complication of instability of most cluster frameworks imposes a major limitation on the catalytic opp or tumt res ,
The typical attempt at catalysis by met al clus t ers
leads to the formation of mononuclear metal complexes and/or colloidal metal.
Only a few examples of catalysis can be confidently attributed to
metal clusters.
One potentially important technological reaction (CO
hydrogenation to give ethylene glycol) may involve metal clusters, but the details of the chemistry are lacking. The prospects for short-term industrial exploitation of cluster catalysis are not good.
The long-term prospects are more attractive; in this
uncharted territory, new chemistry and catalysis will surely be found, but the progress is likely to be slow. Metal clusters provide compelling models of surfaces, and the fields of organometallic chemistry and surface science are destined to intersect broadly.
The insights provided by clusters have already proven to be
valuable in the advancement of surface science.
The metal cluster-metal
surface analogy has impressive validity in terms of structure and bonding--but not reactivity or catalytic activity. Metal clusters are even better models of small metal particles with adsorbates than they are of simple metal surfaces with adsorbates.
The
clusters are truly intermediate structures between the simplest metal complexes and metal particles, and with some metals (e.g., Os), they almost seem to offer a continuum in the large group of carbonyl clusters, with almost every nuclearity between 3 and 10 or more being represented by
608
well-characterized compounds.
This family also offers a wide range of
hydrocarbon ligands, including some that bridge metal centers,
and it
provides tantalizing models of surface organometallic structures yet to be discovered. Metal
clusters
anchored
to
surfaces
may
already
be
the
best-characterized supported metal catalysts in terms of structure, for the structures are often based on definitive crystal and molecular structures of very close analogues.
A few of these materials (e.g.,
HOs3(CO>io-O-M~)
yield dazzlingly simple and understandable spectra and electron micrographs; we can be confident of the structures of the bound clusters and optimistic that characterization of these simple surface structures will help with the development of the physical characterization techniques for surfaces generally and the logical extension of these techniques to the more complicated supported metal aggregates and crystallites of industrial metal catalysts.
The physical characterization techniques of greatest promise
today appear to be the workhorse infrared spectroscopy, the still-difficult but rapidly developing EXAFS and solid-state NMR, and high-resolution transmission electron microscopy. The chemistry of metal clusters on surfaces, especially those on metal oxide surfaces, has arisen and developed strongly in the preceding five years.
Although ill-defined mixtures are the typical result of the reactions
of organometallic compounds with surfaces, there are now numerous surface organometallic analogues of molecular species, and the reactivities of some of these are beginning to be established.
There is an opportunity for rapid
advancement and a need for further development of methodology for quantitative determination of reactivity (e.g., techniques such as temp e r a tur e-p rogrammed decomposition), surfac e spectrosc opy, and synthesis and characterization of exact molecular analogues of surface structures incorporating ligands that mimic the surface. The application of supported molecular metal clusters as catalysts is fraught with all the difficulties of homogeneous catalysis by metal clusters, and more; most of the difficulties are related to the lack of stability of the clusters, complicated by the presence of the support.
The support is a
ligand or group of ligands that may contribute to the lack of stability and limit the ac cess of reactive ligands to bond to the clusters and participate in a catalytic cycle. We see glimmers of hope in the several examples of catalysis by supported clusters, apparently including CO hydrogenation at about 550 K, but we recognize the severe restrictions on preparation of stable cluster catalysts.
Effort might fruitfully be directed at some of the metals giving
609
the most stable clusters (Ru, as, Ir) and at novel supports other than metal oxides, which might
offer stabilizing ligands.
The reactant ligands are
important as well, and the catalytic reactions of stabilizing ligands such as CO are therefore appealing. The great and immediate promise and attraction of
supported metal
clusters is the scientific opportunities they offer for investigation of m ol e cut or c a t a l y s i s a,l «vr f a c e s ,
We stand at a frontier in the design of
catalysts with specific combinations of metals, ligands,
and supports
(themselves ligands)--we have visions of determining the effects of supports as we c an determine the
eff e c t s of ligands in homogeneous cat a l y s i s ,
Molecular catalysts on supports can be characterized in the absence of complicating solvents; with the aid of powerful spectroscopic methods such as
E XAFS, we will some day isolate intermediates and understand the
molecular details of catalytic cycles.
The advancement of understanding of
surface catalytic processes can receive a boost comparable to that provided by the surface science of single crystals--and the restriction
of the
high-vacuum apparatus does not pertain. We foresee development of molecular cluster chemistry and catalysis on surfaces and in cages as a frontier of organometallic chemistry and surface science; the clusters and catalysis are the ties pulling these disciplines together, and the rewards will be a deeper fundamental understanding of catalysis
and
the
technology
that
will ultimately
spring from this
understanding.
B. C. Gates L. Guczi H. Knoz i ng er
607
Metal clusters constitute an extensive new class of compounds, offering new modes of bonding of ligands to metal and subtle new patterns of reactivity.
Most of the literature of metal cluster chemistry still has to do
with synthesis and structure.
The predictive basis of thermochemistry is
still weakly developed, and the understanding of reactivity has only just begun to develop rapidly. A number of assessments of the state of metal cluster chemistry have emphasized the potential value of the clusters as catalysts.
Catalysis has
been a major driving force for the advancement of organometallic chemistry in general, and the major technological successes represented by processes such as alkene hydroformylation and methanol carbonylation loom as monuments justifying research in this field. The forecasts of exciting catalytic routes related to the new bonding and reactivity patterns offered by metal clusters have not been borne out. The clusters are fragile.
A rapid and selective cycle of organometallic
catalysis requires a complex and delicate dynamic balance; the complication of instability of most cluster frameworks imposes a major limitation on the catalytic opp or tumt res ,
The typical attempt at catalysis by met al clus t ers
leads to the formation of mononuclear metal complexes and/or colloidal metal.
Only a few examples of catalysis can be confidently attributed to
metal clusters.
One potentially important technological reaction (CO
hydrogenation to give ethylene glycol) may involve metal clusters, but the details of the chemistry are lacking. The prospects for short-term industrial exploitation of cluster catalysis are not good.
The long-term prospects are more attractive; in this
uncharted territory, new chemistry and catalysis will surely be found, but the progress is likely to be slow. Metal clusters provide compelling models of surfaces, and the fields of organometallic chemistry and surface science are destined to intersect broadly.
The insights provided by clusters have already proven to be
valuable in the advancement of surface science.
The metal cluster-metal
surface analogy has impressive validity in terms of structure and bonding--but not reactivity or catalytic activity. Metal clusters are even better models of small metal particles with adsorbates than they are of simple metal surfaces with adsorbates.
The
clusters are truly intermediate structures between the simplest metal complexes and metal particles, and with some metals (e.g., Os), they almost seem to offer a continuum in the large group of carbonyl clusters, with almost every nuclearity between 3 and 10 or more being represented by
608
well-characterized compounds.
This family also offers a wide range of
hydrocarbon ligands, including some that bridge metal centers,
and it
provides tantalizing models of surface organometallic structures yet to be discovered. Metal
clusters
anchored
to
surfaces
may
already
be
the
best-characterized supported metal catalysts in terms of structure, for the structures are often based on definitive crystal and molecular structures of very close analogues.
A few of these materials (e.g.,
HOs3(CO>io-O-M~)
yield dazzlingly simple and understandable spectra and electron micrographs; we can be confident of the structures of the bound clusters and optimistic that characterization of these simple surface structures will help with the development of the physical characterization techniques for surfaces generally and the logical extension of these techniques to the more complicated supported metal aggregates and crystallites of industrial metal catalysts.
The physical characterization techniques of greatest promise
today appear to be the workhorse infrared spectroscopy, the still-difficult but rapidly developing EXAFS and solid-state NMR, and high-resolution transmission electron microscopy. The chemistry of metal clusters on surfaces, especially those on metal oxide surfaces, has arisen and developed strongly in the preceding five years.
Although ill-defined mixtures are the typical result of the reactions
of organometallic compounds with surfaces, there are now numerous surface organometallic analogues of molecular species, and the reactivities of some of these are beginning to be established.
There is an opportunity for rapid
advancement and a need for further development of methodology for quantitative determination of reactivity (e.g., techniques such as temp e r a tur e-p rogrammed decomposition), surfac e spectrosc opy, and synthesis and characterization of exact molecular analogues of surface structures incorporating ligands that mimic the surface. The application of supported molecular metal clusters as catalysts is fraught with all the difficulties of homogeneous catalysis by metal clusters, and more; most of the difficulties are related to the lack of stability of the clusters, complicated by the presence of the support.
The support is a
ligand or group of ligands that may contribute to the lack of stability and limit the ac cess of reactive ligands to bond to the clusters and participate in a catalytic cycle. We see glimmers of hope in the several examples of catalysis by supported clusters, apparently including CO hydrogenation at about 550 K, but we recognize the severe restrictions on preparation of stable cluster catalysts.
Effort might fruitfully be directed at some of the metals giving
609
the most stable clusters (Ru, as, Ir) and at novel supports other than metal oxides, which might
offer stabilizing ligands.
The reactant ligands are
important as well, and the catalytic reactions of stabilizing ligands such as CO are therefore appealing. The great and immediate promise and attraction of
supported metal
clusters is the scientific opportunities they offer for investigation of m ol e cut or c a t a l y s i s a,l «vr f a c e s ,
We stand at a frontier in the design of
catalysts with specific combinations of metals, ligands,
and supports
(themselves ligands)--we have visions of determining the effects of supports as we c an determine the
eff e c t s of ligands in homogeneous cat a l y s i s ,
Molecular catalysts on supports can be characterized in the absence of complicating solvents; with the aid of powerful spectroscopic methods such as
E XAFS, we will some day isolate intermediates and understand the
molecular details of catalytic cycles.
The advancement of understanding of
surface catalytic processes can receive a boost comparable to that provided by the surface science of single crystals--and the restriction
of the
high-vacuum apparatus does not pertain. We foresee development of molecular cluster chemistry and catalysis on surfaces and in cages as a frontier of organometallic chemistry and surface science; the clusters and catalysis are the ties pulling these disciplines together, and the rewards will be a deeper fundamental understanding of catalysis
and
the
technology
that
will ultimately
spring from this
understanding.
B. C. Gates L. Guczi H. Knoz i ng er