1 Ligand-Free Sub-Nanometer Metal Clusters in Catalysis
17
recent finding that Pt single atoms and clusters can be formed in solution has enabled
the Pt-catalyzed Heck reaction of iodo– and bromo–derivatives [17]. Experimental
studies in combination with DFT calculations strongly support the feasibility of the
coupling mechanism steps on the Pt atoms, thus representing one example where the
formation of ligand-free metal clusters unveils a somehow hidden catalytic behavior
for a particular metal.
Recent theoretical studies on the electronic properties and activation energies
for the three steps of the Suzuki cross-coupling reaction have shown that Pd/Ni
bimetallic clusters supported on defected graphene can be excellent catalysts [72].
In fact, reducing the size of the clusters from Ni 13 to Ni 4 enhances the activity because
of the increased negative charge Pd. Bimetallic Pd/Ni clusters were found to offer
even lower activation energies for all three steps of the Suzuki reaction because of
charge donation from the Ni atoms to the Pd atoms making the bimetallic clusters a
highly active catalyst.
1.3.1.2 Buchner Reaction
The ring-opening cyclopropanation of benzenes with alpha-diazoesters (Fig. 1.16)
is known as the Buchner reaction. This reaction, despite its uniqueness to form
otherwise very difficult to prepare cycloheptatrienes, giving access to a plethora of
advanced organic intermediates, has found little use in industrial organic synthesis
since the only efficient catalysts for the transformation are based on extremely
expensive, soluble and unrecoverable Rh 2 salts [1].
Mixed-valence Pd 4 (0, I) clusters supported within a MOF have shown a high
catalytic activity and selectivity, comparable to Rh 2 salts, for the Buchner reaction,
enabling the performance of the reaction in continuous flow. The Pd 4 clusters have
the appropriate electron density to activate the diazo-compound and generate the
required carbene to open the benzene derivative. It is possible that, in line with
Pd 4
2+
N
N
ROOC
N N
MOF
ROOC
Pd 4
+
MOF
Pd 4
2+
MOF
ROOC
- N 2
6 disrotatory
COOR
Cyclopropanation
Pd carbene formation
COOR
electrocyclic
ring opening
Fig. 1.16 Top: Pd 4 Cluster structure optimized by DFT calculations, on the basis of the real
SC–XRD structure. Bottom: Proposed mechanism for the Pd4-catalyzed intermolecular Buchner
reaction (Fig. from Ref. [20]. Copyright © 2017 by Nature Publishing Group)
17
recent finding that Pt single atoms and clusters can be formed in solution has enabled
the Pt-catalyzed Heck reaction of iodo– and bromo–derivatives [17]. Experimental
studies in combination with DFT calculations strongly support the feasibility of the
coupling mechanism steps on the Pt atoms, thus representing one example where the
formation of ligand-free metal clusters unveils a somehow hidden catalytic behavior
for a particular metal.
Recent theoretical studies on the electronic properties and activation energies
for the three steps of the Suzuki cross-coupling reaction have shown that Pd/Ni
bimetallic clusters supported on defected graphene can be excellent catalysts [72].
In fact, reducing the size of the clusters from Ni 13 to Ni 4 enhances the activity because
of the increased negative charge Pd. Bimetallic Pd/Ni clusters were found to offer
even lower activation energies for all three steps of the Suzuki reaction because of
charge donation from the Ni atoms to the Pd atoms making the bimetallic clusters a
highly active catalyst.
1.3.1.2 Buchner Reaction
The ring-opening cyclopropanation of benzenes with alpha-diazoesters (Fig. 1.16)
is known as the Buchner reaction. This reaction, despite its uniqueness to form
otherwise very difficult to prepare cycloheptatrienes, giving access to a plethora of
advanced organic intermediates, has found little use in industrial organic synthesis
since the only efficient catalysts for the transformation are based on extremely
expensive, soluble and unrecoverable Rh 2 salts [1].
Mixed-valence Pd 4 (0, I) clusters supported within a MOF have shown a high
catalytic activity and selectivity, comparable to Rh 2 salts, for the Buchner reaction,
enabling the performance of the reaction in continuous flow. The Pd 4 clusters have
the appropriate electron density to activate the diazo-compound and generate the
required carbene to open the benzene derivative. It is possible that, in line with
Pd 4
2+
N
N
ROOC
N N
MOF
ROOC
Pd 4
+
MOF
Pd 4
2+
MOF
ROOC
- N 2
6 disrotatory
COOR
Cyclopropanation
Pd carbene formation
COOR
electrocyclic
ring opening
Fig. 1.16 Top: Pd 4 Cluster structure optimized by DFT calculations, on the basis of the real
SC–XRD structure. Bottom: Proposed mechanism for the Pd4-catalyzed intermolecular Buchner
reaction (Fig. from Ref. [20]. Copyright © 2017 by Nature Publishing Group)