1 Ligand-Free Sub-Nanometer Metal Clusters in Catalysis
33
1.4 Conclusions
Ligand-free sub-nanometer metal clusters with a precise number of metal atoms
are extremely active and selective catalysts for a variety of organic reactions
which include carbon–carbon, carbon–heteroatom and heteroatom–heteroatom
bond-forming reactions and hydrogenation reactions, among others.
The synthesis of the tiny metal clusters is simple and can be performed by bottom–
up (from metal salts and complexes) and top–down (from nanoparticles) approaches.
In solution, mild reducing agents such as amide solvents and alcohols or dislodging
agents such as Brönsted acids (HCl, HOTf) are employed, but often, these external
agents are not necessary, and the same organic reagents trigger and organize the
formation of the catalytically active metal clusters during reaction, provided that
the metal is sufficiently diluted to avoid further agglomeration. These soluble metal
clusters can be stored either in amide or alcohol solutions or in solids (polymeric
films, inorganic oxides) to be used on demand for different organic reactions. For the
synthesis of the solid-supported, ligand-free sub-nanometer clusters, much stronger
reducing agents, such as NaBH 4 or H 2 , can be employed since the strong interaction
between the sub-nanometer cluster and the support avoids further agglomeration.
These solid supported clusters are generally more homogeneous in atomicity, spatial
distribution and size than those formed in solution and do not require dilution with
loadings up to 8 wt% in a MOF.
Modern characterization techniques, including SC–XRD and aberration-corrected
TEM, have sufficient technological ability to determine the exact number of atoms,
oxidation state and topological distribution of the metal cluster, in other words, a
complete structural and electronic information. With this in hand, and also in combination with well-advanced theoretical calculations for cluster chemistry, researchers
should now be able to predict the catalytic behavior for a given metal cluster in
different reactions and ultimately design the metal cluster needed for a target (and
perhaps new) reaction.
References
1. Anciaux AJ, Demonceau A, Noels AF et al (1981) Transition-metal-catalyzed reactions of
diazo compounds. 2. Addition to aromatic molecules: catalysis of Buchner’s synthesis of
cycloheptatrienes. J Org Chem 46:873–876. https://doi.org/10.1021/jo00318a010
2. Bayram E, Linehan JC, Fulton JL et al (2011) Is It Homogeneous or Heterogeneous Catalysis Derived from [RhCp*Cl2]2? In Operando XAFS, Kinetic, and Crucial Kinetic Poisoning
Evidence for Subnanometer Rh4 Cluster-Based Benzene Hydrogenation Catalysis. J Am Chem
Soc 133:18889–18902. https://doi.org/10.1021/ja2073438
3. Bittner AM, Wu XC, Balci S, et al (2005) Bottom-up synthesis and top-down organisation of
semiconductor and metal clusters on surfaces. Eur. J. Inorg. Chem. 3717–3728. https://doi.org/
10.1002/ejic.200500388
4. Bogdanovi´ c B, Spliethoff B, Wilke G (1980) Dimerization of propylene with catalysts
exhibiting activities like highly-active enzymes. Angew Chemie Int Ed English 19:622–623.
https://doi.org/10.1002/anie.198006221
33
1.4 Conclusions
Ligand-free sub-nanometer metal clusters with a precise number of metal atoms
are extremely active and selective catalysts for a variety of organic reactions
which include carbon–carbon, carbon–heteroatom and heteroatom–heteroatom
bond-forming reactions and hydrogenation reactions, among others.
The synthesis of the tiny metal clusters is simple and can be performed by bottom–
up (from metal salts and complexes) and top–down (from nanoparticles) approaches.
In solution, mild reducing agents such as amide solvents and alcohols or dislodging
agents such as Brönsted acids (HCl, HOTf) are employed, but often, these external
agents are not necessary, and the same organic reagents trigger and organize the
formation of the catalytically active metal clusters during reaction, provided that
the metal is sufficiently diluted to avoid further agglomeration. These soluble metal
clusters can be stored either in amide or alcohol solutions or in solids (polymeric
films, inorganic oxides) to be used on demand for different organic reactions. For the
synthesis of the solid-supported, ligand-free sub-nanometer clusters, much stronger
reducing agents, such as NaBH 4 or H 2 , can be employed since the strong interaction
between the sub-nanometer cluster and the support avoids further agglomeration.
These solid supported clusters are generally more homogeneous in atomicity, spatial
distribution and size than those formed in solution and do not require dilution with
loadings up to 8 wt% in a MOF.
Modern characterization techniques, including SC–XRD and aberration-corrected
TEM, have sufficient technological ability to determine the exact number of atoms,
oxidation state and topological distribution of the metal cluster, in other words, a
complete structural and electronic information. With this in hand, and also in combination with well-advanced theoretical calculations for cluster chemistry, researchers
should now be able to predict the catalytic behavior for a given metal cluster in
different reactions and ultimately design the metal cluster needed for a target (and
perhaps new) reaction.
References
1. Anciaux AJ, Demonceau A, Noels AF et al (1981) Transition-metal-catalyzed reactions of
diazo compounds. 2. Addition to aromatic molecules: catalysis of Buchner’s synthesis of
cycloheptatrienes. J Org Chem 46:873–876. https://doi.org/10.1021/jo00318a010
2. Bayram E, Linehan JC, Fulton JL et al (2011) Is It Homogeneous or Heterogeneous Catalysis Derived from [RhCp*Cl2]2? In Operando XAFS, Kinetic, and Crucial Kinetic Poisoning
Evidence for Subnanometer Rh4 Cluster-Based Benzene Hydrogenation Catalysis. J Am Chem
Soc 133:18889–18902. https://doi.org/10.1021/ja2073438
3. Bittner AM, Wu XC, Balci S, et al (2005) Bottom-up synthesis and top-down organisation of
semiconductor and metal clusters on surfaces. Eur. J. Inorg. Chem. 3717–3728. https://doi.org/
10.1002/ejic.200500388
4. Bogdanovi´ c B, Spliethoff B, Wilke G (1980) Dimerization of propylene with catalysts
exhibiting activities like highly-active enzymes. Angew Chemie Int Ed English 19:622–623.
https://doi.org/10.1002/anie.198006221