encountered locations (interior, exterior, or core) of NPs (or clusters) for catalysis in
the dendrimer molecule.
(a) Dendrimer-encapsulated NPs (DENs) – Encapsulation of NPs within a single
dendrimer. DENs are the most popular location of NPs with an inner nanospace
and are suitable for generation of the large dendrimers.
(b) Dendrimer-stabilized NPs (DSNs) – Surrounding NPs with peripheral groups of
several dendrimers. The formation of DSNs favors the generation of small
dendrimers, followed by self-assembly of the dendrimer around the NP core.
Therefore, the diameters of DSNs are slightly larger than those of DENs.
(c) NP-cored dendrimers (NCDs) – NPs located in the core by the self-assembly of
dendrons: The coordinating sites (e.g., P, N, and S atoms) at the focal point of the
dendron are required as capping ligands for stabilization of the NPs. As the
dendron size increases from G1 to G5, the number of dendritic units connected
to the core decreases.
In this review, homogeneous and heterogeneous catalysis using NPs and clusters
encapsulated by a series of dendrimers are presented. This review focuses on the
unique catalytic activity, selectivity, and reusability of NPs, which are dependent on
the size and location of the NPs in the dendrimer, as well as multimetallic NP
materials, where the number of atoms and blend ratio of different elements has
been precisely controlled.
2 Poly(amidoamine) (PAMAM) Dendrimers
The PAMAM dendrimers, prepared by Tomalia [10–12], are the most common
dendrimers used to encapsulate NPs; they contain interior amido and amino groups
and hydroxyl or amine groups at the peripheral position. The fourth-generation
hydroxyl-terminated PAMAM dendrimer (G4-OH) are approximately 4.5 nm in
diameter and contain 64 terminal OH groups and 62 tertiary amine groups
(Fig. 4). The molecular weight becomes more than 14,000. The PAMAM dendrimer
families with various generations are commercially available.
Crooks’ group [13–16] pioneered catalysis using Pd DENs (1.3 Æ 0.3 nm,
Fig. 3a) within a hydroxyl-terminated PAMAMGn-OH dendrimer (where Gn represents the nth generation), which possessed high catalytic activity for hydrogenation of unbranched alkene (allyl alcohol) and branched alkene (N-isopropyl
acrylamide) in water [17]. The size-selective catalytic activity during hydrogenation
was dependent on dendrimer size and the reactants. The Pd 40 catalysts within
dendrimers of lower generation (G4-OH), prepared from the ratio of Pd complex
to dendrimer, exhibited the highest activities for the both reactants. Steric crowding
on higher-generation dendrimers (G6-OH, G8-OH) hindered substrate penetration of
the branched alkene, leading to only 10% and 5%, respectively. These results are
possible to control reaction rates and do selective catalysis, which functioned as an
adjustable-mesh “nanofilter” (Fig. 5) [18]. Incorporation of bulky groups
Precise Synthesis of Nanoparticles and Their Catalytic Behavior
135
the dendrimer molecule.
(a) Dendrimer-encapsulated NPs (DENs) – Encapsulation of NPs within a single
dendrimer. DENs are the most popular location of NPs with an inner nanospace
and are suitable for generation of the large dendrimers.
(b) Dendrimer-stabilized NPs (DSNs) – Surrounding NPs with peripheral groups of
several dendrimers. The formation of DSNs favors the generation of small
dendrimers, followed by self-assembly of the dendrimer around the NP core.
Therefore, the diameters of DSNs are slightly larger than those of DENs.
(c) NP-cored dendrimers (NCDs) – NPs located in the core by the self-assembly of
dendrons: The coordinating sites (e.g., P, N, and S atoms) at the focal point of the
dendron are required as capping ligands for stabilization of the NPs. As the
dendron size increases from G1 to G5, the number of dendritic units connected
to the core decreases.
In this review, homogeneous and heterogeneous catalysis using NPs and clusters
encapsulated by a series of dendrimers are presented. This review focuses on the
unique catalytic activity, selectivity, and reusability of NPs, which are dependent on
the size and location of the NPs in the dendrimer, as well as multimetallic NP
materials, where the number of atoms and blend ratio of different elements has
been precisely controlled.
2 Poly(amidoamine) (PAMAM) Dendrimers
The PAMAM dendrimers, prepared by Tomalia [10–12], are the most common
dendrimers used to encapsulate NPs; they contain interior amido and amino groups
and hydroxyl or amine groups at the peripheral position. The fourth-generation
hydroxyl-terminated PAMAM dendrimer (G4-OH) are approximately 4.5 nm in
diameter and contain 64 terminal OH groups and 62 tertiary amine groups
(Fig. 4). The molecular weight becomes more than 14,000. The PAMAM dendrimer
families with various generations are commercially available.
Crooks’ group [13–16] pioneered catalysis using Pd DENs (1.3 Æ 0.3 nm,
Fig. 3a) within a hydroxyl-terminated PAMAMGn-OH dendrimer (where Gn represents the nth generation), which possessed high catalytic activity for hydrogenation of unbranched alkene (allyl alcohol) and branched alkene (N-isopropyl
acrylamide) in water [17]. The size-selective catalytic activity during hydrogenation
was dependent on dendrimer size and the reactants. The Pd 40 catalysts within
dendrimers of lower generation (G4-OH), prepared from the ratio of Pd complex
to dendrimer, exhibited the highest activities for the both reactants. Steric crowding
on higher-generation dendrimers (G6-OH, G8-OH) hindered substrate penetration of
the branched alkene, leading to only 10% and 5%, respectively. These results are
possible to control reaction rates and do selective catalysis, which functioned as an
adjustable-mesh “nanofilter” (Fig. 5) [18]. Incorporation of bulky groups
Precise Synthesis of Nanoparticles and Their Catalytic Behavior
135