compounds. For example, the hydrogenation of aromatic compounds is of great
relevance for industry and organic synthesis [21], since the corresponding cyclohexanes/cyclohexenes are precursors for industrial organic chemistry [22] or key
intermediates in pharmaceuticals [23]. Recent works have demonstrated the capability of MNPs for the catalytic hydrogenation of olefins [24, 25], alkynes [26–28],
arenes [29, 30], ketones [31–33], aldehydes [34–36], nitrocompounds [37, 38] and
carboxylic acids [39, 40], with good levels of chemoselectivities. Among the MNPs
commonly used in catalysis, iridium nanoparticles (Ir NPs) have been widely used
due to their high activity and superior oxidation resistance when exposed to air. In
addition, their large number of oxidation states and the ability to coordinate and
activate a great number of substrates (hydrogen, olefins, carbonyl compounds, etc.)
make them efficient catalyst for hydrogenation reactions [41]. In this chapter we
intend to give an overview of the recent advances of Ir NPs in hydrogenation
reactions. In principle, the Ir NP catalysts will be divided into (1) ligand-stabilised
Ir NPs, (2) IL-stabilised Ir NPs, (3) polymer-stabilised Ir NPs, (4) “naked” Ir NPs and
(5) supported Ir NPs, emphasising the influence of the stabilising agent in the
hydrogenation activity.
2 Hydrogenation Reactions Catalysed by Ligand-Stabilised
Iridium Nanoparticles
The use of organic ligands as stabilisers for the synthesis of metal nanoparticles
(MNPs) with applications in catalysis has been widely explored. These ligands
usually have a simple function as a stabilising agent. However, there are also
many examples of ligand-stabilised MNPs in which the ligand acts both as stabiliser
and as functional ligand playing a role in the catalytic process [42]. In this context,
Claver, Roucoux and coworkers described the synthesis of Ir NPs by H 2 reduction of
bis(1,5-cyclooctadiene)diiridium(I) dichloride ([Ir(cod)Cl] 2 ) in the presence of chiral
diphosphite ligands derived from carbohydrates (Scheme 1a) [43]. These
diphosphite-stabilised Ir NPs were employed as catalysts for methylanisole hydrogenation (Scheme 1b), leading to significant variations in activity depending on the
ligand acting as stabilising agent (Table 1). However, no insights about the factors
that produce these differences in activity were given. In addition, very low enantiomeric excess values (ees) were obtained, although a slight increase in the ees up to
6% was achieved with Rh and Ru NPs prepared this way, which suggest some
influence of the ligand on the hydrogenation process. Finally, no hydrogenation
reaction was observed with Ir NPs prepared from [Ir(cod) 2 ]BF 4 (Table 1). This was
attributed to a poisoning effect caused by the coordination of the counteranion to the
nanoparticle surface.
Claver and coworkers also employed chiral diphosphines for the synthesis of
ligand-stabilised Ir NPs supported on SiO 2 [44]. Ir NPs of 1.6 nm in size ligated by
(2R,4R)-2,4-bis(diphenylphosphino)pentane ((R,R)-BDPP, Scheme 2) were
Iridium Nanoparticles for Hydrogenation Reactions
399
relevance for industry and organic synthesis [21], since the corresponding cyclohexanes/cyclohexenes are precursors for industrial organic chemistry [22] or key
intermediates in pharmaceuticals [23]. Recent works have demonstrated the capability of MNPs for the catalytic hydrogenation of olefins [24, 25], alkynes [26–28],
arenes [29, 30], ketones [31–33], aldehydes [34–36], nitrocompounds [37, 38] and
carboxylic acids [39, 40], with good levels of chemoselectivities. Among the MNPs
commonly used in catalysis, iridium nanoparticles (Ir NPs) have been widely used
due to their high activity and superior oxidation resistance when exposed to air. In
addition, their large number of oxidation states and the ability to coordinate and
activate a great number of substrates (hydrogen, olefins, carbonyl compounds, etc.)
make them efficient catalyst for hydrogenation reactions [41]. In this chapter we
intend to give an overview of the recent advances of Ir NPs in hydrogenation
reactions. In principle, the Ir NP catalysts will be divided into (1) ligand-stabilised
Ir NPs, (2) IL-stabilised Ir NPs, (3) polymer-stabilised Ir NPs, (4) “naked” Ir NPs and
(5) supported Ir NPs, emphasising the influence of the stabilising agent in the
hydrogenation activity.
2 Hydrogenation Reactions Catalysed by Ligand-Stabilised
Iridium Nanoparticles
The use of organic ligands as stabilisers for the synthesis of metal nanoparticles
(MNPs) with applications in catalysis has been widely explored. These ligands
usually have a simple function as a stabilising agent. However, there are also
many examples of ligand-stabilised MNPs in which the ligand acts both as stabiliser
and as functional ligand playing a role in the catalytic process [42]. In this context,
Claver, Roucoux and coworkers described the synthesis of Ir NPs by H 2 reduction of
bis(1,5-cyclooctadiene)diiridium(I) dichloride ([Ir(cod)Cl] 2 ) in the presence of chiral
diphosphite ligands derived from carbohydrates (Scheme 1a) [43]. These
diphosphite-stabilised Ir NPs were employed as catalysts for methylanisole hydrogenation (Scheme 1b), leading to significant variations in activity depending on the
ligand acting as stabilising agent (Table 1). However, no insights about the factors
that produce these differences in activity were given. In addition, very low enantiomeric excess values (ees) were obtained, although a slight increase in the ees up to
6% was achieved with Rh and Ru NPs prepared this way, which suggest some
influence of the ligand on the hydrogenation process. Finally, no hydrogenation
reaction was observed with Ir NPs prepared from [Ir(cod) 2 ]BF 4 (Table 1). This was
attributed to a poisoning effect caused by the coordination of the counteranion to the
nanoparticle surface.
Claver and coworkers also employed chiral diphosphines for the synthesis of
ligand-stabilised Ir NPs supported on SiO 2 [44]. Ir NPs of 1.6 nm in size ligated by
(2R,4R)-2,4-bis(diphenylphosphino)pentane ((R,R)-BDPP, Scheme 2) were
Iridium Nanoparticles for Hydrogenation Reactions
399