IL structure [56, 57]. Interestingly, ILs can operate as electrostatic and/or
electrosteric stabilisers for metallic nanoparticles preventing their agglomeration
[58]. Therefore, MNPs can be generated and immobilised in ILs, allowing the
straightforward separation of products and the recycling of the system
[59, 60]. Since the pioneering work of Dupont et al. [61], in the last two decades,
IL-stabilised MNPs have attracted a large interest by researchers working in catalysis. In this study, iridium nanoparticles (Ir NPs) of 2.0(0.4) nm in size were
generated by H 2 reduction of [Ir(cod)Cl] 2 in 1-butyl-3-methylimidazolium
hexafluorophosphate (bmimPF 6 ). The resulting Ir(0)/bmimPF 6 system was
employed as catalyst for the hydrogenation of several alkenes under mild reaction
conditions (4 bar H 2 and 75
C), showing high activities (TOF ¼ 6,000 h
À1 ) and a
good recycling behaviour. The reported nanocatalysts can be reused for up to at least
seven cycles without any significant loss in activity. This publication was the starting
point for a series of studies by this group on catalytic hydrogenation mediated by
IL-stabilised Ir NPs. Indeed, the Ir(0) NPs stabilised by bmimPF 6 were used, both as
a solid (referred to nanoparticles that were isolated through centrifugation and
washed with organic solvents and subsequently redispersed in the neat substrate)
or redispersed in the ionic liquid (biphasic conditions), for the hydrogenation of
arenes [62] and ketones [63] (Table 7). Similarly, these Ir NPs were successfully
recycled up to 7 and 15 times, respectively.
In a subsequent publication, competitive hydrogenation experiments of
alkylbenzenes (benzene, toluene, ethylbenzene, isopropylbenzene, sec-butylbenzene
and tert-butylbenzene) allowed the researchers to determine the selectivity constants
(S) against toluene of this type of Ir NPs (Table 8) and other bmimPF 6 -stabilised
MNPs, which may be employed to predict the relative reactivity of new
monoalkylbenzenes pairs [64]. The correlation of initial reaction rate constant for
toluene/benzene and toluene/monoalkylbenzene hydrogenation experiments with
the Taft equation indicates that the rates of hydrogenation of alkylbenzenes only
depend on steric impediment of the alkyl substituents. Finally, it was suggested that
bulky substituents slow the hydrogenation rate.
The kinetics for the hydrogenation of 1-decene catalysed by [Ir(cod)Cl] 2 ) in
bmimPF 6 was also investigated, showing an autocatalytic mechanism in which
iridium nanoclusters are formed [65]. The curves obtained in the hydrogenation of
different alkenes were fitted to a model that implies (1) nucleation (A ! B,
k 1 ¼ 2.04 h
À1 ) and (2) autocatalytic surface growth (A + B ! 2B,
k 2 ¼ 5,125 M
À1 h
À1 ). The small and monomodal size of NPs in the Ir/bmimPF 6
system (2.0(0.4) nm) indicates a strong stabilisation by the IL and that (1) is faster
than (2). Indeed, XPS, EXAFS and SAXS studies proved the presence of a protective IL shell with a diameter of ca. 2.8–4.0 nm surrounding the Ir NPs for several
Ir/bmimX systems (X ¼ PF 6 , BF 4 , NTf 2 ) [66]. Additional kinetic experiments on the
hydrogenation of 1-decene by the Ir/bmimPF 6 system demonstrated that the catalyst
follows a monomolecular surface reaction mechanism and thus acts as a heterogeneous catalytic system for alkene hydrogenation [65], that is:
406
L. M. Martínez-Prieto et al.
electrosteric stabilisers for metallic nanoparticles preventing their agglomeration
[58]. Therefore, MNPs can be generated and immobilised in ILs, allowing the
straightforward separation of products and the recycling of the system
[59, 60]. Since the pioneering work of Dupont et al. [61], in the last two decades,
IL-stabilised MNPs have attracted a large interest by researchers working in catalysis. In this study, iridium nanoparticles (Ir NPs) of 2.0(0.4) nm in size were
generated by H 2 reduction of [Ir(cod)Cl] 2 in 1-butyl-3-methylimidazolium
hexafluorophosphate (bmimPF 6 ). The resulting Ir(0)/bmimPF 6 system was
employed as catalyst for the hydrogenation of several alkenes under mild reaction
conditions (4 bar H 2 and 75
C), showing high activities (TOF ¼ 6,000 h
À1 ) and a
good recycling behaviour. The reported nanocatalysts can be reused for up to at least
seven cycles without any significant loss in activity. This publication was the starting
point for a series of studies by this group on catalytic hydrogenation mediated by
IL-stabilised Ir NPs. Indeed, the Ir(0) NPs stabilised by bmimPF 6 were used, both as
a solid (referred to nanoparticles that were isolated through centrifugation and
washed with organic solvents and subsequently redispersed in the neat substrate)
or redispersed in the ionic liquid (biphasic conditions), for the hydrogenation of
arenes [62] and ketones [63] (Table 7). Similarly, these Ir NPs were successfully
recycled up to 7 and 15 times, respectively.
In a subsequent publication, competitive hydrogenation experiments of
alkylbenzenes (benzene, toluene, ethylbenzene, isopropylbenzene, sec-butylbenzene
and tert-butylbenzene) allowed the researchers to determine the selectivity constants
(S) against toluene of this type of Ir NPs (Table 8) and other bmimPF 6 -stabilised
MNPs, which may be employed to predict the relative reactivity of new
monoalkylbenzenes pairs [64]. The correlation of initial reaction rate constant for
toluene/benzene and toluene/monoalkylbenzene hydrogenation experiments with
the Taft equation indicates that the rates of hydrogenation of alkylbenzenes only
depend on steric impediment of the alkyl substituents. Finally, it was suggested that
bulky substituents slow the hydrogenation rate.
The kinetics for the hydrogenation of 1-decene catalysed by [Ir(cod)Cl] 2 ) in
bmimPF 6 was also investigated, showing an autocatalytic mechanism in which
iridium nanoclusters are formed [65]. The curves obtained in the hydrogenation of
different alkenes were fitted to a model that implies (1) nucleation (A ! B,
k 1 ¼ 2.04 h
À1 ) and (2) autocatalytic surface growth (A + B ! 2B,
k 2 ¼ 5,125 M
À1 h
À1 ). The small and monomodal size of NPs in the Ir/bmimPF 6
system (2.0(0.4) nm) indicates a strong stabilisation by the IL and that (1) is faster
than (2). Indeed, XPS, EXAFS and SAXS studies proved the presence of a protective IL shell with a diameter of ca. 2.8–4.0 nm surrounding the Ir NPs for several
Ir/bmimX systems (X ¼ PF 6 , BF 4 , NTf 2 ) [66]. Additional kinetic experiments on the
hydrogenation of 1-decene by the Ir/bmimPF 6 system demonstrated that the catalyst
follows a monomolecular surface reaction mechanism and thus acts as a heterogeneous catalytic system for alkene hydrogenation [65], that is:
406
L. M. Martínez-Prieto et al.