410
L. M. Martínez-Prieto and P. W. N. M. van Leeuwen
catalyst in the hydrolytic dehydrogenation of ammonia borane in water [16]. However, the interactions between amine ligands and Ru NPs are labile. In solution,
the amines coordinate reversibly to the ruthenium surface, leading to a continuous
exchange between coordinated and free ligands. For example, Lee et al. observed that
the interaction between ethylenediamine and the surface of Ru NPs is weak, losing
their solubility in water with time due to the gradual decoordination of the amine
ligand from the metal surface [17]. This fast ligand exchange was experimentally
confirmed by Chaudret and co-workers [18]. A dynamic equilibrium between the
free ligand and the coordinated one was observed by
13 C NMR studies on worm-like
Ru NPs stabilized with high concentrations of long alkyl amines (C 8 , C 12 , and C 16 ).
Depending on the amount and type of amine used as stabilizer, it is also possible to control the morphology of Ru NPs. Tilley et al. obtained hourglass-shaped
Ru NPs by using dodecylamine as stabilizer (Fig. 12.1a–d, left) [19]. Moreover,
by using a mixture of hexadecylamine (HDA) and hexadecanoic acid (PA), Lignier et al. obtained Ru nanostars with large surface area and good dispersion
(Fig. 12.1e–f , right), which were presented as promising non-supported Ru NPs
for catalytic applications, since they were able to activate C–C and C–O bonds [20].
As a whole, we can say that amines are not the most appropriate ligands to obtain
small and spherical Ru NPs, due to their weak coordination to the Ru surface. However, novel nano-objects can be reached with these amine ligands and may present
peculiar reactivities, which demonstrates the high potential of amines in Ru NP
catalysis.
Philippot in collaboration with Zahmakıran stabilized Ru NPs with 3aminopropyltriethoxysilane (APTS), which demonstrated to be highly active,
isolable, and reusable catalysts in the dehydrogenation of dimethylamine borane,
which is considered an efficient hydrogen storage molecule [21, 22]. The coordination of this ligand through the amine group was evidenced by solution NMR studies,
as was previously observed on Ru NPs capped with long alkyl amines [17]. It was
the first example of a reusable heterogeneous catalyst for the dehydrogenation of
dimethylamine borane, the higher the APTS/Ru ratio is, the lower the catalytic activity is; this is a matter of active metal surface coverage. Ru NPs capped with the same
bifunctional amine (APTS) were also used as building blocks to synthesize composite ruthenium-containing silica nanomaterials (RuO 2 @SiO2; Fig. 12.2), which
showed a promising activity in the aerobic oxidation of benzyl alcohol [23].
In 2011, Salas et al. presented a new catalytic system based on Ru NPs stabilized
by ionic liquids (ILs: [C 1 C n H 2n+1 Im][NTf 2 ] (where n = 2; 4; 6; 8; 10) and long alkyl
chain amines (1-octylamine, 1-hexadecylamine) [24]. ILs help the formation of very
small NPs inside their non-polar domains (nanoreactors), while amines increase
their stability. Indeed, one could evidence the influence of the amine ligands in the
control of the size and the stability of these Ru NPs. These NPs showed a high
degree of recyclability in the hydrogenation of toluene (up to 10 cycles), without the
formation of agglomerations or any loss of activity. The presence of amine ligands
increases the NP stability without reducing the catalytic activity, due to their weak
interaction with the NP surface. By using a similar catalytic system (RuNPs stabilized
by in [C 1 C 4 Im][NTf 2 ]) but functionalized with different compounds (1-octylamine,
L. M. Martínez-Prieto and P. W. N. M. van Leeuwen
catalyst in the hydrolytic dehydrogenation of ammonia borane in water [16]. However, the interactions between amine ligands and Ru NPs are labile. In solution,
the amines coordinate reversibly to the ruthenium surface, leading to a continuous
exchange between coordinated and free ligands. For example, Lee et al. observed that
the interaction between ethylenediamine and the surface of Ru NPs is weak, losing
their solubility in water with time due to the gradual decoordination of the amine
ligand from the metal surface [17]. This fast ligand exchange was experimentally
confirmed by Chaudret and co-workers [18]. A dynamic equilibrium between the
free ligand and the coordinated one was observed by
13 C NMR studies on worm-like
Ru NPs stabilized with high concentrations of long alkyl amines (C 8 , C 12 , and C 16 ).
Depending on the amount and type of amine used as stabilizer, it is also possible to control the morphology of Ru NPs. Tilley et al. obtained hourglass-shaped
Ru NPs by using dodecylamine as stabilizer (Fig. 12.1a–d, left) [19]. Moreover,
by using a mixture of hexadecylamine (HDA) and hexadecanoic acid (PA), Lignier et al. obtained Ru nanostars with large surface area and good dispersion
(Fig. 12.1e–f , right), which were presented as promising non-supported Ru NPs
for catalytic applications, since they were able to activate C–C and C–O bonds [20].
As a whole, we can say that amines are not the most appropriate ligands to obtain
small and spherical Ru NPs, due to their weak coordination to the Ru surface. However, novel nano-objects can be reached with these amine ligands and may present
peculiar reactivities, which demonstrates the high potential of amines in Ru NP
catalysis.
Philippot in collaboration with Zahmakıran stabilized Ru NPs with 3aminopropyltriethoxysilane (APTS), which demonstrated to be highly active,
isolable, and reusable catalysts in the dehydrogenation of dimethylamine borane,
which is considered an efficient hydrogen storage molecule [21, 22]. The coordination of this ligand through the amine group was evidenced by solution NMR studies,
as was previously observed on Ru NPs capped with long alkyl amines [17]. It was
the first example of a reusable heterogeneous catalyst for the dehydrogenation of
dimethylamine borane, the higher the APTS/Ru ratio is, the lower the catalytic activity is; this is a matter of active metal surface coverage. Ru NPs capped with the same
bifunctional amine (APTS) were also used as building blocks to synthesize composite ruthenium-containing silica nanomaterials (RuO 2 @SiO2; Fig. 12.2), which
showed a promising activity in the aerobic oxidation of benzyl alcohol [23].
In 2011, Salas et al. presented a new catalytic system based on Ru NPs stabilized
by ionic liquids (ILs: [C 1 C n H 2n+1 Im][NTf 2 ] (where n = 2; 4; 6; 8; 10) and long alkyl
chain amines (1-octylamine, 1-hexadecylamine) [24]. ILs help the formation of very
small NPs inside their non-polar domains (nanoreactors), while amines increase
their stability. Indeed, one could evidence the influence of the amine ligands in the
control of the size and the stability of these Ru NPs. These NPs showed a high
degree of recyclability in the hydrogenation of toluene (up to 10 cycles), without the
formation of agglomerations or any loss of activity. The presence of amine ligands
increases the NP stability without reducing the catalytic activity, due to their weak
interaction with the NP surface. By using a similar catalytic system (RuNPs stabilized
by in [C 1 C 4 Im][NTf 2 ]) but functionalized with different compounds (1-octylamine,