12 Ligand Effects in Ruthenium Nanoparticle Catalysis
425
the ligand and the supramolecular host. It was evidenced that cyclodextrin is able to
modulate the catalytic reactivity of the sulfonated diphosphine-stabilized Ru NPs.
Comparing the reactivity of this nanocatalyst with the diphosphine-stabilized Ru NPs
(without cyclodextrin) in the biphasic hydrogenation of styrene, acetophenone and
m-methylanisole, interesting differences were found. The presence of the cyclodextrin not only increases the catalytic activity, as it acts as phase-transfer promoter,
but also influences the selectivity due to the formation of the inclusion complex
between the cyclodextrin and the phosphine. Philippot and co-workers also reported
another type of water-soluble phosphine-stabilized Ru NPs, but in this case, the NPs
were stabilized by 1,3,5-triaza-7-phosphaadamantane (PTA) [59]. The resulting NPs
showed interesting conversions and selectivities in the hydrogenation of olefins and
arenes in biphasic (water/substrate) catalytic reactions without the necessity of a
phase-transfer agent. The last results demonstrate the potential of stabilizing ligands
not only in the control of selectivity and activity of Ru NPs, but also in the modification of their physical properties such as water solubility, which allow us to carry
out catalysis in a more environmentally friendly way.
12.2.3.2 Secondary Phosphine Oxide Ligands (SPO)
In 2013, van Leeuwen and co-workers developed the use of secondary phosphine
oxides (SPO) for Ru NP stabilization. The obtained SPO-stabilized Ru NPs were
investigated in the hydrogenation of aromatic rings and carbonyl groups, turning out to be efficient nanocatalysts for the hydrogenation of arenes (TOFs up to
2700 mol.h
−1 in the hydrogenation of benzene) [60]. The idea was to induce the
heterolytic cleavage of hydrogen by using SPO ligands for Ru NPs, and in this way
enhance the activity in hydrogenation reactions of polar substrates such as ketones.
Hydrogenation of aromatics was strongly reduced by SPO ligands, but the activity for ketone hydrogenation remained similar. Later, for SPO-stabilized Au and Ir
NPs, they found indeed that H 2 is split through a cooperative action between the
SPO ligand and the neighboring metal center. Air-Stable Au NPs ligated by SPOs
were employed as efficient catalysts in the hydrogenation of numerous substituted
aldehydes, presenting high conversions and chemoselectivities [61]. In addition, a
notable ligand effect was found by using Au NPs ligated by various SPO ligands
with different electronic and steric properties (Fig. 12.18a) [62]. In particular, Au
NPs stabilized with SPO ligands with aromatic groups (1, 2 and 3) are highly selective in the hydrogenation of the carbonyl group, whereas Au NPs that contain alkyl
groups (4 and 5) are less active and selective in the hydrogenation of aldehydes. This
different catalytic behavior is explained through the different coordination forms of
SPOs at the Au surface (Fig. 12.18b), which depend on the type of SPO ligand.
While 1, 2, and 3 mainly coordinate as R 2 P = O anions, 3 and 4 also coordinates as
R 2 P–OH, changing the electronic effect of the catalytic systems and therefore their
catalytic properties. Similar selectivity toward aldehyde hydrogenation was found
for Ir NPs stabilized with SPO ligands 1, 2, and 5 [63, 64]. Lately, chiral Ir NPs
stabilized by an asymmetric secondary phosphine oxide (Fig. 12.18c) was used as
425
the ligand and the supramolecular host. It was evidenced that cyclodextrin is able to
modulate the catalytic reactivity of the sulfonated diphosphine-stabilized Ru NPs.
Comparing the reactivity of this nanocatalyst with the diphosphine-stabilized Ru NPs
(without cyclodextrin) in the biphasic hydrogenation of styrene, acetophenone and
m-methylanisole, interesting differences were found. The presence of the cyclodextrin not only increases the catalytic activity, as it acts as phase-transfer promoter,
but also influences the selectivity due to the formation of the inclusion complex
between the cyclodextrin and the phosphine. Philippot and co-workers also reported
another type of water-soluble phosphine-stabilized Ru NPs, but in this case, the NPs
were stabilized by 1,3,5-triaza-7-phosphaadamantane (PTA) [59]. The resulting NPs
showed interesting conversions and selectivities in the hydrogenation of olefins and
arenes in biphasic (water/substrate) catalytic reactions without the necessity of a
phase-transfer agent. The last results demonstrate the potential of stabilizing ligands
not only in the control of selectivity and activity of Ru NPs, but also in the modification of their physical properties such as water solubility, which allow us to carry
out catalysis in a more environmentally friendly way.
12.2.3.2 Secondary Phosphine Oxide Ligands (SPO)
In 2013, van Leeuwen and co-workers developed the use of secondary phosphine
oxides (SPO) for Ru NP stabilization. The obtained SPO-stabilized Ru NPs were
investigated in the hydrogenation of aromatic rings and carbonyl groups, turning out to be efficient nanocatalysts for the hydrogenation of arenes (TOFs up to
2700 mol.h
−1 in the hydrogenation of benzene) [60]. The idea was to induce the
heterolytic cleavage of hydrogen by using SPO ligands for Ru NPs, and in this way
enhance the activity in hydrogenation reactions of polar substrates such as ketones.
Hydrogenation of aromatics was strongly reduced by SPO ligands, but the activity for ketone hydrogenation remained similar. Later, for SPO-stabilized Au and Ir
NPs, they found indeed that H 2 is split through a cooperative action between the
SPO ligand and the neighboring metal center. Air-Stable Au NPs ligated by SPOs
were employed as efficient catalysts in the hydrogenation of numerous substituted
aldehydes, presenting high conversions and chemoselectivities [61]. In addition, a
notable ligand effect was found by using Au NPs ligated by various SPO ligands
with different electronic and steric properties (Fig. 12.18a) [62]. In particular, Au
NPs stabilized with SPO ligands with aromatic groups (1, 2 and 3) are highly selective in the hydrogenation of the carbonyl group, whereas Au NPs that contain alkyl
groups (4 and 5) are less active and selective in the hydrogenation of aldehydes. This
different catalytic behavior is explained through the different coordination forms of
SPOs at the Au surface (Fig. 12.18b), which depend on the type of SPO ligand.
While 1, 2, and 3 mainly coordinate as R 2 P = O anions, 3 and 4 also coordinates as
R 2 P–OH, changing the electronic effect of the catalytic systems and therefore their
catalytic properties. Similar selectivity toward aldehyde hydrogenation was found
for Ir NPs stabilized with SPO ligands 1, 2, and 5 [63, 64]. Lately, chiral Ir NPs
stabilized by an asymmetric secondary phosphine oxide (Fig. 12.18c) was used as