1 Ligand-Free Sub-Nanometer Metal Clusters in Catalysis
15
current interest, information on structural and electronic properties is often corroborated only by free gas-phase clusters. The stability of MPCs is often understood
within the conceptual framework of superatoms validated in the gas phase on the
basis of electronic shells.
In this respect, we summarize here the research advances in the catalytic activity
of ligand-free metal clusters, either supported or not [14, 34, 38, 66, 70]. For that,
we divide the reactions considering the new bond formed (carbon–carbon, carbon–
heteroatom or heteroatom–heteroatom) and also in hydrogenation reactions. Oxidation reactions have been covered previously, and we do not discuss those here
[29, 30].
1.3.1 Carbon–Carbon Bond–Forming Reactions
1.3.1.1 Heck, Suzuki and Sonogashira Couplings
Pd-catalyzed cross-coupling reactions are recognized as fundamental transformations in synthetic chemistry [71]. The general mechanism for these reactions (also
under ligand-free conditions) involves an oxidative addition–reductive elimination
cycle over a Pd(0) species generated in situ. Despite considerable effort during last
three decades, the exact nature of the Pd
0 catalytic species remains a matter of
debate [13]. It was recently found that Pd 3–4 clusters are formed from either Pd salts,
complexes and nanoparticles in N–methylpyrrolidine (NMP) under heating conditions and are responsible for the catalytic activity during the Heck, Sonogashira,
Stille and Suzuki coupling reactions of different iodo– and bromo–derivatives. The
ligand-free Pd 3–4 clusters can be stored in aqueous solution to be used on demand
and catalyze, for example, the Heck reaction under industrially viable conditions in
high yields and with unprecedented turnover frequencies in some cases (Fig. 1.15).
Despite this high activity, the activation of chloro–derivatives was not possible with
this catalytic system, which highlights the still important role of ligands for particular
molecule activations.
The use of metals in the form of sub-nanometer clusters can lead to a completely
unexpected catalytic behavior for a given metal, and the Heck reaction is an illustrative example, recently uncovered [17]. As commented above, Pd clusters seem
to be very active species for the Heck reaction which, however, is not surprising
in terms of metal nature since Pd is the most common metal catalyst for this type
of C–C coupling reaction. Indeed, the metal above Pd in the group X of the Periodic Table, Ni, is also active for this type of couplings but, in striking contrast,
the metal below in the group X, Pt, is barely reported as a catalyst for C–C crosscoupling reactions. The lack of catalytic activity of Pt for these coupling reactions
has been traditionally ascribed to the difficulties associated to single Pt atoms (in
organometallic complexes) to efficiently shift between two electron oxidation states
and thus promote the oxidative addition and the reductive elimination steps necessary
to perform the coupling, without rapidly aggregating and losing activity. However, the
15
current interest, information on structural and electronic properties is often corroborated only by free gas-phase clusters. The stability of MPCs is often understood
within the conceptual framework of superatoms validated in the gas phase on the
basis of electronic shells.
In this respect, we summarize here the research advances in the catalytic activity
of ligand-free metal clusters, either supported or not [14, 34, 38, 66, 70]. For that,
we divide the reactions considering the new bond formed (carbon–carbon, carbon–
heteroatom or heteroatom–heteroatom) and also in hydrogenation reactions. Oxidation reactions have been covered previously, and we do not discuss those here
[29, 30].
1.3.1 Carbon–Carbon Bond–Forming Reactions
1.3.1.1 Heck, Suzuki and Sonogashira Couplings
Pd-catalyzed cross-coupling reactions are recognized as fundamental transformations in synthetic chemistry [71]. The general mechanism for these reactions (also
under ligand-free conditions) involves an oxidative addition–reductive elimination
cycle over a Pd(0) species generated in situ. Despite considerable effort during last
three decades, the exact nature of the Pd
0 catalytic species remains a matter of
debate [13]. It was recently found that Pd 3–4 clusters are formed from either Pd salts,
complexes and nanoparticles in N–methylpyrrolidine (NMP) under heating conditions and are responsible for the catalytic activity during the Heck, Sonogashira,
Stille and Suzuki coupling reactions of different iodo– and bromo–derivatives. The
ligand-free Pd 3–4 clusters can be stored in aqueous solution to be used on demand
and catalyze, for example, the Heck reaction under industrially viable conditions in
high yields and with unprecedented turnover frequencies in some cases (Fig. 1.15).
Despite this high activity, the activation of chloro–derivatives was not possible with
this catalytic system, which highlights the still important role of ligands for particular
molecule activations.
The use of metals in the form of sub-nanometer clusters can lead to a completely
unexpected catalytic behavior for a given metal, and the Heck reaction is an illustrative example, recently uncovered [17]. As commented above, Pd clusters seem
to be very active species for the Heck reaction which, however, is not surprising
in terms of metal nature since Pd is the most common metal catalyst for this type
of C–C coupling reaction. Indeed, the metal above Pd in the group X of the Periodic Table, Ni, is also active for this type of couplings but, in striking contrast,
the metal below in the group X, Pt, is barely reported as a catalyst for C–C crosscoupling reactions. The lack of catalytic activity of Pt for these coupling reactions
has been traditionally ascribed to the difficulties associated to single Pt atoms (in
organometallic complexes) to efficiently shift between two electron oxidation states
and thus promote the oxidative addition and the reductive elimination steps necessary
to perform the coupling, without rapidly aggregating and losing activity. However, the