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participate in elementary reactions (e.g., oxidative addition, migratory insertion,
reductive elimination…). The reluctance of main group systems to mediate these
types of transformations has precluded their broader implementation into catalytic
systems. Thus, incorporating transition metals into FLPs could offer a whole range
of elementary reactions that could increase the catalytic usefulness of FLPs far
beyond their current use as (mainly) hydrogenation catalysts.
– Structural diversity. The structural diversity of transition metal organometallic
and coordination complexes largely exceeds that of main group systems. Coordination numbers typically range from two to six for the d-block, though they can
reach up to nine (e.g., [ReH 9 ]
2− ) [36]. This leads to a wide variety of structures
and geometries around the metal center that finds no parallel in the main group.
– Tunable properties. The vast amount of ligands with tunable stereoelectronic
properties enables the synthesis of virtually any desired transition metal complex
with precisely defined steric and electronic features, something unattainable for
the main group. Moreover, a precise transition metal element (particularly midTMs) can behave as an acid or a base depending on its oxidation state and ligand
environment, which also contrasts with the main group series, where the same
behavior, though being well-known, is less versatile.
– Synthetic amenability. The methods to prepare transition metal complexes have
been developed over the last decades and in many occasions synthetic protocols
are relatively simple and expeditious. Similarly, the most common basic partners
in traditional FLPs, namely phosphines, are readily prepared and their commercial
catalogue is comprehensive. However, accessing the type of fluorinated boranes
that are typically used as the acid fragment in FLPs is challenging [37], contrasting
with the synthetic amenability of transition metal Lewis acids.
– Broad spectra of affinities. Transition metals exhibit a broader diversity of affinities toward specific elements and interactions than main group compounds. For
instance, while the lighter main group elements are highly oxophilic and even
metalloids such as germanium or tin present medium oxophilicity, the wider
degree of oxophilicity found in the transition metal series is evidenced by the
very high oxophilicity of elements such as titanium or hafnium and the practically inexistent tendency to bind oxygen of gold or palladium [38]. Other examples
include carbophilicity [39], thiophilicity [38], metallophilicity [40], the propensity to form hydrogen bonding, and other types of interactions [41, 42]. Modulating these attributes can be essential for the activation of small molecules, to
trap reactive intermediates or for the development of more efficient catalysts.
For instance, a highly oxophilic element will favour the activation of an oxygencontaining species but if the resulting product becomes a thermodynamic sink,
catalytic turnover may be hampered. Controlling the degree of affinity may thus
be key for enhancing catalytic efficiency.
This Chapter is divided into two Sections. The first covers TMFLPs in which only
one of the two components (acid or basic) is a transition metal, which comprises
the majority of the reported systems. These designs are further categorized into
those constructed around early transition metals and those incorporating mid and
N. Hidalgo et al.
participate in elementary reactions (e.g., oxidative addition, migratory insertion,
reductive elimination…). The reluctance of main group systems to mediate these
types of transformations has precluded their broader implementation into catalytic
systems. Thus, incorporating transition metals into FLPs could offer a whole range
of elementary reactions that could increase the catalytic usefulness of FLPs far
beyond their current use as (mainly) hydrogenation catalysts.
– Structural diversity. The structural diversity of transition metal organometallic
and coordination complexes largely exceeds that of main group systems. Coordination numbers typically range from two to six for the d-block, though they can
reach up to nine (e.g., [ReH 9 ]
2− ) [36]. This leads to a wide variety of structures
and geometries around the metal center that finds no parallel in the main group.
– Tunable properties. The vast amount of ligands with tunable stereoelectronic
properties enables the synthesis of virtually any desired transition metal complex
with precisely defined steric and electronic features, something unattainable for
the main group. Moreover, a precise transition metal element (particularly midTMs) can behave as an acid or a base depending on its oxidation state and ligand
environment, which also contrasts with the main group series, where the same
behavior, though being well-known, is less versatile.
– Synthetic amenability. The methods to prepare transition metal complexes have
been developed over the last decades and in many occasions synthetic protocols
are relatively simple and expeditious. Similarly, the most common basic partners
in traditional FLPs, namely phosphines, are readily prepared and their commercial
catalogue is comprehensive. However, accessing the type of fluorinated boranes
that are typically used as the acid fragment in FLPs is challenging [37], contrasting
with the synthetic amenability of transition metal Lewis acids.
– Broad spectra of affinities. Transition metals exhibit a broader diversity of affinities toward specific elements and interactions than main group compounds. For
instance, while the lighter main group elements are highly oxophilic and even
metalloids such as germanium or tin present medium oxophilicity, the wider
degree of oxophilicity found in the transition metal series is evidenced by the
very high oxophilicity of elements such as titanium or hafnium and the practically inexistent tendency to bind oxygen of gold or palladium [38]. Other examples
include carbophilicity [39], thiophilicity [38], metallophilicity [40], the propensity to form hydrogen bonding, and other types of interactions [41, 42]. Modulating these attributes can be essential for the activation of small molecules, to
trap reactive intermediates or for the development of more efficient catalysts.
For instance, a highly oxophilic element will favour the activation of an oxygencontaining species but if the resulting product becomes a thermodynamic sink,
catalytic turnover may be hampered. Controlling the degree of affinity may thus
be key for enhancing catalytic efficiency.
This Chapter is divided into two Sections. The first covers TMFLPs in which only
one of the two components (acid or basic) is a transition metal, which comprises
the majority of the reported systems. These designs are further categorized into
those constructed around early transition metals and those incorporating mid and