9 Frustrated Lewis Pairs Based on Transition Metals
321
integrating components of FLP systems goes far beyond this simple assumption and
provides a whole range of new chemical possibilities, as will be discussed throughout
the Chapter.
The use of transition metals either as the acid or basic site (or both) in the design
of TM-based FLPs has attracted continuous attention since the pioneering work of
the groups of Wass [9–11] and Erker [12, 13] on zirconium/phosphine pairs. This
emerging field of research has already been reviewed in a reference book Chapter
[14] that precedes the current contribution and in an inspiring perspective reported
by Wass [15]. As such, this Chapter will mainly concentrate on the more recent
achievements accumulated over the last five years. Besides, it will focus on FLP
architectures where the transition metal functions as one (or both) of the active sites
of the cooperative moiety, while those in which it merely acts as a structural pillar will
not be covered [16–18]. The application of intramolecular FLPs as ambiphilic ligands
to tune the reactivity and catalytic behavior of transition metals has already been
reviewed elsewhere [19], and will not be further discussed. Highly informative recent
perspectives have also focused on the analogy between FLPs and metallic cooperative
catalysts in the context of dihydrogen cleavage [20], as well as on systems containing
multiple metal–ligand bonds as masked TMFLPs [21]. The role of gold as the acidic
fragment of TMFLPs has been lately revised [22], including gold nanoparticles.
The latter and other heterogeneous catalysts that seem to operate through FLP-like
mechanisms will not be covered either [23–25].
There is a clear analogy between TMFLPs and other cooperative systems, particularly in the context of hydrogenation/dehydrogenation catalysis. Thus, many of the
most remarkable catalysts in the field, and even the active site of several metalloenzymes such as hydrogenases [26–28], could be described in terms of FLP-like reactivity. This analogy was rapidly noticed by Wass and others and has been thoroughly
discussed before [15]. With this in mind, this Chapter will only cover cooperative
designs in which the authors specifically state their intention to prepare TMFLPs
or where their investigations on reactivity and mechanisms prompted them to identify those as FLP-like systems. Other cooperative entities that could in principle be
understood in terms of FLP-like reactivity, but where the authors have restricted
themselves to the broader term of ‘metal-ligand cooperation’, with no specific claim
of frustration, will not generally be discussed [29–31], as they have been reviewed
and highlighted on several occasions [32–35].
Introducing transition metals into FLP architectures might seem to arguably undermine the utmost benefit of the original systems, namely, their capacity to mediate
catalytic transformations in the absence of transition metals. However, there are a
number of rewards that largely justify intense research in this direction, apart from
the obvious enormous rise of combinatorial possibilities derived from introducing
three series of transition metals (even more if lanthanides and actinides are to be
considered). Among the main advantages of introducing transition metals into FLP
structures, the following should be had in mind:
– Rich reactivity. The rich chemistry of transition metals results from the presence of partly occupied d orbitals with energies that make them amenable to
321
integrating components of FLP systems goes far beyond this simple assumption and
provides a whole range of new chemical possibilities, as will be discussed throughout
the Chapter.
The use of transition metals either as the acid or basic site (or both) in the design
of TM-based FLPs has attracted continuous attention since the pioneering work of
the groups of Wass [9–11] and Erker [12, 13] on zirconium/phosphine pairs. This
emerging field of research has already been reviewed in a reference book Chapter
[14] that precedes the current contribution and in an inspiring perspective reported
by Wass [15]. As such, this Chapter will mainly concentrate on the more recent
achievements accumulated over the last five years. Besides, it will focus on FLP
architectures where the transition metal functions as one (or both) of the active sites
of the cooperative moiety, while those in which it merely acts as a structural pillar will
not be covered [16–18]. The application of intramolecular FLPs as ambiphilic ligands
to tune the reactivity and catalytic behavior of transition metals has already been
reviewed elsewhere [19], and will not be further discussed. Highly informative recent
perspectives have also focused on the analogy between FLPs and metallic cooperative
catalysts in the context of dihydrogen cleavage [20], as well as on systems containing
multiple metal–ligand bonds as masked TMFLPs [21]. The role of gold as the acidic
fragment of TMFLPs has been lately revised [22], including gold nanoparticles.
The latter and other heterogeneous catalysts that seem to operate through FLP-like
mechanisms will not be covered either [23–25].
There is a clear analogy between TMFLPs and other cooperative systems, particularly in the context of hydrogenation/dehydrogenation catalysis. Thus, many of the
most remarkable catalysts in the field, and even the active site of several metalloenzymes such as hydrogenases [26–28], could be described in terms of FLP-like reactivity. This analogy was rapidly noticed by Wass and others and has been thoroughly
discussed before [15]. With this in mind, this Chapter will only cover cooperative
designs in which the authors specifically state their intention to prepare TMFLPs
or where their investigations on reactivity and mechanisms prompted them to identify those as FLP-like systems. Other cooperative entities that could in principle be
understood in terms of FLP-like reactivity, but where the authors have restricted
themselves to the broader term of ‘metal-ligand cooperation’, with no specific claim
of frustration, will not generally be discussed [29–31], as they have been reviewed
and highlighted on several occasions [32–35].
Introducing transition metals into FLP architectures might seem to arguably undermine the utmost benefit of the original systems, namely, their capacity to mediate
catalytic transformations in the absence of transition metals. However, there are a
number of rewards that largely justify intense research in this direction, apart from
the obvious enormous rise of combinatorial possibilities derived from introducing
three series of transition metals (even more if lanthanides and actinides are to be
considered). Among the main advantages of introducing transition metals into FLP
structures, the following should be had in mind:
– Rich reactivity. The rich chemistry of transition metals results from the presence of partly occupied d orbitals with energies that make them amenable to