5 Mechanistic Insight into the Hydrogen Activation by Frustrated Lewis Pairs
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5.2 Types of FLPs
FLP catalysts can be structurally divided into two categories: intra- and intermolecular FLPs. While in intermolecular FLPs, the LA and LB centers reside in separate
molecules, in intramolecular FLPs the LA and LB centers are covalently connected
and thus form a single molecule [13]. The advantage of the latter is the reduced
entropic penalty that has to be overcome to form a catalytically active complex.
Scheme 5.3 shows several examples of experimentally investigated intramolecular (Scheme 5.3a–d) and intermolecular (Scheme 5.3e–g) FLPs. The prototypical
tBu 3 P/BCF pair, shown in Scheme 5.3e, is sterically frustrated due to the bulkiness
of both LA and LB fragments. Scheme 5.3f shows the 2,6-lutidine/BCF pair, which
is thermally frustrated from Lewis-adduct formation and is able to activate H 2 after
dissociation. Also 1,4-dioxane/BCF, shown in Scheme 5.3g, is a thermally frustrated
Lewis pair.
After its initial discovery,the concept of FLP catalysis has been further broadened
to include a range of acceptor (B, Al, Sn, Si, C, P) and donor (P, N, C, O, S, Te)
main-group elements. In the next section, we briefly review several investigations of
the application of these elements toward H 2 activation.
5.2.1 Boron and Aluminum as Lewis Acid Centers
Fluorinated boranes–e.g., B(C 6 F 5 ) 3 (abbreviated as BCF) and its derivatives, which
are commercially available, are the most commonly employed Lewis acids in FLP
chemistry. To further broaden the scope of Lewis acids, other Group 13 and 14
elements have been tried. Without aiming to be exhaustive, here we discuss several
representative Lewis acids based on boron and aluminum as the active center [14].
The use of aluminum-based Lewis acids is an extension of boron-containing
Lewis acids. The first example of Al-based FLP catalysis was reported in 2009 [15].
In several recent studies, intramolecular FLPs including P/Al Lewis pairs have been
considered for activation of H 2 [16]. In addition, geminal FLPs based on N/Al have
been prepared in order to tune the Lewis acidity/basicity of the FLP and consequently
to modify the activity of the system [17].
As an alternative to BCF for FLP activation of H 2 , a series of donor–
acceptor cryptands with pyramidalized donor (azaadamantane) and acceptor
(bora/ala/adamantane) molecules were studied by Timoshkin and coworker in 2019
(shown in Scheme 5.4) [18]. The active sites on the fragments are orientated toward
each other, and the fragments are connected through an aromatic spacer. Using staticDFT calculations, the effectiveness of the constructed cryptands in the heterolytic
splitting of H 2 was predicted and the most promising candidates for experimental
studies were identified. This computational study showed that fluorination of the
bora-adamantane-based group leads to a remarkable decrease of the H 2 splitting
reaction free energy, G, (i.e., 67–73 kcal mol
−1 ) for boron-containing cryptands.
171
5.2 Types of FLPs
FLP catalysts can be structurally divided into two categories: intra- and intermolecular FLPs. While in intermolecular FLPs, the LA and LB centers reside in separate
molecules, in intramolecular FLPs the LA and LB centers are covalently connected
and thus form a single molecule [13]. The advantage of the latter is the reduced
entropic penalty that has to be overcome to form a catalytically active complex.
Scheme 5.3 shows several examples of experimentally investigated intramolecular (Scheme 5.3a–d) and intermolecular (Scheme 5.3e–g) FLPs. The prototypical
tBu 3 P/BCF pair, shown in Scheme 5.3e, is sterically frustrated due to the bulkiness
of both LA and LB fragments. Scheme 5.3f shows the 2,6-lutidine/BCF pair, which
is thermally frustrated from Lewis-adduct formation and is able to activate H 2 after
dissociation. Also 1,4-dioxane/BCF, shown in Scheme 5.3g, is a thermally frustrated
Lewis pair.
After its initial discovery,the concept of FLP catalysis has been further broadened
to include a range of acceptor (B, Al, Sn, Si, C, P) and donor (P, N, C, O, S, Te)
main-group elements. In the next section, we briefly review several investigations of
the application of these elements toward H 2 activation.
5.2.1 Boron and Aluminum as Lewis Acid Centers
Fluorinated boranes–e.g., B(C 6 F 5 ) 3 (abbreviated as BCF) and its derivatives, which
are commercially available, are the most commonly employed Lewis acids in FLP
chemistry. To further broaden the scope of Lewis acids, other Group 13 and 14
elements have been tried. Without aiming to be exhaustive, here we discuss several
representative Lewis acids based on boron and aluminum as the active center [14].
The use of aluminum-based Lewis acids is an extension of boron-containing
Lewis acids. The first example of Al-based FLP catalysis was reported in 2009 [15].
In several recent studies, intramolecular FLPs including P/Al Lewis pairs have been
considered for activation of H 2 [16]. In addition, geminal FLPs based on N/Al have
been prepared in order to tune the Lewis acidity/basicity of the FLP and consequently
to modify the activity of the system [17].
As an alternative to BCF for FLP activation of H 2 , a series of donor–
acceptor cryptands with pyramidalized donor (azaadamantane) and acceptor
(bora/ala/adamantane) molecules were studied by Timoshkin and coworker in 2019
(shown in Scheme 5.4) [18]. The active sites on the fragments are orientated toward
each other, and the fragments are connected through an aromatic spacer. Using staticDFT calculations, the effectiveness of the constructed cryptands in the heterolytic
splitting of H 2 was predicted and the most promising candidates for experimental
studies were identified. This computational study showed that fluorination of the
bora-adamantane-based group leads to a remarkable decrease of the H 2 splitting
reaction free energy, G, (i.e., 67–73 kcal mol
−1 ) for boron-containing cryptands.