9 Frustrated Lewis Pairs Based on Transition Metals
339
In terms of reversibility, only the more congested aryloxide–NHC ligand, namely
the mesityl substituted one, liberates one molecule of CO 2 under dynamic vacuum
(100 °C, 10
–3 mbar). This may be directly connected to the catalytic potential of
these complexes, since only the latter promotes formation of propylene carbonate
from propylene oxide and CO 2 , while the compounds constructed around the less
sterically demanding ligands provided no activity under the same conditions. Triple
activation over a related cerium-based FLP stabilized by a heptadentate N 4 P 3 ligand
was also achieved recently. Capitalizing on the lability of Ce–P bonds, Zhu accomplished the triple activation of isocyanates, isothiocyanates, diazomethane, and azides
[141].
9.3 Frustrated Lewis Pairs and Related Systems based
on Two Transition Metals
The previous section describes a plethora of examples in which transition metal
elements behave as either Lewis acids or bases within a frustrated framework.
However, systems in which the two components are based on transition metals are
rather rare, despite the fact that many polarized heterobimetallic complexes exhibit
cooperative reactivity that is reminiscent of FLPs [142–146]. In this section, we will
first focus on the single currently available example of a genuine transition metal
only FLP (TMOFLP) and its reactivity toward small molecules to later examine
recent results on polarized heterobimetallic species and their connection to the field
of FLPs, particularly in cases where the analogy has been explicitly drawn by the
authors.
9.3.1 Transition Metal Only Frustrated Lewis Pairs
(TMOFLPs)
In a first attempt toward an all-transition metal FLP, the group of Wass anticipated
the use of a phosphinoaryloxide zirconocene as a suitable framework to coordinate an electron rich Pt(0) center through its pendant phosphine. Contrary to the
expected Zr(IV)/Pt(0) FLP, a new heterobimetallic compound is formed instead by
formal insertion of the platinum center into a Zr–C bond [147]. Shortly after, Campos
described the first TMOFLP by combining [(PMe 2 Ar
Dipp2 )Au(I)]NTf 2 (37b; Ar
Dipp2
= C 6 H 3 -2,6-(C 6 H 3 -2,6i Pr 2 ) 2 ; NTf 2
−
= [N(SO 2 CF 3 ) 2 ]
− ) and [Pt(0)(P
t Bu 3 ) 2 ] (38),
motivated by their proven Lewis acidic and basic character, respectively [148].
In these studies, the choice of rather bulky phosphine ligands was essential to
avoid the formation of a metal-only Lewis pair (MOLP) [149]. This was investigated
in detail by modifying the substituents of the terphenyl phosphine ligand that binds
the Au(I) fragment, more precisely by incorporating PMe 2 Ar
Xyl2 and PCyp 2 Ar
Xyl2
339
In terms of reversibility, only the more congested aryloxide–NHC ligand, namely
the mesityl substituted one, liberates one molecule of CO 2 under dynamic vacuum
(100 °C, 10
–3 mbar). This may be directly connected to the catalytic potential of
these complexes, since only the latter promotes formation of propylene carbonate
from propylene oxide and CO 2 , while the compounds constructed around the less
sterically demanding ligands provided no activity under the same conditions. Triple
activation over a related cerium-based FLP stabilized by a heptadentate N 4 P 3 ligand
was also achieved recently. Capitalizing on the lability of Ce–P bonds, Zhu accomplished the triple activation of isocyanates, isothiocyanates, diazomethane, and azides
[141].
9.3 Frustrated Lewis Pairs and Related Systems based
on Two Transition Metals
The previous section describes a plethora of examples in which transition metal
elements behave as either Lewis acids or bases within a frustrated framework.
However, systems in which the two components are based on transition metals are
rather rare, despite the fact that many polarized heterobimetallic complexes exhibit
cooperative reactivity that is reminiscent of FLPs [142–146]. In this section, we will
first focus on the single currently available example of a genuine transition metal
only FLP (TMOFLP) and its reactivity toward small molecules to later examine
recent results on polarized heterobimetallic species and their connection to the field
of FLPs, particularly in cases where the analogy has been explicitly drawn by the
authors.
9.3.1 Transition Metal Only Frustrated Lewis Pairs
(TMOFLPs)
In a first attempt toward an all-transition metal FLP, the group of Wass anticipated
the use of a phosphinoaryloxide zirconocene as a suitable framework to coordinate an electron rich Pt(0) center through its pendant phosphine. Contrary to the
expected Zr(IV)/Pt(0) FLP, a new heterobimetallic compound is formed instead by
formal insertion of the platinum center into a Zr–C bond [147]. Shortly after, Campos
described the first TMOFLP by combining [(PMe 2 Ar
Dipp2 )Au(I)]NTf 2 (37b; Ar
Dipp2
= C 6 H 3 -2,6-(C 6 H 3 -2,6i Pr 2 ) 2 ; NTf 2
−
= [N(SO 2 CF 3 ) 2 ]
− ) and [Pt(0)(P
t Bu 3 ) 2 ] (38),
motivated by their proven Lewis acidic and basic character, respectively [148].
In these studies, the choice of rather bulky phosphine ligands was essential to
avoid the formation of a metal-only Lewis pair (MOLP) [149]. This was investigated
in detail by modifying the substituents of the terphenyl phosphine ligand that binds
the Au(I) fragment, more precisely by incorporating PMe 2 Ar
Xyl2 and PCyp 2 Ar
Xyl2