9 Frustrated Lewis Pairs Based on Transition Metals
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late counterparts. In addition, we have revisited recent systems based on rare-earth
elements. In the second section, we describe transition metal-only frustrated Lewis
pairs (TMOFLP), in which the two components are based on transition metal fragments. Explicit mention to polarized heterobimetallic systems whose behavior could
be understood in terms of FLP-like reactivity will also be covered. These sections
are followed by a summary and outlook of the current state of the field and potential
research avenues for the near future.
9.2 TMFLPs with One Transition Metal Center
9.2.1 Early and Mid-Transition Metals
Electron deficient early transition metal complexes, particularly those in high oxidation states, have been widely used as Lewis acid catalysts for a number of transformations [43]. In an early example, prior to coining the term ‘Frustrated Lewis
Pair’[44], Stephan demonstrated that combining the acidic titanium compound
[CpTi(N P
t Bu 3 )][B(C 6 F 5 ) 4 ], with the sterically demanding P(o-MeC 6 H 4 ) 3 phosphine, did not lead to ligand coordination due to steric clash providing instead cooperative cleavage of a C–Cl bond of a dichloromethane solvent molecule. In a later
study, the group of Wass investigated an intramolecular titanocene–phosphinoaryloxy complex capable of heterolytically activating dihydrogen [45]. This system is
analogous to a zirconium-based counterpart that the same group explored in detail
in previous years [9, 10].
Other intramolecular Ti-based TMFLPs were later reported by Erker. In their first
strategy, the synthesis of a functionalized cyclopentadienyl ligand allowed access
to a series of cationic titanium complexes with a pendant phosphine of general
formula [CpCp
P TiOAr][BPh 4 ] (1) (Cp = ï
5 -C 5 H 5 ; Cp
P
= ï
5 -C 5 H 4 (CMe 2 )PCy 2 )
(Scheme 9.1a) [46]. Although this species was completely unreactive toward gaseous
substrates as H 2 , CO 2 , CO, or C 2 H 2 , it readily reacted with benzaldehyde to yield
the corresponding addition product (2). Interestingly, compound 1 slowly reacts
(9 days) with trans-chalcone to produce a 10-membered titanium macrocyle (3).
More recently, the same group described an intramolecular phosphidotitanocene
cation (4) that revealed FLP reactivity with ferrocene carboxaldehyde to yield the
expected addition product 5 (Scheme 9.1b) [47].
Beckhaus described a series of electrophilic cationic d
0 titanium complexes
with a single ligand framework, more precisely a novel tridentate Cp,N,P system
(Scheme 9.2a) [48]. This design allowed the authors to explore the competence
between the nitrogen termini and the phosphine ligand to cooperate with the acidic
titanium center to activate the C–H bonds in phenylacetylene and acetone (toward
7 and 8, respectively). Although the overall reaction takes place at the Ti–N bond,
as expected for the higher basicity of the nitrogen, a related compound missing
the phosphine group (9) revealed no activity even under harsher conditions, which
323
late counterparts. In addition, we have revisited recent systems based on rare-earth
elements. In the second section, we describe transition metal-only frustrated Lewis
pairs (TMOFLP), in which the two components are based on transition metal fragments. Explicit mention to polarized heterobimetallic systems whose behavior could
be understood in terms of FLP-like reactivity will also be covered. These sections
are followed by a summary and outlook of the current state of the field and potential
research avenues for the near future.
9.2 TMFLPs with One Transition Metal Center
9.2.1 Early and Mid-Transition Metals
Electron deficient early transition metal complexes, particularly those in high oxidation states, have been widely used as Lewis acid catalysts for a number of transformations [43]. In an early example, prior to coining the term ‘Frustrated Lewis
Pair’[44], Stephan demonstrated that combining the acidic titanium compound
[CpTi(N P
t Bu 3 )][B(C 6 F 5 ) 4 ], with the sterically demanding P(o-MeC 6 H 4 ) 3 phosphine, did not lead to ligand coordination due to steric clash providing instead cooperative cleavage of a C–Cl bond of a dichloromethane solvent molecule. In a later
study, the group of Wass investigated an intramolecular titanocene–phosphinoaryloxy complex capable of heterolytically activating dihydrogen [45]. This system is
analogous to a zirconium-based counterpart that the same group explored in detail
in previous years [9, 10].
Other intramolecular Ti-based TMFLPs were later reported by Erker. In their first
strategy, the synthesis of a functionalized cyclopentadienyl ligand allowed access
to a series of cationic titanium complexes with a pendant phosphine of general
formula [CpCp
P TiOAr][BPh 4 ] (1) (Cp = ï
5 -C 5 H 5 ; Cp
P
= ï
5 -C 5 H 4 (CMe 2 )PCy 2 )
(Scheme 9.1a) [46]. Although this species was completely unreactive toward gaseous
substrates as H 2 , CO 2 , CO, or C 2 H 2 , it readily reacted with benzaldehyde to yield
the corresponding addition product (2). Interestingly, compound 1 slowly reacts
(9 days) with trans-chalcone to produce a 10-membered titanium macrocyle (3).
More recently, the same group described an intramolecular phosphidotitanocene
cation (4) that revealed FLP reactivity with ferrocene carboxaldehyde to yield the
expected addition product 5 (Scheme 9.1b) [47].
Beckhaus described a series of electrophilic cationic d
0 titanium complexes
with a single ligand framework, more precisely a novel tridentate Cp,N,P system
(Scheme 9.2a) [48]. This design allowed the authors to explore the competence
between the nitrogen termini and the phosphine ligand to cooperate with the acidic
titanium center to activate the C–H bonds in phenylacetylene and acetone (toward
7 and 8, respectively). Although the overall reaction takes place at the Ti–N bond,
as expected for the higher basicity of the nitrogen, a related compound missing
the phosphine group (9) revealed no activity even under harsher conditions, which