After considering structural limitations in designing functional carbometalated
pincer complexes, Gelman and co-workers initiated a research project devoted to
designing a modular family of three-dimensional PC(sp
3 )P pincer complexes based
on the dibenzobarrelene scaffold [48, 76]. The unique topology of these molecules
allows, in principle, constructing multifunctional catalysts possessing primary and
secondary coordination spheres in reasonable proximity, in such a way that communication between them might affect the catalytic site in a controllable fashion. We
envisioned a straightforward synthesis of these ligands based on the one-step DielsAlder cycloaddition of the desired dienophile to a common, readily available, and
structurally simple anthracene-based precursor that represents a highly modular and
divergent approach to a variety of three-dimensional platforms equipped with
custom-tailored primary and secondary coordination spheres (Scheme 17).
Indeed, a set of bifunctional ligands of this type was prepared using a Diels-Alder
click of 1,8-bis(phosphino)anthracenes with a variety of dienophiles. For instance,
the amine-functionalized (37) [77] was created via a two-step protocol starting
from the quantitative Diels-Alder cycloaddition of the known 1,8-bis(diphenylphosphino)anthracene (35) and fumaronitrile, followed by a reduction of
the adduct (Scheme 18).
PiPr 2
PiPr 2
MeO
31
32
33
NaAlH 4
[IrCl(COD)] 2
6 days, C 6 H 6
reflux
P
C
iPr
iPr
O CH 3
Ir
P iPr
iPr
H
Cl
P
C
iPr
iPr
O CH 3
Ir
P
iPr
iPr
H
H
Scheme 15 Coordination chemistry of the PC(sp
3
)P ligand 31
33
CO
34
base
no reaction
P
C
iPr
iPr
O CH 3
Ir
P
iPr
iPr
H
H
P
C
iPr
iPr
CO
Ir
P
iPr
iPr
H
H
O
H 3 C
Scheme 16 Reactivity of 33
Scheme 17 One-step Diels-Alder cycloaddition approach for the synthesis of three-dimensional
PC(sp
3
)P pincer complexes
106
A. Singh et al.