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DOI: 10.1002/chem.201404782
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Helical Structures
One-Handed Single Helicates of Dinickel(II) Benzenehexapyrrole-
a,w-diimine with an Amine Chiral Source
Chaolu Eerdun, Satoshi Hisanaga, and Jun-ichiro Setsune*[a]
Abstract: Benzenehexapyrrole-a,w-dialdehyde, composed of
a pair of formyltripyrrole units with a 1,3-phenylene linker,
was metallated to give dinuclear single-stranded helicates.
X-ray studies of the bis-nickel(II) complex showed a helical
C2 form with a pair of helical–metal coordination planes of
a 3N+O donor set. The terminal aldehyde was readily con-
verted into the imine by optically active amines, whereby
helix-sense bias was induced. Bis-nickel(II) and bis-palla-
dium(II) complexes of the benzenehexapyrrole-a,w-diimines
were studied to show that an enantiomer pair of the helical
C2 form are interchanged by slow flipping of each coordina-
tion plane and fast rotation around the C(benzene)ÀC(pyr-
role) bond. The helical screw in the bis-nickel(II) complexes
was biased to one side in more than 95% diastereoselec-
tivity, which was achieved by using a variety of optically
active amines, such as (R)-1-cyclohexylethylamine, (S)-1-
phenylethylamine, l-Phe(OEt) (Phe=phenylalanine), and (R)-
valinol. The nickel complexes showed much better diaste-
reoselectivity than the corresponding palladium complexes.
Introduction
sible interplay between a bimetallic chemical process at the
metal sites and the helical-backbone conformation. In fact, the
helix sense was biased by ditopic binding of a chiral guest in
the Zn2L complexes,[8] which is useful for chirality sensing.[9] On
the other hand, predetermined one-handed M2L helicates can
regulate stereochemistry in the chemical process at the metal
sites.
Helicates are one of the most intriguing structural motifs for
asymmetric molecular architecture as they are regarded as
abiotic mimics for helices in biomaterials.[1] In view of their
application to biological systems or chirality-related smart ma-
terials, significant effort in molecular design and synthesis has
focused on inducing absolute helicity.[2] Optically pure double-
and triple-stranded helicates (M2L2 and M2L3) have so far been
synthesized, and their rigorous choice of helical sense is owed
to the chirality introduced at the ligand terminus or in the
linker.[3,4] However, a full bias of helix sense is still difficult to
achieve especially in the case of single-stranded helicates, al-
though they have recently drawn considerable attention due
to their novel dynamic conformational change, such as inver-
sion and transfer of helical chirality.[5] The ML- and M2L-type
helicates of the widely used oligopyridine ligands are also
known,[6,7] but they are racemic, except for a few cases.[7a,b]
Single-stranded helicates tend to be in fast equilibrium be-
tween right- and left-handed helices due to their relatively flex-
ible structure, which also raises problem of the determination
of optical purity. Dinuclear single-stranded M2L complexes in
which both ends of a ligand backbone are folded upon coordi-
nation to metal ions are of special interest because of the pos-
Open-chain oligopyrroles are regarded as a unique scaffold
for helices with intense absorptions in the visible region. Their
hydrogen-bonding ability causes helical conformation to
happen spontaneously[10a,b] or on anion binding,[10c,d] and their
metal-coordinating ability leads to the formation of various
helicates.[11–15] Single-stranded ML and M2L,[11,12] double-
stranded M2L2,[11c,f,13] triple-stranded M2L3,[14] and triangular M3L3
complexes[13f,15] have been reported, but most of these studies
employed tetrapyrroles, and the helicity control was intended
in a limited number of reports;[10a,c,11e–j,13a,b] that is, the chirality
introduced at the end group and at the central part of tetra-
pyrroles was utilized to induce helical-sense bias diastereo-
selectively in the case of ZnL and Zn2L2 helicates.[11e–g,13a,b] It is
remarkable that a M2L-type helicate, in which a pair of zinc(II)
bilinones are coupled by a chiral diol linker, has demonstrated
full helix-sense bias,[16] but helicates of such long-chain oligo-
pyrroles are very rare in spite of their potential applicability
(Figure 1).[17] Helicates of pentapyrrole, hexapyrrole, heptapyr-
role, and octapyrrole have recently been reported, but their
helicity control has remained unexplored.[17] In principle, the
chirality at the linker has a more direct influence on the helical
twist,[2g] but the chirality at the ligand terminus is more con-
venient to introduce. We recently reported that hexapyrrole-
a,w-dialdehyde 1’ forms a helical dinuclear palladium(II) com-
plex 1’Pd2 and its terminal aldehyde group is readily converted
into the imine moiety. The helicate 3’Pd2, which comprises (R)-
[a] Dr. C. Eerdun, S. Hisanaga, Prof. Dr. J.-i. Setsune
Department of Chemistry
Graduate School of Science, Kobe University
1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501 (Japan)
Fax: (+81)78-803-5770
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201404782.
Chem. Eur. J. 2015, 21, 239 – 246
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