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Chirality Transfer from Chiral Monoamines to an m‑Phthalic Diamide-Linked Zinc Bisporphyrinate with a Benzylamide Substituent
journal contribution
posted on 2017-08-12, 13:48 authored by Qingyun Hu, Congcong Zhuo, Yong Wang, Chuanjiang Hu, Jianping LangAn m-phthalic diamide-linked bisporphyrin with
a benzylamide substituent has been designed and synthesized. It has
two types of carbonyl groups. In the solution of this zinc bisporphyrinate,
these carbonyl groups are involved in the formation of two different
Zn–O coordination interactions: one is formed between neighboring
zinc bisporphyrinates; another is formed within zinc bisporphyrinate.
The chirality sensing abilities of this zinc porphyrinate to a number
of chiral monoamines have been examined. When zinc bisporphyrinate
was mixed with a series of chiral monoamines, the signs of the circular
dichroism spectra for the chiral monoamines of the same handedness
with an aryl group as the substituent are just opposite to those with
an alkyl group as the substituent. NMR studies reveal that stepwise
coordinations lead to 1:1 and 1:2 host–guest complexes. The
structure of the 1:1 host–guest complex was confirmed by crystallography,
it is the first time that a 1:1 host–guest complex formed between
zinc bisporphyrinate and a chiral monoamine has been crystallographically
characterized. The structure reveals that there is an intramolecular
hydrogen bond between the amide oxygen and the coordinated NH2. We further investigated the chirality transfer mechanism
by density functional theory calculations. Our studies suggest that
the interactions between the linker and guests in this bisporphyrin
system are crucial in the chirality transfer process, and the nature
of the bulkiest substituent of chiral monoamines makes a difference.
For R-type guests, with an alkyl group, the steric repulsion makes
the conformer A more energetically favorable, which leads to the anticlockwise
twist and negative Cotton effect. However, with an aryl group, the π–π
interaction makes the conformer B more energetically favorable, which
leads to the clockwise twist and positive Cotton effect.