Synthesis, Structure, and Properties of a Series of Chiral Tweezer–Diamine Complexes Consisting of an Achiral Zinc(II) Bisporphyrin Host and Chiral Diamine Guest: Induction and Rationalization of Supramolecular Chirality

We report here the synthesis, structure,
and spectroscopic properties of a series of supramolecular chiral
1:1 tweezer–diamine complexes consisting of an achiral Zn­(II)
bisporphyrin (Zn2DPO) host and five different chiral diamine
guests, namely, (R)-diaminopropane (DAP), (1S,2S)-diaminocyclohexane (CHDA), (S)-phenylpropane diamine (PPDA), (S)-phenyl
ethylenediamine (PEDA), and (1R,2R)-diphenylethylene diamine (DPEA). The solid-state structures are
preserved in solution, as reflected in their 1H NMR spectra,
which also revealed the remarkably large upfield shifts of the NH2 guest protons with the order Zn2DPO·DAP > Zn2DPO·CHDA > Zn2DPO·PPDA> Zn2DPO·PEDA ≫ Zn2DPO·DPEA, which happens to be the order of binding constants
of the respective diamines with Zn2DPO. As the bulk of
the substituent at the chiral center of the guest ligand increases,
the Zn–Nax distance of the tweezer–diamine
complex also increases, which eventually lowers the binding of the
guest ligand toward the host. Also, the angle between the two porphyrin
rings gradually increases with increasing bulk of the guest in order
to accommodate the guest within the bisporphyrin cavity with minimal
steric clash. The notably high amplitude bisignate CD signal response
by Zn2DPO·DAP, Zn2DPO·CHDA, and Zn2DPO·PPDA can be ascribed to the complex’s high
stability and the formation of a unidirectional screw as observed
in the X-ray structures of the complexes. A relatively lower value
of CD amplitude shown by Zn2DPO·PEDA is due to the
lower stability of the complex. The projection of the diamine binding
sites of the chiral guest would make the two porphyrin macrocycles
oriented in either a clockwise or anticlockwise direction in order
to minimize host–guest steric clash. In sharp contrast, Zn2DPO·DPEA shows a very low amplitude bisignate CD signal
due to the presence of both left- (dictated by the pre-existing chirality
of (1R,2R)-DPEA) and right-handed
screws (dictated by the steric differentiation at the chiral center)
of the molecule, as evident from X-ray crystallography. The present
work demonstrates a full and unambiguous rationalization of the observed
chirality transfer processes from the chiral guest to the achiral
host.