Chiral Recognition between Metallohelicates via Strong H Bonds: Homochiral Bishelical Coupling and Mesohelical Polymerization

A rigid,
one-dimensional, and monotopic ligand containing both
a proton donor and a proton acceptor, 2-[(1H-imidazol-2-yl)­methyleneamino]­phenol
(H2L), has been used to attain three hierarchically assembled
supramolecular polymers. The handedness of these hydrogen-bonded metallo-supramolecular
structures is modulated by a selective coupling, via strong H bonds
with significant covalent character. Thus, alternate Δ and Λ
enantiomers of Cd­(HL)2 form a
one-dimensional supramolecular polymer via mutual strong N–H···O
interactions (2.58 Å), which are associated with a hydrogen bond
energy value of ∼11.5
kcal mol–1. In contrast, homochiral self-recognition
of enantiomers of
M­(HL)2 and [M­(H2L)2]2+ (M = CuII and NiII) gives rise to dinuclear
bishelicates, whose metallohelical units are held together by two
strong O–H···O bonds (2.41–2.44 Å),
which are associated with
hydrogen bond energies in the range of 25–35 kcal mol–1.