Structural Characterization of an Enantiopure Hydroxo-Bridged Binuclear Iron(III) Complex with Empty One-Dimensional Helical Channels

A H-bond capable chiral tetradentate ligand, Fe3+, and acetate ion assembles into a hydroxo-bridged binuclear
complex with the formula [FeIII2(μ-OH)(μ-OAc)(S-L)2]·4H2O (1) where H2S-L = S-2-(2-hydroxy-benzylamino)-3-(1H-imidazol-4-yl)-propionic acid. The crystal of 1 contains right-handed one-dimensional (1D) helical channels
with 7.3−9.8 Å diameter. A similar reaction with a ligand having opposite chirality forms the complex with left-handed helical channels (1a). Heating the crystals of 1 at 95 °C under reduced pressure selectively removes three
waters from the channel forming an enantiopure porous crystal with empty channels (solvent accessible voids 18%
v/v). Intermolecular hydrogen bonding between the imidazole N−H and phenolate oxygen in 1−2 forms a C6 symmetric
helix with bridging hydroxo groups pointing inside the channels. All the H-bond capable atoms in the ligand along
with one water molecule form an extended H-bonded network throughout the crystal. Exposing the empty channels
of 2 to iodine vapor indicates partial filling of the channels with iodine. Crystal data for 1·4H2O include the
following:  hexagonal, P61, a = b = 13.164(3) Å, c = 36.305 (11) Å, α = β = 90°, γ = 120°, Z = 6, R1 =
0.0387, wR2 = 0.0842. Crystal data for 1a·2H2O include the following:  hexagonal, P65, a = b = 13.151(4) Å, c
= 36.558(2) Å, α = β = 90°, γ = 120°, Z = 6, R1 = 0.0416, wR2 = 0.1190. Crystal data for 2·H2O include
the following:  hexagonal, P61, a = b = 13.160(7) Å, c = 36.559 (4) Å, α = β = 90°, γ = 120°, Z = 6, R1 =
0.0574, wR2 = 0.1423.