Chiral 27-membered [3 + 3] Schiff-base macrocycles and their reactivity with first-row transition metal ions

27-Membered Schiff-base macrocycles 1 and 2 are prepared via the [3 + 3] template condensation of 2,6-diformyl-4-substituted phenol (R = Cl in 1 and R = CH3 in 2) with enantiomeric [(1R,2R) or (1S,2S)] and racemic trans-cyclohexane-1,2-diamines. Among them two chiral [3 + 3] macrocyclic single-crystals (S,S)-1 and (R,R)-2 in their backbones have been structurally characterized. Both of them have triangular holes (∼42.36 Å2) in which one phenyl ring of the neighboring molecule is inserted as a guest by π–π stacking interactions with the centroid–centroid separation of 3.599 Å. The treatment of aforementioned macrocycles with certain first-row transition metal salts [Cu(II), Zn(II), Fe(III)] with different counterions leads to the cleavage of Schiff-base CN double bonds and degradation of the macrocycles due to the size matching effect of macrocycles as well as the restriction of coordination geometry of the central metals and the counterions used. As a result, [2 + 2], [2 + 1] and [1 + 2] types of complexes ((R,R)-3, 4, 5 and 6) are yielded. In the case of 6, a new type of dimethyl acetal complex is produced in methanol where FeCl3 serves as a reactive Lewis acid catalyst. The intrinsic chiral differences of the macrocycles and their transition metal complexes are demonstrated by circular dichroism (CD) spectra.

Graphical abstractThe reaction of [3 + 3] chiral or achiral Schiff-base macrocycles with certain first-row transition metal salts [Cu(II), Zn(II), Fe(III)] under reflux condition led to the cleavage of Schiff-base CN double bonds and the rearrangement of the macrocycles. We acquired [2 + 2], [2 + 1], [1 + 2] kinds of ring-contraction metal complexes in the process of metal ion complexation.Figure optionsDownload full-size imageDownload as PowerPoint slide