Triple ring contraction of a [2+2] macrocyclic ligand

Abstract

Schiff base condensation of 2,6-diformylpyridine and 1,3-diaminopropan-2-ol in the presence of a BaII template ion yields a complex containing a [2+2] macrocycle, [Ba₂(μ1,2-ClO₄)₂(H₂L1)₂], where H₂L1 is 3,7,15,19,25,26-hexaazatricyclo[19.3.1.1(9,13)]hexacosa-1(25),2,7,9(26),10,12,14,19,21,23-decaene-5,17-diol. On transmetallation with CuII cations, the macrocycle undergoes three successive ring contractions, yielding crystals of (acetato-κO)[26,28-dioxa-3,7,15,19,25,27-hexaazahexacyclo[19.3.1.1(2,5).1(9,13).1(17,10).0(3,8)]octacosa-1(25),9(27),10,12,14,21,23-heptaene-κ⁵N]copper(II) perchlorate, [Cu(CH₃COO)(C₂₀H₂₂N₆O₂)]ClO₄ or [Cu(CH₃COO)(L2)]ClO₄, in which the macrocycle ring size has been reduced from 20 members in H₂L1 to 16 in L2.

Keywords: Schiff base condensation; crystal structure; macrocycles; ring-contraction reactions.