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

Schiff base condensation of 2,6‐diformylpyridine and 1,3‐diaminopropan‐2‐ol in the presence of a Ba^II template ion yields a complex containing a 2+2] macrocycle, Ba₂(μ_1,2‐ClO₄)₂(H₂L1)₂], where H₂L1 is 3,7,15,19,25,26‐hexaazatricyclo19.3.1.1^9,13]hexacosa‐1(25),2,7,9(26),10,12,14,19,21,23‐decaene‐5,17‐diol. On transmetallation with Cu^II cations, the macrocycle undergoes three successive ring contractions, yielding crystals of (acetato‐κO)26,28‐dioxa‐3,7,15,19,25,27‐hexaazahexacyclo19.3.1.1^2,5.1^9,13.1^17,10.0^3,8]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.