Ligand Influence over the Formation of Dinuclear [2+2] versus Trinuclear [3+3] Cu^I Schiff Base Macrocyclic Complexes

The preparation and characterization of three new macrocyclic ligands with pendant arms based on the [2+2] condensation of isophthalaldehyde and the corresponding triamine substituted at the central N-atom is reported. None of these new macrocyclic ligands undergo any equilibrium reaction, based on imine hydrolysis to generate [1+1] macrocyclic formation or higher oligomeric compounds, such as [3+3], [4+4], etc., at least within the time scale of days. This indicates the stability of the newly generated imine bond. In sharp contrast, the reaction of the [2+2] macrocyclic Schiff bases with Cu^I generates the corresponding dinuclear Cu^I complexes [Cu₂(L¹)]²⁺, 1²⁺; [Cu₂(L²)(CH₃CN)₂]²⁺, 2²⁺; and [Cu₂(L³)(CH₃CN)₂]²⁺, 3²⁺, together with their trinuclear Cu^I homologues [Cu₃(L⁴)]³⁺, 4³⁺; [Cu₃(L⁵)(CH₃CN)₃]³⁺, 5³⁺; and [Cu₃(L⁶)(CH₃CN)₃]³⁺, 6³⁺, where the [2+2] ligand has undergone an expansion to the corresponding [3+3] Schiff base that is denoted as L⁴, L⁵, or L⁶. The conditions under which the dinuclear and trinuclear complexes are formed were analyzed in terms of solvent dependence and synthetic pathways. The new complexes are characterized in solution by NMR, UV–vis, and MS spectroscopy and in the solid state by X-ray diffraction analysis and IR spectroscopy. For the particular case of the L² ligand, MS spectroscopy is also used to monitor the metal assisted transformation where the dinuclear complex 2²⁺ is transformed into the trinuclear complex 5³⁺. The Cu^I complexes described here, in general, react slowly (within the time scale of days) with molecular oxygen, except for the ones containing the phenolic ligands 2²⁺ and 5³⁺ that react a bit faster.