Intermolecular Binding Modes in a Novel [1 + 1] Condensation 1<i>H</i>‑Pyrazole Azamacrocycle: A Solution and Solid State Study with Evidence for CO₂ Fixation

The synthesis of a novel cyclophane (<b>L1</b>) consisting of a 1<i>H</i>-pyrazole moiety linked through methylene groups to a 1,5,9,13-tetraazadecane chain is described. As far as we know, this is one of the first reported syntheses of a [1 + 1] condensation 1<i>H</i>-pyrazole azamacrocyclic ligand. The crystal structures of the complexes [Cu₂(H­(H_–1<b>L1</b>))­(H_–1<b>L1</b>)]­(ClO₄)₃·3.75H₂O (<b>1</b>) and ([Cu₂(H­(H_–1<b>L1</b>))_0.5(H_–1<b>L1</b>)_1.5]₂­(ClO₄)₃Br₂·4.2H₂O (<b>2</b>) show that Cu²⁺ coordination leads to formation of 2:2 Cu²⁺:L dinuclear dimeric complexes in which the 1<i>H</i>-pyrazole units lose a proton behaving as bis­(monodentate) bridging ligands. Unlike previously reported complexes of [2 + 2] pyrazole azamacrocycles, the pyrazolate units in <b>1</b> are pointing outward from the macrocyclic cavity to bind the Cu²⁺ ions. Inner coordination with formation of 1:1 Cu²⁺:L complexes is however observed in [1 + 1] pyridine azamacrocycles as shown by the crystal structure here presented of the complex [Cu<b>L2</b>]­(ClO₄)₂ (<b>3</b>). Crystals of [Cu₃(H_–1<b>L1</b>)₂(CO₃)­(H₂O)]­(ClO₄)₂·8H₂O (<b>4</b>) grown by evaporating aqueous solution at pH 9 containing Cu²⁺ and <b>L1</b> in 3:2 molar ratio show the presence of a further Cu²⁺ coordinated to the two free amine groups found in structures <b>1</b> and <b>2</b>. The metal ion fills its coordination sphere capturing atmospheric CO₂ as a η¹,η²-bidentate carbonate anion placed in the equatorial position and an axial water molecule. pH-metric data, UV–vis spectroscopic data, EPR measurements, and HR-ESI-MS data support that the outer coordination mode with formation of 2:2 dinuclear dimeric and 3:2 trinuclear complexes is preserved in aqueous solution