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

The synthesis of a novel cyclophane (L1) consisting of a 1H-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 1H-pyrazole azamacrocyclic ligand. The crystal structures of the complexes [Cu2(H­(H–1L1))­(H–1L1)]­(ClO4)3·3.75H2O (1) and ([Cu2(H­(H–1L1))0.5(H–1L1)1.5]2­(ClO4)3Br2·4.2H2O (2) show that Cu2+ coordination leads to formation of 2:2 Cu2+:L dinuclear dimeric complexes in which the 1H-pyrazole units lose a proton behaving as bis­(monodentate) bridging ligands. Unlike previously reported complexes of [2 + 2] pyrazole azamacrocycles, the pyrazolate units in 1 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 [CuL2]­(ClO4)2 (3). Crystals of [Cu3(H–1L1)2(CO3)­(H2O)]­(ClO4)2·8H2O (4) grown by evaporating aqueous solution at pH 9 containing Cu2+ and L1 in 3:2 molar ratio show the presence of a further Cu2+ coordinated to the two free amine groups found in structures 1 and 2. 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.