Cu(ii) complexation with an acridine-containing macrocycle. Assembly of water cluster chains within the cavity of tetranuclear metallomacrocycles

Abstract

The synthesis and characterisation of a new macrocyclic compound (L), composed of a pentamine chain linking the 2,7 positions of an acridine moiety, is reported. Cu(II) complexation was studied by means of potentiometric, UV-vis and EPR measurements in aqueous solutions. This study reveals that the ligand forms a stable tetranuclear complex with an overall [Cu₄L₂(OH)₄]⁴⁺ stoichiometry in aqueous solution. The crystal structure of the [Cu₈L₄(μ-OH)₈(μ-NO₃)₃](NO₃)₅·32H₂O complex, isolated from neutral aqueous solution, shows that it is formed by tetranuclear clusters [Cu₄L₂(μ-OH)₄(μ-NO₃)_x]^(4−x)+ (x= 1 or 2), composed of two [Cu₂L]⁴⁺ macrocyclic units linked together by four bridging hydroxide and nitrate anions to give an overall metallomacrocyclic structure. Magnetic measurements shows that the hydroxide-bridged Cu(II) ions are ferromagnatically coupled. Inspection of the crystal packing shows that couples of metallomacrocycles are paired by π-stacking interactions between acridine moieties, giving rise to an internal hydrophilic cavity were hexameric or pentameric water clusters are enclosed. The coupled metallomacrocycles assume a columnar disposition growing along the a axis, giving rise to channels passing through the cavities of the metallomacrocycles; these channels are filled by chains of 5- or 6-membered water clusters linked together by [NO₃·H₂O]₂ units.