Crystallographic and ESR studies of π–π stacked binuclear copper(II) complexes of 1,3-bis(quinoline-2-carboxamido) and 1,3-bis(isoquinoline-3-carboxamido)propane

Abstract

Copper(II) binuclear chromophores [{Cu(H 2qpda)} 2] 4+ ( A) and [{Cu(H 2 iqpda)} 2] 4+ ( B) were isolated from an acidic aqueous solution of an equimolar H 2L–CuSO 4 mixture, while H 2iso-1-qpda gave no copper(II) binuclear complex [H 2qpda=1,3-bis(quinoline-2-carboxamido)propane, H 2 iqpda=1,3-bis(isoquinoline-3-carboxamido)propane, H 2iso-1-qpda=1,3-bis(isoquinoline-1-carboxamido)propane]. The crystal structures of A, B and the free ligand H 2qpda were determined. The coordination structures in both A and B are trans-N 2O 2. The copper ions of both complexes are coaxially aligned and hence the heteroaromatic rings are highly stacked. ESR and optical absorption spectroscopies were employed to study their coordination and electronic structures. In the solution, A coexists with aquacopper(II) complex at any [H 2qpda]/[Cu 2+] ratio, and with only a little [Cu(H 2qpda) 2] 2+ mononuclear complex even at [H 2qpda]/[Cu 2+]≥3.0. B coexists with [Cu(H 2 iqpda) 2] 2+ but without the aquacopper(II) complex at [H 2 iqpda]/[Cu 2+]>1.0. A and [Cu(H 2O) 5] 2+ crystallize in an identical crystal isolated from the equimolar H 2qpda–CuSO 4 aqueous solution. The trans-N 2O 2 coordination structure also appears in [Cu(H 2L) 2] 2+ chromophores. The instability of [Cu(H 2qpda) 2] 2+ is ascribed to the trans-N 2O 2 structure, having serious steric repulsion between the carbonyl oxygen and aromatic hydrogen atoms. Stable [Cu(H 2 iqpda) 2] 2+ is free from the steric repulsion. The existence of A possessing two such unstable trans-N 2O 2 coordination units illustrates the important role of the π–π and hydrophobic interactions in forming the binuclear complex. The formation of B is always greater than that of the homologue [{Cu(H 2ppda)} 2] 4+ [ C; H 2ppda=1,3-bis(pyridine -2-carboxamido)propane]. Both Cu⋯Cu distance and interplanar separation between the stacking aromatic rings in B are significantly shorter than those in C. These results endorse the more effective π–π and hydrophobic interactions in B than C.