The first water-soluble polynuclear metallamacrocyclic Sr(ii)–Cu(ii) complex based on simple glycinehydroximate ligands

Abstract

Recently, aminohydroximate ligands have found wide applications in the fascinating class of polynuclear metallamacrocyclic compounds named 15-MC-5 metallacrowns. The enhanced interest in water-soluble aminohydroximate Ln(III)–Cu(II) complexes is largely due to their rich coordination chemistry, diverse properties and ease of synthesis. We examined glycinehydroxamic acid as a simple ligand for the preparation of the first water-soluble polynuclear metallamacrocyclic Sr(II)–Cu(II) compound. The complex Sr(H₂O)₃[15-MC_CuGlyha-5](Cl)₂ was synthesized and characterized structurally and spectroscopically. The single-crystal structure reveals the classic metallamacrocyclic 15-MC-5 configuration. The Sr(II) ion is located at the center of the 15-MC_Cu(II)Glyha-5 ring and coordinated by five oxygen atoms of the cycle in the equatorial plane and an additional three oxygen atoms of the water molecules at apical positions. Detailed DFT and QT AIM studies were carried out for the hydrated isoelectronic Sr[15-MC_CuGlyha-5]²⁺ and Y[15-MC_CuGlyha-5]³⁺ systems. The ionic contribution to the metal–ligand interactions appears to be higher for the Sr(II) derivative despite the smaller charge separation. DFT calculations suggest a thermodynamically favorable substitution of the Sr(II) central ion with Y(III). Indeed, it was shown experimentally that the strontium ion can be easily replaced by yttrium. Preliminary cytotoxic studies revealed the low toxicity of the strontium complex and its yttrium analogue, so the water-soluble Sr(II) and Y(III) metallacrowns can be further investigated as possible platforms for the development of new ⁹⁰Sr and ⁹⁰Y radiotherapy drugs.