Controllable assembly of rectangular macrocycles bearing different numbers of unsaturated sites based on half-sandwich iridium fragments

Abstract

A series of hexanuclear rectangular macrocycles were designed and synthesized by utilizing multifunctional pyrazine-derived (pyrazine-2,3-diamine (H₄L₁)) and quinoxaline-derived ligands (2,3-dihydroxy-quinoxaline (H₂L₂)) featuring two monodentate sites and one pair of chelating sites. X-ray crystallography in combination with ¹H NMR spectroscopy elucidated that both half-sandwich iridium diimine and dihydroxy moieties are located on either side of the rectangular macrocycles, making them centrosymmetric. Thereby, the prepared diimine-functionalised complexes were found to have unsaturated metal sites on account of their strongly bound Ir–N–C–C–N arrangement. However, all the iridium atoms in the rectangular macrocycles containing dihydroxy groups were found to adopt an 18-electron coordination configuration, indicating that the O,O′-bonded iridium centers had bound additional ligands, such as Cl⁻, MeOH, MeCN, etc. Notably, a rare rectangular macrocycle containing a single coordinatively unsaturated metal site was achieved when the ligands H₂L₁²⁻ and L₂²⁻ were introduced simultaneously.