A catalyst-free approach to a novel imidazo [4,5-f][1,10] phenanthroline ligand and its corresponding ruthenium(ii) complex: insights into their applications in colorimetric anion sensing

Abstract

We have designed and synthesized a novel imidazo [4,5-f][1,10] phenanthroline ligand (L) containing an azo-coupled salicylaldehyde scaffold for the first time under catalyst-free conditions using air as an oxidant. L was further utilized to fabricate a mixed-ligand monometallic Ru(II) complex of composition [(bpy)₂Ru(L)](ClO₄)₂ {bpy = 2,2′-bipyridine and L = 2-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)-4-((4-nitrophenyl)diazenyl)phenol}. The anion sensing properties of the synthesized complex as well as the parent ligand have been thoroughly explored using absorption, emission and ¹H NMR spectral measurements. On the basis of these findings, it has been postulated that both synthesized probes act as efficient sensors for colorimetric recognition of F⁻ among halides and AcO⁻ and H₂PO₄⁻ among oxoanions in aqueous media. Significantly, the colorimetric response was considered to be the direct consequence of monoproton transfer from probes to the aforementioned anions. It is evident that the introduction of electron withdrawing groups into the backbone of the prepared complex renders the hydrogen-bond donors appreciably acidic. Thus, the sensor is capable of competing with water molecules to detect the trace amount of anions in an aqueous environment without any spectroscopic instrumentation. Successfully, the current sensory system could as well serve as a practical tool for immediate and qualitative detection of inorganic anions in real samples.