Regioselective Mannich bases of pyrrole-2-carbaldehyde and binuclear copper(II) complexes of bis(iminopyrrolyl) ligand containing the piperazine ring

•Regioselective Mannich base dialdehydes of pyrrole-2-carbaldehyde.•New bis(iminopyrrolyl) ligand containing the piperazine ring spacer.•Three different binuclear copper(II) complexes and their X-ray structures.•The piperazine nitrogens as hydrogen bond acceptors.

Iminopyrrolyl ligands have attracted an attention in the field of coordination chemistry. As every iminopyrrolyl ligand contains at least one acidic pyrrole NH group, it often forms an anionic five-membered chelate ring with a metal. Unlike the familiar pincer framework ligand systems, iminopyrrolyl coordination chemistry is in a developing stage. Hence, we set the synthesis of bis(iminopyrrole) framework ligand containing the piperazine ring as a spacer unit between two mono(iminopyrrolyl) moieties. The Mannich reaction of pyrrole-2-carbaldehyde with piperazine and formaldehyde afforded two new dialdehydes: N,N′-bis(5-formylpyrrol-1-ylmethyl)piperazine 1 and N,N′-bis(5-formylpyrrol-2-ylmethyl)piperazine, 2, which are regioselective products formed in good yields. The pyrrole N aminomethylated dialdehyde 1 was isolated in the absence of an added acid. Conversely, the pyrrole α-C aminomethylated dialdehyde compound 2 was obtained in the presence of a mineral acid. They have different spectroscopic and physical properties. Even they offered different Schiff bases under the same reaction conditions. The Schiff base condensation reaction of 2 with 2,6-diisopropylanilline in the presence of HNO3 gave the expected [1+2] Schiff base in 53% yield after column separation. Conversely, the other dialdehyde 1 yielded the piperazinylmethyl C–N bonds hydrolyzed known 2-iminopyrrole compound. Further, the treatment of the structurally characterized [1+2] bis(iminopyrrolyl) ligand with copper(II) carboxylates [Cu(OOCR)2(H2O)] (R = H, Me and Ph) gave three different binuclear five-coordinate copper(II) complexes. Their structures were determined by single crystal X-ray diffraction studies. Interestingly, the piperazine ring nitrogens are not coordinated, rather involved in intramolecular hydrogen bonding interactions with the coordinated solvent molecules at the copper atom.

Graphical abstractSynthesis and structural characterization of binuclear five-coordinate Cu(II) complexes containing piperazine ring bridged bis(iminopyrrolyl) ligand, which are obtained from the regioselectively formed new dialdehydes.Figure optionsDownload full-size imageDownload as PowerPoint slide