New coordination modes of furosemide anion in copper(II) complexes: synthesis and structural insights

The coordination chemistry of copper(II) was explored using furosemide, a sulphonamide-based diuretic, as a ligand. A series of new copper(II) compounds with furosemide was synthesised and structurally characterised by single crystal X-ray diffraction. Within this series, two novel coordination modes of the deprotonated furosemide (abbreviated as fur − ) were identified. The first one is a bidentate bridging mode ( μ 2 - η 1 : η 1 ), found in dinuclear paddle-wheel (PW) complexes [Cu 2 (fur) 4 (CH 3 CN) 2 ]∙2(CH 3 CH 2 ) 2 O ( 3 ) and [Cu 2 (fur) 4 (CH 3 CH 2 OH) 2 ]∙2CH 3 CH 2 OH∙2H 2 O ( 4 ), where carboxylate oxygens of the furosemide anion link two copper(II) centres. The second one is a unique bridging mode ( μ 3 - η 1 : η 1 : η 1 ), in which the furosemide anion coordinates not only through the carboxylate oxygens but also via the sulphonamide oxygen atom, thereby linking three copper(II) ions to form a one-dimensional coordination polymer [Cu 2 (fur) 4 ] n ∙4 n (CH 3 CH 2 OH)∙2 n (H 2 O) ( 5 ). Introduction of quinaldinate (quin − , anion of quinoline-2-carboxylic acid) into the reaction mixture yielded the cocrystal [Cu(quin) 2 ] n ∙2 n (furH) ( 6 ), consisting of a quinaldinate-based one-dimensional coordination polymer and neutral furosemide molecules attached to the coordination polymer through hydrogen bonding. These findings provide the first structural evidence of furosemide exhibiting both bidentate and tridentate bridging behaviour, highlighting its remarkable flexibility as a ligand. Overall, this work reveals the structural diversity of resulting copper(II) complexes and broadens the understanding of ligand-metal interactions, offering new perspectives for the design of functional coordination compounds based on bioactive ligands. • A series of new copper(II) compounds with furosemide was synthesised. • Two previously unreported coordination modes of the furosemide anion were identified. • Furosemide anion can function as a bridging ligand, linking three copper(II) ions.