Syntheses, crystal structures and magnetic properties of di- and trinuclear croconato-bridged copper(ii) complexes

Abstract

The new croconato-bridged copper(II) compounds [Cu₂(terpy)₂(H₂O)₂(C₅O₅)](NO₃)₂·H₂O (1) and [Cu₃(phen)₅(C₅O₅)₂](CF₃SO₃)₂ (2) (C₅O₅²⁻ = croconate, dianion of 4,5-dihydroxycyclopent-4-ene-1,2,3-trione; terpy = 2,2′:6′,2″-terpyridine; phen = 1,10-phenanthroline) have been prepared, and their crystal structures and variable temperature magnetic susceptibilities determined. The structure of complex 1 consists of croconato-bridged dinuclear [Cu₂(terpy)₂(H₂O)₂(C₅O₅)]²⁺ complex ions, nitrate counter ions and water of hydration. The croconato ligand exhibits an asymmetrical bis-bidentate coordination mode through four of its five oxygen atoms. The two crystallographically independent copper atoms have similar coordination geometries: elongated, distorted octahedral with three terpyridyl nitrogen atoms and one croconate oxygen atom in the equatorial plane, and a second croconate oxygen atom and a water molecule in the axial positions, the stronger axial interaction in each case being to the water molecule. The intramolecular copper–copper separation across the croconate bridge is 6.852(1) Å. The structure of complex 2 is built of croconato-bridged trinuclear [Cu₃(phen)₅(C₅O₅)₂]²⁺ complex ions and triflate counter ions. The terminal copper atoms [Cu(1) and Cu(1a)] have, to the first approximation, a square pyramidal coordination geometry with one croconate oxygen and three phen nitrogen atoms in equatorial positions, and a fourth phen nitrogen atom in the apical position. A second semi-coordinated croconate oxygen atom is located in the remaining axial position. The central copper atom [Cu(2)] is elongated octahedral with two croconate oxygen and two phen nitrogen atoms in equatorial positions, and two oxygen atoms, one from each croconate group, in axial positions. The croconate ligands in 2 adopt an unusual asymmetrical bis-bidentate bridging mode involving three adjacent oxygen atoms, the copper–copper separation across this bridge being 4.177(1) Å. Variable temperature magnetic susceptibility measurements show the occurrence of weak [J = −2.94 cm⁻¹ (1)] and intermediate [J = −47.1 cm⁻¹ (2)] intramolecular antiferromagnetic interactions across the bridging croconato. These values are analyzed in the light of their structures and compared with those previously reported for the parent compounds.