Manganese(II) complexes with croconate and 2-(2-pyridyl)imidazole ligands: Syntheses, X-ray structures and magnetic properties

The mixed-ligand complexes of manganese(II) of formula [Mn(pyim)2(C5O5)] (1) and [Mn(pyim)(H2O)(C5O5)]n · 2.5nH2O (2) [pyim = 2-(2-pyridyl)imidazole and C5O52- = croconate (dianion of 4,5-dihydroxy-4-cyclopentene-1,2,3-trione)] have been prepared and their structures determined by X-ray crystallographic methods. Compound 1 is a tris-chelated mononuclear complex where the manganese atom is six-coordinate: four nitrogen atoms from two pyim molecules and two oxygen atoms from a croconate group build a somewhat distorted octahedral surrounding around the metal atom. The resulting neutral mononuclear units are linked to each other through double bridges which are constituted by the imidazole N–H and the metal-coordinated croconate-oxygen atom, the metal–metal separation through this supramolecular pathway being 7.6856(11) Å. Compound 2 is a croconato-bridged manganese(II) uniform chain with an intrachain metal–metal distance of 7.5118(9) Å. A bidentate pyim group, a water molecule and four oxygen atoms from two bis-bidentate croconate ligands build an irregular seven-coordination polyhedron around each manganese atom in 2. The investigation of the magnetic properties of 2 in the temperature range 1.9–295 K has shown the occurrence of a weak antiferromagnetic interaction [J = −0.066 cm−1 with the Hamiltonian defined as H = −J ∑i Si · Si+1] through the bis-bidentate croconate. The ability of the bis-chelating croconate to mediante magnetic interactions between paramagnetic first-row transition metal ions is discussed and compared to that of the related oxalate ligand.

The reaction of Mn(II), 2-(2-pyridyl)imidazole (pyim) and croconate in aqueous solution affords the tris-chelated mononuclear complex [Mn(pyim)2(C5O5)] (1) and the chain compound [Mn(pyim)(H2O)(C5O5)]n · 2.5nH2O (2). Variable-temperature magnetic measurements on 2 show the occurrence of a weak antiferromagnetic interaction between the manganese(II) ions through bis-bidentate croconato.Figure optionsDownload as PowerPoint slide