Chromium(III) complexes with 2-(2′-pyridyl)imidazole: Synthesis, crystal structure and magnetic properties

Abstract

The preparation, crystal structure and variable temperature-magnetic investigation of three 2-(2′-pyridyl)imidazole-containing chromium(III) complexes of formula PPh4[Cr(pyim)(C2O4)2]·H2O (1), AsPh4[Cr(pyim)(C2O4)2]·H2O (2) and [Cr2(pyim)2(C2O4)2(OH2)2]·2pyim · 6H2O (3) [pyim=2-(2′-pyridyl)imidazole, C2O42-=dianion of oxalic acid, PPh4+=tetraphenylphosphonium cation and AsPh4+=tetraphenylarsonium cation] are reported herein. The isomorphous compounds are made up of discrete [Cr(pyim)(C2O4)2]− anions, XPh4+ cations [X=P (1) and As (2)] and uncoordinated water molecules. The chromium environment in 1 and 2 is distorted octahedral with Cr–N and Cr–O bond distances varying in the ranges 2.040(3)–2.101(3) and 1.941(3)–1.959(3)Å, respectively. The angle subtended by the chromium(III) ion by the two didentate oxalate ligands cover the range 82.49(12)–82.95(12)°, values which are somewhat greater than those concerning the chelating pyim molecule [77.94(13) (1) and 78.50(13)° (2)]. Complex 3 contains discrete centrosymmetric [Cr2(pyim)2(C2O4)2(OH)2] neutral units where the two chromium(III) ions are joined by a di-μ-hydroxo bridge, the oxalate and pyim groups acting as peripheral didentate ligands. Uncoordinated water and pyim molecules are also present in 3 and they contribute to the stabilization of its structure by extensive hydrogen bonding and π–π type interactions. The values of the intramolecular chromium–chromium separation and angle at the hydroxo bridge in 3 are 2.9908(12)Å and 99.60(16)°, respectively. Magnetic susceptibility measurements of 1–3 in the temperature range 1.9–300K show the occurrence of weak inter- (1 and 2) and intramolecular (3) antiferromagnetic couplings. The magnetic properties of 3 have been interpreted in terms of a temperature-dependent exchange integral, small changes of the angle at the hydroxo bridge upon cooling being most likely responsible for this peculiar magnetic behavior.