[Cr(dmbipy)(ox)2]−: a new bis-oxalato building block for metal assembling. Crystal structures and magnetic properties of XPh4[Cr(dmbipy)(ox)2]·5H2O (X = P and As), {Ba(H2O)2[Cr(dmbipy)(ox)2]2}n·17/2nH2O and {Ag(H2O)[Cr(dmbipy)(ox)2]}n·3nH2O

Abstract

The synthesis, X-ray structure and variable-temperature magnetic study of new compounds of formula PPh₄[Cr(dmbipy)(ox)₂]·5H₂O (1), AsPh₄[Cr(dmbipy)(ox)₂]·5H₂O (2), {Ba(H₂O)₂[Cr(dmbipy)(ox)₂]₂}_n·17/2nH₂O (3) and {Ag(H₂O)[Cr(dmbipy)(ox)₂]}_n·3nH₂O (4) (PPh₄⁺ = tetraphenylphosphonium cation; AsPh₄⁺ = tetraphenylarsonium cation; dmbipy = 4,4′-dimethyl-2,2′-bipyridine; ox²⁻ = oxalate dianion) are reported herein. The isomorphous compounds 1 and 2 are made up of discrete [Cr(dmbipy)(ox)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–O and Cr–N bond distances varying in the ranges 1.950(2)–1.9782(12) and 2.047(3)–2.0567(14) Å, respectively. The angles subtended at the chromium atom by the two bidentate oxalate ligands cover the range 82.58(10)–83.11(5)°, and they are somewhat greater than those concerning the chelating dmbipy [79.04(10) (1) and 79.24(5)° (2)]. The [Cr(dmbipy)(ox)₂]⁻ unit of 1 and 2 also occurs in 3 and 4 but it adopts different coordination modes. It acts as a chelating ligand through its two oxalate groups towards the divalent barium cations in 3 affording neutral chains with diamond-shaped units sharing the barium atoms, while the two other corners are occupied by two crystallographically independent chromium atoms. The barium atom in 3 is coordinated by eight oxygen atoms from four oxalate groups and two aqua ligands. The structure of 4 consists of neutral bimetallic layers where the [Cr(dmbipy)(ox)₂]⁻ unit acts as a ligand towards the univalent silver(I) cation through its two oxalate groups, one of them being bidentate and the other bidentate/monodentate (outer). Each silver atom is six-coordinated with a water molecule and five oxygen atoms from three oxalate groups building a highly distorted octahedral environment. Magnetic susceptibility measurements for 1–4 in the temperature range 1.9–300 K show the occurrence of weak ferro- (1 and 2) and antiferromagnetic (3 and 4) interactions which are mediated by π–π stacking between dmbipy ligands through the spin polarization mechanism. A comparative study of the potentiality of the [Cr(AA)(ox)₂]⁻ unit (AA = bidentate nitrogen donor) as a building block for designing heterometallic species is carried out in the light of the available structural information.