[Cr(d pa)(ox)2]−: a new bis-oxalato building block for the design of heteropolymetallic systems. Crystal structures and magnetic properties of PPh4[Cr(dpa)(ox)2], AsPh4[Cr(dpa)(ox)2], Hdpa[Cr(dpa)(ox)2]·4H2O, Rad[Cr(dpa)(ox)2]·H2O and Sr[Cr(dpa)(ox)2]2 ·8H2O (dpa = 2,2′-dipyridylamine)

Abstract

The new complexes of formulae PPh₄[Cr(dpa)(ox)₂] (1), AsPh₄[Cr(dpa)(ox)₂] (2), Hdpa[Cr(dpa)(ox)₂]·4H₂O (3), Rad[Cr(dpa)(ox)₂]·H₂O (4) and Sr[Cr(dpa)(ox)₂]₂·8H₂O (5) [PPh₄  = tetraphenylphosphonium cation; AsPh₄  = tetraphenylarsonium cation; dpa = 2,2′-dipyridylamine; ox = oxalate dianion; Rad = 2-(4-N-methylpyridinium)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl-3-N-oxide] have been prepared and characterised by single-crystal X-ray diffraction. The structures of 1–4 consist of discrete [Cr(dpa)(ox)₂]⁻ anions, tetraphenylphosphonium (1), tetraphenylarsonium (2), monoprotonated Hdpa (3) and univalent radical (4) cations and uncoordinated water molecules (2–4). The chromium environment in 1–4 is distorted octahedral with Cr–O bond distances between 1.982(2)–1.946(2) Å and Cr–N bonds of 2.0716(17)–2.048(3) Å. The angles subtended at the chromium atom by the two oxalates are 83.6(2)–81.71(8)°, whereas the N–Cr–N angles are 87.76(7)–86.24(9)°. The [Cr(dpa)(ox)₂]⁻ unit of 1–4 is also present in 5 but it acts as a chelating ligand through its two oxalato groups towards divalent strontium cations, yielding heterobimetallic zig-zag chains that run parallel to the a axis. Each chain is formed of diamond-shaped units sharing the strontium atoms, while the two other corners are occupied by two crystallographically independent chromium atoms. The [Cr(dpa)(ox)₂]⁻ unit in 5 retains the environment observed in 1–4 and the strontium atom is coordinated to eight oxalate oxygens from four oxalate ligands. The two crystallographycally independent chromium centres within each double chain have opposite chirality. However, the adjacent double chains are related by an inversion centre resulting in achiral layers parallel to the ac plane. The magnetic properties of 1–5 have been investigated in the temperature range 1.9–290 K. A quasi Curie law behaviour is observed for 1–3 and 5 in agreement with their crystal structures, whereas a significant antiferromagnetic interaction between the chromium(III) and the radical centre occurs in the case of 4. The synthetic possibilities offered by the use of the heteroleptic species [CrL(ox)₂]⁻ (L =  α-diimine-type ligand) as a ligand towards metal ions is analysed and discussed in the light of the available structural results.