Magneto-structural study of an oxamato-bridged PdIICo II chain: X-ray crystallographic evidence of a single-crystal-to- single-crystal phase transition

Article abstract of DOI:10.1002/ejic.201200831

Two new mononuclear oxamato-containing palladium(II) complexes of formula K₂[Pd(opba)]·2H₂O (1) and (PPh₄) ₂[Pd(opba)]·2H₂O (2) and the heterodimetallic palladium(II)-cobalt(II) chain {[Co(H₂O)₂Pd(opba)] ·dmso}_n (3) [opba = 1,2-phenylenebis(oxamate), PPh ₄⁺ = tetraphenylphosphonium cation and dmso = dimethyl sulfoxide] have been prepared, and the structures of two of them (compounds 2 and 3) were determined by X-ray diffraction analysis of single crystals. The structure of 2 consists of discrete anions of [Pd(opba)₂] ^2- and PPh₄⁺ cations. Each Pd^II ion in 2 is surrounded by two oxamate nitrogen atoms and two carboxylate oxygen atoms in a square-planar surrounding. Compound 3 is a neutral chain with regular alternating Pd^II and Co^II ions, the [Pd(opba)] ^2- entity acting as a bis(bidentate) ligand towards trans-diaquacobalt(II) fragments, and dmso molecules of crystallization. Compound 3 exhibits a single-crystal-to-single-crystal phase transition between monoclinic C2/c (α phase) and triclinic P1 (β phase) space groups. Within the α phase the dmso molecule of crystallization is disordered, but it becomes ordered below 215 K in the β phase. The ordering process of the dmso molecule is followed by a reduction in the unit cell volume. The magnetic properties of 3 are barely affected by the structural phase transition, and they indicate an important spin-orbit coupling of the high-spin octahedral Co^II ion [α = 1.245, λ =-151 cm ^-1, Δ = 494 cm^-1, the spin Hamiltonian being defined as H =-αλLS + Δ(L_z^2-2/3) + βH(-αL + g_eS)] with an almost negligible intra- [through the diamagnetic square-planar Pd^II centre] and interchain antiferromagnetic interactions (θ =-0.32 K).