FULL PAPER DOI: 10.1002/ejic.201200831 Magneto-Structural Study of an Oxamato-Bridged Pd II Co II Chain: X-ray Crystallographic Evidence of a Single-Crystal-to-Single-Crystal Phase Transition Willian X. C. Oliveira, [a] Marcos A. Ribeiro, [a] Carlos B. Pinheiro, [b] Wallace C. Nunes, [c] Miguel Julve, [d] Yves Journaux, [e,f] Humberto O. Stumpf, [a] and Cynthia L. M. Pereira* [a] Keywords: Palladium / Cobalt / Magnetic properties / Chain structures / X-ray diffraction Two new mononuclear oxamato-containing palladium(II) complexes of formula K 2 [Pd(opba)]·2H 2 O(1) and (PPh 4 ) 2 - [Pd(opba)]·2H 2 O(2) and the heterodimetallic palladium(II)– cobalt(II) chain {[Co(H 2 O) 2 Pd(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 PPh 4 + 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 Introduction Molecular crystals can experience structural phase tran- sitions as a result of external stimuli, such as temperature, pressure or radiation, while maintaining their structural sta- bility and integrity during the transition process. [1] Their physical properties (e.g., optical, magnetic or conducting) can then be modulated in a controlled manner, which sug- gests challenges and opportunities for the development of a crystal engineering approach, referred to as making-crys- [a] Departamento de Química, Instituto de Ciências Exatas, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, MG, Brazil Fax: +55-31-3409-5700 E-mail: cynthialopes@ufmg.br Homepage: http://lattes.cnpq.br/4334534811258685 [b] Departamento de Física, Instituto de Ciências Exatas, Universidade Federal de Minas Gerais, 1270-901 Belo Horizonte, MG, Brazil [c] Instituto de Física, Universidade Federal Fluminense, Niterói, RJ 24210-346, Brazil [d] Departament de Química Inorgânica, Instituto de Ciencia Molecular (ICMol), Universitat de Valencia, C/ Catedrático José Beltrán 2, 46980 Paterna, Valencia, Spain [e] Institut Parisien de Chimie Moléculaire, UPMC Univ. Paris 06, Paris 75005, France [f] CNRS, Institut Parisien de Chimie Moléculaire, UMR 7201, Paris, France Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejic.201200831. Eur. J. Inorg. Chem. 2012, 5685–5693 © 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 5685 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 Ham- iltonian being defined as H =–αλLS + Δ(L z 2– 2/3) + βH(–αL + g e S)] with an almost negligible intra- [through the diamag- netic square-planar Pd II centre] and interchain antiferromag- netic interactions (θ = –0.32 K). tals-from-crystals and making-crystals-by-design strate- gies. [2,3] Along this line, we reported some years ago the synthesis, crystal structure and magnetic properties of the one-dimen- sional (1D) compound of formula [CoCu(opba)(dmso) 3 ] n [opba = 1,2-phenylenebis(oxamate), dmso = dimethyl sulf- oxide]. [4] This compound exhibits two crystalline phases characterized by single-crystal X-ray diffraction at two dif- ferent temperatures, 298 and 100 K. According to birefrin- gence measurements, the structural phase transition takes place at 150K, and it involves an optical bistability, or op- tical hysteresis, which suggests a reversible process. [4] Essen- tially, the phase transition involves the change from a non- organized to an organized state of the dmso molecules when the system is cooled. A study of the magnetic proper- ties of [CoCu(opba)(dmso) 3 ] n showed that it behaves as an isolated 1D ferrimagnetic system with a relatively strong in- trachain antiferromagnetic coupling (a rough estimate for the magnetic interaction is ca. –30 cm –1 ). To better understand the supramolecular and crystal- packing features that can influence the magnetic properties, we reported the related chain compound of formula {[CoCu(opba)]·4H 2 O} n . [5] Unfortunately, the crystal struc- ture of this compound is still unknown because of the diffi- culties of obtaining single crystals suitable for X-ray diffrac-