PAPER www.rsc.org/dalton | Dalton Transactions Ferromagnetic coupling and magnetic anisotropy in oxalato-bridged trinuclear chromium(III)-cobalt(II) complexes with aromatic diimine ligands† Julia Vallejo, a Isabel Castro,* a Laura Ca˜ nadillas-Delgado, b Catalina Ruiz-P´ erez, b Jes´ us Ferrando-Soria, a Rafael Ruiz-Garc´ ıa, a,c Joan Cano, a,c,d Francesc Lloret a and Miguel Julve a Received 30th July 2009, Accepted 24th October 2009 First published as an Advance Article on the web 23rd December 2009 DOI: 10.1039/b915546e Two novel heterotrinuclear chromium(III)-cobalt(II) complexes of formula {[Cr III (bpy)(ox) 2 ] 2 Co II (Me 2 bpy)}·2H 2 O(1) and {[Cr III (phen)(ox) 2 ] 2 Co II (Me 2 bpy)}·1.5H 2 O(2) [ox = oxalato, bpy = 2,2¢-bipyridine, Me 2 bpy = 6,6¢-dimethyl-2,2¢-bipyridine, and phen = 1,10-phenanthroline] have been synthesized using the “complex-as-ligand/complex-as-metal” strategy. The X-ray crystal structure of 2 consists of neutral oxalato-bridged Cr III 2 Co II bent entities formed by the coordination of two anionic [Cr III (phen)(ox) 2 ] - complexes through one of their oxalato groups toward a cationic cis-[Co II (Me 2 bpy)] 2+ complex. The three tris(chelated), six-coordinated metal atoms possess alternating propeller chiralities leading thus to a racemic mixture of heterochiral (K,D,K)- and (D,K,D)-Cr III Co II Cr III triads, whereby the two peripheral chromium(III) ions adopt a trigonal distorted trapezoidal bipyramidal geometry and the central high-spin cobalt(II) ion exhibits a compressed rectangular bipyramidal one. The intermolecular p–p stacking interactions between the enantiomeric pairs of heterochiral Cr III 2 Co II entities through the aromatic diimine terminal ligands lead to a unique two-dimensional supramolecular network. Variable temperature (2.0–300 K) magnetic susceptibility and variable-field (0–5.0 T) magnetization measurements for 1 and 2 reveal the presence of weak but non-negligible intermolecular antiferromagnetic interactions [zj = -0.012 (2a) and -0.08 cm -1 (2b)] between the Cr III 2 Co II molecules possessing a moderately anisotropic S = 9/2 ground state. This results from the moderately weak intramolecular ferromagnetic coupling [J = +2.43 (1) and +2.34 cm -1 (2)] between the two peripheral Cr III (S Cr = 3/2) and the central high-spin Co II (S Co = 3/2) ions across the oxalato bridge as well as the appreciable single-ion axial magnetic anisotropy of the central high-spin Co II (S Co = 3/2) ion [D Co = -2.29 (1) and -2.15 cm -1 (2)]. A simple molecular orbital analysis of the exchange interaction in 1 and 2 identifies the s- and p-type pathways involving the d x 2 –y 2 (Cr)/d xy (Co) and d xz (Cr)/d yz (Co) pairs of orthogonal magnetic orbitals, respectively, as the two main individual contributions responsible for the overall ferromagnetic coupling observed. Introduction Polynuclear complexes of paramagnetic first-row transition metal ions have been subject of a renewed interest in the last years as potential candidates of single molecule magnets (SMMs), i.e., molecules that exhibit slow relaxation of the magnetization below a blocking temperature (T B ). 1 The design and the synthesis of a SMM require a ground state having both a high spin (S) and a large negative axial magnetic anisotropy (D) which result in an energy barrier for the magnetization reversal between the two lowest m S = ±S states [U = -DS 2 and -D(S 2 - 1/4) for integer and half- integer spins, respectively]. 2 These properties ultimately depend on the nuclearity and the topology of the polymetallic species, as a Departament de Qu´ ımica Inorg` anica/Instituto de Ciencia Molecular (ICMol), Universitat de Val` encia, Pol´ ıgono La Coma s/n, E-46980, Paterna, Val` encia, Spain. E-mail: isabel.castro@uv.es b Laboratorio de Rayos X y Materiales Moleculares, Departamento de F´ ısica Fundamental II, Facultad de F´ ısica, Universidad de La Laguna, Avda. Astrof´ ısico Francisco S´ anchez s/n, E-38071, La Laguna, Tenerife, Spain c Fundaci´ o General de la Universitat de Val` encia (FGUV), Spain d Instituci´ o Catalana de Recerca i Estudis Avanc ¸ats (ICREA), Spain †CCDC reference number 742480. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b915546e well as on the nature of the interacting metal centers. Although the so-called “complex-as-ligand/complex-as-metal” approach represents a rational way toward the preparation of SMMs, 3 it has received limited attention compared to serendipitous self- assembly methods. 4 This molecular-programmed self-assembly strategy, whereby a metal complex with potential donor groups (metalloligand) coordinates to another coordinatively unsaturated metal complex, constitutes an appropriate route to gain control on both the nuclearity and the topology of the polymetallic species. 5 Moreover, it allows the straightforward preparation of heteropolymetallic species containing metal ions of different nature. 5 2350 | Dalton Trans., 2010, 39, 2350–2358 This journal is © The Royal Society of Chemistry 2010