supplementary materials

In the field of molecule-based magnetic materials of 3 d and 4f metals with multicarboxylate bridging ligands, a variety of two-dimensional and three-dimensional original structures with interesting magnetic and spectroscopic properties have been discovered (Benelli and Gatteschi, 2002). One-dimensional structures, however, have only been reported in few cases for Ln₂Cu complexes with the trichloroacetate ligand (Kutlu et al.,1997; Legendziewicz et al., 2000)

Aliphatic monocarboxylic acids are expected to have limited bonding ability, since the carboxylate group cannot bridge adjacent large high-nuclear clusters. Monocarboxylates, however, may constitute a route toward one-dimensional 3 d-4f polymers, their main interest being the construction of single chain magnets. We recently used trans-2-butenoic acid in the synthesis and X-ray structure determination of three isostructural Cu^II—Ln^III1-D polymers {[Cu₂Ln₂L₁₀(H₂O)₄].3H₂O}_n(Ln = Gd^III, Er^III, Y^III; HL= trans-2-butenoicacid) (Calvo et al., 2008). The {[Cu₂Ho₂L₁₀(H₂O)₄].3H₂O}_n species had also been synthesized, but no single crystals could be obtained at the time. We now wish to report the crystal data of the isostructural Cu₂Ho₂ complex obtained as well shaped needles. They consist of non-centrosymmetric polymeric chains built in the centrosymmetric P2₁/c group by two distinct blocks: one formed by two Cu^II ions bridged by four carboxylate bridges in the η¹:η¹:µ₂ conformation and the other by two Ho^III ions bridged by two carboxylate oxygen atoms in η²:η¹:µ₂ conformation (See Figure 1). Bonds between Cu and Ho metal centers consist of one covalent tridentate carboxylate oxygen atom and two intrachain H-bonds which enhance the stability of the chains.

Each Cu^II atom of the Cu₂ dinuclear core provides the basal plane for a square pyramidal arrangement, the apical sites being provided by atoms O16 and O19 common to the Ho₂ coordination polyhedra. The Cu₂ block resembles the structure of dinuclear copper acetate monohydrate (Van Niekerk & Schoening, 1953), the main difference being the absence of an embedded inversion symmetry center relating the copper ions, present in the latter. The four independent carboxylato bridges in the Cu₂ structure depart an average of 0.12 Å from a centrosymmetric disposition, as measured by the best fit of the dinuclear unit with its inverted image (XP in the SHELXTL package, Sheldrick, 2008). The four Cu—O—C—O—Cu loops are planar within 0.03 Å, and parallel or perpendicular to each other within a maximum deviation of ca 6°. The Cu···Cu distance within the dinuclear unit is 2.642 (1) Å compared with 2.616 (1) Å in the classical fourfold copper acetate monohydrate. In the Ho₂ block, the metal centers are bridged by two oxygen atoms from two tridentate η²:η¹:µ₂ carboxylates at a Ho···Ho distance of 4.207 (1) Å.The coordination of each holmium (HoO₉) is completed by two chelating carboxylates and two aqua O atoms. Departure from a centrosymmetric arrangement is more important in this block than in the copper one, the mean square deviation from the inverted image being here 0.27 Å. Within a chain, Figure 2, alternate dinuclear units of the same type are related by a unit cell a translation; all the space group symmetry elements are external to the chains and thus relate them into each other. As a consequence of this lack of internal symmetry, alternate Cu—Ho chemical bridges are slightly different: 4.496 and 4.517 Å, respectively. The axis-to-axis separations between a chain and the six neighbouring ones cover the range 10–14 Å. The alkene groups protrude outwards almost normal to the chain direction. Although hydrogen atoms of water molecules could not be found in the late difference Fourier maps, a number of short Ow···OCO₂ and Ow···Ow distances, less than 3.00 Å (Table 2), suggests involvement in H-bonding interactions