2.1 Materials

All manipulations were performed under aerobic conditions, using chemicals as received from Sigma–Aldrich. Type 4 molecular sieves were used to dry the CH₃CN and CH₃NO₂ solvents before use. The precursor (NBu₄)₂[ReBr₄(ox)] was prepared following the synthetic method described in the literature [18,23].

2.2 Synthesis

2.3 Physical measurements

Elemental analysis (C, H, N) were performed using a CE Instruments EA 1110 CHNS analyser. Infrared spectra were recorded using a Thermo-Nicolet 6700 FT-IR spectrophotometer in the 4000–400 cm⁻¹ region. The Re/Cu and Re/Br molar ratios were analysed using a Philips XL-30 scanning electron microscope equipped with a system of X-ray microanalysis from the Central Service for the Support to Experimental Research at the University of Valencia. Magnetic susceptibility measurements of 1 were carried out with a Quantum Design SQUID magnetometer in the temperature range 1.9–300 K and under an applied magnetic field of 0.1 T. All of the experimental magnetic data were corrected for the diamagnetic contributions of the constituent atoms in 1, through the Pascal's constants [24], and also for the sample holder.

2.4 Crystallographic data collection and structure determination

Powder XRD measurements were performed using a PANalytical Empyrean diffractometer with a hybrid monochromator (CuK_α1 radiation), a PIXcel detector and a capillary sample holder. XRD data of a single crystal of 1, with dimensions 0.42 × 0.18 × 0.15, were collected using a Bruker D8 Venture diffractometer with PHOTON II detector and using monochromatized MoK_α radiation (λ = 0.71073 Å). The structure was solved by standard direct methods and subsequently completed by Fourier recycling using SHELXTL [25–28]. The final full-matrix least-squares refinements on F², minimising the function Σw(|F_o| − |F_c|)², reached convergence with values of the discrepancy indices given in Table 1. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms of the MeCN molecule were set in calculated positions and refined as riding atoms. Graphical manipulations were performed using DIAMOND [29]. Main interatomic bond lengths and angles for 1 are given in Table 2. CCDC 1885666 contains the supplementary crystallographic data for compound 1.

3.1 Crystal structure of {[ReBr₄(μ-ox)Cu(pyim)₂]·MeCN}_n (1)

Compound 1 crystallises in the orthorhombic system with space group Pbca (Table 1). The crystal structure is made up of [Cu(pyim)₂]²⁺ cations and [ReBr₄(ox)]²⁻ anions, which are mainly linked through alternating oxalato and bromo bridges generating a Cu^IIRe^IV chain of repeating [ReBr₄(μ-ox)Cu(pyim)₂] units. A MeCN molecule along with a dinuclear [ReBr₄(μ-ox)Cu(pyim)₂] complex forms the asymmetric unit in 1. Although selected bonds lengths and angles are listed in Table 2, a perspective drawing showing the metal-based ions in 1 is given in Fig. 1.

There exist a couple of significant structural differences between 1 and the previously reported [ReCl₄(μ-ox)Cu(pyim)₂] (2) and [ReBr₄(μ-ox)Cu(bpy)₂] (3) compounds that we would like to point out [16,19]. Compounds 2 and 3 crystallise in the monoclinic system with space group P2₁/n, but besides that, 2 and 3 do not contain solvent molecules of crystallisation. Remarkably, the coordination of the oxalate group to the Cu^II ion is through the O(1) atom in 2, whereas it is by means of the O(4) atom in 1 and 3. Only in 3 there exist short halogen⋯halogen contacts connecting the adjacent Cu^IIRe^IV chains [19].

In 1, rhenium(IV) ion is six-coordinate by four bromide anions and two oxygen atoms in a distorted octahedral geometry. The main cause of such a distortion is the reduced bite angle of the oxalate group [the value of the O(1)Re(1)O(2) angle is 78.9(1)°], which exhibits bidentate and monodentate bridging modes towards the Re^IV and Cu^II ions, respectively. The O(1), O(2), Br(2) and Br(3) set of atoms constitute the best equatorial plane around the Re^IV ion, the largest deviation from planarity being 0.070 Å for O(2). The average value of the Re^IVBr [2.479(1) Å] and Re^IVO [2.055(1) Å] bond lengths, and also the bond angles, found in 1 are in agreement with those previously reported for complexes containing the anionic [ReBr₄(ox)]²⁻ entity [18–22].

Each Cu^II ion in 1 is mainly five-coordinate and bonded to four nitrogen atoms from two pyim molecules and an oxygen atom from the oxalate group of the closer [ReBr₄(ox)]²⁻ anion, in a distorted square pyramidal geometry. The Cu^IIO bond length is 2.682(1) Å and the average value of the Cu^IIN bond lengths is 1.997(1) Å. Nevertheless, having into account the Br(1a) ion that would occupy a sixth position generating the one-dimensional motif [the Cu(1)⋯Br(1a) distance is ca. 3.23 Å; (a) = x, 1/2 − y, 1/2 + z], the Cu^II ion could also be seen in a very distorted octahedral environment, as previously described in similar chloro-derivative Cu^IIRe^IV systems [13,16].

The intramolecular Cu^II⋯Re^IV distance through the oxalato bridge is 5.457(1) Å, whereas this intermetallic distance through the bromo bridge is somewhat shorter [Cu(1a)⋯Re(1) distance of 5.263(1) Å; (a) = x, 1/2 − y, 1/2 + z]. The average CC and CN bond length values of the pyim ligand show the expected values for this molecule when coordinated to a metal ion [30–67].

In the crystal packing of 1, the [ReBr₄(μ-ox)Cu(pyim)₂] units are arranged helicoidally forming chains that grow along the c-axis direction (Fig. 2). These Cu^IIRe^IV chains are extended to layers, on the crystallographic bc plane (Fig. 3), by means of bifurcated hydrogen-bonding interactions between oxalate and NH groups of coordinated pyim ligands [the N(6)⋯O(3b) and N(6)⋯O(4b) distances are 2.87(2) and 2.93(2) Å, respectively; (b) = 3/2 − x, 1 − y, −1/2 + z]. Likewise, π⋯π type interactions between the aromatic rings of neighbouring pyim ligands connect the Cu^IIRe^IV chains along the crystallographic ab plane (the shortest intercentroid distance being approximately 3.42 Å). The value of the shortest intermolecular Cu^II⋯Re^IV distance between adjacent chains is 8.822(2) Å [Cu(1)⋯Re(1c), (c) = 3/2 − x, −y, −1/2 + z], whereas the shortest intermolecular Cu^II⋯Cu^II and Re^IV⋯Re^IV distances are 7.385(3) and 9.161(2) Å [Cu(1)⋯Cu(1d) and Re(1)⋯Re(1d), (d) = 3/2 − x, −1/2 + y, z], respectively.

In addition, weak CH⋯Br interactions that vary in the range 3.72–3.78 Å link parallel planes and contribute to stabilising the supramolecular structure of 1.

Finally, the phase purity of the bulk sample of 1 was confirmed through powder XRD patterns (Fig. 4).

3.2 Magnetic properties

Direct current magnetic susceptibility measurements were carried out on a microcrystalline sample of 1 in the 1.9–300 K temperature range and under an external magnetic field of 0.1 T. The χ_MT versus T plot (χ_M being the molar magnetic susceptibility per Cu^IIRe^IV pair) of 1 is shown in Fig. 5. At room temperature the χ_MT value is 1.96 cm³ mol⁻¹ K, which is very close to that expected for a pair of uncoupled Cu^II (3d⁹, S = 1/2 with g = 2.2) and Re^IV (5d³, S = 3/2 with g = 1.8) ions [14–18]. Upon cooling, the χ_MT value decreases slowly with decreasing temperature, more abruptly at approximately 50 K, reaching a minimum value of 0.50 cm³ mol⁻¹ K at 2.0 K. Then, the χ_MT value increases giving a final value of 0.58 cm³ mol⁻¹ K at 1.9 K. No maximum of the magnetic susceptibility is detected in the χ_M versus T plot. The magnetic behaviour observed at higher temperatures would be because of the large zero-field splitting of the Re^IV ion, together with antiferromagnetic interaction between the Re^IV and Cu^II centres, whereas the final increases in the χ_MT value at very low temperatures would account for a ferrimagnetic behaviour for 1 [13–16].

The field dependence of the molar magnetisation (M) plot for 1 at 2.0 K is given in Fig. 6, which exhibits a continuous increase in M with the applied magnetic field and neither saturation nor hysteresis loop was observed. The lack of saturation of M at this temperature is likely because the applied magnetic field would overcome the weak intrachain antiferromagnetic interaction [13,16]. The M versus H plot also supports the presence of antiferromagnetic interactions in 1, given that the maximum value of M per Cu^IIRe^IV pair (ca. 1.19 μ_B) is smaller than that of the mononuclear [ReBr₄(ox)]²⁻ complex isolated as its tetra-n-butylammonium salt (ca. 1.50 μ_B) [23].

Taking into account both the structural description of 1 (see above) and the presence of a minimum at very low temperature in the χ_MT versus T plot, we can consider that compound 1 behaves as a ferrimagnetic chain [13–16]. Thus, to analyse the magnetic properties of 1, the experimental magnetic susceptibility data have been treated through Eqs 1 and 2 and the following spin Hamiltonian (Eq 3):

The approach that we have used to fit the experimental data consists in assuming that the magnetic susceptibility is given by that of the ⁴A_2g term (ground state for a d³ ion in an octahedral environment), including the zero-field splitting, and modulated by a factor predicted from the Ising model of the magnetic exchange with the parameters J and j that would be assigned to the magnetic exchange pathways of the alternating oxalato and bromo bridges, respectively (and defined as indicated in Eqs 1 and 2) [16,17]. This is possible because the magnetic susceptibility of 1 in the high temperature region may be described by two contributions: one from the ⁴A_2g term of the Re^IV ion and the other from the uncoupled Cu^II ion. In the low temperature region it would be described as a chain of S_Re = S_eff = 1/2 with different local g_Cu and g_Re Landé factors [13–16].

To avoid overparameterisation, we have also assumed that g = g_// = g_⊥ for the Cu^II and Re^IV ions. This approach has previously been used for fitting the magnetic data of similar heterometallic Cu^IIRe^IV chains [13–16,58]. A least-squares fit of the experimental data in the 1.9–300 K temperature range afforded the following parameters for 1: J = −6.3, j = −5.4, g_Cu = 2.25, g_Re = 1.87, and |D_Re| = 64.2 cm⁻¹ with R = 6.4 × 1-0⁻⁵ {R being the agreement factor defined as Σi[(χ_MT)_obs(i) − (χ_MT)_calc(i)]²/Σi[(χ_MT)_obs(i)]²}. As shown in Fig. 5, the theoretical curve for 1 (red solid line) matches quite well with the experimental magnetic data in the studied temperature range. The values of the J and j magnetic exchanges that we have obtained by this approach are referred to an S_eff = 1/2 and aiming at comparing them with the D_Re value (which is referred to a real S_Re = 3/2), they should be reduced by a factor of about 3/5 (3J/5 = −3.8 cm⁻¹ and 3j/5 = −3.2 cm⁻¹), as previously reported [13,16,58]. The calculated values for the g_Cu, g_Re and D_Re parameters are in agreement with those previously computed for similar one-dimensional Cu^IIRe^IV systems [13–16,58]. The calculated values of the J and j magnetic exchanges support antiferromagnetic interactions between the Re^IV and Cu^II centres across the two magnetic pathways, that is, through the alternating oxalato and bromo bridges in 1. According to the orthogonality of the involved magnetic orbitals [e_g for Cu^II and t_2g for Re^IV], a priori, a ferromagnetic exchange would be expected. However, this orthogonality is broken because of the asymmetry of the bridges and the distorted coordination geometry of the Cu(II) ion, resulting in a very poor overlap of the magnetic orbitals and, hence, in a weak antiferromagnetic exchange between these metal ions.

Despite showing a chain motif in its crystal structure, compound 3 behaves magnetically as a tetranuclear [Re^IV₂Cu^II₂] species [19]. Compound 3 does not exhibit behaviour of ferrimagnetic chain given that contains in their crystal lattice significant Re–Br⋯Br–Re interactions (Br⋯Br separation of ca. 4.8 Å) between adjacent Re^IVCu^II chains. These intermolecular interactions in the rhenium(IV) chemistry are usually very strong and can easily overcome the intramolecular ones, accounting for the magnetic behaviour observed in the χ_MT versus T variation [68]. Nevertheless, this fact has not been reported for ferrimagnetic chains based on other 5d metal ions [69].

As far as we know, compound 1 is the first system containing the [ReBr₄(ox)]²⁻ metalloligand to exhibit magnetic behaviour of ferrimagnetic chain and, therefore, any comparison would be precluded. Nonetheless, we have tried to compare our results with those obtained for the previously studied Cu^IIRe^IV chains based on the chloro-derivative [ReCl₄(ox)]²⁻ complex [13–17]. Thus, we have plotted the J values versus the Re^IV⋯Cu^II distances and the Re–X–Cu angle (°) (X = Cl and Br) for ferrimagnetic Cu^IIRe^IV chains (Table 3). As shown in Fig. 7, there exists a certain trend in these systems: when the Re···Cu separation shortens and the Re–X–Cu angle decreases, the value of the antiferromagnetic coupling also decreases (red solid line in Fig. 7). Although we cannot talk about a magnetostructural correlation, given that more data of Cu^IIRe^IV systems would be needed to complete the study in detail, this trend could help at least to design new ferrimagnetic Cu^IIRe^IV compounds and similar systems, having into account the high magnetic anisotropy that Re^IV ion exhibits [70–75].