Edinburgh Research Explorer Self-assembly of the tetrachlorido(oxalato)rhenate( iv ) anion with protonated organic cations: X-ray structures and magnetic properties

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As a continuation of our current research activity dealing with halorhenate(IV) salts aimed at analysing the nature and influence of such intermolecular interactions on magnetic properties, we herein report the synthesis and magnetostructural characterization of two novel Re IV  anion and none contains protonated organic cations.The [ReCl4(ox)] 2− anion is a highly anisotropic complex that has been used as a precursor to build both discrete and 1D compounds, in many cases with pre-designed magnetic properties, 3 and there is no doubt that new magnetic systems will be obtained through intermolecular interactions driven by its chloride and oxalate groups, which can be very effective in crystal engineering.

X-ray data collection and structure refinement
X-ray diffraction data on single crystals of dimensions 0.48 x 0.06 x 0.02 (1) and 0.43 x 0.35 x 0.06 mm 3 (2) were collected on a Bruker-Nonius X8APEXII CCD area detector diffractometer with graphite-monochromated Mo-K radiation (λ = 0.71073 Å).Crystal parameters and refinement results for 1 and 2 are summarized in Table 1.2][43] H(2) in 1 and H(2a), H(3a), H(4a), H(6a), H(7a) and H(8a) in 2 were seen in the difference maps before they were geometrically created in the model.The final full-matrix least-squares refinements on F 2 , minimising the function w(Fo-Fc) 2 , reached convergence with the values of the discrepancy indices given in Table 1.The graphical manipulations were performed with the DIAMOND program. 44CCDC 1503662 (1) and 1503663 (2).

Please do not adjust margins
Please do not adjust margins   Two crystallographically independent monoprotonated 2,2′biimidazolium cations charge balance the [ReCl4(ox)] 2-unit in the structure of 2. The protonation involves the N(3) and N( 7) atoms (see Fig. 2).In both cases, the two imidazole rings are essentially coplanar.[66][67][68] In the crystal packing of Please do not adjust margins Please do not adjust margins  MT observed for 1 and 2 is likely due to the presence of intermolecular interactions and/or zero-field splitting (zfs) effects. 3, Theresence of a maximum in the magnetic susceptibility at ca. 12.0 (1) and 17.0 K (2) (see insets of Figs. 9 and 10) unambiguously supports the occurrence of antiferromagnetic exchange interactions between the Re IV ions.No out-of-phase signals in the ac magnetic susceptibility are observed for either 1 or 2. This is in contrast to that previously seen and studied for the parent compound (NBu4)2[ReCl4(ox)], that exhibits single-ion magnet (SIM) behaviour. 29 detailed inspection of the crystal packing of 1 and 2 reveals the existence of short Cl•••Cl contacts between the paramagnetic [ReCl4(ox)] 2-anions in their crystal lattices (Fig. 8).Hence, the presence of relatively important through-space interactions between the spin carriers precludes the occurrence of SIM behaviour.
In order to analyze the magnetic behaviour of 1 and 2, we have employed the Hamiltonian of equation ( 1) and its derived theoretical expression for the magnetic susceptibility, equation   The first term in equation ( 1) corresponds to the zero-field splitting and the second term the Zeeman effects.HF-EPR and theoretical studies performed on the [ReCl4(ox)] 2- moiety, prepared as a magnetically isolated salt, supports a value of D = -53 cm -1 for this unit. 29Thus, D was kept constant during the fitting process.The resulting best least-squares fit parameters, represented as solid red lines in Figures 9 and 10, are g = 1.88 and  = -14.7 K with R = 6.8 x 10 -5 for 1, and g = 1.89 and  = -18.4K with R = 1.3 x 10 -4 for 2 {R being the agreement factor defined as i[(MT)i obs -(MT)i calc ] 2 / [(MT)i obs ] 2 }.The g values calculated for 1 and 2 are in agreement with those previously reported for mononuclear Re IV complexes. 3,7-Please do not adjust margins Please do not adjust margins 36 The negative values of  confirm the presence of relatively strong antiferromagnetic exchange through intermolecular Re-Cl•••Cl-Re pathways (see Fig. 8), with the shorter interactions in 2 resulting in stronger exchange.

Fig. 5 .
Fig. 5. View of a fragment of the crystal packing of 1 through the bc plane, showing an anionic layer connected by Cl•••Cl contacts (dashed lines).Colour code: pink, Re; green, Cl; red, O; blue, N; grey, C; black, H.
, 69 by including a  term (as T-) to account for the observed intermolecular interactions.

Fig. 9 .
Fig. 9. Thermal variation of the MT product for 1.The solid red line represents the best-fit of the experimental data (see text).The inset shows the temperature dependence of the magnetic susceptibility.

Fig. 10 .
Fig. 10.Thermal variation of the MT product for 2. The solid red line represents the best-fit of the experimental data (see text).The inset shows the temperature dependence of the magnetic susceptibility.

Table 1 . Summary of the crystal data and structure refinement parameters for 1 and 2
While crystals of 1 are obtained by standing overnight in the fridge at 4 °C, those of 2 were grown in one week after slow evaporation of the mother liquor at room temperature.It is worth noting that the oxalate dianion remains coordinated to the Re IV metal ion and undergoes neither protonation nor dissociation, a feature which is quite unusual under such reaction conditions.