How to Quench Ferromagnetic Ordering in a CN-Bridged Ni(II)-Nb(IV) Molecular Magnet? A Combined High-Pressure Single-Crystal X-Ray Di ﬀ raction and Magnetic Study

: High-pressure (HP) structural and magnetic properties of a magnetic coordination polymer {[Ni II (pyrazole) 4 ] 2 [Nb IV (CN) 8 ] · 4H 2 O} n (Ni 2 Nb) are presented, discussed and compared with its two previously reported analogs {[Mn II (pyrazole) 4 ] 2 [Nb IV (CN) 8 ] · 4H 2 O} n (Mn 2 Nb) and {[Fe II (pyrazole) 4 ] 2 [Nb IV (CN) 8 ] · 4H 2 O} n (Fe 2 Nb). Ni 2 Nb shows a signiﬁcant decrease of the long-range ferromagnetic ordering under high pressure when compared to Mn 2 Nb, where the pressure enhances the T c (magnetic ordering temperature), or to Fe 2 Nb exhibiting a pressure-induced spin crossover. The di ﬀ erent HP magnetic responses of the three compounds were rationalized and correlated with the structural models as determined by single-crystal X-ray di ﬀ raction.


Introduction
Structural and magnetic measurements under high pressure are the most reliable source of straightforward magneto-structural correlations in crystalline magnetic solids. These types of studies make it possible to fine-tune the structure and physical properties in a continuous manner-a feature that cannot be achieved via chemical modifications, which often introduce unexpected complications (different packing modes, additional intra-and intermolecular contacts). The application of high pressure is known to be extremely useful for enhancing the magnetic ordering temperature of extended coordination systems [1][2][3][4][5], ligand field and magnetic anisotropy tuning of mononuclear complexes [6,7] and control of the spin crossover behavior [8,9]. The combined high-pressure single-crystal X-ray diffraction (scXRD) and SQUID magnetometry make a perfect set of tools to study and understand the changes induced by this type of mechanical stimulus. scXRD structural analysis experiments are commonly performed using diamond anvil cells (DACs) [10] and, in the case of high-pressure SQUID magnetometry, the most common environment chamber is a piston-cylinder cell (PCC) made of diamagnetic copper-beryllium alloy [11].
Magnetic coordination polymers are known for their high responsiveness to mechanical stress and high pressure. In particular, cyanide-bridged Prussian Blue analogs [12] and the related octacyanometallate-based bimetallic assemblies [2] show significant magnetic changes under high pressure.

X-ray Crystal Structure Description under High Pressure
Ni 2 Nb shows some interesting structural distortions when pressurized using a Merrill-Bassett DAC [14]. Its structure under pressure was determined by scXRD ( Figure 1 and Table 1) [13]. Ni 2 Nb crystallizes in a tetragonal space group I4 1 /a and forms a three-dimensional (3-D) CN-bridged skeleton with a flattened diamond-like topology where the niobium and nickel ions are all linked by cyanide ligands. The niobium(IV) centers play the role of the four-fold tetrahedral nodes in the 3-D framework of Ni 2 Nb (Figure 1a,b).

X-Ray Crystal Structure Description under High Pressure
Ni2Nb shows some interesting structural distortions when pressurized using a Merrill-Bassett DAC [14]. Its structure under pressure was determined by scXRD ( Figure 1 and Table 1) [13]. Ni2Nb crystallizes in a tetragonal space group I41/a and forms a three-dimensional (3-D) CN-bridged skeleton with a flattened diamond-like topology where the niobium and nickel ions are all linked by cyanide ligands. The niobium(IV) centers play the role of the four-fold tetrahedral nodes in the 3-D framework of Ni2Nb (Figures 1a,b).
The high-pressure compression of Ni 2 Nb is presented in Table 1 and Figure 2 as the pressure dependence of the normalized unit-cell volume V/V 0 , where V 0 is the unit-cell volume at 1000 hPa, along with the relevant data for the two analogs Mn 2 Nb and Fe 2 Nb published previously [8]. The unit-cell volumes of Mn 2 Nb, Fe 2 Nb and Ni 2 Nb are significantly compressed up to 87.6%, 84.6% and 87.7% of the initial value at ca. 2.4 GPa, respectively. The V/V 0 vs. p dependences were fitted using the third-order Birch-Murnaghan equation of state (BMEOS) [15,16] (Equation (1)): where p-pressure, V 0 -volume at zero pressure, in this case the ambient pressure, V-volume, K 0 -isothermal bulk modulus at zero pressure, K' 0 -dimensionless first derivative of K 0 with respect to pressure. The solid lines in Figure 2, which represent the best fit to Equation (1), match the experimental data for Ni 2 Nb and Mn 2 Nb in the investigated pressure range. In the case of Fe 2 Nb there is a strong deviation from the BMEOS above 1 GPa. The best fit parameters K 0 and K 0 ' are: 10.7 ± 0.9 and 8.7 ± 1.8 GPa for Mn 2 Nb, 10.4 ± 1.6 and 8.2 ± 5.6 GPa for Fe 2 Nb (from the 0-1 GPa range fit) and 11.9 ± 1.2 and 6.9 ± 2.0 GPa in the case of Ni 2 Nb. The bulk modulus K 0 value is identical for all isomorphs within the experimental error and quite similar to other molecule-based coordination compounds [6,17,18]. Noteworthy, the K 0 for the studied coordination polymers are nearly two orders of magnitude smaller than for diamond (440 GPa) and only one order larger than for rubber (1 GPa) [19]. This places the mechanical properties of coordination polymers somewhere between typical inorganic solids and soft matter. isomorphs within the experimental error and quite similar to other molecule-based coordination compounds [6,17,18]. Noteworthy, the K0 for the studied coordination polymers are nearly two orders of magnitude smaller than for diamond (440 GPa) and only one order larger than for rubber (1 GPa) [19]. This places the mechanical properties of coordination polymers somewhere between typical inorganic solids and soft matter. The analysis of the coordination spheres of Ni II and Nb IV in Ni2Nb under high pressure leads to similar observations as for Mn2Nb [8]: the Nb-C and C-N distances as well as Nb-C-N angles remain roughly unchanged, while the Ni-N bonds (Figure 3a) shrink significantly in a linear fashion ( Figure  3b). This fact and the good match between the V/V0(p) dependence and the BMEOS both indicate that no pressure-induced phase transition occurs at room temperature in Ni2Nb and Mn2Nb. In fact, the behavior of these two solids is very similar while that of Fe2Nb is quite different and strongly deviates from BMEOS due to the SCO (SCO -spin crossover) behavior. Also, the Fe-N distances shrink in a non-linear fashion in Fe2Nb above 1.5 GPa.  Figure 3b). This fact and the good match between the V/V 0 (p) dependence and the BMEOS both indicate that no pressure-induced phase transition occurs at room temperature in Ni 2 Nb and Mn 2 Nb. In fact, the behavior of these two solids is very similar while that of Fe 2 Nb is quite different and strongly deviates from BMEOS due to the SCO (SCO -spin crossover) behavior. Also, the Fe-N distances shrink in a non-linear fashion in Fe 2 Nb above 1.5 GPa.  A more detailed analysis of M-N bonds up to 1.5 GPa (Ni2Nb, Mn2Nb and Fe2Nb follow the BMEOS in this pressure range) demonstrates that M-NCN shrinkage is much larger than that of M-Npyrazole (Figure 3b and Table 2). As a result, the elongated octahedral coordination spheres of Fe II in Fe2Nb and Ni II in Ni2Nb become closer to a perfect octahedron at around 0.7 and 1.2 GPa, respectively. A more detailed analysis of M-N bonds up to 1.5 GPa (Ni 2 Nb, Mn 2 Nb and Fe 2 Nb follow the BMEOS in this pressure range) demonstrates that M-N CN shrinkage is much larger than that of M-N pyrazole (Figure 3b and Table 2). As a result, the elongated octahedral coordination spheres of Fe II in Fe 2 Nb and Ni II in Ni 2 Nb become closer to a perfect octahedron at around 0.7 and 1.2 GPa, respectively.

Magnetic Properties under High Pressure
The magnetic properties of Mn 2 Nb and Fe 2 Nb (Figures 4 and 5, respectively) have been discussed in detail in a previous report [8]. It was established that at ambient pressure both Mn 2 Nb and Fe 2 Nb are ferrimagnets with long-range magnetic ordering temperatures (T c ) of 23.4 and 9.4 K, respectively. However, the behavior of these compounds under high pressure is very different. Mn 2 Nb (Figure 4) displays a very strong and linear increase of the T c from 23.4 K at ambient pressure to 36.5 K at 1.03 GPa. The linear fit of the T c (p) dependence (Figure 4c) leads to dT c /dp = 12.4 ± 0.2 K GPa −1 , typical for octacyanidoniobate(IV)-based systems and Prussian Blue analogs [2]. Fe 2 Nb, on the other hand, exhibits almost complete quenching of the long-range magnetic ordering under pressure, resulting in paramagnetic properties above 0.7 GPa ( Figure 5).
Ni 2 Nb is a ferromagnet due to the local ferromagnetic interactions between Nb IV and Ni II ions [13] with the Curie temperature (T C ) of 13.2 K. It exhibits a completely different type of magnetic response to high pressure ( Figure 6) when compared to the other two analogs Mn 2 Nb and Fe 2 Nb. The T C of Ni 2 Nb decreases with increasing pressure, which is characteristic for ferromagnets [20,21], and confirms the presence of local ferromagnetic interactions between Nb IV and Ni II . The T C shift is 1.9 K GPa −1 , as obtained from the linear fit of the T C (p) dependence (Figure 6c, T C is the position of the dχ/dT peak in the χ(T) measurement at 10 Oe). The pressure response of Ni 2 Nb is much weaker and opposite to Mn 2 Nb, where the antiferromagnetic interactions between Nb IV and Mn II are present. Hence, the underlying local magnetic interactions in Ni 2 Nb (ferromagnetic) vs. Mn 2 Nb (antiferromagnetic) are the source of the observed difference. The shortening of the Ni-N CN bonds favors the resonance integral and makes the antiferromagnetic contribution to the total exchange interaction stronger while destabilizing the ferromagnetic one. Overall, the J NiNb coupling constant decreases under pressure.
Ni 2 Nb does not present a loss of the magnetic moment under high pressure, which is confirmed by the pressure-independent magnetization at saturation values of 5.3 Nβ (at 2.0 K and 7 T; Figure 6b) up to 1.10 GPa and the structural pressure response that matches the BMEOS. The magnetization at saturation values are close to 5.2 Nβ, as expected for two Ni II (S = 1) and one Nb IV (S = 1 2 ) coupled ferromagnetically (assuming g Nb = 2.0 and g Ni = 2.1 [22]). Ni 2 Nb also shows interesting pressure-induced changes in the M(H) dependencies in the 0.1-5 T magnetic field range (Figure 6b). Following the initial sharp increase of the magnetization around 0.05 T in each case, there is a clear dependence with M(H) attaining lower values at higher pressure. All M(H) curves "meet" again above 5 T, converging to the same saturation value of 5.3 Nβ. This behavior is most probably related to the changes of the magnetic anisotropy of the Ni II centers arising from the shortening of the Ni-N CN bonds along the CN-Ni-CN axis of the Ni II coordination sphere, as evidenced by high-pressure structural studies (Figure 3b

Materials
Chemicals used in this study were of analytical grade and were obtained from commercial sources (Sigma-Aldrich Co., Avantor, Alfa-Aesar). K 4 [Nb(CN) 8 ]·2H 2 O was prepared according to the newest available procedure [23]. {[Ni II (pyrazole) 4 ] 2 [Nb IV (CN) 8 ]·4H 2 O} n (Ni 2 Nb) was obtained according to the literature procedure and its purity/identity was confirmed by elemental analysis and powder X-ray diffraction, which were identical to those published previously [13].

Single-Crystal X-ray Diffraction under Pressure
The single crystal diffraction data for Ni 2 Nb were collected at room temperature for a single crystal placed in a Merrill-Bassett DAC filled with Fluorinert 77 as the pressure transmitting medium and a chip of ruby. The ruby fluorescence method was used for the pressure determination inside the DAC chamber. The diffraction frames were collected on KUMA KM4-CCD and Xcalibur EOS machines (running Crysalis software; instruments presently manufactured by Rigaku Oxford Diffraction) using a Mo Kα radiation source and a graphite monochromator (λ = 0.71073 Å). The data were corrected for the sample and DAC absorption as well as the for the gasket shadow [24]. Overlaps with the diamond reflections were excluded from the refinement. Non-H atoms were refined anisotropically (weighted full-matrix least-squares on F 2 ) [25]. The summary of crystallographic data can be found in

Magnetic Measurements under Pressure
Magnetic measurements under pressure for Mn 2 Nb and Fe 2 Nb were reported previously [8]. Ni 2 Nb was characterized in a similar fashion using a Quantum Design MPMS3 SQUID-VSM (Sand Diego, USA) magnetometer. A powdered sample of Ni 2 Nb was loaded into the CuBe piston-cylinder cell (manufactured by HMD, Japan; purchased from Quantum Design) with a piece of high-purity lead as the manometer and Daphne 7373 oil as the pressure-transmitting medium. The pressure determination at low temperature was performed with 0.02 GPa accuracy by using the linear pressure dependence of the superconducting transition of Pb (−0.379 K GPa −1 ). Magnetic data were corrected for the diamagnetic contribution of the sample and the pressure cell.

Conclusions
A combined high-pressure magneto-structural study of a magnetic coordination polymer {[Ni II (pyrazole) 4 ] 2 [Nb IV (CN) 8 ]·4H 2 O} n Ni 2 Nb was carried out and revealed a strong magnetic response of this material to mechanical stress. The magnetic ordering temperature of Ni 2 Nb shifted linearly towards lower temperatures at higher pressure due to the ferromagnetic character of the exchange coupling between Nb IV and Ni II ions. Such behavior is completely different from that observed for the two analogs {[Mn II (pyrazole) 4 8 ]·4H 2 O} n Fe 2 Nb. Mn 2 Nb exhibited an opposite effect-a strong enhancement of T c under pressure due to the antiferromagnetic exchange coupling between the constituent magnetic ions. Our study confirms the usefulness of combined high-pressure studies to understand the magnetic properties of molecular magnets and the possibility to fine-tune their properties by applying a mechanical stimulus-namely, high pressure.