How to Make a Better Magnet ? Insertion of Additional Bridging Ligands into a Magnetic Coordination Polymer

A three-dimensional cyanide-bridged coordination polymer based on FeII (S = 2) and NbIV (S = 1/2) {[Fe(H2O)2]2[Nb(CN)8]·4H2O}n (Fe2Nb) was modified at the self-assembly stage by inserting an additional formate HCOO− bridge into its cyanide framework. The resulting mixed-bridged {(NH4)[(H2O)Fe-(μ-HCOO)-Fe(H2O)][Nb(CN)8]·3H2O}n (Fe2NbHCOO) exhibited additional FeII-HCOO-FeII structural motifs connecting each of the two FeII centers. The insertion of HCOO− was possible due to the substitution of some of the aqua ligands and crystallization water molecules in the parent framework by formate anions and ammonium cations. The formate molecular bridge not only shortened the distance between FeII ions in Fe2NbHCOO from 6.609 Å to 6.141 Å, but also created additional magnetic interaction pathways between the magnetic centers, resulting in an increase in the long range magnetic ordering temperature from 43 K for Fe2Nb to 58 K. The mixed-bridged Fe2NbHCOO also showed a much broader magnetic hysteresis loop of 0.102 T, compared to 0.013 T for Fe2Nb.


Introduction
The rational design of crystalline molecular magnets enables the combination of magnetism (ferromagnetism) with other properties within a single material.The additional functionalities co-exist or are strongly coupled with the magnetic ordering, providing a convenient route to multifunctionality [1].In terms of future technological applications, it is crucial to develop new strategies towards higher magnetic ordering temperatures in molecular magnets [2] to eliminate the need for expensive cooling resources like liquid helium.Efforts thus far have focused mainly on Prussian blue analogs [3] and cyanido-bridged materials based on hepta-or octacyanidometallates of 4d and 5d metal ions [4,5].This is due to the ability of CN − to efficiently mediate moderate-to-strong magnetic interactions between the bridged paramagnetic metal ions.Most of the room-temperature molecular magnets are based on V II /V III and Cr III metal ions, where the magnetic coupling was found to be the strongest [6][7][8][9][10][11][12].Unfortunately, these compounds are often obtained as amorphous powders and are air-sensitive, therefore, there is still a high demand for air-stable, crystalline, high-T c, molecule-based magnets.One way to achieve this is changing the metal centers (spins and coupling constants).Here, a slightly different approach is proposed based on increasing the number of the closest magnetic neighbors.This approach is dictated by the molecular field theory Equation (1) [13], where the T c depends on three factors: the coupling constant J AB between the two metal centers A and B, the spin values S A and S B and the number of the closest magnetic neighbors n A and n B (k B is the Boltzmann constant).
The aim of our research was to modify the well-known compound {[Fe II (H 2 O) 2 ] 2 [Nb IV (CN) 8 ]•4H 2 O} n (Fe 2 Nb) [14] by inserting additional bridging ligands connecting the Fe II ions in order to increase the number of the nearest magnetic neighbors, and re-inforce the existing long-range magnetic ordering.The implemented changes were similar to the modification of the Mn II -Nb IV analogue {[Mn II (H 2 O) 2 ] 2 [Nb IV (CN) 8 ]•4H 2 O} n , described previously [15].The insertion of formate into the newly obtained mixed-bridged compound {(NH 4 )[(H 2 O)Fe II -(µ-HCOO)-Fe II (H 2 O)][Nb IV (CN) 8 ]•3H 2 O} n (Fe 2 NbHCOO) led to a very significant increase of the critical temperature T c from 43 K (Fe 2 Nb parent framework) to 58 K, and a larger coercive field.

Synthesis and X-ray Crystal Structure Description
The insertion of additional formate anions into the three-dimensional (3D), CN-bridged parent framework of Fe 2 Nb at the self-assembly stage resulted in the formation of Fe 2 NbHCOO with mixed 3d-4d and 3d-3d metal bridging.The key to its formation was a large excess of ammonium formate in the solution of the building blocks: Mohr's salt and potassium octacyanidoniobate(IV) dihydrate.The formation of the desired product was fully confirmed by single crystal X-ray diffraction structural analysis (sc-XRD; Figure 1 and Table 1), infrared spectroscopy (IR), elemental analysis (EA), and powder X-ray diffraction (PXRD).Insertion of formate anions causes mild changes in the geometry of the CN-framework of Fe2NbHCOO, compared to the parent Fe2Nb.However, a noticeable change occurred between the adjacent Fe II centers, where the formate ion was bound (Figure 1b), the Fe II •••Fe II distance was shortened by 0.468 Å (from 6.609 Å to 6.141 Å).The corresponding change in the Mn2NbHCOO [15] compared to its Mn II -Nb IV parent [16] was smaller, shortening by only 0.337 Å (from 6.590 Å to 6.253 Å).A comparison of the most important distances and angles in Fe2NbHCOO and Fe2Nb is presented in Table 2.The cyanide bridges in Fe2NbHCOO were slightly less bent compared to   [15].The coordination framework of Fe 2 NbHCOO becomes anionic compared to Fe 2 Nb, due to the insertion of negatively charged formate.As such, its electroneutrality is maintained by a simultaneous incorporation of an ammonium cation, which replaces one of the crystallization water molecules present in the parent compound.
Insertion of formate anions causes mild changes in the geometry of the CN-framework of Fe 2 NbHCOO, compared to the parent Fe 2 Nb.However, a noticeable change occurred between the adjacent Fe II centers, where the formate ion was bound (Figure 1b), the Fe II •••Fe II distance was shortened by 0.468 Å (from 6.609 Å to 6.141 Å).The corresponding change in the Mn 2 NbHCOO [15] compared to its Mn II -Nb IV parent [16] was smaller, shortening by only 0.337 Å (from 6.590 Å to 6.253 Å).A comparison of the most important distances and angles in Fe 2 NbHCOO and Fe 2 Nb is presented in Table 2.The cyanide bridges in Fe 2 NbHCOO were slightly less bent compared to Fe 2 Nb, while other structural parameters were quite similar (except for the aforementioned Fe•••Fe distances).NbHCOO was slightly closer to an ideal square antiprism than in Fe 2 Nb, according to the continuous shape measure analysis executed using SHAPE software [17].Conversely, the geometry of the octahedral Fe II centers was much more distorted in Fe 2 NbHCOO due to the strain imposed by the bridging formate anion (Figure 1a,b and Table 3), which might lead to a completely different magnetic anisotropy.The insertion of the additional formate bridge into the parent Fe 2 Nb framework changed the symmetry from a centrosymmetric I4/m to a non-centrosymmetric I4cm.The structure of Fe 2 NbHCOO was refined as an inversion twin with the following scales: 0.49 (10) and 0.51 (10).The asymmetric unit (Figure 2) was comprised of Nb IV and Fe II ions linked with a CN − ligand (C2N2).The second CN − ligand (C1N1) in the asymmetric unit connected the Fe II ion with the next Nb IV ion in the structure.The asymmetric unit also included an aqua ligand coordinated to Fe II (O1) and half of the bridging formate anion (C3O2).There were also two oxygen atoms of the crystallization water molecules (O3 with occupancy 0.5 and O4 with occupancy 1.0), and a nitrogen atom of the ammonium cation (N3 with 0.5 occupancy in the same position as O3).H-atoms could not be located from the Fourier difference map.The presence of the ammonium cation was confirmed by IR spectroscopy (see below) and is consistent with the elemental analysis (EA) results.
in the structure.The asymmetric unit also included an aqua ligand coordinated to Fe II (O1) and half of the bridging formate anion (C3O2).There were also two oxygen atoms of the crystallization water molecules (O3 with occupancy 0.5 and O4 with occupancy 1.0), and a nitrogen atom of the ammonium cation (N3 with 0.5 occupancy in the same position as O3).H-atoms could not be located from the Fourier difference map.The presence of the ammonium cation was confirmed by IR spectroscopy (see below) and is consistent with the elemental analysis (EA) results.

IR Spectroscopy
The infrared (IR) spectrum of Fe2NbHCOO (Figure 3) exhibited bands typical for the parent framework: two bands characteristic for stretching (3576 cm −1 ) and bending (1635 cm −1 ) vibrations of O-H, and a strong band characteristic for stretching vibrations of CN -ligands (2141 cm −1 ).Additional specific bands as compared to the parent framework indicate the presence of the inserted formate anions (1584 cm −1 , 1383 cm −1 , 1365 cm −1 and 812 cm −1 ) and the ammonium cations (3261 cm −1 and 1403 cm −1 ).

IR Spectroscopy
The infrared (IR) spectrum of Fe 2 NbHCOO (Figure 3) exhibited bands typical for the parent framework: two bands characteristic for stretching (3576 cm −1 ) and bending (1635 cm −1 ) vibrations of O-H, and a strong band characteristic for stretching vibrations of CN -ligands (2141 cm −1 ).Additional specific bands as compared to the parent framework indicate the presence of the inserted formate anions (1584 cm −1 , 1383 cm −1 , 1365 cm −1 and 812 cm −1 ) and the ammonium cations (3261 cm −1 and 1403 cm −1 ).

Identity and Purity Confirmation by Powder X-Ray Diffraction
The identity and purity of the bulk sample of Fe2NbHCOO was confirmed by powder X-ray diffraction measurements (Figure 4).The experimental PXRD diffraction patterns were in very good agreement with the simulated results from the sc-XRD structural model obtained at room temperature.

Identity and Purity Confirmation by Powder X-ray Diffraction
The identity and purity of the bulk sample of Fe 2 NbHCOO was confirmed by powder X-ray diffraction measurements (Figure 4).The experimental PXRD diffraction patterns were in very good agreement with the simulated results from the sc-XRD structural model obtained at room temperature.

Identity and Purity Confirmation by Powder X-Ray Diffraction
The identity and purity of the bulk sample of Fe2NbHCOO was confirmed by powder X-ray diffraction measurements (Figure 4).The experimental PXRD diffraction patterns were in very good agreement with the simulated results from the sc-XRD structural model obtained at room temperature.

Magnetic Properties
The shape of the χT(T) dependence for Fe2NbHCOO recorded in the 2-300 K range at 0.1 T magnetic field (Figure 5) suggests the presence of a long-range magnetic ordering in this system below 70 K (discussed below).The experimental χT value at 300 K of 8.31 cm 3 •K•mol -1 was slightly lower than the expected 9.02 cm 3 •K•mol -1 for two Fe II (S = 2), assuming gFe = 2.4, and one Nb IV (S = ½), assuming gNb = 2.0.This was most probably caused by the presence of antiferromagnetic interactions within the Nb IV -CN-Fe II coordination framework.χT increased significantly as the temperature was

Magnetic Properties
The shape of the χT(T) dependence for Fe 2 NbHCOO recorded in the 2-300 K range at 0.1 T magnetic field (Figure 5) suggests the presence of a long-range magnetic ordering in this system below 70 K (discussed below).The experimental χT value at 300 K of 8.31 cm 3 •K•mol -1 was slightly lower than the expected 9.02 cm 3 •K•mol -1 for two Fe II (S = 2), assuming g Fe = 2.4, and one Nb IV (S = 1  2 ), assuming g Nb = 2.0.This was most probably caused by the presence of antiferromagnetic interactions within the Nb IV -CN-Fe II coordination framework.χT increased significantly as the temperature was lowered to reach a maximum value of 889 cm 3 •K•mol -1 at 45 K, confirming the long-range magnetic ordering.The magnetization vs. temperature M(T) curves recorded in the zfc-fc (zero field-cooled, field-cooled) modes at 0.3 mT for Fe2NbHCOO are presented in Figure 6.The most striking feature is the significant increase of the magnetization at 58 K, which constitutes the long-range magnetic ordering temperature Tc for this system.The critical temperature Tc was defined as the zfc-fc bifurcation point and was confirmed by the position of the maximum of the AC (alternating current) magnetic susceptibility signal χ' at 57.5 K (Figure 7).There was essentially no frequency dependence of the in-phase and out-of-phase AC susceptibilities at the magnetic ordering temperature, which The magnetization vs. temperature M(T) curves recorded in the zfc-fc (zero field-cooled, field-cooled) modes at 0.3 mT for Fe 2 NbHCOO are presented in Figure 6.The most striking feature is the significant increase of the magnetization at 58 K, which constitutes the long-range magnetic ordering temperature T c for this system.The critical temperature T c was defined as the zfc-fc bifurcation point and was confirmed by the position of the maximum of the AC (alternating current) magnetic susceptibility signal χ' at 57.5 K (Figure 7).There was essentially no frequency dependence of the in-phase and out-of-phase AC susceptibilities at the magnetic ordering temperature, which supports the claim that the reported compound is a magnetically ordered system.However, a slight frequency drift below 50 K indicates some dynamic processes taking place in the compound.Understanding this behavior will require further measurements using more advanced techniques (i.e., muon spin spectroscopy).The magnetization vs. temperature M(T) curves recorded in the zfc-fc (zero field-cooled, field-cooled) modes at 0.3 mT for Fe2NbHCOO are presented in Figure 6.The most striking feature is the significant increase of the magnetization at 58 K, which constitutes the long-range magnetic ordering temperature Tc for this system.The critical temperature Tc was defined as the zfc-fc bifurcation point and was confirmed by the position of the maximum of the AC (alternating current) magnetic susceptibility signal χ' at 57.5 K (Figure 7).There was essentially no frequency dependence of the in-phase and out-of-phase AC susceptibilities at the magnetic ordering temperature, which supports the claim that the reported compound is a magnetically ordered system.However, a slight frequency drift below 50 K indicates some dynamic processes taking place in the compound.Understanding this behavior will require further measurements using more advanced techniques (i.e., muon spin spectroscopy).The magnetic field dependence of the molar magnetization for Fe2NbHCOO (recorded up to 7 T at T = 2.0 K) is presented in Figure 8.The magnetization curve showed a fast increase to circa 4.5 Nβ at 0.2 T, followed by a further slower increase and a saturation value of ca.8.8 Nβ at 7.0 T. This was similar to the value obtained for the parent Fe2Nb [14], and close to the expected 8.6 Nβ for two high-spin Fe II (S = 2, gFe = 2.4) antiferromagnetically coupled with Nb IV ion (S = ½, gNb = 2.0).This suggests that Fe2NbHCOO exhibits long-range ferrimagnetic ordering.Fe2NbHCOO exhibited a magnetic hysteresis loop (Figure 8, inset) with the coercive field Hc = 102 mT and the remanence MR = 4.8 Nβ.Both values were much larger than the parent framework Fe2Nb (Hc = 13 mT and MR = 1.24Nβ).The significant increase of the magnetic ordering temperature and a larger coercive field/remanence of Fe2NbHCOO compared to the parent Fe2Nb were most likely caused by the inserted bridging formate connecting the neighboring Fe II ions.This formate bridge must promote The magnetic field dependence of the molar magnetization for Fe 2 NbHCOO (recorded up to 7 T at T = 2.0 K) is presented in Figure 8.The magnetization curve showed a fast increase to circa 4.5 Nβ at 0.2 T, followed by a further slower increase and a saturation value of ca.8.8 Nβ at 7.0 T. This was similar to the value obtained for the parent Fe 2 Nb [14], and close to the expected 8.6 Nβ for two high-spin Fe II (S = 2, g Fe = 2.4) antiferromagnetically coupled with Nb IV ion (S = 1  2 , g Nb = 2.0).This suggests that Fe 2 NbHCOO exhibits long-range ferrimagnetic ordering.Fe 2 NbHCOO exhibited a magnetic hysteresis loop (Figure 8, inset) with the coercive field H c = 102 mT and the remanence M R = 4.8 Nβ.Both values were much larger than the parent framework Fe 2 Nb (H c = 13 mT and M R = 1.24Nβ).The significant increase of the magnetic ordering temperature and a larger coercive field/remanence of Fe 2 NbHCOO compared to the parent Fe 2 Nb were most likely caused by the inserted bridging formate connecting the neighboring Fe II ions.This formate bridge must promote the Fe II ions magnetic anisotropy change and most probably additional local ferromagnetic interactions between them, which work in concert with the postulated ferrimagnetic structure of the CN-bridged framework (Figure 9).Nevertheless, an unlikely scenario where strong ferromagnetic ordering within the Nb IV -CN-Fe II framework [14] is accompanied by very weak antiferromagnetic interactions through Fe II -HCOO-Fe II pairs is also possible.Please note that the Mn II -based analog {(NH 4 )[(H 2 O)Mn II -(µ-OOCH)-Mn II (H 2 O)][Nb IV (CN) 8 ]•3H 2 O} n (Mn 2 NbHCOO) that we recently reported [15] showed a significant lowering of the T c from 49 K to 45 K compared to its {[Mn II (H 2 O) 2 ] 2 [Nb IV (CN) 8 ]•4H 2 O} n (Mn 2 Nb) parent framework, due to the antiferromagnetic interactions transmitted through the Mn II -HCOO-Mn II motifs.
The magnetic field dependence of the molar magnetization for Fe2NbHCOO (recorded up to 7 T at T = 2.0 K) is presented in Figure 8.The magnetization curve showed a fast increase to circa 4.5 Nβ at 0.2 T, followed by a further slower increase and a saturation value of ca.8.8 Nβ at 7.0 T. This was similar to the value obtained for the parent Fe2Nb [14], and close to the expected 8.6 Nβ for two high-spin Fe II (S = 2, gFe = 2.4) antiferromagnetically coupled with Nb IV ion (S = ½, gNb = 2.0).This suggests that Fe2NbHCOO exhibits long-range ferrimagnetic ordering.Fe2NbHCOO exhibited a magnetic hysteresis loop (Figure 8, inset) with the coercive field Hc = 102 mT and the remanence MR = 4.8 Nβ.Both values were much larger than the parent framework Fe2Nb (Hc = 13 mT and MR = 1.24Nβ).The significant increase of the magnetic ordering temperature and a larger coercive field/remanence of Fe2NbHCOO compared to the parent Fe2Nb were most likely caused by the inserted bridging formate connecting the neighboring Fe II ions.This formate bridge must promote the Fe II ions magnetic anisotropy change and most probably additional local ferromagnetic interactions between them, which work in concert with the postulated ferrimagnetic structure of the CN-bridged framework (Figure 9).Nevertheless, an unlikely scenario where strong ferromagnetic ordering within the Nb IV -CN-Fe II framework [14] is accompanied by very weak antiferromagnetic interactions through Fe II -HCOO-Fe II pairs is also possible.Please note that the Mn II -based analog {(NH4)[(H2O)Mn II -(μ-OOCH)-Mn II (H2O)][Nb IV (CN)8]•3H2O}n (Mn2NbHCOO) that we recently reported [15] showed a significant lowering of the Tc from 49 K to 45 K compared to its {[Mn II (H2O)2]2[Nb IV (CN)8]•4H2O}n (Mn2Nb) parent framework, due to the antiferromagnetic interactions transmitted through the Mn II -HCOO-Mn II motifs.

Materials
Chemicals of analytical grade were purchased from commercial sources (Sigma-Aldrich Co., St. Louis, MO, USA) and used as received.K4[Nb(CN)8]•2H2O was synthesized according to the last reported procedure [18].All operations were carried out in an ambient atmosphere.

Synthesis of {(NH4)[(H2O)Fe
A water (160 mL) solution of HCOONH4 (1.26 g, 20.0 mmol) with a small amount of ascorbic acid (ca.10-15 mg) was prepared.Half of the volume of this solution was used to dissolve

Materials
Chemicals of analytical grade were purchased from commercial sources (Sigma-Aldrich Co., St. Louis, MO, USA) and used as received.K 4 [Nb(CN) 8 ]•2H 2 O was synthesized according to the last reported procedure [18].All operations were carried out in an ambient atmosphere.

Synthesis of {(NH
A water (160 mL) solution of HCOONH 4 (1.26 g, 20.0 mmol) with a small amount of ascorbic acid (ca.10-15 mg) was prepared.Half of the volume of this solution was used to dissolve K 4 [Nb(CN) 8 ]•2H 2 O (149.1 mg, 0.30 mmol), and the other half was used to dissolve (NH 4 ) 2 Fe(SO 4 ) 2 •6H 2 O (235.3 mg, 0.60 mmol).After adding the greenish solution of Mohr's salt to the yellow solution of potassium octacyanidoniobiate(IV), the resulting orange mixture was left for one day for crystallization.Dark purple crystals (60.2 mg, 35%) of Fe 2 NbOOCH were isolated by repeated decantation with distilled water followed by ethanol (92%), and were dried for a short while in air (the compound is slightly sensitive to oxygen and needs to be stored at low temperature).We found C, 18

Single-Crystal X-ray Diffraction
The single crystal diffraction data for Fe 2 NbHCOO was collected on a Bruker D8 Quest Eco Photon50 CMOS machine equipped with a Mo Kα radiation source and a graphite monochromator (λ = 0.71073 Å).Measurements were performed at ambient temperature (details in Table 1).Data reduction and unit cell determination were carried out using SAINT and SADABS (Apex3 package).Absorption correction using the multi-scan method was applied for all reflection intensities.The structure was solved using direct methods (Apex3 package).Non-hydrogen atoms were refined anisotropically using weighted full-matrix least-squares on F 2 [19,20].The crystallographic data is summarized in Table 1.CCDC 1859396 (Fe 2 NbHCOO) contains the supplementary crystallographic data for this paper.This data can be obtained free of charge from the Cambridge Crystallographic Data Centre via ww.ccdc.cam.ac.uk/data_request/cif.

Magnetic Measurements
Magnetic measurements were performed using a Quantum Design MPMS-3 Evercool magnetometer equipped with a 7 T superconducting magnet.The sample was loaded into a double polypropylene bag and sealed.The magnetic susceptibility was corrected for the diamagnetic contribution of the sample holder and the diamagnetism of the samples themselves using Pascal constants.

Other Physical Measurements and Calculations
Infrared spectra were collected on Nicolet iS 5 FT-IR spectrometer (Thermo Fisher Scientific, Wltham, MA, USA) in the range 4000-650 cm −1 .Powder X-ray diffraction experiments (PXRD) were carried out using PANalytical X'Pert Pro MPD diffractometer (Cu Kα radiation, Malvern PANalytical, Royston, UK) at ambient temperature for dry well-ground samples loaded into a narrow diameter borosilicate-glass capillary (0.7 mm).Elemental analysis was performed using an ELEMENTAR Vario Micro Cube CHNS analyzer (Elementar, Langenselbold, Germany).Continuous shape measure analysis for coordination spheres of Nb IV and Fe II was performed using the SHAPE software [21].The results are summarized in Table 3.

Conclusions
A modification of the structure and magnetic properties of a parent coordination polymer Fe 2 Nb was performed at the self-assembly stage by "forcing" an additional bridging formate anion into its structure.The formate anion formed direct coordination connections with two adjacent Fe II centers within the CN-bridged Fe II -Nb IV framework.The presence of this additional molecular bridge promoting ferromagnetic interactions between iron(II) centers reinforced the ferrimagnetic

Figure 1 .
Figure 1.Structural diagrams showing the potential "cavity" between adjacent Fe II ions in the parent framework Fe2Nb (a), the additional formate bridge occupying the "cavity" in Fe2NbHCOO (b), a slice of Fe2NbHCOO crystal packing parallel to the ab crystallographic plane (c), and a packing diagram presenting the three-dimensional (3D) CN-bridged coordination framework (red) cross-linked by local formate bridges (green) (d).Note: Nb-cyan, Fe-yellow, C-gray, N-blue, and Oformate-red.Hydrogen and oxygen atoms of water molecules have been omitted for the sake of clarity.

Figure 1 .
Figure 1.Structural diagrams showing the potential "cavity" between adjacent Fe II ions in the parent framework Fe 2 Nb (a), the additional formate bridge occupying the "cavity" in Fe 2 NbHCOO (b), a slice of Fe 2 NbHCOO crystal packing parallel to the ab crystallographic plane (c), and a packing diagram presenting the three-dimensional (3D) CN-bridged coordination framework (red) cross-linked by local formate bridges (green) (d).Note: Nb-cyan, Fe-yellow, C-gray, N-blue, and O formate -red.Hydrogen and oxygen atoms of water molecules have been omitted for the sake of clarity.

Figure 4 .
Figure 4. Experimental (red) and simulated from the single-crystal structural model (blue) powder X-ray diffraction patterns for Fe2NbHCOO (the broad peak at 8° is from the glass capillary-the sample holder).

Figure 4 .
Figure 4. Experimental (red) and simulated from the single-crystal structural model (blue) powder X-ray diffraction patterns for Fe 2 NbHCOO (the broad peak at 8 • is from the glass capillary-the sample holder).

Magnetochemistry 2018, 4 ,Figure 5 .
Figure 5. Temperature dependence of the molar magnetic susceptibility and temperature product for Fe2NbHCOO at H = 0.1 T.

Figure 5 .
Figure 5. Temperature dependence of the molar magnetic susceptibility and temperature product for Fe 2 NbHCOO at H = 0.1 T.

Figure 5 .
Figure 5. Temperature dependence of the molar magnetic susceptibility and temperature product for Fe2NbHCOO at H = 0.1 T.

Figure 6 .
Figure 6.Temperature dependence of the molar magnetization measured in the zfc-fc (red and black points, respectively) modes for Fe2NbHCOO.

Figure 6 . 10 Figure 7 .
Figure 6.Temperature dependence of the molar magnetization measured in the zfc-fc (red and black points, respectively) modes for Fe 2 NbHCOO.Magnetochemistry 2018, 4, x FOR PEER REVIEW 7 of 10

Figure 7 .
Figure 7. Temperature dependence of the in-phase and out-of-phase AC magnetic susceptibility of Fe 2 NbHCOO at H AC = 0.1 mT and at three different frequencies (7, 70 and 700 Hz).

Figure 8 .
Figure 8.The magnetic field dependence of the molar magnetization of Fe2NbHCOO recorded at 2.0 K in the 0-7 T range and the magnetic hysteresis loop at the same temperature (inset).

Figure 8 . 10 Figure 9 .
Figure 8.The magnetic field dependence of the molar magnetization of Fe 2 NbHCOO recorded at 2.0 K in the 0-7 T range and the magnetic hysteresis loop at the same temperature (inset).Magnetochemistry 2018, 4, x FOR PEER REVIEW 8 of 10

Figure 9 .
Figure 9. Schematic representation of the postulated ferrimagnetic structure of Fe 2 NbHCOO with an antiparallel alignment of the Nb IV and Fe II magnetic moments.The ferromagnetic interactions transmitted through the newly introduced Fe II -HCOO-Fe II structural motifs work in concert with the antiferromagnetic coupling within the -Nb IV -CN-Fe II -3D skeleton.

Table 1 .
Sc-XRD structural, solution and refinement parameters for

Table 2 .
Comparison of selected distances (Å) and angles ( • ) in the parent framework and modified Fe 2 NbHCOO.Nb IV (CN) 8 ] 4− was connected to eight nearly octahedral [Fe II (CN) 4 (H 2 O)L] moieties (L = H 2 O for Fe 2 Nb and L = formate for Fe 2 NbHCOO).The coordination geometry of Nb IV in Fe 2 In both Fe 2 NbHCOO and Fe 2 Nb, each [

Table 3 .
Analysis of the coordination spheres of Fe II and Nb IV ions in Fe