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

Abstract

A three-dimensional cyanide-bridged coordination polymer based on Fe^II (S = 2) and Nb^IV (S = 1/2) {[Fe^II(H₂O)₂]₂[Nb^IV(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^II-(μ-HCOO)-Fe^II(H₂O)][Nb^IV(CN)₈]·3H₂O}_n (Fe₂NbHCOO) exhibited additional Fe^II-HCOO-Fe^II structural motifs connecting each of the two Fe^II 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 Fe^II 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.

1. 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).

$$T_c=\frac{2\sqrt{n_A{n}_B}\left|J_{AB}\right|\sqrt{S_A(S_A+1)S_B(S_B+1)}}{3k_B},$$

(1)

The aim of our research was to modify the well-known compound {[Fe^II(H₂O)₂]₂[Nb^IV(CN)₈]·4H₂O}_n (Fe₂Nb) [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(H₂O)₂]₂[Nb^IV(CN)₈]·4H₂O}_n, described previously [15]. The insertion of formate into the newly obtained mixed-bridged compound {(NH₄)[(H₂O)Fe^II-(μ-HCOO)-Fe^II(H₂O)][Nb^IV(CN)₈]·3H₂O}_n (Fe₂NbHCOO) led to a very significant increase of the critical temperature T_c from 43 K (Fe₂Nb parent framework) to 58 K, and a larger coercive field.

2. Results and Discussion

2.1. Synthesis and X-ray Crystal Structure Description

The insertion of additional formate anions into the three-dimensional (3D), CN-bridged parent framework of Fe₂Nb at the self-assembly stage resulted in the formation of Fe₂NbHCOO 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. Sc-XRD structural, solution and refinement parameters for Fe₂NbHCOO.

Formula	C9HFe2N9NbO7
Temperature, K	296(2)
λ, Å	0.71073 Å
Molecular weight, g/mol	551.80
Crystallographic system	tetragonal
Space group	I4cm
Unit cell	a = 11.8782(12) Å b = 13.2770(14) Å
Volume V, Å3	1873.3(4)
Z	4
Density ρcalc, g/cm3	1.957
F (000)	1068
θ, deg	3.4–28.0
Abs. coeff. µ, mm−1	2.18
Data/parameters /restrains	1211/81/8
R [F2 > 2σ(F2)]	0.032
wR (F2)	0.064
GOF on F2	1.103
Flack parameter	0.472(25)
max/min resid. density, e· Å−3	1.13/−0.59
Reflections collected	13274
Unique reflections	1211
Rint	0.048
Completeness, %	99.6

), infrared spectroscopy (IR), elemental analysis (EA), and powder X-ray diffraction (PXRD).

Fe₂NbHCOO is isostructural with its Mn^II analogue {(NH₄)[(H₂O)Mn^II-(μ-HCOO)-Mn^II(H₂O)][Nb^IV(CN)₈]·3H₂O}_n (Mn₂NbHCOO) [15]. The coordination framework of Fe₂NbHCOO becomes anionic compared to Fe₂Nb, 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 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. Comparison of selected distances (Å) and angles (°) in the parent framework and modified Fe₂NbHCOO.

Atom Names	Fe2Nb	Fe2NbHCOO
Nb···Fe	5.409 5.508	5.461 5.480
Nb···Feav	5.459	5.471
Fe···Fe	6.609(1)	6.141(1)
Fe-NCN	2.144(2) 2.175(2)	2.119(10) 2.171(10)
Fe-N-C	154.5(2) 167.5(2)	157.8(11) 170.0(9)
Nb-C-N	174.6(2) 176.7(2)	173.0(10) 178.4(11)
Fe-Oaq	2.099(3) 2.130(3)	2.191(9)
Fe-OHCOO	-	1.977(9)

. The cyanide bridges in Fe₂NbHCOO were slightly less bent compared to Fe₂Nb, while other structural parameters were quite similar (except for the aforementioned Fe···Fe distances).

In both Fe₂NbHCOO and Fe₂Nb, each [Nb^IV(CN)₈]⁴⁻ was connected to eight nearly octahedral [Fe^II(CN)₄(H₂O)L] moieties (L = H₂O for Fe₂Nb and L = formate for Fe₂NbHCOO). The coordination geometry of Nb^IV in Fe₂NbHCOO was slightly closer to an ideal square antiprism than in Fe₂Nb, 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 Fe₂NbHCOO due to the strain imposed by the bridging formate anion (Figure 1a,b and

Table 3. Analysis of the coordination spheres of Fe^II and Nb^IV ions in Fe₂Nb and Fe₂NbHCOO.

NbIV	Square antiprism (SAPR-8)	0.192	0.113
FeII	Octahedron (OC-6)	0.161	0.562

), which might lead to a completely different magnetic anisotropy.

The insertion of the additional formate bridge into the parent Fe₂Nb framework changed the symmetry from a centrosymmetric I4/m to a non-centrosymmetric I4cm. The structure of Fe₂NbHCOO 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.

2.2. 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).

2.3. 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.

2.4. 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 cm³·K·mol^‒1 was slightly lower than the expected 9.02 cm³·K·mol^‒1 for two Fe^II (S = 2), assuming g_Fe = 2.4, and one Nb^IV (S = ½), 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 cm³·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 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 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, g_Fe = 2.4) antiferromagnetically coupled with Nb^IV ion (S = ½, g_Nb = 2.0). This suggests that Fe₂NbHCOO exhibits long-range ferrimagnetic ordering. Fe₂NbHCOO 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 Fe₂Nb (H_c = 13 mT and M_R = 1.24 Nβ). 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 T_c 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.

3. Materials and Methods

3.1. 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.

3.2. Synthesis of {(NH₄)[(H₂O)Fe^II-(µ-HCOO)-Fe^II(H₂O)][Nb^IV(CN)₈]·3H₂O}_n (Fe₂NbHCOO)

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 K₄[Nb(CN)₈]∙2H₂O (149.1 mg, 0.30 mmol), and the other half was used to dissolve (NH₄)₂Fe(SO₄)₂∙6H₂O (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 Fe₂NbOOCH 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, 18.81; H, 2.59; N, 21.40. C₉H₁₅Fe₂N₉NbO7 (565.87 g/mol) requires C, 19.10; H, 2.67; N, 22.28%. Slightly lower N content is caused by the air-sensitivity of the compound. IR (ν_max/cm⁻¹): 3576 s ν(OH), 3261 s ν(NH), 2141 s ν(CN), 1635 m δ(OH), 1584 vs ν(COO), 1403 ν(NH₄⁺), 1383 m and 1365 ν(COO), and 812 ν(COO).

3.3. Single-Crystal X-ray Diffraction

The single crystal diffraction data for Fe₂NbHCOO 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 F² [19,20]. The crystallographic data is summarized in Table 1. CCDC 1859396 (Fe₂NbHCOO) 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.

3.4. 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.

3.5. 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.

4. Conclusions

A modification of the structure and magnetic properties of a parent coordination polymer Fe₂Nb 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 structure of the Fe^II-Nb^IV framework, and made Fe₂NbHCOO a much better magnet than the parent Fe₂Nb. Our approach demonstrates a new efficient route towards molecular magnets with higher critical temperatures.