Cyanide-Bridged Polynuclear and One-Dimensional FeIII-MnIII/II Bimetallic Complexes Based-on Pentacyanoferrite(III) Building Block: Synthesis, Crystal Structures, and Magnetic Properties

In this contribution, based-on the structurally confirmed pentacyanometallate (PPh4)2[Fe(CN)5(imidazole)]·(imidazole)·H2O (1) and the manganese compounds [Mn(L)(H2O)2]ClO4 (L = N,N-ethylenebis(3-methoxysalicylideneiminate) or [Mn(MAC)(H2O)Cl]ClO4 (MAC = 2,13-dimethyl-3,6,9,12,18-pentaazabicyclo-[12.3.1]octadeca-1(18),2,12,14,16pentaene), two new cyanide-bridged bimetallic FeIII-MnIII/II complexes {[Mn(L)(H2O)]3[Fe(CN)5(imidazole)]}(ClO4) (2) and {[Mn(MAC)][Fe(CN)5(imidazole)]·CH3OH}n (3) were successfully synthesized and characterized by elemental analysis, IR spectroscopy and X-ray structure determination. Single X-ray diffraction analysis reveals the cationic FeMn3 tetranuclear entity for complex 2, which can be further assembled into supramolecular 1D ladder-like double chain by the strong intermolecular hydrogen bond interactions. For complex 3, it can be structurally characterized as neutral one-dimensional linear single infinite chain. The magnetic investigations discover the ferromagnetic coupling between the FeIII-MnIII units in complex 2 and the antiferromagnetic coupling in complex 3 between the FeIII-MnII units through the bridging cyanide group, respectively.


Introduction
In the past several decades, due to their great potential in high-tech fields including quantum compute, information storage, etc., more and more attention have been paid to the research of the molecular-based magnetism. [1][2][3][4][5][6][7][8] As one of the most important building block for the rational construction of molecular magnetic systems, cyanide-bridged magnetic complexes have all along received intense attention since their readily controlled molecular topological structures and theoretically predicted magnetic properties. [9][10][11][12][13] Up to now, a great deal of cyanide-bridged molecular materials structural ranging from 0-dimensional cluster to three-dimensional beautiful networks have been rationally designed and structurally characterized.

1. General Procedures and Materials
All the reactions were carried out at room temperature under air atmosphere with the solvents and chemicals used reagent grade without additional purification.  (20 ml), and PPh 4 Br (4.0 g, 10 mmol) was added to the solution. Then, the mixture was stirred in the dark overnight before the yellow powder formed was filtered out. Yield: 3.57g (70%). Recrystallization of the powder in MeOH afforded yellow single crystals. Main IR bands(cm -1 ): 2110, 2121(s, ν C≡N ). The elemental analysis (experimental and theoretical) and some physical properties are given in Table 1.
The starting materials used in this paper.

X-ray Data Collection and Structure Refinement
Single crystals with suitable dimensions for complexes 1-3 for X-ray diffraction analysis were mounted on the glass rod and the crystal data were collected on a Bruker APEX II CCD area-detector with a Mo Kα sealed tube (λ = 0.71073 Å) at room temperature using a ω scan mode. The structures were solved by direct method and expanded using Fourier difference techniques with the SHELX-TL-2018/3 program package. 45 While all the hydrogen atoms were introduced as fixed contributors, the non-hydrogen atoms were refined anisotropically with anisotropic displacement coefficients. Hydrogen atoms except some ones from the solvent molecules were assigned isotropic displacement coefficients U(H) = 1.2U(C) or 1.5U (C) and their coordinates were allowed to ride on their respective carbon/nitrogen atoms using SHELXL 2018/3. For the solvent H atoms, they were refined isotropically with fixed U values, during which the DFIX command was used to rationalize the bond parameter. The CCDC 1978692-1978694 for complexes 1-3 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. Details of the crystallographic parameter, data collection, and refinement are summarized in Table 2.

1. Synthesis
With comparison to the widely used hexacyanoiron(III) in cyanide-bridged molecular magnetism field, the pentacyanoiron(III) has been relatively less used to assemble magnetic complex, and only several pentacyanoiron(III)-based heterometallic magnetic systems have been reported. [39][40][41][42]45 By using the three-layers diffusion method, which has been shown to be a powerful way for growing single crystals, 43 The cyanide-bridged heterometallic complexes have been characterized by IR spectroscopy. Compared to the cyanide precursor with only one peak at about 2125 cm -1 , two sharp peaks in the IR spectra of complexes 2 and 3 due to the cyanide-stretching vibration [42,43] were observed at about 2120 and 2160 cm -1 , respectively, indicating the presence of bridging and nonbridging cyanide ligands in these complexes. For complex 2, the strong peak centered at 1100 cm -1 is attributed to the free ClO 4 anions.

2. Crystal Structure Descriptions
The crystal tructure of the cyano precursor: The selected bond lengths and angles for complex 1 are given in Table 3. The molecular structure and the H-bond resulted 1D supramolecular structure are shown in Scheme 2. The preparation diagram for the complex 2 and 3. Table 3. Selected bond lengths (Å) and angles ( o ) for complex 1.

The Crystal Structure of the Complex 2
The cationic tetranuclear structure and the one-dimensional ladder-like double-chain structure formed by the intermolecular hydrogen bonds for complex 2 are shown in Figures 3 and 4, respectively. The selected important bond parameters for complex 2 are listed in Table 5.
The complex 2 is a neutral tetranuclear cluster comprised by the cationic FeMn 3 unit and an additional disordered

4. The Crystal Structure of the Complex 3
The asymmetry binuclear unit and 1D neutral chain structure of complex 3 is presented in Figure 5. The important bond parameters are given in Table 5. As can be found, complex 3 possesses one-dimensional neutral single chain structure comprising of the repeating [-NC-Fe(imidazole)(CN) 3 -CN-Mn(MAC)-] units. In this complex, each [Fe(imidazole)(CN) 5 ] 2unit, functioning as bidentate ligand through its two cyanide groups in trans position, connects the Mn(II) ions of two independent macrocyclic manganese units. Similar to that in compounds 1 and 2, the coordination geometry of the Fe atom is also a slightly distorted octahedral. As listed in Table 5

5. The Magnetic Properties of Complexes 2 and 3
The temperature dependences of the c m T product per Fe III Mn III 3 and Fe III Mn II unit for 2 and 3 measured from 2 to 300 K under an applied magnetic field of 2000 Oe are shown in Figures 6 and 7. The c m T value at room temperature is 9.11 emu K mol -1 (µ eff = 8.57 BM) for 2 and 4.33 emu K mol -1 (µ eff = 5.91 BM) for 3, which are slightly lower than the spin only value of 9.375/4.75 emu K mol -1 for uncoupled three high spin Mn(III) (S = 2)/one high spin Mn(II) ( S = 5/2) and one low spin Fe(III) (S = 1/2) based on g = 2.00, respectively. For these two complexes, the c m T values maintains nearly constant until the temperature lowering to 20 K for 2 and 50 K for 3. After that, the c m T value for complex 2 starts to increase smoothly and reaches its highest value 10.12 emu K mol -1 , and then decreases rapidly to the lowest value about 0.87 emu K mol -1 at 2 K. The suddenly decrease for the c m T value in the low temperature range can maybe attributed to the comparable strong intermolecular H-bond interaction and/or the zero field spitting of the Mn(III) ion. Different from that for complex 2, the c m T value of complex 3 presents the obvious decreasing tendency from 50 K, and attains its minimum value 2.19 emu K mol -1 at 2 K, implying the different coupling nature in these two complexes. The  (1) interactions through cyanide bridges, respectively. On the other hand, according to the method successfully employed to simu-late the magnetic susceptibilities of 1D chain compound with alternating spins 1/2 and 2, 30b the one-dimensional chain structure of the complex 3 can be considered as isotropic Heisenberg chain containing alternating spins 1/2 and 5/2 with two antiferromagnetic exchange interactions J 1 and J 2 . In this case, the magnetic susceptibility of this complex can be calculated rationally based on a closed ring cluster model consisting of five 1/2-5/2 spin pairs,

Scheme 4.
Evaluation of the exchange coupling between the iron(III) ion and manganese(III/II) ions bridged by cyanide group in complexes 2 and 3 are carried out by MAG-PACK program. 47 The best-fit parameters obtained are J 1 = 1.12, J 2 = 1.65, J 3 = 0.91 cm -1 , D = -1.81 cm -1 , g = 2.02, R = Σ(c obsd T-c cald T) 2 /Σ(c obsd T) 2 = 1.61 × 10 -5 for complex 2 and J 1 = -1.34, J 2 = -0.54(1) cm -1 , g = 1.99, R = 3.01 × 10 -5 for complex 3, respectively. All the theoretical fitting results are comparable to those found in the previously reported cyanide-bridged Fe III -Mn III/II complexes. [42][43][44]48 The field-dependent magnetizations measured up to 50 kOe at 2 K for complexes 2 and 3 are shown in Figure 6 and the inset of Figure 7, respectively. The field-dependent magnetization curve for complex 2 has a sigmoid shape, implying maybe the metamagnetic behavior: The magnetization first increases slowly with increasing magnetic field until 20 kOe because of the relatively strong intermolecular hydrogen bond interaction, then increases abruptly for a phase transition at about 20 kOe, and finally attains the highest value about 7.45 Nb , which is slightly higher than the saturated value for three Mn(III) ion (S = 2) and one low spin Fe(III) ion (S = 1/2). For complex 3, the magnetization quickly increases with the field increasing until about 15 kOe, then increases smoothly up to about 3.9 Nb until 50 kOe. This data is very close to the saturated value of 4.0 Nb but obviously lower than the value of uncoupled low spin Fe(III) and Mn(II) based on g = 2.0, confirming again the overall antiferromagnetic coupling interaction between Fe(III) and Mn(II) ions bridged by cyanide group.

Conclusion
In summary, two new heterobimetallic cyanide-bridged complexes have been prepared with pentacyanoiron(III) as building block and manganese(III/II) compounds as the counterpart assemble segment. The single crystal X-ray analysis revealed the cationic tetranuclear FeMn 3 entity or one-dimensional infinite chain structure, respectively. For the polynuclear cluster, it can be self-complementary through coordinated aqua ligand from one complex and the free O 4 compartment from the neighboring complex, therefore giving interesting supramolecular one-dimensional ladders. The experimental and theoretical investigation on their magnetic properties disclose the ferro-or antiferromagnetic coupling in cyanide-bridged Fe III -Mn III or Fe III -Mn II units, respectively. The present results can further enrich the pentacyanometallate-based molecule magnetic system, which is helpful for fully discover the magneto-structural relation from the molecule magnetism.