Tunable Assembly and Magnetic Interactions in Two Cyano-bridged FeIII–CoII Bimetallic Complexes

Using a new semi-rigid tetradentate ligand and two cyanometalate building blocks with different steric hindrance, a Fe2Co triangular unit [(Tp)FeIII(CN)3]2CoII(bpmb)·2H2O [1; Tp = hydrotris(pyrazolyl)borate; bpmb = 1,2-bis(3-(2-pyridyl)pyrazol-1-ylmethyl)benzenes] and a Fe2Co2 square unit {[(pzTp)FeIII(CN)3]2[CoII(bpmb)]2}·[(pzTp)FeIII(CN)3]2·4H2O [2; pzTp = tetrakis(pyrazolyl)borate] were synthesized via tunable assembly. Both compounds adopt cis arrangement because of the structural distorted semi-rigid bpmb. Tetranuclear square of 2 was formed due to larger steric hindrance building block of [(pzTp)FeIII(CN)3]− compared with [(Tp)FeIII(CN)3]− in trinculear 1. Magnetic measurements show that antiferromagnetic interactions dominate in 1 and 2. The fitting results of J values suggest a positive correlation between magnetic orbitals overlap of FeIII and CoII ions and Co–N≡C bond angles. Two cis arrangement cyanometalate [Fe-Co] clusters were synthesized via tunable assembly with a new semi-rigid tetradentate ligand. The tetranuclear cluster was formed because of larger steric hindrance of pzTp. Both compounds exhibit dominant antiferromagnetic couplings. The fitting results of J values suggest a positive correlation between magnetic orbitals overlap of FeIII and CoII ions and Co–N≡C bond angles.


Introduction
Rational design molecules assembly has always been a hot topic in coordination chemistry. In the research of molecule-based magnetic materials, cyano-bridged 1 3 bimetallic assemblies have attracted considerable attention due to their predictable structures and fascinating magnetic properties [1][2][3][4]. In the meanwhile, it becomes an efficient way to control the nuclearity of complexes via introducing rigid ligands to occupy active sites of the metal centers in the heterobimetallic system [5][6][7][8][9]. Using this strategy, lots of compounds with cis or trans arrangement have been prepared, such as cis mode of trinuclear complexes [10] and trans mode of 1D chains [11]. In our previous work, the cis or trans arrangements of Fe 2 Cu complexes have been successfully controlled with different steric hindrance building blocks, wherein ferromagnetic and antiferromagnetic interactions dominate respectively [12]. We intend to control the nuclearity via tune the steric effect. Herein, a semi-rigid tetradentate ligand bpmb (1,2-bis(3-(2-pyridyl)pyrazol-1-ylmethyl)benzenes) was adopted to occupy the four coordination sites of the Co II center with two sites in cis position be vacant. The remained two cis sites were further linked by two building blocks with different steric effect. One trinuclear complex was obtained for building blocks with smaller steric hindrance [(Tp)Fe III (CN) 3 ] − (Tp = hydrotris(pyrazolyl) borate), whereas one tetranuclear compound was obtained for building block with larger steric hindrance effect [(pzTp)Fe III (CN) 3 ] − (pzTp = tetrakis(pyrazolyl)borate). Both complexes were characterized by the spectroscopic, structural, and magnetic measurements. Our results provide an effective strategy to control the nuclearity and magnetic properties utilizing steric effect.

Chemicals and Measurements
Unless the otherwise specified, materials were acquired from the suppliers and could be used with no in-depth purification. 1,2-bis(3-(2-pyridyl)pyrazol-1-ylmethyl) benzenes was synthesized according to method reported previously in the reference.

Description of Crystal Structures
Single-crystal X-ray diffraction analysis collected at 296 K reveals that 1 and 2 both crystallize in a monoclinic P2 1 /c space group. The crystallographic data are presented in Table.S1. The selected bonds and angles are listed in Table. S2 (for compound 1) and Table. S3 (for compound 2). As shown in Fig. 1a, the structure has a cyanide-bridged cis trinuclear core. Two crystallographically independent [(Tp)Fe III (CN) 3 ] − units are linked to [Co II (bpmb)] 2+ unit via cyanide bridges, forming a cis neutral [(Tp) Fe III (CN) 3 ] 2 Co II (bpmb) trinulcear. Uncoordinated water molecules are located between the complexes. In the [(Tp) Fe III (CN) 3 ] − units, each Fe center adopts a distorted octahedral geometry with a C 3v symmetry. Coordination bond lengths of Fe1-C are in the range of 1.918(6)-1.928(7) Å, while those of Fe1-N are 1.976(5)-1.981(5) Å, respectively. The coordination environment of Fe2 ion is very similar to that of Fe1 center, and coordination bond lengths are 1.908(8)-1.916(6) Å and 1.969(5)-1.981(5) Å for Fe2-C and Fe2-N, respectively, in good agreement with those observed in other low-spin Fe (III) compounds [13,14]. The Fe-C≡N linkages are close to linearity with bond angles of 173.4-177.4°. The Cobalt ion in [Co II (bpmb)] 2+ unit is located in the octahedral environment with four nitrogen atoms from tetradentate bpmb and two cyanide nitrogen atoms from the bridging cyanide ions. The Co-N cyanide bonds [2.083(5)-2.095(4) Å] are slightly shorter than the bpmb ones, which range from 2.116 (4)

Magnetic Properties
Temperature-dependent magnetic susceptibilities for 1 and 2 were measured between 300 and 2 K under a dc field of 1000 Oe. The resulting plots of χT versus T for 1 were given in Fig. 2a. As shown in the figure, the χT value is 4.40 cm 3 mol −1 K at room temperature, which is obviously larger than the spin-only value of χT = 2.9 cm 3 mol −1 K for the two magnetically isolated low-spin Fe III (S = 1/2) and one magnetically isolated high-spin Co II (S = 3/2) assuming g Fe = g Co = 2.0. The larger value probably due to the orbital contributions of Co center and afford g values to deviate significantly from 2.0 (normally ca. 2.5) [16]. Later it decreased gradually with lowering temperature and then abruptly dropped to 1.49 cm 3 mol −1 K at T = 2 K. Such a magnetic behavior indicates the dominant antiferromagnetic couplings between the Fe III and Co II ions, which could be observed in other complexes exhibiting similar antiferromagnetic behaviors [17]. The Curie-Weiss law [ χT = C/ (Tθ)] is applied in the temperature range of 2-300 K to afford the Curie constant C = 4.70 cm 3 mol −1 K and Weiss temperature θ = -25.71 K. The negative Weiss temperature further suggests that antiferromagnetic interactions are dominant in the trinuclear complex. According to the distorted trinuclear structure, the magnetic data have been simulated with the MAGPACK program [18] based on an exchange Hamiltonian H = -2J 1 ·S Fe1 ·S Co1 -2J 2 ·S Fe2 ·S Co1 , where J 1 and J 2 represent the exchange magnetic coupling constants between Fe III ions and Co II ions through the cyano-bridges. The best fitting results above 48 K give the J 1 = -10.5 cm -1 , J 2 = -8.4 cm -1 , g = 2.64 with the R = 1.37 × 10 -4 , which is consistent with other Fe III -Co II assemblies based on cyanometalate precursors [19]. The negative J value confirms the antiferromagnetic couplings dominate between Fe III and Co II ions. The M versus H dc data measured at 1.8 K shown in Figure. S4. As the applied magnetic field increased, the magnetization increased steeply and reached to 1.3 Nβ at 50 kOe. Ac susceptibility measurements were performed at various frequencies ( Figure.S5) but no slow relaxation of the magnetization was detected above 2 K, leading to a fast quantum tunneling which may be due to the small energy gap between the ground and excited states [20].
The χT vs T data for 2 show the similar magnetic behavior and also suggest the antiferromagnetic couplings with 1. At 300 K, the χT value is 7.70 cm 3 mol −1 K, corresponding to four low-spin Fe III ions and two high-spin Co II ions with g = 2.31. Firstly this curve gradually decreased to 6.92  for 1 (a) and 2 (b). The red lines represent the best fit of the experimental results (Color figure online) cm 3 mol −1 K at 65 K, then it sharply dropt and reached the minimum value of 4.11 cm 3 mol −1 K at 2 K, indicting the dominant antiferromagnetic behavior. The Curie-Weiss law is also applied in the temperature range of 2-300 K to afford the Curie constant C = 7.93 cm 3 mol −1 K and Weiss temperature θ = -8.72 K. The negative Weiss temperature further suggests the dominant antiferromagnetic interactions exist in the tetranuclear complex. The fitting results above 25 K of the magnetic data for 2 are similar with 1 and the best-fit parameters for χT vs T are J = -10.5 cm -1 , g = 2.44 and R = 3.8 × 10 -5 . The large J value indicates the dominant antiferromagnetic interactions between Fe III and Co II ions. Further confirmation of the dominant antiferromagnetic couplings is obtained in the field-dependent magnetization at 1.8 K (Figure. S6) because the magnetization is 4.7 Nβ at 50 kOe, far from the saturated value of 6 Nβ (g = 2.0). No out-of-phase signal was observed in ac susceptibility studies above 2 K ( Figure. S7), indicating the absence of SMM properties.
Both compounds exhibit cis arrangements due to the fact that large steric exclusion in the distorted [Co II (bpmb)] 2+ unit prevents cyanide nitrogen atoms bridging the Co II center from opposite direction but with an approximate right angle. However, they still form structures with different nuclearities because of distinct steric hindrance of building blocks. In comparison, the Co-N≡C angle of compound 2 is larger than 1, the longer intramolecular Fe···Co and Fe···Fe distances lead to greater interspace between Fe centers, which is beneficial to the formation of square unit. On the other hand, the magnetic simulations were performed to discuss the correlations between structure and magnetic properties. As judged from the magnetic simulation data, the J value in compound 2 is larger than 1, which may be due to more linear Co-N≡C angle. It seems that the overlap degree of the magnetic orbitals between Fe III and Co II ions is positively relative to Co-N≡C bond angles. The larger orbitals overlap will result in the stronger Fe III -CN-Co II magnetic interactions [21].

Conclusions
In summary, a cyano-bridged Fe 2 Co triangular unit 1 and a Fe 2 Co 2 square unit 2 were successfully synthesized via tunable assembly. The semi-rigid ligand bpmb adopts a distorted coordination configuration with Co center, leading to the cis arrangement of two compounds. At the same time, the larger steric exclusion of [(pzTp)Fe III (CN) 3 ] − increases the Fe1···Co1···Fe2 angle, forming tetranuclear square of 2. The magnetic-structural correlations indicate a positive correlation exists between J values and the Co-N≡C bond angles. This strategy provides a feasible way in the tunable assembly process.
Acknowledgements This work was partly supported by the NSFC (No. 21701167) China.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.