View PDF Version
 
DOI: 10.1039/D5DT01007A (Communication) Dalton Trans., 2025, Advance Article

A strongly hydrogen-bonded one-dimensional high-spin dinuclear Fe(II) complex†

Hoa Phan*a, Kieu Thuy Thi Thaia, Nobuto Funakoshib, Huyen Thu Thi Trana, Masahiro Yamashitabc and Michael Shatruk*de
aSchool of Chemistry and Life Sciences, Hanoi University of Science and Technology, 01 Dai Co Viet, Hanoi, Vietnam. E-mail: hoa.phanvan@hust.edu.vn
bInstitute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-Ku, Sendai 980-8577, Japan
cSchool of Chemical Science and Engineering, Tongji University, Siping Road 1239, Shanghai 200092, China
dDepartmenet of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, FL 32306, USA. E-mail: shatruk@chem.fsu.edu
eNational High Magnetic Field Laboratory, 1800 E Paul Dirac Dr, Tallahassee, FL 32310, USA

Received 29th April 2025 , Accepted 24th June 2025

First published on 25th June 2025

Abstract

In this study, we report the synthesis and detailed characterization of a novel chain compound, composed of hydrogen-bonded dinuclear complexes, in which two Fe(II) ions are bridged by a 2,2′-biimidazolate (bim2−) dianion. The crystal structure of [(tpma)Fe(μ-bim)Fe(Hbim)2] (1), where tpma = tris(2-pyridylmethyl)amine, exhibits a 1D zigzag chain architecture, formed through double hydrogen bonds between terminal 1H-2,2′-biimidazolate monoanions (Hbim−), with an N⋯H distance of 1.72 Å. Magnetic susceptibility measurements reveal weak antiferromagnetic coupling between the Fe(II) centers with the exchange constant J = −1.4 cm−1, mediated by the bridging ligand. The directional hydrogen bonding network, combined with π–π and H–π intermolecular interactions, suggests potential for proton dynamics that could lead to ferroelectric behavior. This complex represents the first example of a bim2− ligand bridging two iron ions and the first dimeric complex containing both bim2− and Hbim− ligands, which form strong one-dimensional hydrogen bonds. Investigation of ferroelectric behavior and potential interplay between functional properties are ongoing.

The design and synthesis of polynuclear coordination complexes continue to be a vibrant area of research in modern coordination chemistry, owing to their rich structural diversity and wide-ranging applications in catalysis, magnetism, and materials science. Among these, one-dimensional (1D) systems are particularly attractive due to their anisotropic physical properties and potential applications in advanced magnetic and electronic devices.1,2

Hydrogen bonding—though relatively weak—is highly directional and plays a critical role in guiding the assembly of molecular architectures. It is especially effective in stabilizing well-ordered 1D structures, facilitating polar alignment and, in some cases, ferroelectric behavior. In such systems, directional proton dynamics within the hydrogen-bonding network can give rise to switchable polarization under external electric fields.3–11

In this context, 1H-imidazole, 1H,1′H-2,2′-biimidazole (H2bim), and their deprotonated forms (Hbim− and bim2−) are promising candidates for constructing hydrogen-bonded chains capable of supporting ferroelectricity.12–14 For example, Horiuchi et al.12 demonstrated that certain imidazole derivatives maintain high electric polarization up to temperatures near 400 K. Moreover, these ligands also serve as versatile coordination units, enabling the construction of metal–organic frameworks and polynuclear complexes with diverse functionalities.15–19 Notably, Tadokoro et al.17 and Kagilev et al.19 have shown that Hbim− and related ligands can promote magnetic exchange and redox-active behavior in transition metal complexes.

Inspired by these dual functionalities—hydrogen bonding and metal coordination—we aim to develop multifunctional materials in which an external electric field can modulate magnetic properties such as intra- and intermolecular magnetic coupling, or even induce spin state transitions at metal centers. To this end, we selected the Hbim− ligand to mediate intermolecular hydrogen bonds and bim2− as a bridging ligand to promote intramolecular magnetic coupling. Fe(II) and tris(2-pyridylmethyl)amine (tpma) were chosen as the remaining components (Scheme 1), given that [Fe(tpma)(bim)]2+ is known to exhibit spin-crossover behavior.15


image file: d5dt01007a-s1.tif
Scheme 1 Draw of the target dinuclear complex.

Herein, we report our preliminary findings, including the successful synthesis and structural characterization of the target compound. This compound is notable for featuring a bim2− ligand bridging two Fe(II) ions and incorporating both bim2− and Hbim− ligands in a dinuclear structure, assembling into robust one-dimensional hydrogen-bonded chains. Preliminary structural characterizations and magnetic measurements are presented, while ferroelectric and additional functional characterizations are ongoing and will be detailed in future reports.

Single-crystal X-ray diffraction analysis was employed to determine the crystal structure of the compound, confirming the structure is as the proposed model. The dinuclear iron-based coordination complex crystallizes in the triclinic space group P[1 with combining macron] (Table S1†). The asymmetric unit contains two Fe centers, one TPMA ligand, one 2,2′-biimidazolate dianion (bim2−), and two 1H-2,2′-biimidazolate monoanions (Hbim−), resulting in a neutral dinuclear structure (Fig. 1). Each Fe center exhibits a distorted octahedral coordination geometry with six nitrogen atoms. The Fe1 center is coordinated by four nitrogen atoms of TPMA and two nitrogen atoms from the bridging bim2− dianion. Fe1–N bond lengths range from 2.0937 Å (Fe1–N6) to 2.2477 Å (Fe1–N5), with both the shortest and longest bonds involving the bim2− bridge. The average Fe1–N bond length of 2.186 Å (Table 1) indicates a high-spin state for the Fe(II) center. However, this average value is less than 2.2 Å, suggesting the Fe(II) center might be near the spin-crossover (SCO) regime and its spin state could be sensitive to subtle structural changes or external stimuli.


image file: d5dt01007a-f1.tif
Fig. 1 Crystal structure of the dinuclear complex (iron: yellow, nitrogen: blue, carbon: gray and hydrogen: white) Thermal ellipsoids at 50% probability level.
Table 1 Fe–N, C–C and C–N bond lengths in the dinuclear complex
Fe–N Bim2− Hbim− Hbim−
Bond Length, Å Bond Length, Å Bond Length, Å Bond Length, Å
Fe1–N1 2.2288(18) C21–C22 1.370(3) N10–C26 1.329(3) C32–C31 1.452(3)
Fe1–N2 2.2289(18) C22–N7 1.376(3) N10–C29 1.367(3) N15–C31 1.347(3)
Fe1–N3 2.1751(18) N7–C19 1.340(3) C30–C29 1.346(3) N15–C34 1.374(3)
Fe1–N4 2.1437(18) C19–N5 1.341(3) N12–C30 1.377(3) C34–C33 1.369(3)
Fe1–N5 2.2477(18) N5–C21 1.383(3) C26–N12 1.346(3) N13–C33 1.359(3)
Fe1–N6 2.0937(18) C20–C19 1.445(3) C25–C26 1.458(3) N13–C31 1.344(3)
Fe1–N avr 2.1863167 N8–C20 1.328(3) C25–N11 1.344(3) N16–C32 1.344(3)
Fe2–N7 2.2171(18) N6–C20 1.340(3) N11–C28 1.379(3) N16–C36 1.376(3)
Fe2–N8 2.2247(19) N8–C24 1.380(3) N9–C25 1.347(3) C36–C35 1.360(3)
Fe2–N9 2.1881(19) C23–C24 1.367(3) N9–C27 1.370(3) N14–C35 1.374(3)
Fe2–N10 2.2166(19) N6–C23 1.379(3) C27–C28 1.372(4) N14–C32 1.331(3)
Fe2–N11 2.1889(19) Average 1.368 Average 1.367 Average 1.366
Fe2–N12 2.1673(19)            
Fe2–N avr 2.20045            

The Fe2 center is coordinated by two nitrogen atoms of the bim2− bridge and four nitrogen atoms from two Hbim−. Fe2–N bond lengths span from 2.1673 Å (Fe2–N14) to 2.2247 Å (Fe2–N8). The average Fe–N bond length of 2.200 Å confirms the high-spin state of this Fe(II) center (Table 1). As mentioned above, the bim ligand exhibits two different forms in this structure: the monoanion Hbim− and the dianion bim2−. The latter functions as a bridging ligand, while the former acts as a terminal ligand and forms hydrogen bonds with a Hbim− monoanion from an adjacent dinuclear complex. The average N–C and C–C bond lengths are 1.368 Å, 1.367 Å, 1.366 (Table 1) and 1.359 Å (ref. 15) for the bim2− dianion, the two Hbim− monoanions, and H2bim neutral, respectively. This suggests that the additional electron occupies an antibonding orbital, leading to the observed elongation of the C–N and C–C bonds.

The N–Fe–N bond angles further elucidate the coordination geometries around the Fe centers. For Fe1, angles such as N1–Fe1–N4 = 174.23(7)° and N2–Fe1–N5 = 165.92(7)° approach 180°, indicating nearly trans arrangements. In contrast, smaller angles like N5–Fe1–N6 = 50.85(9)° and N3–Fe1–N4 = 76.25(7)° reveal significant deviations from an ideal octahedral geometry. Similarly, Fe2 exhibits angles such as N11–Fe2–N12 = 158.38(7)° and N9–Fe2–N11 = 76.90(7)°, which also indicate notable distortions. Overall, the extent of distortion is quantified by the parameter ∑, defined as the total deviation of the twelve cis N–Fe–N angles from the ideal value of 90°.20 For the Fe1 and Fe2 centers, ∑ is 115.13° and 84.53°, respectively (Table S2†).

The top view of the crystal packing, illustrated in Fig. 2A, reveals a highly ordered, grid-like arrangement of the binuclear complex within the bc-plane of the triclinic unit cell (a = 9.6149(5) Å, b = 13.1277(7) Å, c = 15.2214(9) Å, α = 95.010(1)°, β = 94.046(1)°, γ = 110.962(1)°). Intermolecular hydrogen bonding creates a zigzag pattern (highlighted in orange). These hydrogen bonds occur between the N–H of the imidazole moiety and the N of the deprotonated imidazolate moiety (Fig. 3A). The N⋯H bond distance is 1.72 Å, with an N⋯N distance of 2.47 Å. These strong, double hydrogen bonds between two Hbim− anions lead to the formation of a chain structure. The crystal packing is characterized by the parallel arrangement of these zigzag chains. Furthermore, several close interchain contacts are observed, including strong π–π interactions between two pyridyl moieties (3.39 Å), H–π interactions between a hydrogen atom of one of the pyridyl moieties and the imidazolate (2.50 Å), and between a hydrogen atom of a pyridyl moiety and the bim2− bridging moiety (2.62 Å) (Fig. 3B). The overall directionality of the hydrogen bonds suggests the potential for ferroelectric behavior.


image file: d5dt01007a-f2.tif
Fig. 2 (A) A top view of the crystal packing of the dinuclear complex, presented along the crystallographic b and c axes, shows the following color scheme: iron (yellow), nitrogen (blue), carbon (gray), and hydrogen (white). The highlighted yellow region emphasizes the role of hydrogen bonding in generating a one-dimensional zigzag structure. (B) A side view of the packing shows segregated layers.

image file: d5dt01007a-f3.tif
Fig. 3 Intermolecular interactions in the crystal structure of the dinuclear complex. (A) Double hydrogen bonds between two adjacent Hbim− moeities. (B) π–π, H–π interactions within a layer, viewed along b and c axes. (C) H–π interactions between layers.

The side view (Fig. 2B) illustrates the stacking of molecular layers along the c-axis (c = 15.2214(9) Å, β = 94.046(1)°). The layers are staggered, with binuclear iron complexes offset between adjacent layers to optimize intermolecular interactions. There are some close contacts between layers, such as H–π interactions between H of pyridyl and neighboring pyridyl and bim2− bridging ligand, which are 2.84 Å and 2.92 Å, respectively (Fig. 3C).

The magnetic properties of the dinuclear Fe(II) complex were studied using SQUID magnetometry on a polycrystalline sample. The temperature dependence of the magnetic susceptibility product (χMT) from 0 K to 300 K is shown in the χMT vs. T plot (Fig. 4A). At higher temperatures (above 150 K), χMT remains relatively constant at approximately 6.4 emu K mol−1, indicating paramagnetic behavior where the magnetic moments of the two iron(II) centers (S = 2) are largely uncorrelated, with g ≈ 2.08, typical for a high-spin Fe(II) ion.21,22 Below 50 K, χMT gradually decreases, followed by a sharp drop to near zero at 2 K. This suggests a weak antiferromagnetic coupling between the two Fe(II) centers through the bim2− bridge. The magnetic exchange can be described by the Heisenberg-Dirac-van Vleck Hamiltonian:

 
H = −2JS1·S2 (1)
where J is the magnetic exchange coupling constant and S1 and S2 are the interacting spin states. This leads to the equation that describes the temperature dependence of χMT for the system of two coupled S1 = S2 = 2 centers.21,22
image file: d5dt01007a-t1.tif
where kB is Boltzmann constant, β: Bohr magneton, and g-factor, g.


image file: d5dt01007a-f4.tif
Fig. 4 (A) Temperature dependence of χMT measured under an applied magnetic field of 1000 Oe (black circles), along with the best fit (red solid line) based on the Bleaney–Bowers equation for a dinuclear complex containing two high-spin Fe(II) centers (S = 2), antiferromagnetically coupled with J = −1.4 cm−1 and g = 2.08. (B) Field-dependent magnetization curves recorded at 2 K (solid black line), 6 K (blue dashed line), and 10 K (red dotted line).

The best fit of the data to this equation yields a magnetic coupling constant, J = −1.4 cm−1, with g = 2.08. This result indicates a weak antiferromagnetic coupling between the two Fe(II) centers. Other bim2− bridging complexes also shows similar behaviors, such as binuclear [(Ni(cyclam))2(μ-bim)]2+, J = −4.6 cm−1,23 [(Ni(tren))2(μ-bim)]2+, J = −2.9 cm−1, [Cu(dpt)(μ-bim)]2+, J = −1.9 cm−1 (ref. 24) and [(Cp2Ti)2(μ-bim)], J = −25.2 cm−1.25

The field-dependent magnetization curves illustrate the magnetization as a function of applied field (−5 T to 5 T) at 2 K, 6 K, and 10 K. The black curve, measured at 2 K, shows a pronounced near-zero magnetization at low field due to the dominance of the antiferromagnetic singlet ground state. The magnetization then increases rapidly below 2 T. Above 2 T, the increase slows and reaches approximately 4.6μB at 5 T. This value is well below the expected saturation magnetization for this system, which is around 8μB, indicating that the spins are not fully aligned at this field strength, due to antiferromagnetic interaction between spin centers. The blue dashed curve, recorded at 6 K, shows a smoother but more gradual rise, reaching 4.3μB at 5 T, a lower value than at 2 K, consistent with increased thermal energy reducing the effectiveness of the applied field. The red dotted curve, measured at 10 K, exhibits an almost linear relationship between magnetization and applied field, reaching 3.7μB at 5 T.

In summary, we proposed and successfully synthesized a potential multifunctional dinuclear Fe(II) complex utilizing 2,2′-biimidazole-based ligands. The single-crystal X-ray diffraction data confirmed the dinuclear structure, with each Fe(II) center adopting a high-spin state within a distorted octahedral coordination environment, bridged by a bim2−. The Fe1 center's proximity to the spin-crossover regime suggests potential sensitivity to subtle structural changes or external stimuli. The complex self-assembles into a one-dimensional zigzag chain through strong double hydrogen bonds between terminal 1H-2,2′-biimidazolate monoanion ligands, further stabilized by π–π and H–π interactions in the crystal packing. Magnetic studies revealed weak antiferromagnetic coupling (J = −1.4 cm−1) between the two Fe(II) centers. The directional hydrogen bonding observed in this 1D system suggests potential proton dynamics and ferroelectric behavior, areas currently under investigations, paving the way for the design of multifunctional materials with applications in magnetism and materials science.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

ESI† provides experimental details for this article. Crystallographic data for the reported structure are available from the CCDC (deposition number 2442579†).

Acknowledgements

H. P. acknowledges financial support from Hanoi University of Science and Technology (T2022-XS-002). M. S. acknowledges support from the Center for Molecular Magnetic Quantum Materials (M2QM), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award DE-SC0019330.

References

  1. I. Castro, M. L. Calatayud, M. Orts-Arroyo, N. Marino, G. De Munno, F. Lloret, R. Ruiz-García and M. Julve, Coord. Chem. Rev., 2022, 471, 214730 CrossRef CAS .
  2. H. A. Elsayed, M. Medhat, A. Hajjiah, H. E. Alfassam, M. R. Abukhadra and A. Mehaney, Sci. Rep., 2025, 15, 1977 CrossRef CAS PubMed .
  3. Y.-A. Xiong, S.-S. Duan, H.-H. Hu, J. Yao, Q. Pan, T.-T. Sha, X. Wei, H.-R. Ji, J. Wu and Y.-M. You, Nat. Commun., 2024, 15, 4470 CrossRef CAS PubMed .
  4. G. Yuan, Y. Kimura, T. Kobayashi, T. Takeda, N. Hoshino and T. Akutagawa, Chem. Sci., 2021, 12, 13520–13529 RSC .
  5. D. P. Karothu, R. Ferreira, G. Dushaq, E. Ahmed, L. Catalano, J. M. Halabi, Z. Alhaddad, I. Tahir, L. Li, S. Mohamed, M. Rasras and P. Naumov, Nat. Commun., 2022, 13, 2823 CrossRef CAS PubMed .
  6. H. S. Choi, S. Li, I.-H. Park, W. H. Liew, Z. Zhu, K. C. Kwon, L. Wang, I.-H. Oh, S. Zheng, C. Su, Q.-H. Xu, K. Yao, F. Pan and K. P. Loh, Nat. Commun., 2022, 13, 794 CrossRef CAS PubMed .
  7. H.-Y. Ye, D.-W. Fu, Y. Zhang, W. Zhang, R.-G. Xiong and S. D. Huang, J. Am. Chem. Soc., 2009, 131, 42–43 CrossRef CAS PubMed .
  8. P. S. Ghosh, D. DeTellem, J. Ren, S. Witanachchi, S. Ma, S. Lisenkov and I. Ponomareva, Phys. Rev. Lett., 2022, 128, 077601 CrossRef CAS PubMed .
  9. Z. Zhang, Y. Ye, S. Xiang and B. Chen, Acc. Chem. Res., 2022, 55, 3752–3766 CrossRef CAS PubMed .
  10. Q. Huang, W. Li, Z. Mao, H. Zhang, Y. Li, D. Ma, H. Wu, J. Zhao, Z. Yang, Y. Zhang, L. Gong, M. P. Aldred and Z. Chi, Chem, 2021, 7, 1321–1332 CAS .
  11. L. Chen, B. Zhang, L. Chen, H. Liu, Y. Hu and S. Qiao, Mater. Adv., 2022, 3, 3680–3708 RSC .
  12. S. Horiuchi, F. Kagawa, K. Hatahara, K. Kobayashi, R. Kumai, Y. Murakami and Y. Tokura, Nat. Commun., 2012, 3, 2322 CrossRef PubMed .
  13. Y. Wang, F.-H. Zhao, A.-H. Shi, Y.-X. Che and J.-M. Zheng, Inorg. Chem. Commun., 2012, 20, 23–26 CrossRef CAS .
  14. Y.-A. Xiong, S.-S. Duan, H.-H. Hu, J. Yao, Q. Pan, T.-T. Sha, X. Wei, H.-R. Ji, J. Wu and Y.-M. You, Nat. Commun., 2024, 15, 4470 CrossRef CAS PubMed .
  15. H. Phan, P. Chakraborty, M. Chen, M. Y. Calm, K. Kovnir, K. J. Keniley, J. Hoyt, S. E. Knowles, C. Besnard, W. M. Meisel, A. Hauser, C. Achim and M. Shatruk, Chem. – Eur. J., 2012, 18, 15805 Search PubMed .
  16. F. Benner and S. Demir, Inorg. Chem. Front., 2023, 10, 4981–4992 RSC .
  17. M. Tadokoro, H. Kanno, T. Kitajima, H. Shimada-Umemoto, N. Nakanishi, K. Isobe and K. Nakasuji, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 4950–4955 CrossRef CAS PubMed .
  18. J. Carranza, J. Sletten, F. Lloret and M. Julve, Polyhedron, 2009, 28(11), 2249–2257 CrossRef CAS .
  19. A. A. Kagilev, Z. N. Gafurov, A. S. Evdokimov, I. Y. F. Sakhapov, A. B. Dobrynin, V. I. Morozov, R. B. Zaripov, E. M. Zueva, A. S. Bogomyakov, A. O. Kantyukov, N. A. Zhukova, O. G. Sinyashin, V. A. Mamedov and D. G. Yakhvarov, Inorg. Chim. Acta, 2025, 579, 122555 CrossRef CAS .
  20. P. Guionneau, M. Marchivie, G. Bravic, J. F. Ltard and D. Chasseau, Top. Curr. Chem., 2004, 234, 97 CrossRef CAS .
  21. M. Julve and O. Kahn, Inorg. Chim. Acta, 1983, 76, L39 CrossRef CAS .
  22. O. Kahn, Angew. Chem., Int. Ed. Engl., 1985, 24(10), 834–850 CrossRef .
  23. M. Tadokoro, K. Sato, D. Shiomi, T. Takui and K. Itoh, Mol. Cryst. Liq. Cryst., 1997, 306, 49 CrossRef CAS .
  24. M. S. Haddad and D. N. Hendrickson, Inorg. Chem., 1978, 17(9), 2622 CrossRef CAS .
  25. B. F. Fieselmann, D. N. Hendrickson and G. D. Stucky, Inorg. Chem., 1978, 17(8), 2078 CrossRef .

Footnote

† Electronic supplementary information (ESI) available. CCDC 2442579. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5dt01007a
This journal is © The Royal Society of Chemistry 2025
Click here to see how this site uses Cookies. View our privacy policy here.