Spin state modulation and chiral hierarchical assembly via amine-driven single-crystal-to-single-crystal transformation

Abstract

We report a mononuclear Fe(II) synthon, Fe-BF₄, with four acidic NH sites and two terminal iodo-groups, for achieving achiral or chiral supramolecular hydrogen- and/or halogen-bond frameworks through amine-driven single-crystal-to-single-crystal (SC–SC) transformation. Et₃N-driven SC–SC transformation enables the formation of both a hydrogen- and halogen-bonded 2D architecture and a halogen-bonded 1D chain, accompanied by a magnetic transition from spin-crossover behavior to the low-spin state. When the optically pure amine, R/S-quinuclidinol, is introduced, the crystal undergoes a transformation from a centrosymmetric to a chiral structure. This successfully generates 1D hydrogen-bonded helical chains with M/P-helicity, which are further orthogonally linked via intermolecular C–I⋯N halogen bonds, forming three individual yet identical networks. Remarkably, these three networks adopt an alternating stacking arrangement, resulting in a three-fold interpenetrated supramolecular architecture. This hierarchical assembly illustrates a controlled transition from an achiral to a chiral configuration. Therefore, the amine-triggered SC–SC transformation in Fe-BF₄ provides an effective strategy for simultaneously controlling the topological architectures and spin states of advanced supramolecular frameworks.

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Introduction

Single-crystal-to-single-crystal (SC–SC) transformation has attracted enormous interest in crystal engineering as it provides an ideal method to modulate physicochemical properties of materials, showing great potential in porous materials,^1,2 molecular magnetism and ferroelectricity,^3–5 and molecular actuators.^6,7 At the molecular level, such transformations induced by light, heat, solvent exchange and static pressure are vital in accurately understanding structure–property–function relationships.^8–10 For example, SC–SC isomerization allowed ab initio calculations for a 1D photochromic dysprosium single-molecule magnet (SMM) before and after visible irradiation, thereby enabling the elucidation of the relaxation mechanism of magnetization in closed and open isomers.¹¹ In addition, SC–SC transformation is a kind of a topochemical synthesis strategy, presenting great advantages in creating novel functional crystalline materials with attractive chiroptical properties, spin-crossover (SCO) behaviors and ferroelectricity.^12–18 However, the number of examples of SC–SC transformation still remains limited as it is usually difficult to retain crystallinity after being subjected to an external stimulation or post-chemical modification, especially in 0D metal coordination complexes.¹⁹ In 2014, Aromí et al. reported a heteroleptic 0D low-spin Fe(II) complex, [Fe(bpp)(H₂L₁)](ClO₄)₂·1.5C₃H₆O (bpp = 2,6-bis(pyrazol-3-yl)pyridine, H₂L₁ = 2,6-bis(5-(2-methoxyphenyl)pyrazol-3-yl)pyridine),²⁰ which showed a three-way SC–SC transformation via absorption, desorption and exchange of small solvent molecules, achieving the full conversion cycle of spin states by virtue of the fact that SCO behavior is highly sensitive to subtle structural changes.^20–23 In 2018, Zheng et al. achieved a reversible SC–SC transformation in a mononuclear Dy(III) complex, Dy(depma)(NO₃)₃(hmpa)₂ (depma = 9-diethylphosphono-methylanthracene, hmpa = hexamethylphosphoramide), via the intermolecular dimerization and de-dimerization between two neighboring anthracene units.²⁴ Such transformations lead to profound changes in magnetization dynamics and luminescent properties before and after photodimerization, paving a new way to construct magneto-optical smart materials. In addition, the SC–SC transformation of a 0D metal coordination complex can induce chiral interconversions, as exemplified by the simple centrosymmetric [Zn(eg)₃]SO₄ (eg = ethylene glycol) complex reported by Tao et al.¹⁷ When the crystal was heated, it eliminated eg via a two-step thermal elimination process, along with preserving its crystallinity, leading to changes in topological structures and molecule symmetries, i.e. from 0D to 1D to 3D accompanied by a centric-chiral-polar symmetric variation.

Apart from the above solvent-, light- and thermal-responsive molecule-based metal complexes, acid–base-responsive species for intelligent molecular devices have sparked great interest in recent years. In general, they possess acidic protons in the prototype compound and show reversible deprotonation/protonation behavior.^25–27 More interestingly, the number of acidic protons can be controlled chemically, providing a unique chance to precisely modulate the quantum states and magneto-optical properties of materials.²⁸ In 2019, Shiga et al. reported a mononuclear homoligand Fe(II) complex, featuring a typical spin-interconversion behavior.²⁹ It had four non-equivalent imidazole and pyrazole sites, exhibiting five switchable electronic states with different ligand-field strength by sequential deprotonation steps. Very recently, such multilevel proton-induced magnetic response has also been achieved in an air-stable cubic Fe^III₈ metal–organic cage with twelve monodeprotonated bis-bidentate ligands, 2,2′-(1H-imidazole-4,5-diyl)bis(1,4,5,6-tetrahydropyrimidine), as edges, which affords 12 switchable spin states via precise deprotonation and protonation at the NH active sites of the ligands.³⁰ However, we notice that the above acid–base controls were all carried out in solution, which somewhat hinders the deep understanding of their structure–property-function relationships.

Herein, we report an Et₃N- or chiral amine-triggered SC–SC transformation in a mononuclear Fe(II) complex, [Fe(H₂L)₂](BF₄)₂ [H₂L = 4-iodo-2,6-di(1H-imidazole-2-yl)pyridine] (Fe-BF₄), via the deprotonation of the NH sites in the imidazole motif (Scheme 1). The imidazole moieties and terminal iodo groups were strategically incorporated to facilitate the formation of extensive N−H⋯N hydrogen bonds and C−I⋯N halogen bonds, which were anticipated to simultaneously maintain crystallinity and direct the supramolecular framework topology.^17,31 We initially investigated Et₃N serving as a proton abstractor to mediate the SC–SC transformation of Fe-BF₄. Interestingly, when excess Et₃N was employed, the system underwent conversion to [Fe(L)₂][Et₃NH], which adopted a 1D chain topology stabilized by halogen bonding. This observation prompted us to explore chiral amines for constructing topologically chiral architectures. Remarkably, employing optically pure R/S-quinuclidinol as a chiral inducer enabled a four-stage hierarchical assembly process, ultimately yielding enantiomeric 3D frameworks with three-fold interpenetration. Moreover, the above transformations were accompanied by a switch in spin states.

Scheme 1 SC–SC transformation protocol of Fe-BF₄.

Results and discussion

Fe-BF₄ was prepared by a room-temperature solution reaction between H₂L and Fe(BF₄)₂·6H₂O in a molar ratio of 2 : 1 in CH₃CN. The solid structure of Fe-BF₄ was determined by single-crystal X-ray diffraction (SC-XRD) at room temperature and is displayed in Fig. 1. The compound crystallized in the orthorhombic space group Pbcn (Table S1), consisting of two H₂L, one Fe(II) ion and two BF₄⁻ ions as non-coordinating counterions. The Fe(II) center coordinated with six nitrogen atoms from two H₂L, forming a distorted octahedral coordination geometry. At 300 K, the Fe–N bond lengths ranged from 1.925(4) to 1.999(4) Å (Table S2), giving an average bond length of 1.971 Å. The average trans N(pyridine)–Fe–N(pyridine) angle ϕ and dihedral twist angle θ ^21,32 were 178.79 and 88.01°, respectively (Table S2). When the temperature was increased to 400 K, the space group remained unchanged, but the unit cell volume expanded from 2949.2 to 3028.1 Å (Table S1). Accordingly, the average length of Fe–N bonds increased from 1.971 to 2.117 Å, although ϕ and θ did not change much; ϕ = 178.39° and θ = 86.99° (Table S2). Notable changes in angular (Σ) and torsional (Θ) distortion parameters^21,33 were also observed: Σ(300 K) = 88.48°, Σ(400 K) = 124.73°, Θ(300 K) = 288.56° and Θ(400 K) = 402.91°. All the above changes implied a temperature-induced spin-crossover behavior in Fe-BF₄. The packing diagram of Fe-BF₄ along the crystallographic b axis revealed that the counter anions BF₄⁻ acted as bridging units between the neighboring molecules, forming a 3D network via N–H⋯F interactions. This arrangement resulted in the shortest intermolecular Fe(II)⋯Fe(II) separation of 8.936 Å at 300 K (Fig. S1).

Fig. 1 Et₃N-driven SC–SC transformation of Fe-BF₄. Color codes: Fe(II)-orange, Fe(III)-black, N-blue, C in Fe(II)/Fe(III) units-gray, C in amine units-green, I-pink, B-yellow, F-dark yellow, and H-turquoise.

Given the acid/base response of the imidazole group, we investigated the acid–base interconversion of Fe-BF₄ among different spin states. Firstly, Et₃N was reacted with the NH protons of the imidazole moiety, and it was found that the compound underwent SC–SC transformations accompanied by the oxidation of Fe(II) to Fe(III) (Fig. 1). It should be clarified that such an SC–SC transformation was not feasible under anaerobic conditions since oxygen, as established in the literature,^34,35 is essential for such oxidation. When three equivalents of Et₃N were used, Fe-BF₄ was successfully transformed into a neutral Fe(III) complex, [Fe(HL)(L)] (Fe-1H), which contained three deprotonated and one protonated imidazole groups. Upon treatment with a large excess of Et₃N, four NH protons in Fe-BF₄ were completely consumed, producing an Fe(III) anionic unit balanced by an [Et₃NH]⁺ cation, [Fe(L)₂]·(Et₃NH)·H₂O (Fe-Et₃N). In addition, Fe-1H could be obtained by adding one equivalent of glacial acetic acid to Fe-Et₃N in an SC–SC transformation process. These transformations from Fe(II) to Fe(III) complexes were further confirmed by the disappearance of the peak centered at ca. 540 nm in the absorption spectra (metal-to-ligand charge transfer process, Fig. 2), the vanishing of a broad peak in the region within 25–40 ppm in the ¹H NMR spectra (intrinsic paramagnetism of Fe(II), Fig. S13 and S14), and the XPS data (Fig. S25 and S26). Moreover, it is worthy to note that the Fe(III) species could be converted back to the Fe(II) species by adding equivalent amounts of hydroxylamine hydrochloride in methanol, as confirmed by UV-Vis spectra, ¹H NMR spectra and magnetic measurements (Fig. S12, S15, S16 and S22). The single crystals of the above reductive compounds were obtained by slow diffusion of diethyl ether into a methanol solution. SC-XRD analysis revealed that all the reductive species were identical, named by Fe-Re. Its solid structure is shown in Fig. S9, and the structural details are presented in Tables S10–S12.

Fig. 2 UV-Vis absorption spectra of Fe-BF₄, Fe-1H, Fe-Et₃N, Fe-R-QNO and Fe-S-QNO in MeOH (c = 10⁻⁵ M).

Compounds Fe-1H and Fe-Et₃N crystallized in the space groups of lower symmetry, P2₁/n and P2₁/c, respectively, than Fe-BF₄ (Table S4). The structural distortion of Fe-1H was smaller than that of Fe-BF₄, as demonstrated by its larger ϕ(179.00°) and θ(89.45°) and smaller Σ(75.97°) and Θ(249.61°) values (Table S5). In Fe-1H, adjacent molecules were connected by orthogonal N–H⋯N hydrogen bonding and C–I⋯·N halogen bonding interactions, producing a 2D network structure (Fig. S3). In comparison, Fe-Et₃N possessed four deprotonated imidazole groups and an [Et₃NH]⁺ for balancing the charge. The average length of Fe–N bonds and angles of ϕ and θ were 1.928 Å, 177.96°, and 88.85°, respectively (Table S5). Compared with Fe-BF₄, the Σ and Θ parameters of Fe-Et₃N reduced from 88.48° and 288.56° to 78.50° and 265.59°, respectively (Table S5). In contrast to Fe-1H, the topological supramolecular architecture of Fe-Et₃N was the C−I⋯N halogen-bonded 1D chain with discrete N−H⋯N hydrogen bonds between [Et₃NH]⁺ and one deprotonated imidazole group (Fig. S5). The shortest intermolecular Fe(III)⋯Fe(III) distances for Fe-1H and Fe-Et₃N were 7.677 and 7.601 Å, respectively (Fig. S2 and S4).

The magnetic susceptibilities of the above three compounds were measured under a 1000 Oe direct current magnetic field over the temperature range of 5–400 K. Thermogravimetric analysis (TGA) curves, as shown in Fig. S10, verified that they were all thermally stable at 400 K. Fe-BF₄ exhibited an incomplete and gradual SCO behavior, while the other two Fe(III) complexes, Fe-1H and Fe-Et₃N, showed the predominant low-spin state of Fe(III) across the entire temperature range, indicating that the amine-driven SC–SC transformation successfully induced a spin-state switch. As shown in the top panel of Fig. 3, the χ_MT value of Fe-BF₄ at 400 K was 2.47 cm³ K mol⁻¹, which corresponded to 82% of the high-spin state of Fe(II) (S = 2, g = 2).³⁰ As the temperature dropped, the χ_MT value showed a noticeable decline and reached 0.1 cm³ K mol⁻¹ at 224 K, suggesting an almost complete low-spin state of Fe(II) (S = 0, g = 2). For Fe-1H and Fe-Et₃N, the χ_MT values at 400 K were ca. 0.58 and 0.72 cm³ K mol⁻¹, respectively, corresponding to the predominant low-spin state of Fe(III) (S = 1/2, g = 2). The larger χ_MT value in the latter implied that further deprotonation of the imidazole group may prompt the Fe(III) center to transition to the high-spin state.³⁰ Their χ_MT values did not change significantly with decreasing temperature, demonstrating the absence of SCO behavior in these two Fe(III) complexes. Furthermore, the magnetic signature of this low-spin state could revert to that of the SCO via reduction with hydroxylamine hydrochloride (Fig. S21). X-band EPR spectroscopy was also employed to further elucidate the spin state of the Fe(III) complexes following the SC–SC transition. Results for both Fe-1H and Fe-Et₃N revealed typical low-spin signals of Fe(III) complexes (S = 1/2) (middle and bottom panels of Fig. 3). Fitting the spectra yielded anisotropic g-tensors of g_x = 2.27, g_y = 2.26, g_z = 1.89 for Fe-1H and g_x = 2.24, g_y = 2.23, g_z = 1.91 for Fe-Et₃N. These values were similar to those of the previously reported low-spin Fe(III) complexes.^36,37

Fig. 3 Top: Temperature dependence of χ_MT values for Fe-BF₄, Fe-1H and Fe-Et₃N. Experimental and simulated EPR spectra of Fe-1H (middle) and Fe-Et₃N (bottom) at room temperature.

Chiral amines serve as crucial structural motifs with broad applications across diverse fields, including drug preparation,^38–40 asymmetric catalysis,⁴¹ and ferroelectric materials.¹⁵ The successful incorporation of Et₃N into the lattice of Fe-BF₄ prompted us to employ chiral amines to drive such transformations for obtaining intriguing chiral supramolecular organic frameworks supported by the hydrogen and/or halogen bonds. Herein, we used optically pure organic amines, R/S-quinuclidinol (R/S-QNO), to drive the SC-SC transformation of Fe-BF₄. When an excess of R/S-QNO was added, the color of the Fe-BF₄ crystals gradually changed from crimson to black without the loss of crystallinity, yielding the homochiral Fe(III) complex, [Fe(L)₂]·(R/S-QNO-H)·3CH₃CN (Fe-R/S-QNO). This complex was characterized by SC-XRD, UV-Vis spectroscopy and ¹H NMR spectroscopy (Fig. 2, 4, S17 and S18). Note that this is a nonbonding method for chiral induction, i.e. the protonated chiral amines serve as inherent sources of chirality, showing hydrogen-bond interaction with the periphery of the Fe(III) coordination sphere. More importantly, the integration of [Fe(L)₂] (achiral “soldiers”) and chiral organic amines (chiral “sergeants”) via weak interactions achieved remote chirality transfer and effective chiral amplification (Fig. 4). The enantiomeric nature of Fe-R-QNO and Fe-S-QNO was unequivocally established by their mirror-symmetric crystal structures. For clarity, we focus our discussion on Fe-R-QNO as a representative example. SC-XRD revealed that Fe-R-QNO crystallized in the chiral space group P2₁2₁2₁ (Table S7). The asymmetric unit of Fe-R-QNO included one iron-centered anion, [Fe(L)₂]⁻, one chiral amine cation, [R-QNO-H]⁺, and three CH₃CN molecules (Fig. S6). Interestingly, the protonated amino group and –OH group of R-QNO simultaneously engaged in hydrogen bonding with [Fe(L)₂]⁻ units, giving rise to a 1D hydrogen-bonded helical chain with M-helicity (Fig. 4 and S7). These 1D chains were further connected by the orthogonal C–I⋯N halogen bonds, giving three 3D hexagonal windows along the crystallographic a axis. Finally, these windows were held together in an alternating pattern via abundant C–H⋯π and C–H⋯N interactions between organic amines and imidazole motifs or two intermolecular imidazole motifs, forming a threefold interwoven topology (Fig. 4). Correspondingly, the supramolecular organic framework of Fe-S-QNO was constructed in the same hierarchical assembly as that of Fe-R-QNO. As shown in Fig. 4 and S8, each stage of the assembly of Fe-S-QNO resulted in a mirror-symmetric structure relative to that of Fe-R-QNO. Compared with the silent CD signal in solution (Fig. S19), the solid-state CD spectra exhibited the most intense signals at ca. 410 nm (Fig. 4), clearly demonstrating the successful transfer of chirality from the chiral amine cations to the supramolecular structure. Such an SC–SC transformation from centrosymmetric crystals to chiral crystals via the nonbonding approach provides an alternative for preparing chiral supramolecular organic frameworks, which is reminiscent of the combination of chiral amine drugs with proteins in living organisms. Additionally, this chiral amine-driven SC–SC transformation induced changes in magnetic properties, from SCO behavior to a low-spin Fe(III) state, as confirmed by the magnetic measurements and EPR spectra (Fig. S20, S21, S23 and S24).

Fig. 4 Synthetic route, chiral superstructures and CD spectra of Fe-R/S-QNO. All solvents are omitted for clarity. Label: 1D hydrogen-bonded helical chains, space-filling model.

Conclusions

In summary, we developed a mononuclear Fe(II) complex functionalized with acid–base-responsive imidazole moieties and terminal iodo groups that serves as a versatile building block for constructing achiral and chiral supramolecular architectures. These structures were obtained through a rare amine-driven SC–SC transformation process. In Fe-1H, a 2D supramolecular network was formed through the cooperative action of orthogonal intermolecular hydrogen and halogen bonds. In contrast, Fe-Et₃N adopted a 1D chain structure mediated by halogen bonding, complemented by discrete intramolecular hydrogen bonds. Remarkably, when optically pure R/S-quinuclidinol was used, we observed a homochiral hydrogen-bonded helical 1D chain with M-/P-helicity. These helical chains were further interconnected through the orthogonal C–I⋯N halogen bonds, constructing a 3D chiral network that eventually packed into a three-fold interpenetrated framework via C–H⋯N/π interactions, achieving a four-stage chiral hierarchical assembly. Moreover, these SC–SC transformations induced corresponding changes in magnetic properties: from SCO behavior in parent Fe-BF₄ to the low-spin state of Fe(III) complexes. This SC–SC transformation via nonbonding interactions paves a new way for the transfer and amplification of chirality and the modulation of topologies and spin states in supramolecular frameworks.

Author contributions

J. Tang conceived the idea. X. Ying, Z. Zhu and P. Xu performed the synthesis and characterization. Z. Zhu, X. Ying, J. Tang and Q. Zhou wrote the manuscript with contributions from all authors.

Conflicts of interest

There are no conflicts to declare.

Data availability

All data that support the conclusion of this study are available in this article and its supplementary information (SI). Supplementary information: synthetic procedures, single-crystal data, characterization and magnetic measurements. See DOI: https://doi.org/10.1039/d5qi01812a.

CCDC 2378478–2378481, 2378488, 2468133 and 2468134 contain the supplementary crystallographic data for this paper.^42a–g

Acknowledgements

We thank the support from the National Natural Science Foundation of China (22201279 and 92261103), the Key Research Program of Frontier Sciences, CAS (ZDBS-LY-SLH023), and the Natural Science Foundation of Jilin Province State Key Laboratory Major Project (SKL202302037) for this work.

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