Fine tuning of intra-lattice electron transfers through site doping in tetraoxolene-bridged iron honeycomb layers

Abstract

The precise control of intra-lattice multiple electron transfers was demonstrated in the solvated and desolvated species of the tetraoxolene-bridged Fe honeycomb layer system, (NPr₄)₂[Fe₂(Cl₂An)₃]·(solv) (Cl₂Anⁿ⁻ = 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinonate; NPr₄⁺ = tetrapropylammonium cation), by the site-doping of the Cl₂Anⁿ⁻ bridging unit using X₂Anⁿ⁻ units with X = Br or F.

---

Exploring new stimuli-controllable functional systems is important in the field of materials science.^1–4 These systems can be obtained by achieving thermally driven electron transfers (TDETs) in a material,⁵ which can successfully induce a state (or phase) change in materials at a transition temperature (T_1/2). Moreover, modifications of the physical properties of materials such as electronic conductivity,⁶ magnetic properties,⁷ elasticity,⁸ and the dielectric property can be attained.⁹ Currently, the relevant systems in the charge-transfer metal complexes, which are composed of electron-donor (D) and -acceptor (A) moieties,^9–20 and organic charge transfer systems,^21–24 have been investigated. Nevertheless, the systematic fine tuning of the variable charge states is still challenging; however, it can potentially be used for the application of these systems in molecular devices and sensors.^1–3

Recently, we have reported two kinds of tetraoxolene-bridged iron honeycomb layered compounds, (NPr₄)₂[Fe₂(Cl₂An)₃]·2(acetone)·H₂O (0; Cl₂Anⁿ⁻ = 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinonate; NPr₄⁺ = tetrapropylammonium cation)¹⁵ and their desolvated compound (NPr₄)₂[Fe₂(Cl₂An)₃] (0-d).¹⁶ The former exhibited a TDET at T_1/2a = 236 K between the phases of [Fe^III₂(Cl₂An²⁻)(Cl₂An˙³⁻)₂]²⁻ at T < T_1/2a and [Fe^IIFe^III(Cl₂An²⁻)₂(Cl₂-An˙³⁻)]²⁻ at T > T_1/2a,¹⁵ while the latter underwent a stepwise TDET at T_1/2b = 317 K and T_1/2c = 354 K which changed phases from [Fe^III₂(Cl₂An²⁻)(Cl₂An˙³⁻)₂]²⁻ at T < T_1/2b to [Fe^II₂(Cl₂An²⁻)₃]²⁻ at T > T_1/2cvia the intermediate state of [Fe^IIFe^III(Cl₂An²⁻)₂(Cl₂An˙³⁻)]²⁻ at T_1/2b < T < T_1/2c (Scheme 1a).¹⁶ In addition, compounds 0 and 0-d are reversible via the solvation and desolvation processes, respectively. Hence, the systematic and continuous tuning of these phases were favorable due to being unprecedented in multiple TDET systems.

Scheme 1 Schematic representations of the investigated tetraoxolene-bridged Fe honeycomb layers. (a) The variations of charge-ordered states in 0 and 0-d, where for 0, T_1/2(1) was T_1/2a (i.e., there was no high-temperature phase for 0), and for 0-d, T_1/2(1) and T_1/2(2) correspond to T_1/2b and T_1/2c, respectively. (b) Schematic route for the syntheses of site-doped solid solution compounds in this work.

Herein, we report a precise tuning of the multiple transition temperatures T_1/2a, T_1/2b, and T_1/2c based on a chemical technique of site-doping for the charge-transfer materials 0 and 0-d. A rational incorporation of the redox metal ions and bridging ligands into the charge-transfer skeleton is one of the efficient strategies in achieving the fine tuning of TDET.^{17,18,25–28} In this work, the Cl₂An^m− unit in 0 and 0-d was replaced by X₂An^m− with X = F or Br in several ratios in the isostructural series where F₂An^m− = 2,5-difluoro-3,6-dihydroxy-1,4-benzoquinonate and Br₂An^m− = 2,5-dibromo-3,6-dihydroxy-1,4-benzoquinonate. Because the F₂An²⁻ and Br₂An²⁻ units act as stronger and weaker electron acceptors than Cl₂An²⁻, respectively, their solid-solution series revealed inverse trends in the doping rate dependence of T_1/2s between the F-doped and Br-doped series, that is, proportional and inversely-proportional, respectively, despite being linearly variable in both cases.

The solvated compound series of the solid solutions (NPr₄)₂[Fe₂(Cl₂An)_(3−3Y)(X₂An)_3Y]·2(acetone)·H₂O (X-Y) as X = Br with Y = 0.10 (Br-0.10), 0.18 (Br-0.18), 0.35 (Br-0.35), 0.65 (Br-0.65) and X = F with Y = 0.06 (F-0.06), 0.12 (F-0.12), 0.17 (F-0.17), 0.24 (F-0.24), 0.31 (F-0.31), 0.42 (F-0.42) were synthesized using an identical procedure for 0. Specifically, H₂Br₂An or H₂F₂An was mixed into an H₂Cl₂An solution in various ratios (Scheme 1b),¹⁵ where the Y value was obtained from the halogen elemental analyses for the X-Y and their desolvated series, X-Y-d (ESI†). The Y doping ratio was approximately identical to the mixing ratio in the synthetic procedures.

All compounds of the solvated form (X-Y) were isostructural to 0 in the Y-doped region of 0.65 ≥ Y and 0.42 ≥ Y for X = Br and F, respectively. The compounds crystalized in the monoclinic P2₁/c space group with an asymmetric unit containing two crystallographically independent Feⁿ⁺ ions, three Cl₂An^m−/X₂An^m− ligands, and two NPr₄⁺ cations (Fig. S1 and Table S1, S2, ESI†). The Fe ions and Cl₂An/X₂An ligands formed a honeycomb anionic layer [Fe₂(Cl₂An/X₂An)₃]²⁻ in the (100) plane. The occupancy ratio (i.e., the X/Cl ratios) in the Cl₂An/X₂An ligands was fixed using the ratio obtained using the halogen elemental analyses (ESI†). The polycrystalline samples of the X-Y series were macroscopically evaluated using the powder X-ray diffractograms (PXRD) recorded at room temperature and compared with the simulated patterns determined using the single crystal XRD (SC-XRD) analyses (Fig. 1). We investigated the highly doped compounds beyond the doping rate of 0.65 < Y and 0.42 < Y for X = Br and F, respectively. The polycrystalline samples for the highly doped compounds and X₂An^m−-pure compounds had different PXRD patterns from that of 0. Unfortunately, the SC-XRD analyses for the highly doped samples did not succeed because of their low quality single crystals.

Fig. 1 PXRD patterns of Br-Y (a) and F-Y (b) measured at room temperature.

The desolvated series X-Y-d were obtained by using the samples of X-Y; (NPr₄)₂[Fe₂(Cl₂An)_(3–3Y)(X₂An)_3Y] (X = Br; Br-0.10-d for Y = 0.10, Br-0.18-d for Y = 0.18, Br-0.35-d for Y = 0.35, Br-0.65-d for Y = 0.65, X = F; F-0.06-d for Y = 0.06, F-0.12-d for Y = 0.12, F-0.17-d for Y = 0.17, F-0.24-d for Y = 0.24, F-0.31-d for Y = 0.31, F-0.42-d for Y = 0.42) (Scheme 1b, Fig. S2, ESI†). The SC-XRD analysis at 103 K revealed that the X-Y-d compounds successfully changed to an isostructural 2D honeycomb layer structure with 0-d (Fig. S3, S4 and Tables S3, S4, ESI†). Both the Br-Y-d and F-Y-d were crystallized in the monoclinic space group, P2₁/n, with an asymmetric unit containing one crystallographically unique Feⁿ⁺ (Fe × 1), one and a half Cl₂An^m−/X₂An^m− ligands, and one NPr₄⁺ cation (Fig. S3, ESI†) NPr₄⁺ was located between the honeycomb anionic layers [Fe₂(Cl₂An/X₂An)₃]²⁻ formed by the Fe ions and Cl₂An^m−/X₂An^m− ligands, which were parallel to the (10–1) plane (Fig. S3, ESI†).

The oxidation states of the Feⁿ⁺ and X₂An^m− ligands for the series of X-Y and X-Y-d at 103 K were estimated from the local bond lengths of Fe–O and C–O in the Cl₂An^m−/X₂An^m− because their bond distances characteristically changed depending on their oxidation states.^15,16,19,29 At 103 K, the electronic states of all the site-doped compounds were estimated to be a low-temperature state, which was similarly observed in 0 and 0-d as ([(Fe^III)₂(Cl₂An²⁻)(Cl₂An˙³⁻)₂]²⁻) (see the ESI†).

The temperature dependence of the magnetic susceptibilities (χ = M/H) of Br-Y (Y = 0.10, 0.18, 0.35, 0.65) and F-Y (Y = 0.06, 0.12, 0.17, 0.24, 0.31, 0.42) was measured by applying a 1 kOe dc field (H_dc) in the temperature range of 300 to 1.8 K (Fig. 2a and 3a and Fig. S5 and S6, ESI†). The χ_mT values at 300 K were in the range of 9.72–10.07 cm³ K mol⁻¹ and 9.89–11.28 cm³ K mol⁻¹ for the series of Br-Y and F-Y, respectively, which were varied in a narrow range (Table S5, ESI†). However, it is difficult to conclude that the variation of χ_mT values at 300 K was caused by the electronic state modulations in the series because of the presence of a strong antiferromagnetic spin coupling between the Fe^III with S = 5/2 and X₂An˙³⁻ with S = 1/2 (Fig. S5, S6 and Table S5, ESI†).^{15,16,29–32} As the original 0 compound showed abrupt increases of χ_m and χ_mT at T_1/2a = 236 K upon cooling concomitant with the one-step transition from the intermediate phase of [Fe^IIFe^III(Cl₂An²⁻)₂(Cl₂An˙³⁻)]²⁻ to a low temperature phase of [Fe^III₂(Cl₂An²⁻)(Cl₂An˙³⁻)₂]²⁻, all doped X-Y compounds exhibited similar transition features in χ_m and χ_mT. However, their T_1/2a shifted to lower and higher temperatures with increasing doping rate Y for Br-Y and F-Y, respectively, compared with T_1/2a = 236 K for 0 (Fig. 2a and 3a); the T_1/2a values reached 184 K (Δ51 K from T_1/2a in 0) and 270 K (Δ34 K) for Br-0.65 and F-0.31, respectively. Specifically, T_1/2as in both Br-Y and F-Y linearly varied whereas the doping rate of Y had inversely proportional and proportional trends on Y, respectively (Fig. 4 and Fig. S7, ESI†), where the T_1/2a temperatures were determined from the peak in the dχ_mT/dT vs. T plots and their error bars were estimated from the half-value width of the peaks (Fig. S8, ESI†). The slopes of the T_1/2a variations were −77.3 and 110.3 K Y⁻¹ for Br-Y and F-Y, respectively. The F-Y series tended to have larger error bars with increasing Y than that of the Br-Y series, and T_1/2a was not assigned in F-0.42, which could be due to the large random domain effect of F-X. Nevertheless, these trends of T_1/2a in Br-Y and F-Y could be due to the site-doping of X₂An^m− with different electron acceptabilities, that is, F₂An²⁻ > Cl₂An²⁻ > Br₂An²⁻ (electronegativity of halogen atom: F > Cl > Br).^33,34 The magnetic profile in the entire temperature range from 300 to 1.8 K was similar to that for 0 excluding T_1/2a; however, the magnetic correlation lengths at low temperatures were slightly different from each other. Br-Y had a tendency to have a longer correlation length in highly doped compounds, whereas it was approximately inversed for F-Y (Fig. S5 and S6, ESI†). Furthermore, the superparamagnetic behavior of single-chain magnets was observed at lower temperatures as shown in the original compound 0 (Fig. S9, ESI†), which can be attributed to the ferrimagnetic chain motif of [–Fe^III–(Cl₂An˙³⁻/X₂An˙³⁻)–] (S_Fe = 5/2, S_rad = 1/2) separated by the diamagnetic Cl₂An²⁻/X₂An²⁻ bridges in the honeycomb layer (low-temperature phase in Scheme 1a).¹⁵

Fig. 2 Temperature dependence of χ_mT for Br-Y (a) and Br-Y-d (b) measured at H_dc = 1 kOe (χ_m = M_m/H_dc), where the narrow temperature ranges of 160–260 K and 260–400 K, respectively, were used.

Fig. 3 Temperature dependence of χ_mT for F-Y (a) and F-Y-d (b) measured at H_dc = 1 kOe (χ_m = M_m/H_dc), where narrow temperature ranges of 220–300 K and 300–400 K, respectively, were used.

Fig. 4 Variations of the T_1/2 value of TDETs for the site-doped rate Y in the solid solution series of X-Y and X-Y-d, whereas the basis of Y = 0 corresponded to 0 and 0-d. The left and right scales represent Br-Y/Br-Y-d and F-Y/F-Y-d, respectively. The solid lines are least-square linear fitting lines for the respective series.

The temperature dependence of the magnetic susceptibility for the doped series of the desolvated compounds X-Y-d was measured in the temperature range of 400–1.8 K. As the χ_m–and χ_mT − T plots for 0-d showed a two-step transition (abrupt increasing of χ_m and χ_mT upon cooling) at T_1/2b = 316 K and T_1/2c = 353 K, corresponding to the variation of the electronic state from [Fe^II₂(Cl₂An²⁻)₃]²⁻ at T > T_1/2c to [Fe^III₂(Cl₂An²⁻)(Cl₂An˙³⁻)₂]²⁻ at T < T_1/2bvia an intermediate state of [Fe^IIFe^III(Cl₂An²⁻)₂(Cl₂-An˙³⁻)]²⁻ at T_1/2b < T < T_1/2c (Scheme 1a),¹⁶ the site-doped X-Y-d series showed similar two-step transitions. The error bar was larger in the highly doped species in F-X-d (Fig. 2b and 3b), owing to the characteristic domain effect. Moreover, the transition temperatures of T_1/2b and T_1/2c in Br-Y-d clearly shifted linearly to lower temperatures. On the other hand, the tendency in F-Y-d was slightly different from that of F-Y. The transition temperature of T_1/2c in F-Y-d tended to shift to higher temperatures in the solid solution species with lower doping rates; however, T_1/2b was approximately constant in the series. As mentioned above, the domain effect was characteristic in the series of F-Y-d and F-Y. Hence, it is difficult to conclude that T_1/2b was not affected by the site-doping of F₂An^m−. Although there was no clear reason, the transition temperatures in F-Y-d were slightly affected by the doping of the site-doping of F₂An^m−. The slope of the T_1/2b and T_1/2c variations versus Y was −41.7 and –52.7 K Y⁻¹ for Br-Y-d and 2.0 and 46.5 K Y⁻¹ and F-Y-d (Fig. S7, ESI†). The difference in tendencies can be explained by the electron-acceptability of F₂An²⁻ > Cl₂An²⁻ > Br₂An²⁻.

The site-doping of F₂An^m− influenced the magnetic phase of the solid-solution materials of F-Y-d. The F-Y-d species with Y ≤ 0.24 have a maximum of χ_m at approximately 23 K followed by a steep decrease after further cooling. This was indicative of an antiferromagnetic ground state as similarly found in 0-d and Br-Y-d (Fig. S5, ESI†); however, the decrease of χ_m did not occur in the cases with higher doping rate. Instead, the superparamagnetic behavior of single-chain magnets was observed in the low-temperature phase of F-Y-d with 0.24 < Y (Scheme 1a).

In summary, we succeeded in the precise tuning of multiple TDET at three T_1/2s transition temperatures through the chemical-site doping of two types of tetraoxolene-bridged Fe-based honeycomb layer compounds. This provided an “on-demand TDET” in a temperature range of 184 to 365 K.

We thank H. Sato (Rigaku Co. Ltd) for his assistance with single-crystal crystallography. This work was supported by a Grant-in-Aid for Scientific Research (No. 18K19050, 18H05208, 19K15518, and 20H00381) from the MEXT, Japan, and E-IMR project. Y. S. is thankful for financial support received from the Shorai Foundation for Science and Technology, and research grant from The Mazda Foundation. J. C. gratefully acknowledges the financial support received from the China Scholarship Council (CSC) and Institute for Materials Research, Tohoku University.

Conflicts of interest

There are no conflicts to declare.

Notes and references

1. O. Sato, Nat. Chem., 2016, 8, 644–656.
2. M. E. Guillermo and E. Coronado, Chem. Soc. Rev., 2018, 47, 533–557.
3. A. Dei, D. Gatteschi, C. Sangregorio and L. Sorace, Acc. Chem. Res., 2004, 37, 827–835.
4. E. Coronado, Nat. Rev. Mater., 2019, 5, 87–104.
5. J. S. Miller and K. S. Min, Angew. Chem., Int. Ed., 2009, 48, 262–272.
6. M. Mitsumi, T. Nishitani, S. Yamasaki, N. Shimada, Y. Komatsu, K. Toriumi, Y. Kitagawa, M. Okumura, Y. Miyazaki, N. Córska, A. Inaba, A. Kanda and N. Hanasaki, J. Am. Chem. Soc., 2014, 136, 7026–7037.
7. M. Nihei, Y. Sekine, N. Suganami, K. Nakazawa, A. Nakano, H. Nakano, Y. Murakami and H. Oshio, J. Am. Chem. Soc., 2011, 133, 3592–3600.
8. K. Boukheddaden, E. D. Loutete-Dangui, E. Codjovi, M. Castro, J. A. Rodriguéz-Velamazán, S. Ohkoshi, H. Tokoro, M. Koubaa, Y. Abid and F. Varret, J. Appl. Phys., 2011, 109, 013520.
9. N. Hoshino, F. Iijima, G. N. Newton, N. Yoshida, T. Shiga, H. Nojiri, A. Nakao, R. Kumai, Y. Murakami and H. Oshio, Nat. Chem., 2012, 4, 921–926.
10. R. M. Buchanan and C. G. Pierpont, J. Am. Chem. Soc., 1980, 102, 4951–4957.
11. D. F. Li, R. Clérac, O. Roubeau, E. Harté, C. Mathonière, R. L. Bris and S. M. Holmes, J. Am. Chem. Soc., 2008, 130, 252–258.
12. D. Kiriya, H. C. Chang and S. Kitagawa, J. Am. Chem. Soc., 2008, 130, 5515–5522.
13. H. Miyasaka, N. Motokawa, T. Chiyo, M. Takemura, M. Yamashita, H. Sagayama and T. Arima, J. Am. Chem. Soc., 2011, 133, 5338–5345.
14. M. Nihei, Y. Okamoto, Y. Sekine, N. Hoshino, T. Shiga, I. P. C. Liu and H. Oshio, Angew. Chem., Int. Ed., 2012, 51, 6361–6364.
15. J. Chen, Y. Sekine, Y. Komatsumaru, S. Hayami and H. Miyasaka, Angew. Chem., Int. Ed., 2018, 57, 12043–12047.
16. J. Chen, Y. Sekine, A. Okazawa, H. Sato, W. Kosaka and H. Miyasaka, Chem. Sci., 2020, 11, 3610–3618.
17. K. Nakabayashi and H. Miyasaka, Chem. – Eur. J., 2014, 20, 5121–5131.
18. K. Nakabayashi, M. Nishio and H. Miyasaka, Inorg. Chem., 2016, 55, 2473–2480.
19. J. A. DeGayner, K. Y. Wang and T. D. Harris, J. Am. Chem. Soc., 2018, 140, 6550–6553.
20. W. Kosaka, Y. Takahashi, M. Nishio, K. Narushima, H. Fukunaga and H. Miyasaka, Adv. Sci., 2018, 5, 1700526.
21. J. B. Torrance, A. Girlando, J. J. Mayerle, J. I. Crowley, V. Y. Lee and P. Batail, Phys. Rev. Lett., 1981, 47, 1747–1750.
22. S. Horiuchi, R. Kumai, Y. Okimoto and Y. Tokura, Chem. Phys., 2006, 325, 78–91.
23. S. Horiuchi, R. Kumai, Y. Okimoto and Y. Tokura, Phys. Rev. B: Condens. Matter Mater. Phys., 1999, 59, 11267–11275.
24. G. Saito and Y. Yoshida, Bull. Chem. Soc. Jpn., 2007, 80, 1–137.
25. M.-H. Zeng, B. Wang, X.-Y. Wang, W.-X. Zhang, X.-M. Chen and S. Gao, Inorg. Chem., 2006, 45, 7069–7076.
26. M. Nishio, N. Hoshino, W. Kosaka, T. Akutagawa and H. Miyasaka, J. Am. Chem. Soc., 2013, 135, 17715–17718.
27. S. Chorazy, J. J. Stanek, W. Nogas, A. M. Majcher, M. Rams, M. Kozieł, E. Juszyńska-Gałązka, K. Nakabayashi, S.-I. Ohkoshi, B. Sieklucka and R. Podgajny, J. Am. Chem. Soc., 2016, 138, 1635–1646.
28. L. Liu, L. Li, M. E. Ziebel and T. D. Harris, J. Am. Chem. Soc., 2020, 142, 4705–4713.
29. J. A. DeGayner, I. R. Jeon, L. Sun, M. Dincă and T. D. Harris, J. Am. Chem. Soc., 2017, 139, 4175–4184.
30. A. Dei, D. Gatteschi, L. Pardi and U. Russo, Inorg. Chem., 1991, 30, 2589–2594.
31. I. R. Jeon, B. Negru, R. P. Van Duyne and T. D. Harris, J. Am. Chem. Soc., 2015, 137, 15699–15702.
32. S. A. Sahadevan, A. Abhervé, N. Monni, D. P. Sáenz, J. R. Galán-Mascarós, J. C. Waerenborgh, B. J. C. Vieira, P. Auban-Senzier, S. Pillet, E. Bendeif, P. Alemany, E. Canadell, M. L. Mercuri and N. Avarvari, J. Am. Chem. Soc., 2018, 140, 12611–12621.
33. M. Atzori, S. Benmansour, G. Mínguez Espallargas, M. Clemente-León, A. Abhervé, P. Gómez-Claramunt, E. Coronado, F. Artizzu, E. Sessini, P. Deplano, A. Serpe, M. L. Mercuri and C. J. Gómez-García, Inorg. Chem., 2013, 52, 10031–10040.
34. R. Murase, B. F. Abrahams, D. M. D'Alessandro, C. G. Davies, T. A. Hudson, G. N. L. Jameson, B. Moubaraki, K. S. Murray, R. Robson and A. L. Sutton, Inorg. Chem., 2017, 56, 9025–9035.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental details associated with this research; Tables S1–S5, Fig. S1–S8. CCDC 2014803–2014822. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0cc03808c

---