Authors: Shihao Liu | Yi‐Fei Deng | Ziyi Chen | Lingyi Meng | Xiaoyong Chang | Zhiping Zheng | Yuan‐Zhu Zhang
Source: CCS Chemistry (article, 2020-10-28)
Field: Materials Science — Electronic, Optical and Magnetic Materials

Secondary Metal Coordination Using a Tetranuclear Complex as Ligand Leading to Hexanuclear Complexes with Enhanced Thermal Barriers for Electron Transfer Open AccessCCS ChemistryRESEARCH ARTICLE1 Sep 2021Secondary Metal Coordination Using a Tetranuclear Complex as Ligand Leading to Hexanuclear Complexes with Enhanced Thermal Barriers for Electron Transfer Shihao Liu†, Yi-Fei Deng†, Zi-Yi Chen, Lingyi Meng, Xiaoyong Chang, Zhiping Zheng and Yuan-Zhu Zhang Shihao Liu† Department of Chemistry, Southern University of Science and Technology (SUSTech), Shenzhen 518055 , Yi-Fei Deng† Department of Chemistry, Southern University of Science and Technology (SUSTech), Shenzhen 518055 , Zi-Yi Chen Department of Chemistry, Southern University of Science and Technology (SUSTech), Shenzhen 518055 , Lingyi Meng Department of Chemistry, Southern University of Science and Technology (SUSTech), Shenzhen 518055 , Xiaoyong Chang Department of Chemistry, Southern University of Science and Technology (SUSTech), Shenzhen 518055 , Zhiping Zheng Department of Chemistry, Southern University of Science and Technology (SUSTech), Shenzhen 518055 and Yuan-Zhu Zhang *Corresponding author: E-mail Address: [email protected] Department of Chemistry, Southern University of Science and Technology (SUSTech), Shenzhen 518055 https://doi.org/10.31635/ccschem.020.202000552 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Postsynthesis of the paramagnetic square-shaped complex {[(Tp*Me)Fe(μ-CN)2(CN)][Co(dmbpy)2]}2(BPh4)2·6MeCN·H2O [ 1, Tp*Me = tris(3,4,5-trimethylpyrazole)-borate; dmbpy = 4,4′-dimethyl-2,2′-bipyridine)] by grafting transition metal(II) thiocyanates via its terminal cyano groups afforded three hexanuclear [Fe2Co2M2] clusters (M = Zn, 2; Co, 3; Cd, 4). The peripheral metal-complex units serving as excellent electron acceptors were found to help stabilize the low-temperature state of FeII,LS–CoIII,LS within the complex core. As a result, the desolvated complexes 2– 4 underwent reversible and sharp thermally induced electron-transfer behavior with the transition temperatures (T1/2) up to 312, 296, and 365 K, respectively, demonstrating an effective means of manipulating thermal barriers of the celebrated cyano-bridged square core. Download figure Download PowerPoint Introduction The engineering of magnetically switchable molecular materials involving both thermally and photoinduced electronic spin transitions is a topical research area aimed at the development of next-generation materials for memory, display, switching, and spintronic devices.1–6 These materials may exhibit magnetic bistability or multistability with sensitive response to external stimuli, such as temperature, light, pressure, and so forth. In addition to the significant changes in magnetism, modulation on spin state may also endow materials with other synergizing functionalities, such as catalytic activity,7–10 negative thermal expansion,11,12 luminescence, and fluorescence.13–16 Among the many complexes studied, cyanometallates featuring cyano-bridged metal centers of different kinds are most prominent; the small linear cyano ligand provides an intimate linkage, allowing for through-bond electron transfer (ET) to occur. Furthermore, using a building block approach, molecular precursors are allowed to self-assemble toward a common structural archetype, which may be further engineered readily and systematically at the atomic level.17–20 The research along this line started with the reports by Sato and co-workers21–24 in which a three-dimensional (3D) Fe/Co analog of Prussian blue (PB) exhibited thermally and photoinduced ET between the paramagnetic FeIII,LS–CN–CoII,HS and the diamagnetic FeII,LS–CN–CoIII,LS configurations [high spin (HS) and low spin (LS)], accompanied by significant color changes. Dunbar and co-workers25,26 subsequently moved the research forward with a series of cyano-bridged trigonal bipyramidal {Fe2Co3} complexes. Later, Holmes and co-workers27 reported the first [Fe4Co4] cubic complex whose tunable magnetic properties mimic that of PB. Since then, with the use of molecular building blocks with judiciously disposed cyano ligand(s), a series of discrete cyanometallate complexes containing different Fe–CN–Co motifs28–34 as well as one-dimensional (1D) chains were obtained,35–38 with which interesting ET behaviors and the underlying electronic structures were thoroughly studied.39–41 Of particular interest amongst them are cyano-bridged [Fe2Co2] squares with Fe-bound terminal cyano ligands because their electronic structures can be systematically tuned either by modifying the coordination environment of individual metal centers or by regulating intermolecular interactions in the complexes. For example, equipping correct ligands with electron-donating functional groups around the Co sites or the Fe ones in the square complexes moved the transition to a higher or lower temperature.42–49 Meanwhile, the intermolecular supramolecular interactions, such as π–π stacking, H-bonding interactions, and even solvent polarity have also been found to have profound effects on the stabilization of low-temperature (LT) states.50–54 Specifically, the H-bonded lattice solvents acting as electron acceptors may pull down the charge density around Fe centers and stabilize the Fe(II) state to some degree, thus leading to a higher thermal barrier for ET. As an example, Oshio and co-workers55 introduced Brønsted acids into a square complex by cocrystallization, and the resulting H-bonded supramolecular assemblies showed multistepped spin transitions toward room temperature. The supramolecular approach, though effective, is inherently limited since such systems are not particularly thermally stable. For example, such systems generally contain a large number of solvent molecules that are readily lost upon heating, therefore eliminating their interesting ET behavior toward possible applications.56–58 In comparison with the supramolecular interactions, covalent bonds are more powerful and thermally stable. Given that a terminal cyano ligand has a free and coordination-capable N atom, postmodification by using the complex as ligand for the coordination of certain secondary (versus the primary metal ions in the parent complex) metal ions may be achieved, but remains very challenging due to the presence of electronic repulsion and steric hindrance.59 In this work, we report our findings along this line by first constructing a tetranuclear complex ( 1) that includes the well-known square-shaped cationic core of [Fe2Co2]2+, followed by its postsynthesis of a series of hexanuclear complexes [Fe2Co2M2] (M = Zn, 2; Co, 3; Cd, 4) with the secondary anionic metal(II) thiocyanates attached to the square core by way of the terminal cyano ligands (Scheme 1). In sharp contrast to 1, all the hexanuclear complexes underwent thermally induced reversible ET events at near room temperature—a key property for possible practical device applications of such materials. The profound electronic effects of postmodification by peripheral metal coordination are discussed below using complex 2 as representative. Related results obtained for complexes 3 and 4 are provided in the Supporting Information. Scheme 1 | Postsynthesis of square to hexanuclear complexes. Download figure Download PowerPoint Experimental Methods Materials and syntheses [NEt4][(Tp*Me)Fe(CN)3] [Tp*Me = tris(3,4,5-trimethylpyrazolyl)borate] was synthesized according to the literature.60 All other chemicals and reagents were commercially available and used without further purification. Synthesis of {[(Tp*Me)Fe(CN)3]2[Co(dmbpy)2]2[BPh4]2}·6MeCN·H2O (1) Treatment of CoCl2·6H2O (23.8 mg, 0.1 mmol), NaBPh4 (68.6 mg, 0.2 mmol), and 4,4′-dimethyl-2,2′-bipyridine (dmbpy, 36.8 mg, 0.2 mmol) in water (1 mL) and acetonitrile (5 mL) at room temperature afforded a clear orange solution that was stirred for 2 h. [NEt4][(Tp*Me)Fe(CN)3] (62.0 mg, 0.1 mmol) in MeCN (10 mL) and H2O (1 mL) were added into the above solution. After being stirred for 5 min, the dark red mixture was filtered and allowed to stand at room temperature for several days. Dark red crystals were obtained and isolated via filtration, then washed quickly with a cold 5∶1 (v∶v) MeCN/MeOH (1 mL) solution. Yield: 79 mg (64.5 %). Anal. Calcd for C150H164B4Co2Fe2N32O ( 1): C, 66.63; H, 6.11; N, 16.58. Found: C, 66.42; H, 5.96; N, 16.72. Infrared (IR) (KBr/Nujol, cm−1; 300 K): 3627 (w), 3424 (w), 2729 (w), 2538 (m), 2250 (w), 2152 (s), 2126 (w), 1637 (w), 1063 (s), 1598 (s), 1577 (m), 1566 (m), 1520 (m), 1490 (m), 1456 (vs), 1442 (vs), 1377 (vs), 1365 (vs), 1312 (s), 1243 (vs), 1173 (s), 1153 (s), 1101 (s), 1062 (m), 1032 (w), 1019 (s), 918 (w), 889 (w), 837 (m), 834 (m), 812 (w), 769 (vs), 735 (vs), 708 (vs), 667 (w), 651 (m), 625 (m), 613 (m), 607 (m). Synthesis of {[(Tp*Me)Fe(CN)3]2[Co(dmbpy)2]2[Zn(NCS)3]}·6DMF·2H2O (2) Treatment of 1 (67.5 mg, 0.025 mmol), Zn(ClO4)2·6H2O (18.5 mg, 0.05 mmol), and NaSCN (12.1 mg, 0.15 mmol) in acetonitrile (5 mL) at room temperature gave green precipitate. The precipitate was dissolved in dimethylformamide (DMF) (3 mL) which was layered by Et2O, and green crystals were obtained after 3 days. Yield: 42.1 mg (61.9%). Anal. Calcd for C114H150B2Co2Fe2N38O8S6Zn2 ( 2): C, 49.70; H, 5.49; N, 19.32; S, 6.98. Found: C, 49.92; H, 5.24; N, 19.15; S, 7.26. Anal. Calcd for the desolvated sample ( 2d, C96H104B2Co2Fe2N32S6Zn2): C, 50.56; H, 4.60; N, 19.65; S, 8.44. Found: C, 50.39; H, 4.46; N, 19.78; S, 8.32. IR (KBr/Nujol, cm−1; 300 K): 3132 (w), 2920 (m), 2860 (m), 2515 (m), 2360 (m), 2341 (m), 2111 (s), 2088 (vs), 2057 (vs), 1662 (vs), 1618 (s), 1558 (m), 1496 (m), 1434 (s), 1411 (m), 1386 (s), 1355 (m), 1323 (w), 1301 (m), 1280 (m), 1245 (s), 1220 (m), 1190 (w), 1172 (m), 1092 (s), 1037 (m), 1002 (m), 960 (w), 926 (m), 889 (w), 871 (m), 831 (s), 730 (m), 696 (m), 660 (m), 634 (m), 613 (w). Physical measurements X-ray crystallographic data The single-crystal data for complexes 1– 4 at 100 K were collected on a Bruker D8 VENTURE diffractometer (Shenzhen, China) with monochromated Mo Kα radiation (λ = 0.71073 Å). Lorentz/polarization corrections were applied during data reduction, and the structures were solved by the direct method (SHELXS-2014).61 Refinements were performed by full-matrix least-squares (SHELXL-2014) on F2, and empirical absorption corrections (SADABS) were applied.62,63 Anisotropic thermal parameters were used for the nonhydrogen atoms. Hydrogen atoms were added geometrically and refined using a riding model. Weighted R factors (wR) and all the goodness-of-fit (S) values are based on F2; conventional R factors (R) are based on F, with F set to zero for negative F2. Notably, the lattice solvent molecules in 2 and 4 could not be properly modeled; therefore, their contributions were partially subtracted by the SQUEEZE program implemented in Olex2 (Durham University, UK). Data collection and structural refinement parameters are given in Supporting Information Table S1, and selected bond lengths and angles are given in Supporting Information Tables S2–S4. CCDC-1998180 ( 1), CCDC-1998181 ( 2), CCDC-1998178 ( 3), and CCDC-1998179 ( 4) contain the crystallographic data that can be obtained via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB21EZ, UK; fax: (+44) 1223-336-033; or [email protected]cam.ac.uk). Powder X-ray diffraction Variable-temperature powder X-ray diffraction (PXRD) measurements were recorded using a Rigaku SmartLab X-ray diffractometer with Cu Kα radiation (45 kV, 200 mA) between 5° and 40° (2θ). The simulated patterns are calculated from the single-crystal data at 100 K. Thermogravimetric analysis The thermogravimetric analysis (TGA) measurements were carried out on freshly filtered crystals using the METTLER TOLEDO TGA2 instrument in an insert argon atmosphere over a temperature range of 30–500 °C with a heating rate of 10 °C /min. Elemental analysis Elemental analyses (EA) (C, H, N, S) were measured by a vario electroluminescence (EL) cube CHNOS Elemental Analyzer Elementar Analysensysteme GmbH. Fourier transform IR spectroscopy Fourier transform IR (FT-IR) spectra were recorded in the range 600–4000 cm−1 on a Bruker TENSOR II spectrophotometer. Variable temperature data were obtained by in situ heating and cooling in the sample holder. Magnetic and photomagnetic measurements Magnetic susceptibility data under 1 kOe direct current (dc) field were collected at temperatures between 2 and 400 K using a SQUID MPMS3 magnetometer. To prevent the loss of lattice solvents, fresh crystals were restrained in a minimum amount of mother liquor, which was loaded into the equipment and quickly cooled down to 200 K before the measurements. Solutions of 2– 4 for magnetic measurements were prepared by dissolving the fresh crystals in DMF solvents. For the photomagnetic experiments, irradiation was performed on the fresh sample at 20 K, and light from a Diode Pumped Solid State laser (808 nm; 10 mW, MDL-III-808 nm, Changchun New Industries Optoelectronics Technology Co., Ltd laser) was guided via a flexible optical fiber (CNI fiber; 5 m length) into the SQUID magnetometer. Magnetic data were corrected for the diamagnetism of the sample holder and for the diamagnetism of the sample using Pascal's constants.64 Results and Discussion Crystal structure Single crystals of 1 were obtained by slow evaporation of a solution mixture of CoCl2·6H2O, NaBPh4, dmbpy, and [NEt4][(Tp*Me)Fe(CN)3] in acetonitrile at room temperature. Similar to other reported square analogues,40 the adjacent [(Tp*Me)Fe(CN)3]− and [Co(dmbpy)2]2+ blocks in 1 that reside in the alternate corners are linked by cyano groups, giving a nearly square planar core of {Fe2(μ-CN)4Co2}2+ with two Fe-bound terminal cyano groups in anti-orientation ( Supporting Information Figures S1 and S2). Each Co ion is surrounded with six N atoms, two of which are from the bridging cyano ligands and the remaining four are from two chelating dmbpy ligands. The average Co–N bond length of 2.074 Å is consistent with the HS state of the CoII ion, thus giving a paramagnetic [FeIII,LS2CoII,HS2] configuration. Treatment of 1 with Zn(ClO4)2·6H2O and NaSCN, first in acetonitrile and then DMF/Et2O, afforded the hexanuclear complex 2. The single-crystal X-ray diffraction (SCXRD) study at 100 K revealed that 2 crystallized in the monoclinic space group P21/n with the cyano-bridged neutral hexanuclear structure [Fe2Co2Zn2] ( Supporting Information Table S1). In comparison with 1, the most salient feature for 2 is the two peripheral [Zn(NCS)3] − fragments, which were grafted on the parent square core of 1 through the Fe-bond terminal cyano groups (Figure 1). As such, each of the Zn(II) centers is situated in a tetrahedron coordination sphere formed by four nitrogen atoms, three from thiocyanates and one from the cyano ligand. Although the surroundings of each Co center are similar to those in 1, the average Co–N bond length of 1.92 Å is significantly shorter (0.15 Å) than that in 1. This value is in good agreement with the LS state CoIII ion, and thus suggests the diamagnetic [FeII,LS2CoIII,LS2ZnII2] state. The Fe–C≡N–Co cyano linkages are nearly linear with the Fe–C≡N and Co–N≡C angles in the range of 175.6–179.1° and 174.2–175.9°, respectively, similar to those in 1. The individual complex units of 2 are fairly separated with the closest intercomplex M···M contacts of 9.8 Å ( Supporting Information Figure S3). No crystal data at above room temperature can be obtained due to the loss of lattice solvents. The TGA studies revealed a weight loss of 16.5% for 2 at below 100 °C ( Supporting Information Figure S8), corresponding to the release of six DMF and two H2O molecules (calculated 17.2%) per formula unit. It is followed by a broad region of thermal stability up to ca. 210 °C, indicating that the structural integrity may be maintained during the desolvation process. Accordingly, the validity of structural models obtained from SCXRD studies for the respective bulk samples was further investigated by means of the in situ variable temperature PXRD method (Figure 2). At 298 K, the PXRD patterns for 2– 4 are in agreement with their SCXRD simulations, indicating the good purity of the samples. On the heating cycle between 298 and 390 K, the PXRD peaks for 2 and 3 underwent significant decrease in intensity at around 6° and 6.6°, accompanied by the enhancement of the peak at ca.7°, possibly due to the phase transition during the desolvation process (Figure 2 and Supporting Information Figures S11). Combined with the EA studies of the desolvated sample, the PXRD characteristics of 2 and 3 may be attributed to the changes in space group rather than the structural framework. For complex 4, the PXRD peaks at ca. 5.1° ({100} facets) disappeared with temperatures at above 320 K ( S12). Given that the {100} facets are located at the interfaces within the packing arrangements of 4 ( Supporting Information Figure S13), such behavior is primarily ascribed to the crystallographic disorders caused by the loss of lattice solvents. Moreover, all the desolvated analogues preserved similar PXRD patterns ( Supporting Information Figure S14). These results support the notion that the structural integrity is valid for the bulk samples of 2– 4 both in solvated and desolvated forms. As the temperature was lowered, the diffraction patterns remained almost the same at 380–250 K, suggesting the excellent thermal stability of the desolvated phase ( 2d–4d). It should be noted that the small shift between the entire experimental and simulated PXRD patterns is ascribed to the differences in the measurement temperatures: the SCXRD data are collected at 100 K, whereas the PXRD patterns are measured at 298 K. And the decreased quality of the experimental PXRD patterns presented as the broadening of the peaks is mainly attributed to the significant vibrations of the powder that produced by the in situ thermal desolvation process in the PXRD instrument. Figure 1 | Molecular structure of 2 at 100 K. Color codes: Fe, green; Zn, cyan; C, gray; N, blue; B, lavender; S, yellow. Hydrogen atoms and interstitial solvent molecules are omitted for clarity. Download figure Download PowerPoint Figure 2 | In situ variable temperature PXRD for 2 (left, heating cycle) and 2d (right, cooling cycle). The simulated pattern is calculated based on the single-crystal data at 100 K. Download figure Download PowerPoint IR spectroscopy The intramolecular ET behavior of 2 was first investigated by variable temperature IR spectra in its solid state (Figure 3), which displayed temperature-dependent fingerprint signals of the cyano vibrations, reminiscent of other structurally related and thermally responsive cyano-bridged Fe/Co complexes.42–48 At 298 K, the IR spectra of 2 exhibit absorptions at 2516 and to the and for the The other two peaks at 2088 and cm−1 are as the of the of and heating to K, the to higher of but the at cm−1 accompanied with the of absorptions and of the paramagnetic bridging cooling to room temperature, the IR spectra of the in situ desolvated sample ( showed a reversible and almost to its suggesting the of reversible thermal between the paramagnetic phase and the diamagnetic Figure 3 | In situ variable temperature spectra for 2 (left, heating and 2d (right, cooling Download figure Download PowerPoint Magnetic properties that ET in 2 was in a series of magnetic Variable temperature magnetic susceptibility data for the fresh samples for 1 and 2 as well as the in situ desolvated of 2d were collected in the applied field of 1 For 1, the of K at 300 K is consistent with the value for the paramagnetic [FeIII,LS2CoII,HS2] state (Figure 4). the to a of K at 10 K, suggesting intramolecular It then to K at 2 K, due to the magnetic intermolecular The as a of the applied field were collected at K ( Supporting Information Figure which quickly at small external and at 2 K and consistent with the [FeIII,LS2CoII,HS2] and intramolecular The is attributed to the magnetic of metal No ET process was indicating that 1 is thermally Figure 4 | magnetic susceptibility of 1 in the dark (1 2 and 2d measured in the dark (1 and after light irradiation (808 nm, 10 at 20 K (10 Download figure Download PowerPoint For the of K at below 300 K is in good agreement with the diamagnetic [FeII,LS2CoIII,LS2ZnII2] state. heating at a rate of 2 the value before a at = K with a value of K at 400 K, which is attributed to a desolvated phase as by the variable and consistent with the paramagnetic in desolvated (Figure 4). Given that the lattice solvents lost in the same temperature the ET was to be induced by the loss of solvent behavior is further by the results of solution studies for which the diamagnetic state was maintained at K ( Supporting Information Figure the sample of 2 was maintained in the under at 400 K for 1 for the desolvated ( the value of 2d remains down to K, and then at = K to a value of K at K, which to K at 2 K. Furthermore, this is reversible heating the sample to 400 K. To the possible of the ET the magnetic properties were further investigated at low temperatures under light a selected laser light (808 nm, 10 and at 20 K ( Supporting Information Figures and the of both 2 and 2d a of and K after 10 respectively, for those Fe(II) charge with the diamagnetic phase being partially in to the value of 1) to the paramagnetic state. As in Figure 4, the for both 2 and 2d exhibited a similar to that in 1 at below K. then readily to the more diamagnetic state at near K ( and K ( These results are with those reported for cyano-bridged Fe/Co this postsynthesis to the parent complex was to the of other secondary metal Specifically, two other hexanuclear complexes ( = = 4, = = were obtained by the same for the of 2. It should be that is to be as and in either or and a of have been In this SCXRD and EA studies more support the of rather than or Given that the of atoms is the be from the bond that may have with the Co or metal The further both complexes 3 and 4 were structurally and magnetically Similar to both complexes 3 and 4 and their desolvated analogues ( and exhibited thermally and induced ET behaviors at different temperatures 1). are in the Supporting Information. Table 1 | The for All the Complexes 2 2d 3 4 365 hexanuclear complexes were obtained by postmodification of the well-known parent of the cyano-bridged square by secondary metal The grafted metal complex by as effective electron are of the electronic structures of the resulting complexes with to their parent complex and therefore, their ET behavior in a temperature In comparison with the electronic caused by supramolecular interactions, direct of the electronic structure of the parent square via secondary metal coordination not the with but also provides the to the electronic and the properties with the peripheral This thus a to the thermal barrier for ET of the celebrated leading to materials with bistability and multistability of Supporting Information Supporting Information is of is of interest to Information This was by the Science of and and from 1. Molecular Materials 2. 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