Acidity-Driven Bidirectional Room-Temperature Spin-State Switch and Fluorescence Modulation of a Mononuclear Fe(II) Complex

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Aug 2021Acidity-Driven Bidirectional Room-Temperature Spin-State Switch and Fluorescence Modulation of a Mononuclear Fe(II) Complex Yi-Shan Ye, Xiu-Qin Chen, Kai-Yan Shen, Ming-Liang Tong and Xin Bao Yi-Shan Ye School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094 , Xiu-Qin Chen School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094 , Kai-Yan Shen School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094 , Ming-Liang Tong School of Chemistry, Sun Yat-Sen University, Guangzhou 510275 and Xin Bao *Corresponding authors: E-mail Address: [email protected] School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094 https://doi.org/10.31635/ccschem.020.202000452 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Room-temperature switchable materials showing multiple responses toward external stimuli are highly desired. Herein, we report bidirectional spin-state switch and fluorescence modulation of an Fe(II) complex ( 1) based on a rhodamine B 2-pyridinecarbaldehyde hydrazone ligand in both the solid state and solution. The complex is predominantly stabilized in the low-spin (LS) state at room temperature due to the strong ligand-field strength imposed by acylhydrazone pyridine. Heating to 400 K results in changes of the spin state to predominantly high spin (HS) due to solvent loss, and the dehydrated sample shows reversible incomplete spin crossover (SCO) in the following thermal cycles. Besides thermally-induced spin-state change, acidification treatment of complex 1 results in iminol–amide tautomerization and dissociation of the complex, and, hence, a switch of the spin state to HS at room temperature. Concurrent fluorescence enhancement was observed and attributed to the intrinsic response of the rhodamine luminophore toward acid perturbation. Reverse switching was achieved upon further alkalization treatment. Download figure Download PowerPoint Introduction Iron(II) spin crossover (SCO) complexes represent a highly promising class of molecule-based switchable materials where the spin state can be reversibly switched between a diamagnetic low-spin (LS) state and a paramagnetic high-spin (HS) state upon external stimuli.1,2 The associated changes of chemical and physical properties give rise to possible applications as memory devices, molecular switches, optical/electronic devices, and sensors.3–5 Bistability at room temperature is clearly a prerequisite for most prospective applications. This will rely on the successful manipulation of both transition temperature and large enough elastic interactions (also called cooperativity) between the magnetic centers, which is a great challenge for synthetic chemists. The SCO properties are not only determined by the first coordination sphere but also strongly depend on secondary noncovalent interactions. Although vast SCO complexes have been reported in recent decades, very rarely do some of them show large enough hysteresis spanning room temperature.6–16 The combination of SCO and fluorescence is highly appealing in the development of new multifunctional materials. Synergetic effects of the two functions is a goal as it allows modulation of fluorescence emission upon spin state changes, and conversely, spin-state "read out" by monitoring the emission intensity. Coordination of SCO active metal ions with fluorescent ligands17–25 and construction of fluorescent-SCO composite24–31 are two well-developed strategies. Abnormal temperature-dependent fluorescence variations were observed in the SCO region due to different energy transfers in the two states and, in most cases, an enhanced fluorescence in the HS state was observed. Spin bistability at a constant temperature will be ideal in maximizing luminescence modulation upon SCO and eliminating thermal quenching interference. Easy modulation of magnetic properties toward pH stimuli at constant temperature is promising for the design of new classes of switchable complexes and sensors. Only a few pH responsive SCO examples reported, and the modulation of magnetic properties is due to the change of the overall ligand-field strength upon ligand protonation/deprotonation.32–36 Nowak et al.37 reported proton-driven coordination-induced spin-state switching (PD-CISSS) of iron(II) complexes in solution and as composite materials. Recently, we also reported successful switching of the spin state of an Fe(II) complex in solution as a result of reversible dissociation–association of the coordination sphere driven by protonation and deprotonation.38 The latter two cases open up a reliable route for pursuing bistable spin states at room temperature. Spin-state manipulation relies on changes in the first coordination sphere rather than uncontrolled intermolecular interactions. Nevertheless, drastic structural modification may bring challenges for reversibility,39 especially in the case of solid-state switching.38,40 Bidirectional switching through grinding in the solid state was unsuccessful in our previous report.38 As a continuation of our work on acidity-driven switchable materials, we aim to develop a new room-temperature switch that combines fluorescence. Rhodamine B derivatives integrated with an acylhydrazone-pyridine N2O coordination site (Scheme 1) were chosen as the ligand for the following reasons: (1) The strong ligand-field strength of acylhydrazone pyridine40–44 is expected to stabilize the desired LS state of the complex at room temperature. (2) The lability of acylhydrazone between iminol and amide tautomers is expected to facilitate the modulation of the first coordination sphere.38 (3) The rhodamine moiety can serve as a pH-responsive luminophore of which the ring-opened form is strongly fluorescent.45,46 It should be noted that Yuan et al.21 recently reported an Fe(II) complex exhibiting synergetic SCO and fluorescence based on a rhodamine 6G hydrazone ligand. However, the spin conversion was induced thermally instead of with pH perturbations. Herein, we report a new Fe(II) complex whose spin state and luminescence intensity are reversibly switched at room temperature in both solution and solid state upon changes in acidity. Experimental Methods General All reagents obtained from commercial sources were used without further purification. Safety note: Perchlorate salts are potentially explosive, and caution should be taken when dealing with such materials. Synthetic procedures Synthesis of 3′,6′-Bis(diethylamino)-2-[(2-pyridinylmethylene)amino]spiro[1H-isoindole-1,9′-[9H]xanthen]-3(2H)-one (L) The ligand was synthesized according to a reported reference.47 The crystals suitable for single-crystal X-ray diffraction were recrystallized in methanol. 1H NMR [500 MHz, methanol-d4/chloroform-d = 2∶1, δ (ppm)]: 8.46 (d, J = 4.4 Hz, 1H), 8.38 (s, 1H), 8.05–7.98 (m, 2H), 7.60 (t, J = 7.7 Hz, 1H), 7.50–7.10 (m, 2H), 7.14–7.10 (m, 2H), 6.55 (d, J = 8.8 Hz, 2H), 6.45 (s, 2H), 6.24 (d, J = 8.8 Hz, 2H), 3.31 (q, J = 7.0 Hz, 8H), 1.14 (t, J = 7.0 Hz, 12H). Synthesis of {[Fe(L)(HL)](ClO4)3}·nH2O (1) To a 11 mL solution of L ligand (0.027 g, 0.05 mmol) in methanol and dichloromethane (ratio 10∶1), 0.18 g (0.5 mmol) of Fe(ClO4)2·6H2O was added, and the resulting reaction mixture was stirred for 10 min. The reaction mixture was then filtered and the dark purple filtrate was evaporated, where well-shaped single crystals were obtained in 2 days in 65% yield. Elemental analyses: calcd for n = 3: C, 54.43; H, 5.17; N, 9.33. Found: C, 54.58; H, 4.84; N, 9.35. Infrared (IR) (cm−1): 3530 (w), 3076 (w), 2977 (w), 1649 (w), 1586 (s), 1531 (w), 1463 (m), 1411 (m), 1334 (s), 1273 (m), 1244 (m), 1198 (w), 1177 (s), 1063 (s), 1008 (m), 974 (m), 919 (m), 818 (w), 754 (w), 679 (s), 620 (s), 579 (w). Synthesis of {[Zn(L)(HL)](ClO4)3}·nH2O (2) 2 was prepared in the same way as 1, but Fe(ClO4)2·6H2O was replaced with Zn(ClO4)2·6H2O. Purple crystals were obtained in 55% yield. Elemental analyses: calcd for n = 6: C, 52.21; H, 5.35; N, 8.95. Found: C, 52.22; H, 5.28; N, 8.86. IR (cm−1): 3450 (m), 3087 (w), 2977 (w), 1646 (w), 1592 (s), 1531 (w), 1470 (m), 1414 (m), 1340 (s), 1274 (m), 1250 (m), 1180 (s), 1157 (w), 1120 (s), 1013 (w), 976 (w), 925 (w), 822 (w), 783 (w), 707 (w), 683 (m), 621 (m), 546 (w). Solid-state grinding The acidified form of 1 was prepared by grinding 1 (100 mg, 0.062 mmol), concentrated HCl (24.8 mg, 0.248 mmol), and 1 mL methanol in an agate pestle. Immediately after grinding started, the sample became a wet reddish-brown paste. Upon continued grinding, the mixture dried to form a reddish-brown powder. Further grinding of the acidified form of 1 (100 mg, 0.057 mmol) with NaOH (9.2 mg, 0.23 mmol) and 1 mL methanol in an agate pestle allowed the recovery of 1. Physical characterization Magnetic susceptibility measurements were recorded with a Quantum Design PPMS-9 (Quantum Design; California, USA), operating with an applied field of 1.0 T at temperatures between 10 and 400 K at a rate of 2 K min−1. Elemental analyses were performed using an Elementar Vario EL Elemental Analyser (elementar Analysensysteme GmbH, German). The IR spectra were recorded on a Perkin-Elmer Spectrum (Perkin-Elmer, USA) in the range of 4000–500 cm−1. Powder X-ray diffraction (PXRD) patterns were recorded on a D8 Advance X-ray diffractometer (Bruker, German) (CuKα radiation, λ = 0.154056 nm). UV–vis spectra were recorded by the Thermo Fisher Evolution 220 (Thermo Fisher Scientific, USA) instrument in the range of 800–200 nm. Thermogravimetric curves were recorded on a Mettler-Toledo TGA/SDTA851e thermoanalyzer (METTLER-TOLEDO, Switzerland) by filling the sample into alumina crucibles under a nitrogen atmosphere within a temperature range of 300–1050 K at a heating rate of 10 K min−1. 1H NMR spectra were recorded at 500 MHz (Bruker Avance III 500) or 300 MHz (Bruker DRX 300) (Bruker, German). 1H, 1H correlation spectroscopy (COSY) NMR spectra were recorded at 400 MHz (Bruker Avance III 400). Fluorescence spectra were conducted on a FL3-Tcspc spectrofluorimeter (HORIBA Jobin Yvon, France). Single-crystal X-ray diffraction Single-crystal X-ray data were collected at 123 K on a Bruker D8 Quest diffractometer using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). A multiscan absorption correction was performed (SADABS, Bruker, 2016). The structures were solved using a direct method (SHELXS) and refined by full-matrix least-squares on F2 using SHELXL48 under the graphical user interface of OLEX2 (Bruker, German).49 Nonhydrogen atoms were refined anisotropically, and hydrogen atoms were placed in calculated positions refined using idealized geometries (riding model) and assigned fixed isotropic displacement parameters. Solvent molecules in 1 and 2 and half of an anion in 1 were severely disordered and could not be properly modeled. The masking procedure implemented in OLEX2 was employed to remove the contribution of the electron density associated with these disordered anions and solvent from the model. In each case, the electron density has been included in the reported formulas as an appropriate number of solvent molecules. The crystals rapidly suffered solvent loss and diffracted poorly at room temperature. CCDC 2017040–2017042 contain the supplementary crystallographic data of this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Solution-phase magnetic measurements A special double-walled NMR tube was used: CD3OD and CDCl3 (ratio 2∶1)/CH2Cl2 mixtures were placed in the inner tube, while the solution of complex 1 was placed in the outer tube. Preparation of the solution of complex 1: Complex 1 was dissolved in CD3OD and CDCl3 (ratio 2∶1) mixtures (25 mmol L−1), and CH2Cl2 was added as the reference. About 2 equiv of D2SO4 and 4 equiv of NaOH were added in the outer tube sequentially. Magnetic susceptibility was determined by the Evans method50,51 using the equation corrected for a superconducting NMR spectrometer52,53: χ g = 3 ( Δ ν ν ) 4 π m + χ 0 The resulting NMR spectrum exhibits two peaks for CH2Cl2: one is due to the CH2Cl2 in the inner tube and the other is due to CH2Cl2 that has been paramagnetically shifted by the paramagnetic sample in the outer tube. The chemical shifts of these two peaks were determined and the separation between them (Δν) is measured in hertz (300 MHz NMR: ν = 3 × 108 Hz). The above equation was then used to calculate the gram susceptibility χg (m is the gram concentration of the solute), which was then converted into molar susceptibility (χM). χ0 is the gram susceptibility of the solvents. Results and Discussion Slow evaporation of the methanol/dichloromethane (10∶1) solution of Fe(ClO4)2·6H2O (10 equiv) and the ligand (1 equiv) gave single crystals of complex {[Fe(L)(HL)](ClO4)3}·nH2O ( 1). The Zn(II) analogue {[Zn(L)(HL)](ClO4)3}·nH2O ( 2) was prepared in the same way by using Zn(ClO4)2·6H2O salt instead. The crystals, especially in the case of 1, rapidly suffered solvent loss at room temperature. The solvent molecules were determined to be n = 3 ( 1) and n = 6 ( 2) for samples stored at room temperature over a week based on TG ( Supporting Information Figures S1 and S2) and elemental analyses (see Experimental Methods section). The values could be different among different batches of samples and their preservation time and temperature. Scheme 1 | Ring-opening and iminol–amide tautomerism of the ligand upon complexation. Download figure Download PowerPoint The crystal structures of 1, 2, and the free ligand were determined at 123 K (crystallographic data and refinement parameters are listed in Supporting Information Tables S1 and S2). 1 and 2 are isostructural and crystallize in the triclinic P-1 space group, while the free ligand crystallizes in the monoclinic P21/c space group. The crystallographic data of 2 are of higher quality, and three ClO4– anions were clearly resolved, indicating that one of the ligands is protonated. In the case of 1, two and a half anions were modeled. Possibly, the rest of the anion is severely disordered and could not be properly modeled. In view of the isostructural Zn(II) analogue as well as the elemental analyses, it is more sensible to include three anions in the chemical formulas of 1. As shown in Figure 1, the metal center is located in a distorted octahedral coordination sphere, composed of two O atoms, two imine N atoms, and two pyridine N atoms from two ligands. The ligands adopt the ring-opened form, in contrast to the ring-closed form of the free ligand ( Supporting Information Figure S3). The xanthene groups wrap the coordination sphere peripherally. The C−O bond lengths in acylhydrazone increase upon coordination [1.203(7) Å in the free ligand vs 1.228(3)–1.281(4) Å in 1 and 2] and the C−N bond lengths decrease [1.396(8) Å in the free ligand vs 1.330(5)–1.361(3) Å in 1 and 2], indicating a tautomerization process of the amide group to an isomer of higher iminolate characteristics. The Fe−N (1.86−1.94 Å) and Fe−O (2.09−2.25 Å) bond lengths are comparable with other LS Fe(II) complexes based on acylhydrazone derived ligands.21,38,40–42,44 The octahedral distortion parameters Σ and Θ (Σ is the sum of deviations from 90° of the 12 cis N−Fe−N angles; Θ is the sum of the deviations from 60° of the 24 possible octahedron twist angles; i.e., Σ = 0 and Θ = 0 are corresponding to a perfect octahedral site)54,55 are 80.7(6)° and 264.6(14)°, respectively. These parameters are also consistent with other Fe(II) complexes in the LS state based on similar acylhydrazone ligands.21,38,40–42,44 Structure determination above room temperature is not possible due to crystallinity loss after desolvation. The Zn−N (2.06−2.14 Å) and Zn−O (1.93−1.96 Å) bond lengths are larger due to the larger size of the Zn(II) ion. One set of intramolecular π–π interactions were observed between a pyridine ring and benzene ring of the peripheral xanthene group within each cation (Figure 1). The xanthene groups were also involved in weak intermolecular π–π interactions, giving rise to one-dimensional (1D) supramolecular chains (Figure 2). Figure 1 | Crystal structure of the cation in complex 1 at 123 K. Thermal ellipsoids are presented at 30% probability. Color code: Fe, red; N, blue, C, two sets of color (gray/yellow) are used for C in two ligands. Hydrogens are omitted for clarity. The rings involved in intramolecular π–π stacking interactions are highlighted. Angle: 165.09(19)°, centroid–centroid distance: 3.958(3) Å, shift distance: 0.283(7) Å. Download figure Download PowerPoint Magnetic susceptibility data were recorded on polycrystalline sample 1 in the following sequence: 300 K → 10 K → 400 K → 10 K. The results are shown in Figure 3 in the form of χMT versus T plots (where χMT is the molar magnetic susceptibility and T is temperature). It deserves to be noted that the data are repeatable from different batches, although the exact contents of solvent molecules may vary among them. The χMT value slightly decreases from 0.6 cm3 K mol–1 at 300 K to 0.24 cm3 K mol–1 at 250 K and then remains almost constant until 30 K. The χMT values at low temperatures indicate Fe(II) in its LS state (S = 0). The slight decrease observed below 30 K is due to the zero-field splitting of the residual HS Fe(II) ions. The curve is reproducible in the subsequent heating process to 300 K. Upon further heating, the χMT value increases to 2.82 cm3 K mol–1 at 400 K, indicating the spin-state change of the Fe(II) centers from LS to HS (S = 2). As the limit temperature of the magnetometer is reached, the completeness of the SCO cannot be observed. A different SCO curve showing a more gradual decrease in χMT values was observed in the following cooling sequence ascribed to the escape of residue solvent molecules. The χMT value is 1.55 cm3 K mol–1 at 50 K, indicating that approximately half of the Fe(II) centers is in the HS state. The curve is reproducible in the following heating–cooling–heating cycles ( Supporting Information Figure S5), indicating a reversible SCO process after solvation. Figure 3 | The χMT versus T curve (sweep mode at 2 K min−1) for 1 measured in the following sequence: 300 K → 10 K → 400 K → 10 K. Download figure Download PowerPoint The LS of 1 at room temperature as well as the lability of the ligand to the response toward acid and with the of switching the spin state at room temperature (Figure The solid-state switching was first by grinding the sample with 4 equiv of concentrated The χMT value of cm3 K mol–1 at K the change of spin state to HS upon acid perturbation. further grinding with 4 equiv of NaOH allows switching to as by the recovery of the χMT value of cm3 K mol–1 at K. A was by cycles of change of the χMT value by grinding with acid and Figure 4 | switching of χMT values of 1 at K upon acid and Download figure Download PowerPoint Magnetic in solution at room temperature were also measured by the Evans 1H NMR χMT value of 1 is solvent The value of cm3 K mol–1 in CDCl3 with the solid-state magnetic In the χMT value of cm3 K mol–1 the HS state of the complex or the of dissociation in solution. The response of 1 to acid and was in a solvent mixture of to after perturbation. The χMT value increases from to cm3 K mol–1 upon of 2 equiv of D2SO4 and upon further of 4 equiv of NaOH cm3 K This a reversible change between the LS and HS by acid and Figure 2 | of 1. intermolecular π–π stacking interactions molecules into a supramolecular The rings involved in π–π stacking are highlighted. Angle: centroid–centroid distance: Å, shift distance: Å. Download figure Download PowerPoint 1H NMR spectra were measured on 2 as a diamagnetic ( Supporting Information Figures of the isostructural Zn(II) complex were based on 1H and NMR The most shifts upon were observed for the in the pyridine especially the shift from to for and from 8.38 to for which were located to the N the peaks not only shifted but also structural The complex between different chemical on the NMR Upon the of chemical shifts of complex 2 were Solid-state IR spectra were recorded at room temperature (Figure A strong at ascribed to was observed for the free ligand. Upon the to with a and a new at a of of the These the to tautomerization of the amide group upon in with the crystallographic In the is to structural change, from to upon complexation. complex 1 with HCl the same spectrum as the free ligand with a from the and dissociation of the complex by It is that is a group than Further grinding with NaOH allows spectrum recovery corresponding to the complex in with the reversible spin-state switching observed from the magnetic Figure | spectra of the free complex 1, and 1 after grinding with HCl equiv) and NaOH Download figure Download PowerPoint The response of the UV–vis absorption and the fluorescence toward acidification and alkalization were in 10 changes were observed for samples prepared by the samples after solid-state grinding (Figure and direct treatment in solution ( Supporting Information Figure The strong at of 1 is of the ring-opened form of rhodamine B intensity increases upon of properties of the rhodamine B The emission at shows enhancement upon acidification in with a slight shift nm). The luminescence for complex 2 was also for and it also shows emission enhancement after This the modulation of the fluorescence should from the response of the rhodamine moiety upon of acidity. However, the enhancement of 2 is than 1 due to a emission of the Zn(II) complex while similar emission intensity after acid treatment. As by magnetic in complex 1 in to a of LS Fe(II) It has been that the absorption of the LS Fe(II) complex and emission of the results in emission The of after acidification the dissociation of the complex, in with the IR the UV–vis and fluorescence spectra are after Figure 6 | UV–vis and fluorescence spectra of 1 and its response toward acid the solution was prepared by the sample after grinding with 4 equiv of and the solution was prepared by the sample after further grinding with 4 equiv of 10 in The shows the corresponding changes and under at Download figure Download PowerPoint The of complex 1 is with our previous which to upon treatment in the solid state. The of bidirectional switching relies on an of the coordination The structural analyses show that the peripheral xanthene groups wrap the coordination sphere and form intramolecular π–π interactions with the pyridine The by the xanthene groups the of the O atoms with outer sphere, which is in contrast to the previous where the O atoms are involved in strong hydrogen with the and coordination are expected to be after acid treatment and give rise to an This is to SCO where are will elastic interactions between switchable The gradual conversion curve of the 1 is an have achieved reversible spin-state switch and fluorescent modulation of an Fe(II) complex based on a rhodamine B 2-pyridinecarbaldehyde hydrazone ligand upon acid and at room temperature. of a center into a coordination sphere has to be a promising for spin-state In such cases, supramolecular interactions are and the of a peripheral is for As a further the of an intrinsic luminophore as the peripheral response with the spin-state These will the design of multifunctional switchable materials, with applications as molecular and sensors. Supporting Information Supporting Information is of is of to Information This work was by the Science of and 1. and Spin In Spin in Spin In Spin in 1, in Spin (SCO) and to for and Data of the Thermal and Spin in the Spin Complex with a Thermal