Photochromic diarylethene ligands featuring 2-(imidazol-2-yl)pyridine coordination site and their iron(II) complexes

A new family of photochromic diarylethene-based ligands bearing a 2-(imidazol-2-yl)pyridine coordination unit has been developed. Four members of the new family have been synthesized. The photoactive ligands feature non-aromatic ethene bridges (cyclopentene, cyclopentenone, and cyclohexenone), as well as closely spaced photoactive and metal coordination sites aiming a strong impact of photocyclization on the electronic structure of the coordinated metal ion. The ligands with cyclopentenone and cyclohexenone bridges show good cycloreversion quantum yields of 0.20–0.32. The thermal stability of closed-ring isomers reveals half-lives of up to 20 days in solution at room temperature. The ligands were used to explore coordination chemistry with iron(II) targeting photoswitchable spin-crossover complexes. Unexpectedly, dinuclear and tetranuclear iron(II) complexes were obtained, which were thoroughly characterized by X-ray crystallography, magnetic measurements, and Mössbauer spectroscopy. The formation of multinuclear complexes is facilitated by two coordination sites of the diarylethene, acting as a bridging ligand. The bridging nature of the diarylethene in the complexes prevents photocyclization.


I. General information
Synthesis and characterization of organic compounds. NMR spectra were recorded in deuterated solvents on Bruker AM-300 spectrometer working at 300.13 MHz for 1 H and 75.77 MHz for 13 C, respectively. Both 1 H and 13 C NMR chemical shifts are referenced relative to the residual solvents signals (CHCl3: δ 7.27 for 1 H NMR and δ 77.16 for 13 C NMR) and reported in parts per million (ppm) at 293 K. Data are represented as follows: chemical shift, multiplicity (s, singlet; d, doublet; m, multiplet; br, broad), coupling constant in hertz (Hz), integration. Melting points (Mp) were recorded on a Boetius hot stage and were not corrected. High-resolution mass spectra were obtained from a TOF mass spectrometer (Bruker micrOTOF) with an ESI source.
All chemicals and anhydrous solvents were purchased from commercial sources and used without further purification. Silica column chromatography was performed using silica gel 60 (70−230 mesh); TLC analysis was conducted on silica gel 60 F254 plates.

Synthesis and characterization of iron(II) complexes.
Elemental analyses were carried out with an EURO EA analyzer from a EuroVector. All starting materials were utilized as received without further purification unless otherwise noted. Pure anhydrous solvents were collected from a solid-state solvent purification system Glass Contour (Irvine, CA). Synthesis was performed under anaerobic conditions using Schlenk techniques.
Photochemistry of diarylethenes. UV−vis spectra were recorded with a Fluorat-Panorama spectrophotometer in 1.0 cm quartz cuvettes. The experimental measurements were performed in the presence of air in solutions of acetonitrile and toluene. The quantum yields of the photocyclization and cycloreversion (ΦAB and ΦBA) were measured by previously reported method [S1]. 1,2-Bis(2-methyl-1-benzothiophen-3-yl)perfluorocyclopentene in hexane solution was used as a chemical actinometer at 313 and 480 nm [S1]. The photon numbers per second at 480 nm are ≈ 7 × 10 15 photons/s; at 313 nm are ≈ 2 × 10 15 photons/s. Molar extinction coefficients of the photogenerated isomers were determined as follows: a diarylethene (1 mg) was dissolved in appropriate deuterated solvent (0.5 ml) and the solution was irradiated with UV-light (365 nm) for 20 minutes in a NMR tube. The obtained deeply colored solution was studied by 1 H NMR and UV-vis spectroscopy studies (in 0.1 cm quartz cuvettes).
Molar extinction coefficients (εc) were calculated by equation (1): where D is the absorption at band maximum of photogenerated isomer; conv is the conversion of diarylethene; c0 is the total concentration of the compound. For the representative 1 H NMR spectra with characteristic signals see Section IV.  [S2]. The structures were solved by direct methods (SHELXTL NT 6.12) [S3] and have been refined by full-matrix least squares procedures on F 2 using SHELXL 2016/6 [S4]. Hydrogen atoms were placed in a position of optimized geometry and their isotropic displacement parameters were tied to those of their corresponding carrier atoms by a factor of 1.2 or 1.5. The crystallographic data, data collection and structure refinement details are summarized in Table S1.
Magnetic studies. Magnetic susceptibility data on solid samples were collected with a Quantum Design MPMS 3 Magnetometer. DC susceptibility data were collected in the temperature range 2-300 K on powder samples restrained within a polycarbonate gel capsule in the applied magnetic field of 1 T at 1 K min -1 heating/cooling rate and 0.5 K intervals between 2-25 K, 5 K intervals between 25-150 K and 10 K intervals between 150-300 K. The magnetic susceptibility data were corrected for diamagnetism using an estimation m, diamag = ½ Mw·10 -6 cm 3 mol -1 with Mw being the molar mass of the compound [S5]. The program PHI was used for the quantitative analysis of magnetic data [S6].

II. Synthesis of photochromic diarylethenes
The synthesis of desired photochromic ligands 3, 4, 6 and 7 are depicted at Scheme S1.
Aniline (4.00 mL, 42.6 mmol) and 2-pyridinecarboxaldehyde (4.20 mL, 42.6 mmol) were added to the solution and the resulting mixture was heated for reflux for 4 h. The reaction mixture was cooled to 10 °C and zinc powder (3.77 g, 58.1 mmol) was added portionwise. The resulting mixture was heated to reflux for 1 h. The resulting solution was poured into water (500 mL) and extracted with ethyl acetate (5 × 50 mL). The combined organic phases were washed with water (500 mL), dried over MgSO4, and evaporated in vacuum. The residue was purified by flash chromatography with petroleum ether/ethyl acetate (2:1) to give the titled product as colorless crystals.

Synthesis of diarylethene 3.
This compound was synthesized in a similar manner as described in [S9].

Synthesis of diarylethene 4.
This compound was synthesized in a similar manner as described in [S10].

Synthesis of diarylethene 7.
This compound was synthesized in a similar manner as described in [S10].
To a solution of diarylethene 6 (0.4 mmol) in ethanol (2 mL) solution of KOH (112 mg, 2.0 mmol) in water (2 mL) was added and the resulting mixture was refluxed until completion of the reaction (TLC control). The resulting solution was poured into water (50 mL), extracted with ethyl acetate (3 × 30 mL), washed with brine (50 mL), dried over MgSO4, and evaporated in vacuum. The residue was purified by column chromatography eluting by petroleum ether/ethyl acetate (2:1).

III. Synthesis of iron(II) complexes
These complexes were synthesized in a similar manner as described in [S11].

Synthesis of iron(II) complexes was performed under anaerobic conditions using Schlenk
techniques. A solution of K[H2B(pz)2] (127 mg, 0.7 mmol) in 3 mL of dry methanol was added to a solution of FeSO4·7H2O (95 mg, 0.3 mmol) in 5 mL of dry methanol and stirred at rt for 10 min.
After centrifuging and filtration, a solution of diarylethene 6 (200 mg, 0.3 mmol) in 15 mL of dry methanol was added dropwise to the filtrate to give a red solution with a red powder. After 2 h, the precipitate was filtered off and washed with methanol (10 mL), toluene (3 mL) and water (20 mL) to give complex 8 as an analytically pure material.
Single crystals of complexes 8 and 9 were obtained in a fritted U-shape tube by the slow diffusion of methanol solutions of freshly prepared Fe([H2B(pz)2])2 and diarylethene 6. In the case of complex 8, the U-tube was stored for 1 month. In the case of complex 9, the U-tube was stored for 2 years.

Complex 8
Yield: 53% (268 mg).  Figure S1. Molecular structure of imidazole 1 at 120 K. The H atoms are omitted for clarity; the thermal ellipsoids are drawn at the 50% probability level.   Table S1. Crystallographic data, data collection and structure refinement details. V. Electronic spectra of photochromic diarylethenes Figure S5. Electronic spectra of diarylethene 3 before and after irradiation with UV light (λ = 313 nm, toluene, c = 4.7 × 10 −5 M). Figure S6. Electronic spectra of diarylethene 3 before and after irradiation with UV light (λ = 313 nm, acetonitrile, c = 4.7 × 10 −5 M). S14 Figure S7. Electronic spectra of diarylethene 4 before and after irradiation with UV light (λ = 313 nm, toluene, c = 4.9 × 10 −5 M). Figure S8. Electronic spectra of diarylethene 4 before and after irradiation with UV light (λ = 313 nm, acetonitrile, c = 4.9 × 10 −5 M). S15 Figure S9. Electronic spectra of diarylethene 6 before and after irradiation with UV light (λ = 313 nm, acetonitrile, c = 3.4 × 10 −5 M). S16 Figure S10. Electronic spectra of diarylethene 7 before and after irradiation with UV light (λ = 313 nm, toluene, c = 3.9 × 10 −5 M). Powder samples of 8 and 9 were used for variable temperature magnetic susceptibility measurements in the temperature range of 2-300 K. 8 shows a nearly constant χT product of 7.63 cm 3 mol -1 K above 125 K ( Figure S17). It increases gradually upon lowering the temperature reaching a maximum of 8.80 cm 3 mol -1 K at 13 K, which is indicative of ferromagnetic coupling between the two iron(II) ions. By lowering the temperature further below 14 K, the χT product decreases sharply due to ZFS (zero-field splitting). The room temperature value of the χT product of 7.63 cm 3 mol -1 is slightly higher than expected spin-only value for two non-interacting HS (SFe = 2) iron(II) centres (χs.o.T = 6.00 cm 3 mol -1 K) due to orbital contribution. To determine the intramolecular coupling constant and g-values, magnetic data were fitted using Hamiltonian (2).

IV. Crystallographic Data
The first term describes the exchange coupling, the second is the electron Zeeman effect, and the third and fourth are axial ZFS terms for each iron(II) ion.
̂= −2̂1̂2 + ( 1̂1 + 2̂2 ) + 1 (̂1 z 2 − 1 3 1 ( 1 + 1)) + 2 (̂2 z 2 − 1 3 2 ( 2 + 1)) (2) Due to the symmetry of the complex, D1 = D2 and g1 = g2 are used in the simulation. The fit affords a ferromagnetic coupling constant J = +0.5 cm -1 and D = 5.0 cm -1 . The determined g1 = g2 = 2.24 are in the expected range for d 6 systems. To get further insight, field dependent magnetic measurements from 0 to 7 T at 2 K were conducted (see Figure S18). At fields > 4 T eight Bohr magnetons per mole (NAµB) correspond to two ferromagnetically coupled HS iron(II) centres. sample at an external magnetic field of 1 T in the heating mode (1 K min -1 , 0.5 K intervals between 2-25 K, 5 K intervals between 25-150 K and 10 K intervals between 150-300 K). See the text for fitting parameters. The χT product of 9 increases to almost constant value of 14.61 cm 3 mol -1 K at room temperature, which is in good agreement with four uncoupled high-spin iron(II) ions (χs.o.T = 12.00 cm 3 mol -1 K) ( Figure S19). A large drop in susceptibility is observed at temperatures below 40 K due to ZFS. The magnetic data were fitted using Hamiltonian (3), with the first term describing the electron Zeeman effect, whereas the second term is due to the ZFS. The simplifications g1 = g2 = g3 = g4 and D1 = D2 = D3 = D4 have been assumed to exclude overparamaterization, which yields the fitting parameters for g = 2.20 and D = 5.9 cm -1 , which is in agreement with a d 6 -system. Field-dependent magnetic measurements show a more gradual increase of magnetization compared to 8. The magnetization does not saturate at 7 T ( Figure S20).
The observed maximum of 14.17 NAµB at 7 T points to four SFe = 2 ions (16 NAµB is expected).  Figure S19: Variable temperature χT product (blue) and χ (green) of 9 measured on a powder sample at an external magnetic field of 1 T in the heating mode (1 K min -1 , 0.5 K intervals between 2-25 K, 5 K intervals between 25-150 K and 10 K intervals between 150-300 K). See the text for fitting parameters.

Mössbauer spectroscopy
Zero-field 57 Fe Mössbauer spectra were obtained on a powder sample of 8 at 77 K and room temperature (rt). The rt spectrum reveals a quadrupole doublet with an isomer shift (δ) of 0.89 mm s -1 and a quadrupole splitting (|ΔEQ|) of 2.64 mm s -1 which is assigned to a HS iron(II) ion ( Figure S21, top) [S12]. Cooling to 77 K induces no change in spin state, as the spectrum ( Fig   S21, bottom) is still appearing phenotypically similar to the room temperature data. The minor changes in the isomer shift (δ) of 1.01 mm s -1 and a quadrupole splitting (|ΔEQ|) of 2.86 mm s -1 are in good agreement with HS iron(II).