Heteroleptic Iron(II) Spin-Crossover Complexes Based on a 2,6-Bis(pyrazol-1-yl)pyridine-type Ligand Functionalized with a Carboxylic Acid.

Two new heteroleptic complexes [Fe(1bppCOOH)(3bpp-bph)](ClO4)2·solv (1·solv, solv = various solvents; 1bppCOOH = 2,6-bis(1H-pyrazol-1-yl)isonicotinic acid; 3bpp-bph = 2,6-bis(5-([1,1'-biphenyl]-4-yl)-1H-pyrazol-3-yl)pyridine) and [Fe(1bppCOOH)(1bppCOOEt)](ClO4)2·0.5Me2CO (2·0.5Me2CO, 1bppCOOEt = ethyl 2,6-bis(1H-pyrazol-1-yl)isonicotinate) were designed and prepared. The heteroleptic compound 1·solv was obtained by the combination of stoichiometric amounts of Fe(ClO4)2, 1bppCOOH, and 3bpp-bph, and it was designed to fine-tune the spin crossover (SCO) properties with respect to the previously reported homoleptic compound [Fe(1bppCOOH)2](ClO4)2. Indeed, the introduction of a new substituted 3bpp ligand induces a weaker ligand field in addition to promoting the formation of π···π and C-H···π intermolecular interactions through the biphenyl groups. For the desolvated counterpart 1, this results in a shift of the SCO curve toward room temperature and the observation of a 13 K hysteresis width. Besides, compound 2·0.5Me2CO, which represents the first example of a heteroleptic complex containing two 1bpp tridentate ligands, stabilizes the LS state at room temperature confirming the same trend observed for the corresponding homoleptic compounds. Interestingly, both 1 and 2·0.5Me2CO heteroleptic complexes exhibit photoswitchable properties when irradiating with a 523 nm laser at 10 K. Preliminary characterization of the deposited complexes on native SiO2 by X-ray absorption measurements suggests oxidation and decomposition of the complexes.

Spin-crossover (SCO) complexes can be reversibly switched between two distinct states by a variety of external inputs such as light, temperature, pressure, analytes or electric fields. 1,2 The most appealing case is that of Fe(II), which switches between the diamagnetic low-spin state (LS, S = 0) and the paramagnetic high-spin state (HS, S = 2) while undergoing changes of the metal-to-donor atom bond distances and of the optical and dielectric properties. Thus, since the SCO phenomenon induces reversible changes of a number of physical properties, these materials constitute an ideal and promising platform for many applications such as sensors, memories or as part of spintronic devices. 3 Bis-chelated iron(II) complexes based on tridentate ligands of the type 2,6-bis(pyrazol-xyl)pyridine (x = 1 or 3, i.e., 1bpp 4-8 or 3bpp 9 ) are very attractive, since they generate the appropriate ligand field to observe the SCO phenomenon in the Fe(II) ion and lead to complexes with a dense network of intermolecular interactions affecting in very interesting ways the dynamics of the magnetic transition. Hence, these complexes usually present abrupt thermal spin transitions at accessible temperatures and the Light-Induced Excited Spin State Trapping (LIESST) effect. 10,11 In addition, the spin state of these systems is known to be very sensitive to solvent exchange through the molecular network 9,12-14 and, consequently, they can act as chemoresponsive materials. We recently discovered that the mixture of an iron(II) salt with two different 3bpp ligands of varying length led to the quantitative crystallization of the heteroleptic complex out of all the possible combinations of two ligands per metal. 15 Later on, we expanded this investigation to other 3bpp derivatives in combination with the 2,6-bis(pyridin-2-yl)pyridine (tpy) and the 2,6-bis(benzimidazol-2-yl)pyridine (2bbp) ligands, 16 confirming a marked tendency of the Fe(II) ion to form heteroleptic [Fe(L)(L′)](ClO4)2 complexes from pairs of chelating tris-imine ligands. This quasi-exclusive formation of heteroleptic adducts has been proven as a prolific source of new compounds with very diverse properties. 17,18 In previous reports, we showed that the solvent-free perchlorate salt of the homoleptic iron(II) complex of the ligand 1bppCOOH (2,6-di(1H-pyrazol-1-yl)isonicotinic acid, Scheme 1) exhibits a hysteretic (3 K) abrupt thermal spin transition at high temperatures (380 K) affected by the presence of a linear network of hydrogen-bonded complexes. 19 Moreover, it has been proved that the introduction of a carboxylic acid group on the 4-pyridyl position promotes the formation of a polynuclear metal complex 20  In this work, we intend to take advantage of the properties of ligand 1bppCOOH while incorporating additional features to its SCO materials by extending the family of its iron(II) complexes to heteroleptic species. We have thus combined it with ligands 3bpp-bph (2,6-bis(5- After the addition was completed, the reaction mixture was heated to reflux, turning from yellow to orange after a few hours, and left overnight. The mixture was then left to cool to room temperature, and some drops of EtOH were added in order to quench any remaining NaH, followed by careful addition of 100 mL of water. HCl 37% (20 mL) was then added changing the pH from 14 to 3 that resulted in the precipitation of a light-yellow solid which was filtered, washed with diethyl ether and dried in air.

Structural characterization.
Single crystals of 1·Me2CO, 1·2Me2CO·0.5Et2O or 2·0.5Me2CO were mounted on a glass fiber using a viscous hydrocarbon oil to coat the crystal and then transferred directly to the cold nitrogen stream for data collection. X-ray data were collected at 120 K for 1·2Me2CO·0.5Et2O, at 120 and 300 K for 1·Me2CO and at 120 and 400 K for 2·0.5Me2CO on a Supernova diffractometer equipped with a graphite-monochromated Enhance (Mo) X-ray Source (λ = 0.71073 Å). Higher temperatures led to the quick loss of crystallinity in 1·Me2CO. The program CrysAlisPro, Oxford Diffraction Ltd., was used for unit cell determinations and data reduction. Empirical absorption correction was performed using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm. The structures were solved with the ShelXT structure solution program 24 and refined with the SHELXL-2013 program, 25 using Olex2. 26 Non-hydrogen atoms were refined anisotropically, and hydrogen atoms were placed in calculated positions refined using idealized geometries (riding model) and assigned fixed isotropic displacement parameters. Crystallographic data are summarized in Table   1. CCDC-1917750-1917754 contain the supplementary crystallographic data for 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. For powder X-Ray diffraction patterns, a 0.5 mm glass capillary was filled with polycrystalline samples of the complexes and mounted and aligned on an Empyrean PANalytical powder diffractometer, using CuKα radiation (λ = 1.54177 Å). A total of 2 scans were collected for each compound at room temperature in the 2θ range 5-40º.  (3) 11.0642 (5) 11.2137 (6) 14.2991 (17) 14.7845 (7) b (Å) 15.2064 (5) 13.7718 (5) 13.7063 (6) 27 The spectra were recorded in TEY method from 4 to 350 K with an energy resolution of ~70 meV, the chamber pressure being lower than 1*10 -10 mbar and photon flux about 5*10 11 s -1 . The XMCD spectra were recorded using alternatively left and right circularly polarized x rays produced by an APPLE II undulator and a superconducting split coil setup generating a magnetic field up to 6 T in the direction of propagation of the incident photons.

Results and discussion
Synthesis and 1·Me2CO, which systematically co-crystallize within the crystallization vial (see experimental section), were successfully determined by single-crystal X-ray diffraction at 120 K (see below). It is important to note that the powder X-ray diffraction (PXRD) pattern of a sample formed by a mixture of the two solvatomorphs at room temperature is consistent with the simulated pattern of 1·Me2CO (see Figure S1). This observation clearly indicates that a partial desolvation of crystals of 1·2Me2CO·0.5Et2O leads to compound 1·Me2CO in contact with air.
Indeed, the crystal structure of 1·Me2CO obtained at 300 K confirms that its acetone molecule is not lost at room temperature. At 400 K, compound 1·Me2CO undergoes, after several minutes, a complete loss of crystallinity due to the desorption of the acetone molecules as confirmed by elemental analysis (see experimental section). Consequently, it was not possible to determine the structure of the SCO-active desolvated phase 1 (see below). Powder X-ray diffraction of the SCO phase 1 shows some changes in the position of the peaks with respect to the solvated form 13 (see Figure S1) which suggests sensible structural modifications accompanying to the acetone desorption.
Regarding the heteroleptic complex 2·0.5Me2CO, the elemental analysis indicates the persistence of the acetone of crystallization found in the structure and also that one water molecule is likely absorbed in contact to air (see experimental section). Besides, the powder Xray diffraction pattern of 2·0.5Me2CO is in good agreement with the diagram simulated from the single crystal X-ray diffraction data (see Figure S2).

Structure of [Fe(bppCOOH)(bppCOOEt)](ClO4)2·0.5Me2CO (2·0.5Me2CO)
This compound crystallizes in the orthorhombic Pbca space group. Its structure was solved at present CH···π contacts between CH groups and aromatic rings from the pyrazolyl groups of ligand 1bppCOOH. This gives rise to chains of complexes running along the a axis (see Figure   S7). These chains, which are surrounded by ClO4anions and acetone molecules, are connected through short contacts involving CO groups from 1bppCOOH and 1bppCOOEt and CH groups from pyridine and pyrazolyl groups from both ligands (see Figure S7). 1·Me2CO is shown in Figure 3. As mentioned above, the PXRD pattern at room temperature indicates that the structure of this sample is that of 1·Me2CO. The value of χMT is close to 3.4-3.6 cm 3 ·K·mol -1 in the 50 to 400 K range, consistent with the HS state of 1·Me2CO at 120 and 300 K, as indicated by the Fe-N distances from the molecular structure (see above). The sharp decrease below 50 K is due to zero-field-splitting as expected for the Fe(II) ion in the HS state.
Quite remarkably, after heating to 400 K, the desolvated phase (1) shows a completely different behavior. Thus, when cooling from 300 to 250 K, a gradual decrease of χMT from 3.3 cm 3 ·K·mol -1 at 300 K to 2.75 cm 3 ·K·mol -1 at 250 K is registered. Upon further cooling, an abrupt decrease of the χMT value is observed down to 0.9 cm 3 ·K·mol -1 at 220 K. Below this temperature, the χMT product decreases gradually registering a value of 0.5 cm 3 ·K·mol -1 at 50 K The temperature dependence of the χ MT value for 2·0.5Me2CO is shown in Figure 4. From 2 to 300 K, χMT values are lower than 0.4 cm 3 ·K·mol -1 , consistent with an iron(II) ion being in the LS state, in agreement with the Fe-N distances found at 120 K (see above). At higher temperatures, a gradual increase of the magnetic signal to reach a χMT value of 1.6 cm 3 ·K·mol -1 20 at 400 K is recorded. Therefore, an spin transition takes place in this compound with a T1/2 well above 300 K and close to 400 K that is not completed at the latter temperature (the highest measured). This behavior is reversible in the cooling cycle after heating to 400 K ( Figure S8). This is consistent with the structural data that indicates that heating at 400 K does not remove the half equivalent of acetone molecules found in the structure.  under soft X-ray sources in low conducting substrates. 54 This effect could be reduced by decreasing the photon flux to around 80 % of its initial intensity (1) and decreasing the 24 temperature in the case of 2. These spectra differ drastically from those obtained in the bulk compound [Fe(1bppCOOH)2](BF4)2 measured at 100 and 350 K, 21 which has been used as reference compound for the LS and HS states of Fe(II), respectively (see Figure S15 and associated text  55 The use of other deposition conditions such as less polar solvents or increased concentrations of the complexes, and the preparation of more robust Fe(II) complexes based on other 1bpp derivatives could be possible strategies to improve these results.