Investigations on the Spin States of Two Mononuclear Iron(II) Complexes Based on N-Donor Tridentate Schiff Base Ligands Derived from Pyridine-2,6-Dicarboxaldehyde

Iron(II)-Schiff base complexes are a well-studied class of spin-crossover (SCO) active species due to their ability to interconvert between a paramagnetic high spin-state (HS, S = 2, T2) and a diamagnetic low spin-state (LS, S = 0, A1) by external stimuli under an appropriate ligand field. We have synthesized two mononuclear FeII complexes, viz., [Fe(L)2](ClO4)2.CH3OH (1) and [Fe(L)2](ClO4)2.2CH3CN (2), from two N6–coordinating tridentate Schiff bases derived from 2,6-bis[(benzylimino)methyl]pyridine. The complexes have been characterized by elemental analysis, electrospray ionization–mass spectrometry (ESI-MS), Fourier-transform infrared spectroscopy (FTIR), solution state nuclear magnetic resonance spectroscopy, 1H and 13C NMR (both theoretically and experimentally), single-crystal diffraction and magnetic susceptibility studies. The structural, spectroscopic and magnetic investigations revealed that 1 and 2 are with Fe–N6 distorted octahedral coordination geometry and remain locked in LS state throughout the measured temperature range


Introduction
Ever since the very first report on spin-crossover (SCO) compounds in early 1931 [1], numerous reports have been devoted to this spectacular field of molecular magnetism [2][3][4]. Among these, octahedral Fe II compounds has received special attention due to the clear discrimination between the paramagnetic high spin state (S = 2, 5 T 2 ) and diamagnetic low spin state (S = 0, 1 A 1 ), occurring with external stimuli in an appropriate ligand field [5][6][7][8]. Intermolecular interactions, such as π-π stacking or hydrogen bonding, usually enhance the SCO behavior with abrupt transitions and hysteresis loops [9,10]. The corresponding complexes 1 and 2 were synthesized by the reaction of the tridentate ligands L 1 and L 2 with Fe(ClO4)2.6H2O in methanol for 1 and acetonitrile for 2, respectively (Scheme 1). Upon slow diffusion of diethylether into mother liquor at room temperature, black block crystals were obtained.
The 1 H and 13  The corresponding complexes 1 and 2 were synthesized by the reaction of the tridentate ligands L 1 and L 2 with Fe(ClO 4 ) 2 ·6H 2 O in methanol for 1 and acetonitrile for 2, respectively (Scheme 1). Upon slow diffusion of diethylether into mother liquor at room temperature, black block crystals were obtained.
The IR peaks were observed at 1644 cm −1 and 1651 cm −1 for L 1 and L 2 , respectively, confirming the presence of CH=N. The IR spectra of L 1 and L 2 exhibit peaks at 1570 and 1594 cm −1 , respectively, corresponding to the pyridine C=N stretching vibration [36]. Additionally, the IR spectra of 1 and 2 show a strong band at 1603, 1528 cm −1 (1) and 1600,  (2), confirming the coordination of the azomethine and pyridine nitrogen atoms to the metal centers [37,38]. Moreover, the elemental analysis and ESI-MS measurements are also in conformity with the molecular formulae assigned, [Fe(L 1 ) 2 ](ClO 4 ) 2 ·CH 3 OH and [Fe(L 2 ) 2 ](ClO 4 ) 2 ·2CH 3 CN for 1 and 2, respectively.

X-ray Crystallographic Analysis
The crystallographic data of the ligand L 1 and complexes 1 and 2 are collated in Table 3; bond lengths (Table 4) and bond angles (Table S3) of 1 and 2 are also presented. For L 1 , X _ ray quality crystals were grown by slow evaporation of its methanolic solution. The compound crystallizes in a triclinic lattice and space group P1 with two symmetrically independent molecules located in the asymmetric part of the unit cell (Z = 4). The crystal structure is shown in Figure 1.

X-ray Crystallographic Analysis
The crystallographic data of the ligand L 1 and complexes 1 and 2 are collated in Table  3; bond lengths (Table 4) and bond angles (Table S3) of 1 and 2 are also presented. For L 1 , X _ ray quality crystals were grown by slow evaporation of its methanolic solution. The compound crystallizes in a triclinic lattice and space group P1 with two symmetrically independent molecules located in the asymmetric part of the unit cell (Z = 4). The crystal structure is shown in Figure 1.  Table 3. Collated crystal parameters data for L 1 , 1 and 2.     Table 4. Coordination bond lengths for 1 and 2 at 120 K.
Hydrogen bonding and π-π stacking were found to have a marked influence on the magnetic properties of crystalline Fe II complexes when they directly bridge individual ligands [42]. Upon inspection of the intermolecular interaction in the molecular packing of 1 and 2, it was found that complex units form weak π-π interaction through phenyl rings in the ligands on the bc plane for 1 and ab plane for 2, with an angle, centroid-centroid distances and shift distances of 3.140 • , 3.715 and 1.810 Å, respectively, for 1, and 2.321 • , 3.840 and 1.676 Å, respectively, for 2 ( Figure S2a,b). Moreover, hydrogen bonding occurs between the oxygen atom of perchlorate counteranion and hydrogen atom of methanol, ClO1· · · H9 with a distance of 1.969(13) Å for 1, whereas for 2, the disordered perchlorate counteranions interact via hydrogen bonding with the CH 2 group of the ligand moiety, O1E· · · H7A with a distance of 2.17(3) Å ( Figure S2c,d). Hydrogen bonding interactions that are mediated by counteranions, as in our case, show negligible effects on the spin state of the metal center and are thereby locked in the LS state [43,44]. As regards the intermolecular interaction of 1, the solvent CH 3 OH forms short contacts with ClO 4 − ions with an average distance of 2.65(3) Å and the molecular packing of 2 shows that weak contact exists through the CH 2 group of the ligand moiety and ClO 4 − ion with an average distance of 2.58(2) Å [45] (Figure S2c,d).

Magnetic Studies
The temperature dependence of the molar magnetic susceptibility for 1 and 2 was measured with a SQUID magnetometer (MPMS XL, Quantum Design) in the DC mode at B DC = 0.1 T. It was converted to the dimensionless product function. Its temperature dependence is shown in Figure 3a (1) and Figure 3b (2). Magnetic susceptibility measurements show that both complexes are completely locked in spin-paired diamagnetic states in the whole temperature range measured. This observation is in accordance with the average Fe-N bond observed-1.9505(15) Å for 1 and 1.950(5) Å for 2-which is characteristic of Fe II in a low spin state [20,27]. However, the very small positive susceptibility values observed in the temperature dependance of the molar susceptibility curves for 1 and 2 may be due to the temperature-independent paramagnetism. The diamagnetic nature of both Fe II complexes is also supported in solution by characteristic, well-resolved 1 H and 13 C NMR peaks. These chemical shifts are in excellent accord with those computed at the TPSSh-D3(BJ)/def2-TZVP level for closed-shell (S = 0) species (see Tables S1 and S2 and Figure S1 in Supplementary Materials). High spin complexes in a quintet (S = 2) state are computed at the same level to be energetically disfavored by 76.0 kJ/mol and 72.9 kJ/mol for 1 and 2, respectively. This relatively large energy gap can explain the locking of these complexes in the closed-shell state (S = 0) over a wide temperature range. A similar spinpaired, diamagnetic state has been observed for a N 6 -coordination (two azomethine and one pyridyl nitrogen from each ligand) in [FeL 2 ] 2+ Schiff base complexes [35]. Our previous investigations revealed that the spin state cannot be changed by introducing an electrondonating methyl group to the ligand system mentioned above with N 6 -coordination [34]. With the same type of hexa coordinate ligand systems, it has been observed that, if the pyridyl nitrogen coordination is replaced by imidazole nitrogen, the compound exhibits SCO behavior [46]. It is worth noting that, with the similar type of coordination environment reported by Alberto et al. [47] and Ishida et al. [25], the HS state and SCO become stabilized by increasing the size of the halogen substituent. Moreover, Gu et al. have shown that the electron-donating methyl substitution, counteranion or solvent have a negligible influence on the SCO behavior [20,39], with the same type of N 6 -ligand field, which is in accordance with the results that we have obtained.
Inorganics 2022, 10, x FOR PEER REVIEW 8 of 13 nitrogen coordination is replaced by imidazole nitrogen, the compound exhibits SCO behavior [46]. It is worth noting that, with the similar type of coordination environment reported by Alberto et al. [47] and Ishida et al. [25], the HS state and SCO become stabilized by increasing the size of the halogen substituent. Moreover, Gu et al. have shown that the electron-donating methyl substitution, counteranion or solvent have a negligible influence on the SCO behavior [20,39], with the same type of N6-ligand field, which is in accordance with the results that we have obtained.

Materials and Methods
All chemicals and reagents were purchased from commercial sources and were of analytical reagent grade, used without further purification. All complexation reactions were carried out under nitrogen atmosphere. FTIR spectra were measured on an Agilent Technologies Cary 630 FTIR spectrometer in the 4000-400 cm −1 range. NMR spectra were recorded with Bruker Ascend TM (Billerica, MA, USA) 400 (400 MHz for 1 H and 101 MHz for 13 C) instruments in CD 3 CN with tetramethylsilane (TMS) as an internal reference. Elemental analyses were carried out on a FLASH elemental analyzer 1112 CHNS-O (Thermo Finnigan Italia, Rodano, Italy). Melting points were determined on a Büchi Melting Point M-565 apparatus. Single crystal XRD measurements were performed with a Bruker D8 VENTURE Kappa Duo diffractometer equipped with a PHOTON III detector and using monochromatic CuK α primary radiation. The phase problem was solved by intrinsic phasing (SHELXT) [48] and the structure model was refined by full-matrix least-squares on F 2 values (SHELXL) [49]. Magnetic susceptibility measurements were carried out on a SQUID magnetometer (Quantum Design MPMS XL, San Diego, CA, USA) operating between 5 and 350 K with magnetic fields 1 kOe. The data were corrected for the intrinsic diamagnetic contributions of the sample and the sample holder. Electrospray ionization-mass spectrometry was performed using ESI-ToF Mass spectrometer (Bruker Daltonics micrOTOF-Q II).
CAUTION. Handling with metal-organic perchlorates is potentially dangerous due to their explosive properties. It should be handled with care in small quantities. Especially high temperature magnetic measurements are risky.

Computational Details
The structures of all systems under investigation were fully optimized (without counteranion) in Turbomole [52] at the TPSSh level of theory, [53] including an atom-pairwise correction for dispersion forces (Grimme's D3 model) with Becke-Johnson (BJ) damping [54,55] and employing the def2-TZVP basis set for all atoms [56]. The optimized structures were characterized as true minima on the potential energy hypersurface by harmonic vibrational frequency analyses. Calculations of NMR nuclear shieldings were performed in the Gaussian 16 program package [57] using gauge-including atomic orbitals (GIAO) at the same level as structure optimization (TPSSh-D3(BJ)/def2-TZVP). In these calculations, bulk solvent effects were simulated by means of the integral equation formalism of the polarizable continuum model (IEF-PCM) [58]. The calculated 1 H and 13 C shieldings were converted to chemical shifts (δ in ppm) relative to the shieldings of tetramethylsilane (TMS).

Conclusions
Two mononuclear Fe II complexes, [Fe(L 1 ) 2 ](ClO 4 ) 2 ·CH 3 OH (1) and [Fe(L 1 ) 2 ](ClO 4 ) 2 · 2CH 3 CN (2), based on two unsymmetrical tridentate Schiff base ligands, were synthesized and characterized. Both complexes show distorted octahedral coordination geometries. Spectroscopic, magnetic and structural studies revealed that the spin states of both complexes remain diamagnetic throughout the measured temperature range. The ligand field created by N 6 -coordination was comprised of four azomethine and two pyridine nitrogen favors, thus showing a low spin Fe II state. It is notable that the variation of solvents (acetonitrile and/or methanol) did not influence the magnetic properties of 1 and 2. Introducing an electron-withdrawing chlorine substituent into the meta position of L 2 did not alter the ligand field, and hence, there is no change in the spin state. We hope the observed results are of particular importance for further design of molecular magnetic materials and encourage the development of new Fe II Schiff base SCO systems. Further investigations by varying the ligand field and making substitutions in the ligand moiety are under way.