inorganics-10-00098

inorganics-10-00098

Inorganics 2022, 10(7), 98; doi:10.3390/inorganics10070098

Article

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

Yosef Bayeh 1,2,3Orcid, Nithin Suryadevara ⁴Orcid, Sören Schlittenhardt ⁵, Róbert Gyepes ⁶Orcid, Assefa Sergawie 1,2, Peter Hrobárik 3,*Orcid, Wolfgang Linert ⁷Orcid, Mario Ruben 4,5,8,* and Madhu Thomas 1,2,*Orcid

¹

Department of Industrial Chemistry, College of Applied Sciences, Addis Ababa Science and Technology University, Addis Ababa P.O. Box 16417, Ethiopia

²

Nanotechnology Center of Excellence, Addis Ababa Science and Technology University, Addis Ababa P.O. Box 16417, Ethiopia

³

Department of Inorganic Chemistry, Faculty of Natural Sciences, Comenius University, Mlynská dolina CH-2, Ilkovičova 6, 84215 Bratislava, Slovakia

⁴

Institute of Quantum Materials and Technologies, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Karlsruhe, Germany

⁵

Institute of Nanotechnology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Karlsruhe, Germany

⁶

Department of Inorganic Chemistry, Faculty of Science, Charles University in Prague, Hlavova 2030/8, 12843 Prague, Czech Republic

⁷

Institute of Applied Physics, Vienna University of Technology, Wiedner Hauptstraße 8-10, 1040 Vienna, Austria

⁸

Centre Européen de Science Quantique (CESQ), Institut de Science et d’Ingénierie Supramoléculaires (ISIS, UMR 7006), CNRS-Université de Strasbourg, 8 Allée Gaspard Monge, BP 70028, CEDEX, 67083 Strasbourg, France

*

Correspondence: peter.hrobarik@uniba.sk (P.H.); mario.ruben@kit.edu (M.R.); madhu.thomas@aastu.edu.et (M.T.)

Academic Editor: László Kótai

Received: 31 May 2022 / Accepted: 6 July 2022 / Published: 8 July 2022

Abstract

:

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, ⁵T₂) and a diamagnetic low spin-state (LS, S = 0, ¹A₁) by external stimuli under an appropriate ligand field. We have synthesized two mononuclear FeII complexes, viz., [Fe(L¹)₂](ClO₄)₂.CH₃OH (1) and [Fe(L²)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 from 5–350 K.

Keywords:

Iron(II) complexes; Schiff base; octahedral; spin-crossover and low spin

1. 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 FeII compounds has received special attention due to the clear discrimination between the paramagnetic high spin state (S = 2, ⁵T₂) and diamagnetic low spin state (S = 0, ¹A₁), 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].

Nano-sized SCO complexes are highly relevant for considering future applications and offer diverse pathways towards multifunctional systems for molecular memory, switches or display devices [10,11,12,13,14,15,16,17,18]. For acquiring a desired property, it is key to tune the ligand field, which could eventually modulate the magnetic properties. It has been observed that a N₆– [19,20] or N₄O₂– [21] coordination environment around FeII can bring about sharp SCO with appreciable hysteresis width [22,23]. The effects of halogen substitution on the SCO behavior have been reported in FeII–N₆ compounds [24,25,26,27,28,29,30], and the spin transition temperature, T1/2, is found to increase upon moving from fluoride to bromide substitution, thereby proving the size effect on the spin state of the complexes [20,24,31].

Previously, we have reported on how N₆–coordination and variation in the ligand field in a series of bispyrazolone derivatives brought abrupt SCO at around room temperature [32]. Additionally, in our previous reviews, the role of azomethine and substituent effects in tuning the SCO behavior and attaining the desired architecture have been inferred [8,33]. In this context, Schiff bases are ideal candidates on account of their fine tunability to the ligand field by varying the substituents in both amine and aldehydic precursors.

Recently, we have observed that N₆–coordination in FeII complexes from four azomethine and two pyridine nitrogens locks the spin state completely to a low spin condition even with an electron-donating methyl group attached to the meta position [34].

On account of the above facts and considering ligand-field and electronic effects, we are herein reporting our investigations on the structural and spin-state of two mononuclear FeII Schiff base complexes having N₆–coordination. The ligands were designed without a substituent in L¹ (2,6-bis[(benzylimino)methyl]pyridine) and with an electron-withdrawing chloro substituent at the meta position in L² (2,6-bis[(3_chlorobenzylimino)methyl]pyridine) in comparison with the electron-donating methyl group of the previous reports [34,35].

2. Results and Discussion

2.1. Schiff Bases L¹ and L² and Their Complexes 1 and 2

Both ligands were prepared by condensation reaction, where benzylamine or 3-chlorobenzylamine were allowed to condense with pyridine-2,6-dicarboxaldehyde in ethanol under reflux (Scheme 1). Single crystals of L¹, appropriate for X-ray diffraction studies, were grown by the slow evaporation of the solvent from a methanolic solution of L¹ at room temperature. However, we were unsuccessful in generating single crystals for L².

The corresponding complexes 1 and 2 were synthesized by the reaction of the tridentate ligands L¹ and L² 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 ¹H and ¹³C NMR spectra exhibit key resonances at 8.42 and 162.4 ppm, respectively, for L¹, and 8.43 and 163.0 ppm, respectively, for L², assigned to the imine (CH=N) group. The ¹H and ¹³C NMR resonances of pyridine moiety are observed between 7.97–7.79 and 154.6–121.8 ppm, respectively, for L¹, whereas for L², they are between 8.00–7.81 and 154.4–122.0 ppm, respectively (see Table 1 and Table 2 for more detailed assignments and Tables S1 and S2 in Supplementary Materials for DFT computed NMR chemical shifts).

Upon coordination of L¹ or L² with FeII, the imine ¹H NMR peaks are low-frequency shifted to 7.73 ppm (1) and 7.84 ppm (2), while the imine carbons are deshielded to 170.1 ppm (1) and 171.1 ppm (2), thus by more than +7.7 ppm. Even larger ¹H shielding effects upon FeII complexation are observed for benzylic CH₂ groups (Δδ(¹H) = ca. −1.1 ppm) and ortho-hydrogens (Δδ(¹H) = ca. −0.8 ppm). Contrarily, the largest coordination-induced ¹H deshieldings (Δδ(¹H)~+0.6 ppm) are seen for hydrogens of the pyridine moiety at the position 4 (py-4). Apart from the imine ¹³C nuclei, the coordination-induced deshieldings are also observed for pyridine-2/-6 and pyridine-3/-5 carbons, while benzylic C-1 carbons on the phenyl ring possess the most pronounced coordination-induced shielding (Δδ(¹³C) = −6 to −7 ppm).

The IR peaks were observed at 1644 cm⁻¹ and 1651 cm⁻¹ for L¹ and L², respectively, confirming the presence of CH=N. The IR spectra of L¹ and L² 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, 1529 cm⁻¹ (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(L¹)₂](ClO₄)₂·CH₃OH and [Fe(L²)₂](ClO₄)₂·2CH₃CN for 1 and 2, respectively.

2.2. X-ray Crystallographic Analysis

The crystallographic data of the ligand L¹ 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 L¹, X_ray quality crystals were grown by slow evaporation of its methanolic solution. The compound crystallizes in a triclinic lattice and space group P 1 ¯ with two symmetrically independent molecules located in the asymmetric part of the unit cell (Z = 4). The crystal structure is shown in Figure 1.

The single crystal X-ray diffraction data for the complexes 1 and 2 were collected at 120 K. Compound 1 crystallizes in a monoclinic crystal lattice with space group C2/c, whereas 2 crystalizes in a triclinic lattice with space group P 1 ¯ . An asymmetric unit of 1 consists of one discrete [FeL₂]²⁺ cation, two ClO₄⁻ ions and one methanol solvent molecule (Figure 2a). However, the asymmetric units of 2 contain one discrete [FeL₂]²⁺ cation, two ClO₄⁻ ions and two acetonitrile solvent molecules (Figure 2b). The unit cell of 1 contains eight complex units, sixteen counter anions and eight solvent molecules, whereas 2 contains two complex units, four counter anions and four solvent molecules (Figure S2c,d).

In both 1 and 2, the coordination sphere is composed of six nitrogen donors. Two pyridine nitrogen atoms of both L¹ and L² occupy the axial positions and the four azomethine nitrogens occupy the equatorial plane (Scheme 1). Two tridentate ligands are wrapped around the FeII metal centers in a distorted octahedral geometry, with average Fe–N bond lengths of 1.9505 Å for 1 and 1.9643 Å for 2 (Table 4). These distances are indicative of LS FeII centers, which is in accordance with magnetic investigations, vide infra [39,40,41]. The average Fe–Npy and Fe–Nimine bond distances are 1.8797(14) Å and 1.9860(15) Å, respectively, for 1, and 1.878(5) Å and 1.987(5) Å, respectively, for 2 (Table 4), which coincide very well with DFT optimized parameters for [Fe(L¹)₂]²⁺ (d(Fe–Npy)avrgd = 1.881 Å; d(Fe–Nimine)avrgd = 1.983 Å) and [Fe(L²)₂]²⁺ (d(Fe–Npy)avrgd = 1.884 Å; d(Fe–Nimine)avrgd = 1.989 Å) in singlet (S = 0) states (see Supplementary Materials for TPSSh-D3(BJ)/def2-TZVP optimized geometries). The axial angles N(2)–Fe(1)–N(5) 178.92(6)° for 1 and 178.8(2)° for 2 indicate a clear distortion from the linear arrangement in both molecules (Table S3).

Hydrogen bonding and π-π stacking were found to have a marked influence on the magnetic properties of crystalline FeII 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 CH₂ 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 CH₃OH forms short contacts with ClO₄⁻ ions with an average distance of 2.65(3) Å and the molecular packing of 2 shows that weak contact exists through the CH₂ group of the ligand moiety and ClO₄⁻ ion with an average distance of 2.58(2) Å [45] (Figure S2c,d).

2.3. 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 BDC = 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 FeII 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 FeII complexes is also supported in solution by characteristic, well-resolved ¹H and ¹³C 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 spin-paired, diamagnetic state has been observed for a N₆–coordination (two azomethine and one pyridyl nitrogen from each ligand) in [FeL₂]²⁺ Schiff base complexes [35]. Our previous investigations revealed that the spin state cannot be changed by introducing an electron-donating methyl group to the ligand system mentioned above with N₆–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 N₆–ligand field, which is in accordance with the results that we have obtained.

3. 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 AscendTM (Billerica, MA, USA) 400 (400 MHz for ¹H and 101 MHz for ¹³C) instruments in CD₃CN 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 CuKα primary radiation. The phase problem was solved by intrinsic phasing (SHELXT) [48] and the structure model was refined by full-matrix least-squares on F² 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.

3.1. Synthesis

3.1.1. Synthesis of the Schiff Bases L¹ and L²

The tridentate Schiff base ligands L¹ ((E,E)-2,6-bis[(benzylimino)methyl)pyridine) were prepared by modified literature procedures [50,51], where a solution of benzylamine (0.65 mL, 6 mmol) and L² ((E,E)-2,6-bis[(3-chlorobenzylimino)methyl]pyridine) 3-chlorobenzylamine (0.90 mL, 6 mmol) in ethanol (15 mL) were mixed with a stirred solution of pyridine-2,6-dicarbaldehyde (0.41 g, 3 mmol) in hot ethanol (15 mL). The mixtures were refluxed for 4 h (Scheme 1), while no precipitation occurred. The resultant solutions were concentrated to 10 mL and agitated thoroughly with a small amount of petroleum ether at 70–80 °C, which led to the precipitation of the ligands. These were then filtered, washed with petroleum ether and dried in vacuum. Pale orange (L¹) and off-white (L²) needle-like crystalline products were collected.

L¹: Yield; 85%; M. P. 75 °C; Anal. C₂₁H₁₉N₃: Calcd. C 80.51, H 6.07, N 13.42; found C 80.46, H 6.08, N 13.51%; ¹H NMR (CD₃CN): δ(ppm) = 8.42 (s, 2H, CH-azomethine), 7.97 (d, J = 7.8 Hz, 2H, pyridine), 7.79 (t, J = 7.8 Hz, 1H, pyridine), 7.30 (m, 4H, Ar), 7.29 (m, 4H, Ar), 7.21 (m, 2H, Ar) and 4.89 (s, 4H, 2 × CH₂). ¹³C NMR (CD₃CN): δ(ppm) = 162.35 (C=N, azomethine), 154.55 (C-N, pyridine), 139.32, 137.49, 128.51, 128.19, 127.05, 121.83, 64.31 (Figure S3a,b). IR: ν ¯ (cm⁻¹) = 3055(w), 3026(w), 2839(w), 1644(s), 1570(s), 1493(m), 1450(s), 1354(w), 1154(m), 1073(m), 1027(s), 991(m), 806(m) and 730(s).

L²: Yield; 47.3%; M. P. 74.3 °C; Anal. C₂₁H₁₇N₃Cl₂: Calcd. C 65.96, H 4.45, N 10.99; found C 65.80, H 4.61 N 10.87%; M. Pt. 63.4; ¹H NMR (CD₃CN): δ(ppm) = 8.43 (s, 2H, azomethine), 8.00 (d, J = 7.8 Hz, 2H, pyridine), 7.81 (t, J = 7.8 Hz, 1H, pyridine), 7.34 (bs, 2H, Ar), 7.29–7.20 (m, 6H, Ar) and 4.77 (s, 4H, 2 × CH₂). ¹³C NMR (CD₃CN): δ(ppm) = 162.99 (C=N, azomethine), 154.43 (C-N, pyridine), 141.83, 137.56, 133.77, 130.13, 127.97, 127.00, 126.56, 122.03, 63.41 (Figure S4a,b). IR: ν ¯ (cm⁻¹) =3060(w), 3026(w), 2839(w), 1651(s), 1594(s), 1571(s), 1472(s), 1429(s), 1335(w), 1200(m), 1157(m), 1074(s), 995(m), 864(m) and 744(s).

3.1.2. Synthesis of the Complex [Fe(L¹)₂](ClO₄)₂·CH₃OH (1)

To a stirred solution of L¹ (0.313 g, 1.0 mmol) in methanol (20 mL), Fe(ClO₄)₂·6H₂O (0.181 g, 0.5 mmol) was added under nitrogen atmosphere. The resultant purple solution was refluxed for 2 h under continuous stirring (Scheme 1). It was then cooled and filtered. Slow diffusion of diethyl ether vapor into the filtered solution for two weeks afforded block-shaped black crystals suitable for X-ray diffraction measurements. The crystals were then filtered and washed with cold methanol and dried subsequently under vacuum overnight. Yield: 0.266 g, 58.3%; Anal. C₄₃H₄₂Cl₂FeN₆O9, Calcd.: C 56.55; H 4.60; N 9.21; Found: C 56.77; H 4.78; N 9.16%. ¹H NMR(CD₃CN): δ(ppm) = 8.38 (t, J = 7.9 Hz, 2H, pyridine), 8.13 (d, J = 7.9 Hz, 4H, pyridine), 7.73 (s, 4H, azomethine), 7.29 (t, J = 7.4 Hz, 4H, Ar), 7.16 (t, J = 7.7 Hz, 8H, Ar), 6.47 (d, J = 7.6 Hz, 8H, Ar) and 3.68 (s, 8H, 4 × CH₂). ¹³C NMR(CD₃CN): δ(ppm) = 170.05, 160.27, 137.43, 133.27, 129.34, 129.28, 128.70, 128.46 and 62.39 (Figure S5a,b). FTIR: (cm⁻¹) = 3021(w),1603(s), 1585(m), 1528(s), 1485(s), 1382(s), 1208(m), 1159(m), 1072(s), 980(m), 821(m), 740(s), 688(s), 620(s) and 450(s). ESI-MS: 781.20 (M⁺-ClO₄⁻), and 681.24 (M²⁺) (Figure S7a).

3.1.3. Synthesis of the Complex [Fe(L²)₂](ClO₄)₂·2CH₃CN (2)

The ligand L² (0.191 g, 0.5 mmol) was dissolved in acetonitrile (20 mL) and this solution was mixed with Fe(ClO₄)₂·6H₂O (0.090 g, 0.25 mmol) under nitrogen atmosphere. The resultant purple solution was refluxed for 2 h under continuous stirring (Scheme 1). The reaction mixture was then cooled and filtered. Slow diffusion of diethyl ether into the filtered solution yielded blocks of black crystals appropriate for X-ray diffraction. These were then filtered off, washed with cold acetonitrile and subsequently dried under vacuum. Yield: 0.213 g, 77.2%; Anal. C₄₆H₄₀Cl₆FeN₈O₈, Calcd.: C, 50.12; H, 3.63; N, 10.17; Found: C, 49.98; H, 3.74; N, 10.18%. ¹H NMR(CD₃CN): δ(ppm) = 8.50 (t, J = 7.9 Hz, 2H, pyridine), 8.23 (d, J = 7.9 Hz, 4H, pyridine), 7.84 (s, 4H, azomethine), 7.33 (d, J = 9.5 Hz, 4H, Ar), 7.17 (t, J = 9.5 Hz, 4H, Ar), 6.48 (bs, 8H, Ar) and 3.74 (s, 8H, 4 × CH₂). ¹³C NMR(CD₃CN): δ(ppm) = 171.14, 160.36, 137.75, 135.49, 134.50, 130.94, 129.61, 128.74, 128.62, 127.26 and 61.79 (Figure S6a,b). FTIR: ν ¯ (cm⁻¹) = 3044(w), 1600(s), 1564(s), 1529(s), 1474(s), 1396(s), 1355(m), 1194(m), 1075(s), 928(m), 852(m), 789(s), 705(s), 678(s), 614(s) and 442(s). ESI-MS: 917.04 (M⁺–ClO₄_) and 818.09 (M²⁺) (Figure S7b).

3.2. 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 ¹H and ¹³C shieldings were converted to chemical shifts (δ in ppm) relative to the shieldings of tetramethylsilane (TMS).

4. Conclusions

Two mononuclear FeII complexes, [Fe(L¹)₂](ClO₄)₂·CH₃OH (1) and [Fe(L¹)₂](ClO₄)₂·2CH₃CN (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 N₆-coordination was comprised of four azomethine and two pyridine nitrogen favors, thus showing a low spin FeII 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 L² 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 FeII Schiff base SCO systems. Further investigations by varying the ligand field and making substitutions in the ligand moiety are under way.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics10070098/s1, Figure S1: Comparison of calculated and experimental NMR shifts in [FeL2]2+ (S = 0) complexes (cf. Tables S1 and S2 for numeric data); Figure S2: Projection of the π-π interaction through phenyl rings of 1 (a) along the bc plane and 2 (b) along the ab plane. ClO4-ions, solvents and hydrogen atoms have been omitted for clarity. Short intermolecular contacts and H-bonding 1 (c) and 2 (d) both in b-direction; Figure S3: (a) 1H NMR and (b) 13C NMR of L1; Figure S4: (a) 1H NMR and (b) 13C NMR of L2; Figure S5: (a) 1H NMR and (b) 13C NMR of 1; Figure S6: (a) 1H NMR and (b) 13C NMR of 2; Figure S7: ESI-MS molecular ion peaks of 1 (a) and 2 (b); Table S1: Experimental and computed 1H NMR shifts (in ppm vs. TMS) in free ligands L1, L2 and corresponding [Fe(L1)2]2+ and [Fe(L2)2]2+ complexes (all in CD3CN); Table S2: Experimental and computed 13C NMR shifts (in ppm vs. TMS) in free ligands L1, L2 and corresponding [Fe(L1)2]2+ and [Fe(L2)2]2+ complexes (all in CD3CN); Table S3: Coordination bond angles for 1 and 2 at 120 K.

Author Contributions

The major work for this article, designing, execution and writing of the original draft, was done by the first author (Y.B.), which is part of his Ph.D. program. The second and third authors, N.S. and S.S. have done SQUID and ESI-MS measurements. The fourth author, R.G. solved the structures. The other authors, W.L., M.R., A.S., P.H. and M.T. were responsible for supervision, editing, and reviewing of this article. All authors have read and agreed to the published version of the manuscript.

Funding

This research project was supported by the National Scholarship Programme of the Slovak Republic 2021 and Addis Ababa Science and Technology University, Addis Ababa, Ethiopia. This work was also supported by the Slovak Research and Development Agency (Grant No. APVV-17-0324) and the Grant Agency of the Ministry of Education of the Slovak Republic (VEGA Project No. 1/0669/22).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

We are thankful to National Scholarship Programme of the Slovak Republic 2021 for a research stay and Woldia University, Ethiopia, for a Ph.D studentship to Y.B. Thanks are due to Patrik Osuský, Department of Inorganic Chemistry, Comenius University, Slovakia, for spectroscopic measurements. We are also grateful to Jozef Noga, Department of Inorganic Chemistry, Comenius University, Slovakia, for his keen interest and support during this work.

Conflicts of Interest

The authors declare no conflict of interest.

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Scheme 1. Synthetic route for ligands L¹, L² and complexes 1 and 2.

Scheme 1. Synthetic route for ligands L¹, L² and complexes 1 and 2.

Figure 1. X-ray crystal structure of L¹ molecule in bc-axis; Color code: blue, N; grey, C; and white, H.

Figure 1. X-ray crystal structure of L¹ molecule in bc-axis; Color code: blue, N; grey, C; and white, H.

Figure 2. Labelled ORTEP drawing of 1 (a) and 2 (b) both in b-axis. Thermal ellipsoids are drawn on 30% probability level. Hydrogen atoms have been omitted for clarity. Color code: Yellow-green, Fe; blue, N; grey, C; green, Cl and red, O.

Figure 2. Labelled ORTEP drawing of 1 (a) and 2 (b) both in b-axis. Thermal ellipsoids are drawn on 30% probability level. Hydrogen atoms have been omitted for clarity. Color code: Yellow-green, Fe; blue, N; grey, C; green, Cl and red, O.

Figure 3. The χMT vs. T magnetic plots for 1 (a) and 2 (b) between 5–350 K.

Figure 3. The χMT vs. T magnetic plots for 1 (a) and 2 (b) between 5–350 K.

Table 1. Experimental ¹H NMR shifts (in ppm vs. TMS) in free ligands L¹, L² and corresponding [Fe(L¹)₂]²⁺ and [Fe(L²)₂]²⁺ complexes (all measured in CD₃CN) a.

Table 1. Experimental ¹H NMR shifts (in ppm vs. TMS) in free ligands L¹, L² and corresponding [Fe(L¹)₂]²⁺ and [Fe(L²)₂]²⁺ complexes (all measured in CD₃CN) a.

| H-Imine | py-3,5 | py-4 | CH₂ | H-2/H-6 | H-3/H-5 | H-4 |
L¹ | 8.42 | 7.97 | 7.79 | 4.79 | 7.30 | 7.29 | 7.21 |
[Fe(L¹)₂]²⁺ | 7.73 | 8.13 | 8.38 | 3.68 | 6.47 | 7.16 | 7.29 |
Δδ(¹H) b | −0.69 | +0.16 | +0.59 | −1.11 | −0.83 | −0.13 | +0.08 |
| H-Imine | py-3,5 | py-4 | CH₂ | H-2 | H-4 | H-5 | H-6
L² | 8.43 | 8.00 | 7.81 | 4.77 | 7.34 | 7.25 | 7.25 | 7.25
[Fe(L²)₂]²⁺ | 7.84 | 8.23 | 8.50 | 3.74 | 6.48 | 7.17 | 7.33 | 6.48
Δδ(¹H) b | −0.59 | +0.23 | +0.69 | −1.03 | −0.86 | −0.08 | +0.08 | −0.77

a See SI for corresponding NMR spectra and computed chemical shifts. b ¹H NMR coordination shifts as a difference between resonance of given ¹H nuclei in complex and the free ligand. See Scheme 1 for atom numbering.

Table 2. Experimental ¹³C NMR shifts (in ppm vs. TMS) in free ligands L¹, L² and corresponding [Fe(L¹)₂]²⁺ and [Fe(L²)₂]²⁺ complexes (all measured in CD₃CN) a.

Table 2. Experimental ¹³C NMR shifts (in ppm vs. TMS) in free ligands L¹, L² and corresponding [Fe(L¹)₂]²⁺ and [Fe(L²)₂]²⁺ complexes (all measured in CD₃CN) a.

| C-Imine | py-2,6 | py-3,5 | py-4 | CH₂ | C-1 | C-2/C-6 | C-3/C-5 | C-4 |  |
L¹ | 162.4 | 154.6 | 121.8 | 137.5 | 64.3 | 139.3 | 128.5 | 128.2 | 127.1 |  |
[Fe(L¹)₂]²⁺ | 170.1 | 160.3 | 128.5 | 137.4 | 62.4 | 133.3 | 128.7 | 129.3 | 129.3 |  |
Δδ(¹³C) b | +7.7 | +5.7 | +6.6 | −0.1 | −1.9 | −6.0 | +0.2 | +1.2 | +2.2 |  |
| C-Imine | py-2,6 | py-3,5 | py-4 | CH₂ | C-1 | C-2 | C-3 | C-4 | C-5 | C-6
L² | 163.0 | 154.4 | 122.0 | 133.8 | 63.4 | 141.8 | 128.0 | 137.6 | 127.0 | 130.1 | 126.6
[Fe(L²)₂]²⁺ | 171.1 | 160.4 | 128.6 | 135.5 | 61.8 | 134.5 | 128.7 | 137.8 | 130.9 | 129.6 | 127.3
Δδ(¹³C) b | +8.1 | +5.9 | +6.6 | +1.7 | −1.6 | −7.3 | +0.8 | +0.2 | +3.9 | −0.5 | +0.7

a See SI for corresponding NMR spectra and computed chemical shifts. b ¹³C NMR coordination shifts as a difference between resonance of given ¹³C nuclei in complex and the free ligand. See Scheme 1 for atom numbering.

Table 3. Collated crystal parameters data for L¹, 1 and 2.

Table 3. Collated crystal parameters data for L¹, 1 and 2.

Parameter
L¹
1
2

Empirical formula
C₂₁H₁₉N₃
C₄₃H₄₂Cl₂FeN₆O₉
C₄₆H₄₀Cl₆FeN₈O₈

Formula weight
313.39
912.44
1101.41

Temperature
120(2) K

Wavelength
0.71073 Å

Crystal system
Triclinic
Monoclinic
Triclinic

Space group
P 1 ¯
C2/c
P 1 ¯

Unit cell dimensions
a = 8.9002(19) Å
b = 10.289(2) Å
c = 19.181(4)Å
α = 92.691(10)°
β = 99.378(9)°
γ = 100.078(10)°
a = 37.8848(9) Å
b = 10.5130(3) Å
c = 21.4945(5) Å
α = 90°
β = 109.7270(10)°γ = 90°
a = 10.1735(10) Å
b = 10.2669(9) Å
c = 23.149(2) Å
α = 92.685(3)°
β = 101.466(3)°
γ = 90.261(3)°

Volume
1701.1(7) Å³
8058.5(4) Å³
2366.9(4) Å³

Z
4
8
2

Calculated density
1.224 g/cm³
1.509 g/cm³
1.545 g/cm³

Absorption coefficient
0.073 mm⁻¹
0.573 mm⁻¹
0.721 mm⁻¹

Crystal size
0.321 × 0.299 × 0.050 mm³
0.248 × 0.220 × 0.104 mm³
0.427 × 0.358 × 0.309 mm³

Theta range for data collection
2.016° to 26.570°
2.020° to 27.493°
2.043° to 27.573°

Limiting indices
−11 ≤ h ≤ 10;
−12 ≤ k ≤ 12;
0 ≤ l ≤ 24
−48 ≤ h ≤ 49;
−13 ≤ k ≤ 13;
−23 ≤ l ≤ 27
−13 ≤ h ≤ 13;
−12 ≤ k ≤ 13;
−30 ≤ l ≤ 30

Reflections collected/unique
7284/7284
54461/9242
71645/10877

Completeness to θ: fraction
25.242°: 100.0%
25.242°: 99.9%
25.242°: 99.7%

Absorption correction
Semi-empirical from equivalents

Max. and
min. transmission
1.00 and 0.65
0.95 and 0.87
0.91 and 0.77

Refinement method
Full-matrix least-squares on F²

Data/restraints/parameters
7284/0/434
9242/6/639
10877/25/683

Goodness-of-fit on F²
1.020
1.047
1.144

Final R indices (I > 2σ(I))
R₁ = 0.0717;
wR₂ = 0.1517
R₁ = 0.0354;
wR₂ = 0.0779
R₁ = 0.1089
wR₂ = 0.2256

R indices (all data)
R₁ = 0.1317;
wR₂ = 0.1819
R₁ = 0.0434;
wR₂ = 0.0818
R₁ = 0.1308;
wR₂ = 0.2359

Largest diff. peak and hole
0.221 and −0.251 × 10⁻³ Å
0.572 and −0.463 × 10⁻³ Å
0.859 and −0.894 × 10⁻³ Å

Spin states
-
LS
LS

CCDC number
2165843
2154904
2128830

Table 4. Coordination bond lengths for 1 and 2 at 120 K.

Table 4. Coordination bond lengths for 1 and 2 at 120 K.

Fe1–N Bond Lengths (Å) at 120 K
1 | 2
Fe(1)–N(2) | 1.8794(14) | Fe(1)–N(2) | 1.875(5)
Fe(1)–N(5) | 1.8799(14) | Fe(1)–N(5) | 1.880(5)
Fe(1)–N(3) | 1.9763(15) | Fe(1)–N(4) | 1.978(5)
Fe(1)–N(4) | 1.9846(14) | Fe(1)–N(3) | 1.981(5)
Fe(1)–N(6) | 1.9891(16) | Fe(1)–N(6) | 1.990(5)
Fe(1)–N(1) | 1.9938(15) | Fe(1)–N(1) | 1.998(5)
Average Fe1–N | 1.9505(15) |  | 1.950(5)

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