Heteroleptic β-Diketonate Fe³⁺ Complex: Spin-Crossover and Optical Characteristics

Abstract

The heteroleptic halogen-substituted Fe³⁺ complex of formula [FeL₂Bipy]Cl, where L is 1-(4-fluoro-phenyl)-3-(4-bromo-phenyl)-propane-1,3-dione and Bipy is 2,2′-bipyridine, was synthesized, and its optic and magnetic properties were studied. Magnetic measurements showed that the complex at high temperatures (T > 75 K) is predominantly in the high-spin (HS) state of Fe³⁺ ions (S = 5/2, γ_HS¹ = 93%) with a small admixture of low-spin (LS) state (S = 1/2, γ_LS¹ = 7%). At T* ≈ 46 K, a partial spin-crossover transition (SCO, 5/2↔1/2) occurs. This process is accompanied not only by a change in the magnetic state (γ_HS² = 76%, γ_LS² = 24%), but also by the appearance of AFM interactions (θ_II = −2.3 K) between neighboring Fe³⁺ ions. A theoretical model was proposed to describe magnetic susceptibility, χ(T). As a result of the analysis of the ground spin state M(H) at 2.0 K, it was established that the majority of the LS states, as well as part of the HS states of Fe³⁺ ions (γ_HS ~ 53%), do not participate in exchange interactions. EPR studies confirmed the presence of HS and LS Fe³⁺ centers and made it possible to isolate I-type and II-type HS centers, corresponding to strong low-symmetry and weak distorted octahedral fields. SCO was also detected and the temperature dependences of the EPR intensities, I(T), of the HS and LS centers were analyzed.

1. Introduction

β-Diketones are distinctive building blocks for the creation of coordination-based assemblies [1]. Coordination chemistry involving β-diketonate ligands have always been a subject of interest to chemists due to their importance in various fields [2,3]. A large number of studies on the synthesis of metal-containing β-diketonate derivatives can be found among scientific publications. Complexes of this type are potential candidates for application as molecular precursors in chemical vapor deposition (CVD) techniques [4]. Another primary field of interest is the potential application as promising precursors for microelectronic and optical devices [5,6]. Most literature in this field is devoted to the lanthanide-metal derivatives. Their intriguing photoluminescence and catalytic features make them outstanding candidates for the development of a range of energy-efficient emissive materials [7] and promising catalysts in many organic syntheses [8]. The use of 3d- metal ions makes it possible to obtain magnetic complexes in addition to photoactive. There are a number of publications on Fe(II) and Fe(III) clusters containing β-diketonate donors, which can show interesting magnetic behavior; although their share is relatively small. The first references to this type of complexes come from the publication of German scientists who showed the possibility of forming metal complexes by β-diketonates [9]. Subsequent work has dealt with the tautomeric equilibrium of β-diketones, their metal complexation and the protonation constants of Fe(II) and Fe(III) complexes [10]. In the following works, it was established that iron (III) forms mainly 1:1 and 1:3 complexes with acetylacetone and with benzoylacetone in DMF [11]. An attempt to study the spin state of a metal was made by the group of Dr. T.Iyoda at the beginning of the millennium, who reported about dinuclear metal complexes of phenylene-linked bis-diketone ligands [12]. Ferro- and antiferromagnetic interactions were observed in case of dimeric complexes of metals [13]. Due to the fact that iron ion can be in different spin states, low-spin (LS) and high-spin (HS), which interconvert under external influences, such as temperature, magnetic field, light irradiation or ligand modification [14,15,16,17,18], cationic SCO Fe complexes are good candidates for the creation of magnetic systems. Another important characteristic of this type of coordination compound is its process ability in film or by wet lithography, which is a key advantage for its technological applications [19,20,21,22].

The aim of this work was to synthesize a new coordination Fe(III) complex formed by β-diketones and nitrogen-containing heterocycle 2,2′-bipyridine, and explore its optical and magnetic properties. Particular attention is paid to a detailed analysis of the magnetic state by combining two experimental methods (EPR spectroscopy and SQUID magnetometry) to obtain consistent information. This is the first attempt to create fundamentally new objects, which assumes our further development of a number of heteroleptic β-diketonate metal(III) complexes by including not only 3d-, but also lanthanide ions. Thus, the creation of a single molecular platform will allow the development of new heterofunctional dendrimer metal complexes of polydentate N,O-ligands with controlled specified properties.

2. Results and Discussion

2.1. Synthesis of Compounds

β-Diketone F-DK-Br was synthesized by the Claisen condensation reaction [23] in a solution of anhydrous THF at a stoichiometric ratio of reagents 4′-fluoroacetophenone/ethyl 4-bromobenzoate of 1/2. The product was recrystallized from MeOH to obtain a light-yellow powder with a yield of 38%.

To obtain a complex based on F-DK-Br, Scheme 1, Fe(III) chloride acting as metal salt and 2,2′-bipyridine as a neutral ligand complementing the coordination sphere were used. Nitrogen-containing heterocycles of this type are strong bidentate ligands in which the N atoms are easily coordinated with the metal center, forming very stable complexes [24]. They are used due to their technological properties, such as photoactivity, high charge transfer capacity, bright luminescence and electroactivity, and are necessary for the modification of target complexes along the axial axis in order to improve their chromophoric and electron-optical properties [25].

Scheme 1. Synthesis of iron(III) bis[1-(4-fluoro-phenyl)-3-(4-bromo-phenyl)propane-1,3-dionate](2,2′-bipyridine) chloride. Although β-diketone derivatives exist in solution in the form equilibrium mixture of enol and keto tautomers [26], for simplicity, only the keto structure is given.

Scheme 1. Synthesis of iron(III) bis[1-(4-fluoro-phenyl)-3-(4-bromo-phenyl)propane-1,3-dionate](2,2′-bipyridine) chloride. Although β-diketone derivatives exist in solution in the form equilibrium mixture of enol and keto tautomers [26], for simplicity, only the keto structure is given.

2.2. Spectral Characteristics

Spectral characteristics of the [FeL₂Bipy]Cl complex and their interpretation are given below. The presence of Fe, Cl and Br ions in the structure of the coordination compound was confirmed by X-ray fluorescence analysis (Figure S1). Based on this, the value of the cationic complex was included in the calculation of the mass spectrum.

A MALDI-ToF mass spectrometry study showed the presence of the molecular ion peak with m/z 903.36 corresponding to the composition [FeL₂Bipy]⁺Cl⁻ stabilized by NH₄⁺, as shown in Figure 1. The signals observed at smaller m/z values 851.19 (calc. 852.47) and 695.31 (calc. 696.28) refer to [FeL₂Bipy]⁺ and [FeL₂]⁺ fragments. Unfortunately, all attempts to obtain a sample suitable for single-crystal X-ray analysis were unsuccessful. The complex sedimented as a red powder in a mixture of solvents and at slow evaporation of the solvent from the concentrated solution of the compound.

In the ¹H NMR spectrum of [FeL₂Bipy]Cl, a broadening of signals in the absence of multiplicity is observed, which indicates the paramagnetic nature of the sample due to the presence of Fe(III) ions.

The distinctive infrared absorption bands of the free para-halogen substituted β-diketone and corresponding iron(III) ternary complex are presented below. The IR spectrum of the uncoordinated F-DK-Br ligand exhibited a broad absorption band at 3546–3416 cm⁻¹, which can be attributed to the stretching vibration of the enolic O-H form (Figure S2). The intense peaks observed at 1607–1586 cm⁻¹ and 1512–1477 cm⁻¹ can be attributed to the stretching vibrations of the C=O and enolic C=C fragments, respectively. However, in the IR spectrum of the [FeL₂Bipy]Cl complex, the enolic O-H form was not observed. The absorption bands attributed to the stretching vibrations of the C=O (1599–1587 cm⁻¹) and enolic C=C (1498–1476 cm⁻¹) bonds in the IR spectrum of complex exhibited a shift at 8–14 cm⁻¹ in comparison to those observed in the corresponding β-diketone ligand (Figure S2). The results indicate that the coordination bands were formed between the β-diketone and the Fe(III) ion. In the meantime, the strong band at 1529 cm⁻¹ in the complex is attributed to stretching vibrations of the C=N bond of the nitrogen heterocyclic ligand (Bipy). The new absorption bands at 423 cm⁻¹ and 356 cm⁻¹ in the spectrum of the complex, shown in Figure 2, were attributed to the stretching vibrations of the coordinated Fe–O and Fe–N, respectively [27]. These findings further corroborate the proposed conformation of the Fe(III) complex with the para-halogen-substituted β-diketone and nitrogen heterocyclic ligands.

Regarding temperature stability and phase behavior, it can be said that in contrast to the crystalline nature of β-diketone F-DK-Br, which is characterized by a melting point of 124 °C, the ternary complex obtained as a result of complexation is an amorphous compound with a decomposition temperature of 306.7 °C (Figure S3). During the heating cycle, the substance transitions to a glassy state at a temperature of 85.9 °C (Figure S4); no further phase transitions are observed.

2.3. Optical Properties

The optical properties of [FeL₂Bipy]Cl were studied for dilute solutions in DCM (C = 10⁻⁶ mol/L). In the UV/vis spectrum of the solutions of [FeL₂Bipy]Cl, two absorption bands of different intensities are observed at 271 nm and 325 nm (Figure 3). These bands arise from π-π* electronic transitions of the chelating β-diketonate and 2,2′-bipyridine ligands. Solutions of [FeL₂Bipy]Cl in DCM exhibit very weak fluorescence (QY_FL = 1%) with an emission maximum at 435 nm (Figure 3). The fluorescence decay curve for [FeL₂Bipy]Cl is described by a biexponential model, with a fluorescence lifetime of 1.96 ns (Figure S5).

2.4. DFT Calculations

In [FeL₂Bipy]Cl, the central atom coordinates with two β-diketonate ligands and one 2,2′-bipyridine ligand, forming a distorted octahedral coordination geometry (Figure 4, Table S1). The average M–O and M–N coordination bond lengths are 1.884 Å and 1.971 Å, respectively (Table S2). The bite angles (O–M–O and N–M–N) are 91.3° and 81.4°, respectively (Table S2). Analysis of intramolecular interactions shows that the complex is characterized by the presence of C–H···Br interactions (3.844 Å) between the bromophenyl fragments of the β-diketonate ligands and weak C–H···π contacts (4.051 Å) between the fluoro-phenyl fragments of the β-diketonate ligands and the 2,2′-bipyridine ligand.

An analysis of the electronic transition nature of [FeL₂Bipy]Cl was performed using TDDFT. The results of the TDDFT analysis showed that the absorption spectrum of [FeL₂Bipy]Cl (Figure S6) is characterized by five vertical excitations with oscillator strengths greater than 0.21. Among these, the most probable HOMO to LUMO electronic transition is associated with the process of charge transfer from the coordination center to 2,2′-bipyridine.

2.5. Magnetic and Resonance Properties

To study the magnetic properties of the complex, EPR and SQUID techniques were used, which made it possible to obtain consistent information and describe the temperature evolution of the magnetic states of the substance. SQUID is an integral magnetic method that allows one to observe the evolution of the total spin response, while EPR is a locally sensitive method that makes it possible to detect individual spin systems, for example, the HS and LS states of Fe(III) ions, and study their temperature dynamics, as well as analyze the influence of the environment on relaxation processes.

2.5.1. SQUID Magnetometry

Static magnetic susceptibility, χ(T), was measured on polycrystalline sample under heating regime using the zero-field cooled (ZFC) protocol. The temperature dependency of the χ in the form of the product χT is depicted in Figure 5. The χT value starts at 4.12 cm³ K/mol and reveals nearly temperature-independent behavior in the range of 90–300 K. The χT value at 300 K differs from the theoretical spin-only value of 4.375 cm³ K/mol, corresponding to 100% amount of HS Fe(III) ions (S = 5/2, g = 2.0). This means that the fully realized HS state is not reached and there is a contribution of LS states. Using the equation (χT)_exp = γ_LS·(χT)_LS^theor + (1 − γ_LS)·(χT)_HS^theor [28], the molar contributions of LS and HS states at room temperature were estimated, which were γ_LS¹ = 7% and γ_HS¹ = 93%, respectively. Below 90 K, there is a gradual decrease in the magnetic moment with the inflection around 50 K, and then, it decreases continuously down to 2.05 cm³ K/mol. The slight increase in the range 30–90 K is most likely due to the occurrence of partial Fe(III) SCO transition (5/2↔1/2). The drastic decrease in the χT value observed at T < 20 K is characteristic of AFM interactions and/or zero-field splitting (ZFS).

The temperature dependence of the inverse magnetic susceptibility, χ⁻¹(T), was plotted and analyzed (Figure 6). It was found that the χ⁻¹ curve is described by two Curie–Weiss approximations on different temperature ranges: I—90 < T < 300 K, II—10 < T < 35 K. The respective Weiss constants are Θ_I = −0.79 K, C_I = 4.11 cm³ K/mol; Θ_II = −2.34 K, C_II = 3.8 cm³ K/mol. This behavior of χ⁻¹(T) indicates a partial SCO transition. To confirm this, the first derivative curve, d(χ⁻¹)/dT, was plotted (inset in the Figure 6). For this purpose, the original χ⁻¹(T) curve was used to smoothly interpolate the experimental data. The d(χ⁻¹)/dT curve shows a pronounced minimum near the T = 50 K, which clearly indicates a transition.

The negative sign of Θ indicates the presence of AFM interactions between Fe(III) cations. The contribution of the ZFS factor to the decrease in χT can be neglected, since the parameter D is small [29]. Thus, in the first temperature range, there are practically non-interacting Fe(III) ions (paramagnetic behavior), and in the second range, strong AFM correlations arise.

To describe the experimental dependence of the χT, three temperature intervals were identified, for each of which a magnetic state was proposed within the framework of a theoretical approach. The first region at T > 75 K corresponds to the paramagnetic Curie law, the second in the range 30–75 K is partial SCO, and the third at T < 30 K is AFM interactions in the Curie–Weiss approximation. SCO transition was modeled by replacing LS Fe³⁺ spin moments S = 1/2 to HS S = 5/2 via Boltzmann activation mechanism [30,31]. We used a symmetric Boltzmann distribution model to estimate a midpoint of the transition, T*. The upper limit of the (χT)_SCO curve was set equal to the experimental level of only the Fe(III) HS states (γ_HS¹ = 93%) χT(300 K) = 4.09 cm³ K/mol, which correspond to PM Curie law at T > 75 K. The fitting parameters were T* and the width of the transition at the levels ±1/2 of the midpoint, ΔT_1/2. Thus, this procedure made it possible to estimate the lower limit of the (χT)_SCO curve corresponding to the partial SCO transition and to determine new molar contributions of LS (γ_LS²) and HS (γ_HS²) states. The fitting of T* was carried out near T = 50 K, corresponding to the minimum on the d(χ⁻¹)/dT curve. A best-fit (χT)_SCO curve is shown in the inset of Figure 5 as a solid red line (include 0.03 cm³ K/mol from γ_LS¹ = 7%). The SCO parameters were T* = 45.9 K and ΔT_1/2 = 30 K. The lower limit of the (χT)_SCO = 3.42 cm³ K/mol, which correspond to γ_LS² = 24% and γ_HS² = 76%. The continuous transition of the (χT)_SCO curve allows to determine the mutual concentrations of the spin moments S = 1/2 and 5/2 of Fe(III) cations at any temperature in the SCO range. The lowest temperature segment at T< 30 K was fitted by a Curie–Weiss law (χT)_C-W with Θ_II = −2.34 K, C_II = 3.8 cm³ K/mol (solid blue line), obtained from the approximation of the χ⁻¹(T) curve (Figure 6). The solid black line in Figure 5 represents the resulting calculated (χT)_theor = (χT)_SCO + (χT)_C-W curve that best fits the proposed theoretical model with the parameters described above.

To determine the ground spin state, the field dependence of the magnetization, M(H), was measured at 2.0 K. The magnetization curve is presented in Figure 7. Saturation of M_s at the highest field was observed. Its value at 70 kOe reaches 2.84 μ_B. Field dependence was perfectly fitted by the superposition of Brillouin functions for S = 5/2 and 1/2 with the corresponding magnitude factors, γ_HS and γ_LS. The fitting curve is presented as a solid line. The basic magnetic characteristics and fitting M(H) parameters for complex at 2.0 K are given in

Table 1. Basic magnetic characteristics and fitting M(H) parameters of [FeL₂Bipy]Cl at 2.0 K.

Compound	Ms, μB	Magnitude Factor γLS (Brillouin Function for S = 1/2)	Magnitude Factor γHS (Brillouin Function for S = 5/2)
[FeL2Bipy]Cl	2.84	0.2	0.53

. The Brillouin shape indicates that some of the Fe(III) spin moments remain in a PM non-interacting state. The magnitude factors γ_HS = 0.53 and γ_LS = 0.2 mean that 53% of HS and 20% of LS Fe(III) moments are effectively enough for the description of the χT curve at helium temperatures (Figure 5). Indeed, the χT value at 2.0 K, corresponding to γ_HS and γ_LS obtained from M(H) measurements, should be 2.3 cm³ K/mol, which is close to the experimentally observed value. The γ_HS = 53% indicates that a noticeable part of the HS Fe(III) moments “disappeared” (~27%) from the total magnetic response, which is a sign of AFM interactions. Moreover, the γ_LS ≈ γ_LS² indicates that that most LS Fe(III) ions do not participate in exchange interactions, although a certain proportion can.

2.5.2. EPR Spectroscopy

EPR measurements of [FeL₂Bipy]Cl were carried at 5.0 K < T < 300 K. The spectra contained three EPR signals: two high-spin (HS, S = 5/2) and one low-spin (LS, S = 1/2) centers. This result is completely consistent with the mixed LS+HS state obtained from magnetic measurements. The EPR spectra of HS Fe(III) centers are described by the spin Hamiltonian:

$$\stackrel{⌢}{H}=g{\mu }_B\stackrel{⌢}{\overrightarrow{H}}\stackrel{⌢}{\overrightarrow{S}}+D\left[{\stackrel{⌢}{S}}_z^2-{\displaystyle \frac{1}{3}}S(S+1)\right]+E({\stackrel{⌢}{S}}_x^2-{\stackrel{⌢}{S}}_y^2)$$

(1)

where D and E represent the fine structure parameters of the EPR spectrum, which quantify the axial (D) and rhombic (E) deviations from octahedral symmetry. The I-type HS centers (g = 4.4) correspond to high-spin HS Fe³⁺ ions subjected to strong low-symmetry crystal fields (D >> hν = 0.3 cm⁻¹, E/D~1/3). In contrast, the broad signal at g~2 arises from high-spin Fe³⁺ ions in octahedral environments with weak distorted crystal field (D <<0.3 cm⁻¹, E = 0), classified as II-type of HS centers. Meanwhile, the EPR spectra of low-spin Fe³⁺ centers (with ²T_2g ground state) are characterized by the spin Hamiltonian exhibiting rhombic symmetry:

$$\stackrel{⌢}{H}={\mu }_B(g_x{\stackrel{⌢}{H}}_x{\stackrel{⌢}{S}}_x+g_x{\stackrel{⌢}{H}}_y{\stackrel{⌢}{S}}_y+g_x{\stackrel{⌢}{H}}_z{\stackrel{⌢}{S}}_z)$$

(2)

A Lorentzian line shape was used to simulate the EPR spectra through the EasySpin—5.2.36 MATLAB toolbox [32,33] (Figure 8 and Figure 9). The large line width, ΔH, from the II-type HS centers leads to inaccurate determination of the baseline. The signals from the II-type HS and LS centers overlap with each other, which makes it difficult to separate them. All this leads to errors in simulations of the spectra and analyses of the integrated intensities. The experimental and simulation spectra of [FeL₂Bipy]Cl measured at T = 40 K are shown in Figure 9.

The experimental spectra were simulated using the following magneto-resonance parameters: g = 2, D = 0.42 cm⁻¹, and E = 0.105 cm⁻¹ (I-type HS Fe(III) centers), 2.02 < g < 2.25, D = 0.013 cm⁻¹, and E = 0 cm⁻¹ (II-type HS Fe(III) centers) and g_x = 2.96, g_y = 1.87, g_z = 1.7 (LS Fe(III) centers).

The temperature dependence of the integral intensity, I(T), in the form of the product I∙T for the whole EPR spectrum of [FeL₂Bipy]Cl is shown in Figure 10. The inverse intensity, 1/I(T), (the inset) is described by the Curie–Weiss law with θ = −0.8 K. Comparing the temperature dependence of product χ·T (Figure 5) and I·T demonstrates a similarity in behavior. This is not surprising, since it is known that the integrated EPR intensity I(T) is proportional to the static magnetic susceptibility χ(T) of the spins participating in the resonance [34].

To examine the magnetic characteristics of individual contributions from HS (types I and II) and LS Fe(III) ions, the corresponding lines in the EPR spectrum were analyzed separately. The temperature dependence of the integral intensity, I(T), in the form of the product I∙T and 1/I (the inset) for I-type HS centers, II-type HS centers and LS Fe(III) centers are presented in Figure 11, Figure 12 and Figure 13, respectively. The temperature dependences of the inverse integral intensities 1/I_I, 1/I_II and 1/I_LS obey the Curie–Weiss law with θ_I = −0.2 K, θ_II = −0.8 K and θ_LS = −0.1 K, respectively. As can be seen from the analysis of the individual contributions of HS (I and II-types) and LS Fe(III) ions, the main contribution is made by I_II(T). The contribution of I-type HS centers remains approximately the same over the entire temperature range and is about 1–2% of the total number of centers. The position, H_res(T), and the EPR line width, ΔH(T), of the signal from I-type HS centers remain unchanged with temperature. It can be assumed that this type of Fe(III) centers are paramagnetic, do not participate in the spin transition and probably belong to single-boundary Fe centers, while a partial spin transition is observed between II-type HS centers and LS centers at low temperatures. This is confirmed by the temperature dependence of the I_LS∙T (Figure 13). At T < 50 K, a sharp increase in the I_LS∙T value is observed, whereas at higher temperatures, it is constant. The contributions of LS and II-type HS states were estimated, which were γ_LS¹ ≈ 9%, γ_HS¹ ≈ 89% at 300 K and γ_LS² ≈ 22%, γ_HS² ≈ 71% at 20 K, respectively. These contributions from II-type HS centers and LS centers are in good agreement with those obtained using SQUID magnetometry (Figure 5). As in the case of I-type HS centers, the g-factor and ΔH_LS(T) from LS centers remain constant with temperature. For this reason, and also in view of the close-to-zero θ_LS value, the LS Fe(III) spins can be treated as non-interacting. This validates the M(H) approximation (Figure 7), which indicates that the ground state of [FeL₂Bipy] at 2.0 K can only be accurately described by assuming the contribution of LS Fe(III) ions remains temperture-independent following the spin-crossover transition—that is, LS states persist in a paramagnetic state.

In contrast to the above-discussed I-type HS and LS Fe(III) centers, H_res(T) and ΔH(T) for II-type HS centers, shown in Figure 14, demonstrate a significant temperature dependence. Both of these parameters change oppositely to each other: the line width increases and the resonance field decreases with decreasing temperature. Thus, the EPR results show that only the II-type HS Fe(III) sub-system is magnetically active. A partial SCO transition occurs in it and, probably, AFM-correlated dimers are formed due to π-π bonding of aromatic groups, while the I-type HS and LS centers are in PM state.

3. Experimental Section

3.1. Materials

All commercially obtained reagents were of chemically pure grade and used as received. Organic solvents were freshly distilled before use.

3.2. Methods

FT-IR spectra were recorded on a Vertex 80V device (Bruker, Ettlingen, Germany) in range of 7500–210 cm⁻¹ in KBr and CsBr pellets. ¹H, ¹³C NMR spectral studies were carried out on Avance 500 (Bruker, Ettlingen, Germany) (500.13 MHz) in CDCl₃ solvents with TMS as an internal standard. Gas chromatography was conducted on Maestro Alfa MS (Interlab, Moscow, Russia). Mass spectra were recorded on a time-of-flight mass spectrometer AXIMA Confidence (Shimadzu Biotech, Kyoto, Japan) with matrix-associated laser desorption; 2,5-dihydroxybenzoic acid was the matrix. The decomposition temperature was obtained on the TG 209 F1 analyzer (NETZCH, Selb, Germany) under argon flow at 10 K/min rate to 500 °C. Differential scanning calorimetry measurements were carried out on the DSC 204 F1 Phoenix device with μ-sensor (NETZCH, Selb, Germany). The scan rate at heating and cooling was 10 K/min in argon atmosphere. The presence of Fe, Br, Cl ions was determined by X-ray fluorescence spectrometer Clever C-31 (Eleran, Electrostal, Russia) in the following conditions: voltage—25 kV, current—600 μA, without filter, time—60 s, air, anode of x-ray tube—rhodium. The elemental analysis (CHN) was carried out using a FlashEA 1112 analyzer (Thermo Scientific, Waltham, MA, USA).

The steady-state absorption spectra of Fe(III) complex in DCM (CH₂Cl₂) solution were recorded using the CM 2203 spectrometer (SOLAR, Minsk, Belarus) within the molar concentration range of 10⁻⁷ to 10⁻⁵ M at T = 298 K. The steady-state fluorescence spectra of Fe(III) complex in DCM solution were measured with the FluoTime 300 (PicoQuant, Berlin, Germany), employing an excitation wavelength of 350 nm. The time-resolved fluorescence measurements were carried out by means of FluoTime 300 (PicoQuant, Berlin, Germany) spectrometer, with a LED at 290 nm (PicoQuant, Germany) as the excitation source. The fluorescence quantum yield (QY_FL) of Fe(III) complex in DCM solution was determined on the FluoTime 300 (PicoQuant, Berlin, Germany), utilizing an integration sphere with an excitation wavelength of 350 nm.

The conformational screening was carried out for the cationic form Fe(III) complex by means of GFN2-xTB in the CREST package [35]. Subsequently, the geometry of the most stable conformer was optimized in the ground state using ωB97X-D/def2-SVP/def2-TZVPP (for Fe) [36,37] in the Gaussian 16 software package [38]. The absence of imaginary frequencies confirmed the energy minima of the optimized geometry. Additionally, TDDFT analysis was performed at the same level of theory, wherein the first 50 singlet excited states were calculated. The effect of chloroform on the spectral characteristics was taken into account in the SMD (solvation model based on density) model. The UCSF Chimera v. 1.15 [39] and ChemCraft 1.8 [40] were used to analyze the results and molecular graphics.

The magnetic properties were measured using MPMS XL7 SQUID magnetometry (Quantum Design, San Diego, CA, USA) at 2.0 < T< 300 K. Temperature dependency of the static magnetic susceptibility, χ, was measured using zero-field cooled (ZFC) protocols within the magnetic field H = 100 Oe. Field dependency of magnetization, M, was obtained at T = 2.0 K within the range of −70 kOe–+70 kOe.

The EPR properties were measured using a CW-EPR ER200 SRC (EMXplus), spectrometer (X-band, 9.41 GHz) (Bruker, Ettlingen, Germany) in the temperature range from 5.0 to 300 K using a cryostat ER 4112HV (Bruker, Ettlingen, Germany) and a digital temperature-control system ER 4131VT (Bruker, Ettlingen, Germany). The temperature dependence of the spin contribution to the magnetic susceptibility was estimated by the double integration of the EPR signal (Schumacher–Slichter method) provided that the sweep width δH_sw > 5ΔH, where ΔH is the EPR linewidth.

3.3. Chemical Synthesis

3.3.1. Synthesis of 1-(4-Fluoro-phenyl)-3-(4-bromo-phenyl)-propane-1,3-dione, F-DK-Br (Ligand) [41]

A suspension of NaH (0.6 g) in mineral oil was brought to a boil in THF (20 mL). Solutions of 4′-fluoroacetophenone (3 g, 21.72 mmol) and ethyl 4-bromobenzoate (9.95 g, 43.44 mmol) were dissolved in THF (30 mL) and slowly added to the reaction mixture. The reaction was carried out at boiling for 4 h and then left at room temperature with stirring for 12 h. Then, the reaction mass was poured into 400 mL of distilled water and acidified with HCl up to pH = 2. The product was extracted with CH₂Cl₂, the organic layer was dried over anhydrous Na₂SO₄, and the solvent was distilled off. The yellow dry residue was recrystallized from MeOH. The product is a light-yellow powder. Yield: 2.7 g (38%). mp 124 °C. Anal.: Calculated (%): C 56.1, H 3.14, O 9.96. C₁₅H₁₀BrFO₂. Found (%): C 56.09, H 3.15, O 10.01. FT-IR, v, cm⁻¹: 3546–3416 (s br, ν(C–OH) of enol), 3102, 3056 (w, C-H_ar), 1607–1586 (s, C=O of enol). ¹H NMR (CDCl₃, δ, ppm): 16.79 (s br, 1H, OH of enol), 8.01–7.98 (m, 2H, Ar-H), 7.84–7.82 (d, 2H, Ar-H), 7.63–7.61 (d, 2H, Ar-H), 7.18–7.15 (t, 2H, Ar-H), 6.75 (s, 1H, C(OH)=CH–CO). ¹³C NMR (CDCl₃, δ_C, ppm): 185.66 (C=O), 184.34 (C(OH)=CH–CO), 166.94 (C_ar–F), 134.57, 132.4, 130.11, 130.04, 128.99, 127.76 (C_ar–Br), 116.38, 116.21, 93.08 (C(OH)=CH–CO). MS, m/z: Calculated 321.14; found 321.1 [M⁺].

3.3.2. Synthesis of Iron(III) Bis[1-(4-fluoro-phenyl)-3-(4-bromo-phenyl)propane-1,3-dionate](2,2′-bipyridine) chloride,[FeL₂Bipy]Cl

In a three-neck flask, a sample of 1-(4-fluoro-phenyl)-3-(4-bromo-phenyl)-propane-1,3-dione (0.52 g, 1.62 mmol) was dissolved in ethanol (20 mL) at reflux. A 1M NaOH solution (2 mL) was added dropwise. A powder form of 2,2′-bipyridine (0.13 g, 0.81 mmol) was added. Then, a solution of FeCl₃ (0.13 g, 0.81 mmol) in ethanol was poured from a dropping funnel. The reaction was carried out with heating (80 °C) for 5 h and then at RT for 12 h. Then, the reaction mass was brought to a boil, filtered on a glass filter and washed with hot ethanol. The product was lyophilized from benzene. The product is a fine dark-red powder. Yield: 0.28 g (40%). Anal.: Calculated (%): C 54.11, H 2.95, N 3.15. C₄₀H₂₆Br₂F₂N₂O₄FeCl. Found (%): C 55.03, H 2.35, N 3.18. MS, m/z: Calculated 905.92; found 903.36 [M+NH₄]⁺.

4. Conclusions

We reported syntheses and structure with physical properties (static magnetic susceptibility and magnetization, EPR and spectral characteristics) of the new Fe³⁺ β-diketonate complex, [FeL₂Bipy]Cl. As demonstrated by differential scanning calorimetry data, the complex is characterized by an amorphous glassy phase devoid of a clearly defined melting point. Solutions of the Fe complex in dichloromethane exhibit weak fluorescence (QY_FL = 1%) with an emission maximum at 435 nm. Furthermore, solid-state emission is exhibited by the substance when irradiated at a wavelength of 356 nm.

Magnetic measurements reveal that at T > 75 K, complex [FeL₂Bipy]Cl is predominantly in the high-spin (HS, S = 5/2) state of Fe³⁺ ions with a small admixture of low-spin (LS, S = 1/2) state. The corresponding molar spin contributions are γ_HS¹ = 93% and γ_LS¹ = 7%. At T* ≈ 46 K, a partial spin-crossover transition (SCO, 5/2↔1/2) was detected. As a result of this process, the new magnetic state is realized with γ_HS² = 76%, γ_LS² = 24%. In the range 90 < T < 300 K, the complex exhibits weak AFM correlations between the neighboring Fe³⁺ ions with Θ_I = −0.79 K; after partial SCO in the range 10 < T < 35 K, an increase in AFM interactions with θ_II = -2.3 K is observed. A theoretical model was proposed to describe χ(T). Analysis of the ground spin state at 2.0 K showed that most of the LS states of Fe³⁺ ions, as well as part of the HS states, the share of which is γ_HS ~53%, do not participate in exchange interactions.

It was proposed that π-π bonding of the aromatic groups of the neighboring SCO complexes is responsible for the formation of AFM-coupled HS Fe³⁺ dimers. It was shown that not all Fe³⁺ ions are involved in AFM interactions in the ground spin state at 2.0 K, as some of them (HS and LS) are in the paramagnetic state.

EPR studies confirmed the presence of LS and HS Fe³⁺ centers and allowed to distinguish I-type (strong low-symmetry field) and II-type HS centers (weak distorted octahedral field). Theoretical modeling of experimental spectra was carried out and the basic magneto-resonance parameters were obtained. The temperature dependences of the EPR intensities, I(T), of the HS and LS centers were analyzed. SCO was detected by a sharp change of the product I_LS∙T. The temperature-independent behavior of the H_res(T) and ΔH(T) of the I-type HS centers, as well as their low concentration ~2%, allowed to assume that they belong to single-boundary Fe centers; as a result, they are paramagnetic and do not participate in the spin transition. Thus, only the II-type HS centers participate in SCO and AFM interactions.