New Branched Iron(III) Complexes in Fluorescent Environment Created by Carbazole Moieties: Synthesis and Structure, Static Magnetic and Resonance Properties

Abstract

The branched complexes of Schiff bases with various iron(III) salts, named G2-[L₂Fe]⁺A⁻ (A⁻ is NO₃⁻, Cl⁻, PF₆⁻), were synthesized using the condensation reaction between carbazole derivatives of salicylic aldehyde and N’-ethylethylenediamine and characterized by various spectroscopic methods (GPC, IR, ¹H NMR, UV/Vis). The studies revealed that the coordination of the two ligand molecules to metal occurs through the nitrogen ions and oxygen atom of azomethine to form a homoleptic system. All the synthesized coordination compounds were examined for their thermal, optical, and magnetic features. Static magnetic measurements showed that only G2-[L₂Fe]Cl was in a single-phase HS state, whereas the Fe(III) ions of G2-[L₂Fe]NO₃ and G2-[L₂Fe]PF₆ at room temperatures were in mixed low-spin (LS, S = 1/2) and high-spin (HS, S = 5/2) states: 58.9% LS/41.1% HS for G2-[L₂Fe]NO₃, 56.1% LS and 43.9% HS for G2-[L₂Fe]PF₆. All G2-[L₂Fe]⁺A⁻ complexes demonstrate antiferromagnetic exchange interactions between neighboring Fe(III) ions. The ground spin state at 2.0 K revealed a Brillouin contribution from non-interacting LS ions and a proportion of the HS Fe(III) ions not participating in AFM interactions: 57%, 18%, and 16% for G2-[L₂Fe]Cl, G2-[L₂Fe]NO₃ and G2-[L₂Fe]PF₆, respectively. EPR measurements confirmed the presence of magnetically active HS and LS states of Fe(III) ions and made it possible to distinguish two HS types-with strong low-symmetry (I-type) and weak, distorted octahedral environments (II-type). It was shown that G2-[L₂Fe]⁺A⁻ complexes are magnetically inhomogeneous and consist of two magnetic sub-lattices: AFM-correlated chains in layers from the I-type HS Fe(III) centers and dynamic short-range AFM ordered LS/II-type HS Fe(III) centers in the paramagnetic phase located between the layers.

1. Introduction

In recent decades, special attention has been paid to the creation of materials with a controlled structure and specified characteristics. In this regard, dendrimer macromolecules are of particular interest. They are capable of acting as complexing agents and encapsulating “guest” molecules into their structure [1,2]; they can serve as molecular antennas for the absorption of light energy [3,4,5] and act as nanoporous materials; they are oriented in a magnetic field [6]; they have mesogenic and emissive properties [7,8,9]; and they have the ability to self-organize into supramolecular assemblies [10,11]. The main feature inherent in dendrimers of various chemical natures is the presence of a certain shape, size, and controlled functionality. Therefore, it is important to study not only the specifics of the structural organization of dendrimer macromolecules, but also their functional properties determined by this structure. By encapsulating paramagnetic metal ions into a dendrimer matrix, it becomes possible to create promising materials with specified molecular sizes and controlled magnetic parameters. Of particular interest is the study of new dendrimer metal complexes with a large internal free volume, which leads to an expansion of the scope of application of liquid crystalline materials: in catalysis, chromatography, and the sensor industry. The combination of the properties of a dendrimer with the specific properties of different metal ions seems very interesting from the point of view of possible applications. Convergence, architecturally driven by “branching effects”, together with the ability of tree structures to control the nanoscale size, shape, and chemical functionality of the system, allows the creation of nanomaterials with new specified properties. The introduction of peripheral substituents into the ligand structure makes it possible to specifically influence the mesomorphic and photoactive properties of the system. Systems of this type are used in heterogeneous catalysis, as components in molecular electronics and photochemical molecular devices for converting solar energy and storing information.

Herein, we report three novel branched Fe³⁺ complexes in fluorescent environment received on Schiff-base ligands modified with carbazole fragments with NO₃⁻, Cl⁻, PF₆⁻ counter-ions. We continue to study a new series of Fe³⁺ dendrimeric complexes. This work is a logical continuation of our previous investigations of first-generation (G1) homoleptic coordination complexes, on which synthetic approaches were developed and spectral and magnetic properties were studied [12]. The goal of this work is to further increase the π-conjugated system by increasing the degree of branching (second-generation, G2) of the complex and introducing additional carbazole fragments. Thus, our research is aimed at searching for and creating novel multifunctional metal-containing self-organizing dendrite-like coordination compounds. In this regard, it is of interest to study such complexes with a high degree of dendronization, which can exist in a low-spin (LS) and high-spin (HS) states, and also experience spin-crossover (SCO) transitions.

2. Experimental

2.1. Instrumental and Physical Measurements

Gel permeation chromatography (GPC) was performed on a Shimadzu 10A liquid chromatographer (Shimadzu, Duisburg, Germany), eluent–tetrahydrofuran. FTIR spectra were obtained using KBr and CsBr disks in the regions of 7500–370 cm⁻¹ and 670–190 cm⁻¹, respectively, on the Vertex 80V (Bruker, Ettlingen, Germany). NMR spectral studies on the ¹H (500.17 MHz) nuclei were recorded on a Avance-500 spectrometer (Bruker, Ettlingen, Germany) in the solvent CDCl₃ relative to TMS as an external reference. Chemical shifts are expressed in δ ppm. Thermal analyses of synthesized metal complexes were carried out in a platinum crucible at a heating rate of 10 K/min between the temperature range 298 and 923 K under argon flow on a TG 209 F1 analyzer (NETZCH, Selb, Germany) as onset gravimetric reduction point of the TG curve. Differential scanning calorimetry measurements were performed on a DSC 204 F1 Phoenix (NETZCH, Selb, Germany) with a μ-sensor. The scan rate at heating and cooling was 10 K/min in an argon atmosphere. The UV-vis spectra were measured on an Agilent 8454 UV-visible spectrophotometer (Agilent, Santa Clara, CA, USA). Steady-state fluorescence complexes in CH₂Cl₂ (10⁻⁶ mol/L) solvents were recorded using a CM 2203 spectrometer (SOLAR, Minsk, Belarus) at room temperature. Steady-state fluorescence complexes in a solid state were measured with a FluoTime 300 (PicoQuant, Berlin, Germany). The fluorescence quantum yield of G2-[L₂Fe]⁺A⁻ in CH₂Cl₂ and solid state was determined on the FluoTime 300 (PicoQuant, Berlin, Germany), utilizing an integration sphere. An excitation wavelength of 350 nm was used to record fluorescence spectra and to determine the quantum yield. The time-resolved fluorescence measurements were carried out by means of the FluoTime 300 (PicoQuant, Berlin, Germany) spectrometer, with an LED at 290 nm (PicoQuant, Berlin, Germany) as the excitation source. The instrument response function (IRF) of the system was determined by measuring the stray light signal of the dilute colloidal silica suspension (LUDOX^®, Germany). The fluorescence decay curves were measured, and the fluorescence lifetimes were obtained by reconvolution of the decay curves using the EasyTau 2 software (version 2.2) package (PicoQuant, Berlin, Germany).

EPR (electron paramagnetic resonance) and SQUID (superconducting quantum interference device) methods were used for magnetic certification. SQUID measurements were carried out on a MPMS-5-XL magnetometer (QUANTUM DESIGN, San Diego, CA, USA). The static magnetic susceptibility, χ(T), was measured at a magnetic field strength H₀ = 1000 Oe at a temperature range of 2.0 ÷ 300 K. The magnetization curves M(H) were measured at 2.0 K in the field range of −50 ÷ +50 kOe. The pure paramagnetic susceptibility was determined by subtracting the diamagnetic contribution of the organic core from the total magnetic response. The EPR spectra (X-band 9.41 GHz) on the powder samples were obtained using a CW-EPR Bruker ER200 SRC (EMXplus) spectrometer (Bruker, Ettlingen, Germany); the modulation frequency was 100 kHz. The temperature of samples was varied in the range 4.2–300 K using a cryostat ER 4112HV (Bruker, Ettlingen, Germany) and a digital ER 4131VT temperature-control system (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 (ΔH is the EPR peak-to-peak linewidth). In this case, the error of the method for the EPR Lorentz line was ~10%.

2.2. Preparations

2.2.1. Synthesis of Ligands

Schiff bases modified with carbazole fragments were prepared using the condensation reaction of the corresponding amines and substituted aldehydes in solution immediately before the complexation. The synthesis of key precursors of the second generation (G2), Figure 1, is discussed in detail in [13].

2.2.2. Synthesis of Complexes

The iron(III) complexes, Figure 2, were synthesized by the following procedure [12]. The compounds 4-[3,5-di-(3,6-bis-tert-butylcarbazol-9-yl-benzoyloxy)benzoyloxy]-2- hydroxybenzaldehyde (0.49 mmol), Di(CrB)aldehyde, dissolved in C₆H₆ (10 mL) at heating, and 3,5-di-[3,6-bis-tert-butylcarbazol-9-yl-benzoyloxy]benzoyloxy-N’-ethyl-N- ethyleneamine (0.49 mmol), Di(CrB)amine, dissolved in EtOH (10 mL) were added to a flask under stirring. The mixture was stirred for 20 min and KOH (1.49 mmol) in 10 mL of EtOH was added. Then, an alcohol solution (15 mL) of an appropriate metal salt—Fe(NO₃)₃·9H₂O (0.25 mmol), FeCl₃ (0.25 mmol), or Fe(NO₃)₃·9H₂O (0.25 mmol)/KPF₆ (0.98 mmol)—was slowly added (drop by drop) to the reaction. After 3 h, the reaction mass was cooled for 12 h in freezer. The formed precipitate was filtered off, washed with ethanol, dissolved, and lyophilized from benzene. The complexes are abbreviated as G2-[L₂Fe]⁺A⁻, where L is 2-[2-[(E)-[4-[3,5-bis[[4-(3,6-di-tert-butyl-carbazol-9-yl)benzoyl]oxy]benzoyl]oxy-2-phenol-ate]methyleneamino]ethylamino]ethyl-3,5-bis[[4-(3,6-di-tert-butyl-carbazol-9-yl)benzoyl]-oxy]benzoate and A⁻ is NO₃⁻, Cl⁻, PF₆⁻ counter-ions. The complexes show a good solubility in solvents such as dichloromethane, chloroform, tetrahydrofuran (THF), toluene, and benzene but they are insoluble in ethanol, water, and hexane. We assumed that the octahedral environment of the iron is due to the metal ion interacting with the nitrogen atoms from the N’-ethyl-N-ethylenediamine fragments and hydroxyl groups from the salicylic aldehyde. The precise manner in which the complexes coordinate remains uncertain. Figure 2 illustrates fac coordination, but the true nature of fac and mer coordination has not been established.

G2-[L₂Fe]NO₃. Product was a light-brown fine-dispersed powder. Yield was 39.12% (0.39 g). FTIR (KBr, ν, cm⁻¹): 3405 (m, NH), 3067 (w, Ph–H), 2959–2864 (s, CH₂, CH₃), 1718 (s, –COO), 1605 (s, HC=N), 1384 (br s, NO₃⁻).

G2-[L₂Fe]Cl. Product was a light-brown fine-dispersed powder. Yield was 35.35% (0.35 g). FTIR (KBr, ν, cm⁻¹): 3396 (m, NH), 3072 (w, Ph–H), 2959–2869 (s, CH₂, CH₃), 1722 (s, –COO), 1601 (s, HC=N).

G2-[L₂Fe]PF₆. Product was a grey fine-dispersed powder. Yield was 25% (0.26 g). FTIR (KBr, ν, cm⁻¹): 3415 (m, NH), 3072 (w, Ph–H), 2959–2865 (s, CH₂, CH₃), 1720 (s, –COO), 1600 (s, HC=N), 834 (br s, PF₆⁻), 560 (s, PF₆⁻).

3. Results and Discussion

The structure and purity of complexes were confirmed by means of various spectroscopic and thermoanalytical techniques including IR and ¹H NMR spectroscopy, gel-permeation chromatography (GPC), mass spectrometry, and thermal analysis (DSC and TGA).

3.1. Spectroscopy Characterization

The purity of the coordination compounds was examined using the GPC method. A comparison of the chromatograms of the initial compounds and the G2-[L₂Fe]Cl complex after isolation and purification is presented in Figure 3a. It can be observed that the complex exhibited a distinct retention time (τ_R = 1327 s), in contrast to Di(CrB)aldehyde (τ_R = 1096 s) and Di(CrB)amine (τ_R = 1051 s). In Figure 3b it can be seen a single peak on each curve corresponding the individual retention time of all complexes.

In accordance with GPC data, the branched complexes were obtained with a high purity.

FTIR spectra of the samples recorded in the region 4000–370 cm⁻¹ with KBr disks demonstrate the broad band around 3400 cm⁻¹ corresponding to the N–H bond stretching vibrations, Figure 4. A high intensity band observed at 1722–1718 cm⁻¹ can be assigned to the υ(C=O) carboxyl stretching vibration. The characteristic stretching frequency of the HC=N azomethine group is detectable at 1605–1600 cm⁻¹ and indicates the formation of a Schiff base. The band that appeared at 1384 cm⁻¹ characterizes the presence of the NO₃ group [14]. A set of weak bands in the far infrared region, around 423 and 280 cm⁻¹, can be attributed to vibrations of Fe–O and Fe–N bonds [15], Figure 4. When considering the IR spectra of compounds in the range of 650–200 cm⁻¹, one can note the appearance of an intense absorption band associated with vibrations of the PF₆⁻ ion (559 cm⁻¹) [16,17]. In addition, a rather broad and intense peak at 834 cm⁻¹ also corresponds to vibrations of the PF₆⁻ ion [14].

A distinctive feature of the IR spectrum of the G2-[L₂Fe]Cl complex in the far IR region is the presence of a medium-intensity vibration band of the one-atom Cl⁻ counter-ion at 214 cm⁻¹ upon its interaction with the cation of the dendrimer complex. A similar band was correlated with the contribution of symmetric and asymmetric stretching of strong I–Cl bonds by Waterland and co-authors [18] in far IR spectra of trihalide salts of [NBu₄]ICl₂. It is likely that we observe this type of vibrational band in the interaction of the δ⁺ proton of the azomethine fragment –N=CH– with the δ⁻ ion Cl⁻ through the induced dipole mechanism.

All three complexes are paramagnetic, as evidenced by the broadened signals in the ¹H NMR spectra, which do not allow a clear determination of the structure of the compounds. This broadening of spectral response is typical for the paramagnetic Fe³⁺ ion in an azomethine environment [12,19]. In this case, the signal width in the spectrum consists of its own, determined by the relaxation time and associated with the inhomogeneity of the magnetic field. As a result of the presence of paramagnetic LS or HS Fe³⁺ ions nearby, their magnetic field leads to additional broadening of the spectral lines. It is also worth noting the contribution of the organic component around the metal ion. A comparative analysis of NMR spectroscopy data from the initial aldehyde and amine [13] and the resulting coordination compound G2-[L₂Fe]NO₃, Figure 5, shows that the complexes under study do not contain the starting reagents in their composition. This is indicated by the absence of a COH (9.84 ppm) signal from the fragment of salicylic branched aldehyde (Di(CrB)aldehyde), which directly enters into a condensation reaction with the amine (Di(CrB)amine). The triplet signal of the amino group of Di(CrB)amine (4.49–4.46 ppm) shifts upfield to 3.26 ppm, indicating the formation of an imine fragment –C=NH–, which is included in the coordination environment of the iron ion. The signal of the OH group (11.21 ppm) in the ortho position to the reactive aldehyde fragment completely disappears, indicating deprotonation with the formation of the O–Fe³⁺ ionic bond as one of the main bonds in the coordination site. The absence of signals from –CH₂– groups from N,N-diamine fragments involved in the direct coordination of iron to form an octahedron is probably due to the strong influence of the paramagnetic metal ion.

3.2. Thermal Properties

Thermal analysis was used to investigate the thermal stability and phase behavior of the compounds obtained. None of the three complexes have a well-defined melting temperature in thermal polarization microscopy which is typical for many representatives of the class of branched dendrimers [20]. Although the substances were synthesized in powder form, no stable thermodynamic transitions between melting and crystallization were observed. It is important to note that the samples were dried under vacuum to a constant weight after separation from the reaction mass to avoid any inaccuracies when studying phase behavior. The temperature stability of the compounds does not exceed 200 °C. A distinctive feature of branched azomethine iron(III) complexes is the formation of stable crystallosolvates with solvent molecules. The DSC curves in the heating cycle of the samples display an endothermic solid-state transition within the temperature range of 120–130 °C, Figure 6a. This transition is visually observed in the contact sample as a softening. However, the melting temperature before the onset of thermal destruction was not observed.

To obtain complexes in a form free from crystallosolvates, we lyophilized the samples from concentrated benzene solutions. DSC analysis data in the heating cycle are shown in Figure 6b. It is evident that the compounds only transitioned to the glassy state, remaining essentially amorphous. This was confirmed by the impossibility of obtaining a clear melting point value and the absence of crystalline phase reflections in the powder X-ray analysis data. This is likely due to steric effects caused by the repulsion of tert-butyl-substituted carbazole derivatives connected through ester benzoate groups with the azomethine nucleus. In contrast to the complexes of the first generation with a similar periphery groups and counter-ions, the glass transition temperature of the samples increased significantly. This suggests that the high branched complexes improved the thermal stability.

3.3. Optical Properties

The UV-vis spectra of complexes are shown in Figure 7. In the absorption spectrum spanning from 200 to 400 nm, multiple peaks are evident, signifying transitions between π–π* and n–π* electronic states, and charge transfer processes. Notably, a prominent absorption band around 240 nm indicates intraligand transitions, while absorptions in the 290–350 nm range suggest charge transfers involving ligand → metal interactions, metal → metal interactions, and interactions within the ligand itself [21,22]. UV-vis spectra for all complexes are similar. The main absorption maxima are presented in the Table S1.

The obtained compounds had a sufficiently high quantum yield in a CH₂Cl₂ solution. The absolute quantum yield values at a concentration of 10⁻⁶ mol/L were 21% (for G2-[L₂Fe]NO₃), 11% (for G2-[L₂Fe]Cl), and 28% (for G2-[L₂Fe]PF₆). The absolute fluorescence quantum yield of solid-state samples did not exceed 3%. The fluorescence spectra in dichloromethane solution, Figure 8, and solid form were similar, Figure S1. The maximum emission in solution was observed at 440 nm, and for the solid state at 435 nm. The value of the Stokes shift was 94 nm.

The fluorescence lifetime (τ_F) of the complexes in CH₂Cl₂ was determined. The values were 9.8, 9, and 10.1 ns for G2-[L₂Fe]⁺A⁻ (A⁻ is NO₃⁻, Cl⁻, PF₆⁻), respectively (Figure 9, Table S2). It was established that the nature of the counter-ion and the number of generations [12] had virtually no effect on τ_F.

3.4. Magnetic and Resonance Properties

3.4.1. SQUID Measurements

The temperature evolutions of the static magnetic susceptibility, χ(T), for G2-[L₂Fe]⁺A⁻ complexes were measured at a wide temperature range of 2.0–300 K. The temperature dependences of the inverse static magnetic susceptibility, χ⁻¹(T), for G2-[L₂Fe]NO₃ (◊), G2-[L₂Fe]Cl (ο), and G2-[L₂Fe]PF₆ (▯) are presented in Figure 10a. All these curves show linear behavior and can be described by the Curie-Weiss law:

$$\chi =C/\left(T-\theta \right)$$

(1)

where C is the Curie constant and θ is the Weiss temperature. Linear approximation of the χ⁻¹(T) data made it possible to determine the fitting parameters, which are presented in

Table 1. Fitting parameters for χ⁻¹(T) for G2-[L₂Fe]⁺A⁻ complexes.

№	Θ, K	C, cm3 K/g
G2-[L2Fe]NO3	−1.0	0.00049
G2-[L2Fe]Cl	−9.3	0.0011
G2-[L2Fe]PF6	−0.8	0.0005

. Good Curie–Weiss fitting indicates the absence of spin-crossover transitions. A negative sign Θ (R-factor = 0.9999) in all cases clearly indicates the presence of antiferromagnetic (AFM) exchange interactions between neighboring Fe(III) ions.

To determine the spin states of Fe(III) ions, the temperature dependences of χ in the form of the product χ·T for G2-[L₂Fe]NO₃ (◊), G2-[L₂Fe]Cl (ο), and G2-[L₂Fe]PF₆ (▯) were plotted and analyzed (Figure 10b). The χ·T(T) curves show similar behavior: temperature-independence at high T, a gradual decrease at 10 < T < 200 K and a sharp decrease at T < 10 K, reaching 1.24, 3.01, 1.19 cm³ K/mol at T = 2.0 K for G2-[L₂Fe]NO₃, G2-[L₂Fe]Cl, and G2-[L₂Fe]PF₆, respectively.

However, the quantitative values of the magnetic moment at high temperatures were different and, in addition, its fluctuation scatter was observed. This is due to the weak magnetic response typical of organometallic materials with a low spin concentration of paramagnetic ions. An increase in the degree of dendronisation leads to a multiple increase in the molecular weight and consequently to an even greater decrease in the specific magnetic moment. Therefore, in the range of 200–300 K, the average temperature-independent values of χ·T were determined to be 2.02, 4.39, and 2.13 cm³ K/mol for G2-[L₂Fe] + A− (A− is NO³⁻, Cl⁻, PF⁶⁻) complexes, respectively. A magnetic response is was expected only from d5 Fe(III) cations, which can be in low-spin (LS, S = 1/2) and/or high-spin (HS, S = 5/2) states. Based on the observed experimental values χ·T, we assumed the existence of mixed HS and LS states. To determine their contributions, Equation (2) was used [23]:

$$\chi T=C=x{C}_{LS}+(1-x)C_{HS}$$

(2)

where C_HS = 4.375 cm³ K/mol is the theoretical value for HS, C_LS = 0.375 cm³ K/mol is the theoretical value for LS Fe(III) (g-factor is 2.0), and x is the contribution of LS states. As a result, the following LS/HS contribution estimates were obtained: 58.9%/41.1% for G2-[L₂Fe]NO₃, 0%/100% for G2-[L₂Fe]Cl and 56.1%/43.9% for G2-[L₂Fe]PF₆. Thus, only G2-[L₂Fe]Cl was in a single-phase magnetic state (100% HS), while G2-[L₂Fe]NO₃ and G2-[L₂Fe]PF₆ were in mixed states (~50% HS/LS).

Note, that the low-temperature magnetic properties for G2-[L₂Fe]⁺ A⁻ complexes demonstrate peculiarity in χ·T data at T ≈ 13.0 K. At this temperature, a local minimum was observed (1.61 (G2-[L₂Fe]NO₃), 3.51 (G2-[L₂Fe]Cl), 1.86 (G2-[L₂Fe]PF₆) cm³ K/mol), after which χ·T increased slightly and reached a local maximum at T = 9.4 K (1.82 (G2-[L₂Fe]NO₃), 3.61 (G2-[L₂Fe]Cl), 1.93 (G2-[L₂Fe]PF₆) cm³ K/mol) with a further sharp drop. The increase in χ·T at T < 13.0 K is probably associated with the appearance of ferri- or ferromagnetic (FM) correlations between Fe(III) ions. The nature of this low-temperature maximum is not yet clear and requires further study.

The observed decrease in χ·T is characteristic of antiferromagnetic correlations and/or zero-field splitting (ZFS). However, because the contribution of the zero-field splitting parameter (D) to the Weiss constant is small [24], ZFS is not significant and we can assume that AFM interactions arose between neighboring monomeric Fe(III) complexes linked to each other by π-stacking of benzene rings. The strongest AFM interactions appeared in G2-[L₂Fe]Cl, where Θ_Cl was −9.3 K. In G2-[L₂Fe]NO₃ and G2-[L₂Fe]PF₆ rather weak correlations were observed, with Θ_NO3 = −1.0 K and Θ_PF6 = −0.8 K, respectively.

To determine the ground spin state for G2-[L₂Fe]⁺A⁻ complexes, measurements of the isothermal field dependence of magnetization, M(H), were carried out at 2.0 K (Figure 11). The applied magnetic field was slowly reversed from +50 to −50 and back to +50 kOe. All M(H) curves have a Brillouin-like behavior, but without the characteristic M_s saturation region in a maximum field of 5.0 T, and tend to a slight linear increase. The absence of M_s could be explained by the process of canting of AFM coupled Fe(III) spins. In this case, the magnetization curve often has a Brillouin dependence and a smooth increase proportional to the magnetic field strength when |J| << k_bT (T = 2.0 K) [25]. As a consequence, the experimental M(H) curves cannot be approximated by simple Brillouin functions for S = 1/2 and S = 5/2 or their combinations, regardless of the scaling factors. The first fact is consistent with χ·T data, according to which G2-[L₂Fe]NO₃ and G2-[L₂Fe]PF₆ are found in mixed spin states. The second fact requires taking AFM interactions into consideration. As a result, the best approximation was provided by a combination of Brillouin functions for S = 1/2 and 5/2 with the corresponding scaling factors, C₁/C₂, and linear functions, kH, taking into account AFM interactions. The corresponding fitting curves are shown as solid lines in Figure 11 and the AFM contributions are presented separately as straight lines. The basic magnetic characteristics and fitting parameters M(H) for G2-[L₂Fe]⁺A⁻ complexes are presented in

Table 2. Basic magnetic characteristics and fitting M(H) parameters for G2-[L₂Fe]⁺A⁻ complexes.

№	Ms, μB	Mr, μB	Hc, Oe	Scaling Factor C1 for the Brillouin Function S = 1/2	Scaling Factor C2 for the Brillouin Function S = 5/2	Linear Factor k, μBK/T
G2-[L2Fe]NO3	1.67	0.011	80	0.589	0.184	0.007
G2-[L2Fe]Cl	3.13	0.0011	4	0	0.57	0.13
G2-[L2Fe]PF6	1.33	0.026	200	0.561	0.16	0.01

. As expected for G2-[L₂Fe]Cl, according to the χ·T data, M(H) was completely described by the contribution from S = 5/2 only. Good agreement between the experimental M(H) and the fitted curves for G2-[L₂Fe]NO₃ and G2-[L₂Fe]PF₆ was obtained with scaling factors C₁ of 0.59 and 0.56, respectively. These coefficients coincide with the estimates of the LS contribution of Fe(III) ions (58.9% (G2-[L₂Fe]NO₃), 56.1% (G2-[L₂Fe]PF₆)) obtained from measurements of the temperature dependence of susceptibility (Figure 10b). Thus, at T = 2.0 K, the contribution from the LS Fe(III) cations for G2-[L₂Fe]NO₃, G2-[L₂Fe]PF₆ does not change, which means they remain in a paramagnetic (non-interacting) state. At the same time, the scale factors C₂ for G2-[L₂Fe]⁺A⁻ complexes equal to 0.184, 0.57, and 0.16, indicate a decrease in the contribution of HS Fe(III) ions to the static magnetic response, which can be explained by the fact that only some of them participate in antiferromagnetic interactions, while the remainder, including the LS Fe(III) ions, remain in a paramagnetic state, at 18%, 57% and 16%, respectively. These estimates are in good agreement with values of χ·T observed at helium temperatures. Indeed, the values of χ·T corresponding to the fraction of non-interacting HS and LS Fe(III) ions extracted from M(H) measurements should be 1.03, 2.49 and 0.91 cm³ K/mol, which is close to those observed experimentally. The small difference is due to the need to take into account the contribution of AFM coupling spins.

The field dependences of magnetization, M(H), for all G2-[L₂Fe]⁺A⁻ complexes demonstrated weak hysteresis with residual magnetization, M_r, and coercive force, H_c, the values of which are given in the Table 2. The smallest value of H_c = 4 Oe was observed in G2-[L₂Fe]Cl, while in G2-[L₂Fe]NO₃, G2-[L₂Fe]PF₆ it was an order of magnitude greater, amounting to 80 and 200 Oe, respectively. The obtained coercive force values are a quantitative characteristic of magnetic anisotropy which can be caused by FM correlations that arise at T = 13.0 K, according to χ⁻·T data (Figure 10b).

3.4.2. EPR Spectroscopy

Since magnetic measurements indicate the existence of mixed spin states in G2-[L₂Fe]⁺A⁻ complexes, EPR spectroscopy was used as a locally sensitive technique to study in detail the individual HS and LS contributions of Fe(III) ions.

In the case of one-electron approximation, the orbital quintet of d-orbitals is split into a doublet and a triplet under the action of the crystal field of the ligands. If the electronic configuration of the central ion contains more than one d-electron, the picture of possible terms and their splitting in the ligand field becomes noticeably more complicated. If the ligand field is not very strong—that is, the splitting of d-orbitals is less than the energy gain achieved by distributing electrons in accordance with Hund’s rules—such states are called high spin. The EPR spectra of HS Fe³⁺ centers (that have a ground state of ⁶A_1g) are described by the spin Hamiltonian:

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

(3)

where D and E are the fine structure parameters of the EPR spectrum which characterize axial (D) and rhombic (E) distortion from octahedral symmetry. The positions of the fine structure lines (g-factor) depend on the value of the ratio between the microwave quantum hν and E/D parameters. Theoretical analysis shows that the signal with g = 4.26 belongs to HS iron ions with strong (D >> hν = 0.3 cm⁻¹) low-symmetry (E/D~1/3) crystal fields (I-type of HS centers), whereas the broad signal with g~2 belongs to HS Fe³⁺ ions in an octahedral environment with weak (D <<0.3 cm⁻¹, E = 0) distorted crystal field (II-type of HS centers). When the ligand field is “strong”, a low-spin state is realized. The EPR spectra of the LS Fe³⁺ centers (that have a ground state of ²T_2g) are described by the spin Hamiltonian with rhombic symmetry:

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

(4)

The G2-[L₂Fe]⁺A⁻ complexes were studied in a temperature range of 5.0 K to 300 K. The spectra contained three signals. The narrow signal in a low field with g_eff = 4.2, whose intensity decreased as temperature increased, is typical of I-type HS Fe(III) centers [26,27,28]. The broad signal with g_eff~2.0, which was most clearly observed in the spectra at temperatures above 120 K, arises from II-type HS Fe(III). The third signal, a narrow, low-intensity signal with g_eff~2.0, observed in the EPR spectra of G2-[L₂Fe]NO₃ and G2-[L₂Fe]PF₆, is associated with LS centers.

The EPR spectra were simulated by Lorentzian line shape by using EasySpin Matlab toolbox [29,30]. Experimental and simulation spectra for G2-[L₂Fe]⁺A⁻ complexes are showed in Figure 12a–c.

Experimental spectra were simulated with the following magneto-resonance parameters: g = 1.96, D = 0.42 cm⁻¹, and E = 0.105 cm⁻¹ (I-type HS Fe(III) centers) and 2.04 < g < 2.15, D = 0.013 cm⁻¹, and E = 0 cm⁻¹ (II-type HS Fe(III) centers).

The temperature dependence of the integral intensity of the EPR spectra I(T) is one of the sources of information about the evolution of the spin system since I(T) ~ χ(T). The magnetic behavior, represented by the temperature dependence of the product I·T(T) for the entire EPR spectrum for G2-[L₂Fe]⁺A⁻ complexes, is shown in Figure 13.

To analyze the magnetic behavior of the individual contributions of HS (I and II-types) and LS Fe(III) ions, the corresponding lines of the EPR spectrum were studied separately. The temperature dependences of the I_I(T) for I-type Fe(III) centers for all G2-[L₂Fe]⁺A⁻ complexes pass through a maximum at T ≈ 7.0 K, above which I_I(T) obey the Curie–Weiss law with Weiss constants, θ: θ_NO3 = −1.3 K, θ_Cl = −8.0 K, θ_PF6 = −1.8 K for G2-[L₂Fe]NO₃, G2-[L₂Fe]Cl, G2-[L₂Fe]PF₆, respectively (Figure 14a–e). This approximation is consistent with the same one obtained for static magnetic susceptibility χ (Figure 10a). Weiss temperatures in both experiments were close in magnitude. Therefore, we can assume that I-type Fe(III) centers are responsible for the AFM exchange interactions. The temperature dependences of the EPR line width, ΔH(T), and its position, H_res(T), for I-type centers for G2-[L₂Fe]⁺A⁻ complexes are constant.

The temperature dependences of the I_II(T) in the form of the product I_II∙T(T) for II-type Fe(III) centers for G2-[L₂Fe]⁺A⁻ complexes are presented in Figure 15. To avoid errors that arise when using the double integration method for broad EPR lines (II-type HS centers), two methods were used: using the form A∙ΔH², where A is the amplitude of the EPR signal; and using the double numerical integration method (Figure 14f). The results were similar. Temperature dependences from product I_II∙T(T) for G2-[L₂Fe]NO₃ and G2-[L₂Fe]PF₆ starts from 120 K and 160 K (Figure 15a,c) because signal from II-type Fe(III) centers had weak intensity at lower temperatures and it was not possible to interpret it unambiguously.

In contrast to I-type Fe(III) centers, the EPR signal from II-type centers gradually shifts towards weaker magnetic fields with decreasing temperature for all G2-[L₂Fe]⁺A⁻ complexes (Figure 16). In this case, the linewidth for G2-[L₂Fe]Cl increases, whereas for G2-[L₂Fe]NO₃ and G2-[L₂Fe]PF₆ it decreases.

Signals from iron LS centers can observed in the spectra for G2-[L₂Fe]NO₃ and G2-[L₂Fe]PF₆. There is no low-spin signal for G2-[L₂Fe]Cl. This result is completely consistent with the LS/HS estimates obtained from the analysis of χ·T (Figure 10b). The LS EPR signal have anisotropic line form with g_x = 2.34, g_y = 2.34, and g_z = 2 for G2-[L₂Fe]NO₃; and g_x = 2.29, g_y = 2.29, and g_z = 1.89 for G2-[L₂Fe]PF₆. The temperature dependence of integrated intensity of LS Fe(III) centers, I_LS(T), for G2-[L₂Fe]NO₃ and G2-[L₂Fe]PF₆ obey the Curie–Weiss law, with Weiss constant θ_LS = −0.14 K and −0.65 K (Figure 17), respectively. Thus, the LS Fe(III) spins can be considered to be non-interacting, which confirms the correctness of the M(H) approximation (Figure 11) according to which the ground state of the G2-[L₂Fe]⁺A⁻ complexes at 2.0 K can only be described taking into account that the contribution from the LS Fe(III) ions does not change with temperature, i.e., LS states remain in a paramagnetic state. As can be seen, the temperature dependence of I_II for HS centers of the II-type behaves differently compared to the integrated intensities I_I for HS centers of the I-type and I_LS of EPR lines for LS.

Comparing the temperature dependence of product χ·T (Figure 10) and I·T (Figure 13) demonstrated a significant difference in behavior. As can be seen from the analysis of the temperature dependences of the individual contributions of HS (I and II-types) and LS Fe(III) ions, the main contribution to this difference was made by I_II(T). It was assumed that the integral EPR intensity I(T) is proportional to the magnetic susceptibility χ(T) of the spins participating in the resonance and the product I·T is correspondingly proportional to the effective magnetic moment. We posit that differences between χ and I(T) data can be explained by the existence of dynamic short-range AFM ordered spins in the paramagnetic phase, which observed only on a short time scale (τ~10⁻¹⁰ s) and thus could be registered only by using the EPR method (~9.4 GHz), not by static magnetic measurements. The first concept of dynamic spin clusters was proposed in [31] and was subsequently confirmed by us in some spin-crossover dendrimeric Fe(III) complexes [32,33].

Thus, the EPR results show that the G2-[L₂Fe]⁺A⁻ complexes are inhomogeneous and consist of two magnetic sublattices. These are Fe(III) centers with a strong rhombic distorted environment (I-type), which probably form AFM-correlated chains (due to π-π stacking of benzene rings) in the layers, and Fe(III) centers with a weak, distorted environment (II-type), which, including LS, are probably located between the layers characterized by short-range AFM correlations that have a fast dynamic (fluctuating) nature [34], and therefore cannot be detected by static magnetic methods.

4. Conclusions

In continuation of our previous investigations of first-generation (G1) homoleptic coordination compounds, a number of metal-containing second-generation (G2) dendrite-like complexes of Schiff bases, named G2-[L₂Fe]⁺A⁻ (A⁻ is NO₃⁻, Cl⁻, PF₆⁻), were synthesized. An increase in the branching degree due to the introduction of additional carbazole fragments led to the appearance of new properties. None of the three complexes had a well-defined melting temperature. A distinctive feature of branched azomethine iron(III) complexes is the formation of stable crystallosolvates with solvent molecules, which leads to the need for freeze drying. But even in this case, the compounds retain a glassy state, remaining essentially amorphous. This is likely due to steric effects caused by the repulsion of tert-butyl-substituted carbazole derivatives connected through ester benzoate groups with an azomethine nucleus. In contrast to the first-generation complexes with similar periphery groups and counter-ions, the glass transition temperature of the samples increased significantly. This suggests that the highly branched complexes have improved thermal stability. The obtained complexes have a sufficiently high absolute quantum yield in a solution of CH₂Cl₂ from 11% (Cl⁻) to 28% (PF₆⁻). The absolute fluorescence quantum yield of solid-state samples did not exceed 3%.

The magnetic and resonance properties of the G2-[L₂Fe]⁺A⁻ complexes were studied using SQUID magnetometry and EPR spectroscopy. Magnetic measurements showed that G2-[L₂Fe]NO₃ and G2-[L₂Fe]PF₆ at high temperatures were in mixed low-spin (LS, S = 1/2) and high-spin (HS, S = 5/2) states. In the range of 200–300 K the G2-[L₂Fe]NO₃ contained 58.9% LS and 41.1% HS; and the G2-[L₂Fe]PF₆ contained 56.1% LS and 43.9% HS Fe(III) ions. Complex G2-[L₂Fe]Cl was in a single-phase HS state. No SCO transitions were detected over the entire temperature range. All G2-[L₂Fe]⁺A⁻ complexes were characterized by antiferromagnetic exchange interactions between neighboring Fe(III) ions, which probably arose due to the π-stacking of benzene rings. Analysis of the ground spin state at 2.0 K showed that it was characterized by the Brillouin contribution from non-interacting LS and a proportion of the HS Fe(III) ions not participating in AFM interactions. For G2-[L₂Fe]Cl, this proportion was 57%; for G2-[L₂Fe]NO₃ and G2-[L₂Fe]PF₆ it was 18% and 16%, respectively.

EPR measurements confirmed the presence of magnetically active HS and LS Fe(III) ions states and made it possible to distinguish two HS types: I-type with strong low-symmetry and II-type with weak, distorted octahedral environments. It was shown that G2-[L₂Fe]⁺A⁻ complexes are magnetically inhomogeneous and consist of two magnetic sub-lattices. The I-type HS Fe(III) centers probably form AFM-correlated chains in layers. The LS and II-type HS Fe(III) centers are in a fluctuating short-range AFM ordered state which is not detectable by static magnetic susceptibility and is likely located between the layers.