New Mononuclear Mn(III) Complexes with Hydroxyl-Substituted Hexadentate Schiff Base Ligands

This paper reports the syntheses, crystal structures and magnetic properties of Mn(III) hexadentate Schiff base complexes [Mn(4-OH-sal-N-1,5,8,12)]NO3(1) and [Mn(4-OH-sal-N-1,5,8,12)]ClO4(2), where (4-OH-sal-N-1,5,8,12)2− (4,4′-((1E,13E)-2,6,9,13-tetraazatetradeca-1,13-diene-1,14-diyl)bis(3-methoxyphenol) is a new hydroxyl-substituted hexadentate Schiff base ligand. The introduction of the (4-OH-sal-N-1,5,8,12)2− ligand induces more hydrogen bonding interactions, in addition to promoting the formation of intermolecular interactions among the cations. However, the close-packing structures of both complexes lead to their stabilization in the high-spin state in the temperature range of 2−300 K.

Schiff bases are used widely because they are readily derivatized and generally easy to prepare. As versatile ligands, they can stabilize a wide range of geometries and oxidation states in transition metal complexes. This great diversity may permit the design of Schiff bases with a suitable ligand field strength to obtain Mn(III) SCO complexes. Notably, the first SCO d 4 system [Mn(pyrol) 3 tren] with a hexadentate N 6 Schiff base ligand was designed in 1981 by Sinn and Sim [12], breaking the conventional wisdom that it was too difficult to generate an amenable ligand-field strength to stabilize an LS state in Mn(III) complexes. Later, the prototypic gradual SCO phenomenon of a Mn(III) hexadentate N 4 O 2 Schiff base complex was reported in 2006 by Morgan et al. [13] Subsequently, another ligand system, a tridentate Schiff base N 2 O ligand provided the third example of Mn(III) SCO compounds [14].
In addition, the report on the 4-position of salicylaldehyde of hexadentate Schiff base ligands is rare; however, a series of Mn(III) SCO compounds [Mn(4-R-sal-N-1,5,8,12)]Y (R = OC 6 H 13 , OC 12 H 25 , OC 18 H 37 ) with gradual and uncompleted SCO behavior were reported by Albrecht and his co-workers [16]. Under this circumstance, we study to obtain complexes that can provide strong cooperation interactions and more complete SCO behaviors. Therefore, the complexes [Mn(4-OH-sal-N-1,5,8,12)]Y (Y = NO 3 − and ClO 4 − ) (Scheme 1) were selected with the following main goals: increase the number of hydrogen bonds through the hydroxyl group, in order to strengthen the connection between anions and cations and improve the cooperativity of the system.  [29], has emphasized the impact of the substituent effects.
In addition, the report on the 4-position of salicylaldehyde of hexadentate Schiff base ligands is rare; however, a series of Mn(III) SCO compounds [Mn(4-R-sal-N-1,5,8,12)]Y (R = OC6H13, OC12H25, OC18H37) with gradual and uncompleted SCO behavior were reported by Albrecht and his co-workers [16]. Under this circumstance, we study to obtain complexes that can provide strong cooperation interactions and more complete SCO behaviors. Therefore, the complexes [Mn(4-OH-sal-N-1,5,8,12)]Y (Y = NO3 − and ClO4 − ) (Scheme 1) were selected with the following main goals: increase the number of hydrogen bonds through the hydroxyl group, in order to strengthen the connection between anions and cations and improve the cooperativity of the system. Herein, we report the synthesis, X-ray crystal structures, and magnetic properties of two isostructural compounds with the general formula [Mn(4-OH-sal-N-1,5,8,12)]Y (Y = NO3 − and ClO4 − ). Correlations between properties and structures are discussed with respect to the alteration of the geometry and hydrogen-bonding acceptor of the anions.

Experimental Section
All of the reagents and chemicals were analytically pure, purchased from commercial sources, and were used without further purification. Although we experienced no problems with the compounds reported in this work, the manganese perchlorate salt with the organic ligand is potentially explosive and should be handled with great care and used in small amounts. Elemental analyses of C, H, and N were performed on a Vario EL III elemental analyzer. IR spectra of the solid samples (KBr tablets) in the range 400−4000 cm −1 were recorded by an FT-IR Perkin Elmer spectrometer. The properties of the Hirshfeld surface for complexes 1 and 2 were generated using a CrystalExplorer 17.5. The Hirshfeld surface was generated using a high resolution and mapped with the dnorm and shape-indexed functions. 2D fingerprint plots were prepared using the same software. Magnetic susceptibility was measured in a sweep mode upon cooling from 300 to 2 K under a 0.1 T applied magnetic field by the use of a Quantum Design MPMS SQUID VSM magnetometer. A freshly prepared crystalline sample was placed in a gelatin capsule holder. Magnetic data were calibrated with the sample holder and diamagnetic corrections Herein, we report the synthesis, X-ray crystal structures, and magnetic properties of two isostructural compounds with the general formula [Mn(4-OH-sal-N-1,5,8,12)]Y (Y = NO 3 − and ClO 4 − ). Correlations between properties and structures are discussed with respect to the alteration of the geometry and hydrogen-bonding acceptor of the anions.

Experimental Section
All of the reagents and chemicals were analytically pure, purchased from commercial sources, and were used without further purification. Although we experienced no problems with the compounds reported in this work, the manganese perchlorate salt with the organic ligand is potentially explosive and should be handled with great care and used in small amounts. Elemental analyses of C, H, and N were performed on a Vario EL III elemental analyzer. IR spectra of the solid samples (KBr tablets) in the range 400−4000 cm −1 were recorded by an FT-IR Perkin Elmer spectrometer. The properties of the Hirshfeld surface for complexes 1 and 2 were generated using a CrystalExplorer 17.5. The Hirshfeld surface was generated using a high resolution and mapped with the d norm and shape-indexed functions. 2D fingerprint plots were prepared using the same software. Magnetic susceptibility was measured in a sweep mode upon cooling from 300 to 2 K under a 0.1 T applied magnetic field by the use of a Quantum Design MPMS SQUID VSM magnetometer. A freshly prepared crystalline sample was placed in a gelatin capsule holder. Magnetic data were calibrated with the sample holder and diamagnetic corrections were estimated from Pascal's constants. Magnetic behavior was primarily analyzed by using variable-temperature magnetic susceptibility measurements.

Crystallographic Studies
Single-crystal X-ray diffraction data for the compounds [Mn(4-OH-sal-N-1,5,8,12)]NO 3 (1) and [Mn(4-OH-sal-N-1,5,8,12)]ClO 4 (2) were collected at 100 and 298 K, respectively. Table 1 presents the most relevant parameters for single-crystal determination.   Moreover, the bond lengths of the complexes at 298 K had no significant change compared with those at 100 K (Table 2).  The octahedral distortion parameters Σ and θ can intuitively reflect the changes of the Mn(III) coordination sphere. For complex 1, θ was from 292.38° at 100 K to 291.98° at 298 K, while the change in Σ was small, with values of 79.59° and 79.41°, respectively. Both parameters, with a little change, further demonstrate that there was no SCO behavior in complex 1. The central Mn 3+ ion of compound 2, which is similar to complex 1, was furnishing a distorted octahedral geometry (θ = 289.93° and Σ = 77.66° at 100 K; θ = 294.06° and Σ = 78.90° at 298 K).
In Figure 3, a pair of [Mn(4-OH-sal-N-1,5,8,12)] + cations form a centrosymmetric dimer through the N-H···O hydrogen bonds between the amino nitrogen atoms and peripheral hydroxy oxygen atoms. Moreover, weak edge-to-edge π···π contacts exist between the two sets of C(1)-C(2) atoms (Figure 4), and stabilize the dimer structure.  The octahedral distortion parameters Σ and θ can intuitively reflect the changes of the Mn(III) coordination sphere. For complex 1, θ was from 292.38° at 100 K to 291.98° at 298 K, while the change in Σ was small, with values of 79.59° and 79.41°, respectively. Both parameters, with a little change, further demonstrate that there was no SCO behavior in complex 1. The central Mn 3+ ion of compound 2, which is similar to complex 1, was furnishing a distorted octahedral geometry (θ = 289.93° and Σ = 77.66° at 100 K; θ = 294.06° and Σ = 78.90° at 298 K).
The anions connected cationic dimers located in chains via the O-H···O hydrogen bonds (Figure 3). The supramolecular chains were relatively independent and extended infinitely in the bc-plane. With the increase of the temperature, there was no significant change in the strength of the hydrogen bonds ( Table 3). The intrachain Mn-Mn separation was 8.337 Å at 100 K and 8.349 Å at 298 K. However, the formation of dimers resulted in the short interchain Mn···Mn distance, which was 6.709 Å at 100 K and 6.749 Å at 298 K, respectively. The close Mn-Mn distance gave the [Mn(4-OH-sal-N-1,5,8,12)] + cation less space to change the coordination geometry.
As for complex 2, increasing the size of the anion from planar NO 3 − to tetrahedral  Figure 5). In addition, the interchain Mn-Mn separation changed from 6.841 at 298 K to 6.740 Å at 100 K because of its bigger anion size. However, this did not give the Mn(III) cation enough space to change its conformation to meet the structural requirements of SCO.  The anions connected cationic dimers located in chains via the O-H···O hydrogen bonds (Figure 3). The supramolecular chains were relatively independent and extended infinitely in the bc-plane. With the increase of the temperature, there was no significant change in the strength of the hydrogen bonds ( Table 3). The intrachain Mn-Mn separation  The anions connected cationic dimers located in chains via the O-H···O hydrogen bonds (Figure 3). The supramolecular chains were relatively independent and extended infinitely in the bc-plane. With the increase of the temperature, there was no significant change in the strength of the hydrogen bonds ( Table 3). The intrachain Mn-Mn separation

Hirshfeld Surface Analysis
To gain deeper insight into the supramolecular contacts in 1 and 2, we undertook Hirshfeld surface analysis using CrystalExplorer 17.5. The Hirshfeld surfaces for the cations of complexes 1 and 2 were mapped with the dnorm function [39,40], which shows several red spots. For 1, the four strongest red spots were due to N-H···O and O-H···O hydrogen bonding interactions, and the weak red spots were due to C-H· · · O interactions ( Figure  6a). The hydrogen bond was one of the major interactions here, contributing to 17% of all interactions in Figure 6c.
For complex 2, owing to the change in anions, the hydrogen bond contributed to

Hirshfeld Surface Analysis
To gain deeper insight into the supramolecular contacts in 1 and 2, we undertook Hirshfeld surface analysis using CrystalExplorer 17.5. The Hirshfeld surfaces for the cations of complexes 1 and 2 were mapped with the d norm function [39,40], which shows several red spots. For 1, the four strongest red spots were due to N-H···O and O-H···O hydrogen bonding interactions, and the weak red spots were due to C-H···O interactions ( Figure  6a). The hydrogen bond was one of the major interactions here, contributing to 17% of all interactions in Figure 6c. In order to more intuitively study the influence of hydroxyl on the hydrogen bond interaction, we introduced Hirshfeld surface analysis on the complex [Mn(4-OC6H13-sal-N-1,5,8,12)]NO3·H 2O [16] (Figure 7). Though it crystallizes as an H2O solvate, the hydrogen bond contributed to only 9.6% of all interactions. All in all, the OH group played a significant role in non-covalent interactions.  For complex 2, owing to the change in anions, the hydrogen bond contributed to 24.8% of all interactions (Figure 6d). This supports the discussion above and suggests that the OH group in the (4-OH-sal-N-1,5,8,12) 2− ligand is critical in linking the [Mn(4-OH-sal-N-1,5,8,12)] + cations together in these structures.
In order to more intuitively study the influence of hydroxyl on the hydrogen bond interaction, we introduced Hirshfeld surface analysis on the complex [Mn(4-OC 6 H 13sal-N-1,5, 8,12)]NO 3 ·H 2 O [16] (Figure 7). Though it crystallizes as an H 2 O solvate, the hydrogen bond contributed to only 9.6% of all interactions. All in all, the OH group played a significant role in non-covalent interactions.
From the Hirshfeld analysis, it was very clear that the hydroxyl group effectively enhanced the hydrogen bonding interactions in both complexes. However, these noncovalent intermolecular forces were insufficient to result in a cooperative SCO. The cation structures were tightly packed, hindering the distortion required to undergo SCO.
gen bond contributed to only 9.6% of all interactions. All in all, the OH group significant role in non-covalent interactions.

Magnetic Characterization
The temperature dependence of the product χ M T (χ M is the molar paramagnetic susceptibility) versus T plots for the crystalline samples of complexes 1 and 2 is shown in Figures 8 and 9, respectively.   At room temperature, the χMT value of 1 was about 2.79 cm 3 K mol −1 , which is typical of an HS Mn 3+ center (S = 2) with a g value of 2.0 ( Figure 7). As the temperature went down, the χMT value was constant until 20 K, when it decreased rapidly because of the ZFS (zero field split) effects of HS Mn 3+ ions. However, within the temperature range, the χMT value did not At room temperature, the χ M T value of 1 was about 2.79 cm 3 K mol −1 , which is typical of an HS Mn 3+ center (S = 2) with a g value of 2.0 ( Figure 7). As the temperature went down, the χ M T value was constant until 20 K, when it decreased rapidly because of the ZFS (zero field split) effects of HS Mn 3+ ions. However, within the temperature range, the χ M T value did not drop to 1.0 cm 3 K mol −1 . The temperature dependence of the χ M −1 of complex 1 is shown in Figure 7. It is linear between 2 and 300 K and a linear least-squares fit yields a Curie constant of 2.84 emu K mol −1 , a Weiss temperature θ of −1.67 K, while the Curie constant and θ value of compound 2 was 2.95 emu K mol −1 and −1.51 K (Figure 8).
The magnetic characterization of [Mn(4-OH-sal-N-1,5,8,12)]ClO 4 was similar to that of complex 1, and increasing the size of the anion from NO 3 − from ClO 4 − seems to have had no effect on the magnetic behavior.

Conclusions
In an effort to synthesize new Mn(III) SCO complexes, Funding: This research received no external funding.

Data Availability Statement:
The data presented in this study are available in this article.