FeIII in the high-spin state in dimethylammonium bis[3-ethoxysalicylaldehyde thiosemicarbazonato(2–)-κ3 O 2,N 1,S]ferrate(III)

The FeIIIS2N2O2 chromophore contains two O,N,S-donating dianionic ligands in perpendicular planes, with the O and S atoms in cis and the N atoms in trans positions. The FeIII ion is in the high-spin state at 100 K. The variable-temperature magnetic susceptibility measurements (5–320 K) are consistent with the presence of a high-spin S = 5/2 FeIII ion.


Introduction
The continuing research and development of switchable magnetic, optical and/or photomagnetic materials seeks to provide solutions for the societal desire towards more advanced electronic devices (e.g. larger data storage capacity and faster data processing) and their miniaturization by offering industry novel magnetic materials that can be implemented in electronic devices for information storage and as displays (Lé tard et al., 2004;Gü tlich & Goodwin, 2004;Halcrow, 2013;Molná r et al., 2018;Senthil Kumar et al., 2017;Rubio-Gimé nez et al., 2019;Tissot et al., 2019;Karuppannan et al., 2021). Spin-crossover materials have attractive physical properties that make them suitable candidates for fulfilling these requirements. Such compounds exhibiting a temperature-dependent crossover between electronic states having a different magnetic moment were first discovered for iron(III) tris(dithiocarbamates) (Cambi & Szegö, 1931, 1933. Since then, two main families of Fe III spin-crossover systems have been extensively studied, i.e. those containing ligands sporting chalcogen donor atoms and those based on multidentate N,Odonating Schiff base-type ligands (van Koningsbruggen et al., 2004;Harding et al., 2016). It has been found that the magnetic interconversion between the low-spin (S = 1/2) and high-spin (S = 5/2) state in Fe III systems can be triggered by a change in temperature or pressure, or by light irradiation (Hayami et al., 2000(Hayami et al., , 2009; van Koningsbruggen et al., 2004;Harding et al., 2016).
In fact, in solution, the free R-salicylaldehyde 4R 0 -thiosemicarbazone ligand (H 2 L) exists in two tautomeric forms, i.e. the thione and thiol forms, as illustrated in Scheme 1. Moreover, the ligand may also be present in its neutral, anionic or dianionic form. We established that the formation of a particular type of Fe III complex unit, i.e. neutral, monocationic or monoanionic, can be achieved by tuning the degree of deprotonation of the ligand through pH variation of the reaction solution during the synthesis (Powell et al., 2014(Powell et al., , 2015(Powell et al., , 2020(Powell et al., , 2022Powell, 2016;Yemeli Tido, 2010;Floquet et al., 2009).

Spectroscopic and magnetic measurements
A room-temperature IR spectrum of 3-ethoxysalicylaldehyde thiosemicarbazone within the range 4000-400 cm À 1 was recorded on a PerkinElmer FT-IR spectrometer Spectrum RXI using KBr pellets. IR spectroscopic measurements of (I) within the range 4000-600 cm À 1 were carried out at room temperature using an ATR (attenuated total reflectance) PerkinElmer FT-IR Frontier spectrometer. 1 H and 13 C NMR spectra were recorded in DMSO-d 6 (dimethyl sulfoxide) using a Bruker cryomagnet BZH 300/52 spectrometer (300 MHz), with the recorded chemical shifts in � (in parts per million) relative to an internal standard of tetramethylsilane (TMS).
Measurements of direct current (dc) magnetic susceptibility, � M , versus temperature, T, were conducted between 5 and 320 K, heating and cooling at a rate of 2 K min À 1 in an applied field, � 0 H, of 0.1 T using a Quantum Design MPMS-5S superconducting quantum interference device (SQUID) magnetometer. The SQUID magnetometer was calibrated using a standard palladium sample. The background due to the sample holder and the diamagnetic signal of the sample, estimated using Pascal's constants (Bain et al., 2008), was subtracted from the measured molar magnetic susceptibility � M .

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. The H atoms of terminal amine atoms N103 and N3 were located in difference Fourier maps and refined with restrained N-H distances of 0.86 (2) Å and with U iso (H) = 1.2U eq (N). The remaining H atoms were included in the refinement in calculated positions and treated as riding on their parent atoms, with N-H distances of 0.91 Å and U iso (H) = 1.2U eq (N) for the amine N atom of the cation, C-H distances of 0.95 Å and U iso (H) = 1.2U eq (C) for aryl (-CH ) H atoms, C-H distances of 0.99 Å and U iso (H) = 1.2U eq (C) for secondary (-CH 2 -) H atoms, and C-H distances of 0.98 Å and U iso (H) = 1.5U eq (C) for methyl (-CH 3 ) H atoms.

Results and discussion
In solution, the free ligand, i.e. 3-ethoxysalicylaldehyde thiosemicarbazone (H 2 L), exists in two tautomeric forms, the thione and the thiol form, as illustrated in Scheme 1. Consequently, in Fe III compounds, the ligand may be present as either one of the possible tautomers, and may be neutral, anionic or dianionic. Referring to the thiol tautomer, neutral H 2 L has H atoms located on the phenol O atom and the thiol S atom. The first deprotonation step involving the phenol group results in the formation of 3-ethoxysalicylaldehyde thiosemicarbazone(1À ) (abbreviated as HL À ). Subsequent deprotonation yields 3-ethoxysalicylaldehyde thiosemicarbazonate(2À ) (abbreviated as L 2À ).
The structure of dimethylammonium bis[3-ethoxysalicylaldehyde thiosemicarbazonato(2À )-� 3 O 2 ,N 1 ,S]ferrate(III), (I) (Fig. 1), was determined at 100 K. Compound (I) crystallized in the monoclinic space group P2 1 /n, with Z = 4. The asymmetric unit consists of one formula unit, [(CH 3 ) 2 NH 2 ][Fe(3-OEt-thsa) 2 ], with no atom on a special position. The Fe III cation is coordinated by the thiolate S, phenolate O and imine N atoms of each of the two dianionic O,N,S-tridentate L 2À ligands. The donor atoms of the ligands are situated in two perpendicular planes, with the O and S atoms in cis positions, and mutually trans N atoms. Selected geometric parameters are listed in Table 2.
167.63 (7) N102-C108-S101 125.65 (18) Koningsbruggen et al., 2004). The bond lengths involving the Fe atom and the donor atoms in (I) correspond with Fe III being in the high-spin state at 100 K. Variable-temperature magnetic susceptibility measurements (5-320 K) confirm that the Fe III ion in (I) is indeed in the high-spin state over this temperature range (Powell, 2016). High-spin Fe III has also been evidenced in the related Cs[Fe(thsa) 2 ] compound at 103 (and 298 K) (Ryabova et al., 1981a). It is significant to note that the Fe-O distances seem to be less sensitive to the change in Fe III spin state than the Fe-N and Fe-S distances, which may be related to the �-acceptor capability of the N-and S-donor atoms as opposed to the �-donor capability of the O-donor atoms. This is of particular significance when Fe III is in the low-spin state, as increased � backbonding will lead to comparatively more pronounced shortening of the Fe-N and Fe-S bonds than of the Fe-O bonds (Powell et al., 2014).
The ligands have been found to be in the dianionic form as no H atoms were located on the phenolate O (O1 and O101) or the thiolate S (S1 and S101) atoms. The charge of the two L 2À ligands is balanced by the presence of the monovalent dimethylammonium cation together with the trivalent iron cation. The tridentate ligands of the present compound are coordinated to the Fe III cation by the thiolate S, phenolate O and imine N atoms, forming six-and five-membered chelate rings. The six-membered chelate ring involves a significantly less restricted bite angle [O1-Fe-N1 = 82.17 (7) � and O101-Fe-N101 = 84.03 (7) � ] than the five-membered chelate ring [S1-Fe-N1 = 78.45 (5) � and S101-Fe-N101 = 78.93 (5) � ]. The r.m.s. deviations from their least-squares plane of atoms of the six-membered chelate ring of both coordinated ligands are 0.197 and 0.177 Å for Fe1/N11/C17/ C11/C12/O11 and Fe1/N101/C107/C101/C102/O101, respectively, and the corresponding values for the five-membered chelate rings are 0.129 and 0.102 Å for Fe1/N11/C12/C18/S11 and Fe1/N101/C102/C108/S101, respectively. It appears that the metal chelate rings deviate slightly from the ideal planar structure. Furthermore, the O-Fe-N and S-Fe-N bite angles of the six-and five-membered chelates are deficient by ca 38 and 30 � , respectively, compared to the angle at the vertex of a regular hexagon (120 � ) or pentagon (108 � ), respectively. In comparison to other (cation + )[Fe(L 2À ) 2 ]·x(solvent) compounds of related ligands (Powell et al., 2014(Powell et al., , 2015(Powell et al., , 2020, the deficiency of the bite angle in both the six-and five-membered chelate rings is larger than expected, though it has been recognized that these other Fe III bis(ligand) compounds contain Fe III in the low-spin state, whereas the present compound contains Fe III in the high-spin state. Consequently, (I) displays longer Fe III -donor atom bond lengths, which are associated with more restricted bite angles. Moreover, the remaining bond angles involving each six-membered chelate ring (Table 2) are, as expected, within ca 5 � of the value of 125 � . However, the C-S-Fe bond angles involving each fivemembered chelate ring are only about 95 � , providing an additional deficiency of 13 � . The additional deficiency can be offset by increasing the other bond angles within this fivemembered chelate ring to ca 120 � . It has been found that the N-N-C angles are <120 � and the N-C-S angles are >120 � ; these values suggest sp 2 hybridization at the C and N atoms.
The stability of the Fe III complex is further enhanced by the high degree of electron delocalization throughout the chelated ligands, which is evident from the geometric parameters. The C-S, C-N and N-N bond lengths of (I) show characteristics of a bond order between 1 (i.e. single bond) and 2 (i.e. double bond). The C8-S1 bond length of 1.746 (3) Å and the C108-S101 bond length of 1.752 (2) Å suggest partial electron delocalization of these C-S bonds. This feature has also been found in the structure of the related high-spin Fe III compound Cs[Fe(thsa) 2 ] at 103 K (Ryabova et al., 1981a), in which the C-S bond lengths of 1.749 (9) and 1.761 (9) Å are indicative of partial electron delocalization.
In addition, the electron delocalization within each of the O,N,S-tridentate ligands is confirmed by a bond order larger than 1 for the C-N bond involving the deprotonated hydra-
The hydrogen-bonding interactions of (I), identified using the default parameters of OLEX2 (Dolomanov et al., 2009), are listed in Table 3 and displayed in Fig. 2. The N atom of the dimethylammonium cation forms two hydrogen bonds: one contact is formed with the phenolate O atom of one ligand, whereas the second contact is formed with the ethoxy O atom of the salicylaldehyde moiety of the other ligand. The N201-H20A� � �O102 and N201-H20B� � �O1 contacts form an intramolecular hydrogen-bonded ring system, giving rise to an R 2 2 (9) ring (Bernstein et al., 1995). Magnetic susceptibility versus temperature measurements for (I) were carried out to investigate the spin state of the Fe III ion. The data collected on heating and cooling coincide over the temperature range studied. The temperature dependence of � M T collected on cooling between 320 and 5 K is displayed in Fig. 3. Above 100 K, � M T is temperature independent with a value of 4.41 (1) cm 3 K mol À 1 [5.94 (1) m B /Fe]. This is just above the expected value of 4.38 cm 3 K mol À 1 (5.92 m B /Fe) for Fe III in its high-spin (S = 5/2) state with an electronic g factor of 2. � M À 1 (T) is linear in T and a fit to a Curie-Weiss law between 100 and 320 K shown in Fig. 4 gives a Weiss temperature of À 3.3 (1) K and an effective moment of 6.00 (1) m B / Fe. � M T drops rapidly below 100 K. This may be due to weak (antiferro)magnetic interactions between neighbouring spins or may reflect a splitting of the S = 5/2 state (O'Connor, 1982). Studies using aligned single crystals are needed to differentiate between these possibilities. For splitting, the spin Hamiltonian can be written as H S = H CEF + H z , where the crystalline electric field (CEF) term H CEF = D[S z 2 À S(S + 1)/ 3] + E(S x 2 -S y 2 ), with D and E being the axial and rhombic zero-field splitting, respectively. The 6 S high-spin state is split into three Kramers doublets. For E = 0, the doublets are separated by 2D and 6D from the lowest energy doublet. The Zeeman energy H z = g� B HS z and the molar susceptibility with a field along z is where X = D/k B T, N A is Avogadro's number and k B is the Boltzmann constant (O'Connor, 1982). A fit gives D = 0.83 (1) cm À 1 with g = 2. D is in the range expected for highspin Fe III (Chen et al., 2002;Yemeli Tido et al., 2007). Fits with a finite E expected for a system with a rhombic distortion are possible, cf. Chen et al. (2002), but these require a knowledge of the ratio � = E/D from other studies, such as electron paramagnetic resonance (EPR) spectroscopy. It is of interest to compare the two Fe III compounds that have so far been reported to contain the 3-ethoxysalicylaldehyde 4-R 0 -thiosemicarbazonate(2À ) dianion. In Cs[Fe(3-OEt-thsa-Me) 2 ]·CH 3 OH (Powell et al., 2014), Fe III is low spin, whereas in the present [(CH 3 ) 2 NH 2 ][Fe(3-OEt-thsa) 2 ] compound, (I), the metal ion adopts the high-spin state. The differences between the two compounds further involve: (i) the relative size of the R 0 substituent on the terminal N atom of the thiosemicarbazide moiety, as (I) contains a H atom, whereas Cs[Fe(3-OEt-thsa-Me) 2 ]·CH 3 OH (Powell et al., 2014)   Hydrogen-bond geometry (Å , � ).

Figure 2
The unit cell of (I), with displacement ellipsoids drawn at the 50% probability level.

Figure 3
� M T versus T for (I). The data were measured while cooling at a rate of 2 K min À 1 in an applied field � 0 H of 0.1 T.
(iii) the presence of a methanol solvent molecule in the crystal lattice of Cs[Fe(3-OEt-thsa-Me) 2 ]·CH 3 OH (Powell et al., 2014). These differences are associated with (I) forming intramolecular ring systems through hydrogen bonds (vide supra), whereas Cs[Fe(3-OEt-thsa-Me) 2 ]·CH 3 OH forms intermolecular hydrogen-bonded ring systems which link neighbouring Fe III entities. These factors determine the arrangement of the Fe III entities within the unit cell, which is further characterized by the space group P2 1 /n, with Z = 4 and V = 2576.35 (17) Å 3 for (I), with a volume of 644.09 Å 3 per high-spin Fe III formula unit, and the space group P1, with Z = 2 and V = 1369.5 (8) Å 3 for Cs[Fe(3-OEt-thsa-Me) 2 ]·CH 3 OH, with a volume of 684.75 Å 3 per low-spin Fe III formula unit (Powell et al., 2014); hence the volume increase associated with Fe III being low-spin compared to high-spin is more than offset by the differences in substituents, composition and crystal packing. Evidently, the intricate interplay between the variation in cation, ligand substituents and associated solvent molecules affects the crystal packing of compounds of this class of (cation + )[Fe(L 2À ) 2 ]·x(solvent) materials and allows for a variation of the spin state of Fe III , with some members displaying temperature-dependent spin-crossover behaviour (van Koningsbruggen et al., 2004;Powell, 2016). Further studies by our group will additionally focus on tuning the spin state of Fe III by varying the degree of deprotonation of the ligand. CrysAlis PRO (Agilent, 2014); program(s) used to solve structure: Superflip (Palatinus & Chapuis, 2007;Palatinus & van der Lee, 2008;Palatinus et al., 2012); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

Dimethylammonium bis[3-ethoxysalicylaldehyde thiosemicarbazonato(2-)-κ 3 O 2 ,N 1 ,S]ferrate(III)
Crystal data (C 2  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. H atoms bonded to N3 and N103 were located in the difference map and then refined with U iso 1.2 times the parent atoms and a geometrical distance restraint