A new mononuclear neutral high-spin iron(III) complex with the different tridentate ligands 5-bromosalicylaldehyde (pyridin-2-yl)hydrazone and 5-bromosalicylaldehyde thiosemicarbazone

The molecular and crystal structure of a new mononuclear neutral high-spin iron(III) complex with two different tridentate ligands has been studied by an X-ray diffraction analysis.


Chemical context
Much attention have been paid to the design and synthesis of Fe III complexes for magnetic materials owing to their interesting thermal-or light-induced spin conversion between the high-spin (HS, S = 5/2) and low-spin (LS, S = 1/2) states (Li et al., 2013;Phonsri et al., 2017;Sato et al., 2007). It is well known that the organic ligands usually play a significant role in the crystal structures and magnetic properties of metal complexes (Ni et al., 2017;Zhang et al., 2016). Up to date, many Fe III complexes with spin-crossover (SCO) behavior have been designed and synthesized through the subtle design and combination of different ligands. Among the many organic ligands, Schiff bases are the most common ligands for new Fe III complexes due to their convenient synthesis and regulation. Compared with homo-ligand complexes, the employment of mixed ligands provides more selection and modification strategies for new magnetic complexes. In previous reports, the ligands 5-bromo-salicylaldehyde-2pyridylhydrazone (5-Br-Hpsal), 5-bromo-salicylaldehydethiosemicarbazone (5-Br-H 2 thsa) and their derivatives have been explored to assembly Fe III and Mn III complexes with SCO behavior (Shongwe et al., 2014). Recently, we obtained the title complex, [(C 20 H 15 N 6 O 2 SBr 2 )Fe] (I), using 5-Br-Hpsal and 5-Br-H 2 thsa ligands. Herein, we report the crystal structure and magnetic property of this iron(III) complex.

Supramolecular features
In the crystal, there are two independent hydrogen bonds (Table 1), which link the complex molecules into layers parallel to (100) (Fig. 2). In addition, there exist relatively stronginteractions between the pyridine and benzene rings of the 5-Br-psal À ligands with a shortest interatomic distance of 3.485 (3) Å (Fig. 2).

Magnetic properties
The magnetic susceptibilities of (I) have been measured in the temperature range 2-350 K under an applied magnetic field strength of 2000 Oe by SQUID magnetometry. A plot of m T versus T is presented in Fig. 3, where m represents the molar magnetic susceptibility per Fe III unit. The m T value is 4.042 emu K mol À1 at room temperature, which is slightly smaller than the expected value of 4.375 emu K mol À1 for the single spin carrier of high-spin Fe III (S = 5/2) based on g = 2.0. The measurement of the magnetic property shows that the Fe III ion is in the high-spin state, which agrees well with the abovementioned bond lengths around the Fe III ion. The m T value keeps nearly constant with decreasing temperature until around 75 K, and then it decreases quickly to a minimum value of 1.12 emu K mol À1 at 2.0 K. This tendency to change of the m T curve indicates the existence of overall weak antiferromagnetic interactions in (I). The magnetic susceptibilities in the range of 2-350 K comply well with the Curie-Weiss law with a negative Weiss constant = À4.28 K, and Curie constant C = 4.08 emu K mol À1 , which further confirms the presence of overall intermolecular antiferromagnetic interactions between neighboring Fe III ions through intermolecular hydrogen bonds andinteractions in complex (I).  Table 1 Hydrogen-bond geometry (Å , ). (4) 2.00 (4) 2.825 (5) 171 (4) N1-H1AÁ Á ÁO2 ii 0.88 2.29 2.987 (4) 136

Figure 1
Molecular structure of (I) with 30% probability displacement ellipsoids.

Synthesis and crystallization
All reactions were conducted in air using reagent grade solvents. The 5-Br-Hpsal and 5-Br-H 2 thsa ligands were synthesized by refluxing equimolar 5-bromosalicylaldehyde with thiosemicarbazone and 2-pyridylhydrazine, respectively, in an ethanol solvent. All other chemicals were purchased from the Sigma Aldrich Chemical Company and used as received.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The amino-H atom of 5-Br-psal À was found from the difference-Fourier map and refined isotropically. All other hydrogen atoms were placed in calcu- The layered structure of (I) formed through hydrogen bonds (green dotted lines) andinteractions. riding model with fixed isotropic displacement parameters [U iso (H) = 1.2U eq (C,N)].

[1-(5-Bromo-2-oxidobenzylidene)thiosemicarbazidato](4-bromo-2-{[2-(pyridin-2yl)hydrazinylidene]methyl}phenolato)iron(III)
Crystal data 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.