Crystal structure of bis(5-bromo-1,10-phenanthroline-κ2 N,N′)bis[dihydrobis(pyrazol-1-yl)borato-κ2 N 2,N 2′]iron(II) toluene disolvate

Each of the two unique metal complex molecules in the title compound shows a distorted N6 coordination set defined by three pairs of chelating ligands.

The structure determination of the title compound was undertaken as part of a project on the modification and synthesis of new spin-crossover (SCO) compounds based on octahedral Fe II bis(pyrazolyl)borate complexes. In the course of these investigations, the compound [Fe(C 6 H 8 BN 4 ) 2 (C 12 H 7 BrN 2 )] was synthesized, for which magnetic measurements revealed an incomplete spincrossover behaviour. Crystallization of this compound from toluene led to the formation of crystals of the toluene disolvate, [Fe(C 6 H 8 N 4 B) 2 (C 12 H 7 N 2 Br)]Á-2C 7 H 8 . Its asymmetric unit comprises two discrete metal complex molecules and two toluene solvent molecules. One of the latter is severely disordered and its contribution to the diffracted intensities was removed using the SQUEEZE routine [Spek (2015). Acta Cryst. C71, 9-18]. In each complex molecule, the Fe II cation is coordinated by the two N atoms of a 5-bromo-1,10-phenanthroline ligand and by two pairs of N atoms of chelating dihydrobis-(pyrazol-1-yl)borate ligands in the form of a slightly distorted octahedron. The discrete complexes are arranged in columns along the a-axis direction with the toluene solvate molecules located between the columns. The 5-bromo-1,10phenanthroline ligands of neighbouring columns are approximately parallel and are slightly shifted relative to each other, indicatinginteractions.

Chemical context
Spin crossover (SCO) occurs in octahedrally coordinated transition-metal complexes with an electron configuration of 3d 4 -3d 7 and is of extraordinary importance in coordination chemistry and the field of molecular magnetism. Such materials are also of interest because of their potential for future applications as molecular switches, in data storage or in spintronics (Gü tlich et al., 2013;Halcrow, 2007Halcrow, , 2013b. SCO compounds can be switched between the paramagnetic highspin (HS, S = 2) and the diamagnetic low-spin state (LS, S = 0) by external stimuli such as temperature or light (Gü tlich et al., 2013). Most compounds reported in the literature are based on Fe II in an octahedral coordination because, in this case, a very long lifetime of the photochemically excited high-spin state is expected. During the spin-transition, the Fe-L (L = ligand) bond lengths and also the unit-cell volume change significantly. Therefore, cooperativity effects are of importance, which frequently lead to abrupt spin transitions, very often associated with a hysteresis or a more complicated SCO behaviour (Halcrow, 2007(Halcrow, , 2013a. Up to date, hundreds of Fe II SCO complexes have been published (Halcrow, 2007). Recently, complexes based on organoborate ligands such as [Fe(H 2 B(pz) 2 ) 2 (L)] (with pz = pyrazole and L = diimine co- ISSN 2056-9890 ligand) have become of particular interest, because they can be evaporated in vacuo and therefore allow a facile preparation of thin films (Ruben & Kumar, 2019;Naggert et al., 2015;Ossinger et al., 2020a).
In our own systematic investigations we are interested how a chemical modification of such Fe organoborate complexes influences the SCO behavior in the bulk material and in thin films. This includes functionalization of the neutral diimine ligand L and the pyrazole ligand in iron(II) complexes with general composition [Fe(H 2 B(pz) 2 ) 2 (L)], which leads to characteristic changes in the spin-transition behaviour in the solid state (Naggert et al., 2015;Ossinger et al., 2019Ossinger et al., , 2020a. Analysis of the crystal structures of these iron(II) bis(dihydrobis(pyrazolyl)borate) complexes reveals that most of them are built up of dimers that are linked by intermolecular interactions between the phenyl rings of the co-ligands L (Ossinger et al., 2020b). We found that the toluene solvate [Fe(H 2 B(pz) 2 ) 2 (4,7-dimephen)]Á0.5C 7 H 8 exhibits a shortintra-dimer distance (3.507 Å at 293 K and 3.483 Å at 200 K) and consequently the complexes are locked in the high-spin state, whereas for [Fe(H 2 B(pz) 2 ) 2 (4,4 0 -dimebipy)] with a long distance (3.753 Å at 293 K and 3.736 Å at 200 K) complete thermal SCO is observed (Ossinger et al., 2020b). Alternatively, for the intermediate distances (3.575 Å at 300 K and 3.508 Å at 140 K) found in [Fe(H 2 B(4-CH 3 -pz) 2 ) 2 (bipy)] an incomplete spin-crossover is observed (Ossinger et al., 2020b). In the course of this project we became interested in the compound [Fe(H 2 B(pz) 2 ) 2 (5-Br-phen)] (pz = pyrazole, 5-Br-phen = 5-bromo-1,10-phenanthroline). Magnetic measurements of this new complex revealed an incomplete SCO in the temperature range from 2 to 300 K with only one step during the spin transition (see Fig. S1 in the supporting information). This compound can easily be crystallized from toluene whereby a toluene disolvate, [Fe(H 2 B(pz) 2 ) 2 (5-Brphen)]Á2C 7 H 8 , is formed. The crystal structure of this solvate shows dimers that are linked by phenanthroline ligands with an intra-dimer distance of 3.465 (6) Å at 200 K, indicating strong intermolecularinteractions (see above). Therefore, the system may be locked in the HS, which would concur with the observed bond lengths at 200 K, reflecting a HS configuration, see Structural commentary. Unfortunately, we were not able to prepare large amounts of pure samples of the title compound for magnetic measurements. On the other hand, a comparison of the experimental XRPD pattern of the ansolvate with the simulated pattern of the title complex based on single-crystal data ( Fig. S2) reveals that the crystal structure of the disolvate is entirely different from that of the ansolvate. Therefore, we have no information as to whetherinteractions are also present in the ansolvate and, if so, how strong these are. Nevertheless, from the observation of thermal spin-crossover in the latter (albeit in an incomplete fashion), we can conclude that the strength of theinteractions must be weaker in the ansolvate than in the solvate.

Structural commentary
The asymmetric unit of the title compound comprises two discrete complexes of [Fe(H 2 B(pz) 2 ) 2 (5-bromo-1,10-phenanthroline)] and two toluene molecules (Fig. 1). One of the solvent molecules shows severe disorder and was not taken into account in the final model (see Refinement). The Fe II cation of each independent complex is distorted octahedrally coordinated (Table 1) by the two N atoms of the chelating 5-bromo-1,10-phenanthroline ligand and by two pairs of N atoms of two chelating dihydrobis(pyrazol-1-yl)borate ligands (Fig. 1). The two complexes are different regarding their individual bond lengths and angles (

Supramolecular features
In the crystal structure, the discrete complexes are arranged into columns that extend along the a-axis direction (Fig. 2). Between these columns, channels are formed in which the toluene solvate molecules are embedded. The planes of the 5-bromo-1,10-phenanthroline ligands of neighbouring columns are approximately parallel with the planes slightly tilted and shifted relative to each other (Fig. 3). The shortest distance between two parallel-aligned carbon atoms C28 and C64(Àx, Ày, Àz + 1) of neighbouring 5-bromo-1,10-phenanthroline planes is 3.465 (6) Å , indicating stronginteractions.

Figure 3
Parts of the crystal structure of the title compound emphasizing the arrangement of the 5-bromo-1,10-phenanthroline ligands.

Synthesis and crystallization
All reactions were carried out in dry solvents and the complexation was carried out under nitrogen-atmosphere using standard Schlenk techniques or in an M-Braun Labmaster 130 glovebox under argon. 1H-Pyrazole, 5-bromo-1,10-phenanthroline and potassium tetrahydroborate were purchased from commercial sources and used without further purification. Iron(II) triflate is also commercial available but was purified by the following method: the compound was dissolved in dry methanol (a few ml for a supersaturated solution), filtered off and afterwards the solvent was removed in vacuo. Solvents were purchased from commercial sources and purified by distilling over conventional drying agents. K[H 2 B(pz) 2 ] was synthesized according to previously reported procedures (Naggert et al., 2015;Ossinger et al., 2019Ossinger et al., , 2020a.
Synthesis of [Fe(H 2 B(pz) 2 ) 2 (5-bromo-1,10-phenanthroline]: To a solution of Fe(OTf) 2 (353 mg, 1.00 mmol) in methanol (3 ml), a solution of K(H 2 B(pz) 2 ) (373 mg, 2.00 mmol) in methanol (5 ml) was added, leading to the formation of a slightly yellow-coloured solution, which was stirred for 15 min at room temperature. A solution of 5-bromo-1,10-phenanthroline (259 mg, 1.00 mmol) in methanol (3 ml) was added dropwise to the reaction mixture. Immediately, the solution turned purple and a purplecoloured precipitate was formed. The solution was stirred for 1 h at room temperature and then the precipitate was filtered off, washed with methanol (7 ml (6) Crystallization: Single crystals of [Fe(H 2 B(pz) 2 ) 2 (5-bromo-1,10-phenanthroline]Á2C 7 H 8 were obtained under a nitrogen atmosphere by dissolving microcrystalline [Fe(H 2 B(pz) 2 ) 2 (5bromo-1,10-phenanthroline] in dry toluene and overlaying with dry n-hexane. After a few days, purple-coloured single crystals were obtained that were collected and dried under reduced pressure. Experimental details: Elemental analyses were performed using a vario MICRO cube CHNS element analyser from Elementar. Samples were burned in sealed tin containers by a stream of oxygen. High-resolution ESI mass spectra were recorded on a ThermoFisher Orbitrap spectrometer. IR spectra were recorded on a Bruker Alpha-P ATR-IR Spectrometer. Signal intensities are marked as s (strong), m (medium), w (weak) and br (broad). For FT-Raman spectroscopy, a Bruker RAM II-1064 FT-Raman Module, a R510-N/R Nd:YAG-laser (1046 nm, up to 500 mW) and a D418-T/R liquid-nitrogen-cooled, highly sensitive Ge detector or a Bruker IFS 66 with a FRA 106 unit and a 35 mW NdYAG-LASER (1064 nm) were used. XRPD experiments were performed with a Stoe Transmission Powder Diffraction System (STADI P) with Cu K radiation ( = 1.5406 Å ) that is equipped with position-sensitive detectors (Mythen-K1). UV/ vis spectra were recorded with a Cary 5000 spectrometer in transmission geometry. The magnetic measurement was performed at 1 T between 300 and 2 K using a physical property measurement system (PPMS) from Quantum Design. Diamagnetic corrections were applied with the use of Pascal's constants (Bain & Berry, 2008).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. C-bound H atoms were positioned with idealized geometry and refined with U iso (H) = 1.2U eq (C) using a riding model. B-bound H atoms were located in a  difference map, their bond lengths were set to ideal values, and finally they were refined with U iso (H) = 1.2U eq (B) using a riding model. The asymmetric unit contains two toluene solvate molecules, of which one is severely disordered. Its contribution to the intensity data was removed using the SQUEEZE (Spek, 2015) routine in PLATON (Spek, 2020). The disordered toluene molecule was not taken into account in the calculation of the molecular formula and the molecular weight.

(5-Bromo-1,10-πhenanthroline-κ 2 N,N′)bis[dihydrobis(pyrazol-1-yl)borato-κ 2 N 2 ,N 2′ ]iron(II) toluene disolvate
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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Fe1 0.01316 (5