Crystal structure of bis{(3,5-dimethylpyrazol-1-yl)dihydro[3-(pyridin-2-yl)pyrazol-1-yl]borato}iron(II)

The FeII atom in the title complex, [Fe{H2B(3,5-(CH3)2-pz)(pypz)}2] (pz = pyrazole, pypz = pyridylpyrazole), is coordinated by two tridentate {H2B(pz)(pypz)}− ligands in form of a distorted N6 octahedron.


Chemical context
Spin-crossover (SCO) complexes of transition-metal cations (3d 4 -3d 7 ) are a fascinating class of functional materials with potential for applications in electronic data storage or in spintronics (Gü tlich et al., 2013;Halcrow, 2013). The transition between the diamagnetic low-spin state (S = 0 for Fe II ) and the paramagnetic high-spin state (S = 2 for Fe II ) of such complexes can be induced via temperature or light as stimuli. In most cases, SCO complexes are based on octahedral [Fe II N 6 ] coordination spheres with chelating or mono-coordinating nitrogen donor ligands, because these combinations lead to the largest metal-ligand bond length differences between the two spin states and the largest lifetimes of the photoexcited spin states (Halcrow, 2007). Whereas hundreds of Fe II SCO complexes have been reported (Halcrow, 2007), only a few of them are based on organoborate ligands such as [Fe(H 2 B(pz) 2 ) 2 (L)] (pz = pyrazole; L = di-imine co-ligand) or tripodal organoborate ligands such as [Fe(HB(pz) 3 ] (pz = pyrazole and derivatives thereof). These compounds are of special interest because most of them, as we and other research groups have shown, are suitable for physical vapour deposition, which is one important requirement for a possible application of these materials (Ruben & Kumar, 2019;Naggert et al., 2015;Ossinger et al., 2020a). Notably, bidentate compounds of the type [Fe(H 2 B(pz) 2 ) 2 (L)] have been found to dissociate into the tetrahedral complex [Fe(H 2 B(pz) 2 ) 2 ] and the free co-ligand (Gopakumar et al., 2013) in the first (sub)monolayer on Au(111), whereas the SCO complex [Fe(HB(3, ) 2 -pz) 3 ) 2 ] supported by a tridentate tris-(pyrazolyl)borate ligand can be adsorbed without fragmen- ISSN 2056-9890 tation on an Au(111) surface in a submonolayer (Bairagi et al., 2016(Bairagi et al., , 2018. Along these lines, we synthesized and characterized the first neutral and vacuum-evaporable SCO complex based on a linear tridentate organoborate ligand. The new complex [Fe{H 2 B(pz)(pypz)} 2 ] was found to crystallize in two polymorphs, I (T 1/2 = $270 K) and II (T 1/2 = $390 K), with form II exhibitinginteractions that are absent in form I (Ossinger et al., 2020c). To investigate a possible correlation between the spin-transition temperature (T 1/2 ) and the presence ofinteractions in more detail, we decided to modify the complex [Fe{H 2 B(pz)(pypz)} 2 ] by replacing 1Hpyrazole with 3,5-dimethyl-pyrazole in the tridentate ligand. This led to the title complex, [Fe{H 2 B(3,5-(CH 3 ) 2 -pz)(pypz)} 2 ], which was characterized by single crystal X-ray diffraction. The corresponding X-ray powder diffraction pattern revealed that the employed synthetic route yields a pure complex (see Fig. 1 in the supporting information). It was found to be suitable for physical vapour deposition, in analogy to the parent system [Fe{H 2 B(pz)(pypz)} 2 ] (Ossinger et al., 2020c). Comparison of the infrared spectra from the bulk and the vacuum-deposited compound shows identical vibrational modes, indicating that no decomposition takes place upon vacuum evaporation and deposition (Fig. S2). Magnetic measurements revealed the presence of the high-spin state in the temperature range from 25 K to 300 K (Fig. S3), in contrast to the parent system and its two polymorphs, which exhibit the low-spin in polymorph II and SCO behaviour in polymorph I. Moreover, the crystal structure of the title compound is devoid ofinteractions, similar to polymorph I of the parent complex [Fe{H 2 B(pz)(pypz)} 2 ]. As the latter shows thermally induced spin crossover, this indicates that the introduction of methyl groups has shifted the magnetic properties of the parent complex into the high-spin regime.

Structural commentary
The asymmetric unit of the title compound consists of one discrete complex in a general position. The central Fe II atom is coordinated by six N atoms of two tridentate mono-anionic {H 2 B(3,5-(CH 3 ) 2 -pz)(pypz)} ligands in a slightly distorted octahedral environment (Fig. 1), as shown by different bond lengths and angles deviating from ideal values ( To characterize the distortion in more detail, the structural parameters AE and Â were calculated with the aid of the program OctaDist (OctaDist, 2019). AE is calculated from the 12 cis-N-Fe-N angles and is a general measure of the deviation from an ideal octahedron. Â is calculated from 24 unique N-Fe-N angles measured on the projection of two triangular faces of the octahedron along their common pseudo-threefold axis and indicates more specifically its distortion from an octahedral towards a trigonal-prismatic structure. For a perfectly octahedral complex AE = Â = 0 is valid (Guionneau et al., 2004;Iasco et al., 2017;Halcrow, 2013).

Figure 1
The molecular structure of the title compound with the atom labelling and displacement ellipsoids drawn at the 50% probability level.

Supramolecular features
In polymorph II of the parent system [Fe{H 2 B(pz)(pypz)} 2 ], individual complexes are pairwise linked to dimers by intermolecularinteractions between the pyridine rings of the ligands of neighbouring complexes (Ossinger et al., 2020c). In the crystal structure of the title compound, no parallel arrangements of pyridine rings and no intermolecularinteractions are observed (Fig. 2), as was the case for poly- Apart from a weak C-HÁ Á ÁN hydrogen bond (

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.
3,5-Dimethylpyrazole, 2-(1H-pyrazol-3-yl)pyridine and potassium tetrahydroborate were purchased from commercial sources and were used without further purification. Iron(II) triflate, which is also commercially available, 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 distillation over conventional drying agents.
Synthesis of K[H 2 B(3,5-(CH 3 ) 2 -pz)(pypz)]: Potassium tetrahydroborate (539 mg, 0.01 mol), 3,5-dimethylpyrazole (961 mg, 0.01 mol) and 2-(1H-pyrazol-3-yl)pyridine (1.45 g, 0.01 mol) were suspended in toluene (20 ml) and refluxed for 17 h. The solution was filtered whilst hot to remove any residual traces of unreacted K [BH 4 ]. The filtrate was allowed to cool to room temperature. A few hours later a white precipitate formed, and after one additional night of crystallization the precipitate was collected by suction filtration and subsequently dissolved in a few ml of acetonitrile. The resulting cloudy solution was again filtered by suction filtration. The solvent was removed in vacuo, and a white precipitate was obtained. Yield 260 mg (859 mmol, 9% vs K[BH 4 ]).
Elemental analysis calculated for C 13    crystalline material in dry toluene that was overlayed with dry n-hexane. This mixture was stored at 278 K, and after a few weeks long orange-coloured needle-like single crystals were formed. Experimental details: NMR spectra were recorded in deuterated solvents on a Bruker DRX500 spectrometer operating at a 1 H frequency of 500 MHz, a 13 C frequency of 125 MHz, and a 11 B frequency of 160 MHz. They were referenced to the residual protonated solvent signal [ 1 H: (CD 3 CN) = 1.94 ppm], the solvent signal [ 13 C: (CD 3 CN) = 118.26 ppm], or an external standard ( 11 B: BF 3 ÁEt 2 O) (Gottlieb et al., 1997;Fulmer et al., 2010). Signals were assigned with the help of DEPT-135 and two-dimensional correlation spectra ( 1 H, 1 H-COSY, 1 H, 13 C-HSQC, and 1 H, 13 C-HMBC). Signal multiplicities are abbreviated as s (singlet), d (doublet), t (triplet), m (multiplet), and br (broad signal). 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 35mW Nd:YAGlaser (1064 nm) were used. XRPD experiments were performed with a Stoe Transmission Powder Diffraction System (STADI P) with Cu K radiation ( = 1.5406 Å ) equipped with a position-sensitive detector (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 3. C-bound hydrogen atoms were positioned with idealized geometry (methyl H atoms allowed to rotate but not to tip) and were refined with U iso (H) = 1.2U eq (C) (1.5 for methyl H atoms) using a riding model. Bbound hydrogen atoms were located in a difference-Fourier map and were refined freely.  (Sheldrick, 2015b), DIAMOND (Brandenburg, 1999) and publCIF (Westrip, 2010  SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: publCIF (Westrip, 2010).

Bis{(3,5-dimethylpyrazol-1-yl)dihydro[3-(pyridin-2-yl)pyrazol-1-yl]borato}iron(II)
Crystal data [Fe(C 13  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.