Crystal structure of bis[dihydrobis(pyrazol-1-yl)borato-κ2 N 2,N 2′](1,10-phenanthroline-κ2 N,N′)zinc(II)

In the crystal structure of the title compound, the ZnII cation is octahedrally coordinated by the N atoms of a chelating phenanthroline ligand and the N atoms of two symmetry-related dihydrobis(pyrazol-1-yl)borate ligands into discrete complexes.

The asymmetric unit of the title compound, [Zn(C 6 H 8 N 4 B) 2 (C 12 H 8 N 2 )], comprises one half of a Zn II cation (site symmetry 2), one dihydrobis(pyrazol-1-yl)borate ligand in a general position, and one half of a phenanthroline ligand, the other half being completed by twofold rotation symmetry. The Zn II cation is coordinated in form of a slightly distorted octahedron by the N atoms of a phenanthroline ligand and by two pairs of N atoms of symmetry-related dihydrobis(pyrazol-1-yl)borate ligands. The discrete complexes are arranged into columns that elongate in the c-axis direction with a parallel alignment of the phenanthroline ligands, indicating weakinteractions.

Chemical context
Spin-crossover transition-metal complexes (3d 4 -3d 7 ) continue to be a fascinating class of functional materials in the field of coordination chemistry and have the potential to play a significant role in electronic data storage or in spintronics (Gü tlich et al., 2013;Halcrow, 2013). Transitions 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 by stimuli such as temperature or light. In most cases, spin-crossover complexes are based on octahedral [Fe II N 6 ] coordination environments with chelating or monocoordinating nitrogen donor ligands. From all metal ions and ligands leading to spin-crossover complexes, the Fe II /nitrogen ligand combination leads to the greatest changes in metalligand bond lengths between the two spin states and so far to the longest-lived photochemical excited spin state (Halcrow, 2007). Since the beginning of this research area some several decades ago, this field has been directed towards applications using the change of the magnetic and electronic properties of the spin-crossover compounds associated with the spin transition. Regarding applications, it might be advantageous to deposit spin-crossover complexes as thin films on substrates. This can be achieved by different methods of which physical vapour deposition is the most practicable because the formation of solvates can be ruled out. In this context, we have deposited various complexes with organoborate ligands mainly based on dihydrobis(pyrazol-1-yl)borate on different substrates (Naggert et al., 2011(Naggert et al., , 2015Ossinger et al., 2017;Gopakumar et al., 2012;Kipgen et al., 2018).
In the course of this project we became interested in the well-known iron spin-crossover complex [Fe(H 2 B(pz) 2 ) 2 -(phen)] ((H 2 B(pz) 2 ) 2 = bis(dihydrobis(pyrazol-1-yl)borate); phen = 1,10-phenanthroline). To make conclusions regarding ISSN 2056-9890 the behaviour of [Fe(H 2 B(pz) 2 ) 2 (phen)] on substrates such as, for example, graphene, quantum-chemical calculations using the xTB program (Grimme et al., 2017;Bannwarth et al., 2019) are useful. We are especially interested in structural details of the high-spin state, but unfortunately for iron(II) complexes the geometry optimization always leads to the low-spin state. To overcome this problem, corresponding compounds with Zn II can be used in the calculation, because their geometry is close to that of Fe II compounds in the high-spin state. This approach is beneficial because the calculation of diamagnetic compounds is simpler and, in addition, diamagnetic compounds can easily be investigated by NMR spectroscopy. Therefore, Zn II complexes are often used as model systems for high-spin iron(II) complexes (Seredyuk et al., 2007;Schenker et al., 2001). The ionic radii (Shannon, 1976) for Zn II cations (3d 10 , 1 S) are nearly the same as for Fe II cations in the highspin state (3d 6 , 5 T 2 ), frequently leading to the formation of isotypic compounds.
With these consideration in mind, [Zn(H 2 B(pz) 2 ) 2 (phen)] was synthesized, crystallized and investigated by single crystal X-ray diffraction. The X-ray powder pattern revealed that a pure compound was obtained (see Fig. S1 in the supporting information) that is suitable for physical vapour deposition, in analogy to the Fe II analogue (Naggert et al., 2011(Naggert et al., , 2015Ossinger et al., 2017). Comparison of the infrared spectra from the bulk and vacuum-deposited Zn II compound shows identical vibrational modes, proving that no decomposition takes place during deposition (Fig. S2).

Structural commentary
[Zn(H 2 B(pz) 2 ) 2 (phen)] is isotypic with the Fe II analogue (Real et al., 1997). The asymmetric unit of the title compound consists of one dihydrobis(pyrazol-1-yl)borate ligand, one half of a Zn II cation located on a twofold rotation axis and one half of a phenanthroline ligand, the other half being completed by application of twofold rotation symmetry. The Zn II cation is coordinated by the N atoms of the chelating phenanthroline ligand and by two pairs of N atoms of two symmetry-related dihydrobis(pyrazol-1-yl)borate ligands, leading to a slightly distorted octahedral coordination environment (Fig. 1), as shown by the different bond lengths and angles deviating from ideal values ( Table 1). The Zn-N bond lengths involving the dihydrobis(pyrazol-1-yl)borate ligand are 2.1454 (18) and 2.1705 (18) Å and thus are significantly shorter than those to the phenanthroline ligand of 2.2101 (19) Å . The planes of the five-membered rings of the dihydrobis(pyrazol-1-yl)borate ligand are rotated with respect to each other by 44.4 (2) .

Supramolecular features
In the crystal structure of the title compound, the discrete complexes are arranged into columns that elongate in the caxis direction (Fig. 2). Within these columns, the phenanthroline ligands are parallel but shifted relative to each other (Fig. 3). The shortest distance between two parallel phenanthroline planes amounts to 3.9341 (11) Å , indicative of weak interactions.

Synthesis and crystallization
1H-pyrazole, potassium tetrahydroborate, zinc perchlorate hexahydrate and 1,10-phenanthroline were purchased and used without further purification. Solvents were purchased and purified by distilling over conventional drying agents. K[H 2 B(pz) 2 ] and [Zn(H 2 B(pz) 2 ) 2 (phen)] were synthesized according to previously reported procedures (Naggert et al., 2011(Naggert et al., , 2015Ossinger et al., 2016Ossinger et al., , 2017. Synthesis of [Zn(H 2 B(pz) 2 ) 2 (phen)]: To a solution of Zn(ClO 4 ) 2 Á6H 2 O (746 mg, 2.00 mmol) in methanol (10 ml) a solution of K[H 2 B(pz) 2 ] (744 mg, 4.00 mmol) in methanol (10 ml) was added. After 15 min of stirring, precipitated KClO 4 was removed by filtration. To the filtrate a solution of 1,10-phenanthroline (361 mg, 2.00 mmol) in methanol (10 ml) was added dropwise, leading to the formation of a colourless precipitate. The mixture was stirred for another hour at room temperature and the precipitate was filtered off, washed with methanol (5 ml) and filtered again by suction filtration (30 min     Crystallization: Single crystals of [Zn(H 2 B(pz) 2 ) 2 (phen)] were obtained under synthetic conditions as described above. After the precipitate was filtered off and washed with methanol, the mother liquor was stored at 278 K. After a few days colourless block-like single crystals had formed.
Experimental details: NMR spectra were recorded in deuterated solvents with a Bruker Avance 400 spectrometer operating at a 1 H frequency of 400 MHz, a 13 C frequency of 100 MHz and a 11 B frequency of 128 MHz. They were referenced to the residual protonated solvent signal [ 1 H: (CDCl 3 ) = 7.26 ppm], the solvent signal [ 13 C: (CDCl 3 ) = 77.16 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, 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 À1064 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 NdYAG-LASER (1064 nm) was 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.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The H atoms were positioned with idealized geometry (C-H = 0.93 Å ) and were refined with U iso (H) = 1.2U eq (C) using a riding model. The B-H hydrogen atoms were located in a difference-Fourier map. Their bond lengths were set to ideal values (B-H = 0.97 Å ) and finally they were refined with U iso (H) = 1.5U eq (B) using a riding model. & Cie, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: publCIF (Westrip, 2010).

Bis[dihydrobis(pyrazol-1-yl)borato-κ 2 N 2 ,N 2′ ](1,10-\ phenanthroline-κ 2 N,N′)zinc(II)
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.