Crystal structure of {N 1,N 3-bis[(1-tert-butyl-1H-1,2,3-triazol-4-yl)methylidene]-2,2-dimethylpropane-1,3-diamine}bis(thiocyanato)iron(II)

The title charge-neutral complex shows a cis-arrangement of the thiocyanate anions, with a severely distorted coordination octahedron. The three-dimensional supramolecular architecture of the lattice is formed by weak C—H⋯C/S/N hydrogen bonds.

The unit cell of the title compound, [Fe II (NCS) 2 (C 19 H 32 N 8 )], consists of two charge-neutral complex molecules. In the complex molecule, the tetradentate ligand N 1 ,N 3 -bis[(1-tert-butyl-1H-1,2,3-triazol-4-yl)methylene]-2,2-dimethylpropane-1,3-diamine coordinates to the Fe II ion through the N atoms of the 1,2,3-triazole and aldimine groups. Two thiocyanate anions, also coordinated through their N atoms, complete the coordination sphere of the central Fe ion. In the crystal, neighbouring molecules are linked through weak C-HÁ Á ÁC/S/N interactions into a three-dimensional network. The intermolecular contacts were quantified using Hirshfeld surface analysis and two-dimensional fingerprint plots, revealing the relative contributions of the contacts to the crystal packing to be HÁ Á ÁH 50.8%, HÁ Á ÁC/CÁ Á ÁH 14.3%, HÁ Á ÁS/SÁ Á ÁH 20.5% and HÁ Á ÁN/NÁ Á ÁH 12.1%. The average Fe-N bond distance is 2.170 Å , indicating the high-spin state of the Fe II ion, which does not change upon cooling, as demonstrated by low-temperature magnetic susceptibility measurements. DFT calculations of energy frameworks at the B3LYP/6-31 G(d,p) theory level were performed to account for the interactions involved in the crystal structure.

Chemical context
An interesting class of coordination compounds exhibiting spin-state switching between low-and high-spin states is represented by Fe II complexes based on Schiff bases derived from N-substituted 1,2,3-triazole aldehydes (Hagiwara et al., 2014(Hagiwara et al., , 2020Hora & Hagiwara, 2017). In all of the chargeneutral mononuclear complexes of this kind described so far, the thiocyanate anions occupy the axial position in the coordination sphere and thus are in a trans-configuration (Hagiwara & Okada, 2016;Hagiwara et al., 2017).

Structural commentary
The Fe II ion of the title complex has a distorted trigonalprismatic N 6 coordination environment formed by the four N atoms of the tetradentate Schiff-base ligand and the two NCS À counter-ions (Fig. 1). The average bond length, <Fe-N> = 2.170 (4) Å , is typical for high-spin complexes with an [FeN 6 ] chromophore (Gü tlich & Goodwin, 2004). The N-Fe-N 0 angle between the cis-aligned thiocyanate N atoms is 91.91 (8) . The average trigonal distortion parameters, AE = AE 1 12 (|90 -' i |), where ' i is the angle N-Fe-N 0 (Drew et al., 1995), Â = AE 1 24 (|60i |), where i is the angle generated by superposition of two opposite faces of an octahedron (Chang et al., 1990) are 127.8 and 438.2 , respectively. The values reveal a great deviation of the coordination environment from an ideal octahedron (where AE = Â = 0), and are significantly larger than those of similar [FeN 6 ] high-spin trans-complexes . With the aid of continuous shape measurements (CShM), the closest shape of a coordination polyhedron and its distortion can be determined numerically (Kershaw Cook et al., 2015). The calculated CShM value relative to ideal O h symmetry is 3.829, while it is 6.709 relative to the ideal D 3h trigonal-prismatic symmetry. Hence, the polyhedron is closer to the former geometry, but is still appreciably distorted, as indicated by the calculated value (for an ideal polyhedron CShM = 0). The volume of the [FeN 6 ] coordination polyhedron is 12.60 Å 3 .

Figure 2
The packing of molecules into the three-dimensional network held together by weak C-HÁ Á ÁC/S bonding (dashed cyan lines).

Figure 1
The molecular structure of the title compound with displacement ellipsoids drawn at the 50% probability level. H atoms have been omitted for clarity.

Hirshfeld surface and 2D fingerprint plots
Hirshfeld surface analysis was performed and the associated two-dimensional fingerprint plots were generated using Crystal Explorer (Turner et al., 2017), with a standard resolution of the three-dimensional d norm surfaces plotted over a fixed colour scale of À0.1141 (red) to 1.9978 (blue) a.u. The pale-red spots symbolize short contacts and negative d norm values on the surface correspond to the interactions described above. The overall two-dimensional fingerprint plot is illustrated in Fig. 3. The Hirshfeld surfaces mapped over d norm are shown for the HÁ Á ÁH, HÁ Á ÁC/CÁ Á ÁH, HÁ Á ÁS/SÁ Á ÁH, and HÁ Á ÁN/ NÁ Á ÁH contacts, and the two-dimensional fingerprint plots are presented in Fig. 4, associated with their relative contributions to the Hirshfeld surface. At 50.8%, the largest contribution to the overall crystal packing is from HÁ Á ÁH interactions, which are located in the middle region of the fingerprint plot. HÁ Á ÁC/ CÁ Á ÁH contacts contribute 14.3%, and the HÁ Á ÁS/SÁ Á ÁH contacts contribute 20.5% to the Hirshfeld surface, both resulting in a pair of characteristic wings. The HÁ Á ÁN/NÁ Á ÁH contacts, represented by a pair of sharp spikes in the fingerprint plot, make a 12.1% contribution to the Hirshfeld surface.

Energy frameworks
The energy frameworks, calculated using the wave function at the B3LYP/6-3G(d,p) level of theory for the title compound, including the electrostatic potential forces (E ele ), the dispersion forces (E dis ) and the total energy diagrams (E tot ), are shown in Fig. 5a. The cylindrical radii, adjusted to the same scale factor of 80, are proportional to the relative strength of the corresponding energies (Turner et al., 2017;Tan et al., 2019). It can be seen that the major contribution to the intermolecular interactions is from Coulomb forces (E ele ), reflecting dipole-dipole interactions of the asymmetric complex cis-molecules in the lattice. According to the calculations, the most repulsive interaction is due to the anion-to-   anion alignment of neighbouring complex molecules (E tot = 65.3 kJ mol À1 ) while the ligand-to-anion alignment gives the most attractive one (E tot = À223.9 kJ mol À1 ) (Fig. 5b). The colour-coded interaction mappings within a radius of 3.8 Å of a central reference molecule for the title compound together with full details of the various contributions to the total energy (E tot ) are given in the Supporting Information.

Magnetic properties
Variable-temperature magnetic susceptibility measurements were performed on single crystals (10 mg) of the title compound using a Quantum Design MPMS2 superconducting quantum interference device (SQUID) susceptometer operating at 1 T. Experimental susceptibilities were corrected for the diamagnetism of the holder (gelatine capsule) and of the constituent atoms by the application of Pascal's constants. The magnetic behaviour of the compound is shown in Fig. 6 in the form of M T versus T ( M is the molar magnetic susceptibility and T is the temperature). At 300 K, the M T value is close to 3.51 cm 3 K mol À1 , and on cooling the value remains constant

Figure 5
(a) The calculated energy frameworks, showing the electrostatic potential forces (E ele ), the dispersion forces (E dis ) and the total energy diagrams (E tot ). Yellow coloured tubes correspond to the repulsive interactions; (b) the strongest repulsive and attractive interactions between neighbouring complex molecules.

Figure 6
M T versus T plot for the title compound.
down to 30 K. The decrease of M T below 30 K is attributed to the zero-field splitting of the high-spin (S = 2) Fe II centres (Kahn, 1993), which corroborates with the observed long average Fe-N bond length and the large geometric distortion of the coordination polyhedron of the central Fe II ion.  (Znovjyak et al., 2020(Znovjyak et al., , 2021. Table 2 collates the distortion parameters AE, Â and CShM for the pseudo-trigonal-prismatic complexes mentioned above.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms were positioned geometrically (C-H = 0.93-0.97 Å ) and refined as riding with U iso (H) = 1.2U eq (C) or 1.5U eq (C-methyl).

{N 1 ,N 3 -Bis[(1-tert-butyl-1H-1,2,3-triazol-4-yl)methylidene]-2,2-dimethylpropane-1,3-diamine}bis(thiocyanato)iron(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.

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