Synthesis, crystal structure, and Hirshfeld surface analysis of 3-ferrocenyl-1-(pyridin-2-yl)-1H-pyrazol-5-amine

The multi-step synthesis, characterization, and structural examination (single-crystal X-ray diffraction and Hirshfeld surface analysis) of 3-ferrocenyl-1-(pyridin-2-yl)-1H-pyrazol-5-amine (C18H16FeN4), are reported. The supramolecular characteristics, including π–π stacking and hydrogen bonding, are discussed, and a database exploration highlights the distinctive combination of molecular components.


Chemical context
We have previously reported a pyrazole-based ligand scaffold, which incorporates groups for both cation and anion coordination, as well as the opportunity for functionalization with other moieties for practical applications, for example, fluorescent tags (Hiscock et al., 2019), or in the area of molecular magnetism.
One step towards achieving magnetic device applications for polynuclear metal-based systems is the strategic design of ligands such that resulting metal assemblies possess some type of 'switch ' [electrochemical, photo-induced, or other (Cador et al., 2019)].As an example, a single ion magnet switching process with a bis-diaminoferrocene-based ligand for Dy III yielded a chemically (iodine) induced one-electron reduction (Dickie et al., 2017).In this reversible process, a change in magnetization dynamics (in the absence of an applied DC field) characterized this system as an 'on/off' switch for slow magnetic relaxation.

Structural commentary
The molecular structure of 1 is shown in Fig. 1.A Mogul geometry search (Cottrell et al., 2012;Bruno et al., 2004) revealed only one unusual bond angle, present in the pyrazole ring, formed by C7-C8-N3.The experimental value reported for this angle in 1 is 112.3 (2) � , while the Mogul search revealed a mean value of 111.28 � with a standard deviation of 0.48 � based on 33 observations in the Cambridge Structural Database (Groom et al., 2016).It is noted that despite being flagged as unusual, the value for 1 lies within three standard deviations of that reported from the Mogul search.

Supramolecular features and Hirshfeld surface analysis
Examination of the crystal packing for 1 reveals short contacts between the mean pz-py planes, parallel to the b axis, with plane-to-plane centroid separations (i.e.shortest distance between planes) of 3.4790 (18) A ˚and a plane-to-plane shift of 2.006 (3) A ˚[measured from molecules generated by symmetry operations (ii) x, y, z to (iii) 1 À x, y À 1 2 , 1 2 À z; Fig. 2].Intermolecular hydrogen bonding from the pyrazole amine group to an adjacent pyrazole nitrogen acceptor (N4-H4A� � �N3 i ; Table 1) yields infinite chains [graph-set notation C 1 1 (5)] parallel to the c axis.Hirshfeld surface analysis (Spackman & Jayatilaka, 2009) was performed using CrystalExplorer17 (Spackman et al., 2021).Examination of the shape-index plot (Fig. 3) shows the same short pz-py planar contacts and perpendicular N-H� � �N hydrogen-bonding interactions, but also additional short contacts, indicated as red hollows (shape-index <1) and blue bumps (shape-index >1) representing complementary The asymmetric unit of 1, shown with displacement ellipsoids at the 50% probability level and hydrogen atoms as fixed-size spheres with radius of 0.15 A ˚.The intramolecular hydrogen bond is represented as a dashed line.Symmetry code: (i) x; À y þ 1 2 ; z À 1 2 .

Figure 2
Packing diagrams for 1, represented with displacement ellipsoids at the 50% probability level; (left) viewed down the c axis to show short contacts between pz-py planes of adjacent molecules; (right) viewed down the b axis to show intermolecular hydrogen-bonding perpendicular to the pz-py interplanar interactions.intermolecular interaction between donors and acceptor groups, respectively (Tan et al., 2019).These interactions are quantitatively summarized as 2D fingerprint plots (Fig. 4).In these plots,

Synthesis and crystallization
1 H NMR and 13 C NMR spectra were recorded on an Agilent Technologies Varian Unity Inova 300 or 400 MHz NMR spectrometer using the indicated deuterated solvents purchased from Sigma-Aldrich.Chemical shifts are reported in � scale in p.p.m. using the residual solvent peak (CDCl 3 , � = 7.260) as reference.2-Hydrazinylpyridine was prepared from 2-bromopyridine using a modified literature procedure (Klingele et al., 2010), as we have previously reported (Hiscock et al., 2019).�-Chloroacetylferrocene (Yang et al., 2007) and 3-oxo-3-(ferrocenyl)propanenitrile (Rao & Muthanna, 2016) were also prepared via modified literature procedures.All other reagents and starting materials were purchased from Sigma-Aldrich and used as purchased.

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 Figure 1

Figure 3 (
Figure 3 (Left) Hirshfeld shape index surface for the asymmetric unit of 1, viewed down the b axis; and (right) with symmetry-related molecules making short contacts with the asymmetric unit.Transparent surface representations, with ball-and-stick molecular model on the left, and with molecular bonds represented as tubes and hydrogen bonds as dashed lines on the right.Hydrogen atoms were generated in normalized neutron X-H positions by CrystalExplorer17 (Spackman et al., 2021).
i is plotted on the x-axis and represents the distance to the nearest nucleus inside the Hirshfeld surface, and d e is plotted on the y-axis, and represents the distance from the Hirshfeld surface to the nearest nucleus outside the surface.These fingerprint plots indicate weak (blue and bluegreen) van der Waals H� � �H contacts as the dominant packing interaction (66.9% of the overall surface) in 1, with C� � �H/ H� � �C contacts [i.e.C-H� � ��/�� � �C-H contacts(Tan et al., 2019)] contributing 12.4% of the Hirshfeld area, and N� � �H/ H� � �N, N� � �C/C� � �N, and C� � �C interactions contributing 7.8%, 6.8% and 6.1% of interactions, respectively.Note that, as expected, these plots are pseudo-mirrored along the diagonal, i.e.where d e and d i have the same value.

A
search of the Cambridge Structural Database (Conquest Version 2023.1.0;CSD version 5.44 with April 2023 updates;Groom et al., 2016) yielded 6635 carbon-functionalized monosubstituted ferrocene structures.Narrowing the search parameters to monosubstituted 3-ferrocenyl-1H-pyrazole structures yielded 96 structures with available coordinates, while further limiting the search to require the presence of a 1Hpyrazol-5-amine group resulted in zero previously reported structures.This demonstrates the unique combination of elements in the molecular structure, each of which have been incorporated for a purpose (redox activity, cation coordination, and hydrogen bonding), which we hope to demonstrate in future studies.

Figure 4
Figure 4Fingerprint plots showing all close contacts in the crystal structure of 1 (top left), and (other plots) the contributions of the total interactions by H� � �H, C� � �H, N� � �H, N� � �C and C� � �C contacts.Plots were generated using CrystalExplorer17(Spackman et al., 2021).

Table 2
Experimental details.