Synthesis, crystal structure and Hirshfeld analysis of trans-bis(2-{1-[(6R,S)-3,5,5,6,8,8-hexamethyl-5,6,7,8-tetrahydronaphthalen-2-yl]ethylidene}-N-methylhydrazinecarbothioamidato-κ2 N 2,S)palladium(II) ethanol monosolvate

The synthesis, crystal structure and Hirshfeld analysis of the first complex with the (R,S)-fixolide 4-methylthiosemicarbazonato ligand is reported. A hydrogen-bonded macrocyclic environment type is observed for the PdII homoleptic complex. In the crystal, the complexes and the ethanol solvate molecules are linked by H⋯S and H⋯O interactions, forming mono-periodic hydrogen-bonded ribbons along [011].

The reaction between the (R,S)-fixolide 4-methylthiosemicarbazone and Pd II chloride yielded the title compound, [Pd(C 20 H 30 N 3 S) 2 ]•C 2 H 6 O {common name: trans-bis[(R,S)-fixolide 4-methylthiosemicarbazonato-� 2 N 2 S]palladium(II) ethanol monosolvate}.The asymmetric unit of the title compound consists of one bis-thiosemicarbazonato Pd II complex and one ethanol solvent molecule.The thiosemicarbazononato ligands act as metal chelators with a trans configuration in a distorted square-planar geometry.A C-H� � �S intramolecular interaction, with graph-set motif S(6), is observed and the coordination sphere resembles a hydrogen-bonded macrocyclic environment.Additionally, one C-H� � �Pd anagostic interaction can be suggested.Each ligand is disordered over the aliphatic ring, which adopts a half-chair conformation, and two methyl groups [s.o.f.= 0.624 (2):0.376(2)].The disorder includes the chiral carbon atoms and, remarkably, one ligand has the (R)-isomer with the highest s.o.f.value atoms, while the other one shows the opposite, the atoms with the highest s.o.f.value are associated with the (S)-isomer.The N-N-C( S)-N fragments of the ligands are approximately planar, with the maximum deviations from the mean plane through the selected atoms being 0.0567 (1) and À 0.0307 (8) A (r.m.s.d.= 0.0403 and 0.0269 A ˚) and the dihedral angle with the respective aromatic rings amount to 46.68 (5) and 50.66 (4) � .In the crystal, the complexes are linked via pairs of N-H� � �S interactions, with graph-set motif R 2 2 (8), into centrosymmetric dimers.The dimers are further connected by centrosymmetric pairs of ethanol molecules, building mono-periodic hydrogen-bonded ribbons along [011].The Hirshfeld surface analysis indicates that the major contributions for the crystal cohesion are [atoms with highest/lowest s.o.f.s considered separately]: H� � �H (81.6/82.0%),H� � �C/C� � �H (6.5/6.4%),H� � �N/N� � �H (5.2/5.0%) and H� � �S/S� � �H (5.0/4.9%).

Chemical context
One of the first reports concerning thiosemicarbazone chemistry was published more than a century ago (Freund & Schander, 1902).These molecules, with the [R 1 R 2 N-N(H)-C( S)-NR 3 R 4 ] functional group, were observed as the major product of the reactions between thiosemicarbazide derivatives [H 2 N-N(H)-C( S)-NR 3 R 4 ] and aldehydes or ketones (R 1 R 2 C O). Indeed, thiosemicarbazides were employed as analytical reagents in the organic chemistry for the detection of the carbonyl group (R 1 R 2 C O). From those early times, thiosemicarbazones emerged as a class of compounds with applications in a wide range of scientific disciplines.A milestone of this chemistry was the report of the biological activity as chemotherapeutic agents against tuberculosis in in vitro essays, published in the mid-1940s (Domagk et al., 1946).
As a result of the huge structural diversity of thiosemicarbazone derivatives, because of the large number of aldehydes and ketones employed in synthesis, several applications for metals, e.g., palladium(II) are observed.The [N-N(H)-C( S)-N] fragment, and its anionic form, are very efficient ligands, since hard (N) and soft (S) Lewis-base behaviors are present in the same atom chain.In addition, the N-N-S-N entity can adopt different geometries, coordinating metal centers in diverse bonding modes (Lobana et al., 2009).
The applications of thiosemicarbazone derivatives in palladium chemistry range from analytical chemistry, e.g., the spectrophotometric determination of Pd II in different matrices, as for example alloys and complexes (Karthikeyan et al., 2011), to their use as reagents for the synthesis of palladium nanoparticles for Suzuki-Miyaura cross-coupling catalysis (Kovala-Demertzi et al., 2008) and the synergetic effect of thiosemicarbazones with palladium(II) has led to the development of catalysts for organic chemistry (Priyarega et al., 2022).Furthermore, in the field of materials science, a palladium(II) coordination compound, with the 4-{bis[4-(p-methoxyphenyl)thiosemicarbazone]}-2,3-butane derivative, has found application in electrocatalytic hydrogen production (Straistari et al., 2018), which is an important topic for energy research today.Finally, bioinorganic chemistry is one of the most relevant approaches for thiosemicarbazone chemistry (Aly et al., 2023;Singh et al., 2023).
Herein, as part of our interest in thiosemicarbazone chemistry, we report the synthesis, crystal structure and Hirshfeld analysis of the first fixolide 4-methylthiosemicarbazonato palladium(II) complex.

Structural commentary
The asymmetric unit of the title compound consists of one bisthiosemicarbazonato Pd II complex and one ethanol solvate molecule.The coordination compound is composed of a palladium(II) center and two (R,S)-fixolide 4-methylthiosemicarbazonato ligands, which act as metal chelators, � 2 N 2 ,Sdonors, and form five-membered metallarings in a trans-configuration.An intramolecular C24-H24C� � �S1 hydrogen bond is observed, with a graph-set motif of S( 6), and the coordination sphere of the metal center resembles a hydrogen-bonded macrocyclic environment (Fig. 1, Table 1).The Pd II metal center is fourfold coordinated in a distorted square-planar geometry: the N3-Pd1-N6 and S1-Pd1-S2 angles are 178.02(5) and 164.63 (2) � , while the maximum deviation from the mean plane through the Pd1/N3/N6/S1/S2 atoms amounts to 0.1722 (4) A ˚for S1 [the r.m.s.d. for the selected atoms is 0.1409 A ˚] and the torsion angles between the N3-N2-C2-S1 and N6-N5-C22-S2 chains amount to À 5.6 (2) and À 1.7 (2) � .Additional structural data concerning the N/N/C/S/N entities are given in Table 2.
In addition, a C24-H24C� � �Pd1 weak anagostic interaction can be suggested (Fig. 2).The angle between the C-H� � �M atoms is 117.78 (2) � and the H� � �Pd distance amounts to 2.8235 (7) A ˚, which lies in the upper limit for these interactions.For an agostic interaction, which involves a covalent or a three-center and two-electron bond, an H� � �M distance of at least 2.3 A ˚is required and the C-H� � �M angle should range between 90 and 140 � .For an anagostic interaction that is assigned with an electrostatic nature, the H� � �M distance should range from 2.3 to 2.9 A ˚and the C-H� � �M angle between 110 and 170 � (Brookhart et al., 2007).For an article  that corroborates with the H24C� � �Pd1 anagostic interaction of the title compound, see also: Derry Holaday et al. (2014).
In the complex, the thiosemicarbazonato ligands are disordered over the aliphatic rings and two of the methyl groups [site-occupancy ratio = 0.624 (2):0.376(2)], with the Alabeled atoms having the highest s.o.f.value and the B-labeled atoms, the lowest (Fig. 1).For both ligands, the disorder includes the carbon chiral atoms (C10 and C30) and thus, (R)and (S)-isomers are observed.The C10A-HA and C10B-HB bonds are in opposite directions, and the (R)isomer is assigned for the A-labeled atoms [s.o.f.= 0.624 (2)].For the case of the C30A-H30A and C30B-H30B bonds, the (R)-isomer is assigned to the B-labeled atoms [s.o.f.= 0.376 (2)].This inverted site-occupancy ratio for the (R,S)isomery in the two ligands is a remarkable feature of the complex structure.Selected structural data parameters are provided in Tables 2 and 3.
Finally, the anionic form of the (R,S)-fixolide 4-methylthiosemicarbazonato ligands is confirmed by the absence of the H acidic hydrazinic atom and by the changes on the bond lengths over the N-N-C-S fragment.In a neutral, noncoordinated, thiosemicarbazone derivative, N-N(H)-C S entity, the H hydrazinic atom is present, the N-N and N-C distances are characteristic for single bonds, while the C S distance indicates a double bond.When the thiosemicarbazone is deprotonated with a base, e.g.NaOH, the negative charge is delocalized over the N-N-C-S chain and the values for the chemical bonds distances tend to intermediate lengths.Thus, the N-N bond length tends to be longer, maintaining single-bond character, the N-C bond lengths tend to be shorter, suggesting a double-bond character and the C-S bond lengths tend to be longer, indicating a single-bond character (Table 4).

Supramolecular features
In the crystal, the coordination compounds are connected through N-H� � �S interactions into centrosymmetric dimers with graph-set R 2 2 (8) (Fig. 3, Table 1).These dimers can be considered subunits of a hydrogen-bonded ribbon, since they are further linked by centrosymmetric pairs of ethanol solvate molecules through N-H� � �O-H� � �S bridges (Fig. 4) into mono-periodic hydrogen-bonded ribbons along [011] (Fig. 5).The O1 atoms serve as hydrogen-atom acceptors and donors and the S1 atoms act as bifurcated hydrogen-atom acceptors.
For the title compound, the Hirshfeld surface analysis (Hirshfeld, 1977), the graphical representations and the twodimensional Hirshfeld surface fingerprint were performed with Crystal Explorer software (Wolff et al., 2012).The Hirshfeld surface analysis of the (R)-isomer structure of the The maximum deviations from the mean plane through the N/N/C/S/N entities, the r.m.s.deviations of the selected atoms and the dihedral angle with the respective aromatic rings for the title compound (A ˚, � ).The maximum deviations from the mean plane through the aliphatic rings and the respective r.m.s.deviations of the selected atoms for the title compound (A ˚).

Figure 3
Graphical representation of the H� � �S intermolecular interactions for the complex of the title compound, forming a graph-set motif of R 2 2 (8) and linking the molecules into centrosymmetric dimers.The solvate molecule is omitted and the figure is simplified for clarity [Symmetry code: (i) title compound indicates that the most relevant intermolecular interactions for crystal cohesion are the following: H� � �H (81.6%),H� � �C/C� � �H (6.5%), H� � �N/N� � �H (5.2%) and H� � �S/S� � �H (5.0%).Just for comparison, the (S)-isomer values amount to H� � �H (82.0%),H� � �C/C� � �H (6.4%), H� � �N/N� � �H (5.0%) and H� � �S/S� � �H (4.9%) and are quite similar to the results for the (R)-isomer.Since no considerable differences between the isomers was observed, the further evaluations and graphics were performed for the (R)-isomer only, which has the highest s.o.f.value.The graphical representations of the Hirshfeld surface for the trans-bis[(R,S)fixolide 4-methylthiosemicarbazonato-� 2 N 2 S]palladium(II) and the ethanol solvate molecule are represented with transparency and using the ball-and-stick model (Fig. 6).The locations of the strongest intermolecular contacts, i.e, the regions around the H1, H3, S1 and S2 atoms are indicated in magenta.These atoms are those involved in the H� � �S interactions shown in previous figures (Figs. 3,4 and 5).The contributions to the crystal packing are shown as twodimensional Hirshfeld surface fingerprint plots (HSFP) with cyan dots (Fig. 7).The  Pd(C 20 H 30 N 3 S) 2 ]•C 2 H 6 O 1169   Table 4 Selected torsion angles for disordered fixolide derivatives ( � ).

Database survey
To the best of our knowledge and from using database tools such as SciFinder (Chemical Abstracts Service, 2023) and the Cambridge Structural Database (CSD, accessed via WebCSD on October 21, 2023; Groom et al., 2016), this work is the first attempt at the synthesis, crystal structure and Hirshfeld analysis of a (R,S)-fixolide-thiosemicarbazonato complex.Thus, three crystal structures with some similarities to the title compound were selected for comparison.
The first selected compound is the (R,S)-fixolide carboxylic acid derivative (Kuhlich et al., 2010).In this structure, only one crystallographically independent molecule is observed in the asymmetric unit, which is disordered over the aliphatic ring and two methyl groups (Fig. 8).The chiral centers are disordered, C10A and C10B, so two isomers are observed, with Aand B-labeled atoms and related to the (R)-and (S)-isomers, as observed for the title compound (Table 4).
The second selected molecule for comparison is the (R,S)fixolide 4-methylthiosemicarbazone ligand (Melo et al., 2023a), which is disordered over the fixolide group (Fig. 9) and was employed in the synthesis of the title compound.The structural similarities and differences between non-coordinated and coordinated molecules are shown in Tables 4 and 5.For the (R,S)-fixolide 4-methylthiosemicarbazone, a distorted geometry is also observed, in particular between the aromatic ring and the thiosemicarbazone entity, with a dihedral angle of 51.77 (1) � .
Finally, a bis-thiosemicarbazonato Pd II complex was chosen for comparison.In the crystal structure of the trans-bis-[cinnamaldehyde 4-phenylthiosemicarbazonato-� 2 N 2 S]palladium(II) compound (Melo et al., 2023b), the molecules are also connected by N-H� � �S intermolecular interactions,

Table 5
Bond lengths for the N-N-C-S entities in the neutral, non-coordinated, and the anionic, coordinated, forms of thiosemicarbazone derivatives (A ˚).(a) The neutral and non-coordinated form of the (R,S)-fixolide 4-methylthiosemicarbazone structure (Melo et al., 2023a); (b) the anionic and coordinated form of the cinnamaldehyde 4-phenylthiosemicarbazone in a Pd II -complex (Melo et al., 2023b); (c) the anionic and coordinated form of the (R,S)-fixolide 4-methylthiosemicarbazone in the Pd II complex of this work.

Synthesis and crystallization
The starting materials are commercially available and were used without further purification.The synthesis of the complex was adapted from a previously reported procedure (Melo et al., 2023b).An ethanolic solution of (R,S)-fixolide 4methylsemicarbazone (4 mmol, 50 mL) was prepared and the ligand was deprotonated with one pellet of NaOH.The solution was stirred for 4 h, until a yellow color could be observed.Simultaneously, an ethanolic suspension of palladium(II) chloride (2 mmol, 50 mL) was prepared under stirring.A yellow-colored mixture of both ethanolic solution and suspension was maintained with stirring at room temperature for 8 h, until the PdCl 2 was consumed.Orange single crystals suitable for X-ray diffraction were obtained by the slow evaporation of the solvent.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 6.The crystallographically independent bis-thiosemicarbazonato Pd II complex is disordered over the fixolide fragments (Fig. 1).Thus, the C9, C10, C16, C18, C29, C30, C37 and C38 atoms were split into two positions labeled A and B, with a refined site-occupancy ratio of 0.624 (2):0.376(2).The EADP command was used to constrain the displacement parameters of the disordered atoms to get a stable refinement.Although the displacement ellipsoids of the C17, C19, C20, C36, C39 and C40 atoms seen to be prolate-like, no additional disorder was indicated by the data analysis.
Hydrogen atoms were located in difference-Fourier maps but were positioned with idealized geometry and refined isotropically using a riding model (HFIX command).Methyl H atoms were allowed to rotate but not to tip to best fit the experimental electron density.So, for the methyl H atoms, with [U iso (H) = 1.5U eq (C)], the C-H bond lengths were set to 0.98 A ˚.The other C-H bond lengths were also set according to the H-atom neighborhood, with [U iso (H) = 1.2U eq (C)].For the phenyl H atoms, the C-H bond lengths were set to 0.95 A ˚, for the H atoms of the disordered -CH 2 -fragments (C9A, C9B, C29A and C29B), the C-H bond lengths were set to 0.99 A ˚and for the H atoms attached to the disordered tertiary C atoms (C10A, C10B, C30A and C30B), the C-H bond lengths were set to 1.00 A ˚. Finally, the N-H bond lengths, with [U iso (H) = 1.2U eq (N)], were set to 0.88 A ˚. Computer programs: APEX3 and SAINT (Bruker, 2015), SHELXT2014/5 (Sheldrick, 2015a), SHELXL2018/3 (Sheldrick, 2015b), DIAMOND (Brandenburg, 2006), Crystal-Explorer (Wolff et al., 2012), WinGX (Farrugia, 2012), publCIF (Westrip, 2010) and enCIFer (Allen et al., 2004).

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 1The molecular structure of the title compound, showing the atom labeling and displacement ellipsoids drawn at the 40% probability level.Disordered atoms are drawn with 40% transparency and A-labeled for the (R)isomer [s.o.f.= 0.624 (2)] and B-labeled for the (S)-isomer [s.o.f.= 0.376 (2)].The ethanol solvate molecule is omitted for clarity.

Figure 2
Figure 2 Graphical representation of the coordination sphere of the title compound showing the H� � �Pd weak anagostic intramolecular interaction.The figure is simplified for clarity.

Figure 5
Figure 5 Crystal structure section of the title compound viewed along [100].The H� � �O and H� � �S intermolecular interactions are drawn as dashed lines and link the molecules into mono-periodic hydrogen-bonded ribbons along [011].

Figure 4 Figure 6
Figure 4 Graphical representation of the H� � �O and H� � �S intermolecular interactions for the title compound drawn as dashed lines.Two ethanol solvate molecules act as bridges connecting two complexes into centrosymmetric dimers.The figure is simplified for clarity.[Symmetry code: (ii) À x + 2, À y + 1, À z + 2.]

Figure 7 The
Figure 7 The Hirshfeld surface two-dimensional fingerprint plot for the title compound showing the intermolecular contacts in detail (cyan dots).The major contributions to the crystal cohesion amount to (a) H� � �H = 81.6%,(b) H� � �C/C� � �H = 6.5%,(c) H� � �N/N� � �H = 5.2% and (d) H � � �S/S� � �H = 5.0%.The d i (x-axis) and the d e (y-axis) values are the closest internal and external distances from given points on the Hirshfeld surface (in A ˚).

Table 6
Experimental details.