Palladium(II) halide complexes of N,N,S-tridentate Schiff bases. Synthesis, structural studies in solution and solid state, and analysis of Hirshfeld surfaces

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Introduction
Benzothiazoline compounds have been used in the synthesis of versatile ligands such as the benzothiazoles and benzothiazines that are widely utilized in Coordination Chemistry.[1,2] Also, these heterocyclic compounds have been described as precursors for the formation in situ of sulfur containing Schiff bases by means of thiazoline opening-ring reactions in presence of metallic ions.[3,4,5] For example, the synthesis of tetrahedral Cd(II) or Zn(II) complexes of N,S-bidentate sulfur Schiff bases with potential luminescent applications has been reported.[6] In the same vein, an anionic Schiff obtained by the opening-ring of a thiazoline ring showed a k 2 NkS-tridentate coordinate mode in the octahedral complexes of Fe(II), that have been proposed for the study of reactivity of new models of metalloenzymes such as the hydrogenase.[5,7] Regarding to halogen Pd(II) complexes of analogue N,S-polydentate Schiff bases, some bromo-and chloro complexes containing five-membered chelate rings with a  2 NS-tridentate coordinate mode have been reported.[8,9,10] In some  2 NS-tricoordinate Pd(II) complexes of analogue sulfur Schiff bases, the antimicrobial and antitumor activity has been described.[11, 12,] However, despite the applications mentioned, there are few reports of chloro Pd(II) complexes of sulfur Schiff bases derived from benzothiazoline precursors.[13] To the best of our knowledge, bromo-and iodo Pd(II) complexes have not been reported.In order to contribute to the synthesis of Pd(II) complexes of N,N,S-tridentate Schiff bases of interest for their possible biological and structural applications, we described the synthesis of the halogen Pd(II) complexes of Schiff bases.Bromo-and iodo Pd(II) complexes were prepared in situ by the reaction of 2-R-2-(2pyridyl)benzothiazolines (1)(2)(3)(4) precursors with the corresponding starting material [Pd(tmeda)X2] (X = Br, I), yielding complexes of general formula [Pd(L n )X] (X = Br, n = 1-4, 1a-4a; X = I, n = 1-4, 1b-4b).The bromo Pd(II) complexes (1a-4a) and iodo Pd(II) complexes (1b-4b) were characterized by infrared spectroscopy, elemental analyses, NMR, and specifically for 3a, 3b, 4a and 4b by single crystal X-Ray diffractions studies.
Analysis of Hirshfeld surfaces for bromo 3a and 3b, and iodo 4a and 4b complexes of anionic Schiff bases {L 3 } − and {L 4 } − allowed identify the main intermolecular contacts.

Experimental Section
All manipulations of air and moisture sensitive materials were carried out in a dinitrogen atmosphere by using Schlenk techniques.The solvents used such as acetonitrile, dimethylsulfoxide, diethyl ether, chloroform, and hexane were dried by standard methods and distilled before their use.The PdCl2, NaBr, and NaI salts, and the N,N,N',N'-tetramethylethylenediamine (TMEDA) were purchased from Aldrich and used as received.2-R-2-(2-pyridyl)benzothiazolines 1-4 were prepared according to previous reports.[13,14] Melting points were determined on a Mel-Temp II apparatus.The elemental analyses were determined in a Perkin-Elmer Series II CHNS/O Analyzer 2400.Infrared spectra were recorded on a FT-IR 200 Perkin-Elmer spectrophotometer in the 4000-400 cm −1 range using KBr pellets.NMR spectra were obtained on a Bruker Ascend 400 spectrometer at 400.13 MHz for 1 H and 100.62 MHz for 13 C{ 1 H}.Chemicals shifts (ppm) of 1 H and 13 C{ 1 H} spectra are relative to the frequency of SiMe4.

Synthesis of [(Pd(tmeda)Br2] and [Pd(tmeda)I2] complexes
The [Pd(MeCN)2X2] (X = Br, I) complexes were obtained by exchange halide reactions, according to previous reports.[15] A mixture of [Pd(tmeda)Cl2] in 25 mL of acetone with the corresponding halogenated sodium salt was stirred at room temperature by five (X = Br) or three (X = I) days.The solvent was removed under vacuum to give stable solids at room temperature.

General Procedure
An equimolar mixture of [Pd(tmeda)X2] (X = Br, I) in 25 mL of acetonitrile with the corresponding 2-R-2-(2pyridyl)benzothiazoline (1-4) was refluxed for 2 h.At the end, the reaction mixture was allowed to reach room temperature, then, was filtered and the solvent was vacuum evaporated to give stable solids at room temperature that were identified as the [Pd(L n )X] complexes.

Computational details for the calculation of Hirshfeld surfaces
The atomic coordinates of 3a, 3b, 4a, and 4b compounds determined by X-ray crystallographic studies at 100 K as well as the atomic coordinates of [Pd(L 3 )Cl] [data taken from reference 13] were used to construct the molecular Hirshfeld surfaces based on the electron distribution calculated as the sum of spherical atomic electron densities (the promolecule), which dominates the corresponding sum over the crystal (the procrystal) yielding an implicit 0.5 value for the promolecule-to-procrystal ratio.The bond lengths involving hydrogen atoms were adjusted to their neutron values as usual.[20] The Hirshfeld surfaces (HS) mapped with dnorm and 2D fingerprint plots were generated using CrystalExplorer21 with a high standard surface resolution.[21] The HS were mapped using a fixed color scale, red-white-blue, where red highlights are for contacts shorter than van der Waals (vdW) radii sum, white for contacts around vdW separation, and blue is for contacts longer than the vdW sum.

Results and discussion
Synthesis of [Pd(L n )X] complexes derived from precursors 1-4.
Scheme 1 Synthesis of complexes Pd(L n )X] (X = Br, I).The scheme numbering for NMR assignment is also shown.

IR Spectroscopy
The (N-H) absorption described for the precursors 1-4 was not observed in the bromo-and iodo Pd(II) complexes 1a-4a and 1b-4b.[13] For all complexes were displayed two absorptions (C=N) in the range of 1602-1589 cm −1 and 1581-1534 cm −1 ; these (C=N) absorptions were attributed to the   NS-tridentate mode of the anionic Schiff bases {L n } − .

NMR Spectroscopy
1 H and 13 C{ 1 H} NMR spectra of complexes 1a-4a and 1b-4b were obtained at room temperature in DMSO-d6 solutions.All complexes were characterized by two-dimensional heteronuclear and mononuclear experiments HSQC, HMBC, and COSY except for the [Pd(L 1 )I] complex due to its low solubility in DMSO-d6 solution.The numbering utilized by NMR assignment is shown in Scheme 1.
In the 1 H spectra of halogen Pd(L n )X] complexes, the NH signal described for the benzothiazolines 1-4 vanished, corroborating the coordination of the corresponding anionic Schiff base {L n } − .In all complexes, the coordination showed the deshielding of the H5 pyridinic proton in comparison with their corresponding precursors.[13] The 1 H NMR spectra of the precursor 2 and complexes 2a and 2b in are displayed in the Supplementary material, Figure 1S.
In addition, the H5 proton in the bromo complexes 1a-4a ( = 8.92-8.73ppm) and iodo complexes 1b-4b ( = 9.15-8.98ppm) was observed at higher frequencies respect to the chloro complexes [Pd(L n )Cl] ( = 8.89-8.55ppm) previously reported [13,15] (see Figs. 1S, 2S and 3S).It is noteworthy that the deshielding of the H5 proton of bromo-(1a-4a) and iodo complexes (1b-4b) is opposite to the electronegativity of the halogen ligands, and to the expected shielding by the heavy-atom effect.[22] Deshielding of signal protons in the order I > Br > Cl for d 8 planar complexes has been evaluated and explained by: i) 1 H NMR through-bond and throughspace interactions, [23] and ii) by NMR and X-ray diffraction by means of filled/filled repulsions between the d orbitals of the metal atom and the lone pair orbitals of the halogen ligands.[24] In this context, noteworthy that the Pd-Cl (R = C6H5, 2.291 Å, and R = C5H4N, 2.293 Å) distances of chloro analogue complexes, [13,15] and those obtained in the present work (see below) for Pd-Br {2.424 (rt); 2.426-2.424(100 K) Å} and for Pd-I {2.575 (rt); 2.577-2.589(100 K) Å} for the bromo complexes 3a and 3b, and the iodo complexes 4a and 4b are smaller than their covalent radii sum (2.41, 2.59 and 2.78 Å), respectively.Thus, we proposed that the deshielding of H5 proton in the [Pd(L n )X] ( X = Br and I) complexes could be attributed to a M→X -donation that encourage the decreasing filled/filled repulsions in the order I > Br > Cl like as was described for halogen Pd(II) complexes of 2,6-(piperidyl-methyl)pyridine.[23] and the iminic nature of C6 carbon were demonstrated by means of its chemical shift toward high frequencies in the range of 171.7-159.4ppm.(See Supplementary material, Figs.4S and 5S).

X-ray crystallographic studies
We obtained suitable crystals of compounds 3a, 3b, 4a, and 4b from slow evaporation of MeCN solutions for their studies by single-crystal X-ray diffraction.3a and 3b were diffracted at 300 and 100 K; 4a and 4b just at 100 K. Crystallographic data and selected bond distances and angles are shown in Tables 1 and 2; there are also listed a selected set of covalent radii sums.Next, we discuss the molecular features as well as the crystal packing of these complexes.

Molecular structures of complexes 3a, 3b, 4a, and 4b
The conformation as well as the structural parameters of the molecular complexes 3a, 4a, and 4b, including the four independent molecules in the asymmetric unit of 3b, are very similar (excepting, evidently, the Pd-X bond distance data).In all compounds, it is observed a  2 NS-tridentate mode of the anionic Schiff base {(C5H4N)C(R)=N(C6H4S)} − , with the formation of two five-membered chelate rings (Fig. 1).Also, the Pd(II) cation displays a square planar geometry, where the X halogen ligands are in trans position respect to the iminic nitrogen atom (N1).Additionally, the Pd(L n )] 1+ core exhibit a practically planar tetracyclic system, with the phenyl and pyridinic rings being almost perpendicular to the tetracyclic system.A more detailed analysis indicates that the Pd-X distances observed at 100 K are 2.4262(2) and 2.4242(5) Å for 3a and 4a (X = Br), and 2.5775(59)mean and 2.5897(2) for 3b and 4b (X = I); they are ca.0.2 Å shorter than the respective covalent radii sum (2.59 and 2.78 Å).This shortening is also observed in compounds with the [Pd II (N  N  E)X] core, where the Pd(II) cation is tetracoordinated.The (N  N  E) system represents a tridentate ligand, where E is any atom, and X is either Br or I.The results of a delimited search in the Cambridge structural database of this [Pd II (N  N  E)X] core are shown in Fig. 2; we found 28 hits, with Pd-X distances ranging from 2.39 to 2.53 Å for bromo compounds and 2.55 to 2.62 Å for the iodo ones, in concordance with those observed in our complexes.Now, the Pd-S mean distances are 2.2408(4), 2.2359(32)mean, 2.2409(9), and 2.2419(4) Å for 3a, 3b, 4a, and 4b, respectively; they agree with the covalent radii sum of these atoms (2.44 Å) and are significantly equal, despite the different nature of the X halogen.Furthermore, we observed that the Pd-Niminic averaged distances are significantly shorter than the Pd-Npyridinic ones; in both cases are also similar to the reported covalent radii sum (2.10 Å).We also carried out a search in the structural database for tetracoordinate palladium(II) compounds coordinated by a Schiff tridentate ligand (Fig. 3).Here, we can analyze two cases; in the first case when the donor atom of the ligand L is a first-row atom (C or N), the Pd-Niminic (blue line) and Pd-Npyridinic (green line) distances are practically equalized.[26,27] On the other hand, when the donor atom is a second-row atom, the Pd-Niminic distances are shorter than the Pd-Npyridinic, a situation that we observed in the four complexes.These two cases are fulfilled regardless the nature of the E atom, at least for the structural data retrieved, where E = O, N or S.
Overall, the similitude in the structural data around the Pd(II) center in 3a, 3b, 4a, and 4b, as well as those reported for the analogous chloro complex [Pd(L 3 )Cl] [13] with reported structural data equal to Pd-S, 2.2369(19) Å; Pd-Niminic, 2.001(4) Å; Pd-Npyridinic, 2.053(5) Å; Pd-Cl, 2.2949( 14) Å, indicates that the rigidity of the [PdNiminicNpyridinicSX] core rules the molecular structural requirements, despite the different nature of the X halogen ligand.The packing differences of these isostructural 3a/3b and 4a/4b pairs prompts us to analyze them on the basis of the Hirshfeld surface approach; for the sake of comparison, we also used the crystal data of the analogous compound [Pd(L 3 )Cl].[13] Hirshfeld surface analysis The Hirshfeld surface (HS) encloses either a molecule or even a cluster of molecules, and its corresponding 2D-fingerprint is unique for a given compound, therefore, the construction of this type of surfaces makes them a useful approach to compare crystal structures.[29] To build these HS, we used the CrystalExplorer21 software with a high standard surface resolution; the surfaces were mapped using a fixed color scale, red-white-blue, where red highlights are for contacts shorter than van der Waals radii sum, white for contacts around vdW separation, and blue is for contacts longer than the vdW sum.[30,21] The HS based on the dnorm of all complexes, including the four-molecules cluster in 3b, clearly exhibit red spots near the sulfur atoms and the aromatic rings (Fig. than the van der Waals radii sum.Here, due to the similar conformations in all complexes, the comparison of superficial areas and volumes of the HS indicated two predictable facts: firstly, the iodo compounds displayed larger values in each 3a/3b and 4a/4b pair, and secondly, in the 3a/4a and 3b/4b pairs it was observed that the complexes with the phenyl groups displayed the larger values, attributable to the larger size of the C-H group as compared with the isoelectronic nitrogen atom.In addition, the fragment patch plots are useful to easily visualize the number of surrounding molecules that interacts with a central one; also, the area data of these patches can be used to find the external major molecular fragments that are closer to a HS given.The number of molecules that interact with a central one ranges from 11 in 3b_Pd 1 to 18 in 4a (see data in Fig. 5).Despite these numbers, just four molecules cover more than the half of the superficial area (3a, 54.6 %; 3b_Pd The 2D-fingerprints of the corresponding Hirshfeld surfaces were plotted to evaluate the contribution of the intermolecular contacts; we show the contribution of all contacts by means of a stacked bar chart, where we considered the reciprocal ones (Fig. 6); here, we included the contributions observed in the crystal structure of [Pd(L 3 )Cl], calculated from the published data [13].The analysis of the intermolecular contact contributions for all compounds showed that H  6).
The absence of strong classical interactions prompts us to analyze the energies for intermolecular interactions; these analyses were carried out based on the energy framework approach.[32] We used again Crystal Explorer 21, considering the four energy components: electrostatic (Eele), polarization (Epol), dispersion (Edis) and exchange-repulsion (Erep).The energies were obtained using the B3LYP/DGDZVP level of theory.
For the visual comparison of magnitudes of interaction energies, they were adjusted to a cylinder scale of 50 with a cutoff value of 2 kJ/mol within the given unit cells (Fig. 7).It is clear that the coulombic and dispersion components make a significant contribution to the supramolecular architecture in the crystal, where the red and green cylinders join the centroids of the [Pd(L n )X] complexes, and are almost parallel in magnitude with the total energy (cylinders in blue color).All complexes displayed the largest total energies associated to the centrosymmetric arrangements of the planar tetracyclic systems (3a, −139.8;−148.8 in 3b_Pd   4).These energetic interactions are associated to the larger surface areas above mentioned (see fragment patches in Fig. 5).It is noteworthy that in 4a and 4b the cylinders with the larger radii are concatenated, whereas in 3a it is just one cylinder with a similar value.In 3b there are both concatenated and isolated cylinders representing the centrosymmetric intermolecular associations.In both cases, the largest energies are associated with the C-H••• interactions and they are quite important in the formation of the crystalline structure.proton in the bromo and iodo complexes could be attributed to the decreasing filled/filled repulsions.The above is agreement with the shortening of the Pd-X bond in the halide 3a, 3b, 4a, and 4b complexes.
5); we have used a superscript on the Pd symbol to indicate the HS of the corresponding molecule in the cluster.We also show the fragment patches projected on the corresponding HS along with an interacting molecule.The red spots correspond to the presence of close contacts due to C-H••• interactions and C-H•••S non-classical hydrogen bonding, i.e., contact distances that are shorter

Fig. 7 The reaction of benzothiazoline precursors 1 - 4
Fig. 7 Energy frameworks along the a axis for 3a (first row), 3b (second row), 4a (third row), and 4b (fourth row) showing the electrostatic energy term (in red), repulsive term (in yellow), dispersion energy term (in green), and total energy (in blue).The energy frameworks were adjusted to the same scale factor of 50 with a cut-off value of 2 kJ/mol

Table 1
Selected crystallographic data for the complexes [Pd(L 3 X] and [Pd(L 4 X]