Crystal structure of bis{2-[5-(3,4,5-trimethoxyphenyl)-4H-1,2,4-triazol-3-yl]pyridine}palladium(II) bis(trifluoroacetate) trifluoroacetic acid disolvate

In the PdII complex, two substituted 3-(pyridin-2-yl)-1,2,4-triazole ligands in the neutral form coordinate to the metal atom through the pyridine-N and triazole-N atoms in a trans-configuration.


Chemical context
Triazoles are five-membered heterocyclic compounds containing three nitrogen atoms and two carbon atoms in the ring.They can exist in different isomeric forms, such as 1,2,3triazole and 1,2,4-triazole.1,2,4-Triazole derivatives are of interest in various research fields ranging from medicinal chemistry and pharmaceuticals (Aggarwal & Sumran, 2020;Leenders et al., 2021) to materials science (Farooq, 2020).Versatile coordination behaviour due to the presence of neutral, anionic or cationic nitrogen donors (N-coordination) as well as carbanionic donors (C-coordination) makes 1,2,4triazoles appealing ligands for the construction of metal complexes with useful functionalities (Song et al., 2019;Feltham et al., 2017;Kumar et al., 2015;Wen et al., 2017).Substitution reactions at the azole ring create a virtually unlimited range of chemical and structural variations to tune the desired characteristics of the resulting complexes.

Structural commentary
The title compound is assembled from discrete [Pd(HL) 2 ] 2+ cations (the Pd II atom is located on a special position with C i site symmetry), CF 3 COO À anions, and CF 3 COOH molecules of crystallization in a 1:2:2 ratio (Fig. 1).Both neutral HL molecules are coordinated to the metal atom as bidentate ligands through the triazole-N2 and pyridine-N1 atoms in a trans-configuration.The square-planar N 4 environment of the Pd II centre is moderately distorted with the two Pd-N distances and two cis N-Pd-N angles differing by 0.046 (2) A ˚and 20.70 (8) � , respectively (Table 1).The [Pd(HL) 2 ] 2+ cation, except for the methoxo groups, is almost planar with the largest deviation from the mean plane being 0.117 A ˚(C11).Two intramolecular hydrogen bonds, C1-H1� � �N3 i and C15-H15B� � �O1, with an S(6) graph-set motif are observed (Fig. 2, Table 2; symmetry code as given in Table 2) (Etter, 1990).The C-H� � �N S(6) rings support the planar configuration of the cation.

Figure 1
Extended view of the asymmetric unit of (I) with the atom labelling and displacement ellipsoids at the 50% probability level showing the coordination environment of the metal atom.

Database survey
More than 1400 crystal structures of metal complexes featuring the 3-(pyridin-2-yl)-1,2,4 Gallagher et al., 2007), which is a cocrystal of a neutral molecule and a zwitterion with a protonated N4 atom.Most similar, but not isomorphous, to the title compound is [Pd(HL 0 ) 2 ](CF 3 COO) 2 •4CF 3 COOH with the neutral ligand HL 0 having a phenyl group instead of the trimethoxyphenyl substituent in (I), which also crystallizes in the triclinic space group P1 (KEFKUF; Zakharchenko et al., 2021b).
Due to very poor solubility of the title compound in organic solvents, it was not possible to obtain its satisfactory 1 H NMR spectrum in CDCl 3 .Only the protons of the methoxy groups are distinctly observed as two singlets in a 2:1 ratio at 4.01 and 3.96 ppm while the aromatic protons in the 10-7 ppm range were indistinguishable from the background.The presence of TFA molecules and trace amounts of water in the solvent leads to significant broadening of the N4H signal to the point of disappearing in the spectrum.On the contrary, in the 1 H NMR spectrum of the free HL ligand in CDCl 3 the acidic N2bound proton appears as a broadened singlet at 13.31 ppm (Zakharchenko et al., 2016).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3.Both trifluoroacetic moieties were found to be disordered over two resolvable positions with a refined occupancy ratio of 0.587 (1):0.413(17) and 0.530 (6):0.470(6) for the protonated and deprotonated forms, respectively.The disorder was restrained using SIMU and RIGU commands in SHELXL for the ten resulting atoms except for C19 and O4 of the trifluoroacetic anion and twelve resulting atoms except for C17 of the trifluoroacetic acid.The four-atom C-COO fragments were restrained to be nearly planar by a FLAT command.Bond distances in the disordered fragments were restrained by the SAME command to be similar in length.Anisotropic displacement parameters were employed for the non-hydrogen atoms.Anisotropic displacement parameters for pairs of the disordered atoms were constrained to be the same.The H atom bound to O was found in difference-Fourier maps, C/N-bound H atoms were included in calculated positions and refined using a riding model with isotropic displacement parameters based on those of the parent atom [C-H = 0.93 A ˚, N/O-H = 0.86 A ˚, U iso (H) = 1.2U eq C for CH, NH and OH; C-H = 0.96 A ˚, U iso (H) = 1.5U eq C for CH 3 ].Idealised methyl groups were refined as rotating groups.

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.

Table 3
Experimental details.