Synthesis, spectroscopic characterization, crystal structure and theoretical investigation of two azo-palladium (II) complexes derived from substituted (1-phenylazo)-2-naphtol

The ortho-substituted (E)-1-((2-methoxyphenyl)diazenyl)naphthalen-2-ol and the meta-substituted (E)-1-((3-methoxyphenyl)diazenyl)naphthalen-2-ol were, respectively, used in the synthesis of two new complexes, bis[1-(2-methoxyphenylazo)-2-naphthoxy]palladium(II) and bis[1-(3-methoxyphenylazo)-2-naphthoxy]palladium(II), noted (I) and (II), respectively. (I) and (II) were characterized by physicochemical and spectroscopic methods, and their molecular structures were determined by X-ray crystallography. Both complexes display a square-planar geometry, which is reproduced by full geometry optimizations at the DFT/B3LYP level. Calculations were also performed on the free ligands (in their precursor form), as well as their para-substituted isomer (E)-1-((4-methoxyphenyl)diazenyl)naphthalen-2-ol and its hypothetical complex bis[1-(4-methoxyphenylazo)-2-naphthoxy]palladium(II) (compound (III). Calculations were also performed on the free p-phenylazo-2-naphthol ligand (p-MoxyPhNap), in order to understand their bonding and to analyze their electronic structure. TD-DFT calculations were also performed on the three complexes to simulate their absorption spectra from and compare to the experimental UV–Vis data of (I) and (II). The main peaks in the spectrum of (I) are assigned to mixed LMCT/LLCT and π–π * (ILCT) transition, while the unique major peak afforded by (II) is assigned to MLCT and LLCT transitions.


Introduction
Transition metal complexes of azo compounds have received increasing interest in both basic and applied research, owing to their peculiar electronic and structural characteristics and their relationships with their use in different crucial fields [1][2][3][4], privileged by their low cost. The transition metal-azo dye complexes are related to the possibility to obtain new compounds with various biological activities [5,6]. This class of complexes is important in the progress of metalbased anticancer agents [7,8]. These different areas of interest fostered inorganic chemists to develop novel approaches to the preparation of non-covalently bound anticancer drugs [9,10].
There is a great coordination affinity of the metals toward chromophoric ligands, leading to the formation of transition metal complexes evidenced by an absorption displacement of their UV-Vis spectra and accompanied by an enhancement of the dye fastness [11][12][13]. Various properties such as structures, chelate formation constants, and stability have been reported [14]. Different classes of chromophoric ligands, particularly those characterized by the presence This paper is dedicated to Dr. Jean-René Hamon at the occasion of his 65th birthday.

Electronic supplementary material
The online version of this article (https ://doi.org/10.1007/s1124 3-020-00425 -5) contains supplementary material, which is available to authorized users. of the azo group (-N = N-), have been applied as chelate indicators to identify metal ions, where the complex formation is related to a change of color [15][16][17][18]. Due to their easy use, several areas of application have emerged into numerous high-technological frontier applications such as high-density memory storages (CD-R and DVD-R) [19][20][21], which is considered as an important and promising branch of chemistry with significant physicochemical and material properties.
Recently, Pd-arylazo complexes have found application in C-C coupling reactions [22,23] interacting strongly with CT DNA [24] and exhibiting photochromic activity [25,26], where most of their electronic and photochromic efficiency have been explained by DFT calculations.
We report two novel Pd(II) coordination complexes obtained using (E)-1-((2-methoxyphenyl)diazenyl)naphthalen-2-ol [27] and (E)-1 ((3methoxyphenyl)diazenyl) naphthalen-2-ol [28] azo-ligands. These two bidentate ligand precursors are isomers differing from the ortho versus meta position of their methoxyphenyl group. In both Pd(II) complexes (I) and (II), respectively, the metal is coordinated to two N atoms and two O atoms. The experimental studies were performed on the meta ligand precursor in both ketonic and enolic forms under variable temperatures. To our knowledge, there are no other complexes with similar ligands bearing methoxy groups reported in the literature. In all the investigated complexes, the metal coordination mode is almost perfectly square-planar.

Materials and methods
Elemental analyses (C, H, N) were performed on a Vario EL III elemental analyzer. UV spectra were recorded on an Agilent UV-VIS spectrophotometer 8453 (spectroscopy system) with G1120A multicell transport and a computer with ChemStation (G Visible Ultra Violet 1120 A). Infrared spectra were recorded with a Fourier transform infrared spectrometer (ALPHA) FTIR from the brand BRUKER controlled by Opus 6.5 software and fitted with an Attenuated Total Reflectance (ATR) accessory in diamond crystal. The 1 H NMR spectra were recorded in deuterated chloroform CDCl 3 with infinite dilution at high fields on a Brücker Avence DPX-type spectrophotometer, 400 MHz, TMS as internal reference, chemical shift in ppm. The diffraction data were collected on a Nonius Kappa CCD diffractometer using graphite monochromatized Mo-Kα radiation ( λ = 0.71073 Å).

Preparation of complex (I)
Complex (I) was prepared as described in Scheme 1. A mixture of Pd(AcO) 2 (0.056 g, 0.25 mmol) was dissolved in hot methanol (10 ml), added to the ligands (E)-1-((2-methoxyphenyl)diazenyl)naphthalen-2-ol (0.33 g,0.5 mmol) and dissolved in hot methanol (25 ml). The color of the solution changed to dark red after the addition. The reaction mixture was refluxed overnight (24 h) and then evaporated until 30% of the total volume of the reaction solution remained at the end of the reaction. The solvent was removed under vacuum and the residue was washed twice with hexane to give dark red solids (70%). The resulting solids were crystallized from CH 2 Cl 2 to yield dark red prismatic-like crystals.

Preparation of complex (II)
Complex (I) was prepared as described in Scheme 2. A mixture of Pd(AcO) 2 (0.056 g, 0.25 mmol) was dissolved in hot methanol (10 ml), added to the ligands (E)-1-((3-methoxyphenyl)diazenyl)naphthalen-2-ol (0.33 g,0.5 mmol) and dissolved in hot methanol (25 ml). The color of the reaction solution changed to dark red after the addition. The reaction mixture was refluxed (24 h) overnight and then evaporated until 30% of the total volume of the reaction solution remained at the end of the reaction. The solvent

X-ray crystallography
Single crystal X-ray structural analysis A single crystal was carefully selected under a polarizing microscope in order to perform its structural analysis by X-ray diffraction. The diffraction data were collected on a Nonius Kappa CCD diffractometer using graphite monochromatized Mo-Kα radiation (λ = 0.71073 Å). The structures were solved by direct methods using SHELXS-2014 [29] and refined against F2 by full-matrix least-squares methods with anisotropic displacement parameters for all non-hydrogen atoms. All calculations were performed using SHELXS-2014 and SHELXL-2014, implemented in the WINGX system of programs [30]. The drawings were performed using the Mercury program [31]. The refinement was done by full-matrix least squares methods (SHELXL-2014 program) and converged to an acceptable final agreement factor. The pertinent experimental details of the structure determination of the new compounds are gathered in Table 1. All hydrogen atoms were placed in idealized positions and constrained to ride on their parent atoms with C-H = 0.93 Å with Uiso (H) = 1.2 Ueq (C) for aromatic hydrogen; C-H = 0.96 Å with Uiso (H) = 1.5Ueq(C) for CH 3 -group. Crystal data and structure refinement parameters for complexes (I) and (II) are listed in Table 1.

Computational methods
All geometry optimizations were performed using the 2016.01 version of the Amsterdam Density Functional (ADF) program [32] developed by Baerends and co-workers [33][34][35][36][37] by means of the hybrid-type B3LYP functional (Becke's three parameter hybrid exchange functional [38] coupled with the Lee-Yang-Parr nonlocal correlation functional) [39]. The atom electronic configurations were described by a triple-ζ Slater-type orbital (STO) basis set for H 1 s, C 2 s and 2p, N 2 s and 2p, O 2 s and 2p augmented with a 3d single-ζ polarization for C, N and O atoms and with a 2p single-ζ polarization for H atoms. A triple-ζ STO basis set was used for Pd 4d and 5 s augmented with a single-ζ 5p polarization function for Pd. The scalar relativistic zero-order regular approximation (ZORA) was used (with the associated optimized valence basis set) for Pd [40][41][42]. Vibrational frequency calculations [43,44] were performed on all the optimized geometries to confirm that these structures are true minima on the potential energy surface. Singlet-triplet excitation energies and the transition dipole lengths were computed using TD-DFT as implemented in the Response [45] code in the ADF package. Solvent effects were considered by using the Conductorlike Screening Model for realistic solvent (COSMO-RS) model developed by Klamt and coworkers [46]. Natural populations (NAO) are obtained by NBO calculations [47,48]. Representations of the molecular structures and molecular orbitals were done using the ADF-GUI software [32].  (Table 2).

Crystal structure of complexes (I) and (II)
In the crystal, the molecules are forming chains along [100] as shown in Fig. 3. The chains are linked by C-H···π interactions, where C(17)-H(17C)…Cg1 corresponds to a distance of 2.720 Å (Table 3) giving rise to slabs lying parallel to (011). The network is characterized by the presence of weak C7-H7···N1 hydrogen bonds with relatively long H7···N1 distance of 2.450 Å, long donor-acceptor C7-N1 distance of 2.770 Å and an angle C-H-N of 100° which strongly deviates from the linearity in accordance with weak interactions.
Compound (II) crystallizes in the monoclinic crystal system (space group P21/c). As observed for compound (I), it is coordinated in a N, O-bidentate manner (Fig. 2). The metal atom is tetra-coordinated to two oxygen atoms in a trans position of the C-O function and two nitrogen atoms in a trans position to the N = N function. The geometry around the metal is almost perfectly square-planar  with distances between Pd(II) and O1, N1 of 2.000(2) and 2.031(3) Å, respectively, and an O1-Pd-N2 bond angle of 86.41(12)° and O1-Pd-N2 i angle of 93.59(12)°; symmetry code: (i) − x + 2, − y, − z + 2 (Fig. 3).
In the crystal, the molecules are linked via C-H···N and C-H···O intermolecular interactions, forming a layer parallel to (101) (Fig. 4). Indeed, C-H···O hydrogen bonds engaging the hydrogen and oxygen atoms of the methoxy groups are characterized by long H···O and C-O distances of 2.540 and 3.374 Å and a bending C-H-O angle of 145°, giving rise to weak interactions. Details of these interactions are given in Table 3.

Geometry optimizations
The full geometry optimization of the meta and para isomers of the free ligand precursor (E)-1-((p-methoxyphenyl) diazenyl)naphthalen-2-ol (p = 3, 4, respectively) gave rise to almost isoenergetic structures, while the ortho isomer (p = 2) is higher in energy by 9.1 kcal/mol. The three structures are planar with comparable geometrical parameters (Fig. 5), but showing a destabilizing interaction between the oxygen atom of the methoxy group and the nitrogen atoms of the azo group that is larger in the ortho case, in line with its Pauli repulsion energy of 448 kcal/mol, compared to those of the meta and the ortho isomers of 418 and 422 kcal/mol, respectively.
In order to provide further understanding of the molecular structures, electronic structure and bonding, full geometry optimizations have been carried out on the Pd complexes (I), (II) and (III), deriving from the three ortho, meta and para precursor isomers (E)-1-((p-methoxyphenyl)diazenyl) naphthalen-2-ol (p = 2, 3, 4), respectively. Whereas (I) and (II) have been characterized (see above), (III) is unknown so far. The three complexes are of C i symmetry and exhibit a square-planar geometry around the metal center (Fig. 6). The two substituted azo ligands are related through the inversion center at the Pd(II) center, which is bonded to two N and two O atoms, in a manner similar to related experimentally characterized complexes [49,50]. In particular, there is a good agreement between the optimized and experimental (see above) structures of (I) and (II). Whereas (II) and (III) are close in energy, (I) is less stable. This instability   (6) 145.00 Symmetry codes: (i) − x + 2, − y, − z + 2; (ii) − x + 2, − y + 1, − z + 2; (iii) − x + 1, − y + 1, − z + 2 Fig. 2 The molecular structure of compound (II), with atom labeling and 50% probability displacement ellipsoids. The unlabeled atoms are related to the labeled atoms by the symmetry operation (− x + 2, − y, − z + 2) 1 3 Fig. 3 A partial view along the b axis of the crystal packing of the title complex (I), displaying the C-H···N and C-H···π interactions as dashed lines (see Table 3)  Table 3) is related to that of the free ligand and also to O···O repulsions that occur between the oxygen atom of each methoxy group (NAO charge = − 0.50) and the oxygen atom directly bound to the Pd center (NAO charge = − 0.65) [47,48]. In (I), the oxygen atoms displaying negative charges are faceto-face, hence, leading to electrostatic repulsions. Due to the described repulsions, (I) is destabilized by 12.0 and 11.5 kcal/mol with respect to (II) and (III), respectively. Indeed in these two isomers, the methoxy and the azo groups are in remote positions to each other decreasing the repulsion interactions. Nevertheless, in the three complexes the metal adopts a nearly square-planar conformation ( Fig. 8 and Table 4), with ONON dihedral angles of 0°, 0°and 2° for (I), (II) and (III), respectively. The square-planar environment Ortho Meta Para   Steric effects can be evidenced by the orientation between the phenyl and naphthyl rings around the azo group. The largest distortion is observed for (I), due to repulsions between the azo groups of both ligands. This distortion decreases for (II) and (III), whose relative stability is depending on the position of the methoxy group. The optimized Pd-N and Pd-O bond distances for (I) and (II) are comparable to those observed experimentally, where the differences do not exceed 0.07 Å (Table 4). They are also comparable to those reported in the literature of related complexes [49,50]. The C-C bond distances ranging from 1.38 to 1.42 Å suggest delocalization within the phenyl and naphthyl. They are comparable to those found in previous works [51][52][53][54][55][56][57].
The large HOMO-LUMO gaps of 1.72, 1.57 and 1.51 eV of (I), (II) and (III), respectively, are consistent with a good kinetic stability. Kohn-Sham MO plots of the three complexes are displayed in Fig. 7

UV-Vis electronic spectra
TD-DFT calculations [45] were carried out on (I) and (II), taking into account solvent (CH 2 Cl 2 ) effects (see computational details). Their simulated electronic spectra (Fig. 8a) show some differences which are somewhat influenced by the methoxy position. They also were found to be extremely sensitive to their environment, such as the solvent polarity, as known for many dye molecules [57][58][59]. The simulated spectra are in good agreement with their experimental counterparts (Fig. 8b). Because of the different conjugative effects in their respective ligands, different electronic transitions are expected to occur in I and II. They were assigned from our TD-DFT results and the orbitals shown in Fig. 7.
The main calculated absorption bands of 0.52 a.u is centered at 348 nm, corresponding to the experimental one at 381 nm. It is an intense HOMO-3(84a g ) → LUMO + 2(84a g ) transition of (d(M) + π (L)) → π *(L) nature. Thus, The main band is the result of a strong intra-ligand charge transfer (ILCT) associated with ligand-to-metal charge transfer (LMCT). The second main band of 0.65 a.u can be identified as a HOMO(86a g ) → LUMO(83a u ) transition of MLCT character. The band found at relatively low energy and centered at 534 nm (experimental at 515 nm) is mainly a HOMO(86a g ) → LUMO (83a u ) transition corresponding to a metal-to-ligand charge transfer (MLCT).
The simulated electronic spectrum of (II) shows only one major broad peak centered at 400 nm, which is less intense than those obtained for (I). This peak is composed of two transitions: one appears at 394 nm and the other one at 408 nm, comparable to the experimental one which is found at 382 nm. The first peak corresponds to a mixture of HOMO-4(84a g ) → LUMO(83a u ) (65%) and HOMO-5(83a g ) → LUMO(83a u ) (36%) transitions.

Conclusion
This paper reports the preparation, the spectroscopic analyses, X-ray characterization and theoretical calculations of two palladium complexes (I) and (II) containing azo dye ligands. Both complexes (I) and (II) exhibit a square planar geometry around the metal, and the solid-state supramolecular structures are maintained by π-stacking interactions. The weak intermolecular interactions observed in the solid state do not distort the molecular structures significantly. The DFT-optimized structures of (I) and (II) reproduce well the structural properties of their experimentally determined X-ray counterparts. Complexes (I), (II) and (III) display large HOMO-LUMO gaps, in line with the strong metal-ligand interactions. Complex (I) is computed to be less stable by 12.0 and 11.5 kcal/mol than (II) and (III), respectively, showing the impact of the methoxy position on the molecular structures, which are planar in (II) and (III), but distorted in (I). The simulated UV-Vis spectra obtained by TD-DFT calculations are comparable to the experimental spectra, putting emphasis on the ILCT and MLCT electronic transitions involving several of the frontier molecular orbitals in the complexes.