Introduction

Azo compounds are very important molecules and have been the subject of several works in both experimental and fundamental research [1, 2], where synthetic textile dyes of the azo family represent an important part of the world production of synthetic dyes and are characterized by the presence of the azo group (–N=N–) [3,4,5].

Azo derivatives and their metal complexes are essential pigments for synthetic leather and vinyl polymers. Furthermore, high-density optical data storage has been a focus of wide research in the past decades as nonlinear and photoelectronics [6]. Lately, metal complex dyes have also attracted increasing attention due to their interesting electronic and geometrical features in connection with their application for molecular memory storages, nonlinear optical elements, printing system, and so on [7,8,9].

Particularly, metal-azo complex dyes are used in the recording layer of DVD-R (Digital Versatile Disc-Recordable) disc, are lighter stable, allow for easier control of the wavelength by selection of the appropriate substituent groups, and have good thermal stability [10,11,12,13,14,15,16]. As known, the metal complex formation on chromophoric ligands leads to remarkable shift of UV–Vis absorption spectrum [9, 17,18,19,20,21,22] accompanied by the increasing of the dye fastness [21, 23].

The focus of this investigation is to give a deeper understanding on the coordination behavior of these azo molecules towards the M(II) (M = Ni, Pd, Pt, Cu, and Ag) metal ions using DFT and TD-DFT methods which are compared to the available experimental data and different analytical tools. Indeed, several experimental works have been the subject of syntheses, spectral characterizations, and X-ray diffraction of metal-azo complex dyes [24,25,26,27,28,29,30,31,32,33,34].

In addition, potentiometric and spectrophotometric investigations of Co2+, Ni2+, and Cu2+ metal cations connected to azo dyes have been reported [35]. Up till now, azo conjugated transition metal complexes have been shown to afford new possibilities en route for redox, optical properties, and magnetic due to the role of the d orbitals [9, 36,37,38].

In recent work, we have studied theoretically the substituent effects on 1-phenylazo-2-naphtol azo molecule substituted at the ortho position of the phenyl ring denoted (PhNap) as shown in Scheme 1 [39], while in this one, we will endeavor to provide an understanding into the N,O-bidentate chelation stabilization of 1-(4-methylphenylazo)-2-naphtol (MePhNap) as ligand in the M(MePhNap)2 transition metal complexes. Furthermore, the relationship between the differences in absorption maximum from free ligand to their metal-azo complexes was thoroughly studied. It is worth noting that the organic ligands containing N and O donors give rise to great potential for fine control over coordination structural arrangements [40,41,42,43,44,45,46].

Scheme 1
scheme 1

the Optimized structure of (1-(4-methylphenylazo)-2-naphtol) denoted (MePhNap) and its atoms numbering

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The density functional theory (DFT) method using the hybrid B3LYP functional or other hybrid and GGA ones is precious in determining the electronic structures, the geometrical parameters, the bonding, and other properties based on various works of monometallic systems [47,48,49,50,51,52,53,54,55,56].

Computational details

DFT calculations have been carried out on all complexes with the 2014.01 version of the Amsterdam Density Functional (ADF) program [57] developed by Baerends and co-workers [58,59,60,61,62]. All calculations have been performed with the hybrid-type B3LYP functional (Becke’s three parameter hybrid exchange functional [63] coupled with the Lee-Yang-Parr nonlocal correlation functional) [64]. The atom electronic configurations were described by a triple-ζ Slater-type orbital (STO) basis set for H 1s, C 2s and 2p, and N 2s and 2p augmented with a 3d single-ζ polarization for C and N atoms and with a 2p single-ζ polarization for H atoms. A triple-ζ STO basis set was used for the first row transition metals Ni and Cu 3d and 4s, for second row Pd and Ag 4d and 5s and for third row Pt 5d and 6s augmented with a single-ζ 4p polarization function for the first row Ni and Cu, a single-ζ 5p polarization function for Pd and Ag and a single-ζ 6p polarization function for Pt. For the systems containing atoms in which Z is greater than 41, the scalar relativistic zero-order regular approximation (ZORA) was used (with the associated optimized valence basis set). Vibrational frequency calculation [65, 66] was performed on all the optimized geometries to verify that these structures are characterized as 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 [67] code in the ADF package of programs.

The solvent effect using the Conductor-like Screening Model for Realistic Solvent (COSMO-RS) developed by Klamt and co-workers [68] was introduced in the single point DFT calculations, where the Cartesian coordinates were extracted from the geometry optimizations.

Representations of the molecular structures were done using the ADF-GUI [57] and the MOLEKEL4.1 [69] programs, respectively.

Results and discussion

Optimized structures and MO diagrams

The studied M(MePhNap)2 compounds consist of slightly distorted square planar geometry of Ci symmetry, allowing considerable orbital mixing and more flexibility to the metal coordination sphere, where some of them are characterized experimentally [31, 32]. The full optimized geometry have been carried out on the diamagnetic Ni, Pd, and Pt metal species, where the two substituted azo ligands are symmetrical through an inversion center occupied by the metal cation which is tetraconnected via one N atom and one O atom of each azo dye molecule. Hence, our analysis has been extended to the paramagnetic Cu and Ag species. For all the studied complexes, the metal cation is connected to two molecules of (1-phenylazo-2-naphtol) through N and O atoms providing a square planar environment of the metal conducting to ML4 complexes. The optimized geometries for Ni, Pd, and Pt complexes showed a perfect square planar arrangement around the M(II) metal cation as sketched in Fig. 1, while the azo ligand undergoes substantial modifications losing its nearly flatness as displayed in Scheme 1 and Fig. 1, because of the fixation of coordinative bonds and the geometric requirement for the central M(II) square plane. The planarity of the different ML4 complexes around the metal is highlighted by the ONON dihedral angle of 0° and the linear O–M–O and N–M–N bond angles of 180° for Ni, Pd, and Pt complexes, in accordance with the 16-MVE (metal valence electrons) configuration of the Ni(II), Pd(II), and Pt(II) as d8 metal cations. The computed M–N bond distances of 1.919, 2.101, and 2.108 Å and M–O bond distances of 1.842, 2069, and 2.077 Å for Ni, Pd, and Pt complexes, respectively, are in harmony with the increasing of the metal radii following the order: Ni < Pd < Pt. It is worth nothing that for the Pd structure, the optimized parameters are comparable to those observed experimentally [31] as gathered in Table 1 and comparable to those reported in recent work [33, 34]. The calculated C–C bond distances ranging from 1.38 to 1.42 Å (Scheme 1) suggest a delocalized scheme within the phenyl and naphtyl rings for Ni, Pd, and Pt complexes as shown in Fig. 1. As clearly shown in Fig. 1, both azo dye molecules are oriented to each other by 105, 98, and 95° for Ni, Pd, and Pt species, respectively, due to the steric repulsions, thus avoiding the planarity of the different complexes. It is important to note that the orientation between the azo ligands is more marked for the 3d Ni transition metal than those of 4d and 5d ones. The N–N bond distance of 1.269 Å for the azo group in the free molecule undergo lengthening for Ni complex becoming equal to 2.272 Å, but slightly shortened in Pd and Pt complexes becoming equal to 2.266 and 2.265 Å, respectively. The MO diagrams for the diamagnetic Ni(MePhNap)2, Pd(MePhNap)2, and Pt(MePhNap)2 complexes are shown in Fig. 2, exhibiting large HOMO-LUMO gaps of 3.15, 3.08, and 3.07 eV, respectively, suggesting good chemical stability. For the three neutral Ni(MePhNap)2, Pd(MePhNap)2, and Pt(MePhNap)2 complexes, the LUMOs are ligand character, while the HOMOs are weakly metallic orbitals showing a contribution not exceeding 10% in each case. It is interesting to note that the orbitals with considerable metal based are located relatively at low energies, particularly, 71ag (31%), 73ag (15%), and 74ag (40%) for Ni(MePhNap)2; 71ag (30%), 73ag (35%), and 74ag (30%) for Pd(MePhNap)2; and 71ag (30%), 71ag (30%), 73ag (20%), and 74ag (20%) for Pt(MePhNap)2 which are principally nonbonding orbitals.

Fig. 1
figure 1

Optimized of M[MePhNaph]2 (M = Ni, Pd, and Pt) singlet structures of 16-MVE configuration

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Table 1 Selected geometrical and energetic parameters for neutral M(MePhNap)2 (M = Ni, Pd, Pt, Cu, Ag), cationic [M(MePhNap)2]+ (M = Cu, Ag), and anionic [Ag(MePhNap)2]− complexes of different metal valence electrons (MVE). The HOMO-LUMO gap is given in (eV), and total bonding energy is given in (kcal/mol)
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Fig. 2
figure 2

MO diagrams of M(MePhNap)2 (M = Ni, Pd, Pt). The metal contributions in (%) are given in parentheses

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The optimized geometry of the paramagnetic Cu(MePhNap)2 and Ag(MePhNap)2 complexes are sketched in Fig. 3 and Fig. 4. For the optimized neutral Cu(MePhNap)2 structure having 17-MVE configuration in its doublet spin state (S = ½), the Cu–N and Cu–O bond distances of 2.020 and 1.921 Å are comparable to the experimental ones of average values of 1.999 and 1.924 Å, respectively, and the N–Cu–N and O–Cu–O bond angles are of 180° [32]. The NONO plane centered by the copper dication of 0° corresponds to ML4 square planar geometry, while the azo dye molecules are oriented to each other by 98°. The one electron oxidation gives rise to the [Cu(MePhNap)2]+ structure of 16-MVE configuration which is isoelectronic with the neutral Pd and Pt complexes and exhibiting a significant HOMO-LUMO gap of 1.66 eV as clearly shown in Fig. 5. This oxidation leads to the Cu–N and Cu–O bond distances lengthening from 2.020 and 1.921 Å to 2.259 and 1.983 Å, respectively, but keeping the same square planar geometry. The neutral Ag(MePhNap)2 17-MVE structure consists of an ML4 square plane, in which the N–Ag–N and O–Ag–O are linear and Ag–N and Ag–O bond distances are of 2.275 and 2.215 Å. The one electron reduction of the square plane Ag(MePhNap)2 structure without any symmetry constraints gives rise to the anionic [Ag(MePhNap)2]− structure undergoing a considerable distortion as shown in Fig. 4, wherein the N–Ag–N and O–Ag–O of 159 and 124° deviate considerably from the linearity, in accordance with the 18-MVE configuration of a tetrahedral ML4 complex around the Ag+ center, in which the N–Ag and O–Ag bond distances undergo considerable lengthening from 2.275 and 2.215 to 2.440 and 2.376 Å, respectively, in agreement with the population of the antibonding 83ag orbital as shown by the MOs diagram of Fig. 5. However, the [Ag(MePhNap)2]+ structure keeps the same square plane geometry, but accompanied by slight shortening of N–Ag and O–Ag bond distances. It is noticeable that the redox property studies of Cu complexes are showing the irreversible character [70].

Fig. 3
figure 3

Optimized [Cu(MePhNaph)2]+ and Cu(MePhNaph)2 structures of doublet (S = 1/2) (a) and singlet (S = 0) (b) spin states, respectively

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Fig. 4
figure 4

Optimized [Ag(MePhNaph)2]+ (a), Ag(MePhNaph)2 (b), and [Ag(MePhNaph)2]− (c) structures of 16-, 17-, and 18-MVEs, respectively

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Fig. 5
figure 5

MO diagrams of [Cu(MePhNap)2]+ and [Ag(MePhNap)2]−. The metal contributions in (%) are given in parentheses

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Electronic spectra

In order to gain a better understanding of the simulated UV–Vis spectra, TD-DFT calculations were carried out on the MePhNap azo dye molecule substituted at the ortho position of the phenyl by CH3 and their transition metal M(MePhNap)2 (M = Ni, Pd, and Pt) complexes. The theoretical electronic spectrum (Fig. 6a) of the substituted azo dye in gas phase displays a major band in the region of 420–445 nm due to the (πC-C + n)→(π*N–N + π*C–O) and πC–C→(π*N–N + π*C–O) transitions corresponding to the HOMO-1→LUMO (81%) and HOMO-LUMO (15%) electronic transitions with an oscillator strength of 0.42 (a.u), implying the naphtyl ring and the –N=N– azo group as intramolecular charge transfer (ILCT). Whereas, the theoretical electronic spectra of the studied azo dye exhibit red shifts in different used solvents (Fig. 6a). Really, one can observe that the polar DMSO and ethanol solvents conduct to similar spectra, where the main pick is moved to the region of 480–512 nm with an oscillator strength of 0.8 (a.u). Since they have sharp UV–Vis spectra that are tremendously sensitive to the immediate environment, like as the solvent polarity phenomenon, which is known for many dye molecules [71, 72]. The electronic spectrum obtained in hexane solvent is slightly shifted to the region of 445–470 nm with oscillator strength of 0.6 (a.u), which is comparable to the experimental results found by V.N. Mkpenie et al., where the main peak is shifted from 450 to 460 nm [73]. It is worth noting that the same orbitals are implied in the electronic transitions both in gas phase and in the different solvents used, but exhibiting differences in their contribution as clearly shown in Table 2.

Fig. 6
figure 6

Electronic spectra of a, b, c, and d obtained in gas phase and different solvents for the uncomplexed azo dye, Ni, Pd, and Pt complexes, respectively

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Table 2 Calculated electronic transitions and their assignments, the dominant contributions to electron excitations in percentages (%), wavelengths λ (nm), oscillator strengths f (au), and excitation energies ΔE (eV) for uncomplexed (MePhNap) species obtained in gas phase and different solvents
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The three electronic spectra of Ni, Pd, and Pt diazo complexes exhibit similar shapes, but with somewhat differences concerning the bands’ positions and their widths (Fig. 6b, c, d). Our spectral analysis is based on the comparison of the theoretical spectra obtained for the uncomplexed and the complexed species in gas phase and in different solvents as shown in Fig. 6, while the identification of the electronic transitions are based on the MOs’ localization. Indeed, different from the electronic spectrum of the (MePhNap) azo dye molecule, one can observe the appearance of new bands and the weakness of others. Because M(MePhNap)2 complexes are constructed from two bidentate π-conjugated (MePhNap) azo ligands connected to one transition metal, different electronic transitions should appear; these can be probably classified into metal to ligand charge transfer (MLCT), ligand to metal CT (LMCT), intrametal d–d* orbitals, intraligand, and interligand transitions. The calculated transitions have been assigned to these classes (Tables 3, 4, 5) by TD-DFT analysis based on the orbital characteristics (Fig. 6). For the Ni complex spectrum (Fig. 6b), the main absorption band appears essentially in the ultraviolet region centered at 370, 374, 382, and 382 nm in gas phase, hexane, ethanol, and DMSO, respectively, corresponding to the intense (d(M) + π)→π* transitions, which are remarkably reinforced (Table 3). These transitions are metal-ligand and intraligand characteristics corresponding to excitations from the metal and the MePhNap’s HOMO (76ag which is made of a mixture of d metallic, nitrogen and oxygen orbitals) to LUMO (76au which is purely ligand). While the relatively weak band appearing in the visible region and centered at 476, 477, 482, and 482 nm in gas phase, hexane, ethanol, and DMSO, respectively, is established between the HOMO (76ag) and the LUMO (76au) electronic transition with and can be classified into metal to ligand charge transfer (MLCT) and intraligand charge transfer (ILCT). It is noteworthy that the electronic transitions are intensified in polar solvents rather than in both nonpolar one and in gas phase.

Table 3 Calculated electronic transitions and their assignments, the dominant contributions to electron excitations in percentages (%), wavelengths λ (nm), oscillator strengths f (au), and excitation energies ΔE (eV) of Ni(MePhNap)2 complexes obtained in gas phase and different solvents. The oscillator strength values less than 0.05 (au) are not listed
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Table 4 Calculated electronic transitions and their assignments, the dominant contributions to electron excitations in percentages (%), wavelengths λ (nm), oscillator strengths f (au), and excitation energies ΔE (eV) of Pd[MePhNap]2 complexes obtained in gas phase and different solvents. The oscillator strength values less than 0.05 (au) are not listed
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Table 5 Calculated electronic transitions and their assignments, the dominant contributions to electron excitations in percentages (%), wavelengths λ (nm), oscillator strengths f (au), and excitation energies ΔE (eV) of Pt[MePhNap]2 complexes obtained in gas phase and in different solvents. The oscillator strength values less than 0.05 (au) are not listed
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For the Pd complex spectrum (Fig. 6c), the main absorption band appears essentially in the ultraviolet region centered at 370, 373, 379, and 379 nm in gas phase, hexane, ethanol, and DMSO, respectively, corresponding to the intense (d(M) + π(L))→π*(L) transitions, which are considerably enhanced in intensity (Table 4) compared to those obtained for the uncomplexed species. These transitions are metal-ligand and intraligand characteristics stemming from the metal and the MePhNap’s HOMO-3 (72ag which is made of a mixture of d metallic, nitrogen, and oxygen orbitals) to LUMO (70au which is purely ligand) featuring the principally transition (72%) and coupled to two others less intense as specified in Table 4. The band found at relatively low energy centered at 482, 484, 489, and 489 nm in gas phase, hexane, ethanol, and DMSO, respectively, occurred chiefly between the HOMO (73ag) and the LUMO (70au) corresponding to the metal to ligand charge transfer (MLCT) and intraligand charge transfer (ILCT), where the λmax is reached for the polar DMSO and ethanol solvents for oscillator strength values of 0.28 and 0.27 (a.u). Finally, in the electronic spectrum of azo-Pt complex (Fig. 6d), the absorption band A in the visible region at 476 nm in gas phase corresponds to the (d(M) + n)→(π*N–N + π*C–O) electronic transition assigned to HOMO→LUMO transition, which is shifted by about 51 nm than that of the free ligand as is aforementioned. In the gas phase, the strong absorption band appears in the ultraviolet region centered at 370 nm.

Also, it is worthwhile to mention the weak ligand to metal charge transfer LMCT occurred from the ligand orbitals to 78ag, 75ag, and 76ag as LUMO+2 for Ni, Pd, and Pt complexes, respectively, which feature considerable metal contributions as shown in Fig. 3.

Conclusion

In this theoretical investigation, we have shown that the geometry of free ligand does not deviate from the planarity which exhibits a delocalized scheme within both phenyl and naphtyl rings connected to each other by the azo –N=N– group.

Once the azo ligand is complexed, it could not maintain the approximate planar structure and the dihedral angles between the phenyl and naphtyl rings become almost perpendicular evidenced by the dihedral angle value of about 95° in the M(MePhNap)2 (M = Ni, Pd, Pt, Cu, and Ag) complexes, which are comparable to the available Pd and Cu experimental data. Whereas, the metal M(II) cation is in a perfect square planar environment as an ML4 fragment obeying the 16-MVE configuration and the different computed structures display large HOMO-LUMO gaps. The reduced [Ag(MePhNap)2]− species underwent a geometrical distortion from ML4 with 17-MVE square planar to ML4 tetrahedral structure obeying the 18-MVE rule. From the electronic spectra obtained for the Ni, Pd, and Pt complexes, one can observe that the main peaks are attributed to the HOMO→LUMO and HOMO-1→LUMO electronic transitions. Based on the obtained HOMO-LUMO gaps and the electronic spectra for the metal-azo complexes, slight bathochromic shifts are observed towards the long wavelengths with diminution of the HOMO-LUMO energy gaps; thus, the enhancement of the wavelengths pursues the following sequence: Ni < Pd < Pt.