The physical and electronic transitions of the complexes are presented in Table 1.
3.1 FT-IR spectra.
The FT-IR spectral data of the complexes are illustrated in Table 2. The peaks of the complexes were compared with that of the free 2-hydrazinopyridine (HPY) and free dithiooxamide (DTO) ligands to monitor the variations in the frequencies of the coordination sites. The spectrum of the free 2-hydrazinopyridine (HPY) ligand showed bands at (3395, 3308 cm-1) which are assigned to the stretching vibrations asymmetrical and symmetrical of the amine groups. The spectrum of the [MoO2(acac)(HPY)] complex showed broad bands at (3089, 3016 cm-1) attributed to asymmetrical and symmetrical (NH2) stretching frequencies; respectively. This is an indication of the coordination between the 2-hydrazinopyridine (HPY) ligand and the Mo(VI) ion through the nitrogen atoms of the amine groups. The band at 1600 cm-1 vibration was assigned for the υ(C=O) stretching frequency in (acac). The spectrum exhibited new bands at (921, 1110 cm-1) that can be attributed to symmetric and asymmetric stretching of υ(O=Mo=O) in cis-configuration [18-19]. The` spectrum of the [MoO2(DTO)(HPY)] complex is illustrated in Fig. 2 showed broad bands at (3618, 3047 cm-1) attributed to asymmetrical and symmetrical (NH2) stretching; respectively [19]. This is confirmed the coordination between the 2-hydeazinopyridine (HPY) ligand and the Mo ion through the nitrogen atoms of the amine groups. The complex with (DTO) spectrum also exhibited a new band at (953, 1153 cm-1) that can be attributed to symmetric and asymmetric stretching of υ(O=Mo=O) in cis-configuration [11, 19]. The thioamide group stretching three bands have appeared in this complex at ( 1527, 1423, and 1191 cm-1), this confirms the coordination of the (DTO) ligand to the Mo ion [11]. The spectrum of the [MoO(acac)(HPY)] complex showed broad bands at (3606, 3101 cm-1) attributed to asymmetrical and symmetrical (NH2) stretching; respectively. The band at 1519 cm-1 assigned for the vibrations of υ(C=O) in (acac). The spectrum showed a band at (952 cm-1) this band refers to the ν( Mo=O) stretching. The spectrum of the [MoO(DTO(HPY)] complex showed broad bands at (3738, 3603 cm-1) attributed to asymmetrical and symmetrical (NH2) stretching. The thioamide group stretching three bands have appeared in this complex at ( 1519, 1427, and 1172 cm-1), this confirms the coordination of the (DTO) ligand to the Mo ion [11]. The spectrum showed a band at the range (952 cm-1) which refers to the ν( Mo=O) stretching [18-19]. The M-N, M-O, and M-S bands appeared in the complexes at ( 450-467 cm-1, 487-510 cm-1, and 570-585cm-1); respectively [12, 21-22]. The experimental FT-IR data of the dioxomolybdenum and oxomolybdenum complexes were compared with the calculated data of optimized complexes structure obtained from the DFT calculation by using (Gaussian 09W software) and it was without any negative value that and it's in good agreement with the experimental ones. The differences between the experimental and the calculated data due to the different methods used to obtain them.
3.2 Mass spectral analysis
The mass spectra of the complexes exhibited the main mass fragmentation peaks which are listed in Table 3. Mass spectrum of the complex [MoO2(acac)(HPY)] (molecular weight 339.01) gave molecular ion peak (M) at m/z = 338.40, a peak at m/z = 238.23 assigned for (M-acac), a peak at m/z = 230.94 assigned for (M-HPY), a peak at m/z = 130.16 attributed to (MoO2). The spectrum showed also peaks at m/z=95.08, 96.0, and 96.79 which are assigned to the stable molybdenum isotopes. Mass spectrum of the complex [MoO2(DTO)(HPY)] (molecular weight equals 358.94) gave molecular ion peak (M) at m/z=(356.66), a peak at m/z=250.40 assigned for (M-HPY), a peak at m/z=235.59 assigned for (M-DTO), a peak at m/z=129.26 assigned for (MoO2), a peak at m/z=119.89 assigned for (DTO), a peak at m/z=110.76 assigned for (HPY) ligand. The spectrum showed also peaks at m/z=97.41, 98.86, and 101.37 which are assigned to the stable molybdenum isotopes. Mass spectrum of the [MoO(acac)(HPY)] complex (molecular weight 323.02) gave molecular ion peak (M) at m/z=323.09, a peak at m/z=227.14 assigned for (M-acac), a peak at m/z = 215.01 assigned for (M-HPY), a peak at m/z=115.64 attributed to (MoO). The spectrum showed also peaks at m/z=95.08, 96.0, and 96.79 which are assigned to the stable molybdenum isotopes. Mass spectrum of the [MoO(DTO)(HPY)] complex (molecular weight equals 342.95) gave molecular ion peak (M) at m/z=(340.25), a peak at m/z=225.93 assigned for (M-DTO), a peak at m/z=235.15 assigned for (M-HPY), a peak at m/z=113.26 assigned for (MoO), a peak at m/z=121.19 assigned for (DTO), a peak at m/z=111.96 assigned for (HPY) ligand. The spectrum of the complex showed also a peak at m/z=99.05, which is assigned to the stable molybdenum isotope [23]. The mass spectrum data of the [MoO2(DTO)(HPY)] complex is presented in Fig. 3 as a represented example. The data of mass spectra for the complexes are presented in (SI).
3.3 1H-NMR spectra
The 1H-NMR spectral data for the free ligands and Mo(VI) complexes in DMSO-d6 are presented in Table 4. The experimental data compared with the calculated spectra that obtained from the DFT calculations. The 1H-NMR spectrum of the 2-hydrazinopyridine (HPY) ligand showed the signals at the range (δ=7.38-8.50 4H) ppm are assigned to the pyridine group protons as multiple peaks. The characteristic signal at (δ=4.36 H) ppm is assigned to the NH proton as a single peak. The signal at (δ=3.66 2H) ppm is assigned to the NH2 proton as a single peak. The 1H-NMR spectrum of the [MoO2(acac)(HPY)] complex showed the signal at the range (7.8-8.7 4H,s) ppm assigned to pyridine group protons as multiple peaks. The signal at (δ=6.1 H) ppm is assigned to CH for the enol form of (acac), a peak at (δ= 3.34 2H) as a singlet peak which is assigned to NH2 protons, and also peak at (δ=1.34 6H) ppm as a singlet peak which is assigned to CH3 of (acac) ligand. The 1H-NMR spectrum of the [MoO2(DTO)(HPY)] complex showed signals at the range (δ=7.15-8.46 4H) ppm which are assigned to the pyridine group protons as multiple peaks. The signals at (δ=9.69-10.21) ppm are assigned to NH2-DTO. The signal at (δ=2.23-3.37) ppm is assigned to NH2 protons as a singlet peak, which is shifted to downfield from those of the free 2-hydrazinopyridine (HPY) ligand. The signal at (δ=4.21-4.30 H) ppm can be assigned for the NH. These data indicate that the 2-hydrazinopyridine (HPY) ligand coordinates with molybdenum(VI) atom by two nitrogen atoms of the NH2 groups and with the dithiooxamide (DTO) ligand by the two sulfur atoms. The calculated 1H-NMR data of the Mo(VI) complexes by use of DFT/LanL2DZ basis set were compared with the experimental data and it is in good agreement with the experimental data; Fig. 4 shows the experimental and calculated 1H-NMR spectra of [MoO2(DTO)(HPY)] complexes [15, 24].
3.4 Electronic spectra
The electronic spectral data of the complexes in the DMSO solutions were recorded in the 200–1100 nm Table 1 and compared with the calculated spectra obtained from the TD-DFT calculations in DMSO as solvent. The experimental UV-Vis. and calculated spectra of the [MoO2(DTO)(HPY)] complexes are given in Fig. 5. The absorption spectrum showed peaks at (281, and 325 nm), which can be assigned to (π-π*) and (n- π*) of the intra-ligand electronic transitions. These peaks were shifted to lower wavenumbers when compared with the peak of free ligands. The spectrum also exhibited a peak at (438 nm) assigned to LMCT from L(pπ)→dMo. The spectrum of the [MoO2(acac)(HPY)] complex showed peaks at (296, and 386 nm), which can be assigned to (π-π*) and (n- π*) of the intra-ligand electronic transitions. These peaks were shifted to lower wavenumbers when compared with the peak of free ligands. The spectrum also exhibited a peak at (407 nm) assigned to LMCT LMCT from L(pπ)→dMo (SI). UV-Vis. spectra of the [MoO(acac)(HPY)] and [MoO(DTO(HPY)] complexes are given in (SI). The absorption spectra showed peaks at (270-268, and 318-369 nm); respectively which can be assigned to (π-π*) and (n- π*) of the intra-ligand electronic transitions. The spectra exhibited peaks at the range (440, and 447 nm) assigned to LMCT from L(pπ)→dMo. The d-d electronic transitions within the octahedral arrangement around Mo(VI) (d0-configuration) have vanished whereas for Mo(IV) (d2-configuration) observed as a weak band at the range (637 and 704 nm); respectively [12, 25]. The experimental UV-Vis. data of the prepared complexes have been compared with the calculated ones by using of TD-DFT/LanL2DZ basis set in DMSO as solevent. There were acceptable differences between the experimental and the calculated data due to the different ways used to determine each one; solid-state for the experimental data and gaseous state for the calculated data.
3.5 Magnetic measurements
The magnetic measurement of the prepared complexes showed that the Mo(VI) complexes were diamagnetic with d0 configuration whereas the Mo(IV) complexes were paramagnetic with d2 (t2g2,eg0) electronic configuration and the values of the μeff for the complexes are (2.9 and 3.1) [MoO2(acac)(HPY)] and [MoO2(DTO)(HPY)]; respectivily[26].
3.6 Theoretical studies
The optimized structures of the 2-hydrazinopyridine (HPY), dithiooxamide (DTO) ligands, dioxomolybdenum (VI), and oxomolybdenum(IV) complexes were carried out by using the B3LYP/LanL2DZ basis sets [27, 28]. The complexes structure with natural bond order (NBO) charges of the molybdenum and binding sites atoms are given in Fig. 6. Selected bond angles and bond lengths are given in Table 5. In Mo(VI) complexes the angles between Mo(VI) atoms and the surrounded atoms are ranged from 71.51 to 107.51 which suggests the distorted octahedral geometry for the complexes [15, 29]. The (N-Mo-N) bonds angle ranged from 71.51 to 72.58, it deviated from the perfect octahedral structure, which supports the suggestion of the distortion in the structure of the complexes. The bond lengths between the Mo(VI) atoms and the nitrogen atoms in the complexes are (2.18-2.44 Aº). The Mo=O bonds length are (1.72 Aº), angles with cis-configuration of (O=Mo=O) are in the range (106.74º-107.51º) and the angles contain the oxygen, and nitrogen atoms are in the range (81.33º-89.55º), whereas the angle between the two sulfur atoms and Mo(VI) is (81.49º). The Mo-S bonds length are (2.45-2.65 Aº) and the angle contains the sulfur and nitrogen atoms is (89.55º). These angles are compared with the reported Mo(VI) complexes havd been prepared prevously, the values are consistent with the values reported (2.43) [31]. Atomic charges are very important to conclude and expected the donor and acceptor atoms in the (HPY) and (DTO) ligands and molybdenum [25, 31]. The charge densities are on the nitrogen atoms in the 2-hydrazinopyridine (HPY) and on the sulfur atoms in the dithiooxamide ligands. Molybdenum with it hexavalent coordinate and positive charge in the complexes acts as the acceptor of the charge. This is a ligand to metal charge transfer (LMCT) from the π orbitals of the (HPY) and (DTO) ligands to the Mod orbital. The Mo(VI) complexes are more polarized than the (HPY) and (DTO) ligands, the dipole moments of the complexes are (8.11-10.80 Debye), whereas for (HPY) and (DTO) are (1.46 and 1.67 Debye); respectively [32-34]. In Mo(IV) complexes the angles between Mo(IV) atoms and the surrounded atoms are (74.85º to 110.61º) which suggests the distorted square pyramidal geometry for the Mo(IV) complexes [35]. The (N-Mo-N) bond angles are (74.85º-77.18º) deviated from the perfect square pyramidal structure and the bond lengths between the Mo(IV) atom and the nitrogen atoms in the complexes are (2.12-2.24 Aº). The Mo=O bond lengths are in the range (1.70-1.71 Aº) for the two complexes, these data have been compared with the reported oxomolybdenum(IV) complexes, and the values are consistent with the value reported (1.68) [36]. The angles contain the oxygen, and nitrogen atoms are in the range (80.10º to 90.39º), whereas the angles between the two sulfur atoms and Mo(IV) is (85.30º). The Mo(IV) complexes are more polarized than the ligands as indicated from the values of the dipole moments values of the complexes (6.36-13.72 Debye) [33-34, 37]. The electronic energy, the atomic charges, and the dipole moments of the ligands, and the complexes are tabulated in Table 6. According to (NBO) analysis for Mo(VI) complexes the electronic configuration of Mo in the [MoO2(acac)(HPY)] complex are:[core] 5s0.21 4d4.05 5p0.47 5d0.05, 35.968 core electrons, 4.732 valence electrons and 0.055 Rydberg electrons, which gives 40.755 total electrons and +0.556 e charge on Mo atom. The occupancies of 4d in orbitals are dxy 0.824; dxz 0.829; dyz 0.867; dx2-y2 0.741 and dz2 0.785. The 4d-electron populations (4.046) are in agreement with the charge transfer from (HPY) and (acac) ligands to dMo. The electronic configuration of the complex [MoO2(DTO)(HPY)] are: [core] 5s0.27 4d4.45 5p0.68 5d0.06, 35.974 core electrons, 5.460 valence electrons and 0.067 Rydberg electrons, which gives 41.501 total electrons and +0.556e charge on Mo atom. The occupancies of 4d orbitals are dxy 0.857; dxz 0.901; dyz 0.824; dx2-y2 1.013 and dz2 0.857. The 4d-electron populations (4.453) are in agreement with the charge transfer from (DTO) and (HPY) ligands to dMo. The oxomolybdenum Mo(IV) complexes, the electronic configuration of [MoO(acac)(HPY)] complex are: [core] 5s0.17 4d4.391 5p0.24 5d0.03 6p0.11, 35.963 core electrons, 4.911 valence electrons and 0.032 Rydberg electrons, which gives 40.906 total electrons and +1.092e charge on Mo atom. The occupancies of 4d orbitals are dxy 1.035; dxz 0.677; dyz 0.650; dx2-y2 1.355 and dz2 0.671. The 4d-electron populations of 4.388 are in agreement with (HPY) and (acac) ligands to dMo electron transfer [38]. The electronic configurations of [MoO(DTO)(HPY)] complex are: [core] 5s0.25 4d4.91 5p0.19 5d0.04 6p0.37, 35.968 core electrons, 5.713 valence electrons and 0.040 Rydberg electrons, which gives 41.721 total electrons and +0.276e charge on Mo atom. The occupancies of 4d orbitals are dxy 1.166; dxz 0.756; dyz 0.760; dx2-y2 1.492 and dz2 0.729. The 4d-electron populations of 4.903 are in agreement with (HPY) and (DTO) ligands to dMo electron transfer. The electronic energies of the Mo(VI) complexes are (-784.07 and -921.99 a.u.); respectively and for the Mo(IV) complexes are (-708.85 and -846.76 a.u.); respectively these values indicate the stability of dioxomolybdenum complexes are more than the oxomolybdenum complexes [38]. HOMO and LUMO orbitals energies of the complexes are given in Table 6. The hardness (η=(I-A)/2) where (I-A)=ΔE=HOMO-LUMO energy levels. The η values and ΔE are given in Table 6. The transitions of electrons are easier in the Mo(VI) complexes than the ligands which are indicated from ΔE of the Mo(VI) complexes (0.105-0.130) whereas for the (HPY) and (DTO) ligands are (0.124 and 0.191); respectively [39-43]. The Mo(VI) complexes are softer (η=(0.052-0.065) than the ligands also (0.062-0.095) [44]. The transitions are also easier in the Mo(IV) complexes than the ligands ΔE of the Mo(IV) complexes are (0.099-0.110) [42]. The Mo(IV) complexes are softer (η=(0.045-0.055) than the ligands also [43]. The negative values of the energies for the HOMO orbitals and the LUMO orbitals in the Mo(VI) and Mo(IV) complexes support the suggestion of their stability [39]. The surface plots of the HOMO and LUMO orbitals for (HPY), (DTO) ligands, Mo(VI), and Mo(IV) complexes are presented in Figs. 7 and 8. The transition energies of the complexes have been calculated from (time-dependent density functional linear response theory) Table 7. The density of the electrons in the (HPY) ligand is localized on the pyridine part and on the nitrogen atoms which may point to a mixed π→π* and n→π* transitions, whereas for the (DTO) ligand the red regions are on the sulfur atoms and the blue regions are on the nitrogen atoms [39]. The HOMO energies (H to H-4), % of the contribution, and the main characters of (HPY), (DTO) ligands, and the molybdenum are calculated for the complexes and tabulated in Table 8. In the [MoO2(acac)(HPY)] complex, % contribution of (HPY) to the HOMOs (H to H-4) in the range from 3% to 99% with the main character is HPY(π). The % contribution of (HPY) to LUMOs (L to L+4) is lower (6%-13%) (except L+4 90%). The % contribution of the (acac) ligand to the HOMOs is low and varies from 0% to 7% (except H-1 90% and H-2 86%) through acac(π) as the main character. The (acac) ligand % contribution to LUMOs is higher than its contribution to HOMOs and varied from 1% to 18% through acac(π*) (except L+3 69%). The Mo % contribution to the HOMOs varied from 0% to 4%, whereas, the % contribution of Mo to the LUMOs varied from 19% to 54% (L+4 7%) by Mo(eg), which states the possibility of LMCT from O(π) and/or coordinated ligands to Mo(eg) [12, 32-34]. In the [MoO(acac)(HPY)] complex, % contribution of (HPY) to the HOMOs (H to H-4), ranged from 4% to 83% with the main character is HPY(π). The % contribution of (HPY) to LUMOs (L to L+4) is higher (13%-87%). The % contribution of the (acac) ligand to the HOMOs is from 12% to 85% through acac(π) as the main character. The (acac) ligand % contribution to LUMOs is lower than its contribution to HOMOs and varied from 2% to 28% through acac(π*) (except L+1 32%). The Mo % contribution to the HOMOs varied from 1% to 6% (except H 74%) whereas, the % contribution of Mo to the LUMOs varied from 9% to 49% by Mo(eg), which support the possibility of LMCT from O(π) and/or coordinated ligands to Mo(eg). The Mo(VI) and Mo(IV) complexes with (DTO) ligand instead of (acac) ligand, the % of (DTO) ligand in HOMOs are much higher than Mo and (HPY). On the other hand, the LUMOs are mainly concentrated on the Mo atoms, which support the LMCT transitions. The % contribution and the main characters of Mo, (acac), (HPY), and (DTO) ligands to the different HOMO and LUMO orbitals in the Mo(VI) complexes are presented in Table 8. The molecular electrostatic potential (MEP) for the ligands and the complexes have been calculated and shown in Fig. 9, the red regions represent an electrophilic reactivity and the blue regions represent a nucleophilic reactivity. The nitrogen atoms of the (HPY) and the sulfur atoms in the (DTO) ligands; with their red regions (negative charge) are the reactive sites for the electrophilic attack, this constitutes the high electronegativity of the two atoms [32-34, 39]. The red regions in the complexes are mainly localized over the oxygen and sulfur atoms.