Dioxomolybdenum (VI) and Oxomolybdenum (IV) Complexes With N, O, and S Bidentate Ligands, Spectral Characterization, and DFT Studies


 Two dioxomolybdenum (VI) complexes with chemical formula [MoO2(acac)(HPY)], and [MoO2(DTO)(HPY)], with another two oxomolybdenum (IV) complexes [MoO(acac)(HPY)], and [MoO(DTO)(HPY)] have been prepared and characterized by different spectral techniques such as (FTIR, UV-Vis., Mass, 1H-NMR) spectra, magnetic susceptibility, and theoretical studies. The bidetentate ligands used in this study were acetylacetone (acac), 2-hydrazinopyridine (HPY), and dithiooximid (DTO). All the spectroscopic data and the theoretical calculations support the suggestion that the dioxomolybdenum(VI) complexes are diamagnetic and have distorted octahedral structures wheras the oxomolybdenum(IV) complexes are paramagnetic and have distorted square pyramidal structures. Theoretical calculations of the free ligands and the prepared complexes have been done by using DFT calculations by using (GAUSSian 09W) software with basis sets (B3YLP/LanL2DZ). The complexes were very stable and their energies ranged from (-708.85 to -921.99 a.u.) and were very different from that of the free (HPY, DTO) ligands (-359.06 and -984.54 a.u.); respectivily. The prepared complexes are polar (8.11-10.80 Debye) for dioxocomplexes(VI), and (6.63-13.72 Debye) for oxocomplexes(IV), its more than the free (HPY and DTO) ligands (1.46-1.67 Debye); respectivly. The HOMO orbitals energies of the dioxocomplexes(VI) are (-0.229, and -0.377 a.u.), respectivily whereas oxocomplexes(IV) are (-0.192, -0.318 a.u.); respectivily while for the ligands are (-0.216, -0.262 a.u.), respectivily. The LUMO orbitals energies of the dioxocomplexes(VI) are (-0.124, and -0.247 a.u.) and for the oxocomplexes are (-0.093, -0.208 a.u.) its obvouse that they are more lower in their energies than that for the (HPY and DTO) ligands.


Introduction
Molybdenum is an essential trace element required in most biological systems [1]. It has many oxidation states (II-VI), the oxidation states (II, III, IV, and V) are air sensitive. The salts of molybdenum are more stable due to its being durability, toughness, and hardness, it was made them alloys and steel [2]. Water can contain another amount of different concentrations from molybdenum and on the other hand, humans, animals, and plants include a vital trace element from molybdenum [3]. The most important oxidation states of molybdenum are (IV) and (VI) states during the binding and reactions. A stable oxomolybdenum(IV) complexes are comparatively scarce than their dioxomolybdenum (VI) coordinate and structurally characterized oxomolybdenum (IV) complexes are excessively rare [4]. The importance of molybdenum complexes in the medicinal applications of dental caries, enhancement of immunological reaction, anticancer and antidiabetic agents, therapeutic, medicinal immense and its effect on different enzymes [5]. Molybdenum can be given oxidation state in oxo complex so determined by the number of oxo groups that to join in center atom [6]. Molybdenum play as a cofactor in three enzyme called molybdoenzymes. This molybdoenzymes has an oxo-group that to believe to responsible for the oxo transferase activity in this enzyme. The oxo-group includes a molybdenum center that can be coordinated by two or more S and N donor ligand [7]. Molybdenum(VI) complexes include a cis-MoO 2 unit that has worked the same as enzyme-like xanthine oxidase and nitrogenase, the complexes are contained a Mo = O unit used as catalysts in the industry because of oxygen atom transfer reactions [8]. The discovery of the presence of NSO donor points around the Mo(VI) center of oxotransferase enzymes like xanthine oxidase and DMSO reductase led to the synthesis and exploration of the oxo transferability of model complexes that mimic the oxotransferase molybdoenzymes [9]. Dithiooximide (DTO) is an external active agent with varied coordination chemistry due to the intense color character, (DTO) can be used in photographic processes, coordination polymers, and histological agents. (DTO) complexes in transition metals have properties as semiconductor, spectroscopic and magnetic [10]. (DTO) has a long history of use as a reagent for the detection and determination of more transition metals due to the presence of two parts of the thioamide in this class of compounds, it plays an important role in chemotherapy because a large number of biologically active compounds contain this part (-N-C = S) [11]. Dioxomolybdenum(VI) complexes with bi-donor ligands and polydonor atoms such as O and N and ligands with donor sets O2N, O2N, SO2N, or S2N can catalyze oxo-atom transfer reactions. Mo(VI) complexes with 2-hydroxyarylidene thiosemicarbazones having N2O or ONS donor sets have important results in biological activity tests [11].
Other complexes of Mo(VI) with aroylhydrazone ligands containing a cis [MoO 2 ] 2+ core have been prepared and extensively studied due to their structural exibility, facile preparation, and their stability.
Mononuclear dioxomolybdenum(VI) complexes [MoO 2 LB] [H 2 L = 2-aminobenzoylhydrazone of benzoyl acetone have been prepared and investigated [12]. Mo(VI) complexes are potential as an anti-diabetic agent but didn't have an understanding of Mo(VI) speciation in biological media. The complex speciation in aqueous solutions of molybdate is further underscored by the formation of several oligomeric species, conversion among oxidation states, and rapidly exchanging forms. This property gives the Mo(VI) complexes to be insulin-mimetic candidates and this can be explained as a result of reversible inhibition of phosphate-dependent enzymes by [MoO 4 ] 2− , similar to that of vanadate ion, or due to the irreversible oxidation of phosphatases by Mo(VI) peroxido complexes similar to V(V) peroxido complexes [13]. Mo(VI) complex of picolinic acid-based metallomicellar catalyst was used in the controlled and chemoselective oxidation of the activated alcohols in the aqueous medium. Interesting metathetic oxidation of 2-butene to acetaldehyde by O 2 gas catalyzed by the immobilized MoO 2 -hydrazone complexes on SiO 2 afforded high yields. The heterogeneous molybdenum-salicylidene 2-picoloyl hydrazone complex, which supported on Fe 3 O 4 nano-particles, showed high catalytic potential in the (ep)oxidation of various ole ns. Molybdenum acetylacetonate complexes, which are supported on functionalized nano-particles, investigated as catalysts in the (ep)oxidation of unsaturated hydrocarbons presenting high potential. Dioxomolybdenum(VI) complexes as durable and highly e cient precatalysts for alkene epoxidation, were investigated recently by Mösch-Zanetti et al. [14]. Mo [15]. The present study is the destination to prepare and characterize Mo(IV), and Mo(VI) ternary complexes with dioxygen, ditholene, and dinitrogen donor atoms ligands. The reactions of the bidentate ligands with bis-(acetylacetonate) dioxomolybdenum(VI) gave the complexes. The structures and NBO charges of 2-hydrazinopyridne (HPY) and dithiooxamide (DTO) ligands are depicted in Fig. 1. Our research group is engage with the preparation and characterization of molybdenum and vanadium complexes in different oxidation states because of their biological activity, many industrial uses, and important role in medicinal chemistry so this complexes will be inter in different applications in the future studies.

Measurements
Infrared measurements of the 2-hydrazinopyridine (HPY), dithiooxamide (DTO) ligands, and the complexes, as KBr pellets, were carried out using a Bruker Tensor 27 FT-IR spectrophotometer in the range 4000-400 cm -1 . 1 H-NMR was recorded using ultra shield Bruker 500 MHNMR at ( 1 H at 500 MHz). Mass spectrometry measurements were carried out using GCMS-QP1000EX. The magnetic susceptibility of the complexes was carried out using a Balance Magnetic Susceptibility Model (MSB-MKI). UV-Vis. spectra were recorded in a 1.0 cm path length quartz cell by using a UV-Vis. spectrophotometer type Shimadzu UV-1800 UV-Vis. in DMSO. The structures of the 2-hydrazinopyridine (HPY), dithiooxamide (DTO) ligands, and the complexes were optimized by the use of [GAUSSIAN 09W] software program at the B3LYP/LanL2DZ basis sets for the complexes and B3LYP/6-31G (d, P) for the ligands.

Results And Discussion
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 2hydrazinopyridine (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 [MoO 2 (acac) (HPY)] complex showed broad bands at (3089, 3016 cm -1 ) attributed to asymmetrical and symmetrical (NH 2 ) stretching frequencies; respectively. This is an indication of the coordination between the 2hydrazinopyridine (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-con guration [18-19]. The` spectrum of the [MoO 2 (DTO)(HPY)] complex is illustrated in Fig. 2 showed broad bands at (3618, 3047 cm -1 ) attributed to asymmetrical and symmetrical (NH 2 ) stretching; respectively [19]. This is con rmed 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-con guration [11,19]. The thioamide group stretching three bands have appeared in this complex at ( 1527, 1423, and 1191 cm -1 ), this con rms the coordination of the (DTO) ligand to the Mo ion [11].  [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.

Mass spectral analysis
The mass spectra of the complexes exhibited the main mass fragmentation peaks which are listed in Table 3 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 [MoO 2 (DTO)(HPY)] complex is presented in Fig. 3 as a represented example. The data of mass spectra for the complexes are presented in (SI).

1 H-NMR spectra
The 1 H-NMR spectral data for the free ligands and Mo(VI) complexes in DMSO-d 6 are presented in Table  4. The experimental data compared with the calculated spectra that obtained from the DFT calculations.
The 1 H-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 NH 2 proton as a single peak. The 1 H-NMR spectrum of the [MoO 2 (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 NH 2 protons, and also peak at (δ=1.34 6H) ppm as a singlet peak which is assigned to CH 3 of (acac) ligand.

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 [MoO 2 (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)→d Mo . The spectrum of the [MoO 2 (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)→d Mo  peaks at (270-268, and 318-369 nm); respectively which can be assigned to (π-π*) and (n-π*) of the intraligand electronic transitions. The spectra exhibited peaks at the range (440, and 447 nm) assigned to LMCT from L(pπ)→d Mo . The d-d electronic transitions within the octahedral arrangement around Mo(VI) (d 0 -con guration) have vanished whereas for Mo(IV) (d 2 -con guration) 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.

Magnetic measurements
The magnetic measurement of the prepared complexes showed that the Mo(VI) complexes were diamagnetic with d 0 con guration whereas the Mo(IV) complexes were paramagnetic with d 2 (t 2g 2 ,e g 0 ) electronic con guration and the values of the μ eff for the complexes are

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]. 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 [MoO 2 (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(e g ), which states the possibility of LMCT from O(π) and/or coordinated ligands to Mo(e g ) [12,[32][33][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(e g ), which support the possibility of LMCT from O(π) and/or coordinated ligands to Mo(e g 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][33][34]39]. The red regions in the complexes are mainly localized over the oxygen and sulfur atoms.

Conclusion
The  Table 2 The most diagnostic FT-IR bands experimental and calculated of the prepared complexes.   Table 5 The bond lengths and bond angles of the Mo(VI) and the Mo(IV) complexes using the DFT/B3LYP/Lan2DZ basis set. The structures and NBO charges of the ligands.      The HOMO and LUMO molecular orbitals and energy gap of the (HPY) and (DTO) ligands. Figure 8