Oxoperoxomolybdenum(VI) complexes of catalytic and biomedical relevance: Synthesis, characterization, antibacterial activity and 3D-molecular modeling of some oxoperoxomolybdenum(VI) chelates in mixed (O,O) coordination environment involving maltol and β-diketoenolates

2.1 Materials

Molybdenum trioxide (99%) (Sisco Chem. Industries, Bombay), 30% hydrogen peroxide (E. Merck, India Ltd., Bombay), 3-hydroxy-2-methyl-4-pyrone (Lancaster, Alfa Aesar, England), o-acetoacetotoluidide and o-acetoacetanisidide (Aldrich Chemical Co., USA), methyl acetoacetate, acetylacetone and DMF (Tomas Baker Chemical Ltd., Mumbai) and ethanol (Bengal Chemical and Pharmaceuticals Ltd., Kolkata) were used as received. All other chemicals used were of analytical reagent (A.R.) grade.

2.2 Preparation of mixed ligands complexes

The mixed-ligand oxoperoxo complexes were prepared according to the following general method.

A suspension of MoO₃ (0.025 mol, 0.359 g) in 30% H₂O₂ (35 mL) was stirred for 24 h at 50 °C giving a yellow solution. This was filtered and a solution of maltol (0.0025 mol, 0.315 g) in ethanol (15 mL) was added to the filtrate followed by the addition of 0.0025 mol of β-diketones: o-actoacetotoluidide (o-aatdH) (0.477 g), o-acetoacetanisidide (o-aansH) (0.517 g), acetylacetone (acacH) (0.26 mL) or methyl acetoacetate (macacH) (0.27 mL) in (20 mL) ethanol. The resulting mixture was stirred for 2 h. while cooling in ice-bath. The light brown precipitate appeared was suction filtered with water–ethanol (1:1) solution and dried in a desiccator over anhydrous calcium chloride.

2.3 Analysis

Carbon, hydrogen, nitrogen were determined micro-analytically at C.D.R.I., Lucknow. The molybdenum content in each chelate was determined (Mohanti et al., 1991) as follows. A weighed amount (∼200 mg) of the chelate was first decomposed by heating with concentrated nitric acid then strongly heating the residue over 500 °C for 45 min until constant weight was obtained.

2.4 Physical methods

The following physical methods were used to determine the structure of the resulting molybdenum(VI) complexes. Thermogravimetric analysis was done by heating the sample at the rate of 10 °C min⁻¹ up to 1000 °C on a thermal analyzer at Sophisticated Analytical Instrument Facility (Formerly RSIC) Indian Institute of Technology, Bombay. Solid-state infrared spectra were obtained using potassium bromide pallets with a Perkin–Elmer model 1620 FT-IR spectrophotometer at C.D.R.I., Lucknow. Conductance measurements were made in dimethylformamide solution using Toshniwal Conductivity Bridge and dip type cell with a smooth platinum electrode of a cell constant 1.02. Magnetic measurements were performed by Gouy’s method using mercury(II)tetrathiocyanatocobaltate(II) as calibrant at Indian Institute Of Technology, Roorkee. The decomposition temperatures of the complexes were recorded by an electrically operated melting point apparatus (Kumar Industries, Bombay) of heating capacity up to 360 °C. Electronic spectra were recorded on Systronics Double Beam UV–vis Spectrophotometer: 2001 at Central Instrument Center, Bio-Science Department, R.D. University, Jabalpur. ¹HNMR spectra in DMSO-d₆, mass spectra and elemental analysis (micro-analytically) of the complexes were determined at Sophisticated Analytical Instrumentation Facility, C.D.R.I., Lucknow. The FAB mass spectrum of a representative complex was recorded on a JEOL SX 102/DA-6000 mass spectrometer/data system using xenon/argon (6 kV, 10 mA) as the FAB gas in the m/z range 94.36–508.17.

2.5 3D-Molecular modeling studies

The 3D-molecular modeling of one of the synthesized compound was carried out on CS Chem 3D Ultra Molecular Modeling and Analysis Programme (www.cambridgesoft.com). It is an interactive graphics program that allows rapid structure building, geometry optimization and molecular display. It has the ability to handle transition metal compounds.

3.1 Infrared spectral studies

The important infrared spectral bands and their plausible assignment are given in Table 2. All synthesized complexes display a strong band at 949–955 cm⁻¹, which has been assigned to the ν(Mo⚌O) mode. The metal peroxo grouping gives rise to three IR active vibrational modes and these are (O–O) stretching (ν₁), the symmetric (Mo–O) stretching (ν₂) and asymmetric (Mo–O) stretching (ν₃). The characteristic (ν₁) (O–O) mode of the complexes appears at 867–879 cm⁻¹, while the (ν₂) and (ν₃) modes appear at 620–626 and 725–729 cm⁻¹, respectively. These observations are in agreement with results reported elsewhere (Carreiro et al., 2006; Islam et al., 1994; Maurya et al., 2008).

β-Diketones and related derivates such as β-diketoesters exhibit keto-enol tautomerism. Due to the presence of two oxygen donor atoms at suitable positions and facile keto-enol tautomerism (Fig. 2) they easily coordinate with metal ions after deprotonating the enolic hydrogen atom and provide stable metal complexes with six-membered chelate ring (Maurya, 2003; Maurya et al., 1991a). Among β-diketones, acetylacetone (acacH) is most common, and the infrared spectra of [M(acac)₂] and [M(acac)₃] type complexes have been studied extensively. The two significant absorption bands for acetylacetone at 1628 and 1553 cm⁻¹, assignable to ν(C⚌O) (acetyl) and ν(C⚌C) modes, respectively, are shifted to lower wave numbers in these complexes on account of the coordinated acetylacetone after deprotonation of the enolic hydrogen and appear in the regions 1600–1530 and 1525–1480 cm⁻¹, respectively (Nakamoto, 1978a; Maurya et al., 1991b, 1995). In complexes of neutral acetylacetone involving coordination of two ketonic groups, ν(C⚌O) bands appear at ∼1700 cm⁻¹ (Nakamoto, 1978b).

Close

Figure 2

Keto-enol forms of β-diketones and related derivatives and their metal chelates.

Export to PPT

The appearance of two strong bands at 1616–1619 and ∼1515 cm⁻¹ [merged with ν(C⚌C) (maltol), see Table 2), respectively, attributed to ν(C⚌O) (acetyl) and ν(C⚌C) modes, respectively, in the complexes (3) and (4), is consistent with the chelating bidentate (O,O) coordination in enol form after deprotonation of enolic hydrogen of acetylacetone and methyl acetoacetate moieties (Nakamoto, 1978a; Maurya et al., 1991b, 1995; Maurya, 1997).

The chelating behavior of o-acetoacetotoluidide (o-aatd) and o-acetoacetanisidide (o-aans) is quite comparable to acetylacetone. Like acetylacetone they coordinate to a metal ion in their most common chelating bidentate fashion in enol form through a ketonic oxygen, and an enolic oxygen after deprotonation (Maurya et al., 1992). The two characteristic absorption bands due to coordinated o-aatd ando-aans anions are ν(C⚌O) (acetyl carbonyl) and ν(C⚌O) (amide carbonyl) (Maurya and Rajput, 2004). These two bands have been observed at ∼1580 (merged with ν(C⚌O) maltol, see Table 2) and 1619–1625 cm⁻¹, respectively. These results are in agreement with the data reported elsewhere (Maurya and Rajput, 2004; Maurya et al., 2008).

The co-ligand, maltol, used in this investigation contains two donor sites: (i) the carbonyl oxygen, (ii) the hydroxyl oxygen. The IR spectrum of this ligandshows eight significant absorption bands at 3260, [3150, 3080], 2970, 1680, [1630, 1575], 1210 cm⁻¹ assignable to ν(OH) (Parajon-Costa and Baran, 2011) (strongly intramolecular hydrogen bonded with carbonyl oxygen) (Zborowski et al., 2005), [ν(CH)], ν(CH₃) (Parajon-Costa and Baran, 2011), ν(C⚌O), [ν(C⚌O) + ν(C⚌C)] (Lamboy et al., 2007), ν(C–O) (Maurya et al., 1998), respectively. When hydroxyl oxygen coordinates with the metal center after deprotonation, ν(OH) observed at 3260 cm⁻¹ in the free ligand should be disappeared in the IR spectrum of all the complexes. Infact, this band is found to be disappeared in all the complexes, suggesting coordination of hydroxyl oxygen after deprotonation. This is further supported by the upward shift (Saatchi et al., 2005; Maurya et al., 1998), of ν(C–O) hydroxyl and appearing at 1272–1276 cm⁻¹ in the complexes compared to ν(C–O) at 1210 cm⁻¹ in free ligand. The coordination of carbonyl oxygen of this ligand to metal centers was inferred by the appearance of ν(C⚌O) at lower wave number (Thompson et al., 2004; Maurya and Mishra, 1991c) (1617–1625 cm⁻¹) in all the complexes compared to ν(C⚌O) of the free maltol at 1680 cm⁻¹. This is further supported by the lower energy shift of the combination bands [ν(C⚌O) + ν(C⚌C)] appearing at 1580–1590 cm⁻¹ and 1510–1520 cm⁻¹ (Lamboy et al., 2007) in the complexes compared to the free maltol combination bands at 1630 cm⁻¹ and 1575 cm⁻¹. These observations suggest that the maltol is acting as a monobasic bidentate (O,O)-donor, forming stable five member chelate ring.

The IR spectra of all the complexes show a broad band centered at 3409–3450 cm⁻¹, attributable to ν(OH) mode possibly due to the presence of lattice water in these complexes. The IR spectra of maltol and compound (3) are given in Figs. 3 and 4, respectively.

Close

Figure 3

IR spectrum of maltol.

Export to PPT

Close

Figure 4

IR spectrum of [MoO(O₂)(ma)(acac)]·H₂O (3).

Export to PPT

3.2 Conductance measurements

The molar conductivities of all the complexes in 10⁻³ M DMF solution lies in the range 12.5–14.6 ohm⁻¹ cm² mol⁻¹ (Table 1) as expected for non-electrolyte (Maurya et al., 2008). Such a non-zero molar conductance value for each of the complex in the present investigation is most probably due to the strong donor capacity of DMF, which may lead to the displacement of anionic ligand and change of electrolyte type.

3.3 Magnetic measurements

The observed magnetic moments of these complexes indicate they are diamagnetic (Maurya et al., 2008) as expected for molybdenum(VI) complexes.

3.4 Mass spectral studies

The FAB mass spectrum of a representative complex, [MoO(O₂)(ma)(acac)]·H₂O (3) (Fig. 5) was recorded assuming almost similar ones for other three complexes. The mass spectral peaks observed at 136,137, 154, 289 and 307 m/z are matrix (m-nitrobenzyl alcohol or NBA) peaks. The spectral peaks observed at, 95, 279, 333, 363, 364, 377, 391, 392, 407 and 491 m/z in the complex might be correlated possibly with the following types of ion associations (Yasumatsu et al., 2007): $$\begin{array}{cc}\hfill & {[\text{MoO}({\text{O}}_2)(\text{ma})(\text{acac})]}^{+}(368)-4{[\text{H}]}^{+}=364\hfill \\ \hfill & {[\text{MoO}(\text{O}2)(\text{ma})(\text{acac})]}^{+}(368)-5{[\text{H}]}^{+}=363\hfill \\ \hfill & {[\text{MoO}({\text{O}}_2)(\text{ma})(\text{acac})]}^{+}(368)-3{[\text{H}]}^{+}-{[\text{O⚌O}]}^{+}(32)=333\hfill \\ \hfill & {[\text{MoO}(\text{ma})(\text{acac})]}^{+}(336)+{[\text{matrix}]}^{+}(154)+{[\text{H}]}^{+}=491\hfill \\ \hfill & {[\text{MoO}(\text{ma})]}^{+}(237)+{[\text{matrix}]}^{+}(154)+{[\text{H}]}^{+}=392\hfill \\ \hfill & {[\text{MoO}(\text{mal})]}^{+}(237)+{[\text{matrix}]}^{+}(154)=391\hfill \\ \hfill & {[(\text{ma})]}^{+}(125)+{[\text{matrix}]}^{+}(154)=279\hfill \\ \hfill & {[(\text{acac})]}^{+}(100)+{[\text{matrix}]}^{+}(307)=407\hfill \\ \hfill & {[\text{MoO}(\text{ma})]}^{+}(237)+{[\text{matrix}]}^{+}(154)-{[{\text{CH}}^3]}^{+}(15)+{[\text{H}]}^{+}=377\hfill \end{array}$$ These results are consistent with the proposed molecular composition of the complex (3).

Close

Figure 5

Mass spectrum of [MoO(O₂)(ma)(acac)]·H₂O (3).

Export to PPT

3.5 Electronic spectral studies

Electronic spectra of all the complexes were recorded in 10⁻³ M DMF solutions. The electronic spectral peaks observed in each of the complexes along with the molar extinction coefficients are given in the Table 3. The bands at 265–277 nm in the complexes are characteristic of intra-ligand n → π^∗/π → π^∗ transition (Litos et al., 2006). The other two bands in each of the complexes are most probably due to ligand to metal charge transfer transitions (LMCT) (Litos et al., 2006) that might explain the colors of complexes obtained. The electronic spectrum of compound (3) is given in Fig. 6.

Close

Figure 6

Electronic spectrum of [MoO(O₂)(ma)(acac)]·H₂O (3).

Export to PPT

3.6 Thermogravimetric studies

The thermogravimetric curves of two representative compounds, namely, [MoO(O₂)(ma)(acac)]·H₂O (3) (Fig. 7) and [MoO(O₂)(ma)(macac)]·H₂O (4) were recorded in the temperature rang 50–1000 °C at the heating rate of 15 °C/min. Looking over the stability of the seven coordinate compound (3) up to 200 °C, and taking into account of the analytical data (Table 1) in favor of the presence of one water molecule in this compound along with the infrared result substantiating the same (vide supra), it appears that this compound would have shown a weight loss corresponding to elimination of one molecule of lattice water below 50 °C. It shows a second weight loss of 38% around 540 °C corresponding to the elimination of one acac and one (O–O) groups from the complex (calcd. weight loss = 38.60%). The final weight loss (obs. = 76.75) at 780 °C corresponds to the elimination of all the ligand groups (calcd. = 75.14%) from the complex. The final residue at ∼900 °C (obs. = 23.25%) roughly corresponds to MoO₃ (calcd. = 37.29%). This difference in observed and calculated weight losses is most probably due to MoO₃ starts volatilizing above 800 °C (melting point 795 °C, Greenwood and Earnshaw, 1984).

Close

Figure 7

TG curve of [MoO(O₂)(ma)(acac)]·H₂O (3).

Export to PPT

Similar to the compound (3), compound (4) is stable up to 210 °C. With the same explanation as given for the compound (3), it again appears that that this compound would have shown a weight loss corresponding to elimination of one lattice water molecule below 50 °C. This compound shows a second weight loss of 41% at 710 °C corresponding to the removal of one macac and one (O–O) moieties from the complex (calcd. 41.05%). The final weight loss of 77.75% observed at 790 °C against a theoretical weight loss of 76.13% corresponds to the elimination of all the ligand moieties from the complex. The final residue at 880 °C (obs. = 22.25%) roughly corresponds to MoO₃ (calcd. 35.81%).

3.7 ¹H NMR spectral studies

The proton NMR spectrum of one of the representative compounds, namely, [MoO(O₂)(ma)(acac)]·H₂O (3) was recorded in DMSO-d₆ using TMS as a reference. This compound displays a proton signals at δ 8.52, δ 6.90, δ 3.33, δ 2.41 and δ 1.23 ppm, which are most probably due to O–CH (a), –CH (b), –CH (c), –CH₃ (d), and –CH₃ (e), respectively, present in this compound. The proton signal at 2.50 ppm is most probably due to solvent (DMSO-d₆) taken. The absence of proton signal at ∼12 ppm in the complex indicates the coordination of enolic/hydroxyl oxygen to the metal ion after deprotonation (Maurya et al., 2008; Williams and Fleming, 1987) The NMR spectrum of compound (3) is given in Fig. 8. The indexing of various proton groups is given in the following Fig. 9.

Close

Figure 8

¹H NMR spectrum of [MoO(O₂)(ma)(acac)]·H₂O (3).

Export to PPT

Close

Figure 9

Indexing of various proton groups in the compound (3).

Export to PPT

3.8 Antibacterial studies

One of the co-ligands maltol and the two representative metal complexes, namely, [MoO(O₂)(ma)(acac)]·H₂O (3) and [MoO(O₂)(ma)(macac)]·H₂O (4) have been screened for their antibacterial activity against Escherichia coli and Vibrio cholera at a concentration of 300 μg cm⁻³ in DMSO by the agar diffusion method (Ong and Martelli, 1994; Ollee et al., 1989) using streptomycin as the standard antibacterial agent. The results so obtained are presented in Table 4. It is observed that both the metal complexes are more potent bactericides than the ligand maltol. The enhancement in the activity of complexes may depend upon the metal ions (Thimmaiah et al., 1985; Franklin and Snow, 1971), viz., size, charge distribution, shape and redox potential of the metal chelates and chelation. Moreover, steric and pharmacokinetic factors also play a decisive role in deciding the potency of an antimicrobial agent. Thus, antibacterial property of metal complexes is an intricate blend of several contributions (Levingson et al., 1978; Murukan and Mohanan, 2006).

3.9 3D-Molecular modeling and analysis

In view of the hepta-coordination of the present complexes (vide infra), and also taking into account of the well established hepta-coordinate pseudopentagonal bipyramidal structure (Burke, 2008) of bis(phenolato-oxazoline)-monoperoxomolybdenum(VI) (Fig. 10) [having (i) two momobasic bidentate (O,N-donor) phenolate-oxazoline ligand similar to momobasic (O,O-donor) maH and LH ligands in the present investigation and (ii) oxo, peroxo and phenolate oxygens of the two phenolate moieties at the equatorial positions, and the tertiary nitrogen of the each oxazoline moiety trnas to each other at the axial positions], the molecular modeling of a representative compound, [MoO(O₂)(mal)(o-aatd)]⋅H₂O (1), is based on its pseudopentagonal bipyramidal structure having oxo, peroxo and two enolate/hydroxyl oxygens of the β-diketone/maltol moieties at the equatorial positions, and the carbonyl oxygens of the of the β-diketone and maltol moieties trans to each other at the axial positions. The details of bond lengths and bond angles as per the 3D structure (Fig. 11) are given in Tables 5 and 6, respectively. For convenience of looking over the different bond lengths and bond angles, the various atoms in the compound in question are numbered in Arabic numerals. In all, 139 measurements of the bond lengths (48 in numbers), plus the bond angles (91 in numbers) are listed. Except few cases, optimal values of both the bond lengths and the bond angles are given in the tables along with the actual ones. The actual bond lengths/bond angles given in Tables are calculated values as a result of energy optimization in CHEM 3D Ultra (44 CS Chem 3D Ultra, www.cambridgesoft.com), while the optimal bond length/optimal bond angle values are the most desirable/ favorable (standard) bond lengths/bond angles established by the builder unit of the CHEM 3D. The missing of some values of standard bond lengths/bond angles may be due to the limitations of the software, which we had already noticed in modeling of other systems (Maurya et al., 2006, 2008, 2010). In most of the cases, the actual bond lengths and bond angles are close to the optimal values, and thus the proposed structures of the compound (1) as well as of the others are acceptable (Maurya et al., 2006, 2008, 2010).

Close

Figure 10

Structure of bis(phenolato-oxazoline)monoperoxomolybdenum(VI).

Export to PPT

Close

Figure 11

3D Structure of compound (1).

Export to PPT