Synthesis, characterization, and 3D-molecular modeling and analysis of some copper(II) chelates in O, N-donor coordination pattern involving Schiff bases derived from 4-butyryl-3-methyl-1-phenyl-2-pyrazolin-5-one and some sulfa drugs

2.1 Materials

3-Methyl-1-phenyl-2-pyrazolin-5-one was the product of Johnson Chemical Co., Mumbai. The sulfa drugs, viz., sulfamethoxazole, sulfadimidine, sulfanilamide, sulfamoxole and sulfaguanidine were products of Sigma Chemical Co., USA. Copper(II) chloride dihydrate was the product of B.D.H. Chemicals, Mumbai, while butyryl chloride was purchased from Aldrich Chemical Company, USA. All other chemicals used were of analytical reagent grade. 4-Butyryl-3-methyl-1-phenyl-2-pyrazolin-5-one (bumphp) was prepared by the method reported elsewhere (Jensen, 1959).

2.2 Synthesis of sulfa drug Schiff bases

The Schiff bases with sulfa drugs were prepared as follows: an ethanolic solution (15 mL) of bumphp (2.44 g, 0.01 mol) was added to the solution of sulphamethoxazol (2.53 g, 0.01 mol) in ethanol, or sulfadimidine (2.78 g, 0.01 mol) in ethanol, or sulfanilamide (1.72 g, 0.01 mol) in acetone or sulfamoxole (2.67 g, 0.01 mol) in ethanol or sulfaguanidine (2.14 g, 0.01 mol) in ethanol along with two drops of HCl. The resulting solution was refluxed with stirring for 5–6 h and then filtered to remove the insoluble sulfa drug, if any. The filtrate so obtained was concentrated on a water bath and left overnight at room temperature when yellow crystals of Schiff bases separated out from their respective solutions. The crystals thus obtained were washed with ethanol and dried in vacuo.

2.3 Synthesis of complexes

All the complexes were prepared by the general method, namely, the salt cupric chloride dihydrate (0.001 mol, 0.170 g) was dissolved in ethanol (10 mL) by heating and the resulting solution was added to a warmed, stirred solution of the corresponding Schiff base, bumphp-smzH (0.002 mol, 0.958 g), bumphp-sdmH (0.002 mol, 1.8 g), bumphp-snmH (0.002 mol, 0.796 g), bumphp-smxH (0.002 mol, 0.986 g), or bumphp-sgdH (0.002 mol, 0.880 g) in 15 mL of ethanol. The resulting solution was refluxed for 4–5 h when the desired compound separated out as a fine precipitate keeping the reaction mixture overnight. It was filtered by suction, washed several times with ethanol and then dried in vacuo over anhydrous calcium chloride.

2.4 Analysis

Microanalysis of carbon, hydrogen, and nitrogen of the complexes was performed on a Carlo Erba 1108 Heracus elemental analyser at CDRI, Lucknow. Copper was estimated as copper salicyladoximate (Vogel, 1962).

2.5 Physical methods

The solid-state infrared spectra were recorded in KBr pellets with a Perkin Elmer FT-IR spectrophotometer at Central Drug Research Institute, Lucknow. Magnetic measurements were carried out at room temperature on vibrating sample magnetometer at RSIC, Indian Institute of Technology, Chennai. Electronic spectra of the complexes were recorded on an ATI Unicam, UV-2-100 UV–Visible Spectrophotometer in our Department. Conductance measurements were performed at room temperature in dimethylformamide using a Toshniwal conductivity bridge and a dip type cell with a smooth platinum electrode of cell constant 1.02. The decomposition temperatures of the complexes were recorded by an electrically operated melting point apparatus, Kumar Industries, Mumbai, with a heating capacity up to 360 °C. The X-Band EPR spectra of the complexes were measured on a Bruker ESP X-band EPR spectrometer using powdered samples at the Regional Sophisticated Instrumentation Centre, Indian institute of Technology, Chennai at the microwave frequency of 9.45 GHz.

2.6 Molecular modeling studies

The 3D molecular modeling of a representative compound was carried out on ChemBio3D Ultra Molecular Modeling and Analysis Program (www.cambridgesoft.com). It is an interactive graphics program that allows rapid structure building, geometry optimization with minimum energy and molecular display. It has the ability to handle transition metal compounds.

3.1 Infrared spectral studies

The sulfa drug based Schiff base ligands used in the present investigation were synthesized by the scheme as shown in Fig. 1. The formation of the Schiff base ligands is consistent with the microanalytical data of the ligands. The C, H, and N data, percentage yields, and important infrared spectral bands are given in Table 1. The formation of the Schiff base ligands is supported by the appearance of a strong band at 1625–1629 cm⁻¹ due to ν(C⚌N) (azomethine) in the IR spectra of these ligands. All of the Schiff base ligands in the present investigation exhibit a broad band centered at 3328–3440 cm⁻¹. This suggests the involvement of the 5-OH group in the intramolecular hydrogen (Maurya et al., 1997) bonding with the lone pair of azomethine nitrogen. It also suggests that the ligands exist in enol form in the solid state (Maurya et al., 1996).

The important infrared spectral bands of the complexes along with their plausible assignments are given in Table 3. All the Schiff base ligands used in this investigation exist in enol form as discussed above. Hence, they possess six potential donor sites: (i) the enolic oxygen, (ii) the cyclic nitrogen N¹ pyrazolone moiety, (iii) the cyclic Nitrogen N² pyrazolone moiety, (iv) the azomethine nitrogen, (v) sulfonamide (SO₂NH) oxygen or nitrogen and (vi) the ring nitrogen of the sulfa drug.

The appearance of a band at 1385–1390 cm⁻¹ in the ligands is assigned (Maurya et al., 2007) to ν_as(O⚌S⚌O). This band remains almost at the same position in the complexes and hence suggests that the sulfonamide oxygen is not taking part in coordination with the metal centre.

The ν(NH) mode(s) of the sulfonamide group/amino group of the free Schiff bases, which is/are observed at 3310–3362 and 3216 cm⁻¹, remain(s) unperturbed in the spectra of their complexes. This suggests that the sulfonamide/amino nitrogen is not taking part in coordination.

All the ligands display a sharp and strong band due to ν(C⚌N) of the azomethine group at 1625–1629 cm⁻¹. The observed low energy shift of this band appearing in the range 1607–1613 cm⁻¹ in the infra red spectra of the complexes suggests the coordination of azomethine nitrogen (Maurya et al., 2007) to the metal centre. The coordination of enolic oxygen of the pyrazolone moiety of the ligands after deprotonation is most likely for the formation of six membered chelate ring including azomethine nitrogen and the metal centre in question, and this should be reflected by the disappearance of the ν(OH) mode of the ligands centered at 3328–3440 cm⁻¹. However, the appearance of ν(OH) mode in all the complexes in the same region suggests the coordination of the enolic oxygen without deprotonation. One of the of the possibilities of the presence of ν(OH) mode in the complexes may be due to lattice or coordinated water in them, but this is ruled out by TG analysis of the complexes wherein no weight loss was observed up to ∼200 °C. The coordination of enolic oxygen without deprotonation has already been observed by us (Maurya et al., 2003a) in a mixed-ligand cyanonitrosyl complexes of molybdenum(II) of the composition,[Mo^II(NO)(CN)₃(BMPHP)], where BMPHP = 4-benzoyl-3-methyl-1-phenyl-2-pyrazolin-5-one, a carbonyl compound similar to 4-butyryl-3-methyl-1-phenyl-2-pyrazolin-5-one, which is one of the component of the Schiff base ligands used in the present studies. The overall IR data conclude that the sulfa Schiff bases behave as neutral bidentate (N, O) ligands.

The ν(C⚌N²) (cyclic) mode arising from the pyrazolone moiety of the ligands appears at 1580–1582 cm⁻¹, and it does not show any appreciable change in its position in the complexes (see Table 3). Similarly, the ν(C⚌N) (cyclic) mode due to the sulpha drug skeleton of all the ligands appearing at ∼1600 cm⁻¹ remains unchanged after complexation, and seems to be merged with ν(C⚌N) (azomethine). These observations indicate the non-participation of the ring nitrogen N² of the pyrazolone moiety and the cyclic nitrogen of the sulfa drug skeleton of the ligands in bonding. Considering the planarity of the ligands, the coordination of the ring nitrogen N² is also unlikely due to being in the backside of the effective donor sites, (i) and (iv) discussed above forming six membered chelate ring including the metal. Such a result has already been reported (Maurya and Rajput, 2006) by us. Likewise, the non-participation of the cyclic nitrogen of the sulfa drug skeleton in bonding may be attributed due to being too far from the suitable donor sites, (i) and (iv) and the central metal. The coordination of the ring nitrogen N¹ is also unlikely due to a steric demand of the bulky phenyl group attached with it.

A single peak for the ν(Cu–Cl) should appear at ∼300 cm⁻¹ in the infrared spectra of all the complexes. Unfortunately, this peak could not be assigned due to recording of the spectra up to 500 cm⁻¹. The two non-ligand bands occurring at 510–557 and 416–490 have been assigned (Maurya et al., 2008) to ν(Cu–O) and ν(Cu–N), respectively. The IR spectrum of a Schiff base ligand, bumphp-snmH (III) and its complex, (3), are given in Figs. 2 and 3, respectively.

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Figure 2

IR spectrum of bumphp-snmH (III).

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Figure 3

IR spectrum of[Cu(bumphp-snm)₂(Cl)₂] (3).

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3.2 Electron spin resonance spectra

The ESR spectra of polycrystalline compounds, namely,[Cu(bumphp-smzH)₂(Cl)₂] (1) and[Cu(bumphp-sdmH)₂(Cl)₂] (2) were recorded at room temperature at the microwave frequency of 9.45 GHz. Both parallel and perpendicular features of ⁶³Cu are resolved in both the spectra, which are characteristic of axial symmetry. Nuclear hyperfine coupling is observed in the $g_{\parallel }$ region with one component clearly resolved. Thus, it appears that the second, third, and fourth components are obscured by the broad $g_{\perp }$ component (Garcio-Lozano et al., 1994). The $g_{\perp }$ , $g_{\parallel }$ , and g_av values for these complexes were calculated as 2.074/2.079, 2.174/2.190, and 2.107/2.116, respectively. The trend $g_{\parallel }$ (2.174/2.190) >  $g_{\perp }$ (2.074/2.079) > g_e (2.0036, free ion value) observed in these two complexes shows that the unpaired electron is in the ${\text{d}}_{x^2-y^2}$ orbital of Cu(II). The $g_{\parallel }$ and $g_{\perp }$ values deviating considerably from the free ion value are close to those reported for a number of distorted copper(II) complexes (Maurya et al., 2003b). Moreover, the observed g values less than 2.3, suggest the covalent (Neiman and Kivelson, 1961) nature of metal-ligand bonds in these complexes. The ESR spectra of compound (1) and (2) are given in Fig. 4a and b, respectively.

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Figure 4

ESR spectrum of ESR spectrum of[Cu(bumphp-smzH)₂(Cl)₂] (1) (a)[Cu(bumphp-sdmH)₂(Cl)₂] (2) (b).

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3.3 Electronic spectra

The electronic spectra of all these compounds were recorded in 10⁻³ molar dimethylformamide solutions in the range 200–800 nm. The λ_max of electronic spectral peaks and respective molar extinction coefficients along with their tentative assignments are given in Table 4. The high intensity peak(s) in all the complexes at 288–326 nm is/are assigned as intra-ligand transition(s). Normally, octahedral copper(II) complexes exhibit a broad band around 714–740 nm described by the transition ²T_2g ← ²E_g. However, due to distortion caused by weak axial ligands, the broad bands split distinctly into two or three bands. Three spin-allowed transitions are theoretically possible and are attributed to ²A_1g ← ²B_1g, ²B_2g ← ²B_1g, and ²Eg ← ²B_1g. But when the difference between ²A_1g and ²B_1g is small, the transition ²A_1g ← ²B_1g could remain unobserved. Based on the energy level diagram (Sathyanarayana, 2001) reported elsewhere, and also considering the low intensity of the remaining one/two d–d transition(s) in the visible region at 400–441 and 680–710 nm is/are assigned to the transitions ²Eg ← ²B₁g and ²B_2g ← ²B_1g, respectively. However, the band at 400–441 nm in these complexes may also be considered as the metal-ligand charge transfer transition as reported elsewhere in case of the axially distorted octahedral (Jaskova et al., 2007) copper(II) complexes. The electronic spectrum of compound (1) is given in Fig. 5.

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Figure 5

Electronic spectrum of[Cu(bumphp-smzH)₂(Cl)₂] (1).

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3.4 Thermogravimetric studies

Thermogravimetric curves of two compounds[Cu(bumphp-smzH)₂(Cl)₂](1) (Fig. 6) and[Cu(bumphp-smxH)₂(Cl)₂] (4) (Fig. 7) were recorded in the temperature range from room temperature to 1300 °C. Both compounds are stable up to ∼200 °C. This excludes the possibility of lattice as well as coordinated water molecule(s) in these two complexes. The compound (4) shows a first weight loss of 43.03% at around 300 °C and this corresponds to the elimination of one molecule of sulfa Schiff base ligand (calcd 43.99%). The second weight loss of 86.5% observed around 600 °C in this compound corresponds to the elimination of second molecule of the sulfa Schiff base ligand (calcd 87.9%). Compound (1) shows weight losses in two steps and the total observed weight loss of 84% around 750 °C roughly corresponds to the elimination of two molecules of the sulfa Schiff base ligand of this complex (calcd 87.68%). Thus, the overall the thermogravimetric results are consistent with the formulation of these complexes along with the conclusion derived from infrared spectral studies (vide supra).

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Figure 6

TG curve of[Cu(bumphp-smzH)₂(Cl)₂] (1).

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Figure 7

TG curve of[Cu(bumphp-smxH)₂(Cl)₂] (4).

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3.5 Magnetic susceptibility

The copper(II) has d⁹ electronic configuration and its complexes contain one unpaired electron in the d-shell. A majority of complexes are formed by the involvement of d-orbital and they are square planar or distorted octahedral. The John Teller distortions contribute a major role in the distortion of the geometry of the complexes. The formation of tetrahedral complexes of copper(II), with no involvement of d-orbital is also reported to be formed. When the Cu(II) complexes display magnetic moment values equivalent to one unpaired electron, the complexes are referred to as magnetically dilute. In such complexes the paramagnetic metal centers are situated apart and the metal ions are surrounded by the ligand molecules in such a way that unpaired spins of the neighboring metal ions remain unaffected. In case of tetrahedral or distorted octahedral Cu(II) complexes the room temperature magnetic moment values are usually observed in the range of 1.8–2.2 BM, which are not affected appreciably by temperature and magnetic field. In case of planar dimeric or polynuclear species, the complexes display subnormal magnetic moments. In practice, compounds whose geometry approaches an octahedral geometry usually exhibit magnetic moments at the lower end, while those approaching a tetrahedral geometry are at the higher end. In the present investigations complexes exhibit magnetic moment 1.81–186 BM. These data indicate the octahedral (Maurya et al., 2003) geometry for these compounds.

3.6 3D Molecular modeling and analysis

In view of the hexa-coordination of the present complexes (vide infra), and also taking into account of the well established hexa-coordinate octahedral structure of[Cu(en)₂(H₂O)₂]²⁺ cation (Jaskova et al., 2007) ([having neutral bidentate (N, N-donor) “en” ligands similar to neutral (O, N)-donor sulfa drug based Schiff base ligands in the present investigation], wherein four nitrogen atoms of the ethylenediamine (en) form the coordination plane around the copper(II) atom with the oxygen atoms of the two water molecules lying in the axial positions, the molecular modeling of a representative compound,[Cu(bumphp-snmH)₂(Cl)₂] (3) with two Cl groups at the axial position trans to each other and the two (O, N)-donor sulfa drug based Schiff base ligands at the equatorial positions in cis-arrangement to each other, is based on its octahedral structure. The details of bond lengths and bond angles as per the 3D structure (Fig. 8) are given in Tables 5 and 6, respectively. For convenience of looking over the different bond lengths and bond angles, the various atoms of the compound in question are numbered in Arabic numerals. In all, 323 measurements of the bond lengths (114 in numbers), plus the bond angles (209 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 (calculated) bond lengths/bond angles given in tables are obtained as a result of energy optimization in CHEM 3D Ultra, 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 optimal bond lengths/bond angles may be due to the limitations of the software, which we had already noticed in modeling of other systems (Maurya and Rajput, 2007; Maurya et al., 2007, 2008). In most of the cases, the calculated bond lengths and bond angles are close to the optimal values, and thus the proposed structure of the compound (3) as well as of the others are acceptable (Maurya and Rajput, 2007; Maurya et al., 2007, 2008, 2011).

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Figure 8

3D Structure of compound (3).

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