Syntheses, Characterization, and Biological Activities of Metal Complexes of N-(2-Carbamoylthienyl)-C-(3′-carboxy-2′-hydroxyphenyl) Azetidin-2-one with Some Di-, Tetra-, and Hexavalent Metal Ions
The cyclization of the Schiff base, N-(2-carbamoylthienyl)-3′-carboxy-2′-hydroxybenzylideneimine with ClCH2COCl in dioxane, in the presence of triethylamine (Et3N) forms N-(2-carbamoylthienyl)-C-(3′-carboxy-2′-hydroxyphenyl) azetidin-2-one, LH3 (I). A methanolic solution of I reacts with Co(II), Cu(II), Zn(II), Zr(OH)2(IV), and MoO2(VI) ions and forms the coordination compounds, [Co(LH)(MeOH)]2 (II), [Cu(LH)]2 (III), [Zn(LH)(MeOH)2] (IV), [Zr(OH)2(LH)(MeOH)] (V), and [MoO2(LH)(MeOH)2] (VI). The compounds have been characterized on the basis of elemental analyses, molar conductance, molecular weight, spectral (IR, NMR, reflectance, and ESR) studies, and magnetic susceptibility measurements. All the coordination compounds are nonelectrolytes ( mho cm2 mol−1) in DMF. I behaves as a dibasic tridentate OON donor ligand in V; a dibasic tetradentate OONO donor ligand in II, III, and VI; and a dibasic tetradentate OONS donor ligand in IV. II and III are dimers, while others are monomers in diphenyl. III exhibits subnormal magnetic moment (1.55 B.M.) and is involved in antiferromagnetic exchange, while the other complexes are magnetically dilute. A square-pyramidal structure for III and an octahedral structure for II, IV, and V are suggested. VI exhibits an eight-coordinate structure. The ligand (I) and its compounds show antibacterial activities towards E. coli (gram negative) and S. aureus (gram positive).
Azetidinones or β-lactams are the 2-carbonyl derivatives of azetidine (a four-membered heterocyclic ring containing N as the heteroatom). These are the most versatile compounds used in medicines. Antibiotics like cephalosporins, nocardicins, and penicillins all possess β-lactam ring and their biological activities are associated with the chemical reactivity of the β-lactam ring .
Azetidinones can be prepared from Schiff’s bases, which are the condensation products of aldehydes and amino compounds. They are considered significant owing to their wide range of biological applications. They are also employed as intermediates in chemical synthesis. Azetidinones are known to possess biological and therapeutic activities such as antibiotic, antidepressant, sedatives, anticancer, antibacterial, and antipyretic [2–5].
Metal ions play a key role in the actions of drugs . They are involved in specific interactions with antibiotics, proteins, membrane components, nucleic acids, and other biomolecules. Some transition metals exert pronounced biological effects, functioning as active centers within important bioactive molecules in living systems. The introduction of β-lactam antibiotics into the health care system in the latter stages of World War II represents one of the most important contributions to medical science in recent history. Today, β-lactams remain the most widely utilized antibiotics owing to their comparatively high effectiveness, low cost, ease of delivery, and minimal side effects.
Many drugs possess modified toxicological and pharmacological properties in the form of metal complexes. Transition metal ions possess an important role in the designing of metal-based drugs and such complexes are more effective against infectious diseases compared to the uncomplexed drugs. The metal which is probably the most widely studied in this respect is the copper(II) ion which has proved beneficial in many diseases such as tuberculosis, gastric ulcers, rheumatoid arthritis, and cancers .
These facts motivate us to explore the coordination behavior of a newly synthesized azetidin-2-one with transition metal ions.
In this report, we describe the syntheses and characterization of N-(2-carbamoylthienyl)-C-(3′-carboxy-2′-hydroxy phenyl) azetidin-2-one, LH3 (I) as shown in Figure 1, and its coordination compounds with Co(II), Cu(II), Zn(II), Zr(OH)2(IV), and MoO2(VI) ions. It is expected that the study of the present azetidinone and its coordination compounds may contribute towards the applications in the fields of biological and pharmaceutical activities.
Copper(II) acetate monohydrate, zinc(II) acetate dihydrate (SD’s Fine); cobalt(II) acetate tetrahydrate, hexadecaaquaoctahydroxotetrazirconium(IV) chloride (BDH); ammonium molybdate(VI) tetrahydrate, acetyl acetone, methanol, triethylamine, chloroacetyl chloride (Ranbaxy); thiophene-2-carboxylic acid hydrazide (Acros Organics (USA)) were used as supplied for the syntheses. Bis(acetylacetonato)dioxomolybdenum(VI), hexadecaaquaoctahydroxotetrazirconium(IV) acetate, and 3-formylsalicylic acid were synthesized by following the mehods as reported earlier .
2.2. Analyses and Physical Measurements
The estimations of metal, chlorine, and the sulphur contents of the respective compounds were carried out by the standard methods as reported . The C, H, and N contents were determined by the Eager analyzer model-300 CHN analyzer. The molar conductances of the coordination compounds were determined in DMSO with the help of a Toshniwal conductivity bridge (CL01-02A) and a dip type cell calibrated with KCl solutions. The molecular weights were determined by Rast’s method using diphenyl as the solvent . The IR spectra were recorded in KBr pellets (4000–400 cm−1) on a Beckman-20 spectrophotometer. The reflectance spectra were recorded on a Beckmann DU spectrophotometer attached with a reflectance arrangement. The magnetic susceptibility measurements were carried out at room temperature, using Hg[Co(NCS)4] as the standard . The diamagnetic corrections were computed using Pascal’s constants. The magnetic susceptibilities were corrected for temperature independent paramagnetism term (TIP)11 using value of units for Co(II) and units for Cu(II) ions.
2.3. Synthesis of N-(2-Carbamoylthienyl)-3′-carboxy-2′-hydroxybenzylideneimine (Schiff Base)
A MeOH solution (30 mL) of thiophene-2-carboxylic acid hydrazide (1.42 g, 10 mmol) was added to a MeOH solution (30 mL) of 3-formylsalicylic acid (1.66 g, 10 mmol). The mixture was refluxed for 2 h and the precipitates formed were suction-filtered, washed with and recrystallized from MeOH, and dried in vacuo at room temperature over silica gel for 24 h. Yield was 80%. The elemental analyses of the title compound gave the satisfactory results.
2.4. Synthesis of N-(2-Carbamoylthienyl)-C-(3′-carboxy-2′-hydroxyphenyl) Azetidin-2-one (I)
The title compound was synthesized by the drop-wise addition of chloroacetyl chloride (2.26 g, 20 mmol) during a period of 2 h to a dioxane solution (50 mL) of the Schiff base (2.90 g, 10 mmol) in the presence of triethylamine (3.03 g, 30 mmol) while stirring constantly. Triethylamine hydrochloride formed was filtered off and the volume of the filtrate was reduced to 50%. The solution was kept aside for 24 h and the solid product formed was suction-filtered, washed with dioxane, and recrystallized from CHCl3. The compound was dried as mentioned above. Yield = 28%. Anal: (I, C15H11N2O5SCl) (obsd: C, 48.80%; H, 2.88%; N, 7.50%; S, 8.50%; Cl, 9.48%. calc.: C, 49.11%; H, 3.00%; N, 7.64%; S, 8.73%; Cl, 9.69%); IR bands (KBr): 2900 cm−1 [ν(O–H)(intramolecular H-bonded)], 1750 cm−1 [ν(C=O)(β-lactam)], 1660 cm−1 [ν(C=O)(amide)], 1536 cm−1 [ν(C–O)(phenolic)], 1414 cm−1 [ν(C–N)(β-lactam)], 748 cm−1 [ν(C–Cl)(β-lactam)] and 645 [ν(C–S)(thiophene)].
2.5. Syntheses of Coordination Compounds of I
A MeOH solution (30–50 mL) of the appropriate metal salt (10 mmol) was added to a MeOH solution (50 mL) of I (3.66 g, 10 mmol) and the mixture was then refluxed for 3-4 h. The solid products formed were suction-filtered, washed with MeOH, and then dried as mentioned above. Yield was 55–75%.
3. Results and Discussion
N-(2-Carbamoylthienyl)-C-(3′-carboxy-2′-hydroxyphenyl) azetidin-2-one, LH3 (I), was synthesized by the reaction of chloroacetyl chloride on the Schiff base, N-(2-carbamoylthienyl)-3′-carboxy-2′-hydroxybenzylideneimine. The reaction of I with appropriate metal salts in 1 : 1 molar ratio in MeOH produces the coordination compounds of the types [Co(LH)(MeOH)]2, [Cu(LH)]2, [Zn(LH)(MeOH)2], [Zr(OH)2(LH)(MeOH)], and [MoO2(LH)(MeOH)2]. The coordination compounds are insoluble in H2O, MeOH, and EtOH. They are partially soluble in CHCl3, Me2CO, and C6H6 and completely soluble in DMSO and DMF. They are nonelectrolytes () in DMF. The analytical data of I and its coordination compounds are given in Table 1. The formation of coordination compounds of I takes place according to Schemes 1 and 2.
3.1. Infrared Spectral Studies
The infrared spectra of the Schiff base, I, and its coordination compounds were recorded in KBr pellets and the prominent peaks (in cm−1) are shown in Table 2. The conversion of the Schiff base to the corresponding azetidinone is confirmed by the disappearance of the (C=N)(azomethine) stretch occurring originally at 1625 cm−1 in the former and the appearance of a new band at 1414 cm−1 due to the (C–N)(β-lactam) stretch  in the latter. The formation of I is further supported by the appearance of a new band at 748 cm−1 due to the (C–Cl)(β-lactam) stretch  in its IR spectrum. I exhibits a strong band at 2900 cm−1 due to the intramolecular H-bonded OH group of phenolic and/or carboxylic acid moieties . This band disappears in the coordination compounds indicating the breakdown of H-bonding and subsequent deprotonation of the OH groups followed by the involvement of phenolic and carboxylic acid O atoms towards coordination. The presence of a broad band between 3350 and 3450 cm−1 due to the (O–H)(MeOH) stretch and the decrease of the (C–O)(MeOH) stretch from 1034 cm−1 to lower energy by 49–74 cm−1 in the coordination compounds of Co(II), Zn(II), Zr(OH)2(IV), and MoO2(VI) ions indicate the involvement of the O atom of MeOH towards coordination . I occurs in the keto-form as evident by the presence of a strong band due to the (C=O)(amide) stretch at 1660 cm−1. This band remains at the same energy in the coordination compounds, IV and V, indicating the noninvolvement of the keto O atom towards coordination. However, this band shifts to lower energy by 25 cm−1 in [Cu(LH)]2 indicating the involvement of the keto O atom towards coordination. II and VI do not display (C=O)(amide) stretch but show a new band at 1255 and 1240 cm−1, respectively, suggesting the conversion of –C(O)NH– moiety (keto-form) into –C (OH)=N– moiety (enol-form) followed by the involvement of enolic O atom towards coordination.
The (C–N)(β-lactam) stretch of I shifts from 1414 cm−1 to lower energy by 24–39 cm−1 in the complexes indicating the involvement of the N atom of β-lactam moiety towards coordination . The (C=O)(β-lactam) stretch  of I occurring at 1750 cm−1 remains unchanged in the coordination compounds suggesting the noninvolvement of the O atom of β-lactam moiety towards coordination. The appearance of two new bands between 1555 and 1580 cm−1 and between 1350 and 1375 cm−1, assignable to the (COO) and (COO) stretches, respectively, of the carboxylate group with an energy difference of 205–230 cm−1 indicates the monodentate nature of the carboxylate moiety . The (C–O)(phenolic) stretch  of I occurring at 1536 cm−1 shifts to higher energy by ≤10 cm−1 in the IV, V, and VI, while it shifts by 20 and 29 cm−1 in II and III, respectively, supporting the involvement of phenolic O atom towards coordination. The Cl atom of β-lactam moiety does not take part in coordination as evident by the occurrence of the (C–Cl)(β-lactam) stretch  (748 cm−1) of I unaltered in the coordination compounds.
The (C–S)(thiophene) stretch  of the ligand shifts from 645 cm−1 to lower energy by 50 cm−1 in IV, while it remains unchanged in the rest of the compounds. The absence of a band in the region 835–955 cm−1 due to the (Zr=O) stretch  in the present Zr(OH)2(IV) compound suggests its formulation as [Zr(OH)2(LH)(MeOH)] and not as [ZrO(H2O)(LH)(MeOH)]. The presence of a broad band at 3450 cm−1 due to the coordinated OH group of H2O and/or MeOH and the appearance of a new medium intense band at 1135 cm−1 due to the (Zr–OH) bending mode  also support the proposed structure, V of the present Zr(OH)2(IV) compound. VI exhibits the (O=Mo=O) and (O=Mo=O) stretches at 950 and 918 cm−1, respectively, . These bands occur in the usual ranges: (O=Mo=O) stretch, 892–964 cm−1, and (O=Mo=O) stretch, 842–928 cm−1, reported for the majority of MoO2(VI) compounds . The presence of two bands due to the (O=Mo=O) stretch is indicative of a cis-MoO2 configuration as the complex with trans-MoO2 structure shows only (O=Mo=O) stretch since the (O=Mo=O) stretch is IR inactive . The cis-structure, VI, forces the tetradentate ligand to coordinate in a nonplanar fashion . The absence of a band at ~775 cm−1in the present compound indicates the absence of an oligomeric chain with interaction . The new nonligand bands in the present coordination compounds in the low frequency region are assigned to the (M–O) (550–575 cm−1), the (M–N) (430–460 cm−1), and the (M–S) (330 cm−1) stretches. These bands are in the expected order of increasing energy: (M–S) < (M–N) < (M–O) .
3.2. NMR Spectral Studies
The NMR spectra of LH3 (I) and [Zn(LH)(MeOH)2] were recorded in DMSO-. The chemical shifts () are expressed in ppm downfield from TMS. The prominent resonance signals of these compounds were compared with the reported peaks . I exhibits a singlet at 17.7 ppm due to the carboxylic proton, a sharp singlet at 13.63 ppm due to phenolic proton, a singlet at 10.6 ppm (due to CONH proton) and at 11.2 ppm (due to HO-C=N, tautomeric protons), and multiplets between 7.34–7.70 ppm due to the aromatic protons. The absence of the signal at 17.5 ppm due to the COOH proton in [Zn(LH)(MeOH)2] indicates the deprotonation of the COOH group, followed by the involvement of its O atom in coordination. The absence of the resonance signal at 13.60 ppm due to the phenolic proton in [Zn(LH)(MeOH)2] indicates the deprotonation of the phenolic OH group followed by its involvement in coordination . The singlet at 10.4 ppm due to CONH proton remains unchanged in the complex and shows its noninvolvement in the coordination. The appearance of resonance signals at 2.82 ppm due to alcoholic proton and at 3.0-3.1 ppm due to methyl protons in the coordination compound supports the presence of MeOH in it.
3.3. Reflectance Spectral Studies
Due to the poor solubility of the coordination compounds in common noncoordinating organic solvents, their solution electronic spectra could not be recorded and, hence, the reflectance spectra were recorded (Table 2). II exhibits three bands at 9260, 13220, and 19280 cm−1 due to 4T1 g (F) → 4T2 g (), 4T1 g (F) → 4A2 g (), and 4T1 g (F) → 4T1 g () transitions, respectively, suggesting an octahedral arrangement of LH3 around Co(II) ions . The / value is 2.08 which lies in the usual range (2.00–2.80), reported for the majority of octahedral Co(II) compounds . The spectral parameters  are , , , , and . The reduction of Racah parameter from the free ion value of 971 cm−1 to 744 cm−1 and the β0 value (23%) indicates the covalent nature of the compound and the strong field nature of the tetradentate ligand . The presence of a shoulder at 18100 cm−1 due to the d−d transitions in [Cu(LH)]2 suggests a square-pyramidal arrangement of LH3 around Cu(II) ions .
3.4. ESR Studies
The ESR spectrum of [Cu(LH)]2 in DMSO at 77 K has been recorded in X-band, using 100 kHz field modulation, and the values are relative to the standard marker tetracyanoethylene (TCNE) (). The spectrum shows seven hyperfine lines () in parallel components due to the intramolecular exchange interaction in the dimer with no superhyperfine lines. It exhibits the half-filled line at ~1600 gauss due to transition and conclusively proves the presence of the magnetic exchange interaction in this coordination compound.
The ESR spectral parameters of [Cu(LH)]2 are as follows: , , , , , , and . The values of , , and obtained are close to the values reported for other dimetallic Cu coordination compounds . For ionic environments, is normally ≥2.3 and is <2.3 for covalent environments. The value (2.26) indicates that the metal-ligand bonding in the compound is covalent. The in-plane covalence parameter () has been calculated using the relation :
where is related to the overlap integral () according to the relation . The smaller the value of , the more covalent the bonding; indicates complete ionic bonding, while indicates complete covalent bonding. On the other hand the larger the value of , the more covalent the bonding; suggests a complete ionic bonding. The observed value of [Cu(LH)]2 is less than unity and this indicates that this compound possesses significant covalent character in the M-L bonding .
3.5. Magnetic Measurements
The room temperature magnetic moments of the compounds are presented in Table 2. The magnetic moment of II is 4.86 B.M. This value lies in the usual ranges, reported for the majority of octahedral Co(II) coordination compounds . The Cu(II) ion belongs to the system and since its spin-orbit coupling constant is negative , due to the presence of orbital contribution the magnetically dilute Cu(II) compound is expected to exhibit magnetic moment higher than the spin-only value (1.73 B.M.) . The magnetic moment (1.55 B.M.) of III is substantially less than the above range of magnetic moment and this indicates the presence of antiferromagnetic exchange interaction . IV, V, and VI are diamagnetic in nature.
3.6. Antibacterial Studies
The antibacterial activity of ligand (I) and its complexes were tested against bacteria, E. coli (gram negative) and S. aureus (gram positive), by using disc diffusion method (as shown in Table 3). Stock solution were prepared by dissolving compounds in DMSO. Under aseptic conditions, plain sterilised discs were soaked in solution of compounds for overnight. Test culture was spread over the plates containing Mueller Hinton Agar (MHA) E. coli and S. aureus by using sterile swab. Inoculated plates were dried for 30 minutes and discs were placed on inoculated plates. The plates were left for 30 minutes at room temperature to allow diffusion. The plates were then incubated at 37°C for 24 hours. After incubation, diameter of zone of inhibition was noted for each disc.
3.7. Determination of Minimum Inhibitory Concentration (MIC)
The stock solution of compounds was prepared using DMSO as diluent. In a set of test tubes having 2 mL of Mueller Hinton Broth, compounds were serially diluted. 2 mL of the test culture was added to all tubes and tubes were incubated at 37°C for 24 hr. Lack of turbidity was noted for the determination of MIC (Table 4).
On the basis of analytical data, molecular weight, valence requirements, and spectral and magnetic studies, it is proposed that I behaves as a dibasic tridentate OON donor ligand in monomeric, six-coordinate, diamagnetic octahedral compound, [Zr(OH)2(LH)(MeOH)] (V); a dibasic tetradentate OONO donor ligand in dimeric, six-coordinate, high-spin octahedral compound, [Co(LH)(MeOH)]2 (II); dimeric, five-coordinate, square-pyramidal compound, [Cu(LH)]2 (III); monomeric, eight-coordinate compound, [MoO2(LH)(MeOH)2] (VI); and a dibasic tetradentate OONS donor ligand in diamagnetic, six-coordinate, octahedral compound, [Zn(LH)(MeOH)2] (IV) as shown in Figure 2.
This paper is the authors’ own work, and the research is original. The results have not been published (in any language or medium), and the paper is not considered and will not be offered elsewhere. Both the authors have read and approved the paper, and due care has been taken to ensure the integrity of the work.
Conflict of Interests
No conflict of interests exists in the submission of the paper.
One of the authors (Dr. Amit Kumar) is grateful to the Director of his institute for financial assistance and encouragement for this work.
F. H. Havaldar and S. K. J. Mishra, “Synthesis of some azetidin-2-ones and thiazolidin-4-ones as potential antimicrobial agents,” Indian Journal of Heterocyclic Chemistry, vol. 13, no. 3, pp. 197–200, 2004.View at: Google Scholar
M. N. Kumaraswamy, V. P. Vaidya, C. Chandrasekhar, D. A. Prathima Mathias, H. Shivakumar, and K. M. Mahadevan, “Synthesis and pharmacological investigations of azetidinone derivatives involving naphtho[2,1-b]furan-2-carboxamide,” Research Journal of Pharmaceutical, Biological and Chemical Sciences, vol. 4, no. 1, p. 90, 2013.View at: Google Scholar
D. H. Brown, W. E. Smith, J. W. Teape, and A. J. Lewis, “Antiinflammatory effects of some copper complexes,” Journal of Medicinal Chemistry, vol. 23, no. 7, pp. 729–734, 1980.View at: Google Scholar
D. Kumar, A. Syamal, A. Kumar, P. K. Gupta, and D. Dass, “Syntheses and characterization of coordination compounds of N-(2-mercaptoethyl)-4-(3′-carboxy-2′-hydroxyphenyl)-2-azetidinone,” Journal of the Indian Chemical Society, vol. 87, no. 4, pp. 417–423, 2010.View at: Google Scholar
A. I. Vogel, A Textbook of Quantitative Inorganic Analysis, ELBS and Longman, London, UK, 1978.
F. G. Mann and B. C. Saunders, Practical Organic Chemistry, Longmans, London, UK, 1961.
R. L. Dutta and A. Syamal, Elements of Magnetochemistry, Affiliated East West Press, New Delhi, India, 2nd edition, 1993.
Z. H. Chohan and C. T. Supuran, “Metalloantibiotics: synthesis, characterization and in-vitro antibacterial studies on cobalt (II), copper (II), nickel (II) and zinc (II) complexes with cloxacillin,” Journal of Enzyme Inhibition and Medicinal Chemistry, vol. 21, no. 4, pp. 441–448, 2006.View at: Publisher Site | Google Scholar
E. Jayachandran, L. V. G. Nargund, A. Roy, G. M. Sreenivasa, and K. Krupanidhi, “Synhesis of 1-[6'-fluoro-7' substituted- (1,3') benzothiazol-2-yl] amino-4-substituted-3 chloro azetidin-2-one for antimicrobial screening,” Oriental Journal of Chemistry, vol. 23, no. 3, pp. 829–835, 2007.View at: Google Scholar
P. J. Bahad, N. S. Bhave, and A. S. Aswar, “Synthesis, structural and electrical studies of cobalt-, nickel-, copper- and zinc(II) polymeric complexes,” Journal of the Indian Chemical Society, vol. 77, no. 8, pp. 363–366, 2000.View at: Google Scholar
D. Kumar, A. Syamal, A. Kumar, D. Dass, and A. Gupta, “Spectral studies on metal complexes of a newly synthesized azetidinone,” Asian Journal of Chemistry, vol. 21, no. 9, pp. 7345–7353, 2009.View at: Google Scholar
Z. H. Chohan, C. T. Supuran, and A. Scozzafava, “Metalloantibiotics: synthesis and antibacterial activity of cobalt(II), copper(II), nickel(II) and zinc(II) complexes of kefzol,” Journal of Enzyme Inhibition and Medicinal Chemistry, vol. 19, no. 1, pp. 79–84, 2004.View at: Publisher Site | Google Scholar
J. R. Anacona and M. Lopez, “Mixed-ligand nickel(II) complexes containing sulfathiazole and cephalosporin antibiotics: synthesis, characterization, and antibacterial activity,” International Journal of Inorganic Chemistry, vol. 2012, Article ID 106187, 8 pages, 2012.View at: Publisher Site | Google Scholar
K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, John Wiley, New York, NY, USA, 4th edition, 1986.
D. Kumar, A. Kumar, and D. Dass, “Syntheses and characterization of the coordination compounds of N-(2-hydroxymethylphenyl)-C-(3′-carboxy-2′-hydroxyphenyl)thiazolidin-4-one,” International Journal of Inorganic Chemistry, vol. 2013, Article ID 524179, 6 pages, 2013.View at: Publisher Site | Google Scholar
D. Kumar, A. Syamal, Jaipal, and L. K. Sharma, “Synthesis, magnetic and spectral studies on polystyrene-anchored coordination complexes of bi-, tri-, tetra- and hexavalent metal ions with unsymmetrical dibasic tetradentate ONNO donor Schiff base derived from 3-formylsalicylic acid, ethylenediamine and 2-benzoylacetanilide,” Journal of Chemical Sciences, vol. 121, no. 1, pp. 57–64, 2009.View at: Publisher Site | Google Scholar
J. R. Ferraro, Low Frequency Vibrations of Inorganic and Coordination Compounds, Plenum Press, New York, NY, USA, 1971.
B. J. K. Nag, S. Pal, and C. Sinha, “Synthesis and characterization of cobalt(II), nickel(II), copper(II), palladium(II) and dioxouranium(VI) complexes of the antipyrine Schiff base of 3-formylsalicylic acid,” Transition Metal Chemistry, vol. 30, no. 5, pp. 523–526, 2005.View at: Publisher Site | Google Scholar
D. Kumar, A. Syamal, A. Gupta, M. Rani, and P. K. Gupta, “Role of pH on the formation of the coordination compounds with the Schiff base derived from 3-formylsalicylic acid and 4-amino-2,3-dimethyl-1-phenyl-3-pyrazolin-5-one,” Journal of the Indian Chemical Society, vol. 87, no. 10, pp. 1185–1197, 2010.View at: Google Scholar
A. B. P. Lever, Inorganic Electronic Spectroscopy, Elsevier, Amsterdam, The Netherlands, 2nd edition, 1984.
J. E. Huheey, Inorganic Chemistry, Principles of Structure and Reactivity, Harper and Row, New York, NY, USA, 3rd edition, 1983.
M. Todokora, H. Saiyama, N. Matsumoto, M. Kodeva, H. Okawa, and S. Kida, “Template synthesis of copper(II)lead(II) complexes of new binucleating macrocycles with dissimilar co-ordination sites,” Journal of the Chemical Society, Dalton Transactions, no. 2, pp. 313–317, 1992.View at: Publisher Site | Google Scholar
P. Athappan, S. Sevagapandian, and G. Rajgopal, “Synthesis and spectral studies of copper(II), nickel(II), cobalt(II) and vanadyl(II) complexes of tridentate Schiff bases of 1,2,3,5,6,7,8,8a-octahydro-3-oxo-N,1-diphenyl-5-(phenylmethylene)-2-naphthalenecarboxamide,” Transition Metal Chemistry, vol. 20, no. 5, pp. 472–476, 1995.View at: Publisher Site | Google Scholar
F. Cotton, A. G. Wilkinson, C. Murillo, and M. Bochmann, Advanced Inorganic Chemistry, John Wiley, New York, NY, USA, 6th edition, 1999.