Ligational Behavior of the Azetidin-2-One Derived From Salicylaldehyde and O-Hydroxyphenylurea towards Some Di-, Tri- and Hexavalent Metal Ions

The 2-carbonyl derivatives of azetidine containing a four-membered heterocyclic ring with N as heteroatom, are known as azetidin-2-one or β-lactam. They are most widely used antibiotic, antimicrobial, anti-inflammatory, anticonvulsant and antitubercular substances[308]. They also act as enzyme inhibitors and are effective on central nervous system[309]. In view of the above importance of azetidin-2-one and in continuation of our earlier work on the metal complexes of azetidin-2-one[310], the syntheses and characterization of the coordination compounds of 8 with Cu(II), Cd(II), Mn(II), Fe(III) and MoO2(VI) ions are presented in this Chapter.

The various chemicals and solvents obtained from the sources mentioned in Appendix-2 were used as received for the syntheses.

The estimation of metal, elemental contents, molar conductivity measurements, spectral (IR, reflectance, ESR) studies and magnetic susceptibility measurements were carried out by the methods described in Appendix-3.

Chloroacetyl chloride (2.26 g, 20 mmol) was added drop wise during a period of 2 h to a continuously stirred dioxane solution (50 mL) of 1 (2.56 g, 10 mmol) in the presence of Et3N (3.03 g, 30 mmol). 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, recrystallized from CHCl3 and then dried as mentioned elsewhere. Yield = 30%; anal. [C16H13N2O4Cl; found(calcd)%; C = 57.44(57.74), H = 3.82(3.91), N = 8.45(8.42), Cl = 10.43(10.68); IR bands (KBr): ν(O––H) (intramolecular H-bonding) (2765 cm-1), ν(C==O)(β-lactam)(1728 cm-1),

A MeOH solution (30-50 mL) of appropriate metal acetate/chloride/complex (10 mmol) was added to a MeOH solution (50 mL) of 8 (3.32 g, 10 mmol). The solution was refluxed for ~ 4 h on a water bath and

L′H3 (8) + Cu(OAc)2·H2O Reflux

MeOH[Cu(OAc)(L'H2)] (9) + AcOH + H2O

L′H3 (8) + Cd(OAc)2·2H2O Reflux

MeOH [Cd(OAc)(L'H2)] (10) + AcOH + 2H2O

L′H3 (8) + Mn(OAc)2·4H2O Reflux

MeOH [Mn(OAc)(L'H2)(MeOH)2] (11) + AcOH + 4H2O

L′H3 (8) + FeCl3 Reflux

MeOH [FeCl2(L'H2)(MeOH)] (12) + HCl

L′H3 (8) + [MoO2(acac)2] Reflux

The coordination compounds are insoluble in common solvents such as H2O, MeOH, EtOH but are soluble in DMF and DMSO. The experimental molar conductance values (5.3–8.3 mho cm2 mol-1 in 10-3 M DMF solution) of these compounds are indicative of their non-electrolytic nature. The analytical data of 8 and its coordination compounds are presented in Table 2.1.

ν(C==N)(azomethine) and ν(C––O) stretches at 1665, 1620 and 1526 cm-1 respectively. The ν(C==N)(azomethine) stretch of 1 disappears in 8 and a new band due to the ν(C––N)(β-lactam) stretch appears at 1410 cm-1 supporting the formation of the corresponding azetidin-2-one[311]. The formation of 8 is further supported by the appearance of a new band at 1728 cm-1 and other at 780 cm-1 due to the ν(C==O)(β-lactam) and the ν(C––Cl)(β-lactam) stretches respectively[312]. The ν(C––O) stretch of 8 occurring at 1510 cm-1 shifts to higher energy by ≤10 cm-1 in 9-13 indicating the involvement of phenolic O atom towards coordination. The magnitude of above shift of the ν(C––O) stretch indicates the monomeric nature of the present coordination compounds. The molecular weight data also suggest their monomeric nature. 8 occurs in keto form as evident by the presence of a band at 1670 cm-1 due to the ν(C==O)(amide) stretch[91]. The persistence of this band at the same energy in 8 as well as in 9-13 indicates the non-involvement of amide O atom towards coordination. The ν(C==O)(β-lactam) stretch of 8 occurring at 1728 cm-1 shifts to lower energy by 12-30 cm-1 indicating the involvement of O atom of β-lactam moiety towards coordination[313]. The Cl atom of the β-lactam ring does not participate in coordination as evident by the presence of the ν(C––Cl)(β-lactam) stretch at the same energy in 8 and 9-13. The appearance of a broad band between 3360-3440 cm-1 due to the ν(O––H)(MeOH) stretch and the decrease of the ν(C––O) (MeOH) stretch from 1034 cm-1 to lower energy by 40-60 cm-1 in 11-13 indicates the presence of coordinated MeOH molecule(s)[314]. The νas(OAc) and νs(OAc) stretches of free acetate ions occur at 1560 and 1416 cm-1 respectively[25]. These bands occur between 1586-1595 and 1372-1380 cm-1 respectively in 9-11. The magnitude of energy separation (∆ν = 211-223) between νas(OAc) and νs(OAc) stretches is >144 cm-1 which indicates the monodentate nature[315] of acetato group in 9-11. The occurrence of two bands, one at 940 cm-1 and other at 880 cm-1 in 13, due to the νs(O==Mo==O) and νas(O==Mo==O) stretches respectively, indicates the presence of a cis-MoO2 configuration in it[208]. Acetylacetone is coordinated here as a monobasic bidentate OO donor ligand as evident by the presence of a band at 1695 cm-1 due to the ν(C==O) stretch[316]. The new non-ligand bands in the present coordination compounds in the low frequency region are assigned as ν(M––O)(550-575 cm-1) and ν(M––N)(430-465 cm-1) and these bands are in the expected order of increasing energy: ν(M––N) < ν(M––O)[297].

The appearance of an asymmetric band at 17450 cm-1 due to the 2B1g → 2A1g, 2B2g and 2Eg transitions in 9 is consistent with the Cu(II) ion in a square-planar environment[30]. 11 exhibits three bands at 16236,

respectively in an octahedral symmetry[300]. 12 exhibits three bands at 12450, 15000 and 19220 cm-1 corresponding to the 6A1g → 4T1g(G), 6A1g → 4T2g(G) and 6A1g → 4A1g(G) transitions respectively in octahedral symmetry[300].

ESR SPECTRAL STUDIES

The X-band ESR spectrum of 9 shows usual anisotropic pattern with two g values, which are characteristic of tetragonal type symmetry. Its spin Hamiltonian parameters are: All = 1.50 × 10-2 cm-1, A┴ = 3.0 × 10-3 cm-1, g|| = 2.8, g= 2.08, κ = 0.49, G = 3.50, Pd = 1.54 × 10-2 cm-1, κPd = 7.55 × 10-2 cm-1, α2Cu = 0.77 and (α')2 = 0.33. The pattern g||  g > 2 is suggestive of dx2-y2 ground state. The value of g|| (2.28) indicates the covalent character in the metal-ligand bonding. The value of G (3.50) is indicative of strong field nature of the ligand in the complex. The values of α2Cu (0.77) and (α')2 (0.33) indicate the covalent nature of 9. The positive value of κ (0.49) suggests that All should be .greater than A┴. The Pd value (1.54 × 10-2 cm-1) is lower than free ion value (3.5 × 10-2 cm-1), which is suggestive of the presence of the covalent character between metal-ligand bonding. The absence of a band at ~1500 gauss due to ∆Ms = 2 transition in 9 rules out the presence of Cu––Cu interaction.

The room temperature magnetic moments of the compounds are presented in Table 2.2. The Cu(II)

ion belongs to S = ½ system and since its spin-orbit coupling constant is negative, the magnetically dilute Cu(II) compounds, due to the presence of orbital contribution are expected to exhibit magnetic moments higher than the spin-only value (1.73 B.M.). The magnetic moment of 9 is 1.87 B.M. in the range expected for the monomeric Cu(II) compounds. The magnetic moment of 11 is 5.81 B.M., which lies in the normal range reported for the magnetically dilute octahedral compounds of Mn(II) ions[107] The magnetic moment of 12 is 5.78 B.M. suggesting a high-spin octahedral environment around Fe(III) ion in the complex[317]. The coordination compounds of Cd(II) and MoO2(VI) ions are diamagnetic as expected.

On the basis of analytical data, valence requirements and spectral studies, it is proposed that 8 behaves as a monobasic tridentate ONO donor ligand in these monomeic and non-electrolyte coordination compounds. The data suggest a square-planar structure for 9, a tetrahedral structure for 10, an octahedral structure for 11 and 12. The compound, 13 is eight-coordinate. A. Syamal, D. Kumar, A.K. Singh, P.K. Gupta, Jaipal and L.K. Sharma (2012). Indian J. Chem., 41A, pp. 1385. A.S. El-Tabl, F.A. El-Saied and A.N. Al-Hakimi (2017). Transition Met. Chem., 32, pp. 689. D. Kumar, P.K. Gupta and A. Syamal (2002). Indian J. Chem., 41A, pp. 2494. D. Kumar, P.K. Gupta, A. Kumar, D. Dass and A. Syamal (2011). J. Coord. Chem., 64, pp. 590. J.R. Anacona and A. Rodriguez (2014). J. Coord. Chem., 57, pp. 1263. K.A.R. Salib, M.F. Ishak, M.A. El-Behairi and H.F.A. El-Halim (2013). Synth. React. Inorg. Met.-org. Chem., 33, pp. 1667. L. Feng, C. Liying, Z. Hongyun, W. Qingan, C. Zhanhua, N. Yunyin and J. Haigang (1995). Transition Met. Chem., 20, pp. 511. M. Gallego, M.V. Garcia and M. Valcarcel (1979). Analyst, 104, pp. 613. M.B.H. Howlader, M.B. Hossain and N. Akhter (2008). Indian J. Chem., 47A, pp. 214.

Research Scholar, Singhania University, Pacheri Bari, Rajasthan