INTRODUCTION:

During the past two decades considerable attention has been paid to the chemistry of metal complexes of Schiff bases containing nitrogen and other donors.^1-8 This may be attributed to their stability, biological activity⁹ and potential applications in many fields such as oxidation catalysis¹⁰ and electrochemistry.¹¹Schiff bases containing

polyfunctional groups produce stable complexes of transition, non-transition, inner-transition and actinide metal ions. Among these, manganese is an important trace element needed for normal physiological functions. Manganese complexes in high oxidation state are potentially used as oxidizing agents, catalysts¹² and electro-catalysts¹³ for oxidation of alcohols, ethers and water. H₂L is also an important Schiff base derived from the condensation of benzoylhydrazide and salicylaldehyde. The high affinity for the complexation of the Schiff bases towards the transition metal ions is utilized in preparing their solid complexes. Monometallic complexes of H₂L with a variety of transition metals have been reported^{14 - 16} but there is no report on complexes of Mn with the titled ligand. The main target of the present article is to study the coordination behaviour of H₂L Schiff base that incorporate several binding sites towards Mn ion.

The choice for Mn ion for complex synthesis has been received from the application of manganese complexes in converting light energy in to fuel in PS-II.¹⁷ In this paper, I am reporting the synthesis and characterization of Mn (II,III,IV) complexes with ligand (H₂L) (Fig. 1). The possible geometry for the complexes has been deduced by the elemental analysis, molar conductance, magnetic susceptibility, IR, electronic and EPR spectral studies.

EXPERIMENTAL:

Materials:

Reagent grade organic solvents were purified and dried by recommended procedures. ¹⁸ Hydrochloric and nitric acids (E-Merck) were used De-ionized water collected from all glass equipments were usually used in all preparations. Organic solvents used included absolute ethanol, diethylether, methanol, dimethylsulphoxide (DMSO) and dimethylformamide (DMF). Manganese(II) acetate tetrahydrate, potassium hydroxide, ethylbenzoate, hydrazine hydrate and salicylaldehyde were GR (E-Merck) or equivalent grade reagents. The ligand ¹⁹ and manganese(III) acetate dehydrate²⁰ have been prepared by literature procedures.

Physical measurements:

The molar conductance values of the complexes at 10^-3M in DMSO solution were measured on a Systronics Direct Reading Conductivity meter-303 with a dip type conductivity cell at room temperature. Magnetic susceptibility measurements were carried out on a Vibrating Sample Magnetometer. UV-Vis. Spectra of the complexes were recorded in DMSO solution (10^-3M) on a Perkin-Elmer Lambda 25 UV–Vis spectrophotometer. Infrared spectra were recorded on a Perkin-Elmer Model spectrum BX-II IR spectrophotometer in the range 4000–450 cm^-1 in KBr discs. The ESR spectra of the complexes were recorded at X-band frequency on a Varian E-112E-line century series ESR spectrometer using TCNE (g = 2.00277) as an internal field marker. Microanalytical ( C, H & N) data were obtained with an Elementar Vario EL 1108 at SAIF, Central Drug Research Institute, Lucknow, India. For determining Mn the complexes were decomposed with aqua regia till the all organic moieties were oxidized. The solution was evaporated to dryness and treated with concentrated HCl. The concentrated HCl solution was also evaporated to dryness and finally the solution was made in distilled water and was used for estimation of manganese gravimetrically following the standard literature procedure.²¹

Synthesis of the ligand (H₂L):

The ligand, [N-(2-hydroxysalicyliden-1-yl) methylenebenzoylhydrazide] (H₂L) was prepared in two steps. In the first step benzoylhydrazide(BH) was prepared by the reaction of ethylbenzoate (48.0 mL) and hydrazine hydrate (16.3 mL) in 1:1 molar ratio under reflux for 4 hours. The product thus isolated was recrystallized from hot benzene.²² Yield: 80%. In the second step, benzoylhydrazide (5.0 g, 36.746 mmol) in ethanol (50 mL) solution was allowed to react with salicylaldehyde(5 mL) in ethanol (20 mL) in 1:1 molar ratio over hot plate at 70 ºC with constant gentle stirring for about 30-45 min. The light yellow polycrystalline precipitate thus obtained was purified by repeated washing with hot ethanol and dried over anhydrous CaCl₂. .[Yield : ~85%]. (m. p. 162 ºC) (Found: C, 70.38; H, 5.02; N, 11.58; Calcd. for C₁₄H₁₂ O₂N₂, C, 70.00; H, 5.0; N, 11.66(%). λ_max(nm), 285 (7200 dm³mol^-1cm^-1), 320 (5030 dm³mol^-1cm^-1) and 378(4714 dm³mol^-1cm^-1); ν(cm^-1) 3500-3100 (sbr), 3468(S), 3307(s), (OH + NH); 1676(s), 1648(m) (C=O); 1623(m), 1613(m)(C=N); 1581(m), 1532(s)[amide II(CO + NH) + (C-O)(phenolic)].

Synthesis of [Mn(L)₂] (1):

H₂Lligand (1.0 g, 4.166 mmol) in methanol ( 30 mL ) was treated with Mn(OAc)₂.4H₂O(0.51 g, 2.08 mmol) in methanol (20 mL) at 70 ºC for ½h. To the resulting suspension KOH(0.466 g, 8.32 mmol) in methanol (10 mL) was added drop wise maintaining the temperature at 70 ºC and finally stirred for another 45 minutes. The solution obtained was filtered and filtrate was allowed to stand at room temperature for 7 days to precipitate red brown solid products which were collected by filtration and dried over anhydrous CaCl_2.[Yield: 70 %]

Synthesis of [Mn(L)(OH)(H₂O)] (2):

To a 50 mL methanolic solution of Mn(OAc)₃.2H₂O(0.56 g, 2.089 mmol), (1.0 g, 4.166 mmol) of H₂L was added and stirred magnetically at about 70 ºC for ½ h. To the resulting suspension, 10 mL methanolic solution of KOH(0.702 g, 12.53 mmol) was added dropwise with constant stirring and finally stirred for 45 /minutes resulting in a blackish green solution. It was filtered and the filtrate was kept for crystallization which yielded blackish green precipitate after 4 days. The precipitate was washed several times with methanol and dried over anhydrous CaCl₂. [Yield: 78 %]

Synthesis of [Mn(L)(py)] (3):

H₂L ligand (1.0 g, 4.166 mmol) in methanol ( 30 mL ) was treated with Mn(OAc)₂.4H₂O(0.51 g, 2.08 mmol) in methanol (20 mL) at 70 ºC for ½h. To the resulting suspension KOH(0.93 g, 16.60 mmol) in methanol (10 mL) was added dropwise maintaining the temperature at 70 ºC and finally stirred for another ½ h. To the resulting solution mixture pyridine(py) (3.87 g, 52.29 mmol) was added and stirred for another 1h. It was filtered and the solution mixture was allowed to stand at room temperature for 5 days to precipitate dark brown solid products which were collected by filtration and dried over anhydrous CaCl_2. [Yield: 72 %]

Synthesis of [Mn(L)(A)] [ Where A = 2-pic (4), (3-pic)] (5) or (4-pic)] (6)]:

The complexes 4-6 were obtained essentially by the same procedure as used for synthesis of [Mn(L)(py)] only by adding 2-picoline, 3-picoline or 4-picoline to the reaction mixture instead of pyridine. The complexes were isolated as usual as dark brown solid products.

RESULTS AND DISCUSSION:

Schiff base characterization:

The Schiff base H₂L, is subjected to elemental analyses. The result of elemental analyses(C, H, N) with molecular formula and the melting points are presented in the preparation section of the ligand. The results obtained are in good agreement with those calculated for the suggested formula. The structure of this Schiff base is also confirmed by IR and the IR spectra of the ligand were given in Fig 2(a) and 2(b). It takes the following structural formula and IUPAC name:

Figure 1 keto- and enol- forms of N-(2-hydroxysalicyliden-1-yl)methylenebenzoylhy- drazide.

Composition and structures of monometallic complexes:

All the complexes are insoluble in water and common organic solvents such as EtOH, MeOH, CH₃COCH_3,CCl_4,CHCl_3,Et₂O, C₆H₆, CH₂Cl₂, CH₃CNexcept in the highly coordinating solvents like DMF (N,N-dimethyl formamide).and DMSO(N,N-dimethyl sulphoxide) and do not melt or decompose up to 300ºC. The weight loss experiments for the complexes were carried out by heating a small amount of sample in glass tube for 4h in an electric oven maintained at 110, 180 and 220-240 ºC. None of the complexes show weight loss at 110 ºC ruling out the presence of lattice water in their structure. But the complex 2 show weight loss corresponding to one water molecule at 180 ºC suggesting the presence of coordinated water molecule to the metal centre. The complexes 3-6 show weight loss in the temperature range 220-240ºC corresponding to one molecule of pyridine, 2-picoline, 3-picoline or 4-picoline respectively. The vapours evolved in this temperature range turned a solution of CHCl₃ containing a drop of 5M NaOH solution red confirming the presence of pyridine bases in coordination sphere of metal ions.²³Expulsion of these molecules at such a high temperature indicates they are coordinated to the metal centre. The isolated complexes comprising of Mn(II), Mn(III) and Mn(IV) were subjected to elemental analyses(C, H, N and metal content), molar conductance, magnetic studies, IR, electronic and ESR, to identify their tentative formulae in a trial to elucidate their molecular structures. The results of elemental analyses listed in Table 1 suggest the formulae [Mn(L)₂](1), [Mn(L)(OH)(H₂O)](2) and [Mn(L)(A)][where A = py(3), 2-pic(4), 3-pic(5) and 4-pic(6)]. The analytical data and stoichiometries of the monometallic complexes are summarized in Table 1.

TABLE 1 The analytical data and physical properties of the complexes

Complex (Colour)

% Yield (Dec. Temp. ºC)

Elemental analysis found (Calcd) (%)

µ_B(B.M)

Λ_M(Ω^-1 cm² mol^-1)

[Mn (L)₂ ] (1)

(red brown)

70.00

( > 300 °C )

10.32

(10.35)

64.10

(63.27)

3.80

(3.76)

10.60

(10.54)

3.90

7.80

[Mn (L) (OH) ( H₂O)] (2)

(blackish green)

78.00

( > 300 °C )

16.74

(16.76)

51.50

(51.21)

4.00

(3.96)

8.80

(8.73)

4.94

10.50

[Mn (L)(py)] (3)

(dark brown)

72.00

( > 300 °C )

13.88

(13.88)

63.66

(63.63)

3.82

(3.78)

10.70

(10.60)

5.98

8.00

[Mn (L)(2-pic)] (4)

(dark brown)

75.00

( > 300 °C )

13.40

(13.41)

64.50

(64.39)

4.20

(4.14)

10.31

(10.24)

5.99

8.40

[Mn (L)(3-pic)] (5)

(dark brown)

75.00

( > 300 °C )

13.38

(13.41)

64.42

(64.39)

4.18

(4.14)

10.28

(10.24)

6.00

7.90

[Mn (L)(4-pic)] (6)

(dark brown)

70.00

( > 300 °C )

13.40

(13.41)

64.50

(64.39)

4.16

(4.14)

10.30

(10.24)

6.00

7.90

Molar conductivity measurements:

The complexes were dissolved in DMF and the molar conductivities of 10⁻³ M of their solutions at 25 °C were measured. Table 1 shows the molar conductance values of the complexes. The molar conductivity values of the complexes fall in the range 7.8 – 10.5 Ω^-1cm²mol^-1and these areindicative of non-electrolytic nature of the complexes [5].

Magnetic susceptibility measurements:

The μ_effvalues for Mn(IV) complexes are reported to lie in the range 3.83 – 4.12 B.M. with d³ configuration(S = 3/2)^5,24 while Mn(III) complexes have μ_effvalues in the range 4.80 – 5.20 B.M. with d⁴ configuration(S = 2).²⁵ The μ_effvalue of 3.90 B.M. for the complex 1 suggested that the complex contains manganese in the + 4 oxidation state while the μ_effvalue of 4.94 B.M. for the complex 2 is consistent with the +3 oxidation of manganese. Complexes 3 - 6 have μ_effvalue in the range 5.98 – 6.00 B.M. which are consistent with the manganese(II) complexes adopting a d⁵ high spin electronic configuration.⁶^,26

Electronic spectral measurements:

UV-Visible spectra of the ligand and the complexes were recorded in DMF solution (10^-3M) and relevant data are given in Table 2. Mn(III) having d⁴ electronic configuration with ⁵D ground state is expected to show only one spin-allowed absorption band (⁵E_g → ⁵T_2g) in the visible region. Mn(III) octahedral complexes generally show a broad band around 500 nm.^27,28 In the present study the ligand shows four absorption bands at 232(7720), 285(7640), 320(5200) and 378 nm (4714 Lmol^-1cm^-1) due to carbonyl and imino chromophores in conjugation with phenyl group. Manganese(IV) complex 1 shows only two bands at 325(6100) and 370 nm (6210 Lmol^-1cm^-1) in the UV region are assigned to intraligand transitions. In addition to ligand bands, the complex shows a very strong band at 530 nm (11500 Lmol^-1cm^-1). This band may be attributed to charge-transfer from phenolate oxygen to Mn(IV).²⁹ On the other hand, complex 2 shows three absorption bands at 410, 428 and 520 nm in addition to other ligand bands. The bands at 410 and 428 nm have Є_max value 2530 and 2670 Lmol^-1cm^-1respectively, which may be assigned to charge-transfer bands probably arising from transfer of charge from phenolate oxygen to Mn(III).⁷ The additional band observed at 520 nm may be assigned to d-d transition band in octahedral stereochemistry around Mn(III) ion. The high spin manganese(II) complexes 3-6 show a band in the region 534-538 nm with molar extinction coefficient value in the region 2400 - 2500 Lmol^-1cm^-1in addition to ligand bands may tentatively be assigned to d-d transition in square-planar geometry around Mn(II) ion. However, d-d bands in Mn(II) complexes are Laporte forbidden as well as spin forbidden.

TABLE 2: UV-Visible spectral data of the ligand and the complexes

Ligand/ Complex	Wave length (λ_{max, nm})( ε_max, L mol^-1 cm^-1)
H₂L(C₁₄H₁₂N₂O₂)	232 (7720), 285 (7640), 320 (5200), 378 (4714)
[Mn(L)] (1)	530(11500), 370 (6210), 325 (6100), 280 (7250), 210 (7400)
[Mn(L)(OH)( H₂O)] (2)	520 (3460), 428 (2670), 410(2530), 390 (6100), 320 (5320), 230 (7600)
[Mn(L)(py)] (3)	535 (2480), 374 (6230), 290 (7400), 220 (7600)
[Mn(L)(2-pic)] (4)	538 (2400), 380 (6250), 296 (7350), 210 (7580)
[Mn(L)(3-pic)] (5)	534 (2420), 378 (6200), 300 (7300), 222 (7620)
[Mn(L)(4-pic)] (6)	536 (2500), 370 (6100), 312 (7320), 218 (7630)

IR spectral studies:

FT-IR spectral data of the ligand and its complexes are given in Table 3.

N-(2-hydroxysalicyliden-1-yl) methylenebenzoylhydrazide (H₂L) has four potential donor sites such as carbonyl oxygen, phenolic oxygen, secondary amine nitrogen and azine nitrogen. The uncoordinatedH₂Lligand shows two strong bands at 3468 and 3307 cm^-1assigned to stretching vibrations due to –NH– and –OH. These bands are replaced by strong broad band in the region 3500 – 3100 cm^-1 in all of its complexes which may be assigned to either ν(O-H) or ν(N-H). The amide-I bands for H₂L are observed at 1676 and 1648 cm^{-1 30} which disappear in manganese complexes suggesting collapse of amide structure probably due to enolisation of ligand in these complexes resulting coordination through the carbonyl oxygen atom in enol form, further supported by the appearance of an intense band at 1548 cm^-1assigned to newly generated ν(NCO) group.³¹

TABLE 3 FT-IR (KBr, cm^-1) spectral data of the ligand and its complexes.

Ligand/Complex	ν(OH) + ν(NH)	ν(C=O)	ν(C=N)	AmideII + ν(C-O) (phenolic)	ν(NCO)	ν(C-O)	ν(M-O)
H₂L(C₁₄H₁₂N₂O₂)	3500-3100(sbr), 3468(s), 3307(s),	1676(s) 1648(m)	1623(m), 1613(m)	1581(m) 1532(s	-	-	-
[Mn (L)₂] (1)	3500-3100 (sbr), 3434(s)	-	1625(s), 1604(s)	1541(vs)	1548(s)	1303(m)	593(msh)
[Mn (L)(OH)( H₂O)] (2)	3500-3100(sbr), 3430(s)	-	1625(s), 1600(s)	1540(vs)	1548(s)	1296(m)	591(msh)
[Mn (L)(py)] (3	3500-3100(sbr), 3409(s)	-	1628(s), 1601(s)	1540(s)	1548(s)	1300(m)	586(w)
[Mn(L)(2-pic)] (4)	3500-3100(sbr), 3410(s)	-	1627(s), 1602(s)	1536(s)	1548(s)	1303(m)	588(w)
[Mn(L)(3-pic)] (5) 1298(m)	3400-3200(sbr), 1600(s)	-	1630(s),	1542(s)	1548(s)	3412(s)	590(m)
[Mn(L)(4-pic)] (6)	3400-3100(sbr), 3409(s)	-	1630(s), 1605(s)	1540(s)	1548(s)	1304(w)	588(m)

The ν(C-O)(phenolic) band observed at 1581 cm^-1in the ligand suffers negative shift in the complexes indicating its bonding to the metal centre.³²Coordination of ligand to manganese in enol-form probably arises from the basic condition of the reaction. Acetic acid produced by the reaction of manganese acetate and ligand reacts with KOH promoting enolization. The ν(C=N) band occurs in the range 1630 – 1610 cm^-1for Schiff bases derived from condensation of hydrazine with aromatic aldehydes or ketones.³³ The hydrazone form of the ligand (H₂L) shows two bands at 1623 and 1613 cm^-1which shifts to higher position in all the complexes suggesting coordination of azomethine nitrogen.³⁴ The aromatic ring vibrations at 1600 cm^-1cannot be distinguished as it overlaps with ν(C=N), amide-II or ν(C-O)(phenolate). The bending vibration bands for coordinated water found in the region 1640 – 1600 cm^-1also overlapped. The C–H in-plane and out-of-plane deformation bands in H₂L appear at 1260, 1205, 1177, 1156 cm^-1and 796, 789, 736 cm^-1, respectively, remain either unaffected or slightly shifted in the complexes. Two strong bands at 1276 and 1261 cm^-1, in the IR spectrum of H₂Ldue to bending vibration of phenolic(C-O) group merge into single band in the complexes and appears at 1303, 1296, 1300, 1303, 1298 and 1304 cm^-1respectively. The positive shifts in these bands suggest that the phenyl electron density flows to the metal centre through phenolate C-O. The IR spectra of the complexes and dehydrated form were recorded under identical conditions to evaluate whether bridging –OH is present in the complexes. The IR spectra of 2 show a medium broad band centered at 3420 cm^-1 due to coordinated water, while dehydrated sample(heated at ~ 200 °C for 4 h) shows a weak broad band at 3200-3500 cm^-1, very similar to that observed in hydroxo complexes.³⁵ Hence, this weak broad band is assigned to stretching vibration of –OH group. It appears that in hydrated 2the band due to stretching of –OH group is masked by coordinated water. Complex 2 in its hydrated and dehydrated forms shows a medium intensity band at 980 cm^-1 assigned to bridging –OH bending. Similar results have been reported for [Mn(L)(OH)(H₂O)], [Cu₂(OH)₂(bipy)₂] SO₄.5H₂O, and others.^7,36,37 This medium intensity band at 980 cm^-1in 2 is not observed in IR spectra of free ligand or other complexes. Hence, this band is assigned to bending vibration of M(µ-OH)₂M, indicating metal-metal interaction through bridging. Non-ligand band observed in the region 593 – 586 cm^-1is assigned to ν(M-O) (phenolate).³⁸ Complexes 3-6 show a weak band in the region 1028- 1030 cm^-1 indicating the coordination of pyridine, 2-picoline, 3-picoline and 4-picoline to the metal centre. The IR spectra of the complexes 1 and 3 are given in the Fig 3(a), 3(b) and 4(a), 4(b) respectively as the representative complexes.

Electron spin resonance spectra:

Complex 1 shows two ESR signals at both RT and LNT in polycrystalline state which appear as a strong one near g = 2.033 and the other as weak near g = 4.326. The g values remain same at both the temperatures which indicate that the structure of the complex does not change on lowering the temperature. The ⁵⁵Mn hyperfine structure is well resolved for the resonance near g = 2.0 in DMSO solution only but not in DMF solution. Depending on the nature and extent of distortion of ligand field of O_hsymmetry, the ESR spectra of a d³ ion can assume different forms.³⁹ The form observed here (strong g = 2.033, and weak g = 4.326 resonances) is characteristic of small axial distortion, 2D« hυ, where D is the axial zero field splitting parameter and hυ is the microwave quantum (0.31/cm^-1 at X-band). When D is small, rhombic splitting is necessarily very small, since D/E ≥3, where E is the rhombic splitting parameter. Large distortion, 2D»hυ, occurs in a number of complexes^40,41 and the 2D«hυ situation for Mn(IV) has also been documented in few cases.⁴² Interestingly the ⁵⁵Mn hyperfine coupling constant for the g = 2.031 signal lies close to 100G for the known 2D « hυ for the species suggesting small axial distortion in O_h symmetry around Mn(IV) ion. Complex 2 was ESR silent consistent with the presence of Mn(III) in the complex. Complexes 3-6 in polycrystalline phase show isotropic spectra. At LNT a six line spectrum is obtained with ⁵⁵Mn hyperfine splitting constant equal to 98G, characteristic of Mn(II) complexes. The ESR spectra of the complex 1 are given in the Fig 5(a) and 5(b) as a representative complex.

CONCLUSION:

Based on stoichiometries and physico-chemical studies, the ligand behaves in enol-form in its complexes with 6-coordinate octahedral for Mn(III), Mn(IV) and 4-coordinate square-planar geometry for Mn(II) complexes have been tentatively proposed as shown in figures 6-8.

Figure 6 Suggested structure of [Mn(L)₂].

Figure 7 Suggested structure of [Mn(L)(OH)(H₂O)].

Figure 8 Suggested structure of [Mn(L)(A)][ where A = py(3), 2-picoline(4), 3-picoline(5) and 4-picoline(6).

ACKNOWLEDGMENTS:

We are thankful to the Department of Inorganic Chemistry, IACS, Jadavpur, Kolkata for magnetic susceptibility measurement, Head, SAIF, NEHU, Shillong in recording IR spectra and Head, SAIF, Central Drug Research Institute, Lucknow, India for Elemental Analysis. Help received from Head, SAIF, Bombay in recording ESR spectra is also acknowledged.

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Received on 25.11.2010 Modified on 12.12.2010

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