Synthesis and properties of mononuclear and binuclear molybdenum complexes derived from bis(2-hydroxy-1-naphthaldehyde)oxaloyldihydrazone

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Spectrochimica Acta Part A 69 (2008) 706–714

Synthesis and properties of mononuclear and binuclear molybdenum complexes derived from bis(2-hydroxy-1-naphthaldehyde)oxaloyldihydrazone Ram A. Lal a,∗ , Debajani Basumatary a , Syamal Adhikari b , Arvind Kumar c a

Department of Chemistry, North-Eastern Hill University, Shillong 793022, Meghalaya, India b Department of Chemistry, Tripura University, Suryamaninagar 799130, Tripura, India c Institute of Chemistry, Academia Sinica, 128 Academia Road, Sec.2, Nankang, Taipei, 115, Taiwan, ROC Received 17 January 2007; received in revised form 9 May 2007; accepted 9 May 2007

Abstract The monomer molybdenum(VI) complex [MoO2 (napoxlhH2 )]·2H2 O (1) has been synthesized from the reaction of MoO2 (acac)2 with bis(2hydroxy-1-naphthaldehyde)oxaloyldihydrazone (napoxlhH4 ) in 1:1 molar ratio in ethanol under reflux. This complex on reaction with pyridine/3picoline/4-picoline yielded the dimer molybdenum(VI) complexes [Mo2 O4 (napoxlhH2 )2 (A)2 ]·2H2 O (A = py (2), 3-pic (3), 4-pic (4)), whereas reaction with isonicotinoylhydrazine (inhH3 ) and salicyloylhydrazine (sylshH3 ) lead to the reduction of the metal centre yielding monomeric molybdenum(V) complexes [Mo(napoxlhH2 )(hzid)]·2H2 O (where hzidH3 = inhH3 (5) and sylshH3 (6)). The complexes have been characterized by elemental analyses, molecular weight determinations, molar conductance data, magnetic moment data, electronic, IR, ESR and 1 H NMR spectroscopic studies. The complexes (5) and (6) are paramagnetic to the extent of one unpaired electron. The electronic spectra of the complexes are dominated by strong charge transfer bands. In all of the complexes, the principal dihydrazone ligand has been suggested to coordinate to the metal centres in the anti-cis-configuration. The complexes (1), (5) and (6) are suggested to have six-coordinate octahedral stereochemistry around molybdenum(VI) and molybdenum(V) metal centres, respectively, while the complexes (2)–(4) are suggested to have eight coordinate dodecahedral stereochemistry around molybdenum(VI) metal centre. © 2007 Elsevier B.V. All rights reserved. Keywords: Mononuclear and binuclear; Molybdenum complexes; Bis(2-hydroxy-1-naphthaldehyde)oxaloyldihydrazone; Magnetic moment; Spectroscopic studies

1. Introduction Owing to their biological relevance for molybdoenzymes [1] and also due to the use of high oxidation state molybdenum species in, for example, olefin metathesis catalysis [2], the coordination chemistry of mononuclear molybdenum(IV), (V) and (VI) complexes continues to attract a great deal of attention. The function of molybdenum core depends on the functionalities present in the ligands [3]. Extended X-ray absorption fine structure (EXAFS) spectroscopic studies have also implicated the presence of a sulphur atom, besides oxygen and nitrogen at the active sites of molybdoenzymes. Further, recently it has been shown that the NNN functionalities gener-

∗

Corresponding author. Tel.: +91 364 2722616; fax: +91 364 2550486. E-mail address: lal [email protected] (R.A. Lal).

1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2007.05.023

ated by a pyrazolylborate ligand [4] also activate the MoO2 2+ unit towards oxo-transfer. However, the possibility of activation of MoO2 2+ unit towards oxo-transfer by other NNO or NOO functionalities cannot be eliminated [5]. All these observations have kindled renewed interest in the coordination chemistry of molybdenum. The chemical information gained in studying the molybdenum coordination complexes, may be transferable to enzyme structure/functions questions not readily solvable by studying the enzyme themselves. Further, the metal complexes derived from polyfunctional ligands exhibit interesting properties as electrocatalysts [6], as models of biological systems [7] and as precursors for electrically conducting polymers [8]. Further, interest has been shown, very recently, in the synthesis of molybdenum complexes of macrocylic ligands with a view to make an assessment of the role that the ligand backbone and side chains of macrocycle may play in determining the redox and spectroscopic properties of a

R.A. Lal et al. / Spectrochimica Acta Part A 69 (2008) 706–714

707

molybdenum centre [5]. Acyl, aroyl and pyridoyl-hydrazones containing amide, azomethine and phenol functions are recently recognized as polyfunctional ligands [9] which can react with metal ions either in keto form and enol form. Although, oxomolybdenum(VI), (V), (IV) complexes of several bi-, tri- and tetra dentate nitrogen, sulphur and oxygen donor system are reported [5,6,10–12], a careful survey of literature has failed to locate any study on mononuclear molybdenum complexes of dihydrazone derived from condensation of acyl-, aroyl-, and pyridoyl-dihydrazines with o-hydroxy aromatic aldehydes and ketones [9]. Further, although some first row transition metal ion and polynuclear molybdenum complexes of dihydrazones have been reported [9,12], yet the mononuclear molybdenum complexes of the ligand bis(2-hydroxy-1-naphthaldehyde)oxaloyldihydrazone are virtually absent in the literature to the best of our knowledge. Accordingly, our interest in the chemistry of oxometal cations ligated to nitrogen and oxygen donors with NO2 and N2 O4 chromophores [13–15] has prompted us to pursue the present work which reports the mononuclear Mo(VI) complex, derived from the title ligand bis(2-hydroxy1-naphthaldehyde)oxaloyldihydrazone (napoxlhH4 ) and its reactivity study with proton and electron donor reagents and the characterization of the resulting compounds.

tivity cell. IR spectra were recorded on a Perkin-Elmer – 983 Spectrophotometer in the range 4000–180 cm−1 in KBr disk. The 1 H NMR spectra of the complexes were recorded on an EM-390, 90 MHz Spectrophotometer using DMSO-d6 solution. Electronic spectra were recorded on a DMR-21 Spectrophotometer in solid state. The ESR spectra of the compounds in powdered form at room temperature and liquid nitrogen temperature were recorded at X-band frequency on a Varian E-112 X/Q-band spectrometer, DPPH was used as an internal field marker. The magnetic susceptibilities were determined by the Faraday method at room temperature using Hg[Co(NCS)4 ] as the calibrant.

2. Experimental

2.2. Preparation of the complexes [Mo2 O4 (napoxlhH2 )2 (A)2 ].2H2 O (A = py(2), 3-pic(3), 4-pic(4))

Ammonium paramolybdate tetrahydrate, acetyl acetone, diethyl oxalate, hydrazine hydrate, 2-hydroxy-1-naphthaldehyde, isonicotinoylhydrazine, pyridine and acetonitrile were E-Merck grade reagents. MoO2 (acac)2 [16] and salicyloylhydrazine [17] were prepared by literature methods. Oxaloyldihydrazine was prepared by reacting ethyloxalate (1 mol) with hydrazine hydrate (2 mols). Bis(2-hydroxy-1naphthaldehyde)oxaloyldihydrazone was prepared by refluxing hot dilute ethanol solution of oxaloyldihydrazone (1 mol) with 2-hydroxy-1-naphthaldehyde (2 mol) by the literature method [21,28]. The yellow precipitate obtained was crystallized from ethanol, dried in an electric oven at ca. 70 ◦ C, m.p. > 300 ◦ C(dec). Found: C, 68.00; H, 4.25; N, 13.32; calcd. For C24 H18 N4 O4 , C, 67.61; H, 4.25; N, 13.15. Molybdenum in the complexes was determined by standard literature method [18]. Carbon, hydrogen and nitrogen were determined by microanalysis. Water molecules were determined by heating the sample at ca. 110, 130, 180 ◦ C, respectively, while pyridine, 3-picoline and 4-picoline molecules were determined by heating the sample at 220 ◦ C. Thermogravimetric studies of the complex was carried out manually by heating the samples at a particular temperature for 1/2 h in the temperature range 80–250 ◦ C at an interval of 5 ◦ C in hot air oven and estimating the weight loss. The vapours were passed through a trap containing anhydrous copper sulphate, a CHCl3 solution containing a drop of 5 N NaOH solution. Molecular weight determinations were carried out in DMSO (spectral grade) by freezing point depression method. The molar conductance of the complexes at 10−3 M dilution in DMSO was measured using a direct reading conductivity meter – 303 with a dip conduc-

2.1. Preparation of the complexes [MoO2 (napoxlhH2 )]·2H2 O (1) To an ethanol solution of MoO2 (acac)2 was added ligand solution in ethanol maintaining the molar ratio at 1:1. The reaction mixture was refluxed for 2 h and cooled which yielded an orange red precipitate. This was filtered and washed with ethanol, hot benzene and finally with ether and air dried over anhydrous CaCl2 and collected.

The complex [MoO2 (napoxlhH2 )]·2H2 O (1) (1.5 gm, 40 cm3 ) was suspended in ethanol accompanied by gentle stirring for 15 min at 70 ◦ C. To this suspension, pyridine was added maintaining the molar ratio 1:16. The reaction mixture was refluxed for 1 h. This precipitated the greenish yellow complex which was filtered, washed with ethanol and finally with ether and air dried over anhydrous CaCl2 . The complexes (3) and (4) were also prepared by essentially following the above method using 3-picoline or 4-picoline instead of pyridine. 2.3. Preparation of the complexes [Mo(napoxlhH2 )(inh)]·2H2 O (5) and [Mo-(napoxlhH2 )(sylsh)]·2H2 O (6) These complexes were prepared under dinitrogen atmosphere by the following general procedure. The complex [MoO2 (napoxlhH2 )]·2H2 O (1) (1.5 gm, 40 cm3 ) was suspended in ethanol accompanied by gentle stirring for 15 min at 70 ◦ C. To this suspension ethanolic solution of inhH3 , was added maintaining the molar ratio at 1:6. The reaction mixture was refluxed for 4 h. This precipitated the red complex which was filtered, washed with ethanol and finally with ether and air dried over anhydrous CaCl2 and collected. The complex [Mo(napoxlhH2 )(sylsh)]·2H2 O was also isolated in a similar manner by employing sylshH3 instead of inhH3 maintaining the molar ratio 1:6. This precipitated the reddish orange complex.

6.

[Mo(napoxlhH2 )(sylsh)]·2H2 O Reddish orange

690 ± 32 (705)

>300

90

9.63 (9.87)

52.42 (52.77)

3.92 (3.97)

11.50 (11.91)

1.70

3.5

340, 360, 371, 450, 600, 710 365, 480, 560, 750 2.7 1.69 14.62 (14.20) 3.96 (3.91) 52.62 (52.17) 95 >300

8.895 (9.24)

355, 445, 575 3.1 Dia 10.31 (10.56) 3.82 (3.77) 53.85 (54.30) 14.95 (14.48) 89

360, 450, 580 2.9

320, 370, 440, 550 365, 440, 560 2.5 2.0 Dia Dia

Dia 10.81 (10.56) 3.72 (3.77) 54.72 (54.30) 14.10 (14.80) 92 >300

N

9.31 (9.52) 10.48 (10.79) 3.35 (3.40) 3.60 (3.54) 50.37 (48.98) 27.30 (26.81) 16.75 (16.33) 14.48 (14.80) 75 90 >300 >300

>300

5.

The complexes (5) and (6) have been characterized by ESR spectroscopy. The ESR spectrum of the complex (6) has been shown in Fig. 2. The X-band ESR spectra of the complexes in the solid state at RT and LNT can be explained by assuming axial

4.

3.3. ESR spectra

3.

The μB values for the complexes (1)–(4) are zero indicating that they are diamagnetic. These facts conspicuously suggest that these complexes contain molybdenum in +6 oxidation state with d0 electronic configuration. On the other hand, the compounds (5) and (6) are paramagnetic to the extent of 1.63 and 1.70 BM, respectively. The μB values for the complexes are close to the spin-only value for a formally d1 system. This feature can be interpreted as being indicative of the effective quenching of the orbital angular momentum by a low symmetry ligand field surrounding the metal centre. The μB values in the present Mo(V) complexes dismiss the possibility of any significant metal-metal interactions.

[MoO2 (napoxlhH2 )]·2H2 O Orange red 620 ± 25 (588) 1240 ± 60 (1298) [Mo2 O4 (napoxlhH2 )2 (py)2 ]·2H2 O Greenish Yellow [Mo2 O4 (napoxlhH2 )2 (3-pic)2 ]·2H2 O 1305 ± 65 (1326) Brownish yellow [Mo2 O4 (napoxlhH2 )2 (4-pic)2 ]·2H2 O 1240 ± 60 (1298) Greenish yellow 700 ± 30 (690) [Mo(napoxlhH2 )(inh)]·2H2 O Dark red

3.2. Magnetic moment

1. 2.

All of the complexes melt with decomposition above 300 ◦ C but complex (1) decomposes without melting. All of the complexes show loss of weight corresponding to two water molecules at 110 ◦ C indicating that they are present in the lattice structure of the complexes. These complexes do not show any loss of weight at 180 ◦ C ruling out the possibility of presence of water molecules in the coordination sphere of the complexes. On the other hand, the complexes (2)–(4) show weight loss at 220 ◦ C corresponding to two pyridine/3-picoline/4-picoline molecules indicating that they are coordinated to the metal centre. The vapours evolved in this temperature range 220–230 ◦ C in the complex (2) turn the CHCl3 and 5 M NaOH solution red confirming that they originate from pyridine molecules indicating that they are present in the first coordination sphere around the metal centre.

H

3.1. Thermal studies

Molecular weight D.P (◦ C) Yield (%) Analysis:found (Calcd)% expt. (theo) Mo C

The complexes isolated in the present study together with their colour, decomposition point, analytical data and molar conductance are set out in Table 1. Based on the analytical data, the complexes are suggested to have the compositions [MoO2 (napoxlhH2 )]·2H2 O (1), [Mo2 O4 (napoxlhH2 )2 (A)2 ]·2H2 O (A = py(2), 3-pic(3), 4-pic(4)), [Mo(napoxlhH2 )(inh)]·2H2 O (5) and [Mo(napoxlhH2 )(sylsh)]·2H2 O (6). These complexes are orange red, yellowish brown, red and reddish orange, respectively. These complexes are air stable. The experimental values of the molecular weight for the complexes (1), (5) and (6) suggested their monomer structure while a dimeric structure for the complexes (2) and (4). These complexes have molar conductance values in the region 2.0–3.5 ohm−1 cm2 mol−1 in DMSO indicating that they are non-electrolyte in this solvent [19].

SI. no. Ligand/complex

3. Results and discussion

Electronic spectral μeff (B.M.) Molar bands (nm) conductance (Scm−1 mol−1 )

R.A. Lal et al. / Spectrochimica Acta Part A 69 (2008) 706–714 Table 1 Molecular weight, decomposition temperature, percentage yield, analytical, magnetic moment, molar conductance and electronic spectral data for Mo complexes of Bis(2-hydroxy-1-naphthaldehyde) oxaloyldihydrazone

708

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709

Table 2 ESR data for the molybdenum(V) Complexes in solid state Complex [Mo(napoxlhH2 )(inh)]·2H2 O (5) [Mo(napoxlhH2 )(sylsh)]·2H2 O (6)

RT gII

g⊥

gav

A||

A⊥

Aav (G)

LNT g||

g⊥

gav

A||

A⊥

Aav (G)

2.055 2.070

1.965 1.950

1.995 1.99

46.0 52.0

40.0 43.0

42.0 46.0

2.041 2.065

1.996 1.962

1.991 1.996

40 49

20 34

26.7 39

symmetry. The various g-parameters are presented in Table 2. The satellite peaks are due to 95,97 Mo (25.15 atom %; I = 5/2). Derived g and A tensor components indicate that the unpaired electron is located in a molybdenum centred orbital of essentially identical composition. Fairly high values of g and A are in conformity with the oxygen and nitrogen coordination in these compounds [20] (Table 2). 3.4. UV-visible spectra The electronic spectrum of napoxlhH4 shows bands at 330 and 370 nm in the solid state which arise due to π → π* and n → π* transitions in dihydrazone. The electronic spectra of the complexes show two to four bands in the region 300–480 nm. While the band in the region 440–480 nm is assigned to ligand band at 370 nm, the additional bands in the region 320–371 are assigned to another ligand bands appearing at 330 nm [21]. The appearance of one or two bands in the region 320–371 nm suggests that the ligands at 330 nm undergoes splitting and shows blue as well as red shift. Such a feature associated with ligand at 330 nm undergoes splitting and shows blue as well as red shift. Such a feature associated with ligand band at 330 nm indicates effect of complexation of the ligand. The complexes show another strong broad band in the region 550–600 nm (sh) in the visible region. Since napoxlhH4 is not expected to be chromophoric in the visible region and the complexes (1) to (4) are diamagnetic, these bands have been assigned as the ligand–metal charge transfer (LMCT) transition on the basis of their high intensity. They may be associated, most probably,

with a ligand-to-metal charge transfer originating from an electronic excitation from the HOMO of naphtholate oxygen to the LUMO of molybdenum [22]. An important feature of the electronic spectra of the complexes (5) and (6) in the solid state is the presence of one distinct weak absorption at 710 and 750 nm, respectively, arising out of the first crystal field transition [23] 2 B → 2 E (d → d , d ) The second crystal field transition 2 xy xz yz . 2 B → 2 B (d → d 2 1 xy x2 −y2 ) is obscured either by the strong ligand band occurring in the region 440–480 nm or charge-transfer transition band occurring in the region 550–600 nm. 3.5. 1 H NMR spectra The 1 H NMR spectrum of napoxlhH4 has been recorded in DMSO-d6 as it is insoluble in CCl4 and CHCl3. The 1 H NMR spectra of the ligand and the complexes (1) and (3) have been shown in Fig. 3. The four and two proton signals observed at ␦ 12.72 ppm and ␦ 9.75 ppm downfield of TMS have been assigned to the ␦OH + δNH and ␦ CH N protons, respectively. The multiplet appearing in the region ␦ 7.15–8.20 ppm has been assigned to naphthyl protons. Because of their diamagnetic character, the complexes (1)–(4) have been studied by 1 H NMR spectroscopy. The complex (1) shows two signals at δ 12.67 and 12.87 ppm whereas the complexes (2)–(4) show a quartet at almost the same position showing an insignificant downfield shift of about 0.05–0.07 ppm. The feature of these signals suggests that they arise due to NH protons. The downfield shift of this signal occurs due to the drainage of the azomethine nitrogen electron density to the metal centre.

Table 3 1 H NMR spectral data (in ␦) for dihydrazones and its complexes Sl. No.

Ligand/complex

␦-naphthyl (m)

␦ CH N–

␦ OH + ␦ NH

napoxlhH4

7.15–8.20

9.75

12.72

1.

[MoO2 (napoxlhH2 )]·2H2 O

7.22–8.42

9.82 10.07

12.67 12.87

2.

[Mo2 O4 (napoxlhH2 )2 (py)2 ]·2H2 O

7.17–8.40

9.79 9.97

12.62 12.68 12.85 12.91

3.

[Mo2 O4 (napoxlhH2 )2 (3-pic)2 ]·2H2 O

7.15–8.35

9.80 9.98

12.65 12.70 12.86 12.93

4.

[Mo2 O4 (napoxlhH2 )2 (4-pic)2 ]·2H2 O

7.20–8.42

9.82 10.05

12.66 12.71 12.85 12.93

710

Table 4 Characteristic IR bands (cm−1 ) for Bis(2-hydroxy-1-naphthaldehyde)oxaloyldihydrazone and its molybdenum complexes SI. no.

ν(OH) + ν(NH)

ν(C O)

ν(C N)

Amide(II) +ν(C O)

ν(C O)

ν(M O)

ν(M O)(phenolic)

ν(M O)(carbonyl)

Other bands

napoxlhH4

3500-3300(sbr); 3382(s), 3332s(s) 3550-3000(sbr); 3462(s), 3181(s) 3550-3000(sbr); 3616(s), 3500(s) 3167(s), 3103(s) 3550-3000(sbr); 3400(s); 3217(s) 3600-3000(sbr); 3450(s); 3270(s) 3550-3000(sbr); 3411(s) 3615(m); 33003000(sbr); 3217(s)

1655(vs)

1616(s)

1528(s)

1278(w)

–

–

–

–

1674(m); 1660(s)

1616(s); 1593(vs)

1545(vs)

1283(s)

944(vs); 916(vs)

590(s) 557(m)

–

–

1677(s); 1658(s)

1618(vs); 1598(s)

1531(s)

1283(m)

952(s); 907(m)

592(w) 556(w)

–

–

1685(m); 1660(s)

1618(s); 1600(s)

1540(vs)

1284(w)

945(vs); 920(m)

590(s) 550(m)

–

–

1680(s); 1662(s)

1616(s); 1596(s)

1535(s)

1285(s)

952(vs); 910(m)

585(m) 540(s)

–

–

1674(m); 1662(s) 1675(m); 1660(s)

1614(s); 1595(vs) 1614(s); 1597(s)

1532(s) 1533(s)

1273(w) 1276(s)

– –

589(m) 531(w) 591(m) 530(m)

444(w) 486(w)

1506(s) 1003(s) 1482(s)

1.

[MoO2 (napoxlhH2 )]·2H2 O

2.

[Mo2 O4 (napoxlhH2 )2 (py)2 ]·H2 O

3.

[Mo2 O4 (napoxlhH2 )(3-pic)2 ]·2H2 O

4.

[Mo2 O4 (napoxlhH2 )(4-pic)2 ]·2H2 O

5. 6.

[Mo(napoxlhH2 )(inh)]·2H2 O [Mo(napoxlhH2 )(sylsh)]·2H2 O

of

Fig. 1. (A) Staggered-configuration. (B) Anti-cis configuration. (c) Syn-cis configuration.

spectrum

The ␦ CH N signal observed at ␦ 9.75 ppm also splits into two resonances and appears in the region ␦ 9.79–10.07 ppm. This signal shows downfield shift of about 0.13–0.19 ppm. The splitting of this signal into two signals shows the effect of coordination of dihydrazone through the azomethine nitrogen atoms to the metal centre. The complexes (1)–(4) show two signals in the region ␦ 9.79–10.02 ppm due to azomethine protons while the complex (1) shows two signals and the complexes

Fig. 2. X-band room temperature solid state ESR [Mo(napoxlhH2 )(syslh)]·2H2 O (6), frequency 9.02 GHz.

R.A. Lal et al. / Spectrochimica Acta Part A 69 (2008) 706–714

Ligand/complex

R.A. Lal et al. / Spectrochimica Acta Part A 69 (2008) 706–714

711

Fig. 3. 1 H NMR spectra of DMSO-d6 solution of (a) napoxlhH4 (b) [MoO2 (napoxlhH2 )2 ]·2H2 O (1). (c) Mo2 O4 (napoxlhH2 )2 (3-pic)2 ]·2H2 O (3).

(2)–(4) show two doublets in the region ␦ 12.62–12.91 ppm due to secondary NH protons, respectively. The splitting of ␦NH and CH N signals indicates that the conformation of dihydrazone is changed from staggered to anti-cis conformation in the complexes [24] (Table 3). The dihydrazone shows a single signal corresponding to ␦ CH N and ␦ NH protons in the free state while the complexes show two signals corresponding to ␦ CH N and two doublets corresponding to ␦ NH protons. This is because the two hydrazone moieties rotate freely about C C single bond in free dihydrazone but in the complexes, the rotation about C C single bond is inhibited by bonding of the two hydrazone groupings to the same metal centre. This introduces steric crowding in the molecules as a result of which one hydrazone arm of dihydrazone remains in the equatorial plane while the other hydrazone arm occupies axial position. In this configuration, the equatorial protons would absorb at higher field (upfield) while the axial protons would absorb at lower field (downfield). The appearance of two doublets corresponding to ␦NH protons in the complexes (2)–(4) indicates strong coupling between axial and equatorial counter parts of secondary NH protons. The signals due to secondary NH protons are broader in the complexes than that observed in free dihydrazone. The broadening of secondary NH protons signal is, most probably, due to coupling of the protons with the quadrupolar nitrogen nucleus [25]. The pattern of the azomethine proton signals is assymetric in nature in the complexes. This is the consequence of interchange between the two types of azomethine groups as a result of nitrogen inversion around the metal centre [26,27]. The lines due to azomethine

protons are also broader than that in the free dihydrazone. The broadening of the lines may be due to the nitrogen inversion at sufficiently slow rate at the metal centre. This may be related to stronger bonding between azomethine nitrogen atoms and metal centre. The fact that one hydrazone part is in the axial position while the other part is in the equatorial position suggests W-configuration of dihydrazone around the metal centre [24]. Methyl protons show signal at ␦ 2.32 and 2.37 ppm in free 3-picoline and 4-picoline, respectively [24,28]. In metal complexes (3) and (4) these signals appear at ␦ 2.30 and 2.07 ppm, respectively, and are thus upfield shifted by 0.02 and 0.30 ppm, respectively. This upfield shift is the result of drainage of electrons density from nitrogen atom to the metal centre causing a decrease in the electronegativity of ring nitrogen atom. Consequently, the electron density on all carbon atoms decreases. While the decrease in electron density on 2- and 4-carbon atoms is maximum due to o- and p-directing influence, that on 3-carbon atom is minimum. Hence, the upfield shift of methyl protons in 4-picoline is more than that in 3-picoline. Further, no signal is observed in the downfield region in the 1 H NMR spectra of the complexes (2)–(4) which may be assigned to 2-pyridyl protons of pyridine/3-picoline/4-picoline molecules [29]. This indicates that the signals due to 2-pyridyl protons of pyridine/3-picoline/4picoline molecules are upfield shifted and merged with naphthyl proton signals. Such features associated with the 2-pyridyl protons of pyridine, 3-picoline and 4-plicoline molecules confirm coordination of ring nitrogen atoms of these donor molecules to the metal centre.

712

R.A. Lal et al. / Spectrochimica Acta Part A 69 (2008) 706–714

3.6. IR spectra The complex (1) shows two very strong bands at 944 and 916 cm−1 of almost equal intensity which are characteristic of cis-MoO2 2+ group. On the other hand, the complexes (2)–(4) show only a strong band in the 944–952 cm−1 region and a medium intensity band in the 907–920 cm−1 region. These bands are assigned to νs and νas stretching vibrations of cisMoO2 2+ group. Further, the complexes show another medium intensity band in the 833–840 cm−1 region which is assigned to stretching vibration of Mo O–Mo linkage [30]. On the other hand, the complexes (5) and (6) do not show strong band in the region 1000–850 cm−1 indicating absence of oxo-group. This suggested condensation of Mo O group with NH2 group of the hydrazide ligand (Table 4). The dihydrazone derived from condensation of o-hydroxy aromatic aldehydes and ketones and acyl-, aroyl- and pyridoyldihydrazine are polyfunctional ligands and can coordinate to the metal centre in staggered (Fig. 1A), anti-cis (Fig. 1B) and syn-cis (Fig. 1C) configurations either in keto, keto-enol or enol forms [31]. Further, salicyloylhydrazine and isonitinoylhydrazine molecules can also coordinate to the metal

centre in the keto and enol form. In addition, they can coordinate to the metal centre accompanied by condensation with oxo-metal ions via their hydrazinic amino groups accompanied by enolization leading eventually to redox reactions. The uncoordinated ligand shows strong bands at 1655 and 1616 cm−1 which are assigned to stretching vibrations of C O and C N group. These bands split into two bands and on average, undergo shift to higher position by 12–18 cm−1 and lower position by 8–11 cm−1 , respectively. The average shift of νC O band to higher frequency by ca 12–18 cm−1 and of νC N band to lower frequency by 8–11 cm−1 in complexes (1)–(6) suggests non-coordination of C O group while coordination of dihydrazone through C N groups, to the metal centre. The band at 1528 cm−1 assigned to amide II + ν(C O)(phenolic) shifts to higher frequency ruling out the possibility of coordination of dihydrazone to the metal centre through C O groups. This further indicates involvement of naphtholate oxygen atom in bonding to the metal centre. The shift of band at 1278 cm−1 to higher frequency by ∼5 cm−1 and appearance of medium to strong bands at 557 and 590 cm−1 confirms the coordination of dihydrazone to the metal centre through

Scheme 1. Complexes [Mo2 O4 (napoxlhH2 )2 (A)2 ] (A = py(2), 3-pic(3) and 4-pic(4)).

R.A. Lal et al. / Spectrochimica Acta Part A 69 (2008) 706–714

713

Scheme 2. Complexes [Mo(napoxlhH2 )(hzid)]·2H2 O (hzidH3 = inhH3 (5), syslhH3 (6)).

naphtholate oxygen atoms [32]. The complexes (2)–(4) show new bands in the region 679–690 and 430–433 cm−1 assigned to in-plane ring deformation and out–of–plane ring deformation mode of pyridine/3-picoline/4-picoline molecules. These bands are considerably shifted to higher position as compared to that in the free pyridine/3-picoline/4-picoline molecules indicating coordination of pyridyl ring nitrogen atom to the metal centre. The complexes (2)–(4) show a new weak band in the region 270–280 cm−1 characteristic of ␯M–N resulted from the coordination of pyridyl nitrogen atom to the metal centre [33]. The IR spectra of the complexes (5) and (6) show different feature as compared to the precursor complex (1) and the complexes (2)–(4). This is evident from the absence of any additional band characteristic of C O and NH2 groups in the region 1660–1630 cm−1 and ν(Mo O) vibration in the region 1000–850 cm−1 and appearance of medium intensity band at 1506 and 1482 cm−1 characteristic of (N N) group which indicate the condensation of –NH2 group of the hydrazide with Mo O group, enolization of C O group and inclusion of salicyloyl- and pyridoyl-diazenido grouping in the molybdenum coordination sphere [34]. The weak bands observed in

the region 2800 cm−1 in the IR spectrum of the free sylshH3 attributed to intramolecular H-bonding between C O and OH group disappears in the complex (6) indicating coordination through carbonyl oxygen atoms. However, a well defined broad band centred at 3217 cm−1 having feature different as compared to that in the complex (5) and similar in feature to those observed in o-hydroxy aromatic aldehyde and ketone suggests the development of a new type of intramolecular H-bonding in the complex. On the other hand, in the complex (5), a strong broad band is observed in the region 3550–3000 cm−1 with its middle point at 3411 cm−1 . This is attributed to νOH of lattice water molecules. These complexes do not show any band in the region 750–650 cm−1 attributable to the rocking mode of the coordinated water molecules ruling out the possibility of presence of coordinated water molecules in the complexes [35].␯OH The νC O and νC N stretching frequencies appear as a couple of bands in these complexes. The difference between these frequencies is of the order of 18–28 cm−1 which is quite reasonable for the existence of C O and C N groups in the axial and equatorial positions and hence the complexes in W-configuration [36].

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4. Conclusion In the present study, we have prepared some monomeric and dimeric molybdenum(V) and molybdenum(VI) complexes and characterized them on the basis of data obtained from physicochemical and spectroscopic studies. The reaction of monomeric molybdenum(VI) complex with proton donor reagents gives monomeric molybdenum(V) complexes while reaction with electron donor reagents gives dimeric molybdenum(VI) complexes. The dihydrazone coordinates to the metal centre through azomethine nitrogen atoms and naphtholate oxygen atoms as a tetradentate N2 O2 donor. The C O and NH groups remain uncoordinated. The dihydrazone exists in the staggered conformation in the free state while it is isomerized to anti-cis conformation in the complexes. The molybdenum centre has six coordinate octahedral geometry in the complexes (1), (5) and (6) while it has eight coordinate dodecahedral geometry [37] in the complexes (2)–(4). In the complexes (2)–(4) the molybdenum centres are bonded as . In all of the complexes, the principal dihydrazone ligand is coordinated to the metal centre in keto form in the anti-cis-configuration. Tentative structures for the complexes have been proposed in Schemes 1 and 2, respectively.

[10] [11] [12] [13] [14]

[15]

[16] [17] [18] [19] [20]

[21] [22]

Acknowledgements The authors are thankful to the Head, RSIC, CDRI, Lucknow for C, H, N analyses and to the Head RSIC, as IIT, Madras for recording esr spectra. One of the authors (RAL) is thankful to the University Grants Commission, New Delhi, India for financial assistance through a research grant.

[23] [24]

[25]

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