Synthesis and properties of mononuclear and binuclear molybdenum complexes derived from...

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Spectrochimica Acta Part A 69 (2008) 706–714

Synthesis and properties of mononuclear and binuclear molybdenumcomplexes 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, Indiab 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

bstract

The monomer molybdenum(VI) complex [MoO2(napoxlhH2)]·2H2O (1) has been synthesized from the reaction of MoO2(acac)2 with bis(2-ydroxy-1-naphthaldehyde)oxaloyldihydrazone (napoxlhH4) in 1:1 molar ratio in ethanol under reflux. This complex on reaction with pyridine/3-icoline/4-picoline yielded the dimer molybdenum(VI) complexes [Mo2O4(napoxlhH2)2(A)2]·2H2O (A = py (2), 3-pic (3), 4-pic (4)), whereaseaction with isonicotinoylhydrazine (inhH3) and salicyloylhydrazine (sylshH3) lead to the reduction of the metal centre yielding monomericolybdenum(V) complexes [Mo(napoxlhH2)(hzid)]·2H2O (where hzidH3 = inhH3 (5) and sylshH3 (6)). The complexes have been characterized

y elemental analyses, molecular weight determinations, molar conductance data, magnetic moment data, electronic, IR, ESR and 1H NMRpectroscopic studies. The complexes (5) and (6) are paramagnetic to the extent of one unpaired electron. The electronic spectra of the complexesre dominated by strong charge transfer bands. In all of the complexes, the principal dihydrazone ligand has been suggested to coordinate to

auvoociti

eywords: Mononuclear and binuclear; Molybdenum complexes; Bis(2-hydrox

. Introduction

Owing to their biological relevance for molybdoenzymes1] and also due to the use of high oxidation state molybde-um species in, for example, olefin metathesis catalysis [2],he coordination chemistry of mononuclear molybdenum(IV),V) and (VI) complexes continues to attract a great deal ofttention. The function of molybdenum core depends on theunctionalities present in the ligands [3]. Extended X-ray absorp-ion fine structure (EXAFS) spectroscopic studies have also

∗ Corresponding author. Tel.: +91 364 2722616; fax: +91 364 2550486.E-mail address: lal 198@rediffmail.com (R.A. Lal).

milpiati

aphthaldehyde)oxaloyldihydrazone; Magnetic moment; Spectroscopic studies

ted by a pyrazolylborate ligand [4] also activate the MoO22+

nit towards oxo-transfer. However, the possibility of acti-ation of MoO2

2+ unit towards oxo-transfer by other NNOr NOO functionalities cannot be eliminated [5]. All thesebservations have kindled renewed interest in the coordinationhemistry of molybdenum. The chemical information gainedn studying the molybdenum coordination complexes, may beransferable to enzyme structure/functions questions not read-ly solvable by studying the enzyme themselves. Further, theetal complexes derived from polyfunctional ligands exhibit

nteresting properties as electrocatalysts [6], as models of bio-ogical systems [7] and as precursors for electrically conductingolymers [8]. Further, interest has been shown, very recently,

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R.A. Lal et al. / Spectrochimic

olybdenum centre [5]. Acyl, aroyl and pyridoyl-hydrazonesontaining amide, azomethine and phenol functions are recentlyecognized as polyfunctional ligands [9] which can react withetal ions either in keto form and enol form. Although, oxo-olybdenum(VI), (V), (IV) complexes of several bi-, tri- and

etra dentate nitrogen, sulphur and oxygen donor system areeported [5,6,10–12], a careful survey of literature has failedo locate any study on mononuclear molybdenum complexesf dihydrazone derived from condensation of acyl-, aroyl-, andyridoyl-dihydrazines with o-hydroxy aromatic aldehydes andetones [9].

Further, although some first row transition metal ion andolynuclear molybdenum complexes of dihydrazones have beeneported [9,12], yet the mononuclear molybdenum complexesf the ligand bis(2-hydroxy-1-naphthaldehyde)oxaloyl-ihydrazone are virtually absent in the literature to the bestf our knowledge. Accordingly, our interest in the chemistryf oxometal cations ligated to nitrogen and oxygen donorsith NO2 and N2O4 chromophores [13–15] has prompted us

o pursue the present work which reports the mononuclearo(VI) complex, derived from the title ligand bis(2-hydroxy-

-naphthaldehyde)oxaloyldihydrazone (napoxlhH4) and itseactivity study with proton and electron donor reagents andhe characterization of the resulting compounds.

. Experimental

Ammonium paramolybdate tetrahydrate, acetyl acetone,iethyl oxalate, hydrazine hydrate, 2-hydroxy-1-naphthalde-yde, isonicotinoylhydrazine, pyridine and acetonitrile were-Merck grade reagents. MoO2(acac)2 [16] and salicy-

oylhydrazine [17] were prepared by literature methods.xaloyldihydrazine was prepared by reacting ethyloxalate

1 mol) with hydrazine hydrate (2 mols). Bis(2-hydroxy-1-aphthaldehyde)oxaloyldihydrazone was prepared by refluxingot dilute ethanol solution of oxaloyldihydrazone (1 mol)ith 2-hydroxy-1-naphthaldehyde (2 mol) by the literatureethod [21,28]. The yellow precipitate obtained was crystal-

ized from ethanol, dried in an electric oven at ca. 70 ◦C,.p. > 300 ◦C(dec). Found: C, 68.00; H, 4.25; N, 13.32; calcd.or C24H18N4O4, C, 67.61; H, 4.25; N, 13.15.

Molybdenum in the complexes was determined by standarditerature method [18]. Carbon, hydrogen and nitrogen wereetermined by microanalysis. Water molecules were determinedy heating the sample at ca. 110, 130, 180 ◦C, respectively, whileyridine, 3-picoline and 4-picoline molecules were determinedy heating the sample at 220 ◦C. Thermogravimetric studies ofhe complex was carried out manually by heating the samplest a particular temperature for 1/2 h in the temperature range0–250 ◦C at an interval of 5 ◦C in hot air oven and estimatinghe weight loss. The vapours were passed through a trap con-aining anhydrous copper sulphate, a CHCl3 solution containing

drop of 5 N NaOH solution. Molecular weight determina-

a Part A 69 (2008) 706–714 707

ivity cell. IR spectra were recorded on a Perkin-Elmer – 983pectrophotometer in the range 4000–180 cm−1 in KBr disk.he 1H NMR spectra of the complexes were recorded on anM-390, 90 MHz Spectrophotometer using DMSO-d6 solution.lectronic spectra were recorded on a DMR-21 Spectropho-

ometer in solid state. The ESR spectra of the compounds inowdered form at room temperature and liquid nitrogen tem-erature were recorded at X-band frequency on a Varian E-112/Q-band spectrometer, DPPH was used as an internal fieldarker. The magnetic susceptibilities were determined by thearaday method at room temperature using Hg[Co(NCS)4] as

he calibrant.

.1. Preparation of the complexesMoO2(napoxlhH2)]·2H2O (1)

To an ethanol solution of MoO2(acac)2 was added ligandolution in ethanol maintaining the molar ratio at 1:1. The reac-ion mixture was refluxed for 2 h and cooled which yielded anrange red precipitate.

This was filtered and washed with ethanol, hot benzene andnally with ether and air dried over anhydrous CaCl2 and col-

ected.

.2. Preparation of the complexesMo2O4(napoxlhH2)2(A)2].2H2O (A = py(2), 3-pic(3),-pic(4))

The complex [MoO2(napoxlhH2)]·2H2O (1) (1.5 gm,0 cm3) was suspended in ethanol accompanied by gentle stir-ing for 15 min at 70 ◦C. To this suspension, pyridine was addedaintaining the molar ratio 1:16. The reaction mixture was

efluxed for 1 h. This precipitated the greenish yellow complexhich was filtered, washed with ethanol and finally with ether

nd air dried over anhydrous CaCl2.The complexes (3) and (4) were also prepared by essen-

ially following the above method using 3-picoline or 4-picolinenstead of pyridine.

.3. Preparation of the complexesMo(napoxlhH2)(inh)]·2H2O (5) andMo-(napoxlhH2)(sylsh)]·2H2O (6)

These complexes were prepared under dinitrogen atmospherey the following general procedure.

The complex [MoO2(napoxlhH2)]·2H2O (1) (1.5 gm,0 cm3) was suspended in ethanol accompanied by gentle stir-ing for 15 min at 70 ◦C. To this suspension ethanolic solutionf inhH3, was added maintaining the molar ratio at 1:6. Theeaction mixture was refluxed for 4 h. This precipitated the redomplex which was filtered, washed with ethanol and finally

The complex [Mo(napoxlhH2)(sylsh)]·2H2O was also iso-ated in a similar manner by employing sylshH3 instead of inhH3aintaining the molar ratio 1:6. This precipitated the reddish

range complex.

7 ica Acta Part A 69 (2008) 706–714

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08 R.A. Lal et al. / Spectrochim

. Results and discussion

The complexes isolated in the present study together withheir colour, decomposition point, analytical data and molaronductance are set out in Table 1. Based on the analytical data,he complexes are suggested to have the compositions [MoO2napoxlhH2)]·2H2O (1), [Mo2O4(napoxlhH2)2(A)2]·2H2OA = py(2), 3-pic(3), 4-pic(4)), [Mo(napoxlhH2)(inh)]·2H2O5) and [Mo(napoxlhH2)(sylsh)]·2H2O (6). These complexesre orange red, yellowish brown, red and reddish orange,espectively. These complexes are air stable.

The experimental values of the molecular weight for theomplexes (1), (5) and (6) suggested their monomer struc-ure while a dimeric structure for the complexes (2) and (4).hese complexes have molar conductance values in the region.0–3.5 ohm−1 cm2 mol−1 in DMSO indicating that they areon-electrolyte in this solvent [19].

.1. Thermal studies

All of the complexes melt with decomposition above 300 ◦Cut complex (1) decomposes without melting. All of theomplexes show loss of weight corresponding to two waterolecules at 110 ◦C indicating that they are present in the lattice

tructure of the complexes. These complexes do not show anyoss of weight at 180 ◦C ruling out the possibility of presence ofater molecules in the coordination sphere of the complexes. On

he other hand, the complexes (2)–(4) show weight loss at 220 ◦Corresponding to two pyridine/3-picoline/4-picoline moleculesndicating that they are coordinated to the metal centre. Theapours evolved in this temperature range 220–230 ◦C in theomplex (2) turn the CHCl3 and 5 M NaOH solution red con-rming that they originate from pyridine molecules indicating

hat they are present in the first coordination sphere around theetal centre.

.2. Magnetic moment

The μB values for the complexes (1)–(4) are zero indicatinghat they are diamagnetic. These facts conspicuously suggest thathese complexes contain molybdenum in +6 oxidation state with0 electronic configuration. On the other hand, the compounds5) and (6) are paramagnetic to the extent of 1.63 and 1.70 BM,espectively. The μB values for the complexes are close to thepin-only value for a formally d1 system. This feature can benterpreted as being indicative of the effective quenching of therbital angular momentum by a low symmetry ligand field sur-ounding the metal centre. The μB values in the present Mo(V)omplexes dismiss the possibility of any significant metal-metalnteractions.

.3. ESR spectra

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

R.A. Lal et al. / Spectrochimica Acta Part A 69 (2008) 706–714 709

Table 2ESR data for the molybdenum(V) Complexes in solid state

Complex RT LNTgII g⊥ gav A|| A⊥ Aav (G) g|| g⊥ gav A|| A⊥ Aav (G)

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Mo(napoxlhH2)(inh)]·2H2O (5) 2.055 1.965 1.995 46.0Mo(napoxlhH2)(sylsh)]·2H2O (6) 2.070 1.950 1.99 52.0

ymmetry. The various g-parameters are presented in Table 2.he satellite peaks are due to 95,97Mo (25.15 atom %; I = 5/2).erived g and A tensor components indicate that the unpaired

lectron is located in a molybdenum centred orbital of essentiallydentical composition. Fairly high values of 〈g〉 and 〈A〉 are inonformity with the oxygen and nitrogen coordination in theseompounds [20] (Table 2).

.4. UV-visible spectra

The electronic spectrum of napoxlhH4 shows bands at 330nd 370 nm in the solid state which arise due to π → π* and→ π* transitions in dihydrazone. The electronic spectra of theomplexes show two to four bands in the region 300–480 nm.hile the band in the region 440–480 nm is assigned to ligand

and at 370 nm, the additional bands in the region 320–371 aressigned to another ligand bands appearing at 330 nm [21]. Theppearance of one or two bands in the region 320–371 nm sug-ests that the ligands at 330 nm undergoes splitting and showslue as well as red shift. Such a feature associated with lig-nd at 330 nm undergoes splitting and shows blue as well ased shift. Such a feature associated with ligand band at 330 nmndicates effect of complexation of the ligand. The complexeshow another strong broad band in the region 550–600 nm (sh)n the visible region. Since napoxlhH4 is not expected to be

able 3H NMR spectral data (in �) for dihydrazones and its complexes

l. No. Ligand/complex �-

napoxlhH4 7

. [MoO2(napoxlhH2)]·2H2O 7

. [Mo2O4(napoxlhH2)2(py)2]·2H2O 7

. [Mo2O4(napoxlhH2)2(3-pic)2]·2H2O 7

. [Mo2O4(napoxlhH2)2(4-pic)2]·2H2O 7

0 42.0 2.041 1.996 1.991 40 20 26.70 46.0 2.065 1.962 1.996 49 34 39

ith a ligand-to-metal charge transfer originating from an elec-ronic excitation from the HOMO of naphtholate oxygen to theUMO of molybdenum [22]. An important feature of the elec-

ronic spectra of the complexes (5) and (6) in the solid state ishe presence of one distinct weak absorption at 710 and 750 nm,espectively, arising out of the first crystal field transition [23]B2 → 2E (dxy → dxz, dyz). The second crystal field transitionB2 → 2B1(dxy → dx2−y2 ) is obscured either by the strong lig-nd band occurring in the region 440–480 nm or charge-transferransition band occurring in the region 550–600 nm.

.5. 1H NMR spectra

The 1H NMR spectrum of napoxlhH4 has been recordedn DMSO-d6 as it is insoluble in CCl4 and CHCl3. The 1HMR spectra of the ligand and the complexes (1) and (3)ave been shown in Fig. 3. The four and two proton signalsbserved at � 12.72 ppm and � 9.75 ppm downfield of TMS haveeen assigned to the �OH + δNH and � CH N protons, respec-ively. The multiplet appearing in the region � 7.15–8.20 ppmas been assigned to naphthyl protons. Because of their dia-agnetic character, the complexes (1)–(4) have been studied

y 1H NMR spectroscopy. The complex (1) shows two sig-als at δ 12.67 and 12.87 ppm whereas the complexes (2)–(4)how a quartet at almost the same position showing an insignif-

naphthyl (m) � CH N– � OH + � NH

.15–8.20 9.75 12.72

.22–8.42 9.82 12.6710.07 12.87

.17–8.40 9.79 12.629.97 12.68

12.8512.91

.15–8.35 9.80 12.659.98 12.70

12.8612.93

.20–8.42 9.82 12.6610.05 12.71

12.8512.93

aletal./Spectrochimica

PartA 69 (2008) 706–714

Table 4Characteristic IR bands (cm−1) for Bis(2-hydroxy-1-naphthaldehyde)oxaloyldihydrazone and its molybdenum complexes

SI. no. Ligand/complex 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)

1655(vs) 1616(s) 1528(s) 1278(w) – – – –

1. [MoO2(napoxlhH2)]·2H2O 3550-3000(sbr); 3462(s),3181(s)

1674(m); 1660(s) 1616(s); 1593(vs) 1545(vs) 1283(s) 944(vs); 916(vs) 590(s) 557(m) – –

2. [Mo2O4(napoxlhH2)2(py)2]·H2O 3550-3000(sbr); 3616(s),3500(s) 3167(s), 3103(s)

1677(s); 1658(s) 1618(vs); 1598(s) 1531(s) 1283(m) 952(s); 907(m) 592(w) 556(w) – –

3. [Mo2O4(napoxlhH2)(3-pic)2]·2H2O 3550-3000(sbr); 3400(s);3217(s)

1685(m); 1660(s) 1618(s); 1600(s) 1540(vs) 1284(w) 945(vs); 920(m) 590(s) 550(m) – –

4. [Mo2O4(napoxlhH2)(4-pic)2]·2H2O 3600-3000(sbr); 3450(s);3270(s)

1680(s); 1662(s) 1616(s); 1596(s) 1535(s) 1285(s) 952(vs); 910(m) 585(m) 540(s) – –

5. [Mo(napoxlhH2)(inh)]·2H2O 3550-3000(sbr); 3411(s) 1674(m); 1662(s) 1614(s); 1595(vs) 1532(s) 1273(w) – 589(m) 531(w) 444(w) 1506(s) 1003(s)6. [Mo(napoxlhH2)(sylsh)]·2H2O 3615(m); 3300-

3000(sbr); 3217(s)1675(m); 1660(s) 1614(s); 1597(s) 1533(s) 1276(s) – 591(m) 530(m) 486(w) 1482(s)

Fig.1.(A

)Staggered-configurfiguration.

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Fig. 3. 1H NMR spectra of DMSO-d6 solution of (a) napoxlhH4 (b) [M

2)–(4) show two doublets in the region � 12.62–12.91 ppm dueo secondary NH protons, respectively. The splitting of �NHnd CH N signals indicates that the conformation of dihy-razone is changed from staggered to anti-cis conformation inhe complexes [24] (Table 3).

The dihydrazone shows a single signal corresponding toCH N and � NH protons in the free state while the com-

lexes show two signals corresponding to � CH N and twooublets corresponding to � NH protons. This is because thewo hydrazone moieties rotate freely about C C single bondn free dihydrazone but in the complexes, the rotation about

C single bond is inhibited by bonding of the two hydrazoneroupings to the same metal centre. This introduces steric crowd-ng in the molecules as a result of which one hydrazone armf dihydrazone remains in the equatorial plane while the otherydrazone arm occupies axial position. In this configuration, thequatorial protons would absorb at higher field (upfield) whilehe axial protons would absorb at lower field (downfield). Theppearance of two doublets corresponding to �NH protons in theomplexes (2)–(4) indicates strong coupling between axial andquatorial counter parts of secondary NH protons. The signalsue to secondary NH protons are broader in the complexes thanhat observed in free dihydrazone. The broadening of secondaryNH protons signal is, most probably, due to coupling of therotons with the quadrupolar nitrogen nucleus [25]. The pattern

(napoxlhH2)2]·2H2O (1). (c) Mo2O4(napoxlhH2)2(3-pic)2]·2H2O (3).

rotons are also broader than that in the free dihydrazone. Theroadening of the lines may be due to the nitrogen inversion atufficiently slow rate at the metal centre. This may be relatedo stronger bonding between azomethine nitrogen atoms andetal centre. The fact that one hydrazone part is in the axial

osition while the other part is in the equatorial position sug-ests W-configuration of dihydrazone around the metal centre24].

Methyl protons show signal at � 2.32 and 2.37 ppm in free-picoline and 4-picoline, respectively [24,28]. In metal com-lexes (3) and (4) these signals appear at � 2.30 and 2.07 ppm,espectively, and are thus upfield shifted by 0.02 and 0.30 ppm,espectively. This upfield shift is the result of drainage of elec-rons density from nitrogen atom to the metal centre causingdecrease in the electronegativity of ring nitrogen atom. Con-

equently, the electron density on all carbon atoms decreases.hile the decrease in electron density on 2- and 4-carbon atoms

s maximum due to o- and p-directing influence, that on 3-carbontom is minimum. Hence, the upfield shift of methyl protons in-picoline is more than that in 3-picoline. Further, no signal isbserved in the downfield region in the 1H NMR spectra of theomplexes (2)–(4) which may be assigned to 2-pyridyl protonsf pyridine/3-picoline/4-picoline molecules [29]. This indicateshat the signals due to 2-pyridyl protons of pyridine/3-picoline/4-icoline molecules are upfield shifted and merged with naphthyl

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.6. IR spectra

The complex (1) shows two very strong bands at 944 and16 cm−1 of almost equal intensity which are characteristic ofis-MoO2

2+ group. On the other hand, the complexes (2)–(4)how only a strong band in the 944–952 cm−1 region and aedium intensity band in the 907–920 cm−1 region. These

ands are assigned to νs and νas stretching vibrations of cis-oO2

2+ group. Further, the complexes show another mediumntensity band in the 833–840 cm−1 region which is assignedo stretching vibration of Mo O–Mo linkage [30]. On the otherand, the complexes (5) and (6) do not show strong band in theegion 1000–850 cm−1 indicating absence of oxo-group. Thisuggested condensation of Mo O group with NH2 group of theydrazide ligand (Table 4).

The dihydrazone derived from condensation of o-hydroxyromatic aldehydes and ketones and acyl-, aroyl- and pyridoyl-ihydrazine are polyfunctional ligands and can coordinate to

Scheme 1. Complexes [Mo2O4(napoxlhH2)2(

ta Part A 69 (2008) 706–714

entre in the keto and enol form. In addition, they canoordinate to the metal centre accompanied by condensa-ion with oxo-metal ions via their hydrazinic amino groupsccompanied by enolization leading eventually to redox reac-ions.

The uncoordinated ligand shows strong bands at 1655 and616 cm−1 which are assigned to stretching vibrations of C Ond C N group. These bands split into two bands and on aver-ge, undergo shift to higher position by 12–18 cm−1 and lowerosition by 8–11 cm−1, respectively. The average shift of νC Oand to higher frequency by ca 12–18 cm−1 and of νC N bando lower frequency by 8–11 cm−1 in complexes (1)–(6) sug-ests non-coordination of C O group while coordination ofihydrazone through C N groups, to the metal centre. Theand at 1528 cm−1 assigned to amide II + ν(C O)(phenolic)hifts to higher frequency ruling out the possibility of coor-ination of dihydrazone to the metal centre through C Oroups. This further indicates involvement of naphtholate oxy-

A)2] (A = py(2), 3-pic(3) and 4-pic(4)).

nnttbtcctt[

ectr11cMsn

tagtbpottbmtioo[

Scheme 2. Complexes [Mo(napoxlhH2)(h

aphtholate oxygen atoms [32]. The complexes (2)–(4) showew bands in the region 679–690 and 430–433 cm−1 assignedo in-plane ring deformation and out–of–plane ring deforma-ion mode of pyridine/3-picoline/4-picoline molecules. Theseands are considerably shifted to higher position as comparedo that in the free pyridine/3-picoline/4-picoline molecules indi-ating coordination of pyridyl ring nitrogen atom to the metalentre. The complexes (2)–(4) show a new weak band inhe region 270–280 cm−1 characteristic of �M–N resulted fromhe coordination of pyridyl nitrogen atom to the metal centre33].

The IR spectra of the complexes (5) and (6) show differ-nt feature as compared to the precursor complex (1) and theomplexes (2)–(4). This is evident from the absence of any addi-ional band characteristic of C O and NH2 groups in theegion 1660–1630 cm−1 and ν(Mo O) vibration in the region000–850 cm−1 and appearance of medium intensity band at506 and 1482 cm−1 characteristic of (N N) group which indi-

alicyloyl- and pyridoyl-diazenido grouping in the molybde-um coordination sphere [34]. The weak bands observed in

·2H2O (hzidH3 = inhH3(5), syslhH3 (6)).

he region 2800 cm−1 in the IR spectrum of the free sylshH3ttributed to intramolecular H-bonding between C O and OHroup disappears in the complex (6) indicating coordinationhrough carbonyl oxygen atoms. However, a well defined broadand centred at 3217 cm−1 having feature different as com-ared to that in the complex (5) and similar in feature to thosebserved in o-hydroxy aromatic aldehyde and ketone suggestshe development of a new type of intramolecular H-bonding inhe complex. On the other hand, in the complex (5), a strongroad band is observed in the region 3550–3000 cm−1 with itsiddle point at 3411 cm−1. This is attributed to νOH of lat-

ice water molecules. These complexes do not show any bandn the region 750–650 cm−1 attributable to the rocking modef the coordinated water molecules ruling out the possibilityf presence of coordinated water molecules in the complexes35].�OH

The νC O and νC N stretching frequencies appear as a cou-le of bands in these complexes. The difference between these

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[36] H. Adams, N.A. Bailey, M.J.S. Dwyer, D.E. Fenton, P.C. Hellier, P.D.

. Conclusion

In the present study, we have prepared some monomeric andimeric molybdenum(V) and molybdenum(VI) complexes andharacterized them on the basis of data obtained from physico-hemical and spectroscopic studies. The reaction of monomericolybdenum(VI) complex with proton donor reagents givesonomeric molybdenum(V) complexes while reaction with

lectron donor reagents gives dimeric molybdenum(VI) com-lexes. The dihydrazone coordinates to the metal centre throughzomethine nitrogen atoms and naphtholate oxygen atoms as aetradentate N2O2 donor. The C O and NH groups remainncoordinated. The dihydrazone exists in the staggered con-ormation in the free state while it is isomerized to anti-cisonformation in the complexes. The molybdenum centre has sixoordinate octahedral geometry in the complexes (1), (5) and (6)hile it has eight coordinate dodecahedral geometry [37] in the

omplexes (2)–(4). In the complexes (2)–(4) the molybdenum

entres are bonded as . In all of the complexes, therincipal dihydrazone ligand is coordinated to the metal centren keto form in the anti-cis-configuration. Tentative structuresor the complexes have been proposed in Schemes 1 and 2,espectively.

cknowledgements

The authors are thankful to the Head, RSIC, CDRI, Lucknowor C, H, N analyses and to the Head RSIC, as IIT, Madras forecording esr spectra. One of the authors (RAL) is thankful to theniversity Grants Commission, New Delhi, India for financial

ssistance through a research grant.

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Synthesis and properties of mononuclear and binuclear molybdenum complexes derived from...

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