rIndian Journal of ChemistryVol. 28A, September 1989, pp. 783-786

Oxomolybdenum(V) complexes with isomeric oxygen and nitrogen donortridentate schiff bases derived from ortho-hydroxybenzyl amine, ortho-\ aminobenzyl alcohol and salicylaldehyde/substituted salicylaldehyde,

A Syarnal", S Ahmed & D KumarDepartment of Applied Sciences and Humanities, Kurukshetra University, Kurukshetra 132 119

and

Department of Chemistry, Regional Engineering College, Kurukshetra 132 119

Received 12 September 1988; revised and accepted 24 November 1988 r

New isomeric oxomolybdenum(V) complexes of the type MoOClL (where LH2 = schiff base derivedfrom o-hydroxybenzyl amine or o-aminobenzyl alcohol and salicylaldehyde, S-chlorosalicylaldehyde,5-bromosalicylaldehyde, 3-methoxysalicylaldehyde or 2-hydroxynaphthaldehyde) have been synthes-

. ized and characterized onthe basis of elemental analysis, electrical conductance, IR and electronic spect-ra, and magnetic susceptibility measurements. The isomeric schiff bases behave as dibasic tridentate li-gands. The complexes are non-electrolytes and exhibit two electronic spectral bands around 13600 and24000 cm -] due to the 2 B2 -> 2 E and 2 B2 -> 2 B] transitions, respectively. The complexes exhibit magne-tic moments in the range 0.57-0.73 B.M. at room temperature and are involved in antiferromagnetic ex-change. On the basis of magnetic susceptibility and IR d,ta, a dimeric octahedral structure involvingphenolic/enolic oxygen atom bridge has been suggested. I·

Inspite of the practical importance of molybdenumand its compounds, and its applications in enzymeactivity', the chemistry of molybdenum (V) schiffbase complexes has been much less explored incomparison to the vast chemistry of schiff base com-plexes of the first transition series metal ions+'. Wereport here the synthesis of some molybdenum(V)coordination compounds of the type MoOCIL withthe dianions of tridentate oxygen and nitrogen don-or isomeric schiff bases (I and II) derived from 0-

hydroxybenzyl amine or o-aminobenzyl alcohol and,

(I)

(II)X =H, S-C HlORO. S-BROMO,

3-METHOXY, S.6-BENZO,

(/

salicylaldehyde or substituted salicylaldehyde. Notmuch work seems to have been reported on oxomo-lybdeimm(V) complexes of tridentate Iigands-".

Materials and MethodsSalicylaldehyde and ammonium molybdate{Vl)

were the products of Sarabhai M. Chemical Co. 0-

Aminobenzyl alcohol was purchased from AldrichChemical Co. (U.S.A.). 5-Bromosalicylaldehyde,3-methoxysalicylaldehyde and 2-hydroxy-l-naph-thaldehyde were purchased from Fluka A.G. (Swit-zerland). 5-Chlorosalicylaldehyde5, o-hydroxyben-zyl amine" and the schiff bases (I and 1l)1·8 were pre-pared according to the published procedures. Di-ammonium oxopentachloromolybdate(V) was pre-pared by following the standard method".

The electrical conductance measurements weremade on a Toshniwal conductivity bridge (type CL01-02A) in DMF solution (O.Oot M). Magnetic sus-ceptibility measurements were made on ~ Gouy bal-ance at room ter,nperature using Hg[Co(NCS)4] as acalibrant. The magnetic susceptibilities were cor-rected for the diamagnetic susceptibility of metalion (x dia = - 14 x 10- 6 C.g.s.units for M05 + ), ligandand temperature-independent paramagnetism termof M05 + using a value of 60 x 10 - 6 c.g.s. units. Elec-tronic spectra of the complexes were recorded innujol mull on a Beckman DMR-21 spectropho-

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tometer in the region 300-1000 nm. Infrared spect-ra were recorded in KBr pellets in the region 600-4000 cm - 1 on a Beckman IR 20 spectrophotome-ter. The metal content was determined gravimetri-cally as bis( 8-hydroxyquinolinato )dioxomolybde-num(VI) after fusing the complex with a mixtue ofNaOH pellets and Na202• Chlorine was determinedgravimetrically as AgCl after fusing the. compoundwith a mixture of Na202 and NaOH pellets. For theestimation of chlorine in MoOCl (5-bromosalicy-laldehyde-o-aminobenzyl alcohol) and MoOCl (5-bromosalicylaldehyde- o-hydroxybenzylamine.), thecomplex was decomposed with equal volumes ofcone. HN03 and H2S04; molybdenum was re-moved as [Mo02(oxine)2] and chlorine was precipi-tated as AgCl, which was filtered, dissolved inNH40H, reprecipitated with dilute HN03 andweighed as AgCl. Nitrogen analyses were done mic-roanalytically.

Method of determination of oxidation state of MoA weighed amount of the compound (- 0.1 g)

was dissolved in 75 rnl of ferric alum solution (0.1N) and 25 rnl of dilute H2S04 solution (2 N) bystirring in a stoppered iodine flask. The mixture wascooled to about 5°C and 25 rnl of Zimmermann-

Reinhardt's solution was added to the mixture. Themixture was then titrated immediately with a stand-ard potassium permanganate solution.

'Synthesis of MoOClLDiammonium oxopentachloromolybdate (1.0 gt

0.003 mol) was .dissolved in absolute ethanol (10 rnl)and the precipitated ammonium chloride was fil-tered off. To the dark reddish brown filtrate, an ab-solute ethanolic solution (10-25 ml) of the appropri-ate schiff base (0.003 mol) was added slowly whilestirring continuously. The mixture was then refluxedon an electric mantle for two hours with continuousmagnetic stirring and the separated precipitateswere suction filtered, washed with absolute ethanoland dried in vacuo; yield = - 40%.

The analytical and other physicochemical dataare presented in Tables 1 and 2.

Results and DiscussionThe complexes were prepared by reacting the

ethanolic solutions of (NH4lz[MoOCls] and theschiff base in 1 : 1 ratio according to Eq. (1),

CH30H(NH4lz[MoOCls] + lli2 • MoOClL

reflux+ 2NH4Cl + 2HCI ... (1)

..

Table 1- Analytical and magnetic moment data of oxomolybdenum(V) complexes of schiff bases="

Sr. No. Complex Stoichiometry Found (calc.) % x"MIT x 10-6, p.eff., B.M.c.g.s: units (Temp,K)

Mo N Cl

1 MoOCl( sal-o-hyba) MoCI4Hl1N03Cl 25.4 3.95 9.8 134 0.57(25.77) (3.76) (9.53) (300)

2 MoOCl( 5-chlorosal-o-hyba) MOCI4HION03Cl2 23.2 3.2 17.2 156 0.61(23.58) (3.44) (17.44) (296)

3 MoOCl( 5-bromosal-o-hyba) MoCI4HION03CIBr 21.4 2.9 7.5 222 0.73(21.26) (3.10) (7.86) (298)

4 MoOCl(3-methoxysal-o-hyba) MoC1sH13N04Cl 23.9 3.5 8.6 158 0.61(23.85) (3.48) (8.82) (297)

5 MoOCl(hydroxy-o-hyba) MoC1SH13N03Cl 22.6 3.2 8.2 67 . 0.40(22.72) (3.31) (.8.40) (298)

6 MoOCl(sal-o-aba) MOC14Hl1N03Cl 25.3 3.6 9.7 171 0.64(25.77) (3.76) (9.53) (297)

7 MoOCl( 5-chlorosal-.o-aba) MOC14HION03Cl2 23.3 3.35 17.1 198 0.69(23.58) (3.44) (17.44) (298)

8 MoOCl(5-bromosal-o-aba) MOC14HION03CIBr 20.93 3.25 7.6 216 0.72(21.26) (3,10) (7.86) (298)

9 MoOCl( 3-methoxysal-o-aba) MoC1sH13N04Cl 23.7 3.6 8.6 204 0.70(23.85) (3.48) (8.82) (297)

10 MoOCl(hydroxy-o-aba) MoC1SH13N03Cl 22.8 3.1 8.2 159 0.62(22.72) (3.31) (8.40) (299)

(a) Abbreviations: sal = salicylaldehyde, 5-chlorosal = 5-chlorosalicylaldehyde, 5-bromosal = 5~bromosalicylaldehyde, 3-methoxy-sal =3-methoxysalicylaldehyde, hydroxy = 2-hydroxy-l-naphthaldehyde, hyba = o-hydroxybenzyl amine and aba = o-aminobenzylalcohol.(b) The magnetic moment was calculated using the Curie equation: p.eff = 2.84 (x~rr x T)112B.M.

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SYAMAL et al: OXOMOLYBDENUM(V) COMPLEXES WITH SCHIFF BASES

Table 2- Infrared and electronic spectral data of oxomolybdenum(V) complexes of schiff bases"

Sf. No. Complex v(C=N) v(C-O) v(C-O) v(Mo=O) vmaxcomplex (phenolic) (alcoholic)(ligand) complex complex

(ligand) (ligand)

MoOCI(sal-o-hyba) 1600 1540 920 13800 sh, 21800(1655) (1530)

2 MoOCl(5-chlorosal-o-hyba) 1630 1530 940 13700 sh, 24500(1660) (1515)

3 MoOCl (5-bromosal- o-hyba) 1610 1550 980 13700,25000(1655) (1515)

4 MoOCl(3-methoxysal-a-hyba) 1630 1550 970 13700 sh, 23500(1640) (1515)

5 MoOCl(hydroxy-a-hyba) 1625 1540 960 13330 sh, 24500(1630) (1530)

6 MoOCI(sal-c-aba) 1600 1535 1230 970 13500,24000(1615) (1530) (1190)

7 MoOCl (5-chlorosal- o-aba) 1615 1535 1245 950 13500,24100(1620) (1530) (1225)

8 MoOCl (5-bromosal- o-aba) 1615 1520 1230 920 13600 sh, 24500(1620) (1515) (1200)

9 MoOCI(5-methoxysal-o-aba) 1600 1520 1230 930 13500,24000(1610) (1515) (1200)

10 MoOCI(hydroxy-o-aba) 1615 1535 1250 935 13500,25000(1620) (1530) (1215)

(0) All spectral bands are in ern - I ; sh = shoulder.

The complexes are insoluble in water, methanol andethanol, and are soluble in coordinating solventslike DMSO and DMF. The electrical conductancemeasurements of the complexes in 10 - 3 M DMFsolutions indicate their non-electrolytic nature(1\ M = 2-10 ohm -1 C0l2 mol " '], The non-electrolyt-ic nature of the complexes indicates that Cl is coord-inated and the complexes do not undergo solvolysisin DMF. The molecular weight measurements onthe complexes could not be done due to their insolu-bility in a suitable solvent. The oxidimetric titrationof complexes with ferric alum indicated the oxida-tion state of molybdenum as + 5 and the oxidationof the complexes to diamagnetic molybdenum(VI)complexes is ruled out.

The v(Mo = 0) mode of the complexes occurs at920-980 em -I which lies in the range (900-1007cm - I) observed for the majority of oxomolybde-num(V) complexes'. The occurrence of thev(Mo = 0) in the normal range excludes the pres-ence of a ..... Mo = 0 ...Mo = 0 interaction whichwould have lowered v(Mo = 0) (< 850 em - 1)10.

The present oxomolybdenum(V) complexes exhi-bit magnetic moments in the range 0.57-0.73 B.M.per molybdenum ion, which are remarkably lessthan those expected (1.68-1.78 B.M.) for magneti-cally dilute oxomolybdenum(V) complexes with

(

s= t system 10. The magnetic moment values in thevicinity of zero indicate the complete neutralisationof the spin paramagnetism due to the complete anti-parallel coupling of the two S = t spins in a dimer.Due to the low magnetic susceptibility of the com-plexes at room temperature, no attempt was madeto record the variable temperature magnetic suscep-tibility data of the complexes. The exchange inte-gral, J, of the complexes, calculated using the roomtemperature magnetic susceptibility data and theBleaney Bowers equation'>, is negative and is in therange 605-875 cm-I. These informations are indi-cative of the presence of strong intramolecular antif-erromagnetic exchange and of the presence of S = 0ground state in the complexes. It is of interest tonote that the corresponding copper(II) and oxova-nadium(IV) complexes (S = t systems) show a par-allel behaviour". The unpaired electron is in the d xyorbital in oxomolybdenum(V) complexes. The d xy

orbital of molybdenum has the proper symmetry fora a type overlap with the dxyorbital of a neighbour-ing molybdenum centre leading to direct Mo-Moexchange. The indirect zr-pathway of super-ex-change interaction, i.e., Mo(dx" -1)O(pz)Mo(dx"-I) would be weak as dxy-dx"-I mixingwill occur only in the higher energy state. Thus, a di-rect mechanism of magnetic exchange through

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INDIAN J CHEM, SEC. A, SEPTEMBER 1989

Mo - Mo interaction is suggested for the presentcomplexes". It is evident from the magnetic pro-perties of the complexes (Table 1) that the electroniceffects of the substituents in the benzene ring on themagnetic properties of oxomolybdenumfv ) com-plexes is relatively unimportant, except for anystructural modifications they may produce in theMo - 0 - Mo bridge via alteration of molecularpacking!'.

The schiff bases (I) exhibit the v( C - 0) (phenolic)at 1530-1550 cm-I. The v(C-O) (alcoholic) is ex-pected to occur at lower energy than the v( C - 0)(phenolic) as in the latter case delocalisation of elec-trons towards benzene ring occurs leading to higherbond order of C - 0 (phenolic) than that of C - 0(alcoholic). The v( C - 0) (alcoholic) and v( C - 0)(phenolic) are observed at 1190-1225 and 1515-1530 em -I respectively in the_schiff bases (II). Incomplexes of schiff bases (I) the formation of bridgemay take place through the phenolic oxygen atomwhile in the complexes of schiff base (II) the bridgeformation may occur via either phenolic oxygen oralcoholic oxygen. A shift in the v( C - 0) (alcoholic)and v(C-O) (phenolic) to higher energy by 10-55em -I and ~ 10 ern -I, respectively, in metal com-plexes has been unambiguously attributed to theformation of an alcoholic oxygen bridge or a phen-olic oxygen bridge 11,13. In Mo(V) complexes ofschiff bases (I), the v(C - 0) (phenolic) shifts tohigher energy by 10-35 em - I indicating the pres-ence of a phenolic oxygen bridge. In the schiff base(I), there are two phenolic oxygen atoms-one be-longing to the salicylaldehyde moiety and the otherto the benzylamino phenol moiety. It is not possibleto distinguish between these two possibilities ofbridge formation on the basis of IR data. We suggestthe benzylamino phenol oxygen atoms are thebridging atoms, similar to the case in Cu(II) and ox-ovanadium(IV) complexes!' of schiff bases (I). Inthe Mo(V) complexes of the schiff bases (II), thev( C - 0) (alcoholic) shifts to higher frequency by20-40 em - 1 and the v( C - 0) (phenolic) shifts tohigher energy by < 10 cm -I. This indicates the pres-ence of alcoholic oxygen atom bridges which is inline with the behaviour of Cu(II) and oxovanadi-um(IV) complexes" of (II). A band at -740 cm-Iin the complexes has been assigned tov{Mo - 0 - Mo) arising out of ligand bridging".The schiff bases (1)and (II) exhibit v(C = N) (azome-

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thine) in the ranges 1630-1660 em -I and 1615-1620 em -I, respectively, and this band undergoes anegative shift of 5-55 em -I in the complexes indi-cating the coordination of nitrogen to Mo. Thus, theIR data suggest the tridentate oxygen, nitrogen don-or behaviour of (I) and (II).

Three electronic spectral transitions eB2 - 2 E, -13000 cm-I; 2B2 - 2BI, - 23000 cm-I; and2 B2 - I A I, - 27000 em - 1 ) are expected for octahe-dral molybdenum(V) complexes. The present com-plexes exhibit two bands around 13000 and 24000cm-I due to the 2B2-2E and 2B2-2BI trans-itions, respectively. The third band due to the spin-forbidden 2 B2 - 1Al transition could not be locatedas it is covered by the intense charge transfer band.It is interesting to note that the present magneticallyconcentrated oxomolybdenum(V) complexes donot exhibit any electronic spectral band which ischaracteristic of M-M interaction. No band charac-teristic of M-M interaction has been observed in thecase of magnetically concentrated copper (II) andoxovanadium(IV) complexes'P-" also.

The isomeric schiff bases (I and II) have producedisomeric oxomolybdenum(V) complexes whichhave comparable chemical, spectral and magneticproperties.

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(London), 20(1966) 103.2 Stiefel E I, Progr inorg Chern, 22 (1977) 1.3 Holm R H, Everett (Jr) G W & Chakravorty A, Progr inorg

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