Synthesis and characterization of some dioxomolybdenum(VI) complexes with O,N,O-Donor schiff bases derived from nicotinoyl hydrazone and substituted salicylaldehyde.
Metal complexes of the Schiff bases have occupied a central role in the development of co-ordination chemistry. A large variety of stable chemical species have been synthesized containing both transition and non-transition metals with multi-dentate ligand systems and occupy important position in coordination chemistry [1-2]. Molybdenum complexes with Schiff base ligands have gained considerable importance due to their potential for biological and physiological activity and the industrial use [3-8]. Most molybdenum complexes contain cis- Mo[O.sub.2.sup.2+] moiety in an octahedral structure. A great deal of work is being carried out in the synthesis and characterization of oxo-, dioxo-molybdenum complexes with one site vacant which might allow binding and displacement of various substrates [9-13].The availability of such a labile site imparts catalytic property to the complex. This can be achieved by using a dibasic tridentate ligand around octahedral molybdenum(VI) [9-13]. We have been interested in five coordinate octahedral cis- Mo[O.sub.2] complexes with nonsymmetrical Schiff bases having mixed sets of donor atoms in which the sixth coordination site is occupied by a solvent molecule [9,13]. The envisaged advantage of such complexes is that an olefin molecule may displace solvent in the sixth coordination and undergo oxidation under suitable experimental conditions. The formal oxidation state of molybdenum cycles between 4+ and 6+ in the reactions with the olefin and oxidant.
In view of this, we report the synthesis and structural characterization of some novel cis-dioxomolybdenum(VI) complexes with Schiff bases derived from ohydroxyacetophenone (hap), o-hydroxypropiophenone (hpp) or ohydroxybenzophenone (hbp) with nicotinic acid hydrazide (NAH). These Schiff bases form mononuclear dioxomolybdenum(VI) complexes having the general formula Mo[O.sub.2](L)(EtOH) (where L[H.sub.2]=Schiff base namely [H.sub.2]hap N'[(1Z)-1-2-hydroxyphenyl)ethylidene]nicotinohydrazide ([H.sub.2]hap-NAH), N'[(1Z)-1-(2hydroxyphenyl)propylidene]nicotinohydrazide ([H.sub.2]hpp-NAH) and N'[(1Z)-1-(2hydroxyphenyl)phenylethylidene]nicotinohydrazide ([H.sub.2]hbp-NAH).
Ammonium molybdate(VI) tetrahydrate was obtained from Sisco Research Laboratory, (Mumbai, India). o-hydroxyacetophenone, o-hydroxypropiophenone,o-hydroxybenzophenone and nicotinic acid hydrazide were procured from Lancaster Synthesis Ltd (UK). Mo[O.sub.2][(acac).sub.2] was prepared according to reported method .
Synthesis of Schiff base ligands (1a-3a)
An ethanolic solution of o-hydroxyacetophenone (1.36g, 10mmol), o-hydroxypropiophenone (1.50g, 10mmol) or o-hydroxybenzophenone (1.98g, 10mmol) was added to a hot ethanolic solution of nicotinic acid hydrazide (1.37g, 10mmol). The mixture was heated under reflux for 2-4 h and then cooled in an ice bath. The yellow crystals that separated out were filtered, washed several times with ethanol and then dried in vacuo. Yield: (1a) 2.32g, 85%; (2a) 2.61 g, 91 % and (3a) 2.91 g, 87%.
Synthesis of complexes (1b-3b)
To the hot ethanolic solution of the appropriate Schiff base ligand (1mmol, (1a) 0.255 g; (2a) 0.269 g; (3a) 0.317 g) was added an ethanolic solution of [cis- Mo[O.sub.2][(acac).sub.2]] (0.326, 1mmol) with vigorous stirring. The reaction mixtures were then refluxed for 6 h. The precipitate that separated out from their respective solution was filtered, washed several times with water and ethanol and then dried in vacuo. Yield : (1b) 0.47 g, 81%; (2b) 0.50 g, 84% ; (3b) 0.57 g, 89 %.
Microanalysis of the Schiff base ligands and complexes were performed on Nicolet Impact 410 FTIR spectrometer. [sup.1]H NMR spectra were recorded on a Cd[Cl.sub.3] on a Bruker-Varian 300 MHz spectrometer. The conductance measurements were done in DMF using a Toshniwal conductivity bridge and a dip type cell calibrated with KCl solution. Electronic spectra were recorded on Perkin Elmer [lambda] 900 UV- Vis spectrophotometer.
Results and Discussion
Bis(acetylacetonato) dioxomolybdenum(VI), Mo[O.sub.2] [(acac).sub.2] undergoes ligand exchange reaction with the Schiff bases (1a-3a) and complexes of the type [Mo[O.sub.2] (L)(EtOH)] (where L[H.sub.2]= Schiff base) are formed as shown in Eq:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)
The analytical data (Table 1) show that the Schiff bases behave as dibasic, tridentate ligands, hence complete replacement of the bidentate acetylacetone occurs under the reaction conditions and the ethanol occupies the sixth coordination position. The analytical data further support the formulation of the complexes as cis-[Mo[O.sub.2] (L)(EtOH)]. (Figure 1; structures (1a-3a) for ligands and (1b-3b) for complexes). The molar conductance measurements of the complexes in DMF were found to be in the range 2.5 to 4.5 [ohm.sup.-1] [cm.sup.-2] [mol.sup.-1], indicating that they are non-electrolytes.
The IR spectra of the complexes (Table 1), exhibit two bands in the regions 935-949 and 908-912 [cm.sup.-1] due to the vsym(O=Mo=O) and vasym(O=Mo=O) respectively, indicating the presence of cis Mo[O.sub.2.sup.2+] moiety [9-13]. The IR spectra of the Schiff base ligands (1a-3a) exhibit a strong band in the range 3026-3180 [cm.sup.-1]and 1672-1679 [cm.sup.-1] due to vNH and vC=O groups respectively, which disappears in the corresponding complexes suggesting the enolization of the -C=O of the aroyl keto group giving rise to the -C=N-N=C moiety [15 ] and consequent deprotonation upon coordination. This enolization is further evidenced by the appearance of a new band due to vC-O, which is observed in the region 1149-1150 [cm.sup.-1]. A strong band in the region 1649-1654 [cm.sup.-1] is assigned to vC=N(azomethine) which undergoes a shift to lower frequency by 50 [cm.sup.-1], suggesting the coordination of azomethine nitrogen to the metal centre.
The IR spectra (Table 1) also reveal that both symmetric and asymmetric stretching of cis-Mo[O.sub.2] in the complex formed with ([H.sub.2]hbp-NAH)(3a) occur at a relatively higher energy (912 and 949[cm.sup.-1]) than the corresponding ([H.sub.2]hppNAH)(2a),(910 and 941 [cm.sup.-1]) and ([H.sub.2]hap-NAH)(1a), (908 and 935 [cm.sup.-1]), respectively. Moreover, the difference between sym and asym frequencies is also less ([DELTA]v =27 and 31 [cm.sup.-1]) for complexes (1b) and (2b), whereas this difference is more ([DELTA]v = 37[cm.sup.-1]) for complex (3b). The differences in stretching frequencies of the cis-Mo[O.sub.2] moiety may be attributed to progressively decreasing steric and electron-withdrawing character of benzyl, ethyl and methyl groups on azomethine carbon in the complexes (3b), (2b) and (1b), respectively.
The electronic spectra of the complexes (Table 2), recorded in DMF displayed absorption maxima in the range 390-400 nm which may be due to ligand to metal charge transfer (LMCT) transition. This is in the range usually observed for Mo[O.sub.2] complexes [16-19 ]. As expected for a 4d[degrees] system, no d-d transitions were observed.
[sup.1]H NMR Spectra
The [sup.1]H NMR spectral data (Table 2) of the ligands exhibit two signals in the range 11.20-11.80 ppm (s,1H each) and 12.20-13.20ppm (s,1H) due to the NH and phenolic OH protons, respectively. The absence of these signals in the spectra of the complexes indicate coordination through the phenolic oxygen and enolization of the -C=O of the aroyl keto group and consequent deprotonation. The Schiff bases display a multiplet in the range 6.65-9.3 ppm due to the aromatic protons and this did not show any appreciable change upon coordination. The EtOH coordination to molybdenum is supported by the presence of the singlet, triplet and quartet of OH, C[H.sub.3] and C[H.sub.2] respectively at 12.91-12.93, 1.06-1.30 and 3.16-3.70 ppm.
The olefin oxidation studies using these complexes (1b-3b) are in progress and will be reported elsewhere.
[FIGURE 1 OMITTED]
As is evident from the above data, the Schiff base ligands(1a-3a) behave as dibasic tridentate ligands and co-ordinate through phenolic oxygen, azomethine nitrogen and enolic oxygen atoms. The corresponding cis-Mo[O.sub.2.sup.2+] (1b-3b) complexes are found to be monomers, non-electrolytes, diamagnetic and six co-ordinated. The solvent molecule (ethanol) occupies the sixth coordination site imparts catalytic property to the complexes.
The authors thank Dr.N.N.Rao (Sr.Scientist, NEERI, Nagpur) and Dr.C.G. Dethe (Principal, PIET, Nagpur) for encouragement and useful discussions.
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M.A Katkar (a), S.N. Rao * (a) and H.D. Juneja (b)
(a) Department of Chemistry, Priyadarshini Institute of Engineering and Technology, Hingna road, Nagpur-440019, India
* E mail: firstname.lastname@example.org
(b) Department of Chemistry, Rashtrasant Tukodoji Maharaj, Nagpur University, Nagpur-440010, India
Table 1: Analytical and IR Spectral Data of Ligands 1a-3a) and Complexes (1b-3b). Elemental Analysis (%) Calculated (found %) Sr. Ligands/ no. complexes C H N 1a [H.sub.2]-hap-NAH 65.88 5.09 16.40 [C.sub.14][H.sub.13] (65.61) (5.08) (16.17) [N.sub.3][O.sub.2] 1b Mo[O.sub.2](hap-NAH) 44.97 3.98 9.83 Mo[C.sub.16][H.sub.17] (45.10) (3.74) (9.70) [N.sub.3][O.sub.5] 2a [H.sub.2]-hpp-NAH 66.90 5.57 15.61 [C.sub.15][H.sub.15] (66.40) (5.60) (15.36) [N.sub.3][O.sub.2] 2b Mo[O.sub.2](hpp-NAH) 42.76 2.37 9.60 Mo[C.sub.17][H.sub.19] (42.26) (2.30) (9.52) [N.sub.3][O.sub.5] 3a [H.sub.2]-hbp-NAH 71.92 4.73 13.24 [C.sub.19][H.sub.15] (71.90) (4.75) (13.25) [N.sub.3][O.sub.2] 3b Mo[O.sub.2](hbp-NAH) 51.54 3.88 8.89 Mo[C.sub.21][H.sub.19] (51.24) (4.02) (8.79) [N.sub.3][O.sub.5] Infrared Data ([cm.sup.-1]) Sr. Ligands/ no. complexes V(OH) V (NH) V(C = N) 1a [H.sub.2]-hap-NAH 3493 3180 1649 [C.sub.14][H.sub.13] [N.sub.3][O.sub.2] 1b Mo[O.sub.2](hap-NAH) 3453 - 1599 Mo[C.sub.16][H.sub.17] 1557 [N.sub.3][O.sub.5] 2a [H.sub.2]-hpp-NAH 3226 3026 1651 [C.sub.15][H.sub.15] [N.sub.3][O.sub.2] 2b Mo[O.sub.2](hpp-NAH) 3108 - 1605 Mo[C.sub.17][H.sub.19] 1556 [N.sub.3][O.sub.5] 3a [H.sub.2]-hbp-NAH 3172 3043 1654 [C.sub.19][H.sub.15] [N.sub.3][O.sub.2] 3b Mo[O.sub.2](hbp-NAH) 3060 - 1600 Mo[C.sub.21][H.sub.19] 1553 [N.sub.3][O.sub.5] Infrared Data ([cm.sup.-1]) Sr. Ligands/ no. complexes V(C = O) V(C-O) V(Mo 0= O) 1a [H.sub.2]-hap-NAH 1672 - - [C.sub.14][H.sub.13] [N.sub.3][O.sub.2] 1b Mo[O.sub.2](hap-NAH) - 1150 935 Mo[C.sub.16][H.sub.17] 908 [N.sub.3][O.sub.5] 2a [H.sub.2]-hpp-NAH 1679 - - [C.sub.15][H.sub.15] [N.sub.3][O.sub.2] 2b Mo[O.sub.2](hpp-NAH) - 1150 941 Mo[C.sub.17][H.sub.19] 910 [N.sub.3][O.sub.5] 3a [H.sub.2]-hbp-NAH 1679 - - [C.sub.19][H.sub.15] [N.sub.3][O.sub.2] 3b Mo[O.sub.2](hbp-NAH) - 1150 949 Mo[C.sub.21][H.sub.19] 912 [N.sub.3][O.sub.5] Table 2: UV-VIS and [sup.1]H NMR Spectral Data of Ligands and Complexes. Sr.No. Ligands/ complexes 1a [H.sub.2]-hap-NAH [C.sub.14][H.sub.13] [N.sub.3][O.sub.2] 1b Mo[O.sub.2](hap-NAH) Mo[C.sub.16][H.sub.17] [N.sub.3][O.sub.5] 2a [H.sub.2]-hpp-NAH [C.sub.15][H.sub.15] [N.sub.3][O.sub.2] 2b Mo[O.sub.2](hpp-NAH) Mo[C.sub.17][H.sub.19] [N.sub.3][O.sub.5] 3a [H.sub.2]-hbp-NAH [C.sub.19][H.sub.15] [N.sub.3][O.sub.2] 3b Mo[O.sub.2](hbp-NAH) Mo[C.sub.21][H.sub.19] [N.sub.3][O.sub.5] Sr.No. [sup.1]H NMR, ppm 1a 2.48 (s,-C[H.sub.3];3H) ; 6.65-9.13(ArH,8H);11.20(NH,1H); 13.20(OH, 1H). 1b 1.06-1.17 (t,-C[H.sub.3], 3H);1.25(s,C[H.sub.3],3H); 3.16-3.31(q,C[H.sub.2], 2H); 6.94-9.39 (ArH,7H); 12.91(s, OH,1H). 2a 1.2-1.3(t,-C[H.sub.3],3H);2.6-2.9 (s,-C[H.sub.2], 2H); 6.8-8.9(ArH, 8H); 11.70(NH,1H)12.80(OH,1H). 2b 1.17-1.28(t,-C[H.sub.3],3H); 2.90-3.61 (q,-C[H.sub.2],4H); 6.9-8.3(ArH-6H); 12.92 (sOH,1H). 3a 7.32-8.34(m,ArH,13H); 12.20(OH, 1H); 11.80(NH,1H). 3b 1.19-1.30(t, C[H.sub.3], 3H);3.41-3.70 (q,C[H.sub.2], 2H) 7.50-8.60 (m,ArH,13H);12.93(s,OH,1H). Sr.No. UV-vis [lambda]([epsilon] [M.sup.-1][cm.sub.-1]) 1a 370 (1.5 x [10.sup.4]) 1b 375 (1.3 x [10.sup.4]) 2a 380 (1.0 x [10.sup.4]) 2b 390 (8.0 x [10.sup.3]) 3a 398 (7.8 x [10.sup.3]) 3b 405 (7.5 x [10.sup.3])
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|Author:||Katkar, M.A; Rao, S.N.; Juneja, H.D.|
|Publication:||International Journal of Applied Chemistry|
|Date:||May 1, 2010|
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