Reactions of Al(OPri)3 with sterically hindered heterocyclic β-diketones,4-acyl-3-methyl-1-phenyl-2-pyrazolone-5-ones
Acylpyrazolones constitute an interesting class of heterocyclic β-diketones, containing a pyrazole ring fused to a chelating arm (Marchetti et al., 2005). The presence of pyrazole moiety stabilizes the metal derivatives by creating an extensive π-conjugate system. Acylpyrazolone ligands have also been used as advantageous metal extractants or chelating reagents in the spectroscopic determination of metals in traces (Zolotov and Kuzmin, 1977). The formation of the metal complexes with acylpyrazolones is also applied for the separation of elements with similar properties, i.e. lanthanides, coinage metals, actinides, early transition metals, etc. (Nishihama et al., 2001). Al(OPri)3 exists in trimeric form (Bradley et al., 1978). The structure of [(OPri)2Al(β-diketone)] was reported earlier to be symmetrical dimer having two five-coordinated aluminum atoms (Mehrotra and Mehrotra, 1961). However, during the last decade reinvestigations on the structure of [(OR)2Al(β-diketone)] using X-ray crystallography revealed that the structure of (OR)2Al(β-diketone) is influenced by the steric hindrance of the alkyl (R) group of alcohol as well as β-diketones. Compounds with least sterically hindered β-diketones, i.e. acetylacetone (R=CH3) are trimeric when acac is replaced by Et2acac then compound is dimeric. Moreover, with the increase in the steric hindrance of alkyl group the oligomerization of the compound decreases. In the compounds where R=i-C3H7, n-C4H9 group are dimeric but when R=Ph3Si group the compound is monomeric (Jeffrey et al., 1986). In view of the above, it is assumed that it would be worthwhile to synthesize aluminum compounds of sterically hindered heterocyclic β-diketones 4-acyl-3-methyl-1-phenyl-2-pyrazolone-5-ones
The aluminum compounds
All these derivatives are white (1) or yellow (2−4) colored solids, which are soluble in common organic solvents such as chloroform, methanol, THF, etc.
A broad band present in the spectra of free ligands in the range 3445−3600 cm−1 due to enolic ν(OH) group is absent in the spectra of compounds 1−4 indicating the deprotonation of -OH group during complexation. This is supported by the appearance of a new band ν (Al-O) in the range 450−840 cm−1. The stretching bands due to ν(C=N), ν(C-N ), and ν(=CH ) groups attached to the phenyl group have been observed at 1560−1585, 1495−1520, and 1580−1610 cm−1, respectively, in the spectra of all the compounds. A new band appeared at 1000−1009 cm−1 in the spectra of all the derivatives which may be attributed to ν(C-O) of the isopropoxy group.
The 1H NMR signals of these compounds are summarized in Table 1. The signal due to enolic proton observed in the spectra of the free
|S.N||1HNMR(ppm), J in Hz||13CNMR (ppm)||27AlNMR (ppm)|
|1||1.21(d, OCH(CH3)2), 3.80−3.83(sep, OCH(CH3)2), 1.09 (d, OCH(CH3)2), 4.32(sep, OCH(CH3)2J=6.00), 1.90 (s,CH3CN), 2.55(s, COCH3), 7.09−8.22 (m, phenyl)||64.8 (OCH)
|2||1.16(d) OCH(CH3)2, 3.99(sep, OCH(CH3)2), 1.08(d,OCH(CH3)2), 4.35 (sep,OCH(CH3)2J=5.80), 1.19(s,CH3CN), 2.38(s, COCH3), 1.18(t,CH2CH3) 7.09−8.22 (m, phenyl)||64.6 (OCH)
|3||1.16(d, OCH(CH3)2), 4.05(sep, OCH(CH3)2), 1.04(d, OCH(CH3)2), 4.51(sep, OCH(CH3)2J=6.40), 1.92(s,CH3CN), 2.22(s, COCH3), 7.20−7.88 (m, phenyl)||64.9 (OCH)
|4||1.16(d, OCH(CH3)2), 3.91(sep, OCH(CH3)2), 1.04(d, OCH(CH3)2), 4.55(sep, OCH(CH3)2J=6.30), 1.93(s,CH3CN), 2.24(s, COCH3), 7.14−8.12 (m, phenyl)||64.7 (OCH)
Two sets of methyl and methine signals for the bridging and isopropoxy groups present in the spectra of these derivatives
13C NMR spectroscopic data of these derivatives are summarized in Table 1. The spectra of all these derivatives exhibit signals for terminal and bridging isopropoxy group. Methyl carbon of bridging and terminal groups appeared at 24.41−25.99 and 23.22−24.9 ppm, respectively. The methine carbons of the bridging and terminal appeared at 64.67−64.92 and 59.70−63.60 ppm, respectively. The signals due to >C=O and >C-O groups appear at 187.19−195.64 and 163.79−164.89 ppm, respectively, and show a small downfield shift as compared to its position in the spectra of ligands which indicates the delocalization of electron in the quasi-aromatic ring during chelation. The signals due to C=C and C=N appear at 104.84−105.96 and 137.70−138.32 ppm, respectively. The methyl and phenyl carbon atoms attached to the heterocyclic ring do not show any shift and appeared at 15.95−16.60 and 119.57−149.15 ppm, respectively.
Two singlets appeared in the spectra of
The FAB mass spectrum of one of the asymmetric aluminum compound (2) recorded, which shows the dimeric nature of the compound. The mass fragmentation pattern of compound (2) is summarized in Table 2.
In view of the presence of bridging and terminal isopropoxy groups and four and six coordinated aluminum atoms an asymmetric structure as shown in Scheme 1 is proposed for these compounds. This structure is in contrast to the symmetrical structure proposed by us earlier (Singh and Rai 1982).
Based on the spectroscopic data an asymmetric dimeric structure containing four and six coordinated aluminum atoms has been proposed for these compounds. This structure is similar to the structure of earlier reported simple β-diketone compounds [(OR)2Al(β-diketone)]2 (Jeffrey et al., 1986). It appears that the steric hindrance of these heterocyclic β-diketones does not have any effect on the dimeric asymmetrical structure which is probably due to the tendency of the aluminum atom to acquire a coordination number six as maximum.
All the reactions have been carried out under anhydrous conditions. Solvents used in the synthesis have been dried by standard procedures (Perrin et al., 1980). Heterocyclic β-diketone ligands (Jensen, 1959) and aluminum isopropoxide have been synthesized by literature method (Mehrotra and Singh, 1997). Aluminum was estimated as oxinate (Vogel, 1989). Isopropyl alcohol liberated during the reaction was removed azeotropically. Molecular weights were determined ebullioscopically using Beckman thermometer. NMR spectra were recorded on a JEOL 400MHz spectrometer (Jeol coorporation, Akishima, Tokyo, Japan).
1H and 13C spectra have been recorded in CDCl3 using TMS as an internal reference whereas 27Al NMR spectra have been recorded in benzene solution using aluminum nitrate as an external reference. IR spectra were recorded on an 8400 SHIMADZU FT-IR spectrophotometer (Kyoto, Japan) as nujol mull on KBr cell in the range 4000−400 cm−1.The FAB mass spectrum of one representative compound was recorded on JEOL-Sx 102/Da-600 mass spectrometer (JEOL Corporation, Akishima, Tokyo, Japan).
All the aluminum derivatives have been synthesized by a similar route and hence the synthesis of only one representative compound is discussed in detail and the synthetic and analytical details of others are summarized and shown in Table 3.
|Compound||R Empirical formula, (yield%)||Color physical State (mp °C)||Reactants g(mmol)||% Analysis found (calcd)||Molecular weight found (calcd)|
|White solid (230)||1.69
|Light yellow solid (179)||1.62
|Light yellow solid (240)||1.44
|Light yellow solid (219)||1.33
One of the authors (Ms Jyoti Bhomia) is thankful to U.G.C. Delhi for financial support in the form of JRF, MNIT Jaipur for NMR spectral studies and USIC, university of Rajasthan, Jaipur, India, for FAB mass study.
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