Jyoti Bhomia , Jyoti Sharma and Yashpal Singh

Synthesis and characterization of asymmetric dinuclear aluminum compounds containing sterically hindered heterocyclic β-diketones

De Gruyter | Published online: October 12, 2016


Reactions of Al(OPri)3 with sterically hindered heterocyclic β-diketones,4-acyl-3-methyl-1-phenyl-2-pyrazolone-5-ones

have been carried out in 1:1 molar ratio in toluene to yield asymmetric dimeric compounds [RC(O)C:C(O)N(Ph)N:CCH 3] 2Al(μ-OPr i) 2Al(OPr i) 2 where 1, R=CH 3; 2, R=C 2H 5, 3, R=C 6H 5; 4, C 6H 4Cl. These derivatives have been characterized by elemental analysis, molecular weight measurements and probable structure of these compounds have been proposed on the basis of IR, 1H, 13C, 27Al NMR spectroscopies and FAB mass spectrometry. Based on the spectroscopic evidence an asymmetric structure with tetrahedral and octahedral aluminum centers has been proposed for these derivatives.


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

and to reinvestigate the structure of these compounds using IR, 1H, 13C and 27Al NMR spectroscopic as well as ESI mass spectrometric data. In this article we report the synthesis of four aluminum compounds of the type [RC(O)C:C(O)N(Ph)N:CCH 3] 2Al(μ-OPr i) 2Al(OPr i) 2. These compounds have been characterized by elemental analysis molecular weight measurements and their probable structure has been proposed on the basis of IR, 1H, 13C, 27Al NMR spectroscopies and FAB mass spectrometry.

Results and discussion

The aluminum compounds

have been synthesized by the reactions of Al(OPr i) 3 with 4-acyl-3-methyl-1-phenyl-2-pyrazolone-5-ones
in 1:1 molar ratio in refluxing toluene ( Scheme 1).

Scheme 1: Synthesis of asymmetric dinuclear aluminium compounds (1−4).

Scheme 1:

Synthesis of asymmetric dinuclear aluminium compounds (14).

All these derivatives are white (1) or yellow (24) colored solids, which are soluble in common organic solvents such as chloroform, methanol, THF, etc.

IR spectroscopy

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.

1H NMR spectroscopy

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

at 12.05−12.35 ppm is absent in all these complexes indicating the deprotonation of this group and formation of an Al−O bond. The signals due to alkyl group (RCO) protons show an upfield shift in spectra of complexes ( 12) due to the involvement of R C O group in the bonding.

Table 1:

1H, 13C, and 27Al NMR spectroscopic data of compounds


S.N 1HNMR(ppm), J in Hz 13CNMR (ppm) 27AlNMR (ppm)
Isopropoxy >C=O −CO Heterocyclicring Phenyl
Bridging Terminal
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)

25.4 (CH3)
59.7 (OCH)

24.9 (CH3)
192.1 162.8 15.9; CH3CN

105.3; C-CH3

137.5; -CN
119.5−148.8 8.0

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)

25.4 (CH3
63.6 (OCH)

23.3 (CH3)
195.3 163.8 16.6; CH3CN

105.0; C-CH3

138.3; -CN
120.7−148.3 6.0

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)

24.4 (CH3)
59.9 (OCH)

24.0 (CH3)
189.1 164.4 16.3; CH3CN

105.1; C-CH3

138.1; -CN
120.2−149.1 7.2

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)

24.4 (CH3)
62.7 (OCH)

23.2 (CH3)
187.1 163.9 16.2; CH3CN

104.8; C-CH3

137.7; -CN
119.9−148.5 8.0


Two sets of methyl and methine signals for the bridging and isopropoxy groups present in the spectra of these derivatives

appeared at 1.2−1.8, 4.32−4.55 and at 1.04−1.09, 3.80−4.05 ppm, respectively, as doublet and septet. The signals due to CH 3 proton and phenyl group protons attached to pyrazolone ring appeared at 2.22−2.55 and 7.07−8.22 ppm, respectively, and do not show any shift in the spectra of complexes as compared to their position in the spectra of free ligands.

13C NMR spectroscopy

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.

27Al NMR spectroscopy

Two singlets appeared in the spectra of

compounds centered at 8.0 and 73.0 ppm, respectively ( Figure 1) indicating the presence of tetra and hexacoordinated aluminum atoms ( Jeffrey et al., 1986). It is interesting to note that the intensity of the signal appearing at 73.00 ppm for tetrahedral aluminum is low as compared to the signal for the hexacordinated aluminum atom which appeared at 8.0 ppm. Similar 27Al NMR signals with different intensities of the two signals, i.e. hexacordinated aluminum atom and tetracoordinated aluminum have also been reported for the derivatives [(β-diketone) 2Al(μ-OPr i) 2Al(OPr i) 2] ( Jeffrey et al., 1986), [(acac) 2Al(μ-OPr i) 2Al(OPr i) 2] ( Dhammani et al., 1997) and [C 6H 4O{CH=N(C 6H 5)}] 2Al(OPr i) 2Al(O-G-O) where G=(CH 2) 2, CH 2CH(CH 3), CH 2CH(C 2H 5),CH(CH 3)CH(CH 3),(CH 2) 5, C(CH 3) 2 CH 2CH(CH 3 ), (CH 2) 6 ( Sharma et al., 2002). One explanation for the lower intensity and broadening of 27Al NMR signal for tetrahedrally coordinated aluminum center may be that there is an exchange between terminal and bridging isopropoxy groups attached to tetrahedral aluminum atom.

Figure 1: 27Al NMR of  (1).

Figure 1:

27Al NMR of


FAB mass spectrometry

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.

Table 2:

FAB mass fragmentation mode of compound


Compound m/z
C38H54N4O8Al2 755.36
C38H53N4O8Al2 754.35
C38H52N4O8Al2 753.35
C37H51N4O8Al2 737.37
C25H41N4O8Al2 568.30
C25H40N4O8Al2 567.30
C22H34N4O7Al2 485.24


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).

Synthesis of the aluminum compounds

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.

Table 3:

Synthetic, physical, and analytical data of compounds


Compound R Empirical formula, (yield%) Color physical State (mp °C) Reactants g(mmol) % Analysis found (calcd) Molecular weight found (calcd)
Al(OPri)3 LHR C H N Al
1 CH3


White solid (230) 1.69




(59. 99)




2 C2H5

C38H54N4 O8Al2

Light yellow solid (179) 1.62








3 C6H5


Light yellow solid (240) 1.44




(65.39 )




4 4-C6H4Cl


Light yellow solid (219) 1.33









Synthesis of (1)

(1.79 g ; 8.27mmol) was added in toluene solution (~50 mL) of Al(OPr i) 3 (1.69 g; 8.27 mmol) and the solution was refluxed on a fractionating coloumn for about 3 h. The isopropanol in the reaction was fractionated out azeotropically with toluene. The product was soluble in toluene. After stripping off the solvent under reduced pressure, a white colored solid was obtained in good yield, which was purified by recrystallization from a mixture of toluene and hexane and on analysis was found to have C, 59.70; H, 7.78; N, 7.46; Al, 7.50. Calc. for C 36H 50N 4O 8Al 2: C, 59. 99; H, 6.99; N, 7.77; Al, 7.48%.


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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Received: 2015-10-7
Accepted: 2016-9-7
Published Online: 2016-10-12
Published in Print: 2016-12-1

©2016 Walter de Gruyter GmbH, Berlin/Boston

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