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Synthesis and Spectral Studiesof Three Neodymium AcetateComplexesZhang Yugeng a & Zhao Guiwen aa Department of Applied Chemistry, University ofScience and Technology of China, Hefei Anhui,230026, People's Republic of China

Version of record first published: 23 Sep 2006

To cite this article: Zhang Yugeng & Zhao Guiwen (1995): Synthesis and SpectralStudies of Three Neodymium Acetate Complexes, Synthesis and Reactivity in Inorganicand Metal-Organic Chemistry, 25:3, 371-381

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SYNTH. REACT. INORG. MET.-ORG. CHEM., 25(3), 371-381 (1995)

SYNTHESIS AND SPECTRAL STUDIES OF THREE NEODYMIUM ACETATE COMPLEXES

* Zhang Yugeng , Zhao Guiwen

Department of Applied Chemistry, University of Science and Technology of China, Hefei, Anhui, 230026

People’s Republic of China

ABSTRACT

Three forms of neodymium acetates Nd(OAc)3-x H,O (x =1.0, 1.5, 3.0), were synthesized at different pH values. According to the IR, thermal properties and X-ray diffraction data, we found that the neodymium acetates are a l l part of the lanthanide acetate system. The photoacoustic (PA) spectra of these three Nd acetate complexes were determined and interpreted.

INTRODUCTION

The so-call ’sol-gel’ process offers new approaches to the synthesis of inorganic materials . This process has been used in the synthesis of’ superconductors, the copper and rare earth complexes as molecular precursors, and has, in general, been widely ~tudied’’~, Molecular precursors lead to the formation of a solid network through hydrolysis and polycondensation reactions. The temperature required for material processing can be notably

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Copyright 0 1995 by Marcel Dekker, Inc.

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372 ZHANGANDZHAO

4 lowered . The molecular precursors are usually inorganic salts or metal alkoxides. They can be chemically modified by chelating ligands such as organic acids or B-diketones , this giving rise to a better control of the process. The so-obtained gels are usually amorphous and, therefore, characterization of their local structure is generally very delicate. Crystalline compounds are very good models to correlate spectroscopic data with structural information.

The most common inner sphere lanthanide complexes are the compounds with carboxylic ligands . As the simplest carboxylic ligands, the spectral data of acetate cosplexes are far from complete. However, the hydrated lanthanide acetates have several coordination numbers , generally 6-10. The acetate ion may be either monodentate, bidentate, or a polymerizing ligand. In this work, we synthesized three crystalline Nd acetates, and determined and interpreted their spectral properties.

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EXPERIMENTAL

Synthesis Neodymium triacetate hydrate Nd(OAc)3*x H,O (x = 1.0, 1.5, 3.0)

single crystals Here prepared from the Nd oxide (Ndz03)-glacial acetic acid solution system. The pH value was controlled by aqueous ammonia at 4 , and slow evaporation at room temperature yielded single, red plate like crystals in about ten days. These crystals were then removed and quickly washed with distilled water and acetone. Elemental analyses of the crystalline compound were carried out. Found: C, 20.61; H, 3.47; calc. for Nd(0A~)~*1.5 H,O (11): C, 20.68; H, 3.45%.

However, when the pH value of the neodymium oxide-glacial acetic acid solution was fixed at 1, fibrous, red crystals grew from the solution at room temperature after three days. These crystals were isolated and washed by acetone. The results of elemental analyses show that crystal (I) is Nd(OAc);H20 ( found: C, 20.77; H, 3.28; calc., C, 21.23; H, 3.24% ).

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NEODYMIUM ACETATE COMPLEXES 373

Two days later a few of the fibrous (the same crystals as above) red crystals and many cubiod ( 7 x 2 x 1.5 mm ) red single crystals grew from the above solution. They were also isolated and washed with acetone. The results of elemental analyses show that crystal (111) is Nd(OAc);3H20 ( found: C, 19.11; H, 4.19; calc., C, 19.19; H, 4.00% ).

Characterization TGA experiments were perforsed on a WRT-3 thermal analyzer under

a 80ml/min N2 atmosphere. The initial and final temperatures were 50 OC and 800 OC with a heating rate of 10 'C/min. IR spectra were recorded on a Sakara-440 spectrometer at a frequency range 4000-400 cm . The samples were studied as powders dispersed in KBr pellets. The powder diffraction patterns were determined by a revolving target X-ray diffractometer. A copper target was used (X = 1.5814), with a 28 angle of 3-60°.

- 1

In the photoacoustic (PA) spectra experiment, the excitation source was a 500 W xenon lamp and the optical system was a CT-3OF aonochromator. The light source was modulated by a variable speed mechanical chopper at a frequency of 12 He. The acoustic signal was detected with the sample placed in a locally-built photoacoustic cell fitted with an EMR 10 electret microphone. After prearplification, output of the microphone was fed to a

lock-in-amplifier to which a reference signal was input from the chopper. The output signal was normalized for changes in lamp intensity using carbon-black . E

RESULTS AND DISCUSSION

Infrared Spectra The I R spectra of Nd(0Ac);x H,O ( x = 1.0, 1.5, 3.0) are very

similar. Their spectra CM be divided into three distinctive regions. High energy bands, around 3400 and 2930 cn-', correspond to ~(0-H) and u(C-H) stretching vibrations, respectively. On the

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374 ZHANGANDZHAO

low energy aide of the spectrum, three sets of bands appear corresponding to rocking p(CH,) (around 1010-1050 CI-' ) , stretching O(C-C) (around 950-960 CP-') and deformation &(COO-) (between 450 and 700 cm-') vibrations, respectively. In the middle energy range, three sets of bands contribute to the IR spectrum. One set located at 1700 or 1660 cm-' correspond to the deformation mode 6(0-H) of the water molecule. Another set is the characteristic strong band of acetate ligands appearing around 1600, 1540 Uas(COO), 1415, 1440, 1465 h(C00). The last set occurred at 1350, 1400 cm-' and corresponds to the deformation mode 6(CH,) of acetates 9 , l O .

Thermal Treatments TGA experiments of these three crystals are shown in Fig. 1.

The weight loss during the TGA experiment clearly indicates the steps of the thermal decomposition mechanism. The results from TGA shows that the three Nd acetate crystals have a different decomposition process. The TGA curves of all three Nd acetates show evidence for three stages of decomposition: the first stage between 100-200 O C indicates that the water molecules were lost in this temperature range. The second stage between 300-400 OC corresponds to the oxycarbonate, Nd,0,(C03), and the third stage corresponds to the formation of the sesquioxide Nd,O,. However, the three crystals have different decomposition temperatures, as shown in Table 1.

In the stage two region, the weight loss in the TGA curves 11,12 indicates a three step thermal deco8poaition mechanism , as

shown below:

Nd(CH,COZ), Nd(CO,)(CH,CO,) t CH,COCH, (1) NdC03(CH,COZ) + 0.5 Nd,(CO,), t 0.5 CH,COCH, Nd,(CO,), ----$ Nd,O,(CO,) + 2 CO,

( 2 )

( 3 )

The weight loss values in the third step for Nd(OAc),*H,O and 1.5 H,O are 19.0, 9.4, 7.7% and 17.1, 8.5, 8 . 5 X , respectively, and they correspond to the above-mentioned mechanism. However, the weight loss values of the third step for Nd(OAc);JH,O are 10.3,

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NEODYMIUM ACETATE COMPLEXES

Sample

Nd(OAc);H,O Nd(OAc);1.5HZ0 N~(OAC)~*~ H,O Sm(OAc);4 H,O

375

Stage I Stage I1 Stage 111

180 329,377,391 716 154 307,360,399 707

118,144,175 321,378,403 710 107,153,176 390,404 665

-----v- I1 1 kf- \ \ I \ I /

120 200 300 400

Fig. 1. The TGA Weight Loss Curves Of Nd(OAc)3-x H,O Crystals x = 1.0 (I), 1.5 (II), 3.0 (111).

Table 1. The Decomposition Temperature of Nd(OAc),*x H,O Crystals ( " C ) ,

13.7, 7.7%, which does not corresponding to the above equations. It is known that four kinds of coordination of acetate groups are generally considered, according to the literature : monodentate, chelating, bridging and polymeric. For the ionic radius decrease in the series of lanthanide ions, the acetate ions may be one bridging and two polymeric ions for the lighter lanthanide acetates and one polymeric and two chelating ions for heavier lanthanide acetates. Usually, for the heavier Ln acetates, the decomposition mechanism consists of two stepsi3:

13

Ln(CH,CO,), ---$ 1/3 Ln,0,C0,(CH3C02) t 2/3 CO, t CH,COCH, ( 4 ) 2 Ln,0,C0,(CH3C0,) 4 3 Ln20,C0, t 2 C02 t 3 CH,COCH, (5)

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376 ZHANGANDZHAO

However, we know that the different weight loss values among the three steps of Nd(0Ac);J H,O are due to the different decomposition mechanisms. We have found that Nd is a transition state in the lanthanide acetates system and the decomposition mechanism involved for N~(OAC)~-~ H,O is either that of eq(l), ( 2 )

and ( 3 ) or eq (4) and (5). By there two different mechanisms, the different weight loss peaks were produced.

In the first stage, weight loss of Nd(OAc);H,O occurred at high temperature (180 "C) , close to that of Pr(OAc);H,O (200 " C ) .

1 4 The weight loss of Nd(OAc);3 H,O occurred in three steps, close to Sm(OAc),*4 H,O at 107, 153, 176 C: A clear conclusion emerges: neodymium acetates have transition states similar to lanthanide acetates systems and they are also intermediate states in the lanthanide acetate system.

Powder X-ray Diffraction The powder x-ray diffraction data of Nd(OAc)3.x H,O (x = 1.0,

1.5, 3.0) are presented in Fig. 2. The experimental data and that of ASTM values are presented in Table 2 .

According to Table 2 , we could see that the XRD data of Nd(OAc);H20 are very similar to those of Pr(OAc);H,O (ASTM I). Their d values and intensities are very similar. These two crystals have a similar structure. This conclusion corresponds to the results of thermal properties. The data of Nd(0Ac);l.B H,O are similar to that of ASTM (11) values. So, this crystal has a triclinic system, space group Pi. But, there are no ASTM values for crystalline N~(OAC)~*J H,O. Compared with ASTM values of Nd(OAc);4H20 crystal, we find that the d values and intensity of

Nd(OAc),*3H20 are quite different from that of Nd(OAc), -4H, 0. Consequently, they have different structures and formulas and this is also in accord with its thermal properties.

Photoacoustic Spectra The PA spectra and the assignment energy levels of Nd(OAc)3.x

H,O ( x = 1.0, 1.5, 3.0) are presented in Table 3 . As a comparison,

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I

I1

10 15 20 25 28

Fig. 2. The Powder XRD Patterns Of Nd(OAc)3.x H,O Complexes

x = 1.0 (I), 1.5 (II), 3.0 (111).

Table 2. The XRD Data of Nd(OAc)3-x H,O Complexes

x = 1.0 1.5 3.0 4.0 exp . ASTM I* exp. ASTM I1 exp. ASTM I11

d, 1/10 d, 1/10 d, 1/10 d, 1/10 d, 1/10 d, 1/10

7.5 100 7.7 , 100 9.72, 100 9.4 , 100 9.8 , 20 9.3 , 100 5.40, 60 5.47, 60 7.50, 25 7.86, 25 7.50, 50 7.92, 80 5.79, 5 5 5.82, 50 8.34, 40 8.4 25 8 . 5 100 7.95, 80

4.191 18 4.20, 10 7.31, 15 6.89, 25 5.50, 25 7.56, 100 3-87, 20 3.86, 20 5.40, 25 5.42, 25 5.40, 25 5-47, 50 3-56, 95 3.56, 90 3.56, 20 3.99, 25 3.90, 45 5.34, 35 3.41, 20 3.41, 15 3.16, 18 3.60, 45

ASTM I: for Pr(0Ac) -H 0: 11: for Nd(OAc)3a1.5 H,O; 111: for Nd(0Ac);j H,O

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378 ZHANG AND ZHAO

Table 3. PA spectra And Assignment Of Different Nd Acetate Complexes (nm)*

E.L. Nd(OAc)3-H20 1.5 H,O

D5/2 359.6(sh) 359.6/39 D3/2 362.7(sh) 362.7/33 PI12 437.6/13 443.7/13 Gill2 469.7/23 468.2/22 G9/2 477.4/24 483.5/22 GS/Z 517.1/45 517.1/38

G712 529.4/56 529.4/52

G 5 f 2 585.0/ 586.5/

4

4

2

4

4

2

4

4

t2G7/2 100 100

H i l l 2 628.7/12 631.8/11 F9/2 680.7/12 682.3/22 S3/2 740.4/79 741.9/71

F712 746.5/79 744.5/68 FS/Z 798.5/80 798.5/75

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4

4

4

~ ~~~ ~

3.0 H20 Nd,O, Hydrated ion(ES)

359.8/53 360(sh) 350/34

364.4/54 364.2/70 353/74 434.9/16 437.6/15 428/16

471.7/32 474.3/35 469/11

482.4/32 483.5(sh) 476/10

519.8/58 517.1/60 512/24

531.5/63 529.4/70 521/24

583.6/ 582.9/ 576/

100 100 100

629.6/18 633.3/10 683.2/32 683.8/20 679/80

744.5/85 732/82

749.1/84 754.1/85 740/101

799.7/87 797.6/80 794/174

YC A/B = nm / relative intensity of PA absorptions

the PA spectra of Nd203 and the absorption spectrum of hydrated Nd ion were also listed in Table 3. Fig. 3 is the PA spectrum of Nd(OAc);1.5 H,O crystals.

According to Fig. 3, the transition absorptions of different J energy levels are clearly shown in their PA spectra. As shown by the data in Table 3, a difference cuong the PA spectra of different Nd complex was not notable. This is due to the special electronic structure of Ln3* ions. Because of the shielding effect of the 5 s , 5p6 electrons, there exists a weak interaction between the f-f transitions and the ligand field. We know that electrons in excited states usually relax by two processes: radiative and non-radiative. PA is a technique which monitors only the energies of non-radiation processes. But the absorption spectrum can monitor all energies of

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NEfjDYMIUM ACETATE COMPLEXES 379

s r+

8. e. Y

400 600 nm

Fig. 3 . The PA spectrum of Nd(OAc)3-1.5 H,O complex.

two relaxation process. So, the difference of intensities and positions between PA spectra and electronic absorption spectra may indicate the fluorescence properties of their energy levels 15 .

Among the energy levels of the Nd(II1) ion, the longest life-time of the excited state is for F3/2 (about 4 x S)16.

The electron in the excited state F3/2 has a large possibility of undergoing a non-radiative relaxation process due to the small energy interval between F3/2 and the next lower energy level ‘ I i5 /2 . The Nd(II1) ion is consequently a weakly fluorescent system. As the electrons are excited to the F5/2, F7/2 state, they generally relax to F3/2 by a non-radiative process (see Figure 4 ) , and then relax by both a non-radiative process and a radiative process (fluorscence). It can be seen that energy levels higher than F3/2 exist as part of a radiative relaxation process. In the PA spectra, it is shown that the intensity is weaker than that of the absorption spectra. From the data of Table 3, it can be seen that the PA relative intensities of F 5 / 2 , F7/2 are clearly

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380 ZHANGANDZHAO

4

4

4

F7/2

I15/2

1131.2

11112

IS/Z

4

4

4

4

z E 3t Fig. 4. The Relaxation Process O f The Excited State In Nd Ion.

weaker than the absorption intensity of the hydrated Nd ion, and these intensity variations were not notable in the higher energy levels.

CONCLUSION

Under different conditions, we obtained three different neodymium acetate crystals, Nd(OAc)3-x H,O (x = 1 .0 , 1.5, 3.0). Their thermal, XRD and IR properties show that Nd acetates are a transition state in the lanthanide acetate system. Nd(0Ac);J H,O crystals have the structures and properties of either lighter or heavy lanthanide acetates. Their PA spectra show that Nd acetate complexes have weak fluorescence energy levels ( 'F3/2, F512,

F7/2), and the PA spectrum clearly shows the f-f transitions of Nd3+ complexes,

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ACKNOWLEDGEMENT We thank of The National Nature Science Fund of the P. R. China

for supporting this program.

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NEODYMIUM ACETATE COMPLEXES 381

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Received: 12 March 1993 Accepted: 28 November 1994

Referee I: D. 3. Casadonte Referee 11: A. W. Apblett

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