HUANG Shen Sysoev Sergey Viktorovich Stnikov Pvel Aleksndrovich Pervukhin Ntli Viktorovn, Korolkov Ili Viktorovich, Mosygin Svetln Andreevn

a (Nikolaev Institute of Inorganic Chemistry, SB RAS, Novosibirsk, Russia)

b (Novosibirsk State Technical University, Novosibirsk, Russia)

1 INTRODUCTION

Coordination compounds of rare earth elements(REE) with β-diketonates are used as materials for electro- and photoluminescent devices[1-3].In addition, volatility of these compounds allows them to be used as precursors for metal-organic chemical vapor deposition (MOCVD) of inorganic coatings.Trisdipivaloylmethanates of REE (Ln(dpm)3) are the most widely used precursors for MOCVD due to their technological properties[4].Promising technological properties of Ln(dpm)3give rise to crystallographic and structure investigations of these complexes since the 70s of last century.The tris-dipivaloylmethanates of REE, with the exception of Tm,Yb and Lu complexes, were crystallographically characterized[5,6].It was demonstrated that the majority of the complexes (from La(dpm)3to Dy(dpm)3) are crystallized in space group P21/n when grown from hexane.Complexes (from Tb(dpm)3to Er(dpm)3) grown from vacuum are crystallized in space groupIt is shown[7]that Tm(dpm)3and Yb(dpm)3are crystallized in space group P1.The first structural investigation of Pr trisdipivaloylmethanate[5]showed that this compound is composed of isolated dimeric molecules [Pr(dpm)3]2.Further structural studies ([La(dpm)3]2[7],[Nd(dpm)3]2, and [Ho(dpm)3]2[8], [Sm(dpm)3]2[9],[Eu(dpm)3]2[10], [Gd(dpm)3]2[11], [Tb(dpm)3]2[12])showed that the majority of the crystals of these complexes are composed of dimeric molecules.Recently, it has been shown that both dimeric[Dy(dpm)3]2and monomeric Dy(dpm)3molecules may be structural units of tris-dipivaloylmethanate Dy(III) crystal[13].Only the crystals of Er(dpm)3[14,15]and Lu(dpm)3[16]are formed by monomers.To date,structures of Tm(dpm)3and Yb(dpm)3have not yet been determined.The present work deals with the studies on the structures of Tm(dpm)3and Yb(dpm)3and the investigation of volatility of a series of lanthanoid tris-dipivaloylmethanates.

2 EXPERIMENTAL

2.1 Materials and instruments

All chemicals and solvents commercially available were analytical reagents and used without further purification.Elemental analyses (C, H) were carried out on a CHNS-vario MICRO cube.The melting point is set on a hot plate table.Thermogravimetric studies were performed using a TG 209 F1 Iris?(NETZSCH) thermobalance.Experimental conditions: sample weight ～7 mg, aluminum crucible,helium atmosphere, gas flow rate 60 mL/min,heating rate 10 deg/min.The results were processed using the standard Proteus Analysis software[17].XRD measurements were performed on a Bruker D8 Advance X-ray diffractometer using a CuKα radiation (λ = 0.154 nm) at 35 kV and 20 mA.Simulated XRD patterns were calculated with the SHELXTL-XP program using the single-crystal data.

2.2 Synthesis

The complexes were synthesized according to the reported procedure[7].Initial reagents were crystal hydrates of the chlorides of lanthanoids, with the purity not less than 99%, dipivaloyl methane from DALKHIM company, with the purity not less than 99%, and NaOH (analytically pure grade).In a glass with 20 mL H2O, 0.75 g (2 mmol) of TmCl3·6H2O was dissolved, and then 0.24 g (6 mmol) of NaOH dissolved in H2O was added.The resulting mixture containing the precipitate was twofold diluted with acetone.The gelatinous precipitate was separated with a paper filter, washed with a mixture of water and acetone (1:1), and then with acetone.The precipitate was added to 1.1 g (6 mmol) of Hdpm placed in an evaporation dish.After drying, the solid powder was sublimed in a vacuum gradient furnace(Р = 10-2torr, Т = 200 ℃).The product was 1.15 g[Tm(dpm)3]2, with the yield of 80%.The complexes are well soluble in the majority of organic solvents.The melting point of Tm(dpm)3is within the 174～175 ℃ range, while for Yb(dpm)3it is 170～171оС,which agrees with the data 170～171.5 and 165～166 ℃[18].For Tm(dpm)3, found (%): С, 55.4; H, 8.2.For TmC33H57О6, calculated (%): С, 55.2; H, 8.0.For Yb(dpm)3, found (%): C, 54.7; H, 8.0.For YbC33H57О6, calculated (%): С 54.8, H 7.9.

Tris-dipivaloylmethenates of rare earth elements do not change for a long time when stored under usual conditions.Their volatility is conserved, and no changes are observed in the IR spectra and in the diffraction patters of powders, which is the evidence of their stability and the possibility to use them as precursors for MO CVD.

2.3 Termogravimetric study

Results of the thermogravimetric measurements are depicted in Fig.1.It is shown that the volatility of lanthanide dipivaloylmethanates increases from[La(dpm)3]2to Yb(dpm)3.This can be explained by the dimeric structure of the complexes of light lanthanides and the monomeric structures of complexes of heavy lanthanides.

Fig.1. Mass loss curve of Ln(III)dipivaloylmethanates from La to Yb

2.4 X-ray crystallographic study

X-ray Phase Analysis of polycrystals was carried out with a Shimadzu XRD-7000 diffractometer(CuKɑ radiation, Ni filter, angle range 5 ～ 50°2θ, step 0.03° 2θ, accumulation 1 s).The powder was ground and deposited on the polished side of a standard quartz cell coated with a thin layer of vaselin oil.A sample of polycrystalline silicon prepared in the same manner was used as an external reference.The diffraction patterns of Tm(dpm)3and Yb(dpm)3are indexed completely over the single crystal data,which means that the samples are single-phase.

2.5 X-ray diffraction analysis

Single crystals of Tm(dpm)3were obtained in vacuum at the temperature gradient of 165～170 ℃.The single crystals of Yb(dpm)3were obtained under slow evaporation of the solution of the complex in heptane in the dry atmosphere.

X-ray intensity data were collected on a Bruker APEX DUO CCD diffractometer at 200(2) K using standard techniques (ω- and ?-scans of narrow frames) and corrected for absorption effects(SADABS)[19].The structure 1 was solved by direct methods[20]and refined by full-matrix least-squares on F2using the SHELX97 program set[21].Compounds 1 and 2 are isostructural and therefore, a model of 1 was used in refinement of structure 2.The crystallographic data and the details of single-crystal diffraction experiments are as follows: C33H57O6Tm(1), Mr= 718.72, orthorhombic, Pmn21, a =17.7509(8), b = 10.5805(4), c = 9.8380(4) ?, V =1847.7(1) ?, Z = 2, Dc= 1.292 g/cm3, μ = 2.437 mm-1,crystal size 0.26 × 0.12 × 0.08 mm3, θ = 2.24～27.10°, S = 1.066, R = 0.0173, wR = 0.0415 for 4221 observed reflections (I ＞ 2σ(I)), absolute structure parameter = 0.687(7); C33H57O6Yb (2), Mr= 722.83,orthorhombic Pmn21, a = 17.7272(6), b =10.5882(3), c = 9.8440(3) ?, V = 1847.7(1) ?3, Z = 2,Dc= 1.299 g/cm3, μ = 2.566 mm-1, crystal size 0.22 ×0.12 × 0.05 mm3, θ = 1.92～30.55°, S = 0.973, R =0.0277, wR = 0.0544 for 5820 observed reflections(I ＞ 2σ(I)), absolute structure parameter = 0.005(9).All non-hydrogen atoms were refined anisotropically.In the structures of complexes 1 and 2, partial disordering of t-Bu-groups is observed.The hydrogen atoms were calculated by geometrical methods.Selected bond lengths and bond angles for complexes 1 and 2 are listed in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°) for Compounds 1 and 2

3 RESULTS AND DISCUSSION

3.1 Crystal structures of Tb(dpm)3 and Yb(dpm)3

Compounds 1 and 2 are isostructural to complexes Er(dpm)3[14,15]and Lu(dpm)3[16].The crystals of 1 and 2 consist of monomeric Ln(dpm)3units (Ln =Tm(III), Yb(III)), and in this unit there are two crystallographically independent chelate rings (Fig.2).The oxygen atoms coordinated to the central atom Ln form a distorted trigonal prism (Table 1), and this coordination polyhedron is the same as that found in Er(dpm)3[14,15]and Lu(dpm)3[16].The packing of the units in the crystal can be described as an almost trigonal array within layers approximately perpendicular to [010], as shown in Fig.3.The bond lengths in the dpm-ligands are as follows: O–C 1.257(3)～ 1.269(4) ?, С(sp2)–С(sp2) 1.397(4)～1.403(5) ?, С(sp2)–С(sp3) 1.535(3)～1.546(3) ?,С(sp3)–С(sp3) 1.28(2)～ 1.633(6) ? for 1; O–C 1.257(4)～ 1.272(6) ?, С(sp2)–С(sp2) 1.388(4)～1.399(6) ?, С(sp2)–С(sp3) 1.540(5)～1.544(6) ?,С(sp3)–С(sp3) 1.36(4)～ 1.593(9) ? for 2.The chelate rings are nearly planar, and the angles between the two chelate rings are about 115.2° (1)and 115.1° (2).The bond lengths and bond angles in compounds 1 and 2 agree well with the known literature data[22].

Fig.2. Structure of the units of Ln(dpm)3 (Ln = Tm(III) (1), Yb(III) (2)).The atoms are represented by 50% probability thermal ellipsoids (Hydrogen atoms are not given)

Fig.3. Molecular packing along the b axis in the structure of Ln(dpm)3 (Ln = Tm(III) (1), Yb(III) (2))

4 CONCLUSION

In summary, this work describes two structures:tris-dipivaloylmethanates of Tm(III) and Yb(III) and completes structural characterization of lanthanide dipivalylmethanate complexes.Thermogravimetry study showed that volatility of lanthanide dipivalylmethanates decreases from Lu(dpm)3to [La(dpm)3)]2that may be explained by the tendency to dimerisation for light REE complexes.

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