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Molecular structures of volatile Ce(IV) tetrafluoroisopropoxide complexes with TMEDA and diglyme. CVD experiments
Polyhedron 21 (2002) 1985 /1990 www.elsevier.com/locate/poly
Molecular structures of volatile Ce(IV) tetrafluoroisopropoxide complexes with TMEDA and diglyme. CVD experiments Stephane Daniele a, Liliane G. Hubert-Pfalzgraf a,*, Monique Perrin b a
b
IRC, Universite´ Claude Bernard-Lyon 1, 2 avenue A. Einstein, 69629 Villeurbanne cedex, France Laboratoire de Cristallographie, Universite´ Claude Bernard-Lyon 1, 69622 Villeurbanne cedex, France Received 8 January 2002; accepted 7 March 2002
Abstract The structures of the cerium tetrafluoroisopropoxide adducts with TMEDA (1) and diglyme [Me(OC2H4)2OMe] (2) were established by single crystal X-ray diffraction (TMEDA /tetramethylethane-1,2-diamine). The cerium atom is hexa- and ˚ ] and are associated heptacoordinated for 1 and 2, respectively. The Ce/O(hfip) bond distances are quite short [2.115(5) /2.152(6) A to large Ce /O(hfip) angles (/1568). Both complexes display a range of short F C contacts, the shortest ones having values of ˚ for 1 and 2, respectively. Decomposition experiments were achieved in a cold-wall reactor. The films deposited on 3.754 and 4.060 A glass substrates were characterized by X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and UV /Vis spectroscopy. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Cerium; Fluoroalkoxides; Structures; Complexes; MOCVD; Films
1. Introduction Cerium oxide is a candidate for various areas of technological applications due to high chemical stability and insulating properties [1], interesting optical properties [2] and lattice parameters close with high Tc superconductors and silicon, and thus its potential value for buffer layers [3]. Various methods have been used for deposition of ceria thin films. Chemical routes, namely sol /gel processing and metal-organic chemical vapor phase deposition (MOCVD) are quite flexible and cheaper than physical methods [4]. MOCVD processes have a high potential for CeO2 layers having the quality required for microelectronics and several types of volatile cerium derivatives, mostly b-diketonates have been used for the growth of pure or doped [5] ceria layers. After the use of Ce(thd)4 (thd /2,2,6,6-tetra-
* Corresponding author. Tel.: /33-4-7244-5322; fax: /33-4-72445399 E-mail address: [email protected]_lyon1.fr (L.G. HubertPfalzgraf).
methylheptane-3,5-dionate) [5,6] and of fluorinated Ce(IV) or Ce(III) derivatives such as Ce(fdh)4 (fdh / 6,6,6-trifluoro-2,2-dimethyl-3,5-hexanedionate) [7], a second generation of precursors was based on adducts such as Ce(fdh)3(o -phen) [8]; [Ce2(fod)6(tetraglyme)] [9a] (fod /heptafluoro-7,7-dimethyl-4,6-octanedionate) and more recently [Ce2(etbd)6(tetraglyme)], [NH4][Ce(etbd)4] (etbd /1-ethoxy-4,4,4-trifluorobutane-1,3-dionate) [9b] and Ce(hfac)3 complexes (hfac /hexafluoroacetylacetonate) with various glymes [10]. Fluorinated alkoxides are another class of volatile derivatives and have been used for either oxide or fluoride films for gallium [11], barium [12], lanthanides or transition metals such as yttrium [13], zirconium [14] and are available for cerium [15]. We wish to report herein the solid state characterization of Ce(IV) hexafluoroisopropoxide adducts namely Ce(hfip)4(TMEDA) (1) and Ce(hfip)4(diglyme) (2), the polydentate Lewis bases being used for stabilization of Ce(hfip)4 toward hydrolysis. These volatile complexes were used as cerium sources in a cold wall CVD reactor. Films were characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and X-ray diffraction.
0277-5387/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 5 3 8 7 ( 0 2 ) 0 1 0 0 5 - 7
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S. Daniele et al. / Polyhedron 21 (2002) 1985 /1990
2. Experimental All manipulations were routinely performed under nitrogen atmosphere using Schlenk tubes and vacuum line techniques with dried and distilled solvents. Hexafluoroisopropanol (hfipH) (Aldrich) was stored over molecular sieves and used as received. The Ce(IV) hexafluoroisopropoxide adducts were obtained as previously reported by us [15]. XPS experiments were performed with an Escalab 200R (VG Scientific) spectrometer using the monochromated Ka Al radiation as excitation source. SEM images were collected on a Hitachi S800 spectrometer. XRD data were collected on a Siemens D 5000 diffractometer using the Cu Ka radiation.
2.1. Crystallography of 1 and of 2 Suitable crystals of 1 were grown directly from the reaction medium, those of 2 were obtained by recrystallization in petroleum ether. The crystals were fixed at a glass fiber with paratone. Data collection was made on a Nonius CCD diffractometer at 123 K. Three thousand seven hundred thirty-eight reflections were measured for 1 and 12 721 reflections of which 8787 were unique were measured for 2. Cell parameters were refined using DENZO [16]. Compound 1 crystallizes in the monoclinic system (C 2/c group) whereas 2 crystallizes in the triclinic system (P/1¯ group). The structures were solved by direct methods and refined by least-squares using SHELXTL [17]. All non-H atoms were refined anisotropically giving the values of R listed in Table 1. Hydrogen atoms position were calculated at theoretical positions and refined isotropically riding on C. The distribution of the C /F bond lengths is quite large due to disorder phenomena which are also illustrated by high thermal factors and a large residual density.
2.2. Decomposition experiments Compounds 1 and 2 were sublimed and decomposed in a hot-wall, dynamic vacuum reactor with argon flow. The precursor reservoir was charged with 200/300 mg of compound and maintained at about 150 8C; the furnace temperature was 550 8C and the pressure was 5 /103 bar, the decomposition took place over a 4/8 h period. About 15% in weight of residue remained in the source reservoir after this time. After removing the residue, a flow of water-saturated argon (water bubbler at room temperature) was introduced and the furnace was maintained at 550 8C for over 8 h.
Table 1 Crystal data and structure refinement for Ce(hfip)4(TMEDA) (1) and Ce(hfip)4(diglyme) (2)
Chemical formula Formula weight Temperature (K) ˚) Wavelength (A Crystal system Space group ˚) a (A ˚) b (A ˚) c (A a (8) b (8) g (8) ˚ 3) V (A Z Dcalc (Mg m 3) Absorption coefficient (mm 1) Range for data collection (8) Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I 2s (I )] R indices (all data) Largest difference ˚ 3) peak and hole (e A
1
2
C18H20CeF24N2O4 924.48 123(2) 0.71073 monoclinic C 2/c 10.026(2) 17.622(4) 17.563(4) 90 91.22(3) 90 3102.3(11) 4 1.979 1.641
C18H18CeF24O7 942.44 123(2) 0.71073 triclinic P/1¯/ 10.003(2) 10.261(2) 16.791(3) 88.39(3) 83.50(3) 65.71(3) 1560.5(5) 2 2.006 1.638
2.31 /28.33
1.22 /30.09
full-matrix leastsquares on F2 3738/0/224
full-matrix leastsquares on F2 8787/0/451
1.034 R1 0.0795, wR2 0.1952 R1 0.1177, wR2 0.2178 2.348 and 1.470
0.765 R1 0.0539, wR2 0.1358 R1 0.1005, wR2 0.1748 0.885 and 1.005
3. Results and discussion 3.1. Synthesis and molecular structures of Ce(hfip)4(TMEDA) (1) and of Ce(hfip)4(diglyme) (2) The alcohol exchange reaction between cerium isopropoxide Ce2(OPri )8(Pri OH)2 and hexafluoroisopropanol (hfipH /OHCH(CF3)2) (1:8 stoichiometry) in THF at room temperature afforded a cerium hexafluoropropoxide THF adduct Ce(hfip)4(THF)2. The high Lewis acidity of fluoroalkoxides makes, however, this adduct extremely prone to hydration. Ligand exchange with polydentate Lewis bases such as TMEDA and diglyme MeO(CH2CH2O)2Me provided more stable adducts. Those were characterized (FT-IR, 1H and 19F NMR) by comparison with authentic samples as previously reported [15]. Their FT-IR spectra are characterized by very intense broad absorption bands around 1290 /1100 cm 1 corresponding to the C /F vibrations. Additional sharp bands due to the coordinated Lewis base are present as well but they are shifted downfield with respect to the free ligand. The absorption bands of the Ce /OR bonds are observed below 600 cm 1. The ligand exchange reactions between Ce(hfip)4(THF)2 and chelating ligands afforded derivatives
S. Daniele et al. / Polyhedron 21 (2002) 1985 /1990
being more air stable, the order of stability being Ce(hfip) 4 (TMEDA) B/ Ce(hfip)4(diglyme) / Ce(hfip)4(bipy)2. All these compounds are volatile and sublime unchanged (no loss of the Lewis base occurs during volatilization). Compound 1 is more favorable than 2 in terms of melting point (155 vs. 234 8C) whereas their volatility properties are comparable (sublimation around 70 8C under 104 mm of Hg). Their solubility properties make them suitable as precursors for liquid injection CVD. By contrast to barium and yttrium derivatives with various ligands [4], no rationale and predictive trends could be established by analysis of the relationship between sublimation temperatures and molecular weight data. However, for derivatives having similar formula, the volatility appears to be in favor of the hfac derivatives with the ranking hfac /hfip / fdh /thd /fod. Ce(hfip)4(diglyme) has a volatility comparable to that of Ce(hfac)3(dedg) (dedg /diethyldiglyme) [10]. Compounds 1 and 2 were characterized by low temperature single crystal X-ray diffraction. Their structures are depicted in Figs. 1 and 2 for 1 and 2, respectively. Selected bond lengths and angles are collected in Tables 2 and 3. The coordination polyhedron of the hexacoordinated cerium atom is quite distorted with angles ranging from 69.6(3)8*/the bite angle of the bidentate ligand*/to 170.5(3)8. The Ce /O ˚ , the distances have values of 2.115(5) and 2.152(6) A shortest ones being trans to the Ce /N coordination ˚ . Data on Ce /N bond distances are bonds of 2.624(7) A scarce. The Ce /N coordination bond observed for 1 are shorter than those reported for [Ce2(Oi Pr)6(m,h2OC2H4NMeC2H4NMe)2] but where the metal is heptacoordinated [18]. The shortest intramolecular cerium / fluorine distance corresponds to Ce F(210) with a
1987
Fig. 1. Molecular structure of Ce(hfip)4(TMEDA) (1) showing the atom numbering scheme (thermal ellipsoids at 20% probability). F(210), F(212), F(111), F(102), F(101) have Ce F interactions in ˚. the range 3.745 /4.332 A
˚ . Four other Ce F /C distances are value of 3.754 A ˚ , each of the fluorfound in the range 4.128 /4.332 A oisopropoxide ligand providing either one or two
Table 2 ˚ ) and angles (8) for Ce(hfip)4(TMEDA) (1) Selected bond lengths (A Bond lengths Ce O(1) Ce O(2) Ce N(1)
2.115(5) 2.152(6) 2.624(7)
Bond angles O(1) Ce O(1)#1 O(1) Ce O(2) O(1) Ce N(1) O(2) Ce N(1) O(2) Ce N(1)#1 O(1)#1 Ce O(2) O(2)#1 Ce O(2) O(1) Ce N(1)#1 O(2)#1 Ce N(1) N(1)#1 Ce N(1) C(1) O(1) Ce C(2) O(2) Ce
108.7(3) 92.3(2) 90.9(2) 87.1(2) 85.1(2) 93.2(2) 170.5(3) 160.4(2) 85.1(2) 69.6(3) 166.5(7) 156.7(7)
Symmetry transformations used to generate equivalent atoms: #1 x1, y , z3/2.
Fig. 2. Molecular structure of Ce(hfip)4(diglyme) (2) showing the atom numbering scheme (thermal ellipsoids at 20% probability). F(2), F(6), F(7), F(8), F(11), F(13), F(17), F(19) and F(23) have Ce F ˚. interactions in the range 4.060 /4.309 A
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S. Daniele et al. / Polyhedron 21 (2002) 1985 /1990
Table 3 ˚ ) and angles (8) for Ce(hfip)4(diglyme) (2) Selected bond lengths (A Bond lengths Ce O(1) Ce O(2) Ce O(7) Ce O(3) Ce O(4) Ce O(5) Ce O(6)
2.580(4) 2.581(5) 2.585(5) 2.132(4) 2.139(5) 2.127(4) 2.122(4)
Bond angles O(1) Ce O(2) O(1) Ce O(3) O(1) Ce O(7) O(1) Ce O(4) O(1) Ce O(5) O(1) Ce O(6) O(2) Ce O(3) O(2) Ce O(4) O(2) Ce O(5) O(2) Ce O(6) O(2) Ce O(7) O(3) Ce O(4) O(3) Ce O(5) O(3) Ce O(6) O(3) Ce O(7) O(4) Ce O(5) O(4) Ce O(6) O(4) Ce O(7) O(5) Ce O(6) O(5) Ce O(7) O(6) Ce O(7) C(3) O(3) Ce C(4) O(4) Ce C(5) O(5) Ce C(6) O(6) Ce
62.54(16) 78.49(16) 62.02(15) 138.72(17) 136.41(17) 83.43(18) 89.26(17) 76.94(17) 160.79(18) 82.39(18) 124.26(16) 94.18(19) 97.50(18) 161.92(19) 85.30(18) 84.61(18) 99.47(19) 158.74(17) 95.55(19) 74.42(17) 86.22(18) 160.2(4) 174.2(5) 149.4(5) 170.3(5)
fluorine for these interactions. Activation of carbon / fluorine bonds by metallic species has been observed in a number of complexes giving short M F /C interactions [19]. The shortest distance is much longer than the Ce /O bond lengths or the sum of the van der Waals ˚ ) and than the interactions found for radii (/3.20 A ˚ ) [12]. The barium derivatives (usually 3 /3.20 A Ce F(210) interaction might however account for the opening of the O(1)/Ce /O(1)# angle [108.7(3)8]. Some intramolecular F N interactions involving F(111), ˚ ). These F(112), F(200) are also observed (4.137 /4.259 A interactions are much longer than the Ce /F bond distances reported for cerium fluoride derivatives such as NH4[CeF2(PO4)] for instance [20]. The molecular structure of the Ce(hfip)4(diglyme) adduct 2 is based on a heptacoordinated metal center since all oxygens of the diglyme are involved in coordination. Such a coordination number was observed for Ce4O(OPri )14 [21]. The Ce /O(hfip) bond ˚] lengths are similar to those of 1 [2.122(4) /2.139(5) A and are much shorter than the Ce /O bond distances ˚ av.]. The stereoinvolving the diglyme ligand [2.582 A
chemistry around the metal corresponds to a trigonal prism capped by one of the oxygen O(1) of the glyme. The metric parameters of the diglyme ligand are in agreement with the literature body [9,10]. The shortest ˚ but Ce F distance [with F(17)] has a value of 4.060 A ˚ additional Ce F distances having values below 4.31 A are observed with eight other fluorine atoms. The Ce /OR bond distances are, for both complexes, quite short, although longer than those observed for ˚ av. non-fluorinated terminal Ce /OR alkoxides (2.088 A i for [Ce2(OPr )8(Pri OH)2]) [22]. They are shorter than those observed for the [Ce(hfip)6]2 anion [2.183(5) / ˚ ]. The Ce /O /(hfip) angles display values of 2.208(5) A [156.7(7)8 /166.5(7)8] and [149.4(5)8/174.2(5)8] for 1 and 2, respectively. Those are large by comparison to most other hexafluoroisopropoxide derivatives [143.3(5)8/ 156.5(5)8 for [Ce(hfip)6]2] and nearly comparable to those observed for non-fluorinated OR ligands which are considered to be more prone to p-bonding [23]. These large Ce /O /C angles do, however, not prevent interaction of the fluorine atoms with the metal center. These interactions shield the positive charge of the metal and reduce its electrophilic character. Its is noticeable that metal/fluorine interactions are generally absent in the solid state structures of fluorinated b-diketonate complexes in which the coordination number of the metal is usually higher than for fluorinated alkoxide adducts [10]. 3.2. CVD experiments Deposition experiments were achieved at various temperatures using an horizontal cold wall reactor under dynamic vacuum (4 /5 /103 bar). Compounds 1 and 2 were transported into the vapor phase at 120/ 140 8C. No deposits were observed below 500 8C and depositions were thus conducted at a temperature of 550 8C for 1 and 2. Films were homogeneous, white and displayed poor mechanical properties. They were characterized by XPS in the range 0/1000 eV and gave similar spectra in terms of peak energies and profile for both compounds (Fig. 3(a) and (b)). The Ce3d spectra displayed peaks due to Ce4 and Ce3 atoms namely those of ceria CeO2 and of fluoride, CeF3, the latter being predominant. Indeed the F1s spectra showed a peak at 684.5 eV and the O1s spectra displayed a broad peak at 530 eV representing the O2 contributions of Ce3 /O and Ce4 /O bonds. Under similar experimental conditions, the amount of cerium oxide was slightly higher for the diglyme adduct (42%) than for the TMEDA one (33%) by measuring the relative intensity of peak u?ƒ in the Ce3d3/2 region [24]. Subsequent heating at 550 8C under a water-saturated argon atmosphere allowed to convert CeF3 into CeO2 as shown by the XPS pattern (Fig. 3(c)). The latter compares with that reported for an authentic CeO2 sample [25] and no
S. Daniele et al. / Polyhedron 21 (2002) 1985 /1990
1989
Fig. 3. Ce3d XPS patterns of films obtained from: (a) Ce(hfip)4(TMEDA) (1), (b) Ce(hfip)4(diglyme) (2), (c) 2 with water-saturated argon treatment at 550 8C.
peaks assigned to fluorine could be detected. These films were homogeneous, colorless with very good transparency (transmittance of 90 /95% in the range 350 /900 nm) and displayed good mechanical properties (tested by applying cellophane tapes to the coated substrate). SEM data featured a smooth film but with small cracks resulting probably from the elimination of the fluorine residues. The thickness was estimated from cross sectional SEM pictures to be around 20 nm and XRD experiments indicated that the films were amorphous. The formation of cerium(III) fluoride indicates cleavage of the C /F bonds. Although fluorine contamination has been observed for systems without M. . .F interactions in the ground-state structure, this process is probably assisted here by the metal /fluorine interactions which weaken the C /F bonds but was not further investigated. It should be noted that while fluorine
contamination of oxide films is generally undesirable, pure metal fluorides have many desirable properties of their own and cerium fluoride is involved in the formulation of g-scintillations [26]. Thus production of pure metal fluoride films has its own relevance.
4. Supplementary material Tables of coordinates, of thermal parameters, of bond lengths and angles have been deposited at the Cambridge Crystallographic Data Base Centre. CCDC reference numbers: 171191 for 1 and 171192 for 2. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 IEZ, UK (fax: /44-1223-336033; e-
S. Daniele et al. / Polyhedron 21 (2002) 1985 /1990
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mail: [email protected] www.ccdc.cam.ac.uk).
or
www:
http:// [11] [12]
Acknowledgements We thank Mr. P. Delichere and Mr. G. Wicker for XPS and SEM experiments, respectively. [13]
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Molecular structures of volatile Ce(IV) tetrafluoroisopropoxide complexes with TMEDA and diglyme. CVD experiments Stephane Daniele a, Liliane G. Hubert-Pfalzgraf a,*, Monique Perrin b a
b
IRC, Universite´ Claude Bernard-Lyon 1, 2 avenue A. Einstein, 69629 Villeurbanne cedex, France Laboratoire de Cristallographie, Universite´ Claude Bernard-Lyon 1, 69622 Villeurbanne cedex, France Received 8 January 2002; accepted 7 March 2002
Abstract The structures of the cerium tetrafluoroisopropoxide adducts with TMEDA (1) and diglyme [Me(OC2H4)2OMe] (2) were established by single crystal X-ray diffraction (TMEDA /tetramethylethane-1,2-diamine). The cerium atom is hexa- and ˚ ] and are associated heptacoordinated for 1 and 2, respectively. The Ce/O(hfip) bond distances are quite short [2.115(5) /2.152(6) A to large Ce /O(hfip) angles (/1568). Both complexes display a range of short F C contacts, the shortest ones having values of ˚ for 1 and 2, respectively. Decomposition experiments were achieved in a cold-wall reactor. The films deposited on 3.754 and 4.060 A glass substrates were characterized by X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and UV /Vis spectroscopy. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Cerium; Fluoroalkoxides; Structures; Complexes; MOCVD; Films
1. Introduction Cerium oxide is a candidate for various areas of technological applications due to high chemical stability and insulating properties [1], interesting optical properties [2] and lattice parameters close with high Tc superconductors and silicon, and thus its potential value for buffer layers [3]. Various methods have been used for deposition of ceria thin films. Chemical routes, namely sol /gel processing and metal-organic chemical vapor phase deposition (MOCVD) are quite flexible and cheaper than physical methods [4]. MOCVD processes have a high potential for CeO2 layers having the quality required for microelectronics and several types of volatile cerium derivatives, mostly b-diketonates have been used for the growth of pure or doped [5] ceria layers. After the use of Ce(thd)4 (thd /2,2,6,6-tetra-
* Corresponding author. Tel.: /33-4-7244-5322; fax: /33-4-72445399 E-mail address: [email protected]_lyon1.fr (L.G. HubertPfalzgraf).
methylheptane-3,5-dionate) [5,6] and of fluorinated Ce(IV) or Ce(III) derivatives such as Ce(fdh)4 (fdh / 6,6,6-trifluoro-2,2-dimethyl-3,5-hexanedionate) [7], a second generation of precursors was based on adducts such as Ce(fdh)3(o -phen) [8]; [Ce2(fod)6(tetraglyme)] [9a] (fod /heptafluoro-7,7-dimethyl-4,6-octanedionate) and more recently [Ce2(etbd)6(tetraglyme)], [NH4][Ce(etbd)4] (etbd /1-ethoxy-4,4,4-trifluorobutane-1,3-dionate) [9b] and Ce(hfac)3 complexes (hfac /hexafluoroacetylacetonate) with various glymes [10]. Fluorinated alkoxides are another class of volatile derivatives and have been used for either oxide or fluoride films for gallium [11], barium [12], lanthanides or transition metals such as yttrium [13], zirconium [14] and are available for cerium [15]. We wish to report herein the solid state characterization of Ce(IV) hexafluoroisopropoxide adducts namely Ce(hfip)4(TMEDA) (1) and Ce(hfip)4(diglyme) (2), the polydentate Lewis bases being used for stabilization of Ce(hfip)4 toward hydrolysis. These volatile complexes were used as cerium sources in a cold wall CVD reactor. Films were characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and X-ray diffraction.
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2. Experimental All manipulations were routinely performed under nitrogen atmosphere using Schlenk tubes and vacuum line techniques with dried and distilled solvents. Hexafluoroisopropanol (hfipH) (Aldrich) was stored over molecular sieves and used as received. The Ce(IV) hexafluoroisopropoxide adducts were obtained as previously reported by us [15]. XPS experiments were performed with an Escalab 200R (VG Scientific) spectrometer using the monochromated Ka Al radiation as excitation source. SEM images were collected on a Hitachi S800 spectrometer. XRD data were collected on a Siemens D 5000 diffractometer using the Cu Ka radiation.
2.1. Crystallography of 1 and of 2 Suitable crystals of 1 were grown directly from the reaction medium, those of 2 were obtained by recrystallization in petroleum ether. The crystals were fixed at a glass fiber with paratone. Data collection was made on a Nonius CCD diffractometer at 123 K. Three thousand seven hundred thirty-eight reflections were measured for 1 and 12 721 reflections of which 8787 were unique were measured for 2. Cell parameters were refined using DENZO [16]. Compound 1 crystallizes in the monoclinic system (C 2/c group) whereas 2 crystallizes in the triclinic system (P/1¯ group). The structures were solved by direct methods and refined by least-squares using SHELXTL [17]. All non-H atoms were refined anisotropically giving the values of R listed in Table 1. Hydrogen atoms position were calculated at theoretical positions and refined isotropically riding on C. The distribution of the C /F bond lengths is quite large due to disorder phenomena which are also illustrated by high thermal factors and a large residual density.
2.2. Decomposition experiments Compounds 1 and 2 were sublimed and decomposed in a hot-wall, dynamic vacuum reactor with argon flow. The precursor reservoir was charged with 200/300 mg of compound and maintained at about 150 8C; the furnace temperature was 550 8C and the pressure was 5 /103 bar, the decomposition took place over a 4/8 h period. About 15% in weight of residue remained in the source reservoir after this time. After removing the residue, a flow of water-saturated argon (water bubbler at room temperature) was introduced and the furnace was maintained at 550 8C for over 8 h.
Table 1 Crystal data and structure refinement for Ce(hfip)4(TMEDA) (1) and Ce(hfip)4(diglyme) (2)
Chemical formula Formula weight Temperature (K) ˚) Wavelength (A Crystal system Space group ˚) a (A ˚) b (A ˚) c (A a (8) b (8) g (8) ˚ 3) V (A Z Dcalc (Mg m 3) Absorption coefficient (mm 1) Range for data collection (8) Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I 2s (I )] R indices (all data) Largest difference ˚ 3) peak and hole (e A
1
2
C18H20CeF24N2O4 924.48 123(2) 0.71073 monoclinic C 2/c 10.026(2) 17.622(4) 17.563(4) 90 91.22(3) 90 3102.3(11) 4 1.979 1.641
C18H18CeF24O7 942.44 123(2) 0.71073 triclinic P/1¯/ 10.003(2) 10.261(2) 16.791(3) 88.39(3) 83.50(3) 65.71(3) 1560.5(5) 2 2.006 1.638
2.31 /28.33
1.22 /30.09
full-matrix leastsquares on F2 3738/0/224
full-matrix leastsquares on F2 8787/0/451
1.034 R1 0.0795, wR2 0.1952 R1 0.1177, wR2 0.2178 2.348 and 1.470
0.765 R1 0.0539, wR2 0.1358 R1 0.1005, wR2 0.1748 0.885 and 1.005
3. Results and discussion 3.1. Synthesis and molecular structures of Ce(hfip)4(TMEDA) (1) and of Ce(hfip)4(diglyme) (2) The alcohol exchange reaction between cerium isopropoxide Ce2(OPri )8(Pri OH)2 and hexafluoroisopropanol (hfipH /OHCH(CF3)2) (1:8 stoichiometry) in THF at room temperature afforded a cerium hexafluoropropoxide THF adduct Ce(hfip)4(THF)2. The high Lewis acidity of fluoroalkoxides makes, however, this adduct extremely prone to hydration. Ligand exchange with polydentate Lewis bases such as TMEDA and diglyme MeO(CH2CH2O)2Me provided more stable adducts. Those were characterized (FT-IR, 1H and 19F NMR) by comparison with authentic samples as previously reported [15]. Their FT-IR spectra are characterized by very intense broad absorption bands around 1290 /1100 cm 1 corresponding to the C /F vibrations. Additional sharp bands due to the coordinated Lewis base are present as well but they are shifted downfield with respect to the free ligand. The absorption bands of the Ce /OR bonds are observed below 600 cm 1. The ligand exchange reactions between Ce(hfip)4(THF)2 and chelating ligands afforded derivatives
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being more air stable, the order of stability being Ce(hfip) 4 (TMEDA) B/ Ce(hfip)4(diglyme) / Ce(hfip)4(bipy)2. All these compounds are volatile and sublime unchanged (no loss of the Lewis base occurs during volatilization). Compound 1 is more favorable than 2 in terms of melting point (155 vs. 234 8C) whereas their volatility properties are comparable (sublimation around 70 8C under 104 mm of Hg). Their solubility properties make them suitable as precursors for liquid injection CVD. By contrast to barium and yttrium derivatives with various ligands [4], no rationale and predictive trends could be established by analysis of the relationship between sublimation temperatures and molecular weight data. However, for derivatives having similar formula, the volatility appears to be in favor of the hfac derivatives with the ranking hfac /hfip / fdh /thd /fod. Ce(hfip)4(diglyme) has a volatility comparable to that of Ce(hfac)3(dedg) (dedg /diethyldiglyme) [10]. Compounds 1 and 2 were characterized by low temperature single crystal X-ray diffraction. Their structures are depicted in Figs. 1 and 2 for 1 and 2, respectively. Selected bond lengths and angles are collected in Tables 2 and 3. The coordination polyhedron of the hexacoordinated cerium atom is quite distorted with angles ranging from 69.6(3)8*/the bite angle of the bidentate ligand*/to 170.5(3)8. The Ce /O ˚ , the distances have values of 2.115(5) and 2.152(6) A shortest ones being trans to the Ce /N coordination ˚ . Data on Ce /N bond distances are bonds of 2.624(7) A scarce. The Ce /N coordination bond observed for 1 are shorter than those reported for [Ce2(Oi Pr)6(m,h2OC2H4NMeC2H4NMe)2] but where the metal is heptacoordinated [18]. The shortest intramolecular cerium / fluorine distance corresponds to Ce F(210) with a
1987
Fig. 1. Molecular structure of Ce(hfip)4(TMEDA) (1) showing the atom numbering scheme (thermal ellipsoids at 20% probability). F(210), F(212), F(111), F(102), F(101) have Ce F interactions in ˚. the range 3.745 /4.332 A
˚ . Four other Ce F /C distances are value of 3.754 A ˚ , each of the fluorfound in the range 4.128 /4.332 A oisopropoxide ligand providing either one or two
Table 2 ˚ ) and angles (8) for Ce(hfip)4(TMEDA) (1) Selected bond lengths (A Bond lengths Ce O(1) Ce O(2) Ce N(1)
2.115(5) 2.152(6) 2.624(7)
Bond angles O(1) Ce O(1)#1 O(1) Ce O(2) O(1) Ce N(1) O(2) Ce N(1) O(2) Ce N(1)#1 O(1)#1 Ce O(2) O(2)#1 Ce O(2) O(1) Ce N(1)#1 O(2)#1 Ce N(1) N(1)#1 Ce N(1) C(1) O(1) Ce C(2) O(2) Ce
108.7(3) 92.3(2) 90.9(2) 87.1(2) 85.1(2) 93.2(2) 170.5(3) 160.4(2) 85.1(2) 69.6(3) 166.5(7) 156.7(7)
Symmetry transformations used to generate equivalent atoms: #1 x1, y , z3/2.
Fig. 2. Molecular structure of Ce(hfip)4(diglyme) (2) showing the atom numbering scheme (thermal ellipsoids at 20% probability). F(2), F(6), F(7), F(8), F(11), F(13), F(17), F(19) and F(23) have Ce F ˚. interactions in the range 4.060 /4.309 A
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S. Daniele et al. / Polyhedron 21 (2002) 1985 /1990
Table 3 ˚ ) and angles (8) for Ce(hfip)4(diglyme) (2) Selected bond lengths (A Bond lengths Ce O(1) Ce O(2) Ce O(7) Ce O(3) Ce O(4) Ce O(5) Ce O(6)
2.580(4) 2.581(5) 2.585(5) 2.132(4) 2.139(5) 2.127(4) 2.122(4)
Bond angles O(1) Ce O(2) O(1) Ce O(3) O(1) Ce O(7) O(1) Ce O(4) O(1) Ce O(5) O(1) Ce O(6) O(2) Ce O(3) O(2) Ce O(4) O(2) Ce O(5) O(2) Ce O(6) O(2) Ce O(7) O(3) Ce O(4) O(3) Ce O(5) O(3) Ce O(6) O(3) Ce O(7) O(4) Ce O(5) O(4) Ce O(6) O(4) Ce O(7) O(5) Ce O(6) O(5) Ce O(7) O(6) Ce O(7) C(3) O(3) Ce C(4) O(4) Ce C(5) O(5) Ce C(6) O(6) Ce
62.54(16) 78.49(16) 62.02(15) 138.72(17) 136.41(17) 83.43(18) 89.26(17) 76.94(17) 160.79(18) 82.39(18) 124.26(16) 94.18(19) 97.50(18) 161.92(19) 85.30(18) 84.61(18) 99.47(19) 158.74(17) 95.55(19) 74.42(17) 86.22(18) 160.2(4) 174.2(5) 149.4(5) 170.3(5)
fluorine for these interactions. Activation of carbon / fluorine bonds by metallic species has been observed in a number of complexes giving short M F /C interactions [19]. The shortest distance is much longer than the Ce /O bond lengths or the sum of the van der Waals ˚ ) and than the interactions found for radii (/3.20 A ˚ ) [12]. The barium derivatives (usually 3 /3.20 A Ce F(210) interaction might however account for the opening of the O(1)/Ce /O(1)# angle [108.7(3)8]. Some intramolecular F N interactions involving F(111), ˚ ). These F(112), F(200) are also observed (4.137 /4.259 A interactions are much longer than the Ce /F bond distances reported for cerium fluoride derivatives such as NH4[CeF2(PO4)] for instance [20]. The molecular structure of the Ce(hfip)4(diglyme) adduct 2 is based on a heptacoordinated metal center since all oxygens of the diglyme are involved in coordination. Such a coordination number was observed for Ce4O(OPri )14 [21]. The Ce /O(hfip) bond ˚] lengths are similar to those of 1 [2.122(4) /2.139(5) A and are much shorter than the Ce /O bond distances ˚ av.]. The stereoinvolving the diglyme ligand [2.582 A
chemistry around the metal corresponds to a trigonal prism capped by one of the oxygen O(1) of the glyme. The metric parameters of the diglyme ligand are in agreement with the literature body [9,10]. The shortest ˚ but Ce F distance [with F(17)] has a value of 4.060 A ˚ additional Ce F distances having values below 4.31 A are observed with eight other fluorine atoms. The Ce /OR bond distances are, for both complexes, quite short, although longer than those observed for ˚ av. non-fluorinated terminal Ce /OR alkoxides (2.088 A i for [Ce2(OPr )8(Pri OH)2]) [22]. They are shorter than those observed for the [Ce(hfip)6]2 anion [2.183(5) / ˚ ]. The Ce /O /(hfip) angles display values of 2.208(5) A [156.7(7)8 /166.5(7)8] and [149.4(5)8/174.2(5)8] for 1 and 2, respectively. Those are large by comparison to most other hexafluoroisopropoxide derivatives [143.3(5)8/ 156.5(5)8 for [Ce(hfip)6]2] and nearly comparable to those observed for non-fluorinated OR ligands which are considered to be more prone to p-bonding [23]. These large Ce /O /C angles do, however, not prevent interaction of the fluorine atoms with the metal center. These interactions shield the positive charge of the metal and reduce its electrophilic character. Its is noticeable that metal/fluorine interactions are generally absent in the solid state structures of fluorinated b-diketonate complexes in which the coordination number of the metal is usually higher than for fluorinated alkoxide adducts [10]. 3.2. CVD experiments Deposition experiments were achieved at various temperatures using an horizontal cold wall reactor under dynamic vacuum (4 /5 /103 bar). Compounds 1 and 2 were transported into the vapor phase at 120/ 140 8C. No deposits were observed below 500 8C and depositions were thus conducted at a temperature of 550 8C for 1 and 2. Films were homogeneous, white and displayed poor mechanical properties. They were characterized by XPS in the range 0/1000 eV and gave similar spectra in terms of peak energies and profile for both compounds (Fig. 3(a) and (b)). The Ce3d spectra displayed peaks due to Ce4 and Ce3 atoms namely those of ceria CeO2 and of fluoride, CeF3, the latter being predominant. Indeed the F1s spectra showed a peak at 684.5 eV and the O1s spectra displayed a broad peak at 530 eV representing the O2 contributions of Ce3 /O and Ce4 /O bonds. Under similar experimental conditions, the amount of cerium oxide was slightly higher for the diglyme adduct (42%) than for the TMEDA one (33%) by measuring the relative intensity of peak u?ƒ in the Ce3d3/2 region [24]. Subsequent heating at 550 8C under a water-saturated argon atmosphere allowed to convert CeF3 into CeO2 as shown by the XPS pattern (Fig. 3(c)). The latter compares with that reported for an authentic CeO2 sample [25] and no
S. Daniele et al. / Polyhedron 21 (2002) 1985 /1990
1989
Fig. 3. Ce3d XPS patterns of films obtained from: (a) Ce(hfip)4(TMEDA) (1), (b) Ce(hfip)4(diglyme) (2), (c) 2 with water-saturated argon treatment at 550 8C.
peaks assigned to fluorine could be detected. These films were homogeneous, colorless with very good transparency (transmittance of 90 /95% in the range 350 /900 nm) and displayed good mechanical properties (tested by applying cellophane tapes to the coated substrate). SEM data featured a smooth film but with small cracks resulting probably from the elimination of the fluorine residues. The thickness was estimated from cross sectional SEM pictures to be around 20 nm and XRD experiments indicated that the films were amorphous. The formation of cerium(III) fluoride indicates cleavage of the C /F bonds. Although fluorine contamination has been observed for systems without M. . .F interactions in the ground-state structure, this process is probably assisted here by the metal /fluorine interactions which weaken the C /F bonds but was not further investigated. It should be noted that while fluorine
contamination of oxide films is generally undesirable, pure metal fluorides have many desirable properties of their own and cerium fluoride is involved in the formulation of g-scintillations [26]. Thus production of pure metal fluoride films has its own relevance.
4. Supplementary material Tables of coordinates, of thermal parameters, of bond lengths and angles have been deposited at the Cambridge Crystallographic Data Base Centre. CCDC reference numbers: 171191 for 1 and 171192 for 2. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 IEZ, UK (fax: /44-1223-336033; e-
S. Daniele et al. / Polyhedron 21 (2002) 1985 /1990
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mail: [email protected] www.ccdc.cam.ac.uk).
or
www:
http:// [11] [12]
Acknowledgements We thank Mr. P. Delichere and Mr. G. Wicker for XPS and SEM experiments, respectively. [13]
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