Preparation and characterization of volatile alkaline-earth metal complexes with multiply coordinated aminoalkoxide ligands

Article abstract of DOI:10.1039/b103310g

The aminoalkoxide complexes Sr(amak)₂ (1) and Ba(amak)₂ (2) were prepared by treatment of a multiply-chelating fluorinated aminoalcohol ligand HOC(CF₃)₂CH₂N(CH₂CH ₂OMe)₂, (amak)H, with the Group 2 metal reagents Sr(OPrⁱ)₂ and BaH₂ respectively. Single crystal X-ray diffraction studies indicate that complex 1 possesses an 8-coordinate distorted bicapped octahedral geometry with all O and N atoms coordinated to the central Sr cation. However, complex 2 adopts a unique 10-coordinate bicapped square antiprismatic structure, of which the coordination number is increased by two fluorine-to-barium dative interactions. Variable temperature ¹⁹F NMR studies show the existence of two inter-convertible isomers in solution and their possible molecular structures are proposed according to their structures in the solid-state. Preliminary investigation suggests that these complexes are good CVD source reagents for depositing SrF₂ and BaF₂ thin films.

Full text of DOI:10.1039/b103310g

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Preparation and characterization of volatile alkaline-earth metal
complexes with multiply coordinated aminoalkoxide ligands
Yun Chi,*a Sudhir Ranjan,a Tsung-Yi Chou,a Chao-Shiuan Liu,*a Shie-Ming Pengb
and Gene-Hsiang Leeb
a Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan,
Republic of China. E-mail: ychi@mx.nthu.edu.tw
b Department of Chemistry and Instrumentation Center, National Taiwan University,
Taipei 10764, Taiwan, Republic of China
Received 12th April 2001, Accepted 14th June 2001
First published as an Advance Article on the web 9th August 2001
The aminoalkoxide complexes Sr(amak)2 (1) and Ba(amak)2 (2) were prepared by treatment of a multiply-chelating
fluorinated aminoalcohol ligand HOC(CF3)2CH2N(CH2CH2OMe)2, (amak)H, with the Group 2 metal reagents
Sr(OPri)2 and BaH2 respectively. Single crystal X-ray diffraction studies indicate that complex 1 possesses an
8-coordinate distorted bicapped octahedral geometry with all O and N atoms coordinated to the central Sr
cation. However, complex 2 adopts a unique 10-coordinate bicapped square antiprismatic structure, of which
the coordination number is increased by two fluorine-to-barium dative interactions. Variable temperature
19F NMR studies show the existence of two inter-convertible isomers in solution and their possible molecular
structures are proposed according to their structures in the solid-state. Preliminary investigation suggests that
these complexes are good CVD source reagents for depositing SrF2 and BaF2 thin films.
There has been a growing interest in the use of Group 2
chemical vapor deposition (CVD) precursors for the prep-
aration of a variety of metal oxide1 or metal fluoride thin
films,2 the latter having potential application for manufacturing
optical wave-guides or solid-state laser devices.3 Enhanced
volatility and high thermal stability are the two most essential
requirements for the better performance of such CVD pre-
cursors. As the fully saturated environment is expected to
exhibit enhanced volatility by minimizing intermolecular
attraction in the condensed phase,4 the challenge in finding new
source reagents is thus centered on how to achieve coordin-
atively saturated metal centers. On the contrary, when the
ligands of the complexes cannot provide a sufficient number
of donor atoms, formation of multimetallic structures is
expected; this situation has been demonstrated by the isolation
of β-diketonate5 or alkoxide6 complexes such as [Sr3(thd)6]
appendage.11 The enhancement of volatility was attributed
to a tendency of the polyether side chain to bind to the same
metal atom in order to favor intramolecular chelation over
intermolecular bonding.
In this paper, we wish to report the synthesis of two new
monomeric Group 2 aminoalkoxide complexes Sr(amak)2 (1)
and Ba(amak)2 (2), (amak)H = HOC(CF3)2CH2N(CH2CH2-
OMe)2. It is notable that the (amak)H ligand not only possesses
an acidic hydroxyl group due to the presence of two electron-
withdrawing CF3 groups, but also contains three additional
donor atoms on the side chains that can provide desirable
chelate interactions, forming a fully saturated coordination
environment around the metal cation. Moreover, as the effective
ionic radius of strontium (1.40 Å for coordination number = 8)
differs greatly from that of barium (1.66 Å for coordination
number = 10),12 the structural identification of these two metal
complexes would allow us to assess the bonding mode of
ligands vs. the ionic radius of metal cations. Preliminary
experiments were also carried out to evaluate their potential to
serve as CVD source reagents for depositing SrF2 and BaF2 thin
films, as these aminoalkoxide complexes were reasonably
volatile.
(thd = 2,2,6,6-tetramethylheptane-3,5-dionate),
[Sr3(thd)6-
(hfac = 1,1,1,5,5,5-hexafluoro-
(Hthd)], [Ba(hfac)2(H2O)]n
pentane-2,4-dionate), [Ba4(thd)8], [Ba(OtBu)2(HOtBu)]4, [H4-
Ba6(µ6-O)(OC2H4OMe)14] and [Ba5(µ5-OH){OCH(CF3)2}9-
(THF)4(H2O)]. Most of these complexes are found to be
thermally unstable or require a higher temperature for evapor-
ation, making them less suitable for CVD experiments.
One successful method of achieving monomeric structures
is to introduce a neutral ancillary ligand possessing a sufficient
number of donor sites. For example, crown ether, glyme,
ethanolamine and polyamine based chelate molecules have
been utilized for stabilizing monomeric structures.7 Some
representative examples involving volatile Group 2 metals
Experimental
General information and materials
Mass spectra were obtained on a JEOL SX-102A instrument
operating in electron impact (EI) mode. 1H, 13C and 19F NMR
spectra were recorded on Varian Mercury-400 or INOVA-500
instruments; chemical shifts are quoted with respect to internal
standard tetramethylsilane for 1H and 13C NMR and CFCl3 for
19F NMR data. BaH2 and Sr(OPri)2 were purchased from Strem
Chemicals and used as received. All reactions were performed
under a nitrogen atmosphere using deoxygenated solvents dried
with an appropriate reagent. Elemental analyses were carried
out at the NSC Regional Instrumentation Center at National
Cheng Kung University, Tainan, Taiwan.
include:
Mg(thd)2(tmeda)
(tmeda = tetramethylethylene-
diamine), Sr(thd)2(triglyme), Ba(hfac)2(18-crown-6) and Ba-
(hfac)2(hmteta) (hmteta = hexamethyltriethylenetetraamine).8
A second approach is to incorporate a chelating or a donor
functionalized feature into the anionic ligand, so that it could
satisfy the multiple coordination requirement of larger cations.9
This strategy has allowed the isolation of several volatile Group
2 metal complexes with oligoether substituted alcoholate
ligands,10 or β-ketoiminate ligands involving a polyether
2462
J. Chem. Soc., Dalton Trans., 2001, 2462–2466
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b103310g

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Table 1 X-Ray structural data for complexes 1 and 2
Synthesis of (amak)H
To a reaction flask equipped with a dry ice condenser,
anhydrous hexafluoroacetone (13.3 g, 0.08 mol) was slowly
added into a diethyl ether solution of diazomethane (3.4 g, 0.08
mol) prepared from Diazald®. The reaction mixture was then
stirred for three hours to allow complete formation of the
1
2
Formula
M
T /K
Crystal system
Space group
a/Å
b/Å
c/Å
C22H36Cl4F12N2O6Sr C20H32BaF12N2O6
881.95
150
Monoclinic
P21/n
9.2804(2)
11.4196(3)
16.8509(4)
104.152(1)
1731.63(7)
2
761.82
295
Monoclinic
P21/n
10.048(3)
14.977(3)
21.031(4)
94.48(3)
3005.4(12)
4
1.684
1.431
5288
371
oxirane compound (CF3)2COCH2. Then 11.7 g (0.088 mol) of
bis(2-methoxyethyl)amine was added dropwise over a period
of 15 minutes and the reaction mixture was stirred at room
temperature overnight. Finally, the reaction was quenched by
the addition of water and washed with a saturated NaHCO3
solution (2 × 20 mL) to remove excess amine. The organic layer
was dried over anhydrous MgSO4, the diethyl ether evaporated
in vacuo and the resultant oily liquid purified by vacuum
distillation, giving 13.3 g (53%) of colorless liquid with bp 60 ЊC
at 1.2 Torr. Selected spectral data: 1H NMR (400 MHz, CDCl3,
β/Њ
V/Å3
Z
Dc/g cmϪ3
1.691
1.968
3934
215
µ(Mo-Kα)/mmϪ1
No. of data in refinement
No. of parameters
R1, wR2 with I > 2σ(I )
3
0.034, 0.073
0.029, 0.072
295 K): δ 6.54 (s, br, 1H, OH), 3.42 (t, JHH = 5.1 Hz, 4H,
OCH2), 3.30 (s, 6H, OMe), 3.16 (s, 2H, CH2), 2.88 (t, 3JHH = 5.1
Hz, 4H, NCH2). 13C NMR (100 MHz, CDCl3, 295 K): δ 123.5
1
CD3CN): δ 126.9 (q, JCF = 293.0 Hz, CF3), 82.4 (sept,
2JCF = 25.0 Hz, (CF3)2C), 69.7 (OCH2), 58.4 (OCH3), 56.7
(CH2), 54.6 (NCH2). 19F NMR (470 MHz, CD2Cl2, 295 K):
δ Ϫ78.9 (6F); 19F NMR (470 MHz, CD2Cl2, 193 K): δ Ϫ78.2
(very broad), Ϫ79.2 (broad). Anal. Calcd for C20H32F12N2O6Ba:
C, 31.53; H, 4.23; N, 3.68. Found: C, 31.51; H, 4.46; N,
3.52%.
1
2
(q, JCF = 286.7 Hz, CF3), 73.2 (sept, JCF = 28.1 Hz, C(CF3)2),
70.0 (OCH2), 58.7 (OMe), 54.5 (CH2), 53.5 (NCH2). 19F NMR
(470 MHz, CDCl3, 295 K): δ Ϫ77.6 (6F).
Reaction of (amak)H with Sr(OPri)2
Strontium isopropoxide (0.512 g, 2.49 mmol) in THF (15 mL)
was taken in a 100 mL flask equipped with a condenser. To this
was added 1.56 g of (amak)H (4.98 mmol). Then 30 mL of
THF was added and the solution was brought to reflux for
40 hours. The solution was filtered to remove any insoluble
impurities and the filtrate evaporated to dryness, giving a white
crystalline powder. Finally, the product was purified by
recrystallization from dichloromethane–heptane, giving 1.3 g
of Sr(amak)2 (1, 1.73 mmol, 75%) as colorless crystalline
material, mp = 196 ЊC. Spectral data of 1: MS (EI, 70 eV, m/eϩ,
L = C10H16F6NO3), observed (actual) {relative abundance}
[assignment]: 818 (819) {4.2} [SrL2 ϩ SrF], 712 (712)
{1.0} [SrL2], 566 (565) {17.2} [SrL2 Ϫ C7H16O2N], 400 (400)
{42.7} [SrL], 330 (331) {37.8} {[SrL Ϫ CF3], 268 (268) {6.9}
[L Ϫ C2H5O], 146 (146) {100} [C7H16O2N], 107 (107) {7.2}
[SrF]. 1H NMR (400 MHz, C6D6): δ 3.49 (s, 6H, OMe), 3.22 (br,
CVD experiments
The thermal CVD reactions were carried out using a horizontal
hot-wall reactor, consisting of a Pyrex tube of 25 mm internal
diameter, placed within an electric temperature-controlled tube
furnace. The precursors were loaded in a glass container, which
served as precursor reservoir and attached at the far end of the
Pyrex tube. The temperature of the sample reservoir was
adjusted to 180–200 ЊC. The carrier gas was introduced through
the sidearm of the container, which became saturated with the
vapor of the source reagent and then entered the hot zone,
where deposition took place. The flow rate of the carrier gas
was typically adjusted to ≈30 mL minϪ1. The deposition tem-
perature was kept at 300–400 ЊC, and the deposition time set
typically between 20 and 40 min. The Si substrates were cleaned
using dilute HF solution, then washed with de-ionized water
and acetone.
m, 4H, OCH2), 2.93 (s, 2H, CH2), 2.44 (br, m, 4H, NCH2). 13
C
NMR (100 MHz, C6D6): δ 127.2 (q, 1JCF = 291.0 Hz, CF3), 82.7
(sept, 2JCF = 25.2 Hz, (CF3)2C), 69.1 (OCH3), 59.1 (OCH2), 56.8
(CH2), 54.4 (NCH2). 19F NMR (470 MHz, CD2Cl2, 295 K):
δ Ϫ79.2 (6F); 19F NMR (470 MHz, CD2Cl2, 193 K): δ Ϫ78.8
(broad), Ϫ79.5 (broad). Anal. Calcd for C20H32F12N2O6Sr: C,
33.73; H, 4.53; N, 3.93. Found: C, 33.75; H, 4.69; N, 3.93%.
X-Ray crystallography
X-Ray diffraction data of complex 1 was measured on a Bruker
SMART CCD diffractometer using λ(Mo-Kα) radiation
(0.7107 Å). The crystallographic data were collected over a
hemisphere of reciprocal space, by a combination of three sets
of exposures. Each set had a different φ angle for the crystal
and each exposure of 10 seconds covered 0.30Њ in ω. The data
collection was executed using the SMART program.13 An
empirical absorption was based on symmetry-equivalent
reflections and applied to the data using the SADABS
program.13 The structure was solved using the SHELXTL-97
program.13 For complex 2, the crystallographic data were
collected on a Nonius CAD-4 diffractometer. Initial lattice
parameters were determined from 25 accurately centered
reflections with 2θ values in the range 18.86 to 30.22Њ. Reflec-
tion data were collected using the θ/2θ scan mode. The θ scan
angle was then determined for each reflection according to
the expression 0.65 ϩ 0.35tanθ. A semi-empirical absorption
correction from psi-scans was applied.
Reaction of (amak)H with BaH2
Barium hydride (0.49 g, 3.53 mmol) was taken in a 100 mL flask
equipped with a condenser. To this was added dropwise 2.1 g of
(amak)H (6.43 mmol). The resulting mixture was first stirred at
40 ЊC for 4 hours until evolution of gas stopped. Then 50 mL of
THF was added and the solution was brought to reflux for 12
hours. The excess of BaH2 was removed by filtration and the
filtrate evaporated to dryness, giving a light yellow powder.
Finally, the product was purified by recrystallization from hot
toluene, giving 2.15 g of Ba(amak)2 (2, 2.82 mmol, 84%) as
colorless crystalline material, mp = 155 ЊC. Spectral data of 2:
MS (EI, 70 eV, m/eϩ, L = C10H16F6NO3), observed (actual)
{relative abundance} [assignment]: 918 (919) {2.3} [BaL2 ϩ
BaF], 762 (762) {1.1} [BaL2], 615 (616) {11.9} [BaL2 Ϫ
C7H16O2N], 450 (450) {61.3} [BaL], 380 (381) {23.2}
[BaL Ϫ CF3], 314 (313) {2.9} [HL], 268 (268) {53.2}
[L Ϫ C2H5O], 157 (157) {7.1} [BaF], 146 (146) {100}
[C7H16O2N]. 1H NMR (400 MHz, CD3CN): δ 3.65 (t, 4H,
3JHH = 4.9 Hz, OCH2), 3.51 (s, 6H, OMe), 2.80 (s, 2H, CH2),
Finally, anisotropic thermal parameters were used for all
non-hydrogen atoms, while the hydrogen atoms were given
fixed isotropic displacement parameters. The crystallographic
refinement parameters of complexes 1 and 2 are summarized in
Table 1.
3
2.77 (t, 4H, JHH = 4.9 Hz, NCH2). 13C NMR (100 MHz,
CCDC reference numbers 163031 and 163032.
J. Chem. Soc., Dalton Trans., 2001, 2462–2466
2463

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See http://www.rsc.org/suppdata/dt/b1/b103310g/ for crystal-
lographic data in CIF or other electronic format.
Results and discussion
The fluorinated aminoalcohol ligand (amak)H was prepared
in good yield by direct treatment of the amine HN(CH2-
CH2OMe)2 and the fluorinated oxirane (CF3)2COCH2 in diethyl
ether solution, the latter is generated in-situ from hexafluoro-
acetone and diazomethane etherate at ambient temperature.14
This aminoalcohol (amak)H is the first fluorinated alcoholate
ligand with a donor-functionalized side chain,6,9 where its
parent fluoroalkoxide ligands formed stable metal complexes
with approximate formula [Ba(ORF)2], RF = CH(CF3)2 and
CMe(CF3)2, but their crystal structures were not determined.15
On the contrary, the related aminoalkoxide ligands have
recently been applied to synthesize the volatile Cu() complexes
[Cu{OC(CF3)2CH2NHR}2], R = Me, CH2CH2OMe, Bui, for
generation of copper metal thin films using chemical vapor
deposition.16
Fig. 1 ORTEP13 drawing of 1 with thermal ellipsoids shown at the
50% probability level. Selected bond lengths (Å): Sr–O(1) = 2.605(2),
Sr–O(2) = 2.671(2), Sr–O(3) = 2.436(2) and Sr–N(1) = 2.933(2).
As indicated in Scheme 1, synthesis of the strontium amino-
Fig. 2 ORTEP13 drawing of 2 with thermal ellipsoids shown at the
30% probability level. Selected bond lengths (Å): Ba–O(1) = 2.562(2),
Ba–O(2) = 2.878(3), Ba–O(3) = 2.862(3), Ba–O(4) = 2.558(2), Ba–O(5) =
2.820(3), Ba–O(6) = 2.832(3), Ba–N(1) = 3.003(3), Ba–N(2) = 3.080(3),
Ba–F(1) = 3.212(3) and Ba–F(7) = 3.132(2). Bond angle (Њ): N(1)–Ba–
N(2) = 169.50(8).
Scheme 1
alkoxide complex Sr(amak)2 (1) was achieved by treatment of
the isopropoxide complex Sr(OPri)2. The related barium
complex Ba(amak)2 (2, 84%) was prepared using BaH2 as
the source reagent. Both complexes are found to be sparingly
soluble in non-polar hydrocarbons such as hexane and cold
toluene but are much more soluble in solvents such as CHCl3,
CH2Cl2 and THF. Finally, as both alkoxide complexes are
found to be moderately sensitive to moisture upon exposure to
air, storage under an inert atmosphere is needed.
The identity of these two complexes was initially revealed by
EI mass analysis, of which the parent molecular ions Mϩ, along
with the fragmentation ions were clearly observed, showing
formation of the expected molecular formula. A subsequent
single crystal X-ray diffraction study indicates that complex 1
contains one monomeric metal entity and two uncoordinated
CH2Cl2 solvates in the asymmetric unit. The strontium atom is
situated on a crystallographic center of inversion and bonded
to all four heteroatoms of the amak ligand, giving a tetra-
dentate chelate interaction encircling the central metal atom
(Fig. 1). The Sr–O(3) distance (2.436(2) Å) is found to be much
shorter than the respective Sr–O(ether) dative interactions,
Sr–O(1) (2.605(2)) and Sr–O(2) (2.671(2) Å), which shows the
presence of the stronger bonding interaction between the Sr2ϩ
cation and the (Ϫ1) negatively charged O(3) atom. As these six
oxygen donor atoms constitute a slightly distorted octahedral
skeletal arrangement, the immediate coordination geometry
of 1 is best described as bicapped octahedron, where the two
capping atoms are the N(1) and N(1A) atoms, located at the
center of the triangular faces defined by the O(1), O(2) and O(3)
and the O(1A), O(2A) and O(3A) atoms. Moreover, the Sr–
N(1) distance of 2.933(2) Å is found to be much longer with
respect to the Sr–O(ether) distances discussed earlier (2.605(2)–
2.671(2) Å). Since this difference in bond distance (≈ 0.3 Å) is
greater than the difference between the radius of the nitrogen
and oxygen atom (0.02–0.05 Å), our observation clearly
indicates that lengthening of the Sr–N distance is not due to the
larger radius of the nitrogen atom, but due to the unfavorable
angle strain imposed by three adjacent C–C–N linkages which
prevents the nitrogen donor atom from getting too close to the
central strontium cation.
In contrast, the barium complex 2 shows a 10-coordinate
environment, of which the donor ligands consist of all available
oxygen and nitrogen atoms and most importantly, two add-
itional fluorine-to-barium contacts from the trifluoromethyl
groups of the aminoalkoxide ligands (Fig. 2). Formation of
Ba ؒ ؒ ؒ F dative interactions with distances 2.87–3.29 Å have
been cited in many literature reports.17 Such Ba ؒ ؒ ؒ F contacts
are observed in complex 2 with Ba–F(1) = 3.212(3) Å and
Ba–F(7) = 3.132(2) Å, both non-bonding distances of which are
much shorter than the shortest Sr ؒ ؒ ؒ F distance observed in
complex 1 (Sr ؒ ؒ ؒ F(6) = 4.218 Å) or the sum of the fluorine van
der Waals’ radius (1.5 Å) and the ionic radius of barium (1.66
Å).5a As a result, the coordination environment is best con-
sidered as a distorted bicapped square antiprism, resembling
that observed in some other 10-coordinate barium complexes.18
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J. Chem. Soc., Dalton Trans., 2001, 2462–2466

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Fig. 3 VT 19F NMR spectra of 1 in CD2Cl2 solution; the signal
marked with an asterisk is due to a small amount of free aminoalcohol.
Scheme 2
The unique capping atoms in 2 are the nitrogen atoms N(1) and
N(2), while the two square faces of the antiprism are defined by
the atoms O(1), O(3), O(2) and F(1) and by the atoms O(4),
O(6), O(5) and F(7) respectively. In addition, the alkoxy termini
of 2 are arranged in a bent position with an angle O(1)–Ba–
O(4) = 124.4Њ. This geometry is in sharp contrast to that of
complex 1, in which the alkoxy termini are located at the trans
positions of the bicapped octahedral geometry.
geometry, i.e. the 1 and 1Ј pair and the 2 and 2Ј pair,
respectively.
Although we cannot rule out either possibility based on the
present 19F NMR evidence, it is important to note that the
coordinated CF3 group of the amak ligands in the 10-
coordinate barium complex 2 would exhibit a fast threefold
rotation about its C–CF3 axis. This rotational motion must be
accompanied by a concurrent breaking and reforming of the
M ؒ ؒ ؒ F dative interactions as well as rapid interchange between
the coordinated and the non-coordinated CF3 substituents, as
only one 19F NMR signal was observed at room temperature,
irrespective of the presence of two chemically distinctive CF3
substituents in the solid-state. The fluxionality is independent
of the polarity of the solvent system and this is revealed by the
result that addition of excess THF to the non-polar CD2Cl2
solution causes no change in the VT 19F NMR spectral pattern.
Moreover, for both complexes 1 and 2, a relatively weak
signal at δ Ϫ77.6 was clearly observed in the 19F NMR spectra
recorded at lower temperatures. This signal cannot be removed
by any means and has been assigned to the dissociated amino-
alcohol ligand (amak)H, which may be produced from
inadvertent hydrolysis during preparation of the NMR
samples. Interestingly, this signal disappeared upon raising the
temperature to 295 K, implying that a rapid intermolecular
exchange with the ligated aminoalkoxide may be occurring
under these conditions.
The 1H and 13C NMR spectra at room temperature have been
measured and all data are consistent with the proposed molec-
ular structures. The variable temperature 19F NMR studies were
next examined, and of particular interest is the 19F NMR spec-
trum of the barium complex 2, which showed one sharp signal
at δ Ϫ78.9 at 295 K in CD2Cl2 solution. This signal begins to
broaden upon lowering the temperature to 213 K, and splits
into two very broad signals centered at δ Ϫ78.2 and Ϫ79.2 at
193 K. The 1H NMR spectrum recorded at this temperature is
not well resolved, but it is basically consistent with the 19F
NMR data. Moreover, a similar fluxional behavior is observed
for the strontium complex 1, showing only one sharp 19F NMR
signal at δ Ϫ79.2 in CD2Cl2 at 295 K, and a broad peak upon
lowering to 233 K which further turned into two sharp 19F
NMR signals at δ Ϫ78.8 and Ϫ79.5 with an approximate ratio
of 1 : 4 at 193 K (Fig. 3). The large difference in the signal
intensities indicated that they were not derived from the non-
equivalent CF3 groups of the ligands in one single complex;
therefore, the co-existence of two inter-convertible isomers was
proposed.
Based on these VT 19F NMR experiments and the X-ray crys-
tal structure established, the observed temperature-dependent
behavior may be interpreted as a cis-to-trans isomerization of
the coordinated aminoalkoxide ligands, giving co-existence of
both cis- and trans-isomers of the bicapped octahedral complex
1 and the bicapped square antiprismatic complex 2, which are
labeled as the 1 and 1Љ pair and the 2 and 2Љ pair in Scheme 2.
This hypothesis is valid only if we assume that both complexes
retain their monomeric nature and that the coordination
number of each metal complex is preserved in solution, i.e. 8-
coordination for 1 and 10-coordination for 2, respectively.
However, if we arbitrarily assume that the 10-coordinate cis-
geometry is associated with the simultaneous presence of the
M ؒ ؒ ؒ F dative interactions, this would eliminate the existence
of the 8-coordinate cis-isomer 1Љ as well as the 10-coordinate
trans-isomer 2Љ, because the corresponding structures have not
been detected by the X-ray structural analysis of 1 and 2. If this
is the case, the fluxional behavior in solution is best explained
as a rapid exchange between the pairs of isomers derived from
the bicapped octahedral and the bicapped square antiprismatic
After completing the spectroscopic identification, the phys-
ical properties important to CVD studies were established. It
showed that both complexes underwent sublimation without
notable decomposition at 150 ЊC under a pressure of 200 mTorr
overnight, as samples obtained before and after sublimation
exhibited identical spectroscopic and physical characteristics.
The thermal behaviors were also investigated by TG analysis
under O2 at 1 atm and a rapid loss of weight was observed at
about 220 ЊC due to a combination of sample sublimation and
decomposition, giving an off-white residue upon increasing the
temperature to 500 ЊC (1: 14.6 wt%, 2: 30.8 wt%). Powder XRD
analysis confirmed that these solid residues contain mainly
polycrystalline metal fluorides SrF2 and BaF2, giving a first
indication that these complexes were suitable CVD precursors.
Preliminary experiments were carried out to probe the
possibility of depositing fluoride thin films from aminoalkoxide
complexes 1 and 2; most of the previous reports utilized
the fluorinated β-diketonate complexes as the CVD source
reagents.2,19 Our data indicated that the BaF2 thin film can be
deposited on both Pyrex glass and Si(100) substrate at 350 ЊC,
J. Chem. Soc., Dalton Trans., 2001, 2462–2466
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Hubert-Pfalzgraf, Polyhedron, 1994, 13, 3323.
Fig. 4 The XRD patterns for the thin film samples obtained from
CVD experiments using complexes 2 and 1 as the source reagents: (a)
BaF2 and (b) SrF2.
7 (a) W. S. Rees, Jr., M. W. Carris and W. Hesse, Inorg. Chem., 1991,
30, 4479; (b) G. Rossetto, A. Polo, F. Benetollo, M. Porchia and
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(d ) K. G. Caulton, M. H. Chisholm, S. R. Drake and K. Folting,
Inorg. Chem., 1991, 30, 1500.
8 (a) J. A. T. Norman and G. P. Pez, J. Chem. Soc., Chem. Commun.,
1991, 971; (b) S. R. Drake, M. B. Hursthouse, K. M. A. Malik and
S. A. S. Miller, J. Chem. Soc., Chem. Commun., 1993, 478; (c) S. R.
Drake, M. B. Hursthouse, K. M. A. Malik, S. A. S. Miller and D. J.
Otway, Inorg. Chem., 1993, 32, 4464; (d ) J. R. Babcock, D. D.
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using 2 as source reagent under a carrier gas of Ar. The XRD
measurement showed formation of a polycrystalline texture on
the Pyrex glass substrate, giving a preferential orientation along
the (200) planes on the Si(100) surface (Fig. 4). The formation
of such highly ordered thin films is not uncommon as it
has been observed in CVD experiments using the Ba(hfac)2-
(tetraglyme) adduct (hfac = 1,1,1,5,5,5-hexafluoropentane-2,4-
dionate).20 Upon switching to complex 1, SrF2 thin films with a
preferred (220) orientation on a Si(100) surface was obtained
using a 1 : 1 mixture of Ar and O2 as carrier gas.
In summary, our results show that the designed synthesis of
alkaline-earth metal fluoroalkoxide complexes has been
achieved, for which the bonding mode of the fluoroalkoxide
ligands in the solid state is determined by the nature of the
central metal cation. All O and N donor atoms on the side
chain are coordinated to the metal cation and in the case of the
barium complex 2 two additional fluorine contacts are clearly
observed. The weak basicity of the alkoxide oxygen and the
coordinative unsaturation of the larger barium center make
these fluorine dative contacts energetically favorable. Moreover,
of particular interest is the formation of two rapidly inter-
convertible isomers in solution, which may be attributed to the
intramolecular cis–trans isomerization or the transformation
between the 8-coordinate bicapped octahedral and the 10-
coordinate bicapped square antiprismatic geometry. The
relatively high volatility of these two complexes makes them
optimum CVD source reagents for growing fluoride thin-films.
Accordingly, preservation of a certain degree of intramolecular
M ؒ ؒ ؒ F dative bonding is possible during precursor evap-
oration and vapor transport, which in turn can be used to
account for the rapid formation of the metal fluorides.
9 (a) W. A. Herrmann, N. W. Huber and O. Runte, Angew. Chem., Int.
Ed. Engl., 1995, 34, 2187; (b) W. A. Herrmann and N. W. Huber,
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10 W. S. Rees, Jr. and D. A. Moreno, J. Chem. Soc., Chem. Commun.,
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11 (a) A. M. Bahl, S. Krishnaswamy, N. G. Massand, D. J. Burkey and
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Pfalzgraf, F. Labrize, C. Bois and J. Vaissermann, Polyhedron, 1994,
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O. Just, B. Obi-Johnson and W. S. Rees, Jr., Chem. Vap. Deposition,
2000, 6, 129.
12 J. E. Huheey, E. A. Keiter and R. L. Keiter, Inorganic Chemistry:
Principles of Structure and Reactivity, Harper Collins College
Publishers, New York, 4th edn., 1993, p. 116.
13 (a) G. M. Sheldrick, SHELXTL, version 5.10, Siemens Analytical
X-ray Instruments Inc., Madison, WI, 1998; (b) SMART and
SAINT, Siemens Analytical X-ray Instruments Inc., Madison, WI,
1995; (c) G. M. Sheldrick, SADABS, University of Göttingen,
Germany, 1996; (d ) C. K. Johnson, ORTEP, Report no. ORNL-
5138, Oak Ridge National Laboratory, Oak Ridge, TN, 1976.
14 I.-S. Chang and C. J. Willis, Can. J. Chem., 1977, 55, 2465.
15 B. Borup, W. E. Streib and K. G. Caulton, Inorg. Chem., 1997, 36,
5058.
16 P.-F. Hsu, Y. Chi, T.-W. Lin, C.-S. Liu, A. J. Carty and S.-M. Peng,
Chem. Vap. Deposition, 2001, 7, 28.
Acknowledgements
17 (a) J. A. Samuels, E. B. Lobkovsky, W. E. Streib, K. Folting, J. C.
Hoffman, J. W. Zwanziger and K. G. Caulton, J. Am. Chem. Soc.,
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F. Labrize, L. G. Hubert-Pfalzgraf, J. C. Daran and S. Halut,
J. Chem. Soc., Chem. Commun., 1993, 1556; ( f ) A. P. Purdy and
C. F. George, Inorg. Chem., 1991, 30, 1970; (g) A. Drozov,
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18 D. A. Neumayer, J. A. Belot, R. L. Feezel, C. Reedy, C. L. Stern,
T. J. Marks, L. M. Liable-Sands and A. L. Rheingold, Inorg. Chem.,
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19 M. L. Hitchman, S. H. Shamlian, D. D. Gilliland, D. J.
Cole-Hamilton, J. A. P. Nash, S. C. Thompson and S. L. Cook,
J. Mater. Chem., 1995, 5, 47.
We thank the National Science Council, Taiwan, Republic of
China for funding (Grant No. NSC 89-2113-M-007-034).
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