Decomposition behavior of M(DPM)n (DPM = 2,2,6,6-tetramethyl-3, 5-heptanedionato; n = 2, 3, 4)

Article abstract of DOI:10.1021/jp064010j

The decomposition behavior of M(DPM)_n (DPM = 2,2,6,6-tetramethyl-3,5-heptanedionato; M = Sr, Ba, Cu, Sm, Y, Gd, La, Pr, Fe, Co, Cr, Mn, Ce, Zr; n = 2-4) was studied in detail with infrared spectroscopy and mass spectrometry. The results indicat

Full text of DOI:10.1021/jp064010j

J. Phys. Chem. A 2006, 110, 13479-13486
13479
Decomposition Behavior of M(DPM)n (DPM ) 2,2,6,6-Tetramethyl-3,5-heptanedionato;
n ) 2, 3, 4)
Yinzhu Jiang, Mingfei Liu, Yanyan Wang, Haizheng Song, Jianfeng Gao,* and
Guangyao Meng
USTC Laboratory for Solid State Chemistry & Inorganic Membranes, Department of Materials Science and
Engineering, UniVersity of Science and Technology of China (USTC), Hefei, 230026,
People’s Republic of China
ReceiVed: June 27, 2006; In Final Form: October 3, 2006
The decomposition behavior of M(DPM) (DPM ) 2,2,6,6-tetramethyl-3,5-heptanedionato; M ) Sr, Ba, Cu,
n
Sm, Y, Gd, La, Pr, Fe, Co, Cr, Mn, Ce, Zr; n ) 2-4) was studied in detail with infrared spectroscopy and
mass spectrometry. The results indicated that the chemical bonds in these compounds dissociate generally
following the sequence of C-O > M-O > C-C(CH
decomposition processes of M(DPM) are strongly influenced by the coordination number and central metal
ion radius. In addition, the decomposed products, in air atmosphere, varied from metal oxides to metal
carbonates associated with different M(DPM)
3 3
) > C-C and C-H at elevated temperatures. The
n
n
.
1. Introduction
stability are still extensively used in preparing many kinds of
materials such as YBCO, CYO, SCYO, YSZ, SDC, and Pd-
Metal-organic chemical vapor deposition (MOCVD), clas-
1,8-10
Pa alloy.
sified according to the use of metallorganics as precursors, is a
well-established, versatile, and widely applied method for
fabricating technologically important thin films, ranging from
metals to semiconductors, insulators, and superconductors with
tailored properties.1 In general, metallorganic precursors
possess lower decomposition or pyrolysis temperatures than
halides, hydrides, or halohydrides. Thus, these advantages enable
the MOCVD process to occur at a lower deposition temperature
than conventional chemical vapor deposition (CVD), which
generally uses halides or hydrides as precursors.4
Knowledge of the decomposition mechanism of metal
complexes, particularly the sequence of the bond dissociation
in the ligand, is of great importance for two reasons. First, it
could provide basic information required to guide us in designing
new metallorganic compounds by modifying the primary ligand
structures to improve its physical properties and/or decomposi-
tion behavior. For instance, in Ba(DPM)2 and Sr(DPM)2, H2O
can coordinate to the highly positively charged metal center to
form oligomeric oxygen/hydroxide bridged structures, such as
-3
11
Ba5(DPM)9(OH)3H2O and Sr2(DPM)3. In this case, the intro-
duction of fluorine and/or bulky ligand groups into the â-dike-
tonate ligand reduces the net positive charge on the metal center,
rendering the compound less sensitive to O2/H2O. Alternatively,
the metal center can be saturated with a polydentate coordinating
ligand, such as polyethers (CH3O(CH2CH2O)nCH3). These form
relatively stable complexes with fluorinated â-diketonates of
the type M(hfac)2(polyether) (M ) Ba, Sr, Ca; polyether )
There are three kinds of metallorganic precursors utilized
commonly in MOCVD processes: metal â-diketonates, metal
alkoxides, and metal alkyls. Metal â-diketonates have been
widely applied in fabrication of electroceramic oxides films,
such as high-Tc superconductors, ferromagnetics, ferroelectrics,
conducting oxide layers, electrochemical devices, high-k di-
electrics, giant magnetoresistant (GMR) oxides, buffer layers,
and alloy thin films.1 â-Diketonate complexes used mainly
include M(DPM)n (DPM ) dipivaloylmethanate ) 2,2,6,6-
tetramethyl-3,5-heptanedionato), M(ACAC)n (ACAC ) 2,4-
,5
1
2
tetraglyme, triglyme), which have higher vapor pressures and
higher ambient stabilities than M(DPM)2 complexes.
6
Second, the information about precursor decomposition
behavior would guide us in optimizing MOCVD process
conditions for obtaining a film of desired properties. Examples
are Mn(DPM)3 and La(DPM)3, of which Nakamura et al. studied
the decomposition mechanism through spectroscopic absorption
pentanedione), M(HFA)n (HFA ) 1,1,1,5,5,5-hexafluoropentane-
7
2
3
,4-dionate), M(DFHD)n(DFHD)1,1,1,2,2,6,6,7,7,7-decafluoro-
3
,5-heptanedione), et al. In comparison with the other kinds
of metallorganic precursors, one of the advantages of metal
â-diketonates is that their physical and chemical properties can
1
3-15
1
spectroscopy.
Meanwhile, the results effectively guide the
be altered by “tailoring” the â-diketonate group. Among these
deposition of lanthanum oxide films and manganese oxide films
in a liquid delivery MOCVD process.
â-diketonate complexes, chelates with large steric hindrance
ligands, such as M(DPM)n, possess somewhat lower volatility
than ones with fluorine. However, the latter is always expensive
and produces toxic gas, which may contaminate the as-deposited
films. Up to today, M(DPM)n, nontoxic, nonfluorinated and
environmentally benign compounds with sufficient volatility and
In the present study, the decomposition behavior of a series
of tris-, bis-, and tetra(dipivaloylmethanate) metal chelates,
where the metals complexed were Sr, Ba, Cu, Sm, Y, Gd, La,
Pr, Fe, Co, Cr, Mn, Ce, and Zr, were systematically studied by
infrared spectroscopy and mass spectrometry. Their possible
decomposition mechanisms and influencing factors were pro-
posed and discussed.
*
Corresponding author. Telephone: +86-551-3601700. Fax: +86-551-
3
607627. E-mail: jfgao@ustc.edu.cn.
1
0.1021/jp064010j CCC: $33.50 © 2006 American Chemical Society
Published on Web 11/24/2006

13480 J. Phys. Chem. A, Vol. 110, No. 50, 2006
Jiang et al.
n 3
Figure 1. IR spectra of M(DPM) at room temperature: (a) Sm(DPM) ,
(
b) Pr(DPM) , (c) Ce(DPM) , and (d) Zr(DPM) .
3
4
4
2. Experimental Section
Some M(DPM)n (M ) Ba, Sr, Gd, Ce) samples were
purchased from Strem; other M(DPM)n (M ) Pr, Mn, Fe, Cr,
Co, Y, and Sm) samples were synthesized in-house, using
corresponding inorganic salts and HDPM (Strem) as starting
materials. The reaction was fulfilled in 50% ethanol/aqueous
mixture solution,16 where the favored pH value was controlled
by sodium hydroxide addition. These M(DPM)n precipitates
were quickly filtered off, dried, and purified by recrystallization
from toluene. Final M(DPM)n chelates were dried by P2O5 in a
desiccator. All the samples were verified to be of high purity
by the former characterization of carbon and hydrogen elemen-
tary analysis (VARIO ELIII), metal atomic absorption spectro-
photometric analysis (Perkin-Elmer 3100), and nuclear magnetic
1
resonance ( H NMR, AV300) for a sample dissolved in CDCl3.
To investigate the mechanisms of the bond dissociation in
M(DPM)n (M ) Sm, Ce, Zr, Pr), we observed changes in the
Fourier transform infrared (FTIR) peak intensity while the
sample was heated to a certain temperature. The FTIR spectrum
was recorded from 400 to 4000 cm- using the Bruker FT-IR
spectrometer, Model Vector-22. The sample was mixed with
KBr powder, pressed into a self-supporting disk, and then placed
at the center of the IR cell for experiments. In addition, a mass
spectrometer (Agilent 6890/Micromass GCT-MS) was used to
obtain more explicit and idiographic oligomerization mecha-
nisms of these metal complex. The cracking patterns of
M(DPM)n and the mass signal changes were monitored as the
sample was heated in a vacuum at the rate of 10 °C/min from
1
3
0 to 400 °C. The samples were ionized by the electron impact
(EI) method within the scanned mass range of 1-800 m/z.
3
. Results and Discussion
.1. FTIR Spectra. To explore the processes of the bond
3
dissociation of the mentioned M(DPM)n, the samples were
heated to different temperatures and the changes in the FTIR
peak intensity were recorded. Figure 1 shows the FTIR spectra
of fresh M(DPM)n (M ) Sm, Ce, Zr, Pr) at room temperature.
All the complexes show similar FTIR spectra, which suggests
that they possess the same ligand DPM. Table 1 lists the
observed wavenumber with band assignments of these com-
plexes and other M(DPM)n chelates. These results are incon-
13,17
sistent with those of other reported M(DPM)n.
In the IR
absorption spectra of fresh samples, the peaks numbered 2, 3,
and 4 represent the stretching vibrational mode of C-O, C-C,
and the bending vibrational mode of C-H in the ring structure,

Decomposition Behavior of M(DPM)n
J. Phys. Chem. A, Vol. 110, No. 50, 2006 13481
Figure 3. Schematic diagram of the proposed thermal decomposition
process of Sm(DPM) in air.
3
Figure 2. IR spectra of Sm(DPM)
preheated at various temperatures: (b) 200, (c) 270, (d) 325, and (e)
00 °C.
3
at room temperature (a) and
4
TABLE 2: Decomposition Temperatures and Decomposed
Products of M(DPM) (M ) Pr, Sm, Ce, Zr)
n
which contribute to the tautomeric equilibrium of the keto and
enol forms. The peaks numbered 15 and 16 are due to the
stretching mode of the M-O bond. These results can be used
decomposition
a
complex
Pr(DPM)
temp (°C)
decomposed products
18
to identify these complexes. A broad peak ranging from 3300
3
∼280
praseodymium oxide and
praseodymium oxide carbonate
-
1
to 3600 cm in each spectrum represents the O-H vibrational
mode in H2O, which was adsorbed by the sample and/or KBr
during the preparation of IR sample.
Sm(DPM)
Ce(DPM)
3
∼200
∼220
∼260
Sm
CeO
ZrO
2
(CO
2
)
3 3
and Sm
O
2 3
4
Zr(DPM)
4
2
Spectra b, c, d, and e in Figure 2show the IR spectra of
Sm(DPM)3 heated at four different temperatures: 200, 270, 325,
a
According to IR analysis.
19
and 400 °C, respectively. These IR results indicate that the
chemical bonds in Sm(DPM)3 decompose in sequence when
the sample is heated. It can be seen that the intensities of the
peaks corresponding to the C-O bond are greatly weakened as
the temperature rises to 200 °C, and disappear eventually when
the sample is heated to 270 °C. Accompanying the disappear-
flexible, resulting in an increase of the coupling of C-C
vibrational modes, and then the intensity of the peaks for C-C
-
1
bonds enhances at around 1540 cm as observed from 270 to
1
7
3
25 °C, which was also reported by Ryu et al. The peaks for
C-C disappear up to 400 °C due to excessive temperature. For
a similar reason, the peak intensity for C-H increases from
200 to 270 °C, then decreases from 270 °C, and finally
disappears at 400 °C.
The IR results indicate that Sm(DPM)3 dissociates in sequence
heated in air. The C-O and the Sm-O bonds decompose easily
at relatively low temperature, and then the C-C(CH3)3 bond
decomposes. The C-H and C-C bonds are the most stable,
dissociating at a higher temperature. The proposed sequence of
-
1
ance of the peak at 1573 cm for the C-O bond, a new peak
-
1
(
numbered 17) appears at 1486 cm above 270 °C, representing
2-
the stretching mode of CO3 , and maintains its intensity up to
00 °C. This result suggests the generation of Sm2(CO3)3. The
4
intensity of the peak for the Sm-O bond is reduced continuously
from low temperatures up to 325 °C, indicating that the chemical
bond between the Sm ion and DPM ligand is also unstable,
similar to the Sr-O bond in Sr(DPM)2.
-
1
the bond dissociation in Sm(DPM) is described schematically
The peaks between 1100 and 1300 cm and those between
3
-
1
in Figure 3.
7
00 and 900 cm , corresponding to the C-C(CH3)3 bond,
decrease slowly above 200 °C, and two new peaks (numbered
8 and 19) assigned to the new vibrational modes of CH3 appear
All chemical bonds of organic groups dissociate completely
1
at 400 °C, and two broad peaks (numbered 20 and 21) at 1486
-
1
-1
-1
at 1079 and 901 cm . When the bond of C-C(CH3)3 is
dissociated partially while the sample is heated at higher
temperature, it could be found that the two new peaks go through
two stages: (1) while all the peaks for C-C(CH3)3 decrease
and the peaks for C-O disappear before 270 °C, the intensity
of the two new peaks increases; (2) when the peaks for
C-C(CH3)3 disappear partially from 270 °C, we can see the
intensity of the two new peaks decreases dramatically, and then
disappear at 325 and 400 °C, respectively. This phenomenon
may be interpreted as by follows: at the first stage, along with
the dissociation of the C-O bond, the ligand structure becomes
more flexible and therefore it is easier for the vibration of CH3;
at the second stage, the bond of C-C(CH3)3 is dissociated
partially, and the amount of the CH3 reduces, too, which results
in the decrease of the two new peaks.
cm and about 480 cm corresponding to the stretching modes
of Sm2(CO3)3 and Sm2O3, respectively, indicate that the residue
of the complex decomposed in air is a mixture of samarium
oxide and samaric carbonate.
A similar evolution of the bond dissociation is demonstrated
through the study on the IR spectra of Zr(DPM)4, Ce(DPM)4,
18
and Pr(DPM)3 heated at different temperatures. However, each
of the complexes exhibits different decomposition temperatures
and decomposed products at elevated temperatures in air (Table
2). It should be pointed out that the decomposition temperatures
determined by IR analysis are generally less than those
determined by differential thermal analysis (DTA) owing to heat
1
9
relaxation. For the sake of optimal conditions, the addition of
extra oxidant, such as oxygen, is needed when MOCVD is
applied for deposition of metal oxides films using M(DPM)n
as precursors. The added oxygen will promote the complete
decomposition of the precursors, as well as prevent carbon and/
or other impurities from remaining in the films. Meanwhile,
By reason of dissociation of the C-O bond at relatively low
temperature, the ligand ring opens so that the ligand is attached
only at one end. Hence the ligand structure becomes more

13482 J. Phys. Chem. A, Vol. 110, No. 50, 2006
Jiang et al.
Figure 5. Possible decomposition mechanism of Sr(DPM)
2
.
Figure 6. Possible decomposition mechanism of Cu(DPM)
2
.
of ionized fragments released from the chelates while the sample
is heated.
3
.2.1. M(DPM)2 (M ) Sr, Ba, Cu). Figure 4 shows the
prevailing mass fragments of M(DPM)2 (M ) Sr, Ba, and Cu).
Each of the complexes exhibits different decomposition behav-
iors.
In the mass spectra of the compounds of Sr(DPM)2 and
Ba(DPM)2 could be found peaks with m/z exceeding 454 and
504 (respective molecular weights of Sr(DPM)2 and Ba(DPM)2).
The results have been noted earlier elsewhere and clearly
ascribed to the oligomerization of Sr(DPM)2 and Ba(DPM)2.
However, the peaks at m/z above 800 in the mass spectrum of
Ba(DPM)2 could not be detected under our current experimental
conditions, although they do exist in the process of oligomer-
+
+
12
ization of Ba(DPM)2, e.g., Ba4(DPM)7 and Ba3(DPM)5. The
other major peaks of the various fragments of Sr(DPM)2 are as
+
follows: The base peak at m/z ) 271 corresponds to Sr(DPM).
+
The peaks at m/z ) 397, 256, and 127 correspond to OCCH2-
+
COC(CH3)3Sr(DPM),
C(CH3)2OCCHCOC(CH3)3Sr, and
+
OCCH2COC(CH3)3. The peaks in the mass spectrum of
Ba(DPM)2 is similar to those of Sr(DPM)2, which indicates
similar decomposition behavior as shown in Figure 5.
In the mass cracking patterns of Cu(DPM)2, the parent peak,
+
i.e., Cu(DPM)2, m/z ) 429, corresponds to the molecular
weight of the complex and thus confirms the formation of the
complex. At the same time, the absence of peaks with m/z
exceeding 429 (molecular weight of the compound) is indicative
of the monomer composition of the gas phase. However,
Turnispeed et al. found the oligomerization of Cu(DPM)2 under
their experimental conditions, e.g., Cu2(DPM)2 and Cu2(DPM)3.12
The diversity may be due to the different synthesis methods of
the Cu(DPM)2 source. The other major peaks at m/z ) 372,
+
+
Figure 4. Typical mass spectrum of M(DPM)
2
(M ) Sr, Ba, and Cu).
315, 246, 189, 127, and 57 correspond to Cu(DPM)2, OCCH2-
+
+
COC(CH3)3Cu(DPM), OCCH2COCCu(DPM), Cu(DPM),
OCCH2COC(CH3)3Cu, OCCH2COC(CH3)3, and C(CH3)3,
+
+
+
the deposition should be processed in the system without water
vapor and CO2, so as to prevent the formation of carbonate
byproduct.
respectively. These mass fragments reveal the possible process
of decomposition in a vacuum, which is described in Figure 6:
In the beginning, the C-O bond dissociates by heating and the
ligand structure becomes more flexible, resulting in the detach-
ment of the two -C(CH3)3 groups successively. Afterward, the
residual -OCCH2CO- group dissociates from the central metal
3.2. Mass Spectroscopic Analysis. Mass spectrometry is one
of the effective tools for the investigation of the dissociation
mechanisms of chemical bonds in an organic compound. In the
present experiments, mass spectrometry was applied for exam-
ining the dissociation processes of M(DPM)n and the properties
+
atom, leading to the formation of Cu(DPM).

Decomposition Behavior of M(DPM)n
J. Phys. Chem. A, Vol. 110, No. 50, 2006 13483
3
Figure 8. Possible decomposition mechanism of Sm(DPM) .
Figure 9. Typical mass spectrum of M(DPM) (M ) La and Pr).
3
COC(CH3)3 group in the same DPM ligand detaches. The
mechanism is consistent with the IR analysis: the C-O bond
dissociates in the beginning by heating and the ligand structure
becomes more flexible, resulting in the detachment of the
-
CC(CH3)3. The strongest peaks appear at m/z ) 127 and 57,
indicating that this process is dominant.
The mass cracking patterns of La(DPM)3 and Pr(DPM)3 are
given in Figure 9. In comparison with the above spectra of
M(DPM)3 (M ) Sm, Y, Gd), the mass fragment peaks are
obviously fewer, and the base peaks at m/z ) 507 and 505
+
correspond to M(DPM)2 (M ) La, Pr).
3
.2.3. M(DPM)3 (M ) Transition Metals, Cr, Mn, Co, Fe).
Figure 10 shows the total ion current of the mass signal of
Mn(DPM)3 and prevailing mass fragments at four different
temperature regions. In the low-temperature region (T < 203
°
C), five major peaks of the highest mass are observed at m/z
Figure 7. Typical mass spectrum of M(DPM)
3
(M ) Y, Sm, and Gd).
+
)
421, 364, 238, 127, and 57, corresponding to Mn(DPM)2,
+
+
+
3
.2.2. M(DPM)3 (M ) Y, Sm, Gd, La, Pr). Figure 7 shows
OCCHCOC(CH3)3Mn(DPM), Mn(DPM), OCCH2COC-
+
typical mass spectra for M(DPM)3 (M ) Sm, Y, Gd), which
exhibit similar fragment distribution. For the mass spectrum of
Sm(DPM)3, six major peaks of highest mass are observed at
(CH3)3, and C(CH3)3, respectively. These mass fragments
reveal the possible process of decomposition under vacuum
(Figure 11). In the beginning, one of the DPM groups detaches
from the Mn atom, leading to the formation of +Mn(DPM)2
(m/z ) 421); then the -C(CH3)3 and the -OCCH2COC(CH3)3
groups dissociate in sequence partially from the DPM ligand
of Mn(DPM)2. The similar dissociation process is also verified
in the investigation of Cr(DPM)3, Co(DPM)3, and Fe(DPM)3,
as shown in Figure 12.
+
m/z ) 644, 518, 127, 99, 85, and 57, corresponding to OCCH2-
+
+
COC(CH3)3Sm(DPM)2, Sm(DPM)2, OCCH2COC(CH3)3,
+
CH2COC(CH3)3, +COC(CH3)3, and C(CH3)3, respectively.
+
+
Based on these mass fragments, a possible process of the
decomposition was proposed in Figure 8. First, the -CC(CH3)3
group detaches from DPM ligand first, and then the -OCCH2-

13484 J. Phys. Chem. A, Vol. 110, No. 50, 2006
Jiang et al.
Figure 10. (a, top) Changes in total ion current of mass signals of
Mn(DPM) with temperature. (b-e, bottom) Mass cracking patterns
of Mn(DPM) in four temperature regions: (b, first zone) 125 °C < T
203 °C, (c, second zone) 203 °C < T < 210 °C, (d, third zone) 210
C < T < 225 °C, and (e, fourth zone) 225 °C < T < 250 °C.
3
3
<
°
Figure 11. Schematic diagram of the proposed decomposition mech-
anism of Mn(DPM) under vacuum.
3
Figure 12. Typical mass spectrum of M(DPM)
Fe).
3
(M ) Cr, Co, and
On the whole, the bond dissociation process in Mn(DPM)3
Figure 11b,c) is similar to the above results and reported ones
(
17-19
of other M(DPM)n:
the C-O bond dissociates by heating
When the temperature increases to the second zone (203-
210 °C; see Figure 10), the intensity of the peak at m/z ) 421
increases to be the strongest peak, indicating that the Mn(DPM)2
fragment becomes uppermost among all the fragments. The
change may be interpreted by the following reasons: when the
temperature is above 203 °C, the complex largely vaporizes,
leading to the acceleration of reaction a (Figure 11a) and then
and the ligand structure becomes more flexible, resulting in
the detachment of the -CC(CH3)3 and -OCCH2COC(CH3)3
successively. However, different from the above results of
M(DPM)n, one of the DPM groups will detach from Mn(DPM)3
+
(Figure 11a) in advance of the following dissociation (Figure
11b,c).

Decomposition Behavior of M(DPM)n
J. Phys. Chem. A, Vol. 110, No. 50, 2006 13485
4
Figure 14. Possible decomposition mechanism of Zr(DPM) .
4
Figure 15. Possible decomposition mechanism of Ce(DPM) .
n
Figure 16. Chemical structure of M(DPM) .
Figure 13. Typical mass spectrum of M(DPM) (M ) Zr and Ce).
4
the vast production of +Mn(DPM)2 fragment. When the tem-
perature increases more, the primary fragments possess lower
activation energy and easier dissociation, which lead to the
appearance of various dissociation reactions and hence various
fragments. The fourth zone (Figure 10) shows the end of the
decomposition of Mn(DPM)3.
It is well-known that the properties of M(DPM)n, especially
the decomposition behaviors, are strongly influenced by the
coordination number and the molecular structure. For example,
in the chemical structure of M(DPM)3, oxygen atoms in DPM
groups are bonded with the central metal atom to form C3 or
2
0
D3 symmetry. Their decomposition properties are influenced
by two factors: the chemical nature of the central metal ion,
such as electronegativity, and the degree of distortion of the
frame structure that is related to the radius of the metal ion.
Figure 17 illustrates each metal ion radius. When the metal ion
radius is larger, for example, Sm (R ) 0.96 Å), the frame
structure will be normal or expand; therefore there are no high
repulsions between the DPM groups, which will result in the
detachment of the -CC(CH3)3 and -OCCH2COC(CH3)3 suc-
cessively. A similar decomposition mechanism is observed in
Y(DPM)3, Gd(DPM)3, La(DPM)3, and Pr(DPM)3. The latter two
possess higher metal ion radii, which may hence exhibit
somewhat different phenomena in the mass spectra. On the other
hand, when the metal ion radius is relatively smaller, such as
3
.2.4. M(DPM)4 (M ) Ce, Zr). Figure 13 shows the mass
spectra of tetra(dipivaloylmethanate) metal chelates Zr(DPM)4
and Ce(DPM)4. The peaks at m/z ) 127 and 57, which
+
+
reprensent OCCH2COC(CH3)3 and C(CH3)3 fragments, are
observed in both spectra. Except for these, the mass spectra of
Zr(DPM)4 and Ce(DPM)4 exhibit a great many diversities in
the mass fragments. The spectrum for Zr(DPM)4 contains
+
+
+
Zr(DPM)3 (m/z ) 640), Zr(DPM)2O (m/z ) 473), and
Zr(DPM)O (m/z ) 290), whereas peaks at m/z ) 690 and 507,
corresponding to Ce(DPM)3 and Ce(DPM)2, appear in the
mass spectrum of Ce(DPM)4. The results reveal the different
possible decomposition behaviors of Zr(DPM)4 and Ce(DPM)4,
which are described in Figures 14 and 15, respectively.
+
+
3+
3
.3. Discussion. In the structure of M(DPM)n molecule
Figure 16), each metal M atom is spherically surrounded by
n oxygen atoms from DPM bidentate ligands. Generally, for
transition metal Mn (R ) 0.66 Å), the DPM groups will shrink
toward the central metal atom, resulting in higher repulsion
between the DPM groups. Therefore, one of the DPM groups
will detach directly from the ligand structure. A similar
dissociation process is also verified in the investigation of
Cr(DPM)3, Co(DPM)3, and Fe(DPM)3, the metal ion radii of
which are 0.63, 0.63, and 0.64 Å, respectively. In addition,
(
2
M(DPM)n (n > 2), there will be no oligomerization due to a
large steric bulk ligand shielding the positively charged metal
center and preventing it from increasing its coordination number
by forming intermolecular bonds.

13486 J. Phys. Chem. A, Vol. 110, No. 50, 2006
Jiang et al.
Acknowledgment. This work was supported by the National
Natural Science Foundation of China under Contract No.
2
0271047.
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(
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different electronic structures of metal ions may be the other
reason for the variation of the decomposition mechanisms.
(
1
(
4. Summary
(
A systematic research on the decomposition behavior of
2
M(DPM)n (DPM ) 2,2,6,6-tetramethyl-3,5-heptanedionato; M
Sr, Ba, Cu, Sm, Y, Gd, La, Pr, Fe, Co, Cr, Mn, Ce, Zr; n )
-4) was provided. The chemical bonds in these compounds
(
)
2
(
begin to dissociate at low temperatures and are sequentially
decomposed at elevated temperatures, according to the order
of C-O > M-O > C-C(CH3)3 > C-C and C-H. The
decomposed products varied from metal oxides to metal
carbonates in air atmosphere. According to mass spectroscopic
analysis at elevated temperature, detailed decomposed mecha-
nisms were proposed and classified in term of coordination
number and metal ion radius.
(
(
(