Stabilization and destabilization of zirconium propoxide precursors by acetylacetone

Article abstract of DOI:10.1039/b406012a

The stabilizing and destabilizing mechanism in the action of acetylacetone on zirconium propoxide precursors is revealed; the nature of heteroleptic intermediates provides an explanation.

Full text of DOI:10.1039/b406012a

Stabilization and destabilization of zirconium propoxide precursors by
acetylacetone†
a
a
a
b
Gerald I. Spijksma, Henny J. M. Bouwmeester, Dave H. A. Blank and Vadim G. Kessler*
a
+
Inorganic Materials Science, Faculty of Science & Technology, and Mesa Research Institute for
Nanotechnology, University of Twente, P.O. Box 217, 7500 AE, Enschede, The Netherlands.
E-mail: G.I.Spijksma@utwente.nl; Fax: +31 53 4894683; Tel: +31 53 489299
Department of Chemistry, SLU, Box 7015, 75007 Uppsala, Sweden. E-mail: Vadim.Kessler@kemi.slu.se;
Fax: +46 18 673476; Tel: +46 18 671541
b
Received (in Cambridge, UK) 21st April 2004, Accepted 2nd June 2004
First published as an Advance Article on the web 6th July 2004
The stabilizing and destabilizing mechanism in the action of
acetylacetone on zirconium propoxide precursors is revealed;
the nature of heteroleptic intermediates provides an explana-
tion.
propoxide precursors were used, only needle shaped crystals were
obtained. XRD and NMR showed these to have the composition
Zr(acac) . The higher precursor reactivity apparently leads to an
4
even less stable tri-substituted precursor structure.
Finally, an attempt to isolate the form with 0.5 mol equivalents
of Hacac was made. The preparation was similar to that of the forms
with 1 and 2 mol equivalents of Hacac. NMR showed the formation
of 1 as shown in Fig. 3, while the isolated crystals were identified
by XRD as unmodified precursor. No indication was found that the
asymmetric 0.5 equivalent Hacac modified structure, as proposed
The application of unmodified zirconium precursors in for instance
MOCVD (Metal Organic Chemical Vapour Deposition) or sol–gel
is a difficult task, due to extreme moisture sensitivity. A generally
applied method to moderate the reactivity is the exchange of the
1
alkoxide ligands by chelating organic ligands. In sol–gel studies,
4
acetic acid and acetylacetone are most often applied for this
by Ribot et al., exists.
2
purpose. Both complexing ligands are bidentate, but acetic acid
NMR measurements as a function of time were performed on the
mixed-ligand system in order to exclude the existence of other
stable forms and to gain information on the stability of the different
structures. An experiment was performed by adding 1 mol
equivalent of Hacac to the mixed-ligand precursor. The composi-
tion of the sample was monitored by NMR as a function of time.
Only minor changes occurred after leaving the sample at room
temperature (21.4 °C) for several days, indicating that the stability
can be bridging or chelating, while acetylacetone is only chelating.
The literature on the application of acetylacetone (Hacac) as a
complexing agent for the controlled processing of zirconium
3
alkoxide precursors is abundant.
To the best of our knowledge, no reports have been published on
zirconium precursor structures formed upon modification. The only
4
proposed model for stabilization is that by Ribot et al. however, it
is extracted from work performed on cerium(IV) isopropoxide. The
authors proposed that the precursor structure in propyl alcohol,
i
i
[
Ce
2 8 2
(O Pr) ( PrOH) ], is altered upon addition of 0.5, 1 or 2 mol
i
i
i-
equivalents of Hacac to give [Ce
2
(O Pr)
7
(acac)( PrOH)], [Ce
2
2
(O
i
i
Pr)
6
(acac)
2
( PrOH)
2
] and [Ce(O Pr)
2
(acac) ] respectively. In the
present study, we attempted to isolate and characterize the
stabilized modifications of different zirconium propoxide pre-
cursors to confirm the mechanism proposed by Ribot et al.4
The precursors utilized in this study are commercially available
zirconium n-propoxide, zirconium isopropoxide, and a mixed-
n
i
i
2 2 6 2
ligand precursor [Zr (O Pr) (O Pr) ( PrOH) ] that was obtained
according to ref. 5. Modification of precursors was performed with
commercial grade acetylacetone. All structures presented in this
communication were obtained using zirconium isopropoxide as
starting reagent. The proton NMR data, however, clearly indicate
that the more reactive n-propoxide and mixed-ligand precursors are
modified by acetylacetone in an analogous manner. In order to
obtain crystals of the 1 : 1 Hacac modified zirconium precursor, the
zirconium isopropoxide was dissolved in a mixture of hexane and
toluene (in a volume ratio of 2 : 1). After addition of an equivalent
amount of acetylacetone the sample was dried and, subsequently,
re-dissolved in hexane. Crystallization occurred during cooling at
i
Fig. 1 Molecular structure of [Zr(O Pr)
3
(acac)]
2
(1).
2
30 °C overnight; a single crystal was mounted in inert oil and
transferred to the diffractometer. The structure was characterized as
i
[
Zr(O Pr)
Following an analogous preparation procedure, but adding two
mol equivalents of Hacac to zirconium isopropoxide, crystals of a
3 2
(acac)] (1),‡ depicted in Fig. 1.
i
new unexpected compound Zr(O Pr)(acac)
depicted in Fig. 2.
3
(2)§ were obtained,
The NMR spectra of the dried crystals of 2 are in agreement with
the determined structure, e.g., the signal of the CH proton of the
central carbon in acetylacetone was nicely split in the expected ratio
of 2 : 1. In cases where normal or mixed-ligand zirconium
†
Electronic supplementary information (ESI) available: summary of 1
H
NMR spectra and CIF files for compounds 1 and 2. See http://www.rsc.org/
suppdata/cc/b4/b406012a/
i
Fig. 2 Molecular structure of Zr(O Pr)(acac)
3
(2).
1
874
C h e m . C o m m u n . , 2 0 0 4 , 1 8 7 4 – 1 8 7 5
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

form rearranges in time to the tetra-substituted one and to free
alkoxide. No indications were found for the existence of a di-
substituted species.
It emerges that the nature of the heteroleptic intermediates on
different zirconium propoxides characterized in this study is not in
agreement with the proposed mechanism and structures of
4
intermediates for the corresponding cerium system. Upon addition
i
of up to 1 mol equivalent of Hacac, the fairly stable [Zr(O Pr)
3
(a-
cac)]
2
is formed. When an additional Hacac is added, the
i
[Zr(O Pr)
3 2
(acac)] structure is supposedly destabilized leading to
i
the presence of the initial precursor and unstable Zr(O Pr)(acac)
Spontaneous rearrangement of Zr(O Pr)(acac) to stable Zr(acac)
3 4
3
.
i
occurs at room temperature. The proposed stabilization and
destabilization mechanism together with the accompanying struc-
tures of the intermediates are schematically depicted in Fig. 3.
The authors are very grateful to Rolf Andersson for performing
the NMR analyses, and to The Swedish Research Council
(Vetenskapsrådet) for the financial support.
Fig. 3 The proposed mechanism and structures for the stabilization of
zirconium propoxide in propanol. Addition of up to 1 mol equivalent of
Hacac to the initial precursor (a) leads to (b). Addition of more than 1 mol
equivalent leads to a reaction of (b) to (a) and (c). Spontaneous
rearrangement of (c) gives (d) and (a). Different reactions are schematically
depicted in the center of the figure.
of the mono-substituted structure is fairly high. The addition of
excess isopropanol, often facilitating ligand exchange, did not lead
6
Notes and references
to significant structural rearrangements upon monitoring the
precursor solution for several days. The stabilization of zirconium
and hafnium isopropoxides with 1 eq. of a bulky b-diketonate
‡ Crystal data: C28H56O10Zr , M = 735.17, triclinic, a = 10.004(16), b =
2
10.696(14), c = 12.27(2) Å, a = 87.74(4)°, b = 67.16(3)°, g = 64.75(5)°,
3
¯
21
V = 1081(3) Å , T = 295 K, space group P1, Z = 1, m = 0.520 mm
582 reflections measured, 4123 unique (Rint = 0.0222 which were used in
all calculations. The final discrepancy factors were R1 = 0.0469; wR2 =
,
5
2,2,6,6,-tetramethylheptanedione giving structural analogs of com-
7
pound 1 for MOCVD applications has been reported earlier, but 1
remains so far the first structurally characterized and relatively
stable intermediate in sol–gel applications.
0
§
=
.1018 for 2261 observed reflections (I > 2s(I)).
Crystal data: C18 Zr, M = 447.62, orthorhombic, a = 8.5444(17), b
30.933(6), c = 8.2339(16) Å, V = 2176.3(7) Å , T = 295 K, space group
28 7
H O
3
After the addition of 2 mol equivalents of Hacac to the mixed-
ligand precursor solution, the initial NMR spectra showed multiple
peaks at around 5.5 ppm, 10 minutes after the sample was prepared.
Distinguishing different structures on the basis of these peaks is
rather complicated, since peaks of the mono- and tetra-substituted
structures partly or completely overlap with one of the peaks of the
tri-substituted derivatives. However, the largest signal was as-
signed to Zr(acac) , since only this peak remained after 16 hours.
4
The two minor peaks were attributed to the intermediate tri-
substituted structure. The presence and disappearance after 16
21
Pna2(1), Z = 4, m = 0.537 mm , 8461 reflections measured, 3517 unique
(Rint = 0.0792) which were used in all calculations. The final discrepancy
factors were R1 = 0.0681; wR2 = 0.1609 for 2279 observed reflections (I
> 2s(I)).
1
J. Livage, M. Henry and C. Sanchez, Prog. Solid State Chem., 1988, 18,
59.
2
2
3
U. Schubert, J. Sol–Gel Sci. Technol., 2003, 26, 47.
J. Livage, F. Babonneau, M. Chatry and L. Coury, Ceram. Int., 1997, 23,
1
3; S. Benfer, U. Popp, H. Richter, C. Siewert and G. Tomandl, Sep.
hours of peaks assigned to normal and isopropoxide CH
3
groups
Purif. Technol., 2001, 22–23, 231; R. J. Vacassy, C. Guizard, J. Palmeri
and L. Cot, Nanostruct. Mater., 1998, 10, 77; C. R. Xia, H. Q. Cao, H.
Wang, P. H. Yang, G. Y. Meng and D. K. Peng, J. Membr. Sci., 1999,
lend support to the conclusion that the tri-substituted precursor is
formed initially in this system. After 16 hours the spectra had also
changed with respect to the position of the peaks corresponding to
the bridging groups.
1
62, 181.
4
5
F. Ribot, P. Toledano and C. Sanchez, Chem. Mater., 1991, 3, 159.
S. V. Pol, V. G. Pol, G. Seisenbaeva, V. G. Kessler and A. Gedanken,
Chem. Mater., 2004, 16, 1793.
The rearrangement from tri- to tetra-substituted structures was
i
also observed on the dried Zr(O Pr)(acac)
3
crystals. After 15 days
6
V. G. Kessler, Chemistry and solution stability of alkoxide precursors, in
Handbook of Sol–Gel Science and Technology, ed. H. Kozuka, Kluwer
Academic Publishers, Boston, 2004, ch. 1, in press.
of storage at room temperature, the NMR data showed that the
majority of the sample had transformed to Zr(acac)
4
.
Recapitulating, upon addition of 2 mol equivalents of Hacac the
tri- and tetra-substituted species are formed. The tri-substituted
7 K. A. Fleeting, P. O’Brian, D. J. Otway, A. J. P. White, D. J. Williams and
A. C. Jones, Inorg. Chem., 1999, 38, 1432.
C h e m . C o m m u n . , 2 0 0 4 , 1 8 7 4 – 1 8 7 5
1875