Preparation of carboxylato-coordinated titanium alkoxides from carboxylic anhydrides: Alkoxido group transfer from metal atom to carbonyl group

Article abstract of DOI:10.1002/ejic.201200296

Reaction of titanium(IV) isopropoxide, Ti(OiPr)₄, with an equimolar amount of phthalic anhydride resulted in the transfer of an isopropoxido group from the metal atom to one carbonyl group of the anhydride and coordination of the thus formed mo

Full text of DOI:10.1002/ejic.201200296

FULL PAPER
DOI: 10.1002/ejic.201200296
Preparation of Carboxylato-Coordinated Titanium Alkoxides from Carboxylic
Anhydrides: Alkoxido Group Transfer from Metal Atom to Carbonyl Group
Matthias Czakler,[a] Christine Artner,[a] and Ulrich Schubert*[a]
Keywords: Titanium / Alkoxides / Carboxylic anhydrides / Carboxylate ligands / Hydrolysis
Reaction of titanium(IV) isopropoxide, Ti(OiPr)4, with an
equimolar amount of phthalic anhydride resulted in the
transfer of an isopropoxido group from the metal atom to one
carbonyl group of the anhydride and coordination of the thus
formed monoester to the titanium atom. One monoester li-
gand in Ti2(OiPr)6(μ2-OOC-C6H4-COOiPr)(η1-OOC-C6H4-
COOiPr)(iPrOH) is bridging and the other is η1-coordinated.
When the reaction is performed in the presence of 1 mol-
equiv. of acetic acid, the oxido cluster Ti6(μ3-O)6(OiPr)6(μ2-
OOC-C6H4-COOiPr)6 was instead obtained. The μ3-oxygen
groups in the latter compound are due to esterification of
acetic acid by the cleaved isopropyl alcohol.
coordinated water molecules were isolated from such reac-
tions, such as Zr6O4(OH)4(isobutyrate)12(H2O)[5] or a series
of hydrated yttrium carboxylates.[6] The carboxylic acids
employed in the reactions with metal alkoxides thus have a
dual role: as a source for the bidentate ligands to cap the
core of the formed clusters and to provide water through
esterification reactions.
In the work reported in this article, we attempted to de-
couple these two functions. To this end, we treated a car-
boxylic anhydride with Ti(OiPr)4. We chose the anhydride
of a dicarboxylic acid (phthalic anhydride), because the
groups formed during the reaction are linked with each
other and thus easier to identify. The results reported in
this article provide new insight into reactions of metal alk-
oxides with carboxylic acids.
Introduction
Carboxylic acids are frequently used to moderate the re-
activity of metal alkoxides, M(OR)n, for sol-gel processing.
This is not a straightforward reaction, however, because ox-
ido clusters of the type MaOb(OH/OR)c(OOCRЈ)d are often
obtained instead of the substitution products M(OR)n–x
-
(OOCRЈ)x.[1,2] The case of Ti(OR)4 has been particularly
well investigated. Spectroscopic analysis of the reaction of
Ti(OBu)4 with an equimolar quantity of acetic acid sug-
gested that [Ti(OR)3(OOCRЈ)]n (n = 2 or 3) was formed and
that the acetate ligands were bridging.[3] A corresponding
structure was found for crystalline Ti2(OCH2CMe3)6(OOC-
CMe3)2, with bulky alkoxido and carboxylato ligands.[4]
The compound Ti2(OiPr)6(μ2-OOC-CMe3)(η1-OOC-CMe3)-
(iPrOH) possibly represents an intermediate state in the co-
ordination of Ti(OR)4 by the second carboxylato ligand.
One carboxylato ligand is only monodentate, and the vac-
ant coordination site at the other titanium atom is occupied
by an isopropyl alcohol molecule hydrogen-bonded to the
dangling oxygen atom of the monodentate carboxylato li-
gand.[4] In most reactions of Ti(OR)4 with carboxylic acids,
however, carboxylato-coordinated oxido/alkoxido clusters
TiaOb(OR)c(OOCRЈ)d have been obtained.[2]
Results and Discussion
Reaction of Ti(OiPr)4 with an equimolar amount of
phthalic anhydride in isopropyl alcohol resulted in the
quantitative formation of crystalline Ti2(OiPr)6(μ2-OOC-
C6H4-COOiPr)(η1-OOC-C6H4-COOiPr)(iPrOH) (1) [Equa-
tion (1)]. The molecular structure of 1 is analogous to that
The most probable sequence of reactions leading to the
formation of such clusters is that one or more alkoxido li-
gands are replaced by carboxylato ligands in the first step
(see above), followed by esterification of the liberated
alcohol. The thus produced water could be the source of
oxido or hydroxido ligands in the clusters. This possibility
is also supported by a few cases, where compounds with
of
Ti2(OiPr)6(μ2-OOC-CMe3)(η1-OOC-CMe3)(iPrOH)
mentioned before.[4] The two titanium atoms in the asym-
metric dimer are bridged by two OiPr ligands and one
phthalate monoester ligand (Figure 1). The octahedral co-
ordination of Ti(1) is completed by coordination of an η1-
phthalate monoester and two terminal OiPr ligands, and
that of Ti(2) by three terminal OiPr ligands.
Because of the bridging carboxylato ligand, the [TiO6]
coordination octahedra are slightly tilted towards this li-
gand. The carbonyl oxygen O(13) of the η1-phthalic ester
and that of the adjacent OiPr ligand O(5) at the neigh-
boring titanium atom are clearly connected through a hy-
[a] Institute of Materials Chemistry, Vienna University of
Technology
Getreidemarkt 9, 1060 Wien, Austria
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M. Czakler, C. Artner, U. Schubert
FULL PAPER
which the proton of HY is transferred to an OR group with
subsequent elimination of ROH (Scheme 1; X = H, for Y
= OOCRЈ). In a similar manner, an alkoxido group could
be transferred to the RЈC(O) moiety of the anhydride with
concomitant formation of an ester (Scheme 1; X = RЈCO).
The resulting carboxylato ligand is coordinated to the tita-
nium atom. If a cyclic anhydride is used, as in the case of
phthalic anhydride, the ester and carboxylate groups remain
of course attached to each other.
(1)
Scheme 1.
No protic compounds are involved in the reaction with
phthalic anhydride. No water or hydroxido groups can
therefore be formed, and hence no oxido/hydroxido clusters.
To gain further insight into the role of carboxylic acids in
reactions with metal alkoxides, we modified the above reac-
tion in a way that an equimolar amount of acetic acid was
added to the isopropyl alcohol solution of Ti(OiPr)4 and
phthalic anhydride. This allowed a competition between a
carboxylic acid and a carboxylic anhydride as potential
sources for carboxylato ligands.
After a long reaction period, crystals of the centrosym-
metric cluster Ti6O6(OiPr)6(OOC-C6H4-COOiPr)6 (2) were
isolated [Equation (2)]. The IR spectrum of the supernatant
solution showed a broad peak at 1723 cm–1 indicating ester
formation. For comparison: the νCO band of isopropyl acet-
ate is at 1735 cm–1 and that of acetic acid at 1710 cm–1. The
cluster core of 2 can be described as a Ti6 octahedron in
which six of the eight triangular faces are capped by μ3-
oxygen atoms or as a slightly distorted hexagonal prism
with alternating titanium and oxygen atoms. The six phthal-
ate isopropyl ester ligands bridge the six four-membered
Ti2O2 rings of the hexagonal prism. Each titanium atom is
octahedrally coordinated by two phthalate ester groups, one
terminal OiPr ligand and three μ3-oxygen atoms. The ter-
minal OR ligands are oriented perpendicular to the slightly
puckered Ti3O3 rings. This structural motif was already
found in other Ti6O6 clusters obtained by reaction of tita-
nium alkoxides with various carboxylic acids[7] or oximes.[8]
The bond lengths and angles observed in 2 are in the same
range as in the reference compounds.
Figure 1. Molecular structure of Ti2(OiPr)6(μ2-OOC-C6H4-CO-
OiPr)(η1-OOC-C6H4-COOiPr)(iPrOH) (1). Selected distances [pm]
and angles [°]: Ti(1)–Ti(2) 322.74(6), Ti(1)–O(1) 177.82(13), Ti(1)–
O(2) 181.07(13), Ti(1)–O(6) 205.37(12), Ti(1)–O(7) 204.92(12),
Ti(1)–O(8) 210.41(11), Ti(1)–O(12) 198.09(12), Ti(2)–O(3)
181.92(12), Ti(2)–O(4) 177.34(12), Ti(2)–O(5) 207.20(12), Ti(2)–
O(6) 202.85(11), Ti(2)–O(7) 202.86(12), Ti(2)–O(9) 206.17(11);
Ti(1)–O(6)–Ti(2) 104.48(5), Ti(1)–O(7)–Ti(2) 104.64(5), O(7)–
Ti(1)–O(6) 72.82(5), O(6)–Ti(1)–O(8) 85.80(5), O(7)–Ti(1)–O(8)
84.57(5), O(1)–Ti(1)–O(6) 167.30(5), O(2)–Ti(1)–O(7) 163.25(5),
O(12)–Ti(1)–O(8) 175.49(5), O(4)–Ti(2)–O(6) 168.56(5), O(3)–
Ti(2)–O(7) 166.26(5), O(6)–Ti(2)–O(7) 73.78(5), O(9)–Ti(2)–O(5)
171.52(5).
drogen bond [O(5)···O(13) 260.3(2) pm]. The hydrogen
atom was located in difference Fourier maps, and was close
to O(5) of the OiPr ligand [O(5)–H(5D) 81(3) pm] with an
O(5)–H(5D)–O(13) angle of 175(3)°, corresponding to a co-
ordinated iPrOH molecule. This can also be concluded
from the Ti(2)–O(5) distance [207.20(12) pm], which is
much longer than that of the Ti–O distances of the other
terminal OiPr ligands (177.8–181.9 pm). Furthermore, the
C(33)–O(13) distance is 124.2(2) pm, which is only slightly
longer than that of the ester CO group in the bridging li-
gand [C(29)–(O11) 120.4(2) pm]. The stabilization of an η1-
carboxylato ligand by means of a hydrogen bond to the
oxygen atom of an adjacent ligand was also observed in
Zr6O4(OH)4(isobutyrate)12(HX) (HX = H2O or BuOH).[5]
Solution NMR spectroscopic data are in line with the solid-
state structure, but a clear assignment of individual signals
(especially in the phenyl and OiPr areas) is not possible due
to the different groups with similar chemical shifts.
(2)
Although the general structure type represented by 1 has
been observed before (see above), the formation of 1 is re-
Compound 2 only contains phthalate monoester ligands
markable. Substitution of metal alkoxides is usually per- but no acetato ligand (Figure 2). This is remarkable, be-
formed by reaction with protic compounds (HY) during cause two potential sources for carboxylato ligands were
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Carboxylato-Coordinated Titanium Alkoxides
Figure 2. Molecular structure of Ti6O6(OiPr)6(OOC-C6H4-COOiPr)6 (2). Selected distances [pm] and angles [°]: Ti(1)–O(1) 190.3(4),
Ti(1)–O(2) 187.0(4), Ti(1)–O(3) 213.7(4), Ti(1)–O(4) 176.0(4), Ti(1)–O(7) 205.6(5), Ti(1)–O(11) 206.4(4), Ti(2)–O(1) 215.5(4), Ti(2)–O(2)
192.1(4), Ti(2)–O(3) 187.3(4), Ti(2)–O(5) 174.7(4), Ti(2)–O(8) 205.0(5), Ti(2)–O(15) 207.7(4), Ti(3)–O(1) 188.5(4), Ti(3)–O(2) 216.8(4),
Ti(3)–O(3) 192.5(4), Ti(3)–O(6) 176.6(4), Ti(3)–O(12) 206.3(5), Ti(3)–O(16) 208.2(4); O(1)–Ti(1)–O(2) 103.0(2), O(1)–Ti(1)–O(3) 77.5(2),
O(1)–Ti(1)–O(4) 99.7(2), O(1)–Ti(1)–O(7) 86.0(2), O(1)–Ti(1)–O(11) 158.6(2), O(1)–Ti(2)–O(2) 78.0(2), O(1)–Ti(2)–O(3) 77.7(2), O(1)–
Ti(2)–O(5) 177.8(2), O(1)–Ti(2)–O(8) 88.5(2). O(1)–Ti(2)–O(15) 88.1(2), O(1)–Ti(3)–O(2) 78.4(2), O(1)–Ti(3)–O(3) 101.4(2), O(1)–Ti(3)–
O(6) 105.3(2), O(1)–Ti(3)–O(12) 157.9(2), O(1)–Ti(3)–O(16) 86.5(2).
present in the reaction mixture (in a 1:1 ratio). It is rather
When the system was modified in a way that slow in-
obvious that the phthalate monoester ligands were formed ternal water production was possible (with otherwise the
by the same reaction as in 1. On the other hand, the μ3- same reaction conditions), the oxido/alkoxido cluster 2 was
oxygen atoms in 2 must be due to esterification of acetic obtained instead. The added acetic acid is the only possible
acid. Since formation of 2 was much slower than that of 1, source of the oxido groups. If one assumes that formation
it can be assumed that reaction of the anhydride (giving the of 1 is the first step in the formation of 2 (because forma-
phthalate monoester ligands) is faster than that of acetic tion of 1 is very fast), two OiPr ligands must be sub-
acid (resulting in partial hydrolysis). Although we could not sequently be replaced by one oxido groups, while the Ti/
identify other species, which may be present in the reaction OOCR ratio is retained. The formal equation 2 iPrO–
+
solution, it appears that the role of acetic acid was to pro- CH3COOH Ǟ CH3COOiPr + iPrOH + O2– shows that
vide the water for hydrolysis of part of the alkoxido groups 1 mol-equiv. of acetic acid per titanium is sufficient to ex-
(through formation of isopropyl acetate), while phthalic an- plain the outcome of the overall reaction.
hydride served to replace part of the alkoxido groups.
Experimental Section
Conclusions
General: All compounds were handled under argon by using stan-
dard Schlenk techniques. Isopropyl alcohol was dried by distilling
The results reported in this paper strongly support the
twice from sodium, acetic acid was freshly distilled from P2O5 prior
notion that the first step of the reaction of Ti(OR)4 with
to use, Ti(OiPr)4 and phthalic anhydride were used as received from
carboxylic acids is the formation of carboxylato-coordi-
nated titanium alkoxides Ti(OR)n–x(OOCRЈ)x (x = 1 or 2).
In the case studied here, i.e. the reaction of a carboxylic
anhydride, the carboxylato ligand was formed by a route
that excludes the formation of water or OH species and thus
the formation of oxido/hydroxido clusters. The carboxylato
ligand, which eventually was coordinated to the titanium
Aldrich.
Ti2(OiPr)6(μ2-OOC-C6H4-COOiPr)(η1-OOC-C6H4-COOiPr)-
(iPrOH) (1): Ti(OiPr)4 (1.08 g, 3.8 mmol) was added to a suspen-
sion of phthalic anhydride (560 mg, 3.8 mmol) in isopropyl alcohol
(291 μL, 3.8 mmol). The mixture was heated until a clear solution
was obtained. Crystals were obtained at room temperature within
1
24 h. Yield 1.86 g (100%). H NMR (CD2Cl2, 300 MHz): δ = 1.29
atom, was generated by OR group transfer from the metal
atom to the other CO group of the anhydride. Formation
of 1 was a surprisingly fast and quantitative process.
(d, J = 6.1 Hz, 42 H, CH3), 1.39 (d, J = 6.0 Hz, 12 H, CH3, iso-
propyl ester), 4.60–5.20 (7 H, CH, OiPr), 5.25 (m, 2 H, CH, iso-
propyl ester), 7.3–7.7 (m, 6 H, CH, Ph), 8.12 (2 H, CH-C-COOTi)
Eur. J. Inorg. Chem. 2012, 3485–3489
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M. Czakler, C. Artner, U. Schubert
FULL PAPER
Table 1. Crystallographic data for 1 and 2.
1
2
Empirical formula
Mr
C43H71O15Ti2
923.8
C84H108O36Ti6
1981.1
Crystal system
Space group
triclinic
P1
triclinic
P1
¯
¯
a [pm]
1148.9(2)
1290.95(7)
b [pm]
1177.70(18)
1368.20(8)
c [pm]
1944.0(3)
1458.24(7)
α [°]
103.090(8)
85.570(4)
β [°]
97.570(9)
66.340(4)
γ [°]
99.780(8)
2484.7(7)
78.120(4)
2308.6(2)
V [pm3ϫ106]
Z
2
1
Dx [Mgm–3]
1.236
0.382
1.425
0.579
μ [mm–1]
Crystal size [mm]
No. measured, independent, observed refl. [I Ͼ 2σ (I)]
Rint
0.4ϫ0.32ϫ0.3
72908, 14419, 11404
0.0356
0.2ϫ0.18ϫ0.14
11300, 6535, 3965
0.0476
θmax [°]
30.01
23.25
R [F2 Ͼ 2σ(F)], wR (F2), S
No. reflections/parameters
Weighting scheme
δρmax, δρmin [eÅ–3]
0.0439, 0.1206, 1.028
14419/635
0.0670, 0.2028, 1.027
6535/580
w = 1/[σ2(Fo2) + (0.0529P)2 + 1.8451P][a]
1.312, –0.693
w = 1/[σ2(Fo2) + (0.1108P)2 + 1.4297P][a]
0.946, –0.602
2
2
[a] P = (Fo + 2Fc )/3.
ppm. 13C NMR (CD2Cl2, 75 MHz): δ = 21.4 (CH3, ester), 24.71
(CH3, iPrOH, H-bond), 25.55 (CH3, OiPr), 68.4 (CH, ester, H-
bonded), 69.3 (CH, ester), 76.2–78.2 (CH, OiPr, terminal), 79–81
(CH, OiPr, bridging), 127–136 (CH, aryl), 166.6 (COO, ester),
tified in the electron density map. The carbon atoms of almost all
OiPr ligands of 1 were disordered, especially that of the non-bridg-
ing OiPr. Their positions were refined with two sites, with about
50% occupancy each. CCDC-862908 (for 1) and -862909 (for 2)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
167.8 (COO, ester, H-bonded), 182.5 (COO-Ti) ppm. IR: ν = 2972
˜
(w, C–H), 1723 (m, C=O, ester), 1550 (m, C=O, acetate), 1493 (w),
1403 (m), 1290 (m), 1109 (s), 1076 (m), 1011 (m), 851 (w), 822 (w)
cm–1.
Ti6O6(OiPr)6(OOC-C6H4-COOiPr)6
4.25 mmol) was added to a suspension of phthalic anhydride
(630 mg, 4.25 mmol) in mixture of acetic acid (255 mg,
(2):
Ti(OiPr)4
(1.2 g,
Acknowledgments
a
4.25 mmol) and isopropyl alcohol (1 mL, 17 mmol). The suspen-
sion was heated until a clear solution was obtained. Colorless crys-
tals were obtained at room temperature after 10 weeks. Yield
This work was supported by the Fonds zur Förderung der wissen-
schaftlichen Forschung (FWF), Austria (Project P22536).
1
600 mg (43%). H NMR (CD2Cl2, 250 MHz): δ = 0.8–1.7 (72 H,
CH3), 4.5–5.3 (12 H, CH), 7.1–8.1 (24 H, aryl) ppm. 13C NMR
(CD2Cl2, 63 MHz): δ = 21–25 (CH3, OiPr), 68.5–69.5 (CH, ester),
79–80 (CH, OiPr), 128–135 (C, aryl), 166–168 (COOiPr, ester) 174–
[1] U. Schubert, Acc. Chem. Res. 2007, 40, 730–737.
[2] U. Schubert, J. Mater. Chem. 2005, 15, 3701–3715.
[3] S. Barboux-Doeuff, C. Sanchez, Mat. Res. Bull. 1994, 29, 1–
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[4] T. J. Boyle, R. P. Tyner, T. M. Alam, B. L. Scott, J. W. Ziller,
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[5] F. R. Kogler, M. Jupa, M. Puchberger, U. Schubert, J. Mater.
Chem. 2004, 14, 3133–3138.
180 (COO, acetate) ppm. IR: ν = 2972 (w, C–H), 1726 (m, C=O,
˜
ester), 1555 (m, C=O, acetate), 1400 (s), 1275 (m), 1108 (m), 1075
(m), 1008 (m), 950 (m), 840 (m) cm–1.
X-ray Structural Analyses: All measurements were performed at
100 K by using Mo-Kα (λ = 0.71073 Å) radiation. Data was col-
lected with a Bruker AXS SMART APEX II four-circle dif-
fractometer with κ-geometry. Data were collected with φ- and ω-
scans and 0.5° frame width. The data were corrected for polariza-
tion and Lorentz effects, and an empirical absorption correction
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PLATON.[11] Details of the X-ray investigations are given in
Table 1. The structures were solved by the Patterson method
(SHELXS-97).[12] Refinement was performed by the full-matrix
least-squares method based on F2 (SHELXL-97)[13] with aniso-
tropic thermal parameters for all non-hydrogen atoms. Hydrogen
atoms were inserted in calculated positions and refined riding with
the corresponding atom, those bonded to oxygen atoms were iden-
[6] H. Fric, M. Jupa, U. Schubert, Monatsh. Chem. 2006, 137, 1–
6.
[7] This cluster type obtained from titanium alkoxides and carbox-
ylic acids: R. Papiernik, L. G. Hubert-Pfalzgraf, J. Vaisser-
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Alam, B. L. Scott, J. W. Ziller, B. G. Potter, Jr., J. Am. Chem.
Soc. 1999, 121, 12104–12112; P. S. Ammala, S. R. Batton,
C. M. Kepert, L. Spiccia, A. M. Van der Bergen, B. O. West,
Inorg. Chim. Acta 2003, 353, 75–81; P. Piszczek, A. Grodzicki,
M. Richert, A. Wojtczak, Inorg. Chim. Acta 2004, 357, 2769–
2775.
[8] S. O. Baumann, M. Bendova, H. Fric, M. Puchberger, C. Visi-
nescu, U. Schubert, Eur. J. Inorg. Chem. 2009, 3333–3340.
[9] SADABS, Bruker AXS Inc., Madison, Wisconsin, USA, 2001.
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Carboxylato-Coordinated Titanium Alkoxides
[10] SAINT PLUS, Bruker AXS Inc., Madison, Wisconsin, USA,
2007.
[11] A. L. Spek, J. Appl. Crystallogr. 2003, 36, 7–13.
[12] G. M. Sheldrick, SHELXS-97, Program for Crystal Structure
Determination, University of Göttingen, Göttingen, Germany,
1997.
[13]
G. M. Sheldrick, SHELXL-97, Program for Crystal Structure
Determination, University of Göttingen, Göttingen, Germany,
1997.
Received: March 23, 2012
Published Online: June 13, 2012
Eur. J. Inorg. Chem. 2012, 3485–3489
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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