Preservation of the cluster core upon formation of Ti3O(OPri)8(benzoate)2 from Ti3O(OR)10

Article abstract of DOI:10.1039/b208255a

The cluster Ti₃O(OPrⁱ)₈(benzoate)₂ is obtained upon reaction of Ti₃O(OPrⁱ)₁₀ or Ti₃O(OPrⁱ)₉(OMe) with benzoic acid. The remarkable feature of this substitution reaction is that the cluster structure is approximately preserved. Ti₃O(OPrⁱ)₈(benzoate)₂ undergoes a dynamic exchange processes of the propoxy groups in solution.

Full text of DOI:10.1039/b208255a

View Article Online / Journal Homepage / Table of Contents for this issue
Preservation of the cluster core upon formation of
Ti3O(OPri)8(benzoate)2 from Ti3O(OR)10
Ivan Mijatovic, Guido Kickelbick, Michael Puchberger and Ulrich Schubert*
Institute of Materials Chemistry, Vienna University of Technology, Getreidemarkt 9,
A-1060, Wien, Austria. E-mail: uschuber@mail.zserv.tuwien.ac.at
L e t t e r
Received (in Montpellier, France) 22nd August 2002, Accepted 25th September 2002
First published as an Advance Article on the web 7th November 2002
The cluster Ti3O(OPri)8(benzoate)2 is obtained upon reaction
of Ti3O(OPri)10 or Ti3O(OPri)9(OMe) with benzoic acid. The
remarkable feature of this substitution reaction is that the cluster
structure is approximately preserved. Ti3O(OPri)8(benzoate)2
undergoes a dynamic exchange processes of the propoxy groups
in solution.
are the same as those of the leaving groups. For example, when
Ti16O16(OEt)32 was reacted with small proportions of car-
boxylic acids, a fraction of the bridging OEt groups was
replaced by bridging carboxylate groups; the resulting cluster
has not been structurally characterized. For higher carboxyl-
ate:Ti proportions, the cluster was degraded.2 However, the
presence of bridging alkoxo ligands does not guarantee that
substitution by bidentate ligands will take place. Reaction of
Ti7O4(OEt)20 with benzoic acid resulted in the formation of
the new cluster Ti6O4(OEt)14(OOCPh)2 with concomitant
major rearrangement of the cluster core, despite the presence
of bridging OR groups in the starting cluster.3 Even complete
degradation may occur, as observed for the reaction of
Zr4O2(OMc)12 with acetylacetone.4
We now report a rare example where the general structure of
a titanium oxo alkoxo cluster was preserved upon substitution
of alkoxo ligands by carboxylates. When a toluene solution of
the cluster Ti3O(OPri)10 (1a) or Ti3O(OPri)9(OMe) (1b)5 was
reacted with 1.5 equivalents of benzoic acid, the new cluster
Ti3O(OPri)8(OOCPh)2 (2) was obtained in high yield (eqn. 1).
A higher portion of benzoic acid (such as 2 equivalents as
required by eqn. 1) leads to a degradation of 2 and to lower
yields.
Organically modified transition-metal oxide clusters
(OMTOCs) are interesting building blocks for the synthesis
of inorganic–organic hybrid materials.1 The most general
approach is to use complexing multidentate ligands, such
as carboxylates, sulfonates, phosphonates, b-diketonates,
etc., to bind the organic groups, which may carry functional
groups, directly to the metal atoms at the surface of the cluster.
OMTOCs with multidentate organic ligands can be pre-
pared by two strategies. The multidentate groups can either
be grafted to a pre-formed cluster (‘‘surface modification’’
method) or introduced during the cluster synthesis (‘‘in-situ’’
method). The latter method has been successfully used for
the preparation of a variety of carboxylate- or b-diketonate-
substituted metal oxide clusters by reaction of metal alkoxides
with carboxylic acids or b-diketones,1 where concomitant ester
formation results in the formation of oxo and hydroxo ligands.
The surface modification method is less general. The reason
is that the post-synthesis modification of the cluster surface
by organic groups requires (i) reactive surface groups, such
as OH, Cl or OR, and (ii) the simultaneous balancing of
charges and co-ordination numbers upon substitution of these
groups. Substitution can therefore only be expected to proceed
without major difficulties, if both the number of the occupied
co-ordination sites and the charges of the entering ligands
Ti3OðORÞ10 þ 2 PhCOOH !
1
Ti3OðOPriÞ8ðOOCPhÞ2 þ 2 ROH ð1Þ
2
The molecular structure of 2 was determined by an X-ray
structure analysis. Fig. 1 shows the comparison with the struc-
ture of the starting cluster 1b (1a is isostructural5). The three
titanium atoms in 1 are capped by one oxygen atom and one
alkoxy group. Three propoxy groups bridge the edges of the
Fig. 1 A comparison of the structures of Ti3O(OPri)9(OMe) (1b) (left) and Ti3O(OPri)8(OOCPh)2 (2) (right).
DOI: 10.1039/b208255a
New J. Chem., 2003, 27, 3–5
3
This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2003

View Article Online
Ti3 triangle, and the remaining propoxy groups are terminally
bonded to the titanium atoms. In 2, one of the benzoate
ligands (O9/O11) replaces a bridging alkoxy group. According
to the general considerations made above, this is an expected
reaction because both the co-ordination numbers and charges
are preserved. However, the group substituted by the second
benzoate ligand (O30/O32) is the m3 alkoxy group capping
the Ti3 triangle. Because a tridentate ligand is replaced by a
bidentate ligand with the same charge, one of the co-ordina-
tion sites at one of the titanium atoms must be left empty.
One titanium atom (Ti2) does indeed change its co-ordination
number from 6 to 5.
Clusters of this structural type were already obtained, and
structurally characterized, upon reaction of Ti(ONp)4
(Np ¼ CH2CMe3) with acetic acid or tert-butylacetic acid;6
the acetate derivative was postulated as an intermediate in
the formation of Ti6O4(OPri)12(acetate)4 from Ti(OPri)4 and
acetic acid.7
The dynamic behavior of compound 2 in CD2Cl2 solution
was studied at different temperatures by 1D and 2D NMR
spectroscopy. The proton spectrum at 298 K showed reso-
nances in three distinct regions, i.e. 8.3–7.1 ppm (C6H5), 5.3–
4.3 ppm (very broad and unstructured, CH) and 1.6–0.9
ppm (CH3). In the HSQC and HMBC spectra six resolved
OiPr signals were observed, five assigned to OiPr groups from
the cluster and one from free isopropanol.8 In the EXSY spec-
trum at room temperature (Fig. 2), exchange signals were
observed between all the different OiPr groups.
By cooling the solution to 233 K, a sharpening of the broad
proton signals of the CH groups of the OiPr groups was
observed. When the solution was further cooled to 193 K,
the CH signals of the OiPr groups showed again a broaden-
ing and a further splitting. In contrast, the aromatic region
remained nearly unchanged. The HSQC (Fig. 3) and HMBC
spectra at this temperature showed now signals of eight dis-
tinct isopropanolate groups, seven from the cluster and one
of the free isopropanol. At this temperature, exchange of the
OiPr groups was no longer observed.
Fig. 3 HSQC spectrum of 2 (CD2Cl2) at 193 K (CH region).
carboxylate ligands. We found that only Ti3O(OR)10 under-
goes substitution with concomitant preservation of the general
cluster structure (this work). Ti7O4(OEt)20 also reacts with
benzoic acid, but the cluster structure is completely re-orga-
nized.3 When Ti11O13(OiPr)18 is reacted with benzoic acid,
complete degradation is observed. The same was observed
for Ti12O16(OiPr)16
2
.
Experimental
Titanium oxo alkoxo clusters (with only oxo and alkoxo
substituents) range in size from Ti3O(OR)10 to Ti17O24-
All reactions were carried out using standard Schlenk techni-
ques in an argon atmosphere. Argon was purified using Cu
9
(OiPr)20
.
A general reactivity pattern emerges. While some
˚
and molecular sieve (4 A). The glass equipment was flame-
of the alkoxo ligands in the bigger clusters (starting with
Ti11O13(OiPr)1810) can be replaced by other alkoxo groups,
the opposite is true for the exchange of alkoxo groups against
dried. The solvents were made oxygen-free and water-free by
standard methods. The starting clusters 1a and 1b were pre-
pared according to the literature.5 The NMR spectra were
recorded on a Bruker Avance 300, 300.13 MHz {1H} and
75.47 MHz {13C}, equipped with a 5 mm broadband head
and a z-gradient unit. The 2D spectra were measured accord-
ing to Bruker standard pulse sequences: HSQC (Heteronuclear
Single Quantum Correlation), HMBC (Heteronuclear Multi-
ple Bond Correlation), EXSY (Exchange Spectroscopy); a
mixing time of 1 s was used.
Synthesis of 2 from 1
The solution of 0.41 g (3.37 mmol) of benzoic acid in 5 ml
toluene was added dropwise to a stirred solution of 1.73 g
(2.25 mmol) of 1a in 15 ml of toluene. Stirring was continued
at room temperature for 14 h, then all volatile compounds
were removed in vacuo. The solid residue was dissolved in 4
ml of toluene, and 20 ml of acetonitrile were added. Storage
of the solution at ꢀ30 ꢁC resulted in the crystallization of 2
within two weeks (1.20 g, 61% yield relative to the employed
amount of 1a).
The same cluster was obtained from 0.73 g (1.00 mmol) of
1b and 0.18 g (1.51 mmol) of benzoic acid by the same proce-
dure (0.60 g, 69% yield relative to the employed amount of 1b).
Compound 2. NMR (CD2Cl2 , 193 K, 1H/13C; d/ppm):
Fig. 2 EXSY spectrum of 2 (CD2Cl2) at room temperature.
benzoate (12 signals from 14 possible 13C signals resolved):
4
New J. Chem., 2003, 27, 3–5

View Article Online
8.12/130.2, 8.00/129.5, 7.52/132.5, 7.44/128.4, 7.31/129.0,
7.23/128.4, 7.17/129.1, 7.15/125.4, 134.8, 138.7, 170.2, 171.6;
CH of OiPr (8 signals from 9 possible resolved): 5.22/78.3,
Acknowledgements
This work was supported by the the Jubila¨umsstiftung der
Stadt Wien.
i
5.16/80.0, 5.00/76.8, 4.92/78.7, 4.76/77.6 (free PrOH), 4.75/
74.6, 4.53/74.3, 4.35/70.7; CH3 of OiPr (9 signals): 1.42/
24.4, 1.40/23.2, 1.29/25.7, 1.25/23.6, 1.18/26.4 (free PrOH),
i
References
1.17/25.3, 1.11/25.1, 1.09/23.4, 0.96/25.1. Anal. calc. for
C38H66O13Ti3 : C 52.2, H 7.6, Ti 16.4. Found: C 50.3, H 7.6,
Ti 16.4%.
1
F. Ribot and C. Sanchez, Comments Inorg. Chem., 1999, 20, 327;
G. Kickelbick and U. Schubert, Monatsh. Chem., 2001, 132, 13;
U. Schubert, Chem. Mater., 2001, 13, 3487; C. Sanchez, G. J. de
A. A. Soler-Illia, F. Ribot, T. Lalot, C. R. Mayer and V. Cabuil,
Chem. Mater., 2001, 13, 3061; G. Kickelbick, Progr. Polymer Sci.,
in press.
X-Ray crystallography
2
G. J. de A. A. Soler-Illia, L. Rozes, M. K. Boggiano, C. Sanchez,
C.-O. Turrin, A.-M. Caminade and J.-P. Majoral, Angew. Chem.,
Int. Ed., 2000, 39, 4248; G. J. de A. A. Soler-Illia, E. Scolan, A.
Loius, P.-A. Albouy and C. Sanchez, New J. Chem., 2001, 25, 156.
I. Mijatovic, G. Kickelbick and U. Schubert, Eur. J. Chem., 2001,
1933.
B. Moraru, G. Kickelbick, M. Batistella and U. Schubert,
J. Organomet. Chem., 2001, 636, 172.
V. W. Day, T. A. Eberspacher, Y. Chen, J. Hao and W. G.
Klemperer, Inorg. Chim. Acta, 1995, 229, 391.
T. J. Boyle, R. P. Tyner, T. M. Alam, B. L. Scott, J. W. Ziller and
B. G. Potter Jr., J. Am. Chem. Soc., 1999, 121, 12 104.
N. Steunou, F. Robert, K. Boubekeur, F. Ribot and C. Sanchez,
Inorg. Chim. Acta, 1998, 279, 144.
The sample used for NMR spectroscopy still contained some
isopropanol from the preparation procedure. Residual isopropa-
nol is difficult to remove by drying, even in high vacuum, possibly
due to hydrogen bonding. Only crystallization yields isopropanol-
free samples of 2.
Single crystal data were collected on a Siemens SMART dif-
fractometer with a CCD area detector (Mo-Ka radiation,
l ¼ 71.073 pm). The data were corrected for polarization
and Lorentz effects, and an empirical absorption correction
(SADABS) was applied. The cell dimensions were refined with
all unique reflections. The structure was solved by direct meth-
ods (SHELXS86). Refinement was carried out with the full-
matrix least-squares method based on F2 (SHELXL93) with
anisotropic thermal parameters for all non-hydrogen atoms.
Hydrogen atoms were inserted in calculated positions and
refined riding with the corresponding atom.
CCDC reference number 188068. See http://www.rsc.org/
suppdata/nj/b2/b208255a/ for crystallographic data in CIF
or other electronic format.
Crystallographic data for 2: C38H66O13Ti3 : M ¼ 874.6, mono-
clinic, space group P21/c, a ¼ 1191.42(7), b ¼ 3793.0(2),
c ¼ 1140.83(7) pm, b ¼ 115.129(1), V ¼ 4667.6(6) ꢂ 106 pm3,
Z ¼ 4, T ¼ 294 K, m ¼ 0.555 mmꢀ1, 11 463 independent
reflections, Rint ¼ 0.0253, R1 ¼ 0.0517, wR2 ¼ 0.1178.
3
4
5
6
7
8
9
N. Steunou, G. Kickelbick, K. Boubekeur and C. J. Sanchez,
J. Chem. Soc., Dalton Trans., 1999, 3653.
10 V. W. Day, T. A. Eberspacher, W. G. Klemperer and C. W. Park,
J. Am. Chem. Soc., 1993, 115, 8469.
New J. Chem., 2003, 27, 3–5
5