An oxygen-centered titanium square embedded in a cuboctahedron of iodine in the salt K4[{Ti4O}I12]

Article abstract of DOI:10.1002/anie.200453739

A round peg in a square hole: The unusual bonding situation within the cluster ion [{Ti₄O}I₁₂]^4- (see picture) arises through the optimization of strong Ti-Ti and Ti-O σ bonding as well as to the absence of Ti-Ti multiple (π) bonding and the avoidance of Ti-I antibonding interactions.

Full text of DOI:10.1002/anie.200453739

Angewandte
Chemie
Cluster Compounds
Four out of six faces of the cube of an iodine cuboctahe-
dron with shortest I–I separations of 396 to 407 pm are
centered by titanium atoms. This square of titanium atoms is
itself centered by an oxygen atom, so that an isolated ion
[{Ti4O}I12]4À is created (Figure 1). These anions are arranged
An Oxygen-Centered Titanium Square
Embedded in a Cuboctahedron of Iodine in the
Salt K4[{Ti4O}I12]**
Liesbet Jongen, Anja-Verena Mudring, Angela Möller,
and Gerd Meyer*
The di- and triiodides of titanium, TiI2 and TiI3, are excellent
and chemically simple examples with which to study the
formation of metal–metal bonds. According to single-crystal
data TiI3 undergoes a reversible one-dimensional Peierls
transition at about 300 K.[1] The low-temperature structure
corresponds to that of ZrI3,[2] with Ti Ti bond lengths of
À
309.6(2) pm at 293 K. TiI2 exhibits the same anionic structure,
that is, the hexagonal closest-packing of iodine ions. At high
temperatures, half of all the octahedral interstices are
occupied by Ti2+ ions just as in the CdI2 type of structure.
The TiI2 structure undergoes a two-dimensional Peierls
distortion upon cooling, at about 240 K, with a large
hysteresis of about 110 K, according to magnetic data.[1]
Thereby triangular clusters {Ti3}6+ are formed with single
Figure 1. Crystal structure of K4[{Ti4O}I12].
À
Ti Ti bonds of 300 pm. These triangles are also known from
Ti7Cl16 =^ Ti[{Ti3}Cl8]2,[3] KTi4Cl11,[4] and Na2[{Ti3}Cl8][5] with
bond lengths of 295.4, 295.5, and 299.6 pm, respectively. To
some extent, the formation of the carbon-stabilized octahe-
in a tetragonal body-centered fashion and held together by
potassium cations with K+–IÀ separations ranging from
347.0(1) to 404.5(4) pm (Figure 1).
The Ti–Ti separations within the {Ti4O} square are
unusually short, only 286.0(2) pm at 293 K. This situation
also accounts for short Ti–O separations of 202.5(1) pm, a
distance that is well known from Ti2O3.[10] With an oxidation
[6]
dral cluster in {Ti6C}Cl14 can be interpreted as a three-
dimensional Peierls distortion. In this case the Ti Ti bonds
are between 300 and 310 pm.
À
We have now found a titanium square with a central
oxygen atom, {Ti4O}, in the salt K4[{Ti4O}I12]. This salt was
first obtained in the pursuit of crystal growth of the complex
salt KTiI3. We are now able to produce pure samples of
K4[{Ti4O}I12] when potassium iodide, titanium(iv) iodide,
titanium, and titanium dioxide are allowed to react in the
appropriate proportions in a sealed tantalum container at
798 K.[7] K4[{Ti4O}I12] is obtained as good-quality black
crystals with metallic luster. The crystal structure has a
tetragonal body-centered symmetry, space group I4/m. No
structural changes are observed between 300 and 10 K,
according to X-ray powder diffractometry and single-crystal
structure determinations at 293 and 170 K.[8]
À
state of + 2.5 for titanium, the short Ti Ti separation may
À
either be the result of strong Ti Ti bonding (1.5 electrons can
À
be allocated to each of the four Ti Ti bonds in the square) or
2À
of strong Ti2.5+
bonding. Naturally a quasi one-dimen-
À
O
sional Peierls distortion of the square to a rectangle would be
expected if such a simple picture should hold some truth (see
below for a better theoretical description). Ti–I separations
are within a narrow range between 282 and 287 pm, averaging
to 284 pm. This mean distance is in good agreement with the
286 pm found in CsTi4.3I11.[4] All Ti-O-Ti, O-Ti-I and I-Ti-I
angles are 90 and 1808, or close to these ideal values deviating
by 28 at most.
Quantum mechanical calculations for solid K4[{Ti4O}I12]
(relativistic density functional theory and extended Hückel
calculations) and for the isolated cluster ion [{Ti4O}I12]4À
(extended Hückel calculations) were performed.[11] Inde-
pendent of the method used, a consistent picture of the
electronic structure results.
[*] Dr. L. Jongen,Dr. A.-V. Mudring,Priv.-Doz. Dr. A. Möller,
Prof. Dr. G. Meyer
Institut für Anorganische Chemie
Universität zu Köln
Greinstrasse 6,50939 Köln (Germany)
Fax: (+49)221-470-5083
E-mail: gerd.meyer@uni-koeln.de
The band structure obtained from DFT-LMTO studies
(Figure 2, left) is characterized by comparatively flat bands
indicative of very well localized electronic levels. The narrow
bands around the Fermi level suggest an insulating behavior.
The density of states can be divided into several distinctive
regions. A projection of the density of states which allows for
the partitioning of the total density of states into contributions
of certain atoms or atomic orbitals (Figure 2, right) shows—
from lower to higher energies—mainly oxygen p character,
[**] This work has been supported by the Deutsche Forschungsge-
meinschaft,Bonn (Sonderforschungsbereich 608 “Komplexe Über-
gangsmetallverbindungen mit Spin- und Ladungsfreiheitsgraden
und Unordnung”) and by the Alexander von Humboldt-Stiftung
through a postdoc stipend to L.J. We are also grateful to Dr.
Vladislav Kataev and Dipl.-Phys. Markus Kriener for magnetic
measurements.
Angew. Chem. Int. Ed. 2004, 43, 3183 –3185
DOI: 10.1002/anie.200453739
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Communications
Figure 2. Band structure (left) and density of states (DOS,right) for
K4[{Ti4O}I12] obtained from LMTO studies.
followed by a broad region stemming predominantly from
iodine p orbitals. At even higher energies the titanium
d levels follow which are only partially occupied. Fatband
analysis of the valence bands of K4[{Ti4O}I12] shows that the
valence levels are made up predominantly of titanium dz2 and
dxy states (to comply with the local point-group symmetry,
angular-dependent parts of the wave functions are given with
respect to the local cluster coordinate system). A graphical
interpretation of the crystal overlap Hamiltonian population
(COHP) reveals that these levels are responsible for strong
Figure 4. Energy level diagram for the valence orbitals of [{Ti4O}I12]4À
(left) and corresponding orbitals (right) obtained from extended
Hückel calculations.
{Ti4O} unit is necessary for the optimization of the bonding in
this compound. Other interstitial atoms, such as carbon or
nitrogen, would lead to less stable radical compounds.
Furthermore, a comparison of the total energies obtained
from theoretical calculations (extended Hückel) show that
the total energy of the oxygen compound is about 10% higher
than that of a hypothetical oxygen-free compound. Note also
À
Ti Ti bonding interactions (Figure 3a). These interactions lie
right below the Fermi level and are locally of the same
magnitude as titanium–iodine interactions (Figure 3b).
À
the highly bonding COHPvalues of the Ti O bond (Fig-
À
ure 3c). As a result, in K4[{Ti4O}I12] the shortest Ti Ti
distance ever observed is found owing to the strong stabiliza-
tion by the oxygen atom. Without the interstitial atom,
energetically higher cluster orbitals which no longer have
À
s symmetry with respect to the Ti Ti bond would have to be
occupied. To avoid this situation, incorporation of the oxygen
atom in accord with the general tendency—well known for
complex compounds and solids—that transition metals (espe-
cially the early ones) of the first series, in strong contrast to
their heavier homologues try to avoid multiple metal-metal
bonds.
Figure 3. Crystal overlap Hamiltonian population (COHP) analysis of
the data from LMTO studies for the a) Ti–Ti,b) Ti–I,and c) Ti–O inter-
actions in K4[{Ti4O}I12].
First magnetic measurements[12] reveal a small temper-
ature independent paramagnetic (TIP) behavior for
K4[{Ti4O}I12], in perfect accord with the calculated band
structure. Furthermore, X-band ESR measurements between
2 and 300 K do not show a bulk signal. Below 20 K a very
small signal with a g value of approximately 1.91 is observed
which can be assigned to a TiIII (d1) impurity. The ESR results
confirm the presence of oxygen in this cluster compound,
since carbon and nitrogen should give a bulk signal (e.g. g ꢀ
2.0) owing to the odd total number of electrons and the
resulting paramagnetic behavior.
For a more localized analysis of the bonding interactions,
extended Hückel calculations were carried out for a distinct
[{Ti4O}I12]4À cluster. The resulting molecular levels correlate
directly with the states in the extended solid.
A closer inspection of the frontier orbitals of this cluster
À
(Figure 4) reveals that three Ti Ti s-bonding HOMOs of b2g
and eu symmetry exist. Additionally, the eu orbital set has an
oxygen p-orbital contribution.
À
Directly above the Fermi level a slightly bonding Ti Ti
level of a1g symmetry is found (c.f. Figure 3a and Figure 4b).
Population with electrons up to this energy, however,
In conclusion, with K4[{Ti4O}I12] we have a salt that
contains an anion consisting essentially of a {Ti4O}8+ cluster
À
becomes unfavorable as the resulting weak Ti Ti bonding is
embedded in
a cuboctahedron of iodide ions. This
overcompensated by a strongly antibonding Ti I interaction
[{Ti4O}I12]4À ion is an excellent example of an optimized
À
À
À
(see Figure 3b). Thus, the electron count of six electrons per
bonding situation where strong Ti Ti and Ti O s bonding as
3184
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Chemie
À
trum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany
well as the absence of Ti Ti multiple (p) bonding and,
(fax: (+ 49)7247-808-666; e-mail: crysdata@fiz-karlsruhe.de),
on quoting the depository numbers CSD-413504 and CSD-
413505.
À
furthermore, the avoidance of unfavorable Ti I antibonding
interactions stabilize the cluster anion.
[9] W. Klemm, L. Grimm, Z. Anorg. Allg. Chem. 1942, 249, 198.
[10] W. R. Robinson, J. Solid State Chem. 1974, 9, 255.
Received: January 12, 2004 [Z53739]
[11] Theoretical calculations: Semi-empirical extended Hückel cal-
culations (EH) of the whole solid K4[{Ti4O}I12] were performed
with the program package CAESAR [J. Ren, W. Liang, M.-H.
Whangbo, CAESAR, PrimeColor Software Inc., Raleigh, NC,
USA, 1998] with the following parameters: Hij [eV] (z1): O: 2s
À32.29999 (2.275), 2p À14.8 (2.275), I: 5s À18.0 (2.679), 5p À12.7
(2.322) K: 4s À4.34 (0.874), 4p À2.73 (0.874); Ti: 4s À8.97
(1.075), 4p À5.44 (0.675) for Ti-3d a double zeta-function was
used Hij [eV], z1, coefficient 1, z2, coefficient 2: À10.81, 4.55,
0.4206, 1.4, 0.7839. The program package CACAO [C. Mealli,
D. M. Proserpio, A. Iienco, CACAO, A package of programs for
Molecular Orbital Analysis, Instituto per lo Studio della Stereo-
chimica ed Energetic dei Composti di Coordinazione, Florence
(Italy), D. M. Proserpio, J. Chem. Educ. 1990, 67, 399 – 402] was
used for extended Hückel calculations of the electronic structure
and interpretation of bonding for an isolated [{Ti4O}I12]4À cluster.
Mulliken overlap populations [R. S. Mulliken, J. Chem. Phys.
1955, 23, 1397] are given for selected interactions. For graphical
display of the cluster valence orbitals Hyperchem [HyperChem,
Hypercube Inc., Gainesville, Florida, USA, 2001] was used.
Density functional studies were performed with the Stuttgarter
LMTO program [R. W. Tank, O. Jepsen, A. Burckhardt, O. K.
Andersen, TB-LMTO-ASA Program, Vers. 4.7, Max-Planck-
Institut für Festkörperforschung, Stuttgart, Germany, 1998]
based on the linear-muffin-tin approximation using the local
density functional LDA as the exchange-correlation functional
and the atomic sphere approximation ASA [H. L. Shriver, The
LMTO Method, Springer, Berlin, 1984; O. Jepsen, M. Snob,
O. K. Andersen, Linearized Band-Structure Methods in Elec-
tronic Band-Structure and its Applications, Springer Lecture
Note, Springer, Berlin, 1987; O. K. Anderson, O. Jepsen, Phys.
Rev. Lett. 1984, 53, 2571]. Interstitial spheres are introduced to
achieve space filling. The ASA radii as well as the positions and
radii of additional empty spheres are calculated automatically.
Reciprocal-space integrations are carried out using the tetrahe-
dron method [O. K. Andersen, O. Jepsen, Solid State Commun.
1971, 9, 1763; P. Blöchl, O. Jepsen, O. K. Andersen,Phys. Rev. B
1994, 34, 16223] The basis set of short ranged, atom-centered
TB-LMTOs contained for O 3s, 2p, 3d, for I 6 s, 5p, 5d, and 4f, for
K 4s, 4p, 3d, and 4f and for Ti 4s, 4p, and 3d. For O the 3s and 3d,
for I the 6s, 5d, and 4f, for K the 4p, 3d, and 4f and for Ti 4s, 4p,
and 3d orbitals were treated with the downfolding technique
[W. R. L. Lambrecht, O. K. Andersen, Phys. Rev. B 1986, 34,
2439; O. Jepsen, O. K. Andersen, Z. Phys. B 1995, 97, 35; G.
Krier, O. Jepsen, O. K. Andersen, Max-Planck-Institut für
Festkörperforschung, Stuttgart, Germany, unpublished] The
crystal orbital Hamiltonian population (COHP) method is
used for bond analysis [R. Dronskowski, P. Blöchl, J. Phys.
Chem. 1993, 97, 8617]. Note that the values are negative for
bonding and positive for antibonding interactions, the reverse of
the signs used in crystal orbital overlap population (COOP; [S.
Wijeyesekera, R. Hoffmann, Organometallics 1984, 3, 949])
diagrams in the semi-empirical Hückel treatment. This discrep-
ancy emerges because, to obtain the COOPthe DOS gets
multiplied by the overlap population while for weighting the
DOS in case of the COHPthe corresponding element of the
Hamiltonian is used.
Keywords: cluster compounds · electronic structure · iodine ·
titanium
.
[1] T. Gloger, Dissertation, Universität zu Köln, 1998.
[2] A. Lachgar, D. S. Dudis, J. D. Corbett, Inorg. Chem. 1998, 37,
2242.
[3] B. Krebs, G. Henkel, Z. Anorg. Allg. Chem. 1981, 474, 149.
[4] J. Zhang, R.-I. Qi, J. D. Corbett, Inorg. Chem. 1991, 30, 4794.
[5] D. J. Hinz, G. Meyer, T. Dedecke, W. Urland, Angew. Chem.
1995, 107, 97; Angew. Chem. Int. Ed. Engl. 1995, 34, 71.
[6] D. J. Hinz, G. Meyer, J. Chem. Soc. Chem. Commun. 1994, 125.
[7] Synthesis: KI (Aldrich, 99%) was dried under vacuum at 473 K.
TiI4 was prepared from titanium (chips, Chempur, 99.6%) and
iodine (Riedel de Häen, 99.8%), according to the procedure
described in ref. [9]. TiO2 was used as obtained (Kronos Titan).
All storage and manipulation of all starting materials and
products were handled under dry box conditions (MBraun,
Garching). The reaction was carried out in a sealed tantalum
container, jacketed with a silica tube under argon at a temper-
ature of 798 K. The compound formed as black shiny crystals of
good quality. X-ray powder diffraction measurements at room
temperature revealed the product to consist of only one phase,
K4[{Ti4O}I12]. No phase transition was observed by X-ray
powder diffraction in the temperature range between 293 K
and 10 K. Indexed X-ray powder diffractograms of K4[{Ti4O}I12]
were measured at room temperature on a transmission powder
diffraction system, Guinier Image Plate Camera G670 (Huber,
Rimsting), using monochromatic MoKa radiation (cell parame-
ters at 298 K refined from powder data: a = 1373.0(3) pm, c =
807.1(2) pm). Low temperature X-ray powder diffractograms
were measured on a Guinier diffractometer G645 (Huber,
Rimsting, Germany), using monochromatic CuKa radiation, (cell
parameters at 150 K refined from powder data: a =
1369.1(4) pm, c = 808.8(3) pm).
[8] Crystal data and structure refinement: A suitable single crystal
(0.2 ” 0.2 ” 0.1 mm) was mounted in a glass capillary. Intensity
data were collected on an IPDS diffractometer (Stoe, Darm-
stadt) at 170 K and 293 K. 170 K: Tetragonal, space group I4/m;
a = 1364.5(1) pm, c = 801.51(7) pm, V= 1.4923(2) nm3; Z = 2;
1calcd = 4.199 gcmÀ3; 2.11 < q < 29.978; IPDS II, MoKa radiation
(l = 71.073 pm); T= 170(2) K; F(000) = 1616; m = 13.152 mmÀ1
;
13964 reflections were measured, of which 1169 are unique, 1104
are considered as observed. R1 = 0.0196 and wR2 = 0.0426 for
[I0 > 2s(I0)]. 293 K: Tetragonal, space group I4/m; a =
1373.9(2) pm, c = 810.1(2) pm, V= 1.5293(5) nm3; Z = 2; 1calcd
=
4.097 gcmÀ3; 2.92 < q < 25.988; IPDS I, MoKa radiation (l =
71.073 pm); T= 293(2) K; F(000) = 1616; m = 13.658 mmÀ1
;
6080 reflections were measured, of which 804 are unique, 536
are considered as observed. R1 = 0.0261 and wR2 = 0.0385 for
[I0 > 2s(I0)]. The data were processed with the program systems
SHELX-97 [G. M. Sheldrick, SHELX-97, Universität Göttin-
gen, 1997]. Scattering factors were taken from the International
Tables for Crystallography, Volume C [A. J. C. Wilson, Interna-
tional Tables for Crystallography, Vol. C, Kluwer Dordrecht, the
Netherlands, 1995]. Numerical absorption correction after
crystal shape optimization was performed using the programs
XRED and XSHAPE [Stoe, XRED 1.01 and XSHAPE 1.01,
Darmstadt, 1996]. Further details on the crystal structure
investigations may be obtained from the Fachinformationszen-
[12] M. Kriener, V. Kataev, A. Freimuth, unpublished results.
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