Reversible bromide-chloride exchange in zirconium cluster compounds: Synthesis by means of ionic liquids, structures, and some spectral data of (EMIm)4[(Zr6Z)Br18] cluster phases (Z = Be, Fe)

Article abstract of DOI:10.1021/ic061624u

The solid-state precursor cluster chlorides Na₄[(Zr ₆Be)Cl₁₆] and K[(Zr₆Fe)Cl₁₅] readily dissolve in Lewis-basic ionic liquids consisting of mixtures of EMIm-Br and AlBr₃ (EMIm: 1-ethyl-3-methyl-imidazolium) to give dark colored solutions. From these solutions, the cluster phases (EMIm)₄[(Zr ₆Fe)Br₁₈] (1) and (EMIm)₄[(Zr ₆Be)Br₁₈] (2) were obtained in acceptable yields. Crystallographic data of the isostructural phases are the following: monoclinic, P2₁/c, Z = 2. The data for 1 follow: a = 10.5746(4) A, b = 22.6567(9) A, and c = 13.0260(5) A, β = 111.279(2)°. The data for 2 follow: a = 10.574(2) A, b = 22.681(4) A, and c = 13.041(2) A, β = 111.31(2)°. Compound 1 is the first detailed structurally characterized molecular Fe-centered zirconium bromide cluster phase. In the bromide based ionic liquid, a complete exchange of all the outer and inner chlorides by bromide takes place. Since the inverse reaction, the exchange of all bromides by chlorides, was reported before, this complete ligand exchange can be considered as reversible, with the equilibrium being largely determined by the free ligand concentration. The electronic spectra of a chloride supported cluster precursor in different ionic liquids were measured and analyzed.

Full text of DOI:10.1021/ic061624u

Inorg. Chem. 2007, 46, 6197−6203
Reversible Bromide
Compounds: Synthesis by Means of Ionic Liquids, Structures, and
Some Spectral Data of (EMIm)4[(Zr6Z)Br18] Cluster Phases (Z Be, Fe)
−Chloride Exchange in Zirconium Cluster
)
Johannes B. Willems, Henning W. Rohm, Christine Geers, and Martin Ko1ckerling*
Institut fu¨r Chemie, Anorganische ChemiesFestko¨rperchemie, UniVersita¨t Rostock,
Albert-Einstein-Str. 3a, D-18051 Rostock, Germany
Received August 29, 2006
The solid-state precursor cluster chlorides Na4[(Zr6Be)Cl16] and K[(Zr6Fe)Cl15] readily dissolve in Lewis-basic ionic
liquids consisting of mixtures of EMIm-Br and AlBr3 (EMIm: 1-ethyl-3-methyl-imidazolium) to give dark colored
solutions. From these solutions, the cluster phases (EMIm)4[(Zr6Fe)Br18] (1) and (EMIm)4[(Zr6Be)Br18] (2) were
obtained in acceptable yields. Crystallographic data of the isostructural phases are the following: monoclinic, P21/
c, Z
)
2. The data for 1 follow: a
)
10.5746(4) Å, b
)
22.6567(9) Å, and c
)
13.0260(5) Å, â ) 111.279(2)
°.
The data for 2 follow: a
)
10.574(2) Å, b
)
22.681(4) Å, and c
)
13.041(2) Å, â ) 111.31(2)°
. Compound 1
is the first detailed structurally characterized molecular Fe-centered zirconium bromide cluster phase. In the bromide
based ionic liquid, a complete exchange of all the outer and inner chlorides by bromide takes place. Since the
inverse reaction, the exchange of all bromides by chlorides, was reported before, this complete ligand exchange
can be considered as reversible, with the equilibrium being largely determined by the free ligand concentration.
The electronic spectra of a chloride supported cluster precursor in different ionic liquids were measured and analyzed.
Introduction
halogen atoms between cluster units, on the magnitude of n
and x, and also on the atom sizes of A, Z, and X, more than
30 distinct structure types with over 250 examples have been
realized to date. This solid-state chemistry has been expanded
in recent years to mixed-halide cluster phases.8-12
Besides the solid-state approach, a chemical solution
approach has been developed to zirconium cluster phases
through the discovery that the cluster core [(Zr6Z)Cl12] can
be extracted from suitable solid-state materials and remains
intact in solution.13-15 The outer halogen atoms, located on
Inorganic compounds containing transition metal atoms
in lower oxidation states form quite often metallic octahedral
clusters with metal-metal distances as short as in the
elemental metal.1,2 An especially interesting group of cluster
materials is found in the reduced zirconium halides. A large
variety of such cluster phases was discovered via solid-state
chemical reactions in the laboratories of J. D. Corbett. The
octahedral Zr6 units are centered by an interstitial heteroatom
Z. Most of these cluster phases fall within the general formula
AI,IIx[(Zr6Z)Xi12Xan], with AI,II ) group 1 or 2 cation, Z )
H, Be-N, Al-P, Mn-Ni; X ) Cl, Br, or I, Xi ) inner,
edge bridging halide, Xa ) outer halide,3 and 0 e x, n e
6.4-7 Depending on various types of bridging modes of
(6) Corbett, J. D. In Modern PerspectiVes in Inorganic Crystal Chemistry;
Parthe´, E., Ed.; Kluwer: Dordrecht, The Netherlands, 1992; p 27.
(7) Corbett, J. D. Inorg. Chem. 2000, 39, 5178-5191.
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1443.
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1439-1444.
* To whom correspondence should be addressed. E-mail:
Martin.Koeckerling@chemie.unirostock.de. Phone: ++49-(0)381-498-
6390. Fax: ++49-(0)381-498-6382.
(11) Ko¨ckerling, M. J. Solid State Chem. 2003, 170, 273-280.
(12) Ko¨ckerling, M. In Inorganic Chemistry in Focus III; Meyer, G.,
Naumann, D., Wesemann, L., Eds.; Wiley-VCH: New York, 2006;
pp 61-77 and references cited therein.
(1) Metal Clusters in Chemistry; Braunstein, P., Oro, L. A., Raithly, P.
R., Eds.; Wiley-VCH: Weinheim, 1999.
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7910.
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10.1021/ic061624u CCC: $37.00
Published on Web 06/30/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 15, 2007 6197

Willems et al.
the exo positions of the (Zr6Z) octahedra, can be exchanged
by other inorganic or organic neutral or charged ligands (L)
to form novel zirconium cluster compounds of the type
[(Zr6Z)Cli12La6]m (charge m depending on Z). Structurally
characterized so far are phases with L ) primary amines,15-17
pyridine,18 phosphines,19,20 2,6-dimethylphenyl isocyanide,21
and cyanide,22 besides other halides.14,23,24 No oligomeric Zr
cluster phase with bifunctional ligands is known to date.
and some spectroscopic data of the iron centered phase in
different ionic liquids.
Experimental Section
Materials. Because of the air and moisture sensitivity of at least
some of the reactants, especially those in solution, all manipulations
were carried out in a high quality argon glovebox or with Schlenk
techniques. All glassware was oven dried (150 °C) overnight before
use. Preparation and handling details of some starting materials,
the use of sealed tantalum or niobium ampules, and the phase
identification and yield estimation of solid-state cluster materials
by means of Guinier powder patterns have been described previ-
ously.10,11 The solid-state precursors Na4[(Zr6Be)Cl16] and K[(Zr6-
Fe)Cl15] were prepared by heating stoichiometric amounts of
thoroughly dried NaCl (or KCl), Zr-powder, sublimed ZrCl4, and
Be (or Fe), which were sealed under argon in niobium or tantalum
ampules.39,40 Aluminum tribromide was synthesized from the
elements and purified by means of high vacuum sublimations.
1-Ethyl-3-methyl-imidazolium bromide was synthesized according
to ref 41, recrystallized from acetonitrile, and dried in high vacuum.
The ionic liquid used in these studies was obtained by slow addition
(because of the exothermic reaction) of AlBr3 to 1-ethyl-3-methyl-
imidazolium bromide in a 3:5 molar ratio. Similar procedures were
used to prepare EMIm-Cl/AlCl3 (5:3) and EMIm-I/AlI3 (5:3) ionic
liquids for UV-vis investigations.
Because of the strong reduction potential of the Zr cluster
phases, the number of possible solvents for the solution
chemistry is limited. Nevertheless, reports exist about the
excision of cluster units into strongly acidic and/or halide-
rich aqueous or alcoholic solutions, where they seem to be
kinetically protected against oxidative decomposition.25-27
Ionic liquids, defined as molten salts with melting points
below ∼100 °C, have emerged during the past decade as
convenient solvents in chemistry with a large potential for
scientific and industrial use.28-32 With respect to cluster
chemistry, some of them proved to be especially useful
because of their good solubility for ionic compounds and
their large electrochemical windows.33-34 Recently, the
excision of soluble hexanuclear zirconium halide cluster units
from solid-state materials using ionic liquids was reported.
These reports also include the exchange of bromide by
chloride ions using EMIm-Cl/AlCl3 mixtures (EMIm: 1-ethyl-
3-methyl-imidazolium).14,35-38
Caution! The handling of beryllium or beryllium compounds
inVolVes hazards because they are seVere poisons and are
carcinogens.
Syntheses. (EMIm)4[(Zr6Fe)Br18] (1). In a typical experiment
a glass ampule was filled with 20 mg (0.017 mmol) of K[(Zr6Fe)-
Cl15]40 and 3 mL of the Lewis-basic ionic liquid EMIm-Br/AlBr3
(3:5 molar ratio) within the glove box. After the ampule was
sealed off under vacuum, the mixture was heated for 36 h at
100-120 °C. First, a dark blue solution was obtained, which then
changed to a color of slightly blue-violet. Afterward the ampule
was opened, and the carefully decanted liquid was diluted with 1
mL acetonitrile. At room temperature, diisopropyl ether vapor was
allowed to diffuse slowly into the solution. Within a few weeks,
dark blue-violet crystals are formed which were suitable for single-
crystal X-ray structure determination. Yield: 22 mg (EMIm)4[(Zr6-
Fe)Br18] (may contain some diisopropyl ether) (yield: 0.009 mmol,
53%, with respect to pure cluster compound). Anal. Calcd for
C24H44Br18N8FeZr6: C, 11.60; H, 1.78; N, 4.51. Found: C, 12.18;
H, 1.97; N, 4.74.
(EMIm)4[(Zr6Be)Br18] (2). The synthesis of 2 was similar to
that of 1. As starting material, 20 mg (0.008 mmol) of Na4[(Zr6-
Be)Cl16]39 was used. The diffusion of diisopropyl ether into a
solution of the cluster phase in the EMIm-Br/AlBr3/acetonitrile
mixture gave dark brown crystals of 2, suitable for single-crystal
X-ray diffraction experiments.
Crystal Structure Studies. Suitable crystals of the two phases
were selected inside of an Ar-filled drybox, and flame-sealed within
thin-wall glass capillaries. X-ray intensity data were collected at
room temperature with the aid of a Bruker-Nonius Apex-X8-
diffractometer, equipped with a CCD detector (1) and a Siemens
P4 four-circle diffractometer (2). Graphite monochromated Mo KR-
radiation (λ ) 0.71073 Å) was used in both cases. Crystal data,
In this paper, we report the excision of chloride based Zr
cluster units from solid-state starting materials, their trans-
formation to bromides using EMIm-Br/AlBr3 ionic liquids,
the crystal structures of (EMIm)4[(Zr6Z)Br18] (Z ) Be, Fe),
(16) Rogel, F.; Zhang, J.; Payne, M. W.; Corbett, J. D. In Electron Transfer
in Biology and the Solid State; Johnson, M. K., King, R. B., Kurtz,
D. M., Jr., Kutal, C., Norton, M. L., Scott, R. A., Eds.; American
Chemical Society: Washington, DC, 1990; Vol. 226, p 367.
(17) Rogel, F. Ph.D. Thesis, Iowa State University, Ames, IA, 1990.
(18) Xie, X.; Riebenspies, J. H.; Hughbanks, T. J. Am. Chem. Soc. 1998,
120, 11391-11400.
(19) Cotton, F. A.; Kibala, P. A.; Roth, W. J. J. Am. Chem. Soc. 1988,
110, 298-300.
(20) Chen, L.; Cotton, F. A.; Wojtczak, W. A. Inorg. Chem. 1996, 35,
2988-2994.
(21) Chen, L.; Cotton, F. A. J. Cluster Sci. 1998, 9 (1), 63-91.
(22) Beauvais, L. G.; Long, J. R. Inorg. Chem. 2006, 45, 236-243.
(23) Chen, L.; Cotton, F. A.; Wojtczak, W. A. Inorg. Chim. Acta 1996,
252, 239-250.
(24) Hughbanks, T. J. Alloys Compd. 1995, 229, 40-53.
(25) Xie, X.; Hughbanks, T. Angew. Chem. 1999, 111, 1894-1897.
(26) Xie, X.; Hughbanks, T. Solid State Sci. 1999, 1, 463-471.
(27) Xie, X.; Hughbanks, T. Inorg. Chem. 2000, 39, 555-561.
(28) Antonietti, M.; Kuang, D.; Smarsly, B.; Zhou, Y. Angew. Chem., Int.
Ed. 2004, 43, 4988-4992.
(29) Ionic Liquids in Synthesis; Wasserscheid, P., Welton, T., Eds.; Wiley-
VCH: Weinheim, 2002.
(30) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772-
3789.
(31) Welton, T. Chem. ReV. 1999, 99, 2071-2083.
(32) Sheldon, R. Chem. Commun. 2001, 2399-2407.
(33) (a) Quigley, R.; Barnard, P. A.; Hussey, C. L.; Seddon, K. R. Inorg.
Chem. 1992, 31, 1255. (b) Hussey, C. L.; Quigley, R.; Seddon, K. R.
Inorg. Chem. 1995, 34, 370.
(34) Das, B.; Carlin, R. T.; Ostryoung, R. A. Inorg. Chem. 1989, 28, 421-
426.
(35) Hughbanks, T. J. Alloys Compd. 1995, 229, 40-53.
(36) Harris, J. D.; Hughbanks, T. J. Am. Chem. Soc. 1997, 119, 9449-
9459.
(37) Sun, D.; Hughbanks, T. Inorg. Chem. 1999, 38, 992-997.
(38) Sun, D.; Hughbanks, T. Inorg. Chem. 2000, 39, 1964-1968.
(39) Ziebarth, R. P.; Corbett, J. D. Inorg. Chem. 1989, 28, 626.
(40) Ziebarth, R. P.; Corbett, J. D. J. Am. Chem. Soc. 1987, 109, 4844-
4850.
(41) Avent, A. G.; Chaloner, P. A.; Day, M. P.; Seddon, K. R.; Welton, T.
J. Chem. Soc., Dalton Trans. 1994, 105, 3405.
6198 Inorganic Chemistry, Vol. 46, No. 15, 2007

Br-Cl Exchange in Zr Clusters
Table 1. Crystal Data and Structure Refinement Parameters for
(EMIm)4[(Zr6Be)Br18] and (EMIm)4[(Zr6Fe)Br18]
(EMIm)4[(Zr6Be)Br18]
(EMIm)4[(Zr6Fe)Br18]
formula
fw (g/mol)
T, K
C24H44BeBr18N8Zr6
2439.38
293(2)
C24H44Br18FeN8Zr6
2486.22
293(2)
cryst syst
space group, Z
a, Å
b, Å
c, Å
monoclinic
P21/c, 2
monoclinic
P21/c, 2
10.574(2)
22.681(4)
13.041(2)
111.31(2)
2913.8(9)
2.780
13.404
0.0411
0.0855
0.0288, 0.0
0.721
10.5746(4)
22.6567(9)
13.0260(5)
111.279(2)
2908.1(2)
2.839
13.670
0.0450
0.1261
0.0523, 17.11
1.036
â, deg
V, Å3
F
calcd, g‚cm-1
µ, mm-1
R1(F)a
wR2(F2)b
weighting, A, Bc
GOF
Figure 1. Molecular structure of the [(Zr6Fe)Br18]4- cluster anion in
crystals of (EMIm)4[(Zr6Fe)Br18] with 50% thermal probability ellipsoids
showing the atom labeling scheme.
2 2
a R1 ) ∑||Fo| - |Fc||/∑|Fc|. b wR2 ) [∑{w(Fo2 - Fc2)2}/∑{w(Fo ) }]1/2
.
2
2
2
c w ) 1/[(σ2(Fo ) + (AP)2 + BP]; P ) (Fo + 2Fc )/3.
and the space group (monoclinic, P21/c), the Zr coordinates of 1
were used to solve the structure of 2. The positions of all the other
atoms were obtained from difference Fourier maps. Anisotropic
refinement of all the non-hydrogen atoms resulted in R1(F) )
0.0411, and wR2(F2) ) 0.0855.
Elemental Analysis. The elemental analysis (C, H, and N) was
carried out with a CHNS-analyzer Thermoquest Flash EA 1112.
Ion Chromatography. In order to confirm the structural result,
that less than 3 atom % of the bromine sites are occupied by chlorine
atoms, another elemental analysis using ion chromatography was
conducted. A 16.4 mg sample of selected crystals of 1 was dissolved
in deionized water to give 25 mL solution after addition of halide-
free sodium citrate. Results are 9.87 mg Br and 0.138 mg Cl. The
ion chromatography measurements on Cl and Br were done by
Bayer Industry Services GmbH and Co. OHG, Leverkusen,
Germany.
Spectroscopic Studies. UV-vis-spectra of three different cluster
solutions were recorded at room temperature in the region
420-850 nm. Carefully weighed amounts (13-31 mg) of
K[(Zr6Fe)Cl15] were added to 3 mL of each of the ionic liquids
EMIm-Cl/AlCl3 (5:3), EMIm-Br/AlBr3 (5:3), or EMIm-I/AlI3
(5:3). Each mixture was separately sealed in a glass ampule and
heated at 110 °C for 2 days in a sand bath. The solid-state cluster
material almost completely dissolved in the ionic liquids. Clear
cluster solutions were transferred into a quartz cell of 0.1 cm
thickness. The reference cell was filled with the cluster-free ionic
liquid. Spectra were recorded using a Perkin-Elmer Lambda 5
spectrometer.
data collection, and refinement parameters for both compounds are
given in Table 1. Crystallographic files are available free of charge
via the CCDC data center, CCDC-206532 for (EMIm)4[(Zr6Fe)-
Br18] and CCDC-206533 for (EMIm)4[(Zr6Be)Br18].
(EMIm)4[(Zr6Fe)Br18]. First unit cell dimensions of 1 were
determined from the reflections of 60 frames measured in 3 different
crystal directions. After data collection up to 61.4° (in 2θ), the unit
cell parameters were refined to values of a ) 10.5746(4) Å, b )
22.6567(9) Å, c ) 13.0260(5) Å, and â ) 111.279(2)° for the
monoclinic cell using the positions of 9659 reflections. Data
collection was done using the Bruker-Nonius APEX-2, version 1.6-8
software,42 and the data reduction including corrections for
background, Lorentz, and polarization effects using the SAINT
program, version 7.06A.42 A multiscan absorption correction was
applied with the aid of SADABS42 giving a data set with Rint
)
0.0597. The structure of 1 was solved via direct methods in the
monoclinic space group P21/c; the structural model was completed
using difference Fourier maps and refined by full-matrix least-
squares methods on F2 using the SHELX-97 program package.43
All atoms except hydrogen were refined using anisotropic thermal
parameters. Hydrogen atoms on the imidazolium cations were fixed
on idealized positions and refined with isotropic thermal parameters
based on the bonded atom. The final refinements converged at R1-
(F) ) 0.0450, and wR2(F2) ) 0.1261.
In order to test for any remaining chloro ligands, which would
indicate a noncomplete halide exchange, the occupancy parameters
of the nine independent Br sites were refined. They converged with
maximum deviations of (2.8 atom % from the corresponding full
occupancy with bromine. Similar results were obtained by another
structure refinement on a crystal from a different reaction batch.
(EMIm)4[(Zr6Be)Br18]. With the aid of a conventional four-
circle diffractometer (Siemens P4), the unit cell dimensions of 2
were determined from 32 carefully centered reflections up to 35°
(in 2θ) to be a ) 10.574(2) Å, b ) 22.681(4) Å, c ) 13.041(2) Å,
and â ) 111.31(2)°. A total of 6904 reflections were collected up
to 54° (in 2θ) from which 6273 were symmetry independent. Data
reduction included Lorentz and polarization corrections as well as
an empirical absorption correction using one ψ-scan reflection to
give Rint ) 0.0345. Because of the similarity of the lattice constants,
Results and Discussion
(A) Structural Aspects. Both title compounds crystallize
isostructurally in the monoclinic space group P21/c with 2
cluster units per unit cell. Besides discrete 1-ethyl-3-methyl-
imidazolium cations, the structures contain isolated molecular
[(Zr6Z)Br18]4- ions (Z ) Be and Fe) located on inversion
centers. Important bond distances (average values and ranges)
are listed for both compounds in Table 2. The molecular
cluster anions consist of octahedral (Zr6Z) units with the 12
edges capped by bromide ligands (denoted as inner ligands
Bri). Six further bromides occupy the apical positions (outer
bromides Bra).3 The structures of the cluster anions are shown
in Figures 1 and 2, respectively. Neither the thermal
(42) Apex-2, Version 1.6-8, Saint, Version 6.25a, SADABSsSoftware for
the CCD Detector System; Bruker-Nonius Inc.: Madison, WI, 2003.
(43) Sheldrick, G. M. SHELX97sPrograms for Crystal Structure Analysis
(Release 97-2); University of Go¨ttingen: Go¨ttingen, 1997.
Inorganic Chemistry, Vol. 46, No. 15, 2007 6199

Willems et al.
precise values of bond lengths are not obtainable from the
atom coordinates of the chloride structures and the lattice
constants of the bromides, because the Zr atoms are located
on general positions and the pronounced matrix effect
changes the Zr-Zr and Zr-Z distances largely on going from
Z ) C to Z ) Fe without changing the lattice constants to
the same degree. One way to see at least if the found
distances in 1 are within a reasonable expectation range is
to apply the same percentage of increase in Zr-Zr and Zr-Z
distances on going from C to Fe in comparable chloride
phases to the C-centered zirconium bromide, e.g., K4[(Zr6C)-
Br18]. A comparison of the Zr-Zr and Zr-Z distances in
(EMIm)4[(Zr6C)Cl18]38 and (Et4P)4[(Zr6Fe)Cl18]‚2CH3CN15,17
(Et ) ethyl) shows that an increase of 6.4% is found on
going from the C to the Fe containing phase. Applying the
same increase to the distances in K4[(Zr6C)Br18] we get 3.479
Å for Zr-Zr and 2.460 Å for Zr-Fe. These values are close
enough to the measured ones to conclude that no unusual
structural effects are present. The Zr6 core in 1 is expanded
relative to comparable chloride phases15 due to steric factors,
called “matrix effects”, which describe a compromise
between attractive M-M and M-Z bonds and Xi-Xi closed-
shell repulsion forces.4,48 This expansion can also be
expressed by the trans Bri-Zr-Bri angle, which measures
170.58° (average), compared to 173.8° in (EMIm)4[(Zr6Fe)-
Cl18].14
Figure 2. Molecular structure of the [(Zr6Be)Br18]4- cluster anion in
crystals of (EMIm)4[(Zr6Be)Br18] with 50% thermal probability ellipsoids
showing the atom labeling scheme.
ellipsoids, nor results from refinements of occupancies, allow
for the conclusion that more than a marginal amount of
chloride is present on the halide sites of the cluster anions.
These cluster anions are the first completely structurally
characterized examples of Be- and Fe-centered bromide
supported molecular zirconium clusters, which were obtained
by cluster excision techniques using ionic liquids. So far,
only one example of a bromide supported zirconium cluster
is known, which is obtained by solution chemical methods:
(H3O)3[(Zr6B)Br16(H2O)2] containing coordinated water mol-
ecules.26 Further compounds, containing molecular bromide
cluster anions, obtained by solid-state chemical procedures,
are K4[(Zr6C)Br18] (an isostructural iron centered version is
also mentioned, but not structurally fully characterized)44 and
(AI4Br)2[(Zr6Z)Br18] with AI ) Na-Cs, Z ) H, Be, B, Mn.45
Contrary to the bromide supported cluster systems, compa-
rable chlorides have been much further investigated, and a
much larger variety of different compounds with different
structures is known.46,47
As with all the Zr-cluster systems, the Zr-Zr and Zr-Z
distances depend strongly on the type of halide and inter-
stitial, as well as on the number of cluster bonding electrons
(CBEs).45,46,48 The average Zr-Zr and Zr-Fe distances in 1
are 3.4920 and 2.4692 Å, respectively. Comparing these
values with those of other iron centered, bromide supported
zirconium clusters proved rather difficult, because no detailed
structural data has been published so far on any Fe-centered
zirconium bromide (to the best of the author’s knowledge).
The only phases mentioned so far in the literature are
[(Zr6Fe)Br14],4 Cs[(Zr6Fe)Br15],40 and the above referenced
K4[(Zr6Fe)Br18].44
Contrary to the Zr-Zr and Zr-Fe distances, the Zr-Bri
ones depend much less on the type of interstitial, but more
on the oxidation state of the cluster core.15,47 Therefore, the
average Zr-Bri distance in 1 (2.6992 Å, see Table 2)
matches that of other cluster bromides with a closed shell
configuration, but also with different interstitials, for ex-
ample, as found in (H3O)3[(Zr6B)Br16(H2O)2] (2.692 Å).45
The same applies to the slightly longer Zr-Bra distances
(see Table 2), which vary by 1.1% between 2.7292(8) and
2.7600(8) Å.
The narrow range of the Zr-Zr and Zr-Fe distances
indicates a small asymmetry in the [(Zr6Fe)Br12]2+ unit. A
similar asymmetry is found in the comparable chloride, e.g.,
(EMIm)4[(Zr6Fe)Cl18].15-17
The second title phase, the beryllide (EMIm)4[(Zr6Be)-
Br18] (2), is an isostructural variant of 1. Therefore, we find
many structural similarities. Both the Zr-Bri and Zr-Bra
distances (average 2.700 and 2.738 Å, see Table 2) compare
well in 1 and 2. Notable features are the rather long Zr-Zr
and Zr-Be, respectively, distances (average values 3.474
and 2.456 Å) compared for example to those in Cs4[(Zr6-
Be)Br16]49 (3.355 and 2.372 Å). This phase contains cluster
units with 14 CBEs. The expansion of the cluster core in 2
is evidently due to the lower number of CBEs, 12 instead of
14. The two electrons missing are removed from a bonding
molecular orbital of t2g symmetry, which covers basically
Zr contributions. Therefore, the Zr-Zr bonding is weakened,
Lattice constants of [(Zr6Fe)Br14], and Cs[(Zr6Fe)Br15] were
obtained by powder diffraction (Guinier) methods on the
basis of the isostructural chloride analogues. Unfortunately,
(44) Qi, R.-Y.; Corbett, J. D. J. Solid State Chem. 1998, 139, 85-92.
(45) Qi, R.-Y.; Corbett, J. D. Inorg. Chem. 1997, 36, 6039-6044.
(46) Xie, X.; Jones, J. N.; Hughbanks, T. Inorg. Chem. 2001, 40, 522-
527.
(47) Hughbanks, T. Inorg. Chem. 2002, 41, 1824-1830 and references
cited therein.
(48) Ziebarth, R. P.; Corbett, J. D. J. Am. Chem. Soc. 1989, 111, 3272-
3280.
(49) Qi, R.-Y. Ph.D. Thesis, Iowa State University, Ames, IA, 1993.
6200 Inorganic Chemistry, Vol. 46, No. 15, 2007

Br-Cl Exchange in Zr Clusters
Figure 3. Arrangement of the [(Zr6Z)Br18]4- cluster anions and the 1-ethyl-3-methyl-imidazolium cations in crystals of (EMIm)4[(Zr6Z)Br18] (Z ) Fe, Be).
View down [100] (Zr thermal ellipsoids with 50% probability, for Z ) Fe; Z, regularly dotted; Br and N, irregularly dotted; C, shaded; H, open circles).
Table 2. Selected Bond Distances (Å) within the Cluster Units of (EMIm)4[(Zr6Fe)Br18] and (EMIm)4[(Zr6Be)Br18]
(EMIm)4[(Zr6Fe)Br18]
range
(EMIm)4[(Zr6Be)Br18]
range
average
average
Zr-Zr
3.4920
2.4692
2.6992
2.7463
3.4870(7)-3.5064(8)
2.4626(5)-2.4781(5)
2.6890(8)-2.7105(8)
2.7292(8)-2.7600(8)
3.474
2.456
2.700
2.738
3.464(1)-3.491(1)
2.447(1)-2.466(1)
2.687(1)-2.711(1)
2.719(1)-2.750(1)
Zr-M (Be; Fe)
Zr-Bri
Zr-Bra
and the distances increase on oxidation.47,50 Unfortunately,
no detailed structural data of an oxidized Zr-Br-Be cluster
has been published so far, which would allow for direct
comparison. It is worth mentioning the phase Rb5[(Zr6Be)-
Br15] here, which contains reduced clusters with 16 CBEs,
that are arranged in a tunnel-like structure.51 The percentage
increase of the distances in the chloride-beryllides going
from the 14 CBE compound [(Zr6Be)Cl12(EtNH2)6]‚8CH3-
CN to (Et4N)4[(Zr6Be)Cl18]‚8CH3CN (12 CBEs)15,17 (Et )
ethyl) is 3.3% for both the Zr-Zr and Zr-Be distances.
Applying this increase to the values of the 14 CBE containing
Cs4[(Zr6Be)Br16], we obtained 3.456 and 2.443 Å. These
values are close to those observed in 2.
The three-dimensional arrangement of the cluster units and
the imidazolium cations within the unit cell of both phases
is depicted in Figure 3 in a view down the crystallographic
c direction. All the cluster anions are well separated from
each other. The shortest distance between neighboring cluster
anions is 4.105 Å (Br-Br), much longer than twice the
nonbonding Shannon radius for six-coordinate Br-, which
is 3.92 Å.54 In between the cluster anions are located the
1-ethyl-3-methyl-imidazolium cations, which more or less
surround the cluster units.
(B) Synthetic Aspects. An efficient synthetic procedure
of molecular zirconium cluster phases is the excision of
suitable cluster units from solid-state precursors. In first
attempts to develop such a solution chemistry, Rogel and
Corbett used aprotic solvents, like CH3CN, pyridine, and
DMF.15-17 In more recent investigations, ionic liquids,
composed of mixtures of 1,3-dialkyl-imidazolium chloride
and aluminum trichloride, were successfully used.14,38 These
solvents have the great advantage of large electrochemical
windows and high solubility for ionic compounds. Such
liquids were used before in spectroscopic and electrochemical
investigations on a variety of metal cluster systems.33,34 The
formation of molecular units starting from interlinked clusters
in the solid precursors requires the disruption of Cla-a-Zr
bonds (or others) and the addition of halide ions to fill the
thereby created free coordination sites on the metal atoms.
In the precursor phases, which were used in the studies
reported here, the intercluster connectivity can be expressed
40
as [(Zr6Fe)Cli12Cla-a
]
and [(Zr6Be)Cli12Cla2Cla-a4/2],39
6/2
respectively.3 For the excision process, the breakage of 6
and 4, respectively, Cla-a-Zr bonds is needed. Furthermore,
3 and 2, respectively, additional halides are needed to fill
the free coordination sites. Therefore, the use of Lewis basic
mixtures is inevitable. With the imidazolium based haloalu-
(50) Cotton, F. A.; Hughbanks, T.; Runyan, C. E., Jr.; Wojtczak, W. A.
Halide Supported Octahedral Clusters of Zirconium: Structural and
Bonding Questions. In Early Transition Metal Clusters with Pi-Donor
Ligands; Chisholm, M. H., Ed.; VCH Publishers: New York, 1995;
pp 1-26.
(51) Qi, R.-Y.; Corbett, J. D. Inorg. Chem. 1995, 34, 1646-1651.
Inorganic Chemistry, Vol. 46, No. 15, 2007 6201

Willems et al.
minates, this is easily achieved by an excess of the imida-
zolium halide over the AlX3.14,38
Whereas studies so far used only chloride supported ionic
liquids, we report here for the first time studies with bromide
supported systems.
The most striking feature of the excision reactions leading
to the title phases is the complete exchange of chloride by
bromide, according to
[(Zr6Fe)Cl15]- +
EMIm-Br/AlBr3
18Br-
[(Zr6Be)Cl16]4-
18Br-
8 [(Zr6Fe)Br18]4- + 15Cl- (1)
+
EMIm-Br/AlBr3
8 [(Zr6Be)Br18]4- + 16Cl- (2)
-2e-
Figure 4. UV-vis spectra of the zirconium cluster phase K[(Zr6Fe)Cl15],
Not only the outer chlorides of the solid-state precursors but
also all the inner chlorides are replaced by bromides from
the ionic liquid. The inverse reaction, in which a complete
exchange of Br by Cl (starting from [(Zr6Fe)Br14]) was
observed, has been reported before.14 For both halide
exchange reactions, it is clear that with respect to yields larger
than 50% the exchange of all the inner halides does not
necessarily lead to a decomposition of the metal cluster core.
The exchange of Br by Cl and vice versa appears to be
reversible and can be driven forth and back using the
concentration of the other halide, which is present in excess
in the ionic liquid.
Chlorine admixture on the bromine sites has been shown
to be less than 3 atom %, independently by refinement of
occupancies and Cl/Br ion chromatography.
This result, the reversibility of the exchange of ligands
on inner and outer cluster coordination sites, could open up
large opportunities for the development of new octahedral
cluster compounds with the inner ligands being other than
halides.
dissolved in three different ionic liquids.
surprisingly [FeCl4{K(18-crown-6)}4][Fe4S4Cl4], were ob-
tained. The last is a reaction product of K[(Zr6Fe)Cl15],
KSCN, and 18-crown-6 in acetonitrile solution.53 With no
oxidant present, disproportionization reactions leading to
oxidized cluster phases have also been discussed.15,17 The
electrochemistry of chloride-based zirconium cluster species
using cyclovoltammographic techniques has been investi-
gated thoroughly.37,38,47 On the contrary, results on bromide
and iodide supported systems are so far rather sparse.
Further investigations to expand the solution chemistry of
zirconium cluster phases are underway in our laboratories.
(C) Spectroscopic Aspects. On dissolution of iron cen-
tered, chloride supported zirconium clusters in ionic liquids,
royal blue solutions were obtained.14,17 Acetonitrile solutions
maintain initially the same color as long as the cluster
material exists intact. Upon dissolving the chloride supported
cluster material in bromide and iodide based EMIm-X/AlX3
mixtures and heating to ∼120 °C, the initially intensive blue
color changed slightly to blue-violet. This color change can
be traced as a red shift in the electronic spectra. Figure 4
shows the electronic spectra of solutions which were obtained
on dissolving K[(Zr6Fe)Cl15] in the chloride, bromide, and
iodide, respectively, supported ionic liquids. All recorded
spectra show two strong absorption peaks in the region
between 14000 and 21000 cm-1. The absorptions observed
in this region can be assigned to the electronic transitions
1t1u f 2t2g and 1t1u f 3t2g.55 For the cluster in an EMIm-
Cl/AlCl3 melt, the absorption peaks are found at 15800 and
20000 cm-1, respectively. They diverge slightly from the
previously reported values.14 This fact can be attributed to
the different molar ratio of EMIm-Cl and AlCl3, and also to
the resulting higher Lewis-acidic character of the ionic melt
used in our studies. The ratio of absorbance of the two peaks
compares very well to the previously reported ones. In the
case of the bromide melt, these transitions are shifted by
about -700 and -800 cm-1 to 15100 and 19200 cm-1,
As indicated by formula 2, a two-electron oxidation takes
place during the formation of the beryllium centered cluster.
This leaves two fewer cluster base electrons (CBEs) than
are required for optimal Zr-Zr and Zr-Be bonding. In the
comparable beryllium centered chloride systems, a wide
range of redox chemistry has been uncovered. The range
goes from oxidized cluster phases with 12 CBEs in
(Et4Y)4[(Zr6Be)Cl18]‚2CH3CN (Y ) N or P),16 over 13 CBEs
in (Et4N)5[(Zr6Be)Cl18]‚3CH3CN52 and those with closed shell
configurations (14 CBEs), for example [(Zr6Be)Cl12(H2N-
i
R)6]‚8CH3CN (R ) Et, Pr),15,17 K3[(Zr6Be)Cl15],5 and
Na4[(Zr6Be)Cl16],48 to the reduced solid-state phases Rb5[(Zr6-
Be)Br15] (and Cs4.6[(Zr6Be)Br15])51 with 16 (15.6) CBEs. The
fact that quite often oxidized cluster phases are obtained from
solution chemical reactions has been explained by the
accidental presence of water, but also by an inherent
instability of the cluster with 14 CBEs with respect to the
present solvent. Iron centered zirconium cluster compounds
cannot be handled in CH3CN solutions for long periods of
time. Only cluster oxidation products, like Fe[ZrCl6]15 and
(53) Willems, J. B.; Ko¨ckerling, M. Chem. Commun. 2001, 1380-1381.
(54) Shannon, R. D. Acta Crystallogr. 1976, A32, 751-767.
(55) Bond, M. R.; Hughbanks, T. Inorg. Chem. 1992, 31, 5015-5021.
(52) Tian, Y.; Hughbanks, T. Z. Anorg. Allg. Chem. 1996, 622, 425.
6202 Inorganic Chemistry, Vol. 46, No. 15, 2007

Br-Cl Exchange in Zr Clusters
respectively. These peaks can be assigned to a cluster,
completely surrounded by bromo ligands. The peak positions
as well as the absorbance ratio compare well with the
absorptions measured for Cs[(Zr6Fe)Br15] in acetonitrile
solution.14 The above-mentioned maximum admixture of
chlorine on the bromine sites of less than 3 atom % is not
assumed to have any major influence on the absorption
bands. With transitions at 15300 and 19500 cm-1, the cluster
in the iodine melt shows only a shift of -500 cm-1 compared
to the Cl-supported cluster. This transition shows a slight
blue shift compared to the Br-supported cluster species. We
would expect the absorption maxima to be even more red-
shifted, with respect to the bromide supported cluster, with
completely substituted cluster species, where all the inner
and outer chlorides are substituted by iodide. This could
suggest that the complete ligand substitution does not happen
in the iodide based ionic liquid, but instead when clusters
with mixtures of chloride and iodide ligands are present. The
shift and change of intensity of the absorption peak at lower
energy, with respect to that at higher energy, indicates a
change in the environment of the iron centered cluster. The
assumption that no cluster anion of the type [(Zr6Fe)I18]4-
is formed during dissolution is supported by results of recent
investigations on Zr-cluster iodides, in which phases of the
type [(Zr6Z)I12]n+ (Z ) B, C; n ) 1, 2) were obtained, but
no [(Zr6Fe)I18]4- type clusters.
Acknowledgment. Support from the Deutsche Fors-
chungsgemeinschaft (DFG), from the Fonds der Chemischen
Industrie (FCI), and from the state North Rhine-Westphalia
(Bennigsen-Foerder Award for M.K.) is gratefully acknowl-
edged. We thank Prof. Dr. A. Mewis, Prof. Dr. W. Frank,
and Dr. H. Wunderlich (University of Du¨sseldorf); Prof. Dr.
G. Henkel (University of Paderborn); Prof. Dr. H. Reinke
(University of Rostock); Markus Neumann; and Natasha
Catlin for all their valuable support.
Supporting Information Available: X-ray crystallographic file
of (EMIm)4[(Zr6Be)Br18] and (EMIm)4[(Zr6Fe)Br18], in CIF format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
IC061624U
Inorganic Chemistry, Vol. 46, No. 15, 2007 6203