Technetium tetrachloride as A precursor for small technetium(IV) complexes

Article abstract of DOI:10.1021/ic060896u

Polymeric technetium tetrachloride reacts with monodentate donor ligands such as THF, acetonitrile, DMSO, thioxane (1-oxa-4-thiacyclohexane), PMe ₂Ph, PPh₃, OPPh₃, or OH₂ via cleavage of the polymeric network and the formation of [TcCl₄(L) ₂] complexes. The configuration of the products is dependent on the donor atoms such that trans coordination is established with 'soft' donor atoms such as sulfur or phosphorus, while cis-[TcCl₄(L)₂] complexes are formed with the 'harder' donors oxygen or nitrogen. The ambivalent thioxane binds to technetium via the sulfur atom. The trans products are air stable and resistant to hydrolysis. The cis complexes, however, undergo stepwise hydrolysis, during which complexes of the composition [Cl₃(L) ²TcOTc(L)₂Cl₃] (L = CH₃CN, DMSO, or OH₂) are formed. They are the first representatives of a new class of technetium(IV) complexes with a bridging oxo ligand. The Tc-O bond lengths in these bridges are between 1.803(1) and 1.823(2) A.

Full text of DOI:10.1021/ic060896u

Inorg. Chem. 2006, 45, 7331−7338
Technetium Tetrachloride as A Precursor for Small Technetium(IV)
Complexes
Adelheid Hagenbach, Eda Yegen, and Ulrich Abram*
Institut fu¨r Chemie und Biochemie, Freie UniVersita¨t Berlin, Fabeckstrasse 34-36,
D-14195 Berlin, Germany
Received May 23, 2006
Polymeric technetium tetrachloride reacts with monodentate donor ligands such as THF, acetonitrile, DMSO, thioxane
(1-oxa-4-thiacyclohexane), PMe2Ph, PPh3, OPPh3, or OH2 via cleavage of the polymeric network and the formation
of [TcCl4(L)2] complexes. The configuration of the products is dependent on the donor atoms such that trans
coordination is established with ‘soft’ donor atoms such as sulfur or phosphorus, while cis-[TcCl4(L)2] complexes
are formed with the ‘harder’ donors oxygen or nitrogen. The ambivalent thioxane binds to technetium via the sulfur
atom. The trans products are air stable and resistant to hydrolysis. The cis complexes, however, undergo stepwise
hydrolysis, during which complexes of the composition [Cl3(L)2TcOTc(L)2Cl3] (L
formed. They are the first representatives of a new class of technetium(IV) complexes with a bridging oxo ligand.
The Tc O bond lengths in these bridges are between 1.803(1) and 1.823(2) Å.
) CH3CN, DMSO, or OH2) are
−
Introduction
crystallizes as polymeric chains, in which the Tc atoms are
bridged by chloro ligands giving edge-sharing, distorted
The oxidation state ‘+4’ is one of the most stable for
technetium, but its coordination chemistry is scarcely
explored.1 The 4d3 electronic configuration of the Tc(IV)
complexes implies a ‘borderline’ situation, which lies
between that of complexes stabilized by π-donors and
π-acceptors. Technetium(IV) complexes often tend to hy-
drolyze or to form polymeric compounds. This also holds
true for the most common Tc(IV) starting materials, the
[TcX6]2- (X ) Cl, Br, I) complexes. They hydrolyze under
final formation of the polymeric ‘TcO2‚nH2O’.1-5 Ligand-
exchange reactions starting from hexahalotechnetates often
yield mixtures of products.
octahedra (1) with three distinct Tc-Cl bond lengths.6a The
longest one (2.49 Å) is associated with the bridging chlorine
atoms perpendicular to the chain. These bonds should be
easily cleaved by donor solvents and yield cis-[TcCl4(L)2]
complexes, as was concluded from spectroscopic data in
reactions with acetonitrile and isocyanides in an early
report.6b Very recently, we could isolate cis-[TcCl4(CH3CN)2]
(2) in crystalline form and elucidate its crystal structure.7
A hitherto rarely used Tc(IV) starting material is the
neutral technetium tetrachloride. The dark red compound
* To whom correspondence should be addressed. Tel: (49) 30 83854002.
Fax: (49) 30 83852676. E-mail: abram@chemie.fu-berlin.de.
(1) Alberto, R. In Technetium, ComprehensiVe Coordination Chemistry
II, McCleverty, J. A., Meyer, T. J., Eds.; Elsevier: Amsterdam, 2003;
Vol. 5, p 140.
(2) (a) Gorski, B.; Koch, H. J. Inorg. Nucl. Chem. 1969, 31, 3665. (b)
Canterford, J. H.; Colton, R. Halides of the Second and Third
Transition Row Metals; Wiley-Interscience: London, 1968; p 272.
(3) (a) Mu¨nze, R.; Noll, B. Isotopenpraxis 1975, 11, 190. (b) Grossmann,
B. Mu¨nze, R. Int. J. Appl. Radiat. Isot. 1982, 33, 189. (c) Fiser, M.;
Brabec, V.; Dragoun, O.; Kovalik, A.; Rysavy, M.; Dragounova, N.
Appl. Radiat. Isot. 1988, 39, 943.
(4) Neck, V.; Fangha¨nel, T.; Kim, J. I. Scientific Report Institute Nuclear
Technique; Research Centre Karlsruhe: Karlsruhe, Germany, 1999.
(5) Hess, N. J.; Yuanxian Xin, Rai, D.; Conradson, S. D. J. Solution Chem.
2004, 33, 199.
(6) (a) Elder, M.; Penfold, B. R. Inorg. Chem. 1966, 5, 1197. (b) Abram,
U.; Wollert, R.; Hiller, W. Radiochim. Acta 1993, 63, 145.
(7) Yegen, E.; Hagenbach, A.; Abram, U. Chem. Commun. 2005, 5575.
10.1021/ic060896u CCC: $33.50
Published on Web 08/08/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 18, 2006 7331

Hagenbach et al.
analysis: C8H16O2S2Cl4Tc (449.2); Tc, 21.9 (calc. 22.0)%. IR (νmax
/
The ready availability of 2 and its potential as a precursor
in Tc(IV) chemistry prompted us to study reactions of
(TcCl4)n with other donor solvents such as THF, DMSO,
and dioxane, or simple, monodentate ligands such as PPh3,
PMe2Ph, OPPh3, or thioxane (1-thia-4-oxa-cyclohexane).
cm-1): 352, 344 (Tc-Cl), 264 (Tc-S).
trans-[TcCl4(PPh3)2] (7). (TcCl4)n (0.1 mmol, 24 mg) was
dissolved in 2 mL of THF, and PPh3 (0.2 mmol, 52 mg) was added.
The mixture was stirred at room temperature for 2 h. During this
time, the product precipitated as a sparingly soluble, green powder.
Yield: 57 mg, 75%. The spectroscopic and analytical data are
identical with those previously reported for 7.9
Experimental Section
General Considerations. All reactions were performed using
standard Schlenk technique. Solvents were analytical grade, flushed
with argon, and carefully dried prior to use. (TcCl4)n was prepared
from Tc metal and Cl2 following the standard procedure of Colton.8
Particular care was taken to avoid any breakthrough of volatile
radioactive material by several washings of the exhausting gas
stream with aqueous KOH solution.
IR spectra between 400 and 4000 cm-1 were measured as KBr
pellets on a Shimadzu FTIR spectrometer. Far IR measurements
were done in Nujol using polyethylene windows on a Nicolet Nexus
FTIR spectrometer. Raman spectra were measured on a Bruker RFS
100/S spectrometer. Technetium elemental analyses were done by
liquid scintillation measurements.
Radiation Precautions. 99Tc is a weak â- emitter (Emax ) 0.292
MeV) with a half-life of 2.12 × 105 a. Normal glassware provides
adequate protection against the weak â radiation when milligram
amounts are used. Secondary X-rays (bremsstrahlung), however,
play a role when larger amounts of 99Tc are handled. All
manipulations were done in a laboratory approved for the handling
of radioactive materials. Particular care was taken during the
manipulation of volatile materials to avoid any contamination (see
above).
Reaction of (TcCl4)n with PMe2Ph. Reactions of (TcCl4)n with
PMe2Ph gave the same products as were obtained starting from
[TcCl6]2- 10
This means the addition of 2 equiv of the phosphine
.
to a suspension of the tetrachloride in benzene and stirring at room-
temperature resulted in a slow dissolution of (TcCl4)n and the
formation of a clear, dark green solution, from which trans-
[TcCl4(PMe2Ph)2] (8) could be isolated in almost quantitative yield.
The use of a 10-fold excess of PMe2Ph resulted in the reduction of
technetium, and the orange-red mer-[TcCl3(PMe2Ph)3] (9) was
isolated in a yield of 80%.
The well-known complexes were characterized by their IR
spectra and elemental analyses and compared with literature values.
mer-[TcCl3(PMe2Ph)3] was additionally characterized by the de-
termination of the unit cell dimensions of the obtained single crystals
(monoclinic, a ) 10.935 Å, b ) 39.191 Å, c) 13.738 Å, â )
107.3°), which were identical with the previously published values.10
[{TcCl3(CH3CN)2}2O] (10). The synthesis was performed as
described for 2, but after the complete dissolution of (TcCl4)n, the
mixture was exposed to air. After 1 day, golden-brown crystals
deposited. Yield: almost quantitative. Elemental analysis: C8H12N4-
OCl6Tc2 (590.9); Tc, 33.2 (calcd 33.5)%. IR (νmax, cm-1) 2318,
2291 (CtN); Raman (νmax/cm-1): 340 (Tc-Cl), 2316, 2292 (Ct
N).
cis-[TcCl4(CH3CN)2] (2). The synthesis of 2 is reported else-
where.7 Elemental analysis of the solvent-free sample: C4H6N2-
Cl4Tc (322.9); Tc, 29.9 (calcd 30.7)%. Raman (νmax/cm-1): 2313,
2288 (CtN); 369, 322 (Tc-Cl).
cis-[TcCl4(THF)2] (3). (TcCl4)n (0.1 mmol, 24 mg) was sus-
pended in 2 mL of carefully dried THF and heated at reflux for 2
h. The obtained green solution was reduced in volume to 1 mL
and kept at -20 °C overnight, giving yellow crystals. Yield: 17
mg, 44%. Elemental analysis: C8H16O2Cl4Tc (385.0); Tc, 25.4
(calcd 25.7)%. IR (νmax/cm-1): 364, 348 (Tc-Cl).
cis-[TcCl4(OH2)2]‚2dioxane (4‚2dioxane). The synthesis of the
compound is described in ref 7. Technetium analysis did not give
satifactory values due to partial decomposition of the isolated
crystals upon drying. IR (νmax/cm-1): 360, 339 (Tc-Cl), 436 (Tc-
O), 2870 (O-H).
[{TcCl3(OH2)2}2O]‚6dioxane (11‚6dioxane). (a) (TcCl4)n (24
mg, 0.1 mmol) was dissolved in 3 mL of dioxane, which was not
dried prior use, and the mixture was stirred at room temperature
for 3 h. The volume of the resulting dark green solution was reduced
in a vacuum to about 1 mL. Golden-brown crystals precipitated
upon standing overnight at room temperature. Yield: 33 mg, 65%.
(b) Alternatively, the dioxane solution of 4 was exposed to air.
The color of the solution changed from dark green to brown, and
single crystals of 11‚6dioxane could be recovered after concentra-
tion. Yield: 28 mg, 55%. IR (νmax, cm-1): 798 (Tc-O-Tc), 375,
351 (Tc-Cl), 575, 472 (Tc-Owater), 2858 (O-H, H-bonded).
Technetium analysis did not give satisfactory results due to partial
decomposition of the isolated crystals upon drying.
[{TcCl3(DMSO)2}2O] (12). (TcCl4)n (24 mg, 0.1 mmol) was
dissolved in 1 mL of dried DMSO and exposed to air. After 1 day,
green crystals of 12 appeared. Yield: 15 mg, 20%. Elemental
analysis: C8H24Cl6O5S4Tc2 (739.2); Tc, 26.9 (calcd 26.8)%. IR
(νmax/cm-1): 890 (SdO), 352, 344 (Tc-Cl).
[{TcCl3(DMSO)3]2[TcCl6] (13). (a) (TcCl4)n (24 mg, 0.1 mmol)
was dissolved in 1 mL of dried DMSO. The resulting green mixture
was treated with 4 mL of acetonitrile, which resulted in a color
change to yellow-green, and yellow crystals of 13 precipitated upon
standing overnight at room temperature. Yield: 56 mg, 47%. (b)
The same product could be isolated from the reaction between trans-
[TcCl4(PPh3)2] and DMSO after the addition of acetonitrile as
cis-[TcCl4(OPPh3)2] (5). (TcCl4)n (0.1 mmol, 24 mg) was
dissolved in 2 mL of THF, and OPPh3 (0.2 mmol, 56 mg) was
added. The mixture was stirred at room temperature for 2 h.
Reduction of the volume of the resulting dark green solution to 1
mL and standing overnight gave large greenish-yellow crystals (21
mg, 30%). More of the product was recovered by further concentra-
tion of the mixture in a vacuum as a microcrystalline solid. Overall
yield: 63 mg, 90%. Elemental analysis: C36H30O2P2Cl4Tc (797.4);
Tc, 12.2 (calc. 12.4)%. IR (νmax/cm-1): 350, 336 (Tc-Cl), 1122
(OdP).
trans-[TcCl4(thioxane)2] (6). (TcCl4)n (0.1 mmol, 24 mg) was
suspended in 2 mL of neat thioxane. Stirring at room temperature
for 2 h resulted in the formation of a clear red solution. The reaction
mixture was halved in volume and placed overnight in a refrigerator.
Dark red crystals were obtained. Yield: 30 mg, 60%. Elemental
(9) Mazzi, U.; de Paoli, G.; di Bernardo, P.; Magon, L. J. Inorg. Nucl.
Chem. 1976, 38, 721.
(10) (a) Mazzi, U.; De Paoli, G.; Rizzardi, G.; Magon, L. Inorg. Chim.
Acta 1974, 10, L2. (b) Bandoli, G.; Clemente, D. A.; Mazzi, U. J.
Chem. Soc., Dalton Trans. 1976, 125.
(8) Colton, R. Nature 1962, 193, 872.
7332 Inorganic Chemistry, Vol. 45, No. 18, 2006

TcCl4 as a Precursor for Small Technetium(IV) Complexes
Table 1. X-ray Structure Data Collection and Refinement Parameters
cis-[TcCl4(THF)2] cis-[TcCl4(OPPh3)2] trans-[TcCl4(thioxane)2]‚ [{TcCl3(CH3CN)2}2O] [{TcCl3(DMSO)2}2O] [{TcCl3(DMSO)3]2-
(3)
(5)
thioxane (6‚thioxane)
(10)
(11)c
[TcCl6](13)
formula
Mw
cryst syst
a/Å
b/Å
c/Å
C8H16Cl4O2Tc
385.01
orthorhombic
7.737(1)
12.658(2)
13.941(1)
90
90
90
1365.4(2)
P212121
4
C36H30Cl4O2P2Tc
797.34
C12H24Cl4O3S3Tc
553.29
C8H12Cl6N4OTc2
590.92
C8H24Cl6O5S4Tc2 C12H36Cl12O6S6Tc3
739.21
monoclinic
11.335(2)
9.463(1)
13.116(2)
90
113.13(1)
90
1293.8(3)
P2/c
1191.17
triclinic
8.703(1)
8.968(1)
13.909(2)
104.28(1)
100.53(1)
93.59(1)
1027.7(3)
P1h
monoclinic
13.896(1)
12.993(1)
19.412(2)
90
95.82(1)
90
3486.7(5)
C2/c
triclinic
6.470(1)
6.469(1)
12.831(3)
93.95(2)
93.87(2)
104.46(2)
516.8(2)
P1hb
hexagonal
10.072(1)
10.072(1)
44.724(2)
90
90
120
3929.1(2)
P6122
6
R/°
â/°
γ/°
V/Å3
space group
Z
4
1
2
1
D
calcd/g cm-3
1.868
1.816
4482
1.517
0.843
12 299
1.775
1.524
5348
1.493
1.667
56 221
1.892
1.920
µ/mm-1
2.026
2.106
no. of reflns
2095
6136
no. independent reflns 3086
4661
2737
2795
2095
3248
no. params
R1/wR2a
GOF/Flack
temp/K
137
204
116
98
114
178
0.0518/0.1425
1.037/-0.05(7)
173
0.0450/0.1166
0.858/-
200
0.0390/0.1046
0.796/-
200
0.0466/0.1206
1.091/0.1(1)
200
0.0969/0.2701
0.953/-
200
0.0609/0.1529
1.040/-
200
CCDC ref
CCDC 613257
CCDC 613258
CCDC 613259
CCDC 613260
CCDC 613261
CCDC 613262
a R1 ) ∑(|Fo - Fc|)/∑|Fo| for observed reflections; wR2 ) [∑w(Fo - Fc )2/∑(wFo )]-1/2 for all reflections. b The structure of the complex can also be
refined in the higher symmetric space group C2/m. This results, however, in lower R values since the disordered solvent thioxane does not fit this space
group. c Poor crystal quality and some twinning give comparably high R values.
2
2
2
Scheme 1
described before.11 Elemental analysis: C12H36O6S6Cl12Tc3 (1191.2);
Tc, 25.3 (calcd 24.9)%. IR (νmax/cm-1): 895 (SdO), 353, 340 (Tc-
Cl).
X-ray Crystallography. The intensities for the X-ray determina-
tions were collected on a STOE IPDS 2T instrument with Mo KR
radiation (λ ) 0.71073 Å). Standard procedures were applied for
data reduction and absorption correction. Structure solution and
refinement were performed with SHELXS97 and SHELXL97.12
Hydrogen atom positions were calculated for idealized positions
and treated with the ‘riding model’ option of SHELXL, except of
those H-atoms bonded to nitrogen atoms and/or involved in
hydrogen bonds. Their positions were derived from the final Fourier
maps and refined. More details on data collections and structure
calculations are contained in Table 1.
longest (weakest) Tc-Cl bonds in 1 and the occupation of
the vacant coordination positions by the donor solvent.
The general trend of this reaction is confirmed when 1 is
dissolved in THF. Yellow crystals of 3 deposit from the
obtained green solution upon standing overnight in a
refrigerator. The compound is extremely moisture-sensitive
and decomposes on contact with moist air under formation
of a brownish, oily product. In dry air, 3 is stable and can
be stored without decomposition. The moisture sensitivity
is unlike the behavior of the analogous rhenium(IV) complex,
which is more resistant to moisture and can be handled for
a short time in air.13
Additional information on the structure determinations have been
deposited with the Cambridge Crystallographic Data Centre.
Results and Discussion
The red, neutral technetium tetrachloride readily dissolves
in a series of donor solvents and forms neutral Tc(IV)
complexes of the composition [TcCl4(L)2] (Scheme 1). The
products can be isolated as crystalline solids when the
presence of water is strictly avoided during all manipulations.
Recently, this has been successfully demonstrated for the
reaction with acetonitrile, and the crystal structure of the
resulting 2 was reported in a preliminary communication.7
2 is a yellow solid, which is stable in dry air. Traces of
moisture, however, result in a progressive darkening of the
large crystals and slow hydrolysis. The formation of the cis
product is not unexpected in light of the solid-state structure
of the polymeric 1. It corresponds with the cleavage of the
A single-crystal structure analysis of 3 confirms the cis
arrangement of the THF ligands. The molecular structure of
the compound is shown in Figure 1. Selected bond lengths
and angles are compared with the values for other cis-[TcCl4-
(L)2] complexes in Table 2. The technetium atom possesses
an almost perfect octahedral coordination sphere. As is to
be expected, two different Tc-Cl bond lengths are observed
as a consequence of the corresponding trans ligands. In all
cases, the trans-labilizing effect of the chloro ligand is larger
than that of the oxygen- or nitrogen-donor ligands. The
Tc-N and Tc-O bonds in 2 and 3 are in the typical range
of single bonds.15 Although THF is a common solvent in
the synthetic coordination chemistry of technetium, only a
few examples exist where this cyclic ether acts as a ligand,1
(11) Breikss, A. I.; Davison, A.; Jones, A. G. Inorg. Chim. Acta 1990,
170, 75.
(12) Sheldrick, G. M. SHELXS-97 and SHELXL-97, Programs for the
solution and refinement of crystal structures; University of Go¨ttin-
gen: Go¨ttingen, Germany, 1997.
(13) Swidersky, H.-W.; Pebler, J.; Dehnicke, K.; Fenske, D. Z. Naturforsch.
1990, B45, 1227.
(14) Farrugia, L. J. Appl. Cryst. 1999, 32, 837.
Inorganic Chemistry, Vol. 45, No. 18, 2006 7333

Hagenbach et al.
Figure 1. Molecular structure of 3.14
Figure 2. Molecular structure of 5.14
Table 2. Selected Bond Lengths (Å) and Angles (deg) in 3 and 5a
Scheme 2
3
5
2‚CH3CNb 4‚2dioxaneb
Tc-Cl1
2.320(1)
2.315(1)
2.286(1)
2.283(1)
2.111(4)
2.110(4)
2.334(1)
2.334(1)c
2.308(1)
2.308(1)c
2.029(3)
2.029(3)c
88.6(2)c
2.323(2)
2.308(2)
2.279(2)
2.279(2)d
2.123(5)
2.123(5)d
88.1(3)
2.331(2)
2.331(2)e
2.283(3)
2.283(3)e
2.096(7)
2.096(7)e
84.9(5)
Tc-Cl2
Tc-Cl3
Tc-Cl4
Tc-O1/N1
Tc-O2/N2
O1/N1-Tc-O2/N2 84.3(2)
Cl1-Tc-Cl2
176.39(5) 174.07(5)c 171.09(9)
170.8(1)
a The values of the previously published compounds 2 and 4 are given
for comparison. The atomic labeling schemes for 2 and 4 are adopted to
that of the THF complex in Figure 1. b Values taken from ref 7. c Symmetry
related atoms, symmetry operation: 1 - x, y, -0.5 - z. d Symmetry related
atoms, symmetry operation: x, 1.5 - y, z. e Symmetry related atoms,
symmetry operation: 0.5 - x, -0.5 - y, z.
(OPMe2Ph)], contains a monodentate phosphine oxide (Tc-O
bond length: 2.097 Å).16a The NS+ ligand, however, is
known to cause a perceptible trans influence, which explains
the observed difference. The other two examples, [TcCl2-
{Ph2P(O)py-N,O}2](PF6) and [TcCl2(dppmO-P,O)2](PF6)
(dppmO ) bis(diphenylphosphinomethane monoxide),16b
contain chelate-bonded phosphine oxides and, hence, are not
suitable for comparison.
A completely different reaction pattern was observed for
the reaction of (TcCl4)n with dioxane. The donor abilities of
this cyclic ether were apparently not good enough to stabilize
the corresponding cis-[TcCl4(L)2] complex. Instead, the
technetium tetrachloride reacted with traces of water, which
were present even in carefully dried dioxane (Scheme 2).
Details of this hydrolysis have been published before7 and
shall not be outlined here in detail. Isolation of molecular
intermediates from aqueous solutions (although suggested
by model calculations)4 has so far failed.
From the stepwise hydrolysis of (TcCl4)n in moist dioxane,
we were able to isolate two molecular intermediates of this
reaction in crystalline form, the monomeric 4‚2dioxane and
the oxo-bridged dimer 11‚6dioxane. The unexpected stability
of the aqua complexes 4 and 11 is explained by their solid-
state structures. Both compounds co-crystallize with dioxane
solvate molecules. Hydrogen bonds between the aqua ligands
and the surrounding dioxane molecules arrange the complex
molecules such that they are completely encapsulated by
dioxane. The three-dimensional networks prevent polymer-
ization and the final formation of ‘hydrolyzed TcO2’ as long
and 3 is the first example of such a compound which has
been studied crystallographically.15
The THF ligands in 3 are only weakly bonded, and thus,
the complex can be used as a precursor for the synthesis of
other Tc(IV) complexes. Addition of 2 equiv of OPPh3 to
the yellow solution of 3 in THF results in a green reaction
mixture, and greenish-yellow crystals of 5 are obtained after
concentration. The compound can be handled in air for some
minutes without decomposition. Longer exposure to mois-
ture, however, results in subsequent darkening of the surface
of the crystals and slow hydrolysis, which finally gives
‘hydrolyzed TcO2’ as an insoluble, almost black solid. The
IR band of the OPPh3 ligand is observed at 1122 cm-1. This
corresponds to a slight bathochromic shift with respect to
the noncoordinated phosphine oxide (1195 cm-1).
The single-crystal structure of 5 confirms the cis coordina-
tion of the bulky OPPh3 ligands. A graphical representation
of the molecular structure is given in Figure 2. Selected bond
lengths and angles are contained in Table 2. All main
structural features discussed for 3 are also valid for 5. It is
remarkable that despite the steric bulk of the phosphine oxide
ligands the O-Tc-O angle is smaller than 90°. The
structural trans influence of OPPh3 is slightly larger than for
acetonitrile or THF in the compounds 2 and 3, respectively.
The observed Tc-O bond length in 5 is smaller than the
values in the only three previously published complexes of
technetium with phosphine oxide ligands. Only one of them,
the Tc(II) thionitrosyl compound [Tc(NS)Cl2(PMe2Ph)-
(15) (a) Bandoli, G.; Tisato, F.; Dolmella, A.; Agostini, S. Coord. Chem.
ReV. 2006, 250, 561 and references therein. (b) Cambridge Crystal-
lographic Database, The Cambridge Crystallographic Data Centre,
Cambridge, UK, version 5.27, update November 2005.
(16) (a) Kaden, L.; Lorenz, B.; Kirmse, R.; Stach, J.; Behm, H.; Beurskens,
P. T. Inorg. Chim. Acta, 1990, 43, 169. (b) Freiberg, E.; Davis, W.
M.; Nicholson, T.; Davison, A.; Jones, A. G. Inorg. Chem. 2002, 41,
5667.
7334 Inorganic Chemistry, Vol. 45, No. 18, 2006

TcCl4 as a Precursor for Small Technetium(IV) Complexes
Figure 3. Molecular structures of the oxo-bridged complexes (a) 10 and (b) 12.
Table 3. Selected Bond Lengths (Å) and Angles (deg) in
able electron donation from the px and py orbitals of the
oxygen atom to dπ orbitals of the metal ions. This is mainly
indicated by the relatively short Tc-O-Tc bonds. The same
holds true for the bridging oxygen ligands in 10, 11, and
12. With values between 1.803(1) and 1.823(2) Å, these bond
lengths are between those expected for common Tc-O single
and double bonds. The Tc-O-Tc bond angles are essentially
linear with angles between 168.3(9)° and 180°. The structural
trans influence due to the bridging oxo ligands is slightly
larger than that of a chloro ligand. Compounds 10, 11, and
12 represent the first examples of complexes with an oxygen
bridge between two Tc(IV) centers. Structural information
about similar systems were hitherto restricted to complexes
of Tc(V),17 Tc(III),18 and some mixed-valent complexes,
which also contain bridging oxo ligands.19
Complex 12 is the first technetium complex with oxygen-
bonded DMSO ligands to be studied by X-ray crystal-
lography. Its isolation in crystalline form from neat DMSO
was successful due to its relatively low solubility in this
solvent. Up to now, we have not been able to isolate single
crystals of its ‘parent complex’ cis-[TcCl4(DMSO)2], but we
have no doubt that this compound exists in the yellow
solution, which is formed after the dissolution of [TcCl4]n
in dry DMSO. The synthesis of this neutral, monomeric
complex was described in a previous paper by the reaction
of trans-[TcCl4(PPh3)2] with DMSO in boiling acetonitrile
and subsequent precipitation of the product as a yellow
powder with a large excess of diethyl ether.11 The compound
was characterized by elemental analysis, IR, and NMR
spectroscopy. Main support for its composition comes from
FAB+ mass spectrometry in nitrobenzyl alcohol (NBA),
where the molecular ion of [TcCl4(DMSO)2(NBA)]+ was
registered at m/z ) 548 with the correct isotopic pattern for
[TcCl3(L)2OTcCl3(L)2] Complexesa
10b
12c
11d,e
Tc-Cl1
2.346(2)
2.341(2)
2.307(2)
1.803(1)
2.128(6)
2.135(5)
2.384(5)
2.344(5)
2.340(5)
1.823(2)
2.06(1)
2.12(1)
172.7(4)
168.3(9)
2.344(2)
2.346(2)
2.317(2)
1.812(1)
2.116(4)
2.132(3)
178.67(9)
180
Tc-Cl2
Tc-Cl3
Tc-O10
Tc-N1/O1
Tc-N2/O2
O10-Tc-N2/O2
Tc-O10-Tc′
176.6(2)
171.8(3)
a The atomic labeling scheme for 11 is adopted to that of the other
complexes. b Symmetry related atoms, symmetry operation: x, x - y, 1/6
- z. c Symmetry related atoms, symmetry operation: 2 - x, y, 3/2 - z.
d Symmetry related atoms, symmetry operation: 1 - x, -y, 1 - z. e Values
taken from ref 7.
as the compounds are stored under dioxane. In air, however,
the single crystals rapidly decompose under formation of an
oily residue. The same type of decomposition was observed
during attempts to obtain pure samples for elemental analysis
by drying the crystals in a vacuum. We have not yet been
able to isolate defined oxo-bridged compounds with more
than two technetium atoms.
The formation of oxo-bridged technetium(IV) dimers is
not restricted to the hydrolysis product of the aqua complex
described above. Reactions of moist acetonitrile or DMSO
with (TcCl4)n also results in a partial hydrolysis of the
intermediately formed cis-[TcCl4(L)2] complexes. This is
indicated by a subsequent darkening of the yellow-green
reaction mixtures and the formation of golden-brown pre-
cipitates. The corresponding reaction with acetonitrile is
almost quantitative and yields 10 as compact single crystals,
while the analogous reaction mixture with DMSO must be
concentrated almost to dryness to yield 12 as thin crystal
plates. The IR and Raman spectra of the products agree with
the structures of 10 and 12. They show Tc-Cl bands between
340 and 355 cm-1 and the CtN vibrations of 10 are detected
as strong bands at 2316 and 2292 cm-1. Figure 3 illustrates
the molecular structures of the dimeric complexes 10 and
12. Selected bond lengths and angles are summarized in
Table 3 and compared with the values in the previously
published aqua complex, 11. Oxo bridges are common in
the coordination chemistry of technetium(V), where the
almost linear [OdTc-O-TcdO]4+ core is perfectly suitable
for the stabilization of neutral complexes with bidentate,
monoanionic or tetradentate, dianionic ligands.1 The bonding
situation in such compounds is characterized by a consider-
(17) (a) Bandoli, G.; Nicolini, M.; Mazzi, U.; Refosco, F. J. Chem. Soc.,
Dalton Trans. 1984, 2505. (b) Baldas, J.; Colmanet, S. F.; Mackay,
M. F. J. Chem. Soc., Dalton Trans. 1988, 1725. (c) Tisato, F.; Refosco,
F.; Mazzi, U.; Bandoli, G.; Dolmella, A. Inorg. Chim. Acta, 1989,
164, 127. (d) Pillai, M. R. A.; John, C. S.; Lo, J. M.; Schlemper, E.
O.; Troutner, D. E. Inorg. Chem. 1990, 29, 1850. (e) Pietzsch, H.-J-;
Spies, H.; Leibnitz, P.; Reck, G.; Beger, J.; Jacobs, R. Polyhedron,
1993, 12, 187. (f) Abram, U.; Abram, S.; Ma¨cke, H. R.; Koch, P. Z.
Anorg. Allg. Chem. 1995, 621, 854.
(18) (a) Tisato, F.; Refosco, F.; Mazzi, U.; Nicolini, M. Inorg. Chim. Acta,
1989, 157, 227. (b) Jun Lu, Hiller, C. D.; Clarke, M. J. Inorg. Chem.
1993, 32, 1417.
(19) (a) Kastner, M. E.; Fackler, P. H.; Podbielski, L.; Chakoudian, J.;
Clarke, M. J. Inorg. Chim. Acta 1986, 114, 11. (b) Clarke, M. J.;
Kastner, M. E.; Podbielski, L. A.; Fackler, P. H.; Schreifels, J.;
Meinken, G.; Srivastava, S. C. J. Am. Chem. Soc. 1988, 110, 1818.
Inorganic Chemistry, Vol. 45, No. 18, 2006 7335

Hagenbach et al.
Scheme 3
Scheme 4
water into such solutions. Interestingly, no ligand-exchange
products with the donor solvent acetonitrile were identified
during these manipulations.
Figure 4 contains the molecular structure of the complex
cation of 13 with three DMSO ligands in facial arrangement.
Since the [TcCl6]2- anions have the expected octahedral
structure and no intermolecular contacts are established
between the ion pairs, the counterions have been omitted
for clarity. Table 4 contains selected bond lengths and angles.
The Tc-O and Tc-Cl bond lengths show no unusual
features, but it is remarkable that the coordination octahedron
is distorted such that the angles between the facially
coordinated DMSO ligands are considerably smaller than
90°. The same feature is observed in the solid-state structure
of 12, where the O1-Tc-O2 angle of 82.3(5)° can be
explained by a partial double-bond character of the trans-
situated Tc-O bond. In 13, however, the DMSO ligands
are trans to chloro ligands, which does not explain the
observed distortion. A possible explanation may be weak
interactions between the oxygen atoms, leading to interatomic
distances between 2.709 and 2.759 Å, which is shorter than
the sum of the van der Waals radii. Markedly longer metal-
oxygen distances (3.165-3.213 Å) are observed in another
second-row transition metal complex with a {MCl3(DMSO)3}
coordination sphere, the polymeric [{(CdCl2)5(DMSO)7}n],20
while shorter M-O bonds are observed in a series of hexakis
DMSO complexes of first-row transition elements.15b
All of the reactions of (TcCl4)n with the nitrogen and
oxygen donor atom ligands OH2, THF, DMSO, or OPPh3
discussed above result in the formation of cis-[TcCl4(L)2]
products. This can be understood from the polymeric
structure of technetium tetrachloride, which provides two cis
positions for coordination after cleavage of the longest Tc-
four chloro ligands. Our attempts to obtain crystalline [TcCl4-
(DMSO)2] by layering acetonitrile/diethyl ether over a
DMSO solution of [TcCl4(DMSO)2] resulted in the precipita-
tion of single crystals of the ionic compound 13. Single
crystals of the same composition were also obtained from
the [TcCl4(PPh3)2]/DMSO/CH3CN reaction mixture described
above.11 Obviously, the DMSO ligands are only weakly
bonded to technetium and an equilibrium is established,
which involves species such as [TcCl3(DMSO)3]+ and
[TcCl6]2- ions (Scheme 3). Addition of acetonitrile then
results in the separation of single crystals of the ion pair 13,
while the dimeric compound 12 is formed after diffusion of
Figure 4. Molecular structure of the complex cation in 13.
Table 4. Selected Bond Lengths (Å) and Angles (deg) in the Complex
Cation of 13
Tc1-Cl1
2.304(3)
2.304(3)
2.309(3)
1.558(6)
1.544(8)
93.4(1)
Tc1-O1
Tc1-O2
Tc1-O3
O2-S2
2.036(7)
2.035(7)
2.053(6)
1.560(6)
Tc1-Cl2
Tc1-Cl3
O1-S1
Figure 5. Molecular structure of complex 6.
O3-S3
Cl1-Tc1-Cl2
Cl1-Tc1-O1
Cl1-Tc1-O3
Cl2-Tc1-O1
Cl2-Tc1-O3
Cl3-Tc1-O2
O1-Tc1-O2
O2-Tc1-O3
Cl1-Tc1-Cl3
Cl1-Tc1-O2
Cl2-Tc1-Cl3
Cl2-Tc1-O2
Cl3-Tc1-O1
Cl3-Tc1-O3
O1-Tc1-O3
93.4(1)
92.0(2)
93.8(1)
92.0(2)
90.4(2)
90.7(2)
84.5(3)
Table 5. Selected Bond Lengths (Å) and Angles (deg) in 6
90.9(2)
173.9(2)
173.8(2)
90.9(2)
171.9(2)
83.4(3)
Tc-Cl1
2.320(1)
2.239(0)
89.95(4)
90.9(2)
Tc-S1
2.476(1)
Tc-Cl2
Cl1-Tc-Cl2
Cl2-Tc-S1
Cl1-Tc-S1
90.01(4)
90.05(4)
Cl1-Tc-Cl2′a
83.5(3)
a Symmetry related atoms, symmetry operation: (′) 1 - x, -y, -z.
7336 Inorganic Chemistry, Vol. 45, No. 18, 2006

TcCl4 as a Precursor for Small Technetium(IV) Complexes
Scheme 5
Cl bonds of the polymer. Nevertheless, a trans arrangement
of the incoming ligands is observed with phosphorus or sulfur
donor ligands such as PPh3, Me2PhP, or thioxane.
in mind that no direct coordination with dioxane was
observed in the corresponding experiments). Stirring of
(TcCl4)n in neat thioxane at room-temperature results in the
formation of a clear red solution, from which red crystals of
6 can be isolated upon concentration and cooling. In contrast
to the cis complexes described above, 6 is stable even in
moist air and no further reactions, e.g., the formation of oxo-
bridged dimers, were observed. The compound is soluble in
common organic solvents such as benzene or chloroform.
The trans coordination of the two thioether ligands was
confirmed by X-ray crystallography. Figure 5 shows the
molecular structure of 6. Selected bond lengths and angles
are summarized in Table 5. The technetium atom is located
on a center of inversion, and its coordination environment
can be described as an almost perfect octahedron. Thus, the
structure of 6 is very similar to that of trans-[TcCl4(THT)2]
(THT ) tetrahydrothiophene), which has been prepared
recently from K2[TcCl6].21 It must be noted that attempts to
obtain 6 by an analogous reaction between K2TcCl6 and
thioxane failed and gave only intractable dark-brown oils,
A dark green powder begins to precipitate immediately
after the addition of PPh3 to a solution of (TcCl4)n in THF.
This product was identified as the well-known 7, which can
alternatively be prepared from [TcCl6]2- or [TcO4]-/HCl
mixtures.9,10 A similar reaction course is observed with
Me2PhP, which results in the formation of the green 8 when
2 equiv of the phosphine are used, while reduction and
formation of the orange-red 9 are observed with an excess
of Me2PhP (Scheme 4). This behavior is not completely
unexpected in light of reactions of various Tc(VII), Tc(V),
and Tc(IV) precursors with tertiary phosphines,1,9,10 but in
contrast to the formation of the cis adducts, formed with
ligands having ‘hard’ donor atoms (vide supra).
The cyclic ether/thioether 1-oxa-4-thiacyclohexane (thiox-
ane) provides a ‘soft’ sulfur donor atom and a ‘hard’ oxygen
donor and principally allows for the formation of both
isomers depending on the donor atom involved (also bearing
(20) Nieuwenhuyzen, M.; Wilins, C. J. J. Chem. Soc., Dalton Trans. 1993,
2673.
(21) Hagenbach A.; Abram, U. Inorg. Chem. Commun. 2004, 7, 1142.
Inorganic Chemistry, Vol. 45, No. 18, 2006 7337

Hagenbach et al.
while (TcCl4)n reacts with THT in benzene under formation
of the known adduct trans-[TcCl4(THT)2].
A new class of Tc(IV) complexes having the {Tc-O-
Tc}6+ core is obtained by partial hydrolysis of the cis-[TcCl4-
(L)2] compounds. The stability of the oxo-bridged complexes
is dependent on the incoming ligands. While the correspond-
ing acetonitrile compound 10 is mostly stable against further
hydrolysis and can be stored in air for a few hours without
decomposition, the corresponding aqua compound 11 is only
stable in a matrix of hydrogen-bonded dioxane solvents
molecules, which keep the complex molecules apart from
each other thus preventing further hydrolysis and the final
formation of TcO2‚nH2O.
Conclusions
The polymeric (TcCl4)n is an excellent precursor for the
synthesis of small technetium(IV) molecules. Its versatility
and the fact that different classes of Tc(IV) complexes can
be prepared by simple reactions with common donor solvents
justify the relatively cumbersome synthesis of technetium
tetrachloride by chlorination of the metal. Scheme 5 repre-
sents a summary of the products, which were obtained by
direct or subsequent reactions starting from (TcCl4)n.
(TcCl4)n reacts by well-defined procedures with dry
solvents having ‘hard’ donor atoms such as acetonitrile or
THF to form cis-[TcCl4(L)2] complexes, which can be
isolated as crystalline solids. Such products are promising
candidates as precursors for further ligand exchange reactions
with chelating ligands since they are more stable than
(TcCl4)n but much more reactive than other Tc(IV) precursors
such as the sparingly soluble trans-[TcCl4(PPh3)2] or salts
of the [TcCl6]2- anion. Such reactions are currently under
study in our laboratory and may contribute to the further
exploration of the hitherto relatively undexplored coordina-
tion chemistry of Tc(IV).
In contrast to the cis-[TcCl4(L)2] complexes, corresponding
trans products, which can be obtained with ligands having
‘soft’ donor atoms such as PPh3 or thioxane, are stable
against hydrolysis.
Acknowledgment. This work was supported by the
Deutsche Forschungsgemeinschaft (AB 67/08-01). We also
thank Dr. Johann Spandl (FU Berlin) for Raman measure-
ments.
Supporting Information Available: Crystallographic data in
cif format. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC060896U
7338 Inorganic Chemistry, Vol. 45, No. 18, 2006