Journal of Nuclear and Radiochemical Sciences, Vol. 4, No. 1, pp. N1-N2, 2003

Notes
X-ray Crystallographic Determination of the Structure of trans-Dichloro[N,N'-ethylenebis(3,5-dimethylsalicylideneiminato)]technetium(IV)Tsutomu Takayama,* Ryouji Koudo, Tsutomu Sekine, and Hiroshi Kudo
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
Received: May 12, 2003; In Final Form: June 19, 2003
A novel technetium(IV) complex of a tetradentate Schiff base ligand was synthesized from the reaction ofH2dmsalen [dmsalen; N,N'-ethylenebis(3,5-dimethylsalicylideniminato)] and trans-99TcIVCl4(PPh3)2 [PPh3; triphenylphosphin]. The complex 99TcIVCl2(dmsalen), the structure of which was determined by X-ray crystallography, has an octahedral coordination geometry. The dmsalen ligand coordinates to the technetium atom throughtwo N and two O atoms of the tetradentate ligand in the equatorial plane. Two chloro ligands are in the transapical position with an average Tc-Cl bond length of 2.35 Å.

*Corresponding author. E-mail: takayama@mail.cc.tohoku.ac.jp.
FAX: +81-22-217-6596.

5.0

Current/µA

Technetium complexes with Schiff base ligands attract
interest not only for application in nuclear medicine,1 but also
in coordination chemistry because the flexibility of Schiff baseligands in geometry and coordination sites makes it possible tosynthesize several types of technetium complexes differing in
the chemical properties.2 Schiff base complexes of technetium
reported so far are in oxidation states of Tc(III), Tc(V) andTc(VI),3−7 but little is known for Tc(IV). We have synthesizeda technetium(IV) complex with a tetradentate Schiff baseligand for the first time using 99Tc (T1/2 = 2.1 × 105 y). In thisnote, the crystal structure of trans-TcIVCl2(dmsalen) is reported
together with a redox property of the complex.
The TcIVCl2(dmsalen) complex was synthesized in a manner
described below. An ethanol solution (10 mL) of H2dmsalen(110 µmol) was added to suspensions of trans-TcIVCl4(PPh3)2(105 µmol)8 in benzene (50 mL). This mixture was stirred for
3 h at 60°C. The color of the mixture gradually turned fromgreen to brown. The reaction mixture was filtered, and thefiltrate was evaporated to dryness. The residue was dissolvedin CH2Cl2, and then charged on a silica gel column (φ 2 cm ×20 cm). The product was eluted with CH2Cl2. The effluent
with a purple band in the column was collected, and the dark
purple crystals were obtained by concentration, the solvent
being slowly evaporated to dryness. Yield 16.0 µmol (15%).
Anal. Found: C, 48.35; H, 4.30; N, 5.81; Tc, 19.91%. Calcd
for C20H22Cl2N2O2Tc : C, 48.79; H, 4.50; N, 5.69; Tc, 20.11%.
UV-vis spectrum {EtOH, λmax / nm (ε / M−1cm−1)}: 262 (2.22 ×105), 345 (7.19 × 104), 456 (7.47 × 103).
The magnetic moment of TcCl2(dmsalen) was measured by
the Evans method9 at 293 K in CDCl3 solvent to determine theoxidation state of Tc. The observed value of 3.82 µB indicatesthat spin state is S = 3/2. The result indicates that Tc of thiscomplex is in the +4 oxidation state with three unpaired electrons.
A stability of the complex toward oxidation-reduction reactions was examined by cyclic voltammetry in a CH3CN solution. Cyclic voltammograms were recorded with a Yanaco
voltammetric analyzer P-1000 equipped with a standard threeelectrode configuration at 298 K. Figure 1 shows the cyclic
voltammogram of TcIVCl2(dmsalen) observed in CH3CN. This
complex showed two quasi-reversible redox waves at +0.825
V and –0.522 V, suggesting that this Tc(IV) complex is stabletoward the addition and removal of one electron.

0.0-0.522 V-5.0+0.825 V+1.00.0+0.5-0.5-1.0

Potential/VFigure 1. Cyclic voltammogram of 1.9 mM trans-TcIVCl2(dmsalen).
Counter electrode, platinum wire; working electrode, glassy carbon;
reference electrode, Ag/Ag+ [0.01 M AgNO3/CH3CN containing 0.1 M(C4H9)4NClO4]; supporting electrolyte, 0.1 M (C4H9)4NClO4.

A single crystal (0.20 × 0.20 × 0.06 mm3) of TcIVCl2(dmsalen)
was subjected to X-ray crystallography. The diffraction data
were collected at 160 K using a Rigaku/MSC Mercury CCD
diffractometer with graphite monochromated MoKα radiation(λ = 0.71070 Å). A total of 27007 reflections was collected inthe range 6.2 < 2 θ < 55.0 degrees. The structure of thecomplex was solved by the direct method (SIR92). Allremaining non-hydrogen atoms were refined by the full-matrix
least-squares method (SHELXL-97)10 with anisotropic thermal
parameters. The coordinates of the hydrogen atoms werededuced from the difference Fourier map and refined using
isotropic temperature factors. All of the calculations weremade using the teXsan crystallographic software package.11
Crystal data: formula = C 2 0 H 2 2 Cl 2 N 2 O 2 Tc, fw = 490.31,
orthorhombic, space group Pna21 (No. 33), a = 13.309(4) Å, b= 11.760(3) Å, c = 12.880(3)Å, V = 2015(1) Å3, Z = 4, Dcalcd
1.662 g/cm3, R1 = 0.024, Rw = 0.064 for 3404 unique reflections (I > 2σ (I)).
The ORTEP drawing of Tc IV Cl 2 (dmsalen) is shown in
Figure 2 with thermal ellipsoids at the 30% probability level.
The technetium atom is in an octahedral coordination geometry. Two chloro ligands are trans to each other in the axialposition. The tetradentate dmsalen ligand coordinates to thetechnetium atom through two nitrogen atoms of imine and twoanionic oxygen atoms of phenol in the equatorial plane. The
symmetrical coordination of the dmsalen ligand in the equatorial plane is the same as that in technetium complexes of salen
type ligands reported previously.3−6© 2003 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 06/30/2003

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Takayama

J. Nucl. Radiochem. Sci., Vol. 4, No. 1, 2003
IVFigure 2. ORTEP diagram of trans-TcIVCl2(dmsalen). Selected bondlengths (Å): Tc1–Cl1, 2.352(1); Tc1–Cl2, 2.3465(9); Tc1–O1,1.968(2); Tc1–O2, 1.960(2); Tc1–N1, 2.067(2); Tc1–N2, 2.061(3).
Selected bond angles (degree): O1–Tc1–O2, 100.1(1); N1–Tc1–N2,81.1(1); O1–Tc1–N1, 89.1(1); O2–Tc1–N2, 89.7(1); Cl1–Tc1–Cl2,174.73(3).

An average bond length between the anionic oxygen
atoms and the technetium atom is 1.96 Å. The Tc–O bondlength is short compared with that of other technetium
complexes with Schiff base ligands such as TcVO(salen)Cl[salen; N,N'-ethylenebis(salicylideniminato)],4 TcVN(dmsalen),6and trans-[Tc III (acacen)(PPh 3 ) 2 ]PF 6 [acacen; N,N'-ethylenebis(acetylacetonato)] 5 (Table 1). The fact indicates thestrong σ donation of the dmsalen ligand to the Tc(IV) atom
through the anionic oxygen atoms. The Tc–Cl bond length oftrans-Tc IV Cl 2 (dmsalen) is extremely shorter than that ofTc V O(salen)Cl. This is due to a difference of the ligand
species at the trans position of the chloro ligand. Two chloroligands are at the trans position each other in transTcIVCl2(dmsalen), while an oxo ligand is at the trans positionin TcVO(salen)Cl. The electron donation of the chloro ligand
is weaker than that of the oxo ligand, and the Tc–Cl bondlength of trans-Tc I V Cl 2 (dmsalen) is shorter than that inTcVO(salen)Cl.
TABLE 1: Selected Bond Lengths and Bond Angles ofTechnetium Complexes with Tetradentate Schiff Base
Ligands
Bond length/Å Bond angle/degree Ref.
Tc–O Tc–N Tc–Cl N–Tc–N O–Tc–Otrans-TcIVCl2(dmsalen)
V

1.96

2.06

2.3581.1

100.1 This work

Tc O(salen)Cl

1.99

2.02

2.53

81.9

79.6

4TcVN(dmsalen)

2.04

2.07

–

79.6

87.8

6

[Tc (acacen)(PPh3)2]PF6 2.02

2.06

–

82.8

99.6

5

III

The N imine –Tc–N imine angle of trans-Tc Cl 2 (dmsalen) iscomparable with that of other technetium complexes with
Schiff base ligands as listed in Table 1. The O–Tc–O angle oftrans-Tc I V Cl 2 (dmsalen) is comparable with that of[TcIII(acacen)(PPh3)2]PF6, but larger than that of TcVO(salen)Cland TcVN(dmsalen). The difference in the O–Tc–O angle isattributed to a difference in the location of the technetium atom
relative to the equatorial coordination plane. The technetium
atom of trans-Tc IV Cl 2 (dmsalen) is on the O–N–N–O plane
defined by two coordinating nitrogen atoms and two coordinating oxygen atoms. This structural feature of transTc I V Cl 2 (dmsalen) is the same as that of [Tc I I I (acacen)(PPh3)2]PF6, while in Tc(V)-oxo and Tc(V)-nitrido complexes
the technetium atom is displaced toward the oxo or nitrido
ligand from the O–N–N–O plane.4−6
Acknowledgment. The authors thank Prof. Dr. Ryohei
Amano of Kanazawa University for providing us Tc-99. This
work was partly supported by the Ministry of Education,
Culture, Sports, Science and Technology, Japan (a Grant-inAid for Scientific Research).
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