Rhenium and technetium complexes with tridentate N-[(N″,N″- dialkylamino)(thiocarbonyl)]-N′-substituted benzamidine ligands

Article abstract of DOI:10.1021/ic702344s

N-[(Dialkylamino)(thiocarbonyl)]benzimidoyl chlorides react with functionalized amines such as 2-aminophenol, 2-methylaminopyridine, and 2-aminobenzoic acid in clean and high-yield procedures with the formation of the novel tridentate N-[(N″,N″-dialkylamino)(thiocarbonyl)]-N′- substituted benzamidine ligands H₂L¹, HL², and H₂L³. By starting from (NBu₄)[MOCl₄] (M = Re, Tc) or [ReOCl₃(PPh₃)₂] and H ₂L¹, a series of oxorhenium(V) and oxotechnetium(V) complexes of the composition [MOCl(L¹)] were synthesized and characterized by spectroscopic methods and X-ray crystallography. The monomeric, five-coordinate compounds are air-stable and bind (L¹)^2- tridentate in the equatorial coordination sphere. Dimeric products of the compositions [{ReOCl(L²)}₂O] and [ReOCl(L ³)]₂ were isolated during reactions with HL² and H₂L³. While dimerization in [{ReOCl(L ²)}₂O] is established via an oxo bridge, the metal atoms in [ReOCl(L³)]₂ are connected by the carboxylic group of the ligand, and the product represents the first example of a high-oxidation state rhenium complex displaying such a bonding feature.

Full text of DOI:10.1021/ic702344s

Inorg. Chem. 2008, 47, 5136-5144
Rhenium and Technetium Complexes with Tridentate
N-[(N′′,N′′-Dialkylamino)(thiocarbonyl)]-N′-substituted Benzamidine
Ligands
Nguyen Hung Huy, Jacqueline Grewe, Jennifer Schroer, Bernd Kuhn, and Ulrich Abram*
Institute of Chemistry and Biochemistry, Freie UniVersita¨t Berlin, Fabeckstrasse 34-36,
D-14195 Berlin, Germany
Received December 1, 2007
N-[(Dialkylamino)(thiocarbonyl)]benzimidoyl chlorides react with functionalized amines such as 2-aminophenol,
2-methylaminopyridine, and 2-aminobenzoic acid in clean and high-yield procedures with the formation of the novel
tridentate N-[(N′′,N′′-dialkylamino)(thiocarbonyl)]-N′-substituted benzamidine ligands H2L1, HL2, and H2L3. By starting
from (NBu4)[MOCl4] (M ) Re, Tc) or [ReOCl3(PPh3)2] and H2L1, a series of oxorhenium(V) and oxotechnetium(V)
complexes of the composition [MOCl(L1)] were synthesized and characterized by spectroscopic methods and X-ray
crystallography. The monomeric, five-coordinate compounds are air-stable and bind (L1)2- tridentate in the equatorial
coordination sphere. Dimeric products of the compositions [{ReOCl(L2)}2O] and [ReOCl(L3)]2 were isolated during
reactions with HL2 and H2L3. While dimerization in [{ReOCl(L2)}2O] is established via an oxo bridge, the metal
atoms in [ReOCl(L3)]2 are connected by the carboxylic group of the ligand, and the product represents the first
example of a high-oxidation state rhenium complex displaying such a bonding feature.
Introduction
various cores such as [MdO]3+,4 [MtN]2+,5 and [M(CO)3]+
(M ) Re, Tc).6 In most of the known examples, they bind
as mononegative, bidentate ligands.
Since N-[(dialkylamino)(thiocarbonyl)]benzimidoyl chlo-
rides (1) were first synthesized and incorporated into the
synthesis of corresponding benzamidines,1,2 numerous bi-
dentate dialkylaminothiocarbonylbenzamidine ligands (HL0)
have been synthesized, and their coordination chemistry with
transition metal ions such as Ni2+, Pd2+, Pt2+, Co3+, Cu2+,
Ag+, and Au+ has been extensively studied.3 With techne-
tium and rhenium, dialkylaminothiocarbonylbenzamidines
HL0 (R3 ) H, aryl) act as versatile ligands and stabilize
In principle, the general synthetic procedure of the ligand
class HL0, in which benzimidoyl chlorides react with
ammonia or primary aromatic amines, should also be well
suitable for reactions with other, functionalized primary
amines as well as secondary amines. This, however, has only
* To whom correspondence should be addressed. E-mail: abram@
chemie.fu-berlin.de. Fax: 0049 30 83852676. Phone: 0049 30 83854002.
(1) Beyer, L.; Widera, R. Tetrahedron Lett. 1982, 23, 1881.
(2) Beyer, L.; Hartung, J.; Widera, R. Tetrahedron 1984, 40, 405.
(3) (a) Hartung, J.; Weber, G.; Beyer, L.; Szargan, R. Z. Anorg. Allg.
Chem. 1985, 523, 153. (b) del Campo, R.; Criado, J. J.; Garcia, E.;
Hermosa, M. R.; Jimenez-Sanchez, A.; Manzano, J. L.; Monte, E.;
Rodriguez-Fernandez, E.; Sanz, F. J. Inorg. Biochem. 2002, 89, 74.
(c) Hernandez, W.; Spodine, E.; Richter, R.; Hallmeier, K.-H.;
Schro¨der, U.; Beyer, L. Z. Anorg. Allg. Chem. 2003, 629, 2559. (d)
Schro¨der, U.; Richter, R.; Beyer, L.; Angulo-Cornejo, J.; Lino-Pacheco,
M.; Guillen, A. Z. Anorg. Allg. Chem. 2003, 629, 1051. (e) Guillon,
E.; Dechamps-Olivier, I.; Mohamadou, A.; Barbier, J.-P. Inorg. Chim.
Acta 1998, 268, 13. (f) Richter, R.; Schro¨der, U.; Kampf, M.; Hartung,
J.; Beyer, L. Z. Anorg. Allg. Chem. 1997, 623, 1021. (g) Braun, U.;
Sieler, J.; Richter, R.; Hettich, B.; Simon, A. Z. Anorg. Allg. Chem.
1988, 557, 134.
(4) (a) Abram, U.; Hu¨bener, R. Inorg. Chim. Acta 1993, 206, 231. (b)
Dilworth, J. R.; Lewis, J. S.; Miller, J. R.; Zheng, Y. Polyhedron 1993,
12, 221. (c) Abram, U.; Mu¨nze, R.; Hartung, J.; Beyer, L.; Kirmse,
R.; Ko¨hler, K.; Stach, J.; Behm, H.; Beurskens, P. T. Inorg. Chem.
1989, 28, 834.
(5) (a) Abram, U.; Schmidt-Bru¨cken, B.; Ritter, S. Polyhedron 1999, 18,
831. (b) Abram, U.; Braun, M.; Abram, S.; Kirmse, R.; Voigt, A.
J. Chem. Soc., Dalton Trans. 1998, 231. (c) Ritter, S.; Abram, U.
Inorg. Chim. Acta 1994, 215, 159.
(6) (a) Abram, U.; Abram, S.; Alberto, R.; Schibli, R. Inorg. Chim. Acta
1996, 248, 193. (b) Braband, H.; Abram, U. J. Organomet. Chem.
2004, 689, 2066.
5136 Inorganic Chemistry, Vol. 47, No. 12, 2008
10.1021/ic702344s CCC: $40.75  2008 American Chemical Society
Published on Web 05/13/2008

Re and Tc Complexes with Tridentate Benzamidine Ligands
Chart 1. Ligands Used in This Study
impact (EI) mass spectra were measured with a Varian MAT711
(60 °C, 70 eV) mass spectrometer. All MS results are given in the
following form: m/z, assignment. The elemental analysis of carbon,
hydrogen, nitrogen, and sulfur was determined using a Heraeus
vario EL elemental analyzer. The 99Tc values were determined by
standard liquid scintillation counting. NMR spectra were taken with
a JEOL 400 MHz multinuclear spectrometer.
Syntheses of the Ligands. N-(N′′,N′′-Dialkylaminothiocarbo-
nyl)-N′-(2-hydroxyphenyl)benzamidines, H2L1. N-(N′,N′- Dialkyl-
aminothiocarbonyl)benzimidoyl chloride (5 mmol) was dissolved
in 10 mL of dry acetone and slowly added to a stirred mixture of
2-aminophenol (545 mg, 5 mmol) and NEt3 (1.51 g, 15 mmol) in
10 mL of dry acetone. The mixture was stirred for 2 h at 40 °C
and then cooled to 0 °C. The formed precipitate of NEt3 ·HCl was
filtered off, and the filtrate was evaporated under reduced pressure.
The resulting residue was redissolved in 4 mL of MeOH. Dieth-
ylether (10 mL) was added, and the mixture was stored at -20 °C.
The pale yellow solid of H2L1, which deposited from this solution
over a period of two days, was filtered off, washed with diethylether,
and dried under vacuum. The resulting compounds were used for
the synthesis of the complexes without further purification.
Data for H2L1a (R1 ) R2 ) Et) (2a). Yield: 78% (1.275 g).
Anal. Calcd for C18H21N3OS: C, 66.06; H, 6.42; N, 12.84; S, 9.79%.
Found: C, 65.80; H, 6.40; N, 13.22; S, 9.02%. IR (ν in cm-1):
been demonstrated for a small number of amino acid esters,7
diamines,8 and tris(aminoethyl)amine.9 Furthermore, no
metal complexes of the newly proposed tridentate ligands
have hitherto been isolated and structurally characterized.
Only two structural reports of Cu(II) and Ni(II) with
tetradentate benzamidinates exist.8
Here, we introduce the syntheses of novel, tridentate
dialkylamino(thiocarbonyl)benzamidine ligands and some of
their rhenium and technetium complexes. These new ligands
(Chart 1) are the first representatives of a novel family of
multidentate systems, which are primarily intended as strong
chelators in the radiopharmaceutical chemistry of technetium
and the radioactive rhenium isotopes 186Re and 188Re.10 In
addition, these multidentate systems should also be of interest
in the complexation of other transition metal ions.
1
3421 m (NsH), 3060 m, br (OsH), 1620 s (CdN). H NMR
(CDCl3; δ, ppm): 1.21 (t, J ) 7.1 Hz, 3 H, CH3), 1.32 (t, J )
7.3 Hz, 3 H, CH3), 3.69 (q, J ) 7.2 Hz, 2 H, CH2), 3.87 (q, J )
7.1 Hz, 2H, CH2), 6.60 (t, J ) 6.9 Hz, 1 H, PhOH), 6.86 (d, J )
6.7 Hz, 1 H, PhOH), 6.89 (t, J ) 6.4 Hz, 1 H, PhOH), 7.04 (d,
J ) 7.5 Hz, 1 H, PhOH), 7.24 (t, J ) 7.5 Hz, 2 H, Ph), 7.33 (t, J
) 7.4 Hz, 1 H, Ph), 7.43 (d, J ) 7.2 Hz, 2 H, Ph). 13C NMR
(CDCl3; δ, ppm): 12.01, 13.45 (CH3), 46.01, 46.17 (CH2), 116.89,
120.00, 126.27, 126.94, 127.60, 128.24, 128.54, 128.90, 130.67,
and 134.53 (Ph + PhOH), 149.36 (CdN), 187.06 (CdS).
Experimental Section
Materials. All reagents used in this study were reagent grade
and used without further purification. The solvents were dried and
used freshly distilled prior to use unless otherwise stated.
(NBu4)[ReOCl4], (NBu4)[TcOCl4], and [ReOCl3(PPh3)2] were
prepared by standard procedures.11 The syntheses of the N-
[(dialkylamino)(thiocarbonyl)]benzimidoyl chlorides were per-
formed by the standard procedure of Beyer and Widera.1
Radiation Precautions. 99Tc is a weak ꢀ- emitter. All manipu-
lations with this isotope were performed in a laboratory approved
for the handling of radioactive materials. Normal glassware provides
adequate protection against the low-energy beta emission of the
technetium compounds. Secondary X-rays (bremsstrahlung) play
an important role only when larger amounts of 99Tc are used.
Physical Measurements. IR spectra were measured as KBr
pellets on a Shimadzu FTIR-spectrometer between 400 and
4000 cm-1. FAB+ mass spectra were recorded with a TSQ
(Finnigan) instrument using a nitrobenzyl alcohol matrix, positive
electrospray ionization mass spectra (ESI-MS) were measured with
a Agilent 6210 ESI-TOF (Agilent Technologies), and electron-
Data for H2L1b (R1, R2 ) Morph) (2b). Yield: 80% (1.364 g).
Anal. Calcd for C18H19N3O2S: C, 63.34; H, 5.61; N, 12.32; S,
9.38%. Found: C, 60.60; H, 6.4; N, 12.67; S, 9.10%. IR (ν in cm-1):
1
3367 m (NsH), 3051 m, br (OsH), 1612 s (CdN). H NMR
(CDCl3; δ, ppm): 3.63 (t, J ) 4.8 Hz, 2 H, NsCH2), 3.69 (t, J )
4.7 Hz, 2 H, NsCH2), 3.93 (t, J ) 4.8 Hz, 2 H, OsCH2), 4.14 (t,
J ) 4.8 Hz, 2 H, OsCH2), 6.66 (t, J ) 7.6 Hz, 1 H, PhOH), 6.86
(d, J ) 7.4 Hz, 1 H, PhOH), 6.93 (t, J ) 7.7 Hz, 1 H, PhOH), 7.00
(d, J ) 8.0 Hz, 1 H, PhOH), 7.28 (t, J ) 7.5 Hz, 2 H, Ph), 7.37 (t,
J ) 7.4 Hz, 1 H, Ph), 7.44 (d, J ) 7.7 Hz, 2 H, Ph). 13C NMR
(CDCl3; δ, ppm): 48.06, 48.70 (NsCH2), 65.99, 66.51 (OsCH2),
117.29, 120.50, 124.60, 127.00, 127.63, 128.40, 128.49, 128.92,
131.04, and 134.24 (Ph + PhOH), 149.45 (CdN), 187.57 (CdS).
EI MS (m/z): 341 (49%, M+), 255 (30%, M - Morph), 233 (35%,
M - Ph(OH)NH).
(7) (a) Criado, J. J.; Rodriguez-Fernandez, E.; Garcia, E.; Hermosa, M. R.;
Monte, E. J. Inorg. Biochem. 1998, 69, 113. (b) Rodriguez-Fernandez,
E.; Garcia, E.; Hermosa, M. R.; Jimenez-Sanchez, A.; Mar Sanchez,
M.; Monte, E.; Criado, J. J. J. Inorg. Biochem. 1999, 75, 181. (c)
Hartung, J.; Weber, G.; Beyer, L.; Kirmse, K.; Stach, J. J. Prakt. Chem.
1990, 332, 359.
N-(N′′,N′′-Diethylaminothiocarbonyl)-N′-picolylbenzamid-
ine, HL2 (3). A solution of N-(N′,N′-diethylaminothiocarbonyl)-
benzimidoyl chloride (1.018 g, 4 mmol) in 10 mL of dry acetone
was added dropwise to a mixture of 2-methylaminopyridine
(436 mg, 4 mmol) and triethylamine (606 mg, 6 mmol) in 5 mL of
dry acetone over a period of 5 min. After a few minutes, a colorless
precipitate of NEt3 ·HCl began to deposit. The mixture was stirred
for 2 h and then cooled to 0 °C. The formed precipitate of NEt3 ·HCl
was filtered off, and the solvent was removed under vacuum. The
resulting residue was recrystallized from methanol to obtain pale
yellow crystals of HL2. Yield: 85% (1.108 g) Anal. Calcd for
C18H22N4S: C, 66.26; H, 6.75; N, 17.18; S, 9.82%. Found: C, 65.72;
H, 6.58; N, 16.82; S, 9.05%. IR (ν in cm-1): 3217 m (NsH), 1608 s
(CdN). 1H NMR (CDCl3; δ, ppm): 1.18 (t, J ) 7.0 Hz, 3 H, CH3),
(8) (a) Richter, R.; Sieler, J.; Beyer, L.; Yanovskii, A. I.; Struchkov, Y. T.
Z. Anorg. Allg. Chem. 1989, 570, 84. (b) Lessmann, F.; Beyer, L.;
Hallmeier, K.-H.; Richter, R.; Sieler, J.; Strauch, P.; Voigt, A. Z.
Naturforsch. B: Chem. Sci. 2000, 55, 253.
(9) Lessmann, F.; Quas, L.; Beyer, L.; Dietze, F.; Sieler, J. Z. Anorg.
Allg. Chem. 2000, 626, 722.
(10) Abram, U.; Alberto, R. J. Braz. Chem. Soc. 2006, 17, 1486.
(11) (a) Alberto, R.; Schibli, R.; Egli, A.; Schubiger, P. A.; Herrmann,
W. A.; Artus, G.; Abram, U.; Kaden, T. A. J. Organomet. Chem. 1995,
217, 492. (b) Johnson, N. P.; Lock, C. J. L.; Wilkinson, G. Inorg.
Synth. 1967, 145. (c) Thomas, R. W.; Davison, A.; Trop, H. S.;
Deutsch, E. Inorg. Chem. 1980, 19, 2840.
Inorganic Chemistry, Vol. 47, No. 12, 2008 5137

Nguyen et al.
1.25 (t, J ) 7.0 Hz, 3 H, CH3), 3.64 (q, J ) 7.0 Hz, 2 H, CH2),
3.93 (q, J ) 7.0 Hz, 2 H, CH2), 4.73 (s, 2 H, CH2sPy), 7.21 (t,
J ) 6.1 Hz, 1 H, Py), 7.38-7.45 (m, 4 H, Ph + Py), 7.52 (d, J )
6.8 Hz, 2 H, Ph), 7.70 (t, J ) 7.5 Hz, 1 H, Py), 8.53 (d, J ) 4.8
Hz, 1 H, Py).
N-(N′′,N′′-Diethylaminothiocarbonyl)-N′-(2-carboxyphenyl)-
benzamidine, H2L3 (4). H2L3 was synthesized by a procedure
similar to that of the method described for H2L1 except that
2-aminobenzoic acid was used instead of 2-aminophenol. Yield:
40% (711 mg). Anal. Calcd for C19H21N3O2S: C, 64.23; H, 5.92;
N, 11.83; S, 9.01%. Found: C, 64.61; H, 5.81; N, 11.73; S, 10.33%.
IR (ν in cm-1): 3163 m, br (OsH), 1682 s (CdO), 1635 s (CdN).
1H NMR (CDCl3; δ, ppm): 1.2-1.4 (m, 6 H, CH3), 3.4-4.0 (m, 4
H, CH2), 5.78 (s, br, 1 H, NH), 7.3-7.6 (m, 6 H, Ph + PhCOOH),
7.63 (t, J ) 8.1 Hz, 1 H, PhCOOH), 7.78 (t, J ) 7.2 Hz, 1 H,
PhCOOH), 8.25 (t, J ) 8.0 Hz, 1 H, PhCOOH).
6.5-6.6 (m, 2 H, PhOH), 6.91 (t, J ) 7.5 Hz, 1 H, PhOH), 7.25
(d, J ) 7.4 Hz, 1 H, PhOH), 7.40 (t, J ) 7.1 Hz, 2 H, Ph), 7.54 (t,
J ) 7.3 Hz, 1 H, Ph), 7.70 (d, J ) 7.7 Hz, 2 H, Ph).
[ReO(L1a)(gly)] (7). Method 1. HL1a (33 mg, 0.1 mmol) in
3 mL of MeOH was added to a solution of (NBu4)[ReOCl4] (58
mg, 0.1 mmol) in 3 mL of CH2Cl2. After stirring at room
temperature for 15 min, solid glycine (8.2 mg, 0.11 mmol) and
three drops of NEt3 were added, and the mixture was heated under
reflux for 3 h. This resulted in a complete dissolution of glycine
and the formation of a dark red solution. The solvent was removed
under vacuum, and the residue was washed with cold methanol
and recrystallized from CH2Cl2/MeOH to yield big red crystals of
7 suitable for X-ray diffraction.
Method 2. Glycine (8.2 mg, 0.11 mmol), three drops of NEt3,
and 5a (57 mg, 0.1 mmol) were heated under reflux in a CH2Cl2/
MeOH mixture (1/1) (10 mL) for 3 h, whereupon glycine
completely dissolved. The volume of the mixture was reduced to
approximately 2 mL, and the red solid, which precipitated upon
cooling to room temperature, was filtered off, subsequently washed
with cold MeOH, and dried under vacuum. Yield: 73% (44 mg)
for method 1, 80% (48 mg) for method 2. Anal. Calcd for
C20H23N4O4SRe: C, 39.92; H, 3.83; N, 9.31; S, 5.32%. Found: C,
40.61; H, 3.68; N, 10.09; S, 5.73%. IR (ν in cm-1): 3425 m (NsH),
1651 vs (CdO), 1543 vs (CdN), 972 s (RedO). 1H NMR (CDCl3;
δ, ppm): 1.25-1.31 (m, 6 H, CH3), 3.50 (m, 1 H, CH2), 3.86 (m,
2 H, CH2), 3.9-4.1 (m, 1 HR-CH2 + 2 HCH2), 5.87 (bs, 1 H, NH2),
6.28 (t, J ) 7.0 Hz, 1 H, PhOH), 6.36 (d, J ) 6.8 Hz, 1 H, PhOH),
6.85 (bs, 1 HPhOH + 1 HNH2), 6.95 (d, J ) 7.7 Hz, 1 H, PhOH),
7.30 (t, J ) 7.3 Hz, 2 H, Ph), 7.35 (t, J ) 7.2 Hz, 1 H, Ph), 7.57
(d, J ) 6.9 Hz, 2 H, Ph). 13C NMR (CDCl3; δ, ppm): 13.41 (CH3),
47.42, 47.81 (CH2), 61.91 (R-CH2, glycine), 116.98, 118.92, 120.83,
125.24, 128.40, 130.55, 131.54, and 134.90 (Ph + PhOH), 146.96
(CPhOHsN), 164.43 (CPhOHsO), 167.88 (CdN), 174.34 (CdS),
177.28 (CdO). FAB+ MS (m/z): 602, 36% [M + H]+; 527, 28%
[M - Gly]+.
Syntheses of Complexes. [ReO(L1)Cl] (5). HL1 (0.11 mmol)
dissolved in 3 mL of MeOH was added dropwise to a stirred
solution of (NBu4)[ReOCl4] (58 mg, 0.1 mmol) in 2 mL of MeOH.
The color of the solution immediately turned to deep red, and a
red precipitate deposited within 30 min. The red powder was filtered
off,washedwithcoldmethanol,andrecrystallizedfromCH2Cl2/MeOH.
Data for 5a. Yield: 87% (49 mg). Anal. Calcd for
C18H19ClN3O2SRe: C, 38.33; H, 3.38; N, 7.46; S, 5.69%. Found:
C, 38.49; H, 3.41; N, 7.08; S, 5.63%. IR (ν in cm-1): 1527 vs
1
(CdN), 991 s (RedO). H NMR (CDCl3; δ, ppm): 1.34 (t, J )
7.6 Hz, 3 H, CH3), 1.38 (t, J ) 7.6 Hz, 3 H, CH3), 3.80 (m, 1 H,
CH2), 3.84 (m, 1 H, CH2), 4.21 (m, 1 H, CH2), 4.43 (m, 1 H, CH2),
6.5-6.6 (m, 2 H, PhOH), 6.85 (t, J ) 7.3 Hz, 1 H, PhOH), 7.26
(d, J ) 7.4 Hz, 1 H, PhOH), 7.36 (t, J ) 7.7 Hz, 2 H, Ph), 7.49 (t,
J ) 7.4 Hz, 1 H, Ph), 7.68 (d, J ) 7.3 Hz, 2 H, Ph). 13C NMR
(CDCl3; δ, ppm): 13.20, 13.32 (CH3), 47.80, 48.57 (CH2), 116.79,
117.69, 120.64, 124.55, 128.99, 130.63, 133.25 (Ph + PhOH),
145.12 (CPhOHsN), 165.26, (CPhOHsO), 165.18 (CdN), 173.98
(CdS). Positive ESI-MS (m/z): 560 (50%, [M - Cl + MeOH]+),
528 (5%, [M - Cl]+.
Data for 5b. Yield: 83% (48 mg). Anal. Calcd for
C18H17ClN3O3SRe: C, 37.45; H, 2.95; N, 7.28; S, 5.55%. Found:
C, 37.41; H, 2.83; N, 7.40; S, 5.39%. IR (ν in cm-1): 1527 vs
(CdN), 995 s (RedO). 1H NMR (CDCl3; δ, ppm): 3.7-4.1 (m, 4
H, NsCH2), 4.2-4.5 (m, 4 H, OsCH2), 6.56 (t, J ) 7.8 Hz, 1 H,
PhOH), 6.61 (d, J ) 7.0 Hz, 1 H, PhOH), 6.93 (t, J ) 6.6 Hz, 1
H, PhOH), 7.32 (d, J ) 7.6 Hz, 1 H, PhOH), 7.43 (t, J ) 7.8 Hz,
2 H, Ph), 7.55 (t, J ) 7.4 Hz, 1 H, Ph), 7.73 (d, J ) 7.3 Hz, 2 H,
Ph). 13C NMR (CDCl3; δ, ppm): 50.20, 50.41 (NsCH2), 66.53,
66.57 (OsCH2), 116.93, 117.95, 120.70, 124.98, 129.04, 130.77,
133.11, 133.56 (Ph + PhOH), 144.82 (CPhOHsN), 166.48
(CPhOHsO), 165.4 (CdN), 172.26 (CdS). Positive ESI-MS (m/z):
574 (100%, [M - Cl + MeOH]+), 542 (5%, [M - Cl]+.
X-ray quality single crystals of 5b were obtained by slow
evaporation of a CH2Cl2/isopropanol solution.
[{ReOCl(L2)}2O] (8). Method 1. HL2 (36 mg, 0.11 mmol) in
3 mL of acetone and three drops of NEt3 were added to a stirred
suspension of [ReOCl3(PPh3)2] (83 mg, 0.1 mmol) in 3 mL of
CH2Cl2. The mixture was heated under reflux for 30 min,
whereupon the precursor complex completely dissolved and the
color of the reaction mixture changed from yellow-green to violet.
The solvent was removed under reduced pressure, and the resulting
residue was washed with methanol and recrystallized by slow
evaporation of a CH2Cl2/acetone solution to yield violet blocklike
crystals of 8.
Method 2. HL2 (36 mg, 0.11 mmol) was dissolved in 3 mL of
acetone and three drops of NEt3 were added. This solution was
added dropwise to a solution of (NBu4)[ReOCl4] (58 mg, 0.1 mmol)
in 2 mL of acetone. The mixture was stirred at ambient temperature
for 2 h and the solvent was removed. The residue was carefully
washed with water and extracted with CH2Cl2. Recrystallization
from CH2Cl2/acetone yielded violet crystals. Yield: 91% (52 mg)
for method 1, 40% (23 mg) for method 2. Anal. Calcd for
C36H42Cl2N8O3S2Re2: C, 37.85; H, 3.68; N, 9.81; S, 5.61%. Found:
C, 37.52; H, 3.44; N, 10.02; S, 5.35%. IR (ν in cm-1): 1488 s
(CdN), 949 w (RedO), 683 s (ResOsRe). 1H NMR (CDCl3; δ,
ppm): 0.99 (t, J ) 7.0 Hz, 3 H, CH3), 1.06 (t, J ) 7.0 Hz, 3 H,
CH3), 3.19 (m, 1 H, CH2), 3.39 (m, 1 H, CH2), 3.95 (m, 1 H, CH2),
4.03 (m, 1 H, CH2), 5.03 (d, J ) 19.2 Hz, 1 H, PyCH2), 5.99 (d,
J ) 19.2 Hz, 1 H, PyCH2), 7.39-7.48 (m, 6 H, Ph), 7.37 (t, J )
7.4 Hz, 1 H, Ph), 8.79 (d, J ) 5.6 Hz, 2 H, Py). 13C NMR (CDCl3;
δ, ppm): 48.06 48.70 (NsCH2), 65.99, 66.51 (OsCH2), 117.29,
[TcO(L1a)Cl] (6a). The technetium complex was prepared from
(NBu4)[TcOCl4] by the procedure described previously for its
rhenium analogue. Single crystals of 6a suitable for X-ray analysis
were obtained by slow evaporation of a CH2Cl2/isopropanol
solution. Yield: 86% (41 mg). Anal. Calcd for C18H19ClN3O2STc:
Tc, 20.8%. Found: Tc, 19.9%. IR (ν in cm-1): 1524 vs (CdN),
1
972 s (TcdO). H NMR (CDCl3; δ, ppm): 1.42 (m, 6 H, CH3),
3.89 (m, 2 H, CH2), 4.12 (m, 1 H, CH2), 4.27 (m, 1 H, CH2),
5138 Inorganic Chemistry, Vol. 47, No. 12, 2008

Re and Tc Complexes with Tridentate Benzamidine Ligands
Table 1. X-ray Structure Data Collection and Refinement Parameters
H2L3 ·MeOH
[ReO(L1b)Cl]
[TcO(L1a)Cl]
[ReO(L1a)(Gly)]
[{ReOCl(L2)}2O]
[ReO(L3)Cl]2
4·MeOH
C20H25N3O3S
387.49
monoclinic
21.356(2)
10.809(1)
18.663(2)
90
102.37(1)
90
4208.1(7)
C2/c
5b
6a
7
8
9
formula
mol wt
cryst syst
a/Å
b/Å
c/Å
C18H17ClN3O3ReS
577.06
C18H19ClN3O2STc
474.87
monoclinic
11.406(1)
12.878(1)
13.007(1)
90
93.06(1)
90
1907.8(3)
P21/n
C20H23N4O4ReS
601.70
monoclinic
13.529(1)
14.421(1)
10.887(1)
90
91.11(1)
90
2123.6(3)
P21/c
C36H42Cl2N8O3Re2S2
1142.20
monoclinic
10.003(1)
10.630(1)
19.441(2)
90
99.54(1)
90
2038.5(3)
P21/n
C38H38Cl2N6O6Re2S2
1182.16
monoclinic
8.865(1)
13.704(1)
16.230(1)
90
90.92(1)
90
1971.4(3)
P21/c
monoclinic
10.310(1)
12.389(1)
15.295(1)
90
103.25(1)
90
1901.6(3)
P21/n
R/deg
ꢀ/deg
γ/deg
V/Å3
space group
Z
8
4
4
4
2
2
Dcalcd/g cm-3
µ/mm-1
no. of reflns
no. of independent
no. params
R1/wR2
GOF
1.223
0.178
14200
5641
247
0.0633/0.1079
1.077
2.016
6.665
16231
5099
244
0.0475/0.1183
1.089
1.653
1.022
9486
5054
235
0.0502/0.1219
0.954
1.882
5.855
12394
4451
295
0.0306/0.0643
1.040
1.861
6.212
7811
4181
241
0.0341/0.0648
0.949
1.991
6.431
14571
5306
253
0.0532/0.0940
0.914
Scheme 1
120.50, 124.60, 127.00, 127.63, 128.40, 128.49, 128.92, 131.04,
and 134.24 (Ph + Py), 149.45 (CdN), 187.57 (CdS). FAB+ MS
(m/z): 544, 32% [ReO2(L2)]+; 528, 30% [ReO(L2)]+.
Additional information on the structure determinations has been
deposited with the Cambridge Crystallographic Data Center.
[ReO(L3)Cl]2 (9). (NBu4)[ReOCl4] (58 mg, 0.1 mmol) was added
to a solution of H2L3 (36 mg, 0.1 mmol) in 5 mL of MeOH. The
mixture was stirred at room temperature for 15 min and reduced
in volume to about 2 mL. X-ray quality green crystals of 9 deposited
from this solution within several days. Yield: 87% (49 mg). Anal.
Calcd for C38H38Cl2N6O6S2Re2: C, 38.60; H, 3.22; N, 7.11; S,
5.42%. Found: C, 38.75; H, 2.98; N, 6.52; S, 5.27%. IR (ν in cm-1):
Results and Discussion
Preparation of the Ligands. N-[N′,N′-Dialkylamino(thio-
carbonyl)]benzimidoyl chlorides (1) are reactive building
blocks, which can readily be prepared by reactions of N,N-
dialkyl-N′-benzoylthioureatonickel(II) chelates with SOCl2.1,2
Modifications of the residues R1 and R2 allow for the
variation of basic properties of the products such as solubility,
polarity, and lipophilicity. Furthermore, the fine-tuning of
such properties requires a third molecular position. We have
now synthesized the first representatives of a novel class of
tridentate N-[(N′′,N′′-dialkylamino)(thiocarbonyl)]-N′-sub-
stituted benzamidine ligands by reactions of the appropriate
N-[N′,N′-dialkylamino(thiocarbonyl)]benzimidoyl chlorides
and functionalized primary amines such as 2-aminophenol,
2-aminomethylpyridine, and 2-aminobenzoic acid in dry
acetone (Scheme 1). In the presence of the supporting base
NEt3, such reactions proceed quickly under mild conditions.
They can be performed at room temperature as in the case of
the preparation of HL2 or in warm acetone (40 °C) as in the
case of the synthesis of HL1. The progress of the reaction can
readily be checked by thin-layer chromatography on alumina
and is indicated by the formation of a colorless precipitate of
1
1542 vs, br (CdN, CdO), 1002 s (RedO). H NMR (DMSO; δ,
ppm): 1.31-1.38 (m, 6 H, CH3), 3.80-4.20 (m, 4 H, CH2), 6.51
(d, J ) 8.1 Hz, 1 H, PhCOO), 6.95 (t, J ) 8.0 Hz, 1 H, PhCOO),
7.16 (t, J ) 7.5 Hz, 1 H, PhCOO), 7.34 (m, 3 H, Ph), 7.63 (d, J )
8.1 Hz, 1 H, Ph), 8.18 (d, J ) 8.2 Hz, 2 H, Ph).
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. More
details on data collections and structure calculations are contained
in Table 1.
(12) Sheldrick, G. M. SHELXS-97 and SHELXL-97, programs for the
solution and refinement of crystal structures; University of Go¨ttingen:
Go¨ttingen, Germany, 1997.
Inorganic Chemistry, Vol. 47, No. 12, 2008 5139

Nguyen et al.
at the N5 atom. The configuration of the H2L3 molecule is
best described as E, dZ, which has also been found in some
related ligands.3b,7b,17 The E, dZ configuration is obtained
due to the intramolecular hydrogen bond between N5 and
O58, which is more stable than a potential hydrogen bonding
between N5 and S1.
Complexes of HL1. Reactions of H2L1 with the common
rhenium(V) precursor (NBu4)[ReOCl4] in methanol at room
temperature yield red solids of the composition [ReOCl(L1)]
(5) in excellent yields. Corresponding reactions with the less
soluble [ReOCl3(PPh3)2] give the same products but with
significantly lower yields. This is mainly due to the formation
of side products with remaining PPh3 ligands, which cause
difficulties during the isolation of the complexes. The
products are sparingly soluble in methanol and acetone and
almost insoluble in pentane or hexane. IR spectra of
complexes of 5 exhibit strong bands near the 1525 cm-1
region but no absorptions in the range between 1612 and
1620 cm-1, where the νCdN stretches in the spectra of the
noncoordinated benzamidines typically appear.1 This corre-
sponds to a bathochromic shift of about 100 cm-1 and
indicates chelate formation with a large degree of π-electron
delocalization within the chelate rings. The absence of
absorptions in the regions around 3350 cm-1 and 3150 cm-1,
which correspond to νNH and νOH, respectively, in the
uncoordinated H2L1, indicates the expected double depro-
tonation of the ligands during complex formation. Intense
bands appear between 991 and 995 cm-1, which can be
assigned to the RedO vibrations.14
Figure 1. Molecular structure of H2L3 (4) together with the intramolecular
hydrogen bond (N5-H5, 0.86 Å; H5 · · ·O58, 1.93 Å; N5 · · ·O58, 2.655(3)
Å; >N5-H5· · ·O58, 140.8°).
Table 2. Selected Bond Lengths (Å) and Angles (deg) in 4
S1-C2
1.678(3)
1.330(4)
1.383(4)
1.290(4)
1.493(5)
120.4(3)
124.0(2)
123.7(3)
121.1(3)
C4-N5
1.363(4)
1.402(4)
1.305(4)
1.220(4)
C2-N6
N5-C51
C57-O57
C57-O58
C2-N3
N3-C4
C4-C31
S1-C2-N3
S1-C2-N6
C2-N3-C4
N3-C4-N5
C4-N5-C51
N5-C51-C56
C56-C57-O58
130.8(3)
118.1(3)
124.6(3)
NEt3 ·HCl, which is almost insoluble in acetone. The ligands
were obtained as crystalline solids in high yields. They were
characterized by elemental analysis and spectroscopic methods
such as IR, 1H NMR, and 13C NMR.
The ligands H2L1 (2) and HL2 (3) have good solubility in
methanol and acetone. Their 1H NMR spectra are character-
ized by two sets of well-separated signals corresponding to
their alkyl residues, which are due to the hindered rotation
around of the C-NEt2 or C-N(CH2)2O bonds. This finding
has also been observed with their parent N,N-dialkylben-
zoylthioureas.13 However, the C-NR1R2 bonds of N,N-
dialkylbenzoylthioureas seem to be more flexible, as indi-
The NMR spectra of complexes of 5 provide additional
evidence for the proposed composition and molecular
structure of the complexes. The two expected triplet signals
of the methyl groups are observed in the 1H NMR spectrum
1
cated by broad signals in their H NMR spectra.
The synthesis of the anthranilic acid derivative H2L3 (4)
follows the general route described previously for the other
dialkylamino(thiocarbonyl)benzamidines. The compound is
separated as a yellow oil upon concentration of the reaction
mixture. This compound, which is analytically pure H2L3
according to IR and NMR spectra, can be crystallized from
a concentrated MeOH solution stored in a deep freezer.
Slightly yellow crystals of the methanol solvate H2L3 ·MeOH
deposit over a period of several days. Figure 1 illustrates
the structure of H2L3 together with the intramolecular
hydrogen bond. Additional hydrogen bonds are established
between S1 and O57 and the solvate MeOH molecule.
Selected bond lengths and angles are summarized in Table
2. While the C4-N3 bond of 1.290 Å is within the expected
range of CdN double bonds, the C4-N5 bond of 1.363 Å
has only partial double-bond character. This is found in all
similar ligands and suggests that the proton is mainly located
1
of 5a. Furthermore, H NMR reflects the rigid structure of
the tertiary amine nitrogen atom. The appearance of four
proton signals of the methylene groups shows that they are
magnetically not equivalent with respect to their axial and
equatorial positions. This leads to complex coupling patterns
in the proton spectra of 5a and 5b. 13C NMR spectra of
complexes 5a and 5b exhibit a pattern similar to the spectra
of the noncoordinated ligands, in which a couple of signals
for the CH2 and CH3 groups appears in each. Deprotonation
of the aromatic Car-NH and Car-OH groups during the
complex formation of the H2L1 ligands results in a strong
shift from their respective 13C NMR chemical shifts of
145 ppm and 165 ppm.
Figure 2 depicts the molecular structure of compound
5b as a prototype compound for this type of complexes.
(13) (a) Fitzl, G.; Beyer, L.; Sieler, J.; Richter, R.; Kaiser, J.; Hoyer, E. Z.
Anorg. Allg. Chem. 1977, 433, 237. (b) Irving, A.; Koch, K. R.;
Matoetoe, M. Inorg. Chim. Acta 1993, 206, 193. (c) Sacht, C.; Datt,
M. S.; Otto, S.; Roodt, A. J. Chem. Soc., Dalton Trans. 2000, 727.
(14) Abram, U. Rhenium. In ComprehensiVe Coordination Chemistry II;
McCleverty, J. A., Mayer, T. J., Eds.; Elsevier: Amsterdam, The
Netherlands, 2003; Vol. 5, p 271.
5140 Inorganic Chemistry, Vol. 47, No. 12, 2008

Re and Tc Complexes with Tridentate Benzamidine Ligands
Figure 3. Molecular structure of [TcOCl(L1a)] (6a). H atoms have been
Figure 2. Molecular structure of [ReOCl(L1b)] (5b) and a perspective view
omitted for clarity.
of the six-membered chelate ring. H atoms have been omitted for clarity.
strongly agrees with the 1H NMR spectrum of the
compound, which indicates a rigid arrangement of the
morpholine ring in the complex.
Table 3. Selected Bond Lengths (Å) and Angles (deg) in 5b and 6a
5b
6a
5b
6a
An analogous technetium complex was prepared by the
reaction of (NBu4)[TcOCl4] with H2L1a in methanol. The
reaction was performed at room temperature, and a red,
crystalline product of the composition [TcOCl(L1a)] (6a)
precipitated directly from the reaction mixture in good yields.
The IR spectrum of 6a exhibits a ν(TcdO) frequency at
972 cm-1 and indicates a strong bathochromic shift of the
CdN band, as a consequence of the complex formation. The
1H NMR spectral features previously described for 5a also
apply to the Tc compound. The same holds true for the main
structural features of the solid-state structures of the com-
pounds. Figure 3 depicts the molecular structure of 6a, and
the corresponding bond lengths are compared to those of
5b in Table 3. As discussed for the rhenium compound, the
Tc atom has a distorted square-pyramidal geometry and is
located 0.552 Å above the basal plane formed by S1, N5,
O57, and Cl. The Tc-O10 distance of 1.642 Å is within the
typical Tc-O double bond range.
The chloro ligands in the coordination spheres of the
compounds of 5 and complex 6a are sufficiently labile to
allow further ligand exchange. Thus, the reaction of 5a with
glycine, Hgly, in a CH2Cl2/MeOH mixture yields the dark
red mixed-ligand complex [ReO(L1a)(gly)] (7) in good yields.
The same product is formed in a one-pot reaction starting
from (NBu4)[ReOCl4] and 1 equiv of HL1a in a CH2Cl2/
MeOH mixture followed by the addition of a slight excess
of Hgly.
In both cases, the addition of a base such as NEt3 supports
the deprotonation of glycine. The IR spectrum of 7 shows a
medium broad band at 3425 cm-1 and a strong absorption
at 1651 cm-1, which can be assigned to νNH and νCO stretches
of the chelating glycinate ligand. While the absorption of
the RedO vibration shifts to longer wavelengths by about
20 cm-1 with respect to 5a, the absorption of the CdN stretch
moves to the shorter wave region by about 15 cm-1. The
chelate formation of the glycinate ligand is indicated by the
detection of magnetically unequal protons of methylene and
amino groups in the NMR spectrum of the compound. Thus,
two multiplet signals for each proton of the CH2 group are
MdRe MdTc
MdRe MdTc
MsO10
MsCl
MsS1
MsN5
MsO57
S1sC2
1.665(7) 1.642(6) C2sN3
2.344(2) 2.351(2) N3sC4
2.289(2) 2.296(2) C4sN5
1.999(7) 1.991(5) N5sC51
1.957(6) 1.994(4) C56sO57
1.786(9) 1.758(8) C2sN6
104.4(3) 106.1(2) C2sN3sC4
1.33(1) 1.347(9)
1.31(1) 1.304(8)
1.37(1) 1.386(8)
1.42(1) 1.435(7)
1.35(1) 1.374(9)
1.33(1) 1.321(9)
127.5(8) 126.1(6)
123.8(8) 124.0(6)
117.4(5) 118.1(4)
O10sMsCl
O10sMsS1 108.6(3) 111.2(2) N3sC4sN5
O10sMsN5 106.1(3) 104.8(2) C4sN5sM
O10sMsO57 114.8(3) 114.0(2) C4sN5sC51 125.0(7) 123.4(5)
MsS1sC2
S1sC2sN3
S1sC2sN6
106.7(3) 106.6(2) N5sC51sC56 111.6(7) 109.2(6)
121.9(7) 123.5(5) C56sO57sM 116.7(5) 115.5(4)
118.6(7) 119.0(5)
Selected bond lengths and angles are given in Table 3.
The rhenium atom exhibits a distorted square-pyramidal
environment with the oxo ligand in the apical position.
The square plane defined by the donor atoms of the
tridentate ligand and the chloro ligand is slightly distorted,
with a main distortion of 0.124(3) Å from a mean least-
squares plane for atom O57. The Re atom is situated by
0.537 Å above this plane toward the oxo ligand. All
O10-Re-X angles (X ) equatorial donor atom) fall in
the range between 104 and 114°. This corresponds with
the typical bonding situation of square-pyramidal ReVO
complexes.15 The RedO distance of 1.665(7) Å is within
the expected range of a rhenium-oxygen double bond.14
Despite the fact that the six-membered chelate ring is not
planar (see also the wireframe model of the chelate ring
in Figure 2), a considerable delocalization of π-electron
density is indicated by the observed bond lengths. The
C-S and C-N bonds inside the chelate ring fall within
the range between carbon-sulfur and carbon-nitrogen
single and double bonds. This bond length equalization
is even extended to the C2-N6 bond of 1.330(4) Å, which
is significantly shorter than expected for a single bond.
The partial transfer of electron density into this bond
(15) (a) Hansen, L.; Xu, X.; Lipowska, M.; Taylor, A, Jr.; Marzilli, L. G.
Inorg. Chem. 1999, 38, 2890. (b) O’Neil, J. P.; Wilson, S. R.;
Katzenellenbogen, J. A. Inorg. Chem. 1994, 33, 319.
Inorganic Chemistry, Vol. 47, No. 12, 2008 5141

Nguyen et al.
Figure 4. Molecular structure of [ReO(L1a)(gly)] (7). H atoms have been
observed due to the combination of the geminal coupling
and the vicinal coupling with the adjacent NH2 group. One
of them is well resolved at 3.50 ppm, and the other is partially
covered by the NCH2 signals of the L1a ligand in the range
between 3.9 and 4.1 ppm. Two broad signals of the NH2
protons appear at 5.87 ppm and 6.85 ppm.
omitted for clarity.
Table 4. Selected Bond Lengths (Å) and Angles (deg) in 7
ResO10
ResS1
ResN5
ResO57
ResN11
ResO15
S1sC2
C2sN3
1.680(5)
2.348(2)
1.989(5)
2.029(5)
2.189(6)
2.120(5)
1.760(7)
1.330(9)
1.304(8)
98.4(2)
104.7(2)
100.3(2)
90.5(2)
164.6(2)
103.5(2)
123.3(5)
119.6(5)
127.6(6)
C4sN5
C2sN6
1.353(8)
1.328(9)
1.419(8)
1.354(8)
1.46(1)
1.52(1)
1.210(9)
1.300(8)
N5sC51
C56sO57
N11sC12
C12sC13
C13sO14
C13sO15
The molecular structure of 7 is depicted in Figure 4. This
structure confirms the results of the spectroscopic studies.
The rhenium atom is placed in a distorted octahedral
coordination environment, which is formed by the oxo
oxygen, an equatorially coordinated (L1a)2- ligand and a
bidentate glycinate. The amino group occupies the remaining
equatorial position, whereas the carboxylate group coordi-
nates trans to the oxo ligand. With respect to the structure
of the five-coordinate 5a, the rhenium atom expectedly drops
toward the equatorial plane. While the Re-O10 distance is
slightly lengthened, all C-N and C-S bonds in the chelate
ring are slightly shortened. This is in agreement with the IR
spectroscopic results. Nevertheless, the Re-O10 bond length
of 1.680 Å falls into the typical range of RedO double
bonds. The Re-N11 (2.189 Å) and Re-O15 (2.120 Å) bond
lengths are about 0.1 Å longer than the Re-N5 and Re-O57
bonds, which reflects the coordination of the NH2 group as
an amine and the trans influence of the oxo ligand. Selected
bond lengths and angles of 7 are given in Table 4.
N3sC4
O10sResS1
O10sResN5
O10sResO57
O10sResN11
O10sResO15
ResS1sC2
S1sC2sN3
S1sC2sN6
C2sN3sC4
N3sC4sN5
124.8(6)
122.4(5)
113.4(6)
117.6(6)
113.2(4)
114.6(4)
112.0(6)
113.5(6)
123.5(4)
C4sN5sC51
N5sC51sC56
C51sC56sO57
C56sO57sRe
ResN11sC12
N11sC12sC13
C12sC13sO15
C13sO15sRe
Complex Formation with HL2. Treatment of
[ReOCl3(PPh3)2] with HL2 in CH2Cl2/acetone at room
temperature results in the immediate formation of a green
solution, which slowly changes to a brown color when an
alcohol is added. Up until now, each of our attempts to isolate
crystalline products from such solutions failed. However, the
green color of the solution described above immediately turns
to violet after the addition of a base such as NEt3. From this
solution, the crystalline violet complex [{ReOCl(L2)}2O] 8
was isolated in high yield. The same compound can be
prepared from (NBu4)[ReOCl4] but with lower yields. The
formation of an oxo-bridged dimer is not unexpected with
this monoanionic, tridentate ligand and is frequently observed
when traces of water are present. Furthermore, charge
compensation more favorably allows for the formation of a
neutral dimeric rhenium-oxo complex rather than a complex
cation.14
The IR spectrum of 8 exhibits a strong bathochromic shift
of the CdN vibration of about 120 cm-1 and the absence of
the medium broad N-H stretch band at 3217 cm-1, with
respect to the IR spectrum of the uncoordinated HL2.
Moreover, the formation of an oxo-bridged dimeric com-
pound is strongly indicated by a weak absorption of the
RedO stretch at 949 cm-1 and a strong absorption of the
5142 Inorganic Chemistry, Vol. 47, No. 12, 2008

Re and Tc Complexes with Tridentate Benzamidine Ligands
Table 5. Selected Bond Lengths (Å) and Angles (deg) in 8
Re-O10
1.692(4)
2.156(5)
2.038(5)
2.332(2)
2.458(2)
1.916(3)
1.745(6)
1.325(9)
88.3(2)
94.7(2)
98.2(2)
91.0(2)
168.9(1)
80.0(2)
N3-C4
C4-N5
C2-N6
N5-C6
C6-C51
C51-N52
C4-C31
1.336(8)
1.329(7)
1.352(8)
1.485(8)
1.502(8)
1.345(9)
1.502(8)
Re-N52
Re-N5
Re-S1
Re-Cl
Re-O20
S1-C2
C2-N3
O10-Re-N52
O10-Re-N5
O10-Re-S1
O10-Re-Cl
O10-Re-O20
N52-Re-N5
N5-Re-S1
S1-Re-Cl
N52-Re-Cl
N52-Re-S1
N5-Re-Cl
O20-Re-N52
O20-Re-N5
O20-Re-S1
O20-Re-Cl
93.9(2)
172.1(1)
171.5(2)
81.1(1)
86.8(2)
92.64(4)
86.36(4)
94.84(14)
90.59(5)
(9), which can be isolated as a green, microcrystalline solid
Figure 5. Molecular structure of [{ReOCl(L2)}2O] (8). H atoms have been
omitted for clarity.
directly from the reaction mixture.
16
1
ResOsRe′ unit at 683 cm-1. In the H NMR spectrum
of 8, the protons of the ethyl residues show similar patterns
as those observed in 5a. As expected, the two methylene
protons at C6 of the (L2)- ligand are magnetically non-
equivalent. FAB+ mass spectra show no evidence of the
molecular ion peak. Preferably, the Re-O-Re bond is
cleaved as evidenced by peaks corresponding to the
[ReO2(L2)]+ (m/z ) 544) and/or [ReO(L2)]+ (m/z ) 528)
fragments.
X-ray quality single crystals of 8 were obtained by the
slow evaporation of a CH2Cl2/acetone solution. Figure 5
depicts the molecular structure of 8, showing a dimer with
O20 as a center of inversion. The coordination sphere of
the metal atom is best described as a slightly distorted
octahedron with trans angles between 168.9(1) and 172.1(1)°.
The tridentate organic ligand is singly deprotonated and
coordinates equatorially to the Re center. The established
six-membered and five-membered chelate rings are almost
planar and show a maximum deviation from planarity for
atom S1 (0.182 Å). The Re atom is only slightly (0.098 Å)
shifted out of the mean least-squares plane, which is formed
by the atoms S1, N5, N52, and Cl. The described coordina-
tion mode leaves less space for an “in-plane” arrangement
of the phenyl ring. Consequently, the plane of this ring stands
almost perpendicular to the plane of the adjacent chelate ring,
and conjugation of π-electron density between these rings
is prevented, as can be seen in the relatively long C4-C31
bond length of 1.502(8) Å. Additional bond lengths and
angles of 8 are given in Table 5.
The IR spectrum of 9 shows the disappearance of a
broad band at 3163 cm-1 of the O-H vibration of 4 and
a very strong shift of the CdO stretch from 1683 cm-1 in
the uncoordinated H2L3 to 1542 cm-1 in the complex. This
is commonly observed when carboxylato ligands coordi-
nate to metal centers via both oxygen atoms.18 A strong
band at 1002 cm-1 is assigned as the RedO stretching
band. The poor solubility of 9 in organic solvents, even
in DMSO, causes difficulties in the elucidation of its
1
structure by spectroscopic methods. The H NMR spec-
trum of 9 in DMSO-d6 is less intense, and the signals are
not sufficiently resolved to allow a detailed analysis of
the coupling patterns. Nevertheless, all protons of the
ligand can be assigned and integrated in the required ratio.
Green blocks of 9 are stable in the air and could be studied
by X-ray diffraction without noticeable decomposition.
Figure 6 illustrates the dimeric structure of the compound.
Selected bond lengths and angles are summarized in Table
6. Each of the organic ligands establishes a tridentate
equatorial coordination at one ReO unit. The remaining
Complex Formation with H2L3. The reaction of
(NBu4)[ReOCl4] with an equivalent amount of H2L3 in
methanol results in ligand exchange and the formation of
an oxorhenium(V) complex of the composition [ReOCl(L3)]2
(16) (a) Lawrence, A. B.; Paul, E. W.; Grant, N. H. Inorg. Chim. Acta
1997, 255, 149. (b) Benny, P. D.; Barnes, C. L.; Piekarski, P. M.;
Lydon, J. D.; Jurisson, S. S. Inorg. Chem. 2003, 42, 6519. (c) Gerber,
T. I. A.; Tshentu, Z. R.; Garcia-Granda, S.; Mayer, P. J. Coord. Chem.
2003, 56, 1093.
(18) (a) Rouschias, G.; Wilkinson, G. J. Chem. Soc. A 1966, 5, 465. (b)
Ouchi, A.; Suzuki, Y.; Ohki, Y.; Koizumi, Y. Coord. Chem. ReV. 1988,
92, 29.
(17) Braun, U.; Sieler, J.; Richter, R.; Leban, I.; Golic, L. Cryst. Res.
Technol. 1988, 23, 39.
Inorganic Chemistry, Vol. 47, No. 12, 2008 5143

Nguyen et al.
well suited for the formation of stable complexes with rheni-
um(V) and technetium(V) centers. Substitutions in their pe-
riphery allow for the control of the coordination mode of the
metal. The stability of the complexes and the high synthetic
potential of benzamidine ligands justify the use of this new class
of compounds in further experiments, particularly, such as the
synthesis of bioconjugates with 99mTc or 186,188Re. Of particular
interest, the monomeric complexes of 5 may be adopted for a
mixed-ligand (3 + 2 or 3 + 1 + 1) radiopharmaceutical design.
When designing radioactive bioconjugates using this new
class of compounds, a more generalized route for the
synthesis of the ligands described in this communication is
advisable: it can easily be extended to amines with three or
four additional potential donor atoms.
Figure 6. Molecular structure of [{ReOCl(L3)}2] (9). H atoms have been
omitted for clarity.
Table 6. Selected Bond Lengths (Å) and Angles (deg) in 9a
Re-O10
1.655(8)
2.376(3)
2.295(3)
2.040(8)
2.051(7)
2.271(7)
1.75(1)
96.4(3)
101.6(3)
97.4(4)
C2-N3
1.34(1)
1.33(1)
1.33(1)
1.34(1)
1.44(1)
1.28(1)
1.25(1)
130.9(9)
125.1(9)
117.6(9)
124.0(9)
120.0(9)
122.1(6)
117.6(9)
Re-Cl
C2-N6
Re-S1
N3-C4
Re-N5
C4-N5
Re-O57
N5-C51
Re-O58′
C57-O57
S1-C2
C57-O58
O10-Re-Cl
O10-Re-S1
O10-Re-N5
O10-Re-O57
O10-Re-O58′
S1-C2-N3
S1-C2-N6
C2-N3-C4
N3-C4-N5
C4-N5-C51
N5-C51-C56
C56-C57-O58
C57-C58-Re′
C4-N5-C51
88.5(3)
Such multidentate ligand systems can be readily attached
to an anchor group for couplings with peptides or proteins.
Corresponding experiments (e.g., with the multidentate
benzamidines H3L4 and with bioconjugatable derivatives such
as H3L5) are being performed in our laboratory.
175.8(3)
124.3(9)
118.2(9)
a Symmetry operation: (′) -x, 1 - y, -z.
position in the basal plane of the resulting square pyramid
is occupied by a chloro ligand. Dimerization is achieved
by coordination of the oxygen atom O58 of a second
{ReO(L3)Cl} unit trans to the oxo oxygen. Thus, each of
the rhenium atoms has a distorted octahedral environment,
and the resulting dimeric molecule possesses inversion
symmetry. The Re · · · Re′ distance is 5.324 Å, and binding
interactions between the metal atoms can be excluded.
To the best of our knowledge, such a bridging coordination
of a carboxylic group in a rhenium complex with the
transition metal in one of its high oxidation states is
without precedent. Most of the hitherto structurally
characterized rhenium complexes with bridging carboxy-
late coordination establish metal-metal bonds19 or contain
the tricarbonylrhenium(I) core.20
Acknowledgment. H.H. Nguyen gratefully acknowledges
grants from the government of Vietnam and the DAAD. We
would also like to express our thanks to Jason Heier for his
valuable help during the preparation of the manuscript.
Supporting Information Available: X-ray crystallographic data
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
IC702344S
(19) Walton, R. A. Rhenium Compounds. In Multiple Bonds between Metal
Atoms, 3rd ed.; Cotton, F. A., Murillo, C. A., Walton, R. A., Eds.;
Springer: New York, 2005; p 271.
(20) (a) Cotton, F. A.; Dikarev, E. V.; Petrukhina, M. A. Inorg. Chim.
Acta 1999, 284, 304. (b) Brand, U.; Shapley, J. R. Inorg. Chem. 2000,
39, 32. (c) Klausmeyer, K. K.; Beckles, F. R. Inorg. Chim. Acta 2005,
358, 1041. (d) Alberto, R.; Schibli, R.; Angst, D.; Schubiger, P. A.;
Abram, U.; Abram, S.; Kaden, T. A. Transition Met. Chem. 1997,
22, 597. (e) Cotton, F. A.; Foxman, B. M. Inorg. Chem. 1968, 7, 1784.
(f) Bera, J. K.; Fanwick, P. E.; Walton, R. A. Inorg. Chem. 2001, 40,
2914.
Conclusions
The tridentate benzamidines described above are the first
representatives of a novel, versatile class of ligands, which are
5144 Inorganic Chemistry, Vol. 47, No. 12, 2008