Osmium NAMI-a analogues: Synthesis, structural and spectroscopic characterization, and antiproliferative properties

Article abstract of DOI:10.1021/ic700405y

The osmium(III) complex [(DMSO)₂H][trans-Os^IIICl ₄(DMSO)₂] (1) has been prepared via stepwise reduction of OsO₄ in concentrated HCl using N₂H₄·2HCl and SnCl₂·2H₂O in DMSO. 1 reacts with a number of azole ligands, namely, indazole (Hind), pyrazole (Hpz), benzimidazole (Hbzim), imidazole (Him), and 1H-1,2,4-triazole (Htrz), in organic solvents, affording novel complexes (H₂ind)[Os^IIICl₄(Hind)(DMSO)] (2), (H₂pz)[Os^IIICl₄(Hpz)(DMSO)] (3), (H ₂bzim)-[Os^IIICl₄(Hbzim)(DMSO)] (4), (H ₂im)[Os^IIICl₄(Him)(DMSO)] (6), and (H ₂trz)[Os^IIICl₄(Htrz)(DMSO)] (7), which are close analogues of the antimetastatic complex NAMI-A. Metathesis reaction of 4 with benzyltriphenylphosphonium chloride in methanol led to the formation of (Ph₃PCH₂Ph)[Os^IIICl₄(Hbzim)(DMSO)] (5). The complexes were characterized by IR, UV-vis, ESI mass spectrometry, ¹H NMR spectroscopy, cyclic voltammetry, and X-ray crystallography. In contrast to NAMI-A, 2-4, 6, and 7 are kinetically stable in aqueous solution and resistant to hydrolysis. Surprisingly, they show reasonable antiproliferative activity in vitro in two human cell lines, HT-29 (colon carcinoma) and SK-BR-3 (mammary carcinoma), when compared with analogous ruthenium compounds. Structure-activity relationships and the potential of the prepared complexes for further development are discussed.

Full text of DOI:10.1021/ic700405y

Inorg. Chem. 2007, 46, 5023−5033
Osmium NAMI-A Analogues: Synthesis, Structural and Spectroscopic
Characterization, and Antiproliferative Properties
Berta Cebria´n-Losantos, Artem A. Krokhin, Iryna N. Stepanenko, Rene Eichinger, Michael A. Jakupec,
Vladimir B. Arion,* and Bernhard K. Keppler*
Institute of Inorganic Chemistry, Faculty of Chemistry, UniVersity of Vienna, Wa¨hringerstrasse 42,
A-1090 Vienna, Austria
Received March 2, 2007
The osmium(III) complex [(DMSO)2H][trans-OsIIICl4(DMSO)2] (1) has been prepared via stepwise reduction of OsO4
in concentrated HCl using N2H4 2HCl and SnCl2 2H2O in DMSO. 1 reacts with a number of azole ligands, namely,
‚
‚
indazole (Hind), pyrazole (Hpz), benzimidazole (Hbzim), imidazole (Him), and 1H-1,2,4-triazole (Htrz), in organic
solvents, affording novel complexes (H2ind)[OsIIICl4(Hind)(DMSO)] (2), (H2pz)[OsIIICl4(Hpz)(DMSO)] (3), (H2bzim)-
[OsIIICl4(Hbzim)(DMSO)] (4), (H2im)[OsIIICl4(Him)(DMSO)] (6), and (H2trz)[OsIIICl4(Htrz)(DMSO)] (7), which are close
analogues of the antimetastatic complex NAMI-A. Metathesis reaction of 4 with benzyltriphenylphosphonium chloride
in methanol led to the formation of (Ph3PCH2Ph)[OsIIICl4(Hbzim)(DMSO)] (5). The complexes were characterized
by IR, UV
contrast to NAMI-A, 2
they show reasonable antiproliferative activity in vitro in two human cell lines, HT-29 (colon carcinoma) and SK-
−
vis, ESI mass spectrometry, 1H NMR spectroscopy, cyclic voltammetry, and X-ray crystallography. In
4, 6, and 7 are kinetically stable in aqueous solution and resistant to hydrolysis. Surprisingly,
−
BR-3 (mammary carcinoma), when compared with analogous ruthenium compounds. Structure
and the potential of the prepared complexes for further development are discussed.
−activity relationships
Introduction
One of the reasons is an optimal chemical reactivity of these
compounds under physiological conditions. Certain isostruc-
tural osmium analogues, for example [(η6-biphenyl)Os(en)-
Cl]BF4, which enter substitution reactions much more slowly
than the corresponding ruthenium complexes, were found
to be inactive in A2780 ovarian cancer cells.4
Quite recently Sadler et al.5 have demonstrated that the
reactivity of osmium complexes, particularly the hydrolysis
and nucleobase binding, can be effectively tuned by sys-
tematic variation of ancillary ligands. The same authors
expressed the opinion that in using this approach the design
and synthesis of osmium complexes with potential antitumor
properties can be realized. Nevertheless, studies on the
antitumor activity of osmium complexes are still very
scarce.4,6-8
Current interest in ruthenium antitumor drugs was stimu-
lated by recent phase I clinical studies of two complexes,
namely, (H2im)[trans-RuCl4(Him)(DMSO)] (NAMI-A)1 and
(H2ind)[trans-RuCl4(Hind)2] (KP1019, FFC14),2 the first as
an agent, which inhibits the process of metastasis formation,
and the second as an effective means against primary tumors
and metastases, in particular, among others, colon carcino-
mas. The quest for novel ruthenium compounds with
potential antitumor properties has led to synthesis of a large
variety of arene complexes with piano-stool structure of the
type [(η6-arene)Ru(L-L)Cl]X, where arene is biphenyl,
benzene, or p-cymene, and L-L is ethylenediamine or
acetylacetone, and X is a counterion.3 The complexes showed
antitumor activity both in vitro and in vivo, which in some
cases was comparable with that of cisplatin and carboplatin.
(3) (a) Morris, R. E.; Aird, R. E.; Murdoch, P. D. S.; Chen, H.; Cummings,
J.; Hughes, N. D.; Parsons, S.; Parkin, A.; Boyd, G.; Jodrell, D. I.;
Sadler, P. J. J. Med. Chem. 2001, 44, 3616-3621. (b) Aird, R. E.;
Cummings, J.; Ritchie, A. A.; Muir, M.; Morris, R. E.; Chen, H.;
Sadler, P. J.; Jodrell, D. I. Br. J. Cancer 2002, 86, 1652-1657.
(4) Dorcier, A.; Dyson, P. J.; Gossens, C.; Rothlisberger, U.; Scopelliti,
R.; Tavernelli, I. Organometallics 2005, 24, 2114-2123.
(5) Peacock, A. F. A.; Habtemariam, A.; Fernandez, R.; Walland, V.;
Fabbiani, F. P. A.; Parsons, S.; Aird, R. E.; Jodrell, D. I.; Sadler, P.
J. J. Am. Chem. Soc. 2006, 128, 1739-1748.
* To whom correspondence should be addressed. E-mail:
vladimir.arion@univie.ac.at (V.B.A.), bernhard.keppler@univie.ac.at
(B.K.K.).
(1) Alessio, E.; Mestroni, G.; Bergamo, B.; Sava, G. Curr. Top. Med.
Chem. 2004, 4, 1525-1535.
(2) Hartinger, C. G.; Zorbas-Seifried, S.; Jakupec, M. A.; Kynast, B.;
Zorbas, H.; Keppler, B. K. J. Inorg. Biochem. 2006, 100, 891-904.
10.1021/ic700405y CCC: $37.00
Published on Web 05/12/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 12, 2007 5023

Cebria´n-Losantos et al.
Attempts to extend the synthetic work onto synthesis of
NAMI-A type complexes with metal ions other than ruthe-
nium have also been undertaken and are well documented
in the literature. In particular, complexes of rhodium(III) and
iridium(III) have been synthesized and their properties have
been investigated.9,10
Biochemical and cell biological studies show reactivity
and cytotoxicity profiles different from those of ruthenium-
(III) NAMI-A type compounds. In particular, the complexes
neither bind strongly to bovine serum albumin nor inhibit
markedly the proliferation of human tumor cell lines.
Surprisingly, osmium analogues of NAMI-A have not been
reported so far, although their synthesis and comparison with
ruthenium compounds appears to be more desirable from
the point of view of the position of these metals in the
periodic table. The evaluation of NAMI-A analogues with
metals other than ruthenium, specifically those of osmium-
(III), has been mainly hindered by synthetic limitations. The
synthesis of NAMI-A and related rhodium(III) and iridium-
(III) compounds was straightforwardly realized because of
the availability of starting compounds [(DMSO)2H] [trans-
MCl4(DMSO)2] (M ) Ru, Rh, Ir).11,12 The reaction of these
compounds with azole ligands in organic solvents enabled
the synthesis of the desired compound families in good
yields. Although evidence for the formation of [trans-OsCl4-
(DMSO)2]- (1) in solution was reported,13 the compound
has not been isolated as a solid so far. Being intrigued by
this fact, we directed our efforts on (i) the isolation of this
osmium compound and its full characterization, (ii) the study
of the reactivity of [(DMSO)2H][trans-OsCl4(DMSO)2]
toward azole ligands and full characterization of the resulting
osmium analogues of NAMI-A type compounds (Figure 1),
(iii) cytotoxicity assays of the novel osmium(III) compounds,
and (iv) comparison of hydrolytic stability of the prepared
Figure 1. Anticancer osmium(III) NAMI-A analogues; underlined com-
plexes have been characterized in this work by X-ray crystallography.
Figure 2. Heterocyclic azole ligands.
osmium complexes with those of the NAMI-A family of
compounds.
We show in this article that [(DMSO)2H][trans-OsIIICl4-
(DMSO)2] can be efficiently prepared by stepwise reduction
of OsO4 with hydrazine dihydrochloride in concentrated HCl
to OsIV 14 and then with SnCl2‚2H2O/DMSO to OsIII and
OsII.15 We also describe the nearest congeners, resulting from
reactions of [(DMSO)2H][trans-OsCl4(DMSO)2] with azole
ligands (Figure 2), that are closely related to the well-
characterized NAMI-A type ruthenium(III) compounds16-18
with a d5 low-spin electronic configuration and show that
osmium(III) complexes are also worth being investigated as
potential antitumor drugs.
(6) Dorcier, A.; Ang, W. H.; Bolan˜o, S.; Gonsalvi, L.; Juillerat-Jeannerat,
L.; Laurenczy, G.; Peruzzini, M.; Phillips, A. D.; Zanobini, F.; Dyson,
P. J. Organometallics 2006, 25, 4090-4096.
Experimental Section
Chemicals. OsO4 (99.8%) was purchased from Johnson Matthey
in 1 g ampoules, N2H4‚2HCl from Fluka, and SnCl2‚2H2O from
Merck. The azole ligands, indazole, pyrazole, imidazole, benzimi-
dazole, and 1H-1,2,4-triazole, were from Aldrich. 9-Methyladenine
(MeAde) was prepared as reported in the literature.19
Synthesis of Complexes. trans-OsIICl2(DMSO)4 and [(DMSO)2-
H][trans-OsIIICl4(DMSO)2] (1). N2H4‚2HCl (0.42 g, 4.00 mmol)
was added to OsO4 (1.00 g, 3.93 mmol) in HCl (37%, 25 mL) and
(7) (a) Craciunescu, D. G.; Molina, C.; Parrondo-Iglesias, E.; Alonso, M.
P.; Doadrio-Villarejo, J. C.; Gutierrez Rios, M. T.; De Frutos, M. I.;
Certad Fombona, G.; Gaston de Iriarte, E. An. Real Acad. Farm. 1991,
57, 221-240. (b) Doadrio, A.; Craciunescu, D.; Ghirvu, C.; Nuno, J.
C. An. Quim. 1977, 73, 1220-1223. (b) Craciunescu, D. G.; Parrondo
Iglesias, E.; Alonso, M. P.; Molina, C.; Doadrio Lopez, A.; Gomez,
A.; Mosquerra, R. M.; Ghirvu, C.; Gaston de Iriarte, E. An. R. Acad.
Farm. 1988, 54, 16-45. (c) Craciunescu, D. G.; Molina, C.; Parrondo-
Iglesias, E.; Alonso, M. P.; Doadrio-Villarejo, J. C.; Gutierrez Rios,
M. T.; De Frutos, M. I.; De Iriarte, E.; Gaston Fombona, G. C.; Ercoli,
N. An. R. Acad. Farm. 1992, 58, 207-231.
(8) Peacock, A. F. A.; Parsons, S.; Sadler, P. J. J. Am. Chem. Soc. 2007,
129, 3348-3357.
(9) Mestroni, G.; Alessio, E.; Sessanta o Santi, A.; Geremia, S.; Bergamo,
A.; Sava, G.; Boccarelli, A.; Schettino, A.; Coluccia, M. Inorg. Chim.
Acta 1998, 273, 62-71.
(14) Brauer, G. Handbuch der Pra¨paratiVen Anorganischen Chemie; Bd.
III, 1981; pp 1742-1744.
(15) McDonagh, A. M.; Humphrey, M. G.; Hockless, D. C. R. Tetrahe-
dron: Asymmetry 1997, 8, 3579-3583.
(16) (a) Reisner, E.; Arion, V. B.; Guedes da Silva, M. F. C.; Lichtenecker,
R.; Eichinger, A.; Keppler, B. K.; Kukushkin, V.; Yu Pombeiro, A. J.
L. Inorg. Chem. 2004, 43, 7083-7093.
(10) Messori, L.; Marcon, J.; Orioli, P.; Fontani, M.; Zanello, P.; Bergamo,
A.; Sava, G.; Mura, P. J. Inorg. Biochem. 2003, 95, 37-46.
(11) (a) Alessio, E.; Balducci, G.; Calligaris, M.; Costa, G.; Attia, W. M.;
Mestroni, G. Inorg. Chem. 1991, 30, 609-618. (b) Alessio, E.;
Sessanta o Santi, A.; Faleschini, P.; Calligaris, M.; Mestroni, G. J.
Chem. Soc., Dalton Trans. 1994, 13, 1849-1855. (c) Haddad, Y. M.
Y.; Henbest, H. B.; Trocha-Grimshaw, J. J. Chem. Soc., Perkin Trans.
1 1974, 5, 592-595.
(17) (a) Alessio, E.; Balducci, G.; Lutman, A.; Mestroni, G.; Calligaris,
M.; Attia, W. M. Inorg. Chim. Acta 1993, 203, 205-217. (b) Mura,
P.; Camalli, M.; Messori, L.; Piccioli, F.; Zanello, P.; Corsini, M. Inorg.
Chem. 2004, 43, 3863-3870.
(18) Alessio, E.; Mestroni, G.; Pacor, S.; Sava, G.; Spinelli, S. PCT Int.
Appl. WO 9013553, 1990.
(19) (a) Talman, E. G.; Bruening, W.; Reedijk, J.; Spek, A. L.; Veldman,
N. Inorg. Chem. 1997, 36, 854-861. (b) Charland, J. P.; Phan Viet,
M. T.; St-Jacques, M.; Beauchamp, A. L. J. Am. Chem. Soc. 1985,
107, 8202-8211.
(12) Jaswal, J. S.; Rettig, S. J.; James, B. R. Can. J. Chem. 1990, 68, 1808-
1817.
(13) (a) Komozin, P. N. Zh. Neorg. Khim. 2000, 45, 662-674. (b) Komozin,
P. N. Zh. Neorg. Khim. 1998, 43, 547-553.
5024 Inorganic Chemistry, Vol. 46, No. 12, 2007

Osmium NAMI-A analogues
water (3 mL). The solution was stirred at room temperature for 7
days. Afterward, the solvent was removed by rotary evaporation
under reduced pressure at 60 °C. To the red residue, SnCl2‚2H2O
(0.80 g, 3.54 mmol) and DMSO (8 mL) were added, and the
solution was held at 87 °C for 1 h. The precipitated yellow solid
trans-OsIICl2(DMSO)4 was filtered off, washed with acetone (4 ×
15 mL) and Et2O (4 × 15 mL), and dried in vacuo. The IR spectrum
of this compound was identical to that of trans-OsCl2(DMSO)4
prepared as described in the literature.15 The filtrate was allowed
to stand at room temperature overnight, producing yellow-orange
crystals of the product. These were separated by filtration and dried
in vacuo. Yield: trans-OsIICl2(DMSO)4 0.34-0.56 g, 15-25%;
[(DMSO)2H][trans-OsIIICl4(DMSO)2] 0.50-0.76 g, 20-30%. Anal.
Calcd for C8H25Cl4O4OsS4 (Mr ) 645.59 g/mol): C, 14.88; H, 3.90;
S, 19.87; Cl, 21.97. Found: C, 14.63; H, 3.79; S, 19.67; Cl, 21.67.
ESI-MS in MeOH (negative): m/z 488 [OsIIICl4(DMSO)2]-, 410
ether (5 mL), and dried in vacuo. Yield: 0.06 g, 60%. Anal. Calcd
for C16H19Cl4N4OOsS (Mr ) 647.45 g/mol): C, 29.68; H, 2.96; N,
8.65; S, 4.95. Found: C, 29.61; H, 3.04; N, 8.47; S, 4.82. ESI-
MS in MeOH (negative): m/z 528 [OsIIICl4(Hbzim)(DMSO)]-, 410
[OsIIICl4(DMSO)]-, 332 [OsIIICl4]-. IR (KBr), selected bands, cm-1
:
426 (w), ν(Os-S); 1016 (s), F(CH3); 1067 (vs), ν(SdO). UV-vis
(MeOH), λmax, nm (ꢀ, M-1 cm-1): 377 (1186), 334 (6324), 279
(13 747) sh, 273 (19 763), 267 (20 553). UV-vis (H2O), λmax, nm
(ꢀ, M-1 cm-1): 406 (599) sh, 370 (1367), 330 (5096), 277 (11690),
270 (13 488), 250 (11 091) sh.
(Ph3PCH2Ph)[trans-OsIIICl4(Hbzim)(DMSO)] (5). Benzyl-
triphenylphosphonium chloride (0.06 g, 0.15 mmol) was added to
a solution of 4 (0.05 g, 0.10 mmol) in methanol (20 mL), and the
mixture was stirred at room temperature for 2 h. Slow removal of
solvent by rotary evaporation under reduced pressure led to
precipitation of a yellow solid, which was filtered off, washed with
diethyl ether (2 × 5 mL), and dried in vacuo. Yield: 0.05 g, 60%.
Anal. Calcd for C34H34Cl4N2OOsPS (Mr ) 881.73 g/mol): C, 46.31;
H, 3.89; N, 3.18. Found: C, 46.61; H, 3.64; N, 3.17. ESI-MS in
MeOH (negative): m/z 528 [OsIIICl4(Hbzim)(DMSO)]-, 410
[OsIIICl4(DMSO)]-, 332 [OsIIICl4]-. ESI-MS in MeOH (posi-
tive): m/z 353, [Ph3PCH2Ph]+. X-ray diffraction-quality crystals
were grown by slow diffusion of methanol solutions of 4 and
benzyltriphenylphosphonium chloride in an H-tube.
[OsIIICl4(DMSO)]-, 332 [OsIIICl4]-. IR (KBr), selected bands, cm-1
:
416 (w), ν(Os-S); 909 (vw), 916 (vw), 936 (w), 947 (w), 969 (vw),
977 (vw), 1017 (s), F(CH3); 1080 (s), ν(SdO); 1292 (vw), 1304
(vw), 1318 (w), δ(CH3). UV-vis (MeOH), λmax, nm (ꢀ, M-1 cm-1):
419 (351) sh, 348 (4189), 268 (5077) sh. UV-vis (H2O), λmax, nm
(ꢀ, M-1 cm-1): 410 (666) sh, 374 (2516) sh, 344 (6162), 300 (757)
sh, 257 (1174). Suitable crystals for X-ray diffraction study were
selected directly from the reaction vessel.
(H2ind)[trans-OsIIICl4(Hind)(DMSO)] (2). Indazole (0.04 g,
0.31 mmol) was added to 1 (0.10 g, 0.15 mmol) in dry ethanol (10
mL), and the mixture was held at 53 °C for 2 h. The orange-red
solution was filtered to remove solid impurities and then evaporated
almost completely under reduced pressure. Addition of hexane to
the remaining residue led to the formation of red crystals, which
were filtered off, washed with diethyl ether (5 mL), and dried in
vacuo. Yield: 0.05 g, 50%. Anal. Calcd for C16H19Cl4N4OOsS (Mr
) 647.45 g/mol): C, 29.68; H, 2.96; N, 8.65; S, 4.95. Found: C,
29.26; H, 2.96; N, 8.35; S, 4.83. ESI-MS in MeOH (negative):
m/z 528 [OsIIICl4(Hind)(DMSO)]-, 410 [OsIIICl4(DMSO)]-, 332
[OsIIICl4]-. IR (KBr), selected bands, cm-1: 432 (w), ν(Os-S);
1023 (m), F(CH3); 1058 (vs), ν(SdO). UV-vis (MeOH), λmax, nm
(ꢀ, M-1 cm-1): 378 (1650), 332 (7317), 298 (21 255) sh. UV-vis
(H2O), λmax, nm (ꢀ, M-1 cm-1): 408 (168) sh, 370 (330), 330 (815),
296 (1361) sh, 285 (1485), 258 (1260). X-ray diffraction-quality
crystals were grown by vapor diffusion of n-hexane into a saturated
EtOH solution of the complex.
(H2pz)[trans-OsIIICl4(Hpz)(DMSO)] (3). Pyrazole (0.025 g, 0.37
mmol) was added to 1 (0.10 g, 0.15 mmol) in dry methanol (10
mL), and the mixture was held at 53 °C for 2 h. Afterward the
yellow solution was evaporated under reduced pressure, and diethyl
ether was added dropwise. The resulting red crystals were filtered
off, washed with diethyl ether (5 mL), and dried in vacuo. Yield:
0.05 g, 60%. Anal. Calcd for C8H15Cl4N4OOsS (Mr ) 547.34
g/mol): C, 17.55; H, 2.76; N, 10.24; S, 5.86. Found: C, 17.62; H,
2.69; N, 10.23; S, 5.95. ESI-MS in MeOH (negative): m/z 478
[OsIIICl4(Hpz)(DMSO)]-, 410 [OsIIICl4(DMSO)]-, 332 [OsIIICl4]-.
IR (KBr), selected bands, cm-1: 436 (w), ν(Os-S); 1016 (s),
F(CH3); 1056 (s), ν(SdO). UV-vis (MeOH), λmax, nm (ꢀ, M-1
cm-1): 377 (330), 335 (5280). UV-vis (H2O), λmax, nm (ꢀ, M-1
cm-1): 408 (237) sh, 372 (1168), 330 (4865), 236 (6568). Suitable
crystals for X-ray diffraction study were selected directly from the
reaction vessel.
(H2im)[trans-OsIIICl4(Him)(DMSO)] (6). Imidazole (0.03 g,
0.45 mmol) was added to a suspension of 1 (0.10 g, 0.15 mmol) in
acetone (10 mL), and the mixture was held at 53 °C for 20 h. The
reaction mixture was cooled to room temperature, and the yellow
solid was filtered off, washed with acetone (2 × 5 mL) and diethyl
ether (5 mL), and dried in vacuo. Yield: 0.07 g, 80%. Anal. Calcd
for C8H15Cl4N4OOsS (Mr ) 547.34 g/mol): C, 17.55; H, 2.76; N,
10.24; S, 5.85; Found: C, 17.61; H, 2.64; N, 10.18; S, 5.47. ESI-
MS in MeOH (negative): m/z 478 [OsIIICl4(Him)(DMSO)]-, 410
[OsIIICl4(DMSO)]-, 332 [OsIIICl4]-. IR (KBr), selected bands, cm-1
:
435 (m), ν(Os-S); 1016 (s), F(CH3); 1065 (vs), ν(SdO). UV-vis
(MeOH), λmax, nm (ꢀ, M-1 cm-1): 373 (869), 334 (3913). UV-
vis (H2O), λmax, nm (ꢀ, M-1 cm-1): 409 (192) sh, 371 (1275), 330
(4996), 254 (2025). X-ray diffraction-quality crystals of 6·0.5EtOH
were grown by vapor diffusion of diethyl ether into an ethanol
solution of the complex.
(H2trz)[trans-OsIIICl4(Htrz)(DMSO)] (7). 1-H-1,2,4-Triazole
(0.03 g, 0.45 mmol) was added to a suspension of 1 (0.10 g, 0.15
mmol) in acetone (10 mL), and the mixture was held at 53 °C for
1 h. A clear solution formed on heating produced a yellow solid,
which was filtered off, washed with acetone (2 × 5 mL) and diethyl
ether (5 mL), and dried in vacuo. Yield: 0.05 g, 60%. Anal. Calcd
for C6H13Cl4N6OOsS (Mr ) 549.31 g/mol): C, 13.12; H, 2.38; N,
15.30; S, 5.84. Found: C, 13.32; H, 2.31; N, 15.61; S, 5.81. ESI-
MS in MeOH (negative): m/z 479 [OsIIICl4(Htrz)(DMSO)]-, 410
[OsIIICl4(DMSO)]-, 332 [OsIIICl4]-. IR (KBr), selected bands, cm-1
:
434 (m), ν(Os-S); 1026 (br), F(CH3); 1057 (vs), ν(SdO). UV-
vis (MeOH), λmax, nm (ꢀ, M-1 cm-1): 370 (2778), 336 (5555).
UV-vis (H2O), λmax, nm (ꢀ, M-1 cm-1): 409 (329) sh, 371 (2983),
333 (5786), 256 (2026). X-ray diffraction-quality crystals were
grown by vapor diffusion of diethyl ether into a methanol solution
of the complex.
Physical Measurements. Elemental analyses were carried out
at the Microanalytical Service of the Institute of Physical Chemistry
of the University of Vienna. Infrared spectra were obtained from
KBr pellets with a Perkin-Elmer 370 FTIR 2000 instrument (4000-
400 cm-1). UV-vis spectra were recorded on a Perkin-Elmer
Lambda 20 UV-vis spectrophotometer using samples dissolved
(H2bzim)[trans-OsIIICl4(Hbzim)(DMSO)] (4). Benzimidazole
(0.05 g, 0.45 mmol) was added to a suspension of 1 (0.10 g, 0.15
mmol) in acetone (10 mL), and the mixture was held at 53 °C for
2 h. The clear solution formed on heating produced a yellow solid,
which was filtered off, washed with acetone (2 × 5 mL), diethyl
Inorganic Chemistry, Vol. 46, No. 12, 2007 5025

Cebria´n-Losantos et al.
Table 1. Crystal Data and Details of Data Collection for 1-3 and 5-7
complex
empirical
1
2
3
5
6·0.5EtOH
7
C8H25Cl4O4OsS4
C16H19Cl4N4OOsS
C8H15Cl4N4OOsS
C34H34Cl4N2OOsPS C9H18Cl4N4O1.5OsS C6H13Cl4N6OOsS
formula
fw
645.59
647.45
547.34
881.73
570.37
549.31
space group
a (Å)
b (Å)
P1h
P21/n
P21/c
P21/n
P21/c
P1h
9.1652(4)
13.8866(14)
16.4116(8)
90.201(5)
90.130(2)
100.651(1)
2052.8(2)
4
10.5903(4)
8.6393(3)
22.6247(9)
8.4670(2)
14.9489(4)
13.0000(3)
10.6249(17)
15.416(3)
20.809(3)
10.1758(4)
16.1069(7)
20.9749(9)
7.4444(4)
8.0206(4)
13.1209(7)
92.973(3)
103.329(3)
93.293(3)
759.36(7)
2
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
103.068(2)
97.585(1)
93.677(10)
92.605(3)
2016.39(13)
4
1631.04(7)
4
3401.5(9)
4
3434.2(2)
8
λ (Å)
0.71073
2.089
0.71073
2.133
0.71073
2.229
0.71073
1.722
0.71073
2.206
0.71073
2.402
F
calcd (g cm-3
)
cryst size (mm3) 0.20 × 0.10 × 0.08 0.26 × 0.20 × 0.06 0.14 × 0.14 × 0.14 0.36 × 0.26 × 0.14
0.16 × 0.05 × 0.02
0.18 × 0.14 × 0.12
T (K)
µ (cm-1
R1a
100
100
296
296
100
100
)
71.47
0.0320
0.0845
1.094
69.73
0.0212
0.0577
1.048
85.97
0.0183
0.0358
1.026
42.03
0.0271
0.0617
1.005
81.73
92.37
0.0201
0.0451
1.010
0.0328
0.0627
1.029
wR2b
GOFc
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) {∑[w(Fo - Fc )2]/∑[w(Fo )2 ]}1/2
.
c GOF ) {∑[w(Fo - Fc )2]/(n - p)}1/2, where n is the number of reflections,
2
2
2
2
2
and p is the total number of parameters refined.
in methanol or in water at 298 K. The aqueous solution behavior
with respect to hydrolysis of 2-4, 6, and 7 was studied at 298 K
over 48 h by UV-vis spectroscopy. Electrospray ionization mass
spectrometry was carried out with a Bruker Esquire 3000 instrument
(Bruker Daltonic, Bremen, Germany) in methanol and methanol/
water (30/70). Expected and experimental isotope distributions were
compared. The 1H NMR spectra were recorded at 400.13 MHz on
a Bruker DPX400 (Ultrashield Magnet) spectrometer. The solution
methods and refined by full-matrix least-squares techniques. Non-
hydrogen atoms were refined with anisotropic displacement pa-
rameters. H atoms were placed at calculated positions and refined
as riding atoms in the subsequent least-squares model refinements.
The isotropic thermal parameters were estimated to be 1.2 times
the values of the equivalent isotropic thermal parameters of the
non-hydrogen atoms to which hydrogen atoms are bonded. The
following computer programs were used: structure solution,
SHELXS-97;22 refinement, SHELXL-97;23 molecular diagrams,
ORTEP;24 computer, Pentium IV. Scattering factors were taken from
the literature.25
Cell Lines and Culture Conditions. Human HT-29 (colon
carcinoma) and SK-BR-3 (mammary carcinoma) cells were kindly
provided by Brigitte Marian (Institute of Cancer Research, Medical
University of Vienna, Austria) and Evelyn Dittrich (General
Hospital, Medical University of Vienna, Austria), respectively. All
of the cell culture media and supplements were purchased from
Sigma-Aldrich, Vienna, Austria, unless indicated otherwise. Cells
were grown in 75 cm2 culture flasks (Iwaki/Asahi Technoglass,
Gyouda, Japan) as adherent monolayer cultures in complete culture
medium, that is, minimal essential medium (MEM) supplemented
with 10% heat-inactivated fetal bovine serum (Invitrogen, Paisley,
U.K.), 1 mM sodium pyruvate, 4 mM l-glutamine, and 1% non-
essential amino acids (100×). Cultures were maintained at 37 °C
in a humidified atmosphere containing 5% CO2.
1
behavior of 6 was studied by H NMR in aqueous solution (D2O,
pH ∼5.5, 37 °C) for 2 days and in physiological medium (phosphate
buffer 0.05 M, pH 7.4; 0.15 M NaCl, 37 °C) for 3 days. The
interaction of 6 with 9-methyladenine (1:1.5 molar ratio) in
phosphate buffer (pH 6.0, 37 °C) was followed by 1H NMR
spectroscopy for 4 days. Acetone (1 µL/mL) was used as reference
for the NMR experiments. Cyclic voltammograms were measured
in a two-compartment three-electrode cell using a 1.0-mm-diameter
glassy-carbon-disk working electrode, probed by a Luggin capillary,
and connected to a silver-wire pseudoreference electrode and a
platinum auxiliary electrode. Measurements were performed at room
temperature using an EG & G PARC 273A potentiostat/galvanostat.
Deaeration of solutions was accomplished by passing a stream of
argon through the solution for 10 min prior to the measurements
and then maintaining a blanket atmosphere of argon over the
solution during the measurements. The potentials were measured
in 0.2 M phosphate buffer solutions (pH 7.0), using methyl viologen
ïx
(E ) -0.44 V vs NHE in water)20 as an internal standard and
Cytotoxicity Tests in Cancer Cell Lines. Cytotoxicity was
determined by means of the colorimetric MTT assay (MTT )
3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide;
purchased from Fluka, Vienna, Austria). HT-29 and SK-BR-3 cells
were harvested from culture flasks by trypsinization and seeded
into 96-well microculture plates (Iwaki/Asahi Technoglass, Gyouda,
Japan). A seeding density of 4 × 103 cells/well was chosen to ensure
exponential growth throughout drug exposure. After a 24 h
1/2
are quoted relative to NHE.
Crystallographic Structure Determination. X-ray diffraction
measurements were performed on an X8APEX II CCD diffracto-
meter at 100 or 296 K. Single crystals were positioned at 37.5, 40,
37.5, 40, 40, and 40 mm from the detector, and 1793, 5247, 1134,
948, 3201, and 1973 frames were measured, each for 20, 5, 60, 8,
50, and 3 s over a 1° scan width for 1-3 and 5-7, correspondingly.
The data were processed using SAINT software.21 Crystal data,
data collection parameters, and structure refinement details for 1-3
and 5-7 are given in Table 1. The structures were solved by direct
(22) Sheldrick, G. M. SHELXS-97, Program for Crystal Structure Solution;
University of Go¨ttingen, Germany, 1997.
(23) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure Refine-
ment; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(24) Johnson, C. K. Report ORNL-5138; Oak Ridge National Laboratory:
Oak Ridge, TN, 1976.
(25) International Tables for X-ray Crystallography; Kluwer Academic
Press: Dordrecht, The Netherlands, 1992; Vol. C, Tables 4.2.6.8 and
6.1.1.4.
(20) Guedes da Silva, M. F. C.; Pombeiro, A. J. L.; Geremia, S.; Zangrando,
E.; Calligaris, M.; Zinchenko, A. V.; Kukushkin, V. Yu. J. Chem.
Soc., Dalton Trans. 2000, 1363-1371.
(21) SAINT-Plus, version 7.06a and APEX2; Bruker-Nonius AXS Inc.:
Madison, WI, 2004.
5026 Inorganic Chemistry, Vol. 46, No. 12, 2007

Osmium NAMI-A analogues
preincubation, cells were exposed to solutions of the test compounds
in 200 µL/well complete culture medium for 96 h. At the end of
exposure, drug solutions were replaced by 100 µL/well RPMI1640
culture medium (supplemented with 10% heat-inactivated fetal
bovine serum) plus 20 µL/well MTT solution (5 mg/mL) in
phosphate-buffered saline. After incubation for 4 h, the medium/
MTT mixtures were removed, and the formazan crystals formed
by the mitochondrial dehydrogenase activity of vital cells were
dissolved in 150 µL DMSO per well. Optical densities at 550 nm
were measured with a microplate reader (Tecan Spectra Classic),
using a reference wavelength of 690 nm to correct for unspecific
absorption. The quantity of vital cells was expressed in terms of
T/C values by comparison to untreated control microcultures, and
IC50 values were calculated from concentration-effect curves by
interpolation. Evaluation is based on the means from at least three
independent experiments, each comprising six microcultures per
concentration level.
Figure 3. ORTEP diagram of the structure of 1 with atom labeling scheme
showing two crystallographically independent complex anions and one
[(DMSO)2H]+ cation. The second cation present in the asymmetric unit
was omitted for clarity; thermal ellipsoids were drawn at 50% probability
level. Selected bond lengths (Å) and angles (deg): Os1-S1 2.3301(9), Os1-
S2 2.3345(9), Os1-Cl1 2.3693(11), Os1-Cl2 2.3629(7), Os1-Cl3 2.3632-
(10), Os1-Cl4 2.3569(7), Os2-S3 2.3303(10), Os2-S4 2.3364(10), Os2-
Cl5 2.3726(9), Os2-Cl6 2.3496(8), Os2-Cl7 2.3490(9), Os2-Cl8 2.3704(8),
S1-O1 1.466(3), S2-O2 1.469(3), S3-O3 1.479(3), S4-O4 1.469(3), S5-
O5 1.538(3), S6-O6 1.567(3).
Results and Discussion
Synthesis and Characterization of the Complexes. 1 was
synthesized by two consecutive reductions. The first one is
the reduction of OsVIIIO4 to [OsIVCl6]2- by hydrazine
dihydrochloride in the presence of concentrated HCl, and
the second is the reduction of [OsIVCl6]2- to OsIII and OsII
by SnCl2‚2H2O in the presence of dimethyl sulfoxide with
the formation of [(DMSO)2H][OsIIICl4(DMSO)2] (20-30%)
and trans-OsIICl2(DMSO)4 (15-25%). trans-OsIICl2-
(DMSO)4,15 which precipitated first, was filtered off, whereas
the filtrate produced nice orange needles of 1, which were
suitable for X-ray crystallography (vide infra). It should be
also noted that the related ruthenium(III) complex [(DMSO)2H]-
[RuIIICl4(DMSO)2] was originally isolated from the mother
liquor of cis-RuIICl2(DMSO)4.26 2-4, 6, and 7 were synthe-
sized by reaction of [(DMSO)2H][trans-OsIIICl4(DMSO)2]
(1) with an excess of the corresponding azole ligand in
acetone, ethanol, or methanol by using the destabilizing trans
effect of the two axially S-bound π-accepting DMSO ligands
in 1, whereas 5 was obtained by metathesis reaction of (H2-
bzim)[trans-OsIIICl4(Hbzim)(DMSO)] (4) and Ph3PCH2PhCl
in methanol. The reaction of [(DMSO)2H][OsIVCl5(DMSO)]27
with pyrazole in 1:2 molar ratio in isoamylalcohol at 105
°C for 1 h yielded a mixture of different products, one of
which was shown by X-ray diffraction to be (H2pz)[trans-
OsIIICl4(Hpz)(DMSO)] (3).
Figure 4. ORTEP view of 2 with atom labeling scheme; the thermal
ellipsoids are shown at 50% probability level. Selected bond lengths (Å)
and bond angles (deg): Os1-N1 2.090(2), Os1-Cl1 2.3530(7), Os1-Cl2
2.3511(7), Os1-Cl3 2.3603(7), Os1-Cl4 2.3928(7), Os1-S1 2.2690(7),
S1-O1 1.495(2) Å, N1-Os1-S1 176.02(6), Cl3-Os1-N1-N2
-39.1(2)°.
between 1600 and 1100 cm-1 and an intense broad band
between 900 and 600 cm-1. In 1, this intense band was found
at 844 cm-1, very similar to that for [(DMSO)2H]2[OsCl6]27
(845 cm-1) and was blue shifted when compared to that
observed for [(DMSO)2H][trans-RuIIICl4(DMSO)2] at 730
cm-1. DMSO possesses rather strong π-acceptor properties
and, compared to azole ligands, shows the greater trans-
labilizing effect along the vector S-Os-N. As a result, the
positions of stretching vibrations Os-S (426-436 cm-1),
SdO (S-bonded DMSO) (1056-1067 cm-1), and deforma-
tion vibrations C-H (1016-1026 cm-1) in IR spectra of
2-4, 6, and 7 do not change significantly after the replace-
ment of one azole by another. In the case of starting
compound 1, a mutual labilization of DMSO ligands on the
vector S-Os-S expressed in Os-S bond lengthening (2.33
Å (1), 2.26-2.27 Å (2-7)) (Figures 3-8) resulted in the
decrease of its strength as evidenced by lowering the
absorption frequency ν(Os-S) to 416 cm-1 (1). The marginal
The infrared spectrum of 1 showed one SdO (S-bonded
DMSO) stretching vibration at 1080 cm-1 typical for a trans
configuration. The lighter congener [(DMSO)2H][trans-RuIII-
Cl4(DMSO)2] revealed this vibration at 1082 cm-1.12 Band
assignment was also confirmed by replacement of DMSO
with DMSO-d6. The hydrogen-bridged cation [(DMSO)2H]+
has been previously discovered for [(DMSO)2H][trans-MCl4-
(DMSO)2], where M ) Ru,12 Rh.28 The IR spectra of these
complexes display a very broad band of medium intensity
(26) Alessio, E.; Mestroni, G.; Nardin, G.; Attia, W. M.; Calligaris, M.;
Sava, G.; Zorzet, S. Inorg. Chem. 1988, 27, 4099-4106.
(27) Rudnitskaya, O. V.; Buslaeva, T. M.; Lyalina, N. N. Zh. Neorg. Khim.
1994, 39, 922-924.
(28) (a) James, B. R.; Morris, R. H.; Einsten, F. W. B.; Willis, A. J. Chem.
Soc., Chem. Commun. 1980, 1, 31-32. (b) James, B. R.; Morris, R.
H. Can. J. Chem. 1980, 58, 399-408.
Inorganic Chemistry, Vol. 46, No. 12, 2007 5027

Cebria´n-Losantos et al.
Figure 5. ORTEP view of 3 with atom labeling scheme; the thermal
ellipsoids are shown at 50% probability level. Selected bond lengths (Å)
and bond angles (deg): Os1-N1 2.109(2), Os1-Cl1 2.3542(6), Os1-Cl2
2.3646(6), Os1-Cl3 2.3591(6), Os1-Cl4 2.3640(6), Os1-S1 2.2656(7),
S1-O1 1.482(2) Å, N1-Os1-S1 179.09(5), Cl4-Os1-N1-N2 -40.65-
(18).
Figure 8. ORTEP view of a centrosymmetric fragment of the crystal
structure of 7 with atom labeling scheme; the thermal ellipsoids are
shown at 50% probability level. Selected bond lengths (Å) and bond angles
(deg): Os1-N1 2.095(3), Os1-Cl1 2.3548(8), Os1-Cl2 2.3724(8), Os1-
Cl3 2.3574(8), Os1-Cl4 2.3497(8), Os1-S1 2.2592(8), S1-O1 1.492(2),
N1-N2 1.383(4), N2-C3 1.302(4), C3-N4 1.350(4), N4-C5 1.353(4),
C5-N1 1.307(4) Å, N1-Os1-S1 177.71(7), Cl3-Os1-N1-N2
39.13(22)°.
IR spectra of 2-7 compares well with those of ruthenium
NAMI-A analogues.
The electronic absorption spectra of 1 in water and
methanol are very similar. The band with an absorption
maximum at 348 nm in methanol is slightly blue shifted (344
nm) when water is used as solvent. These results agree well
with the spectra for the lighter congener [(DMSO)2H][trans-
RuIIICl4(DMSO)2],11a which shows a maximum absorption
at 402 (methanol) and 396 nm (water). The bands at 194
and 204 nm for 1 in water are due to the π-π* transitions
of the DMSO ligands. UV-vis spectra of the aqueous
solutions of 2-4, 6, and 7 are characterized by four
absorption bands with maxima near 200 nm (π-π* transi-
tions of the ligands) within 236-298, 323-336, 370-378
nm and a shoulder at 406-409 nm. In addition, the second
band of 2 and 4 is typical for a free or annelated benzene
ring and has an oscillatory character29 (Table S1). Very weak
d-d transitions of octahedral osmium(III) complexes, which
are solvent independent, can be observed very rarely because
they are obscured by more intense charge transfer absorp-
tions.30 The observed solvatochromism (a hypsochromic shift
(blue) with increasing solvent polarity, in our case the change
from methanol to water and a change of band intensity)
indicates the presence of metal-ligand charge-transfer
(MLCT) bands, which also overlap with intraligand transi-
tions in coordinated azole and DMSO ligands. Taking into
consideration the presence of a hole in the t2g subshell of
low-spin OsIII and the presence of ligands with electron-
donating properties in its coordination sphere, the appearance
of low-energy LMCT bands is expected. However, there is
no clear dependence of charge-transfer energy from the net
electron-donor character of the azole ligands (the third and
fourth bands for 2-4, 6, and 7 show very close maxima).
Just as for [(DMSO)2H][trans-MIIICl4(DMSO)2], upon re-
Figure 6. Fragment of the crystal structure of 5 showing the formation of
interanionic bifurcated hydrogen bond. Atoms marked with i are at the
symmetry positions (-x + 1.5, y + 0.5, -z + 1.5). Selected bond lengths
(Å) and angles (deg): Os1-N1 2.120(3), Os1-Cl1 2.3536(9), Os1-Cl2
2.3704(8), Os1-Cl3 2.3604(9), Os1-Cl4 2.3634(8), Os1-S1 2.2632(9),
S1-O1 1.475(3) Å, N1-Os1-S1 177.74(8), Cl3-Os1-N1-C7 41.2(3)°.
Figure 7. Part of the crystal structure of 6 showing the bridging role of
one of the imidazolium cations between two [trans-RuCl4(Him)(DMSO)]-
anions. Atoms marked with i are at the symmetry positions (-x + 1.5, y +
0.5, -z + 1.5). Selected bond lengths (Å) and angles (deg): Os1-N1 2.097-
(4), Os1-Cl1 2.3626(11), Os1-Cl2 2.3494(11), Os1-Cl3 2.3640(11),
Os1-Cl4 2.3748(11), Os1-S1 2.2648(11), S1-O1 1.482(3) Å, N1-Os1-
S1 177.55(11), N3-Os2-S2 178.96(11), Cl1-Os1-N1-C3 -38.8(4),
Cl8-Os2-N3-C6 -37.4(4)°.
(29) Lambert, J. B.; Shurvell, H. F.; Lightner, D. A.; Cooks, R. G. Organic
Structural Spectroscopy; Prentice Hall: Upper Saddle River, NJ, 1998;
pp 568.
(30) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd Ed.; Elsevi-
er: Amsterdam, The Netherlands, 1984; p 862.
shortening of SdO bond length (1.47-1.48 Å (1), 1.48-
1.49 Å (2-7)) (Figures 3-8) did not affect the position of
the stretching vibration ν(SdO). The general character of
5028 Inorganic Chemistry, Vol. 46, No. 12, 2007

Osmium NAMI-A analogues
placement of RuIII by OsIII in all of the NAMI-A analogues,
a blue shift of all of the absorption bands with preserved
general character of the spectra is observed. This is probably
due to the stronger ligand field splitting for third row
transition-metal complexes (Table S1).
2.3304(19), and 2.3365(10) Å] with DMSO ligands trans to
each other. Replacement of one of the DMSO ligands by an
indazole or pyrazole ligand results in a shortening of the
other Os-DMSO bond because of a diminished π back-
bonding competition between the two trans ligands or
decreased σ-directed trans influence of the nitrogen ligand.
The SdO bond length of 1.495(2) Å in 2 is equal within 3σ
with that of metal-free DMSO [1.492(1) Å], whereas it is
5σ shorter in 3 [1.482(2) Å]. It should, however, be noted
that the X-ray diffraction studies for 2 and 3 were performed
at different temperatures (Table 1). The crystal structures of
2 and 3 consist of azolium cations and complex anions, which
are involved in hydrogen bonding interactions. Two such
hydrogen bonds, a weak N3-H‚‚‚Cl3 [N3-H ) 0.88 Å,
H‚‚‚Cl3 ) 2.672 Å, N3‚‚‚Cl3 3.134 Å, N3HCl3 ) 113.94°]
and a strong N4-H‚‚‚O1 [N4-H ) 0.88 Å, H‚‚‚O1 ) 1.905
Å, N4‚‚‚O1 ) 2.750 Å, N4HO1 ) 160.41°] in 2, are shown
in Figure 4, whereas two others, N6-H‚‚‚O1 [N6-H ) 0.86
Å, H‚‚‚O1 ) 1.874 Å, N6‚‚‚O1 ) 2.733 Å, N6HO1 )
176.27°] and a bifurcated hydrogen bond involving N7H as
the donor and Cl1 and Cl2 as acceptors [N7-H ) 0.86 Å;
H‚‚‚Cl1 ) 2.722 Å, N7‚‚‚Cl1 ) 3.378 Å, N7HCl1 )
134.13°; H‚‚‚Cl4 ) 2.876 Å, N7‚‚‚Cl4 ) 3.319 Å, N7HCl4
)113.88°], in 3 are shown in Figure 5. The results of X-ray
diffraction studies of the crystal structures of 5 and 6 are
shown in Figures 6 and 7, respectively. Selected bond
distances (Å) and bond angles (deg) are quoted in the legends
to Figures 6 and 7.
The asymmetric unit of 5 contains an essentially octahedral
anion [trans-RuCl4(Hbzim)(DMSO)]- and the (Ph3PCH2Ph)+
cation, whereas that of 6 consists of two crystallographically
distinct [trans-RuCl4(Him)(DMSO)]- anions, two imidazo-
lium cations, and a disordered ethanol molecule. The
osmium(III) atoms in 5 and 6 are bound to four chlorido
ligands in the equatorial plane and to DMSO and benzimi-
dazole or imidazole molecules in axial positions. The N2-H
group of the benzimidazole ring in 5 appears to form a
bifurcated hydrogen bond involving O1i as well as Cl4i
(N2-H 0.86 Å, H‚‚‚O1i ) 2.029 Å, N2‚‚‚O1i ) 2.795 Å,
N2HO1i ) 147.81°; N2-H 0.86 Å, H‚‚‚Cl4i ) 2.827 Å,
N2‚‚‚Cl4i ) 3.412 Å, N2HCl4i ) 126.82°) (Figure 6).
Bond lengths and angles in the (Ph3PCH2Ph)+ cation are
similar to those reported in the literature.35
Crystal Structures. The structures of the complexes
reported herein are of interest because of the paucity of
documented X-ray diffraction data for osmium(III)-sulfoxide
complexes31 and lack of such data for osmium(III)-azole
derivatives.32 1 crystallizes in the triclinic space group P1h
with cell parameters very close to those of related Ru, Ir, or
Rh compounds, the structures of which were solved in the
monoclinic space group P2/n. The crystal structure of 1
consists of [(DMSO)2H]+ cations and [trans-OsCl4(DMSO)2]-
anions. Figure 3 displays a perspective view of two crys-
tallographically independent complex anions and one of the
two independent [Me2SO‚‚‚H‚‚‚OSMe2]+ cations. Selected
bond lengths (Å) and angles (deg) are quoted in the legend
to Figure 3. The structure of the [(DMSO)2H]+ cation is the
same as in the complexes [(DMSO)2H][trans-MCl4(DMSO)2],
where M ) Ru, Ir, Rh or in [(DMSO)2H][trans-OsCl4-
(DMSO)NO].33
The osmium(III) ions in [trans-OsCl4(Hind)(DMSO)]- of
2 and in [trans-OsCl4(Hpz)(DMSO)]- of 3 have the expected
distorted octahedral coordination geometry (Figures 4 and
5), with four chloride ligands in the equatorial positions and
a DMSO molecule bound through its sulfur atom trans to
the indazole or pyrazole ligand in axial positions. The Os1-
Cl4 and Os1-Cl3 bonds in 2 are significantly longer than
Os1-Cl1 and Os1-Cl2 (legend to Figure 4). The elongation
of the first two bonds is probably due to the involvement of
both Cl4 and Cl3 as proton acceptors in two intermolecular
and one intramolecular hydrogen-bonding interactions, re-
spectively, namely, Cl4‚‚‚H2-N2(-x + 1, -y + 1, -z +
1) [Cl4‚‚‚N2 3.336 Å], Cl4‚‚‚H3-N3(-x + 1, -y + 1, -z
+ 1) [Cl4‚‚‚N3 3.226 Å] and Cl3‚‚‚H3-N3 (the parameters
of this H bond are quoted below). The average value of the
Os-Cl bond length in 3 [2.361(2) Å] is close to those found
in [(DMSO)2H][trans-OsCl4(DMSO)2] (1) [2.3617(32) Å]
and in mer-[OsCl3(NH3)2(Me2S)] [2.368(8) Å].20 The Os-
N1 bond of 2.090(2) and 2.109(2) Å in 2 and 3, correspond-
ingly, compares well with Ru-N1 in the related complex
(H2ind)[trans-RuCl4(Hind)(DMSO)] at 2.0901(13) Å16a and
is significantly shorter than similar bonds trans to sulfur-
bound DMSO in the osmium(II) complexes trans,cis,cis-OsII-
Cl2(Hind)2(DMSO)2 and trans,cis,cis-OsIICl2(Hpz)2(DMSO)2
at 2.130(4), 2.137(4) and 2.1202(17), 2.1365(17) Å, respec-
tively.34 The Os-S1 bond lengths of 2.2690(7) Å (2) and
2.2656(7) Å (3) (sulfur being trans to the indazole or pyrazole
nitrogen)aremarkedlyshorterthanthosefoundin[(DMSO)2H]-
[OsCl4(DMSO)2] [2.3300(9), 2.3343(9),
A fragment of the solid-state structure of 7 shown in Figure
8 features an S-bonded DMSO ligand trans to the N-donor
atom of Htrz. The Os1-S1 bond distance is 2.2592(8) Å,
and the S1-O1 bond length is 1.492(2) Å. The first bond is
shorter than Ru1-S1 [2.2730(8) Å] in (Ph3PCH2Ph)[trans-
RuCl4(Htrz)(DMSO)].16 The latter bond is practically identi-
cal to that in free DMSO [1.492(1) Å].36 The distribution of
electron density over the triazole ring indicates a pre-
vailing double bond character of N1-C5 and N2-C3. The
analysis of the crystal structure of 7 showed the presence of
hydrogen bonding interactions with participation of triazole-
(31) (a) Alessio, E. Chem. ReV. 2004, 104, 4203-4242.
(32) Chiorescu, I.; Stepanenko, I. N.; Arion, V. B.; Krokhin, A. A.; Scaffidi-
Domianello, Y. Y.; Keppler, B. K., manuscript in preparation.
(33) Rudnitskaya, O. V.; Buslaeva, T. M.; Stash, A. I.; Kisin, A. V. Russ.
J. Coord. Chem. 1995, 21, 136-140.
(34) Stepanenko, I. N.; Cebria´n-Losantos, B.; Arion, V. B.; Krokhin, A.
A.; Nazarov, A. A.; Keppler, B. K. Eur. J. Inorg. Chem. 2007, 400-
411.
(35) Cifuentes, M. P.; Waterman, S. M.; Humphrey, M. G.; Heath, G. A.;
Skelton, B. W.; White, A. H.; Perera, M. P. S.; Williams, M. L. J.
Organomet. Chem. 1998, 565, 193-200.
(36) Calligaris, M.; Carugo, O. Coord. Chem. ReV. 1996, 153, 83-154.
Inorganic Chemistry, Vol. 46, No. 12, 2007 5029

Cebria´n-Losantos et al.
Table 2. Cyclic Voltammetric Data for Complexes 2-4, 6 and 7 in
Aqueous Phosphate Buffer (pH 7)
E
1/2 (OsII/III)a,
E
1/2 (OsIII/IV)a,
complex
(∆Ep)b
(∆Ep)b
ΣEL/V vs NHE pKa(H2azole+)
2
3
4
6
7
0.17 (60)
0.16 (60)
0.12 (70)
0.12 (60)
0.15 (80)
1.41 (70)
1.40 (70)
1.27 (60)
1.33 (80)
1.35 (70)
-0.23
-0.29
-0.39
-0.37
-0.31
1.25
2.64
5.63
6.65
2.55
a Potentials E1/2 (E1/2 ) (Epa + Epc)/2, where Epa and Epc are the anodic
and cathodic peak potential, respectively) are given in V and measured
at a scan rate 0.2 V s-1 in aqueous phosphate buffer (pH 7) using
ïx
1/2
methyl viologen (E ) -0.44 V vs NHE in water) as internal standard
and are quoted relative to NHE. b ∆Ep values (∆Ep ) Epa - Epc) are given
in mV.
Figure 9. Multiple cyclic voltammogram of 0.5 mM (H2trz)[trans-OsCl4-
(Htrz)(DMSO)] in aqueous phosphate buffer at pH 7 at a carbon-disk
working electrode and at a scan rate of 0.2 V/s, starting with a scan in
cathodic direction.
ring nitrogen atoms N2 and N4. The first one acts as an
acceptor in the hydrogen bond N8-H‚‚‚N2 with the follow-
ing parameters: N8-H ) 0.88 Å, H‚‚‚N2 ) 2.089 Å, N8‚
‚‚N2 ) 2.811 Å, N8HN2 ) 138.71°. The second acts as
a donor in the hydrogen bond N4-H‚‚‚Cl2 (-x + 1, -y,
-z) with the following parameters: N4-H ) 0.88 Å, H‚‚
‚Cl2 ) 2.538 Å, N4‚‚‚Cl2 ) 3.350 Å, N4HCl2 ) 153.91°.
All of this indicates that, in contrast to [trans-RuCl4(Htrz)-
(DMSO)]- where 1H-1,2,4-triazole is coordinated to ruthe-
nium(III) via N4, the triazole ligand in 7 is stabilized as a
4H tautomer. Hence, coordination of triazole via N1 (or N2
in the nomenclature used for 1H- or 4H-1,2,4-triazole)
is proposed. The N2 atom in triazole is less basic than
N4,37 therefore such behavior is rather unexpected. 1,2,4-
Triazole behaves as a monodentate ligand coordinating to
the first-row or second-row transition metal ion in the
majority of cases via N4 ([MnII(SO4)(Htrz)(H2O)4],38 [Cd-
(NCS)2(Htrz)2],39 [FeCl3(bpy)(Htrz)],40 or Zn(II) in human
carbonic anhydrase.41 Only in two cases, namely, (H2trz)-
[cis-RuCl4(Htrz)2] and (Ph3PCH2Ph)[trans-RuCl4(Htrz)2], has
crystallographic evidence for coordination via N2 been
provided.42
> EL(Him) > EL(Hbzim)] and their basicity [pKa(H2ind+)
<pKa(H2trz+)<pKa(H2pz+)<pKa(H2bzim+)<pKa(H2im+)]43-46
(Table 2). The reversible responses are characterized by a
peak-to-peak separation (∆Ep) of 60-80 mV and an anodic
peak current (ipa) that is almost equal to the cathodic peak
current (ipc), as expected for reversible electron-transfer
processes. The one-electron nature of electron-transfer
processes was verified by comparing the peak-current height
(ip) with that of standard methyl viologen couples under
identical experimental conditions.
The linear relationships between the redox potentials for
the OsIII f OsII and OsIII f OsIV processes and ∑EL (EL-
(Cl) ) -0.24,47 EL(DMSO) ) 0.47,48 EL(Hind) ) 0.26,16
EL(Hpz) ) 0.20,48 EL(Htrz) ) 0.18,48 EL(Hbzim) ) 0.1,47
EL(Him) ) 0.1248), expressed by general Lever’s equation48
(eq 1)
E ) SM E + I
(1)
∑
L
M
and was obtained from the plots in Figures 10 and 11. This
expression enabled for the first time the estimate of the SM
and IM values, which are dependent upon the metal and redox
couple, the spin state, and the stereochemistry for the OsII/III
Electrochemical Behavior in Aqueous Phosphate Buffer
at pH 7. The cyclic voltammograms of 2-4, 6, and 7 in 0.2
M phosphate buffer (pH 7) at a carbon-disk working
electrode and at a scan rate of 0.2 V/s display one reversible,
single-electron oxidation wave (Iïx) assigned to the OsIII f
OsIV process with E1ï/x2 potential values ranging from 1.27 to
(eq 2) and OsIII/IV
(eq 3) redox couples in aqueous phosphate
buffer at pH 7 as follows:
1.41 V and one reversible single-electron reduction wave
E(OsII/III) ) 0.35 E + 0.26
(2)
(3)
red
(Ired) attributed to the OsIII f OsII process with E
∑
L
1/2
E(OsIII/IV) ) 0.82 E + 1.61
potential values ranging from 0.12 to 0.17 V versus NHE
(Table 2). The redox potentials of the complexes are in the
following rank order: E1/2(2) > E1/2(3) > E1/2(7) > E1/2(6)
g E1/2(4) (in the case of OsIII f OsII process E1/2(4) )
E1/2(6)), which agrees well with the relative electron-donor
character of the N-ligands [EL(Hind) > EL(Hpz) > EL(Htrz)
∑
L
(43) Reedijk, J. Heterocyclic Nitrogen-Donor Ligands. In ComprehensiVe
Coordination Chemistry; Wilkinson, G., Gillard, R. D., McCleverty,
J. A., Eds.; Pergamon Press: Elmsford, NY, 1987; Vol. 2, pp 73-98.
(44) Potts, K. T. Chem. ReV. 1961, 61, 87-127.
(45) Catala´n, J.; Claramunt, R. M.; Elguero, J.; Laynez, J.; Mene´ndez, M.;
Anvia, F.; Quian, J. H.; Taagepera, M.; Taft, R. W. J. Am. Chem.
Soc. 1988, 110, 4105-4111.
(46) Reisner, E.; Arion, V. B.; Eichinger, A.; Kandler, N.; Giester, G.;
Pombeiro, A. J. L.; Keppler, B. K. Inorg. Chem. 2005, 44, 6704-
6716.
(37) Meot-Ner, M.; Liebman, J. F.; Del Bene, J. E. J. Org. Chem. 1986,
51, 1105-1110.
(38) Gorter, S.; Engelfriet, D. W. Acta Crystallogr. 1981, B37, 1214-
1218.
(39) Haasnoot, J. G.; De Keyzer, G. C. M.; Verschoor, G. C. Acta
Crystallogr. 1983, C39, 1207-1209.
(47) (a) Lever, A. B. P. Inorg. Chem. 1990, 29, 1271-1285. (b) Lever, A.
B. P.; Dodsworth, E. S. Inorganic Electronic Structure and Spectros-
copy; Wiley: New York, 1999; pp 227-290.
(48) (a) Bacac, M.; Hotze, A. C. G.; van der Schilden, K.; Haasnoot, J.
G.; Pacor, S.; Alessio, E.; Sava, G.; Reedijk, J. J. Inorg. Biochem.
2004, 98, 402-412. (b) Bouma, M.; Nuijen, B.; Jansen, M. T.; Sava,
G.; Flaibani, A.; Bult, A.; Beijnen, J. H. Int. J. Pharm. 2002, 248,
239-246.
(40) Driessen, W. L.; De Graaff, R. A. G.; Vos, J. G. Acta Crystallogr.
1983, C39, 1635-1637.
(41) Mangani, S.; Liljas, A. J. Mol. Biol. 1993, 232, 9-14.
(42) Arion, V. B.; Reisner, E.; Fremuth, M.; Jakupec, M. A.; Keppler, B.
K.; Kukushkin, V. A.; Pombeiro, A. J. L. Inorg. Chem. 2003, 42,
6024-6031.
5030 Inorganic Chemistry, Vol. 46, No. 12, 2007

Osmium NAMI-A analogues
Figure 10. Plot of E1/2(OsII/III) for 2-4, 6, and 7 in aqueous phosphate
buffer (pH 7) against ∑EL (in V vs NHE). E(OsII/III) ) 0.35∑EL + 0.26 (r
) 0.97) (eq 2). (H2ind)[OsIIICl4(Hind)(DMSO)] 2; (H2pz)[OsIIICl4(Hpz)-
(DMSO)] 3; (H2bzim)[OsIIICl4(Hbzim)(DMSO)] 4; (H2im)[OsIIICl4(Him)-
(DMSO)] 6; (H2trz)[OsIIICl4(Htrz)(DMSO)] 7, where EL(Hind) ) 0.26;
EL(Hpz) ) 0.20; EL(Hbzim) ) 0.10; EL(Him) ) 0.12; EL(Htrz) ) 0.18.
Figure 12. 1H NMR spectra of 6 (5.5 mM) in phosphate buffer (0.05 M
phosphate, 0.15 M NaCl), pH 7.4, 37 °C for t ) 0 and 3 days with chemical
formula and atom numbering scheme. For signal integration, acetone (1
µL/mL) was added to the buffer solution as reference (2.16 ppm).
NMR spectra of 6 in D2O (pH ∼5.5, 37 °C) were monitored
over 2 days. The spectra obtained immediately after dis-
solution of 6 and after 2 days were almost identical. A very
small signal (<2%) at 2.63 ppm was attributed to free
DMSO. Addition of 1 µL of DMSO resulted in an increase
of this signal. The integration of the DMSO signal observed
for a freshly prepared solution of 6 after 2 days showed an
increase of only 3%, indicating that a very small amount of
the expected hydrolysis product [OsCl4(Him)(H2O)]- is
presumably formed. In contrast, NAMI-A under similar
conditions hydrolyzes to a larger extent with formation of
the complex [RuCl4(Him)(H2O)]-. However, no hydrolysis
Figure 11. Plot of E1/2(OsIII/IV) for 2-4, 6, and 7 in aqueous phosphate
buffer (pH 7), against ∑EL (in V vs NHE). E(OsIII/IV) ) 0.82ΣEL + 1.61
(r ) 0.93) (eq 3). (H2ind)[OsIIICl4(Hind)(DMSO)] 2; (H2pz)[OsIIICl4(Hpz)-
(DMSO)] 3; (H2bzim)[OsIIICl4(Hbzim)(DMSO)] 4; (H2im)[OsIIICl4(Him)-
(DMSO)] 6; (H2trz)[OsIIICl4(Htrz)(DMSO)] 7, where EL(Hind) ) 0.26;
EL(Hpz) ) 0.20; EL(Hbzim) ) 0.10; EL(Him) ) 0.12; EL(Htrz) ) 0.18.
1
of Ru-Cl bonds was found. The H NMR spectrum of the
anion [OsCl4(Him)(DMSO)]- showed a broad signal centered
at -20 ppm attributed to the protons of S-bonded DMSO
(by comparison with the spectrum of 1), two broad reso-
nances around -17 and -15 ppm (Him, H4,5), and a sharper
resonance at -5.26 ppm (Him, H2). Two sharp signals at δ
8.61 and 7.39 ppm (1:2 integration ratio) correspond to H2
and H4,5 of the H2im+ counterion, respectively. Other peaks
Resistance to Hydrolysis in Aqueous and Physiological
Media. The aqueous solution behavior with respect to
hydrolysis of 2-4, 6, and 7 was studied at 298 K over 48 h
by UV-vis spectroscopy and 1H NMR spectroscopy. All of
the complexes were quite stable in aqueous solution, as can
be seen from their electronic absorption spectra (Figures S2-
S6). Immediate hydrolysis can be excluded by comparison
of UV-vis spectra of the complexes in methanol and water.
In addition, the negative ion ESI mass spectra of complexes
2-4, 6, and 7 in methanol/water (30/70) showed a similar
pattern with three different peaks, which can be attributed
to [OsIIICl4(L)(DMSO)]-, [OsIIICl4(DMSO)]-, and [OsIIICl4]-,
where L is the respective azole ligand. The presence of the
parent peak due to [OsIIICl4(azole)(DMSO)]- provides further
evidence against immediate hydrolysis of the complexes
studied in aqueous solution (Figure S8).
1
observed in the H NMR spectrum of 6 have not been
identified (a sharp peak at 7.67 ppm shifted to 7.38 ppm
after 2 days, a very broad signal at about 3.38 ppm, a peak
at 3.58, and a sharp signal at 2.13 ppm). All of these peaks
were of marginal intensity in the spectrum of freshly prepared
solutions of 6 and did not change their intensity after 2 days.
1H NMR spectra of 6 in physiological medium (phosphate
buffer 0.05 M, pH 7.4; 0.15M NaCl, 37 °C) were monitored
over 3 days. The 1H NMR spectra recorded immediately after
dissolution (t ) 0) and after 3 days were practically identical,
as can be seen in Figure 12. Acetone (1 µL/mL) was used
as reference, as it gives a sharp signal at 2.16 ppm that does
not overlap with the signals of 6. The integration of the
DMSO signal observed (2.66 ppm) for a freshly prepared
solution of 6 after 3 days showed an increase of only 5%.
The spectrum showed a broad signal centered at -20 ppm
for the protons of S-bonded DMSO, two broad bands around
-17 and -15 ppm (Him, H4,5), and a sharper peak at -5.18
ppm (Him, H2). The signals of the H2im+ counterion seen
at 8.33 and 7.30 ppm (1:2 integration ratio) correspond to
H2 and H4,5, respectively. Two other minor peaks at 3.42
1H NMR experiments in aqueous solution (D2O, pH ∼5.5,
37 °C) and in physiological medium (phosphate buffer 0.05
M, pH 7.4; NaCl 0.15 M, 37 °C) were carried out with 6 for
direct comparison with the earlier reported data for
[RuIIICl4(Him)(DMSO)]- (NAMI-A).48 Because of the para-
magnetism of the OsIII ion (d5, low spin), a full identification
of the NMR signals was not accomplished. The main peaks
observed were assigned taking into account the NMR spectra
of NAMI-A49 and the osmium(II) complexes trans,cis,cis-
OsCl2(Him)2(DMSO)2 and cis,fac-OsCl2(Him)(DMSO)3.35 1
H
Inorganic Chemistry, Vol. 46, No. 12, 2007 5031

Cebria´n-Losantos et al.
Table 3. Antiproliferative Effects of 1-4, 6, and 7 and Three
Ruthenium(III) Analogues in Two Human Cancer Cell Lines
compound
L
IC50 (µM)a
HT-29
75 ( 11
21 ( 1
SK-BR-3
241 ( 57
119 ( 41
325 ( 141
194 ( 19
930 ( 46
>1000
1
2
3
4
6
7
DMSO
Hind
Hpz
Hbzim
Him
39 ( 10
36 ( 2
103 ( 15
214 ( 61
Htrz
Ru analoguesb
Hind
Him
Htrz
212 ( 22
339 ( 68
322 ( 32
169 ( 10
472 ( 25
415 ( 48
a Fifty percent inhibitory concentrations after exposure for 96 h in the
MTT assay. Values are means ( standard deviations from at least three
independent experiments. b Data taken from the previous publication.49
dazole results in increased antiproliferative activity, whereas
replacement by imidazole or triazole weakens it. A com-
parison with ruthenium analogues (presented in a previous
paper)49 reveals that replacement of ruthenium(III) by
osmium(III) increases the activity in HT-29 cells in all three
pairs of complexes (L ) Hind, Him, or Htrz). Whereas IC50
values >100 µM indicate that the potencies of the ruthenium
analogues are rather modest, osmium confers a reasonable
activity with IC50 values mostly in the 10-5 M range in these
cells. This effect is most pronounced in the case of 2 (L )
Hind), which is 10 times more potent than its ruthenium
congener. These tendencies are not paralleled by the findings
in cell line SK-BR-3, however. But generally, the variation
of the azole ligand seems to have greater consequences for
biological activity in the osmium series than in the ruthenium
series. The observed effects are all the more remarkable
because of the inertness of these osmium(III) complexes in
general and especially their resistance to hydrolysis (even
in chloride-free solution) and lack of reactivity toward
9-methyladenine, which sharply contrasts with the behavior
of ruthenium analogues, in particular NAMI-A, under
comparable experimental conditions.49 Whereas the antime-
tastatic activity of NAMI-A in vivo has been attributed
specifically to hydrolyzed species, the lack of cytotoxicity
observed with various osmium(II) arene compounds has been
attributed to either too slow or too rapid hydrolysis, in the
latter case resulting in the predominant formation of stable
hydroxo-bridged dimers.5 Even though the latter compounds
differ distinctly from those presented here in terms of
oxidation state and ligand sphere, it is quite remarkable that
our findings conversely suggest that hydrolysis is not at all
an essential prerequisite for antiproliferative activity of this
class of osmium complexes. This also contrasts with conclu-
sions drawn from the behavior of rhodium and iridium
analogues of NAMI-A, the inactivity of which has been
attributed to their inertness to hydrolysis.9,10
Figure 13. Antiproliferative effects of 1-4, 6, and 7 in the human cancer
cell lines HT-29 (A) and SK-BR-3 (B). Concentration-effect curves were
obtained by the MTT assay; values are means ( standard deviations from
at least three independent experiments.
and 1.37 ppm have not been identified, but remained
unchanged over 3 days. The 1H NMR spectrum of NAMI-A
(-15 ppm for S-bonded DMSO, -3.5, -5.6, and -7.8 ppm
for coordinated Him, and 8.69 and 7.49 ppm for H2im+)
showed notably downfield-shifted signals when compared
1
to 6. Moreover, the H NMR spectrum of NAMI-A in
physiological medium was time dependent and showed fast
replacement of the coordinated chloride and DMSO by water,
as evidenced by the appearance of signals at -10 ppm (S-
bonded DMSO) and at 0.6 ppm (Him) for mer-[RuCl3(Him)-
(H2O)(DMSO)] and free DMSO (2.7 ppm) within a few
minutes. NAMI-A underwent chloride and DMSO hydrolysis
responsible for the disappearance of NAMI-A after only 15
min.49a In contrast, the osmium NAMI-A analogue remained
intact after 3 days in physiological medium.
1H NMR spectra of a mixture of 6 and 9-methyladenine
(as a DNA model base) in 1:1.5 molar ratio in phosphate
buffer (pH 6.0, 37 °C) were measured. The absence of a
new set of signals expected for coordinated 9-methyladenine
even after 4 days indicated inertness of 6 toward the purine
nucleobase under the conditions employed (Figure S7).
Antiproliferative Activity. 1-4, 6, and 7 were tested for
their antiproliferative activity in two human cancer cell lines
using a colorimetric MTT assay. Concentration-effect curves
are depicted in Figure 13, and IC50 values are listed in
comparison with three ruthenium(III) analogues in Table 3.
Structure-activity relationships are similar in both cell lines.
In the more sensitive cell line HT-29 (colon carcinoma),
antiproliferative activity decreases in the following rank
order: 2 > 4 ≈ 3 > 1 g 6 > 7. In the less sensitive cell
line SK-BR-3 (mammary carcinoma), the rank order is as
follows: 2 > 4 g 1 g 3 > 6 g 7. Taking 1 as a reference,
replacement of one DMSO ligand by indazole or benzimi-
NAMI-A reduces the formation and growth of metastases
in experimental tumor models but has little impact on the
growth of primary tumors.50 This behavior is in line with a
modest cytotoxicity but a pronounced and rapid impact on
(49) Groessl, M.; Reisner, E.; Hartinger, C. G.; Eichinger, R.; Semenova,
O.; Timerbaev, A. R.; Jakupec, M. A.; Arion, V. B.; Keppler, B. K.
J. Med. Chem. 2007, 50, 2185-2193.
(50) Sava, G.; Zorzet, S.; Turrin, C.; Vita, F.; Soranzo, M. R.; Zabucchi,
G.; Cocchietto, M.; Bergamo, A.; DiGiovone, S.; Pezzoni, G.; Sartor,
L.; Garbisa, S. Clin. Cancer Res. 2003, 9, 1898-1905.
5032 Inorganic Chemistry, Vol. 46, No. 12, 2007

Osmium NAMI-A analogues
the interactions of tumor cells with the extracellular matrix,
suggesting a target located in the cell membrane. In
particular, NAMI-A increases actin-dependent cell adhesion
by a mechanism probably involving integrin activation,51,52
it inhibits matrix degradation by reducing the release of
matrix metalloproteinases rather than by its direct inhibitory
effect on these enzymes,53 and it reduces cell invasiveness
and migration,50,53,54 altogether manifesting in a less malig-
nant cell phenotype. In endothelial cells, inhibition of
proliferation, chemotactic behavior, and matrix metallopro-
teinase secretion have been observed, suggesting a contribu-
tion of antiangiogenic effects to the antimetastatic proper-
ties.55 Furthermore, NAMI-A is capable of inducing a
transient cell cycle arrest55 and binding to DNA to a certain
degree,56 but neither of these effects is likely to account for
the antimetastatic activity.
NAMI-A is transformed under physiological conditions
into several different ruthenium species, but their individual
contributions to the pharmacological effects remain elu-
sive.48a,57 Rapid hydrolysis to polyoxo species in physiologi-
cal buffer, their higher cellular uptake, and their unaltered
effects on the cell cycle of metastatic cells have prompted
investigators to attribute the activity of NAMI-A to hydroly-
sis products.48a Furthermore, reduction to ruthenium(II)
species does not alter the effects on cell cycle distribution
and metastasis growth to a meaningful extent.57 Because it
remains unclear which molecular effects account for the
antimetastatic activity and whether they depend on hydrolysis
and/or reduction, it is impossible to predict whether this
activity is preserved in the osmium analogues. Experiments
in suitable rodent tumor models with reliable metastasizing
propensity are planned in order to examine these compounds
for their antimetastatic capacity, along with their impact on
primary tumors in vivo. From a therapeutical point of view,
a compound endowed with a dual mechanism combining
inhibitory effects on the processes of tumor cell invasion
and migration with a cytotoxic component in primary as well
as secondary sites of malignant tumors would be particularly
attractive for further development. In any case, the suggestion
of Messori et al. to use analogues not amenable to hydrolysis
as model compounds for studying the biodistribution profile
of unhydrolysed NAMI-A10 is applicable also to the osmium
complexes presented here.
Final Remarks. Investigation of stepwise reduction of
OsO4 with N2H4‚2HCl and SnCl2‚2H2O/DMSO enabled the
preparation and characterization of [(DMSO)2H][trans-OsIII-
Cl4(DMSO)2], the compound which remained elusive (at least
in terms of solid-state isolation) for quite a long time. The
latter proved to be a suitable precursor for the synthesis of
osmium NAMI-A analogues, which showed reasonable
antiproliferative activity in vitro. The full characterization
of prepared compounds, both in the solid state and in
solution, permitted the elucidation of essential differences
between related osmium and ruthenium complexes. In
particular, we found that osmium complexes, which are
markedly more inert than related ruthenium compounds
toward substitution reactions (hydrolysis, interaction with
DNA bases), in accord with their position in the periodic
table, partially show a higher antiproliferative activity than
the related ruthenium compounds. This is even more intrigu-
ing, if we take into account that other related compounds
based on iridium and rhodium do not exhibit any antipro-
liferative activity.
The slope, SM, and intercept, IM, of Lever’s equation were
determined for the OsII/OsIII and OsIII/OsIV couples in aqueous
medium, which will allow the prediction of the redox
potential of other osmium complexes with the same redox
couples.
The work also made a notable crystallographic contribu-
tion. 2, 3, and 5-7 expand the relatively small class of OsIII-
DMSO-azole complexes characterized by X-ray diffraction.
The binding mode of triazole (monofunctional via N2) and
the tautomer stabilized (4H) are distinct from those found
in related ruthenium compounds (coordination via N4 and
1H tautomer).
(51) Sava, G.; Frausin, F.; Cocchietto, M.; Vita, F.; Podda, E.; Spessotto,
P.; Furlani, A.; Scarcia, V.; Zabucchi, G. Eur. J. Cancer 2004, 40,
1383-1396.
(52) Frausin, F.; Scarcia, V.; Cocchietto, M.; Furlani, A.; Serli, B.; Alessio,
E.; Sava, G. J. Pharmacol. Exp. Ther. 2005, 313, 227-233.
(53) Pacor, S.; Zorzet, S.; Cocchietto, M.; Bacac, M.; Vadori, M.; Turrin,
C.; Gava, B.; Castellarin, A.; Sava, G. J. Pharmacol. Exp. Ther. 2004,
310, 737-744.
Acknowledgment. We thank A. Roller for collection of
X-ray data and Dr. S. Shova for discussion of the crystal-
lographic part of the work.
(54) Zorzet, S.; Bergamo, A.; Cocchietto, M.; Sorc, A.; Gava, B.; Alessio,
E.; Iengo, E.; Sava, G. J. Pharmacol. Exp. Ther. 2000, 295, 927-
933.
Supporting Information Available: UV-vis spectra of com-
plexes [H2L][MCl4(HL)(DMSO)], where M ) Os, Ru, UV-vis
spectrum of 1 in water, UV-vis spectra of 2-4, 6, and 7 in water
(55) Vacca, A.; Bruno, M.; Boccarelli, A.; Coluccia, M.; Ribatti, D.;
Bergamo, A.; Garbisa, S.; Sartor, L.; Sava, G. Br. J. Cancer 2002,
86, 993-998.
(56) Pluim, D.; van Waardenburg, R. C. A. M.; Beijnen, J. H.; Schellens,
J. H. M. Cancer Chemother. Pharmacol. 2004, 54, 71-78.
(57) Sava, G.; Bergamo, A.; Zorzet, S.; Gava, B.; Casarsa, C.; Cocchietto,
M.; Furlani, A.; Scarcia, V.; Serli, B.; Iengo, E.; Alessio, E.; Mestroni,
G. Eur. J. Cancer 2002, 38, 427-435.
1
monitored over 48 h, H NMR spectrum of 6 with 9-MeAde, ESI
mass spectrum of 6 in methanol/water (30/70), X-ray crystal-
lographic files in CIF format for 1-3, 5, 6, and 7. This material is
available free of charge via the Internet at http://pubs.acs.org.
IC700405Y
Inorganic Chemistry, Vol. 46, No. 12, 2007 5033