Chloro half-sandwich osmium(II) complexes: Influence of chelated N,N-ligands on hydrolysis, guanine binding, and cytotoxicity

Article abstract of DOI:10.1021/ic062350d

Relatively little is known about the kinetics or the pharmacological potential of organometallic complexes of osmium compared to its lighter congeners, iron and ruthenium. We report the synthesis of seven new complexes, [(η⁶-arene)Os(NN)Cl]⁺, containing different bidentate nitrogen (N,N) chelators, and a dichlorido complex, [(η⁶-arene)- Os(N)Cl₂]. The X-ray crystal structures of seven complexes are reported: [(η⁶-bip)Os(en)Cl]PF₆ (1PF₆), [(η⁶-THA)Os(en)Cl]BF₄ (2BF₄), [(η⁶-p-cym)Os(phen)Cl]PF₆ (5PF₆), [(η⁶-bip)Os(dppz)Cl]PF₆ (6PF₆), [(η⁶-bip)Os(azpy-NMe₂)Cl]PF₆ (7PF ₆), [(η⁶-p-cym)Os(azpy-NMe₂)Cl]PF ₆ (8PF₆), and [(η⁶-bip)Os(NCCH ₃-N)Cl₂] (9), where THA = tetrahydroanthracene, en = ethylenediamine, p-cym = p-cymene, phen = phenanthroline, bip = biphenyl, dppz = [3,2-a: 2′,3′-c]phenazine and azpy-NMe₂ = 4-(2-pyridylazo)-N,N-dimethylaniline. The chelating ligand was found to play a crucial role in enhancing aqueous stability. The rates of hydrolysis at acidic pH* decreased when the primary amine N-donors (NN = en, t_1/2 = 0.6 h at 318 K) are replaced with π-accepting pyridine groups (e.g., NN = phen, t_1/2 = 9.5 h at 318 K). The Os^II complexes hydrolyze up to 100 times more slowly than their Ru^II analogues. The pK _a*. of the aqua adducts decreased with a similar trend (pK _a* = 6.3 and 5.8 for en and phen adducts, respectively). [(η⁶-bip)Os(en)Cl]PF₆/BF₄ (1PF ₆/BF₄) and [(η⁶-THA)Os(en)Cl]BF₄ (2BF₄) were cytotoxic toward both the human A549 lung and A2780 ovarian cancer cell lines, with IC₅₀ values of 6-10 μM, comparable to the anticancer drug carboplatin. 1BF₄ binds to both the N7 and phosphate of 5′-GMP (ratio of 2:1). The formation constant for the 9-ethylguanine (9BG) adduct [(η⁶-bip)M(en)(9EtG)]²⁺ was lower for Os^II (log K= 3.13) than Ru^II (log K = 4.78), although the Os^II adduct showed some kinetic stability. DNA intercalation of the dppz ligand in 6PF₆ may play a role in its cytotoxicity. This work demonstrates that the nature of the chelating ligand can play a crucial role in tuning the chemical and biological properties of [(η⁶-arene)Os(NN)Cl]⁺ complexes.

Full text of DOI:10.1021/ic062350d

Inorg. Chem. 2007, 46, 4049−4059
Chloro Half-Sandwich Osmium(II) Complexes: Influence of Chelated
N,N-Ligands on Hydrolysis, Guanine Binding, and Cytotoxicity
Anna F. A. Peacock, Abraha Habtemariam, Stephen A. Moggach, Alessandro Prescimone,
Simon Parsons, and Peter J. Sadler*
School of Chemistry, UniVersity of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, U.K.
Received December 8, 2006
Relatively little is known about the kinetics or the pharmacological potential of organometallic complexes of osmium
compared to its lighter congeners, iron and ruthenium. We report the synthesis of seven new complexes, [(η6
arene)Os(NN)Cl]+, containing different bidentate nitrogen (N,N) chelators, and a dichlorido complex, [( 6-arene)-
Os(N)Cl2]. The X-ray crystal structures of seven complexes are reported: [(
6-bip)Os(en)Cl]PF6 (1PF6),
[(
6-THA)Os(en)Cl]BF4 (2BF4), [( 6-p-cym)Os(phen)Cl]PF6 (5PF6), [( 6-bip)Os(dppz)Cl]PF6 (6PF6), [( 6-bip)Os(azpy-
NMe2)Cl]PF6 (7PF6), [(
6-p-cym)Os(azpy-NMe2)Cl]PF6 (8PF6), and [( 6-bip)Os(NCCH3-N)Cl2] (9), where THA
tetrahydroanthracene, en ethylenediamine, p-cym p-cymene, phen phenanthroline, bip biphenyl, dppz
[3,2-a: 2 ,3 -c]phenazine and azpy-NMe2 4-(2-pyridylazo)-N,N-dimethylaniline. The chelating ligand was found
to play a crucial role in enhancing aqueous stability. The rates of hydrolysis at acidic pH* decreased when the
primary amine N-donors (NN en, t1 0.6 h at 318 K) are replaced with -accepting pyridine groups (e.g., NN
phen, t1
9.5 h at 318 K). The OsII complexes hydrolyze up to 100 times more slowly than their RuII analogues.
6.3 and 5.8 for en and phen adducts,
6-THA)Os(en)Cl]BF4 (2BF4) were cytotoxic toward both
10 M, comparable to the
-GMP (ratio of 2:1). The formation
3.13) than RuII
-
η
η
η
η
η
η
η
η
)
)
)
)
)
)
′
′
)
)
)
π
/
2
)
)
/2
The pK* of the aqua adducts decreased with a similar trend (pK*
)
a
a
respectively). [(η η
6-bip)Os(en)Cl]PF6/BF4 (1PF6/BF4) and [(
the human A549 lung and A2780 ovarian cancer cell lines, with IC50 values of 6
−
µ
anticancer drug carboplatin. 1BF4 binds to both the N7 and phosphate of 5
′
+
constant for the 9-ethylguanine (9EtG) adduct [(
(log K
6PF6 may play a role in its cytotoxicity. This work demonstrates that the nature of the chelating ligand can play a
crucial role in tuning the chemical and biological properties of [(
6-arene)Os(NN)Cl]+ complexes.
η
6-bip)M(en)(9EtG)]2 was lower for OsII (log K
)
)
4.78), although the OsII adduct showed some kinetic stability. DNA intercalation of the dppz ligand in
η
Introduction
complexes has been little explored. In general, not much is
known about the chemistry of osmium arene complexes
compared to those of iron and ruthenium. A SciFinder search
revealed ∼8 times fewer publications on osmium arene
complexes.
Ruthenium and platinum complexes containing chelating
N,N-heterocyclic ligands, for example, phenanthroline (phen),
bipyridine (bipy), and phenylazo-pyridine (azpy), have been
extensively studied, and some have been reported to show
anticancer activity.7-9 Romeo et al. investigated the ligand
exchange kinetics of platinum complexes containing different
The design and development of organometallic drugs is a
relatively new but rapidly developing field.1,2 Organometallic
complexes of the group 8 elements ruthenium and iron are
receiving increasing attention as potential anticancer agents,3-6
yet the pharmacological potential of organometallic osmium
* To whom correspondence should be addressed. E-mail P.J.Sadler@
ed.ac.uk.
(1) Fish, R. H.; Jaouen, G. Organometallics 2003, 22, 2166-2177.
(2) Melchart, M.; Sadler, P. J. In Bioorganometallics; Jaouen, G., Ed.;
Wiley-VCH: Weinheim, Germany, 2006; Vol. 1, pp 39-64.
(3) Allardyce, C. S.; Dorcier, A.; Scolaro, C.; Dyson, P. J. Appl.
Organomet. Chem. 2005, 19, 1-10.
(4) Yan, Y. K.; Melchart, M.; Habtemariam, A.; Sadler, P. J. Chem. Comm.
2005, 4764-4776.
(7) Novakova, O.; Kasparkova, J.; Vrana, O.; van Vliet, P. M.; Reedijk,
J.; Brabec, V. Biochemistry 1995, 34, 12369-78.
(5) Hillard, E.; Vessieres, A.; Le, Bideau, F.; Plazuk, D.; Spera, D.; Huche,
M.; Jaouen, G. ChemMedChem. 2006, 1, 551-559.
(8) Bloemink, M. J.; Engelking, H.; Karentzopoulos, S.; Krebs, B.;
Reedijk, J. Inorg. Chem. 1996, 35, 619 - 627.
(6) Hartinger, C. G.; Nazarov, A. A.; Arion, V. B.; Giester, G.; Jakupec,
M.; Galanski, M.; Keppler, B. K. New J. Chem. 2002, 26, 671-673.
(9) Velders, A. H.; Kooijman, H.; Spek, A. L.; Haasnoot, J. G.; De Vos,
D.; Reedijk, J. Inorg. Chem. 2000, 39, 2966-2967.
10.1021/ic062350d CCC: $37.00
Published on Web 04/19/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 10, 2007 4049

Peacock et al.
[(η6-bip)Os(en)Cl]BF4 (1BF4). A solution of [(η6-bip)OsCl2]2
(187 mg, 0.22 mmol) in 12 mL of methanol was heated under reflux
for 80 min under argon; ethylenediamine (32 µL, 0.48 mmol) was
added, and the reaction mixture was heated under reflux for a further
40 min. The mixture was filtered through a 0.45 µm pore size filter
while still hot. NH4BF4 (390 mg, ca. 8 mol equiv) was added; the
mixture was stirred, and the solvent was removed on a rotary
evaporator. Soxlett extraction using dichloromethane was carried
out for 5.5 h. The solvent volume was reduced to ∼5 mL, and the
sample was stored at 253 K overnight. The yellow microcrystalline
product was recovered by filtration, washed with dichloromethane
(10 mL) and diethyl ether (10 mL), and air-dried. Yield: 129 mg
(54%). Anal. Calcd for C14ClH18N2OsBF4 (526.79): C, 31.92; H
bidentate nitrogen-chelating ligands,10 and the DNA inter-
calation ability of platinum and ruthenium complexes
containing this class of ligands has also received attention.11-14
Osmium complexes containing such heterocyclic bidentate
nitrogen chelates have been investigated as DNA photo-
reagents;15-17 however these complexes were coordinately
saturated, containing no exchangeable ligand, and so pro-
vided no information on ligand exchange rates on osmium
complexes containing different nitrogen chelators. However,
it is clear that bidentate nitrogen chelators such as bipy form
characteristically stable osmium complexes.18 This is poten-
tially important for possible anticancer activity, since we have
found that dissociation of oxygen-chelating ligands from
osmium at micromolar concentrations can deactivate com-
plexes under biological testing conditions.19,20
1
3.44; N 5.32%. Found: C, 32.05; H, 3.20; N, 5.07%. H NMR
(DMSO-d6): δ 7.69 (d, 2H, J ) 7.2 Hz), 7.49 (t, 2H, J ) 7.6 Hz),
7.44 (t, 1H, J ) 7.3 Hz), 7.07 (b, 2H), 6.42 (d, 2H, J ) 5.7 Hz),
6.13 (t, 1H, J ) 5.0 Hz), 6.03 (t, 2H, J ) 5.3 Hz), 4.79 (b, 2H),
2.44 (m, 2H), 2.20 (m, 2H).
Here, we explore the solid-state structures, kinetics, and
solution behavior of OsII arene complexes containing a series
of different bidentate nitrogen chelators, including diamines
and azopyridines, and attempt to relate this to their activity
toward cancer cell lines. This work may also be relevant to
the design of new catalysts.
[(η6-bip)Os(en)Cl]PF6 (1PF6). Synthesis was performed in the
same manner as for 1BF4, using NH4PF6 instead of NH4BF4.
Yield: 139 mg (67%). ESI-MS Calcd for C14ClH18N2Os: m/z 442.1.
1
Found: m/z 441.4. H NMR (DMSO-d6): δ 7.69 (d, 2H, J ) 7.2
Hz), 7.49 (t, 2H, J ) 7.4 Hz), 7.44 (t, 1H, J ) 7.3 Hz), 7.07 (b,
2H), 6.42 (d, 2H, J ) 5.5 Hz), 6.13 (t, 1H, J ) 5.1 Hz), 6.03 (t,
2H, J ) 5.4 Hz), 4.80 (b, 2H), 2.45 (m, 2H), 2.20 (m, 2H). Crystals
of 1PF6 suitable for X-ray diffraction were obtained by slow
evaporation of a dichloromethane solution at ambient temperature.
Experimental Section
Materials. OsCl3‚nH2O was purchased from Alfa Aesaer;
ethylenediamine and 4-(2-pyridylazo)-N,N-dimethylaniline (azpy-
NMe2) were obtained from Sigma-Aldrich. 9-Ethylguanine and
guanosine 5′-monophosphate were purchased from Sigma, and
ammonium tetrafluoroborate, ammonium hexafluorophosphate,
2-picolylamine (ampy), 2,2′-bipyridine (bipy), 1,10-phenanthroline
(phen), and deuterated solvents came from Aldrich. The dimers
[(η6-bip)OsCl2]2, [(η6-p-cym)OsCl2]2, [(η6-THA)OsCl2]2, ligand
[3,2-a: 2′,3′-c]phenazine (dppz), and the ruthenium complex, [(η6-
bip)Ru(en)Cl]PF6, were prepared by previously reported pro-
cedures.19,21-24 Ethylenediamine and methanol/ethanol were distilled
over sodium and magnesium/iodine, respectively, prior to use.
Preparation of Complexes. Complexes 1PF6/BF4-9 were
synthesized from the dimer precursor [(η6-arene)OsCl2]2.
[(η6-THA)Os(en)Cl]BF4 (2BF4). A solution of [(η6-THA)-
OsCl2]2 (27.1 mg, 0.03 mmol) in methanol (5 mL) was refluxed
for 1.5 h under an argon atmosphere; ethylenediamine (5 µL, 0.075
mmol) was added, and the reaction mixture was refluxed for a
further 40 min. The mixture was filtered through a glass wool plug
while still hot. A filtered solution of NH4BF4 (29 mg, ∼9 mol equiv)
in methanol (2 mL) was added; the mixture was stirred, and the
solvent was removed in vacuo. Soxlett extraction using dichlo-
romethane was carried out on the solid residue for 6.5 h. The solvent
was allowed to evaporate slowly overnight at ambient temperature
(∼0.5 mL). The crystalline product was recovered by filtration,
washed with diethyl ether (10 mL), and air-dried. Yield: 6.4 mg
(19%). Anal. Calcd for C16ClH22N2OsBF4 (554.82): C, 34.63; H
1
(10) Romeo, R.; Scolaro, L. M.; Nastasi, N.; Arena, G. Inorg. Chem. 1996,
35, 5087-5096.
4.00; N 5.05%. Found: C, 34.33; H, 3.44; N, 4.87%. H NMR
(DMSO-d6): δ 6.84 (b, 2H), 5.85 (dd, 2H, J ) 4.0 and 1.8 Hz),
5.78 (s, 2H), 5.73 (dd, 2H, J ) 3.9 and 1.9 Hz), 4.72 (b, 2H), 3.34
(m, 4H, J ) 175.1 and 16.4 Hz), 2.65 (s, 4H), 2.45 (m, 2H), 2.19
(m, 2H). Crystals suitable for X-ray diffraction were obtained by
slow evaporation of a dichloromethane solution of 2BF4 at ambient
temperature.
(11) Lippard, S. J. Acc. Chem. Res. 1978, 11, 211-217.
(12) Blasius, R.; Moucheron, C.; Kirsch-De Mesmaeker, A. Eur. J. Inorg.
Chem. 2004, 20, 3971-3979.
(13) Patel, K. K.; Plummer, E. A.; Darwish, M.; Rodger, A.; Hannon, M.
J. J. Inorg. Biochem. 2002, 91, 220-229.
(14) van der Schilden, K.; Garcia, F.; Kooijman, H.; Spek, A. L.; Haasnoot,
J. G; Reedijk J. Angew. Chem., Int. Ed. 2004, 43, 5668-70.
(15) Holmlin, R. E.; Barton, J. K. Inorg. Chem. 1995, 34, 7-8.
(16) Content, S.; Mesmaeker, A. K.-D. J. Chem. Soc., Faraday Trans. 1997,
93, 1089-1094.
[(η6-p-cym)Os(ampy)Cl]PF6 (3PF6). 2-Picolylamine (14 µL,
0.13 mmol) was added to a solution of [(η6-p-cym)OsCl2]2 (49.7
mg, 0.06 mmol) in 4 mL of methanol. The reaction mixture was
shielded from light and stirred at ambient temperature for 20 h. A
filtered solution of NH4PF6 (100 mg, ∼5 mol equiv) in 2 mL of
MeOH was added, and the solvent volume was reduced on a rotary
evaporator until a yellow precipitate began to form (∼2 mL). The
reaction vessel was stored at 253 K for 2 h. The product was
recovered by filtration, washed with methanol (4 mL) and diethyl
ether (10 mL), and air-dried. Yield: 60.6 mg (79%). Anal. Calcd
for C16ClH22N2OsPF6 (614.07): C, 31.35; H 3.62; N 4.57%.
Found: C, 30.91; H, 3.33; N, 4.60%. 1H NMR (MeOD-d4): δ 9.06
(d, 1H, J ) 5.7 Hz), 7.97 (dd, 1H, J ) 8.5 and 7.6 Hz), 7.65 (d,
1H, J ) 7.9 Hz), 7.48 (dd, 1H, J ) 7.6 and 6.6 Hz), 6.09 (d, 1H,
J ) 5.5 Hz), 6.06 (d, 1H, J ) 5.5 Hz), 5.91 (d, 1H, J ) 5.5 Hz),
(17) Mishima, Y.; Motonaka, J.; Ikeda, S. Anal. Chim. Acta 1997, 345,
45-50.
(18) Dwyer, F. P.; Goodwin, A.; Gyarfas, E. C. Aust. J. Chem. 1962, 16,
42-50.
(19) Peacock, A. F. A.; Habtemariam, A.; Ferna´ndez, 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.
(20) Peacock, A. F. A.; Melchart, M.; Deeth, R. J.; Habtemariam, A.;
Parsons, S.; Sadler, P. J. Chem.sEur. J. 2007, 13, 2601-2613.
(21) Stahl, S.; Werner, H. Organometallics 1990, 9, 1876-1881.
(22) Chen, H.; Parkinson, J. A.; Parsons, S.; Coxall, R. A.; Gould, R. O.;
Sadler, P. J. J. Am. Chem. Soc 2002, 124, 3064-3082.
(23) Yamada, M.; Tanaka, Y.; Yoshimoto, Y.; Kuroda, S.; Shimao, I. Bull.
Chem. Soc. Jpn. 1992, 65, 1006-1011.
(24) 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.
4050 Inorganic Chemistry, Vol. 46, No. 10, 2007

Chloro Half-Sandwich Osmium(II) Complexes
5.88 (d, 1H, J ) 5.5 Hz), 4.41 (m, 2H), 2.71 (sep, 1H, J ) 6.9
Hz), 2.17 (s, 3H), 1.25 (d, 3H, J ) 6.8 Hz), 1.19 (d, 3H, J ) 6.6
Hz).
precipitate. A solution of azpy-NMe2 (4-(2-pyridylazo)-N,N-dim-
ethylaniline) (35.3 mg, 0.16 mmol) in 5 mL of MeOH was added
dropwise to the resulting pale yellow filtrate, which resulted in an
instant color change to blue. The resulting mixture was stirred at
ambient temperature for 1 h under argon. The solvent volume was
reduced to ∼10 mL; NH4PF6 (114 mg, >10 mol equiv) added, and
the mixture was left at 253 K overnight. The blue precipitate was
recovered by filtration, washed with ethanol (20 mL) and diethyl
ether (10 mL), and air-dried. The product was recrystallized from
dichloromethane. Yield: 66.4 mg (73%). ESI-MS Calcd for C25-
[(η6-p-cym)Os(bipy)Cl]PF6 (4PF6). A solution of 2,2′-bipyridine
(21.1 mg, 0.14 mmol) and [(η6-p-cym)OsCl2]2 (51.1 mg, 0.07
mmol) in 4 mL methanol was stirred for 19 h at ambient
temperature. It was then filtered through a glass wool plug, and a
filtered solution of NH4PF6 (100 mg, ∼5 mol equiv) in 2 mL of
MeOH was added. The solvent volume was reduced on a rotary
evaporator to ∼3 mL. A yellow precipitate formed as the solution
cooled to ambient temperature. The product was recovered by
filtration, washed with methanol (4 mL) and diethyl ether (10 mL),
and air-dried. Yield: 63.2 mg (74%). Anal. Calcd for C20ClH22N2-
OsPF6 (661.05): C, 36.34; H 3.35; N 4.24%. Found: C, 36.19; H,
1
ClH24N4Os: m/z 607.1. Found: m/z 607.4. H NMR (CDCl3): δ
8.57 (d, 1H, J ) 5.7 Hz), 8.31 (d, 1H, J ) 7.6 Hz), 8.00 (d, 2H,
J ) 8.7 Hz), 7.94 (t, 1H, J ) 7.9 Hz), 7.50 (m, 5H), 7.32 (t, 1H,
J ) 7.9 Hz), 6.64 (d, 2H, J ) 8.7 Hz), 6.50 (t, 2H, J ) 6.8 Hz),
6.23 (t, 2H, J ) 5.7 Hz), 6.15 (t, 1H, J ) 5.7 Hz), 3.30 (s, 6H).
Crystals suitable for X-ray diffraction were obtained by diffusion
of diethyl ether into a solution of 7PF6 in methanol at ambient
temperature.
1
3.36; N, 4.08%. H NMR (CDCl3): δ 9.24 (d, 2H, J ) 5.6 Hz),
8.26 (d, 2H, J ) 8.0 Hz), 8.09 (dd, 2H, J ) 8.7 and 7.8 Hz), 7.67
(dd, 2H, J ) 7.8 and 6.6 Hz), 6.11 (d, 2H, J ) 5.8 Hz), 5.86 (d,
2H, J ) 5.7 Hz), 2.57 (sep, 1H, J ) 6.9 Hz), 2.32 (s, 3H), 1.05 (d,
6H, J ) 6.9 Hz).
[(η6-p-cym)Os(azpy-NMe2)Cl]PF6 (8PF6). The dropwise addi-
tion of a solution of azpy-NMe2 (31.6 mg, 0.14 mmol) in 4 mL of
MeOH to a solution of [(η6-p-cym)OsCl2]2 (55.5 mg, 0.07 mmol)
in 15 mL of MeOH, resulted in an instant color change to blue.
The resulting mixture was stirred at ambient temperature for 6.5 h
under argon, and the solvent volume was then reduced to ∼5 mL
on a rotary evaporator. Blue crystals formed overnight at 253 K
after the addition of NH4PF6 (100 mg, ∼5 mol equiv) to the
solution. The product was recovered by filtration, washed with
ethanol (20 mL) and diethyl ether (10 mL), air-dried, and recrys-
talized from dichloromethane. Yield: 92.1 mg (90%). Anal. Calcd
for C23ClH28N4OsPF6 (731.12): C, 37.78; H 3.86; N 7.66%.
[(η6-p-cym)Os(phen)Cl]PF6 (5PF6). A solution of 1,10-phenan-
throline (23.6 mg, 0.13 mmol) in 1 mL of methanol was added to
a solution of [(η6-p-cym)OsCl2]2 (49.1 mg, 0.06 mmol) in 3 mL of
methanol, and the mixture was stirred at 323 K for 1 h under argon.
The solution was filtered through a glass wool plug, and the solvent
volume was reduced to ∼2 mL on a rotary evaporator. A filtered
solution of NH4PF6 (100 mg, ∼5 mol equiv) in 2 mL of MeOH
was added, and the solvent was removed in vacuo. The residue
was sonicated in ethanol to give a bright yellow suspension. The
yellow powder was recovered by filtration, washed with ethanol
(10 mL) and diethyl ether (10 mL), and air-dried. Yield: 67.4 mg
(79%). Anal. Calcd for C22ClH22N2OsPF6 (685.04): C, 38.57; H
1
Found: C, 37.81; H, 3.74; N, 7.67%. H NMR (CDCl3): δ 9.18
1
(d, 1H, J ) 5.8 Hz), 8.32 (d, 1H, J ) 7.9 Hz), 8.06 (d, 2H, J ) 9.0
Hz), 8.00 (t, 1H, J ) 7.7 Hz), 7.62 (t, 1H, J ) 6.6 Hz), 6.77 (d,
2H, J ) 9.2 Hz), 6. 25 (d, 1H, J ) 5.7 Hz), 6.03 (d, 1H, J ) 5.8
Hz), 5.93 (d, 1H, J ) 5.8 Hz), 5.82 (d, 1H, J ) 5.6 Hz), 3.32 (s,
6H), 2.34 (s, 3H), 2.33 (sept, 1H, J ) 7.0 Hz), 0.96 (d, 3H, J )
7.0 Hz), 0.87 (d, 3H, J ) 6.9 Hz). Crystals suitable for X-ray
diffraction were obtained by slow evaporation of a methanol
solution of 8PF6 at ambient temperature.
3.24; N 4.09%. Found: C, 38.61; H, 3.15; N, 3.96%. H NMR
(CDCl3): δ 9.59 (d, 2H, J ) 5.3 Hz), 8.57 (d, 2H, J ) 8.2 Hz),
8.07 (s, 2H), 8.03 (dd, 2H, J ) 8.2 and 5.3 Hz), 6.27 (d, 2H, J )
5.9 Hz), 6.03 (d, 2H, J ) 5.8 Hz), 2.57 (sep, 1H, J ) 6.9 Hz), 2.29
(s, 3H), 1.00 (d, 6H, J ) 6.9 Hz). Crystals suitable for X-ray
diffraction were obtained by diffusion of hexane into a solution of
5PF6 in chloroform at ambient temperature.
[(η6-bip)Os(dppz)Cl]PF6 (6PF6). A solution of [(η6-bip)OsCl2]2
(22.3 mg, 0.03 mmol) was refluxed in MeOH (5 mL)/ H2O (2.5
mL) for 2 h under argon and then filtered through a cotton wool
plug to remove a dark black precipitate. A solution of dppz (15.8
mg, 0.06 mmol) in 5 mL of MeOH was added to the resulting pale
yellow filtrate. The reaction mixture was stirred at 323 K for 80
min under argon. The mixture was filtered through a glass wool
plug, and the solvent volume was reduced on a rotary evaporator
(to ∼5 mL). The addition of a filtered solution of NH4PF6 (90 mg,
∼10 mol equiv) in 1 mL of MeOH gave rise to a yellow precipitate
which was recovered by filtration, washed with methanol (5 mL)
and diethyl ether (10 mL), and air-dried. Yield: 32.5 mg (75%).
Anal. Calcd for (6 + CH3OH) C31ClH24N4OOsPF6 (839.20): C,
44.37; H 2.88; N 6.68%. Found: C, 43.61; H, 2.43; N, 7.15%. 1H
NMR (DMSO-d6): δ 9.80 (d, 2H, J ) 5.6 Hz), 9.73 (d, 2H, J )
8.4 Hz), 8.56 (dd, 2H, J ) 6.6 and 3.5 Hz), 8.24 (dd, 2H, J ) 6.7
and 3.4 Hz), 8.21 (dd, 2H, J ) 8.3 and 5.6 Hz), 7.60 (d, 2H, J )
7.9 Hz), 7.56 (t, 1H, J ) 7.1 Hz), 7.40 (t, 2H, J ) 7.7 Hz), 7.02
(d, 2H, J ) 6.0 Hz), 6.86 (t, 2H, J ) 5.8 Hz), 6.49 (t, 1H, J ) 5.6
Hz). Crystals suitable for X-ray diffraction were obtained by
diffusion of diethyl ether into a solution of 6PF6 in methanol at
ambient temperature.
[(η6-bip)Os(NCCH3-N)Cl2] (9). Acetonitrile (0.5 mL) was added
to a suspension of [(η6-bip)OsCl2]2 (98 mg, 0.12 mmol) in
dichloromethane (6 mL), and the reaction mixture was heated (320
K) with stirring under argon, shielded from light, for 3 h. The
reaction mixture was filtered; the volume of the filtrate was reduced
(to 4 mL) on a rotary evaporator, and diethyl ether was added until
yellow crystals began to form. After the mixture stood at 253 K
overnight, the yellow crystals were recovered by filtration, washed
with diethyl ether (5 mL) and hexane (5 mL), and air-dried. Yield:
80.7 mg (76%). Anal. Calcd for C14Cl2H13NOs (456.40): C, 36.84;
1
H 2.87; N 3.07%. Found: C, 36.24; H, 2.69; N, 2.80%. H NMR
(DMSO-d6): δ 7.70 (d, 2H, J ) 7.9 Hz), 7.45 (m, 3H), 6.66 (d,
2H, J ) 5.5 Hz), 6.36 (t, 1H, J ) 5.1 Hz), 6.32 (t, 2H, J ) 5.1
Hz), 2.07 (s, 3H). Crystals of 9 suitable for X-ray diffraction were
obtained by diffusion of diethyl ether into an aliquot of the original
reaction mixture, at ambient temperature in the dark.
Instrumentation. X-ray Crystallography. All diffraction data
were collected using a Bruker (Siemens) Smart Apex CCD
diffractometer equipped with an Oxford Cryosystems low-temper-
ature device operating at 150 K. Absorption corrections for all data
sets were performed with the multiscan procedure SADABS;25
[(η6-bip)Os(azpy-NMe2)Cl]PF6 (7PF6). A solution of [(η6-bip)-
OsCl2]2 (50.7 mg, 0.06 mmol) in 10 mL of MeOH was refluxed
for 3 h under argon and hot-filtered to remove a dark black
(25) Sheldrick, G. M. SADABS; University of Go¨ttingen: Germany, 2001-
2004.
Inorganic Chemistry, Vol. 46, No. 10, 2007 4051

Peacock et al.
structures were solved using direct methods (SHELXL,26 SIR92,27
or DIRDIF28). The complexes were refined against F2 using
SHELXTL, and the H-atoms were placed in geometrically calcu-
lated positions. X-ray crystallographic data for complexes 1PF6,
2BF4, and 5PF6-9 are available as Supporting Information and
have been deposited in the Cambridge Crystallographic Data Centre
under the accession numbers CCDC 630298, 630299, 630300,
630301, 630303, 630302, and 630304, respectively.
NMR spectra were recorded. The chemical shifts of the arene ring
protons were plotted against pH*. The pH* titration curves were
fitted to the Henderson-Hasselbalch equation, with the assumption
that the observed chemical shifts are weighted averages according
to the populations of the protonated and deprotonated species. These
pK*values can be converted to pKa values by use of the equation
a
pKa ) 0.929pK* + 0.42 as suggested by Krezel and Bal,30 for
a
comparison with related values in the literature.
NMR Spectroscopy. 1H NMR spectra were acquired on a Bruker
AVA 600 (1H ) 600 MHz) spectrometer. 1H NMR spectra in D2O
were typically acquired with water suppression by Shaka29 or
Interactions with Nucleobases. The reaction of complex 3PF6
with 9-ethylguanine (9EtG) at pH* 7.4 and 310 K was monitored
over time using H NMR spectroscopy by mixing equal amounts
1
1
of equilibrium D2O solutions (1 mM) of 3PF6 and 9EtG. The pH*
values of these solutions were adjusted with dilute NaOH to remain
close to physiological pH* 7.4, and the solutions were allowed to
presaturation methods. H NMR chemical shifts were internally
referenced to 1,4-dioxane (3.75 ppm) for aqueous solutions,
(CHD2)(CD3)SO (2.50 ppm) for DMSO-d6, CD2HOD (3.34 ppm)
for methanol-d4, and CHCl3 (7.26 ppm) for chloroform-d1 solutions.
All data processing was carried out using XWIN-NMR, version
3.6 (Bruker U.K. Ltd.).
Mass Spectrometry. Electrospray ionization mass spectra (ESI-
MS) were obtained on a Micromass Platform II Mass Spectrometer
and D2O/H2O solutions were infused directly. The capillary voltage
was 3.5 V, and the cone voltage was varied between 20 and 45 V
depending on sensitivity. The source temperature was 353 K. Mass
spectra were recorded with a scan range of m/z 200-1000 for
positive ions.
pH* Measurement. The pH* (pH meter reading without
correction for effects of D on glass electrode) values of NMR
samples in D2O were measured at ∼298 K directly in the NMR
tube, before and after the NMR spectra were recorded, using a
Corning 240 pH meter equipped with a microcombination electrode
calibrated with Aldrich buffer solutions at pH 4, 7, and 10.
Methods. Kinetics of Hydrolysis. The kinetics of the hydrolysis
of complexes 1BF4-5PF6 were monitored at various temperatures
by preparation of 0.8 mM solutions, prepared in methanol-d4 (5%
final), followed by subsequent dilution (1:20 v/v) with D2O (pH*
≈ 2, so that deprotonation of the aqua ligand is prevented) at the
required temperature. Samples were filtered and placed in the NMR
spectrometer preset at the desired temperature, and spectra were
recorded at various time intervals. Data, based on peak integrals,
were fitted to first-order kinetics to give the rate constant (k). The
Arrhenius activation energies (Ea), activation enthalpies (∆Hq), and
activation entropies (∆Sq) were determined from the slopes of the
Arrhenius and (intercepts of) Eyring plots. The kinetics of hydrolysis
and pKa* values of coordinated water of complexes 6PF6, 7PF6,
and 8PF6 were not determined because of the poor aqueous
solubility of these complexes.
1
equilibrate at 310 K before mixing. Their H NMR spectra were
recorded at 310 K after various time intervals.
Similarly, equal volumes of equilibrium solutions of 1BF4 and
5′-GMP in D2O were mixed at 298 K (final concentration 1.6 mM,
pH* 6.9), and 1H NMR spectra recorded at various time intervals.
To determine the OsII binding sites on 5′-GMP, a pH* titration
1
over the range 2-12 was monitored by H NMR spectroscopy,
and the chemical shifts of the H8 signals were plotted against pH*.
Solutions containing 1BF4 or the ruthenium analogue, [(η6-bip)-
Ru(en)Cl]PF6 (1R), and 1 mol equiv 9EtG were prepared in D2O
at various concentrations (1 mM, 500, 250, 100, and 50 µM), and
after incubation at 310 K for 24 h (to reach equilibrium), their 1H
NMR spectra were recorded at 298 K. Binding constants (log K)
for 9EtG were obtained from the slopes of plots of [bound 9EtG]/
[free 9EtG] versus [free metal complex], based on peak integrals.
In addition, a 1:1 stock solution of 1BF4 and 9EtG (1 mM), which
had already been allowed to reach equilibrium (24 h at 310 K),
was diluted to give solutions of 500, 250, 100, and 50 µM. These
1
were incubated at 310 K for a further 24 h before their H NMR
spectra were recorded at 298 K.
Cytotoxicity Toward A2780 and A549 Human Cancer Cells.
After they were plated, human ovarian A2780 cancer cells were
treated with OsII complexes on day 3, and human lung A549 cancer
cells were treated on day 2, at concentrations ranging from 0.1 to
100 µM. Solutions of the OsII complexes were made up in 0.125%
DMSO to assist dissolution. Cells were exposed to the complexes
for 24 h, washed, supplied with fresh medium, and allowed to grow
for three doubling times (72 h), and then the protein content
measured (proportional to cell survival) using the sulforhodamine
B (SRB) assay.31
Stability Studies. The stabilities of complexes 1PF6, 1BF4, and
2BF4 were investigated by preparation of 1 mM solutions in 5%
MeOD-d4/95% isotonic saline solution (150 mM NaCl) by dis-
solution of the complex in MeOD-d4, followed by rapid dilution
To study the aqueous stability of complex 9, a 2 mM solution
1
was prepared in D2O, and its H NMR spectrum was recorded
directly after sample preparation (∼15 min).
1
with the isotonic saline solution. H NMR spectra were recorded
Determination of pK*Values. For determination of pK*values
a
a
after various time intervals (10 min, 1 week, 2 weeks, and 2 months)
during which time the sample was stored at ambient temperature
in the dark.
(pKa values for D2O solutions), the pH* values of solutions of the
aqua complexes 2A-5A in D2O (formed in situ by hydrolysis of
the parent chloro complexes 2BF4-5PF6) were varied from pH*
The stability of 1BF4 in DMSO under various conditions was
also investigated. Two solutions (∼4 mM) of 1BF4 were prepared
1
∼2 to 10 by the addition of dilute NaOD and HNO3, and the H
1
in DMSO-d6, and their H NMR spectra were recorded directly
(26) Sheldrick, G. M.. SHELXL-97, Program for the refinement of crystal
structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(27) Altomare, A.; Cascarano, G.; Giacovazzo, G.; Guagliardi, A.; Burla,
M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435-
435.
(28) Beurskens, P. T.; Beurskens, G.; Bosman, W. P.; de Gelder, R.; Garcia-
Granda, S.; Gould, R. O.; Israel, R.; Smits, J. M. M. DIRDIF;
Crystallography Laboratory, University of Nijmegen: Nijmegen, The
Netherlands, 1996.
after sample preparation (<10 min). One sample was stored at
ambient temperature, stoppered in the dark for 24 h, and one was
exposed to heat, sonication, sunlight, and air for 24 h, before its
1H NMR spectrum was recorded.
(30) Krezel, A.; Bal, W. J. Inorg. Biochem. 2004, 98, 161-166.
(31) Skehan, P.; Storeng, R.; Scudiero, D.; Monks, A.; McMahon, J.;
Vistica, D.; Warren, J. T.; Bokesch, H.; Kenney, S.; Boyd, M. R. J.
Natl. Cancer Inst. 1990, 82, 1107-1112.
(29) Hwang, T. L.; Shaka, A. J. J. Magn. Reson., Ser. A 1995, 112, 275-
279.
4052 Inorganic Chemistry, Vol. 46, No. 10, 2007

Chloro Half-Sandwich Osmium(II) Complexes
Figure 1. Osmium(II) arene complexes containing N-donor ligands studied
in this work.
Results
Figure 2. X-ray structures and atom numbering schemes for complexes
(A) [(η6-bip)Os(en)Cl]PF6 1PF6, (B) [(η6-THA)Os(en)Cl]BF4 2BF4, (C)
[(η6-p-cym)Os(phen)Cl]PF6 5PF6, (D) [(η6-bip)Os(dppz)Cl]PF6 6PF6, (E)
[(η6-bip)Os(azpy-NMe2)Cl]PF6 7PF6, (F) [(η6-p-cym)Os(azpy-NMe2)Cl]-
PF6 8PF6, and (G) [(η6-bip)Os(NCCH3-N)Cl2] 9 (50% probability el-
lipsoids). The H atoms and counterions have been omitted for clarity.
Monofunctional (chlorido) osmium(II) arene complexes
containing neutral diamine or azopyridine N,N-chelating
ligands, as well as the bifunctional complex [(η6-bip)Os-
(NCCH3-N)Cl2] (9), Figure 1, were synthesized in good
yields via the Cl-bridged dimers, [(η6-arene)OsCl2]2 where
arene ) biphenyl (bip), tetrahydroanthracene (THA), or
p-cymene (p-cym). They were characterized by elemental
analysis or ESI-MS, NMR spectroscopy, and for seven
complexes, X-ray crystallography.
molecules crystallize with their arenes parallel, see Figure
S1. The distance between the THA ligands is 3.3 Å, and the
rings are separated by 4.13, 4.10, and 4.13 Å at dihedral
angles of 32, 27, and 32°, respectively. Further intermolecular
arene ring stacking is observed in the crystal structures of
complex 5PF6 and the twinned complex 6PF6, between one
of the bound pyridine rings and the related pyridine of a
second molecule (3.72 and 4.75 Å at dihedral angles of 37.0
and 40.0°, respectively), Figure S2. The biphenyl ligands in
the crystal structures of 6PF6 and 7PF6 are twisted by 33
and 32°, respectively. Arene ring stacking was also observed
in the crystal structure of 7PF6, between the coordinated
pyridine and the phenyl ring attached to the azo group of an
adjacent molecule (4.7 Å at a dihedral angle of 50°), but
not in that of complex 8PF6. Complex 9, in which the
biphenyl is twisted by 42°, crystallizes such that dimers link
four independent molecules through chloride ligand (Cl1)
of one molecule and the arene protons C8H81 and C5H51
of two others (C8‚‚‚Cl1 ) 3.513(5) Å and C5‚‚‚Cl1 3.680-
X-ray Crystal Structures. The X-ray crystal structures
of [(η6-bip)Os(en)Cl]PF6 (1PF6), [(η6-THA)Os(en)Cl]BF4
(2BF4), [(η6-p-cym)Os(phen)Cl]PF6 (5PF6), [(η6-p-cym)Os-
(dppz)Cl]PF6 (6PF6), [(η6-bip)Os(azpy-NMe2)Cl]PF6 (7PF6),
[(η6-p-cym)Os(azpy-NMe2)Cl]PF6 (8PF6), and [(η6-bip)Os-
(NCCH3-N)Cl2] (9) were determined. Their structures and
atom numbering schemes are shown in Figure 2. The
complexes adopt the expected pseudo-octahedral “three-leg
piano-stool” geometry with the osmium π-bonded to the
arene ligand (1.65-1.71 Å to centroid of ring), σ-bonded to
a chloride (2.41-2.37 Å) and the two nitrogen atoms of the
chelate (2.02-2.15 Å), which constitute the three legs of
the piano stool. Crystallographic data and selected bond
lengths and angles are shown in Tables 1 and 2.
-
-
(6) Å, respectively), Figure S3. The PF6 and BF4 coun-
terions in these crystal structures are extensively involved
in H-bonding to the chelated ligand or arene protons.
Kinetics of Hydrolysis. The rate of hydrolysis of complex
3PF6 at 298 K in aqueous solution (D2O) was monitored at
pH* values of 8, 6 and 2. The rate of hydrolysis, based on
peak integrals for p-cymene peaks, of the intact chloro
complex 3 (5.93 and 5.91 ppm) and the aqua complex 3A
Complex 1PF6 crystallizes with 2 independent molecules
in the unit cell. The biphenyl unit is twisted (by 27 or 43°),
and intermolecular arene ring stacking is observed between
the coordinated arene in two independent molecules (4.34
Å at a dihedral angle of 32.4°). In contrast, the coordinated
THA ligand in complex 2BF4 is almost planar, being slightly
bent upward (away from the legs) by 6.5°. Adjacent
Inorganic Chemistry, Vol. 46, No. 10, 2007 4053

Peacock et al.
Table 1. Crystallographic Data for [(η6-bip)Os(en)Cl]PF6, 1PF6, [(η6-THA)Os(en)Cl]BF4, 2BF4, [(η6-p-cym)Os(phen)Cl]PF6, 5PF6,
[(η6-p-cym)Os(dppz)Cl]PF6, 6PF6, [(η6-bip)Os(azpy-NMe2)Cl]PF6, 7PF6, [(η6-p-cym)Os(azpy-NMe2)Cl]PF6, 8PF6, and [(η6-bip)Os(NCCH3-N)Cl2], 9
1PF6
2BF4
5PF6
6PF6
7PF6
8PF6
9
formula
C14H18ClF6-
N2OsP
C16H22BClF4-
N2Os
C22H22ClF6-
N2OsP
C30H21ClF6-
N4OsP
C50H48Cl2F12-
N8Os2P2
C46H56Cl2F12-
N8Os2P2
C14H13Cl2NOs
MW
584.92
554.82
685.04
807.13
751.11
731.12
456.37
cryst
colorless block
yellow plate
yellow block
yellow lath
dark green prism
black block
yellow plate
description
cryst size
(mm)
0.08 × 0.08 × 0.34 0.09 × 0.15 × 0.44 0.14 × 0.16 × 0.36 0.13 × 0.16 × 0.61 0.17 × 0.22 × 0.26 0.33 × 0.52 × 0.55 0.17 × 0.42 × 0.81
λ (Å)
temp (K)
cryst
syst
space
group
a (Å)
b (Å)
0.71073
150
monoclinic
0.71073
150
monoclinic
0.71073
150
orthorhombic
0.71073
150
monoclinic,
twinned
P121/c1
0.71073
150
triclinic
0.71073
150
triclinic
0.71073
150
monoclinic
P121/c1
P121/n1
Pbca
P1h
P1h
P121/c1
19.5853(8)
9.0300(4)
23.2542(8)
90
112.608(2)
90
3796.6(3)
8
0.0913
0.0972
0.6814
12.722(3)
7.9298(16)
18.142(4)
90
105.28(3)
90
1765.6(7)
4
0.0307
0.0907
0.9498
12.7769(3)
13.3958(2)
26.6395(6)
90
90
90
4559.53(16)
8
0.048
0.066
0.599
11.8153(12)
8.7125(9)
26.175(3)
90
102.952(4)
90
2626.0(5)
4
0.043
8.1226(3)
10.0165(4)
17.3516(6)
101.105(2)
96.365(2)
109.271(2)
1284.28(9)
1
8.2549(3)
12.2787(4)
12.8556(5)
84.792(2)
82.980(2)
80.680(2)
1272.84(8)
1
10.0020(15)
7.009(1)
19.717(3)
90
102.447(2)
90
1349.8(3)
4
0.0285
0.0802
1.0434
c (Å)
R (deg)
â (deg)
γ (deg)
vol (Å3)
Z
2
R(Fo )
0.039
0.077
0.941
0.031
0.065
0.665
2
Rw(Fo )
GOF
0.045
1.056
Table 2. Selected Bond Lengths (Å) and Angles (deg) for [(η6-bip)Os(en)Cl]PF6, 1PF6, [(η6-THA)Os(en)Cl]BF4, 2BF4, [(η6-p-cym)Os(phen)Cl]PF6,
5PF6, [(η6-p-cym)Os(dppz)Cl]PF6, 6PF6, [(η6-bip)Os(azpy-NMe2)Cl]PF6, 7PF6, [(η6-p-cym)Os(azpy-NMe2)Cl]PF6, 8PF6, and
[(η6-bip)Os(NCCH3-N)Cl2], 9a
1PF6
2BF4
5PF6
6PF6
7PF6
8PF6
9
Os-C(arene)
2.232(7)
2.208(7)
2.195(7)
2.176(8)
2.169(7)
2.194(7)
2.136(6)
2.152(6)
2.3918(19)
2.156(5)
2.180(5)
2.224(4)
2.171(5)
2.218(5)
2.200(5)
2.127(4)
2.140(4)
2.4107(12)
2.196(3)
2.208(3)
2.187(3)
2.224(3)
2.194(4)
2.178(3)
2.091(3)
2.094(3)
2.4062(9)
2.215(8)
2.192(7)
2.195(5)
2.139(7)
2.173(8)
2.257(5)
2.117(5)
2.073(5)
2.3958(14)
2.220(4)
2.207(5)
2.189(5)
2.219(5)
2.211(5)
2.201(5)
2.055(3)
2.071(4)
2.3962(11)
2.278(4)
2.209(4)
2.214(3)
2.220(3)
2.175(3)
2.231(4)
2.055(3)
2.021(3)
2.3711(8)
2.178(4)
2.193(5)
2.181(4)
2.170(4)
2.183(5)
2.186(4)
2.057(4)
2.4139(12)
2.3976(11)
Os-N1
Os-X2
Os-Cl
N1-Os-X2
N1-Os-Cl
Cl-Os-X2
78.7(2)
83.47(17)
83.65(18)
78.48(16)
82.88(12)
81.87(11)
77.22(11)
83.17(8)
83.06(8)
76.98(17)
85.14(17)
84.69(17)
74.89(14)
85.10(10)
83.94(10)
74.99(12)
83.26(8)
87.17(8)
85.01(11)
84.00(11)
86.14(4)
a X2 ) second nitrogen donor group and N1 ) pyridine (and X2 ) azo) in 7PF6 and 8PF6. For complex 9, X2 ) Cl2.
(5.76 and 5.74 ppm) was rapid at pH* 8, with a half-life of
∼0.11 h. At pH* ∼6, the rate of hydrolysis was slower (t1/2
) 0.66 h), with less hydrolysis of the complex and at pH*
∼2, even slower. A fit to pseudo-first-order kinetics yielded
a rate constant of k ) 0.13 h-1 (t1/2 ) 5.3 h, Figure 3A) at
pH* 2.
pK* Determination. Peaks for coordinated arene-ring
a
protons of the aqua complexes, [(η6-arene)Os(NN)(OD2)]2+
2A-5A, shifted to high field with increase in pH*. The
chemical shifts were plotted against pH* (Figure S5), and
the data were fitted to the Henderson-Hasselbalch equation,
from which the pK* values of the coordinated water were
a
A comparison of the rates of hydrolysis of complexes
1BF4, 2BF4, 3PF6, 4PF6, and 5PF6 under acidic aqueous
conditions (D2O, pH* ∼2), to prevent deprotonation of the
coordinated water (Figure 3B and Table 3), showed that the
rates decrease in the order 3PF6 > 2BF4 > 1BF4 . 4PF6 ≈
5PF6. From the data obtained at various temperatures, the
Arrhenius activation energies (Ea), activation enthalpies
(∆Hq), and activation entropies (∆Sq) were determined,
Figure S4 and Table 3. The large negative ∆Sq values for
complexes 1BF4, 2BF4, and 3PF6 are notable.
determined. This yielded a value of 5.8 for complexes
containing two π-acceptor pyridine groups as N-donors (4A/
5A) and 6.3 for complexes containing a primary amine as
N-donor (2A/3A), see Table 4.
Guanine Adducts. New peaks, assignable to the adduct
[(η6-p-cym)Os(ampy)(9EtG)]2+ 3G, appeared in the 1H NMR
spectrum of 3PF6 and 9EtG (1:1, 0.5 mM) over time and
under biologically relevant conditions (pH* ∼7.4, 310 K),
Figure 5A. On the basis of peak integration, the reaction
half-life was ∼3.2 h (Figure 5B).
1
The H NMR spectrum of a solution of complex 9, [(η6-
Similarly, new peaks appeared over time in the H8 region
1
bip)Os(NCCH3)Cl2], in D2O (2 mM) showed peaks for a
number of minor unidentified species and major arene peaks
(∼60% of species) at 6.46, 6.25, and 6.01 ppm assignable
to the trihydroxo-bridged dimer, [(η6-bip)Os(µ-OD)3Os(η6-
bip)]+, Figure 4.19
in H NMR spectra of 1:1 solutions of 1BF4 and 5′-GMP
(1.6 mM, D2O, pH* 6.9, 298 K). After 30 h, ∼40% of the
5′-GMP had reacted, Figure 6A. The H8 peak for free 5′-
GMP (8.15 ppm, b), decreased in intensity with an ap-
proximate half-life of ∼8.5 h, and peaks for new species a
4054 Inorganic Chemistry, Vol. 46, No. 10, 2007

Chloro Half-Sandwich Osmium(II) Complexes
1
Figure 6. (A) Low-field region of the H NMR spectrum of an aqueous
solution of 1BF4 (1.6 mM) containing 1 mol equiv 5′-GMP after 30 h at
298 K. (B) Time dependence, based on 1H NMR peak integrals, of decay/
formation of peaks a, b, and c from the above reaction. (C) pH* titrations
of the reaction mixture. Plots of H8 chemical shift vs pH*. Solid curves
are computer fits giving pK*a values for a (5.94 and 8.09), b (2.55, 6.57,
and 9.86), and c (0.95 and 9.66). Peak assignments (see structures): a,
Os-N7GMP; b, free 5′-GMP; c, Os-O(PO3)GMP; x, possible dinuclear
complex Os-O(PO3)GMPN7-Os.
Figure 3. Time dependence for formation of the aqua complex 3A (based
on 1H NMR peak integrals) during hydrolysis of 3PF6 [(η6-p-cym)Os-
(ampy)Cl]PF6 (A) at 298 K in D2O at various pH* values, 8 (9), 6 (b),
and 2 (2), giving half-lifes of ∼0.11, 0.66 and 5.3 h, respectively. (B)
Hydrolysis of 3PF6 in acidic D2O (pH* ∼2) at 298 K (9), 304 K (b), 310
K (2), and 316 K (1).
Figure 4. Formation of hydroxo-bridged dimer [(η6-bip)Os(µ-OD)3Os-
(η6-bip)]+ from [(η6-bip)Os(NCCH3-N)Cl2] 9 in aqueous solution. Low-
1
field region of the H NMR spectrum, showing the coordinated biphenyl
Figure 7. Determination of the apparent formation constants for the
osmium and ruthenium analogues, [(η6-bip)Os(en)(9EtG)]2+ (1G) and [(η6-
bip)Ru(en)(9EtG)]2+ (1RG). The extent of binding was determined by the
integration of 1H NMR peaks (see Figure S5). From the slopes of the plots
of [(η6-bip)M(en)(9EtG)]2+/[free 9EtG] vs [(η6-bip)M(en)(OD2)]2+, the
binding constants log K ) 3.13 ( 0.03 and 4.78 ( 0.09 were determined
for OsII (1G) and RuII (1RG), respectively.
arene ring protons, of 9 in D2O within 15 min of sample preparation.
Assignments: x corresponds to the hydroxo-bridged dimer, which accounts
for ∼60% of {(η6-bip)Os}2+ species present in solution. Other peaks have
not been assigned.
Figure 8. Cytotoxicity of [(η6-bip)Os(en)Cl]PF6 (1PF6, s), [(η6-bip)Os-
(en)Cl]BF4 (1BF4, - -), and [(η6-THA)Os(en)Cl]BF4 (2BF4, - - -) toward
(A) human A549 lung cancer cells and (B) human A2780 ovarian cancer
cells. The IC50 values (concentrations that inhibit cell growth by 50%)
obtained from these curves are given in Table 5.
established by monitoring a pH* titration of the mixture by
1H NMR spectroscopy (Figure 6C). Peak b for free 5′-GMP
was assigned by addition of excess 5′-GMP to the mixture
and shifted with pH* to give three pK*a values of 2.55, 6.57,
and 9.86, corresponding to N7H, the phosphate backbone,
and N1H deprotonation, respectively. Peak a shifted with
1
Figure 5. (A) Low-field region of the H NMR spectrum of an aqueous
solution of 3PF6 (0.5 mM) after reactiion with 1 mol equiv 9EtG for 15 h
(pH* ≈ 7.4 and 310 K). Peak assignments: [(η6-p-cym)Os(ampy)(OD)]+
3A, H8 of free 9EtG G and the 9EtG bound product [(η6-p-cym)Os(ampy)-
(9EtG)]2+ 3G. (B) Time dependence, based on 1H NMR peak integrals,
for formation of [(η6-p-cym)Os(ampy)(9EtG)]2+ (3G) in the above reaction.
two associated pK*values of 5.94 and 8.09 (phosphate and
a
N1H deprotonation), consistent with N7-bound 5′-GMP, [(η6-
bip)Os(en)(5′-GMP-N7)]. Peak c shifted with two associated
(8.45 ppm) and c (8.03 ppm) appeared with half-lives of
∼8.5 and 9.0 h, respectively, with c forming before a (Figure
6B). Peaks were assigned and binding sites on 5′-GMP were
pK* values of 0.95 and 9.66 assignable to N7H and N1H
a
Inorganic Chemistry, Vol. 46, No. 10, 2007 4055

Peacock et al.
Table 3. Rate Data for the Aquation of Complexes 1BF4-5PF6 at Various Temperatures
T
(K)
k
t1/2
(h)
Ea
∆Hq
∆Sq
(h-1
)
(kJ mol-1
)
(kJ mol-1
)
(J K-1 mol-1
)
1BF4
2BF4
3PF6
4PF6
5PF6
298
310
318
333
298
310
318
333
298
304
310
316
323
328
333
343
318
328
333
347
0.046 ( 0.007
0.250 ( 0.001
0.615 ( 0.014
3.439 ( 0.151
0.082 ( 0.001
0.423 ( 0.009
1.205 ( 0.052
5.105 ( 0.232
0.131 ( 0.002
0.308 ( 0.007
0.545 ( 0.009
1.203 ( 0.037
0.150 ( 0.003
0.266 ( 0.007
0.643 ( 0.016
1.217 ( 0.039
0.073 ( 0.001
0.343 ( 0.006
0.609 ( 0.012
2.105 ( 0.125
15.11 ( 0.22
2.80 ( 0.08
1.13 ( 0.03
0.20 ( 0.01
8.47 ( 0.13
1.64 ( 0.03
0.58 ( 0.03
0.14 ( 0.01
5.29 ( 0.08
2.25 ( 0.05
1.27 ( 0.02
0.58 ( 0.02
4.63 ( 0.10
2.61 ( 0.07
1.08 ( 0.03
0.57 ( 0.02
9.45 ( 0.14
2.02 ( 0.04
1.14 ( 0.02
0.33 ( 0.02
101.2 ( 2.0
97.8 ( 4.1
94.3 ( 4.6
98.3 ( 14.1
105.4 ( 9.8
98.6 ( 2.0
95.1 ( 4.2
91.7 ( 4.6
95.5 ( 14.1
102.6 ( 9.8
-60.6 ( 6.3
-54.2 ( 13.3
-46.2 ( 15.2
-35.1 ( 42.5
-56.9 ( 29.6
Table 4. pK*and pKa Valuesa for the Deprotonation of the
Table 5. In Vitro Growth Inhibition of Human A549 Lung and A2780
a
Coordinated D2O in Hydrolyzed Complexes 1A-5A
Ovarian Cancer Cell Lines
complex
pK*
a
pKa
IC50 (µM)
[( η6-bip)Os(en)(OD2)]2+
1A
2A
3A
4A
5A
6.37b
6.36
6.32
5.81
5.80
6.34b
6.33
6.29
5.82
5.81
complex
A549
A2780
[(η6-THA)Os(en)(OD2)]2+
[(η6-p-cym)Os(ampy)(OD2)]2+
[(η6-p-cym)Os(bipy)(OD2)]2+
[(η6-p-cym)Os(phen)(OD2)]2+
1PF6
1BF4
2BF4
3PF6
4PF6
5PF6
6PF6
7PF6
8PF6
9
10
10
6.4
7.0
7.6
9.4
>100
>100
>100
a
>100
>100
>100
a pKa values calculated from pK*according to ref 30. b Ref 19.
a
b
deprotonation, and consistent with phosphate bound 5′-GMP,
[(η6-bip)Os(en)(5′-GMP-O)]. Peak x may be the result of a
dinuclear complex in which two {(η6-bip)Os(en)}2+ frag-
ments bind to N7 and phosphate of the same 5′-GMP unit.32
Solutions of 1BF4 or the RuII analogue [(η6-bip)Ru(en)-
Cl]PF6 1R (1 mM to 50 µM) containing 1 mol equiv 9EtG
a <40 µM, see text. <2 µM, see text.
b
Anticancer Activity. The cytotoxicity of complexes
1BF4-9 toward the human lung A549 cancer cell line and
of complexes 1BF4, 1PF6, 2BF4, and 6PF6 toward the human
ovarian A2780 cancer cell line was investigated.33 Complexes
3PF6-5PF6 and 7PF6-9 were nontoxic up to the highest
test concentration (50 µM). The IC50 (concentration inhibiting
cell growth by 50%) values are therefore likely to be >100
µM, and the complexes are deemed inactive. However,
complexes 1BF4, 1PF6, 2BF4, and 6PF6 were active (Figure
8 and Table 5). Complexes 1BF4/PF6 and 2BF4 displayed
IC50 values of 6-10 µM. The IC50 values for complex 6PF6
were estimated to be <40 µM for A549 cells and <2 µM
for A2780 cells. Tests on 6PF6 (as well as 7PF6 and 8PF6)
were complicated by its poor solubility in water. It was
initially dissolved in DMSO and then rapidly diluted with
water but could only be administered to the cells as a
suspension34 to maintain a low exposure of the cells to
DMSO (0.25%).
1
were incubated at 310 K, and their H NMR spectra were
recorded. Peaks for free 9EtG (H8, 8.10 ppm) increased in
intensity with a decrease in metal concentration, Figure S6.
Species distribution plots based on peak integrals, Figure 7,
yielded apparent formation constants of log K ) 3.13 ( 0.03
and 4.78 ( 0.09 for the formation of [(η6-bip)Os(en)-
(9EtG)]2+ (1G) and [(η6-bip)Ru(en)(9EtG)]2+ (1RG), re-
spectively.
Similar solutions were prepared by dilution of an equi-
librium stock solution of 1BF4 and 9EtG (1 mM) to give
500-50 µM solutions, which were incubated at 310 K for
24 h before their 1H NMR spectra were recorded. Based on
1H NMR peak integration of H8 peaks for bound 9EtG (8.21
ppm) and free 9EtG (8.10 ppm), no significant change in
ratio occurred with decrease in concentration (Figure S7).
Binding of 1 to 9EtG was further confirmed by ESI-MS.
An equilibrium NMR sample (containing equal amounts of
1BF4 and 9EtG), gave peaks at m/z 294.1 and 585.7
consistent with {(η6-bip)Os(en)(9EtG)}2+ (calcd m/z 292.6)
and {(η6-bip)Os(en)(9EtG)-H}+ (calcd m/z 584.2), respec-
tively.
(33) The IC50 values for cisplatin were 2.6 and 0.5 µM for these human
A549 lung and A2780 ovarian cancer cell lines, respectively, as
reported previously: S. M. Guichard, R. Else, E. Reid, B. Zeitlin, R.
Aird, M. Muir, M. Dodds, H. Fiebig, P. J. Sadler, D. I. Jodrell Biochem.
Pharmacol. 2006, 71, 408-415.
(34) Drugs are sometimes administered as suspensions, although usually
in peanut oil and not as a suspension in water as used here. Cells
were washed after removal of complex, in an attempt to remove
unreacted particles.
(32) Chen, H.; Parkinson, J. A.; Morris, R. E.; Sadler, P. J. J. Am. Chem.
Soc. 2003, 125, 173-186.
4056 Inorganic Chemistry, Vol. 46, No. 10, 2007

Chloro Half-Sandwich Osmium(II) Complexes
Stability Studies. Since complex 1BF4 had previously
been found to have a low cytotoxicity,19 its stability during
the sample preparation process was investigated. This initially
involved dissolution in DMSO and subsequent dilution with
water. Solutions of 1BF4 in DMSO were prepared; one
sample was stored stoppered at ambient temperature in the
dark, and the second sample was exposed to the air, heat,
sonication, and light, all of which may have been encountered
during the earlier sample preparation. Directly after sample
preparation the solution was clear and pale yellow color.
After 24 h, the first sample appeared unchanged, and no
the flexible extended arene ring system of bip may mean
that intercalation via both dppz and bip can contribute to
the cytotoxicity of 6PF6. Finally, the crystal structures of
the highly colored complexes containing azopyridine type
ligands in 7PF6 and 8PF6, also show π-π stacking. The
crystal structure of the ruthenium analogue of 8PF6 has
previously been determined and again appears to be structur-
ally similar.36 These appear to be the first reported X-ray
crystal structures of osmium arene complexes, containing
coordinated dppz or azopyridine ligands.
Rates of Hydrolysis and Acidity of Coordinated Water.
The incorporation of a chelating ligand into these OsII arene
complexes is crucial for maintaining their aqueous stability.
Complex 9, which contains only monodentate non-arene
ligands and a single donor nitrogen ligand, is largely
deactivated in water to form predominantly the inert hydroxo-
bridged dimer, [(η6-bip)Os(µ-OD)3Os(η6-bip)]+ (Figure 4).
The rate of hydrolysis of complex 3, [(η6-p-cym)Os-
(ampy)Cl]+, was found to be markedly pH*-dependent,
increasing with increase in pH*. Two equilibria are involved
(eq 1), the first of which is hydrolysis and the second is
deprotonation of coordinated water. Because of the relatively
1
additional peaks were present in the H NMR spectrum.
However, the second sample (exposed to extreme conditions)
was now dark red, and new peaks were seen in the 1H NMR
spectrum (Figure S8).
1H NMR spectra of 1 mM solutions of the active
complexes, 1PF6/BF4 and 2BF4 in isotonic saline solution
(150 mM NaCl) were recorded directly after sample prepara-
tion and after storage at ambient temperature in the dark for
1 week, 2 weeks, and 2 months. Small peaks for the aqua
complexes (<10%) were present. The only changes in the
1H NMR spectra were loss of peaks assigned as the NH
protons of en because of the exchange with deuterium
(Figure S9).
low pK* values of water coordinated to osmium in these
a
complexes, the hydroxo species is formed even at neutral
pH*. This shifts the equilibrium to the right, effectively
increasing the rate of hydrolysis. To measure the rate of the
hydrolysis step, alone, the pH* was lowered.
1
The H NMR spectrum of a 50 µM solution of 3PF6 in
D2O, ∼10 min after sample preparation, gave rise to
p-cymene arene peaks predominantly for the aqua complex
3A (78%) with minor peaks present for the intact chloro
species 3 (12%). No new peaks appeared after incubation
of the sample at 310 K for 24 h (Figure S10).
+H2O
+
{Os-Cl}+ y+Cl-z {Os-OH2}
y
-H z {Os-OH}+
(1)
2+
+H+
Discussion
The strong dependence of the rate of hydrolysis of 3PF6
on pH*, and the presence of exchangeable NH protons on
the chelated ligand, suggest that a conjugate base mechanism
may be involved.
Eight new osmium arene complexes with the general
formula [(η6-arene)Os(NN)Cl]+, where NN ) bidentate
nitrogen-chelating ligands, 1PF6-8PF6 and [(η6-bip)Os-
(NCCH3)Cl2] (9) were synthesized via the Cl-bridged dimers,
[(η6-arene)OsCl2]2. Complex 1BF4, [(η6-bip)Os(en)Cl]BF4,
had been synthesized previously,19 but an improved synthesis
is reported here.
X-ray Crystal Structures. We determined the X-ray
crystal structure of the PF6- salt of complex 1 for comparison
with that of the BPh4- salt which we reported previously.19
They are structurally very similar. The X-ray structure of
complex 2BF4 appears to be the first of an osmium complex
bound to tetrahydroanthracene (THA). In 2BF4, the arene,
THA, is planar, as it is in the ruthenium analogue of 2, and
it has been proposed that the increase in cytotoxicity of [(η6-
arene)Ru(en)Cl]+, on replacing bip with THA may be partly
the result of the ability of coordinated THA to intercalate
into DNA.22 The ability of the THA ligand to π-π stack is
evident in the crystal structure of the osmium complex 2BF4,
Figure S1; π-π stacking is also evident in the crystal
structures of complexes 5PF6 and 6PF6, especially for 6PF6
involving the dppz ligand, which has been extensively studied
as a DNA intercalator.15,35 This feature in combination with
The rate of hydrolysis of complexes 1BF4 and 2BF4
containing a chelating ligand with two primary amine donors,
en, is faster than that for complexes containing two π-ac-
ceptor pyridine groups, bipy and phen, (∼5.5 and 8 times,
respectively). This is attributable to the reduced electron
density on OsII. Intriguingly, complex 3PF6, which contains
the unsymmetrical chelated ligand, 2-picolylamine, hydro-
lyzed the most rapidly, despite replacement of one of the
amine groups with a pyridine. This is contrary to the expected
trend and that observed for square-planar platinum com-
plexes.10 The rate of hydrolysis of complex 3PF6 was also
the most sensitive to changes in pH*. A direct comparison
between complexes 1BF4-2BF4 and 3PF6-5PF6 cannot be
made because of the different nature of the arenes, with
p-cymene being more electron-rich than either biphenyl (bip)
or tetrahydroanthracene (THA). Complex 2BF4, containing
THA as the arene, hydrolyzed almost twice as fast as
complex 1BF4, in which the arene is bip, in agreement with
observations made for the ruthenium analogues.37
(36) Dougan, S. J.; Melchart, M.; Habtemariam, A.; Parsons, S.; Sadler,
P. J. Inorg. Chem. 2006, 45, 10882-10894.
(37) Wang, F.; Chen, H.; Parsons, S.; Oswald, I. D. H.; Davidson, J. E.;
Sadler, P. J. Chem.sEur. J. 2003, 9, 5810-5820.
(35) Frodl, A.; Herebian, D.; Sheldrick, W. S. J. Chem. Soc., Dalton Trans.
2002, 19, 3664-3673.
Inorganic Chemistry, Vol. 46, No. 10, 2007 4057

Peacock et al.
Osmium complexes are generally more inert than those
of their lighter congener ruthenium, and the data reported
here show that complexes 1BF4 and 2BF4 hydrolyze 100
times more slowly than their ruthenium analogues, [(η6-bip/
THA)Ru(en)Cl]+.37 Hydrolysis of the osmium complex [(η6-
bz)Os(en)Cl]+ and anation of the aqua adduct [(η6-bz)Os-
(en)(OH2)]2+ have also been reported to occur ∼100 times
more slowly (khydro ) 1.2 × 10-5 s-1 and kCl ) 0.13 × 10-2
M-1 s-1, 298 K)38 than for the related ruthenium complexes,
[(η6-bip)Ru(en)(Cl/OH2)]+/2+ (khydro ) 1.3 × 10-3 s-1 and
kCl ) 0.15 M-1 s-1, 298 K).37 The activation parameters for
hydrolysis of the complexes [(η6-bip)M(en)Cl]+, where M
) Os or Ru, can now be compared.37 As expected the
Arrhenius activation energy, Ea, for hydrolysis of the osmium
complex is higher than that for its ruthenium analogue (∆Ea
≈ 26 kJ mol-1) as is the enthalpy of activation, ∆Hq, (∆∆Hq
≈ 26 kJ mol-1) with similar entropies of activation, ∆Sq.
The ∆Ea of 26 kJ mol-1 is in good agreement with reported
theoretical calculations, which give a ∆Ea value of 20 kJ
mol-1 for the hydrolysis of these OsII and RuII complexes.20,39
The large negative ∆Sq values for the hydrolysis of osmium
complexes 1BF4-3PF6 are indicative of an associative
process (Table 3). The errors associated with the ∆Sq values
for the hydrolysis of complexes 4PF6 and 5PF6 are too large
to allow comparisons.
nucleotide, one to the N7 and the other to the phosphate
group, [(η6-bip)Os(en)(OP-5′-GMP-N7)Os(en)(η6-bip)]2+; how-
ever its concentration was too low to follow by 1H NMR in
the pH* titration of the reaction mixture. The time depen-
dence of the species distribution (Figure 6B) within the first
5 h suggests that phosphate binding occurs first, followed
by N7 binding. This would be consistent with the migration
mechanism proposed by Chen et al.32 for the ruthenium
analogue 1R. The migration of OsII from the phosphate to
N7 of 5′-GMP appears to be slower than that for the
ruthenium analog. After 22 h at 298 K, all of the RuII was
bound to the N7 of 5′-GMP,32 compared to 66% of the bound
GMP for OsII (Figure 6B).
The binding constant for formation of the OsII adduct with
9EtG [(η6- bip)Os(en)(9EtG)]+ (1G) of log K ) 3.13 ( 0.03
is an order of magnitude lower than that for the RuII analog
(log K ) 4.78 ( 0.09), Figure 7. Intriguingly, the stronger
binding to Ru(II) is opposite to that observed for the maltolate
(mal) complexes [(η6-p-cym)M(mal)(9EtG)]+, for which log
K ) 4.41 (Os) and 3.87 (Ru).20 Despite the lower formation
constant compared to RuII, the kinetic stability of the OsII
complex 1G is high: 9EtG did not readily dissociate, even
in 50 µM solutions incubated at 310 K for 24 h. This suggests
that once formed on DNA (or RNA), {(η6-bip)Os(en)}2+
adducts with G bases may persist long enough to interfere
with downstream DNA processing (including protein rec-
ognition and excision/repair).
Another indication of the electron density on osmium and
of the Os-O bond strength, is the acidity of the coordinated
water. When the pK*values of the aqua complexes 1A, 2A,
a
Cancer Cell Cytotoxicity. We reported previously that
complex 1BF4, [(η6-bip)Os(en)Cl]BF4, was inactive against
the human ovarian A2780 cancer cell line (IC50 >100 µM).19
Because the chemical properties of the complex (rate of
hydrolysis and binding to guanine) appeared to be compatible
with activity, we reinvestigated its cytotoxicity. Apart from
the potential variability of cell culture media (especially fetal
calf serum) and the cell population, a possible explanation
of the previous finding of inactivity (IC50 > 100 µM) and
the present finding of activity can be related to partial
decomposition of the complex in stock solutions prepared
in DMSO. We have shown here that solutions of 1BF4 in
DMSO can decompose after sonication, heating, and a longer
term of standing in air and direct sunlight, as is evident from
color change from yellow to dark red (Figure S8A/B).
However, such solutions are stable when stoppered and stored
at ambient temperature in the dark, and then exhibit good
activity, as does the PF6 salt (1PF6), giving IC50 values of
7-10 µM against the human ovarian A2780 and lung A549
cancer cell lines (Table 5). The change of the arene from
biphenyl in 1BF4 to the extended tetrahydroanthracene gave
rise to a similar potency (complex 2BF4, IC50 values of 6-9
µM, Table 5), whereas for the ruthenium analogues this
change in arene increases the activity by 10-fold.41 Further
work will be required to investigate whether Os-bound
extended arenes can intercalate into DNA in a manner similar
to Ru-bound arenes.42
3A, 4A, and 5A (Table 4) are compared, it is evident that
water coordinated to osmium bound to two π-acceptor groups
such as pyridines is more acidic (pK*) 5.8) than when one
a
of the nitrogen donors is a primary amine (pKa*) 6.3). Water
bound to ruthenium analogues is less acidic but shows the
same trend: pKa values of 7.71 and 7.2 for [(η6-bip)Ru(en)-
(OH2)]2+ and [(η6-p-cym)Ru(bipy)(OH2)]2+, respectively.37,40
Binding to Guanine. The kinetics for the reactions of
3PF6 and 1BF4 with 9EtG and 5′-GMP, respectively, were
investigated. In addition, the stability constants of [(η6-bip)-
Os(en)(9EtG)]2+ 1G and its ruthenium analogue [(η6-bip)-
Ru(en)(9EtG)]2+ 1RG were determined.
The reaction of the bip/en complex 1BF4 with 5′-GMP at
298 K resulted in the formation of three OsII-GMP adducts
1
(2 major and 1 very minor) as judged by the H8 H NMR
signals. From reported work on the binding of the ruthenium
analogue [(η6-bip)Ru(en)Cl]PF6 1R to 5′-GMP32 and from
pH* titrations of the products, the major species can be
assigned as N7-bound 5′-GMP, [(η6-bip)Os(en)(5′-GMP-
N7)], with the phosphate-bound 5′-GMP adduct [(η6-bip)-
Os(en)(5′-GMP-OP)] also being formed in significant amounts
(ratio of 2:1 for N7/phosphate-bound adducts, after 30 h at
298 K). The minor product (labeled x in Figure 6) may be
a dinuclear adduct, with two osmium units bound to the
(38) Hung, Y.; Kung, W.; Taube, H. Inorg. Chem. 1981, 20, 457-463.
(39) Wang, F.; Habtemariam, A.; Geer, E. P. L. v. d.; Fernandez, R.;
Melchart, M.; Deeth, R. J.; Aird, R.; Guichard, S.; Fabbiani, F. P. A.;
Lozano-Casal, P.; Oswald, I. D. H.; Jodrell, D. I.; Parsons, S.; Sadler,
P. J. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 18269-18274.
(40) Dadci, L.; Elias, H.; Frey, U.; Hornig, A.; Koelle, U.; Merbach, A.
E.; Paulus, H.; Schneider, J. S. Inorg. Chem. 1995, 34, 306-315.
(41) 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.
4058 Inorganic Chemistry, Vol. 46, No. 10, 2007

Chloro Half-Sandwich Osmium(II) Complexes
The p-cym/ampy complex 3PF6 was non-cytotoxic toward
A549 cells despite its similar rate of hydrolysis to the bip/
en and p-cym/en complexes 1PF6/BF4 and 2BF4, stability
at micromolar concentrations (50 µM), and the ability to react
with 9EtG under biologically relevant conditions with an
associated half-life of 3.2 h. The reasons for the inactivity
are therefore not clear. However the other complexes tested
which have pyridyl ligands (4PF6 and 5PF6) were also
inactive, as are some of the RuII arenes with unsubstituted
pyridyl ligands.43
The inactivity of complexes 4PF6, 5PF6, 7PF6, and 8PF6
can be correlated with their slow kinetics of hydrolysis and
higher acidity of coordinated water. Complex 9, which
contains only monodentate non-arene ligands, is deactivated
in water by formation of the inert hydroxo-bridged dimer.
The dppz complex 6PF6 is probably too substitution-inert
to bind effectively to G bases on DNA, but both the arene
(bip) and chelating ligand (dppz) have the ability to
intercalate into DNA15,35 (vide supra), which may contribute
to the mechanism of cytotoxicity of 6PF6 against both the
A549 and A2780 cancer cell lines.
chelator hydrolyze relatively rapidly (t1/2 ) 1.6 h at 310 K
for complex 2BF4), the introduction of π-acceptor pyridines
as the N-donors, considerably slows down hydrolysis (e.g.,
by an order of magnitude for complex 5PF6), and a large
window of reactivity can therefore be accessed. The intro-
duction of an π-acceptor N-donor also increases the acidity
of the aqua adduct. The en complexes [(η6-bip)Os(en)Cl]-
PF6/BF4 and [(η6-THA)Os(en)Cl]BF4 are as cytotoxic to
human A549 lung and A2780 ovarian cancer cells as the
drug carboplatin, whereas those complexes containing a
pyridine N-donor were inactive. An exception is the dppz
complex with biphenyl as the arene [(η6-bip)Os(dppz)Cl]-
PF6 (6PF6), which can also act as a DNA intercalator.
Although {(η6-bip)Os(en)}2+ adducts with G nucleobases
appear to be less thermodynamically stable than similar
adducts thought to be responsible for the anticancer activity
of the RuII analogues, once formed the OsII adducts have
some kinetic stability with respect to dissociation. It is unclear
why the complex [(η6-p-cym)Os(ampy)Cl]PF6 (3PF6) ex-
hibits low cytotoxicity, since it hydrolyzes at a similar rate,
has a similar pK*for its aqua adduct, and a similar rate of
a
binding to G nucleobases as the active complexes 1PF6/BF4
and 2BF4.
Despite the instability of complex 1BF4 in DMSO,
solutions of the active complexes 1PF6/BF4 and 2BF4 were
found to be stable in isotonic saline solutions (150 mM NaCl)
stored at ambient temperature in the dark over a period of 2
months. This suggests that drug formulation in aqueous
solution would be feasible for these complexes.
Thus we have shown that half-sandwich OsII arene
complexes containing N,N-chelating ligands can be designed
which have cancer cell cytotoxicity comparable to their RuII
analogs, but the reactivities are 100 times less. Such a range
of kinetic effects can be useful for balancing cytotoxicity
and unwanted side-effects of anticancer drugs, as illustrated
by the clinical profiles of cisplatin and the less-labile second
generation drug carboplatin. Some of the OsII complexes
prepared here appear to be stable under conditions relevant
to drug formulation (e.g., isotonic saline) and are interesting
candidates for further investigation.
Conclusions
Although organometallic complexes offer a versatile
platform for the design of new medicines, and of anticancer
agents in particular, their aqueous solution chemistry has
been relatively little explored. An understanding of how
ligand design can be used to control their redox and ligand
substitution reactions under conditions of biological relevance
is vital for the establishment of structure-activity relation-
ships and for understanding their mechanisms of action.
Although osmium complexes are often considered to be
kinetically inert, we have shown that the reactivity of half-
sandwich OsII arene complexes depends strongly on the other
ligands in the complex, which can play both steric and
electronic roles. Here we have shown that although [(η6-
arene)Os(N,N)Cl]+ complexes containing en as the N,N-
Acknowledgment. We thank the EPSRC and The Uni-
versity of Edinburgh (studentship for A.F.A.P.), Dr. Michael
Melchart (Edinburgh) for the gift of the dppz ligand, Rhona
E. Aird and Professor Duncan Jodrell (Western General
Hospital/Cancer Research U.K. Centre) for advice and
assistance with cell culture, and members of EC COST
groups D20 and D39 for stimulating discussions.
Supporting Information Available: Details of the crystal-
lographic data (Figures S1-S3), aqueous and nucleobase binding
studies (Figures S4-S7) and stability studies (Figure S8-S10), and
X-ray crystallographic data in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
(42) Liu, H.-K.; Berners-Price, S. J.; Wang, F.; Parkinson, J. A.; Xu, J.;
Bella, J.; Sadler, P. J. Angew. Chem., Int. Ed. 2006, 45, 8153-8156.
(43) Habtemariam, A.; Melchart, M.; Ferna´ndez, R.; Parsons, S.; Oswald,
I.; Parkin, A.; Fabbiani, F. P. A.; Davidson, J.; Dawson, A.; Aird, R.
E.; Jodrell, D. I.; Sadler, P. J. J. Med. Chem. 2006, 49, 6858-6868.
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