Osmium complexes arene

We have recently extended our interest to the analogous halfsandwich osmium-arene complexes and are exploring the chemical and biological properties of [Os(r 6-arene)(XY)Z]ra 1 complexes (Fig. 25) (105). Both the aqueous chemistry and the biological activity of osmium complexes have been little studied. Third-row transition metals are usually considered to be more inert than those of the first and second rows. Similar to the five orders of magnitude decrease in substitution rates of Pt(II) complexes compared to Pd(II), the [Os(ri6-arene)(L)X]"+ complexes were expected to display rather different kinetics than their Ru(II)-arene analogs. A few other reports on the anticancer activity of osmium-arene complexes have also appeared recently (106-108). 

Fig. 26. Bar charts relate the influence of different chelates in [Os(r 6-arene)Cl(XY)]n+ (XY = NJV- N,0- or 0,0-) on cytotoxicity, stability with respect to hydroxido-dimer formation, hydrolysis rates, and pKa of the aqua adduct for osmium-arene complexes. Shading indicates the range in observed values. Adapted from Ref. (III).

In line with expectations of kinetic inertness for third-row transition metals, little interest has been vested in the development of osmium anticancer drugs, as ligand-exchange rates did not seem favorable on the timescale of cellular processes. Our work, however, shows that the kinetic lability of such complexes can be timed to such extent that anticancer activity comes within range. We have demonstrated how rational chemical design can thus be applied to osmium-arene complexes resulting in specific... 

The University of Edinburgh (former employer of Peter J. Sadler) has filed patent applications relating to the ruthenium-arene, and platinum diazido complexes and University of Warwick for osmium-arene complexes under study in the Peter J. Sadler laboratory. 

The crystal structure of (ij4-cyclooctatetraene)(hexamethylbenzene)ruthenium (16) indicates bonding as a tetrahapto ligand60. For this complex and similar iron-, ruthenium- and osmium-(ij4-cyclooctatetraene)(arene) complexes, their XH and 13C NMR spectra exhibit only a single signal for the cyclooctatetraene ligand at temperatures as low as —145 °C. Using this temperature, the barrier-to-metal migration is estimated to be <6.6 kcal mol 1. 

Aquocoppert I) complexes, 17 117, 118 Aquoxides, 5 215 Archibald technique, 19 256 Arenecyclopentadiene-iron complexes, 4 92 Arenediazo complexes, from oxomolybdenum complexes and hydrazines, 23 225-226 h -Arene ligands, osmium, 37 244 Arenes... 

C-M bond addition, for C-C bond formation, 10, 403-491 iridium additions, 10, 456 nickel additions, 10, 463 niobium additions, 10, 427 osmium additions, 10, 445 palladium additions, 10, 468 rhodium additions, 10, 455 ruthenium additions, 10, 444 Sc and Y additions, 10, 405 tantalum additions, 10, 429 titanium additions, 10, 421 vanadium additions, 10, 426 zirconium additions, 10, 424 Carbon-oxygen bond formation via alkyne hydration, 10, 678 for aryl and alkenyl ethers, 10, 650 via cobalt-mediated propargylic etherification, 10, 665 Cu-mediated, with borons, 9, 219 cycloetherification, 10, 673 etherification, 10, 669, 10, 685 via hydro- and alkylative alkoxylation, 10, 683 via inter- andd intramolecular hydroalkoxylation, 10, 672 via metal vinylidenes, 10, 676 via SnI and S Z processes, 10, 684 via transition metal rc-arene complexes, 10, 685 via transition metal-mediated etherification, overview,... 

Consequently, organometallic ruthenium(II) and osmium(II) arene complexes have recently attracted interest as anticancer agents [51]. The presence of a 7i-bonded arene in Ru11 (and Os11) complexes can have a dramatic influence on their chemical reactivity. There is a delicate balance between electron donation from the arene into the empty Ru 4d orbitals and back-donation from the filled 4d6 orbitals into vacant arene orbitals. This is influenced by the donor-acceptor power of the arene (e.g. hexamethylbenzene as a strong donor, in contrast to biphenyl which may act as acceptor) and by the other ligands on Ru11 which can influence the... 

Once the ruthenium or osmium arene organometallic complex is activated with the formation of the aqua species, [Ru/Os(r 6-arene)(OH2)(X)(Y)] (Chart 3), the metal becomes a potential centre for nucleophilic attack by biomolecules. The binding of Run/Osn arene complexes to nucleobases is of special interest, since DNA could be the ultimate target for this class of organometallic complexes. A number of studies have confirmed this postulate [86, 87] and investigated in detail such interactions [53, 54, 72, 88-93]. DNA interactions of Ru and Os arene complexes have recently been reviewed [94],... 

Keppler et al. have synthesised Ru11 and Os11 arene complexes with paullones as ligands to confer solubility on these otherwise insoluble cyclin-dependent kinase (CDK) inhibitors [152]. No dramatic differences between the ruthenium and the osmium complexes were found in the IC50 values against A549, CHI and SW480 cancer cell lines. 

Peacock AFA, Habtemariam A, Fernandez R, Walland V, Fabbiani FPA, Parsons S, Aird RE, Jodrell DI, Sadler PJ (2006) Tuning the reactivity of osmium(II) and ruthenium(II) arene complexes under physiological conditions. J Am Chem Soc 128 1739-1748... 

Peacock AFA, Parsons S, Sadler PJ (2007) Tuning the hydrolytic aqueous chemistry of osmium arene complexes with N,O-chelating ligands to achieve cancer cell cytotoxicity. J Am Chem Soc 129 3348-3357... 

Fig. 7 Examples of ruthenium(II)-arene and osmium(II)-arene paullone complexes with high in vitro anticancer activity...

Schmid WF, John RO, Arion VB, Jakupec MA, Keppler BK (2007) Highly antiproliferative ruthenium(II) and osmium(II) arene complexes with paullone-derived ligands. Organome-tallics 26 6643-6652... 

Two arene cyclopentadienyl osmium complexes of type 191 have been prepared in moderate yields (17-43%) by reacting precursor 144 with thallium cylopentadienide in acetonitrile (73). 

The synthesis and reactivity of arene cyclooctatetraene osmium(O) complexes of type 216 (arene = benzene, mesitylene) (13,132) and arene dicyclopentadiene osmium(O) complexes 212 (arene = mesitylene) (102) have been discussed previously (Section III,A,2). Arene osmium(O) complexes of type Os( 7j6-C6H6)(PR3) (L) (230) have been prepared by reduction... 

Protonation of 322 with tetrafluoroboric acid in diethyl ether gives the cyclohexadienyl derivative 325 in 70% yield. Treatment of 325 with lithium aluminum hydride yields the biscyclohexadienyl osmium(II) complex 326. Treatment of 322 with PMe3 at 60°C gives the hydridophenyl osmium-(II) complex 181, rather than the expected arene bistrimethylphosphine osmium(O) compound, via intramolecular C—H bond activation of the benzene ligand (192,193) (Scheme 38). Compound 181 as well as the analogous ruthenium complex (92) have also been obtained directly by cocondensation of osmium or ruthenium atoms with benzene and tri-methylphosphine (62) [Eq. (44)]. 

Osmium forms a wide variety of alkyl and aryl complexes including homoleptic alkyl and aryl complexes and many complexes with ancillary carbonyl (see Carbonyl Complexes of the Transition Metals), cyclopentadienyl (see Cyclopenta-dienyl), arene (see Arene Complexes), and alkene ligands (see Alkene Complexes). It forms stronger bonds to carbon and other ligands than do the lighter elements of the triad. Because of this, most reactions of alkyl and aryl osmium complexes are slower than the reactions of the corresponding ruthenium complexes. However, because osmium is more stable in higher oxidation states, the oxidative addition (see Oxidative Addition) of C-H bonds is favored for osmium complexes. The rate of oxidative addition reactions decreases in the order Os > Ru Fe. 

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