Aqua complexes, osmium

No unsubstituted aqua complexes of osmium seem to have been reported, though we have already referred to a number of species in which H20 is a ligand. Species such as [0s(H20)6]3+ and [0s(H20)6]4+ would be expected to exist. 

There has also been little work on osmium hydroxo complexes apart from the now well-characterized czs-[0s04(0H)2]2 (p. 592), tra j-[0s02(0H)4]2 (p. 581) and the two /i-hydroxo species structurally characterized, M[0s2(0H)08] (M = Rb, Cs) (p. 596). The [Os(OH)6]2- species should certainly exist but does not seem to have been mentioned in the literature even [Os(OH)6]3 might be expected to be stable in the absence of oxygen. Other hydroxo (and aqua) complexes are considered in sections dealing with the other ligands present. 

Osmium and ruthenium polypyridine complexes initially received much attention from Dwyer and coworkers because the M(II), M(III), and M(IV) oxidation states are substitution inert.Interest in them has been renewed because of their photochemical reactions and the role they play in the study of reactions of coordinated ligands " and of mixed valence ions and in the preparation of electroactive polymer films. The aqua complexes " also have important potential applications in the selective oxidation of organic molecules and water. We found that trifluoromethanesulfonato (triflato) complexes are convenient synthetic intermediates in the preparation of aqua and oxo species, " and we describe the syntheses of the cii-bis(2,2 -bipyridine) complexes here. 

The (2,2 -bipyridine-A(,A( )(2,2 6 2"-terpyridine-A(,yV, yV"),(bpy)(trpy) complexes of ruthenium and osmium, like the bis(2,2 -bipyridine-A(,N ) series, were first studied by Dwyer and coworkers " and are interesting because more than one oxidation state is substitution inert. More recently, these (bpy)(trpy) species have generated interest because the M(IV) oxo complexes are useful oxidants of organic molecules. " The (bpy)(trpy) series of complexes have also proved useful in the study of the redox chemistry of coordinated ligands. The (bpy)(trpy) and bis(bpy) complexes containing the coordinated trifluoromethanesulfonato (triflato) ligands are useful precursors to a variety of complexes. The syntheses of these species and the aqua complexes derived from them are reported here. 

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).

The inert hydroxo-bridged species were also a product of (very fast) hydrolysis of p-cymene osmium complexes with glycinate, L-alaninate, a-aminobutyrate and p-alaninate. However, complexes with picolinate as the chelating ligand, [Os(r 6-/> cym)Cl(pic)] 8 and [Os(r 6-biph)Cl(pic)] 9, with pyridine as /V-donor and carboxy-late as O-donor, hydrolyzed with half-lives of 0.20 and 0.52 h (298 K), and aqua adduct pKa values (pk L value for solutions in D20) of 6.67 and 6.33, respectively. Complexes 8 and 9 were cytotoxic towards A2780 human ovarian cancer cells, with IC50 values of 8 and 4.2 pM, respectively [64],... 

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],... 

Since osmium was first isolated as the tetroxide, one of its most important and celebrated coordination complexes, a brief account of the history of its coordination chemistry is not out of place. The tetroxide, and subsequently the metal, was first isolated in 1803 by Smithson Tennant1 (1761-1815) by distillation with nitric acid of the black material derived after aqua regia treatment of platinum metal concentrates. Of the tetroxide Tennant wrote ... 

The 0x0 (0 ) ligand is dominant in the coordination chemistry of osmium, participating in the VIII to IV oxidation states inclusive. The tetroxide OSO4 is the single most important compound of osmium OSO4 and the recently discovered [0s04] ion arc tetrahedral. The /ra 5-[0=0s —O] osmyl moiety displays an extensive chemistry, comparable with that of the uranyl 0=U=0 unit, and there is an extensive and unique cyclic oxo-ester chemistry (p. 584). There is surprisingly little information on hydroxo, aqua, sulfato, nitrato or phosphato complexes, but much recent work has been carried out on carboxylato species, and clearly much work remains to be done on the O-donor chemistry of the element. There are a reasonable number of sulfur-donor complexes but few with selenium or tellurium. 

The aqua osmium(II) complex is prepared by reduction of the aqua osmium(III) complex (prepared as in Section E). Aqua(2,2 -bipyridine-A, A )(2,2 6, 2"-ter-pyridine-yV,A, iV")osmium(III) trifluoromethanesulfonate dihydrate (0.5(X) g, 4.063 mmol) is dissolved in 0.33 M CF3SO3H (30 mL), and the solution is filtered. Two pieces of freshly prepared zinc amalgam are added. The solution is stirred for 5 hr. The zinc is removed, and the solution is cooled in an ice bath. The resulting black crystals are filtered off, washed with ice cold water (2x2 mL), and dried under vacuum. Yield 0.224 g (53%). A second crop may be obtained by addition of fresh zinc amalgam to the filtrate. 

Ruthenium is produced mainly from an anode slime yielded when crude copper or crude nickel, obtained from nickel sulfide ores, is electrolytically refined. The anode slime contains precious metal elements. It is treated with hot aqua regia and platinum, palladium and gold are separated as their chloro complexes. Then, by nitric acid treatment, fusion treatment with NaHS04, and fusion treatment with Na202, silver, rhodium and iridium are separated. The residual ruthenium and osmium salts are dissolved in water, and the osmium is separated by treatment with chlorine, hydrochloric acid and nitric acid. The ruthenium salt is treated with ammonium chloride to afford a ruthenium salt ((NH4)3RuCl6), and the reduction with hydrogen yields ruthenium powder [1,4-6]. 

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