Osmium reactivity

CONTROLLING PLATINUM, RUTHENIUM, AND OSMIUM REACTIVITY FOR ANTICANCER DRUG DESIGN... 

Controlling Platinum, Ruthenium,and Osmium Reactivity for Anticancer Drug Design... 

Ruthenium dissolves anodically in alkaline solutions, as predicted by Pourbaix but its corrosion resistance when made anodic in acid solutions is variable. Under some conditions the volatile and toxic tetroxide is evolved. Osmium is even more reactive anodically than ruthenium. 

A technologically important effect of the lanthanide contraction is the high density of the Period 6 elements (Fig. 16.5). The atomic radii of these elements are comparable to those of the Period 5 elements, but their atomic masses are about twice as large so more mass is packed into the same volume. A block of iridium, for example, contains about as many atoms as a block of rhodium of the same volume. However, each iridium atom is nearly twice as heavy as a rhodium atom, and so the density of the sample is nearly twice as great. In fact, iridium is one of the two densest elements its neighbor osmium is the other. Another effect of the contraction is the low reactivity—the nobility —of gold and platinum. Because their valence electrons are relatively close to the nucleus, they are tightly bound and not readily available for chemical reactions. 

The chemical reactivity of the organoruthenium and -osmium porphyrin complexes varies considerably, with some complexes (M(Por)R2, M(Por)R and Os(OEP)(NO)R) at least moderately air stable, while most are light sensitive and Stability is improved by handling them in the dark. Chemical transformations directly involving the methyl group have been observed for Ru(TTP) NO)Me, which inserts SO2 to form Ru(TTP)(N0) 0S(0)Me and Ru(OEP)Me which undergoes H- atom abstraction reactions with the radical trap TEMPO in benzene solution to yield Ru(OEP)(CO)(TEMPO). Isotope labeling studies indicate that the carbonyl carbon atom is derived from the methyl carbon atom. "" Reaction of... 

The most significant and widely studied reactivity of the ruthenium and osmium porphyrin carbene complexes is their role in catalyzing both the decomposition of diazoesters to produce alkenes and the cyclopropanation of alkenes by diazoesters. Ethyl diazoacetate is used to prepare the carbene complex 0s(TTP)(=CHC02Et)... 

Both rhodium and osmium porphyrins are active for the cyclopropanation of alkenes. The higher activity of the rhodium porphyrin catalysts can possibly be attributed to a more reactive, cationic carbene intermediate, which so far has defied isolation. The neutral osmium carbene complexes are less active as catalysts but the mono- and bis-carbene complexes can be isolated as a result. 

A photo-induced dihydroxylation of methacryamide by chromium (VI) reagent in aqueous solution was recently reported and may have potential synthetic applications in the syn-dihydroxylation of electron-deficient olefins.63 Recently, Minato et al. demonstrated that K3Fe(CN)6 in the presence of K2C03 in aqueous rm-butyl alcohol provides a powerful system for the osmium-catalyzed dihydroxylation of olefins.64 This combination overcomes the disadvantages of overoxidation and low reactivity on hindered olefins related to previous processes (Eq. 3.14). 

Oxidative cleavage of alkenes using sodium periodate proceeds effectively in a monophasic solution of acetic acid, water, and THF with very low osmium content or osmium-free. The orders of reactivity of alkenes are as follows monosubstituted trisubstituted >1,2 disub-stituted > 1,1-disubstituted > tetrasubstituted alkynes.100 Cleavage with polymer-supported OSO4 catalyst combined with NaI04 allows the reuse of the catalyst.101... 

In spite of the rich chemistry developed starting from the OsHCl(CO)(P Pr3)2 complex, the presence of a carbonyl group in its coordination sphere is probably a limitation for some subsequent developments. In this context it seems important to mention the encouraging reactivity of the related osmium(IV) complex, OsH2Cl2(P Pr3)2, that in methanol afford OsHCl(CO)(P Pr3)2. We believe that both interrelated osmium complexes present not only a rich chemistry but also a promising future as starting materials in organometallic chemistry. 

Thus the reactivity of transition metal-carbene complexes, that is, whether they behave as electrophiles or nucleophiles, is well explained on the basis of the frontier orbital theory. Studies of carbene complexes of ruthenium and osmium, by providing examples with the metal in either of two oxidation states [Ru(II), Os(II) Ru(0), Os(O)], help clarify this picture, and further illustrations of this will be found in the following sections. 

The effect of metal basicity on the mode of reactivity of the metal-carbon bond in carbene complexes toward electrophilic and nucleophilic reagents was emphasized in Section II above. Reactivity studies of alkylidene ligands in d8 and d6 Ru, Os, and Ir complexes reinforce the notion that electrophilic additions to electron-rich compounds and nucleophilic additions to electron-deficient compounds are the expected patterns. Notable exceptions include addition of CO and CNR to the osmium methylene complex 47. These latter reactions can be interpreted in terms of non-innocent participation of the nitrosyl ligand. 

The characteristic reactivity of neutral dg alkylidene complexes of Ru, Os, and Ir is with electrophilic reagents. The osmium methylene 47 reacts with the widest range of electrophiles, the most significant reactions being summarized in Scheme 2. 

Carbyne complex chemistry of osmium and ruthenium is discussed in this section. These studies demonstrate clearly the parallels that exist between the metal-carbon bonds in carbene and carbyne complexes and again emphasize the importance of metal basicity in determining complex reactivity. 

Furthermore, kinetic analysis of the decay rate of anthracene cation radical, together with quantum yield measurements, establishes that the ion-radical pair in equation (76) is the critical reactive intermediate in osmylation reaction. Subsequent rapid ion-pair collapse then leads to the osmium adduct with a rate constant k 109 s 1 in competition with back electron-transfer, i.e.,... 

In our laboratory we have examined the reactivity pattern of [0s3(y-H)2(C0)10], an unsaturated cluster which can be represented as possessing an osmium-osmium double bond in its classical valence bond representation. We find (2,3) that this compound undergoes a number of reactions with metal carbonyls which in some cases can be formulated as proceeding through intermediates analogous to metal olefin complexes ... 

Fig. 25. (a) General aqueous reactivity of osmium(II)-arenes the formation of an hydroxido-bridged dimer can play a major role in the aqueous chemistry of osmium(II)-arenes, especially if the chelate XY is an 0,0-bidentate ligand (b) molecular structures of the osmium-arene anticancer agents 26-30. 

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