Osmium complexes substituted

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). 

No terminal fluoromethylidyne complexes have been reported, but triply bridging CF ligands are observed in the cobalt complex 14 (45), in various substituted analogues (46-49), and in the osmium complex 15 (50). An iron cluster 16 containing two triply bridging CF ligands has also been reported... 

We have already alluded to the diversity of oxidation states, the dominance of oxo chemistry and the cluster carbonyls. Brief mention should be made too of the tendency of osmium (shared also by ruthenium and, to some extent, rhodium and iridium) to form polymeric species, often with oxo, nitrido or carboxylato bridges. Although it does have some activity in homogeneous catalysis (e.g. of m-hydroxylation, hydroxyamination or animation of alkenes, see p. 558, and occasionally for isomerization or hydrogenation of alkenes, see p. 571), osmium complexes are perhaps too substitution-inert for homogeneous catalysis to become a major feature of the chemistry of the element. The spectroscopic properties of some of the substituted heterocyclic nitrogen-donor complexes may yet make osmium an important element for photodissociation energy research. 

There are also bis(dinitrogen) complexes of osmium(II). The first to be made was cis-[Os(N2)2(NH3)4]2+ by a diazotization procedure using [Os(N2)(NH3)5]2" and nitrite in acid solution.270 Vibrational spectroscopic data using both 2H and 15N substitution were obtained for the salts,271 which are useful precursors for other osmium complexes involving pyrazine (see p. 536). A similar preparative procedure involving [Os(N2)(NH3)4L]2+ gives m-[Os(N2)2(NH3)3L]2+ (L = isonicotinamide).98... 

The 7] -osmium complexes of a-unsubstituted thiophenes undergo Lewis acid-promoted addition with acetals at C-2 to give the thiophenium complexes in good yields <19990M2988>. These can be deprotonated to give the 2-substituted thiophene complexes. The electrophile attacks the substrate on the rivn-face (Scheme 83). 

The synthesis and chemistry of an 7] -selenophene osmium complex 50 has been studied <19990M1559>. Protonation and electrophilic substitution with acetaldehyde diethyl acetal occurred at C-2. Methylation of complex 50 with methyl triflate gave 51 which upon treatment with tetrabutylammonium borohydride (TBAB) led to the selenophene ring-opened complex 52 (Scheme 5). 

Table VI lists the known trifluoroacetato complexes of iron, ruthenium, and osmium. Photochemical substitution reactions of various tricovalent phosphorus-donor ligands with (7r-allyl)Fe(C0)2(02CCF3) and (iT-Cp)Fe(C0)3(02CCF3), analogous to those described in the preceding for Mn(C0)5(02CCF3), have afforded (ir-allyl)Fe(CO)(cis-Ph2PCH=CHPPh2)(02CCF3), (7r-Cp)Fe(C0)(PR3)(02CCF3) (where...

Iron complex (55) also reacts with H2 to produce methane and ethene to afford propene <80JA1752>. Both reactions appear to involve insertion into a metal-carbon bond followed by elimination. When osmium complex (56) adds ethene, the diosmacyclopentane which results from ethene addition is isolated. When terminal alkynes react with (55), an alkene-substituted ring carbon results... 

Figure 5.11 Scheme for the synthesis of a pyridinylimidazolyl ligand, its copolymerization with acrylic acid (AA) and butyl acrylate (BA), and subsequent ligand substitution reaction with an osmium complex to yield a redox polymer. From [147] with permission from Elsevier. 

Reduction of conjugated iminum salts. Amines are obtained in this reduction in acidic methanol. Thus, by reduction and demetalation, the alkylation product of an osmium complex of A/ Af-dimethylaniline is transformed into the 4-substituted cyclohex-2-enylamine. 

There are considerable possibilities inherent in addition of organolithiums to osmium complexes of quinoline for the synthesis of 5-substituted quinolines. 

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