Osmium!VII complexes

Osmium(VII) complexes bisimido, 559 fluorides, 612 hexaoxo,588 imido,558 monooxo, 588... 

All four halide ions from octahedral complexes with the metal. Fluorides or fluoro complexes are known for osmium(VII) to osmium(IV) inclusive, with rather tenuous evidence for OsFj OsF is a rare example of seven coordination for the metal. The other halides prefer the IV and III states, though osmium(V) chloride and chloro complexes have recently been made and also [OsBr ]". Although [RuFs] " is well known there is no osmium analogue, in keeping with the general tendency of third-row elements to display higher oxidation states than their second-row partners. 

Osmium(iv).—Group VII Donors. Simple syntheses of hexafluoro-complexes of some noble metals have been reported, e.g. 

No imido complexes of ruthenium(VII) have been reported and there are only two examples for osmium. 

These early successes with carbonyl complexes of rhenium encouraged me to undertake systematic research on the carbon monoxide chemistry of the heavy transition metals at our Munich Institute during the period 1939-45, oriented towards purely scientific objectives. The ideas of W. Manchot, whereby in general only dicarbonyl halides of divalent platinum metals should exist, were soon proved inadequate. In addition to the compounds [Ru(CO)2X2] (70), we were able to prepare, especially from osmium, numerous di- and monohalide complexes with two to four molecules of CO per metal atom (29). From rhodium and iridium (28) we obtained the very stable rhodium(I) complexes [Rh(CO)2X]2, as well as the series Ir(CO)2X2, Ir(CO)3X, [Ir(CO)3]j (see Section VII,A). With this work the characterization of carbonyl halides of most of the transition metals, including those of the copper group, was completed. 

Osmium(n).—Group VII Donors. Hydrido- and halogeno-carbonyl complexes. Carbonylation of [OsHX(CO)(PCy3)2] has produced the novel dicarbonyl hydride [OsHX(CO)2(PCy3)2] (X = Cl or Br Cy = tricyclohexyl).17 I.r. and n.m.r. data indicate the geometry (17). Addition of other ligands to the five-co-ordinate... 

In the absence of tertiary amines, osmium tetroxide reacts with alkenes via 1,3-dipolar addition to generate a monomeric Os(VI) ester such as 252,352 where L is a ligand that can be a solvent molecule or an added substrate such as pyridine. Sharpless et al. proposed that hydroxylation proceeds by an allowed [2-1-2]- cycloaddition reaction, producing an Os(VII) intermediate, followed by reductive insertion of the Os—C bond into an Os=0 bond.353 This complex can be decomposed in aqueous or alcoholic solution, but the hydrolysis is... 

Preparation of dihydroxytiagabine (VII) was accomplished by the method shown in Scheme 29.19. Synthesis of the 9-0-(4 -Methyl-2 -quinolyl) ether of dihydroquinidinol proved difficult in that the central double bond is hindered and proved to be refractory to attack by reagents such as m-chloroperbenzoic acid and hydrogen peroxide. The putative epoxide was not detected under a variety of reaction conditions a complex mixture of products was always obtained. Reaction with osmium tetroxide/pyridine/V-methylmorpholine-V-oxide was slow and yielded the requisite diol in low yield, but extraction of the product from water proved to be a problem. 

Early reports on interactions between redox enzymes and ruthenium or osmium compounds prior to the biosensor burst are hidden in a bulk of chemical and biochemical literature. This does not apply to the ruthenium biochemistry of cytochromes where complexes [Ru(NH3)5L] " , [Ru(bpy)2L2], and structurally related ruthenium compounds, which have been widely used in studies of intramolecular (long-range) electron transfer in proteins (124,156-158) and biomimetic models for the photosynthetic reaction centers (159). Applications of these compounds in biosensors are rather limited. The complex [Ru(NHg)6] has the correct redox potential but its reactivity toward oxidoreductases is low reflecting a low self-exchange rate constant (see Tables I and VII). The redox potentials of complexes [Ru(bpy)3] " and [Ru(phen)3] are way too much anodic (1.25 V vs. NHE) ruling out applications in MET. The complex [Ru(bpy)3] is such a powerful oxidant that it oxidizes HRP into Compounds II and I (160). The electron-transfer from the resting state of HRP at pH <10 when the hemin iron(III) is five-coordinate generates a 7i-cation radical intermediate with the rate constant 2.5 x 10 s" (pH 10.3)... 

As already mentioned, tris-diimine complexes of ruthenium(II) and osmium(II) [M(LL)3] (LL = bpy or phen) could be ideal mediators for GO and HRP due to their inertness to substitution and almost diffusion-controlled self-exchange rates (Section IV) providing high reactivity with GO (Table VII). But the redox potentials of the... 

There will be circumstances other than those I have described here in which "high oxidation state" organometallic chemistry of rhenium in a catalytic reaction will be viable, although it is becoming clear that the balance necessary to achieve this feat is more difficult to maintain as one moves to the right in the transition metal series, and that some of the d rhenium chemistry in fact may look like chemistry of dP osmium species. On this basis it would seem unlikely that the principles that have been used to prepare Re(VII) alkylidyne and alkylidene complexes (a hydrogen migration reactions) can be extended further (to technetium, or especially osmium or ruthenium), at least in a routine fashion. 

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