Osmium dimers

Only one example of an NHC-containing olefin metathesis catalyst containing a transition metal other than ruthenium has been reported in the literature. The NHC-osmium complexes 53a and 53b (Scheme 2) are synthesized from the dichloro(i]6-p-cymene)osmium dimer by addition of the NHC prepared in situ and abstraction of the chloride, followed by introduction of the ben-zylidene moiety with phenyl diazomethane. 

The use of isotopes, as well as the appropriate control experiments required for valid interpretation, is illustrated well by the reductive elimination studies of Norton.Reductive elimination is often a key step in proposed mechanisms and is generally presumed to be intramolecular in nature however, crossover experiments have shown that intermolecular processes can be involved. Thus, the observation that mixed elimination species and an osmium dimer, Os2 CO)sR2, 10, are found requires an intermolecular reaction (17,18). 

Though in moderate yield, the reported conversion of ethanol to ethyl acetate by complex 14 and a related osmium dimer complex [37] by Gusev generated interest in the catalytic synthesis of ethyl acetate from ethanol because it is a widely used fine chemical. Very recently, Beller et al. [38] screened the catalytic activity of various known pincer complexes for this transformation and found thatTakasago s complex 15, known as Ru-MACHO catalyst, is very efficient and the reaction in the presence of a base resulted in very good TON (Table 1.4). 

Table 5.3. Relative enthalpies for coordination of various phosphorus iigands to ruthenium and osmium dimers.

It is a volatile orange-red crystalline solid (m.p. 30°C), stable to over 100°C. On reduction with tertiary phosphines or sodium amalgam, Os(NBu )3 is formed, which is dimeric (ButN)2Os(/x-NBut)2Os(NBu )2. This can be oxidized to the osmium(VII) dication with concomitant shortening in the Os-Os distance from 3.1 to 2.68 A. 

Imamura (11,20,21) synthesized several similar perpendicular dimers exploiting axial coordination of the 4-pyridyl free-base porphyrin to Ru(II)CO (3) and Os(II)CO (4) porphyrins (Fig. 1). The pyridine-ruthenium and pyridine-osmium interactions are much stronger than the pyridine-zinc interaction, and the complexes are perfectly stable in solution and can be isolated by precipitation. One of the ruthenium dimers was characterized by FAB-MS (11). Complexation is accompanied by characteristic changes in JH NMR chemical shift, indicating... 

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. 

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).

Porphyrazines have also been used to prepare metal-metal bonded dimers with ruthenium and osmium, specifically homometallic dimers comprised of two metallic octaethylporphyrazine and heterometallic dimers comprised of one metallic oc-taethylporhyrazine ligand and one metallic octaethylporphyrin ligand have been reported (189-191). The major bonding interactions that occur in these systems are directly between the two metal centers. 

The tetranuclear osmium(VI) oxo imido complex [(Bu N)20s(/u-NBu )20s(NBu )(/u-0)]2 (2) is obtained as a side product in the reaction of [OSO4] with neat NHBu (SiMe3) (Scheme 1). Its structure can be described as a dimer of dimers with Cjh symmetry. ... 

A number of /.t-oxo-osmium(IV) porphyrin complexes have been prepared by aerial oxidation of [Os OEP)(CO)(MeOH)] in the presence 2,3-dimethylindole in CH2Cl2. " Cyclic voltammetric studies show that [0s2(0)(0EP)2(0Me)2] can undergo reduction to give an Os —O—Os dimer. The X-ray crystal structure of [0s(0EP)(0Me)]2( -0) shows a linear Os—O—Os backbone with Os—O and Os—OCH3 distances of 1.808 A and 1.997 A, respectively. 

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