Ruthenium imido complexes

A nickel-imido complex and a ruthenium-imido complex boimd by an ancillary porphyrin ligand also react with olefins, in this case to generate aziridine products (Equations 13.74 and 13.75). A pathway for the formation of an aziridine that occurs by a [2+2] addition of the olefin across the metal imido unit, followed by reductive elimination of the aziridine, was proposed for the reaction of the nickel complex, although the azametaUacycle was not observed directly. Because of the lack of coordination site cis to the imido group in the ruthenium-porphyrin system, the transfer of the imido group likely occurs by direct reaction of the imido group with the olefin. ... 

By reaction of Fe3(CO)i2 with aromatic nitro compounds in benzene, derivatives such as 1 and 3 (R = Aryl) have been obtained [59, 60]. Similarly, nitrobenzene reacts with Ruj(CO)i2 in boiling benzene to give the ruthenium homologues of 1 and 5 [61]. The ruthenium imido complex 5 (M = Ru, R = Ph) gives the bis-imido complex 1 (M = Ru, R = Ph) by reaction with excess nitrobenzene in boiling benzene [61]. On the other hand, even the thermal disproportionation of 5 under an inert atmosphere and in solution leads to the formation of 1 and other uncharacterised products [62]. It has been proposed that the complexes 1-5 are connected via the sequence of reactions reported in Scheme 9 [56] ... 

Over the last decade a number of high oxidation state ruthenium porphyrin complexes containing 0x0 or imido ligands have been reported and have been thoroughly studied for their role in oxidation and atom-transfer chemistry. Although comparisons can be drawn with organometallic species (carbene, imido. and 0x0 ligands are formally isolobal) the chemistry of the 0x0 and imido complexes is beyond the scope of the review and will not be covered here. 

Inter- and intramolecular (cyclometallation) reactions of this type have been ob-.served, for instance, with titanium [408,505,683-685], hafnium [411], tantalum [426,686,687], tungsten [418,542], and ruthenium complexes [688], Not only carbene complexes but also imido complexes L M=NR of, e.g., zirconium [689,690], vanadium [691], tantalum [692], or tungsten [693] undergo C-H insertion with unactivated alkanes and arenes. Some illustrative examples are sketched in Figure 3.37. No applications in organic synthesis have yet been found for these mechanistically interesting processes. 

Together with Schrock s molybdenum-imido compound 50 ° the ruthenium-phosphine complexes 51 and especially 52 developed by Grubbs " proved to be an outstanding achievement in the development of molecular catalysts for olefin metathesis reactions (Scheme 10). 

There have also been significant advances in the imido chemistry of ruthenium and osmium. A variety of imido complexes in oxidation states +8 to +6 have been reported. Notably, osmium (VIII) imido complexes are active intermediates in osmium-catalyzed asymmetric aminohydroxyl-ations of alkenes. Ruthenium(VI) imido complexes with porphyrin ligands can effect stoichiometric and catalytic aziridination of alkenes. With chiral porphyrins, asymmetric aziridination of alkenes has also been achieved. Some of these imido species may also serve as models for biological processes. An imido species has been postulated as an intermediate in the nitrite reductase cycle. " ... 

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

A highly reactive CM-bis(imido)ruthenium(VI) complex has been suggested in the oxidation of cii-[Ru(bpy)2(tmen)] (tmen = 2,3-dimethylbutane-2,3-diamine) by Ce in aqueous solutions. This Ru species decomposes rapidly in water to give [Ru(bpy)2 (ONCMe2CMe2NO)] ". ... 

No mononuclear imido complexes of Ru(IV) have yet been isolated. The recently reported imido-bridged dimer [Ru(t)-C6H6)(2V-2,6-Pr2 C6H3)]22 has ruthenium in an oxidation state of +11 (98). 

The ruthenium sulfoximido complex HRu3(C0)9 NS(0)MePh is of note because it formally has two electrons fewer than the normal tetrahedral structure [Eq. (64)].170 Upon addition of CO it is converted into an electron-precise /a-imido complex [Eq. (65)170]. 

The synthetic procedure is very critical. In our case, we believe that the imido-lithium compound (Li2NR) is present in the solution of butyl lithium and para-toluidine, in diethyl ether, as reported for the dilithiated a-naphtylamine. This is a noticeable difference in comparison to typical preparations of ruthenium, osmium, and iridium imido complexes, " in which a dichlorometal complex and the monolithium salt (LiNHR) in a molar ratio 1 2, appropriate for a ftA-amido precursor, are used. In these cases a subsequent removal of amine, or a dehydro-halogenation step with LiNHR, is required to afford the products and free amine. Equation (1) summarizes our synthetic procedure ... 

The selectivity of the synthesis of 2 was rather poor (ca 32%). Because it is known that aromatic nitro compounds react with Ru3(CO)i2 to give imido complexes in which the nitrogen atom is coordinated to all of the three ruthenium atoms of the cluster skeleton, the imido complex 4, a possible intermediate in the reaction, was synthesized (Eq. 13) and its crystal structure determined ... 

The cluster Ru3(CO)i2 is known to react in the presence of bases to afford, under certain experimental conditions, [HRu3(CO)u] [42]. Since the corresponding iron cluster is known to react with nitrobenzene to yield an hydrido imido cluster [HFe3(CO)9(NPh)] , which can be protonated and afford small amounts of aniline [43], a catalytic cycle was proposed, and supported by some model reactions, for the ruthenium-catalysed reduction of nitroarenes by CO/H2O, which includes the intermediate formation of the trinuclear hydrido cluster [44-46]. However, one of us has recently shown that [HFe3(CO)n] and the corresponding imido complex play no role in the Fe3(CO)i2-promoted reduction of nitrobenzene [6, 47] and the proposal of an active role of [HRu3(CO)ii] appears to be questionable. 

A further mthenium(IV) complex, [Ru(ti -TsN—N=N-NTs)(S2CNEt2)2] (270), has been prepared in 35% yield from the reaction of excess tosylazide and ciJ-[Ru(PPh3)2(S2CNEt2)2] (268) (287). The structure is confirmed crystal-lographically and the authors speculate that it results from initial formation of a ruthenium(IV) imido complex, [Ru(NTs)(S2CNEt2)2] (269), which reacts further with a second equivalent of azide (Eq. 123). 

The reaction of [Ru(NH3)s(OHa)] + with hydrazoic acid has as its first step an easy substitution at ruthenium to give [Ru(NH3)5(N3H)] +. This decomposes to [Ru(NH3)6(NH)] + plus dinitrogen. The main interest here, and in other similar cases, is whether this species behaves as an imido (19) or a nitrene (20) derivative. The chemical characteristics of the present intermediate indicate that it acts as an imido-complex. ... 

Olefin-metathesis is a useful tool for the formation of unsaturated C-C bonds in organic synthesis.186 The most widely used catalysts for olefin metathesis include alkoxyl imido molybdenum complex (Schrock catalyst)187 and benzylidene ruthenium complex (Grubbs catalyst).188 The former is air- and moisture-sensitive and has some other drawbacks such as intolerance to many functional groups and impurities the latter has increased tolerance to water and many reactions have been used in aqueous solution without any loss of catalytic efficiency. 

The vast majority of ruthenium(VI) and osmium(VI) complexes are those stabilized by metal-ligand multiple bonds. These are mainly imido, nitrido, and 0x0 species. 

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