High-valent imidos

Edwards NY, Eikey RA, Loring MI, Abu-Omar MM (2005) High-valent imido complexes of manganese and chromium corroles. Inorg Chem 44 3700-3708... 

High-valent iron-imido complexes have also been proposed as reaction intermediates in several reactions of the iron catalysis. Que and coworkers have provided evidence for Fe(IV) imide as a reaction intermediate in the reaction of [(6-PhTPA)-Fe°(CH3CN)2] " with PhI=NTs. Borovik and coworkers have also reported the formation of an amide product involving the generation of a putative iron(IV) imide [36] (Scheme 7). 

Little related work on the technetium analog has been completed. Much of the synthetic work with high-valent Tc has centered on imido complexes,54 or on complexes for use in radiopharmaceuticals.55 Methyl technetium trioxide has been prepared.8 This species is thermodynamically a powerful enough oxidant to cycloadd to alkenes cyclohexene gave (after hydrolysis) c/s-l,2-cyclohexanediol. No chemistry with H202 has been reported it is expected to behave in a similar manner to MTO, unless the change in the metal-oxo bond strength perturbs the overall thermodynamics. 

While inorganic complexes of osmium in oxidation states +4 through +8 have been known for many years, the study of high-valent alkyl and aryl complexes of osmium is much more recent. The organometallic complexes include homoleptic see Homoleptic Compound) alkyls and aryls, oxo alkyls and aryls, nitrido/imido alkyls and aryls, and cyclopentadienyl see Cyclopentadienyl) alkyls and aryls. The majority of these are complexes of osmium(VI). 

Nitrogen ligands are particularly versatile in high-valent, early transition metal chemistry. Deprotonation of amines, amides, and hydrazines generates multiply bonding ligands which help to stabilize the high oxidation states. Aspects of metal-amido and metal-imido chemistry have been reviewed.79,80... 

At least two reasonable mechanisms for copper-catalyzed aziridination using PhI=NTs can be postulated (Fig. 3). In one, copper is a redox catalyst and aziridination proceeds through a discrete, high-valent copper-imido intermediate (mechanism a). Alternatively, the copper catalyst may serve only as a Lewis acid... 

Investigations by both Katsuki and Che have highlighted the application of chiral Mn(III) salen complexes for asymmetric N-atom transfer reactions (Fig. 17) [79-81]. In the former case, the use of [Mn(salen)PF6] together with PhI= Ts affords modest product yields and enantiomeric ratios for amination of simple benzylic and allylic starting materials. A high-valent Mn imido species is presumed to be the active oxidant. Methodological and mechanistic studies conducted by Che have found that the same types of Mn(III) salen complexes will induce oxidative cyclization of sulfamate esters. Although these catalysts afford... 

As noted above, the complexes of corroles with metals of the first transition series comprise the majority of reported metallocorroles. Many investigations have focused on the usual bioinorganic suspects, Cu and Fe. A number of computationally heavy reports on the electronic and molecular structures and spectroscopic properties of Cu corroles have been produced over the last few years, while the field has seen numerous attempts to formalize consensus electronic structural descriptions of Co and Fe corroles. There is also a rich literature describing the catalytic utility of high-valent Cr and Mn complexes with oxo, imido, or nitrido ligands, and a small amount of work has been performed on Ti and V corroles. Ni corroles have been reported in the literature as well, but investigations of their properties have often been folded into larger studies. 

Mn corroles have been isolated as Mn(III) [22, 60, 89, 90], Mn(IV) halogenato [91], and Mn(V) oxo [75, 92-94], imido [81, 95], and nitrido complexes [96, 97] (oxidation of (nitrido)Mn(V) correlates to Mn(VI) complexes has been reported as well). Mn(III) corroles represent rare examples of four-coordinate Mn(III) complexes. Mn(IV) corroles are unusually stable in fact, Mn(dpoec)(I) (dpoec = 5,15-diphenyl-2,3,7,8,12,13,17,18-octaethylcorrolate) was the first well-characterized Mn(IV)-I complex reported in the literature [98]. Meanwhile, the high-valent oxo, imido, and nitrido complexes have proven to be competent catalysts for... 

Scheme 2 Synthetic routes for the synthesis of high-valent organouranium-bis (imido) complexes and their reductive reactivity with dihydrogen...

A very elegant and simple procedure for the generation of high-valent bis(imido) organouranium (VI) complexes 13, and 15 was described starting from the organo-metallic uranium (111) complex 16. The reaction involves the direct reduction of diazenes or azides (6) [54]. 

Well defined high valent Schrock type carbene complexes of Mo or W with imido ligands are nowadays favoured as very active and selective catalysts for Ring opening Metathesis Polymerization (ROMP) and Acyclic Diene Metathesis Condensation (ADMET). Low valent Fischer type carbene complexes are less active and often need cocatalysts to enhance activities [1]. 

Zdilla MJ, Abu-Omar MM (2006) Mechanism of catalytic aziridination with manganese corrole the often postulated high-valent Mn(V) imido is not the group transfer reagent. J Am Chem Soc 128(51) 16971-16979... 

Such complexes have been referred to as nitrene complexes but, strictly, nitrenes are RN species and while zero valent NR compounds have been reported, none has been isolated. Imido compounds are commonly found in transition metal complexes with oxidation states 3 and above. The high capacity for electron donation by the imido group does of course, like that of O2-, act to stabilize high oxidation states prime examples of this are Os(NBu )4 vs. 0s04 and (Bu N)3MnCl vs. 03MnCl. 

The reaction of [Ru3(GO)n] dianion with molybdenum bis(imido) complexes [Mo(NAr)2Gl2(dme)] led to the isolation of new high-low valent clusters Ru3(GO)i2 Mo(NAr)2 (Ar = G6H3Me2-2,6 163). These compounds are unstable in polar solvents, such as THF, which may reflect the strong polarization of the Ru-Mo bonds and the electron deficiency at the molybdenum center. 

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