Imido complexes formation from

The metathetical reaction of C02 with metal (Ti(IV), U(V)) arylimido-complexes led to an arylisocyanate and a metal-oxo species [4g, 115a]. In these processes, which take place under very mild conditions (ambient temperature and pressure), the metal center acts as the sink for unrequired oxygen. A common feature which characterizes these transformations from a mechanistic point of view is the intermediate formation of a metal carbimato-species through the [2+2] cycloaddition of C02 with the metal-imido complex (Scheme 6.23). By cycloreversion, the four-membered aza-metalla-cycle then converts into isocyanate and a stable metal-oxo complex. These processes, at least formally, are reminiscent of the reaction of imino-phosphoranes with C02 to give isocyanates and carbodiimides [115b,c]. 

Lu atom (Lu-N 2.352(4) A). The formation of the dianion complex apparently proceeds by nucleophilic addition of the imido unit to the C N group of benzoni-trile, which demonstrates that a lanthanide-imido bond, even in a bridging form, is very reactive. This is in contrast with what was observed previously for bonds between transition metals and bridging imido ligands, which are usually robust and unreactive. Moreover, the resultant product represented the first metal complex of an amidinate dianion, in contrast to the well-known complexes of various metals with amidinate monoanions [RNC(R0NR] as ligands [6,7]. The dianion complex and related complexes derived from imido lanthanide species react with excess of benzonitrile under selective formation of the cyclotrimerization product 2,4,6-triphenyl-1,3,5-triazine [72]... 

Many routes to 0x0 and imido compounds have been developed. Transition metal-oxo complexes are coimnonplace and, in many cases, are the most stable form of an element in air. Some imido compoimds, including the first imido complex, have been prepared from metal-oxo complexes (Equation 13.50). The reactions of some of the later metal-oxo compounds with a primary amine generate an imido species by extrusion of water. In other cases, metal-imido complexes are prepared from 0x0 complexes by the reaction of phosphmimines (Equation 13.51) or from 0x0 or halo complexes by the reactions of silylamines (Equations 13.52 and 13.53). The driving force for these reactions is the formation of phosphine oxide, silyl ethers, or silyl halides. 

The mechanism by which isocyanates are formed by catalytic carbonylation of organic nitro compoimds is reasonably well understood only in a few cases (see Chapter 6). The intermediate formation of a nitrene (imido) complex has been very often proposed to intermediately occur, being these derivatives formed in some cases from nitro compounds and transition metals carbonyl complexes (2.1.3.). Moreover the synthesis and reactivity of this class of compounds is now well developed [14-16]. However, only in a few cases a clean reaction between a metal-nitrene complex and carbon monoxide giving an isocyanate derivative has been reported. For example, the monomeric terminal imido complex Cp IrNR (Cp = q -CsMes, R = Bu ) reacts under 600 torr of carbon monoxide, affording the corresponding r -isocyanate complex (eq. 2) [17, 18] ... 

Apart from the isolation of discrete imido complexes from the reaction mixtures, the main evidence for the intermediate formation of nitrene species has been represented by the formation of carbazole when o-nitrobiphenyl is used as a substrate. It has been known for a long time [258, 259] that o-azidobiphenyl yields carbazole when photochemically activated and the reaction is believed to occur via an intermediate nitrene (Scheme 23) ... 

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