Imido complexes formation from isocyanates

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]. 

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] ... 

Given the evidence from DFT calculations and the results from I lN-labeling studies, it appears the mechanism for the formation of 9 involves the [2 - - 2] cycloaddition of the aryl isocyanate C = N bond across the U = N imido moiety. These results are quite surprising given the thermodynamic and kinetic stability of U = O bonds. The further reaction at elevated temperature gives a mixed oxo-imido complex 8 which is consistent with the relative energies determined by DFT calculations. 

Reaction of CO with the tautomeric mixture of the two aforementioned rhodium complexes (several / flra-substituted imidoaryl groups were tested) afforded a unique bridging isocyanate complex Rh2(CO)2(ii -N,T] -C, x-ArNCO)(p-DPPM)2. The CO insertion is irreversible. Since the two initial tautomers are in equilibrium in solution, insertion of CO may in principle proceed by either of the two (Scheme 20)(next page). However, evidence was given in favour of the amido-path (path b in the Scheme), based on the fact that the cationic complex [Rh2(p-NHPh)(CO)2(DPPM)2] rapidly reacted with CO. No complex could be isolated from this last reaction, but the formation of PhNCO was detected. Two features of this mechanism are worth of note. The first is the contrast between the conclusion reached for this system (amido complex more reactive than imido one in the insertion reaction of CO) and the one reached by Bhaduri et al. [161] for the trinuclear complex Ru3(p-H)(p-NHPh)(CO)io, which, upon deprotonation of the amido group by OH, affords the inserted product [Ru3(p-H)(T] -N,ii -C,p3-PhNCO)(CO)9]. The difference is likely due to the fact that, in this latter case, the complex is trinuclear, so that the inserted CO is already coordinated to the third ruthenium atom and, especially, the formation of the new C-N bond does not require the breaking of any of the pre-existing Ru-N bonds. 

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