Imido clusters

Similar reactions lead to other oligomers depending on the size of the R groups and the conditions of the reaction, e.g. cyc/o-(Me2AlNMe2)2 (structure 1) and the imido-clusters (PhAlNPh)4, (HAlNPr )4 or 6,... 

Imidazolium ligands, in Rh complexes, 7, 126 Imidazolium salts iridium binding, 7, 349 in silver(I) carbene synthesis, 2, 206 Imidazol-2-ylidene carbenes, with tungsten carbonyls, 5, 678 (Imidazol-2-ylidene)gold(I) complexes, preparation, 2, 289 Imidazopyridine, in trinuclear Ru and Os clusters, 6, 727 Imidazo[l,2-a]-pyridines, iodo-substituted, in Grignard reagent preparation, 9, 37—38 Imido alkyl complexes, with tantalum, 5, 118—120 Imido-amido half-sandwich compounds, with tantalum, 5,183 /13-Imido clusters, with trinuclear Ru clusters, 6, 733 Imido complexes with bis-Gp Ti, 4, 579 with monoalkyl Ti(IV), 4, 336 with mono-Gp Ti(IV), 4, 419 with Ru half-sandwiches, 6, 519—520 with tantalum, 5, 110 with titanium(IV) dialkyls, 4, 352 with titanocenes, 4, 566 with tungsten... 

This year has also seen a variety of new clusters and aggregates bridged by oxo, hydroxo and nitrogen ligands. Of particular interest are a pair of apparent imido clusters. But unsupported terminal imido ligands remain the field s araba fenice ... 

Reaction of [Rh(/u,-Cl)(COD)]2 with / -toluidine and butyl lithium forms the tetrametallic imido cluster Rh4(/u,-NR)2(C0D)4 (R = j9-tol) that is converted to PPN[Rh3(/u,3-NR)2(C0)6] on treatment with CO and then [PPNjCl. Oxidation of the carbonyl cluster with one equivalent of ferricinium hexafluorophosphate yields [Rh3(/u.3-NR)2(CO)6]2 whose... 

In the managanese system [Mn3(i7-C5Me5)3(/i,-NO)3(/i,3-NO)] (279), very similar chemistry is observed but in addition, reduction of the nitrosyl group occurs, yielding the imido cluster [Mn3(i7-C5Me5)3(/u,3-NH)(/i2-NO)3] in low yield. Such a reaction undoubtedly proceeds by a nitrido intermediate, though the exact mechanism is, at present, unknown. 

Further reactions of the imido cluster complexes with nitriles are possible. Depending on the nature of the organonitriles, either mixed imido clusters are formed or amidinate complexes are produced as a result of nucleophilic addition of the R-N (R = Lu, Y) imido unit to a C-N triple bond, as shown in Figure 56 (Cui et al., 2005b). 

FIGURE 56 Different reactivity of tetranuclear rare-earth imido clusters toward phe-nylnitrile (redrawn after Cui et al., 2005b). 

Gp Ru)3(/i3-H)(/i-H)3 catalyzes hydrogenation of azobenzene, forming aniline and 1,2-diphenyl hydrazine. At 373 K and 100 atm, 68% conversion was obtained. Stoichiometric reactions allowed the characterization of a /i3-imido cluster, which may be an intermediate. ... 

Our approach to the preparation of new imido clusters of rhodium consists of the reactions of equimolar quantities of [ Rh( -Cl)(diolefin) 2] with a solution, prepared in situ, by the addition of butyl lithium to ara-toluidine in a 2 1 molar ratio in diethyl ether. At first glance, a hypothetical trinuclear cluster of formulation [Rh3(/i-N-p-tolyl)2(diolefin)3] could be expected from these reactions on the basis of the formation the isoelectronic thiolate complexes [Rh3(//-SPh)2(diolefin)3]+. However, unexpectedly, the results of these one-pot reactions were the formation of the novel tetranuclear clusters [Rli4( u-N-/7-tolyl)2(diolefin)4] diolefin = 1,5-cyclooctadiene (cod) (1), tetrafluorobenzobarrelene (tfb) (2), isolated as dark-red crystalline solids in good yields. 

It was also tentatively proposed the chloride attacks the imido cluster generating the equivalent of an acyl chloride, which might then react more easily with methanol. Protonation of the imido nitrogen and reductive elimination would yield the final product, regenerating Ru3(CO)i2. 

Other studies, again conducted by Geoffrey s group, also led to the discovery of interesting promotion effects of halides. It was found that the addition of [PPN][X] (PPN " = (PPh3)2N+, X = Cl, Br, I) strongly accelerates the reaction between Ru3(CO)i2 and nitrosobenzene, to afford an imido cluster in which the... 

Although in all of Geoffroy s papers it was always remarked that no firm evidence existed that the reactions reported were of any relevance to the actual catalytic cycle, this prudent statement is not always found in other authors papers and seems anyway to have been overlooked by many readers, so that the involvement of imido clusters as intermediates in the catalytic carbonylation reactions of nitro-arenes catalyzed by Ru3(CO)i2 seems now to be taken for granted by many researchers in the field. 

We next analyzed the role of the chloride anion. We have already mentioned that chloride has been reported to accelerate the reaction of nitrosoarenes with Ru3(CO)i2 and also to promote the formation of phenyl isocyanate from a trinu-clear imido-halide cluster. However, neither of these effects seems relevant to the effect observed in catalysis. In fact, apart from the problem of the cluster nature of the compounds involved, nitrosoarenes are always much more reactive than their nitro counterparts and the reaction of a free nitroso intermediate should be very fast relative to the other steps in the catalytic cycle, so that its further acceleration should not be influential. The promotion effect on the isocyanate-producing step, on the other hand, cannot be relevant to the catalytic cycle because it has been shown that the eorresponding imido clusters are not involved. So we must look for other effects. 

Force-field calculations have shown that when there is a spacer between the two phenyl rings, such as in (2-nitrophenyl) phenyl thioether and 2-nitrobenzophenone, the distances between the nitrene nitrogen and the ortho carbon, where cyclization should occur in the hypothetical imido clusters, are much longer than in compound This explains why these substrates afforded the corresponding amines, with only trace amounts of the desired six-membered heterocycles being observable, when subjected to reaction conditions the same as those used for Eq. (12). ... 

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. 

Since, as already mentioned, it was known that Ru3(CO)i2 reacts with nitrobenzene to afford a mixture of the two imido clusters (G) and (H) [104] and since the monoimido cluster could also be used as a catalyst precursor, a catalytic cycle was tentatively proposed, which included the formation of (G) as an intermediate (Scheme 9). 

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