Heptahexaenylidene Complexes

An example of the next higher homolog in the series of odd-numbered cumuleny-lidene complexes, heptahexaenylidene complexes, was already proposed in 1996 as an intermediate in the formation of the alkenyl(dimethylamino)pentatetraenylidene tungsten complex 27 (Scheme 3.21) in the sequential reaction of triyne Me3Si-(C = C)3-C(NMe2)3 with LiMe LiBr, (CO)5W(thf)], and BFj OEtz [37]. The putative heptahexaenylidene complex 26 could neither be isolated nor spectroscopically... 

Scheme 3.21 Synthesis of the first heptahexaenylidene complex and its reaction with dimethylamine.

The reactivity of heptahexaenylidene complexes is reminiscent of that of the corresponding pentacarbonyl pentatetraenylidene complexes. Like 23, complex 26... 

The stabilizing effect of the 7t-donor dimethylamino group is clearly reflected in the preparation of the heptahexaenylidene complexes M C C C C C C C (NMe2)2 (CO)5] (M = Cr, W) (75), which represent the longest metallacumulenylidenes isolated to date (Scheme 23) [69,70]. They were synthesized from triyne Me SiiCsCj SiMe by a sequential five step reaction (overall yield 3%), analogous to that discussed previously in the preparation of their pentatetraenylidene counterparts 72. 

The longest cumulenylidene ligand reported to date is the heptahexaenylidene complex shown in Figure 13.49. As in the case of extended organic pi systems, the difference in energy between the HOMO and the LUMO decreases as the length of the cumulene ligand increases. ... 

However, by very careful control of the reaction conditions and the purity of the triyne, using lower temperatures and substituting SiO, for BF3-OEt2 it was recently possible to synthesize, isolate and characterize the first heptahexaenylidene tungsten complex 26 and its chromium analog [38]. 

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