Pentatetraenylidene complexes

In contrast, the chemistry of higher metaUacumulenylidene complexes containing longer odd carbon chains has been comparatively less studied due to the synthetic difficulties in preparing such species. This fact relies on the increased reactivity of the unsaturated carbon chains towards electrophilic and nucleophilic attacks. In fact, only a limited number of stable pentatetraenylidene complexes have been described, while others have been proposed as highly reactive transient intermediates. 

Scheme 45 Synthesis of the first isolated pentatetraenylidene complex...

Scheme 47 Synthesis of the pentatetraenyhdene-rhenium(I) complexes 137 Tahle 3 Structural data of pentatetraenylidene complexes 132 and 134...

Although the chemistry of pentatetraenylidene complexes [M]=C(=C)3=CR R has not received as much attention as that of aUenylidenes, there is ample experimental evidence to confirm the electrophilic character of the C, Cy and carbons of the cumulenic chain [26-29, 31]. Thus, treatment of tra s-[RuCl(=C=C=C=C=CPh2) (dppe)2][PFg] (132) with alcohols or secondary amines resulted in addition of the nucleophilic solvent across the Cy=Cs double bond to give alkenyl-allenylidenes 138 (Scheme 48) [358]. In chloroform, electrophilic cyclization with one of the Ph groups occurred to give 139. This transformation is actually the parent of the later observed allenylidene to indenylidene intramolecular rearrangement (Scheme 15). 

The number of pentatetraenylidene complexes (x= 5) reported in the literature is even less. All in all, 11 complexes have been described. Complexes with an even larger number of carbon atoms in the chain are unknown. Therefore, cumulenylidene complexes with more than three carbon atoms in the chain are still rather elusive classes of metallacumulenes [1] and their involvement in a catalytic process has not yet been observed. 

Until now, the structures of only three butatrienylidene [2-5] and four pentatetraenylidene complexes [6-9] (Chart 3.2) have been established by X-ray structure analysis. 

Chart3.2 Mononuclear butatrienylidene and pentatetraenylidene complexes characterized by X-ray structure analysis. 

In the majority of pentatetraenylidene complexes prepared or generated so far, the pentatetraenylidene ligand is derived from suitable C5 precursors. Usually penta-1,3-diynyl derivatives like the alcohol HC = C—C = C—CPh20H, its trimethylsilyl ether, or the 5,5,5-tris(dimethylamino)-substituted penta-l,3-diyne are employed. 

The formation of other mono- [27-29] or even bis[alkoxy(alkenyl)allenylidene[ ruthenium complexes [28, 30] from the corresponding ruthenium chlorides and 5,5 -diphenyl-penta-1,3 -diynyl alcohol or trimethylsilyl ether in the presence of methanol (Scheme 3.13) and of the allenylidene complex 18 in the absence of methanol (Scheme 3.13) [30, 31] was also suggested to proceed via pentatetraenylidene intermediates. Neither one of these pentatetraenylidene complexes could be isolated or spectroscopically detected although their formation as an intermediate was very likely. 

The first neutral pentatetraenylidene complex was obtained by Werner et al. Treatment of a solution of the butadiynyl(hydrido) complex 21 at —78°C with an equimolar amount of (Cp3S02)20 followed by addition of two equivalents of NEt3 at... 

Scheme 3.14 Synthesis of the first isolable cationic pentatetraenylidene complex.

Scheme 3.17 Synthesis of n-donor substituted pentatetraenylidene complexes.

The reaction of these complexes with ynamine allowed the modification of the pentatetraenylidene ligand. Insertion of the C = C bond of the alkyne into the C4=C5 bond of the chain afforded alkenyl(amino)pentatetraenylidene complexes [9]. 

The LUMO in d pentatetraenylidene complexes is predominantly localized on the odd carbon atoms and to a lesser extent on the metal. The coefficients on Cl and C3 are very similar, independent of the metal-ligand fragment and the terminal substituent. The coefficient at C5 is somewhat larger. In square-planar d rhodium and iridium complexes the coefficient at the metal is comparable to that on C5 and is larger than those on Cl and C3. Thus, a nucleophilic attack at the metal of d complexes has also to be taken into account. 

Experimental information on the reactivity of pentatetraenylidene complexes is still rather rare. The isolated pentatetraenylidene complexes [Cl (dppe)2Ru=C=C=C=C=CPh2]+ (20) [7] and [(CO)5M=C=C=C=C=C(NMe2)2] (M = Cr, W) (22) [8] tvere found to read with secondary amines by addition of the amine across the C3=C4 bond, very likely via an initial nucleophilic attack at C3, to give alkenyl(amino)allenylidene complexes (Scheme 3.32). Analogously, methanol... 

Scheme 3.32 Addition of amines and methanol to the C3—C4 bond of pentatetraenylidene complexes.

In contrast, soft carbon nucleophiles attack at C5. The reaction of 23 with diethylaminopropyne yields alkenyl(amino)pentatetraenylidene complexes (34) by insertion of the C = C bond of the alkyne into the C4=C5 bond of the pentatetrae-nylidene ligand [9]. The reaction is initiated by a nucleophilic attack of the ynamine at C5 followed by ring closure and electrocyclic ring opening (Scheme 3.34). Complexes 34 are obtained as mixtures of s-cis/s-trans isomers. 

Other C-nucleophiles such as Me , Ph , [C = CPh], likewise add to C5 of 23 forming diynyl metallates. Subsequent Si02-induced elimination of [NMe2] gives new dimethylamino(organyl)pentatetraenylidene complexes (Scheme 3.35) [56]. This substitution route affords a variety of pentatetraenylidene complexes so far not accessible by other routes. 

The attack of a nucleophile at Cl of the chain in an isolated pentatetraenylidene complex has been observed only once. The reaction of CH2N2 with trans-[Cl (PPr 3)2Rh=C=C=C=C=CPh2j proceeded by addition of CH2 to Cl, N2 elimination and rearrangement of the thus-formed diphenylhexapentaene ligand to form a C2,C3-bonded 6,6-diphenylhexapentaene complex (Scheme 3.36) [33]. 

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