Pentatetraenylidenes

Keywords Allenylidene ligands Carbene complexes Cumulenylidene ligands Pentatetraenylidene ligands Propargylic alcohols... 

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). 

Otherwise, reaction of 132 with a hydride source such as NaBH4 afforded the neutral allenylethynyl complex 140 by regioselective addition at the Cy atom (Fig. 23) [372]. In contrast, reduction with one equivalent of cobaltocene, and subsequent trapping of the resulting radical with Ph3SnH, yielded the isomeric 5,5-diphenylpenta-l,3-diynyl complex 141, suggesting that the unpaired electron of the pentatetraenylidene radical is predominantly localized on the Ce atom [372]. 

Extension of these studies to other ruthenium fragments, such as [RuCl2( k" (A, P, P,P)-N(CH2CH2PPh2)3 ] [36], cis-[RuCl2(dppe)2] [37] and cA-[RuCl2(dppm)2] [38, 39], led to the related mono(alkenyl-allenylidenes) 24-26 and the bis(alke-nyl-allenylidene) 27 (Fig. 5). Pentatetraenylidene derivatives were in all cases... 

Cp carbon atom of the ynamine at the electrophilic terminal pentatetraenylidene atom, followed by cycloreversion. These observations seem to indicate a marked preference of soft nucleophiles for the Ce carbon atom and hard nucleophiles for the central carbon of the unsaturated chain. 

Complexes 75 are remarkably stable at room temperature in the solid state and, when heated, they start to decompose only at about 130 °C (Cr) or 145 °C (W). Such a thermal stability is undoubtedly associated with their strongly dipolar nature, in which six possible ylide-type resonance forms contribute to the bonding (Fig. 12). As expected, analysis of the electronic structure of complex [W (=C=C=C=C=C=C=C(NMe2)2 (CO)5] by DPT methods showed that the LUMO is mostly localized on the odd carbon atoms of the chain, whereas the HOMO is on the even carbons. In accord with these electronic features, it was found that [W =C=C=C=C=C=C=C(NMe2)21(00)5] readily adds dimethylamine across the 05=05 bond, to give the isolable alkenyl-pentatetraenylidene derivative [W =C=C=C=C=C(NMe2)CH=C(NMe2)21(00)5] [69, 70]. 

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. 

---