Iron allenylidene

In contrast to ruthenium and osmium, the reactivity of iron allenylidenes remains almost unexplored. Only the behavior of the cationic diphenylallenylidene-Fe(II) derivative frans-[FeBr(=C=C=CPh2)(depe)2]" has been studied in detail. Thus, it has been found that this complex reacts exclusively at Cy with both neutral (amines, phosphines) and anionic (H , MeO , CN ) nucleophiles [105-107]. This behavior contrasts with that of the neutral Fe(0) derivative [Fe =C=C=C(f-Bu)2 (CO)5] which undergoes PPhs-attack at Co- to afford the zwitterionic phosphonio-allenyl species [Fe C(PPh3)=C=C(f-Bu)2 (CO)5] [104]. 

Nucleophilic additions of alcohols, amines, thiols, and selenols to Group 8 buta-trienylidene intermediates [M]=C=C=C=CR2 have also been used in the preparation of stable heteroatom-conjugated allenylidene complexes. Thus, activation of trimethylsilyl-l,3-butadiyne HC=C-C=CSiMe3 by the iron(II) complex [FeClCp (dppe)], in methanol and in the presence of NaBPh4, resulted in the high-yield formation of the methoxy-allenylidene [FeCp =C=C=C(OMe)Me ... 

Scheme 7 Synthesis of methoxy-allenylidene iron(II) complex 30...

Finally, it should be mentioned that the reaction of the dianion derived from alkynol HC=CC(OH)f-Bu2 with [Fe(CO)4(NMe3)] led to a complex mixture of mono- and polynuclear products, from which the iron(0)-allenylidene complex 42 (Fig. 7) could be isolated [55]. 

Reaction of Fe3(CO)12 with 3-pentyn-l-ol gives the hydrido cluster 4, formed by coupling of an allenylidene unit with a coordinated carbonyl ligand and methoxy group. In contrast, reaction with the isomeric alkyne 2-methyl-3-butyn-2-ol affords a binuclear iron complex.18 Treatment of Fe3(p3-S)2(CO)9 with Me3NO affords the vinylferrocene complex 5, whereas the similar reaction with Fe3(p3-Te)2(CO)9 gives no cluster products.19... 

The use of Me3NO to induce substitution of dppm (bis(diphenylphosphino)-methane) for CO molecules on dinuclear iron complexes led to insertion of CO into C-C bonds of alkyne-derived metallacycles. Similar behavior was observed when [PPNJCl salts were used to favor the formation of alkyne-substituted triruthenium dppm-containing clusters.I This behavior should be compared with the insertion of CO into allenylidene and phosphido-bridging ligands occurring when dppm coordinates to binuclear ruthenium complexes as shown in Fig. 3. This reaction is a nucleophilic attack of the coordinated allenylidene and phosphido groups on a coordinated CO (see Section 2.8.2.2). 

Also, bi- and trimetallic cumulene complexes, such as M=C=M, M=M=C, M=M=M and M=C=C=M are known. Cationic ruthenium allenylidene complexes are used as catalysts for ring closing metathesis reactions. Nonlinear optical properties have been measured for the Group 6 cumulenylidene complexes. Also, cationic chromium or iron vinylidene complexes undergo [2-1-2] cycloaddition reactions across imines to give fi-lactams. This reaction is useful for the synthesis of j8-lactam antibiotics. ... 

The chemistry of transition metal complexes containing alkylidene, allenylidene, and cumulenylidene ligands, [L M]=C(=C) =CRR n > 2), has been reviewed.An extensive series of Cp (dppe)Fe compounds with an end-bound hydrocarbon ligand have been synthesized, and the iron-carbon bonding studied using Fe Mossbauer spectroscopy. 

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