Metal complexes of cyclobutadiene

Several transition-metal complexes of cyclobutadiene have been prepared, and this is all the more remarkable because of the instability of the parent hydrocarbon. Reactions that logically should lead to cyclobutadiene give dimeric products instead. Thus, 3,4-dichlorocyclobutene has been de-chlorinated with lithium amalgam in ether, and the hydrocarbon product is a dimer of cyclobutadiene, 5. However, 3,4-dichlorocyclobutene reacts with diiron nonacarbonyl, Fe2(CO)9, to give a stable iron tricarbonyl complex of cyclobutadiene, 6, whose structure has been established by x-ray analysis. The 7r-electron system of cyclobutadiene is considerably stabilized by complex formation with iron, which again attains the electronic configuration of krypton. 

To date there are no examples of the very highly strained cyclopropadiene (288), 1,2-cyclobutadiene (289), or 1,2-cyclopentadiene (290), either free or complexed to transition metals. Complexes of a valence isomeric form of cyclopropadiene (291) have been prepared,110 but as with the free hydrocarbon,111112 there is no evidence for any converting to the allene form [Eq. (46)]. 

Ni(r -C4Ph4)2 is notable as the first metal sandwich based on two C4 rings [completing the series Cr(r -C6R6)2, Fe(rj-C5R5)2 and Ni(rj-C4R4)2] and also because its isolation (m.p. 404 °C ) defied theoretical calculations suggesting that d10 complexes of cyclobutadienes would not be... 

Transition-metal complex of benzyne, It is well known that some very unstable compounds (e.g., cyclobutadiene) can be isolated as complexes with transition... 

Cyanides do form metal complexes, and are important as linking groups between metalsAn important reaction of cyanides is with a molecule already complexed to a metal. This reaction is quite similar to the reaction reported by Hogeveen between aluminium halide a complexes of cyclobutadiene and cyanides, yielding a mixture of pyridines. 

The unfused cyclobutadiene system is stable in complexes with metals (see Chapter 3), but in these cases electron density is withdrawn from the ring by the metal and there is no aromatic quartet. In fact, these cyclobutadiene-metal complexes can be looked upon as systems containing an aromatic duet. The ring is square planar, the compounds undergo aromatic substitution, and NMR spectra of monosubstituted derivatives show that the C-2 and C-4 protons are equivalent. ... 

Catenated Organic Compounds of the Group IV Elements, 4,1 Conjugate Addition of Grignard Reagents to Aromatic Systems, 1, 221 Cyclobutadiene Metal Complexes, 4, 95 Cyclopentadienyl Metal Compounds, 2, 365 Diene-Iron Carbonyl Complexes, 1, 1... 

Crystal structure data15 indicate that in the vast majority of (cyclobutadiene)metal complexes (4) the cyclobutadiene ligand is approximately square-planar with nearly equal C—C bond distances (ca 1.46 A) and bond angles of ca 90°. Within a given complex the cyclobutadiene carbon-to-metal distances are roughly equal. 

While one of the first preparations of a cyclobutadiene-metal complex involved the cyclodimerization of diphenylacetylene in the presence of Fe(CO)5 at high temperature212, the thermal reaction of alkynes with Fe(CO)s gives predominantly cyclopentadienone complexes (Section IV.E.l.b). The cyclization of alkynes by a wide variety of metal complexes has been reported (Scheme 59)l 5-21 A—222... 

The (tetraphenylcyclobutadiene)PdX2 dimer reacts with a variety of metal complexes via transfer of the cyclobutadiene ligand to another metal. These reactions and other ligand transfer reactions have been reviewed by Efraty15. 

The reaction of (cyclobutadiene)metal complexes with X2 results in the oxidative decomplexation to generate either dihalocyclobutenes or tetrahalocyclobutanes. In comparison, substitution of (cyclobutadiene)MLn complexes 223 [MLn = Fe(CO)3, CoCp, and RhCp] with a variety of carbon electrophiles has been observed (equation 34)15. Electrophilic acylation of 1-substituted (cyclobutadiene)Fe(CO)3 complexes gives a mixture of regioisomers predominating in the 1,3-disubstituted product and this has been utilized for the preparation of a cyclobutadiene cyclophane complex 272 (equation 35)246. For (cyclobutadiene)CoCp complexes, in which all of the ring carbons are substituted, electrophilic acylation occurs at the cyclopentadienyl ligand. 

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