Cyclobutadiene rings, coordinated

POLYMERS CONTAINING METALLACYCLOPENTADIENE RINGS AND CYCLOBUTADIENE RINGS COORDINATED TO CYCLOPENTADIENYL METAL COMPLEXES... 

The X-ray study 170, 171) established a planar structure for the cyclobutadiene ring with C-—C distance equal to 1.46 A and angles of 90°. All the M—C distances are equivalent and close to those observed in ferrocene. The phenyl and methyl substituents are distorted from the ring plane and bent towards the metal atom. If one assumes that cyclobutadiene occupies two coordination sites then in the known tetraphenylcyclobutadiene-nickel and -palladium complexes the metal atom has a coordination number of 5. This suggests coordinative unsaturation for the metal and a priori one may expect an associative substitution for such complexes. 

Figure 2.28 Coordinate system (top), energy levels (left) and real Huckel MOs (right) for cyclobutadiene (ring with N = 4)...

Bunz and co-workers reported the preparation of polymers containing cyclopentadienyl-cobalt moieties coordinated to cyclobutadiene rings, using diethylnyl and dipropynyl cyclobu-... 

The sandwich structure of a benzene ring and cyclobutadiene ring is shown in Figure 21.6 [99]. The outer electrons by coordination of these two rings become 20-electrons. Therefore the platinum atom becomes Pt " [99]. 

Tomita, Endo, and coworkers have reported the synthesis of thermotropic liquid crystalline polymers containing cyclopentadienylcobalt moieties coordinated to cyclobutadiene rings. Scheme 37 shows the polymerization of monomers 138... 

The structure of compound 223 was ascertained by a crystal structure analysis. Whereas the thermal treatment of 223 afforded the expected phenylene 224, under UV irradiation, a cyclobutadiene ring was formed, 225. The UV light was most probably responsible for the dissociation of the alkyne -bound to Co in 223, thus producing a coordinatively unsaturated Co derivative which is well known to lead to such cyclobutene compounds via reductive elimination (see Section 7.01.4.3). 

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