Planar bridged acetylene

The dipyrrylacelylenedicarbaldehyde 18, which already contains, along with the acetylene moiety, two of the two-carbon bridges of the final macrotetracycle, can be prepared by a Heck-type coupling of 5-iodopyrrole-2-carbaldehydes with acetylene.8a,b The main conjugation pathway with 22n electrons is already present in the acetylene-cumulate systems 19. The expected planarity of this chromophore has been confirmed by X-ray structure analysis.8a b... 

Unfortunately, the only available theoretical study on lithium-acetylene species is that of Apeloig and coworkers on dilithioethylene, in which the most stable isomer of Li2C2H2 was found to have a C2v geometry with two lithium atoms bridging the C—C bond and the C2H2 group in a cis planar position. This seems to indicate structure 5 more probable than structure 4. Consequently, the structure, which meets best all conditions, is 5 (Scheme 3). ... 

It is not known if cw-addition occurs first followed by c/s-Zra j-isomerization. The infrared spectrum of the primary product of lithium atoms and acetylene molecules in an argon matrix at 15 K speaks for a planar structure LiC Hj with lithium bridging the n system and the hydrogen atoms situated at the opposite side in a cM-arrangement... 

The molecule [Fe2(CO)4(CioHis)2] (18) contains two Fe(CO)2 groups held together by a metal-metal bond and by two bridging hydrocarbon ligands." The Fe2(CO)4 unit is planar and perpendicular to the plane of the four acetylenic carbon atoms. The t-butyl groups are bent away from the... 

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