An unexpected carbon-carbon bond formation in the reaction between [Co(.eta.5-C5H5)(CO)2] and hexafluorobut-2-yne. X-ray crystal structure of [{(.eta.5-C5H5)Co}2{.mu.-.eta.4,.eta.4-C4(CF3)4C10H12C4(CF3)4}].cntdot.0.5CH2Cl2

Theoretical Study of the Acetylene Trimerization with CpCo

“…Substitution of the remaining phosphine in 2 for an acetylene molecule results in the formation of bisacetylene adduct 3 , which has been proposed as an intermediate along the acetylene cyclotrimerization pathway on a number of occasions. ,,37a, The B3LYP-optimized geometry of 3 is also shown in Figure . The acetylene ligands are found to exhibit the characteristics of moderately bound alkynes with Co−C acetylene bond lengths of 2.00 Å, elongated acetylenic C−C bonds measuring 1.25 Å, and complements to the C−C−H bond angles of ∼25°.…”

“…Formation of the Cobaltacyclopentadiene. Oxidative coupling of the acetylene ligands in 3 results in the formation of cobaltacyclopentadiene 4 a coordinatively unsaturated, 16-electron species which has been proposed as an intermediate in the catalytic cycle by a number of authors. ,,37a, Metallacyclopentadiene rings containing d 6 or d 8 metal centers such as 4 are well-known to exhibit π-bond localization despite the presence of lone pairs on the metal that could contribute to the formation of an aromatic sextet. Using extended Hückel calculations, Thorn and Hoffmann found that the required interaction of a filled metal d-orbital with the in-phase combination of olefinic π * -orbitals suffered from ineffective overlap, preventing electronic delocalization.…”

“…An alternative way for 4 to achieve saturation is to coordinate a third acetylene molecule. The resulting cobaltacyclopentadiene complex, 5 , has been proposed as an intermediate leading to arene formation on a number of occasions. ,,37a,, Early in the development of the mechanism shown in Scheme , the most significant evidence arguing for the intermediacy of 5 was the observation that blocking the empty coordination site in 4 with an ancillary ligand slowed the rate of arene formation, implying that the third acetylene coordinated to the Co atom prior to arene construction . Recently, Vollhardt and co-workers 13 supplied more concrete evidence for the appearance of 5 in the catalytic cycle with their isolation of 23 from the intramolecular cyclotrimerization of bis(2-ethynylphenyl)ethyne with CpCo(C 2 H 4 ) 2 , providing the first experimentally isolated example of this structural motif in CpCo chemistry.…”

“…Although the exact mechanism for the formation of 7 is unknown, three pathways are logical candidates. Schore's mechanism 9 envisions the insertion of the coordinated alkyne ligand of 5 into a Co−C α σ-bond of the cobaltacyclopentadiene ring to generate cobaltacycloheptatriene 6 , a frequently proposed intermediate. ,37a, Subsequent reductive elimination of the bidentate C 6 H 6 ligand in 6 then generates 7 . The likelihood of this pathway suffers from two drawbacks.…”

Reactions of bis(diphenylphosphine)acetylene with metal carbonyl complexes

“…Our interest in alkyne chemistry [1][2][3][4][5][6][7] prompted us to investigate the behavior of bis(diphenylphosphine)acetylene, (DPPA), towards some metal-carbonyl complexes. DPPA is a potentially tridentate ligand, whose coordination capability has been extensively studied [8][9][10][11].…”