Tetramethyleneethane as a Bis(allylic) Ligand: 16-Electron Cobalt Configurations in Preference to Cobalt-Cobalt Bonding

Recently (2010) O'Hare and collaborators have synthesized and structurally characterized a tetramethyleneethane (TME) cobalt carbonyl complex, namely, trans-(eta(3),eta(3)-TME)Co-2(CO)(6), by reaction of NaCo(CO)(4) with 2,3-bis(bromomethyl)butadiene (TMEBr2). Motivated by this experimental discovery, the structures and energetics of the series of TMECo2(CO)(n) derivatives (n = 6, 5, 4, 3, 2) have now been investigated by density functional theory. The lowest energy TMECo2(CO)(6) structure is this experimentally known trans-(eta(3),eta(3)-TME)Co-2(CO)(6) structure consisting of two (eta(3)-allyl)Co(CO)(3) units linked by a C-C bond joining the central carbons of the allylic ligands in each half. However, the corresponding cis-TMECo2(CO)(6) structure lies only similar to 3 kcal/mol above the global minimum. The lowest energy TMECo2(CO)(5) and TMECo2(CO)(4) structures consist of (eta(3)-allyl)Co(CO)(3) and/or (eta(3)-allyl)Co(CO)(2) building blocks linked through the central allyl carbon atoms to give the TME ligand. The cobalt atoms in the (eta(3)-allyl)Co(CO)(2) portions,of these structures have only a 16-electron configuration rather than the normally favored 18-electron configuration. Such TMECo2(CO)(n) structures (n = 6, 5, 4) are found with both cis and trans stereochemistries, corresponding to locations of the cobalt atoms on the same or opposite sides, respectively, of the plane of the TME ligand. In the only low-energy TMECo2(CO)(3) structure a Co -> Co dative bond from the Co(CO)(2) cobalt to the Co(CO) cobalt gives both cobalt atoms a 16-electron configuration. For TMECo2(CO)(2) both of the low-energy structures have bis(tetrahapto) eta(4),eta(4)-TME ligands in which the two central carbon atoms of the TME ligand bridge both cobalt atoms.