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

Abstract

Recently (2010) O’Hare and collaborators have synthesized and structurally characterized a tetramethyleneethane (TME) cobalt carbonyl complex, namely, trans-(η3,η3-TME)Co2(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 knowntrans-(η3,η3-TME)Co2(CO)6 structure consisting of two (η3-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 ∼3 kcal/mol above the global minimum. The lowest energy TMECo2(CO)5 and TMECo2(CO)4 structures consist of (η3-allyl)Co(CO)3 and/or (η3-allyl)Co(CO)2 building blocks linked through the central allyl carbon atoms to give the TME ligand. The cobalt atoms in the (η3-allyl)Co(CO)2 portions o...