A binuclear trimethylenemethane cobalt carbonyl providing the first example of a low-energy perpendicular structure with acyclic hydrocarbon ligands

Abstract

Trimethylenemethane (TMM) has long been known as a ligand in metal carbonyl complexes such as [η⁴-(CH₂)₃C]Fe(CO)₃ and [η⁴-(CH₂)₃C]Cr(CO)₄. The prospects for synthesizing currently unknown TMM complexes of cobalt carbonyl have been explored by a density functional theory study of the binuclear complexes [(CH₂)₃C]₂Co₂(CO)_n (n = 2, 3, 4, 5, 6). The dicarbonyl is unexpectedly found to favor by more than ∼12 kcal mol⁻¹ a perpendicular structure [μ-(CH₂)₃C]₂Co₂(CO)₂ with bridging η⁴-TMM ligands, terminal CO groups, and a relatively short Co–Co distance of ∼2.29 Å. This is the first example of an energetically favorable perpendicular binuclear metal complex structure having bridging acyclic hydrocarbon ligands. The other viable species is [(CH₂)₃C]₂Co₂(CO)₄, which is a substitution product of the well-known Co₂(CO)₈ with terminal η⁴-TMM ligands. Similar to Co₂(CO)₈, doubly bridged and unbridged [(CH₂)₃C]₂Co₂(CO)₄ structures have approximately equal energies within ∼2 kcal mol⁻¹ and predicted Co–Co distances of ∼2.5 and ∼2.7 Å, respectively. Carbonyl-rich [(CH₂)₃C]₂Co₂(CO)_n (n = 5, 6) structures are also found with dihapto η²-TMM ligands. However, these are predicted to be unstable with respect to CO dissociation to give [(CH₂)₃C]₂Co₂(CO)₄.