Reductive coupling of carbon monoxide to glycolaldehyde and hydroxypyruvaldehyde polyanions in binuclear cyclopentadienyl lanthanum and lutetium derivatives: analogies to cyclooctatetraene thorium chemistry

Abstract

Cloke and coworkers have recently (2006–2012) shown that reaction of carbon monoxide with organouranium compounds results in reductive coupling to yield the oligomeric anions C _n O ²⁻ _n (n = 2, 3, 4). In order to explore the possibilities of similar reductive coupling of carbon monoxide in organolanthanide systems, the structures and thermochemistry of the cyclopentadienyllanthanide carbonyls Cp₂Ln₂(CO) _n (n = 2, 3, 4, 5) have been investigated using lanthanum and lutetium, which are diamagnetic in the favored +3 oxidation state. All of these Cp₂Ln₂(CO) _n structures have long Ln···Ln distances exceeding 4.2 Å for La and 3.6 Å for Lu, indicating the lack of direct metal–metal bonding and suggesting the normally favored +3 oxidation state for these lanthanides. In the dicarbonyls Cp₂Ln₂(CO)₂, the two CO groups couple to form a bridging µ-C₂O₂ ⁴⁻ ligand, which can be derived by removal of four protons from glycolaldehyde (hydroxyacetaldehyde). Similarly, in the tricarbonyls, the three CO groups couple to form a bridging µ-C₃O₃ ⁴⁻ ligand, which can be derived by removal of four protons from hydroxypyruvaldehyde. However, the lowest energy structures for the tetracarbonyls Cp₂Ln₂(CO)₄ (by more than 13 kcal/mol) have four separate η²-µ-CO ligands bonded to the central Ln₂ unit through both their carbon and oxygen atoms. Thermochemistry of the Cp₂Ln₂(CO) _n systems suggests viability of Cp₂Ln₂(CO)₂ and Cp₂Ln₂(CO)₄. However, Cp₂Ln₂(CO)₃ is predicted to be disfavored relative to disproportionation into Cp₂Ln₂(CO)₂ + Cp₂Ln₂(CO)₄.