Reductive coupling of carbon monoxide to glycolaldehyde and hydroxypyruvaldehyde polyanions in binuclear cyclopentadienyl lanthanum and lutetium derivatives: analogies to cyclooctatetraene thorium chemistry
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 CnOn2− (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-C2O24− 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-C3O34− 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.
Carbon monoxide coupling Lanthanides Lutetium Lanthanum Cyclopentadienylmetal derivatives Reductive coupling Glycolaldehyde Hydroxypyruvaldehyde Density functional theory
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We acknowledge the support of the Chinese National Natural Science Foundation (Grant Nos. 11447228 and 11174236), the Natural Science Foundation of the Department of Education in Sichuan Province of China (Grant No. 15ZB0129), the Funds for Sichuan Distinguished Scientists (Grant No. 2015JQ0042—China), the Funds for the Youth Innovation Team of Sichuan Province (Grant No. 14TD0013—China), the Program of the Key Scientific Research in Xihua University (Grant No. z1313320), Undergraduate Training Programs for Innovation and Entrepreneurship of Sichuan Province (Grant No. 05020732—China), and the US National Science Foundation (Grants CHE-1057466 and CHE-1361178).