文摘
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 n 2− (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.