Facile Insertion of CO2 into Tetra- and Pentamethylcyclopentadienyl Lanthanide Moieties To Form (C5Me4RCO2)- Carboxylate Ligands (R = H, Me)

Recent efforts to extend the reduction chemistry of LnZ3/M and LnZ2Z‘/M (Ln = lanthanide; Z = C5Me5, C5Me4H; Z‘ = BPh4, Cl; M = alkali metal) to substrates beyond N2 have identified unexpectedly facile insertion of CO2 into lanthanide cyclopentadienyl linkages to make carboxylate ligands. The insertion was initially identified in [(C5Me4H)La(THF)]3(μ-η1:η1-O2CC5Me4H)3(μ3-η2:η2:η2-CO3)(μ3-Cl), 1, a product of a (C5Me4H)3La/Na/CO2 reaction in the presence of chloride. In this case, both insertion and reduction of CO2 to carbonate was observed. A [(C5Me5)2La][(μ-Ph)2BPh2]/KC8/CO2 reaction also provided a CO2 insertion product, [(C5Me5)La(μ-η1:η1-O2CC5Me5)(μ-η1:η2-O2CC5Me5)]2, 2, but in this case a CO2 reduction product was not identified. CO2 insertion also occurs in the absence of an alkali metal. Hence, the (C5Me4H)3Ln complexes (Ln = La, Nd, Sm, Gd) react with CO2 to form mixtures of tetramethylcyclopentadienyl carboxylate products. In the La complex, single crystals of {[(C5Me4H)2La](μ-η1:η1-O2CC5Me4H)}2, 3, could be isolated and structurally characterized. Crystallographic characterization of a rare example of a tetramethylcyclopentadienyl lanthanide ate salt, [(μ-C5Me4H)(C5Me4H)LaCl(μ-Cl)K(18-crown-6)]n, 4, was also obtained as part of this study, and it was found to have a chain structure in the solid state involving bridging methyl groups as well as bridging chloride ligands.