Reaction Chemistry of Sterically Crowded Tris(pentamethylcyclopentadienyl)samarium¹

Although (C₅Me₅)₃Sm is an extremely sterically crowded molecule, it displays high reactivity with a variety of substrates including CO, THF, ethylene, hydrogen, nitriles, isonitriles, isocyanates, 1,3,5,7-cyclooctatetraene, azobenzene, and Ph₃PE (E = O, S, Se). The reactions include polymerization, insertion, ring-opening, and reduction. Depending on the substrate, (C₅Me₅)₃Sm can react (1) as if it were a bulky alkyl complex of formula (C₅Me₅)₂SmR in which R is an η¹-C₅Me₅ group or (2) as if it were the zwitterion [(C₅Me₅)₂Sm]⁺[C₅Me₅]^- in which the [C₅Me₅]^- component is a one-electron reductant. In the former mode, this compound (a) reacts with CO to form (C₅Me₅)₂Sm(O₂C₇Me₅), which has a ligand containing a nonclassical carbocationic center, (b) undergoes hydrogenolysis with H₂ to form [(C₅Me₅)₂Sm(μ-H)]₂, (c) ring-opens THF to form (C₅Me₅)₂Sm[O(CH₂)₄(C₅Me₅)](THF), (d) inserts PhCN to form (C₅Me₅)₂Sm[NC(Ph)(C₅Me₅)](NCPh), 2, and (e) reacts with PhNCO to form 3, a product which can be rationalized by a C−N coupling between a coordinated PhNCO and a PhNCO unit inserted into a Sm(η¹-C₅Me₅) bond. On the other hand, (C₅Me₅)₃Sm reduces (a) Ph₃PO to form PPh₃, (C₅Me₅)₂, and [(C₅Me₅)₂Sm]₂(μ-O), (b) Ph₃PE (E = S, Se) to form PPh₃, (C₅Me₅)₂, and a complex which adds THF to form [(C₅Me₅)₂Sm(THF)]₂(μ-E), (b) cyclooctatetraene to form (C₅Me₅)Sm(C₈H₈) and (C₅Me₅)₂, (c) azobenzene to form (C₅Me₅)₂Sm(N₂Ph₂) and (C₅Me₅)₂, and (d) Me₃CNC to form [(C₅Me₅)₂Sm(μ-CN)(CNCMe₃)]₃, 4. (C₅Me₅)₃Sm also initiates the polymerization of ethylene. This reaction chemistry is described here as well as structural data on 2−4, each of which has a formal eight-coordinate bent metallocene geometry around samarium.