Reaction Chemistry of Sterically Crowded Tris(pentamethylcyclopentadienyl)samarium¹

Although (C 5 Me 5 ) 3 Sm 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 Ph 3 P E (E = O, S, Se). The reactions include polymerization, insertion, ring-opening, and reduction. Depending on the substrate, (C 5 Me 5 ) 3 Sm can react (1) as if it were a bulky alkyl complex of formula (C 5 Me 5 ) 2 SmR in which R is an η 1 -C 5 Me 5 group or (2) as if it were the zwitterion [(C 5 Me 5 ) 2 Sm] + [C 5 Me 5 ] - in which the [C 5 Me 5 ] - component is a one-electron reductant. In the former mode, this compound (a) reacts with CO to form (C 5 Me 5 ) 2 Sm(O 2 C 7 Me 5 ), which has a ligand containing a nonclassical carbocationic center, (b) undergoes hydrogenolysis with H 2 to form [(C 5 Me 5 ) 2 Sm(μ-H)] 2, (c) ring-opens THF to form (C 5 Me 5 ) 2 Sm[O(CH 2 ) 4 (C 5 Me 5 )](THF), (d) inserts PhCN to form (C 5 Me 5 ) 2 Sm[NC(Ph)(C 5 Me 5 )](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(η 1 -C 5 Me 5 ) bond. On the other hand, (C 5 Me 5 ) 3 Sm reduces (a) Ph 3 P O to form PPh 3, (C 5 Me 5 ) 2, and [(C 5 Me 5 ) 2 Sm] 2 (μ-O), (b) Ph 3 P E (E = S, Se) to form PPh 3, (C 5 Me 5 ) 2, and a complex which adds THF to form [(C 5 Me 5 ) 2 Sm(THF)] 2 (μ-E), (b) cyclooctatetraene to form (C 5 Me 5 )Sm(C 8 H 8 ) and (C 5 Me 5 ) 2, (c) azobenzene to form (C 5 Me 5 ) 2 Sm(N 2 Ph 2 ) and (C 5 Me 5 ) 2, and (d) Me 3 CNC to form [(C 5 Me 5 ) 2 Sm(μ-CN)(CNCMe 3 )] 3, 4 . (C 5 Me 5 ) 3 Sm 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.