Open samarocene and ytterbocenes and their adducts with N-heterocyclic carbene (NHC) and imidazolin-2-thiones

Reaction of the open samarocene [(eta(5)-pdl')(2)Sm(thf)(2)] (1-Sm; pdl' = 2,4-(Bu2C5H5)-Bu-t) towards IMes (= 1,3-dimesitylimidazolin-2-ylidene), (IBu)-Bu-t (= 1,3-di-tert-butylimidazolin-2-ylidene) and (IPr2Me2)-Pr-i (= 1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene) forms the corresponding NHC adducts, [(pdl')(2)Sm(IMes)] (2-Sm), [(pdl')(2)Sm((IBu)-Bu-t)] (3-Sm) and [(pdl')(2)Sm((IPr2Me2)-Pr-i)] (4-Sm), respectively in good to excellent yields. Diverging reactivity patterns emerge when we attempted to prepare the related adducts using the open ytterbocene [(pdl')(2)Yb(thf)] (1-Yb). Attributed to the smaller ionic radius of Yb-II no adduct is observed for IMes. However, for the smaller ItBu ligand isobutene elimination occurs to yield the imidazole Yb-II adduct (3'-Yb). For the slightly less sterically encumbered (IPr2Me2)-Pr-i two products were isolated, albeit in low yield: [(pdl')(2)Yb((IPr2Me2)-Pr-i)] (4-Yb) and [{(pdl')Yb((IPr2Me2)-Pr-i)(mu-S)}(2)] (5-Yb). The surprising formation of the mu-sulfido bridged species 5-Yb suggested that this product might result from trace impurities of imidazoline-2-thioneoriginating from the synthesis of (IPr2Me2)-Pr-i. To verify this hypothesis the first imidazoline-2-thioneadducts in organolanthanide chemistry were prepared by the reaction of 1-Sm and 1-Yb towards S= IMe2Me2 and S=(IPr2Me2)-Pr-i, respectively. All of these adducts were structurally authenticated. However, the imidazoline-2-thione adducts slowly degrade in solution with the Yb derivatives beingless stable. Analyses of the organic degradation products suggest that reduction of imidazoline-2thione to imidazoline-2-ylidene can indeed be accomplished by the lanthanide metal (providing oneelectron) as well as the (pdl')(-) ligand (also delivering one electron).