Open samarocene and ytterbocenes and their adducts with

Abstract

Reaction of the open samarocene [(η5-pdl′)2Sm(thf)2] (1-Sm; pdl′ = 2,4-tBu2C5H5) towards IMes (= 1,3-dimesitylimidazolin-2-ylidene), ItBu (= 1,3-di-tert-butylimidazolin-2-ylidene) and IiPr2Me2 (= 1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene) forms the corresponding NHC adducts, [(pdl′)2Sm(IMes)] (2-Sm), [(pdl′)2Sm(ItBu)] (3-Sm) and [(pdl′)2Sm(IiPr2Me2)] (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′)2Yb(thf)] (1-Yb). Attributed to the smaller ionic radius of YbII no adduct is observed for IMes. However, for the smaller ItBu ligand iso-butene elimination occurs to yield the imidazole YbII adduct (3′-Yb). For the slightly less sterically encumbered IiPr2Me2 two products were isolated, albeit in low yield: [(pdl′)2Yb(IiPr2Me2)] (4-Yb) and [{(pdl′)Yb(IiPr2Me2)(μ-S)}2] (5-Yb). The surprising formation of the μ-sulfido bridged species 5-Yb suggested that this product might result from trace impurities of imidazoline-2-thione originating from the synthesis of IiPr2Me2. To verify this hypothesis the first imidazoline-2-thione adducts in organolanthanide chemistry were prepared by the reaction of 1-Sm and 1-Yb towards S=IMe2Me2 and S=IiPr2Me2, respectively. All of these adducts were structurally authenticated. However, the imidazoline-2-thione adducts slowly degrade in solution with the Yb derivatives being less stable. Analyses of the organic degradation products suggest that reduction of imidazoline-2-thione to imidazoline-2-ylidene can indeed be accomplished by the lanthanide metal (providing one electron) as well as the (pdl′)− ligand (also delivering one electron).