New Dinuclear 5-ansa-Lanthanocene Derivatives. Crystal Structures of [Ln{2,6-(CH2C5H4)2C5H3N}μ-O2SOCF3]2 (Ln = Nd, Y, Yb) and a VT Solution NMR (1H, 19F) Study of the Fluxional Yttrium Homologue

Five homologues of the new complex series [LnL(μ-OTf)] 2 with Ln = Pr ( 1 ), Nd ( 2 ), Sm ( 3 ), Y ( 4 ), Yb ( 5 ) have been prepared from [Ln(OTf) 3 ] and Na 2 L, where L designates two cyclopentadienyl rings tethered by a 2,6-bis(methylene)pyridyl unit and −OTf the triflate (trifluoromethanesulfonate) anion. According to crystal structure analyses of 2, 4, and 5, each triflate ligand bridges two Ln ions (Ln−O−S(O)−O‘−Ln‘), while, owing to substantial Ln−N(pyridyl) bonding, the LnL fragments turn out to be notably rigid entities. Nevertheless, the variable-temperature (VT) 1 H NMR spectra of all complexes reveal notable fluxionality. According to a detailed VT 19 F NMR study of 4, two dinuclear isomers, differing in the trans and cis orientations, respectively, of their two L fragments, interconvert with a negligibly small entropy of activation Δ S ⧧ . A motion reminiscent of the Berry turnstile would most plausibly explain the experimental findings.