Cationic rare-earth metal trimethylsilylmethyl complexes supported by THF and 12-crown-4 ligands: Synthesis and structural characterization

Article abstract of DOI:10.1021/ic0511165

To expand the limited range of rare-earth metal cationic alkyl complexes known, a series of mono- and dicationic trimethylsilylmethyl complexes supported by THF and 12-crown-4 ligands with [BPh₄]^-, [BPh ₃(CH₂SiMe₃)]^-, [B(C ₆F₅)₄]^-, [B(C₆F ₅)₃(CH₂SiMe₃)]^-, and [Al(CH₂SiMe₃)₄]^- anions were prepared from corresponding neutral precursors [Ln(CH₂SiMe ₃)₃L_n] (Ln = Sc, Y, Lu; L = THF, n = 2 or 3; L = 12-crown-4, n = 1) as solvent-separated ion pairs. The syntheses of the monocationic derivatives [Ln(CH₂SiMe₃)₂(12- crown-4)_n(THF)_m]⁺[A]^- are all high yielding and proceed rapidly in THF solution at room temperature. A one pot procedure using the neutral species directly for the syntheses of a number of lutetium and yttrium dicationic derivatives [Ln(CH ₂SiMe₃)(12-crown-4)_n(THF)_m] ²⁺[A]^-₂ with a variety of different anions, a class of compounds previously limited to just a few examples, is presented. When BPh₃ is used to generate the ion triple, the presence of 12-crown-4 is required for complete conversion. Addition of a second equiv of 12-crown-4 and a third equiv of [NMe₂PhH]⁺[B(C₆F ₅)₄]^- abstracts a third alkyl group from [Ln(CH₂SiMe₃)(12-crown-4)₂(THF) _x]²⁺[B(C₆F₅)₄] ^-₂ (Ln = Y, Lu). X-ray crystallography and variable-temperature (VT) NMR spectroscopy reveal a structural diversity within the known series of neutral 12-crown-4 supported tris-(trimethylsilylmethyl) complexes [Ln(CH₂SiMe₃)₃(12-crown-4)] (Ln = Sc, Y, Sm, Gd-Lu) in the solid and solution states. The X-ray structure of [Sc(CH₂SiMe₃)₃(12-crown-4)] exhibits incomplete 12-crown-4 coordination. VT NMR spectroscopy indicates fluxional 12-crown-4 coordination on the NMR time scale. X-ray crystallography of only the second structurally characterized dicationic rare-earth metal alkyl complex [Y(CH ₂SiMe₃)(12-crown-4)(THF)₃] ²⁺[BPh₄]^-₂ shows exocyclic 12-crown-4 coordination at the 8-coordinate metal center with well separated counteranions. ¹¹B and ¹⁹F NMR spectroscopy of all mono- and dicationic rare-earth metal complexes reported demonstrate that the anions are symmetrical and noncoordinating on the NMR time scale. A series of trends within the ¹H and ¹³C{¹H} NMR resonances arising from the Ln-CH₂ groups and, in the case of yttrium, the ¹J_YC coupling constants at the Y-CH₂ group and the ⁸⁹Y chemical shift values are discussed.