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

To expand the limited range of rare-ea h metal cationic alkyl complexes known, a series of mono- and dicationic trimethylsilylmethyl complexes supported by THF and 12-crown-4 ligands with [BPh4](-), [BPh3(CH2SiMe3)](-), [B(C6F5)(4)](-), [B(C6F5)(3)(CH2SiMe3)](-), and [Al(CH2SiMe3)(4)](-) anions were prepared from corresponding neutral precursors [Ln(CH2SiMe3)(3)L-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(CH2SiMe3)(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(C6F5)(4)](-) abstracts a third alkyl group from [Ln(CH2SiMe3)(12-crown-4)(2)(THF)](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-ea h metal alkyl complex [Y(CH2SiMe3)(12-crown-4)(THF)(3)](2+)[BPh4](-)(2) shows exocyclic 12-crown-4 coordination at the 8-coordinate metal center with well separated counteranions. B-11 and F-19 NMR spectroscopy of all mono- and clicationic rare-earth metal complexes reported demonstrate that the anions are symmetrical and noncoordinating on the NMR time scale. A series of trends within the H-1 and C-13{H-1} NMR resonances arising from the Ln-CH2 groups and, in the case of yttrium, the (1)J(YC) coupling constants at the Y-CH2 group and the Y-89 chemical shift values are discussed.