Formation and Spectroscopic Characterization of Chelated d0Yttrium(III)−Alkyl−Alkene Complexes

The yttrium hydride dimer (Cp* 2 YH) 2 ( 2 ) reacted rapidly with 3,3-dimethyl-1,4-pentadiene in methylcyclohexane- d 14 at −78 °C to form the d 0 yttrium(III) pentenyl chelate complex Cp* 2 Y[η 1,η 2 -CH 2 CH 2 C(CH 3 ) 2 CH CH 2 ] ( 4 ). Low-temperature 1 H, 13 C, and 1 H NOESY NMR spectroscopy of 4 established bonding of the tethered alkene to the d 0 metal center. Quantitative analysis of the NOESY time course using the Conformer Population Analysis method demonstrates that the dominant conformers in solution are interconverting pairs of chelated complexes, one in a twist-boat conformation and the other a chair conformer. No significant contribution by a free alkene conformation is required to explain the spectroscopic data. Addition of THF to pentenyl chelate 4 at −78 °C displaced the alkene and formed the yttrium(III) pentenyl THF adduct Cp* 2 Y[η 1 -CH 2 CH 2 C(CH 3 ) 2 CH CH 2 ](THF) ( 5 ), in which there is no interaction between the pendant alkene and the d 0 metal center. Yttrium hydride dimer 2 also reacted with either 1,4-pentadiene or methylenecyclobutane to form the same pentenyl chelate complex Cp* 2 Y[η 1,η 2 -CH 2 CH 2 CH 2 CH CH 2 ] ( 6 ). The interconversion of deuterium-labeled chelate complexes 6 -1,1,5,5- d 4 and 6 -3,3,5,5- d 4 at −78 °C (Δ G ⧧ = 14.4 ± 0.2 kcal mol - 1 ) establishes that intramolecular alkene insertion is occurring under the reaction conditions.