Selective Intermolecular C–H Bond Activation: A Straightforward Synthetic Approach to Heteroalkyl Yttrium Complexes Containing a Bis(pyrazolyl)methyl Ligand

Abstract

The reactions of bis(pyrazolyl)methanes CH2(C3HN2R2-3,5)2 (R = Me, tBu) with Y(CH2SiMe3)3(THF)2 and LY(CH2SiMe3)2(THF)n (L = amidopyridinate (Ap′), amidinate (Amd), tridentate amidinate bearing 2-methoxyphenyl pendant in a side arm (AmdOMe) and pentamethylcyclopentadienyl (Cp*); n = 0, 1) were investigated. CH2(C3HN2tBu2-3,5)2 turned out to be inert in these reactions, while less bulky CH2(C3HN2Me2-3,5)2 easily undergoes metalation by yttrium alkyls. The reaction of Y(CH2SiMe3)3(THF)2 with CH2(C3HN2Me2-3,5)2 regardless of the molar ratio of the reagents affords a homoleptic tris(alkyl) species, Y[CH(C3HN2Me2-3,5)2]3 (1). However, the reactions of equimolar amounts of LY(CH2SiMe3)2(THF)n and CH2(C3HN2Me2-3,5)2 occur selectively with replacement of a sole CH2SiMe3 fragment and afford the related heteroalkyl complexes LY(CH2SiMe3)[CH(C3HN2Me2-3,5)2](THF)n (L = Ap′, n = 1 (6); Amd, n = 0 (7); AmdOMe, n = 1 (8); Cp*, n = 1 (9)) in good yields. The second equivalent of CH2(C3HN2Me2-3,5)2 does not react with heteroalkyl yttrium complexes. The X-ray studies revealed that in complexes 1 and 6–9 the bis(pyrazolyl)methyl ligands are bound to the yttrium centers in a similar fashion via one covalent Y–C and two coordination Y–N bonds. Thermal decomposition of complexes 6–9 (C6D6, 80 °C) as evidenced by 1H NMR spectroscopy resulted in SiMe4 elimination, while no activation of the C–H bonds of bis(pyrazolyl)methyl ligands was detected. When 6 was treated with an equimolar amount of PhSiH3, only the YCH2SiMe3 bond selectively underwent σ-bond metathesis and a dimeric yttrium alkyl-hydrido complex, {Ap′Y[CH(C3HN2Me2-3,5)2](μ2-H)}2 (10), was formed. The reaction of 6 with 2,6-diisopropylaniline also resulted in the selective protonation of the YCH2SiMe3 bond and cleanly afforded alkyl-anilido complex Ap′Y(NHC6H3iPr2-2,6)[CH(C3HN2Me2-3,5)2](THF) (11). The ternary catalytic systems 6–9/borate/AliBu3 (borate = [HNMe2Ph][B(C6F5)4], [Ph3C][B(C6F5)4]; [Ln]:[borate]:[AliBu3] = 1:1:10) demonstrated moderate catalytic activity in isoprene polymerization; they allow quantitative conversion into polymer of up to 1000 equiv of monomer in 2–4 h. The best activity and 1,4-cis selectivity (83.5%) were demonstrated by amidinato complex 8.