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

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 (Amd^OMe) 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); Amd^OMe, 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.