Synthesis of Cationic Silaamidinate Germylenes and Stannylenes and the Catalytic Application for Hydroboration of Pyridines

The N-heterocyclic germylenes and stannylenes LSi(NAr)₂EX (L = PhC(NtBu)₂, Ar = 2,6-iPr₂C₆H₃; E = Ge, Sn; X = Cl, CF₃SO₃, BPh₄) supported by the bulky silaamidinate ligand [LSi(NAr)₂]^- have been synthesized and fully characterized. The germylene triflate LSi(NAr)₂GeOTf (3b) and dimeric borate [LSi(NAr)₂Ge]₂ClBPh₄ (3a) enabled highly regio- and chemoselective catalytic hydroboration of pyridines and may represent the most active catalytic system for the transformation. DFT calculations disclosed that the cationic germylene [LSi(NAr)₂Ge]⁺ with a low-lying LUMO energy initiated the catalytic process. In contrast, the analogous amidinate germylene triflates are almost inactive, indicating the silaamidinate ligand is essential for the stabilization of cationic species.