Magnesium(ii) complexes supported by indole- and pyrrole-containing Schiff-base ligands: syntheses, structures, and applications as precatalysts in the hydroboration of nitriles and alkynes†

Abstract

The application of organometallic magnesium(II) compounds in the catalytic dihydroboration of nitriles and hydroboration of alkynes has been reported. Five compounds [(L)MgCl(THF)₃] (1), [(L)MgBr(THF)₃] (2), [(L)₂Mg₂Cl₂(THF)]·0.2Tol (3·0.2Tol), [(L)₂Mg₂Br₂(THF)] (4) and [(L1)₂Mg₂] (5) (HL = 2-(2-((((1H-indol-2-yl)methylene)amino)methyl)-1H-pyrrol-1-yl)-N,N-dimethylethan-1-amine, H₂L1 = N-((1-(2-(dimethylamino)ethyl)-1H-pyrrol-2-yl)methyl)-1-(1H-indol-2-yl)pentan-1-amine) were prepared. Mononuclear complexes 1 and 2 were synthesized by the 1 : 1 reaction of HL with THF solution of MeMgCl and MeMgBr, respectively. When the reactions were conducted in toluene, two dinuclear compounds, 3·0.2Tol and 4, were obtained. When the magnesium source was changed to ⁿBu₂Mg, the nucleophilic addition of ⁿBu₂Mg to the CN bond of the HL ligand occurred, affording the new ligand [L1]²⁻via the transformation of the CN bond into a C–N bond and the generation of a new C–C_Bu bond. Complexes 1–5 are active precatalysts for the hydroboration of nitriles and alkynes, and 3·0.2Tol and 4 exhibited higher activities. The catalytic reactions possess the features of excellent functional group tolerance and good chemo- and regioselectivity.