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

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)(2)Mg2Cl2(THF)]<middle dot>0.2Tol (3<middle dot>0.2Tol), [(L)(2)Mg2Br2(THF)] (4) and [(L1)(2)Mg-2] (5) (HL = 2-(2-((((1H-indol-2-yl)methylene)amino)methyl)-1H-pyrrol-1-yl)-N,N-dimethylethan-1-amine, H(2)L1 = 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<middle dot>0.2Tol and 4, were obtained. When the magnesium source was changed to (Bu2Mg)-Bu-n, the nucleophilic addition of (Bu2Mg)-Bu-n 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<middle dot>0.2Tol and 4 exhibited higher activities. The catalytic reactions possess the features of excellent functional group tolerance and good chemo- and regioselectivity.