| Abstract |
Four paramagnetic 14-electron tetracoordinated Fe(II) complexes of 5-substituted-2-iminopyrrolyl ligands of the type [Fe\kappa N-2, N -5-R-NC4H2-2-C(H)=N.2,6-iPr(2)-C6H3)\(Py)Cl], with R = 2,6-Me-2-C6H3 (1a), 2,4,6-Pr-i(3)-C6H2 (1b), 2,4,6-Ph-3-C6H3 (1c) and CPh3 (1d), were synthesized in moderate yields by reacting the respective 5-substituted-2-iminopyrrolyl potassium salts KLa-d with FeCl2(Py)(4) in toluene. Complexes 1a-d were characterized by H-1 NMR, FTIR spectroscopies, elemental analysis and by the Evans method, the corresponding effective magnetic moments showing a high-spin electronic nature. X-ray diffraction studies on complexes 1a and 1c showed distorted tetrahedral coordination geometries. Complexes 1a-c, activated with K(HBEt3), were efficient catalyst systems for the hydroboration of several terminal alkenes with pinacolborane in good to high yields (50-90\%). This system mainly yielded the respective anti-Markovnikov addition products, except when styrenes were used. A screening of the hydroboration of styrene catalyzed by complexes 1a-c activated with K(HBEt3) showed that the selectivity in the Markovnikov product increased with increasing steric bulkiness of the R group, exhibiting selectivities up to 91\%. Additionally, the stoichiometric reaction of complex 1b with K(HBEt3) over 30 minutes yielded the mixture of hydride species 2 and 2(2) (mixture I). On the other hand, when reacting the same components over 16 h, the Fe(I) complex 3 was also identified in the mixture, in addition to 2 + 2(2) (mixture II). These mixtures were characterized in solution by the Evans method and in the solid state by elemental analysis, Fe-57 Mossbauer and FTIR spectroscopies, compounds 2(2) and 3 being also analyzed by X-ray diffraction. These results suggest that the corresponding catalytic cycle follows the borane oxidative addition route to a Fe(I) species. |
