Modification of Iron(II) Tridentate Bis(imino)pyridine Complexes by a Boryl Group for the Production of α-Olefins at High Temperature

A new series of boryl-substituted bis(imino)pyridine ligands [o-PinacolB-Ph-NC(Me)-Py-C(Me)N-Ph-o-BPinacol], Pinacol = (−O-(Me)2C−C(Me)2-O−) (6), [2,4,6-tri-Me-Ph-NC(Me)-Py-C(Me)N-Ph-o-BPinacol] (11), [4-bromo-2,6-di-Me-Ph-NC(Me)-Py-C(Me)N-Ph-o-BPinacol] (12), [2,4,6-tri-Me-Ph-NC(Me)-Py-C(Me)N-Ph-m-BPinacol] (14), [2,4,6-tri-Me-Ph-NC(Me)-Py-C(Me)N-Ph-p-BPinacol] (17), and [4-Bromo-2,6-di-Me-Ph-NC(Me)-Py-C(Me)N-Ph-p-BPinacol] (18), and their corresponding Fe(II) complexes [{o-PinacolB-Ph-NC(Me)-Py-C(Me)N-Ph-o-BPinacol}FeCl2] (3), [{2,4,6-tri-Me-Ph-NC(Me)-Py-C(Me)N-Ph-o-BPinacol}FeCl2] (19), [{4-bromo-2,6-di-Me-Ph-NC(Me)-Py-C(Me)N-Ph-o-BPinacol}FeCl2] (20), [{2,4,6-tri-Me-Ph-NC(Me)-Py-C(Me)N-Ph-m-BPinacol}FeCl2] (21), [{2,4,6-tri-Me-Ph-NC(Me)-Py-C(Me)N-Ph-p-BPinacol}FeCl2] (22), and [{4-bromo-2,6-di-Me-Ph-NC(Me)-Py-C(Me)N-Ph-p-BPinacol}FeCl2] (23) were synthesized. According to X-ray analysis, the introduction of boryl groups in the ortho-positions of the complex as in 3 caused the shortening of the axial Fe−N bond lengths (up to 0.02 Å) vs the analogous Fe(II) complex with ortho-methyl groups [{o-Me-Ph-NC(Me)-Py-C(Me)N-Ph-o-Me}FeCl2], 1. The comparisons of the axial bond lengths in ortho-boryl-substituted isomer 19 with para-boryl-substituted 22 revealed that these bonds are shorter in the ortho-isomer (2.201(11) Å) than in the para-isomer (2.232(2) Å). An interesting structural feature of 3 is that it exists in the unexpected “up−up” conformer in the solid state, despite reasonable bulkiness of dioxaboranyl substituents in ortho-positions. Complexes 21–23 afforded very productive catalysts for the production of α-olefins with a more linear Schultz−Flory distribution and with the least amounts of the heavier insoluble fractions of α-olefins than the parent methyl-substituted Fe(II) complex 1.