ic401432m_si_001.cif (151.49 kB)
Synthesis, Structures, and Dearomatization by Deprotonation of Iron Complexes Featuring Bipyridine-based PNN Pincer Ligands
dataset
posted on 2016-02-19, 00:36 authored by Thomas Zell, Robert Langer, Mark A. Iron, Leonid Konstantinovski, Linda J. W. Shimon, Yael Diskin-Posner, Gregory Leitus, Ekambaram Balaraman, Yehoshoa Ben-David, David MilsteinThe synthesis and characterization
of new iron pincer complexes bearing bipyridine-based PNN ligands
is reported. Three phosphine-substituted pincer ligands, namely, the
known tBu-PNN (6-((di-tert-butylphosphino)methyl)-2,2′-bipyridine) and the two new iPr-PNN (6-((di-iso-propylphosphino)methyl)-2,2′-bipyridine)
and Ph-PNN (6-((diphenylphosphino)methyl)-2,2′-bipyridine)
ligands were synthesized and studied in ligation reactions with iron(II)
chloride and bromide. These reactions lead to the formation of two
types of complexes: mono-chelated neutral complexes
of the type [(R-PNN)Fe(X)2] and bis-chelated
dicationic complexes of the type [(R-PNN)2Fe]2+. The complexes [(R-PNN)Fe(X)2] (1: R = tBu, X = Cl, 2: R = tBu, X = Br, 3: R = iPr, X = Cl, and 4: R = iPr, X = Br) are readily prepared from reactions of FeX2 with the free R-PNN ligand in a 1:1 ratio. Magnetic susceptibility
measurements show that these complexes have a high-spin ground state
(S = 2) at room temperature. Employing a 2-fold or
higher excess of iPr-PNN, diamagnetic
hexacoordinated dicationic complexes of the type [(iPr-PNN)2Fe](X)2 (5: X =
Cl, and 6: X = Br) are formed. The reactions of Ph-PNN
with FeX2 in a 1:1 ratio lead to similar complexes of the
type [(Ph-PNN)2Fe](FeX4) (7: X
= Cl, and 8: X = Br). Single crystal X-ray studies of 1, 2, 4, 6, and 8 do not indicate electron transfer from the FeII centers to the neutral bipyridine unit based on the determined bond
lengths. Density functional theory (DFT) calculations were performed
to compare the relative energies of the mono- and bis-chelated complexes. The doubly deprotonated complexes
[(R-PNN*)2Fe] (9: R = iPr, and 10: R = Ph) were synthesized by reactions
of the dicationic complexes 6 and 8 with
KOtBu. The dearomatized nature of the
central pyridine of the pincer ligand was established by X-ray diffraction
analysis of single crystals of 10. Reactivity studies
show that 9 and 10 have a slightly different
behavior in protonation reactions.