Synthesis, structures, and dearomatization by deprotonation of iron complexes featuring bipyridine-based PNN pincer ligands

The 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 Fe^II 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 KO ^tBu. 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.