Arene Coordination in Bis(imino)pyridine Iron Complexes: Identification of Catalyst Deactivation Pathways in Iron-Catalyzed Hydrogenation and Hydrosilation

The phenyl-substituted bis(imino)pyridine iron bis(dinitrogen) complex (iPrPhPDI)Fe(N2)2 (iPrPhPDI
= 2,6-(2,6-iPr2-C6H3NCPh)2C5H3N) was prepared by sodium amalgam reduction of the corresponding
ferrous dichloride precursor under 4 atm of dinitrogen. Comparison of the infrared stretching frequencies
of the bis(dinitrogen), mono(dinitrogen), and related dicarbonyl derivatives to those of the corresponding
bis(imino)pyridine iron compounds bearing the methyl-substituted ligand, (iPrPDI)Fe(L)n (iPrPDI
= 2,6-(2,6-iPr2-C6H3NCMe)2C5H3N; L = CO, n = 2; L = N2, n = 1, 2), established a more electrophilic
iron center for the phenyl-substituted cases. Comparing the productivity of (iPrPhPDI)Fe(N2)2 to
(iPrPDI)Fe(N2)2 in the catalytic hydrogenation and hydrosilation of 1-hexene demonstrated higher turnover
frequencies for (iPrPhPDI)Fe(N2)2. For more hindered substrates such as cyclohexene and (+)-(R)-limonene, the opposite trend was observed, where the methyl-substituted precursor, (iPrPDI)Fe(N2)2,
produced more rapid conversion. The difference in catalytic performance resulted from competitive,
irreversible formation of η6-aryl and -phenyl compounds with the phenyl-substituted complex. Addition
of coordinating solvents such as cyclohexene or THF resulted in exclusive formation of the η6-phenyl
derivative. When alkoxide substituents are introduced in the bis(imino)pyridine ligand backbone, the
formation of η6-aryl compounds was exclusive, as alkali metal reduction of (iPrROPDI)FeBr2 (iPrROPDI
= 2,6-(2,6-iPr2-C6H3NC(OR))2C5H3N, R = Me, Et) yielded only the catalytically inactive η6-aryl species.