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 ((PrPhPDI)-Pr-i)Fe(N-2)(2) ((PrPhPDI)-Pr-i = 2,6-(2,6-Pr-i(2)-C6H3N=CPh)(2)C5H3N) 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, ((PrPDI)-Pr-i)Fe(L)(n) ((PrPDI)-Pr-i = 2,6-(2,6-Pr-i(2)-C6H3N=CMe)(2)C5H3N; L = CO, n = 2; L = N-2, n = 1, 2), established a more electrophilic iron center for the phenyl-substituted cases. Comparing the productivity of ((PrPhPDI)-Pr-i) Fe(N-2)(2) to ((PrPDI)-Pr-i)Fe(N-2)(2) in the catalytic hydrogenation and hydrosilation of 1-hexene demonstrated higher turnover frequencies for ((PrPhPDI)-Pr-i)Fe(N-2)(2). For more hindered substrates such as cyclohexene and (+)-(R)limonene, the opposite trend was observed, where the methyl-substituted precursor, ((PrPDI)-Pr-i) Fe(N-2)(2), produced more rapid conversion. The difference in catalytic performance resulted from competitive, irreversible formation of A6-aryl and -phenyl compounds with the phenyl-substituted complex. Addition of coordinating solvents such as cyclohexene or THF resulted in exclusive formation of the A6-phenyl derivative. When alkoxide substituents are introduced in the bis(imino) pyridine ligand backbone, the formation of eta(6)-aryl compounds was exclusive, as alkali metal reduction of ((PrROPDI)-Pr-i)FeBr2 ((PrROPDI)-Pr-i) 2,6-(2,6-Pr-i(2)-C6H3N=C(OR))(2)C5H3N, R = Me, Et) yielded only the catalytically inactive eta(6)-aryl species.