Prev. Article Next Article Table of Contents

Article

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

Supporting Info

Andrew M. Archer, Marco W. Bouwkamp, Maria-Patricia Cortez, Emil Lobkovsky, and Paul J. Chirik*

Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853

Organometallics, 2006, 25 (18), pp 4269–4278

DOI: 10.1021/om060441c

Publication Date (Web): August 5, 2006

To whom correspondence should be addressed. E-mail: pc92@cornell.edu.

Abstract

The phenyl-substituted bis(imino)pyridine iron bis(dinitrogen) complex (ⁱPrPhPDI)Fe(N₂)₂ (ⁱPrPhPDI = 2,6-(2,6-ⁱPr₂-C₆H₃NCPh)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-C₆H₃NCMe)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-C₆H₃NC(OR))₂C₅H₃N, R = Me, Et) yielded only the catalytically inactive η⁶-aryl species.