Experimental Evidence of syn H–N–Fe–H Configurational Requirement for Iron-Based Bifunctional Hydrogenation Catalysts

Abstract

Iron hydrides supported by a pincer ligand of the type HN(CH2CH2PR2)2 (RPNHP) are versatile hydrogenation catalysts. Previous efforts have focused on using CO as an additional ligand to stabilize the hydride species. In this work, CO is replaced with isocyanide ligands, leading to the isolation of two different types of iron hydride complexes: (RPNHP)FeH(CNR')(BH4) (R = iPr, R' = 2,6-Me2C6H3, tBu; R = Cy, R' = 2,6-Me2C6H3) and [(iPrPNHP)FeH(CNtBu)2]X (X = BPh4, Br, or a mixture of Br and BH4). The neutral iron hydrides are capable of catalyzing the hydrogenation of PhCO2CH2Ph to PhCH2OH, although the activity is lower than for (iPrPNHP)FeH(CO)(BH4). The cationic iron hydrides are active hydrogenation catalysts only for more reactive carbonyl substrates such as PhCHO, and only when the NH and FeH hydrogens are syn to each other. The cationic species and their synthetic precursors [(iPrPNHP)FeBr(CNtBu)2]X (X = BPh4, Br) can have different configurations for the isocyanide ligands (cis or trans) and the H-N-Fe-H(Br) unit (syn or anti). Unlike tetraphenylborate, the bromide counterion participates in a hydrogen-bonding interaction with the NH group, which influences the relative stability of the cis,anti and cis,syn isomers. These structural differences have been elucidated by X-ray crystallography, and the geometric isomerization processes have been studied by NMR spectroscopy.