Understanding the Reactivity and Decomposition of a Highly Active Iron Pincer Catalyst for Hydrogenation and Dehydrogenation Reactions
Julia B. Curley, Nicholas E. Smith, Wesley H. Bernskoetter, Mehmed Z. Ertem, Nilay Hazari, Brandon Q. Mercado, Tanya M. Townsend, Xiaoping Wang
Abstract
The iron pincer complex (iPrPNP)Fe(H)(CO) (1, iPrPNP– = N(CH2CH2PiPr2)2–) is an active (pre)catalyst for many hydrogenation and dehydrogenation reactions. This is in part because 1 can reversibly add H2 across the iron-amide bond to form (iPrPNHP)Fe(H)2(CO) (2, iPrPNHP = HN(CH2CH2PiPr2)2). However, rapid decomposition limits the catalytic performance of 1 and related complexes. We explored the pathways through which catalytic intermediates related to 1 and 2 undergo decomposition. This involved characterizing the unstable and previously unobserved complexes [(iPrPNHP)Fe(H)(CO)(L)]+ (5-L; L = THF or N2) and [(iPrPNHP)Fe(H)(H2)(CO)]+ (8), which are proposed as intermediates when 1 and 2 are used as catalysts. Compound 8 was synthesized through the reaction of (iPrPNHP)Fe(H)(CO)(PF6) (6) with H2, and the solid-state structure was established using both X-ray and neutron diffraction. As part of our studies on understanding the reactivity of 5-L, we determined the thermodynamic hydricity of 2, which is valuable for predicting its reactivity as a hydride donor. Further, it is shown that species such as 5-L decompose to the same inactive species observed in catalysis using 1 and 2, and theoretical calculations suggest that this likely occurs via a bimolecular pathway. To provide support for this hypothesis, we isolated the dimeric species [{(iPrPNHP)Fe(H)(CO)}2{μ-CN}]+ (11) and [{(iPrPNHP)Fe(H)(CO)}2{μ-OC(H)O}]+ (12), which show that catalytic intermediates ligated by iPrPNHP can form dimeric species. Our results provide general strategies for improving catalysis using 1 and 2, and we used this information to rationally increase the performance of 1 in formic acid dehydrogenation.