Understanding the Reactivity and Decomposition of a Highly Active Iron Pincer Catalyst for Hydrogenation and Dehydrogenation Reactions

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.