Synthesis, Reactivty, and Catalytic Applications of Iron Complexes Featuring Pyrrole-Based Pincer Ligands




Thompson, C. Vance

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Pincer ligand frameworks have become popular targets to study coordination and organometallic chemistry of transition metals. They are particularly advantageous due to the kinetic stability provided by chelation, as well as the mediation of open coordination sites about the metal. We have synthesized and characterized a class of Fe complexes featuring the pyrrole-based pincer ligand RPNP (R = Cy, Ph, tBu) in multiple oxidation states (FeII, Fe1, and Fe0) , spin states (S = 0, ½, 1, and 2), and coordination numbers (4, 5, and 6). Stabilization of several catalytically relevant Fe species such as square-planar Fe alkyls [FeR(CyPNP)] (R = Me, Ph, Bn) and octahedral Fe hydrides [FeH(L)2(CyPNP)] (L = CO, Bipy, N2, PR3) have generated considerable mechanistic insight into several hydrofucnctionalizations including alkyne dimerization, alkyne hydrosilylation, and carbonyl hydrosilylation. The active species for these reactions feature an Fe-heteroatom bond as opposed to M-H bonds featured in many precious metal systems. The active alkyne dimerization catalyst [FeCCPh(CyPNP)] appears to proceed through a metal acetylide-insertion mechanism. Hydrosilylation catalysts of the type [FeSiR3(CyPNP)] proceed through a modified Chalk-Harrod and peripheral mechanism for alkynes and carbonyl substrates, respectively although disparate mechanisms appear operative under certain conditions. In depth kinetic investigations of carbonyl hydrosilylation have revealed complex multiple equilibiria that control the rate-limiting step, while mechanistic investigations of alkyne hydrosilylation have revealed potential side reactions and catalyst decomposition pathways including hydrogenation and silane disproportionation. Fe complexes supported by RPNP have been investigated and proven valuable as tools to study Fe-based catalysis.


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Alkyne, Catalysis, Dimerization, Hydrosilylation, Iron, Pincer