Mechanistic investigation of iron-catalyzed cross-coupling using sterically encumbered β-diketonate ligands

Abstract

Iron β-diketonate compounds are reported using the hindered 2,6-dimesitylbenzoyl pinacolone (Aracac), which sterically precludes formation of tris-Aracac complexes. Fe(Aracac)2 (1), Fe(Aracac)2Cl (2) were synthesized and characterized using crystallographic, spectroscopic, elemental analysis, and electrochemical techniques. Iron complexes with only one Aracac ligand form dimers [Fe(Aracac)(THF)2]2(μ-OTf)2 (3) and [(Aracac)Fe(μ-mesityl)]2 (4). Complexes 1–4 were evaluated in a Kumada cross-coupling reaction and compared with the parent FeIII(acac)3 complex. Whereas the bis-acac complexes 1 and 2 show only modest activity (∼50 % yield after 60 min), the mono-acac complexes 3 and 4 are nearly as effective as the parent compound (∼90 % yield). DFT computations, in conjunction with catalytic results and electrochemical observations, suggest a catalytic cycle involving an oxidative addition of an aryl-chloride to Fe(II) to produce a high-spin Fe(IV) complex that could alternately be described as a high-spin Fe(II) stabilizing two coordinated organoradicals. These results suggest that a unique aspect of acac as a ligand for this reaction is its ability to support high-spin organoiron complexes, which can provide low-barrier pathways for C–Cl bond cleavage and CC bond formation reactions.