Synthesis and Electronic Structure of Bis(imino)pyridine Iron Metallacyclic Intermediates in Iron-Catalyzed Cyclization Reactions

The bis­(imino)­pyridine
iron dinitrogen compound, (iPr(TB)PDI)­Fe­(N2)2 (iPr(TB)PDI = 2,6-(2,6-iPr2-C6H3-NC-(CH2)3)2(C5H1N)) is
an effective precatalyst for the [2π + 2π] cycloaddition
of diallyl amines as well as the hydrogenative cyclization of N-tosylated
enynes and diynes. Addition of stoichiometric quantities of amino-substituted
enyne and diyne substrates to (iPr(TB)PDI)­Fe­(N2)2 resulted in isolation of catalytically competent bis­(imino)­pyridine
iron metallacycle intermediates. A combination of magnetochemistry,
X-ray diffraction, and Mössbauer spectroscopic and computational
studies established S = 1 iron compounds that are
best described as intermediate-spin iron­(III) (SFe = 3/2) antiferromagnetically coupled to a chelate radical
anion (SPDI = 1/2). Catalytically competent
bis­(imino)­pyridine iron diene and metallacycles relevant to the [2π
+ 2π] cycloaddition were also isolated and structurally characterized.
The combined magnetic, structural, spectroscopic, and computational
data support an Fe­(I)–Fe­(III) catalytic cycle where the bis­(imino)­pyridine
chelate remains in its one-electron reduced radical anion form. These
studies revise a previous mechanistic proposal involving exclusively
ferrous intermediates and highlight the importance of the redox-active
bis­(imino)­pyridine chelate for enabling catalytic cyclization chemistry
with iron.