Di- and Mononuclear Iron Complexes of N,C,S-Tridentate Ligands Containing an Aminopyridyl Group: Effect of the Pendant Amine Site on Catalytic Properties for Proton Reduction

A series
of diiron complexes of N,C,S-tridentate ligands containing
a 6-, 5-, or 4-amino-2-pyridyl group, [{Fe­(μ-L-κ3N,C,S)­(CO)2}­Fe­(CO)3] (2, L = o-apyBPT; 3, L = m-apyBPT; 4, L = p-apyBPT),
was synthesized: apyBPT is a doubly deprotonated form of 3′-(amino-2″-pyridyl)-1,1′-biphenyl-2-thiol.
Complexes 2–4 were converted to the
mononuclear iron­(II) complexes trans-[Fe­(L-κ3N,C,S)­(CO)­(PMe2Ph)2] (6, L = o-apyBPT; 7,
L = m-apyBPT; 8, L = p-apyBPT). In 2 and 6, the o-amino group is close to Fe bound to the aminopyridyl group. Cyclic
voltammograms of 2–4 exhibit two
consecutive one-electron reduction events, and catalytic current for
proton reduction appears in the presence of acetic acid. The reduction
potentials of 2–4 are similar to
each other, while the overpotential for proton reduction with o-amino complex 2 is ca. 0.2 V lower than those
with 3 and 4. In the mononuclear complexes 6–8, the redox potentials for the FeIII/FeII couple are dependent on the position of
the amino group in the pyridine ring, which is described by electronic
and steric effects of the amino group. Such effects on the redox potentials
are suppressed in the diiron complexes because the reduction occurs
at the diiron core with π-accepting CO ligands, which is supported
by DFT calculations. The lower overpotential in 2 compared
with 3 and 4 is attributed to the concerted
effect of the amino group proximal to the iron center. The amino group
probably acts as a proton acceptor and assists the formation of the
H–H bond from a hydride on the iron centers and a proton bound
to the amino group.