Iron(II) Complexes with Scorpiand-Like Macrocyclic Polyamines: Kinetico-Mechanistic Aspects of Complex Formation and Oxidative Dehydrogenation of Coordinated Amines

The
Fe­(II) coordination chemistry of a pyridinophane tren-derived scorpiand type ligand containing a pyridine ring in the
pendant arm is explored by potentiometry, X-ray, NMR, and kinetics
methods. Equilibrium studies in water show the formation of a stable
[FeL]2+ complex that converts to monoprotonated and monohydroxylated
species when the pH is changed. A [Fe­(H–2L)]2+ complex containing an hexacoordinated dehydrogenated ligand
has been isolated, and its crystal structure shows the formation of
an imine bond involving the aliphatic nitrogen of the pendant arm.
This complex is low spin Fe­(II) both in the solid state and in solution,
as revealed by the Fe–N bond lengths and by the NMR spectra,
respectively. The formation rate of [Fe­(H–2L)]2+ in aqueous solutions containing Fe2+ and L (1:1
molar ratio) is strongly dependent on the pH, the process being completed
in times that range from months in acid solutions to hours in basic
conditions. However, detailed kinetic studies show that those differences
are caused, at least in part, by the effect of pH on the rate of formation
of the unoxidized [FeL]2+ complex. In this sense, the protonation
of the donor atoms in the pendant arm of the scorpiand ligand leads
to the formation of protonated species resistant to oxidative dehydrogenation.
Complementary studies in acetonitrile solution indicate that the initial
stage in the oxidative dehydrogenation process is the oxidation of
the starting complex to form a [FeL]3+ complex, which then
undergoes disproportionation into [Fe­(H–2L)]2+ and [FeL]2+. Experiments starting with Fe­(III)
have allowed us to determine that disproportionation occurs with first
order kinetics both in water and acetonitrile solutions. However,
whereas a significant acceleration is observed in water when the pH
is increased, no effect of the addition of acid or base on the rate
of disproportionation is observed in acetonitrile. Oxidative dehydrogenation
of the Fe­(II) complex formed in experiments starting with an Fe­(III)
salt is slower than that occurring when an Fe­(II) salt is used, an
observation that can be explained in terms of the formation of two
different Fe­(III) complexes, one of them with a structure unable to
evolve directly toward the product of oxidative dehydrogenation.