The dinuclear species
[(trispicMeen)ClFeIIIOFeIIICl(trispicMeen)]Cl(OH)(H2O)7
(1) (trispicMeen =
N,N,N‘-tris(2-pyridylmethyl)-N‘-methylethane-1,2-diamine) was
synthesized. It crystallizes in the monoclinic space
group
C2/c with a = 33.87(2) Å,
b = 17.42(2) Å, c = 23.41(5) Å,
β = 132.88(5)°, V = 10 121(25)
Å3, and Z = 8. It
contains an almost linear unit (Fe−O−Fe angle =
177.4(7)°). The potentially pentadentate ligand is in fact
only
tetracoordinated with one pyridine not bound to the metal ion. The
octahedral coordination of Fe(III) is completed
by one chloride ion. The structure of
[(bispicMeen)ClFeOFeCl(bispicMeen)]Cl2·CH3COCH3·
2H2O (2)
(bispicMeen =
N,N‘-bis(2-pyridylmethyl)-N‘-methylethane-1,2-diamine)
was also determined. It crystallizes in
the monoclinic space group C2/c with a
= 11.124(4) Å, b = 22.769(9) Å, c
= 15.874(6) Å, β = 97.79(4)°, V
=
3984(3) Å3, and Z = 4. The main
difference from 1 is that, in 2, the Fe−O−Fe
unit is bent with an FeOFe angle
= 152.3(3)°. In cyclic voltammetry, 1 exhibits
two reduction peaks at −0.230 and −0.960 V/SCE.
They
correspond respectively to the reduction to the
FeIIFeIII and
FeIIFeII states. Cyclic voltammetry
shows that the
mixed-valent form
[(trispicMeen)ClFeIIOFeIIICl(trispicMeen)]+
(E° = −0.175 V/SCE) is in equilibrium with
another
species (E° = +0.065 V/SCE) proposed to be
[(trispicMeen)FeIIOFeIIICl(trispicMeen)]2+
in which a chloride ion
has been displaced by the originally unbound pyridine. The
equilibrium constant was estimated to be 90
M-1,
and the rate of the recombination of chloride to the
[(trispicMeen)FeIIIOFeIIICl(trispicMeen)]3+
complex was
found equal to 3 × 105 M-1
s-1. Controlled potential electrolysis
of an acetonitrile solution of 1 allowed the
preparation of the mixed-valent Fe(II)−O−Fe(III) form
which gives an almost isotropic EPR signal similar to
that already observed with oxo-bridged model compounds (Holz, et al.
Inorg. Chem.
1993, 32, 5844.
Hartman,
et al. J. Am. Chem. Soc.
1987, 109,
7387) but different from the rhombic one observed in the mixed valent
form
of MMO. The mixed-valent forms slowly disproportionate to a
mixture of FeIIIFeIII and Fe(II)
forms. The mixed-valent forms could be generated by radiolysis at 77 K, and an EPR study
of the mixed-valent forms obtained by
this procedure demonstrated that these species could not be protonated.
Radiolysis of 1 at 77 K afforded the
EPR spectrum of
[(trispicMeen)ClFeIIOFeIIICl(trispicMeen)]+;
upon annealing at 200 K, the solution gave an
EPR spectrum very similar to that observed on the electrochemically
reduced solution. This is in agreement with
the observation of substitution of a chloride ligand. The
mixed-valent form was not detected with the analogous
complexes of the bispicen family:
[(bispicMeen)ClFeOFeCl(bispicMeen)]Cl2·CH3COCH3·
2H2O (2) and
[(bispicMe2en)ClFeOFeCl(bispicMe2en)]Cl2
(3). Upon reduction, these complexes quickly (at CV
time scale)
decompose to mononuclear species.
