Two novel
low-spin complexes,
[Et4N][FeL2]·1.5H2O
(1) and
[Et4N][CoL2]·2H2O
(2) (L is a deprotonated bis-amide ligand), have been
synthesized and characterized. Four relatively longer equatorial
M−Namide bonds and two significantly shorter axial
M−Npy bonds are the noteworthy features of their X-ray
structures. EPR and (34−300 K) magnetic studies of 1
confirm its low−spin character. Cyclic voltammetric studies
reveal a highly stabilized M(III) state. For 1 a linear
correlation between the Fe(III)−Fe(II) reduction potentials
and the reciprocal of solvent dielectric constants is
obtained.
Non-heme Fe(II) proteins form stable nitrosyl adducts. However, there are few synthetic examples of structurally
characterized non-heme iron nitrosyl complexes, especially those that are coordinatively unsaturated. Presented
herein is a series of non-heme {Fe−NO}7 complexes with trigonal bipyramidal coordination geometry. These
complexes were synthesized with tripodal ligands derived from tris(N-R-carbamoylmethy)amine [1
R]3- where R
= isopropyl (
i
Pr), cyclopentyl (cyp), or 3,5-dimethylphenyl (dmp) groups. The R groups of these [1
R]3- ligands
form cavities around the metal ion that can influence structural and functional properties. The nitrosyl complexes
were synthesized by treating the Fe(II) precursors [Fe1
R]- with NO at room temperature. These iron nitrosyl
complexes have similar molecular structures as determined by X-ray diffraction methods. They differ in their
Fe−N−O angles which range from 178.2(5)° in [Fe1
i
Pr(NO)]- to 160.3(2)° in [Fe1
dmp(NO)]-. The observed
difference in angle is related to the cavity size: [Fe1
i
Pr(NO)]- has the most restricted cavity with a small diameter
(∼ 3 Å) while [Fe1
dmp(NO)]- is more flexible and larger (∼8 Å diameter). This angular difference is supported
by EPR measurements which shows that [Fe1
dmp(NO)]- has a significantly more rhombic spectrum than that
found for [Fe1
i
Pr(NO)]-. Magnetic moment, Mössbauer and EPR data on these {Fe−NO}7 complexes agree
with a electronic configuration of [Fe3+ d5 HS (S = 5/2)−NO- (S = 1)] that lead to an S = 3/2 ground state via
antiferromagnetic coupling.
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