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sorption predicted at 299 nm additionally involves an UMO lo-
cated at one of the meso-aryl substituents, that is, indicating
additional CT character. Thus, the spectral changes that occur
upon NO binding can be assigned to CT transitions from the
corrole to NO.
However, in the case of pyridine solutions, no spectral
change could be observed, indicating that an NO complex had
not formed (see the Supporting Information). Contrary to
Kadish et al. who reported conversion of an electron-rich oc-
taethyl-substituted iron(III) corrole ([FeIII(oec)]) with an apical
pyridine ligand to the corresponding nitrosyl compound,[43] no
such reaction was observed for our less-electron-rich complex
1. Even small amounts of pyridine in methanolic solutions
(1:30 v/v) inhibit NO coordination of 1 as pyridine binds signifi-
cantly stronger to FeIII than NO. This inhibition can be con-
trolled by the electronic properties of the corrole: the affinity
of the electron-rich [FeIII(oec)] with pyridine is lower than that
of our less-electron-rich 1, as also reflected by the fact that
[FeIII(oec)] coordinates just one pyridine ligand,[69] whereas 1 co-
ordinates two (cf. the (TD-)DFT calculations of 1 with two ace-
tonitrile ligands).[60] Thus, 1 has a higher affinity for pyridine
and is deactivated with respect to NO detection, whereas
[FeIII(oec)] would likely not get deactivated in the presence of
pyridine.
Figure 5. The UV/Vis spectra of 1 measured in acetonitrile under exclusion
of air before and after NO addition, that is, 1 (black dashed line) and 2 (blue
solid thin line). The B3LYP-derived spectrum of 2 is shown as a blue solid
thick line with underlying stick spectra. The B3LYP-derived spectrum of 1 is
shown for comparison (thick dashed gray line). Doming of the corrole is de-
picted in the molecular structures.
suggests a ligand exchange that leads to the formation of the
nitrosyl complex 2 (Figure 5), as similar Fe–nitrosyl complexes
possess Soret bands at virtually identical wavelengths, as
shown in Table 1.[42,54,63] Similar spectral responses upon NO
binding are reported for structurally similar iron(III) porphyr-
ins.[75]
The reversibility of the NO binding was investigated by de-
gassing methanolic solutions of 2 by using three freeze–
pump–thaw cycles (solutions of 2 were frozen in liquid nitro-
gen with subsequent evacuation for 5 min and melting under
reduced pressure). After performing this procedure in the dark,
the spectrum of 1 was again measured, as degassing leads to
NO release from the iron(III) corrole complex 2.
It has been reported that the NO complex is sensitive to
light,[42,76] therefore we investigated light-induced NO cleavage
by simply performing subsequent UV/Vis spectroscopic meas-
urements, thereby utilizing the light source of the spectrome-
ter for NO cleavage. As shown in Figure 7, after 10 subsequent
measurements, the absorption spectrum of 2 was converted to
the characteristic UV/Vis absorption spectrum of 1. Further-
more, the ability to restore the spectrum of 1 demonstrates
that no irreversible reactions, such as nitration of the macrocy-
cle, occur under aerobic conditions if NO is added.
Although the experimental absorption spectra show essen-
tially one prominent spectral change in the Soret region upon
NO binding, the involved electronic changes are pronounced
(Figure 5). Not only is NO a different ligand compared to the
aforementioned coordinating solvents, but its binding changes
the coordination sphere from octahedral to quadratic pyrami-
dal, thus yielding a closed shell instead of the open-shell
system of 1 and causing doming of the corrole.[72] As shown in
Figure 5, our B3LYP calculations strongly overestimate the
spectral changes due to NO binding in the spectral region be-
tween 500 and 800 nm (two prominent Q bands at 558 and
553 nm for 2). However, the experimentally observed spectral
changes at wavelengths shorter than 500 nm are reproduced
reliably. In agreement with the experiment, rather small spec-
tral changes due to NO binding are predicted for the Soret
bands (420 and 407 nm for 2) and two new bands are predict-
ed at higher photon energies than the Soret bands (340 and
299 nm for 2). In Figure 6 it is shown that the domed closed-
shell system can be essentially understood using Gouterman’s
four-orbital model,[31,71,72] as the Q and Soret bands originate
from the HOMO p257 and the OMO p256, respectively, and
the Q bands involve the UMO pNO259, whereas the Soret
bands both involve the UMO p262. The B3LYP-predicted ab-
sorptions at 340 and 299 nm are more complex than the typi-
cal corrole-related absorptions. These absorptions (340,
299 nm) involve three UMOs with strong NO amplitudes
(pdz2 NO258, pNO259, pNO261) and various lower-energy
OMOs without significant amplitudes at NO (Figure 6). The ab-
In the presence of oxygen, many iron(III) corroles undergo
oxidation and form oxo dimers,[68] which have in part been
characterized by crystal structure analysis.[60,69] Such oxo
dimers typically show pronounced and characteristic absorp-
tion signals at approximately 380 nm (Table 1). However, such
absorption characteristics were not observed in our experi-
ments despite the fact that the samples were stored under
aerobic conditions. This finding might be rationalized by con-
sidering that all isolated oxo dimers reported in the literature
have been synthesized in non-coordinating solvents such as
benzene, toluene or dichloromethane.[60,68,69] Thus, it seems
plausible that the blocking of axial coordination sites inhibits
the oxidation with aerobic oxygen (1 is insoluble in non-coor-
dinating solvents).
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