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tion produces a mixed IrIV–corrole radical, with similar EPR
is the most likely counterion for these cationic species, but
positive identification proved elusive.
and spectroelectrochemical signatures (see the Supporting
Information, Figures S-12 and S-13) to the one-electron
oxidized form of 1-Ir(tma)2.[10] We reasoned that the second
oxidation process might result in formation of an IrV complex,
and we attempted chemically to oxidize the corrole. Treat-
ment with N-bromosuccinimide (NBS) in the presence of
ammonium hydroxide (which we hoped would remove
protons from the ammine ligands) led to a rapid color
change from green to red-purple and the appearance of new
TLC (thin layer chromatography) spots.
Azaporphyrins, particularly those containing iron(III),[13]
possess high-energy Soret bands and broad Q-band systems.
Our iridium monoazaporphyrins (Scheme 2),[14] which exhibit
similarly energetic Soret absorptions, are unique in that they
have unsubstituted b-positions but are fully substituted at the
meso positions, whereas most other azaporphyrins are heavily
b-substituted but have no meso substituents.[15]
Three different compounds, with an overall yield of about
50%, were isolated from the reaction mixture by gradient
column chromatography (the Danish “dry column” tech-
nique[12] is recommended) in 1–5% CH3OH/CH2Cl2. The
most polar of these complexes, dubbed [2-Ir(NH3)2]+, has a
mass corresponding to that of 1-Ir(NH3)2 plus one nitrogen
atom; its 1H NMR spectrum (Figure 3) demonstrates that the
complex retains pseudo-C2v symmetry, the ammine ligands
remain attached to iridium, and all b-pyrrole protons are
intact. The two other complexes exhibit similar NMR spectra
and their mass spectra are consistent with replacement of one
{[2(Br)-Ir(NH3)2]+} and two {[2(Br2)-Ir(NH3)2]+} protons by
bromines. The UV/Vis spectral signatures of these pink
compounds (Figure 4), taken in combination with their NMR
and mass spectra, point unambiguously to the formation of
monoazaporphyrin complexes. We hypothesize that bromide
Scheme 2. Synthesis of [2-Ir(NH3)2]+ and its brominated derivatives.
In order to gain insight into the mechanism of formation
of [2-Ir(NH3)2]+ and its brominated derivatives, we ran the
NBS/NH4OH reaction using 15N-labeled ammonium hydrox-
ide; additionally, we attempted to drive the reaction to the
hypothetical end product, octabromo(tris)pentafluorophenyl-
monoazaporphyrinatoiridium(III) (bis)ammine, using both
large excesses of NBS (in which case bromination is still
halted at the [2(Br2)-Ir(NH3)2]+ stage) and elemental bro-
mine (resulting in an inseparable mixture of variously
1
brominated analogues). The H NMR spectra of the mono-
azaporphyrins display a singlet resonance far upfield assigned
to the ammine ligands; substitution by 15N would be expected
to produce a doublet due to 15N-1H coupling. In addition, the
15N-1H HMBC NMR spectrum of [15N-2(Br2)-Ir(NH3)2]+ (see
the Supporting Information) shows a strong signal corre-
sponding to coupling between the N atom of the azaporphyrin
and the protons on the C2 and C18 atoms of the ring. This
HMBC signal also confirms the assignment of the bromine
atoms to positions 3 and 17 on the corrole ring; if they were at
positions 2 and 18, the other possibility, no 3-bond 15N-1H
coupling would be observed.
Figure 3. 1H NMR spectrum of [2-Ir(NH3)2]+ in [D6]dmso.
The implication of the labeling studies is that ammonium
hydroxide acts as the source of nitrogen that is eventually
inserted into the corrole framework during the reaction, so we
thought it should be possible to convert other iridium corroles
to azaporphyrins. However, attempted reactions for both 1-
Ir(tma)2 and 1-Ir(py)2 were unsuccessful. Given that the
ammine ligands play no active role in formation of 2, we
tentatively suggest that the ease with which 1-Ir(NH3)2
undergoes nitrogen insertion stems from its low redox
potential; the ammine-ligated corrole is more than 100 mV
easier to oxidize than either 1-Ir(tma)2 or 1-Ir(py)2.
We propose that nitrogen insertion in the oxidatively
generated metal/p-cation radical state involves nucleophilic
attack by ammonia in solution. The initial intermediate is
likely a ring-opened, brominated biladiene of the sort that has
Figure 4. UV/Vis absorption spectra of [2-Ir(NH3)2]+, [2(Br)-Ir(NH3)2]+,
and [2(Br2)-Ir(NH3)2]+ in CH3CN.
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 9433 –9436