Angewandte
Communications
Chemie
432 to 455 nm without significant peak broadening in the UV/
Vis SEC spectra (Figure 1). These spectroscopic features from
constant potential electrolysis strongly indicated a metal–NO-
centered reduction of 1, in which case it is understandable
that reduction of the relatively electron rich CTPPMe2À
moiety requires a more negative potential. Significantly, no
other stable {Co(NO)}9 cobalt–nitrosyl porphyrinoid derived
from the electrochemical reduction of the corresponding
{Co(NO)}8 complex without denitrosylation and with a large
DnNO value has been reported to date. Inspired by the CVand
IR SEC studies, we attempted to chemically reduce 1 to the
{Co(NO)}9 complex 2. The chemical reduction of 1 with
[Co(Cp*)2] (1.2 equiv) in neat THF under anaerobic condi-
tions generated complex 2, which could be isolated as
a crystalline product (89.7%) by saturating the solution of 2
in THF with ether. Similar to the value of 95 cmÀ1 obtained
from IR SEC studies (CH2Cl2), the powders of 1 and the
chemically reduced compound 2 showed DnNO = 83 cmÀ1
(KBr) in the IR spectra.
between the alcohols, and thus the difference in the strength
of the H-bonding interactions exerted on the reduced form of
2, should also be considered. The IR spectrum of 2 in
[D8]THF/[D4]MeOH (1:1, v/v) strongly supports the presence
of an H-bonding interaction between the coordinating NO
molecule and methanol, as it shows two distinguishable nNO
humps at 1542 and 1513 cmÀ1 (Figure 2), which were assigned
The quantitative production of 2 with high stability by
a chemical reduction reaction in aprotic solvents under
anaerobic conditions allowed us to examine its reactivity
and the NO reduction intermediates one step at a time. A
proton source is proposed to be essential for catalytic NO
reduction by NOR. However, the addition of a proton donor
(NH4PF6) to 2 resulted in H2 evolution exclusively, accom-
panied by the regeneration of {Co(NO)}8 1, in analogy with
protonation reactions of {Fe(NO)}8 heme–nitrosyl complexes.
We then serendipitously observed that the addition of
methanol or water to the solution of 2 in THF (protic
solvent/THF, 1:1, v/v) is necessary to promote the NO-to-N2O
conversion in association with the disappearance of the
characteristic nNO peak (see Figure S9). N2O production was
confirmed by gas chromatography (GC) of samples taken
from the headspace of the reaction vessel (see Figure S10).
N2O production was further corroborated by IR analysis of
the headspace gas, which showed characteristic N2O(g)
stretching frequencies at 2236 and 2213 cmÀ1. These peaks
shifted to 2167 and 2144 cmÀ1 when 1-15NO was used as the
starting compound, which further confirmed N2O production
from axial NO on the cobalt NCP (see Figure S11). Since the
addition of NH4PF6 generated H2(g) exclusively, the pathway
of metalloporphyrinoid-irrelevant HNO dimerization to N2O
and H2O was not the preferred pathway. Although a route to
HNO/N2O generation through proton-coupled nucleophilic
attack by aromatic alcohols or ascorbate has been reported,
no HNO or N2O generation with MeOH has been de-
scribed.[19]
Figure 2. Time–course IR spectra (2–180 min) of 2 dissolved in
a) [D8]THF/[D4]MeOH (1:1, v/v) and b) THF/methanol (1:1, v/v; inset
for N2O detection).
as 2 and 2···CD3OD and which correspond to forms with and
without H-bonding interactions between 2 and CD3OD,
respectively. A shift in the nNO value from 1542 to 1513 cmÀ1
in the presence of H-bonding interactions with 2 is consistent
with the observed back-donation of electron density from
a copper–substrate complex to a p*NO orbital polarized by
a H-bonding interaction.[7] In bioengineered non-heme/heme
NOR, the ONO atom of heme–NO electrostatically interacts
with a distal metal ion, such as Zn2+ or Fe2+, in the non-heme
pocket, which also decreases the nNO value up to 50 cmÀ1.[20]
As DFT calculations on heme–copper oxidoreductase
À
showed that the NO reduction proceeds through an N N
coupling phase facilitated by the protonation of heme–NO to
À
form an HN2O2 intermediate, we postulated that treatment
with CD3OD induced the formation of 2···CD3OD (shifted to
nNO = 1513 cmÀ1) through H bonding. It is this polarization
that stimulates the interaction between 2 and 2···CD3OD and
À
allows N N coupling to proceed and form a plausible [N2O2]-
bridged intermediate.
To obtain more data to support the formation of the
[N2O2]-containing intermediate and to monitor the spectro-
scopic changes in the whole reaction process for 2 and MeOH,
we collected time-resolved IR measurements. When 2 was
dissolved in THF/MeOH (1:1, v/v) a new IR peak at
1622 cmÀ1 appeared immediately and continuously increased
during the initial 30 min, after which it decreased (Figure 2b).
The use of 2-15NO[10] shifted this peak from 1622 to 1577 cmÀ1
(Dn = 45 cmÀ1; see Figure S13). This isotope-sensitive peak at
1622 cmÀ1 is close to the value of nNO ꢀ 1605 cmÀ1 observed
for bridging neutral [N2O2] coordination in diruthenium
complexes that produce N2O by protonation-induced electron
transfer from the diruthenium core to the [N2O2] unit.[4a,b]
Furthermore, the concomitant decrease in the bands at 1542
Besides methanol and water, more sterically bulky
alcohols were also applied to the NO-to-N2O conversion
under the same conditions. The replacement of MeOH with
EtOH resulted in a much slower N2O formation rate, which
implied direct participation of the alcohol in N2O formation.
Intriguingly, the use of sterically bulky tBuOH did not result
in N2O generation (see Figure S10). We therefore concluded
that this NO reduction occurred via an intermediate that
combined two molecules of 2. Presumably, a nonplanar NCP
core in 2 and sterically bulky tBuOH prohibited the dimeri-
zation of the two molecules of 2, although the pKa difference
5192
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 5190 –5194
Chuang, Chuan-Hung
