Proton-Coupled Electron Transfer Enhances the Electrocatalytic Reduction of Nitrite to NO in a Bioinspired Copper Complex

Article abstract of DOI:10.1021/acscatal.8b00361

The selective and efficient electrocatalytic reduction of nitrite to nitric oxide (NO) is of tremendous importance, both for the development of NO-release systems for biomedical applications and for the removal of nitrogen oxide pollutants from the environment. In nature, this transformation is mediated by (among others) enzymes known as the copper-containing nitrite reductases. The development of synthetic copper complexes that can reduce nitrite to NO has therefore attracted considerable interest. However, there are no studies describing the crucial role of proton-coupled electron transfer during nitrite reduction when such synthetic complexes are used. Herein, we describe the synthesis and characterization of two previously unreported Cu complexes (3 and 4) for the electrocatalytic reduction of nitrite to NO, in which the role of proton-relaying units in the secondary coordination sphere of the metal can be probed. Complex 4 bears a pendant carboxylate group in close proximity to the copper center, while complex 3 lacks such functionality. Our results suggest that complex 4 is twice as effective an electrocatalyst for nitrite reduction than is complex 3 and that complex 4 is the best copper-based molecular electrocatalyst for this reaction yet discovered. The differences in reactivity between 3 and 4 are probed using a range of electrochemical, spectroscopic, and computational methods, which shed light on the possible catalytic mechanism of 4 and implicate the proton-relaying ability of its pendant carboxylate group in the enhanced reactivity that this complex displays. These results highlight the critical role of proton-coupled electron transfer in the reduction of nitrite to NO and have important implications for the design of biomimetic catalysts for the selective interconversions of the nitrogen oxides.

Full text of DOI:10.1021/acscatal.8b00361

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Proton-Coupled-Electron Transfer Enhances the Electrocatalytic
Reduction of Nitrite to NO in a Bioinspired Copper Complex
Giacomo Cioncoloni, Isolda Roger, Paul S. Wheatley, Claire
Wilson, Russell E. Morris, Stephen Sproules, and Mark D Symes
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00361 • Publication Date (Web): 25 Apr 2018
Downloaded from http://pubs.acs.org on April 25, 2018
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ProtonꢀCoupledꢀElectron Transfer Enhances the
Electrocatalytic Reduction of Nitrite to NO in a
Bioinspired Copper Complex
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†,§
†,§
‡
†
Giacomo Cioncoloni, Isolda Roger, Paul S. Wheatley, Claire Wilson, Russell E.
‡
†
,†
Morris, Stephen Sproules, and Mark D. Symes*
†
WestCHEM, School of Chemistry, University of Glasgow, University Avenue, Glasgow,
G12 8QQ, United Kingdom
‡
EaStCHEM School of Chemistry, University of St Andrews, Purdie Building, St Andrews,
KY16 9ST, United Kingdom
§
These authors contributed equally to this work
*Email: mark.symes@glasgow.ac.uk
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ABSTRACT
The selective and efficient electrocatalytic reduction of nitrite to nitric oxide (NO) is of
tremendous importance, both for the development of NOꢀrelease systems for biomedical
applications and for the removal of nitrogen oxide pollutants from the environment. In nature,
this transformation is mediated by (amongst others) enzymes known as the copperꢀcontaining
nitrite reductases. The development of synthetic copper complexes that can reduce nitrite to
NO has therefore attracted considerable interest. However, there are no studies describing the
crucial role of protonꢀcoupledꢀelectron transfer during nitrite reduction when using such
synthetic complexes. Herein, we describe the synthesis and characterization of two
previously unreported Cu complexes (3 and 4) for the electrocatalytic reduction of nitrite to
NO, in which the role of protonꢀrelaying units in the secondary coordination sphere of the
metal can be probed. Complex 4 bears a pendant carboxylate group in close proximity to the
copper center, whilst complex 3 lacks such functionality. Our results suggest that complex 4
is twice as effective an electrocatalyst for nitrite reduction than is complex 3, and that
complex 4 is the best copperꢀbased molecular electrocatalyst for this reaction yet discovered.
The differences in reactivity between 3 and 4 are probed using a range of electrochemical,
spectroscopic and computational methods, which shed light on the possible catalytic
mechanism of 4 and implicate the protonꢀrelaying ability of its pendant carboxylate group in
the enhanced reactivity that this complex displays. These results highlight the critical role of
protonꢀcoupledꢀelectron transfer in the reduction of nitrite to NO and have important
implications for the design of biomimetic catalysts for the selective interconversions of the
nitrogen oxides.
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KEYWORDS
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Nitrite reduction, nitric oxide, copper nitrite reductase mimic, electrocatalysis, Protonꢀ
CoupledꢀElectron transfer
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INTRODUCTION
−
The reduction of nitrite (NO
2
) to nitric oxide (NO) in the bloodstream is potentially a
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physiological source of NO, which plays an important role in neurotransmission and
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vasodilation. Nitrite reduction to NO is also a key step in the natural nitrogen cycle, being
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part of the process of bacterial denitrification. There are a number of classes of enzyme that
perform this transformation, including those using haems, molybdenum and copper centers as
the catalytic species. In the case of Cu, the enzymes performing this reaction are known
collectively as the copperꢀcontaining nitrite reductases (CuNIR), and the reaction that they
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mediate is summarized in Eq. 1.
−
+
Cu(I) + NO
2
+ 2H → Cu(II) + NO + H
2
O
(Eq. 1)
In 1991, Adman and coꢀworkers were the first to determine the crystal structure of a CuNIRꢀ
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−
2
type enzyme, which showed that the active site for the reduction of NO consists of a Cu(II)
ion coordinated by three histidine ligands and a single water molecule, giving a distorted
tetrahedral geometry at the copper center. These results have been confirmed by subsequent
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studies, which have also given insight into the nature of nitrite binding during catalysis. At
the present time, there is some debate over the precise order of events during nitrite reduction
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to NO with CuNIR.
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One possibility is that nitrite displaces the bound water from the Cu(II) center in CuNIR,
−
forming a complex in which the NO
2
is bound to Cu(II) through both of its oxygen atoms
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,10
(and hence the Cu(II) adopts a geometry akin to a squareꢀbased pyramid, see Scheme 1).
Subsequent electron and proton transfer steps then lead to N–O bond scission and ultimately
the loss of one of the nitriteꢀderived oxygens as water. Alternatively, reduction of the copper
center may occur before nitrite binds. In any case, a nitriteꢀbound Cu(I) center is obtained,
from which NO is liberated with concomitant regeneration of the original Cu(II) complex (in
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1,12
its waterꢀbound form), ready to reꢀenter the catalytic cycle.
Scheme 1. A simplified proposed mechanism for nitrite reduction to nitric oxide by CuNIR.
The histidine ligands are abbreviated as “His” and are shown in grey.
On account of the comparatively simple active site structure of CuNIR suggested by the
crystallographic studies, a number of groups have developed simplified analogs of this
1
1,13ꢀ45
enzyme based on tripodal Nꢀdonor ligands coordinated to a Cu center.
In many cases,
these analogs also encapsulate the functionality of CuNIR (and so can mediate the reduction
of nitrite to NO), normally with the use of stoichiometric sacrificial electron donors. There is
also a distinct subset of such CuꢀN donor complexes that can mediate the electrocatalytic
4
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reduction of nitrite to NO.
In these latter cases in particular, the role of solution pH (or
the presence of additional proton sources in nonꢀaqueous media) has been shown to be
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critical, with acidic regimes being essential for catalytic reduction of nitrite to NO. This is
perhaps not unsurprising, given the dependence of Eq. 1 on the presence of protons. More
surprising to us, especially considering that protons are required for this reduction, was the
relative dearth of studies on the role of protonꢀrelay units that might mediate protonꢀcoupledꢀ
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2ꢀ56
electron transfer (PCET)
during catalytic nitrite reduction with such CuNIR mimics.
In the natural CuNIR class of enzymes, an aspartate residue sits in close proximity to the
bound nitrite and helps to mediate proton transfer to one of the nitrite oxygens, which is
5,10
eventually lost as water.
The importance of nearby protonꢀdonating residues in the
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mechanism of nitrite reduction by CuNIR has been highlighted by recent crystallographic
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and siteꢀdirected mutagenesis studies on the relevant enzymes. There have also been three
seminal papers in recent years showcasing the importance of hydrogen bonding in the
secondary coordination sphere of synthetic systems for nitrite reduction based on both
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iron
and copper, although none of these systems was able to demonstrate catalytic
turnover. However, to the best of our knowledge, there exist no examples of synthetic, smallꢀ
molecule platforms for the catalytic reduction of nitrite to NO where the role of proton relays
has been addressed explicitly, as it has been, for example, with synthetic platforms for
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catalytic proton reduction,
water oxidation
2
and CO reduction.
Herein we report the synthesis and characterization of two new copper complexes (3 and 4)
which allow the role of PCET in the electrocatalytic reduction of nitrite to NO with synthetic,
smallꢀmolecule platforms to be addressed for the first time. Both complexes contain a copper
center ligated by a tetradentate tris(2ꢀmethylpyridyl)amine (TMPA) ligand and both are
competent for the electrocatalytic reduction of nitrite to NO in nonꢀaqueous solvent.
However, complex 4 (in which a carboxylate group is positioned in close proximity to nitrite
bound to the copper center), is roughly twice as effective a catalyst for this transformation as
complex 3, which lacks any such protonꢀrelaying groups. We probe and rationalize these
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differences in reactivity using a range of electrochemical, spectroscopic and computational
methods, which together shed light on the possible mechanism of reaction. Our results
suggest that the incorporation of PCETꢀcompetent groups in the secondary coordination
sphere of the metal center gives rise to a considerable enhancement in the rate of
electrocatalytic nitrite reduction, and hence give insights into the workings of the natural
CuNIR enzymes and suggest new avenues for the development of biomimetic catalysts for
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applications such as the removal of NO
2
from wasteꢀwater streams and NOꢀrelease
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systems for biomedical applications.
RESULTS AND DISCUSSION
Synthesis and Crystallography.
The general synthetic route for the formation of complexes 3 and 4 is shown in Scheme 2.
Hence, 6ꢀchloromethylpyridineꢀ2ꢀcarboxylic acid methyl ester was prepared via the
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procedure of MatoꢀIglesias et al. The synthesis of compound 1 (see Figure S1) was then
achieved by reaction of this methyl ester with bis(2ꢀpyridylmethyl)amine according to the
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procedure used by Kotani et al. for the preparation of the allied ethyl ester. Compound 1
76,77
could then be hydrolyzed to yield the known acid (compound 2)
by treatment with
NaOH. Metallation of the acid and ester ligands was then achieved accordingly to adapted
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2+
+
literature procedures,
2 2
generating the new complexes [3ꢀH O] and [4ꢀH O] (see
Experimental Section).
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N
N
H
N
N
N
Cl
O
N
N
O
NaOH, EtOH
N
N
O
N
N
N
Na CO , CH CN
2
3
3
OMe
OMe
OH
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2
.
.
Cu(ClO ) 6H O
Cu(ClO ) 6H O
4 2 2
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EtOH
EtOH
OH2
OH2
2+
+
N
N
N
N
O
CuII
CuII
N
N
N
COOMe
O]2+
3-H
N
O
[
2
[4-H
2
O]+
2+
+
Scheme 2. The synthetic route to obtain complexes [
3
ꢀH
2
O] and [
4
ꢀH
2
O] .
After dissolution in CH CN, dark blue crystals of the acetonitrile adduct of complex
3 [3ꢀ
3
3 4 2
CH CN](ClO ) were obtained after standing at room temperature for 3 weeks. These crystals
proved to be of suitable quality for Xꢀray diffraction, allowing the structure shown in Figure
1 to be obtained.
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3 4 2
Figure 1. The molecular structure of [3ꢀCH CN](ClO ) . Further crystallographic details can
be found in the Supporting Information. Ellipsoids are drawn at 50% probability level. Color
scheme: C = grey, N = blue, O = red, Cl = green, Cu = purple. For selected bond lengths and
angles, see Table 1.
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The presence of two perchlorate anions per Cu complex indicated that the copper center
remained in the +II oxidation state. Typically, Cu(II) complexes with tris(2ꢀ
methylpyridyl)amine (TMPA) ligands adopt a distorted trigonal bipyramidal geometry,
whereby the Cu–N(pyridyl)TMPA bonds define the triangular plane (with typical lengths between
2.0 and 2.1 Å) and the apices of the bipyramid consist of a rather short Cu–Nacetonitrile distance
43,79ꢀ81
(~2.0 Å) and a comparatively longer Cu–N(alkyl)TMPA interaction (~2.4 Å).
However, the
2+
3
geometry of [3ꢀCH CN] is more akin to a squareꢀbased pyramid (τ value ≈ 0.1 on the scale
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of Addison and Reedijk ), with N2 as its apex (see selected bond lengths and angles in Table
1). This gives Cu–N bond lengths in the square plane of between 2.045(2) and 1.972(2) Å,
whilst the Cu–N2 interaction appears to be only very weak (Cu1–N2 = 2.473(2) Å). That
said, Figure 1 shows that the lone pair on the N2 nitrogen is clearly directed towards the
Cu(II) center. It is noteworthy that this elongation in the Cu–N2 distance occurs for the
pyridine which bears the methyl ester substituent adjacent to the Nꢀdonor. This arrangement
is rather reminiscent of the geometry previously observed by Tanaka and coꢀworkers for the
+
complex [CuCl(Me TMPA)] , where the TMPA pyridine bearing the methyl group exhibited
1
4
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a much longer Cu–N interaction than the unsubstituted pyridines (2.337 Å vs. ~1.99 Å). On
account of the differing electronic properties of methyl and methyl ester substituents, it
therefore seems likely that this common bond elongation effect is the result of steric
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crowding brought about by the close proximity of the substituents to the Nꢀdonor atom.
Similarly long Cu–N interactions have also been noted before in Cu(II)ꢀtris(2ꢀ
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methylpyridyl)amine complexes bearing bulky substituents next to the Nꢀdonors (albeit
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where the ligands were more symmetricallyꢀsubstituted) by Reinaud and coꢀworkers. The
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shortest distance between the Cu center in [3ꢀCH CN] and any of the oxygens on the
nearest perchlorate anion is 2.745 Å, suggesting only a very weak interaction between the Cu
and perchlorate and suggesting that the metal center is best considered as fiveꢀcoordinate.
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Table 1. Experimental Bond Distances (Å) and Angles (°) in [3ꢀCH CN]
3
Cu1–N1
Cu1–N2
Cu1–N3
Cu1–N4
Cu1–N5
2.045(2)
2.473(2)
1.972(2)
1.981(2)
1.984(3)
N1–Cu1–N2
N3–Cu1–N1
N3–Cu1–N2
N3–Cu1–N4
N3–Cu1–N5
N4–Cu1–N1
N4–Cu1–N2
N4–Cu1–N5
N5–Cu1–N1
N5–Cu1–N2
77.49(9)
84.5(1)
79.14(9)
161.9(1)
96.6(1)
82.6(1)
110.26(9)
92.9(1)
166.0(1)
116.45(9)
When tetrabutylammonium nitrite (TBAꢀNO
2
) is added to solutions of complexes 3 and 4 in
acetonitrile, the initially blue solutions immediately turned green (see Figure 2), indicating
4
2
coordination of the Cu(II) center to nitrite in both cases.
Figure 2A. Overlay of the electronic spectra of complex
3
in acetonitrile (black line) and of
(also in acetonitrile, red line).
this complex after the addition of 10 equivalents of TBAꢀNO
2
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2B. Overlay of the electronic spectra of complex
4
in acetonitrile (black line) and of this
(also in acetonitrile, red line).
complex after the addition of 10 equivalents of TBAꢀNO
2
Green crystals of complex
4
bound to nitrite were isolated from an acetonitrile solution of
and one equivalent of benzoic
1
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this complex in the presence of 5 equivalents of TBAꢀNO
2
acid. Xꢀray diffraction on these crystals revealed a structure in which nitrite is bound to the
copper center through nitrogen, whilst the carboxylate group remains coordinated to Cu,
giving an overall neutral species (with no further counterions present), which we formulate as
2
[4ꢀNO ] (see Figure 3). Hence, although acid was added to the crystallization medium, there
is no evidence that the complex is protonated under these conditions. We note that Kojima
and coꢀworkers have previously noted the tendency for this carboxylate group to coordinate
76,77
to the metal center in their allied Ru and Cr complexes,
reported a Co(III) complex of ligand in which the carboxylate group also coordinates to the
metal center, giving a distorted octahedral geometry at the metal.
and Lonnon et al. have similarly
2
8
5
Figure 3. The molecular structure of [
2
4ꢀNO ]. Further crystallographic details can be found
in the Supporting Information. Ellipsoids are drawn at 30% probability level. Color scheme:
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C = grey, H = white, N = blue, O = red, Cu = purple. A minor disorder component for bound
–
NO
2
has been omitted. For selected bond lengths and angles, see Table 2.
The geometry around the copper center in [4ꢀNO ] is probably best described as distorted
2
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octahedral, with a JahnꢀTeller elongation along the N1–Cu–O1 axis. This leads to a rather
long Cu–N1 distance (2.384(2) Å), whilst the CuꢀNpyridyl lengths are all significantly shorter
(2.047(2) – 2.031(2) Å) and more similar to the CuꢀN1N distance between the copper center
and the bound nitrite (1.983(3) Å). It is these CuꢀNpyridyl and CuꢀN1N interactions that define
the equator of the distorted octahedron (with roughly 90° angles between them, see Table 2),
whilst the second axial position is occupied by O1 (Cu–O1 = 2.205(2) Å). However, as the
bond angles in Table 2 show, O1 does not sit directly over the center of the equatorial plane,
but rather is skewed towards N2.
Two somewhat allied Cu(II) complexes that incorporate a TMPA ligand bearing a
carboxylate group that binds to the metal center have been reported by Suzuki and coꢀ
8
6,87
workers,
although in both of these structures the metal has only five coordination
interactions (i.e. there is no coordinated nitrite, or similar small molecule) and so the Cu(II)
adopts square pyramidal geometry. Meanwhile, Comba and coꢀworkers have reported a
number of structures in which Cu(II) is supported by a TMPAꢀlike ligand (which also bears a
88,89
carboxylate group which coordinates to the metal),
but where the linkage between the
pyridine groups is a bispidine unit. This then affords structures in which the metal center
forms six coordination interactions (one to each of the three pyridines, one to the carboxylate
oxygen and one to each of the two amines in the bispidine system), giving distorted
octahedral geometries. Again, however, there is no interaction between the metal center and
any small molecules (such as the nitrite in Figure 3). Hence the structure reported here for [4ꢀ
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NO
2
] appears to be unique in showing a Cu(II) center ligated by a TMPAꢀderived ligand, and
which is also coordinated to both nitrite and carboxylate.
2
Table 2. Experimental Bond Distances (Å) and Angles (°) in [4ꢀNO ]
Cu01–O1
Cu01–N1
Cu01–N2
Cu01–N3
Cu01–N4
2.205(2)
2.384(2)
2.031(2)
2.047(2)
2.039(2)
O1–Cu01–N1
N2–Cu01–O1
N2–Cu01–N1
N2–Cu01–N3
N2–Cu01–N4
N3–Cu01–O1
N3–Cu01–N1
N4–Cu01–O1
N4–Cu01–N1
N4–Cu01–N3
N1N–Cu01–O1
N1N–Cu01–N1
N1N–Cu01–N2
N1N–Cu01–N3
N1N–Cu01–N4
154.03(7)
76.67(7)
77.37(8)
88.49(7)
92.37(7)
101.32(8)
77.29(8)
105.68(8)
76.05(8)
152.5(1)
98.8(1)
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Cu01–N1N 1.983(3)
107.1(1)
175.2(1)
90.9(1)
90.3(1)
EPR and IR Spectroscopy of Complexes 3 and 4.
1
Complexes
in CH
the hallmark fourꢀline pattern resulting from the hyperfine coupling of the Cu(II) electron
3
and
4
are paramagnetic with an S = /
2
ground state. Their EPR spectra recorded
3
CN solution at room temperature are very similar (Figures S2a and S3a), exhibiting
3
63,65
spin with the I = /
2
nuclear spin of the
Cu isotopes (100% natural abundance). Their
similarity is underscored by the near identical spinꢀHamiltonian parameters obtained from
–4
–1
spectral simulation, with giso = 2.122 and Aiso = 57 × 10 cm for 3, and giso = 2.123, Aiso =
–
4
–1
55 × 10 cm for 4. The large linewidth obscures any additional splitting from the spinꢀ
14
active N (I = 1, 99.7% natural abundance) atoms in the ligands, and derives from the rates
of tumbling of the molecules in solution. Given the presence of a coordinated acetonitrile in
2+
the crystal structure (vide supra), we assign these species as being [3ꢀNCCH3] and [4ꢀ
+
NCCH
3
] complex ions in solution, though there will likely be species without the solvent
ligand that will have similar parameters that contribute to the linewidth. Interestingly, treating
1
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these samples with an excess of benzoic acid (Figures S2b and S3b) does not alter the
spectral profile, confirming that the carboxylate group in complex
protonated under these conditions (i.e. the bulk of the sample is the carboxylateꢀbound
version of complex ).
Upon treating solutions of [
resulting EPR spectra exhibit rather different profiles (Figures S2c and S3c). In the case of
4 does not become
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2
+
+
3
ꢀNCCH3] and [4ꢀNCCH ] with excess TBAꢀNO , the
3 2
complex
3, the EPR spectrum that is obtained shows only a slight change from that of [
3
ꢀ
2+
–4
–1
NCCH
3
]
(giso = 2.130 and Aiso = 53 × 10 cm ), with the increase in gꢀvalue being
–
commensurate with replacement of CH CN by NO . Hence the EPR data suggest that nitrite
3
2
+
+
is bound to give a species of the form [
excess TBAꢀNO
3
ꢀNO ] . In contrast, when [
4
ꢀNCCH ] is treated with
2
3
2
2
, the resulting spectrum of [4ꢀNO ] is featureless with rampant line
broadening that drowns out the hyperfine features (Figure S3c). This is either the result of
attenuated molecular tumbling in this solvent medium or overlapping spectra from various
isomers and conformations of two or more species.
To gain more information on the nature of complexes
3
and
4
in solution in the presence of
2+
3
acid and nitrite, dichloromethane was added to the solution samples of [3ꢀNCCH ] and [4ꢀ
+
NCCH
3
] to produce a homogeneous frozen glasses for measurements at 150 K. The
2
+
spectrum of [
3
ꢀNCCH
3
]
is quintessentially that of a Cu(II) species with an unpaired
electron in the dx2ꢀy2 orbital (Figure S4a). The spectrum yielded axial spinꢀHamiltonian
–
4
–1
parameters, g = (2.054, 2.083, 2.226) and ACu = (ꢀ6, ꢀ6, 180) × 10 cm , where the average
–
4
–1
values, 〈g〉 = 2.121 and 〈A〉 = 56 × 10 cm , nicely match the isotropic parameters and so
confirm that a single dominant species is present at both 150 K and room temperature. The
2
+
benzoic acidꢀtreated sample of [
3
ꢀNCCH
3
]
reveals some differences in the frozen solution
spectrum which we ascribe to the presence of a minor second species and aggregation effects
common in Cu(II) EPR spectra (Figure S4b).
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+
The frozen glass EPR spectrum of [
3
ꢀNO
2
] is shown in Figure S4c and is dominated by a
–
4
–1
signal with g = (2.044, 2.102, 2.247) and ACu = (10, 14, 144) × 10 cm . These values are
consistent with nitrite binding to copper through oxygen in a quasiꢀbidentate manner (i.e. as
2
[
Cu(κ ꢀONO)(TMPAꢀCO Me)]). The spectrum in Figure S4c also has a 5% contribution
2
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2
from a species containing two nitrites, [Cu(κ ꢀNO )(κ ꢀONO)(TMPAꢀCO CH )]. This second
2
2
3
–
4
–1
subspectrum has g = (2.053, 2.059, 2.312); ACu = (0, 24, 162) × 10 cm ; A
N
= (0, 20, 0) ×
–
4
–1
–4
–1
10 cm ; A
N
= (0, 18, 0) × 10 cm and is identical to those we previously reported for the
1
2
42
complex [Cu(κ ꢀNO
2
)(κ ꢀONO)(6ꢀmethylꢀtris(2ꢀpyridyl)methanol methyl ester)].
The
occurrence of a similar species in the current case is readily apparent due to its narrow lines
14
in the frozen glass that is indicative of anisotropic coupling to two different N nuclei, one
–
for the Nꢀbound NO
2
ligand and the other for the pyridylꢀN trans to the nitrite. Figure S4d
shows that when benzoic acid is added to this sample, very little change is observed in the
EPR signal, suggesting that the coordination geometry remains unchanged.
+
The frozen solution spectrum of [
4
ꢀNCCH ] in CH CN/CH Cl is more elaborate and
3
3
2
2
consists of a least two different monocopper complexes present in the solution in addition to
aggregation of these complexes in the frozen solution (Figure S5a). This is confirmed by
using DMF as a glassing solvent (Figure S5b), which leads to an almost featureless spectrum
as this more basic solvent mixture promotes greater aggregation of the Cu(II) species. The
absence of a halfꢀfield signal rules out dimerization of this complex, for example through the
+
pendant carboxylate group of the TMPA ligand. Treating [
4
ꢀNCCH ] with excess benzoic
3
acid (Figures S5c and S5d) shows that one of the two monocopper species is favored over the
other and also diminishes the effects of aggregation. Furthermore, these frozen solution
spectra suggest that slow tumbling (rather than multiple overlapping signals) produces the
+
broad line observed in the room temperature spectrum of [
acid.
4
ꢀNCCH ] treated with benzoic
3
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2 3 2 2
Figure 4. Xꢀband EPR spectrum of [4ꢀNO ] recorded in 5:1 CH CN/CH Cl solution at 150
K (experimental conditions: frequency, 9.430 GHz; power, 6.3 mW; modulation, 0.1 mT).
Experimental data are represented by the black line. The simulation is composed of three
subspectra with relevant weighting shown in orange, violet and green lines; the sum is given
by the red trace overlaid on the experimental spectrum. Guidelines map the contribution of
each subspectrum to the A|| features of the experimental spectrum.
2 3 2 2
The spectrum of [4ꢀNO ] recorded in a CH CN/CH Cl frozen glass is shown in Figure 4 and
Figure S5e. Simulation of this spectrum reveals that it is composed of three subspectra (lower
panels in Figure 4). Hence the major species (82% of the spectrum, orange line in Figure 4)
–
4
–1
has been simulated with g = (2.058, 2.081, 2.295) and ACu = (10, 10, 147) × 10 cm , where
the average gꢀvalue, = 2.145, matches that from the fluid solution spectrum, giso = 2.142,
〈g〉
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suggesting that the nitrite is bound to the copper center through nitrogen. The two minor
species have been included in the fit as their features are clearly observed, and are highly
+
analogous to the spectra observed for [
3
ꢀNO
2
] (Figure S4c). Hence there is a component
accounting for 15% of the spectrum with g = (2.044, 2.102, 2.247) and ACu = (10, 14, 144) ×
1
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2
2
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3
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4
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0
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0
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0
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2
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
–
4
–1
2
10
cm , which we assign to a quasiꢀbidentate Oꢀbound species, [Cu(κ ꢀONO)(TMPAꢀ
CO
2
H)] (green trace in Figure 4). Meanwhile, by analogy to Figure S4c, the smallest of the
1
2
simulated components corresponds to [Cu(κ ꢀNO
2 2
)(κ ꢀONO)(TMPAꢀCO H)] (purple trace in
Figure 4). Isomerization to the weaker nitrito ligand will account for the lower g||ꢀvalue, and
1
2
is corroborated by calculations that show the κ ꢀNO and κ ꢀONO structures are isoenergetic
2
(
vide infra).
The spectrum of [
that for [ ꢀNO ], except that it has a slightly different ratio of 88:10:2 of the three subspectra
under discussion. The dominant species is thus again assigned as [ ꢀNO ], where the
4
2
ꢀNO ] treated with excess benzoic acid (Figure S5f) is almost identical to
4
2
4
2
carboxylate remains bound to the Cu ion in a sixꢀcoordinate environment. Despite the lack of
an observable hyperfine splitting, we nevertheless assign this complex with an Nꢀbound
nitrite ligand based on computational analysis (vide infra). The second subspectrum has
+
features that closely resemble those of the frozen solution spectrum of [
3
ꢀNO
2
] (Figure S4c),
2
suggesting that there is a 10% component from [Cu(κ ꢀONO)(TMPAꢀCO
2
H)] and a 2%
1
2
component from [Cu(κ ꢀNO )(κ ꢀONO)(TMPAꢀCO H)].
2
2
Hence these EPR data support the assumption that nitrite binds to complex
4 in solution
primarily through nitrogen, in a similar fashion to that observed in the solid state (Figure 3),
although with significant minority species where the nitrite is bound through oxygen.
Furthermore, the data suggest that addition of acid to [
2
4ꢀNO ] does not lead to a significant
change in the coordination geometry at the copper center, implying that species where the
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carboxylate group remains bound to the metal dominate in solution over those where the
pendant carboxylate is protonated and hence not bound to copper.
+
The infrared spectrum of complex [
3
ꢀNO
2
] in acetonitrile solution is shown in Figure S6.
−1
There is an intense band at 1097 cm which is characteristic of the antisymmetric NꢀO
0
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30,46
stretch previously observed for nitrite bound to Cu(II) centers in a κ ꢀONO fashion.
solutionꢀphase IR spectrum is therefore in agreement with the EPR data, which also suggest
that nitrite binds to copper through its oxygens in a quasiꢀbidentate manner in complex . The
This
3
−
1 (43)
corresponding symmetrical stretch is expected to come between 1450 and 1400 cm , but
this is obscured by signals arising from acetonitrile in the same region. Meanwhile, a band at
−1
1
736 cm can be assigned to the C=O stretch in the methyl ester. The solid state infrared
spectrum of complex [
2
4ꢀNO ] is shown in Figure S7. This spectrum shows strong
−1
absorbances at 1362 and 1328 cm , which are assigned to the νasym(NO
2
2
) and νsym(NO )
2
7,30,43
bands of Nꢀbound nitrite respectively by comparison with the literature.
Hence the IR
data are consistent with the EPR spectra and solid state structures in suggesting that nitrite is
+
predominantly Oꢀbound in complex [
NO ].
3
ꢀNO ] and predominantly Nꢀbound in complex [
4
ꢀ
2
2
Electrocatalysis of nitrite reduction.
The potential electrocatalytic activity of complexes
was then explored by cyclic voltammetry. Figure 5A (black trace) shows the behavior of
complex in acetonitrile containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAꢀ
PF ). A reversible redox wave is evident at E ≈ –0.29 V (vs. ferrocenium/ferrocene), which
3
and 4 for the reduction of nitrite to NO
3
6
1/2
we assign as the Cu(II)/Cu(I) couple. This wave shifts to significantly more cathodic
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potentials (E1/2 = –0.69 V) upon the addition of tetrabutylammonium nitrite (red trace; see
also Figure S8 for a comparison over a larger voltage window). The solution also turns green
as nitrite is added. Both the color change and the shift in the position of the reversible redox
wave are consistent with nitrite binding to the Cu(II) center and displacing the coordinated
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acetonitrile found in the crystal structure (Figure 1); the replacement of a neutral CH CN
–
ligand by a more electronꢀrich NO ligand would be expected to make the reduction of the
2
Cu(II) center more difficult. There is, however, no catalytic wave that would suggest electroꢀ
reduction of nitrite mediated by the complex under these conditions.
Figure 5A. Cyclic voltammograms of a 0.6 mM acetonitrile solution of complex
3 containing
0.1 M TBAꢀPF run under the conditions detailed in the Experimental Section. The black
6
trace shows the behavior of the complex on its own, and the red trace shows the behavior in
the presence of 50 equivalents of tetrabutylammonium nitrite. 5B: Overlay of the cyclic
6
voltammograms of a 0.6 mM acetonitrile solution of complex 3 containing 0.1 M TBAꢀPF in
the presence of 50 equivalents of tetrabutylammonium nitrite, and adding benzoic acid as
indicated: 0 equivalents (black line), 1 equivalent (red line), 5 equivalents (green line), 10
equivalents (blue line) and 20 equivalents (orange line).
Figure 5B then shows the effect of adding the proton source benzoic acid to a solution of
complex
3 containing 50 equivalents of tetrabutylammonium nitrite. Hence it is apparent that
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even at 20 equivalents of protons added to the solution of complex 3, the only effect on the
wave is a slight cathodic shift to E1/2 = –0.71 V, with no catalytic wave for nitrite reduction
evident.
However, the situation changes when the methyl ester on complex
complex . In Figure 6A, the CV of complex in acetonitrile containing 0.1 M
3 is deprotected to give
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tetrabutylammonium hexafluorophosphate is shown (black trace). This presents a reversible
redox wave for the Cu(II)/Cu(I) redox couple at E1/2 = –0.82 V, the position of which changes
very little upon addition of nitrite (red trace in Figure 6A, and see also Figure S9 in the
+
Supporting Information for cyclic voltammograms of [
3
ꢀNO ] and [
4
ꢀNO ]). The much
2
2
more cathodic position of the Cu(II)/Cu(I) redox wave in comparison to that seen in complex
is further evidence that the deprotonated carboxylic acid group remains coordinated to the
3
copper center in solution (increasing the effective electron density at the metal and making
reduction harder), as seen in the solid state (Figure 3), and also as suggested by EPR, mass
spectrometry and CHN analysis (see Experimental Section). In contrast to the case for
complex
3
, it is, therefore, very difficult to gauge nitrite binding by observing the shift in the
. However, the color change of the solution from blue
Cu(II)/Cu(I) redox wave for complex
4
to green suggests that nitrite is indeed bound, as do the EPR and crystallographic data.
Further evidence for this comes in the shape of the red trace in Figure 6A, which now shows
some of the features characteristic of a catalytic wave.
Intrigued by this waveꢀshape, we added benzoic acid to solutions of
4 in the presence of 50
equivalents of tetrabutylammonium nitrite, as shown in Figure 6B. A clear trend was
apparent, in which adding larger amounts of the proton source gave rise to a more and more
pronounced catalytic wave at the potential of the Cu(II)/Cu(I) redox couple. There was also a
slight anodic shift in the position of the peak current for this wave with increasing acid
concentration.
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Figure 6A. Cyclic voltammograms of a 1 mM acetonitrile solution of complex
4 containing
6
0.1 M TBAꢀPF run under the conditions detailed in the Experimental Section. The black
trace shows the behavior of the complex on its own, and the red trace shows the behavior in
the presence of 50 equivalents of tetrabutylammonium nitrite. 6B: Cyclic voltammograms of
a 1 mM acetonitrile solution of complex
6
4 containing 0.1 M TBAꢀPF in the presence of 50
equivalents of tetrabutylammonium nitrite, and adding benzoic acid as indicated: 0
equivalents (black line), 2 equivalents (red line), 5 equivalents (purple line), 10 equivalents
(blue line) and 22 equivalents (green line).
In order to determine the product(s) of this electrocatalytic activity, we conducted bulk
electrolyses in sealed electrochemical cells at a fixed potential –0.91 V vs.
ferrocenium/ferrocene under a variety of conditions (see Experimental Section for details).
Representative current density vs. time curves are shown in Figure 7 for electrolyses using 5
mM benzoic acid, 5 mM TBAꢀNO and complex
3
(blue line), 5 mM benzoic acid, 5 mM
(red line), and a control containing just 5 mM benzoic acid and 5
(black line). Hence it is apparent that considerably more current flows when
is employed compared to when complex is used, or compared to the background
2
TBAꢀNO and complex
4
2
mM TBAꢀNO
complex
2
4
3
reaction. In order to determine if any NO was produced during these bulk electrolyses, we
90ꢀ93
used the known NOꢀcomplexation agent mesoꢀtetraphenylporphyrin cobalt(II) (CoꢀTPP),
2
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which undergoes a highly characteristic shift in the position of its visible absorption band
29,94
(λmax ≈ 530 nm) upon NO binding.
A calibration curve (Figures S10 and S11) was
constructed, which allowed the amount of NO in the reaction vessel headspace to be
determined on the basis of the position of the CoꢀTPP absorption band (see Experimental
Section for details). By assessing the shifts produced in the CoꢀTPP absorption band in bulk
electrolyses that passed various amounts of charge, a Faradaic yield for the production of NO
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by complex
4 of 97.3 ± 3.6% was obtained.
In an extendedꢀrun bulk electrolysis with complex
4
(Figure S12), 0.225 C of charge were
passed for the reduction of nitrite to NO over 22000 s, without any apparent diminution of the
current density. Given a concentration of catalyst of 7 ꢁM in 15 mL of solution, this
corresponds to around 22 turnovers of the catalyst, thus proving that complex
as a genuine catalyst for this reaction.
4 is functioning
Figure 7. Bulk electrolysis in 0.2 M TBAꢀPF
6
in acetonitrile of a 7 ꢁM solution of complex 3
(
blue line) a 7 ꢁM solution of complex (red line) and in the absence of any catalyst (black
4
line) at a fixed potential of –0.91 V vs. ferrocenium/ferrocene. The working electrode was a
2
glassy carbon foil electrode (area ~4.6 cm ). An Ag/AgNO
3
pseudo reference electrode and a
Pt wire counter electrode were used. In each case, the electrolyte also contained 5 mM
benzoic acid and 5 mM TBAꢀNO2.
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Inspection of Figure 7 suggests that current appreciably above the background level is also
observed with complex
2
3 in the presence of benzoic acid and TBAꢀNO . Moreover, CoꢀTPPꢀ
based colorimetric NO determination assays revealed that this current was associated with
NO formation (Faradaic yield = 88.2 ± 7.3%). Given the apparent absence of electrocatalytic
activity evident in Figure 5, this was initially surprising to us. We therefore conducted further
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cyclic voltammetry experiments with complex
3
in the presence of benzoic acid and TBAꢀ
–1
NO
2
at the slower scanꢀrate of 10 mV s , as shown in Figure S13. This indicated that a
catalytic wave is evident when the scanꢀrate is slowed. Coupled with the aboveꢀbackground
currents observed by bulk electrolysis (and the associated detection of NO), this suggests that
complex
3
is indeed competent for nitrite reduction electrocatalysis, but that the catalysis
is less effective than it is with complex . Indeed, in an extendedꢀrun bulk
could be induced to turn over 10 times in a 17000 s time period (see
with complex
3
4
electrolysis, complex
Figure S12).
3
To further confirm the production of NO during bulk electrolysis, NO release profiles were
obtained using a Sievers NOA 280i chemiluminescence nitric oxide analyzer (see
Experimental Section). Figure 8 shows one such experiment with complex
4 in which a bulk
electrolysis was performed for an hour at a fixed potential of –0.91 V vs.
ferrocenium/ferrocene in a sealed cell (panel A), after which time the contents of the
headspace were flushed into the detector of the Sievers instrument using a stream of nitrogen.
The main panel in Figure 8B then shows the total (cumulative) amount of NO measured by
the detector after termination of the electrolysis, whilst the inset shows the instantaneous
level of NO detected. On the basis of this instantaneous data, one can conclude that after
about 300 s, the level of NO detected has decayed back to basal levels. The slope of the graph
in the main panel after 300 s therefore allows the rate of background NO production due to
spontaneous processes to be calculated. Subtracting this value from the overall amount of NO
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produced gives a Faradaic yield for NO production by complex
4 using this method of 100 ±
1%, in good agreement with the values obtained by colorimetric tests. As expected, an
electrolysis under the same conditions but in the absence of any catalyst produces
significantly less NO (see Figure S14).
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Figure 8A: Charge vs. time curve for the bulk electrolysis of complex
of –0.91 V vs. ferrocenium/ferrocene in acetonitrile/0.2 M TBAꢀPF
4
at a fixed potential
6
as per the conditions
detailed in the Experimental Section. 8B: (main panel) total NO measured upon flushing cell
headspace into the NO detector after termination of electrolysis and (inset) instantaneous NO
detection.
The above tests serve to establish that both complexes
electrocatalytic nitrite reduction to NO with essentially full Faradaic efficiency, but that
complex appears to be the more effective of the two. In order to gain insight into these
3 and 4 are competent for
4
apparent differences in catalytic activity, further electrochemical investigations were
undertaken.
Firstly, Tafel analysis of
3
and
and benzoic acid was found to give slopes of 138 and 128 mV
decade respectively if a transfer coefficient of 0.5 was assumed (Figure S15). These are
4 in 0.1 M TBAꢀPF /acetonitrile in the presence of 30
6
equivalents of both TBAꢀNO
2
–1
2
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1
close to the theoretical value for a rateꢀlimiting oneꢀelectron process (120 mV decade ), in
agreement with a reaction such as that given in Eq. 1 for the oneꢀelectron reduction of nitrite
to NO.
Next, we investigated the kinetics of the nitrite reduction reaction by cyclic voltammetry.
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Hence the peak current for electrocatalytic nitrite reduction by both complex
3 and complex 4
was measured as a function of nitrite concentration in the presence of a large excess (300
–
1
equivalents relative to the catalyst complexes) of benzoic acid at a scanꢀrate of 10 mV s (at
which scanꢀrate there is evidence of an electrocatalytic wave with both complexes; see
above). As the Faradaic yield for NO production is essentially unity, and as the reduction of
nitrite to NO is a oneꢀelectron process, these current densities can be converted to a rate of
nitric oxide production per unit area of electrode (V[NO]) by dividing the current density by
Faraday’s constant. These data (Figures S16ꢀS18), can then be fitted to MichaelisꢀMentenꢀ
type kinetics as per Eq. 2:
–
2
–
V[NO] = Vmax[NO ]/(K + [NO ]) (Eq. 2)
M
2
which allows the maximal rate of reaction (Vmax) for complex
4
to be estimated as being 14.7
–
1
–2
±
1.5 nmol s cm , with a Michaelis constant (K ) of 11 ± 3 mM (see Figure 9). Rates of
M
NO formation are shown as negative in Figure 9 as these derive from cathodic (reductive)
currents in the CVs. The value of Vmax can be deꢀconvoluted to yield the MichaelisꢀMenten
k
cat (the turnover frequency) by dividing Vmax by the concentration of catalyst molecules. In
our case, this concentration corresponds to the number of catalyst molecules participating in
the electrochemical reaction, which (given that the reaction medium is not stirred in these CV
measurements) can be determined by integrating the charge passed in the reversible
Cu(II)/Cu(I) redox wave in the absence of substrate. When this integration is performed, an
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–
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–2
electrochemicallyꢀactive surface concentration of catalyst molecules of 1.1 × 10 mol cm is
obtained, which in turn gives a value for kcat (the turnover frequency) for complex of 13.4 ±
4
–
1
1
.4 s . From this value, the catalytic efficiency of the catalyst can be expressed by the ratio
–1 –1
k
M
cat/K , which gives a value of around 1200 M s . These values can be compared to those
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previously reported by Orain and coꢀworkers and Yamaguchi and coꢀworkers for CuꢀTMPA
complexes without any pendant acid/base units immobilized on the surface of an electrode
47,50
and performing the reduction of nitrite to NO in aqueous solution.
Hence, the best metrics
–
1
achieved by Orain and coꢀworkers were kcat = 1.39 s with K
M
M
= 70 mM (giving kcat/K =
–
1
–1
–1
1
7.4 M s ), whilst Yamaguchi and coꢀworkers reported kcat = 0.11 s with K
M
= 15.8 mM
–1 –1
(
M
giving kcat/K = 7.0 M s ). Thus our system is the most effective small molecule Cuꢀbased
–
platform for electrocatalysis of NO
2
reduction to NO yet reported. For comparison, the
CuNIR enzyme from Alcaligenes faecalis has been estimated to have a turnover frequency of
–1
7
–1 –1 (95)
M M
around 2000 s and a K of around 100 ꢁM, giving kcat/K ≈ 2 × 10 M s .
–
Figure 9: The dependence of the rate of electrochemical NO generation on [NO
complexes (red circles) and (black squares) in the presence of 300 equivalents of
benzoic acid (PhꢀCO H) in acetonitrile/0.2 M TBAꢀPF as per the conditions detailed
in the Experimental Section. The green squares show analogous data for complex
but in deuterated acetonitrile/0.2 M TBAꢀPF in the presence of 300 equivalents of
2
] for
3
4
2
6
4
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deuterated benzoic acid (PhꢀCO
Menten equation.
2
D). The blue dashed lines show fits to the Michaelisꢀ
To discriminate between the effects of the pendant acid/base group and the change in solvent
and reaction conditions in this apparent improvement in catalytic efficiency relative to the
previous studies with TMPAꢀbased Cu complexes, we also performed a similar Michaelisꢀ
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Menten analysis on complex
analogous conditions (Figures 9 and S18). The resulting MichaelisꢀMenten plot allowed the
3 (which lacks any the pendant acid/base groups) under
–
1
–2
maximal rate of reaction (Vmax) to be estimated as 8.0 ± 0.5 nmol s cm , with a Michaelis
constant (K ) of 12 ± 2 mM. Taking the electrochemicallyꢀactive surface concentration of
M
–9
–2
–1
catalyst molecules to be 1.1 × 10 mol cm , the value of kcat is then 7.3 ± 0.5 s , giving
–1
–1
k
cat/K
M
≈ 600 M s . Hence there is a demonstrable enhancement in the catalytic
performance of complex compared to complex over and above that attributable to the
change in solvent medium relative to the previous studies.
The nature of the protonꢀcoupledꢀelectron transfer event occurring during nitrite reduction
with complex was then probed by comparing the kinetics in a protic environment with
those observed under deuterated conditions. Hence the solvent medium was switched to
CD CN and a deuterated benzoic acid source (PhꢀCO D) was used. Figure 9 (green squares)
shows the results of this deuteration. Vmax remains largely unaltered with respect to protic
4
3
47,50
4
3
2
–
1
–2
–1
conditions, at 13.1 ± 1.1 nmol s cm (giving k = 11.9 ± 1.0 s ). However, K increases
cat
M
significantly to 25 ± 4 mM, suggesting that higher concentrations of substrate are required to
reach this maximal turnover frequency. This in turn gives a value for the catalytic efficiency
–1 –1
of complex
4 under deuterated conditions of around 470 M s , implying a significant
kinetic isotope effect of ~2.5. The fact that such a pronounced effect can be seen upon
9
6
moving to deuterated solvents suggests that the proton and electron transfers are concerted.
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Calculations.
Based on the experimental observations derived from spectroscopic and electrochemical data,
a mechanism for the catalytic reduction of nitrite to nitric oxide was developed using DFT
calculations. Optimizations were performed at the BP86 level of theory using the def2ꢀSVP
basis set for all atoms and acetonitrile as the solvent continuum. Singleꢀpoint calculations
were performed on the optimized coordinates at the PBE0 level of theory with tripleꢀζ basis
sets.
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3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
In these simulations, we found that nitrite readily displaces a coordinated solvent molecule
+
(H
2
O or CH
3
CN) from complexes
3
and
4
to give [
3
ꢀNO
2
] and [
4
ꢀNO
2
]. The coordination
2
mode of the nitrite ligand in [4ꢀNO ] favors the Nꢀbound (nitro) linkage isomer by 2.9 kcal
–1
mol over that of the nitrito (Oꢀbound) variant. Although this is very close to the limit of
energies distinguishable by DFT, it does match the solid state structure (Figure 3), and the
major solutionꢀphase species suggested by EPR (Figure 4). Additionally, no optimized
2
complex retained a chelating κ ꢀONO ligand when combined with
4
. For
3
, the absence of the
pendant acid/base group gave isoenergetic complexes with nitro or chelating nitrito ligands
–
1
(
1.4 kcal mol in favor of the latter). The latter is proposed to be the complex that provides
+
the largest contribution to the EPR spectrum of [
compared with [
3
ꢀNO
2
] on account of the sizeable gꢀshift
4
ꢀNO ] (see EPR discussion above and Figure S4). These isomers were 5.6
2
–
1
kcal mol more favorable energetically than the complex with a monodentate nitrito ligand.
Having established the most likely entry point into the catalytic cycle as being [
NO ], further computational analysis (Scheme 3) reveals that protonation of [ ꢀNO
to give [
discussed above in relation to the EPR data) there is no evidence to support
protonation of the carboxylate group in [ ꢀNO ] in the presence of excess benzoic
4
ꢀ
]
2
4
2
+
–1
4
2
HꢀNO ] would afford a 17.2 kcal mol increase in stability, but (as
4
2
2
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Page 29 of 68
ACS Catalysis
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2
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acid. On the other hand, reduction of [
4
ꢀNO
2
] by one electron (to give the Cu(I)
–
–1
species [
4
ꢀNO
2
] ) leads to a decrease in stability of 32.7 kcal mol . This is
unsurprising as sixꢀcoordinate species are not favored by Cu(I). However, subsequent
–
protonation of the carboxylate in [
4
ꢀNO ] to give [
4
HꢀNO ] (in which the carboxylate
2
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
–1
unit is no longer coordinated to copper), affords a 26.9 kcal mol increase in stability
–
relative to [
4
ꢀNO
2
] . This DFTꢀmodelled PCET is therefore consistent with the
experimental observation of concerted proton and electron transfer, in which the
reduction of the Cu(II) complex is concomitant with the release and protonation of the
carboxylate, leading directly from [4ꢀNO ] to [4HꢀNO ] and avoiding the high energy
2 2
–
[
4
ꢀNO ] intermediate.
2
Computational optimization of [
carboxylate proton had migrated to the adjacent nitro ligand to give a coordinated nitrous acid
group, [ ꢀHNO ] (Scheme 3). This underscores the key role played by the protonꢀrelaying
2
4HꢀNO ] then leads spontaneously to a structure where the
4
2
group in nitrite reduction, as the in silico process follows an analogous path to that observed
experimentally. This is not the case for the nitrito isomers of this Cu(I) complex, which are
less stable than the nitro isomer. This further corroborates our assignment of the nitro isomer
as the catalytically competent species in complex
less effective electrocatalyst for this process than complex
its lack of any protonꢀrelaying units and to the different nature of the dominant nitrite
coordination mode in complex (i.e. predominantly through oxygen), which will also impact
upon the NꢀO bond cleavage pathway that is followed with this complex.
4
. Meanwhile, complex
3 is found to be a
4
, which can be attributed both to
3
2
9
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