Dicopper μ?oxo, μ?nitrosyl complex from the activation of nO or nitrite at a dicopper center

Article abstract of DOI:10.1021/jacs.9b03635

Treatment of a dicopper(I,I) complex with nitric oxide produces a dicopper μ-oxo, μ-nitrosyl complex [LCu₂(μ-O)(μ-NO)]²⁺, representing the first structurally characterized μ-oxo, μ-nitrosyl metal complex. This compound can also be synthesized from the reaction of nitrite with an [LCu^IICu^I]³⁺ synthon. Full characterization of the thermal-sensitive [LCu₂(μ-O)(μ-NO)]²⁺ complex with IR, EPR, and X-ray crystallography suggests a localized mixed-valent Cu^III, Cu^II, O^2?, NO^? formulation. The [Cu₂(μ-O)(μ-NO)]²⁺ core efficiently oxidizes exogenous substrates, such as phosphine, cyclohexadienes, and isochroman to afford phosphine oxide, benzene, and 1-isochromanone. Since both nitrite and nitric oxide are proposed oxidants in denitrifying methane oxidation, the oxidative reactivity of [Cu₂(μ-O)(μ-NO)]²⁺ core is potentially relevant to anaerobic methane oxidation observed in methanotro hic archaea

Full text of DOI:10.1021/jacs.9b03635

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Communication
Dicopper µ-Oxo, µ-Nitrosyl Complex from the
Activation of NO or Nitrite at a Dicopper Center
Wenjie Tao, Jamey K. Bower, Curtis E. Moore, and Shiyu Zhang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b03635 • Publication Date (Web): 20 Jun 2019
Downloaded from http://pubs.acs.org on June 20, 2019
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Dicopper -Oxo, -Nitrosyl Complex from the Activation of NO or
Nitrite at a Dicopper Center
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Wenjie Tao, Jamey K. Bower, Curtis E. Moore, Shiyu Zhang*
Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210,
United States
Supporting Information Placeholder
for methane selective monooxygenation in Cu-ZSM-5,9,10 nitrite-
derived dicopper -oxo species could be synthetically useful in
oxidation/oxygenation reactions. Additionally, activation of NO2
toward C-H hydroxylation is mechanistically and functionally
relevant to anaerobic methane oxidation observed in denitrifying
ABSTRACT: Treatment of a dicopper(I,I) complex with nitric
oxide produces a dicopper -oxo, -nitrosyl complex [LCu (-
2

2+
O)(-NO)] , representing the first structurally characterized -
oxo, -nitrosyl metal complex. This compound can also be
II
I 3+
synthesized from the reaction of nitrite with a [LCu Cu ] synthon.
Full characterization of the thermal-sensitive [LCu (-O)(-
NO)] complex with IR, EPR, and X-ray crystallography suggests
archaea, a metabolic process that couples reduction of nitrite with
2

+ 8H+  3CO
2
+
methane oxidation: 3CH
4
+ 8NO
2
2
+ 4N
2
+
1
1
III
II
2−
−
2
10H O. Although the biochemical mechanism for denitrifying
a localized mixed-valent Cu , Cu , O , NO formulation. The
(-O)(-NO)]2+
methane oxidation is still under investigation, the involvement of
[Cu
2
core efficiently oxidizes exogenous
particulate methane monooxygenase (pMMO)  an enzyme that
substrates, such as phosphine, cyclohexadienes, and isochroman to
afford phosphine oxide, benzene, and 1-isochromanone. Since both
nitrite and nitric oxide are proposed oxidants in denitrifying
1
2–16
17,18
contains copper active sites
– has been proposed.
Scheme 1.
methane oxidation, the oxidative reactivity of [Cu
(-O)(-NO)]2+
2
Cytochrome c oxidase
core is potentially relevant to anaerobic methane oxidation
observed in methanotrophic archaea.
 NO
FeII
CuII + NO2
A
B
FeIII
O
CuII
FeII CuI + 2 NO
 N O
2
Heme/non-heme and flavo-diiron NO reductases
) at
2
The interconversion of nitric oxide (NO•) and nitrite (NO

N O
2
FeIII
FeIII
biological metal centers has important implications in
FeII FeII + 2 NO
O
1
neurotransmission and vasodilation. For example, the heme-
CuII
a
3
/Cu
B
site in cytochrome c oxidase (CcO) reduces NO 
2
to NO•
{CuNO}11 + {CuNO}11
O
C
N
CuII
O
N
(Scheme 1A) under hypoxic conditions, triggering a series of
2
,3
 N2O
CuII
biochemical responses leading to cellular O
heme-a /Cu site can also detoxify NO• by promoting the reductive
coupling of two NO• molecules to form nitrous oxide (N O),
2
restoration. The
3
B

NO
O
CuII
CuI or Cu NO
II
CuII
2
N
2
O
III
II
O
leaving behind a Fe -O-Cu moiety (Scheme 1A). Similar
4
reductive coupling of NO• occurs at heme/non-heme and flavo-
diiron NO reductases (NOR, Scheme 1B). Based on modeling
This work
5
CuI
CuII
O
N
D
+ NO2
2+
studies, it appears that bimetallic centers can promote the coupling


of adjacent nitrosyl ligands to form hyponitrite ( O-N=N-O )
preceding the release of N O. For instance, reductive coupling of
CuI
CuI + 3 NO
Cu
Cu
O

N O
2
2
1
1
two {CuNO}
moieties generates
a
fully characterized
2
O to form copper(II)
dicopper(II,II) hyponitrite that releases N
nitrite ([Cu ]-NO
complex (Scheme 1C).
A common intermediate shared in the interconversion of NO• and
In this report, we describe a new nitrite activation mechanism that
can be harnessed for oxidative reactivity. Cleavage of an O-NO
bond at a dicopper(I,II) center provides an unprecedented
dicopper(II,III) -oxo, -nitrosyl complex ([Cu
Scheme 1D). This complex can also be prepared from the reaction
(-O)(-NO)]2+ core is
of dicopper(I,I) with NO•. The [Cu
reminiscent of the active species in methane-oxidizing copper-
and cleanly oxidizes phosphine,
cyclohexadienes, and isochroman to afford phosphine oxide,
benzene, and 1-isochromanone respectively. Such oxidative
II
2
), presumably through a dicopper(II,II) -oxo
6–8
2
+
2
(-O)(-NO)] ,

NO
2
is the bimetallic -oxo (M-O-M) motif, which could be
generated from either reductive coupling of two NO• molecules or
2
activation of the O-N bond in NO 
2
2
(Scheme 1A and 1B). Owing
19–23
, we were curious
exchanged zeolite,
to our interests in the oxidative reactivity of NO
2
if a dicopper(II,II) -oxo could be formed from the activation of

2
NO
2 3 B
at a dicopper center in a manner similar to heme-a /Cu site.

reactivity of NO
2
at dicopper centers represents a strategy
Given that dicopper(II,II) -oxo core is proposed as the active site
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HO
OH
3 NO
2
+
N
N
N
O
N
1
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 N O
2
Cl

2 NCMe
O
O
[LCu 2(NCMe)2][BArF4]2
I
N
N
L
BArF4]2
2
Cu
Cu
[
K CO
80 to 40 °C
2
3
O
2-BAr
N
MeCN
F
1
4
L 67%
stable up to 40 °C
3
2
eq
Exp
Sim
acetone 80 to 40 °C
THF 20 K
[
Cu(MeCN)4]  6 MeCN
[BArF4]
2-BArF
2
1
0
4
2
+
2+
Me Me
Cu
N
Cu
C
N
Cu
C
N
N
N
N
N
N
0
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PhC CH
2 MeCN
Cu
N
O
O
N
O
O

250
275
300
325
350
375
350
450
550
650
750
850
Field (mT)
BArF4]2
Wavelength (nm)
[
[BArF4]2
Figure 2. Reaction scheme (top) and corresponding UV-Vis spectra
X-ray structure of
1•phenylacetylene
F
(
left) for the formation of 2-BAr (−40 °C, 0.75 mM [Cu
2
], acetone).
1
81%
Right: frozen EPR spectrum and simulation of 2 (THF, 20 K, 0.5 mM).
87%
thermostability of 2-OTf and 2-BArF
spectroscopy. The NO of 2-OTf is comparable to that of
dicopper(II,II) -nitrosyl complexes [(XYL-O)Cu (-NO )]
(1536 cm ) and lower than that of the mixed-valent [Cu Cu (-
4
observed by UV-Vis
Figure 1. Synthesis of Py
4
DMB ligand (L), 1-BArF
4
and
F
1
•phenylacetylene-BAr
4
. Solid-state structure of 1•phenylacetylene-
with thermal ellipsoids shown at 50% probability level. Two
anions and all hydrogen atoms are omitted for clarity. Selected
II
 2+
F
BAr
BAr
4
2
F
1 28
I
II
4
interatomic distances (Å): Cu-Cu 2.8249(5) Å, Cu-O (average) 2.757
Å.
1 2+ 29
NO•)] (1670 cm ) .
X-ray diffraction analysis of single crystals of 2-BArF
4
grown from

available to Nature to utilize NO
anaerobic conditions.
2
and NO• as oxidants under
a mixture of THF and pentane showed a dicopper -oxo, -nitrosyl
F
complex [LCu
2
(-O)(-NO)][BAr
4
]
2
(Figure 3). Although it is
…
24
difficult to distinguish -O2 ligand from -OH ligand

To template a dicopper complex with a close Cu Cu distance, we
linked four pyridine ligands using a rigid 1,2-dimethoxylbenzene
crystallographically, further EPR spectroscopy study supports the
assignment of -O2 (vide infra). The structure of 2-BAr
suggested that the coupling of two {CuNO} resulted in the
O and a presumed [Cu -O-Cu ] species, which
was then trapped by another equivalent of NO•. Complex 2-BAr
is the first structurally characterized -oxo, -nitrosyl metal
complex. The -nitrosyl ligand is disordered over two positions
(see supporting information). In the major occupancy site (60 %),
the N-O bond distance is 1.154(19) Å slightly shorter than that in
(DMB) linker. The Py
yl)methoxy)benzene, L)
4
DMB ligand (1,2-bis(di(pyridin-2-
could be prepared from
F
4
11
dipyridylchloromethane and catechol in 67% yield (Figure 1).
Insertion of copper(I) ions to L proceeds upon treatment of L with
two equivalents of [Cu (MeCN)
bis(trifluoromethyl)phenyl)borate) or [Cu (MeCN)
trifluoromethanesulfonate), affording yellow powders with the H
and C NMR signals expected for [LCu
) in 81% yield or [LCu
yield respectively. Complex 1-BArF
II
II 2+
formation of N
2
F
4
I
F
F
4 4 4
]BAr (BAr = tetrakis(3,5-
I
4
]OTf (OTf =
1
1
3
I
F
2
(MeCN)
][OTf] (1-OTf) in 86%
can be stored as a solid at 40
2 4 2
][BAr ] (1-
F
I
BAr
4
2
(MeCN)
2
2
(XYL-O)CuII
 2+
[
2
(-NO )] (1.176(1) Å), the only other structurally
4
28
characterized dinuclear copper nitrosyl complex. Each copper
center also coordinates to one THF solvent molecule with a long
Cu-O bond (Cu-O 2.395(6) and 2.406(9) Å), completing a pseudo-
square pyramidal geometry. The Cu-Cu distance is 2.844(2) Å,
shorter than that observed in [(XYL-O)Cu
Å). Close examination of the structural parameters of 2-BAr
°
C without decomposition up to several months. Over prolonged
I
2+
periods in solution, however, [LCu
2
(MeCN)
(CF )]BAr
which is structurally reminiscent of the dicopper -aryl complexes
2
]
abstracts an aryl
4
(Figure S45),
F
I
F
group from BAr
4
to form [LCu
2
(C
6
H
3
3
)
2
II
 2+
2
(-NO )] (3.140(1)
25,26
reported by Tilley et. al..
To confirm the binucleating nature of
28
F
4
I
(phenylacetylene)][BArF
F
1, [LCu
2
4
]
2
(1•phenylacetylene-BAr
4
)
was prepared in 87% yield by treating 1 with one equivalent of
phenylacetylene, which was used to mimic a bridging  donor.
The X-ray structure of 1•phenylacetylene-BAr showed a Cu Cu
4
distance of (2.8249(5) Å) and long Cu-O distances (average 2.757
Å), indicating minimal Cu-O interaction.
2
7
F
…
Treatment of 1-BArF
with four equivalents of NO• in acetone
resulted in the formation of a dark green complex, which quickly
converted to a red species 2-BAr
= 2300 M cm ) at 40 °C. Similar UV-Vis spectroscopic features
were observed when 1-OTf was treated with NO• (Figure S19).
Formation of 1 eq N O per dicopper complex was detected in 98%
2
yield with GC analysis of the reaction headspace (Figure S62-S65).
The reaction of 1-OTf and NO• was monitored with low-
temperature solution IR spectroscopy. A new band at 1554 cm
4
F
4
(Figure 2 left, max = 515 nm; 

1
1
-
1
assigned to 2-OTf grew in at ‒40 °C and was found to be sensitive
to 15N labeling (∆NO14N−15N = −30 cm , Figure S50). The 1554
cm-1 N-O stretch was persistent up to 40 °C, matching the
Figure 3. Solid-state structure (150 K) of 2-BArF
ellipsoids was shown at 30% probability level. Two BAr anions, the
minor components of disorder, and all H atoms are omitted for clarity.
with thermal
−1
2
F
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indicates the [Cu
2
(-O)(-NO)]2+ core is unsymmetric.
square planar complex (Figure S46). To our delight, we found that
I
II 3+
II
1
2
3
4
5
6
7
8
9
the in situ generated [LCu Cu ] synthon (1 eq [LCu ]OTf
+ 1eq
Specifically, both the -nitrosyl and -oxo are closer to Cu2 (Cu2-
N1 1.987(13) Å; Cu2-O1 1.869(8) Å) than Cu1 (Cu1-N1 2.030(12)
Å; Cu1-O1 1.892(8) Å), hinting at a valence localized electronic
2
I
[Cu (MeCN)
4
]OTf]) reacted with TBANO
2
(TBA
=
tetrabutylammonium) in a 1:1 mixture of acetone and THF solvent
3
0
(chosen to maximize the solubility of [LCuII]OTf ) at −40 °C to
structure. We also synthesized and crystalized 2-OTf by treating
1-OTf with NO•. X-ray diffraction data of 2-OTf show that the
2
afford the [LCu
at 515 nm (Figure 4 right). The formation of [LCu
was also confirmed with solution IR study (Figure S52). A plot of
the yield of 2-OTf vs. different equivalent of TBANO shows a
per [LCu Cu ] , indicating
a 1:1 stoichiometry (Figure S26-S27). Interestingly, addition of
TBANO to the dicopper(I,I) complex 1-OTf did not produce any
2
(-O)(-NO)]2+ core as identified by a strong band
(-O)(-NO)]2+ core is bisected by a mirror plane that goes
(-O)(-NO)]2+
[Cu
2
2
through both coppers (Figure S49) and crystallographically
overlays the -oxo with the -nitrosyl, which somewhat limits the
insights into the precise bond metrics. Nonetheless, 2-OTf also
2
I
II 3+
maximum at one equivalent TBANO
2
(-O)(-NO)]2+ core (Cu1-O(NO) =
shows an unsymmetric [Cu
.943(5) Å; Cu2-O(NO) = 1.927(5) Å) and likewise suggests a
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
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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
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9
0
1
2
change in the UV-Vis spectroscopy (Figure S28-S29). The lack of
valence-localized electronic structure.
Further evidence for the valence-localized [Cu
(-O)(-NO)]2+

reaction between 1-OTf and NO
2
is perhaps due to the low Lewis
2
acidity of Cu(I) compared to Cu(II). Typically, the activation of
core was obtained by EPR analysis. A frozen solution EPR

NO
2
is strongly influenced by the Lewis acidity while rather
F
spectrum of 2-BAr
axial signal (g
couplings to one
4
in THF at 20 K (Figure 2 right) displayed an
= 2.055, g = 2.270) with major hyperfine
Cu (I = 3/2) nucleus (A (Cu) = 520 MHz). The
2,3,39–41
independent of the reducing ability of the bimetallic centers.
x
= g
3/65
y
z
6
z
Scheme 2.
additional superhyperfine structures around 325 mT could be
+
O
Ph
OTf
Cu

O
N
TBANO2
2+
modeled in several ways (see supporting information). No
F
I
difference is observed between the EPR spectra of 2-BAr
4
and
N
N
N
[Cu (MeCN)4]OTf
Cu
Cu
OTf
15
F
O
NO-labeled 2-BAr
4
(Figure S58), indicating that the N atom on
N
O
Ph
3
not observed

-NO contributes little to the superhyperfine structures. The EPR
F
III
II
4+
features of 2-BAr
complex, the only other reported mixed-valent dicopper(II,III)
complex. The S = 1/2 EPR spectrum lends further support to the
assignment of a -O instead of a -OH ligand, since both
4
is very similar to that of [Cu Cu (-OH)]
31
2+ CuCu
4.3 Å
2+ CuCu
2.8 Å
O
O
2+
O
2

CuI
CuII
N
O
N
O
N
Cu
Cu
Cu
Cu
II

II 2+
I
II 2+
[Cu (-OH)(-NO )Cu ]
and
[Cu (-OH)(-NO•)Cu ]
O
species are likely to be EPR silent due to the antiferromagnetic
9
9
32
coupling of two Cu d centers or Cu d and NO• centers. Together,
solution IR, frozen EPR, and X-ray crystallography data for
complex 2 indicate a Cu , Cu , O , NO formulation.
1
2
2
2
II
III
2
−
- : -NO
- : -NO2
-O, -NO
2
II
L + Cu [OTf]2
To determine the role that the binucleating ligand L may have on
3

40 °C
, we prepared a mononuclear analog 3 by
with two equivalents of 2,2'-
the activation of NO
complexing Cu OTf
(phenoxymethylene)dipyridine (Scheme 2). X-ray structure of 3
indicates the interaction of copper with both axial O donors are
quite weak (Figure S47, Cu1-O1(OPh) 2.522 Å, Cu1-O1A(OTf)
2
+
OTf
1:1 acetone:THF
II
2
2
1
0
N
N
N
CuII
N
O
O
2-OTf
OTf
2
.393(1) Å). Treatment of 3 with an equimolar amount of
I
[Cu (MeCN)
4
OTf] and TBANO
2
did not produce any spectroscopic
TBANO
2
TBAOTf
I
350
450
550
650
750
850
(-O)(-NO)]2+ core (Scheme 2,
[
Cu (MeCN) ]OTf
changes expected for a [Cu
2
4
Wavelength (nm)
Figure S30-S32). Molecular orbital considerations in Scheme 2
outline the importance of bimetallic cooperativity in the activation
of the O-NO bond. The dimethoxylbenzene linker in L templates a
2
-OTf
Figure 4. Reaction scheme (left) and corresponding UV-Vis spectra
right) (−40 °C, 0.75 mM [Cu
-OTf via activation of NO
(
2
2
], 1:1 THF:acetone) for the synthesis of
…
.
close Cu Cu distance (ca. 2.8 Å) and aligns the two Cu d
x2-y2
2
2
2
orbitals with the nitrite * orbital in the - : -NO
which weakens the ON-O bond in NO
2
binding mode,
. While the - : -NO
3
3–36
6–8
In light of the seminal findings by Tolman
and Karlin,
11
a

2
2
2
2
II
II 2+
[
Cu -O-Cu ] species produced by {CuNO} coupling is
binding mode is also possible with mononuclear analog 3, the
II
expected to react with NO• to form [Cu ]-NO
2
or mixed-valent
1
2
2
commonly observed - : -NO binding mode is perhaps more
II
I 2+
37,38
[
Cu (NO
the [LCu
2
)Cu ] (Scheme 1C).
(-O)(-NO)]2+ core could support a nitrosyl and oxo
ligand at the same time without reductively eliminating NO
Given that the elimination of NO
envisioned that a [Cu
from the activation of NO
Figure 4). To generate a [LCu Cu ] synthon, we first prepared a
It is therefore surprising that
37,38,42
favorable,
since it allows the soft Cu(I) to interact with N
2
atom and the hard Cu(II) to interact with O atoms. Since the
.
2
1
2
otherwise favored - : -NO
2
binding mode requires an

2
2
is not favored in 2, we
intermetallic distance of at least 4.3 Å,
prevents the formation of the inactive - : -NO
37,38,42
the binuclear L
sink, therefore
+
1
2
2
(-O)(-NO)] species could be accessed
2

2
by Cu(II) and Cu(I) templated by L
making the O-NO bond cleavage reactivity somewhat unique to the
linked bimetallic system.
I
II 3+
(
II
II
II
Cu precursor [LCu ][OTf]
2
by treating L with Cu OTf
characterization of [LCu ][OTf] , including UV-Vis, EPR, and X-
ray crystallography, is included in the supporting information. The
2
. Full

Given that NO
2
and NO• are proposed as active oxidants in
II
2
denitrifying methane oxidation,18 we are interested if the [Cu
O)(-NO)]2+ core derived from NO
(-
2

and NO• could engage in
II
II
2
2
Cu ion in [LCu ][OTf] occupies four pyridine donors to form a
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oxidative reactions, such as oxygen atom transfer (OAT), hydrogen
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1
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2
2
2
2
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atom abstraction (HAA), and C-H hydroxylation. As followed by
UV-Vis and NMR spectroscopy, we found that the complex 2-OTf
readily transferred an oxygen atom to tricyclohexylphosphine
Notes
The authors declare no competing financial interests.
(PCy
3 3
) to afford tricyclohexylphosphine oxide (OPCy ) in
ACKNOWLEDGMENT
quantitative yield. Reactions of 2-OTf with hydrogen atom donor
We are grateful for financial supports from OSU and ACS-PRF
(59036-DNI3). We thank the lab of Prof. T.V. RajanBabu (OSU)
for assistance with solution IR measurement and Prof. Hannah
Shafaat (OSU) for helpful discussion on EPR analysis. Prof.
Christine Thomas (OSU) is thanked for valuable discussion.
1
,3-cyclohexadiene (BDE
=
74.3 kcal/mol) and 1,4-
cyclohexadiene (BDE = 76.0 kcal/mol)43 at 40 °C furnished
benzene in 43% and 86% yield respectively. Finally, treatment of
-OTf with isochroman44,45 at 40 °C gave oxidized product 1-
2
isochromanone in 78% yield, assuming the formation of each 1-
isochromanone consumed two equivalents of 2-OTf (Scheme 3).
These results point to the ability of 2 to perform hydroxylation
reaction through sequential HAA and radical rebound sequence.
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(7)
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•
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The Supporting Information is available free of charge on the ACS
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Experimental details, and characterization data for complex 1-
F
F
F
BAr
4
,
1•phenylacetylene-BAr
LCu (OTf) , and 3 including X-ray crystallographic data for
•phenylacetylene-BAr
4
,
1-OTf, 2-BAr
4
,
2-OTf,
II
2
F
I
F
1
(
4
(1905396), [LCu
(1905394), 2-OTf (1905395), LCu OTf
2
(C
6
H
3
(CF
3
)
2
)]BAr
4
2
(
13)
14)
F
II
1905393), 2-BAr
4
(1905397), and 3 (1905392).
(
AUTHOR INFORMATION
Corresponding Author
* zhang.8941@osu.edu
(15)
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Journal of the American Chemical Society
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2
+
O
N
2+
2+
N
N
N
N
CuI
N
N
NO2
[CuI]
N
CuI
O
N
CuII
Cu
Cu
O
N
3 NO
N
N
N
O
O
O
O
_
O
N2O
dicopper(II,III)
-oxo, -nitrosyl
NO activation
NO2 activation
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