Copper complexes relevant to the catalytic cycle of copper nitrite reductase: Electrochemical detection of NO(g) evolution and flipping of NO 2 binding mode upon CuII → CuI reduction

Article abstract of DOI:10.1021/ic401295t

Copper complexes of the deprotonated tridentate ligand, N-2-methylthiophenyl-2′-pyridinecarboxamide (HL1), were synthesized and characterized as part of our investigation into the reduction of copper(II) o-nitrito complexes into the related copper nitric oxide complexes and subsequent evolution of NO(g) such as occurs in the enzyme copper nitrite reductase. Our studies afforded the complexes [(L1)Cu^IICl] _n (1), [(L1)Cu^II(ONO)] (2), [(L1)Cu^II(H ₂O)](ClO₄)·H₂O (3·H ₂O), [(L1)Cu^II(CH₃OH)](ClO₄) (4), [(L1)Cu^II(CH₃CO₂)]·H₂O (5·H₂O), and [Co(Cp)₂][(L1)Cu^I(NO ₂)(CH₃CN)] (6). X-ray crystal structure determinations revealed distorted square-pyramidal coordination geometry around Cu^II ion in 1-5. Substitution of the H₂O of 3 by nitrite quantitatively forms 2, featuring the κ²-O,O binding mode of NO ₂^- to Cu^II. Reduction of 2 generates two Cu^I species, one with κ¹-O and other with the κ¹-N bonded NO₂^- group. The Cu ^I analogue of 2, compound 6, was synthesized. The FTIR spectrum of 6 reveals the presence of κ¹-N bonded NO₂^-. Constant potential electrolysis corresponding to Cu^II → Cu ^I reduction of a CH₃CN solution of 2 followed by reaction with acids, CH₃CO₂H or HClO₄ generates 5 or 3, and NO(g), identified electrochemically. The isolated Cu^I complex 6 independently evolves one equivalent of NO(g) upon reaction with acids. Production of NO(g) was confirmed by forming [Co(TPP)NO] in CH ₂Cl₂ (λ_max in CH₂Cl ₂: 414 and 536 nm, ν_NO = 1693 cm^-1).

Full text of DOI:10.1021/ic401295t

Article
pubs.acs.org/IC
Copper Complexes Relevant to the Catalytic Cycle of Copper Nitrite
Reductase: Electrochemical Detection of NO(g) Evolution and
Flipping of NO2 Binding Mode upon CuII → CuI Reduction
Ram Chandra Maji,† Suman Kumar Barman,‡ Suprakash Roy,† Sudip K. Chatterjee,† Faye L. Bowles,§
Marilyn M. Olmstead,§ and Apurba K. Patra*,†
†Department of Chemistry, National Institute of Technology−Durgapur, Mahatma Gandhi Avenue, Durgapur 713 209, India
‡Department of Chemistry, Indian Institute of Technology−Kanpur, Kanpur 208 016, India
§Department of Chemistry, University of California-Davis, Davis, California 95616, United States
S
* Supporting Information
ABSTRACT: Copper complexes of the deprotonated tridentate
ligand, N-2-methylthiophenyl-2′-pyridinecarboxamide (HL1), were
synthesized and characterized as part of our investigation into the
reduction of copper(II) o-nitrito complexes into the related copper
nitric oxide complexes and subsequent evolution of NO(g) such as
occurs in the enzyme copper nitrite reductase. Our studies afforded
the complexes [(L1)CuIICl]n (1), [(L1)CuII(ONO)] (2), [(L1)-
CuII(H2O)](ClO4)·H2O (3·H2O), [(L1)CuII(CH3OH)](ClO4)
(4), [(L1)CuII(CH3CO2)]·H2O (5·H2O), and [Co(Cp)2][(L1)-
CuI(NO2)(CH3CN)] (6). X-ray crystal structure determinations
revealed distorted square-pyramidal coordination geometry around
CuII ion in 1−5. Substitution of the H2O of 3 by nitrite
quantitatively forms 2, featuring the κ2-O,O binding mode of
NO2− to CuII. Reduction of 2 generates two CuI species, one with κ1-O and other with the κ1-N bonded NO2− group. The CuI
analogue of 2, compound 6, was synthesized. The FTIR spectrum of 6 reveals the presence of κ1-N bonded NO2 . Constant
−
potential electrolysis corresponding to CuII → CuI reduction of a CH3CN solution of 2 followed by reaction with acids,
CH3CO2H or HClO4 generates 5 or 3, and NO(g), identified electrochemically. The isolated CuI complex 6 independently
evolves one equivalent of NO(g) upon reaction with acids. Production of NO(g) was confirmed by forming [Co(TPP)NO] in
CH2Cl2 (λmax in CH2Cl2: 414 and 536 nm, νNO = 1693 cm−1).
shown in Scheme 1. One of the N−O bond cleavages of CuI
INTRODUCTION
■
bound NO2− in the presence of protons (Asp 98) generates an
Denitrification is an important biological process that involves
intermediate copper nitrosyl that eventually releases NO(g)
and returns to the water bonded CuII resting state, thus
sustaining the catalytic cycle.2
the stepwise reduction of NO3− to N2, as in NO3− → NO2
→
−
NO → N2O → N2, which assists in the maintenance of an
optimum level of nitrogen in the atmosphere.1 The enzyme,
Several nitrite bound CuII (κ1-O, κ2-O,O, and κ1-N bonded
copper nitrite reductase (CuNIR),2 catalyzes the proton-
3,4
−
NO2 ) complexes and a few CuI (κ1-O or κ1-N bonded
coupled one-electron reduction of NO2− → NO (NO2
+
−
5
−
NO2 ) model complexes are reported, whereas the reactive
and unstable copper nitrosyls are rarely isolated in the solid
state. Only one example of a structurally characterized CuII−
NO·, i.e., {Cu−NO}10 species6 and a few CuI−NO·, i.e., {Cu-
NO}11 species7 are known. As the copper nitrosyls are reactive,
in solution either the organic solvent8 or the copper-
coordinated ligand frame9 is nitrosylated and the CuI species
is depleted.
2H+ + e− → NO + H2O) and thus participates in the
denitrification pathway. The resting-state enzyme has a CuII
center bonded to three His-N and a H2O molecule in a
−
tetrahedral fashion, capable of binding NO2 by replacing
H2O.2d From X-ray structures and spectroscopic results, it is
believed that the key reaction intermediate for CuNIR activity
is a CuI−NO2 species that may be envisioned to form via either
the reduction of CuII−NO2 (Scheme 1, Path-I) or reduction of
the resting-state CuII aquo complex, followed by NO2− binding
(Scheme 1, Path-II). In the oxidized CuII state, an asymmetric
bidentate κ2-O,O binding of nitrite occurs, and in the reduced
CuI state, either κ1-N (according to Averil et al.),2a,b or κ2-O,O
(according to Suzuki et al.),2c or κ3-O,N,O (according to
Hasnain et al.)2d binding mode of NO2− has been proposed, as
As a part of an investigation into whether the CuI−NO2
complexes mimic functionally as CuNIR, it is necessary to
explore their reactivity with acids in solution and to detect the
Received: May 27, 2013
Published: September 25, 2013
© 2013 American Chemical Society
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ONO]− and [(L1)Cu(NO2)]−, where κ1-O and κ1-N binding
fashion of NO2 are involved, respectively, and confirmed
Scheme 1. Proposed Mechanistic Pathway for Nitrite
Reduction via Copper Nitrite Reductase (CuNIR)
a
−
electrochemically. Coulometrically reduced sample of 2, or
isolated CuI complex 6 itself in CH3CN, reacts with CH3CO2H
or HClO4 that quantitatively generates NO(g) and the CuII
complexes 5 or 3, respectively, detected and quantified
electrochemically. Simultaneous determination of NO(g) and
the product metal complex (formed after NO liberation, e.g., 5
or 3) using any of the above-mentioned three conventional
methods (such as gas chromatography−thermal conductivity
detection (GC-TCD), UV-vis of metal porphirinate nitrosyls,
and NO-sensing electrode) has not been previously reported.
In GC-TCD, the headspace NO(g) is used for sampling and its
quantification.4e,5a−g Using a metal-porphyrin NO scavenger,
diffusion of the generated headspace NO(g) from one vessel
(where NO will generate) to the other (containing a solution of
metal-porphyrin complex) is carried out.5h,j Both of these
methods demand extra caution to account for the dissolved
NO(g) left in the solvent that does not reach the headspace
before its quantification. No such precaution is necessary for
the electrochemical detection of NO(g) in solution, because it
will dissolve immediately after its formation and will be readily
available to the working electrode surface for its detection and
quantification, up to a certain concentration.
a
E = enzyme.
generated intermediates such as copper nitrosyl (Cu-NO) or
NO(g), using various spectroscopic techniques. For example,
gas chromatography (GC) was used to detect and quantify
evolved NO(g) from several model complexes.4e,5a−g A few
examples are known where the liberated NO(g) has been
allowed to react with (OEP)Fe,5h (EDTA)Fe,3e or (TPP)Co5j
solution and then detected using ultraviolet-visible (UV-vis)
spectroscopy of the respective stable metal nitrosyls. Lippard
and co-workers and others have used fluorescence spectrosco-
py10 for the chemical detection of NO. The use of NO-sensitive
electrodes for the measurement of NO in biological systems is
also known.11 On the other hand, to detect the unstable Cu-
NO adduct, Mondal and co-workers9b−d and others4a,5i,7d have
used UV-vis and Fourier transform infrared (FTIR) spectros-
copy. Herein, we report the synthesis, structure, electro-
chemical, and spectral properties of CuII/I complexes of a
tridentate carboxamide ligand, N-2-methylthiophenyl-2′-pyridi-
necarboxamide (HL1; see Scheme 2). The deprotonated
EXPERIMENTAL SECTION
■
Materials and Reagents. Pyridine-2-carboxylic acid, 2-
(methylthio)aniline, triphenylphosphite, sodium hydride, sodium
perchlorate, (n-Bu4N)NO2, Co(Cp)2, (n-Bu4N)ClO4, CD3CN,
pyrrole, benzaldehyde, propanoic acid were purchased from Aldrich
Chemical Co. and used without further purification. CH3CN, CH3OH,
C2H5OH, CHCl3, CH2Cl2, C5H5N (pyridine), (C2H5)2O (diethyl
ether), and n-hexane used either for spectroscopic studies or for
syntheses were purified and dried following standard procedures prior
to use. The ligand HL1 has been synthesized following a reported
procedure.12 AgClO4·C6H6 was prepared by recrystallization of
AgClO4 from hot benzene. Tetraphenyl porphyrin (TPP) was
synthesized following a reported procedure (J. Org. Chem. 1967, 32
(2), 476).
Synthesis Safety Note: Transition-metal perchlorates are hazardous
and explosive upon heating; therefore, these should be handled cautiously.
No explosion occurred in the present study.
Syntheses of Complexes. [(L1)CuCl]n (1). Method A. To a stirred
solution of HL1 (0.3 g, 1.233 mmol) in 100 mL of CH3OH was added
solid NaH (29.6 mg, 1.233 mmol). The light yellow solution of NaL1
was added dropwise to a stirred solution of anhydrous CuCl2 (165.8
mg, 1.233 mmol) in 30 mL of CH3OH. The color changed to dark
green. The solution was stirred for 8 h then allowed to slowly
evaporate. After 4−5 days, dark green needle-shaped crystals of 1 were
obtained, filtered off, washed with ether, and vacuum-dried (294 mg,
70%). Elemental analysis calcd for C13H11N2OSClCu, 1: C 45.61, H
3.24, N 8.18; Found: C 45.55, H 3.15, N 8.12; Selected IR frequencies
(KBr disk, cm−1): 3066(w), 3017(w), 2924(w), 1629(vs, νCO),
1598(vs), 1581(s), 1570(m), 1467(s), 1438(m), 1411(w), 1375(s),
1358(s), 1315(w), 1299(m), 1265(w), 1150(w), 1090(w), 1047(w),
1037(w), 1027(w), 977(w), 932(w), 911(w), 859(w), 809(w),
757(vs), 684(m), 652(w), 600(w), 516(w), 475(w), 437(w).
Method B. A CH3OH solution (7 mL) of 35% HCl (19.2 mg, 0.184
mmol) was added dropwise to a stirred solution of 2 (synthesis of 2:
vide supra, 31 mg, 0.088 mmol) in 7 mL of CH3OH. The resulting
green solution was stirred for 4 h and kept for slow evaporation. After
several days, green crystals of 1 were obtained, filtered off, washed with
ether, and vacuum-dried (yield = 26 mg, 86%).
Scheme 2. Chemdraw Depiction of Ligand HL1 and
Deprotonated Na+L1−
amidato N− is a strong σ-donor ligand, expected to coordinate
CuII/I ion by proper tuning of its charge via delocalization and
therefore, be amenable to production of (L1)CuI−NO2 species
and subsequent proton-coupled NO(g) evolution.
The synthesized and characterized CuII/I complexes relevant
to the catalytic cycle of CuNIR are shown in Scheme 3.
Complexes 1−5 were structurally characterized to reveal L1−
coordination via its amidato N− donor. The H2O or CH3OH
−
ligated complexes, 3 and 4, readily react with NO2 to form 2,
[(L1)Cu(ONO)] (2). Method A. To a stirred solution of 1 (200 mg,
0.584 mmol) in 35 mL of CH3OH was added solid NaNO2 (403 mg,
5.84 mmol). The resulting reaction mixture was stirred for 2 days and
filtered. The filtrate was allowed to slowly evaporate and afforded
where the NO2 is coordinated to CuII in an asymmetric κ2-
−
O,O fashion. Complex 2 in CH3CN upon reduction converts to
the corresponding CuI−NO2 complexes such as [(L1)Cu-
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Article
a
Scheme 3. Interconversion of Copper Complexes and Their Reactivities, Relevant to the Catalytic Cycle of CuNIR
a
Steps (i) and (ii) are when CH3CO2H and HClO4 acids, respectively, are used as the proton source.
needle-shaped dark bluish green crystals of 2 after 5 days. The crystals
were filtered off, washed with ether, and vacuum-dried (134.6 mg,
65%). Elemental analysis calcd for C13H11N3O3S1Cu1, 2: C 44.25, H
3.14, N 11.91; Found: C 44.21, H 3.08, N 11.81; Selected IR
frequencies (KBr disk, cm−1): 3054(w), 3028(w), 2928(w), 1632(vs,
νCO), 1601(vs), 1580(s),1570(m), 1467(vs), 1438(m), 1420(m),
1379(s), 1362(vs, ν(NO2)), 1298(m), 1266(m, ν(NO2)), 1244(w),
1150(w), 1116(m, ν(NO2)), 1048(w), 1038(w), 1028(w), 992(w),
970(w), 937(w), 903(w), 872(w), 832(w), 807(w), 767(vs), 752(vs),
723(w), 687(s), 653(m), 600(w), 516(w), 473(w), 437(w), 412(w).
Method B. To a stirred solution of HL1 (50 mg, 0.206 mmol) in 14
mL CH3OH was added solid NaH (5.43 mg, 0.226 mmol). The yellow
solution of NaL1 was added dropwise to a solution of [Cu-
(CH3CN)4](ClO4)2 (88 mg, 0.206 mmol) and NaNO2 (21.3 mg,
0.309 mmol) in 6 mL CH3OH. The resulting bluish green solution
was stirred for 4 h and then kept for slow evaporation. After a few
days, the bluish green crystals of 2 were obtained, filtered off, washed
with ether, and vacuum-dried (yield = 52.4 mg, 72%).
changed from green to bluish green. The resulting solution was stirred
for 2 h then kept for slow evaporation. After 4 days, bluish green
crystals of 4 were obtained, filtered off, washed with ether, and
vacuum-dried (yield = 52 mg, 83.7%). Elemental analysis for
C14H15N2O6S1Cl1Cu1 (4): Calcd: C 38.36, H 3.45, N 6.39; Found:
C 38.32, H 3.39, N 6.33; Selected IR frequencies (KBr disk, cm−1):
3306(s, ν(OH)), 3106(w), 3059(w), 2997(w), 1654(s, overtone band
of ν(OH)), 1626 (s, ν(CO)), 1597(vs), 1580(m), 1570(m),
1535(m), 1482(m), 1466(s), 1436(w), 1379(w), 1364(s), 1314(w),
1296(w), 1261(w), 1105(vs), 1076(s), 1025(w), 951(w), 933(w),
915(w), 866(w), 813(w), 757(s), 687(w), 677(w), 646(w), 623,
519(w), 452(w).
[(L1)Cu(CH3CO2)].H2O (5·H2O). Method A. To a stirred solution of
HL1 (50 mg, 0.205 mmol) in 5 mL CH3OH was added dropwise an
aqueous solution (2 mL) of Cu(OAc)2·H2O (41 mg, 0.205 mmol).
The resulting green solution was stirred for 1 h and, to this solution,
10 mL of ether was layered and kept at 4 °C overnight. A green solid
precipitated out and was filtered off, washed with ether and vacuum-
dried (68 mg, 86%). The crystals suitable for XRD studies were grown
from slow evaporation of a CH3OH solution of this green solid.
Elemental analysis for C13H11O4N3S1Cu1 (5·H2O): Calcd: C 46.93, H
4.20, N 7.30; Found: C 46.76, H 4.09, N 7.27; Selected IR frequencies
(KBr disk, cm−1): 3524(vs, ν(OH)), 3440(vs, ν(OH)), 3064(w),
3014(w), 2928(w), 1618(vs, ν(CO) + νa(COO)), 1594(vs),
1579(vs), 1469(vs), 1438(vs), 1392(vs), 1364(vs), 1336(vs),
1298(vs, νs(COO)), 1269(w), 1242(w), 1150(m), 1088(m),
1047(m), 1032(m), 1019(m), 981(m), 953(w), 934(m), 904(w),
879(w), 813(w), 774(s), 760(vs), 722(w), 689, 653, 618(w), 601(w),
518(w), 474(w).
Method B. To a stirred solution of 2 (50 mg, 0.141 mmol) in 10
mL of CH3OH was added dropwise a solution of CH3CO2H (51 mg,
0.846 mmol) in 15 mL of CH3OH. The color changed from bluish
green to green. Distilled water (0.5 mL) was added and stirred for 4 h.
The volume of the resulting reaction mixture was reduced to ∼10 mL
by rotary evaporation and kept for slow evaporation. After 7 days,
needle-shaped blue crystals of 5·H2O were obtained, filtered, washed
with ether, and vacuum-dried (yield = 42.5 mg, 79%).
[Co(Cp)2][(L1)Cu(NO2)(CH3CN)] (6). The reaction was performed
inside a glovebox under N2 atmosphere using dry degassed (freeze−
pump−thaw) solvents. To a stirred greenish blue solution of 2 (50 mg,
0.141 mmol) in 4 mL of CH2Cl2 was added dropwise 4 mL of CH2Cl2
solution of Co(Cp)2 (29 mg, 0.155 mmol). The solution color
changed to brownish yellow and was stirred for 30 min and then 20
mL hexane was layered on top of the reaction mixture. After 3 h, the
Method C. To a stirred CH3CN solution (6 mL) of 3 (50 mg, 0.113
mmol) or 4 (50 mg, 0.114 mmol) was added a CH3CN solution (5
mL) of (n-Bu4N)NO2 (33 mg, 0.114 mmol). The resulting green
solution was stirred for 2 h and kept for slow evaporation. After 4 days,
green crystals formed that were filtered off, washed with water and
ether, then vacuum-dried (yield = 36 mg, 90%).
[(L1)Cu(H2O)](ClO4)·H2O (3·H2O). To a stirred solution of 1 (50
mg, 0.146 mmol) in 10 mL CH3OH was added 2 mL of a CH3OH
solution of AgClO4.C6H6 (36 mg, 0.127 mmol) dropwise. The
resulting blue solution was stirred overnight. AgCl was filtered through
a Celite pad and filtrate was evaporated to dryness using a rotary
evaporator that afforded a blue solid that was washed with water, ether,
and vacuum-dried. The dry blue solid was then redissolved in crude
CH3CN, and to this solution, ether diffusion at 4 °C yielded dark blue
crystals suitable for X-ray diffraction (XRD). Crystals were filtered off
and washed with ether and vacuum-dried (yield = 49 mg, 79%).
Elemental analysis calcd for C14H15N2 O6S1Cl1Cu1, 3·H2O: C 38.36, H
3.45, N 6.39. Found: C 38.21, H 3.42, N 6.27. Selected IR frequencies
(KBr disk, cm−1): 3450(br, ν(O−H)), 3108(w), 3067(w), 2924(m),
2856(w), 1630 (vs, ν(CO)), 1597(vs), 1581(m), 1570(w),
1467(vs), 1438(w), 1375(s), 1359(s), 1299(m), 1265(w), 1147(s),
1100(s), 1079(s), 1048(w), 980(w), 932(w), 810(w), 756(s), 684(m),
655(w), 636(m), 625(w), 519(w), 474(w).
[(L1)Cu(CH3OH)](ClO4) (4). To a stirred solution of 2 (50 mg, 0.142
mmol) in 9 mL CH3OH was added dropwise a CH3OH solution (15
mL) of 70% perchloric acid (40.5 mg, 0.283 mmol). The color
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Article
Table 1. Data Collection and Structure Refinement Parameters for Complexes 1−5
complex
formulas
1
2
3·H2O
4
5·H2O
C13H11N2OSClCu
342.29
C13H11N3O3SCu
352.85
C13H15N2O7SClCu
442.32
C14H15N2O6SClCu
438.33
C14H16N2O4SCu
383.90
mol wt
cryst. system
color
orthorhombic
green
triclinic
orthorhombic
blue
monoclinic
blue-green
P21/n
monoclinic
brown
purple
space group
a, Å
Pca21
P1
̅
Pbca
P21/c
20.580(2)
4.1325(4)
15.3910(15)
90.0
7.070(4)
10.986(7)
17.650(11)
105.695(14)
92.008(6)
93.611(4)
1315.2(14)
4
7.6549(7)
19.104(2)
22.017(7)
90.0
7.6173(4)
19.0453(11)
11.2557(7)
90.0
7.032(2)
17.395(5)
13.098(4)
90.0
b, Å
c, Å
α, deg
β, deg
γ, deg
V, Å3
90.0
90.0
97.839(5)
90.0
99.315(7)
90.0
90.0
90.0
1308.9(2)
4
3219.8(11)
8
1617.65(16)
4
1581.0(8)
4
Z
d
calc, g cm−3
1.737
1.782
1.825
1.800
1.613
θ range, deg
μ, mm−1
1.98−27.98
2.022
2.5375−32.261
1.832
2.53−39.93
2.141
2.90−31.50
1.680
1.963−30.593
1.534
F(000)
692
716
1800
892
788
reflections/parameters
unique reflections
11210/173
2448
19569/381
6067
51872/243
6184
18900/232
5171
25338/242
4638
a
R1 [I > 2σ(I)]
0.0329
0.0343
0.0355
0.0274
0.0283
b
wR2 [I > 2σ]
0.0603
0.0844
0.0841
0.0758
0.0635
goodness of fit, GOF
1.000
1.033
1.054
1.092
1.187
−3
res. dens., eÅ
0.255/−0.427
1.028/−0.600
0.633/−1.095
1.059/−1.107
0.419/−0.341
a
b
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = {∑ [w(Fo − Fc2)2]/∑w[(Fo )2]}1/2
.
2
2
microcrystalline solid precipitated out and was filtered, washed with
hexane, and dried (55 mg, 72%). Selected IR frequencies (KBr disk,
cm−1): 3429 (vs, ν(OH)), 3100 (m), 3017 (w), 2923(w), 2130 (w,
ν(CN)), 1620 (vs, ν(CO)), 1594 (vs), 1577 (s), 1567 (s), 1465
(vs) 1437 (w), 1413 (s), 1365 (vs), 1269 (s), 1092 (br, vs), 1010 (s),
864 (m), 814 (m), 761 (m), 690 (m), 620 (w), 460 (s); Elemental
analysis for C25H24ON3SCuCo (6): C 55.91, H 4.50, N 7.82; Found:
C 54.87, H 4.79, N 7.54. 1H NMR (500 MHz, CD3CN): δ 5.64 (1H, s,
Cp proton), 5.42 (6H, s, of which 3H for Cp and 3H for either py or
phenyl ring), 3.42 (2H, dd of py ring), 3.27 (6H, d, Cp), 2.15 (3H, s,
−SCH3), 1.939 (s, 3H, CH3CN), 1.09 (3H, t, py, or phenyl ring
proton). ESI mass spectrum m/z (%) negative mode: 439.97
[{(L1)Cu(NO2)2(CH3CN) + 1H}, 98], 393.97 [{(L1)Cu(NO2)-
(CH3CN) + 1H}, 100], 377.92 [{(L1)Cu(NO)(CH3CN) + 1H}, 30];
positive mode: 189.008 [Co(Cp)2, 100].
Physical Measurements. The FTIR spectra of the ligand and the
complexes were recorded on a Thermo Nicolet iS10 or Bruker Vector
22 spectrometer, using KBr pellets, in the range of 4000−400 cm−1.
The electronic spectra were recorded on an Agilent 8453 diode array
spectrophotometer. Elemental analyses were carried out on a Perkin−
Elmer 2400 series-II CHNS Analyzer. Electron paramagnetic
resonance (EPR) spectra were obtained in CH3CN using a Bruker-
EMX-1444 EPR spectrometer at 77 K. Mass spectra were recorded on
Waters-HAB213 spectrometer. Solution conductivity was measured
using CHEMILINE conductivity meter CL220, 1H NMR spectra were
recorded on JEOL JNM LA 500. Redox potentials were measured
using CHI 1120A potentiometer. For constant potential electrolysis
experiments, a platinum mesh working electrode was used and the
solute concentration was kept at ∼(1−2) × 10−3 M.
X-ray Crystallography. Crystals of 1, 2, 4, and 5 were grown via
the slow evaporation of CH3OH solution of the corresponding
complexes at room temperature. Diethyl ether diffusion at low
temperature (4 °C) to the CH3CN solution of 3 yielded X-ray-quality
crystals. Single-crystal intensity measurements for 1 were collected at
120(2) K, while those for 2−5 were collected at 90(2) K with a Bruker
Smart APEX II CCD area detector, using either Mo Kα radiation (λ =
0.71073 Å) with a graphite monochromator (for 1, 3, 4, and 5) or
synchrotron radiation with a Si(111) monochromator (for 2). The cell
refinement, indexing and scaling of the dataset were carried out using
SAINT and Apex2 program.13 All structures were solved by direct
methods with SHELXS, and refined by full-matrix least-squares based
on F2 with SHELXL.14 Other calculations were performed using
WinGX, Ver 1.80.05.15 The crystal of 2 shows two molecules in the
asymmetric unit. The perchlorate anions present in 3 and 4 are not
disordered. The Cu ion and the phenyl ring C atoms with its S-Me
group is 7.1% disordered in 5. The positions of the C-bound H atoms
were calculated assuming ideal geometry and refined using a riding
model. Figures showing displacement parameteres were created using
the program XP.16 Crystal data for the complexes 1−5 are summarized
in Table 1. Additional crystallographic data and refinement details are
available in CIF format in the Supporting Information.
Crude Product (n-Bu4N)[(L1)Cu(NO2)]. The reaction was per-
formed inside a glovebox under N2 atmosphere using dry degassed
(freeze−pump−thaw) solvents. Solid NaH (6 mg, 0.25 mmol) was
added pinchwise to a stirred solution of HL1 (50 mg, 0.205 mmol) in
8 mL CH3CN. A light yellow solution was generated that was stirred
for 5 min and then solid [Cu(CH3CN)4](ClO4) (68 mg, 0.208 mmol)
was added. A dark red color developed immediately. Then, solid (n-
Bu4N)NO2 (60 mg, 0.208 mmol) was added to this solution. The red
color changes to brownish black. It was then stirred for 5 min and the
solvent was removed completely to isolate a brownish black solid.
Longer reaction times generate CuII via disproportionation: 2CuI →
CuII + Cu0. Selected IR frequencies (KBr disk, cm−1): 3056 (w), 2964
(s), 2935(m), 2876 (m), 2015 (w, νCN of CH3CN), 1621 (νCO, s),
1596 (s), 1578 (m), 1567 (m), 1466 (vs), 1438 (m), 1360 (br, s),
1295 (w), 1262 (m), 1091 (νCl−O of ClO4 present as an impurity), 882
(m), 815 (br, m), 756 (s), 690 (m), 647 (w), 622 (s), 516 (w), 456
1
(w), 412 (w). H NMR (500 MHz, CD3CN): δ 5.42 (2H, s, py ring
ortho + py or phenyl), 3.41 (4H, dd, i.e. py (2H) + phenyl (2H) of 4
and 5 positions ring protons), 3.25 (d, 2H, Bu), 3.05 (6H, m, -Bu),
2.16 (3H, s, -SMe), 1.57 (8H, d, −Bu), 1.33 (6H, m, −Bu), 1.08 (6H,
m, phenyl + py + −Bu proton), 0.94 (10H, t, −Bu). ESI mass
spectrum m/z (%) negative mode: 440.17 [{(L1)Cu(NO2)2(CH3CN)
+ 1H}, 100]; positive mode: 242.27 [n-Bu4N, 100].
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Supporting Information) taken in CH3CN features the
molecular ion peak at 440.1779 (ESI negative) and at
242.2786 (ESI positive), which respectively correspond to the
anion and cation of a complex of probable formulas (n-
Bu4N)[CuI(L1)(NO2)(HNO2)(CH3CN)]. 1H NMR in
CD3CN (sample prepared under N2) features sharp signals
(see the Supporting Information) within δ = 0−10 ppm. All
these spectroscopic data reveals the formation of CuI species
RESULTS AND DISCUSSION
■
1. Synthesis and Characterization. Ligand HL1 has been
prepared, following reported procedures,12 and characterized by
1H NMR and FTIR spectra (νN−H at 3278 cm−1 and νCO at
1682 cm−1). A solution of NaH in CH3OH was used to
deprotonate the ligand. Abstraction of chloride from 1 with the
use of AgClO4·C6H6 in CH3OH, formed 3 and 4. The absence
of νN−H and the red-shifted νCO of 1 (1629 cm−1), 2 (1632
cm−1), 3 (1629 cm−1), 4 (1624 cm−1), 5 (1618 cm−1), and 6
(1620 cm−1) confirms the amidato N− ligation to Cu ions.
Nitrite ligation occurred by addition of an equivalent amount of
(n-Bu4N)NO2 to yield 3 or 4; also, the reaction of excess
NaNO2 to 1 in CH3CN yielded 2 in high yield. From the X-ray
structure of 2 (vide supra), there are two molecules in the
asymmetric unit. In one molecule, the N−O distances of the
bound nitrite are 1.276(3) Å and 1.232(3) Å and in other
molecule these are 1.282(3) Å and 1.228(2) Å. Interestingly, 2
displays strong bands in its IR spectrum (Supporting
Information) at 1362 cm−1 that correspond to νa(NO2) and
at 1266 cm−1 that correspond to νs(NO2) due to the chelating
nitro group (κ2-O,O) in first molecule and the corresponding
two bands for other molecule are observed at 1380 cm−1
(νNO) and 1116 cm−1 (νN−O).17 Casella and co-workers3e
reported similar IR spectral behavior of the CuII-bound nitrite
group (κ2-O,O and κ1-O bonded). Complex 2 reacts with HCl
or CH3CO2H in CH3OH and forms, quantitatively, 1 and 5,
respectively. Complex 5 can also be prepared from the reaction
of an aqueous solution of Cu(CH3CO2)2·H2O to the ligand
−
with bound NO2 group. In solution, this CuI sample is not
stable enough for crystallization. Longer reaction time even
under N2 develop green color CuII species with a red color
scum on the side wall of the reaction vessel, which is possibly
due to the disproportionation reaction (2CuI → CuII + Cu0).
From the green solution, the solid isolated is EPR active that
display a strong signal at g = 2.0922 (see the Supporting
Information), confirming that it is CuII.
(b). Synthesis of LCuI(NO2) via Reduction of LCuII(NO2)
Using Co(Cp)2 (i.e., Complex 6). The reaction of CH2Cl2
solution of 2 with a dilute CH2Cl2 solution of Co(Cp)2,
followed by precipitation using dry hexane, changes to
brownish black solid. The solid display stretches at 3100
cm−1 (νCH), 2130 cm−1 (νCN of CH3CN), 1620 cm−1 (νCO),
1413 cm−1 (νa(NO2)), 1365 cm−1 (νs(NO2)), 814 cm−1
(δ(ONO)), 620 cm−1(ρw(NO2)) and 460 cm−1 (νCuN(nitrite)
)
(see the Supporting Information) that corroborates the κ1-N
binding mode of NO2 to CuI. Mass spectrum (CH3CN)
−
features peaks at 439.97 [(L1)Cu(NO2)(HNO2)(CH3CN),
98%], 393.97 [(L1)Cu(HNO2)(CH3CN)], 100%], and 377.92
[(L1)Cu(HNO)(CH3CN), 30%] in ESI negative mode and at
189.008 correspond to [Co(Cp)2]+ in positive mode (see the
Supporting Information). The 1H NMR spectrum of the
isolated solid in CD3CN, given in the Supporting Information,
display sharp signals within δ = 0−10 ppm. The fact that the
aromatic ring protons are shifted to higher field than δ ∼7 ppm
indicates the nonplanarity of pyridine and phenyl rings to each
other. Thus, the aromatic ring current of one, with respect to
the other, opposes the applied magnetic field and shifts the
resonance to higher δ.18 Appreciably higher field shift of phenyl
ring protons (in the range 0.4−2.6 ppm and 1.4−3.3 ppm) are
known in the literature.18b It is noteworthy that a 1:1 mixture of
−
solution in CH3OH (CH3CO2 acts as deprotonating agent
and as a ligand). Strong stretching vibrations at 3520 cm−1 and
3440 cm−1 (νOH), 1618 cm−1 (νa(COO)) and 1298 cm−1
(νs(COO)) have been observed in the IR spectrum of 5 due to
the H2O and CuII coordinated CH3CO2 ion.
−
According to the proposed mechanisms,2a−d the key reaction
intermediate; CuI−NO2 , may be envisioned to form either via
−
the reduction of the nitrite bound CuII or via the reduction of
resting-state CuII followed by nitrite binding, mentioned as
Path-I and Path-II, respectively, in Scheme 1. Therefore, we
have attempted to synthesize the NO2− bound CuI analogue of
2 via two possible ways (syntheses done in side glovebox using
1
2 and Co(Cp)2 in CD3CN also display exactly the same H
dry degassed solvents): synthesis of LCuI(NO2) via NO2
−
NMR like the isolated solid, which confirms the integrity of the
samples in solution and isolated solid 6.
binding to (L1)CuI and synthesis of LCuI(NO2) via reduction
of LCuII(NO2) using Co(Cp)2, i.e., complex 6.
Solution conductivity measurement in CH3CN reveals that 1,
2, and 5 are nonelectrolytic (Λ ranges from 4 Ω mol−1 cm−1 to
10 Ω mol−1 cm−1), whereas 3 and 4 behave as 1:1 electrolytes
(Λ ranges from 120 Ω mol−1 cm−1 to 145 Ω mol−1 cm−1).19
Mass spectral results, microanalytical data, 1H NMR, and other
information mentioned above of 1−6 support their formula-
tions as stated.
Structures of the Complexes, 1−5. Perspective full
molecule views of 1, 2, 5 and the cationic views of 3 and 4 are
shown in Figures 1−5, along with their atom labeling schemes.
Distorted square pyramidal coordination geometry around CuII
has been observed in all cases. In 1, the Cl− anion of one
molecule coordinates axially to the other CuII ion, thus making
a polymeric chain compound in the solid state. Three donors of
the amide ligand occupy three sites of the square plane of which
the amidato N− donor is trans to the fourth site of the plane.
Stronger coordination of the amidato N− to CuII is evident in
all cases; therefore, the CuII−Namide distances (in the range of
1.930−1.946 Å) in 1−5 are shorter than the CuII−Npy distances
(in the range of 1.9747−1.992 Å) in the respective complexes.
(a). Synthesis of LCuI(NO2) via NO2 Binding to (L1)CuI.
−
Reaction of L1− with [CuI(CH3CN)4](ClO4) followed by
addition of (n-Bu4N)NO2 in solvents such as (CH3)2CO,
CH3CN, CH3OH (separate syntheses) generates a yellow
solution. Solvent removal under vacuum yielded brownish black
solid that was subjected to various spectroscopic studies such as
FTIR, mass, and 1H NMR spectroscopies. The solid in its FTIR
spectrum display the νCO at 1621 cm−1, red-shifted from that
of 2 (1632 cm−1) which is expected as relatively electron-rich
CuI pushes the electron density to the carbonyl group. Intense
stretch at 2964 cm−1 is due to νC−H of Bu4N+ and the stretches
at 1466 cm−1 (νa(NO2)), 1360 cm−1 (νs(NO2)), 1267, 815
cm−1 (δ(ONO)) and 456 cm−1 (νCuN(nitrite)) are due to the κ1-
N bound NO2 group17 (see the Supporting Information).
−
Similar IR stretches for the structurally characterized κ1-N
bound CuI−NO2 and15N isotope labeled CuI‑15NO2 complexes
have been reported recently by Fujii and co-workers5h where
δ(ONO) and νCu−N(nitrite) have been observed at around 800
cm−1 and 400 cm−1, respectively. The mass spectrum (see the
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Figure 1. Thermal ellipsoid (probability level = 30%) plot of
[(L1)CuCl] (1), along with the atom labeling scheme. H atoms are
omitted for the sake of clarity.
Figure 4. Thermal ellipsoid (probability level = 50%) plot of
[(L1)Cu(CH3OH)](ClO4) (4), along with the atom labeling scheme.
Figure 2. Thermal ellipsoid (probability level = 50%) plot of
[(L1)Cu(ONO)] (2), along with the atom labeling scheme. H
atoms are omitted for the sake of clarity. O3 and O6 occupy axial
positions.
Figure 5. Thermal ellipsoid (probability level = 50%) plot of
[(L1)Cu(CH3CO2)]·H2O (5·H2O), along with the atom labeling
scheme.
the longer Cu−OONO in the range of 2.19−2.4 Å.21 Model
complexes with tripodal TEPA3a (where TEPA = triethyl-2-
pyridylamine), various substituted Tp3b−d (where Tp =
hydrotris(pyrazolyl)borate), 1-BB (bis[(1-methylbenzimidazol-
2-yl)methyl]amine), and 2-BB (bis[2-(1-methylbenzimidazol-2-
yl)ethyl]amine) ligands3e−g are known, where the longer Cu−
OONO distance ranges from 2.031 Å to 2.678 Å. The angle ϕ
between the O−N−O and O−Cu−O planes are very small
(<1°) like other model complexes, which is in sharp contrast to
that observed in native CuNIR that usually vary over the range
of 60°−84° (the smallest known angle is 6°). The Cu−NONO
distances of 2.678(3) Å (Cu1−N3) and 2.737(3) Å (Cu2−N6)
in the two molecules of the asymmetric unit of 2 (Figure 2) are
closer to the longer Cu−OONO distances of 2.554 Å (Cu1−O3)
and 2.667 Å (Cu2−O6), respectively, than the shorter Cu−
OONO distances of 1.9759 Å (Cu1−O2) and 1.9675 Å (Cu2−
O5), respectively, similar to the trend observed in the oxidized
CuNIR protein structures,21 where these distances of Cu−
NONO and longer Cu−OONO are found as 2.15 Å and 2.19 Å
(for AcNIR), 2.31−2.36 Å and 2.29−2.38 Å (for AfNIR), 2.32
Å and 2.42 Å (for D98N AfNIR), and 3.21 Å and 3.60 Å (for
H255N AfNIR), respectively. In 3 the water molecule present
trans to the amidato N− is strongly hydrogen-bonded to
another water molecule present as a solvent of crystallization
with an O2···O7 distance of 2.642(2) Å. In the case of the
resting-state CuNIR, the CuII-bound water molecule is
hydrogen-bonded to the O atom of the Asp(98) side chain
(O of CuII coordinated H2O to O of Asp-98 distance is 2.80
Figure 3. Thermal ellipsoid (probability level = 50%) plot of
[(L1)Cu(H2O)](ClO4)·H2O (3·H2O), along with the atom labeling
scheme.
The CuII ions are situated almost in the square plane, only
0.101, 0.099, 0.137, and 0.080 Å out of the plane toward the
axial ligands in complexes 2−5, respectively. The axial CuII−
ligand bond is much longer than the other four, because of
pseudo-Jahn−Teller distortion, which is commonly observed in
CuII complexes.20 Selected bond distances and angles of 1−5
are tabulated in Table 2.
In 2, two molecules are present in the asymmetric unit. An
average shorter CuII−OONO (in plane) and longer CuII−OONO
(axial) distances of 1.9717(19) Å and 2.611(2) Å, respectively,
strongly reveal the asymmetric κ2-O,O binding fashion of the
nitrite ion to CuII center(s) in both molecules (see Figure 2).
Several reported structures of the oxidized state of CuNIRs
−
reveal a typical asymmetric binding of the NO2 ion to CuII
with a shorter Cu−OONO distance in the range 1.98−2.2 Å and
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Å).2d The trigonality index of τ = 0.071 for 1, τ = 0.13 and
0.016 for two molecules of 2, τ = 0.11 for 3, and τ = 0.20 for 4
and 5 are comparable to those reported for square pyramidal
CuII complexes of the N2S donor ligand.20
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
Complexes 1−5
Electronic Spectra. The electronic spectra of 1−5 were
taken in CH3OH, and the electronic spectra of 6 were taken in
CH3CN. The data are listed in Table 3. All complexes except 6
[(L1)CuCl] (1)
Bond Distances, Å
Cu(1)−N(1)
Cu(1)−N(2)
Bond Angles, deg
N(1)−Cu(1)−N(2)
N(1)−Cu(1)−Cl(1)
N(2)−Cu(1)−Cl(1)
1.984(3)
1.947(3)
Cu(1)−S(1)
Cu(1)−Cl(1)
2.2881(9)
2.2442(9)
Table 3. Electronic Absorption Spectral Data of 1−6 and
HL1 Complexes at 298 K
83.36(11)
97.14(8)
172.12(9)
N(1)−Cu(1)−S(1)
N(2)−Cu(1)−S(1)
S(1)−Cu(1)−Cl(1)
167.84(8)
85.96(8)
92.65(4)
compound
λmax, nm (ε, M−1 cm−1
)
1
2
CH3OH 258 (10400), 299 (5500), 310 (5590), 326 sh (5426),
455 sh (81), 642 (103)
[(L1)Cu(ONO)] (2)
Molecule 1
Molecule 2
CH3OH 257 (14480), 298 (8540), 310 (8280), 327 sh (7730),
613 (157)
Bond Distances, Å
Cu(1)−N(1)
Cu(1)−N(2)
Cu(1)−S(1)
Cu(1)−O(2)
1.992(2)
Cu(2)−N(4)
Cu(2)−N(5)
Cu(2)−S(2)
Cu(2)−O(5)
Cu(2)−O(6)
1.984(2)
CH3CN
261 (15540), 301 sh (8263), 311 sh (7930),
330 sh (7080), 589 (133).
1.930(2)
1.932(2)
2.3004(12)
1.9759(19)
2.554(2)
2.2875(11)
1.9675(19)
2.667(2)
3
CH3CN
H2O
260 (12740), 300 (8160), 312 sh (7630), 330 sh (6450)
, 573 (190)
Cu(1)−O(3)
Bond Angles, deg
N(1)−Cu(1)−N(2)
N(1)−Cu(1)−S(1)
N(1)−Cu(1)−O(2)
N(2)−Cu(1)−S(1)
N(2)−Cu(1)−O(2)
S(1)−Cu(1)−O(2)
O(2)−N(3)−O(3)
253 (15320), 295 sh (9640), 306 (9880), 322 sh (9200)
, 451 sh (90), 625 (125)
83.62(9)
168.27(6)
95.21(8)
87.05(7)
176.09(7)
93.63(7)
115.1(2)
N(4)−Cu(2)−N(5)
N(4)−Cu(2)−S(2)
N(4)−Cu(2)−O(5)
N(5)−Cu(2)−S(2)
N(5)−Cu(2)−O(5)
S(2)−Cu(2)−O(5)
O(5)−N(6)−O(6)
83.46(9)
165.59(6)
95.61(8)
87.25(7)
164.86(8)
96.21(7)
115.1(2)
CH3OH 258 (13000), 285 (7915), 297 (8050), 310 sh (7810),
330 sh (6940), 455 sh (110), 626 (150)
4
5
CH3OH 259 (15855), 285 (10990), 296 sh (10540),
310 sh (9300), 328 (6900), 460 sh (87), 627 (110)
CH3OH 259 (15420), 296 sh (8920), 310 sh (8500), 330 (7600)
, 627(143)
[(L1)Cu(H2O)](ClO4)3·H2O
Bond Distances, Å
Cu(1)−N(1)
Cu(1)−N(2)
6
CH3CN
CH2Cl2
267 (25870), 335 (6300)
268 (5080), 287 (4000)
1.9798(13)
1.9367(12)
2.3907(12)
Cu(1)−S(1)
Cu(1)−O(2)
C(6)−O(1)
2.3126(5)
1.9551(11)
1.2459(16)
HL1
Cu(1)−O(3)
Bond Angles, deg
N(1)−Cu(1)−N(2)
N(1)−Cu(1)−S(1)
N(1)−Cu(1)−O(2)
N(2)−Cu(1)−S(1)
N(2)−Cu(1)−O(2)
83.36(5)
167.79(4)
93.37(5)
86.02(4)
174.49(5)
S(1)−Cu(1)−O(2)
O(3)−Cu(1)−N(1)
O(3)−Cu(1)−O(2)
O(3)−Cu(1)−N(2)
O(3)−Cu(1)−S(1)
96.73(4)
92.45(5)
79.45(4)
105.07(5)
96.12(4)
display a broad absorption band in the range of 613−642 nm,
because of a d−d electronic transition (ε < 200 M−1 cm−1). In
the 300-nm region for 1−5, three have been observed
overlapped transitions (∼300, ∼310, and ∼330 nm), and the
spectral profiles for all these complexes are similar. Among
these transitions, the relatively lower energy transition at ∼330
nm is due to charge transfer from the monodentate ligand
(-trans to amidato N) to CuII (see Figure 6). Other higher-
energy transitions are due to the intraligand n−π* (300 and
310 nm) and π−π* transition (260 nm).
[(L1)Cu(CH3OH)](ClO4) (4)
Bond Distances, Å
Cu(1)−N(1)
Cu(1)−N(2)
1.9747(15)
1.9395(15)
1.9627(13)
Cu(1)−S(1)
Cu(1)−O(1)
C(6)−O(5)
2.2991(5)
2.4264(14)
1.249(2)
Cu(1)−O(6)
Bond Angles, deg
N(1)−Cu(1)−N(2)
N(1)−Cu(1)−S(1)
N(1)−Cu(1)−O(6)
N(2)−Cu(1)−S(1)
N(2)−Cu(1)−O(6)
To check whether or not the water molecule ligated to CuII
83.69(6)
164.60(5)
94.35(6)
86.80(5)
176.74(6)
S(1)−Cu(1)−O(6)
O(1)−Cu(1)−N(1)
O(1)−Cu(1)−N(2)
O(1)−Cu(1)−O(6)
O(1)−Cu(1)−S(1)
94.55(4)
92.33(6)
93.43(6)
89.26(5)
100.32(4)
−
in 3 is easily replaceable by the externally added NO2 ion
(required for the catalytic activity of CuNIR), we have titrated
the CH3CN solution of 3 with CH3CN solution of (n-
Bu4N)(NO2). The UV-vis traces obtained during titration are
shown in Figure 7. The band position at 573 nm of 3 shifts to
579 nm, corresponding to the d−d band of 2. The isosbestic
points at 629, 478, 378, 332, and 291 nm clearly reveal the neat
transformation of 3 → 2. It is noteworthy, in the case of
CuNIR, that the resting-state CuII-0bound water molecule
[(L1)Cu(CH3CO2)]5·H2O
Bond Distances, Å
Cu(1)−N(1)
Cu(1)−N(2)
1.9875(13)
1.9319(13)
2.7766(13)
Cu(1)−S(1)
Cu(1)−O(2)
C(6)−O(1)
2.3101(7)
1.9391(12)
1.2367(17)
−
Cu(1)−O(3)
provides the entry of NO2 ion in the catalytic cycle by
Bond Angles, deg
N(1)−Cu(1)−N(2)
N(1)−Cu(1)−S(1)
N(1)−Cu(1)−O(2)
N(2)−Cu(1)−S(1)
N(2)−Cu(1)−O(2)
replacing itself. To the best of our knowledge, at least one
example of a CuII−OH2 complex is reported in relation to the
CuNIR modeling, where the bound water molecule has been
83.89(5)
167.52(4)
96.46(5)
86.87(4)
179.54(6)
S(1)−Cu(1)−O(2)
O(3)−Cu(1)−N(1)
O(3)−Cu(1)−O(2)
O(3)−Cu(1)−N(2)
O(3)−Cu(1)−S(1)
92.74(4)
90.94(5)
52.54(4)
127.87(5)
101.36(4)
shown to be replaced by NO2 ion in solution.4c To check the
−
−
possibility of back replacement of NO2 by water, complex 2
was dissolved in CH3OH/H2O in the presence of NaClO4 and
slow evaporation yielded back 2. The electronic spectrum of 6
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electron-rich CuI to NO2 charge transfer, similar to the other
reported CuI−NO2 model complexes.5
−
Redox Properties and Nitrite Reductase Activity. For
CuNIR activity, the easy access to the CuI−NO2− intermediate
from its CuII precursor is necessary, which subsequently
−
reduces the bound NO2 to NO. Therefore, the cyclic
−
−
voltammograms of either CuII−NO2 or CuI−NO2 model
complexes that highlight the accessibility of the CuII/I oxidation
states and their relative stability in solution in the designed
ligand donor atoms environment are important. To investigate
the susceptibility of the CuII center in complexes 1−5 toward
oxidation or reduction, the cyclic voltammograms were
recorded in CH3CN using same cell setup. A three-electrode
cell setup (such as platinum, saturated calomel electrode
(SCE), and a platinum wire as the working, reference, and
auxiliary electrodes, respectively) has been used to measure the
potentials. Results are summarized in Table 4. The cyclic
voltammograms of 1 in CH3CN feature two irreversible
reductions (see Figure 8): one at Epc = −0.35 V and another
Figure 6. Electronic absorption spectra of 1 (black), 4 (blue), and 5
(red) in CH3OH.
Figure 7. Electronic absorption spectral changes during transformation
of 3 (black trace) → 2 (pink trace) when titrating a CH3CN solution
of 3 with a CH3CN solution of (n-Bu4N)NO2.
Figure 8. Cyclic voltammograms of 1 in CH3CN containing 0.1 M
[with (n-Bu)4N]ClO4 as a supporting electrolyte at 298 K, at a
platinum working electrode, at a scan rate of 50 mV s−1, using SCE as
the reference electrode.
taken in CH3CN displays (see the Supporting Information) an
intense shoulder at 335 nm, possibly due to the relatively
b
Table 4. Electrochemical Data for Complexes 1−5 (Measured in CH3CN Solution), Other CuNIR Model Complexes with
a
Ligands Containing a Negative Charge, and a Few Native CuNIRs
Reductive Response
Oxidative Response
b
c
d
complex
Epc, V
E1/2 , V (ΔEp , mV), Es
Epa, V
ref
1
−0.35
−0.41
+0.03, −0.33 V
−0.30
−0.53
−0.38 (−0.95)
−0.58 (195), −0.27
−0.51 (110), −0.33 −0.62 (80), −0.28
−0.50 (130), −0.33
−0.59 (110), −0.32
−0.56 (247), −0.22
+1.27
+1.38
+1.40
+1.28
+1.30
this work
this work
this work
this work
this work
this work
this work
this work
5g
2
e
3
4
5
f
6
−0.62 (90), −0.32
HL1
NaL1 + Zn2+
[CuI(HB(3,5-Me2Pz)3)(NO2)]−
[(TtztBu,Me)CuII(NO2)]
Al. Xylosoxidans
+1.45
+1.34
g
+0.08
h
−1.15
+0.240
5j
i
22a
i
Ac. Cycloclastes IAM 1013
Ps. Chlororaphis DSM 50135
+0.250
22c
i
+0.172
22d
a
b
Potentials are vs SCE (Fc/Fc+ couple in CH3CN, E1/2 = +0.43 V), scan rate 50 mV/s, supporting electrolyte: n-Bu4NClO4 (0.1 M). E1/2 = (Epc
+
=
c
d
e
f
Epa)/2. ΔEp = Epa − Epc. Es is the stripping potential of the oxidation reaction Cu0 → Cu2+. Broad shoulder. E1/2 for Co(Cp)2 /0 couple (Epa
+
g
h
i
−0.91 V, Epc = −0.99 V, ΔEp = 80 mV). Epa vs Fc/Fc+, measured in CH2Cl2. Epc vs SCE, measured in CH2Cl2. E vs NHE.
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at E1/2 = −0.58 V (Epc = −0.67 V, Epa = −0.48 V, ΔEp = 190
mV) correspond to the CuII → CuI and CuI → Cu0
transformation, respectively. Epa at −0.48 V represents the
transformation of the generated Cu0 species to CuI, which is
only partial (ipc/ipa ratio ≠ 1). Full transformation of Cu0 →
CuII takes place at the anodic stripping potential of −0.27 V.
This response is only observed when the scan has been
performed up to −1.2 V that generates Cu0 species in the
vicinity of the working electrode surface. If the scan from 0.0 V
up to −0.5 V is performed, then no Cu0 is formed and, hence,
no 2e− reoxidation of Cu0 → CuII is possible; therefore, this
stripping potential at −0.27 V is absent (black trace in Figure
8). One irreversible oxidative response at +1.27 V found for 1 is
due to the ligand centered oxidation and not for the CuII →
CuIII conversion, as similar irreversible oxidation at +1.34 V has
been observed from the CV scan of an equimolar solution of
ZnCl2 and NaL1 (see the Supporting Information) in CH3CN
(free HL1 response is at +1.45 V, Supporting Information).
Similar redox behaviors of the complexes 3, 4, and 5 (see the
Supporting Information) have been observed in CH3CN (see
Table 4).
Figure 10. Cyclic voltammograms of 2 in CH3CN containing 0.1 M
[(n-Bu)4N]ClO4 as a supporting electrolyte at 298 K, at a platinum
working electrode, at various scan rates (given in the legend inset),
using SCE as the reference electrode.
Complex 2, with a CuII bound nitrite anion displays three
irreversible reductive responses at Epc values of −0.41, −0.57,
and −0.66 V, as shown in Figure 9. Among these, the reduction
up to 400 mV/s or lower, the formation of both CuI species
was observed, while, above that, the reduction wave that
corresponds to Epc at −0.57 V, responsible for CuI → Cu0
−
reduction of [(L1)Cu(NO2)]− species with κ1-N bound NO2 ,
has vanished. The potential values clearly reveal that the donor
strength of the κ1-N bound NO2− group (Epc = −0.57 V) is less
than that of the κ1-O bound NO2 group (Epc = −0.66 V),
−
which is expected, since the negative charge is delocalized to
−
the three atoms of the NO2 group, unlike the κ1-O bound
NO2− group, where the charge is more localized on one of the
O atoms of the bound NO2 group. Tanaka and co-workers4c
−
reported the redox behavior of [Cu(H2O)(tpa)]2+ (where tpa is
tris[(2-pyridyl)methyl]amine) in CH3CN, which shows a
reversible CuII/CuI response at E1/2 = +0.07 V vs Ag/AgCl
(vs SCE, it is +0.025 V), which is comparable to the Epc value of
+0.03 V observed for 3 in CH3CN. Interestingly, they have also
reported the electrochemical evidence of formation of two CuII
species such as [Cu(ONO)(tpa)]+ and [Cu(NO2)(tpa)]+ at
−25 °C that display the anodic responses at Epa = −0.20 V and
−0.03 V (vs Ag/AgCl) responsible for the [Cu(ONO)(tpa)]+/0
and [Cu(NO2)(tpa)]+/0 couples, where κ1-O and κ1-N bonded
NO2− are present, respectively. With neutral tridentate ligands,
the reported CuI−NO2 model complexes feature an irreversible
oxidative response at E1/2 ranging from +0.10 V − +0.27 V vs
SCE (CuI→CuII) which is more anodic than the model
complexes containing ligands with a negative charge that may
occur at very high negative potential e.g. at −1.15 V vs SCE
Figure 9. Cyclic voltammograms of 2 in CH3CN containing 0.1 M
[(n-Bu)4N]ClO4 as a supporting electrolyte at 298 K, at a platinum
working electrode, at a scan rate of 50 mV s−1, using SCE as the
reference electrode.
at −0.41 V is due to the [(L1)CuII(NO2)] → [(L1)-
CuI(NO2)]− transformation; the reverse scan from −0.5 V
toward 0.0 V does not show any response that corresponds to
the stripping potential(s) at −0.28 V or −0.33 V, observed
when the scans have been performed beyond the second or
third reduction waves at Epc = −0.57 V and −0.66 V,
respectively. Therefore, these two reductions at higher
potentials are due to the CuI → Cu0 transformations of two
different CuI species generated in situ in the solution (most
possibly, one is κ1-O bound [(L1)CuI(ONO)]− and the other
is κ1-N bound [(L1)CuI(NO2)]−).
(CuII→CuI),5j as mentioned in Table 4. Complex 6 displays the
+/0
Co(Cp)2 response at E1/2 = −0.95 V (Epa = −0.91 V, Epc
=
=
−0.99 V, ΔEp = 80 mV) and the other two responses, at Epc
−0.66 V and −0.32 V, similar to 2 (see the Supporting
Information), as expected. Note that the reported redox
potential for the CuII/CuI couple of the type 2 Cu center in
CuNIR of A. Xylosoxidans measured from pulse radiolysis
studies found is at 240 mV22a or 230 mV22b vs NHE (−0.014
mV or −0.004 mV vs SCE). This potential reported is at 250
mV for Ac. Cycloclastes IAM 1013,22c at 280 mV for Al.
Xylosoxidans GIFU 105122c and at 172 mV for Ps. Chlororaphis
DSM 50135.22d The more cathodic CuII/CuI potential of 2
than CuNIR is due to the amidato N− coordination of L1− to
CuII, whereas in CuNIR, neutral NHis are coordinated to the
nitrite bound CuII ion.
To flip the NO2 binding mode from κ1-O → κ1-N requires
time; hence, the cyclic voltammetry measurement with high
scan rate should not show the reduction wave that corresponds
to the CuI → Cu0 transformation responsible for the κ1-N
bound CuI−NO2 species. To prove this, variable-scan-rate
cyclic voltammetry (CV) measurements of a CH3CN solution
of 2 were performed, as shown in Figure 10. With a scan rate
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According to the mechanisms proposed by Averil, Suzuki,
and Hasnain2a−d for the catalytic nitrite reduction by CuNIRs,
the key reaction intermediate is CuI−NO2, which, in the
presence of protons, converts NO2− → NO(g) via an unstable
copper nitrosyl. To gain insight to the proton-coupled
reduction of nitrite ion, bound to either CuII or CuI, we have
investigated the reaction of CH3CO2H to the CuII complex 2
and to its coulometrically reduced CuI form, as well as 6 in
CH3CN. It has been observed that CH3CO2H or HCl reacts
with 2 in CH3CN to quantitatively form 5 or 1, respectively,
used for plotting the calibration curve (see the Supporting
Information)). The identity of this released NO(g) has further
been confirmed by allowing it to react with a CH2Cl2 solution
of [Co(TPP)] to generate [Co(TPP)NO],25 which is
characterized via UV-vis (the soret and Q-bands at λmax
=
414 and 536 nm, respectively (see the Supporting Informa-
tion)) and FTIR spectroscopy (after the removal of CH2Cl2,
the solid obtained displays a value of νNO = 1693 cm−1; see the
Supporting Information). From the calibration curve (Imax (μA)
vs ppm of NO(g), given as Figure S18 in the Supporting
Information), the concentration of NO(g) liberated has been
calculated that is equivalent to the Cu ion concentration.
Furthermore, the almost-equal cathodic current peaks at −0.41
V of the CV scan of 2 (black trace), at −0.70 V of the CV scan
of the generated 5 (blue trace), and at −0.93 V for that of the
in-situ-formed NO(g) clearly indicate that an equivalent
amount of NO(g) and 5 has been formed from the reaction
of CH3CO2H with the reduced CuI analogue of 2 in CH3CN.
The isosbestic points of the CV scans shown in Figure 11 at
−0.24, −0.60, and −0.69 V reveal a neat transformation of
reduced 2 to 5 and NO(g), according to the reaction
−
possibly via simple replacement of the CuII bound NO2 ion
and no formation of NO(g) has been observed, electrochemi-
cally, after the immediate addition of CH3CO2H (no redox
response within −0.8 V to −1.4 V). The sulfanilamide
spectrophotometric method23 (described in the Supporting
−
Information) supports this notion of NO2 liberation.
To check whether the reduced form of 2 in CH3CN can
mimic the nitrite reduction activity functionally, we have
coulometrically reduced a solution of 2 (0.96 mmol) at a
constant potential of −0.48 V (see the Supporting Information
for the UV traces taken during the coulometric reduction), and
to this reduced solution was added 2.2 equiv (with respect to 2)
of CH3CO2H. During a period of 2−3 min, a cathodic response
at −0.93 V was observed, which was obtained due to the
reduction of liberated free NO(g) in solution; the response
increased to a maximum ipc (scans b−e were taken during this
time, at a scan rate of 300 mV/s), as shown in Figure 11, that
[2]− + 2CH3CO2H → 5 + NO(g) + H2O
A similar observation of the CV scans has been observed when
HClO4 is used as a proton source (see the Supporting
Information). The new cathodic response at −0.83 V is found
for the liberated NO gas (NO reduction potential is −0.82
0.2 V, vs NHE);26 then, the purging argon vanishes and the
remaining responses at Epc = −0.57, −0.36, and ∼0.0 V
observed (see the Supporting Information) are identical to
those found for independently synthesized complex 3
(Supporting Information) in CH3CN. Therefore, when
CH3CO2H is used the end product is the acetate bound CuII
complex 5 (Scheme 3, step “i”), whereas when HClO4 is used,
then the catalytic cycle is sustained by reforming 3 (see Scheme
3, step “ii”). Similarly, NO(g) evolution from the reaction of 6
with CH3CO2H has been observed electrochemically (see
Figure S22 in the Supporting Information; the observed NO
reduction potential is −0.86 V). The reactivity of acids to the
CH3CN solution of either 2 or its reduced form or 6 does not
deplete the Cu ion from the ligand, because of the strong σ-
donor capability of the monoanionic L1−, and makes amenable
this nitrite reductase activity and also stabilizes other complexes
Figure 11. Cyclic voltammograms in CH3CN containing 0.1 M [(n-
Bu)4N]ClO4 as a supporting electrolyte at 298 K, at a platinum
working electrode at a scan rate of 300 mV s−1, using SCE as a
reference electrode: of 2 (0.96 mmol solution, black trace, a), gradual
development of 0.93 V response (traces b → e) upon coulometric
reduction, followed by CH3CO2H reaction, after N2 purging for 2 min
(blue trace, f).
−
with bound H2O, CH3OH, and CH3CO2 , which are relevant
to the catalytic cycle of CuNIR.
CONCLUSIONS
■
CuII/I complexes relevant to the catalytic cycle of CuNIR have
been synthesized and characterized. The principal findings are
as follows: the X-ray structure of 2 reveals an asymmetric κ2-
persists for a few minutes; after that, this response started to
decrease slowly, because of the loss of NO(g) from the solution
to the headspace (∼1 mL) of anaerobically closed coulometric
cell. After obtaining the maximum ipc value, argon gas was
purged for 2 min with stirring and then repeated scans from 0.0
V to −1.2 V do not display the response at −0.93 V, as shown
in Figure 11 (blue trace, f) confirming that the reaction product
is gaseous (and, most probably, NO(g)). If the (L1)Cu(NO)
adduct forms, then amidato N− will be trans to the NO, which
may accelerate this NO loss.24 The liberated NO(g) has been
confirmed from the CV measurement, using the same cell setup
and scan rate of a CH3CN solution of dissolved pure NO(g)
that displayed the cathodic response at the same potential (see
the CV scans of dissolved NO(g) of various concentrations,
O,O binding mode of NO2 to CuII. In 3, the CuII is
−
coordinated by H2O that is hydrogen-bonded to a solvate H2O
(in CuNIR, the CuII-coordinated H2O is hydrogen-bonded to
−
the Asp(98) side chain). This H2O is replaceable by the NO2
ion in CH3CN solution. Variable-scan-rate cyclic voltammetry
−
(CV) experiments on 2 suggest the flipping of the NO2
binding mode from κ2-O,O to κ1-N, similar to that observed in
CuNIR, according to Averil’s mechanism. FTIR supports
flipping of the NO2 binding mode from κ2-O,O in 2 → κ1-
−
N in 6. Using Co(Cp)2, the CuI complex 6 was synthesized
from precursor 2 (Path-I, Scheme 1). (L1)CuI−NO2 formation
from the reaction of NO2 with (L1)CuI has also been shown
−
(Path-II, Scheme 1). A coulometrically reduced sample of 2 and
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independently synthesized CuI complex 6 react efficiently with
CH3CO2H or HClO4 to form equivalent amounts of 5 or 3,
respectively, and NO(g). With a supporting carboxamide
ligand, neither the CuII−NO2 nor CuI−NO2 model complex
of CuNIR is reported. The present set of complexes mimic
many of the catalytic steps of CuNIR. The electrochemical
technique is used to detect and quantify NO(g).
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■
S
* Supporting Information
FTIR, mass, and 1H NMR spectra of [(n-Bu4N)[(L1)-
CuI(NO2)(CH3CN)] (Figures S1−S3); EPR of CuII formed
from disproportionation reaction (Figure S4) during the
synthesis of [(n-Bu4N)[(L1)CuI(NO2)(CH3CN)]; FTIR,
1
mass, H NMR, and electronic spectra of 6 (Figures S5−S8);
cyclic voltammogram (CV) of ZnCl2 + NaL1 mixture and HL1
itself (Figures S9 and S10); CV of 3, 4, 5, 5 in the presence of
CH3CO2H, and 6 (Figures S11−S14); calibration curve and
−
experimental details used to determine [NO2 ] via a
spectrophotometric sulfanilamide method (Figure S15); UV−
vis traces during the coulometric reduction of 2 (Figure S16);
CV of NO(g) and standardization plot (Figures S17 and S18);
electronic absorption spectra of [Co(TPP)] and [Co(TPP)-
NO] (Figure S19); FTIR spectra of [Co(TPP)] and [Co-
(TPP)NO] (Figure S20); CV of NO(g) generation from the
reaction of 2 with HClO4 (Figure S21) and from the reaction
of CH3CO2H with 6 (Figure S22). X-ray crystallographic data
of 1−5 are available as a CIF file. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: apurba.patra.nitdgp@gmail.com.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
Financial support from the Department of Science and
Technology (DST), Govt. of India (SR/S1/IC-35/2007) is
gratefully acknowledged. R.C.M. and S.K.C. acknowledge the
support of UGC and CSIR, respectively, for fellowship. We
sincerely acknowledge the AvH Foundation, Germany, for the
equipment donation grant to procure the CHI-1120A
spectroelectrochemical analyzer. We thank the Advanced
Light Source, Lawrence Berkeley Lab, for use of beamline
11.3.1 for X-ray data collection. The Advanced Light Source is
supported by the Director, Office of Science, Office of Basic
Energy Sciences, of the U.S. Department of Energy (under
Contract No. DE-AC02-05CH11231). Authors are grateful to
R. N. Saha, Department of Chemistry, National Institute of
Technology, Durgapur, India, for his help with the
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132, 14336−14337.
−
spectrophotometric analysis of [NO2 ].
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DEDICATION
■
Dedicated to Prof. R. Mukherjee, IIT Kanpur, on the occasion
of his 60th birthday.
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