Characterization of a new copper(I)-nitrito complex that evolves nitric oxide

Article abstract of DOI:10.1021/ic100083b

The complexes [Cu(κ²-Ph₂PC₆H ₄(o-OMe))₂(CH₃CN)](BF₄) (1) and [CuCl(Ph₂PC₆H₄(o-OMe))₂] (2) have been prepared by treating [Cu(CH₃CN)₄](BF₄) or CuCl with two equivalents o-(diphenylphosphino)anisole (Ph₂PC ₆H₄(o-OMe)) at room temperature, respectively. The reaction of 1 and (PPN)(NO₂) in acetonitrile solution affords a neutral compound [Cu(Ph₂PC₆H₄(o-OMe)) ₂(ONO)] (3). In contrast to the synthesis of 3, mixing NaNO ₂ and 1 in MeOH yielded a unique dicopper(I) cationic species, [((Ph₂PC₆H₄(o-OMe))₂Cu) ₂(μ-NO₂)]⁺ (4) after ether/CH ₂Cl₂ crystallization. The molecular structures of 1-4 have been determined by an X-ray diffraction study. The copper(I)-nitrito adduct 3 containing phosphine-ether ligands forms nitric oxide gas from the reaction with acetic acid, suggesting the first example and model compound in the asymmetric O-bound copper(I) nitrite intermediate microenvironment of copper nitrite reductases (Cu-NIRs).

Full text of DOI:10.1021/ic100083b

Inorg. Chem. 2010, 49, 5377–5384 5377
DOI: 10.1021/ic100083b
Characterization of A New Copper(I)-Nitrito Complex That Evolves Nitric Oxide
Wan-Jung Chuang, I-Jung Lin, Hsing-Yin Chen, Yu-Lun Chang, and Sodio C. N. Hsu*
Department of Medicinal and Applied Chemistry and Center of Excellence for Environmental Medicine,
Kaohsiung Medical University, Kaohsiung 807, Taiwan
Received October 29, 2009
The complexes [Cu(κ2-Ph2PC6H4(o-OMe))2(CH3CN)](BF4) (1) and [CuCl(Ph2PC6H4(o-OMe))2] (2) have been
prepared by treating [Cu(CH3CN)4](BF4) or CuCl with two equivalents o-(diphenylphosphino)anisole (Ph2PC6H4-
(o-OMe)) at room temperature, respectively. The reaction of 1 and (PPN)(NO2) in acetonitrile solution affords a
neutral compound [Cu(Ph2PC6H4(o-OMe))2(ONO)] (3). In contrast to the synthesis of 3, mixing NaNO2 and 1 in
MeOH yielded a unique dicopper(I) cationic species, [((Ph2PC6H4(o-OMe))2Cu)2(μ-NO2)]þ (4) after ether/CH2Cl2
crystallization. The molecular structures of 1-4 have been determined by an X-ray diffraction study. The copper(I)-
nitrito adduct 3 containing phosphine-ether ligands forms nitric oxide gas from the reaction with acetic acid, suggest-
ing the first example and model compound in the asymmetric O-bound copper(I) nitrite intermediate microenvironment
of copper nitrite reductases (Cu-NIRs).
Introduction
the results of numerous spectroscopic and crystallographic
studies.10-21 Those reaction mechanisms involve the forma-
tion of a copper(I)-nitro or -nitrito core species which
forms from the reduction of oxidized Cu-NIRs, followed by
nitrite binding or by binding of nitrite to oxidized Cu-NIRs
followed by reduction. Recently, Soloman and co-workers
suggested that the bidentate nitrito binding (O-bound) to
copper is calculated to play a major role in the activating of
the nitrite bond based on the density functional theory (DFT)
calculated reaction coordinated for nitrite reduction in the
reduced type II copper site.20 The key challenge on the
bioinorganic modeling area for a synthetic inorganic chemist
is the synthesis of a copper(I)-nitro or -nitrito core adduct.
Nitrite anion may coordinate to a mononuclear copper
cation in one of five ways, as demonstrated in Chart 1.9,22
According to thehard-soft acid-base theory (HSAB), in the
Copper-containing nitrite reductases (Cu-NIRs) active site
contains a type I electron transfer center that is coupled via a
His-Cys bridge to a type II catalytic center.1-9 The reaction
mechanisms of Cu-NIRs have been proposed on the basis of
*Corresponding author. E-mail: sodiohsu@kmu.edu.tw.
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2010 American Chemical Society
Published on Web 05/19/2010
pubs.acs.org/IC

5378 Inorganic Chemistry, Vol. 49, No. 12, 2010
Chuang et al.
Chart 1. Bonding Modes of Copper-Nitrite and Copper-Nitrito
Complexes
from the spectroscopic titration of the copper(I)-nitro com-
plex reduction process.38 Recently, a series of copper(I)-
nitro complexes modeling a system based on sterically hide-
red tris(1-pyrazolyl)methane and tris(4-imidazolyl)-carbinol
ligands is reported and shows the steric and electronic effects
of these tridentate ligands.27 However, the copper(I)-nitrito
(O-bound) complexes are seldom reported, only two known
examples are the symmetric η2-O,O-coordination on [(PPh3)2-
Cu(NO2-O,O)]39 and asymmetric η2-O,O-coordination on
[(2,6-(Ph2P(o-C6H4)CHdN)2C5H3N)Cu(NO2-O,O0)],40 which
both contain the typical soft-base phosphine ligands. Recon-
sidering the bonding characteristic of copper nitrite bonding
modes, it may not simply referto the hard or soft ability of the
local metal ion. The binding modes of nitrites may be influen-
ced by the surrounding ligands environment of cooper(I)
center. The phosphine ligands and nitrogen-donor ligands
will provide different electronic properties to the copper(I)
core that results in different NO2-bonding behavior. To
examine this thought, we chose another typical soft base
o-(diphenylphosphino)anisole as a ligand environment to
compare with other known type II copper(I)-nitro model
complex containing aromatic and/or aliphatic nitrogen
ligands,23,25-27 which are classified into borderline bases
and also compare with the only two known copper(I)-nitrito
phosphine complexes [(PPh3)2Cu(NO2-O,O)]39 and [(2,6-(Ph2-
P(o-C6H4)CHdN)2C5H3N)Cu(NO2-O,O0)],40 the latter com-
plex was reported very recently. In this paper, we report the
synthesis and crystal structures of four copper(I) phosphine-
ether ligand complexes with or without NO2 bonding. The
copper(I)-nitrito adduct [Cu(Ph2PC6H4(o-OMe))2(ONO)]
(3) containing phosphine-ether ligands and possessing an
asymmetric η2-O,O0 bounded nitrito ligation (bonding mode
C0) forms nitric oxide gas from the reaction with acetic acid,
suggesting the first example and model compound in the
asymmetric O-bound copper(I) nitrite intermediate micro-
environment of copper nitrite reductases (Cu-NIRs).
nitrite anion (NO2-), it should be expected that the soft Cu(I)
cation would prefer N-bound (A; nitro),23-27 while the harder
Cu(II) cation would prefer O-bound in monodentate bond-
ing mode (B; nitrito).22,28-30 On the other hand, the bind-
ing of the nitrite anion in a bidentate fashion to a copper
center would yield either an oxygen, O-bound species, η2-O,O
(C),28,31-34 or a mixed nitrogen/oxygen-coordinated side on
complex, η2-N,O (D).16,18 Additionally, the possibility of an
asymmetric η2-O,O0-bound moiety35-37 (C0; similar to C or
between B and C) containing only one formal Cu-O bond
and a second “long-contact” Cu-O interaction (generally with
˚
a Cu-O length >2.5 A) should also be considered. Bonding
mode D may have not been reported in monocopper coordi-
nation chemistry but has been proposed in Cu-NIRs protein
catalyzed cycles or observed in Cu-NIR protein crystallo-
graphic studies.16,18 According to literature searching, nearly
all of the known mononuclear Cu(II)-NO2 compounds crys-
tal structures contain an O-ligation nitrito moiety,22,28-37
and the Cu(I)-NO2 adducts are N-ligation nitrites.23-27 An
interesting solvent-induced interconversion between η1-N-
bound copper(II)-nitro and asymmetric O-bound copper(II)-
nitrito bonding modes containing a tris(2-pyridylmethyl)-
amine (TMPA) ligand was reported to show the possible
nitrite-reduced intermediates in electrochemical reduction.29
Regarding to copper(I)-nitro or -nitrito core species, there
were only a few copper(I) nitrite coordination complexes that
were characterized by a single crystal X-ray method.23,25-27
Tolman and co-workers developed the first functional, mono-
nuclear biomimetic model of Cu-NIRs, which is a nitro-
bonding copper(I) complex [LiPr3Cu(NO2)] (LiPr3 = 1,4,7-
triisopropyl-1,4,7-triazacyclononane) and its analogue binuclear
complex [(LiPr3Cu)2(μ-NO2)]þ as structural and functional
models for the nitrite-binding type II copper site of Cu-NIRs.23
Other researchers recently have proposed an intermediate
Results and Discussion
Synthesis. The reaction of hemilabile phosphine-ether
ligand41,42 o-(diphenylphosphino)anisole (Ph2PC6H4-
(o-OMe)) with [Cu(CH3CN)4](BF4) in CH2Cl2 under N2
gives a copper(I) complex [Cu(κ2-Ph2PC6H4(o-OMe))2-
(CH3CN)](BF4) (1), which is isolated as a colorless solid.
In the analogy to the preparation of 1, we also examined
the reaction of Ph2PC6H4(o-OMe) ligandwithCuCltoafford
a neutral compound [CuCl(Ph2PC6H4(o-OMe))2] (2).
Treatment of an acetonitrile solution of 2 with an equi-
molar amount of AgBF4 under N2 affords 1, and a white
precipitate silver chloride was formed as side product.
The reaction of 1 and (PPN)(NO2) in acetonitrile solution
affords a colorless mononuclear neutral compound
[Cu(Ph2PC6H4(o-OMe))2(ONO)] (3), which has an asym-
metric η2-O,O0 bounded nitrito ligation. In contrast to the
synthesis of 3, addition of NaNO2 to a solution of 1 in
MeOH yielded a yellow crystal after crystallization from
ether/dichloromethane, which was characterized crystal-
lographically as being a unique NO2 bridge dicopper(I)
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Article
Inorganic Chemistry, Vol. 49, No. 12, 2010 5379
Scheme 1. Synthesis of Copper(I)-Phosphine Complexes with or without NO2 Binding
Figure 1. ORTEP representation of the molecular structure of 1 (50%
ellipsoids, the hydrogen atoms are not shown, and the BF4 anion is
artificially omitted for clarity).
-
Figure 2. ORTEP representation of the molecular structure of 2 (50%
ellipsoids and hydrogen atoms are not shown for clarity).
cationic species, [((Ph2PC6H4(o-OMe))2Cu)2(μ-NO2)]þ
(4). To establish the formation of 4, the recrystallization
of 3 in ether/dichloromethane was also performed, which
results 3 and did not convert to 4. However, binuclear
species 4 converts to 3 in acetonitrile or dimethyl sulf-
oxide (DMSO) solution. All reactions are summarized in
Scheme 1.
a
˚
Table 1. Selected Bond Lengths (A) and Angles (°) for 1 and 2
1
2
Cu(1)-P(1)
2.2223(7)
2.2449(8)
1.969(3)
2.2326(6)
2.2387(6)
Cu(1)-P(2)
Cu(1)-N(1)
Cu(1)-Cl(1)
2.2240(7)
N(1)-C(39)
1.140(6)
1.447(5)
129.12(3)
122.16(8)
108.538
166.7(3)
X-ray Structural Investigations. Single crystals of 1 and
2 suitable for single crystal X-ray diffraction analysis were
grown by laying a CH3CN or CH2Cl2 solution of the
compound with ether at 273 K, respectively. The crystallo-
graphic analysis structures of 1 and 2 are shown in
Figures 1 and 2. Selected bond distances and angles of 1
and 2 are reported and compared in Table 1. Complexes 1
and 2 both contain two phosphine-ether ligands and
afford a three-coordinated copper environment with co-
ordinated CH3CN or Cl. Previously, the importance of
steric constraints in the related diphosphine-copper(I)
complexes were well established,43 so that the two
Ph2PC6H4(o-OMe) ligands and coordinated CH3CN or
halide forming the trigonal complexes 1 and 2 are likely
for steric reasons. The oxygen atoms of phosphine-ether
ligands in 1 have short Cu O contacts (Cu O=2.721
C(39)-C(40)
P(1)-Cu(1)-P(2)
P(1)-Cu(1)-N(1)
P(2)-Cu(1)-N(1)
Cu(1)-N(1)-C(39)
P(1)-Cu(1)-Cl(1)
P(2)-Cu(1)-Cl(1)
129.46(2)
115.36(3)
113.26(3)
3.349
Cu(1) O(1)
2.721
2.749
3 3 3
Cu(1) O(2)
3.525
3 3 3
a Estimated standard deviation is given in parentheses. Atoms are
labeled as indicated in Figures 1 and 2.
˚
and 2.749 A) that are less than the sum of the van der
˚
Waal radii (1.40(Cu) þ 1.52 (O) = 2.92 A) and may be
considered as a dipole-ion interaction,44 which are really
different from 2, while the nonbonded Cu O distances
˚
of 2 are 3.349 and 3.525 A. These features imply complex
3 3 3
3 3 3
3 3 3
(43) Bowmaker, G. A.; Engelhardt, L. M.; Healy, P. C.; Kildea, J. D.;
Papasergio, R. I.; White, A. H. Inorg. Chem. 1987, 26, 3533–3538.
(44) Yeh, W.-Y.; Liu, Y.-C.; Peng, S.-M.; Lee, G.-H. Inorg. Chim. Acta
2005, 358, 1987–1992.

5380 Inorganic Chemistry, Vol. 49, No. 12, 2010
Chuang et al.
Figure 3. ORTEP representation of the molecular structure of 3 (50%
ellipsoids, hydrogen atoms are not shown for clarity).
a
Figure 4. ORTEP representation of the molecular structure of 4 (50%
ellipsoids, the hydrogen atoms are not shown, and the anion has artifici-
ally omitted for clarity).
˚
Table 2. Selected Bond Lengths (A) and Angles (°) for 3 and 4
Cu(Ph2PC6H4(o-OMe))2(ONO) (3)
asymmetric η2-O,O0-bound moiety (bonding mode C0)
here is a striking example of a neutral Cu(I)-nitrito com-
plex bearing two phosphine-ether ligands. Particularly
remarkable features of 3 have a wide P-Cu-P angle of
129.45(5)° and a smaller O-Cu-O angle of 55.38(17)°
compared to related copper(I)-phosphine complexes,
[(PPh3)2Cu(NO2-O,O)]39 (127.75(7)° and 56.7(2)°), which
is more similar to the very recent example [(2,6-(Ph2P(o-
C6H4)CHdN)2C5H3N)Cu(NO2-O,O0)]40 (129.86(4)° and
56.7(2)°), respectively. The asymmetric nitrito-O,O0 co-
Cu(1)-P(1)
2.2333(13) Cu(1)-P(2)
2.2358(13)
2.344(4)
1.245(7)
Cu(1)-O(3)
N(1)-O(3)
2.116(4)
1.227(6)
129.45(5)
Cu(1)-O(4)
N(1)-O(4)
P(1)-Cu(1)-P(2)
P(1)-Cu(1)-O(3) 117.98(12)
P(2)-Cu(1)-O(3) 112.47(13)
O(3)-Cu(1)-O(4)
55.38(17)
O(3)-N(1)-O(4)
114.8(5)
[((Ph2PC6H4(o-OMe))2Cu)2(μ-NO2)](NO2) (4)
Cu(1)-P(1)
Cu(1)-N(1)
Cu(2)-O(3)
2.2241(16) Cu(2)-P(2)
2.2249(17)
2.924
1.228(6)
2.037(9)
2.230(6)
Cu(1)...O(2)
N(1)-O(3)
P(1)-Cu(1)-P(1)i 128.68(9)
P(1)-Cu(1)-N(1) 115.66(5)
56.1(3)
P(2)-Cu(2)-P(2)i 127.58(10)
O(3)-N(1)-O(3)i 117.4 (9)
O(3)-Cu(2)-P(2) 111.38(13)
˚
ordinated with a Cu-O distance of 2.116(4) and 2.344(4) A,
O(3)-Cu(2)-O(3)i
O(3)-Cu(2)-P(2)i 114.52(13)
which is significantly shorter and longer than the Cu(I)-
Onitrito distance of the known copper(I) complexes [(PPh3)2-
Cu(NO2-O,O)] (2.191(4)A) and[(2,6-(Ph2P(o-C6H4)CHd
a Estimated standard deviation are given in parentheses. Atoms are
labeled as indicated in Figures 3 and 4.
39
˚
40
N)2C5H3N)Cu(NO2-O,O0)] (2.131(4) and 2.197(4) A),
˚
suggesting the asymmetric nitrito-O,O0 coordination leads
to one Cu(I)-Onitrito interaction in 3 that is rather weak.
The significant structural bonding mode difference of the
known symmetric η2-O,O-coordination of [(PPh3)2Cu(NO2-
O,O)],39 the slight asymmetric η2-O,O0-coordination
of [(2,6-(Ph2P(o-C6H4)CHdN)2C5H3N)Cu(NO2-O,O0)]40
and 3 may derive from the more sterically hindered
phosphine-ether ligand in 3 compared with other phos-
phine ligands.
An X-ray crystal structure determination confirmed
the dimeric formulation for 4, which represents a sym-
metric NO2 bridge η1-N; η2-O,O dicopper cationic species
[((Ph2PC6H4(o-OMe))2Cu)2(μ-NO2)]þ (4) with a nitrite
counteranion. The ORTEP drawing for the complex 4 is
depicted in Figure 4. Selected bond distances and angles
of 4 are reported in Table 2. There is the molecular two-
fold rotational axis through Cu(1), N(1), and Cu(2). A
tridentate bridging mode in which the NO2- ion is chela-
ted to Cu(2) through two oxygen atoms and bridged to
Cu(1) through nitrogen atom of nitrite anion. The co-
ordination environment around Cu(1) of 4 is quite similar
1 may be more reactive than 2 in the substitution reaction
of NaNO2. Indeed, the reaction of 1 with (PPN)(NO2) in
acetonitrile solution leads to the substitution of an CH3CN
ligand of 1 to afford a colorless neutral compound [Cu-
(Ph2PC6H4(o-OMe))2(ONO)] (3). On the other hand, com-
plex 2 does not react with NO2- to form compound 3 at
ambient temperatures.
Single crystals of 3 were grown by the diffusion of diethyl
ether into an acetonitrile solution of the compound. The
single crystal X-ray diffraction analysis shows the struc-
ture of 3 also consists of two phosphine-ether ligands
and a novel O-bound nitrito ligand, in which an asym-
metric η2-O,O0-bound C0 bonding mode forms. The ORTEP
drawing for the complex is depicted in Figure 3. Selected
bond distances and angles of 3 are reported in Table 2. In
contrast to the trigonal planar geometry in 1 and 2, the
copper atom of 3 is in a mildly distorted trigonal pyr-
˚
amidal environment with the Cu atom lying 0.087 A
above the plane (toward the O(4) atom) of the P(1),
P(2), and O(3) atoms. A literature search of published
data on Cu(I)-NO2 complexes revealed that there were
only two symmetric η2-O,O bound moiety (bonding
mode C) structures known, one featuring two PPh3 ligand
complexes [(PPh3)2Cu(NO2-O,O)] and another one that is
˚
to that of 1. The Cu(1) O(2) contacts in 4 (2.924 A) are
3 3 3
almost equal to the sum of the van der Waal radii of
copper and oxygen atoms, which may be better described
as a consequence of packing constrains instead of bond-
ing interaction. The coordination about the Cu(2) center
a dicopper cationic species [(LiPr3Cu)2(μ-NO2)]2þ (LiPr3
1,4,7-triisopropyl-1,4,7-triazacyclononane),23,24,39 thus the
=

Article
Inorganic Chemistry, Vol. 49, No. 12, 2010 5381
Figure 6. Spectral changes observed during the anaerobic reaction of
3 with acetic acid in CH2Cl2 at 298 K.
Figure 5. Electronic absorption spectra of 1 (solid line), 3 (dash line),
and 4 (dotted line) in CH2Cl2 at room temperature.
of 4 can be described as a distorted tetrahedral symmetry
with a nitrito symmetric η2-O,O bound moiety. The Cu(1)-
induced behavior between 3 and 4, it gives different results.
In CD2Cl2, the 31P{1H} NMR spectrum (Figure S10 in
the Supporting Information) of 4 has two peaks at δ -7.00
and -14.18 for the phosphine ligands, which are consis-
tent with a C2 symmetry in crystallographically results of 4.
In contrast, only one resonance (δ -14.85) appeared in the
31P{1H} NMR spectrum (Figure S10 in the Supporting
Information) of 3. The spectra of 3 and 4 in CD2Cl2 remain
unchanged even in low temperature (223 K) 31P{1H}
NMR experiments. Therefore, based on the synthesis
observations and the spectroscopic results, they indicate
that the solvent-induced formation of binuclear species 4
may attribute to different reaction solvents and suggest
that a solvent-induced isomerization may happen when 4
converts to 3 in DMSO or MeCN solutions.
˚
˚
Nnitrite (2.037(9)A) andCu(2)-Onitrito (2.230(6)A) distances
in 4 are significantly longer than those of other related known
copper(I) complexes containing such a bond, 3 (Cu-
O
nitrito=2.116(4)A), [LiPr3Cu(NO2)] (LiPr3=1,4,7-triiso-
˚
23
˚
propyl-1,4,7-triazacyclononane) (Cu-Nnitrite=1.903(2)A),
[(LiPr3Cu)2(μ-NO2)]2þ (Cu-Nnitrite = 1.899(2)A and Cu-
˚
Onitrito =1.968(2)A),24 and [(PPh3)2Cu(NO2-O,O)] (Cu-
˚
O
nitrito =2.191(4)A),39 suggesting the bonding interactions
˚
between two copper and bridging nitrite anions in 4 are
rather weak.
Spectral Characterizatons. The positive-ion electro-
spray mass spectra of 1-3 all gave a peak at m/z = 647,
representing the parent ion with a loss of the coordinated
CH3CN, Cl-, or NO2- ion from the metal. The dicopper(I)
cationic species 4 can also confirm by positive-ion high-
resolution electrospray mass specrum with a peak at m/z=
1342.3201 (Figures S1 and S2 in the Supporting Infor-
mation). The relative abundances of its isotope patterns
are in good agreement with those of a simulated one. The
electronic absorption spectra of 1-4 in CH2Cl2 are shown
in Figure 5 and Figure S3 in the Supporting Information.
These copper(I)-phosphine complexes all give signifi-
cant π-π* transition bands due to the phenyl groups of
the phosphine ligands and have no absorption band in the
visible region. Complexes 1 and 2 each exhibit intense
broad bands around 258 and 288(sh) nm (for 1) and 236
and 288 nm (for 2), which could be assigned to MLCT
transitions. Complexes 3 and 4 both also exhibit an intense
broad band around 288 nm (for 3), 264 and 288(sh) nm
(for 4), which is assigned to a Cu(I) f NO2- MLCT tran-
sition. Tolman et al. have reported that [LiPr3Cu(NO2)]
(308 nm) and its analogue binuclear complex [(LiPr3Cu)2-
(μ-NO2)]þ (338 and 380 nm) both exhibit intense metal-
to-ligand charge transfer (MLCT) band(s) in CH2Cl2.23
The shift of the MLCT band could be due to the different
ligand electronic environments and NO2 binding modes.
The DMSO-d6 solution state 31P{1H} NMR spectrum
of 1 and 2 both appears only as one broad singlet at
δ -9.96 and -14.39, respectively, which is also consistent
with the crystallographic results. In addition, the DMSO-
d6 (or CD3CN) solution state 31P{1H} NMR of the yellow
crystal 4 only shows one peak, which is similar with the
31P{1H} NMR of 3 in the same d-solvent (Figure S6 and
S7 in the Supporting Information), which does not agree
with the X-ray structure of 4. This result may imply there
is a solvent-induced transformation that occurs from 4 to 3.
By using CD2Cl2 as the d-solvent to investigate the solvent-
Reactivity with Acid and NO Generation. Previously,
the best models for Cu-NIRs active site included the
nitrite-bonding copper(I) complexes containing triden-
tate ligands; the nitrite is in the N-bound monodentate
bonding mode (A).23,25-27 In 3, the microenvironment of
an asymmetric η2-O,O0-bound moiety to copper(I) is repli-
cated and may react with acid to give nitric oxide. There-
fore, the reactivity of 3 with a proton source was inves-
tigated. When six equivalents of acetic acid were added to
a CH2Cl2 solution of 3 or 4 at room temperature, the color
of the mixture solution changes from colorless to green-
blue, and the evolution of NO gas (∼89 for 3 and ∼42%
for 4; see Table S1 in the Supporting Information) was
identified by GC-TCD (Figure S12 in the Supporting
Information) following the literature method.23,25 Accor-
ding to the structure difference between 3 and 4, the half
NO generation yield for 4, which is compared with 3, may
imply a catalyzing ability only on coordinated nitrites,
not for the nitrite counterion. These results can be com-
pared with the known copper(I)-nitro nitrogen ligand
environment complexes (∼100 for [LiPr3Cu(NO2)]24 (LiPr3
=
1,4,7-triisopropyl-1,4,7-triazacyclononane); ∼95 for [Cu-
(Me2bpa)(NO2)]25 (Me2bpa = bis(6-methyl-2-pyridyl-
methyl)amine); and ∼70% for [(iPr-TIC)Cu(NO2)]27
(iPr-TIC = tris(3,5-diisopropyl-1-pyrazoyl)methane) and
indicate that the copper(I)-nitrito complexes with phos-
phine ligands (3 and 4) are also good functional Cu-NIRs
models that produce known copper(II) acetate complexes
and NO with acetic acid under anaerobic conditions. The
known blue copper(II) product [Cu2(CH3COO)4(H2O)2]
was isolated from the NO generation reaction mixtures
and demonstrated by crystallographic characterization.
The reaction of 3 with acetic acid was monitored by
recording either the UV-vis absorption spectra at 5 min

5382 Inorganic Chemistry, Vol. 49, No. 12, 2010
Chuang et al.
Table 3. Rate Constants for the Nitrite Reduction of 3a
lated and analyzed. It was found that the nitrite binds to
[Cuþ(Ph2PC6H4(o-OMe))2] more strongly in the η2-O,O0
bonding mode (-117.60 kcal mol-1) than in the η1-N bond-
ing mode (-112.41 kcal mol-1), in line with the relative
energies between the two conformers. These results can
be compared with the known calculation results on the
known copper(II)-nitrito complex [TpCuII(η2-NO2-O,
O)] (Tp = hydrotris(pyrazolyl)borate), which suggests
the O-bound nitrito-bonding mode is favorable for the
copper(II) case with the nitrogen-donor ligands.22 Inter-
estingly, the energy decomposition analysis, which parti-
tions the bonding energy into physically meaningful
terms of Pauli repulsion and electrostatic interaction as
well as orbital interaction (including polarization and
charge-transfer contributions), reveals that it is the smal-
ler Pauli repulsion rather than electrostatic and orbital
interactions accounting for the relatively larger stability
for η2-O,O0 conformation (Table S3 in the Supporting
Information). The smaller Pauli repulsion in the η2-O,O0
conformer relative to the η1-N conformer is a conse-
temp(K)
(105)kobs, M-1 -1
s
253
273
283
298
9.296 ( 0.16
40.34 ( 1.19
85.35 ( 1.59
217.0 ( 10.79
a The fit indicates ΔH‡ of 40.89 (0.32) kJ mol-1 and ΔS‡ -158.48 (1.18)
J/K-1 mol-1 (numbers in parentheses indicate standard deviation).
intervals or the absorption vs time at a fixed wavelength
(288 nm). At 298 K, the 288 nm band of 3 was observed to
be a sharp peak, as shown in Figure 6. Adding acetic acid
to the solution of 3 under N2 induced a decrease in the
absorbance at 288 nm and a slight increase in the absor-
bance at about 675 nm. The reaction of 3 with acetic
acid was found to be first order both in [3] and in [acid].
This observation implies the rate-determining step only
involves one protonation process. The temperature de-
pendence of the rate of the reaction was examined at four
temperatures over the range 253-298 K (Table 3). The
analysis of these results using the usual Eyring approach
(Figure S13 in the Supporting Information) gave ΔH‡ of
40.89 ( 0.32 kJ mol-1 and ΔS‡ of -158.48 ( 1.18 J/K-1
mol-1. Comparison of the activation parameters for 3
and the known copper(I)-nitro complex [Cu(Me2bpa)-
(NO2)]25 (Me2bpa = bis(6-methyl-2-pyridylmethyl)amine;
quence of the fact that the Cu O contacts (2.185 and
˚
2.316 A) in the former are considerably longer than in the
3 3 3
˚
Cu N contact (2.024 A) in the latter (Table S3 in the
Supporting Information).
3 3 3
To investigate the influences of phosphine-ether ligands
on bonding between copper(I) and nitrite, we have carried
out the corresponding calculations and analysis for the
ΔH‡=25 ( 1 kJ mol-1 and ΔS‡=-51 ( 3 J/K-1 mol-1
)
bare Cuþ NO2- complex. As observed in complex 3, the
3
reveals a significantly larger ΔH‡ and a larger negative
ΔS‡ in the protonated process. Since a negative value of
ΔS‡ generally corresponds to the additional loss of free-
dom of motion by the solvating molecule, the larger
negative ΔS‡ value in 3 means a nearly complete proton-
ation (association) process in the rate-determining step.
This result may be attributable to the different ligand
environment and coordination mode of NO2. Therefore,
the protonated 3 will be an active intermediate. The larger
positive ΔH‡ strongly suggests that 3 is more stable than
protonated 3.
bidendate O-bound configuration is more stable than the
monodentate N-bound one; this can be, once again, attri-
buted to the longer contact between copper and nitrite,
which, in turn, leads to smaller Pauli repulsions in the
η2-O,O0 configuration than in the η1-N configuration
(Tables S2 and S3 in the Supporting Information).
Although the phosphine-ether ligand does not alter the
bonding mode of nitrite, the presence of it, however,
significantly attenuates the bonding between nitrite and
copper; the bonding energies decrease from -190.69 to
-117.60 kcal mol-1 and from -187.73 to -112.41 kcal
mol-1 for η2-O,O0 and η1-N configurations, respectively
(Table S3 in the Supporting Information). The energy
decomposition analysis clearly evidenced that the less
attractive electrostatic interaction is responsible for the
lowering of bonding energy caused by phosphine-ether
ligands (Table S3 in the Supporting Information). This
result is not surprising since a certain amount of electron
transfer would occur from phosphine-ether ligands to
copper(I) and, thus, reduces the effective positive charge
on copper. As a result, the electrostatic stabilization
Computational Results. Since the η1-N or η2-O,O0
bonding characteristic of copper nitrite bonding modes
may not simply refer to the hard or soft ability of the local
metal ion. It should be considered in the surrounding
ligand environment of copper(I) center. Therefore, den-
sity functional theory (DFT) calculations have been
performed to get insight into the bonding characteristic
in complex 3. The optimized geometry of bidentate η2-O,
O0 conformer of complex 3 is reasonably in agreement
with the crystal structure (Table 2 and S2 in the Support-
ing Information); the average values of absolute devia-
between nitrite and copper would be smaller in complex
.
3 than in the bare Cuþ NO2
-
˚
tion for bond length and angle are 0.049 A and 3.3°,
respectively.
3
Besides the η2-O,O0 conformer, the calculations reveal
that the monodentate η1-N conformation is also a local
minimum on the potential energy surface. However, the
η1-N conformer was found to be less stable than the η2-O,
O0 conformer by 3.07 and 3.69 kcal mol-1, respectively, in
terms of total electronic energy and free energy (Table S3
in the v), which is consistent with the observation that
complex 3 crystallize in η2-O,O0 bonding form. To further
elucidate why the η2-O,O0 bonding form is more stable
than the η1-N bonding form, the bonding energies be-
tween [Cuþ(Ph2PC6H4(o-OMe))2] and nitrite were calcu-
Conclusion
The known type II copper(I)-NO2 model complexes with
aromatic and/or aliphatic nitrogen-donor ligands prefer to
have a N-bound bonding mode.23,25-27 However, nearly all of
the crystal structures of the known mononuclear copper(II)-
NO2 complexes with similar nitrogen-donor ligand sets
contain an O-bound conformation on the nitrite-bonding
modes.22,28-37 Since the binding modes of nitrite may be in-
fluenced by the surrounding ligand environment of the copper
ion center. The nitrogen-donor ligands (hard or borderline
bases) and phosphine ligands (soft base) would provide

Article
Inorganic Chemistry, Vol. 49, No. 12, 2010 5383
different electronic environments around the copper ion
core, which may induce dissimilar NO2 bonding behavior.
An O-bound copper(I)-nitrito complex 3 containing two
phosphine-ether ligands has been prepared and revealed its
asymmetric η2-O,O0-bound moiety around the copper(I) atom
by crystallographic analyses in this study. Phosphine-
ether ligand (Ph2PC6H4(o-OMe)) is a typical hemilabile
ligand41,42 which bonds to the copper(I) ion by the soft base
side (phosphorus atom). Therefore, the phosphine ligands
may induce the nitrite-bonding mode change from N-bound
to O-bound on the copper(I) ion, which is significantly
different from other known copper(I)-NO2 complex contain-
ing nitrogen donor ligand sets. The DFT calculations also
confirmed that the O-bound configuration is indeed more
stable than the N-bound for Ph2PC6H4(o-OMe) containing
copper(I)-NO2 complexes. Complex 3 is a striking example
with significant asymmetric η2-O,O0 nitrito structural-bond-
(15 mL) solution of Ph2PC6H4(o-OMe) (0.2 g, 0.686 mmol).
A white solid precipitate forms immediately. The resulting white
powder was recrystallized from CH2Cl2 to yield the product as
colorless needles of [CuCl(Ph2PC6H4(o-OMe))2] (2). Yield: 95%
(0.223 g, 0.326 mmol). Anal. calcd for C38H34ClCuO2P2: C, 66.76;
H, 5.01. Found: C, 66.77; H, 5.04. 1H NMR (DMSO-d6): δ 3.38-
(s, 6H, -OCH3), 6.69-7.53(m, 28H, Ph). 13C{1H} NMR (DMSO-
d6): δ 55.58(OCH3), 120.95-169.95(Ph), 111.56-160.27(Ph).
31P{1H} NMR (DMSO-d6): δ -14.39(s). UV-vis absorption
(CH2Cl2, λmax, nm)(ε/M-1cm-1) 236 (9600), 288 (6100). ESI-
MS: 647.08 (100%) [M-Cl]þ.
[Cu(Ph2PC6H4(o-OMe))2(ONO)] (3). A solution of 1 (0.2 g,
0.258 mmol) in CH3CN (8 mL) was added to a stirring CH3CN
(7 mL) solution of (PPN)(NO2) (0.151 g, 0.258 mmol). The resul-
ting solution allowed staying at -20 °C for one day to obtain
colorless crystals [Cu(Ph2PC6H4(o-OMe))2(ONO)] (3). Yield:
75% (0.133 g, 0.193 mmol). Anal. calcd for C38H34CuNO4P2: C,
65.75; H, 4.94. Found: C, 65.72; H, 4.93. 1H NMR (DMSO-d6):
δ 3.45(s, 6H, -OCH3), 6.60-7.50(m, 28H, Ph). 13C{1H} NMR
(DMSO-d6): δ 55.59(OCH3), 111.44-160.35(Ph). 31P{1H} NMR
(DMSO-d6): δ -13.00(s). 1H NMR (CD2Cl2): δ 3.38(s, 6H,
-OCH3), 6.58-7.36(m, 28H, Ph). 31P{1H} NMR (CD2Cl2): δ
-
ing mode for a recently conjectural biological Cu(I)-NO2
intermediate.20 The protonation of 3 to release NO gas is of
interest within the context of the chemistry of thoroughly
examining the copper center of Cu-NIRs during the nitrite
reduction process, which is important in order to enhance our
understanding of the biological denitrification process.
-14.85(s). UV-vis absorption (CH2Cl2, λmax, nm)(ε/M-1cm-1
)
236 (18 400), 288 (14 300). FAB-MS: 647.13 [M-NO2]þ. ESI-
MS: 647.08 (100%) [M-NO2]þ.
[((Ph2PC6H4(o-OMe))2Cu)2(μ-NO2)](NO2) (4). A solution
of 1 (0.7 g, 0.903 mmol) in MeOH (3 mL) was added to a stirring
MeOH (18 mL) solution of NaNO2 (0.0623 g, 0.903 mmol). The
resultant mixture was stirred for 1 h to give a yellow solid
precipitate, which was recrystallized from CH2Cl2/Et2O to yield
a yellow crystal product as [((Ph2PC6H4(o-OMe))2Cu)2(μ-NO2)]-
(NO2) (4). Yield: 86% (0.54 g, 0.402 mmol). 1H NMR (DMSO-
d6): δ 3.30(s, 12H, -OCH3), 6.45-7.45(m, 56H, Ph). 31P{1H}
NMR (DMSO-d6): δ -11.25. 1H NMR (CD2Cl2): δ 3.32(s, br,
12H, -OCH3), 6.38-7.65(m, br, 56H, Ph). 31P{1H} NMR (CD2-
Experimental Section
All manipulations were carried out under an atmosphere
of purified dinitrogen with standard Schlenk techniques.
Chemical reagents were purchased from Aldrich Chemical
Co. Ltd., Lancaster Chemicals Ltd., or Fluka Ltd. All the
reagents were used without further purification, apart from
all solvents that were dried over Na (Et2O, THF) or CaH2
(CH2Cl2, CH3CN) and then thoroughly degassed before use.
The salt [Cu(CH3CN)4](BF4)45 andligandPh2PC6H4(o-OMe)41
were prepared as described in the literature. IR spectra were
recorded on a Perkin-Elmer System 2000 FT-IR spectro-
meter. UV-vis spectra were recorded on an Agilent 8453
Cl2): δ -7.00 (s), -14.18(s). UV-vis absorption (CH2Cl2, λmax
,
nm)(ε/M-1cm-1) 238 (21 800), 264 (17 500), 288sh (14 800). ESI-
MS: 1342.03 (3%) [M-NO2]þ, 939.03 (60%) [M-NO2-
Cu(Ph2PC6H4(o-OMe))]þ, 646.96 (100%) [M-NO2-Cu(Ph2P-
1
spectrophotometer. H NMR, 13C NMR, and 31P NMR
þ
C6H4(o-OMe))2]þ. HRMS (ESIþ) m/z for C76H68Cu2NO6P4
[M-NO2]þ, calcd: 1342.2590. Found: 1342.2569.
spectra were acquired on a Varian Gemini-200 proton/carbon
FT NMR or a Varian Gemini-500 proton/carbon FT NMR
spectrometer. ESI mass spectra were collected on a Waters
ZQ 4000 mass spectrometer. Elemental analyses were per-
formed on a Heraeus CHN-OS Rapid Elemental Analyzer.
Gas chromatography thermal conductivity detector (GC-
TCD) experiments were performed by using a Varian CP-
3800 gas chromatography, Porpak Q column (6 ft, 20 mL/min
flow rate, 30 °C, nitrogen carrier gas), and TCD detector.
[Cu(K2-Ph2PC6H4(o-OMe))2(CH3CN)](BF4) (1). A solution
of [Cu(CH3CN)4](BF4) (0.100 g, 0.318 mmol) in CH2Cl2 (8 mL)
was added dropwise to a stirring CH2Cl2 (15 mL) solution of
Ph2PC6H4(o-OMe) (0.185 g, 0.636 mmol). The reaction mixture
was stirred overnight and concentrated to about 3 mL, then
Et2O (8 mL) was laying on the top and allowed to stay at room
temperature for one day to obtain colorless crystals of [Cu(κ2-
Ph2PC6H4(o-OMe))2(CH3CN)](BF4) (1). Yield: 90% (0.225 g,
0.285 mmol). Anal. calcd for C40H37O2P2NCuBF4: C, 61.91;
Measurement of NO Generated from 3. A solution of 3 (33.3 mg,
0.048 mmol) in CH2Cl2 (0.9 mL) was prepared in a small vial
capped with a rubber septum. A solution of acetic acid (10.8 μL) in
CH2Cl2 (0.1 mL) was then introduced with a syringe at room
temperature. The solution changed immediately from colorless to
blue. Analysis of the headspace gas by a thermal conductivity
detector indicated that NO had been generated (88.8 ( 1.6 for 3).
The NO generation data are obtained by five different experiments
(see Table S1 and Figure S12, Supporting Information). NO
concentration was performed by calibrating curve response with
known concentrations of NO gas mixed with N2 (120, 100, 80, 60,
(46) Frisch, M. J. T., G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; Montgomery, J. A.; Vreven, T., Jr.; Kudin, K. N.; Burant,
J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.;
Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.;
Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian,
H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.;
Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.;
Ochterski, J.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain,
M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski,
J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin,
R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.;
Gonzalez, C.; Pople, J. A.; Gaussian 03, , revision E, 1st ed.; Gaussian, Inc.:
Wallingford, CT, 2004.
1
H, 4.81; N, 1.80. Found: C, 61.77; H, 4.81; N, 1.95. H NMR
(DMSO-d6): δ 2.07(s, 3H, CH3CN), 3.41(s, 6H, -OCH3), 6.69-
7.53(m, 28H, Ph). 13C{1H} NMR (DMSO-d6): δ 1.157(CH3-
CN),55.58(OCH3), 118.907(CH3CN), 111.56-160.27(Ph). 31P{1H}
NMR (DMSO-d6): δ -9.96(s). UV-vis absorption (CH2Cl2,
λ
max, nm)(ε/M-1cm-1) 230 (16 900), 258 (8500), 288sh (5900).
ESI-MS: 647.08 (100%) [M-CH3CN]þ.
[CuCl(Ph2PC6H4(o-OMe))2] (2). A solution of CuCl (0.034 g,
0.343 mmol) in CH3CN (10 mL) was added to a stirring CH3CN
(45) Kubas, G. J. Inorg. Synth. 1990, 28, 68–70.

5384 Inorganic Chemistry, Vol. 49, No. 12, 2010
Chuang et al.
Table 4. Crystallographic Data for Complexes 1- 4
1
2
3
4
empirical formula
formula weight
T (K)
C40H37BCuF4NO2P2
776.00
C38H34ClCuO2P2
683.58
200(2)
0.40 ꢀ 0.28 ꢀ 0.22
monoclinic
C2/c
22.2512(3)
16.0416(2)
19.2459(3)
90
105.1710(10)
90
C38H34Cu NO4P2
694.14
200(2)
0.34 ꢀ 0.25 ꢀ 0.09
monoclinic
C2/c
22.2503(6)
15.9491(5)
19.8515(7)
90
105.5320(10)
90
C76H68Cu2N2O7P4
1372.28
200(2)
0.3 ꢀ 0.27 ꢀ 0.15
orthorhombic
Pncn
18.0326(4)
20.1099(4)
20.5000(5)
90
90
90
200(2)
0.54 ꢀ 0.36 ꢀ 0.15
cystal size (mm)
crystal system
space group
triclinic
P-1
˚
a (A)
10.7243(2)
13.0834(2)
14.5137(2)
92.9650(10)
97.8830(10)
107.9950(10)
1908.91(5)
2
˚
b (A)
˚
c (A)
R (°)
β (°)
γ (°)
3
˚
V (A )
6630.31(16)
8
1.370
6788.3(4)
8
1.358
7434.0(3)
4
1.226
Z
Dcalcd (g cm-3
μ mm-1
)
1.350
0.710
0.869
0.779
0.709
reflns mcasd/indep
data/rcstraints/params
GOF
23 106/6655
6655/0/462
1.095
20 951/5834
5834/0/397
1.180
21 302/5957
5957/0/415
1.073
38 987/6606
6606/0/408
1.175
Rint
0.0463
0.0274
0.0812
0.0807
R1 [I>2σ] (all data)
Rw [I>2σ] (all data)
max. pcak/hole (e / A )
0.0447 (0.1221)
0.0575 (0.1351)
0.903/-0.644
0.0294 (0.0845)
0.0407 (0.1034)
0.540/-0.681
0.0608 (0.1415)
0.1083 (0.1718)
0.522/-0.729
0.0851(0.2434)
0.1367(0.2701)
1.519/-2.213
-
3
˚
and 40 ppm of NO in N2); molar quantities were calculated using
the ideal gas equation.
Nonius Kappa CCD diffractometer. The structures were solved
with Direct Methods (program SHELXS-97). Refinement was
performed with SHELXL-97 against F2 of all reflections. Non-
hydrogen atoms were refined with anisotropic displacement
parameters. All hydrogen atoms were introduced in calculated
positions and refined with a riding model. Geometry calcula-
tions and checking for higher symmetry was performed with the
PLATON program. Further details are given in Table 4. The
NO2- counterion and solvent molecule (diethylether, CH2Cl2)
of 4 was disordered and in addition could not be completely
located. All attempts to model satisfactorily this disorder were
unsuccessful. Therefore, the disordered solvents were squeezed
with the program PLATON prior to final refinement. The resi-
dual electron density in a two-fold symmetric unit was assigned
to a whole NO2- counteranion.
Kinetics. The kinetics studies of nitrite reduction of 3 in
CH2Cl2 were carried out by monitoring the intensity decrease
of the 288 nm band. The absorbance was detected by an Agilent
8453 spectrophotometer equipped with a custom-designed fiber-
optics immersion quartz probe of 1 cm path length (Hellma) and
fitted to a Schlenk flask containing 50 mL CH2Cl2 solution of 3
(8.7 mg, 0.25 mmol) under N2 atmosphere. The reaction was
started with the addition of 360 μL of acetic acid, which had
been degassed with N2 before use.
Computational methods. Geometry optimizations for com-
plex 3 in η2-O,O0 and η1-N bonding modes were carried out at
B3LYP/6-31þG* level. Vibrational frequency calculations were
then performed to confirm that both optimized structures are
local minima on the potential energy surface. These calculations
were achieved by using the Gaussian 03 package.46 The bonding
Acknowledgment. Financial support of the National Science
Council (Taiwan) and the Center of Excellence for Environ-
mental Medicine in the KMU are highly appreciated. We thank
Mr. Ting-Shen Kuo of National Taiwan Normal University for
X-ray structural determinations and the National Center for
High-Performance Computing for computer time and facilities.
-
energies between Cu1þ(Ph2PC6H4(o-OMe))2 and NO2 were
evaluated by fragment orbital approach and analyzed by energy
decomposition analysis implemented in ADF program.47 In
these calculations, B3LYP functional was used in combination
with the basis sets of triple-ζ quality (TZP) and double-ζ quality
(DZP) augmented by a set of polarization functions for copper
and remaining elements, respectively.
Supporting Information Available: Text containing additional
figures (Figure S1-S13), tables (Table S1-S3), and crystallo-
graphic data in CIF format for the structure determinations of
1, 2, 3 and 4. These material are available free of charge via the
Internet at http://pubs.acs.org.
X-ray crystal structure determinations. All X-ray reflections
˚
were measured with Mo KR radiation (λ = 0.71073 A) on a
(47) ADF 2009.01; SCM, Theoretical Chemistry Amsterdam, Vrije Universiteit:
Amsterdam, The Netherlands, http://www.scm.com.