Article
pubs.acs.org/IC
Alkoxide Migration at a Nickel(II) Center Induced by a π‑Acidic
Ligand: Migratory Insertion versus Metal−Ligand Cooperation
Seohee Oh, Seji Kim, Dayoung Lee, Jinseong Gwak, and Yunho Lee*
Department of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon 34141, Republic of Korea
S Supporting Information
*
ABSTRACT: Two pathways of alkoxide migration occurring at a nickel(II)
center supported by a PPP ligand (PPP− = P[2-PiPr2-C6H4]2−) are
presented in this Article. In the first route, the addition of a π-acidic ligand to
a (PPP)Ni alkoxide species reveals the formation of a P−O bond. This
reaction occurs via metal−ligand cooperation (MLC) involving a 2-electron
reduction at nickel. To demonstrate a P−O bond formation, a nickel(II)
isopropoxide species (PPP)Ni(OiPr) (4) was prepared. Upon addition of a
π-acidic isocyanide ligand CNtBu, a nickel(0) isocyanide species (PPOiPrP)Ni(CNtBu) (6b) was generated; P−O bond formation occurred via
reductive elimination (RE). When CO is present, migratory insertion
(MI) occurs instead. The reaction of 4 with CO(g) results in the formation
of (PPP)Ni(COOiPr) (5), representing an alternative pathway. The corresponding RE product (PPOiPrP)Ni(CO) (6a) can be
independently produced from the substitution reaction of {(PPOiPrP)Ni}2(μ-N2) (3) with CO(g). While two different
carbonylation pathways in 4 seem feasible, C−O bond forming migratory insertion singly occurs. Regeneration of a (PPP)Ni
moiety via a P−O bond cleavage was demonstrated by treating 3 with CO2(g). The formation of (PPP)Ni(OCOOiPr) (7)
clearly shows that an isopropoxide group migrates onto the bound CO2 ligand, and a P−Ni moiety is regenerated.
■
INTRODUCTION
Metal−ligand cooperation (MLC) has become a popular
synthetic approach applied in various transition metal-based
catalytic reactions in recent years.1 Relative to classical catalysis,
in this approach both the metal center and a ligand are actively
involved in the key transformations, such as bond activation
and redox processes. In cytochrome P450, a well-known MLC
system, the intermediate species known as Compound I plays a
crucial role in C−H bond activation.2 A high-valent Fe(IV)O
species is generated subsequent to O2 activation, which requires
additional electron donation from a redox-noninnocent
porphyrin macrocycle. Several research groups have developed
various catalytic systems to demonstrate the synergic
collaboration between a central metal ion and a noninnocent
ligand in bond forming reactions.3,4 While most noninnocent
ligands employ various π-conjugated systems, there are few
MLC systems utilizing a P atom as a key component of
noninnocent behavior. The Thomas group reported a series of
pincer complexes of Pt, Pd, Cu, and Co employing an Nheterocyclic phosphenium (NHP) donor as a noninnocent
moiety.5 The interconversion occurs between NHP+ and
NHP−, which can act as a Z- and X-type ligand, respectively,
via a two-electron redox process on a central metal ion nicely
coupled with its local geometry.5 As a major noninnocent
component, such an MLC system requires a P donor
possessing a wide redox window.6 As a related example of a
redox active P atom, the Radosevich group reported a reversible
PIII/PV redox cycle based on various tricoordinate phosphorus
compounds utilized in several catalytic hydrogenations.7
© 2016 American Chemical Society
Recently, our group reported a unique example of MLC by
employing a (PPP)Ni scaffold (PPP− = P[2-PiPr2-C6H4]2−).8,9
The reversible conversion of a phosphide-Ni(II) and a
phosphinite-Ni(0) species occurs at a P−Ni moiety acting as
a team to achieve group transfer (Scheme 1).9 A square planar
Scheme 1
nickel(II) alkoxide stabilized by a Pphosphide−Ni bond can be
converted to a pseudo tetrahedral nickel(0) species from its
reaction with a π-acidic ligand, such as N2 and CO. The
reaction involves a P−OR bond formation, which is crucial in
both transferring two electrons to Ni and accommodating the
necessary geometry of a zerovalent nickel species. As a key
mechanistic step of this process, we initially proposed that
reductive elimination (RE) occurs at a P−Ni(OR) moiety to
give a phosphinite-Ni(0) species in the presence of CO. This
postulate was successfully demonstrated by the carbonylation of
(PPP)Ni(OPh) to generate the corresponding zerovalent
nickel monocarbonyl species (PPOPhP)Ni(CO).9 More recently, we also reported an analogous reaction of (PPP)NiReceived: September 14, 2016
Published: December 2, 2016
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and thus it can be isolated as a clean solid product. The solution
behavior of 2 and dinuclear 3 is similar to the formation of a
previous analogous pair of compounds, (PPOMeP)Ni(N2) and
{(PPOMeP)Ni}2(μ-N2), also supporting the production of two
dinitrogen adducts.9 After the careful purification process, both
N2 adducts were cleanly isolated as 3 in 16% yield, vide infra. In
the 31P NMR spectral data in C6D6, a set of triplet and doublet
appears at 126.78 ppm (t, Jpp = 98.8 Hz) and 59.63 ppm (d, Jpp
= 97.2 Hz) corresponding to 3 along with another set of signals
centered at 132.47 ppm (t, Jpp = 84.2 Hz) and 61.63 ppm (d, Jpp
= 84.2 Hz) corresponding to compound 2. The identity of 3
was further confirmed by the X-ray crystallography. Its solidstate structure clearly shows a dinuclear nickel(0) dinitrogen
species revealing distorted tetrahedral nickel centers in its
dimeric structure (τ4 = 0.68 and 0.72, Figure 1 and Table 1).13
(SPh), which involves thiolate group transfer to the phosphorus
for a new example of P−S bond formation.10
According to our previous observations, upon addition of a
methoxy group to (PPP)NiCl (1), both zerovalent nickel-N2
species, {(PPOMeP)Ni}2{μ-N2} and (PPOMeP)Ni(N2), were
generated under N2 atmosphere. Unfortunately, even under
Ar atmosphere, the detection and isolation of (PPP)Ni(OMe)
was unsuccessful from the same reaction. Thus, its mechanistic
understanding is still limited. Currently, at this stage, our
mechanistic postulates are as follows: an associative pathway is
operating involving a 5-coordinate nickel(II) intermediate
species,11 and a dissociative pathway is accessible involving a
3-coordinate nickel(0) species.12 Presently, there is no
indication of a (PPERP)Ni(0) species without any π-acidic
ligand. Thus, an associative reaction might occur at a (PPP)Ni
scaffold through accepting a π-acidic ligand as the fifth ligand in
the presence of an ER donor (E = O or S, R = aryl or alkyl). By
this pathway, a 5-coordinate species undergoes RE as a part of a
MLC process. However, RE is subject to competition with a
well-known CO migratory insertion (MI) pathway, vide infra.
To study such reaction pathways, isolating a stable alkoxide
species of a (PPP)Ni scaffold is crucial. To access and stabilize
such species, we have employed an isopropoxide group,
Scheme 2. In this Article, we report a divalent nickel
Scheme 2
Figure 1. Displacement ellipsoid (50%) representation of {(PPOiPrP)Ni}2(μ-N2) (3). Hydrogen atoms are omitted for clarity.
A bridging N2 ligand is weakly bound to two nickel ions with
the Ni−N bond distances of 1.847(3) and 1.837(3) Å. Its N−N
bond distance of 1.133(4) Å is fairly similar to those of other
analogous nickel(0) dinitrogen species {(PPRP)Ni}2(μ-N2) (R
= OMe or Me, dN−N = 1.112(5) and 1.124(3) Å).9,14a
From the reaction mixture, a nickel(II) alkoxide species,
(PPP)NiOiPr (4), was cleanly isolated as a major product with
a higher yield of >55% displaying a triplet at 101.21 ppm (Jpp =
10.5 Hz) and a doublet at 45.85 ppm (Jpp = 9.7 Hz) in the 31P
NMR spectrum in C6D6 (Figure 2a). The small P−P coupling
constant (∼10 Hz) between a central phosphide P atom and
two flanking phosphorus atoms of a PPP ligand as compared to
that of a typical (PPRP)nickel(0) species exhibiting their JPP
values in the range of 50−100 Hz indicates that the product is a
square planar nickel(II) species similar to (PPP)Ni(EPh) (E =
O or S).9,10 In the 1H NMR spectrum shown in Figure 2a, the
methine peak of an isopropoxide group appears as a septet at
4.09 ppm (JHH = 5.7 Hz) in C6D6, with no indication of H−P
coupling, vide infra. The same species is cleanly generated from
the reaction of 1 in the absence of N2. Under argon
atmosphere, the addition of NaOiPr to a solution of 1 in
THF affords a quantitative conversion to a nickel(II) species 4.
Although the structural data have not yet been acquired, the
identity of 4 was assured by multinuclear NMR spectral data
and further confirmed by elemental analysis.
P−O Bond Forming Reaction via Metal Ligand
Cooperation of (PPP)Ni(OiPr): Reductive Elimination
versus Migratory Insertion. Because the isolation of
(PPP)Ni(OMe) was unsuccessful, our mechanistic investigations were limited. As alternative studies, a Ni(II)-phenoxide
and analogous thiolato species were employed.9,10 The
carbonylation of (PPP)Ni(SAr) showed pseudo-first-order
kinetics suggesting an intramolecular reaction pathway with
excess CO(g).10 Thus, one can anticipate that (PPP)Ni(OR)
isopropoxide species and its reactivity toward a π-acidic ligand,
such as carbon monoxide and tert-butyl isocyanide. Unlike
previous examples with a phenoxide and phenyl thiolate group,
CO migratory insertion (MI) occurs with (PPP)Ni(OiPr) (4)
resulting in the formation of (PPP)Ni(COOiPr) (5). However,
in the case of the reaction with tert-butyl isocyanide, similar
cooperative behavior occurs to generate a P−O bond along
with the two-electron reduction of Ni.
■
RESULTS AND DISCUSSION
Synthesis of {(PPOiPrP)Ni}2(μ-N2) and (PPP)Ni(OiPr). To
introduce an isopropoxide group to a (PPP)Ni scaffold, we
conducted the reaction of (PPP)NiCl (1) with sodium
isopropoxide under N2 at room temperature. According to
the crude 31P NMR spectrum, three species were generated
from the reaction; two nickel(0) dinitrogen adducts, (PPOiPrP)Ni(N2) (2) and {(PPOiPrP)Ni}2(μ-N2) (3), and one nickel(II)
alkoxide species, (PPP)Ni(OiPr) (4); see the Experimental
Section. While 2 and 3 are in equilibrium under N2 atmosphere,
a single species 3 remains under vacuum in a C6D6 solution,
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Table 1. Selected Bond Distances and Angles of 3, 5, 6a, 6b, and 7
Ni−P(1) (Å)
Ni−P(2,3) (Å)
Ni−La (Å)
L−Xb (Å)
P(1)−Ni−L (deg)
P(2)−Ni−P(3) (deg)
τ4
2.1688(9), 2.1678(9)
2.1746(9), 2.1756(9)d
2.1729(9), 2.1689(9)
1.847(3)
1.837(3)e
1.911(3)
1.133(4)
(PPP)Ni(COOiPr) (5)
2.1248(9)
2.1200(9)c
2.1644(9)
134.59(8)
130.27(8)f
168.0(1)
129.59(4)
128.87(4)g
162.53(4)
0.68
0.72
0.21
(PPOiPrP)Ni(CO) (6a)
(PPOiPrP)Ni(CNtBu) (6b)
(PPP)Ni(OCOOiPr) (7)
2.1225(6)
2.1028(6)
2.1551(6)
2.1841(6), 2.1839(6)
2.1508(6), 2.1617(5)
2.1792(6), 2.2058(6)
1.745(2)
1.815(2)
1.946(1)
127.17(7)
124.87(6)
173.86(5)
121.64(2)
129.87(2)
163.55(2)
0.79
0.75
0.16
OiPr
{(PP
P)Ni}2(μ-N2) (3)
1.208(4)h
1.391(4)i
1.153(3)
1.170(2)
1.268(2)
a
L = N for 3, O for 4 and 7, C for 5a, 5b, and 6. bX = N for 3, C for 4 and 7, O for 5a, 5b, and 6. cDistance for Ni(2)−P(4). dDistance for Ni(2)−
P(5), Ni(2)−P(6). eDistance for Ni(2)−N(2). fAngle for P(4)−Ni(2)−N(2). gAngle for P(5)−Ni(2)−P(6). hDistance for C(1)−O(1). iDistance
for C(1)−O(2).
Figure 3. Displacement ellipsoid (50%) representation of (a)
(PPP)Ni(COOiPr) (5) and (b) (PPOiPrP)Ni(CO) (6a). Hydrogen
atoms are omitted for clarity.
Figure 2. 31P (left) and 1H (right with simulations in blue) NMR
spectra of (a) (PPP)Ni(OiPr) (4) and (b) (PPOiPrP)Ni(CO) (6a) in
C6D6.
Following this, the possibility of generating a corresponding
nickel(0) species was contemplated. To access such a species,
(PPOiPrP)Ni(CO) (6a) was separately synthesized by carbonylation of {(PPOiPrP)Ni}2(μ-N2) (3). Upon addition of CO(g)
at ambient pressure, the clean conversion of 3 to 6a occurred
within 1 h indicated by a color change from brown to orange.
In the 31P NMR data, peaks at 154.90 ppm (t, Jpp = 59.9 Hz)
and 70.24 ppm (d, Jpp = 59.9 Hz) display features similar to
those of found for 3. The chemical shifts and the coupling
constants in the 31P NMR spectrum are clearly different from
those of a nickel(II) species (PPP)Ni(COOiPr) (5); thus the
nickel(0) species 6a was suggested. In the 1H NMR spectrum
in C6D6, the methine peak of an isopropoxide group appears as
a doublet of septets centered at 4.50 ppm (JPH = 9.5 Hz, JHH =
6.2 Hz) showing a H−P coupling, which is not present in the
spectrum for (PPP)Ni(OiPr) (4) (Figure 2b). This suggests
that an isopropoxide group remains bonded to a central P atom
as a phosphinite moiety. Its X-ray structural data show a POiPr
moiety is bound to a nickel(0) center possessing a CO
coordination with the Ni−C bond distance of 1.745(2) Å,
which is similar for the analogous species, (PPERP)Ni−CO; see
Figure 3b and Table 1.9,10 The C−O bond distance of 1.153(3)
Å and the carbonyl vibration of 1905 cm−1 indicate that backdonation almost identical to that found in (PPOMeP)Ni(CO) is
present in 6a.9
Although a nickel(0) carbonyl species, (PPOiPrP)Ni(CO)
(6a), is accessible, the reaction of (PPP)Ni(OiPr) (4) with
CO(g) shows that MI has occurred instead of reductive metal−
ligand cooperation. According to our DFT computational
results, both isomeric species are close in energy; see the
Supporting Information. MI found in the reaction of 4 is clearly
different from the previous reports on the carbonylation of
(PPP)Ni(OPh) or (PPP)Ni(SAr).9,10 To verify the two
different pathways with an isopropoxide group, alkyl isocyanide
was utilized as an alternative isoelectronic π-acidic ligand.16
can undergo a similar reaction pathway, such as reductive
elimination in the presence of a π-acidic ligand. With successful
isolation of a stable nickel(II) alkoxide species, the mechanistic
pathway of the MLC process occurring at a P−Ni moiety can
be evaluated.
With a stable (PPP)Ni(II)-alkoxide complex in hand, we
further explored the metal ligand cooperation by addition of
various π-acidic ligands. For the first attempt, the reactivity of
(PPP)Ni(OiPr) (4) toward CO was investigated to generate a
corresponding zerovalent nickel monocarbonyl adduct via the
reductive pathway. To a brown solution of 4 in benzene was
added CO(g) at ambient pressure, resulting in the formation of
an unexpected product, (PPP)Ni(COOiPr) (5), obtained with
a high yield of >90%. The reaction was completed within 1 h at
room temperature. The 31P NMR spectrum of 5 in C6D6
exhibits a fairly similar feature as compared to that of 4 showing
a triplet at 98.53 ppm (Jpp = 13.3 Hz) and a doublet at 61.45
ppm (Jpp = 14.6 Hz). The IR spectrum indicates the presence
of an alkoxycarbonyl moiety with a carbonyl vibration at 1614
cm−1 clearly different from that of a nickel(0)-CO species,
suggesting that compound 5 is a nickel(II) species. Its X-ray
structural analysis confirms a distorted square planar geometry
(τ4 = 0.21)13 about a nickel(II) center along with a coordinated
isopropoxycarbonyl ligand (Figure 3a). Two asymmetric C−O
bond distances of 1.208(4) and 1.391(4) Å clearly support the
presence of an alkoxycarbonyl moiety coordinated to a nickel
ion with a Ni−C bond (dNi−C = 1.911(3) Å); see Table 1.
Although MI of CO commonly occurs with group 10 metalalkoxide and alkyl species,15 current CO migratory insertion
occurring with 4 is unlike the reaction of (PPP)Ni(EPh) with
CO(g), which results in the formation of (PPEPhP)Ni(CO) (E
= O or S) via a metal−ligand cooperation pathway.9,10
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isopropoxide versus phenoxide and phenylthiolate groups,23
which needs to be further investigated by detailed kinetic
experiments and computational studies. A π-acidic ligand also
affects the relative reaction rates of the two pathways.22,24
P−O Bond Cleavage via Metal Ligand Cooperation of
{(PPOiPrP)Ni(0)}2(μ-N2). To explore the metal−ligand cooperation involving P−O bond cleavage, the reaction of a
(PPP)Ni(0)-dinitrogen species (3) toward CO2 was compared
to that of (PPP)Ni(OiPr) (4). Upon addition of CO2(g), a dark
brown solution of 4 immediately became red, suggesting that
CO2 insertion at the Ni(II) center has occurred. The product
(PPP)Ni(OCOOiPr) (7) was isolated in a good yield of 92%,
and its C6D6 solution exhibits a triplet at 99.06 ppm (Jpp = 8.9
Hz) and a doublet at 51.06 ppm (Jpp = 9.7 Hz) in the 31P NMR
spectrum. A 13C NMR spectrum of the labeled product
7-13CO2 shows a resonance at 157.0 ppm. The v(CO) of 7
are 1674, 1638, and 1294 cm−1, whereas those for 7-13CO2 are
1630, 1603, and 1269 cm−1 (calculated value: 1637, 1601, and
1265 cm−1). The solid-state structure shows a distorted square
planar geometry (τ4 = 0.16)13 along its nickel center
coordinated by an isopropoxycarbonate moiety (Figure 4b).
A structurally characterized nickel(II) methylcarbonato complex supported by a tetraazacycloalkane ligand was previously
reported by Ito and co-workers as a single example of a nickel−
OCOOMe species.25 A compound with a structure almost
identical to 7, (PPP)Ni−OCOOMe, was recently reported by
our group.9
When CO2(g) was exposed to an acetonitrile/benzene
solution of {(PPOiPrP)Ni}2(μ-N2) (3), the formation of 7
occurred, suggesting the MLC process operates. However, this
reaction is much slower and more complicated than direct
addition of CO2 to 4. Upon addition of CO2, a Ni−CO2 adduct
was formed revealing the 31P signals at 161.39 ppm (t, Jpp =
13.0 Hz) and 55.37 ppm (d, Jpp = 13.0 Hz). This species is
suggested to be (PPOiPrP)Ni(η2-CO2) according to our
previous study reporting (PPOMeP)Ni(η2-CO2)9 and (PPMeP)Ni(η2-CO2).14a This Ni−CO2 product is in equilibrium with 3
close to a 1:1 ratio (see the Supporting Information). However,
when compound 3 is dissolved in acetonitrile/C6D6 (1:4), a
green nickel(0) solvento species was formed exhibiting a set of
a triplet at 113.82 ppm (t, Jpp = 97.2 Hz) and a doublet at 60.32
ppm (d, Jpp = 82.1 Hz) in the 31P NMR spectrum (see the
Supporting Information). To the green acetonitrile/C6D6
solution of 3, CO2(g) was charged resulting in a slow but
quantitative conversion to 7 over 24 h. Thus, it is achievable to
regenerate a (PPP)Ni(II) species by the migration of an
isopropoxide group from a phosphinite moiety via a P−O bond
cleavage.
Treatment of 4 with 2.3 equiv of tert-butyl isocyanide furnished
a corresponding zerovalent nickel complex, (PPOiPrP)Ni(CNtBu) (6b), displaying the 31P NMR peaks at 146.50 ppm
(t, Jpp = 69.7 Hz) and 69.86 ppm (d, Jpp = 69.8 Hz) similar to
those for 6a. From the reaction of 3 and tert-butyl isocyanide,
an identical product was also synthesized and isolated as a dark
red solid in 98% yield.
X-ray-quality single crystals were obtained from a cold
saturated pentane solution of 6b at −35 °C. According to the
XRD data, both zerovalent nickel species, 6a and 6b, share
similar structural features including a pseudotetrahedral nickel
center (τ4 = 0.79 and 0.75, respectively)13 and POiPr−Ni bond
distances of 2.1225(6) and 2.1028(6) Å, respectively; see
Figures 3b and 4a. The P−O bond length of 6b is 1.661(1) Å,
Figure 4. Displacement ellipsoid (50%) representation of (a)
(PPOiPrP)Ni(CNtBu) (6b) and (b) (PPP)Ni(OCOOiPr) (7). Hydrogen atoms are omitted for clarity.
which is also close to that of 6a (1.644(2) Å), consistent with
those in other transition metal monophosphinite complexes
(1.58−1.68) Å.17 A phosphinite donor moiety is widely utilized
in the transition metal coordination chemistry.18 Transformation of an osmium diphenylphosphine complex to the
corresponding phosphinite species was previously reported by
the Esteruelas group.17a It is suggested that a P−O bond can be
formed from the reaction of a phosphide intermediate species
with alcohol. Such additions of methanol and water to a MP
bond were previously known to access such phosphine
coordination.17b In addition, the Müller group reported the
phosphinine metal complexes of Pt, Pd, Rh, and Ir to show
both P−O bond formation and cleavage by nucleophilic
insertion and elimination of MeOH.17c,d
The X-ray structure of 6b shows a pseudo tetrahedral
nickel(0) species with an isocyanide coordination via a Ni−C
bond of 1.815(2) Å; see Figure 4a and Table 1. The C−N bond
distance of 1.170(2) Å and its stretching vibration of 2012 cm−1
shifted from a free tert-butyl isocyanide vibration at 2136 cm−1
suggest that a significant back bonding is present in 6b.19 There
are few examples of structurally characterized mononuclear
nickel(0) complexes with a terminally bound monoisocyanide
ligand.20 In fact, isocyanide can undergo MI at a Ni(II) center
showing the formation of C−C, C−O, and C−N bonds, etc.21
However, as compared to the carbonylation of 4, its reaction
with isocyanide favors a reductive elimination (RE) pathway,
presumably due to its less polarized CN triple bond relative
to that for CO.22 Thus, two pathways are feasible for both
reactions of 4 with CO and isocyanide. Both MI and RE
compete at a nickel(II) center, which needs to be carefully
tuned to obtain a desired product. The difference in reactivity
toward π-acidic ligands, such as CO or CNR, is presumably
originating from a different nucleophilic character of alkoxide,
■
CONCLUSIONS
We present a P−O bond formation/cleavage occurring at a
nickel center supported by a PPP ligand. By utilizing an
isopropoxide group along with a (PPP)Ni scaffold, an unusual
metal−ligand cooperative (MLC) transformation was investigated. In fact, a successful isolation of (PPP)Ni(OiPr) (4)
allowed us to evaluate two feasible pathways, migratory
insertion (MI) versus reductive elimination (RE) with MLC.
Upon addition of CO(g) to 4, a nickel(II) alkoxycarbonyl
species (PPP)Ni(COOiPr) (5) was produced via a MI pathway.
However, in the case of CNtBu, P−O bond formation occurs
nicely coupled with a two-electron reduction of a nickel center,
resulting in the formation of a Ni(0) species, (PPOiPrP)Ni(CNtBu) (6b), via metal−ligand cooperation. Thus, two
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color change to dark brown. The solution was stirred for 1 h, and all
volatiles were removed under vacuum. The resulting residue was
dissolved in 10 mL of pentane and cooled to −35 °C. After the
solution was filtered through Celite and dried under vacuum, a
pentane soluble product (PPP)Ni(OiPr) (4, 324 mg, 0.605 mmol,
55.5%) was isolated as a dark brown solid. The pentane insoluble solid
was washed with additional portions of cold pentane. After being dried
under vacuum, the resulting solid was dissolved in benzene and filtered
through Celite. The product {(PPOiPrP)Ni}2(μ-N2) (3, 98 mg, 0.089
mmol, 16%) was isolated as a dark brown solid after drying under
vacuum. Characterization data for 4: 1H NMR (400 MHz, C6D6, ppm)
δ 7.85−7.83 (m, 2H), 7.09−7.03 (m, 4H), 6.94−6.90 (m, 2H), 4.09
(sep, J = 6.0 Hz, 1H), 2.45−2.36 (m, 4H), 1.68 (d, J = 4.0 Hz, 6H),
1.47 (q, J = 8.0 Hz, 12H), 1.16 (q, J = 6.7 Hz, 12H); 13C NMR (151
MHz, C6D6, ppm) δ 157.9 (dt, J = 31.6, 21.1 Hz, Ar−C), 134.8 (dt, J =
24.1, 21.1 Hz, Ar−C), 131.0 (d, J = 3.0 Hz, Ar−C), 130.4 (s, Ar−C),
129.2 (q, J = 8.5 Hz, Ar−C), 125.0 (t, J = 3.0 Hz, Ar−C), 70.4 (t, J =
3.0 Hz, NiOCH(CH3)2), 31.8 (s, NiOCH(CH3)2), 25.7 (t, J = 9.0 Hz,
CH(CH3)2), 18.9 (s, CH(CH3)2), 18.2 (s, CH(CH3)2); 31P NMR
(162 MHz, C6D6, ppm) δ 101.21 (t, J = 10.5 Hz, 1P), 45.85 (d, J = 9.7
Hz, 2P). UV−vis [THF, nm (L mol−1 cm−1)]: 405 (2900), 530
(1200). IR (KBr pellet, cm−1): vAr = 1568. Anal. Calcd for
C27H43NiOP3: C, 60.59; H, 8.10; N, 0.00. Found: C, 60.25; H, 8.03;
N, 0.02. Characterization data for 3: 1H NMR (400 MHz, C6D6, ppm,
under vacuum) δ 8.25−8.22 (m, 4H), 7.29 (d, J = 8.0 Hz, 4H), 7.19 (s,
4H), 7.08−7.05 (m, 4H), 4.71 (dsep, J = 8.0, 6.0 Hz, 2H), 2.40−2.32
(m, 8H), 1.48 (q, J = 6.7 Hz, 12H), 1.31−1.25 (m, 24H), 1.18 (q, J =
6.7 Hz, 12H), 0.89 (q, J = 6.7 Hz, 12H); 13C NMR (101 MHz, C6D6,
ppm, under vacuum) δ 153.7 (dt, J = 36.2, 22.6 Hz, Ar−C), 142.7 (dt,
J = 48.3, 14.1 Hz, Ar−C), 129.3−129.0 (m, Ar−C), 70.9 (d, J = 16.1
Hz, P−OCH(CH3)2), 28.9 (t, J = 8.6 Hz, P−OCH(CH3)2), 25.3 (d, J
= 4.0 Hz, CH(CH3)2), 24.9 (d, J = 22.1 Hz, CH(CH3)2), 20.9 (s,
CH(CH3)2), 20.5 (t, J = 6.5 Hz, CH(CH3)2), 20.2 (s, CH(CH3)2),
19.1 (s, CH(CH3)2); 31P NMR (162 MHz, C6D6, ppm, under
vacuum) δ 126.78 (t, J = 96.4 Hz, 2P), 59.63 (d, J = 98.8 Hz, 4P).
UV−vis [THF, nm (L mol−1 cm−1), under N2 atmosphere (1 atm)]:
370 (10 000), 500 (2400). IR (KBr pellet, cm−1): vAr = 1462, 1447.
Anal. Calcd for C54H86N2Ni2O2P6: C, 59.04; H, 7.89; N, 2.55. Found:
C, 59.04; H, 7.88; N, 2.21. Dark brown crystals suitable for X-ray
diffraction were obtained by cooling a saturated pentane solution of 3
to −35 °C. Characterization data for (PPOiPrP)Ni(N2) (2): 1H NMR
(400 MHz, C6D6, ppm) δ 8.16 (t, J = 6.0 Hz, 2H), 7.29 (d, J = 8.0 Hz,
2H), 7.13 (s, 2H), 7.08−7.05 (m, 2H), 4.45−4.37 (m, 1H), 2.30−2.20
(m, 4H), 1.41 (q, J = 8.0 Hz, 12H), 1.22 (d, J = 4.0 Hz, 6H), 1.13−
1.08 (m, 12H); 31P NMR (162 MHz, C6D6, ppm) δ 132.47 (t, J = 84.2
Hz, 1P), 61.63 (d, J = 84.2 Hz, 2P).
Synthesis of (PPP)Ni(OiPr) (4). To a solution of 1 (103 mg, 0.201
mmol) in 5 mL of THF was added dropwise a solution of NaOiPr (25
mg, 0.30 mmol) in 3 mL of THF at −35 °C in the drybox filled with
Ar. The reaction mixture was slowly warmed to room temperature and
stirred for 1 h. Volatiles were removed under vacuum, and a resulting
dark brown solid was dissolved in pentane. The solution was filtered
through Celite, and volatiles were removed under vacuum. The
product (PPP)Ni(OiPr) (4, 72 mg, 0.13 mmol, 67%) was isolated as a
dark brown solid after washing with cold pentane and drying under
vacuum. Analytically pure compound was obtained from the
recrystallization of the concentrated pentane solution of 4 at −35
°C. The identity of the product 4 was confirmed by comparing its 1H
and 31P NMR data.
Synthesis of (PPP)Ni(COOiPr) (5). In a 25 mL Schlenk tube, the
solution of 4 (118 mg, 0.221 mmol) in 10 mL of benzene was
degassed by three freeze−pump−thaw cycles on the Schlenk line and
then exposed to CO(g) at ambient pressure. The reaction mixture was
stirred for 1 h at room temperature, causing a color change from
brown to orange brown. After solution was filtered through Celite, the
volatiles were removed under vacuum. The resulting solid was then
washed with pentane and dried under vacuum. The product
(PPP)Ni(COOiPr) (5, 113 mg, 0.201 mmol, 90.8%) was isolated as
an orange solid. 1H NMR (600 MHz, C6D6, ppm) δ 7.96 (d, J = 6.0
Hz, 2H), 7.15 (t, J = 12.0 Hz, 2H), 7.08−7.07 (m, 2H), 6.92 (d, J = 6.0
pathways compete at a nickel(II) center supported by a PPP
ligand. The difference in reactivity toward a π-acidic ligand
presumably originates from the nucleophilicity of an ER group
(E = O or S, R = aryl or alkyl) and the polarization of a π-acidic
ligand. Tuning the nucleophilicity and employing a different πacidic ligand thus help to guide the reaction pathway. Detailed
kinetic experiments and computational studies are currently
under investigation.
■
EXPERIMENTAL SECTION
General Considerations. All manipulations were carried out using
standard Schlenk or glovebox techniques under a N2 or Ar
atmosphere. Unless otherwise noted, solvents were deoxygenated
and dried by thoroughly sparging with Ar gas followed by passage
through an activated alumina column. Nonhalogenated solvents were
tested with a standard purple solution of sodium benzophenone ketyl
in tetrahydrofuran to confirm effective oxygen and moisture removal.
All reagents were purchased from commercial vendors and used
without further purification unless otherwise stated. PPPCl8 and
(PPP)NiCl (1)26 were prepared according to the literature procedures.
Elemental analyses were carried out at KAIST Research Analysis
Center on a Thermo Scientific FLASH 2000 series instrument.
Deuterated solvents were purchased from Cambridge Isotope
Laboratories, Inc., or Euriso-top, degassed, and dried over activated
4 Å molecular sieves prior to use.
X-ray Crystallography. The diffraction data of 3, 5, and 6a were
collected on a Bruker SMART 1000. The diffraction data of 6b and 7
were collected on a Bruker D8 QUEST. A suitable size and quality of
crystal was coated with Paratone-N oil and mounted on a MiTeGen
MicroLoop. The data were collected with graphite-monochromated
Mo Kα radiation (λ = 0.71073 Å) at 120 K. Cell parameters were
determined and refined by SMART program.27 Data reduction was
performed using SAINT software.28 An empirical absorption
correction was applied using the SADABS program.29 The structures
were solved by direct methods, and all non-hydrogen atoms were
subjected to anisotropic refinement by full-matrix least-squares on F2
by using the SHELXTL/PC package.30 Unless otherwise noted,
hydrogen atoms were placed at their geometrically calculated positions
and refined riding on the corresponding carbon atoms with isotropic
thermal parameters. Crystal data and details of refinements are given in
the Supporting Information.
Spectroscopic Measurements. A Bruker AVHD-400 spectrometer and an Agilent Technologies DD2 600 spectrometer were used to
measure 1H NMR spectra. The chemical shifts for 1H NMR spectra
are quoted in parts per million (ppm) referenced to residual solvent
peaks. Coupling constants, J, are reported in hertz unit (Hz). 13C
NMR spectra were recorded on a Bruker AVHD-400 spectrometer
and an Agilent Technologies DD2 600 spectrometer. 13C NMR
chemical shifts are quoted in parts per million (ppm) referenced to
internal solvent peaks. 31P NMR spectra were recorded on a Bruker
400 spectrometer and were decoupled by broad band proton
decoupling. The chemical shifts for 31P NMR spectra are quoted in
parts per million (ppm) referenced to external phosphoric acid. The
following abbreviations are used to describe peak splitting patterns
when appropriate: s = singlet, d = doublet, t = triplet, q = quartet, sex
= sextet, sep = septet, m = multiplet, dd = doublet of doublet, dt =
doublet of triplet, dsep = doublet of septet, ddd = doublet of doublet
of doublet, ddt = doublet of doublet of triplet, td = triplet of doublet,
sepd = septet of doublet. UV−vis spectra were measured by an Agilent
Cary 60 UV−vis spectrophotometer using a 1 cm two-window quartz
spectrophotometer cell sealed with a screw-cap purchased from
Hellma Analytics (117.100-QS). Infrared spectra were recorded in KBr
pellets by Bruker VECTOR 33. Frequencies are given in reciprocal
centimeters (cm−1), and only selected absorbances are reported.
Synthesis of {(PPOiPrP)Ni}2(μ-N2) (3) and (PPP)Ni(OiPr) (4). To
a violet solution of 1 (559 mg, 1.09 mmol) in 30 mL of THF was
added a suspension dropwise of NaOiPr (140 mg, 1.72 mmol) in 10
mL of THF at −35 °C in the drybox filled with N2. The reaction
mixture was gradually warmed to room temperature, causing a slow
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Hz, 2H), 5.46 (sep, J = 6.0 Hz, 1H), 2z), 2.55−2.51 (m, 4H), 1.33−
1.30 (m, 18H), 1.08 (q, J = 8.0 Hz, 12H); 13C NMR (151 MHz, C6D6,
ppm) δ 208.8 (dt, J = 48.0, 25.5 Hz, Ni−C), 157.3 (dt, J = 27.1, 23.3
Hz, Ar−C), 135.5 (dt, J = 30.1, 21.1 Hz, Ar−C), 131.0 (d, J = 4.5 Hz,
Ar−C), 129.8 (q, J = 10.0 Hz, Ar−C), 124.6 (s, Ar−C), 62.2 (s,
NiCOOCH(CH3)2), 25.7 (t, J = 12.1 Hz, CH(CH3)2), 23.5 (s,
NiCOOCH(CH3)2), 19.0 (s, CH(CH3)2), 18.2 (s, CH(CH3)2); 31P
NMR (243 MHz, C6D6, ppm) δ 98.53 (t, J = 13.3 Hz, 1P), 61.45 (d, J
= 14.6 Hz, 2P). UV−vis [THF, nm (L mol−1 cm−1)]: 300 (8100), 345
(8200), 465 (1700). IR (KBr pellet, cm−1): vCO = 1614, vAr = 1566.
Anal. Calcd for C28H43NiO2P3: C, 59.71; H, 7.70; N, 0.00. Found: C,
59.84; H, 7.65; N, 0.01. Orange crystals suitable for X-ray diffraction
were obtained by cooling a saturated pentane/toluene solution of 5 to
−35 °C.
Synthesis of (PPOiPrP)Ni(CO) (6a). In a 25 mL Schlenk tube was
degassed the solution of 3 (99 mg, 0.090 mmol) in 8 mL of THF by
three freeze−pump−thaw cycles on the Schlenk line, which was then
exposed to CO(g) at ambient pressure. The reaction mixture was
stirred for 1 h at room temperature, causing a color change from
brown to orange brown. After all volatiles were removed by vacuum,
the resulting solid was dissolved in benzene, and the solution was
filtered through Celite. The product (PPOiPrP)Ni(CO) (6a, 92 mg,
0.16 mmol, 91%) was isolated as an orange solid after drying under
vacuum. Analytically pure compound was obtained from the
recrystallization of the concentrated pentane solution of 6a at −35
°C. 1H NMR (400 MHz, C6D6, ppm) δ 8.12 (t, J = 6.0 Hz, 2H), 7.29
(t, J = 4.0 Hz, 2H), 7.15 (t, J = 8.0 Hz, 2H), 7.03 (t, J = 8.0 Hz, 2H),
4.50 (dsep, J = 8.0, 6.0 Hz, 1H), 2.26 (sex, J = 4.8 Hz, 2H), 2.14 (sex, J
= 4.8 Hz, 2H), 1.40 (q, J = 8.0 Hz, 6H), 1.16 (d, J = 8.0 Hz, 6H), 1.06
(q, J = 6.7 Hz, 6H), 0.97−0.93 (m, 12H); 13C NMR (151 MHz, C6D6,
ppm) δ 207.4 (q, J = 5.5 Hz, Ni−C), 152.6−152.0 (m, Ar−C), 144.9
(ddt, J = 49.7, 27.1, 9.0 Hz, Ar−C), 129.3 (d, J = 4.5 Hz, Ar−C), 129.2
(d, J = 13.6 Hz, Ar−C), 129.1 (t, J = 7.5 Hz, Ar−C), 128.8 (d, J = 1.5
Hz, Ar−C), 128.6 (s, Ar−C), 128.4 (s, Ar−C), 72.0 (d, J = 12.1 Hz,
POCH(CH3)2), 28.6 (td, J = 12.1, 3.0 Hz, CH(CH3)2), 26.5 (dt, J =
16.6, 5.3 Hz, CH(CH3)2), 24.6 (d, J = 6.0 Hz, CH(CH3)2), 20.6 (t, J =
3.0 Hz, CH(CH3)2), 20.0 (t, J = 3.8 Hz, CH(CH3)2), 19.7 (t, J = 5.3
Hz, CH(CH3)2), 19.1 (t, J = 3.0 Hz, CH(CH3)2); 31P NMR (162
MHz, C6D6, ppm) δ 154.90 (t, J = 59.9 Hz, 1P), 70.24 (d, J = 59.9 Hz,
2P). UV−vis [THF, nm (L mol−1 cm−1)]: 310 (5400), 450 (1200). IR
(KBr pellet, cm−1): vCO = 1905, vAr = 1460, 1448. Anal. Calcd for
C28H43NiO2P3: C, 59.71; H, 7.70; N, 0.00. Found: C, 60.08; H, 7.72;
N, 0.03. Orange crystals suitable for X-ray diffraction were obtained by
cooling a saturated pentane solution of 6a to −35 °C.
Synthesis of (PPOiPrP)Ni(CNtBu) (6b). Method A: To a solution
of 3 (56 mg, 0.051 mmol) in 5 mL of benzene was added CNtBu (15
μL, 0.13 mmol) dropwise with an immediate color change from dark
brown to red, and the solution was stirred at room temperature for 1 h.
After the reaction mixture was filtered through Celite, all volatiles were
removed under vacuum. The product (PPOiPrP)Ni(CNtBu) (6b, 61
mg, 0.099 mmol, 98%) was isolated as a reddish brown solid after
drying under vacuum. Analytically pure compound was obtained from
the recrystallization of the concentrated pentane solution of 6b at −35
°C. Method B: In an NMR tube equipped with a J-Young valve,
CNtBu (5 μL, 0.044 mmol) and triphenylphosphine oxide (10 mg,
0.036 mmol, internal standard) were added to a solution 4 (10 mg,
0.019 mmol) in 0.5 mL of C6D6. An NMR tube was taken out of the
drybox and shaken for 10 min. The yield of the resulting product,
(PPOiPrP)Ni(CNtBu) (6b, 84%), was determined by integration of the
corresponding signal in the 31P NMR spectrum. 1H NMR (400 MHz,
C6D6, ppm) δ 8.27 (t, J = 4.0 Hz, 2H), 7.40 (d, J = 4.0 Hz, 2H), 7.20
(t, J = 6.0 Hz, 2H), 7.10 (t, J = 8.0 Hz, 2H), 4.57−4.47 (m, 1H), 2.36
(sex, J = 7.2 Hz, 2H), 2.27 (sex, J = 8.0 Hz, 2H), 1.50 (q, J = 8.0 Hz,
6H), 1.28−1.12 (m, 6H), 1.21 (s, 9H), 1.19−1.12 (m, 12H), 0.98 (q, J
= 6.7 Hz, 6H); 13C NMR (101 MHz, C6D6, ppm) δ 172.0 (s, Ni−C),
153.6 (dt, J = 34.2, 22.6 Hz, Ar−C), 146.3 (ddd, J = 52.1, 12.1, 15.1
Hz, Ar−C), 128.9−128.8 (m, Ar−C), 128.6 (s, Ar−C), 70.0 (d, J =
15.1 Hz, P−OCH(CH3)2), 54.3 (s, Ni−CNC(CH3)3), 31.2 (s, Ni−
CNC(CH3)3), 29.7 (td, J = 11.1, 3.0 Hz, P−OCH(CH3)2), 26.7 (dt, J
= 20.1, 3.0 Hz, CH(CH3)2), 24.8 (d, J = 4.0 Hz, CH(CH3)2), 20.8 (t, J
= 3.5 Hz, CH(CH3)2), 20.7 (t, J = 5.5 Hz, CH(CH3)2), 20.2 (td, J =
5.5, 2.0 Hz, CH(CH3)2), 19.6 (t, J = 3.0 Hz, CH(CH3)2); 31P NMR
(162 MHz, C6D6, ppm) δ 146.51 (t, J = 69.7 Hz, 1P), 70.02 (d, J =
69.7 Hz, 2P). UV−vis [THF, nm (L mol−1 cm−1)]: 330 (sh, 3000),
520 (520). IR (KBr pellet, cm−1): vCN = 2012, vAr = 1460, 1445, 1426.
Anal. Calcd for C32H52NNiOP3: C, 62.15; H, 8.48; N, 2.27. Found: C,
62.10; H, 8.57; N, 2.26. Dark red crystals suitable for X-ray diffraction
were obtained by cooling a saturated pentane solution of 6b to −35
°C.
Synthesis of (PPP)Ni(OCOOiPr) (7). In a 25 mL Schlenk tube was
degassed the solution of 4 (62 mg, 0.12 mmol) in 10 mL of benzene
by three freeze−pump−thaw cycles on the Schlenk line, and then it
was exposed to CO2(g) at ambient pressure. The reaction mixture was
stirred for 1 h at room temperature, causing a color change from
brown to red. After the solution was filtered through Celite, all
volatiles were removed under vacuum. The product (PPP)Ni(OCOOiPr) (7, 62 mg, 0.11 mmol, 92%) was isolated as a red solid
after washing with pentane and drying under vacuum. 1H NMR (400
MHz, C6D6, ppm) δ 7.77 (d, J = 4.0 Hz, 2H), 7.06 (t, J = 8.0 Hz, 2H),
6.97−6.95 (m, 2H), 6.90 (t, J = 8.0 Hz, 2H), 5.18 (sep, J = 6.0 Hz,
1H), 2.45−2.36 (m, 4H), 1.46 (q, J = 8.0 Hz, 12H), 1.36 (d, J = 4.0
Hz, 6H), 1.09 (q, J = 7.0 Hz, 12H); 13C NMR (151 MHz, C6D6, ppm)
δ 157.3 (s, NiOCOOCH(CH3)2), 157.0 (dt, J = 30.2, 21.1 Hz, Ar−C),
133.1 (q, J = 22.7 Hz, Ar−C), 130.8 (d, J = 43.8 Hz, Ar−C), 129.2 (dt,
J = 10.6, 7.6 Hz, Ar−C), 128.4 (s, Ar−C), 125.5 (t, J = 2.3 Hz Ar−C),
124.7 (s, CO2), 67.7 (s, NiOCOOCH(CH3)2), 25.1 (s, CH(CH3)2),
23.1 (s, NiOCOOCH(CH3)2), 18.8 (s, CH(CH3)2), 18.0 (s,
CH(CH3)2); 31P NMR (162 MHz, C6D6, ppm) δ 99.06 (t, J = 8.9
Hz, 1P), 51.06 (d, J = 9.7 Hz, 2P). UV−vis [THF, nm (L mol−1
cm−1)]: 313 (7500), 400 (sh, 2100), 520 (1200). IR (KBr pellet,
cm−1): v(RCO2−) = 1674, 1638, 1294, vAr = 1566. Anal. Calcd for
C28H43NiO3P3: C, 58.06; H, 7.48; N, 0.00. Found: C, 58.04; H, 7.45;
N, 0.01. Red crystals suitable for X-ray diffraction were obtained by
cooling a saturated diethyl ether/toluene solution of 7 to −35 °C.
Synthesis of (PPP)Ni(O13COOiPr) (7-13CO2). In an NMR tube
with a J-Young valve, the brown solution of 4 (10 mg, 0.020 mmol) in
0.5 mL of C6D6 was taken out of the drybox and degassed by three
freeze−pump−thaw cycles on the Schlenk line. After 13CO2 gas was
added at ambient pressure, the solution was shaken for 1 h at room
temperature, causing an immediate color change from brown to red.
All volatiles were removed by vacuum, and the resulting product
(PPP)Ni(O13COOiPr) (7-13CO2, 11 mg, 0.020 mmol, >99%) was
isolated as a red solid without a further purification process.
Spectroscopic features in the 31P NMR spectrum were identical to
those for 7. 1H NMR (400 MHz, C6D6, ppm) δ 7.78 (dd, J = 8.0, 4.0
Hz, 2H), 7.06 (t, J = 8.0 Hz, 2H), 6.97−6.88 (m, 4H), 5.18 (sepd, J =
6.0, 4.0 Hz, 3H), 2.45−2.36 (m, 4H), 1.47 (q, J = 8.0 Hz, 12H), 1.37
(d, J = 8.0 Hz, 6H), 1.10 (q, J = 6.7 Hz, 12H); 13C NMR (101 MHz,
C6D6, ppm) δ 157.0 (d, J = 2.0 Hz, NiO13COOCH(CH3)2); 31P NMR
(162 MHz, C6D6, ppm) δ 99.06 (t, J = 8.9 Hz, 1P), 51.07 (d, J = 9.7
Hz, 2P). IR (KBr pellet, cm−1): v(R13CO2−) = 1630, 1603, 1269.
Reaction of {(PPOiPrP)Ni}2(μ-N2) (3) with CO2. In an NMR tube
equipped with a J-Young valve, the dark brown solution of 3 (5 mg,
0.005 mmol) in 0.5 mL of CH3CN/C6D6 (1:4) was taken out of the
drybox and degassed by three freeze−pump−thaw cycles on the
Schlenk line. After CO2(g) was added, the reaction mixture was shaken
at room temperature. The reaction was monitored for 24 h by 31P
NMR spectroscopy.
■
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02226.
Figures, tables, and CIF files giving characterization data
for 2, 3, 4, 5, 6a, 6b, and 7 (PDF)
X-ray crystallographic data for compound 3 (CIF)
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■
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work has been supported by C1 Gas Refinery Program
through the National Research Foundation of Korea (NRF
2015M3D3A1A01064880) and TJ Park Fellowship of POSCO
TJ Park Foundation. This work was also supported by the
Supercomputer Center/Korea Institute of Science and
Technology (KSC-2015-S1-0005).
■
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