Mechanistic Study on C-C Bond Formation of a Nickel(I) Monocarbonyl Species with Alkyl Iodides: Experimental and Computational Investigations

Article abstract of DOI:10.1021/acs.organomet.5b00548

An open-shell reaction of the nickel(I) carbonyl species (PNP)Ni-CO (1) with iodoalkanes has been explored experimentally and theoretically. The initial iodine radical abstraction by a nickel(I) carbonyl species was suggested to produce (PNP)Ni-I (4) and

Full text of DOI:10.1021/acs.organomet.5b00548

Article
pubs.acs.org/Organometallics
Mechanistic Study on C−C Bond Formation of a Nickel(I)
Monocarbonyl Species with Alkyl Iodides: Experimental and
Computational Investigations
Changho Yoo,† Manjaly J. Ajitha,‡ Yousung Jung,*,†,‡ and Yunho Lee*,†
†Department of Chemistry and ‡Graduate School of EEWS, Korea Advanced Institute of Science and Technology, Daejeon 305-701,
Republic of Korea
S
* Supporting Information
ABSTRACT: An open-shell reaction of the nickel(I) carbonyl
species (PNP)Ni-CO (1) with iodoalkanes has been explored
experimentally and theoretically. The initial iodine radical
abstraction by a nickel(I) carbonyl species was suggested to
produce (PNP)Ni-I (4) and the concomitant alkyl radical,
according to a series of experimental indications involving
stoichiometric controls employing iodoalkanes. Corresponding
alkyl radical generation was also confirmed by radical trapping experiments using Gomberg’s dimer. Molecular modeling supports
that the nickel acyl species (PNP)Ni-COCH3 (2) can be formed by a direct C−C bond formation between a carbonyl ligand of 1
and a methyl radical. As an alternative pathway, the five-coordinate intermediate species (PNP)Ni(CO)(CH3) (5) that involves
both CO and CH3 binding at a nickel(II) center is also suggested with a comparable activation barrier, although this pathway
energetically favors the formation of (PNP)Ni-CH3 (3) via a barrierless elimination of CO over a CO migratory insertion. Thus,
our present work supports that the direct C−C bond coupling occurs between an alkyl radical and the carbonyl ligand at a
monovalent nickel center in the generation of an acyl product.
Scheme 1
INTRODUCTION
■
Monovalent nickel chemistry is drawing much attention in both
organometallic and bioorganometallic chemistry.1−3 With aryl
and alkyl halides, the nickel(I/III) mechanism was recently
proposed for nickel-catalyzed cross-coupling reactions, distinc-
tive from a well-known Ni(0) to Ni(II) cycle.2 Carbon
monoxide activation mediated by nickel(I) is also crucial in
the reaction of the biological acetyl-CoA synthase (ACS), in
which a nickel(I) carbonyl species is proposed as an active
intermediate according to the paramagnetic mechanism.3
Although an open-shell nickel(I) carbonyl intermediate was
proposed in this C−C bond formation, its reactivity is currently
unexplored. While a radical carbonylation of free CO with an
alkyl radical is well established,4 the open-shell C−C bond
formation of transition-metal-coordinated CO is uncommon.5
We have recently reported the reaction of a nickel(I)
carbonyl complex (PNP)Ni-CO (1) supported by a PNP
scopic analysis and density functional theory (DFT) studies,
those redox processes involved in the reaction originate not
from a metal center but from noninnocent ligands. Thus, the
alkyl radical formation in those systems occurs via a ligand-
based electron transfer process.7a,c,d To date, reports of
reactions involving genuine nickel(I) species with alkyl halide
are fairly limited.8
In addition, the formation of two methylated products,
(PNP)Ni-COCH3 (2) and (PNP)Ni-CH3 (3), raises a
question regarding their reaction pathways. The five-coordinate
alkyl metal carbonyl species (PNP)Ni(CO)(CH3) (5) can be
anticipated as a reaction intermediate (Scheme 2). However,
we cannot rule out direct C−C bond formation between a
methyl radical and a CO ligand in the formation of a nickel
acetyl species (Scheme 2). While direct alkylation at a
coordinated CO was recently proposed, there have been only
a few literature precedents.5,9
ligand (PNP− = N[2-PiPr2-4-Me-C6H3]2 ) with iodomethane,
−
revealing the formation of the nickel(II) acetyl species
(PNP)Ni-COCH3 (2) along with (PNP)Ni-CH3 (3) and
(PNP)Ni-I (4) (Scheme 1).6 We postulated that a nickel(I)
carbonyl species initiates the reaction through iodine radical
abstraction with the concomitant formation of a corresponding
methyl radical. This reaction is evocative of C−X bond
activation mediated by nickel(I) species; such transformations
are proposed in nickel cross-coupling reactions.2,7 The initial
C−X bond activation mediated by nickel(I) was corroborated
with nickel(I) complexes supported by pyridine-based ligands.7
According to electron paramagnetic resonance (EPR) spectro-
Received: June 23, 2015
© XXXX American Chemical Society
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Scheme 2
Table 1. Reaction of (PNP)Ni-CO (1) with Alkyl Iodides10
yield (%)
RI
amt (equiv)
Ni-I (4)
Ni-COR
Ni-R
tBuI
tBuI
MeI
MeI
CF3I
1
100
1
53
73
50
50
64
47
27
39
11
11
19
100
25
39
1811
reaction dominantly favors iodine radical abstraction at the first
stage followed by the production of 6 via radical coupling
between 1 and a tert-butyl radical (Scheme 3). The generation
Herein, we report the detailed reaction mechanism of
(PNP)Ni-CO (1) with iodoalkanes on the basis of both
experimental and theoretical investigations. The initial alkyl
radical generation was demonstrated by stoichiometric control
experiments and resulting product analysis, which were well
supported by the results of DFT calculations. The alkyl radical
generation was further confirmed by radical trapping experi-
ments. Theoretical evaluations on both the methylated
products (PNP)Ni-COCH3 (2) and (PNP)Ni-CH3 (3) and
the possible five-coordinate intermediate (PNP)Ni(CO)(Me)
(5) suggest that a direct C−C bond formation occurs between
a methyl radical and a coordinated CO ligand.
Scheme 3
t
of 6 (27%), even with a large excess of BuI, suggests that the
radical coupling reaction between a tert-butyl radical and a
carbonyl ligand of 1 is more rapid than either (i) the
dimerization of tert-butyl radicals or (ii) the reaction of 1
RESULTS AND DISCUSSION
■
t
with BuI.
We recently reported nickel(II/I/0) monocarbonyl species
supported by an anionic tridentate PNP ligand and their
reactivity toward iodoalkanes.6 While the divalent nickel
monocarbonyl species {(PNP)Ni-CO}{BF4} does not show
any reactivity toward MeI, the zerovalent congener (PNP)-
Ni0(CO)Na undergoes oxidative addition and CO elimination,
resulting in the formation of a nickel(II) methyl species.6a
Interestingly, (PNP)Ni-CO (1) reveals its unique capability to
produce a nickel(II) acetyl species via C−C bond coupling,
according to the reaction product distribution: Ni-COCH3 (2),
39%; Ni-CH3 (3), 11%; Ni−I (4), 50% (Scheme 1). This is
comparable to those of reactions with corresponding zerovalent
and divalent congeners. In fact, the closed-shell reaction of 1
with MeOTf (OTf = trifluoromethanesulfonate) results in no
product formation. Therefore, the corresponding acyl for-
mation at the monovalent nickel center presumably occurs
through a radical pathway, rather than the typical SN2 type
reaction.6a
According to our DFT calculations, the activation barrier
(ΔG⧧, the energy difference between the transition state and
the corresponding prereaction complex) for the formation of
(PNP)Ni-I (4) by the homolytic cleavage of tert-butyl iodide
(via TS1tBu) is 12.3 kcal/mol, whereas that of (PNP)Ni-COtBu
(6) (via TS2tBu) is 19.2 kcal/mol (Figure 1). The lower
At the initial stage of a radical pathway occurring at a
monovalent nickel center, two possible routes can be suggested:
initial iodine radical abstraction by nickel(I) followed by
concomitant generation of an alkyl radical and initial alkyl
radical abstraction with iodine radical generation. To evaluate
these two routes and the role of a monovalent nickel ion, we
have pursued both experimental and theoretical investigations
involving the reaction of alkyl iodides. Since the reaction of
(PNP)Ni-CO (1) with 1 equiv of tert-butyl iodide reveals the
formation of (PNP)Ni-I (4; 53%) and (PNP)Ni-COtBu (6;
47%) (Table 1), iodine radical abstraction might be favorable
over tert-butyl radical abstraction due to its steric effect. In
order to test this hypothesis, we have conducted the same
reaction using excess alkyl iodides, where the exclusive
Figure 1. Optimized geometries of the two transition states TS1tBu and
TS2tBu. Distances are given in Å. ΔG⧧ values are given in kcal/mol.
Color code: black, C; blue, N; green, P; yellow, Ni; pink, I; red, O.
Hydrogen atoms are omitted for clarity.
activation barrier of TS1tBu in comparison to that of TS2tBu
indicates that the formation of 4 occurs prior to the tert-butyl
radical abstraction, in agreement with the experimental
observation. The calculated spin density of 1 indicates a 67%
contribution at a nickel center; most of the remaining amount
is equally shared between the atoms directly attached to a
nickel center (Supporting Information). In TS1tBu, however, the
spin density was dramatically decreased at the metal center
(21%) with a significant increment of the tert-butyl radical
character (75%). The five-coordinate intermediate species
t
production of 4 is expected. To a THF solution of BuI (100
equiv) was added a dark green solution of 1 at room
temperature, resulting in an immediate color change to yellow,
revealing the higher yield of the formation of 4 (73%) (Table 1
and the Experimental Section). This result indicates that the
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(PNP)Ni(CO)(I), formed from the initial iodine radical
abstraction, undergoes a fast CO elimination reaction to yield
4. In the case of the second reaction pathway, since TS2tBu
possessing a direct C−C bond interaction between a CO ligand
and tert-butyl iodide requires ∼7 kcal/mol higher energy than
TS1tBu, the slower reaction is expected. Direct coupling of the
tert-butyl radical with a nickel center may also be considered as
an alternative pathway, but calculation of this species
(PNP)Ni(CO)(tBu) was not able to be carried out and always
yielded 6, presumably due to the steric bulkiness of a tert-butyl
group.
However, the energy difference between iodine radical vs
methyl radical abstraction is smaller (4.4 kcal/mol) than that
found in the tert-butyl case (6.9 kcal/mol) due to the relative
stability of the two organic radicals.
In order to provide evidence for the initial alkyl radical
generation, we attempted to trap a radical species by employing
Gomberg’s dimer, (CPh3)2, in which a trityl radical (Ph3C•)
can be generated by equilibrium dissociation of the dimer in the
solution state.12 In the presence of Gomberg’s dimer,
t
(PNP)Ni-CO (1) was treated with BuI, revealing dramatic
changes in the product formation: 91% of 4 and 9% of 6 (Table
2 and the Experimental Section). Control experiments
To compare the result obtained from the reaction of
t
(PNP)Ni-CO (1) with BuI, we also pursued studies with
methyl iodide. The different alkyl radical species (methyl vs
tert-butyl) impart a steric factor that might affect the reaction
pathway. In order to discriminate between the two different
reaction routes, iodine radical abstraction vs methyl radical
abstraction, we conducted the same experimental and
theoretical evaluations on the reaction of (PNP)Ni-CO (1)
and MeI. Unlike the reactions with tBuI, the yields of (PNP)Ni-
I (4) consistently stood at ∼50% from the reactions of
(PNP)Ni-CO (1) with both stoichiometric and excess (100
equiv) amounts of MeI (Table 1 and the Experimental
Section). This might be due to the instability of a methyl
radical, resulting in an instant radical coupling reaction with 1.
To evaluate the electronic consequence with a minimum steric
effect, an electron-deficient methyl iodide was utilized. The
reaction of 1 with CF3I (25 equiv) resulted in the increased
formation of 4 (64% yield) (Table 1 and the Experimental
Section), suggesting the effect of a relatively stable CF3
radical.11
Table 2. Reaction of (PNP)Ni-CO (1) with 1 Equiv of Alkyl
Iodides in the Presence of Gomberg’s Dimer
yield (%)
RI
amt of (CPh3)2 (equiv) temp (°C) Ni-I (4) Ni-COR Ni-R
tBuI
MeI
MeI
MeI
0.5
0.5
0.5
5
room temp
room temp
−35
91
60
60
77
9
30
29
17
10
11
6
room temp
t
confirmed that Gomberg’s dimer does not react with 1, BuI,
or MeI under the same reaction conditions. It seems that the
tert-butyl radical was trapped by a trityl radical, resulting in the
formation of Ph3CtBu rather than reaction with 1 (Scheme 4).
The identity of Ph3CtBu isolated via silica gel column
chromatography (38% yield) (see the Experimental Section)
1
was confirmed by H NMR spectroscopic data.
Since the generation of an iodide radical still cannot be
excluded through this additional experimental evidence, we also
conducted DFT calculations on the reaction of (PNP)Ni-CO
(1) with MeI. Similar to the result of the reaction with tBuI, the
iodine radical abstraction via the homolytic cleavage of MeI also
occurs with a lower activation barrier (11.6 kcal/mol) via TS1Me
in comparison to that for TS2Me (ΔG⧧ = 16.0 kcal/mol), in
which the carbonyl carbon of 1 is found to directly interact with
the carbon atom of MeI (Figure 2, vide infra). In TS1Me, 98% of
the spin density occurs at a departing methyl moiety,
confirming the radical nature of a leaving CH3 group.
Therefore, the theoretical evaluation also suggests that iodine
radical abstraction occurs prior to methyl radical generation.
Scheme 4
To confirm the existence of the corresponding methyl radical
generation, the reaction of (PNP)Ni-CO (1) with MeI was also
conducted in the presence of Gomberg’s dimer. To a mixture of
1 and 0.5 equiv of (CPh3)2·(pentane) was added 1 equiv of
MeI, and the mixture was stirred for 10 min at room
temperature. The resulting solution possessed three species,
(PNP)Ni-COCH3 (2; 30%), (PNP)Ni-CH3 (3; 10%), and
(PNP)Ni-I (4; 60%); the corresponding yields were established
1
by H and 31P NMR data (Table 2). The effect of Gomberg’s
t
dimer was not as significant as the case of the BuI reaction,
presumably due to the instability of a methyl radical. Reducing
the reaction temperature (−35 °C) turned out to give
insignificant trapping of a methyl radical. However, in the
presence of 5 equiv of Gomberg’s dimer, the yield of 4
increased to 77% (Table 2 and the Experimental Section).
Ph3CMe was isolated by column chromatography in 32% yield,
and its identity was confirmed by 1H NMR analysis. The results
obtained from the radical trapping experiments with Gomberg’s
dimer clearly reveal the preference of initial methyl radical
generation over iodine radical production.
Figure 2. Optimized geometries of the two transition states TS1Me and
TS2Me. Distances are given in Å. ΔG‡ values are given in kcal/mol.
Color code: black, C; blue, N; green, P; yellow, Ni; pink, I; red, O.
Hydrogen atoms are omitted for clarity.
The nickel(I)-mediated alkyl radical generation from
iodoalkane was initially proposed for the mechanism of
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Figure 3. Free energy (kcal/mol) profiles of different pathways from (PNP)Ni-CO (1) via direct Ni−C bond coupling toward the formation of
(PNP)Ni-COCH3 (2) and (PNP)Ni-CH3 (3) through the five-coordinate intermediate (PNP)Ni(CO)(CH3) (int, 5) and via direct C−C bond
coupling as an alternative route to produce 2. Distances are given in Å. Color code: black, C; blue, N; green, P; yellow, Ni; pink, I; red, O. Hydrogen
atoms are omitted for clarity.
nickel-catalyzed cross-coupling reactions.2,7 Vicic and co-
workers isolated the nickel(I) methyl species (typ)Ni(CH3)
with a terpyridine ligand and examined its reaction with
cyclohexyl iodide. The formation of a corresponding monoiodo
nickel(I) complex with methylcyclohexane suggested the
migration is unlikely to occur with 5, since the CO insertion
into a Ni−C bond in (PNP)Ni-CH3 (3) requires a higher
activation barrier. According to our experimental results
obtained from the reaction of 3 with CO, the formation of
the corresponding acetyl species 2 via migratory insertion
requires a much longer reaction time (40 h) at room
temperature, while the acyl formation from 1 with MeI takes
less than 1 min under the same reaction conditions. In fact, the
reaction of the corresponding zerovalent nickel carbonyl
species (PNP)Ni(CO)Na with MeI, where the formation of
the same five-coordinate intermediate is expected via oxidative
addition, resulted in the clean formation of a nickel methyl
species (3).6a Therefore, the possibility of another pathway is
engendered, such as direct C−C bond coupling.
According to DFT calculations, the five-coordinate inter-
mediate (PNP)Ni(CO)(CH3) (5) generated by Ni−C bond
formation can undergo CO migratory insertion with an
activation barrier of 12.8 kcal/mol (TS5Me; Figure 3).
Alternatively, CO elimination occurs to generate a nickel
methyl product (3) via a barrierless CO elimination process
(TS4Me, ΔG⧧ = < 1 kcal/mol; Figure 3). This clearly indicates
that the initial Ni−C bond formation, followed by CO
elimination to produce (PNP)Ni-CH3 (3), is energetically
favorable over CO migration. In fact, the yield of 2 increases
with elevated temperature, indicating that a portion of 5
overcomes the activation barrier to reveal limited, but
reasonable, CO migratory insertion (Table 3). Although the
contribution of 5 to the formation of 2 cannot be neglected,
and interestingly the yield of 2 is always higher than that of 3,
the formation of (PNP)Ni-COCH3 (2) might require another
pathway bypassing the formation of 5.
nickel(I)-mediated reaction pathway.7e Car
́
denas and co-
workers conducted the nickel-catalyzed cross-coupling reaction
of iodoalkanes with alkyl zinc halides utilizing a pybox-type
ligand. In fact, the coupling reactions of several radical clocks,
revealing the cyclization/ring opening of unsaturated/cyclic
alkyl iodides, indicate the existence of an alkyl radical as a
reaction intermediate.7c However, a series of EPR and DFT
studies suggest that the unpaired electron is located on the
ligand, not the metal center.7a,c,d Thus, the alkyl radical
formation by a nickel(I) species supported by a pyridine-based
ligand occurs substantially via a ligand-based electron transfer
reaction.7c,d According to our previous experiments, the
monovalent nickel carbonyl species (PNP)Ni-CO (1) possess-
ing significant spin density on a nickel center at 71.2%, exhibits
a NiI/II couple at −1.23 V vs Fc/Fc+.6a The redox properties of
1 cannot reduce methyl iodide via electron transfer.13 Thus,
iodine radical abstraction is presumably mediated by a nickel(I)
center, which is fairly rare.8
After the generation of a methyl radical, two methylated
products, the nickel acetyl species (PNP)Ni-COCH3 (2) and
the nickel methyl species (PNP)Ni-CH3 (3), are produced. In
both product formations, the five-coordinate intermediate
species (PNP)Ni(CO)(CH3) (5) is thought to be involved.
While a methyl species can be easily produced by CO
elimination from the nickel(II) center of 5, a nickel acetyl
species can be generated by alkyl migration onto a CO ligand in
5, for which the reaction is well established.14 However, this
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of a nickel(II) iodide species (4) when excess amount of
iodoalkanes were employed. Corresponding alkyl radical
generation was confirmed by trapping methyl and tert-butyl
radicals by utilizing Gomberg’s dimer. DFT calculations on the
reaction of 1 with MeI suggest that the nickel-acyl formation
(2) may not include the five-coordinate intermediate species
(PNP)Ni(CO)(CH3) (5) but suggest direct C−C bond
formation between a methyl radical and a coordinated CO
ligand at a monovalent nickel center.
Table 3. Reaction of (PNP)Ni-CO (1) with MeI at Varying
Temperatures
yield (%)
temp (°C)
Ni-I (4)
Ni-COCH3 (2)
Ni-CH3 (3)
−35
room temp
50
50
50
50
36
39
41
43
14
11
9
60
100
7
DFT calculations indicate that, apart from the superior
radical nature at a nickel center (67%), the rest of the spin
density is distributed almost equally among the donor atoms
directly coordinated to the Ni center (7% of the spin density is
located on the carbon atom of a CO ligand; see the Supporting
Information). Direct C−C bond formation between the carbon
atom of a CO ligand and a methyl radical cannot be excluded as
an alternative pathway. Due to the difficulties in modeling the
biradical nature of a reacting complex and the transition state of
the present system, a radical approach of 1 toward a methyl
radical is modeled using the unrestricted formalism of DFT for
the hypothesized reaction of 1 with MeI. Although the latter
modeling certainly does not describe the desired radical−radical
reaction as suggested in experiments, we believe it can at least
capture the relative reactivity of the methyl species toward Ni vs
carbonyl carbon. The generation of 2 occurs via a direct C−C
bond formation with TS2Me (ΔG⧧ = 16.0 kcal/mol; Figure 3).
A small methyl group can also directly couple with a nickel
center to produce a five-coordinate intermediate (5) via TS3Me
(ΔG⧧ = 16.6 kcal/mol; Figure 3). This intermediate (5) then
energetically favors the formation of a nickel methyl species (3)
rather than the C−C bond coupled product (2). In other
words, the desired C−C bond formation may occur mainly via
direct coupling, instead of involving five-coordinate species.
Due to a small difference in rate-determining activation barriers
for the two pathways (direct C−C coupling vs through five-
coordinate intermediate species), however, the formation of
both 2 and 3 seems to be quite possible to similar extents.
The reaction of free CO and alkyl radicals to form acyl
radicals is considerably utilized in organic synthesis.4 However,
direct C−C bond formation between a coordinated CO ligand
and an alkyl radical is fairly rare. Hughes and co-workers
proposed direct fluoroalkylation at a CO ligand in the reaction
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were carried out using
standard Schlenk or glovebox techniques under a N2 atmosphere.
Solvents were deoxygenated and dried by thoroughly sparging with Ar
gas followed by passage through an activated alumina column. Solvents
were tested with a standard purple solution of sodium benzophenone
ketyl in tetrahydrofuran in order to confirm effective oxygen and
moisture removal. All reagents were purchased from commercial
vendors and used without further purification unless otherwise stated.
(PNP)Ni-CO (1)6a and (CPh3)2·(pentane)12b were prepared
according to literature procedures. Deuterated solvents were
purchased from Cambridge Isotope Laboratories, Inc., and Euriso-
top, degassed, and dried over activated 4 Å molecular sieves prior to
use.
Spectroscopic Measurements. A Bruker AVHD-400 spectrom-
1
eter was used to measure H, 19F, and 31P NMR spectra. 13C NMR
spectra were recorded on Agilent 600 spectrometers. The chemical
1
shifts for H NMR and 13C NMR spectra are quoted in parts per
million (ppm) referenced to residual solvent peaks. The chemical
shifts for 19F NMR spectra are referenced to external α,α,α-
trifluorotoluene as −63.72 ppm. 31P NMR spectra 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 as 0.0 ppm. The following abbreviations are used to
describe peak splitting patterns when appropriate: d = doublet, t =
triplet, q = quartet, m = multiplet, qt = quartet of triplets. Coupling
constants, J, are reported in hertz (Hz). The N values corresponding
to (1/2)[JAX + JA’X] are provided when virtual couplings are observed
in the 13C NMR spectra.
Computational Details. All of the geometries were optimized at
the B3LYP/Gen1 level, where Gen1 stands for the 6-31G* basis set
for C, H, N, O, and P atoms, the 6-311G* basis set for an I atom, and
the LanL2DZ basis set for a Ni atom, as implemented in the Q-CHEM
quantum chemistry package.16 The radical species were treated with
the spin-unrestricted formalism. The transition state searches were
done using the Hessian mode following option incorporated into the
Baker’s eigenvector following algorithm.17 Transition states (first-order
saddle points) were characterized by a single imaginary frequency
(negative eigenvalue for the Hessian) pertaining to the desired
reaction coordinates. Energies were further refined at the B3LYP/
Gen2 level, where Gen2 stands for the 6-311+G** basis set for C, H,
N, O, and P atoms, the 6-311G** basis set for an I atom, and the
LanL2DZ basis set with f polarization function for a Ni atom along
with the Grimme’s DFT-D3 corrections18 for nonbonding interactions
and single-point solvation energy correction (the solvent is THF)
using the IEF-PCM method.19 The discussions presented in the main
text are based on the relative Gibbs energies at 298.15 K and 1 atm
with respect to the infinitely separated reactants.
i
of CpRh(PMe3) (CO) with C3F7I, but they provided limited
evidence for a radical pathway.9 Goldman and co-workers
proposed alkyl radical addition to d8 metal carbonyls for the
photocatalytic carbonylation of cyclohexane.15 This system was
later reevaluated theoretically, revealing that an alkyl radical
directly attacks at a coordinated CO ligand, as suggested by
Hasanayn and co-workers.5 Our experimental and theoretical
evaluations on the reaction of (PNP)Ni-CO (1) with alkyl
iodides support that the reaction undergoes alkyl radical
formation followed by its direct attack on a coordinated CO
ligand. Obviously, the radical C−C bond coupling occurs at the
carbon atom coordinated to a monovalent nickel ion with an
even higher spin density located on a metal center.
Reaction of 1 with RI. To a solution of 1 (20 mg, 0.039 mmol) in
10 mL of THF was added RI (0.040 mmol) at room temperature. The
reaction mixture was stirred for 10 min, resulting in a color change
from dark green to greenish yellow. Volatiles were removed under
vacuum. The yields of products were determined by single-pulse 31P
NMR spectroscopy.
Reaction of 1 with Excess RI. To a solution of RI (3.90 mmol) in
5 mL of THF was added a solution of 1 (20 mg, 0.039 mmol) in 5 mL
of THF dropwise at room temperature, resulting in a color change
from dark green to yellow. The reaction mixture was stirred for 10
CONCLUSIONS
■
The reaction mechanism of a nickel(I) monocarbonyl species
(1) supported by a pincer type PNP ligand with iodoalkanes
was experimentally and theoretically investigated, revealing the
formation of compounds 2−4. The initial iodine radical
abstraction by 1 was suggested from the increase in the yield
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min, and volatiles were removed under vacuum. The yields of products
were determined by single-pulse 31P NMR spectroscopy.
NMR spectra, X-ray crystallographic data for (PNP)Ni-
CF3, and details of DFT calculations (PDF)
Reaction of 1 with CF3I. A solution of 1 (10 mg, 0.019 mmol) in
5 mL of THF in a sealed Schlenk tube (total volume 17 mL) was
degassed by freeze−pump−thaw cycles on a Schlenk line. CF3I was
charged under ambient conditions, resulting in an immediate color
change from dark green to brown. The reaction mixture was stirred for
10 min, and volatiles were removed under vacuum. The yields of
products were determined by single-pulse 31P NMR spectroscopy.
Synthesis of (PNP)Ni-CF3. To a mixture of (PNP)Ni-Cl (157 mg,
0.300 mmol) and AgF (190 mg, 1.50 mmol) in 10 mL of THF was
added Me3SiCF3 (213 mg, 1.50 mmol). The green reaction mixture
was stirred for 12 h at 45 °C in the dark, resulting in a color change to
red. After the volatiles were removed under vacuum, the resulting
residue was dissolved in pentane. The reddish solution was filtered
through Celite, and the volatiles were removed under vacuum. The
resulting product (PNP)Ni-CF3 (101 mg, 0.182 mmol, 60.5%) was
isolated as a red solid after washing with cold pentane (−35 °C) and
X-ray crystallographic data for (PNP)Ni-CF3 (CIF)
Cartesian coordinates for calculated structures (XYZ)
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail for Y.J.: ysjn@kaist.ac.kr.
*E-mail for Y.L.: yunholee@kaist.ac.kr.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work has been financially supported by KAIST and
Aramco Overseas Company.
1
drying under vacuum. H NMR (400 MHz, benzene-d6): δ 7.55 (d,
JHH = 8.6 Hz, 2H), 6.93 (s, 2H), 6.76 (d, JHH = 8.5 Hz, 2H), 2.36−2.22
(m, 4H), 2.14 (s, 6H), 1.38 (q, JHH = 7.7 Hz, 12H), 1.12 (q, JHH = 7.2
Hz, 12H). 13C NMR (151 MHz, benzene-d6): δ 161.99 (virtual t, N =
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12.1 Hz), 140.14 (qt, JCF = 354 Hz, JCP= 31.9 Hz), 132.58, 131.96,
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(10) Product yields were determined by relative determination of
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organic products were not evaluated.
reaction mixture was stirred for 10 min, resulting in a color change
from dark green to greenish yellow. Volatiles were removed under
vacuum. Ph3CtBu (11 mg, 0.037 mmol, 38%) was obtained as a white
solid after purification via silica gel column chromatography using EA/
hexane 1/40 v/v as eluent.
Synthesis of Ph3CMe. Author: To a mixture of 1 (50 mg, 0.097
mmol) and (CPh3)2·(pentane) (271 mg, 0.485 mmol) in 10 mL of
THF was added MeI (50 μL, 2.0 M solution in tBuOMe, 0.10 mmol)
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resulting in a color change from dark green to greenish yellow.
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solution of 1 (20 mg, 0.039 mmol) in 10 mL of toluene was added
t
MeI (20 μL, 2.0 M solution in BuOMe, 0.040 mmol) at different
(11) Three products were obtained from the reaction of (PNP)Ni-
CO (1) with CF3I. The formation of (PNP)Ni-I (64%) and (PNP)Ni-
CF3 (19%) was confirmed by comparing their 31P NMR data. The
other product (18%) displaying a 31P NMR signal at 42.13 ppm is
assigned as (PNP)Ni-COCF3 (see the Supporting Information).
Independently, (PNP)Ni-CF3 was synthesized and characterized (see
the Experimental Section).
temperatures (−35, 25, 60, and 100 °C). The reaction mixture was
stirred for 10 min (12 h for −35 °C), and volatiles were removed
under vacuum. The yields of products were determined by single-pulse
31P NMR spectroscopy.
ASSOCIATED CONTENT
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* Supporting Information
(b) Jang, E. S.; McMullin, C. L.; Kaβ, M.; Meyer, K.; Cundari, T. R.;
̈
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00548.
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