Reactivity of the diphosphinodithio ligated nickel(0) complex toward alkyl halides and resultant nickel(i) and nickel(ii)-alkyl complexes

Article abstract of DOI:10.1039/C8DT03188F

Diphosphinodithio ligated complexes of nickel(0), nickel(i) and nickel(ii)-alkyl with a reactivity relevant to the C-C bond formation were described. Stoichiometric reactions of the nickel(0) complex, [(P₂S₂)Ni] ([1]⁰, P₂S₂ = (Ph₂PC₆H₄CH₂S)₂(C₂H₄)), with alkyl halides (RX) such as C₆H₅CH₂Br, C₂H₃CH₂Br, C₂H₅I and (CH₃)₂CHI were investigated, from which the products were found to be highly dependent on the nature of RX used. Oxidative addition of C₂H₃CH₂Br to [1]⁰ provides the stable Ni(ii)-alkyl complexes [1-allyl]⁺. The reaction of [1]⁰ with C₆H₅CH₂Br proceeds through a radical pathway resulting in the formation of the nickel(i) complex [1]⁺ and an organic homo-coupled product 1,2-diphenylethane. Oxidative addition of C₂H₅I or (CH₃)₂CHI to [1]⁰ can be achieved but it competes with the halogen atom abstraction reaction as found for C₆H₅CH₂Br. [1]⁰ was shown to be an active catalyst for the coupling reactions of primary halides and alkyl Grignard reagents.

Full text of DOI:10.1039/C8DT03188F

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PAPER
Reactivity of the diphosphinodithio ligated
nickel(0) complex toward alkyl halides and
resultant nickel(I) and nickel(II)–alkyl complexes†
Cite this: Dalton Trans., 2018, 47,
15757
Ailing Zhang, Congxiao Wang, Xiaoyu Lai, Xiaofang Zhai, Maofu Pang,
Chen-Ho Tung and Wenguang Wang
*
Diphosphinodithio ligated complexes of nickel(0), nickel(I) and nickel(II)–alkyl with a reactivity relevant to
the C–C bond formation were described. Stoichiometric reactions of the nickel(0) complex, [(P
2
S
2
)Ni]
I and
CHI were investigated, from which the products were found to be highly dependent on the nature
0
(
[1] , P
2
S
2
= (Ph
2
PC
6
H
4
2
CH S)
2
(C
2
H
4
)), with alkyl halides (RX) such as C
6
H
5
CH
2
2
Br, C H
3 2 2
CH Br, C H
5
(CH
3
)
2
0
+
of RX used. Oxidative addition of C
2
H
3
CH
2
Br to [1] provides the stable Ni(II)–alkyl complexes [1-allyl] .
0
The reaction of [1] with C
6
H
5
CH
2
Br proceeds through a radical pathway resulting in the formation of the
+
Received 4th August 2018,
Accepted 15th October 2018
nickel(I) complex [1] and an organic homo-coupled product 1,2-diphenylethane. Oxidative addition of
0
C
2
H I or (CH
5
3
)
2
CHI to [1] can be achieved but it competes with the halogen atom abstraction reaction as
DOI: 10.1039/c8dt03188f
0
found for C
6
H
5
CH
2
Br. [1] was shown to be an active catalyst for the coupling reactions of primary
rsc.li/dalton
halides and alkyl Grignard reagents.
tallation and reductive elimination to provide the product
Introduction
9
(
Chart 1, (a) and (b)). The Ni(II)–alkyl complex can undergo
With their high abundance, relatively low cost and environ- Ni–C homolytic cleavage to form Ni(I) species and alkyl rad-
mentally friendly properties, versatile Ni-based catalysts have icals (c), and this competes with the transmetallation process.
1
been extensively studied over the last decade. They have In particular, Ni-mediated alkyl–aryl couplings can proceed
demonstrated high efficiency in a wide range of valuable, intri- through a radical chain pathway that involves a Ni(I)/Ni(II)/Ni(III)
1
,2
10
guing and challenging transformations. In particular, nickel cycle. It has been proposed that the Ni(I) complex reacts with
5
b,c
catalysis in cross-coupling reactions, such as aryl–aryl coup- alkyl electrophiles to release an alkyl radical (d),
ling, aryl–alkyl coupling, and alkyl–alkyl coupling reactions,
which then
3
4
5
adds to the Ni(II)–aryl species generated by oxidative addition
has attracted significant interest. Recent advances in character- of aryl halides to Ni(0) or by the transmetallation from aryl
ization techniques and computational studies have provided
insight into Ni-mediated alkyl–alkyl cross-coupling reactions,
in which Ni–alkyl complexes are considered to be the key
5
–8
intermediates.
Owing to their propensity to exist in various oxidation
states, Ni(0), Ni(I), Ni(II) and even Ni(III) species can be invoked
in the catalysis of cross-coupling reactions. The general nickel-
based elementary reactions involved in Ni catalysis are shown
in Chart 1. A typical Ni(0)/Ni(II)-based catalytic cycle includes
the elementary steps of oxidative addition of RX to Ni(0)
species affording Ni(II)–alkyl complexes, followed by transme-
Key Lab for Colloid and Interface Chemistry of Education Ministry,
School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100,
PR China. E-mail: wwg@sdu.edu.cn
†
Electronic supplementary information (ESI) available: Experimental pro-
cedures, characterization data and NMR spectra. CCDC 1840915–1840917. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c8dt03188f
Chart 1 General Ni-centered elementary reactions involved in catalytic
alkyl–alkyl and alkyl–aryl couplings.
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5
b,c
Grignard reagents to Ni(II) halides (e).
The resulting Ni(III)
species undergoes reductive elimination to give the coupling
7
b,11
product, regenerating the Ni(I) complex (f).
In the cases of
aryl–aryl couplings catalyzed by nickel complexes bearing
1
2–14
bulky NHC ligands,
it was proposed that aryl halides react
0
I
with Ni (NHC)
pound. Transmetallation from the Grignard reagent R
NHC) Ni(I)X provides (NHC) Ni(I)-R intermediates, followed
n
species to generate the (NHC)
n
Ni –X com-
1
MgX′ to
(
n
n
1
2
by oxidative addition of aryl halide R X forming Ni(III) species (g).
Apparently, there are multiple reaction pathways in Ni-cata-
lyzed reactions due to the redox active nature of the nickel
0
1
b,10
Fig. 1 Solid-state structure of [1] . Thermal ellipsoids are shown at the
0% probability level, and hydrogen atoms are omitted for clarity.
Selected distances (Å) and angles (°): Ni1–S1 = 2.1939(6), Ni1–S2 =
.1933(6), Ni1–P1 = 2.1315(6), Ni1–P2 = 2.1205(6), S2–Ni1–S1 = 93.57(2),
complexes,
and the mechanism is dependent on the
5
ligand employed. Therefore, it is of obvious interest and
importance to synthesize nickel complexes in various oxi-
2
dation states and examine their reactivity as it is related to the P1–Ni1–S1 = 101.15(2), P1–Ni1–S2 = 110.90(2), P2–Ni1–S1 = 113.26(2),
fundamental steps of Ni-mediated C–C couplings.
P2–Ni1–S2 = 101.30(2), P2–Ni1–P1 = 130.73(3).
In our previous studies, we found that the four-coordinate
2
+
dicationic [(P
reacts with CH
2
S
2
)Ni] (P
S
2 2
= (Ph
2
PC
6
H
4
CH
2
S)
2 2 4
(C H )) complex
+
3
MgBr forming the [(P
2
S
2
)Ni–CH
3
]
complex soft/soft interaction of Ni(0) and P vs. the borderline hard/
1
5
16,17
which is active towards CO insertion. In the present paper, borderline soft interactions of Ni(II) and S.
we report the reactivity of [(P S )Ni ] toward alkyl halides and
2 2
0
the resultant nickel(I) and nickel(II)–alkyl complexes, which are
relevant to the C–C bond formation.
I
2 2
[(P S )Ni ]
Addition of one equivalent of benzyl bromide (BnBr) to a solu-
tion of [1] in THF at room temperature resulted in a rapid
0
color change from red to brown (eqn (1)). The organonickel
complex was crystallized from a CH Cl /hexane solution over-
Results and discussion
2
2
0
[
(P S )Ni ]
night at −30 °C to afford yellow crystals, and the molecular
2
2
+
0
0
structure was identified as [(P S )Ni]BPh ([1] ) by X-ray crystal-
The precursor complex [(P S )Ni ] ([1] ) was synthesized
2 2 4
2
2
1
5
lographic analysis.
according to a published method with modifications. Direct
treatment of a solution in THF of P with a THF solution of
2 2
S
Ni(COD) provided the desired product, which was isolated as
2
0
a red powder. Alternatively, complex [1] was prepared by the
reduction of the Ni(II) complex [(P S )Ni] ([1] ) with two
2 2
equivalents of Cp Co. The P NMR spectrum of [1] exhibits a
2
+
2+
ð1Þ
3
1
0
2
phosphorus resonance at δ 21.3, compared to δ −16.9 for the
1
free ligand P
2
S
2
. In the H NMR spectrum of P
2
S
2
, the four
+
In the solid state, the framework of [1] is very similar to
PhCH protons were displayed as a singlet at 3.89 ppm, which
0
2
that of the nickel(0) precursor [1] , and the Ni(I) center also
adopts a tetrahedral geometry (Fig. 2). However, the structure
of [1] is less compact than that of [1] , and this is reflected by
after complexation with Ni splits into two doublets at 3.06 and
3
.51 ppm in THF-d
X-ray quality crystals of [1] were grown from a THF/hexane
8
.
+
0
0
0
solution. In the solid structure of [1] , the Ni(0) center is in a
distorted tetrahedral geometry suggested by the angles of
9
3.57(2)° for ∠S2–Ni1–S1 and 130.73(3)° for ∠P2–Ni1–P1
Fig. 1). The two Ni–S bonds have almost identical bond
lengths of 2.1939(6) and 2.1933(6), which compare well with
(
0
16
2
.205 Å in [(Ph PC H SMe) Ni] . The Ni–P distances of
2 6 4 2
2
.1315(6) and 2.1205(6) Å are shorter than the Ni–S distances.
II
2+
2 2
In our earlier report, the nickel center in Ni (P S ) ([1] )
adopts a distorted tetrahedral geometry. Surprisingly, the
average Ni–S distance in complex [1] is only about 0.007 Å
longer when compared to that in [1] . In contrast to Ni–S >
1
5
0
2
+
0
II
Ni–P in [1] , in (P S )Ni the Ni–P distances are longer than
2
2
+
Fig. 2 Structure of [1] . Selected distances (Å) and angles (°): Ni1–S1 =
the Ni–S distances. Such differences agree with the changes
2.2240(6), Ni1–S2 = 2.2160(7), Ni1–P1 = 2.2209(6), Ni1–P2 = 2.2095(6),
reported for sulfur/phosphorus supported Ni(II) and Ni(0)
species, which probably are due to the better match of the P2–Ni1–S1 = 126.90(3), P2–Ni1–S2 = 98.44(2), P2–Ni1–P1 = 117.97(2).
S2–Ni1–S1 = 90.56(3), P1–Ni1–S1 = 96.48(2), P1–Ni1–S2 = 126.15(3),
1
6
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Fig. 3 Cyclic voltammogram for [1](BF
4
)
2
and [1]BPh
4 2 2
in CH Cl .
+
Fig. 4 Structure of [1-allyl] . Selected distances (Å) and angles (°):
Ni1–S1 = 2.4446(7), Ni1–S2 = 2.2314(7), Ni1–P2 = 2.1913(7), Ni(1)–C(1) =
2
1
9
−1
Conditions: 1 mM sample, 0.1 M Bu NPF ; scan rate, 100 mV s ; poten-
4 6
+
/0
tials vs. Fc
.
.096(2), Ni(1)–C(2)
.393(4), C(2)–C(3) = 1.420(4), S2–Ni1–S1 = 87.90(2), P2–Ni1–S1 =
8.55(2), P2–Ni1–S2 = 97.02(3), C(1)–C(2)–C(3) = 117.4(3).
= 1.997(3), Ni(1)–C(3) = 2.025(3), C(1)–C(2) =
an increase in the average Ni–S distance and the Ni–P distance
by 0.026 and 0.089 Å, respectively, together with the enlarged
∠
P1–Ni1–S2 (126.15(3)°). The solution magnetic moment of
showed a characteristic peak at 6.02 ppm corresponding to the
+
[1] is 1.79μB at room temperature, which is consistent with
24
allyl proton (Fig. S8†). Two phosphorus signals (20.2 ppm
9
18
the expected d configuration.
The cyclic voltammogram of [1] was recorded in CH
compare its redox properties with those of [1] (Fig. 3).
31
and −18.2 ppm) were displayed in the P NMR spectrum,
+
2
2
Cl to
suggesting the decoordination of one of the P atoms from the
2
+
Ni(II) center. Crystallographic analysis confirmed the antici-
+
Complex [1] exhibits two reversible redox peaks at 0.02 V
+
pated structure of [1-allyl] in which the allyl group is bound
(
ipc/ipa = 1.00) and −1.05 V (ipc/ipa = 1.02). Cyclic voltammetry
3
to nickel in a η mode. The Ni–C distances are nearly equal
2
+
of [1] demonstrated two similar reversible electrochemical
processes for [1] . Accordingly, the first redox event at 0.02 V is
assigned to the Ni /Ni couple and the second one at −1.05 V
to the Ni /Ni couple. The stability of [1] and [1] is reflected
by the readily reversible redox properties.
Reactions of Ni(0) species with aryl or alkyl halides can
proceed in a multiplicity of pathways such as electron transfer
and oxidative addition, affording a diversity of products. For
example, reactions of Ni(PEt and aryl halides produce a
mixture of Ni(I) and the oxidative addition product ArNiX.
(
Ni1–C1 2.096(2) Å, Ni1–C2 1.997(3) Å and Ni1–C3 2.025(3) Å),
and the C–C bond lengths in the allyl group differ by only
.027 Å (dC1–C2 = 1.393(4) Å vs. dC2–C3 = 1.420(4) Å). The Ni(II)
+
II
I
0
I
0
+
0
center exhibits an 18-electron configuration. In addition to the
allyl group, the nickel center is also coordinated to the two
S atoms and one P atom of the P S ligand. It is worth noting
2
2
that in our previously reported Ni(0)–CO system, one of the
1
9
15
S atoms is not coordinated to the metal center. These
3 4
)
present results reveal the coordination flexibility of the P2S
2
3
c
ligand in the stabilization of nickel complexes.
According to the ESI-MS spectral analysis, the organonickel
product for the stoichiometric reaction between [1] and
PhCH Br was [1] ; however, the formation of the oxidative
addition product [1-CH Ph] was not evidenced. In addition to
It is well known that metal–allyl complexes undergo dynamic
0
3
1
25,26
conversions between η - and η -forms.
The dynamic behav-
+
2
+
ior of the allyl group in [1-allyl] was demonstrated by VT NMR
+
2
experiments. As seen in Fig. 5, lowering the temperature of the
+
[1] , the organic product is 1,2-diphenylethane, which was
+
1
CD Cl solution of [1-allyl] led to decalescence of the H NMR
1
2
2
identified by H NMR spectroscopy and GC-MS analysis
spectrum, suggesting that a dynamic process is involved in the
system. The central proton H in the allyl group appeared as a
0
(
Fig. S5 and S6†). We proposed that the reaction of [1] and
PhCH Br proceeds through a radical pathway resulting in the
formation of [1] and a benzyl radical. We also found that
a
2
characteristic quintet at 6.02 ppm at room temperature, and the
chemical shift is not temperature-dependent, which is consist-
+
20
+
2+
[1] can further react with PhCH Br to give [1] and 1,2-diphe-
3
1
2
ent with the dynamic interconversion of a η –η rearrangement
3
1
nylethane, which were confirmed respectively by P NMR
spectroscopy and GC-MS analysis. A similar one-electron trans-
fer type mechanism has been reported for the macrocyclic
26
reported for the Ni(II)–allyl systems. At room temperature, the
syn protons (H
yielded broad signals.
to 213 K, the syn and anti protons were resolved into four sharp
b
and H′
b
) and anti protons (H
When the temperature was decreased
c
and H′
c
) together
24,25
+
19
nickel(I) complex [Ni(tmc)] with alkyl halides.
3
1
doublets at δ 3.14, 2.86, 2.79 and 2.58. In the P NMR spectra,
+
[1-allyl]
two broad peaks were observed at room temperature, and they
0
Treatment of [1] with allyl bromide in THF followed by became sharper at lower temperatures. It is likely that one
anion exchange with NaBPh
resulted in the formation of P atom remains uncoordinated in the η complex and this is
1
4
1
31
[1-allyl]BPh
4
, identified by H NMR, P NMR spectroscopy probably due to the steric hindrance from the phenyl groups
1
and X-ray crystallography (Fig. 4). The H NMR spectrum and the alkyl group.
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i
+
1
with the calculated value for [1- Pr] (Fig. S26†). In the H NMR
i
+
spectrum of [1- Pr] , the methine proton Ni–CH(CH ) exhibits
3
2
a well-resolved septet at δ 2.39, while the methyl groups appear
as a doublet at δ 0.96 (JH–H = 5.2 Hz). Intensive efforts to
obtain high-quality single crystals suitable for X-ray crystallo-
graphic analysis that would provide the solid-state structure of
i
+
+
[
1- Pr] or [1-Et] , however, were unsuccessful. According to
3
1
31
i
+
+
P NMR spectra, the P resonances of [1- Pr] and [1-Et] are
similar to the patterns observed for [1-Me] at low tempera-
ture. The P NMR spectrum of [1- Pr] showed phosphorus
resonances as two doublets at δ 44.06 and 12.76 with J
07.8 Hz, which are comparable to the P signals at δ 40.45
and 9.81 with JP–P = 201.1 Hz for [1-Et] . Although the phos-
phorus resonances of [1-Me] were displayed as a broad peak
δ 12.98) at room temperature, it split into two peaks at
+
1
5
31
i
+
=
P–P
3
1
2
+
+
1
5
(
δ 16.78 and δ 7.38 when the temperature was decreased to
3
1
2
53 K. At a lower temperature of 213 K, the P signals were
well resolved as two doublets with JP–P = 177.5 Hz (Fig. S29†),
consistent with the pseudo-trigonal–bipyramidal geometry
1
2 2 4
Fig. 5 VT H NMR (500 MHz, CD Cl ) of [1-allyl]BPh
(○ = diethyl
ether).
+
reported for the solid-state structure of [1-Me] . The two non-
+
equivalent phosphorus atoms in [1-Me] are also reflected by
the striking differences in the Ni–P distances (Δ(Ni–P) =
1
5
0
0.1 Å).
In addition to the expected oxidative addition product
Reactions of [(P S )Ni ] with C H I and (CH ) CHI
2
2
2
5
3 2
0
The reactivity of [1] toward less activated alkyl halides such as
iodoethane and 2-iodopropane was also examined. The reac-
+
0
[
1-R] , it is more likely that reactions of [1] with C
2 5
H I and
(
CH ) CHI also involve halogen atom abstraction to form
0
3 2
tions of [1] with C
2
H
5
I or (CH
3
)
2
CHI were performed in THF
22
nickel(I) species and alkyl radicals (Scheme 1b). The reaction
mixture of [1] and the alkyl iodide was always NMR silent,
indicating the production of paramagnetic species.
Recrystallization of the reaction residue at −30 °C provided
single crystals and some of these were confirmed to be [1] by
crystallographic analysis. Although homolytic cleavage of the
Ni–C bond of Ni(II)–alkyl complexes is known,
be noted that the decomposition of [1-Et] only provided
Ni(II)–H hydride species and C H , at least according to
followed by treatment with NaBPh . ESI-MS studies of the reac-
tion solutions indicated the production of organonickel
species [1-Et] and [1- Pr] , respectively (Scheme 1a). For the
reaction of [1] with CD CD I, the ESI-MS spectrum analysis
0
4
+
i
+
0
3
2
+
+
featured an ionic peak at m/z 734.1768 for [1-C
2
D
5
]
vs. m/z
29.1457 for [1-Et] . The isotopic distribution agrees well with
the calculated values. The product was precipitated by diluting
+
7
19,20,22
it should
+
1
the reaction solution with Et
2
O. The H NMR spectrum of
CH signals at δ 1.97 (q, 2H) and
δ 1.03 (t, 3H), compared to the H resonances at δ 1.95 and 0.99
+
2
4
[
1-Et] displayed the Ni–CH
2
3
1
+
H NMR spectroscopic studies. Unlike [1-Et] , the degeneration
of [1- Pr] to [1-H] was not observed by H NMR spectral
studies (Fig. S27†).
2
i
+
+
1
+
observed for the deuteride [1-C
1-Et] underwent degeneration to provide [1-H] and ethylene
2
D
5
] in CH
2 2
Cl . We found that
+
+
[
(
1
eqn (2), Fig. S16†). As monitored by the H NMR spectrum, Reactivity of Ni(II)–alkyl complexes towards Grignard reagents
2
1
the production of ethylene was signaled at δ 5.40, while the
characteristic hydride signal of [1-H] was displayed at δ −13.8.
+
The stoichiometric reaction of [1-Et] with PhMgBr was sub-
sequently examined. [1-Et] was formed in situ by the reaction
+
15
+
0
31
of CH
after which the Grignard reagent was added. The formation of
the desired C–C coupled product C was analysed by
3 2
CH I with [1] monitored by P NMR spectroscopy,
þ CD2Cl2
þ
þ
½
ðP2S2ÞNi‐Etꢀ ꢁ! ½ðP2S2ÞNi‐Hꢀ þ C2H4
ð2Þ
þ
rt
½1‐Etꢀ
½1‐Hꢀ
6
5 2 5
H C H
0
1
0
ESI-MS spectral analysis for the reaction solution of [1]
H NMR and GC-FID, while the regeneration of [1] was con-
with (CH ) CHI showed a peak at m/z 743.1600 matching well firmed by P NMR spectroscopy (Fig. S31†). The results indi-
3
1
3
2
+
cate that [1-Et] is an active intermediate in the C–C bond for-
mation through transmetallation. Based on the stoichiometric
0
reaction, we found that [1] is capable of catalysing the coup-
ling reaction between CH
3
CH
CH
pleted in 15 min, as suggested by the disappearance of proton
2
I and PhMgBr. With 2 mol% of
0
[
1] , the reaction of CH
3
2
I and PhMgBr in d -THF com-
8
1
signals of CH
addition to C
3
CH
2
I in the H NMR spectrum (Fig. S36†). In
0
H C H (60% yield), the reaction indeed pro-
Scheme 1 Possible reaction pathways for [1] with iodoethane and
-iodopropane.
6
5 2 5
2
duces butane with the characteristic peaks at δ 1.27 (m) and
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soft and flexible coordination properties of the diphosphino-
dithio tetradentate ligand are reflected by its character in the
stabilization of nickel complexes with oxidation states of 0,
I and II. It should be noted that transition metal–methyl
bonds tend to be stronger than those of higher alkyl ligands,
and the metal–alkyl bond strength is significantly affected by
2
7
steric factors.
0
Experimental section
Scheme 2 The proposed catalytic cycle for alkyl–aryl coupling by [1] .
2
All manipulations were conducted under a N atmosphere
using standard Schlenk techniques or in a glovebox, unless
2
2,28
otherwise stated. All reagents were purchased from Sigma-
δ 0.87 (t, JH–H = 6.9 Hz).
By contrast, the conversion of
0
Aldrich, and used as received. (P
according to our previous methods. Et
by distillation over sodium), MeCN (dried by distillation over
CaH ) and CH Cl for general use were of AR grade and stored
under an atmosphere of N . CD Cl was dried using activated
2
S
2
)Ni ([1] ) was prepared
CH CH I for the reaction of CH CH I and PhMgBr without
1] is only 16%, and the production of butane was not
detected. These results confirm that halogen atom abstraction
by [1] competes with oxidative addition of CH CH I to [1]
3
2
3
2
0
O, pentane, THF (dried
2
[
0
0
2
2
2
3
2
2
2
2
(Scheme 2).
molecular sieves (4 Å) and degassed with three thaw–freeze
cycles. C was dried over CaH and distilled by vacuum
transfer. NMR spectra were recorded on a Bruker Avance 500
6
D
6
2
1
13
31
ð3Þ spectrometer in J. Young NMR tubes. H, C and P NMR
chemical shifts are referenced to the residual solvent peak of
the deuterated solvent and external H PO , respectively. Cyclic
3
4
voltammetry was performed under N
2
at room temperature
It is well known that alkyl–aryl coupling is more favoured
than alkyl–alkyl coupling in classic Pd and Ni-catalyzed cross-
using a CHI 760e electrochemical workstation (Shanghai Chen
Hua Instrument Co., Ltd) with a glassy carbon working elec-
trode, Pt wire counter electrode, and the pseudo-reference elec-
trode Ag wire. HRMS were recorded on a commercial instru-
ment (ESI Source).
1
a
0
coupling reactions. Notably, [1] can also promote the acti-
vation of Csp –X bonds and can catalyse the cross-coupling
reactions of primary alkyl halides with alkyl Grignard reagents
eqn (3)). TMEDA (tetramethylethylenediamine) was added to
the reaction mixture to increase the stability of the functiona-
3
(
0
Synthesis of (P S )Ni, [1]
2
3
0
2
2
lized Grignard reagent. With only 2 mol% of [1] , the reac-
tion of (2-iodoethyl)-benzene and n-BuMgCl proceeded suc-
cessfully in 2 h at 0 °C, giving the desired C–C coupled
product in 74% yield (eqn (3)). Switching the substrate to an
0
Method a. [1] was prepared according to our published
1
5
methods. A solution of Ni(COD)2 (0.141 g, 0.51 mmol) in
THF (10 mL) was added dropwise to solution of
Ph PC CH S) (C ) (0.300 g, 0.47 mmol) in THF (20 mL)
a
(
2
6
H
4
2
2
2 4
H
alkyl ether iodide C
desired product in a reasonable yield (67%). Utilization of a
primary alkyl bromide C OC Br resulted in a significantly
6 5 3 6
H OC H I, the catalysis also provided the
over the course of 5 min. The mixture was stirred for 10 min,
and then the solvent was removed under vacuum. The residue
was washed with Et O (3 × 5 mL), and then dried under
2
vacuum to give the product as a dark red solid (0.278 g, 85%
yield). Single crystals suitable for X-ray diffraction were
6
H
5
3 6
H
decreased yield of 42% because of the difficulty of C–Br bond
activation compared to a C–I bond (entry 3, Table S1†).
0
Although [1] is an active catalyst for the alkyl–aryl coupling
obtained by layering hexane into a saturated THF solution of
reactions, the competing radical pathway of halogen atom
abstraction leading to the formation of a alkyl–alkyl by-
product is considered to be a major impediment to efficient
alkyl electrophile cross-coupling (Scheme 2).
1
the product at −30 °C. H NMR (500 MHz, THF-d ): δ
8
7
.63–6.79 (m, 28 × ArH), 3.51 (br, 2H, SCH
SCH Ph), 2.53 (br, 2H, SCH CH S), 1.40 (s, 2H, SCH
P{ H} NMR (202 MHz, toluene, H PO as internal standard):
2
Ph), 3.06 (br, 2H,
2
1
2
2
2
CH S).
2
3
1
3
4
2 2
δ 21.28 (s). Anal. Calcd for C40H36NiP S : C, 68.49; H, 5.17.
0
Found: C, 68.55; H, 5.29. ESI-MS: calcd for [1] : 700.1087;
Conclusions
found: 700.1071.
2
+
Reactions of the nickel(0) compound with alkyl halides were
Method b. [(P
2
S
2
)Ni] was prepared according to literature
1
5
examined, and the products were found to depend on the procedures. Cp
2
Co (0.086 g, 0.46 mmol) in THF (10 mL) was
0
2+
nature of RX used. [(P S )Ni ] is an active catalyst for coupling added to
a
solution of complex [(P S )Ni]
(0.200 g,
2
2
2 2
reactions with primary halides and Grignard reagents, for 0.23 mmol) in THF (15 mL). The mixture was stirred at room
II
which the Ni –alkyl species formed by oxidative addition of temperature until the Ni(II) compound was reduced, and the
alkyl halides are envisioned to be the key intermediates. The color of the solution changed from red to deep red. The
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2 2 2
mixture was immediately filtered through a short pad of CD Cl ): δ 7.31–6.86 (m, 48 × ArH), 3.23 (br, 4H, SCH Ph), 2.86
Celite, and the filtrate was dried under vacuum. The product (br, 2H, SCH CH S), 2.21 (br, 2H, SCH CH S), 1.97 (q, J =
2
2
2
2
3
1
1
was extracted by toluene (3 × 5 mL) and dried under vacuum 7.1 Hz, 2H, CH
2
CH
3
), 1.03 (t, J = 7.1 Hz, 3H, CH
Cl ): δ 40.45 (d, JPP = 201.1 Hz), 9.81 (d,
JPP = 201.1 Hz). Anal. Calcd for NiP S C H B: C, 75.51; H,
2 3
CH ). P{ H}
to afford (P
2
S
2
)Ni as a dark red solid. (0.130 g, 81% yield).
NMR (202 MHz, CD
2
2
2
2 66 61
0
Reaction of [(P S )Ni ] with benzyl bromide
2
2
5
.86. Found: C, 75.63; H, 5.95.
ESI-MS: calcd for [1-C
+
2
Benzyl bromide (18.6 μL, 0.16 mmol) was added into a solu-
2 5
D ] : 734.1792; found, 734.1768. H
0
tion of [1] (0.100 g, 0.14 mmol) in THF, resulting in a rapid NMR (CH Cl ): δ 1.95 (br, Ni–CD CD ), 0.99 (br, Ni–CD CD ).
2
2
2
3
2
3
3
1
1
color change from red to brown. Then, a solution of NaBPh
4
2 2
P{ H} NMR (202 MHz, CH Cl ): δ 40.52 (d, JP–P = 198.2 Hz),
(
0.059 g, 0.17 mmol) in THF (5 mL) was added to the reaction 9.53 (d, JP–P = 198.2 Hz).
mixture. After being vigorously stirred for 10 min, the reac-
tion mixture was filtered through Celite, and the filtrate was
subjected to mass spectroscopy (ESI-MS and GC-MS) and The reaction of [(P
NMR spectroscopy analyses. Finally, the filtrate was dried a similar procedure utilized for C H I with [(P S )Ni ]. Yield:
under vacuum, and the residue was recrystallized from 0.093 g (61%). ESI-MS: calcd for [1- Pr] : 743.1635; found,
0
Reaction of [(P S )Ni ] with (CH ) CHI
2
2
3 2
0
2 2
S )Ni ] with 2-iodopropane was followed by
0
2
5
2 2
i
+
1
CH
2
+
Cl
2
/hexane overnight at −30 °C, providing [(P
2
S
2
)Ni]BPh
4
743.1600. H NMR (500 MHz, CD
2
Cl
2
): δ 7.60–6.86 (m, 48 ×
(
[1] ) as yellow crystals (0.131 g, 90% yield). μeff (Evans): ArH), 3.32 (br, 2H, SCH Ph), 2.88 (d, 2H, SCH Ph), 2.39 (sept,
2
2
1
B 2 2 3 2 2 2
.79μ . Anal. Calcd for C64H56BNiP S : C, 75.31; H, 5.53. 1H, CH(CH ) ), 2.15 (br, 2H, SCH CH S), 1.66 (br, 2H,
+
31
1
Found: C, 75.45; H, 5.62. ESI-MS: calcd for [1] : 700.1087; SCH
found: 700.1068. CD Cl ): δ 44.06 (d, J
P–P
2
CH
2
S), 0.96 (d, 6H, CH(CH
3
)
2
)
P{ H} NMR (202 MHz,
= 207.8 Hz), 12.76 (d, JP–P = 207.8 Hz).
2
1
2
3
1
P{ H} NMR (202 MHz, CD
96.8 Hz), 13.48 (d, J = 196.8 Hz). C{ H} NMR (126 MHz,
Allyl bromide (13.6 μL, 0.16 mmol) was added into a solution CD Cl ): δ 164.20, 136.34, 133.94, 133.85, 133.70, 132.39,
2 2
Cl , 213 K): δ 48.14 (d, J =
0
Reaction of [(P S )Ni ] with allyl bromide
13
1
2
2
1
2
2
0
of [1] (0.100 g, 0.14 mmol) in THF at room temperature. 132.30, 132.18, 132.12, 131.79, 131.73, 131.58, 131.46, 131.01,
Then, a solution of NaBPh (0.059 g, 0.17 mmol) in THF 129.92, 129.34, 129.14, 128.05, 128.00, 127.89, 127.69, 126.68,
5 mL) was added to the reaction mixture. After being vigor- 125.91, 122.05, 37.72, 37.59, 35.30, 35.16, 29.91, 27.04, 22.38.
4
(
ously stirred for 10 min, the reaction mixture was filtered Anal. Calcd for C67
through Celite. The filtrate was subjected to mass spec- 75.63; H, 6.10.
troscopy (ESI-MS) and NMR spectroscopy analyses. The fil-
2 2
H63BNiP S : C, 75.65; H, 5.97. Found: C,
General procedures for catalytic cross-coupling reactions
trate was subsequently dried under vacuum, and the residue
was extracted with CH Cl . The resulting CH Cl
2
2
2
2
solution was A mixture of alkyl halide (0.5 mmol), (P )Ni (0.007 g,
2 2
S
concentrated, layered with hexane and cooled at −30 °C to 0.01 mmol), and TMEDA (22.6 µL, 0.15 mmol) was dissolved
1
give dark red crystals. Yield: 124 mg (82%). H NMR in THF (1 mL) in a glovebox. 1,3,5-Trimethoxybenzene (10 mg)
(
2 2
500 MHz, CD Cl , 213 K): δ 8.38–6.57 (m, 48 × ArH), 6.02 (m, was added to this solution as an internal standard. RMgX
1
H, allyl H ), 3.70 (m, 4H, SCH Ph), 3.14 (d, 1H, allyl H ), 2.86 (0.5 mmol) was added dropwise to the reaction mixture at
a
2
b
(
d, 1H, allyl H′
b
), 2.79 (d, 1H, allyl H
c
c
), 2.58 (d, 1H, allyl H′ ), 0 °C. The reaction mixture was stirred for 2 h, after which the
2
1
.21 (t, 2H, SCH
2
CH
2
S), 2.12 (dd, 1H, SCH
2
CH
2
S), 0.47 (m, NMR sample was prepared using 0.05 mL of the reaction
3
1
1
H, SCH CH S). P{ H} NMR (202 MHz, CD Cl , 213 K): mixture in 0.5 mL of CDCl3.
2
2
2
2
3
1
1
δ 20.24 (s), −20.58 (s). P{ H} NMR (202 MHz, CD
2
2
Cl
2
,
1
3
1
98 K): δ 20.21 (s), −18.20 (s). C{ H} NMR (126 MHz,
CD Cl ): δ 103.74 (s, allyl C2), 62.96 (br s, allyl C1), 37.68 (br Conflicts of interest
2
2
s, allyl C3). Anal. Calcd for C67
Found: C, 75.88; H, 5.87. ESI-MS: calcd for [1-allyl] :
2 2
H61BNiP S : C, 75.79; H, 5.79.
+
There are no conflicts to declare.
7
41.1478; found: 741.1458.
0
Reaction of [(P S )Ni ] with C H I or C D I
Acknowledgements
2
2
2
5
2 5
0
[
1] (0.100 g, 0.14 mmol) and iodoethane (12.6 μL, 0.16 mmol)
were dissolved in THF (15 mL). The mixture was then was
treated with NaBPh (0.059 g, 0.17 mmol) and stirred at room
temperature for 15 min. The reaction mixture was filtered
through Celite, and the filtrate was subjected to mass spec-
troscopy (ESI-MS) and NMR spectroscopy analyses. The filtrate
was subsequently dried under vacuum, and the residue was
This work is supported by the “Thousand Plan” Youth
Program and the National Natural Science Foundation of
China (21871166 and 91427303).
4
Notes and references
extracted with CH
2
Cl
2
. The product was obtained as a red solid
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+
1
for [1-Et] : 729.1478; found, 729.1457. H NMR (500 MHz,
15762 | Dalton Trans., 2018, 47, 15757–15764
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