One-Pot Synthesis of 1,3-Bis(phosphinomethyl)arene PCP/PNP Pincer Ligands and Their Nickel Complexes

Article abstract of DOI:10.1021/acs.organomet.5b00671

A one-pot synthesis of arene-based PCP/PNP ligands has been developed. The reaction of 1,3-bis(bromomethyl)benzene or 2,6-bis(bromomethyl)pyridine with various chlorophosphines in acetonitrile afforded bis-phosphonium salts. These salts can then be reduced by magnesium powder to yield PCP or PNP ligands. In comparison to traditional synthetic methods for making PCP/PNP ligands involving the use of secondary phosphines, this new alternative method allows for the use of chlorophosphines, which are cheaper, safer to handle, and have a broader range of commercially available derivatives. This is especially true for the chlorophosphines with less bulky alkyl groups. Moreover, the one-pot procedure can be extended to allow for the direct synthesis of PCP/PNP nickel complexes. By using nickel powder as the reductant, the resulting nickel halide was found to directly undergo metalation with the PCP or PNP ligand to generate nickel complexes in high yields.

Full text of DOI:10.1021/acs.organomet.5b00671

Article
pubs.acs.org/Organometallics
One-Pot Synthesis of 1,3-Bis(phosphinomethyl)arene PCP/PNP Pincer
Ligands and Their Nickel Complexes
Wei-Chun Shih and Oleg V. Ozerov*
Department of Chemistry, Texas A&M University, 3255 TAMU, College Station, Texas 77842, United States
S
* Supporting Information
ABSTRACT: A one-pot synthesis of arene-based PCP/PNP ligands has
been developed. The reaction of 1,3-bis(bromomethyl)benzene or 2,6-
bis(bromomethyl)pyridine with various chlorophosphines in acetonitrile
afforded bis-phosphonium salts. These salts can then be reduced by
magnesium powder to yield PCP or PNP ligands. In comparison to
traditional synthetic methods for making PCP/PNP ligands involving the
use of secondary phosphines, this new alternative method allows for the
use of chlorophosphines, which are cheaper, safer to handle, and have a
broader range of commercially available derivatives. This is especially true for the chlorophosphines with less bulky alkyl groups.
Moreover, the one-pot procedure can be extended to allow for the direct synthesis of PCP/PNP nickel complexes. By using
nickel powder as the reductant, the resulting nickel halide was found to directly undergo metalation with the PCP or PNP ligand
to generate nickel complexes in high yields.
phosphines of modest molecular weight are pyrophoric liquids,
and the selection of commercially available secondary
phosphines is rather limited. An equally reliable PCP/PNP
synthesis utilizing chlorodiorganyl phosphines (ClPR2) would
be preferable because ClPR2 are generally not pyrophoric
(although still quite air- and moisture-sensitive), because a
somewhat large variety of ClPR2 compounds is commercially
available, and because ClPR2 compounds tend to be cheaper
than their HPR2 counterparts. The latter is understandable
since most HPR2 compounds are prepared by LiAlH4 reduction
of ClPR2,10 a rather hazardous procedure on many counts.
We surmised that if ClPR2 could successfully undergo an SN2
reaction with halomethylarenes, then there should be a practical
way to reduce the resultant halophosphonium salt to the
desired phosphine (Scheme 1, eq 2). ClPR2 should possess
reduced nucleophilicity compared to HPR2, but the reaction
still appeared plausible. This report describes its successful
implementation leading to a reliable method of preparation of a
series of PCP/PNP pincer ligands that obviates the need for
secondary phosphines. We also describe the incorporation of
this method into one-pot synthesis of PCP and PNP nickel
complexes, inspired by the one-pot synthesis of related
POCOP complexes of Ni by Zargarian et al.11
INTRODUCTION
■
Pincer ligands are broadly used in organometallic chemistry.1,2
They are typically defined as ligands that preferentially bind to
transition metals in a tridentate, meridional fashion. Pincer
complexes often possess exceptional thermal stability, which
may permit homogeneous catalysis at unusually high temper-
atures.3 They are also prized for the opportunity to position the
desired types of donors about the metal center in a predictable
and persistent fashion. Among the many families of pincers,
PCP and PNP ligands based on the bis(phosphinomethyl)-
benzene or -pyridine (Scheme 1, eq 1, simply PCP and PNP
Scheme 1. Synthesis of PCP or PNP Ligands
from here on) frameworks hold a particularly venerated place.
This is in part due to their historical significance,4 but also to
the number of groundbreaking discoveries in which they have
been instrumental.5
The traditional synthesis of these ligands involves an SN2
reaction of a secondary phosphine (HPR2) with bis-
(bromomethyl)benzene or -pyridine, followed by deprotona-
tion of the resultant bis(phosphonium) salt (Scheme 1, eq
1).1,6−9 This type of synthesis works very well, but the reliance
on the HPR2 starting material is not ideal. Secondary
RESULTS AND DISCUSSION
■
Synthesis of Benzylphosphonium Salts (3-X2) and
Benzylphosphines (3). To the best of our knowledge, the
only precedent for the preparation of quaternary phosphonium
salts by reactions of halodiorganyl phosphines with alkyl halides
is the succinct report of Kabachnik et al.,12 in which
Received: August 3, 2015
© XXXX American Chemical Society
A
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iododiisopropylphoshine was shown to react with methyl, allyl,
and benzyl halides. That work reported that chloro- and
bromodialkylphosphines did not react, but without providing
any details. We were eager to explore this reaction in greater
depth in the context of the proposed phosphine ligand
synthesis. Although alkyl chlorides are typically less expensive,
many simple alkyl bromides and iodides are also commercially
available. On the other hand, iododialkylphosphines are not,
and the ability to use chlorodialkylphosphines would be
strongly advantageous.
exchange with sodium iodide to form iodo-substituted
compounds 1-I and 2a-I. Since 1-I is a better SN2 electrophile,
access to it and regeneration of iodide anion upon SN2
substitution is likely the mechanism of acceleration.
On the preparative scale (2 mmol), thermolysis of 1-Br and
2a-Cl for 15 h at 100 °C led to 3a-BrCl in 85% isolated yield.
Analogously, the reaction of 1-Br with diisopropylchlorophos-
phine 2b-Cl under the same conditions led to 3b-BrCl in 87%
isolated yield (Scheme 2). Available literature evidence
Instead of the bis(halomethyl)arenes required for pincer
synthesis, we chose to explore benzyl halides (PhCH2X, 1-X) as
simpler model substrates. We initially focused on their
reactions13 with di-tert-butylchlorophosphine (2a-Cl), which
we expected to be more challenging compared to smaller
dialkylhalophosphines (Table 1). Gratifyingly, the reaction of
Scheme 2. Synthesis of Phosphonium Salts (3-X2) from
Chlorodialkylphosphines (2-Cl)
Table 1. Screening and Optimization of the Reaction of
a
Benzyl Halide with Halodi-tert-butylphosphine
indicates that R3PX2 (R = alkyl and X = Cl, Br, or I) are
ionic compounds as [R3PX][X] with approximately tetrahedral
phosphonium centers.16 We assume 3a-BrCl and 3b-BrCl are
also ionic because of their poor solubility in benzene, toluene,
diethyl ether, and THF.
3a-BrCl showed two resonances at 115.8 and 117.1 ppm in a
1:9 ratio in the 31P NMR spectrum, which are presumed to be
isomeric products with either a bromine or chlorine atom
attached to the phosphorus atom. 3b-BrCl showed only one
broad resonance at 111.2 ppm. This may be due to a stronger
preference for one halide or a more rapid halide exchange. The
observation of doublet signals for the benzylic protons of 3a-
b
entry
X1
X2
additive T (°C) t (h)
solvent
3a-X2 (%)
1
2
3
4
5
6
Br
Br
Cl
I
Cl
Cl
Cl
Cl
I
50
50
50
50
rt
15
15
15
15
15
15
15
15
20
15
C6D6
22
90
CD3CN
CD3CN
CD3CN
CD3CN
CD3CN
CD3CN
CD3CN
CD3CN
CD3CN
2
100
100
100
86
I
2
Br
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
100
50
50
50
50
BrCl (δ 4.69 ppm, JP−H = 7.3 Hz) and 3b-BrCl (δ 4.86 ppm,
c
2JP−H = 10 Hz) in the 1H NMR spectra was in agreement with
the formation of benzyl C−P bonds.
7
NaI
NaI
NaI
LiBr
d
8
e
100
26
9
Pure 3a-BrCl and 3b-BrCl were reduced by the reactions of
3-X2 with magnesium powder in acetonitrile at 0 °C for 2 h.
The reactions were allowed to stir at room temperature for
another 2 h, affording benzyldialkylphosphines 3a and 3b in
77% and 81% isolated yields, respectively (Scheme 3, eq 1). A
c
10
66
a
The reaction was carried out using 1-X (0.2 mmol) and 2a-X (0.2
b
mmol) in C6D6 or CD3CN in a J. Young NMR tube. The conversion
c
was determined by 31P NMR and 1H NMR. 1.0 equiv of additive was
d
e
added. 2.0 equiv of additive was added. 0.1 equiv of additive was
added.
Scheme 3. Synthesis of Benzyldialkylphosphines (3) from
Phosphonium Salts (3-X2)
PhCH2Br (1-Br) with 2a-Cl proceeded to give the product of
SN2 addition in both CD3CN and C6D6 as solvents (entries 1
and 2). The product formation was accelerated in the more
polar acetonitrile (consistent with Hughes−Ingold rules for SN2
reactions),14 and further optimization was conducted in this
solvent. Using benzyl chloride (1-Cl) as substrate resulted in a
much lower conversion than with benzyl bromide (1-Br) or
benzyl iodide (1-I) (entries 2−4). The highest reactivity can be
achieved by the reaction of benzyl iodide (1-I) with di-tert-
butyliodophosphine (2a-I)15 to afford 100% conversion in 15 h
at room temperature (entry 5). By using commercially available
1-Br and 2a-Cl, the reaction can be brought to completion in
15 h at 100 °C (entry 6). Addition of 1 equiv of sodium iodide
or lithium bromide accelerated the reaction between 1-Cl and
2a-Cl, leading to 86% or 66% conversion (entries 7 and 10) vs
2% without additives (entry 3). The use of 2 equiv of sodium
iodide further improved the conversion under the same
conditions to 100% (entry 8). Addition of 0.1 equiv of sodium
iodide gave lower conversion, but the observed 26% yield
(entry 9) indicated that NaI accelerates the reaction as a
catalyst. Both chlorides 1-Cl and 2a-Cl can undergo halide
73% overall isolated yield of 3b was obtained by the reaction of
1-Br with 2b-Cl followed by reduction in situ (Scheme 3, eq 2),
showing that isolation of pure 3a-BrCl/3b-BrCl was not
necessary. Pure products 3a and 3b were obtained simply by
pentane extraction from the crude mixture in acetonitrile. Two
immiscible layers were formed, and the acetonitrile layer
fortuitously retained not only magnesium halides but also the
small amounts of various unidentified side products.
B
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Synthesis of PCP and PNP Ligands (5). Having
optimized the synthesis of benzyldialkylphosphines, we set
out to tackle the one-pot synthesis of PCP or PNP pincer
ligands from chlorophosphines 2-Cl (Table 2). The reaction of
yield of 5d is due to lower conversion to 5d and the need for an
extra workup step to separate it from 5d′.
Synthesis of PCP and PNP Nickel Complexes (6 and
7). We envisaged that metallic powders other than Mg might be
suitable for reduction of phosphonium salts and through it
generate precursors for the formation of pincer complexes. This
line of thought was inspired by the similar one-pot concept of
synthesis of P°COP-type nickel complexes from metallic nickel
powder, resorcinol, and 2b-Cl reported by the Zargarian group
(Scheme 4).11 In the Zargarian synthesis, the interaction of the
a
Table 2. Synthesis of Various PCP and PNP Ligands
Scheme 4. Proposed Mechanism of (P°COP)NiCl Formation
in the Study by Zargarian et al.11
ligand precursors can be viewed to produce HCl equivalents,
which react with Ni(0) to give NiCl2 for the metalation. On the
other hand, our syntheses (Scheme 5) here can be viewed to
produce formal dihalogen (X2) equivalents that then give rise to
NiX2.
a
General conditions: a mixture of 4 (2.0 mmol) and 2-Cl (4.0 mmol)
in CH3CN was stirred at 100 °C for 15 h, and then Mg powder (4.0−
5.0 mmol) was added to the mixture. See Experimental Section for
procedure details. Isolated yield. Sodium iodide (8.0 mmol) was
added. Larger scale preparation: a mixture of 4a-Cl2 (20.0 mmol) and
b
c
Scheme 5. Proposed Mechanism of (PCP)NiX Formation
d
2a-Cl (40.0 mmol) in CH3CN with NaI was refluxed for 15 h, and
then Mg powder (40.0 mmol) was added to the mixture. See
Experimental Section for procedure details.
1,3-bis(bromomethyl)benzene (4a-Br2) with 2b-Cl in acetoni-
trile at 100 °C for 15 h produced a copious amount of a poorly
soluble product, which is assumed to be the corresponding bis-
phosphonium salt 5a-Br2Cl2; it resonated at a frequency (δ
110.9 ppm) similar to 3b-BrCl by 31P NMR spectroscopy.
Reduction of this bis-phosphonium salt with magnesium
powder in situ at 0 °C followed by gradual warming to
ambient temperature gave the iPr(PCP)H ligand 5a in 70%
overall isolated yield (entry 1). The analogous synthesis of the
bulkier tBu(PCP)H (5b) from 4a-Br2 and 2a-Cl afforded a
yellow, viscous oil in 76% yield, but only with ∼75% purity.17
Attempts to purify 5b by chromatography or recrystallization
were unsuccessful. However, reaction of 4a-Cl2 and 2a-Cl with
4 equiv of sodium iodide followed by reduction with Mg led to
a pure off-white powder of 5b in 64% isolated yield (entry 2,
0.5 g scale). On a larger scale, analogous preparation yielded 4.1
g as a 52% isolated yield of 5b. The less sterically imposing
ligand Et(PCP)H (5c) was successfully isolated in 78% yield
starting from 4a-Br2 and 2c-Cl (entry 3). Benzyl C−P bond
cleavage was observed in the synthesis of Ph(PCP)H 5d and
iPr(PNP) 5e, which gave rise to the formation of (3-
methylbenzyl)diphenylphosphine (5d′) and 5e′ as side
products.17 The oily nature of 5e and 5e′ precluded their
separation (entry 5), but it proved possible to obtain pure 5d in
42% yield by recrystallization (entry 4). The lower isolated
When the mixture of 4a-Br2, 2b-Cl, and metallic nickel
powder in acetonitrile was heated at 100 °C for 40 h, the
metalated products 6-Cl18 and 6-Br19 were isolated in 50%
combined yield. The reaction yield was increased to 70% by the
addition of 1.0 equiv of 2,6-lutidine as base to the mixture of 5a,
2c, and Ni prior to thermolysis (Scheme 6, eq 1). The ratio of
6-Br and 6-Cl was ca. 9:1 based on the 31P NMR spectrum (6-
Br: δ 61.7 ppm, 6-Cl: δ 60.6 ppm). The mixture could be fully
converted to 6-Br by treatment with 2 equiv of lithium bromide
in toluene at room temperature overnight. Most likely, the
bis(halophosphonium) salt 5a-Br2Cl2 is formed first and then
was reduced by Ni metal to give rise to 5a, which undergoes
metalation to 6 with NiX2 formed in the reduction of 5a-Br2Cl2
(Scheme 5). The analogous [iPr(PNP)NiBr][Br] complex 7 was
also synthesized by the reaction of 4b-Br2 and 2b-Br20 without
C
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1
(ppm). For H and 13C NMR, the residual solvent peak was used to
reference the spectra (1H NMR: δ 7.16 for C6D6, 7.26 for CDCl3, 5.32
for CD2Cl2, and 1.94 for CD3CN; 13C NMR: δ 128.06 for C6D6, 77.16
for CDCl3, 53.84 for CD2Cl2, and 118.26 for CD3CN).
Scheme 6. Synthesis of iPr(PCP)NiX (6) and
[iPr(PNP)NiBr][Br] (7)
Synthesis of Di-tert-butyliodophosphine (2a-I). Method 1: In
a 25 mL Schlenk flask, to the toluene solution (10 mL) of di-tert-
butylchlorophosphine (2a-Cl, 1.80 g, 10.0 mmol) was added sodium
iodide (3.00 g, 20.0 mmol), and the reaction mixture was stirred at
room temperature for 24 h. After the solution was qucikly filtered
through a short pad of Celite and silica gel, the filtrate was collected
and the volatiles were removed under vacuum, yielding a yellow liquid
that appeared ca. 95% pure by NMR spectroscopy. Isolated yield: 2.24
g, 82%. Method 2: In a 10 mL Schlenk flask, to the pentane solution (1
mL) of di-tert-butylchlorophosphine (2a-Cl, 90 mg, 0.50 mmol) was
added trimethylsilyl iodide (200 mg, 1.00 mmol), and the reaction
mixture was stirred at room temperature for 10 min. After the solution
was filtered through a pad of Celite, the filtrate was collected and the
volatiles were removed under vacuum, yielding a yellow liquid (129
base (Scheme 6, eq 2). In control experiments, Ni powder did
not react with 2b-Cl in CD3CN (100 °C, 15 h) and gave only
ca. 30% conversion of 4a-Br2 under analogous conditions to
benzyl radical coupling products containing Ar−CH2−CH2−Ar
1
mg, 95%) that appeared >98% pure by NMR spectroscopy. H NMR
(500 MHz, C6D6): δ 1.19 (d, JH−P = 12.0 Hz, 18 H). 13C{1H} NMR
(126 MHz, C6D6): δ 33.1 (d, JP−C = 47.0 Hz), 29.7 (d, JP−C = 16.4
Hz). 31P{1H} NMR (202 MHz, C6D6): δ 135.8.
1
moieties (as evidenced by H NMR resonances in the 2.8−3.0
Synthesis of Bromodiisopropylphosphine (2b-Br). In a 25 mL
Schlenk flask, to the toluene solution (10 mL) of chlorodiisopropyl-
phosphine (2b-Cl, 1.53 g, 10.0 mmol) was added lithium bromide
(1.74 g, 20.0 mmol), and the reaction mixture was stirred at room
temperature for 24 h. After the solution was filtered through a pad of
Celite and silica gel, the filtrate was collected and the volatiles were
removed under vacuum, yielding a colorless liquid. Isolated yield: 1.58
ppm region). These products were not observed in the one-pot
syntheses of 6. It is worth noting that Ni powder can be added
to the reaction mixture from the start, while Mg has to be
added only after the formation of halophosphonium salts is
allowed to take place.
g, 80%. 1H NMR (500 MHz, C6D6): δ 1.70 (m, 2 H), 0.98 (dd, JH−H
=
CONCLUSION
■
13 Hz, JH−P = 6.8 Hz, 12 H). 13C{1H} NMR (126 MHz, C6D6): δ 28.1
(d, JP−C = 34 Hz), 18.6 (d, JP−C = 14 Hz). 31P{1H} NMR (202 MHz,
C6D6): δ 134.7. HRMS (ESI): calcd for C6H14BrLiP (M + Li)+
203.0177, found 203.0178.
The synthesis of PCP- and PNP-type pincer ligands and their
nickel complexes was achieved via utilization of inexpensive and
commercially available chlorophosphines. Due to the electron-
withdrawing nature of the chlorine atom on the phosphorus,
chlorophosphines are weaker nucleophiles but safer reactants
than secondary phosphines, particularly for the phosphines
containing less sterically imposing alkyl groups. Moreover, we
discovered metallic nickel powder was not only a suitable
reagent for the reduction of halophosphonium salts to
phosphine ligands but also the nickel precursor for the
formation of PCP and PNP nickel complexes. This synthetic
strategy permits one-pot synthesis of PCP and PNP nickel
complexes from commercially available, nonpyrophoric21
materials.
Synthesis of 3a-BrCl. In a screw-cap culture tube, to the
acetonitrile solution (10 mL) of benzyl bromide (342 mg, 2.00
mmol) was added 2a-Cl (361 mg, 2.00 mmol), and the reaction
mixture was stirred at 100 °C for 15 h. After cooling to room
temperature, the volatiles were removed under vacuum, and the
product was washed with pentane, yielding a white solid. Isolated
yield: 597 mg, 85%. The product was a mixture of two isomers in ca.
1
9:1 ratio based on 31P NMR (117.1 and 115.8 ppm) and H NMR
spectra. The spectroscopic data for the major isomer follow. 1H NMR
(500 MHz, CD3CN): δ 7.61 (m, 2 H), 7.41 (m, 3 H), 4.69 (d, JP−H
=
7.3 Hz, 2 H), 1.52 (d, JP−H = 19 Hz, 18 H). 13C{1H} NMR (126 MHz,
CD3CN): δ 132.2 (d, JP−C = 5.7 Hz), 130.0 (s), 129.7 (m), 129.5 (d,
JP−C = 11 Hz), 42.3 (d, JP−C = 23 Hz), 29.5 (d, JP−C = 27 Hz), 27.0 (d,
JP−C = 4.8 Hz). 31P{1H} NMR (202 MHz, CD3CN): δ 117.1. HRMS
(MALDI): calcd for C15H25ClP (M − Br)+ 271.1376, found 271.1360.
Synthesis of 3b-BrCl. In a screw-cap culture tube, to the
acetonitrile solution (10 mL) of benzyl bromide (342 mg, 2.00 mmol)
was added 2b-Cl (305 mg, 2.00 mmol), and the reaction mixture was
stirred at 100 °C for 15 h. After cooling to room temperature, the
volatiles were removed under vacuum, and the product was washed
EXPERIMENTAL SECTION
■
General Considerations. Unless otherwise noted, all manipu-
lations were performed under argon, using standard glovebox or
Schlenk line techniques. Screw-cap culture tubes with PTFE-lined
phenolic caps were used to perform reactions. Toluene, THF, pentane,
and diethyl ether were dried and deoxygenated (by purging) using a
solvent purification system (Innovative Technology Pure Solv MD-5
solvent purification system) and stored over molecular sieves in an Ar-
filled glovebox. C6D6 was dried over NaK/Ph2CO/18-crown-6,
distilled, and stored over molecular sieves in an Ar-filled glovebox.
Acetonitrile, CD3CN, CDCl3, CH2Cl2, CD2Cl2, and 2,6-lutidine were
dried over CaH2, distilled or vacuum transferred, and stored over
molecular sieves in an Ar-filled glovebox. Benzyl chloride, benzyl
bromide, and benzyl iodide were degassed by freeze−pump−thaw and
stored over molecular sieves in an Ar-filled glovebox. Sodium iodide
was purified by recrystallization in acetone and dried under vacuum at
70 °C for 12 h. Celite and silica gel were dried at 200 °C overnight
under vacuum and then stored inside a glovebox. All other chemicals
were used as received from commercial vendors. NMR spectra were
recorded on a Varian Inova 300, Mercury 300 (1H NMR, 299.952
MHz; 13C NMR, 75.421 MHz; 31P NMR, 121.422 MHz), and Inova
NMR 500 (1H NMR, 499.703 MHz; 13C NMR, 125.697 MHz; 31P
NMR, 202.265 MHz) spectrometers. Chemical shifts are reported in δ
1
with pentane, yielding a white solid. Isolated yield: 560 mg, 87%. H
NMR (500 MHz, CD3CN): δ 7.57 (m, 2 H), 7.40 (m, 3 H), 4.86 (d,
JP−H = 10 Hz, 2 H), 3.43 (m, 2 H), 1.32 (dd, JP−H = 21 Hz, JH−H = 7.0
Hz, 12 H). 13C{1H} NMR (126 MHz, CD3CN): δ 131.8 (d, JP−C = 5.9
Hz), 130.2 (d, JP−C = 2.7 Hz), 129.8 (m), 128.4 (d, JP−C = 10 Hz),
31.4 (d, JP−C = 32 Hz), 28.4 (d, JP−C = 33 Hz), 16.5 (s). 31P{1H} NMR
(202 MHz, CD3CN): δ 111.2 (br s).
Synthesis of PhCH2PtBu2 (3a). In a screw-cap culture tube, to the
acetonitrile solution (5 mL) of 3a-BrCl (352 mg, 1.00 mmol) was
added magnesium powder (37 mg, 1.50 mmol), and the reaction
mixture was stirred at 0 °C for 2 h. The reaction was allowed to warm
to room temperature and stirred for another 2 h. The product was
extracted with pentane (3 × 10 mL), and the volatiles were removed
1
under vacuum, yielding a yellow oil. Isolated yield: 191 mg, 81%. H
NMR (500 MHz, CDCl3): δ 7.32 (d, JH−H = 7.5 Hz, 2 H), 7.21 (t,
JH−H = 7.6 Hz, 2 H), 7.09 (t, JH−H = 7.2 Hz, 1 H), 2.83 (d, JP−H = 3.0
D
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Hz, 2 H), 1.12 (d, JP−H = 11 Hz, 18 H). 13C{1H} NMR (126 MHz,
CDCl3): δ 141.5 (d, JP−C = 12 Hz), 129.5 (d, JP−C = 8.5 Hz), 128.2,
125.3 (d, JP−C = 2.1 Hz), 31.8 (d, JP−C = 22 Hz), 29.8 (d, JP−C = 13
Hz), 28.5 (d, JP−C = 24 Hz). 31P{1H} NMR (202 MHz, CDCl3): δ
35.9. NMR spectroscopic data were consistent with those previously
reported in the literature.8,22
26 Hz). 31P{1H} NMR (202 MHz, C6D6): δ 33.6. NMR spectroscopic
data were consistent with those previously reported in the literature.10a
Larger Scale Preparation of tBu(PCP)H (5b). In a 250 mL
Schlenk flask, to the acetonitrile solution (100 mL) of m-xylylene
dichloride (4a-Cl2) (3.50 g, 20.0 mmol) were added 2a-Cl (7.23 g,
40.0 mmol) and sodium iodide (12.0 g, 80.0 mmol), and the reaction
mixture was refluxed under argon for 15 h. After cooling to room
temperature, magnesium powder (0.97 g, 40.0 mmol) was added
under argon, and the reaction mixture was stirred at room temperature
for 36 h. The reaction was filtered through a pad of silica gel, and the
product was extracted with pentane (3 × 75 mL). The volatiles were
removed under vacuum, yielding an off-white solid. Isolated yield: 4.10
Synthesis of PhCH2PiPr2 (3b). In a screw-cap culture tube, to the
acetonitrile solution (5 mL) of 3b-BrCl (324 mg, 1.00 mmol) was
added magnesium powder (37 mg, 1.50 mmol), and the reaction
mixture was stirred at 0 °C for 2 h. The reaction was allowed to warm
to room temperature and stirred for another 2 h. The product was
extracted with pentane (3 × 5 mL), and the volatiles were removed
under vacuum, yielding a colorless oil. Isolated yield: 161 mg, 77%. 1H
NMR (500 MHz, CD2Cl2): δ 7.29 (m, 4 H), 7.17 (m, 1 H), 2.82 (d,
JP−H = 1.6 Hz, 2 H), 1.78 (dsp, JP−H = 2.0 Hz, JH−H = 7.1 Hz, 2 H),
1.09 (m, 12 H). 13C{1H} NMR (126 MHz, CD2Cl2): δ 141.0 (d, JP−C
= 8.5 Hz), 129.8 (d, JP−C = 6.9 Hz), 128.7 (d, JP−C = 0.8 Hz), 125.8 (d,
JP−C = 2.2 Hz), 30.1 (d, JP−C = 21 Hz), 23.9 (d, JP−C = 15 Hz), 20.0 (d,
JP−C = 14 Hz), 19.5 (d, JP−C = 11 Hz). 31P{1H} NMR (202 MHz,
CD2Cl2): δ 12.2. NMR spectroscopic data were consistent with those
previously reported in the literature.23
g, 52%. 1H NMR (500 MHz, C6D6): δ 7.59 (br s, 1 H), 7.26 (d, JP−H
7.5 Hz, 2 H), 7.14 (m, 1 H), 2.76 (d, JP−H = 2.0 Hz, 4 H), 1.06 (d, JP−H
= 11 Hz, 36 H). 13C{1H} NMR (126 MHz, C6D6): δ 141.7 (d, JP−C
=
=
12 Hz), 131.5 (vt, JP−C = 8.4 Hz), 128.4 (s), 127.2 (dd, JP−C = 9.2 Hz,
JP−C = 1.8 Hz), 31.8 (d, JP−C = 25 Hz), 30.1 (d, JP−C = 14 Hz), 29.2 (d,
JP−C = 26 Hz). 31P{1H} NMR (202 MHz, C6D6): δ 33.6. NMR
spectroscopic data were consistent with those previously reported in
the literature.10a
Synthesis of Et(PCP)H (5c). In a screw-cap culture tube, to the
acetonitrile solution (10 mL) of 4a-Br2 (528 mg, 2.00 mmol) was
added diethylchlorophosphine (2c-Cl, 498 mg, 4.00 mmol), and the
reaction mixture was stirred at 100 °C for 15 h. After cooling to room
temperature, magnesium powder (122 mg, 5.00 mmol) was added
under argon, and the reaction mixture was stirred at 0 °C for 2 h. The
reaction was allowed to warm to room temperature and stirred for
another 2 h. The product was extracted with pentane (3 × 20 mL),
and the volatiles were removed under vacuum, yielding a colorless oil.
One-Pot Synthesis of PhCH2PiPr2 (3b). In a screw-cap culture
tube, to the acetonitrile solution (10 mL) of benzyl bromide (342 mg,
2.00 mmol) was added 2b-Cl (305 mg, 2.00 mmol), and the reaction
mixture was stirred at 100 °C for 15 h. After cooling to room
temperature, magnesium powder (73 mg, 3.00 mmol) was added
under argon, and the reaction mixture was stirred at 0 °C for 2 h. The
reaction was allowed to warm to room temperature and stirred for
another 2 h. The product was extracted by pentane (3 × 20 mL), and
the volatiles were removed under vacuum, yielding a yellow oil.
Isolated yield: 305 mg, 73%. 1H NMR (500 MHz, CD2Cl2): δ 7.29 (m,
1
Isolated yield: 440 mg, 78%. H NMR (500 MHz, C6D6): δ 7.11 (t,
JH−H = 8.0 Hz, 1 H), 7.08 (m, 1 H), 6.97 (m, 2 H), 2.61 (s, 4 H), 1.21
(m, 8 H), 0.96 (t, JH−H = 7.1 Hz, 6 H), 0.93 (t, JH−H = 7.1 Hz, 6 H).
4 H), 7.17 (m, 1 H), 2.82 (d, JP−H = 1.6 Hz, 2 H), 1.78 (dsp, JP−H
=
13C{1H} NMR (126 MHz, C6D6): δ 138.8 (dd, JP−C = 3.9 Hz, JP−C
=
2.0 Hz, JH−H = 7.1 Hz, 2 H), 1.09 (m, 12 H). 13C{1H} NMR (126
MHz, CD2Cl2): δ 141.0 (d, JP−C = 8.5 Hz), 129.8 (d, JP−C = 6.9 Hz),
128.7 (d, JP−C = 0.8 Hz), 125.8 (d, JP−C = 2.2 Hz), 30.1 (d, JP−C = 21
Hz), 23.9 (d, JP−C = 15 Hz), 20.0 (d, JP−C = 14 Hz), 19.5 (d, JP−C = 11
Hz). 31P{1H} NMR (202 MHz, CD2Cl2): δ 12.2. NMR spectroscopic
data were consistent with those previously reported in the literature.23
Synthesis of iPr(PCP)H (5a). In a screw-cap culture tube, to the
acetonitrile solution (10 mL) of m-xylylene dibromide (4a-Br2) (528
mg, 2.00 mmol) was added 2b-Cl (610 mg, 4.00 mmol), and the
reaction mixture was stirred at 100 °C for 15 h. After cooling to room
temperature, magnesium powder (122 mg, 5.00 mmol) was added
under argon, and the reaction mixture was stirred at 0 °C for 2 h. The
reaction was allowed to warm to room temperature and stirred for
another 2 h. The product was extracted with pentane (3 × 20 mL),
and the volatiles were removed under vacuum, yielding a yellow oil.
Isolated yield: 472 mg, 70%. 1H NMR (300 MHz, C6D6): δ 7.47 (br s,
1.2 Hz), 130.4 (vt, JP−C = 5.4 Hz), 128.5 (vt, JP−C = 1.0 Hz), 126.8
(dd, JP−C = 5.3 Hz, JP−C = 2.0 Hz), 34.3 (d, JP−C = 19 Hz), 19.3 (d,
JP−C = 15 Hz), 10.0 (d, JP−C = 14 Hz). 31P{1H} NMR (202 MHz,
C6D6): δ −15.9. HRMS (ESI): calcd for C16H29P2 (M + H)+
283.1745, found 283.1746.
Synthesis of Ph(PCP)H (5d). In a screw-cap culture tube, to the
acetonitrile solution (10 mL) of 4a-Br2 (528 mg, 2.00 mmol) was
added diphenylchlorophosphine (2d-Cl, 882 mg, 4.00 mmol), and the
reaction mixture was stirred at 100 °C for 20 h. After cooling to room
temperature, magnesium powder (97.2 mg, 4.00 mmol) was added
under argon, and the reaction mixture was stirred at 0 °C for 2 h. The
reaction was allowed to warm to room temperature and stirred for
another 2 h. The volatiles were removed under vacuum, and then the
product was extracted with THF and filtered through Celite. The
filtrate was layered with pentane and placed in the freezer (−35 °C)
overnight, yielding a white crystalline solid. Isolated yield: 400 mg,
1 H), 7.18 (m, 3 H), 2.70 (br s, 4 H), 1.62 (dsp, JP−H = 1.9 Hz, JH−H
=
1
7.1 Hz, 4 H), 1.04 (d, JH−H = 7.0 Hz, 12 H), 1.00 (dd, JP−H = 0.9 Hz,
42%. H NMR (500 MHz, C6D6): δ 7.35 (m, 8 H), 7.05 (m, 13 H),
JH−H = 7.0 Hz, 12 H). 13C{1H} NMR (126 MHz, C6D6): δ 140.5 (d,
6.87 (t, JH−H = 7.5 Hz, 1 H), 6.80 (m, 2 H) 3.20 (s, 4 H). 13C{1H}
JP−C = 7.4 Hz), 130.9 (vt, JP−C = 7.0 Hz), 128.6 (s), 127.0 (dd, JP−C
=
NMR (126 MHz, C6D6): δ 139.2 (d, JP−C = 17 Hz), 137.9 (dd, JP−C
=
7.2 Hz, JP−C = 2.1 Hz), 30.3 (d, JP−C = 22 Hz), 23.8 (d, JP−C = 16 Hz),
19.9 (d, JP−C = 14 Hz), 19.5 (d, JP−C = 12 Hz). 31P{1H} NMR (121
MHz, C6D6): δ 10.3. NMR spectroscopic data were consistent with
those previously reported in the literature.24
8.2 Hz, JP−C = 1.6 Hz), 133.3 (d, JP−C = 19 Hz), 131.0 (vt, JP−C = 7.1
Hz), 128.8 (s), 128.6 (d, JP−C = 6.5 Hz), 128.4 (s), 127.4 (dd, JP−C
=
6.9 Hz, JP−C = 2.5 Hz), 36.3 (d, JP−C = 17 Hz). 31P{1H} NMR (202
MHz, C6D6): δ −9.9. NMR spectroscopic data were consistent with
those previously reported in the literature.25
Synthesis of tBu(PCP)H (5b). In a screw-cap culture tube, to the
acetonitrile solution (10 mL) of m-xylylene dichloride (4a-Cl2) (350
mg, 2.00 mmol) were added 2a-Cl (723 mg, 4.00 mmol) and sodium
iodide (1.20 g, 8.00 mmol), and the reaction mixture was stirred at 100
°C for 15 h. After cooling to room temperature, magnesium powder
(97 mg, 4.00 mmol) was added under argon, and the reaction mixture
was stirred at room temperature for 36 h. The product was extracted
with pentane (3 × 20 mL), and the volatiles were removed under
Synthesis of iPr(PNP) (5e). In a screw-cap culture tube, to the
acetonitrile solution (10 mL) of 2,6-bis(bromomethyl)pyridine (5b-
Br2, 532 mg, 2.00 mmol) was added 2b-Cl (610 mg, 4.00 mmol), and
the reaction mixture was stirred at 100 °C for 15 h. After cooling to
room temperature, magnesium powder (97.2 mg, 4.00 mmol) was
added under argon, and the reaction mixture was stirred at 0 °C for 2
h. The reaction was allowed to warm to room temperature and stirred
for 2 h. The product was extracted with pentane (3 × 20 mL), and the
volatiles were removed under vacuum, yielding a yellow oil. The
product was a mixture of 5e and 5e′, and the ratio was about 9:1 based
1
vacuum, yielding an off-white solid. Isolated yield: 507 mg, 64%. H
NMR (500 MHz, C6D6): δ 7.59 (br s, 1 H), 7.26 (d, JP−H = 7.5 Hz, 2
H), 7.14 (m, 1 H), 2.76 (d, JP−H = 2.0 Hz, 4 H), 1.06 (d, JP−H = 11 Hz,
36 H). 13C{1H} NMR (126 MHz, C6D6): δ 141.7 (d, JP−C = 12 Hz),
1
on the H NMR spectrum. Total yield: 519 mg, 79%. The NMR
131.5 (vt, JP−C = 8.4 Hz), 128.4 (s), 127.2 (dd, JP−C = 9.2 Hz, JP−C
1.8 Hz), 31.8 (d, JP−C = 25 Hz), 30.1 (d, JP−C = 14 Hz), 29.2 (d, JP−C
=
=
spectroscopic data for 5e follow. 1H NMR (500 MHz, CDCl3): δ 7.42
(t, JH−H = 8.0 Hz, 1 H), 7.07 (d, JH−H = 8.0 Hz, 2 H), 2.92 (d, JP−H
=
E
DOI: 10.1021/acs.organomet.5b00671
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
2.2 Hz, 4 H), 1.77 (dsp, JP−H = 2.0 Hz, JH−H = 7.0 Hz, 4 H), 1.03 (m,
24 H). 13C{1H} NMR (126 MHz, CDCl3): δ 159.9 (d, JP−C = 9.0 Hz),
136.2 (s), 120.5 (d, JP−C = 7.7 Hz), 32.5 (d, JP−C = 21 Hz), 23.6 (d,
JP−C = 14 Hz), 19.8 (d, JP−C = 15 Hz), 19.2 (d, JP−C = 11 Hz). 31P{1H}
NMR (202 MHz, CDCl3): δ 12.7. NMR spectroscopic data were
consistent with those previously reported in the literature.26
of Education of Taiwan (Study Abroad Scholarship to W.-C.S.).
We are grateful to Prof. David Milstein and Dr. Moran Feller of
the Weizmann Institute of Science for insightful discussions.
REFERENCES
■
Synthesis of iPr(PCP)NiBr (6-Br). In a screw-cap culture tube, to
the acetonitrile solution (10 mL) of 4a-Br2 (528 mg, 2.00 mmol) were
added nickel powder (294 mg, 5.00 mmol), 2b-Cl (610 mg, 4.00
mmol), and 2,6-lutidine (214 mg, 2.00 mmol). The reaction mixture
was stirred at 100 °C for 40 h. After cooling to room temperature,
diethyl ether (3 × 10 mL) was added and the mixture was filtered
through silica gel. The filtrate was collected, and the volatiles were
removed under vacuum, yielding a yellow solid. Isolated yield: 659 mg,
70%. The product was a mixture of 6-Br and 6-Cl, and the ratio was
about 9:1 based on the 31P NMR spectrum (61.7 ppm for 6-Br and
60.6 ppm for 6-Cl). The mixture can be fully converted to 6-Br by
reacting the mixture with 2 equiv of lithium bromide in toluene at rt
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1
overnight. The NMR spectroscopic data for 6-Br follow. H NMR
(500 MHz, CD2Cl2): δ 6.91 (br s, 3 H), 3.09 (vt, JP−H = 4.0 Hz, 4 H),
2.37 (m, 4 H), 1.45 (dvt, JH−H ≈ JP−H ≈ 7.8 Hz, 12 H), 1.18 (dvt, JH−H
≈ JP−H ≈ 7.0 Hz, 12 H). 13C{1H} NMR (126 MHz, CD2Cl2): δ 159.5
(vt, JP−C = 16 Hz), 152.8 (vt, JP−C = 13 Hz), 125.5 (s), 122.5 (vt, JP−C
= 8.9 Hz), 33.1 (vt, JP−C = 13 Hz), 24.2 (vt, JP−C = 11 Hz), 19.1 (s),
18.3 (s). 31P{1H} NMR (202 MHz, CD2Cl2): δ 61.7. NMR
spectroscopic data were consistent with those previously reported in
the literature.19c
(5) (a) Goldman, A. S.; Roy, A. H.; Huang, Z.; Ahuja, R.; Schinski,
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the acetonitrile solution (2 mL) of 4b-Br2 (53 mg, 0.20 mmol) were
added nickel powder (29 mg, 0.50 mmol) and 2b-Br (90 mg, 0.40
mmol). The reaction mixture was stirred at 100 °C for 15 h. After
cooling to room temperature, the volatiles were removed under
vacuum, and the product was extracted with CH2Cl2 and filtered
through Celite. The filtrate was collected, and the volatiles were
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1
mg, 80%. H NMR (500 MHz, CD3CN): δ 8.09 (br, 1 H), 7.78 (d,
JH−H = 6.7 Hz, 2 H), 3.79 (s, 4 H), 2.50 (br s, 4 H), 1.53 (dvt, JH−H
≈
1
(10) (a) Gusev, D. G.; Madott, M.; Dolgushin, F. M.; Lyssenko, K.
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JP−H ≈ 8.0 Hz, 12 H), 1.38 (dvt, JH−H ≈ JP−H ≈ 7.3 Hz, 12 H). H
NMR (500 MHz, CDCl3): δ 11.23 (br s, 2 H), 9.72 (br s, 1 H), 5.17
(br s, 4 H), 2.99 (br s, 4 H), 2.09 (br s, 12 H), 1.92 (br s, 12 H).
13C{1H} NMR (126 MHz, CDCl3): δ 165.4, 147.5, 125.5, 36.2, 25.6
(vt, JP−C = 13 Hz), 25.0, 20.3. 31P{1H} NMR (202 MHz, CD3CN): δ
50.5. 31P{1H} NMR (202 MHz, CDCl3): 50.3. NMR spectroscopic
data were similar to [iPr(PNP)NiCl][Cl] previously reported in the
literature.27
(12) Kabachnik, M. M.; Khomutova, Y. A.; Beletskaya, I. P. Russ. J.
Org. Chem. 1999, 35, 26−27.
(13) Halophosphonium salts are easily hydrolyzed, trialkylphoshines
are easily oxidized by dioxygen, and halodialkylphosphines are
sensitive to both H2O and O2. Therefore all manipulations involving
these materials were carried out under dry argon.
(14) Hughes, E. D.; Ingold, C. K. J. Chem. Soc. 1935, 244−255.
(15) Di-tert-butyliodophosphine (2a-I) is not commercially available
and was prepared from di-tert-butylchlorophosphine (2a-Cl) and
sodium iodide. See Experimental Section.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00671.
■
S
Additional experimental details and pictorial NMR
spectra (PDF)
(16) (a) Bricklebank, N.; Godfrey, S. M.; Lane, H. P.; McAuliffe, C.
A.; Pritchard, R. G.; Moreno, J.-M. J. Chem. Soc., Dalton Trans. 1995,
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3815−3818.
AUTHOR INFORMATION
Corresponding Author
*E-mail: ozerov@chem.tamu.edu.
Notes
■
(17) See the Supporting Information.
The authors declare no competing financial interest.
(18) Boro, B. J.; Duesler, E. N.; Goldberg, K. I.; Kemp, R. A. Inorg.
Chem. 2009, 48, 5081−5087.
ACKNOWLEDGMENTS
■
́
́
(19) (a) Campora, J.; Palma, P.; del Río, D.; Alvarez, E.
We are grateful for the support of this research by the TAMU-
Weizmann Collaborative Research Program, the U.S. National
Science Foundation (grant CHE-1300299 to O.V.O.), the
Welch Foundation (grant A-1717 to O.V.O.), and the Ministry
Organometallics 2004, 23, 1652−1655. (b) Castonguay, A.;
Beauchamp, A. L.; Zargarian, D. Organometallics 2008, 27, 5723−
5732. (c) Martínez-Prieto, L. M.; Melero, C.; del Río, D.; Palma, P.;
́
Cam
́
pora, J.; Alvarez, E. Organometallics 2012, 31, 1425−1438.
F
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(20) Bromodiisopropylphosphine (2b-Br) needs to be separately
prepared from Chlorodiisopropylphosphine (2b-Cl) and lithium
bromide. See Experimental Section for procedure details.
(21) Very finely divided metal powders can be pyrophoric, but we
were able to use Mg powder (40−80 mesh) and Ni powder (APS 2.2−
3.0 μm), which are not pyrophoric.
(22) Ionkin, A. S.; Marshall, W. J.; Adelman, D. J.; Fones, B. B.; Fish,
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(27) Vogt, M.; Rivada-Wheelaghan, O.; Iron, M. A.; Leitus, G.;
Diskin-Posner, Y.; Shimon, L. J. W.; Ben-David, Y.; Milstein, D.
Organometallics 2013, 32, 300−308.
G
DOI: 10.1021/acs.organomet.5b00671
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