Nickel(II) complexes containing bidentate diarylamido phosphine chelates: Kumada couplings kinetically preferred to β-hydrogen elimination

Article abstract of DOI:10.1021/om5006353

A series of divalent nickel complexes containing diarylamido phosphine ligands of the type (o-ArNC₆H₄PR₂)^- (1a, Ar = 2,6-C₆H₃Me₂, R = Ph; 1b, Ar = 2,6-C₆H₃iPr₂, R = Ph; 1c, Ar = 2,6-C₆H₃Me₂, R = iPr; 1d, Ar = 2,6-C₆H₃iPr₂, R = iPr) have been prepared and structurally characterized. The dimeric nickel chloride derivatives {[1b-d]Ni(-Cl)}₂ (2b-d) were isolated as brick red microcrystals in high yields from the reactions of NiCl₂(DME) with either Li[1b-d](solv)_x or H[1b-d] in the presence of NEt₃. Similar reactions employing [1a]^-, however, generated homoleptic Ni[1a]₂ (3a) as paramagnetic, dark red prisms in high yield. Addition of trimethylphosphine to red solutions of 2b,c in THF at room temperature afforded emerald crystals of [1b,c]NiCl(PMe₃) (4b,c). Interestingly, solution NMR spectroscopic and X-ray crystallographic data of these PMe₃ adducts reveal the exclusive formation of cis-4b and trans-4c, as defined by the mutual orientation of the two phosphorus donors incorporated. Metathetical reactions of 4b,c with RMgCl (R = Me, CH₂SiMe₃, Ph) in THF at -35 °C produced high yields of red or brownish red crystalline [1b,c]NiR(PMe₃) (R = Me (5b,c), CH₂SiMe₃ (6b,c), Ph (7b,c)). Analogous reactions of 4c with EtMgCl or nBuMgCl, however, led instead to the isolation of the hydrido species [1c]NiH(PMe₃) (8c) in quantitative yield. Solution NMR data of the methyl complexes 5b,c indicate the presence of both cis and trans isomers; the major component of 5b is cis whereas that of 5c is trans. In contrast, complexes 6b,c, 7b,c, and 8c all exist exclusively in the trans form. The chloro complexes 2b-d are all active catalyst precursors for Kumada couplings under mild conditions. In particular, this catalysis is compatible with alkyl nucleophiles that contain β-hydrogen atoms, even in reactions with chlorobenzene. The X-ray structures of 2d, 3a, 4c, 5c, 6b,c, 7c, and 8c are presented.

Full text of DOI:10.1021/om5006353

Article
pubs.acs.org/Organometallics
Nickel(II) Complexes Containing Bidentate Diarylamido Phosphine
Chelates: Kumada Couplings Kinetically Preferred to β‑Hydrogen
Elimination
Ming-Tsz Chen,†,§ Wei-Ying Lee,†,§ Tzung-Ling Tsai,† and Lan-Chang Liang*,†,‡
†Department of Chemistry and Center for Nanoscience & Nanotechnology, National Sun Yat-sen University, 70 Lien-Hai Road,
Kaohsiung 80424, Taiwan
‡Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 80708, Taiwan
S
* Supporting Information
ABSTRACT: A series of divalent nickel complexes containing
diarylamido phosphine ligands of the type (o-ArNC6H4PR2)−
(1a, Ar = 2,6-C6H3Me2, R = Ph; 1b, Ar = 2,6-C6H3iPr2, R = Ph;
1c, Ar = 2,6-C6H3Me2, R = iPr; 1d, Ar = 2,6-C6H3iPr2, R = iPr)
have been prepared and structurally characterized. The dimeric
nickel chloride derivatives {[1b−d]Ni(μ-Cl)}2 (2b−d) were
isolated as brick red microcrystals in high yields from the
reactions of NiCl2(DME) with either Li[1b−d](solv)x or
H[1b−d] in the presence of NEt3. Similar reactions employing
[1a]−, however, generated homoleptic Ni[1a]2 (3a) as
paramagnetic, dark red prisms in high yield. Addition of
trimethylphosphine to red solutions of 2b,c in THF at room temperature afforded emerald crystals of [1b,c]NiCl(PMe3) (4b,c).
Interestingly, solution NMR spectroscopic and X-ray crystallographic data of these PMe3 adducts reveal the exclusive formation of
cis-4b and trans-4c, as defined by the mutual orientation of the two phosphorus donors incorporated. Metathetical reactions of 4b,c
with RMgCl (R = Me, CH2SiMe3, Ph) in THF at −35 °C produced high yields of red or brownish red crystalline [1b,c]NiR(PMe3)
(R = Me (5b,c), CH2SiMe3 (6b,c), Ph (7b,c)). Analogous reactions of 4c with EtMgCl or nBuMgCl, however, led instead to the
isolation of the hydrido species [1c]NiH(PMe3) (8c) in quantitative yield. Solution NMR data of the methyl complexes 5b,c
indicate the presence of both cis and trans isomers; the major component of 5b is cis whereas that of 5c is trans. In contrast,
complexes 6b,c, 7b,c, and 8c all exist exclusively in the trans form. The chloro complexes 2b−d are all active catalyst
precursors for Kumada couplings under mild conditions. In particular, this catalysis is compatible with alkyl nucleophiles
that contain β-hydrogen atoms, even in reactions with chlorobenzene. The X-ray structures of 2d, 3a, 4c, 5c, 6b,c, 7c, and 8c
are presented.
diarylamido phosphine complexes17−28 have led us to discover
INTRODUCTION
■
that nickel chloride complexes of PNP (Figure 1) are active
Tremendous efforts have been made in the past decades by
catalyst precursors for Kumada coupling reactions, including
synthetic chemists to find appropriate methods to facilitate
those involving β-hydrogen-containing alkyls.29 The success of
C−C bond-forming reactions.1,2 Transition-metal-catalyzed
this catalysis is ascribed, in part, to the unusual inherent nature
cross-coupling reactions of Grignard reagents with (pseudo)-
of organonickel complexes of PNP, wherein β-hydrogen
halogenated hydrocarbon electrophiles, generally referred to as
elimination is a thermodynamically uphill process,30 as corrobo-
Kumada couplings, have proven to be some of the most
rated by its reverse reactions in view of the principle of
powerful and valuable protocols for organic syntheses.3−5
microscopic reverse: i.e., insertion reactivity of [PNP]NiH with
Though widely employed, cross-coupling reactions, broadly
defined, often suffer from incompatibilities with alkyl building
blocks that contain β-hydrogen atoms, for which β-hydrogen
H2CCHR to give [PNP]NiCH2CH2R that is markedly
thermally stable even at elevated temperatures. In this particular
study, the nondissociative characteristics of the phosphorus donors
elimination is usually a kinetically more accessible process.6,7
incorporated are believed to be crucial.31 Upon the inclusion of
Moreover, the hydrido complexes thus produced are generally
an ethylene tethered amino donor that is characteristic of having
more thermodynamically stable than their corresponding
metal−alkyl precursors, in turn giving unwanted hydro-
Special Issue: Catalytic and Organometallic Chemistry of Earth-
dehalogenated instead of the desired cross-coupled products.
Abundant Metals
Several examples are now known to facilitate Kumada
couplings compatible with β-hydrogen-containing alkyl building
Received: June 15, 2014
blocks.8−16 Our interests in exploratory chemistry employing
© XXXX American Chemical Society
A
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to Kumada-type catalysis. Factors to determine thermodynamically
favored stereochemistry that these four-coordinate complexes
prefer, i.e., cis versus trans isomers, are also discussed.
RESULTS AND DISCUSSION
■
Equation 1 illustrates the synthetic strategies to prepare nickel
chloride complexes 2. Compounds 2c,d were both prepared in
a manner similar to that established previously for 2b33 as brick
red microcrystals in high yields by treating NiCl2(DME)34 with
either Li[1c−d](solv)2 (solv = THF for 1c, 0.5 DME for 1d)35
or H[1c−d]35 in the presence of NEt3. The solution NMR data of
these chloride complexes are all consistent with a structure having
a reflection symmetry. For instance, the 1H NMR spectrum of 2c
in C6D6 reveals a multiplet resonance at 1.61 ppm for the CHMe2
groups bound to the phosphorus donor and a singlet at 2.60 ppm
for the ortho N-aryl methyl groups. The phosphorus donor in
these nickel chloride complexes generally resonates significantly
downfield in comparison to those of their corresponding protio or
lithio precursors: e.g., 62.6 ppm for 2c versus −17.4 ppm for
H[1c]35 and −7.9 ppm for [1c]Li(THF)2.35 Compounds bearing
P-alkyl substituents, including protio, lithium, and nickel
derivatives, typically give rise to relatively downfield 31P signals
in comparison with their P-arylated counterparts: e.g., 62.6 ppm
for 2c and 60.2 ppm for 2d versus 32.5 ppm for 2b.33 Selected
NMR data are summarized in Table 1.
Figure 1. Representative examples of o-phenylene-derived amido
phosphine ligands.
higher hemilability, the PNN complexes of nickel alkyls also resist
β-elimination at elevated temperatures and facilitate effectively
catalytic Kumada couplings compatible with β-hydrogen-contain-
ing alkyl building blocks.32 A notable discrepancy in Kumada
activities between PNP- and PNN-ligated complexes concerns the
comparatively higher reactivity and selectivity of the latter.32 In line
with these intriguing results and the fundamental concepts
concerning dynamic κ2 and κ3 coordination modes of PNN, we
became interested in exploratory chemistry of nickel complexes
containing bidentate NP chelates,33 in particular their compatibility
with catalytic Kumada couplings involving β-hydrogen-containing
alkyls. In this contribution, we show evidence that β-hydrogen
elimination does occur in this particular system for thermody-
namic reasons, but complexes derived herein are yet superior
catalysts for β-hydrogen-containing alkyl Grignard reagents to
couple with aryl halides, including inherently more challenging
chloro electrophiles. We describe herein the synthesis and
structural characterization of nickel complexes supported by a
series of NP ligands [1a−d]− and their reactivity study with respect
NiCl2(DME), NEt3
H[1b−d] ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→⎯ {[1b−d]Ni(μ‐Cl)}2
2b−d
NiCl2(DME)
←⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ [1b−d]Li(solv)2
(1)
a
Table 1. Selected NMR Spectroscopic Data
NP
PMe3
compd
δP
δP
2JPP
δHα
3JPHα
δCα
2JPCα
b
H[1a]
−19.3
−13.7
32.0
c
[1a]Li(THF)2
d
{[1a]Ni(μ-Cl)}2 (2a)
e
H[1b]
−20.1
−12.0
32.5
e
[1b]Li(THF)2
c
{[1b]Ni(μ-Cl)}2 (2b)
c
[1b]NiCl(PMe3) (4b)
47.3
−17.0
−7.5
−18.0
−22.5
−22.3
88
25
cf
,
[1b]NiMe(PMe3) (5b)
[1b]NiMe(PMe3) (5b)
35.6
−0.12
−0.42
−0.70
4.0, 7.5
11.1
39, 59
cg
,
h
h
39.0
301
318
288
7.5, 12.0
N/A
N/A
i
[1b]NiCH2SiMe3(PMe3) (6b)
34.4
N/A
−13.4
151.9
18, 25
34, 39
c
[1b]NiPh(PMe3) (7b)
33.0
j
H[1c]
−17.4
−7.9
62.6
j
[1c]Li(THF)2
{[1c]Ni(μ-Cl)}2 (2c)
[1c]NiCl(PMe3) (4c)
51.1
−24.2
−20.0
−9.1
−26.3
−24.3
−19.9
314
296
22
f
[1c]NiMe(PMe3) (5c)
45.5
−0.69
−0.42
−0.93
7.0, 13.0
3.5, 7.5
−18.5
N/A
23, 34
g
h
h
[1c]NiMe(PMe3) (5c)
47.7
N/A
[1c]NiCH2SiMe3(PMe3) (6c)
[1c]NiPh(PMe3) (7c)
42.9
305
278
263
10.5, 15.5
−18.9
150.9
17, 30
34, 41
40.6
k
k
[1c]NiH(PMe3) (8c)
63.3
−20.2
72.5, 82.0
j
H[1d]
−17.2
−7.3
60.2
j
[1d]Li(DME)
{[1d]Ni(μ-Cl)}2 (2d)
a
b
f
Unless otherwise noted, all spectra were recorded in C6D6 at room temperature. Chemical shifts are given in ppm and coupling constants in Hz.
c
d
e
Data selected from ref 36. Data selected from ref 33. Tentative assignment (see context for details), recorded in THF. Data selected from ref 37.
g
Data summarized correspond to the major component of the two geometric isomers produced. Data summarized correspond to the minor
h
i
j
component of the two geometric isomers produced. Signal intensities are too low to confirm. Too broad to determine. Data selected from ref 35.
k
2
3
Chemical shift δH (instead of δHα) and coupling constants JPH (instead of JPHα) correspond to the nickel-bound hydride ligand, not Hα.
B
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Attempts to grow crystals of 2b,c for X-ray diffraction
analysis were not successful. X-ray-quality crystals of 2d,
however, were obtained by layering pentane on a concentrated
diethyl ether solution at −35 °C. As deduced in the previous
report,33 complexes 2 exist as chloride-bridged dimers in the
solid state. Figure 2 depicts the X-ray structure of 2d. The
selectively isolate the presumed 2a were not successful. Both
electronic and steric factors were considered to rationalize the
higher reactivity of 2a in comparison to that of 2b−d. On
electronic grounds, the nickel center in the former is more
electron deficient due to the incorporation of electronically less
releasing substituents at the NP donor atoms. Sterically, these
substituents are not bulky enough to prevent 2a from further
reactions that give ultimately the homoleptic 3a. This result is
consistent with what was demonstrated previously on the
overall steric size of these NP ligands following the order 1a <
1b < 1c < 1d.35
Figure 3 depicts the molecular structure of this homoleptic
complex. The coordination geometry of 3a is severely distorted
Figure 2. Molecular structure of 2d with thermal ellipsoids drawn at
the 35% probability level. Selected bond distances (Å) and angles
(deg): Ni1−N1, 1.881(7); Ni1−P1, 2.131(2); Ni1−Cl1, 2.196(3);
Ni1−Cl1A, 2.267(2); N1−Ni1−P1, 86.1(2); N1−Ni1−Cl1, 178.0(2);
P1−Ni1−Cl1, 92.3(1); N1−Ni1−Cl1A, 96.1(2); P1−Ni1−Cl1A,
177.4(1); Cl1−Ni1−Cl1A, 85.5(1); Ni1−Cl1−Ni1A, 94.5(1).
Figure 3. Molecular structure of 3a with thermal ellipsoids drawn at
the 35% probability level. Selected bond distances (Å) and angles
(deg): Ni1−N1, 1.965(2); Ni1−P1, 2.1588(8); N1−Ni1−N1A,
105.6(1); N1−Ni1−P1A, 153.00(7); N1−Ni1−P1, 83.34(7); P1A−
Ni1−P1, 100.32(4).
coordination geometry about nickel in 2d is best described as
distorted square planar, in which the Cl1A is datively bound to
Ni1, as evidenced by its relatively longer bond length of 0.071 Å
in comparison to that of Ni1−Cl1. The two datively bound
donors at nickel are thus mutually trans to each other, so are
the two formally anionic donors. The two o-phenylene rings lie
approximately on the same plane as the Ni2Cl2 rectangle,
wherein the two nickel atoms are separated by 3.277 Å. The N-
bound aryl and P-bound isopropyl groups are oriented virtually
axially or nearly perpendicular to the o-phenylene rings, thereby
creating significant steric protection for the nickel center at axial
positions. Other structural parameters are unexceptional. A
similar structure was also reported previously for a bromide-
bridged molecule.38
from both square planar and tetrahedral, having a dihedral
angle between two N1−Ni1−P1 planes of 40.89°: i.e., a
structure approximately halfway between these idealized
extremes. The same conclusion is also derived from Houser’s
40
four-coordinate geometry index τ4 of 0.38 for this molecule.
Such a distortion from the ideal square planar or tetrahedral
structures likely has something to do with the π−π stacking
(Figure S1; see the Supporting Information) that involves two
amido aryls (ring centroid distance 3.484 Å) or P-bound phenyl
groups (ring centroid distance 3.464 Å). Though bis-ligated
nickel complexes of [1b−d]− were not produced under the
conditions examined, these ligands, on the other hand, do show
a much higher propensity to give exclusively homoleptic
derivatives of zinc35 upon reactions of their corresponding
lithium complexes with ZnX2 (X = Cl, OAc), regardless of the
stoichiometry of the starting materials employed. In particular,
Zn[1d]2 rather than {[1d]Zn(μ-Cl)}2 or {[1d]Zn(μ-OAc)}2
was isolated, though this homoleptic complex is severely congested
sterically, as demonstrated by X-ray diffraction analysis.35 The
remaining structural parameters of 3a are unexceptional.
In contrast to 2b−d, complex 2a remains elusive.
33
Surprisingly, reactions employing H[1a]36 or Li[1a](THF)2
with strategies illustrated in eq 1 led instead to high-yield
isolation of dark red prismatic Ni[1a]2 (3a), as indicated by
X-ray diffraction analysis. Compound 3a is 31P NMR silent,
reflective of its paramagnetic nature. In principle, the presumed
nickel chloride 2a was also produced from the reactions
attempted. We reason that 2a, however, is transient and
inherently much more reactive than 2b−d to react with a
second equivalent of their corresponding protio or lithium
precursors, even if the reactions were conducted with
equimolar starting materials. Close scrutiny of the reaction
progress by 31P{1H} NMR spectroscopy revealed no observable
intermediates by the lithium route39 but occurrence of an
intermediate in the other strategy in <30 min, as evidenced by
a signal found at 32.0 ppm, presumably 2a. Attempts to
Direct alkylation of 2b−d with nonchelating41 Grignard
reagents (1 equiv per nickel) was attempted. Though reactions
involving those lacking β-hydrogen atoms appear to proceed
smoothly (e.g., treating 2c with MeMgCl in THF at −35 °C led
to quantitative formation of a new species whose 31P{1H}
NMR spectrum shows a singlet at 55 ppm), identification of the
products produced is thus far inconclusive. Similar reactions
employing β-hydrogen-containing Grignard reagents, on the
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Scheme 1. Synthesis of PMe3 Adducts of Nickel Chlorides 4b,c and Methyls 5b,c
other hand, gave mixtures of products as indicated by 31P{1H}
NMR studies. Characterization or purification of these mixtures
proved problematic.
species 8c could alternatively be prepared from the reaction of 4c
with LiHBEt3.
Though a wide variety of exogenous Lewis bases are
synthetically useful to facilitate the preparation of organonickel
complexes in this study, the inclusion of a phosphine, e.g.,
PMe3 in the current case, in these four-coordinate species
provides an excellent probe to investigate their thermodynamic
The most effective strategy in these pursuits involves the
participation of an exogenous Lewis base. Upon addition of
PMe3 in THF at room temperature, the red solutions of 2b,c
turned green, from which emerald crystals of [1b,c]NiCl(PMe3)
(4b,c) were isolated in high yields (Scheme 1). Subsequent
alkylation of 4b,c with RMgCl (R = Me, CH2SiMe3, Ph) in THF
at −35 °C successfully afforded high yields of [1b,c]NiR(PMe3)
(R = Me (5b,c), CH2SiMe3 (6b,c), Ph (7b,c)) as red or
brownish red crystals. Interestingly, treating 4c with EtMgCl or
nBuMgCl under similar conditions yielded instead the hydrido
species [1c]NiH(PMe3) (8c) quantitatively; analysis of the
reaction progress with time by 31P{1H} NMR spectrometry in
THF revealed in each of these reactions the formation of an
intermediate that we tentatively formulated as [1c]NiEt(PMe3)42
and [1c]NinBu(PMe3),43 respectively, in view of the resem-
blance of their chemical shifts and phosphorus−phosphorus
coupling constants to what was observed for alkylated major-5c
and 6c (Table 1). Attempts to isolate the presumed [1c]NiEt-
(PMe3) or [1c]NinBu(PMe3) were not successful due to the
inevitably concomitant presence of either 4c or 8c. The
generation of hydrido compound 8c is apparently a consequence
of β-hydrogen elimination of these organonickel species. Note
that this β-elimination is facile, as 8c begins to evolve in <15 min,
as indicated by 31P{1H} NMR studies. This result is in sharp
contrast to what was found in the PNP and PNN systems, wherein
β-hydrogen-containing organonickel complexes are thermally
stable29−32 even at elevated temperatures (e.g., 70 °C). The
reactivity of hydrido species 8c with respect to olefin insertion was
also examined. In the presence of 1-hexene (3 equiv, 25−60 °C in
C6D6), 8c appears to remain intact, as indicated by 31P{1H} NMR
2
preference in stereochemistry by the JPP values acquired from
31P{1H} NMR spectra of reaction aliquots. In principle, two
possible geometric isomers may be produced, i.e., cis and trans,
as defined by the orientation of the two phosphorus donors
incorporated. Interestingly, 31P{1H} NMR studies of all
complexes described herein revealed the presence of only one
of these possible isomers in solution, except for the methyl
complexes 5b,c. Though both isomers were observed for 5b,c,
the cis:trans ratio of the former is ca. 9:1 whereas that of the
latter is <1:11, as estimated by the relative intensities of two
pairs of doublet resonances observed in their corresponding
31P{1H} NMR spectra. As summarized in Table 1, the major
isomer of 5b exhibits in the 31P{1H} NMR spectrum two
doublet resonances with a coupling constant of 25 Hz, while
that of 5c shows a 2JPP value of 296 Hz. For those existing as a
single isomer, the chloro 4b is the only cis-disposed complex,
whereas the others are all trans. In other words, all [1c]−-
derived PMe3 adducts of nickel reported herein are trans-
disposed except for the minor isomer of the methyl species 5c.
With the coordination of [1b]−, the trimethylsilyl compound 6b,
phenyl compound 7b, and the minor isomer of methyl 5b are
trans but the chloro species 4b and the major isomer of 5b are
cis. As a result, the isomeric preference of these nickel complexes
is clearly a function of the identity of both NP and the
monodentate anionic ligands (i.e., Cl, alkyl, Ph, H) incorporated.
1
The H and 13C{1H} NMR data are also informative in
determining the solution structures of these PMe3 adducts. In
general, these complexes are characteristic of having a time-
averaged Cs-symmetric structure; for instance, only one set of
signals is observed for the two o-alkyl groups in the amido
substituents. The hydrido ligand in 8c and the Hα/Cα atoms in
complexes 5−7 generally give rise to a diagnostic doublet of
doublets resonance; for instance, the former ligand resonates
characteristically at −20 ppm with 2JHP values of 73 and 82 Hz.
These data are all quite comparable to those of [PNP]NiH,
particularly the species that carries different substituents at the
two phosphorus donors,30 thus emphasizing the cis-disposed
orientation of the hydride ligand in 8c with respect to these
phosphorus atoms. Collectively, these NMR data are all consistent
with a solution structure having square-planar geometry.
1
studies. Examination of these reaction mixtures by H NMR and
GC/MS analyses, however, confirmed the production of oligo-
(1-hexene), thereby revealing the catalytic nature of 8c in 1-hexene
oligomerization that involves 1-hexene polyinsertion followed by
β-hydrogen elimination. Note that the presumed [1c]Ni(hexyl)-
(PMe3) was not observed spectroscopically. Collectively,
β-elimination of [1c]Ni(CH2CH2R)(PMe3) to produce 8c is
thus thermodynamically downhill. In comparison with those
compounds derived from PNP and PNN, this present result is
attributable to the lack of a third donor atom in the amido
phosphine chelates incorporated. Though overall four-coordinate in
conformation, the nickel center in 8c and its presumed olefin
insertion products possesses, upon PMe3 dissociation,44,45 a vacant
d orbital that is low-lying in energy available to either olefin
coordination or β-hydrogen elimination, respectively. The hydrido
X-ray studies of complexes 4−8 were attempted to elucidate
the solid-state structures of these molecules and to evaluate the
D
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possibilities of resolving the decisive factors that determine
their preferred stereochemistry. Methods to grow X-ray-quality
crystals of these complexes are addressed in the Experimental
Section. The molecular structures of 4c, 5c, 6b,c, 7c, and 8c are
depicted in Figures 4−9, while those of 4b, 5b, and 7b were
Figure 7. Molecular structure of 6c with thermal ellipsoids drawn at
the 35% probability level.
Figure 4. Molecular structure of 4c with thermal ellipsoids drawn at
the 35% probability level.
Figure 8. Molecular structure of 7c with thermal ellipsoids drawn at
the 35% probability level.
Figure 5. Molecular structure of 5c with thermal ellipsoids drawn at
the 35% probability level.
Figure 9. Molecular structure of 8c with thermal ellipsoids drawn at
the 35% probability level.
defined by four donor atoms, whereas that in trimethylsilylmethyl
species 6b,c and phenyl species 7b,c is displaced by 0.142, 0.179,
0.052, and 0.017 Å, respectively. This phenomenon is ascribable
to the distinct steric sizes of Cl, Me, CH2SiMe3, and Ph
incorporated in these molecules. In particular, the core geometry
of the trimethylsilylmethyl species 6b,c is severely distorted from
the ideal square planar, with P1−Ni−P2 angles of 151.36(4) and
155.22(3)°, respectively.
Figure 6. Molecular structure of 6b with thermal ellipsoids drawn at
Though severely distorted, the X-ray structures of 6b,c
signify clearly their preferred stereoisomer in the ground state.
It has been demonstrated that the steric size of substituents
in these o-phenylene-derived ligands follows the order PiPr2 >
N(C6H3iPr2-2,6) > N(C6H3Me2-2,6)/PPh2.35 Assuming a struc-
ture with lessened steric congestion is preferred, complex 6c
would have adopted an orientation wherein the sterically
demanding trimethylsilylmethyl group and the comparatively
large PiPr2 moiety are trans, leaving in turn the two phosphorus
donors being cis. The fact that the phosphorus donors in 6c are
trans-disposed is thus ascribable to electronic reasons: the two
formally anionic donors prefer a trans orientation, as do the two
datively bound phosphorus atoms. This preferred stereochemical
orientation appears to be general in this series of compounds, as
the 35% probability level.
reported previously.33 For a comprehensive comparison,
selected bond distances and angles are summarized in Tables 2
and 3, respectively. Consistent with what was deduced from the
solution NMR studies, the coordination geometry of these PMe3
adducts is best described as distorted square planar, wherein the
two phosphorus donors in 4c, 5c, 6b,c, 7b,c, and 8c are mutually
trans while those in 4b and 5b are cis. The resolved X-ray
structures of methyl species 5b and 5c agree well with their
corresponding major isomers observed in solution. In general,
the nickel center in chloro compounds 4b,c and methyl
compounds 5b,c lies perfectly on the mean coordination plane
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Table 2. Selected Bond Distances (Å) for [1]NiX(PMe3) (4−8)
a
compd
X
Ni−N
Ni−P1
Ni−P2
Ni−X
b
4b
Cl
1.923(3)
1.939(3)
1.956(3)
1.947(5)
1.893(2)
1.956(3)
1.972(2)
1.948(5)
1.915(2)
2.156(1)
2.201(1)
2.1578(9)
2.149(2)
2.1744(8)
2.153(1)
2.1556(8)
2.171(2)
2.1304(9)
2.194(1)
2.148(1)
2.2162(9)
2.238(2)
2.2979(8)
2.242(1)
2.2549(8)
2.242(2)
2.1712(9)
2.201(1)
2.035(4)
1.980(3)
1.923(7)
b
5b
Me
6b
CH2SiMe3
b
7b
Ph
4c
5c
6c
7c
8c
Cl
2.1957(9)
1.980(4)
1.979(3)
1.916(7)
1.4489
Me
CH2SiMe3
Ph
H
a
b
Data reported involve the donor atom of the X ligand specified. Data selected from ref 33.
Table 3. Selected Bond Angles (deg) for [1]NiX(PMe3) (4−8)
a
a
a
compd
X
N−Ni−P1
N−Ni−X
N−Ni−P2
P1−Ni−X
P1−Ni−P2
P2−Ni−X
b
4b
Cl
85.21(9)
85.0(1)
93.54(9)
92.2(2)
175.2(1)
173.5(1)
104.97(8)
105.1(2)
102.86(8)
102.64(9)
98.22(7)
102.3(2)
108.69(8)
177.68(5)
176.4(1)
90.0(1)
85.7(2)
87.32(3)
87.0(1)
88.43(9)
87.7(2)
85.8
97.51(4)
99.36(5)
83.85(5)
83.7(1)
88.3(1)
85.1(2)
84.37(3)
85.1(1)
91.13(9)
85.9(2)
78.9
b
5b
Me
6b
CH2SiMe3
83.81(8)
85.5(2)
162.3(1)
167.6(3)
172.07(8)
171.8(1)
169.5(1)
168.8(3)
172.3
151.36(4)
163.76(9)
171.68(3)
170.69(4)
155.22(3)
168.58(8)
164.20(4)
b
7b
Ph
4c
5c
6c
7c
8c
Cl
85.46(8)
85.10(9)
85.19(7)
85.4(2)
Me
CH2SiMe3
Ph
H
86.70(8)
a
b
Data reported involve the donor atom of the X ligand specified. Data selected from ref 33.
exemplified by dimeric chloro species 2d and all PMe3 adducts,
except 4b and major-5b. Consistent with this rationale, Alcock
also argued that primary bonds (e.g., Ni−N and Ni−X in this
study, X = Cl (4), Me (5), CH2SiMe3 (6), Ph (7), H (8)) are
more stereochemically active than secondary bonds46 (e.g., Ni−P
in this study) in dictating the primary coordination geometry.
Though electronic factors predominate in dictating the cis/trans
geometry of these four-coordinate species, the steric nature of
ligands incorporated therein deserves more comment. While
electronic and steric factors play contrary roles for 6c in this
regard, these considerations lead instead to the same preference
in 6b, as both formally anionic donors, i.e., CH2SiMe3 and
N(C6H3iPr2-2,6), are characteristic of the largest and the
second largest in this molecule. In contrast, the sterically much
smaller Cl and Me ligands in 4b and major-5b, respectively,
take on a position that is sterically favored; i.e., the two
relatively large N(C6H3iPr2-2,6) and PMe3 donors are trans,
rendering two datively bound phosphorus donors cis-disposed.
Such a preference in cis-disposed phosphine ligands appears
exceptional. In a separate study, Klein et al. also demonstrated
that trans isomers of chloro species 4e,f and methyl compound
5e−h (eq 2) are dominant in solution.44 A similar preference for
The reactivity of chloro species 2b−d with respect to
Kumada couplings was examined. Table 4 summarizes the survey
of reaction parameters with an n-butyl nucleophile and a phenyl
electrophile as prescribed building blocks. We chose to assess the
compatibility of this model reaction with both polar and nonpolar
solvents under ambient and elevated temperature conditions. In
general, these chloro complexes are all competent catalyst
precursors for Kumada couplings, yielding n-butylbenzene as the
desirable product. Regardless of the identity of NP, reactions
conducted in THF and DME at 60 °C are consistently superior to
the others investigated (Table 4) in view of higher reactivity and
selectivity. On the other hand, complex 2d (entry 24) outperforms
2b (entry 4) and 2c (entry 14), producing n-butylbenzene nearly
quantitatively and selectively. The high-yield production of
n-butylbenzene in this catalysis is particularly remarkable, given
the fact that β-elimination is a facile thermodynamic process, as
indicated by the stoichiometric chemistry established (vide supra).
Led by the results concluded in Table 4, we attempted to
probe the reaction scope employing catalytic 2d in THF at
60 °C. As presented in Table 5, both alkyl and aryl Grignard
reagents are compatible nucleophiles to couple with phenyl
chloride, bromide, and iodide efficiently and selectively. In
particular, chlorobenzene, which is characteristic of a syntheti-
cally more challenging electrophile, could be successfully
coupled with 4-tolyl, 2-tolyl, and n-butyl Grignard reagents
(entries 3, 6, and 9, respectively). Interestingly, this inherently
inert electrophile gives reactivity and selectivity comparable to
that of bromo- and iodobenzene upon reaction with 4-tolyl
nucleophile (entries 1−3). Steric repulsion imposed by ortho
substituents in tolyl Grignard reagents does not appear to affect
catalysis much (entries 1 and 2 versus entries 4 and 5) except
those involving chlorobenzene (entry 3 versus entry 6) in terms
of conversion rates. Nevertheless, the selectivities to produce
cross-coupled products were found to be virtually identical in
these reactions (entries 1−6).
the trans configuration was also reported for their o-phosphino-
phenolate congeners.45 The reason why sterics dominates in 4b and
major-5b is not clear at this stage. It is likely that, however, the
energy differences in these cis/trans isomers could be fairly small,
allowing the cis/trans equilibria to be established readily in solution.
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Table 4. Survey of Reaction Parameters for Kumada
Couplings by Catalytic 2b−d
is remarkable, as β-elimination in this system is thermodynami-
cally favorable due to the lack of a third donor such as what
PNP and PNN possess. These intriguing results, however,
strongly argue that the kinetic rate of catalytic Kumada
couplings is much higher than that of β-elimination in this
NP system. The kinetic rates of olefin insertion involving nickel
hydrides should be even slower, and thus the possibilities of
cross-couplings involving β-elimination followed by olefin
reinsertion47 can be ruled out. The high-yield production of
these alkylated arenes also strongly implies that monoalkyl
nickel complexes of the type [1]NiCH2CH2R(L) (L = solvent
or other datively bound donors available in reaction solutions)
are not key intermediates responsible for this cross-coupling
catalysis, particularly taking into account that the couplings
were conducted at elevated temperatures (60 °C in this case)
that in principle should facilitate β-elimination. To gain more
insights, controlled experiments employing catalytic PMe3-
bound 4c (1 mol %) were conducted under conditions
otherwise identical with those for reactions catalyzed by dimeric
2c (Table 4, entry 14). Interestingly, though 4c and 2c gave
virtually identical selectivities in n-butylbenzene production
(87% versus 85%), the former apparently reacts much more
slowly than the latter (23% versus 100% conversion), likely
reflecting the necessity of dissociation or nucleophilic
substitution of the coordinated PMe3 in the former. Never-
theless, the nearly identical selectivities of 4c and 2c imply
possibly the same operative mechanism. It is also interesting to
compare the catalytic activity of 4c with those of [PNP]NiCl29
in Kumada couplings, given the fact that both catalysts are
composed of two trans-disposed phosphorus donors. In this
regard, 4c was found to have much lower turnover rates but
significantly higher selectivities in n-butylbenzene formation,
clearly indicating that the Kumada couplings by NP- and
PNP-derived nickel catalysts are mechanistically distinctive.
We reason instead that the double-alkylated ate complexes
{[1]Ni(CH2CH2R)2}− play an important role in this Kumada
coupling in view of the potentially facile alkylation rates of
[1]NiCH2CH2R(L) and the lack of an empty, low-lying d
orbital in {[1]Ni(CH2CH2R)2}− that discourages β-elimina-
tion. In line with this, reactions of dimeric 2c with 4 equiv of
either nBuMgCl or Me3SiCH2MgCl in THF at 25 °C were
attempted. Unfortunately, these reactions gave a mixture of
a
temp
conversn
nBuPh/biphenyl
b
b
entry
2
solvent
Et2O
(°C)
(%)
selectivity
1
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2c
2c
2c
2c
2c
2c
2c
2c
2c
2c
2d
2d
2d
2d
2d
2d
2d
2d
2d
2d
25
2
83
24
98
70
98
15
82
0
71/29
42/58
84/16
82/18
73/27
85/15
95/5
2
Et2O
reflux
25
3
THF
4
THF
60
5
DME
25
6
DME
60
7
1,4-dioxane
1,4-dioxane
toluene
toluene
Et2O
25
8
60
83/17
0/0
9
25
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
60
10
2
83/17
60/40
52/48
71/29
85/15
72/28
77/23
93/7
25
Et2O
reflux
25
35
59
100
62
100
16
30
1
THF
THF
60
DME
25
DME
60
1,4-dioxane
1,4-dioxane
toluene
toluene
Et2O
25
60
92/8
25
56/44
83/17
98/2
60
8
25
11
26
16
100
28
94
24
82
0
Et2O
reflux
25
91/9
THF
91/9
THF
60
98/2
DME
25
80/20
88/12
89/11
76/24
0/0
DME
60
1,4-dioxane
1,4-dioxane
toluene
toluene
25
60
25
60
5
83/17
a
All reactions were carried out with 0.11 M phenyl bromide and 0.12 M
n-butylmagnesium chloride in 2 mL of solvent at the prescribed
b
temperature. Determined by GC, based on phenyl bromide; average
of two runs.
a
Table 5. Kumada Couplings by Catalytic 2d
1
products as evidenced by H and 31P{1H} NMR studies, from
which the product conformations could not be deduced
conclusively. The 31P{1H} NMR spectra of these reaction
aliquots, however, clearly indicated that these products are
conformationally distinctive from those derived from analogous
reactions employing 2 equiv of corresponding Grignard
reagents, consistent with the proposal that the monoalkyl
species [1c]NiR(L) reacts with another 1 equiv of RMgX to
give a nickelate complex. The participation of anionic nickelate
species in cross-coupling catalysis has also been proposed and
discussed in several studies.48−51 With the accumulated
negative charge, the ate complexes {[1]Ni(CH2CH2R)2}− are
in principle much better nucleophiles than [1]NiCH2CH2R(L)
to react with halo electrophiles, thus bypassing the
thermodynamically favorable β-elimination of [1]NiCH2CH2R-
(L) found in stoichiometric reactions.
b
b
entry
R
X
conversn (%)
R-Ph/Ph-Ph selectivity
c
c
c
1
2
4-tolyl
4-tolyl
4-tolyl
2-tolyl
2-tolyl
2-tolyl
n-Bu
I
94
89
95/5
93/7
93/7
99/1
99/1
95/5
82/18
98/2
100/0
70/30
Br
Cl
I
3
89
4
92
5
Br
Cl
I
93
6
65
7
100
100
75
8
n-Bu
Br
Cl
I
9
n-Bu
10
Et
100
a
Unless otherwise noted, all reactions were carried out with 0.11 M
phenyl halide and 0.12 M Grignard reagent in 2 mL of THF.
Determined by GC, based on phenyl halides; average of two runs.
Alternatively, the presumed {[1]Ni(CH2CH2R)2}− may
undergo reductive elimination to give {[1]Ni}− that contains
a formally zerovalent nickel, which in turn reacts with PhX to
generate [1]NiPh and X− via oxidative addition. Subsequent
nucleophilic attack of RMgX to [1]NiPh then possibly affords
b
c
4-Tolylmagnesium bromide was used.
The success in satisfactorily yielding cross-coupled products with
β-hydrogen-containing building blocks (entries 7−10 in Table 5)
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the ate complex intermediates {[1]Ni(Ph)(R)}− ready for R-Ph
production and {[1]Ni}− regeneration. To briefly assess the
validity of this hypothesis, controlled experiments employing 7c
with 1 equiv of nBuMgCl or 4-tolylMgBr in THF at 60 °C
were conducted. Indeed, these reactions produced in 12 h
n-butylbenzene in nearly quantitative yield and 4-methylbi-
phenyl in 95% selectivity, respectively, as evidenced by GC/MS
analyses on organic products of reaction aliquots. Interestingly,
these selectivity results are quite comparable to those obtained
under similar but catalytic conditions (Table 5, entries 8 and 9
and entries 1−3, respectively).
The somewhat superior reactivity and selectivity of 2d in
comparison to those of 2b,c (entries 4, 14, and 24 in Table 4)
thus likely reflect the electronic discrepancy of the presumed
nickelate complexes {[1b−d]Ni(CH2CH2R)2}− or {[1b−d]Ni}−.
Perhaps more importantly, the nucleophilicity of {[1d]Ni-
(CH2CH2R)2}− or {[1d]Ni}− is sufficiently capable of activating
the intrinsically challenging C(sp2)−Cl bond (entries 3, 6, and 9 in
Table 5), giving selectively the desirable cross-coupled products.
Notably, these results are in sharp contrast to what was observed
for both PNP and PNN, wherein Kumada catalysis employing
chloro electrophiles proceeds barely or unsatisfactorily.29,32
Mechanistically, the participation of radicals in this catalysis was
considered. In the presence of typical radical inhibitors such as
2,6-di-tert-butyl-4-hydroxytoluene or 2,2,6,6-tetramethyl-1-piperi-
dinyloxy (1 equiv to halo electrophiles), the catalytic activity and
selectivity remain virtually identical (e.g., entry 10 in Table 5),
thereby precluding radical involvement in this process.
EXPERIMENTAL SECTION
■
General Procedures. Unless otherwise specified, all experiments
were performed under nitrogen using standard Schlenk or glovebox
techniques. All solvents were reagent grade or better and were purified
by standard methods. The compounds NiCl2(DME),34 H[1a],36
H[1b],37 H[1c],35 H[1d],35 [1a]Li(THF)2,33 [1b]Li(THF)2,37
[1c]Li(THF)2,35 [1d]Li(DME),35 2b,33 4b,33 5b,33 and 7b33 were
prepared according to literature procedures. All other chemicals were
obtained from commercial vendors and used as received. All NMR
spectra were recorded at room temperature in specified solvents on
Varian Unity or Bruker AV instruments. Chemical shifts (δ) are listed
as parts per million downfield from tetramethylsilane, and coupling
constants (J) and peak widths at half-height (Δv1/2) are in hertz.
Routine coupling constants are not listed. 1H NMR spectra are
referenced using the residual solvent peak at δ 7.16 for C6D6. 13C
NMR spectra are referenced using the internal solvent peak at δ
128.39 for C6D6. The assignment of the carbon atoms for all
compounds is based on DEPT 13C NMR spectroscopy. 31P NMR
spectra are referenced externally using 85% H3PO4 at δ 0. The
Kumada coupling reactions were analyzed by GC on a Varian
Chrompack CP-3800 instrument equipped with a CP-Sil 5 CB chrompack
capillary column or by GC/MS on a Varian 450-GC/240-MS instrument
equipped with a Restek MXT-5 column. The identity of the cross-coupling
products was confirmed by comparison with authentic samples. GC yields
were quantified by signal integrals relative to those of prescribed amounts
of eicosane as an internal standard. Elemental analysis was performed on a
Heraeus CHN-O Rapid analyzer.
X-ray Crystallography. Crystallographic data for 2d, 3a, 4c, 5c,
6b,c, 7c, and 8c (CCDC reference numbers 999526−999533) are
summarized in the Supporting Information. Data were collected on a
diffractometer with graphite-monochromated Mo Kα radiation (λ =
0.7107 Å). Absorption correction was applied using SADABS.52
Structures were solved by direct methods and refined by full-matrix
least-squares procedures against F2 using SHELXL-97.53 All full-weight
non-hydrogen atoms were refined anisotropically. Hydrogen atoms
were placed in calculated positions. The structure of 2d contains
disordered CH2Cl2. Attempts to obtain suitable disorder models failed.
The SQUEEZE procedure of the Platon program was used to obtain a
new set of F2 (hkl) values without the contribution of solvent
molecules.54
CONCLUSIONS
■
We have prepared and structurally characterized a series of
divalent nickel complexes containing bidentate diarylamido
phosphine ligands. Of the complexes characterized in this
study, 3a is the only homoleptic species that is paramagnetic in
nature. As elucidated by X-ray crystallography, the coordination
geometry of 3a is approximately halfway between ideal
tetrahedral and square planar. With [1b−d]− ligands that are
sterically larger and electronically more releasing than [1a]−, all
other nickel derivatives are diamagnetic mono-NP complexes
whose core structure is best described as square planar, as
confirmed by solution NMR and single-crystal X-ray studies.
Incorporation of an exogenous PMe3 in these square-planar
species facilitates the synthesis of organonickel complexes and
the identification of their possible cis/trans stereoisomers in
solution. Electronic factors were found to play a major role in
dictating cis/trans preferences of these square-planar species:
the two formally anionic donors prefer a trans orientation, as do
the two datively bound atoms. In contrast to what was found
for PNP and PNN systems,29−32 β-hydrogen elimination is thermo-
dynamically favorable in this study. As a result, organonickel
derivatives were all found not to contain β-hydrogen atoms, as
exemplified by complexes 5−8. Nevertheless, the chloro complexes
2 are all competent catalyst precursors for Kumada couplings,
including those employing β-hydrogen-containing building blocks.
Remarkably, the synthetically more challenging chlorobenzene is
also a compatible electrophile in this catalysis. The NP complexes
are thus superior catalysts to their PNP or PNN congeners29,32
in this regard. We propose that the formation of anionic nickelate
complexes {[1]Ni(CH2CH2R)2}− and {[1]Ni}− is crucial to
facilitate such intriguing reactivity; i.e., catalytic Kumada couplings
are kinetically preferred to thermodynamically favorable β-hydrogen
elimination.
Synthesis of {[1c]Ni(μ-Cl)}2 (2c). Method 1. A THF solution
(5 mL) of H[1c] (500 mg, 1.60 mmol) and neat NEt3 (243 mg,
2.40 mmol, 1.5 equiv) was sequentially added to a suspension of
NiCl2(DME) (352 mg, 1.60 mmol) at room temperature. After being
stirred at room temperature for 1 h, the reaction mixture was
evaporated to dryness under reduced pressure. The residue was
triturated with pentane (3 mL × 2), and benzene (16 mL) was added.
The benzene solution was filtered through a pad of Celite and
evaporated to dryness in vacuo. The solid thus obtained was washed
with pentane and dried in vacuo to give the desired product as a brick
red solid; yield 605 mg (93%).
Method 2. Solid NiCl2(DME) (48.3 mg, 0.22 mmol) was
suspended in THF (3 mL) and cooled to −35 °C. To this was
added dropwise a solution of [1c]Li(THF)2 (100 mg, 0.22 mmol) in
THF (3 mL) at −35 °C. Upon addition, the reaction mixture turned
red and the suspended NiCl2(DME) dissolved. The solution was
stirred at room temperature overnight. All volatiles were removed in
vacuo. The resulting residue was dissolved in benzene (6 mL). The
benzene solution was passed through a pad of Celite and evaporated to
dryness in vacuo to afford the product as a brick red solid; yield
51.2 mg (58%). 1H NMR (C6D6, 500 MHz): δ 7.04 (br s, 6, Ar), 6.71
(m, 4, Ar), 6.23 (t, 2, Ar), 5.70 (m, 2, Ar), 2.60 (s, 12, ArCH3), 1.61
(m, 4, CHMe2), 1.36 (m, 12, CHMe2), 1.22 (m, 12, CHMe2). 31P{1H}
NMR (C6D6, 202.3 MHz): δ 62.6 (Δv1/2 = 27). 13C{1H} NMR (C6D6,
125.5 MHz): δ 167.3 (d, JCP = 17.8, C), 149.0 (s, C), 138.0 (s, C),
134.2 (s, CH), 131.3 (s, CH), 129.0 (s, CH), 124.8 (s, CH), 113.5 (d,
JCP = 4.0, CH), 112.5 (s, CH), 110.0 (d, JCP = 49.8, C), 25.9 (d, 1JCP
24.4, CHMe2), 19.8 (s, CHMe2), 18.6 (s, CHMe2), 17.8 (s, ArCH3).
=
H
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Organometallics
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Anal. Calcd for (C40H54Cl2N2Ni2P2)(C6H6)(C4H8O)2: C, 62.62; H,
7.40; N, 2.71. Found: C, 62.85; H, 7.42; N, 2.95.
CHMe2), 19.7 (s, ArCH3), 19.3 (d, 2JCP = 4.5, CHMe2), 18.4 (d, 2JCP
=
1
3
1.4, CHMe2), 13.6 (dd, JCP = 21.5, JCP = 1.9, P(CH3)3), −18.5 (dd,
2JCP = 33.9 and 23.3, NiCH3). Minor isomer (cis):55 1H NMR (C6D6,
500 MHz) δ 7.33 (d, Ar), 6.40 (m, Ar), 6.09 (dd, Ar), 2.45 (s, 6,
ArCH3), 1.93 (m, 2, CHMe2), 1.18 (dd, 6, CHMe2), 1.07 (dd, 6,
CHMe2), 0.82 (d, 9, 2JHP = 8.5, P(CH3)3), −0.42 (dd, 3, 3JHP = 3.5 and
7.5, NiCH3); 31P{1H} NMR (C6D6, 202.3 MHz) δ 47.7 (d, 2JPP = 21.7,
NP), −9.1 (d, 2JPP = 21.7, PMe3); 13C{1H} NMR (C6D6, 125.5 MHz)
δ 136.5 (s, C), 133.4 (s, C), 132.4 (s, C), 129.8 (s, CH), 123.6 (s,
CH), 113.2 (dd, JCP = 10.0 and 2.3, C), 111.5 (d, JCP = 5.5, C), 25.5
Synthesis of {[1d]Ni(μ-Cl)}2 (2d). The experimental procedures
were similar to those of 2c, employing H[1d] (in CH2Cl2 instead of
THF) and [1d]Li(DME) in place of H[1c] and [1c]Li(THF)2,
respectively: yield of method 1 74%, yield of method 2 79%. 1H NMR
(C6D6, 500 MHz): δ 7.17 (m, 6, Ar), 6.67 (t, 2, Ar), 6.63 (t, 2, Ar),
6.06 (t, 2, Ar), 5.65 (d, 2, Ar), 4.04 (septet, 4, CHMe2), 1.88 (d, 12,
CHMe2), 1.64 (br m, 4, CHMe2), 1.43 (m, 12, CHMe2), 1.19 (d, 24,
CHMe2). 31P{1H} NMR (C6D6, 202.3 MHz): δ 60.2. 13C{1H} NMR
(C6D6, 125.5 MHz): δ 168.7 (d, JCP = 21.0, C), 147.9 (s, C), 145.6 (s,
C), 133.5 (s, CH), 131.2 (s, CH), 125.9 (s, CH), 124.1 (s, CH), 115.4
(d, JCP = 8.7, CH), 112.5 (s, CH), 109.4 (d, JCP = 51.2, C), 29.1 (s,
CH), 25.7 (d, JCP = 26.1, CH), 24.9 (s, CH3), 24.9 (s, CH3), 18.4 (s,
CH3), 17.5 (s, CH3). Anal. Calcd for C48H70Cl2N2Ni2P2: C, 62.28; H,
7.63; N, 3.03. Found: C, 62.24; H, 7.70; N, 3.08.
1
2
2
(d, JCP = 7.3, CHMe2), 20.3 (d, JCP = 7.4, CHMe2), 19.7 (d, JCP
=
1
3
10.2, CHMe2), 18.7 (s, ArCH3), 18.0 (dd, JCP = 30.1, JCP = 3.6,
(PCH3)3), NiMe was not found due to low signal intensity. Anal.
Calcd for C24H39NNiP2: C, 62.34; H, 8.51; N, 3.03. Found: C, 61.87;
H, 8.56; N, 2.92.
Synthesis of [1b]NiCH2SiMe3(PMe3) (6b). Red crystals suitable for
Synthesis of Ni[1a]2 (3a). The experimental procedures were
similar to those of 2c, employing H[1a] and [1a]Li(THF)2 in place of
H[1c] and [1c]Li(THF)2, respectively: yield of method 1 60%, yield
of method 2 78%. After multiple attempts even with X-ray-quality
crystals, we were not able to obtain satisfactory analysis data for this
compound. Anal. Calcd for C52H46N2NiP2: C, 76.20; H, 5.66; N, 3.42.
Found: C, 76.16; H, 5.55; N, 2.97.
X-ray diffraction analysis were grown by layering pentane on a
1
concentrated diethyl ether solution at −35 °C; yield 88%. H NMR
(C6D6, 500 MHz): δ 7.85 (m, 4, Ar), 7.26 (t, 1, Ar), 7.13 (s, 4, Ar),
7.08 (m, 5, Ar), 6.88 (t, 1, Ar), 6.34 (t, 1, Ar), 6.01 (dd, 1, Ar), 3.66 (br
s, 2, CHMe2), 1.13 (m, 6, CHMe2), 1.06 (m, 6, CHMe2), 0.62 (d, 9,
2JHP = 7.5, PMe3), 0.14 (s, 9, NiCH2SiMe3), −0.70 (br s, 2,
2
NiCH2SiMe3). 31P{1H} NMR (C6D6, 202.3 MHz): δ 34.4 (d, JPP
=
Synthesis of [1c]NiCl(PMe3) (4c). Solid 2c (74 mg, 0.08 mmol)
was dissolved in THF (3 mL), and PMe3 (0.16 mL, 1.0 M in THF,
0.16 mmol) was added at room temperature. The reaction mixture was
stirred at room temperature overnight. All volatiles were removed in
vacuo. The residue thus obtained was extracted with diethyl ether
(6 mL), and the solution was filtered through a pad of Celite, which
was further washed with diethyl ether (1 mL × 2) until the washings
became colorless. The diethyl ether filtrate and washings were
combined and evaporated to dryness under reduced pressure to afford
the product as an emerald solid; yield 68.2 mg (89%). Emerald crystals
suitable for X-ray diffraction analysis were grown from a pentane
2
318.3, NP), −22.5 (d, JPP = 317.5, PMe3). 13C{1H} NMR (C6D6,
125.5 MHz): δ 168.9 (dd, JCP = 29.2 and 2.8, C), 153.8 (s, C), 145.9
(s, C), 134.5 (d, JCP = 10.0, CH), 133.5 (s, CH), 133.0 (s, CH), 130.2
(d, JCP = 2.3, CH), 128.6 (d, JCP = 13.3, CH), 124.9 (s, CH), 124.7 (s,
CH), 116.3 (d, JCP = 11.8, CH), 112.9 (d, JCP = 1.9, C), 112.5 (d, JCP
=
1.4, C), 112.3 (d, JCP = 6.4, CH), 25.3 (s, CHMe2), 24.0 (s, CHMe2),
15.0 (dd, JCP = 21.5 and 1.8, PMe3), 3.9 (s, CH2Si(CH3)3), −13.4 (dd,
2JCP = 18.2 and 25.2, NiCH2Si(CH3)3). After multiple attempts even
with X-ray-quality crystals, we were not able to obtain satisfactory
analysis data for this compound. Anal. Calcd for C37H51NNiP2Si: C,
67.47; H, 7.81; N, 2.13. Found: C, 67.75; H, 7.50; N, 1.35.
1
solution at −35 °C. H NMR (C6D6, 500 MHz): δ 6.99 (m, 3, Ar),
Synthesis of [1c]NiCH2SiMe3(PMe3) (6c). Red crystals suitable for
X-ray diffraction analysis were grown from a concentrated pentane
solution at −35 °C; yield 75%. 1H NMR (C6D6, 500 MHz): δ 7.13 (t,
1, Ar), 7.07 (d, 1, Ar), 6.93 (t, 1, Ar), 6.90 (t, 1, Ar), 6.32 (t, 1, Ar),
5.84 (dd, 1, Ar), 2.51 (s, 6, ArCH3), 2.25 (m, 2, CHMe2), 1.28 (dd, 6,
6.92 (m, 1, Ar), 6.77 (t, 1, Ar), 6.25 (t, 1, Ar), 5.69 (dd, 1, Ar), 2.47 (s,
6, CH3), 2,34 (m, 2, CHMe2), 1.62 (dd, 6, CHMe2), 1.34 (dd, 6,
2
CHMe2), 0.57 (d, 9, JHP = 8, P(CH3)3). 31P{1H} NMR (C6D6, 202.3
2
2
MHz): δ 51.1 (d, JPP = 313.5, NP), −24.24 (d, JPP = 313.5, PMe3).
13C{1H} NMR (C6D6, 125.5 MHz): δ 168.4 (dd, JCP = 26.5, 3.26, C),
153.5 (d, JCP = 4.5, C), 137.4 (s, C), 133.6 (s, CH), 131.7 (s, CH),
129.6 (s, CH), 124.8 (s, CH), 113.1 (d, JCP = 12.3, CH), 112.4 (d, JCP
2
CHMe2), 1.25 (dd, 6, CHMe2), 0.58 (d, 9, JHP = 7.5, P(CH3)3), 0.27
3
(s, 9, CH2SiMe3), −0.93 (dd, 2, JHP = 10.5 and 15.5, NiCH2SiMe3).
2
31P{1H} NMR (C6D6, 202.3 MHz): δ 42.9 (d, JPP = 305.4, NP),
1
= 6.0, CH), 112.4 (d, JCP = 40.8, C), 25.9 (d, JCP = 26.6, CHMe2),
2
−26.3 (d, JPP = 305.4, PMe3). 13C{1H} NMR (C6D6, 125.5 MHz): δ
19.6 (s, CH3), 19.2 (d, 2JCP = 2.3, CHMe2), 18.1 (s, CHMe2), 13.2 (d,
1JCP = 22.8, PMe3). Anal. Calcd for C23H36ClNNiP2: C, 57.24; H, 7.52;
166.8 (dd, JCP = 24.7 and 4.5, C), 154.8 (s, C), 136.5 (s, C), 132.7 (s,
CH), 131.0 (s, CH), 129.3 (s, CH), 123.5 (s, CH), 113.6 (d, JCP
=
=
N, 2.90. Found: C, 57.29; H, 7.59; N, 2.86.
38.4, C), 112.9 (d, J = 11.0, CH), 110.8 (d, J = 5.5, CH), 24.4 (d, 1JCP
General Procedures for the Synthesis of [1b,c]NiR(PMe3) (R
= Me (5b,c), CH2SiMe3 (6b,c), Ph (7b,c)). One equivalent of RMgCl
was added dropwise to a solution of 4b,c in THF at −35 °C. The
reaction mixture was naturally warmed to room temperature and
stirred overnight. All volatiles were removed in vacuo. The residue
thus obtained was triturated with pentane to give a red or brownish
red solid. Benzene was added. The benzene solution was filtered
through a pad of Celite, which was further washed with benzene until
the washings became colorless. The filtrate and washings were
combined and evaporated to dryness under reduced pressure to afford
the product as a red or brownish red crystalline solid.
2
23.9, CHMe2), 20.6 (s, CHMe2), 20.1 (d, JCP = 3.6, CHMe2), 18.7 (s,
ArCH3), 15.1 (dd, 1JCP = 19.2, 3JCP = 1.9, P(CH3)3), 4.2 (s, SiMe3), −18.9
2
(dd, JCP = 17.4 and 30.2, NiCH2). Anal. Calcd for C27H47NNiP2Si: C,
60.67; H, 8.87; N, 2.62. Found: C, 60.52; H, 8.93; N, 2.52.
Synthesis of [1c]NiPh(PMe3) (7c). Brownish red crystals suitable for
X-ray diffraction analysis were grown from a concentrated diethyl ether
solution at −35 °C; yield 83%. 1H NMR (C6D6, 500 MHz): δ 7.45 (d,
2, Ar), 7.13 (d, 2, Ar), 7.04 (t, 1, Ar), 7.00 (t, 1, Ar), 6.93 (t, 3, Ar),
6.85 (t, 1, Ar), 6.36 (t, 1, Ar), 5.93 (dd, 1, Ar), 2.50 (s, 6, ArCH3), 2.31
(m, 2, CHMe2), 1.12 (dd, 6, CHMe2), 0.95 (dd, 6, CHMe2), 0.23 (d, 9,
2JHP = 8.0, P(CH3)3). 31P{1H} NMR (C6D6, 202.3 MHz): δ 40.6 (d,
Synthesis of [1c]NiMe(PMe3) (5c). Ruby crystals suitable for X-ray
2
2JPP = 278.0, NP), −24.3 (d, JPP = 278.0, PMe3). 13C{1H} NMR
diffraction analysis were grown from a concentrated pentane solution
1
(C6D6, 125.5 MHz): δ 167.8 (dd, JCP = 25.6 and 4.1, C), 155.4 (s, C),
150.9 (dd, JCP = 33.9 and 40.8, C), 139.2 (t, JCP = 4.1, CH), 136.6 (s,
C), 133.3 (d, JCP = 1.4, CH), 131.9 (t, JCP = 1.8, CH), 129.5 (s, CH),
126.5 (t, JCP = 3.1, CH), 123.8 (s, CH), 122.5 (t, JCP = 2.8, CH), 112.0
(d, JCP = 8.5, CH), 111.8 (d, JCP = 37.4, C), 111.4 (d, J = 5.5, CH),
at −35 °C; yield 77%. Major isomer (trans): H NMR (C6D6, 500
MHz) δ 7.14 (td, 1, Ar), 7.09 (d, 2, Ar), 6.97 (t, 1, Ar), 6.92 (t, 1, Ar),
6.34 (t, 1, Ar), 5.92 (dd, 1, Ar), 2.43 (s, 6, ArCH3), 2.18 (m, 2,
CHMe24), 1.28 (dd, 6, CHMe2), 1.24 (dd, 6, CHMe2), 0.48 (dd, 9, 2JHP
3
= 7.5, JHP = 1.5, P(CH3)3), −0.69 (dd, 3, JHP = 13.0 and 7.0,
1
2
2
NiCH3); 31P{1H} NMR (C6D6, 202.3 MHz) δ 45.5 (d, JPP = 295.7,
23.6 (d, JCP = 26.5, CHMe2), 19.6 (s, ArMe), 18.0 (d, JCP = 3.6,
CHMe2), 17.3 (s, CHMe2), 13.7 (dd, 1JCP = 23.9, 3JCP = 1.4, P(CH3)3).
Anal. Calcd for C29H41NNiP2: C, 66.42; H, 7.89; N, 2.67. Found: C,
66.14; H, 7.90; N, 2.53.
2
NP), −20.0 (d, JPP = 295.7, PMe3); 13C{1H} NMR (C6D6, 125.5
MHz) δ 167.6 (dd, JCP = 26.1, JCP = 4.6, C), 155.9 (dd, JCP = 0.9 and
1.9, C), 136.3 (s, C), 133.1 (d, JCP = 1.4, CH), 131.8 (t, JCP = 1.9,
CH), 129.3 (s, CH), 123.5 (s, CH), 112.7 (dd, JCP = 38.0 and 1.4, C),
111.9 (d, JCP = 10.9, CH), 110.8 (d, J = 5.5, CH), 24.7 (d, 1JCP = 24.7,
Synthesis of [1c]NiH(PMe3) (8c). Method 1. Solid 4c (50 mg,
0.10 mmol) was dissolved in THF (4 mL) and cooled to −35 °C. To
I
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Organometallics
Article
this was added LiHBEt3 (0.10 mL, 1 M in THF, 0.10 mmol) dropwise.
The reaction mixture was naturally warmed to room temperature and
stirred overnight. All volatiles were removed in vacuo. The brown
residue thus obtained was triturated with pentane (2 mL × 2), and
diethyl ether (8 mL) was added. The diethyl ether solution was filtered
through a pad of Celite, which was further washed with diethyl ether
(1 mL × 2) until the washings became colorless. The diethyl ether
filtrate and washings were combined and evaporated to dryness under
reduced pressure to afford the product as a red solid. Red crystals
suitable for X-ray diffraction analysis were grown from a concentrated
diethyl ether solution at −35 °C; yield 37.9 mg (82%).
(2) de Mejeire, A.; Diederich, F. Metal-Catalyzed Cross-Coupling
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Method 2. Experiments were conducted with procedures similar to
those of method 1, except that 1 equiv of EtMgCl or nBuMgCl was
used in place of LiHBEt3, leading to the formation of 8c quantitatively.
1H NMR (C6D6, 500 MHz): δ 7.14 (s, 2, Ar), 7.09 (td, 1, Ar), 6.99 (m,
1, Ar), 6.97 (tt, 1, Ar), 6.37 (t, 1, Ar), 6.04 (dd, 1, Ar), 2.40 (s, 6,
ArCH3), 2.08 (m, 2, CHMe2), 1.27 (dd, 6, CHMe2), 1.14 (dd, 6,
CHMe2), 0.59 (dd, 9, 2JHP = 8.0, 4JHP = 1.5, P(CH3)3), −20.24 (dd, 1,
2JHP = 72.5 and 82.0, NiH). 31P{1H} NMR (C6D6, 202.3 MHz): δ 63.3
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2
2
(d, JPP = 262.6, NP), −19.9 (d, JPP = 262.6, PMe3). 13C{1H} NMR
(C6D6, 125.5 MHz): δ 168.4 (dd, JCP = 27.0 and 4.1, C), 155.3 (d,
JCP = 1.9, C), 135.7 (s, C), 133.6 (d, JCP = 1.4, CH), 132.1 (t, JCP = 1.9,
CH), 129.1 (s, CH), 123.3 (s, CH), 113.6 (d, JCP = 34.8, C), 111.4 (d,
JCP = 11.9, CH), 111.1 (d, JCP = 5.4, CH), 25.1 (dd, JCP = 29.2 and 1.8,
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CHMe2), 20.2 (d, JCP = 5.5, CHMe23), 19.4 (s, ArCH3), 18.7 (d, JCP
=
1
1.0, CHMe2), 17.0 (dd, JCP = 24.7, JCP = 2.3, P(CH3)3). Anal. Calcd
for C23H37NNiP2: C, 61.62; H, 8.33; N, 3.13. Found: C, 61.36; H,
8.42; N, 3.00.
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General Procedures for Catalytic Kumada Couplings. A vial
was sequentially charged with 2 (1.0 mg for each single experiment,
corresponding to 0.5 mol %), THF (2 mL), Grignard reagent
(1.1 equiv), phenyl halide (1.0 equiv), and a magnetic stir bar. The
solution was stirred at the prescribed temperature in a temperature-
controlled bath for 12 h and quenched with deionized water. The
products were extracted with diethyl ether. The diethyl ether solution
was separated from the aqueous layer, dried over MgSO4, and filtered
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ASSOCIATED CONTENT
* Supporting Information
■
S
Tables, a figure, and CIF files giving X-ray crystallographic data
for 2d, 3a, 4c, 5c, 6b,c, 7c, and 8c (CCDC reference numbers
999526−999533). This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for L.-C.L.: lcliang@mail.nsysu.edu.tw.
■
Author Contributions
§These authors contributed equally to this study.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(32) Liang, L.-C.; Lee, W.-Y.; Hung, Y.-T.; Hsiao, Y.-C.; Cheng, L.-
C.; Chen, W.-C. Dalton Trans. 2012, 41, 1381−1388.
(33) Liang, L.-C.; Lee, W.-Y.; Yin, C.-C. Organometallics 2004, 23,
3538−3547.
■
We thank the Ministry of Science and Technology of Taiwan
for financial support (NSC 99-2113-M-110-003-MY3 and NSC
102-2113-M-110-002-MY3), Mr. Ting-Shen Kuo (NTNU) for
assistance with X-ray crystallography, and the National Center
for High-performance Computing (NCHC) for the access to
chemical databases. M.-T.C. thanks the Ministry of Science and
Technology for a postdoctoral fellowship (NSC 102-2811-M-
110-009).
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2166−2174.
(37) Liang, L.-C.; Lee, W.-Y.; Hung, C.-H. Inorg. Chem. 2003, 42,
5471−5473.
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