Catalytic C - S cross-coupling reactions employing Ni complexes of pyrrole-based pincer ligands

Article abstract of DOI:10.1021/cs500874z

A series of catalytic C - S coupling reactions utilizing well-defined Ni(II) PNP pincer complexes as precatalysts are reported (PNP = anion of 2,5-bis[(dialkyl/aryl-phosphino)methyl]pyrrole, abbreviated as P₂^RPyr). Coupling reactions employing a variety of aryl iodides and thiols in the presence of base and DMF proceed in good to excellent yield at 80°C with low catalyst loadings. Aryl bromides were found to result in substantially lower yields, and aryl chlorides were found to be unreactive under the catalytic conditions. In an effort to further understand the reactivity of the nickel PNP precatalysts, complexes of Ni(II) containing amide, alkoxide, hydroxide, thiolate, and hydrosulfide ligands have been prepared and examined in stoichiometric reactions relevant to carbon-heteroatom coupling. Full characterization of each nickel complex is provided, including solid-state structures. The results of stoichiometric reactions implicate a reduced Ni(I) species as the active catalyst, which forms by reduction of the Ni(II) precatalyst in the presence of excess thiolate. The facility in forming Ni(I) species is invoked to rationalize the observed activity among different Ni PNP precatalysts. (Chemical Equation Presented).

Full text of DOI:10.1021/cs500874z

Research Article
pubs.acs.org/acscatalysis
Catalytic C−S Cross-Coupling Reactions Employing Ni Complexes of
Pyrrole-Based Pincer Ligands
Gopaladasu T. Venkanna, Hadi D. Arman, and Zachary J. Tonzetich*
Department of Chemistry, University of Texas at San Antonio, San Antonio, Texas 78249, United States
S
* Supporting Information
ABSTRACT: A series of catalytic C−S coupling reactions utilizing well-defined
Ni(II) PNP pincer complexes as precatalysts are reported (PNP = anion of 2,5-
R
bis[(dialkyl/aryl-phosphino)methyl]pyrrole, abbreviated as P2 Pyr). Coupling
reactions employing a variety of aryl iodides and thiols in the presence of base
and DMF proceed in good to excellent yield at 80 °C with low catalyst loadings.
Aryl bromides were found to result in substantially lower yields, and aryl
chlorides were found to be unreactive under the catalytic conditions. In an effort
to further understand the reactivity of the nickel PNP precatalysts, complexes of Ni(II) containing amide, alkoxide, hydroxide,
thiolate, and hydrosulfide ligands have been prepared and examined in stoichiometric reactions relevant to carbon-heteroatom
coupling. Full characterization of each nickel complex is provided, including solid-state structures. The results of stoichiometric
reactions implicate a reduced Ni(I) species as the active catalyst, which forms by reduction of the Ni(II) precatalyst in the
presence of excess thiolate. The facility in forming Ni(I) species is invoked to rationalize the observed activity among different Ni
PNP precatalysts.
KEYWORDS: C−S cross-coupling, nickel catalysis, PNP ligands, nickel(II), mechanistic studies
3
in detail for related POCOP pincers containing a central Csp
donor atom.27 Thus, the question remains if intact phosphine-
based Ni pincer complexes are capable of effecting catalytic C−
X bond formation at all, or if all observed catalytic activity is an
artifact of pincer ligand decomposition. Such a scenario is
counterintuitive given the thermal stability traditionally
associated with pincer ligands28 yet logical given the precedent
for C−S cross-coupling with simple binary Ni(0) systems.29,30
Herein we describe the chemistry of Ni(II) complexes of the
INTRODUCTION
■
Carbon−heteroatom (C−X) bond forming reactions have
emerged as a very important class of synthetic methods that
now rival C−C bond forming reactions in their scope and
simplicity. By in large, C−N, C−O, and C−S couplings are of
primary focus due to the prevalence of these linkages in natural
products and pharmaceutically important small molecules.1,2
A
variety of catalyst systems have been reported for these
transformations, with the majority incorporating late transition
metals such as Ni, Cu, or Pd.3−8 The success of these metals in
mediating C−X bond-forming reactions can be traced to the
unique aspects of M−X bonding within their complexes, a topic
of considerable theoretical interest.9−15
R
R
pyrrole-based pincer ligand, P2 Pyr (P2 Pyr = (2,5-
{R2PCH2}2C4H2N)−; R = Ph, Cy, t-Bu), relevant to C−X
cross-coupling catalysis. Of the C−X cross-coupling reactions
examined (X = N, O, and S), only C−S couplings were found
to proceed. The nature of the active species in these reactions
has been probed by investigating the stoichiometric chemistry
of well-defined Ni(II) precatalysts. The results support an
important role for Ni(I) in the catalytic cycle.
As part of our continuing investigations into new PNP pincer
ligands that feature a central pyrrole group,16−19 we sought to
examine the catalytic efficacy of Ni PNP complexes in C-
heteroatom cross-coupling reactions. We have previously
demonstrated that such Ni complexes are efficient precatalysts
for a variety of Kumada-type C−C cross-coupling reactions
under mild conditions.20 Moreover, examples of catalytic C−X
cross-coupling reactions with Ni pincer complexes where X =
N, O, or S are quite rare, with the best-studied examples
involving formation of C−S bonds.21−24 Guan has reported
elegant mechanistic work concerning C−S cross-coupling
reactions catalyzed by Ni pincer complexes containing the
bis(phosphinite) POCOP ligand.25,26 These studies delineated
the thermodynamic role of pincer ligand substituent and thiol
in stoichiometric C−S bond forming steps and demonstrated
that lower valent Ni complexes resulting from decomposition of
the pincer ligand are likely the true catalytically active species.
Zargarian has also studied such ligand decomposition pathways
RESULTS AND DISCUSSION
■
Synthesis of the P2tBuPyr Ligand. Previous reports of
pyrrole-diphosphine pincer ligands from our group and others
feature either phenyl or cyclohexyl substitution at the
phosphine arms.16−18,20 In order to compare the catalytic
reactivity of a nickel(II) complex possessing a more bulky
ligand, we pursued the synthesis of the di-tert-butylphosphino
analog (Chart 1). Bulky tert-butyl phosphine substituents in
nickel pincers bearing pyridine and aryl central units have
Received: June 20, 2014
Revised: July 22, 2014
© XXXX American Chemical Society
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electrophile and either aniline, phenol, or thiophenol as the
nucleophile. Under several conditions utilizing THF, toluene,
or DMF as solvent and KOtBu, Cs2CO3, or KOH as base at
elevated temperatures (60−110 °C), no products of C−N or
C−O cross-coupling were detected. Such an observation is
consistent with the absence of any examples of C−N or C−O
cross-coupling with Ni pincer catalysts reported in the
literature. In contrast, however, the C−S coupling reaction
was found to proceed to the coupled diphenylsulfide product.
The catalytic C−S coupling reaction was optimized using 4-
iodotoluene and thiophenol with precatalysts 1, 2, and 3 as
displayed in Table 1. Of the conditions examined, only DMF
Chart 1. PNP Ligands Used in This Study
proven effective in supporting complexes active toward
catalysis.31,32 Reaction of 2,5-bis-(dimethylaminomethyl)-
pyrrole with neat di-tert-butylphosphine at 140 °C afforded
the desired pyrrole-diphosphine, H(P2tBuPyr), in good yield
after removal of all volatiles (Scheme 1). Metalation to give
Table 1. C−S Coupling of Thiophenol with 4-Iodotoluene
Scheme 1. Preparation of of H(P2tBuPyr) and
[NiCl(P2tBuPyr)]
a
under Various Conditions
b
entry
catalyst
solvent
base
% yield
c
1
none
1
DMF
DMF
DMF
DMF
DMF
Toluene
KOH
0
2
None
0
[NiCl(P2tBuPyr)] (3) was accomplished by combining H-
(P2tBuPyr) with anhydrous nickel(II) chloride in the presence of
triethylamine in identical fashion to that reported for the
P2PhPyr and P2CyPyr ligands.
3
1
KOH
97
64
55
0
4
2
KOH
5
3
KOH
6
1
KOH
d
Much like the diphenyl- and dicyclohexylphosphine analogs,
the P2tBuPyr ligand gives rise to a thermally stable (decomp. >
250 °C), diamagnetic nickel(II) chloride complex. The 31P
NMR resonance of complex 3 is found at 64.9 ppm,
demonstrating a trend to more downfield chemical shifts
upon moving to more electron-donating phosphine substitu-
ents on the PNP ligand (cf. 30.3 ppm for [NiCl(P2PhPyr)] (1)
and 51.1 ppm for [NiCl(P2CyPyr)] (2)). The solid-state
structure of complex 3 is depicted in Figure 1 and displays
7
1
THF
KOH
0
8
1
DMSO
DMF
DMF
DMF
DMF
DMF
KOH
trace
30
trace
0
9
1
K3PO4
Cs2CO3
KOtBu
NaOH
NaOMe
10
11
12
13
1
1
1
89
40
1
a
Reaction conditions: Ni cat., 0.8 mol %; 4-iodotoluene, 1.1 mmol;
thiophenol, 1.0 mmol; base, 2.0 mmol; solvent, 3 mL; 80 °C; N2
atmosphere. Isolated yield. Reaction time up to 6 h. 60 °C.
b
c
d
was found to result in substantial quantities of the coupled
thioether product. Both KOH and NaOH were found to be
effective bases, although limited yields were also obtained with
NaOMe and K3PO4. We note that the role of base has
previously been demonstrated to play an important role in C−S
cross-coupling reactions mediated by Cu catalysts.33 Complex 1
was found to be superior to either 2 or 3 as a precatalyst for the
C−S cross-coupling reaction (entries 4 and 5). The better
performance of 1 may stem from the decreased steric bulk of
the P2PhPyr ligand versus P2CyPyr and P2tBuPyr; however,
stoichiometric reactions with the nickel precatalysts also point
to an important role for the NiII/NiI redox couple (vida infra).
Having identified 1 as an effective precatalyst for the coupling
of 4-iodotoluene and thiophenol, we next examined the
substrate scope of the reaction. Examining several different
haloarenes quickly identified aryl iodides as the best electro-
philic coupling partners (Table 2, entries 1−3). Bromobenzene
was found to afford diphenylsulfide, albeit in lower isolated
yields and only after additional reaction time. In contrast, aryl
chlorides were found to be completely unsuitable electrophiles
for cross-coupling reactions. Aryl chlorides containing electron-
donating substituents were found to be unreactive, and those
containing electron-withdrawing substituents such as CN, CF3,
and NO2 were found to react without added catalyst in control
experiments.
Figure 1. Thermal ellipsoid drawing (50%) of [NiCl(P2tBuPyr)] (3).
Hydrogen atoms omitted for clarity. Selected bond distances (Å) and
angles (deg): Ni(1)−Nl(1) = 1.860(5); Ni(1)−P(1) = 2.2352(18);
Ni(1)−P(2) = 2.2190(18); Ni(1)−Cl(1) = 2.1830(18); N(1)−
Ni(1)−Cl(1) = 178.64(15); P(1)−Ni(1)−P(2) = 168.08(6).
the expected square-planar geometry about nickel. Despite the
larger steric bulk of the tert-butyl groups, the distances and
angles about the nickel atom are comparable to those observed
for complexes 1 and 2.
Catalytic Reactions. Complex 1 was used to test for
catalytic activity in C−N, C−O, and C−S coupling reactions in
the presence of base employing bromobenzene as the
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a
Table 2. Nickel Catalyzed C−S Coupling Reactions Employing 1
a
b
Reaction conditions: Ni cat., 0.8 mol %; electrophile, 1.1 mmol; thiol, 1.0 mmol; KOH, 2.0 mmol; DMF, 3 mL; 80 °C; N2 atmosphere. Isolated
yield.
The results in Table 2 demonstrate effective cross-coupling
for a variety of aryl iodides and thiols. Both electron-
withdrawing and electron-donating substituents on the electro-
phile were tolerated as well as substitution at the 2-position.
Aryl iodides containing halide substituents were also successful
coupling partners (entries 8 and 9), yielding selective
substitution of the iodide to generate halogenated thioethers.
In addition to arene thiols, aliphatic thiols were found to yield
the desired thioethers in good yields (entries 13 and 14). In
sum, C−S cross-coupling reactions of thiols with aryl iodides
employing 1 as a precatalyst demonstrate good efficacy for a
range of different substrates.
position of phosphinite-based pincer ligands during C−S
coupling,25 we next investigated the synthesis and reactivity
of several Ni(II) complexes relevant to the catalytic reaction to
examine if similar processes are responsible for the activity
observed in our PNP system. Since catalytic activity was
observed for nickel(II) complexes of each of the three ligands
displayed in Chart 1, we elected to focus on P2CyPyr, as
complexes of this ligand have proven to be more stable and
easier to work with in terms of solubility and crystallinity.
The synthesis of Ni(II) complexes containing heteroatom
ligands proceeded in good yield from 2 via simple salt
metathesis (4−6, Scheme 2). Related complexes have been
reported previously for both Ni-PCP and Ni-PNP sys-
tems.26,34−36 Several of the compounds display intense colors
Synthesis of Nickel(II) Complexes Containing N, O,
and S-Bound Ligands. Given the precedent for decom-
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Scheme 2. Preparation of [NiX(P2CyPyr)] Complexes
Scheme 4. Synthesis of [Ni(OH)(P2CyPyr)]
toluene.40 The Brønsted basicity of 4 also permitted the
synthesis of the rare terminal hydrosulfide complex, 8, through
an analogous reaction with H2S (Scheme 5). Despite the
resulting from transitions in the visible region of the spectrum
(see the Supporting Information). Alternatively, the phenolate
and thiophenolate complexes, 5 and 6, could be prepared by
protonolysis reactions of the amide complex 4 with the
corresponding conjugate acids (Scheme 3). The synthetic
procedures in Scheme 2 were also amenable to complexes of
the P2PhPyr ligand for E = O and S (see the Supporting
Information).
Scheme 5. Protonolysis of [Ni(NHPh)(P2CyPyr)] Complexes
by H2O and H2S
Scheme 3. Protonolysis of [Ni(NHPh)(P2CyPyr)] by HOPh
and HSPh
successful synthesis of both nickel hydroxide and hydrosulfide
complexes with the P2CyPyr ligand, attempts to prepare a nickel
hydroxide complex bearing the P2PhPyr ligand were unsuccess-
ful.
Complexes 7 and 8 display sharp upfield 1H NMR
resonances for the EH protons at −4.79 and −2.26 ppm,
respectively. The resonances appear as triplets, consistent with
coupling to the two phosphorus atoms of the PNP ligand. The
solid-state structures of 7 and 8 are displayed in Figure 3. The
Ni−X distances are shorter than those in the corresponding
alkoxide and thiolate complexes (Figure 2). Inspection of the
packing diagrams for both 7 and 8 demonstrates no
intermolecular interactions suggestive of hydrogen bonding in
the solid state.
Stoichiometric Reactions Relevant to C−S Cross-
Coupling. In order to gain more information about the nature
of the active nickel species present during C−S cross-coupling
catalysis, we next examined several stoichiometric reactions
with the well-defined Ni pincer complexes. We first investigated
the reaction of 6 with 4-iodotoluene in DMF-d7 at 80 °C to
determine if the Ni(II) thiolate species is capable of effecting
C−S bond formation under conditions present during catalysis.
Prolonged heating (20 h) of 6 and 4-iodotoluene at 80 °C led
The solid-state structures of complexes 4−6 are displayed in
Figure 2. Each structure contains a square-planar Ni(II) ion
with PNP ligand metrics similar to those observed for
previously reported nickel halide and alkyl complexes of
P2CyPyr.20 Complexes 4 and 5 feature short Ni−X bond
distances consistent with reports of other amide and alkoxide
complexes of Ni(II) PNP compounds.26,34,37 The N-bound
hydrogen atom in 4 could not be identified from the electron
density map and the location of this atom in Figure 2
represents a calculated position.
The requirement of KOH or NaOH in the C−S cross-
coupling reactions prompted us to explore the preparation of a
terminal Ni(II) hydroxide complex because of the possible
involvement of such a species during catalysis. Treatment of 2
with NaOH or KOH in THF was found to produce the
hydroxide complex, [Ni(OH)(P2CyPyr)] (7), after 24 h at room
temperature (Scheme 4).38,39 Alternatively, complex 7 could be
prepared from the amide complex 4 by treatment with H2O in
Figure 2. Thermal ellipsoid drawings (50%) of [Ni(NHPh)(P2CyPyr)] (4), [Ni(OPh)(P2CyPyr)] (5), and [Ni(SPh)(P2CyPyr)] (6). Hydrogen atoms
and cocrystallized solvent molecules omitted for clarity. Selected bond lengths (Å) and angles (deg) for 4: Ni(1)−N(1) = 1.8509(16); Ni(1)−N(2)
= 1.8891(18); Ni(1)−Pavg = 2.193(1); N(1)−Ni(1)−N(2) = 176.50(8); P(1)−Ni(1)−P(2) = 169.28(2); for 5: Ni(1)−N(1) = 1.850(2); Ni(1)−
O(1) = 1.867(2); Ni(1)−Pavg = 2.199(1); N(1)−Ni(1)−O(1) = 173.22(10); P(1)−Ni(1)−P(2) = 169.32(3); for 6: Ni(1)−N(1) = 1.873(3);
Ni(1)−S(1) = 2.1956(12); Ni(1)−Pavg = 2.202(1); N(1)−Ni(1)−S(1) = 171.51(10); P(1)−Ni(1)−P(2) = 168.38(4).
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Figure 3. Thermal ellipsoid drawings (50%) of [Ni(OH)(P2CyPyr)] (7) and [Ni(SH)(P2CyPyr)] (8). Hydrogen atoms and minor disordered
component of 7 omitted for clarity. Selected bond distances (Å) and angles (deg) for 7: Ni(1)−N(1) = 1.843(3); Ni(1)−O(1) = 1.789(9); Ni(1)−
Pavg = 2.188(2); N(1)−Ni(1)−O(1) = 173.7(4); P(1)−Ni(1)−P(2) = 165.13(4); for 8: Ni(1)−N(1) = 1.8635(13); Ni(1)−S(1) = 2.1719(4);
Ni(1)−Pavg = 2.189(1); N(1)−Ni(1)−S(1) = 175.65(4); P(1)−Ni(1)−P(2) = 165.294(16).
to no formation of 4-tolylphenylsulfide as judged by NMR
spectroscopy (see Supporting Information, Figure S12). In
addition, no decomposition of 6 was apparent during the
reaction. This result demonstrates that direct reaction of a
Ni(II) thiolate species with aryl iodide is not responsible for
C−S bond formation during catalysis. Such a finding is
consistent with the observations of Guan with the nickel
POCOP system, where only minor amounts of coupled C−S
products were detected from the reaction of a nickel thiolate
complex with iodobenzene.25
complex with this ligand and with the previously reported
failure to generate a nickel hydride complex of P2PhPyr.17,20
Control experiments with 2 in DMF at 80 °C in the absence
of NaOH lead to no detectable change after several hours
demonstrating that NaOH is necessary for formation of 9.
Likewise, DMF is also essential, as reaction of 2 with NaOH in
benzene-d6 at 80 °C leads only to formation of the hydroxide
complex, 7. One possible mechanism for the transformation in
Scheme 6 is decarbonylation of a transient formate species
formed from DMF and OH− under the reaction conditions.
However, the resonance for Ni−H in Figure 4 appears to be
predominantly protio and not deutero, arguing against the acyl
C−H(D) bond of DMF as the source of the hydride ligand.
Furthermore, reactions of excess NaO2CH with 2 in benzene-d6
The lack of reactivity of 6 with iodotoluene suggested that
decomposition of precatalyst 2 to give a new nickel species
might be responsible for the catalytic activity observed in our
system in similar fashion to that reported for Ni POCOP.
Therefore, we next examined the stability of 2 in the presence
of NaOH and DMF-d7 at 80 °C. Unexpectedly, 2 was not
found to decompose appreciably under these conditions but
rather to generate the nickel(II) hydride species, [NiH-
1
or acetonitrile-d3 at 80 °C failed to produce 9 as judged by H
NMR spectroscopy. An alternative mechanism for formation of
9 is insertion of CO, formed from DMF in the presence of base
at elevated temperature,41 into 7 to give a hydroxycarbonyl
complex that rapidly undergoes β-H elimination (Scheme 7).
1
(P2CyPyr)] (9), as judged by H and 31P NMR spectroscopy
́
Campora and Palma have demonstrated formation of a bridged
(Scheme 6 and Figure 4). A small amount of decomposition
hydroxycarbonyl species upon treatment of a related [(PCP)-
Ni(OH)] (PCP = anion of C6H3-2,6-(CH2PiPr2)2) with CO in
hexanes.38 Likewise, Lee has recently reported formation of a
hydroxycarbonyl species and its subsequent decarbonylation
with a PNP Ni(II) system.42 In order to discern if similar
reactivity is responsible for the observed formation of 9, we
examined the reaction of excess CO(g) with 7 in benzene-d6 in
the presence and absence of added NaOH. Very little reaction
was evident at room temperature for either case as judged by
1H NMR, but heating to 80 °C for 2 h did lead to complete
consumption of the OH resonance at −4.79 ppm. Of the
multitude of compounds formed in the reaction, however, none
corresponded to 9. Therefore, if CO is responsible for
formation of 9, DMF must play an important role in selecting
for formation of the hydride.
Scheme 6. Reaction of [NiCl(P2CyPyr)] with NaOH in DMF
does appear to occur in tandem with hydride formation as
1
resonances for the free ligand can be observed in both H and
31P NMR spectra (Figure 4). However, this decomposition
amounts to less than 5%. Similar results were obtained when
complex 3 was heated in DMF-d7 in the presence of NaOH. In
contrast, however, subjecting complex 1 to the aforementioned
reaction conditions was found to lead to no detectable nickel
Although unlikely, given the activity of 1, we nonetheless
examined the reactivity of 9 with 4-iodotoluene to determine if
the hydride complex could play a role in catalysis. Upon
prolonged heating (20 h) of 9 with 4-iodotoluene in DMF-d7 at
80 °C a new nickel(II) species was observed to form. This new
species was identified as the iodide complex, [NiI(P2CyPyr)]
1
hydride species as judged by H and 31P NMR. Unassignable
resonances due to decomposition were observed, but, as with
complex 2, these peaks accounted for only a small fraction of
the total species in solution, with complex 1 remaining as the
major species. Lack of reactivity with 1 under these conditions
is consistent with our inability to isolate a nickel hydroxide
1
(10), based on its H and 31P NMR spectra, and on the
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Figure 4. 500 MHz H NMR spectrum of the reaction of [NiCl(P2CyPyr)] with NaOH in DMF-d7 after heating to 80 °C for 2 h. † = DMF-d7
solvent; ‡ = H(P2CyPyr); * = residual toluene from purification. Inset displays corresponding 31P NMR.
1
Scheme 8, but we note that Agapie has observed a related
hydride transfer to arene with a nickel(II) phosphine complex
at elevated temperature, which proceeds through the
intermediacy of a η3-aryl.43 A second nickel(II) species was
also identified during the reaction shown in Scheme 8 albeit in
very minor quantities and only after heating for 20 h (Figure
S13). The 31P resonance for this species is very similar to that
observed for [Ni(Ph)(P2CyPyr)], and therefore we assign this
complex as the putative nickel tolyl. Nonetheless, the reaction
of an intermediate hydride complex with 4-iodotoluene appears
to be too slow to account for any observed catalytic activity.
As a final experiment, we investigated the reaction of 2 with
excess NaSPh to determine if thiolate is capable of reducing 2
under catalytic conditions and providing access to Ni(I) or
Ni(0) species. Morales-Morales has proposed that both Ni(I)
and Ni(0) are involved in C−S cross-coupling reactions
employing a Ni(II) bis(imino)pyridine precatalyst.22 In the
Morales-Morales system, however, external reductant is present
during catalysis. Gratifyingly, reaction of 2 with 3 equiv of
NaSPh in DMF-d7 at 80 °C for 2 h produced several new
species (Figure 5). Complex 2 is completely consumed, and the
major Ni species in solution is 6. In addition to 6 and free
Scheme 7. Possible Pathway for Formation of
[NiH(P2CyPyr)] (9) from [NiCl(P2CyPyr)] (2)
observation that addition of KI to a DMF-d7 solution of 2
produces the identical compound (Figures S13). The iodide
complex forms concomitantly with toluene demonstrating that
hydride transfer from 9 has taken place (Scheme 8). At this
time we do not know the mechanism for the reaction in
1
NaSPh, peaks in the H NMR spectrum assignable to PhSSPh
and a new species (−11.6 ppm) are also present in minor
1
amounts. Based on the H chemical shift and our inability to
identify a 31P NMR signal for this new species, we tentatively
assign it as a paramagnetic Ni(I) complex. Addition of 4-
iodotoluene to this mixture followed by additional heating for 2
h led to complete consumption of both free NaSPh and the
species responsible for the peak at −11.6 ppm (Figure 5).
Furthermore, peaks due to the expected C−S coupling product,
4-tolylphenylsulfide, were observed to form. Throughout the
Scheme 8. Reaction of [NiH(P2CyPyr)] with 4-Iodotoluene
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Figure 5. 500 MHz 1H NMR spectra of the reaction of [NiCl(P2CyPyr)] (2) with 3 equiv of NaSPh in DMF-d7 after heating to 80 °C for 2 h (black)
and after addition of 4-iodotoluene and additional heating (red). † = DMF-d7 solvent; ° = [Ni(SPh)(P2CyPyr)] (6); ‡ = PhSSPh; § = new Ni species;
* = 4-iodotoluene.
Scheme 9. Proposed Catalytic Cycle for C−S Cross-Coupling with PNP Ni Complexes
reaction with 4-iodotoluene, resonances attributable to 6
remained unchanged. An analogous reaction of 1 with excess
NaSPh in DMF-d7 at 80 °C produced a similar resonance at
−11.6 ppm consistent with a closely related active species for
both catalysts (Figure S14). Cyclic voltammetry experiments
with 6 identified an irreversible reduction event at potentials
below 2 V (i.e., −2.61 V in THF and −2.33 in DMF; see the
SI). This low value indicates that simple outer-sphere electron
transfer to generate a Ni(I) species is unlikely and that
coordination of additional PhS− to 6 may be required for
reduction.
The results of stoichiometric reactions with well-defined
nickel complexes suggest that the active species in C−S cross-
coupling reactions employing Ni precatalysts 1−3 is a PNP
Ni(I) species formed by reduction in the presence of thiolate. A
proposed catalytic cycle based on this proposal that
incorporates the results of other stoichiometric reactions is
shown in Scheme 9. The catalytic cycle in Scheme 9 accounts
for several observations. First, the inactivity of thiolate complex
6: this species most likely represents a dormant state for the
catalyst. Second, the superiority of complex 1 as a precatalyst:
complex 1 is both the easiest to reduce to Ni(I) based on
measured reduction potentials (see the SI) and is incapable of
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forming a Ni−H species.16,20 Third, the inability to observe
cross-coupling with anilines or phenols: these nucleophiles are
incapable of effecting reduction to Ni(I) under the catalytic
conditions. Finally, the electrophile scope of the catalytic
reaction: the reducing potential of the Ni(I) species is modest
and only capable of undergoing oxidative addition with
activated electrophiles (aryl iodides).
Saturn 724 CCD fitted with Mo Kα radiation (λ = 0.71073 Å).
Data collection and unit cell refinement were performed using
Crystal Clear software.48 Data processing and absorption
correction, giving minimum and maximum transmission factors,
were accomplished with Crystal Clear and ABSCOR,
respectively.49 All structures were solved by direct methods
and refined on F2 using full-matrix, least-squares techniques
with SHELXL-97.50,51 All non-hydrogen atoms were refined
with anisotropic displacement parameters. All carbon bound
hydrogen atom positions were determined by geometry and
refined by a riding model. Crystallographic data and refinement
parameters can be found in the Supporting Information.
H(P2tBuPyr). A sealable flask was charged with 1.08 g (6.0
mmol) of 2,5-bis-(dimethylaminomethyl)pyrrole and 2.2 mL
(12 mmol) of di-tert-butylphosphine. The contents of the
sealed flask were heated to 140 °C and allowed to stir for 20 h.
The reaction mixture was cooled to room temperature, and all
volatiles were removed in vacuo to afford 2.0 g (88%) of the
desired compound as a white solid. The material was used
immediately for nickel chemistry without further purification.
CONCLUSIONS
■
In this contribution we have described the catalytic activity of a
series of Ni PNP complexes in C−S cross-coupling reactions.
The most successful electrophilic coupling partners were found
to be aryl iodides, which demonstrated good substrate scope for
a variety of substitution patterns. Coupling reactions were
successful with both aryl and alkyl thiols with isolated yields
between 76 and 97%. Synthesis of several PNP nickel
complexes containing heteroatom ligands has permitted an
examination of the nickel species responsible for catalytic
activity. The results of these studies support the proposal that
intact PNP Ni pincer complexes are capable of catalyzing C−S
cross-coupling reactions and that a cycle involving Ni(I) is
likely operative. Such a conclusion is consistent with Ni-
catalyzed C−C cross-coupling reactions where well-defined
paramagnetic complexes are now believed to play an important
role.44−46
1H NMR: δ 8.59 (br s, 1 pyr-NH), 6.08 (d, 2 pyr−CH, JHH
=
2.7), 2.73 (s, 4 CH2), 1.04 (d, 36 t-Bu, JHP = 10.5). 13C{1H}
NMR: δ 129.24 (d, JCP = 12.6), 107.27 (m), 31.73 (d, JCP
=
23.0), 30.09 (d, JCP = 12.8), 21.08 (d, JCP = 23.5). 31P{1H}
NMR: δ −7.66. ESI-MS (positive mode): calcd for [M + H]+
m/z 384.3; found for [M + H]+ m/z 384.1.
[NiCl(P2tBuPyr)]. A flask was charged with 1.14 g (2.97 mmol)
of H(P2tBuPyr) and 30 mL of THF. To the solution was added
0.388 g (2.99 mmol) of NiCl2 and 0.83 mL (6.0 mmol) of
Et3N. The mixture was allowed to stir for 12 h at room
temperature during which time it became dark red. All volatiles
were removed in vacuo leaving a dark red residue, which was
extracted into 50 mL of benzene. The benzene extract was
filtered through a pad of Celite to remove Et3NHCl. The
resulting solution was evaporated to dryness to afford 1.20 g
(85%) of the desired complex as a red solid. Crystals suitable
for X-ray diffraction were grown by cooling of a saturated
EXPERIMENTAL SECTION
■
General Comments. Unless otherwise noted, all manipu-
lations were performed under an atmosphere of nitrogen gas
using standard Schlenk technique or in a Vacuum Atmospheres
glovebox. Tetrahydrofuran, diethyl ether, pentane, dichloro-
methane, and toluene were purified by sparging with argon and
passage through two columns packed with 4 Å molecular sieves.
Benzene and benzene-d6 were dried over sodium ketyl and
vacuum-distilled prior to use. Melting points were obtained in
glass capillaries sealed with a plug of silicone grease and are
reported as uncorrected values. NMR spectra were recorded in
benzene-d6 on a Varian spectrometer operating at 500 MHz
1
diethyl ether solution at −35 °C. Mp: 260 °C (decomp). H
NMR: δ 6.22 (s, 2 pyr−CH), 2.64 (app t, 4 CH2, JHP = 4.2),
1.37 (app t, 36 t-Bu, JHP = 6.5), 13C{1H} NMR: δ 138.67 (t, JCP
(1H). Chemical shift values were referenced to the residual H
1
(7.16 ppm) or 13C (128.39 ppm) resonance of solvent, except
in the case of 31P NMR, where an external standard of 85% aq
H3PO4 (0.00 ppm) was used. Coupling constants (J) are
reported in units of Hz. FT-IR spectra were recorded with a
ThermoNicolet iS 10 spectrophotometer as thin films on a
NaCl plate or as solid samples pressed into KBr disks. UV−vis
spectra were recorded in toluene on a Cary-60 spectropho-
tometer in airtight Teflon-capped quartz cells. Elemental
analyses were performed by Atlantic Microlab of Norcross, GA.
Materials. 2,5-Bis(dimethylaminomethyl)pyrrole and the
nickel complexes [NiCl(P2PhPyr)], [NiCl(P2CyPyr)], [NiH-
(P2CyPyr)], and [Ni(Ph)(P2CyPyr)] were prepared by published
procedures.16,20,47 Aniline, phenol, thiophenol, triethylamine,
sodium phenoxide, sodium thiophenolate, and sodium/
potassium hydroxide were purchased from commercial
suppliers and used as received. Hydrogen sulfide was purchased
from Praxair and used as received. LiNHPh was prepared by
deprotonation of aniline with n-BuLi in THF.
= 6.8), 105.53 (t, JCP = 4.8), 35.42 (t, JCP = 6.7), 29.61 (t, JCP
=
1.9), 23.60 (t, JCP = 9.7). 31P{1H} NMR: δ 64.93. Anal. Calcd
for C22H42ClNNiP2: C, 55.43; H, 8.88; N, 2.94. Found: C,
55.70; H, 8.88; N, 2.90.
[Ni(NHPh)(P2CyPyr)]. A flask was charged with 0.116 g (0.200
mmol) of [NiCl(P2CyPyr)] and 10 mL of THF. To the
resulting solution was added 0.030 g (0.30 mmol) of solid
LiNHPh. The reaction mixture was allowed to stir for 6 h at
room temperature during which time it slowly darkened from
red to purple. All volatiles were removed in vacuo, and the
remaining residue was extracted into 10 mL of benzene. The
purple extract was filtered through a pad of Celite and the
benzene evaporated to dryness. The residue was then washed
with pentane and dried in vacuo to afford 0.109 g (86%) of the
desired compound as a purple microcrystalline solid. Crystals
suitable for X-ray diffraction were grown by cooling of a
saturated diethyl ether solution of the complex at −30 °C. Mp:
1
184−186 °C. H NMR: δ 7.19 (m, 2 m-NHPh), 6.97 (d, 2 o-
NHPh), 6.50 (t, 1 p-NHPh), 6.36 (s, 2 pyr−CH), 2.67 (app t, 4
CH2, JHP = 4.2), 2.23 (app d, 4 Cy−CH), 1.77 (m, 4 Cy−CH),
1.69−1.45 (overlapping m, 24 Cy−CH), 1.12 (m, 8 Cy−CH),
1.01 (m, 4 Cy−CH), −1.20 (s, 1 NHPh). 13C{1H} NMR: δ
160.91, 137.67 (t, JCP = 7.5), 129.24, 117.80, 110.72, 105.87 (t,
X-ray Data Collection and Structure Solution Refine-
ment. Crystals suitable for X-ray diffraction were mounted in
Paratone oil onto a glass fiber and frozen under a nitrogen cold
stream maintained by a X-Stream low-temperature apparatus.
The data were collected at 98(2) K using a Rigaku AFC12/
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JCP = 5.0), 33.85 (t, JCP = 9.6), 28.91, 28.60, 27.58 (t, JCP = 5.9),
27.39 (t, JCP = 4.8), 26.85, 23.23 (t, JCP = 10.4). 31P{1H} NMR:
δ 42.71. IR (KBr) ν, cm−1: 3352 (νNH). UV−vis λ, nm, (ε, M−1
cm−1): 350 (6500), 446 (500), 561 (1400). Anal. Calcd for
C36H56N2NiP2: C, 67.83; H, 8.85; N, 4.39. Found: C, 67.81; H,
9.09; N, 4.26.
benzene evaporated to dryness. The residue was washed with
pentane and dried in vacuo to afford 0.099 g (89%) of the
desired compound as a yellowish-orange solid.
Procedure B. A flask was charged with 0.116 g (0.200 mmol)
of [NiCl(P2CyPyr)] and 10 mL of THF. To the resulting
solution was added 0.056 g (1.0 mmol) of KOH as a ground
solid. The reaction mixture was allowed to stir for 24 h at room
temperature during which time the red color slowly lightened
to yellowish-orange. All volatiles were removed in vacuo, and
the remaining residue was extracted into 10 mL of benzene.
The orange extract was filtered through a pad of Celite and the
benzene evaporated to dryness. The residue was then washed
with pentane and dried in vacuo to afford 0.079 g (71%) of the
desired compound as a yellowish-orange solid. Crystals suitable
for X-ray diffraction were grown by cooling of a saturated
[Ni(OPh)(P2CyPyr)]. A flask was charged with 0.116 g (0.200
mmol) of [NiCl(P2CyPyr)] and 10 mL of THF. To the
resulting solution was added 0.034 g (0.29 mmol) of NaOPh.
The reaction mixture was allowed to stir for 8 h at room
temperature during which time the red color brightened
slightly. All volatiles were removed in vacuo, and the remaining
residue was extracted into 10 mL of benzene. The red extract
was filtered through a pad of Celite and the benzene evaporated
to dryness. The residue was then washed with pentane and
dried in vacuo to afford 0.122 g (96%) of the desired
compound as a bright red microcrystalline solid. Crystals
suitable for X-ray diffraction were grown by cooling of a
saturated diethyl ether solution of the complex at −30 °C. Mp:
191−193 °C. 1H NMR: δ 7.34 (d, 2 o-OPh), 7.25 (t, 2 m-OPh),
6.68 (t, 1 p-OPh), 6.31 (s, 2 pyr−CH), 2.61 (app t, 4 CH2, JHP
= 4.5), 2.32 (app d, 4 Cy−CH), 1.72 (m, 16 Cy−CH), 1.62
(app d, 4 Cy−CH), 1.56 (m, 8 Cy−CH), 1.16 (m, 8 Cy−CH),
1
diethyl ether solution of the complex at −30 °C. H NMR: δ
6.35 (s, 2 pyr−CH), 2.69 (app t, 4 CH2, JHP = 4.2), 2.41 (app d,
4 Cy−CH), 1.81 (m, 4 Cy−CH), 1.74 (m, 16 Cy−CH), 1.66
(m, 4 Cy−CH), 1.59 (m, 4 Cy−CH), 1.16 (m, 8 Cy−CH),
1.07 (m, 4 Cy−CH), −4.79 (t, 1 OH, JHP = 5.8). 13C{1H}
NMR: δ 137.43 (t, JCP = 7.5), 105.73 (t, JCP = 5.0), 32.80 (t, JCP
= 9.4), 28.90, 28.56, 27.55 (t, JCP = 6.2), 27.39 (t, JCP = 4.9),
26.91, 23.03 (t, JCP = 10.8). 31P{1H} NMR: δ 43.32. IR (KBr)
ν, cm−1: 3355 (νOH). UV−vis λ, nm, (ε, M−1 cm−1): 293
(2800), 317 (3000), 421 (1000), 514 (150). Anal. Calcd for
C30H51NNiOP2·H2O: C, 62.08; H, 9.20; N, 2.41. Found: C,
61.68; H, 8.78; N, 2.39.
1.01 (m, 4 Cy−CH). 13C{1H} NMR: δ 169.37, 138.08 (t, JCP
=
7.2), 129.21, 121.62, 114.05, 106.27 (t, JCP = 4.8), 33.49 (t, JCP
= 9.3), 28.53, 28.45, 27.53 (t, JCP = 6.1), 27.38 (t, JCP = 5.0),
26.81, 22.09 (t, JCP = 10.9). 31P{1H} NMR: δ 42.33. UV−vis λ,
nm, (ε, M−1 cm−1): 304 (12000), 335 (sh), 477 (1300). Anal.
Calcd for C36H55NNiOP2: C, 67.72; H, 8.68; N, 2.19. Found:
C, 68.00; H, 8.67; N, 2.36.
[Ni(SH)(P2CyPyr)]. A flask was charged with 0.127 g (0.199
mmol) of [Ni(NHPh)(P2CyPyr)] and 10 mL of toluene. The
flask was sealed with a septum, and H2S gas (∼1 atm) was
added to the headspace of the reaction vessel. Upon addition of
H2S, the purple solution immediately changed to reddish-
brown. The reaction mixture was allowed to stir for a further 2
min at room temperature before all volatiles were removed in
vacuo. The remaining solid was washed with pentane to afford
0.112 g (97%) of the desired compound as brown micro-
crystals. Crystals suitable for X-ray diffraction were grown by
vapor diffusion of pentane into a saturated benzene solution at
[Ni(SPh)(P2CyPyr)]. A flask was charged with 0.116 g (0.200
mmol) of [NiCl(P2CyPyr)] and 10 mL of THF. To the
resulting solution was added 0.039 g (0.30 mmol) of NaSPh.
The reaction mixture was allowed to stir for 8 h at room
temperature during which time the color changed slowly from
red to pinkish-red. All volatiles were removed in vacuo, and the
remaining residue was extracted into 10 mL of benzene. The
pinkish-red extract was filtered through a pad of Celite and the
benzene evaporated to dryness. The residue was then washed
with pentane and dried in vacuo to afford 0.119 g (92%) of the
desired compound as a pinkish-red microcrystalline solid.
Crystals suitable for X-ray diffraction were grown by cooling of
a saturated diethyl ether solution of the complex at −30 °C.
Mp: 187−189 °C. 1H NMR: δ 7.91 (d, 2 o-SPh), 7.01 (t, 2 m-
SPh), 6.94 (t, 1 p-SPh), 6.44 (s, 2 pyr−CH), 2.78 (app t, 4 CH2,
JHP = 4.2), 2.32 (app d, 4 Cy−CH), 1.71 (m, 12 Cy−CH),
1.64−1.52 (overlapping multiplets, 16 Cy−CH), 1.13 (m, 8
Cy−CH), 1.00 (m, 4 Cy−CH). 13C{1H} NMR: δ one peak
obscured by solvent, 147.56 (t, JCP = 6.4), 137.34 (t, JCP = 7.2),
135.62, 123.55, 105.72 (t, JCP = 5.1), 33.95 (t, JCP = 10.6),
29.14, 28.67, 27.53 (overlapping triplets), 26.86, 24.46 (t, 10.7).
31P{1H} NMR: δ 50.02. UV−vis λ, nm, (ε, M−1 cm−1): 294
(19000), 336 (sh), 424 (1000), 540 (730). Anal. Calcd for
C36H55NNiP2S: C, 66.06; H, 8.47; N, 2.14; S, 4.90. Found: C,
65.96; H, 8.57; N, 2.06; S, 5.00.
1
room temperature. Mp: 183−185 °C. H NMR: δ 6.43 (s, 2
pyr−CH), 2.79 (app t, 4 CH2, JHP = 4.5), 2.31 (app d, 4 Cy−
CH), 1.84 (m, 4 Cy−CH), 1.74−1.44 (overlapping m, 24 Cy−
CH), 1.13 (m, 8 Cy−CH), 1.04 (m, 4 Cy−CH), −2.26 (t, 1
SH, JHP = 19.5). 13C{1H} NMR: δ 137.05 (t, JCP = 7.1), 105.38
(t, JCP = 5.2), 33.46 (t, JCP = 10.9), 28.91, 28.57, 27.55 (t, JCP
=
6.2), 27.36 (t, JCP = 5.1), 26.84, 25.07 (JCP = 11.1). 31P{1H}
NMR: δ 55.36. IR (KBr) ν, cm−1: 2539 (νSH). UV−vis λ, nm,
(ε, M−1 cm−1): 322 (8200), 387 (390), 521 (250). Anal. Calcd
for C30H51NNiP2S: C, 62.29; H, 8.89; N, 2.42; S, 5.54. Found:
C, 62.77; H, 8.89; N, 2.49; S, 5.49.
General Procedure for C−S Coupling Reactions. A
Schlenk tube was charged with 5 mg (0.008 mmol) of
[NiCl(P2PhPyr)] and 3 mL of DMF. To the solution was added
1.1 mmol of electrophile, 1.0 mmol of thiol, and 2.0 mmol of
KOH. The reaction mixture was allowed to stir at 80 °C for 3−
8 h as indicated. After this time, the reaction mixture was
allowed to cool to room temperature, exposed to the ambient
atmosphere, and extracted into ethyl acetate. The organic layer
was washed with water and dried over sodium sulfate. The
solvent was removed by rotary evaporation to afford crude
product. The crude product was then purified by column
chromatography on Al2O3 (2% EtOAc in hexanes). NMR and
LC-MS analysis provided confirmation of the coupled thioether
products (see the Supporting Information). Control reactions
[Ni(OH)(P2CyPyr)]. Procedure A. A flask was charged with
0.127 g (0.199 mmol) of [Ni(NHPh)(P2CyPyr)] and 10 mL of
toluene. To the resulting solution was added 0.1 mL of H2O via
syringe. The reaction mixture was allowed to stir for 12 h at
room temperature during which time the color darkened from
red to brown. All volatiles were removed in vacuo, and the
remaining residue was extracted into 10 mL of benzene. The
brown extract was filtered through a pad of Celite and the
2949
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(22) Baldovino-Pantaleon
́
, O.; Hernan
́
dez-Ortega, S.; Morales-
without catalyst were performed for each substrate to ensure
that uncatalyzed nucleophilic aromatic substitution was not
responsible for the observed reactivity.
Morales, D. Adv. Synth. Catal. 2006, 348, 236−242.
(23) The Chemistry of Pincer Compounds; Morales-Morales, D.,
Jensen, C. G. M., Eds.; Elsevier B. V.: Amsterdam, 2007.
(24) Adhikary, A.; Guan, H. Nickel-Catalyzed Cross-Coupling
Reactions. In Pincer and Pincer-Type Complexes; Wiley-VCH Verlag
GmbH & Co. KGaA: 2014; p 117−148.
ASSOCIATED CONTENT
■
S
* Supporting Information
(25) Zhang, J.; Medley, C. M.; Krause, J. A.; Guan, H. Organo-
metallics 2010, 29, 6393−6401.
Additional experimental procedures, figures, crystal structures,
tabulated crystallographic data, and crystallographic informa-
tion (cif) files. This material is available free of charge via the
Internet at http://pubs.acs.org.
(26) Zhang, J.; Adhikary, A.; King, K. M.; Krause, J. A.; Guan, H.
Dalton Trans. 2012, 41, 7959−7968.
́
(27) Hao, J.; Mougang-Soume, B.; Vabre, B.; Zargarian, D. Angew.
Chem., Int. Ed. 2014, 53, 3218−3222.
AUTHOR INFORMATION
(28) van Koten, G. Top. Organomet. Chem. 2013, 40, 1−20.
(29) Zhang, Y.; Ngeow, K. C.; Ying, J. Y. Org. Lett. 2007, 9, 3495−
3498.
■
Corresponding Author
*E-mail: zachary.tonzetich@utsa.edu.
(30) Xu, X.-B.; Liu, J.; Zhang, J.-J.; Wang, Y.-W.; Peng, Y. Org. Lett.
2013, 15, 550−553.
Notes
The authors declare no competing financial interest.
(31) Zargarian, D.; Castonguay, A.; Spasyuk, D. Top. Organomet.
Chem. 2013, 40, 131−173.
́
(32) Szabo, K. Top. Organomet. Chem. 2013, 40, 203−241.
ACKNOWLEDGMENTS
■
(33) Chen, H.-Y.; Peng, W.-T.; Lee, Y.-H.; Chang, Y.-L.; Chen, Y.-J.;
Lai, Y.-C.; Jheng, N.-Y.; Chen, H.-Y. Organometallics 2013, 32, 5514−
5522.
This work was supported by a grant from the Welch
Foundation (AX-1772). G.T.V. also acknowledges partial
support from the UTSA TRAC program. The authors thank
Mr. Jacob Przyojski and Mr. Daniel Meininger for assistance
with NMR measurements.
(34) Liang, L.-C.; Chien, P.-S.; Lee, P.-Y.; Lin, J.-M.; Huang, Y.-L.
Dalton Trans. 2008, 3320−3327.
(35) van der Vlugt, J. I.; Lutz, M.; Pidko, E. A.; Vogt, D.; Spek, A. L.
Dalton Trans. 2009, 1016−1023.
(36) Martínez-Prieto, L. M.; Melero, C.; del Río, D.; Palma, P.;
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