Discrete Air-Stable Nickel(II)-Palladium(II) Complexes as Catalysts for Suzuki-Miyaura Reactions

Article abstract of DOI:10.1021/acs.organomet.7b00176

Four novel bidentate phosphine-Pd complexes coordinated by NiN₂S₂ metallodithiolate ligands were synthesized and characterized, including XRD analysis; these redox-active complexes are stable in air, and they survive column chromatography on silica gel. The Ni-Pd bimetallic complexes are demonstrated to be precatalyst for Suzuki-Miyaura cross-coupling reactions. The best of these has an overall yield of over 99% under only 1% catalyst loading. The role of NiN₂S₂ as a stabilizing bidentate ligand that is also oxidizable may account for its efficacy in the support of the catalysis.

Full text of DOI:10.1021/acs.organomet.7b00176

Article
pubs.acs.org/Organometallics
Discrete Air-Stable Nickel(II)−Palladium(II) Complexes as Catalysts
for Suzuki−Miyaura Reactions
Tiankun Zhao,† Pokhraj Ghosh,† Zachary Martinez,† Xufeng Liu,†,‡ Xianggao Meng,†,§
and Marcetta Y. Darensbourg*,†
†Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States
‡College of Materials and Chemical Engineering, Ningbo University of Technology, Ningbo 315016, China
§Key Laboratory of Pesticide & Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University,
Wuhan 430079, China
S
* Supporting Information
ABSTRACT: Four novel bidentate phosphine-Pd complexes
coordinated by NiN2S2 metallodithiolate ligands were
synthesized and characterized, including XRD analysis; these
redox-active complexes are stable in air, and they survive
column chromatography on silica gel. The Ni−Pd bimetallic
complexes are demonstrated to be precatalyst for Suzuki−
Miyaura cross-coupling reactions. The best of these has an
overall yield of over 99% under only 1% catalyst loading. The
role of NiN2S2 as a stabilizing bidentate ligand that is also
oxidizable may account for its efficacy in the support of the
catalysis.
(cys-gly-cys)NiII supported CO/CH3 insertion or coupling
reactivity in ACS, we might expect a similar demonstration in
INTRODUCTION
■
The acetyl-coA synthase (ACS) enzyme active site offers the
intriguing prospect that the tight binding site with contiguous
palladium chemistry, choosing the copolymerization of ethylene
and CO as a prototype.10 Indeed the (NiN2S2)Pd(CH3)Cl
S−N−N−S donors for one of the required pairs of nickels
complex shown in Figure 1 was shown to take up and insert
renders a cysteine-glycine-cysteine tripeptide motif into a
metallo-cis-dithiolate, NiN2S2, bidentate metalloligand.1−3 The
second nickel, the catalytic center, is bound by three bridging
thiolate sulfurs, two from the NiN2S2 and an additional one to a
4Fe-4S cluster; there is as well in the protein crystal structure
an unknown ligand, X.4,5 Mechanistic possibilities for the
overall C−C coupling reactions to yield acetyl co-A include
+
CH3 transfer from cobalt in the vitamin B12 cofactor to the
Figure 1. NiN2S2-coordinated Pd complex for CO and ethylene
active nickel and CO insertion into a Ni−CH3 bond, with
nucleophilic attack on the Ni−C(O)−CH3 from the thiolate
of co-A reductively releasing the acetyl-coA, thus regenerating
the low-valent nickel into the cycle.1,4,6 Whether the NiN2S2
metallodithiolate retains bidentate binding to the active nickel
or shifts into a monodentate binding mode is uncertain.7
We have pursued the development of MN2S2 complexes as a
class of ligands in which the cis-dithiolate sulfur donors, tamed
by transition metal ions such as Ni2+, [Fe(NO)]2+, or
[Co(NO)]2+, can be viewed as analogues of diphosphines,
with similar electronic donor properties.8,9 They may be used
to strategically design bi- and multimetallic complexes. The
second lone pair on each sulfur generates a hinge at the
bridging dithiolate sulfurs and a butterfly core. This steric
feature positions metals just beyond M−M′ bonding distances
in bimetallic systems but sufficiently close to accommodate
bridging ligands. Earlier we suggested that since the natural
insertion reactions.10 Crystal structure of [(NiN2S2)Pd(CH3)(Cl)] is
overlaid to illustrate the butterfly core.9
CO in the Pd−CH3 bond and also initiate the formation of
short chains of (−CO)CH2CH2−)n attached to PdII,
analogous to the well-known copolymerization process.10
With the above results in mind, we have directed attention
toward designing Ni−Pd complexes for the purpose of
exploring their potential for catalyzing the Suzuki−Miyaura
reaction. As one of the most widely applied methods for
constructing a C−C bond,11 extensive studies have focused on
palladium and the selection of appropriate ligands for specific
reactions. Multimetallic complexes have found application in
Received: March 7, 2017
© XXXX American Chemical Society
A
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Figure 2. Synthesis of Ni−Pd bimetallic complexes I−IV.
catalysis, either as biomimetic complexes of enzyme active
sites12−14 or as catalysts in organic reactions.15,16 For the latter,
most of the reported multimetallic catalysts are in the form of
nanoparticles16 and metal−organic frameworks.17−19 There are
a few examples of discrete, molecular heterobimetallic
complexes such as N-heterocyclic carbene-supported Ir−Pd,
Ir−Ir, and Pd−Pd complexes that were found to catalyze
tandem dehalogenation/transfer hydrogenation or Suzuki/
transfer hydrogenation reactions.20 An impressive hydro-
defluorination of organic molecules was found to require
both Rh and Pd in a discrete heterobimetallic catalyst.21
Another example uses the iron-modified triaryl phosphine as
ligand, (η2-dppe)(η5-C5Me5)Fe(CC-Aryl-PPh2), coordinated
to a Pd(II) complex, which was competent for Stille-type cross-
coupling reactions.22 These few examples, disparate as they are
in molecular makeup, have not thus far led to design principles
that might define the possible role of two metal centers in
proximity.
Figure 3. Solid-state molecular structures presented as a mixture of
thermal ellipsoids (50% probability level) and wire frames for the
phenyl groups. [Ni(bme-dach*)Pd(dppe)](PF6)2, I, [Ni(bme-dach*)-
Pd(dppp)](PF6)2, II, [Ni(bme-dach)Pd(dppe)](PF6)2, III, and [Ni-
(bme-dach)Pd(dppp)](PF6)2, IV. The PF6 counterions, hydrogen
atoms, and any cocrystallized solvent molecules are omitted for clarity.
Building on the success of the NiN2S2·PdII complex for CO/
olefin copolymerization described above, we have further
explored the synthesis and characterization of a group of
[NiN2S2]-PdII(diphos) complexes as stable, efficient, well-
soluble catalysts or pro-catalysts for the Suzuki−Miyaura
reaction. Mechanism studies establish the requirement for the
NiN2S2 metalloligand that we maintain serves as a convenient
platform for palladium to enter the catalytic cycle.
−
Table 1. Selected Metric Data Bond Length [Å] and Angles
[deg] for Complexes I−IV
I
II
III
IV
a
N−Ni /Å
1.906(3)
2.143(6)
2.410(8)
2.267(12)
2.883(1)
77.5(1)
94.15(4)
81.24(3)
85.02(3)
96.73(3)
93.43(8)
118.3
1.902(5)
2.138 (1)
2.421 (1)
2.283 (9)
2.962(3)
77.9(2)
94.80(5)
81.00(5)
90.34(5)
93.84(5)
93.11(14)
128.3
1.926(4)
2.156 (7)
2.381(6)
2.282 (2)
3.079 (9)
83.50(16)
90.74(5)
80.26(4)
83.50(4)
98.03(4)
92.12(12)
131.3
1.928(3)
2.154 (3)
2.394(2)
2.294 (3)
3.111(1)
83.14(1)
90.01(4)
79.05(3)
90.07(3)
95.13(3)
92.58(9)
132.8
a
S−Ni /Å
RESULTS AND DISCUSSION
a
■
S−Pd /Å
a
Synthesis. Nickel dithiolates (NiN2S2) built into the six-
and seven-membered diazacycles, 2,2′-(piperazine-1,4-diyl)bis-
(ethane-1-thiol), Ni(bme-dach*), and 2,2′-(1,4-diazepane-1,4-
diyl)bis(ethane-1-thiol), Ni(bme-dach),23 were chosen to be
used in this study. The former was synthesized in the same
manner as the latter,23 and its X-ray crystal structure is provided
in the Supporting Information, SI. Palladium(II) was added in
the form of Pd(dppe)Cl2 (dppe = 1,2-bis-diphenylphosphino-
ethane) or Pd(dppp)Cl2 (dppp = 1,2-bis-diphenyl-
phosphinopropane) to the NiN2S2 unit in CH3CN solution
also containing NH4PF6. The resulting Ni−Pd bimetallic
complexes I−IV were obtained as red solids following solvent
removal. Their stability to air and moisture permitted
purification by flash chromatography on silica gel; isolated
yields are given in Figure 2.
P−Pd /Å
Ni−Pd/Å
N−Ni−N/deg
S−Ni−S/deg
S−Pd−S/deg
P−Pd−P/deg
S−Pd−P /deg
N−Ni−S /deg
a
a
b
Hinge Angle /deg
c
Ni−N2S2 disp. / Å
Pd−S2P2 disp. / Å
0.164
0.144
0.167
0.178
d
0.083
0.200
0.098
0.171
a
b
Average value. The angle between the N2S2 best plane and S2P2 best
c
d
plane. Displacement of Ni from the N2S2 best plane. Displacement
of Pd from the S2P2 best plane.
Solid-State Structures. Single crystals suitable for X-ray
diffraction analysis were obtained by slow evaporation of a
concentrated CH3CN solution of complexes I−IV. Their
molecular structures are shown in Figure 3, and listings of
metric data are given in Table 1. Full structure reports are
deposited in the Cambridge Crystallographic Data Centre;
further refinement parameters and relevant crystal data are in
the SI. Complexes I−IV crystallized with NiII and PdII in almost
regular square-planar geometries, bridged by thiolate sulfurs.
The intersection of the N2S2 best plane with the P2S2 best
plane, defined as the hinge angle in the bimetallic structures, is
118.27° for I, increasing by 10° for complex II to 128.3°;
complexes III and IV have hinge angles of 131.3° and 132.8°,
respectively. This increase is accompanied by an overall
increase in the Ni···Pd distance of 0.23 Å.
The S−Ni−S bite angles of the free metallodithiolate ligands,
Ni(bme-dach*), Ni(bme-dach),23 and Ni(bme-daco) (bme-
daco is bis-mercaptoethane diazacyclooctane),24 are presented
in Figure 4 for comparison to the Ni−Pd complexes. Note that
B
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[tBu4N][PF6] as the supporting electrolyte. All potentials are
referenced to added Fc0/+ at E1/2 = 0.0 V. Figure 5 shows the
Figure 4. Molecular structures of Ni(bme-dach*), Ni(bme-dach),23
and Ni(bme-daco)24 as thermal ellipsoids (50% probability level).
Select metric parameters are shown in the table.
the constriction of the two nitrogen atoms in the mesodiaza-
cycle leads to an increase in the S···S distance due to
enlargement of the ∠S−Ni−S; that is, the ∠N−Ni−N angle
varies inversely as the ∠S−Ni−S angle with an overall
expansion of ca. 12°.8 In addition, on binding a second metal
center, the ∠S−Ni−S is further constricted by ca. 5−6° in the
bimetallic complexes (I−IV). Whether there is a role for bite
angles in the NiN2S2 metallodithiolates that might govern
possible hemilability in the opening of reactive sites on the
palladium metal awaits further experimentation.
Figure 5. Cyclic voltammetry study of 2 mM complexes of I (blue), II
−
(red), III (green), and IV (purple) as PF6 salts under argon in
CH3CN solutions containing 0.1 M [tBu4N][PF6] as supporting
electrolyte at 200 mV/s.
full scans of all complexes. The CVs were initiated as noted and
scanned in the negative direction at a 200 mV/s scan rate; scan
reversal was at the solvent window. The results are summarized
in Table 3. The four complexes showed similar redox patterns,
The ∠S−Pd−S angles vary very slightly across the four
complexes (79.05−81.24°). The ∠P−Pd−P differs by 5°
according to the two- or three-carbon linker in dppe (ca. 84°
or 85°) or dppp (ca. 90°); see Table 1. The six-membered
metallocycles such as NiN2C3 in III and IV and PdP2C3 in II
and IV Ni(bme-dach*)/(bme-dach) and Pd(dppp)/(dppe)
components are found in chair conformations.
Table 3. Cathodic Potentials for Reductions (Epc) of
a
Complexes I−IV
entry
complex
Epc(PdII/I)
Epc(PdI/0
)
Epc(NiII/I
)
1
2
3
4
I
−1.25 V
−1.27 V
−1.28 V
−1.29 V
−1.59 V
−1.61 V
−1.61 V
−1.59 V
−2.38 V
−2.38 V
−2.48 V
−2.49 V
Comparison of 31P{1H} NMR Chemical Shifts. All
II
1
complexes were characterized by H, 13C, 19F, and 31P{1H}
III
NMR spectroscopy (see the SI). The 31P{1H} NMR shifts,
summarized in Table 2, show the dppe-containing complexes,
IV
a
All potentials were referenced to Fc0/+ in CH3CN solutions in 0.1 M
n-Bu4NPF6 electrolyte using a glassy carbon working electrode.
Table 2. 31P{1H} NMR Chemical Shift Data in Dioxane for
Complexes I−IV, the Free Diphosphine Ligands, and Pd-
Diphosphine Analogues
with two irreversible reduction events separated by ca. 300 mV
with nearly the same Epc values for each complex. Likewise
there is a more negative quasi-reversible redox event at ca. −2.4
V for each.
a
a
complex
31P{1H} shift
reference complex
31P{1H} shift
I
68.03
4.28
dppe/dppp
−11.84/−16.02
68.36
The cyclic voltammogram for [Pd(dppp)2]Cl2 in dichloro-
methane solution was recorded (see the SI, Figures S9, S10) for
comparison to those of complexes I−IV. Two irreversible redox
events at −1.28 and −1.60 V are assigned as PdII/I and PdI/0
reductions, respectively. On the basis of the similarity in pattern
and position, the first two reduction events of the Ni−Pd
complexes were assigned to the NiII(N2S2)PdII/I(P2) and
NiII(N2S2)PdI/0(P2) couples, at ca. −1.27 and ca. −1.60 V,
respectively. Electrochemical studies of analogous NiIINiII
complexes with similar coordination geometry suggested the
likelihood of NiII(N2S2)NiI(P2) and NiII(N2S2)Ni0(P2) reduced
species.25 Although phosphine ligands support low oxidation
states, the possibility of describing the more reduced species as
NiINiI rather than NiIINi0 was raised by others.26,27
The quasi-reversible, more negative cathodic event in the
CVs of I−IV displays greater dependency on the nature of the
N2S2 and is assigned to the NiII/I(N2S2)Pd0(P2) couple. With
increasing scan rate, this event becomes more reversible for all
complexes; however the less negative events assigned to the
palladium reductions are unchanged (see the SI, Figure S14).
Overall the E1/2 values of the NiII/I(N2S2)Pd0(P2) couple of
II
Pd(dppe)Cl2
Pd(dppp)Cl2
[Pd(dppp)2]2+
III
65.03
13.72
IV
3.67
11.89
a
H3PO4 (0 ppm) was used as reference for the 31P NMR shift; values
are given in ppm units.
including Pd(dppe)Cl2, to be in the 65−68 ppm range. In
contrast, the 31P chemical shifts for complexes II and IV
containing the dppp unit are much smaller, around 4 ppm;
consistently, their precursor Pd(dppp)Cl2 showed a 31P{1H}
resonance at 13.72 ppm. Resonances for the free diphosphine
ligands, dppe and dppp, are at −11.84 and −16.02 ppm,
respectively. Resonances from other nuclei are consistent with
their chemical environment represented by the solid-state
structures.
ELECTROCHEMISTRY
■
Cyclic Voltammetry. Cyclic voltammograms (CVs) of
complexes I−IV were recorded at 22 °C under Ar in CH3CN
using a glassy carbon working electrode with 0.1 M
C
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such as W(CO)4,29 [(η5-C5H5)Fe(CO)]+,7 [(η5-C5Me5)Fe-
(CO)]+,30 or Me+ (as in the alkylation of the thiolate
generating S-bound, NiII thioether complexes),28 shift the
NiII/I reduction to considerably more positive potentials. The
absence of such positive shifts at Ni in the cases reported here
suggests that the reduced palladium has little effect on the NiII/I
reduction. While one might argue that the NiN2S2 metal-
loligand dissociates from the reduced palladium, there was little
evidence of complex degradation in the electrochemical cell;
deposition on the electrode surface was minor, as indicated by
consistency of multiple scans over the reductive region (see the
SI, Figure S12−S14).
Table 4. Reaction Optimization of Suzuki Coupling Reaction
Catalyzed by Complexes I−IV
Ni−Pd-Catalyzed Suzuki Coupling Reactions. Optimiz-
ing Conditions. In order to establish the possibility that the
NiN2S2 metalloligands might serve as a diphosphine surrogate,
we have examined complexes I through IV according to a
typical protocol for Suzuki coupling, i.e., establishing optimal
conditions for simple phenyl substrates and the scope or
tolerance of the reaction to various aryl substituents. In addition
the requirement for the NiN2S2 metalloligand was established
via several control experiments. In general the reactions were
analyzed for product following only one experiment; however
entry 6 of Table 4 was performed three times with the same
result, while the results of entries 2, 9, and 11 are duplicates.
Scope of Reaction. With the optimized reaction conditions
in hand, we extended the reaction to include a range of
commercially available aryl halides and phenylboronic acids. As
listed in Table 5, phenylboronic acid 2a readily reacted with
bromobenzene 1b and gave the corresponding product 3a in
excellent yield (entry 2). Reaction with 4-chlorotoluene 1c
proceeded slowly; after 4 days, product 3b was isolated in 21%
yield with recovery of the starting materials (entry 3). Entry 4
demonstrates the expected selectivity of iodine over fluorine.
The reaction slowly proceeded with substrates bearing the OTf
group, 1e; after 5 days product 3d was obtained in 34% yield
(entry 5). This catalytic system is also compatible with
heteroaryl halides such as 2-bromopyridine 1f (entry 6).
Several substituted phenylboronic acids were also tested for
the reaction scope. Reaction with p-tolylboronic acid 2b and 4-
tert-butylphenyl boronic acid 2c proceeds smoothly, giving the
corresponding products 3b and 3f in excellent yields (entries 7
a
b
entry
cat.
solvent
base
temp (°C)
yield (%)
1
I
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
toluene
THF
Na2CO3
K2CO3
reflux (101)
reflux
17
75
96
13
2
I
3
I
Cs2CO3
Et3N
reflux
4
I
reflux
c
5
I
Pyridine
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
reflux
0
6
II
II
II
II
II
III
IV
−
reflux
>99
39
85
26
14
92
95
7
reflux (111)
reflux (66)
reflux (82)
60
8
9
CH3CN
dioxane
dioxane
dioxane
dioxane
10
11
12
13
reflux (101)
reflux
c
reflux
0
a
Reactions were carried out by using aryl halide (0.3 mmol, 1 equiv),
phenylboronic acid (0.36 mmol, 1.2 equiv), Ni−Pd complex (0.003
mmol, 1 mol % equiv), and base (0.6 mmol, 2 equiv) in 2 mL of
solvent under reflux for 15 h. Isolated yield. Starting material was
recycled.
b
c
complexes III and IV, (the Ni(bme-dach) derivatives) at −2.48
V are about 100 mV more negative than those of complexes I
and II (with Ni(bme-dach*)), at −2.38 V.
Interestingly the E1/2 values assigned to the nickel in the
NiN2S2 binding site of the Ni−Pd complexes are close to that
of the NiII/I reduction in the free metalloligands (see the SI,
Figure S15).28 A compilation of NiII/I redox couples in a series
of NiN2S2M′ complexes finds that better Lewis acid receivers,
Table 5. Catalyst II and the Scope of Cross Coupling of Aryl Halides (1a−e) and Arylboronic Acids (2a−e)
a
b
entry
R1
X
R2
time
product
yield (%)
1
2
H
I
1a
1b
1c
1d
1e
1f
H
2a
2a
2a
2a
2a
2a
2b
2c
2d
2e
15 h
16 h
4 d
3a
3a
3b
3c
3d
3e
3b
3f
>99
94
H
Br
H
c
3
4-Me
4-F
Cl
H
21
4
I
H
19 h
5 d
83
34
81
85
87
93
5
4-OMe
OTf
H
6
2-bromopyridine
Br
I
H
20 h
12 h
14 h
12 h
13 h
7
H
H
H
H
1a
1a
1a
1a
4-Me
4-tBu
4-Cl
4-OMe
8
I
9
I
3g
3d
d
10
I
88
a
Reactions were carried out by using aryl halide (0.3 mmol, 1 equiv), boronic acid (0.36 mmol, 1.2 equiv), compound II (0.003 mmol, 1% equiv),
b
c
d
and Cs2CO3 (0.6 mmol, 2 equiv) in 2 mL of solvent under reflux. Isolated yield. Starting material was recycled. 92% NMR yield by using p-
toluidine as an internal reference.
D
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and 8). Boronic acid bearing phenyl-substituted Cl 2d and
OMe groups 2e also react well under standard conditions
(entries 9 and 10).
The mechanism for this reaction is assumed to be typical in
that the NiN2S2 is a surrogate for bidentate diphosphines,
Figure 6. We hypothesize that the needed Pd(0) species is
Catalyst Control Experiments. To further explore the role
of the NiN2S2 unit, several control reactions were carried out, as
shown in Table 6, for comparison to the standard reaction
Table 6. Control Reaction of Ni−Pd Complex Catalyzed
Suzuki−Miyaura Reaction
a
b
entry
cat.
loading
1 mol %
yield (%)
1
2
3
4
5
6
7
8
[Ni(bme-dach*)Pd(dppp)](PF6)2
Ni(bme-dach*)
[Pd2(Ni(bme-dach))4]4+
Pd(dppp)Cl2
>99
c
1 mol %
0
c
1 mol %
0
1 mol %
43
94
91
[Pd(dppp)2]Cl2
1 mol %
Pd(dppp)Cl2/Ni(bme-dach*)
Pd(dppp)Cl2/bme-dach
Pd(dppp)Cl2/bme-dach
1 mol % (each)
c
1 mol % (each)
d
0
c
0
a
Reactions were carried out by using iodobenzene (0.3 mmol, 1
equiv), phenylboronic acid (0.36 mmol, 1.2 equiv), Cs2CO3 (0.6
mmol, 2 equiv), and catalyst (see entries) in 2 mL of 1,4-dioxane
b
c
d
under reflux for 15 h. Isolated yield. Recycle of starting material. 1%
Pd(dppp)Cl2 and 10% bme-dach ligand (as dithiol) were used.
Figure 6. Presumed mechanism of discrete air-stable nickel−
palladium(II) complexes catalyzed Suzuki−Miyaura reactions.
conditions (entry 1). The use of 1 mol % Ni(bme-dach*) as a
potential catalyst in the reaction did not give any biaryl
compound 3a (entry 2). It is known that the NiN2S2 complexes
combine with Pd2+ to form thermodynamically stable
hexametallic [Pd2(NiN2S2)4]4+ complexes of a paddle-wheel-
type structure.31 This complex was independently prepared as
its chloride salt with Ni(bme-dach) “paddles”.31 This aggregate
was found to be inactive as a catalyst under the conditions
employed. Thus, we conclude that the NiN2S2 unit is
catalytically inactive in this reaction and becomes an active
bimetallic Ni−Pd catalyst only in the presence of a palladium-
diphosphine. On using 1 mol % Pd(dppp)Cl2 as catalyst in the
absence of nickel, a 43% yield of the 3a product was obtained;
the use of [Pd(dppp)2]Cl2 gave 3a in 94% yield. This result
suggests that the diphosphine ligand is required for the catalysis
by palladium, and we assume that an excess of the diphosphine
reduces Pd(II) to Pd(0).32,33 When a combination of 1 mol %
Pd(dppp)Cl2 and 1 mol % Ni(bme-dach*) was used as catalyst,
3a was isolated in 91% yield. A possible explanation is that the
NiN2S2 unit might serve as a reducing agent, affording the
[Pd(0)(dppp)] species as the “real” catalyst. This hypothesis
was somewhat supported by a MS analysis obtained on a
sample derived from a crude reaction mixture using 20% of II
as catalyst. While the formation of disulfide is possible, a
prominent signal of mass 32 greater than that for Ni(bme-
dach*) was observed and may be interpreted as a NiN2S2·O2
product (a bis-sulfoxide or sulfenate).28 Control reactions were
also carried out employing a combination of Pd(dppp)Cl2 with
1 mol % and 10 mol % of the free bme-dach ligand; neither
reaction gives the product 3a. This result suggests that the free
bme-dach ligand does not reduce the Pd(II); however it might
competitively bind to the Pd(II) center and deactivate the
catalyst.
generated on oxidation of the NiN2S2 unit; the release of thiol-
oxidized NiN2S2 could be mediated by a trace amount of water.
Such an in situ generated Pd(0)(dppp) is expected to proceed
with the oxidative addition of iodobenzene. In fact, the
expected intermediate, [PhPdI(dppp)], was indicated as a
pair of douplets (d, 11.2 ppm, J = 52.3 Hz) and (d, −9.3 ppm, J
= 52.3 Hz) in the time-resolved 31P NMR spectra recorded on
crude reaction mixtures in dioxane, using 20% catalyst II (see
the SI, Figure S7). For comparison the literature reported
values in d6-acetone are (d, 11.94 ppm, J = 53.6 Hz) and (d,
−9.05 ppm, J = 53.6 Hz).34 In accordance with the accepted
Suzuki−Miyaura reaction mechanism, transmetalation and
reductive elimination lead to the biaryl product and
regeneration of the Pd(0) species, thus completing the catalytic
cycle. There is no expectation, nor is there evidence, of the re-
formation of the Ni−Pd precatalyst.
In summary, we have developed a series of NiN2S2
metalloligand-coordinated Pd(II) complexes; their structures
were characterized by NMR spectroscopy and X-ray diffraction
analysis. Analogous to the well-known (dppp)2Pd2+, the Ni−Pd
complexes are precatalysts for Suzuki cross-coupling reactions
with benefits in terms of solubility, stability, and ease of
preparation. The reaction scope and the requirement of the
NiN2S2 metallodithiolates have been established; preliminary
results show that sulfoxygenation might account for the needed
reduction of Pd(II) to Pd(0). As the possibilities for modifying
the MN2S2 metallodithiolate ligands, both at M and in the
organic framework, are extensive,8 our study provides a
foundation for further exploration of this new class of ligands
in catalyst design and mechanism study.
E
DOI: 10.1021/acs.organomet.7b00176
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00176.
■
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1H, 13C, and 31P{1H} NMR spectra for all complexes and
other details of syntheses and extensive electrochemical
studies (PDF)
Crystallographic data for complexes I−IV and Ni(bme-
dach*) (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail (M. Y. Darensbourg): Marcetta@chem.tamu.edu.
ORCID
Marcetta Y. Darensbourg: 0000-0002-0070-2075
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
■
We are grateful for financial support from the National Science
Foundation (CHE-0910679 and CHE-1266097) and the R. A.
Welch Foundation (A-0924). We thank the National Science
Foundation for continued support of our supercomputing
facilities (CHE-0541587).
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DOI: 10.1021/acs.organomet.7b00176
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