Ranking Ligands by Their Ability to Ease (C6F5)2NiIIL → Ni0L + (C6F5)2Coupling versus Hydrolysis: Outstanding Activity of PEWO Ligands

Article abstract of DOI:10.1021/acs.inorgchem.0c02831

The NiII literature complex cis-[Ni(C6F5)2(THF)2] is a synthon of cis-Ni(C6F5)2 that allows us to establish a protocol to measure and compare the ligand effect on the NiII → Ni0 reductive elimination step (coupling), often critical in catalytic processes. Several ligands of different types were submitted to this Ni-meter comparison: Bipyridines, chelating diphosphines, monodentate phosphines, PR2(biaryl) phosphines, and PEWO ligands (phosphines with one potentially chelate electron-withdrawing olefin). Extremely different C6F5-C6F5 coupling rates, ranging from totally inactive (producing stable complexes at room temperature) to those inducing almost instantaneous coupling at 25 °C, were found for the different ligands tested. The PR2(biaryl) ligands, very efficient for coupling in Pd, are slow and inefficient in Ni, and the reason for this difference is examined. In contrast, PEWO type ligands are amazingly efficient and provide the lowest coupling barriers ever observed for NiII complexes; they yield up to 96% C6F5-C6F5 coupling in 5 min at 25 °C (the rest is C6F5H) and 100% coupling with no hydrolysis in 8 h at-22 to-53 °C.

Full text of DOI:10.1021/acs.inorgchem.0c02831

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Ranking Ligands by Their Ability to Ease (C6F5)2NiIIL → Ni0L + (C6F5)2
Coupling versus Hydrolysis: Outstanding Activity of PEWO Ligands
́
́
Jaime Ponce-de-Leon, Estefania Gioria, Jesus M. Martínez-Ilarduya, and Pablo Espinet*
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ABSTRACT: The NiII literature complex cis-[Ni(C6F5)2(THF)2] is a synthon
of cis-Ni(C6F5)2 that allows us to establish a protocol to measure and compare
the ligand effect on the NiII → Ni0 reductive elimination step (coupling), often
critical in catalytic processes. Several ligands of different types were submitted to
this Ni-meter comparison: bipyridines, chelating diphosphines, monodentate
phosphines, PR2(biaryl) phosphines, and PEWO ligands (phosphines with one
potentially chelate electron-withdrawing olefin). Extremely different C6F5−C6F5
coupling rates, ranging from totally inactive (producing stable complexes at room
temperature) to those inducing almost instantaneous coupling at 25 °C, were
found for the different ligands tested. The PR2(biaryl) ligands, very efficient for
coupling in Pd, are slow and inefficient in Ni, and the reason for this difference is
examined. In contrast, PEWO type ligands are amazingly efficient and provide
the lowest coupling barriers ever observed for NiII complexes; they yield up to
96% C6F5−C6F5 coupling in 5 min at 25 °C (the rest is C6F5H) and 100% coupling with no hydrolysis in 8 h at −22 to −53 °C.
elimination (coupling) step, which closes the cross-coupling
catalytic processes, in competence with detrimental hydrolysis.
In C−C Pd coupling, and presumably in Ni as well, the
reductive elimination gets more difficult in the order aryl−aryl
< aryl−alkyl < alkyl−alkyl < C6F5−C6F5 < aryl−CF3 ≪ CF3−
CF3.8 In the early 1970s, seminal studies of A. Yamamoto’s
group had already found that butane formation from
[NiII(Et)2(bipy)] was induced by addition of electron-attractor
moieties such as electron-withdrawing olefins (EWOs),9
aromatic compounds, and phosphines.10 They proposed that
coordination of an EWO molecule formed a 5-coordinate NiII
complex and reduced the Et−Et coupling barrier from 68 kcal
× mol−1 in [NiII(Et)2(bipy)] to 20 kcal × mol−1 in
[NiII(Et)2(bipy)(EWO)]. For perfluoroaryl groups, the cou-
pling barriers turned inaccessible, and EWO addition failed to
promote coupling in [NiII(C6F5)2(bipy)] at any temperature.11
In this line, the group of Doyle has reported an electron-
deficient olefin Fro-DO (with E-R−C(O)−CHCH−C(O)−
R structure) that enables for efficient nickel-catalyzed cross-
coupling of 1,1-disubstituted aziridines with organozinc
reagents to generate quaternary centers at room temperature.12
INTRODUCTION
■
The interest in Ni-catalyzed cross-coupling reactions has
boomed in the last two decades.1 Compared to the deeply
studied Pd processes, the available information to select
appropriate ligands for Ni-catalyzed C−C couplings is still
scarce. For identical structures of group 10 metals, the
activation energy of C−C reductive elimination follows the
trend Ni < Pd < Pt.2 It looks, however, that often the
efficiencies of identical ligands do not run parallel for Pd and
Ni. For instance, bulky PR3 ligands and Buchwald type biaryl
phosphines are very efficient in Pd-catalyzed processes, but
there are few examples of their successful use in Ni catalysis.3,4
In general, ligand extrapolation from Pd to Ni is likely to fail
due to different factors. One reason analyzed by Doyle is that
due to the smaller radius of Ni the cone angle θ, proposed long
ago by Tolman,5 for the same ancillary ligand is smaller on Pd
than that on Ni.6 This reduces the volume available to
accommodate the reacting groups coordinated to Ni.7 If the
groups defining the cone angle are remote from the small Ni
center, then some percentage of the cone angle in the spatial
neighborhood of the Ni atom is not buried, and it is still
available to be accessed when required by the transformations
in a catalytic cycle. In fact, ligands designed with large θ but
low %Vbur make operative some Ni catalyzed couplings that fail
with ligands such as PtBu3 or JohnPhos.7
Received: September 24, 2020
Published: December 8, 2020
The nonburied volume requirement is a necessary but not
sufficient condition throughout a catalytic process. Addition-
ally, for a catalytic cycle to work it has to have accessible
activation barriers at each catalytic step. Our specific interest
here is how to get information on the rate of reductive
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There are not many computational studies so far. One
comparing the effects of 42 diphosphines on the reductive
elimination barrier of Ph−CF3 from [Ni(CF3)Ph(P−P)] is
available, but access to parallel experimental data was limited
to two cases because of synthetic problems.13 There is also a
theoretical study showing that dimethyl fumarate facilitates the
nickel-catalyzed conjunctive cross-coupling of alkenyl amides
with aryl iodides and aryl boronic esters because it lowers the
aryl-alkyl coupling activation energy from ΔG⧧ = 14.6 kcal ×
mol−1 with ethylene to only ΔG⧧ = 2.6 kcal × mol−1 with
dimethyl fumarate.14 Much earlier, using Me−Me coupling as a
model, our studies on Pd had confirmed experimentally and
explained theoretically the dramatic facilitation of coupling by
coordination of EWOs.15
product (C6F5−C6F5) and the hydrolysis byproduct
(C6F5H),16 presumably due to easy THF dissociation to the
three-coordinate [NiII(C6F5)2(THF)] that undergoes reduc-
tive elimination more quickly than the four-coordinate 1 and
competitive substitution to [NiII(C6F5)2(THF)(OH2)] fol-
lowed by hydrolysis (Scheme 1).24
Scheme 1. Coupling versus Hydrolysis from the NiII-Meter
More recently, we proposed the use of complex cis-
[Pd(C6F5)2(THF)2] as a PdII-meter to rank experimentally
the different ligands according to their ability to facilitate the
difficult C6F5−C6F5 coupling. The procedure consists of
measuring the activation energy for C6F5−C6F5 formation
(ΔG⧧(C6F5−C6F5)Pd) upon addition of the ligand being tested
to cis-[Pd(C6F5)2(THF)2].16 Although the C6F5−C6F5 cou-
pling barrier from Pd is difficult,17 the percentages of
competing hydrolysis are moderate in Pd, which is not
oxophilic. The work here aims at extending this idea to a NiII-
meter that might provide information to compare the coupling
barriers in NiII (ΔG⧧(C6F5−C6F5)Ni) with different ligands in
similar conditions. We expect hydrolysis to be more
competitive with coupling in Ni than it was in Pd, which
may prevent to quantify this coupling barrier, except for the
most active ligands, yet valuable semiquantitative information
on the ligand ability of coupling should be obtained for all the
cases.
The protocol for the coupling/hydrolysis measurements
shown in eq 2
(where each L stands for one potentially bidentate ligand or for
two monodentate ligands) is as follows: The reactions are
monitored by 19F NMR. The addition of L is made at lower
temperature, and the NMR tube is brought to the coupling
temperature once the ligand coordination has reached
equilibrium. Excess ligand (L/Ni molar ratio: 2:1 for chelating
ligands, 4:1 for monodentate ligands) is used to stabilize the
reduction product as [Ni0Ln] (n = 2−4 depending on L), in
order to prevent initially formed Ni0 from sequestering a
portion of the L needed for completion of Ni0 complex. We
confirmed for L chelates that using L/Ni = 1:1 or 2:1 does not
change the initial coupling rate. The CH2Cl2 solvent in all the
experiments reported was as supplied by our solvent-
purification system (SPS) PS-MD-5. In the experimental
reagent concentrations used (limited by its solubility), the 1/
H2O molar ratio was approximately 1:0.7. In toluene, used
occasionally, it was approximately 1:0.3 (see the Supporting
Information).
Ni-meter 1 itself undergoes coupling and hydrolysis
(Scheme 1) and can be taken as reference. It reveals
immediately much higher hydrolysis:coupling ratio (53:47
mol %) at 25 °C than observed for the equivalent Pd-meter.16
In general, higher participation of hydrolysis is found in Ni
compared to Pd for all the quested ligands, consistent with the
more favorable coordination of water and its higher acidity on
the harder NiII center than on the softer PdII. In the presence
of D2O the hydrolysis product was enriched in C6F5−D, as
expected (see the Supporting Information for details).
RESULTS AND DISCUSSION
■
The NiII-meter complex should be a fairly stable synthon of cis-
[NiII(C6F5)2] that is easy to prepare and handle, where
addition of the examined ligand at low temperature should lead
to fast coordination before any significant coupling occurs, in
order to avoid deceptive interferences when measuring
coupling and hydrolysis rates. After a few trials with more
stable complexes that did not facilitate fast ligand substitution
( c i s - [ N i I I ( C 6 F 5 ) 2 ( N C P h ) 2 ] 1 8 o r t r a n s -
[NiII(C6F5)2(SbPh3)2]),19 we found cis-[Ni(C6F5)2(THF)2]
(1) as the one fulfilling reasonably our requirements. On the
basis of the two reported syntheses of 1,20 we have developed a
more direct procedure: Commercially available [NiX2(DME)]
(X = Br and Cl; DME = 1,2-dimethoxyethane) reacts with
Ag(C6F5)21 in THF (1 h at −40 °C) to produce 1:
The ligands tested are grouped into five types (Scheme 2):
(a) bipyridines; (b) chelating diphosphines; (c) monodentate
phosphines; (d) dialkylbiaryl (Buchwald type) phosphines;
and (e) PEWO type ligands (phosphine-EWO ligands). Upon
addition of these ligands to 1, results spanning from formation
of totally inert complexes to instantaneous C6F5−C6F5
coupling, as well as diverse C6F5−C6F5:C6F5H ratios, were
observed at 25 °C. X-ray diffraction structures were solved
when the stable complexes allowed for crystallization.
The bipy ligands (2 and 3) immediately form the X-ray
characterized [NiII(C6F5)2L] chelated complexes (18 and 19)
(Figure 1, above). Symmetrical diphosphine ligands 4 and 5
also lead to stable [NiII(C6F5)2L] chelated complexes 20 and
Evaporation, extraction with Et2O, and filtration to remove the
insoluble silver salts, followed by evaporation to dryness affords
1 as an orange solid with >97% purity.22
Complex 1 is stable for several months in the fridge under
inert atmosphere, but it is somewhat sensitive to atmospheric
oxygen and water at room temperature.23 Solutions of 1 in
freshly distilled dry THF are stable enough for comfortable
quick handling at room temperature and can be stored in the
freezer for a few hours, although decomposition (turbidity) is
eventually observed. In noncoordinating solvents, complex 1
decomposes noticeably quickly to a mixture of the coupling
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Scheme 2. Ligand Types Tested with the NiII-Meter
Figure 2. X-ray diffraction structure of trans-[Ni(C6F5)2(XantPhos)]·
toluene (22·toluene). Solvent molecule and H atoms omitted for
clarity.
(C6F5)2(Xantphos)] (23) was initially formed (Scheme 3)
and then isomerized to the thermodynamically favored trans-
Scheme 3. Reaction Sequence Observed at −40 °C in the
Formation of trans-[Ni(C6F5)2(XantPhos)] (22)
[Ni(C6F5)2(Xantphos)] (22). This supports that isomerization
of the initially formed cis complex at 25 °C is considerably
faster than coupling at the same temperature. The cis- and
trans-chelation ability of XantPhos is well-known,25 but only
trans coordination of XantPhos to NiII had been reported so
far.26,27
In contrast to the stability of the precedent complexes,
coupling (C6F5−C6F5) and hydrolysis (C6F5H) was observed
at 25 °C for all the other ligands in Scheme 2. These two
evolutions are comparatively slow for most ligands of groups
(c) and (d) but extremely fast for ligands of group (e). Table 1
summarizes all the results, including the following: (i) the
complexes formed in solution by reaction of 1 with L, when
they are stable, or NMR data on the complex formed in
solution, before coupling/hydrolysis occurs (column 2); (ii)
the conversion produced in the time specified; and (iii) the
C6F5−C6F5/C6F5H ratio formed, as percentage of products.
Some specific reactions with addition of p-benzoquinone
(bzq), a strongly electron-deficient olefin (EDO) with a strong
electron-withdrawing effect upon coordination (EWO) are also
included, as specified in the first column of Table 1.15
For complex 1, the reactions in Scheme 1 are sensitive to 2:1
addition of bzq (entry 2 versus 1). Presumably the coupling
occurs on NiII species with at least one coordinated bzq in
equilibrium with free bzq, which accelerates the coupling
reaction and noticeably improves the coupling/hydrolysis
ratio. Consistently, increasing the bzq concentration (entry 3
in Table 1) has a larger positive effect on the percentage of
substitution and on the coupling rate, as well as on the
coupling/hydrolysis ratio. Obviously, neither THF nor bzq are
hoped to maintain the Ni center active throughout a catalytic
cycle, but these initial entries of Table 1 are interesting to
illustrate the potential positive coupling effect of an EWO
ligand at the coupling step when facing difficult couplings.
Monodentate phosphines with different molecular sizes
totally replace THF. The small PPh3 (7) and PCy3 (8) form
stable trans-[Ni(C6F5)2(PR3)2] complexes 24 and 25 (entries
Figure 1. Top left: [Ni(C6F5)2(CO2Et-bipy)] (18). Top right:
[Ni(C6F5)2(tBu-bipy)] (19). Bottom left: [Ni(C6F5)2(dppe)]·1/2
CH2Cl2 (20·1/2 CH2Cl). Bottom right: [Ni(C6F5)2(dppf)]·n-
pentane (21·n-pentane). All solvent molecules and H atoms omitted
for clarity.
21, respectively, which were isolated and X-ray characterized
(Figure 1, below). As reported by Yamamoto for the
[NiII(C6F5)2(2,2′-bipy)] complexes,11 coupling on 18−21
does not occur at 25 °C. Moreover, all these complexes are
stable not only at 25 °C in CH2Cl2 but also at 80 °C in
dioxane.
The reaction of 1 with Xantphos in CH2Cl2 at 25 °C
produced trans-chelate complex 22. Its X-ray diffraction
structure confirmed the expectations from the NMR studies,
and it is shown in Figure 2. Monitoring of the reaction of 1
with Xantphos at −40 °C showed that cis-[Ni-
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Complex 26 undergoes fast evolution at 25 °C, predom-
inantly to C6F5H. The reaction was complete in less than 2 h,
which precluded obtaining single crystals of the complex in
solution. Since two nonequivalent C6F5 groups are observed in
the 19F NMR spectrum, it is not excluded that complex 26 in
THF solution could be [Ni(C6F5)2(PtBu3)(THF)]. The
efficient hydrolysis shows that complex 26 allows for water
coordination and subsequent hydrolysis. This result discour-
ages using PtBu3 for difficult couplings on Ni, in contrast with
its successful utilization in Pd-catalyzed processes.
Table 1. Conversion (%) of 1 (Equation 2) and Coupling/
Hydrolysis Ratio with Ligands in Scheme 2
a
compounds in
solution and NMR
signals
(C6F5)2/
C6F5H
entry/L
1. THF
t
conv. %
(mol %)
2 h
4 h
85
100
49:51
47:53
complex 1
equilibrium mixture
(text)
equilibrium mixture
(text)
complex 24 +
2 free L
complex 25 +
2 free L
complex 26 +
3 free L
b
c
2. bzq
3. bzq
4 h
100
100
66:34
89:11
1.5 h
24 h
24 h
2 h
Biaryl phosphines 10−12 (group (d)) are very efficient in
Pd-catalyzed processes, but there are few examples of good
performance in Ni catalysis.3,4 In the reaction with the Pd-
meter, they were very fast for coupling, or at least very efficient
to hinder undesired hydrolysis.16 With the Ni-meter, however,
they are very inefficient (entries 7−12, Table 1): It took 6−9.5
h at 25 °C to reach 47−77% conversion in CH2Cl2, showing
slower coupling than hydrolysis with only 20−24% of coupling
product (entry 8). The reaction in toluene instead of
dichloromethane (entry 9) was not any faster, but it produced
a better (still bad) coupling/hydrolysis ratio of 42:58 in 6 h
and 59:41 in 9.5 h, possibly due to the lower water content in
the toluene. The late improvement of this ratio possibly occurs
when the initial H2O content in toluene has been practically
exhausted. Thus, in sharp contrast with their good behavior on
PdII, these PR2(biaryl) phosphines not only are quite
inefficient for C6F5−C6F5 coupling on Ni but are also unable
to protect the NiII center and prevent hydrolysis. The slowness
of the coupling reactions and the double dependence of water
and L prevented to measure (ΔG⧧(C6F5−C6F5)Ni) for these
ligands, although some qualitative information can be surmised
from the conversion time and products time evolution. The
possible cooperative effect of addition of p-benzoquinone
(L:bzq = 1:1) was checked with ligand 11, hoping for a higher
rate of the coupling reaction and better protection against
hydrolysis, but no improvement was observed (entry 10).
Recently reported fluorinated biaryl ligand 13,28 where the
fluorinated aryl might somehow resemble an EWO (see later),
was also tested. Its reactivity was slow and produced somewhat
better but still very unsatisfactory coupling/hydrolysis ratio
(entries 12 and 13).
d
4. 7
d
5. 8
df
,
6. 9
100
78
11:89
20:80
cis-1:1 complex, 2
nonequiv C6F5, 1 L
b
7. 10
6 h
cis-1:1 complex, 2
nonequiv. C6F5,
1 free L
6 h
47
77
29:71
24:76
b
8. 11
9. 11
9.5 h
6 h
9.5 h
45
68
42:58
59:41
cis-1:1 complex, 2
nonequiv C6F5, 1 L
f
cis-1:1 complex, 2
b
10. 11 + bzq
18 h
76
83
20:80
23:77
40:60
43:57
95:5
nonequiv C6F5, 1 L
cis-1:1 complex, 2
nonequiv C6F5, 1 L
cis-1:1 + trans-1:2
complexes + free L
cis-1:1 + trans-1:2
complexes + 1
complex 27 +
1 free L
complex 28 +
1 free L
complex 29 +
1 free L
complex 30 +
1 free L
b
11. 12
6 h
b
12. 13
28 h
100
100
100
100
100
e
13. 13
28 h
b
14. 14
<5 min
<5 min
<5 min
<5 min
b
15. 15
82:18
89:11
96:4
b
16. 16
b
17. 17
100
a
b
c
In CH2Cl2; T = 25 °C. L/Ni molar ratio = 2:1. L/Ni molar ratio =
d
e
f
20:1. L/Ni molar ratio = 4:1. L/Ni molar ratio = 1:1. In toluene.
4 and 5, Table 1). The trans arrangement of the two C6F5
groups prevents reductive elimination. Figure 3 shows the
Finally, concerning PEWO ligands 14−17 of group (e) in
Scheme 2, they produce complexes 27−30 in CD2Cl2 (entries
14−17). Single crystals for X-ray diffraction of these complexes
could not be obtained due to very fast conversion to C6F5−
C6F5, but their P−olefin chelate coordination in solution is
unambiguously confirmed by observation of four non-
equivalent Fortho signals in their 19F NMR spectra.29 In a
square planar NiII complex the P−olefin chelate coordination
leads to two nonequivalent C6F5 groups. The Ni coordination
plane is not a symmetry plane, and the restriction to rotation of
the C6F5 groups produces nonequivalence of their two Fortho
atoms affording four Fortho signals. The upfield shifts of the
olefinic protons (e.g., δ = 7.32 and 5.97 ppm in 29 compared
to δ = 8.35 and 7.17 ppm in free PhPEWO-H phosphine 16)30
further support the coordination of the CC group and the
proposed geometry. PEWO ligands 14−17 show an excep-
tional power to induce C6F5−C6F5 coupling due to the effect
of coordination of the EWO olefin group.15 All of them beat
any other ligand in this respect: Total conversion of complex at
25 °C occurs in a scale of seconds, rather than hours!
Furthermore, the conversion at this temperature affords very
predominantly coupling product (82−96%). To the best of our
Figure 3. X-ray diffraction structure of trans-[Ni(C6F5)2(PCy3)2]
(25). H atoms omitted for clarity.
structure of trans-[Ni(C6F5)2(PCy3)2]. In contrast, the NMR
spectra show that only one molecule of PtBu3 (9) can
coordinate to the NiII center, producing the reactive cis-
[Ni(C6F5)2(PtBu3)] (26), and leaving the other three PtBu3
uncoordinated (entry 6).
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knowledge, these coupling rates are the fastest ever observed in
NiII → Ni0 reductive elimination processes.31 In fact, they are
too fast to be measured by NMR at 25 °C by applying the
initial rates method.
Consistent with the overall evidence and these computa-
tional results, it is reasonable to propose the structures in
solution depicted in Figure 4. For Pd, the structure in CH2Cl2
The dramatically different behavior of PR2(biaryl) and
PEWO ligands in the Ni-meter, in contrast to their similar
behavior in the Pd-meter, must probably have a structural
origin. In Pd, both types of ligand behave as chelating: For
PEWO ligands several X-ray diffraction PdII and Pd0 structures
of chelated complexes with E- or Z-coordinated olefin have
been reported,32 and many PdII and Pd0 complexes with
PR2(biaryl) ligands show Pd−Cipso chelating interactions with
the distal aryl (Pd−Cipso distances in the range of 2.19−2.60
Å).33 These interactions help to stabilize [Pd(aryl)XL] or
[Pd(aryl)R′L] intermediates in the catalytic cycles and support
their chelating coordination along the C−C coupling process
for both kinds of ligand.15
Figure 4. Proposed structures in CH2Cl2 solution for
[Pd(C6F5)2{PR2(biaryl)}] (A), [Ni(C6F5)2{PR2(biaryl)}(THF)]
(B), and [Ni(C6F5)2(R-PEWO)] (C).
solution of the complex with JohnPhos must be A, as observed
in the solid state by X-ray diffraction studies. For Ni and the
cis-(C6F5)2 structures in entries 10−17 (Table 2), the distal
aryl would be unable to chelate Ni in competence with the
smaller and harder THF (or eventually water), and structure B
is preferred, even in the presence of only low concentration of
THF or water. The lack of coordination of the distal aryl is
very detrimental for coupling, which becomes slow and allows
for faster hydrolysis. Finally, the chelate coordination of
PEWO (Table 2, entries 14−17) affords structure C for the
complexes in solution.
It is worth noting that the coupling power of the PEWO
ligands is much higher in Ni than in Pd, to the point that the
quantitative kinetic studies required the use of very low
temperatures (−22 to −53 °C, instead of 0 °C in Pd). This is
due to the gain in stability of the EWO olefin as the coupling
evolution starts, which is higher for a hard NiII → soft Ni0
process than for a soft PdII → soft Pd0 coupling. The measured
ΔG⧧(C6F5−C6F5)Ni barriers at the corresponding experimental
temperature used in each case (Table 3, column 2), are
There is no similar structural X-ray information available for
Ni complexes with these ligands, and we also have failed to
obtain single crystals in this work. The NMR spectra for these
complexes in CH2Cl2, in Ni/L = 1:2 solutions, always show
one free L and one coordinated L (entries 7−17). For the
PEWO complexes (entries 14−17) coordination of the olefin
group is clearly seen in the 1H NMR spectra, but the
PR2(biaryl) complexes with ligands 10−12 (entries 7−11)
1
show ill-defined broad H spectra perhaps associated to slow
conformational changes. The fact is that their chemical
behavior is very similar to that of PtBu3: formation of Ni/P
= 1:1 complexes in solution, NMR observation of 2 chemically
nonequivalent C6F5 groups, slow conversion, and much less
coupling than hydrolysis. This suggests that the PR2(biaryl) Ni
complexes are behaving in cis-[Ni(C6F5)2{PR2(biaryl)}] or cis-
[Ni(C6F5)2{PR2(biaryl)}(THF)] as bulky monodentate li-
gands, allowing for easy coordination of water and hydrolysis.
As an exception, fluorinated biaryl phosphine 13 in solution
forms a mixture of cis-[Ni(C6F5)2{PR2(biaryl)}(THF)] and
some trans-[Ni(C6F5)2{PR2(biaryl)}2] (entries 12 and 13), the
latter with two P-coordinated phosphines.28
In the lack of access to other experimental information, we
performed DFT calculations on the stabilization of the
potential Pd and Ni complexes formed by reaction of one
molecule of the ligand JohnPhos (10) to complex [M-
(C6F5)2(THF)2] (M = Pd and Ni), taking as the starting
complex in each case zero energy. The results in Table 2 show
Table 3. Experimental ΔG⧧(C6F5−C6F5)Ni (kcal × mol−1)
a
for Reductive Elimination of cis-[NiII(C6F5)2(THF)2] (1)
L
T (°C)
(C6F5)2/C6F5H (%)
ΔG⧧ at T
ΔG⧧ at 0 °C
THF
14
15
16
17
10
−53
−52
−22
−36
61:39
100:0
100:0
100:0
100:0
21.9
16.7
17.0
19.5
18.1
17.7
18.1
19.9
18.8
a
Promoted by PEWO ligands in Scheme 2, at the indicated
temperature.
Table 2. DFT Calculations for the Thermodynamic Effect of
Displacing 1 or 2 THF upon Addition of Ligand 10
a
compound
ΔΔG0
compound
ΔΔG0
collected in Table 3, column 4. Additionally, we determined
ΔH⧧(C6F5−C6F5)Ni and ΔS⧧(C6F5−C6F5)Ni for ligand 16 in
an experimental variable-temperature study. Assuming that the
ΔS⧧(C6F5−C6F5)Ni contribution is unlikely to change much
from one PEWO ligand to another, we could work out a
unified comparative scale at 0 °C (Table 3, column 5).
Comparing the data in Table 3 with those at 25 °C in Table
1, it is clear that lower temperatures increase the conversion
times but favor higher C6F5−C6F5:C6F5H ratios. The reference
complex 1 already shows this cooling effect, and lowering the
work temperature from 25 to 10 °C improves this ratio from
47:53 (in 4 h) in Table 1 to 61:39 (in 6 h) in Table 3. For the
PEWO ligands their reactions, carried out at temperatures
below −22 °C, are complete in about 8 h and do not show any
sign of hydrolysis.
[Pd(C6F5)2(THF)2]
[Pd(C6F5)2(THF)(L)]
[Pd(C6F5)2(L)]
In CH2Cl2 solution. L = JohnPhos. ΔΔG0 in kcal × mol−1.
0.0
−11.8
−6.6
[Ni(C6F5)2(THF)2]
[Ni(C6F5)2(THF)(L)]
[Ni(C6F5)2(L)]
0.0
−6.7
4.5
a
that in dichloromethane the replacement of one or the two
THF ligands in Pd produces more stable complexes,
supporting plausibility of chelation along the coupling
evolution. On the contrary, for Ni in dichloromethane the
calculations show that displacement of one THF is clearly
favorable, but displacement of the second is very disfavored,
supporting a monodentate coordination of JohnPhos, like
PtBu3, along the process.
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Article
The temperature-unified column in Table 3 shows clearly
that the more electron deficient the olefin group, the faster the
couplings: (i) PEWO-F ligands (14 and 15) are faster than
PEWO-H ligands (16 and 17). (ii) PEWO-F ligand 14 is faster
than its PEWO-H homologue 16. (iii) PEWO-H ligand 17
(with two CO2Me substituents) is faster than PEWO-H ligand
16 (with only one CO2Ph group). However, contrary to Pd,
for Ni PhPEWO-F (14) is faster than o-TolPEWO-F (15),
suggesting that bulkier R groups on phosphorus can be
beneficial in Pd but are detrimental in Ni. Although this fits
well the steric expectations, it should be taken with caution
until more cases are available for comparison. Figure 5
summarizes graphically, for representative ligands, the most
significant experimental results of this study.
The coupling results of Ni and Pd with PEWO versus
PR2(biaryl) ligands show the reasons behind the scarce success
in Ni catalysis of some structurally sophisticated ligands
efficient in Pd catalysis. For structurally simple ligands, ΔG⧧
trends in group 10 metals run often parallel to ΔH⧧ trends
because they give rise to similar structures along the reaction
pathway.2 However, this structural match becomes particularly
unlikely when combining metals of unequal radii and volumes,
such as Ni and Pd, with structurally sophisticated ligands
tailored for one of them. This is the case for the direct (one-
step) coupling process studied here and the PR2(biaryl)
ligands. The complexes of Ni and Pd with the flexible and
adaptable PEWO ligands have identical ground-state struc-
tures, and Ni shows, as expected from the enthalpy trends,
lower ΔG⧧(C6F5−C6F5)M activation energy. In contrast, the
PR2(biaryl) complexes of Ni and Pd have different ground-
state structures, and Ni happens to show (against the enthalpy
trend in this case) noticeably higher ΔG⧧(C6F5−C6F5)M
activation energy than Pd for reductive elimination.
Last but not least, the family of PEWO ligands is
impressively efficient in Ni, inducing hydrolysis protection
and fast coupling even at very low temperatures. The coupling
that was reported impossible from [NiII(C6F5)2(bipy)] at any
temperature10 occurs from [NiII(C6F5)2(PEWO)] complexes
in only 8 h at −50 °C, or in seconds at room temperature!!
Investigation of the application of PEWO ligands in Ni
catalysis is ongoing.
EXPERIMENTAL SECTION
Experimental details are given in the Supporting Information.
■
Figure 5. L and T dependence of coupling versus hydrolysis.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications Web site. The Supporting Information is
available free of charge at https://pubs.acs.org/doi/10.1021/
acs.inorgchem.0c02831.
CONCLUSIONS
■
In conclusion, we have established a protocol to evaluate
qualitatively, and quantitatively when possible, ligand effects on
the NiII → Ni0 reductive elimination rate and the competitive
hydrolysis, using cis-[Ni(C6F5)2(THF)2] (1) as Ni-meter at
room or lower temperatures. Experimental determination of
ΔG⧧(C6F5−C6F5)Ni can be achieved easily for the more
efficient ligands. An additional bonus is that NMR monitoring
of the process can provide plausible clues to understand the
unsatisfactory coupling performance of PR2(biaryl) ligands in
Ni.
General methods, synthesis, experimental coupling/
hydrolysis studies, kinetic and variable-temperature
NMR studies, computational methods and results, X-
ray crystallographic data, NMR spectra, additional
references (PDF)
Optimized gas phase structures of the Ni−Pd species in
Table 2 (XYZ)
Competitive hydrolysis on the hard NiII center can be a
serious problem for survival of the NiII catalysts, unless strict
dryness conditions are used or the high activity of the
nucleophile (e.g., LiR, ZnR2, etc.) guarantees solvent dryness.
The hydrolysis/coupling ratio found in this work using SPS
quality solvents shows that water contents acceptable for Pd
can be unacceptable for Ni. In the case of PR2(biaryl) ligands,
this undesired hydrolysis competence is due in part to the
small nonburied volume in the cis-Ni(C6F5)2 fragment, which
prevents their protecting chelate coordination but allows
coordination of small O-donor ligands such as water with
subsequent fast hydrolysis. In addition to the slowness of
coupling, this very negatively affects the performance of
PR2(biaryl) phosphines in Ni. The different entropy depend-
ence of the two processes (hydrolysis must be at least
bimolecular) helps to reduce the percentage of hydrolysis at
low temperatures.
Accession Codes
CCDC 1907218, 1907224, 1907226, 1907227, 1907229, and
1963893 contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
Pablo Espinet − IU CINQUIMA/Química Inorgánica,
Facultad de Ciencias, Universidad de Valladolid, 47071
Valladolid, Spain; orcid.org/0000-0001-8649-239X;
Email: espinet@qi.uva.es
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Inorganic Chemistry
Authors
Jaime Ponce-de-Leon − IU CINQUIMA/Química
Inorgánica, Facultad de Ciencias, Universidad de Valladolid,
47071 Valladolid, Spain; orcid.org/0000-0001-7464-
2469
Estefania Gioria − IU CINQUIMA/Química Inorgánica,
Facultad de Ciencias, Universidad de Valladolid, 47071
Valladolid, Spain; orcid.org/0000-0002-3258-7390
pubs.acs.org/IC
Article
108, 6115−6128. (d) Ananikov, V. P.; Musaev, D. G.; Morokuma, K.
Organometallics 2005, 24, 715−723. (e) Zuidema, E.; Van Leeuwen,
P. W. N. M.; Bo, C. Organometallics 2005, 24, 3703−3710.
(f) Yandulov, D. V.; Tran, N. T. J. Am. Chem. Soc. 2007, 129,
1342−1358. (g) Ananikov, V. P.; Musaev, D. G.; Morokuma, K. Eur.
J. Inorg. Chem. 2007, 2007, 5390−5399.
(9) EWO or EDO applies to the same kind of olefin, electron-
deficient (EDO) as free, or electron-withdrawing (EWO) as metal-
coordinated.
(10) (a) Yamamoto, T.; Yamamoto, A.; Ikeda, S. Organo (dipyridyl)
nickel complexes. I. Stability and activation of the alkyl-nickel bonds
of dialkyl (dipyridyl) nickel by coordination with various substituted
olefins. J. Am. Chem. Soc. 1971, 93, 3350−3359. (b) Yamamoto, T.;
Yamamoto, A.; Ikeda, S. Organo (dipyridyl) nickel complexes. II.
Stabilities of olefin-nickel bonds in olefin-cooridinated dipyridylnickel
and dialkyl (dipyridyl) nickel complexes. J. Am. Chem. Soc. 1971, 93,
3360−3364. (c) Yamamoto, T.; Abla, M. Reductive elimination of
EtEt from NiEt2(bpy) promoted by electron-accepting aromatic
compounds. J. Organomet. Chem. 1997, 535, 209−211. (d) Komiya,
S.; Abe, Y.; Yamamoto, A.; Yamamoto, T. Phosphine-induced
reductive elimination from cis-arylmethylnickel(II) complexes having
a 1,2-bis(dimethylphosphino)ethane ligand. Organometallics 1983, 2,
1466−1468.
(11) Yamamoto, T.; Abla, M.; Murakami, Y. Promotion of Reductive
Elimination Reaction of Diorgano(2,2′-bipyridyl)nickel(II) Com-
plexes by Electron-Accepting Aromatic Compounds, Lewis Acids,
and Brønsted Acids. Bull. Chem. Soc. Jpn. 2002, 75, 1997−2009.
(12) Huang, C.-Y.; Doyle, A. G. Electron-Deficient Olefin Ligands
Enable Generation of Quaternary Carbons by Ni-Catalyzed Cross-
Coupling. J. Am. Chem. Soc. 2015, 137, 5638−5641. and references
therein.
́
́
Jesus M. Martínez-Ilarduya − IU CINQUIMA/Química
Inorgánica, Facultad de Ciencias, Universidad de Valladolid,
47071 Valladolid, Spain
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c02831
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors thank the Spanish MINECO (projects CTQ2017-
89217-P and CTQ2016-80913-P) and the Junta de Castilla y
■
́
Leon (projects VA051P17 and VA062G18) for financial
support. J.P.-de-L. gratefully acknowledges the Spanish
MECD for a FPI scholarship (BES-2017-080726).
REFERENCES
■
(1) (a) Tasker, S. Z.; Standley, E. A.; Jamison, T. F. Recent advances
in homogeneous nickel catalysis. Nature 2014, 509, 299−309.
(b) Han, F.-S. Transition-metal-catalyzed Suzuki−Miyaura cross-
coupling reactions: a remarkable advance from palladium to nickel
catalysts. Chem. Soc. Rev. 2013, 42, 5270−5298. (c) Correa, A.
Editorial. Top. Curr. Chem. 2016, 374, 71.
(13) Jover, J.; Miloserdov, F. M.; Benet-Buchholz, J.; Grushin, V. V.;
Maseras, F. On the Feasibility of Nickel-Catalyzed Trifluoromethy-
lation of Aryl Halides. Organometallics 2014, 33, 6531−6543.
(14) Derosa, J.; Kleinmans, R.; Tran, V. T.; Karunananda, M. K.;
Wisniewski, S. R.; Eastgate, M. D.; Engle, K. M. Nickel-Catalyzed 1,2-
Diarylation of Simple Alkenyl Amides. J. Am. Chem. Soc. 2018, 140,
17878−17883.
(2) Ananikov, V.; Nickel, P. The “Spirited Horse” of Transition
Metal Catalysis. ACS Catal. 2015, 5, 1964−1971.
(3) Ni-catalyzed Suzuki−Miyaura cross-coupling of tertiary benzylic
and allylic sulfones works well with (ArBO)3 but requires BrettPhos
(12%) and Ni(COD)2 (10%) in toluene at 85 °C for 16 h: Ariki, Z.
T.; Maekawa, Y.; Nambo, M.; Crudden, C. M. Preparation of
Quaternary Centers via Nickel-Catalyzed Suzuki−Miyaura Cross-
Coupling of Tertiary Sulfones. J. Am. Chem. Soc. 2018, 140, 78−81.
(4) The coupling of aryl styrene oxides with various boronic acids
requires stringent conditions (toluene at 100 °C, 4 h, 10% Ni(COD)2,
20% BrettPhos) to afford yields in the range of 50−77%: Nielsen, D.
K.; Doyle, A. G. Nickel-Catalyzed Cross-Coupling of Styrenyl
Epoxides with Boronic Acids. Angew. Chem., Int. Ed. 2011, 50,
6056−6059.
́
(15) Perez-Rodríguez, M.; Braga, A. A. C.; García-Melchor, M.;
́
Perez-Temprano, M. H.; Casares, J. A.; Ujaque, G.; de Lera, A. R.;
́
Alvarez, R.; Maseras, F.; Espinet, P. C−C Reductive Elimination in
Palladium Complexes, and the Role of Coupling Additives. A DFT
Study Supported by Experiment. J. Am. Chem. Soc. 2009, 131, 3650−
3657.
(16) Gioria, E.; del Pozo, J.; Martínez-Ilarduya, J. M.; Espinet, P.
Promoting Difficult Carbon-Carbon Couplings: Which Ligand Does
Best? Angew. Chem., Int. Ed. 2016, 55, 13276−13280.
(17) This coupling from Pd has been also dealt with latterly in a
couple of contributions: (a) Gu, L.; Wolf, L. M.; Thiel, W.; Lehmann,
C. W.; Alcarazo, M. Reductive Elimination of C6F5−C6F5 from Pd(II)
Complexes: Influence of α-Dicationic Chelating Phosphines. Organo-
metallics 2018, 37, 665−672. (b) Budiman, Y. P.; Jayaraman, A.;
Friedrich, A.; Kerner, F.; Radius, U.; Marder, T. B. Palladium-
Catalyzed Homocoupling of Highly Fluorinated Arylboronates:
Studies of the Influence of Strongly vs. Weakly Coordinating Solvents
on the Reductive Elimination Process. J. Am. Chem. Soc. 2020, 142,
6036−6050.
(5) (a) Tolman, C. A. Phosphorus ligand exchange equilibriums on
zerovalent nickel. Dominant role for steric effects. J. Am. Chem. Soc.
1970, 92, 2956−2965. (b) Tolman, C. A.; Seidel, W. C.; Gosser, L.
W. Formation of three-coordinate nickel(0) complexes by phosphorus
ligand dissociation from NiL4. J. Am. Chem. Soc. 1974, 96, 53−60.
(c) Tolman, C. A. Steric effects of phosphorus ligands in
organometallic chemistry and homogeneous catalysis. Chem. Rev.
1977, 77, 313−348.
(6) Since the early estimations in physical models (by the way, using
Ni as the metal center), other methods have been incorporated,
including the modern application of computational methods to
different coordination geometries and metal centers: Jover, J.; Cirera,
J. Computational assessment on the Tolman cone angles for P-
ligands. Dalton Trans. 2019, 48, 15036−15048.
(7) Wu, K.; Doyle, A. G. Parameterization of phosphine ligands
demonstrates enhancement of nickel catalysis via remote steric effects.
Nat. Chem. 2017, 9, 779−784.
(18) Lopez, G.; Garcia, G.; Sanchez, G.; Garcia, J.; Ruiz, J.;
Hermoso, J. A.; Vegas, A.; Martinez-Ripoll, M. A.; Vegas, A.;
́
Martinez-Ripoll, M. Hydroxo and azolate derivatives of pentafluor-
ophenyl-nickel(II) complexes. Crystal structure of [NBu4]2[{Ni-
(C6F5)2(μ-OH)}2] and [NBu4]2[{Ni(C6F5)2}2(μ-OH)(μ-pyrazola-
to)]. Inorg. Chem. 1992, 31, 1518−1523.
(19) Casares, J. A.; Espinet, P.; Martín-Alvarez, J. M.; Martínez-
Ilarduya, J. M.; Salas, G. Stable Nickel Catalysts for Fast Norbornene
Polymerization: Tuning Reactivity. Eur. J. Inorg. Chem. 2005, 2005,
3825−3831.
(8) Combining observations from different sources: (a) Tatsumi, K.;
Hoffmann, R.; Yamamoto, A.; Stille, J. K. Bull. Chem. Soc. Jpn. 1981,
54, 1857−1867. (b) Low, J. J.; Goddard, W. A. Organometallics 1986,
5, 609−622. (c) Low, J. J.; Goddard, W. A. J. Am. Chem. Soc. 1986,
́
́
(20) Fornies, J.; Martín, A.; Martín, L. F.; Menjon, B.; Kalamarides,
H. A.; Rhodes, L. F.; Day, C. S.; Day, V. W. Synthesis and
18293
https://dx.doi.org/10.1021/acs.inorgchem.0c02831
Inorg. Chem. 2020, 59, 18287−18294

Inorganic Chemistry
pubs.acs.org/IC
Article
Characterization of a New Family of Square-Planar Nickel(II)
Carbonyl Derivatives. Chem. - Eur. J. 2002, 8, 4925−4934.
(21) Kuprat, M.; Lehmann, M.; Schulz, A.; Villinger, A. Synthesis of
Pentafluorophenyl Silver by Means of Lewis Acid Catalysis: Structure
of Silver Solvent Complexes. Organometallics 2010, 29, 1421−1427.
(22) The purity of complex 1 was evaluated by NMR (see the
Supporting Information).
Amines Using Aryl Chlorides. J. Am. Chem. Soc. 2008, 130, 13552−
13554.
(23) Complex 1 should be handled in a glove-box or under N2 or Ar
atmosphere. Prolonged vacuum evacuation should be avoided due to
elimination of the weakly coordinated THF ligand. The solid product
should be kept at −20 °C.
(24) For hydrolysis by adventitious water, see Casares, J. A.; Espinet,
P.; Martínez-Ilarduya, J. M.; Mucientes, J. J.; Salas, G. Study of the
replacement of weak ligands on square-planar organometallic
nickel(II) complexes. Organo-nickel aquacomplexes. Inorg. Chem.
2007, 46, 1027−1032.
(25) (a) Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Reek, J. N. H.
Wide Bite Angle Diphosphines: Xantphos Ligands in Transition
Metal Complexes and Catalysis. Acc. Chem. Res. 2001, 34, 895−904.
(b) Yin, J.; Buchwald, S. L. Pd-Catalyzed Intermolecular Amidation of
Aryl Halides: The Discovery that Xantphos Can Be Trans-Chelating
in a Palladium Complex. J. Am. Chem. Soc. 2002, 124, 6043−6048.
(c) Zuideveld, M. A.; Swennenhuis, B. H. G.; Boele, M. D. K.; Guari,
Y.; van Strijdonck, G. P. F.; Reek, J. N. H; Kamer, P. C. J.; Goubitz,
K.; Fraanje, J.; Lutz, M.; Spek, A. L.; van Leeuwen, P. W. N. M. The
coordination behaviour of large natural bite angle diphosphine ligands
towards methyl- and 4-cyanophenyl-palladium (II) complexes. J.
Chem. Soc., Dalton Trans. 2002, 2308−2317.
(26) Standley, E. A.; Smith, S. J.; Muller, P.; Jamison, T. F. A
̈
Broadly Applicable Strategy for Entry into Homogeneous Nickel(0)
Catalysts from Air-Stable Nickel(II) Complexes. Organometallics
2014, 33, 2012−2018.
(27) Hirata, Y.; Tanaka, M.; Yada, A.; Nakao, Y.; Hiyama, T.
Alkynylcyanation of alkynes and dienes catalyzed by nickel.
Tetrahedron 2009, 65, 5037−5050.
́
(28) Perez-Iglesias, M.; Infante, R.; Casares, J. A.; Espinet, P.
Intriguing Behavior of an Apparently Simple Coupling Promoter
Ligand, PPh2(p-C6H4−C6F5), in Their Pd Complexes. Organometallics
2019, 38, 3688−3695.
(29) For a review of 19F NMR studies of organometallic complexes
́
with fluorinated aryls, see Espinet, P.; Albeniz, A. C.; Casares, J. A.;
Martínez-Ilarduya, J. M. 19F NMR in organometallic chemistry:
Applications of fluorinated aryls. Coord. Chem. Rev. 2008, 252, 2180−
2208.
(30) Luo, X.; Zhang, H.; Duan, H.; Liu, Q.; Zhu, L.; Zhang, T.; Lei,
A. Superior Effect of a π-Acceptor Ligand (Phosphine−Electron-
Deficient Olefin Ligand) in the Negishi Coupling Involving Alkylzinc
Reagents. Org. Lett. 2007, 9, 4571−4574.
(31) The rates obtained are comparable to those reported by
Sanford et al. for paramagnetic NiIII complexes in the following:
Bour, J. R.; Camasso, N. M.; Meucci, E. A.; Kampf, J. W.; Canty, A. J.;
Sanford, M. S. Carbon−Carbon Bond-Forming Reductive Elimination
from Isolated Nickel(III) Complexes. J. Am. Chem. Soc. 2016, 138,
16105−16111.
́
(32) Penas-Defrutos, M. N.; Velez, A.; Gioria, E.; Espinet, P. E−Z
̃
Isomerization of Phosphine-Olefin (PEWO-F4) Ligands Revealed
upon PdCl2 Capture: Facts and Mechanism. Organometallics 2019,
38, 4701−4707. and references therein.
(33) (a) Milner, P. J.; Maimone, T. J.; Su, M.; Chen, J.; Muller, P.;
̈
Buchwald, S. L. Investigating the Dearomative Rearrangement of
Biaryl Phosphine-Ligated Pd(II) Complexes. J. Am. Chem. Soc. 2012,
134, 19922−19934. (b) Christmann, U.; Pantazis, D.; Benet-
Buchholz, A. J.; McGrady, J. E.; Maseras, F.; Vilar, R. Experimental
and Theoretical Investigations of New Dinuclear Palladium
Complexes as Precatalysts for the Amination of Aryl Chlorides. J.
Am. Chem. Soc. 2006, 128, 6376−6390. (c) Fors, B. P.; Watson, D. A.;
Biscoe, M. R.; Buchwald, S. L. A Highly Active Catalyst for Pd-
Catalyzed Amination Reactions: Cross-Coupling Reactions Using
Aryl Mesylates and the Highly Selective Monoarylation of Primary
18294
https://dx.doi.org/10.1021/acs.inorgchem.0c02831
Inorg. Chem. 2020, 59, 18287−18294