Comparison of dppf-Supported Nickel Precatalysts for the Suzuki-Miyaura Reaction: The Observation and Activity of Nickel(I)

Article abstract of DOI:10.1002/anie.201505699

Ni-based precatalysts for the Suzuki-Miyaura reaction have potential chemical and economic advantages compared to commonly used Pd systems. Here, we compare Ni precatalysts for the Suzuki-Miyaura reaction supported by the dppf ligand in 3 oxidation states, 0, I and II. Surprisingly, at 80 °C they give similar catalytic activity, with all systems generating significant amounts of Ni^I during the reaction. At room temperature a readily accessible bench-stable Ni^II precatalyst is highly active and can couple synthetically important heterocyclic substrates. Our work conclusively establishes that Ni^I species are relevant in reactions typically proposed to involve exclusively Ni⁰ and Ni^II complexes.

Full text of DOI:10.1002/anie.201505699

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International Edition: DOI: 10.1002/anie.201505699
German Edition: DOI: 10.1002/ange.201505699
Cross-Coupling
Comparison of dppf-Supported Nickel Precatalysts for the Suzuki–
Miyaura Reaction: The Observation and Activity of Nickel(I)
Louise M. Guard, Megan Mohadjer Beromi, Gary W. Brudvig, Nilay Hazari,* and
David J. Vinyard
Abstract: Ni-based precatalysts for the Suzuki–Miyaura
reaction have potential chemical and economic advantages
compared to commonly used Pd systems. Here, we compare Ni
precatalysts for the Suzuki–Miyaura reaction supported by the
dppf ligand in 3 oxidation states, 0, I and II. Surprisingly, at
808C they give similar catalytic activity, with all systems
generating significant amounts of NiI during the reaction. At
room temperature a readily accessible bench-stable NiII
precatalyst is highly active and can couple synthetically
important heterocyclic substrates. Our work conclusively
establishes that NiI species are relevant in reactions typically
proposed to involve exclusively Ni0 and NiII complexes.
this system, here, we compare the catalytic performance and
properties of CinNiII with a series of related dppf-supported
complexes (Figure 1) and study the speciation of Ni during
C
ross-coupling reactions, such as the Suzuki–Miyaura
Figure 1. Precatalysts studied in this work.
reaction, are commonly used to construct carbon–carbon
bonds.[1] Currently, the most active systems are based on the
precious metal Pd.[1f] The development of catalysts containing
earth-abundant first row transition metals such as Ni could
result in more affordable systems.[2,3] Furthermore, owing to
its smaller size, increased nucleophilicity[3a] and weaker
binding to coordinating atoms in heterocycles, there are also
often chemical advantages to using Ni. For example, Ni-based
systems are superior to Pd for performing Suzuki–Miyaura
reactions involving carbamate,[4] carbonate,[4b] sulfamate,[4b,d,5]
acyliminium,[6] and sp3-based substrates.[7] Nevertheless, in
general, Ni-catalyzed Suzuki–Miyaura reactions require high
temperatures and high catalyst loadings, long reaction times,
and have limited substrate scope. Frequently, the cost benefit
of using Ni over Pd is offset by one or more of these problems.
Additionally, there have been few mechanistic studies of Ni
based precatalysts for the Suzuki–Miyaura reaction,[4d,8]
which complicates the rational design of improved systems.
Against this background, a remarkable advance was made
in 2012, when Ge and Hartwig reported the highly active Ni
Suzuki–Miyaura reactions. We show that, at elevated temper-
ature, all of the precatalysts form a catalytically active NiI
complex, and propose pathways for the formation of this NiI
species. At high temperatures Ni precatalysts in three differ-
ent oxidation states, 0, I and II, give comparable activity,
whereas at room temperature (RT), a bench-stable NiII
precatalyst is very efficient. It is so active that it is able to
perform Suzuki–Miyaura reactions involving heterocyclic
substrates relevant to the synthesis of pharmaceuticals at
RT; this marks the first time these reactions have been
performed under such mild conditions. Overall, our studies
establish that NiI species are relevant in Suzuki–Miyaura
reactions previously proposed to solely involve complexes in
the Ni0 and NiII oxidation states.[3b, 10]
To compare CinNiII to other commonly used precatalyst
motifs, a family of Ni dppf-supported complexes were
prepared using literature methods.[11] The NiII species
(dppf)Ni(o-tol)(Cl)[11c] otolNiII) is a representative example
(
precatalyst,
(dppf)Ni(h3-cinnamyl)(Cl)
(dppf = 1,1’-bis
from the commonly used Suzuki–Miyaura precatalyst scaffold
(PR3)2Ni(Ar)(Cl).[12] It is a particularly attractive precatalyst
as, unlike CinNiII, its synthesis does not require the use of the
expensive precursor Ni(cod)2 (cod = 1,5-cyclooctadiene) and
it is bench-stable. The air and moisture sensitive Ni0
(diphenylphosphino)ferrocene) (CinNiII),[9] which can perform
Suzuki–Miyaura reactions with synthetically useful hetero-
cycles at low catalyst loadings (0.5 mol%) and moderate
temperatures (50–808C). In order to gain further insight into
precatalysts (dppf)Ni(C2H4)[11b]
(
(
C2H4Ni0) and (dppf)2Ni[11a]
dppfNi0) were synthesized as representative Ni0 species and
presumably activate via ligand dissociation, whereas CinNiII
and otolNiII most likely activate via transmetalation.[9,13]
Numerous attempts to crystalize CinNiII under different
conditions resulted in the crystallographic characterization of
the air and moisture sensitive NiI complex[14] (dppf)Ni(Cl)
(ClNiI) (see Supporting Information (SI)).[15] Assuming there
may be a facile pathway to convert CinNiII to ClNiI, we
independently synthesized ClNiI and included it in our
[*] Dr. L. M. Guard, M. Mohadjer Beromi, Prof. G. W. Brudvig,
Prof. N. Hazari, Dr. D. J. Vinyard
The Department of Chemistry, Yale University
P.O. Box 208107, New Haven, CT 06520 (USA)
E-mail: nilay.hazari@yale.edu
Supporting information for this article (including experimental
procedures, X-ray information, and details about DFTcalculations) is
available on the WWW under http://dx.doi.org/10.1002/anie.
201505699.
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precatalyst comparison. ClNiI was isolated in good yield
(78%) from the comproportionation of C2H4Ni0 with (dppf)Ni-
(Cl)2 (Cl2NiII) in diethyl ether [Eq. (1)]. It was characterized by
paramagnetic 1H NMR, UV-Vis and EPR spectroscopy,
electrochemistry and elemental analysis (see SI). A similar
synthetic route has previously been used to synthesize
(PiPr3)2Ni(Cl).[16]
species formed in catalytic amination and trifluoromethyl-
thiolation reactions using Ni(cod)2/dppf, suggesting different
roles for NiI in Suzuki–Miyaura couplings compared to other
related reactions.[15,18]
Remarkably, all of the dppf-supported systems are so
active that they give complete conversion at RT in 12 h or less
when the catalyst loading is increased from 0.5 mol% to
2.5 mol% (Table 1, entries 6–10). These are the mildest
conditions reported to date for Ni-catalyzed Suzuki–Miyaura
reactions as there are only two other Ni precatalysts that are
active for the coupling of aryl halides and boronic acids at RT.
However both require significantly higher catalyst loadings
and longer reaction times.[12b,19] The Ni0 complex dppfNi0 gives
complete conversion faster than any other precatalyst inves-
tigated in this study (entry 9). We believe that the improved
catalytic activity of dppfNi0 compared to C2H4Ni0 is related to the
second dppf ligand in dppfNi0 binding less tightly to Ni than the
ethylene ligand in C2H4Ni0 (Table 1, entries 3, 4, 8, 9, see SI).
There is a disparity in the catalytic performance of the NiII
precatalysts CinNiII and otolNiII at 808C (Table 1, entries 1 and
2) despite literature precedent that both activate rapidly.[9,13]
This is almost certainly related to the relative stability of
CinNiII and otolNiII at elevated temperature. It was previously
noted that otolNiII is unstable in solution,[11c] and decomposi-
tion at 808C could explain its inferior performance. This
hypothesis is supported by RT results, where otolNiII gives
significantly better activity than CinNiII (entries 5 and 6).
Given the exceptional activity of the dppf-supported
precatalysts, we probed the species present under catalytic
conditions to gain information about their relative perfor-
mance. Although the finding that precatalysts in three
different oxidation states give similar catalytic activity at
808C is consistent with all systems forming the same active
species, the differences in the relative activity of the
precatalysts as a function of temperature (see SI) could be
indicative of a more complicated situation. The speciation of
Ni both during and at the end of catalysis was probed using
The catalytic performance of the family of dppf-supported
precatalysts for the Suzuki–Miyaura reaction was compared
at a range of temperatures, using 2-chloronaphthalene
(
2ClNap) and 4-methoxyphenylboronic acid (4OMePhB(OH)2)
as the substrates (Table 1). Our conditions are related to those
Table 1: Yields[a] of product for the Suzuki–Miyaura reaction[b] catalyzed
by dppf-supported Ni complexes.
% Yields for precatalysts
Entry T [8C]
Compound
0.5 h
1 h
1.5 h
1
2
3
4
5
80[c]
CinNiII
otolNiII
C2H4Ni0
dppfNi0[c]
ClNiI
94
66
81
>99
69
>99
95
>99
–
–
>99
–
–
–
>99
1 h
11
41
22
66
2 h
45
57
55
85
4 h
52
>99
57
>99
25
8 h
>99
–
74
–
12 h
–
–
>99
–
>99
6
7
8
9
RT[d]
CinNiII
otolNiII
C2H4Ni0
dppfNi0[c]
ClNiI
1
paramagnetic H NMR spectroscopy (Table 2). In catalytic
reactions at 808C, under our standard conditions, the
predominant Ni species present at the end of the reaction is
the NiI complex ClNiI, regardless of which precatalyst was
utilized (entries 1–5). The identity of the NiI complex was
unambiguously confirmed using EPR spectroscopy. Further-
more, when reactions using the Ni0 and NiII precatalysts were
monitored by 1H NMR spectroscopy during catalysis, it is
clear that ClNiI forms while the catalytic reaction is still
occurring (entries 1–4, 0.25 h). Interestingly, otolNiII is con-
verted completely to ClNiI very early in the reaction (entry 2),
which presumably explains the near identical catalytic
performance of otolNiII and ClNiI at 808C. In contrast for CinNiII
(entry 1), relatively little ClNiI is formed initially and the
concentration of ClNiI rises drastically at complete conversion.
At RT the Ni0 and NiII precatalysts also form NiI, but in
lower amounts (Table 2, entries 5–10). When ClNiI was used as
the precatalyst, it was detected in essentially unchanged
amounts at all stages of the reaction at both RT and 808C
(entries 5 and 10). These results suggest that the Ni0 and NiII
precatalysts all have a facile pathway to form ClNiI under
catalytic conditions, but that ClNiI is relatively stable and does
10
<5
<5
66
[a] Yields were calculated using gas chromatography and are the average
of two runs. [b] Reaction conditions: 0.2 mmol 2-chloronaphthalene,
0.4 mmol 4-methoxyphenylboronic acid, 0.8 mmol K3PO4, 0.2 mmol
naphthalene (internal standard), 0.5 or 2.5 mol% precatalyst, 340 mL
1,4-dioxane and 160 mL benzene. [c] 0.5 mol% catalyst was utilized.
[d] 2.5 mol% catalyst was utilized.
reported by Ge and Hartwig,[9] although we find that there is
an improvement in catalytic performance using 2:1 1,4-
dioxane:benzene as the solvent instead of neat ethereal
solvent. At 808C all precatalysts give complete conversion
within 1.5 h (entries 1–5). In particular, the finding that the
NiI species ClNiI gives comparable activity to Ni0 and NiII
complexes is significant (entry 5), as there are only two
previous reports of NiI precatalysts that are active for the
Suzuki–Miyaura reaction.[8a,17] In both examples complete
conversion did not occur even with high catalyst loadings
(10 mol%) and the use of a strong base (KOtBu). Interest-
ingly, ClNiI has previously been implicated as an inactive
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Table 2: Amount[a] of ClNiI observed both during and at the end of
consistent with previous studies demonstrating that the
reaction of (NHC)2Ni (NHC = IPr, IMes) with various aryl
halides generates NiI products and biaryl species,[8a,25] indicat-
ing that this observation could be broadly applicable to other
ligand sets.
catalysis[b] using different dppf-supported Ni precatalysts.
% of total Ni present in the form of ClNiI
Entry T [8C] Compound 0.25 h
0.5 h
End[d]
1
2
3
4
5
80
CinNiII
otolNiII
C2H4Ni0
dppfNi0[c]
ClNiI
26
>99
51
41
>99
23
>99
63
85
>99
68 (1 h)
>99 (1.5 h)
62 (1 h)
85 (0.5 h)
96 (1)
In contrast, no reaction was observed between otolNiII and
2ClNap. However, the reaction between otolNiII and 1 equiv of
4OMePhB(OH)2 and K3PO4 generated ClNiI in 50% yield by
2 h
6
35
34
12
97
End[d]
6
7
8
9
RT[e]
CinNiII
otolNiII
C2H4Ni0
dppfNi0[c]
ClNiI
26 (8 h)
44 (4 h)
61 (12 h)
22 (4 h)
94 (12 h)
1
paramagnetic H NMR spectroscopy [Eq. (3)]. Additionally,
4’-methoxy-2-methyl-1,1’-biphenyl (4OMePh-2MePh) was also
formed in 42% yield. Presumably, in this reaction half an
equivalent of otolNiII initially undergoes transmetalation and
reductive elimination to yield the biaryl product and
(dppf)Ni. The latter then comproportionates with the remain-
ing otolNiII to give ClNiI and (dppf)Ni(Ar), in an analogous
fashion to Equation (1) above. Although (dppf)Ni(Ar) was
not observed, it is expected to be unstable[11c,26] and some
decomposition was noted during the reaction.[27] Further
support for this mechanism is provided through the reaction
of otolNiII and C2H4Ni0 in C6D6 which forms 50% ClNiI at both
RT and 808C, as well as decomposition products (see SI).[28]
Additionally, when catalysis is performed using otolNiII a small
peak correlating to 4OMePh-2MePh is observed by GC. The
stoichiometric reactivity of CinNiII was similar to otolNiII. The
reaction of CinNiII with 1 equiv 4OMePhB(OH)2 and K3PO4
formed ClNiI in 50% yield (see Figure S8). The detection of
the related organic fragment was more complicated but
products consistent with transmetalation and reductive elim-
ination were observed by GC-MS. As with otolNiII, the reaction
of CinNiII with C2H4Ni0 generated 50% ClNiI and decomposition
products.
10
[a] Amounts of ClNiI were calculated using paramagnetic 1H NMR
integrations against a standardized capillary containing Cp2Co.
[b] Reaction conditions 0.6 mmol 2-chloronaphthalene, 1.2 mmol 4-
methoxyphenylboronic acid, 2.4 mmolK3PO4, 0.5 or 2.5 mol% precata-
lyst, 1020 mL 1,4-dioxane and 480 mL benzene. 750 mL of the reaction
mixture was removed, evaporated and dissolved in 500 mL C6D6. [c] The
complex dppfNi0 formed a different NiI complex, [(dppf)Ni(Cl)]2(m-dppf)
(see SI). Capillaries were standardized accordingly. [d] Time at which full
conversion to product was observed. [e] 2.5 mol% catalyst was utilized.
not form appreciable quantities of Ni0 or NiII species (< 5%).
To the best of our knowledge, this is the first time that a NiI
species has been directly observed and quantified during a Ni-
catalyzed Suzuki–Miyaura reaction, although NiI complexes
have been postulated as catalytically relevant intermediates
in both Negishi[21] and Kumada[22] reactions. However,
notably, there does not appear to be any direct correlation
between the amount of NiI formed and catalytic activity.
If our proposed pathway for the formation of NiI from
CinNiII is correct, then the amount of NiI formed in catalysis
could be affected by the concentration of the boronic acid. It
is plausible that by reducing the concentration of boronic
acid, the starting precatalyst will undergo slower transmeta-
lation and is thus more likely to comproportionate with Ni0
that has been formed via transmetalation and reductive
elimination (see SI). When only 1 equiv 4OMePhB(OH)2 was
used in catalysis with CinNiII at 808C, the amount of ClNiI
produced after 15 min was approximately double than when
2 equiv boronic acid were used (58% and 26%, respectively).
The total amount of NiI after 1 hour was also elevated in the
1 equiv case vs the 2 equiv case (87% vs 68%). This could
have implications for the development of reaction conditions;
in systems where NiI complexes are formed and are active, it
may be possible to reduce the amount of boronic acid and still
obtain high conversions. Conversely, in systems where NiI is
less active, both increasing the equivalency of boronic acid
and ensuring it is fully soluble in the reaction mixture could
result in more effective catalysis.
To determine how NiI forms during catalysis, a series of
stoichiometric reactions were performed. The complex
C2H4Ni0 was unreactive towards 4OMePhB(OH)2 and/or K3PO4
at 808C, but ClNiI formed quantitatively upon addition of
1 equiv 2ClNap to a 2:1 [D8]dioxane/C6D6 solution of C2H4Ni0 at
808C [Eq. (2)]. The biaryl 2,2’-binaphthalene (Nap-Nap) was
also detected (93% yield). At this stage the elementary steps
to form ClNiI and Nap-Nap are unclear, possible mechanisms
include bimetallic oxidative addition,[23] comproportiona-
tion[16] or a radical-based pathway[23a,24] (see SI). However,
consistent with this reaction being the route for NiI formation
during catalysis, a small amount of Nap-Nap is also detected
in catalytic reactions. Similar reactivity was observed with
dppfNi0 (see Figure S6 in the SI). Our observations are
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Our stoichiometric reactions confirm that Ni0 and NiII
precatalysts can be readily converted into ClNiI. We also
wanted to test if ClNiI could undergo disproportionation into
Ni0 and NiII species. Exposure of ClNiI to 1 atm of CO[24c] in
C6D6 resulted in the formation of (dppf)Ni(CO)2 and
(dppf)NiCl2 [Eq. (4)]. However, under catalytic conditions,
there is no ligand which can p-backbond to Ni0 as effectively
as CO. Treatment of ClNiI with dppf, the species present in
catalysis that most likely could stabilize Ni0, did not lead to
disproportionation (see SI). This was supported by DFT
calculations which indicate that disproportionation of a sim-
plified version of ClNiI into simplified versions of (dppf)2Ni
and (dppf)NiCl2 is uphill by 5.0 kcalmolÀ1 (see SI). These
results are in agreement with previous studies highlighting the
thermodynamic stability of NiI complexes formed by com-
proportionation.[8a,29]
Figure 2. Isolated yields[a] of product for Suzuki–Miyaura reactions[b] at
RT catalyzed by otolNiII. [a] Yields are the average of two runs.
[b] Reaction conditions: 0.2 mmol ArX, 0.4 mmol Ar’B(OH)2,
0.8 mmolK3PO4, 0.005 mmol catalyst, 340 mL 1,4-dioxane and 160 mL
benzene.
substrate instead of aryl chlorides. The use of aryl sulfamate
substrates is of particular interest because they can be
The quantitative detection of ClNiI at the end of the
reaction when it was used as a precatalyst and the thermody-
namic difficulty of disproportionation raises the possibility
that NiI species may be part of the catalytic cycle. However,
no reaction was observed when either 1 equiv 2ClNap or
4OMePhB(OH)2 and K3PO4 were added to a benzene solution
of ClNiI and heated at 808C for 2 h. Consistent with this
observation DFT calculations suggest that both oxidative
addition of chlorobenzene or transmetallation of 4OMePhB-
(OH)2 with ClNiI are thermodynamically unfavorable (see SI).
When 1 equiv of all three of these reagents were present,
4OMePh-Nap was formed and ClNiI was regenerated in > 99%
selectively functionalized using directed C H activation
À
prior to cross-coupling.[4b,d,5] Using 2.5 mol% otolNiII, 4OMePhB-
(OH)2 and naphthalen-1-yl dimethylsulfamate are quantita-
tively coupled at RT in 4 h [Eq. (6)]. Previous examples of this
reaction have typically required catalyst loadings between 5–
10 mol%, temperatures ranging from 80–1108C and extended
reaction times.[4b,d,5]
1
yield as determined by paramagnetic H NMR spectroscopy
Overall, although our studies clearly indicate that a NiI
compound is forming during catalysis, at this stage the exact
role of the NiI species is unclear. For example, it could be
acting as a catalyst resting state, which releases a very small
amount of a highly active Ni0 or NiII species into the reaction
mixture or it may be directly part of the catalytic cycle, with
the elementary steps being thermodynamically unfavorable
and thus unobservable. Understanding the specific role of NiI
compounds in Suzuki–Miyaura reactions will be the focus of
ongoing work in our laboratory. Nevertheless, we have
conclusively established that Ni0 and NiII complexes can
readily form NiI species under the catalytic conditions; the
first time this has been demonstrated for Suzuki–Miyaura
reactions. In general, the formation of NiI species will be
problematic in catalytic reactions in which NiI species are not
active because it is difficult for NiI to undergo disproportio-
nation. However, the formation of NiI can be suppressed by
the addition of a greater number of equivalents of the
transmetalating agent. In the case of the Suzuki–Miyaura
reactions studied in this work, a NiI species that forms readily
from Hartwig and Geꢀs seminal CinNiII precatalyst is highly
active, however we suggest that the easily accessible otolNiII is
a more active and practical catalyst. In the future we will
explore whether our results are relevant to Ni complexes
supported by other ligands and ascertain if well-defined NiI
precatalysts can lead to milder conditions for other cross-
coupling reactions.
[Eq. (5)], in agreement with our catalytic results.
Finally, to show the utility of our systems, we explored the
substrate scope using otolNiII. This precatalyst was selected for
three reasons: 1) it is highly active, 2) it is both air and
moisture stable, which makes it considerably more practical
than other Ni precatalysts such as dppfNi0, and 3) it can be
synthesized directly from NiCl2 without using costly Ni-
(cod)2.[20] A variety of synthetically valuable substrates
containing heterocycles were successfully coupled at RT
using otolNiII (Figure 2). This includes 2-benzofuran and 2-
benzothiophene boronic acids, and pyridyl and pyrazyl
chlorides, which are problematic for Pd-based systems. It
should be noted that the more challenging substrate 2-
furanylboronic acid still required slightly elevated temper-
ature (408C), and gave a slightly lower yield when coupled
with 2-chloroquinoline. Furthermore, otolNiII is competent for
Suzuki–Miyaura reactions using aryl sulfamates as the
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2427; b) X. H. Fan, L. M. Yang, Eur. J. Org. Chem. 2011, 1467;
c) E. A. Standley, T. F. Jamison, J. Am. Chem. Soc. 2013, 135,
1585.
Acknowledgements
We acknowledge support from the NSF through Grant CHE-
1150826. The EPR spectroscopy work was supported by the
Department of Energy, Office of Basic Energy Sciences,
Division of Chemical Sciences grant DE-FG02-05ER15646
(G.W.B. and D.J.V.). N.H. is a Camille and Henry-Dreyfus
Foundation Teacher Scholar. We are grateful to Dr. Paul
Anastasꢀ group for use of their GC instrument, Dr. Alec
Durrell for assistance with electrochemistry and Professor
James Mayer for valuable discussions.
[13] B. T. Guan, Y. Wang, B. J. Li, D. G. Yu, Z. J. Shi, J. Am. Chem.
Soc. 2008, 130, 14468.
[14] Based on evidence from paramagnetic NMR spectroscopy we
believe that the NiI complex, (dppf)Ni(Cl) (ClNiI), is present as
a minor impurity in the synthesis of (dppf)Ni(h3-cinnamyl)(Cl)
(
CinNiII). We were able to repeatedly crystallize this minor
impurity. Pure samples of CinNiII can be prepared by recrystal-
lization.
[15] While this work was in progress ClNiI was reported in G. Yin, I.
Kalvet, U. Englert, F. Schoenebeck, J. Am. Chem. Soc. 2015, 137,
4164.
Keywords: cross-coupling · homogeneous catalysis · nickel ·
reaction mechanism · Suzuki–Miyaura coupling
[16] R. Beck, M. Shoshani, J. Krasinkiewicz, J. A. Hatnean, S. A.
Johnson, Dalton Trans. 2013, 42, 1461.
[17] J. Wu, A. Nova, D. Balcells, G. W. Brudvig, W. Dai, L. M. Guard,
N. Hazari, P. H. Lin, R. Pokhrel, M. K. Takase, Chem. Eur. J.
2014, 20, 5327.
How to cite: Angew. Chem. Int. Ed. 2015, 54, 13352–13356
Angew. Chem. 2015, 127, 13550–13554
[18] It was proposed that ClNiI is inactive for the amination of
unactivated aryl chlorides (see: S. Ge, R. A. Green, J. F. Hartwig,
J. Am. Chem. Soc. 2014, 136, 1617 – 1627). However, no synthetic
details or characterizing data for ClNiI was reported.
[19] Z. Y. Tang, Q. S. Hu, J. Org. Chem. 2006, 71, 2167.
[20] The complex otolNiII is commerically available from Aspira
Scientific (http://www.aspirasci.com) with compound number
300926.
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Received: June 20, 2015
Published online: September 11, 2015
13356 www.angewandte.org
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