Axial Donor Effects on Oxidatively Induced Ethane Formation from Nickel-Dimethyl Complexes

Article abstract of DOI:10.1021/acs.organomet.9b00438

Tetradentate pyridinophane ligands have been shown to stabilize uncommon high-valent palladium and nickel organometallic complexes. Described herein are the synthesis and detailed characterization of a series of Ni^II- and Ni^III-dimethyl complexes supported by modified tetradentate pyridinophane ligands in which one or both of the N-methyl substituents were replaced with electron-withdrawing p-toluenesulfonyl groups, thus reducing the amine N atom donicity and favoring the formation of Ni complexes with lower coordination numbers. The corresponding Ni^II-dimethyl complexes exhibit accessible oxidation potentials, and their oxidation generates Ni^III species that were characterized by EPR and X-ray crystallography. Moreover, the Ni^II-dimethyl complexes exhibit selective ethane formation upon oxidatively induced reductive elimination using various oxidants - including O₂ and H₂O₂, without the generation of any C-heteroatom products. Overall, these results suggest that the (^RN4)Ni^IIMe₂ complexes with more weakly donating axial ligands are more reactive toward ethane formation, likely due to destabilization of the corresponding high-valent Ni intermediates and formation of 5- and 4-coordinate conformations for these Ni species.

Full text of DOI:10.1021/acs.organomet.9b00438

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Axial Donor Effects on Oxidatively Induced Ethane Formation from
Nickel−Dimethyl Complexes
Sofia M. Smith,† Nigam P. Rath,§ and Liviu M. Mirica*,†,‡
‡Department of Chemistry, University of Illinois at Urbana−Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801,
United States
†Department of Chemistry, Washington University in St. Louis, One Brookings Drive, St. Louis, Missouri 63130-4899, United States
§Department of Chemistry and Biochemistry, University of Missouri−St. Louis, One University Boulevard, St. Louis, Missouri
63121-4400, United States
S
* Supporting Information
ABSTRACT: Tetradentate pyridinophane ligands have been
shown to stabilize uncommon high-valent palladium and
nickel organometallic complexes. Described herein are the
synthesis and detailed characterization of a series of NiII− and
NiIII−dimethyl complexes supported by modified tetradentate
pyridinophane ligands in which one or both of the N-methyl
substituents were replaced with electron-withdrawing p-
toluenesulfonyl groups, thus reducing the amine N atom
donicity and favoring the formation of Ni complexes with
lower coordination numbers. The corresponding NiII−
dimethyl complexes exhibit accessible oxidation potentials, and their oxidation generates NiIII species that were characterized
by EPR and X-ray crystallography. Moreover, the NiII−dimethyl complexes exhibit selective ethane formation upon oxidatively
induced reductive elimination using various oxidantsincluding O2 and H2O2, without the generation of any C−heteroatom
products. Overall, these results suggest that the (RN4)NiIIMe2 complexes with more weakly donating axial ligands are more
reactive toward ethane formation, likely due to destabilization of the corresponding high-valent Ni intermediates and formation
of 5- and 4-coordinate conformations for these Ni species.
electron-withdrawing and sterically demanding toluenesulfonyl
(tosyl, Ts) groups. The oxidatively induced reductive
elimination reactivity studies with various oxidants suggest
that the less coordinating the axial ligand is, the more reactive
(and less stable) is the corresponding high-valent Ni species,
thus leading to faster reductive elimination. Among the three
systems investigated, the (TsN4)NiIIMe2 complex is shown to
be the most reactive and generates ethane selectively at low
temperatures, even when oxidants such as O2 or H2O2 were
employed.
INTRODUCTION
■
Palladium and nickel catalysts have been widely employed in
C−C and C−heteroatom bond formation reactions, such as
Negishi, Kumada, and Suzuki cross-coupling reactions.1−10 Pd
catalysts are more commonly used in these transformations, in
part due to a thorough mechanistic understanding of the Pd-
catalyzed reactions that proceed through PdII/Pd0 catalytic
cycles. By comparison, the mechanism of Ni-catalyzed cross-
coupling reactions is less understood, especially since Ni is
more prone to undergoing one-electron redox reactions.11−22
In this context, many studies strongly suggest that para-
magnetic NiIII species are key intermediates in C−C and C−
heteroatom bond formation reactions.6−10,22−30
In the past several years, we have employed tetradentate
pyridinophane ligands to stabilize uncommon organometallic
PdIII/IV and NiIII/IV complexes31−40 and have shown that these
high-valent complexes are capable of C−C and/or C−
heteroatom bond formation reactions. For example, we have
recently reported the use of N,N′-dimethyl-2,11-diaza[3.3]-
(2,6)pyridinophane (MeN4) ligand to stabilize high-valent NiIII
complexes that undergo C−C bond formation reactions.40
Herein, we report the use of modified pyridinophane ligands to
probe the effect of axial amine donicity on the reactivity of the
corresponding Ni−dimethyl complexes. As such, we have
replaced one or both of the N−Me groups in MeN4 with more
RESULTS AND DISCUSSION
■
Synthesis and Characterization of NiII/III Complexes.
The ligands N,N′-dimethyl-2,11-diaza[3.3](2,6)pyridinophane
(MeN4), N,N′-tosylmethyl-2,11-diaza[3.3](2,6)pyridinophane
(
TsMeN4), and N,N′-ditosyl-2,11-diaza[3.3](2,6)pyridinophane
(TsN4) (Chart 1) were synthesized according to a literature
procedure.41 Complexes (MeN4)NiIIMe2, 1, and [(MeN4)-
NiIIIMe2, 1+, were synthesized as reported previously.40
The dark orange complex (TsMeN4)NiIIMe2, 2, was prepared
in 86% yield from the precursor (TsMeN4)NiIIBr2 via
transmetalation with methylmagnesium chloride (Scheme 1).
Received: June 29, 2019
© XXXX American Chemical Society
A
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Chart 1. Ligands Used for Stabilizing High-Valent Nickel
Complexes
The single crystal X-ray structure of 2 reveals a square planar
geometry for the Ni center that is bound to two pyridyl
nitrogen atoms from the TsMeN4 ligand and two methyl groups,
with an average equatorial Ni−Npyridyl bond length of 1.947 Å
and an average Ni−C bond length of 1.929 Å (Figure 1). As
expected, 2 is diamagnetic, likely due to the strong σ-donor
methyl groups that favor the low-spin square planar geometry,
in contrast to the (RN4)NiIIBr2 precursors that adopt
octahedral geometries. The cyclic voltammogram (CV) of 2
exhibits an oxidation wave at −890 mV vs Fc+/Fc (Figure 2)
that is tentatively assigned to the NiII/III redox couple by
comparison to the analogous (MeN4)NiIIMe2 complex 1,40
followed by ill-defined oxidation events at higher potentials.42
Using the same process as described above, we were able to
prepare the orange complex (TsN4)NiIIMe2, 3, in 38% yield
from the precursor (TsN4)NiIIBr2. Similar to 2, the single
crystal X-ray structure of 3 reveals a square planar geometry for
the Ni center, with an average equatorial Ni−Npyridyl bond
length of 1.974 Å and an average Ni−C bond length of 1.943 Å
(Figure 1). Complexes 2 and 3 were both characterized by 1H
and 13C NMR.42 The CV of 3 exhibits an oxidation process at
−430 mV vs Fc+/Fc (Figure 2) that is tentatively assigned to
the NiII/III redox couple,40 followed by ill-defined oxidation
events at higher potentials.42 Notably, when comparing the
NiII/III oxidation potentials for 1,40 2, and 3, it seems that these
values increase by ∼500 mV for every tosyl group that replaces
a methyl group as the N-substituent, a trend that was
previously observed for similar Pd complexes and was
attributed to the interaction of the axial N donors with the
metal center.35
Figure 1. ORTEP representation of 2 (left) and 3 (right), with 50%
probability thermal ellipsoids. Selected bond distances (Å): 2: Ni1−
C1, 1.934(11); Ni1−C2, 1.923(12); Ni1−N1, 1.976(8); Ni1−N2,
1.971(9). 3: Ni1−C8, 1.929(4); Ni1−C8i, 1.929(4); Ni1−N1,
1.974(3); Ni1−N1i, 1.974(3).
octahedral geometry, with the average equatorial Ni−Npyridyl
bond lengths being similar to the previously reported
organometallic (RN4)NiIII complexes,39,40 while the axial Ni−
N
Ts distance (2.456 Å) is longer than the Ni−NMe distance of
2.154(11) Å (Figure 3), due to the more electron deficient
nature of the tosyl group that weakens the donicity of the N
donor, as well as the stronger trans influence of the NMe group.
Moreover, the 2.154(11) Å Ni−NMe distance in 2+ is shorter
than the average Ni−NMe distance of 2.246 Å of [(MeN4)-
NiIIIMe2], 1+.40 While a Ni−NTs interaction is present in the
solid-state structure, it is possible that 2+ adopts a 5-coordinate
geometry in solution (vide infra). Finally, despite several
attempts, we were not able to obtain X-ray quality crystals for
complex 3+ due to its limited stability.
The EPR spectrum of 2+ reveals a pseudoaxial signal in
THF:2-methyl-THF (MeTHF), and superhyperfine coupling
to the two axial N donors (I = 1) was observed in the gz
direction (Figure 4). The optimal simulation of the EPR
spectrum was obtained when different coupling constants were
used for the axial NMe (Az = 17.5 G) and NTs donors (Az = 6
G), and the values are in line with the stronger Ni−NMe vs Ni−
NTs interaction observed by X-ray crystallography and
consistent with the more electron-withdrawing nature of the
Both 2 and 3 can be oxidized using 1 equiv of silver
hexafluoroantimonate (AgSbF6) in THF at −50 °C to generate
the species [(TsMeN4)NiIIIMe2]SbF6, 2+, and [(TsN4)-
NiIIIMe2]SbF6, 3+, respectively. Single crystal X-ray character-
ization of 2+ reveals a six-coordinate NiIII center in a distorted
Scheme 1. Synthesis of (RN4)NiMe2 Complexes
B
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Figure 2. CV of 2 (top) in 0.1 M nBu4NPF6/MeCN at RT (100 mV/
s scan rate) and CV of 3 (bottom) in 0.1 M nBu4NPF6/MeCN at RT
(100 mV/s scan rate).
Figure 4. Top: experimental (1:3 THF:MeTHF, 77 K) and simulated
EPR spectra of 2+ using the following parameters: gx = 2.270 (Ax =
12.0 G), gy = 2.243 (Ay = 12.0 G), gz = 2.011 (Az (N) = 17.5 G and Az
(N) = 6 G). Center: experimental (1:3 MeCN:PrCN, 77 K) and
simulated EPR spectra of 2+ using the following parameters: gx =
2.222, gy = 2.202, gz = 2.012 (Az (2N) = 15.0 G). Bottom:
experimental (1:3 MeCN:PrCN, blue line, and 1:3 THF:MeTHF,
blue dashed line, 77 K) and simulated EPR spectra of 3+ using the
following parameters: gx = 2.365, gy = 2.304, gz = 2.002 (Az (2N) = 9.0
G).
Figure 3. ORTEP representation of 2+ with 50% probability thermal
ellipsoids. Selected bond distances (Å), 2+: Ni1−C1, 1.932(3); Ni1−
C2, 1.925(3); Ni1−N1, 1.968(2); Ni1−N2, 1.965(2); Ni1−N3,
2.154(11); Ni1−N4, 2.456.
tosyl group, as well as the stronger trans influence of the NMe
donor. Interestingly, when the EPR spectrum of 2+ was
obtained in MeCN:PrCN, the signal was best simulated using
superhyperfine coupling to the two axial N donors with
C
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identical coupling constants (Az = 15.0 G) and giving rise to a
quintet with a 1:2:3:2:1 intensity ratio,42 suggesting that a
solvent molecule (MeCN or PrCN) replaced the NTs axial
ligand and interacts with the Ni center. By comparison, the
EPR spectrum of 3+ in MeCN:PrCN shows a rhombic signal
with weak superhyperfine coupling (Az = 9.0 G) in the gz
region to two N atoms (Figure 4). Since the EPR spectrum of
3+ in THF:MeTHF is similar to the one obtained in
MeCN:PrCN (although 3+ is less stable in the former solvent
mixture, likely due to a more exposed NiIII center), we propose
that the observed weak coupling in the gz region should
correspond to the weakly interacting axial ligands, either the
NTs donors or the nitrile solvent molecules.42
decreased the yield of methane (Scheme 2). The most
pronounced difference was observed for 1, which gave ethane
in 88% yield after only 30 min at RT. This result is similar to
what was observed previously, and this was proposed to be due
to a change in the mechanism of C−C bond formation that
will likely involve a NiIV intermediate upon two-electron
oxidation.40 By comparison, for complexes 2 and 3, the yield of
ethane increased only slightly to 90 and 69%, respectively.42
Overall, these results suggest a more facile oxidatively induced
C−C reductive elimination for the NiMe2 complexes
R
supported by the N4 ligands containing NTs donors, which
only weakly interact axially with the Ni center. Such weak
interactions destabilize the corresponding NiIII species, as
described above, and decoordination of the NTs donors would
allow the formation of 5- and 4-coordinate geometries that
should rapidly undergo reductive elimination. As such, the
TsMeN4 ligand provides the optimal effect of supporting a NiIII
species while also allowing for rapid and selective ethane
elimination.
Additional mechanistic studies were performed to probe the
observed C−C bond formation reactivity for complexes 1, 2,
and 3. Crossover experiments using a 1:1 mixture of
(RN4)NiIIMe2 and (RN4)NiII(CD3)2 in CD3CN revealed two
types of results upon addition of 1 equiv of AcFcBF4. As
observed previously for complex 1, a 1:1 mixture of CH3CH3
and CH3CD3 was generated in ∼25% yield for each product
after 8 h (Scheme 3), which suggests a methyl group transfer
C−C Bond Formation Reactivity of (RN4)NiIIMe2
Complexes. The organometallic reactivity of 1, 2, and 3
was investigated and compared under different reaction
conditions. First, the oxidatively induced reductive elimination
from 1, 2, or 3 in CD3CN was probed using AcFcBF4 as an
outer-sphere oxidant, and the yields of ethane (and methane as
side product) were monitored via 1H NMR over time until the
reaction was complete (Scheme 2 and Table 1);42 the
Scheme 2. C−C Bond Formation Reactivity of the
(RN4)NiIIMe2 Complexes with AcFcBF4
Scheme 3. Crossover Reactivity Studies of the (RN4)NiIIMe2
Complexes
involving two Ni centers, followed by a rapid rearrangement of
the methyl groups between the axial and equatorial positions
before the reductive elimination step (Scheme 4a).40 By
comparison, for either mixture of 2:2-d6 or 3:3-d6, no
crossover product CH3CD3 was observed, with CH3CH3 being
formed in 48 and 32% yield, respectively. The formation of an
equivalent amount of CD3CD3 (46% yield) was also confirmed
by 2H NMR for the reaction of the 2:2-d6 mixture. Moreover,
a similar distribution of products was observed when 2 equiv of
AcFcBF4 was used for the oxidation of either mixture of 2:2-d6
or 3:3-d6.42
On the basis of these reactivity studies, we propose that
complexes 2 and 3 proceed through a different oxidatively
induced C−C bond formation mechanism than complex 1
(Scheme 4).40 It is likely that, upon one-electron oxidation of
the NiII complexes 2 and 3, the generation of NiIII species that
adopt 5- or 4-coordinate geometries leads to fast intra-
molecular reductive elimination to generate ethane and a NiI
species (Scheme 4b). This NiI species can then undergo
disproportionation to form a (RN4)NiII−solvento complex,42
formation of methane is proposed to occur upon hydrogen
atom abstraction from the solvent or ligand molecules by the
methyl radicals generated upon homolysis of Ni−Me
bonds.38,40 When 1 equiv of AcFcBF4 was added to MeCN
solutions of 1, 2, and 3, a general trend was observed in which
R
complexes 2 and 3 supported by the N4 ligands with the
more electron-withdrawing N-substituents yielded higher
yields of ethane, while the formation of methane was also
minimized for ligands bearing tosyl groups.40 Complex 2 gave
the highest yield of ethane (82%), while the TsN4 complex 3
gave ethane in 61% yield; however, the reaction had to be
performed at −20 °C for the latter system to prevent the
formation of Ni0. Interestingly, addition of 2 equiv of AcFcBF4
to solutions of 1, 2, or 3 increased the yield of ethane and
D
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complex 1 generates a more stable 6-coordinate NiIII species,
followed by a subsequent methyl group transfer between two
Ni centers to generate a transient (RN4)NiIVMe3 species that
will undergo reductive elimination to yield ethane, including
the crossover product CH3CD3 (Scheme 4a). Moreover, since
complex 1 exhibits a NiIII/IV oxidation potential at ∼0 mV vs
Fc/Fc+, addition of 2 equiv of AcFcBF4 (with an oxidation
potential of ∼250 mV vs Fc/Fc+) is expected to generate a
transient NiIV species that directly eliminates ethane, without
the need for methyl group transfer.40 Since for 2 and 3 the NiIV
oxidation state is not easily accessible due to the weak donating
ability of the axial NTs donors,42 addition of 2 equiv of AcFcBF4
should not change the mechanism of ethane elimination.
Finally, as we have shown previously for both (RN4)PdMe2
and (RN4)NiMe2 complexes,35,38,40 the ethane elimination
reactivity is similar when other outer-sphere oxidants with
comparable oxidation potentials were employed.
Scheme 4. (a) Previously Reported Mechanism for the
Oxidatively Induced Ethane Formation from
a
(MeN4)NiIIMe2, 1, and (b) Proposed Mechanism for
Oxidatively Induced Ethane Formation from
b
(
TsMeN4)NiIIMe2, 2
We have also probed alternate oxidants, including ones that
could proceed through an inner sphere mechanism, to
compare the C−C vs C−heteroatom bond formation
reactivity. Excitingly, complexes 1, 2, and 3 can be readily
oxidized with oxidants such as O2 or H2O2 to generate ethane
in yields up to 70% (Scheme 5 and Table 1), while no C−O
Scheme 5. C−C Bond Formation Reactivity of the
(RN4)NiIIMe2 Complexes with O2
products were observed for any of these complexes.
Interestingly, in the presence of O2, the yield of ethane
increases from 1 to 2 to 3, while the yield of the side product
methane decreases in the same direction, further suggesting
that the presence of N-Ts substituents in the RN4 ligands
promotes more facile C−C bond formation reactivity. Thus,
even though complex 3 exhibits a NiII/NiIII redox potential at
−505 mV vs Fc+/Fc, it can rapidly react with O2 or H2O2 to
selectively generate ethane. In addition, complexes 1, 2, and 3
can also react with other oxidants such as 1-fluoro-2,4,6-
trimethylpyridinium triflate (NFTPT), (diacetoxyiodo)-
benzene, and 5-(trifluoromethyl)dibenzothiophenium trifluor-
omethanesulfonate (TDTT) to generate ethane in up to 96%
yield, with no C−F, C−O, or C−CF3 bond formation products
being observed in any of these cases (Table 1).42 The same
trend was observed for these oxidants, with a higher yield of
ethane being obtained for 3 over 2 over 1. This trend parallels
the decrease in stability of the corresponding NiIII species and
does not seem to correlate with the oxidation potential or the
outer-sphere/inner-sphere nature of the oxidant employed.
Overall, these oxidatively induced ethane formation reactions
confirm that employing RN4 ligands that contain electron-
withdrawing tosyl N-substituents promotes more facile C−C
reductive elimination from Ni centers. Compared with the
a
The methyl groups marked in red are meant to represent isotopic
labeling.40 A similar mechanism could be envisioned for (TsN4)-
b
NiIIMe2 (3).
free ligand, and Ni0, which were isolated or observed at the end
of the reaction. By comparison, oxidation by one electron of
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Table 1. Oxidatively Induced Reactivity of Complexes 1, 2, and 3
b
(
MeN4)NiIIMe2 (1)
CH3−CH3 (%) CH3−H/D (%)
(
TsMeN4)NiIIMe2 (2)
CH3−CH3 (%) CH3−H/D (%)
(TsN4)NiIIMe2 (3)
a
oxidant
CH3−CH3 (%)
CH3−H/D (%)
1 equiv of AcFcBF4
2 equiv of AcFcBF4
O2
H2O2
NFTPT
67
88
41
69
49
74
79
1
1
1
3
3
2
1
11
1
42
7
9
11
20
2
1
2
2
2
1
2
82
90
48
70
87
85
86
4
3
1
1
1
1
2
4
3
19
13
6
2
2
2
1
1
2
1
61
69
62
61
96
88
88
2
1
2
1
1
2
1
0
c
3
0
0
0
8
0
1
PhI(OAc)2
TDTT
6
13
1
a
b
c
Typical reaction conditions: MeCN, RT, 2−8 h. Reactions performed at −20 °C. 30 min.
MeN4 ligand, which stabilizes to a greater extent NiIII centers
and even NiIV species, the TsMeN4 and especially the TsN4
ligand are proposed to destabilize the high-valent Ni species
due to the weaker axial Ni−NTs interactions, while allowing for
the formation of 5- and 4-coordinate conformations that can
rapidly undergo reductive elimination. Importantly, all of the
(RN4)NiMe2 complexes described herein react with a range of
oxidants, including O2 and H2O2, to cleanly generate ethane
and without the formation of any C−heteroatom bond
formation products.
complexes and investigate their oxidatively induced C−C
and C−heteroatom bond formation reactivity. The (RN4)-
NiIIMe2 complexes can be oxidized with a mild oxidant to
generate [(RN4)NiIIIMe2]+ species that were characterized by
X-ray crystallography or EPR spectroscopy. Moreover, these
NiII−dimethyl complexes exhibit selective ethane formation
upon oxidatively induced reductive elimination using various
oxidants, including O2 and H2O2, without the formation of any
C−heteroatom bond formation products. Compared with the
MeN4 ligand, which stabilizes to a greater extent the NiIII
centers and even NiIV species, the TsMeN4 and TsN4 ligands are
proposed to destabilize the high-valent Ni species due to the
weaker axial Ni−NTs interactions, while allowing for the
formation of 5- and 4-coordinate conformations that can
rapidly undergo reductive elimination.
Catalytic Reactivity of (RN4)NiIIBr2 Complexes. In
addition to stoichiometric C−C and C−X bond formation
studies, we also investigated the ability of the precursor
(RN4)NiBr2 complexes to catalyze the Kumada cross-coupling
reaction. Indeed, these complexes are efficient catalysts for the
coupling of aryl iodides with aryl Grignard reagents, with
TsMeN4)NiBr2 being the optimal catalyst to generate the aryl−
ASSOCIATED CONTENT
* Supporting Information
(
■
aryl coupled product in 94% yield (Table 2).42 However, these
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00438.
Table 2. Kumada Cross-Coupling Reactions Catalyzed by
(RN4)NiIIBr2
R
II
Synthetic details, spectroscopic characterization, stoi-
chiometric and catalytic reactivity studies, and crystallo-
graphic data (PDF)
5mol%( N4)Ni Br
2
R−X + R′−MgBr ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ R−R′
THF,RT,2h
a
ligand
Accession Codes
R−X
R′MgX
MeN4
TsMeN4
TsN4
60
17
5
CCDC 1895832−1895834 and 1895836−1895837 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.
p-tolyl-I
p-tolyl-I
octyl-I
PhMgBr
hexylMgBr
PhMgBr
80
2
1
2
95
38
15
2
2
2
2
2
1
36
14
a
Yields (%) were determined by GC-FID vs decane as internal
standard; no coupled products were observed in these reactions in the
absence of (RN4)NiBr2.
catalysts gave only modest yields for the coupling of aryl
iodides with alkyl Grignard reagents or alkyl iodides with aryl
Grignard reagents. While these results are not outstanding,
they do suggest that the (RN4)Ni complexes described herein
are relevant to classical cross-coupling reactions, which are
commonly proposed to involve NiIII transient intermedi-
ates.7,8,24,25 As we have shown previously, the reaction of
(RN4)Ni complexes with Grignard reagents and aryl/alyl
halides generates in situ detectable NiIII species, which are
responsible for C−C bond formation upon reductive
elimination.34
AUTHOR INFORMATION
Corresponding Author
*E-mail: mirica@illinois.edu.
ORCID
Sofia M. Smith: 0000-0002-7005-4034
Liviu M. Mirica: 0000-0003-0584-9508
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Science Foundation (NSF CHE-
1255424 and CHE-1925751) for support. The purchase of the
Bruker EMX-PLUS EPR spectrometer was supported by the
National Science Foundation (MRI, CHE-1429711). We also
thank Dr. Jason W. Schultz for the EPR measurements.
CONCLUSION
■
In conclusion, we report herein the use of three pyridinophane
RN4 ligands with either methyl or electron-withdrawing tosyl
N-substituents to synthesize a series of (RN4)NiMe2
F
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(21) Spasyuk, D. M.; Zargarian, D.; van der Est, A. New POCN-
Type Pincer Complexes of Nickel(II) and Nickel(III). Organo-
metallics 2009, 28 (22), 6531−6540.
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DOI: 10.1021/acs.organomet.9b00438
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DOI: 10.1021/acs.organomet.9b00438
Organometallics XXXX, XXX, XXX−XXX