Aerobic C-C and C-O bond formation reactions mediated by high-valent nickel species

Article abstract of DOI:10.1039/c9sc03758f

Nickel complexes have been widely employed as catalysts in C-C and C-heteroatom bond formation reactions. While Ni(0), Ni(i), and Ni(ii) intermediates are most relevant in these transformations, recently Ni(iii) and Ni(iv) species have also been proposed to play a role in catalysis. Reported herein is the synthesis, detailed characterization, and reactivity of a series of Ni(ii) and Ni(iii) metallacycle complexes stabilized by tetradentate pyridinophane ligands with various N-substituents. Interestingly, while the oxidation of the Ni(ii) complexes with various other oxidants led to exclusive C-C bond formation in very good yields, the use of O₂ or H₂O₂ as oxidants led to formation of appreciable amounts of C-O bond formation products, especially for the Ni(ii) complex supported by an asymmetric pyridinophane ligand containing one tosyl N-substituent. Moreover, cryo-ESI-MS studies support the formation of several high-valent Ni species as key intermediates in this uncommon Ni-mediated oxygenase-type chemistry.

Full text of DOI:10.1039/c9sc03758f

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Aerobic C–C and C–O bond formation reactions
mediated by high-valent nickel species†
Cite this: Chem. Sci., 2019, 10, 10366
b
c
c
Laura Gomez,d Nigam P. Rath,e Xavi Ribas
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Sofia M. Smith,
and Liviu M. Mirica
Oriol Planas,
*
´
ab
Nickel complexes have been widely employed as catalysts in C–C and C–heteroatom bond formation
reactions. While Ni(0), Ni(I), and Ni(II) intermediates are most relevant in these transformations, recently
Ni(III) and Ni(IV) species have also been proposed to play a role in catalysis. Reported herein is the
synthesis, detailed characterization, and reactivity of a series of Ni(II) and Ni(III) metallacycle complexes
stabilized by tetradentate pyridinophane ligands with various N-substituents. Interestingly, while the
oxidation of the Ni(II) complexes with various other oxidants led to exclusive C–C bond formation in very
good yields, the use of O2 or H2O2 as oxidants led to formation of appreciable amounts of C–O bond
formation products, especially for the Ni(II) complex supported by an asymmetric pyridinophane ligand
containing one tosyl N-substituent. Moreover, cryo-ESI-MS studies support the formation of several
high-valent Ni species as key intermediates in this uncommon Ni-mediated oxygenase-type chemistry.
Received 25th July 2019
Accepted 17th September 2019
DOI: 10.1039/c9sc03758f
rsc.li/chemical-science
C–heteroatom bond formation reactions. In addition, we have
recently reported the use of the ligand 1,4,7-trimethyl-1,4,7-
triazacyclonane (Me3tacn) to stabilize high-valent NiIII/IV
Introduction
The formation of C–C and C–heteroatom bonds plays a funda-
mental role in organic transformations, and C–C cross-coupling
reactions are among the most powerful tools for the construc-
tion of new C–C bonds.1–3 In this context, nickel catalysts have
been commonly used for more than four decades in cross-
coupling reactions such as Negishi, Kumada and Suzuki
couplings.4–9 The most common oxidation states involved in
these catalytic transformations are Ni0, NiI, and NiII, although
more recent studies show that NiIII and NiIV oxidation states can
also play a role in C–C bond formation.10–20 By comparison,
stoichiometric and catalytic Ni-mediated C–heteroatom bond
formation reactions have been developed mostly in the past two
decades.21–36
complexes that undergo C–C and C–heteroatom bond forma-
tion reactions.35 With this ligand, small amounts of C–O bond
formation products (ꢀ5%) were observed upon oxidation of
(Me3tacn)NiII(CH2CMe2-o-C6H4) with O2.35 Herein, we report the
use of modied pyridinophane ligands that directly affects that
stability and reactivity of the corresponding high-valent Ni
complexes. We tested the effect of the amine N-substituents by
replacing the methyl groups with a more electron-withdrawing
toluenesulfonyl (tosyl, Ts) group or a bulkier tert-butyl (tBu)
group. By employing a Ni-cycloneophyl structure motif, we have
been able to isolate NiIII species and also perform uncommon
oxidatively-induced C–C and C–O bond formation reactions
using O2 or H2O2 as the oxidants. The ability to control the
relative reactivity of C–C vs. C–heteroatom bond formation
should have important implications in various organic
transformations.
In the past several years we have employed tetradentate
pyridinophane ligands to stabilize uncommon organometallic
PdIII/IV and NiIII/IV complexes,37–44 which can undergo C–C and
aDepartment of Chemistry, University of Illinois at Urbana-Champaign, 600 S.
Mathews Avenue, Urbana, Illinois 61801, USA. E-mail: mirica@illinois.edu
bDepartment of Chemistry, Washington University in St. Louis, One Brookings Drive,
Results and discussion
Synthesis and characterization of NiII/III complexes
The ligands N,N0-dimethyl-2,11-diaza[3.3](2,6)pyridinophane
St. Louis, Missouri 63130-4899, USA
c
´
´
`
Departament de Quımica, Institut de Quımica Computacional i Catalisi (IQCC),
Universitat de Girona, Campus de Montilivi, Girona E-17003, Catalonia, Spain
Serveis Tecnics de Recerca (STR), Universitat de Girona, Parc Cientıc i Tecnologic,
(
(
MeN4),
N,N0-tosylmethyl-2,11-diaza[3.3](2,6)pyridinophane
d
`
´
`
TsMeN4), N,N0-ditosyl-2,11-diaza[3.3](2,6)pyridinophane (TsN4)
Girona E-17071, Catalonia, Spain
eDepartment of Chemistry and Biochemistry, University of Missouri-St. Louis, One
and N,N0-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane (tBuN4)
were synthesized according to literature procedures.45,46 With
these ligands a series of cycloneophyl complexes were synthe-
sized through a ligand exchange of (py)2NiII(cycloneophyl).47,48
The orange NiII complex (MeN4)NiII(cycloneophyl), 1, was
University Boulevard, St. Louis, Missouri 63121-4400, USA
† Electronic supplementary information (ESI) available: Synthetic details,
spectroscopic characterization, stoichiometric and catalytic reactivity studies,
and crystallographic data. CCDC 1943288–1943290. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c9sc03758f
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Table
complexes
1
Cyclic voltammetry (CV) data for (RN4)Ni(cycloneophyl)
prepared in a 72% yield (Scheme 1),49 while the yellow NiII
complex (TsMeN4)NiII(cycloneophyl), 2, was prepared in 81%
yield and fully characterized.50 The cyclic voltammetry (CV)
characterization of 2 reveals two main oxidation events at
ꢁ990 mV and 750 mV vs. Fc+/Fc, along with a smaller oxidation
wave at 70 mV (Table 1).50 The two main oxidation events are
tentatively assigned to the NiIII/II and NiIV/III redox couples,
while the oxidation wave at 70 mV is proposed to correspond to
a conformer of complex 2 in which the TsMeN4 ligand adopts
a k3 binding mode, while the oxidation wave at ꢁ990 mV likely
corresponds to the conformation of 2 with TsMeN4 in a k4
binding mode.29,35 The third complex, (TsN4)NiII(cycloneophyl),
3, was prepared in a 67% yield and was fully characterized by 1H
Complex
1
2
3
4
E(NiII/III)a (mV)
ꢁ1700
ꢁ990
ꢁ400
200
a
E(NiII/III) values reported vs. Fc+/Fc 0.1 M nBu4NPF6/MeCN at RT,
100 mV sꢁ1 scan rate.
unknown NiIII impurity is present in complex 3+, and simula-
tion of the experimental spectrum suggests this impurity
amounts to ꢀ5% of the entire EPR signal. Complex 4+ also
exhibits a rhombic EPR signal, with superhyperne coupling to
two axial N donors in the gz direction (A2N ¼ 11 G), suggesting
that the N-t-butyl groups interact with the Ni center to a greater
extent than the N-Ts groups in 3+.
13
NMR and C NMR.50 Its CV shows an oxidation potential at
ꢁ400 mV, which is tentatively assigned to the NiIII/II couple.
Finally, the yellow NiII complex (tBuN4)NiII(cycloneophyl), 4, was
1
prepared in 73% yield and was also characterized by H NMR
C–C and C–X bond formation reactivity of (RN4)
and 13C NMR.50 The CV of 4 shows a rst oxidation potential at
200 mV vs. Fc+/Fc, which is higher than those observed for 2 and
3, likely due to the steric bulk of the t-butyl group, and as
observed previously.37,39–41,44
NiII(cycloneophyl) complexes
With the new (RN4)NiII(cycloneophyl) complexes in hand, we set
out to probe their reactivity in C–C and C–heteroatom bond
formation reactions. First, we studied the oxidation of
complexes 1–4 with 1-uoro-2,4,6-trimethylpyridinium triate
Given their accessible oxidation potentials, complexes 1–4
can easily be oxidized with one equivalent of acetylferrocenium
tetrauoroborate
(
AcFcBF4) or silver hexauoroantimonate
(NFTPT),
5-(triuoromethyl)dibenzothiophenium
tri-
(AgSbF6) in THF at ꢁ50 ꢂC to yield [(TsMeN4)NiIII(cycloneo-
phyl)]+, 2+, [(TsN4)NiIII(cycloneophyl)]+, 3+, and [(tBuN4)NiIII(cy-
cloneophyl)]+, 4+. The X-ray structures of both 2+ and 3+ reveal
six-coordinate NiIII centers in distorted octahedral geometries
uoromethanesulfonate (TDTT), xenon diuoride (XeF2) and
(diacetoxyiodo)benzene (PhI(OAc)2), since these oxidants have
been commonly employed in high-valent transition metal
chemistry, including Ni complexes.18,19,51–54 While no C–hetero-
atom bond formation was detected, the C–C coupled product
1,1-dimethylbenzocyclobutene was obtained in various yields
(Table 2).50 Comparing the reactivity of complexes 1–4, it
appears that the presence of electron withdrawing N-Ts groups
leads to higher C–C product yields, with up to 99% conversion
(Fig. 1). For 2+, the Ni–NTs bond length 2.527 A is larger than the
˚
˚
Ni–NMe bond length 2.199(4) A, given that the N-tosyl group is
more electron withdrawing vs. the N-methyl group. The Ni–N1
˚
bond length of 2.182(5) A is larger than the Ni–N2 bond of
3
˚
1.867(4) A due to a stronger trans effect of the C(sp ) vs. the
C(sp2) donor atom. Similar trends are observed for 3+, while the
Ni–NTs distances are signicantly longer than the axial inter-
actions observed for 2+ (Fig. 1).
The EPR spectrum of 2+ exhibits a rhombic signal with
a superhyperne coupling observed in the gz direction due to
one axial N donors (I ¼ 1) coupling to the NiIII center (Fig. 2),
suggesting that the NTs atom only weakly coordinates in the
axial position. By comparison, complex 3+ exhibits a rhombic
signal with a weak superhyperne coupling to two N atoms in
the gz direction (A2N ¼ 8 G), suggesting that both NTs atoms
weakly bind to the Ni center (Fig. 2). Also, a small amount of
Fig. 1 ORTEP representation of 2+ (left) and 3+ (right) with 50%
probability thermal ellipsoids. Selected bond distances (A˚), 2+: Ni1–C1,
1.938(9); Ni1–C4, 1.938(4); Ni1–N1, 2.182(5); Ni1–N2, 1.867(4); Ni1–
N3, 2.199(4); Ni1–N4, 2.527; 3+: Ni1–C1, 1.933(3); Ni1–C8, 1.982(3);
Ni1–N1, 1.993(3); Ni1–N2, 2.010(3); Ni1–N3, 2.360(3); Ni1–N4,
2.436(3).
0
Scheme 1 Synthesis of (RR N4)Ni(cycloneophyl) complexes.
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observed when 3 was oxidized with XeF2. This reactivity trend is
likely due to the coordination of the amine arms to create a six-
coordinate high-valent Ni species that is less reactive towards
reductive elimination than a ve- or four-coordinate Ni species
formed in the presence of the ligands with weakly interacting N-
Ts groups. Since the reagents employed above are considered to
usually act as two-electron oxidants, we tentatively propose the
formation of transient (RN4)NiIV(cycloneophyl) species that
then undergo C–C reductive elimination to generate the 1,1-
dimethylbenzocyclobutene product and a (RN4)NiII-bis-solvento
species, similar to what we observed recently for the (Me3tacn)
NiIV(cycloneophyl) system.35
We have also investigated the oxidation of complexes 1–4
with O2 and H2O2. The use of O2 and H2O2 as an oxidant obvi-
ates the requirement for strong and hazardous oxidants which
generate undesired stoichiometric byproducts, and thus
making O2 and H2O2 “greener” reagents for C–C and C–O bond
formation reactions. The exposure of solutions of complexes 1–
4 to O2 quickly generates orange or red solutions. To our
ꢂ
delight, heating these reaction mixtures for 14 hours at 70 C
followed by acidic workup and GC-MS analysis reveals the
formation of C–O bond formation products. For example,
oxidation of 1 and 3 with O2 generates C–O bond formation
products in 12–17% combined yield, while complex 2 shows up
to 41% combined yield of C–O products (Table 3). By compar-
ison, no C–O bond formation was formed for 4, suggesting that
the steric bulk of the tert-butyl groups precludes exogenous
reactivity, and thus 4 was not employed in any further reactivity
studies. The GC-MS analysis also conrmed the presence of C–C
coupled product 1,1-dimethylbenzocyclobutene for all four
complexes, with up to 69% yield observed for 3. In addition, the
oxidants H2O2 and m-chloroperoxybenzoic acid (mCPBA) are
also capable of promoting C–C and C–O bond formation,
although lower C–O product yields were observed,50 likely due to
the formation of more stable high-valent Ni species that
undergo reductive elimination more slowly or undergo unspe-
cic decomposition (see below). The lower C–O and C–C bond
formation product yields might be attributed to the presence of
water in the H2O2 solution that likely leads to unspecic
decomposition of the high-valent Ni species before being able to
undergo reductive elimination.50 Overall, complex 2 seems to be
the optimal system to generate the highest yields of C–O
products, likely by providing the sweet spot of stability vs.
reactivity to allow for the oxidation by O2, and the formation of
high-valent Ni species that persists long enough to undergo C–O
reductive elimination.
To further probe the mechanism of oxidation, the reaction of
(RN4)NiII(cycloneophyl) complexes 1–3 with O2 or H2O2 was
monitored at ꢁ50 ꢂC in 9 : 1 acetone-d6 : D2O. While no NiIV
intermediates were observed by 1H NMR, the formation of more
than one NiIII species was suggested by EPR spectroscopy. In
addition, cryo-electrospray mass spectrometry (cryo-ESI-MS)
Fig. 2 Experimental (1 : 3 MeCN : PrCN, 77 K) and simulated EPR
spectra of 2+ ((A) top) using the following parameters: gx ¼ 2.277, gy ¼
2.235, gz ¼ 2.009 (Az(N) ¼ 18.0 G). Experimental (1 : 3 MeCN : PrCN, 77
K) and simulated EPR spectra of 3+ ((B) middle) using the following
parameters for sim 1: gx ¼ 2.409, gy ¼ 2.318, gz ¼ 2.000 (Az(2N) ¼ 8.0
G) and sim 2: gx ¼ 2.253, gy ¼ 2.220, gz ¼ 2.010 (Az(N) ¼ 18.0 G).
Simulations 1 and 2 were added in a 19 : 1 ratio respectively to simulate
the experimental spectra properly. Experimental (1 : 3 MeCN : PrCN,
77 K) and simulated EPR spectra of 4+ ((C) bottom) using the following
parameters: gx ¼ 2.345, gy ¼ 2.273, gz ¼ 2.006 (Az(2N) ¼ 11.0 G).
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Table 2 Oxidation of (RN4)NiII(cycloneophyl) with various oxidants in
MeCN
transient species that are observed are tentatively assigned to
a NiIV–hydroperoxo and a NiIV–hydroxo complex (A, m/z
631.1884, and B, m/z 615.1934, Fig. 3).50 The decay of the two
transient species leads to the formation of a persistent species
that likely corresponds to a hydroxylated Ni-cycloneophyl
species (C, m/z 614.1856). This NiIII species
C is then
proposed to undergo reductive elimination to form a cationic
NiI species E, which was detected by ESI-MS, and the cyclized
C–O product (Scheme 2). Alternatively, the NiIV species B can
Yielda (%)
Complex
PhI(OAc)2
NFTPT
TDTT
XeF2
1
2
3
4
4; 22
24; 13
42; 10
NDb
17; 39
32; 27
96; 4
7; 19
33; 15
42; 9
6; 9
20; 40
50; 16
99; 1
NDb
14; 44
a
Reaction conditions: MeCN, 70 ꢂC, 14 h. Yields (%) were determined
by GC-FID vs. 1,3,5-trimethoxybenzene as internal standard. The rst
yield shown is for 1,1-dimethylbenzocyclobutene, while the second
yield is for is tert-butylbenzene. b Not determined.
Table
3
Oxidation of (RN4)NiII(cycloneophyl) with O2 in 9 : 1
MeCN : H2O
Total
yield (%)
Complex
Yieldsa (%)
1
2
3
4
35
41
69
27
12
8
2
2
0
3
17
5
6
22
5
64
94
93
33
12
12
6
0
0
a
Reaction conditions: 9 : 1 MeCN : H2O, 70 ꢂC, 14 h. Yields (%) were
determined by GC-FID vs. 1,3,5-trimethoxybenzene as internal standard.
was employed in an attempt to detect any high-valent Ni inter-
mediates formed during the oxidation process.30,55 First, the
oxidation of (TsMeN4)NiII(cycloneophyl) complex 2 with O2 was
probed by cryo-ESI-MS at ꢁ80 ꢂC in 9 : 1 acetone : H2O, followed
by addition of HClO4 as the proton source. The most intense
signal corresponds to the [(TsMeN4)NiIII(cycloneophyl)]+ species
2+ (molecular formula C32H36N4NiO2S, m/z 598.1912), conrm-
ing the detection of NiIII species by EPR. In addition, two less
intense species were observed that correspond to the mass of
species 2+ plus two or one O atoms, C32H36N4NiO4S (m/z
630.1811), and C32H36N4NiO3S (m/z 614.1856), respectively.
While we cannot unambiguously assign the structure of these
species,50 they are strongly suggesting an inner-sphere aerobic
oxidation mechanism that eventually leads to formation of the
detected C–O bond formation products.
Fig. 3 Cryo-ESI-MS spectra (top) and simulations (bottom) showing
the isotopic patterns of cationic NiIII and NiIV intermediates found
during the oxidation of 2 with H2O2 in 10% H2O/acetone and HClO4 at
ꢁ80 ꢂC. From top to bottom: A, [(TsMeN4)NiIV(cycloneophyl)(OOH)]+
m/z 631.1884 (transient species, simulated as a 49 : 51 mixture with
a 2+ + O2 species with m/z 630.1805), B, [(TsMeN4)NiIV(cycloneophy-
l)(OH)]+ m/z 615.1934 (transient species, simulated in a 39 : 61 ratio
with species C) and C, [(TsMeN4)NiIII(–CH2CMe2-o-C6H4–O)]+ m/z
614.1856 (persistent species).50
Interestingly, oxidation of 2 at ꢁ80 ꢂC with H2O2 in 9 : 1
acetone : H2O generates a number of transient and persistent
species that can be observed by cryo-ESI-MS. The rst two
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those cases, the use of appropriate multidentate N-donor
ligands was also essential in promoting the oxidation of
organometallic Pt57–59 and Pd39,40,42,60,61 complexes with O2 and
H2O2 to generate isolable or detectable high-valent ‘oxygenated’
transition metal species, however their C–heteroatom reductive
elimination reactivity is limited.62 While the mechanistic
studies reported herein are preliminary and no high-valent
oxygenated Ni species have been isolated, this report strongly
suggests that appropriately-designed organometallic Ni
complexes can be easily oxidized by oxidants such as O2 and
H2O2 in a similar manner to the Pt and Pd species, yet the high-
valent Ni species should exhibit increased C–heteroatom
reductive elimination reactivity that can be synthetically useful.
Conclusion
In conclusion, we report herein the use of four different pyr-
idinophane ligands to stabilize organometallic Ni complexes,
including high-valent Ni species. All isolated complexes were
fully characterized and their C–C and C–heteroatom bond
formation reactivity investigated. The (RN4)NiII(cycloneophyl)
derivatives have accessible oxidation potentials and thus can
easily be oxidized with a variety of oxidants. Interestingly,
oxidation of (RN4)NiII(cycloneophyl) complexes with O2 and
H2O2 generates both C–C and C–O bond formation products.
For example, the (TsMeN4)NiII(cycloneophyl) complex yields C–O
bond formation products in 41% yield, along with the C–C
coupled product for an overall conversion of 94%. Cryo-ESI-MS
studies were employed to detect high-valent Ni–oxygen species
and a mechanism was proposed for the oxidatively-induced C–C
and C–O bond formation reactions. Overall, the (TsMeN4)NiII(-
cycloneophyl) complex seems to be at the sweet spot of stability
vs. reactivity for C–O bond formation, while the (TsN4)NiII(cy-
cloneophyl) complex gives the highest yield of C–C coupled
product. All these results are very promising, suggesting that
organometallic Ni complexes supported by multidentate
ligands can exhibit bioinspired aerobic oxidation chemistry,
which could be further developed into catalytic aerobic oxida-
tive transformations.
Scheme 2 Proposed mechanism for the oxidatively-induced C–C and
C–O bond formation of 2 with H2O2 in 10% H2O/acetone and HClO4
as a proton source.
undergo C–C reductive elimination to generate the 1,1-dime-
thylbenzocyclobutene product and the [(TsMeN4)NiII(MeCN)2]2+
complex,35 which is observed in the ESI-MS as the [(TsMeN4)
NiII(ClO4)]+ species D. Analysis by cryo-ESI-MS of the oxidation
of complexes 1 and 3 with H2O2 also reveals the formation of the
persistent Ni complexes, although no high-valent Ni interme-
diates were observed in these cases, likely due to their decreased
stability.50 When comparing the product yields for complexes 1–
3 and the various oxidants employed, the general trend is
observed in which the less stable the high-valent Ni species are,
the higher are the yields of C–C and/or C–O bond formation
products. Overall, these results are very promising, suggesting
that with an appropriately tailored multidentate ligand the
corresponding organometallic Ni complexes can be oxidized
with mild oxidants such as O2 and H2O2 and promote C–C and
C–heteroatom bond formation reactivity that is usually possible
only under stringent anaerobic and anhydrous conditions.
Interestingly, high-valent Ni–oxo species supported by the
multidentate amine ligands have also been reported recently,56
lending support to the transient species proposed above. It is
also important to note that similar reactivity with O2 and H2O2
has been observed previously for both Pt and Pd complexes. In
Conflicts of interest
The authors declare no competing nancial interest.
Acknowledgements
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) and we thank Dr
Michael B. Watson for the EPR measurements. X. R. thanks the
`
Generalitat de Catalunya for an ICREA-Academia Award, and
`
Serveis Tecnics de Recerca, Universitat de Girona is acknowl-
edged for experimental support.
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26 V. Pandarus and D. Zargarian, Chem. Commun., 2007, 978–
980, DOI: 10.1039/b613812h.
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