Ni(I)-Alkyl Complexes Bearing Phenanthroline Ligands: Experimental Evidence for CO2Insertion at Ni(I) Centers

Article abstract of DOI:10.1021/jacs.0c04695

Although the catalytic carboxylation of unactivated alkyl electrophiles has reached remarkable levels of sophistication, the intermediacy of (phenanthroline)Ni(I)-alkyl species - complexes proposed in numerous Ni-catalyzed reductive cross-coupling reactions - has been subject to speculation. Herein we report the synthesis of such elusive (phenanthroline)Ni(I) species and their reactivity with CO2, allowing us to address a long-standing question related to Ni-catalyzed carboxylation reactions.

Full text of DOI:10.1021/jacs.0c04695

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Communication
Ni(I)–Alkyl Complexes Bearing Phenanthroline Ligands:
Experi-mental Evidence for CO2 Insertion at Ni(I) Centers
Rosie J Somerville, Carlota Odena, Marc Obst, Nilay Hazari, Kathrin H Hopmann, and Ruben Martin
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c04695 • Publication Date (Web): 10 Jun 2020
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Ni(I)–Alkyl Complexes Bearing Phenanthroline Ligands: Experimental
Evidence for CO2 Insertion at Ni(I) Centers
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Rosie J. Somerville,†¶ Carlota Odena,†¶‡ Marc F. Obst,¥‡ Nilay Hazari,£ Kathrin H. Hopmann¥* and Ru-
ben Martin†§*
† Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Av. Països Cata-
lans 16, 43007 Tarragona, Spain
¶ Departament de Química Analítica i Química Orgànica, Universitat Rovira i Virgili, c/Marcel·lí Domingo, 1, 43007 Tarra-
gona, Spain
§ ICREA, Passeig Lluís Companys, 23, 08010 Barcelona, Spain
¥ Hylleraas Center for Quantum Molecular Sciences, Department of Chemistry, UiT The Arctic University of Norway, N-
9307 Tromsø, Norway
£ Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, United States
Supporting Information Placeholder
Scheme 1. Carboxylation of Unactivated Alkyl Electro-
philes.
ABSTRACT: Although the catalytic carboxylation of unacti-
vated alkyl electrophiles has reached remarkable levels of so-
phistication, the intermediacy of (phenanthroline)Ni(I)-alkyl
species – complexes proposed in numerous Ni-catalyzed reduc-
tive cross-coupling reactions – has been subject to speculation.
Herein, we report the synthesis of such elusive (phenanthro-
line)Ni(I) species and their reactivity with CO2, allowing us to
address a long-standing question related to metal-catalyzed car-
boxylation reactions.
unactivated alkyl
(pseudo)halide
CO2 + 2 e-
CO2 + 2 e-
CO2H
X
Ha
Ha
n
CO2H
n
Ni
Ni
n
reaction at
initial site
reaction at
remote site
X = Br,Cl,OMs
catalytically
relevant ligands
non-catalytically
relevant ligands
R2
R2
P
R
R
N
N
vs
R
R
R
PR2
PR2
N
N
Over the last decade, Ni-catalyzed reductive carboxylation re-
actions involving organic (pseudo)halides and carbon dioxide
(CO2) have received considerable attention as methodologies
for the preparation of a range of synthetically useful carboxylic
acids.1 Amongst the wide variety of Ni-catalyzed reductive car-
boxylation reactions developed to date, the carboxylation of un-
activated alkyl (pseudo)halides possessing b-hydrogens was
found to be particularly challenging.2 This is likely due to the
propensity of the alkyl nickel intermediates that are formed via
C(sp3)–X scission (X = Br, Cl, OSO2R) to undergo unproduc-
tive reduction, b-hydride elimination, and homocoupling reac-
tions.3 Although nickel catalysts supported by (di)phosphine or
N-heterocyclic carbene ligands are routinely employed in a
myriad of Ni-catalyzed C–C and C–heteroatom bond-forming
reactions,4 only finely-tuned 1,10-phenanthroline derivatives
(phen ligands) have enabled the carboxylation of unactivated
alkyl electrophiles at either the initial C(sp3)–X site or at remote
C(sp3)–H bonds via chain-walking of the Ni catalyst along the
alkyl side chain (Scheme 1).2,5 Furthermore, a careful analysis
of the literature indicates that phen ligands are also crucial for a
wide number of Ni-catalyzed cross-couplings of unactivated al-
kyl halides, indicating that the importance of these ligands ex-
tends beyond carboxylation reactions.4a,6
R1
R1
Despite the significant advances realized in methodology de-
sign, the mechanism of the Ni-catalyzed reductive carboxyla-
tion of unactivated alkyl (pseudo)halides with CO2 is poorly un-
derstood. At present, our knowledge is primarily based on stud-
ies using aryl (pseudo)halides as substrates. These suggest a
pathway involving CO2 insertion into a (phen)Ni(I)–alkyl com-
plex as a crucial elementary step (Scheme 2, left).7,8 However,
it is worth noting that none of these (phen)Ni(I)–alkyl interme-
diates have been structurally characterized or even observed
spectroscopically, probably due to the fleeting nature and high
reactivity of these paramagnetic Ni(I) species.9 Elegant efforts
towards this goal have recently been described by Diao, culmi-
nating in the characterization of well-defined (diphos-
phine)Ni(I)–alkyl complexes and investigations into their reac-
tivity with CO2.10,11 Unfortunately, diphosphine ligands have
not been shown to facilitate the Ni-catalyzed carboxylation of
unactivated alkyl (pseudo)halides (Scheme 1).2,12 Therefore, a
study aimed at preparing well-defined Ni(I)–alkyl complexes
bearing catalytically-relevant phen ligands would represent: (a)
an opportunity to study the reactivity of elusive Ni(I)–alkyl
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complexes supported by nitrogen-donor ligands, (b) a founda-
chain walking carboxylation of alkyl bromides.2c We envi-
sioned that (L)Ni(I)–alkyl species could be accessed by alkyla-
tion of inner-sphere Ni(I)–halide complexes with an appropriate
organometallic reagent. However, at the outset of our investiga-
tions, it was unclear whether an inner-sphere (L)Ni(I)–halide
precursor could be obtained, as the most closely-related re-
ported species bearing a phen ligand was the outer-sphere hal-
ide complex [Ni(L)2]Cl, formed via oxidation of Ni(0)L2 (L =
2,9-dimethyl-phen) with AgCl.8,14,16 In order to avoid the syn-
thesis of Ni(0)L2 complexes and circumvent the purification
steps required to remove oxidation byproducts, we hypothe-
sized that inner sphere (L)Ni(I)X (X = Br, Cl) might be obtained
via comproportionation of (L)NiX2 with [Ni(COD)2] in the
presence of 1 equivalent of bulky L.15,16 This was indeed the
case, and deep blue (L)Ni(I)X species were obtained in high
yields (Figure 1, left). The presence of the inner-sphere halide
ligand was confirmed by X-ray crystallographic analysis of 1-
Cl and 2-Cl. In addition, axial EPR spectra of the four
(L)Ni(I)X complexes at 77 K support the presence of a Ni-cen-
tered radical. These results are noteworthy as they represent ex-
amples of Ni(I) complexes bearing phen ligands with the halide
directly coordinated to the Ni center.16,17 With a reliable route
to (L1,L2)Ni(I)X in hand, we turned our attention to accessing
the targeted Ni(I)–alkyl complexes via an alkylation event. An
initial survey of the stability of the resulting Ni(I)–alkyl prod-
ucts was carried out by monitoring these reactions using EPR
spectroscopy. As expected, the identity of the alkyl group, the
reaction temperature, and the ligand employed all influenced
the reaction outcome. For example, reactions with EtMgBr and
MeMgCl resulted in negligible amounts of new metal-centered
tion for investigating the mechanistic intricacies of catalytic re-
ductive carboxylation reactions, and (c) a starting point for un-
derstanding the speciation of Ni catalysts supported by phen lig-
ands in related cross-coupling processes and chain-walking re-
actions.4 Herein, we report the realization of these goals through
the synthesis and isolation of Ni(I)–alkyl complexes bearing
phen ligands, which has enabled us to obtain experimental evi-
dence for rapid CO2 insertion at Ni(I)–carbon bonds (Scheme 2,
right). These results not only shed light on a long-speculated
mechanistic step, but support efforts to exploit and expand the
reactivity of (phen)Ni(I)–alkyl intermediates through photore-
dox or electrochemical methodologies.2a,2b,4,13
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Scheme 2. Proposed Reductive Carboxylation Mechanism
via CO2 Insertion at Phen-Ligated Ni(I)–Alkyl Species.
this work: filling a gap
in our understanding of
carboxylation reactions
LnNiII
reduction
LnNiI
X
X
R2
R1
LnNi0
insertion
at Ni(I)–C
N
NiI
O
O
N
R2
CO2
R1
LnNiI
O
synthesis of LnNiI–alkyl
CO2 insertion at NiI–C
reduction
O
Our study began by establishing a route to Ni(I)–halide species
bearing phen ligands L1 or L2. The choice of these ligands was
not arbitrary, as having substituents adjacent to the nitrogen do-
nor atoms is critical in Ni-catalyzed reductive carboxylation re-
actions of unactivated alkyl (pseudo)halides.2 Steric shielding
from the bulky mesityl substituents of L1 may help to stabilize
our targeted Ni(I)–alkyl complexes, which are likely highly re-
active.9,14 Additionally, L2 is employed in the Ni-catalyzed
1
radicals, if any. Analysis of these reactions by H NMR spec-
troscopy indicated the presence of Ni(0)Ln complexes, suggest-
ing decomposition pathways arising from β-hydride elimina-
tion, reduction, and/or homolytic cleavage.18
R2
R1
Ni
R2
R1
Ni
R2
R1
Ni
MgBr
(1–2 equiv)
[Ni(COD)2]
L (1 equiv)
N
N
N
N
N
N
X
X
X
1-Br: Et2O/dioxane
−35 ºC, 10 min
2-Br: toluene
THF, rt
10 min – 1 h
R1
R1
R2
R2
R2
R1
<−100 °C, <1 min
L1 (R1=Mes; R2=H)
L2 (R1=C6H13; R2=Ph)
3 (R1=Mes; R2=H), 68%a (50%)
4 (R1=C6H13; R2=Ph), 57%a
L1: 78% 1-Br, 77% 1-Cl
L2: 95% 2-Br, 78% 2-Cl
X-ray structure
spin-density plot
1-Cl
2-Cl
3
Figure 1. Synthesis of Ni(I)–X and Ni(I)–Alkyl Complexes Bearing Phen Ligands. a Yield determined by EPR spectroscopy against
Cu(II) standards. All other yields are isolated yields (0.015 mmol scale for 3). X-ray structures with thermal ellipsoids drawn at 50% proba-
bility levels (see SI for details). Selected distances (Å) , angles (°): 1-Cl: Ni1–Cl1 2.1064(6). N1–Ni1–Cl1 140.24(6). N2–Ni1–Cl1 136.32(6).
N2–Ni1–N1 83.44(8). 2-Cl: Ni–Cl1 2.1417(9). N1–Ni–Cl1 133.80(8). N2–Ni–Cl1 142.61(9). 3: Ni–C1 1.961(3) Å, N1–Ni–C1 156.74(14),
C1–Ni–N2 114.25(13), N1–Ni–N2 82.69(11). Bottom right: calculated spin-density plot of 3 with spin population of 0.94 on Ni (PBE-
D3BJ/def2-TZVP, isovalue=0.01, Figure S33).
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Gratifyingly, the reactions of 1-Br and 2-Br with neopen-
of 3 to a T-shaped geometry is similar to that observed in a re-
lated diphosphine species [(dtbpe)Ni(CH2tBu)] (110.97(8)° and
157.82(8)°, dtbpe = 1,2-bis(di-tert-butylphosphino)ethane).20a
This geometry is electronically favored for a range of three-co-
ordinate Ni(I) complexes and differs from the Y-shaped geom-
etry of 1-Cl and 2-Cl.21,22 At present, we propose that the ge-
ometry of the latter complexes is due to the π-donating nature
of the chloride ligand that has been shown to favor Y-shaped
complexes.22 Alkylation of 2-Br at low temperature gave
[(L2)Ni(I)CH2tBu] (4) in a 57% yield as estimated by EPR
spectroscopy against a Cu(II) standard. Unfortunately, the ther-
mal instability of 4 prevented its isolation or characterization by
X-ray diffraction.
tylMgBr resulted in new rhombic EPR spectra, suggesting that
the desired alkylation may have taken place.19 Low-temperature
crystallization (–35 ºC, Et2O/pentane) furnished deep green
crystals suitable for X-ray diffraction and allowed us to identify
three-coordinate [(L1)Ni(I)CH2tBu] (3) (Figure 1, right). DFT
calculations support the Ni(I) description, with one unpaired
electron centered on Ni (Figure 1, Figure S33). The synthesis of
3 is particularly noteworthy; to the best of our knowledge, it
represents the first Ni(I)–alkyl complex to be obtained with a
catalytically relevant phen ligand. The Ni–C bond distance of
1.961(3) Å is similar to that of Ni(I) complexes bearing phos-
phine or NHC ligands.10,20 The Ni coordination plane is offset
by ca. 23° from the mean plane through L1, presumably due to
the steric bulk of the neopentyl ligand. Interestingly, the N–Ni–
C angles in 3 are 114.25(13)° and 156.74(14)°. The distortion
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O
KO
(2 equiv)
N
N
N
N
N
O
CO2
Ni Cl
Ni
Ni
2-MeTHF, rt, 30 min
2-MeTHF
O
N
thawed under
1 bar CO2
1-Cl
5
3
5 - anion exchange
5 - CO2 insertion
Figure 2. CO2 Insertion at Ni(I) Centers. Top: Anion metathesis reaction (left) and CO2 insertion into 3 (right) to generate 5; Bottom:
Changes to 77 K X-band EPR spectra of 1-Cl (left, gx = 2.084, gy = 2.119, gz = 2.461) after anion metathesis; and after CO2 insertion at 3
(right, gx = 2.065, gy = 2.145, gz = 2.519) to form 5 (center, gx = 2.299, gy = 2.272, gz = 2.064). See SI for simulated spectra.
Next, we turned our attention to investigating CO2 insertion into
the Ni(I)–C bond en route to Ni(I) carboxylate complexes, pro-
posed to be the key elementary step in the catalytic carboxyla-
tion of alkyl (pseudo)halides (Figure 2).7,8 Prior to these inser-
tion experiments, however, an anion metathesis reaction be-
tween 1-Cl and tBuCH2CO2K was performed to obtain refer-
ence EPR and IR spectra of the proposed CO2 insertion product
(Figure 2, left). Gratifyingly, spectroscopic analysis of the reac-
tion mixture showed the formation of a complex distinct from
both 3 and 1-Cl and supported the formation of Ni(I)–carbox-
ylate complex 5. For example, the band in the IR spectrum at
1543 cm-1 is suggestive of a νasym carboxylate stretch.23 Further-
more, although repeated attempts to crystallize 5 did not pro-
vide crystals suitable for X-ray diffraction, the observed stretch-
ing frequency combined with the absence of signals between
1200-1400 cm-1 suggests κ2-coordination of the carboxylate to
the Ni(I) center.23a This was supported by DFT calculations that
suggest a pseudotetrahedral geometry for 5 (Figure 3, right),
with a computed stretching frequency of 1484 cm-1 (Figure
S36). With these results in hand, we next investigated the reac-
tion between 3 and CO2 (1 bar) at –60 ºC (Figure 2, right). Anal-
ysis by EPR spectroscopy (77 K) showed the disappearance of
the rhombic signal of 3 and the appearance of a new pseudoax-
ial signal with gx, gy > gz that very closely resembles the spec-
trum of 5 (Figure 2, center). Importantly, comparison of the IR
spectrum of the product of direct CO2 insertion with the anion
metathesis product showed an identical νasymCO2 stretch at 1543
cm-1. Particularly illustrative was the disappearance of this sig-
nal and appearance of new signals at lower wavenumber when
the reaction was performed with 13CO2, evidence that 5 is
formed via CO2 insertion into the Ni(I)–C bond. Calculations
predict a 34 cm-1 shift to lower wavenumbers upon incorpora-
tion of 13C, consistent with the observed shift of 38 cm-1 to a
band at 1505
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Figure 3. Optimized TS geometries of the inner versus outer sphere insertion of CO2 into 3, and geometry of 5 (PBE-D3BJ/def2-
TZVP/IEFPCM, hydrogen atoms omitted for clarity, distances in Å, energies in kcal mol-1 relative to 3 + free CO2, 298.15 K).
cm-1 (Figure S18, S37). Insertion was also corroborated indi-
rectly by quenching in situ-generated 5 with dilute HCl and ob-
serving a 52% yield of tert-butylacetic acid (Scheme 3, top).
These observations are consistent with DFT calculations that
indicate facile CO2 insertion into the Ni(I)–C bond of 3, with a
free energy barrier of 7.7 kcal mol-1 relative to 3 and free CO2
(Figure 3, middle). Calculations argue against the formation of
a stable Ni-CO2 adduct before insertion (Figure S38), with in-
teractions between Ni and CO2 first becoming significant at the
carboxylation transition state (TS) where CO2 is significantly
bent (137°) and is interacting with Ni in an h2(C,O) fashion
(Figure 3, middle). Notably, an alternative outer sphere inser-
tion pathway where CO2 does not interact with the Ni center in
the transition state is predicted to have a barrier of 22.7 kcal
mol-1, a 15.0 kcal mol-1 penalty compared to the inner sphere
pathway (Figure 3, center).24 Although an inner sphere pathway
has been calculated for the Ni/PCp3-catalyzed reductive carbox-
ylation of benzyl halides,25 our data contrast with the outer
sphere pathway suggested for (Xantphos)Ni(I)-methyl and pin-
cer(PCP)-Ni(II)-methyl complexes.9,11 While one might argue
that the bulky neopentyl group in 3 disfavors an outer sphere
pathway, we note that the inner sphere pathway at the less-ste-
rically encumbered [(L1)Ni(I)Me] was still found to be favored
computationally, by 6.6 kcal mol-1 (Figure S39). This finding is
important as it supports the notion that the ancillary ligand can
influence the mechanism of CO2 insertion.24,26
Scheme 3. CO2 Insertion en route to tBuCH2CO2H.
from 3 + MgBr2
CO2 (1 bar)
1-Br
Et2O, −60 °C to rt
then
2M HCl
R2
R1
Ni
52% yield
N
N
CH2tBu
tBuCH2CO2H
from 4 + MgBr2
R2
R1
CO2 (1 bar)
Et2O, −60 °C to rt
3 (R1=Mes; R2=H)
4 (R1=C6H13; R2=Ph)
then
2M HCl
from (L)NiBr + tBuCH2MgBr (1:1)
2-Br
50% yield
In conclusion, we have investigated the synthesis and CO2 in-
sertion reactivity of Ni(I)–alkyl complexes bearing catalytically
relevant phen ligands. We have obtained experimental evidence
for the rapid insertion of CO2 into Ni(I)–C bonds, a long-pre-
sumed elementary step in the reductive carboxylation of alkyl
(pseudo)halides. Given the widespread use of phen ligands in
many Ni-catalyzed reactions, these results are expected to guide
new investigations into the catalytic relevance of Ni(I)–alkyl
complexes. Further investigations along these lines are cur-
rently underway in our laboratories.
ASSOCIATED CONTENT
Given the relevance of L2 in Ni-catalyzed carboxylation re-
actions, CO2 insertion at 4 was also studied. Although the sen-
sitivity of 4 prevented workup to remove MgBr2 – the byprod-
uct obtained by reacting (L2)Ni(I)Br with 1 equivalent of neo-
pentylMgBr – a 50% yield of tert-butylacetic acid was obtained
upon exposure of a cold solution of 4 initially to CO2 (1 bar)
and then to HCl (Scheme 3, bottom). Interestingly, this reaction
mixture rapidly turned blue upon CO2 addition and only 2-Br
was observed by EPR spectroscopy. This suggested that the L2
carboxylate complex [(L2)Ni(I)O2CCH2tBu] (6) resulting from
CO2 insertion at Ni(I) had undergone halide exchange with
MgBr2 to form blue 2-Br. This was confirmed by the addition
of MgBr2 to salt-free 5. Given the wide number of Ni-catalyzed
reductive coupling reactions that employ MgX2 (X = Br, Cl)
additives,27 the formation of 2-Br from in situ generated 4 pro-
vides support for the formation of Ni(I) halide complexes prior
to reduction to the propagating Ni(0)Ln species.28
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website.
Experimental procedures, spectra, crystallographic details, and
computational details. (PDF)
Cartesian coordinates of calculated structures (.xyz)
Crystallographic data for 1-Cl (.cif)
Crystallographic data for 2-Cl (.cif)
Crystallographic data for 3 (.cif)
AUTHOR INFORMATION
Corresponding Author
* rmartinromo@iciq.es; kathrin.hopmann@uit.no
Author contributions
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‡ These authors contributed equally.
Aliphatic Hydrocarbons with Carbon Dioxide. Nature
2017, 545, 84. (d) Börjesson, M.; Moragas, T.; Martin,
R. Ni-Catalyzed Carboxylation of Unactivated Alkyl
Chlorides with CO₂. J. Am. Chem. Soc. 2016, 138, 7504.
(e) Wang, X.; Liu, Y.; Martin, R. Ni-Catalyzed Divergent
Cyclization/Carboxylation of Unactivated Primary and
Secondary Alkyl Halides with CO2. J. Am. Chem. Soc.
2015, 137, 6476. (f) Liu, Y.; Cornella, J.; Martin, R. Ni-
Catalyzed Carboxylation of Unactivated Primary Alkyl
Bromides and Sulfonates with CO₂. J. Am. Chem. Soc.
2014, 136, 11212.
Funding Sources
No competing financial interests have been declared.
ACKNOWLEDGMENT
R.M., R.J.S., and C.O. thank ICIQ, FEDER/MICIU − AEI/
PGC2018- 096839-B-100, and funding from “la Caixa” Foun-
dation (ID 100010434) under the agreement LCF/BQ/
SO15/52260010 for financial support. R.J.S. sincerely thanks
“la Caixa” for a predoctoral fellowship. NH acknowledges sup-
port from the NIHGMS under Award Number R01GM120162.
K.H.H and M.O. acknowledge the Research Council of Norway
(Grant 262695), the Tromsø Research Foundation (Grant
TFS2016KHH), NordForsk (Grant No. 85378) and Notur
(Grants nn9330k, nn4654k). We also gratefully acknowledge
the X-ray diffraction Research Support Area at ICIQ, Dr. Bran-
don Q. Mercado at Yale University for solving the structure of
1-Cl, and Dr. Georgiana Stoica from the Spectroscopy and Ma-
terial Characterization Unit at ICIQ for her help with EPR stud-
ies. We also thank Dr. Megan Mohadjer Beromi and Dave
Charboneau for insightful discussions.
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3. For selected reviews on the use of unactivated alkyl
electrophiles in cross-coupling reactions, see: (a) Kambe,
N.; Iwasaki, T.; Terao, J. Pd-Catalyzed Cross-Coupling
Reactions of Alkyl Halides. Chem. Soc. Rev. 2011, 40,
4937. (b) Hu, X. Nickel-Catalyzed Cross-Coupling of
Non-Activated Alkyl Halides:
A
Mechanistic
Perspective. Chem. Sci. 2011, 2, 1867. (c) Jana, R.;
Pathak, T. P.; Sigman, M. S. Advances in Transition
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4. For excellent authoritative reviews on Ni catalysis, see:
(a) Diccianni, J. B.; Diao, T. Mechanisms of Nickel-
Catalyzed Cross-Coupling Reactions. Trends Chem.
2019, 1, 830. (b) Richmond, E.; Moran, J. Recent
Advances in Nickel Catalysis Enabled by Stoichiometric
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Ananikov, V. P. Nickel: The “Spirited Horse” of
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Tasker, S.; Standley, E.; Jamison, T. Recent Advances in
Homogeneous Nickel Catalysis. Nature 2014, 509, 299.
5. For a recent review on Ni-catalyzed chain-walking
reactions: Janssen-Müller, D.; Sahoo, B.; Sun, S.-Z.;
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6. Selected reviews on Ni-catalyzed cross-coupling
reactions including those that make use of phen-type
ligands: (a) Goldfogel, M.J., Huang, L. and Weix, D.J.
(2019). Cross-Electrophile Coupling. In Nickel Catalysis
in Organic Synthesis, S. Ogoshi (Ed.), Wiley Online
Library, 2019; pp 183–222. (b) Gu, J.; Wang, X.; Xue,
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Alkyl Halides with Other Electrophiles: Concept and
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1411. (c) Hu, X. Nickel-Catalyzed Cross Coupling of
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17. 2-Br and L1 were competent as precatalyst and ligand,
respectively, for the chain-walking carboxylation of 2-
bromoheptane. See Supporting Information and ref. 2c
for details.
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Mediated by a (Xantphos)NiI–Alkyl Species. Angew.
Chem. Int. Ed. 2019, 58, 13865.
18. It is worth noting that Ni(L2)2 was detected when L2
complexes were employed (see ref. 2c). For reactions
with L1-bearing complexes, unusual [Ni(L1)]3 and
[Ni(L4)]4 trimers and tetramers crystallised from the re-
action mixtures (see the Supporting Information for de-
tails).
11. For examples of CO2 insertion at PCP and PCN
(pincer)Ni(II)–Me complexes, see: (a) Mousa, A. H.;
Polukeev, A. V.; Hansson, J.; Wendt, O. F.
Carboxylation of the Ni−Me Bond in an Electron-Rich
Unsymmetrical PCN Pincer Nickel Complex.
Organometallics, 2020, 39 (9), 1553. (b) Mousa, A. H.;
Bendix, J.; Wendt, O. F. Synthesis, Characterization, and
Reactivity of PCN Pincer Nickel Complexes.
Organometallics 2018, 37 (15), 2581–2593. (c)
Jonasson, K. J.; Wendt, O. F. Synthesis and
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Reactivity Pathways. J. Am. Chem. Soc. 2005, 127,
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a Family of POCOP Pincer
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Nickel Complexes. Chem. Commun. 2011, 47, 1824.
12. For control experiments using tBuXantphos in the
catalytic carboxylation of unactivated alkyl halides, see
the Supporting Information.
13. For reviews on Ni-catalyzed reductive cross-coupling
reactions: (a) Diccianni, J.; Lin, Q.; Diao, T. Mechanisms
of Nickel-Catalyzed Coupling Reactions and
Applications in Alkene Functionalization. Acc. Chem.
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Considerations. Org. Chem. Front. 2015, 3, 1411. (c)
Weix, J. D. Methods and Mechanisms for Cross-Elec-
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philes. Acc. Chem. Res. 2015, 48, 1767. (d) Moragas, T.;
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actions between Two Electrophiles Chem. Eur. J. 2014,
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U.; Meder, M.; Ganz, A.; Michel, C.; Herderich, M.
Synthesis of Sterically Encumbered 2,9-Diaryl
Substituted Phenanthrolines. Key Building Blocks for
the Preparation of Mixed (Bis-Heteroleptic)
Phenanthroline Copper(I) Complexes. Heterocycl.
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Stable Mixed Phenanthroline Copper(I) Complexes. Key
Building Blocks for Supramolecular Coordination
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15. During the course of our studies, a comproportionation
route to inner-sphere Ni(I)-halide complexes was
reported: Zarate, C.; Yang, H.; Bezdek, M. J.; Hesk, D.;
Chirik, P. J. Ni(I)–X Complexes Bearing a Bulky α-
Diimine Ligand: Synthesis, Structure, and Superior
Catalytic Performance in the Hydrogen Isotope
Exchange in Pharmaceuticals. J. Am. Chem. Soc. 2019,
141, 5034.
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Group 9 and 10 Metal-Element σ Bonds. Inorg. Chem.
2017, 56, 13655.
25. Sayyed, F. B.; Sakaki, S. The crucial roles of MgCl2 as a
non-innocent additive in the Ni-catalyzed carboxylation
of benzyl halide with CO2 Chem. Commun. 2014, 50,
13026.
26. For an example where the nature of the ligand affects the
CO2 insertion pathway, see ref. 7a.
27. Selected examples where MgX2 additives are employed
in Ni-catalyzed reductive cross-coupling reactions: (a)
Ye, Y.; Chen, H.; Yao, K.; Gong, H. Iron-Catalyzed
Reductive Vinylation of Tertiary alkyl Oxalates with
Activated Vinyl Halides. Org. Lett. 2020, 22, 2070 (b)
Gao, M.; Sun, D.; Gong, H. Ni-Catalyzed Reductive C–
O Bond Arylation of Oxalates Derived from a-Hydroxy
Esters with Aryl Halides. Org. Lett. 2019, 21, 1645. (c)
Wang, X.; Ma, G.; Peng, Y.; Pitsch, C. E.; Moll, B. J.;
Ly, T. D.; Wang, X.; Gong, H. Ni-Catalyzed Reductive
16. During the course of our studies, an inner-sphere bromide
complex bearing a bulky bipyridine ligand was reported
via reduction of a Ni(II) complex: Lin, Q.; Diao, T.
Mechanism
of
Ni-Catalyzed
Reductive
1,2-
Dicarbofunctionalization of Alkenes. J. Am. Chem. Soc.
2019, 141, 17937.
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Coupling of Electron-Rich Aryl Iodides with Tertiary
Carboxylation of Benzyl Halides J. Am. Chem. Soc.
2013, 135, 1221.
28. Charboneau, D. J.; Brudvig, G. W.; Hazari, N.; Lant, H.
M. C.; Saydjari, A. K. Development of an Improved
System for the Carboxylation of Aryl Halides through
Mechanistic Studies. ACS Catal. 2019, 9, 3228.
Alkyl Halides. J. Am. Chem. Soc. 2018, 140, 14490. (d)
Fujihara, T.; Horimoto, Y.; Mizoe, T.; Sayyed, F. B.;
Tani, Y.; Terao, J.; Sakaki, S.; Tsuji, Y. Nickel-
Catalyzed Double Carboxylation of Alkynes Employing
Carbon Dioxide. Org. Lett. 2014, 16, 4960. (e) Leon, T.;
Correa, A.; Martin, R. Nickel-Catalyzed Direct
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Ni(I)–alkyl complexes
R1
X
X-ray, IR
EPR
R1
R
inner-sphere
CO2 insertion
R2
CO2
H
R
+
stoichiometric
reactions
theoretical
calculations
R2
CO2
CO2 insertion at Ni(I)
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