Synthesis and Reactivity of (N2P2)Ni Complexes Stabilized by a Diphosphonite Pyridinophane Ligand

Article abstract of DOI:10.1021/acs.organomet.1c00003

A series of (N2P2)Ni^IIcomplexes (N2P2 =P,P′-ditertbutyl-2,11-diphosphonito[3.3](2,6)pyridinophane) stabilized by a modified tetradentate pyridinophane ligand containing two phosphonite groups were synthesized and characterized. Cyclic voltammetry (CV) studies revealed the accessibility of the Ni^Ioxidation state at moderate redox potentials for these Ni^IIcomplexes.In situEPR, low-temperature UV-vis, and electrochemical studies were employed to detect the formation of Ni^Ispecies during the reduction of Ni^IIprecursors. Furthermore, the [(N2P2)Ni^I(CNt-Bu)](SbF₆) complex was isolated upon reduction of the Ni^IIprecursor with 1 equiv of CoCp₂and was characterized by EPR and X-ray photoelectron spectroscopy (XPS). Finally, the (N2P2)Ni^IIBr₂complex acts as an efficient catalyst for the Kumada cross-coupling of an aryl halide with an aryl or alkyl Grignard, suggesting that the N2P2 ligand can support the various Ni species involved in the catalytic C-C bond formation reactivity.

Full text of DOI:10.1021/acs.organomet.1c00003

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Article
Synthesis and Reactivity of (N2P2)Ni Complexes Stabilized by a
Diphosphonite Pyridinophane Ligand
Kei Fuchigami, Michael B. Watson, Giang N. Tran, Nigam P. Rath, and Liviu M. Mirica*
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ABSTRACT: A series of (N2P2)NiII complexes (N2P2 = P,P′-
ditertbutyl-2,11-diphosphonito[3.3](2,6)pyridinophane) stabilized by
a modified tetradentate pyridinophane ligand containing two
phosphonite groups were synthesized and characterized. Cyclic
voltammetry (CV) studies revealed the accessibility of the NiI
oxidation state at moderate redox potentials for these NiII complexes.
In situ EPR, low-temperature UV−vis, and electrochemical studies
were employed to detect the formation of NiI species during the
reduction of NiII precursors. Furthermore, the [(N2P2)NiI(CNt-
Bu)](SbF6) complex was isolated upon reduction of the NiII precursor
with 1 equiv of CoCp2 and was characterized by EPR and X-ray
photoelectron spectroscopy (XPS). Finally, the (N2P2)NiIIBr2
complex acts as an efficient catalyst for the Kumada cross-coupling of an aryl halide with an aryl or alkyl Grignard, suggesting
that the N2P2 ligand can support the various Ni species involved in the catalytic C−C bond formation reactivity.
tetradentate ligand P,P′-ditertbutyl-2,11-diphosphonito[3.3]-
(2,6)pyridinophane (N2P2) and investigated the redox
properties and reactivity of its Ni complexes. Reported herein
are the synthesis, characterization, and reactivity studies of a
series of (N2P2)Ni complexes, including a rare [(N2P2)-
NiI(CNt-Bu)](SbF6) (6) complex. Interestingly, we have also
observed that the (N2P2)NiIIBr2 (1) complex can catalyze the
Kumada cross-coupling of an aryl halide with an aryl or alkyl
Grignard, suggesting that the N2P2 ligand can support the
various Ni species that are involved in the catalytic C−C bond
formation reactivity.
INTRODUCTION
■
In addition to Pd-catalyzed transformations, Ni-based catalytic
systems have recently been developed that can promote
Negishi, Kumada, and Suzuki cross-coupling reactions.1−10
Contrary to the extensively studied Pd-mediated trans-
formations that employ diamagnetic intermediates, Ni has
been found to undergo both one- and two-electron redox
reactions leading to additional reaction pathways. Although
there is evidence that Pd can also undergo similar redox
reactions, Ni has been more commonly accepted to involve the
+1 and +3 oxidation states in the aforementioned cross-
coupling reactions.11−20 The presence of these paramagnetic
species has made the characterization of these reactive species
more difficult,21−25 yet a detailed understanding of these
paramagnetic Ni complexes will lead to the development of
more efficient and selective catalysts for Ni-mediated cross-
coupling reactions.
EXPERIMENTAL DETAILS
■
Reagents and Materials. All manipulations were carried out
under a nitrogen atmosphere using standard Schlenk and glovebox
techniques if not indicated otherwise. All reagents for which the
synthesis is not given were commercially available from Sigma-
Aldrich, Fischer, or Strem and were used as received without further
purification. Solvents were purified prior to use by passing through a
column of activated alumina using an MBRAUN SPS. Ni(DME)Br2
(DME = ethylene glycol dimethyl ether) was prepared according to a
modified literature procedure.42
In the past several years we have reported the isolation and
characterization of mononuclear organometallic NiIII com-
plexes stabilized by tetradentate N4 ligands (RN4 = N,N′-
R
i
dialkyl-2,11-diaza[3.3](2,6)pyridinophane, R = Me, Pr, or
tBu) as well as its C-donor derivative RN3C− ligands (R = tBu,
tert-butyl, or neopentyl (Np)) that can undergo C−C and C−
heteroatom bond formation reactions.26−36 These results
suggest that NiIII complexes are more common than previously
anticipated; thus, it prompted us to now target the low-valent
NiI species using a slightly modified ligand system. Inspired by
the many examples of PONOP-type pincer metal com-
plexes,37−41 we have developed the N- and P-donor
Special Issue: Organometallic Solutions to Challenges
in Cross-Coupling
Received: January 2, 2021
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Physical Measurements. 1H (300.121 MHz) NMR spectra were
recorded on a Varian Mercury-300 spectrometer. Solution magnetic
susceptibility measurements were obtained by the Evans method43
using coaxial NMR tubes at 293 K. Diamagnetic corrections were
applied as previously described.44 UV−vis spectra were recorded on a
Varian Cary 50 Bio spectrophotometer and are reported as λmax, nm
(ε, M−1 cm−1). The low-temperature UV−vis measurements were
performed using a fiber-optic immersion probe (Hellma, path length 1
mm or 10 mm). EPR spectra were recorded on a JEOL JES-FA X-
band (9.2 GHz) EPR spectrometer at 77 or 298 K. EPR spectra
simulation and analysis were performed using Bruker WINEPR
SimFonia program, version 1.25. ESI-MS experiments were performed
using a Thermo FT or Bruker Maxis Q-TOF mass spectrometer with
an electrospray ionization source. Elemental analyses were carried out
by the Columbia Analytical Services Tucson Laboratory. Cyclic
voltammetry (CV) experiments were performed with a BASi EC
Epsilon electrochemical workstation or a CHI 660D Electrochemical
Analyzer.
resulting solid was washed with ether and dried in vacuo. The product
[(N2P2)Ni(MeCN)2](SbF6)2 was isolated as a green solid. Yield: 66
mg, 61%. Elemental analysis: found, C 26.27, H 2.93 N 5.70%.
Calculated C22H30F12N4NiO4PsSb2, C 26.25, H 3.00, N 5.57%.
Synthesis of [(N2P2)Ni(OTf)2] (3). The complex was prepared
under N2. To a solution of (N2P2)NiBr2 (15 mg, 0.024 mmol) in
MeCN was added AgOTf (13 mg, 0.048 mmol). After stirring for 1 h,
the solution turns green and was filtered and concentrated. Toluene
was layered on the solution, and slow evacuation resulted in purple
crystals. The crystals were washed with ether, and the isolated crystals
were dried in vacuo. The product [(N2P2)Ni(OTf)2] was isolated as a
purple solid. Yield: 17 mg, 90%. Elemental analysis: found, C 31.76, H
2.88 N 3.48%. Calculated C20H24F6N2NiO10P2S2, C 31.98, H 3.22, N
3.73%.
Synthesis of [(N2P2)Ni(CNt-Bu)](SbF6)2 (4). The complex was
prepared under N2. Into a solution of [(N2P2)Ni(MeCN)2](SbF6)2
(42 mg, 0.0435 mmol) in MeCN, t-BuNC (4.9 μL, 0.0435 mmol) was
added. After stirring for 30 min, the solution turns a darker green and
was filtered. Ether was added to the solution to crash out pink/purple
crystals. The crystals were washed with ether, and the isolated crystals
were dried in vacuo. The product [(N2P2)Ni(CNt-Bu)](SbF6)2 was
isolated as a pink solid. Yield: 33.6 mg, 77%. 1H NMR (CD3CN, 300
MHz), δ (ppm): 1.87 (s, 27H, t-Bu), 7.20 (d, 4H, Py Hmeta), 8.14 (br,
2H, Py Hpara). Elemental analysis: found, C 27.09, H 2.95 N 4.21%.
Calculated C23H33F12N3NiO4P2Sb2, C 27.41, H 3.30, N 4.17%.
EPR Studies of the NiI Species. An EPR tube was charged with a
solution of the Ni complex in 1:3 MeCN/PrCN (v/v) and immersed
into liquid nitrogen. Another solution containing 1 equiv of chemical
reductant in the same solvent mixture was quickly added, mixed, and
frozen in the EPR tube. An initial EPR spectrum was taken at 77 K.
The sample was then carefully warmed up for 10−30 s to allow the
two layers to further mix, quickly refrozen, and the EPR spectrum was
recorded. The warming up step was repeated if necessary.
Electrochemical Measurements. Electrochemical-grade
Bu4NClO4 (Fluka) was used as the supporting electrolyte. Electro-
chemical measurements were performed in the N2-filled glovebox or
under a blanket of nitrogen, and the analyzed solutions were
deaerated by purging with nitrogen. A glassy carbon disk electrode (d
= 1.6 mm) was used as the working electrode, and a Pt wire was used
as the counter electrode. A Ag wire pseudoreference electrode or a
Ag/0.01 M AgNO3/MeCN electrode was used as the reference
electrode. The nonaqueous reference electrode was calibrated against
Cp2Fe (Fc). Caution! Perchlorate salts are potentially explosive and
should be handled with appropriate care only in small quantities.
Synthesis of N2P2 Ligand. 2,6-Dihydroxypyridine hydrochloride
(Aldrich, 500 mg, 3.388 mmol) and 100 mL of THF was combined in
a Schlenk flask equipped with a rubber septum. The solution was
precooled. To the above mixture, n-butyl lithium (Aldrich 2.5 M in
hexane, 4 mL, 10.00 mmol) was added dropwise while stirring. The
reaction mixture was brought to room temperature and stirred for 1.5
h. After the solution became cloudy, a phosphine solution of p,p-
dichloro-tertbutylphosphine (Aldrich 1.0 M in Et2O, 3.2 mL, 3.227
mmol) in 100 mL of THF was prepared and added dropwise. The
reaction mixture was further stirred for 24 h. The solvent was then
removed under vacuum, leaving an oily yellow solid. The solid was
extracted into pentane and filtered. The filtrate collected was a
mixture of ∼6:1 dimer (N2P2)/tetramer (N4P4) based on NMR
(Figure S1), while the undissolved solid was mostly tetramer. The
solvent of the filtrate was removed via vacuum to give a white solid as
the crude product. The crude product can be further purified by
another extraction in n-pentane to give the final product that contains
Low-Temperature UV−Vis Studies of the [(N2P2)NiI(CNt-
Bu)]+ (6) Complex. A 5 mL aliquot of 4 (0.5 mM) in MeCN was
prepared, and UV−vis spectra were recorded initially only for 4 and
monitored immediately after addition of 1 equiv of CoCp2 in order to
observe 6 in solution.
X-ray Structure Determination. Crystals of appropriate
dimensions were mounted on MiTeGen cryoloops in random
orientations. Preliminary examination and data collection were
performed using a Bruker Kappa Apex II or SMART Apex II Charge
Coupled Device (CCD) Detector system single-crystal X-ray
diffractometers equipped with an Oxford Cryostream LT device. All
data were collected using graphite monochromated Mo Kα radiation
(λ = 0.71073 Å) from a fine-focus sealed-tube X-ray source.
Preliminary unit cell constants were determined with a set of 36
narrow frame scans. Typical data sets consist of combinations of ω
and ϕ scan frames with typical scan width of 0.5 and counting time of
15−30 s/frame at a crystal to detector distance of 3.5−5.0 cm. The
collected frames were integrated using an orientation matrix
determined from the narrow frame scans. Apex II and SAINT
software packages45 were used for data collection and data integration.
Analysis of the integrated data did not show any decay. Final cell
constants were determined by global refinement of xyz centroids for
the complete data set. The collected data were corrected for
systematic errors using SADABS45 based on the Laue symmetry
using equivalent reflections. Crystal data and intensity data collection
parameters as well as additional details of structure refinement are
given in the Supporting Information. Structure solution and
refinement were carried out using the SHELXTL-PLUS software
package.46 The structure was solved by direct methods or Patterson
method and refined successfully in the space groups listed below. Full
1
at least 95% N2P2 (Figure S2). Yield: 488 mg, 73%. H NMR (300
MHz, (CD3)2CO) δ (ppm): 1.23 (d, J = 13.1 Hz, 18H, t-Bu), 6.73 (d,
J = 7.8 Hz, 4H, Py Hmeta), 7.81 (t, J = 7.8 Hz, 2H, Py Hpara). 13C NMR
(500 MHz, CDCl3) δ (ppm): 24.09 (t-Bu), 24.24 (t-Bu), 104.84
(Py), 141.64 (Py), 159.72 (Py). 31P NMR (500 MHz, CDCl3) δ
(ppm): 159.69. ESI-MS (m/z): 395.1304. Calculated for
[C18H24N2O4P2]H+: 395.1290.
Synthesis of (N2P2)NiBr2 (1). The complex was prepared under
N2. A solution of Ni(DME)Br2 (21.2 mg, 0.068 mmol) was added to
a solution of N2P2 (31 mg, 0.079 mmol) in DCM. The mixture
turned dark red. After stirring for 1 h, the solution was filtered and
concentrated for recrystallization by pentane diffusion. Black crystals
formed, and the supernatant was removed. The resulting solid was
washed with pentane and dried in vacuo. The product (N2P2)NiBr2
1
was isolated as a black solid. Yield: 28 mg, 67%. H NMR (CD3CN,
300 MHz), δ (ppm): 1.51 (s, 18H, t-Bu), 6.79 (d, 8H, Py Hmeta), 7.82
(br, 2H, Py Hpara). Elemental analysis: found, C 34.35, H 4.06 N
4.00%. Calculated C18H26Br2N2NiO5P2, C 34.27, H 4.15, N 4.44%.
Synthesis of [(N2P2)Ni(MeCN)2](SbF6)2 (2). The complex was
prepared under N2. A solution of AgSbF6 (76 mg, 0.221 mmol) was
added to a solution of (N2P2)NiBr2 (68 mg, 0.111 mmol) in MeCN.
After stirring for 15 min, the mixture turned green and the solution
was filtered and concentrated for recrystallization by ether diffusion.
Green crystals formed, and the supernatant was removed. The
2
matrix least-squares refinement was carried out by minimizing Σw(FO
− FC )2. The non-hydrogen atoms were refined anisotropically to
2
convergence.
General Procedure for the Kumada Cross-Coupling with
(N2P2)NiBr2, 1. Inside a nitrogen-filled glovebox, a small vial
equipped with a magnetic stir bar was charged with the corresponding
alkyl halide or aryl halide substrate (0.1 mmol), decane as internal
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standard, and 1 (3 mg, 0.05 equiv) in THF (5.0 mL). To the stirring
solution, the Grignard reagent (1.2 equiv) was added slowly over 1 h
via syringe and the resulting solution was allowed to stir at room
temperature for 24 h. At 1, 2, and 24 h an aliquot (500 μL) was taken.
Then, each reaction mixture was quenched with 5 mL of saturated
NH4Cl solution, extracted into diethyl ether (15 mL), and the organic
layer was separated and dried over MgSO4. The yield of product(s)
was obtained by GC/FID using the internal standard for calibration
and is an average of at least 3 independent runs, and the identity of
the products was confirmed by GC/MSD.
bromide abstraction by 2 equiv of AgSbF6 from (N2P2)NiBr2
(1). The complex [(N2P2)Ni(OTf)2] (3) could not be
synthesized by reacting Ni(OTf)2 with N2P2, but instead, it
was prepared through bromide abstraction by 2 equiv of
AgOTf from 1. The product obtained by the reaction of 2 with
1 equiv of t-BuNC in MeCN was found to be the
monoisocyanide complex [(N2P2)Ni(CNt-Bu)](SbF6)2 (4)
in the solid state, according to single-crystal X-ray analysis
(Figure 1). Interestingly, the addition of 2 equiv of t-BuNC did
not yield the bisisocyanide complex, and instead it resulted in
the decomposition of the complex, likely due to the loss of the
N2P2 ligand. All complexes were isolated and purified by
recrystallization and characterized by spectroscopic methods.
The effective magnetic moments were measured in CDCl3 for
1 and CD3CN for 2 and 3 at room temperature and their
values were consistent with the expected value for a high-spin
octahedral complex. Complexes 1, 2, and 3 each had a
magnetic moment of 2.4 μB, 3.0 μB, and 2.3 μB, respectively.
Complexes 1 and 2 were further characterized by elemental
analysis.
RESULTS AND DISCUSSION
■
Synthesis and Structural Characterization of (N2P2)
Ni Complexes 1−4. Tetradentate diphosphonite pyridino-
phane ligand N2P2 was synthesized using modified procedures
reported for noncyclic PNP pincer ligands.37,47−49 The crude
product was obtained as a mixture of the tetramer N4P4 and
dimer N2P2 (Figure S1), and purification by multiple
extractions into n-pentane yielded desired pure N2P2 (Figure
S2).50 The (N2P2)NiII complexes 1−4 (Scheme 1) were
Scheme 1. Synthesis of (N2P2)NiII Complexes
X-ray quality crystals for complexes 1−4 were obtained by
slow diffusion of pentane into concentrated solution of CH2Cl2
for 1 and slow diffusion of Et2O into concentrated solutions in
MeCN for 2−4. The ORTEP plots and selected metrical
parameters are given in Figure 1. The X-ray crystal structures
of 1−3 reveal a distorted octahedral coordination at the Ni
center, with the axial Ni−P bonds tilted toward the ligand (P−
Ni−P = 148.9°, 1; 150.46°, 2; 152.04°, 3). The average Ni−N
bond lengths for 1−3 are 2.102, 2.080, and 2.054 Å,
respectively. This trend of decreasing metal−ligand bond
length is due to the decreasing strength of trans influence of
the ligands (Br > MeCN > OTf). The X-ray crystal structure of
4 revealed a distorted trigonal bipyramidal coordination.
Similar to 1−3, the axial Ni−P bonds tilt toward the ligand
(P−Ni−P = 157.8°), albeit to a lesser extent given the reduced
coordination number that leads to less steric repulsion.
Electrochemical Studies. The electrochemical properties
of (N2P2)NiII complexes were studied by CV (Table 1 and
Figures S6−S9).50 CV data of 1−4 in 0.1 M Bu4NClO4/
MeCN show quasi-reversible redox waves for NiI/II (Table 1
and Figures S6−S9). On the basis of these reduction
potentials, complex 4 seems to be the easiest to be reduced,
followed by 2, 1, and 3, which is in line with the increasing
synthesized in 60−90% yield by using common NiII precursors
and synthetic procedures. The complex (N2P2)NiBr2 (1) was
obtained through reaction of Ni(DME)Br2 with 1 equiv of
N2P2 in CH2Cl2/pentane. The complex [(N2P2)Ni-
(MeCN)2](SbF6)2 (2) was conveniently synthesized by
Figure 1. ORTEP representations (50% thermal ellipsoids) of the X-ray crystal structures of 1−4. Selected bond distances (Å): 1, Ni1−Br1 =
2.4869(5), Ni1−Br1_i = 2.4871(5), Ni1−N1 = 2.102(2), Ni1−N1_i = 2.102(2), Ni1−P1 = 2.3138(8), Ni1−P1_i = 2.3137(8); 2, Ni1−N3 =
2.0374(13), Ni1−N4 = 2.0544(14), Ni1−N1 = 2.0881(12), Ni1−N2 = 2.0717(12), Ni1−P1 = 2.3271(4), Ni1−P2 = 2.3301(4); 3, Ni1−O3 =
2.0429(14), Ni1−O3_i = 2.0428(14), Ni1−N1 = Ni1 = N1_i = 2.0535(16), Ni1−P1 = Ni1−P1_i = 2.3140(5); 4, Ni1−C10 = 1.80(2), Ni1−N1
= Ni1−N1_i = 1.963(9), Ni1−P1 = Ni1−P1_i = 2.145(3).
C
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Table 1. Cyclic Voltammetry (CV) Data for (N2P2)Ni
Complexes
complex
1
2
3
4
a
EI/II1/2, V
−0.875
−0.750
−1.170
−0.630
ΔE, mV
150
220
140
220
a
EI/II1/2, V (ΔEp, mV); Epa values reported vs Fc at room temperature,
100 mV/s scan rate.
charge of the NiII complex and the soft nature of the CNt-Bu
ligand.
Electron Paramagnetic Resonance (EPR) Studies. The
one-electron reduction of complex 3 was carried out by
chemical reduction with 1 equiv of cobaltocene (CoCp2) to
generate the NiI species [(N2P2)NiI(OTf)(MeCN)] (5). The
EPR spectrum (77K, 1:3 MeCN/PrCN glass) of 5 reveals a
pdeudo-axial signal corresponding to a NiI, d9 metal center
with a S = 1/2 ground state where the unpaired electron has
predominant dz2 character and gave = 2.05940 (Figure 2). The
Figure 2. EPR spectrum (red line) of [(N2P2)NiI(OTf)(MeCN)]
(5) in 3:1 PrCN/MeCN glass at 77 K, and the simulated EPR
spectrum (blue line) using the following parameters: gx = 2.007 (A2P
= 21 G; A1N = 22 G), gy = 1.994 (A2P = 11 G; A1N = 33 G), gz = 2.177
(A2P = 100 G; A1N = 10 G). Frequency: 9096.337 MHz.
Figure 3. (a) EPR spectrum (red line) of [(N2P2)Ni(CNt-
Bu)](SbF6) (6) in 3:1 PrCN:MeCN glass, 77 K, and the simulated
EPR spectrum (blue line) using the following parameters: gx = 2.098
(A2P = 182 G), gy = 2.060 (A2P = 199 G), gz = 2.017 (A2P = 198 G).
Frequency: 9092.747 MHz. (b) EPR spectrum (red line) of
[(N2P2)Ni(CNt-Bu)](SbF6) (6) in 3:1 PrCN:MeCN glass, room
temperature, and the simulated EPR spectrum (blue line) using the
following parameters: gave = 2.070 (A2P = 186 G). Frequency:
8879.542 MHz. (c) DFT-calculated spin density for 6, shown as a
0.03 isodensity contour plot.
EPR spectrum revealed superhyperfine coupling (A2P = 11−
100 G, I = 1/2) to the two axial P donors, and also to the
single equatorial N donor (A1N = 10−33 G, I = 1). It is
expected that in solution the triflate anions can be replaced by
MeCN solvent molecules, which will provide the N atom that
can undergo superhyperfine coupling to the NiI center. Thus,
we propose the formation of a 5- or 4-coordinate [(N2P2)-
NiI(MeCN)]+ species in solution, with the N2P2 ligand bound
in a κ3 or κ4 conformation.
One-electron reduction of complex 4 was also carried out by
chemical reduction with 1 equiv of CoCp2 to generate the NiI
species [(N2P2)NiI(CNt-Bu)]+ (6). The EPR spectrum (77K,
1:3 MeCN:PrCN glass) of 6 reveals a rhombic signal
corresponding to a NiI, d9 metal center with an S = 1/2
ground state where the unpaired electron has predominant dz2
character and gave = 2.0585 (Figure 3a). The EPR spectrum
revealed strong superhyperfine coupling (A2P = 182−199 G, I
= 1/2) to the two axial P donors, suggesting an appreciable
delocalization of the unpaired electron onto the phosphorus
atoms of the N2P2 ligand, as supported by the DFT-calculated
spin density plot (Figure 3c). Similarly, large phosphorus
coupling ranging from 18−31 G has been reported for nickel
complexes stabilized by PCP pincer ligand, where Kozhanov et
al. speculates that the phosphorus coupling reflects the amount
of contribution from a group metal−pincer orbital to the
orbital containing the unpaired electron.51,52 Therefore, the
strong accepting t-BuNC ligand leads to a greater super-
hyperfine coupling constant versus the OTf or MeCN ligands
in 5. The stronger interaction with the P atoms could also lead
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to a preferred trigonal bipyramidal geometry for this NiI
species.
The NiI EPR of 6 can be observed for up to 1 h at room
temperature, although at 45 min the signal has decreased
significantly (>80%). A room-temperature EPR (298 K, 1:3
MeCN/PrCN glass) was also prepared where the sample
prepared at low-temperature was quickly thawed to room
temperature and was immediately put into the instrument. The
room temperature EPR also revealed strong superhyperfine
coupling (A2P = 185.8 G) to the two P donors with g =
2.06992 (Figure 3b).
Isolation of the (N2P2)NiI Complex 6. The surprising
stability of 6 allowed for its isolation. Into a solution of 4, 1
equiv of CoCp2 was added at −70 °C, and the solution was
stirred for 30 min as its color changed from pink to purple. The
dark purple solution was filtered, and a dark purple solid was
precipitated using diethyl ether. The resulting solid was dried
in vacuo, and the product of a NiI species was confirmed via
EPR. While X-ray quality crystals of 6 could not be obtained
despite several attempts, the complex was further analyzed by
X-ray photoelectron spectroscopy (XPS). The resulting XPS
spectrum shows a decrease in the Ni 2p3/2 and 2p1/2 binding
energies of ∼1.39 eV between 4 and 6 and is consistent with
the presence of a more reduced Ni center in 6 versus 4 (Figure
4).53
Figure 5. (a) In situ UV−vis spectra of the reduction of 4 (0.5 mM,
black line) with 1 equiv of CoCp2 in MeCN at −45 °C: (blue line)
initial spectrum before adding CoCp2 at −45 °C; (red line) spectrum
at room temperature after adding CoCp2. (b) Experimental and
normalized calculated TD-DFT UV−vis spectrum of 6.
2
unpaired electron has predominant dz character (Figure 3c),
in line with the observed EPR spectrum.
Kumada Cross-Coupling Reactivity. Since the CV
studies revealed that 1 can be oxidized and reduced in situ
to the NiIII and NiI species, respectively (Figure S6),50 we
proposed that 1 may be an active catalyst for the Kumada
cross-coupling of aryl iodides with aryl or alkyl Grignard
reagents. Excitingly, the reaction of iodotoluene with phenyl-
magnesium bromide or 1-hexylmagnesium bromide affords the
corresponding coupled products in 99 and 95% unoptimized
yields, respectively (Scheme 2). By contrast, a lower yield of
Figure 4. X-ray photoelectron spectra of the Ni binding energy region
for complexes 4 (black line) and 6 (red line). Selected binding
energies (eV), 4: 2p3/2, 854.88; 2p1/2, 872.18; 6: 2p3/2, 853.66; 2p1/2
,
870.94.
To further probe the formation of a NiI species, the
reduction reaction of 4 to yield 6 was carried out in MeCN at
low temperature and monitored by UV−vis spectroscopy. The
spectrum reveals two bands at 543 and 468 nm that appear
after the addition of CoCp2 at −45 °C, followed by their decay
during the next few minutes as the cell was warmed to room
temperature (Figure 5a). The time-dependent density func-
tional theory (TD-DFT) calculated UV−vis spectrum and
transitions of 6 show similar transitions to the experimental
spectrum (Figure 5b) providing support for the proposed
trigonal bipyramidal geometry of 6 with N2P2 ligand binding
in a κ4 conformation.
Scheme 2. Kumada Cross-Coupling Reactions Catalyzed by
a
1
Further insight into the electronic properties of 6 was
obtained by DFT calculations. The geometry optimized
structure of 6 reveals a NiI center in a trigonal bipyramidal
geometry that adopts an S = 1/2 ground state in which the
a
Yields were determined using GC-FID with decane as the internal
standard; no coupled products were observed in the absence of 1.
E
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Article
Notes
17% was observed for the reaction of octyl-iodide with
hexylMgBr, while the reaction of chlorotoluene with phenyl-
magnesium bromide gave the coupled product in 41% only.50
Similar results were observed recently using (MeN4)NiMe2
complex,36 but we were pleasantly surprised to see complex 1
had a yield (95%) for the cross-coupling reaction with
hexylMgBr much higher than the 60% yield observed with
the (MeN4)NiMe2 complex. Further mechanistic studies are in
progress to further probe the catalytic reactivity of these
(N2P2)Ni complexes.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Department of Energy’s BES Catalysis Science
Program (DE-SC0006862) for financial support. We thank
Sagnik Chakrabarti for assistance with the synthesis of N2P2
and Bailey Bouley with the rerefinement of the X-ray structure
of 4.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.1c00003.
■
sı
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AUTHOR INFORMATION
Corresponding Author
Liviu M. Mirica − Department of Chemistry, University of
Illinois at Urbana−Champaign, Urbana, Illinois 61801,
United States; orcid.org/0000-0003-0584-9508;
Email: mirica@illinois.edu
■
Authors
Kei Fuchigami − Department of Chemistry, Washington
University, St. Louis, Missouri 63130-4899, United States
Michael B. Watson − Department of Chemistry, Washington
University, St. Louis, Missouri 63130-4899, United States
Giang N. Tran − Department of Chemistry, University of
Illinois at Urbana−Champaign, Urbana, Illinois 61801,
United States
Nigam P. Rath − Department of Chemistry and Biochemistry,
University of Missouri-St. Louis, St. Louis, Missouri 63121-
4400, United States
Complete contact information is available at:
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