Synthesis, Characterization, and Reactivity of PCN Pincer Nickel Complexes

Article abstract of DOI:10.1021/acs.organomet.8b00333

New diamagnetic nickel(II) complexes based on an unsymmetrical (1-(3-((ditert-butylphosphino)methyl)phenyl)-N,N-dimethyl-methanamine) (PCN) pincer ligand were synthesized and characterized by ¹H, ³¹P{¹H}, and ¹³C{¹H} NMR spectroscopy. Their molecular structures were confirmed by X-ray diffraction. Oxidation to high-valent paramagnetic Ni(III) dihalide complexes was achieved through straightforward reaction of the corresponding diamagnetic halide complexes with anhydrous CuX₂ (X = Cl, Br). In agreement with this, the complexes are active in Kharasch addition of CCl₄ to olefins. The reaction of the hydroxo complex (8) and the amido complex (11) with CO₂ produced the hydrogen carbonate and carbamate complexes, respectively. The hydrogen carbonate complex was converted to the dinuclear nickel carbonate complex (10). The methyl (13), phenyl (14), and p-tolylacetylide (15) complexes are also described in the current study providing the first example of the hydrocarbyl nickel complexes based on an unsymmetric aromatic pincer ligand. Furthermore, the reactivity of the methyl complex toward different electrophiles has been investigated, showing that C-C bond formation is possible with aryl halides, whereas the reaction with CO₂ is sluggish.

Full text of DOI:10.1021/acs.organomet.8b00333

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthesis, Characterization, and Reactivity of PCN Pincer Nickel
Complexes
Abdelrazek H. Mousa,† Jesper Bendix,‡ and Ola F. Wendt*,†
†Centre for Analysis and Synthesis, Department of Chemistry, Lund University, P.O. Box 124, S-221 00 Lund, Sweden
‡Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark
S
* Supporting Information
ABSTRACT: New diamagnetic nickel(II) complexes based
on an unsymmetrical (1-(3-((ditert-butylphosphino)methyl)-
phenyl)-N,N-dimethyl-methanamine) (PCN) pincer ligand
were synthesized and characterized by 1H, 31P{1H}, and
13C{1H} NMR spectroscopy. Their molecular structures were
confirmed by X-ray diffraction. Oxidation to high-valent
paramagnetic Ni(III) dihalide complexes was achieved
through straightforward reaction of the corresponding
diamagnetic halide complexes with anhydrous CuX2 (X =
Cl, Br). In agreement with this, the complexes are active in
Kharasch addition of CCl4 to olefins. The reaction of the hydroxo complex (8) and the amido complex (11) with CO2
produced the hydrogen carbonate and carbamate complexes, respectively. The hydrogen carbonate complex was converted to
the dinuclear nickel carbonate complex (10). The methyl (13), phenyl (14), and p-tolylacetylide (15) complexes are also
described in the current study providing the first example of the hydrocarbyl nickel complexes based on an unsymmetric
aromatic pincer ligand. Furthermore, the reactivity of the methyl complex toward different electrophiles has been investigated,
showing that C−C bond formation is possible with aryl halides, whereas the reaction with CO2 is sluggish.
ligand, which was extensively studied by Zargarian and more
recently by Miller.40−44 The more electron-rich PCN pincer
ligand45,46 with a phosphine arm is expected to undergo more
facile cyclometalation with nickel compared to the POCN
ligand, a hypothesis which is supported by a previous study
showing that phosphine based ligands undergo faster cyclo-
metalation of nickel compared to that of phosphinite based
ones.34 The presence of bulky substituents on the phosphine
arm could enhance the stability of the resulting nickel
complexes and help to isolate catalytically relevant inter-
mediates, e.g., hydrocarbyl complexes, which surprisingly have
not been reported with unsymmetric aromatic pincer nickel
complexes. It could also influence the hemilability of the
nitrogen arm. Thus, we here continue our work on
unsymmetric pincer ligands45,46 and expand the family of
unsymmetric nickel pincer complexes to include also PCN
complexes, cf. Chart 1. The new complexes undergo rapid
oxidation reactions to give the corresponding air and moisture
stable nickel(III) complexes, and we present the first examples
of hydrocarbyl, hydroxide, and amide nickel complexes with an
unsymmetric aromatic pincer architecture. The reactivity of
these complexes with small molecules and other electrophiles
is also reported.
INTRODUCTION
■
Late transition metal complexes incorporating pincer ligand
architectures have been established as an important class of
organometallic compounds.1−12 At the early stage of develop-
ment of these complexes, nickel complexes based on aromatic
PCP ligands13 have been reported followed by NCN pincer
nickel complexes14−18 and aliphatic PCP complexes.19−23 On
the basis of the difference in electronic and steric properties of
PCP and NCN ligands the preparation of the corresponding
nickel complexes were based on different strategies: PCP
nickel complexes are accessible through direct C−H activation
of the corresponding PCP ligand, while lithiation of the NCN
ligand is required to get the NCN nickel complexes. The
differences between the two types of ligands extended further
and were shown to include the outcome of both stoichiometric
and catalytic reactions. For example, the use of an NCN pincer
ligand enabled the isolation of high-valent nickel complexes
and greatly enhanced the catalytic reactivity of their
corresponding divalent nickel halide complexes in the
Kharasch addition reaction of polyhalogenated hydrocarbons
to olefins,14−18,24 while PCP pincer nickel complexes engaged
in the activation of small molecules as a result of the strong
coordination of the PCP ligand.22,23,25−27 Consequently, a
variety of symmetric pincer nickel complexes have been
synthesized and employed in homogeneous catalysis.28−39 In
contrast, unsymmetric aromatic pincer nickel complexes are
less represented, and the reported examples are mainly based
on the unsymmetric POCN-type phosphinite amine pincer
Received: May 18, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00333
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Chart 1. This and Previous Work
the solvent evaporated under reduced pressure yielding 434 mg
(96%) of complex 3 as a yellowish green solid. Single crystals suitable
for X-ray diffraction analysis were prepared by slow diffusion of n-
hexane into a DCM solution of 3 at 5 °C. 1H NMR (C6D6): δ = 7.00
(td, 3JHH = 7.4 Hz, 1.1 Hz, 1H), 6.78 (d, 3JHH = 7.5 Hz, 1H), 6.58 (d,
3JHH = 7.4 Hz, 1H), 3.29 (s, 2H), 2.81(d, 2JHP = 8.9 Hz, 2H), 2.49 (d,
4JHP = 1.4 Hz, 6H), 1.39 (d, 3JHP = 13.0 Hz, 18H). 13C{1H} NMR: δ
EXPERIMENTAL SECTION
■
General Procedures and Materials. All experiments were
carried out under an atmosphere of nitrogen or argon using glovebox,
Schlenk, or high-vacuum-line techniques unless otherwise noted.47
Solvents were vacuum-transferred to the reaction vessel from sodium/
benzophenone ketyl radical, except for methanol, which was dried
over magnesium activated with iodine and dichloromethane, obtained
from an MBRAUN (MB-SPS 800) dry solvent dispenser. 1H,
13C{1H}, and 31P{1H} NMR spectra were recorded on a Varian
Unity INOVA 500 spectrometer operating at 499.77 MHz (1H) or a
Bruker Avance 400 FT-NMR spectrometer operating at 400.1 MHz
(1H). Chemical shifts are given in ppm downfield from TMS using
residual solvent peaks (1H and 13C) or H3PO4 (31P) as reference.
Multiplicities are abbreviated as follows: (s) singlet, (d) doublet, (t)
triplet, (q) quartet, (m) multiplet. Elemental analyses were performed
2
3
= 156.9 (d, JCP = 28.4 Hz), 149.8 (s), 149.6 (d, JCP = 17.1 Hz),
125.1 (s), 121.6 (d, 3JCP = 16.9 Hz), 119.2 (d, 4JCP = 1.2 Hz), 71.9 (d,
3JCP = 1.8 Hz), 49.6 (d, 3JCP = 1.2 Hz), 35.3 (d, 1JCP = 30.7 Hz), 34.6
1
2
(d, JCP = 14.0 Hz), 29.7 (d, JCP = 3.0 Hz). 31P{1H} NMR: δ =
86.1(s). Anal. Found (calcd for C18H31BrNPNi): C, 50.36 (50.16); H,
7.47 (7.25); N, 3.13 (3.25).
Synthesis of [PCN]Ni(III)−Cl2 (4). To a solution of 11.6 mg (0.03
mmol) of 1 in 2 mL of DCM was added 4.0 mg (0.03 mmol) of
anhydrous CuCl2 immediately forming a red colored solution and
white precipitate. The reaction was left stirring for 1 h. Filtration over
Celite and evaporation of the solvent under reduced pressure yielded
11.8 mg (93%) of the product as a red solid. Single crystals suitable
for X-ray diffraction analysis were prepared by slow diffusion of n-
hexane into a DCM solution of 4 at 5 °C. Anal. Found (calcd for
C18H31Cl2NPNi): C, 51.10 (51.23); H, 7.16 (7.40); N, 3.12 (3.32).
Synthesis of [PCN]Ni (III)−Br2 (5). To a solution of 12.9 mg
(0.03 mmol) of 3 in 2 mL of DCM was added 6.7 mg (0.03 mmol) of
anhydrous CuBr2 immediately forming a deep brown colored solution
and white precipitate. The reaction was left stirring for 1 h. Filtration
over Celite and evaporation of the solvent under reduced pressure
yielded 11 mg (71.8%) of the product as a fluffy violet crystalline
solid. Single crystals suitable for X-ray diffraction analysis were
prepared by slow diffusion of n-hexane into a DCM solution of 5 at 5
°C. Anal. Found (calcd for C18H31Br2NPNi): C, 42.19 (42.31); H,
6.39 (6.12); N, 2.69 (2.74).
by H. Kolbe Microanalytisches Laboratorium, Mulheim an der Ruhr,
̈
Germany. The t-butyl- and methyl-substituted PCN-H ligand ((1-(3-
((di-tert-butylphosphino)methyl)phenyl)-N,N-dimethylmethanamine,
henceforth called PCN-H) was synthesized following the synthetic
procedures we published previously.45
Synthesis of [PCN]Ni−Cl (1). Method A. DMAP (128.2 mg, 1.05
mmol) was added, in the glovebox, to a stirred mixture of the (PCN)-
H ligand (308 mg, 1.05 mmol) and anhydrous NiCl2 (203.9 mg, 1.57
mmol) in toluene (15 mL) in a Straus flask. The flask was closed, and
the reaction mixture stirred for 24 h at 100 °C. After cooling to room
temperature, the mixture was filtered over Celite leaving a green
colored solid and the filtrate evaporated under reduced pressure
giving 257 mg (63%) of complex 1 as a yellow crystalline solid.
Method B. Et3N (0.28 mL, 2.04 mmol) was added, in the glovebox,
to a stirred mixture of the (PCN)-H ligand (286 mg, 0.97 mmol) and
anhydrous NiCl2 (126 mg, 0.97 mmol) in THF (15 mL) in a Straus
flask. The flask was closed, and the reaction mixture stirred for 24 h at
85 °C. After cooling to room temperature, the mixture was filtered
over Celite and the filtrate evaporated under reduced pressure giving
325 mg (86.5%) of complex 1 as a yellow crystalline solid. Single
crystals suitable for X-ray diffraction analysis were prepared by slow
Synthesis of [PCN]Ni−ONO2 (6). 1 (77.3 mg, 0.20 mmol) and
AgNO3 (51.0 mg. 0.30 mmol) were dissolved in 15 mL of THF. The
mixture was stirred for 48 h, and the flask was protected from light by
aluminum foil. After evaporation of the solvent, the residue was
dissolved in benzene, filtered, and the filtrate evaporated yielding 71.9
mg (87%) of the product as an orange solid. Single crystals suitable
for X-ray diffraction analysis were prepared by slow diffusion of
1
diffusion of n-hexane into a DCM solution of 1 at 5 °C. H NMR
3
3
(C6D6): δ = 6.99 (dd, JHH = 7.4, 1H), 6.77 (d, JHH = 7.4 Hz, 1H),
6.58 (d, 3JHH = 7.1 Hz, 1H), 3.31 (s, 2H), 2.80 (d, 2JHP = 8.7 Hz, 2H),
2.46 (s, 6H), 1.39 (d, 3JHP = 12.9 Hz, 18H). 13C{1H} NMR: δ = 155.4
1
pentane into a benzene solution of 6 at 5 °C. H NMR (500 MHz,
3
3
2
3
C6D6) δ = 6.94 (td, JHH = 7.4, 0.9 Hz, 1H), 6.65 (d, JHH = 7.5 Hz,
(d, JCP = 28.9 Hz), 149.9 (s), 149.7 (d, JCP = 17.4 Hz), 124.9 (s),
1H), 6.47 (d, 3JHH = 7.3 Hz, 1H), 3.16 (s, 2H), 2.63 (d, 2JHP = 8.8 Hz,
121.7 (d, 3JCP = 16.8 Hz), 119.1 (s), 72.0 (s), 48.9 (s), 34.9 (d, 1JCP
=
4
3
1
2
30.7 Hz), 34.4 (d, JCP = 13.8 Hz), 29.5 (d, JCP = 2.9 Hz). 31P{1H}
NMR: δ = 85.3 (s). Anal. Found (calcd for C18H31ClNPNi): C, 55.87
(55.93); H, 8.15 (8.08); N, 3.65 (3.62).
2H), 2.24 (d, JHP = 1.2 Hz, 6H), 1.20 (d, JHP = 13.2 Hz, 18H).
13C{1H} NMR: δ = 150.2 (d, JCP = 16.7 Hz), 149.7 (s), 149.5 (d,
3
2JCP = 28.9 Hz), 125.4 (s), 122.0 (d, 3JCP = 17.0 Hz), 119.7 (d, 4JCP
=
2.0 Hz), 70.3 (d, 3JCP = 2.2 Hz), 47.8 (d, 3JCP = 1.4 Hz), 34.0 (d, 1JCP
= 13.1 Hz), 33.0 (d, 1JCP = 32.8 Hz), 28.9 (d, 2JCP = 3.9 Hz). 31P{1H}
NMR: δ = 83.8(s). Anal. Found (calcd for C18H31N2NiO3P): C,
52.31 (52.33); H, 7.37 (7.56); N, 6.50 (6.78).
Synthesis of [PCN]Ni−Br (3). To a solution of the (PCN)-H
ligand (308 mg, 1.05 mmol) in 20 mL of THF was added (DME)
NiBr2 (324.1 mg, 1.05 mmol) inside the glovebox, immediately
forming an intense green color. Then, Et3N (0.3 mL, 2.1 mmol) was
added to the reaction mixture. The Straus flask was sealed, and the
reaction mixture stirred for 24 h at 70 °C. After cooling to room
temperature, THF and all volatiles were removed under reduced
pressure and the solid dissolved in benzene, filtered over Celite, and
Synthesis of [PCN]Ni−TFA (7). 1 (58 mg, 0.015 mmol) and
AgTFA (35 mg, 0.016 mmol) were dissolved in 10 mL of THF. The
mixture was stirred for 48 h, and the flask was protected from light by
aluminum foil. After evaporation of the solvent, the residue was
B
DOI: 10.1021/acs.organomet.8b00333
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
dissolved in Et2O, filtered, and the filtrate evaporated yielding 69 mg
(99%) of the product as a yellow solid. Single crystals suitable for X-
ray diffraction analysis were obtained from Et2O at −20 °C. 1H NMR
(400 MHz, C6D6) δ = 6.94 (td, 3JHH = 7.4, 1.3 Hz, 1H), 6.67 (d, 3JHH
= 7.5 Hz, 1H), 6.48 (d, 3JHH = 7.4 Hz, 1H), 3.13 (s, 2H), 2.63 (d, 2JHP
Synthesis of [PCN]Ni−OCONH2 (12). In a J. Young NMR tube,
10 mg of 11 was dissolved in 0.5 mL of C6D6 inside the glovebox. The
tube was degassed (three freeze−pump−thaw cycles) on the high-
vacuum line, and the solution was pressurized with 4 atm of CO2
giving the carbamate complex and an unknown side product.
Complex 12 could not be isolated pure. 1H NMR (400 MHz,
C6D6): δ = 6.99 (td, 3JHH = 7.4, 1.1 Hz, 1H), 6.74 (d, 3JHH = 7.4 Hz,
1H), 6.56 (d, 3JHH = 7.3 Hz, 1H), 4.00 (s, 2H), 3.32 (s, 2H), 2.73 (d,
4
3
= 8.9 Hz, 2H), 2.26 (d, JHP = 1.5 Hz, 6H), 1.22 (d, JHP = 13.2 Hz,
18H). 13C{1H} NMR: δ = 161.7 (d, 2JCP = 35.2 Hz, CO), 150.5 (d,
3JCP = 29.2 Hz), 150.0 (d, 3JCP = 16.7 Hz), 149.6 (s), 125.3 (s), 121.9
3
4
1
4
3
2JHP = 8.7 Hz, 2H), 2.57 (d, JHP = 0.8 Hz, 6H), 1.38 (d, JHP = 12.9
(d, JCP = 16.9 Hz), 119.5 (d, JCP = 2.0 Hz), 117.1 (q, JCF = 291.0
Hz, CF3), 70.6 (d, 3JCP = 2.2 Hz), 48.1 (s), 33.9 (d, JCP = 13.1 Hz),
1
2
Hz, 18H). 13C{1H} NMR: δ = 162.9 (s), 152.9 (d, JCP = 30.2 Hz),
33.2 (d, 1JCP = 32.9 Hz), 28.9 (d, 2JCP = 3.7 Hz). 31P{1H} NMR: δ =
84.4(s). 19F NMR: δ = −75.0(s). Anal. Found (calcd for
C20H31F3NNiO2P): C, 51.95 (51.76); H, 6.83 (6.73); N, 3.11 (3.02).
Synthesis of [PCN]Ni−OH (8). 6 (61.9 mg, 0.15 mmol) and
ground KOH (168.3 mg, 3 mmol) were dissolved in 20 mL of THF.
The mixture was sonicated for 5 h and then was left stirring overnight.
After evaporating the solvent, the residue was dissolved in benzene,
filtered inside the glovebox, and the filtrate evaporated yielding 49 mg
(89%) of the product 8 as a pale yellow solid. Further reactivity
indicates the presence of NaNO3 as an impurity in 8 (see the “Results
4
3
150.1 (s), 150.0 (d, JCP = 2.4 Hz), 124.7 (s), 121.7 (d, JCP = 16.7
1
Hz), 119.3 (s), 71.1 (s), 48.2 (s), 34.0 (d, JCP = 12.4 Hz), 33.7 (d,
1JCP = 32.1 Hz), 29.2 (d, 2JCP = 3.8 Hz). 31P{1H} NMR: δ = 81.8 (s).
Synthesis of [PCN]Ni−Me (13). Method A. MeLi (40 μL of a 1.6
M solution in Et2O, 0.064 mmol, 2 equiv) was transferred to a J.
Young NMR tube inside the glovebox, and the solvent was evaporated
on the high-vacuum line. The tube was introduced again to the
glovebox where 0.5 mL of C6D6 and 12.3 mg of 1 were added. The
methyl complex was formed immediately. Quenching the excess of
the MeLi by water led to decomposition of the product.
1
and Discussions” section for further details). H NMR (500 MHz,
Method B. A solution of 51.4 mg (0.12 mmol) of 3 in 10 mL of
THF was cooled down to −78 °C, and a slight excess of MeMgCl (3
M in THF) was added dropwise. The reaction was left stirring
overnight. After evaporation of the solvent, the residue was
redissolved in n-hexane, filtered over Celite inside the glovebox, and
the filtrate evaporated giving the product as a yellow solid. For further
purification, the product was redissolved in n-hexane and washed with
degassed water under a nitrogen atmosphere to remove any traces of
inorganic salts. The organic layer was extracted and dried over
anhydrous Na2SO4 followed by filtration and evaporation of the
solvent. Single crystals suitable for X-ray diffraction analysis were
prepared from hexane at −20 °C inside the glovebox. Yield: 39 mg
3
3
C6D6) δ = 7.02 (td, JHH = 7.4, 1.0 Hz, 1H), 6.83 (d, JHH = 7.5 Hz,
1H), 6.66 (d, 3JHH = 7.3 Hz, 1H), 3.49 (s, 2H), 2.83 (d, 2JHP = 8.8 Hz,
4
3
2H), 2.61 (d, JHP = 1.3 Hz, 6H), 1.29 (d, JHP = 12.6 Hz, 18H),
−2.56 (s, 1H). 13C{1H} NMR: δ = 160.2 (d, JCP = 27.8 Hz), 149.9
2
(d, 4JCP = 1.9 Hz), 149.1 (d, 3JCP = 17.2 Hz), 123.7 (s), 121.3 (d, 3JCP
= 17.1 Hz), 118.8 (d, 4JCP = 1.7 Hz), 72.4 (d, 3JCP = 1.9 Hz), 48.0 (d,
3JCP = 2.0 Hz), 35.5 (d, 1JCP = 32.1 Hz), 33.7 (d, 1JCP = 12.3 Hz), 29.4
(d, JCP = 4.0 Hz). 31P{1H} NMR: δ = 84.9(s) . Anal. Found (calcd
2
for C18H32NNiOP): C, 59.37 (58.73); H, 8.99 (8.76); N, 3.48 (3.80).
Synthesis of [PCN]Ni−OCO2H (9). In a J. Young NMR tube, 10
mg of (PCN)Ni−OH (7) was dissolved in 0.5 mL of C6D6 inside the
glovebox. The tube was degassed (three freeze−pump−thaw cycles)
on the high-vacuum line, and the solution was pressurized with 4 atm
1
3
(89%). H NMR (500 MHz, C6D6) δ = 7.13 (td, JHH = 7.3, 1.2 Hz,
3
3
1H), 7.05 (d, JHH = 7.4 Hz, 1H), 6.86 (d, JHH = 7.2 Hz, 1H), 3.54
1
of CO2 giving bicarbonate complex 9 and 7% nitrate complex 6. H
(s, 2H), 3.10 (d, 2JHP = 8.6 Hz, 2H), 2.26 (d, 4JHP = 1.3 Hz, 6H), 1.28
3
NMR (500 MHz, C6D6): δ = 13.26 (bs, 1H), 6.94 (t, JHH = 7.3 Hz,
(d, JHP = 12.2 Hz, 18H), −0.70 (d, JHP = 2.2 Hz, 3H). 13C{1H}
3
3
3
3
1H), 6.69 (d, JHH = 7.4 Hz, 1H), 6.50 (d, JHH = 7.3 Hz, 1H), 3.24
NMR: δ = 174.3 (d, 2JCP = 22.4 Hz), 148.1 (d, 4JCP = 1.5 Hz), 148.0
(s, 2H), 2.69 (d, 2JHP = 8.7 Hz, 2H), 2.43 (s, 6H), 1.34 (d, 3JHP = 13.0
3
3
(d, JCP = 16.9 Hz), 124.0 (s), 120.7 (d, JCP = 16.2 Hz), 118.3 (d,
Hz, 18H). 13C{1H} NMR: δ = 162.8 (s), 151.7 (d, JCP = 29.8 Hz),
2
4JCP = 1.2 Hz), 75.3 (d, JCP = 1.2 Hz), 48.2 (d, JCP = 1.3 Hz), 39.7
3
3
3
3
150.2 (d, JCP = 17.2 Hz), 149.9 (s), 124.9 (s), 121.7 (d, JCP = 16.8
(d, 1JCP = 30.9 Hz), 34.4 (d, 1JCP = 12.9 Hz), 29.9 (d, 2JCP = 3.7 Hz),
Hz), 119.3 (d, 4JCP = 1.5 Hz), 70.9 (d, JCP = 1.9 Hz), 48.2 (s), 34.0
3
−7.9 (d, JCP = 24.2 Hz). 31P{1H} NMR: δ = 90.1(s). Anal. Found
2
(d, 1JCP = 12.7 Hz), 33.4 (d, 1JCP = 32.2 Hz), 29.2 (d, 2JCP = 3.8 Hz).
31P{1H} NMR: δ = 82.5 (s).
(calcd for C19H34NPNi): C, 63.15 (62.33); H, 10.14 (9.36); N, 3.60
(3.83).
Synthesis of {[PCN]Ni}2(μ-CO2) (10). Decarboxylated product 10
was obtained either by crystallization of the bicarbonate complex at
−20 °C from pentane after evaporation of C6D6 or by decarboxylation
of complex 9 on the high-vacuum line (in C6D6) and removing the
volatiles. 1H NMR (400 MHz, C6D6): δ= 7.01 (t, 3JHH = 6.7 Hz, 2H),
Synthesis of [PCN]Ni−Ph (14). Method A. In a J. Young NMR
tube, 20 μL (0.04 mmol, 2.0 M in THF) of PhMgCl was added to 8.6
mg (0.02 mmol) of complex 3 in 0.5 mL of C6D6 inside the glovebox,
and the reaction was monitored by 31P{1H} NMR spectroscopy until
all the starting material was consumed, giving 14 as the sole product
according to the 31P{1H} NMR spectra. Quenching the excess of the
Grignard reagent by water led to decomposition of the product.
Method B. A solution of 43.1 mg (0.1 mmol) of 3 in 10 mL of
THF was cooled down to −78 °C, and a slight excess of PhMgCl (2.0
M in THF) was added dropwise. The reaction was left stirring
overnight. After evaporating the solvent, the residue was dissolved in
n-hexane, filtered over Celite inside the glovebox, and the filtrate
evaporated giving 38.5 mg (90%) of 14 as a yellow solid. Single
crystals suitable for X-ray diffraction analysis were prepared from n-
hexane at −20 °C inside the glovebox. 1H NMR (500 MHz, C6D6): δ
3
3
6.78 (d, JHH = 6.7 Hz, 2H), 6.63 (d, JHH = 6.6 Hz, 2H), 3.44 (s,
2
3
4H), 2.93 (br. s, 12H), 2.75 (d, JHP = 8.3 Hz, 4H), 1.47 (d, JHP
=
12.6 Hz, 36H). 13C{1H} NMR: δ = 166.1 (s), 154.6 (d, JCP = 30.7
2
4
3
Hz), 150.4 (d, JCP = 2.7 Hz), 150.2 (s), 124.3 (s), 121.5 (d, JCP
=
1
16.4 Hz), 118.9 (s), 72.0 (s), 48.8 (s), 33.9 (s), 33.8 (d, JCP = 12.2
Hz), 29.5 (d, JCP = 3.3 Hz). 31P{1H} NMR: δ = 79.6 (s).
2
Synthesis of [PCN]Ni−NH2 (11). 3 (43.1 mg, 0.1 mmol) and
NaNH2 (39.03 mg, 1 mmol) were mixed inside the glovebox in 20
mL of THF and vacuum transferred to the reaction flask. The mixture
was sonicated for 5 h and then was left stirring overnight. After
evaporation of the solvent, the residue was dissolved in benzene,
filtered, and the filtrate evaporated giving the product as a red sticky
3
3
= 8.08 (d, JHH = 6.7, 2H), 7.27 (t, JHH = 7.4, 2H), 7.11 (m, 2H),
3
3
1
3
7.04 (d, JHH = 7.4 Hz, 1H), 6.82 (d, JHH = 7.2 Hz, 1H), 3.46 (s,
solid in 95% yield. H NMR (400 MHz, C6D6): δ = 7.06 (td, JHH
=
2
3
7.4, 1.3 Hz, 1H), 6.92 (d, 3JHH = 7.4 Hz, 1H), 6.75 (d, 3JHH = 7.3 Hz,
1H), 3.49 (s, 2H), 2.95 (d, 2JHP = 8.7 Hz, 2H), 2.37 (d, 4JHP = 1.3 Hz,
6H), 1.32 (d, 3JHP = 12.4 Hz, 18H), −1.80 (s, 2H). 13C{1H} NMR: δ
2H), 3.12 (d, JHP = 8.4 Hz, 2H), 1.95 (s, 6H), 1.19 (d, JHP = 12.5
Hz, 18H). 13C{1H} NMR: δ = 174.8 (d, JCP = 25.0 Hz), 171.9 (d,
2
3JCP = 23.9 Hz), 149.3 (d, JCP = 1.7 Hz), 148.6 (d, JCP = 16.6 Hz),
140.3 (s), 125.9 (d, 4JCP = 1.4 Hz), 124.6 (s), 121.9 (s), 121.0 (d, 3JCP
= 16.1 Hz), 118.5 (d, 4JCP = 1.6 Hz), 75.0 (d, 3JCP = 1.5 Hz), 49.2 (d,
3JCP = 1.6 Hz), 39.1 (d, 1JCP = 31.4 Hz), 35.2 (d, 1JCP = 13.7 Hz), 30.1
(d, 2JCP = 3.7 Hz). 31P{1H} NMR δ = 88.3(s). Anal. Found (calcd for
C24H36NPNi): C, 66.74 (67.32); H, 8.65 (8.47); N, 3.25 (3.27).
4
3
2
3
= 165.9 (d, JCP = 26.6 Hz), 148.42 (d, JCP = 17.4 Hz), 148.30 (d,
4JCP = 1.3 Hz), 123.44 (s), 121.06 (d, 2JCP = 16.9 Hz), 118.70 (d, 4JCP
3
1
= 1.1 Hz), 73.33 (d, JCP = 1.3 Hz), 48.31 (s), 37.35 (d, JCP = 32.1
Hz), 34.02 (d, JCP = 12.0 Hz), 29.63 (d, JCP = 3.9 Hz). 31P{1H}
1
2
NMR: δ = 86.7 (s).
C
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Scheme 1. Synthesis of (PCN)Ni−X
Synthesis of [PCN]Ni−p-tolylacetylide (15). To a mixture of
19.0 mg (0.046 mmol) of 6 and 32.0 mg (0.23 mmol) of K2CO3 in 5
mL of THF was added 11.7 μL (0.092 mmol) of p-tolylacetylene. The
mixture was left stirring for 48 h at room temperature. The solvent
was evaporated, the residue dissolved in n-hexane, filtered over Celite,
and the filtrate evaporated yielding 19.0 mg (90%) of 15 as a yellow
solid. Crystallization from hexane at −20 °C gave the product as
1 mM concentration of the halide complexes (1 or 3). The solutions
were purged with nitrogen before conducting the experiments.
Magnetic Susceptibility and Magnetization. The magnetic
data were acquired on a Quantum-Design MPMS-XL SQUID
magnetometer. Susceptibility data were acquired in a static field of
2.0 kOe. Magnetization data were obtained with selected fields from 0
to 50 kOe at 2 K. The polycrystalline samples were measured on a
compacted powder sample. The diamagnetic contribution to the
sample moment from the sample holder and sample was corrected
through background measurements and Pascal constants, respectively.
EPR Spectroscopy. The EPR spectra were recorded with a Bruker
Elexsys E500 equipped with a Bruker ER 4116 DM dual-mode cavity,
an EIP 538B frequency counter, an ER035 M NMR Gauss meter, and
an Oxford Instruments iTC cryocontroller. The spectra were recorded
at X-band frequencies (ν ≈ 9.63 GHz). The spectra were simulated
using lab-written software considering an electronic spin of 1/2 and
taking into account only the experimentally resolvable interactions
with the nuclear spins of one bromide (I = 3/2) and one phosphorus
ligand (I = 1/2). No distinction was made between the naturally
occurring bromine isotopes. Simulation parameters are given in the
legends pertinent to the individual spectra.
1
3
yellow needles. H NMR (500 MHz, C6D6) δ = 7.56 (d, JHH = 7.8,
2H), 7.08 (t, 3JHH = 7.4 Hz, 1H), 7.01 (d, 3JHH = 7.5 Hz, 2H), 6.95 (d,
3
3JHH = 7.5 Hz, 1H), 6.75 (d, JHH = 7.2 Hz, 1H), 3.47 (s, 2H), 3.03
(d, 2JHP = 8.6 Hz, 2H), 2.67 (s, 6H), 2.12 (s, 3H), 1.42 (d, 3JHP = 13.0
2
Hz, 18H). 13C{1H} NMR: δ = 168.2 (d, JCP = 26.4 Hz), 150.9 (s),
149.4 (d, 3JCP = 18.0 Hz), 134.0 (s), 131.0 (s), 129.10 (s), 127.6 (s),
125.0 (s), 124.1 (d, J = 30.0 Hz), 121.2 (d, 3JCP = 17.6 Hz), 118.8 (d,
4JCP = 1.7 Hz), 114.6 (d, 4JCP = 2.7 Hz), 73.9 (d, 3JCP = 1.8 Hz), 50.3
(d, 3JCP = 1.7 Hz), 37.5 (d, 1JCP = 30.1 Hz), 34.4 (d, 1JCP = 14.9 Hz),
2
29.7 (d, JCP = 3.3 Hz), 21.3 (s). 31P{1H} NMR: δ = 97.4(s). Anal.
Found (calcd for C27H38NNiP): C, 69.71 (69.55); H, 8.20 (8.21); N,
2.89 (3.00).
Reaction of [PCN]Ni−Me(13) with CO2. In a J. Young NMR
tube, 10 mg of 13 was dissolved in 0.5 mL of C6D6 inside the
glovebox. The tube was degassed (three freeze−pump−thaw cycles)
on the high-vacuum line, and the solution was pressurized with 4 atm
of CO2. The tube was heated to 100−150 °C.
Reaction of [PCN]Ni−Me(13) with PhBr. In a J. Young NMR
tube, 10 mg of 13 was dissolved in 0.5 mL C6D6 inside the glovebox.
The tube was degassed (three freeze−pump−thaw cycles) on the
high-vacuum line and introduced to the glovebox where PhBr (2
equiv) was added. The tube was heated to 100−150 °C.
Crystallography. Intensity data were collected with an Oxford
Diffraction Excalibur 3 system, using ω-scans and Mo Kα (λ =
0.71073 Å) radiation.48 The data were extracted and integrated using
Crysalis RED.49 The structures were solved by direct methods and
refine by full-matrix least-squares calculations on F 2 using SHELXT,50
SHELXL,51 and OLEX2.52 Molecular graphics were generated using
Crystal Maker 8.7.53
Kharasch Addition of CCl4 to Styrene. To a mixture of CCl4
(2.5 mL, 26 mmol) and styrene (0.80 mL, 7 mmol) in Schlenk flask
were added the nickel complex (0.0227 mmol) and 3 mL of MeCN.
The flask was purged with nitrogen for 15 min. The golden yellow
color of the reaction mixture turned into orange or red upon stirring
at room temperature overnight. The temperature was increased to
80−85 °C and the color changed to brown. The reaction was stopped
RESULTS AND DISCUSSIONS
■
Synthesis of (PCN)Ni Complexes with Halide Ligands.
Cyclometalation of the PCN ligand45 with anhydrous NiCl2 in
toluene in the presence of 4-dimethylaminopyridine (DMAP)
at 100 °C led to formation of complex 1 as a yellow solid in
63% yield as shown in Scheme 1. We previously used a similar
procedures to prepare aliphatic pincer nickel halide com-
plexes.21,22 The 31P{1H} NMR spectrum of the product
1
after 24 h, and the reaction mixture was analyzed by GC-MS and H
NMR spectroscopy.
Electrochemical Measurements. Cyclic voltammetry measure-
ments were carried out at room temperature using an Autolab
PGSTAT 30 (Eco Chemie, Utrecht, The Netherlands) potentiostat
equipped with GPES software and a 0.1 M solution of (Bu4N)PF6 in
DCM as the electrolyte. The measurements were performed in a
three-electrode cell with a graphite working electrode, a Ag/AgCl
reference electrode and a platinum wire as an auxiliary electrode using
1
showed a singlet peak at δ = 85.3 ppm and the H NMR
spectrum is divided into two regions where the aliphatic region
includes the tert-butyl protons as a doublet, the NMe2 protons
as a singlet, the CH2P protons as a doublet, and the CH2N
ones as a singlet, and the aromatic region contains three
protons reflecting the tridentate chelation of the PCN ligand to
D
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the nickel center. The moderate yield is due to formation of a
paramagnetic byproduct (2), which was completely NMR-
silent. Attempts to isolate 2 in pure form were not successful.
However, single crystals suitable for X-ray diffraction were
obtained after many attempts giving the molecular structure of
2 as shown in Figure 1. The nickel center has a tetrahedral
synthetic procedures of Campora for (i‑PrPCP)Ni−Br,54
(PCN)Ni−Br (3) was obtained in 96% yield as a yellowish
green solid. Its spectroscopic features are similar to those of 1.
The higher yield achieved in the latter case could be attributed
to the higher solubility and/or reactivity of molecular
(DME)NiBr2 compared to the inorganic salt NiCl2.
To the best of our knowledge, complexes 1 and 3 are the
first nickel complexes based on an unsymmetric PCN pincer
ligand with a phosphine arm. Examples of nickel complexes
based on the electron-deficient POCN analogue are known in
the literature.40−44
The molecular structures of the new halide complexes 1 and
3 are shown in Figure 2 together with selected bond distances
and angles. The square planar coordination geometry around
the nickel is slightly distorted.
The N1−Ni1−P1 angle in 3 (164.96(5)) is slightly larger
than that in (i‑PrPOCNMe)Ni−Br (163.81(6)),40 which we
attribute to the presence of the oxygen atom in the latter
pushing the phosphinite arm toward the aromatic ring.
Furthermore, the Ni−Br bond length in 3 is slightly longer
than that in (i‑PrPOCNMe)Ni−Br (2.3818(3) vs 2.3407(5))
indicating the expected stronger trans influence of the PCN
system in comparison to the more electron deficient POCN
system.
Figure 1. Molecular structure of complex 2 at the 50% probability
level. Hydrogen atoms (except on N1) are omitted for clarity.
Selected bond lengths (Å) and bond angles (deg): Ni1−Cl1 =
2.2865(8), Ni1−Cl2 = 2.2629(7), Ni1−Cl3 = 2.2500(7), Ni1−P1 =
2.3764(6), Cl1−Ni1−Cl2 = 102.65(3), Cl1−Ni1−Cl3 = 116.56(3),
Cl2−Ni1−Cl3 = 110.99(3), Cl1−Ni1−P1 = 110.28(3), Cl2−Ni1−P1
= 108.12(2), Cl3−Ni1−P1 = 107.94(2).
Cyclic voltammetry measurements of complexes 1 and 3 in
DCM (Figure 3) showed irreversible oxidation peaks at E1/2
=
0.837 and 0.797 V for 1 and 3, respectively. These values of the
oxidation potentials are lower than the corresponding values
for POCN nickel complexes (ca. 1.0 V), which is in line with
the expected, higher donicity of the PCN ligand. Thus, the
high valent PCN nickel(III) complexes could be readily
available through oxidation of the corresponding PCN
nickel(II) complexes.
Indeed, reaction of complexes 1 and 3 with anhydrous CuX2
salts (X = Cl, Br) in DCM produced the corresponding Ni(III)
complex (Scheme 2). Within seconds after the addition of the
oxidizing agent at room temperature, the yellow solution of the
halide complexes turned a deep red or violet color with
precipitation of white CuX salts. 1H and 31P{1H} NMR spectra
of the isolated products displayed no signal due to their
paramagnetic nature.
geometry where it coordinates three chloride ligands and the
remaining coordination site is occupied by the protonated
PCN ligand through the phosphorus side arm in a
monodentate fashion. Such byproducts have been observed
or suggested in the cyclometalation of both aliphatic and
aromatic pincer ligands with nickel precursors, but no
molecular structure has been reported.19,21,22,40 The low
yield of complex 1 induced us to further optimize the reaction
conditions by (i) using 1:1 stoichiometry of the PCN ligand
and the nickel precursor (NiCl2), (ii) using THF as a solvent
to enhance the solubility of the nickel precursor, and (iii)
replacing the weak aromatic DMAP base with the stronger
aliphatic Et3N base to avoid the protonation of the amine arm
of the PCN ligand. Indeed, the new reaction conditions helped
to improve the yield, and 1 was obtained in 86% yield without
formation of 2. When (DME)NiBr2 was used as a nickel
precursor instead of anhydrous NiCl2 following the modified
The molecular structures of complexes 4 and 5 were
confirmed by X-ray diffraction measurements and the
molecular structures are given in Figure 4. Both of the
Figure 2. Molecular structures of complex 1 and 3 at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å)
and bond angles (deg): For complex 1 Ni1−C1 = 1.881(3), Ni1−P1 = 2.1643(9), Ni1−N1 = 2.028(3), Ni1−Cl1 = 2.2329(9), C1−Ni1−Cl1 =
177.41(10), C1−Ni1−P1 = 83.98(10), C1−Ni1−N1 = 83.66(12), N1−Ni1−P1 = 164.87(8), P1−Ni1−Cl1 = 98.04(4), N1−Ni1−Cl1 =
94.58(8). For complex (3) Ni1−C1 = 1.8921(18), Ni1−P1 = 2.1826(5), Ni1−N1 = 2.0435(16), Ni1−Br1 = 2.3818(3), C1−Ni1−Br1 =
177.06(6), C1−Ni1−P1 = 83.83(6), C1−Ni1−N1 = 83.71(7), N1−Ni1−P1 = 164.96(5), P1−Ni1−Br1 = 98.278(16), N1−Ni1−Br1 = 94.48(5).
E
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these complexes, complex 5 was subjected to a magnetic
susceptibility and magnetization measurement and EPR
analysis. The oxidation and spin-state is unequivocally
demonstrated by the magnetic data, with the χT product
behaving as expected for a low-spin d7 system with gaverage
>
2.00 (cf. Figure 5). The expected value for a perfectly isolated
Figure 3. Cyclic voltammogram of 1 mM solutions of complexes 1
and 3 in DCM containing 0.1 M (Bu4N)PF6 at a scan rate of 0.1 V/s
on a glassy carbon working electrode.
Scheme 2. Synthesis of (PCN)Ni−X2
Figure 5. Magnetic susceptibility, represented by the χT product in
the temperature range 2−300 K for complex 5. The inset shows
magnetization data for the same sample recorded at 2 K.
system with gaverage = 2.19 is 0.450 cm3 mol−1, and this is shown
as the blue line in Figure 5. There is a small but distinct upturn
in the χT value at around 30−10 K followed by a steep
decrease at the lowest temperatures. This could indicate a weak
ferromagnetic interaction in pairs; indeed, there is a weak
nonclassical hydrogen bond between the apical bromide and
the meta C−H group on an adjacent phenyl ring, (cf. Figure
S26). It is overtaken by an overall antiferromagnetic order
below ca. 5K. The magnetization is close to linear in field and
does not reach saturation even at 2 K.
complexes display distorted square pyramidal coordination
geometries around nickel with a halide in the apical position.
The distinct elongation in the Ni−halide bond lengths (0.04
and 0.06 Å for 4 and 5, respectively) along the apical direction
indicates an electronic structure with the unpaired electron
located in the dz orbital directed along Ni1−Cl1 and Ni1−
Br1, respectively, as corroborated by EPR spectroscopy (vide
infra).
2
EPR Spectroscopy. An EPR spectrum of complex 5 was
recorded on an undiluted solid sample at 11 K (cf. Figure S24).
The featureless spectrum can be well-represented as an axial S
= 1/2 spectrum with g⊥ = 2.206 > g∥ = 2.065 (gaverage = 2.159).
Magnetic Susceptibility and Magnetization. To probe
the paramagnetic properties and the electronic structure of
Figure 4. Molecular structures of complexes 4 and 5 at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths
(Å) and bond angles (deg): For complex (4) Ni1−C1 = 1.928(3), Ni1−P1 = 2.2667(8), Ni1−N1 = 2.072(2), Ni1−Cl1 = 2.2888(8), Ni1−Cl2 =
2.2496(9), C1−Ni1−Cl1 = 90.41(8), C1−Ni1−Cl2 = 163.41(8), C1−Ni1−P1 = 81.79(8), C1−Ni1−N1 = 84.06(11), N1−Ni1−P1 = 156.57(8),
P1−Ni1−Cl1 = 103.43(3), P1−Ni1−Cl2 = 95.10(3). For complex (5) Ni1−C1 = 1.928(5), Ni1−P1 = 2.2802(15), Ni1−N1 = 2.079(5), Ni1−Br1
= 2.4430(9), Ni1−Br2 = 2.3818(9), C1−Ni1−Br1 = 89.66(16), C1−Ni1−Br2 = 165.49(16), C1−Ni1−P1 = 82.09(17), C1−Ni1−N1 = 84.0(2),
N1−Ni1−P1 = 155.76(15), P1−Ni1−Br1 = 104.71(5), P1−Ni1−Br2 = 94.97(5).
F
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At room temperature, a CH2Cl2 solution furnishes an isotropic
spectrum (giso = 2.172) with a resolvable hyperfine structure
(Aiso = 0.0053 cm−1), stemming from strong coupling to one I
= 3/2 nucleus (cf. Figure S25). As >98% of the naturally
occurring Ni isotopes have I = 0, this coupling must be a
nickel(III) complexes in addition to their stability in the
solid state for a long time at room temperature points toward
their potential application as catalyst in Kharasch addition
reaction (Scheme 3). Thus, the addition of CCl4 to styrene in
2
hyperfine coupling from the unpaired electron in the dz orbital
Scheme 3. Kharasch Addition of CCl4 to Styrene Using 1
and 3 as Catalyst
to a single bromide ligand corroborating a solution structure
similar to the solid-state structure. A further resolved spectrum
is obtained in a frozen glass (CH2Cl2/toluene; 2:1, T = 20K)
as shown in Figure 6. In this spectrum, the extreme anisotropy
DCM at room temperature was screened using complex 1 as a
catalyst under reaction conditions the same as those reported
by van Koten. There was no reaction at room temperature as
1
seen by GC-MS and H NMR spectroscopy, but a change in
the color from yellow to red was observed upon stirring the
reaction mixture overnight probably due to the oxidation of the
nickel complex to its corresponding trivalent state. Changing
the solvent to acetonitrile had no effect at room temperature,
but heating the reaction mixture to 80−85 °C for 24 h led to a
change in the color from orange to brown. Analyzing the
reaction mixture by GC-MS and 1H NMR spectroscopy
confirmed the formation of the expected 1:1 anti-Markovnikov
Kharasch addition product (100% selectivity). Bromo complex
1
3 was more efficient giving 95% conversion based on the H
NMR spectroscopy compared to 87% conversion using 1
(Table 1). A similar difference in the catalytic activity between
Figure 6. Experimental (black) and simulated (red) X-band EPR
spectra of 5 in a frozen CH2Cl2/toluene (2:1) glass at T = 20 K. The
blue curve is the derivative of the experimental spectrum indicating
superhyperfine splitting also from coupling to one phosphorus
nucleus. The spectrum was recorded with P = 6.325 mW; modulation
amplitude = 3.0 G, modulation freq = 100 kHz. Simulation
Table 1. Catalytic Kharasch addition reaction of CCl4 to
styrene using PCN nickel complexes 1 and 3
cat. loading
(%)
conversion
(%)
selectivity
(%)
entry catalyst
T (°C) t (h)
Br
parameters: g1 = 2.301; g2 = 2.208; g3 = 2.000, Azz = 0.0132 cm−1,
1
2
3
4
1
3
1
3
0.32
0.32
0.32
0.32
25
25
80−85
80−85
24
24
24
24
0
0
87
95
P
Azz = 0.0015 cm−1, Lorentzian derivative line shape, fwhh = 15.5 G.
100
100
of the hyperfine coupling to the bromide ligand is evident with
a completely dominating Azz = 0.0132 cm−1, amounting to
almost three times the Aiso determined from the isotropic
solution spectrum at RT. Strongly anisotropic hyperfine
coupling in low-spin nickel(III) systems with Azz ≫ Aiso is in
accordance with previous observations on e.g. heteroleptic
cyano complexes and Jahn−Teller distorted homoleptic
systems.55,56 Furthermore, a weak coupling to the phosphine
ligand (I = 1/2) can be resolved in the parallel direction
halide complexes has been observed earlier with NCN nickel
complexes17 and is in agreement with the oxidation potentials
which show that 3 is more easily oxidized. The catalyst loading
in the current study is 0.32%, six times smaller than that
reported for the POCN nickel pincer complex which gave a
similar result.40 This result allows an estimation of the catalytic
efficiency of nickel pincer complexes in the Kharasch addition
P
yielding Azz = 0.0015 cm−1. The simulated frozen solution
reaction and the following order appears: NCN > POCsp OP >
3
spectrum requires, in accordance with the solid-state structure,
a nonaxial g-tensor (g1 = 2.301; g2 = 2.208; g3 = 2.000) in order
to achieve a satisfactory reproduction of the experimental
spectrum.
PCN > POCN.40
Synthesis and Reactivity of Hydroxo and Amido
Complexes. In general, hydroxo and amido nickel pincer
complexes are not common in the literature compared to the
corresponding halide complexes, and the reported examples
only involve PCP pincer ligands.23,25,27,57,58 Thus, we were
interested in preparing these complexes and study their
reactivity toward small molecules to investigate the influence
of the weak amine arm in these transformations.
Catalytic Activity in Kharasch Addition Reaction. The
aromatic NCN nickel pincer complexes developed by van
Koten are known to be efficient catalysts for Kharasch addition
reaction of polyhalogenated alkanes to olefins.16,17,24 In
contrast, the aromatic PCP nickel pincer analogues are not
suitable as catalysts in this reaction mainly because they do not
produce the corresponding trivalent nickel complexes, which
are considered to be important intermediates in the catalytic
cycle of the Kharasch addition reaction.17 The Kharasch
addition is synthetically important because it generates a new
C−C bond and introduces functionality in two positions. The
facile preparation and successful isolation of the PCN
Synthesis of (PCN)Ni−OH and Its Reaction with CO2.
The hydroxo complex (8) was synthesized through two
synthetic steps as shown in Scheme 4 following the procedures
reported by Campora and us.23,25,46,59,60 The OH proton
1
appears as a singlet peak at −2.56 ppm in the H NMR
spectrum. Although an excess of silver nitrate was used to
prepare complex 6, no further oxidation to Ni(III) complex
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Scheme 4. Synthesis of (PCN)Ni−OH, 8
was observed as reported in the case of (NCN)Ni−ONO2 and
This observation suggests that the carboxylation reaction could
be carried out at lower pressures. Complex 9 was also prepared
in situ with 96% purity based on 31P NMR spectroscopy using
another batch of the hydroxo complex (originally prepared by
the reaction of (PCN)Ni−TFA and NaOH and which was
contaminated by 4% of the (PCN)Ni−TFA complex).
Removing the volatiles from complex shows that the CO2
insertion is reversible, and in line with this, crystallization of 9
gave the dimeric carbonate complex, 10 (cf. Scheme 5).
Similar results were previously reported for the palladium
analog but in that case the decarboxylation required heating
under vacuum.46 Under CO2 pressure there is no tendency for
9 to interconvert to 10. The low steric hindrance imposed by
the presence of smaller substituents on the nitrogen donor
atom apparently favors the dimerization. Complex 10 gave X-
ray quality crystals, and the molecular structure of the dimeric
complex is displayed in Figure 8.
(
i‑PrPCP)Ni−ONO2.14,54 Complex 8 is also accessible through
reaction of complex 7 with NaOH in THF.
Unfortunately, attempts to grow crystals of complex 8 were
not successful, but a single crystal of 6 was obtained by slow
diffusion of n-hexane into a concentrated benzene solution of 6
at 5 °C. The molecular structure of complex 6 is shown in
Figure 7.
Figure 7. Molecular structure of 6 at the 50% probability level.
Hydrogen atoms are omitted for clarity. Selected bond lengths (Å)
and bond angles (deg): Ni1−C1 = 1.8782(13), Ni1−P1 = 2.1863(7),
Ni1−N1 = 2.0242(19), Ni1−O1 = 1.9762(18), O1−N2 = 1.308(3),
O2−N2 = 1.226(3), O3−N2 = 1.240(3), C1−Ni1−P1 = 84.19(5),
N1−Ni1−P1 = 164.42(6), O1−Ni1−P1 = 97.46(6), O1−Ni1−N1 =
93.60(8), C1−Ni1−O1 = 177.17(8).
Reaction of 8 with 4 atm of CO2 produced, within minutes
at room temperature, a new pincer complex, 9, as seen from
Figure 8. Molecular structure of 10 at the 50% probability level.
Selected bond lengths (Å) and bond angles (deg): Ni1−P1 =
2.1518(10), Ni1−N1 = 2.014(3), Ni1−O1 = 1.902(2), Ni2−P2 =
2.1470(10), Ni2−N2 = 2.008(3), Ni2−O2 = 1.901(2), O1−Ni1−P1
= 94.25(8), O1−Ni1−N1 = 97.69(11), N1−Ni1−P1 = 168.02(9),
O2−Ni2−P2 = 94.04(8), O2−Ni2−N2 = 98.37(11), N2−Ni2−P2 =
165.17(9).
the 31P{1H} and H NMR spectra (cf. Scheme 5). The H
NMR spectrum of 9 features a broad singlet at 13.26 ppm
which was assigned as an acidic hydrogen carbonate proton.
The hydrogen carbonate group −OCO2H was also observed as
a singlet at 162.8 ppm in the 13C{1H} NMR spectrum, and
complex 9 was unambiguously assigned as the hydrogen
carbonate complex. Complex 8 was fully consumed, and in
addition to 9 a small amount of 6 was seen in the 31P NMR
spectrum. Complex 6 is not visible in the 31P NMR spectrum
of 8, and this indicates that there is small amount of nitrate
source present in 8 and that this readily displaces the hydrogen
carbonate once 9 is formed. Free CO2 was also observed in the
13C{1H} NMR spectrum at 124.8 ppm as a sharp singlet.61
1
1
Synthesis of (PCN)Ni−NH2 and Its Reaction with CO2.
Amido complex 11 was obtained directly from 3 as described
in Scheme 6 through a salt metathesis reaction with NaNH2
following literature procedures.58 The complex is extremely air-
and moisture-sensitive. It produces 8 in contact with trace
amounts of water; this can be used as an alternative method for
Scheme 5. Reaction of CO2 with 8
H
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Scheme 6. Insertion of CO2 into Ni−NH2 Bond of 11
preparing hydroxide complexes as previously reported for
Attempts to isolate the methyl complex using aqueous workup
was not successful, and instead, complex 3 was reacted with
MeMgCl in THF at −78 °C producing 13, which was
successfully extracted by n-hexane to remove excess of the
Grignard reagents and the in situ formed salt. The phenyl
complex 14 was obtained in an analogous way.
These hydrocarbyl complexes are stable in air in the solid
state at room temperature for several days. Their molecular
structures (Figures 9 and 10) were corroborated using X-ray
(
t‑BuPCP)Ni−OH.27 The amido protons appear as a singlet in
1
the H NMR spectrum at −1.80 ppm.
The reaction of 11 with 4 atm of CO2 in C6D6 was
monitored by 31P{1H}- and 1H NMR spectroscopy and
showed the immediate consumption of 11 and the formation
of two products in a 93:7 ratio based on 31P NMR spectrum.
The major product shows a downfield singlet peak at 3.49 ppm
1
in the H NMR spectrum assigned to the two protons of a
carbamate group, and the complex was assigned as the
carbamate complex, 12. The carbamate group −COONH2
appeared as a sharp singlet peak at 162.9 ppm in the 13C{1H}
spectrum. Attempts to identify the minor product were
unsuccessful.
Synthesis of (PCN)Ni Hydrocarbyl Complexes.
Although a few examples of unsymmetric aromatic pincer
nickel halide complexes have been published, the correspond-
ing hydrocarbyl complexes have not been reported yet.
Therefore, we were interested in preparing such complexes
particularly to investigate their reactivity toward electro-
philes.22,26 Previous attempts from Zargarian and co-workers
to react a POCN nickel chloride complex with a secondary
amine arm with MeLi did not give the corresponding methyl
complex.41 In contrast to this, a small scale reaction of 1 with
MeLi immediately and cleanly produced the methyl complex
13 based on NMR spectroscopy (cf. Scheme 7). The methyl
Figure 9. Molecular structure of 13 at the 50% probability level.
Hydrogen atoms are omitted for clarity. Selected bond lengths (Å)
and bond angles (deg): Ni1−C1 = 1.902(2), Ni1−P1 = 2.1302(6),
Ni1−N1 = 2.0391(17), Ni1−C7 = 1.983(2), C1−Ni1−C7 =
175.64(9), C1−Ni1−P1 = 83.77(6), C1−Ni1−N1 = 82.96(8),
N1−Ni1−P1 = 164.57(5), C7−Ni1−P1 = 99.63(7), C7−Ni1−N1
= 94.03(9).
Scheme 7. Synthesis of (PCN)Ni−R
diffraction. Both complexes have the expected square-planar
structures, and the phenyl ligand is approximately perpendic-
ular to the coordination plane in 14. Complex 13 has a slightly
shorter Ni−Me bond distance than (t‑BuPCP)Ni−Me and
(
t‑BuPOCyOP)Ni−Me complexes: 1.983(2) Å vs 2.026 (2) and
2.016(3).22,26
protons appeared at −0.7 ppm as a doublet with 3JHP= 2.2 Hz
Reaction of 3 with EtMgCl in C6D6 did not produce the
expected ethyl complex, and, instead, a characteristic signal for
ethylene was observed in the 1H NMR spectrum at 5.25 ppm61
strongly suggesting a β-hydride elimination pathway. However,
no hydride complex was observed, and the free PCN ligand
was formed based on NMR spectroscopy, which could be
attributed to further decomposition where the in situ generated
hydride complex undergoes reductive elimination. Similarly,
bromide complex 3 reacts with hydride sources, e.g., LiAlH4,
1
in the H NMR spectrum. No other complexes were formed,
indicating that the nickel methyl complex is inert toward
further substitution reactions as reported for the palladium
methyl complex based on the same ligand framework, which
underwent subsequent reaction with MeLi to displace the
nitrogen arm and form the dimethyl complex.45 This indicates
that the hemilability of the (PCN)Ni system is lower than that
of the corresponding palladium complexes (Scheme 8).
Scheme 8. Reaction of (PCN)Ni−Br Complex with an Excess of MeLi
I
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Figure 10. Molecular structures of 14 at the 50% probability level.
Hydrogen atoms are omitted for clarity. Selected bond lengths (Å)
and bond angles (deg): Ni1−C1 = 1.915(5), Ni1−P1 = 2.1583(14),
Ni1−N1 = 2.047(4), Ni1−C19 = 1.950(5), C1−Ni1−C19 =
176.3(2), C1−Ni1−P1 = 83.05(17), C1−Ni1−N1 = 83.6(2), N1−
Ni1−P1 = 166.62(15), C19−Ni1−P1 = 100.08(15), C19−Ni1−N1 =
93.3(2).
Figure 11. Molecular structure of 15 at the 50% probability level.
Hydrogen atoms are omitted for clarity. Selected bond lengths (Å)
and bond angles (deg): Ni1−C1 = 1.902(3), Ni1−P1 = 2.1535(8),
Ni1−N1 = 2.035(3), Ni1−C19 = 1.908(3), C20−C19 = 1.218(4),
C21−C20 = 1.452(4), C1−Ni1−C19 = 176.60(12), C1−Ni1−P1 =
84.28(9), C1−Ni1−N1 = 84.10(11), N1−Ni1−P1 = 163.29(8),
C19−Ni1−P1 = 97.37(9), C19−Ni1−N1 = 94.88(11), C19−C20−
C21 = 178.1(3).
NaBH4, and KH, not giving the expected hydride complex but
rather the free PCN ligand together with a black precipitate.
Using stronger hydride sources, e.g., LiEt3BH and Ph2SiH2,
gave the same result, and an immediate change in the color
from yellow to brown after addition of the hydride source was
observed in all the reactions. Decomposition to the free ligand
was observed previously in an attempt to prepare (c‑HexPCP)-
Ni−H by reaction of the corresponding chloride complex with
LiAlH4.62 Furthermore, Hu found that the complex (MeN2N)-
Ni−H decomposed through two routes including intra-
molecular reductive elimination to form the ligand.63 The
facile decomposition of the ethyl and the hydride complexes
with the current PCN framework could be explained by the
small steric hindrance of the nitrogen side arm of the PCN
ligand, which could speed up the decomposition. Using more
bulky substituents in the nitrogen donor atom could solve this
problem and make these complexes more accessible.
toluene, which was observed in the 1H NMR spectrum after 24
h. 88% conversion of the starting complex was calculated based
on the 31P{1H} NMR spectroscopy after 3 days.
Carboxylation of the methyl complex using 4 atm of CO2
was monitored by NMR spectroscopy. A mixture of different
products was formed at 120 °C, and due to the slow progress
of the reaction, the temperature was increased to 150 °C and
the reaction monitored over 6 days at this temperature. At that
point, a small amount of black precipitate had formed
prompting discontinuation of the reaction. Only the remaining
methyl complex and the expected carboxylated product, the
acetate complex were identified from the mixture of the formed
compounds. The acetate complex, 16, was prepared
independently from the reaction of the corresponding chloride
and silver acetate. The electron deficiency of our system
together with the potential decoordination of the amine arm at
high temperature could explain the competing decomposition.
However, the carboxylation of the more electron-rich aliphatic
and aromatic PCP pincer nickel complexes did not achieve full
conversion to the carboxylated product and in both cases other
unknown products were observed probably due to the harsh
condition.22,26 Overall, the current results corroborate the
sluggish reactivity of the Ni−C bond toward CO2.
The acetylide complex, 15, is expected to be more stable
than the methyl and phenyl complexes and was obtained
through the straightforward synthetic method shown in
Scheme 9; the same method was previously used for the
Scheme 9. Synthesis of (PCNMe)Ni−p-tolyl acetylide
CONCLUSIONS
■
New nickel(II) complexes based on the unsymmetric PCN
pincer ligand were synthesized and characterized. As previously
reported for similar compounds, the synthesis is hampered by
the formation of a tetrahedral Ni(II) complex, and this was
characterized for the first time by X-ray diffraction. High-valent
paramagnetic Ni(III) complexes were successfully isolated, and
the Ni(III)(Br)2 system was characterized by magnetic
measurements and EPR spectroscopy, giving insight into the
electronic structure of the Ni(III) complexes. The PCN
framework gives a more electron-rich metal center compared
to that with previously reported POCN ligands, and this gives
a higher reactivity in the Kharasch addition. The reactivity of
the halide complexes toward Grignard reagents allowed the
isolation of the corresponding methyl and phenyl complexes
that represent the first example of alkyl and aryl nickel
complexes based on an unsymmetric aromatic pincer ligand.
corresponding palladium complex and the use of base is
crucial.46 The identity of 15 was confirmed using X-ray
crystallography, and the molecular structure is given in Figure
11.
Reactivity of the Methyl Complex toward Electro-
philes. The reaction of the methyl complex with PhBr in C6D6
was studied (Scheme 10). The reaction was carried out in a J.
1
Young NMR tube and monitored by 31P{1H} and H NMR
spectroscopy. There was no reaction at room temperature, and
increasing the temperature to 50 °C gave the same result.
However, reaction took place at 120 °C based on the 31P{1H}
NMR spectra, giving 3 and the cross coupling product,
J
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Scheme 10. Reactivity of Complex 13 with Electrophiles
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Reaction of the nickel-methyl bond with CO2 is sluggish as
previously reported for similar nickel complexes, but the
reaction with aryl bromide is straightforward giving the C−C
coupling product.
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Platform: Coordination Chemistry & Applications; Springer Interna-
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ASSOCIATED CONTENT
* Supporting Information
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́
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Accession Codes
CCDC 1842118−1842127 contain the supplementary crys-
tallographic 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.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: ola.wendt@chem.lu.se.
■
ORCID
Jesper Bendix: 0000-0003-1255-2868
Ola F. Wendt: 0000-0003-2267-5781
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the Swedish Research Council, the
Knut and Alice Wallenberg Foundation and the Royal
Physiographic Society in Lund is gratefully acknowledged.
We would like to thank Galina Pankratova for assistance in the
electrochemical measurements.
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