Synthesis and Reactivity of Reduced α-Diimine Nickel Complexes Relevant to Acrylic Acid Synthesis

Article abstract of DOI:10.1021/acs.organomet.8b00350

The aryl-substituted α-diimine (DI) nickel vinyl complex (^iPrDI)Ni(CH=CH₂) (^iPrDI = [2,6-(ⁱPr)₂C₆H₃N=C(CH₃)]₂) was synthesized and structurally characterized. The complex is dimeric in the solid state and has a distorted-square-planar geometry at nickel. A combination of single-crystal X-ray diffraction, EPR, magnetic susceptibility, and NMR analyses was used to elucidate the electronic structure of the compound, and it is best described as a low-spin Ni(II) derivative with a singly reduced α-diimine chelate. Addition of CO₂ to the nickel vinyl complex resulted in insertion into the nickel-carbon bond to yield the corresponding nickel acrylate (^iPrDI)Ni(κ²-O₂CCH=CH₂). EPR spectroscopy coupled with DFT calculations established that the S = 1/2 product maintains the nickel(II) oxidation state with an α-diimine-centered radical. Addition of acrylic acid to (^iPrDI)Ni(CH=CH₂) induced rapid, net bimetallic reductive elimination to release butadiene and produced the metastable olefin-bound acrylic acid complex (^iPrDI)Ni(κ²-CH₂=CHCO₂H). Over the course of 2 h at 23 °C, this complex underwent a net oxidation to produce (^iPrDI)Ni(κ²-O₂CCH=CH₂), with concomitant loss of H₂.

Full text of DOI:10.1021/acs.organomet.8b00350

Communication
Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthesis and Reactivity of Reduced α‑Diimine Nickel Complexes
Relevant to Acrylic Acid Synthesis
†
Matthew V. Joannou,† Mate J. Bezdek, Khalid Albahily,‡ Ilia Korobkov,‡ and Paul J. Chirik*,†
́
́
†Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States of America
‡SABIC Corporate Research & Development, Fundamental Catalysis, Thuwal 23955-6900, Saudi Arabia
S
* Supporting Information
ABSTRACT: The aryl-substituted α-diimine (DI) nickel vinyl complex
(iPrDI)Ni(CHCH2) (iPrDI = [2,6-(iPr)2C6H3NC(CH3)]2) was
synthesized and structurally characterized. The complex is dimeric in the
solid state and has a distorted-square-planar geometry at nickel. A
combination of single-crystal X-ray diffraction, EPR, magnetic suscepti-
bility, and NMR analyses was used to elucidate the electronic structure of
the compound, and it is best described as a low-spin Ni(II) derivative with
a singly reduced α-diimine chelate. Addition of CO2 to the nickel vinyl
complex resulted in insertion into the nickel−carbon bond to yield the
corresponding nickel acrylate (iPrDI)Ni(κ2-O2CCHCH2). EPR spectros-
copy coupled with DFT calculations established that the S = 1/2 product
maintains the nickel(II) oxidation state with an α-diimine-centered radical.
Addition of acrylic acid to (iPrDI)Ni(CHCH2) induced rapid, net bimetallic reductive elimination to release butadiene and
produced the metastable olefin-bound acrylic acid complex (iPrDI)Ni(η2-CH2CHCO2H). Over the course of 2 h at 23 °C,
this complex underwent a net oxidation to produce (iPrDI)Ni(κ2-O2CCHCH2), with concomitant loss of H2.
tilization of carbon dioxideas an inexpensive and abundant
C1 source in synthesis has received considerable attention
U
in organometallic chemistry and homogeneous catalysis.1
Distinct from methodologies such as hydrocarboxylation, the
direct, catalytic coupling of CO2 and ethylene (and other
commodity α-olefins) to produce acrylic acid and other α,β-
unsaturated carboxylic acids has been a sought-after “dream
reaction” by industrial and academic chemists alike.1a Currently,
acrylic acid is produced on a 4 million ton/year scale by
successive oxidations of propylene and is used as precursors for
polymers utilized in absorbents, cosmetics, adhesives, and paints
(Figure 1, top).1e While ethylene−CO2 coupling to acrylic acid
would be an atom-economical process, it is thermodynamically
unfavorable under standard conditions, with ΔG° = +14 kcal
mol−1 in the gas phase and +5 kcal mol−1 in THF.2 To overcome
this thermodynamic limitation, chemists have relied on linking
CO2−ethylene coupling to other thermodynamically favorable
processes, usually acrylate salt formation via deprotonation with
an external base.1a
Reports by Hoberg and co-workers described a stoichio-
metric, nickel-promoted ethylene−CO2 coupling with nickel by
Figure 1. Strategies for the direct coupling of ethylene and CO2:
metallalactone intermediates vs direct metal−vinyl addition to CO2.
oxidative cyclization of the two molecules to form the
metallalactone (dbu)2Ni(κ2-O,C-O(O)(CH2)2), which proved
stable to β-hydride elimination and was isolated and
characterized (Figure 1, middle).3 Carmona and co-workers
reported a similar coupling with molybdenum and tungsten
phosphine complexes, yet metallalactones in these reactions
underwent rapid β-hydride elimination to form bridged
acrylate−hydride complexes that were inert to successive
Special Issue: In Honor of the Career of Ernesto Carmona
Received: May 23, 2018
© XXXX American Chemical Society
A
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reactivity relevant to catalytic turnover.4 Vogt5 and Limbach6
have recently developed nickel-catalyzed methods for the
carboxylation of ethylene and other α-olefins, with the latter
method producing sodium acrylate in up to 107 TON from
ethylene and CO2. Both procedures utilize stoichiometric Lewis
acid additives to activate the metallalactone and facilitate β-
hydride elimination, as well as external base to drive the reaction
forward and remove acrylate from the catalyst.
The inert nature of metallalactones inspired exploration into
alternative pathways for carbon−carbon bond formation from
ethylene and CO2. Carbon−carbon bond formation from
organometallic reagents and CO2 has been known for over a
century and ranges from Grignard’s discovery of the fixation of
CO2 with alkylmagnesium halides to current methods that
catalytically generate and add transition-metal−carbon bonds to
CO2.1c Regeneration of the metal−vinyl and acrylic acid release
could, in principle, be accomplished by concerted metalation−
deprotonation (CMD), of which there is theoretical precedent
with ethylene (Figure 1, bottom).7 Due to the prevalence of
nickel in both catalytic and stoichiometric carboxylation and
hydrocarboxylation chemistry,1c,3,5,6 our studies focused on
nickel vinyl complexes. Here we describe the synthesis and
electronic structure of an α-diimine nickel vinyl complex and its
reactivity toward CO2 and acrylic acid.
NiBr]210 followed by warming to room temperature, stirring for
20 min, filtering, and recrystallizing furnished a dark purple,
paramagnetic solid identified as (iPrDI)Ni(CHCH2) in 85%
yield (Scheme 1, bottom).11 The benzene-d6 H NMR spectrum
1
of (iPrDI)Ni(CHCH2) exhibits resonances for the iPrDI ligand
and the vinyl group between 0 and 47 ppm. A solid-state
magnetic moment of 2.3 μB was measured at 23 °C by magnetic
susceptibility balance, consistent with an S = 1/2 ground state for
each nickel complex with negligible antiferromagnetic coupling
between them. This is a rare example of an unsubstituted nickel
vinyl species that is isolable; to our knowledge, the only other
example is a nickel(II) bis-phosphine bromide vinyl complex
synthesized by Lu and co-workers.12
The X-band EPR spectrum of (iPrDI)Ni(CHCH2) (Figure
2, left) collected in fluid benzene solution at 296 K exhibits an
Because of the rich olefin insertion chemistry known with α-
diimine nickel complexes,8 a nickel vinyl complex supported by
the 2,6-diisopropylaryl diimine iPrDI (iPrDI = [2,6-
(iPr)2C6H3NC(CH3)]2) was targeted. Two equivalents of
vinylmagnesium bromide dissolved in THF was added to a
thawing pentane slurry of (iPrDI)NiBr2 and the mixture warmed
to ambient temperature over 20 min with stirring. Following
filtration and removal of the volatiles in vacuo, the formally
nickel(0) butadiene complex (iPrDI)Ni(η4-C4H6) was isolated in
92% yield as a diamagnetic, dark mauve solid (Scheme 1, top).
Figure 2. (left) X-band EPR spectrum of (iPrDI)Ni(CHCH2) at 296
K in fluid benzene. (right) Solid-state representation of (iPrDI)Ni-
(CHCH2) with 30% probability ellipsoids. Hydrogen atoms have
been omitted for clarity.
Scheme 1. Addition of Vinyl Metal Reagents to (iPrDI)Ni
Bromides in Different Oxidation States
isotropic signal with g1,2,3 = 2.021, 2.012, 2.015, with coupling to
both 14N nuclei in the α-diimine chelate (Aiso(14N, n = 2) = 20
MHz). The deviation of the calculated g value from that of a free
electron (g = 2.002) and the presence of 14N nuclei coupling
indicate that the unpaired electron in (iPrDI)Ni(CHCH2) is
ligand-based, with detectable contributions from Ni d orbitals.
This likely arises from spin−orbit coupling due to d−π* orbital
mixing resulting from the highly π acidic character of the ligand
(see the Supporting Information for details). For comparison,
the X-band EPR spectrum of [(iPrDI)NiBr]2 in fluid benzene
solution at 23 °C, which was shown to have a completely Ni-
based SOMO, exhibits an isotropic signal with g1,2,3 = 2.20, 2.24,
2.17 (see the Supporting Information for details).
Single crystals suitable for X-ray diffraction were obtained by
slow evaporation of a pentane solution of (iPrDI)Ni(CHCH2)
at −35 °C over 48 h. A representation of the dimeric solid-state
structure is presented in Figure 2 (right), where the geometry is
best described as distorted square planar at each nickel (τ4 =
0.16). The Cimine−Nimine and Cimine−C′imine bond distances are
established metrics for establishing the redox state of the α-
diimine ligand as a consequence of the transition metal.13 For
(iPrDI)Ni(CHCH2), Cimine−Nimine distances of 1.343(3) and
1.338(4) Å and a Cimine−C′imine value of 1.403(4) Å are
consistent with a singly reduced form of the chelate. Together
On the basis of work by Cardaci and co-workers, the expected
nickel bis-vinyl complex is likely formed at the outset of the
reaction, but rapid reductive coupling of the two vinyl groups
produces the reduced nickel butadiene complex.9
Because the proposed [(iPrDI)Ni(CHCH2)2] intermediate
underwent rapid C−C reductive coupling below ambient
temperature, lower oxidation nickel halide precursors were
targeted. Addition of a diethyl ether solution containing 2 equiv
of vinyllithium to a thawing pentane slurry containing [(iPrDI)-
B
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with the EPR data, (iPrDI)Ni(CHCH2) is best described as a
to produce a new product, which exhibited an isotropic signal
with g1,2,3 = 2.008, 1.994, 2.003 and sharp/resolved coupling to
both 14N nuclei in the α-diimine chelate (Aiso(14N, n = 2) = 14
MHz) (Figure 3, bottom). The hyperfine coupling pattern, along
with the observed g value that does not appreciably deviate from
that of a free electron, suggests that the unpaired electron is
almost entirely ligand based, consistent with a singly reduced α-
diimine chelate. Full-molecule density functional theory studies
(see the Supporting Information for details) were conducted on
(iPrDI)Ni(κ2-O2CCHCH2) to simulate and verify the
experimentally determined EPR parameters. The DFT-com-
puted g values of 2.009, 1.995, and 2.004 are in excellent
agreement with the experimentally determined values. A
Mulliken spin density plot (Figure 3, bottom) was also generated
and supports spin density almost exclusively on the α-diimine
chelate, with only minimal involvement of the metal, which
corroborates the EPR measurements. The combined EPR
measurements and DFT computations demonstrate that
(iPrDI)Ni(κ2-O2CCHCH2) is best described as a low-spin
Ni(II) center bound to a singly reduced α-diimine ligand. The
localization of spin density almost exclusively on the α-diimine
ligand likely arises from the weaker ligand field at nickel, a result
of replacing the vinyl group for a κ2-bound carboxylate.
Ni(II) center, supported by a singly reduced α-diimine.
While (iPrDI)Ni(CHCH2) is dimeric in the solid state, the
observed EPR spectrum and paramagnetically shifted signals of
the 1H NMR spectrum suggest either formation of a detectable
amount of monomer in solution or a persistent dimeric structure
in solution with negligible coupling between the two metal
complexes (Figure 2, bottom).14 While there is close contact
between the α-vinylic carbon atoms in the X-ray structure
(1.482(3) Å), the experimental data do not support the existence
of a carbon−carbon bond. This was supported by degradation
analysis: ethane was observed upon addition of 4 atm of H2 to the
complex, and ethylene was observed upon addition of MeOH.
tom Dieck and co-workers have isolated and characterized
bimetallic α-diimine nickel complexes of similar atomic
composition,15 yet these are distinctly different from [(iPrDI)-
Ni(CHCH2)]2, as they are derived from alkyne, contain
planar C4 bridging ligands, and are diamagnetic.
With an isolated nickel−vinyl complex in hand, its reactivity
with CO2 was examined. Addition of 1 atm of CO2 to a benzene-
d6 solution containing (iPrDI)Ni(CHCH2) was monitored by
1H NMR spectroscopy (Figure 3, top). Over the course of 2 h,
To understand how nickel vinyl and acrylate complexes would
behave in a potential catalytic setting, their reactivity with acrylic
acid18 was studied. A frozen benzene-d6 solution of (iPrDI)Ni-
(CHCH2) was charged with 1 equiv of acrylic acid and
monitored by 1H NMR spectroscopy after warming to ambient
temperature. After 10 min, free butadiene was observed along
with >98% conversion to a new, diamagnetic Cs symmetric
complex, which was identified as (iPrDI)Ni(η2-CH2
CHCO2H) (Scheme 2, top). This implies that coordination of
Scheme 2. Reaction of (iPrDI)Ni(CHCH2) with Acrylic
Acid: Observation of a Ni(0) Acrylic Acid Complex
Figure 3. (top) Addition of (iPrDI)Ni(CHCH2) to CO2. (bottom
left) X-band EPR spectrum of (iPrDI)Ni(κ2-O2CCHCH2) at 296 K in
fluid benzene. (bottom right) Spin density plot of computed
(iPrDI)Ni(κ2-O2CCHCH2). Mulliken spin populations: iPrDI, 0.94;
Ni, −0.05 + 0.05.
resonances corresponding to the nickel−vinyl complex dis-
1
appeared and produced a featureless H NMR spectrum. To
elucidate the identity of the new NMR-silent complex, HCl·
(dioxane) was added to the reaction mixture. After the solution
was degassed, the volatile components were vacuum-distilled
and analyzed by 1H NMR spectroscopy, which established the
formation of acrylic acid, supporting the insertion of CO2 into
the nickel−vinyl bond. The infrared spectrum of the product
contained a C−N stretch of the diimine ligand at ν(KBr) 1641
cm−1, consistent with other (iPrDI)Ni complexes of this type.16 A
C−O stretch of the acrylate ligand was located at ν(KBr) 1621
cm−1, a value that shifted to 1584 cm−1 when the reaction was
conducted with 13CO2. These values are consistent with other κ2-
acrylate and carboxylate ligands,17 which taken together with the
other IR and degradation data suggest that the product of the
reaction is (iPrDI)Ni(κ2-O2CCHCH2).
acrylic acid to (iPrDI)Ni(CHCH2) induces net bimetallic
reductive elimination of the vinyl groups to form butadiene.
Coordination-induced reductive elimination has been demon-
strated with nickel using electron-poor olefins and other π-
accepting ligands.19
1H NMR resonances for the imine methyl groups of
(iPrDI)Ni(η2-CH2CHCO2H) were located at δ 0.13 and
−0.04 ppm, characteristic of Cs-symmetric Ni(0) α-diimine
The reaction was also analyzed by X-band EPR spectroscopy,
and revealed >98% consumption of the starting material after 2 h
C
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olefin complexes.20,21 The carboxylic acid proton resonance
appears at δ 12.6 ppm, while the olefinic protons of acrylic acid
are upfield shifted to δ 2.45, 2.00, and 1.51 ppm. (iPrDI)Ni(η2-
CH2CHCO2H) was also formed in >98% conversion when
(iPrDI)Ni(η4-C4H6) was exposed to 1 equiv of acrylic acid by
straightforward ligand substitution. (iPrDI)Ni(η2-CH2
CHCO2H) is metastable and underwent clean oxidation to
(iPrDI)Ni(κ2-O2CCHCH2) (with observed release of H2)
over the course of 2 h in benzene-d6 at ambient temperature. The
identity of the resulting metal complex was confirmed by both
1H NMR and EPR spectroscopy. Due to the limited stability of
(iPrDI)Ni(η2-CH2CHCO2H), the methyl acrylate derivative,
previously characterized by Brookhart and co-workers,21 was
synthesized for comparison and exhibited remarkably similar 1H
NMR resonances and corroborates the assignment of the η2-
acrylic acid complex (see the Supporting Information for
synthetic details). Single crystals of (iPrDI)Ni(η2-CH2
CHCO2Me) suitable for X-ray diffraction were obtained from
a concentrated toluene solution at −35 °C for 48 h. A
representation of the solid-state structure is presented in Scheme
2, where, on the basis of the bond distances, the nickel is bound
to a neutral form of the α-diimine chelate. The C(sp2)−C(sp2)
bond of the η2-bound methyl acrylate ligand is significantly
elongated to 1.431(3) Å, indicating a high degree of π back-
donation from nickel.
In summary, an α-diimine nickel vinyl complex was
synthesized and spectroscopically and structurally characterized
and is a rare example of an isolable unsubstituted transition-
metal−vinyl compound. Its reactivity with CO2 and acrylic acid
was demonstrated where insertion of CO2 into (iPrDI)Ni(CH
CH2) yielded (iPrDI)Ni(κ2-O2CCHCH2), which was also
characterized and its electronic structure determined. Addition
of acrylic acid to either (iPrDI)Ni(CHCH2) or (iPrDI)Ni(η4-
C4H6) produced metastable (iPrDI)Ni(η2-CH2CHCO2H),
which slowly converted to the nickel acrylate by a currently
unknown oxidative pathway that releases H2. Efforts to remove
acrylate from the nickel center to render this process catalytic are
currently under investigation.
Paul J. Chirik: 0000-0001-8473-2898
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank SABIC for financial support.
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ASSOCIATED CONTENT
* Supporting Information
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00350.
Experimental procedures and data, NMR spectra, crystal
data, and details of the calculations (PDF)
Cartesian coordinates of calculated structures (TXT)
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data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
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AUTHOR INFORMATION
Corresponding Author
*E-mail for P.J.C.: pchirik@princeton.edu.
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ORCID
Matthew V. Joannou: 0000-0002-0079-7107
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Mate J. Bezdek: 0000-0001-7860-2894
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