Oxidative C-C bond formation reactivity of organometallic Ni(II), Ni(III), and Ni(IV) complexes

Article abstract of DOI:10.1021/jacs.6b10303

The use of the tridentate ligand 1,4,7-trimethyl-1,4,7-triazacyclononane (Me₃tacn) and the cyclic alkyl/aryl C-donor ligand-CH₂CMe₂-o-C₆H₄-(cycloneophyl) allows for the synthesis of isolable organometallic Ni^II, Ni^III, and Ni^IV complexes. Surprisingly, the fivecoordinate Ni^III complex is stable both in solution and the solid state, and exhibits limited C-C bond formation reactivity. Oxidation by one electron of this Ni^III species generates a six-coordinate Ni^IV complex, with an acetonitrile molecule bound to Ni. Interestingly, illumination of the Ni^IV complex with blue LEDs results in rapid formation of the cyclic C-C product at room temperature. This reactivity has important implications for the recently developed dual Ni/photoredox catalytic systems proposed to involve high-valent organometallic Ni intermediates. Additional reactivity studies show the corresponding Ni^II species undergoes oxidative addition with alkyl halides, as well as rapid oxidation by O₂, to generate detectable Ni^III and/or Ni^IV intermediates and followed by C-C bond formation.

Full text of DOI:10.1021/jacs.6b10303

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Communication
Oxidative C-C Bond Formation Reactivity of
Organometallic Ni(II), Ni(III) & Ni(IV) Complexes
Michael B. Watson, Nigam P. Rath, and Liviu M. Mirica
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 20 Dec 2016
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Oxidative C-C Bond Formation Reactivity of Organometallic Ni(II), Ni(III)
& Ni(IV) Complexes
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†
Michael B. Watson†, Nigam P. Rath‡, and Liviu M. Mirica,*,
†Department of Chemistry, Washington University, One Brookings Drive, St. Louis, Missouri 63130-4899, United States
‡Department of Chemistry and Biochemistry, University of Missouri-St. Louis, One University Boulevard, St. Louis, Missouri 63121-
4400, United States
Supporting Information Placeholder
formation reactivity, the six-coordinate NiIV complex generates
ABSTRACT: The use of the tridentate ligand 1,4,7-trimethyl-1,4,7-
quantitatively the cyclic C-C product upon illumination with blue
triazacyclononane (Me3tacn) and the cyclic alkyl/aryl C-donor ligand -
LEDs, which is relevant to the recently reported dual Ni/photoredox
CH2CMe2-o-C6H4- (cycloneophyl) allows for the synthesis of isolable
catalytic systems proposed to involve high-valent organometallic Ni
organometallic NiII, NiIII, and NiIV complexes. Surprisingly, the five-
intermediates.10 In addition, (Me3tacn)NiII(cycloneophyl) undergoes
coordinate NiIII complex is stable both in solution and the solid state,
oxidative addition with alkyl halides, as well as rapid oxidation by O2, to
and exhibits limited C-C bond formation reactivity. Oxidation by one
generate detectable NiIII and NiIV intermediates followed by reductive
electron of this NiIII species generates a six-coordinate NiIV complex,
elimination to form new C-C bonds. Overall, these studies set the stage
with an acetonitrile molecule bound to Ni. Interestingly, illumination
for detailed investigations of the reactivity of high-valent
of the NiIV complex with blue LEDs results in rapid formation of the
organometallic Ni complexes relevant to Ni-catalyzed reactions.
cyclic C-C product at room temperature. This reactivity has important
implications for the recently developed dual Ni/photoredox catalytic
systems proposed to involve high-valent organometallic Ni intermedi-
ates. Additional reactivity studies show the corresponding NiII species
undergoes oxidative addition with alkyl halides, as well as rapid oxida-
tion by O2, to generate detectable NiIII and/or NiIV intermediates and
followed by C-C bond formation.
Scheme 1. Synthesis of (Me3tacn)Ni(CH2CMe2-o-C6H4)
Complexes
Nickel-based catalysts are commonly employed in a wide range of
C-C and C-heteroatom bond formation reactions.1 While traditionally
these processes have been considered to involve Ni0, NiI, and NiII
intermediates,2 many studies proposed high-valent NiIII species as key
intermediates in the C-C/C-heteroatom bond formation step.3
Moreover, NiIV species have also emerged as potential intermediates
involved in catalytic reactions.3g,4 However, the number of
The yellow NiII complex (Me3tacn)NiII(CH2CMe2-o-C6H4), 1, was
synthesized through ligand exchange of (py)2NiII(CH2CMe2-o-C6H4)11
with Me3tacn in pentane (Scheme 1).12 The single crystal X-ray
structure of 1 reveals a square planar geometry around the NiII center
with Me3tacn acting as a bidentate ligand (Figure 1). Moreover, 1 is
diamagnetic and its 1H NMR spectrum reveals only one singlet
corresponding to the N-Me groups, suggesting a rapid exchange
between the unbound and bound N-donor atoms, as seen before for
analogous (Me3tacn)PdII complexes.8
The cyclic voltammogram (CV) of 1 in 0.1 M (nBu4N)PF6/MeCN
shows a pseudoreversible redox event at E1/2 = - 1358 mV vs ferrocene
(Fc), followed by a smaller oxidation wave at -530 mV and a reversible
oxidation at E1/2 = - 361 mV (Figure 2). Since the presence of two
anionic carbon ligands is expected to stabilize high-valent Ni centers,
the observed reversible oxidations are tentatively assigned to the NiIII/II
and NiIV/III redox couples, respectively. The oxidation wave at -530 mV
is proposed to correspond to the conformation of 1 in which the
Me3tacn ligand adopts a κ2 binding mode, and in line with a similar
oxidation potential observed by Sanford et al. for the
(bpy)NiII(cycloneophyl) complex supported by a bidentate ligand.7f
As such, the reversible NiIII/II redox couple observed at -1358 mV likely
corresponds to the conformation of 1 with Me3tacn in a κ3 binding
5,6
7
organometallic NiIII and NiIV complexes isolated to date is limited,
which hinders detailed reactivity studies to probe their involvement in
catalysis.
We have recently reported the use of the ligand 1,4,7-trimethyl-
1,4,7-triazacyclononane (Me3tacn) to stabilize high-valent
organometallic Pd complexes.8 In addition, the C-donor cyclic ligand -
CH2CMe2-o-C6H4- (cycloneophyl) has been employed starting more
than two decades ago by Carmona et al.9 to stabilize organometallic Ni
complexes, since the corresponding Ni(cycloneophyl) species exhibit
limited reductive elimination and β-hydride elimination reactivity.
Recently, Sanford et al. have elegantly employed the cycloneophyl
ligand to isolate novel NiIV complexes and investigate their C-
heteroatom bond formation reactivity.7f Herein, we report the
synthesis, characterization, and initial reactivity studies of NiII, NiIII,
and NiIV complexes supported by Me3tacn and containing the
cycloneophyl ligand (Scheme 1). To the best of our knowledge, these are
the first organometallic Ni complexes supported by
a 1,4,7-
triazacyclononane-derived ligand. While the isolated five-coordinate NiIII
mode. Indeed,
1
can be readily oxidized with ferrocenium
complex is uncommonly stable and exhibits limited C-C bond
1
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Page 2 of 5
hexafluorophosphate (FcPF6) to yield the orange product
Figure 2. CV of 1 in 0.1 M nBu4NPF6/MeCN at RT (100 mV/s scan
[(Me3tacn)NiIII(CH2CMe2-o-C6H4)]PF6, [2]PF6 (Scheme 1). The X-
ray structure of [2]PF6 shows a five-coordinate NiIII center in a square
pyramidal geometry (Figure 1B), as supported by the calculated trigo-
nality index parameter τ = 0.14.13 Interestingly, the Ni-N and Ni-C
bond lenghts in 2+ are longer that those in 1, which may be due to an
increased steric hindrance at the Ni center of 2+, and a likely decrease
in orbital overlap upon moving the C/N donor atoms out of the
equatorial plane. The five-coordinate geometry is maintained in MeCN,
since the EPR spectrum of 2+ in 1:3 MeCN:PrCN reveals a rhombic
signal with super-hyperfine coupling to only one nitrogen atom in the
gz direction, suggesting that the nitrile solvent does not coordinate to
the NiIII center (Figure 3). The EPR spectrum also suggests the
rate). Redox potentials: E1/2(NiIII/II) = –1358 mV; E1/2(NiIV/III) = –361
1
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mV.
gx = 2.311
9
gy = 2.263
Exp
Sim
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presence of a NiIII d7 center with a dz
ground state, which is supported
2
gz = 2.102
Az(1N) = 17.5 G
by the DFT-calculated spin density for 2+ (Figure 3, inset).12 Finally, 2+
is stable at RT for weeks as a solid and for several days in MeCN, which
is surprising for a five-coordinate organometallic NiIII complex that is
usually expected to undergo rapid reductive elimination.3
2600
2800
3000
3200
3400
Field (G)
Figure 3. Experimental (1:3 MeCN:PrCN, 77 K) and simulated EPR
spectra of 2+. The following parameters were used for the simulation: gx
= 2.311, gy = 2.263, gz = 2.102 (A1N = 17.5 G). Inset: DFT-calculated
spin density for 2+, shown as a 0.05 isodensity contour plot.
A)
B)
Oxidation of the NiIII complex [2]BF4 with acetylferrocenium
tetrafluoroborate (AcFcBF4) at -30 °C in MeCN generates a red,
diamagnetic product (Scheme 1).14 NMR analysis suggests the
formation of a NiIV species (Figures S16-S20), as exemplified in 1H
NMR by the downfield shift to 5.5 ppm of the peak corresponding to
NiIV-CH2- protons vs. the 2.1 ppm value observed for 1. Structural
characterization confirms the formation of [(Me3tacn)NiIV(CH2CMe2-
o-C6H4)(MeCN)](BF4)2, [3](BF4)2, in which the NiIV center adopts
an octahedral geometry with an MeCN molecule as the sixth ligand.
Surprisingly, the Ni-C bonds lengths in 32+ are longer than those in
both 2+ and 1, likely due to an increased steric hindrance, while the
axial Ni-Namine bond length is shorter than that in 2+, due to the
preference of a NiIV d6 center to adopt an octahedral geometry. While
32+ is the first dicationic NiIV complex isolated to date and it would be
expected to be highly electrophilic,7f its uncommon stability is likely due to
its coordinatively saturated nature. Overall, 1, 2+, and 32+ establish an
uncommon series of isolable, organometallic NiII, NiIII, and NiIV complexes
that can be used to compare the reactivity of these Ni oxidation states in
organometallic reactions.15
C)
Figure 1. ORTEP representation (50% probability thermal ellipsoids)
of 1 (A), 2+ (B), and 32+ (C, 30% probability thermal ellipsoids). Se-
lected bond lengths (Å) and angles (°): 1, Ni1-C1 1.904(3); Ni1-C2
1.930(3); Ni1-N1 2.059(3); Ni1-N2 2.069(2); C1-Ni1-N1 178.0(2);
C2-Ni1-N2 175.1(1); 2+, Ni1-C1 1.976(1); Ni1-C2 1.965(1); Ni1-
N1 2.100(1); Ni1-N2 2.100(1); Ni1-N3 2.083(1); C1-Ni1-N1
173.70(3); C2-Ni1-N2 165.33(3); 32+, Ni1-C1 2.032(8); Ni1-C2
1.973(8); Ni1-N1 2.094(6); Ni1-N2 2.098(6); Ni1-N3 1.977(6); Ni1-
N4 1.858(7); C1-Ni1-N1 175.0(3); C2-Ni1-N2 172.4(3). The space
filling model of 2+ is also shown – as viewed from the bottom along the
axial axis.
With these organometallic NiII, NiIII, and NiIV complexes in hand, we
set out to probe their reactivity in C-C and C-heteroatom bond
formation reactions. First, we studied the ability of the NiII complex 1
to undergo oxidative addition with organic halides, since such a step
has been proposed to occur in several catalytic reactions.3g,4
Gratifyingly, the addition of iodomethane to 1 led to an immediate
formation of a deep red solution that faded to pale yellow after three
hours at RT, and analysis of the reaction mixture by GC-MS upon
acidic workup revealed the formation of three C-C coupled products
that account for 98% of the neophyl ligand (Scheme 2).12 The 2-
methyl-t-butylbenzene product likely forms upon oxidative addition of
MeI to 1 followed by subsequent Csp2-Csp3 reductive elimination. The
dimeric methylated product formed in 30% yield may result upon
bimetallic ligand exchange occuring after the first Csp2-Csp3 reductive
elimination, followed by a Csp3-Csp3 reductive elimination to yield the
observed product. Moreover, the formation of a dimethylated product
in 6% yield suggests that more than one oxidative addition of MeI can
occur at the same Ni center, and sequential reductive eliminations can
form two new C-C bonds. Alternatively, a methyl group transfer at a
-1395 mV
0.5
-410 mV
0.0
-1321 mV
-530 mV
-0.5
-312 mV
-0.5
-1.0
-1.5
Potential vs Fc (V)
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high-valent Ni center, followed by reductive elimination, could also
organometallic NiII complex have been shown to undergo oxidation by
O2 and subsequent C-C bond formation through a high-valent Ni
intermediate, although in those cases no C-O bond formation was
reported.7d,16
account for the dimethylated product.16 Performing the addition of
MeI to 1 in presence of the radical trap TEMPO leads to no detectable
methylation products, instead the cyclized product 1,1-
dimethylbenzocyclobutene was formed in 23% yield.12 This result
1
2
3
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6
7
8
Having isolated both NiIII and NiIV complexes 2+ and 32+, we then
evaluated their C-C bond formation reactivity. Upon heating 2+ or 32+
in MeCN at 80 °C, 1,1-dimethylbenzocyclobutene was generated in
only 42% and 34% yield, respectively (Scheme 2).12 This is in contrast
to related NiIV(cycloneophyl) complexes reported recently by Sanford
et al. that undergo almost quantitative formation of the cyclized
product, and this may be due to the dicationic nature of 2+ and 32+ that
leads to less selective reactivity.7f Interestingly, the yield of C-C
coupled product was found to be dependent on the presence of light,
and exposure of 32+ in MeCN to blue LEDs (470 nm) leads to
formation of 1,1-dimethylbenzocyclobutene in 91% yield in 30 min at
RT. Performing the photolysis in the presence of TEMPO does not
affect the yield of product significantly, sugesting that no free radicals
are generated during the reaction.12 We tentatively assign the observed
photoreactivity to an LMCT excitation of an electron into an
antibonding MO that leads to the dissociation of the axial N donor to
suggests
a single electron transfer (SET) oxidative addition
mechanism, as proposed previously for other Ni systems.3-4 Finally,
addtion of either iodobenzene or ethyl bromide to 1 leads to the
corresponding 2-substituted t-butylbenzene products in 50% and 23%
yields, respectively, suggesting that the proposed oxidative addition to
1 can occur for a range of organic halides.12
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Scheme 2. Reactivity of Complexes 1, 2+, and 32+ (the Bonds
Shown in Bold Represent New Bonds Formed in the Reaction)
yield
a 5-coordinate intermediate, which then undergoes rapid
reductive elimination.17 By comparison, exposure of 2+ to blue LEDs
does not increase the yield of coupled product vs. the thermolysis
reaction (Scheme 2). Overall, the observed photoreactivity is
particularly relevant to the recently reported C-C/C-X bond formation
reactions employing dual Ni/photoredox catalysis,10 and suggests that
organometallic Ni complexes can be directly activated by light and
undergo rapid reductive elimination.
Finally, the reactivity of both 2+ and 32+ in the presence of various
nucleophiles was also investigated. Surprisingly, addition of (Et4N)Cl,
(Et4N)OH, (nBu4N)OAc, (nBu4N)N3, or (Me4N)NHMs to a CD3CN
solution of 2+ did not lead to formation of any C-heteroatom bond
formation product even upon heating at 80 °C, only the C-C product
1,1-dimethylbenzocyclobutene being observed in yields between 24%
and 73% (Table S3).12 The lack of any C-X products could be attribut-
ed to the sterically hindered Ni center (Figure 1B). Indeed, no change
in the EPR spectra of these solutions was observed upon addition of
the nucleophile, supporting the lack of coordination to the NiIII center.
By comparison, addition of the same nucleophiles to a CD3CN solu-
tion of 32+ leads to a rapid disappearance of the NiIV complex, yet the
yields of C-C coupled products are diminished to 10-25%. The EPR
spectra of the reaction mixtures reveal the formation of the NiIII
complex 2+ in up to 50% yield, that could form upon the
comproportionation of 32+ with a reductive elimination NiII product.12
However, the addition of (Et4N)Cl to 32+ also generates the products
2-chloro-t-butylbenzene and neophyl chloride in 16% and 13% yield,
respectively. While the yields are low, these results suggest that both
Cl-Csp2 and Cl-Csp3 bond formation reactions can occur, either through
coordination of Cl– to the NiIV center followed by concerted reductive
elimination, or through an SN2-type reductive elimination, the latter
reaction pathway being also observed by Sanford et al.7f Overall, the
limited C-heteroatom bond formation reactivity of both 2+ and 32+ may
be due to their sterically hindered coordination environment, despite
being quite electrophilic cationic centers.
Next, we explored the oxidatively-induced reactivity of 1 in the
presence of mild oxidants. Excitingly, exposure of 1 to O2 in 9:1
CD3CN:D2O led to rapid formation of a deep red solution and analysis
1
by H NMR revealed the formation of 1,1-dimethylbenzocyclobutene
in 40% yield (Figure S26). GC-MS analysis confirmed the presence of
the C-C coupled product, along with 5% of the C-O product 2-tert-
butylphenol (Scheme 2).12 Performing the oxidation of 1 with O2 in
9:1 d6-acetone:D2O at -30 °C allowed the detection by 1H NMR of
chemical shifts that are tentatively assigned to a NiIV complex similar to
2+, formed in 40% yield (Figure S29). Furthermore, analysis of the
In conclusion, reported herein are the first organometallic Ni
complexes supported by a triazacyclononane-derived ligand. Moreover,
use of the alkyl/aryl C-donor cycloneophyl ligand allowed the isolation
of uncommon organometallic NiII, NiIII, and NiIV complexes. The five-
coordinate NiIII complex is suprisingly stable both in solution and the
solid state and exhibits limited C-C bond formation reactivity. In
contrast, the six-coordinate NiIV complex has an MeCN molecule
bound to Ni and it undergoes rapid C-C bond formation upon
3
reaction mixture by EPR shows the formation of the NiIII species in 50%
yield (Figure S30). While detailed mechanistic studies of this aerobic
reactivity are currently underway, these initial studies suggest O2 is
capable of rapidly oxidizing 1 to generate both NiIII and NiIV species,
followed by formation of C-C and C-O coupled products, and thus can
be potentially employed in catalytic Ni-mediated aerobic
transformations. To the best of our knowledge, only two other
3
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Zoet, R.; van der Linden, J. G. M.; Legters, J.; Schmitz, J. E. J.; Murrall, N.
exposure to blue LEDs. This photoreactivity may have important
mechanistic implications for the recently developed dual
Ni/photoredox catalytic systems proposed to involve high-valent
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1
2
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organometallic
Ni
intermediates.
Furthermore,
the
(Me3tacn)NiII(cycloneophyl) complex undergoes oxidative addition
with alkyl halides, as well as rapid oxidation by O2, to generate
detectable NiIII and NiIV intermediates and followed by reductive
elimination to form new C-C bonds. Current studies are focused on
taking advantage of the ability of the Me3tacn ligand to stabilize high-
valent Ni species and employ such organometallic Ni complexes in
transformations involving rapid oxidative addition and/or aerobic
oxidation steps for catalytic C-C and C-heteroatom bond formation
reactions
9
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ASSOCIATED CONTENT
Supporting Information
Synthetic details, spectroscopic characterization, reactivity studies, and
crystallographic data. This material is available free of charge via the
Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
mirica@wustl.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We thank the National Science Foundation (CHE-1255424) for sup-
port. The purchase of a Bruker EMX-PLUS EPR spectrometer was
supported by the National Science Foundation (MRI, CHE-1429711).
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(17) Detailed photoreactivity and compuational studies of these
organometallic high-valent Ni complexes are currently underway.
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