Experimental and Computational Studies of High-Valent Nickel and Palladium Complexes

Article abstract of DOI:10.1021/acs.organomet.7b00613

This article describes a detailed comparison of the organometallic chemistry of high-valent nickel and palladium model complexes supported by tris(pyrazolyl)borate and cycloneophyl ligands. The accessibility of the M^III and M^IV oxidation states with each metal is investigated through electrochemical and chemical oxidation of the M^II precursors. These studies show that the Ni^II precursor readily undergoes both one- and two-electron oxidations to generate stable Ni^III and Ni^IV products. In contrast, under the conditions examined, the Pd^II analogue undergoes exclusively two-electron-oxidation reactions to form Pd^IV. Reactivity studies of isolated Ni^IV and Pd^IV complexes show that both participate in C(sp³)-heteroatom coupling reactions and that the reactions at Ni^IV are approximately 2 orders of magnitude faster than those at Pd^IV. Experimental and computational mechanistic studies implicate outer-sphere S_N2-type pathways for these processes. With most nucleophiles (e.g., phenoxide, acetate, thiophenoxide), the C(sp³)-heteroatom coupling reaction yields a TpM^II(σ-aryl) product. However, with azide as the nucleophile, the Ni^II product of initial C(sp³)-N₃ coupling undergoes a subsequent C(sp²)-N insertion reaction. Computations implicate an anionic Ni^III-nitrene intermediate in this process and show that the Pd analogue of this species is a much higher energy species. Overall, the combined experimental and computational studies demonstrate remarkable similarities in the chemistry of Ni^IV and Pd^IV but an enhanced role for Ni^III in enabling reactivity which is distinct from that of palladium.

Full text of DOI:10.1021/acs.organomet.7b00613

Article
pubs.acs.org/Organometallics
Experimental and Computational Studies of High-Valent Nickel and
Palladium Complexes
Nicole M. Camasso, Allan J. Canty,* Alireza Ariafard,‡ and Melanie S. Sanford*,†
†
,‡
†Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, United States
‡School of Physical Sciences, University of Tasmania, Hobart, Tasmania 7001, Australia
*
S Supporting Information
ABSTRACT: This article describes a detailed comparison of
the organometallic chemistry of high-valent nickel and
palladium model complexes supported by tris(pyrazolyl)borate
III
IV
and cycloneophyl ligands. The accessibility of the M and M
oxidation states with each metal is investigated through
II
electrochemical and chemical oxidation of the M precursors.
II
These studies show that the Ni precursor readily undergoes
III
both one- and two-electron oxidations to generate stable Ni
IV
and Ni products. In contrast, under the conditions examined,
the Pd analogue undergoes exclusively two-electron-oxidation reactions to form Pd . Reactivity studies of isolated Ni and Pd
complexes show that both participate in C(sp )−heteroatom coupling reactions and that the reactions at Ni are approximately
orders of magnitude faster than those at Pd . Experimental and computational mechanistic studies implicate outer-sphere S 2-
II
IV
IV
IV
3
IV
IV
2
N
3
type pathways for these processes. With most nucleophiles (e.g., phenoxide, acetate, thiophenoxide), the C(sp )−heteroatom
II
II
3
coupling reaction yields a TpM (σ-aryl) product. However, with azide as the nucleophile, the Ni product of initial C(sp )−N
3
2
III
coupling undergoes a subsequent C(sp )−N insertion reaction. Computations implicate an anionic Ni −nitrene intermediate in
this process and show that the Pd analogue of this species is a much higher energy species. Overall, the combined experimental
IV
IV
III
and computational studies demonstrate remarkable similarities in the chemistry of Ni and Pd but an enhanced role for Ni in
enabling reactivity which is distinct from that of palladium.
INTRODUCTION
have begun to investigate the synthesis and reactivity of
■
III
IV
organometallic Ni and Ni complexes, and these inves-
tigations have provided support for their involvement in
challenging bond-forming reactions. However, despite these
contributions, very little work has directly compared the relative
reactivity of high-valent Ni to that of its group 10 congener,
Over the past several decades, fundamental organometallic
studies of high-valent Pd complexes have helped to establish
IV
that Pd intermediates play a key role in a variety of important
1
,2
IV
transformations. This work has demonstrated that Pd can
be accessed under mild reaction conditions using a diverse suite
6
g,13
1
,2
IV
Pd.
A systematic comparison of the organometallic
of two-electron oxidants. Furthermore, the Pd intermedi-
ates can undergo challenging reductive elimination reactions
chemistry of high-valent Ni to the more established chemistry
of high-valent Pd would provide insights into the similarities
and differences between these systems. This could ultimately
inform the rational development of new group 10 metal
catalyzed reactions.
II
that are often complementary to those occurring from Pd
1
−3
III
centers.
While Pd complexes are less common in the
literature, studies by Ritter, Mirica, and others have
demonstrated the competency of both monomeric and dimeric
III
This article describes a detailed comparison of the organo-
metallic chemistry of high-valent Pd and Ni complexes bearing
tris(pyrazolyl)borate (Tp) and cycloneophyl ligands (Figure 1).
We systematically investigate: (i) the accessibility of high-valent
Pd intermediates for mediating carbon−carbon and carbon−
4
heteroatom bond-forming reactions. Overall, these fundamen-
tal studies have driven advances in the field of high-valent Pd
catalysis, which is now widely used for transformations such as
C−H functionalization and alkene difunctionalization.
In recent years, tremendous progress has also been made in
the field of nickel catalysis. In addition to being a sustainable
and low-cost alternative to palladium, new developments in
organonickel chemistry have demonstrated that nickel enables
transformations that are often not accessible with palladium
e.g., cross-coupling reactions that utilize tertiary alkyl halides
IV
III
5
Pd (Pd and/or Pd ) complexes relative to their Ni analogues,
(
ii) the relative reactivity and selectivity of carbon−carbon and
IV
6
carbon−heteroatom bond-forming reactions at Pd versus
IV
Ni , and (iii) the mechanistic pathways of these C−C/C−X
coupling reactions. Overall, our experimental and computa-
tional studies demonstrate that Ni undergoes one-electron
III
7
redox chemistry more readily than Pd to access isolable Ni
(
8
9
or phenol derivatives as electrophiles). However, in
comparison to Pd, the organometallic chemistry of high-valent
Ni remains underdeveloped. Our laboratory and others
Received: August 10, 2017
1
0
11,12
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00613
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Article
Figure 1. Proposed model systems of analogous high-valent Pd and Ni
model complexes.
and NiIV species. The accessibility of these distinct oxidation
states enables transformations and mechanistic pathways not
seen at the palladium analogues.
RESULTS AND DISCUSSION
Electrochemistry. We first studied the accessibility of high-
■
valent Ni and Pd complexes by comparing the electrochemistry
II
of the M complexes 1-Ni and 1-Pd. Prior studies from our
1
r,10c−g,14
12g,15
group,
and others,
have demonstrated that
tris(pyrazolyl)borate (Tp) and cycloneophyl ligands are
particularly stabilizing to high-valent group 10 metal complexes.
II
Thus, we targeted M complexes bearing these ligands for the
investigations described herein. As shown in Scheme 1a,
II
Scheme 1. Synthesis of M Precursors (a)
II
K[(Tp)Ni (CH CMe -o-C H )] (1-Ni) and (b)
2
2
6
4
II
NMe [(Tp)Pd (CH CMe -o-C H )] (1-Pd)
4
2
2
6
4
Figure 2. Cyclic voltammograms of MII precursors 1-Ni and 1-Pd.
Conditions: [Ni] = 0.01 M in MeCN, [NBu PF ] = 0.1 M, scan rate
4
6
1
00 mV/s; [Pd] = 0.005 M in MeCN/pyr, [NBu PF ] = 0.1 M, scan
4 6
rate 100 mV/s.
1
7,18
complexes and related Pd complexes.
The propensity for
II
II
two-electron redox at Pd and Pt has been attributed to the
instability of the corresponding M species.
III
17−19
The large peak separation between the most positive
oxidation and reduction waves in both CVs can be rationalized
on the basis of the molecular reorganization that accompanies
an octahedral/square-pyramidal or octahedral/square-planar
17,18,20
interconversion.
Overall, the CVs of 1-Ni and 1-Pd
complex 1-Ni was obtained in 93% yield from the reaction of
suggest that while Ni can access distinct one-electron-redox
pathways, the analogous Pd complex preferentially undergoes
II
10c
II
(
PMe ) Ni (CH CMe -o-C H ) with KTp.
Similarly, the
3
2
2
2
6
4
treatment of (COD)Pd(CH CMe -o-C H ) (COD = cyclo-
two-electron-redox events. Moreover, while the +4 oxidation
states of Ni and Pd can be accessed at relatively similar onset
2
2
6
4
octadiene) with 1 equiv of NMe Tp afforded 1-Pd in 89%
4
isolated yield (Scheme 1b).
potentials (approximately +0.10 and −0.10 V, respectively), the
II/III
Figure 2 displays the cyclic voltammograms (CVs) of 1-Ni
and 1-Pd in acetonitrile with tetrabutylammonium hexafluor-
ophosphate (NBu PF ) as the supporting electrolyte. The CV
Ni
redox event occurs at a significantly lower onset potential
(approximately −1.1 V). The difference between the
accessibilities of the high oxidation states for nickel and
4
6
IV
of 1-Ni reveals two quasi-reversible redox couples with onset
palladium mirror the trends seen in the literature, in which Pd
+
III
III
IV
poentials of approximately −1.1 and +0.10 V versus Fc/Fc . We
and Ni complexes are more common than their Pd and Ni
II/III
III/IV
1,4,10−12
assign these to the Ni
and Ni
couples, respectively.
counterparts.
II/III
Notably, the onset potential associated with the Ni
among the lowest reported for an organometallic Ni complex.
This is likely due to the strong electron-donor properties of the
Tp and cycloneophyl ligands. The observation of two distinct
redox couples in this system highlights the propensity of nickel
to undergo single-electron-transfer chemistry.
couple is
Chemical Oxidations. The chemical oxidation of 1-Ni and
1-Pd was next evaluated using acetylferrocenium tetrafluor-
III
oborate (AcFcBF ) as an outer-sphere one-electron oxidant.
4
16
This oxidant was selected due to its suitable redox potential (E°
+
= +0.27 V versus Fc/Fc ) as well as its high solubility under the
21
reaction conditions. The treatment of 1-Ni with 1 equiv of
II
In comparison, the CV of Pd complex 1-Pd (Figure 2)
shows a single oxidation wave with an onset potential of
AcFcBF resulted in complete consumption of the diamagnetic
4
II
1
11
Ni starting material and the formation of H and B NMR
signals consistent with a paramagnetic product (Figure 3). The
observed resonances correspond to those that were previously
approximately −0.10 V, along with a corresponding reduction
+
II/IV
at −0.80 V vs Fc/Fc . We assign these features to a Pd
III
10e,22
redox couple. CVs showing net two-electron-transfer processes
reported for the Ni complex 2-Ni.
In contrast, the use of
have been reported previously for mer-coordinated Pt
2 equiv of AcFcBF4 under otherwise identical conditions
B
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Article
1
Figure 4. H NMR spectra of the reaction of 1-Pd with 1 or 2 equiv of
AcFcBF4.
next pursued neutral analogues of 3-Ni and 3-Pd containing the
trifluoromethyl ligand in order to conduct a detailed
IV
comparison of the structures and reactivities of the Pd and
IV
Ni congeners.
The neutral Ni trifluoromethyl complex 4-Ni was prepared
via the treatment of 1-Ni with the electrophilic trifluorome-
thylating reagent S-(trifluoromethyl)dibenzothiophenium tri-
flate (Umemoto’s reagent) and subsequent purification by silica
1
IV
Figure 3. H NMR spectra of the reaction of 1-Ni with 1 or 2 equiv of
III
AcFcBF , generating the paramagnetic Ni complex 2-Ni or the
diamagnetic Ni complex 3-Ni.
4
IV
10c
gel chromatography (Scheme 2a). The Pd analogue 4-Pd
resulted in the quantitative formation of a diamagnetic produ2c3t
that we assign as the cationic NiIV complex 3-Ni (Figure 3).
We next examined the analogous chemical oxidations of 1-
Scheme 2. Synthesis of (a) 4-Ni and (b) 4-Pd
Pd. The treatment of 1-Pd with 2 equiv of AcFcBF in MeCN
4
led to a complex mixture of products. However, the addition of
5
equiv of pyridine-d to this reaction mixture resulted in the
5
IV
formation of the cationic pyridine-ligated Pd complex 3-Pd in
quantitative yield, as determined by H NMR spectroscopy
1
2
4
(Figure 4). The pyridine ligand appears to stabilize the
IV
octahedral Pd product. The treatment of 1-Pd with 1 equiv of
AcFcBF under otherwise identical conditions resulted in a
4
IV
II
5
0:50 mixture of Pd complex 3-Pd and the unreacted Pd
starting material 1-Pd (Figure 4). In marked contrast to the Ni
III
system in Figure 3, no evidence for Pd products/
intermediates was observed by NMR or EPR spectroscopy.
This observation is consistent with the CV of 1-Pd, which
suggests the propensity of this Pd complex to undergo selective
two-electron-oxidation events.
IV
IV
Comparison of Ni and Pd (Synthesis and Charac-
terization). The oxidation reactions shown in Figures 3 and 4
provide evidence that the combination of the Tp and
II
was prepared from Pd precursor 1-Pd via a similar procedure
(Scheme 2b). Complexes 4-Ni and 4-Pd were characterized by
H, C, B, and F NMR spectroscopy. The H NMR spectra
of these complexes are remarkably similar and display
resonances consistent with a κ -tris(pyrazolyl)borate ligand
bound to an octahedral M center. One notable difference
between the two spectra is the chemical shift of the
IV
IV
1
13
11
19
1
cycloneophyl ligands support both Ni and Pd species.
IV
However, efforts to study the reactivity of the cationic M
3
complexes 3-Ni and 3-Pd were complicated by the presence of
labile solvent ligands as well as the propensity of 3-Pd to
decompose into complex mixtures of products. As such, we
IV
C
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Article
3
2
diastereotopic methylene protons in 4-Ni (two resonances
between 4.7 and 4.9 ppm) and 4-Pd (two resonances between
Scheme 4. Inertness of Complex 4-Pd to C(sp )−C(sp )
Coupling over Weeks at 70 °C
4
.1 and 4.2 ppm). The greater deshielding of the α-protons in
-Ni suggests a comparatively more electrophilic M −σ-alkyl
IV
4
carbon.
Crystals of 4-Ni were obtained by slow evaporation of a
methanol solution, while those of 4-Pd were obtained from a
concentrated acetone solution. The solid-state structure of each3
complex is shown in Figure 5. The Tp ligand binds in a κ
The treatment of 4-Ni and 4-Pd with NMe X (where X =
4
3
OPh, OAc, SPh, N ) led to highly selective C(sp )−heteroatom
3
II
28
coupling to form M products 6a−d (Scheme 5). No
3
IV
Scheme 5. C(sp )−Heteroatom Coupling from M
II
Complexes 4-Ni/4-Pd To Form M Products 6a−d
Figure 5. ORTEP diagrams of (a, left) 4-Ni and (b, right) 4-Pd.
Thermal ellipsoids are drawn at 50% probability. Hydrogen atoms and
disorder in the trifluoromethyl group of 4-Pd have been omitted for
clarity.
fashion to both metal centers, forming the anticipated
octahedral geometries. The Pd analogue has significantly longer
bond distances, as is expected for a second-row versus a first-
2
5
row transition metal. For example, the Pd−CF bond length
3
(
2.036 Å) is approximately 0.1 Å longer than that of the Ni
analogue (1.941 Å) and is comparable to that of related Pd −
CF complexes reported in the literature.
Comparison of Ni and Pd Reactivity. Heating Ni
complex 4-Ni at 70 °C in MeCN resulted in slow C(sp )−
C(sp ) bond-forming reductive elimination to afford the
a
b
IV
Isolated yield. NMR conversion yield; 6d-Ni was not sufficiently
c
26,27
stable for isolation. NMR conversion yield; reaction required 5 equiv
of NR X.
3
IV
IV
IV
4
3
2
3
2
2
products derived from C(sp )−C(sp ) or C(sp )−heteroatom
−9
benzocyclobutane product 5 (r = 9.1 × 10 M/s at 70 °C
with [Ni] = 0.011 M; Scheme 3b). Notably, this reaction
0
coupling were observed under any of the conditions examined
with either Ni or Pd. Kinetic studies show that these reactions
are second order overall: first order in [M ] and first order in
29
IV
3
2
Scheme 3. Initial Rate Data for C(sp )−C(sp ) Coupling
[
nucleophile]. Plots of the Swain−Scott nucleophilicity
from (a) 3-Ni and (b) 4-Ni
parameters (n ) versus the initial rates of these reactions (r )
x
0
show a linear correlation for both Ni and Pd (0.944 and 0.974,
30
respectively, Figure 6). Overall, these data are consistent with
an S 2-type mechanism for this transformation. This is
N
proceeds approximately 100-fold more slowly than the
IV
analogous transformation from the cationic Ni complex 3-
−
7
Ni (r = 9.0 × 10 M/s at 70 °C with [Ni] = 0.011 M; Scheme
0
IV
3
a). This is presumably due to stabilization of Ni by the CF
ligand. As shown in Scheme 4, the Pd analogue 4-Pd proved
even more stable and did not undergo detectable C(sp )−
C(sp ) bond-forming reductive elimination to form 5 over
3
IV
Figure 6. Swain−Scott plot relating the relative nucleophilicities (n )
x
3
with the initial rate (r ) of C−heteroatom coupling. Starting
0
2
conditions: [Ni] = 0.0044 M, [X] = 0.0054 M, T = 23 °C; [Pd] =
0.011 M, [X] = 0.057 M, T = 60 °C.
several weeks at 70 °C.
D
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generally the lowest energy pathway for C(sp )−heteroatom
Article
3
attached to the MIV center. We next pursued DFT32,33
calculations to validate this mechanism and to explore several
14b−e,31
couplings from high-valent group 10 metal centers.
While 4-Ni and 4-Pd exhibit comparable selectivity for
C(sp )−heteroatom coupling, the rates of these processes differ
dramatically as a function of the metal. For example, in the
key features of these processes, including (i) whether an open
3
IV
coordination site at M is necessary for S 2-type C−
N
heteroatom coupling and (ii) how the mechanistic pathways
3
IV
IV
IV
presence of 5 equiv of NMe OPh, C(sp )−O coupling at Ni
and transition states at Pd compare to those at Ni . At the
4
proceeds approximately 2 orders of magnitude more quickly
than the analogous reaction at Pd (r = 2.0 × 10 M/s versus
.3 × 10 M/s at 30 °C; [M] = 0.011 M). The initial rates of
C(sp )−O bond-formation from 4-Ni and 4-Pd were also
examined as a function of temperature (for Ni, − 10 to 40 °C;
for Pd, 30 to 70 °C). The resulting Eyring plots are shown in
Figure 7, and the activation parameters from this analysis are
outset, we noted that similar S 2-type mechanisms have been
N
IV
−6
3
0
proposed for C(sp )−heteroatom coupling reactions from
other cycloneophyl Pd complexes.
−8
IV
14b−e
1
However, in these
3
latter systems, the dissociation of a ligand (usually the
nucleophile, X) is typically required prior to nucleophilic
attack. Indeed, the vast majority of reductive elimination
IV
IV
reactions at Pd and Pt are believed to occur via five-
3
coordinate intermediates. In the present system, C(sp )−
heteroatom coupling at M complexes 4-Ni and 4-Pd could
IV
proceed via dissociation of a pyrazole group (pathway A) or by
direct nucleophilic attack at the six-coordinate complex
(
pathway B), a transformation that has much less precedent
34
at octahedral group 10 centers (Figure 8).
Figure 8. Possible mechanisms for carbon−heteroatom coupling from
4
-Ni/Pd.
3
Two S 2-type transition states for C(sp )−X coupling from
N
4
-Ni and 4-Pd were examined. The first involves pre-
3
IV
Figure 7. Eyring plot for C(sp )−O coupling from M complexes 4-
Ni and 4-Pd. Conditions: [Ni] = 0.011 M, [NMe OPh] = 0.055 M, T
equilibrium dissociation of one pyrazole group to give a five-
4
IV
coordinate M center (pathway A). The second involves direct
=
−10 to +40 °C; [Pd] = 0.011 M, [NMe OPh] = 0.055 M, T = +30
4
nucleophilic attack on the sp3 carbon (pathway B).
Computations were carried out for X = OPh, OAc, SPh, N3,
and a summary of the results is provided in Table 2. Reaction
to +70 °C.
−
coordinates for phenoxide (PhO ) are also shown as
3
representative examples in Figure 9.
Table 1. Activation Parameters for C(sp )−O Coupling from
4
3
The transition states for C(sp )−OPh bond formation at the
-Ni and 4-Pd
IV
Pd center via pathways A (19.3 kcal/mol) and B (19.6 kcal/
4
-Ni
4-Pd
mol) are substantially higher in energy than the analogous
⧧
IV
ΔH (kcal/mol)
17.7
22.0
processes at Ni (12.8 and 11.0 kcal/mol, respectively),
⧧
ΔS (cal/(mol K))
−12.0
21.3
−8.09
24.5
consistent with the experimental rates for these processes. In
⧧
3
ΔG303 K (kcal/mol)
both systems, the barriers for C(sp )−OPh coupling via the
five-coordinate versus six-coordinate pathways are remarkably
⧧
⧧
provided in Table 1. Negative entropy of activation values
similar in energy (for Pd, ΔΔG = 0.3 kcal/mol; for Ni, ΔΔG
⧧
(
ΔS ) were obtained for both Ni and Pd (−12.0 and −8.09 eu,
= 1.8 kcal/mol). This suggests that ligand dissociation to
respectively), indicating fewer degrees of freedom in the
generate a five-coordinate intermediate is not essential for S 2-
N
3
IV
IV
transition states for both C(sp )−O coupling reactions. These
results are further consistent with a mechanism involving
type coupling at these Pd and Ni centers.
The transition structures and mechanistic pathways for the
two complexes are more distinct with the more nucleophilic
thiophenoxide as the coupling partner (Figure 10). For
palladium, the calculations show similar barriers for five-
IV
14d,31
nucleophilic attack of phenoxide on the M −alkyl carbon.
Overall, the Eyring parameters implicate similar pathways for
C−O coupling at the two metal centers, with the Pd system
being a significantly higher energy process.
⧧
coordinate (ΔG = 17.8 kcal/mol, pathway A) and six-
⧧
Computational Studies. The experimental data implicate
coordinate (ΔG = 18.0 kcal/mol, pathway B) mechanisms.
a mechanism involving S 2-type attack on the methylene group
However, the pathway B transition structure exhibits a
N
E
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3
Table 2. Barriers for C(sp )−X Bond Formation from 4-Ni
a
and 4-Pd via Pathways A and B
ΔG⧧
Nuc (X)
pathway A
pathway B
4
-Ni
-Pd
OPh
OAc
12.8
14.7
11.3
13.3
11.0
14.0
7.6
b
SPh
N3
12.1
4
OPh
OAc
SPh
N3
19.3
23.9
17.8
20.8
19.6
23.3
18.0
21.5
a
b
Energies are given in kcal/mol at 298 K. The activation barrier is
dissociation of pz trans to CH2.
significantly lengthened Pd···N distance trans to the site of
Figure 10. Energy schemes for the reaction of thiophenoxide with 4-
Ni and 4-Pd via a five-coordinate intermediate (pathway A) or direct
nucleophilic attack (pathway B). For nickel, the barrier for the five-
coordinate mechanism is determined by the TS for dissociation of the
axial pz group A-Ni-diss-TS as shown in Figure 9; the barrier for SPh
attack is 10.7 kcal/mol. Energies ΔG are given in kcal/mol.
3
nucleophilic attack (Pd···N = 2.937 Å, for X = SPh). This long
Pd···N distance appears to represent a weak interaction (sum of
35
van der Waals radii M + N = 3.18 Å). Wiberg bond indices
1
2
3
−
(
WBI) were computed to compare the Pd−N and Pd−N
bonds (trans to CF (0.345) and Ph (0.368)) with the Pd···N
3
interaction (0.132). These values are consistent with a five-
coordinate-like transition state in the “pathway B” Pd system.
In contrast, for Ni, the six-coordinate mechanism (pathway
3
SPh, Ni···N ). These data are consistent with a more “six-
coordinate-like” transition state in the Ni system (see Table S4
in the Supporting Information for full details). As discussed
above, this represents an unusual example of direct C(sp )−X
coupling at an octahedral group 10 metal center. We attribute
⧧
B, Figure 10) is clearly the lower energy pathway, with ΔΔG =
3
3
.7 kcal/mol. In this case, the Ni···N bond distances are
35
significantly shorter than those at Pd, resulting in comparatively
IV
larger Wiberg bond indices (e.g., 2.603 Å and 0.165 for X =
this pathway to the highly electrophilic Ni −alkyl carbon,
Figure 9. Energy profiles and Gaussview diagrams of the transition states for the reaction of phenoxide with (a) 4-Ni and (b) 4-Pd via a five-
coordinate intermediate (mechanism A, blue), or direct nucleophilic attack (mechanism B, black). Square-pyramidal species following A-M-diss-TS
and encounter complexes before A-M-OPh-TS are omitted for clarity. Energies ΔG (ΔH) are given in kcal/mol.
F
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−
which renders direct attack by the strongly nucleophilic SPh
to be lower in energy than pyrazole dissociation.
for the conversion of 6d-Ni to 7 involves a singlet open-shell
mechanism with a Ni -imido intermediate (Figure 11, blue
IV
II
Reactivity and Mechanism of M −Alkyl Azides. A final
set of experimental and computational studies focused on the
distinct reactivity of tetrabutylammonium azide (NBu N ). The
treatment of 4-Ni with 1 equiv of NBu N initially led to the
profile). This pathway proceeds via the formation of an initial
weak Ni···N interaction to generate intermediate S-I. The
highest energy transition structure (S-TS-I) involves the
4
3
IV
4
3
transformation of S-I to the Ni -imido complex S-III, with a
II
formation of Ni alkyl azide 6d-Ni (Scheme 6). This product
barrier of 38.4 kcal/mol. Complex S-III can then undergo
2
C(sp )−N coupling to generate 7.
II
Scheme 6. Distinct Reactivity of M −Alkyl Azides 6d-Ni and
An alternative reaction manifold involving triplet intermedi-
6d-Pd
⧧
ates was found to be substantially lower in energy (ΔΔG = 9.2
kcal/mol for the highest energy transition state of each
process). As shown in the black profile in Figure 11, this
pathway involves an initial minimum energy crossing point
(
MECP) from S-I to the triplet structure T-II. Intermediate T-
II then extrudes N to form the anionic nitrene intermediate
2
(
T-III) via triplet transition structure T-TS-I. This step has the
highest barrier of the sequence (29.2 kcal/mol). C−N coupling
from T-III proceeds via intermediate T-IV and T-TS-II to form
intermediate T-V. Collapse of T-V to generate Ni product 7
then occurs via a second MECP (ΔE = 13.1 kcal/mol).
Gaussview diagrams of the triplet species together with bonding
models for these intermediates are shown in Figure 12.
The initially formed triplet intermediate (T-II) has a
can be detected by NMR spectroscopic analysis of the crude
reaction mixture. However, it reacts within 15 h at room
1
2
3
nonlinear N −N −N unit (129°) with spin density located
temperature to extrude N and form 3,3-dimethylindoline via
2
−
3
−
II
36
II
primarily on Ni (1.09 e ) and N (0.79 e ), consistent with a
Ni intermediate 7. In contrast, the corresponding Pd
III
Ni formulation (Figure 12). Loss of dinitrogen gives T-III,
where the spin density is now localized primarily at N (N ,
complex 6d-Pd is remarkably stable, with no decomposition
or indoline product observed even upon heating at 70 °C for
several weeks.
1
1
−
−
1
.31 e ; Ni, 0.65 e ). As shown in Figure 12, intermediate T-III
III
•− 38
We turned to DFT calculations to better understand the
reactivity differences between 6d-Ni and 6d-Pd. The
can be viewed as a Ni nitrene or imidyl species (NR ) in
II
resonance with a neutral Ni nitrene species. Structure T-IV
can be represented similarly (N , 1.15 e ; Ni, 0.85 e ).
Overall, the calculations suggest that the distinct reactivity of
II
1
−
−
39
mechanism for the conversion of 6d-Ni to Ni intermediate
37
7
was explored using CAM-B3LYP. A first pathway identified
II
Figure 11. Energy profiles computed for the formation of Ni indolinide complexes from 6d-Ni via singlet (blue) and triplet states (black). Energies
ΔG (ΔH) in kcal/mol are referenced to 6d, except for the minimum energy crossing points (MECP) computed as ΔE 3.1 kcal/mol above T-II, and
ΔE 13.1 kcal/mol above T-V, at the BS1 level.
G
DOI: 10.1021/acs.organomet.7b00613
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
In contrast to Pd, the +3 oxidation state for Ni is readily
11,40
accessible.
The propensity of Pd to undergo two-electron-
redox chemistry and for Ni to readily promote one-electron-
transfer processes is confirmed via electrochemical analyses,
oxidation studies monitored by NMR spectroscopy, and the
II
distinct reactivity profiles of M alkyl azide derivatives.
Computations carried out on the last system suggest that a
2
Ni-mediated C(sp )−N insertion process occurs via a transient
III
Ni intermediate. This pathway is not energetically accessible
at the analogous Pd complex. Overall, these results demonstrate
the importance of the metal and oxidation state on reactivity
and selectivity. Additionally, they show the potential for similar
IV
IV
III
roles of Ni /Pd and a complementary role for Ni in
catalysis.
EXPERIMENTAL SECTION
General Procedures. All experiments and manipulations were
carried out under an inert nitrogen atmosphere using standard
glovebox or Schlenk techniques unless otherwise indicated. NMR
■
Figure 12. Gaussview diagrams and bonding models of triplet
intermediates T-II, T-III, and T-IV.
1
spectra were obtained on a Varian VNMR 700 (699.76 MHz for H;
the Ni alkyl azide 6d results from the accessibility of the NiIII
II
13
1
1
75.95 MHz for 1 C), a Varian VNMR 500 (500.09 MHz for H;
9
oxidation state.
470.56 MHz for F), or a Varian VNMR 400 spectrometer (399.54
1
11
1
13
Analogous pathways at Pd were found to involve
substantially higher energy transition states in comparison to
MHz for H; 128.187 for B). H and C chemical shifts are reported
in parts per million (ppm) relative to TMS, with the residual solvent
peak as an internal reference. 19F chemical shifts and B chemical
shifts are reported in ppm and are referenced on a unified scale, where
the single primary reference is the frequency of the residual solvent
11
⧧
the Ni system (minimum ΔΔG = 6.4 kcal/mol). This is
consistent with experimental studies showing that 6d-Pd does
not undergo conversion to the analogue of 7, even after
prolonged heating (Scheme 6). Interestingly, while the triplet
1
peak in the H NMR spectrum. Abbreviations used in the NMR data:
s, singlet; d, doublet; dd, doublet of doublets; t, triplet; td, triplet of
doublets; m, multiplet; br, broad signal; bq, broad quartet. Cyclic
voltammetry was performed using a CHI600C potentiostat from CH
instruments. The electrodes were obtained from BASi. Mass spectral
data were obtained on a Micromass magnetic sector mass
spectrometer in electrospray ionization mode. X-ray crystallographic
data were collected on a Rigaku AFC10K Saturn 944+ CCD-based X-
ray diffractometer. Flash chromatography was conducted using a
Biotage Isolera One system with cartridges containing high-perform-
III
IV
Ni pathway was favored over the singlet Ni -imido
⧧
mechanism (ΔΔG = 9.2 kcal/mol), the analogous processes
⧧
for Pd were indistinguishable (ΔΔG = 1.9 kcal/mol; Figure
1
3). Overall, these DFT studies show that the ability of Ni to
undergo single-electron chemistry leads to reactivity that is not
readily accessible at the Pd analogue.
1
0c
10e
10c
ance silica gel. Complexes 1-Ni, 2-Ni, 4-Ni, and 6a-Ni−6d-
1
0c
Ni were prepared from literature procedures. The NMR spectra of
1
8
complex 3-Pd was consistent with the literature report.
II
Synthesis of NMe [(Tp)Pd (CH CMe -o-C H )] (1-Pd). A 250
4
2
2
6 4
mL round-bottom flask was charged with (COD)Pd(CH CMe -o-
2
2
C H ) (300 mg, 0.864 mmol, 1.0 equiv). The yellow solid was
6
4
dissolved in dichloromethane (50 mL), and NMe Tp (260 mg, 0.907
4
mmol, 1.1 equiv) was added at room temperature. The light tan
solution was stirred for 2 h. The crude reaction mixture was then
concentrated to a tan solid, washed several times with ether (3 × 10
II
Figure 13. Transition structures computed for loss of N from Ni
mL), and dried under vacuum to afford 1-Pd as a white solid (404 mg;
2
II
⧧
⧧
1
intermediate 6d-Ni and Pd complex 6d-Pd. Energies ΔG (ΔH ) are
89% yield). H NMR (700 MHz, CD CN, 23 °C): δ 7.90 (br, 3H),
3
given in kcal/mol referenced to 6d-Ni or 6d-Pd.
7.63 (br, 3H), 7.27 (d, JHH = 7.2 Hz, 1H), 6.80 (t, JHH = 7.2 Hz, 1H),
6
4
.72 (t, JHH = 7.2 Hz, 1H), 6.69 (d, JHH = 7.2 Hz, 1H), 6.17 (br, 3H),
.73 (bq, B-H), 3.04 (s, 12H), 1.96 (s, 2H), 1.33 (s, 6H). C NMR
1
3
(
1
176 MHz, CD CN, 23 °C): δ 168.40, 161.91, 140.44, 135.95, 134.72,
3
11
22.92, 121.46, 121.10, 103.62, 55.1, 47.31, 40.72, 33.59. B NMR
CONCLUSIONS
■
(
225 MHz, CD CN, 23 °C): δ −1.83 (d, J = 112 Hz, B-H). HRMS-
3
BH
This report describes a detailed comparison of Ni and PdIV
complexes bearing identical supporting ligands. A combination
of experimental and computational studies reveal many
IV
−
electrospray (m/z): [M − NMe ] calcd for C H BN Pd, 451.1034;
found, 451.1067.
Synthesis of [(Tp)Ni (CH CMe -o-C H )(MeCN)]BF (3-Ni). In
the glovebox, a 20 mL vial was charged with K[(Tp)Ni (CH
C H )] (150 mg, 0.34 mmol, 1.0 equiv). The yellow solid was
dissolved in acetonitrile (10 mL), and a solution of AgBF (134 mg,
0.69 mmol, 2.0 equiv) in acetonitrile (5 mL) was added at −35 °C.
The orange solution immediately turned dark red, with concomitant
4
19 22
6
IV
2
2
6
4
4
IV
IV
II
similarities in the chemistry of Ni and Pd but a role for
Ni in enabling reactivity that is distinct from that of palladium.
CMe -o-
2 2
III
6
4
4
In particular, electrochemical analyses and chemical oxidations
II
IV
IV
of Tp-ligated M precursors demonstrate that the Ni and Pd
species can be accessed under comparable conditions.
Reactivity and mechanistic studies of isolated Ni and Pd
complexes show that both undergo C(sp )−heteroatom bond-
forming reactions, with the Ni system reacting under milder
conditions.
0
precipitation of Ag . The crude reaction mixture was then filtered
IV
IV
through a Celite plug. The plug was washed with acetonitrile (5 mL),
and the filtrates were combined and concentrated to approximately 2
mL. Red-orange crystals precipitated from the solution over the course
of 15 min. These crystals were collected, washed with acetonitrile (5
3
H
DOI: 10.1021/acs.organomet.7b00613
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
mL), and dried under vacuum to afford 3-Ni as a red-orange solid (91
100 mV/s. After the CV was obtained for each complex, ferrocene was
added as an internal reference.
Experimental Procedure for NMR Oxidation Studies: Nickel.
1
mg, 51% yield). H NMR (700 MHz, CD CN, 23 °C): δ 8.18 (d, J
3
HH
=
2.0 Hz, 1H), 8.13 (d, JHH = 2.0 Hz, 1H), 7.99 (d, JHH = 2.3 Hz, 1H),
7
7
6
2
6
1
1
1
.97 (d, JHH = 2.3 Hz, 1H), 7.86 (d, JHH = 2.3 Hz, 1H), 7.26 (t, JHH
.7 Hz, 1H), 7.19 (d, JHH = 7.7 Hz, 1H), 6.94 (t, JHH = 7.7 Hz, 1H),
.77 (d, JHH = 2.3 Hz, 1H), 6.74 (d, JHH = 7.7 Hz, 1H), 6.55 (d, JHH
.3 Hz, 1H), 6.49 (s, 1H), 6.22 (s, 1H), 6.09 (d, JHH = 3.8 Hz, 1H),
=
A 4 mL vial was charged with 1-Ni (4.2 mg, 0.0096 mmol, 1.0 equiv)
and CD CN (0.5 mL). This light tan solution was transferred to a
3
=
screw-cap NMR tube. A solution of the corresponding amount of
acetylferrocenium tetrafluoroborate (AcFcBF ; 3.0 mg, 0.0096 mmol,
4
.00 (d, JHH = 3.8 Hz, 1H), 4.61 (bq, B-H), 2.36 (s, 3H), 1.68 (s, 3H),
1.0 equiv or 6.0 mg, 0.0192 mmol, 2 equiv) in CD CN was added. The
3
.59 (s, 3H). 13C NMR (176 MHz, CD CN, 0 °C): δ 154.20, 151.08,
3
tube was quickly capped and shaken vigorously, and the contents were
1
43.77, 141.80, 141.62, 138.08, 136.50, 136.07, 132.52, 128.97, 127.94,
analyzed by H NMR spectroscopy after <5 min at room temperature.
11
IV
27.49, 107.86, 107.15, 106.96, 87.40, 48.24, 31.38, 28.50. B NMR
225 MHz, CD CN, 23 °C): δ −1.18 (s, BF ), −4.38 (d, J = 98 Hz,
In the presence of 2 equiv of AcFcBF , Ni complex 3-Ni was formed
4
(
in 95% NMR yield. In the presence of 1 equiv of AcFcBF , analysis by
3
4
BH
4
19
1
11
B-H). F NMR (471 MHz, CD CN, 23 °C): δ −151.95. HRMS-
electrospray (m/z): [M] calcd for C H BN Ni, 444.1612; found,
H NMR and B NMR spectroscopy revealed the formation of a
paramagnetic species that we previously characterized as Ni complex
3
+
III
21
25
7
1
0e
444.1613.
2
-Ni.
IV
Synthesis of [(Tp)Pd (CH CMe -o-C H )(CF )] (4-Pd). A 20 mL
2
2
6
4
3
Experimental Procedure for NMR Oxidation Studies:
Palladium. A 4 mL vial was charged with 1-Pd (5.0 mg, 0.0096
II
vial was charged with NMe [(Tp)Pd (CH CMe -o-C H )] (1-Pd;
4
2
2
6
4
2
(
0
90 mg, 0.55 mmol, 1.0 equiv). The solid was dissolved in acetonitrile
15 mL). S-(Trifluoromethyl)dibenzothiophenium triflate (288 mg,
.72 mmol, 1.3 equiv) was added at room temperature, and the light
mmol, 1.0 equiv), pyridine-d (4 μL, 0.05 mmol, 5.2 equiv), and
5
CD CN (0.5 mL). This light tan solution was transferred to a screw-
3
cap NMR tube. A solution of the corresponding amount of
tan solution immediately turned orange-brown. The solvent was
removed by rotary evaporation. The crude brown solid was purified by
flash chromatography on silica gel (mobile phase ethyl acetate/hexanes
acetylferrocenium tetrafluoroborate (AcFcBF ; 3.0 mg, 0.0096 mmol,
4
1.0 equiv or 6.0 mg, 0.0192 mmol, 2 equiv) in CD CN was added. The
3
tube was quickly capped and shaken vigorously, and the contents were
1
with a gradient from 90/10 to 70/30). Compound 4-Pd was isolated
analyzed by H NMR spectroscopy after <5 min at room temperature.
1
as a light tan solid (245 mg, 86% yield). H NMR (700 MHz, CD CN,
In the presence of 2 equiv of AcFcBF , Pd complex 3-Pd was formed
3
4
2
(
3 °C): δ 8.04 (d, JHH = 1.9 Hz, 1H), 7.89 (d, JHH = 2.3 Hz, 1H), 7.87
d, JHH = 2.3 Hz, 1H), 7.83 (d, JHH = 1.9 Hz, 1H), 7.79 (d, JHH = 2.3
Hz, 1H), 7.26 (m, 1H), 7.18 (m, 1H), 7.10 (dd, JHH = 7.7, 1.7 Hz,
H), 6.93 (td, JHH = 7.7, 1.7 Hz, 1H), 6.86 (d, JHH = 2.2 Hz, 1H), 6.42
t, JHH = 2.2 Hz, 1H), 6.38 (t, JHH = 2.2 Hz, 1H), 6.14 (d, JHH = 2.2
in approximately quantitative yield against acetylferrocene as the
1
internal H NMR standard. In the presence of 1 equiv of AcFcBF , Pd
4
complex 3-Pd was formed in approximately 50% yield against
1
1
(
acetylferrocene as the internal H NMR standard with 50% of
unreacted 1-Pd remaining.
Hz, 1H), 4.63 (bq, B-H), 4.19−4.13 (multiple peaks, 2H), 1.51 (s,
General Procedure for Determining Initial Rates of C−C
Coupling. In the glovebox, complex 3-Ni or 4-Ni (0.0059 mmol, 1.0
equiv) was placed in a J. Young valve NMR tube equipped with an O-
1
3
3
1
1
H), 1.50 (s, 3H). C NMR (176 MHz, CD CN, 23 °C): δ 161.23,
3
54.38, 141.48, 140.93, 140.40, 136.09, 136.05, 135.62, 130.95, 126.71,
26.11, 126.08, 125.85 (Pd-CF , shift for CF group extracted from
3
3
ring seal and then dissolved in CD CN (0.5 mL) at room temperature.
3
1
9
13
F− C HMBC NMR spectrum), 105.96, 105.88, 105.72, 67.02,
DMSO (0.014 mmol, 2.4 equiv) was added as an internal proton
standard. The NMR sample was taken out of the glovebox and
immediately placed in a liquid nitrogen/ethyl acetate bath
(approximately −84 °C). The frozen sample was then placed in the
NMR spectrometer that was preheated to 70 °C. Reductive
1
9
4
5.94, 31.63, 31.53. F NMR (377 MHz, CD CN, 23 °C): δ −18.39.
3
1
1
B NMR (225 MHz, CD CN, 23 °C): δ −3.54 (d, J = 102 Hz, B-
3
BH
H).
II
Synthesis of NMe [(Tp)Pd (C H -o-CMe CH OPh)(CF )] (6a-
Pd). A 20 mL vial equipped with a magnetic stir bar was charged with
(Tp)Pd (CH CMe -o-C H )(CF )] (4-Pd; 50 mg, 0.094 mmol, 1.0
4
6
4
2
2
3
1
elimination from 3-Ni or 4-Ni was monitored by H NMR
IV
[
spectroscopy at this temperature. Concentration versus time data
were acquired from the integration of the methylene proton signals of
5 and 3-Ni/4-Ni with respect to the internal standard. Initial rate
values were obtained from the slope of a linear-fit line corresponding
to the growth of 5.
2
2
6
4
3
equiv) and dissolved in acetonitrile (8 mL). NMe OPh (17 mg, 0.10
4
mmol, 1.1 equiv) was added, and the resulting solution was stirred at
7
0 °C for 32 h. The reaction mixture was cooled to room temperature,
and solvent was removed by rotary evaporation. The resulting yellow
residue was washed with diethyl ether (3 × 10 mL). The solids were
General Procedure for Determining Initial Rates of C−X
IV
dried under vacuum to afford complex 6a-Pd as a yellow solid (35 mg,
Coupling. In the glovebox, Pd complex 4-Pd (3.0 mg, 0.0057 mmol,
1
5
6% yield). H NMR (700 MHz, CD CN, 23 °C): δ 7.89 (d, J = 7.4
1.0 equiv) was placed in a J. Young valve NMR tube equipped with an
3
HH
Hz, 1H), 7.77−7.66 (multiple peaks, 3H), 7.49 (d, J = 2.3 Hz, 1H),
O-ring seal. The respective nucleophile, NR X, where X = OPh, OAc,
HH
4
7
6
7
4
1
.41 (s, 1H), 7.24 (d, J = 7.9 Hz, 1H), 7.15 (t, J = 7.9 Hz, 2H),
SPh, N (0.0288 mmol, 5 equiv), along with the internal standard 4,4′-
HH
HH
3
.87 (t, JHH = 7.5 Hz, 1H), 6.83 (t, JHH = 7.5 Hz, 1H), 6.76 (d, JHH
=
difluorobiphenyl (∼2 mg) was weighed into a 4 mL vial and then
.8 Hz, 3H), 6.60 (s, 1H), 6.28 (s, 1H), 6.16 (s, 1H), 5.85 (s, 1H),
dissolved in CD CN (0.5 mL). The resulting solutions were placed in
3
.75 (bq, B-H) 4.35 (d, JHH = 8.6 Hz, 1H), 4.26 (br, 1H), 3.07 (s,
the NMR tube at room temperature and taken out of the glovebox.
The tube was then placed into an NMR spectrometer that had been
preheated to 60 °C. The rates of reductive elimination were
determined by monitoring the first 10−40% of the reaction progress
by 19F NMR spectroscopy at this temperature. Concentration versus
2H), 1.78 (s, 3H), 1.47 (s, 3H). 13C NMR (176 MHz, CD CN, 23
3
°
C): δ 159.80, 154.43, 150.59, 141.25, 136.98, 136.83 (Pd-CF , shift
3
for CF3 group extracted from 19F− C HMBC NMR spectrum),
13
1
1
35.52, 135.20, 134.06, 129.07, 126.12, 122.56, 121.79, 119.65, 114.48,
03.90, 103.88, 103.61, 77.53, 55.18, 39.64, 27.03, 27.02. 11B NMR
time data were acquired from the integration of the CF signals of 4-
3
(
225 MHz, CD CN, 23 °C): δ −2.04 (d, J = 113 Hz, B-H). 19
F
Pd and 6-Pd with respect to the internal standard. Initial rate values
were obtained from the slope of a linear-fit line corresponding to the
decay of 4-Pd (see Figures S8−S11 in the Supporting Information).
Experimental Procedure for Determining Activation Param-
eters of C−O Coupling. The activation parameters for C−O
coupling at 4-Ni and 4-Pd were determined through an Eyring plot in
the temperature ranges of −10 to +40 °C and +30 to +70 °C,
respectively. In the glovebox, complex 4-Ni or 4-Pd (0.0055 mmol, 1.0
3
BH
NMR (471 MHz, CD CN, 23 °C): δ −18.75. HRMS-electrospray (m/
3
−
z): [M − NMe ] calcd for C H BF N PdO, 613.1326; found,
4
26 27
3
6
613.1344.
General Procedure for Cyclic Voltammetry Studies. Cyclic
voltammetry on complexes 1-Ni and 1-Pd were performed in a three-
electrode cell consisting of a 3 mm glassy-carbon-disk working
+
electrode, a Ag/Ag reference electrode with a Ag wire in a fritted
1
9
chamber containing a solution of AgBF (0.01 M) and NBu PF (0.1
equiv), NMe OPh (0.027 mmol, 5.0 equiv), and the F NMR
4
4
6
4
M) in acetonitrile, and a Pt-wire counter electrode. A 2 mL solution of
the complex (0.01 M) and NBu PF (0.1 M) in acetonitrile was added
to the electrochemical cell. Cyclic voltammetry scans were taken at
standard 4,4-difluorobiphenyl (∼2 mg) were weighed into a 4 mL vial.
CD CN (0.5 mL) was added at −35 °C, and the resulting solution was
4
6
3
transferred to a J. Young valve NMR tube equipped with an O-ring
I
DOI: 10.1021/acs.organomet.7b00613
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
seal at this temperature. The NMR tube was taken out of the glovebox
and immediately flash frozen in an ethyl acetate/liquid nitrogen bath
AUTHOR INFORMATION
Corresponding Authors
E-mail for A.J.C.: Allan.Canty@utas.edu.au.
E-mail for M.S.S.: mssanfor@umich.edu.
ORCID
Allan J. Canty: 0000-0003-4091-6040
Alireza Ariafard: 0000-0003-2383-6380
Melanie S. Sanford: 0000-0001-9342-9436
■
(−84 °C). The sample was placed into an NMR spectrometer where
*
*
the probe had been preset to the respective temperature. The rate of
reductive elimination was determined by monitoring approximately
the first 10% of the reaction by 19F NMR spectroscopy at various
temperatures. Concentration versus time data were acquired from the
integration of the CF signals of 4-Ni/-Pd and 6a-Ni/-Pd with respect
3
to the internal standard. Initial rate values were obtained from the
IV
slope of a linear-fit line corresponding to the decay of the M
Notes
complexes. The activation parameters for C−O coupling were
extracted from the resulting Eyring plot. See the Supporting
Information for full details.
The authors declare no competing financial interest.
34a
Computational Methods. Gaussian 09
was used for DFT
ACKNOWLEDGMENTS
■
calculations at the B3LYP level for optimization, using the Stuttgart/
This work was supported by the U.S. National Science
Foundation (CHE 1361542 and CHE 1664961 to M.S.S. and
CHE 0840456 for X-ray instrumentation), Australian National
Computing Infrastructure, and the Australian Research
Council. We also thank Dr. Jeff Kampf for solving the X-ray
crystal structures of 4-Ni and 4-Pd.
34b
Dresden ECP (SDD) basis set for Pd and the 6-31G(d) basis set for
other atoms (referred to as basis set BS1). Single-point calculations
3
3c,d
were performed at the B3LYP-D3 level,
utilizing the quadruple-ξ
33e
valence polarized def2-QZVP basis set on Ni and Pd along with the
corresponding ECP and the 6-311+G(2d,p) basis set on other atoms
(
basis set BS2). All calculations were carried out for acetonitrile as
solvent with the IEFPCM (SCRF) model. All thermodynamic data
were calculated at the standard state (298.15 K and 1 atm), and
entropy calculations were adjusted by the method proposed by
REFERENCES
■
(1) For select fundamental organometallic studies at high-valent Pd,
see: (a) Uson, R.; Fornies, J.; Navarro, R. J. Organomet. Chem. 1975,
96, 307−312. (b) Byers, P. K.; Canty, A. J.; Skelton, B. W.; White, A.
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L.; Engel, P. F.; Hogerheide, M. P.; Copijn, M.; Spek, A. L.; van Koten,
G. Organometallics 1993, 12, 1831−1844. (d) Markies, B. A.; Canty, A.
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(e) van Asselt, R.; Rijnberg, E.; Elsevier, C. J. Organometallics 1994, 13,
33f
Okuno. This computational procedure has been benchmarked for
palladium when it is applied to C···C coupling from the closely related
IV
2
,2′-bipyridine (bpy) cation [Pd (CH CMe -o-C H -C,C’)(F)(bpy-
2
2
6
4
+
14d
N,N’)] in acetonitrile. The triflate (OTf) salt of this cation has a
⧧
ΔG value of 24.7 kcal/mol, in comparison with values for experiment
⧧
⧧
(
2
ΔG = 23.8 kcal/mol) and different computation procedures (ΔG =
14d
3.3 kcal/mol) for a sulfonamide (Tf N) salt.
All transition
2
7
06−720. (f) van Belzen, R.; Hoffmann, H.; Elsevier, C. J. Angew.
structures contained one imaginary frequency, exhibiting atom
displacements consistent with the anticipated reaction pathway. The
nature of transition structures was confirmed by intrinsic reaction
coordinate (IRC) searches, vibrational frequency calculations, and
Chem., Int. Ed. Engl. 1997, 36, 1743−1745. (g) Cotton, F. A.; Gu, J.;
Murillo, C. A.; Timmons, D. J. J. Am. Chem. Soc. 1998, 120, 13280−
1
3281. (h) Dick, A. R.; Kampf, J. W.; Sanford, M. S. J. Am. Chem. Soc.
4
1
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DOI: 10.1021/acs.organomet.7b00613
Organometallics XXXX, XXX, XXX−XXX