Comprehensive Study of the Reactions between Chelating Phosphines and Ni(cod)2

Article abstract of DOI:10.1021/acs.organomet.8b00438

A comprehensive study of the reactions of chelating phosphines with Ni(cod)₂ to form (phosphine)Ni(cod), (phosphine)₂Ni, or mixtures thereof is presented. A series of (phosphine)Ni(cod) complexes were isolated and characterized. The structural differences between the (phosphine)Ni(cod) and (phosphine)₂Ni complexes were examined using X-ray crystallography and ¹H and ³¹P NMR spectroscopy. In addition, the effects of ring size, rigidity, and bulk of the phosphine backbone on the formation of either (phosphine)Ni(cod) or (phosphine)₂Ni were investigated. These studies show that the Ni-P bond lengths in both the (phosphine)Ni(cod) and (phosphine)₂Ni complexes and the size of the ring formed by the chelating phosphine and Ni are crucial in determining whether or not (phosphine)Ni(cod) complexes can be isolated. Other factors such as π-stacking interactions were found to have marginal influence.

Full text of DOI:10.1021/acs.organomet.8b00438

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Cite This: Organometallics XXXX, XXX, XXX−XXX
Comprehensive Study of the Reactions Between Chelating
Phosphines and Ni(cod)2
Andrew L. Clevenger,† Ryan M. Stolley,† Nicholas D. Staudaher,† Noman Al,† Arnold L. Rheingold,‡
Ryan T. Vanderlinden,† and Janis Louie*,†
†Department of Chemistry, The University of Utah, 315 South 1400 East, Room 2020, Salt Lake City, Utah 84112, United States
‡Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093,
United States
S
* Supporting Information
ABSTRACT: A comprehensive study of the reactions of chelating
phosphines with Ni(cod)2 to form (phosphine)Ni(cod), (phosphine)2Ni, or
mixtures thereof is presented. A series of (phosphine)Ni(cod) complexes
were isolated and characterized. The structural differences between the
(phosphine)Ni(cod) and (phosphine)2Ni complexes were examined using X-
1
ray crystallography and H and 31P NMR spectroscopy. In addition, the
effects of ring size, rigidity, and bulk of the phosphine backbone on the
formation of either (phosphine)Ni(cod) or (phosphine)2Ni were investigated.
These studies show that the Ni−P bond lengths in both the (phosphine)-
Ni(cod) and (phosphine)2Ni complexes and the size of the ring formed by
the chelating phosphine and Ni are crucial in determining whether or not
(phosphine)Ni(cod) complexes can be isolated. Other factors such as π-stacking interactions were found to have marginal
influence.
somewhat surprising considering the different reactivity
relative to monodentate phosphines that chelating phosphines
impart. For example, nickel intermediates with chelating
phosphines are less likely to undergo β-H elimination side
reactions.7 These intermediates are also crucial for effective
stereo- and regiocontrol of products.4b,8 Two general methods
exist for utilizing chelating phosphines in nickel(0) catalysis.
One uses the in situ reduction of a (phosphine)Ni(II)X2
species (X = Cl, Br) to form the active Ni(0) catalyst. The
other involves the in situ formation of the precatalyst through
the combination of Ni(cod)2 and a phosphine ligand. This
precatalyst is generally (phosphine)Ni(cod). The formation of
this complex is followed by the displacement of COD by the
desired substrate in the reaction. This research focuses on the
second of these methods, which has the advantage of avoiding
reducing conditions and having a Ni source readily available.
While catalytic reactions exist that use Ni(II) under oxidative
conditions, these reactions are not as common as those that
use oxidation states between Ni(0) and Ni(II).
INTRODUCTION
■
Transition metal catalyzed reactions are predominated by use
of second- or third-row transition metals as catalysts. Of the
late transition metals, the prevalence of palladium has seen
exponential growth, primarily in cross-coupling reactions. This
is owing to the fact that palladium is very efficient at
performing the two-electron transformations required for most
organic reactions.1 However, palladium-catalyzed cross cou-
pling reactions have limitations, such as a propensity for β-H
elimination.2 In addition, oxidative addition reactions at
palladium are more sluggish when using electrophilic
substrates.3 As a result, new catalytic cross-coupling reactions
of two electrophilic substrates using nickel catalysts have been
on the rise in recent years,4 mirroring a similar path of
palladium. Nickel is intriguing due to its ability to access a wide
range of oxidation states from 0 to +4, which allows for both
single-electron and two electron reactivity. Single electron
reactivity can mitigate problems with β-H elimination and
provide for more facile transmetalation, which is a critical step
in cross-coupling chemistry.2 Ligands used in nickel catalysis
mirror those used in palladium catalysis. In particular,
monodentate phosphines are very popular and widely utilized.
Not surprisingly, the electronic and steric requirements for the
phosphine when ligated to nickel are subtly but noticeably
different than when bound to palladium, and those needs are
just beginning to be understood.5
Although systematic variation of Ni(cod)2/phosphine ratios
are commonly examined, knowledge of the actual nickel
precatalyst that forms in solution is often absent and the
secondary ligand effects on these reactions are regularly
negated or underexplored. As a result, the cause of an
unsuccessful reaction may be the incompatible stereoelectronic
properties of the phosphine or the formation of an unreactive
Chelating phosphines have had less success than mono-
dentate phosphines when utilized in nickel catalysis.6 The
scarcity of catalytic reactions involving chelating phosphines is
Received: June 27, 2018
© XXXX American Chemical Society
A
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catalyst sink, such as a saturated Ni phosphine complex, among
myriad other problems. Thus, chelating phosphines may be
underutilized, despite their potential to greatly expand the
scope of nickel-based catalytic chemistry, because of their
propensity to form undesired (phosphine)2Ni.9
Scheme 1. Formation of (Xantphos)Ni(3-hexyne) through
Use of Benzonitrile to Displace one Xantphos Ligand from
(Xantphos)2Ni
One chelating phosphine that has successfully been used in a
catalytic system with Ni(cod)2 is 1,1-bis(diphenylphosphino)-
ferrocene (DPPF). When reacted in a 1:1 ratio with Ni(cod)2,
DPPF only forms (dppf)Ni(cod) with no (dppf)2Ni
observed.9a,b,10 This unique reactivity has been exploited in
both catalytic and stoichiometric reactions. For example,
Schoenebeck and co-workers reported a successful nickel-
catalyzed trifluoromethylthiolation using a Ni(cod)2/DPPF
system. However, when 1,2-bis(diphenylphosphino)ethane
(DPPE) was employed as a ligand in this reaction, the
formation of (dppe)2Ni doubled the energy barrier of the key
oxidative addition step.9a,b This barrier was doubled due to the
need to dissociate one of the DPPE ligands from (dppe)2Ni in
order to facilitate oxidative addition. The amount of active
catalyst was also reduced by the formation of (dppe)2Ni. The
formation of this unwanted catalyst sink prevented evaluation
of the effectiveness of DPPE in nickel-catalyzed trifluorome-
thylthiolation and narrowed the ligand scope of the reaction.
In terms of stoichiometric transformations, our group
recently published syntheses and full characterizations of a
library of (dppf)Ni(ketene) complexes. A screen of chelating
phosphines showed that most formed (phosphine)2Ni when
reacted with Ni(cod)2 regardless of the presence or absence of
ketene. This problem was further exacerbated by the fact that
formation of (phosphine)2Ni is concomitant with the existence
of unreacted Ni(cod)2, which promoted unwanted side
reactions with the starting materials.10
chelating phosphine and promotes the conversion of catalyti-
cally inactive (Xantphos)2Ni into the catalytically active species
(Xantphos)Ni(π-L) was, to our knowledge, the first of its kind
and provides a launching pad for the discovery of new Ni-
catalyzed reactions utilizing chelating phosphines. This
phenomenon has also been observed by Hartwig and co-
workers using BINAP and Ni(cod)2 for a Ni-catalyzed
arylation of ketones. Addition of benzonitrile to the mixture
of phosphine and Ni(cod)2 was essential to the formation of
the active catalyst.6f,h
In a prior report, we showed that the combination of the
chelating phosphine Xantphos9e and Ni(cod)2 leads to the
precipitation of insoluble (Xantphos)2Ni. However, addition of
benzonitrile and a π-substrate (eq 1) resulted in the formation
of a variety of (Xantphos)Ni(π-L) complexes 1−5 (Figure
1).11 The proposed mechanism (Scheme 1) involves
Herein, we report our investigations into the general use of
nitrile to promote the formation of a family of (phosphine)-
Ni(cod) complexes, which may serve as more active
precatalysts and allow for the expanded use of these potentially
powerful class of compounds. In order to quantify the
usefulness of this method, the effect of the addition of nitrile
on the ratio of (phosphine)Ni(cod)/(phosphine)2Ni com-
plexes in a reaction was investigated. These results give greater
insight into the formation of the precatalysts in nickel-
catalyzed reactions.
RESULTS
Figure 1. Synthesized (Xantphos)Ni(π-L) complexes 1−5
■
The ligands in this study (Table 1, columns 1 and 2) were
chosen in order to perform a systematic survey of the effects of
changing the size and rigidity of the phosphine backbone on
the equilibrium between the heteroleptic (phosphine)Ni(cod)
complex and the homoleptic (phosphine)2Ni complex. These
studies started by using bidentate chelating phosphines with
aliphatic backbones. The backbone size ranged from one
carbon (DPPM) to five carbons (DPPPentane). Complemen-
tary ligands such as DPPMB and DPPBenz allowed
comparison of backbone rigidity, as these phosphines have
phenyl rings in their backbones. DPEPhos and Xantphos vary
by the added rigidity of the xanthene backbone in Xantphos.
As previously stated, other research has shown that DPPF only
forms (dppf)Ni(cod) when mixed with Ni(cod)2 in a 1:1
ratio.9a,b,10
replacement of an arm of the chelating phosphine by the
nitrile binding through the lone pair on the terminal nitrogen,
followed by a hapticity shift to η2-NC that is concurrent with
dissociation of one ligand. This process and its reverse reaction
allows for equilibrium between (Xantphos)2Ni and
(Xantphos)Ni(η2-NC(Ph)). In order to form (Xantphos)Ni-
(π-L), benzonitrile is replaced by a stronger binding π
substrate in solution where the strength of the nickel-π
interaction drives the equilibrium forward. Importantly, a
reaction of Xantphos, Ni(cod)2, and π-substrate gave only
(Xantphos)2Ni with no (Xantphos)Ni(π-L) observed, demon-
strating the need for benzonitrile to bind to Ni and start the
process of removing a phosphine from (Xantphos)2Ni. The
discovery that benzonitrile traps ligand dissociation of a
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a
Table 1. Chelating Phosphine Ligands Studied for This Work
a
Ratios of (phosphine)Ni(cod)/(phosphine)2Ni when the phosphine and Ni(cod)2 are mixed in a 1:1 ratio with no additives, of same experiment
b
with differing amounts of benzonitrile, and of same experiment with 1.3 equiv of nitrile and differing amounts of COD. Ratio is
(phosphine)Ni(cod)/(phosphine)2Ni/(phosphine)Ni(η2-NC(Ph)).
Initial mixture of the chelating phosphine ligands in a 1:1
ratio with Ni(cod)2 confirmed and extended the findings of
our group and others, namely, that each ligand besides DPPF
and DPEPhos formed (phosphine)2Ni along with
(phosphine)Ni(cod) (Scheme 2a) in various molar equiv-
alencies (Table 1, column 3, entries 1−13).
Previously, the addition of nitrile in the presence of a π-
substrate was sufficient to remove one Xantphos ligand from
(Xantphos)2Ni and form (Xantphos)Ni(π-L) complexes as the
final product of the ligand substitution. However, the potential
of nitrile to begin the process of the removing a phosphine
from other (phosphine)2Ni complexes in the presence of COD
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a
Scheme 2. Experiments for Which the Results Are Shown in Table 1
a
(a) Mixture of 1 equiv of Ni(cod)2 and 1 equiv of a chelating phosphine. (b) Addition of benzonitrile. (c) Addition of excess COD.
to form (phosphine)Ni(cod) has not been investigated. To
gather information about this effect, differing amounts of
benzonitrile were added to mixtures of 1:1 Ni(cod)2/
phosphine. The addition of 0.15, 1.3, and 4 equiv of
benzonitrile to these mixtures (Scheme 2b; Table 1, columns
4−6, entries 1−13) was generally not effective forming more
(phosphine)Ni(cod). The cases where the addition of nitrile
had the largest effect were with DPPMB (entry 8) and BINAP
(entry 9), but a substantial amount of (phosphine)Ni(η2-
NC(Ph)) complex was observed along with the desired
(phosphine)Ni(cod) complex. These results showed that
addition of more benzonitrile was not sufficient to isolate
(phosphine)Ni(cod) complexes without the addition of more
COD.
Table 2. Reaction Conditions for the Synthesis of
(Phosphine)Ni(cod) Complexes
a
entry
phosphine
COD (equiv)
days
yield
1
2
3
4
DPEPhos (B)
DPPB (B)
DPPMB (T)
rac-BINAP (T)
0
10
65
20
1
1
4
1
85% (6)
95% (7)
89% (8)
65% (9)
a
All reactions used 1.3 equiv of benzonitrile. B = benzene as solvent.
T = THF as solvent.
required 65 equiv of COD and 4 days of stirring in THF before
it could be isolated.
The ratios from the reaction of 1 equiv of Ni(cod)2, 1 equiv
of phosphine, 1.3 equiv of benzonitrile, and either 2, 10, or 20
equiv of COD (Scheme 2c; Table 1, columns 7−9, entries 1−
13) allow for a few conclusions to be drawn. If the
(phosphine)2Ni complex is susceptible to benzonitrile binding
to facilitate ligand removal (e.g., DPPB (entries 3−5), DPPMB
(entry 8), and BINAP (entry 10)) then the addition of more
COD helps to favor the (phosphine)Ni(cod) complex. If the
(phosphine)2Ni complex is not very susceptible to benzonitrile
(e.g DPPE (entry 1), DPPP (entry 2)), then the addition of
COD does not have a large effect on the ratio of
(phosphine)Ni(cod)/(phosphine)2Ni.
While it was difficult to determine trends in much of these
data, one aspect that stood out was that the three phosphines
that gave the most promising results (DPPB, DPPMB, and
BINAP) also were ligands that form seven-membered rings
with Ni. The respective (phosphine)2Ni complexes of these
phosphines were the most likely to be susceptible to removal of
a phosphine using benzonitrile, and DPPB formed about 20
times more (dppb)Ni(cod) than (dppb)2Ni in most of the
experiments (Table 1, entry 4). As a result, these phosphines
were found to be the most suitable for a synthesis and isolation
of their respective (phosphine)Ni(cod) complexes.
(DPEPhos)Ni(cod) (6) was isolated in an 85% yield by
simply mixing 1 equiv of DPEPhos and Ni(cod)2. Isolation of
other (phosphine)Ni(cod) complexes required the addition of
nitrile and COD, as shown by the NMR scale experiments in
Table 1. Isolations of (dppb)Ni(cod) (7) (95% yield),
(dppmb)Ni(cod) (8) (89% yield), and (rac-binap)Ni(cod)
(9) (65% yield) were carried out by using the reaction
conditions shown in Table 2 (eq 2). The solvent was switched
to THF for the reactions to form 8 and 9 to assist the solubility
of the starting materials. None of these reactions required
more than 1.3 equiv of benzonitrile. The synthesis of complex
7 needed 10 equiv of COD to push the reaction to completion,
while the synthesis of 8 needed 20 equiv of COD. Complex 9
Derivatives of DPPB (1,4-bis(bis(4-fluorophenyl)-
phosphino)butane ((p-F)DPPB), Table 1, column 2, entry 3
and 1,4-bis(bis(4-methoxyphenyl)phosphino)butane ((p-
OMe)DPPB), Table 1, column 2, entry 5) were synthesized
in order to examine the effects of changing the electronic
properties of the ligand. Significant changes were not observed
with added p-OMe and p-F groups to the substituents of the
phosphines of DPPB (Table 1, entries 3−5).
Benzonitrile was used for all of the initial studies due to its
previous use in the syntheses of (Xantphos)Ni(π-L)
complexes.11 For completeness, other nitriles, varying in both
their steric and electronic properties, were screened. The ratios
between (phosphine)Ni(cod) and (phosphine)2Ni were
examined when DPPE, DPPP, and DPPMB were reacted
under the same conditions as those in Table 1, but with
electronically and sterically modified nitriles replacing
benzonitrile (eq 3). The phosphines were chosen in order to
inspect a range of reactivity of the (phosphine)2Ni complexes
with nitrile. Previous studies showed that (dppe)2Ni and
(dppp)2Ni were not reactive with benzonitrile (Table 1, entries
1 and 2), while (dppmb)2Ni was reactive and (dppmb)Ni-
(COD) was isolated (Table 1, entry 8; Table 2, entry 3).
These results are shown in Table 3. As with the previous
experiments involving COD, if the (phosphine)2Ni complex
was not susceptible to removal of one ligand by benzonitrile,
then the change of either the electronics or sterics of the nitrile
did not have a significant effect on the ratio. Both (dppe)2Ni
and (dppp)2Ni were not reactive with benzonitrile and
changing the nitrile did not result in an appreciable change
in the ratio of (phosphine)Ni(cod)/(phosphine)2Ni. With
DPPMB, a more electron donating nitrile (4-methoxybenzoni-
trile, Table 3, entry 2) slightly increased the amount of
(dppmb)Ni (cod) as compared to the reaction with just
benzonitrile (Table 3, entry 1). When the electron deficient
D
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DPPP, and DPPB) and by the rigidity of the backbone (such
as DPPB and DPPMB). ORTEP diagrams of complexes 10−
13 are shown in Figure 2. The average Ni−P bond lengths and
average P−Ni−P bond angles of complexes 10−12, 14, and 15
are shown in Table 4.
Table 3. Results from Experiments with Different Nitriles
Table 4. Average Bond Lengths and Ligand Bite Angles for
a
Complexes 10−12, 14, and 15
entry
complex
average Ni−P bond length ligand bite angle
1
2
3
4
5
(dppbenz)2Ni (10)
(dppmb)2Ni (11)
(dppb)2Ni (12)
(dppe)2Ni (14)
(dppp)2Ni (15)
2.157(2) Å
2.185(3) Å
2.2031(6) Å
2.164(5) Å12
2.177(1) Å13
90.6(3)°
106.7°
104.7°
90.5(3)°12
99.52°13
a
Complex 13 not included due to disorder.
These five complexes are all pseudotetrahedral about the Ni.
The measured bite angle of DPPE in complex 14 is 90.5(3)°.
The bite angle of DPPP in 15 increases to 99.52°. In complex
12, the bite angle of DPPB is 104.7°. The Ni−P bond lengths
also increased as more carbons were added to the backbone of
the phosphine ligand. The average bond length for complex 14
was 2.164(5) Å, followed by 2.177(1) Å in 15 and 2.2031(6) Å
in 12.
a
Ratio is (phosphine)Ni(cod)/(phosphine)2Ni/(phosphine)Ni(η2-
NC(Ar)).
Next, the (phosphine)2Ni complexes that had ligands
varying by the rigidity of their backbones were compared.
The first pair of (phosphine)2Ni complexes were (dppe)2Ni
(14) and (dppbenz)2Ni (10). The average Ni−P bond length
in 10 was about ∼0.01 Å shorter than the average bond length
for 14. The bite angles of the ligands in each complex were
very similar: 90.6(3)° for 10 and 90.5(3)° for 14.
We compared the crystal structures of (dppb)2Ni (12) and
(dppmb)2Ni (11). DPPB and DPPMB both form seven-
membered rings when bound to Ni. The bite angle of DPPB in
12 is 104.7°. In complex 11, the bite angle of DPPMB is
106.7°. The average Ni−P bond lengths are ∼0.018 Å shorter
in 11 as compared to 12.
3,5-difluorobenzonitrile was tested, a significant amount of
(dppmb)Ni(η2-NC((3,5-F)Ph)) formed alongside (dppmb)-
Ni(cod) and (dppmb)2Ni (Table 3, entry 3). Alkyl nitriles
such as ethylnitrile (Table 3, entry 4), isopropylnitrile (Table
3, entry 5), and tert-butylnitrile (Table 3, entry 6) did not have
a significant effect on the ratios of (dppmb)Ni(cod)/
(dppmb)2Ni.
Crystallography: (Phosphine)2Ni Complexes. Single
crystals of (dppbenz)2Ni (10), (dppmb)2Ni (11), (dppb)2Ni
(12), and a disordered crystal of (DPEPhos)2Ni (13) were
analyzed. The solid state structures of (dppe)2Ni12 (14) and
(dppp)2Ni13 (15) have been previously determined. These
data allowed comparisons between the structural properties of
(phosphine)2Ni complexes with ligands that differed by the
amount of carbons in the aliphatic backbone (such as DPPE,
Although the single crystal for complex 13 was disordered,
two bonds to one ligand were significantly shorter than the two
bonds to the opposite ligand. This difference was not observed
in any of the other (phosphine)2Ni complexes, where all of the
Ni−P bond lengths were nearly identical.
Crystallography: (Phosphine)Ni(cod) Complexes. Sin-
gle crystals of (DPEPhos)Ni(cod) (6), (dppb)Ni(cod) (7),
Figure 2. ORTEP diagrams: (dppbenzene)2Ni (10) (a), (dppmb)2Ni (11) (b), (dppb)2Ni (12) (c), and (DPEPhos)2Ni (13) (d). Ellipsoids set at
50% probability. Hydrogen atoms omitted for clarity.
E
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Figure 3. ORTEP diagrams: (DPEPhos)Ni(cod) (6) (a), (dppb)Ni(cod) (7) (b), and (dppmb)Ni(cod) (8) (c). Ellipsoids set at 50% probability.
Hydrogen atoms omitted for clarity.
and (dppmb)Ni(cod) (8) (ORTEP diagrams shown in Figure
3) were also studied. Average bond lengths and angles are
shown in Table 5. These crystal structures allowed for
were attributed to conglomerates for DPPM.14 The large,
floppy backbone of DPPPentane means that it is a likely
candidate to bind to two Ni centers instead of forming an
eight-membered ring with just one Ni center. DPPB hardly
formed any (dppb)2Ni when mixed with Ni(cod)2 (Table 1,
column 3, entry 4), while DPPP and DPPE formed significant
amounts of (phosphine)2Ni along with (phosphine)Ni(cod)
(Table 1, column 3, entries 1 and 2). This result indicated that
the seven-membered ring formed when DPPB binds to Ni is
less stable than the six- and five-membered rings formed,
respectively, when DPPP and DPPE bind to Ni. In addition,
when 15% or 1.3 equiv of benzonitrile was added to the
mixture of Ni(cod)2 and DPPB, (dppb)Ni(cod) was observed
exclusively while the ratios of (phosphine)Ni(cod)/(phosphi-
ne)2Ni for DPPE and DPPP under the same conditions did
not significantly change (Table 1, columns 4−6, entries 1, 2,
and 4). The successful isolation of (dppmb)Ni(cod) and (rac-
binap)Ni(cod) was concurrent with the observation that
phosphines forming seven-membered rings with Ni were more
susceptible to removal through addition of benzonitrile.
Turning to the literature, a report from 2017 by Sauthier and
co-workers found that a system of Ni(cod)2 and either DPPB
or DPPMB was efficient at performing hydroalkoxylation of
alkenes. DPPE and DPPP were not effective in this reaction,
and calculations supported the conclusion that (dppe)2Ni and
(dppp)2Ni were stable and unlikely to dissociate a
phosphine.15 This conclusion is also supported by literature
reports of experiments using chiral phosphines. Kempe and co-
workers16 developed an enantioselective nickel hydrosilyation
catalyst system where they utilized Ni(cod)2 and the chiral
chelating phosphine DIOP (Figure 4), which has four carbons
Table 5. Average Ni−P Bond Lengths and P−Ni−P Bond
Angles for Complexes 6−8
average Ni−P bond
ligand bite
angle
entry
complex
length
1
2
3
(DPEPhos)Ni(cod)(6)
(Dppb)Ni(cod) (7)
(Dppmb)Ni(cod) (8)
2.168(5) Å
2.152(1) Å
2.167(1) Å
105.5°
101.8°
105.5°
comparisons between these complexes and also between
each of them and their (phosphine)2Ni counterparts. The
first pair of complexes compared were (dppb)Ni(cod) (7) and
(dppb)2Ni (12). Replacement of one DPPB ligand in 12 with
COD, forming 7, decreases the Ni−P bond length by ∼0.05 Å.
The P−Ni−P angle also decreases by ∼2.9°.
Replacement of one of the DPPMB ligands in 11 with a
COD ligand to form 8 decreased the Ni−P bond lengths by
∼0.02 Å. The bite angle of the ligand also decreased by ∼1.2°.
These changes are analogous to the changes observed between
complexes 7 and 12.
Finally, complexes 7 and 8 were compared directly (Table 5,
entries 2 and 3). The P−Ni−P angle increased by 3.8° in 8 as
compared to 7. The Ni−P bond lengths also increased by
∼0.02 Å.
DISCUSSION
■
The conclusion previously reached by our group and others
that most chelating phosphines form (phosphine)2Ni com-
plexes when mixed with Ni(cod)2 was confirmed. Attempts to
quantify this complex formation and perform comparisons
between ligand sets were met with significant challenges. In the
beginning of this study, we believed that trends between the
different ligands would present themselves. These trends
would vary based on the amount of carbons in the backbone of
the chelating phosphine, along with the rigidity of the
backbone. When we tried to vary the backbones of the ligands
using an aliphatic backbone size of one (DPPM), two (DPPE),
three (DPPP), four (DPPB), and five (DPPPentane), DPPM
and DPPPentane proved to be unsuitable for analysis under
the normal conditions due to the formation of multiple side
products beyond just (phosphine)Ni(cod) and (phosphi-
ne)2Ni, as demonstrated by the observance of multiple
resonances in the 31P NMR spectra. These different signals
Figure 4. Chiral (S,S)-DIOP
in the backbone. The active catalyst was suspected to be
(diop)Ni(cod) and (diop)2Ni was isolated and shown to be
catalytically inactive. Finally, the report by Hartwig and co-
workers mentioned in the introduction uses benzonitrile to
eliminate (binap)2Ni in catalytic reactions. These methods and
their results mirror our findings.6f,h
Trends following the variance of rigidity of the ligand
backbones are harder to discern. Four pairs of phosphines were
F
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chosen to try and observe trends: DPPE/DPPBenz, DPPB/
DPPMB, BIPHEP/BINAP, and DPEPhos/Xantphos. How-
ever, each of these pairs had different relationships. DPPE and
DPPBenz, being smaller ligands, made (phosphine)2Ni
complexes that were not reactive with nitrile. The more rigid
DPPBenz formed (dppbenz)Ni(cod) in an 8:1 ratio with
(dppbenz)2Ni, while the less rigid DPPE formed (dppe)Ni-
(cod) in a 1:2 ratio with (dppe)2Ni under the same conditions
(Table 1, entries 1 and 7). For DPPB/DPPMB, this
relationship was reversed. The less rigid DPPB formed
(dppb)Ni(cod) in a 15:1 ratio with (dppb)2Ni (Table 1,
column 3, entry 4), while the more rigid DPPMB formed
(dppmb)Ni(cod) in a 1:6 ratio with (dppmb)2Ni (Table 1,
column 3, entry 8). BIPHEP and BINAP had similar ratios in
column 3. (binap)2Ni was reactive with nitrile, while
(biphep)2Ni was not. Xantphos and DPEPhos react very
differently, despite the only difference being the extra dimethyl
connection between the benzene rings on the xanthene
backbone. Xantphos forms an insoluble red solid when
mixed with Ni(cod)2, which is (Xantphos)2Ni. DPEPhos
forms (DPEPhos)Ni(cod) exclusively. A reason for this change
in reactivity could be the addition of more steric bulk around
the nickel by the less rigid DPEPhos, which can flex and move
to ensure more favorable steric and π-stacking17 interactions
with itself rather than with an opposite ligand. Examination of
the solid-state structure of complex 13 shows only one π-
stacking interaction between the opposite ligands, which may
not give enough stabilization to overcome the steric clashes
that occur when two DPEPhos ligands bind to Ni.
While we were unable to acquire an ordered solid-state
structure of (Xantphos)2Ni, the more rigid nature of the ligand
will leave more space around the Ni. This allows for an
opposite ligand to bind and gives enough room for favorable π-
stacking interactions to occur. Xantphos is also interesting in
that (Xantphos)2Ni can be activated for catalysis using
benzonitrile.11 However, experiments using only benzonitrile
and COD were largely unsuccessful at forming any
(Xantphos)Ni(cod). This illustrates the need for a ligand
that binds strongly to Ni (such as an alkyne or strongly
backbonding alkene) opposite Xantphos. Other ligands have
not shown this effect. DPPB, for example, forms a more stable
Ni(0) complex with COD than with a strongly backbonding
alkene such as stilbene.
A unique feature was observed with the crystal structure of
complex 13. Despite the differing exact bond lengths between
different structures in the unit cell and the toluene disorder,
the data were clear that one DPEPhos ligand had shorter bond
lengths to the Ni than the opposite DPEPhos ligand. As
previously stated, DPEPhos only forms (DPEPhos)Ni(cod)
when mixed with Ni(cod)2. A possible reason could be that
more stability exists in forming shorter Ni−P bonds in
(DPEPhos)Ni(cod) rather than forming longer bonds to form
(DPEPhos)2Ni. Less steric clashing occurs in complex 6 as
compared to that in complex 13, resulting in the favored
formation of complex 6.
Another correlation observed was that the formation of
(phosphine)Ni(cod) was more favorable in a specific range of
ligand bite angles. The ligands with bite angles under 100°,
such as DPPE, DPPP, and DPPBenz, formed (phosphine)2Ni
complexes that were not reactive with benzonitrile. Ligands
slightly above this threshold in the range of 102−110° were
much more likely to form (phosphine)Ni(cod) or have a
(phosphine)2Ni complex that was reactive with benzonitrile.
Above a bite angle of 110°, the (phosphine)2Ni complex was
reactive with benzonitrile, but the (phosphine)Ni(cod)
complex was not as stable as the (phosphine)2complex. The
most prominent example was Xantphos (bite angle of 112°).18
Nitrile can activate (Xantphos)2Ni, but (Xantphos)Ni(cod)
was not isolable. While many other factors affect the ligand
exchange, the bite angle provides a starting point toward
categorizing the ligands. An exception is BINAP. The bite
angle of BINAP is 93°, but (binap)2Ni was reactive with nitrile.
The steric bulk of the ligand could be destabilizing (binap)2Ni.
DPPF also has a bite angle of 96°, and its unique reactivity
could be a result of the stereoelectronic properties imparted by
the ferrocene backbone.
The variation of the steric and electronic properties of the
nitrile also gave key information. With DPPE and DPPP, the
use of the electron-withdrawing nitrile did not change the ratio
of (phosphine)Ni(cod)/(phosphine)2Ni (Table 3, entry 3).
With DPPMB, a significant amount of (dppmb)Ni(η2-NC(3,5-
F)Ph) was observed. Since 3,5-difluorobenzonitrile is electron-
deficient, it is much more π-acidic than benzonitrile and thus
more suited for nucleophilic attack by the Ni. These results
showed that the 3,5-difluorobenzonitrile was causing the
removal of a ligand from (dppmb)2Ni in order to form the
more stable complex (dppmb)Ni(η2-NC(3,5-F)Ph).
These studies also showed that the addition of nitrile had
very little effect on (dppe)2Ni and (dppp)2Ni because no η2-
NC bound nitrile complex was observed even with 3,5-
difluorobenzonitrile. These results further confirm that the
distribution of (phosphine)Ni(cod)/(phosphine)2Ni for
DPPE and DPPP is governed by the initial reaction of the
phosphine with Ni(cod)2. Once (dppe)2Ni and (dppp)2Ni are
formed, they are unlikely react with any nitrile to form
(phosphine)Ni(cod).
CONCLUSIONS
■
We have extended our previous research into (Xantphos)Ni(π-
L) complexes11 to inspect the reactions between Ni(cod)2 and
many different chelating phosphines beyond Xantphos. The
desired precatalyst or active species in reactions utilizing
Ni(cod)2 and a chelating phosphine is (phosphine)Ni(cod),
and the amount of this complex in a reaction can be increased
with the addition of benzonitrile.
The syntheses and full characterizations of more (phosphi-
ne)2Ni complexes allowed us to carry out valuable average
bond length studies that gave information regarding the
relationships between the bond lengths and angles and the
propensity of each ligand to form (phosphine)Ni(cod) when
reacted with Ni(cod)2. We have previously shown that
(Xantphos)2Ni is a moderately air-stable complex that can be
activated for catalysis by the addition of benzonitrile.11 The
other (phosphine)2Ni complexes could potentially act the
same way.19
Our studies suggest the role of nitrile in the displacement of
a phosphine from a given (phosphine)2Ni complex (Scheme 1
and Table 3) is quite complex. Experimental and theoretical
routes to explore this mechanism are currently underway.
These studies expand the scope of ligands that can be used
for Ni(0) catalysis. Complexes 6−9 have been isolated and can
be used for catalysis with the firm knowledge that no
detrimental (phosphine)2Ni or Ni(cod)2 will be present in
the reaction. With other ligands, the addition of benzonitrile to
the reaction increases the amount of catalytically active
(phosphine)Ni(cod). The new knowledge about the extent
G
DOI: 10.1021/acs.organomet.8b00438
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
mixed in a 1:1 ratio in benzene, followed by addition of a nitrile (1.3
equiv) after 30 s. The solutions were analyzed by 31P NMR after 2 h
and the following day.
of active precatalyst will allow for an expansion of the ligand
scope in these catalytic reactions and pave the way for new
reaction discovery. Efforts are currently underway to use these
new complexes and the new knowledge of the effects of adding
benzonitrile to these reactions.
(DPEPhos)Ni(cod) (6). DPEPhos (300.2 mg, 0.557 mmol) and
Ni(cod)2 (153.3 mg, 0.557 mmol) were added to a 20 mL
scintillation vial equipped with a stir bar. Benzene (2 mL) was
added, and the solution was stirred for 12 h. Pentane (15 mL) was
then added, and the vial was stored at −38 °C overnight. Subsequent
filtration gave an orange solid which was complex 6 (334.5 mg, 473.5
mmol, 85%). Crystals suitable for X-ray diffraction were grown from
the layering of 1 mL of pentane over a saturated solution of the
complex in 0.5 mL of benzene. 1H NMR (500 MHz, C6D6): δ 7.57 (t,
J = 5 Hz, 8H), 7.11 (t, J = 5 Hz, 2H), 7.00 (m, 12H), 6.74 (m, 4H),
6.52 (t, J = 5 Hz, 2H), 4.61 (d, J = 5 Hz, 4H, vinylic COD), 1.86 (t, J
= 10 Hz, 4H, aliphatic COD), 1.76 (s, 4H). 13C{1H} NMR (125
MHz, C6D6): δ 160.5 (d, J = 12.5 Hz), 137.6 (dd, J = 25, 5 Hz), 133.7
(d, J = 12.5 Hz), 133.7 (s), 129.5 (s), 123.2 (d, J = 1.25 Hz), 121.7 (t,
J = 2.5 Hz), 84.7 (d, J = 6.25 Hz), 29.7 (d, J = 3.75 Hz). 31P{1H}
NMR (C6D6): δ 33.1 (s). IR (nujol, NaCl): δ 3071 (m), 3048 (m),
2872 (m), 2726 (m), 2669 (w), 1959 (w), 1653 (m), 1586 (s), 1562
(m), 1309 (w), 1259 (m), 1204 (w), 1095 (w), 1069 (m), 743 (w),
772 (m), 525 (m). Anal. Calcd for C44H40NiP2O: C,74.96; H, 5.71.
Found: C, 74.68; H, 5.73.
(dppb)Ni(cod) (7). DPPB (77.5 mg, 0.182 mmol) and Ni(cod)2
(50 mg, 0.182 mmol) were added to a 20 mL scintillation vial
equipped with a stir bar. Benzene (1 mL) was added, and the reaction
was stirred for 5 min. COD (288 μL, 2.35 mmol) was added, followed
by benzonitrile (24.3 μL, 0.236 mmol). The heterogeneous yellow
solution was stirred overnight. Pentane (15 mL) was added, and the
vial was stored overnight at −38 °C. The solution was then filtered
through a medium frit, yielding 7 as a bright yellow solid (102.1 mg,
0.172 mmol, 95%). Yellow crystals suitable for X-ray diffraction were
grown by layering 1 mL of pentane on to a saturated solution of the
complex in 0.5 mL of benzene. 1H NMR (500 MHz, C6D6): δ 7.46 (t,
J = 10 Hz, 8H), 7.15 (t, J = 10 Hz, 8H), 7.08 (t, J = 10 Hz, 4H), 4.39
(d, J = 10 Hz, 4H), 2.16 (s, 4H), 1.84 (t, J = 10 Hz, 4H), 1.58 (s, 4H),
1.17 (s, 2H), 1.13 (s, 2H). 13C{1H} NMR (125 MHz, C6D6) δ 141.2
(d, J = 30 Hz), 132.4 (t, 6.25 Hz), 83.5 (t, 2.5 Hz), 67.4 (s), 35.2 (m),
29.8 (s), 25.4 (s) 24.8 (s). 31P{1H} NMR (121 MHz, C6D6): δ 35.9
(s). IR (nujol, NaCl, cm−1): 3071 (m), 3050 (m), 3018 (s), 2029
(w), 1959 (m), 1585 (w), 1537 (s), 1377 (m), 1323 (w), 1300 (m),
1180 (m), 1155 (m), 1098 (w), 1025 (w), 719 (br), 623 (w). Anal.
Calcd for C36H40NiP2: C. 72.87; H, 6.79. Found: C, 72.45; H, 6.81.
(dppmb)Ni(cod) (8). DPPMB (258 mg, 0.543 mmol) and Ni(cod)2
(149 mg, 0.543 mmol) were added to a 20 mL scintillation vial
equipped with a stir bar. THF (10 mL) was added, and the reaction
was stirred for 5 min. COD (4.2 mL, 34.3 mmol) was added, followed
by benzonitrile (76 μL, 0.591 mmol). The homogeneous yellow
solution was then stirred for 4 days. The volatiles were removed under
vacuum. Pentane (15 mL) was added, and the vial was stored
overnight at −38 °C. The solution was filtered through a medium frit,
yielding 8 as a bright yellow solid (299.4 mg, 0.466 mmol, 86%).
Crystals suitable for X-ray diffraction were grown by layering 1 mL of
pentane on to a saturated solution of the complex in 0.5 mL of
benzene. 1H NMR (C6D6): δ 7.56 (t, J = 10 Hz, 8H), 7.15 (t, J = 10
Hz, 12H), 6.44 (m, 2H), 6.11 (m, 2H), 4.41 (d, J = 10 Hz, 4H), 3.65
(d, J = 5 Hz, 4H), 1.86−1.64 (m, 8H). 13C{1H} NMR (C6D6) δ 139.8
(dd, J = 22.5, 3.75), 134.2 (s) 132.0 (d, 30 Hz), 132.0 (s), 129.5 (t, 5
Hz), 127.5 (s) 124.0 (s) 84.9 (t, 6.25 Hz), 84.9 (d, 6.25 Hz), 39.8 (d,
14 Hz), 33.1 (s), 29.1 (t, 2.5 Hz), 27.1 (s), 21.4 (s), 13.0 (s). 31P{1H}
NMR (C6D6) δ 27.4 (s). Anal. Calcd for C40H40NiP2: C. 74.90 H.
6.28. Found: C. 74.63 H. 5.97.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were carried out in a
glovebox under an atmosphere of N2. All glassware was dried in an
oven overnight. Benzene, pentane, and THF were sparged with
nitrogen, dried over neutral alumina, and deoxygenated over Q5
under N2 using a Grubbs type purification system. DPPM, DPPE,
DPPP, DPPB, DPPPentane, DPPMB, rac-BINAP, DPEPhos,
BIPHEP, DPPBenz, DPPF, and Xantphos, were purchased from
commercial sources and used as received. Ni(cod)2 was purchased
from Strem Chemicals or prepared according to the literature
procedure.20 Deuterated benzene (Cambridge Isotope Laboratories),
deuterated THF (Cambridge Isotope Laboratories), and 1,5-cyclo-
octadiene (COD) were distilled from CaH2 and degassed using three
freeze−pump−thaw cycles. Screw-cap NMR tubes were used for all
NMRs. The tubes were cleaned by rinsing with acetone, sonicating
with 1:1 THF/concentrated HCl for 1 h, and rinsing again with
1
acetone. NMR spectra were recorded on Varian spectrometers. H
experiments were recorded at 500 MHz. 13C experiments were
recorded at 125 MHz. 31P experiments were recorded at 121 MHz.
31P NMR spectra were referenced to external 85% H3PO4 (0 ppm).
1
13C and 31P experiments were proton-decoupled. H and 13C NMRs
were referenced to the residual solvent peak for either benzene-d6 (δ
7.13 and δ 128.6, respectively) or THF-d8 (δ 3.76 and δ 68.0,
respectively). IR spectra were recorded on a Bruker Tensor 27 FT-IR
spectrometer. Elemental analyses were performed by Midwest
Microlab LLC. The discussed ORTEP diagrams of complexes 6−8
and 10−13 were created using Mercury.21
X-ray Structure. Single crystal X-ray crystallography data were
collected and analyzed by Dr. Arnold L. Rheingold at the University
of California, San Diego and by Dr. Ryan T. Vanderlinden at the
University of Utah.
University of California, San Diego (A.L.R.). The diffraction data
were collected on a Bruker Ultra mini rotating anode (Mo) with an
Apexil detector and microfocusing optics. The OLEX222 software
suite was used to manage the data. All data were collected at 100 K.
PLATON SQUEEZE23 was used to account for severely disordered
solvent molecules that are not represented in the structure.
University of Utah (R.T.V.). The diffraction data were collected on
a Nonius KappaCCD diffractometer equipped with Mo KR radiation
(λ = 0.71073 Å) and a BRUKER APEXII CCD detector. The
APEX324 software suite was used to manage data collection,
integration, scaling, absorption correction by the multiscan method
(SADABS),25 structure determination via direct methods
(SHLEXT),26 and model refinement (SHELXL).27 All data were
collected at 103(2) K.
General Procedure for Reactions of Ni(cod)2 and a
Phosphine in a 1:1 Ratio (Table 1, Column 3). The phosphine
ligand and Ni(cod)2 were mixed in a 1:1 ratio in 0.75 mL of benzene-
1
d6. The solution was analyzed by 31P and H NMR after 2 h and the
following day.
General Procedure for Reactions of Ni(cod)2, Phosphine,
and Benzonitrile (Table 1, Columns 4−6). The phosphine and
Ni(cod)2 were mixed in a 1:1 ratio in benzene, followed by addition
of benzonitrile (15%, 1.3 equiv, or 4 equiv) after 30 s. The solutions
were analyzed by 31P NMR after 2 h and the following day.
General Procedure for Reactions of Ni(cod)2, Phosphine,
Benzonitrile, and Excess COD (Table 1, Columns 7−9). The
phosphine and Ni(cod)2 were mixed in a 1:1 ratio in benzene,
followed by addition of COD (2, 10, or 20 equiv) after 30 s and
addition of benzonitrile (1.3 equiv) after another 30 s. The solutions
were analyzed by 31P NMR after 2 h and the following day.
General Procedure for Reactions of Ni(cod)2, Phosphine,
and Various Nitriles (Table 3). The phosphine and Ni(cod)2 were
(rac-binap)Ni(cod) (9). BINAP (72.1 mg, 0.115 mmol) and
Ni(cod)2 (31.8 mg, 0.115 mmol) were weighed into a 20 mL
scintillation vial. Benzene (3 mL) was added, followed by benzonitrile
(15.4 μL, 0.149 mmol) and COD (141 μL, 1.15 mmol). The solution
went from yellow to gray to black, and was stirred for 18 h, after
which the volatiles were removed. Pentane (15 mL) was added, and
the vial was stored overnight at −38 °C. Subsequent filtration through
a medium frit yielded 9 as a brown/black solid (61.6 mg, 0.078 mmol,
H
DOI: 10.1021/acs.organomet.8b00438
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
67%). 1H NMR (500 MHz, THF-d8): δ 8.28 (t, J = 10 Hz, 2H), 7.96
(t, J = 10 Hz, 4H), 7.77 (d, J = 10 Hz, 2H), 7.60 (m, 8H), 7.24 (t, J =
10 Hz, 2H), 7.11 (m, 4H), 6.83 (t, J = 10 Hz, 2H), 6.59 (s, 6H), 4.75
(s, 2H, vinylic COD), 4.53 (s, 2H, vinylic COD), 1.96 (s, 2H, under
solvent peak, aliphatic COD), 1.90 (s, 2H, under solvent peak,
aliphatic COD), 1.74 (s, 2H, aliphatic COD), 1.55 (s, 2H, aliphatic
COD). 13C NMR (125 MHz, THF-d8): δ 140.0 (s), 139.7 (s), 138.6
(s), 138.4 (s), 138.1 (s), 137.9 (s), 135.4 (d, J = 12.5 Hz), 135.1 (d, J
= 12.5 Hz), 134.9 (s), 134.0 (s), 128.7 (s), 128.5 (d, J = 12.5 Hz),
128.2 (s), 128.1 (s), 126.7 (s), 126.6 (s), 126.4 (s), 126.2 (s), 86.1
(s), 83.8 (s), 30.4 (s), 30.1 (s), 29.4 (s). 31P NMR (121 MHz, THF-
d8): δ 33.6 (s). IR (nujol, NaCl, cm−1): 3168 (w), 3142 (m), 3052
(s), 2726 (m), 2668 (m), 1585 (w), 1303 (m), 1156 (m), 1088 (w),
1027 (m), 965 (m), 890 (w), 722 (m), 696 (w), 673 (w), 524 (w).
Anal. Calcd for C52H44NiP2: C. 79.10; H, 5.62. Found: C, 78.74; H,
5.76.
ORCID
Nicholas D. Staudaher: 0000-0002-5720-0949
Janis Louie: 0000-0003-3569-1967
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the National Institute of Health
(GM076125) for financial support. We also thank Mia Yu for
assistance in final synthesis and characterization of the new
complexes.
REFERENCES
■
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1
solution sit undisturbed for 24 h. H NMR (500 MHz, THF-d8): δ
7.48 (s, 4H), 7.43 (s, 20H), 7.18 (t, J = 10 Hz, 8H), 7.03 (t, J = 10
Hz, 16H), 13C{1H} NMR (125 MHz, THF-d8): δ 150.7 (m), 140.2
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31P{1H} NMR (121 MHz, C6D6): 48.02 (s). Anal. Calcd for
C60H48NiP4: C, 75.73; H, 5.08. Found: C, 75.56; H, 5.15.
(dppb)2Ni (12). DPPB (153.8 mg, 0.369 mmol) and Ni(cod)2
(50.8 mg, 0.185 mmol) were weighed into a 20 mL scintillation vial.
Benzene (1 mL) was added, and the solution was stirred for 24 h.
Pentane (4 mL) was then added to the heterogeneous yellow
solution. The reaction was stored at −40 °C overnight. The liquid was
removed using a pipet, and the solid was washed 2 times with 5 mL of
pentane and then dried under high vacuum. Complex 12 was
recovered as a yellow solid (150.4 mg, 0.169 mmol, 91%). Yellow
crystals suitable for X-ray diffraction analysis were grown by dissolving
a small-scale version of the reaction in benzene and letting the
solution sit undisturbed for 24 h. 1H NMR (500 MHz, C6D6): δ 7.55
(s, 16H), 7.01 (m, 24H), 2.03 (s, 8H), 1.24 (s, 8H). 13C{1H} NMR
(125 MHz, C6D6): δ 134.1 (s), 128.9 (s), 128.7 (s), 128.5 (s), 128.2
(s), 128.1 (s), 31.7 (q, J = 7.5 Hz), 24.5 (s). 31P{1H} NMR (121
MHz, C6D6): δ 19.36 (s). Anal. Calcd for C56H56NiP4: C, 73.78; H,
6.19. Found: C, 73.52; H, 6.26.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00438.
■
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2016, 138, 6711−6714. (d) Xiao, J.; Yang, J.; Chen, T.; Han, L.-B.
Nickel-Catalyzed α-Benzylation of Arylacetonitriles via C-O Activa-
CCDC 1850155−1850160 and 1850175 contain the supple-
mentary crystallographic 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.
AUTHOR INFORMATION
Corresponding Author
*E-mail: louie@chem.utah.edu.
■
I
DOI: 10.1021/acs.organomet.8b00438
Organometallics XXXX, XXX, XXX−XXX

Organometallics
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DOI: 10.1021/acs.organomet.8b00438
Organometallics XXXX, XXX, XXX−XXX