Addition of a B-H Bond across an Amido-Cobalt Bond: CoII-H-Catalyzed Hydroboration of Olefins

Article abstract of DOI:10.1021/acs.organomet.8b00114

This paper describes a well-defined cobalt(II) half-sandwich complex bearing a phosphinoaminato ligand, Cp Co(1,2-Ph₂PC₆H₄NH) (1), that can activate pinacolborane (HBpin) for catalytic terminal hydroboration of olefins. Th

Full text of DOI:10.1021/acs.organomet.8b00114

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Addition of a B−H Bond across an Amido−Cobalt Bond: CoII−H-
Catalyzed Hydroboration of Olefins
Maofu Pang,†,‡ Chengjuan Wu,† Xuewen Zhuang,† Fanjun Zhang,† Mincong Su,† Qingxiao Tong,*,‡
Chen-Ho Tung,† and Wenguang Wang*,†
†Key Lab of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong
University, Jinan, 250100, People’s Republic of China
‡Department of Chemistry, Shantou University, 243 University Road, Shantou, Guangdong 515063, People’s Republic of China
S
* Supporting Information
ABSTRACT: This paper describes a well-defined cobalt(II) half-sandwich
complex bearing a phosphinoaminato ligand, Cp*Co(1,2-Ph2PC6H4NH)
(1), that can activate pinacolborane (HBpin) for catalytic terminal
hydroboration of olefins. The cooperative cobalt(II)−amido reactivity in 1
enables the B−H bond cleavage, affording the 17-electron cobalt(II) hydride
Cp*Co(1,2-Ph2PC6H4NH(Bpin)), abbreviated H1(Bpin), in which the
borenium ion is captured by the uncoordinated nitrogen atom of the
phosphinoaminato ligand. Hydroboration of the CC bond can be
promoted by a heteroatom such as N or O at the β-position of terminal
alkenes. The mechanism of such hydroboration was established by various
stoichiometric reactions based on the cobalt(II) hydride. With cooperative
CoII−N reactivity for the B−H bond cleavage, our catalysis depends on the
CoII−H hydride generated by the system itself.
Chart 1. Examples of Cobalt Complexes for Catalytic
Hydroboration of Alkenes by HBpin
INTRODUCTION
■
Transition-metal-catalyzed hydroboration of alkenes and
alkynes is a particularly powerful method in organic synthesis
because it provides valuable organoboron reagents for a range
of cross-coupling transformations.1,2 In this type of catalysis,
activation of the B−H bond of hydroboranes such as HBpin is a
fundamental step, essential for the rational design and
optimization of a novel homogeneous catalyst with high
activity and selectivity.3 The elementary reaction between an
organometallic catalyst and a hydroborane depends upon the
nature of the metal center and its ligand scaffold.4 The
traditional modes of B−H bond activation have been the
domain of precious-metal-based oxidative addition,5 as
exemplified by HRh(PiPr3)2(Bcat)Cl6 and [Ir(C5Me5)(PMe3)-
(Bpin)H],7 and σ-bond metathesis.8
In recent years, use of more earth-abundant metals for
homogeneous catalysis has attracted growing interest.9,10 In the
context of metal-catalyzed hydroborations, cobalt complexes
have been developed to pursue high-efficiency catalysis with
excellent selectivity under mild reaction conditions.11 For
example, Chirik et al. showed that (pyridyldiimine)cobalt
complexes catalyze anti-Markovnikov hydroboration of termi-
nal and internal olefins (Chart 1, I and II).12 Related catalysis
has also been reported by the Huang11 and Lu13 groups based
on (bipyridyl-phosphine)CoCl2 (IV) and (iminopyridine-
oxazoline)CoCl2 (V) systems, which are activated in situ with
NaBHEt3. Notably, Thomas et al. and Chirik et al. have found
that the cobalt(II) precursors (VII and VIII) can be activated
Received: February 23, 2018
14
by BuOK and LiOMe.15 The B−H bond is almost activated
t
© XXXX American Chemical Society
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by oxidative addition of HBpin to the Co(I) compound (III)16
or via σ-bond metathesis based on low-valent cobalt hydride or
alkyl active species.11a,12
In addition to metal-centered activation, the H−B bond can
also be activated by cooperative metal−ligand reactivity.17 In
such cases, the metal center accepts the hydride while the basic
site of the ligand captures the remaining borenium ion.18 This
strategy has been successfully applied to Ru-19 and Ir-based20
systems for catalytic hydroborations. However, the examples of
cobalt-based systems are sparse. It is noteworthy that
Stradiotto, Turculet et al. have recently reported a three-
coordinate (N-phosphinoamidinate)(amido)cobalt(II) precata-
lyst (IX) for terminal hydroboration of branched alkenes.21 By
the synergism of cobalt−amido reactivity, the systems operate
in the absence of external activation.
Figure 1. Structure of 1 showing 50% probability ellipsoids. For clarity,
hydrogen atoms are omitted, and the two phenyl groups bonded at the
phosphorus atom are drawn as lines. Selected bond distances (Å):
Co−N, 1.857(4); Co−P, 2.161(1).
Generation of CoII−H species by direct addition of a
hydroborane to a cobalt(II) entity for catalytic hydroboration
has not been previously reported. In this paper, we report a
cobalt(II) half-sandwich complex (1) bearing a phosphinoami-
nato ligand that, with HBpin, achieves catalytic terminal
hydroboration of olefins. The reaction is initiated by addition
of the B−H bond across an N−Co bond (Scheme 1), affording
a
Table 1. Cobalt(II)-Catalyzed Hydroboration of Olefins
Scheme 1. Addition of HBpin across an N−Co Bond To
Generate CoII−H for Catalytic Hydroborations
a 17-electron cobalt(II) hydride, H1(Bpin), that is responsible
for the catalysis. On the basis of the isolation and character-
ization of such a well-defined cobalt(II) hydride, the
mechanism of catalytic hydroboration was established by
various stoichiometric reactions.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of 1. By the reaction of
[Cp*CoCl]2 with lithium (2-(diphenylphosphino)phenyl)-
amide at −78 °C in THF, 1 was prepared. After evaporation
of the reaction mixture, the product was extracted into toluene
and isolated in high yield as a pink solid. The magnetic moment
of 2.02 μB established for 1 by the Evans method is consistent
with a low-spin d7 configuration (μeff = 2.2 μB).22 Crystallo-
graphic analysis confirmed its structure as a five-coordinate
neutral cobalt(II) compound, in which the phosphinoaminato
group is chelated at the cobalt center with a 17-electron
configuration (Figure 1).
a
Reaction conditions: olefin substrate (0.5 mmol), HBpin (0.5 mmol),
b
and 1 (2 mol %) were stirred in C6D6 (0.6 mL) at 40 °C. Isolated
yields are provided.
hexene, and trans-2-hexene, only a single product was obtained,
the terminal alkylboronic ester (3b, entries 2, 5, and 6).
Endocyclic alkenes such as cyclohexene and norbornylene,
which are more challenging in hydroboration reactions,
required prolonged reaction times but were ultimately
converted to the desired products (3f, 52%; 3g, 43%).
Stepwise Hydroboration of CC and CN Bonds.
Chemoselective hydroboration of alkene substrates in the
presence of carbonyl or imine functional groups is always
challenging.16,23 We were therefore encouraged to find that 1
catalyzed selective hydroboration of N-allylimines (Table 2).
With 1 mol % of 1, the reaction of N-allylbenzylideneamine and
Hydroboration of Alkenes. The activity of 1 as a
hydroboration catalyst was demonstrated with unactivated
aliphatic alkenes (Table 1). Terminal alkenes such as
vinylcyclohexane, 1-hexene, 1-octene, and 1-isopentene (entries
1−4) underwent anti-Markovnikov hydroboration with HBpin.
High-yield conversions were achieved in 4 h with only 2 mol %
of catalyst at 40 °C. The corresponding terminal alkylboronic
esters were obtained in ∼90% yields. Internal alkenes, including
trans- and cis-alkenes, are all amenable to this transformation,
affording the isomerization−hydroboration products, again in
good yields (entries 5−8). For a mixture of 1-hexene, cis-3-
B
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Table 2. Cobalt(II)-Catalyzed Stepwise Hydroboration of
CC and CN Bonds
Table 3. “Heteroatom-Promoted” Hydroboration of
Alkenes
a b
,
a
a
Reaction conditions: olefin substrate (0.5 mmol), HBpin (0.5 mmol),
and 1 (1 mol %) were stirred in C6D6 (0.6 mL) at room temperature.
b
Isolated yields are provided. Two equivalents of HBpin was used.
presence of 2 equiv of HBpin, allylamine was fully converted to
the doubly borylated compound (7b) in 15 min. A similar
reaction with a secondary or a tertiary N-allylamine gave the
expected hydroborated products (7c,d) in perfect yields within
30 min. However, the hydroboration reactions became slow in
the presence of an external amine. For instance, the yield of 7d
decreased to 33% when the catalysis was carried out with the
addition of 0.2 equiv of diethylamine into the system.
Interestingly, 4-pentenenitrile afforded the CC borylated
product (7e), indicating that the catalyst was not deactivated by
the cyano group. In these cases, the presence of the nitrogen
atom actually promoted the alkene hydroborations, which were
achieved under milder conditions in comparison with the
simple olefins in Table 1. Furthermore, the presence of an
oxygen atom also promoted such cobalt-catalyzed alkene
hydroboration, as demonstrated by the formation of 7f,g in
high yields.
Mechanistic Studies. For typical cobalt-based hydro-
borations, the catalysis involves B−H bond cleavage via
oxidative addition or σ-bond metathesis by Co(I) active
species.11,12,16 To understand the mechanism of our cobalt-
operated system, several stoichiometric reactions were carefully
examined. Although complex 1 is inactive toward olefins, it
reacts readily with HBpin, affording the cobalt(II) hydride
H1(Bpin).
a
Reaction conditions: olefin substrate (0.5 mmol) and 1 (1 mol %)
were stirred in C6D6 (0.6 mL) at room temperature. Yields are based
b
on 1H NMR analysis. One equivalent of HBpin (0.5 mmol) was used
for selective CC bond hydroborations (5), and 2 equiv of HBpin
was used for the fully hydroborated product 5′.
1 equiv of HBpin produced the terminal boronic ester (5a)
exclusively in 10 min at room temperature. With an extra 1
equiv of HBpin added into the system, the substrate was
converted to the fully hydroborated product (5a′) in 4 h. In the
case of benzaldimines, both electron-donating and electron-
withdrawing substituents at the para position of the phenyl ring
are all compatible with the highly selective stepwise hydro-
boration of CC and CN bonds (5a−5e, 5a′−5e′).
Significantly, the variation of the substituents in the imine
moiety from aryl to cyclohexyl or ferrocenyl also allowed
regioselective hydroboration of the CC bonds by 1 equiv of
HBpin in perfect yields (5f,g).
In Rh-based catalytic systems, the amide group in the alkene
substrates was found to facilitate the CC bond hydro-
boration, achieving “carbonyl-directed” hydroboration.24 Bind-
ing of the substrate carbonyl to rhodium during the catalysis
improves the level of regioselectivity. In our case of
regioselective hydroboration of the CC bonds for N-
allylimines, the imine moiety may also serve as a “directing”
group. The hypothesis of substrate−catalyst interactions
through nitrogen−cobalt interaction was further tested by the
results of hydroboration of N-allylamines (see below).
“Heteroatom-Promoted” Hydroboration of Alkenes.
Using 1 mol % of 1, terminal alkenes containing a nitrogen or
oxygen atom at the β position all underwent rapid hydro-
boration under the mild catalytic conditions (Table 3). The
yields for these terminal boronic esters with functional groups
are good to excellent. In addition to imine groups, amine
groups were also found to accelerate alkene hydroboration. For
example, 1 catalyzed the hydroboration of allyamine to γ-
aminoboronic ester (7a). Crystallographic characterization of
7a revealed the formation of a spirocyclic amine−borane. In the
By treatment of a benzene solution of 1 with HBpin, the
reaction was indicated by an instantaneous color change from
pink to green. The product H1(Bpin) was identified by ESI-MS
(Figure S3 in the Supporting Information). In its IR spectrum,
H1(Bpin) exhibits the νCo−H band at 1902 cm−1, comparable to
the CoII−H frequency of 1935 cm−1 for HCo(PMe3)4Cl.25
H1(Bpin) is also paramagnetic with the measured magnetic
moment μeff = 2.62 μB, which is consistent with the 2.01 μB
observed for [HCo(P(CH2CH2PPh2)3]PF6.26 Crystallographic
analysis reveals the two-legged piano-stool complex Cp*CoH-
(P-NH(Bpin)). The Me5C5 ring and the P atom are
−
coordinated to the Co(II) center, while the N atom is
uncoordinated and connected to the borenium moiety (Figure
2). The hydride does not interact with the borylated amine, and
C
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Interestingly, 7d was produced when extra HBpin was added to
the reaction solutions (eq 2, Figure S8).
In the presence of HBpin, the reaction of 6d with D1(Bpin)
generated by treatment of 1 with DBpin gave unambiguously
the deuteride product (d1-7d) according to 1H NMR
spectroscopic and GC-MS analysis. ESI-MS studies for the
reaction solution indicated that the organocobalt species is
H1(Bpin), and the formation of d1-7d was also verified (Figure
3). In contrast, H1(Bpin) reacts with 6d and DBpin, giving 7d
Figure 2. Structure of H1(Bpin) showing 50% probability ellipsoids.
Selected bond distances (Å): Co−H, 1.37(3); Co−P, 2.115(2); N−B,
1.416(3).
its position was refined. The Co−H distance of 1.37(3) Å is
reasonable when it is compared to 1.34(8) Å for HCo-
(PCy3)2(BH4)27 and 1.42 Å for HCo(PPh3)3(BH3CN).28
On the basis of H1(Bpin) formed by addition of HBpin to 1,
we propose a Co(II)-based cycle for alkene hydroboration
(Scheme 2). The CC bond of the substrate is proposed to
Scheme 2. Proposed Catalytic Cycle for “Heteroatom-
Promoted” CC Bond Hydroboration
Figure 3. ESI-MS spectroscopic analysis of the reaction solutions of
D1(Bpin) with 6d and HBpin.
rather than d1-7d (eq 3, Figure S11). The kinetic analysis was
performed for the catalytic hydroboration of 6d, which showed
that the reaction rate is first order with respect to HBpin
(Figure S12). Especially, a kinetic isotope effect (KIE, Figure
S13) of 3.08 was obtained by using DBpin, indicating that the
B−H bond cleavage is the rate-determining step in the catalysis.
Control experiments showed that each of the three
components is necessary for the formation of 7d. Hence, it is
reasonable to argue that the catalytic reaction proceeds via an
intermolecular pathway (path b) by the reaction of XI with
extra amounts of hydroboranes.
interact with the metal center of CoII−H (X). When the
substrate contains a heteroatom (X = N, O), substrate−catalyst
interactions increase the reaction rate significantly.24d Such an
interaction facilitates 1,2-insertion of CoII−H into terminal
olefins, leading to the formation of a cobalt(II)−alkyl species
(XI). There are two possible pathways for the formation of
hydroborated products from XI. The intramolecular pathway
(path a) involves the recovery of the N−Co bond releasing the
product and proceeds back to compound 1. The second
pathway (path b) involves reaction of XI with another HBpin
molecule, affording the organocobalt product H1(Bpin).
More stoichiometric reactions were conducted at room
temperature to further probe the mechanism of “heteroatom-
promoted” hydroboration. In the stoichiometric reaction of
H1(Bpin) with N,N-dimethylallylamine (6d), the hydroborated
product (7d) was not observed (eq 1). These results indicate
that the intramolecular pathway (path a) can be excluded.
CONCLUSIONS
■
Generating CoII−H species through direct activation of a
simple hydroborane by cooperative metal−ligand reactivity is
attractive. The catalysis established in such a manner requires
no additives, because the borane is the activator. The addition
of the B−H bond across the Co−N bond is driven in part by
the stability of the N−B(pin) bond. Although several cobalt(II)
hydrides have been structurally identified,25−28 the reactivity of
such complexes has rarely been reported.29 H1(Bpin) displays
excellent activity for olefin hydroborations. It should be noted
that examples of cobalt-catalyzed isomerization−hydroboration
D
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Yield: 105 mg (80%). ESI-MS: calcd for D1(Bpin)+, 599.2519; found,
599.2505. Anal. Calcd for C34H42DBO2NPCo: C, 68.12; H, 7.40; N,
2.34. Found: C, 68.21; H, 7.28; N, 2.19.
Synthesis of N-Allylimines. In a typical experiment, anhydrous
sodium sulfate (1.0 g) and allylamine (11.0 mmol, 1.1 equiv) were
added to a stirred solution of the aldehyde (10.0 mmol, 1.0 equiv) in
CH2Cl2 (12 mL). After 12 h, the reaction mixture was filtered through
a sintered-glass funnel. The solvent was evaporated, and the residue
was kept under high vacuum for 3 h. The product was obtained as a
colorless oil and analyzed by a 1H NMR spectrum (Supporting
Information).
General Procedure for the Hydroboration of Olefins.
Complex 1 (4.7 mg, 10 μmol) was dissolved in C6D6 (0.6 mL) in a
J. Young NMR tube. Alkene (0.5 mmol) was then added, followed by
HBpin (73 μL, 0.5 mmol). The reaction at 40 °C was monitored by
1H NMR until complete conversion of the reactants. The product was
isolated by chromatography on silica gel with EtOAc/hexane as eluent.
General Procedure for “Heteroatom-Promoted” Hydro-
boration of Alkenes. In a J. Young NMR tube, the substrate (0.5
mmol) and 1 (2.3 mg, 5 μmol) were dissolved in 0.6 mL of C6D6.
Then, HBpin (73 μL, 0.5 mmol) was added. The reaction at room
temperature was monitored by 1H NMR until complete conversion of
the substrate. The product was isolated by chromatography on silica
gel with EtOAc/hexane as eluent.
of internal alkenes have recently been reported by the groups of
Chirik12a,15b and Stradiotto.21 In comparison to the reported
catalytic systems, our CoII−H-based system proceeds at 40 °C
and with a longer reaction time for olefin hydroborations;
however, the catalysis is able to provide the terminal
hydroborated products in good to excellent yields.
The work was extended to chemoselective hydroboration of
N-allylimines. We propose that CC bond hydroborations are
promoted by the heteroatoms in N-allylimines, allylamines, allyl
ethers, and allyl esters. In particular, the addition of an external
small amine or ether, i.e. diethylamine and diethyl ether, slows
down the hydroboration reactions. Stoichiometric reactions
were carried out to establish the catalytic mechanism of
heteroatom-promoted CC hydroboration based on the
CoII−H. Our study demonstrates the possibility of using
cobalt(II) hydride catalysts in hydroboration reactions.
EXPERIMENTAL SECTION
■
General Information. All manipulations were conducted under a
N2 atmosphere on Schlenk techniques or in a glovebox, unless
otherwise stated. All reagents were purchased from Sigma-Aldrich and
used without further purification; d1-pinacolborane (DBpin)30 was
synthesized according to literature procedures. Et2O, pentane, hexane,
toluene, benzene and THF were purified using a Glass Contour
solvent purification system consisting of neutral alumina, copper
catalyst, and activated molecular sieves and then passed through an in-
line, 2 μm filter immediately before being dispensed. C6D6, toluene-d8,
and THF-d8 were dried over CaH2 and purified by vacuum transfer.
Stoichiometric Reaction of N,N-Dimethylallylamine (6d),
H1(Bpin), and HBpin. A scintillation vial containing a magnetic
stir bar was charged with 0.100 g of N,N-dimethylallylamine (6d) and
a stoichiometric amount of H1(Bpin). The reaction mixture was
stirred at room temperature and monitored by GC-MS and ESI-MS
spectroscopy. The hydroborated product 7d was not observed.
Subsequently, 1 equiv of HBpin was added to the mixture, and the
production of 7d was analyzed by GC-MS and ESI-MS spectroscopy
(Figures S7 and S8).
31
32
1,2-Ph2PC6H4NH2 and [Cp*CoCl]2 were prepared according to
published methods.
NMR spectra were recorded on a Bruker Avance 500 spectrometer
in J. Young NMR tubes at room temperature. 1H and 13C NMR
chemical shifts are referenced to the proton signal of the deuterated
solvent. Carbons that are directly attached to boron atoms were not
observed due to quadrupolar relaxation. 11B NMR is referenced to the
signal of H3BO3. Infrared spectra were recorded on a PerkinElmer FT-
IR Spectrum Two spectrometer. GC-MS spectra were obtained on a
Shimadzu GCMS-QP2010 SE spectrometer. Single crystals with
appropriate dimensions were chosen under an optical microscope and
quickly coated with high-vacuum grease (Dow Corning Corp.) to
prevent decomposition. Intensity data and cell parameters were
recorded at 100 or 173 K on a Bruker Apex II single-crystal
diffractometer, employing Mo Kα or Cu Kα radiation and a CCD area
detector.
Preparation of Cp*Co(1,2-Ph2PC6H4NH) (1). n-BuLi (2.5 mol/
L) (0.29 mL, 0.72 mmol) was added to a solution of 1,2-
Ph2PC6H4NH2 (200 mg, 0.72 mmol) in THF (20 mL) at −78 °C.
After it was stirred for 6 h, the solution was warmed to room
temperature, and then [Cp*CoCl]2 (165 mg, 0.36 mmol) was added.
The mixture immediately turned from yellow to rose Bengal. The
solvent was removed under vacuum, and the residue was extracted
with hexane (3 × 10 mL). After recrystallization from hexane at −30
°C, compound 1 was obtained as red microcrystals. Yield: 298 mg
(88%). ESI-MS: calcd for [1]+, 470.1447; found, 470.1437. Anal. Calcd
for C28H30CoNP: C, 71.48; H, 6.43; N, 2.98. Found: C, 71.52; H,
6.48; N, 3.07. μeff = 2.02 μB (THF-d8, 23 °C).
Deuteroboration Experiments. In a N2-filled glovebox, a
scintillation vial was charged with 0.100 g of N,N-dimethylallylamine
in C6H6 (10 mL) and a stoichiometric amount of H1(Bpin) (or
D1(Bpin)) and then with 1 equiv of DBpin (or HBpin). The vial was
capped, and the mixture was stirred at the room temperature for 1 h.
The mixture was divided into two portions. One portion was filtered
through a plug of silica gel and then analyzed by 2H NMR
spectroscopy (Figure S9). The remaining portion was analyzed by
ESI-MS spectroscopy (Figures S10 and S11).
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00114.
Detailed synthesis procedures, characterization data such
as NMR and IR spectra, and crystallographic data (PDF)
Accession Codes
CCDC 1561201, 1561234, and 1561243 contain the
supplementary 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.
Preparation of Cp*CoH(1,2-Ph2PC6H4NH(Bpin)) (H1(Bpin)).
In a N2-filled glovebox, pinacolborane (34 mg, 0.26 mmol) was added
to a solution of 1 (100 mg, 0.22 mmol) in THF (10 mL). The mixture
turned yellow-green. The solvent was then removed under vacuum
and the residue was extracted with hexane (3 × 10 mL). H1(Bpin) was
obtained as pale yellow crystals by recrystallization from Et2O at −30
°C. Yield: 109 mg (83%). ESI-MS: calcd for H1(Bpin)+, 598.2456;
found, 598.2441. Anal. Calcd for C34H43BO2NPCo: C, 68.24; H, 7.24;
N, 2.34. Found: C, 68.36; H, 7.32; N, 2.23. FT-IR (νCo−H): 1902 cm−1.
μeff = 2.62 μB (THF-d8, 23 °C).
AUTHOR INFORMATION
Corresponding Authors
*E-mail for Q.T.: qxtong@stu.edu.cn.
*E-mail for W.W.: wwg@sdu.edu.cn.
■
ORCID
Qingxiao Tong: 0000-0002-8125-9684
Chen-Ho Tung: 0000-0001-9999-9755
Compound D1(Bpin) was prepared by the reaction of 1 and DBpin
following the same procedure as in the preparation of H1(Bpin).
E
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Wenguang Wang: 0000-0002-4108-7865
Notes
The authors declare no competing financial interest.
(15) (a) Schaefer, B. A.; Margulieux, G. W.; Small, B. L.; Chirik, P. J.
ACKNOWLEDGMENTS
■
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This work was supported by the “Thousand Plan” Youth
Program, the Natural Science Foundation of China (21402107,
91427303, and 51673113). We thank Prof. Di Sun for
assistance with the X-ray crystallography and Prof. Zhenghu
Xu for discussions.
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DOI: 10.1021/acs.organomet.8b00114
Organometallics XXXX, XXX, XXX−XXX