Formation of cationic intermediates upon the activation of bis(imino)pyridine nickel catalysts

Article abstract of DOI:10.1021/om400061p

The activation of bis(imino)pyridine nickel(II) dichloride complexes [LNiCl₂] with methylalumoxane (MAO) and AlMe₃/[CPh ₃][B(C₆F₅)₄] was studied by multinuclear NMR spectroscopy. Formation

Full text of DOI:10.1021/om400061p

Article
pubs.acs.org/Organometallics
Formation of Cationic Intermediates upon the Activation of
Bis(imino)pyridine Nickel Catalysts
Artem A. Antonov,† Denis G. Samsonenko,‡,§ Evgenii P. Talsi,†,§ and Konstantin P. Bryliakov*,†,§
†Boreskov Institute of Catalysis, Pr. Lavrentieva 5, Novosibirsk 630090, Russian Federation
‡Nikolaev Institute of Inorganic Chemistry, Pr. Lavrentieva 3, Novosibirsk 630090, Russian Federation
§Novosibirsk State University, Ul. Pirogova 2, Novosibirsk 630090, Russian Federation
S
* Supporting Information
ABSTRACT: The activation of bis(imino)pyridine nickel(II)
dichloride complexes [LNiCl2] with methylalumoxane (MAO)
and AlMe3/[CPh3][B(C6F5)4] was studied by multinuclear
NMR spectroscopy. Formation of similar outer-sphere ion
pairs of the types [LNiMe]+[MeMAO]− and [LNiMe]+[B-
(C6F5)4]−, capable of promoting norbornene polymerization, was observed.
he area of catalytic ethylene polymerization with bis-
(imino)pyridine iron and cobalt catalysts boomed in the
T
late 1990s and early 2000s, inspired by the mainframe works of
Brookhart1a and of Gibson.1b,c Such polymerizations have been
studied extensively, and the nature of the precursors of
catalytically active sites in bis(imino)pyridine iron2 and cobalt3
systems has been reliably established. In contrast, bis(imino)-
pyridine nickel catalysts have remained relatively little studied.
Norbornene polymerization over bis(imino)pyridine nickel
complexes in the presence of methylalumoxane was studied in
a series of papers and was reported to proceed with low to
moderate activities.4a−c Recently, we have demonstrated that the
introduction of one or more electron acceptors into the aryl rings
drastically enhances the polymerization activity (up to 1.16 × 107
g of PNB/((mol of cat.) h)), affording high-molecular-weight
polynorbornene (Table 1).4d To date, no published data related
Figure 1. Complexes 1a−e used in this study.
followed by NMR spectroscopy. The parent complexes are
paramagnetic (high-spin d8 configuration), exhibiting 1H NMR
spectra in the range of −10 to +100 ppm.4d In contrast, the
interaction of complexes 1a−e with MAO in toluene-d8/1,2-
difluorobenzene or chlorobenzene solutions even at relatively
low at Al/Ni ratios (≥5) converted the paramagnetic starting
catalysts into the new diamagnetic nickel(II) species 2a−e.5
However, to ensure quantitative conversion of 1a−e into 2a−e,
Al/Ni ratios of 30−60 were used in further experiments. 1H and
13C NMR spectra of complexes 2a,b are presented in Figure 2. 1H
spectra of 2a,b both exhibit sharp 1H NMR peaks, suggesting that
bulky oligomeric MAO does not enter into the first coordination
sphere of nickel. Interestingly, while the spectrum of the 2b/
MAO system witnesses the formation of a major and minor
species in a ca. 87/13 ratio, the activation of 1a with MAO results
in the formation of a 54/46 mixture of isomeric nickel complexes.
Table 1. Norbornene Polymerization on Bis(imino)pyridine
Nickel Complexes in Chlorobenzene (from Ref 4d)
Ni
T,
reaction
time, min
PNB
yield, g
activity, 106 g/((mol of
Ni) h))
complex NB/Ni °C
1a
1b
1c
1d
1e
50000
50000
50000
50000
50000
60
60
60
60
60
15
15
15
15
15
2.91
0.17
0.95
1.82
2.35
11.6
0.68
3.81
7.30
9.43
1
Both isomers exhibit the H and 13C NMR peaks of Ni−CH3
groups (Table 2). Their positions and coupling constants (1JCH
=
129−131 Hz) are close to those reported for cationic α-diimine
nickel(II) methyl intermediates.6 The cationic nature of species
of type 2 was confirmed by separate experiments in which
AlMe3/[CPh3][B(C6F5)4] was used as the activator instead of
MAO; the resulting intermediates exhibited NMR patterns of the
cationic parts, very close to those in the systems with MAO, along
to the nature of active sites in bis(imino)pyridine nickel based
1
systems are available. Herewith, we present a H, 13C, and 19F
NMR spectroscopic study of the activation of various bis-
(imino)pyridine nickel catalysts with Lewis acidic cocatalysts
(MAO and AlMe3/[CPh3][B(C6F5)4]).
RESULTS AND DISCUSSION
The activation of the five bis(imino)pyridine nickel catalysts 1a−
e (Figure 1) with MAO and AlMe3/[CPh3][B(C6F5)4] has been
■
Received: January 23, 2013
© XXXX American Chemical Society
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Figure 2. 1H and 13C NMR spectra (toluene-d8/1,2-diflourobenzene, 10 °C (2b) or 2 °C (2a)) of complexes 2a (a, b) and 2b (c, d) obtained by reaction
of 1a or 1b with solid MAO ([Ni] = 1.9 × 10−3 M, Al/Ni = 40). Asterisks denote minor impurities in MAO: oxygenates and vacuum grease.
Table 2. Selected 13C and 1H NMR Chemical Shifts (δ, ppm), Multiplicities, and Coupling Constants (1JCH, Hz) for Complexes
a
2a−e
1H
Ar-H
13C
Ni complex T, °C Al/Ni Py-Hp Py-Hm
NC(Me) Ni−CH3 NC(Me)
Ni-CH3
19F
major/minor isomer
54/46
2a
2
21.5
10
10
10
5
40
30
40
30
40
40
40
7.77
7.23
7.09, 6.47
6.41
b
1.89
1.83
1.67
1.69
1.84
1.79
1.74
1.71
1.82
1.74
1.82
1.75
1.80
−0.39
−0.44
−0.50
−0.47
−0.50
b
17.56
17.40
17.03
17.04
17.67
17.78
17.32
17.46
18.08
b
5.53 (J = 131)
5.76 (J = 131)
5.21 (J = 131)
5.18 (J = 131)
7.68 (J = 129)
7.21
−123.17
−122.79
c
2a′
7.57
7.52
7.80
b
53/47
87/13
90/10
94/6
59/41
e
2b
7.32
7.19
7.36
7.29
b
c
2b′
7.66
7.69
7.66
6.48
7.41
6.4
−0.56
−0.48
−0.54
b
7.17 (J = 129)
b
d
2c
5.11 (J = 130.5)
5.07
2d
2e
7.80
7.84
7.63
−0.46
−0.52
−0.43
17.57
17.50
17.68
5.04 (J = 128.2)
4.95 (J = 129.2)
2.43 (J = 128.6)
−110.01
−109.99
−104.69
−115.75
10
6.33
a
Conditions unless otherwise stated: in toluene-d8/1,2-difluorobenzene, using solid MAO as the activator. Chemical shifts of minor isomers are
b
c
d
given below those for major isomers. Not found. With AlMe3/[CPh3][B(C6F5)4] as the activator (Al/B/Ni = 30/1.2/1.0). In chlorobenzene.
e
Not applicable.
with the NMR signals of outer-sphere tetrakis-
(perfluorophenylborate) anion (Experimental Section).
on the aryl rings. It was found (see Table 2 and the Supporting
Information) that for complex 1c with bulky o-CF3 substituents
the ratio of major/minor isomers was 94/6, while for complex 1d
with a smaller o-Cl substituent this ratio was 59/41. This led us to
a suggestion that isomers of these types could differ in the relative
orientations of the ortho-substituents (syn and anti, Scheme 1).
Assignment of syn and anti conformers of species of the type 2 is
not straightforward. On the one hand, one could expect that the
presence of a bulky ortho substituent in the aryl ring (CF3, iPr)
would favor the dominance of one particular (syn or anti) isomer.
However, while X-ray studies exhibit a pentacoordinate syn
structure for complex 1c in the solid state,4d complex 1b
One can conclude that bis(imino)pyridine nickel dichloride
species react with MAO or with AlMe3/[CPh3][B(C6F5)4]
according to Scheme 1. We note that, while analogous iron
catalysts are converted by those activators into species of the type
[LFe(μ-Me)2AlMe2]+[X]− (where X = MeMAO−, [B-
(C6F5)4]−),2c,d,j no evidence for the formation of heterobinuclear
species was gained in the case of Ni catalysts, apparently due to
the smaller ionic radius of Ni(II)7 and hence to greater steric
crowding at the Ni center. The origin of major and minor isomers
was studied using complexes 1c−e bearing various substituents
B
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Scheme 1. Interaction of Complexes of Type 1 with MAO and AlMe3/[CPh3][B(C6F5)4]: syn and anti Conformers of Complex 2a
crystallizes from CH3CN as a hexacoordinate acetonitrile adduct
featuring the anti conformation (Figure 3). Additional
independent data are needed to assign the conformations of
the major and minor cationic intermediates 2a−c.
accumulation of some new paramagnetic species. EPR spectra of
those samples documented the presence of nickel(0) (Support-
ing Information). At the same time, precipitation of metallic
nickel was observed, and formation of CH4 (δ 0.16) and C2H6 (δ
0.78) was detected.
In summary, precursors of active species of norbornene
polymerization with bis(imino)pyridine nickel complexes,
activated by MAO or AlMe3/[CPh3][B(C6F5)4], have been
detected and identified. They are ion pairs of the types
[LNiMe]+[MeMAO]− and [LNiMe]+[B(C6F5)4]−, representing
a rare example of spectroscopically characterized polymerization-
active cationic nickel alkyl species.6 As distinct from bis(imino)-
pyridine iron catalyst systems, formation of cationic binuclear
adducts with AlMe3 was not detected in their nickel counterparts
at Al/Ni ratios of 5−60. The cationic parts of some of those
intermediates exist as mixtures of syn and anti conformers, the
presence of bulky ortho substituents (o-iPr, o-CF3) favoring the
prevalence of only one conformer. Ion-pair intermediates
[LNiMe]+[MeMAO]− and [LNiMe]+[B(C6F5)4]− demonstrate
the capability to polymerize norbornene in an NMR tube; upon
prolonged storage in the presence of norbornene, their
decomposition was detected, accompanied by reduction of
Ni(II) to Ni(0).
Figure 3. Molecular structure of the complex 1b·CH3CN, with thermal
ellipsoids at the 30% probability level. Hydrogen atoms are omitted for
clarity.
For the cationic intermediate 2e (featuring 2,4,6-F3-
substituted aryl rings), no syn and anti conformers could be
expected; indeed, formation of a single species was observed by
NMR (Table 2 and the Supporting Information). Full NMR data
of complexes 2a−e are given in the Experimental Section and in
the Supporting Information.
EXPERIMENTAL SECTION
■
General Experimental Details. Manipulations with air- and/or
moisture-sensitive compounds were performed under an atmosphere of
argon using glovebox, break-sealed or standard Schlenk techniques.
Toluene and toluene-d8 were dried over molecular sieves (4 Å) and
distilled over sodium metal under argon. 1,2-Difluorobenzene was dried
over molecular sieves (4 Å), degassed, and stored in a glovebox.
Chlorobenzene was dried over P2O5 and distilled under argon. Other
solvents were used as received without further purification. Complexes
1a−e were prepared as previously described.4d MAO (commercial
sample) was purchased from Crompton as a toluene solution (total Al
content 1.8 M, Al as AlMe3 ∼0.5 M). For the NMR experiments, solid
MAO with a total Al content of 40 wt % and ca. 5 mol % of Al as AlMe3
(obtained by removal of the solvent under vacuum at 20 °C from the
commercial MAO) was used.2j
In the absence of monomer, ion-pair species 2a−e are stable
for at least several hours at room temperature. The reactivity of
these intermediates toward norbornene was probed as follows: 4
equiv of norbornene (dissolved in 0.1 mL of chlorobenzene) was
injected at room temperature into an NMR sample containing
species 2a or 2b in chlorobenzene, and the sample was placed in
an NMR probe thermostated at 20 °C. The temperature was
1
increased (in 10 °C increments), and H NMR spectra were
measured at each temperature. Intermediate 2b exhibited rather
low reactivity; the formation of a significant amount of
polynorbornene could be detected only after storing the sample
for several hours at +60 °C (Supporting Information). In
contrast, intermediate 2a polymerized norbornene within several
minutes already at 50 °C, in accordance with a much higher
reactivity of catalyst system 1a/MAO as compared to 1b/MAO
(Table 1). It is reasonable to conclude that intermediates of the
type 2 are the closest precursors of the chain propagating species
in the catalyst systems based on bis(imino)pyridine nickel
complexes.8 Further storing the samples 2a/NB and 2b/NB led
to a drastic broadening of all 1H NMR peaks, apparently due to
1H, 13C, and 19F NMR spectra were measured on a Bruker Avance
400 MHz NMR spectrometer at 400.130 MHz, 100.613, and 376.468
MHz, respectively, using 5 mm o.d. glass NMR tubes, either in toluene-
d8/1,2-difluorobenzene (1/1) or in chlorobenzene.
Activation of Catalyst 1a with MAO: Species 2a. Calculated
quantities of complex 1a (generally 11.4 μmol) and MAO (generally 31
mg, such that Al/Ni = 40) were weighed in the glovebox in an NMR
tube, and the tube was closed with a septum stopper. Then, toluene-d8/
1,2-difluorobenzene (1/1) or chlorobenzene (to achieve an overall
C
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volume of 0.6 mL) was injected outside the glovebox using gastight
syringes in the flow of argon upon appropriate cooling to <0 °C.
Addition of norbornene (4−6 equiv as a 0.45 M solution in
chlorobenzene) to the above sample (if nesessary) was performed at
20 °C, using a gastight syringe.
6H, NCMe); 1.18 (d, 6H, J = 6.6 Hz, CH(CH3)2); 0.95 (d, 6H, J = 6.8
Hz, CH(CH3)2); −0.56 (s, 3H, Ni−CH3); minor conformer, δ 7.69 (t, J
= 7.7 Hz, PyHp); 2.99 (sept, J = 6.7 Hz, CH(CH3)2); 1.71 (s, NCMe);
−0.48 (s, Ni−CH3). 13C NMR (toluene-d8/1,2-difluorobenzene, 10
°C): major conformer, δ 177.47 (NCMe); 149.25 (B(C6F5)4, o-C,
1JFC = 244 Hz); 143.36 (ArC); 141.04 (PyCγ, 1JCH = 170.1 Hz); 140.44
(ArC); 140.26 (B(C6F5)4, p-C, 1JFC = 244 Hz); 137.17 (B(C6F5)4, m-C,
JFC = 244 Hz); 127.15 (ArCH); 121.59 (ArCH, 1JCH = 159.0 Hz); 29.24
1H NMR (toluene-d8/1,2-difluorobenzene, 2 °C): major conformer,
δ 7.77 (dt, 1H, J = 7.8 Hz, PyHp); 7.23 (d, 2H, J = 7.8 Hz, PyHm); 7.09
(d, 2H, J = 7.6 Hz, ArH); 6.47 (t, 2H, J = 7.6 Hz, ArH); 1.89 (s, 3H, N
CMe); −0.39 (s, Ni−CH3); minor conformer, δ 7.77 (dt, 1H, J = 7.8 Hz,
PyHp); 7.23 (d, 2H, J = 7.8 Hz, PyHm); 7.09 (d, 2H, J = 7.6 Hz, ArH);
6.47 (t, 2H, J = 7.6 Hz, ArH); 1.83 (s, 3H, NCMe); −0.44 (s, Ni−
CH3). 13C NMR (toluene-d8/1,2-difluorobenzene, 2 °C): major
1
1
(CH(CH3)2, JCH = 124.9 Hz); 23.79 (CH(CH3)2, JCH = 126.0 Hz);
1
1
22.44 (CH(CH3)2, JCH = 126.0 Hz); 17.32 (NCMe, JCH = 130.6
Hz); 7.17 (Ni−CH3, 1JCH = 129.0 Hz); minor conformer, δ 177.80 (N
CMe); 143.47 (ArC); 127.32 (ArCH); 121.43 (ArCH); 29.05
(CH(CH3)2); 22.62 (CH(CH3)2); 17.46 (NCMe).
1
conformer, δ 179.70 (NCMe); 153.03 (ArCF, JFC = 248.0 Hz);
141.34 (PyCγ, 1JCH = 169.8 Hz); 133.08 (ArC); 130.29 (ArCH); 126.05
X-ray Measurements. X-ray quality crystals of 1b·CH3CN were
grown from CH3CN/EtOAc (1.5/1) upon slow diffusion of Et2O at +4
°C. Single-crystal diffraction data for 1b·CH3CN were collected on a
Bruker X8Apex four-circle diffractometer equipped with an area
detector (Mo Kα, graphite monochromator, φ and ω scans). The
absorption correction was applied on the basis of intensities of
equivalent reflections with the use of the SADABS program.9a The
structure was solved by direct methods and refined anisotropically
(except for the H atoms) by the full-matrix least-squares method using
the SHELX-97 program package.9b The asymmetric unit contains the
complex trans-[Ni(C27H31N3)(CH3CN)Cl2], in which the Ni(II) cation
has a distorted-octahedral coordination environment formed by four
nitrogen atoms of a bis(imino)pyridine ligand and an acetonitrile
molecule in equatorial positions, and two chloride anions in axial
positions. There is also a solvate acetonitrile molecule disordered over
two orientations around the 2-fold axis (0.25 molecules per formula
unit). Hydrogen atoms of organic ligands were placed geometrically and
refined using a rigid model. Hydrogen atoms of the disordered
acetonitrile molecule were not located. The crystal data and details of
the diffraction experiment and the structure refinement are shown in
Table S1 (Supporting Information). Selected bond distances and angles
are listed in Table S2 (Supporting Information). CCDC 910068
contains supplementary crystallographic data for this paper (complex
1b·CH3CN). These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
1
1
(ArCH); 17.56 (NCMe, JCH = 131.0 Hz); 5.53 (Ni−CH3, JCH
=
129.5 Hz); minor conformer, δ 179.50 (NCMe); 153.03 (ArCF, 1JFC
1
= 248.0 Hz); 141.18 (PyCγ, JCH = 170.0 Hz); 133.21 (ArC); 130.22
(ArCH); 125.95 (ArCH); 17.40 (NCMe, JCH = 131.0 Hz); 5.76 (Ni−
1
CH3, JCH = 129.5 Hz). 19F NMR (toluene-d8/1,2-difluorobenzene, 2
°C): major conformer, δ −123.25 (m, 1F, o-F); minor conformer, δ
−122.73 (t, 1F, J = 8.6 Hz, o-F).
Activation of Catalyst 1b with MAO: Species 2b. The synthesis
was performed as described above for catalyst 1a.
1H NMR (toluene-d8/1,2-difluorobenzene, 10 °C): major conformer,
δ 7.80 (t, 1H, J = 8.0 Hz, PyHp); 7.32 (d, 2H, J = 7.9 Hz, PyHm); 3.15
(sept, 2H, J = 6.6 Hz, CH(CH3)2); 1.84 (s, 6H, NCMe); 1.21 (d, 6H, J
= 6.6 Hz, CH(CH3)2); 1.01 (d, 6H, J = 6.5 Hz, CH(CH3)2); −0.50 (s,
3H, Ni−CH3); minor conformer, δ 3.00 (sept, J = 6.5 Hz, CH(CH3)2);
1.79 (s, NCMe). 13C NMR (toluene-d8/1,2-difluorobenzene, 10 °C):
major conformer, δ 177.45 (NCMe); 143.36 (ArC); 141.36 (PyCγ,
1JCH = 169.9 Hz); 140.29 (ArC); 127.35 (ArCH); 127.20 (ArCH);
121.80 (ArCH, 1JCH = 160.0 Hz); 29.31 (CH(CH3)2, 1JCH = 127.7 Hz);
24.02 (CH(CH3)2, 1JCH = 126.5 Hz); 22.55 (CH(CH3)2, 1JCH = 126.1
Hz); 17.67 (NCMe, 1JCH = 130.5 Hz); 7.68 (Ni−CH3, 1JCH = 129.3
Hz); minor conformer, δ 143.45 (ArC); 141.32 (PyCγ); 140.41 (ArC);
127.41 (ArCH); 127.25 (ArCH); 121.46 (ArCH); 29.11 (CH(CH3)2);
23.97 (CH(CH3)2); 22.75 (CH(CH3)2); 17.78 (NCMe); 7.21 (Ni−
CH3, 1JCH = 129.3 Hz).
Activation of Catalyst 1a with AlMe3/[CPh3][B(C6F5)4]: Species
2a′. Calculated quantities of complex 1a (generally 11.4 μmol) and
[CPh3][B(C6F5)4] (1.2 equiv) were weighed in the glovebox in an NMR
tube, and the tube was closed with a septum stopper. Then, 30 equiv of
AlMe3 was injected (0.5 mL of a 0.68 M toluene solution of AlMe3)
using gastight syringes in the flow of argon upon appropriate cooling to
<0 °C. The mixture was stored for ca. 30 min at −10 °C, during which
time an oily precipitate formed in the bottom of the NMR tube. The
upper (organic) phase containing toluene, AlMe3, and Ph3CMe was
carefully removed. Into the tube containing the oily phase was injected
0.5 mL of toluene-d8/1,2-difluorobenzene (1/1) with a gastight syringe.
Addition of norbornene (4−6 equiv as a 0.45 M solution in
chlorobenzene) to the above sample (if nesessary) was performed at
20 °C, using a gastight syringe.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text and figures giving NMR data and representative spectra for
complexes 2c−e, 2a′, 2b′, figures giving NMR spectra of samples
containing intermediates of the type 2 with added norbornene
and EPR spectra measured after degradation of cationic
intermediates in the presence of norbornene, and a CIF file
and tables giving X-ray data for 1b·CH3CN. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*K.P.B.: tel, +7-383-3269578; fax, +7 383 3308056; e-mail,
bryliako@catalysis.ru.
1H NMR (toluene-d8/1,2-difluorobenzene, 21.5 °C): major con-
former, δ 7.57 (t, 1H, J = 8.0 Hz, PyHp); 6.41 (t, 1H, J = 7.9 Hz, ArH);
1.67 (s, 3H, NCMe); −0.50 (s, Ni−CH3); minor conformer, δ 7.52 (t,
■
1H, J = 8.0 Hz, PyHp); 1.69 (s, 3H, NCMe); −0.47 (s, Ni−CH3). 13
C
NMR (toluene-d8/1,2-difluorobenzene, 21.5 °C): major conformer, δ
179.50 (NCMe); 154.92 (ArCF, 1JFC = 248.0 Hz); 149.29 (B(C6F5)4,
o-C, 1JFC = 244 Hz); 140.91 (PyCγ, 1JCH = 169.5 Hz); 140.32 (B(C6F5)4,
Notes
The authors declare no competing financial interest.
1
1
p-C, JFC = 244 Hz); 137.19 (B(C6F5)4, m-C, JFC = 243 Hz); 133.13
(ArC); 130.21 (ArCH); 126.4 (ArCH); 124.2 (ArCH); 116.96 (ArCH);
17.03 (NCMe, 1JCH = 131.2 Hz); 5.21 (Ni−CH3, 1JCH = 131.2 Hz);
ACKNOWLEDGMENTS
This work was supported by the Russian Foundation for Basic
Research (Grant No. 12-03-91159).
■
1
minor conformer, δ 179.61 (NCMe); 154.94 (ArCF, JFC = 248.0
Hz); 133.27 (ArC); 130.14 (ArCH); 116.77 (ArCH); 17.04 (NCMe,
1JCH = 131.2 Hz); 5.18 (Ni−CH3, 1JCH = 131.2 Hz).
REFERENCES
■
Activation of Catalyst 1b with AlMe3/[CPh3][B(C6F5)4]: Species
2b′. The synthesis was performed as described above for catalyst 1a.
1H NMR (toluene-d8/1,2-difluorobenzene, 10 °C): major conformer,
δ 7.66 (t, 1H, J = 7.7 Hz, PyHp); 7.19 (d, 2H, J = 7.7 Hz, PyHm); 6.48 (d,
2H, J = 6.8 Hz, ArH); 3.14 (sept, 2H, J = 6.7 Hz, CH(CH3)2); 1.74 (s,
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dx.doi.org/10.1021/om400061p | Organometallics XXXX, XXX, XXX−XXX