Titanium bis(amidinates) bearing electron donating pendant arms as catalysts for stereospecific polymerization of propylene

Article abstract of DOI:10.1021/om401165g

A series of titanium bis(amidinate) complexes containing pendant arms as one of the amidinate N substituents have been prepared and studied in the polymerization of propylene after their activation with MAO and other cocatalysts. The type of pendant arm g

Full text of DOI:10.1021/om401165g

Communication
pubs.acs.org/Organometallics
Titanium Bis(amidinates) Bearing Electron Donating Pendant Arms
as Catalysts for Stereospecific Polymerization of Propylene
Tatyana Elkin,† Mark Botoshansky,†,§ Robert M. Waymouth,‡ and Moris S. Eisen*,†
†Schulich Faculty of Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel
‡Department of Chemistry, Stanford University, Stanford, California 94305-5080, United States
S
* Supporting Information
ABSTRACT: A series of titanium bis(amidinate) complexes
containing pendant arms as one of the amidinate N substituents
have been prepared and studied in the polymerization of propylene
after their activation with MAO and other cocatalysts. The type of
pendant arm greatly influences the reactivity and stereospecificity of
the resulting polymers. The effect of the cocatalyst nature, its amount,
and the time of the reaction have a dramatic effect on the reactivity of
a titanium bis(amidinate) bis(dimethylamido) precatalyst containing a
furyl group at the pendant arm.
he use of amidinates and benzamidinates as ancillary
Tligands for group 4 complexes has been studied extensively
over the past few years.1,2 These ligands are considered to be
sterically equivalent to cyclopentadienyl ligands; however, the
amidinates donate only four electrons to the metal in comparison
to the six electrons donated by the cyclopentadienyl moiety and
are thereby expected to increase the Lewis acidity of the metal in
the resulting complex.3 In addition, amidinates have exhibited an
attractive alternative to cyclopentadienyl ligands due to their
synthetic accessibility, allowing facile tuning of their steric and
electronic properties.4−6
Much of our current research has focused on the investigation
of group 4 bis(amidinate) complexes as catalytic precursors for
the polymerization of ethylene and propylene. For propylene, it
was found that when titanium bis(amidinate) and bis-
(benzamidinate) complexes were activated with methylalumox-
ane (MAO), the resulting catalytic mixture yielded a blend of
stereospecific and elastomeric polypropylenes.7 Thorough
examination of the mechanism revealed that each substituent
of the amidinate ligand plays an important role in the formation
of the resulting polymer. We have observed that, during the
activation of bis(arylamidinate) group 4 precursors, one of the
amidinate ligands migrates from the group 4 metal center to an
aluminum in MAO. The remaining ligand of the resulting
mono(arylamidinate) complex rearranges and reattaches to the
metal center through the aromatic ring, creating a κ6
coordination in addition to one nitrogen of the amidine moiety.
The catalytic activity of the resulting complex was found to be
very sensitive to the substitution on the aryl ring. Large steric
hindrance of the substituent at the para position of the aromatic
moiety, in comparison to hydrogen, decreases the activity of the
resulting complex and induces an increase of the molecular
weight of the obtained polymers.8
induce higher polymerization activity,9 as opposed to the
reduced activity observed when electron-donating groups were
utilized.10 This electronic tailoring of the ligand has been found
to lead to the formation of high-molecular-weight atactic
polypropylene due to the complete loss of the symmetry of the
active site involved in the monomer insertion.7a,9,11 Attempts to
restrict this dynamic behavior have been unsuccessful, and as a
result, we have consistently obtained mixtures of elastomeric and
stereospecific polymers.
In light of these observations, more elaborate ligand systems
were studied to limit ligand lability and migration. Among the
candidate ligands, amidines with pendant pyridine12,13 and
dimethylamine13 moieties have been used to stabilize various
metals such as vanadium,14 titanium,15 and palladium12 as
mono(amidinate) complexes and zirconium13 as bis(amidinate)
complexes. Hence, here we report the synthesis of titanium
bis(benzamidinates) containing pyridine, dimethylamine, and
furyl pendant moieties. These complexes were found to be active
in the polymerization of propylene, producing highly isotactic
polypropylenes. The stereoregularity of the polymers was found
to be strongly dependent on the nature of the pendant arm
moiety of the amidinate ligands and on the cocatalyst used.
Unexpectedly, the stereochemistry of the polymer indicates that
the stereoregularity of the resulting polymer was induced via a
chain-end control mechanism.
The syntheses of ligands 1, 2, and 4 (eq 1) and of ligand 3 (eq
2) were performed according to previously published method-
ologies.12,13
The titanium bis(benzamidinates) were prepared by proto-
nolysis using 2 equiv of the amidine with Ti(NMe2)4 or
Ti(NMe2)2Cl2 at room temperature (eq 3).
In addition, group 4 bis(benzamidinate) complexes bearing
Received: December 1, 2013
electron-withdrawing groups at the aromatic N substituents
© XXXX American Chemical Society
A
dx.doi.org/10.1021/om401165g | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
Figure 3. Mercury representation of complex 9 (thermal ellipsoids with
25% probability). Hydrogen atoms are omitted for clarity. Representa-
́
tive bond lengths (Å) and angles (deg): Ti(1)−N(5) = 1.901(4),
Ti(1)−N(6) = 1.908(4), Ti(1)−N(2) = 2.090(3), Ti(1)−N(4) =
2.095(3), Ti(1)−N(1) = 2.225(3), Ti(1 −N(3) = 2.228(3), N(4)−
C(2) = 1.324(5), N(3)−C(2) = 1.332(5), N(1)−C(1) = 1.336(5),
N(2)−C(1) = 1.311(5); N(5)−Ti(1)−N(6) = 96.88(16), N(6)−
Ti(1)−N(1) = 161.65(14), N(6)−Ti(1)−N(4) = 103.11(16), N(6)−
Ti(1)−N(2) = 101.98(15), N(5)−Ti(1)−N(3) = 161.44(14), N(5)−
Ti(1)−N(2) = 103.94(16), N(5)−Ti(1)−N(4) = 101.84(14).
of the five possibilities of arranging the four ligands around the
metal center, complexes 5, 7, and 9 possess a close C2 symmetry,
with the amidinate nitrogen atoms bearing the pendant arm
disposed opposite to each other (trans), similarly to the
complexes containing four-electron-donor asymmetric ligands
previously reported in the literature.17 As expected, due to the
trans influence, the bonds between the nitrogens with the
pendant arms and the titanium are shorter than the bonds
between the titanium metal and the second amidinate nitrogens
that are disposed trans to the dimethylamido moieties. The solid-
state structures revealed that in the neutral complexes 5, 7, and 9
the pendant arm is not interacting with the metal center. It is
important to point out that the structures of the complexes in the
solid state correlate with the structures observed by NMR in
solution, at room temperature.
Complexes 5−9 were reacted with MAO at the appropriate
ratio to generate the active catalytic mixture for the polymer-
ization of propylene (see Table 1). The mixtures of catalysts 5−9
with MAO are ESR silent, indicating that under the reaction
conditions there is no Ti(III), and we did not observe any ligand
migration to the MAO. The nature of the pendant arm plays an
important role in the yield and in the stereospecificity of the
resulting polypropylenes. We have observed that the activity of
the complexes increases as the nucleophilicity of the arm moiety
decreases. Hence, the lowest reactivity was observed for complex
5, with the NMe2 motif and the most nucleophilic group, while
the highest reactivity was exhibited by complex 9, bearing the
least nucleophilic furyl moiety.18 We attribute this behavior to
the interaction of the pendant arm with the metal center;
however, we cannot rule out any interaction with the recently
reported intermediate AlMe2+ formed when using MAO.19
The isotacticities of the obtained polymers were found to
follow the trend opposite to the reactivity of the complexes, in
which the polymer with the highest stereoregularity was obtained
by complex 5.
Crystal structures of the complexes 5, 7, and 9 are presented in
Figures 1−3, respectively. For the metal complexes, in all cases,
Figure 1. Mercury representation of complex 5 (thermal ellipsoids with
25% probability). Hydrogen atoms are omitted for clarity. Representa-
́
tive bond lengths (Å) and angles (deg): Ti(1)−N(4) = 1.882(5),
Ti(1)−N(2) = 2.224(4), Ti(1)−N(1) = 2.090(4), N(1)−C(7) =
1.304(5), N(2)−C(7) = 1.344(5); N(4)−Ti(1)−N(1) = 99.58(19),
N(4)−Ti(1)−N(2) = 159.2(2), N(4)−Ti(1)−N(2)* = 95.0(2).
Figure 2. Mercury representation of complex 7 (thermal ellipsoids with
25% probability). Hydrogen atoms are omitted for clarity. Representa-
́
tive bond lengths (Å) and angles (deg): Ti(1)−N(3) = 2.258(5),
Ti(1)−N(6) = 1.915(5), Ti(1)−N(1) = 2.151(4), Ti(1)−N(2) =
2.240(6), Ti(1)−N(5) = 1.893(5), Ti(1)−N(4) = 2.069(4), N(1)−
C(1) = 1.364(7), N(2)−C(1) = 1.299(7), N(4)−C(22) = 1.352(7),
N(3)−C(22) = 1.328(7); N(5)−Ti(1)−N(6) = 99.3(3), N(5)−
Ti(1)−N(4) = 102.2(2), N(5)−Ti(1)−N(2) = 156.6(2), N(6)−
Ti(1)−N(1) = 104.5(2), N(6)−Ti(1)−N(3) = 158.7(2).
Surprisingly, the analysis of the resulting isotactic polymers
revealed that the stereochemistry of the polymerization was
induced through a chain-end control mechanism (only the
mmmr and mmrm pentads are present, and the mrrm pentad at δ
19.7−20.0 is not observed). The chain-end control has been
achieved at low temperatures for various catalytic systems,
producing isotactic polymers20 and in some instances leading to
the titanium is positioned between six nitrogen atoms: four from
two amidinate moieties and two from the dimethylamido groups.
The Ti−N bonds are within the range of those for previously
reported titanium bis(amidinate) complexes.16 Interestingly, out
B
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Communication
a
Table 1. Data for the Polymerization of Propylene with Complexes 5−9 Activated by Various Cocatalysts
b
c
d
d
d
d
cat.
cat.:MAO:TTPB
time (h)
activity
isotactic fraction (%)
mmmm (%)
Mw
PDI
mp (°C)
5
5
6
7
8
9
9
9
9
9
9
1:1000:0
1:1000:0
1:1000:0
1:1000:0
1:1000:0
1:1000:0
1:50:0
3
12
3
2.6
100
87
96
94
77
76
77
269500
2280000
24000
2.3
1.1
2.6
3.0
2.5
152.3
151.9
148.2
147.3
147.5
g
6.1
13.7
4.3
56
3
57
80000
3
20.9
30.0
traces
5.4
59
91000
e
e
e
e
3
0
10
34700
3.0
n.d.
2
n.d.
n.d.
n.d.
n.d.
e
e
e
e
1:50:1
2
0
12
629000
1.5
2.2
2.3
f
g
1:100:0
1:100:1
1:400:0
2
5.0
100
traces
83
f
100500
156.2
g
e
e
e
2
230.0
48.3
9
18000
3
f
f
158.6
a
Reaction conditions: 10 mg of complex, 6 mL of toluene, 30 mL of propylene, T = 298 K, P = 10 bar. Abbreviations: n.d., not determined; mp,
b
c
melting point. In units of kg of polymer (mol of catalyst)−1 h−1 for all fractions. Percentage by weight of the isotactic fraction from the total
amount of obtained polymer. The complementary fraction is the atactic polymer. The values presented are for the heptane-insoluble stereoregular
fraction of the polymer blend. Data for the elastomer formed. The percent mmmm of the resulting polymer could not be measured due to the
extremely low solubility of the polymers. The melting point was not measured due to the low stereoregularity of the polymer.
d
e
f
g
syndiotactic polypropylene.21 To the best of our knowledge, this
is the first case in which an isotactic polypropylene is obtained via
chain-end control at room temperature. The same stereocontrol
has been observed for all the stereoregular fractions generated by
the complexes presented in this paper (see Figure 4 and the
polymer. Only 56% of the resulting polypropylene is stereo-
regular (mmmm 77%). Elongation of the linker between the
amidinate backbone and the pyridine moiety from −CH2−
(complex 6) to −CH2CH2− (complexes 7 and 8) does not
influence the stereoregularity of the resulting polymers
dramatically in terms of distribution of the fractions in the
resulting blend and percent mmmm. On the basis of our previous
observations we assumed that the C6F5 ring that has been
introduced into the second nitrogen atom of the amidinate
instead of C6H5 does not play a major role in the polymerization
stereoregularity.9 Interestingly, the longer linker yields polymers
with higher molecular weights (Table 1). Changing the metal
ligand from dimethylamido (7) to chloride (8) results in a more
active catalyst, similarly to the previously reported trend for the
titanium bis(amidinate) complexes.10 In contrary to behavior
described previously in the literature, both dimethylamido (7)
and dichloride (8) complexes yield polymers with similar
molecular weights and isotacticities, corroborating the sugges-
tion that these complexes lead to the same cationic species with a
minimum counterion effect.
Higher loadings of MAO (MAO:M = 2000 or 3000) did not
lead to higher activities of the catalytic mixtures generated by
complexes 5−9; however, decreasing the loading of MAO to 400
equiv resulted in complete loss of activity for complexes 5−8, and
an unexpected peculiar behavior was observed for complex 9.
The reactivity of the catalytic mixture generated by complex 9
together with 400 mol equiv of MAO increased, yielding highly
isotactic polypropylene (melting point 158.6 °C) as the major
fraction of the resulting polypropylene blend (83%, Table 1).
Increasing the reaction time did not improve the yield of the
reaction. In order to further investigate the nature of the active
species generated by complex 9, the precatalyst was reacted with
100 equiv of MAO, followed by the addition of 1 equiv of
[Ph3C]+[B(C6F5)4]− (TTPB). The addition of TTPB resulted in
a dramatic increase in the activity (Table 1); however, the
stereoregularity of the polymerization was completely lost,
yielding only an atactic elastomeric material. It is evident that, at
the cationic complex, MAO serves as a counterion at low
loadings (9:MAO 1:100), allowing the furyl pendant arm to
interact with the metal center and inducing the formation of
isotactic polypropylene. Increasing the amount of MAO reduces
the ability of furyl to interact with the metal, hence leading
toward an atactic polymer. It is plausible that, when the borate
Figure 4. 13C NMR of polypropylene obtained with a catalytic mixture
of 5 and MAO (1:1000): (a) full spectrum; (b) enlarged methyl region.
Supporting Information). In order to estimate the influence of
the reaction time on the reactivity and stereospecificity, complex
5 was mixed with MAO in a 1:1000 ratio and the polymerization
was allowed to run for 12 h.
Increasing the time of the reaction led to larger amounts of
polypropylene with a much higher molecular weight (Mw
=
2280000, PDI = 1.1 for 12 h, after fractionalization, as compared
to the single fraction obtained with Mw = 269500, PDI = 2.3 for 3
h) together with about ∼12% of an atactic polypropylene (Mw =
12600, PDI = 2.0). The longer reaction time for complexes 6
(activity 11.3 kg of polymer (mol of catalyst)−1 h−1 after 8 h, 41%
isotactic fraction; mmmm 76%) and 8 (activity 18.6 kg of
polymer (mol of catalyst)−1 h−1 after 10 h, 46% isotactic fraction;
mmmm 77%) led to similar activities and fraction distributions
among the elastomeric and isotactic components. This result
indicates that the active complexes are long lived, and it seems
that there are no major additional intermediates as a function of
time influencing the polymerization.
Changing the arm from NMe2 to pyridine increases the
reactivity of complex 6 but reduces the stereoselectivity of the
C
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Organometallics
Communication
(b) Wedler, M.; Knosel, F.; Edelmann, F. T.; Behrens, U. Chem. Ber.-
Recl. 1992, 125, 1313−1318.
counterion is introduced, its interaction with the metal induces
the removal of the furyl from the metal to the MAO, again
allowing the formation of an atactic polymer. Surprisingly, the
addition of the cocatalyst TTPB to the catalytic mixtures
generated by complexes 5−8 with MAO (M:MAO = 1:1000) did
not influence the activity of the complexes or the stereoregularity
of the resulting polypropylenes. Unfortunately, our attempts to
isolate titanium complexes with two amidinates as ancillary
ligands and alkyls as labile groups (−CH2Ph, −Me), in order to
further investigate the catalyst behavior, led to a rapid
decomposition of the resulting complexes. Switching to the
more stable metal precursor Ti(CH2TMS)4 yielded no reactions
at room temperature, and higher reaction temperatures caused
decomposition of the resulting species.
(3) Duchateau, R.; van Wee, C.; Meetsma, A.; van Duijnen, P. Th.;
Teuben, J. H. Organometallics 1996, 15, 2279−2290.
(4) (a) Edelmann, F. T. Adv. Organomet. Chem. 2008, 57, 183−352.
(b) Collins, S. Coord. Chem. Rev. 2011, 255, 118−138. (c) Edelmann, F.
T. Struct. Bonding (Berlin) 2010, 137, 109−163.
(5) (a) Volkis, V.; Shmulinson, M.; Averbuj, C.; Lisovskii, A.;
Edelmann, F. T.; Eisen, M. S. Organometallics 1998, 17, 3155−3157.
(b) Sita, L. R.; Babcock, J. R. Organometallics 1998, 17, 5228−5230.
(c) Zhang, Y.; Sita, L. R. Chem. Commun. 2003, 2358−2359.
(d) Kissounko, D. A.; Zabalov, M. V.; Brusova, G. P.; Lemenovskii, D.
A. Rus. Chem. Rev. 2006, 75, 351−374.
(6) Volkis, V.; Nelkenbaum, E.; Lisovskii, A.; Hasson, G.; Semiat, R.;
Kapon, M.; Botoshansky, M.; Eishen, Y.; Eisen, M. S. J. Am. Chem. Soc.
2003, 125, 2179−2194.
Titanium bis(amidinate) complexes bearing different elec-
tron-donating motifs as a pendant arm were prepared and
demonstrated stereospecific behavior in the polymerization of
propylene. The complexes showed a correlation among the
nucleophilicity of the pendant arm, the activity of the formed
catalyst, and the stereoregularity of the resulting polymers. The
highest activity and lowest percent mmmm was obtained for the
complex containing a furyl pendant arm (9), followed by
pyridine (6−8), reaching the maximum stereospecificity and the
lowest activity for the complex with the NMe2 pendant arm (5).
The mechanism responsible for the stereoregularity of the
resulting polymers was chain-end control, leading to isotactic
polypropylene at room temperature. Decreasing the ratio
between complex 9 and MAO led to an increase of the complex
activity, and the major fraction of the resulting polypropylene
was obtained as a highly isotactic material. Addition of the TTPB
cocatalyst to the active mixture of 9 and MAO results in a
dramatic increase in catalyst reactivity, leading however to
stereoirregular elastomeric polypropylene as a single product.
(7) (a) Volkis, V.; Lisovskii, A.; Tumanskii, B.; Shuster, M.; Eisen, M. S.
Organometallics 2006, 25, 2656−2666. (b) Aharonovich, S.; Volkis, V.;
Eisen, M. S. Macromol. Symp. 2007, 260, 165−171.
(8) (a) Aharonovich, S.; Botoshansky, M.; Balazs, Y. S.; Eisen, M. S.
Organometallics 2012, 31 (8), 3435−3438. (b) Aharonovich, S. Hetero-
aza-allyl Complexes of Li, Ti, Zr, and V: Structure, Reactivity and
Catalytic Propylene Polymerization. Ph.D Thesis, Technion-Israel
Institute of Technology, 2010.
(9) Elkin, T.; Aharonovich, S.; Botoshansky, M.; Eisen, M. S.
Organometallics 2012, 31, 7404−7414.
(10) Elkin, T.; Kulkarni, N.; Botoshansky, M.; Tumanskii, B.; Eisen, M.
S. Organometallics 2013, 32, 6337−6352.
(11) (a) Volkis, V.; Aharonovich, S.; Eisen, M. S. Macromol. Res. 2010,
18, 967−973. (b) Volkis, V.; Lisovskii, A.; Tumanskii, B.; Shuster, M.;
Eisen, M. S. Organometallics 2006, 25, 2656−2666.
(12) Wu, K.-M.; Huang, C.-A.; Peng, K.-F.; Chen, C.-T. Tetrahedron
2005, 61, 9679−9687.
(13) Kincaid, K.; Gerlach, C. P.; Giesbrecht, G. R.; Hagadorn, J. R.;
Whitener, G. D.; Shafir, A.; Arnold, J. Organometallics 1999, 18, 5360−
5366.
(14) Brandsma, M. J. R.; Brussee, E. A. C.; Meetsma, A.; Hessen, B.;
Teuben, J. H. Eur. J. Inorg. Chem. 1998, 1867−1870.
(15) Boyd, C. L.; Guiducci, A. E.; Dubberley, S. R.; Tyrrell, B. R.;
Mountford, P. Dalton Trans. 2002, 4175−4184.
ASSOCIATED CONTENT
■
S
* Supporting Information
(16) (a) Smolensky, E.; Kapon, M.; Woollins, J. D.; Eisen, M. S.
Organometallics 2005, 24 (13), 3255−3265. (b) Ward, B. D.; Risler, H.;
Weitershaus, K.; Bellemin-Laponnaz, S.; Wadepohl, H.; Gade, L. H.
Inorg. Chem. 2006, 45 (19), 7777−7787. (c) Cortright, S. B.; Huffman, J.
C.; Yoder, R. A.; Coalter, J. N.; Johnston, J. N. Organometallics 2004, 23
(10), 2238−2250. (d) Dawson, D. M.; Walker, D. A.; Thornton-Pett,
M.; Bochmann, M. Dalton Trans. 2000, 4, 459−466.
(17) Makio, H.; Fujita, T. Acc. Chem. Res. 2009, 42 (10), 1532−1544.
(18) Berionni, G.; Maji, B.; Knochel, P.; Mayr, H. Chem. Sci. 2012, 3,
878−882.
(19) Ghiotto, F.; Pateraki, C.; Tanskanen, J.; Severn., J. R.; Luehmann,
N.; Kusmin, A.; Stellbrink, J.; Linnolahti, M.; Bochmann, M.
Organometallics 2013, 32, 3354−3362.
(20) (a) Ewen, J. A. J. Am. Chem. Soc. 1984, 106, 6355−6364.
(b) Small, B. L.; Brookhart, M. Macromolecules 1999, 32, 2120−2130.
(c) Azoulay, J. D.; Gao, H.; Kortez, Z. A.; Kehr, G.; Erker, G.; Shimizu,
F.; Galland, G. B.; Bazan, G. C. Macromolecules 2012, 45, 4487−4493.
(d) Coates, G. W. Chem. Rev. 2000, 100, 1223−1252.
(21) Furuyama, R.; Saito, J.; Ishii, S.-I.; Mitani, M.; Matsui, S.; Tohi, Y.;
Makio, H.; Matsukawa, N.; Tanaka, H.; Fujita, T. J. Mol. Catal. A: Chem.
2003, 200, 31−42.
Text, figures, tables, and CIF files giving details of the syntheses
and characterization data for the ligands, complexes, and
polymers and crystallographic data for the ligands and
complexes. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for M.S.E.: chmoris@tx.technion.ac.il.
Notes
The authors declare no competing financial interest.
§Deceased on September 25, 2013.
ACKNOWLEDGMENTS
■
This research was supported by the USA-Israel Binational
Science Foundation under Contract 2010109.
REFERENCES
■
(1) (a) Bai, S.-D.; Guan, F.; Hu, M.; Yuan, S.-F.; Guo, J.-P.; Liu, D.-S.
Dalton Trans. 2011, 40, 7686−7688. (b) Multani, K.; Stanlake, L. J. E.;
Stephan, D. W. Dalton Trans. 2010, 39, 8957−8966. (c) Zhang, W.; Sita,
L. J. Am. Chem. Soc. 2008, 130, 422−443. (d) Sun, J.-F.; Chen, S.-J.;
Duan, Y.; Li, Y.-Zh; Chen, X.-T.; Xue, Z.-L. Organometallics 2009, 28,
3088−3092.
(2) (a) Recknagel, A.; Knosel, F.; Gornitzka, H.; Noltemeyer, M.;
Edelmann, F. T.; Behrens, U. J. Organomet. Chem. 1991, 417, 363−375.
D
dx.doi.org/10.1021/om401165g | Organometallics XXXX, XXX, XXX−XXX