Zirconium and titanium propylene polymerization precatalysts supported by a fluxional C 2-symmetric bis(anilide)pyridine ligand

Article abstract of DOI:10.1021/om201262h

Titanium and zirconium complexes supported by a bis(anilide)pyridine ligand (NNN = pyridine-2,6-bis(N-mesitylanilide)) have been synthesized and crystallographically characterized. C₂-symmetric bis(dimethylamide) complexes were generated from aminolysis of M(NMe₂)₄ with the neutral, diprotonated NNN ligand or by salt metathesis of the dipotassium salt of NNN with M(NMe₂)₂Cl₂. In contrast to the case for previously reported pyridine bis(phenoxide) complexes, the ligand geometry of these complexes appears to be dictated by chelate ring strain rather than metal-ligand π bonding. The crystal structures of the five-coordinate dihalide complexes (NNN)MCl₂ (M = Ti, Zr) display a C ₁-symmetric geometry with a stabilizing ipso interaction between the metal and the anilido ligand. Coordination of THF to (NNN)ZrCl₂ generates a six-coordinate C₂-symmetric complex. Facile antipode interconversion of the C₂ complexes, possibly via flat C_2v intermediates, has been investigated by variable-temperature ¹H NMR spectroscopy for (NNN)MX₂(THF)_n (M = Ti, Zr; X = NMe ₂, Cl) and (NNN)Zr(CH₂Ph)₂. These complexes were tested as propylene polymerization precatalysts, with most complexes giving low to moderate activities (10²-10⁴ g/(mol h)) for the formation of stereoirregular polypropylene.

Full text of DOI:10.1021/om201262h

Article
pubs.acs.org/Organometallics
Zirconium and Titanium Propylene Polymerization Precatalysts
Supported by a Fluxional C2-Symmetric Bis(anilide)pyridine Ligand
Ian A. Tonks, Daniel Tofan, Edward C. Weintrob, Theodor Agapie, and John E. Bercaw*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena, California 91125,
United States
S
* Supporting Information
ABSTRACT: Titanium and zirconium complexes supported
by a bis(anilide)pyridine ligand (NNN = pyridine-2,6-bis(N-
mesitylanilide)) have been synthesized and crystallographically
characterized. C2-symmetric bis(dimethylamide) complexes
were generated from aminolysis of M(NMe2)4 with the
neutral, diprotonated NNN ligand or by salt metathesis of the
dipotassium salt of NNN with M(NMe2)2Cl2. In contrast to
the case for previously reported pyridine bis(phenoxide)
complexes, the ligand geometry of these complexes appears to
be dictated by chelate ring strain rather than metal−ligand π
bonding. The crystal structures of the five-coordinate dihalide
complexes (NNN)MCl2 (M = Ti, Zr) display a C1-symmetric geometry with a stabilizing ipso interaction between the metal and
the anilido ligand. Coordination of THF to (NNN)ZrCl2 generates a six-coordinate C2-symmetric complex. Facile antipode
interconversion of the C2 complexes, possibly via flat C2v intermediates, has been investigated by variable-temperature 1H NMR
spectroscopy for (NNN)MX2(THF)n (M = Ti, Zr; X = NMe2, Cl) and (NNN)Zr(CH2Ph)2. These complexes were tested as
propylene polymerization precatalysts, with most complexes giving low to moderate activities (102−104 g/(mol h)) for the
formation of stereoirregular polypropylene.
by an ONO (ONO = bis(phenolate)pyridine) ligand has
revealed that the preference for ligand geometry (either Cs or
C2) is controlled by a delicate competition between phenolate
oxygen to metal π bonding (favoring Cs) and six-membered-
ring strain (favoring C2) within the (ONO)M chelate
fragment.35c The barrier for interconversion between geo-
metries was found to be low (<5 kcal/mol) for (ONO)Ir
complexes.39 We have therefore begun investigating related
ligand frameworks, where either electronic factors or ring strain
might completely dominate and thus dictate a more rigid
geometry. Catalysts with better defined and predictable
geometric preferences should allow for enantiomorphic site-
controlled propylene polymerization. Herein we report the
synthesis and structural characterization of group 4 complexes
supported by a bis(anilide)pyridine ligand and preliminary
results of their use as propylene polymerization precatalysts.
INTRODUCTION
■
The chemistry of the early transition metals has, to a large
extent, been advanced by the use of bent-metallocene
frameworks. However, there has been increased interest in
using well-defined mono- and polydentate “non-metallocene”
or “post-metallocene” ligand sets to support a diverse range of
organometallic complexes and transformations, catalysis, and
small-molecule activation studies.1−33
Recently, our group34,35 and others36−38 have been
investigating the use of arene- and heterocycle-linked bis-
(phenolate)donor ligand sets (heterocycle = pyridine, furan,
thiophene) to support titanium, zirconium, and vanadium
polymerization catalysts34 and as ancillary ligands for other
early transition metals35 to explore other organometallic
transformations. These nonmetallocene ligands are connected
through rigid sp2−sp2 aryl−aryl linkages instead of more
flexible sp3−sp3 linkages, imparting increased rigidity of the
backbone, which could result in more thermally robust catalysts
that are less prone to undergo ligand C−H activation.
RESULTS AND DISCUSSION
■
Preparation and Characterization of (NNN)MX2 Com-
plexes. The bis(aniline)pyridine ligand 1-H2 was synthesized
by following a previously reported40 two-step reaction
sequence. Both titanium and zirconium bis(dimethylamide)
complexes of 1 can be generated through salt metathesis of the
potassium salt 1-K2, with MCl2(NMe2)2(THF)n (M = Ti, n =
We have found that transition-metal complexes based on this
class of ligands are capable of adopting C1, Cs, C2v, or C2
symmetry.34,35 While these symmetries are observed in the
solid state, propylene polymerization using C2-symmetric
bis(phenolate) early-transition-metal precatalysts nonetheless
yields stereoirregular, essentially atactic (or mixtures of atactic
and isotactic) polypropylene.34 An investigation into the
electronics of six-coordinate tantalum complexes supported
Received: December 20, 2011
Published: February 16, 2012
© 2012 American Chemical Society
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0; M = Zr, n = 2) (eq 1). X-ray-quality orange-red crystals of
(NNN)Ti(NMe2)2 (2) and (NNN)Zr(NMe2)2 (3) were
aryl rings. DFT calculations performed on 2 and 3 agree with
this assessmentthere was no obvious M−N π bonding in any
of the molecular orbitals, and in both the Ti and Zr cases we
could identify orbitals containing the nitrogen lone pair
delocalized across the aryl rings rather than into the metal
center (Figure 2).
obtained by layering pentane over concentrated THF solutions
of 2 or 3 and cooling to −30 °C overnight. The bis(anilide)-
pyridine ligand in 2 is bound in a highly twisted, C2-symmetric
fashion, with the two anilide arms occupying the axial positions
of a distorted trigonal bipyramid (Figure 1). The dihedral angle
between the two anilide aryl rings connected to the pyridine is
81.8°, which is substantially larger (by 30−40°) than those
observed in C2-symmetric complexes of the related ONO
ligand system (ONO = pyridine-2,6-bis(4,6-di-tert-butylphe-
noxide)).35c Although the anilide nitrogens are essentially
planar, the titanium−anilide distances (2.078(2) and 2.080(2)
Å) are substantially longer than a typical titanium amide double
bond. By comparison, the Ti−N distances for the doubly
bonded dimethylamido ligands in 2 are significantly shorter
(1.881(2) and 1.892(2) Å). As a result, there is likely no Ti−N
double-bond character in the titanium−anilide bond and the
nitrogen planarity is due to the steric crowding of the bulky
diarylamine and resonance of the nitrogen lone pair into the
Figure 2. DFT calculated HOMO of (NNN)Ti(NMe2)2 (2; top-down
view) showing nitrogen lone pair delocalization into the aryl backbone.
Interestingly, the complex (ONO)Ti(NMe2)2, the bis-
(phenoxide) analogue of 2, exhibits a Cs symmetry in its
crystal structure.41 We attribute this difference to the steric bulk
of the N-mesityl groups, which likely forces 2 to adopt the more
sterically accommodating C2 symmetry. The overall structural
features of the Zr analogue 3 are very similar to those described
above for 2.
Figure 1. Drawings of 2 (left) and 3 (right) with thermal ellipsoids at the 50% probability level. Selected bond lengths (Å): complex 2, Ti1−N1 =
2.0781(23), Ti1−N2 = 2.1170(19), Ti1−N3 = 2.0803(24), Ti1−N4 = 1.8807(19), Ti1−N5 = 1.8922(20); complex 3, Zr1−N1 = 2.2096(10), Zr1−
N2 = 2.3140(10), Zr1−N3 = 2.1799(10), Zr1−N4 = 2.0264(10), Zr1−N5 = 2.0281(10). H atoms are omitted for clarity.
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The Zr complex (NNN)Zr(NMe2)2 (3) could alternately be
generated via aminolysis between the protonated ligand 1-H2
and Zr(NMe2)4; however, (NNN)Ti(NMe2)2 could not be
synthesized via this pathway, even under significant heating and
extended reaction times (eq 2). By switching to a less bulky
solid-state geometry; thus, 6 further confirms our hypothesis
that the new bis(anilide)pyridine ligand set imparts greater
geometric control. Binding of THF in 6 is reversible, and
successive washings of 6 with toluene remove the THF to
generate 5. The analogous Ti complex, (NNN)TiCl2(THF), is
not generated upon reaction of 2 with excess Me3SiCl in THF,
likely a result of a smaller titanium center that does not readily
accommodate a sixth ligand in its coordination sphere.
Reaction of 1-H2 with ZrBn4 or TiBn4 led only to
decomposition products. However, protonolysis of ZrBn4
with the less bulky (tBuNNN)H2 yielded (tBuNNN)ZrBn2 (7)
after heating at 90 °C for 24 h (eq 4). 7 is light sensitive and
ligand (tBuNNN)H2, where the mesityl groups are replaced by
(3,5-tBu2-C6H3), the aminolysis is successful for both Ti and Zr
and proceeds at much lower (55 °C) temperatures.
Reaction of 2 or 3 with 2.3 equiv of Me3SiCl in toluene
quantitatively generates the dichloride complexes (NNN)TiCl2
(4) and (NNN)ZrCl2 (5) (eq 3). Both 4 and 5 are highly
decomposes over the course of days when exposed to ambient
light. 7 was crystallized from a saturated pentane solution and is
roughly Cs symmetric in the solid state (Figure 5). However, in
1
solution the structure appears fluxional; the H NMR for the
benzylic protons of 7 appear as a singlet at room temperature
(vide infra). The related (ONO)ZrBn2 complex is, in contrast,
C2 symmetric in solution and the solid state, and this
unexpected discrepancy shows that the energy differences
between the possible conformations of the (tBuNNN) frame-
work are likely very small in comparison to the more bulky
mesitylene-substituted (NNN) framework.
insoluble and crystallize out of the reaction mixture. 4 and 5
have been characterized by X-ray crystallography (Figure 3). As
is apparent, the NNN ligand for 4 is bound unsymmetrically. In
addition to the typical tridentate LX2 binding of the NNN
ligand, there is a stabilizing ipso interaction between one of the
anilide arms and the metal center. The Ti1−C17 bond length is
2.584(1) Å, more than 0.4 Å shorter than the distance to the
ipso carbon on the other anilide arm (Ti1−C1 = 3.009 Å). This
ipso interaction is likely necessary to stabilize the highly
electrophilic 14-electron Ti center. The Zr analogue 5 is
structurally very similar to 4 and also contains an unsymmetri-
cally bound NNN ligand with an ipso interaction. Complexes 2
and 3, on the other hand, are significantly more electron-rich as
a consequence of the π-donating dimethylamido ligands and do
not require the ipso interaction to stabilize the metal center.
Since both 4 and 5 appear symmetric by NMR spectroscopy, it
is likely that in solution the ipso interaction is rapidly
interchanging between the two anilide arms.
If 3 is treated with 2.3 equiv of Me3SiCl in THF rather than
toluene, the THF adduct (NNN)ZrCl2(THF) (6) crystallizes
(eq 3). Unlike the case for 4 and 5, the solid-state structure of 6
is C2 symmetric and has no stabilizing ipso interaction with the
NNN ligand aryl rings (Figure 4), likely a result of the
increased electron count around the THF-coordinated Zr. The
C2-symmetric structure of 6 is significant because, in the related
bis(phenoxide)pyridine complexes, the addition of a sixth
ligand to the coordination sphere shifts the solid-state geometry
of the complexes from C2 to Cs. In the bis(anilide)pyridine case,
however, a change in coordination number does not affect the
NMR Studies of the Mechanism of Fluxionality. In
complexes 2−6, the o-methyl groups of the N-mesityl
substituents appear as a broad signal in the 1H NMR at
room temperature. In a C2-symmetric molecule, the o-methyl
groups on each mesityl ring are pairwise equivalent (Scheme 1)
with two pointing toward the X substituents and two pointing
more toward the pyridine. The broadening could be a result of
two possible processes: (1) inversion at the metal center
effecting interconversion between the two C2 antipodes thereby
exchanging the sites of the o-methyls and (2) rotation about the
N−mesityl bond (without inversion) to interconvert the two o-
methyls.
1
Variable-temperature H NMR studies were carried out on
complexes 2−5 and 7 to determine the nature of the o-methyl
fluxionality and barriers for methyl interconversion. Figure 6
1
shows the VT H NMR spectra of 5 in toluene-d8 from 25 to
−90 °C; similar VT NMR spectra are observed for 2−4. At
room temperature, the mesityl o-methyls appear as a singlet at
1.80 ppm. Upon cooling, this singlet broadens, disappears at
−60 °C, and at −90 °C splits into two equal-intensity singlets
at 2.59 and 1.05 ppm. From the coalescence rate constant (kc)
and coalescence temperature (Tc), the free energy of activation,
ΔG⧧, was determined to be 9.2 kcal/mol at the coalescence
temperature for 5 (213 K). All four complexes (2−5) exhibit
activation free energies between 9 and 11 kcal/mol (Table 1).42
1
In complex 7, which is ligated by tBuNNN, the H NMR
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Figure 3. Drawings of 4 (left) and 5 (right) with thermal ellipsoids at the 50% probability level. Selected bond lengths (Å): complex 4, Ti1−N1 =
1.891(1), Ti1−N2 = 2.129(1), Ti1−N3 = 1.9263(9), Ti1−Cl1 = 2.3717(1), Ti1−Cl2 = 2.3323(1), Ti1−C17 = 2.584(1); complex 5, Zr1−N1 =
2.033(1), Zr1−N2 = 2.262(1), Zr1−N3 = 2.066(1), Zr1−Cl1 = 2.4692(1), Zr1−Cl2 = 2.4638(1), Zr1−C17 = 2.654(1). Solvents of crystallization
and H atoms are omitted for clarity.
Figure 4. Drawings of 6 with thermal ellipsoids at the 50% probability level showing front (left) and top-down views (right). Selected bond lengths
(Å): Zr−N1 = 2.133(1), Zr−N2 = 2.3103(8), Zr−N3 = 2.152(1), Zr−Cl1 = 2.4498(3), Zr−Cl2 = 2.4496(3), Zr−O1 = 2.2432(7). H atoms are
omitted for clarity.
spectrum displays a singlet at room temperature for the
benzylic hydrogens (Figure 7). When a toluene sample of 7 is
cooled to −95 °C, the peak corresponding to the benzylic
protons shifts upfield, broadens, and disappears into the
baseline.
As shown in Scheme 1, both possible mechanisms for o-
methyl exchange of the anilide mesityl groups for 2−5 involve
some degree of rotation about the N−Cipso bond. Thus, a clear
distinction between the two processes is difficult. However,
some evidence may support antipode interconversion (process
1) as the operative fluxional process. For example, the
measured barriers are substantially less than those reported
by Mislow and co-workers42 for trimesitylamine and related
amines (18−22 kcal/mol) for N−Cipso rotation. Additionally,
Carpentier has observed barriers similar to those measured for
the (NNN)MX2 complexes for the interconversion of rac and
meso stereoisomers of some related group 4 bis(naphthoxy)-
pyridine complexes.38b Moreover, for the (NNN)MX2
complexes partial rotation coupled to inversion at metal
might well be more facile, if substantial steric clashes of the
methyls with the X substituents contribute to the barrier for
N−Cipso rotation. Space-filling and DFT models of the N−Cipso
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Figure 5. Drawings of 7 with thermal ellipsoids at the 50% probability level. The C53 benzyl group has been darkened for contrast. Selected bond
lengths (Å): Zr−N1 = 2.079(1), Zr−N2 = 2.385(1), Zr−N3 = 2.031(1), Zr−C46 = 2.261(2), Zr−C53 = 2.322(2). Solvent of crystallization and H
atoms are omitted for clarity.
Scheme 1
bond rotation in 2 reveal that the mesitylene ring cannot rotate
through a full 180° (i.e., through a N−M−N plane of the NNN
ligand framework), because the o-methyl group movement is
blocked by an NMe2 group. Thus, we favor C2 antipode
interconversion (process 1) as the mechanism for o-methyl
interconversion.
In Cs-symmetric 7, the two benzyl groups should be
chemically inequivalent on the basis of the solid-state structure.
However, the benzyl groups are found to be averaged in the 1H
NMR spectra at room temperature. Furthermore, the chemical
shift of the averaged benzylic hydrogen peak changes with
temperature, suggesting that another stereoisomer, possibly the
C2 isomer, may be in a temperature-dependent equilibrium
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Figure 6. Variable-temperature NMR spectra of (NNN)ZrCl2 (5) in toluene-d8. The decoalesced o-methyl peaks appear at 2.60 and 1.05 ppm at
−90 °C.
Table 1. Experimentally Determined Barriers of o-Methyl
Interconversion in Complexes 2−5
most of the precatalysts yielded poor to moderate (102−104 g/
(mol h)) activities for polymerization.
Surprisingly, the most active catalyst tested was the
bis(amide)pyridine titanium complex 2, which yielded activities
on the order of 104 g/(mol h). This result is in contrast to the
bis(phenolate)pyridine-based catalyst systems, for which the Zr
precatalysts were typically 3 orders of magnitude more active
than their Ti analogues.34 As 2 was the most active precatalyst,
the polymers generated from it were examined to determine
tacticity, molecular weight distribution, and activator effects.
Activities for propylene polymerization using 2 are maintained
over the course of at least 3 h and appear to increase slightly
with increasing amounts of MAO. Increasing the catalyst
loading does not lead to increased activity, indicating that there
is not an initial sacrifice of catalyst when these highly sensitive
precatalysts are introduced to the reaction mixture.
Polymers generated by 2 activated with 500, 1000, or 2000
equiv of MAO were examined by 13C NMR. The polymers are
stereoirregular and essentially atactic with slight enrichment of
the mmmm pentad. There are no isobutyl or olefinic peaks
visible by NMR, and changing the concentration of MAO does
not appreciably change the spectrum. The lack of visible end
groups in the 13C NMR spectrum is due to the extremely high
molecular weights of the polymersGPC analysis shows
molecular weights of 106, albeit with very broad PDIs. Such
high PDIs are atypical of single-site catalysis, and as such
precatalyst 2 may generate a multimetallic species or
decompose into a mixture of different active species upon
activation with MAO. We were optimistic that by employing
precatalysts which could fluctuate between C2-symmetric
complex
Tc (K)
ΔG⧧ (kcal/mol)
kT (s−1
)
Tc
c
(NNN)Ti(NMe2)2
(NNN)Zr(NMe2)2
(NNN)TiCl2
243
233
253
213
10.8
10.4
10.9
9.2
900
900
1900
1700
(NNN)ZrCl2
with Cs 7. Much like the case for 2−5, there are a number of
potential mechanisms that would lead to an averaging of the
benzyl groups in 7. For example, there could be inversion at the
metal center of (possibly through a C2v intermediate) that
reverses the sense of folding of the Cs isomers and hence
exchanges the inequivalent benzyl groups. Alternatively, Berry
pseudorotations of five-coordinate 7 could exchange the
positions of the benzyl groups. However, unlike the case for
2−5, the benzylic averaging could not be a result of solely N−
Cipso bond rotation as depicted in Scheme 1 and must involve
some sort of geometric reorganization of the NNN framework.
Since the rates of interconversion of 2−5 and 7 are all similarly
fast at room temperature, the observation of NNN geometry
distortion in 7 could provide further support for C2 antipode
interconversion (process 1) as the fluxional process operative
for the (NNN)MX2 complexes.
Propylene Polymerization. Complexes 2−6 were tested
as propylene polymerization precatalysts. Test polymerizations
were run in 35 mL of propylene at 0 °C with 500−2000 equiv
of MAO as an activator (Table 2). Under these conditions,
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Figure 7. Variable-temperature NMR spectra of (tBuNNN)ZrBn2 (7) in toluene-d8.
Table 2. Propylene Polymerization Results Obtained with Precatalysts 2−6
a
entry
precat.
cat. loading (mmol)
MAO (equiv)
time (h)
mass of polymer (mg)
activity (104 g/(mol h))
Mw (106)
Mn (106)
PDI
1
2
2
2
2
2
2
3
4
5
5
6
0.007
0.007
0.007
0.05
1000
1000
2000
500
0.5
3
34.9
242.2
239.9
474
0.997
1.15
2
3
3
1.14
4
3
0.316
0.928
1.78
1.037
0.821
1.151
0.03322
0.1674
0.2062
31.2
4.9
5
0.05
1000
2000
1000
1000
1000
2000
1000
2.5
2
1160
1780
n.d.
6
0.05
5.6
7
0.007
0.007
0.007
0.007
0.007
1
n.d.
8
1
6
0.0857
0.253
0.413
0.024
9
1
17.7
28.9
1.7
10
11
1
1
a
Conditions: 0 °C, 30−40 mL of propylene, 2.7 mL of toluene distilled from “Cp2TiH”.
enantiomers we could generate stereoblock isotactic poly-
propylene,43 but it appears that the active species may be
significantly changed from the precatalyst.
CONCLUSIONS
■
A series of group 4 complexes based on a bis(anilide)pyridine
ligand set have been synthesized. These complexes are fluxional
in solution, likely as a consequence of inversion at the metal
center, thus interconverting the C2-symmetric antipodes, with
barriers of approximately 10 kcal/mol. Unlike the related
bis(phenolate) complexes these complexes do not exhibit Cs
symmetry upon coordination of a sixth ligand, which indicates
that the bis(anilide)pyridine ligand imparts a greater geometric
preference for the C2-symmetric isomer as compared to the
bis(phenolate)pyridine analogues. These complexes were tested
as propylene polymerization precatalysts and yielded poor to
moderate (102−104 g/(mol h)) polymerization activities.
Disappointingly, the most active precatalyst (NNN)Ti(NMe2)2
yielded only slightly isotactically enriched high-molecular-
weight polypropylene with a very broad polydispersity index.
Despite the limited success in using these complexes for
propylene polymerization, understanding the solution-state
fluxional processes that these complexes undergo will allow
While the 13C NMR of polypropylene generated by 2
appears stereoirregular, the polymers are solids that exhibit
thermoplastic elastomeric properties, and the slight mmmm
enrichment does not appear to result in any measurable
crystallinity. The elastomeric nature of these polymers could be
due to entanglement from their high molecular weights, or it
could be a result of a significant amount of regioerrors that are
evident in the 13C NMR between 30 and 46 ppm (2,1- and 3,1-
enchainments) that might give rise to methylene runs long
enough to give some crystallites in the polymeryl chain. Further
investigation into these interesting macroscopic polymer
properties and their relationship to the polymer microstructure
are ongoing using related ligand sets.
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concentrated THF solution that was layered with pentane and cooled
to −30 °C overnight. Yield: 346 mg of 3 as yellow crystals (65%). 1H
NMR (300 MHz, CD2Cl2; δ, ppm): 1.82 (br s, 12H, o-CH3), 2.09 (s,
12H, N(CH3)2), 2.18 (s, 6H, p-CH3), 6.33 (dd, J = 8.5, 1.0 Hz, 2H),
6.70 (dd, J = 7.9, 1.0 Hz, 4H), 6.77 (s, 4H, m-mesityl), 7.13 (ddd, J =
8.5, 7.0, 1.6 Hz, 2H), 7.64 (d, J = 8 Hz, 2H, 3,5-py), 7.71 (dd, J = 7.9,
1.6 Hz, 2H), 7.96 (t, J = 7.9 Hz, 1H, p-py). 13C NMR (125 MHz,
C6D6; δ, ppm): 19.1 (CH3), 21.3 (CH3), 39.5 (NCH3), 116.6, 118.0,
122.6, 124.2, 129.3, 130.3, 131.9, 132.6, 134.9, 137.4, 149.0, 150.7,
154.8 (aryl). Anal. Calcd for C39H45N5Zr: C, 69.39; H, 6.72; N, 10.37.
Found: C, 69.14; H, 6.73; N, 10.09.
Synthesis of (NNN)TiCl2 (4). Me3SiCl (210 μL, 1.65 mmol) was
added by syringe to a CH2Cl2 solution (20 mL) containing bisamide 2
(200 mg, 0.317 mmol). The reaction was stirred for 3 h, and then
volatiles were removed in vacuo. The red residue was recrystallized
from a concentrated toluene solution cooled to −30 °C, giving 175 mg
(90%) of 4 as red crystals. 1H NMR (300 MHz, CD2Cl2; δ, ppm): 1.49
(br s, 12H, o-CH3), 2.29 (s, 6H, p-CH3), 6.51 (d, J = 8.3 Hz, 2H), 6.71
(s, 4H, m-mesityl), 7.38 (t, J = 7.6 Hz, 2H), 7.51 (t, J = 7.7 Hz, 2H),
8.09 (d, J = 8.1 Hz, 2H), 8.15 (d, J = 7.9 Hz, 2H), 8.26 (t, J = 7.8 Hz,
1H, p-py). 13C NMR (125 MHz, CD2Cl2; δ, ppm): 20.1 (CH3), 26.0
(CH3), 120.1, 123.3, 123.9, 126.5, 129.6, 130.1, 133.4, 136.9, 138.7,
139.8, 142.2, 143.6, 151.3 (aryl). Satisfactory combustion analysis
could not be obtained for this compound due to cocrystallized solvent
molecules.
for the development of new catalysts for stereoselective
transformations with well-understood geometric preferences.
EXPERIMENTAL SECTION
■
General Considerations and Instrumentation. All air- and
moisture-sensitive compounds were manipulated using standard high-
vacuum and Schlenk techniques or manipulated in a glovebox under a
nitrogen atmosphere. Solvents for air- and moisture-sensitive reactions
were dried over sodium benzophenone ketyl and stored over
titanocene where compatible or dried by the method of Grubbs.44
(NNN)H2 (1),40 TiCl2(NMe2)2,45 and ZrCl2(NMe2)2(THF)2 were
46
prepared following literature procedures. Benzene-d6, toluene-d8, and
THF-d8 were purchased from Cambridge Isotopes and dried over
sodium benzophenone ketyl. CD2Cl2 was distilled from CaH2 and run
1
through a plug of activated alumina prior to use. H and 13C spectra
were recorded on Varian Mercury 300 or Varian INOVA 500
spectrometers, and chemical shifts are reported with respect to residual
protio solvent impurity for 1H (s, 7.16 ppm for C6D5H; t, 5.32 ppm for
CDHCl2) and solvent carbons for 13C (t, 128.39 ppm for C6H6; p,
53.84 ppm for CD2Cl2).
Computational Details. Density functional calculations were
carried out using Gaussian 03 Revision D.01.47 Calculations were
performed using the nonlocal exchange correction by Becke48,49 and
nonlocal correlation corrections by Perdew,50 as implemented using
the B3LYP51,52 keyword in Gaussian. The following basis sets were
used: LANL2DZ53−55 for Ti and Zr atoms and 6-31G** basis set for
all other atoms. Pseudopotentials were utilized for Ti and Zr atoms
using the LANL2DZ ECP. All optimized structures were verified using
frequency calculations and did not contain any imaginary frequencies.
Isosurface plots were made using the Gaussian 03 Revision D.01
program.47
Synthesis of (NNN)ZrCl2 (5). Me3SiCl (210 μL, 1.65 mmol) was
added via syringe to a CH2Cl2 solution (20 mL) containing bisamide 3
(223 mg, 0.330 mmol). The reaction mixture was stirred for 10 min,
and then volatiles were removed in vacuo to yield a yellow residue.
Recrystallization from a toluene solution of 5 at −30 °C yielded yellow
1
crystals of 5 (179 mg, 83% yield) over the course of 2 days. H NMR
(300 MHz, CD2Cl2; δ, ppm): 1.59 (s, 12H, o-CH3), 2.30 (s, 6H, p-
CH3), 6.61 (d, J = 8.3 Hz, 2H), 6.78 (s, 4H, m-mesityl), 7.26 (t, J = 7.6
Hz, 2H), 7.46 (t, J = 7.7 Hz, 2H), 7.97 (d, J = 8.1 Hz, 2H), 8.06 (d, J =
8.1 Hz, 2H), 8.19 (t, J = 7.8 Hz, 1H, p-py). 13C NMR (125 MHz,
CD2Cl2; δ, ppm): 19.4 (CH3), 21.1 (CH3), 122.4, 122.7, 123.7, 124.3,
130.3, 131.1, 133.8, 138.4, 139.7, 140.2, 143.0, 152.4 (aryl).
Satisfactory combustion analysis could not be obtained for this
compound due to cocrystallized solvent molecules.
X-ray Crystal Data: General Procedure. Crystals were removed
quickly from a scintillation vial to a microscope slide coated with
Paratone N oil. Samples were selected and mounted on a glass fiber
with Paratone N oil. Data collection was carried out on a Bruker
KAPPA APEX II diffractometer with a 0.710 73 Å MoKα source. The
structures were solved by direct methods. All non-hydrogen atoms
were refined anisotropically. Details regarding refined data and cell
parameters are available in Table S1 in the Supporting Information.
Synthesis of (NNN)Ti(NMe2)2 (2). A 10 mL toluene solution of
bis(aniline)pyridine 1 (602.5 mg, 1.210 mmol) was added to a 20 mL
toluene suspension of KBn (314.6 mg, 2.420 mmol) and stirred for 2
h. After 2 h, a 10 mL toluene solution of TiCl2(NMe2)2 (251 mg,
1.210 mmol) was added. The mixture was stirred for 12 h, and the
resulting dark red solution was filtered through Celite to remove salts.
The toluene was then removed in vacuo, giving 687 mg (90%) of 2 as
a red solid. Further purification and X-ray-quality crystals were
obtained by layering pentane over a saturated THF solution of 2 and
Synthesis of (tBuNNN)ZrBn2 (7). (tBuNNN)H2 (44.2 mg, 0.07
mmol) and ZrBn4 (31.6 mg, 0.07 mmol) were dissolved in 0.7 mL of
C6D6 and sealed in a J. Young NMR tube. The tube was shielded from
light using aluminum foil and heated to 90 °C in an oil bath. Reaction
progress was monitored via NMR, and after 24 h volatiles were
removed in vacuo, yielding a red-orange residue. This residue was
dissolved in a minimal amount of pentane and cooled to −30 °C,
1
yielding yellow crystals of 7 (45%). H NMR (500 MHz, CD2Cl2; δ,
ppm): 1.16 (s, 36H, C(CH3)3), 1.97 (s, 4H, PhCH2), 6.34 (d, J = 7.1
Hz, 4H), 6.61 (t, J = 7.3 Hz, 2H), 6.73 (t, J = 2.3 Hz, 4H, o-
C6H3(tBu)2), 6.79 (t, J = 7.7 Hz, 4H), 7.01 (dd, J = 8.3, 1.1 Hz, 2H),
7.07 (t, J = 1.7 Hz, 2H, p-C6H3(tBu)2), 7.15 (dt, J = 12.7, 2.9 Hz, 2H),
7.36 (m, 2H), 7.48 (dd, J = 7.9, 1.6 Hz, 2H), 7.59 (d, J = 8.0 Hz, 2H,
m-py), 7.91 (t, J = 8.0 Hz, 1H, p-py). 13C NMR (125 MHz, CD2Cl2; δ,
ppm): 31.2 (C(CH3)3), 34.7 (C(CH3)3), 68.4 (PhCH2), 117.8, 119.4,
120.1, 123.0, 123.5, 125.7, 126.7, 127.7, 129.3, 131.0, 131.4, 139.2,
142.7, 148.0, 148.1, 151.6, 154.6 (aryl). Satisfactory combustion
analysis could not be obtained for this compound due to cocrystallized
solvent molecules.
General Polymerization Protocol. A high-pressure glass reactor
was charged with solid MAO (0.207−0.828 mg, 500−2000 equiv), and
toluene (3 mL, distilled from “Cp2TiH2”) was added. The vessel was
sealed and attached to a propylene tank and purged. Upon cooling to 0
°C, propylene (35−39 mL) was condensed in. Zirconium or titanium
precatalysts were added via syringe as a toluene solution (0.7 mL).
The reaction mixture was stirred vigorously at 0 °C for the desired
amount of time. Excess propylene was carefully vented. Then the cold
bath was removed, and a MeOH/HCl solution (10:1, 50 mL) was
added slowly. The resulting mixture was transferred to an Erlenmeyer
flask, additional MeOH/HCl solution was added (50 mL), and the
mixture was stirred at room temperature overnight. The methanol
1
cooling to −30 °C overnight. H NMR (500 MHz, C6D6; δ, ppm):
1.97 (br s, 12H, NCH3), 2.18 (s, 6H, p-CH3), 2.50 (s, 12H, o-CH3),
6.56 (dd, J = 8.5, 1.1 Hz, 2H) 6.70 (ddd, J = 8.0, 6.9, 1.2 Hz, 2H), 6.82
(s, 4H, m-mesityl), 7.08 (ddd, J = 8.5, 6.9, 1.7 Hz, 2H), 7.16, (t, J = 7.9
Hz, 1H, p-py), 7.26 (d, J = 7.9 Hz, 3,5-py, 2H), 7.60 (dd, J = 7.9, 1.6
Hz, 2H). 13C NMR (125 MHz, C6D6; δ, ppm): 19.4 (CH3), 21.3
(CH3), 44.4 (NCH3), 116.5, 116.6, 123.5, 123.6, 129.2, 129.4, 131.1,
132.7, 134.8, 136.4, 151.4, 151.4, 154.6 (aryl). Anal. Calcd for
C39H45N5Ti: C, 74.15; H, 7.18; N, 11.09. Found: C, 73.37; H, 7.31; N,
10.15.
Synthesis of (NNN)Zr(NMe2)2 (3). Route A: Salt Metathesis.
Using a procedure identical to the synthesis of 2 starting with
Zr(NMe2)2Cl2(THF)2 yielded 3 as yellow crystals.
Route B: Aminolysis. In a glovebox, a 100 mL bomb fitted with a
Kontes valve was charged with a stirbar, 393 mg (0.79 mmol) of 1, and
211.6 mg (0.79 mmol) of Zr(NMe2)4 and the vessel was then
evacuated on a high-vacuum line. Twenty milliliters of benzene was
vacuum-transferred onto the solid mixture, and then the vessel was
heated to 90 °C and stirred overnight. The vessel was degassed and
then heated at 90 °C for a further 12 h. Solvent was then removed in
vacuo, and the resulting yellow residue was recrystallized from a
1972
dx.doi.org/10.1021/om201262h | Organometallics 2012, 31, 1965−1974

Organometallics
Article
solution was decanted, and the polymer was rinsed with methanol.
When the methanol was decanted, the polymer was transferred to a
vial, and volatile materials were removed by placing the vial under
vacuum and heating to 80 °C overnight. The resulting materials were
investigated by NMR spectroscopy and GPC. Polymer NMR
spectroscopy data were acquired at 120 °C in tetrachloroethane. No
polymer was formed without titanium or zirconium precatalyst
addition or in just the presence of the NNN ligand.
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ASSOCIATED CONTENT
■
(16) Tshuva, E. Y.; Goldberg, I.; Kol, M. J. Am. Chem. Soc. 2000, 122,
10706−10707.
S
* Supporting Information
Text, figures, tables, and CIF files giving full experimental and
crystallographic details for the synthesis of (tBuNNN)H2,
spectroscopic data and variable-temperature NMR spectra for
2−5, and 7, 13C NMR and GPC traces of polymers generated
from 2, bond lengths, angles, and anisotropic displacement
parameters for the presented solid-state structures, and X-ray
crystallographic data of compounds 2−7. This material is
available free of charge via the Internet at http://pubs.acs.org.
The crystallographic data for compounds 2−7 have also been
deposited with the Cambridge Crystallographic Data Center
and can be obtained by requesting the deposition numbers.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: bercaw@caltech.edu.
(25) Freundlich, J. S.; Schrock, R. R.; Davis, W. M. J. Am. Chem. Soc.
1996, 118, 3643−3655.
Notes
(26) Baumann, R.; Davis, W. M.; Schrock, R. R. J. Am. Chem. Soc.
1997, 119, 3830−3831.
The authors declare no competing financial interest.
(27) Baumann, R.; Stumpf, R.; Davis, W. M.; Liang, L. C.; Schrock,
R. R. J. Am. Chem. Soc. 1999, 121, 7822−7836.
ACKNOWLEDGMENTS
■
(28) Liang, L. C.; Schrock, R. R.; Davis, W. M.; McConville, D. H. J.
Am. Chem. Soc. 1999, 121, 5797−5798.
We thank Lawrence Henling and Dr. Michael Day for
assistance with the X-ray studies. The Bruker KAPPA APEXII
X-ray diffractometer was purchased via an NSF CRIF:MU
award to the California Institute of Technology, CHE-0639094.
DFT calculations were carried out using the Molecular
Graphics and Computation Facility, College of Chemistry,
University of California, Berkeley, CA, with equipment support
from NSF Grant No. CHE-0233882. We acknowledge Dr. Jo
Ann Canich (Exxon) for the GPC data and for experimental
insight. This work has been supported by the USDOE Office of
Basic Energy Sciences (Grant No. DE-FG03-85ER13431) and
by the KAUST Center-In-Development at King Fahd
University of Petroleum and Minerals (Dhahran, Saudi Arabia).
(29) Mehrkhodavandi, P.; Bonitatebus, P. J.; Schrock, R. R. J. Am.
Chem. Soc. 2000, 122, 7841−7842.
(30) Mehrkhodavandi, P.; Schrock, R. R. J. Am. Chem. Soc. 2001, 123,
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