Generation of TiII alkyne trimerization catalysts in the absence of strong metal reductants

Article abstract of DOI:10.1021/acs.organomet.7b00096

Low-valent Ti^II species have typically been synthesized by the reaction of Ti^IV halides with strong metal reductants. Herein we report that Ti^II species can be generated simply by reacting Ti^IV imido complexes with 2 equiv of alkyne, yielding a metallacycle that can reductively eliminate pyrrole while liberating Ti^II. In order to probe the generality of this process, Ti^II-catalyzed alkyne trimerization reactions were carried out with a diverse range of Ti^IV precatalysts.

Full text of DOI:10.1021/acs.organomet.7b00096

Article
pubs.acs.org/Organometallics
Generation of TiII Alkyne Trimerization Catalysts in the Absence of
Strong Metal Reductants
Xin Yi See, Evan P. Beaumier, Zachary W. Davis-Gilbert, Peter L. Dunn, Jacob A. Larsen, Adam J. Pearce,
T. Alex Wheeler, and Ian A. Tonks*
Department of Chemistry, University of Minnesota−Twin Cities, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United
States
S
* Supporting Information
ABSTRACT: Low-valent TiII species have typically been
synthesized by the reaction of TiIV halides with strong metal
reductants. Herein we report that TiII species can be generated
simply by reacting TiIV imido complexes with 2 equiv of
alkyne, yielding a metallacycle that can reductively eliminate
pyrrole while liberating TiII. In order to probe the generality of
this process, TiII-catalyzed alkyne trimerization reactions were
carried out with a diverse range of TiIV precatalysts.
Scheme 1. Previously Reported TiII/TiIV Redox Catalytic
Reactions
INTRODUCTION
■
Low-valent Ti reagents have played an important role in many
molecular transformations over the past 50 years.1 In particular,
TiII intermediates have been invoked in a rich and varied range
of stoichiometric and catalytic reactions: N2 fixation,2 McMurry
coupling,1b,3 alkyne cyclotrimerization,1c,4 Pauson−Khand
cycloaddition,5 cyclopropanation,6 and oxidative addition
reactions with many other electrophiles.7 Formally, TiII
coordination complexes are fairly uncommon because of the
extremely high thermodynamic stability of the TiIV oxidation
state. Nevertheless, discrete TiII complexes have been isolated
and characterized across diverse ligand sets: cyclopentadie-
nyl,2c,8 calixarene,4,9 isocyanide,10 porphyrin,7a,11 pyridine,7b
and 1,2-bis(dimethylphosphino)ethane.12 Similarly, “masked”
TiII complexes have also been reported in which the low-valent
similar catalyst classes, we have drawn several qualitative
conclusions and empirical trends.
state is stabilized by strong π-acceptors such as alkyne,13
alkene,14a−c and carbonyl.14d
Given that they are highly reducing, TiII reagents have
RESULTS AND DISCUSSION
We began our investigation by synthesizing a diverse set of Ti
■
typically been formed from the reaction of TiIV halide
precursors with powerful reductants: KC8,2,7b LiAlH4,11 Na/
imido complexes based mostly on ligand architectures that are
Hg,15 and Mg.16 Recently, we have reported several catalytic
well-established in titanium-catalyzed hydroamination and
oxidative C−N bond forming reactions that proceed through a
formal TiII/TiIV redox couple (Scheme 1). These reactions
presumably generate a TiII intermediate in the absence of a
strong metal reductant by coupling a TiIV imido unit with two
alkynes (or an alkyne and an alkene).17
polymerization reactions (Figure 1). These ligands can be
divided into several categories: first, complexes varying in
simple monoanionic X-type ligands: halides (1−3),18 pyrrolides
(4 and 5),19 and aryloxides (6 and 7);20 second, polyhapto
ligands: 1,4-bis(trimethylsilyl)cyclooctatetraene (8),21 cyclo-
pentadienyl Cp (9 and 10),22 and pentamethylcyclopentadienyl
Cp* (11);22 third, LX-type bidentate ligands: amidate (12),23
phenoxyimino (13),24 and amidinate (14 and 15)25 that may
be hemilabile; and last, a Zr analogue (16) included in the
series to allow for reactivity comparison within the group 4
Intrigued by this unusual and mild route to form TiII species
in situ, we have set out to demonstrate the generality of forming
TiII species from the reaction of alkynes with TiIV imido
complexes. These transient TiII complexes were then examined
for their competency as catalysts for alkyne trimerization.
Herein we report that many diverse Ti imido precatalyst
structures are capable of generating TiII intermediates and
subsequently trimerizing alkynes. By comparing structurally
Received: February 7, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00096
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Article
diverse range of molecular structures capable of catalyzing this
reaction.
Catalytic alkyne trimerization reactions were carried out with
5 mol % of each Ti imido precatalyst and either 1-hexyne or 3-
hexyne in C6D5Br at 115 °C for 16 h. Unsymmetrical 1-hexyne
can yield two alkyne trimer regioisomers (1,3,5- and 1,2,4-tri-n-
butylbenzene A and B, respectively) and three pyrrole
regioisomers (2,4-, 2,5-, and 3,4-di-n-butylpyrrole, C−E,
respectively) (Table 1). The expected statistical distribution
between the two alkyne trimer products is 1:3 (A/B),30c while
the distribution of pyrrole products is 2:1:1 (C/D/E). The 2,4-
and 2,5-disubstituted pyrroles were the major products in the
1
reactions reported herein, and no significant additional H
NMR peaks that could plausibly be assigned as the 3,4-
regioisomers were observed in any of the catalytic experiments.
Where possible, the pyrrole regioisomers were independently
synthesized via alternate routes to confirm their character-
ization, although some are inaccessible using modern synthetic
techniques (See Table 1 and Supporting Information for
details). 3-Hexyne can only yield hexaethylbenzene F and
2,3,4,5-tetraethylpyrrole G (Table 2).
Given that the pyrrole byproduct formation is stoichiometric
with respect to TiII formation, the amount of TiIV activated
toward catalysis can be determined by quantifying the amount
of pyrrole formed in a given reaction. Additionally, control
experiments with structurally analogous TiIV halide precatalysts
yield no trimerization (see Table S2), indicating that a Lewis
acid mechanism for trimerization can be ruled out and that all
productive catalysis most likely occurs through TiII. As a result,
one can gain further insight into catalyst activity by calculating a
“real” TON for each catalyst: the amount of trimer generated
per activated Ti center. While this number may not truly reflect
actual turnover given that catalyst dis-/comproportionation or
ligand redistribution may occur, it is nonetheless instructive in
qualitatively comparing catalyst systems.
Figure 1. Ti and Zr imido precatalysts investigated for the generality
of forming MII intermediate.
triad. In contrast to titanium, ZrII is considerably harder to
access.26
Based on our previous mechanism for TiII/TiIV redox
catalysis, TiII species capable of alkyne trimerization can be
generated by the coupling of 2 alkynes with a TiIV imido and
subsequent elimination of one equivalent of pyrrole per Ti
(Scheme 2, top).17 This activation process occurs through a [2
Scheme 2. Catalytic Alkyne Trimerization with TiIV Imido
Catalysts
All precatalysts examined were active for trimerization
catalysis with 1-hexyne, albeit with starkly different degrees of
activation and rates of catalysis (Table 1), demonstrating the
generality of accessing a TiII intermediate from a TiIV imido
unit. Most of the catalysts examined did not deviate
significantly from the statistical distribution of alkyne trimers,
although the ratio of pyrrole byproduct regioisomers varied
widely. In general, precatalysts with poor to moderate
trimerization yields were observed to have poor mass balances
that may be a result of alkyne oligomerization,31 catalyst
decomposition, or off-cycle/arrested alkyne-bound Ti com-
plexes (metallacyclopropene, metallacyclopentadiene, and η6-
arene)32 that were either incapable of or slower at catalytic
turnover.
Results with catalysts bearing simple monoanionic ligands
(1−3) indicate that electron-poor metal centers with weaker
donor ligands (as measured by Odom via ligand donor
parameterization) are better for 1-hexyne trimerization: I (3)
> Br (2) > Cl (1).33 The apparent TON also increases as the
metal center becomes more electron-poor. Most strikingly,
even though only 5% of 3 was activated, it yielded 94%
trimerization at room temperature in less than 5 min. In
contrast, 2 slowly trimerized at room temperature, while 1 (and
other catalysts) required high temperatures for productive
catalysis to occur. The regioselectivity of both 1-hexyne
trimerization and pyrrole formation trends toward sterically
favored products A and C as the electrophilicity of the catalyst
increases.
+ 2] cycloaddition27 of the TiIV imido with an alkyne, followed
by insertion of the second alkyne into the metallacycle28 and
finally reductive elimination of pyrrole to yield the TiII
trimerization catalyst.29 While there may be catalyst-to-catalyst
variation in the mechanistic details of trimerization catalysis,
TiII is generally understood30 to trimerize alkynes by first
oxidatively coupling two alkynes to yield a metallacyclopenta-
diene that can then undergo [4 + 2] cycloaddition with a third
alkyne to yield a titananorbornadiene intermediate, which is
then displaced by alkyne to liberate the trimerized product
(Scheme 2, bottom). Alternately, the metallacyclopentadiene
could insert the third equivalent of alkyne to produce a
metallacycloheptatriene, which upon reductive elimination
yields the trimerized product. In general, the [4 + 2]
mechanism appears to be more broadly invoked,4a although a
universal mechanism for trimerization is unlikely given the
B
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Article
a
Table 1. 1-Hexyne Trimerization Data
b
c
[Ti]
% trimer yield
quant.
A/B
% [Ti] act.
C/D
% conversion
TON
18
1
2
27:73
35:65
39:61
24:76
39:61
31:69
37:63
33:67
34:66
27:73
37:62
15:85
17:83
39:61
21:79
42:58
37
6
4
2
1
3
86:14
78:23
100:0
75:25
60:40
86:14
65:35
100
89
0
3
2
4
0
0
2
87
94
61
85
1
4
5
2
94 31
d
3
5
98
132 38
4
32
93
12
2
5
20 10
100
100
28 14
6
quant.
30
31
36
11
61
33
4
8
9
1
3
4
21
9
4
4
4
4
0
1
2
3
4
1
7
41
62
53
71
50
34
40
18
65
24
8
64 16
e
8
4
2
2
4
9
5
3
5
2
100:0
100
0
3
0
4
7
11
32
7
e
9
100:0
91
100
75
e
10
11
12
13
14
15
16
100:0
e
100:0
10
4
57 19
37:63
69:31
31:69
65:35
64
15
7
2
1
1
0
55 14
56 13
76 12
17
17
11
38
0
53
5
a
b
1
Conditions: 5 mol % [Ti], 0.4 M 1-hexyne, C6D5Br, 16 h, 115 °C, average of 2−4 runs. Quantitation determined by in situ H NMR. % Ti
c
d
e
activated = (yield of C + D)/[Ti]tot. TON = (yield of A + B)/Ti activated. <5 min, room temperature | D (R = NtBu) could not be independently
synthesized/characterized, although raw spectra indicate only formation of C.
a
for 2,4-disubstituted pyrrole activation product A. In fact, the
overall activation and catalytic profile of 6 are very similar to
those of 1. This similarity may indicate that some ligand
redistribution/disproportionation occurs in these monoaryl-
oxide complexes.
Table 2. 3-Hexyne Trimerization Data
Varying the substituents on cyclopentadienyl-supported Ti
catalysts 9−11 significantly affected both the amount of catalyst
activated and the degree of alkyne trimerization. Cp2-
substituted 9 exhibits the lowest amount of catalyst activation
but has the highest apparent TON of all three Cp-substituted
catalysts, indicating that the bulkier and more electron-rich
Cp2TiII active species may be more long-lived and/or more
reactive than the monoligated analogues. More sterically
encumbered Cp* derivative 11 has a lower degree of activation
than Cp counterpart 10, although the apparent TONs for both
are similar, indicating that in these systems the initial activation
of the CpTi(≡NtBu) fragment by incoming alkynes is more
sensitive to sterics than alkyne trimerization by a putative
CpTiII species.
Catalysts 12−15, supported by bidentate potentially hemi-
labile ligands, did not react to full conversion under standard
catalytic conditions. Similar to Cp derivatives 9−11, bis-ligated
bidentate ligands undergo lower catalyst activation but have
higher apparent TONs, indicating that although mono ligation
may aid in catalyst activation due to the steric sensitivity of
[2+2+1] pyrrole formation it may also lead to less stable and/
or less active trimerization catalysts. Unfortunately, there is no
correlation between selectivity in pyrrole activation and 1-
hexyne trimerization in any of these systems.
b
c
[Ti]
% trimer yield
% [Ti] act.
% conversion
TON
1
2
62 13
55 21
80 12
57 12
73 17
77 12
4
5
2
2
d
3
83
24
6
6
8
5
2
3
7
0
0
0
0
0
2
0
0
17
33 18
42
48 11
70
74 14
7
81
54 16
15
3
35 14
4
5
1
5
0
6
1
0
1
0
3
1
1
0
1
0
0
0
0
5
6
0
6
38
3
72 18
49 11
7
9
8
68
1
87
12
7
2
0
2
8
9
8
1
2
0
7
4
0
4
1
0
10
11
12
13
14
15
16
0
3
41
7
49
0
42 13
42 11
20 15
0
0
2
6
2
0
2
0
1
25
0
22
35
6
4
1
a
Conditions: 5 mol % [Ti], 0.4 M 3-hexyne, C6D5Br, 16 h, 115 °C,
b
average of 2−4 runs. Quantitation determined by in situ 1H NMR. %
c
Ti activated = (yield of G)/[Ti]tot. TON = (yield of F)/Ti activated.
d
Room temperature.
Catalysts 6 and 7 allow for a direct comparison of steric
effects on activation and catalysis. While the amount of
precatalyst activated is approximately the same for both,
catalysis with more sterically encumbered 7 is incomplete under
standard conditions, yielding significantly less trimerization
product than that with 6. Intriguingly, although there are
marginal differences in the regioselectivity of trimerization
between both, catalyst 6 is significantly more selective than 7
Remarkably, catalysts 12 and 14 show preference for the
selective formation of 2,5-disubstituted pyrrole activation
product D, which results from a Markovnikov [2 + 2] addition
of 1-hexyne to the Ti≡NTol fragment followed by 2,1-insertion
of 1-hexyne into the resulting azametallacycle. This regiose-
lectivity is surprising given that hydroamination of terminal
aliphatic alkynes with aniline23b,34 by catalyst 12 favors the
C
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opposite [2 + 2] products, in a ratio of approximately 1.6:1
anti-Markovnikov to Markovnikov. While these results are
apparently contradictory, they are likely a result of different
rate-determining steps of catalysis (for example, in Schafer’s
hydroamination report, [2 + 2] addition is reversible and
protonolysis by aniline is rate determining) or a function of a
change in catalyst speciation. In hydroamination catalysis, there
is a large excess of Lewis basic amine present that may
coordinate to Ti throughout the catalytic cycle;34 however, in
these trimerization experiments, no such strong Lewis base
exists.
such as 12 and 14 demonstrated selectivity toward 2,5-
disubstituted pyrroles.
More generally, these trimerization reactions illustrate an
important design principle for early transition metal catalysis
involving redox at the metal: stabilization of low-valent states.
While there have been significant recent advances in the use of
redox noninnocent ancillary ligands36 to modulate similar
transformations, one may also consider that redox noninnocent
reactants or products can play a similar role; in this case, π-
backdonation from TiII into arenes and alkynes is certainly
critical to accessing low-valent states and for catalysis. Similarly,
π-backdonation into CO in Pauson−Khand reactions,5 alkenes
in the Kulinkovich reaction,6 and azobenzene in [2+2+1]
pyrrole syntheses17 is integral for productive reactivity and
catalyst stability. This research will provide new potential access
points to carry out various other stoichiometric and catalytic
reactions with low-valent Ti under mild conditions and a future
platform for new catalyst development toward selective pyrrole
syntheses.
Given successful catalysis with a diverse range of Ti catalysts,
we synthesized and tested a Zr imido analogue, 16.
Interestingly, 16 trimerized 1-hexyne in poor yield in the
absence of any detectable pyrrole activation byproduct. This
result leads to one of two possible conclusions: (1)
Trimerization by Zr occurred through a Lewis acid mechanism
such as that reported by Floriani et al.35 for ZrCl4. (2) Small
amounts of Zr≡NPh were activated in a quantity undetectable
1
by H NMR and GC/MS, and catalysis occurred in a manner
similar to Ti. While ZrCl4 is known to trimerize alkynes
through the Lewis acid pathway, control experiments with
ZrCl4 under our specific reaction conditions yielded no alkyne
trimerization. (Arene coordination to ZrCl4, both from C6D5Br
and 1,3,5-trimethoxybenzene, inhibits Lewis acid catalysis. See
Table S2.) Thus, neither pathway can be fully ruled out.
With an internal alkyne such as 3-hexyne, alkyne
trimerization becomes more challenging (Table 2). In all
cases examined, the apparent TON and yield of hexaethylben-
zene were lower than those in the 1-hexyne reactions.
Interestingly, the amount of pyrrole formed via [2+2+1] of 3-
hexyne was typically higher than the reactions with 1-hexyne.
This is likely the result of the relative rates of activation versus
trimerization: In most 1-hexyne reactions, trimerization rapidly
depletes the amount of alkyne available for further catalyst
activation, whereas with 3-hexyne, catalyst activation can
effectively compete with slower alkyne trimerization.
Trends in catalyst activity similar to those observed in 1-
hexyne reactions can also be observed in the 3-hexyne
reactions. For example, the most electron-deficient simple
halide 3 substantially outperforms other monoanionic ana-
logues 1, 2, 4, and 5, despite a lower degree of catalyst
activation. Increasing the steric profile of monoligated
complexes also suppresses productive catalysis, as 6 gives
moderate yields of hexaethylbenzene while the bulkier 7 only
yields trace product. Disappointingly, bidentate ligands 9−15
yielded no productive catalysis despite some pyrrole production
and instead led to poor mass balances.
EXPERIMENTAL SECTION
■
General Considerations. All air- and moisture-sensitive reactions
were carried out in a nitrogen-filled glovebox. Standard solvents for air-
and moisture-sensitive reactions were either deoxygenated by sparging
with N2 and dried by passing through activated alumina columns of a
Pure Process Technology solvent purification system (benzene, ether,
pentanes, hexanes, THF, or CH2Cl2) or vacuum-transferred from Na/
Ph2CO (C6D6) or CaH2 (CDCl3). C6D5Br was synthesized following a
literature procedure,37 degassed, dried over CaH2, and filtered through
basic alumina prior to use. Commercial PhCF3 was vacuum transferred
from CaH2 and filtered through basic alumina prior to use.
Ti(NtBu)Cl2py3,18 precatalysts 1,18 8,21 and 9−1122 were
synthesized according to a literature procedure. Dimeric [Ti(NPh)-
Cl2py2]2 was prepared by extended heating of Ti(NPh)Cl2py318 under
vacuum. Liquid alkynes and other reagents were freeze−pump−thaw
degassed three times and passed through activated basic alumina prior
to use.
1H and 13C NMR spectra were recorded on Bruker Avance III HD
400 and 500 MHz spectrometers. Chemical shifts were referenced to
the residual protio-solvent impurity for 1H (s, 7.16 ppm for C6D5H; s,
7.26 for CHCl3; s 7.30, 7.02, and 6.94 ppm for C6D4HBr)38 and
solvent carbons for 13C (t, 128.1 ppm for C6D6; t, 77.2 ppm for
CDCl3). X-ray data were collected using a Bruker Photon 100 CMOS
diffractometer for data collection at 123(2) K using Cu Kα radiation
(normal parabolic mirrors). The data intensity was corrected for
absorption and decay (SADABS). Final cell constants were obtained
from least-squares fits of all measured reflections and the structure was
solved and refined using SHELXL-2014/7.39 Details regarding refined
data and cell parameters are available in Table S3. CCDC entries
1524349−1524352 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, Cambridge Crystallo-
graphic Data Center, 12 Union Road, Cambridge CB2 1EZ, United
Kingdom, fax: (+44) 1223-336-033, or e-mail:deposit@ccdc.cam.ac.uk
Synthesis of Ti(N(p-tolyl))(C5H5N)3Br2 (2). TiBr4 (604 mg, 1.64
mmol, 1.0 equiv), N-(p-tolyl)-N,N-bis(trimethylsilyl)amine (404 mg,
1.61 mmol, 1.0 equiv), and 8 mL of CH2Cl2 were added to a 20 mL
scintillation vial equipped with a small stirbar in a N2-filled glovebox.
This was then sealed with a Teflon screw cap, heated to 60 °C, and
stirred for 0.5 h. After cooling to room temperature the mixture was
left to stir for 1.5 h before diluting with 5 mL of hexanes. The reaction
mixture was then filtered through a medium frit and washed with
hexanes (3 × 3 mL). The resulting solid was then dissolved with 4 mL
of pyridine and 2 mL of CH2Cl2. After stirring for 15 min, the solution
was further diluted with 10 mL of CH2Cl2, filtered through a plug of
Celite, layered with hexanes, and placed in a −35 °C freezer overnight.
The resulting tan/green solid was collected and washed with hexanes
CONCLUSIONS
■
In summary, we have demonstrated the generality of obtaining
a reduced “TiII” intermediate by the coupling of a TiIV imido
and 2 equiv of alkyne, generating a stoichiometric amount of
pyrrole as a byproduct. Remarkably, a very diverse range of
catalyst structures are reasonably efficient at generating TiII
intermediates via [2+2+1]. The degrees of catalyst activation
and catalyst activity are highly dependent on the structure of
the Ti complex. In general, electron-poor Ti complexes, such as
those derived from Ti(NTol)(THF)3I2 precatalyst 3, are far
superior for alkyne trimerization compared to other electron-
rich and/or multidentate ligands. Additionally, while most
precatalysts predominantly generated 2,4-disusbstituted pyr-
roles on activation with 1-hexyne, hemilabile ligand scaffolds
D
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to give 2 (356 mg, 40% yield). Elemental analysis was not attempted as
complex decomposition would occur under prolonged drying on the
vacuum line. 1H NMR (400 MHz, CDCl3): δ 9.14 (br s, 4H, o-py-H),
8.86 (br s, 2H, axial o-py-H), 7.84 (t, 3JHH = 7.6 Hz, 2H, p-py-H), 7.72
(t, 3JHH = 7.6 Hz, 1H, axial p-py-H), 7.36 (t, 3JHH = 6.7 Hz, 4H, m-py-
This was then sealed with a Teflon screw cap and stirred overnight at
room temperature. The reaction mixture changed to a dark red color
over this period of time. The solvent was removed in vacuo and the
resulting solid was dissolved in benzene, filtered through Celite and
the filtrate was lyophilized to give 5 as an oily red solid (130 mg, 70%
yield). 1H NMR (400 MHz, C6D6): δ 8.15 (d, 3JHH = 5.0 Hz, 4H, o-py-
H), 7.84−7.82 (m, 2H, Ar-H), 7.32−7.31 (m, 4H, Ar-H), 7.13 (d, 3JHH
3
H), 7.26 (br s, 2H, axial m-py-H), 6.99 (d, JHH = 7.8 Hz, 2H, m-
NTol-H), 6.88 (d, 3JHH = 8.0 Hz, 2H, o-NTol-H), 2.25 (s, 3H, NC6H4-
CH3). 13C NMR (101 MHz, CDCl3): δ 152.1, 151.3 (br), 138.8, 137.1
(br), 132.3, 128.8, 124.2, 124.1, 123.9 (br, 2C), 21.2 (NC6H4-CH3).
Synthesis of Ti(N(p-tolyl))THF3I2 (3). TiI4 (442 mg, 0.796 mmol,
1.0 equiv), N-(p-tolyl)-N,N-bis(trimethylsilyl)amine (200 mg, 0.795
mmol, 1.0 equiv), and 5 mL of toluene were added to a 20 mL
scintillation vial equipped with a small stirbar in a N2-filled glovebox.
This was then sealed with a Teflon screw cap, heated to 75 °C, and
stirred for 2 h. After cooling to room temperature, the mixture was
diluted with 5 mL of hexanes. The reaction mixture was then filtered
through a medium frit and washed with hexanes (3 × 2 mL). The solid
was collected, treated with 5 mL of THF, and heated to 60 °C until all
the solids dissolved to give a red solution. The solution was then
layered with 5 mL of hexanes and placed in a −35 °C freezer overnight
to give 3 as X-ray quality red block crystals which were washed with
cold hexanes (165 mg, 33% yield). Elemental analysis was not
attempted as complex decomposition would occur under prolonged
3
= 8.2 Hz, 2H, m-NTol-H), 6.89 (d, JHH = 8.1 Hz, 2H, o-NTol-H),
6.39 (t, 3JHH = 7.6 Hz, 2H, p-py-H), 5.99 (t, 3JHH = 6.7 Hz, 4H, m-py-
H), 2.53(s, 6H, −CH3), 2.09 (s, 3H, NC6H4-CH3). 13C NMR (126
MHz, C6D6): δ 161.1, 150.4, 138.3, 130.9, 130.5, 129.4, 128.4, 124.5,
122.9, 121.6, 119.6, 118.6, 111.7, 21.0 (NC6H4-CH3), 10.5 (−CH3).
Synthesis of [Ti(μ-NPh)(C5H5N)(2,6-iPr2PhO)Cl]2 (6). This
procedure was adapted from that used for synthesis of a similar
compound with a different imido substituent.20a 2,6-Diisopropylphe-
nol (17.4 g, 97.6 mmol, 1.0 equiv) and 40 mL of THF were added to a
100 mL round-bottomed flask equipped with a stirbar in a N2-filled
glovebox and cooled in the glovebox freezer to −35 °C. Solid NaH
(2.66 g, 111 mmol, 1.1 equiv) was added slowly to the stirring cooled
solution. Caution: This reaction will exotherm. The mixture turned deep
green in color. Upon full addition, the mixture was warmed to room
temperature and stirred for 2 h. The mixture was then filtered through
a Celite plug, washed with THF, and the filtrate solvents were
removed in vacuo to give 2,6-iPr2PhONa as a white solid.
1
drying on the vacuum line. H NMR (400 MHz, CDCl3): δ 6.93 (d,
3
3JHH = 8.3 Hz, 2H, m-NTol-H), 6.84 (d, JHH = 8.1 Hz, 2H, o-NTol-
Next, 2,6-iPr2PhONa (700 mg, 3.50 mmol, 2.5 equiv) was dissolved
in 6 mL of THF in a 20 mL scintillation vial in a N2-filled glovebox.
The solution was then added dropwise to a suspension of
[Ti(NPh)Cl2py2]2 (1.03 g, 1.40 mmol, 1.0 equiv) in 2 mL of THF
in a separate 20 mL scintillation vial equipped with a stirbar. This was
then sealed with a Teflon screw cap and stirred at room temperature
for 16 h before removal of solvents in vacuo. The solids were extracted
into 5 mL of CH2Cl2, filtered through a Celite plug to remove NaCl,
layered with 5 mL of hexanes, and cooled in a −35 °C freezer. The
dark red/black crystalline material was collected and washed with
hexanes to give 6. (470 mg, 48% yield). 1H NMR (400 MHz, CDCl3):
δ 8.57 (d, 3JHH = 4.9 Hz, 4H, o-py-H), 7.47 (t, 3JHH = 7.6 Hz, 2H, p-py-
H), 7.10 (d, 3JHH = 7.6 Hz, 4H, Ar-H), 7.01−6.95 (m, 6H, Ar-H), 6.77
(t, 3JHH = 7.8 Hz, 4H, Ar-H), 6.54−6.49 (m, 6H, Ar-H), 3.76 (br s, 4H,
H), 4.54 (br s, 8H, 2,5-THF-H), 3.77 (br s, 4H, axial 2,5-THF-H),
2.25 (s, 3H, NC6H4-CH3), 2.13 (br s, 8H, 3,4-THF-H), 1.85 (br s, 4H,
axial 3,4-THF-H). 13C NMR (101 MHz, CDCl3): δ 160.3, 133.7,
128.8, 123.6, 76.6 (br-s, 1C), 68.4 (br-s, 1C), 25.5 (br-s, 1C), 21.2
(NC6H4-CH3).
Synthesis of Ti(N(p-tolyl))(C5H5N)3(C4H4N)2 (4). First, Li-
(C4H4N) was prepared. Pyrrole (2.50 g, 37.3 mmol, 1.0 equiv) and
10 mL of toluene were added to a 20 mL scintillation vial equipped
with a small stirbar in a N2-filled glovebox. This was then cooled in the
n
glovebox coldwell to −75 °C. BuLi (2.5 M, 18 mL, 44.7 mmol, 1.2
equiv) was added dropwise to the vial over 15 min. The reaction was
allowed to stir while warming to room temperature. Afterward, excess
hexanes were added to precipitate out the lithium pyrrolide salt, which
was collected on a medium frit, washed with more hexanes, and dried
overnight under vacuum to ensure removal of toluene.
3
i
iPr2C-H), 1.17 (d, JHH = 6.8 Hz, 24H, Pr-H). 13C NMR (101 MHz,
CDCl3): δ 163.3, 161.4, 149.8, 138.3, 138.1, 127.6, 124.2, 123.2, 122.7,
122.0, 118.8, 26.8, 23.9.
Li(C4H4N) (200 mg, 2.77 mmol, 4.0 equiv), 1 (318 mg, 0.69 mmol
1.0 equiv), and 2 mL of THF were added to a 20 mL scintillation vial
equipped with a small stirbar in a N2-filled glovebox. This was then
sealed with a Teflon screw cap and stirred overnight at room
temperature. The reaction mixture changed to a dark red color over
this period of time. Solvent was removed in vacuo, and the remaining
solid was dissolved in benzene and filtered through Celite. The filtrate
was lyophilized to give 4 (350 mg, 92% yield). Elemental analysis for
C30H30N6Ti (calcd, found): C (68.97, 68.88), H (5.79, 5.79), N
(16.09, 16.02). 1H NMR (400 MHz, C6D6): δ 8.34 (br s, 2H, axial o-
py-H), 8.13 (br s, 4H, o-py-H), 7.53 (br s, 4H, o-NC5H4-H), 6.96 (d,
3JHH = 7.8 Hz, 2H, m-NTol-H), 6.88 (br s, 1H, axial p-py-H), 6.82 (d,
3JHH = 7.8 Hz, 2H, o-NTol-H), 6.76 (br s, 4H, m-NC5H4-H), 6.59−
6.56 (m, 4H, axial m-py-H and p-py-H), 6.31 (br s, 4H, m-py-H), 2.06
(s, 3H, NC6H4-CH3). 13C NMR (101 MHz, C6D6): δ 151.0, 138.1,
130.6, 129.2, 128.6, 127.2, 124.4, 123.4, 108.8, 21.0 (NC6H4-CH3).
Synthesis of Ti(N(p-tolyl))(C5H5N)2(skatolide)2 (5). First, Li
skatolide was synthesized. Skatole (2.50 g, 19.1 mmol, 1.0 equiv) and
10 mL of toluene were added to a 50 mL round-bottomed flask
equipped with a stirbar in a N2-filled glovebox. This was then cooled in
the glovebox coldwell to −75 °C. nBuLi (2.5 M, 9 mL, 22.9 mmol, 1.2
equiv) was added dropwise to the round-bottomed flask over 15 min.
The reaction was allowed to stir while warming to room temperature.
Afterward, excess hexanes was added to precipitate out the lithium
skatolide salt which was collected on a medium frit, washed with more
hexanes and dried overnight under vacuum to ensure removal of
toluene.
Synthesis of Ti(NPh)(C5H5N)2(2,6-tBu2PhO)Cl (7). This proce-
dure was adapted from that used for synthesis of a similar compound
with a different imido substituent.20a 2,6-Di-tert-butylphenol (1.00 g,
4.85 mmol, 1.0 equiv) and 10 mL of THF were added to a 20 mL
scintillation vial equipped with a small stirbar in a N2-filled glovebox.
Caution: This reaction will exotherm. Solid NaH (150 mg, 6.25 mmol,
1.3 equiv) was added slowly to the solution, and the resulting mixture
was left to stir uncapped for 30 min to allow for the evolution of H2.
This was then sealed with a Teflon screw cap and stirred at room
temperature for 16 h. Afterward, the mixture was filtered through a
Celite plug to remove residual NaH and dried in vacuo to give
2,6-tBu2PhONa as a white solid.
2,6-tBu2PhONa (227 mg, 0.994 mmol mmol, 2.4 equiv, 1.2 equiv
per Ti center) was dissolved in 6 mL of THF in a 20 mL scintillation
vial in a N2-filled glovebox. This solution was added dropwise to a
suspension of [Ti(NPh)Cl2py2]2 (305 mg, 0.414 mmol, 1.0 equiv) in 2
mL of THF in a separate 20 mL scintillation vial equipped with a small
stirbar. This was then sealed with a Teflon screw cap and stirred at
room temperature for 16 h followed by removal of solvents in vacuo.
The solids were extracted into 5 mL of CH2Cl2, filtered through a
Celite plug to remove NaCl, layered with 5 mL of hexanes, and cooled
in a −35 °C freezer. The globular solids were collected, crushed, and
1
dried in vacuo to yield 7 (150 mg, 48% yield). H NMR (400 MHz,
CDCl3): δ 9.27 (br s, 4H, o-py-H), 7.86 (br s, 2H, p-py-H), 7.44 (br s,
3
t
4H, m-py-H), 7.32 (d, JHH = 7.7 Hz, 1H, m-C6H3 Bu2-H), 6.92 (t,
t
3JHH = 7.6 Hz, 2H, o-NPh-H), 6.85 (t, 3JHH = 7.8 Hz, 1H, p-C6H3 Bu2-
Li skatolide (100 mg, 0.729 mmol, 2.2 equiv), 1 (150 mg, 0.325
mmol, 1.0 equiv) and 2 mL of THF were added to a 20 mL
scintillation vial equipped with a small stirbar in a N2-filled glovebox.
H), 6.64 (t, 3JHH = 7.2 Hz, 1H, p-NPh-H), 6.36 (d, 3JHH = 8.0 Hz, 2H,
t
m-NPh-H), 1.53−1.21 (m, 18H, Bu-H).
E
DOI: 10.1021/acs.organomet.7b00096
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Synthesis of Ti(N(p-tolyl))(C5H5N)((N-2′,6′-iPr2Ph)-
phenylamidate)2 (12). N-(2′,6′-Diisopropylphenyl) (phenyl)-
(amide)40 (186 mg, 0.661 mmol, 2.2 equiv), KBn (86 mg, 0.660
mmol, 2.2 equiv), and 2 mL of benzene were added to a 20 mL
scintillation vial equipped with a small stirbar in a N2-filled glovebox.
This was stirred at room temperature for 10 min until a colorless
solution formed. The colorless solution was added directly into a
suspension of 1 (139 mg, 0.301 mmol, 1.0 equiv) and 8 mL of benzene
in a separate 20 mL scintillation vial equipped with a small stirbar. This
was then sealed with a Teflon screw cap and stirred at room
temperature for 1 h before passing through a plug of Celite and drying
the filtrate in vacuo to give a brown solid. The crude product was
dissolved in 10 mL of ether and filtered through a glass fiber filter
paper fitted in a pipet. Then, 10 mL of hexanes was layered onto the
ether solution, and the mixture was placed in a −35 °C freezer to yield
12 as brown crystals. (132 mg, 55% yield). 1H NMR (400 MHz,
C6D6): δ 9.34 (d, 3JHH = 4.4 Hz, 2H, o-py-H), 7.86 (d, 3JHH = 7.2 Hz,
4H, Ar-H), 7.23 (br s, 4H, Ar-H), 7.19 (br s, 2H, Ar-H), 6.96−6.87
with 5 mL of hexanes, and left to stand at room temperature to yield
more of 14 as a brown solid (300 mg, 56% yield from both the powder
and solid). X-ray quality crystals were grown from a 2:1 ether/hexanes
1
layering mixture. H NMR (400 MHz, C6D6): δ 8.29 (s, 2H, C-H),
7.07 (s, 12H, Ar-H), 6.94 (d, 3JHH = 8.2 Hz, 2H, m-NTol-H), 6.79 (d,
i
3JHH = 8.1 Hz, 2H, o-NTol-H), 4.58 (br s, 4H, Pr2C-H), 3.09 (br s,
i
i
4H, Pr2C-H), 2.09 (s, 3H, NC6H4-CH3), 1.01 (br s, 48H, Pr). 13C
NMR (101 MHz, C6D6): δ 164.9, 162.0, 143.6, 131.3, 128.6, 126.0,
i
i
124.7, 124.0, 28.2 (iPr), 25.7 (br, Pr-CH), 23.4 (br, Pr-CH), 21.0
(NC6H4-CH3).
S y n t h e s i s o f T i ( N ( p - t o l y l ) ) ( C 5 H 5 N ) ( N , N ′ -
(2,6-i Pr2 Ph)2 formamidinate)Cl (15). First, N,N′-
(2,6-iPr2Ph)2formamidine (201 mg, 0.551 mmol, 1.2 equiv), KBn
(72 mg, 0.551 mmol, 1.2 equiv), and 4 mL of THF were added to a 20
mL scintillation vial equipped with a small stirbar in a N2-filled
glovebox. This was stirred at room temperature for 10 min until a
colorless solution formed. The colorless solution was added directly
into a suspension of 1 (211 mg, 0.457 mmol, 1.0 equiv) and 10 mL of
THF in a separate 20 mL scintillation vial equipped with a small
stirbar. This was then sealed with a Teflon screw cap and stirred at
room temperature for 3 h before filtering through a plug of Celite and
drying in vacuo to give a brown solid. The solid was dissolved in 15 mL
of ether, and insoluble material was removed via filtration through a
glass fiber filter paper fitted in a pipet. Then, 5 mL of hexanes were
layered upon the ether solution, and the mixture was left to stand at
room temperature to yield X-ray quality brown crystals of 15 (133 mg,
46% yield). 1H NMR (400 MHz, C6D6): δ 8.73 (d, 3JHH = 4.9 Hz, 2H,
o-py-H), 8.11 (s, 1H, C-H), 7.10 (br s, 5H, Ar-H), 7.08 (d, 3JHH = 8.4
(m, 6H, Ar-H), 6.74 (d, 3JHH = 8.1 Hz, 2H, m-NTol-H), 6.70 (t, 3JHH
=
3
7.6 Hz, 1H, p-py-H), 6.59 (d, JHH = 8.2 Hz, 2H, o-NTol-H), 6.41 (t,
3JHH = 6.4 Hz, 2H, m-py-H), 4.19 (br s, 2H, iPr2C-H), 3.59 (br s, 2H,
i
iPr2C-H), 2.02 (s, 3H, NC6H4-CH3), 1.29−1.19 (m, 12H, Pr-H),
i
i
1.12−1.10 (m, 6H, Pr-H), 0.78 (br s, 6H, Pr-H). 13C NMR (101
MHz, C6D6): δ 158.0, 151.6, 143.1, 142.1, 139.1, 133.5, 131.4, 130.3,
129.7, 128.9, 128.0, 125.8, 125.0, 124.2, 124.1, 28.6, 28.2, 24.6, 23.9,
21.0 (NC6H4-CH3).
Synthesis of Bis(2,6-iPr2Ph-salycilaldimino)Ti(N(p-tolyl)) (13).
First, 2,6-iPr2Ph-salycilaldimine41 (170 mg, 0.604 mmol, 1.2 equiv),
KBn (79 mg, 0.607 mmol, 1.2 equiv), and 4 mL of benzene were
added to a 20 mL scintillation vial equipped with a small stirbar in a
N2-filled glovebox. This was stirred at room temperature for 15 min
until a yellow solution formed. The yellow solution was added
dropwise to a suspension of 1 (234 mg, 0.507 mmol, 1.0 equiv) and 10
mL of benzene in a separate 20 mL scintillation vial equipped with a
small stirbar. This was then sealed with a Teflon screw cap and stirred
at room temperature for 3 h before filtering through a plug of Celite
and drying the filtrate in vacuo. The orange-red solid was dissolved in
15 mL of ether and filtered through a glass fiber filter paper fitted in a
pipet. The ether filtrate was concentrated to 2.5 mL before layering
with 2.5 mL of hexanes. The solution was placed in a −35 °C freezer
to yield 13 as a mixture of fine X-ray quality orange crystals and orange
powder (160 mg from three crops of recrystallization, 45% yield).
Elemental analysis for C45H51N3O2Ti (calcd, found): C (75.72, 74.12),
H (7.20, 6.96), N (5.89, 5.69). 1H NMR (400 MHz, C6D6): δ 8.15 (s,
2H, H-CN), 7.34−7.27 (m, 4H, Ar-H), 7.18 (d, 3JHH = 2.1 Hz, 1H,
3
Hz, 2H, m-NTol-H), 6.82 (d, JHH = 8.1 Hz, 2H, o-NTol-H), 6.53 (t,
3JHH = 7.7 Hz, 1H, p-py-H), 6.23 (t, 3JHH = 8.0 Hz, 2H, m-py-H), 4.03
(br s, 4H, iPr2C-H), 2.05 (s, 3H, NC6H4-CH3), 1.25 (d, 3JHH = 6.0 Hz,
i
24H, Pr-H). 13C NMR (101 MHz, C6D6): δ 164.7, 160.7, 149.9,
144.5, 143.3, 138.6, 131.5, 128.9, 128.6, 126.0, 124.3, 124.1, 123.7, 28.1
i
(iPr), 24.8 (br, Pr-CH), 21.0 (NC6H4-CH3).
Synthesis of [Zr(μ-NPh)THF2Cl2]2 (16). 16 was synthesized via a
modification of the literature procedure, starting from ZrBn4 instead of
Zr(CH2TMS)4.43 ZrCl4(THF)2 (4.47 g, 11.8 mmol, 1.0 equiv) and
100 mL of THF were added to a 250 mL round-bottomed flask
equipped with a stirbar in a N2-filled glovebox. Separately, ZrBn4 (5.40
g, 11.8 mmol, 1.0 equiv) was dissolved in 25 mL of THF in a 50 mL
round-bottomed flask. The ZrBn4 solution was added in dropwise to
the THF solution of ZrCl4 with rapid stirring. The flask was sealed,
covered in aluminum foil, and stirred at room temperature for 5 h to in
situ generate ZrCl2Bn2. Afterward, aniline (2.21 g, 23.7 mmol, 2.0
equiv) in 10 mL of THF was added dropwise to the reaction mixture.
The reaction was stirred for 13 h at room temperature while covered
in aluminum foil. Volatiles were then removed under vacuum, and the
residual brown-yellow solid was dissolved in a minimal amount of 5:1
CH2Cl2/THF, transferred into two 20 mL vials, and layered with an
equal volume of pentane. The solutions were placed in a −35 °C
freezer for 3 days to afford 16 as a yellow crystalline solid (8.68 g, 92%
yield). 1H NMR (400 MHz, CDCl3): δ 7.18 (t, 3JHH = 7.8 Hz, 4H, m-
NPh-H), 7.10 (d, 3JHH = 7.2 Hz, 4H, o-NPh-H), 6.76 (t, 3JHH = 7.2 Hz,
2H, p-NPh-H), 3.99 (br s, 16H, 2,5-THF-H), 1.71 (br s, 16H, 3,4-
THF-H). 13C NMR (101 MHz, CDCl3): δ 152.9, 128.5, 121.7, 121.0,
72.8 (br s), 25.5.
3
Ar-H), 7.16 (br s, 1H, Ar-H), 7.11 (dd, JHH = 7.0 Hz, 4JHH = 1.7 Hz,
1H, Ar-H, 7.09 (dd, 3JHH = 6.9 Hz, 4JHH = 1.6 Hz, 1H, Ar-H), 7.04 (dd,
4
3
3JHH = 7.8 Hz, JHH = 1.7 Hz, 2H, Ar-H), 6.56 (d, JHH = 7.9 Hz, 4H,
Ar-H), 6.46 (d, 3JHH = 8.3 Hz, 2H, m-NTol-H), 6.22 (d, 3JHH = 8.1 Hz,
3
i
2H, o-NTol-H), 3.89 (hept, JHH = 6.7 Hz, 2H, Pr2C-H), 2.60 (hept,
3JHH = 6.8 Hz, 2H, iPr2C-H), 1.95 (s, 3H, NC6H4-CH3), 1.15 (d, 3JHH
= 6.9 Hz, 6H, iPr), 1.11 and 1.10 (d, 3JHH = 6.9 Hz, 12H, iPr), 0.95 (d,
3JHH = 6.9 Hz, 6H, Pr). 13C NMR (101 MHz, C6D6): δ 168.9, 167.5,
i
160.1, 149.6, 142.4, 141.5, 136.0, 134.3, 129.7, 126.9, 124.5, 124.0,
123.3, 122.3, 120.3, 117.7, 29.5, 28.3, 25.3, 24.9, 23.8, 22.9, 21.0
(NC6H4-CH3).
Synthesis of Ti(N(p-tolyl))(N,N′-(2,6-iPr2Ph)2formamidinate)2
(14). First, N,N′-(2,6-iPr2Ph)2formamidine42 (454 mg, 1.25 mmol, 2.1
equiv), KBn (162 mg, 1.25 mmol, 2.1 equiv), and 2 mL of THF were
added to a 20 mL scintillation vial equipped with a small stirbar in a
N2-filled glovebox. This was stirred at room temperature for 10 min
until a colorless solution formed. The colorless solution was added
directly into a suspension of 1 (280 mg, 0.607 mmol, 1.0 equiv) and 4
mL of THF in a separate 20 mL scintillation vial equipped with a small
stirbar. This was then sealed with a Teflon screw cap and stirred at
room temperature for 2 h before filtering through a plug of Celite and
drying the filtrate under vacuum to give a brown solid. Then, 10 mL of
ether was added to the solid, and the suspension was filtered through a
medium frit. The powder residue was dried in vacuo for 3 h to give 14.
The ether filtrate was further concentrated in vacuo to 5 mL, layered
General Procedure for Catalytic Alkyne Trimerization.
Precatalyst (5 mol % Ti, 0.01 mmol, 0.02 M) and 0.5 mL of stock
solution were added to a Teflon-tape-lined screw-cap NMR tube in a
N2-filled glovebox. This was then sealed with a Teflon screw cap and
heated at 115 °C for 16 h. The stock solution was prepared by adding
either 3-hexyne or 1-hexyne (0.4 M) to C6D5Br with 1,3,5-
trimethoxybenzene (0.04 M) acting as an internal standard.
1
Quantitative H NMR spectra of the catalytic mixture were recorded
before and after heating on the Bruker Avance III HD 400 MHz
spectrometers (Acquisition time = 5 s; delay time = 30 s; dummy
scans = 0; number of scans = 8). The initial precatalyst quantity of
0.01 mmol was used for catalyst activation calculations. Ti(NTol)-
THF3I2 (3) was an exception to the general procedure: Trimerization
F
DOI: 10.1021/acs.organomet.7b00096
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
of 1-hexyne was completed at room temperature in less than 5 min,
while 3-hexyne was completed at room temperature over 16 h.
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Bylikin, S. Y.; Carr, S. D.; Watts, R.; Parkin, S. J. Organomet. Chem.
2004, 689, 502−514.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00096.
(10) Allen, J. M.; Ellis, J. E. J. Organomet. Chem. 2008, 693, 1536−
1542.
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Full NMR and XRD data, as well as experimental
controls and characterization of all products of catalysis-
(PDF)
Crystallographic information file for 3 and 13−15 (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: itonks@umn.edu.
■
ORCID
Ian A. Tonks: 0000-0001-8451-8875
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support was provided by the University of Minnesota
(start-up funds, Heisig/Gleysteen fellowship for T.A.W.), the
ACS Petroleum Research Fund (ACS-PRF 54225-DNI3) and
the National Institutes of Health (1R35GM119457). Depart-
mental equipment was purchased via grants from the NSF
(MRI 1224900) (Bruker-AXS D8 Venture diffractometer) and
the NIH (S10OD011952) (NMR Facility) with matching funds
from the University of Minnesota.
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