Highly selective molybdenum ONO pincer complex initiates the living ring-opening metathesis polymerization of strained alkynes with exceptionally low polydispersity indices

Article abstract of DOI:10.1021/ja510919v

The pseudo-octahedral molybdenum benzylidyne complex [TolC≡Mo(ONO)(OR)]·KOR (R = CCH₃(CF₃)₂) 1, featuring a stabilizing ONO pincer ligand, initiates the controlled living polymerization of strained dibenzocyclooctynes at T > 60 °C to give high molecular weight polymers with exceptionally low polydispersities (PDI ～ 1.02). Kinetic analyses reveal that the growing polymer chain attached to the propagating catalyst efficiently limits the rate of propagation with respect to the rate of initiation (k_p/k_i ～ 10^-3). The reversible coordination of KOCCH₃(CF₃)₂ to the propagating catalyst prevents undesired chain-termination and -transfer processes. The ring-opening alkyne metathesis polymerization with 1 has all the characteristics of a living polymerization and enables, for the first time, the controlled synthesis of amphiphilic block copolymers via ROAMP.

Full text of DOI:10.1021/ja510919v

Article
pubs.acs.org/JACS
Highly Selective Molybdenum ONO Pincer Complex Initiates the
Living Ring-Opening Metathesis Polymerization of Strained Alkynes
with Exceptionally Low Polydispersity Indices
Donatela E. Bellone,† Justin Bours,† Elisabeth H. Menke,† and Felix R. Fischer*,†,‡,§
†Department of Chemistry, University of California Berkeley, Berkeley, California 94720, United States
‡Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
§Kavli Energy NanoSciences Institute at the University of California Berkeley and the Lawrence Berkeley National Laboratory,
Berkeley, California 94720, United States
S
* Supporting Information
ABSTRACT: The pseudo-octahedral molybdenum benzylidyne
complex [TolCMo(ONO)(OR)]·KOR (R = CCH3(CF3)2) 1,
featuring a stabilizing ONO pincer ligand, initiates the controlled
living polymerization of strained dibenzocyclooctynes at T > 60
°C to give high molecular weight polymers with exceptionally
low polydispersities (PDI ∼ 1.02). Kinetic analyses reveal that
the growing polymer chain attached to the propagating catalyst
efficiently limits the rate of propagation with respect to the rate
of initiation (kp/ki ∼ 10−3). The reversible coordination of KOCCH3(CF3)2 to the propagating catalyst prevents undesired chain-
termination and -transfer processes. The ring-opening alkyne metathesis polymerization with 1 has all the characteristics of a
living polymerization and enables, for the first time, the controlled synthesis of amphiphilic block copolymers via ROAMP.
1
to catalyst loading. H NMR experiments show that only a
fraction of the catalyst is activated and contributes to the linear
chain growth, indicating that the rate of propagation is larger
than the rate of initiation (kp/ki > 1). The poor selectivity of
alkyne metathesis catalysts for strained over unstrained alkynes
in the growing polymer chain leads to significant broadening of
the polydispersity index (PDI) through chain-transfer processes
and “backbiting” to form cyclic structures.
In this study, we report the synthesis and the detailed
mechanistic investigation of the first molecularly defined
living ring-opening alkyne metathesis catalyst [TolC
Mo(ONO)(OR)]·KOR (R = CCH3(CF3)2, ONO = 6,6′-
(pyridine-2,6-diyl)bis(2,4-di-tert-butylphenolate)) 1 (Figure 1).
In solution, a rapid equilibrium between the -ate complex 1 and
the pentacoordinate 14-electron complex 2 is observed
(electron count does not include potential π-donation of
electron density from alkoxide lone pairs). While the reversible
association of a free alkoxide prevents undesired side reactions,
the dissociation of 1 does not represent a rate-limiting step
during the propagation. Kinetic studies reveal that the growing
polymer chain efficiently limits the rate of propagation with
respect to the rate of initiation (kp/ki ∼ 10−3). We herein
demonstrate the outstanding control over molecular weight and
polydispersity achieved in living ROAMP with 1 and the first
synthesis of block copolymers through alkyne metathesis.
INTRODUCTION
■
Since its discovery in the mid-1960s, the development of stable,
well-defined, and functional-group-tolerant olefin metathesis
catalysts has greatly influenced the fields of organic synthesis
and polymer and materials science.1,2 Although alkene
metathesis has found a wide range of applications, alkyne
metathesis has only recently become the focus of attention.3−10
Moreover, living ring-opening olefin metathesis polymerization
(ROMP) has had a great impact in the areas of biomimetic
synthetic polymers, self-assembled nanomaterials, and mono-
lithic supports.1 Despite recent synthetic advances toward
highly functionalized ring-strained alkynes,11a−d the application
of ring-opening alkyne metathesis polymerization (ROAMP) to
the field of polymer synthesis has remained limited due to the
lack of commercially available well-behaved catalysts.12−17
Presently, poly(arylene ethynylene)s, used in applications
ranging from molecular photonics, electronics, to sensing, can
be accessed through acyclic diyne metathesis (ADIMET)
polymerization of diynes using highly active molybdenum and
tungsten catalysts.18−22 However, this step-growth process
provides only very limited control over the polydispersity,
length, and modality of the polymer product. Previous attempts
at synthesizing polymers using ring-opening of strained alkynes
showed polydispersities ranging from 1.1 to 7.0.12,14,23 While
polymers with polydispersities as low as 1.1 have been obtained,
the active catalyst species is poorly defined, and the reaction
requires low temperatures and rigorous air-free conditions.15
Polymers resulting from these catalysts tend to have higher
molecular weights than predicted on the basis of the monomer
Received: October 23, 2014
© XXXX American Chemical Society
A
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crystal structure of 1 confirms that only one alkoxide in 3 has
been displaced by the ONO pincer ligand. The Mo−O
distances are 2.0038(16) and 2.2475(16) Å for the hexafluoro-
tert-butoxide cis, Mo(1)−O(2), and trans, Mo(1)−O(4), to the
carbyne, respectively. The elongated Mo(1)−O(4) bond for
the alkoxide trans to the carbyne suggests a weak interaction
with an oxygen lone pair.
Crystals of 1 are stable in air for hours and can be stored for
indefinite time under an atmosphere of nitrogen. In the absence
of moisture and air, a solution of 1 in toluene-d8 shows less
than 5% decomposition after one month at 24 °C. In toluene-
d8, the pseudo-octahedral -ate complex 1 is in dynamic
equilibrium with the dissociated pentacoordinate complex
[TolCMo(ONO)(OR)] (R = CCH3(CF3)2) 2 (Supporting
Information Figure S1). In THF-d8 the alkoxide trans to the
carbyne is replaced by the solvent, and only a single species,
corresponding to a THF bound hexacoordinate complex, is
1
observed by H and 19F NMR.
We studied the ROAMP of 3,8-dihexyloxy-5,6-dihydro-
11,12-didehydrodibenzo[a,e][8]annulene (5a) (Scheme 1), a
Scheme 1. ROAMP of 5a,b with Catalyst 1
Figure 1. Synthesis of ROAMP catalyst 1. ORTEP representation of
the X-ray crystal structure of 1. Thermal ellipsoids are drawn at the
50% probability level. Color coding: C (gray), O (red), N (blue), F
(green), Mo (turquoise). Hydrogen atoms are omitted for clarity.
Diisopropyl ether was refined isotropically.
RESULTS AND DISCUSSION
■
Catalyst 1 was synthesized through ligand exchange from the
trisalkoxy molybdenum benzylidyne complex [TolC
Mo(OR)3(dme)] 3.24−27 While structurally related 12-electron
molybdenum and tungsten complexes have been reported as
catalysts for alkyne cross-metathesis and ring-closing meta-
thesis, these highly active complexes are unsuitable for
controlled ROAMP. Extensive chain transfer reactions lead to
undesired broad weight distributions (PDI > 2).6,14,28−35 In an
effort to increase the selectivity of our catalyst for the activation
of strained monomers over unstrained alkynes in the growing
polymer chain, we incorporated a permanent electron donating,
sterically demanding ONO pincer ligand 4.24,36a−f This
tridentate ligand stabilizes the high oxidation state of the
molybdenum benzylidyne complex, prevents its dimerization in
solution,12 and irreversibly blocks one of the catalyst’s active
sites.37,38 Deprotonation of the ONO pincer ligand 4 with
potassium benzyl followed by addition to [TolC
Mo(OR)3(dme)] in toluene quantitatively converted 3 to the
readily accessible highly solubilized ring-strained alkyne, with 1.
Addition of 1 to a solution of 5a in toluene ([5a]/[1] = 10) at
24 °C does not lead to the formation of polymeric species
1
within 24 h. H and 19F NMR indicate that the ROAMP
catalyst 1 quantitatively initiates with a half-life of t1/2 < 5 min
with 1 equiv of 5a to form the initiated complex 6 (n = 1)
(Scheme 1). At 90 °C, however, the initiation reaction is
instantaneous, and the living ROAMP of monomer 5a (10
equiv) in toluene is completed in less than 2 h, as determined
1
by H NMR spectroscopy. In the absence of monomer, the
molybdenum catalyst attached to the propagating polymer
chain remains active and continues to incorporate equivalents
of monomer added sequentially to the reaction mixture
(Supporting Information Figure S2). Precipitation of the
resulting polymers in MeOH affords poly-5a in greater than
90% isolated yield. GPC analysis for various monomer/catalyst
loadings at 90 °C in toluene shows a PDI of ∼1.02, the lowest
value ever reported for ROAMP (Figure 2, Table 1). Extended
reaction times do not lead to a deterioration of the PDI. The
molecular weights of poly-5a determined by GPC, calibrated to
polystyrene standards, scale linearly with the conversion of
monomer (Supporting Information Figure S2), are propor-
tional to the initial [5a]/[1] loading, and show a unimodal
distribution (Figure 2). No evidence for branching or the
1
desired product 1, by H and 19F NMR spectroscopy.
Dark brown crystals of 1 were isolated in 36% yield after
recrystallization from diisopropyl ether at −35 °C. The
geometry at the metal center is pseudo-octahedral. X-ray
crystallography of 1 (Figure 1) confirms the presence of a
C(1)Mo(1) triple bond with bond length of 1.760(2) Å and
C(2)−C(1)−Mo(1) angle of 176.91(19)°. The tridentate
ONO pincer ligand adopts a skewed conformation featuring
typical Mo(1)−O(1) and Mo(1)−O(3) distances of
1.9876(16) and 2.0010(16) Å, respectively. The Mo(1)−
N(1) distance of 2.2227(19) Å corresponds to a neutral L-type
N−Mo bond, indicating the presence of an interaction between
the lone pair of the pyridine ring and the metal center. The
presence of two alkoxides and one potassium cation in the
1
formation of cyclic polymers could be observed by H NMR
analysis and mass spectrometry (Supporting Information
1
Figure S3). H NMR end-group analysis of the tolyl group
reveals that GPC overestimates the Mn of poly-5a. A correction
factor ∼0.7−1.0 correlates well with the degree of polymer-
ization determined by NMR analysis and the expected
molecular weight based on the [5a]/[1] loading.
B
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faster than the rate of initiation/propagation. (iv) Catalyst 1
initiates quantitatively. It is thus reasonable to assume that,
during the polymerization, the concentration of 7 reaches
steady state. The resulting rate law for the polymerization is
kp[C]0[M]
d[M]
dt
−
=
= kp,obs[C]0[M]
⎛
⎞
[KOR]
⎜
⎝
⎟
⎠
+ 1
K
diss,p
(1)
where [M] is the concentration of monomer 5a, [C]0 is the
starting concentration of 1, and Kdiss,p is the dissociation
constant of 6. Since the rate of initiation of complex 2 is very
fast at the temperatures used throughout the polymerization,
we herein rely on an approximation based on initial rates of
reaction
Figure 2. GPC traces for poly-5a produced through ROAMP of 5a
with catalyst 1 at variable loadings of [5a]/[1] = 100 (red), 50 (blue),
20 (green), 10 (black) (T = 90°), calibrated to polystyrene standards.
d[Ci]
dt
ki[C]0[M]
=
= ki,obs[C]0[M]
[KOR]
Table 1. Molecular Weight Analysis of poly-5a
+ 1
(
)
Kdiss
(2)
a
a
b
a
[5a]/[1] T (°C) Mn theory Mn GPC Mw GPC
Xn
PDI GPC
where [Ci] is the concentration of all initiated species 6/7, and
Kdiss is the dissociation constant of 1.
10/1
10/1
10/1
10/1
20/1
50/1
100/1
60
70
80
90
90
90
90
4000
7200
7700
1.07
1.07
1.04
1.08
1.03
1.02
4000
7300
7800
Experimental data are consistent with the proposed rate laws.
Plots of ln([M]/[M]0) over time (Figure 3) are linear
4000
9100
9500
4000
6100
6600
11
23
47
99
8100
11 400
21 500
40 600
11 800
22 100
41 500
20 200
40 400
1.02
a
b
Calibrated to narrow polydispersity polystyrene standards. Degree
1
of polymerization determined by H NMR end-group analysis.
The proposed kinetic scheme for the polymerization of a
ring-strained monomer 5a with catalyst 1 is depicted in Scheme
2. In a fast initiation reaction, 1 equiv of 5a reacts with 2 to
Scheme 2. Kinetic Scheme for the ROAMP of 5a
Figure 3. Kinetic studies of the rate of polymerization of 5a and 5b by
1 at various temperatures.
throughout the entire polymerization and fit a rate law first
order in monomer. The concentration of propagating species is
constant throughout the reaction, and irreversible termination
processes are absent. The observed rate of propagation shows a
linear dependence on the catalyst loading (Supporting
Information Figure S4). A plot of 1/([KOR]/Kdiss,p+1) versus
kp,obs at 90 °C shows a linear correlation between the rate of
propagation and the inverse of the concentration of KOR
(Supporting Information Figure S5). Similarly, the rate of
initiation shows a linear dependence on the concentration of
monomer [M] and catalyst [C]0. Excess KOR added to the
reaction mixture slows the rate of initiation. The observed rate
constants ki,obs and kp,obs at various temperatures are
summarized in Table 2. The standard activation enthalpy for
form the initiated complex 7 (n = 1). Binding of KOR to 7
stabilizes the initiated complex and reversibly blocks the active
site. Dissociation of KOR from 6 regenerates the active
propagating species that undergoes linear chain-growth
polymerization with further equivalents of 5a to form extended
living polymer chains.
To meet the stringent criteria for a living polymerization the
initiation of the catalyst must be fast and quantitative (ki > kp),
the concentration of propagating species has to remain constant
throughout the reaction, all propagating chains have to grow at
the same rate, and irreversible termination and chain-transfer
processes should be absent.39a,b The rate laws for both the
initiation and the propagation reaction are derived employing
the following assumptions: (i) The release of ring-strain stored
in the cyclic monomer 5a makes the initiation and the
propagation irreversible. (ii) The rate of propagation kp is
comparable for all propagating species irrespective of the
degree of polymerization. (iii) The dissociation equilibria are
the initiation (ΔH⧧ = 20.7
1.2 kcal mol−1) and for the
propagation reaction (ΔH⧧ = 23.0 1.2 kcal mol−1) can be
derived from Eyring analysis (Supporting Information Figures
S6−S7).
From Eyring analysis we conclude that, for the polymer-
ization of 5a with catalyst 1 at 90 °C, the rate of initiation is
∼103 times faster than the rate of propagation. Quantitative
initiation of catalyst 1 is practically instantaneous upon addition
to a solution of the monomer. All propagating species
C
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Figure 4 summarizes the experimentally determined kinetic
and thermodynamic parameters for the initiation and the
Table 2. Observed Rates of Initiation kobs,i and Rates of
Propagation kobs,p at Different Temperatures
a
T (°C)
ki,obs (M−1 s−1
)
T (°C)
kp,obs (M−1 s−1
)
10
15
20
25
30
35
0.0158
0.0407
0.1210
0.2197
0.4066
0.8650
60
70
80
90
0.0227
0.0787
0.1642
0.5271
a
The rate of initiation at T > 40 °C is too fast to be monitored by 19
NMR.
F
incorporate monomer 5a at comparable rates (kp) to give
polymers with exceptionally narrow weight distributions.
We studied the role of the weakly coordinating alkoxide
ligand during the initiation and the polymerization reaction. At
elevated temperatures (T > 60 °C) a rapid equilibrium is
established between the -ate complexes 1 and 6, and the
dissociated complexes 2 and 7, respectively (Supporting
Information Figure S1). The dissociation constants of 1
(Kdiss) and 6 (Kdiss,p) at selected temperatures are summarized
in Table 3. Van’t Hoff analysis reveals that the changes in
Figure 4. Reaction coordinate diagram for the initiation (black) and
the propagation reaction (red) at 25 °C.
propagation reaction at standard conditions. The association of
KOR is a fast pre-equilibrium to the rate-determining step. The
rate of initiation is faster than the rate of propagation even
though the equilibrium concentration of 2 is lower than the
concentration of the propagating species 7. The rate-
determining transition state for the propagation is 3.6 kcal
mol−1 higher than the transition state for the initiation reaction.
The observed difference in metathesis activity between 2 and 7
can be rationalized by a combination of electronic and steric
effects imposed by the growing polymer chain. The steric bulk
associated with the ortho-substituted polymer backbone
increases the barrier for the incorporation of the next ring-
strained monomer. The initiated catalyst 7 features an electron
donating hexyloxy substituent on the benzylidyne that further
stabilizes the Mo(VI) complex as compared to the CH3 group
in 2 (Scheme 1).
Table 3. Dissociation Constants (Kdiss, Kp,diss) and Selected
Rate Constants (k1, k2) for 1 and 6 at Different
Temperatures
T (°C)
Kdiss (M)
k1 (s−1
)
T (°C)
Kp,diss (M)
k2 (s−1
)
30
40
50
60
70
80
0.78 × 10−3
1.22 × 10−3
1.98 × 10−3
3.40 × 10−3
4.66 × 10−3
6.84 × 10−3
7.9
15.1
27.7
43.2
a
10.0
15.0
20.0
25.0
27.5
30.0
32.5
35.0
3.70 × 10−3
4.26 × 10−3
5.98 × 10−3
6.62 × 10−3
7.13 × 10−3
8.07 × 10−3
9.00 × 10−3
9.94 × 10−3
1.2
1.9
2.9
4.3
4.9
6.4
7.1
8.2
a
To expand the substrate scope of ROAMP with catalyst 1 we
synthesized ring-strained monomer 5b (Scheme 1) featuring
solubilizing triethylene glylcol chains. Even though the ether
oxygen atoms in the side chains compete with the free alkoxide
and the ring strained monomer for binding to the propagating
molybdenum species 7, the Mn and the PDIs for polymers
obtained from the ring opening of 5b are comparable to 5a and
are summarized in Table 4. The observed rate constant for the
ROAMP of 5b at 90 °C is slower (kp,obs = 0.144 M−1 s−1) than
for 5a resulting in a t1/2 ∼ 38 min (Figure 3).
With two chemically distinct monomers at hand we studied
the performance of ROAMP catalyst 1 in the synthesis of
amphiphilic block copolymers. At 90 °C, 10 (20) equiv of 5a
were reacted with 1 for 30 min. Prior to the addition of 10 (20)
equiv of 5b, an aliquot was removed from the reaction mixture
and analyzed by GPC. After the consumption of all monomers,
as judged by 1H NMR spectroscopy, the reaction was quenched
a
The resonance signals in the 19F NMR are broadened and could not
be inverted for SIR experiments.
standard free enthalpy (ΔH° = 7.1
0.2 kcal mol−1) and
entropy (ΔS° = 13.8 0.6 eu) associated with the dissociation
of KOR from 6 are smaller than the respective changes
observed for the dissociation of 1 (ΔH° = 9.5 0.5 kcal mol−1,
ΔS° = 16.8 2.1 eu) (Supporting Information Figures S8−S9).
The rates of dissociation (k1 and k2) at various temperatures
were measured by selective inversion recovery (SIR) 19F NMR
experiments (Table 3, Supporting Information Figures S10−
S12).40,41 The standard activation enthalpies for the dissocia-
tion of 1 (ΔH⧧ = 10.4 0.6 kcal mol−1) and 6 (ΔH⧧ = 12.8
0.4 kcal mol−1) were derived from Eyring plots (Supporting
Information Figures S13−S14). To highlight the importance of
the KOR dissociation equilibrium for the performance of
ROAMP catalyst 1, we polymerized 5a in the presence of
varying amounts of a Lewis acid. Addition of 2 equiv of BPh3 to
a solution of 1 in toluene efficiently shifts the dissociation
equilibrium toward the pentacoordiante complex 2 (2 equiv of
a Lewis acid are required to trap the labile hexafluoro-tert-
butoxide and the isopropyl ether found in the crystal unit cell of
1). Polymers formed in the absence of free hexafluoro-tert-
butoxide feature broad weight distributions (PDI > 1.3) and Mn
values that do not reflect the initial [5a]/[1] loading
(Supporting Information Figure S15).
with MeOH. Unlike poly-5a, low molecular weight (Mn
=
8000) poly-5a-block-poly-5b is soluble in MeOH and only
precipitates from concentrated solutions as a pale orange solid
in >90% yield. GPC analysis reveals an increase in Mn upon
addition of 5b to the living chains of poly-5a (Supporting
Information Figure S16). The PDI of poly-5a-block-poly-5b is
exceptionally low (1.08) and matches the catalyst performance
achieved for the respective homopolymers. End-group analysis
reveals that the ratio of monomers in poly-5a-block-poly-5b
scales linearly with the monomer loading.
D
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Table 4. Molecular Weight Analysis of poly-5b and Block Copolymers poly-5a-block-poly-5b
a
a
b
a
[5a]/[5b]/[1]
T (°C)
Mn theory
Mn GPC
Mw GPC
Xn[5a]/Xn[5b]
PDI GPC
0/10/1
90
90
90
90
90
5400
4000
8300
8100
13 400
5700
6100
0/9
1.08
1.15
1.07
1.04
1.07
c
10/0/1
3300
3800
10/0
11/12
20/0
20/20
10/10/1
11 000
14 400
25 400
11 800
15 000
27 200
c
20/0/1
20/20/1
a
b
c
Calibrated to narrow polydispersity polystyrene standards. Degree of polymerization determined by 1H NMR end-group analysis. Sample taken
from the reaction mixture after t = 30 min.
combustion analyzer (values are given in %). Gel permeation
chromatography (GPC) was carried out on a LC/MS Agilent 1260
Infinity set up with a guard and two Agilent Polypore 300 mm × 7.5
mm columns at 35 °C and calibrated to narrow polydispersity
polystyrene standards ranging from Mw = 100 to 4 068 981. X-ray
crystallography was performed on APEX II QUAZAR, using a
Microfocus Sealed Source (Incoatec IμS; Mo Kα radiation), Kappa
Geometry with DX (Bruker-AXS build) goniostat, a Bruker APEX II
detector, QUAZAR multilayer mirrors as the radiation monochroma-
tor, and Oxford Cryostream 700 for 1. Crystallographic data were
refined with SHELXL-97, solved with SIR-2007, visualized with
ORTEP-32, and finalized with WinGX. 4,24 and KBn43 were
synthesized following literature procedures.
CONCLUSION
■
In summary, we have described the synthesis of the first
molecularly well-defined 16-electron ROAMP catalyst based on
a molybdenum benzylidyne ONO pincer complex [TolC
Mo(ONO)(OR)]·KOR (R = CCH3(CF3)2) 1. The incorpo-
ration of a permanent electron donating tridentate ligand
irreversibly blocks one of the catalyst’s active sites, prevents
undesired alkyne polymerization reactions, and significantly
increases its stability toward air and moisture. The catalyst is
capable of selectively ring-opening strained alkynes in a
controlled polymerization to yield high molecular weight
polymers with exceptionally low PDIs (1.02). Mechanistic
studies reveal that the ROAMP catalyst 1 meets all the criteria
for a controlled living polymerization: the initiation reaction is
quantitative and ∼103 times faster than the propagation (ki >
kp), the concentration of catalytically active complex is constant
throughout the reaction, and all propagating chains grow at the
same rate. The reversible coordination of KOR to the
propagating catalyst prevents undesired chain termination and
bimolecular decomposition of the catalyst. We demonstrate for
the first time the synthesis of structurally well-defined block
copolymers through a controlled living ROAMP. The catalyst
developed herein provides an unprecedented control and access
to functionalized homo- and block copolymers derived from
ring-strained alkynes with potential applications in advanced
thin-film electronics/photonics, molecular sensing, and nano-
patterning.
Preparation of [TolCMo(ONO)(OCCH3 (CF3 )2 )]·
KOCCH3(CF3)2·iPr2O (1). A 25 mL vial was charged with 4 (88 mg,
0.18 mmol, 1.0 equiv) in dry toluene (3 mL). A suspension of KBn
(48 mg, 0.37 mmol 2.05 equiv) in dry toluene (8 mL) was added
dropwise and the reaction mixture stirred for 15 min at 24 °C. The
resulting suspension was added dropwise to a solution of 3 (164 mg,
0.2 mmol, 1.1 equiv) in toluene (7 mL). An immediate color change to
dark brown was observed, and the reaction mixture was stirred for 30 h
at 24 °C. The suspension was filtered, and the solvent was removed
under dynamic vacuum. The precipitate was dissolved in cold CH2Cl2/
i
pentane (3:2, 4 mL) and filtered through a precooled frit. Pr2O (1
mL) was added to the solution, and the solvent was removed under
vacuum. The residue was recrystallized from iPr2O (2 mL) (−35 °C),
to yield pure 1 (78 mg, 36%) as a dark brown crystalline solid. Crystals
i
for X-ray analysis were grown from saturated Pr2O solutions at −35
°C. In toluene, 1 is in equilibrium with the dissociated pentacoordinate
complex 2 and free KOC(CF3)2CH3. 1H NMR (500 MHz, Tol-d8, 22
°C) δ = 7.70 (2), 7.63 (s, 2H, Ar-H), 7.42 (s, 2H, Ar-H), 7.27 (2), 7.20
(d, J = 8.0 Hz, 2H, 3,5-NC5H2H), 6.91 (t, J = 8.0 Hz, 1H, 4-NC5H2H),
6.58 (d, J = 7.6 Hz, 2H, C6H2 H2CH3), 6.44 (2), 6.30 (d, J = 7.6 Hz,
2H, C6H2H2CH3), 6.26 (2), 2.01 (s, 3H, C6H4−CH3), 1.93 (s, 3H,
OC(CF3)2CH3), 1.71 (2), 1.64 (s, 18H, tBu-H), 1.46 (s, 18H, tBu-H),
1.37 (2), 1.00 (s, 3H, K-OC(CF3)2CH3) ppm. 19F NMR (470 MHz,
Tol-d8, 22 °C) δ = −76.79 (2), −77.80, −78.26, −81.18 (dissociated
KOC(CF3)2CH3) ppm. In THF, only the dissociated species 2·THF is
observed, resulting in the presence of free KOC(CF3)2CH3. 1H NMR
(500 MHz, THF-d8, 22 °C) δ = 7.92 (t, J = 8.0 Hz, 1H, 4-NC5H2H),
7.70 (d, J = 8.0 Hz, 2H, 3,5-NC5H2H), 7.52 (d, J = 2.3 Hz, 2H, Ar-H),
7.46 (d, J = 2.3 Hz, 2H, Ar-H), 6.74 (d, J = 7.9 Hz, 2H, C6H2 H2CH3),
6.12 (d, J = 7.9 Hz, 2H, C6H2H2CH3), 2.20 (s, 3H, C6H4CH3), 1.78 (s,
3H, OC(CF3)2CH3), 1.52 (s, 18H, tBu-H), 1.39 (s, 18H, tBu-H) ppm.
{1H}13C NMR (126 MHz, THF-d8, 22 °C) δ = 307.5, 166.2, 155.6,
141.5, 140.5, 139.1, 138.8, 137.5, 136.8, 130.3, 127.6, 126.0, 125.4,
124.9, 123.1, 84.2, 36.0, 34.8, 32.4, 30.8, 23.5, 21.6 ppm. 19F NMR
(470 MHz, THF-d8, 22 °C) δ = −76.92 ppm. FTMS (ESI-TOF) (m/
z): [[TolCMo(ONO)(OCCH3(CF3)2)] + H]+ calcd [C45H54-
F6MoNO3], 868.3056; found 868.3076. Anal. Calcd for [[TolC
Mo(ONO)(OCCH3(CF3)2)2]KOiPr2]2·iPr2O: C, 56.21; H, 6.26; N,
1.13. Found: C, 56.04; H, 6.40; N, 1.38. Crystal data: CCDC no.,
998197; formula, C60.5H83F12KMoNO6.25; fw, 1297.32 g mol−1; temp,
100(2) K; cryst syst, monoclinic; space group, P21/n; color, black; a,
12.751(5) Å; b, 29.140(5) Å; c, 17.008(5) Å; α, 90.000(5)°; β,
93.406(5)°; γ, 90.000(5)°; V, 6308(3) Å3; Z, 4; R1, 0.0367; wR2,
0.0818; GOF, 1.051.
EXPERIMENTAL SECTION
■
Materials and General Methods. Unless otherwise stated, all
manipulations of air and/or moisture sensitive compounds were
performed in oven-dried glassware, under an atmosphere of Ar or N2.
Solvents were dried by passing through a column of alumina and were
degassed by vigorous bubbling of N2 or Ar through the solvent for 20
min. All 1H, {1H}13C, and 19F NMR spectra were recorded on Bruker
AV-600, DRX-500, AV-500, and AV-900 MHz spectrometers, and are
referenced to residual solvent peaks (CDCl3 1H NMR δ = 7.26 ppm,
13C NMR δ = 77.16 ppm; C6D6 1H NMR δ = 7.16 ppm, 13C NMR δ =
128.06 ppm; Tol-d8 1H NMR δ = 2.08 ppm; THF-d8 1H NMR δ =
1.78 ppm, 13C NMR δ = 67.21 ppm) or trifluorotoluene (19F NMR δ
= −63.72 ppm). The concentrations of 1, 2, 6, 7, and KOCCH3(CF3)2
were determined by 19F NMR using the ERETIC method against an
external standard of 13.6 mM trifluorotoluene in Tol-d8.42 The
concentration of monomer 5a,b was verified by 1H NMR applying the
ERETIC method against an external standard of 19.4 mM of
hexamethyldisiloxane in Tol-d8.41 Selective inversion recovery (SIR)
experiments were performed using TopSpin for data acquisition, and
fitted with CIFIT.40,41 The temperature in all VT NMR experiments is
calibrated to ethylene glycol or MeOH standards. ESI mass
spectrometry was performed on a Finnigan LTQFT (Thermo)
spectrometer in positive ionization mode. MALDI mass spectrometry
was performed on a Voyager-DE PRO (Applied Biosystems Voyager
System 6322) in positive mode using a matrix of dithranol. Elemental
analysis (CHN) was performed on a PerkinElmer 2400 Series II
E
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Journal of the American Chemical Society
Article
Preparation of poly-3,8-Dihexyloxy-5,6-dihydro-11,12-
didehydrodibenzo[a,e][8]annulene (poly-5a). A 10 mL resealable
Schlenk tube was charged with a stock solution of 5a (220 mM) in
toluene. If required, the solution was diluted with additional dry
toluene to reach a total of 0.5 mL. A stock solution of 1 (11 mM, 100
μL) in toluene was added, and the reaction mixture was heated in a
bath at 90 °C for 2 h. The reaction mixture was cooled, and polymers
were precipitated with MeOH (2 mL). The precipitate was filtered,
washed with MeOH (2 mL), and dried in vacuum to yield poly-5a
(92% isolated yield) as a pale brown solid. 1H NMR (600 MHz,
CDCl3, 22 °C) δ = 7.40 (d, J = 8.4 Hz, 2H, Ar-H), 6.77−6.52 (m, 4H,
Ar-H), 3.67 (t, J = 6.5 Hz, 4H, OCH2), 3.19 (s, 4H, CH2), 1.69−1.58
(m, 4H, O(CH2)5CH3), 1.41−1.19 (m, 12H, O(CH2)5CH3), 0.87 (t, J
= 7.0 Hz, 6H, CH3) ppm. {1H}13C NMR (151 MHz, CDCl3, 22 °C) δ
= 159.2, 145.3, 133.5, 115.3, 114.6, 113.0, 90.5, 67.9, 36.6, 31.8, 29.4,
25.9, 22.8, 14.2 ppm.
52722-DNI7, Berkeley NMR Facility is supported in part by
NIH Grant SRR023679A, and X-ray Facility is supported in
part by NIH Shared Instrumentation Grant S10-RR027172.
D.E.B. acknowledges fellowship support through the Abra-
hamson Foundation, E.H.M. acknowledges fellowship support
through the German Academic Exchange Service (DAAD).
The authors acknowledge Prof. Alex D. Bain for helpful
discussions relating to the SIR experiments, Dr. Christian
Canlas for support with NMR acquisition, Dr. Antonio
DiPasquale for assistance with X-ray analysis, and Dr. Rita
Nichiporuk for assistance with mass spectrometry.
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Corresponding Author
ffischer@berkeley.edu
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ACKNOWLEDGMENTS
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Research was supported by the Donors of the American
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