Switchable Polymerization of Norbornene Derivatives by a Ferrocene-Palladium(II) Heteroscorpionate Complex

Article abstract of DOI:10.1002/ejic.201501295

The ferrocene-chelating heteroscorpionate complex [(fc(PPh₂){BH{(3,5-Me)₂pz}₂})PdMe] {(fc^P,B)PdMe, fc = 1,1′-ferrocenediyl, pz = pyrazole} catalyzes the addition polymerization of norbornene and norbornene derivatives upon oxidation with [^AcFc][BAr^F] {acetyl ferrocenium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate}. In situ reduction of [(fc^P,B)PdMe][BAr^F] in the presence of a substituted norbornene results in significant decrease of catalytic activity. Addition of one equivalent of oxidant restores the activity.

Full text of DOI:10.1002/ejic.201501295

DOI: 10.1002/ejic.201501295
Full Paper
CLUSTER
ISSUE
Polymerization of Norbornene
Switchable Polymerization of Norbornene Derivatives by a
Ferrocene-Palladium(II) Heteroscorpionate Complex
Mark Abubekerov,[a] Scott M. Shepard,[a] and Paula L. Diaconescu*[a]
Abstract: The ferrocene-chelating heteroscorpionate complex phenyl)borate}. In situ reduction of [(fcP, B)PdMe][BArF] in the
[(fc(PPh2){BH{(3,5-Me)2pz}2})PdMe] {(fcP, B)PdMe, fc = 1,1′-ferro-
presence of a substituted norbornene results in significant de-
cenediyl, pz = pyrazole} catalyzes the addition polymerization
of norbornene and norbornene derivatives upon oxidation with
[AcFc][BArF] {acetyl ferrocenium tetrakis(3,5-bis(trifluoromethyl)-
crease of catalytic activity. Addition of one equivalent of oxidant
restores the activity.
Introduction
hemilabile supporting ligands has been reported.[28] Weakly co-
ordinating components of these ligands can be readily dis-
placed by more nucleophilic olefins. On the other hand, the
use of a tridentate supporting ligand would severely limit the
availability of an empty coordination site. A possible solution is
the incorporation of a labile component into the ligand frame-
work, similar to those in hemilabile ligands, resulting in an ef-
fective polymerization system without the need for a co-cata-
lyst. Additionally, controlling the lability of a supporting ligand
can be accomplished by a method similar to the one utilized in
redox-switchable hemilabile ligands.[29] An appropriate place-
ment of a redox-active group into a ligand framework provides
a method of influencing both the electronic and steric proper-
ties of the transition metal complex through oxidation of the
ligand. Because of our interest in redox active ferrocene-based
chelating ligands,[30–34] we set out to investigate the effects of
the redox switch on the lability of the ferrocene-chelating
heteroscorpionate ligand in the methylpalladium complex,
[(fc(PPh2){BH{(3,5-Me)2pz}2})PdMe] {(fcP, B)PdMe, fc = 1,1′-ferro-
cenediyl, pz = pyrazole}, in the presence of norbornene deriva-
tives. Herein, we report the first palladium system supported by
a tridentate ligand modified by an on/off redox switch for the
addition polymerization of norbornene and norbornene deriva-
tives.
Vinyl polynorbornenes are important materials for optical appli-
cations due to their excellent thermal stability, high glass transi-
tion temperatures, small birefringence, and dielectric loss.[1–5]
However, polynorbornenes are brittle, even at high molecular
weights, display poor adhesion to common substrates, poor sol-
ubility in common organic solvents, and cannot be melt proc-
essed due to decomposition.[6–9] Modifying the norbornene
monomers with functional groups increases the solubility of
the resulting polymers and improves their processability.[10] The
substitution of norbornenes, typically at the 5-position, can be
achieved via a Diels–Alder reaction, in some cases, directly from
dicyclopentadiene and a dienophile.[11] Polymers obtained from
the substituted monomers show improved adhesive and me-
chanical properties while retaining similar optical properties as
the unsubstituted polynorbornenes.[1,4,12]
Amongst transition metal catalysts, palladium-based systems
are highly active toward norbornene polymerization and pro-
duce high molecular weight vinyl polynorbornenes.[1,13–20] Most
palladium catalysts feature bidentate or monodentate support-
ing ligands since an open coordination sphere is necessary for
monomer coordination.[9,10,13–15,21–25] Commonly employed
systems also require the use of strong Lewis acids/alkylating
reagents (i.e. methylalumoxane, fluorinated arylboranes/alkyl-
aluminum complexes) as co-catalysts to generate a metal-alkyl
fragment and an empty coordination site.[1,26] The resulting cat-
alytic species are typically not stable in the absence of mono-
mer, and in certain cases, have further complications arising
from the possible monomer/co-catalyst interactions.[15,27] Alter-
natively, in cases when an open coordination sphere cannot be
readily obtained, such as allyl palladium complexes, the use of
Results and Discussion
Synthesis and Characterization of Palladium Complexes
The palladium complex (COD)PdMeCl was prepared via trans-
metallation of (COD)PdCl2 with trimethylaluminum, instead of
using alkyl tin reagents,[35] in a mixture of THF and diethyl ether,
similarly to the synthesis of [Pd2(μ-Cl)2Me2(PEt3)2].[35] The char-
acterization of the desired product was accomplished by 1H
NMR spectroscopy (Figure S1), which was consistent with previ-
ously published results.[36]
[a] Department of Chemistry and Biochemistry, University of California,
Los Angeles,
607 Charles E. Young Drive East, Los Angeles, CA 90095, USA
E-mail: pld@chem.ucla.edu
http://copper.chem.ucla.edu/pldgroup/index.htm
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejic.201501295.
Addition of (fcP, B)Li(THF)2 to (COD)PdMeCl in THF led to the
isolation of (fcP, B)PdMe·(C7H8) as red crystals in 77.1 % yield; see
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Equation (1). The solid-state molecular structure of (fcP, B)PdMe tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ([AcFc][BArF]) re-
was determined using single-crystal X-ray diffraction (Figure 1). sults in the appearance of paramagnetically shifted and broad-
1
The coordination environment around the palladium center is ened signals in the H NMR spectrum (Figure S8). The 11B NMR
in a distorted square-planar geometry (τ = 0.16)[37] similar to spectrum (Figure S10) shows a significant upfield shift of the
the nickel analogue, (fcP, B)NiMe.[38] The metal-donor atom dis- signal corresponding to the (fcP, B)PdMe complex, from –7.6 to
tances [P(1)–Pd(1), 2.2107(4) Å; C(1)–Pd(1), 2.0454(18) Å; N(3)– –65.2 ppm, upon oxidation. However, the oxidized complex is
Pd(1), 2.0938(14) Å; N(1)–Pd(1), 2.1097(14) Å] are comparable 31P NMR silent. Reduction with one equivalent of cobaltocene
with values reported for similar complexes: 1-(mesityl)-3-(2- (Cp2Co) restores the original complex, (fcP, B)PdMe, with no ap-
hydroxyphenyl)-4,5-dihydro-imidazolyl methyl triphenylphos- pearance of decomposition or side products (Figures S6 and
phine palladium(II) [Pd–P, 2.2858(9) Å; Pd–C, 2.041(3) Å],[39] S7). On a larger scale, the addition of (fcP, B)PdMe to a suspen-
(TpMe,Me)Pd(methallyl) [TpMe,Me
=
hydrotris(3,5-dimethylpyr- sion of [AcFc][BArF] in toluene resulted in the isolation of
azolyl)borate, Pd–N, 2.090(2) and 2.086(2) Å].[40] However, unlike [(fcP, B)PdMe][BArF] as dark brown solids in 84 % yield; see Equa-
for the nickel analogue, the resonance signals in the corre- tion (2). The presence of the [BArF] counterion was confirmed
sponding 1H NMR spectrum (Figure S2) are sharp, easily dis- by the presence of a singlet at δ = –6.1 and –62.2 ppm in
cernible, and are consistent with a diamagnetic complex.
the 11B and 19F NMR spectra (Figures S10 and S9), respectively.
Attempts to grow X-ray quality crystals from various neat sol-
vents and solvent combinations were unsuccessful and only
dark brown oils were obtained. However, besides spectroscopic
characterization, elemental analysis also agrees with the formu-
lation of [(fcP, B)PdMe][BArF].
(2)
(1)
Figure 2. Cyclic voltammogram recorded with a glassy carbon electrode at
100 mV/s in THF, 0.10
M [nBu4N][PF6] containing 5.0 mM
(fcP, B)PdMe.
Knowing the strength of the palladium–phosphorus and pal-
ladium–nitrogen bonds in (fcP, B)PdMe is necessary for under-
standing its catalytic activity. Addition of four equivalents of
tert-butyl isocyanide (tBuNC) to (fcP, B)PdMe resulted in the isola-
tion of (fcP, B)Pd[C(Me)NtBu](CNtBu)·(C7H8) as an orange crystal-
line material; see Equation (3). The new compound is the prod-
uct of a migratory insertion of one equivalent of tBuNC into
the palladium–carbon bond and the coordination of a second
equivalent of tBuNC to the palladium metal center.[41,42] The
two pyrazoles are not chemically equivalent in the 1H NMR
Figure 1. Molecular structure drawing of (fcP, B)PdMe with thermal ellipsoids
at 50 % probability; hydrogen atoms are omitted for clarity. Selected distan-
ces [Å] and angles [°]: P(1)–Pd(1), 2.2107(4); C(1)–Pd(1), 2.0454(18); N(1)–Pd(1),
2.1097(14); N(3)–Pd(1), 2.0938(14); P(1)–Pd(1)–C(1), 88.41(5); C(1)–Pd(1)–N(3)
91.71(7); P(1)–Pd(1)–N(1), 95.98(4); N(1)–Pd(1)–N(3), 84.99(5); C(1)–Pd(1)–
N(1),174.78(6); C(1)–Pd(1)–P(1), N(3)–Pd(1)–P(1), 161.95(4).
Electrochemical studies performed on (fcP, B)PdMe show a spectrum (Figure 3), suggesting that only one of them is coordi-
quasi-reversible curve with a redox potential of –0.27 V vs. Fc/ nated to palladium. The phosphine is in a solution state equilib-
Fc+ (Figure 2), suggesting that ferrocenium salts may be used rium between a free and palladium bound phosphine as ob-
as chemical oxidants. On an NMR scale, the oxidation of the served by two peaks at –14.9 and 9.4 ppm, respectively, in the
palladium complex with one equivalent of acetyl ferrocenium 31P NMR spectrum (Figure S15).[43] However, no reaction occurs
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upon the addition of benzonitrile or ferrocenyldiphenylphos-
The oxidation of the ferrrocene backbone with [AcFc][BArF]
phine to (fcP, B)PdMe, suggesting that weaker nucleophiles can- turns “on” the activity of the (fcP, B)PdMe complex toward the
not displace the supporting ligand. Similarly, addition of aceto- polymerization of norbornene. The polymerization of 100 equiv.
nitrile to [(fcP, B)PdMe][BArF] results in no reaction. However, of norbornene (NB) reaches completion within an hour at 70 °C.
oxidation (fcP, B)PdMe with [AcFc][BArF] in the presence of ferro- However, the poor solubility of norbornene homopolymers in
cenyldiphenylphosphine results in the disappearance of the common organic solvents prevents their characterization by gel
two 31P NMR singlets at δ = 30.5 and –16.1 ppm (Figure S11) permeation chromatography (GPC) on regular instruments and
that correspond to the two ferrocene compounds, and the ap- complicates the in situ investigation of the “on/off” redox
pearance of a singlet at δ = 24.0 ppm (Figure S12) correspond- switch. To overcome these complications, norbornene deriva-
ing to a palladium bound FcPPh2.[44]
tives (Figure 4) 5-butylbicyclo[2.2.1]hept-2-ene (NBa), 5-(trieth-
oxysilyl)bicyclo[2.2.1]hept-2-ene (NBb), 5-(trimethylsily)bicyclo-
[2.2.1]hept-2-ene (NBc), and 5-cyclohexylbicyclo[2.2.1]hept-2-
ene (NBd) were prepared and their polymerization results are
summarized in Table 1. The polymerization of ca. 100 equiv. of
1
the substituted norbornenes was monitored by H NMR spec-
troscopy, until no further monomer conversion was observed.
The obtained polymers were characterized by gel permeation
chromatography (GPC). A large discrepancy was found between
the experimental and theoretical values of the molecular
weights, suggesting a poorly controlled polymerization process,
with the PDI (PDI = Mw/Mn) values ranging from 1.5–3.6. A simi-
lar wide distribution of PDI values is observed in the literature
for the polymerization of norbornenes by group 10 met-
als.[1,6,11,13,14,25] The polymerization activity for palladium com-
plexes toward norbornene is largely dependent on the coordi-
nation environment around the metal center and the substitu-
ents on the monomer and can vary greatly from just a few
numbers to several millions.[16,18,24,45] By comparison, the
oxidized ferrocene heteroscorpionate complex shows a rela-
tively low activity with turnover frequencies ranging from 12.5
(3)
–1
to 81 molnb molPd h–1.
Figure 3. 1H NMR spectrum (C6D6, 500 MHz, 298 K) of
(fcP, B)Pd[C(Me)NtBu](CNtBu).
Figure 4. Norbornene derivatives NBa-d.
Polymerization of Norbornene and Norbornene
Derivatives
The in situ redox switch was performed with 5-triethoxysilyl
norbornene.
Starting
with
the
oxidized
complex
The reduced ferrocene complex, (fcP, B)PdMe, in the presence of [(fcP, B)PdMe][BArF] in the presence of 100 equiv. of monomer
norbornene showed no polymerization activity even when results in the polymerization of 35 equiv. in 20 min at 70 °C
heating up to 100 °C in benzene. This absence of activity sug- (Table 1, entry 6). The addition of one equivalent of Cp2Co rela-
gests that norbornene is too weak of a nucleophile to compete tive to the palladium complex severely reduces the catalytic
for a coordination site with the supporting ligand. On the other activity. The polymerization of 5 equiv. of NBb observed during
hand, in the presence of a stronger nucleophile, such as tert- this time can be attributed to a small number of catalytically
butyl isocyanide, the phosphine of the supporting ligand is al- active complex present in solution, due to the slow rate of re-
most entirely displaced. Reducing the nucleophilic character of duction of the palladium compound by Cp2Co, which takes up
the supporting ligand through the oxidation of the ferrocene to 30 min at ambient temperature to reach completion. No fur-
moiety, as was observed in the case of ferrocenyldiphenylphos- ther conversion upon heating of the reaction solution at 70 °C
phine coordination, promotes monomer coordination.
for 20 min is observed once the reduction process is complete.
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Table 1. Polymerization of norbornene derivatives by in situ generated
[(fcP, B)PdMe][BArF].[a]
The involvement of alternative mechanisms in the polymeri-
zation of norbornene was also considered. In principle, “naked”
Pd2+ might be generated in situ through loss of the supporting
ligands; such species yield high molecular weight polynorborn-
enes.[48] However, the influence of the redox switch on the po-
lymerization of norbornene suggests that the supporting ligand
remains attached to palladium throughout the polymerization
process. Alternatively, polymerization of norbornene by a cati-
onic mechanism yields oligomeric materials with low molecular
weights.[49] The reaction of norbornenes with [AcFc][BArF] (Fig-
ure S25–S28) under similar polymerization conditions as those
used for [(fcP, B)PdMe][BArF] yielded no isolable polymeric mate-
rial, suggesting that cationic polymerization by the oxidant is
not a viable mechanism in this system.
Entry
Monomer Time
[h]
Conv.
[%]
Mn
theo
Mn
exp
PDI
TOF
1
2
3
4
5
6
NB
1
1
1
0.5
2
1.33
78
79
58
33
27
49
–
–
–
64
81
58
70
12.5
38.3
NBa
NBb
NBc
NBd
NBb
12.2
14.9
5.8
4.4
13.0
30.6
37.0
8.8
1.4
30.5
3.63
2.52
2.10
1.46
3.04
[a] Conditions: Monomer (0.50 mmol), catalyst (0.005 mmol), oxidant
(0.005 mmol), [D6]benzene as solvent (0.5 mL) and 1,3,5-tri-
a
methoxybenzene (0.10 mmol) as an internal standard. All experiments were
performed at 70 °C, except for entry 5, performed at 100 °C. Mn are reported
in 103 g/mol; PDI = Mw/Mn, TOF = (molNB in polymer) molPd–1 h–1. In the case
of entry 1, the isolated yield is reported instead of conversion.
Conclusions
The oxidation with [AcFc][BArF] restores the catalytic activity
(Figure 5). The slow rate of reduction of the catalyst would ac-
count for the shoulder peak observed in the GPC graph of the
isolated polymer (Figure S37).
The application of a ferrocene-chelating heteroscorpionate li-
gand in palladium-catalyzed norbornene polymerization was
investigated. Compound [(fcP, B)PdMe][BArF] shows a reversible
on/off switch toward the polymerization of norbornene deriva-
tives. While the reduced state of the catalyst, (fcP, B)PdMe, shows
no activity, the oxidized complex, [(fcP, B)PdMe][BArF], exhibits
activity toward norbornene polymerization.
Experimental Section
General Considerations: All reactions were performed using stan-
dard Schlenk techniques or in an MBraun drybox (< 1 ppm O2/H2O)
unless noted otherwise. All glassware, cannulae, and Celite were
stored in an oven at > 425 K before being brought into the drybox.
Solvents were purified using a two-column solid-state purification
system by the method of Grubbs[50] and transferred to the glovebox
without exposure to air. NMR solvents were obtained from Cam-
bridge Isotope Laboratories, degassed, and stored over activated
molecular sieves prior to use. NMR spectra were recorded at ambi-
ent temperature on Bruker AV-300, AV-400, AV-500, and DRX-500
spectrometers unless otherwise noted. Proton and carbon chemical
shifts are given relative to residual solvent peaks. Phosphorus,
boron, and fluorine chemical shifts are given relative to external
standards, H3PO4, Et2O·BF3, and trifluoroacetic acid, respectively.
(fcP, B)Li(THF)2[38] was prepared using a literature procedure, and, un-
less otherwise noted, all reagents were acquired from commercial
sources and used as received. Elemental analyses were performed
on an Exeter Analytical, Inc. CE-440 Elemental Analyzer. Molecular
weights of the polymers were determined by GPC-LLS (Gel Permea-
tion Chromatography, Laser Light Scattering). GPC-LLS uses an Agi-
lant liquid chromatograph equipped with an Agilant 1200 series
pump and autosampler, three Phenogel 5 μm Narrow Bore columns,
a Wyatt Optilab differential refractometer, Wyatt Tristar miniDAWN
and a Wyatt Viscostar viscometer. A flow rate of 1.0 mL/min was
used and samples were dissolved in chloroform with 0.25 % triethyl-
amine. Results were calibrated to narrow molecular weight poly-
styrene standards.
Figure 5. Comparison of 5-(triethoxysilyl)-bicyclo[2.2.1]hept-2-ene polymeri-
zation using
a redox switch and not using a redox switch using
[(fcP, B)PdMe][BArF] as a catalyst.
To gain a better understanding of how the redox switches
affect the polymerization reaction, the influence of ligand
oxidation upon electron-donating abilities of individual ferro-
cene substituents was considered. The oxidation of ferrocenyl-
diphenylphosphine reduces the electron-donating ability of the
phosphine donor, resulting in the increase of the CO stretching
frequency by several wave numbers in rhenium carbonyl com-
plexes.[46] Alternatively, oxidation of a ferrocene containing
tris(pyrazolyl)borate carbonylmolybdenum complex resulted in
virtually no change in the stretching frequency of the carbonyl
ligands.[47] Based on these literature reports and our experi-
mental results, we propose that the oxidation of the ferrocene-
chelating heteroscorpionate increases the lability of the phos-
phine moiety allowing monomer coordination and migratory
insertion into the alkyl–metal fragment, while the (pyrazolyl)-
borate portion remains unaffected. Since the displacement of
the phosphine in the reduced complex by norbornene does not
occur, the polymerization process is halted with this catalyst.
(COD)PdMeCl: A suspension of (COD)PdCl2 (247.7 mg, 0.868 mmol)
in 6 mL of Et2O/THF (1:2 vol %) was cooled to –78 °C. Trimethylalu-
minum (125.1 mg, 1.735 mmol) in 1 mL of hexanes was added
slowly, the reaction mixture was removed from the cold well, and
stirred at ambient temperature until the solution became colorless.
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Methanol was added to solvolyze the aluminum by-products and
solved in 1.5 mL of toluene containing 2 drops of tBuNC. Hexanes
volatile substances were removed under reduced pressure. The de- (1 mL) was layered on top and the solution was stored overnight
sired product was extracted with THF and filtered through Celite.
THF was removed under reduced pressure and the resulting off-
white solids were washed with Et2O (2 × 2 mL) to afford the final
product (185.3 mg, 80.5 %). This procedure was carried out multiple
times in similar scales with yields ranging from 50–80 %. 1H NMR
(CDCl3, 500 MHz, 298 K): δ = 1.18 (s, 3 H, PdCH3), 2.47 (m, 4 H, CH2),
at –35 °C. Decanting of the solution and washing of the remaining
solids with hexanes (3 × 1 mL) yielded the final product as an
orange crystalline material (97.5 mg, 71.2 %). Crystals of
(fcP, B)Pd[C(Me)NtBu](CNtBu) always contain one molecule of toluene
per molecule of compound as supported by NMR spectroscopic
data. 1H NMR (C6D6, 500 MHz, 298 K): δ = 0.79 [s, 9 H, C(CH3)3], 1.76
2.58 (m, 2 H, CH2), 2.65 (m, 2 H, CH2), 5.15 (t, 2 H, CH), 5.91 (t, 2 H, [s, 9 H, C(CH3)3], 2.00 (s, 3 H, CH3), 2.15 (s, 3 H, CH3), 2.32 (s, 3 H,
CH) ppm.
CH3), 2.48 (s, 3 H, CH3), 2.49 (s, 3 H, CH3), 3.97 (m, 1 H, Cp-H), 4.05
(m, 1 H, Cp-H), 4.24 (m, 3 H, Cp-H), 4.32 (m, 1 H, Cp-H), 4.48 (m, 1
H, Cp-H), 4.52 (m, 1 H, Cp-H), 5.62 (s, 1 H, CH), 5.80 (s, 1 H, CH), 7.07
(m, 6 H, m-Ph, p-Ph), 7.62 (m, 4 H, o-Ph) ppm. 13C NMR (C6D6,
126 MHz, 298 K): δ = 13.7 (s, CCH3), 14.8 (s, CCH3), 15.0 (s, CCH3),
15.8 (s, CCH3), 29.7 [s, C(CH3)3], 32.2 [s, C(CH3)3], 34.4 [s, C(CH3)=
NC(CH3)3], 56.5 [s, C(CH3)3], 68.7 [s, C(CH3)=NC(CH3)3], 71.0 (s, Cp-C),
72.7 (d, Cp-C), 73.0 (d, Cp-C), 73.6 (s, Cp-C), 73.8 (s, Cp-C), 74.8 (s,
Cp-C), 74.9 (s, Cp-C), 75.3 (s, Cp-C), 75.6 (s, Cp-C), 105.3 (s, CH), 106.5
(s, CH), 134.3 (s, aromatic), 134.4 (s, aromatic), 134.5 (s, aromatic),
134.6 (s, aromatic), 145.0 (s, CCH3), 145.4 (s, CCH3), 147.0 (s, CCH3),
149.1 (s, CCH3), 166.4 [C(CH3)=NC(CH3)3] ppm. 11B NMR (C6D6,
161 MHz, 298 K): δ = 7.2 (br. s) ppm. 31P[1H] NMR (C6D6, 121 MHz,
298 K): δ = –14.9 (s), 9.4 (s) ppm. Anal. Calcd: (fcP, B)Pd[C(Me)-
NtBu](CNtBu)·(C7H8) (C50H62BFeN6PPd) C, 63.14; H, 6.57; N, 8.84;
found C, 62.62; H, 6.49; N, 8.86.
(fcP, B)PdMe: To (COD)PdMeCl (155.1 mg, 0.585 mmol) in 10 mL of
THF, (fcP, B)Li(THF)2 (384.3 mg, 0.532 mmol) in 5 mL of THF was
added dropwise. The reaction solution was stirred for 16 h at ambi-
ent temperature. Volatile substances were removed under reduced
pressure and the desired product was extracted into 10 mL of tolu-
ene and filtered through Celite. Reduction in volume of the toluene
solution to 4 mL and layering with 4 mL of hexanes afforded red
crystalline material after 24 h at –35 °C. Decanting of the solution
and washing of the remaining solids with hexanes (3 × 2 mL) and
cold toluene (3 × 1 mL) yields the product as a red crystalline mate-
rial (321.9 mg, 77.1 %) in two crops. Crystals of (fcP, B)PdMe always
contain one molecule of toluene per molecule of compound as
supported by NMR spectroscopic data. X-ray quality crystals were
1
obtained from toluene at –35 °C. H NMR (C6D6, 500 MHz, 298 K):
δ = 0.99 (d, 3 H, PdCH3), 1.65 (s, 3 H, CCH3), 2.30 (s, 3 H, CCH3), 2.35
(s, 3 H, CCH3), 2.42 (s, 3 H, CCH3), 3.18 (s, 1 H, Cp-H), 3.95 (s, 1 H,
Cp-H), 4.04 (s, 1 H, Cp-H), 4.18 (s, 1 H, Cp-H), 4.21 (s, 1 H, Cp-H), 4.30
(s, 1 H, Cp-H), 4.43 (s, 1 H, Cp-H), 5.04 (s, 1 H, Cp-H), 5.56 (s, 1 H,
CH), 5.73 (s, 1 H, CH), 6.92 (t, 2 H, p-Ph), 7.05 (m, 4 H, m-Ph), 7.41 (t,
2 H, o-Ph), 7.57 (t, 2 H, o-Ph) ppm. 13C NMR (C6D6, 126 MHz, 298 K):
δ = –0.98 (d, PdCH3), 14.2 (s, CCH3), 14.3 (d, CCH3), 14.4 (s, CCH3),
15.3 (s, CCH3), 68.9 (s, Cp-C), 69.5 (s, Cp-C), 70.8 (d, Cp-C), 72.4 (d,
Cp-C), 73.8 (d, Cp-C), 74.7 (s, Cp-C), 74.9 (d, Cp-C), 80.2 (s, Cp-C),
105.8 (s, -CH-), 106.7 (d, -CH-), 132.2 (s, aromatic), 132.6 (s, aromatic),
133.6 (s, aromatic), 133.7 (s, aromatic), 133.9 (s, aromatic), 134.0 (s,
aromatic), 134.1 (s, aromatic), 134.2 (s, aromatic), 145.4 (s, CCH3),
146.4 (d, CCH3), 147.9 (s, CCH3), 148.8 (d, CCH3) ppm. 11B NMR
(C6D6, 161 MHz, 298 K): δ = –7.6 (br. s) ppm. 31P[1H] NMR (C6D6,
121 MHz, 298 K): δ = 30.5 (s) ppm. Anal. Calcd: (fcP, B)PdMe·(C7H8)
(C40H44BFeN4PPd) C, 61.21; H, 5.65; N, 7.14; found C, 60.82; H, 5.00;
N, 6.56.
General Procedure for Synthesis of Substituted Norbornenes: A
pressure reactor was charged with dicyclopentadiene (20.0 mmol)
and the corresponding olefin (44.0 mmol). The reactor was purged
with nitrogen and sealed. The reactor was then heated to 220–
240 °C for two hours before being cooled to ambient temperature.
The reactor was opened and the resulting tan oil was distilled at
2.5 Torr to afford a colorless liquid.
5-Butylbicyclo[2.2.1]hept-2-ene (NBa): Distilled at 35 °C and
2.5 Torr to afford a colorless liquid (44.0 %), 18.2 % exo, 81.8 % endo.
1H NMR (CDCl3, 500 MHz, 298 K): δ = 0.47–0.50 (m, 1 H), 0.87 (m, 3
H), 1.07 (m, 2 H), 1.20–1.46 (m, 6 H), 1.82 (m, 1 H), 1.96 (m, 1 H),
2.50 (s, 1 H), 2.75 (s, 1 H), 5.54–6.11 (m, 2 H, olefinic) ppm. 13C NMR
(CDCl3, 500 MHz, 298 K): δ = 137.1, 137.0, 136.3, 133.4, 132.6, 132.5,
54.4, 49.7, 48.3, 46.5, 45.8, 45.6, 45.4, 42.7, 42.1, 41.6, 38.9, 36.9, 36.5,
34.7, 33.2, 32.6, 31.3, 31.1, 23.1, 14.2 ppm.
[(fcP, B)PdMe][BArF]: To [AcFc][BArF] (75.9 mg, 0.069 mmol) was
added (fcP, B)PdMe·(C7H8) (49.6 mg, 0.063 mmol) in 5 mL of toluene.
The reaction solution was stirred for 10 min and filtered through
Celite. The solution volume was reduced to 1 mL, diluted to 10 mL
with hexanes, and stored at –35 °C for 30 min. Decanting of the
solution and washing of the remaining brown oil with cold hexanes
(2 × 4 mL) yields the product as a brown solid after an hour under
reduced pressure (84.3 mg, 81.2 %). 1H NMR (C6D6, 500 MHz, 298 K):
δ = –9.71 (br. s), –3.28 (s), –1.82 (s), 1.68 (s), 3.58 (br. s), 4.68 (s), 4.84
(s), 7.42 (s), 7.91 (s), 8.60 (s), 10.04 (s), 12.43 (br. s) ppm. 11B NMR
(C6D6, 161 MHz, 298 K): δ = –65.2 (br. s), –6.1 (s) ppm. 19F NMR
5-(Triethoxysilyl)bicyclo[2.2.1]hept-2-ene (NBb): Distilled at
65 °C and 2.5 Torr to afford a colorless liquid (46.8 %) as a mixture
of exo and endo isomers. 1H NMR (CDCl3, 500 MHz, 298 K): δ = 0.46
(m, 1 H), 1.07 (m, 1 H), 1.23 (m, 9 H), 1.35 (m, 1 H), 1.76 (m, 1 H),
1.88 (m, 1 H), 2.92 (m, 2 H), 3.76–3.86 (m, 6 H), 5.92–6.13 (m, 2 H,
olefinic) ppm. 13C NMR (CDCl3, 500 MHz, 298 K): δ = 137.9, 135.4,
134.8, 133.8, 58.6, 58.4, 47.1, 44.3, 42.9, 42.6, 42.3, 27.1, 26.4, 20.9,
20.4, 18.5, 18.4 ppm.
5-(Trimethylsilyl)bicyclo[2.2.1]hept-2-ene (NBc): Distilled at 30 °C
and 2.5 Torr to afford a colorless liquid (37.7 %) as a mixture of exo
and endo isomers. 1H NMR (CDCl3, 500 MHz, 298 K): δ = –0.09–0.00
(s, 9 H), 0.31 (m, 1 H), 0.90–1.14 (m, 2 H), 1.29–1.51 (m, 2 H), 1.85
(m, 1 H), 2.72–2.93 (m, 1 H), 3.21 (m, 1 H), 5.48–6.16 (m, 2 H, olefinic)
ppm. 13C NMR (CDCl3, 500 MHz, 298 K): δ = 138.7, 135.6, 134.1,
51.7, 47.0, 44.9, 43.0, 42.62, 42.57, 27.3, 27.1, 25.4, 25.3, –1.3, –1.8
ppm.
(C6D6, 282 MHz, 298 K):
δ = –62.2 (s) ppm. Anal. Calcd:
[(fcP, B)PdMe][BArF]·(C7H8) (C72H56B2F24FeN4PPd) C, 52.47; H, 3.43; N,
3.40; found C, 51.97; H, 2.74; N, 3.25.
(fcP, B)Pd[C(Me)NtBu](CNtBu): To (fcP, B)PdMe·(C7H8) (113.3 mg,
0.144 mmol) in 5 mL of dichloromethane at –78 °C was added
CNtBu (0.065 mL, 0.577 mmol) via syringe. The reaction mixture was
stirred for 1 h at –78 °C before volatile substances were removed 5-Cyclohexylbicyclo[2.2.1]hept-2-ene (NBd): Distilled at 76 °C at
under reduced pressure. The desired product was extracted in 3 mL
2.5 Torr to afford a colorless liquid (7.53 %) 1H NMR (CDCl3,
of diethyl ether and filtered through Celite. Diethyl ether was re- 500 MHz, 298 K): δ = 0.60 (m, 2 H), 0.83–1.00 (m, 2 H), 1.12–1.20
moved under reduced pressure and the remaining solids were dis- (m, 4 H), 1.26 (s, 1 H), 1.36 (m, 1 H), 1.65 (m, 4 H), 1.77 (m, 2 H),
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2.74–2.87 (s, 2 H), 5.90–6.10 (m, 2 H, olefinic) ppm. 13C NMR (CDCl3,
500 MHz, 298 K): δ = 137.1, 136.5, 132.4, 49.4, 46.0, 45.8, 45.5, 44.0,
43.1, 43.0, 42.5, 42.4, 41.9, 33.0, 32.8, 32.6, 32.5, 31.6, 31.2, 26.95,
29.93, 26.7, 26.6, 26.5, 26.5, 26.4 ppm.
Keywords: Homogeneous catalysis · Polymerization ·
Palladium · Scorpionate ligands · Redox chemistry
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This work was supported by US National Science Foundation
(NSF) (grant numbers 1362999, grant to P. L. D., and CHE-
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Received: November 7, 2015
Published Online: February 12, 2016
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© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim