Polymerization of phenylacetylenes using rhodium catalysts coordinated by norbornadiene linked to a phosphino or amino group

Article abstract of DOI:10.1021/om301147n

The novel rhodium (Rh) catalysts [{nbd-(CH₂)₄-X}RhR] (1, X = PPh₂, R = Cl; 2, X = NPh₂, R = Cl; 3, X = PPh ₂, R = triphenylvinyl; nbd = 2,5-norbornadiene) were synthesized, and their catalytic activities were examined for the polymerization of phenylacetylene (PA) and its derivatives. Rh-103 NMR spectroscopy together with DFT calculations (B3LYP/6-31G-LANL2DZ) indicated that catalyst 1 exists in a mononuclear 16-electron state, while 2 exists in dinuclear states. Catalyst 1 converted PA less than 1% in the absence of triethylamine (Et₃N). Addition of Et₃N and extension of the polymerization time enhanced the monomer conversion. On the other hand, catalysts 2 and 3 quantitatively converted PA in the absence of Et₃N to afford the polymer in good yields. Catalyst 3 achieved two-stage polymerization of PA.

Full text of DOI:10.1021/om301147n

Article
pubs.acs.org/Organometallics
Polymerization of Phenylacetylenes Using Rhodium Catalysts
Coordinated by Norbornadiene Linked to a Phosphino or Amino
Group
Naoya Onishi,† Masashi Shiotsuki,*,‡ Toshio Masuda,§ Natsuhiro Sano,⊥ and Fumio Sanda*,†
†Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura Campus, Nishikyo-ku, Kyoto
615-8510, Japan
‡Molecular Engineering Institute, Kinki University, Kayanomori, Iizuka, Fukuoka 820-8555, Japan
§Department of Environmental and Biological Chemistry, Faculty of Engineering, Fukui University of Technology, 3-6-1 Gakuen,
Fukui 910-8505, Japan
⊥R&D Division, Nippon Chemical Industrial Co., Ltd., 11-1, 9-Chome, Kameido, Koto-ku, Tokyo 136-8515 , Japan
S
* Supporting Information
ABSTRACT: The novel rhodium (Rh) catalysts [{nbd-(CH2)4-X}RhR] (1, X =
PPh2, R = Cl; 2, X = NPh2, R = Cl; 3, X = PPh2, R = triphenylvinyl; nbd = 2,5-
norbornadiene) were synthesized, and their catalytic activities were examined for the
polymerization of phenylacetylene (PA) and its derivatives. Rh-103 NMR spectros-
copy together with DFT calculations (B3LYP/6-31G*-LANL2DZ) indicated that
catalyst 1 exists in a mononuclear 16-electron state, while 2 exists in dinuclear states.
Catalyst 1 converted PA less than 1% in the absence of triethylamine (Et3N). Addition
of Et3N and extension of the polymerization time enhanced the monomer conversion.
On the other hand, catalysts 2 and 3 quantitatively converted PA in the absence of
Et3N to afford the polymer in good yields. Catalyst 3 achieved two-stage
polymerization of PA.
dienophiles bearing target substituents. However, substitution
on the reagents sometimes causes unsuccessful results in the
Diels−Alder reaction due to steric hindrance.3 Another method
for the preparation of modified bicyclic diene ligands is direct
introduction of functional groups on dienes. nbd derivatives
linked to a phosphino or amino group by (CH2)4 can be
synthesized from nbd-(CH2)4-Br (Scheme 1), which is
prepared by the reaction of nbd and Br-(CH2)4-Br.4 This
synthetic method allows the introduction of functional groups
on nbd by substitution of the bromo group with nucleophiles,
leading to easy synthesis of nbd derivatives linked to various
functional groups. In particular, a Rh catalyst bearing the
triphenylvinyl group, [(nbd)Rh{C(Ph)CPh2}{P(4-F-
C6H4)3}], polymerizes phenylacetylene (PA) in a controlled
fashion.5 The polymerization is initiated quantitatively by
insertion of the triple bond of the monomer between the Rh
center and a vinylic carbon atom.
INTRODUCTION
■
Substituted polyacetylenes have received considerable attention
due to the useful properties resulting from their stiff π-
conjugated main chains, including photoconductivity, electro-
luminescence, and gas permeability.1 They are commonly
synthesized by polymerization of the corresponding monomers
catalyzed by transition-metal complexes, with rhodium (Rh)
catalysts being the most widely used, due to their high
functional-group tolerance and cis stereoregularity of the main
chains of the resulting polymers. Most Rh catalysts for
acetylene polymerization contain a bicyclic diene ligand such
as 2,5-norbornadiene (nbd), because of the high stability of the
rigidly chelated structures. Masuda and co-workers have
reported that the π-acidity of bicyclic diene ligands largely
affects the catalytic activity of Rh catalysts.2 For example, Rh
catalysts bearing the highly π-acidic tetrafluorobenzobarrelene
(tfb) ligand polymerize acetylene monomers much more
quickly than their nbd counterparts. The high π-acidity of the
tfb ligand induces a considerable back-donation from filled 4d
orbitals of Rh to the LUMO of tfb, resulting in a more facile
coordination of a monomer to the Rh center. Thus, it is
expected that more sophisticated bicyclic diene ligands may
further improve catalytic activity.
In the present study, we report the synthesis of novel Rh
catalysts 1 and 2 coordinated by nbd derivatives bearing
phosphino/amino groups, characterization of the structures by
103Rh NMR spectroscopy, and polymerization of PA with 1 and
2. We also transformed catalyst 1 into 3, bearing a
One method for the synthesis of substituted bicyclic diene
ligands is the Diels−Alder reaction of dienes and/or
Received: November 27, 2012
Published: January 24, 2013
© 2013 American Chemical Society
846
dx.doi.org/10.1021/om301147n | Organometallics 2013, 32, 846−853

Organometallics
Article
Scheme 1. Structures of nbd-(CH2)4-Br and Rh Complexes 1−3
triphenylvinyl group, and studied the polymerization behavior
using 3. The paper discusses the mechanistic aspects of the
polymerization on the basis of DFT calculations. As far as we
know, catalyst 3 is the first well-defined Rh complex with a
tridentate ligand enabling polymerization of substituted
acetylene monomers.
RESULTS AND DISCUSSION
■
Synthesis of Novel Rh Complexes 1−3. Novel nbd
derivatives bearing -(CH2)4PPh2 and -(CH2)4NPh2 were
synthesized by the reaction of nbd-(CH2)4-Br with the
corresponding phosphine and amine derivatives, respectively,
1
in moderate yields. The structures were confirmed by H and
13C NMR spectroscopy and elemental analysis. The resulting
compounds were reacted with [(C2H4)2RhCl]2 in CH2Cl2 1to
afford complexes 1 and 2,2c as racemic mixtures. In the H
NMR spectra, olefinic proton signals assignable to the olefin
moieties appeared around 6.7 and 6.0 ppm, which shifted 3−4
ppm upfield in comparison to those of the dienes before the
reaction, due to coordination to the Rh center.
Figure 1. ORTEP drawing of the molecular structure of 3 (50%
probability ellipsoids). Selected bond distances (Å) and angles (deg):
Rh−C1 = 2.062, Rh−C2 = 2.169, Rh−C3 = 2.161, Rh−C4 = 2.223,
Rh−C5 = 2.229, Rh−P = 2.288; C2−Rh−C5 = 66.68, C3−Rh−C4 =
65.50, C1−Rh−P = 94.64.
Complex 1 was converted into 3, which bears the -C(Ph)
CPh2 group by the reaction with [BrMg{C(Ph)CPh2}], by a
synthesis similar to that for other Rh-C(Ph)CPh2 complex-
es.2a,5,6 It is expected that 3 enables controlled polymerization
of PA, because the structure is analogous to the propagating
species in acetylene polymerization. Figure 1 shows the
ORTEP structure of 3 obtained by single-crystal X-ray analysis.
The -(CH2)4-PPh2 group coordinates to the Rh center, as
expected. We also tried unsuccessfully to synthesize a Rh-
C(Ph)CPh2 complex from 2.
complex 2 exhibited two singlet signals at −7051.9 and
−7089.7 ppm (Δ = 476 Hz)7 with an integration ratio of 1.1:1.
Contamination by a Rh-containing impurity is very unlikely,
because the elemental analysis matches the calculated value
with an error ≤0.06%.
Various isomeric structures are possible for complex 2 due to
the readily dissociative amine of the tridentate ligand. As shown
in Scheme 1, one isomer is a 16-electron (16e) complex
coordinated by the diene and amino groups. The second one is
a 14-electron (14e) complex coordinated by the diene only.
There are also several possible dinuclear 16e complexes,
possibly consisting of syn-(1S,4R)-(1′S,4′R), syn-(1S,4R)-
(1′R,4′S), syn-(1R,4S)-(1′R,4′S), anti-(1S,4R)-(1′S,4′R), anti-
(1S,4R)-(1′R,4′S), and anti-(1R,4S)-(1′R,4′S) isomers. It is
considered that the complexes are transformable and are in
Rh-103 NMR spectra of complexes 1 and 2 were measured
in CD2Cl2 together with that of [(nbd)RhCl]2. The last
compound exhibited one signal at −7056.9 ppm, while complex
1 exhibited two signals (Figure 2). These two resonances are
assignable to a doublet at −7748.2 ppm with JRh−P = 166 Hz,
because the 31P NMR spectrum shows a doublet coupled with
Rh with the same coupling constant. On the other hand,
1
equilibrium in the solution state. We also examined the H
847
dx.doi.org/10.1021/om301147n | Organometallics 2013, 32, 846−853

Organometallics
Article
flexibility of the mononuclear 14e complexes is larger than that
of the mononuclear 16e counterparts. The Rh−N interatomic
distance of 2 (16e, mononuclear) is 2.39 Å in CH2Cl2 (Table
2). This value is somewhat longer than the Rh−P distance of 1
Table 2. Interatomic Distances between Rh and P/N of 1
and 2 at Mononuclear 16e and 14e States Calculated by the
a
DFT Method (B3LYP/6-31G*-LANL2DZ)
interatomic distance between Rh and P/N (Å)
b
b
complex
no solvent
in CH2Cl2
in THF
1 (16e) mononuclear
1 (14e) mononuclear
2 (16e) mononuclear
2 (14e) mononuclear
3 (16e)
2.44
7.88
2.52
7.44
2.39
8.78
2.34
7.86
2.39
7.50
2.40
8.67
2.35
7.86
2.39
7.50
2.40
3A (14e)
8.67
Figure 2. 103Rh NMR (12.6 MHz) spectra of [(nbd)RhCl]2, 1, and 2
measured in CD2Cl2.
a
b
Basis sets: 6-31G* for C, H, N, P, and Cl; LANL2DZ for Rh. SCRF-
IEFPCM method.
NMR spectrum of 2 but could not obtain clear information
about the isomers.
(16e, mononuclear) (2.34 Å), even though the van der Waals
radius of nitrogen (1.55 Å) is smaller than that of phosphorus
(1.80 Å). The lower stability of the nitrogen complex may be
responsible for the unsuccessful preparation of the nitrogen
analogue of 3 mentioned above. It should be noted that the
dinuclear 16e complexes of 2 are energetically much more
favorable than the mononuclear 16e complex (ΔH, ranging
from −69.9 to −85.0 kJ/mol; ΔG, ranging from −61.5 to
−79.0 kJ/mol). It is very likely that complex 2 exists in
dinuclear forms, exhibiting 103Rh NMR chemical shifts very
close to that of [(nbd)RhCl]2 in CD2Cl2, as shown in Figure 2.
The two 103Rh NMR signals are assignable to syn and anti
isomers and/or isomers with various combinations of (1S,4R)-
and (1R,4S)-nbd derivatives.
The energy differences between the 16e and 14e complexes
of 3 are small (ΔH, from +15.2 to +18.3 kJ/mol; ΔG, from
−1.6 to −7.3 kJ/mol) in comparison with those between the
mononuclear 16e and 14e complexes of 1 (ΔH, from +75.7 to
+92.5 kJ/mol; ΔG, from +59.9 to +72.8 kJ/mol). The 14e state
(3A) seems to be stabilized by the triphenylvinyl group, which
According to DFT calculations, the mononuclear 16e
complex of 1 is more stable than the mononuclear 14e isomer
by 76.3 and 62.1 kJ/mol in enthalpy (H) and Gibbs free energy
(G) in CH2Cl2, respectively (Table 1). All syn- and anti-
dinuclear 16e complexes of 1 are energetically unfavorable in
comparison with the mononuclear 16e complex, indicating that
1 exists as a mononuclear 16e complex ligated by PPh2, as
observed by 103Rh and 31P NMR spectroscopy. The presence of
a 14e complex is also unlikely, judging from the energy values
that are much higher than those of the 16e mononuclear
complex. On the other hand, the mononuclear 16e complex of
2 is less stable than the mononuclear 14e isomer by 10.0 and
28.3 kJ/mol in H and G, respectively, in CH2Cl2. Similar trends
are observed in THF. These calculation results provide
evidence of the less stable chelation of complex 2, which
contains an amino group, in comparison to complex 1 having a
phosphino group. The entropy (S) gains by dechelation are
(4.76−6.13) × 10−2 kJ/(mol K) in CH2Cl2, indicating that the
a
Table 1. Relative Energies of Various Possible Complexes of 1−3 Calculated by the DFT Method (B3LYP/6-31G*-LANL2DZ)
energy difference (kJ/mol)
b
b
no solvent
in CH2Cl2
in THF
c
c
c
complex
1 (16e) mononuclear
ΔH
0.0
ΔG
ΔH
0.0
ΔG
ΔH
0.0
ΔG
0.0
+24.7
+23.7
+22.9
+20.9
+72.8
0.0
0.0
+42.8
+41.6
+41.9
+47.9
+62.1
0.0
0.0
+39.3
+37.5
+38.5
+44.6
+59.9
0.0
1 (16e) syn-(1S,4R)-(1′S,4′R) dinuclear
1 (16e) syn-(1S,4R)-(1′R,4′S) dinuclear
1 (16e) anti-(1S,4R)-(1′S,4′R) dinuclear
1 (16e) anti-(1S,4R)-(1′R,4′S) dinuclear
1 (14e) mononuclear
+12.2
+12.0
+12.1
+12.4
+92.5
0.0
+28.3
+28.1
+28.8
+26.5
+76.3
0.0
+26.2
+26.0
+26.7
+24.4
+75.7
0.0
2 (16e) mononuclear
2 (16e) syn-(1S,4R)-(1′S,4′R) dinuclear
2 (16e) syn-(1S,4R)-(1′R,4′S) dinuclear
2 (16e) anti-(1S,4R)-(1′S,4′R) dinuclear
2 (16e) anti-(1S,4R)-(1′R,4′S) dinuclear
2 (14e) mononuclear
−84.9
−84.8
−84.7
−85.0
+7.9
−77.6
−79.0
−77.5
−78.6
−10.1
0.0
−70.3
−70.1
−69.9
−70.4
−10.0
0.0
−63.2
−63.0
−61.5
−62.3
−28.3
0.0
−71.1
−71.0
−70.7
−71.2
−9.6
−63.5
−64.2
−62.5
−63.4
−25.4
0.0
3 (16e)
0.0
0.0
3A (14e)
+18.3
−1.6
+15.2
−7.2
+15.2
−7.3
a
Basis sets: 6-31G* for C, H, N, P, and Cl; LANL2DZ for Rh. Energy standard: 16e mononuclear complex of each compound. The energy values of
b
c
dinuclear complexes are divided by 2. SCRF-IEFPCM method. At 298.15 K and 1 atm.
848
dx.doi.org/10.1021/om301147n | Organometallics 2013, 32, 846−853

Organometallics
Article
has electron-donating ability, and overlap of the π orbital of one
β-phenyl group with the Rh d orbital, as discussed later in this
paper.
Polymerization. The polymerization of PA was carried out
by using catalysts 1−3 (Scheme 2). Catalyst 1 bearing a
Scheme 2. Polymerization of PA with Rh Catalysts 1−3
a
Table 3. Polymerization of PA with Rh Catalysts 1−3
polymer
Figure 3. Conversion/time and first-order plots for the polymerization
of PA with Rh catalyst 3. Conditions: [M]0 = 0.50 M, [M]0/[Rh] =
500 in THF at 30 °C.
b
c
time
(h)
conversn
(%)
yield
(%)
M /
wd
d
run catalyst additive
Mn
Mn
1
2
3
4
5
1
1
1
2
3
none
Et3N
Et3N
none
none
24
24
72
24
24
<1
11
70
67
83
94
69,000
240,000
85,000
1.7
1.7
1.3
100
100
a
Conditions: [M]0 = 0.50 M, [M]0/[Rh] = 500, [Et3N]/[Rh] = 0 or
b
10 in THF at 30 °C. Determined by GC ([tert-butylbenzene] = 50
mM as an internal standard). MeOH-insoluble part. Estimated by
SEC (polystyrene standard).
c
d
phosphino group gave less than 1% conversion of the monomer
(Table 3, run 1), indicating very low catalytic activity. The
monomer conversion increased to 11% when Et3N was added
as a cocatalyst (run 2). Extension of polymerization time from
24 to 72 h enhanced the monomer conversion to afford the
polymer with an Mn value of 69,000 in a moderate yield (67%)
(run 3). On the other hand, catalyst 2 bearing an amino group
quantitatively converted PA under the same conditions as those
of run 1 to afford the polymer in 83% yield (run 4). The vinyl
type complex 3 converted PA quantitatively to give a polymer
in 94% yield (run 5). The molecular weight distribution was
narrow (Mw/Mn = 1.3), in comparison to that for the polymers
obtained by the polymerization using 1 and 2 (Mw/Mn = 1.7).
The difference in catalytic activities of 1−3 is considered as
follows. The amino group of 2 seems to induce the formation
of an active 14e mononuclear complex by coordinating to the
Rh center of a dinuclear complex in a manner similar to that for
Et3N in the [(nbd)RhCl]2/Et3N catalyst system.8 It is likely
that the population of this active species is small, because the
initiation efficiency is calculated to be as small as 21% (run 4 in
Table 3). In contrast, both complexes 1 and 3 exist as
mononuclear states. In the case of 3, a PA molecule is smoothly
inserted between the Rh and the triphenylvinyl group, resulting
in polymerization. On the other hand, some prereactions such
as formation of Rh-CCPh species are necessary to form an
initiating species in the case of 1. As a result, 3 is much more
active than 1, even though both catalysts contain a phosphine
ligand.
Figure 4. Conversion/time and first-order plots for the polymerization
of PA after the first and second feeds with Rh catalyst 3. Conditions:
[M]0 = [M]second = 0.50 M, [M]0/[Rh] = 500 in THF at 30 °C.
mixture. The polymerization again occurred satisfactorily to
convert the second monomer feed quantitatively (Figure 4).
Interestingly, the first-order plot was linear during the
polymerization after the second monomer feed. The reason
for these results is discussed later in this paper.
Mechanism of Polymerization. Scheme 3 and Figure 5
depict a plausible mechanism for the polymerization of PA
catalyzed by Rh complex 3 and the potential map for the
pathway, respectively.9 The energies were determined by DFT
calculations performed in the presence of a solvent (THF) by
placing the solute in a cavity within the solvent reaction field.10
In the present study, we considered the coordination−insertion
pathway to give cis-transoidal poly(PA), which was confirmed
by a 13C labeling study by Noyori and co-workers for Rh-
catalyzed PA polymerization11 and supported by a detailed
DFT study by Morokuma and co-workers.12 First, the
phosphine moiety of 16e complex 3 is dissociated from the
Rh center to form the 14e complex 3A, having a vacant site.
This process is unfavorable regarding ΔH (+15.2 kJ/mol) while
favorable regarding ΔG (−7.3 kJ/mol), likely due to the
entropy gain by dissociation of the PPh2 group from the Rh
center. As mentioned above, the analogous transformation from
the 16e to 14e complex of 1 is largely endothermic both in ΔH
and ΔG. This difference is explainable by the through-space
The monomer conversion/time relationship was monitored
by GC in order to obtain further information about vinyl
catalyst 3 (Figure 3). The monomer conversion reached 100%
at 180 min. The first-order plot was nonlinear in the early stage
and became linear after about 80 min. After complete monomer
consumption, the second portion of PA was fed to the reaction
849
dx.doi.org/10.1021/om301147n | Organometallics 2013, 32, 846−853

Organometallics
Article
Scheme 3. Plausible Mechanism for the Polymerization of PA by Rh Vinyl Catalyst 3
Figure 6. Overlap of the Rh (blue) d and phenyl π orbitals of 3A and
3C: (left) with the β-phenyl group of 3A, MO 151; (right) with the γ-
phenyl group of 3C, MO 186. Geometries were fully optimized by
B3LYP/6-31G*-LANL2DZ using SCRF-IEFPCM (solvent THF).
The red and green parts represent MO surfaces.
Figure 5. Potential map for Scheme 3 illustrated using the H (blue
line) and G values (red line) of 3 as standards, calculated by B3LYP/6-
31G*-LANL2DZ using SCRF-IEFPCM (solvent THF). The sums
with three molecules of PA (3, 3A), two molecules of PA (3B, 3TS(B-
C), 3C), and one molecule of PA (3D, 3TS(D-E), 3E) were used to
adjust the total atom numbers of the intermediates.
this step ΔG is also positive (+65.0 kJ/mol), due to the entropy
loss by PA coordination. This unfavorable ΔG value explains
the presence of an induction period in the early stage of PA
polymerization with the catalyst.13 Another reason for the lower
stability of 3B in comparison to 3A is loss of the overlap
between the Rh d orbital and Ph π orbital of the triphenylvinyl
group of 3A, which is mentioned above. The third reason is the
steric repulsion between the phenyl group of the coordinating
monomer and nbd moiety of 3B, which is understood from the
capture of the conformation shown in the Supporting
Information (p S36). It is also possible for a THF molecule
to coordinate to the Rh center at the vacant site of 3A, as
illustrated in Scheme 4. In fact, the ΔH and ΔG values from 3A
to 3B are +17.5 and +65.0 kJ/mol, while those for THF
electron donation from the phenyl group of 3A to the Rh
center. Namely, the π orbital of one β-phenyl group of 3A
overlaps with the Rh d orbital at the vacant site to stabilize the
conformer, as shown in Figure 6, left. The overlap of the Rh d
orbital with the π orbital of nbd is observed as well, leading to
effective stabilization by nbd-Rh-Ph electron delocalization.
Next, a PA molecule (red) is coordinated to the Rh center at
the vacant site to give 3B. This step is also endothermic, and in
850
dx.doi.org/10.1021/om301147n | Organometallics 2013, 32, 846−853

Organometallics
Article
coordination to 3A are −3.4 and +38.0 kJ/mol in THF,
respectively.
group of 3A mentioned above. The second monomer insertion
takes place via the metallacyclobutane transition state 3TS(D-
E) to give 3E in a similar fashion with the first monomer
insertion. After that, the third PA monomer coordinates to the
Rh center of 3E to give 3F, and successive PA coordination−
insertion reactions give poly(PA) having -C(Ph)CPh2 and a
Rh residue at the chain ends. The chain end Rh is converted
into H upon addition of acetic acid.6c
Scheme 4. Coordination of THF to 3A
Polymerization of PA Derivatives. The vinyl catalyst 3
was also used for the polymerization of PAs substituted with
trimethylsilyl (TMS), t-Bu, and F at the para positions. As given
in Table 4, monomers bearing p-TMS and p-t-Bu groups
Table 4. Polymerization of PA Derivatives HCCC6H4-p-R′
Then, the PA is inserted between the Rh and vinylic carbon
atom to give 3C via the metallacyclobutane transition state
3TS(B-C). In this step, 2,1-insertion is preferred to 1,2-
insertion, because the π orbital of the benzene ring of PA
significantly interacts with the d orbital of Rh during 2,1-
insertion to stabilize the transition state, while it does not
during 1,2-insertion.12 PA is inserted in a cis manner, as
commonly observed in acetylene polymerization with Rh
catalysts, consistent with the cyclobutane transition state
3TS(B-C). The activation energies were calculated to be
ΔH⧧ = 29.3 kJ/mol and ΔG⧧ = 39.8 kJ/mol from 3B. The
transformation from 3B to 3C is largely exothermic (ΔH =
−146.1 kJ/mol, ΔG = −132.8 kJ/mol), because PA is inserted
and a new C−C σ bond is created while a weaker π bond of the
alkyne is lost. This is energetically quite favorable and is the
reason why polymerization of PA is exothermic. Another reason
is extension of conjugation from Rh-C(Ph)CPh2 to Rh-
C(Ph)CHC(Ph)CPh2, which is supported by the lower
ΔH and ΔG values of 3C in comparison to those of 3A.
The other possible reason for the high stability of 3C in
comparison to 3A and 3B is through-space electron donation
from the phenyl group to the Rh center, as shown in Figure 6,
right. In this case, the γ-phenyl group participates in orbital
overlap with Rh in a manner different from the case for 3A. The
distances between the Rh atom and the centers of the β- and γ-
phenyl groups are 3.29 and 2.90 Å for 3A and 3C, respectively.
A Rh−phenyl interaction can exist even though these distances
are longer than those of η6-phenyl−Rh complexes (X-ray
crystallographic data 1.8−1.9 Å),14 and 3C is more stabilized by
the through-space interaction than 3A. Commonly in Rh-
catalyzed polymerizations, PA derivatives are more polymer-
izable than acetylene monomers having no phenyl group on the
acetylene triple bond. The Rh−γ-phenyl through-space
interaction is possible throughout the propagation process, as
long as the monomer insertion takes place in a cis manner.
Thus, the high polymerizability of PA derivatives is explained
by this interaction as well as the high coordination ability due to
the conjugation of the triple bond and the phenyl group. The
absence of an induction period during the polymerization of the
second monomer feed can also be understood from this
mechanistic aspect. The reaction is already in the propagation
process when the second feed is added.
a
with Rh Catalyst 3
polymer
b
c
d
d
R′
conversn (%)
yield (%)
Mn
Mw/Mn
TMS
t-Bu
F
92
76
44
91
74
40
75,000
45,000
35,000
2.2
1.6
1.7
a
Conditions: [M]0 = 0.50 M, [M]0/[Rh] = 500 in THF, 30 °C, 24 h.
Determined by GC ([tert-butylbenzene] = 50 mM as an internal
b
c
d
standard). MeOH-insoluble part. Estimated by GPC (polystyrene
standard).
underwent polymerization to give the corresponding polymers
in good yields (91 and 74%), while the monomer bearing p-F
gave the polymer in a low yield (40%). It is proposed that the
low reactivity of the last monomer is caused by the low
coordination ability of the triple bond due to the electron-
withdrawing character of F and the low through-space
interaction between the Rh center and the phenyl group at
the inserted monomer unit.
CONCLUSIONS
■
We have developed novel Rh complexes ligated with nbd linked
to a phosphino or amino group. These complexes showed
catalytic activity for the polymerization of PA and its
derivatives. Et3N was necessary as an additive for catalyst 1,
which has a phosphino group. On the other hand, catalyst 2,
bearing an amino group, polymerized PA without the assistance
of Et3N. NMR spectroscopic studies and DFT calculations
revealed that 1 exits as a mononuclear complex, while 2 exists
as dinuclear complexes. It seems that the dinuclear species of 2
is dissociated by the assistance from the amino group to form a
14e mononuclear complex. This species should be active, and it
polymerizes PA smoothly in comparison to 1, in which the Rh
is tightly coordinated by the phosphine ligand. Vinyl catalyst 3
polymerized PA in a controlled manner in the late stage and
achieved complete consumption of the second monomer feed.
Catalyst 3 is the first well-defined Rh complex coordinated by a
tridentate ligand for controlled polymerization of acetylene
monomers.
As shown in Scheme 3, the second PA (blue) coordinates to
the Rh center of 3C at the site next to the nbd double bond
without substituting -(CH2)4PPh2 to give 3D. This process is
endothermic (ΔH = +37.5 kJ/mol, ΔG = +87.1 kJ/mol),
showing a trend similar to, although larger in magnitude, the
analogous step from 3A to 3B. This result provides support for
the larger through-space interaction between Rh and the γ-
phenyl group in 3C in comparison to the Rh and the β-phenyl
EXPERIMENTAL SECTION
■
Instruments. Monomer conversions were determined by GC
(Shimadzu GC-14B, capillary column (CBP10-M25−025)): column
temperature 125 °C, injection temperature 250 °C, flame-ionization
detector, internal standard tert-butylbenzene. Number- and weight-
average molecular weights (Mn and Mw, respectively) and polydisper-
sity indices (Mw/Mn) of polymers were measured by SEC with a
JASCO PU-980/RI-930 chromatograph: 40 °C, eluent THF, columns
851
dx.doi.org/10.1021/om301147n | Organometallics 2013, 32, 846−853

Organometallics
Article
KF-805 (Shodex) × 3, molecular weight limit up to 4 × 106, flow rate
a solution of [(C2H4)2RhCl]2 (264 mg, 0.68 mmol) in CH2Cl2 (8.0
mL) under Ar at room temperature, and the resulting mixture was
stirred overnight. After the solvent was removed in vacuo, the orange
residue was purified by column chromatography (Al2O3) using toluene
as the eluent. The resulting yellow oil was dried in vacuo. Yield: 365
1
1 mL/min, calibrated with polystyrene standards. H NMR spectra
(400 MHz), 13C NMR (100 MHz), and 31P NMR (161 MHz) were
recorded on a JEOL EX-400 spectrometer with chemical shifts
1
referenced to the internal standards CDCl3 (7.24 ppm for H NMR),
1
CD2Cl2 (5.32 ppm, for H NMR), CDCl3 (77.0 ppm for 13C NMR),
1
mg (60%). H NMR (CDCl3): δ 7.26−7.21 (m, 4H), 6.99−6.90 (m,
and CD2Cl2 (53.0 ppm, for 13C NMR) and the external standard
P(OMe)3 (140 ppm, for 31P NMR). 103Rh NMR spectra (12.6 MHz)
were recorded on a JNM-ECX400 spectrometer with chemical shifts
referenced to the external standard Rh(acac)3 (0 ppm). Elemental
analyses were performed at the Microanalytical Center of Kyoto
University. X-ray crystal structure data were collected on a Rigaku
RAXIS RAPID imaging plate area detector with graphite-monochro-
mated Mo Kα radiation (λ = 0.71075 Å). Crystals of suitable size were
mounted on a nylon loop and then transferred to a goniostat for
characterization and data collection. The structure was solved by direct
methods and expanded using Fourier techniques. All calculations were
performed using the crystallographic software package of Rigaku Corp.
and Rigaku/MSC CrystalStructure version 3.7.
6H), 3.96 (s, 1H), 3.90 (s, 1H), 3.84 (br s, 1H), 3.81 (s, 1H), 3.75−
3.69 (m, 3H), 3.51−3.49 (m, 2H), 2.17−2.03 (m, 2H), 1.74−1.55 (m,
4H). 103Rh NMR (CD2Cl2): δ −7052, −7090 ppm. Anal. Calcd for
C23H25NClRh: C, 60.87; H, 5.55; N, 3.09. Found: C, 60.86; H, 5.54;
N, 3.03.
Synthesis of [{nbd-(CH2)4-PPh2}Rh(Ph)CCPh2] (3). BrMg{C-
(Ph)CPh2} (0.58 M in THF), was prepared from Mg (0.60 g, 24.7
mmol) with triphenylvinyl bromide (4.19 g, 12.5 mmol) in THF (12
mL), and a portion (0.65 mL, 0.38 mmol) was added to a solution of
Rh complex 1 (60 mg, 0.13 mmol) in THF (1.0 mL) under Ar. The
resulting mixture was stirred at 50 °C for 24 h. It was concentrated,
and the residue was purified by Al2O3 column chromatography with
Et2O as eluent, followed by recrystallization from CH2Cl2/pentane to
give a yellow solid. Yield: 45 mg (51%). 1H NMR (CD2Cl2): δ 8.01−
7.97 (m, 2H), 7.52−6.47 (m, 23H), 4.37 (s, 1H), 4.00−3.59 (m, 4H),
3.08−3.01 (m, 2H), 2.48−2.28 (m, 2H), 1.90 (m, 2H), 1.67−1.25 (m,
4H), 1.76−1.42 (m, 4H), 1.25−1.10 (m, 2H). 31P NMR (CD2Cl2): δ
26.3. Anal. Calcd for C43H40PRh: C, 74.78; H, 5.84. Found: C, 74.65;
H, 5.91.
Polymerization. All the polymerizations were carried out in a
Schlenk tube equipped with a three-way stopcock under Ar. A THF
solution of PA (c = 1.0 M) was added to a catalyst solution ([Rh] =
2.0 mM). The resulting mixture was stirred at 30 °C for a set time and
poured into a large amount of methanol containing a drop of acetic
acid to precipitate the polymer. The product was isolated by filtration
with a PTFE membrane (pore size 0.2 μm) and dried under vacuum to
constant weight.
Materials. PA and its derivatives were purchased (Aldrich) and
distilled over CaH2 under reduced pressure before use. KPPh2 (0.5 M
in THF, Aldrich) and HNPh2 (TCI) were purchased and used without
further purification. All solvents were distilled by standard procedures.
15
[(C2H4)2RhCl]2 and nbd-(CH2)4-Br16 were synthesized according
to the literature.
Synthesis of nbd-(CH2)4-PPh2. KPPh2 (0.5 M in THF, 12 mL,
6.0 mmol) was added dropwise to a solution of nbd-(CH2)4-Br (1.30
g, 5.72 mmol) in THF (2.0 mL) under Ar at 0 °C. Then, the mixture
was warmed to room temperature and stirred for 1.5 h. Water (0.5
mL) was added to the reaction mixture to quench the reaction, and the
resulting mixture was evaporated under vacuum. The residual yellow
oil was purified by silica gel column chromatography with hexane as
eluent under Ar to give nbd-(CH2)4-PPh2 as a colorless oil (1.41 g,
1
79%). H NMR (CDCl3): δ 7.42−7.25 (m, 10H), 6.70 (s, 2H), 6.07
(s, 1H), 3.45 (s, 1H), 3.23 (s, 1H), 2.20−2.16 (m, 2H), 2.07−2.02 (m,
2H), 1.92−1.89 (br s, 2H), 1.57−1.49 (m, 2H), 1.44−1.36 (m, 2H).
13C NMR (CDCl3): δ 158.3, 143.7, 142.3, 139.0, 138.9, 133.5, 132.7,
132.5, 128.3, 73.4, 53.3, 49.9, 31.0, 28.8, 27.8, 25.4. 31P NMR
(CDCl3): δ −15.5. Anal. Calcd for C23H25P: C, 83.10; H, 7.58. Found:
C, 83.11; H, 7.58.
Computation. All calculations were performed with the
GAUSSIAN 09 program,17 EM64L-G09 Rev C.01, running on the
supercomputer system of the Academic Center for Computing and
Media Studies, Kyoto University. The enthalpy (H) and Gibbs free
energy (G) at 298.15 K and 1 atm were calculated by the DFT18
method with the B3LYP functional19 in conjunction with the
LANL2DZ20 basis set for Rh and the 6-31G* basis set for the other
elements. In some cases, the integral equation formalism variant
(IEFPCM) method was employed for the self-consistent reaction field
(SCRF) to take into account solvent effects.21 The feasibility of
transition states was checked by the presence of only one imaginary
frequency, with the vibrational mode corresponding to the reaction
coordinate. Further, intrinsic reaction coordinate (IRC)22 calculations
were carried out using the geometries of the transition states as the
initial structures to confirm the connection with reactants and
products.
Synthesis of nbd-(CH2)4-NPh2. NaH (540 mg, 22.5 mmol) was
added to a solution of nbd-(CH2)4-Br (520 mg, 2.29 mmol) and
HNPh2 (1.37 g, 8.09 mmol) in CH3CN (10 mL). The resulting
mixture was heated at 50 °C for 20 h. The reaction mixture was cooled
to room temperature and then quenched with water (20 mL) and
extracted with CHCl3 (20 mL × 3 times). The combined organic
phase was dried over anhydrous sodium sulfate. After filtration, the
filtrate was concentrated in vacuo. The residue was purified by silica
gel column chromatography with hexane/EtOAc 100/1 (v/v) as
1
eluent to give nbd-(CH2)4-NPh2 as a colorless oil (570 mg, 79%). H
NMR (CDCl3): δ 7.25−7.15 (m, 10H), 6.71 (s, 2H), 6.10 (s, 1H),
3.68 (m, 2H), 3.47 (s, 1H), 3.24 (s, 1H), 2.20−2.18 (m, 2H), 1.92 (br
s, 2H), 1.63−1.42 (m, 4H). 13C NMR (CDCl3): δ 158.6, 145.8, 143.8,
142.3, 133.3, 128.3, 126.7, 126.5, 73.4, 58.3, 53.4, 50.0, 31.3, 30.0, 25.0.
Anal. Calcd for C23H25N: C, 87.57; H, 7.99; N, 4.44. Found: C, 87.41;
H, 7.75; N, 4.36.
Synthesis of [nbd-(CH2)4-PPh2]RhCl (1). A solution of nbd-
(CH2)4-PPh2 (103 mg, 0.31 mmol) in CH2Cl2 (3.0 mL) was added to
a solution of [(C2H4)2RhCl]2 (55 mg, 0.14 mmol) in CH2Cl2 (4.0
mL) under Ar at room temperature, and the resulting mixture was
stirred for 3 h. After the solvent was removed in vacuo, the resulting
orange solid was purified by recrystallization from CH2Cl2/Et2O.
Yield: 95 mg (83%). 1H NMR (CDCl3): δ 7.79−7.77 (m, 2H), 7.69−
7.66 (m, 2H), 7.38 (m, 6H), 5.33 (s, 1H), 5.22 (s, 1H), 3.78 (s, 1H),
3.56 (s, 1H), 3.36 (s, 1H), 2.37−2.07 (m, 4H), 1.76−1.42 (m, 4H),
1.25−1.10 (m, 2H). 31P NMR (CDCl3): δ 23.7 (d, JRh−P = 166 Hz).
Anal. Calcd for C23H25ClPRh: C, 58.68; H, 5.35. Found: C, 58.73; H,
5.36.
ASSOCIATED CONTENT
■
S
* Supporting Information
A figure giving a potential map for Scheme 3 under vacuum,
Cartesian coordinates for stationary points of the compounds
shown in Schemes 1, 3, and 4, and crystallographic data for 3.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: mshiotsuki@moleng.fuk.kindai.ac.jp (M.S.); sanda.
fumio.5n@kyoto-u.ac.jp (F.S.).
Notes
Synthesis of [nbd-(CH2)4-NPh2]RhCl (2). A solution of nbd-
(CH2)4-NPh2 (457 mg, 1.45 mmol) in CH2Cl2 (5.0 mL) was added to
The authors declare no competing financial interest.
852
dx.doi.org/10.1021/om301147n | Organometallics 2013, 32, 846−853

Organometallics
Article
(9) Initiation mechanism with Rh−Cl complexes. It is commonly
considered that the Rh−CCPh species is first formed by the
reaction of the Rh−Cl complex with HCCPh (monomer), and then
a monomer molecule is inserted into the Rh−C bond to initiate
polymerization. However, the mechanism for initiation is still
controversial and could potentially include Rh−hydride and Rh−
vinylidene species. The necessity of a prereaction prior to the initiation
is one possible reason for the activity of 1 being lower than that of 3.
See: (a) Kishimoto, Y.; Eckerle, P.; Miyatake, T.; Ikariya, T.; Noyori,
ACKNOWLEDGMENTS
■
This research was supported by a Grant-in-Aid for Scientific
Research from the Japan Society for Promotion of Science
(22750108), the Global COE Program, “International Center
for Integrated Research and Advanced Education in Materials
Science” from the Ministry of Education, Culture, Sports,
Science and Technology of Japan, The Ogasawara Foundation
for the Promotion of Science & Engineering, and The
Sumitomo Foundation. We are grateful to Prof. Keiji
Morokuma at Kyoto University/Emory University for offering
the Cartesian coordinates of related compounds, Prof. Kenneth
B. Wagener and Dr. Kathryn R. Williams at the University of
Florida for their helpful suggestions and comments, and Mr.
Osamu Kamo and Mr. Keiichi Yoshida at JEOL RESONANCE
Inc. for measuring the 103Rh NMR spectra.
R. J. Am. Chem. Soc. 1994, 116, 12131−12132. (b) Jimen
́
ez, M. V.;
Perez-Torrente, J. J.; Bartolome, M. I.; Vispe, E.; Lahoz, F. J.; Oro, L.
A. Macromolecules 2009, 42, 8146−8156.
́
́
(10) The energies in the absence of a solvent were also calculated and
depicted in Figure S1 (Supporting Information). The difference
between the energies in THF and vacuum for each intermediate was
0.9−14.1 kJ/mol.
(11) Kishimoto, Y.; Eckerle, P.; Miyatake, T.; Kainosho, M.; Ono, A.;
Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1999, 121, 12035−12044.
(12) Ke, Z.; Abe, S.; Ueno, T.; Morokuma, K. J. Am. Chem. Soc. 2011,
133, 7926−7941.
(13) The transformation from 3 to 3B via 3A possibly accompanies
reverse reactions, because the coordination of Rh with PA and PPh3 is
competitive. The polymerization of PA with [(nbd)RhCl]2/BuLi/
PPh3 is retarded progressively with increasing PPh3 concentration. See:
Kanki, K.; Misumi, Y.; Masuda, T. Macromolecules 1999, 32, 2384−
2386.
(14) (a) Onishi, N.; Shiotsuki, M.; Sanda, F.; Masuda, T.
Macromolecules 2009, 42, 4071−4076. (b) Liu, Z.; Yamamichi, H.;
Madrahimov, S. T.; Hartwig, J. F. J. Am. Chem. Soc. 2011, 133, 2772−
2782. (c) Shi, Y.; Blum, S. A. Organometallics 2011, 30, 1776−1779.
(d) Grochowski, M. R.; Morris, J.; Brennessel, W. W.; Jones, W. D.
Organometallics 2011, 30, 5604−5610. (e) Hooper, J. F.; Chaplin, A.;
́
́
Gonzalez-Rodrıguez, C.; Thompson, A. L.; Weller, A. S.; Willis, M. C.
J. Am. Chem. Soc. 2012, 134, 2906−2909. (f) Chaplin, A. B.; Hooper, J.
F.; Weller, A. S.; Willis, M. C. J. Am. Chem. Soc. 2012, 134, 4885−
4897.
REFERENCES
■
(1) Reviews: (a) Choi, S.-K.; Gal, Y.-S.; Jin, S.-H.; Kim, H. K. Chem.
Rev. 2000, 100, 1645−1681. (b) Nagai, K.; Masuda, T.; Nakagawa, T.;
Freeman, B. D.; Pinnau, I. Prog. Polym. Sci. 2001, 26, 721−798.
(c) Masuda, T.; Sanda, F. Polymerization of Substituted Acetylenes. In
Handbook of Metathesis; Grubbs, R. H., Ed., Wiley-VCH: Weinheim,
Germany, 2003; Vol. 3, Chapter 3.11, pp 375−406. (d) Aoki, T.;
Kaneko, T.; Teraguchi, M. Polymer 2006, 47, 4867−4892. (e) Masuda,
T. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 165−180. (f) Masuda,
T.; Sanda, F.; Shiotsuki, M. Polymerization of Acetylenes. In
Comprehensive Organometallic Chemistry III; Crabtree, R., Mingos,
M., Eds., Elsevier: Oxford, U.K., 2007; Vol. 11, Chapter 16, pp 557−
593. (g) Yashima, E.; Maeda, K.; Iida, H.; Furusho, Y.; Nagai, K. Chem.
Rev. 2009, 109, 6102−6211. (h) Akagi, K. Chem. Rev. 2009, 109,
5354−5401. (i) Liu, J.; Lam, J. W. Y.; Tang, B. Z. Chem. Rev. 2009,
109, 5799−5867. (j) Masuda, T.; Shiotsuki, M.; Sanda, F. Product
Class 31: Macromolecular Conjugated Polyenes. In Science of Synthesis.
Houben-Weyl Methods of Molecular Transformations; Siegel, J. S., Tobe,
Y., Eds.; Georg Thieme: Stuttgart, New York, 2010; Category 6, Vol.
45b, pp 1421−1439. (k) Shiotsuki, M.; Sanda, F.; Masuda, T. Polym.
Chem. 2011, 2, 1044−1058.
(2) (a) Saeed, I.; Shiotsuki, M.; Masuda, T. Macromolecules 2006, 39,
8567−8573. (b) Saeed, I.; Shiotsuki, M.; Masuda, T. Macromolecules
2006, 39, 8977−8981. (c) Onishi, N.; Shiotsuki, M.; Sanda, F.;
Masuda, T. Macromolecules 2009, 42, 4071−4076. (d) Shiotsuki, M.;
Onishi, N.; Sanda, F.; Masuda, T. Chem. Lett. 2010, 39, 244−245.
(e) Shiotsuki, M.; Onishi, N.; Sanda, F.; Masuda, T. Polym. J. 2011, 43,
51−57.
(3) Reviews: (a) Sauer, J. Angew. Chem., Int. Ed. 1967, 6, 16−33.
(b) Sauer, J.; Sustmann, R. Angew. Chem., Int. Ed. 1980, 19, 779−807.
(c) Pindur, U.; Lutz, G.; Otto, C. Chem. Rev. 1993, 93, 741−761.
(4) Yip, C.; Handerson, S.; Tranmer, G. K.; Tam, W. J. Org. Chem.
2001, 66, 276−286.
(5) Miyake, M.; Misumi, Y.; Masuda, T. Macromolecules 2000, 33,
6636−6639.
(15) Cramer, R.; Mccleverty, J. A.; Bray, J. Inorg. Synth. 1974, 15, 14−
18.
(16) Yip, C.; Handerson, S.; Tranmer, G. K.; Tam, W. J. Org. Chem.
2001, 66, 276−286.
(17) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Keith,T.; Kobayashi, R.; Normand, J.;
Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.;
Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.;
Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador,
P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
Revision C.01; Gaussian, Inc., Wallingford, CT, 2010.
(6) (a) Onitsuka, K.; Yamamoto, M.; Mori, T.; Takei, F.; Takahashi,
S. Organometallics 2006, 25, 1270−1278. (b) Shiotsuki, M.;
Kumazawa, S.; Onishi, N.; Sanda, F. J. Polym. Sci., Part A: Polym.
Chem. 2011, 49, 4921−4925. (c) Kumazawa, S.; Rodriguez Castanon,
J.; Onishi, N.; Kuwata, K.; Shiotsuki, M.; Sanda, F. Organometallics
2012, 31, 6834−6842.
(18) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and
Molecules; Oxford University Press: Oxford, U.K., 1989.
(19) (a) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37,
785−789. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652.
(20) (a) Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical
Chemistry; Schaefer, H. F., III, Ed.; Plenum: New York, 1976; Vol. 3,
pp 1−28. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270−
283. (c) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284−298.
(d) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299−310.
(21) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105,
2999−3093.
(7) The 103Rh−15N coupling constants of N-liganded Rh complexes
are less than 21 Hz. See: (a) Bose, K. S.; Abbott, E. H. Inorg. Chem.
1977, 16, 3190−3193. (b) Meij, R.; Stufkens, D. J.; Vrieze, K. J.
Organomet. Chem. 1979, 164, 353−370. (c) Pergola, R. D.;
Cinquantini, A.; Diana, E.; Garlaschelli, L.; Laschi, F.; Luzzini, P.;
Manassero, M.; Repossi, A.; Sansoni, M.; Stanghellini, P. L.; Zanello, P.
Inorg. Chem. 1997, 36, 3761−3771. (d) Rentsch, G. H.; Kozminski,
W.; von Philipsborn, W.; Asaro, F.; Pellizer, G. Magn. Reson. Chem.
1997, 35, 904−907.
(22) Fukui, K. Acc. Chem. Res. 1981, 14, 363−368.
(8) Tabata, M.; Yang, W.; Yokota, K. Polym. J. 1990, 22, 1105−1107.
853
dx.doi.org/10.1021/om301147n | Organometallics 2013, 32, 846−853