RhI-complexes of ditopic bis(pyrazol-1-yl)borate ligands: Assessment of their catalytic activity towards phenylacetylene polymerization

Article abstract of DOI:10.1016/j.jorganchem.2008.09.049

Two dinuclear Rh^I-cyclooctadiene complexes [1,4-(cod)Rh(B(R')pz₂)-C₆H₄-(B(R')pz₂)Rh(cod)], linked by a ditopic scorpionate ligand, have been prepared and fully characterized (R′ = Ph (2), C₆F₅ (2^F); pz = pyrazolide). Both compounds were tested as catalysts for phenylacetylene polymerization but showed no catalytic activity. Attempts at the synthesis of corresponding complexes of the sterically more demanding ligands [1, 4 - (B (R^′) pz₂^Ph) - C₆ H₄ - (B (R^′) pz₂^Ph)]^{2 -} (R′ = Ph (4), C₆F₅ (4^F); pz^Ph = 3-phenylpyrazolide) resulted in B-N bond cleavage and formation of the dinuclear complex [(cod)Rh(μ-pz^Ph)₂Rh(cod)] (5). Complex 5 proved to be an efficient catalyst for the preparation of highly stereoregular head-to-tail cis-transoidal poly(phenylacetylene).

Full text of DOI:10.1016/j.jorganchem.2008.09.049

Journal of Organometallic Chemistry 693 (2008) 3878–3884
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
RhI-complexes of ditopic bis(pyrazol-1-yl)borate ligands: Assessment of their
catalytic activity towards phenylacetylene polymerization
*
Thorsten Morawitz, Songsong Bao, Michael Bolte, Hans-Wolfram Lerner, Matthias Wagner
Institut für Anorganische Chemie, Goethe-Universität Frankfurt, Max-von-Laue-Straße 7, D-60438 Frankfurt (Main), Germany
a r t i c l e i n f o
a b s t r a c t
Two dinuclear RhI-cyclooctadiene complexes [1,4-(cod)Rh(B(R’)pz2)-C6H4-(B(R’)pz2)Rh(cod)], linked by a
ditopic scorpionate ligand, have been prepared and fully characterized (R0 = Ph (2), C6F5 (2F); pz = pyrazo-
lide). Both compounds were tested as catalysts for phenylacetylene polymerization but showed no cata-
lytic activity. Attempts at the synthesis of corresponding complexes of the sterically more demanding
Article history:
Received 19 June 2008
Accepted 29 September 2008
Available online 2 October 2008
2ꢁ
ligands ½1; 4-ðBðR0Þpz2PhÞ-C6H4-ðBðR0Þpz2PhÞꢀ (R0 = Ph (4), C6F5 (4F); pzPh = 3-phenylpyrazolide) resulted
in B–N bond cleavage and formation of the dinuclear complex [(cod)Rh(l
-pzPh)2Rh(cod)] (5). Complex
5 proved to be an efficient catalyst for the preparation of highly stereoregular head-to-tail cis-transoidal
poly(phenylacetylene).
Keywords:
Boron
Ligand design
N-ligands
Poly(phenylacetylene)
Rhodium
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
The catalytic performance of rhodium scorpionates as phenyl-
acetylene polymerization catalysts is peculiar in several respects
For decades, metal-mediated homogeneous catalysis relied al-
most exclusively on mononuclear metal complexes as catalytically
active species [1], even though numerous metalloenzymes are
known in nature for which the added value of a second cooperating
metal ion in the active center is clearly proven [2]. Only in the last
couple of years reports on bimetallic catalysis have been published
which impressively demonstrate that synergistic effects between
metal ions can also be achieved in artificially designed oligonuclear
systems [3–11].
[26]. Firstly, both bis- and tris(pyrazol-1-yl)borate complexes of
Rh(cod) have been successfully employed, even though in the case
of ½ðHBpzR3;RÞRhðcodÞꢀ polymerization is hampered by the fact that
an equilibrium exists [39] between the presumed catalytically ac-
tive j2 form and the j3 form, which is most likely catalytically
inactive. Secondly, the catalytic activity is strongly affected by
the substituents R at the 3-positions of the pyrazolyl groups.
Thirdly, the polymerization proceeds in a highly stereoregular
manner for a variety of different phenylacetylene derivatives to
give macromolecules with a head-to-tail cis-transoid structure.
Given this background, we decided to study whether the incor-
poration of two Rh(cod)-scorpionate complexes into the same cat-
alyst molecule influences the performance of the catalyst and/or
the stereoregularity of the polymerization process.
Our group has a long-standing interest in the development of
oligotopic poly(pyrazol-1-yl)borate (‘‘scorpionate” [12,13]) ligands
[14–20] that are able to bring two metal ions into sufficiently close
proximity so that they can act simultaneously on the same
substrate molecule [21–25]. To identify promising applications
of ditopic scorpionates of the general formula ½1; 4-ðBðR0Þpz2RÞ-
2ꢁ
C6H4-ðBðR0ÞpzR2Þꢀ (pzR: 3-substituted pyrazolide) in homogeneous
2. Results and discussion
catalysis we chose to investigate the Rh-mediated polymerization
of phenylacetylene, because (i) mononuclear complexes
½ðHBðR0ÞpzR2;RÞRhðcodÞꢀ (cod: cyclooctadiene) reveal a high activity
in this reaction [26], (ii) some benefit of two cooperating metal
centers has already been confirmed for homogeneous olefin poly-
merization [27–31], and (iii) stereoregular poly(phenylacetylene)
derivatives possess interesting and useful optoelectronic proper-
ties [32–38].
We selected the dinuclear Rh(cod)-complexes 2 and 2F (Scheme
1) to assess the impact of the electronegativity of the non-coordi-
nating substituent R0 on the stability of the molecular framework
and the reactivity of the RhI-ions. Moreover, we wanted to investi-
gate whether the presence of the second Rh-center leads to cata-
lytic activity even though bulky substituents in the 3-positions of
the pyrazolyl rings are missing. For a direct comparison of the di-
and the mononuclear catalysts it was also necessary to include
complexes of the ditopic ligands 4 and 4F, bearing sterically
demanding 3-phenylpyrazolyl donors, in our study (Schemes 2
and 3).
* Corresponding author. Tel.: +49 69 798 29152; fax: +49 69 798 29260.
E-mail address: Matthias.Wagner@chemie.uni-frankfurt.de (M. Wagner).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.09.049

T. Morawitz et al. / Journal of Organometallic Chemistry 693 (2008) 3878–3884
3879
Ph
2 Li
2 M
N
N
N
N
N
N
R'
R'
Ph
N
N
B
B
B
B
N
N
N
N
N
N
N
Ph
Ph
R'
R'
N
1 : R' = Ph
1F : R' = C6F5
4 : R' = Ph; M = Li
4F : R' = C6F5; M = K
(i) + Rh2Cl2(cod)2
(i) + Rh2Cl2(cod)2
N
N
N
N
Ph
R'
B
N
N
(cod)Rh
B
Rh(cod)
Ph
N
N
N
R'
N
N
Rh(cod)
(cod)Rh
N
5
2 : R' = Ph
2F : R' = C6F5
(ii) + 2 LipzPh
Scheme 1. Synthesis of the dinuclear Rh(cod)-complexes 2 and 2F, (i) 2: r.t., THF;
2F: r.t., toluene.
Rh2Cl2(cod)2
Scheme 3. Degradation of
pyrazolide-bridged complex 5; targeted synthesis of 5, (i) r.t., THF; (ii) r.t., toluene.
4
and 4F by Rh2Cl2(cod)2 to give the dinuclear
Me2N
C6F5
C6F5
B
B
NMe2
A targeted synthesis of complex 5 was achieved by reaction of
Rh2Cl2(cod)2 with 2 equivalents of LipzPh (Scheme 3).
3
The proton NMR spectra of 2 and 2F exhibit only one set of three
signals for all four pyrazolyl rings which points towards highly
symmetric structures in accord with a successful complexation of
both bis(pyrazol-1-yl)borate moieties. This interpretation is further
supported by the integral values of the scorpionate resonances on
one hand and the cyclooctadiene signals on the other (discorpio-
nate:cod = 1:2). The cod-ligands give rise to three multiplets with
integral values of 4H, 4H, and 8H in the aliphatic region of the pro-
ton spectrum together with two resonances (each integrating 4H)
in the typical range of olefinic protons (2: d = 3.68–3.75, 4.01–
4.08; 2F: d = 3.69–3.73, 4.35–4.43). The concomitant 13C NMR res-
onances appear at 30.3, 31.2, 80.9, 81.2 ppm (2) and 31.0, 31.2,
82.0, 82.8 ppm (2F); signals belonging to coordinated olefinic car-
bon atoms are split into doublets due to Rh–C coupling (1JRhC in
the range between 11.9 Hz and 13.0 Hz).
(i) + 2 KpzPh / 2 HpzPh
Ph
Ph
2 K
N
N
C6F5
B
N
N
B
N
N
Ph
Ph
C6F5
N
N
4F
Scheme 2. Synthesis of the sterically demanding ditopic scorpionate ligand 4F, (i)
r.t., toluene.
2.1. Synthesis and spectroscopical characterization
The 11B NMR resonance of the potassium scorpionate 4F appears
at 0.4 ppm, thereby testifying to the presence of tetra-coordinated
boron nuclei [43]. Similar to 2 and 2F, all pyrazolyl rings of 4F are
magnetically equivalent and the 1,4-C6H4 fragment gives rise to
one singlet proton resonance. The proton integral ratios confirm
the presence of four 3-phenylpyrazolyl fragments per 1,4-pheny-
lene linker. Three multiplets at ꢁ163.5, ꢁ157.3, and ꢁ131.5 ppm
appear in the 19F NMR spectrum of 4F, which is the typical signal
pattern of C6F5 substituents in related scorpionates (cf. the 19F
NMR spectrum of 1F: d = ꢁ167.9, ꢁ163.3, ꢁ135.8 [20]).
2 and 2F are available in high yield from the reaction of the
bis(pyrazol-1-yl)borates 1 [40] and 1F [20] with Rh2Cl2(cod)2
(Scheme 1).
The 3-phenylpyrazolyl-substituted scorpionate 4 is already
known in the literature [24]. Its partially fluorinated congener 4F
was synthesized similar to 4 employing the aminoborane 3 [20]
and 2 equivalents of a 1:1 mixture of KpzPh and HpzPh (Scheme 2).
Further treatment of 4 and 4F with Rh2Cl2(cod)2 in THF, how-
ever, did not give the corresponding RhI-complexes but rather
led to a multiplicity of products (NMR spectroscopical control) of
which the doubly pyrazolide-bridged complex 5 could be isolated
and structurally characterized (Scheme 3). These results are
remarkable in the light of the successful synthesis of 2 and 2F
The integral ratio of the 3-phenylpyrazolide rings and the cod-
ligands in complex 5 is 1:1. In contrast to 2 and 2F, we find 8 cyclo-
octadiene resonances both in the 1H and in the 13C NMR spectrum
of 5, which is in agreement with an average C2-symmetric struc-
ture similar to the configuration of 5 that has been established
by X-ray crystallography (see Fig. 3). However, it has to be men-
tioned that the NMR data in THF-d8 would also be in accord with
a monomeric solution structure of the form (thf)(pzPh)Rh(cod).
For this reason, we have also recorded the 1H NMR spectrum of 5
in the non-coordinating solvent CDCl3 and found an identical sig-
nal pattern, which indicates that the dimeric structure of 5 remains
intact in solution. Moreover, the mass spectrum of 5 revealed a
and
given
the
background
that
the
compound
½ðHBpzP3h;MeÞRhðcodÞꢀ, in which a scorpionate ligand with 3-phe-
nyl-5-methylpyrazolyl substituents coordinates to the RhI-ion in
a j
2-mode, is also readily accessible [41]. We attribute our failure
to prepare RhI-complexes of 4 and 4F to the facts that bis(pyrazol-
1-yl)borate ligands tend to have a lower stability than tris(pyrazol-
1-yl)borates [40] and that sterically encumbered poly(pyrazol-1-
yl)borates are generally more prone to B–N bond cleavage than less
bulky derivatives [13,42].

3880
T. Morawitz et al. / Journal of Organometallic Chemistry 693 (2008) 3878–3884
peak at m/z = 708, corresponding to the molecular mass of the
dinuclear species [5]+.
ideal value of 109.5° is found for C(31)–B(1)–C(41), which is wid-
ened to 114.5(3)°.
Single crystals of the partially fluorinated complex 2F (incorpo-
rating 2 equivalents of CH2Cl2; 2F ꢂ 2 CH2Cl2) were grown from
CH2Cl2/hexane at r.t. (Fig. 2, Table 1). Different from 2 ꢂ 2 THF,
2F ꢂ 2 CH2Cl2 adopts a conformation with a mirror plane that coin-
cides with the 1,4-phenylene spacer. Moreover, the two C6F5 sub-
stituents point into the same direction, while the Ph-rings of 2 ꢂ 2
THF are in a trans-arrangement with respect to the B–B-axis. Com-
pared to 2 ꢂ 2 THF, there is no significant effect of the electron-
withdrawing C6F5 fragments on the B–N, B–C, Rh–N, or Rh–C
bonds. The B–C6F5 bonds (1.657(4) Å, 1.662(4) Å), however, are
somewhat longer than the B–Ph bonds in 2 ꢂ 2 THF (1.618(6) Å).
Moreover, the intramolecular distances between the Rh-centers
and the ipso-carbon atoms of the aryl substituents are significantly
shorter in 2F ꢂ 2 CH2Cl2 (Rh(1)ꢂ ꢂ ꢂC(51) = 3.290 Å; Rh(2)ꢂ ꢂ ꢂC(41) =
3.169 Å) than in 2 ꢂ 2 THF (Rh(1)ꢂ ꢂ ꢂC(41) = 3.377 Å). This feature
is likely attributable to a weak interaction between the electron-
2.2. X-ray crystal structure determinations
The RhI-complex 2 crystallizes from THF with two equivalents
of non-coordinating THF molecules (2 ꢂ 2 THF; Fig. 1, Table 1). In
the solid state, the molecule possesses a center of inversion located
at the midpoint of the 1,4-phenylene bridge. The two Rh–N bond
lengths of 2.091(3) Å and 2.103(4) Å, as well as the distances be-
tween the Rh-ions and the centroids (COGs) of the olefinic cyclo-
octadiene bonds (2.021 Å, 2.028 Å) compare well to the
corresponding bond lengths in the mononuclear scorpionate com-
plex [(pzBpz3)RhI(cod)] [44]. The most important bond angles
about the Rh-ion in 2 ꢂ 2 THF are N(12)–Rh(1)–N(22) = 88.7(1)°
and COG[C(1)@C(2)]–Rh(1)–COG[C(5)@C(6)] = 87.7°. With respect
to the scorpionate ligand, we observe all B–N and B–C bonds in
the same range as in the lithium derivative 1 [40]. The most pro-
nounced deviation of a X-B(1)–Y bond angle in 2 ꢂ 2 THF from the
poor C6F5- rings and the charge density localized in the RhI-d2
z
orbital.
Fig. 1. Structure of 2 ꢂ 2 THF in the crystal. Displacement ellipsoids are drawn at the 30% probability level. H atoms and the THF molecules are omitted for clarity. Selected
bond lengths (Å), bond angles (°): B(1)–N(11) = 1.573(6), B(1)–N(21) = 1.573(6), B(1)–C(31) = 1.625(6), B(1)–C(41) = 1.618(6), Rh(1)–N(12) = 2.091(3), Rh(1)–
N(22) = 2.103(4), Rh(1)–COG[C(1) = C(2)] = 2.028, Rh(1)–COG[C(5) = C(6)] = 2.021; N(11)–B(1)–N(21) = 107.3(3), C(31)–B(1)–C(41) = 114.5(3), N(12)–Rh(1)–N(22) = 88.7(1),
COG[C(1) = C(2)]–Rh(1)– COG[C(5) = C(6)] = 87.7.
Fig. 2. Structure of 2F ꢂ 2 CH2Cl2 in the crystal. Displacement ellipsoids are drawn at the 30% probability level. H atoms and the CH2Cl2 molecules are omitted for clarity.
Selected bond lengths (Å), bond angles (°): B(1)–N(11) = 1.570(2), B(1)–C(31) = 1.620(4), B(1)–C(51) = 1.662(4), B(2)–N(21) = 1.568(3), B(2)–C(34) = 1.625(4), B(2)–
C(41) = 1.657(4), Rh(1)–N(12) = 2.098(2), Rh(1)–COG[C(1) = C(2)] = 2.034, Rh(2)–N(22) = 2.097(2), Rh(2)–COG[C(5) = C(6)] = 2.033; N(11)–B(1)–N(11#) = 105.9(2), N(21)–
B(2)–N(21#) = 105.4(2), N(12)–Rh(1)–N(12#) = 87.6(1), N(22)–Rh(2)–N(22#) = 86.6(1). #Symmetry transformation used to generate equivalent atoms: x, ꢁy + 1, z.

T. Morawitz et al. / Journal of Organometallic Chemistry 693 (2008) 3878–3884
3881
Fig. 3. Structure of 5A in the crystal. Displacement ellipsoids are drawn at the 50% probability level. H atoms are omitted for clarity. Selected bond lengths (Å), bond angles (°),
and torsion angles (°): Rh(1)–N(11) = 2.085(3), Rh(1)–N(32) = 2.116(3), Rh(1)–COG[C(1) = C(2)] = 2.041, Rh(1)–COG[C(5) = C(6)] = 2.021, Rh(2)–N(12) = 2.093(3), Rh(2)–
N(31) = 2.079(3), Rh(2)–COG[C(51) = C(52)] = 2.036, Rh(2)–COG[C(55) = C(56)] = 2.018, Rh(1)–Rh(2) = 3.207(1); N(11)–Rh(1)–N(32) = 87.4(1), N(12)–Rh(2)–N(31) = 87.3(1);
Rh(1)–N(11)–N(12)–Rh(2) = ꢁ17.5(3), Rh(1)–N(32)–N(31)–Rh(2) = ꢁ17.2(3).
Table 1
Two pseudopolymorphs of 5 (5 ꢂ THF/5) crystallized from the
reaction mixtures of 4/4F and Rh2Cl2(cod)2, even though the crys-
tallization conditions were rather similar (binary solvent mixture
of THF and hexane; r.t.). The asymmetric unit of 5 contains two
crystallographically independent molecules 5A and 5B. Since the
key structural parameters of 5 ꢂ THF, 5A and 5B are the same within
the experimental error margins, only 5A is discussed here. The
molecular framework of 5A consists of two RhI(cod)-fragments
bridged by two 3-phenylpyrazolide ligands in a head-to-tail
arrangement (Fig. 3, Table 1).
Selected crystallographic data of 2 ꢂ 2 THF, 2F ꢂ 2 CH2Cl2, and 5
2 ꢂ 2 THF
2F ꢂ 2 CH2Cl2
5
Formula
C46H50B2N8Rh2ꢂ2
C
46H40B2F10N8Rh2ꢂ2 C34H38N4Rh2
C4H8O
CH2Cl2
Formula weight
Color, shape
Temperature (K)
Crystal system
Space group
a (Å)
1086.59
1292.15
708.50
Orange, block
173(2)
Triclinic
P1
13.3910(6)
14.4836(6)
15.3410(7)
89.883(4)
87.149(4)
87.225(3)
2968.2(2)
4
Light yellow, plate Light yellow, block
173(2)
Monoclinic
P21/c
173(2)
Monoclinic
C2/m
ꢀ
13.8489(15)
9.1972(10)
20.676(3)
90
18.6778(7)
15.3032(7)
18.4775(7)
90
b (Å)
c (Å)
Both RhI-ions are located in a square-planar ligand environment
with Rh–N bond lengths ranging from 2.079(3) Å to 2.116(3) Å. The
longest bonds are observed for Rh(1)–N(32) = 2.116(3) Å and
Rh(2)–N(12) = 2.093(3) Å, which suffer most from the steric repul-
sion between the cod-ligands and the phenyl rings (the unsubsti-
a
(°)
b (°)
103.502(9)
90
106.265(3)
90
c
(°)
V (Å3)
2560.7(5)
2
5070.0(4)
4
Z
tuted parent compound [(cod)Rh(l-pz)2Rh(cod)] features Rh–N
Dcalcd. (g cmꢁ3
F(000)
)
1.409
1.693
1.585
1440
1.142
bond lengths of 2.074(8) Å [45]). The Rh–COG(olefin) distances fall
in the interval between 2.018 Å and 2.041 Å. Again, these differ-
ences are attributable to different degrees of steric congestion.
All in all, no significant differences between the bond lengths
and angles about RhI are evident in 5A as compared to 2 or 2F. How-
ever, we note that the intramolecular Rh–Rh distance of 3.207(1) Å
1124
2584
l
(mmꢁ1
)
0.693
0.943
Crystal size (mm)
Reflections
collected
Independent
reflections
0.18 ꢃ 0.15 ꢃ 0.09 0.52 ꢃ 0.48 ꢃ 0.46
0.51 ꢃ 0.48 ꢃ 0.42
29512
34177
56499
4404(0.0862)
4853(0.0376)
13610(0.0709)
[R(int)
Data/restraints/
params
]
in 5A is are slightly shorter than in [(cod)Rh(l-pz)2Rh(cod)]
(3.267(2) Å [45]).
4404/0/308
1.020
4853/7/387
1.079
13610/0/721
1.157
Goodness-of-fit
(GOF) on F2
2.3. Polymerization reactions
R1, wR2 (I > 2
r
(I))
0.0426, 0.0990
0.0625, 0.1074
0.614, ꢁ0.631
0.0278, 0.0720
0.0281, 0.0722
0.491, ꢁ0.436
0.0440, 0.1084
0.0461, 0.1096
1.320, ꢁ1.654
R1, wR2 (all data)
Largest difference
peak and hole
In our initial attempts, we added small quantities of the dinu-
clear complexes 2 and 2F to solutions of phenylacetylene in CH2Cl2.
None of these experiments led to the formation of poly(phenyl-
acetylene) (PPA).
(e Åꢁ3
)
We then employed the crude products of the reactions of 4 and
4F with Rh2Cl2(cod)2 as phenylacetylene polymerization catalysts.
After 24 h at r.t., PPA was obtained in yields exceeding 90%, which
is comparable to the results that have been reported for the bis-
½ðHBpzR3;RÞRhðcodÞꢀ with sterically demanding substituents R in
the 3-positions of the pyrazolyl rings [26]. Our polymeric samples
gave well-resolved 1H NMR spectra with resonances at 5.84 ppm
and tris(pyrazol-1-yl)borate complexes ½ðH2BpzR;RÞRhðcodÞꢀ and
2

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T. Morawitz et al. / Journal of Organometallic Chemistry 693 (2008) 3878–3884
(@CH) and 6.62–6.65/6.92–6.96 ppm (Ph), in excellent agreement
with published data for a stereoregular head-to-tail cis-transoidal
structure [26]. With respect to the elucidation of the stereostruc-
ture of our PPA sample, the vinyl signal at 5.84 ppm is particularly
revealing, because it is characteristic to the absorption of a cis pro-
ton, whereas the vinyl proton in trans-poly(phenylacetylene) ab-
sorbs at 6.78 ppm [34].
After we had identified complex 5 as one major constituent of
the product mixture obtained from 4/4F and Rh2Cl2(cod)2, we also
examined this compound with regard to catalytic activity. Again,
PPA formed in almost quantitative yield after a 24 h reaction time
(CH2Cl2, r.t.), and its NMR spectra indicated a very high degree of
stereoregularity (head-to-tail, cis-transoidal). A preliminary molec-
ular weight determination using gel permeation chromatography
gave values of Mw = 100000 g molꢁ1 and Mw/Mn = 4.6. It is reveal-
ing to put the results obtained with 5 in context with a previous
report by Ardizzoia on the reactivity of the related RhI-complex
sieves column prior to use. All other solvents were freshly distilled
under argon from Na/benzophenone. NMR: Bruker AMX 250, AMX
300, AMX 400, Bruker DPX 250. 1H and 13C NMR shifts are reported
relative to tetramethylsilane and were referenced against residual
solvent peaks. 11B and 19F NMR shifts are reported relative to exter-
nal BF3 ꢂ Et2O and CFCl3, respectively. Abbreviations: s = singlet,
d = doublet, tr = triplet, vtr = virtual triplet, m = multiplet,
n. o. = signal not observed, i = ipso, o = ortho, m = meta, p = para,
pz = pyrazolide, pzPh = 3-phenylpyrazolide. All NMR spectra were
run at room temperature. Elemental analyses were performed by
the microanalytical laboratory of the Goethe-University Frankfurt,
Germany. Compounds 1 [40], 1F [20], 3 [20], and 4 [24] were syn-
thesized according to published procedures.
4.2. Synthesis of 2
Rh2Cl2(cod)2 (0.148 g, 0.30 mmol) in THF (20 mL) was added to
a solution of 1 (0.160 g, 0.30 mmol) in THF (20 mL). The clear yel-
low mixture was stirred at r.t. for 12 h. All volatiles were removed
under reduced pressure and the product was extracted into CH2Cl2
(2 ꢃ 40 mL). The extract was evaporated and the solid residue
dried in vacuo. Yield of 2 ꢂ CH2Cl2: 0.218 g (71%). Single crystals
of 2 ꢂ 2 THF suitable for X-ray analysis were grown by storing a sat-
[Ph3P(C2H4)Rh(l-dcmpz)2Rh(C2H4)PPh3]
(Hdcmpz = 3,5-dica-
rbomethoxypyrazole) towards phenylacetylene and acetylene
[46]. With phenylacetylene as substrate, trimerization reactions
to 1,2,4-triphenylbenzene (3%) and 1,3,5-triphenylbenzene (14%)
took place, together with the formation of head-to-head dimers
(Ph–CC–CH@CH–Ph; 52%) and head-to-tail dimers (Ph–CC–
C(Ph)@CH2; 31%). Parent acetylene, however, was polymerized to
give a cis/trans polyacetylene mixture.
urated THF solution at r.t. 11B NMR (96.3 MHz, CDCl3): 2.2 (h1/2
=
520 Hz). 1H NMR (300.0 MHz, CDCl3): 1.47–1.53 (m, 4H, cod-
CH2), 1.69–1.73 (m, 8H, cod-CH2), 2.22–2.30 (m 4H, cod-CH2),
3.68–3.75, 4.01–4.08 (2 ꢃ m, 2 ꢃ 4H, cod-CH), 6.15 (vtr, 4H, pzH-
4), 6.74–6.79 (m, 4H, Ph), 6.98 (s, 4H, C6H4), 7.23–7.28 (m, 6H,
3. Conclusion
3
Ph), 7.42, 7.48 (2 ꢃ d, 2 ꢃ 4H, 2 ꢃ JHH = 2.1 Hz, pzH-3,5). 13C
The purpose of this paper was to evaluate the poten0tial of
dinuclear RhI-cyclooctadiene complexes ½1; 4-ðcodÞRhðBðR ÞpzR2Þ-
C6H4-ðBðR0ÞpzR2ÞRhðcodÞꢀ as catalysts for the preparation of
poly(phenylacetylene) (PPA; R0 = Ph, C6F5; pzR = pyrazolide, 3-
phenylpyrazolide). To this end, we first tested the derivatives 2
(R0 = Ph; R = H) and 2F (R0 = C6F5; R = H) which, however, showed
no catalytic activity at all. Subsequent attempts at the synthesis
of corresponding complexes of the sterically more hindered ligands
NMR (75.5 MHz, THF-d8): 30.3, 31.2 (cod-CH2), 80.9, 81.2 (2 ꢃ d,
1JRhC = 12.3 Hz, 12.9 Hz, cod-CH), 104.7 (pzC-4), 126.7, 127.7,
134.4 (Ph), 135.3 (C6H4), 138.1, 140.2 (pzC-3,5), n.o. (CB). Elemen-
tal Anal. Calc. for C46H50B2N8Rh2 [942.37] ꢃ CH2Cl2 [84.93]: C,
54.95; H, 5.10; N, 10.91. Found: C, 54.96; H, 5.43; N, 11.15% (the
relative amount of CH2Cl2 present in the sample was confirmed
by 1H NMR spectroscopy).
2ꢁ
½1; 4-ðBðR0Þpz2PhÞ-C6H4-ðBðR0Þpz2PhÞꢀ (R0 = Ph (4), C6F5 (4F)) resulted
4.3. Synthesis of 2F
in the formation of mixtures of several products. These crude mix-
tures performed very well in the polymerization of phenylacety-
lene and provided highly stereoregular head-to-tail cis-transoidal
PPA. A closer investigation of the catalytically active samples re-
vealed that they contained the 3-phenylpyrazolide-bridged dinu-
Neat Rh2Cl2(cod)2 (0.237 g, 0.48 mmol) was added to a solution
of 1F (0.344 g, 0.48 mmol) in toluene (50 mL). The resulting clear
yellow solution was stirred at r.t. for 12 h, whereupon a colorless
solid precipitated. The precipitate was removed by filtration and
the filtrate evaporated to dryness. The solid residue was extracted
into CH2Cl2 (2 ꢃ 80 mL) and dried in vacuo. Yield of 2F ꢂ 0.5 CH2Cl2:
0.399 g (71%). X-ray quality crystals of 2F ꢂ 2 CH2Cl2 were obtained
by gas-phase diffusion of hexane into a saturated CH2Cl2 solution.
11B NMR (96.3 MHz, THF-d8): 0.1 (h1/2 = 360 Hz). 1H NMR
(300.0 MHz, THF-d8): 1.76–1.78 (m, 8H, cod-CH2), 1.84–1.93,
2.47–2.56 (2 ꢃ m, 2 ꢃ 4H, cod-CH2), 3.69–3.73, 4.35–4.43 (2 ꢃ m,
2 ꢃ 4H, cod-CH), 6.17 (vtr, 4H, pzH-4), 7.29 (s, 4H, C6H4), 7.48,
clear species [(cod)Rh(l
-pzPh)2Rh(cod)] (5). The formation of 5
requires B-N bond cleavage in 4 and 4F. This degradation pathway
can obviously not be suppressed by increasing the electronegativ-
ity of the substituent R0 (i.e. Ph versus C6F5), but depends to a great
extent on the steric bulk of the pyrazolyl rings (i.e. pz versus pzPh
)
as evidenced by the successful formation of 2 and 2F.
An authentic sample of 5, prepared from Rh2Cl2(cod)2 and
LipzPh, showed essentially the same catalytic behavior as the heter-
ogeneous mixture of Rh-compounds described above. This leads to
the conclusion that easily accessible dinuclear complexes of type 5
have promising potential for the homogeneous polymerization of
phenylacetylene. We are currently studying the influence of the
substitution pattern of the pyrazolide bridges on the polymeriza-
tion kinetics as well as on the structure and the molecular weight
distribution of the produced poly(phenylacetylene).
3
7.55 (2 ꢃ d, 2 ꢃ 4H, JHH = 2.4 Hz, 2.0 Hz, pzH-3,5). 13C NMR
(62.9 MHz, THF-d8): 31.0, 31.2 (cod-CH2), 82.0, 82.8 (2 ꢃ d,
1JRhC = 11.9 Hz, 13.0 Hz, cod-CH), 105.6 (pzC-4), 135.6 (C6H4),
138.0, 141.0 (pzC-3,5), n.o. (CB, CF). 19F NMR (282.3 MHz, THF-
d8): ꢁ166.9 (m, 4F, F-m), ꢁ161.3 (m, 2F, F-p), ꢁ137.3 (m, 4F, F-o).
Elemental Anal. Calc. for C46H40B2F10N8Rh2 [1122.24] ꢃ 0.5 CH2Cl2
[84.93]: C, 47.95; H, 3.55; N, 9.62. Found: C, 48.28; H, 3.84; N,
9.50% (the relative amount of CH2Cl2 present in the sample was
confirmed by 1H NMR spectroscopy).
4. Experimental
4.4. Synthesis of 4F
4.1. General considerations
All reactions and manipulations of air-sensitive compounds
were carried out in dry, oxygen-free argon using standard Schlenk
ware. CH2Cl2 and CDCl3 were passed through a 4 Å molecular
A solution of 3 (0.87 g, 1.68 mmol) in toluene (30 mL) was
added to a suspension of KpzPh (0.61 g, 3.36 mmol) and HpzPh
(0.48 g, 3.36 mmol) in toluene (20 mL) and stirred at r.t. for 10 h.

T. Morawitz et al. / Journal of Organometallic Chemistry 693 (2008) 3878–3884
3883
The solvent was removed in vacuo and the resulting colorless res-
idue was washed with hexane (50 mL) and dried in vacuo. Yield:
1.28 g (70%). For further purification, the entire product was dis-
solved in a small amount of THF (5 mL) and precipitated into hex-
ane (30 mL). 11B NMR (96.3 MHz, C6D6): 0.4 (h1/2 = 590 Hz). 1H
polymer as a yellow powder, which was washed with hexane
(2 ꢃ 20 mL) and dried in vacuo. Yield: 0.145 g (92%). 1H NMR
(250.1 MHz, CDCl3): 5.84 (s, 1H, @CH), 6.62–6.65 (m, 2H, Ph-o),
6.92–6.96 (m, 3H, Ph-m/p). 13C NMR (75.5 MHz, CDCl3): 126.7
(Ph-p), 127.6, 127.8 (Ph-o,m), 131.8 (@CH), 139.3 (Ph-i), 142.9
~
3
NMR (300.0 MHz, C6D6): 6.39 (d, 4H, JHH = 2.2 Hz, pzH-4), 7.00–
(PhC@). IR(KBr, cmꢁ1):
m
¼ 3053 (s), 1596 (s), 1489 (s), 1444 (s),
7.05 (m, 12H, Ph), 7.40 (s, 4H, C6H4), 7.44–7.48 (m, 8H, Ph), 7.74
1073 (s), 1028 (s), 882 (br), 753 (shoulder), 737 (s), 695 (s).
3
(d, 4H, JHH = 2.2 Hz, pzH-5). 13C NMR (62.9 MHz, C6D6): 102.8
(pzC-4), 126.9, 128.5, 129.0 (Ph), 134.0 (C6H4), 135.5 (Ph-i), 138.3
(pzC-5), 153.9 (pzC-3), n. o. (CB, CF). 19F NMR (282.3 MHz, C6D6):
ꢁ163.5 (m, 4F, F-m), ꢁ157.3 (m, 2F, F-p), ꢁ131.5 (br, 4F, F-o). Ele-
mental Anal. Calc. for C54H32B2F10K2N8 [1082.63]: C, 59.91; H, 2.98;
N, 10.35. Found: a decent elemental analysis was not obtained as a
result of varying amounts of coordinated THF and H2O. However,
according to NMR spectroscopy, the ligand was pure and could
thus be used for further complexation studies.
4.9. Polymerization of phenylacetylene II
The reaction protocol was the same as above and the polymer
obtained showed identical NMR and IR spectra. Amounts em-
ployed:
5
(0.040 g, 0.056 mmol), phenylacetylene (0.577 g,
5.65 mmol), CH2Cl2 (2 mL). Yield: 0.520 g (90%).
4.10. X-ray crystallography
4.5. Reaction of 4 with Rh2Cl2(cod)2
Data collections were performed on a Stoe IPDS-II two-circle
diffractometer with graphite-monochromated Mo K
a radiation.
Rh2Cl2(cod)2 (0.094 g, 0.19 mmol) in THF (15 mL) was added to
a solution of 4 (0.159 g, 0.19 mmol) in THF (15 mL). After the clear
yellow solution had been stirred at r.t. for 18 h, all volatiles were
removed under reduced pressure. The 1H NMR spectrum of the so-
lid residue showed a complex mixture of products. Few orange sin-
gle crystals of 5 ꢂ THF were grown by gas-phase diffusion of hexane
into a saturated THF solution of the product mixture.
Empirical absorption corrections with the MULABS option [47] in
the program PLATON [48] were performed. Equivalent reflections
were averaged. The structures were solved by direct methods
[49] and refined with full-matrix least-squares on F2 using the pro-
gram SHELXL-97 [50]. Hydrogen atoms were placed on ideal posi-
tions and refined with fixed isotropic displacement parameters
using a riding model.
Compound 2 crystallizes together with two equivalents of non-
coordinating THF (2 ꢂ 2 THF). Compound 2F crystallizes together
with two equivalents of CH2Cl2 (2F ꢂ 2 CH2Cl2). The solvent mole-
cules are disordered over two positions with occupancy factors of
0.53(1) and 0.47(1). In the disordered CH2Cl2 molecules, all C–Cl
bond lengths on one hand as well as all Clꢂ ꢂ ꢂCl distances on the
other were restrained to have the same values with an effective
esd of 0.02 Å. Compound 5 contains two crystallographically inde-
pendent molecules in the asymmetric unit (5A, 5B). Although 5 is
an achiral compound, 5 ꢂ THF crystallizes in a chiral space group
and crystals of both enantiomorphous structures have been inves-
tigated, i.e. 5ꢂTHF-P65 and 5 ꢂ THF-P61. The absolute structures
have been confirmed by the Flack-x-parameter (ꢁ0.07(4) for
5 ꢂ THF-P65; ꢁ0.06(3) for 5 ꢂ THF-P61).
4.6. Reaction of 4F with Rh2Cl2(cod)2
The reaction was performed similar to the case of 4 and
Rh2Cl2(cod)2 using Rh2Cl2(cod)2 (0.039 g, 0.08 mmol) and 4F
(0.086 g, 0.08 mmol). Few orange single crystals of 5 were grown
by gas-phase diffusion of hexane into a saturated THF solution of
the product mixture.
4.7. Synthesis of 5
Toluene (20 mL) was added to a mixture of neat LipzPh (0.066 g,
0.44 mmol) and neat Rh2Cl2(cod)2 (0.108 g, 0.22 mmol) at r.t. The
resulting pale orange solution was stirred for 18 h. After the sol-
vent had been removed in vacuo the product was extracted into
C6H6 (40 mL). After filtration, the filtrate was evaporated to dry-
ness. Yield: 0.145 g (93%). Orange crystals of 5 were grown from
a saturated THF solution at r.t. by slow evaporation of the solvent.
1H NMR (400.1 MHz, THF-d8): 1.51–1.60 (m, 2H, cod-CH2), 1.85–
2.00 (m, 4H, cod-CH2), 2.01–2.13 (m, 4H, cod-CH2), 2.66–2.76 (m,
4H, cod-CH2), 2.89–2.99 (m, 2H, cod-CH2), 3.40–3.44 (m, 2H, cod-
CH), 4.05–4.10 (m, 4H, cod-CH), 4.70–4.74 (m, 2H, cod-CH), 6.23
Acknowledgments
M.W. is grateful to the ‘‘Deutsche Forschungsgemeinschaft”
(DFG) and the ‘‘Fonds der Chemischen Industrie” (FCI) for financial
support. T.M. wishes to thank the ‘‘Fonds der Chemischen Indust-
rie” (FCI) for a Ph.D. grant.
3
3
(d, 2H, JHH = 2.0 Hz, pzH-4), 7.28 (tr, 2H, JHH = 7.8 Hz, Ph-p), 7.43
Appendix A. Supplementary material
3
(vtr, 4H, Ph-m), 7.49 (d, 2H, JHH = 2.0 Hz, pzH-5), 8.29 (d, 4H,
3JHH = 7.4 Hz, Ph-o). 13C NMR (100.6 MHz, THF-d8): 30.1, 31.6,
CCDC 691989, 691990, 691991, 691992 and 692179 contain the
supplementary crystallographic data for 2 ꢂ 2 THF, 2F ꢂ 2 CH2Cl2, 5,
5 ꢂ THF-P65 and 5ꢂTHF-P61. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.jorganchem.2008.09.049.
1
32.7, 32.9 (cod-CH2), 78.0, 82.5, 83.2, 83.8 (4 ꢃ d, JRhC = 11.7 Hz,
11.9 Hz, 13.6 Hz, 11.4 Hz, cod-CH), 104.7 (pzC-4), 127.8 (Ph-p),
128.1 (Ph-o), 128.8 (Ph-m), 136.0 (Ph-i), 138.6 (pzC-5), 152.5
(pzC-3). ESI-MS: m/z = 708 [5]+. Elemental Anal. Calc. C34H38N4Rh2
[708.50]: C, 57.64; H, 5.41; N, 7.91. Found: C, 57.79; H, 5.43; N,
8.00%.
4.8. Polymerization of phenylacetylene I
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via syringe, whereupon the reaction mixture immediately turned
red. The mixture was stirred for 24 h to complete polymerization.
MeOH (30 mL) was added to the viscous liquid to precipitate the
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