Chelation-assisted carbon-halogen bond activation by a rhodium(I) complex

Article abstract of DOI:10.1021/ic801878e

Chelation-assisted activation of C-X bonds (X = Cl, Br, and I) took place in the reaction of [Rh(PPh₃)₂(acetone)₂] ⁺ and 2-(2-halophenyl)pyridine or 10-halobenzo[h]quinoline. A series of 16-electron five-coordinate cationic rhodium(III) monohalide complexes was synthesized at room temperature. Neutral octahedral rhodium(III) dihalide complexes were obtained when the corresponding cationic monohalide complexes were further heated in acetone. Rhodium and iridium diiodides with these cyclometalation motifs could be alternatively synthesized from the reaction of the corresponding cyclometalated hydride complexes and I₂. X-ray crystal structures of representative monohalide and dihalide complexes are reported. Octahedral rhodium(III) dihalide complexes are active catalysts for the dimerization of terminal alkynes, while 16-electron cationic rhodium(III) monohalides are inactive.

Full text of DOI:10.1021/ic801878e

Inorg. Chem. 2009, 48, 1198-1206
Chelation-Assisted Carbon-Halogen Bond Activation by a Rhodium(I)
Complex
Shanshan Chen,‡ Yongxin Li,‡ Jing Zhao,*,† and Xingwei Li*,§,‡
State Key Laboratory of Pharmaceutical Biotechnology, School of Life Sciences, Nanjing
UniVersity, Nanjing 210093, China, and DiVision of Chemistry and Biological Chemistry, School
of Physical and Mathematical Sciences, Nanyang Technological UniVersity, Singapore 637371
Received October 2, 2008
Chelation-assisted activation of C-X bonds (X ) Cl, Br, and I) took place in the reaction of [Rh(PPh3)2(acetone)2]+
and 2-(2-halophenyl)pyridine or 10-halobenzo[h]quinoline. A series of 16-electron five-coordinate cationic rhodium(III)
monohalide complexes was synthesized at room temperature. Neutral octahedral rhodium(III) dihalide complexes
were obtained when the corresponding cationic monohalide complexes were further heated in acetone. Rhodium
and iridium diiodides with these cyclometalation motifs could be alternatively synthesized from the reaction of the
corresponding cyclometalated hydride complexes and I2. X-ray crystal structures of representative monohalide and
dihalide complexes are reported. Octahedral rhodium(III) dihalide complexes are active catalysts for the dimerization
of terminal alkynes, while 16-electron cationic rhodium(III) monohalides are inactive.
Introduction
sociation energy.4,5 As a contrast to the tremendously ample
examples of carbon-halogen oxidative addition to Pd(0),
Caryl-halogen oxidative addition is less common for Co(I),
Rh(I), and Ir(I) complexes.6-16 The generation of such group
Haloarenes are ubiquitously used as organic substrates in
catalysis. The interactions between aryl halides and low-valent
transition metals often entail carbon-halide bond oxidative
addition, which is a key step in transition-metal-catalyzed cross-
coupling reactions involving haloarenes.1 Oxidative addition
reactions of aryl halides have been studied mostly for group
10 metals, particularly for Pd(0).2,3 Catalytic functionalization
of Caryl-Cl bonds has also been achieved in cross-coupling
reactions, although Caryl-Cl bonds have relatively high dis-
(4) Csok, Z.; Vechorkin, O.; Harkins, S. B.; Scopelliti, R.; Hu, X. J. Am.
Chem. Soc. 2008, 130, 8156.
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10066.
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2004, 23, 269.
* Authors to whom correspondence should be addressed. E-mail:
jingzhao@nju.edu.cn J.Z.), xli@scripps.edu (X.L.).
†
Nanjing University.
Nanyang Technological University.
Current address: The Scripps Research Institute in Florida, Jupiter,
‡
§
Florida 33458.
(1) Reviews: (a) Suzuki, A. Chem. Commun. 2005, 4759. (b) Hartwig,
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1198 Inorganic Chemistry, Vol. 48, No. 3, 2009
10.1021/ic801878e CCC: $40.75  2009 American Chemical Society
Published on Web 12/23/2008

Chelation-Assisted Carbon-Halide Bond ActiWation
Scheme 1. Synthesis of Five-Coordinate Rh(III) Halide Complexes
9 metal aryl species followed by further functionalization entails
synthetically important rhodium-catalyzed C-C coupling reac-
tions,17 which is typically achievable with Pd(0) complexes.
Recent examples of Caryl-Cl oxidative addition for rhodium
and iridium are almost limited to (PNP)M,7 [Ir(CO)(PPh3)2]+,8
(Phebox)Rh(H2O)Cl2,9 RhCl3(py)3,10 Rh(PCy3)2(H)(Cl)2,11 Rh-
(diiminate)(COD),12 Rh(PPh3)3F,13 [Rh(coe)2Cl]2,14 (NNN)-
RhX,15 cyclometalated phosphinimine rhodium(I) complexes,16
and a low-coordinate Rh(I) phosphine system.18
chemistry,7,23 the addition of a hydrogen acceptor such as
tert-butylethylene (TBE) to this rhodium(III) dihydride
should reversibly strip the hydride ligands and afford the
proposed [Rh(PPh3)2(acetone)2]+ complex in situ, although
an alternative access to this complex is also known.19 Here,
only [Rh(PPh3)2(acetone)2]+ generated in situ was utilized
throughout this work.
Attempts to activate the C-X bonds in iodobenzene or
2-bromopyridine by reacting it with [Rh(H)2(PPh3)2-
(acetone)2]PF6 and TBE in acetone resulted in a mixture of
unidentifiable species. Thus, we focused on C-X activation
with chelation assistance. Stirring a mixture of 10-chlo-
robenzo[h]quinoline (1A-Cl), TBE (20 equiv), and
[Rh(H)2(PPh3)2(acetone)2]PF6 in acetone slowly led to a
yellow precipitate (2A-Cl) at ambient temperature (Scheme
1). The resulting complex 2A-Cl was isolated in 89% yield
and was characterized by 1H, 13C, and 31P NMR spectroscopy
Weller and co-workers18 have recently reported intermo-
lecular Caryl-Cl activation that took place on [Rh(PiBu3)2L2]-
F
BAr4 (L ) weakly coordinating ligands or solvent mol-
ecules), which is generated in situ from the 14-electron
[Rh(H)2(PiBu3)2]BAr4F precursor. The high reactivity of this
system likely results from the electron-rich character of the
metal and the readily accessible coordination sites. It has
been recently reported that [Rh(PPh3)2(acetone)2]+,
[Rh(H)2(PPh3)2(acetone)2]+, and [Ir(H)2(PPh3)2(acetone)2]+
could mediate the intramolecular C-H activation of arenes,
provided there is sufficient chelation assistance from chelat-
ing groups such as imines.19,20 In this context, we are
interested in the [Rh(PPh3)2(acetone)2]+ system for the
activation of Caryl-halogen bonds. We feel that this rhod-
ium(I) system might also achieve this goal owing to the low
valence of this rhodium complex and high lability of acetone
ligands. It is also important to investigate the selectivity of
Caryl-H versus Caryl-X activation when these bonds are both
accessible. Selective C-H activation in the presence of
Caryl-Cl or Caryl-Br bonds has been reported for Pd, Rh, Ir,
and Au systems.7,21
1
and elemental analysis. In the H NMR spectrum, neither
hydride resonance signal nor any signal of ligated acetone
could be detected. In the 13C NMR spectrum, a characteristic
1
quaternary carbon signal at δ 154.2 (dt, JRhC ) 27.7 Hz,
2JPC ) 9.5 Hz) was detected and was assigned to Rh-C,
suggestive of the metalation of the ligand, likely at the C10
position facilitated by chelation assitance. In the 31P NMR
1
spectrum, a doublet signal (δ 13.87, JRhP ) 95.0 Hz) was
(17) (a) Wang, X.; Lane, B. S.; Sames, D. J. Am. Chem. Soc. 2005, 127,
4996. (b) Ueura, K.; Satoh, T.; Miura, M. Org. Lett. 2005, 7, 2229.
(c) Ishiyama, T.; Hartwig, J. F. J. Am. Chem. Soc. 2000, 122, 12043.
(18) Douglas, T. M.; Chaplin, A. B.; Weller, A. S. Organometallics 2008,
27, 2918.
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24, 1445. (b) Ezhova, M. B.; Patrick, B. O.; James, B. R. Organo-
metallics 2005, 24, 3753. (c) Marcazzan, P.; Ezhova, M. B.; Patrick,
B. O.; James, B. R. C. R. Chim. 2002, 5.
We now report activation of Caryl-X (X ) Cl, Br, and I)
bonds by [Rh(PPh3)2(acetone)2]+ under mild conditions by
oxidative addition assisted by prior substrate coordination.
Different rhodium(III) halide products were obtained under
different reaction conditions.
(20) (a) Li, X.; Vogel, T.; Incarvito, C. D.; Crabtree, R. H. Organometallics
2005, 24, 62. (b) Li, X.; Chen, P.; Faller, J. W.; Crabtree, R. H.
Organometallics 2005, 24, 4810.
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(b) Kalyani, D.; Deprez, N. R.; Desai, L. V.; Sanford, M. S. J. Am.
Chem. Soc. 2005, 127, 7330. (c) Chotana, G. A.; Rak, M. A.; Smith,
M. R., III J. Am. Chem. Soc. 2005, 127, 10539. (d) Ben-Ari, E.;
Gandelman, M.; Rozenberg, H.; Shimon, L. J. W.; Milstein, D. J. Am.
Chem. Soc. 2003, 125, 4714. (e) Tellers, D. M.; Yung, C. M.; Arndtsen,
B. A.; Adamson, D. R.; Bergman, R. G. J. Am. Chem. Soc. 2002,
124, 1400–1410. (f) Ishiyama, T.; Takagi, J.; Ishida, K.; Miyaura, N.;
Anastasi, N. R.; Hartwig, J. F. J. Am. Chem. Soc. 2002, 124, 390–
391. (g) Cho, J.-Y.; Tse, M. K.; Holmes, D.; Maleczka, R. E., Jr.;
Smith, M. R., III Science 2002, 295, 305. (h) Fuchita, Y.; Utsunomiya,
Y.; Yasutake, M. J. Chem. Soc., Dalton Trans. 2001, 2330.
(22) Schrock, R. R.; Osborn, J. A. J. Am. Chem. Soc. 1971, 93, 2397.
(23) (a) Zhang, X.; Kanzelberger, M.; Emge, T. J.; Goldman, A. S. J. Am.
Chem. Soc. 2004, 126, 13192. (b) Crabtree, R. H.; Mihelcic, J. M.;
Quirk, J. M. J. Am. Chem. Soc. 1979, 101, 7738.
Results and Discussion
The [Rh(PPh3)2(acetone)2]+ complex is the dehydrogena-
tion product of [Rh(H)2(PPh3)2(acetone)2]+, which could be
synthesized by bubbling H2 through a suspension of [Rh(N-
BD)(PPh3)2]+ (NBD ) norbornadiene) in acetone.22 No
attempt was made to isolate the dihydride product due to its
poor stability. On the basis of related iridium dihydride
Inorganic Chemistry, Vol. 48, No. 3, 2009 1199

Chen et al.
Table 1. Crystallographic Data for Complexes 2A-Br, 2B-I·0.5CH2Cl2, and 4A-Br2 ·0.5MeCN·Et2O
2A-Br
2B-I ·0.5CH2Cl2
4A-Br2 ·0.5MeCN·Et2O
empirical formula
C48H38BrF6NP3Rh
1030.54
Mo KR, 0.71073 Å
173(2)
C47.5 H39ClF6INP3Rh
1095.97
Mo KR, 0.71073 Å
173(2)
C54H49.5Br2N1.5OP2Rh
1060.12
Mo KR, 0.71073 Å
173(2)
fw (g mol-1
radiation, λ
T (K)
)
cryst syst
space group
a (Å)
b (Å)
c (Å)
orthorhombic
Pbca
19.7286(5)
19.7688(6)
22.2111(7)
90
8662.6(4)
4
1.580
monoclinic
P21/n
monoclinic
P21/c
12.6321(3)
20.3275(4)
17.1309(4)
91.5330(10)
4397.28(17)
4
12.0657(3)
18.4792(4)
20.7142(5)
96.1040(10)°
4592.35(19)
4
ꢀ (deg)
V (Å3)
Z
Dcalcd (g cm-3
)
1.655
1.321
1.533
2.223
µ (mm-1
)
1.490
cryst size (mm)
total, unique no. of rflns
Rint
0.30 × 0.20 × 0.10
473175, 13285
0.0864
0.30 × 0.20 × 0.05
106611, 13627
0.0467
0.20 × 0.16 × 0.10
60594, 14723
0.0720
data, restraints, params
R, Rw (all data)
GOF
13285, 0, 550
0.0361, 0.0900
1.128
13627, 0, 550
0.0555, 0.1044
1.122
14723, 0, 561
0.1106, 0.1568
1.052
-3
min., max. resid dens (eÅ
)
-1.283, 1.098
-1.264, 1.060
-1.377, 1.059
observed for two equivalent phosphines, indicating their trans
orientation. The haloarene starting materials can be extended
to the bromo analogue (1A-Br) and 2-(2-halophenyl)py-
ridines (1B-Br and 1B-I), and the corresponding monohalide
products were isolated in 86-92% yield. All of these
rhodium monohalide complexes are soluble in CH2Cl2 and
MeCN but are sparingly soluble in acetone. Complexes 2A-
Br, 2B-Br, and 2B-I are relatively stable in CH3CN, but
they decompose in CH2Cl2 solutions to give dihalide
complexes among other products (vide infra).
Single crystals of 2A-Br and 2B-I suitable for X-ray
crystallographic studies were obtained by the slow diffusion
of diethyl ether to their dichloromethane solutions at -20
°C. X-ray crystallography unambiguously confirmed the
identity of 2A-Br and 2B-I as rather rare five-coordinate
rhodium(III) halide complexes (Table 1 and Figures 1 and
2). In the crystal structures of 2A-Br and 2B-I, the PPh3
ligands are trans to each other and the coordination sphere
involves five regular bonds, while the sixth coordination site
is empty and is trans to the high trans-effect aryl ligand.
This type of five-coordinate binding mode has been observed
mostly for Rh, Ir, and Ru pincer complexes.7,18,24 The Rh-N
[2.047(2) Å] and Rh-C [1.991(3) Å] distances in the
metalacycles of 2A-Br are nearly identical to the corre-
sponding ones in 2B-I. The phenyl and the pyridyl rings in
2B-I are essentially coplanar so that there is negligible
difference between the N(1)-Rh(1)-C(19) angle of these
two complexes [81.43(3)o for 2A-Br and 79.49(9)o for 2B-
I]. The C(19)-Rh(1)-Br(1) angle in 2A-Br [105.59(8)o]
deviates significantly from 90°, and this is even more
pronounced for 2B-I [C(19)-Rh(1)-I(1) ) 114.44(7)o],
which is possibly due to the steric repulsion caused by the
bulky iodide ligand.
We noted that both ortho-C-X and ortho-C-H bonds in
1B-Br or 1B-I can be potentially activated. However, only
Figure 1. Molecular structure (ORTEP) of the cation of 2A-Br with thermal
ellipsoids shown at 50% probability. Selected bond lengths (Å) and angles
(deg): Rh1-Br1, 2.4465(3); Rh1-C19, 1.991(3); Rh1-N1, 2.047(2);
Rh1-P1,2.3675(7);Rh1-P2,2.3737(7);P1-Rh1-P2,177.01(3);N1-Rh1-C19,
81.43(3); C19-Rh1-Br1, 105.59(8).
Figure 2. Molecular structure (ORTEP) of the cation of 2B-I·0.5CH2Cl2
with thermal ellipsoids shown at 50% probability. Selected bond lengths
(Å) and angles (deg): Rh1-I1, 2.6399(2); Rh1-C19, 1.990(2); Rh1-N1,
2.053(2); Rh1-P1, 2.3605(7); Rh1-P2, 2.3747(7); P1-Rh1-P2, 176.97(2);
N1-Rh1-C19, 79.49(9); C19-Rh1-I1, 114.44(7).
1200 Inorganic Chemistry, Vol. 48, No. 3, 2009

Chelation-Assisted Carbon-Halide Bond ActiWation
Scheme 2. Synthesis of Rhodium Hydride Complexes
the C-X activation was observed. To explore the activation
of C-H bonds, benzo[h]quinoline and 2-phenylpyridine were
allowed to react with [Rh(H)2(PPh3)2(acetone)2]+. The ac-
tivation of C-H bonds in these arenes did proceed, but only
at elevated temperatures (>50 °C), to afford isolable hydride
complexes 3A and 3B in 86% and 87% yields, respectively
(Scheme 2), which have been fully characterized. In the 1H
NMR spectrum of 3A, the hydride resonates at δ -12.29
(dt, JRhH ) 16.6 Hz, JPH ) 11. 0 Hz), and the ligated
acetone was also detected. These results indicate that the
activation of the ortho C-H bonds in 1B-Br and 1B-I likely
has a higher barrier. The differences between the coordination
numbers of 2A-Cl and -Br (or 2B-Br, I) and 3A (or 3B)
seem accountable to steric effects: since electronically one
would predict that complexes 2A-Cl and -Br (or 2B-Br, I)
should be more electrophilic than 3A (or 3B) and should
have a stronger tendency of acetone binding.
a neutral complex. The 31P NMR spectrum (CD2Cl2) shows
that the PPh3 ligands are equivalent (δ 14.6, d, 1JRhP ) 91.6
Hz). Analogously, neutral rhodium(III) dihalides 4B-Br2 and
4B-I2 could be synthesized from 1B-Br and 1B-I with 38%
and 42% yields, respectively (Scheme 4). These dihalide
complexes have poor solubility in common organic solvents
and are sparingly soluble in CH2Cl2.
Alternatively, rhodium(III) diiodide complexes (such as
4B-I2) could be synthesized from the reaction of the
corresponding rhodium hydride (such as 3B) and I2 in THF
(eq 1), and the products obtained via these two routes have
identical spectroscopic data. The scope of this reaction could
also be extended to its iridium analogue (complex 5, eq 2).
One possible mechanism to account for this transformation
is that I2 oxidatively adds to the metal center to give a
rhodium(V) or iridium(V) hydride diiodide complex, fol-
lowed by proton abstraction. Conversion of transition metal
hydrides to metal halides has been reported using various
halogenating reagents.26
1
2
Heating an acetone solution of 1A-Br (1.2 equiv),
[Rh(H)2(PPh3)2(acetone)2]+, and TBE (20 equiv) under reflux,
however, afforded a precipitate, and product 4A-Br2 was
isolated in analytically pure form in 40% yield (Scheme 3).
Complex 4A-Br2 could also be obtained in similar yield by
heating an acetone suspension of 2A-Br. These results
indicate that 2A-Br is an intermediate leading to the
decomposition product 4A-Br2. 31P NMR analysis of the
acetone solution from which 4A-Br2 precipitated shows a
2
major peak at δ 25.2 (d, JRhP ) 100 Hz), and the ESI-MS
spectrum of this acetone solution gives a peak of m/e )
391.62. Both this m/e value and the isotopic pattern agree
with a structure of [(N^C)Rh(PPh3)2]2+ in mass spectrometry.
The corresponding species in acetone solution is tentatively
assigned to [(N^C)Rh(PPh3)2(acetone)]2+. However, no such
analytically pure product could be isolated. We propose that
2A-Br reversibly dimerizes to a dicationic bridging bromide
intermediate which undergoes an irreversible ligand redis-
tribution process to give these two products as a major
decomposition pathway (Scheme 4). Similar reactions of the
shifting of two bridging halide ligands to one metal center
have been reported for Rh, Ir, and Pd.25
Single crystals of complex 4A-Br2 were obtained by the
slow diffusion of diethyl ether into its acetonitrile solution.
(25) (a) Khalimon, A. Y.; Dorogov, K. Y.; Churakov, A. V.; Kuzmina,
L. G.; Lemenovskii, D. A.; Howard, J. A. K.; Nikonov, G. I. Dalton
Trans. 2007, 2440. (b) Gauntlett, J. T.; Mann, B. E.; Winterd, S.
J. Chem. Soc., Dalton Trans. 1991, 1427. (c) Shin, J. H.; Churchill,
D. G.; Bridgewater, B. M.; Pang, K.; Parkin, G. Inorg. Chim. Acta
2006, 359, 2942. (d) Kovacs, I.; Pearson, C.; Shaver, A. J. Organomet.
Chem. 1999, 584, 347. (e) Wang, X.; Chen, H.; Li, X. Organometallics
2007, 26, 4684.
(26) (a) Kennedy, D. F.; Messserle, B. A.; Smith, M. K. Eur. J. Inorg.
Chem. 2007, 80. (b) Sui-Seng, C.; Enright, G. D.; Zargarian, D. J. Am.
Chem. Soc. 2006, 128, 6508. (c) Ricci, A.; Angelucci, F.; Chiarini,
M.; Sterzo, C. L.; Masi, D.; Giambastiani, G.; Bianchini, C. Orga-
nometallics 2008, 27, 1671. (d) Esteruelas, M. A.; Forcen, E.; Olivan,
M.; Onate, E. Organometallics, 2008, 27, 6188.
Complex 4A-Br2 has been unambiguously identified by
NMR spectroscopy, X-ray crystallography, and elemental
analysis. In the 31P or 19F NMR spectrum, no resonance
-
signal of PF6 could be detected, indicating that 4A-Br2 is
(24) (a) Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. J. Am. Chem.
Soc. 2005, 127, 10840. (b) van der Boom, M. E.; Milstein, D. Chem.
ReV. 2003, 103, 1759.
Inorganic Chemistry, Vol. 48, No. 3, 2009 1201

Chen et al.
Table 2. Screening of Rhodium Catalysts for the Dimerization of
the same for rhodium catalysts with the same dihalides but
with a different metallacyclic motif (entry 3 vs entry 5; entry
2 vs entry 4). The highest NMR yield obtained for the anti-
enyne is 70%. To our surprise, 16-electron ionic monohalide
complexes 2A-Br and 2B-I showed very poor NMR yields
(entries 8 and 10). This is consistent with the result obtained
when a combination of 4A-Br2 and NaBAr4F was used in a
1 to 1.2 ratio (entry 9). Here, the addition of NaBAr4F inhibits
this reaction, and this system is in fact equivalent to the cation
of 2A-Br generated in situ: the bromide trans to the aryl
ligand should be readily removed under these conditions.
We also noted that this dimerzation reaction is inhibited by
the addition of PPh3 (entry 11, Table 2).
Complex 4B-I2 was then retained to further study the
scope of this type of catalytic reaction (eq 4) in CH2Cl2,
and NMR yields are given in Table 3. Terminal alkyne
substrates with electron-rich or electron-poor aryl substituents
all dimerize, with the lowest trans to gem selectivity (87:
13) observed for para-NO2(C6H4)CtCH (entry 5). Alkynes
with electron-poor aryl groups tend to have low reactivity
but still give yields comparable to that of phenylacetylene
(entries 4-5). For EtOC(O)CtCH, the dimerization is highly
selective, although only 65% NMR yield was obtained (entry
6).
Phenylacetylenea
temp selectivity
NMR yield of
entry
catalyst
4A-Br2
4A-Br2
4A-I2
4B-Br2
4B-I2
4B-I2
4B-I2
2A-Br
solvent
acetone
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
acetone
(°C) (trans:gem) trans product (%)b
1
2
3
4
5
6
7
8
9
56
40
40
40
40
56
72:28
82:18
90:10
77:23
90:10
70:30
65:35
40
56
69
49
70
48
31
CH2ClCH2Cl 83
CH2Cl2
40
40
40
40
<5
<5
<5
<5
4A-Br2/NaBArF4 CH2Cl2
CH2Cl2
11 4B-I2/PPh3 (1:6) CH2Cl2
10 2B-I
a Conditions: 0.8 mmol phenylacetylene, 0.008 mmol rhodium catalyst,
4 mL solvent, 12 h, reflux. b NMR yield using 1,3,5-trimethoxybenzene as
an internal standard.
One equivalent of ether and half an equivalent of acetonitrile
cocrystallized with 4A-Br2. X-ray crystallography unambigu-
ously confirmed the identity of this complex (Figure 3 and
Table 1). The Rh(1)-C(1) distance [2.028(4) Å] here is
slightly longer than the corresponding one in 2A-Br [1.991(3)
Å]. In complex 4A-Br2, the Rh(1)-Br(1) length is signifi-
cantly longer (ca. 0.1 Å longer) than that of the Rh(1)-Br(2)
bond, undoubtedly due to the significantly high trans
influence of the aryl ligand compared with the quinoline N
atom. Either of the Rh-Br bonds in 4A-Br2 is at least 0.05
Å longer than the Rh-Br bond in 2A-Br. In contrast to
complex 2A-Br, the C(1)-Rh(1)-Br(2) angle [90.13(12)o]
in 4A-Br2 is essentially 90°.
Rhodium complexes have been widely used in the dimer-
ization of terminal alkynes, and the most common dimer-
ization products are trans-enynes, although cis- or gem-
enynes are also known.27 Our investigations of the applications
of these rhodium monohalide and dihalide complexes started
from the catalytic dimerization of phenylacetylene (eq 3).
Both rhodium monohalide and dihalide complexes have been
screened with 1 mol % loading under refluxing conditions,
and the results are given in Table 2.
Compared with other representative systems of catalytic
alkyne dimerization,16 the rhodium dihalides show lower
catalytic activity. This is likely due to the 18-electron
structures of these complexes with active species. Instead, a
vacant coordination site should be generated to accommodate
an incoming alkyne substrate. This could be achieved by (i)
the dissociation of a halide ligand, particularly the labile
halide that is trans to the aryl ligand, (ii) dechelation of the
nitrogen arm, or (iii) dissociation of one of the phosphine
ligands. Dissociation of a labile halide ligand (such as in
4A-Br2) seems unlikely since the resulting 16-electron
cationic species is in fact the cation part of the corresponding
rhodium monohalide (such as 2A-Br), which proved to be
inactive in catalysis (entries 8-9, Table 2). The likelihood
of the dissociation of the nitrogen arm of the chelating ligand
can be evaluated by comparing the activity of rhodium
complexes with different cyclometalated motifs. It is known
that dissociation of the nitrogen arm in a flexible chelating
system (such as in 2-phenylpyridyl) should have a lower
barrier than that in a rigid one (such as in benzo-
[h]quinolyl).28 In fact, both activity and selectivity are
essentially the same29 for rhodium catalysts with different
metalacyclic motifs (entries 3 and 5, Table 2), which
indicates that nitrogen arm dissociation is unlikely.
The 18-electron neutral rhodium dihalide complexes are
active to different extents in catalyzing the dimerization of
phenylacetylene, and in all cases the trans-enyne is the major
product and the gem-enyne is the only minor product based
on 1H NMR analysis. Catalyst screening in different solvents
under refluxing conditions indicates that dichloromethane
seems to be the optimal solvent (Table 2). Screening of
rhodium catalysts with the same metallacycle entity but with
different dihalides shows that diiodide complexes give higher
activity and selectivity than dibromides (entry 2 vs entry 3;
entry 4 vs entry 5). The activity and selectivity are essentially
To examine the likelihood of phosphine dissociation, we
then compared complexes 2B-I and 4B-I2 for phosphine
exchange reactions. In two separate reactions, 2B-I or
equimolar 4B-I2 was allowed to react with 10 equiv of tris(p-
fluorophenyl)phosphine in CH2Cl2 for 1 h (Scheme 5). The
ratios of starting materials, monophosphine substitution
(27) (a) Weng, W.; Guo, C.; Celenligil-Cetin, R.; Foxman, B. M.; Ozerov,
O. V. Chem. Commun. 2006, 197. (b) Lee, C.-C.; Lin, Y.-C.; Liu,
Y.-H.; Wang, Y. Organometallics 2005, 24, 136. (c) Katagiri, T.;
Tsurugi, H.; Satoh, T.; Miura, M. Chem. Commun. 2008, 3405. (d)
Nishimura, T.; Guo, X.-X.; Ohnishi, K.; Hayashi, T. AdV. Syn. Catal.
2007, 349, 2669. (e) Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. ReV.
2002, 102, 1731.
1202 Inorganic Chemistry, Vol. 48, No. 3, 2009

Chelation-Assisted Carbon-Halide Bond ActiWation
Scheme 3. Synthesis of 18-Electron Rhodium Dihalide Complexes
Scheme 4. Formation of Complex 4A-Br2
Table 3. Dimerization of Terminal Alkynes Catalyzed by Complex
2B-I and 4B-I2 agrees with the hypothesis that the five-
coordinate species derived from the dissociation of a PPh3
ligand is the most likely active catalyst. This hypothesis is
also consistent with the fact that rhodium diiodides possess
higher catalytic activity than rhodium dibromides (entries 4
and 5, Table 2): the dissociation of a PPh3 ligand from a
rhodium diiodide complex is favored for both electronic and
steric reasons. Electronically, the metal center is relatively
more electron-rich for diiodide complexes. Sterically, the
large bulk of iodides provides steric repulsion to facilitate
the departure of a phosphine ligand. Both of these factors
will favor the dissociation of a phosphine ligand to generate
an active 16-electron intermediate.
a
4B-I2
time
(h)
Selectivity
(trans: gem)
NMR yield of
entry
R
trans product (%)b
1
2
3
4
5
6
Ph-
12
12
12
24
24
12
90:10
91:9
94:6
98:2
87:13
100:0
70
88
94
71
70
65
p-CH3(C6H4)-
p-CH3O(C6H4)-
p-CF3(C6H4)-
p-NO2(C6H4)-
EtO(O)C-
a Conditions: 0.8 mmol alkyne, 0.008 mmol rhodium catalyst, 4 mL
solvent, 12 h, reflux.. b NMR yield using 1,3,5-trimethoxybenzene as an
internal standard.
Conclusions
We have achieved nitrogen coordination-assisted activation
of C-X bonds (X ) Cl, Br, and I) in the reaction of 2-(2-
halophenyl)pyridine or 10-halobenzo[h]quinoline and
[Rh(PPh3)2(acetone)2]+. A series of rather rare five-coordinate
16-electron cationic rhodium(III) halides has been synthe-
sized under mild conditions. However, reactions of these
haloarenes and [Rh(PPh3)2(acetone)2]+ in acetone under
reflux afforded neutral octahedral rhodium(III) dihalide
complexes. Alternatively, rhodium and iridium diiodides with
these cyclometalation motifs could be synthesized from the
corresponding hydride complexes via hydride-iodide ex-
change. X-ray crystal structures of representative mono- and
dihalide complexes have been reported. The aryl ligands are
trans to vacant coordination sites in these five-coordinate
monohalide complexes.
The catalytic activity of these rhodium(III) halide com-
plexes has been assessed for the dimerization of terminal
alkynes. The 18-electron rhodoium dihalides are much more
active catalysts, while cationic rhodium monohalide com-
plexes are essentially inactive. Screening of catalysts shows
that such rhodium diiodide complexes possess higher cata-
lytic activity than the dibromide complexes. A five-coordinate
phosphine dissociation product of a dihalide complex has
been proposed as the active species in the catalytic cycle.
Figure 3. Molecular structure (ORTEP) of 4A-Br2 ·0.5MeCN ·C4H10O with
thermal ellipsoids shown at 50% probability. Selected bond lengths (Å)
and angles (deg): Rh1-Br1, 2.6022(5); Rh1-Br2, 2.5053(5); Rh1-C1,
2.028(4); Rh1-N1, 2.061(3); Rh1-P1, 2.3725(10); Rh1-P2, 2.3913(11);
P1-Rh1-P2, 179.49(4); N1-Rh1-C1, 81.85(15); C1-Rh1-Br2,
90.13(12).
products, and diphosphine substitution products were ana-
lyzed by 31P NMR spectroscopy and are given in Scheme 5.
The phosphine substitution reaction of 4B-I2 is significantly
faster than that of 2B-I. The correlation between rates of
phosphine substitution and catalytic activity of complexes
Inorganic Chemistry, Vol. 48, No. 3, 2009 1203

Chen et al.
Scheme 5
zo[h]quinoline (179 mg, 0.694 mmol) gave a yellow powder, 2A-
Experimental Section
1
Br. Yield: 513 mg (0.497 mmol, 86%). H NMR (δ, 400 MHz,
General Considerations. All manipulations were carried out
using standard Schlenk techniques. All chemicals were used as
received without any further purification unless otherwise specified.
All solvents were degassed and dried by standard techniques. NMR
spectra were obtained on a Bruker DPX 300, AMX 400 or 500, or
JEOL ECA 400 spectrometer. The chemical shift is given as
CD3CN, 298 K): 6.91 (dd, 3J ) 8.0 Hz, 3J ) 5.6 Hz, 1H),
3
7.05-7.12 (m, 24H, PPh3), 7.21 (t, J ) 7.8 Hz, 1H), 7.25-7.29
3
3
(m, 6H, PPh3), 7.36 (d, J ) 8.8 Hz, 1H), 7.63 (d, J ) 7.6 Hz,
1H), 7.78 (m, 2H), 7.85 (d, 3J ) 8.0 Hz, 1H), 8.19 (d, 3J ) 5.2 Hz,
1H). 31P {1H} NMR (δ, 162 MHz, CD3CN, 298 K): -144.62 (m,
1
1JF-P ) 706.7 Hz, PF6), 14.62 (d, JRh-P ) 94.6 Hz, PPh3). 13C
1
dimensionless δ values and is referenced relative to SiMe4 for H
{1H} NMR (δ, 100 MHz, CD3CN, 298 K): 122.09 (s), 122.98 (s),
123.63 (s), 127.17 (s), 127.75 (t, JP-C ) 4.88 Hz, PPh3), 128.77
and 13C NMR spectroscopy. Elemental analyses were performed
at the Division of Chemistry and Biological Chemistry, Nanyang
Technological University. X-ray crystallographic analyses were
performed on a Bruker X8 APEX diffractometer.
1
(t, JP-C ) 23.8 Hz, ipso-PPh3), 128.79 (s), 129.82 (s), 130.41 (s,
p-PPh3), 133.31 (s), 133.93 (t, JP-C ) 5.0 Hz, PPh3), 136.29 (s),
138.96 (s), 138.44 (s), 139.31 (s), 149.82 (s), 152.48 (s), 154.38
Compounds 1A-Cl, 1A-Br, 1B-Br, and 1B-I were prepared on
the basis of a recent report.30 Some of the 13C NMR spectra of
dihalide complexes could not be obtained with acceptable signal-
to-noise ratios due to their poor solubility in common solvents.
Compound 2A-Cl. To a suspension of [Rh(NBD)(PPh3)2]PF6
(100 mg, 0.116 mmol) in acetone (5 mL) was bubbled H2 for 15
min at 0 °C, during which time the color of the solution changed
from red to yellow. tert-Butylethylene (0.5 mL) and 10-chloroben-
zo[h]quinoline (30 mg, 0.139 mmol) were then added to the solution
and were stirred at room temperature for 2 h. The resulting
precipitate was then filtered and washed with 2 × 20 mL diethyl
ether to give a yellow powder, 2A-Cl. Yield: 102 mg (0.103 mmol,
89%). 1H NMR (δ, 400 MHz, CD3CN, 298 K): 6.85 (dd, 3J ) 8.0
1
2
(dt, JRh-C ) 27.8 Hz, JP-C ) 10.5 Hz, CRh). Anal. calcd for
C49H38BrF6NP3Rh (1030.55): C, 57.11; H, 3.72; N, 1.36. Found:
C, 57.19; H, 3.27; N, 1.31.
Compound 2B-Br. Complex 2B-Br was synthesized by directly
following the procedure for the preparation of 2A-Cl. The reaction
of [Rh(NBD)(PPh3)2]PF6 (100 mg, 0.116 mmol) and 2-(2-bro-
mophenyl)pyridine (32 mg, 0.139 mmol) gave 2B-Br as a yellow
powder. Yield: 107 mg (0.107 mmol, 92%). 1H NMR (δ, 400 MHz,
3
3
CD3CN, 298 K): 6.52 (t, J ) 6.6 Hz, 1H), 6.72 (t, J ) 7.6 Hz,
1H), 7.08 (t, 3J ) 7.6 Hz, 2H), 7.16-7.19 (m, 12H, PPh3),
7.23-7.25 [m, 13H, 12H (PPh3) + 1H (C11H8N)], 7.26 (m, 6H,
3
3
PPh3), 7.30 (d, J ) 8.0 Hz, 1H), 7.35 (d, J ) 8.0 Hz, 1H), 7.88
(d, J ) 6.0 Hz, 1H). 31P {1H} NMR (δ, 162 MHz, CD3CN, 298
3
3
Hz, J ) 5.6 Hz, 1H), 7.05-7.09 (m, 12H, PPh3), 7.12-7.21 [m,
1
1
K): -144.62 (m, JF-P ) 705.8 Hz, PF6), 14.95 (d, JRh-P ) 95.8
Hz, PPh3). 13C {1H} NMR (δ, 100 MHz, CD3CN, 298 K): 119.92
(s), 123.64 (s), 123.82 (s), 124.12 (s), 128.00 (t, JP-C ) 4.8 Hz,
PPh3), 129.06 (t, 1JP-C ) 23.9 Hz, ipso-PPh3), 130.51 (s, p-PPh3),
130.85 (s), 131.54 (s), 134.28 (t, JP-C ) 4.8 Hz, PPh3), 137.96 (s),
13H, 12H (PPh3) + 1H (C13H8N)], 7.26 (m, 6H, PPh3), 7.33 (d, 3J
3
) 8.0 Hz, 1H), 7.58 (d, J ) 7.8 Hz, 1H), 7.71-7.74 (m, 2H),
3
3
7.80 (d, J ) 8.0 Hz, 1H), 8.15 (d, J ) 5.5 Hz, 1H). 31P {1H}
1
NMR (δ, 162 MHz, CD3CN, 298 K): -144.62 (m, JF-P ) 706.5
1
Hz, PF6), 13.87 (d, JRh-P ) 95.0 Hz, PPh3). 13C {1H} NMR (δ,
1
140.92 (s), 143.73 (s), 150.56 (s), 157.93 (dt, JRh-C ) 26.7 Hz,
100 MHz, CD3CN, 298 K): 122.08 (s), 122.38 (s), 123.64 (s),
127.02 (s), 127.90 (t, JP-C ) 4.8 Hz, PPh3), 128.70 (t, JP-C
23.9 Hz, ipso-PPh3), 128.81 (s), 129.43 (s), 130.48 (s, p-PPh3),
133.29 (s), 133.74 (t, JP-C ) 4.8 Hz, PPh3), 136.25 (s), 137.55 (s),
2JP-C ) 9.5 Hz, CRh). Anal. calcd for C47H38BrF6NP3Rh (1006.53):
C, 56.08; H, 3.81; N, 1.39. Found: C, 56.48; H, 4.22; N, 1.42.
Compound 2B-I. Complex 2B-I was synthesized by directly
following the procedure for the preparation of 2A-Cl. The reaction
of [Rh(NBD)(PPh3)2]PF6 (500 mg, 0.578 mmol) and 2-(2-iodophe-
nyl)pyridine (195 mg, 0.694 mmol) gave 2B-I as an orange powder.
Yield: 548 mg (0.520 mmol, 90%). 1H NMR (δ, 300 MHz, CD3CN,
1
)
1
139.27 (s), 150.03 (s), 152.56 (s), 154.18 (dt, JRh-C ) 27.7 Hz,
2JP-C ) 9.5 Hz, CRh). Anal. calcd for C49H38ClF6NP3Rh (986.10):
C, 59.68; H, 3.88; N, 1.42. Found: C, 59.66; H, 3.50; N, 1.73.
Compound 2A-Br. Complex 2A-Br was synthesized by directly
following the procedure for the preparation of 2A-Cl. The reaction
of [Rh(NBD)(PPh3)2]PF6 (500 mg, 0.578 mmol) and 10-bromoben-
3
3
298 K): 6.60 (t, J ) 6.4 Hz, 1H), 6.72 (t, J ) 8.5 Hz, 1H),
7.13-7.24 [m, 26H, 24H (PPh3) + 2H (C11H8N)], 7.31-7.40 [m,
3
7H, 6H (PPh3)+1H (C11H8N)], 7.45 (d, J ) 9.2 Hz, 1H), 7.74
(d, 3J ) 8.5 Hz, 1H), 7.94 (d, 3J ) 6.4 Hz, 1H). 31P {1H} NMR (δ,
162 MHz, CD3CN, 298 K): -143.99 (m, 1JF-P ) 703.5 Hz, PF6),
(28) Dick, A. R.; Kampf, J. W.; Sanford, M. S. J. Am. Chem. Soc. 2005,
127, 12790.
(29) We have also confirmed that the yields and selectivity for 4A-Br2
and 4B-Br2 are essentially the same for the dimerization of pheny-
lacetylene after 6 h.
(30) Kalyani, D.; Dick, A. R.; Anani, W. Q.; Sanford, M. S. Tetrahedron
2006, 62, 11483.
16.15 (d, JRh-P ) 94.0 Hz, PPh3). 13C {1H} NMR (δ, 100 MHz,
1
CD3CN, 298 K): 120.19 (s), 123.76 (s), 123.95 (s), 124.05 (s),
127.85 (t, JP-C ) 4.8 Hz, PPh3), 129.56 (t, 1JP-C ) 23.95 Hz, ipso-
PPh3), 130.48 (s, p-PPh3), 131.39 (s), 134.59 (t, JP-C ) 5.8 Hz,
1204 Inorganic Chemistry, Vol. 48, No. 3, 2009

Chelation-Assisted Carbon-Halide Bond ActiWation
PPh3), 138.06 (s), 143.77 (s), 144.11 (s), 150.10 (s), 158.06 (dt,
1JRh-C ) 26.8 Hz, 2JP-C ) 10.5 Hz, CRh), 163.61 (s). Anal. calcd
for C47H38IF6NP3Rh (1053.53): C, 53.58; H, 3.64; N, 1.33. Found:
C, 53.17; H, 3.86; N, 1.44.
C49H38Br2NP2Rh (965.49): C, 60.96; H, 3.97; N, 1.45. Found: C,
60.69; H, 4.01; N, 1.47.
Compound 4A-I2. To a suspension of rhodium hydride 3A (100
mg, 0.116 mmol) in THF was added I2 (50 mg, 0.231 mmol), and
the mixture was stirred at 0 °C for 4 h. The precipitate was then
filtered and was washed with 2 × 20 mL diethyl ether to give 4A-
I2 as an orange powder. Analytically pure 4A-I2 could be obtained
after recrystallization from the diffusion of diethyl ether into a
dilution of 4A-I2 in CH2Cl2. Yield: 87 mg (0.084 mmol, 73%). 1H
Complex 3A. To a suspension of [Rh(NBD)(PPh3)2]PF6 (500
mg, 0.578 mmol) in acetone (8 mL) was bubbled H2 for 15 min at
0 °C, during which time the color of the solution changed from
red to light yellow. tert-Butylethylene (0.8 mL) and benzo[h]quino-
line (124 mg, 0.694 mmol) were then added to the solution, and
the mixture was refluxed for 5 h. The solution was concentrated to
0.5 mL under reduced pressure. The residue was washed with
diethyl ether (2 × 20 mL) to give 3A as a white powder. Yield:
502 mg (0.497 mmol, 86%). 1H NMR (δ, 300 MHz, CD3COCD3,
295 K): -12.29 (dt, 1JRh-H ) 16.6 Hz, 2JP-H )11.0 Hz, 1H, RhH),
3
3
NMR (δ, 400 MHz, CD2Cl2, 298 K): 6.55 (dd, J ) 7.6 Hz, J )
5.8 Hz, 1H), 6.85 (t, 3J ) 7.6 Hz, 1H), 6.90-6.93 (m, 12H, PPh3),
7.10-7.16 (m, 18H, PPh3), 7.42 (d, J ) 8.7 Hz, 1H), 7.50 (d, J
) 7.9 Hz, 1H), 7.70 (d, J ) 7.7 Hz, 1H), 7.75 (d, J ) 7.9 Hz,
3
3
3
3
1H), 7.78 (d, J ) 8.8 Hz, 1H), 9.32 (d, J ) 5.5 Hz, 1H). 31P
{1H} NMR (δ, 162 MHz, CD2Cl2, 298 K): 11.98 (d, 1JRh-P ) 94.9
Hz, PPh3). No 13C NMR spectra could be obtained with an
acceptable signal-to-noise ratio due to the compound’s poor
solubility in common NMR solvents. Anal. calcd for C49H38I2NP2Rh
(1035.47): C, 55.55; H, 3.62; N, 1.32. Found: C, 55.53; H, 3.86;
N, 1.40.
3
3
3
2.06 (s, 6H, acetone), 6.96 (t, J ) 7.7 Hz, 1H), 7.05-7.38 [m,
32H, 2H (C13H8N) + 30H (PPh3)], 7.43 (d, 3J ) 7.7 Hz, 1H), 7.57
(d, 3J ) 8.8 Hz, 1H), 7.78 (dd, 3J ) 8.0 Hz, 3J ) 5.0 Hz, 1H), 8.16
(d, 3J ) 7.9 Hz, 1H), 9.79 (d, 3J ) 4.9 Hz, 1H). 31P {1H} NMR (δ,
1
121 MHz, CD3COCD3, 295 K): -143.56 (m, JF-P ) 704.0 Hz,
1
PF6), 39.91 (d, JRh-P ) 114.3 Hz, PPh3). 13C {1H} NMR (δ, 100
Compound 4B-I2. Complex 4B-I2 was synthesized by following
either the procedure for 4A-Br2 or that for 4A-I2. The reaction of
[Rh(NBD)(PPh3)2]PF6 (100 mg, 0.116 mmol) and 2-(2-iodophe-
nyl)pyridine (65 mg, 0.232 mmol) gave 4B-I2 as an orange powder.
Yield: 50 mg (0.049 mmol, 42%). 1H NMR (δ, 300 MHz, CD2Cl2,
MHz, CD3COCD3, 295 K): 120.52 (s), 122.44 (s), 122.83 (s),
126.83 (s), 127.31 (s), 128.08 (t, JP-C ) 4.7 Hz, PPh3), 128.60 (t,
1JP-C ) 23.0 Hz, ipso-PPh3), 130.30 (s, p-PPh3), 133.36 (t, JP-C
)
5.6 Hz, PPh3), 134.73 (s), 135.73 (s), 138.44 (s), 139.65 (s), 148.19
1
2
(s), 150.71 (s), 153.08 (dt, JRh-C ) 31.0 Hz, JP-C ) 11.2 Hz,
CRh), 205.26 (CO). Anal. calcd for C54H45F6NOP3Rh (1009.74):
C, 61.85; H, 4.49; N, 1.39. Found: C, 61.54; H, 4.95; N, 1.43.
Complex 3B. Complex 3B was obtained as a white powder by
directly following the synthesis of 3A. Yield: 428 mg (0.44 mmol,
3
3
298 K): 6.17 (t, J ) 6.0 Hz, 1H), 6.38 (t, J ) 8.0 Hz, 1H),
6.92-7.04 (m, 12H, PPh3), 7.13-7.25 [m, 9H, 6H (PPh3) + 3H
(C11H8N)], 7.35-7.44 [13H, 12H (PPh3) + 1H (C11H8N)], 7.53
(d, 3J ) 8.4 Hz, 1H), 9.16 (d, 3J ) 5.8 Hz, 1H). 31P {1H} NMR (δ,
162 MHz, CD2Cl2, 298 K): 12.23 (d, 1JRh-P ) 95.7 Hz, PPh3). No
13C NMR spectra could be obtained with an acceptable signal-to-
noise ratio due to the compound’s poor solubility in common NMR
solvents. Anal. calcd for C47H38I2NP2Rh (1035.47): C, 54.52; H,
3.70; N, 1.35. Found: C, 54.09; H, 4.04; N, 1.37.
1
87%). H NMR (δ, 300 MHz, CD3COCD3, 295 K): -12.26 (dt,
1
2
1H, JRh-H ) 15.6 Hz, JP-H )10.9 Hz, Rh-H), 2.06 (s, 6H,
acetone), 6.50 (t, 3J ) 6.7 Hz, 1H), 6.87 (t, 3J ) 7.6 Hz, 1H), 7.03
3
(d, J ) 7.7 Hz, 1H), 7.12-7.42 [m, 33H, 3H (C11H8N) + 30H
(PPh3)], 7.62 (t, J ) 6.8 Hz, 1H), 9.46 (d, J ) 5.5 Hz, 1H). 31P
3
3
Compound 4B-Br2. Complex 4B-Br2 could be synthesized by
directly following the procedure for the preparation of 4A-Br2. The
reaction of [Rh(NBD)(PPh3)2]PF6 (100 mg, 0.116 mmol) and 2-(2-
bromophenyl)pyridine (54 mg, 0.232 mmol) gave 4B-Br2 as a
{1H} NMR (δ, 121 MHz, CD3COCD3, 295 K): -143.58 (m, 1JF-P
) 705.1 Hz, PF6), 39.91 (d, JRh-P ) 115.8 Hz, PPh3). 13C {1H}
1
NMR (δ, 75 MHz, CD3CN, 295 K): 119.31 (s), 121.99 (s), 122.22
(s), 124.64 (s), 128.54 (s), 128.16 (t, JP-C ) 4.9 Hz, PPh3), 129.93
(t, 1JP-C ) 23.4 Hz, ipso-PPh3), 130.30 (s, p-PPh3), 133.39 (t, JP-C
) 5.8 Hz, PPh3), 136.85 (s), 142.10 (s), 144.03 (s), 149.33 (s),
161.69 (s), 159.56 (dt, 1JRh-C ) 28.4 Hz, 2JP-C ) 10.3 Hz, Rh-C),
206.45 (CO). Anal. calcd for C50H45F6NOP3Rh (985.72): C, 60.92;
H, 4.60; N, 1.42. Found: C, 60.52; H, 4.96; N, 1.51.
1
yellow powder. Yield: 41 mg (0.044 mmol, 38%). H NMR (δ,
300 MHz, CD2Cl2, 298 K): 6.17 (t, 3J ) 7.6 Hz, 1H), 6.41 (t, 3J )
7.3 Hz, 1H), 6.89 (t, 3J ) 7.2 Hz, 1H), 7.01-7.06 (m, 12H, PPh3),
7.16-7.21 [m, 7H, 6H (PPh3) + 1H (C11H8N)], 7.26-7.33 (m,
2H), 7.37-7.45 [13H, 12H (PPh3) + 1H (C11H8N)], 8.92 (d, 3J )
5.9 Hz, 1H). 31P {1H} NMR (δ, 121 MHz, CD2Cl2, 298 K): 13.66
(d, 1JRh-P ) 96.2 Hz, PPh3). No 13C NMR spectra could be obtained
with an acceptable signal-to-noise ratio due to the compound’s poor
solubility in common NMR solvents. Anal. calcd for
C47H38Br2NP2Rh (941.47): C, 59.96; H, 4.07; N, 1.49. Found: C,
59.99; H, 4.06; N, 1.39.
Compound 4A-Br2. To a suspension of [Rh(NBD)(PPh3)2]PF6
(100 mg, 0.116 mmol) in acetone (5 mL) was bubbled H2 for 15
min at 0 °C, during which time the color of the solution changed
from red to yellow. tert-Butylethylene (0.5 mL) and 10-bromoben-
zo[h]quinoline (60 mg, 0.231 mmol) were then added to the
solution, which was refluxed for 10 h. The precipitate was then
filtered and washed with acetone, cold CH2Cl2, and then diethyl
ether to give 4A-Br2 as a yellow powder. Yield: 45.6 mg (0.047
mmol, 41%). 1H NMR (δ, 400 MHz, CD2Cl2, 298 K): 6.53 (dd, 3J
) 8.0 Hz, 3J ) 5.7 Hz, 1H), 6.87 (t, 3J ) 7.5 Hz, 1H), 6.94-6.97
Compound 6. Iridium hydride 5 was synthesized by following
a literature report.20a To a suspension of 5 (100 mg, 0.091 mmol)
in THF was added I2 (46 mg, 0.182 mmol), and the mixture was
stirred at 0 °C for 4 h. The precipitate was then filtered and was
washed with diethyl ether (2 × 20 mL) to give 6 as a yellow
powder. Yield: 88 mg (0.077 mmol, 85%). 1H NMR (δ, 400 MHz,
CD2Cl2, 298 K): 6.52 (dd, 3J ) 7.7 Hz, 3J ) 5.8 Hz, 1H), 6.89 (t,
3J ) 7.8 Hz, 1H), 6.93-6.97 (m, 12H, PPh3), 7.14-7.17 (m, 18H,
3
(m, 12H, PPh3), 7.14 (t, J ) 7.4 Hz, 6H, PPh3), 7.19-7.24 (m,
12H, PPh3), 7.41 (d, 3J ) 8.4 Hz, 2H), 7.56 (d, 3J ) 7.7 Hz, 1H),
7.69 (d, 3J ) 8.0 Hz, 1H), 7.74 (d, 3J ) 8.7 Hz, 1H), 9.02 (d, 3J )
5.4 Hz, 1H). 31P {1H} NMR (δ, 162 MHz, CD3CN, 298 K): 13.25
(d, 1JRh-P ) 96.9 Hz, PPh3). No 13C NMR spectra could be obtained
with an acceptable signal-to-noise ratio due to the compound’s poor
solubility in common NMR solvents. Single crystals of 4A-Br2
could be obtained by the diffusion of diethyl ether into a dilute
solution of 4A-Br2 in CH2Cl2 or MeCN. Anal. calcd for
3
3
PPh3), 7.41 (d, J ) 8.7 Hz, 1H), 7.49 (d, J ) 7.7 Hz, 1H), 7.62
(m, 2H), 7.80 (d, 3J ) 8.7 Hz, 1H), 9.26 (d, 3J ) 5.6 Hz, 1H). 31
P
{1H} NMR (δ, 162 MHz, CD2Cl2, 298 K): -26.54 (s, PPh3). 13C
{1H} NMR (δ, 100 MHz, CD2Cl2, 298 K): 120.14 (s), 122.04 (s),
122.92 (s), 126.38 (s), 126.68 (t, JP-C ) 4.8 Hz, PPh3), 128.77 (s),
129.25 (s, p-PPh3), 130.63 (t, 1JP-C ) 26.7 Hz, ipso-PPh3), 131.19
Inorganic Chemistry, Vol. 48, No. 3, 2009 1205

Chen et al.
(s), 133.21 (s), 135.12 (t, JP-C ) 4.8 Hz, PPh3), 138.08 (s), 140.66
(t, 2JP-C ) 8.4 Hz, Ir-C), 153.51 (s), 155.94 (s). One CH resonance
signal is missing due to signal overlapping. Anal. calcd for
C49H38I2IrNP2 (1148.81): C, 51.23; H, 3.33; N, 1.22. Found: C,
51.08; H, 3.76; N, 1.25.
General Method for Alkyne Dimerization. A rhodium catalyst
(0.01 mmol, 1 mol %) was added to a dichloromethane solution (3
mL) of an alkyne (1 mmol) in a Schlenk flask, and the mixture
was then heated under reflux for 12 h. The reaction mixture was
cooled to room temperature, and all volatiles were removed under
reduced pressure to give a residue, which was then chromatographed
on silica gel to give the enyne product. The ratio of trans- to gem-
enyne was analyzed by 1H NMR spectroscopy for the crude product.
The trans- to gem-enyne ratios and the NMR yields of the major
products are listed in Table 3. The identities of the dimerization
products of entries 1,31 2,27b 3,31 4,31 and 531 were confirmed by
comparisons with reported NMR data.
135.42 (s, CdC), 153.15 (s, CO), 164.72 (s, CO). Anal. calcd for
C10H12O4 (196.2): C, 61.21; H, 6.16. Found: C, 61.38; H, 6.31.
Crystallographic Analysis. X-ray-quality single crystals of
complexes 2A-Br, 2B-I, and 4A-Br2 were obtained by the slow
diffusion of diethyl ether into their dichloromethane or MeCN
solutions. Complex 2B-I cocrystalized with half an equivalent of
ether. Complex 4A-Br2 cocrystalized with half an equivalent of
MeCN and 1 equiv of Et2O. Crystal data were collected on a Bruker
X8 Kappa CCD diffractometer at 173 K using graphite monochro-
mated Mo KR radiation (λ ) 0.71073Å). The APEX2 Software
Suite (Bruker, 2005) was used for data acquisition, structure
solution, and refinement. Absorption corrections were applied using
SADABS. The structure was solved by direct method and refined
by full-matrix least-squares method on F2 using Xshell.
Acknowledgment. We thank the School of Physical and
Mathematical Sciences, Nanyang Technological University
for financial support and the Johnson Matthey Co. for a loan
of iridium and rhodium chloride. Jing Zhao thanks Nanjing
University for financial support. We also thank Prof. Robert
Crabtree (Yale University) for helpful discussions.
Dimerization Product of EtOC(O)CtCH (entry 6, Table
1
3
3). H NMR (δ, 400 MHz, CDCl3, 298 K): 1.27 (t, J ) 7.1 Hz,
3
3
3H, CH3), 1.29 (t, J ) 7.2 Hz, 3H, CH3), 4.20 (q, J ) 7.1 Hz,
2H, CH2), 4.24 (q, 3J ) 7.1 Hz, 2H, CH2), 6.42 (d, 3J ) 16
Hz, 1H), 6.74 (d, J ) 16 Hz, 1H). 13C {1H} NMR (δ, 100 MHz,
3
CDCl3, 298 K): 13.99 (s, CH3), 14.13 (s, CH3), 61.34 (s, CH2),
62.49 (s, CH2), 81.50 (s, CtC), 87.02 (s, CtC), 121.54 (s, CdC),
Supporting Information Available: Crystal data of complexes
2A-Br, 2B-I, and 4A-Br2 in PDF and CIF formats. This material
is free of charge via the Internet at http://pubs.acs.org.
(31) Bassetti, M.; Pasquini, C.; Raneri, A.; Rosato, D. J. Org. Chem. 2007,
72, 4558.
IC801878E
1206 Inorganic Chemistry, Vol. 48, No. 3, 2009