N-metallacycles from [CpMX2]2 and alkynylpyridines: Synthesis, reaction pathway, and aromaticity

Article abstract of DOI:10.1021/om401154c

The reaction of [CpMX₂]₂ (M = Rh or Ir, X = Cl, Br, or I) with alkynylpyridines afforded halogen-substituted N-metallacyclic complexes. The reaction pathway has been examined through deuterium labeling and other experiments and compu

Full text of DOI:10.1021/om401154c

Article
pubs.acs.org/Organometallics
N‑Metallacycles from [Cp*MX2]2 and Alkynylpyridines: Synthesis,
Reaction Pathway, and Aromaticity
Jing Wei Teo,† Venugopal Shanmugham Sridevi,‡ and Weng Kee Leong*,†
†Division of Chemistry and Biological Chemistry, Nanyang Technological University, 21 Nanyang Link, Singapore 637371
‡Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543
S
* Supporting Information
ABSTRACT: The reaction of [Cp*MX2]2 (M = Rh or Ir,
X = Cl, Br, or I) with alkynylpyridines afforded halogen-substituted
N-metallacyclic complexes. The reaction pathway has been
examined through deuterium labeling and other experiments
and computational studies and is proposed to proceed via
halide dissociation followed by attack at the alkyne. These
N-metallacycles exhibit aromaticity and undergo Sonogashira
coupling reactions.
Scheme 1
INTRODUCTION
■
Cyclometalated complexes have attracted interest because
of their involvement in many carbon−carbon1 and carbon−
heteroatom2 bond-forming reactions, often under relatively
mild conditions. More recently, N-metallacyclic complexes,
especially those of iridium, have been demonstrated to promote
the oxidation of water to oxygen,3 selective C−H bond oxida-
tion,4 conversion of alkanes to alkenes,5 and transfer hydro-
genation of ketones and amines.6 Conjugated metallacyclic
complexes, in particular, are also of interest, as they are known
to display structural and spectroscopic characteristics of aro-
maticity,7 and a wide variety of aromatic metallacycles have
been synthesized and their physical and chemical properties
investigated over the years.8
A variety of synthetic methodologies to metallacycles have
been developed. These include ortho C−H activation of aro-
matic amines9 or imines,10 and transmetalation,11 often involv-
ing the use of additives such as a base,12 an acid,13 or a silver
salt.14 In the course of our investigations into the chemistry of
the complex [Cp*MX2]2, 1, we have serendipitously uncovered
a simple, one-pot route to a class of aromatic N-iridacycles.
by a single-crystal X-ray diffraction study; the ORTEP plot
showing the molecular structure, together with selected bond
parameters, is shown in Figure 1.
The Ir−C bond length of the metallacycle is rather short
(Ir(1)−C(7) = 2.036(4) Å), comparable to that in similar
N-iridacyclic compounds, and suggests some IrC double-
bond character.15 This would also suggest that there is
delocalization of electrons within the metallacycle and hence
some degree of aromaticity. Consistent with this is that, for
all the complexes 3 in this study, the resonance for the proton
on the metallacycle is highly deshielded (δH ≈ 8.5−9.2 ppm).
This can be compared to the more upfield shift reported for an
α-CH of a vinyl group in transition metal vinyl complexes (δH =
5.84 ppm).16 We do note, however, that the differences in the
C−C bond lengths within the ring of 3a-Cl (C(6)−C(7) =
1.313(6) Å and C(5)−C(6) = 1.440(6) Å) suggest the dominance
of one resonance structure.
RESULTS AND DISCUSSION
■
The reaction between 1 and alkynylpyridine 2 afforded the
N-iridacyclic complexes 3 in essentially quantitative yields
(Scheme 1). This reaction works well with both terminal and
internal alkynes, and the electronic properties of R and R′ do
not show much effect on the yield.
The reaction could also work with other halogeno derivatives
(3a-Br and 3a-I), starting from [Cp*IrX2]2 (X = Br or I), and
could also be extended to other N-heterocycles with a similar
structural motif, such as quinoline and pyrimidine. Similar N-
rhodacyclic complexes could also be synthesized starting from
[Cp*RhCl2]2, 1′ (Scheme 2).
The products 3 have been characterized spectroscopically
and analytically, and the structure of 3a-Cl has been confirmed
Received: November 27, 2013
Published: February 24, 2014
© 2014 American Chemical Society
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Scheme 2
Scheme 3
Figure 2. Computed HOMO−28 of 3a-Cl showing delocalization of
the orbital within the pyridine and N-iridacycle.
isomer, such as those shown in Scheme 4. A negative ISE
indicates the presence of a stabilization that may be regarded
as aromatic stabilization energy (ASE) on moving from the
nonaromatic complex I, III, V, and VII to the aromatic
derivative II, IV, VI, and VIII. The ISE from complex I to II is
not appropriate for an assessment of the aromaticity within the
metallacyclic ring, as the ene functionality in I is not conjugated
with the pyridine ring. We have thus calculated the ISE for
complex III and similar analogues in which the location of the
Cl has to be, unfortunately, different from that in the complexes
3. The figures indicate the presence of aromaticity in the
iridacyclic system, which are about two-thirds that of pyrrole
(−63 and −92 kJ mol−1, respectively); the rhodium analogue
has a slightly lower value (−45 kJ mol−1). In contrast to organic sys-
tems, for example, pyrrole and indole (−92 and −197 kJ mol−1,
respectively), however, the presence of the six-membered
pyridine ring does not help to further stabilize the iridacycle
(−63 and −66 kJ mol−1 for II and IV, respectively).
The second measure of aromaticity is the nucleus-
independent chemical shift (NICS),18 in which the absolute
magnetic shielding at the center of the ring is computed. A
negative NICS value indicates aromaticity, while a positive
value indicates antiaromaticity. In order to reduce the influence
of the σ-bonding framework, NICS values calculated are at
points some distance from the plane of the ring.19 Some groups
have recommended scanning the NICS values over a range of
distances for more complicated systems, arguing that the use of
a single NICS value may not be adequate.20 Our calculations on
both complexes I and II gave negative values (Table S2), which
was unexpected since there should not be any aromaticity in
the metallacycle of I, as corroborated by the ISE calculations.
Figure 1. ORTEP plot of 3a-Cl. All H atoms have been omitted.
Thermal ellipsoids are plotted at the 50% probability level. Selected
bond lengths (Å) and angles (deg): Ir(1)−Cl(1) = 2.3997(12); Ir(1)−
C(7) = 2.036(4); Ir(1)−N(1) = 2.077(4); C(6)−C(7) = 1.313(6);
C(5)−C(6) = 1.440(6); C(5)−N(1) = 1.358(6); C(7)−Ir(1)−N(1) =
77.62(17).
A useful feature of the complexes 3 is the presence of a
halogen substituent on the metallacycle. This allows for fur-
ther synthetic elaboration. For instance, we have found that we
can carry out Sonogashira coupling reactions to afford alkyne-
substituted analogues (Scheme 3).
Aromaticity Studies. Besides the structural and NMR
characteristics mentioned above, the aromaticity of the
metallacycle is also indicated by the computationally optimized
structure of 3a-Cl, which possesses a π-type molecular orbital,
the HOMO−28, which is delocalized over both the pyridine
ring and the five-membered iridacycle (Figure 2).
We also examined the use of two different computational
methods to verify the aromaticity of the metallacycle. The first
of these is the isomerization stabilization energy (ISE) proposed
by Schleyer and co-workers.17 This involves computation of
the total energy difference between two species, the proposed
aromatic system and its nonaromatic exocyclic methylene
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Scheme 4
Table 1. NICS Values of 3a-Cl Calculated at Various
a
Distance from the Center of the Metallacyclic Plane
distance of Bq from the geometric center
(n Å)
NICS(n) NICS(0) NICS(−n)
0.5
1.0
1.5
2.0
2.5
3.0
−5.7
−3.1
−1.9
−1.2
−0.9
−0.8
−9.3
−9.3
−9.3
−9.3
−9.3
−9.3
−10.8
−9.3
−7.9
−7.9
−8.8
−7.3
a
The position of Bq, at which the NICS value is calculated, is indicated
by the purple sphere.
Scheme 5
We have also calculated the values of NICS(n) and NICS(−n),
with n ranging from 0 to 3 Å, for the complex 3a-Cl (Table 1).
While the NICS(n) values were dependent on the distance of
Bq from the geometric center of the ring, such a trend was not
observed for the NICS(−n) values. We have attributed this to
Bq approaching the Cl atom for the NICS(−n) values. For such
complexes, therefore, NICS may not be a good indicator of
aromaticity.
Reaction Pathway. An isotopic labeling experiment with
d1- alkynylpyridine 2m shows that the deuterium is incor-
porated into the iridacycle 3m-Cl, confirming that the C−H
bond is not cleaved (Scheme 5).
Table 2. Ratio of Products from the Reactions of 1-X with
2a in the Presence of 10 Equivalents (wrt 1-X) of [Et4N]Y
(X, Y = Cl or Br)
To verify if the Cl on the metallacyclic ring is the result of
external attack by a dissociated chloride or via an intramolecular
transfer, two NMR-scale reactions were carried out: (1) the
reaction of 1-Cl with 2a in the presence of 10 equivalents (wrt
1-Cl) of [Et4N]Br and (2) the reverse reaction of 1-Br with
2a in the presence of 10 equivalents (wrt 1-Br) of [Et4N]Cl.
Each of the reactions gave all four possible products,
3a-X-Y (X, Y = Cl or Br), but in different ratios (Table 2
and Figure S2).
The observations were not consistent with an intramolecular
halide transfer and point instead to external attack by halide.
Such halide ion functionalization of alkynes to form metalla-
cycles has been reported.21 The possibility of halide exchange in
the dimeric reactant prior to subsequent reaction with 2a was
ruled out; although the reaction of 1-Cl with four equivalents of
[Et4N]Br led rapidly (<1/2 h) to the formation of 1-Br, the
reverse reaction did not proceed even overnight. This suggests
that 1-Br is more stable than 1-Cl. On the other hand, the
ratios of 3a-Cl-Y:3a-Br-Y obtained for the reactions in Table 2
reaction
3a-Cl-Cl 3a-Br-Cl 3a-Cl-Br 3a-Br-Br
(1) 1-Cl + 2a + [Et4N]Br
(2) 1-Br + 2a + [Et4N]Cl
1.0
1.0
0.9
0.2
3.7
0.6
3.7
0.1
(∼1:1 and 5:1 for reactions 1 and 2, respectively) point clearly
to the preference for a Cl over a Br ligand in the mononuclear
products. We have also verified this through the reaction of
Cp*Ir(py)(Cl)2 with five equivalents of [Et4N]Br, which afforded
Cp*Ir(py)(Cl)2, Cp*Ir(py)(Br)(Cl), and Cp*Ir(py)(Br)2 in a
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Scheme 6
ratio of 1:3:2, while the reverse reaction of Cp*Ir(py)(Br)2 with
five equivalents of [Et4N]Cl afforded only Cp*Ir(py)(Cl)2 and
Cp*Ir(py)(Br)(Cl) in a ratio of 4:1 (Figure S2). These latter
observations can also account for the fact that the ratio of 3a-X-
Cl:3a-X-Br for reaction 2 in Table 1 is not simply the reverse of
that in reaction 1; the greater tendency for Cl− to displace
metal-bound Br− from a mononuclear intermediate in reaction
2 would lead to a larger proportion of 3a-X-Br than may be
expected from a simple reversal of reaction 1. That the reaction
did not require a dimeric intermediate was ruled out through
the observation that 3a-Cl was also obtained from the reaction
of Cp*Ir(py)(Cl)2 with 2a. Attempts to exploit this reaction to
obtain non-halogen-substituted analogues have not been
successful so far: reactions of 1-Cl and 2a in a large excess
of NaOMe or [Et4N]OH gave only 3a; no MeO- or OH-
substituted analogues were observed.
On the basis of the above, we have proposed the reaction
pathway shown in Scheme 6. The Gibbs free energy for the
various steps has been computed with density functional
theory, and the computed energies (in kJ mol−1) are also
given. For this pathway, the PCM model with CH2Cl2 as the
reaction solvent was used in the computation. The initial
binding of the nitrogen to the iridium metal center led to the
formation of A, followed by the dissociation of a chloride
anion. External attack by chloride on the CC moiety led to
the formation of the final product. We have modeled the
transition state (TS) for this last step, which lies +40 kJ mol−1
above B (Figure S3).
EXPERIMENTAL SECTION
■
General Procedures. All reactions and manipulations, except
for TLC separations, were performed under an argon atmosphere by
using standard Schlenk techniques. Solvents used were of AR grade.
The starting materials 1,22 1′,23 and 1-X (X = Br, I)24 were prepared
according to published methods. All other chemicals were from
commercial sources and used as supplied without further purification.
1H and 13C{1H} NMR spectra were recorded on a 300 or 400 NMR
spectrometer. Chemical shifts reported are with respect to the residual
1H resonance in CDCl3. High-resolution mass spectra (HRMS) were
recorded in ESI mode on a TOF mass spectrometer.
Reaction of 1 with the Alkynylpyridines. In a typical reaction,
to a solution of 1-Cl (20 mg, 25 μmol) in dichloromethane (3 mL)
was added 2a (5.1 μL, 50 μmol). The reaction mixture was stirred for
3 h, and the solvent was then removed under reduced pressure to
afford 3a-Cl as a dark brown solid.
An analogous procedure was used for all the other complexes. The
1
yields and H and 13C NMR data are collected in Table 3.
Sonogashira Coupling Reaction. A mixture of complex 3a-Br
(40 mg, 0.068 mmol), Pd(PPh3)4 (2 mg, 2 mol %), CuI (1 mg,
2 mol %), diisopropylamine (10 μL), and trimethylsilylacetylene
(15 μL, 0.075 mmol) in THF (5 mL) was heated to 50 °C for 18 h.
The mixture was filtered, and solvent was removed under vacuum. The
crude product was purified by TLC using Hex/DCM (1:3, v/v) as
eluent, and a yellow band of 5-Br was isolated. An analogous pro-
cedure was used for the reaction of complex 3a-I and trimethylsily-
lacetylene.
Data for 5-Br. 1H NMR (δ, CDCl3): 0.24 (s, 9H, CH3), 1.84
(s, 15H, Cp*), 6.85−6.89 (m, 1H, aromatic), 7.52−7.61 (m, 2H,
aromatic), 8.49 (d, 1H, 2JHH = 5.7 Hz, aromatic), 9.65 (s, 1H, Ir-C).
HRMS: 529.1779; calc for [M − Br + H]+ 529.1777. Yield: 3.6 mg
(9%).
Data for 5-I. 1H NMR (δ, CDCl3): 0.24 (s, 9H, CH3), 1.84 (s, 15H,
Cp*), 6.85−6.89 (m, 1H, aromatic), 7.51−7.61 (m, 2H, aromatic),
8.50 (d, 1H, 2JHH = 5.7 Hz, aromatic), 9.66 (s, 1H, H−C). HRMS:
529.1711; calc for [M − I + H]+ 529.1722. Yield: 5.7 mg (13%).
Deuterium Labeling Experiment. Complex 1-Cl (0.025 mmol,
20 mg) and 2m (0.05 mmol, 5 μL) were dissolved in DCE (2 mL),
and the mixture was stirred at rt for 18 h. Excess solvent was removed
under reduced pressure to yield 3m-Cl.
CONCLUDING REMARKS
■
In this study, we have reported a facile synthetic route to
N-iridacycles from the reaction of alkynylpyridines with the
readily available dinuclear species [Cp*MX2]2. A reaction
pathway involving dissociation of a halide ion followed by
external attack on the alkyne moiety has been proposed on the
basis of experimental and computational studies. We have also
demonstrated that these complexes can be further derivatized at
the metallacycle via Sonogashira coupling; they are almost
certainly susceptible to other similar coupling reactions as well.
The aromaticity of the metallacycles has been verified by a
number of methods, among which our results suggest that
nucleus-independent chemical shifts are not good indicators for
such complexes.
1
Data for 3m-Cl. H NMR (δ, CDCl3): 1.74 (s, 15H, Cp*), 7.01−
7.05 (m, 1H, aromatic), 7.55 (d, 1H, J = 7.8 Hz, aromatic), 7.61−7.65
(m, 1H, aromatic), 8.47 (d, 1H, J = 5.5 Hz, aromatic), 8.61 (s, 0.09H,
H-C, 90% deuteration).
Halide Exchange Reaction. In a typical reaction, to a solution
of 1-Cl (15 mg, 19 μmol) and NEt4Br (40 mg, 0.19 mmol) in
dichloromethane (3 mL) was added 2a (4 μL, 38 μmol). The reaction
mixture was stirred for 1 h, and the solvent was then removed under
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Table 3. Yields, HRMS, and H and 13C{1H} NMR Data for 3
Article
1
product alkyne
yield
HRMS
1H NMR (δ, CDCl3)
13C{1H} NMR data
a
3a-Cl
2a
25 mg, 99% Found: 467.0989; Calcd [M − Cl + H]+: 1.73 (s, 15H, Cp*), 7.01−7.04 (m, 1H,
9.2 (CH3, Cp*), 89.3 (C, Cp*), 120.6, 121.2,
124.4, 137.8, 151.3, 164.0 (C and CH,
aromatic), 165.3 (Ir-C)
467.0992
aromatic), 7.54 (d, 1H, J = 7.9 Hz,
aromatic), 7.61−7.65 (m, 1H, aromatic),
8.47 (d, 1H, J = 5.3 Hz, aromatic), 8.61
(s, 1H, H-C)
3a-Br
3a-I
2a
2a
2b
29 mg, 99% Found: 511.0471; Calcd [M − Br + H]+: 1.76 (s, 15H, Cp*), 6.94−6.97 (m, 1H,
9.4 (CH3, Cp*), 89.5 (C, Cp*), 110.4, 120.9,
121.6, 137.8, 151.8, 164.5 (C and CH,
aromatic), 169.9 (Ir-C)
511.0487
aromatic), 7.60−7.63 (m, 2H, aromatic),
8.46 (d, 1H, J = 5.7 Hz, aromatic), 8.94
(s, 1H, H-C)
34 mg, 99% Found: 559.0348; Calcd [M − I + H]+: 1.82 (s, 15H, Cp*), 6.79−6.82 (m, 1H,
9.9 (CH3, Cp*), 90.0 (C, Cp*), 120.2, 123.5,
137.7, 152.7, 165.9 (C and CH, aromatic),
180.3 (Ir-C)
559.0349
aromatic), 7.54−7.57 (m, 1H, aromatic),
7.67−7.69 (m, 1H, aromatic), 8.46 (d, 1H,
J = 5.6 Hz, aromatic), 9.51 (s, 1H, H-C)
3b-Cl
29 mg, 99% Found: 543.1298; Calcd [M − Cl + H]+: 1.42 (s, 15H, Cp*), 7.01−7.05 (m, 1H,
8.5 (CH3, Cp*), 89.3 (C, Cp*), 119.9, 120.9,
120.9, 126.2, 127.2, 127.4, 128.4, 132.0 137.5,
146.0, 150.8, 165.0 (C and CH aromatic),
174.9 (Ir-C)
543.1305
aromatic), 7.20 (t, 1H, J = 7.4 Hz,
aromatic), 7.35 (t, 2H, J = 7.8 Hz,
aromatic), 7.49 (d, 2H, J = 7.2 Hz,
aromatic), 7.64−7.68 (m, 2H, aromatic),
8.52 (d, 1H, J = 5.6 Hz, aromatic)
3c-Cl
2c
30 mg, 99% Found: 573.1408; Calcd [M − Cl + H]+: 1.42 (s, 15H, Cp*), 3.84 (s, 3H, OCH3),
8.6 (CH3, Cp*), 55.2 (OCH3), 89.4 (C, Cp*),
112.5, 120.7, 129.1, 137.5, 138.6, 150.7, 158.1,
165.1 (C and CH aromatic), 175.1 (Ir-C)
573.1411
6.89 (d, 2H, J = 8.8 Hz, aromatic),
6.99−7.03 (m, 1H, aromatic), 7.48 (d, 2H,
J = 8.8 Hz, aromatic), 7.62−7.66 (m, 2H,
aromatic), 8.50 (d, 1H, J = 5.6 Hz,
aromatic)
3d-Cl
3e-Cl
2d
2e
30 mg, 99% Found: 561.1201; Calcd [M − Cl + H]+: 1.42 (s, 15H, Cp*), 7.02−7.08 (m, 3H,
8.9 (CH3, Cp*), 89.4 (C, Cp*), 114.0, 114.2,
121.0, 121.1, 129.2, 129.3, 137.6, 150.8, 164.9
(C and CH aromatic), 173.5 (Ir-C)
561.1211
aromatic), 7.48−7.52 (m, 2H, aromatic),
7.65−7.66 (m, 2H, aromatic), 8.52 (d, 1H,
J = 5.6 Hz, aromatic)
30 mg, 98% Found: 588.1144; Calcd [M − Cl + H]+: 1.43 (s, 15H, Cp*), 7.09−7.13 (m, 1H,
8.6 (CH3, Cp*), 89.4 (C, Cp*), 121.4, 121.6,
121.8, 122.9, 127.9, 137.8, 145.9, 150.9, 153.7,
164.2 (C and CH aromatic), 170.5 (Ir-C)
588.1156
aromatic), 7.63−7.65 (m, 2H, aromatic),
7.70−7.71 (m, 2H, aromatic), 8.21−8.23
(m, 2H, aromatic), 8.56 (d, 1H, J = 5.6 Hz,
aromatic)
3f-Cl
2f
29 mg, 97% Found: 573.1401; Calcd [M − Cl + H]+: 1.43 (s, 15H, Cp*), 3.85 (s, 3H, OCH3),
8.5 (CH3, Cp*), 53.5 (OCH3), 89.6 (C, Cp*),
109.8, 120.3, 120.7, 127.2, 128.0, 135.6, 137.4,
150.7, 154.5, 164.7 (C and CH aromatic),
171.3 (Ir-C)
573.1411
6.91−7.01 (m, 3H, aromatic), 7.17−7.22
(m, 1H, aromatic), 7.52−7.54 (m, 1H,
aromatic), 7.58−7.69 (m, 2H, aromatic),
8.50 (d, 1H, J = 7.6 Hz, aromatic)
3g-Cl
3h-Cl
3i-Cl
3j-Cl
2g
2h
2i
38 mg, 98% Found: 481.1140; Calcd [M − Cl + H]+: 1.68 (s, 15H, Cp*), 2.96 (s, 3H, CH3), 7.06 9.5 (CH3, Cp*), 27.4 (CH3), 89.2 (C, Cp*),
2
481.1148
(d, 1H, JHH = 7.2 Hz, aromatic), 7.36 (d,
1H, J = 7.5 Hz, aromatic), 7.47−7.50 (m,
1H, aromatic), 8.56 (s, 1H, H-C)
117.5, 120.2, 137.8 (C and CH aromatic),
164.8 (Ir-C)
26 mg, 98% Found: 497.1100; Calcd [M − Cl + H]+: 1.72 (s, 15H, Cp*), 4.00 (s, 3H, OCH3),
9.7 (CH3, Cp*), 57.3 (OCH3), 89.3 (C, Cp*),
101.1, 113.2, 125.1, 140.8, 163.2, 164.2 (C and
CH aromatic), 165.6 (Ir-C)
497.1098
6.51−6.54 (m, 1H, aromatic), 7.17−7.20
(m, 1H, aromatic), 7.58−7.63 (m, 1H,
aromatic), 8.57 (s, 1H, H-C)
25 mg, 98% Found: 485.0911; Calcd [M − Cl + H]+: 1.74 (s, 15H, Cp*), 7.40−7.46 (m, 1H,
9.1 (CH3, Cp*), 89.5 (C, Cp*), 121.2, 121.2,
125.2, 125.4, 139.3, 139.6, 155.7, 158.2, 160.7,
164.3 (C and CH aromatic), 164.3 (Ir-C)
485.0898
aromatic), 7.52−7.57 (m, 1H, aromatic),
8.37−8.39 (m, 1H, aromatic), 8.57 (s, 1H,
H-C)
2j
27 mg, 98% Found: 517.1133; Calcd [M − Cl + H]+: 1.70 (s, 15H, Cp*), 7.51−7.54 (m, 1H,
9.5 (CH3, Cp*), 90.1 (C, Cp*), 118.6, 125.4,
126.2, 126.4, 128.1, 128.6, 130.8, 138.8, 146.0,
165.0 (C and CH aromatic), 172.2 (Ir-C)
517.1148
aromatic), 7.67−7.75 (m, 3H, aromatic),
8.01 (d, 1H, J = 8.7 Hz, aromatic), 8.41 (d,
1H, J = 8.7 Hz, aromatic), 8.98 (s, 1H, H-
C)
3k-Cl
2k
2a
29 mg, 99% Found: 468.0923; Calcd [M − Cl + H]+: 1.74 (s, 15H, Cp*), 7.00−7.04 (m, 1H,
9.2 (CH3, Cp*), 90.0 (C, Cp*), 116.7, 123.0,
158.4, 158.5, 171.3 (C and CH aromatic),
172.5 (Ir-C)
468.0944
aromatic), 8.59−8.70 (m, 2H, aromatic),
9.16 (s, 1H, H-C)
b
1
3′a
20 mg
(98%)
Found: 377.0428; Calcd [M − Cl + H]+: 1.67 (s, 15H, Cp*), 7.08−7.12 (m, 1H,
377.0418
9.3 (CH3, Cp*), 96.2 (d, JRhC = 6.18 Hz, C,
aromatic), 7.51 (d, 1H, J = 8.0 Hz,
aromatic), 7.70−7.74 (m, 1H, aromatic),
8.56 (d, 1H, J = 5.2 Hz, aromatic), 8.60 (s,
1H, H-C)
Cp*), 120.9, 121.2, 137.8, 151.0 (C and CH,
1
aromatic), 178.8 (d, JRhC = 35.14 Hz, Rh-
C)
1
3′b
2b
24 mg, 99% Found: 453.0734; Calcd [M − Cl + H]+: 1.37 (s, 15H, Cp*), 7.08−7.12 (m, 1H,
8.8 (CH3, Cp*), 96.4 (d, JRhC = 6.61 Hz, C,
453.0731
aromatic), 7.19−7.23(m, 1H, aromatic),
7.32−7.36 (m, 2H, aromatic), 7.59−7.61
(m, 2H, aromatic), 7.64−7.66 (m, 1H,
aromatic), 7.71−7.75(m, 1H, aromatic),
8.74 (d, 1H, J = 5.2 Hz)
Cp*), 120.9, 121.0, 126.3, 127.1, 127.7, 137.6,
144.8, 150.8, 163.6 (C and CH aromatic),
1
186.8 (d, JRhC = 32.96 Hz, Rh-C)
a
b
Anal. Calcd for C17H20Cl2NIIr: C, 40.72; H, 4.02; N, 2.79. Found: C, 40.70; H, 4.27; N, 2.55. Anal. Calcd for C17H20Cl2NRh: C, 49.54; H, 4.89; N,
3.40. Found: C, 50.05; H, 5.29; N, 2.99.
reduced pressure. The crude 1H NMR spectrum showed the presence
of 3a-Cl-Cl, 3a-Cl-Br, 3a-Br-Cl, and 3a-Br-Br.
An analogous procedure was used for the other salts.
Crystallographic Studies. A diffraction quality crystal of 3a-Cl
was grown by slow diffusion of hexane into a dichloromethane
solution and then mounted onto a quartz fiber. X-ray data were
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Organometallics
Article
collected on a Bruker AXS APEX system, using Mo Kα radiation,
at 223 K with the SMART suite of programs.25 Data were processed
and corrected for Lorentz and polarization effects with SAINT26 and
for absorption effects with SADABS.27 Structural solution and
refinement were carried out with the SHELXTL suite of programs.28
The structure was solved by direct methods to locate the iridium atom,
followed by difference maps for the light, non-hydrogen atoms.
Hydrogen atoms were placed in calculated positions and refined with a
riding model.
Computational Studies. Geometry optimizations were carried
out by DFT utilizing Becke’s three-parameter hybrid function29 and
Lee, Yang, and Parr’s gradient-corrected correlation function
(B3LYP).30 The LANL2DZ (Los Alamos effective core potential
double-ζ) basis set, together with an f polarization function,31 was
employed for the Ir atom; the 6-311+G(2d,p) basis set was used for all
other atoms. For calculations involving solvated species, the polarized
continuum model (PCM) using dichloromethane as the solvent was
employed to approximate a solvent field. Spin-restricted calculations
were used for geometry optimization, and harmonic frequencies were
then calculated to characterize the stationary points as equilibrium
structures with all real frequencies and to evaluate zero-point energy
(ZPE) corrections. The transition state was characterized with one
imaginary frequency, which corresponded to the expected reaction
trajectory. NICS(0) was evaluated by placing a ghost atom (Bq) at the
geometric mean of the ring atoms, and NICS(n) and NICS(−n) were
evaluated with Bq at n Å above and below the geometric center,
respectively (Table 1). The NICS values are the isotropic chemical
shifts of the respective Bq’s. All calculations were performed using the
Gaussian 09 suite of programs.32
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ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic data of 3a-Cl in CIF format, experimental
details and characterization for the complexes, and details of
deuterium labeling experiments and of computational studies.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Nanyang Technological University
and the Ministry of Education (Research Grant No. M4011017).
J.W.T. and V.S.S. thank their respective universities for Research
Scholarships.
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