Rhodium(I) and iridium(I) imidazo[1,5-a]pyridine-1-ylalkylalkoxy complexes: Synthesis, characterization and application as catalysts for hydrosilylation of alkynes

Article abstract of DOI:10.1016/j.jorganchem.2015.05.045

Abstract Rh(I) and Ir(I) complexes bearing imidazo[1,5-a]pyridine-1-ylalkyl alcohol as chelating N??O-monoanionic ligands were prepared as thermally stable compounds. Their spectroscopic properties and structures were determined based on IR and NMR spectr

Full text of DOI:10.1016/j.jorganchem.2015.05.045

Journal of Organometallic Chemistry 794 (2015) 76e80
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Rhodium(I) and iridium(I) imidazo[1,5-a]pyridine-1-ylalkylalkoxy
complexes: Synthesis, characterization and application as catalysts
for hydrosilylation of alkynes
, b,
Toshiaki Murai a *, Eri Nagaya a, Fumitoshi Shibahara a, Toshifumi Maruyama a,
Hiroshi Nakazawa c
a Department of Chemistry and Biomolecular Science, Faculty of Engineering, Gifu University, Yanagido, Gifu 501-1193, Japan
b JST, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
c Department of Chemistry, Graduate School of Science, Osaka City University, Osaka 558-8585, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Rh(I) and Ir(I) complexes bearing imidazo[1,5-a]pyridine-1-ylalkyl alcohol as chelating N^O-monoanionic
ligands were prepared as thermally stable compounds. Their spectroscopic properties and structures
were determined based on IR and NMR spectra and X-ray analyses. The Rh(I) complexes were used as
catalysts for the hydrosilylation of alkynes. The catalytic activities of the complexes were highly influ-
enced by the substituents on their aromatic rings. Highly E-selective formation of vinylsilanes was
achieved at higher temperatures for longer reaction times.
Received 8 April 2015
Received in revised form
21 May 2015
Accepted 23 May 2015
Available online 24 June 2015
© 2015 Elsevier B.V. All rights reserved.
Keywords:
Imidazo[1,5-a]pyridine
Rhodium(I) complex
Ir(I) complex
Hydrosilylation
1. Introduction
pyridine-1-ylalkylalcohols with Rh(I) and Ir(I) metals. The catalytic
activities of the resulting Rh(I) complexes were also tested in the
The formation of metal complexes with hard ancillary ligands
such as N^N- and chelating N^O-monoanionic species has been of
interest [1]. As examples of the latter compounds with an alkoxy
group, pyridylalkyl alcohols have been used for the complexation
with Rh [2] and Ir [3] metals. Meanwhile, increasing attention has
recently been paid to metal complexes of imidazo[1,5-a]pyridines,
which possess ring-fused pyridines and imidazoles, as nitrogen-
hydrosilylation reaction of alkynes.
2. Results and discussion
2.1. Synthesis and characterization of Rh(I) and Ir(I) complexes
Initially, imidazo[1,5-a]pyridine-1-ylalkyl alcohol 1a reacted
with Rh(acac) (CO)2 in toluene for 2 h (Table 1).
containing ligands and have 10-
characteristic features of imidazo[1,5-a]pyridines is that a range of
substituents can be introduced to to six carbon atoms within the
p electron systems [4]. One of
The reaction mixture was concentrated, and the residue was
purified by flash column chromatography on silica gel to give Rh(I)
complex 2a as a yellow solid. X-ray analyses of the starting alcohol
1a and Rh(I) complex 2a were carried out. Their ORTEP drawings
are shown in Figs. 1 and 2 along with selected bond lengths, angles
and torsion angles.
These results clearly showed the formation of the unprece-
dented Rh(I) complex 2a with a new type of N^O ligand. For 1a, the
hydroxy group was oriented at the gauche position with respect to
the plane of N1eC2eC1, whereas the oxygen atom in 2a was
involved in the nearly same plane composed of Rh1eN2eC16eC3.
Coordination of the nitrogen atom to Rh atom had almost no effect
a
ring, and this enables us to finely tune the electronic and steric
circumstances of the ring. In this regard, we recently developed
imidazo[1,5-a]pyridyl-based N^O ligands for the first time [5] and
tested their ability to coordinate to main group elements such as a
boron atom [6]. We report herein the preparation and character-
ization of the first transition metal complexes of imidazo[1,5,-a]
* Corresponding author. Department of Chemistry and Biomolecular Science,
Faculty of Engineering, Gifu University, Yanagido, Gifu 501-1193, Japan.
E-mail address: mtoshi@gifu-u.ac.jp (T. Murai).
http://dx.doi.org/10.1016/j.jorganchem.2015.05.045
0022-328X/© 2015 Elsevier B.V. All rights reserved.

T. Murai et al. / Journal of Organometallic Chemistry 794 (2015) 76e80
77
Table 1
Syntheses of Rh(I) and Ir(I) complexes of imidazo[1,5-a]pyridyldiarylmethanol.
Ar0
Ar
M
Yield
1a
1b
1c
1d
1e
1a
C6H4Me-4
C6H4Me-4
C6H4Me-4
C6H4OMe-4
C6H4Me-4
C6H4Me-4
Ph
C6H4CI-4
C6H4OMe-4
Ph
a
Rh
Rh
Rh
Rh
Rh
Ir
2a
2b
2c
2d
2e
3a
74%
69%
71%
68%
76%
74%
Ph
a
Fig. 2. ORTEP drawing of 2a. Selected bond lengths (Å) and angles (ꢁ): N2eC22
1.387(3), N2eC16 1.399(3), C16eC3 1.541(3), C3eO2 1.458(3), N2eRh1 2.110(2),
Rh1eC2 1.885(3), Rh1eC1 1.883(3), C2eO2 1.159(4), C1eO1 1.556(3), C22eN2eC16
109.2(2), N2eC16eC3 117.0(2), C16eC3eO3 107.06(17), N2eRh1eO3 79.71(7),
.
C2eRh1eC1
88.27(13),
N2eRh1eC2
172.75(9),
O3eRh1eC1
177.45(10),
C28eC23eC22eN2 ꢀ48.0(4), 03eC3eC16eN2 24.6(3), C1eRh1eN2eC22 ꢀ23.8(3),
on the bond angles involving the nitrogen atom (bond angle of
CeNeC: 107.4(2) for 1a and 109.2(2) for 2a). The deviation of the 4-
methylphenyl group from the imidazopyridyl ring in 2a was greater
than that in 1a (distorted angle: ꢀ42.1(4) for 1a and ꢀ48.0(4) for
2a). Rh atom adopted a slightly distorted square planar structure.
Two CO ligands in 2a were oriented almost linearly toward the
nitrogen atom (N2) and the oxygen atom (O3), respectively. The
bond lengths of Rh1eC1 and Rh1eC2 and those of C1eO1 and
C2eO2 were nearly identical despite the fact that different atoms,
i.e., nitrogen and oxygen atoms, were oriented to the trans position
of each CO ligand.
A similar reaction of the alcohols 1be1d with Rh(acac)(CO)2 also
proceeded with high efficiency to give the corresponding Rh(I)
complexes 2 in 68e71% yields (Table 1). The chlorine atom and
methoxy group in 1be1d did not affect the efficiency of the reac-
tion. The reaction of the alcohol 1e with a fluorenyl group showed
similar efficiency, and the corresponding Rh(I) complex 2e was
obtained in 76% yield. Finally, instead of Rh(acac)(CO)2, Ir(a-
cac)(CO)2 was used to prepare the Ir(I) complex. Due to the lower
solubility of Ir(acac)(CO)2 in toluene, CH2Cl2 was used as a solvent
O3eRh1eN2eC22 158.1(3).
to give the corresponding Ir(I) complex 3a in 74% yield. The for-
mation of Ir(I) complex 3a was also unequivocally determined by X-
ray analysis. The ORTEP drawing of 3a is shown in Fig. 3.
Ir(I) complex 3a was isostructural to Rh(I) complex 2a, and bond
lengths except for the bonds involving metals are the nearly same.
The melting points and characteristic spectroscopic properties
of 2 and 3 are shown in Table 2.
The data of Rh(I)-quinolinolato complex 4 are also shown for
comparison [7]. While 2 has thermal stability lower than Rh(I)
complex 4 and Ir(I) complex 3a, 2 is thermally stable below 100 ꢁC,
but begins to decompose when the temperature is raised higher
than 100 ꢁC. This result is in marked contrast to the known Rh
diarylalkoxide complexes with phosphine ligands, which readily
Fig. 3. ORTEP drawing of 3a. Selected bond lengths (Å) and angles (ꢁ): N2eC22
1.324(9), N2eC16 1.376(9), C16eC3 1.516(10), C3eO3 1.414(8), N2eIr1 2.066(6), IreC2
1.829(8), IreC1 1.852(9), C2eO2 1.148(10), C1eO1 1.146(10), C22eN2eC16 108.9(6),
N2eC16eC3 114.8(6), C16eC3eO3 108.0(6), N2eIr1eO3 78.5(2), C2eIr1eC1 87.7(3),
Fig. 1. ORTEP drawing of 1a. Selected bond lengths (Å) and angles (ꢁ): N1eC8 1.32(3)7,
N1eC2 1.378(3), C1eC2 1.516(3), C1eO1 1.422(3), C8eN1eC2 107.4(2), N1eC2eC1
117.9(2), C2eC1eO1 109.79(19), N1eC8eC9eC14 ꢀ42.1(4), O1eC1eC2eN1 ꢀ68.1(3).
N2eIr1eC2
172.6(3),
O3eIr1eC1
176.9(2),
N2eC22eC23eC28
50.1(10),
O3eC3eC16eN2 ꢀ24.7(7), C1eIr1eN2eC22 24.6(8), O3eIr1eN2eC16 2(2).

78
T. Murai et al. / Journal of Organometallic Chemistry 794 (2015) 76e80
Table 2
Typical properties of Rh(I) and Ir(I) complexes 2 and 3a.
Rh(I) and Ir(I) complexes
m.p. (dec.)
IR2jðCOÞ (cmꢀ1
)
13C NMR 2xCO (ppm)
2a
132e134 ꢁC (yellow to brownish)
2063, 1991
71.36 cmꢀ1
2065, 1989
76.18 cmꢀ1
2062, 1993
68.46 cmꢀ1
2063, 1986
77.14 cmꢀ1
2065, 1989
76.18 cmꢀ1
2044, 1968
76.17 cmꢀ1
2078, 2004
74 cmꢀ1
184.5 (JRhꢀC ¼ 62.9 Hz)
185.7 (JRhꢀC ¼ 75.3 Hz)
184.2 (JRhꢀC ¼ 62.9 Hz)
185.3 (JRhꢀC ¼ 74.4 Hz)
184.6 (JRhꢀC ¼ 62.9 Hz)
185.7 (JRhꢀC ¼ 75.3 Hz)
184.5 (JRhꢀC ¼ 62.9 Hz)
185.7 (JRhꢀC ¼ 75.3 Hz)
184.6 (JRhꢀC ¼ 62.7 Hz)
185.2 (JRhꢀC ¼ 74.4 Hz)
171.3
2b
2c
2d
2e
3a
138e140 ꢁC (deep yellow to brownish)
105e108 ꢁC (light yellow to brownish)
111e119 ꢁC (yellow to brownish)
122e128 ꢁC (yellow to brownish)
186e190 ꢁC (light yellow to brownish)
227 ꢁC
174.5
186.5 (JRhꢀC ¼ 71.8 Hz)
187.3 (JRhꢀC ¼ 65.8 Hz)
4
undergo b
-elimination at around 50 ꢁC to give PheRh complexes
Table 3
Rh(I) complex 2-catalyzing hydrosilylation of alkyne 5 with HSiMe2Pha.
and ketones (Scheme 1) [8]. In the IR spectra of 2, two vibrational
frequencies due to the carbonyl groups were observed at around
2000 cmꢀ1. In the 13C NMR spectra, the signals due to the carbon
atom of CO of 2 were observed at around 185 ppm, and their
chemical shifts were not affected by aryloxy groups. These ten-
dencies in the spectra of 2 are nearly the same as those for 4. Two
different coupling constants between Rh atoms and the carbon
atoms of CO were observed for 2, although as shown above, the
bond lengths of the two Rh(I)eC bonds were almost identical.
These spectroscopic features of 2 were similar to those of 4. While
complex 3a showed IR spectra similar to those of 2, in the 13C NMR
spectra, the signals due to the carbon atoms of CO for 3a were
shifted to higher fields by more than 10 ppm compared to those for
2.
Entry CatalystX (mol%) Conditions Conversion (%) Ratio of E-6:Z-6:7
1
2
3
4
5
6
7
8
9
2a
2b
2b
2b
2c
2d
2e
2b
2b
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.1
0.1
rt, 3 h
rt, 0.5 h
rt, 1 h
rt, 3 h
rt, 3 h
100
58
92
96
87
88
88
99
38:43:19
52:28:20
59:21:20
76:4:20
34:48:18
74:6:20
62:19:19
33:57:10
84:6:10
rt, 3 h
rt, 3 h
60 ꢁC, 1 h
60 ꢁC, 48 h 100
2.2. Rh(I) complexes-catalyzed hydrosilylation of alkynes
a
To CDCI3 were added 5, HSiMe2Ph, and Rh complex 2, and the mixture was
stirred. The conversion and ratio of the products were determined with integral
ratios in 1H NMR spectra.
The catalytic activities of Rh(I) complexes 2 were examined in
the hydrosilylation [9] of 4-methylphenylacetylene (5) (Table 3).
The addition of HSiMe2Ph to 5 was highly efficiently catalyzed
by all of the Rh complexes 2 (1.0 mol%) to give vinylsilanes E-6, Z-6,
and 7 [10] in different ratios. However, the ratios of E- and Z-iso-
mers 6 were dependent on the substituents on the aromatic rings of
2. Among them, Rh(I) complex 2b, in which the chlorine atom was
introduced to the aromatic ring, showed high conversion with
better selectivity even after 1 h to give E-6, Z-6, and 7 in a ratio of
59:21:20 (entry 3). When the reaction was allowed to proceed for
an additional 2 h, the isomerization of Z-6 to E-6 proceeded to give
E-6 more selectively (entry 4). In contrast, the ratio between 6 and 7
did not change during the reaction (entries 2e4). The catalytic re-
action was then performed with 2b (0.1 mol%) at 60 ꢁC. The reaction
for 1 h gave vinylsilanes E-6, Z-6, and 7 in a ratio of 33:57:10 (entry
8). Higher temperature facilitated the addition of the silyl group to
the terminal carbon atom of 5 to give 6 in higher yields than 7
probably because 6 appears to be thermodynamically more stable
than 7. During the reaction for 48 h, the isomerization of Z-6 to E-6
also proceeded to give E-6 as a major product (entry 9). This is in a
marked contrast to the known transition metal-catalyzed hydro-
silylation in which E-isomers are initially formed, and the mecha-
nistic detail of our reaction is being studied. As an example of the
reaction of internal alkynes, 3-hexyne (8) was also subjected to
hydrosilylation in the presence of 2 (0.1 mol%) to give vinylsilane E-
9 [11] as a sole product (Scheme 2).
In conclusion, we have demonstrated the use of imidazo[1,5-a]
pyridine-1-ylalkyl alcohols as chelating N^O-monoanionic ligands
for Rh and Ir metals. The obtained complexes were characterized
spectroscopically, and their structures were clearly determined by
Scheme 1.
b-Elimination of rhodium alkoxide.
Scheme 2. Rh complex 2becatalyzing hydrosilylation of alkyne 8.

T. Murai et al. / Journal of Organometallic Chemistry 794 (2015) 76e80
79
X-ray analyses. Rh(I) complexes showed high catalytic activities in
the hydrosilylation of alkynes, and their efficiencies were depen-
dent on the substituents on the aromatic groups of the ligands.
Further applications of imidazopyridine derivatives [12] and their
metal complexes are in progress.
kept at 60 ꢁC in a thermostatic bath, and stirred for an appropriate
period. The reaction mixture was concentrated in vacuo. The res-
idue was purified by column chromatography on silica gel (n-
hexane only) to give hydrosilylated products in 82% yield. The re-
action product was identified by NMR by comparison of their
spectroscopic data with data reported in the literature.
3. Experimental
Acknowledgments
3.1. Characterization
This work was partially supported by a Grant-in-Aid for Scien-
tific Research from the MEXT Innovative Areas (Stimuli-responsive
Chemical Species, No. 15H00933).
The IR spectra were obtained on a JASCO FT-IR 410 spectro-
photometer. 1H NMR (399.7 MHz), 13C (100.4 MHz) NMR spectra
were measured on a JNM-
shifts are reported in values referred to tetramethylsilane or CDCl3
a
400 spectrometer. 1H and 13C chemical
d
Appendix A. Supplementary data
as an internal standard, respectively. All spectra were acquired in
the proton-decoupled mode. The mass spectra (MS) and high res-
olution mass spectra (HRMS) were taken on JMS-700 mass spec-
trometers. Melting points were determined by using a Yanaco MP-
S2 micro melting point apparatus and are uncorrected.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2015.05.045.
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80
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