Noninnocent Ligand in Rhodium(III)-Complex-Catalyzed C-H Bond Amination with Tosyl Azide

Article abstract of DOI:10.1021/acs.inorgchem.8b00289

Six-coordinate rhodium(III) complexes coordinated by a planar trianionic ligand (L^3-) are synthesized. One of the axial positions is occupied by chloride (Cl^-), bromide (Br^-), or iodide (I^-), and another axial position is coordinated by a solvent molecule such as acetonitrile (AN), water (H₂O), tetrahydrofuran (THF), or pyridine (PY) to complete an octahedral rhodium(III) center; [Rh^III(L^3-)(X)(Y)]^? (1^X/Y X = Cl^-, Br^-, or I^-, Y = AN, H₂O, THF, or PY). Coordination of the AN, H₂O, and THF ligands to the metal center is rather weak, so that these solvent molecules are easily replaced by PY to give [Rh^III(L^3-)(Cl)(PY)]^?. In the electrochemical measurements, all complexes have two reversible redox couples based on the ligand-centered oxidation L^3- to L^·2- and to L^-, as reflected by the very similar redox potentials regardless of the different axial ligands. The rhodium(III) complexes catalyze C-H bond amination of xanthene with tosyl azide (TsN₃). Because the yields of the aminated product are nearly the same among the complexes, replacement of the axial solvent ligands with TsN₃ readily occurs to give a nitrene-radical-bound rhodium(III) complex, [Rh^III(L^·2-)(N^·Ts)(X)]^?, as an active oxidant, which is generated by one-electron transfer from the trianionic L^3- to the nitrene nitrogen atom. Generation of such a diradical intermediate was substantiated by the direct reaction of 1^Cl/AN with TsN₃ in the absence of the substrate (xanthene). In this case, a rhodium(III) iminosemiquinone complex, 2, was generated by the intramolecular reaction between the nitrene-radical moiety and the radical moiety of the ligand L^·2-.

Full text of DOI:10.1021/acs.inorgchem.8b00289

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Noninnocent Ligand in Rhodium(III)-Complex-Catalyzed C−H Bond
Amination with Tosyl Azide
Daiki Fujita, Hideki Sugimoto,* Yuma Morimoto, and Shinobu Itoh*
Department of Material and Life Science, Division of Advanced Science and Biotechnology, Graduate School of Engineering,
Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
*
S Supporting Information
ABSTRACT: Six-coordinate rhodium(III) complexes coordi-
3
−
nated by a planar trianionic ligand (L ) are synthesized. One
−
of the axial positions is occupied by chloride (Cl ), bromide
−
−
(
Br ), or iodide (I ), and another axial position is coordinated
by a solvent molecule such as acetonitrile (AN), water (H O),
2
tetrahydrofuran (THF), or pyridine (PY) to complete an octa-
III 3−
−
X/Y
hedral rhodium(III) center; [Rh (L )(X)(Y)] (1 ; X =
−
−
−
Cl , Br , or I , Y = AN, H O, THF, or PY). Coordination of
2
the AN, H O, and THF ligands to the metal center is rather
2
weak, so that these solvent molecules are easily replaced by PY
III 3−
−
to give [Rh (L )(Cl)(PY)] . In the electrochemical mea-
surements, all complexes have two reversible redox couples
3−
•2−
−
based on the ligand-centered oxidation L to L and to L ,
as reflected by the very similar redox potentials regardless of the different axial ligands. The rhodium(III) complexes catalyze
C−H bond amination of xanthene with tosyl azide (TsN ). Because the yields of the aminated product are nearly the same among
3
the complexes, replacement of the axial solvent ligands with TsN readily occurs to give a nitrene-radical-bound rhodium(III)
3
III •2−
•
−
3−
complex, [Rh (L )(N Ts)(X)] , as an active oxidant, which is generated by one-electron transfer from the trianionic L to
the nitrene nitrogen atom. Generation of such a diradical intermediate was substantiated by the direct reaction of 1
Cl/AN
with
TsN in the absence of the substrate (xanthene). In this case, a rhodium(III) iminosemiquinone complex, 2, was generated by the
3
•
2−
intramolecular reaction between the nitrene-radical moiety and the radical moiety of the ligand L
.
INTRODUCTION
Scheme 1. Schematic Classification of Nitrene-Bound
Octahedral Metal Complexes and πd−p Interaction
■
Nitrene-bound transition-metal complexes are involved as the
key reactive intermediates in C−H bond functionalization,
1
−10
CC aziridination, and electron-transfer oxidation reactions.
A better understanding of the relationships between the elec-
tronic structure and reactivity of the nitrene complexes is a pre-
requisite in the development of efficient and selective catalysts
for such reactions. When nitrene binds to early-transition-metal
elements, the resultant complexes feature a metal−nitrogen
multiple bond, namely, metal imido (MNR) complexes,
because the early-transition-metal ions have suitable empty
d orbitals that can make a π-bonding interaction with a filled
11−16
p orbital of the nitrene ligand (type 1 in Scheme 1).
Thus,
there are a large number of examples of metal imido complexes
seven valence electrons, which can be generated via one-
of early-transition-metal elements that exhibit electrophilic reac-
electron transfer from the metal center to the nitrene moiety
1
1−16
tivity.
On the other hand, late-transition-metal nitrene
18,19
(
type 3 in Scheme 1).
Such nitrene-radical complexes are
complexes with octahedral structure have filled d orbitals, so
π
expected to have radical character. A typical example is the well-
characterized nitrene-radical cobalt(III) porphyrin complexes
that the π-bonding interaction with filled p orbitals of the
nitrene ligand is virtually impossible (Scheme 1). Thus, the
nitrene-bound late-transition-metal complexes tend to show
single-bond character (M−NR; type 2), exhibiting nucleophilic
Special Issue: Applications of Metal Complexes with Ligand-
Centered Radicals
1
7
reactivity.
The third class of nitrene-bound metal complexes are
Received: February 1, 2018
nitrene-radical complexes having a nitrogen atom with formally
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b00289
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
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(
aniline)
2
(aniline)
2
Scheme 2. (Above) Catalytic C−H Amination of Xanthene with TsN by 1
and (Below) Structures of LH , 1
, and
3
3
the Diradical Intermediate DR for C−H Bond Amination
that can be adopted in aziridination, C−H bond amination, and
2
0−22
C−H bond amidation.
complexes are very limited.
However, such nitrene-radical
2
3−27
Recently, the combination
of a redox-active ligand and a redox-innocent metal ion was
employed for accessing nitrene-radical metal complexes (type 4
8
−10,28−30
in Scheme 1).
In this case, one-electron transfer takes
place from the supporting ligand to the nitrene nitrogen atom
without changing the metal oxidation state. For instance, a
Figure 1. ChemDraw structures of rhodium(III) complexes 1X/Y
.
palladium(II) complex with a redox-active NNO pincer ligand
commercial products of the highest available purity and were used
as received without further purification, unless otherwise noted. All of
the solvents used in this study were purified by the standard
LNNO2 ) induces intramolecular cyclization of 4-azidobutylben-
−
(
II
•−
•−
zene to 2-phenylpyrrolidine via a Pd (L
mediate [R = −(CH ) Ph].
)(RN ) inter-
NNO
3
3
I
−
(aniline)
2
28,29
procedure. Ligand LH , [Rh (LH )(cod)], and 1
thesized according to the reported procedures.
were syn-
H NMR and C
3
2
2
4
3
0,31
1
13
Recently, we have demonstrated an intermolecular C−H bond
spectra were recorded at 400 MHz and 101 MHz, respectively, on a
JEOL-ECP400 or a JEOL-ECS400 spectrometer. Electrospray
ionization mass spectrometry (ESI-MS) spectra were recorded with a
Bruker micrOTOF II spectrometer. Electron paramagnetic resonance
amination reaction using a rhodium(III) complex as the catalyst
and tosyl azide (TsN ) as the nitrene source. In this reaction,
we employed a redox-active trianionic tetradentate ligand (L )
consisting of a β-diketiminate framework and two phenolate
moieties (Scheme 2).
30
3
3−
(
EPR) spectra were taken on a Bruker X-band spectrometer (EMX-Micro)
3
1,32
III 3−
The reaction of [Rh (L )(aniline) ]
) and TsN generates a nitrene-radical rhodium(III)
under nonsaturating microwave power conditions (1.0 mW) operating at
9.7 GHz. Elemental analysis was carried out with a Yanaco CHN-Corder
MT-5. Electrochemical measurements were performed with a Hokuto
Denko HZ-7000. A set of carbon working, saturated calomel reference,
and platinum counter electrodes was employed in these experiments.
2
(aniline)
2
(
1
3
III •2−
•−
complex supported by the ligand radical [Rh (L )(TsN )-
aniline)] (DR in Scheme 2), which induces a catalytic amination
reaction of xanthene (Scheme 2). Density functional theory
DFT) studies have suggested that the diradical (DR) electronic
(
Synthesis. nBu N[Rh (L )(Cl)(CH CN)] (1
III 3−
Cl/AN
). Toluene (2 mL)
4
3
(
I
−
III •2−
•−
was added to a mixture of [Rh (LH
and tetra-n-butylammonium chloride ( Bu NCl; 16.7 mg, 0.60 μmol),
2
)(cod)] (50.0 mg, 0.68 μmol)
configuration of [Rh (L )(TsN )(aniline)] with single-bond
character between the metal center and the nitrene-radical nitro-
gen atom is a plausible reactive intermediate in the C−H bond
abstraction and nitrogen-rebound mechanism. However,
characterization of the reaction intermediate was hampered
because of a slow ligand exchange reaction between the axial
n
4
and the resulting solution was stirred at room temperature in air. After
5
h, n-hexane (40 mL) was added to the solution with vigorous
30
stirring. An orange powder was precipitated from the solution, which
was collected by filtration (48.5 mg). Then, the orange powder (48.5 mg)
was dissolved in CH CN. Diffusion of diethyl ether into the CH CN
3
3
30
solution gave an orange microcrystalline powder, which was collected
aniline ligand and TsN , being the overall rate-limiting step.
3
1
by filtration and dried in air. Yield: 21.9 mg (34%). H NMR
In this study, we have developed a series of rhodium(III)
3−
(CD
2.80 Hz), 7.09 (d, 2H, J = 2.0 Hz), 3.12−3.02 (m, 8H), 1.97 (s, 3H),
.65−1.51 (m, 26H), 1.42−1.28 (m, 26H), 0.936 (t, 12H, J = 7.2 Hz).
3
CN, 400 MHz): δ 9.20 (d, 2H, J = 1.60 Hz), 7.58 (d, 2H, J =
complexes supported by the same ligand L but different axial
III 3−
−
−
−
ligands [Rh (L )(X)(Y)], where X = Cl , Br , or I and Y =
acetonitrile (AN, CH CN), water (H O), tetrahydrofuran
1
13
3
2
C NMR (CD CN, 101 MHz): δ 166.5, 139.5, 139.3, 137.7, 135.4,
3
(
THF), or pyridine (PY) (Figure 1). Because coordination of
1
22.1, 109.4, 59.27, 36.10, 34.87, 32.17, 30.08, 24.27, 20.30, 13.78.
the axial solvent molecule (Y) to the rhodium(III) center is
weaker than that of aniline, we could examine the direct reac-
Cl/AN
Decomposition temperature: 180−190 °C. Anal. Calcd for 1
(C H ClN O Rh): C, 62.44; H, 8.66; N, 7.43; Found: C, 62.42; H,
8.99; N, 7.79. ESI-MS(−): m/z 658.2 ([M − CH CN] ).
Ph P[Rh (L )(Cl)(CH CN)] was prepared by the same method as
4
9
81
5
4
−
tion between the rhodium(III) complex and TsN , providing
3
3
III 3−
more insight into the reaction mechanism of the C−H bond
amination reaction (Scheme 2). The fundamental properties as
well as the catalytic activity in the C−H bond amination
reaction of the rhodium(III) complexes are reported.
4
3
Cl/AN
n
that described for 1
except using Ph PCl instead of Bu NCl
4 4
1
(35.9 mg from 51.1 mg of the precipitate). H NMR (CD CN,
3
4
00 MHz): δ 9.20 (d, 2H, J = 1.6 Hz), 7.98−7.86 (m, 4H), 7.81−7.63
(
m, 16H), 7.58 (d, 2H, J = 2.4 Hz), 7.09 (d, 2H, J = 2.0 Hz), 1.58
(
s, 18H), 1.37 (s, 18H). 13C NMR (CD CN, 101 MHz): δ 166.4,
3
EXPERIMENTAL SECTION
Materials and Methods. General Procedures. The reagents used
in this study, except the compounds mentioned below, were
■
139.5, 139.3, 137.7, 136.4, 135.7, 135.6, 135.5, 131.4, 131.2, 122.1, 109.4,
36.09, 34.87, 32.16, 30.08. Decomposition temperature: 168−185 °C.
Anal. Calcd for C H ClN O RhP·Et O: C, 65.79; H, 6.79; N, 5.03;
57
65
4
4
2
B
DOI: 10.1021/acs.inorgchem.8b00289
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
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Found: C, 65.61; H, 6.74; N, 5.10. ESI-MS(−): m/z 658.2
1Cl/AN (50.0 mg, 53.0 μmol), and the solution was stirred for 1 h at
room temperature. The mixture was poured into n-hexane (40 mL)
−
(
[M − CH CN] ).
3
n
III 3−
Br/AN
Bu N[Rh (L )(Br)(CH CN)] (1
). This complex was prepared by
with vigorous stirring to yield a red powder, which was collected by
4
3
Cl/AN
n
1
the same method as that described for 1
except using Bu NBr
centrifugation. Yield: 33.5 mg (60%). H NMR (CD CN, 400 MHz):
4
3
n
1
instead of Bu NCl. Yield: 20.6 mg (31%). H NMR (CD CN,
δ 9.23 (d, 2H, J = 2.4 Hz), 8.54 (d, 1H, J = 21 Hz), 7.56 (d, 2H, J =
2.0 Hz), 7.47 (dtd, 1H, J = 21.0, 6.0, and 2.0 Hz), 7.16−7.03 (m, 5H),
6.76 (d, 2H, J = 6.8 Hz), 3.08−2.99 (m, 8H), 2.07−1.97 (m, 4H),
1.62−1.40 (m, 28 H), 1.40−1.20 (m, 30 H), 0.935 (t, 12H, J =
4
3
4
00 MHz): δ 9.18 (d, 2H, J = 1.6 Hz), 7.58 (d, 2H, J = 2.4 Hz), 7.09
(
(
(
d, 2H, J = 2.4 Hz), 3.11−3.02 (m, 8H), 1.97 (s, 3H), 1.66−1.51
13
m, 26H), 1,44−1.28 (m, 26H), 0.963 (t, 3H, J = 7.2 Hz). C NMR
CD CN, 101 MHz): δ 166.5, 139.6, 139.5, 137.8, 135.4, 122.1, 109.3,
8.0 Hz), 0.859 (t, 2H, J = 7.2 Hz). 13C NMR (CD CN, 101 MHz):
3
3
5
9.28, 36.10, 34.87, 32.17, 30.07, 24.27, 20.31, 13.78. Decomposition
δ 182.7, 166.0, 141.9, 139.3, 139.2, 137.2, 135.4, 129.3, 129.2, 126.8,
122.2, 109.3, 59.26, 37.01, 36.11, 34.85, 34.51, 32.17, 30.16, 27.34,
24.26, 20.30, 13.78. Decomposition temperature: 173−185 °C. Anal.
Calcd for C H Cl N O Rh: C, 65.28; H, 8.75; N, 6.68. Found: C,
temperature: 165−171 °C. Anal. Calcd for C H BrN O Rh·Et O·
CH CN: C, 59.93; H, 8.60; N, 7.62. Found: C, 59.93; H, 8.75; N, 7.47.
ESI-MS(−): m/z 702.1 ([M − CH CN] ).
49
81
5
4
2
3
−
3
57 91
1
5
4
n
III 3−
I/AN
−
Bu N[Rh (L )(I)(CH CN)] (1 ). This complex was prepared by
65.27; H, 9.18; N, 6.47. ESI-MS(−): m/z 805.3 ([M] ), 658.2 ([M −
4
3
Cl/AN
n
−
the same method as that described for 1
instead of Bu NCl. Yield: 35.3 mg (50%). H NMR (CD CN,
except using Bu NI
(NHCH(CH ) Ph)] ).
4
2 3
n
1
[Rh (L )(NHCH(CH ) Ph)(p-methoxyaniline)] (3aniline‑OMe).
III 3−
4
3
2 3
4
00 MHz): δ 9.14 (d, 2H, J = 1.6 Hz), 7.58 (d, 2H, J = 2.0 Hz), 7.07
4-Phenylbutaneazide (35.4 mg, 0.202 mmol) was added to a toluene
3
−
(
(
(
d, 2H, J = 2.4 Hz), 3.11−3.02 (m, 8H), 1.97 (s, 3H), 1.65−1.51
solution (8 mL) of [Rh(L )(p-methoxyaniline) ] (160.0 mg,
2
13
m, 26H), 1.41−1.28 (m, 26H), 0.963 (t, 12H, J = 7.2 Hz). C NMR
0.184 mmol), and the resulting solution was refluxed for 18 h. The
solvent was removed under reduced pressure, and the crude product
CD CN, 101 MHz): δ 166.6, 139.9, 139.7, 138.0, 135.3, 122.0, 109.2,
3
5
9.27, 36.06, 34.86, 32.17, 30.03, 24.26, 20.30, 13.77. Decomposition
was purified by silica gel chromatography (1:25 ethyl acetate/CHCl )
3
1
temperature: 150−159 °C. Anal. Calcd for C H IN O Rh·
Et O·0.7CH CN: C, 57.47; H, 8.25; N, 7.02. Found: C, 57.83; H, 8.41;
N, 7.01. ESI-MS(−): m/z 750.1 ([M − CH CN] ).
(44.8 mg, 27%). H NMR (CDCl , 400 MHz): δ 9.15 (d, 2H, J =
4
9
81
5
4
3
1.6 Hz), 7.77 (d, 1H, J = 21.2 Hz), 7.46 (d, 2H, J = 2.0 Hz), 7.38 (dt,
1H, J = 21.2 and 4.0 Hz), 7.20 (d, 2H, J = 2.0 Hz), 7.18−7.08 (m, 3H),
6.79 (d, 2H, J = 6.4 Hz), 6.34 (d, 2H, J = 9.2 Hz), 6.04 (d, 2H, J =
8.8 Hz), 3.63 (s, 3H), 2.22 (t, 2H, J = 7.2 Hz), 2.05 (q, 2H, J =
2
3
−
3
n
III 3−
2
Cl/H O
Bu N[Rh (L )(Cl)(H O)] (1
). This complex was obtained by
4
2
Cl/AN
the same method as that described for 1
except using H O and
2
13
acetone for crystallization instead of CH CN. Yield: 43.0 mg (69%).
6.8 Hz), 1.63−1.49 (m, 20H), 1.24 (s, 18H). C NMR (CDCl3,
101 MHz): δ 183.3, 169.3, 157.51, 139.9, 139.8, 137.4, 137.1, 136.9,
130.3, 128.6, 128.4, 127.5, 126.4, 122.4, 121.3, 113.7, 109.1, 55.48,
37.67, 35.81, 34.57, 33.99, 32.08, 29.89, 25.91. Mp: 196−198 °C. Anal.
3
1
H NMR (acetone-d , 400 MHz): δ 9.37 (d, 2H, J = 1.2 Hz), 7.65
6
(
1
(
d, 2H, J = 2.0 Hz), 7.05 (d, 2H, J = 2.8 Hz), 3.42−3.33 (m, 8H),
.83−1.71 (m, 8H), 1.59 (s, 18H), 1.45−1.33 (m, 26H), 0.953
t, 12H, J = 7.6 Hz). 13C NMR (acetone-d , 101 MHz): δ 166.7, 139.6,
Calcd for C48
7.09. Found: C, 62.38; H, 7.55; N, 7.48.
Amination of Xanthene by TsN . A toluene solution (1 mL) of
TsN (6.91 mg, 35.0 μmol) was added to the complex (3.50 μmol,
H N O Rh·2H O·(CH ) CO: C, 61.99; H, 7.55; N,
6
64 5 5 2 3 2
1
2
39.2, 137.3, 134.4, 121.7, 108.8, 59.28, 35.99, 34.74, 32.39, 24.36,
0.31, 13.86. Decomposition temperature: 192−197 °C. Anal. Calcd
3
for C H ClN O Rh: C, 61.39; H, 8.77; N, 6.09. Found: C, 61.00; H,
3
4
7
80
4
5
−
8.87; N, 6.07. ESI-MS(−): m/z 658.2 ([M − H O] ).
10 mol %) and xanthene (6.38 mg, 35.0 μmol) in a 5 mL vial covered
with aluminum foil. After dioxygen was removed by bubbling dinitro-
gen (5 min), the mixture was stirred at 100 °C for 24 h in the dark.
All volatiles were evaporated in vacuo, and the crude products were
2
n
III 3−
Cl/THF
Bu N[Rh (L )(Cl)(THF)] (1
). This complex was obtained by
4
Cl/AN
the same method as that described for 1
except using diethyl ether
and THF for crystallization instead of CH CN. Yield: 39.4 mg (60%).
3
1
analyzed by 1H NMR spectroscopy [CDCl
3
H NMR (acetone-d , 400 MHz): δ 9.37 (d, 2H, J = 1.6 Hz), 7.65
, 400 MHz, internal
standard; 1,4-dimethoxybenzene (4.84 mg, 35.0 μmol)].
6
(
d, 2H, J = 2.0 Hz), 7.04 (d, 2H, J = 2.4 Hz), 3.66−3.58 (m, 4H),
Cl/AN
2
Cl/H O
X-ray Crystallography. Single crystals of 1
and 1
suitable
3
.47−3.38 (m, 8H), 1.86−1.75 (m, 12H), 1.59 (s, 18H), 1.48−1.33
13
for X-ray crystallographic analysis were obtained by the slow diffusion
(
m, 26H), 0.969 (t, 12H, J = 7.2 Hz). C NMR (acetone-d ,
6
Cl/AN
of diethyl ether into a CH CN solution for 1
solution for 1
and a H O/acetone
1
01 MHz): δ 166.7, 139.6, 139.2, 137.24, 134.3, 121.2, 108.8, 68.05,
3
2
2
Cl/H O
. Single crystals of 2 suitable for X-ray crystal-
5
9.29, 36.00, 34.75, 32.40, 26.16, 24.34, 20.32, 13.85. Decomposition
lographic analysis were obtained by the slow diffusion of n-hexane into
temperature: 175−183 °C. Anal. Calcd for C H ClN O Rh: C,
51
86
4
5
aniline‑OMe
an acetone solution of the complex. Single crystals of 3
were
6
2.92; H, 8.90; N, 5.75. Found: C, 62.78; H, 9.34; N, 5.62. ESI-
−
obtained by the slow evaporation of an H O/acetone solution of the
MS(−): m/z 658.2 ([M − THF] ).
2
n
III 3−
Cl/PY
complex. Each single crystal was mounted on a loop with a mineral oil,
and all X-ray data were collected at −165 °C on a Rigaku R-AXIS
RAPID diffractometer using filtered Mo Kα radiation. The structures
were solved by direct methods (SIR-2008) and expanded using Fourier
techniques. The non-hydrogen atoms were refined anisotropically by
Bu N[Rh (L )(Cl)(py)] (1 ). This complex was prepared by the
4
Cl/AN
same method as that described for 1
except using diethyl ether
and PY for crystallization instead of CH CN. Yield: 30.6 mg (46%).
3
1
H NMR (acetone-d , 400 MHz): δ 9.42 (d, 2H, J = 1.6 Hz), 8.16
6
(
2
(
d, 2H, J = 6.4 Hz), 7.65 (tt, 1H, J = 1.2 and 7.6 Hz), 7.56 (d, 2H, J =
.4 Hz), 7.21 (t, 2H, J = 7.2 Hz), 6.96 (d, 2H, J = 2.0 Hz), 3.41−3.31
m, 8H), 1.76 (qu, 8H, J = 8.0 Hz), 1.57 (s, 18H), 1.38 (se, 8H, J =
2
full-matrix least squares on F . The hydrogen atoms were attached at
idealized positions on carbon atoms and were not refined. All struc-
tures in the final stage of refinement showed no movement in the atom
position. The calculations were performed using Single-Crystal Struc-
ture Analysis Software, version 3.8. The crystallographic parameters
are summarized in Table 1.
13
8
1
1
1
.0 Hz), 1.29 (s, 18H), 0.951 (t, 12H, J = 7.2 Hz). C NMR (acetone-d ,
6
01 MHz): δ 166.3, 152.7, 150.7, 139.0, 138.9, 138.8, 136.2, 134.3,
28.0, 125.0, 121.4, 108.6, 59.25, 36.08, 34.67, 32.31, 24.36, 20.31,
3.87. Decomposition temperature: 250−265 °C. Anal. Calcd for
Theoretical Calculations. DFT calculations were performed with
C H ClN O Rh: C, 63.69; H, 8.53; N, 7.14. Found: C, 63.61; H, 8.68;
52
83
5
4
34
−
−
an “ultrafine” grid by using Gaussian 09, revision D.01. Geometry
N, 7.20. ESI-MS(−): m/z 737.3 ([M] ), 658.2 ([M − PY] ).
Complex 2. TsN (190 mg, 0.991 mmol) was added to a CH Cl
2
solution (5 mL) of Ph P[Rh (L )(Cl)(CH CN)] (50.0 mg,
optimizations were carried out at the UB3LYP/def2-TZVP level of
theory. Graphical outputs of the computational results were generated
3
2
III 3−
4
3
35
with the GaussView software program.
48.1 μmol), and the resulting solution was stirred for 30 min. The
reaction solution was added to n-hexane (40 mL) with vigorous stirring
to give a brown solid, which was collected by centrifugation. Yield:
RESULTS AND DISCUSSION
■
2
9.7 mg (53%). Decomposition temperature: 169−174 °C. Anal.
Synthesis, Characterization, and Crystal Structures of
Calcd for C H ClN O PRhS: C, 63.64; H, 6.24; N, 4.57. Found: C,
6
6
5
76
4
7
30
I
−
−
Rhodium(III) Complexes. [Rh (LH )(cod)] was employed
3.57; H, 6.40; N, 4.57. ESI-MS(−): m/z 828.2 ([M] ).
2
n
III 3−
Cl
Bu N[Rh (L )(Cl)(NHCH(CH ) Ph)] (3 ). 4-Phenylbutaneazide
as a starting material for the synthesis of a series of
4
2 3
X/Y
(
186 mg, 1.06 mmol) was added to a CH Cl solution (3 mL) of
rhodium(III) complexes 1
(see Figure 1). The treatment
2
2
C
DOI: 10.1021/acs.inorgchem.8b00289
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Inorganic Chemistry
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Table 1. Crystallographic Data for 1Cl/AN, 1Cl/H O·CH CN,
2
and PY. Thus, crystallization of the orange solid using these
3
aniline‑OMe
2·acetone, and 3
·acetone·H O
coordinating solvents gave the corresponding solvent-coordi-
2
Cl/Y
nated rhodium(III) complexes 1
(Y = AN, THF, or PY) as
1Cl/AN·CH
CN
1Cl/H O
2
3
n
2
Cl/H O
Bu N salts. Complex 1
(Y = H O) was also obtained using
acetone/H O for crystallization. When Bu NBr or Bu NI was
used instead of Bu NCl, the corresponding halide-coordinating
(X = Br or I ) was obtained.
H NMR spectra of the complexes in (CD ) CO displayed
4
2
formula
fw
C
51
H
84ClN
6
O
4
Rh
C
47
H80ClN
4
O
5
Rh
n
n
2
4
4
983.62
919.53
n
4
cryst syst
space group
a, Å
triclinic
triclinic
X/AN
−
−
rhodium(III) complex 1
P1
̅
(No. 2)
̅
P1 (No. 2)
1
3
2
11.4013(5)
15.5803(8)
17.5584(8)
107.535(8)
106.156(8)
104.252(7)
2664.7(4)
2
13.909(2)
14.837(2)
14.992(3)
114.508(3)
97.402(4)
109.859(4)
2512.7(7)
2
sharp signals in a diamagnetic region, δ 0.0−10.0 (see the
b, Å
1
Experimental Section). The number of observed H signals indi-
c, Å
cates that the complexes have C symmetry, having a mirror plane
α, deg
β, deg
s
passing through the two axial ligands, the rhodium(III) center,
and the carbon atom connected to the nitro group.
γ, deg
V, Å3
Parts a and b of Figure 2 show ORTEP drawings of the
Cl/AN
2
Cl/H O
Z
crystal structures of the anionic parts of 1
and 1
,
−
3
ρ, g cm
μ, cm−
1.226
1.215
respectively. Table 2 lists the selected bond lengths and angles
1
Cl/AN
4.161
4.369
of the complexes. The Rh1 atom of 1
is coordinated by
reflns obsd
12094
11330
two oxygen atoms, O1 and O2, and two nitrogen atoms, N1
and N2, from the tetradentate ligand L , AN nitrogen atom
N4, and chloride anion Cl1. The N4−Rh1−Cl1 bond angle is
3−
no. of param
667
602
a
R1
0.0457
0.0885
b
wR2
0.1301
0.2420
1
76.77(6)°, indicating that the rhodium center adopts an octahe-
3−
GOF
1.593
1.605
3aniline‑OMe·acetone·H
dral geometry. The structure of the tetradentate ligand moiety L
is quite close to those of the reported Bu N[M (L )] (M = Ni,
Cu, or Pd) complexes.
2
·acetone
2
O
n
II 3−
4
31,32
formula
fw
C
65
H
76ClN
4
O
7
PRhS
C
51
H
72
N
5
O
7
Rh
Brown proposed the metrical oxida-
1226.73
triclinic
950.03
monoclinic
P2 (No. 4)
tion states that represent the oxidation states of 2-amidophenolate
rings calculated from the crystal structures, where dianionic
amidophenolate, namely, a fully reduced state, exhibits the
cryst syst
space group
a, Å
P1
̅
(No. 2)
1
11.1686(9)
8.5178(4)
20.9407(7)
13.8465(5)
36
metric oxidation state of 1.6− to 2.3−. The present complex
b, Å
11.8470(10)
24.898(2)
77.387(6)
79.345(6)
78.883(6)
3119.2(5)
2
Cl/AN
1
1
showed metric oxidation states of 1.76(16)− and
c, Å
.58(17)− for the two aminophenolate rings, consisting of the
aminophenolate structure of both aromatic rings of the ligand.
α, deg
β, deg
γ, deg
V, Å3
Z
97.830(1)
2
Cl/H O
The overall structure and dimensions of 1
are close to those
2
Cl/AN
Cl/H O
of 1
, and the metric oxidation states of the ligand in 1
2446.77(16)
2
are 1.55(26)− and 1.91(18)−. These results clearly indicate
3−
ρ, g cm−
3
1.306
1.289
that the tetradentate ligand takes the trianionic form L .
In the ESI-MS measurements of an acetone solution, 1
X/AN
μ, cm−1
4.296
4.020
Cl/Y
and 1
(X = Cl, Br, or I; Y = CH CN, H O, or THF),
reflns obsd
14219
11151
645
3 2
Cl/PY
besides 1
, exhibited a peak cluster corresponding to the
no. of param
Flack parameter
721
III 3−
−
−
chemical formula of [Rh (L )(X )] lacking the coordinating
solvent molecule (Y; see the Experimental Section). The results
suggest that the coordinating solvent easily dissociated from the
rhodium(III) center under the ESI-MS measurement conditions.
0.46(2)
0.0431
0.1101
a
R1
0.0630
0.1746
1.067
b
wR2
GOF
1.272
2
1
Cl/AN
a
b
2
2
Figure 3 shows a H NMR spectral change of 1
upon the
R1 = ∑(|F | − |F |)/∑|F |. wR2 = (∑(w(Fo − F ) )/
o
c
o
c
2
2 1/2
∑
w(F ) )
.
addition of PY in acetone-d . Although six-coordinate Werner-
6
o
type rhodium(III) complexes are known to be inactive in ligand
I
−
n
37
of [Rh (LH )(cod)] with Bu NCl in toluene in air gave an
substitution reactions, axial solvent ligand substitution with PY
2
4
orange solid, which was insoluble in non-coordinating or weakly
coordinating solvents such as toluene, n-hexane, and diethyl
ether but soluble in coordinating solvents such as AN, THF,
readily occurred. The resulting spectrum was the same as that of
Cl/PY
1
, clearly demonstrating that the axial solvent ligand (AN)
was replaced by PY. Similar spectral changes were observed with
Figure 2. Crystal structures of the anionic parts of complexes 1Cl/AN (a) and 1Cl/H (b) showing 50% ellipsoids. Hydrogen atoms, countercations,
2
O
and solvent molecules are omitted for clarity.
D
DOI: 10.1021/acs.inorgchem.8b00289
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Inorganic Chemistry
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Cl/AN
Table 2. Selected Bond Lengths (Å) and Angles (deg) and Metric Oxidation States of the Aminophenol Moieties of 1
1
and
2
Cl/H O
1Cl/AN
1Cl/H
2.279(2)
2
O
Rh1−Cl1
Rh1−O1
Rh1−O2
Rh1−N1
Rh1−N2
Rh1−N4/O5
C1−N1
2.3103(6)
2.0294(15)
2.0468(18)
1.985(2)
2.044(5)
2.044(3)
1.961(4)
1.981(6)
2.118(4)
1.302(10)
1.323(9)
1.9755(18)
2.0218(17)
1.300(3)
C3−N2
1.307(3)
Cl1−Rh1−N4/O5
O1−Rh1−O2
O1−Rh1−N1
176.77(6)
178.02(15)
98.93(17)
82.0(2)
98.87(7)
82.32(7)
N1−Rh1−N2
96.80(8)
97.5(2)
metric oxidation state
−1.76(16)/−1.58(17)
−1.55(26)/−1.91(18)
Figure 3. (a) H NMR spectrum of 1Cl/AN in acetone-d and (b) that
1
6
obtained upon the addition of a slight excess amount of PY. Asterisks
indicate the H signals of free PY.
1Cl/THF and 1Cl/H O by the addition of PY in acetone-d6
2
Figure 4. Cyclic voltammograms of complexes (a) 1Cl/AN, (b) 1Br/AN
,
(
Figure S1). Ligand exchange with PY was readily completed
I/AN
Cl/Y
and (c) 1 n (1.0 mM) at room temperature in CH CN solution
when 1 equiv of PY was added to an acetone solution of 1
Y = AN, H O, or THF). Thus, the binding constant of
3
containing Bu NPF (0.10 M). Working electrode: glassy carbon.
(
4
6
2
−1
Counter electrode: platinum. Scan rate: 100 mV s .
rhodium(III) with PY is significantly larger than those of
Cl/PY
rhodium(III) with the solvent molecules. In fact, 1
did not
give free PY when it was dissolved in CD CN. Attempts to
suggest that the two redox couples of 1X/Y are due to the
3
Cl/Y
3−
•2−
monitor the solvent-exchange reactions between 1
complexes
consecutive ligand-centered oxidation from L to L
and
−
(
Y = AN, H O, or THF) to obtain the relative binding constants
L as in the case of the consecutive ligand-based oxidation of
2
1
II 3−
−
II •2−
II
−
+
of Y were unsuccessful because the H NMR and UV−vis spectra
in acetone of these complexes were nearly identical (indis-
tinguishable).
[M (L )] to [M (L )] to [M (L )] (M = Ni, Cu, and
31,32
Pd).
for 1
The appearance of a small peak around 0.2 V observed
I/AN
−
may be due to the oxidation of I dissociated from the
n
Electrochemical Properties of Rhodium(III) Com-
plexes. The electrochemical properties of rhodium(III) com-
plexes were examined by cyclic voltammetry in AN. 1Cl/AN
showed two reversible redox waves at E = 0.03 and 0.45 V
complex because the cyclic voltammogram of Bu NI also shows
4
an oxidation peak at a similar potential. The previously reported
(aniline)
2
dianiline complex 1
exhibited two redox couples at 0.22
+
and 0.62 V versus Fc/Fc , which are relatively positive com-
1
/2
+
X/Y
versus Fc/Fc (ferrocene/ferrocenium ion) and an irreversible
oxidation peak at E = +1.20 V (Figure S2). The cyclic volta-
pared to those of 1 (Table 3). This is probably due to the
pa
difference in the net charge of the complexes. Namely, the
(
aniline)
2
mmogram in the range from −0.2 to +0.7 V is shown in Figure 4a.
The redox potentials of the first two reversible waves are quite
close to those of the ligand-based consecutive oxidation of the
monoanionic nickel(II), copper(II), and palladium(II) com-
dianiline rhodium(III) complex 1
is a neutral complex,
whereas 1 are monoanionic complexes.
To gain further insight into the ligand-based oxidation of the
X/Y
Cl/AN
rhodium(III) complex, chemical oxidation of 1
bromophenyl) N ][SbCl ] (Magic Blue, MB) was monitored
with [(4-
II 3−
−
II 3−
−
•+
plexes [M (L )] reported previously; [M (L )] to
3
6
II •2−
II
−
+
31,32
[
M (L )] to [M (L )] (Table 3).
rhodium(III) complexes, 1
, showed very close E1/2 values regardless of whether
they had different axial ligands, Br , I , H O, THF, and PY
Furthermore, other
by UV−vis spectrometry at −60 °C (Figure 5). A gradual addi-
Br/AN
, 1I/AN, 1Cl/H , 1
Cl/THF
2
O
Cl/AN
, and
tion of MB to an acetone solution of 1
caused a spectral
Cl/PY
1
change, as shown in Figure 5a, where the intense absorption
−
−
Cl/AN
band at 470 nm due to 1
disappeared with the con-
2
(Figures 4b,c and S3 and Table 3). All of these results strongly
comitant appearance of new absorption bands at 540 nm
E
DOI: 10.1021/acs.inorgchem.8b00289
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Inorganic Chemistry
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Table 3. Redox Potentials (E ) of 1X/Y, 1(aniline) , and
2
Table 4. Catalytic C−H Bond Amination of Xanthene with
1
/2
II 3−
−
a
[M (L )]
TsN
3
E1 (V)
a
E2 (V)
a
1
/2
1/2
1Cl/AN
b
b
0.03
0.45
0.45
0.44
0.45
0.42
0.45
0.62
0.46
0.54
Br/AN
1
0.03
I/AN
b
1
1
1
1
1
0.01
Cl/H
2
O
c
c
−0.02
−0.03
−0.03
0.22
Cl/THF
Cl/PY
aniline)
conversion yield (%)
c
entry
additive
A
K
(
2
d
Cu (L3−)]−
NiII(L3−)]−
PdII(L3−)]−
II
e
1
2
3
4
5
6
7
none
none
0
0
[
[
[
−0.06
0.04
(
aniline)
f
0
78
71
67
65
9
0
7
f
Boc O
2
0.07
0.54
none
none
none
none
6
a
f
+
b
c
d
e
Versus Fc/Fc . In CH CN. In acetone, Reference 30. Reference 31.
Reference 32.
3
5
5
ε = 17000 M−1 cm ) and 660 nm (3500 M cm ), together
−1
−1
−1
14
(
a
In toluene in the dark in the presence of 10 mol % catalyst.
with a broad absorption band above 800 nm. Such a spectral
change was completed when an equimolar amount of MB was
added (Figure 5c), suggesting that the generated spectrum was
together with xanthone as a minor product (entry 4). The
amine product (A) was obtained in nearly identical yields in the
case of 1
III •2−
due to the one-electron-oxidized product, [Rh (L )(Cl)].
Cl/H
2
O
Cl/THF
The appearance of the absorption band at 660 nm and the
and 1
(67 and 65% conversion yields,
broad absorption band in the near-IR (NIR) region can be
respectively; entries 5 and 6). These results indicate that one of
ascribed to formation of the ligand radical species L•
2− 31,32
.
The
the axial solvent ligands (Y) can be replaced with TsN easily
3
III •2−
(aniline)
2
addition of MB to the resulting solution of [Rh (L )(Cl)]
resulted in a further spectral change (Figure 5b), where a new
absorption band at 580 nm (ε = 11000 M cm ) increased
compared to the case of the aniline ligand in 1
to start
the catalytic cycle for the amination reaction. Because the
product yields in entries 4−6 are nearly the same in spite of
having different axial ligands (Y), it can be said that the axial
−1
−1
with a concomitant decrease of the absorption band at 540 nm
III •2−
due to [Rh (L )(Cl)]. In this process, the NIR band above
solvent ligand (Y = AN, H O, or THF) is readily replaced by
2
8
00 nm further increased. The spectral change was completed
TsN , giving the same reactive intermediate. On the other
3
Cl/PY
when another 1 equiv of MB was added (in total 2 equiv of MB
against 1
be due to the two-electron-oxidized complex [Rh (L )(Cl)] .
The broad absorption band in the NIR region is indicative of a
quinonoid structure, in which the anion charge is delocalized
hand, the catalytic activity of 1
was much lower than those
Cl/AN
; Figure 5c). Thus, the generated spectrum may
of the former complexes because of the stronger binding of PY
to the metal center (entry 7).
III
−
+
Direct Reaction of a Rhodium(III) Complex with
Cl/AN
Azides. Because the axial CH CN ligand of 1
easily replaced with azide, a direct reaction of 1
can be
3
−
Cl/AN
onto the whole molecule of ligand L .
and TsN3
C−H Bond Amination Reaction Catalyzed by Rhodium-
was examined to gain further insight into the reaction mech-
anism. In a preparative-scale reaction, the brown powder
product 2 was obtained from the reaction in 40% yield as
(
III) Complexes. We have already reported an amination
(
aniline)
2
reaction of xanthene with TsN using 1
as the catalyst
3
3
0
n
+
(
Scheme 2). Without the addition of the rhodium(III) com-
plex, no amination reaction took place (Table 4, entry 1).
a Bu N salt, and the yield was increased to 71% when Ph P
4
4
30
was used as the countercation (see the Experimental Section).
Single crystals suitable for X-ray crystallographic analysis
were obtained by recrystallization of the brown powder
product from acetone/n-hexane. The crystal structure of
Di-tert-butyl dicarbonate [(Boc) O] was required to induce
2
ligand-exchange reaction between the axial aniline ligand and
30
Cl/AN
TsN (entries 2 and 3). With 1
as the catalyst, on the
3
other hand, C−H bond amination proceeded efficiently
the anionic part of the Ph P salt of 2 is shown in Figure 6,
together with the selected bond lengths and angles listed in
Table 5.
4
without the addition of (Boc) O to give the aminated product
2
N-xanthyl-p-toluenesulfonamide (A) in a 71% conversion yield
Figure 5. (a) Spectral change observed upon the addition of MB (1 equiv) to 1Cl/AN (0.033 mM) in acetone at −60 °C. (b) Spectral change
observed upon the addition of another 1 equiv of MB to the resulting solution in part a. (c) Absorbance change at 540 nm in the titration
experiment.
F
DOI: 10.1021/acs.inorgchem.8b00289
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Figure 6. (left) Crystal structure of the anionic part of complex 2
showing 50% ellipsoids. Hydrogen atoms and countercation are
omitted for clarity. (right) ChemDraw structure of 2.
Table 5. Selected Bond Lengths (Å) and Angles (deg) of 2
and 3
Figure 7. DFT (UB3LYP/def2-TZVP)-calculated spin-density plot for
complex 2. Blue represents an excess of α spin density, and green
represents an excess of β spin density.
aniline‑OMe
3aniline‑OMe
2
Rh1−Cl1/N3
Rh1−O1
Rh1−O2
Rh1−N1
Rh1−N2
Rh1−N4
2.3318(8)
2.031(2)
2.000(2)
2.005(3)
1.972(2)
2.073(2)
2.021(2)
2.032(3)
2.049(3)
1.969(4)
1.989(4)
2.104(2)
Cl1/N3−Rh1−N4
O1−Rh1−O2
O1−Rh1−N1
N1−Rh1−N2
N1−Rh1−O2
173.10(7)
87.32(9)
176.44(10)
99.45(13)
81.66(15)
95.89(16)
178.89(14)
94.72(10)
93.54(10)
177.95(9)
As can be clearly seen, one of the phenolate moieties of L3− is
detached from the β-diketiminate unit, and the detached
phenolate is aminated by the TsN group. Thus, the Rh1 center
is coordinated with two nitrogen atoms, N1 and N2, and one
oxygen atom, O2, from the original ligand and one nitrogen
atom, N4, and one oxygen atom, O1, from the newly formed
Figure 8. Absorption spectral change of 1Cl/AN (0.054 mM) upon the
addition of TsN (2.5 mM) at room temperature in acetone.
3
(dotted line), as shown in Figure 8. The UV−vis spectrum of
−
aminated phenolate moiety. The Cl anion remained on the
the brown powder of 2 was identical with that of a product
Cl/AN
rhodium center to make a slightly distorted octahedral geom-
etry. With respect to the structural parameters of the aminated
phenolate moiety, the C9−O1 bond length of 1.289(3) Å is in
the range of bond order of 1.5 and the sum of the bond angles
comprising S1−N4−C4, C4−N4−Rh1, and S1−N4−Rh1 is
generated in situ by treating 1
TsN
with an equal amount of
(Figure 8, dashed line). The absorption band at 550 nm
3
and the broad band in the NIR region can be ascribed to the
formation of the iminosemiquinone radical of 2 because the
Cl/AN
one-electron-oxidized complex of 1
has similar absorption
3
59.8°. The metric oxidation state of the aminated phenolate
bands at 660 nm and the NIR region (Figure 5a). ESI-MS of
Cl/AN
moiety is 0.97(12)−. These data indicate that the aminated
phenolate moiety can be best described as an iminosemiqui-
none radical, which is the origin of the paramagnetic character
of the complex, as indicated by the EPR spectrum (Figure S4).
DFT calculations supported the iminosemiquinone-radical struc-
ture. As is clearly shown in Figure 7, most of the spin is located
on the aminated phenolate moiety, indicating the iminosemi-
quinone-radical structure.
the solution containing 1
and an equal amount of TsN
3
exhibited a peak cluster at m/z 828.2, which is consistent with
t
the molecular formula of 2, [Rh + Cl + Bu
C
H
O(T
C H O(β-diketiminate)] . The EPR spectrum of a reaction
2 6 2
N)] +
2
6
2
S
t
−
Bu
solution of 1
organic radical (g = 2.00; Figure S4) as mentioned above.
Cl/AN
and TsN showed a typical signal for an
3
Cl/AN
Because 2 was formed from 1
by the reaction with TsN3
in the absence of the substrate, 2 might be a degradation prod-
uct of the active species in the C−H bond amination reaction.
On the basis of DFT calculations, we have proposed that the
active species for C−H amination is the diradical complex
Regarding the original ligand moiety, the C23−O2 and
C18−N2 bond lengths of 1.323(4) and 1.433(4) Å, respec-
tively, are in the range of the C−O and C−N single bonds that
3
−
−
II
II
II
III
•2−
•
30
are observed in the [M(L )] (M = Cu , Ni , or Pd ) com-
plexes.
[Rh (X)(L )( NTs)] (DR in Scheme 2). Thus, 2 might be
3
1,32
The metric oxidation state of the ring is 1.64(23)−,
formed from the DR intermediate via intramolecular radical
•
2−
which is consistent with a value for an aminophenolate ring.
The dianionic state of the tridentate ligand moiety may be
stabilized because of the strong electron-withdrawing nature of
coupling between the radical moiety of L and the nitrene-
•
radical moiety (N Ts), as shown in Scheme 3.
•
To generate 2 from DR, hydrogen-atom (H ) transfer or
+
−
the nitro group at C2.
When the reaction of 1
proton-coupled electron-transfer (H /e ) reduction should occur
Cl/AN
(0.054 mM) and TsN (2.5 mM)
after intramolecular C−N bond formation. To find such a
3
Cl/AN
was monitored by UV−vis spectroscopy, the absorption band at
reductant, the reaction of 1
and TsN was carried out in
3
Cl/AN
470 nm due to 1
(solid line) disappeared with the con-
acetone containing D O. In this case, a monodeuterated com-
2
comitant appearance of a new broad absorption band at 550 nm
plex, 2-D, was obtained, as confirmed by ESI-MS (Figure S5).
G
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Scheme 3. Possible Mechanism for the Formation of 2 from the DR Intermediate
2
Thus, H O contained in the solvent might be a reductant for
the formation of 2 from DR (Scheme 3).
However, when the methoxyaniline derivative of 1(aniline‑OMe)
was treated with 4-phenylbutaneazide, we were able to isolate
2
aniline‑OMe
1
Interestingly, the catalytic amination of xanthene also pro-
red single crystals of the product 3
. The H NMR
also showed one doublet at δ 7.77 with
J = 21.2 Hz and one doublet of triplet at δ 7.38 with J = 21.2 Hz,
X/Y
aniline‑OMe
ceeded using 2 instead of 1 as the catalyst, giving A (49%)
spectrum of 3
X/Y
and B (5%). Thus, the amination reactions by 1 may involve
Cl
aniline‑OMe
as in the case of 3 . The crystal structure of 3
is shown
not only the diradical intermediate DR but also a different
active intermediate derived from 2, although mechanistic details
are not clear at present.
in Figure 9. The Rh1 atom is coordinated with two oxygen
atoms, O1 and O2, and two nitrogen atoms, N1 and N2, from
the tetradentate ligand L3 in the basal plane and one aniline
nitrogen atom, N4, and one imine nitrogen atom, N3, from the
axial positions, forming a distorted octahedral structure. As can
be clearly seen, an intramolecular amination−cyclization of the
azide did not occur, but an imine product having an E con-
−
van der Vlugt et al. reported an intramolecular amination−
cyclization of 4-phenylbutaneazide by the reaction with
palladium(II) of an aminophenolate-based pincer ligand in
2
8,29
the presence of (Boc) O, giving a pyrrolidine derivative.
we also examined the direct reaction of 1
Thus,
2
Cl/AN
Cl
and 4-phenyl-
1
figuration was formed, as suggested by the H NMR data. The
butaneazide. In this reaction, red powder of 3 was isolated in
a 60% yield. In the H NMR spectrum in CD CN, the prod-
1
N3−C32 bond length of 1.275(4) Å is consistent with that of
an NC double bond. The dimensions of the tetradentate
,
3
uct showed one doublet at δ 8.54 with J = 21.2 Hz and one
doublet of triplet at δ 7.52 with J = 21.2 and 6.0 Hz (see the
Experimental Section). The appearance of such doublets having
a large coupling constant suggests imine bond formation adopting
(aniline)2
Cl/AN
2
Cl/H O
ligand are very close to those of 1
, 1
, and 1
and the metric oxidation states of the two aminophenolate rings
are 1.61(23)− and 2.03(13)−, indicating that the ligand has a
trianionic form. Thus, it is safely concluded that 3 generated
Cl
Cl
an E configuration. We could not obtain single crystals of 3 .
Cl/AN
from 1
also has an imine ligand derived from 4-phenyl-
aniline‑OMe
butaneazide at the axial position, as in the case of 3
.
Because Lewis acid catalyzed decomposition of alkyl azides to
38
Cl/AN
imines is well-known, the rhodium(III) complex 1
might
work as a Lewis acid in this reaction.
CONCLUSION
■
Six-coordinate rhodium(III) complexes coordinated by a redox-
active tetradentate ligand L were synthesized and charac-
terized. The remaining axial coordination sites were occupied
3−
−
Figure 9. (Left) Crystal structure of the anionic part of complexes
with halide anions (X ) and solvent molecules (Y) to complete
aniline‑OMe
III 3−
3
showing 50% ellipsoids. Hydrogen atoms and countercations
a distorted octahedral rhodium(III) center, [Rh (L )(X)-
aniline‑OMe
are omitted for clarity. (Right) ChemDraw structure of 3
.
−
(
Y)] . The complexes underwent consecutive ligand-based
Scheme 4. Reaction Pathways in the Catalytic Amination Reaction and Formation of 2
H
DOI: 10.1021/acs.inorgchem.8b00289
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Forum Article
oxidation to form [Rh (L•2−)(X)(Y)] and [Rh (L )(X)(Y)] ,
as revealed by spectroscopic and electrochemical studies. The
coordinating solvent molecules Y were easily replaced with PY,
and, hence, the present complexes worked as catalysts for the
III
III
−
+
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ASSOCIATED CONTENT
■
*
S
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.inorgchem.8b00289.
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(
CCDC 1820897, 1820898, 1820899, and 1821135 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/data_
request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by
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AUTHOR INFORMATION
■
3
(
Corresponding Authors
E-mail: sugimoto@mls.eng.osaka-u.ac.jp.
E-mail: shinobu@mls.eng.osaka-u.ac.jp.
*
Transition-Metal Complexes of Groups 9−11. Eur. J. Inorg. Chem.
*
2013, 2013 (22−23), 3784−3807.
(18) Suarez, A. I. O.; Lyaskovskyy, V.; Reek, J. N. H.; van der Vlugt, J.
ORCID
I.; de Bruin, B. Complexes with Nitrogen-Centered Radical Ligands:
Classification, Spectroscopic Features, Reactivity, and Catalytic
Applications. Angew. Chem., Int. Ed. 2013, 52 (48), 12510−12529.
(19) Kuijpers, P. F.; van der Vlugt, J. I.; Schneider, S.; de Bruin, B.
Nitrene Radical Intermediates in Catalytic Synthesis. Chem. - Eur. J.
Hideki Sugimoto: 0000-0002-4887-1740
Yuma Morimoto: 0000-0002-5209-266X
Shinobu Itoh: 0000-0002-3711-2378
Notes
2
(
017, 23 (56), 13819−13829.
The authors declare no competing financial interest.
20) Kuijpers, P. F.; Tiekink, M. J.; Breukelaar, W. B.; Broere, D. L. J.;
van Leest, N. P.; van der Vlugt, J. I.; Reek, J. N. H.; de Bruin, B.
Cobalt-Porphyrin-Catalysed Intramolecular Ring-Closing C−H Ami-
nation of Aliphatic Azides: A Nitrene-Radical Approach to Saturated
Heterocycles. Chem. - Eur. J. 2017, 23 (33), 7945−7952.
ACKNOWLEDGMENTS
■
This work was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas “Stimuli-responsive Chemical
Species for the Creation of Functional Molecules (No. 2408)”
JSPS KAKENHI Grant JP24109015) by CREST, JST (Grant
61052502).
(
21) Lyaskovskyy, V.; Suarez, A. I. O.; Lu, H.; Jiang, H.; Zhang, X. P.;
de Bruin, B. Mechanism of Cobalt(II) Porphyrin-Catalyzed C−H
Amination with Organic Azides: Radical Nature and H-Atom
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