Metal-pyrazole bifunction in half-sandwich C - N chelate iridium complexes: Pyrazole-pyrazolato interconversion and application to catalytic intramolecular hydroamination of aminoalkene

Article abstract of DOI:10.1002/chem.200902827

A study was conducted to demonstrate metal-pyrazole bifunction in half-sandwich C-N chelate iridium complexes, pyrazole-pyrazolato interconversion, and application to catalytic intramolecular hydroamination of aminoalkene. The chloro-pyrazole complex was obtained by acetate-promoted cyclometalation of 3,5-diphenylpyrazole and the ¹H NMR spectrum exhibited a low-field NH singlet that disappeared upon treatment with D ₂O. The Bronsted acidity of the coordinated pyrazole in the chloro-pyrazole complex was indicated by X-ray analysis, revealing the presence of intermolecular hydrogen bonds between the pyrazole proton and the chloro ligand. It was observed that the chloro-pyrazole complex underwent smooth dehydrochlorination with an equivalent amount of a base in toluene. Partial dehydrochlorination took place to afford the chloro-bridged pyrazolato-pyrazole complex when the chloro complex was treated with only 0.5 equiv of base.

Full text of DOI:10.1002/chem.200902827

DOI: 10.1002/chem.200902827
À
Metal–Pyrazole Bifunction in Half-Sandwich C N Chelate Iridium
Complexes: Pyrazole–Pyrazolato Interconversion and Application to
Catalytic Intramolecular Hydroamination of Aminoalkene
Yohei Kashiwame, Shigeki Kuwata,* and Takao Ikariya*[a]
Metal–ligand bifunctional molecular catalysts, in which
the ligand cooperates in the substrate binding and activation
through secondary interactions, such as hydrogen bonding
and proton transfer, have attracted increased attention in
synthetic organic chemistry. Among the most successful ex-
amples are the Noyoriꢀs hydrogenation and Noyori and Ikar-
iyaꢀs transfer-hydrogenation catalysts; these catalysts have
chelate amine ligands to provide a protic NH group at the
a-position to the metal (Scheme 1a,b).[1,2] The nitrogen
ligand works as a hydrogen-bond donor to the substrate in
the protonated amine form, and as an internal Brønsted
base in the deprotonated amido form, to realize efficient
catalytic reactions . Both amine and amido complexes are
isolable in some cases, allowing for detailed mechanistic in-
vestigations. Shvoꢀs catalyst[3] and Milsteinꢀs pincer-type
complexes[4] (Scheme 1c,d) represent an additional class of
bifunctional catalysts, in which an operationally protic group
is situated at a place more distant from the metal. As an ex-
tension of our extensive studies on the bifunctional chelate
amine complexes, such as Scheme 1b,e,[2,5,6] we recently syn-
thesized a chelating N-heterocyclic carbene (NHC) complex,
À
1, bearing a protic b-NH group and revealed facile C O
bond cleavage of allyl alcohol, which was ascribed to the
metal–NH bifunction.[7] Still, bifunctional catalysis based on
the b-NH group in ligated N-heteroaromatics remains in its
infancy if compared with the a-NH analogues. For example,
full characterization of interconvertible couples linked with
the reversible addition of H2 or pronucleophiles has rarely
been achieved for the protic NHC complexes[7–9] and for the
related, and more accessible, pyrazole-based chelate com-
plexes.[10–12] We report here the C N chelating protic pyra-
À
zole complex 2, which is isoelectronic with 1, as a new entry
of b-NH-based bifunctional catalysts (Scheme 1g). The well-
defined deprotonation–reprotonation network involving 2,
as well as catalytic hydroamination with 2 is described.[13]
The chloro–pyrazole complex 2 was readily obtained by
acetate-promoted cyclometalation[6,14] of 3,5-diphenylpyra-
zole [Eq. (1)]. The 1H NMR spectrum exhibits a low-field
Scheme 1. Examples of bifunctional transition metal catalysts with opera-
tionally protic ligands.
[a] Y. Kashiwame, Prof. Dr. S. Kuwata, Prof. Dr. T. Ikariya
Department of Applied Chemistry
Graduate School of Science and Engineering
Tokyo Institute of Technology
O-okayama, Meguro-ku, Tokyo 152-8552 (Japan)
Fax : (+81)3-5734-2637
E-mail: skuwata@apc.titech.ac.jp
NH singlet at d=11.58 ppm, which disappears upon treat-
ment with D2O. The Brønsted acidity of the coordinated
pyrazole in 2 was further indicated by X-ray analysis
tikariya@apc.titech.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902827.
766
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 766 – 770

COMMUNICATION
(Figure 1),[15] which revealed the presence of intermolecular
hydrogen bonds between the pyrazole proton and the chloro
ligand (Cl1···N1*, 3.215(7) ꢂ) similar to those observed for
complex 1.[7]
Figure 1. Dimeric structure of 2 connected with hydrogen bonds. Hydro-
gen atoms, except for the pyrazole proton, are omitted for clarity. Select-
À
À
À
ed interatomic distances [ꢂ]: Ir1 Cl1 2.417(2), Ir1 N2 2.065(7), N1 N2
À
À
1.363(9), Cl1 N1* 3.215(7), Cl1 H1* 2.495.
Scheme 2. Interconversion between pyrazole and pyrazolato complexes.
The chloro–pyrazole complex 2 underwent smooth dehy-
drochlorination with an equivalent amount of a base in tolu-
À
ene, similar to the C N chelate amine complexes
When the chloro complex 2 was treated with only
0.5 equiv of base, partial dehydrochlorination took place to
afford the chloro-bridged pyrazolato–pyrazole complex, 4,
exclusively (Scheme 2). Formally, 4 is an adduct of the satu-
rated complex 2 and the coordinatively unsaturated dehy-
drochlorinated pyrazolato complex; however, electronic de-
localization is suggested by the X-ray analysis of 4, which re-
vealed a symmetrical structure with an approximate C2 axis
passing through the bridging chloride and the midpoint of
the two non-coordinated N atoms (Figure 2b).[15] Although
the NH proton linking the two chelate ligands with a hydro-
gen bond[18] could not be found in the difference Fourier
[Cp*IrCl(C6H4CR2NH2)] (R=Ph, CH3), which contain an a-
NH group.[6] Product 3 is a dimer of the expected 16-elec-
tron, coordinatively unsaturated pyrazolato complex, as
shown in Scheme 2. The X-ray analysis of 3 revealed a pyra-
zolato-bridged dinuclear structure with a crystallographically
imposed inversion center (Figure 2a).[15] The Ir-N-N-Ir link-
age is significantly twisted with the torsion angle of 85.4(3)8
À
to give the chair-form Ir2N4 six-membered ring. The Ir N
bond lengths outside the chelate ring (Ir1 N1*, 2.195(4) ꢂ)
are much longer than those in the chelate (Ir1 N2,
2.072(3) ꢂ). The severe deviation from symmetric and
planar m-pyrazolato structures[17] is ascribed to the three-
legged piano-stool configuration of the metal centers with
the rigid chelate ligand.
À
À
À
map, its presence is supported by the short N1 N3 distance
of 2.736(10) ꢂ as well as the 1H NMR spectrum of 4 at
À908C, showing a broad singlet at d=16.04 ppm.[19] A di-
Figure 2. Structures of a) 3, b) 4, and c) 5. Hydrogen atoms except for the pyrazole proton and bridging hydride are omitted for clarity. Selected intera-
À
À
À
À
À
À
À
tomic distances [ꢂ] for 3: Ir1 N1* 2.195(4), Ir1 N2 2.072(3), N1 N2 1.374(4). For 4: Ir1 Ir2 4.2737(5), Ir1 Cl1 2.419(2), Ir1 N2 2.095(9), Ir2 Cl1
À
À
À
À
À
À
À
À
2.427(2), Ir2 N4 2.084(7), N1 N2 1.365(12), N3 N4 1.383(11), N1 N3 2.736(10). For 5: Ir1 Ir2 3.0372(2), Ir1 N2 2.080(5), Ir2 N4 2.084(6), N1 N2
À
À
1.352(8), N3 N4 1.359(8), N1 N3 2.799(7).
Chem. Eur. J. 2010, 16, 766 – 770
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
767

S. Kuwata, T. Ikariya and Y. Kashiwame
nuclear complex topologically related to 4,[12a] and a mono-
nuclear Cp*Ir complex[12b] bearing hydrogen-bonded
pyrazolato and pyrazole ligands has been reported.
À
Table 1. Intramolecular hydroamination catalyzed by C N chelate com-
plexes.[a]
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
Notably, dehydrochlorination of 2 with base in 2-propanol
as a hydrogen donor solvent resulted in the formation of the
hydrido-bridged pyrazolato–pyrazole complex 5 (Scheme 2).
The crystal structure shown in Figure 2c[15] closely resembles
that of the chloro-bridged complex 4 except for the bridging
À
hydrido ligand associated with the shorter Ir Ir distance
(3.0372(2) ꢂ). The m-hydride and hydrogen-bonded NH
proton, which have not been located crystallographically but
would lie on the approximate C2 axis, give rise to high- and
low-field resonances at d=À19.75 and 18.21 ppm in the
Entry
Substrate
Cat.
Base
t [h]
Yield [%][b]
1
2
3
4
5
6
7
8
6a
6a
6a
6a
6b
6a
6a
6a
2
2
2
3
3
7
8
KOtBu
KOtBu
none
none
none
KOtBu
none
KOtBu
6
15
6
15
15
6
75
98
29
96
96
20
0
1H NMR spectrum at À508C.[19] The two Cp*Ir(C N che-
À
late) moieties are again equivalent in both 1H NMR and
crystal-structure criteria.
The partially or fully deprotonated pyrazolato ligands in 3
and 4 were found to undergo reprotonation coupled with
ligand coordination to the metal (Scheme 2). Stepwise addi-
tion of diphenylamine hydrochloride to the unsaturated pyr-
azolato dimer 3 sequentially afforded the chloro-bridged
complex 4 and mononuclear chloro–pyrazole complex 2.
Complex 3 also reacts with 2-propanol to afford the hydri-
do-bridged complex 5. The reactivities summarized in
Scheme 2 are comparable to those found in the Shvoꢀs com-
plex,[3] as well as those of chelate amine complexes bearing
an a-NH group.[1,2,20] It should be pointed out that the trans-
formations in Scheme 2 are not associated with any formal
redox of the metal center unlike the Shvoꢀs catalyst.
15
6
none
0
[a] Reaction conditions: 6:Ir=20:1, [6]=0.2m. [b] Determined by
1H NMR spectroscopy.
vation have been established.[22] Following the olefin activa-
tion mechanism, the reaction would involve the nucleophilic
attack of the amine to the coordinated olefin, which is as-
sisted by the secondary interaction with the basic pyrazolato
ligand (Scheme 3, path a). Subsequent proton transfer from
Having established the protic pyrazole and basic pyrazol-
ACHTUNGTRENNUNGato interconversion linked to the metal-centered reactivities,
we next applied the bifunctional nature of these complexes
to the catalytic hydroamination of alkenes. The binary cata-
lyst composed of an equimolar mixture of the chloro–pyra-
zole complex 2 and KOtBu proved to promote intramolecu-
lar hydroamination of the w-alkenic primary amine, 6a, to
give the cyclization product (Table 1). Yields as high as 98%
were achieved after 15 h (entry 2), whereas the reaction rate
significantly decreased in the absence of the external base
(entry 3). As expected, the dehydrochlorinated pyrazolato
dimer 3 catalyzed the reaction with almost the same effi-
ciency as the binary catalyst (entry 4). The secondary amine
6b was also quantitatively converted to the corresponding
cyclic amine under the same conditions (entry 5). Notably
complex 7[21] bearing a g-NH group, as well as the amido
complex 8,[6] have a much lower or no catalytic activity, sug-
gesting that the catalysis is affected by the relative orienta-
tion of the metal and NH functionalities, as well as the
ligand basicity (entries 6 and 7).
Hydroamination of alkenes and alkynes catalyzed by late
transition metals provides an practical and atom-economical
access to nitrogen-containing compounds with functional-
group tolerance.[22] However, only a few successful examples
of the addition of simple primary and secondary amines to
unactivated alkenes have been reported.[23] For the catalytic
hydroamination of alkene with late transition metals, mech-
anisms initiated by either olefin coordination or amine acti-
Scheme 3. Possible mechanisms for cyclization of 6 catalyzed by pyrazol-
AHCTUNGTERGaNNUN to complexes. [Ir]=Cp*Ir.
À
the pyrazole nitrogen in A would lead to Ir C bond cleav-
age and the consequent release of the cyclization product.
Whereas, initial addition of the amino group to form an
amido–pyrazole intermediate in a bifunctional manner (path
b) appears less plausible, because the next step would need
an vacant site for olefin on the saturated metal center,[24]
and hence require additional processes such as Cp* ring slip-
page. It is to be emphasized here that the b-nitrogen group
in the bifunctional pyrazole/pyrazolato complexes should
play significant roles in the present hydroamination regard-
less of which mechanism is operative.
768
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Chem. Eur. J. 2010, 16, 766 – 770

Bifunctional Iridium Complexes
COMMUNICATION
[9] Quite recently, Albrecht and co-workers reported reversible addi-
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Inorg. Chem. 2009, 1871–1881.
In summary, we have unequivocally characterized a series
of pyrazole and pyrazolato complexes (2–5) with structural
diversity and revealed their bifunctional addition–elimina-
tion chemistry, which is essential to realize the efficient cat-
alysis. The intramolecular hydroamination of unactivated
aminoalkenes catalyzed by 2 and 3 also suggested the impor-
tance of the acidic b-NH functionality. Further elucidation
of the mechanism and substrate scope of the catalytic hydro-
amination is under way.
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Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Research on
Priority Areas (No. 18065007, “Chemistry of Concerto Catalysis”) from
Ministry of Education, Culture, Sports, Science and Technology, Japan.
Keywords: catalysts · cooperative effects · hydrogen bonds ·
iridium · N ligands
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[15] Crystal data for 2: C25H26ClIrN2, Mr =582.17, crystal dimensions
0.2ꢇ0.1ꢇ0.1 mm,
monoclinic,
P21/n,
a=8.4629(11) ꢂ,
b=
22.439(2) ꢂ, c=11.3728(11) ꢂ, b=93.830(5)8, V=2154.9(4) ꢂ3, Z=
4, 1calcd =1.794 gcmÀ3, F000 =1136, m=6.351 mmÀ1, reflections mea-
sured 19706, independent reflections 4905, Rint =0.081, R1=0.038
[I>2s(I)], wR2=0.135 (all data), residual electron density 2.38/
À2.62 eꢂÀ3. For 3: C50H50Ir2N4, Mr =1091.41, crystal dimensions
¯
0.3ꢇ0.2ꢇ0.05 mm, triclinic, P1, a=9.517(2) ꢂ, b=10.894(2) ꢂ, c=
11.288(3) ꢂ, a=65.774(6)8, b=89.253(9)8, g=71.762(7)8, V=
1004.5(4) ꢂ3, Z=1, 1calcd =1.804 gcmÀ3, F000 =532, m=6.677 mmÀ1
,
reflections measured 7463, independent reflections 4254, Rint =0.030,
R1=0.026 [I>2s(I)], wR2=0.078 (all data), residual electron densi-
ty 1.24/À1.85 eꢂÀ3. For 4: C50H51ClIr2N4, Mr =1127.87, crystal di-
¯
mensions 0.3ꢇ0.2ꢇ0.1 mm, triclinic, P1, a=10.812(3) ꢂ, b=
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2178.
13.866(4) ꢂ, c=14.830(4) ꢂ, a=83.457(11)8, b=81.461(9)8, g=
74.813(8)8, V=2115.3(10) ꢂ3, Z=2, 1calcd =1.771 gcmÀ3, F000 =1100,
m=6.406 mmÀ1, reflections measured 16730, independent reflections
9224, Rint =0.052, R1=0.052 [I>2s(I)], wR2=0.142 (all data), resid-
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¯
Mr =1136.52, crystal dimensions 0.2ꢇ0.2ꢇ0.2 mm, triclinic, P1, a=
10.463(2) ꢂ, b=10.938(3) ꢂ, c=19.844(5) ꢂ, a=87.060(6)8, b=
84.563(6)8,
g=80.218(6)8,
000 =1118, m=6.029 mmÀ1
int =0.045, R1=0.037 [I>
V=2226.3(9) ꢂ3,
Z=2,
1calcd =
1.695 gcmÀ3
,
F
,
reflections measured
17530, independent reflections 9689,
R
2s(I)], wR2=0.112 (all data), residual electron density 1.74/
À1.24 eꢂÀ3. Data were collected at 193 K by using a Rigaku Saturn
CCD area detector with graphite-monochromated MoKa radiation
Chem. Eur. J. 2010, 16, 766 – 770
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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769

S. Kuwata, T. Ikariya and Y. Kashiwame
(l=0.7107 ꢂ) to a maximum 2q value of 558. Intensity data were
corrected for Lorentz polarization effects and for absorption. Struc-
ture solution and refinements were performed with the CrystalStruc-
ture program package.[16] The structures were refined against F2 with
anisotropic temperature factors for all non-hydrogen atoms. The
carbon atoms in the solvating n-hexane molecule in 5·0.5C6H14 were
refined with constraint geometries and fixed isotropic parameters.
The hydrogen atoms except for the hydride and pyrazole hydrogen
atoms in 4 and 5·0.5C6H14 were included in the refinements by using
a riding model. CCDC-743358 (2), 743359 (3), 743360 (4), 743361
(5·0.5C6H14) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_-
request/cif.
broadened at room temperature, which may suggest the presence of
fluxional processes.
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Received: October 29, 2009
Published online: November 27, 2009
770
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