Synthesis of A Pincer-IrV Complex with A Base-Free Alumanyl Ligand and Its Application toward the Dehydrogenation of Alkanes

Article abstract of DOI:10.1002/anie.201909009

A pincer-iridium complex bearing a Lewis-base-free X-type alumanyl ligand has been synthesized. X-ray diffraction, NMR and IR spectroscopy, as well as XANES analysis confirmed its tetrahydrido-Ir^V structure and Lewis acidity at the Al center as supported by DFT calculations. The resulting complex was applied as a catalyst for the transfer dehydrogenation of cyclooctane.

Full text of DOI:10.1002/anie.201909009

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Accepted Article
Title: Synthesis of A Pincer-Ir(V) Complex with A Base-Free Alumanyl
Ligand and Its Application toward Dehydrogenation of Alkanes
Authors: Shogo Morisako, Seiya Watanabe, Satoru Ikemoto, Satoshi
Muratsugu, Mizuki Tada, and Makoto Yamashita
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201909009
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10.1002/anie.201909009
Angewandte Chemie International Edition
COMMUNICATION
Synthesis of A Pincer-Ir(V) Complex with A Base-Free Alumanyl
Ligand and Its Application toward Dehydrogenation of Alkanes
Shogo Morisako,[a] Seiya Watanabe,[b] Satoru Ikemoto,[c] Satoshi Muratsugu,*[c] Mizuki Tada,*[c] and
Makoto Yamashita*[a]
Abstract: A pincer-iridium complex bearing a Lewis-base-free X-type
alumanyl ligand was synthesized. X-ray diffraction study, NMR, IR
and XANES analysis confirmed its tetrahydrido-Ir(V) structure and
Lewis acidity at the Al center as supported by DFT calculations. The
complex D bearing a terpyridine-coodrinated alumanyl ligand
N,N,NPAlP pincer ligand) and cesium pivalate for hydrosilylation of
(
CO2.11f,j Thus, there has been no example for a homogeneous
catalyst with Lewis-base-free alumanyl ligand. Herein, we report
the synthesis of a PAlP-pincer iridium complex bearing a Lewis
base-free alumanyl ligand. The structure, electronic property,
reactivity toward Lewis base, and catalytic activity for transfer
dehydrogenation of cyclooctane were also investigated.
resulting complex was applied as
dehydrogenation of cyclooctane.
a
catalyst for transfer
PXP pincer ligands, consisting of two phosphine-tethers and
an anionic X-ligand, have widely been applied in organometallic
chemistry by using the characteristic electronic features of the
central X atom and high thermal stability.1 Recently, group 13
elements have attracted attention as the anionic X-ligand of PXP
pincer ligand in the viewpoint of its stronger σ-donating property
due to their small electronegativity.2 For example, our group
reported the synthesis, various bond cleavage reactions, and
catalytic application of group 8-10 transition metal complexes
bearing a phosphine-tethered boryl (PBP pincer) ligand (A in
Figure 1).3 Peters and Lin utilized the lability of boryl ligand in
PBP-Co/Ni complexes for catalytic hydrogenation of olefin.4
Ozerov explored chemistry of PBP*-pincer complexes B having a
carbon-based backbone, including oxidative addition of C–B bond
and its reverse reaction, catalytic dehydrogenation of alkanes,
and stoichiometric C–H activation of pyridine.5
By contrast to the rich chemistry of anionic X-type boryl ligand
for transition metal complexes,6 chemistry of anionic X-type
alumanyl ligand7 has been behind. In fact, only three transition
metal complexes having an unsupported X-type alumanyl ligand
have been reported to date.8-10 Addition of Lewis base11 and
bridging coordination mode to two metals12 have also been known
to synthesize alumanyl complexes. Among them, only two
examples of base-stabilized alumanyl complexes have been used
Me
PPh2
PR2
MLn
PR2
PR2
MLn
PR2
N
N
Cl
N
N
N
B
B
Al Rh(nbd)
N
Al Pd Cl
Cl
PiPr2
PiPr2
N
N
PPh2
A
B
C
D
N,N,NPAlP]Pd(Cl)
[PBP]MLn
[PBP*]MLn
[NPAlP]Rh(nbd)
[
Figure 1. Selected examples of transition metal complexes bearing anionic
group 13 ligands. R = alkyl, aryl; M = metal; L = ligand.
To introduce a larger Al atom than B atom, we focused on the
long-tethered scaffold of previously reported PBP-pincer iridium
complex.14 The PAlP pincer ligand precursor 4 was synthesized
with a slightly modified procedure for the preparation of the PBP
pincer ligand precursor (Scheme 1).14 Reduction of
phosphinoborane-tethered phenylenediamine
1 with LiAlH4
followed by hydrolysis afforded 2. Deprotection of the phosphine–
borane moiety of 2 with N-acetylethylenediamine resulted in the
formation of 3. Reaction of 3 with AlH3·NMe2Et provided
phosphine-tethered Al–H precursor 4. The IR spectrum of 4
showed a characteristic absorption band at 1861 cm−1, being
assignable to an Al–H stretching vibration, which was similar to
those of previously reported diaminoaluminum hydrides.15
as
a homogeneous catalyst. Nakao reported C2-selective
monoalkylation of pyridine by using Rh complex C bearing a base-
stabilized NPAlP-pincer ligand as a catalyst,11h in which the
alumanyl ligand increases electron density on the Rh center and
provides a Lewis acidity leading to the high selectivity.13 Takaya
and Iwasawa reported a highly active catalyst consisting of Pd
PtBu2•BH3
PtBu2•BH3
O
NH
NH
NH
NH
LiAlH4
THF
[a]
Dr. S. Morisako, Prof. Dr. M. Yamashita
PtBu2•BH3
PtBu2•BH3
O
Department of Molecular and Macromolecular Chemistry, Graduate
School of Engineering, Nagoya University, Furo-cho, Chikusa-ku,
Nagoya, 464-8603 Aichi (Japan)
E-mail: makoto@oec.chembio.nagoya-u.ac.jp
S. Watanabe
Department of Applied Chemistry, Faculty of Science and
Engineering, Chuo University, 1-13-27, Kasuga, Bunkyo-ku, 112-
8551 Tokyo (Japan)
S. Ikemoto, Prof. Dr. S. Muratsugu, Prof. Dr. M. Tada
Department of Chemistry, Graduate School of Science & Research
Center for Materials Science (RCMS) & Integrated Research
Consortium on Chemical Science (IRCCS), Nagoya University,
Furo-cho, Chikusa-ku, Nagoya, 464-8603 Aichi (Japan)
E-mail: smuratsugu@chem.nagoya-u.ac.jp
1
2
O
PtBu2
NH2
PtBu2
PtBu2
N
H
[b]
[c]
NH
NH
AlH3•NMe2Et
N
N
Al
H
toluene
PtBu2
3
4
Scheme 1. Synthesis of the PAlP Pincer Ligand Precursor 4.
mtada@chem.nagoya-u.ac.jp
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10.1002/anie.201909009
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COMMUNICATION
Complexation of 4 with [Ir(cod)Cl]2 (cod = 1,5-cyclooctadiene)
in toluene at 100 °C led to an isolation of [PAlP]Ir(H)4 5 (Scheme
2).16 The 1H NMR spectrum of 5 showed two different signals each
having 2H integral ratio in the hydride region (Figure S12): a broad
singlet signal at dH −11.40 ppm (fwhm = 23 Hz, T1 = 420 ms) and
a broad triplet of triplet signal at dH −12.52 ppm (2JHH = 16 Hz, 2JPH
= 3 Hz, T1 = 362 ms), which are assignable to the four hydride
ligands around the Ir center.17,18 The long relaxation time T1 for
both signals also supported four hydrogen atoms attached to the
Ir center as hydride ligand rather than dihydrogen ligand.19,20 Two
phosphorus atoms in 5 resonated as one singlet signal at dP 64.5
ppm in the 31P{1H} NMR spectrum. The IR spectrum also
confirmed the existence of four hydride ligands around the Ir
center (νIrH 1802 and 2110 cm−1).21 Complex 5 was thermally
stable even in o-xylene solution at 150 °C as judged by the 1H and
31P{1H} NMR spectroscopy. The solid-state structure of 5 is shown
in Figure 2. All four hydride ligands around the Ir center were
assigned to the residual peaks in the differential Fourier map. The
Al–Ir bond [2.3819(14) Å] is apparently shorter than the sum of
covalent radii of Al and Ir atoms (2.51 Å),22 and is shortest hitherto
reported for Ir complexes bearing an Al atom in the coordination
sphere.23 The slightly shorter N–Al bonds [1.812(4), 1.818(4) Å]
in 5 than those of previously reported NPAlP-Rh complexes
(1.822–1.884 Å)11h indicated pp-pp interaction between N and Al
atoms in 5.11h However, a significant Lewis acidity of the Al atom
in 5 was confirmed by a coordination of an external Lewis base.
The reaction of 5 with DMAP immediately proceeded at room
temperature to form 6 quantitatively (Scheme 2).24 In the solid-
state structure of 6 (Figure 3), the Al–Ir bond length elongated to
2.4779(11) in comparison with that of 5 by the coordination of
DMAP, caused by re-hybridization of the Al atom from sp2 in 5 to
sp3 in 6. The bond length between the coordinated nitrogen atom
N3 and Al atom [2.037(3) Å] was comparable to those observed
in [NPAlP]Rh(CO)2 (2.071(3), 2.094(3) Å).9
PtBu2
PtBu2
H
H
N
H
Ir
N
N
H
H
[Ir(cod)Cl]2
toluene
DMAP
H
N
N
4
Al
Al
Ir
H
PtBu2
toluene
N
H
PtBu2
5
6
Scheme 2. Synthesis of tBu-Substituted [PAlP]Ir(H)4 5 and its DMAP adduct.
Figure 3. Molecular structure of 6 obtained by single-crystal X-ray diffraction
analysis. Thermal ellipsoids are displayed at 50 % probability. All hydrogen
atoms except for hydride ligand are omitted for clarity. Selected bond distances
(Å) and angles (°): Al1–Ir1 2.4779(11), P1–Ir1 2.3119(10), P2–Ir1 2.3179(10),
N1–Al1 1.863(3), N2–Al1 1.857(3), N3–Al1 2.037(3), P1–Ir1–P2 165.54(4), N1–
Al1–N2 88.86(14), N1–Al1–N3 95.66(13), N2–Al1–N3 98.88(14)
1
The H NMR spectroscopic studies and DFT calculations on
5 suggested that the Ir center of 5 has an oxidation number of five.
The oxidation state of 5 was experimentally confirmed by X-ray
absorption near-edge structure (XANES). The Ir LIII-edge XANES
spectrum for 5 is presented in Figure 4 with those for Ir powder,
Cp*Ir(H)4 (Cp* = C5Me5),25 and Ir(acac)3 as reference compounds
of Ir(0), Ir(V), and Ir(III), respectively. The edge energies of the Ir
LIII-edge XANES spectra were sensitive to the oxidation state of
Ir, and differences in the edge energies of Ir(V)-, Ir(0)-, and Ir(III)-
reference compounds were obvious. The XANES spectrum of 5
was similar to that of Cp*Ir(H)4, indicating that 5 has Ir(V) center.
Considering the corresponding boron-containing PBP-pincer
ligand possessing the same o-phenylenediamine backbone led to
a formation of Ir(III) dihydride rather than Ir(V) tetrahydride,26 it is
reasonable to attribute that the reason why 5 was isolable in
higher oxidation state is due to the stronger σ-donicity of the
alumanyl ligand than that of boryl ligand.
Figure 2. Molecular structure of 5 obtained by single-crystal X-ray diffraction
analysis. Thermal ellipsoids are displayed at 50 % probability. All hydrogen
atoms except for hydride ligand are omitted for clarity. Selected bond distances
(Å) and angles (°): Al1–Ir1 2.3819(14), P1–Ir1 2.3232(11), P2–Ir1 2.3275(11),
N1–Al1 1.812(4), N2–Al1 1.818(4), P1–Ir1–P2 170.50(4), N1–Al1–N2 90.75(19).
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10.1002/anie.201909009
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COMMUNICATION
cyclooctane (COA)30 was also explored (Scheme 3). The highest
TON of 31 was obtained at 180 °C using 1-hexene as a hydrogen
acceptor (Table S1). This result demonstrated the first example of
dehydrogenation of an alkane using a homogeneous catalyst with
alumanyl ligand.31
[PAlP]Ir(H)4 5 (S/C = 3000)
180 °C, 6 h
nBu
nBu
COA
COE
TON 31
Scheme 3. Catalytic transfer dehydrogenation of COA using the Lewis-base-
free PAlP pincer iridium complex 5.
In conclusion, we synthesized and characterized the Lewis-
base-free X-type PAlP pincer-Ir complex 5. The intrinsic strong σ-
donation and Lewis acidity of the X-type Al ligand were
experimentally and theoretically elucidated. The thermal stability
of pincer-Ir complex 5 was applied to the catalytic transfer
dehydrogenation of cyclooctane with a moderate activity.
Figure 4. Normalized Ir LIII-edge XANES spectra for 5 (black solid line),
Cp*Ir(H)4 (gray solid line), Ir(acac)3 (gray dotted line) and Ir powder (black
dashed line)..
To obtain details about electronic structure of 5, we conducted
computational studies on the optimized structure opt-5. The
significantly long atomic distances among hydride ligands (>2.24
Å) in opt-5 supported that 5 should be hydride complex rather
than dihydrogen complex, as judged by long T1 experimentally.
The molecular orbital of opt-5 corresponding to two-center two-
electron Al–Ir σ-bond appears as HOMO−8 (Figure 5a). LUMO
shows the contribution of empty 3p orbital on the Al atom (Figure
5b). These shapes of molecular orbitals indicated that opt-5 is
viewed as alumanyl-Ir(V) tetrahydride, [PAlP]Ir(H)4, rather than
the structure with bridging hydride, [PAlP](µ-H)2Ir(H)2. This
structural model [PAlP]Ir(H)4 was also confirmed by AIM
analysis27 for opt-5 using AIMALL software package28 (Figure
S27). A bond path was found between Al and Ir atoms to indicate
the existence of Al–Ir σ-bond, in addition, no bond path was found
between internal hydride ligands and the Al atom.
Acknowledgements
This research was supported by CREST 14529307 in
Establishment of Molecular Technology towards the Creation of
New Functions, Japan. Theoretical calculations were carried out
using resources of the Research Center for Computational
Science, Okazaki, Japan. XAFS measurements were performed
by PF-PAC (2017G533).
Keywords: alumanyl ligand • pincer complex • iridium • catalysis
• XANES
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COMMUNICATION
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10.1002/anie.201909009
Angewandte Chemie International Edition
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PAlP pincer ligand: A pincer-iridium
complex bearing a Lewis-base-free X-
type alumanyl ligand was synthesized.
X-ray diffraction study, NMR, IR, and
XANES analysis confirmed its
tetrahydrido-Ir(V) structure and Lewis
acidity at the Al center as supported
by DFT calculations. The resulting
complex was applied as a catalyst for
transfer dehydrogenation of
Shogo Morisako,[a] Seiya Watanabe,[b]
Satoru Ikemoto,[c] Satoshi Muratsugu,*[c]
Mizuki Tada,*[c] and Makoto
Yamashita*[a]
PtBu2
H
H
N
N
Al Ir
H
H
Page No. – Page No.
PtBu2
PAlP-Ir(V)
Synthesis of Pincer-Ir(V) Complex
Having A Base-Free Alumanyl Ligand
and Its Application toward
Dehydrogenation of Alkanes
cyclooctane.
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Title
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