Synthesis, structure, and catalytic behavior of a PSiP pincer-type iridium(III) complex

Article abstract of DOI:10.1016/j.inoche.2011.05.018

The synthesis and characterization of a novel cyclometalated Iridium(III) complex [IrCl(H)(PSiP)] containing monoanionic, tridentate coordinating PSiP-pincer ligands [κ³-(2-^tBu₂PC ₆H₄)₂SiMe

Full text of DOI:10.1016/j.inoche.2011.05.018

Inorganic Chemistry Communications 14 (2011) 1306–1310
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, structure, and catalytic behavior of a PSiP pincer-type iridium(III) complex
Yong-Hua Li a,c, ,1, Yuan Zhang b, Xue-Hua Ding a,1
⁎
a
Jiangsu Key Laboratory of Organic Electronics & Information Displays, and Institute of Advanced Materials (IAM), Nanjing University of Posts and Telecommunications, Nanjing,
210046, PR China
b
Department of Applied Chemistry, Nanjing College of Chemical Technology, Nanjing 211189, PR China
Research Institute for Innovation in Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba,
c
Ibaraki 305-8565, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and characterization of a novel cyclometalated Iridium(III) complex [IrCl(H)(PSiP)] containing
monoanionic, tridentate coordinating PSiP-pincer ligands [κ3-(2-tBu2PC6H4)2SiMe]− ([PSiP]) is reported.
Complex (3) is one of the few examples of bis(phosphino)silyl(hydrido)iridium(III) complexes structurally
characterized by single crystal X-ray analysis. This compound has also been shown to catalyze the transfer
hydrogenation of ketones to the corresponding secondary alcohols moderately with 2-propanol as the
hydrogen source instead of using molecular dihydrogen gas or hazardous reducing agents (e.g., NaBH4 and
Received 26 April 2011
Accepted 19 May 2011
Available online 27 May 2011
Keywords:
Iridium
Silyl
Pincer
t
LiAlH4), and BuOK as the base.
© 2011 Elsevier B.V. All rights reserved.
Transfer hydrogenation
Tridentate ligands
Cyclometalated phosphine-based “PCP” pincer complexes of the
transition-metals have been the subject of intense research since the
initial investigations of this type of “PCP” ligands by Shaw et al. [1],
owing to the remarkable stoichiometric and catalytic reactivity
exhibited by such complexes [2,3]. Recently, significant effort has
been devoted to the synthesis of structurally and/or electronically
related systems where strategic alterations have been introduced to
the pincer ligand architecture, including variation of the central and
peripheral donor fragments, as well as the ancillary ligand backbone
[4]. However, the “PSiP” pincer-like transition metal complexes have
rarely been reported [5]. Although metal–silicon chemistry is well-
precedented across the transition series, relatively little attention has
been given to the incorporation of silyl donor fragments into the
framework of a preformed tridentate and tetradentate ancillary
ligand. Silyl ligands have strong σ-donating characters and show a
stronger trans influence than do commonly used ligands in transition
metal chemistry [6]. Silyl ligands would make an electron-rich metal
center and coordinatively unsaturated species by its strong trans-
labilizing effect. Therefore, “ancillary” silyl ligands would provide
transition metal complexes having unique reactivities useful for
catalysis. We have been working on the reaction of chelating disilyl
compounds with group 10 transition metal complexes and obtained a
number of unusual complexes bearing chelating silyl ligands [7]. Over
the course of the research, we found simple silyl ligands usually
have high reactivity and cannot stay on transition metals as “ancillary”
ligands. Incorporation of silyl group in a multidentate ligand frame-
work would be a useful strategy to make “ancillary” silyl ligands.
There are two types of approaches for this kind of silyl ligands: 1)
incorporation of one silyl group at the center of muntidentate
framework [8], and 2) attachment of two silyl groups in a rigid
multidentate framework. The second approach is so far rather limited
and the xanthsil ligand by Tobita and co-workers is a representative
example [9]. Recently, transition metal complexes bearing other
tridentate N2Si, and S2Si type ligands as well as tetradentate P3Si and
S3Si type ligands have also been reported [10].
Hydrogen transfer catalysis is an attractive protocol for the
reduction of ketones to alcohols in both academic and industrial
research. The use of a hydrogen donor (e.g., 2-propanol) instead of
using molecular dihydrogen gas or hazardous reducing agents (e.g.,
NaBH4 and LiAlH4) has a potential advantage in terms of mild reaction
conditions and excellent regioselectivity.11 Many pincer-like transi-
tion metal complexes of Ru, Ir, and Rh have been found to be active
catalysts in (a)symmetric hydrogen transfer reactions of polar groups
(e.g., ketones and imines) [11]. In recent years, a number of studies
appeared on the successful use of cyclometalated ruthenium(II)
complexes containing tridentate, cyclometalated PCP−, and NCN− as
catalyst precursors in hydrogen transfer reactions [12]. The great
interest in the use of E,C,E-pincer ligands (E=N, P) arises from the
remarkable stability of the corresponding metal complexes and the
⁎
Corresponding author at: Jiangsu Key Laboratory of Organic Electronics &
Information Displays, and Institute of Advanced Materials (IAM), Nanjing University
of Posts and Telecommunications, Nanjing, 210046, PR China. Tel.: +86 25 83335779;
fax: +86 25 83335882.
E-mail address: liyhnju@hotmail.com (Y.-H. Li).
Fax: +86 25 83335882.
1
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.05.018

Y.-H. Li et al. / Inorganic Chemistry Communications 14 (2011) 1306–1310
1307
possibility to modulate the reactivity of the metal center by fine-
tuning and control of the electronic and steric properties of the ligand
framework [13].
We are interested in [{2-(R2P)C6H4}2MeSi]– ligands (=PSiP-R,
t
i
R=Cy, Bu, Pr), which would have high electron donating property
and higher rigidity than do [(Ph2P(CH2)n)2MeSi]– ligands [14]. Herein
we report the synthesis and structure of a novel cyclometalated iridium
(III) complex [IrCl(H)(PSiPtBu)] (3) containing monoanionic, tridentate
coordinating PSiP-pincer ligands [κ3-(2-tBu2PC6H4)2SiMe]− ([PSiPtBu])
(Fig. 1), and its catalytic activity in the transfer hydrogenation of ketones
with 2-propanol as the hydrogen source and tBuOK as the base. Dialkyl
(aliphatic and cyclic), alkyl aryl, and diaryl ketones were all reduced in a
moderate yield by this PSiP Ir(III) complex (3).
Scheme 1. Synthetic route of organosilicon complex Ir-(κ3-PSiP) (3).
Treatment of the parent tertiary silane, [PSiP]H (2) with 0.5 equiv
of [IrCl(COD)]2 (cod=1,4-cyclooctadiene) in dry toluene at 80 °C
resulted in oxidative addition of the Si–H bond to the Iridium
(I) center to give a 16-electron Ir(III)-(κ3-PSiP) complex in high yields
(Scheme 1; isolated yield ca. 90%) [15]. Crystallization of compound
(3) from benzene afforded X-ray quality single crystals, and its
structure was unambiguously confirmed by single-crystal X-ray struc-
ture analysis (Fig. 1) [16,17]. To the best of our knowledge, there are
only more than 10 examples of bis(phosphino)silyl(hydrido) iridium
(III) complexes structurally characterized by single crystal X-ray
analysis [14]. Complex (3) crystallizes in the monoclinic group P21/c
(Table 1). The pincer-like title compound contains two stable five-
membered cyclometalated rings with the P–Ir–Si angles of 85.92(3)
and 85.70(4) °. The Ir atom is coordinated by two P atoms, one Si
atom, one Cl atom and one H atom in a distorted square-pyramidal
geometry, in which the silyl group occupies the apical coordination
site, while the remaining phosphine arms of the [PSiP] ligand, the Cl
and H atoms occupy basal sites. The bond distances of Ir1-Si1 and Ir1-
Cl1 are 2.2668(9) and 2.3994(8) Å, respectively, which are similar to
the other Ir analogue with pincer-like tridentate PSiP ligand, Ir(H)
[SiMe(CH2CH2CH2 PPh2)2]Cl [14]. The two P donor atoms are almost in
a trans arrangement with a P1-Ir1-P2 angle of 161.99(2) °, the methyl
group on Si1 donor in compound (3) is positioned trans to hydrido
group, and cis to Cl, with a Cl(1)–Ir(1)–H(52) angle of 163.0(13)°.
The two phenyl rings are, of course, planar, which are oriented at a
dihedral angle of 89.49 (2)° (Table 2). 1H NMR spectroscopy of
compound (3) showed the Ir-H signal as triplets with small 2J(P,H)
value of 14 Hz at around −23 ppm suggesting a cis relationship of the
H and the two P atoms.
reactivity, given the strong electron donating and trans-labilizing
abilities of Si. Reduction by means of hydrogen-transfer reactions has
recently attracted much attention because of its practical simplicity
and potential use at ambient pressure. Furthermore, the use of an
alternative source of hydrogen may result in different reactivity
patterns [18]. Recently, several Ru(II) PCP−, NCN−, CNC−, and CNN−
pincer complexes have been shown to catalyze the transfer
hydrogenation of ketones [19], and it has been proposed that the
metal-C σ-bond plays an important role in the formation of long-lived,
catalytically active species [20]. In this context, we became interested
in surveying the catalytic activity of iridium(III) complex [Ir(H)Cl
(PSiP)] (3) containing monoanionic, tridentate coordinating PSiP-
pincer ligand in the transfer hydrogenation of ketones, employing
basic iPrOH as the hydrogen source. When employing 0.2 mol% of
compound (3) with 5 mol% of KOtBu at 80 °C, moderate conversion to
the corresponding secondary alcohols was observed for several
ketone substrates, including diaryl, dialkyl, and alkyl/aryl ketones
(Scheme 2). Although conditions have not yet been optimised, it is
obvious that the activity of compound (3) as a catalyst in this reaction
is comparable to that observed with phosphinosilyl complexes
that feature an aliphatic or benzylic ligand backbone, and we
found that reduced conformational rigidity associated with the
flexible o-tertbutyl backbone of [PSiP] could provide moderate
stability and selectivity in metal-mediated substrate transformations
and the representative catalytic data obtained in our preliminary
survey are summarized in Table 3. The reactions are slow at room
Table 1
Based on our synthetic investigations of [PSiP]-ligated metal
complexes, we have begun to examine the utility of such species as
catalysts in a range of substrate transformations. In particular, we are
interested in exploring how the substitution of Si for C in a rigid
tridentate ancillary ligand framework influences metal-mediated
Crystal data and structure refinement parameters for 3.
Structure parameters
3
Empirical formula
fw
cryst syst
space group
a (Å)
C
29H48ClIrP2Si
714.35
monoclinic
P 21/c
12.291(3)
15.050(2)
16.500(3)
90.00
98.57(3)
90.00
b (Å)
c (Å)
α (o)
β (o)
γ (o)
V (Å3)
Z
3017.8(11)
4
T(K)
153(2)
F(000)
1440
1.572
4.675
0.941
ρ (g cm−3
)
absorption coefficient (mm−1
goodness of fit on F2
)
total no. of data collected
no. of unique data
21955
6935
R indexes [IN2σ(I)]
R1 =0.0217
wR2 =0.0435
R1 =0.0263
wR2 =0.0441
0.099 and −0.515
R (all data)
largest diff map hole and peak (e Å−3
R1=∑||Fo|−|Fc||/∑|Fo|; wR2=[∑w(Fo2 −Fc2)2/∑w(Fo2)2]1/2
)
Fig. 1. The structure of 3, showing the coordination environment of Ir atom. The
hydrogen atoms are omitted for clarity.
.

1308
Y.-H. Li et al. / Inorganic Chemistry Communications 14 (2011) 1306–1310
Table 2
Acknowledgments
Selected bond lengths and angles for 3.
This work was financially supported by the National Natural
Science Foundation of China (Grant Nos. 20901016, (6507040007)
and the National Basic Research Program of China(2009CB930601).
Ir(1)–P(1)
Ir(1)–Cl(1)
Si(1)–C(1)
Si(1)–C(8)
2.3251(7)
2.3994(8)
1.886(3)
Ir(1)–P(2)
Ir(1)–Si(1)
Si(1)–C(2)
Ir(1)–H(52)
2.3270(8)
2.2668(9)
1.898(3)
1.43(3)
1.897(3)
C(2)–Si(1)–C(8)
P(1)–Ir(1)–Cl(1)
C(1)–Si(1)–C(2)
P(2)–Ir(1)–Si(1)
C(2)–Si(1)–Ir(1)
105.04(15)
99.19(3)
104.63(12)
85.70(4)
P(1)–Ir(1)–P(2)
P(2)–Ir(1)–Cl(1)
P(1)–Ir(1)–Si(1)
C(1)–Si(1)–Ir(1)
Cl(1)–Ir(1)–H(52)
161.99(2)
98.60(3)
85.92(3)
120.62(10)
163.0(13)
Appendix A. Supplementary data
CCDC-821325 contains the supplementary crystallographic data for
compound (3). These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.
uk/data_request/cif. Supplementary data associated with this article can
be found in the online version, at doi:10.1016/j.inoche.2011.05.018.
106.29(9)
temperature but proceed at good rates at 80 °C and the catalyst
system hydrogenates aliphatic ketones faster that aromatic ones. As is
the case for most metal-catalyzed transfer hydrogenation processes
conducted in iPrOH, less than 5% conversion was observed in the
absence of KOtBu as base. The preformed Ir complex (3) was similarly
inactive for transfer hydrogenation of cyclohexanone in the absence of
added KOtBu, although 82% conversion was obtained when using
5 mol% KOtBu along with 0.2 mol% 3 (entry 6, Table 3). Furthermore,
these data show that the stability provided by the chelate ligand, and
the potential for electronic/coordinative unsaturation are possible
reasons for the activity observed. These preliminary results establish
[PSiP]Ir complexes as a promising class of precatalysts for transfer
hydrogenation. Further mechanistic studies of this reaction, as well as
catalytic studies featuring these and other [PSiP] derivatives, are
currently in progress.
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cyclopentanone
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[15] (a) Preparation of (2-BrC6H4)But2P (1):
A mixture of Pd(OAc)2 (4.5 mg,
0.02 mmol), 1,1'-bis(diisopropylphosphino)ferrocene (10 mg, 0.024 mmol) and
Cs2CO3 (393 mg, 1.2 mmol) in 1,4-dioxane (3 mL) was stirred for 1 h at room
temperature under N2. Then 2-bromo-iodobenzene (284 mg, 1.0 mmol) and
tBu2PH (146 mg, 1.0 mmol) (tBu=tert-butyl) were added by syringe. The
mixture was stirred at 80 °C for 3 days, then allowed to cool to room temperature,
filtered and concentrated. The crude product was purified by flash column
chromatography on silica gel to afford the title compound 1 (150 mg, 50%) as a
colorless solid. 1H NMR (C6D6, 499.1 MHz,): δ 1.15 (d, 18H, J=11 Hz), 6.73 (t, 1H,
J=7 Hz), 6.91 (t, 1H, J=7 Hz), 7.53 (dd, 1H, J=8 Hz, J=4 Hz), 7.64 (d, 1H,
J=7 Hz). 31P{1H}NMR (C6D6, 202.0 MHz): δ 32.29 (s). Calcd (%) for C14H22PBr: C,
55.83; H, 7.36. Found: C, 56.09; H, 7.47. (b) Preparation of [2-( tBu2P)C6H4]2SiHMe
([PSiP]H, 2): n-BuLi (7.2 mL, 1.59 M in hexane, 11.3 mmol) was added dropwise
to an Et2O solution (30 mL) of 2- tBu2PC6H4Br (3.40 g, 11.3 mmol) (tBu=tert-
butyl) at −78 °C, resulting in a red-orange colored solution . The resulting
solution was allowed to warm to room temperature over the course of 2 h, over
the course of which a white precipitate was observed and the solution became
yellow-orange in color. Then the mixture was once again cooled to −78 °C and
MeHSiCl2 (0.58 mL, 5.7 mmol) was added via syringe. The resulting pale yellow-
orange colored reaction mixture was allowed to warm to room temperature and
continue stirring for an additional 14 h at room temperature. Volatiles were then
removed under vacuum, and the remaining residue was extracted with benzene
(3×10 mL). The benzene extracts were filtered through Celite and the benzene
was removed in vacuo to afford a viscous, air- sensitive colorless liquid 2 (2.64 g,
95%). 1H NMR (C6D6, 499.1 MHz): δ 0.93 (d, 3H, J=4 Hz), 1.17 (d, 18H, J=3 Hz),
1.19 (d, 18H, J=3 Hz), 6.45 (quasi octet, 1H, J(H,Si)=216 Hz), 7.13-7.19 (m, 4H),
7.72-7.78 (m, 4H). 31P{1H} NMR (C6D6, 202.0 MHz): δ 23.2 (s). 29Si{1H} NMR
(C6D6, 99.1 MHz): δ –24.0 (t, J(P,Si)=22 Hz). Calcd (%) for C37H56P2Si: C, 75.21;
H, 9.55. Found: C, 74.75; H, 9.58. (c) Preparation of [2-( tBu2P)C6H4]2SiIr(H)ClMe
([PSiP] Ru(H)ClMe, 3): A solution of [2-( tBu2P)C6H4]2SiHMe (1.83 g, 3.75 mmol)
(tBu=tert-butyl) in dry toluene (15 ml) was added to a slurry of [IrCl(COD)]2
(1.27 g, 1.88 mmol) (cod=1,4-cyclooctadiene) in toluene (10 ml) at room
temperature. The resulting wine colored solution was allowed to stir at room
temperature for 1 h, and then was heated at 80 °C for 24 h. The volatile
components were removed in vacuo, and the residue was washed with hexane
(3×5 mL) to give the pure product 3 (2.41 g, 90%) as a colorless solid. Colorless
crystals suitable for X-ray diffraction were obtained by slow evaporation of a
benzene solution (5 mL) of the compound (20 mg) after 2 d. 1H NMR (C6D6,
499.1 MHz): δ -23.7 (t, 1H, J=14 Hz), 0.84 (s, 3H), 1.39 (t, 18H, J=7 Hz), 1.55 (t,
18H, J=7 Hz), 7.03 (t, 2H, J=8 Hz), 7.15 (t, 2H, J=7 Hz), 7.66 (br d, 2H, J=8 Hz),
7.98 (d, 2H, J=8 Hz). 31P{1H} NMR (C6D6, 202.0 MHz): δ 78.59 (s). 29Si{1H} NMR
(C6D6, 99.1 MHz): δ 7.56 (s). Calcd (%) for C29H48ClIrP2Si: C, 48.76; H, 6.77. Found:
C, 48.97; H, 6.69. (d) Typical procedure for the catalytic hydrogen-transfer
reaction: To a mixture of ketone (2 mmol), the catalyst 3 (0.004 mmol, 2 mL of a
stock solution of 3 in iPrOH) and 2 mL of iPrOH was added a solution of KOtBu
(195 μL of a 5 wt % solution in iPrOH, 0.1 mmol) under nitrogen. The resulting
brown solution was stirred at room temperature for 10 min. The mixture was
then heated at 80 °C. The mixture turned pale yellowish after the reaction was
complete. The yields were determined by GC analysis with n-undecane as internal
standard, and the identities of the hydrogenation products were confirmed by
comparison to authentic samples.
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