Iridium complexes of new NCP pincer ligands: Catalytic alkane dehydrogenation and alkene isomerization

Article abstract of DOI:10.1039/c3cc46851h

Iridium complexes of novel NCP pincer ligands containing pyridine and phosphinite arms have been synthesized. One Ir complex shows good catalytic activity for alkane dehydrogenation, and all complexes are highly active for olefin isomerization. A combination of the Ir complex and a (PNN)Fe pincer complex catalyzes the formation of linear alkylboronates selectively from internal olefins via sequential olefin isomerization-hydroboration. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3cc46851h

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Iridium complexes of new NCP pincer ligands:
catalytic alkane dehydrogenation and alkene
isomerization†
Cite this: Chem. Commun., 2014,
50, 11056
Received 7th September 2013,
Accepted 29th July 2014
Xiangqing Jia, Lei Zhang, Chuan Qin, Xuebing Leng and Zheng Huang*
DOI: 10.1039/c3cc46851h
www.rsc.org/chemcomm
Iridium complexes of novel NCP pincer ligands containing pyridine
The synthesis of the NCP ligands and (NCP)IrHCl complexes
and phosphinite arms have been synthesized. One Ir complex is outlined in Scheme 1. Suzuki coupling between (3-methoxy-
shows good catalytic activity for alkane dehydrogenation, and all phenyl)boronic acid and 2-bromopyridines gave 2-(3-methoxy-
complexes are highly active for olefin isomerization. A combination phenyl)pyridines. Deprotection of the methoxy groups with
of the Ir complex and a (PNN)Fe pincer complex catalyzes the HBr, followed by the reaction with NaH and di-tert-butyl-
formation of linear alkylboronates selectively from internal olefins chlorophosphine formed the NCP ligands 2a–c. Metalation of
via sequential olefin isomerization–hydroboration.
2a–c with [Ir(COD)Cl]2 under a hydrogen atmosphere produced
t
the (RNCP)IrHCl (R = H, 1a; Me, 1b; Bu, 1c) complexes.
Transition metal complexes of pincer ligands are currently a
The Ir complexes were characterized by NMR spectroscopy
subject of great interest.1 One of the potential applications of such and single crystal X-ray diffraction. The characteristic hydridic
complexes is the pincer iridium-catalyzed alkane dehydrogena- IrH resonances appear at À39.24 ppm for 1a and À33.27 ppm
tion.2 This transformation converts low-value alkanes into olefins for 1b in the 1H NMR spectra, consistent with that observed for
that are important synthetic intermediates. Despite the fact that a pentacoordinate Ir hydridochloride species with the hydride
a wide range of pincer Ir complexes with various donors are trans to a vacant coordination site. In contrast, the IrH reso-
now known,1d,e,3 only bis(phosphine)-ligated (PCP)Ir2b,4,5 and nance for 1c appears at À21.89 ppm, implying that the site
bis(phosphinite)-ligated (POCOP)Ir6 pincer complexes exhibit good trans to the hydride is occupied. The solid structure of 1c
t
catalytic activity for alkane dehydrogenation.7 Several (CCC)Ir reveals an agostic interaction between the Bu group on the
complexes of dicarbene pincer ligands have also been investigated, pyridine ring and the Ir centre (Ir-H13A, 2.045 Å), and it shows
but they are significantly less effective than the (PCP)Ir and that the hydride is indeed trans to Cl (Fig. 1). The structure of
(POCOP)Ir catalysts in catalytic alkane dehydrogenation.8
1b adopts a severely distorted square pyramidal geometry with
Driven by our interest in developing a new type of pincer the hydride being at the apical site. Different to the structures
complex for catalytic alkane functionalization, herein we report of related Ir(III) hydridochloride pincer complexes,6,9 the Cl
the synthesis and catalytic properties of Ir complexes supported atom in 1b significantly deviates from the C–P–Ir–N plane,
by new NCP pincer ligands with pyridine and phosphinite most likely due to the steric repulsion between the Me group
arms. The dehydrogenation activity of one (NCP)Ir complex is
comparable to that of the (PCP)Ir system. More notably, these
complexes are exceptionally active precatalysts for olefin
isomerization. A dual-catalyst system has been developed for
regioselective conversion of internal olefins to linear alkylbor-
onate esters via tandem Ir-catalyzed olefin isomerization and
Fe-catalyzed olefin hydroboration.
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, 345 Lingling Road, Shanghai, 200032, China.
E-mail: huangzh@sioc.ac.cn; Fax: +86 5492 5533; Tel: +86 5492 5522
† Electronic supplementary information (ESI) available: Experimental procedure,
NMR data, and crystallographic data for 1a(H2O), 1b, and 1c. CCDC 957716–
957718. For ESI and crystallographic data in CIF or other electronic format see
Scheme 1 Synthesis of the NCP ligands and (NCP)Ir complexes.
DOI: 10.1039/c3cc46851h
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Table 2 TONs and octene distributions (concentrations in mM) from
Ir-catalyzed transfer–dehydrogenation of n-octane and TBE
Fig. 1 Crystal structures of complexes 1a(H2O), 1b, and 1c. The key bond
distances and angles are listed in the ESI.†
Entry
1
[Ir]
Time (min)
TONsa
1-Octb (%)
trans-2
cis-2
1a
5
30
60
27
68
78
2 (7)
3 (4)
3 (4)
6
7
12
7
11
12
on the pyridine ring and the Cl atom. An attempt to grow crystals
of 1a in a toluene–pentane solution resulted in the formation of
crystals of a H2O-bound six-coordinate Ir species 1a(H2O)
(Fig. 1). The least sterically hindered complex 1a appears to be
more subjected to water coordination than 1b and 1c.10
2
1b
1c
5
30
60
7
21
28
0.5 (7)
1.3 (6)
1.6 (6)
3
4
5
2
6
7
3
5
30
60
5
14
19
0.5 (10)
0.9 (6)
1.1 (6)
3
5
5
1
4
8
We initially examined the catalytic activity of 1a–c for
transfer–dehydrogenation of cyclooctane (COA) with tert-
butylethylene (TBE) as the hydrogen acceptor. The results are
summarized in Table 1. Upon activation with NaOtBu, 1a
exhibits good activity, but 1b and 1c are weakly effective.
A COA solution containing 1.0 mM [Ir], 2.2 mM NaOtBu and
0.5 M TBE was heated at 150 1C. After 12 h, the process with 1a
converted 93% of TBE to tert-butylethane (TBA) (466 TONs),
while the processes with 1b and 1c gave only 6 TONs. It is worth
noting that reactions with higher concentration of TBE resulted
in lower initial turnover frequency. Dehydrogenation with 1a
using 0.5, 1.0, 2.0, and 2.5 M TBE gave 168, 123, 101, and 71
TONs after 10 min at 150 1C, respectively (see the ESI† for
details). The data indicate that catalysis can be inhibited by
TBE at high [TBE], as found for the (PCP)Ir system.4a
a
TONs were calculated based on conversion of TBE determined by GC.
Values in parentheses are the percentage of 1-octene relative to total
b
octenes.
The selective formation of a-olefin via alkane dehydrogena-
tion is desirable, but remains challenging. Goldman et al. have
shown that (PCP)Ir complexes dehydrogenate n-alkane regio-
selectively to give a-olefin as the major kinetic product, but the
a-olefin is eventually isomerized to the internal isomers.4c The
kinetic selectivity for a-olefin formation with our (NCP)Ir
complexes is significantly lower compared to that reported for
the (PCP)Ir system.4c After 5 min, catalysis with 1a, 1b, and 1c
gave a small amount of 1-octene, constituting only 7, 7, and
10% of the total octenes, respectively (Table 2).
The distribution of octenes in the dehydrogenation reaction is
determined by the regioselectivity for dehydrogenation and the
rate of olefin isomerization. To assess the isomerization activity of
the (NCP)Ir complexes, Ir-catalyzed isomerization of 1-octene was
studied. After 30 min at 60 1C, catalysis with only 0.2% of 1a, 1b and
1c converted 73, 90 and 87% of 1-octene into internal octenes. For
comparison, the related (PCP)Ir and (POCOP)Ir systems isomerized
only 27 and 13% of 1-octene to internal octenes under the same
conditions (see the ESI† for details).11 The isomerization of
1-octene over time was monitored. As shown in Fig. 2, the reaction
Next we evaluated the (NCP)Ir complexes for dehydrogena-
tion of linear alkanes. The results are summarized in Table 2.
The dehydrogenation of n-octane with 1a as the precatalyst gave
78 TONs after 1 h at 150 1C (entry 1). As observed in reactions
with COA, 1b and 1c are less active than 1a in this transforma-
tion (entries 2 & 3). However, 1b and 1c showed enhanced
activity in dehydrogenation of n-octane compared to the reac-
tions with COA. For example, the dehydrogenation of n-octane
with 1b gave 21 TONs after 30 min at 150 1C, while the reaction
with COA gave only 3 TONs under the otherwise identical
t
conditions. As shown in Fig. 1, the Me and Bu substituents
on the pyridine rings in 1b and 1c create a crowded steric
environment around the Ir center. We attribute the higher
reactivity of n-octane versus COA in catalysis with 1b and 1c
to the reduced steric demand of the linear alkane relative to the
cyclic alkane.
Table 1 TONs for transfer–dehydrogenation of COA/TBE catalyzed by
1a–c upon activation with NaOtBua
Time
1a
1b
1c
10 min
30 min
12 h
168
257
466
2
3
6
1
2
6
Fig. 2 Isomerization of 1-octene catalyzed by 1b/NaOtBu at 60 1C. The
reaction was performed with 2.5 mM 1b, 5.5 mM NaOtBu and 1.25 M
1-octene in 1.6 mL mesitylene.
a
TONs were calculated based on conversion of TBE determined by GC.
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with 1b nearly reached equilibrium in 1 h. A substantial produc- this system is completely ineffective for hydroboration of
tion of 3- and 4-octenes was observed at the early stage of the internal olefins.14 To our delight, the combination of 1b and
reaction, suggesting that the rate of isomerization of 2-octenes to 3 efficiently catalyzes the formation of linear alkylboronate
3-octenes is comparable to the rate of isomerization of 1-octenes esters from internal olefins (Table 3). With NaBHEt3 (11%) as
to 2-octenes.
the catalyst activator, the reactions of cis-3-hexene and cis-4-
Given the high activity of the (NCP)Ir complexes in olefin octene with HBPin using 0.5% of 1b and 5% of 3 in toluene at
isomerization, we envisioned that a combination of the Ir 23 1C formed the linear alkylboronate products 4 and 5 in
catalyst with a second catalyst that effects selective hydrobora- 90 and 92% isolated yield, respectively; no branched boronates
tion of a-olefin would provide a possibility of converting inter- were detected (entries 1 and 2).15 The reactions of trans olefins,
nal olefin mixtures into linear alkylboronates that are useful such as trans-3-hexene, trans-3-octene, and trans-4-octene, also
synthetic intermediates.12 Such a transformation would be of provided the linear products in high yields (entries 3–5). The
value because internal olefin mixtures are economically more hydroboration of a mixture of cis-2-hexene and trans-2-hexene
attractive and more easily available than the pure terminal afforded 4 cleanly in 82% yield (entry 6). A cis and trans mixture
isomers. Notably, related terminal functionalization of internal of branched olefin, 5,7,7-trimethyl-2-octene, was converted to
olefins via tandem isomerization and hydroformylation has 6 selectively in 84% isolated yield (entry 7). Treatment of
been well established.13
1,4-hexadiene with 2 equiv. of HBPin selectively formed the
We recently reported that a pincer precatalyst (PNN)FeCl2 (3, terminal substituted diboronate 7 in 80% yield (entry 8). More
see Table 3) is highly active and selective for anti-Markovnikov interestingly, acetal functionalities are tolerated as shown by
hydroboration of a-olefins with pinacolborane (HBPin); and the isolation of 8 in 71% yield (entry 9). Lastly, the reaction
with (E)-6-chloro-2-hexene afforded the linear hydroboration
product 9 in 74% isolated yield (entry 10), albeit with a
Table 3 Isomerization–hydroboration of internal olefins catalyzed by 1b/
3/NaBHEt3
relatively high loading of the Fe catalyst (20%) (entry 10).16,17
a
Iridium complexes with new NCP pincer ligands have been
prepared. The least sterically hindered complex 1a is substan-
tially more active than 1b and 1c in dehydrogenation of both
linear and cyclic alkanes. All complexes are highly active for
olefin isomerization. We have developed a tandem system
consisting of the (NCP)Ir catalyst for olefin isomerization and
a (PNN)Fe catalyst for a-olefin hydroboration. This system
provides a novel approach to produce synthetically valuable
alkylboronates regioselectively from internal olefin mixtures,
which would be more accessible than the pure terminal olefins
by means such as alkane dehydrogenation.
Entry Olefin
Time (h) Product
2
1
2
3
4
5
6
cis-3-Hexene
cis-4-Octene
2
We gratefully acknowledge the financial support from the
National Natural Science Foundation of China (No. 21272255,
21121062) and the Science and Technology Commission of
Shanghai Municipality (No. 13QA1404200, 13JC1406900).
trans-3-Hexene
trans-3-Octene
trans-4-Octene
2
0.5
2
Notes and references
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0.5
´
(c) N. Selander and K. J. Szabo, Chem. Rev., 2010, 111, 2048–2076;
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C. Jensen, Elsevier, Amsterdam, 2007; (e) W. Leis, H. A. Mayer and
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4
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NaBHEt3 in 1 mL toluene at 23 1C. Using 2 equiv. of HBPin. With
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b
c
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¨
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