Hydrogenolysis of carbon-carbon σ-bonds using water catalysed by semi-rigid diiridium(iii) porphyrins

Article abstract of DOI:10.1039/c8nj05664a

Semi-rigid diiridium(iii) porphyrin alkyls with m-xylyl and p-xylyl diether linkers were synthesized. They were found to be catalysts for the carbon-carbon σ-bond hydrogenolysis of [2.2]paracyclophane under neutral conditions using water as the hydrogen s

Full text of DOI:10.1039/c8nj05664a

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DOI: 10.1039/C8NJ05664A
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Hydrogenolysis of carbon-carbon σ-bond using water catalysed by
semi-rigid diiridium(III) porphyrins
Ching Wai Fonga, Ho Yin Yua, Claude P. Grosb and Kin Shing Chan*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Semi-rigid diiridium(III) porphyrins alkyls with m-xylyl and p-xylyl state with two molecules of rhodium porphyrin and one
diether linkers were synthesized. They were found to be catalysts molecule of PCP.7 The iridium porphyrin is also assumed to
for
the
carbon-carbon
σ-bond
hydrogenolysis
of behave likewise. A covalent linked dimetalloporphyrin would
[2.2]paracyclophane in neutral conditions using water as the be expected to promote catalytic efficiency through reduction
hydrogen source. The ether linkage in semi-rigid diiridium(III) of entropic penalty.
porphyrins are unstable and undergoes clevage during the
reaction.
Furthermore, iridium porphyrins have been shown to be
better catalysts in the hydrogenolysis (Chart 1, Scheme 1) with
higher yield of hydrogenation product
6
and little
Bimetallic complexes have been studied extensively in various
chemical reactions for their synergistic effect in chemical
transformation.1-3 Bis-rhodium complexes have shown to have
large rate enhancement in the hydroalkoxylation of alkynes
over the mono-rhodium complexes in equivalent catalytic
amount of rhodium.4 The restriction in conformational
freedom using a linker is the key of the synergistic rate
enhancement.
dehydrogenation product 7 formed. The superior performance
of iridium porphyrins is attributed to the efficient bimetallic
reductive elimination of the iridium benzyl and iridium hydride
intermediates to give 6.
H
H
H
H
[M] (10 mol%)
benzene
200°C
+ 1/2 "O2"
+
+ H2O
N2
dark
5
6
7
Wayland has reported the rate enhancement in the
hydrogen-hydrogen bond activation and carbon-hydrogen
bond activation of methyl-hydrogen and benzyl-hydrogen
bonds using covalently linked dirhodium porphyrin,5 when
compared with their monomeric analogues.6
Entry #
Metalloporphyrin catalysta
RhIII(ttp)Me (1)
Time (h)
54
50
53
41
6b (%)
78
68
94
98
7b (%)
<20
<20
0
17
28
39
49
RhIII(ttp)iPr (2)
IrIII(ttp)Me (3)
IrIII(ttp)iPr (4)
0
N
Rh
N
N
Rh
N
N
Ir
N
Ir
iPr
Me
N
iPr
Me
N
a ttp = tetra-p-tolylporphyrinato dianion b NMR yield
N
N
N
N
N
N
N
N
Scheme 1 Catalyzed carbon-carbon σ-bond hydrogenolysis of PCP with H2O with
metalloporphyrin catalysts with various ligand effect for rhodium and iridium
Rh(ttp)iPr
2
Ir(ttp)iPr
4
Rh(ttp)Me
1
Ir(ttp)Me
3
We therefore embarked to synthesize covalently linked
diiridium porphyrin complexes and examine the catalytic
efficiently in the hydrogenolysis reaction.
Chart
1
Structures of mono-rhodium and mono-iridium porphyrins 1-4,
Rh(ttp)Me, Rh(ttp)iPr, Ir(ttp)Me and Ir(ttp)iPr
We have reported the rhodium7-8 and iridium porphyrin9
catalyzed carbon-carbon σ-bond hydrogenolysis of [2,2]-
paracyclophane (PCP) with water. (Chart 1, Scheme 1). A key
step in the catalytic cycle involves a termolecular transition
Diporphyrin
Porphyrin
[MII]2
[MII]
[MII]
[MIII
]
[MIII
]
[MIII
]
[MIII
]
2[MII]
[MII]
[MII]
H2O
-[MIII]-OH
-"H2"
H2O
a. Department of Chemistry, The Chinese University of Hong Kong, Shatin, New
Territories, Hong Kong SAR, The People’s Republic of China. E-mail:
ksc@cuhk.edu.hk
-"H2"
[M] = Rh(ttp), Rh(tmp),
Ir(ttp)
[M] = Rh(ttp), Rh(tmp),
Ir(ttp)
-[MIII]-OH
[MIII]-H
= m-xylyl or p-xylyl
diether linker
H-[MIII
]
[MIII]-H
b. ICMUB (UMR CNRS 6302), Université de Bourgogne Franche-Comté, 21 000 Dijon,
France.
† Electronic Supplementary Information (ESI) available: Experimental section and
NMR spectra. See DOI: 10.1039/x0xx00000x
Scheme 2 Partial catalytic cycle of the carbon-carbon σ-bond activation of PCP
with group 9 metalloporphyrin and proposed group 9 dimetalloporphyrin
MeI
NaBH4
THF/aq. NaOH
70°C, 2 h, N2
IrIII2(dpmx)(ttp)Me2
11
73%
Br
Br
[Ir(COD)Cl]2
,'-dibromo-m-xylene
O
IrIII2(dpmx)(ttp)(CO)2Cl2
H4(dpmx)(ttp)
This journal is © The Royal Society of Chemistry 20xx
New J. Chem., 2018, 00, 1-3 | 1
p-xylene
reflux
2 d
K2CO3 (20 equiv)
H
10
63%
9
58%
DMF
r.t
1 d
iPrI
HO
IrIII2(dpmx)(ttp)iPr2
NaBH4
12
62%
+
THF/aq. NaOH
70°C, 2 h, N2
O
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H2(ttp-OH)
H
3
CH3CH2COOH
Scheme 3 Synthesis route of diiridium porphyrins, Ir2(dpmx)(ttp)R2 and Ir2(dppx)(ttp)R2 (R = Me, iPr)
8

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Different classes of linkers for diporphyrins according to Ir2(dppx)(ttp)iPr2 16 (56% yield) were prepared by reductive
their rigidity have been reported. Examples of rigid linkers alkylation ligand replacement of the diiridium porphyrin
include anthracene,10,11 biphenylene10,12 and enediyne13 carbonyl chlorides 10 and 14 with methyl iodide (10 to 11 and
linkers. Examples of semi-rigid linkers incorporate 14 to 15) or isopropyl iodide (10 to 12 and 14 to 16),
calix[4]arene,14 dibenzofuran,15 glycoluril,16,17 xanthene,18 respectively, with sodium borohydride as the reductant.
diphenylether19 and BODIPY20 linkers. Examples of flexible
To investigate the catalytic efficiency of catalytic
linkers comprise of ethane21-23 and alkyl diether24 linkers. Here, hydrogenolysis of carbon-carbon σ-bond with water, the newly
semi-rigid m-xylyl24 and p-xylyl diether linkers are selected, as synthesized diiridium(III) porphyrin alkyl precatalysts were
the rigidity of the benzene ring provides some degree of pre- examined under previously reported conditions (Table 1).9
organization of the catalyst-substrate complex transition state,
while the ether linkage provides some degree of freedom to catalysts for the hydrogenolysis of PCP with water. The
adapt its cavity size according to the substrate size. catalysis of PCP 5 with 11 (Table 1, entry 3) resulted in 27% of
The diiridium(III) porphyrin complexes were found to be
Herein, we report the synthesis of diiridium porphyrin hydrogenation product 6 in 40 h, while the catalysis with 12
alkyls featuring m-xylyl and p-xylyl diether linkers, (Table 1, entry 4) required the same reaction time but gave a
Ir2(dpmx)(ttp)R2 and Ir2(dppx)(ttp)R2 (Chart 2), (dpmx
=
higher yield of 6 of 40% yield. The catalysis of PCP with 15
diporphyrinato m-xylyl diether, dppx = diporphyrinato p-xylyl (Table 1, entry 5) yielded 59% of 6 in 40.5 h, while the catalysis
diether and ttp = tetra-p-tolylporphyrinato dianion) and the with 16 (Table 1; entry 6) required the shortest reaction time
results of catalytic hydrogenolysis of PCP using these of 31.5 h and gave the highest yield of 6 of 66%.
diiridium(III) porphyrin alkyls.
Table 1 Catalytic hydrogenolysis of PCP with various iridium precatalyst
H
H
H
H
[Ir] (10 mol%)
+ 1/2 "O2"
+
+ H2O
C6D6
200°C
(100 equiv.)
dark
sealed tube
7a
6
5
Hydrogenation
Product
Dehydrogenation
Product
Entry
Metalloporphyrin
Precatalystb
Ir(ttp)Me (3)
Ir(ttp)iPr (4)
Ir2(dpmx)(ttp)Me2 (11)
Ir2(dpmx)(ttp)iPr2 (12)
Ir2(dppx)(ttp)Me2 (15)
Ir2(dppx)(ttp)iPr2 (16)
mol
(%)
10
10
5
5
5
5
Time
(h)
53
41
40
40
40.5
31.5
5c
(%)
0
0
0
0
9
6
6c
(%)
94
98
27
40
59
66
#
19
29
3
4
5
Chart 2 Structure of diiridium porphyrins, Ir2(dpmx)(ttp)L2 and Ir2(dppx)(ttp)L2
The synthesis of semi-rigid diiridium(III) porphyrins follows
the route of dirhodium porphyrin reported by Wayland24 with
modification of substitution positions of the linker. (Scheme 3)
The starting material A3B-type porphyrin H2(ttp-OH) 8 was
prepared via the mixed aldehyde condensation approach of 4-
hydroxybenzaldehyde, 4-tolylaldehyde and pyrrole in 5% yield.
The free base diporphyrins H4(dpmx)(ttp) 9 and H4(dppx)(ttp)
13 were prepared by nucleophilic substitution of linker α,α’-
dibromo-m-xylene and α,α’-dibromo-p-xylene with 8 in 55%
and 63% yields, respectively, with improved yields at room
temperature. Adapting from the procedure of the preparation
of the mono-iridium porphyrin analogue,25 the diiridium
porphyrin carbonyl chlorides Ir2(dpmx)(ttp)(CO)Cl 10 and
Ir2(dppx)(ttp)(CO)Cl 14 were prepared by metalation of the
free base diporphyrins 9 and 13 with [Ir(COD)Cl]2 in 63% and
65% yields, respectively. Similar to the preparation of mono-
iridium porphyrin alkyl analogues,26 the diiridium porphyrin
alkyl Ir2(dpmx)(ttp)Me2 11 (73% yield), Ir2(dpmx)(ttp)iPr2 12
6
a < 3% NMR yield for all reaction b dpmx = diporphyrinato m-xylyl diether, dppx =
diporphyrinato p-xylyl diether and ttp = tetra-p-tolylporphyrinato dianion NMR
yield
c
Diiridium(III) complexes with isopropyl ligand 4, 12 and 16
(Table 1, entries 2, 4 and 6) reacted with a shorter reaction
time and gave higher yields of the hydrogenation product 6
than the methyl analogue (Table 1; entries 1, 3 and 5). This can
be accounted by the formation of more active iridium(III)
porphyrin hydride via an additional faster β-hydrogen
elimination with isopropyl ligand instead of slower hydrolysis
with methyl ligand as reported earlier in the mononuclear
iridium porphyrin complexes.9
Diiridium(III) complexes (15 and 16) with the p-xylyl diether
linker (Table 1, entries 5 and 6) produced a higher yield of the
hydrogenation product 6 when compared with complexes (11
and 12) with the m-xylyl diether linker (Table 1, entries 3 and
(62% yield), Ir2(dppx)(ttp)Me2 15
(50% yield) and
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4). The larger cavity size from the p-xylyl diether linker likely without PCP (Scheme 4). The reaction was monitored with H
conforms better to the PCP substrate in the transition state.
View Article Online
NMR spectroscopy. The tether xylyl proDtoOnI:s10a.1t0δ39=/C85N.0J205p66p4mA
When the Ir2(dppx)(ttp)iPr2 precatalyst 16 was used, the decreased in signal intensity with time. The iridium hydride
reaction time was reduced from 41 h (Table 1, entry 2) to 31.5 signal at δ = -57.26 ppm appeared quickly after 30 min and
h (Table 1, entry 6), albeit only yielded 66% of 6. The low reached the maximum intensity at 16 h before further
material balance is accounted by the formation of unknown reduction in signal intensity. Correspondingly, a new iridium
insoluble precipitates in the course of the reaction. hydride signal appeared at δ = -57.30 ppm in 6 h and increased
Precipitates were also observed in all reactions with further in intensity. Furthermore,
diiridium(III) precatalysts. aldehyde signal at δ = 9.71 ppm appeared, supporting the
Though the reaction with p-xylyl diether linker catalyst structural assignment of 20 in Scheme 4.
a new characteristic
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yielded a faster reaction, the very facile degradation of catalyst
Further MALDI/TOF-MS analysis of the reaction mixture
led to the incomplete consumption of starting material PCP 5 revealed that the same two species with m/z of 863, 981 were
and diminished the yield of the product 6.
The results in Table reveal that the semi-rigid
observed, supporting the fragmentation of the ether linkage.
1
4-tol
diiridium(III) complexes are active catalysts of PCP
hydrogenolysis, however no rate enhancement was observed.
The reason behind the lack of synergy in semi-rigid
diiridium(III) complexes is likely due to degradation of
diiridium(III) porphyrins leading to the incomplete
consumption of PCP (Table 1, entries 5 and 6).
H
N
N
O
iPr
N
O
4-tol
Ir
O
O
4-tol
4-tol
N
4-tol
N
Ir
H2O
N
Ir
H
N
N
4-tol
4-tol
N
+
N
C6D6
200°C
dark
H
N
N
4-tol
4-tol
4-tol
N i
Pr
N
HO
Ir
19
N
20
4-tol
To gain more insight in the catalysis, the progress of
N
12
1
reaction was monitored by H NMR spectroscopy. Throughout
4-tol
Tentatively assigned products
the reaction, the amount of starting material 5 decreased over
time while the hydrogenation product 6 increased over time.
The amount of dehydrogenation product 7 is less than 3% and
remains constant throughout the reaction.
Scheme 4 The ether linkage cleavage of diiridium porphyrins
4-tol
In the catalysis with semi-rigid diiridium(III) complexes 11,
12, 15 and 16 as catalysts (Table 1, entries 3-6), two iridium
hydride signals were observed at δ = -57.3 ppm respectively at
the end of the reactions and are similar to the previously
reported Ir(ttp)H signal at δ = -57.4 ppm. This is distinctly
different from the identical chemical shift of rhodium hydride
in the reported semi-rigid dirhodium porphyrin hydrides.5 We
suspected that these two differentiated hydride signals are
due to the ether cleaved mono-iridium(III) porphyrin hydrides
during the catalysis.
N
R
= [Ir] (R = Me, iPr or H)
Ir
N
N
N
4-tol
4-tol
HO
[Ir]
HO
[Ir]
O
[Ir]
19
H2O
19
Hydrolysis
- H2
+
O
[Ir]
+
OH
Oxidation
O
H
O
[Ir]
O
[Ir]
12
21
20
To validate our proposal, the reaction mixture was
subjected to MALDI/TOF-MS. Two new species with the m/z of
863 and 981 were observed, which are consistent with the
ether linkage cleavage products with following structures
(Chart 3).
Scheme 5 Proposed decomposition mechanism of diiridium porphyrins
Scheme
5 depicts the proposed mechanism of the
hydrolytic ether cleavage of iridium porphyrin 12. Nucleophilic
attack of the benzylic moiety yields an iridium porphyrin
benzylic alcohol 21 and another iridium porphyrin phenol 19.
21 then undergoes dehydrogenation to give aldehyde 20. This
type of dehydrogenation has been shown to be catalysed by
iridium complexes27 and rhodium porphyrin complexes,28 thus
iridium porphyrin complexes are also assumed to behave in a
similar fashion.
In conclusion, the semi-rigid diiridium(III) porphyrins with
different conformations and axial ligands were synthesised.
They were found to be catalysts for the carbon-carbon σ-bond
hydrogenolysis of [2.2]paracyclophane in neutral conditions
with water as the hydrogen source. The diiridium porphyrins
with p-xylyl diether linker are more efficient catalysts than
those with m-xylyl diether linker. The semi-rigid diiridium(III)
porphyrins undergo degradation in catalysis due to the ether
H
OH
O
O
N
N
N
Ir
N
N
Ir
N
N
N
17
Exact Mass: 863.2356
18
Exact Mass: 981.2775
Chart 3 Assigned products of ether linkage cleavage of diiridium porphyrins
An independent experiment was conducted to verify the
hydrolytic decomposition of IrIII2(dpmx)(ttp)iPr2 at 200°C
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23 S. K. Ghosh, R. Patra and S. P. Rath, Inorg. CheVmiew., A2rt0ic1le0O,n4lin9e,
linkage cleavage via hydrolysis. Construction of semi-rigid
linker porphyrins stable towards hydrolysis is underway.
3449-3460.
DOI: 10.1039/C8NJ05664A
24 X. X. Zhang, and B. B. Wayland, Inorg. Chem., 2000, 39, 5318-
5325.
25 K. S. Chan, X. M. Chen and T. C. Mak, Polyhedron, 1992, 11,
2703-2706.
26 H. Ogoshi, J. Setsune, T. Omura and Z. Yoshida, J. Am. Chem.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
The authors thank the financial support from the General
Research Fund of Hong Kong (No. 14301717) and the
PROCORE France/Hong Kong Joint Research Scheme 2016/17
(F-CUHK 404/16).
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