Highly stable and porous cross-linked polymers for efficient photocatalysis

Article abstract of DOI:10.1021/ja109166b

Porous cross-linked polymers (PCPs) with phosphorescent [Ru(bpy) ₃]²⁺ and [Ir(ppy)₂(bpy)]⁺ building blocks were obtained via octacarbonyldicobalt (Co₂(CO) ₈)-catalyzed alkyne trimerization

Full text of DOI:10.1021/ja109166b

COMMUNICATION
pubs.acs.org/JACS
Highly Stable and Porous Cross-Linked Polymers for Efficient
Photocatalysis
Zhigang Xie, Cheng Wang, Kathryn E. deKrafft, and Wenbin Lin*
Department of Chemistry, CB#3290, University of North Carolina, Chapel Hill, North Carolina 27599, United States
S Supporting Information
b
organic precursors and represent a new class of robust, nanopor-
ous materials.5 Like their porous metal-organic framework (MOF)
ABSTRACT: Porous cross-linked polymers (PCPs) with
phosphorescent [Ru(bpy)3]2þ and [Ir(ppy)2(bpy)]þ build-
ing blocks were obtained via octacarbonyldicobalt (Co2-
(CO)8)-catalyzed alkyne trimerization reactions. The resul-
tant Ru- and Ir-PCPs exhibited high porosity with specific
surface areas of 1348 and 1547 m2/g, respectively. They are
thermally stable at up to 350 °C in air and do not dissolve or
decompose in all solvents tested, including concentrated
hydrochloric acid. The photoactive PCPs were shown to be
highly effective, recyclable, and reusable heterogeneous
photocatalysts for aza-Henry reactions, R-arylation of bro-
momalonate, and oxyamination of an aldehyde, with cata-
lytic activities comparable to those of the homogeneous
[Ru(bpy)3]2þ and [Ir(ppy)2(bpy)]þ photocatalysts. This
work highlights the potential of developing photoactive
PCPs as highly stable, molecularly tunable, and recyclable
and reusable heterogeneous photocatalysts for a variety of
important organic transformations.
counterparts,6 PCPs have shown great promise in gas storage and
separation and light harvesting.7 Just like recently developed
catalytic MOFs,8 PCPs can serve as an ideal platform for incorpo-
rating molecular catalytic modules into highly stable, recyclable,
and reusable heterogeneous catalyst systems by taking advantage
of their permanent porosity and the ability to tune their com-
positions and properties at the molecular level.9 PCPs are advan-
tageous over MOFs as photocatalysts since many MOFs are
constructed from metal connecting points with unfilled d orbitals
which can readily quench the photoexcited states of the photo-
catalytic building units.10 In this work, we report successful
incorporation of the Ru and Ir phosphors into robust PCPs by
cobalt carbonyl-mediated trimerization of the end alkyne groups
on the monomers.11 The resulting PCPs are highly active in
catalyzing visible light driven aza-Henry reactions between nitro-
methane or nitroethane and tertiary aromatic amines, R-arylation of
bromomalonate via intermolecular C-H functionalization, and
oxyamination of an aldehyde with 2,2,6,6-tetramethylpiperidinyl-
1-oxy (TEMPO). The PCP photocatalysts can be readily re-
cycled and reused for these reactions without a decrease in
catalytic efficiency.
Phosphorescent Ir monomer [(ppy)2Ir(debpy)]Cl (debpy
=5,50-diethynyl-2,20-bipyridine) and Ru monomer [(bpy)2Ru-
(debpy)]Cl2 were synthesized by allowing debpy to react with
[Ir(ppy)2Cl2]2 and Ru(bpy)2Cl2 at 90 °C overnight, respective-
ly. Copolymerization of the monomer [(ppy)2Ir(debpy)]Cl or
[(bpy)2Ru(debpy)]Cl2 with tetra(4-ethynylphenyl)methane was
achieved through Co2(CO)8-mediated trimerization of the end
alkyne groups of the monomers in dioxane or dichloroethane at
115 °C for 10 min (Scheme 1). The resulting brown solids were
stirred in concentrated hydrochloric acid at room temperature
for 2 h to remove all the Co species and then washed with various
solvents to afford Ir-PCP and Ru-PCP in 97% yields. The Ir- and
Ru-PCPs were characterized by thermogravimetric analysis (TGA),
inductively coupled plasma mass spectrometry (ICP-MS), infra-
red spectroscopy (IR), nitrogen adsorption, scanning electron
microscopy (SEM), transmission electron microscopy (TEM),
and powder X-ray diffraction (PXRD).
rganic transformations driven by visible light are gaining
O
increasing interest from synthetic chemists because of their
generally mild conditions for substrate activation, the ability to
afford desired products without generating unwanted byproducts,
and the potential to mediate thermodynamically uphill reactions by
harvesting energy from the sunlight in these reactions.1 Photocata-
lysts are often required in visible light driven organic reactions since
the majority of organic substrates in these reactions do not readily
absorb photons in the visible region. [Ru(bpy)3]2þ (bpy = 2,20-
bipyridine) is undoubtedly one of the most extensively studied pho-
tocatalysts since the pioneering work of Meyer and Whitten in the
1970s.2 Taking advantage of the long lifetime of the 3MLCT state
of this Ru-based phosphor and its propensity to undergo redox
quenching, a variety of new photocatalytic organic reactions, ranging
from [2þ2] cycloaddition to radical chemistry, have been devel-
oped recently.3 Stephenson et al. and MacMillan et al. also reported
the use of an Ir complex, Ir(ppy)2(dtbbpy)PF6 (ppy = 2-phenyl-
pyridine; dtbbpy =4,40-di-tert-butyl-2,20-bipyridine), as a photo-
redox catalyst in tin-free dehalogenation, aza-Henry reactions,
and R-trifluoromethylation of aldehydes.4 Like other precious
metal catalyzed reactions, it is highly desirable to develop recyclable
and reusable heterogeneous photocatalytic systems based on the Ir
and Ru phosphors. The ability to recover and reuse such hetero-
geneous photocatalysts can not only eliminate the contamination of
organic products by trace amounts of heavy metals but also reduce
processing and waste disposal costs in large-scale reactions.
Porous cross-linked polymers (PCPs), including crystalline
covalent-organic frameworks, can be built from well-designed
Morphology of the PCPs was examined by SEM and TEM
(Figure 1a,b). Particles on the order of several micrometers in
size displayed rather rough surfaces and appear to be aggregates
of much smaller particles with dimensions of around 10 nm. The
Received: October 12, 2010
Published: January 28, 2011
r
2011 American Chemical Society
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|

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Scheme 1. Synthesis of Ir-PCP and Ru-PCP
PXRD patterns indicated that the Ir-PCP and Ru-PCP are amor-
phous in nature (Figure S1, Supporting Information (SI)).
The Ir-PCP and Ru-PCP are insoluble in water, concentrated
hydrochloric acid and all of the organic solvents tested. The PCPs
are stable in air up to 350 °C, as revealed by TGA (Figure 1c).
The Ir and Ru catalyst loadings for Ir-PCP and Ru-PCP were
determined by ICP-MS to be 4.5 and 2.2 wt %, respectively. The
absence of the carbon-hydrogen stretching peak of the CtC—H
group around 3300 cm-1 in the IR spectra of the Ir-PCP and
Ru-PCP suggested that most of the alkyne groups in the starting
materials have been consumed to form benzene rings in the PCPs,
consistent with a high degree of polymerization (Figure 1d).
Porosity of the PCPs was revealed by nitrogen sorption mea-
surements at 77 K. The PCPs exhibit large reversible adsorptions
at low pressure (P/P0 < 0.1) and hysteresis loops at higher pre-
ssure, suggesting the coexistence of micro- and mesopores in the
framework (Figure 1e).12,13 The BET surface areas were deter-
mined to be 1547 and 1348 m2/g for Ir-PCP and Ru-PCP in the
P/P0 = 0.03-0.2 range, respectively. Both of the PCPs have wide
pore size distributions, with the pore widths centering around
7.5, 11.0, 13.5, and 16.5 Å, as calculated by the nonlocal density
functional theory method (NLDFT) (Figure S2, SI).
Figure 1. Characterization of Ir-PCP and Ru-PCP: (a) SEM and (b)
TEM images of Ir-PCP. The scale bars represent 200 nm. (c) TGA, (d)
FT-IR, (e) nitrogen adsorption isotherms at 77 K, and (f) uncorrected
emission spectra. The Ir-PCP emission spectrum (red line) was taken
when excited at 380 nm. The broad emission around 458.5 nm comes
from fluorescence of the framework. The Ru-PCP emission spectrum
(blue line) was taken when excited at 450 nm. The broad emission around
522.5 nm comes from fluorescence of the framework.
to a Stern-Volmer constant of 23 M-1, which is comparable to
that of the monomeric [(bpy)2Ru(debpy)]Cl2 complex (20
3
M-1). This result indicates effective quenching of the MLCT
phosphorescence of the Ru-PCP by the amine. In the case of Ir-
PCP, the quenching effect was also observed but could not be
quantified spectroscopically owing to the interference of the
framework fluorescence (Figure 1f). The quenching result sug-
gested that good photocatalytic performance of the PCPs should
be expected.
With the Ir/Ru-based phosphors covalently integrated into
the skeletons, we hypothesized that the synthesized PCPs can act
as solid-state photosensitizers by utilizing the 3MLCT states of
the Ir/Ru complexes. Phosphorescence measurements were per-
formed on a stirred suspension of the PCPs in CH3NO2, showing
peaks originating from 3MLCT f GS transitions around 550 nm
for the Ir-PCP and 602 nm for the Ru-PCP (Figure 1f). We
intended to explore the utility of the Ir-PCP and Ru-PCP in
catalyzing visible light driven organic reactions such as the aza-
Henry reaction between nitromethane or nitroethane and aro-
matic tertiary amines. Before the catalysis test of the materials, a
preliminary quenching experiment was carried out to confirm
that the 3MLCT state of the phosphors in the PCPs can be reduc-
tively quenched by the amine substrate, which is the very first
step of the photocatalytic cycle of homogeneous aza-Henry
Catalytic activities of the Ir-PCP and Ru-PCP toward photo-
catalytic aza-Henry reaction were evaluated with tetrahydroiso-
quinoline (1a) as the amine substrate and CH3NO2 as solvent.
The reaction was carried out in the presence of air with a common
fluorescent lamp (26 W) as the light source. The reaction was
stopped after 8 h by filtering off the PCP catalysts. Conversions of
1
the reactions were determined by integrating the peaks of H
NMR spectra of the crude reaction mixtures (Supporting In-
formation). As shown in Table 1 (entry 1), both the Ir- and Ru-
PCPs are highly effective photocatalysts for the aza-Henry reaction
between 1a and nitromethane, with >90% conversions. These
conversions are comparable to those of the homogeneous coun-
terparts. We have also explored the scope of substrates for this
photocatalytic reaction (Table 1). The Ir- and Ru-PCPs catalyzed
aza-Henry reactions between nitromethane and a variety oftertiary
aromatic amines with different functional groups (Table 1, entries
1-4). The PCPs also catalyzed aza-Henry reactions between
nitroethane and tertiary aromatic amines (Table 1, entries 5-8).
reactions as proposed by Stephenson and co-workers.4b
A
Stern-Volmer plot for the Ru-PCP was obtained by plotting the
intensity ratio I0/I at 602 nm vs the amine concentration, leading
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COMMUNICATION
Table 1. Photocatalytic Aza-Henry Reactionsa
Scheme 2. Ru-PCP-Catalyzed R-Arylation of Bromomalo-
nate and Oxyamination of 3-Phenylpropanala
conv (%) with different catalystsb
entry amine substrate product Ir-M
Ir-PCP
Ru-M
Ru-PCP
a The Ru-PCP catalyst was readily recovered and reused for these
reactions.
1c
2c
3c
4c
5d
6d
7d
8d
1a
2a
3a
4a
1a
2a
3a
4a
1b
2b
3b
4b
5b
6b
7b
8b
99
99
81
79
94
73
86
66
94
97
91
98
94
75
97
86
85
84
90
65
81
57
62
76
90
87
recycling four times (Table S2, SI). Furthermore, UV-vis analysis
of the supernatant showed no sign of Ir/Ru complex leaching into
the solution. This is further supported by the absence of Ir or Ru in the
supernatants of the PCP-catalyzed reaction mixtures by ICP-MS.
We have also demonstrated the applicability of the PCP
photocatalysts in other visible light driven reactions. As shown
in Scheme 2, Ru-PCP efficiently catalyzed the R-arylation of
bromomalonate via direct intermolecular C-H functionalization3e
as well as oxyamination of 3-phenylpropanal,14 with isolated pro-
duct yields comparable to those of the homogeneous control
catalyst. We also showed that the Ru-PCP catalyst could be
readily recovered and reused for all of these reactions with no or
little drop in the product isolated yields (Scheme 2). These
results illustrate the generality of the phosphorescent PCP mat-
erials in catalyzing photodriven organic transformations.
In conclusion, Ir and Ru phosphors have been successfully
integrated into porous cross-linked polymers via cobalt-catalyzed
alkyne trimerization. The resultant framework materials are stable in
various solvents, including concentrated hydrochloric acid, and
are thermally and oxidatively stable in air up to 350 °C. These
phosphorescent Ir- and Ru-based PCPs were shown to be highly
active, recyclable, and reusable heterogeneous photocatalysts in
aza-Henry reactions of a variety of amine substrates. This work
highlights the potential of using PCPs as a stable and molecularly
tunable platform for developing highly efficient heterogeneous
photocatalysts for a number of important organic transformations.
>99
85
84
78
95
91
a All the reactions were run at room temperature for 8 h with 1 mol %
Ir-based catalyst or 0.2 mol % Ru-based catalyst. b Conversions were
determined by integrating the 1H NMR spectra. Ir-M and Ru-M are Ir
monomer [(ppy)2Ir(debpy)]Cl and Ru monomer [(bpy)2Ru(debpy)]
Cl2, respectively. c With nitromethane as solvent. d With nitroethane
as solvent.
Interestingly, most of the PCP-catalyzed aza-Henry reactions
have higher conversions than those catalyzed by their homoge-
neous counterparts.
A number of control experiments were carried out to demon-
strate the heterogeneous and photocatalytic nature of the reac-
tions. The reaction of 1a in the dark yielded negligible amounts
of aza-Henry products (<5%, Table S1, SI), demonstrating the
necessity of light in this reaction. On the other hand, the back-
ground reaction in the absence of the catalysts but in the presence
of light showed only around 10% conversions after 8 h (Table
S1), verifying that the PCPs played a catalytic role in the reac-
tions. These observations are consistent with those for the homo-
geneous system reported by Stephenson and co-workers.4b In
addition, a crossover experiment was carried out to prove the
heterogeneity of the PCP catalyst. Substrate 1a was used in the Ir-
PCP-catalyzed aza-Henry reaction, and 94% conversion was
achieved after 8 h. The PCP catalyst was then removed by filtering
through Celite, and another substrate 3a was added to the
supernatant solution. After the solution was stirred under light
for 8 h, only 12% conversion was observed for the second
substrate (3a). This low conversion, comparable to that of the
background reaction with visible light but without a photocata-
lyst, proved that the supernatant of the Ir-PCP reaction mixture is
inactive in photocatalysis. This result supports the heteroge-
neous nature of the PCP photocatalysts.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures and
b
characterization data. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
wlin@unc.edu
’ ACKNOWLEDGMENT
We have examined recyclability and reusability of the PCP
catalysts. Ir-PCP and Ru-PCP were readily recovered from the
reaction mixtures via simple filtration. The recovered catalyst show-
ed no deterioration of conversion for aza-Henry reactions after
We thank DOE (DE-SC0001011) and NSF for financial support.
Z.X .and K.E.d. were supported by the DOE grant and C.W. was
supported by the NSF grants.
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