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1H NMR (300 MHz, DMSO d6) d 10.02 (d, J = 9.0 Hz, 1H), 9.42 (d,
J = 5.1 Hz, 1H), 8.24 (d, J = 8.4 Hz, 1H), 7.78 (dd, J = 8.7, 5.1 Hz, 1H),
6.98 (d, J = 8.4 Hz, 1H), 3.65 (s, 6H). 13C NMR (75 MHz, MeOD-d4) d
151.9, 145.0, 142.3, 141.7, 130.6, 130.3, 128.7, 122.4, 121.1, 46.5.
195Pt NMR (64 MHz, MeOD-d4) d ꢁ2768.9 (s).
2.2. Synthesis of heterogeneous catalyst MSN-AP-Pt1, MSU-2-AP-Pt1
and SBA-15-AP-Pt1
Scheme 1. Synthesis of complex Pt1.
First, a buffer of NaCl (2.92 g, 50 mmol) and morpholinoethane
sulfonic acid monohydrate (MES, 2.13 g, 10 mmol) in 100 mL of
Milli-Q water was prepared. Then, 40 mg of Pt1 was dissolved in
10 mL of DMSO and subsequently added to the buffer. Next, N-(3
ꢁ2768 ppm which is in agreement with chemical shifts reported
for similar platinum(II) complexes [6,9].
In order to explore the catalytic performance of the catalyst Pt1
in a heterogeneous system, MSN, MSU-2 and SBA-15 materials
have been chosen as supports for the immobilization of the
platinum complex [7]. These three silica-based mesoporous mate-
rials present the same chemical composition but different particle
size, pore size and morphology, and different arrangements of the
porous structure. Whereas MSN are silica spheres with hexagonal
arrangement of the pores, SBA-15 has hexagonal arrangement of
the pores too, but a bigger pore size than that of MSN. In addition,
the SBA-15 particle shape is different to that of MSN as SBA-15
consists of elongated spheres of up to 700 nm and MSN in spheres
of ca. 80 nm diameter. Moreover, the silica material MSU-2, with
spherical morphology and wormhole-like arrangement of the
pores, was also selected as support for this catalytic study. The
structural differences of the different silica-based supports will
allow us to study the influence of the textural and morphological
properties of the silica materials into the catalytic performance
of the photocatalyst. The syntheses of the silica-based materials
were carried out following reported procedures [7] and were char-
acterized by IR, X-ray powder diffraction (XRD), solid physisorp-
tion (BET), SEM and TEM, obtaining data which are in agreement
with those previously described in the literature for MSN, SBA-15
or MSU-2, respectively (see S.I. for all the data). Subsequently,
the mesoporous materials were functionalized with (3-
aminopropyl)triethoxysilane (APTS) in order to incorporate amino-
propyl (AP) groups, which will serve as linkers for the subsequent
anchoring of the complex Pt1 to the materials. The functionaliza-
tion with APTS was successfully achieved for MSN, SBA-15 and
MSU-2 by heating APTS and the corresponding material in toluene
at high temperature (Scheme 2) [8]. The incorporation of the AP
group to the materials was confirmed by IR analysis, as the charac-
teristic stretching vibration band around 3000 cmꢁ1 attributed to
the presence of N-H and C-H was observed in the IR spectra of each
functionalized material (see S.I.).
-dimethylaminopropyl)-N0-ethylcarbodiimide
hydrochloride
(EDC, 40 mg, 0.26 mmol), N-hydroxysuccinimide (NHS, 60 mg,
0.52 mmol), 2-mercaptoethanol (0.17 mL, 2.42 mmol) and the cor-
responding aminopropyl-functionalized material (400 mg) were
added. The reaction mixture was stirred for 2 h at rt and then
hydroxylamine hydrochloride (83 mg, 2.52 mmol) was added. A
yellow solid was formed which was isolated by centrifugation
and washed with Milli-Q water (3 ꢀ 30 mL) and diethyl ether
(3 ꢀ 30 mL).
2.3. Photocatalytic debromination of organic bromides
An oven-dried 10 mL glass vial was loaded with the correspond-
ing heterogeneous Pt complex (1 mol% of Pt). Then, 1 mL of abso-
lute ethanol, N,N-diisopropylethylamine (0.3 mmol) and the
corresponding organic bromide 1 (0.1 mmol) were added to the
vial. The reaction mixture was degassed by three cycles of
freeze-pump-thaw. Next, the reaction was stirred under irradiation
using a custom-made ‘‘light box” (see S.I.) at rt. After 24 h, the reac-
tion mixture was filtered over CeliteÒ and the crude was purified
by flash chromatography to afford the corresponding debromi-
nated product 2.
2.4. Photocatalytic a-alkylation of aldehydes
To an oven-dried 10 mL glass vial were added MSN-AP-Pt1
(1 mol% of Pt), diethyl 2-bromomalonate 1a (0.1 mmol), the corre-
sponding aldehyde 3 (0.2 mmol) and MacMillan’s imidazolidinone
catalyst (20 mol%). Then, 200 lL of anhydrous DMF and 2,6-
lutidine (0.2 mmol) were added to the vial. The vial was sealed
with a PTFE/rubber septum and the reaction mixture was degassed
by three cycles of freeze-pump-thaw. Afterwards, the reaction
mixture was stirred and irradiated using blue LED irradiation
(see S.I. for more details) at rt. The reaction progress was moni-
tored by 1H NMR.
In the last synthetic step, the covalent anchoring of the photo-
catalyst Pt1 was carried out by using a well-established coupling
methodology [12] between the amino group of the AP linker of
each silica-based material and the carboxylic acid of the Pt1 cata-
lyst (Scheme 2).
The characterization of these new synthesized materials with
the platinum complex Pt1 was conducted using different tech-
niques: IR analysis, X-ray fluorescence (XRF), BET, XRD, SEM and
TEM (see S.I.). The IR spectra of the three Pt-functionalized materi-
als showed the specific bands of the mesoporous framework
together with the corresponding bands of the aminopropyl linker.
In addition, the appearance of a new characteristic band corre-
sponding to an amide C@O stretching vibration at ꢂ1650 cmꢁ1 cor-
roborates the tethering of the Pt(II) complex onto the material.
Moreover, the IR band corresponding to the unmodified carboxylic
acid was not detected, indicating that all the Pt catalyst is bound to
the silica-based material. The BET isotherms of the three Pt-
functionalized materials exhibited the typical type IV isotherm
with hysteresis loops of mesoporous solids (Fig. 2). Nevertheless,
the covalent anchoring of the catalyst led to lower surface areas,
3. Results and discussion
3.1. Synthesis and characterization of the heterogeneous catalyst
The platinum photocatalyst Pt1 was prepared according to a
procedure previously reported by us [6,9], by the complexation
of the 8-hydroxyquinoline derivative ligand (L1) [10] and the plat-
inum precursor [PtCl2(dmso)2] [11] using NaOH as base in a mix-
ture of methanol/acetone at room temperature. Following these
conditions, the sodium carboxylate complex Pt1 was isolated in
78% yield (Scheme 1). The 1H and 13C NMR chemical shifts of the
complex Pt1 showed a downfield shift of most of the 1H or 13C sig-
nals compared to those of the free L1, which confirmed the com-
plexation of the ligand L1 to the metal. In addition, complex Pt1
was also characterized by 195Pt NMR, observing a signal at ca.