TiO2Nanoparticles Functionalized with Non-innocent Ligands Allow Oxidative Photocyanation of Amines with Visible/Near-Infrared Photons

Article abstract of DOI:10.1021/jacs.8b07539

Photosynthesis is an efficient mechanism for converting solar light energy into chemical energy. We report on a strategy for the aerobic photocyanation of tertiary amines with visible and near-infrared (NIR) light. Panchromatic sensitization was achieved

Full text of DOI:10.1021/jacs.8b07539

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Article
TiO2 Nanoparticles Functionalized with Non-Innocent Ligands Allow
Oxidative Photocyanation of Amines with Visible/Near-Infrared Photons
Alexander M. Nauth, Eugen Schechtel, Rene Dören, Wolfgang Tremel, and Till Opatz
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b07539 • Publication Date (Web): 28 Sep 2018
Downloaded from http://pubs.acs.org on September 28, 2018
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TiO2 Nanoparticles Functionalized with Non-Innocent Ligands Al-
low Oxidative Photocyanation of Amines with Visible/Near-Infrared
Photons
Alexander M. Nauth,a,† Eugen Schechtel,b,† René Dören,b Wolfgang Tremel,*,b and Till Opatz*,a
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a Institut für Organische Chemie, Johannes Gutenberg-Universität Mainz, Duesbergweg 10-14, D-
55128 Mainz, Germany
b Institut für Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-Universität
Mainz, Duesbergweg 10-14, D-55128 Mainz, Germany
†These authors contributed equally to this work.
Abstract
Photosynthesis is an efficient mechanism for converting solar light energy into chemical energy.
We report on a strategy for the aerobic photocyanation of tertiary amines with visible and near-
infrared light. Panchromatic sensitization was achieved by functionalizing TiO2 with a 2-methyli-
soquinolinium chromophore, which captures essential features of the extended p-system of 2,7-
diazapyrenium (DAP2+) dications or graphitic carbon nitride. Two phenolic hydroxy groups make
this ligand highly redox-active and allow for efficient surface binding and enhanced electron trans-
fer to the TiO2 surface. Non-innocent ligands have energetically accessible levels that allow redox
reactions to change their charge state. Thus, the conduction band is sufficiently high to allow pho-
tochemical reduction of molecular oxygen even with near IR light. The catalytic performance (up
to 90% chemical yield for NIR excitation) of this panchromatic photocatalyst is superior to all
photocatalysts known thus far, enabling oxidative cyanation reactions to the corresponding a-cya-
nated amines to proceed with high efficiency. The discovery that the surface-binding of redox ac-
tive ligands exhibiting enhanced light harvesting in the red and near-IR region opens up the way to
improve the overall yields in heterogeneous photocatalytic reactions. Thus, this class of function-
alized semiconductors provides the basis for the design of new photocatalysts containing non-in-
nocent donor ligands. This should increase the molar extinction coefficient, permitting a reduction
of nanoparticle catalyst concentration and an increase of the chemical yields in photocatalytic re-
actions.
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INTRODUCTION
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Sunlight is an abundant source of energy which can drive chemical reactions in an eco-friendly
fashion and is most likely to play a key role in the global energy supply of the future. Catalytic
systems using visible (VIS) or infrared (IR) light are highly attractive because ultraviolet (UV)
radiation accounts only for a small fraction of the sun’s energy spectrum.1–4 In fact, the maximum
photon flux of the sun is centered around 880 nm, while its maximum power output is centered
around 500 nm.5 Most plants depend on sunlight and use the visible portion of the solar spectrum
with the aid of photosynthetic pigments (e.g. tetrapyrroles and carotenoids).6
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Chemists attempt to mimic the photosynthetic conversion of solar to chemical energy with photo-
redox systems and sensitizers. Irradiation leads to a photoexcited state that serves as a source of
electrochemical potential.4,7,8 The lifetime of the photoexcited state, the quantum efficiency of its
formation, and the chemical stabilities of ground and excited state, have been exploited in the de-
sign of systems for solar energy conversion, either directly into electrical current, but also for elec-
tron-transfer reactions, where substrates can be oxidized or reduced, depending on the conditions.
TiO2 is a stable, non-toxic, low-cost material for photocatalysis,9,10 whose photocatalytic efficiency
is dictated by the charge separation efficiency, surface area, and exposed reactive facets.11 Photo-
reduction with unmodified TiO2 nanoparticles (NPs) requires UV light which reduces the effi-
ciency of unmodified TiO2 in solar applications.12–14 Therefore, transition metal complexes,15–17
organic dyes18,19 or semiconductor nanoparticles20 have been used as sensitizers in TiO2-based so-
lar cells. These surface-bound chromophores not only serve to shift the light absorption of unmod-
ified TiO2 particles from the ultraviolet to the visible region, but – in the case of “black dyes”21–23
– achieve a panchromatic sensitization.24 Among the concepts of TiO2 sensitization via surface-
bound chromophores, one particular approach has become popular in the last few years: Photosen-
sitization through ligand-to-metal charge transfer (LMCT) on TiO2 surfaces,25 which has also been
employed in dye-sensitized solar cells (DSSCs).26,27
In inorganic photocatalysis, the semiconductors ZnO,28 ZnS 29
, CdQ (Q = S, Se, Te) quantum
dots,30,31 carbon nitride,32 BiOBr,33 or PbAO2X (A = Sb, Bi, X = Cl, Br34) with absorption in the
visible range were used to maximize the efficiency of light utilization. Likewise, organic dyes35,36
and inorganic pigments21,37 were used to stretch to absorption into the visible range. Crucial for the
TiO2 catalyst performance are light-absorbing chromophores, which must be (I) thermo-, photo-
and chemically stable. In addition, an efficient photocatalyst should (II) absorb strongly in the
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ultraviolet–visible (UV–VIS) region to produce electrons and holes, (III) separate the electrons and
holes in space to prevent their recombination, (IV) carry robust anchor groups for surface binding,
(V) have a ground-state potential positive enough to facilitate hole transfer to the catalyst, and an
excited-state potential more negative than the edge of the semiconductor conduction band to allow
interfacial electron transfer.32 It is difficult to fulfill all criteria (I)-(V) simultaneously.
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2+
TiO2 nanoparticles sensitized with a variety of Ru(bpy)3 dyes are prototypical heterogeneous
photocatalysts.22–24,38 Although organic dyes find wide application as pigments, colorants, or pho-
toreceptors, only a few have been utilized in photocatalysis, because they suffer from photodegra-
dation on TiO2 under visible-light irradiation.39,40 Perylene bisimides41 or metalated porphyrins42
fulfill the stability requirements, optical and redox criteria,38 when surface-bound via different an-
chor groups (e.g. carboxylic acids, phosphonic acids, hydroxamic acids, or silatranes).43 However,
these groups are typically redox-inert and do not allow efficient charge transfer. Polymeric carbon
nitride (CNx)-TiO2 organic–inorganic heterojunction photocatalysts have shown promising photo-
catalytic activity for the degradation of organic dyes44 or the photocatalytic generation of H2.45
However, their visible light harvesting capabilities are limited because they suffer from a weak
interaction and poor electron transfer across the TiO2 and the CNx interface.
Here, we demonstrate that a novel redox-active chromophore bound to TiO2 nanoparticles through
strong electronic coupling fulfills all performance criteria and permits the panchromatic TiO2 sen-
sitization for efficient aerobic photocyanation of tertiary amines. To this end, we synthesized the
6,7-dihydroxy-2-methylisoquinolinium (DHMIQ) ligand containing the 2-methylisoquinolinium
chromophore equipped with a redox-active catechol surface anchor group (Figure 1a). This new
ligand combines the essential features of two known chromophores: (i) quinolinium cations (e.g.
N-methyl-quinolinium (NMQ+)) and (ii) 2,7-diazapyrenium dications (DAP2+). NMQ+ cations are
powerful photocatalysts with strongly oxidizing and long-lived excited singlet states,46,47 but their
UV absorption range and their susceptibility to attack by nucleophiles in the 2- and 4-position
render them unsuitable for photocyanation. 2,7-Diazapyrenium dications (DAP2+)13,48 with an ex-
tended isoquinolinium-type p-system absorb in the visible range and do not suffer from nucleo-
philic attack on the chromophore. The catechol surface anchor makes the ligand highly redox-
active (“non-innocent”49–51) by facilitating efficient electron transfer to/from the metal oxide sur-
face.52,53
Nature employs non-innocent ligands in various metalloenzymes where the active site contains a
redox-active ligand that works in synergy with a metal center.54,55 These ligands have more
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energetically accessible levels that allow redox reactions to change their charge state. The func-
tionalized TiO2 nanoparticles exhibit a substantially improved vis-NIR spectral light harvesting
performance compared to pure CNx44 or unfunctionalized naked TiO2 nanoparticles. Neither un-
functionalized particles nor the sensitizer itself show any appreciable light absorption in the visible
spectral range. The surface-functionalized particles, however, allow direct photocatalysis down to
the near-infrared region.56,57
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The catechol-bound isoquinolinium chromophore is particularly suited because (i) amines and cy-
anides do not absorb visible light and therefore cannot be excited directly, (ii) direct oxidation of
organic substrates with molecular oxygen is kinetically unfavored.58 (iii) The redox potentials of
functionalized TiO2 particles are comparable or even higher than those of typical photoredox cat-
2+ 59
alysts like Ru(bpy)3 . This enables the use of oxygen as terminal oxidant and circumvents the
use of stronger or kinetically faster oxidants like ClO2, H2O2, t-BuOOH, or carbenium ions. (iv)
The redox potential of the TiO2-DHMIQ system is utilized for single-electron oxidations60 of the
amines and the subsequent addition of cyanide to the iminium ion resulting from hydrogen abstrac-
tion.
We have applied these new photocatalysts for the synthesis of a-aminonitriles by oxidative cya-
nation. a-Aminonitriles are an important class of compounds for the synthesis of nitrogen-contain-
ing bioactive compounds like α-amino acids and alkaloids or heterocycles.61–63 Their various
modes of reactivity make them versatile and widely applicable intermediates in organic chemistry64
since the nitrile function can either be transformed to carboxy, amide, or aminomethyl function or
be completely replaced by alkyl or aryl groups in single operation.65 This also makes α-aminoni-
triles ideal intermediates in the postfunctionalization of amines. In this paper, we demonstrate ox-
idative cyanation of tertiary amines catalyzed by TiO2 particles functionalized with the novel re-
dox-active chromophore, the scope of the reaction and the basic reaction mechanism.
RESULTS AND DISCUSSION
First, the 6,7-dihydroxy-2-methylisoquinolinium chloride surface ligand (Figure 1a) was synthe-
sized from homoveratrylamine (1), which was converted with formaldehyde in a combined Pictet-
Spengler/Eschweiler-Clarke reaction to 6,7-dimethoxy-2-methyl-1,2,3,4-tetrahydroiso-quinoline
(2) (Figure 1b).66 After oxidation with 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) to 6,7-di-
methoxy-2-methylisoquinolinium chloride (3), DHMIQ was obtained by acidic cleavage of the
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methyl ethers. In the second step, TiO2 nanoparticles were prepared hydrothermally from tita-
nium(IV) butoxide with oleic acid and oleylamine in ethanol,67 yielding nanoparticles of spherical
to elongated shape and a typical size on the order of 10 nm (Figure S1). Powder X-ray diffraction
confirmed that the particle cores consist of the anatase phase of TiO2 (Figure S2). The as-synthe-
sized TiO2 particles carry oleate capping ligands (TiO2-OA) and are perfectly soluble in apolar
solvents like CHCl3, n-hexane or tetrahydrofuran (THF). The outstanding solubility allows for nu-
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clear magnetic resonance (NMR) investigation of the nanoparticle’s ligand sphere. Thus, the H
NMR spectrum of as-synthesized TiO2-OA NPs (Figure S3) confirms the presence of oleic acid as
surface ligand. The surface-bound ligands give rise to signal broadening.68 Re-functionalization of
TiO2-OA with the positively charged DHMIQ ligand rendered the nanoparticles water-soluble, in-
ducing a polarity umpolung of the nanoparticle’s ligand sphere (Figure 1c). The DHMIQ-function-
alized TiO2 nanoparticles were purified by repeated cycles of dissolution, precipitation and drying
to remove displaced OA and excess DHMIQ molecules. No significant change in size, shape, or
phase of the particles was observed after the ligand exchange (Figures S1 and S2). 1H NMR inves-
tigation of surface-modified TiO2-DHMIQ NPs in D2O (Figure S4) revealed a virtually complete
displacement of native OA ligands, indicated by the loss of the aliphatic signal in the high-field
regime of the spectrum. The spectrum is dominated by two intense and rather broad signals, which
can be assigned to the partially coalesced signals of DHMIQ. The NMR data reveal that the ligand
spheres of the particles consist predominantly of single molecule species before and after surface
modification. Therefore, it is possible to estimate the surface coverage of the ligands simply by
quantifying the organic fraction of the nanoparticles via thermogravimetric analysis (TGA, Figure
S5). By combining the TGA data with the particle dimensions (Figure S1), ligand densities of 2.4
± 1.0 nm-2 for TiO2-OA and 2.0 ± 0.9 nm-2 for TiO2-DHMIQ NPs were obtained (Table S1). The
surface density of the OA ligand is lower than for other oleate-stabilized NPs due to the purification
(removel of unbound ligand) of the as-synthesized TiO2 NPs.69 The same applies for the DHMIQ-
modified NPs since most ligand exchange studies with catechol derivatives have yields usually
twice as high.70
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Figure 1. Design and synthesis of DHMIQ-sensitized TiO2 nanoparticles. (a) Combination of the organic photocatalyst
(N,N’-dimethyl-2,7-diazapyrenium (DAP2+) or N-methyl-quinolinium (NMQ+)) and the complexing agent catechol to
yield the photosensitive surface ligand DHMIQ. (b) Synthetic route for the preparation of DHMIQ from homovera-
trylamine (1). (c) Sensitization of TiO2 NPs via postsynthetic ligand exchange of oleic acid (OA) with DHMIQ.
The large specific surface area (~150 m2g-1, Table S1) of the anatase nanoparticles aids in the for-
mation of a photocurrent, similar as in DSSCs that operate based on similar principles.38,39 The
functionalized TiO2 nanoparticles show a substantially enhanced solar light harvesting perfor-
mance compared to “naked” TiO2 nanoparticles. Band gap excitation of TiO2 efficiently utilizes
the optical spectrum (band gap of 3.2 eV for anatase TiO2 with the conduction band starting close
to the redox potential of the normal hydrogen electrode).71 A significant portion of the visible and
even the NIR spectrum (Figure 2a) is utilized with the TiO2 surface-bound isoquinolinium ligand.
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Upon photo-excitation of the ligand, electron transfer occurs from the ligand LUMO to the TiO2
conduction band (Figure 2b).
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Figure 2. Light-absorbing properties of DHMIQ-sensitized TiO2 nanoparticles. (a) UV/Vis absorbance spectrum of
TiO2-DHMIQ NPs (red line) in comparison with ligand-stripped TiO2 NPs (with tetramethylammonium (TMA) coun-
terions, blue line) and a pure DHMIQ reference (green line). The inset shows the enlarged Vis-NIR absorption range
where the charge transfer in TiO2-DHMIQ NP takes place. (b) Proposed excitation scheme for DHMIQ-sensitized
TiO2 nanoparticles (CB: conduction band, VB: valence band, LUMO: lowest unoccupied molecular orbital, HOMO:
highest occupied molecular orbital). (c) Visual appearance of DHMIQ-sensitized TiO2 NPs (left), ligand-stripped TiO2
NPs (middle), and DHMIQ reference (right) in aqueous solutions. Nanoparticle concentrations of TiO2-DHMIQ and
TiO2-TMA: 100 mg/mL, DHMIQ reference concentration: 12 mg/mL.
In order to demonstrate that the wide-ranging absorption of TiO2-DHMIQ nanoparticles arises spe-
cifically from the surface complexation, a reference batch of ligand-stripped, water-soluble TiO2
NPs was prepared by reacting TiO2-OA NPs with tetramethylammonium hydroxide (TMAH).72,73
As can be seen by NMR spectroscopy (Figure S6), the hydroxide treatment displaced the native
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oleate ligand quantitatively, leaving a negatively charged TiO2 nanoparticle surface, stabilized
electrostatically by the counterion tetramethylammonium (TMA). Due to the non-coordinating na-
ture of the TMA cation, TiO2-TMA (Figure 2a) exhibits no charge-transfer band at ~360 nm, and
strong absorption only occurs below 350 nm for TiO2-TMA, which is a typical feature of non-
sensitized TiO2 nanoparticles.74 The unbound DHMIQ ligand has two absorption bands centered
at 251 and 351 nm. The DHMIQ-functionalized TiO2 particles show absorption throughout the
visible range and tailing into the near-infrared (Figure 2c), while non-sensitized TiO2 particles or
the unbound ligand show no significant absorption in the visible range (inset in Figure 2a). The
catechol group of the DHMIQ ligand acts as the docking site for binding to the TiO2 surface be-
cause the absorption spectrum is red-shifted after the binding of DHMIQ, which indicates the elec-
tronic interaction of the chromophore with the TiO2 NPs. The broad absorption of the TiO2-
DHMIQ NPs is attributed to diverse interactions across the ligand-NP interface: (i) The non-inno-
cent, redox-active nature of DHMIQ allows for a multitude of electronic and associated redox states
with diverse electronic transitions. (ii) Multiple facets of the nanocrystals and a non-negligible
density of intrinsic surface defects in hydrothermally synthesized TiO2 NPs75 lead to a multitude
of complexation geometries with different excitation energies.
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The energy conferred by infrared light excitation is limited by the energy of the absorbed photons.
The energy of near-infrared photons (800 nm) of 1.6 eV (149 kJ/mol) is the maximum theoretical
energy threshold between the photocatalyst and the acceptor. Since a part of this energy is lost due
to intersystem crossing or reorganization of the excited states of the photocatalyst by non-radiative
pathways, the maximum available energy of TiO2-DHMIQ is in the order of 100 kJ/mol and still
sufficient for oxidizing tertiary amines. The a-cyanation of tertiary amines reported here is one of
the few36,76 catalytic applications based on photoinduced interfacial electron transfer between a
non-innocent ligand covalently bound to a TiO2 NP surface.
The catalytic activity for the oxidative cyanation of different amines in the presence of trimethylsi-
lyl cyanide (TMSCN) was examined. In a typical reaction, an aliquot of the aqueous catalyst solu-
tion was transferred into a reaction vial and freeze dried. The dried catalyst was suspended in ace-
tonitrile, the amine substrate was added, and the solution was saturated with oxygen. After adding
TMSCN as a cyanide source, the solution was irradiated (Figure S7). The crude cyanation product
was isolated by extraction and purified by column chromatography. The minimum amount of cat-
alyst required for nearly quantitative yield was determined by successively increasing the catalyst
loading in a series of otherwise identical reactions (Figure S8). A catalyst loading of 2.5 mg TiO2-
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DHMIQ NPs, corresponding to 0.6 mol% of the catalytically active LMCT complex with respect
to the standard amount (0.24 mmol) of substrate, was found to be sufficient. Trace amounts of
water in the reaction mixture caused considerable losses in yield. A systematic study revealed that
increasing amounts of water caused an exponential decay in reaction yield, levelling out at 20%
(Figure S9). The most plausible explanation for this behavior is the formation of a hydration shell
around the polar nanoparticles in the otherwise aprotic environment of acetonitrile which passiv-
ates the surface and leads to an energy barrier for the surface reactions.77 Since the hydration shell
has a finite maximum thickness, the conversion is not completely suppressed for higher amounts
of water (see Section 5.2 in SI for a more thorough discussion).
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Table 1. Yields of the photocatalytic oxidative cyanation of tributylamine, using as-synthesized TiO2-A NPs, ligand-
stripped TiO2-TMA NPs, DHMIQ-sensitized TiO2 NPs and unbound DHMIQ as photocatalysts for different excitation
wavelengths (LED source) and excitation powers.
LED blue
462 nm, 100 W
43%[a]
LED green
LED yellow
LED red
LED IR 1
730 nm, 51 W
LED IR 2
850 nm, 3.7 W
n.t.
Light source
520 nm, 100 W 592 nm, 52 W 635 nm, 67 W
TiO2-OA
TiO2-TMA
13%[a]
2%[a]
<1%[a]
û
û
û
û
û
4%[a]
n.t.
19%[a]/61%[a,
TiO2-DHMIQ
96%isolated
93%[a]
56%[a]
43%[a]
2%[a]/12%[a, c]
b]/90%[d]
DHMIQ
Background
84%[a]
15%[a]
22%[a]
3%[a]
2%[a]
û
<1%[a]
û
û
û
û
û
Conditions: Bu3N (0.239 mmol, 1.0 eq.) and TiO2-DHMIQ NPs (2.5 mg) were dissolved in CH3CN (4.0 mL) the solution was
saturated with O2 and TMSCN (3.0 eq.) was added. Unless stated otherwise, irradiation time was 3 h. Yield was determined by 1H
NMR using 1,4-bis(trimethylsilyl)benzene as an internal standard. [a]Incomplete conversion. [b]14 h reaction time. [c]16 h reaction
time. [d]62 h reaction time.
Different excitation wavelengths were tested to excite the TiO2-DHMIQ photocatalyst. Nearly
quantitative yields were achieved after 3 h reaction upon irradiation with blue and green light (96%
and 93%, see Table 1). The conversion decreased for excitation with yellow, red and NIR (730 nm)
light. However, this could be compensated by using longer irradiation times (90% yield after 62 h
irradiation for NIR). It is unusual and quite extraordinary that even IR radiation at 850 nm from a
relatively weak 3.7 W LED module resulted in a conversion of 12% after a 16 h reaction period.
This is consistent with the very broad absorption spectrum of the TiO2-DHMIQphotocatalyst. Con-
trols with free DHMIQ gave much lower yields under identical conditions and for ligand-stripped
TiO2-TMA NPs, the yields were almost zero. This demonstrates that covalently functionalized
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TiO2 nanoparticles are essential for the oxidative cyanation reaction under visible and NIR light.
Finally, recyclability studies revealed that the heterogeneous photocatalyst could be recovered and
reused several times, with the yield decreasing only moderately from 95% to 82% after four cycles
of photocatalysis (Figure S10).
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Figure 3. Visible-light/NIR induced oxidative cyanation of amines into a-aminonitriles with O2 on DHMIQ-sensitized
1
TiO2 NPs. Yields were isolated. Reaction conditions: [a]3 h. [b]16 h. [c]24 h. Structures verified by H and 13C NMR
(Figures S11-S43).
To demonstrate the substrate scope of TiO2-DHMIQ, the catalyst was applied to a variety of dif-
ferent tertiary amines (Figure 3). Besides simple aliphatic amines like tributylamine, complex nat-
ural products like nicotine and atropine were also converted into their corresponding a-aminoni-
triles and isolated in remarkable yields. The hydroxyl group of atropine was TMS-protected due to
the effect of TMSCN as a silylating agent. Reactions with the TiO2-DHMIQ catalyst led to different
cyanation products compared to those using the organic dye rose bengal as photocatalyst. For nic-
otine, cyanation occurred in the benzylic position with TiO2-DHMIQ while rose bengal favors a
reaction at the methyl group.19 For N,N-dimethyltetradecylamine, the unexpected formation of 2-
(dimethlyamino)pentadecannitrile was observed with TiO2-DHMIQ, and the yield of the cyanation
of the N-methyl groups of gramine was approx. 4.5 times higher than that with rose bengal (89%
vs. 20%).19 This differential selectivity could prove useful in further synthetic endeavors. In
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addition, reactions with the TiO2-DHMIQ catalyst appeared to be milder than those with rose ben-
gal, as the isolated crude products were of superior purity.
With the catalytic activities, the results of the control experiments and the structure and properties
of molecular Ti(IV) complexes78 and TiO2 particles functionalized with catechol ligands79 a tenta-
tive mechanism of visible-NIR light photoredox catalysis with DHMIQ-sensitized TiO2 nanoparti-
cles is proposed in Figure 4. The DHMIQ chromophore is anchored on the TiO2 particle surface
with a catechol group to form a stable LMCT complex. Upon irradiation, DHMIQ is excited and a
charge transfer occurs to the TiO2 nanoparticle, which is associated with an injection of an electron
into the TiO2 conduction band. In the next step, the electron reacts with O2 at the TiO2 surface to
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form a hyperoxide radical anion O2 . The O2•- radical anion displaces CN– from TMSCN to form
a TMS-peroxo species which then abstracts a H-atom from the amine radical cation to generate an
iminium ion.80,81 To elucidate the fate of the TMS-peroxo species, a reaction mixture was investi-
gated by 29Si NMR spectroscopy after irradiation. The 29Si NMR spectrum (Figure S44) revealed
the presence of four major Si-containing products, all of them different from TMSCN (Table S6).
The two most prominent signals can be safely assigned to trimethylsilanol TMSOH and its con-
densation product hexamethyldisiloxane (TMS)2O. The other two species are most probably
bis(trimethylsilyl) peroxide (TMSO)2 and trimethylmethoxysilane (Table S6). As a mechanistic
alternative for iminium formation, α-amino radical intermediates are often discussed in the litera-
ture.82 These might be formed by α-deprotonation of the amine radical cations83,84 or through H-
atom transfer85 from the parent amine. Since neither the addition of diethylsilane (whose Si–H
bonds are slightly weaker than the Cα–H bonds of trialkylamines) nor the addition of ethyl acrylate
as a known trap for α-amino radicals86–88 interferes with the clean photocyanation of tributylamine
(see the SI for details), both alternatives appear unlikely. Increasing the size of the silyl group from
TMS to tert-butyldimethylsilyl (TBDMS) slows down the photocyanation, but only leads to minor
changes in the regioselectivity (16/17 and 19/20, see Section 7.2 in the SI for details). Further
experimental and theoretical work is needed to obtain a deeper mechanistic insight.
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Figure 4. Proposed mechanism for the visible-light-induced oxidative cyanation of tertiary amines with O2, catalyzed
by DHMIQ-sensitized TiO2 nanoparticles.
From a mechanistic point of view, the TiO2-DHMIQ system is not only responsible for visible light
harvesting, but is also the driving force for the oxidative transformation. Compared to similar aer-
obic oxidation reactions, this double functionality obviates the need for an additional redox medi-
ator such as (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO)36,76,89 or a radical initiator such as
azobisisobutyronitrile (AIBN).63 Furthermore, the heterogeneous nature of the catalyst enables an
easy recovery, which makes the TiO2-DHMIQ system a superior photocatalyst for oxidative cya-
nation and a promising candidate for other oxidative transformations.
CONCLUSION
A redox-active DHMIQ ligand was prepared for surface sensitization of TiO2 nanoparticles. It al-
lows panchromatic sensitization for photocatalytic oxidative cyanation reactions using molecular
oxygen as oxidant. This photo-active DHMIQ ligand was adsorbed onto TiO2 nanoparticles via
postsynthetic quantitative ligand exchange of oleic acid, yielding a DHMIQ ligand density of 2.0
± 0.9 nm-2. The ligand, when anchored to TiO2 nanocrystals, achieves very efficient sensitization
over the whole visible range extending into the near-IR region down to 800 nm. The spectral re-
sponse in the red and near-IR region is greatly enhanced compared to chemically stable standard
semiconductors.10,90,91 On the basis of its catalytic performance (up to 90% chemical yield for NIR
excitation), this panchromatic photocatalyst is superior to all photocatalysts known thus far. It
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enables oxidative cyanation reactions to the corresponding a-aminonitriles to proceed with high
efficiency. a-Aminonitriles are useful intermediates for synthesis of a wide variety of compound
classes. The reaction formally represents a photoassisted cross-dehydrogenative coupling (CDC)92
of amines and HCN through sp3 C–H bond activation adjacent to nitrogen followed by carbon-
carbon bond formation under oxidative conditions. The discovery of enhanced light harvesting in
the red and near-IR region through surface-binding of redox active ligands opens up the way to
improve the overall yields in photocatalytic reactions. Thus, this class of functionalized semicon-
ductors provides the conceptual basis for the design of new photocatalysts containing non-innocent
donor ligands. This increases the molar extinction coefficient, permitting a reduction of nanoparti-
cle catalyst concentration and an increase of the overall yields in photocatalytic reactions. Further
improvements could be achieved by the use of microflow technology or through immobilized TiO2
surfaces in continuous flow operations.93
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EXPERIMENTAL SECTION
Materials. Unless otherwise stated, all chemicals and solvents were obtained from commercial
suppliers and used without further purification. Oleic acid (technical grade, 90%) was obtained
from Sigma-Aldrich. Titanium(IV) n-butoxide (99%) was purchased from Acros Organics. Oleyla-
mine (>50.0% (GC)) was acquired from TCI. Deionized water (18.2 MΩ·cm) was obtained from a
Milli-Q water purification system (Millipore Synergy 185).
Synthesis of Oleic Acid-Stabilized TiO2 Nanoparticles (TiO2-OA). TiO2 nanoparticles were
synthesized according to a previously reported procedure.67 Titanium(IV) butoxide (2.89 g,
2.89 mL, 8.5 mmol, 1.0 eq.) was added to a mixture of oleic acid (14.41 g, 16.24 mL, 51 mmol,
6.0 eq.), oleylamine (9.09 g, 11.19 mL, 34 mmol, 4.0 eq.) and ethanol (7.83 g, 9.93 mL, 170 mmol,
20.0 eq., absolute grade). The mixture was stirred for 15 min and transferred into a 50 mL Teflon
vessel. This vessel was placed into a 250 mL Teflon-lined stainless steel autoclave, already con-
taining 34 mL of the hydrolysis solution of ethanol and water (96:4 v/v). The sealed autoclave was
then heated at 180 °C for 18 h. After opening the cooled autoclave, the content of the inner vessel
was decanted into a 50 mL centrifuge tube and the crude solid product was isolated by centrifuga-
tion (9000 rpm, 10 min) and dissolved in n-hexane (10 mL). The white solid residue at the bottom
of the inner vessel was also dissolved in n-hexane (10–15 mL), transferred into a 50 mL centrifuge
tube and precipitated by addition of ethanol (30–35 mL, technical grade). The white precipitate
was isolated by centrifugation (9000 rpm, 10 min) and redissolved in n-hexane (5 mL).The com-
bined NP solutions were further purified by repeated cycles of precipitation and dissolution (3 x
30/5 mL EtOH/n-hexane). The last dissolution step was usually carried out in CHCl3 (3 mL) to
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obtain a concentrated stock solution in a solvent that can be easily displaced for further reactions.
The stock solution was stored in a tightly sealed amber glass vial with screw top, protected from
sunlight, to extend its shelf life (stable solutions for at least up to a year). The exact concentration
of the NP solution was determined from the remaining mass of an evaporated 100 µL aliquot (after
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several hours of drying at 80 °C). H NMR (400 MHz, CDCl3): δ = 5.7–5.0 (m, 2H, -CH=CH-),
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2.4–0.6 (m, 31H, -CH2- & -CH3).
Synthesis of 6,7-Dimethoxy-2-methyl-1,2,3,4-tetrahydroisoquinoline (2). Homoveratrylamine
(10.0 g, 55.2 mmol, 1.0 eq.), formic acid (16.5 g, 20.8 mL, 552 mmol, 10 eq.) and aqueous formal-
dehyde solution (37 wt.%, 20.5 mL, 5.0 eq.) were combined and heated for 6 h to 100 °C. The
solution was made alkaline with sodium hydroxide solution (2 M) and extracted with ethyl acetate
(3 x 50 mL). The combined organic layers were dried over sodium sulfate and concentrated in
vacuo to yield the crude product. After purification by column chromatography (ethyl ace-
tate/methanol = 2:1) the product was isolated as slightly yellow solid (10.96 g, 52.87 mmol, 96%).
mp: 75.9–78.2 °C. Lit.: 78–90 °C.66 Rf = 0.23 (ethyl acetate/methanol = 2:1). IR (ATR): n = 2936
(s), 2834 (m), 2768 (m), 1518 (vs), 1463 (s), 1374 (s), 1258 (vs), 1228 (vs), 1138 (vs), 1104 (s),
1013 (m) cm–1. 1H NMR, COSY (300 MHz, CDCl3): δ = 6.59 (s, 1H, H-5), 6.51 (s, 1H, H-8), 3.84
(s, 3H, OCH3), 3.83 (s, 3H, OCH3). 3.50 (s, 2H, H-1), 2.84 (t, 3JH-4, H-3 = 5.9 Hz, 2H, H-4), 2.66 (t,
3JH-3, H-4 = 5.9 Hz, 2H, H-3), 2.45 (s, 3H, NCH3). 13C NMR, HMBC, HSQC (75 MHz, CDCl3): δ =
147.6, 147.3 (C-6, C-7), 126.7 (C-8a), 125.8 (C-4a), 111.5 (C-5), 109.4 (C-8), 57.7 (C-1), 56.1,
56.0 (2 x OCH3), 53.1 (C-3), 46.2 (NCH3), 29.0 (C-4). ESI-MS: m/z = 207.0 ([M+H+], 100%). The
spectral data match those reported in the literature.66,94
Synthesis of 6,7-Dimethoxy-2-methylisoquinolinium Chloride (3). 6,7-Dimethoxy-2-methyl-
1,2,3,4-tetrahydroisoquinoline (1.00 g, 4.83 mmol, 1.0 eq) was dissolved in acetonitrile (80 mL)
and DDQ was added (2.19 g, 9.65 mmol, 2.0 eq.). The reaction mixture was refluxed and every
24 h additional DDQ (2.0 eq) was added. After 4 days and 8.0 eq. of DDQ added, LCMS indicated
full conversion. The solvent was evaporated in vacuo and the solid residue was dissolved in diluted
hydrochloric acid (2 M, 50 mL). The aqueous solution was extracted with diethyl ether in a
Kutscher-Steudel apparatus for two days (note: ethyl acetate, chloroform or dichlormethan are not
suitable in this case). The organic extract was dried over sodium sulfate and concentrated in vacuo
to yield the pure iminium salt (1.04 g, 4.35 mmol, 90%). mp: 301–311 °C (decomposition). Lit.
(iodide salt): 235–238 °C.95 1H NMR, COSY (300 MHz, D2O): δ = 9.14 (s, 1H, ), 8.21 (dd, 3JH-3,
H-4 = 6.8 Hz, 4JH-3, H-1 = 1.4 Hz, 1H, H-3), 8.03 (d, 3JH-4, H-3 = 6.8 Hz, 1H, H-4), 7.48 (s, 1H, H-8),
7.40 (s, 1H, H-5), 4.37 (s, 3H, NCH3), 4.00 (s, 3H, C6-OCH3), 3.97 (s, 3H, C7-OCH3). 13C NMR,
HMBC, HSQC (75 MHz, D2O): δ = 157.0 (C-6), 152.1 (C-7), 145.1 (C-1), 135.3 (C-4a), 133.7 (C-
3), 124.0 (C-8a), 123.5 (C-4), 106.6 (C-8), 105.4 (C-5), 56.6 (C6-OCH3), 56.2 (C7-OCH3), 47.1
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(NCH3). ESI-MS: m/z = 204.1 ([M+], 100%). The spectral data match those reported in the litera-
ture.96
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Synthesis of 6,7-Dihydroxy-2-methylisoquinolinium Chloride (DHMIQ). 6,7-Dimethoxy-2-
methylisoquinolinium chloride (890 mg, 3.71 mmol, 1.0 eq.) was dissolved in conc. hydrochloric
acid (20 mL) and heated in a monomode microwave reactor (8 h, 140 °C, 20 bar, 45 W; ramp:
20 min). The solution was concentrated into dryness in vacuo to yield DHMIQ (786 mg,
3.71 mmol, quant.). mp: 265–271 °C (decomposition). IR (ATR): n = 2970 (wB), 1624 (w), 1529
(w), 1483 (m), 1436 (m), 1391 (w), 1371 (w), 1298 (s), 1173 (s), 1156 (s), 871 (s), 628 (m), 576
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(m), 471 (vs) cm–1. 1H NMR, COSY (300 MHz, D2O): δ = 8.96 (s, 1H, H-1), 8.04 (dd, 3JH-3, H-4
=
6.8 Hz, 4JH-3, H-1 = 1.3 Hz, 1H, H-3), 7.84 (d, 3JH-4, H-3 = 6.8 Hz, 1H, H-4), 7.30 (s, 1H, H-8), 7.18
(s, 1H, H-5), 4.30 (s, 3H, NCH3). 13C NMR, HMBC, HSQC (75 MHz, D2O): δ = 155.2 (C-6), 149.3
(C-7), 144.7 (C-1), 134.5 (C-4a), 132.6 (C-3), 123.6 (C-8a), 122.9 (C-4), 110.5 (C-8), 108.7 (C-5),
46.9 (NCH3). ESI-HRMS: calcd for [C10H10NO2]+: m/z = 176.0706, found: 176.0711.
Synthesis of TiO2-DHMIQ Nanoparticles. First, an aliquot of the titania stock solution, corre-
sponding to a NP amount of 100 mg, was transferred into a 2 mL microcentrifuge tube, precipitated
with excess acetone, centrifuged (14800 rpm, 3 min) and redissolved in THF (1.0 mL). The
DHMIQ ligand (50 mg, 0.24 mmol) was dissolved in a ternary solvent mixture of water (1.0 mL),
MeOH (0.5 mL) and THF (1.5 mL). The NP solution was added to the ligand solution, forming an
opaque suspension, which was then sonicated (280 W) at 40 °C for 1 h. The reaction mixture was
centrifuged (9000 rpm, 15 min), the yellow supernatant was discarded and the brownish-red pellet
was dried in rough vacuum (30 mbar, 10 min). The dried NPs were redissolved in a minimum
amount of water (0.2 mL) with the aid of vortexing and sonication. After precipitation with excess
THF (3.0 mL), the NPs were separated by centrifugation (9000 rpm, 10 min) and dried in low vac-
uum. The DHMIQ-functionalized TiO2 NPs were further purified by repeated cycles of dissolution,
precipitation and drying as described above (6 iterations in total). To obtain an 1H NMR spectrum
of the DHMIQ-functionalized TiO2 NPs that is not obscured by large residual signals of the un-
deuterated solvents, the penultimate dissolution step was performed in D2O, followed by the ulti-
mate precipitation step with acetone-d6 and redissolution of the dried NPs in D2O. 1H NMR (400
MHz, D2O): δ = 9.4–5.2 (m, 5H, Ar-H), 5.2–2.5 (m, 3H, NCH3).
Synthesis of Ligand-Stripped TiO2 Nanoparticles (TiO2-TMA). First, an aliquot of the titania
stock solution, corresponding to a NP amount of 200 mg, was precipitated with excess acetone,
centrifuged (14800 rpm, 3 min) and redissolved in THF (1.0 mL). Tetramethylammonium hydrox-
ide pentahydrate (TMAH·5H2O, 1.0 g, 5.5 mmol) was dissolved in 1.0 mL H2O and added to the
NP solution. The combined solution was shaken and sonicated (280 W) at 40 °C for 1 h to afford
a biphasic clear mixture. Addition of MeOH (2.0 mL) caused the phases to merge and the NPs
were then precipitated with excess THF (15 mL). The mixture was centrifuged (9000 rpm, 10 min),
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the yellow supernatant was discarded and the whitish pellet was dried in rough vacuum (30 mbar,
10 min). The dried NPs were redissolved in a minimum amount of water (0.2 mL) with the aid of
vortexing and sonication. After precipitation with excess THF (3.0 mL), the NPs were again sepa-
rated by centrifugation (9000 rpm, 10 min) and dried in rough vacuum. The ligand-stripped TiO2-
TMA NPs were further purified by repeated cycles of dissolution, precipitation and drying as de-
scribed above (4 iterations in total). To obtain an 1H NMR spectrum of the TiO2-TMA NPs that is
not obscured by large residual signals of the undeuterated solvents, the penultimate dissolution step
was performed in D2O, followed by the ultimate precipitation step with acetone-d6 and redissolu-
tion of the dried NPs in D2O. 1H NMR (400 MHz, D2O): d = 3.6–2.8 (s, 12H, N(CH3)4).
General Procedure for the Photocyanation of Amines. An aliquot of the TiO2-DHMIQ solution
(2.5 mg catalyst) was transferred in a reaction vial and freeze dried. The dried catalyst was sus-
pended with acetonitrile (4.0 mL) and the substrate (0.239 mmol, 1.0 eq.) was added. Oxygen was
bubbled through the solution for one minute to saturate the solution. Trimethylsilyl cyanide
(3.0 eq.) was added and the vial was closed tight. The solution was irradiated, if not stated other-
wise, for 3 h, poured in concentrated NaHCO3 solution (30 mL) and extracted with CH2Cl2 (3 x
20 mL). The combined organic layers were dried over Na2SO4 and the solvent was evaporated
under reduced pressure to obtain the crude product. The pure product was isolated by filtration
through a plug of aluminum oxide (basic) using CH2Cl2 as the eluent or by column chromatog-
raphy.
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Further details of the experimental setup and substrate-specific procedures are provided in the Sup-
porting Information.
ASSOCIATED CONTENT
(1) Synthesis of TiO2-DHMIQ, (2) nanoparticle characterization, (3) general procedure and setup,
(4) analytical data, (5) reaction screening, (6) additional NMR spectra.
AUTHOR INFORMATION
Corresponding Authors: opatz@uni-mainz.de and tremel@uni-mainz.de. The authors declare no
competing financial interest.
ACKNOWLEDGEMENTS
E. Schechtel is recipient of a Max Planck Graduate Center (MPGC) fellowship and a collegiate of
the MAINZ Graduate School of the State of Rhineland-Palatinate. This work was supported by the
Rhineland-Palatinate Natural Products Research Center. We thank R. Jung-Pothmann for X-ray
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diffraction measurements. The microscopy equipment is operated by the electron microscopy cen-
ter, Mainz, (EMZM) supported by the Johannes Gutenberg-University.
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