Luminescent palladium(II) complexes with π-extended cyclometalated [R-C^N^N-R′] and pentafluorophenylacetylide ligands: Spectroscopic, photophysical, and photochemical properties

Article abstract of DOI:10.1002/asia.201301059

A series of cyclometalated Pd^II complexes that contain π-extended R-C^N^N-R′ (R-C^N^N-R′=3-(6′-aryl-2′- pyridinyl)isoquinoline) and chloride/pentafluorophenylacetylide ligands have been synthesized and their photophysical and photochemical prop

Full text of DOI:10.1002/asia.201301059

FULL PAPER
DOI: 10.1002/asia.201301059
Luminescent Palladium(II) Complexes with p-Extended Cyclometalated
ꢀ
ꢀ
[R C^N^N R’] and Pentafluorophenylacetylide Ligands: Spectroscopic,
Photophysical, and Photochemical Properties
Pui-Keong Chow,[a] Wai-Pong To,[a] Kam-Hung Low,[a] and Chi-Ming Che*[a, b]
Abstract: A series of cyclometalated
PdII complexes that contain p-extended
pentafluorophenylacetylide
ligand
In the presence of 0.15 mol% PdII
complex, the oxidative cyanation of
tertiary amines could be performed
with product yields up to 91%. The
PdII complexes have also been used to
sensitize photochemical hydrogen pro-
duction with a three-component system
that comprises the PdII complex,
show emission with lmax at 585–602 nm
upon an increase in the complex con-
centration in solutions. These PdII com-
plexes can act as photosensitizers for
the light-induced aerobic oxidation of
amines. In the presence of 0.1 mol%
PdII complex, secondary amines can be
oxidized to the corresponding imines
with substrate conversions and product
yields up to 100 and 99%, respectively.
ꢀ
ꢀ
ꢀ
ꢀ
R C^N^N R’ (R C^N^N R’=3-(6’-
aryl-2’-pyridinyl)isoquinoline) and
chloride/pentafluorophenylacetylide li-
gands have been synthesized and their
photophysical and photochemical prop-
erties examined. The complexes with
the chloride ligand are emissive only in
the solid state and in glassy solutions at
77 K, whereas the ones with the penta-
fluorophenylacetylide ligand show
phosphorescence in the solid state
(lmax =584–632 nm) and in solution
(lmax =533–602 nm) at room tempera-
ture. Some of the complexes with the
[CoACTHNUTRGENUG(N dmgH)2(py)Cl] (dmgH=dimethyl-
glyoxime; py=pyridine), and trietha-
nolamine, and a maximum turnover of
hydrogen production of 175 in 4 h was
achieved. The excited-state electron-
transfer properties of the PdII com-
plexes have been examined.
Keywords: luminescence
dium · photochemistry · photophy-
sics · sensitizers
·
palla-
Introduction
phosphorescence of transition-metal complexes at room
temperature. Thus there has been a surge of interest in de-
veloping new classes of phosphorescent organometallic com-
plexes by using C-deprotonated cyclometalated ligand(s)
with C- and N-donor atoms. The deprotonated carbon
donor atom is a strong s donor that destabilizes the d–d ex-
cited state, thus leading to enhanced emission proper-
ties.[1,7,8] This strategy has been proven successful in the
design of strongly luminescent IrIII and PtII complexes.[7,8] In
the literature, a number of cyclometalated PdII complexes
such as those that contain a SCS’ pincer ligand (SCS’=3-thi-
ophosphorylbenzoic acid thioamides), C-deprotonated C^N
ligands (C^N=2-phenylpyridine), and C-deprotonated
C^N^N (C^N^N=6-phenyl-2,2’-bipyridine) ligands have
been reported,[5,9] most of which exhibit phosphorescence
only in 77 K glassy solutions or in the solid state at low tem-
perature. Notable examples of luminescent organopalladi-
um(II) complexes in solution at room temperature include
those that contain both C-deprotonated 2-phenylpyridine
and 5-substituted 8-hydroxyquinoline ligands, which show
structureless emission at 493–570 nm with emission quantum
yields (Fem) of 0.2–0.8% in CH2Cl2 at room temperature.[5a]
Very recently, Borisov and co-workers reported a series of
PdII complexes that contain donor–acceptor Schiff base li-
gands. These PdII Schiff base complexes show phosphores-
cence with Fem of 1–2% in the near-infrared region.[10] We
have recently reported strongly phosphorescent PdII com-
Luminescent platinum-group metal (PGM) complexes such
as that of ReI, OsII, PtII, and IrIII are well-documented triplet
emitters with profound applications in organic light-emitting
diodes (OLEDs), photocatalysis, luminescent bioprobes, and
cell imaging.[1–4] In contrast, reports on luminescent PdII
complexes that contain nonporphyrin ligands are compara-
tively sparse.[5] The quenching of emissive excited states by
the thermally accessible, structurally distorted d–d excited
state(s) has been proposed to account for the non- and/or
weakly emissive behavior of PdII complexes.[6] In this con-
text, increasing the energy gap between the d–d excited
state and the emissive state(s) is a means to switch on the
[a] P.-K. Chow, Dr. W.-P. To, Dr. K.-H. Low, Prof. Dr. C.-M. Che
State Key Laboratory of Synthetic Chemistry
Institute of Molecular Functional Materials
HKU-CAS Joint Laboratory on New Materials
Department of Chemistry, The University of Hong Kong
Pokfulam Road, Hong Kong (P.R. China)
Fax : (+852)2915-5176
E-mail: cmche@hku.hk
[b] Prof. Dr. C.-M. Che
HKU Shenzhen Institute of Research and Innovation
Shenzhen 518053 (China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201301059.
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Chi-Ming Che et al.
plexes with emission quantum yields up to 22% and emis-
tempts were low (ꢁ30–40%). By treating [Pd
ACHTUNGRTEN(NUNG PhCN)2Cl2]
sion peak maxima in the range of 517–543 nm using dianion-
ic tetradentate ligands with mixed O-, N-, and C-donor
atoms.[11]
In recent years, the photophysics and material applica-
tions of PtII complexes that contain substituted C-deproton-
ated C^N^N ligands with extended p conjugation such as
with ligands L1–L4 in acetone, the product yields of the PdII
complexes could be increased to 72–81%. By using 1a–4a
as precursors and employing Sonogashiraꢁs conditions,[14]
complexes 1b–4b were synthesized with yields of 52–60%.
It is noteworthy that the amount of Et3N and CuI in the re-
action could greatly affect the product yield. If over
40 equivalents of Et3N were used, the reaction mixture
would darken. Also, the use of excessive amounts of CuI (>
10 mol%) resulted in the formation of insoluble side prod-
uct(s). All the complexes were characterized by 1H and
19F NMR spectroscopy, MS (FAB), FTIR spectroscopy, and
elemental analyses. The positive FAB mass spectra showed
prominent molecular ion peaks [M+] for all the metal com-
plexes. The atom-numbering schemes of all the metal com-
plexes for NMR spectroscopic assignments are depicted in
the Supporting Information. The proton assignments are
based on the results of 1H,1H COSY and NOESY NMR
spectroscopic experiments. All the complexes have good sol-
ubility in common organic solvents, including CH2Cl2,
CHCl3, DMF, and DMSO.
ꢀ
ꢀ
R C^N^N (R C^N^N=3-(6’-(2’’-naphthyl)-2’-pyridyl)iso-
quinoline compounds) have been extensively studied.[12] The
ꢀ
reduced excited-state structural distortion of [Pt
G
C^N^N)(Cl)] complexes as revealed by time-dependent
density functional theory (TD-DFT) calculations accounts
for their high emission quantum yields.[13] The use of the s-
aryl/alkyl acetylide ligand also enhances the emission of PtII
complexes on account of the destabilization of the s*[Pt-
ACHTUNGTRENNUNG
ꢀ
ꢀ
plexes that contain p-extended C-deprotonated R C^N^N
R’ ligands (1a–4a and 1b–4b; Scheme 1). Some of these
1H NMR Spectroscopic Study of Complex 2b in Solution
With reference to previous works as well as the assignment
of proton signals of ligand L2 and a PtII analogue of 2b (see
the Supporting Information),[12] the most downfield 1H
signal of 2b in CDCl3 at 4ꢂ10ꢀ3 m was assigned to H-13.
The COSY spectrum (see the Supporting Information)
shows the coupling between H-13 and an H signal at d=
1
7.97 ppm. This H signal was assigned to H-15. Protons H-16
(d=7.70 ppm), H-17 (dꢁ7.86 ppm), H-18 (d=7.93 ppm),
and H-20 (d=8.35 ppm) were accordingly assigned on the
basis of the couplings shown in the COSY spectrum. Cross-
peaks between the signals of H-20 and another H at d=
7.87 ppm are shown in the NOESY spectrum (see the Sup-
porting Information); the latter was assigned to H-10. Cou-
plings between H-10 and H-8 at d=7.51 ppm are shown in
the COSY spectrum. Based on the cross-peaks at d=
7.41 ppm (between H-8 and H-6) in the NOESY spectrum
and couplings shown in the COSY spectrum, H-3 (d
ꢁ7.86 ppm), H-4 (dꢁ7.03 ppm), H-5 (dꢁ7.03 ppm), and H-
6 (d=7.41 ppm) were assigned accordingly.
ꢀ
ꢀ
Scheme 1. Chemical structures of [Pd(R C^N^N R’)(X)] complexes.
complexes show aggregation-induced emission behavior in
solution at room temperature[15] and have been found to cat-
alyze photochemical aerobic oxidative C H functionaliza-
tion of secondary and tertiary amines, as well as the produc-
tion of hydrogen from a H2O/CH3CN mixture with a three-
component system that comprises the PdII complex, [Co-
ꢀ
ACHTUNGTRENNUNG
1
As shown in Figure 1, the H signals of 2b are concentra-
1
tion-dependent. H signals of 2b in CDCl3 (5ꢂ10ꢀ4 m) at d=
7.11 (H-4), 7.19 (H-5), 7.54 (H-6), 7.70 (H-8), 8.04 (H-3),
8.13 (H-15), and 9.66 ppm (H-13) are upfield-shifted upon
increasing the complex concentration. This finding is remi-
niscent of the presence of intermolecular p–p stacking inter-
actions between the ligand moieties of the metal complexes
at elevated complex concentration.[16]
Results and Discussion
Synthesis
Ligands L1–L4 were prepared according to literature meth-
ods[12,14] (see the Supporting Information), and the syntheses
generally involve 1) base-catalyzed Aldol condensation and
Heck reaction for the synthesis of ligand precursors, and
2) Krçhnkeꢁs synthesis in the ligand syntheses. The [Pd(R
C^N^N R’)Cl] complexes (1a–4a) were initially prepared
X-ray Crystal Structure
ꢀ
ꢀ
Crystals of 1b were obtained by slow evaporation of its
CH2Cl2/ethyl acetate solution. As depicted in Figure 2, the
Pd atom adopts a distorted square-planar coordination ge-
by treating the ligands, L1–L4, with K2PdCl4 or PdCl2 in gla-
cial acetic acid. However, the product yields of these at-
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Chi-Ming Che et al.
are comparable to those of the reported [PdACTHNURGTNE(NUG C^N^N)(X)]
complexes (X=Cl, PPh3, or 1,1-bis(diphenylphosphino)me-
(m-dppm)).[9a,b,17]
The
Pd Cacetylide
distance
ꢀ
thane
(1.973(9) ꢃ) is similar to those of s-alkynyl–PdII complex-
es.[6b] The acetylenic unit deviates from linearity as revealed
by the angles of Pd1-C27-C28 (174.0(8)8) and C27-C28-C29
(171.2(9)8). Figure 2 shows that the molecules of 1b are in
a head-to-head arrangement. An intermolecular Pd···Pd con-
tact of 4.043 ꢃ is observed, thereby suggesting the absence
of Pd···Pd interactions (the sum of van der Waals radii of
PdII ions is 3.26 ꢃ).[18] The interplanar distances of 3.220–
3.273 ꢃ allow for the intermolecular p···p interactions.[19]
ꢀ
ꢀ
Extensive intermolecular C H···F C interactions with the
shortest distance of 2.555 ꢃ are revealed in the crystal struc-
ture.[14]
Figure 1. 1H NMR spectra of 2b in CDCl3 at various concentrations at
298 K.
Spectroscopic and Photophysical Properties
Photophysical data of 1a–4a and 1b–4b including their ab-
sorption and emission maxima, emission lifetimes (t), and
emission quantum yields (Fem) are summarized in Table 1.
The UV/Vis absorption spectrum of the free ligand together
with the absorption and emission spectra of 2a and 1b in
different solvents have been examined. The photophysical
properties of PtII analogues of 2a and 2b (Pta and Ptb, re-
spectively) have also been studied for comparison.
UV-Visible Absorption
The absorption spectra of 1a–4a in CH2Cl2 resemble each
other and display intense high-energy absorption bands with
molar extinction coefficients (e) in the order of 104 mꢀ1 cmꢀ1
accompanied by tailings with moderate intensity in the spec-
tral region of 340–430 nm (Figure 3). It is worth noting that
the e value of 4a is larger than that of the other complexes,
which is attributable to the extended p conjugation of the
Figure 2. X-ray crystal structure of 1b: a) Perspective view of 1b. For
clarity, all the hydrogen atoms are omitted. Thermal ellipsoids are drawn
at the 35% probability level. b) Molecular arrangement of 1b viewed
along the b axis (note: interplanar distances of 3.273 and 3.220 ꢃ).
ometry with Ccyclometalated-Pd-N(3ꢀisoquinoline) angle of 161.3(3)8.
ꢀ
ꢀ
The Pd Npyridine and Pd N(3ꢀisoquinoline) distances (1.975(6)–
ꢀ
ꢀ
ꢀ
2.131(6) ꢃ), as well as the Pd Cphenyl distances (1.998(8) ꢃ)
R C^N^N R’ ligand of 4a. The high-energy absorption
Table 1. Photophysical data of all of the complexes.
UV/Vis absorption, labs [nm][a,b] (e [mꢀ1 cmꢀ1])
Emission
Solution: lmax [nm];[a,c,d]
Solid state, RT:
Solid state, 77 K:
Glassy solution, 77 K:
t [ms]; Fem [%][a,c,e]
lmax [nm]; t [ms][d]
lmax [nm]; t [ms][d]
lmax [nm]; t [ms][d,f]
[g]
[g]
1a 245 (4.42), 263 (3.45), 304 (br, 3.85), 321 (sh, 3.07),
344 (sh, 1.80), 359 (sh, 1.48), 401 (sh, 0.17)
2a 248 (4.20), 261 (3.89), 294 (4.59), 340 (sh, 1.98), 361
(1.19), 398 (sh, 0.19)
3a 248 (4.68), 261 (sh, 3.79), 299 (br, 4.70), 344 (sh,
1.87), 361 (1.26), 398 (sh, 0.19)
–
–
538, 578; 48.4
497, 514, 540, 582 (sh);
99.5
[g]
[g]
–
–
505, 548, 598; 22.5 497, 538, 585, 637; 1270
[g]
[g]
–
–
531, 614; 16.2
495, 537, 584, 632 (sh);
1890
[g]
[g]
4a 251 (4.86), 261 (5.12), 298 (4.55), 338 (sh, 2.36), 358
(sh, 1.32), 394 (sh, 0.26)
–
–
571, 619, 678 (sh); 514, 557, 606, 658 (sh);
147
3940
1b 262 (4.94), 305 (3.90), 320 (sh, 3.13), 358 (sh, 1.89),
372 (sh, 0.91), 400 (0.23)
2b 249 (4.92), 263 (4.69), 302 (4.57), 347 (sh, 1.87), 364
(sh, 1.20), 375 (sh, 0.73), 393 (0.26)
3b 250 (4.85), 261 (5.11), 298 (4.56), 338 (sh, 2.37), 359
(sh, 1.31), 371 (sh, 0.93), 396 (0.20)
4b 249 (7.98), 277 (5.78), 312 (6.09), 368 (sh, 1.76), 381
(sh, 1.35), 407 (0.39)
533, 566 (sh); 0.3; 0.6
619; 0.5
632; 0.3
584; 0.6
627; 1.8
539, 614; 2.8
497, 536, 579; 224
503, 540, 600 (br);
<0.2; 0.05
498, 539, 585 (br); 0.5;
0.11
519, 564, 602 (br); 1.6;
0.29
527, 574, 629; 1.9
610; 3.5
495, 535, 585; 1560
492, 534, 579, 631; 1550
511, 555, 600; 4080
545, 621; 3.9
[a] Determined in degassed CH2Cl2. [b] Absorption maxima. [c] At 4ꢂ10ꢀ5 m. [d] Emission maxima. [e] Emission quantum yield was measured by the op-
tical dilution method with [RuACTHNURTGENNUG(bpy)3]AHCUTNGTREN(NUGN PF6)2 (bpy=2,2’-bipyridine) in degassed CH3CN as standard (Fem =0.062). [f] In 2-MeTHF. [g] Not measurable.
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Chi-Ming Che et al.
(from 340 to 430 nm) absorption bands of 1b are assigned
1
to intra
li
E
respectively. It is noteworthy that complexes 1b–4b display
higher e values in the spectral region of approximately 368–
400 nm than 1a–4a (Figure 4 and the Supporting Informa-
tion). These are tentatively assigned to the contribution of
1LLCT (pacetylide!p*Rꢀ
(LLCT=ligand-to-ligand
charge transfer) in addition to the 1MLCT transitions.
Emission in Solutions
The complexes with a chloride ligand, 1a–4a, are nonemis-
sive in solutions at room temperature. This phenomenon is
commonly observed for cyclometalated PdII complexes that
contain a weak-field ancillary ligand. The thermally accessi-
ble structurally distorted d–d excited state can readily
quench the emissive excited states of most of the reported
PdII complexes that contain a nonporphyrin ligand. In con-
trast, Pta (lem =530 nm, Fem =0.78, t=7.6 ms) is highly emis-
sive in solutions at room temperature (see the Supporting
Information). This is consistent with the general observation
that the d–d excited states of PtII complexes are higher in
energy than those of their PdII counterparts,[21] thereby the
nonradiative decay pathways in the case of Pta would be less
pronounced.
Figure 3. Absorption spectra of solutions of complexes 1a–4a in CH2Cl2
at room temperature (4ꢂ10ꢀ5 m).
1
bands are assigned to intra
G
gand (IL) p!p* transitions of
ꢀ
ꢀ
the R C^N^N R’ ligands. This assignment is supported by
similar absorption bands present in the free ligand and its
PtII analogue Pta (see the Supporting Information).[12,14] Ex-
amination of the solvent effect on the absorption spectrum
of 2a reveals negative solvatochromic shifts for the absorp-
tion tail (labs >360 nm) (see the Supporting Information).
This reveals that the ground state of 2a is more stabilized
than its excited state in polar solvents, which is reminiscent
of the emissive charge-transfer excited states of PtII com-
plexes.[20] Thus the moderately intense bands at 340–430 nm
are assigned to electronic transitions with mixed 1IL p!
p*/1MLCT characters (MLCT=metal-to-ligand charge
transfer). This assignment is further supported by the batho-
chromic shift observed at labs >400 nm in the absorption
spectrum of Pta (see the Supporting Information).[9a] Com-
plexes 1b–4b in CH2Cl2 show similar absorption bands as
1a–4a (Figure 4 and the Supporting Information). On the
basis of the negative solvatochromic shifts of the absorption
bands of 1b recorded in various solvents (see the Supporting
Information) as well as the similar absorption bands ob-
served with the free ligand (see the Supporting Informa-
tion), the high-energy (from 240 to 340 nm) and low-energy
Complexes 1b–4b with
a pentafluorophenylacetylide
ligand show emissions with lem at 533–602 nm and t in the
sub-microsecond to microsecond time regime (Figure 5 and
Figure 5. Emission spectra of 1b–4b in CH2Cl2 (4ꢂ10ꢀ5 m) at room tem-
perature (excitation wavelength, lex =350–372 nm).
Table 1). In contrast to 1a–4a, the success in switching on
the phosphorescence of 1b–4b upon the incorporation of
the pentafluorophenylacetylide ligand suggests that the d–d
excited states of these complexes are destabilized, and thus
the nonradiative decay pathways are minimized. A similar
enhancement in emission properties has also been observed
by replacing the chloride ligand of Pta with the pentafluoro-
phenylacetylide ligand to give Ptb (lem =521 nm, Fem ꢁ1, t=
5.5 ms); the latter is a much better emitter than Pta.
Complexes 2b–4b display a high-energy emission at 498–
519 nm and a low-energy emission at 585–602 nm (Figure 5
Figure 4. Absorption spectra of 1a and 1b in CH2Cl2 at room tempera-
ture (4ꢂ10ꢀ5 m).
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Chi-Ming Che et al.
and Table 1), both emissions are weak. The excitation spec-
tra for these two emissions of 2b are not similar to each
other (see the Supporting Information), thereby revealing
that the two emissions come from electronic excited states
of different emissive species. The high-energy one with vi-
brational spacings of approximately 1600 cmꢀ1 (Figure 5),
which could be attributed to C=C/C=N stretchings of the li-
to the radiative decay of an excited state formed directly
from optical excitation of intermolecular aggregates in the
ground state.
In contrast, 1b shows a structured emission with lmax at
533 nm, Fem of 0.6%, and t of 0.3 ms (Figure 5 and Table 1).
The solvent effect on the emission of 1b has been investigat-
ed. Complex 1b has its emission maximum shifted slightly
(ꢂ6 nm) upon changing the solvent polarity (see the Sup-
porting Information). Together with the vibrational progres-
sions of 1100–1500 cmꢀ1 that correspond to the C=C/C=N
stretchings of the ligand, the emission of 1b is tentatively as-
3
gands, are assignable to IL p!p* emissions.[14] To examine
the origin of the low-energy emission, nanosecond time-re-
solved emission (ns-TRE) measurements with a solution of
2b in CH2Cl2 at concentrations of 1ꢂ10ꢀ4 and 2ꢂ10ꢀ4
m
3
were undertaken. As depicted in Figure 6, the ns-TRE spec-
signed to a metal-perturbed IL p!p* excited state.[14]
Notably, the Fem of 1b (0.6%) is higher than those of 2b–
4b (0.05–0.29%) in dilute solutions (4ꢂ10ꢀ5 m). This may be
accounted for by the electron-donating methoxy group,
which increases the electron density of the deprotonated
carbon-donor atom. The energy gap between the d–d state
and the emissive state is enlarged, thus disfavoring a nonra-
diative decay pathway caused by the d–d state. Complex 1b
shows no emission at around 600 nm even at a complex con-
centration of 1ꢂ10ꢀ4 m in CH2Cl2 (see the Supporting Infor-
mation). Presumably, the steric effect offered by the me-
thoxy group disfavors the ground-state intermolecular p–p
interactions in solution.
Aggregation-Induced Emission (AIE) Behavior
The low-energy emissions of 2b–4b are enhanced upon
either increasing the complex concentration or lowering the
temperature. For example, a solution of 4b in CH2Cl2 has
a 100-fold increase in emission intensity at 600 nm when its
Figure 6. ns-TRE of 2b in CH2Cl2 (1ꢂ10ꢀ4 m) with gate delays of 0–
200 ns after laser excitation (355 nm).
concentration is increased from 2ꢂ10ꢀ5 to 2ꢂ10ꢀ3
m
(Figure 7), whereas a solution of 2b in DMF has about a 10-
fold increase in emission intensity at 600 nm when the tem-
perature is decreased from 85 to 58C (Figure 8). These find-
ings are attributed to the intermolecular aggregation, which
is favored at high complex concentration and at low temper-
ature.[24] The Fem of 2b–4b are also enhanced upon increas-
ing the complex concentration (see the Supporting Informa-
tion). With increased concentration from 2ꢂ10ꢀ5 to 1ꢂ
10ꢀ4 m, the Fem values of 2b, 3b, and 4b exhibit 2.3-, 3.0-,
trum obtained at 1ꢂ10ꢀ4 m features a high-energy structured
emission at 500 nm and a low-energy structureless emission
at 607 nm, whereas the spectrum obtained with a 2ꢂ10ꢀ4
m
solution only shows a low-energy structureless emission at
609 nm (see the Supporting Information). The high-energy
emission vanishes within 50 ns, whereas the t value of the
low-energy one is estimated to be 134 (1ꢂ10ꢀ4 m) and 161 ns
(2ꢂ10ꢀ4 m) (Figure 6 and the Supporting Information). This
finding is indicative of different electronic origin for these
two emissions. In the cases of PtII complexes such as [Pt-
ACHTUNGTRENNUNG
(N^C^N)(Cl)] (N^CH^N=1,3-di(pyridin-2-yl)benzene),[22]
the low-energy emissions that emerged at around 600 nm
are often attributed to excimer formed between an excited
molecule and a ground-state molecule. However, this ex-
planation may not be applicable to the PdII complexes in
this work, based on 1) the excitation spectra for the high-
and low-energy emissions being different, and this is contra-
ry to that expected for emission from an excimer (see the
Supporting Information);[23] 2) the absence of an induction
time period in the plot of emission intensity versus time for
excimer formation (see the Supporting Information).[23]
Along with the findings from the NMR spectroscopic stud-
ies that intermolecular ligand p–p interactions of the PdII
complexes are favorable at high complex concentrations
(10ꢀ4–10ꢀ3 m; Figure 1), the low-energy emission is assigned
Figure 7. Emission spectra of 4b in CH2Cl2 at concentrations from 1ꢂ
10ꢀ5 to 1ꢂ10ꢀ4 m at room temperature.
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Chi-Ming Che et al.
Figure 8. Emission spectra of 2b in DMF (2ꢂ10ꢀ3 m) at various tempera-
tures.
Figure 9. Solid-state emissions of 1b–4b at room temperature (lex
=
350 nm).
and 3.7-fold increases, respectively. We conceive that upon
formation of intermolecular aggregates, the excited state(s)
of dimer/aggregates of the PdII complexes are at lower-lying
energy than the d–d excited state(s), thereby minimizing the
nonradiative decay. This is reminiscent of the phenomenon
of AIE observed for organic systems in which cases the so-
lution becomes more emissive at higher complex concentra-
tion at which aggregation occurs. Reports on AIE behavior
of square-planar transition-metal complexes are scarce as
aggregation-caused quenching (ACQ) is commonly encoun-
tered.[15] In this work, 4b, which has a p-extended ligand
moiety, shows the greatest enhancement in Fem at high com-
plex concentration. In contrast to most of the reported AIE
that a ꢄnonsolventꢁ is commonly used to induce aggregation
by means of increasing the local luminophore concentra-
tion,[15] the use of such a solvent is not necessary in the pres-
ent study.
the emissions of 1b, 2b, and 4b are blueshifted in energy
and become structured, whereas the emission of 3b is red-
shifted (see the Supporting Information). The observation
for 3b can be ascribed to the occurrence of lattice contrac-
tion at 77 K, in which case the p–p interactions are en-
hanced and the energy levels of emissive excited state(s) of
dimer/aggregates are lowered.[25] In glassy solutions at 77 K,
the emissions of 1b–4b are blueshifted relative to their solid
emission and display a structured emission profile with emis-
sion maxima at 492–514 nm (Table 1 and the Supporting In-
formation). The glassy solutions of 2b–4b show slight blue-
shifted emission (ꢁ2–3 nm) relative to those of 2a–4a.
These observations are in accordance with the finding of
[Pt
CH2Cl2, which shows lmax at 565 nm,[12b] whereas [Pt-
(C^N^N)
A
(C^N^N=6-phenyl-2,2’-bipyridine)
in
A
E
at 560 nm.[14] On account of the vibrational spacings of ap-
proximately 1300–1600 cmꢀ1 and emission lifetimes on the
order of 10–103 ms, their emissions are assigned primarily to
3p!p* excited states.[14]
Photophysical Properties in the Solid State and in 77 K
Glassy Solutions
Complexes 1a–4a are luminescent in the solid state and in
glassy solutions (2-MeTHF) at 77 K but are nonemissive in
the solid state at room temperature (Table 1 and the Sup-
porting Information). These findings are common for PdII
complexes as the thermal nonradiative processes would be
minimized at low temperature.[6] In the solid state at 77 K,
complexes 1a, 2a, and 4a all show a structured emission
with lmax at 505–571 nm (see the Supporting Information),
whereas 3a shows a low-energy structureless emission at
614 nm, which is assumed to come from aggregates. In
glassy solutions at 77 K, 1a–4a show a blueshifted and well-
resolved emission band (see the Supporting Information).
The long emission lifetimes (99.5–3940 ms) and vibrational
progressional spacings of 1200–1600 cmꢀ1 suggest that the
Comparison Between PdII and PtII Systems
Comparison of the photophysical properties between PdII
complexes with PtII analogues provides insight into the ex-
cited-state properties, which is useful for molecular design
study. When compared to 2a and 2b, the PtII analogues Pta
and Ptb show redshifted absorption maxima at labs >400 nm
(see the Supporting Information). Since PdII 4d orbitals are
lower in energy than the PtII 5d orbitals,[9a] the redshifted ab-
sorption maxima are indicative of MLCT character in the
emissive excited states of the latter system.
For square-planar d8 metal complexes, their aggregation is
mainly driven by metal–metal and/or p–p interactions.
Metal–metal interaction is a combined effect of the axial
overlapping between the valence ndz2 orbitals of two metal
complexes and the symmetry-allowed mixing of dz2 with
(n+1) metal s and pz orbitals.[26] In general, closed-shell
metal–metal interactions of PdII complexes are significantly
weaker than that of their PtII analogues. This is attributed to
the energy gap between the 4dz2 and 5s/5p orbitals in the
3
emissions originate from an IL p!p* excited state.[14]
Complexes 1b–4b are luminescent in the solid state at
both 77 K and room temperature. At room temperature,
1b–4b exhibit a broad and structureless emission with lmax
at 584–632 nm and t in the sub-microsecond to microsecond
time regime (Figure 9 and Table 1). Upon cooling to 77 K,
Chem. Asian J. 2014, 9, 534 – 545
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Chi-Ming Che et al.
former being significantly larger than that between 5dz2 and
6p/6s orbitals in the latter; therefore, the mixing between
4dz2 and 5s/5p orbitals in PdII complexes is less favorable.[27]
In this work, complexes 2b–4b show distinct low-energy
emission from intermolecular aggregates when the complex
concentration is elevated from 1ꢂ10ꢀ5 to 1ꢂ10ꢀ4 m. In con-
trast, such emission for Ptb is not observable even at a con-
centration of 1ꢂ10ꢀ4 m (see the Supporting Information). In
principle, intermolecular aggregation should be more favor-
able in the solution of Ptb, which has the same ligand set as
2b. This is because PtII ions should be more prone for inter-
molecular metal–metal interactions. Thus, the failure to ob-
serve emission from intermolecular aggregation in the case
of Ptb could be due to the large difference in emission quan-
tum yields of its monomeric and aggregate forms; the latter
could be much less emissive in the case of PtII systems. On
the contrary, complexes 2b–4b show enhanced emission
upon aggregation since the emission from the monomer is
quenched by the low-lying d–d excited state.
grams of 1a, 2a, 4a, 1b, and 4b in DMF show one quasi-re-
versible reduction couple with E1/2 at ꢀ1.86 to ꢀ1.88 V and
an irreversible oxidation wave at Epa of 0.56 to 0.79 V versus
Ag/AgNO3, respectively. For the complexes with a chloride
ligand (1a, 2a, and 4a), a cathodic wave is observed at
around ꢀ0.37 V if the anodic scan is performed prior to the
cathodic scan. Such a wave is not observed with the scans in
reverse sequence in the case of 2a (see the Supporting In-
formation). Thus, the additional wave can be attributed to
the oxidized species of the metal complexes. All the com-
plexes display similar E1/2 values for the reduction couple.
Together with the similar reduction potentials shown by Pta
(ꢀ1.89 V) and Ptb (ꢀ1.84 V) (see the Supporting Informa-
tion), the reduction of these PdII complexes is tentatively as-
sumed to be ligand-centered.
The chloride-containing complexes 1a, 2a, and 4a have
similar Epa values of 0.76–0.79 V, thereby suggesting that
ꢀ
ꢀ
modification of the cyclometalated R C^N^N R’ ligand
has a minor effect on the HOMO. The Epa values of penta-
fluorophenylacetylide-containing complexes 1b and 2b are
less anodic than that of 1a and 2a (0.56 V versus 0.76–
0.79 V), thereby suggesting that the HOMO is destabilized
by the strong s-donating pentafluorophenylacetylide
ligand.[14] It is worth noting that the Epa values of 2a and 2b
are more anodic than those of Pta (0.37 V) and Ptb (0.12 V)
(see the Supporting Information), thereby suggesting that
metal-center oxidation is involved in the PtII complexes.
Electrochemical Properties
The electrochemical data of the PdII complexes in DMF
(0.1m nBu4NPF6 as supporting electrolyte) are summarized
in Table 2. As depicted in Figure 10, the cyclic voltammo-
red
Table 2. Electrochemical data (Eox, E ), 0–0 transition energy (E0–0),
1=2
and estimated excited-state potentials (E1=r2ed) of 1a, 2a, 4a, 1b, and 2b.
*
red[a]
1=2
red
Eox[a]
E
E0–0(M+)[b]
E1=2
*
Photochemical Properties
1a
2a
4a
1b
2b
0.76
0.78
0.79
0.56
0.56
ꢀ1.87
ꢀ1.86
ꢀ1.86
ꢀ1.88
ꢀ1.88
2.54
2.53
2.46
2.55
2.54
0.67
0.67
0.60
0.67
0.66
The excited-state reduction potentials of 1a, 2a, 4a, 1b, and
2b were estimated from the electrochemical and spectro-
scopic data using Equation (1):[28]
red
red
*
[a] Determined in DMF at 298 K with 0.1m nBu4NPF6 as supporting elec-
trolyte. Scan rate: 50 mVsꢀ1. Values are versus Ag/AgNO3 (0.1 m in
CH3CN) reference electrode. Cp2Fe+/0 occurs at 0.03–0.04 V versus Ag/
AgNO3 (0.1 m in CH3CN) reference electrode. [b] Estimated from the
onset of triplet emission band of metal complexes.
E1=2 ¼ E1=2 þ E0ꢀ0
ð1Þ
red
*
E1=2 denotes the excited-state reduction potential of the
red
complexes.[28] The value of E can be obtained as the E1/2
1=2
value of the quasi-reversible reduction couple from the
cyclic voltammogram of the complexes (Table 2). The 0–0
transition energy, E0–0, can be estimated from the onset of
the emission band of the complexes (Table 2 and the Sup-
red
porting Information).[28] The E1=2 of 1a, 2a, 4a, 1b, and 2b
*
are in the range of 0.60 to 0.67 V versus Ag/AgNO3, thereby
revealing that the complexes in the excited states are
a strong oxidant (Table 2).
Complexes 1a, 2a, 1b, and 2b were found to catalyze
aerobic oxidation of secondary amines to imines upon light
irradiation (l>320 nm) at ambient temperature (Scheme 2).
With 0.1 mol% of 1b and an irradiation time of 4.5 h, 100%
conversion of N-(tert-butyl)-4-chlorobenzylamine (D1) to the
corresponding imine (D2) was reached with a product yield
of 99%. The outstanding performance of 1b among these
four complexes appears to correlate with its higher Fem. The
high catalytic activity of 1b prompted us to extend the study
Figure 10. Cyclic voltammograms of complexes 1a, 2a, 4a, 1b, and 2b in
DMF (0.1m nBu4NPF6 as supporting electrolyte) at 298 K. Scan rate:
ꢀ
to oxidative C H functionalization of tertiary amines. Cyan-
50 mVsꢀ1
.
ation of tertiary amines is a useful reaction for organic syn-
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Chi-Ming Che et al.
The mechanism of the photochemical reactions could be
revealed from a thermodynamic perspective. The excited-
state potential of 1b, E1=r2ed, is 0.67 V versus Ag/AgNO3
*
(0.1m in DMF) (Table 2), and the oxidation potential of E1
is 0.58 V against the same reference (see the Supporting In-
formation). Thus, the oxidation of E1 by 1b in its excited
state is only slightly thermodynamically favorable (+
0.09 V). This suggests that the quenching of the excited state
of 1b by E1 might not be due to an electron-transfer process,
and this is in line with the absence of species that could be
attributed [1b]ꢀ in ns-TA spectroscopic measurements.
Therefore, 1b could only drive the photochemical reaction
by the generation of singlet oxygen upon light excitation.
Scheme 2. Conversions and product yields of secondary-amine oxidation
by 1a, 1b, 2a, and 2b. [a] Reaction conditions: substrate (0.08 mmol),
1a/1b/2a/2b (0.1 mol%), CH3CN (1.6 mL), O2 bubbling, light (l>
320 nm), 4.5 h; conversions and yields were calculated on the basis of the
consumption of the substrates. [b] Reaction conditions are the same as
[a] except the irradiation time was 3 h. [c] Reaction conditions are the
same as [a] except the irradiation time was 2.5 h.
ꢀ
We propose that upon photolysis, the a-C H of the sub-
1
strate reacts with O2 sensitized by the excited state of 1b,
thereby producing the iminium ion, which reacts with NaCN
in the presence of acetic acid to give the corresponding a-
substituted product (Scheme 4).[28,32]
Scheme 3. Conversions and product yields of oxidative cyanation of terti-
ary amines catalyzed by 1b. [a] Reaction conditions: substrate
(0.107 mmol), NaCN (0.214 mmol), HOAc (0.161 mmol), 1b
(0.15 mol%), CH3CN/MeOH solvent mixture (v/v=1:1, 1.6 mL), O2 bub-
bling, light (l>320 nm), 1.5 h; conversions and yields were determined
by 1H NMR spectroscopic analysis. [b] Reaction conditions were the
same as [a] except the irradiation time was 4 h.
thesis because a-aminonitriles obtained are versatile starting
materials for a-amino acids, aldehydes, ketones, b-amino al-
cohol, and 1,2-diamines.[29] As depicted in Scheme 3, in the
presence of 0.15 mol% of 1b, all the N-aryltetrahydroiso-
quinoline substrates reacted with NaCN with the highest
product yield of 91% achieved for product G2 (Scheme 3)
after irradiation (l>320 nm) for 1.5 h.
With reference to previous works, the light-induced aero-
bic oxidation reaction can proceed through singlet-oxygen
oxidation,[30] photoredox reactions,[31] or both of these path-
ways. To provide insight into the reaction mechanism, nano-
second time-resolved absorption (ns-TA) and cyclic voltam-
metric measurements were performed.
The ns-TA spectrum of a solution 1b in CH2Cl2 (see the
Supporting Information) shows two positive absorption
bands with lmax at 394 and 553 nm, which is similar to the
ns-TA spectrum of a solution that contained 1b and E1
upon laser excitation at 355 nm (see the Supporting Infor-
mation). The major difference between these two ns-TA
spectra is that absorption bands decay at a faster rate when
E1 is present. This suggests that the excited state(s) of 1b is
not reductively quenched by the amine; otherwise, a new
absorption band that corresponds to [1b]ꢀ should be pro-
duced. The Stern–Volmer plot reveals that the absorption
signal at 394 nm is quenched by E1 with a rate constant of
1.18ꢂ108 dm3 molꢀ1 sꢀ1 (see the Supporting Information).
Scheme 4. Proposed reaction pathway for photochemical aerobic oxida-
tion of E1 catalyzed by 1b.
Studies on photoinduced ET reactions of transition-metal
complexes have led to many important findings, such as the
photoreduction of water using multicomponent systems, [Pt-
AHCTUNGTRENNUNG
(terpy)(arylacetylide)]+/TEOA/MV2+/colloidal Pt (terpy=
2,2’:6’,2’’-terpyridine;
TEOA=triethanolamine)[33]
(NHC)](OTf)/TEOA/[Co-
ꢀ
and
III
ꢀ
[Au (R Cnp^N^Cnp)
G
ACHTUNGTRENNUNG
A
2-yl-pyridine; R=C6H4-4-OCH3 or H; NHC=1,3-dimethyli-
midazol-2-ylidene).[28] To demonstrate the capability of the
PdII complexes to undergo photoinduced ET, the photoin-
duced ET reaction of 1b with methyl viologen (MV2+) was
investigated by means of emission quenching, nanosecond
transient absorption measurements, and cyclic voltammetry.
The 534 nm emission of 1b in CH3CN is readily quenched
by MV2+ following Stern–Volmer behavior with a kq value
of 1.2ꢂ1010 mꢀ1 sꢀ1 (see the Supporting Information). Such
a quenching reaction is not attributed to energy transfer
from the excited state of 1b to MV2+ on account of the lack
of spectral overlap between the emission of 1b and absorp-
tion spectrum of MV2+ (see the Supporting Information).
The excited state of 1b may be oxidatively quenched by
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Chi-Ming Che et al.
MV2+. To gain insight into this, the driving force (DGFET
)
for the oxidative quenching of 1b by MV2+ can be estimat-
ed by the following equation [Eq. (2)]:[34]
DGFET ¼ EoxðDÞꢀEredðAÞꢀE0ꢀ0ꢀCSE
ð2Þ
The excited-state energy (E0–0) of 1b is estimated to be
2.55 eV (Table 2). Eox(D) and Ered(A) are the potentials for
the couples (1b+/1b) and (MV2+/MV+), respectively. Since
the couple (1b+/1b) is irreversible, Eox(D) is estimated to
be (0.56–0.05)=0.51 V. The 0.56 V is the Epa value of (1b+
/1b), and the 0.05 V is derived from the difference in poten-
tials (100 mV) between the Epa and Epc of the ferrocenium/
ferrocene (Cp2Fe+/Cp2Fe) couple. CSE is the coulombic sta-
bilization energy of the charge-separated species, which is
neglected in the present case on account of its small value
(<0.1 eV). Thus, DGFET of 1b was estimated to be ꢀ1.2 eV,
thereby revealing that electron transfer from the excited
state of 1b to MV2+ is thermodynamically favorable.
Figure 12. Left: Plot of turnover of hydrogen versus irradiation time of
4a (l>300 nm). Right: Proposed reaction mechanism for hydrogen pro-
duction ([Pd] denotes complex 1a, 4a, 1b, or 2b; [CoIII] denotes [Co-
AHCTUNGTERG(NNUN dmgH)2(py)Cl]).
ed to be 1.2ꢂ1010 mꢀ1 sꢀ1 (see the Supporting Information).
It is worth noting that after ns-TA measurements the solu-
tion turned from pale yellow to bluish green. Also, absorp-
tion maxima at 395 and 605 nm that are characteristic of
MV+C were developed in the UV-visible absorption spec-
trum of the solution (see the Supporting Information).[35]
This finding could be attributed to the lack of back electron
transfer (BET) reaction responsible for recovering MV2+
and 1b. Presumably this is due to the instability of [1b]+ as
also revealed by the irreversibility of electrochemical oxida-
tion of 1b as depicted in Figure 10.
To examine the light-induced excited-state ET process,
ns-TA spectral measurements upon flashing degassed solu-
tions of 1b in CH3CN that contain MV2+ at different con-
centrations were performed. With the solution that con-
tained MV2+ at low concentration (2.5ꢂ10ꢀ5 m) (see the
Supporting Information), the TA spectrum is similar to the
one without the presence of MV2+ (see the Supporting In-
formation). However, the decay kinetics for the absorbance
The application of the PdII complexes as a photosensitizer
for hydrogen production has been examined and the find-
ings are depicted in Figure 12. The photochemical reactions
were performed with a three-component system that com-
at 385 nm showed the presence of a major short-lived (tobs
=
prised the PdII complex (1a/4a/1b/2b), [Co
ACTHUNGTERNU(NG dmgH)2(py)Cl],
198 ns) and a minor long-lived (tobs >300 ms) component.
The former has a tobs consistent with that of the emission at
534 nm and is therefore ascribed to the absorption of the
emissive excited state of 1b, whereas the latter is attributed
to the MV+· cation radical, which is stable in the absence of
oxygen.[35]
The time-resolved absorption difference spectra of the so-
lution that contained MV2+ at 4.7ꢂ10ꢀ4 m showed the pres-
ence of a long-lived MV+C cation radical characterized by its
absorptions at 394 and 605 nm (Figure 11). The rate con-
stant (kq) for the oxidative quenching reaction was estimat-
and TEOA (see the Supporting Information).[2,28] In the
course of the reaction, a bright yellow color developed
within 30 s after photolysis, thus signifying the reduction of
CoIII to CoII. Quenching experiments showed that the emis-
sion of 1b is oxidatively quenched by [CoACTHNURTGNENG(U dmgH)2(py)Cl]
with a rate constant of 2.16ꢂ1010 mꢀ1 sꢀ1 (see the Supporting
Information). However, reductive quenching by TEOA is
not observed (see the Supporting Information), thereby im-
plying that TEOA acts as a sacrificial donor to reduce 1b+
generated after 1b is oxidatively quenched by [Co-
AHCTUNGTRENNUNG
(dmgH)2(py)Cl] (Figure 12).[28] It is worth noting that the
PdII complexes with a chloride ligand were more active pho-
tocatalysts than those with a pentafluorophenylacetylide
ligand. Similar findings have also been reported for their PtII
analogues, [Pt
yl)],[36] in which cases more turnover of hydrogen production
were found with [Pt(C^N^N)Cl] as photocatalyst. The plot
ACHUTGTNRENNUG(C^N^N)(Cl)] and [PtACHTUNGTREN(NUNG C^N^N)(s-alkyn-
AHCTUNGTRENNUNG
of turnover of hydrogen versus irradiation time in the case
of 4a (Figure 12) shows that the maximum turnover of hy-
drogen production was 175 after irradiation over 4 h with
the use of a xenon lamp. To the best of our knowledge, this
is the first example of a PdII complex being used as a photo-
sensitizer for the photogeneration of hydrogen.[37]
Conclusion
Figure 11. Time-resolved absorption difference spectra of 1b (1ꢂ10ꢀ4 m)
and MV2+ (4.7ꢂ10ꢀ4 m) in degassed CH3CN monitored at 0–640 ns (at in-
tervals of 80 ns) after the laser excitation (baseline is shown as a dotted
line).
The PdII complexes that contained the C-deprotonated R
C^N^N R’ and pentafluorophenylacetylide ligands exhibit
ꢀ
ꢀ
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Chi-Ming Che et al.
the subscripts s and r refer to sample and reference standard solution, re-
spectively, n is the refractive index of the solvents, D is the integrated in-
tensity, and Fem is the luminescence quantum yield. The refractive indices
of the solvents at room temperature were taken from standard sources.
Errors for Fem values (ꢄ10%) were estimated. The nanosecond time-re-
solved emission (ns-TRE) measurements were performed with a LP920-
KS Laser Flash Photolysis Spectrometer (Edinburgh Instruments Ltd.,
Livingston, UK). The excitation source was the 355 nm output (third har-
monic) of an Nd:YAG laser (Spectra-Physics Quanta-Ray Lab-130
Pulsed Nd:YAG Laser). The signals were processed by a PC plugin con-
troller with L900 software. The preparation of samples for the measure-
ments was exactly the same as those for steady-state emission measure-
ments.
phosphorescence in the solid state and in solution at room
temperature. The photophysical properties of these com-
plexes have been studied, with some of them exhibiting AIE
behavior in solution. These newly prepared PdII complexes
could act as a photosensitizer in light-induced aerobic oxida-
ꢀ
tive C H functionalization of amines. Mechanistic studies
revealed that these photochemical reactions proceed by sin-
glet oxygen oxidation. The PdII complexes have also been
used as a photosensitizer for light-induced hydrogen produc-
tion. The present work reveals the photochemistry of lumi-
nescent PdII complexes to be a rich area that has yet to be
developed.
General Procedure for Light-Induced Aerobic Oxidation of Secondary
Amines
Secondary amine (0.08 mmol) and PdII complex (1a, 2a, 1b, or 2b)
(0.1 mol%) were dissolved in acetonitrile (1.6 mL) in a quartz cell. The
solution in the quartz cell was then bubbled with solvent-saturated
oxygen gas (1 atm) and irradiated at l>320 nm by a 300 W xenon lamp
throughout the experiment at room temperature. After irradiation for
2.5–4.5 h, the solvent was removed under reduced pressure. The conver-
sions and product yields were calculated on the basis of the consumption
Experimental Section
Materials and Measurements
Palladium(II) chloride and the starting materials for the synthesis of li-
gands (purchased from Sigma–Aldrich) were used as received. Unless
specified, solvents of analytical grade were used in the syntheses. THF
was distilled over sodium/benzophenone under a nitrogen atmosphere
prior to use.
1
of the substrate by H NMR spectroscopic analysis.
General Procedure for Light-Induced Oxidative Cyanation of N-Aryl-
1,2,3,4-tetrahydroisoquinoline by 1b
1H NMR spectroscopic experiments were performed with
a Bruker
Avance 500, 400, or DPX-300 FT-NMR spectrometer. 19F NMR spectro-
scopic experiments were performed with a Bruker Avance 400 FT-NMR
spectrometer. Chemical shifts [ppm] reported were calibrated to the sol-
vent residual peak(s) or tetramethylsilane (TMS) as internal reference.
Peak assignments were based on 1H,1H COSY and NOESY NMR spec-
troscopic experiments. EI mass spectra were recorded with a Finnigan
MAT 95 mass spectrometer. Elemental analyses of the complexes were
performed at the Institute of Chemistry of the Chinese Academy of Sci-
ence, Beijing. UV/Vis absorption spectra were recorded with a Hewlett–
Packard 8453 diode array spectrophotometer or a Perkin-Lambda 19
UV/Vis spectrophotometer. X-ray diffraction data of single crystals were
collected with a Bruker X8 Proteum diffractometer. The diffraction
images were interpreted and the diffraction intensities were integrated by
using the program SAINT. The crystal structure was solved by direct
methods employing the SHELXS-97 program and refined by full-matrix
least-squares using the SHELXL-97 program.[38] Cyclic voltammetric
measurements were performed with a Princeton Applied Research elec-
trochemical analyzer (potentiostat/galvanostat Model 273A) with a con-
ventional three-compartment cell. nBu4NPF6 (TBAH; 0.1m) in DMF was
used as a supporting electrolyte for the electrochemical measurements at
room temperature. All solutions used in electrochemical measurements
were deaerated with pre-purified argon gas. Ag/AgNO3 (0.1m in acetoni-
trile), a glassy carbon electrode, and a platinum wire were used as refer-
ence electrode, working electrode, and counter electrode, respectively.
Ferrocene was used as an internal reference.
The experimental procedure of photoinduced cyanation of tertiary
amines was similar to that of photoinduced secondary-amine oxidation.
Tertiary amine (0.107 mmol), sodium cyanide (0.213 mmol), acetic acid
(0.160 mmol), and 1b (0.15 mol%) were dissolved in a mixture of aceto-
nitrile and methanol (1.6 mL, v/v=1:1). The solution in a quartz cell was
bubbled with solvent-saturated oxygen gas (1 atm) and irradiated at l>
320 nm by a 300 W xenon lamp throughout the experiment at room tem-
perature. The solvent was then removed under reduced pressure. The
conversions and product yields were calculated on the basis of 1H NMR
spectroscopic analysis using 4,4’-dimethyl-2,2’-bipyridine as an internal
standard. The yield was calculated based on the conversion.
General Procedure for Photochemical Hydrogen Experiments
Complexes 2a, 4a, 1b, and 4b were used as photosensitizers for light-in-
duced hydrogen production. PdII complex (2ꢂ10ꢀ5 m), [Co-
AHCTUNGTRENNUNG
(dmgH)2(py)Cl] (2ꢂ10ꢀ4 m), and TEOA (0.1m), were dissolved in a mix-
ture of acetonitrile and water (10 mL, v/v=4:1). The reaction mixture
was prepared in a test tube (24 mm internal diameter, 170 mm tall)
equipped with a magnetic stirrer and a rubber septum. The solution was
then degassed by bubbling nitrogen gas through the solution for 15 min
in the dark. The test tube was immersed in a 500 mL beaker of water to
keep the reaction temperature at about 208C throughout the experiment.
A 300 W xenon lamp was used as the light source, and a piece of optical
filter cutoff at 300 nm was used to ensure blocking of light at l<300 nm.
To detect the formation of hydrogen from the reaction mixture, 50 mL of
the headspace of the test tube was taken out by a pressure-lock syringe
and injected into a GC with thermal conductivity detector (TCD) using
a 5 ꢃ molecular sieve column and argon as carrier gas and reference gas.
The volume of the hydrogen produced was calculated by comparing the
integrated area of the signal of hydrogen and nitrogen gas with a calibra-
tion curve. The injector and detector temperatures were set to be 80 and
1508C. The retention times of hydrogen, oxygen, and nitrogen were 1.8,
2.1, and 2.8 min, respectively.
Photophysical Measurements
Steady-state excitation and emission spectra were obtained with a SPEX
Fluorolog-3 spectrophotometer. All solutions for photophysical measure-
ments were degassed with no less than five successive freeze–pump–thaw
cycles prior to the measurements. For emission measurements with
a solid sample at room temperature/77 K, or 77 K glass solution, the
sample was placed in a 5 mm-diameter quartz tube. The emission spec-
trum at 77 K was recorded after the sample-loaded tube was immersed in
a Dewar flask (equipped with a spectral window) that contained liquid
nitrogen. The emission spectra were corrected for monochromator and
photomultiplier efficiency and for xenon-lamp stability. Emission lifetime
measurements were carried out with a Quanta Ray GCR 150-10 pulsed
Nd:YAG laser system (pulse output: 355 nm, 5–6 ns). Luminescence
quantum yields were measured by the optical dilution method with [Ru-
Synthesis of Complexes 1a–4a
ꢀ
ꢀ
The [Pd(R C^N^N R’)Cl] complexes were prepared according to a liter-
ature method.[9] [Pd
AHCTUNGTRENNUNG
the corresponding ligand (L1–L4) in acetone (ꢁ30 mL). The reaction
mixture was heated at reflux for 12 h to give the crude product as
a yellow/off-white suspension. The solid was filtered and washed with n-
hexane, methanol, deionized water, and diethyl ether consecutively. The
A
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
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Chi-Ming Che et al.
crude product was then purified by flash column chromatography on
a silica gel column using an n-hexane/ethyl acetate mixture (7:3 v/v) as
eluent. The solvent was removed under reduced pressure, thereby giving
the desired complex as yellow/off-white crystalline solids.
3.76 (s, 3H; OMe), 6.62 (dd, J=2.7, 8.3 Hz, 1H; H4), 7.01 (d, J=2.7 Hz,
1H; H6), 7.46 (s, 1H; H8), 7.52 (s, 2H; H22), 7.61 (s, 1H; H24), 7.69–
7.75 (m, 2H; H3 and H16), 7.84–7.88 (m, 2H; H10 and H17), 7.93–7.97
(m, 2H; H15 and H18), 8.36 (s, 1H; H20), 9.40 ppm (s, 1H; H13);
19F NMR (376 MHz, CDCl3, 258C): d=ꢀ139.13, ꢀ160.70, ꢀ164.45 ppm;
IR (KBr): n˜ =2102 cmꢀ1 (CꢃC); FAB-MS (+ve): m/z: 796 [M+]; ele-
mental analysis calcd (%) for C43H35F5N2OPd·0.5CH2Cl2: C 62.23, H
4.32, N 3.34; found: C 62.20, H 4.22, N 3.49. CCDC-903458 (1b) contains
the supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Compound 1a
Prepared from L1 (0.46 g, 0.92 mmol) and [PdACTHNURGTENUNG(PhCN)2Cl2] (0.39 g,
1.01 mmol). Yellow crystalline solid. Isolated yield: 77%. 1H NMR
(500 MHz, CDCl3, 258C): d=1.48 (s, 18H; tBu), 3.78 (s, 3H; OMe), 6.54
(d, J=8.2 Hz, 1H; H4), 6.83 (s, 1H; H6), 7.32 (s, 1H; H8), 7.53 (d, J=
8.3 Hz, 1H; H3), 7.60–7.62 (m, 3H; H16 and H22), 7.65 (t, J=1.7 Hz,
1H; H24), 7.73–7.76 (m, 2H; H15 and H17), 7.88 (d, J=1.1 Hz, 1H;
H10), 7.92 (d, J=8.6 Hz, 1H; H18), 8.37 (s, 1H; H20), 9.16 ppm (s, 1H;
H13); FAB-MS (+ve): m/z: 640 [M+]; elemental analysis calcd (%) for
C35H35ClN2OPd·0.5H2O: C 64.62, H 5.58, N 4.31; found: C 64.55, H 5.58,
N 4.29.
Compound 2b
Prepared from 2a (0.22 g, 0.36 mmol). Off-white solid. Isolated yield:
59%. 1H NMR (500 MHz, CDCl3, 258C): d=1.44 (s, 18H; tBu), 7.00–
7.06 (m, 2H; H4 and H5), 7.41 (dd, J=1.5, 7.3 Hz, 1H; H6), 7.51 (m,
3H; H8 and H22), 7.62 (t, J=1.6 Hz, 1H; H24), 7.70 (t, J=8.1 Hz, 1H;
H16), 7.84–7.87 (m, 3H; H3, H10, and H17), 7.93 (d, J=8.1 Hz, 1H;
H15), 7.97 (d, J=8.2 Hz, 1H; H18), 8.35 (s, 1H; H20), 9.41 ppm (s, 1H;
H13); 19F NMR (376 MHz, CDCl3, 258C): d=ꢀ138.94, ꢀ160.44,
ꢀ164.49 ppm; IR (KBr): n˜ =2106 cmꢀ1 (CꢃC); FAB-MS (+ve): m/z: 766
[M+]; elemental analysis calcd (%) for C42H33F5N2Pd: C 65.76, H 4.34, N
3.65; found: C 65.83, H 4.41, N 3.65.
Compound 2a
Prepared from L2 (0.55 g, 1.17 mmol) and [PdACTHNURGTENUNG(PhCN)2Cl2] (0.49 g,
1.29 mmol). Off-white crystalline solid. Isolated yield: 81%. 1H NMR
(600 MHz, CDCl3, 258C): d=1.50 (s, 18H; tBu), 6.72 (dt, J=1.1, 7.5 Hz,
1H; H4), 6.89 (dt, J=1.0, 7.4 Hz, 1H; H5), 7.07 (d, J=1.0 Hz, 1H; H6),
7.13 (s, 1H; H8), 7.45–7.46 (m, 3H; H3, H15, and H16), 7.60–7.65 (m,
4H; H17, H22, and H24), 7.84–7.85 (m, 2H; H10 and H18), 8.36 (s, 1H;
H20), 8.87 ppm (s, 1H; H13); FAB-MS (+ve): m/z: 612 [M+]; elemental
analysis calcd (%) for C34H33ClN2Pd: C 66.78, H 5.44, N 4.58; found: C
66.59, H 5.61, N 4.47.
Compound 3b
Prepared from 3a (0.31 g, 0.48 mmol). Off-white crystalline solid. Isolat-
1
ed yield: 66%. H NMR (600 MHz, CDCl3, 258C): d=1.45 (s, 18H; tBu),
7.30 (mixed with CHCl3 solvent peak signal, 1H; H3), 7.50 (m, 3H; H8
and H22), 7.64 (s, 1H; H24), 7.69 (t, J=9.4, 1H; H6), 7.79 (t, J=7.4 Hz,
1H; H16), 7.88 (s, 1H; H10), 7.92 (t, J=7.0 Hz, 1H; H17), 8.03 (d, J=
8.0 Hz, 1H; H18), 8.07 (d, J=7.7, 1H; H15), 8.39 (s, 1H; H20), 9.55 ppm
(s, 1H; H13); 19F NMR (376 MHz, CDCl3, 258C): d=ꢀ133.25, ꢀ138.94,
ꢀ143.91, ꢀ159.82, ꢀ164.26 ppm; IR (KBr): n˜ =2118 cmꢀ1 (CꢃC); FAB-
MS (+ve): m/z: 802 [M+]; elemental analysis calcd (%) for
C42H31F7N2Pd: C 62.81, H 3.89, N 3.49; found: C 62.57, H 4.10, N 3.47.
Compound 3a
Prepared from L3 (0.48 g, 0.95 mmol) and [PdACTHNURGTENUNG(PhCN)2Cl2] (0.40 g,
1.04 mmol). Off-white crystalline solid. Isolated yield: 72%. 1H NMR
(400 MHz, CDCl3, 258C): d=1.49 (s, 18H; tBu), 6.95 (dd, J=7.3,
11.1 Hz, 1H; H6), 7.11 (s, 1H; H8), 7.24 (mixed with CHCl3 solvent peak
signal, 1H; H3), 7.57–7.60 (m, 2H; H15 and H16), 7.63 (d, J=1.7 Hz,
2H; H22), 7.66 (t, J=1.6 Hz, 1H; H24), 7.78 (ddd, J=2.2, 5.8, 8.1 Hz,
1H; H17), 7.85 (d, J=1.1 Hz, 1H; H10), 7.93 (d, J=8.2 Hz, 1H; H18),
8.32 (s, 1H; H20), 8.97 ppm (s, 1H; H13); 19F NMR (376 MHz, CDCl3,
258C): d=ꢀ133.21, ꢀ144.03 ppm; FAB-MS (+ve): m/z: 647 [M+]; ele-
mental analysis calcd (%) for C34H31ClF2N2Pd·0.5H2O: C 62.20, H 4.91,
N 4.27; found: C 62.18 H 4.87, N 4.21.
Compound 4b
Prepared from 4a (0.20 g, 0.30 mmol). Yellow crystalline solid. Isolated
yield: 64%. 1H NMR (500 MHz, CDCl3, 258C): d=1.46 (s, 18H; tBu),
7.29–7.47 (m, 2H; H6 and H7), 7.49 (d, J=8.0 Hz, 1H; H8), 7.53 (d, J=
1.7 Hz, 2H; H26), 7.62–7.66 (m, 3H; H12, H20, and H28), 7.71 (d, J=
7.6 Hz, 1H; H5), 7.80–7.86 (m, 4H; H3, H14, H21, and H19), 7.94 (d, J=
7.9 Hz, 1H; H22), 8.12 (s, 1H; H10), 8.30 (s, 1H; H24), 9.39 ppm (s, 1H;
H17); 19F NMR (376 MHz, CDCl3, 258C): d=ꢀ139.02, ꢀ160.64,
ꢀ164.42 ppm; IR (KBr): n˜ =2105 cmꢀ1 (CꢃC); FAB-MS (+ve): m/z: 816
[M+]; elemental analysis calcd (%) for C46H35F5N2Pd: C 67.61, H 4.32, N
3.43; found: C 67.52, H 4.13, N 3.41.
Compound 4a
Prepared from L4 (0.52 g, 1.0 mmol) and [PdACTHNUTGRNEUNG(PhCN)2Cl2] (0.42 g,
1.10 mmol). Yellow crystalline solid. Isolated yield: 75%. 1H NMR
(600 MHz, CDCl3, 258C): d=1.51 (s, 18H; tBu), 7.32–7.34 (m, 2H; H6
and H8), 7.47 (t, J=4.9 Hz, 1H; H7), 7.55 (s, 1H; H12), 7.59 (t, J=
7.3 Hz, 1H; H20), 7.65 (s, 2H; H26), 7.68 (s, 1H; H28), 7.73–7.79 (m,
4H; H5, H10, H19, and H21), 7.87 (s, 1H; H14), 7.96 (d, J=8.1 Hz, 1H;
H22), 7.98 (s, 1H; H3), 8.34 (s, 1H; H24), 9.30 ppm (s, 1H; H17); FAB-
MS (+ve): m/z: 660 [M+]; elemental analysis calcd (%) for
C38H35ClN2Pd·0.5H2O: C 68.06, H 5.41, N 4.18; found: C 68.10, H 5.35, N
4.17.
Acknowledgements
Synthesis of Complexes 1b–4b
This work was supported by the Hong Kong Research Grant Council
(HKU 700812P), the CAS-Croucher Foundation Funding Scheme for
Joint Laboratories, the Area of Excellence Program (AoE/P-03/08), the
National Key Basic Research Program of China (2013CB834802), and
the Guangdong Special Project of the Introduction of Innovative R&D
Teams, China.
Complexes 1b–4b were synthesized from their respective precursor com-
plexes by employing Sonogashiraꢁs conditions.[14] A solution of penta-
fluorophenylacetylene (1.5 equiv) in acetonitrile was added dropwise to
a mixture of the corresponding complex (1.0 equiv) and CuI (5 mol%) in
CH2Cl2. Triethylamine (20.0 equiv) was added to the mixture. The reac-
tion mixture was stirred for 12 h under a nitrogen atmosphere. The sol-
vent was removed under reduced pressure. The crude was washed with n-
hexane, methanol, deionized water, and diethyl ether consecutively. Fur-
ther purification of the crude was performed by flash chromatography on
a silica gel column using n-hexane/ethyl acetate/CHCl3 =1:1:1 as eluent.
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Received: August 7, 2013
Revised: September 19, 2013
Published online: November 7, 2013
Chem. Asian J. 2014, 9, 534 – 545
545
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