Palladium(II)- and platinum(II)phenyl-2,6-bis(oxazole) pincer complexes: Syntheses, crystal structures, and photophysical properties

Article abstract of DOI:10.1039/c1dt10369e

Phenyl-2,6-bis(oxazole) ligands have been explored for the synthesis of novel palladium(ii) and platinum(ii) pincer complexes. The materials were characterized by spectroscopic methods and by X-ray crystallography. Investigations of the photophysical properties revealed that the lowest triplet states of the materials are largely centred at the bis(oxazole) ligands. The platinum(ii) compounds are moderately emissive in fluid solution at ambient temperature. Introduction of both strong donors and strong acceptors leads to a significant red shift of the emission. Due to the facile synthesis of bis(oxazole) based complexes with electronically tuneable oxazole moieties, these materials might be promising alternatives to the well-established phenyl-2,6-bipyridyl systems. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1dt10369e

View Article Online / Journal Homepage / Table of Contents for this issue
This article is published as part of the Dalton Transactions themed issue entitled:
Pincers and other hemilabile ligands
Guest Editors Dr Bert Klein Gebbink and Gerard van Koten
Published in issue 35, 2011 of Dalton Transactions
Image reproduced with permission of Jun-Fang Gong
Articles in the issue include:
PERSPECTIVE:
Cleavage of unreactive bonds with pincer metal complexes
Martin Albrecht and Monika M. Lindner
Dalton Trans., 2011, DOI: 10.1039/C1DT10339C
ARTICLES:
Pincer Ru and Os complexes as efficient catalysts for racemization and deuteration of alcohols
Gianluca Bossi, Elisabetta Putignano, Pierluigi Rigo and Walter Baratta
Dalton Trans., 2011, DOI: 10.1039/C1DT10498E
Mechanistic analysis of trans C–N reductive elimination from a square-planar macrocyclic aryl-
copper(III) complex
Lauren M. Huffman and Shannon S. Stahl
Dalton Trans., 2011, DOI: 10.1039/C1DT10463B
CSC-pincer versus pseudo-pincer complexes of palladium(II): a comparative study on
complexation and catalytic activities of NHC complexes
Dan Yuan, Haoyun Tang, Linfei Xiao and Han Vinh Huynh
Dalton Trans., 2011, DOI: 10.1039/C1DT10269A
Visit the Dalton Transactions website for more cutting-edge inorganic and organometallic research
www.rsc.org/dalton

Dynamic Article Links
Dalton
Transactions
Cite this: Dalton Trans., 2011, 40, 8800
www.rsc.org/dalton
PAPER
Palladium(II)- and platinum(II)phenyl-2,6-bis(oxazole) pincer complexes:
Syntheses, crystal structures, and photophysical properties†
Gang Xu,‡a Qunli Luo,‡a Stefan Eibauer,‡a Andreas F. Rausch,b Sabine Stempfhuber,c Manfred Zabel,c
Hartmut Yersin*b and Oliver Reiser*a
Received 3rd March 2011, Accepted 21st April 2011
DOI: 10.1039/c1dt10369e
Phenyl-2,6-bis(oxazole) ligands have been explored for the synthesis of novel palladium(II) and
platinum(II) pincer complexes. The materials were characterized by spectroscopic methods and by
X-ray crystallography. Investigations of the photophysical properties revealed that the lowest triplet
states of the materials are largely centred at the bis(oxazole) ligands. The platinum(II) compounds are
moderately emissive in fluid solution at ambient temperature. Introduction of both strong donors and
strong acceptors leads to a significant red shift of the emission. Due to the facile synthesis of
bis(oxazole) based complexes with electronically tuneable oxazole moieties, these materials might be
promising alternatives to the well-established phenyl-2,6-bipyridyl systems.
ties by introducing electron donating or electron withdrawing
substituents, following our previously developed synthetic route
A Introduction
The robustness of metal pincer complexes1 has been invaluable
for their applications in catalysis or material science. Important
subclasses of pincer complexes are those with NCN-ligands,2
to ligand 2a that served as a precursor for the corresponding
palladium complex 6a.6
Due to the relatively strong spin–orbit coupling (SOC) induced
by the central metal ion7 and the high molecular rigidity -
both being important for high emission quantum yields -
platinum(II) pincer compounds with structurally closely related
dipyridinebenzene (dpyb) ligands have been established as effi-
cient phosphorescent emitters in organic light emitting diodes
(OLEDs).8,9 Further, several Pt(II) complexes with CNC10,11 and
NNC ligands12,13 have been reported to exhibit efficient room
temperature phosphorescence. The latter materials were also
successfully applied as OLED emitters.13 In order to evaluate the
potential of the here reported bis(oxazole) compounds and to
study the effects of donor/acceptor-substitution, investigations of
the photophysical properties of the new complexes were carried
out.
featuring for example amines, pyridines, and most relevant to
this study oxazolines3 as nitrogen donors. The latter moiety has
proved to be especially versatile in the coordination of metals,4
moreover, its readily accessibility from the abundant pool of amino
acids allows their synthesis in a large structural variety.5 However,
the inherent instability of oxazolines against Brønsted and Lewis
acids hampers their application as materials when longevity is a
requirement.
The oxazole moiety offers the same advantages like oxazolines
with respect to availability and variability, however, its use as a
ligand for metal complexes has been scarcely explored.6 In this
work, we utilized such ligands to synthesize new palladium(II)
and platinum(II) bis(oxazole) complexes. We reasoned that the
oxazole moiety could be easily tuned in its electronic proper-
B Results and Discussion
aUniversita¨t Regensburg, Institut fu¨r Organische Chemie, 93053, Regensburg,
Germany. E-mail: Oliver.Reiser@chemie.uni-regensburg.de; Fax: +49 941
943 4121; Tel: +49 941 943 4631
B1 Synthesis of 1-bromo-phenyl-2,6-oxazole ligands
Starting from 2-bromoisophthaloyl dichloride 114 amination with
ethyl glycinate hydrochloride or 1-amino-3,3-dimethylbutan-2-
one hydrochloride followed by dehydrative cyclisation smoothly
gave rise to bis-ethoxy substituted 2a and bis-t-butyl substituted
2b, respectively (Scheme 1). Likewise, the analogous bis-phenyl
substituted bis(oxazole) can be obtained, however, this ligand
turned out to be sparsely soluble in most common organic solvents
and was therefore not evaluated further. Following the same
sequence but starting from isophthaloyl dichloride, we also could
successfully synthesise the debrominated analogues to 2a and 2b,
bUniversita¨t Regensburg, Institut fu¨r Physikalische und Theoretische
Chemie, 93053, Regensburg, Germany. E-mail: hartmut.yersin@chemie.uni-
regensburg.de; Fax: +49 941 943 4488; Tel: +49 941 943 4464
cUniversita¨t Regensburg, Institut fu¨r Anorganische Chemie, 93053, Regens-
burg
† Electronic supplementary information (ESI) available: Details on the
X-ray structure analyses, absorption spectra of the ligands 2a–2c at
ambient temperature, emission spectra of the palladium(II) compounds
6a–6c and the platinum(II) compounds 7a–7c at 77 K. CCDC reference
numbers 630898, 817026, 817027 and 817028. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c1dt10369e
‡ These authors contributed equally to this work.
8800 | Dalton Trans., 2011, 40, 8800–8806
This journal is
The Royal Society of Chemistry 2011
©

yields. All six complexes could be fully characterized by standard
spectroscopic methods, moreover, X-ray structure analyses could
be performed for 6a,6,18 6c,18 7a,18 and 7c18 (Scheme 3, Fig. 1, Table
1 and 2).
Scheme 1 Synthesis of 1-bromo-phenyl-2,6-bis(oxazole) ligands 2a and
2b with strong and weak donor substituents in the oxazole moieties.
which also turned out to be difficult to be further processed due
to low solubility.
In order to arrive at a donor–acceptor substituted bis(oxazole)
ligand (Scheme 2), we reacted in a one pot procedure a 1 : 1 : 1
mixture of 1a, ethyl glycinate hydrochloride and methyl serinate
hydrochloride to obtain the desired unsymmetrical coupling
product 3 in 40% yield along with the two symmetrical coupling
products (15 and 17% respectively). Selective activation of the
serinate amide with DAST to the oxazoline 4 followed by oxidation
Scheme 3 Synthesis of palladium(II) and platinum(II) phenyl-2,6-oxazole
complexes 6 and 7.
15
with DBU/BrCCl3 established the acceptor-substituted oxazole
5. The donor substituted oxazole ring was readily set up by
dehydrative cyclisation of 5 along the lines already demonstrated
in the synthesis of 2a to arrive at the targeted ligand 2c.
Fig. 1 Molecular structures of palladium complexes 6a/6c (top) and plat-
inum complexes 7a/7c (bottom) as determined by X-ray crystallography;
hydrogens are omitted for clarity.
All four complexes are very similar in their geometry, displaying
an only slightly distorted square planar geometry around the metal
centre. Bond lengths and angles are well within the range of Pd-
and Pt-NCN pincer complexes reported before.19 The differences
between 6a/6c and 7a/7c due to electronic differences caused by
the substituents in the oxazole moieties are reflected in the change
of the metal-nitrogen bond lengths: Donor substitution causes
a shortening of those bonds, suggesting a stronger coordination
between that part of the ligand and the metal. Moreover, the
nitrogen-metal-bromine angle is significantly smaller with the
nitrogen belonging to the donor substituted (N2) with respect
to the acceptor substituted (N1) oxazole moiety, being most likely
a consequence of avoiding 1,3-allylic strain (Pd–N1–C(oxazole)
representing an allylic system) between the ester and the bromine
substituent.
Scheme 2 Synthesis of 1-bromo-phenyl-2,6-bis(oxazole) ligand 2c with
electron donor and acceptor substituents in the oxazole moieties.
B2 Synthesis and crystal structures of palladium(II)- and
platinum(II)phenyl-2,6-oxazole complexes
Ligands 2 proved to be suitable precursors for palladium(II)
and platinum(II) by oxidative addition with Pd(0) or Pt(0) salts
in contrast to their debrominated analogs, which could not
be converted into metal complexes by direct cyclometalation
following a previously established protocol16 for the synthesis
of NCN pincer complexes.17 Thus, 2a–2c were treated with
Pd(dba)2 or with Pt2(dipdba)3, respectively, to cleanly give the
palladium complexes 6a–c and platinum complexes 7a–7c in good
B3 Photophysical properties
Several platinum(II) pincer compounds with dipyridinebenzene
(dpyb) ligands show remarkably high emission quantum yields20
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 8800–8806 | 8801
©

Table 1 Crystal data and structure refinements for compounds 6a, 6c, 7a and 7c
6a
6c
7a
7c
Formula
Mr
Crystal system
C16H15BrN2O4Pd
485.62
Monoclinic
C16H13BrN2O5Pd
499.60
C16H15BrN2O4Pt
574.28
Monoclinic,
C16H13BrN2O5Pt
588.26
Triclinic
Triclinic
¯
¯
Space group
P21/c
P1
C2/c
P1
˚
a/A
7.5601(8)
9.3101(6)
24.686(3)
9.656(1)
10.061(1)
10.590(1)
107.99(1)
112.14(1)
100.06(1)
855.5(2)
2
33.697(4)
5.2354(4)
19.994(2)
9.696(1)
10.073(1)
10.436(1)
107.82(1)
111.73(1)
10.00(1)
852.7(2)
2
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
90.83(1)
96.70(1)
3
˚
V/A
1737.4(3)
4
173(1)
3503.2(6)
8
173(1)
Z
Temperature (K)
Reflections
173(1)
173(1)
collected/unique
Rint
19590/3360
0.0360
0.969
0.0226/0.0560
0.0272/0.0571
10880/3756
0.0326
0.963
0.0238/0.0580
0.0293/0.0596
6524/1688
0.0834
1.021
0.0461/0.1225
0.0513/0.1272
10760/3422
0.0339
1.029
0.0233/0.0563
0.0265/0.0572
GOF on F2
R1, wR2[I > 2s(I)]a
R1, wR2 (all data)a
ꢀ
ꢀ
ꢀ
ꢀ
2
1/2
a R1 =
(|Fo| - |Fc|)/ |Fo|; wR2 = [ w (|Fo| - |Fc|)2/ w Fo
]
Table 2 Selected bond lengths and angles of Pd(II)- and Pt(II) pincer
complexes with ligands 2a and 2c
6a6
6c
˚
Pd–N1 [A]
2.0591(18)
2.0605(18)
1.959(2)
2.5125(4)
158.81(7)
79.40(7)
79.41(7)
101.15(5)
100.04(5)
0.1(3)
2.138(2)
2.045(2)
1.959(2)
2.5382(5)
157.94(8)
78.89(10)
79.06(10)
111.11(6)
90.94(6)
2.0(4)
˚
Pd–N2 [A]
˚
Pt–C [A]
˚
Pd–Br [A]
N1–Pd–N2 [◦]
C–Pd–N1 [◦]
C–Pd–N2 [◦]
N1–Pd–Br [◦]
N2–Pd–Br [◦]
N1–Pd–N2–C [◦]
7a
7c
˚
Pt–N1 [A]
2.027(9)
2.024(10)
1.975(13)
2.4973(17)
159.2(4)
79.1(4)
80.1(4)
101.8(3)
99.0(3)
1.2(8)
2.097(4)
2.024(4)
1.937(5)
2.5267(6)
158.47(16)
79.17(19)
79.30(19)
110.67(11)
90.86(12)
0.1(6)
˚
Pt–N2 [A]
˚
Pd–C [A]
˚
Pt–Br [A]
N1–Pt–N2 [◦]
C–Pt–N1 [◦]
C–Pt–N2 [◦]
N1–Pt–Br [◦]
N2–Pt–Br [◦]
N1–Pt–N2–C [◦]
Fig. 2 Absorption spectra of (a) 6a–6c and (b) 7a–7c at ambient
temperature in CH2Cl2.
1
compounds, the maxima of the lowest MLCT states are found
between 363 and 370 nm, while the corresponding bands in the
Pt(II) compounds lie between 385 and 393 nm. At the applied
concentration of ª 10-5 mol L-1, absorptions from the singlet
ground state to the lowest triplet state T1 could not be observed for
any of the studied materials, indicating relatively weak spin–orbit
coupling between the T1 and higher lying MLCT states in both
the Pd(II) and Pt(II) compounds. This fits to the assignment of the
T1 states as being largely centred at the bis(oxazole) ligands.
The emission properties of the materials are summarized
in Table 3. The Pt(II) pincer compounds 7a–7c are emissive
in solution at ambient temperature (Fig. 3). The spectra are
structured and show a series of resolved bands. The high-energy
bands with maxima between 508 and 542 nm correspond to the
electronic transition from the lowest triplet state T1 to the singlet
ground state, while the bands at lower energy represent vibrational
and therefore have been employed as efficient emitters in OLEDs.8
Consequently, it seems attractive to investigate the photophysical
properties of the reported bis(oxazole) pincer compounds.
Fig. 2 shows the absorption spectra of (a) the palladium(II)
compounds 6a–6c and (b) the platinum(II) compounds 7a–7c.
Both the Pd(II) and the Pt(II) materials exhibit intense absorp-
tion bands below ª 325 nm, which can also be found in the
absorption spectra of the respective free ligands (see Supporting
Information†). Thus, these bands can be assigned to correspond to
singlet states which are largely centred at the bis(oxazole) ligands
(1LC states). The intense absorptions at longer wavelengths,
on the other hand, are absent in the spectra of the ligands.
Consequently, they represent transitions to singlet states of strong
metal-to-ligand charge-transfer (MLCT) character. In the Pd(II)
8802 | Dalton Trans., 2011, 40, 8800–8806
This journal is
The Royal Society of Chemistry 2011
©

Table 3 Emission properties of the Pd(II) (6a–6c) and Pt(II) (7a–7c)
decay times, the radiative and nonradiative deactivation rates kr
and knr can be calculated (see Table 3). The radiative rates, lying
between 1.1 ¥ 104 (7c) and 3.4 ¥ 104 s-1 (7b), are significantly
lower than those of some Pt(II) pincer compounds with dpyb
ligands, which exhibit kr values of up to 1 ¥ 105 s-1.20 Thus, spin–
orbit coupling to higher lying singlet MLCT states, facilitating
the radiative decay from the T1 state to the singlet ground state,7 is
weaker in the reported bis(oxazole) compounds than in the related
dipyridinebenzene complexes.
The nonradiative decay rate of the Pt(II) compounds increases
with decreasing electronic transition energy in the order 7b →
7a → 7c. Furthermore, the intensities of the vibrational emission
bands increase in the same order, indicating increasing geometrical
distortions of the T1 state with respect to the singlet ground
state. Since the experimental results are in agreement with the
energy gap law for radiationless transitions, which predicts an
increase of knr with decreasing transition energy and increasing
geometrical distortions upon excitation,23 nonradiative decay for
7a–7c at ambient temperature can be assumed to occur mainly via
vibrational coupling to ground state modes.
bis(oxazole) compounds at 298 K in CH2Cl2 and at 77 K in ethanol
Compound
6a
6b
6c
7a
7b
7c
lem (298 K) [nm]a
t (298 K) [ms]
fPL
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
535
3.0
0.05
508
8.0
0.27
542
1.8
0.02
kr [s-1]b
1.7 ¥ 104 3.4 ¥ 104 1.1 ¥ 104
knr [s-1]b
32 ¥ 104
9.1 ¥ 104 54 ¥ 104
504
10.4
lem (77 K) [nm]a
t (77 K) [ms]
520 488 534 532
45
538
3.3
149 33
4.1
a Electronic T1
→
S0 transition. b Calculated according to
kr
fPL
=
= t ⋅kr
knr + kr
At 77 K in glassy ethanol, the emission spectra of the plat-
inum(II) bis(oxazole) compounds 7a–7c are more structured, while
the decay times are only slightly longer (see Table 3 and Supporting
Information†). The good correspondence of the decay times at
ambient temperature and at 77 K supports the assumption that the
nonradiative decay is mainly governed by vibrational coupling to
ground state modes, while the thermal population of higher lying
metal-centered dd* states, as observed for several blue emitting
Pt(II) compounds,7,24 can probably be neglected even at ambient
temperature.
Fig. 3 Emission spectra of 7a–7c at ambient temperature in CH2Cl2.
For Pd(II) compounds, spin–orbit coupling is much weaker
than for Pt(II) compounds.7,25 As a consequence and due to lower
lying metal centred dd* states (which quench the emission), no
phosphorescence at ambient temperature can be observed for the
bis(oxazole) complexes 6a–6c. However, at 77 K, the materials
are emissive (see Table 3 and Supporting Information†). With
respect to the transition energies, the same trend as found for
the Pt(II) compounds can be observed, i.e. the T1 → S0 energies
decrease in the order 6b → 6a → 6c. The decay times of the
complexes are about one order of magnitude longer than those
of the respective Pt(II) compounds. This is in agreement with the
weaker SOC induced by Pd(II) compared to Pt(II).
Interestingly, the materials coordinated by ligand 2c, i.e. 6c and
7c, have a strong tendency to form intermolecular aggregates in
frozen solution even at low concentrations, leading to intense and
broad low energy emission bands centred at 750 nm (6c) and 690
nm (7c).26 For the compounds with the ligands 2a and 2b, on the
other hand, corresponding low energy emission bands at 77 K are
completely absent (see Supporting Information†). Discussion of
the aggregate formation at low temperature is beyond the focus of
this contribution.
satellites corresponding to ground state modes as well as to
combinations and/or progressions of these modes. For all three
compounds, spacings of ª 1500 cm-1 can be observed, indicating
that the vibrational bands mainly stem from stretching vibrations
of the aromatic rings.21
Similarly as observed for the absorptions to the lowest 1MLCT
states (vide supra), the emission energy of the compounds decreases
in the order 7b → 7a → 7c. However, the energy shifts in
emission are much larger than in absorption, indicating an
emitting triplet state of largely ligand centred character with
both the HOMO and the LUMO strongly influenced by the
electronically different substituents in the oxazole moieties. A
large red shift of ª 1000 cm-1 is observed after the introduction of
stronger donors by replacing the t-butyl groups by ethoxy groups
(7b → 7a). An additional red shift of ª 250 cm-1 is found when
going to the donor–acceptor substituted compound (7a → 7c).
For Pt(II) pincer compounds with dpyb ligands, on the other
hand, a blue shift of the emission was reported after inserting
electron-donating groups at the pyridine rings.22 This different
behaviour that the electronic structures of the emitting triplet
states in the bis(oxazole) and dipyridinebenzene compounds seem
to exhibit significant dissimilarities. However, without further
experimental data or the results of quantum chemical calculations,
no reasonable explanation for the observed behaviour can be given.
The emission quantum yields in deaerated solutions amount
to 0.05 (7a), 0.27 (7b), and 0.02 (7c), with decay times of 3.0 ms
(7a), 8.0 ms (7b), and 1.8 ms (7c). From the quantum yields and
C Conclusion
Bis(oxazole)phenyl ligands of type 2 can be readily synthesized
in a modular fashion, allowing the variation of their electronic
properties by introducing donor and/or acceptor substituents
into the oxazole moieties. This provides an alternative to the
more commonly employed electronic tuning of such types of
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 8800–8806 | 8803
©

ligand in the central ring.19,20 As representative examples, three
different palladium(II) and platinum(II) pincer complexes could
be synthesized having two weak donors, two strong donors, and
one strong donor and one strong acceptor (push-pull approach)
present in the oxazole moieties. The energies of absorptions to
1MLCT states are only slightly affected by those substitutions,
but the emission energies are distinctly red shifted by two strong
donors, with a further slight red shift achieved by substitution
of one strong donor by one strong acceptor. Phosphorescence of
the Pd(II) compounds can be observed only at 77 K, while the
Pt(II) compounds are moderately emissive in ambient temperature
solution, exhibiting quantum yields of up to 0.27. A comparison
of the photophysical properties of the reported platinum(II)
bis(oxazole) compounds with related materials coordinated by
dipyridinebenzene pincer ligands shows that the latter ones exhibit
stronger spin–orbit coupling, leading to higher radiative decay
rates and higher emission quantum yields. Nevertheless, further
modifications of the reported bis(oxazole) ligands might lead to
materials with improved emission properties.
2-Bromo-N-(1-hydroxymethyl-2-oxo-2-methoxyethyl)-N¢-(2-oxo-
2-ethoxyethyl)isophthalamide (3)
Isophthaloyl dichloride (1, 2.0 g, 7 mmol), methyl serinate
hydrochloride (1.1 g, 7.1 mmol) and ethyl glycinate hydrochloride
(0.99 g, 7.1 mmol) were suspended in 100 mL of dry CH2Cl2, and
Et3N (4.8 mL, 34 mmol) was added dropwise at -15 ◦C. After
being stirred for 12 h with gradual warming to room temperature,
the mixture was concentrated to dryness, the residue was purified
by column chromatography on silica: I (DCM/EtOAc 1 : 1):
2-bromo-N,N¢-bis(2-oxo-2-ethoxyethyl)isophthalamide (510 mg,
1
yield 17%). -II (EtOAc): 3 (1.24 g, 40%, white solid)- H NMR
(300 MHz, CDCl3): d = 7.47 (d, J = 7.7 Hz, 1H), 7.36 (dd, J = 6.9,
8.0 Hz, 2H), 7.26 (dd, J = 6.7, 8.4 Hz, 1H), 7.19 (t, J = 5.4 Hz, 1H),
4.71–4.66 (m, 1H), 4.19 (q, J = 7.1 Hz, 2H), 4.13 (d, J = 5.2 Hz,
2H), 4.01–3.90 (m, 2H), 3.74 (s, 3H), 3.39 (brs, 1H), 1.28 (t, J =
7.1 Hz, 3H). 13C NMR (75 MHz, CDCl3): d = 170.4, 169.3, 168.1,
167.8, 138.5, 130.1, 127.7, 116.3, 62.6, 61.7, 55.3, 52.7, 41.8, 14.2.
MS (ESI, CH2Cl2/MeOH + 10 mmol L-1 NH4OAc): m/z (%) =
450.1 (55), 448.1 (56) [M+NH4 ], 433.0 (100), 431.0 (98) [MH+]-.
+
III: (EtOAc/MeOH 5 : 1): 2-bromo-N,N¢-bis(1-hydroxymethyl-
2-oxo-2-methoxyethyl)isophthalamide (483 mg, yield 15%)- H
D Experimental
1
The synthesis of ligand 2a and palladium complex 6a has been
NMR (300 MHz, CD3OD): d = 7.56–7.48 (m, 3H), 4.72 (t, J =
4.7 Hz, 1H), 3.98 (dd, J = 4.8, 11.4 Hz, 2H), 3.91 (dd, J = 4.4,
11.3 Hz, 2H), 3.79 (s, 6H). 13C NMR (75 MHz, CDCl3): d =
171.9, 170.5, 140.7, 130.9, 128.8, 117.5, 62.8, 56.7, 52.9. MS (ESI,
MeOH + 10 mmol L-1 NH4OAc): m/z (%) = 466.0 (26), 464.0 (27)
described before.6
5,5¢-Di-tert-butyl-2,2¢-m-(2-bromo-phenylene)-bis-oxazole (2b)
To a stirred mixture of 2-bromo-1,3-benzenedicarbonyl dichloride
(1) (0.705 g, 2.5 mmol, 1.0 equiv) in 8 ml of dry pyridine was
added in portions 1-amino-3,3-dimethyl-butan-2-one hydrochlo-
ride (0.758 g, 5.0 mmol, 2.0 equiv) under a nitrogen atmosphere.
The mixture was heated to reflux for 30 min. After cooling to
room temperature, the mixture was diluted with water. Extraction
with CH2Cl2 afforded an orange oil (crude: 0.872 g), which was
dissolved in CH2Cl2 and washed with 10-4 mol l-1 aqueous HCl
solution to remove pyridine. The organic layer was washed with
brine, dried (MgSO4) and concentrated. The resulting 2-bromo-
N,N¢-bis-(3,3-dimethyl-2-oxo-butyl)-isophthalamide (0.715 g of
+
[M+NH4 ], 449.0 (100), 447.0 (98) [MH+].
2-Bromo-3-(4-methoxycarbonyloxazolin-2-yl)-N-(2-oxo-2-
ethoxyethyl)benzamid (4)
Diethylaminosulfur trifluoride (DAST, 0.16 mL, 1.2 mmol in 1 mL
◦
of dry CH2Cl2) was added dropwise to a solution (-78 C) of 3
(431 mg, 1 mmol) in 14 mL of dry CH2Cl2. After stirring for 1
h at -78 ◦C, anhydrous K2CO3 (207 mg, 1.5 mmol) was added
in one portion and the mixture was allowed to warm to room
temperature. The reaction was quenched with saturated aqueous
NaHCO3 (20 mL). The biphasic mixture was extracted with
EtOAc (three portions of 40 mL). The combined organic extract
was washed with water and saturated aqueous NaCl, dried over
anhydrous MgSO4 and concentrated. The residue was purified by
column chromatography (silica, EtOAC/hexanes 1 : 1 to EtOAc)
to afford 4 (290 mg, 70%) as a colorless oil. 1H NMR (300 MHz,
CDCl3): d = 7.66 (dd, J = 1.9, 7.7 Hz, 1H), 7.53 (dd, J = 1.8, 7.5 Hz,
1H), 7.38 (t, J = 7.7 Hz, 1H), 7.79 (t, J = 4.8 Hz, 1H), 4.98(dd,
J = 8.0, 10.7 Hz, 1H), 4.75–4.59 (m, 2H), 4.23 (q, J = 7.1 Hz, 2H),
4.27–4.19 (m, 2H), 3.81 (s, 3H), 1.30 (t, J = 7.1 Hz, 3H). 13C NMR
(75 MHz, CDCl3): d = 171.1, 169.5, 167.5, 166.0, 139.4, 132.6,
131.2, 130.6, 127.4, 118.9, 70.1, 68.5, 61.7, 52.8, 41.8, 14.2. MS
(EI-MS): m/z (%) = 413.9 (10), 412.0 (7) [M+], 354.9 (100), 353.0
(88), 311.9 (61), 309.9 (47).
1
an orange solid, 1.6 mmol, H-NMR (300 MHz, CDCl3): d =
7.56–7.52 (m, 2H), 7.42 (dd, 1H, J = 8.5, 6.6 Hz), 6.81–6.73
(m, 2H), 4.49 (d, 4H, J = 4.1 Hz), 1.24 (s, 18H) was used
without further purification. A mixture of 2-bromo-N,N¢-bis-(3,3-
dimethyl-2-oxo-butyl)-isophthalamide (400 mg, 0.91 mmol, 1.0
equiv) and phosphorus oxybromide (1.82 g, 6.35 mmol, 7.0 equiv)
was heated to 140 ◦C for 2 h. The mixture was allowed to cool to
roomtemperatureupon water was addedslowly. The aqueous layer
was extracted twice with ethyl acetate, the combined organic layers
were washed with brine, dried (MgSO4) and concentrated. The
residue was purified on silica (hexanes/EtOAc 5 : 1) afforded 2b as
colourless oil (0.222 g, 0.55 mmol, 39%). Rf (SiO2, hexanes/EtOAc
5 : 1) = 0.14 (UV); 1H-NMR (300 MHz, CDCl3): 1H-NMR
(300 MHz, CDCl3): d = 7.88 (d, 2H, J = 7.9 Hz), 7.48 (t, 1H,
J = 7.9 Hz), 6.88 (s, 2H), 1.37 (s, 18H); 13C-NMR (75.5 MHz,
CDCl3): d = 161.9, 159.1, 132.6, 131.5, 127.3, 120.8 (2 C), 31.9,
28.8; IR (KBr): 3120, 3060, 2960, 2930, 2900, 2860, 1640, 1580,
1545, 1450, 1390, 1360, 1350, 1320, 1275, 1250, 1200, 1120, 1105,
1060, 1030, 980, 960, 935, 825, 805, 755, 730, 690 cm-1; MS (EI-
MS): m/z (%) = 402.1 (36) [M+], 387.1 (100) [M+-CH3], 187.1
(43), 55.1 (44), 41.1 (42), 28.1 (75); C20H23BrN2O2 (403.31): calc.
C 59.56, H 5.75, N 6.95; found C 58.94, H 5.56, N 6.93.
2-Bromo-3-(4-methoxycarbonyloxazol-2-yl)-N-(2-oxo-
2-ethoxyethyl)benzamide (5)
To a solution of 4 (170 mg, 0.41 mmol) in 4 mL of dry CH2Cl2
cooled to 0 ◦C was added DBU (0.070 mL, 0.47 mmol) and then
8804 | Dalton Trans., 2011, 40, 8800–8806
This journal is
The Royal Society of Chemistry 2011
©

dropwise BrCCl3 (0.055 mL, 0.55 mmol). The mixture was stirred
overnight while warming to room temperature. Saturated aqueous
NH4Cl solution (7 mL) was added, and the biphasic mixture was
extracted with EtOAc (three portions of 10 mL). The combined
organic extract was washed with water and saturated aqueous
NaCl, dried over anhydrous MgSO4 and concentrated. The residue
was purified on silica (hexanes/EtOAc) 1 : 1 to EtOAc) to give 5
(166 mg, 98%) of a colorless oil. 1H NMR (300 MHz, CDCl3): d =
8.39 (s, 1H), 7.96 (dd, J = 1.7, 7.7 Hz, 1H), 7.60 (dd, J = 1.8, 7.5 Hz,
1H), 7.49 (t, J = 7.7 Hz, 1H), 6.44 (t, J = 4.7 Hz, 1H), 4.28 (q, J =
7.2 Hz, 2H), 4.27 (d, J = 4.9 Hz, 2H), 3.96 (s, 3H), 1.33 (t, J =
7.1 Hz, 3H). 13C NMR (75 MHz, CDCl3): d = 169.5, 167.5, 161.4,
160.9, 144.6, 140.0, 134.2, 133.4, 131.1, 129.1, 127.7, 118.8, 61.8,
52.4, 41.9, 14.2. MS (EI-MS): m/z (%) = 411.9 (8), 410.0 (8) [M+],
338.8 (11), 336.9 (10), 309.8(100), 307.9 (97). Element Analysis for
C16H15BrN2O6: Found C46.68, H 3.73, N 6.73, Calcd. C 46.73, H
3.68, N 6.81.
Synthesis of 6c
Under a N2 atmosphere, a 25 mL Schlenk flask was charged
with 2-bromo-1-(4-methoxycarbonyl- oxazol-2-yl)-3-(5-ethoxy-
oxazol-2-yl) benzene (89 mg, 0.226 mmol), Pd(dba)2 (130 mg,
0.226 mmol) and 8 mL of dry benzene. The reaction mixture
was heated to reflux until the purple color faded (30 min, from
purple to dark green or yellow), then cooled to room temperature
and stirred for further 2 h, followed by filtration to remove the
precipitate. To the filtrate was slowly added hexanes (10 mL) to
precipitate the complex. Filtration and subsequent washing with
hexanes (three times of 1 mL) gave a yellow powder, which was
dissolved in acetone (ca 50 mL) and filtered to remove residual
palladium black. The filtrate was concentrated, the residue was
recrystallised from CH2Cl2/EtOAc to yield 86 mg of 6c (76%) as
yellow crystals. 1H NMR (300 MHz, CDCl3): d = 8.20(s, 1H), 7.31
(dd, J = 1.5, 7.3 Hz, 1H), 7.18(dd, J = 1.4, 7.7 Hz, 1H), 7.13 (t,
J = 7.4 Hz, 1H), 6.74(s, 1H), 4.24 (q, J = 7.0 Hz, 4H), 3.99 (s,
3H), 1.48 (t, J = 7.1 Hz, 6H). 13C NMR (75 MHz, CD2Cl2): d =
169.2, 162.4, 159.2, 159.1, 158.4, 142.4, 133.9, 120.6, 128.3, 125.2,
123.3, 123.1,100.5, 69.6, 53.0, 14.5. MS (FI-FDMS): m/z (%) =
918.1 (20) [2M+-HBr], 500.2 (100) [M+], 456.2 (9) [M+-CO2], 419.2
(63) [M+-HBr]. Elemental Analysis for C16H13BrN2O5Pd: Found
C 38.52, H 2.36, N 5.58, Calcd. C 38.46, H 2.62, N 5.61.
2-Bromo-1-(4-methoxycarbonyl-oxazol-2-yl)-3-(5-ethoxy-oxazol-
2-yl) benzene (2c)
A mixture of 5 (408 mg, 0.99 mmol) and P2O5 (4.8 g, 33 mmol)
in 40 mL of dry toluene was refluxed for 3 h. After cooling
to room temperature, the mixture was diluted with 30 ml of
Et2O, and then carefully neutralized with cold 10% KOH aqueous
solution (50 mL) followed by extraction with Et2O (two portions
of 200 mL). The combined organic layers were washed with water
and saturated aqueous NaCl, dried over anhydrous MgSO4, and
concentrated. The residue was purified on silica (hexanes/EtOAc
1 : 1) to afford 2c (280 mg, 71%) as a pale yellow solid. 1H NMR
(300 MHz, CDCl3): d = 8.40 (s, 1H), 7.94 (dd, J = 1.8, 7.8 Hz,
1H), 7.84 (dd, J = 1.8, 7.8 Hz, 1H), 7.48 (t, J = 7.7 Hz, 1H), 6.32
(s, 1H), 4.23 (q, J = 7.1 Hz, 2H), 3.96 (s, 3H), 1.48 (t, J = 7.1 Hz,
3H). 13C NMR (75 MHz, CDCl3): d = 160.5, 160.4, 159.1, 149.3,
143.5, 133.1, 132.2, 132.1, 129.5, 129.4, 126.4, 119.5, 99.9, 67.3,
51.3, 13.4. MS (EI-MS): m/z (%) = 394.0 (34), 392.0 (33) [M+],
309.8 (97), 307.9 (100), 283.0(17), 280.9 (19). Element Analysis for
C16H13BrN2O5: Found C 48.58, H 3.46, N 6.85, Calcd. C 48.88, H
3.33, N 7.12.
Synthesis of 7a
Under a N2 atmosphere, a 25 mL Schlenk flask was charged with 2a
(227 mg, 0.6 mmol), Pt2(dipdba)3 (459 mg, 0.34 mmol, 1.1 equiv)
and 9◦mL of dry THF. The reaction mixture was stirred overnight
at 60 C, and then cooled to room temperature. To the reaction
mixture, 10 mL of petroleum ether was slowly added under N2-
pretection, and yellow solid was precipitated. After filtration and
washing with hexanes (three portions of 2 mL each), 270 mg (78%)
of 7a were obtained. 1H NMR (300 MHz, CDCl3): d = 7.32 (dd, J =
7.1, 8.2 Hz, satellite JPt–H = 7.8 Hz, 2H), 7.16 (dd, J = 6.9, 8.5 Hz,
1H), 6.71(s, satellite JPt–H = 9.2 Hz, 2H), 4.27 (q, J = 7.1 Hz, 4H),
1.50 (t, J = 7.1 Hz, 6H). 1H NMR (300 MHz, acetone-d6): d = 7.41
(dd, J = 7.1, 8.2 Hz, satellite JPt–H = 7.8 Hz, 2H), 7.29 (dd, J = 6.9,
8.5 Hz, 1H), 6.70(s, satellite JPt–H = 9.2 Hz, 2H), 4.45 (q, J = 7.0 Hz,
4H), 1.51 (t, J = 7.1 Hz, 6H). 13C NMR (75 MHz, CDCl3): d =
164.6, 159.0, 157.2, 128.1, 123.4, 121.4 (satellite JPt–C = 18.7 Hz),
100.2 (satellite JPt–C = 21.3 Hz), 67.0, 14.5. MS (FI-FDMS): m/z
(%) = 1147.3 (41) [2M+], 1068.6 (60) [2M+-HBr], 573.9 (100) [M+].
Element Analysis for C16H15BrN2O4Pt: Found C 33.55, H 2.48, N
4.76, Calcd. C 33.46, H 2.63, N 4.88.
Synthesis of 6b
Pd(dba)2 (223 mg, 0.388 mmol, 1.0 equiv) and 2b (157 mg,
0.389 mmol, 1.0 equiv) were dissolved in dry benzene (14 ml).
The solution was degassed (3 ¥ freeze-pump-thaw cycles) and
heated to reflux until the purple colour had faded (20 min). The
reaction mixture was concentrated, the residue was purified on
silica (hexanes/EtOAc 3 : 1) to afforded 6b (166 mg, 0.326 mmol,
84%) as a yellow solid. Rf (SiO2, hexanes/EtOAc 3 : 1) = 0.18 (UV);
Synthesis of 7b
To a solution of 2a (60 mg, 0.15 mmol, 1.0 eq) in 3 ml of THF was
added Pt2(dipdba)3 (161 mg, 0.12 mmol, 1.6 equiv “P◦t”) under
a nitrogen atmosphere. The reaction was stirred at 60 C for 20
h. The mixture was concentrated, and the residue was purified
on silica (hexanes/EtOAc 3 : 1) to afford 7b as an orange solid
(77 mg, 0.13 mmol, 86%). Rf (SiO2, hexanes/EtOAc 3 : 1) = 0.30
(UV); M.p. > 295 C (decomp.); H-NMR (300 MHz, CDCl3):
d = 7.52–7.43 (m, 2H), 7.31–7.18 (m, 3H), 1.38 (s, 18H); 13C-NMR
(75.5 MHz, CDCl3): d = 172.2, 162.0, 157.3, 128.0, 123.3, 123.0,
119.8, 32.2, 28.4; IR (KBr): 3140, 3060, 2970, 2870, 1590, 1520,
1460, 1430, 1395, 1365, 1320, 1280, 1210, 1150, 1130, 1110, 1025,
◦
1
M.p. > 290 C (decomp.); H-NMR (300 MHz, MeOH-d4): d =
7.25–7.20 (m, 2H), 7.12 (dd, 1H, J = 8.5, 6.6 Hz), 6.89 (s, 2H),
1.42 (s, 18H); 13C-NMR (75.5 MHz, MeOH-d4): d = 168.6, 164.7,
162.7, 131.0, 126.0, 123.9, 121.0, 33.1, 29.0; IR (KBr): 3137, 3058,
2966, 2906, 2870, 2369, 1591, 1459, 1397, 1364, 1281, 1211, 1152,
1126, 1030, 1004, 946, 824, 724, 681 cm-1; MS (PI-FDMS): m/z
(%) = 939.5 (50) [2M+-Br], 510.4 (100) [M+], 429.4 (20) [M+-Br];
C20H23BrN2O2Pd (509.73): calc. C 47.13, H 4.55, N 5.50; found C
46.99, H 4.68, N 5.44.
◦
1
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 8800–8806 | 8805
©

1005, 940, 815, 720, 680 cm-1; MS (PI-FDMS): m/z (%) = 1117.1
(40) [2M+-Br], 598.4 (100) [M+]; C20H23BrN2O2Pt (598.39): calc.
C 40.14, H 3.87, N 4.68; found C 40.34, H 3.90, N 4.74.
3 C. Bolm, K. Weickhardt, M. Zehnder and T. Ranff, Chem. Ber., 1991,
124, 1173; S. E. Denmark, R. A. Stavenger, A.-M. Faucher and J. P.
Edwards, J. Org. Chem., 1997, 62, 3375; M. A. Stark and C. J. Richards,
Tetrahedron Lett., 1997, 38, 5881; M. A. Stark, G. Jones and C. J.
Richards, Organometallics, 2000, 19, 1282.
4 R. Rasappan, D. Laventine and O. Reiser, Coord. Chem. Rev., 2008,
252, 702.
5 G. Desimoni, G. Faita and K. A. Jorgensen, Chem. Rev., 2006, 106,
3561.
6 Q. Luo, S. Eibauer and O. Reiser, J. Mol. Catal. A: Chem., 2007, 268,
65.
Synthesis of 7c
Following the same procedure as for 7a, 187 mg (53%) of 7c were
obtained: 1H NMR (300 MHz, CD2Cl2): d = 8.25(s, satellite JPt–H
=
3.4 Hz, 1H), 7.45(d, J = 7.7 Hz, satellite JPt–H = 3.2 Hz, 1H),
7.33(d, J = 6.7 Hz, satellite JPt–H = 3.1 Hz, 1H), 7.16 (t, J = 6.9 Hz,
1H), 6.82(s, satellite JPt–H = 10.5 Hz, 1H), 4.30 (q, J = 7.0 Hz,
2H), 3.99 (s, 3H), 1.50(t, J = 7.1 Hz, 3H). 13C NMR (75 MHz,
CD2Cl2): d = 175.4, 164.1, 159.5, 159.1, 156.6, 142.2 (satellite
JPt–C = 15.0 Hz), 134.2, 129.0, 126.8, 123.9, 123.6, 123.3, 99.9
(satellite JPt–C = 22.8 Hz), 70.5, 53.4, 14.7. MS (FI-FDMS): m/z
(%) = 1176.0 (16) [2M+], 588.3 (100) [M+]. Elemental Analysis for
C16H13BrN2O5Pt: Found C 32.78, H 2.18, N 4.74, Calcd. C 32.67,
H 2.23, N 4.76.
7 A. F. Rausch, H. H. H. Homeier and H. Yersin, Top. Organomet. Chem.,
2010, 29, 193.
8 J. A. G. Williams, S. Develay, D. L. Rochester and L. Murphy, Coord.
Chem. Rev., 2008, 252, 2596; M. Cocchi, D. Virgili, V. Fattori, D. L.
Rochester and J. A. G. Williams, Adv. Funct. Mater., 2007, 17, 285; X.
Yang, Z. Wang, S. Madakuni, J. Li and G. E. Jabbour, Adv. Mater.,
2008, 20, 2405.
9 H. Yersin, (Ed.), Highly Efficient OLEDs with Phosphorescent Materi-
als, Wiley-VCH, Weinheim, 2008.
10 M. V. Kulikova, K. P. Balashev and H. Yersin, Russ. J. Gen. Chem.,
2003, 73, 1839.
11 H. Yersin, A. F. Rausch, R. Czerwieniec, T. Hofbeck and T. Fischer,
Coord. Chem. Rev., 2011, DOI: 10.1016/j.ccr.2011.01.042.
12 D. Ravindranathan, D. A. K. Vezzu, L. Bartolotti, P. D. Boyle and S.
Huo, Inorg. Chem., 2010, 49, 8922.
Spectroscopy
For ambient temperature measurements, the materials were dis-
solved in CH2Cl2, while ethanol was used as solvent at 77 K.
All solvents were of spectroscopic grade, the concentration of the
solutions was ª 10-5 mol/L. Absorption spectra were recorded
with a Varian Cary 300 double beam spectrometer. Emission
spectra at 300 and at 77 K were measured with a steady-state
fluorescence spectrometer (Jobin Yvon Fluorolog 3). Lumines-
cence quantum yields were determined with a commercially
available system for the measurements of absolute quantum yields
(Hamamatsu Photonics C9920-02).27 The estimated relative error
of the quantum yields is about 10%. Fluid solutions were degassed
by at least three pump–freeze–thaw cycles with a final vapor
pressure at 77 K of ª 10-5 mbar. A pulsed diode laser (PicoQuant
PDL 800-B) with a pulse width of about 500 ps (excitation
wavelength of 372 nm) or a nitrogen laser (MNL100, Lasertechnik
Berlin) with a pulse width of 3 ns (excitation wavelength of 337 nm)
were applied as excitation sources for decay time measurements.
Decay times were registered using a FAST Comtec multichannel
scaler PCI card with a time resolution of 250 ps.
13 W. Lu, B. X. Mi, M. C. W. Chan, Z. Hui, C. M. Che, N. Zhu and S.
T. Lee, J. Am. Chem. Soc., 2004, 126, 4958; H. F. Xiang, S. W. Lai,
P. T. Lai, C. M. Che, in: Highly Efficient OLEDs with Phosphorescent
Materials (Ed.: H. Yersin), Wiley-VCH, Weinheim, 2008, p. 259.
14 Y. Motoyama, M. Okano, H. Narusawa, N. Makihara, K. Aoki and
H. Nishiyama, Organometallics, 2001, 20, 1580.
15 D. R. Williams, P. D. Lowder, Y.-G. Gu and D. A. Brooks, Tetrahedron
Lett., 1997, 38, 331.
16 D. J. Ca´rdenas, A. M. Echavarren and M. C. Ram´ırez de Arellano,
Organometallics, 1999, 18, 3337.
17 More recent work has demonstrated that cyclopalladated and cyclo-
platinated N,C,N-pincer complexes of related pyridine ligands can
be prepared from the debrominated ligands under different reaction
conditions, see B. Solo, S. Stoccoro, G. Minghetti, A. Zucca, M. A.
Cinellu, M. Manassero and S. Gladiali, Inorg. Chim. Acta, 2006, 359,
1879; B. Solo, S. Stoccoro, G. Minghetti, A. Zucca, M. A. Cinellu, S.
Gladiali, M. Manassero and M. Sansoni, Organometallics, 2005, 24, 53
We thank one of the referees for pointing us towards this work.
18 Details on the X-ray structures† can be obtained from the Cambridge
Crystallographic Data Centre.
19 M. Q. Slagt, G. Rodriguez, M. M. P. Grutters, R. Gebbink, W. Klopper,
L. W. Jenneskens, M. Lutz, A. L. Spek and G. van Koten, Chem.–Eur.
J., 2004, 10, 1331.
20 A. F. Rausch, L. Murphy, J. A. G. Willians and H. Yersin, Inorg.
Chem., 2009, 48, 11407; J. A. G. Williams, A. Beeby, F. S. Davies, J. A.
Weinstein and C. Wilson, Inorg. Chem., 2003, 42, 8609; S. J. Farley, D.
L. Rochester, A. L. Thompson, J. A. K. Howard and J. A. G. Williams,
Inorg. Chem., 2005, 44, 9690; Z. Wang, E. Turner, V. Mahoney, S.
Madakuni, T. Groy and J. Li, Inorg. Chem., 2010, 49, 11276.
21 H. Yersin and D. Donges, Top. Curr. Chem., 2001, 214, 81.
22 V. Fattori, J. A. G. Williams, L. Murphy, M. Cocchi and J. Kalinowski,
Photonics Nanostruct., 2008, 6, 225; M. Cocchi, J. Kalinowski, L.
Murphy, J. A. G. Williams and V. Fattori, Org. Electron., 2010, 11,
388.
Acknowledgements
This work was supported by the Alexander v. Humboldt and the
Friedrich Neumann foundation, the EU-Asia link program, the
Fonds der Chemischen Industrie, and the Bundesministerium fu¨r
Bildung und Forschung (BMBF).
23 E. M. Kober, J. V. Caspar, R. S. Lumpkin and T. J. Meyer, J. Phys.
Chem., 1986, 90, 3722; D. J. Stufkens and A. Vlcˇek Jr., Coord. Chem.
Rev., 1998, 177, 127.
Notes and references
1 D. Morales-Morales, C. M. Jensen, The Chemistry of Pincer Com-
pounds, Elsevier, Oxford, U. K., 2007; M. Albrecht and G. van Koten,
Angew. Chem., Int. Ed., 2001, 40, 5000; F. Neve, A. Crispini, C. D. Pietro
and S. Campagna, Organometallics, 2002, 21, 3511; Y. Wakatsuki,
H. Yamazaki, P. A. Grutsch, M. Santhaman and C. Kutal, J. Am.
Chem. Soc., 1985, 107, 8153; I. Aiello, D. Dattilo, M. Ghedini and A.
Golemme, J. Am. Chem. Soc., 2001, 123, 5598.
24 J. Brooks, Y. Babayan, S. Lamansky, P. I. Djurovich, I. Tsyba, R. Bau
and M. E. Thompson, Inorg. Chem., 2002, 41, 3055.
25 S. L. Murov, J. Carmichael, G. L. Hug, Handbook of Photochemistry,
2nd Ed. Marcel Dekker, New York, 1993, p. 340.
26 Cf . H. Yersin, D. Donges, W. Humbs, J. Strasser, R. Sitters and M.
Glasbeek, Inorg. Chem., 2002, 41, 4915.
2 H. Lang, R. Packheiser and B. Walfort, Organometallics, 2006, 25,
1836; M. Q. Slagt, D. A. P. van Zwieten, A. Moerkerk, R. Gebbink
and G. van Koten, Coord. Chem. Rev., 2004, 248, 2275; H. Nishiyama,
Chem. Soc. Rev., 2007, 36, 1133.
27 K. Suzuki, A. Kobayashi, S. Kaneko, K. Takehira, T. Yoshihara, H.
Ishida, Y. Shiina, S. Oishi and S. Tobita, Phys. Chem. Chem. Phys.,
2009, 11, 9850; H. Ishida, S. Tobita, Y. Haegawa, R. Katoh and K.
Nozaki, Coord. Chem. Rev., 2010, 254, 2449.
8806 | Dalton Trans., 2011, 40, 8800–8806
This journal is
The Royal Society of Chemistry 2011
©