Microwave-assisted synthesis and complexation of luminescent cyanobipyridyl-zinc(ii) bis(thiolate) complexes with intrinsic and ancillary photophysical tunability

Article abstract of DOI:10.1039/c001898h

A series of cyanobipyridine-derived zinc(ii) bis(thiolate) complexes are prepared rapidly and efficiently by a microwave-assisted cross-coupling/ complexation sequence and display luminescence that can be modulated using intrinsic functionality and ancillary ligands. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c001898h

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Microwave-assisted synthesis and complexation of luminescent
cyanobipyridyl-zinc(II) bis(thiolate) complexes with intrinsic and ancillary
photophysical tunability†
Mark C. Bagley,* Zhifan Lin and Simon J. A. Pope*
Received 16th December 2009, Accepted 4th February 2010
First published as an Advance Article on the web 12th February 2010
DOI: 10.1039/c001898h
A series of cyanobipyridine-derived zinc(II) bis(thiolate) com-
plexes are prepared rapidly and efficiently by a microwave-
assisted cross-coupling/complexation sequence and display
luminescence that can be modulated using intrinsic function-
ality and ancillary ligands.
received increasing attention in recent years,8 in particular for
transition metal-mediated processes.9 In this manuscript we report
on the photophysical properties of a series of new cyanobipyridines
prepared and modified by microwave-assisted methods for rapid
access to structural variants. The addition of a second pyridyl unit
provides the basic chromophoric framework with an additional
dimension for modulating luminescent behaviour, through com-
plexation with a metal ion. Furthermore, simply by changing
the ligands coordinating to the metal it was hoped that the
photophysical properties of these novel chromophores could be
further controlled.
The starting point for this study (Scheme 1) was cyanobipyridine
1a, prepared by our previously reported method3,10 in good
yield. This versatile precursor contains a bromide substituent
that it was envisaged could be transformed rapidly and efficiently
by cross-coupling methods in order to provide good varia-
tion in electronic properties. Two different methods for bipyri-
dine functionalization were investigated under microwave dielec-
tric heating: a Pd-mediated Suzuki cross-coupling reaction or
Cu-mediated N-arylation. Microwave irradiation of bromide
1a and phenylboronic acid at 150 ◦C for 30 min in DMSO
in the presence of Pd(PPh3)4 under basic conditions gave
(biphenyl)bipyridine 3a, whereas reaction with diphenylamine at
120 ◦C for 1 h in toluene using the pre-formed Cu(I) catalyst
Cu(neocup)(PPh3)Br11 under basic conditions, according to our
established procedure,4 gave (aminophenyl)bipyridine 3b, both
reactions proceeding in excellent yield (Scheme 1).
Convenient protocols for synthesizing functionalized chro-
mophores with tunable optical properties are highly desirable for
delivering new materials for wide-ranging applications such as
photovoltaics, OLEDs and fluorescence imaging microscopy.1,2
Recently we described the synthesis of a new series of solva-
tochromic 3-cyanopyridine-derived chromophores 1 (Fig. 1) with
visible absorption and charge transfer (CT)-based emission prop-
erties, significant Stokes shifts, excellent quantum yields and usable
nanosecond fluorescent lifetimes.3 Our route to this motif was
both rapid and efficient and employed a novel microwave-assisted
tandem oxidation/Bohlmann-Rahtz heteroannulation to establish
the central tetrasubstituted pyridine. Furthermore, incorporated
into this scaffold was capability for two-dimensional tunability
through modification of substituents, and this was varied in
order to modulate photophysical behaviour. Further application
of this work provided a series of highly-fluorescent p-extended tri-
arylpyrimidines 2 (Fig. 1) that display solvent-dependent emission
wavelengths.4 These reports demonstrate that the photophysical
properties of chromophoric frameworks can be readily tuned via
control of electronic properties by structural modification in order
to develop materials with unusual optical properties.5-7
Fig. 1 Electro-luminescent frameworks with tunable photophysical and
solvatochromic properties prepared by microwave-assisted synthesis.
Central to our approach has been the use of controlled
microwave dielectric heating in order to gain rapid access to
compounds for study. Applications of microwave technology have
School of Chemistry, Main Building, Cardiff University, Park Place, Cardiff,
UK CF10 3AT. E-mail: Bagleymc@cardiff.ac.uk, Popesj@cardiff.ac.uk;
Fax: +44(0)29 2087 4030; Tel: +44(0)29 2087 4029
† Electronic supplementary information (ESI) available: Details of ex-
perimental procedures, characterization data and photophysical measure-
ments. See DOI: 10.1039/c001898h
Scheme 1 Synthesis of zinc bipyridine complexes 4a-f and 5. Reagents
& conditions: (i) PhB(OH)2, Pd(PPh3)4 (10 mol%), Na2CO3, DMSO,
microwaves, 150 ◦C, 30 min; (iv) diphenylamine, Cu(neocup)(PPh3)Br
(10 mol%), tBuOK, PhMe, microwaves, 120 ◦C, 1 h; (iii) Zn(ClO4)2·6H2O,
CH2Cl2, SiC PHE, microwaves, 120 ◦C, 10 min.
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Table 1 Conditions for the synthesis of ZnII complex 4a
Table 2 Synthesis of ZnII complexes 4a-f under microwave dielectric
heating and at reflux
Entry
Conditionsa
Yield%b
Yield% aat
reflux
Microwave
Yield%b
25 ◦C, 24 h
36
44
82
94
Entry
3
4
R
Ar
1
2
3
4
Conductive heating, reflux, 24 h
Microwaves, 120 ◦C, 10 minc
Microwaves, SiC passive heating element, 120 ◦C,
10 min
1
2
3
4
5
6
a
a
a
b
b
b
a
b
c
d
e
f
Ph
Ph
Ph
NPh2
NPh2
NPh2
Ph
44
52
50
54
48
56
94
97
96
96
95
98
4-MeOC6H4
2-naphthyl
Ph
4-MeOC6H4
2-naphthyl
a Microwaves denotes microwave irradiation in a sealed tube at the given
temperature, monitored by the in-built IR sensor, using a CEM DiscoverꢀR
single-mode microwave synthesizer by moderating the initial microwave
power (150 W). b Isolated yield. c Set temperature; recorded temperature
was actually lower (see Fig. 2).
a Isolated yield of 4a-f after heating a solution of 3a,b, the corresponding
thiol (2 equiv) and Zn(OAc)2 (1 equiv) in EtOH at reflux for 24 h;
b Isolated yield of 4a-f after microwave irradiation of a solution of 3a,b, the
corresponding thiol (2 equiv) and Zn(OAc)2 (1 equiv) in EtOH using a SiC
passive heating element in a sealed tube at 120 ◦C (measured by the in-built
IR sensor) for 10 min with a CEM DiscoverꢀR microwave synthesizer by
moderation of the initial microwave power (150 W); all products exhibited
satisfactory characterization data by IR, 1H and 13C NMR spectroscopy,
and low and high resolution mass spectrometry.
The formation of cyanobipyridyl-zinc(II) bis(thiolate) complex
4a was first investigated by stirring bipyridine 3a, Zn(OAc)2 and
thiophenol (2 equiv) in EtOH at room temperature for 24 h
(Scheme 2, Table 1) but surprisingly only proceeded in poor yield.
Furthermore, the efficiency of this process was improved only a
little by carrying out the complexation at reflux over a prolonged
period (entry 2). The use of controlled microwave dielectric heating
to promote the formation of metallo complexes has hitherto
been reported only recently, for example in the synthesis of
RuII coordination complexes,12 despite long standing recognition
of the benefits of microwave-assisted autoclave reactions for
bipyridine complexation by Greene and Mingos.13 Gratifyingly,
under microwave dielectric heating at greatly elevated temperature
(entry 3), ZnII complex 4a was formed in high yield without the
need for further purification.
In order to investigate if this dramatic improvement in chemical
yield was attributed to Arrhenius behaviour, or whether a specific
microwave effect14 was in evidence, the reaction was repeated
under microwave irradiation in the presence of a SiC passive
heating element (PHE)15,16 (entry 4). Under these conditions, the
thermal profile of the reaction improved (Fig. 2), an observation
that we have made before in microwave-assisted tandem oxidation
processes,16 and ZnII complex 4a was formed in excellent yield.
Although firm conclusions regarding rate accelerations under
microwave dielectric heating would require much more detailed
experimentation, we believe these findings are indicative of
Arrhenius behaviour for this complexation reaction and not
specific, or indeed non-thermal, microwave effects in contrary to
other suggestions.
bis(bipyridyl) perchlorate complex 5, prepared in high yield by
microwave-mediated complexation, in order to establish the role
of the metal, intrinsic functionality and the ancillary ligands upon
CT character. In so doing, the high efficiency of both microwave-
assisted complexation procedures was established by comparison
with conductive heating methods. Microwave irradiation of bipyri-
dine 3a,b, Zn(OAc)2 (1 equiv) and the corresponding thiophenol
(2 equiv) in EtOH at 120 ◦C in a sealed tube in the presence of a
SiC passive heating element (Scheme 1) gave a series of complexes
4a-f suitable for photophysical study in excellent (94-98%) yield
(Table 2); far in excess of the chemical yield at reflux in EtOH
after 24 h. Similarly, irradiation of bipyridine 3b (2 equiv) and
zinc perchlorate in CH2Cl2 at 120 ◦C gave complex 5 in excellent
yield (96%) after 10 min whereas a known conductive heating
method17 was poorly efficient (42% yield after 24 h).
Electronic absorption spectra were obtained for each of the
complexes in chloroform (10-6 M). Comparison with the free
ligands 3a-b revealed the presence of ligand-centred transitions
together with an additional broadened low-energy shoulder at ca.
390 (4a-c) and ca. 485 nm (4d-f). In accordance with previous
reports this transition was assigned to an inter-ligand thiolate-
to-bipyridine charge transfer (LLCT),18 further evidenced by a
subtle wavelength dependence upon the electron donating ability
of the coordinated thiolate.19 For 4d-f the LLCT band was red-
shifted, presumably as a consequence of the greater dipolar CT
character of the parent bipyridine (3b). Further confirmation was
provided with the homoleptic complex 5, which gave an absorption
spectrum lacking the LLCT feature at ca. 490 nm.
The emission characteristics of the Zn(II) complexes (see Table 3)
were probed in both solid and solution-states. Room temperature
measurements on solid samples showed a featureless fluorescence
band (< 1 ns in all cases) in the visible region, which was
independent of excitation wavelength and was therefore attributed
to emission from a LLCT excited state. However, in chloroform
solution each of the complexes’ emission profiles were dependent
upon the wavelength of excitation: for 4a-c two emission bands at
ca. 400 and ca. 510 nm resulted from excitation at 330 and 400 nm
respectively (Fig. 3 clearly shows the emission profile dependence
of 4a upon variable excitation wavelength). The corresponding
excitation spectra showed that the longer wavelength emission was
Fig. 2 Heating profile under microwave irradiation in the presence
(orange) or absence (black) of a SiC passive heating element.
A series of bipyridyl-zinc(II) bis(thiolate) complexes 4a-f were
prepared using this approach and compared with the zinc(II)
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Table 3 Absorption and photophysical properties of complexes 4a-f
Entry
l4
labs (loge)/nma
l
em/nmb
l
em/nma (t/ns)
l
em/nma (t/ns)
(f)a,g
1
2
3
4
5
6
a
b
c
d
e
f
325 (4.37), 390 (4.03)
322 (4.40), 392 (3.96)
328 (4.32), 388 (4.06)
392 (4.18), 486 (3.86)
390 (4.20), 490 (3.78)
394 (4.14), 484 (3.90)
584
625
512
590
615
627
410c (2.7)
402c (2.5)
398c (1.6)
540d (3.9)
539d (3.1)
524d (6.9)
520e (1.6)
510e (2.6)
512e (7.2)
604f (6.5)
618f (7.6)
612f (< 1)
0.24
0.2
0.28
0.05
0.01
0.08
a aerated CHCl3 solution; b solid; c lex = 320 nm; d lex = 390 nm; e lex = 390 nm; f lex = 500 nm; g obtained for LLCT emission.
the microwave-assisted complexation of cyanobipyridine 3a-b,
a thiophenol and Zn(OAc)2 using a passive heating element.
The complexes display LLCT fluorescence in the solid-state,
but tunable dual emission in chloroform, arising from co-
emissive excited states. In general, the luminescence from the
complexes can be tuned through changes in bipyridine function-
ality and subtly modulated by changes in the ancillary thiolate
co-ligands.
We thank EPSRC for funding together with Cardiff University,
Professor C. Oliver Kappe for helpful discussions and Steve Singh
at Anton Paar Ltd (Hertford, UK) for the provision of the SiC
passive heating elements.
Fig. 3 Main: Emission profiles for 4a in CHCl3 (lex = 330 (blue), 350
(pink), 370 (yellow), 390 (turquoise), 400 (purple) and 410 (brown) nm.
Inset: excitation spectra for 4a lem = 410 nm (grey) and 520 nm (black).
Notes and references
1 (a) D. R. Kanis, M. A. Ratner and T. J. Marks, Chem. Rev., 1994, 94,
195; (b) M. M. M. Raposo, A. M. R. C. Sousa, G. Kirsch, P. Cardoso,
M. Belsley, E. D-, M. Gomes and A. M. C. Fonseca, Org. Lett., 2006,
8, 3681.
2 (a) A. D’Aleo, G. Pompidor, B. Elena, J. Vicat, P. L. Baldeck, L. Toupet,
R. Kahn, C. Andraud and O. Mauryr, ChemPhysChem, 2007, 8, 2125;
(b) J. H. Yu, D. Parker, R. Pal, R. A. Poole and M. J. Cann, J. Am.
Chem. Soc., 2006, 128, 2294.
3 M. C. Bagley, Z. Lin and S. J. A. Pope, Chem. Commun., 2009, 5165.
4 M. C. Bagley, Z. Lin and S. J. A. Pope, Tetrahedron Lett., 2009, 50,
6818.
5 (a) V. P. Rao, A. K-, Y. Jen and J. B. Caldwell, Tetrahedron Lett., 1994,
35, 3849; (b) A. Facchetti, L. Beverina, M. E. Van-der Boom, E. G.
Dutta, G. A. Pagani and T. J. Marks, J. Am. Chem. Soc., 2006, 128,
2142; and references cited therein.
6 Z. R. Grabowski, K. Rotkiewicz and W. Rettig, Chem. Rev., 2003, 103,
3899.
7 B. C. Thompson, K. A. About, J. R. Reynolds, K. Nakatani and
P. Audebert, New J. Chem., 2005, 29, 1128.
8 (a) S. Caddick and R. Fitzmaurice, Tetrahedron, 2009, 65, 3325; (b) C. O.
Kappe, Chem. Soc. Rev., 2008, 37, 1127; (c) C. O. Kappe, Angew. Chem.,
Int. Ed., 2004, 43, 6250; (d) C. O. Kappe, A. Stadler, Microwaves in
Organic and Medicinal ChemistryWiley-VCH: Weinheim, 2005.
9 P. Nilsson, K. Olofsson and M. Larhed, Top. Curr. Chem., 2006, 266,
103.
10 A. L. K. Shi Shun, E. T. Chernick, S. Eisler and R. R. Tykwinski,
J. Org. Chem., 2003, 68, 1339.
11 D. Venkataraman, R. K. Gujadhur and C. G. Bates, Org. Lett., 2001,
3, 4315.
associated with a unique excitation peak at ca. 400 nm, consistent
with the LLCT assignment in the corresponding absorption
spectra. Similar observations were noted for compounds 4d-f,
although with notable red-shifts in all cases. Time-resolved
luminescence lifetime measurements showed that each of the
emission peaks were relatively short-lived (< 10 ns) and indicative
of a fluorescence in each case, whilst the quantum yields of the low
energy emissions are significantly reduced for 4d-f.
Taken together with the data obtained for the corresponding free
ligands 3a-b, and the solid-state fluorescence (assigned to a LLCT
emitting state), these results indicate that the longer wavelength
emission in chloroform solution can again be attributed to the
LLCT excited state, but that the shorter wavelength band probably
arises from an intra-ligand charge transfer (ILCT) associated with
the cyanobipyridine. In this context, the extent of restricted motion
of the coordinated thiol can be invoked to explain disparities in
single (LLCT) or dual emission.20
The solvatochromic behaviour of 4f was probed and revealed
that the emission in acetonitrile (lem = 573 nm) and DMSO
(lem = 593 nm) was independent of excitation wavelength (390,
450 or 500 nm). A comparison with the free ligand 3b (lem
=
12 (a) T. J. Anderson, J. R. Scott, F. Millett and B. Durham, Inorg. Chem.,
2006, 45, 3843; (b) Y. Sun, M. L. Machala and F. N. Castellano, Inorg.
Chim. Acta, 2010, 363, 283; and references cited therein.
13 D. L. Greene and D. M. P. Mingos, Transition Met. Chem., 1991, 16,
71.
576 nm (MeCN); 580 nm (DMSO)) suggests that in polar
solvent the red-shifted emission of 4f is likely to be domi-
nated by (Zn(II)-perturbed) IL(bpy)CT character. This observation
is consistent with our previous studies into solvatochromic
cyanopyridines3 and related reports in the literature into classical
N,N-dimethylaminobenzonitrile (DMABN) species.6
´
14 A. De la Hoz, A. D´ıaz-Ortiz and A. Moreno, Chem. Soc. Rev., 2005,
34, 164.
15 For the use of passive heating elements in microwave-assisted organic
synthesis, see reference 16 and the following: (a) R. Hoogenboom,
T. F. A. Wilms, T. Erdmenger and U. S. Schubert, Aust. J. Chem., 2009,
In conclusion, a series of cyanobipyridine-derived Zn(II) bisthi-
olate complexes can be prepared rapidly and in high yield by
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62, 236; (b) T. Razzaq, J. M. Kremsner and C. O. Kappe, J. Org. Chem.,
2008, 73, 6321; (c) J. M. Kremsner and C. O. Kappe, J. Org. Chem.,
2006, 71, 4651.
18 K. A. Truesdell and G. A. Crosby, J. Am. Chem. Soc., 1985, 107, 1787;
V. J. Koester, Chem. Phys. Lett., 1975, 32, 575.
19 A. M. Galin, Y. V. Razskazovsky and M. Y. Mel’nikov, J. Photochem.
Photobiol., A, 1993, 72, 35; T. W. Ngan, C. C. Ko, N. Zhu and V. W. W.
Yam, Inorg. Chem., 2007, 46, 1144.
16 M. C. Bagley, Z. Lin, D. J. Phillips and A. G. Graham, Tetrahedron
Lett., 2009, 50, 6823.
17 W.-F. Fu, Q.-Q. Xu, D.-H. Wang, S.-M. Chi and X. Gan, Inorg. Chim.
Acta, 2009, 2529.
20 P. J. Gronland, J. A. Burt and W. F. Wacholtz, Inorg. Chim. Acta, 1995,
234, 13.
3166 | Dalton Trans., 2010, 39, 3163–3166
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