Synthesis, structures, and photoluminescent properties of cyclometalated platinum(II) complexes bearing upper-rim phosphinated calix[4]arenes

Article abstract of DOI:10.1021/om800969z

A series of upper-rim diphosphinated calixarene receptors bearing various R and R' substituents at the lower rim (Lⁿ; n = 1-5) were synthesized, and their reactions with [Pt(Thpy)(HThpy)Cl] afforded the phosphorescent platinum(II)-modified cali

Full text of DOI:10.1021/om800969z

34
Organometallics 2009, 28, 34–37
Communications
Synthesis, Structures, and Photoluminescent Properties of
Cyclometalated Platinum(II) Complexes bearing Upper-Rim
Phosphinated Calix[4]arenes
Siu-Wai Lai,* Queenie K.-W. Chan, Jie Han, Yong-Gang Zhi, Nianyong Zhu, and
Chi-Ming Che*
Department of Chemistry, HKU-CAS Joint Laboratory on New Materials, and Open Laboratory of
Chemical Biology of the Institute of Molecular Technology for Drug DiscoVery and Synthesis,
The UniVersity of Hong Kong, Pokfulam Road, Hong Kong SAR, People’s Republic of China
ReceiVed October 9, 2008
to be incorporated into the calixarene systems. Signaling re-
sponses toward targeted guests involving calixarene receptors
are commonly analyzed by NMR titration experiments5 and
electrochemistry,6 yet reporting means using emission measure-
ments have predominantly focused on [Ru(bpy)3]2+-bearing
molecules.7,8 Examples of optical sensors with luminescent
metal complexes integrated into the framework of calixarenes
are sparse.2d,9
Summary: A series of upper-rim diphosphinated calixarene
receptors bearing Various R and R′ substituents at the lower
rim (Ln; n ) 1-5) were synthesized, and their reactions with
[Pt(Thpy)(HThpy)Cl] afforded the phosphorescent platinum(II)-
modified calixarene receptors [(PtThpyCl)2Ln]. The structure
of [{PtThpy(CH3CN)}2L1](ClO4)2 and [(PtThpyCl)2L5] were
determined by X-ray crystallographic analysis, and the photo-
physical properties of [(PtThpyCl)2Ln] were inVestigated.
The square-planar geometry of platinum(II) complexes con-
fers photophysical and photochemical properties that are dif-
ferent from those of octahedral [RuII(bpy)3]2+ derivatives due
to the open Pt(II) coordination sites. Intrusion of guest molecules
induces changes in its local environment, leading to a profound
impact upon the photoluminescent properties.10 There is a
growing interest in employing Pt(II) luminophores as a signaling
moiety in the development of biomolecular chemosensors.
Previous reports on cyclometalated platinum(II) complexes
bearing Thpy ligands (HThpy ) 2-(2′-thienyl)pyridine) revealed
that they exhibit intriguing photoluminescent properties (visible
emission, long emission lifetime) which are sensitive to the local
environment.11 These properties are advantageous to the de-
velopment of luminescent sensors.
Calixarenes1 have been extensively documented as ion
carriers, chemical sensors, and models for in vivo reactions of
enzymes.2 Functionalization at the upper rim of calixarenes has
expanded this class of molecules into transition-metal-bearing
derivatives through metal-ligand coordination,3,4 which enables
the signaling properties conferred by transition-metal complexes
* To whom correspondence should be addressed. E-mail: swlai@hku.hk
(S.-W.L.); cmche@hku.hk (C.-M.C.). Fax: (+852) 2857 1586.
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Cambridge, U.K., 1989. (b) Gutsche, C. D. In Calixarenes ReVisited,
Monographs in Supramolecular Chemistry; Stoddart, J. F., Ed.; The Royal
Society of Chemistry: Cambridge, U.K., 1998. (c) Calixarenes 2001; Asfari,
Z., Bo¨hmer, V., Harrowfield, J., Vicens, J., Eds.; Kluwer Academic:
Dordrecht, The Netherlands, 2001.
(2) (a) Ikeda, A.; Shinkai, S. Chem. ReV. 1997, 97, 1713–1734. (b)
Molenveld, P.; Engbersen, J. F. J.; Reinhoudt, D. N. Chem. Soc. ReV. 2000,
29, 75–86. (c) Bakirci, H.; Koner, A. L.; Dickman, M. H.; Kortz, U.; Nau,
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10.1021/om800969z CCC: $40.75
2009 American Chemical Society
Publication on Web 12/08/2008

Communications
Organometallics, Vol. 28, No. 1, 2009 35
Scheme 1. Syntheses of L1-5
Scheme 2. Synthesis of [Pt(Thpy)]+-Incorporated Diphosphinated Calixarene Receptors, 1-5
In this work, a series of upper-rim diphosphinated calixarene
receptors bearing various R and R′ substituents at the lower
rim were synthesized (Scheme 1), and their subsequent reactions
with [Pt(Thpy)(HThpy)Cl] resulted in coordination of cyclom-
etalated Pt(II) luminophores, [Pt(Thpy)]+, at the upper rim of
the calixarenes, giving phosphorescent platinum(II)-modified
calixarene receptors (Scheme 2).
25,26,27,28-Tetrahydroxycalix[4]arene was prepared by the
reaction of p-tert-butylcalix[4]arene with phenol and AlCl3 (step i
in Scheme 1),12 and its reaction with benzyl bromide, 1-
iodopropane, or 1-iodobutane (R-Br; step ii in Scheme 1) in
the presence of 1 equiv of K2CO3 in acetonitrile afforded
dibenzyloxy- (LBn), dipropoxy- (LC3), or dibutoxycalix[4]arene
(LC4), respectively.13 Bromination was carried out at the para
positions of hydroxylated aromatic rings using N-bromosuc-
cinimide (NBS) and methyl ethyl ketone (MEK) (step iii in
Scheme 1).14 This was followed by further reaction with benzyl
or alkyl bromide (R′-Br; step iv in Scheme 1) to afford various
R and R′ substituents at the lower rim of the calixarene ligand.15
Phosphination using Ph2PCl at the upper rim of the calixarenes
afforded L1-5 (step v in Scheme 1).15
The reaction of L1-L5 with [Pt(Thpy)(HThpy)Cl]11b in a 1:1
molar ratio in acetonitrile under reflux for 4 h gave
[(PtThpyCl)2Ln] (1-5), which are isolated as orange solids in
moderate to high yields (Scheme 2). Complexes 1-5 were
characterized using 1H, 195Pt, 13C{1H}, and 31P{1H} NMR
spectroscopy, ESI or FAB mass spectrometry, and X-ray
crystallography (Supporting Information). They are air- and
moisture-stable at room temperature.
1
The H NMR spectra of 1-5 show signals consistent with
their chemical formulation; the peak integration shows that the
1
ratio of hydrogens for [Pt(Thpy)]+:Ln is 2:1. The H NMR
spectra feature characteristic doublets at δ 2.87-3.05 and
4.15-4.40 due to the AB spin system of ArCH2Ar in the
calixarene ligand. Typically broad 195Pt satellites at ∼9.61 ppm
were observed for 1-5. The 31P{1H} NMR spectra of 1-5
reveal one singlet at δ 17.43-18.83 with 195Pt satellites due to
one-bond coupling (1JPtP ) 4226-4232 Hz). This is comparable
to that of [Pt(Thpy)PPh3Cl] (Supporting Information) at δ 18.62
1
with JPtP ) 4236 Hz, revealing that the phosphorus atoms in
[(PtThpyCl)2L1-5] and [Pt(Thpy)PPh3Cl] are in similar environ-
ments. The 195Pt NMR spectra of 1-5 exhibit a doublet of peaks
at ca. δ -4250 with 1JPPt coupling constants of 4220-4232 Hz.
The 13C{1H} NMR spectra of 1-5 are well resolved with peaks
at δ 117.0-161.0 corresponding to the aryl protons in the
[Pt(Thpy)]+ and Ln (n ) 1-5) moieties. Their ESI or FAB
mass spectra show the [(PtThpy)2LnCl]+ molecular ion. The
(12) Percec, V.; Bera, T. K.; De, B. B.; Sanai, Y.; Smith, J.; Holerca,
M. N.; Barboiu, B.; Grubbs, R. B.; Fre´chet, J. M. J. J. Org. Chem. 2001,
66, 2104–2117.
(13) van Loon, J.-D.; Arduini, A.; Coppi, L.; Verboom, W.; Pochini,
A.; Ungaro, R.; Harkema, S.; Reinhoudt, D. N. J. Org. Chem. 1990, 55,
5639–5646.
(14) Shimizu, S.; Shirakawa, S.; Sasaki, Y.; Hirai, C. Angew. Chem.,
Int. Ed. 2000, 39, 1256–1259.
(15) Takenaka, K.; Obora, Y.; Jiang, L. H.; Tsuji, Y. Organometallics
2002, 21, 1158–1166.
Figure 1. Perspective view of [(PtThpyCl)2L5] (30% probability
ellipsoids).

36 Organometallics, Vol. 28, No. 1, 2009
Communications
Figure 2. Perspective view of the cation in [{PtThpy(CH3CN)}2L1](ClO4)2 (30% probability ellipsoids).
high-resolution ESI-MS of [(PtThpyCl)2L4] in acetonitrile
reveals peak clusters centered at m/z 1706.4 corresponding
to [(PtThpy)2L4Cl]+, plus additional peaks centered at m/z
856.2 with a peak separation of 0.5 mass unit, which is
attributed to [(PtThpy)2L4(CH3CN)]2+ (Figure S2 in the Sup-
porting Information).
Crystals of [(PtThpyCl)2L5] were obtained by layering of
n-hexane into a dichloromethane solution. However, upon
layering of n-hexane into a dichloromethane solution of
[(PtThpyCl)2L1] in the presence of LiClO4 in acetonitrile,
crystals of [{PtThpy(CH3CN)}2L1](ClO4)2 were obtained. As
depicted in the crystal structures (Figures 1 and 2), the
platinum atom adopts an approximately square planar ge-
ometry. The N-Pt-C angle of ∼80.6° is typical for related
complexes.11b The Pt-N (2.087-2.124 Å) and Pt-C (∼2.01
Å) distances of the [Pt(Thpy)]+ units agree with those
reported for [Pt(Thpy)PPh3Cl] (2.118(6) and 1.987(8) Å,
respectively). For [{PtThpy(CH3CN)}2L1]2+, the Nt CCH3
groups are nearly linear (Pt-N-C ) 174°; N-C-C ) 178°),
whereas the Pt-NCCH3 distances of 2.069(9) and 2.094(8)
Å are slightly longer than that of 1.996(14) Å in [Pt-
(trpy)MeCN]2+ (trpy ) 2,2′:6′,2′′-terpyridine),16 due to the
trans effect of the thienyl carbanion (Figure 2). The
Pt-P-C(calixarene) angles of 111.3(3)-119.8(1)° allow
good separation between the two phenyl rings in PPh2 and
the [Pt(Thpy)]+ moiety, in order to avoid steric repulsion.
The bond lengths and angles are consistent with those
observed for metal complexes featuring such calix[4]arene
scaffolds.15 The four n-BuO (Figure 1) or PhH2CO moieties
(Figure 2) remain at the lower rim to afford modified
calixarenes in the cone conformation, and the four oxygen
atoms are directed toward the cavity of the calixarenes. There
are weak intermolecular interactions including PhCH2 · · ·
CH(Thpy) (2.698-2.886 Å), (L5)Ar2CH2 · · · CH(Ph2P) (2.771
Å), and (L5)HC · · · HC(Ph2P) (2.852 Å) in the crystal structure
of [(PtThpyCl)2L5] (Figure 1). Similarly, there are intermo-
lecular interactions between the [{PtThpy(CH3CN)}2L1]2+
cations as well as between the [{PtThpy(CH3CN)}2L1]2+ and
perchlorate ions in the crystal structure of [{PtThpy-
(CH3CN)}2L1](ClO4)2. These weak interactions include
CH3CN · · · HC(Ph2P) (2.627 Å), NCCH3 · · · OClO3 (2.648 Å),
PhCH2 · · · OClO3 (2.497 Å), and (pyridyl)CH · · · OClO3 (2.486 Å).
The UV-visible absorption and emission properties of 1-5
have been studied and are listed in the Supporting Information.
As examples, the absorption spectra of 3 and 4 in acetonitrile
are depicted in Figures 3 and S3 (Supporting Information),
Figure 3. Absorption (left) and emission (right; λex ) 400 nm,
concentration 1.55 × 10-5 M) spectra of 3 in acetonitrile at 298 K.
respectively. The high-energy, intense absorption bands with
λmax at 295-300 nm (ε ) 23 800-29 400 M-1 cm-1) are
attributed to spin-allowed intraligand 1IL: π(Thpy) f π*(Thpy)
and π(Ln) f π*(Ln) transitions. The moderately intense low-
energy band with λmax at 402-406 nm (ε ) 8200-9600 M-1
cm-1) is assigned to spin-allowed metal-to-ligand charge transfer
1MLCT: Pt(5d) f π*(Thpy) transition,11b which closely re-
1
sembles the MLCT absorption of [Pt(Thpy)PPh3Cl] with λmax
at 398 nm (ε ) 3800 M-1 cm-1). Solvatochromic effects for
the absorption spectrum of 4 were investigated. The high-energy
absorption maximum slightly shifts from 293 nm (ε ) 29 100
M-1 cm-1) in MeOH to 302 nm (ε ) 31 600 M-1 cm-1) in
THF, whereas the low-energy band with λmax at 398 nm (ε )
10 000 M-1 cm-1) in MeOH is red-shifted to 402 nm (ε ) 9400
M-1 cm-1) in CH3CN and 409 nm (ε ) 9900 M-1 cm-1) in
THF.
The emission spectra of 3 and 4 in acetonitrile at 298 K
exhibit a low-energy structured emission with peak maxima at
∼558 and 602 nm (Figures 3 and S3). The microsecond
emission lifetimes (∼1.20-1.43 µs) of 1-5 in dichloromethane
solutions indicate the emissions to be phosphorescence. Similar
emission properties of analogous [Pt(Thpy)]+ complexes11b and
of [Pt(Thpy)PPh3Cl]17 have been reported. In comparison with
the absorption data, solvatochromic effects on the emission
energy of 4 is minor, as the emission λmax does not alter upon
(16) Bu¨chner, R.; Field, J. S.; Haines, R. J.; Cunningham, C. T.;
McMillin, D. R. Inorg. Chem. 1997, 36, 3952–3956.
(17) Kulikova, M. V.; Balashev, K. P.; Kvam, P. I.; Songstad, J. Russ.
J. Gen. Chem. 1999, 69, 1521–1527.

Communications
Organometallics, Vol. 28, No. 1, 2009 37
changing the solvent from dichloromethane to acetonitrile.
Instead, the lifetime increases from 0.77 µs in CH3CN to 2.09
µs in DMSO, and the emission quantum yield increases from
0.0074 in CH3CN to 0.045 in CH3OH. The emission data reveal
that the emissive excited state is largely 3IL in nature, although
energy of the d-d excited state, which consequently leads
to increases in the emission intensity and lifetime.
In conclusion, we have prepared phosphorescent platinu-
m(II)-modified calixarene receptors by coordination of [Pt-
(Thpy)]+ moieties by the pendant phosphine groups at the
upper rim of calix[4]arenes. The electronic and spectroscopic
properties of the [Pt(Thpy)]+ units can be systematically
varied by changing the bidentate cyclometalated ligands.
Furthermore, the chloride and acetonitrile ligands in [(Pt-
ThpyCl)2L5] and [{PtThpy(CH3CN)}2L1](ClO4)2, respectively,
would allow modification of the platinum(II)-incorporated
calixarene receptors by ligand substitution reactions. All these
3
mixing with MLCT cannot be excluded.
The solid-state emission spectra of 1-5 at 298 K show
structured emission bands with peaks at ∼559, 580, and 603
nm, which become more resolved with slightly blue-shifted
λmax to 557 nm upon cooling to 77 K. Importantly, the emission
lifetimes of solid samples of 1-5 significantly increase to
13.9-21.2 µs at 77 K. The emission spectra of 1-5 in
butyronitrile glassy solution show vibronically resolved peaks
at 550-655 nm and emission lifetimes of 42.1-44.1 µs. The
significant temperature dependence for the emission lifetimes
of solid samples of 1-5 is consistent with the d-d excited states
being responsible for the nonradiative decay of the emission.
To examine the effects of auxiliary ligands upon the
emission properties of [PtThpy]+ luminophores, we synthe-
sized [Pt(Thpy)PPh3(CH3CN)]ClO4 (6) by reacting [Pt(Thpy)-
PPh3Cl] with AgClO4 in acetonitrile. The low-energy 1MLCT
absorption band of 6 at 389 nm (ε ) 2400 M-1 cm-1) is
blue-shifted from that (398 nm; ε ) 3800 M-1 cm-1) in
[Pt(Thpy)PPh3Cl] (Supporting Information). Interestingly, the
emission λmax values of [Pt(Thpy)PPh3Cl] and 6 in acetonitrile
solutions are virtually the same. However, both the emission
quantum yield and lifetime of 6 remarkably increase to 0.17
and 26.2 µs, respectively, from [Pt(Thpy)PPh3Cl] (Φ ) 0.020;
τ ) 6.8 µs). Similar phenomena were observed when
[(PtThpyCl)2L4] (4) was treated with AgClO4 in acetonitrile;
3
can significantly affect the mixed MLCT/3IL emissive ex-
cited states, together with the deactivating d-d states, leading
to a new class of luminescent calix[4]arene-based sensory
molecules. Our studies on the binding reactions of
[(PtThpyCl)2Ln] with neutral and anionic hydrophobic ana-
lytes are in progress.
Acknowledgment. We are grateful for financial support
from the Research Grant Council of the Hong Kong SAR,
People’s Republic of China (HKU 7030/06P), The University
of Hong Kong (URC-administered Seed Funding Grant
200411159082), and the National Natural Science Foundation
of China/Research Grants Council Joint Research Scheme
(N_HKU 752/08). We thank Dr. Kwan-Ming Ng for per-
forming high-resolution ESI-MS experiments and for helpful
discussions.
1
namely, a blue-shifted MLCT absorption band (402 to 390
Supporting Information Available: Text, figures, tables, and
CIF files giving experimental details, spectroscopic data, crystal-
lographic data, and selected photophysical spectra. This material
is available free of charge via the Internet at http://pubs.acs.org.
nm) and substantial enhancement in emission quantum yield
(0.0074 to 0.21) and lifetime (0.77 to 28.9 µs) were detected.
We postulate that the increased cationic charge on the
[Pt(Thpy)PPh3]+ luminophore, upon replacing the coordinated
chloride ligand by acetonitrile, causes an increase in the
OM800969Z