The first example of a Pt...Pt interaction in platinum(II) complexes bearing bulky tri-tert-butyl-2,2′:6′,2″-terpyridine pendants via conformational control of the calix[4]arene moiety

Article abstract of DOI:10.1039/b712645j

An unprecedented short Pt...Pt contact between sterically bulky Pt(^tBu₃trpy) alkynyl moieties has been observed in the X-ray crystal structure of a dinuclear platinum(ii) complex bridged by a diethynylcalix[4]arene derivative; the complex in its crystalline state showed a red shift in the emission maxima at 298 K and 77 K relative to its powder form, which has been attributed to the presence of a metal-metal interaction in the crystal lattice. The Royal Society of Chemistry.

Full text of DOI:10.1039/b712645j

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The first example of a Pt · · · Pt interaction in platinum(II) complexes bearing
bulky tri-tert-butyl-2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridine pendants via conformational control
of the calix[4]arene moiety†‡
Hiu-Suet Lo, Sung-Kong Yip, Nianyong Zhu and Vivian Wing-Wah Yam*
Received 16th August 2007, Accepted 21st August 2007
First published as an Advance Article on the web 6th September 2007
DOI: 10.1039/b712645j
An unprecedented short Pt · · · Pt contact between sterically
bulky Pt(tBu3trpy) alkynyl moieties has been observed in the
X-ray crystal structure of a dinuclear platinum(II) complex
bridged by a diethynylcalix[4]arene derivative; the complex in
its crystalline state showed a red shift in the emission maxima
at 298 K and 77 K relative to its powder form, which has been
attributed to the presence of a metal–metal interaction in the
crystal lattice.
been commonly observed in platinum(II) terpyridyl complexes
with no or bulk-free substituents, as it is believed that the
presence of bulky groups on the terpyridyl ligands will hinder the
molecules from coming into close proximity with each other.7c,e,f ,h
Recently, we showed that utilization of calix[4]arene-containing
ligands in a pre-determined and pre-organised geometry could
generate highly novel tetranuclear gold(I) assemblies with short
Au · · · Au contacts.4d We believed that by using appropriately
designed calix[4]arene in a pre-organised geometry as the clipping
ligand, two platinum metal centres can be brought together at
the lower rim to give rise to a short Pt · · · Pt contact. Herein, we
report the synthesis and structural characterisation of dinuclear
platinum(II)–alkynylcalix[4]arene assemblies and demonstrate the
existence of a short Pt · · · Pt contact for the very first time in
platinum(II) terpyridyl complexes bearing sterically-demanding
tri-tert-butyl groups; their photophysical properties associated
with metal · · · metal and p · · · p interactions will also be discussed.
Complexes 1 and 2 were synthesised according to a modification
of a literature procedure for the synthesis of platinum(II) terpyridyl
alkynyl complexes,7g in which [Pt(tBu3trpy)(MeCN)](OTf)2 and
[Pt(trpy)(MeCN)](OTf)2, respectively, were allowed to react with
the calix[4]arene-containing bis-alkynyl ligand, L in MeOH in the
presence of KF (Scheme 1). Subsequent diffusion of diethyl ether
vapour into a dichloromethane solution of 1 and an acetonitrile
solution of 2 gave the corresponding complexes as orange-red
crystals and a dark-red solid; the crystals of 1 obtained were
suitable for crystallographic analysis.§
Over the past decade, the design and construction of discrete
functional supramolecular species has been an area of intense
interest.1 Calix[n]arenes, in addition to their well known ability
to serve as ion receptors,2 are one of the most important
building blocks in supramolecular chemistry and an attractive
candidate for molecular design strategies, owing to their unique
molecular structures, simple one-pot preparations, easy chemical
transformability on the lower and upper rims, as well as their
“tunable” molecular shapes and conformations.3 By combining
all these intrinsic properties, calix[4]arenes may serve as versatile
ligands for the construction of luminescent transition-metal
alkynyl supramolecular assemblies.4 Since d8–d8 metal complexes
with well defined metal–metal distances are well-known to exhibit
unique spectroscopic and luminescence features associated with
these metal–metal interactions,5–7 the square-planar coordination
geometry of d8 platinum(II) polypyridyl complexes has therefore
attracted a lot of interest and led to their increasing utilization as
versatile building blocks for self-assembly.6,7a–c,e,f
Recently, our group has synthesised a number of platinum(II)
terpyridyl alkynyl complexes which show drastic colour changes
and luminescence enhancement upon intermolecular aggregation
and oligomerisation.7a–e Intramolecular self-association has also
been demonstrated in a dinuclear platinum(II) terpyridyl system.7f
The reason behind these remarkable spectroscopic changes is
suggested to be a result of the formation of inter- or intra-
molecular Pt · · · Pt and p · · · p interactions in solution induced by
nucleic acids,7a polymers,7b gelation,7c solvents7d,e or intramolecu-
lar self-association7f of the metal complexes, leading to a change
in its conformation and microenvironment. These changes have
Centre for Carbon-Rich Molecular and Nano-Scale Metal-Based Materials
Research, and Department of Chemistry, The University of Hong Kong,
Pokfulam Road, Hong Kong, P. R. China. E-mail: wwyam@hku.hk;
Fax: +852 2857-1586; Tel: +852 2859-2153
† CCDC reference number 654512. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b712645j
‡ Electronic supplementary information (ESI) available: Detailed char-
acterisation, UV-vis absorption spectra and emission spectra. See DOI:
10.1039/b712645j
Scheme 1
4386 | Dalton Trans., 2007, 4386–4389
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Fig. 1 depicts the structure of the complex cation of 1. The
calix[4]arene adopts the “pinch-cone” conformation, which can
be clearly revealed in the structure where the aryl rings substituted
with ester groups within the calix[4]arene moieties are placed
almost parallel to each other (interplanar angle 9.08◦), and are
tilted so as to increase the separation between the two tert-butyl
pyridyl ligands (N(1)–Pt(1)–N(3) 161.5–161.9◦; N(1)–Pt(1)–N(2)
and N(2)–Pt(1)–N(3) 80.2–81.5◦). The two platinum(II)–terpyridyl
moieties are connected to the calix[4]arene by the alkynyl groups
in a linear fashion with Pt–C≡C angles in the range of 178.1–
◦
˚
˚
178.9 . The Pt–C (1.964–1.977 A) and C≡C (1.203–1.228 A) bond
lengths are comparable to that found in the related platinum(II)–
alkynyl systems.7e–i,9 The phenyl rings directly attached to the
alkynyl ligands are almost parallel to the planes of the platinum–
terpyridine with dihedral angles of 4.29◦ and 7.22◦. The two
[Pt(tBu3trpy)] coordination planes are essentially parallel, as
revealed by the interplanar angle of 11.25◦; and the interplanar
˚
groups leading to an O(3) · · · O(4) separation of 4.90 A. The two
phenolic rings in the calix[4]arene are also tilted (interplanar angle
74.55◦) so as to place the hydroxyl groups inside the cavity, with
˚
an O(1) · · · O(2) separation of 3.15 A. This arrangement allows
for easy hydrogen bond formation between the proximal hydroxy
groups and the ester functionalities. This hydrogen bonding,
together with the four bulky tert-butyl groups on the upper rim of
calix[4]arene help stabilise and lock the calix[4]arene in the cone
conformation.8 The dihedral angles between the plane containing
the four methylene carbon atoms linking the aromatic rings and
the planes of the aryl rings bearing ester groups are found to be
91.07◦ and 97.96◦, with a difference of close to 7◦ between the
two angles. Compared with other calix[4]arene systems devoid of
sterically bulky groups in the lower rims, the difference between
these two angles is about 2◦. This deviation is probably due to the
presence of the sterically demanding tri-tert-butyl groups on each
of the terpyridyl ligands.8
˚
distance of 3.53 A, as calculated from the average distances of
the atoms on the two least squares planes, suggested the existence
of a p · · · p stacking interaction. Surprisingly, unlike the other
related system,7f ,h,9,10a the two terpyridyl groups are found to be
in a staggered conformation. With this arrangement, the mutual
repulsions between the sterically bulky tri-tert-butyl terpyridyl
ligands can be minimised. Astonishingly, the intramolecular
˚
Pt · · · Pt distance is found to be 3.272 A, which is shorter than
˚
the sum of van der Waals’ radii of Pt (3.4 A) and is suggestive of
an intramolecular interaction between the two metal centres. Both
the Pt · · · Pt and p · · · p stacking interactions are thought to play a
role in providing the driving force for locking the Pt(tBu3trpy) units
together. It is interesting to note that mechanical grinding of the
orange-red crystals would give rise to a light orange powder. This
is probably due to the breakage of Pt · · · Pt and p · · · p stacking
interactions caused by the collapse of crystal lattices and the loss
of solvents of recrystallisation.5l,7d
Dissolution of 1 and 2 in acetonitrile gave a yellow solu-
tion, with similar UV-vis absorption patterns. Both spectra are
dominated by high-energy intraligand absorption bands at ca.
294–314 nm, and an admixture of low-energy metal-to-ligand
charge-transfer (MLCT) and alkynyl-to-terpyridine ligand-to-
ligand charge-transfer (LLCT) bands at ca. 448–456 nm in the
visible region (Fig. S2‡ ), typical of monomeric platinum(II)
terpyridyl complexes.7e,i,10 A slight red shift of the MLCT/LLCT
band was observed in 2 relative to 1. This has been attributed
to the electron-richness of the tert-butyl substituents on the
tBu3trpy units, which raise the energy of the p*(tBu3trpy) orbital.
The photophysical data are summarised in Table 1.
Fig. 1 Perspective drawing of the complex cation of 1 with selected atomic
numbering scheme. Hydrogen atoms and solvent molecules have been
omitted for clarity. Thermal ellipsoids are shown at the 30% probability
level.
Upon excitation at k > 400 nm, both 1 and 2 exhibited
luminescence at about 641–742 nm in acetonitrile solution at
room temperature and butyronitrile glass at 77 K. With refer-
ence to the spectroscopic studies on other related platinium(II)–
terpyridyl systems,7e–i,10 the origin of the luminescence is assigned
The two platinum(II) metal centres adopt a distorted square-
planar geometry, with distortion from the idealised 90◦ and
180◦ due to the coordination constraints imposed by the ter-
Table 1 Photophysical spectral data for 1 and 2
Complex
Medium (T/K)
Absorption kmax/nm (emax/dm3 mol−1 cm−1
)
Emission kmax/nm (s0/ls)
1
MeCN (298)
294 (88 380), 314 sh (82 590), 448 (16 180)
693 (0.6)
640 (0.2)
663 (0.3)
626 (1.5)
652 (2.0)
641 (2.3)
742 (<0.1)
834 (0.1)
825 (0.3)
654 (2.3)
Powder solid (298)
Crystalline solid (298)
Powder solid (77)
Crystalline solid (77)
Glass (77)
MeCN (298)
Solid (298)
2
294 (65 490), 312 sh (51 160), 456 (10 445)
Solid (77)
Glass (77)
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as being derived from states of predominantly 3MLCT [dp(Pt) →
p*(tBu3trpy)] character, mixed with some intraligand 3LLCT
[p(C≡CR) → p*(tBu3trpy)] character. Again, the red shift in
the emission bands of 2 relative to that of 1 is consistent with
the electron-donating ability of the tri-tert-butyl groups on the
terpyridyl ligands.
16661 unique reflections collected at 253 K with Mo-Ka radiation (k =
0.71073 A) equal to 2hmax = 48.84◦ on a single crystal X-ray MAR image-
˚
plate diffractometer, the structure was solved on a PC using the SIR-
97 programme and refined by full-matrix least squares methods. The
refinement converged to final R1 = 0.0553 and wR2 = 0.1491 with a
goodness-of-fit of 0.91; the largest difference peak and hole 1.812 and
−3
˚
−0.809 e A respectively.
Both the crystalline and powder solids of 1 exhibited rich
luminescent properties at 298 K and 77 K, with the 3MLCT
emissive state energy of the crystalline solid slightly lower than
that of the powder form (Fig. S3‡). This may be attributed to
the existence of a weak Pt · · · Pt interaction and little partial
p · · · p stacking as observed in its crystal lattice. However, the
emission maxima of the two forms do not vary to a large extent
as is observed in the related platinum(II)–terpyridyl complexes.7e
This may probably be attributed to the staggered arrangement
of the tri-tert-butylterpyridine units, leading to poor spatial
overlap. The solid-state emission of 2 showed emission maxima
at k > 820 nm at 298 K and 77 K. With reference to previous
spectroscopic studies on platinum(II)–terpyridyl complexes,7a–g
these low-energy emissions are assigned as originating from triplet
states of MMLCT character, associated with the presence of
Pt · · · Pt and p · · · p stacking interactions. It is likely that in the
absence of the bulky tert-butyl groups on the terpyridine unit in 2,
stronger Pt · · · Pt and p · · · p stacking interactions can exist. This,
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t
together with the lower p* orbital energy of trpy than Bu3trpy,
would give rise to a low-energy MMLCT emission at k > 800 nm
in the solid state.
In conclusion, a new class of luminescent dinuclear platinum-
(II)–terpyridyl calix[4]arene-containing bis-alkynyl complexes has
been demonstrated to possess Pt · · · Pt and p · · · p interactions
in the solid state. An unprecedented Pt · · · Pt close contact of
˚
3.272 A was observed in the dinuclear platinum(II) calix[4]arene-
bis-alkynyl complex, representing the very first example of a
Pt · · · Pt interaction in platinum(II) complexes containing the bulky
tri-tert-butyl terpyridine ligand. This probably arises from the
unique conformational arrangement of the calix[4]arene moiety
as well as the propensity of Pt metal centres and p-aromatic
ligands for non-covalent interactions that lock the two Pt–
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V. W.-W. Y. acknowledges support from the URC Seed Funding
for Strategic Research Theme on Organic Optoelectronics and the
Faculty Development Fund of The University of Hong Kong.
The work described in this paper has been supported by a Central
Allocation Vote (CAV) Grant from the Research Grants Council
of Hong Kong Special Administrative Region, China (Project No.
HKU 2/05C). H.-S. L. acknowledges the receipt of a postgraduate
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Notes and references
§ Crystallographic data. 1: C127.5H155Cl3F6N6O15Pt2S2, Mr = 2686.23,
˚
˚
monoclinic, space group ◦P21/c, a = 26.746(5) A, b = 22.659(5) A, c =
22.886(5) A, b = 92.19(3) , V = 13860(5) A , Z = 4, qcalcd = 1.287 g cm−3
,
3
˚
˚
l(Mo-Ka) = 2.170 mm−1. 57034 reflections measured. With the use of
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