Terpyridine-platinum(ii) acetylide complexes bearing pendent coordination units

Article abstract of DOI:10.1039/b603603a

Platinum(ii) complexes bearing various alkyne-pyrene, alkyne-4′-terpyridine and alkyne-dibutoxyphenylacetylide-terpyridine units were constructed in a step-by-step procedure based on copper-promoted cross-coupling reactions with preconstructed modules; formation of bis(ligand) complexes of Fe(ii) and Zn(ii) by binding of the pendent terpyridine units provided heterotrinuclear derivatives, all of which exhibit highly structured absorption features in solution and display a rich electrochemistry due to the presence of various redox active modules. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b603603a

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www.rsc.org/dalton | Dalton Transactions
Terpyridine-platinum(II) acetylide complexes bearing pendent
coordination units†
Raymond Ziessel,*a Ste´phane Diringa and Pascal Retailleaub
Received 9th March 2006, Accepted 19th May 2006
First published as an Advance Article on the web 31st May 2006
DOI: 10.1039/b603603a
Platinum(II) complexes bearing various alkyne–pyrene,
alkyne–4ꢀ-terpyridine and alkyne–dibutoxyphenylacetylide–
terpyridine units were constructed in a step-by-step procedure
based on copper-promoted cross-coupling reactions with
preconstructed modules; formation of bis(ligand) complexes
of Fe(II) and Zn(II) by binding of the pendent terpyridine units
provided heterotrinuclear derivatives, all of which exhibit
highly structured absorption features in solution and display
a rich electrochemistry due to the presence of various redox
active modules.
yields (48%) and microsecond triplet excited state lifetimes have
been measured in Pt(II)–bipyridine–bis-silylacetylide complexes.18
However, very few Pt(II)–terpyridine complexes bearing an
acetylide connected to uncomplexed polypyridine fragments or
large chromophoric centres have been described.
Herein, we detail the synthesis of terpyridine-based, func-
tionalized-alkyne platinum(II) complexes and report some pre-
liminary optical and electrochemical measurements. For 1a and
1b, we chose unsubstituted [(terpy)Pt(Cl)]+ (terpy = 2,2ꢀ:6ꢀ,6ꢀꢀ-
terpyridine) as the starting material. CuI-catalysed cross-coupling
with 1-ethynylpyrene and 4ꢀ-ethynyl-terpy afforded the desired
complexes in good yields. To solve solubility problems arising with
this first set of complexes, we introduced tert-butyl substituents
onto the terpy ligands to produce complexes 2a and 2b under
similar conditions (Scheme 1).
Currently there is great interest in the development of square-
ꢀ
planar d8-complexes of Pt(II) containing aromatic a,a -diimine
ligands, mainly due to their interesting spectroscopic and lumi-
nescence behavior.1–5 Complexes which possess functional groups
have proved to be particularly effective as DNA intercalators6
and molecular probes for biological macromolecules.7 Paramount
in the design of luminescent molecules, polymers and devices
is the need to incorporate transition metals such as Pt(II) into
the backbone in order to relax the spin selection rules for
phosphorescence through spin–orbit coupling.8 This has proven
to be important in the design of light emitting diodes,9 non-linear
optical materials10 and conductance switches.11
Monomeric oligopyridine platinum(II) complexes display re-
markably rich luminescence properties arising from the ligand (so-
called intra-ligand, IL, emission), from metal-to-ligand-charge-
transfer (MLCT) or in some cases from ligand field (LF) states.12
Platinum(II) terpyridine complexes have been shown to display
especially intriguing spectroscopic and luminescence properties
depending on the metal–metal interactions and p–p stacking of the
surrounding ligands.13 In particular, solvent induced aggregation
and marked solvato-chromism have been found14 and the grafting
of pyrenyl fragments to terpyridine has provided exceptionally
long excited state lifetimes (e.g. 64 ls).15
Lately, there has been a considerable surge of interest in
developing platinum(II) r-alkynyl complexes, as the p-orbitals on
the ligand can be fully conjugated with the dxy orbitals of the metal.
This creates favourable structural and electronic conditions for fast
electron/hole or exciton migration, a sine qua non condition for
efficient electron/hole recombination and the regeneration of the
ground state through luminescence.16,17 Remarkably high quantum
Scheme 1
Alternatively, the solubility problem was circumvented by
using a 1,4-dialkoxyphenyl substituted alkyne fragment as a
spacer between the platinum and the uncomplexed terpy units
(Scheme 2). The synthesis of complex 7 was accomplished in eight
steps commencing with 1,4-hydroquinone. Alkylation followed
by bromination provided a good yield of 3, which was readily
converted to its dipropargylic alcohol derivative. The key step is the
mono-deprotection of one of the propargylic functions by means
of NaOH under anhydrous benzene. With compound 4 in hand,
cross coupling with 4ꢀ-[{(trifluoromethyl)sulfonyl}oxy]-2,2ꢀ:6ꢀ,2ꢀꢀ-
terpyridine and deprotection of the resulting alkyne gave the
target ligand 5. Compound 5 flanked by two butoxy solubilizing
groups can be cross-coupled with unsubstituted or substituted
(terpy)platinum chloro-salts in the presence of catalytic amounts
of CuI, leading respectively to complexes 6 and 7 (Scheme 2).
The functionalisation of the pendent terpy units in 6 and 7 is
not such as to obstruct their complexation and reaction of 7 with
Fe(ClO4)2·6H2O or Zn(OTf)2·4H2O afforded the heterotrinuclear
aLaboratoire de Chimie Mole´culaire, Ecole de Chimie, Polyme`res,
´
Mate´riaux (ECPM), Universite´ Louis Pasteur (ULP), 25 rue Becquerel,
67087, Strasbourg Cedex 02, France. E-mail: ziessel@chimie.u-strasbg.fr
bLaboratoire de Cristallochimie, ICSN-CNRS, Baˆt 27-1 avenue de la
Terrasse, 91198, Gif-sur-Yvette, Cedex, France
† Electronic supplementary information (ESI) available: Table S1 for p–p
interactions; Fig. S1 for the packing. See DOI: 10.1039/b603603a
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Scheme 2 (i) C4H9Br, NaOH, DMF, 150 ◦C, 18 h, 75%; (ii) Br2, CCl4, reflux, 70 h, 80%; (iii) 2-methyl-3-butyn-2-ol, Pd(PPh3)4 (12 mol%), nPrNH2,
65 ◦C, 70 h, 86%; (iv) NaOH, C6H6, 70 ◦C, 14 h, 45%; (v) terpy-OTf, Pd(PPh3)4 (6 mol%), iPr2NH, Tol, 70 ◦C, 24 h, 62%; (vi) NaOH, Tol, 120 ◦C, 18 h,
86%; (vii) [(terpy)PtCl]BF4, CuI (10 mol%), TEA, DMF, NaBF4, rt, 70 h, 38%; (viii) [(tertBu)3-terpy)PtCl]BF4, CuI (10 mol%), TEA, DMF, NaPF6, rt,
70 h, 43%.
complexes 8 and 9, while complex 10 resulted from the reaction of
6 with Fe(II) salts (Scheme 3).
potassium carbonate in DMF in the presence of the [(terpy)PtCl]+
complex (Scheme 4). Copper catalysis appears not to be essential
to reach the target and it is surmised that the chloro ligand is
readily substituted by the deprotonated alkyne. Although the
use of a biphasic mixture (benzene/water) in the presence of a
phase transfer catalyst has previously been found useful for the
preparation of sophisticated tritopic ligands,20 this does not appear
to be of any benefit here.
For all complexes, the IR spectra exhibit an intense mPtC≡C
stretching vibration around 2100 cm−1. A second mC≡C absorption
is observed around 2200 cm−1 whenever a C≡C unit is present to
link different organic fragments (such as in 6, 7 and 10).
Having achieved such syntheses, we then evaluated the possibi-
lity of cross-coupling additional luminophores to the platinum(II)–
terpy centre. It turns out that the deprotected form of the
boraindacene 11 only slowly reacts with [(terpy)PtCl]+ in the
presence of CuI in DMF at rt and has a strong tendency to
give an oxidative homocoupling reaction leading to butadiene-
bridged bis-indacene compounds. Catalysis of this homocoupling
by both Pd and Cu salts has been observed previously.19 After
several failures to achieve coupling to the Pt complex, success was
achieved simply by deprotecting compound 11 with anhydrous
Scheme 3
3286 | Dalton Trans., 2006, 3285–3290
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Table 1 Selected data for the novel Pt(II) complexes
Complex Yield (%) mC≡Ca/cm−1 ES-MSb
kmaxc/nm (e/M−1cm−1
)
1a
1b
2a
2b
6
90
82
92
74
38
43
87
2103
2101
2099
2122
653.1ꢁ+
684.1ꢁ+
821.4ꢁ+
852.4ꢁ+
492 (5800)
408 (5600)
474 (7300)
403 (8450)
2201/2111 928.3ꢁ+
2197/2106 1096.4ꢁ+
478 (9550)
7
466 (13 000)
584 (29 700)
425 (sh, 9800)
375 (21 300)
590 (49 000)
425 (40 000)
375 (49 000)
524 (70 300)
402 (sh, 12 000)
8
2099
440.3ꢁ4+
620.2ꢁ3+
10
12
65
96
2194/2106 478.3ꢁ4+
831.3ꢁ+
Scheme 4 (i) Anhydrous K2CO3 excess, DMF, [(terpy)PtCl]BF4, rt, 1 day.
2111
X-Ray structure of complex 7
a One drop of CH2Cl2 containing the sample was evaporated on a KBr
disk. b Nature of the cluster determined by electrospray mass spectroscopy
in methanol/acetonitrile at Vc = 110 V, except for analysis of compound
10, where Vc = 200 V. The molecular peaks correspond either to [M −
BF4]+, [M − PF6]+ or to [M − ClO4]+. For compounds 8, 9 and 10 the 3+
The coordination geometry around platinum is essentially square
planar but with the central Pt–N2 distance (1.959(5) A) slightly
shorter than those to the two outer nitrogen atoms (Pt–N1
˚
peaks correspond to [M − 3ClO4]3+ and the 4+ peaks to [M − 4ClO4]4+
.
˚
˚
2.016(5) A and Pt–N3 2.013(5) A) and the Ncentral–Pt–Nouter bond
All isotopic profiles correspond to the expected sequence. c Averaged value
determined from at least two different solutions of non-degassed DMF.
angles <90◦, as expected from the restrictions of the terpy ligand
˚
structure (Fig. 1).‡ The Pt–C16 distance of 1.965(7) A compares
well with values reported in the literature,21 and the C≡C bond
˚
length of 1.203(9) A is comparable to those in related Pt(II)
acetylide complexes. An important feature of the crystal structure
is the quasi-planarity of the molecule (Fig. 1b) with insignificant
dihedral angles between φB and φPt (2.87◦), φB and φT (3.53◦), φPt
and φT (1.52◦) (φB denotes the average plane of the central benzene,
φPt the one containing the Pt and φT the one containing the
uncomplexed terpy). Analysis of the crystal packing of 7 suggests
pronounced intermolecular interactions between the complexes
12, the strong absorption at 524 nm is due to the S0 → S1
transition of the indacene fragment19 and that at 402 nm is assigned
as PtLCT
Complex 2b showed two quasi reversible couples at −0.97
n
and −1.48 V vs. ferrocene (+0.38 V) in CH2Cl2 and BuNPF6
as supporting electrolyte. The first reduction must arise from
a terpy-based reduction and the second reduction is tentatively
assigned to a metal-centered reduction (Fig. 3).22 However, we
could not exclude based on our experimental evidence that
the second reduction is also localized on the complexed tert-
butyl-terpy. The potential difference of about 500 mV would be
acceptable for electron pairing into the same orbital as previously
hypothetized in more complex architectures.23,24 In the trinuclear
Fe/Pt2 complex 8, similar terpy-Pt and Pt centered reduction
waves are found at −0.96 and −1.54 V (two electron exchange
process) and two additional single electron reversible processes
are found at −1.27 and −1.40 V (Fig. 3b). These are probably
due to successive reductions of the two terpy units coordinated
to iron. In addition, a reversible one electron oxidation corre-
sponding to an Fe(II)/(III) couple is observed at +1.09 V. No
such process is observed in complex 2b. However, addition of
anhydrous iron salts to a solution of 2b results in the instantaneous
formation of the deep-violet Fe/Pt2 complex and the emergence
of new redox waves identical to those found for the isolated
complex 8.
˚
by revealing p–p stacking distances around 3.6 A between the
different ring moieties of a molecule in general position x, y, z and
four neighbouring complexes (Table S1 for p–p interactions† and
Fig. 1c for the packing).
In DMF solution at 298 K, all the complexes show intense
multiple absorption peaks in the range 250–600 nm (Fig. 2). On the
basis of earlier studies of related compounds,15,18 the high-energy
structured absorption bands are ascribed to p–p* transitions of
the terpy units and of the phenylalkynyl moiety. In cases 1a and
2a, the intense additional absorption centred at 370 nm is due to
absorption of the pyrene moieties. These well defined transitions
are not observed when unbound terpy is substituted for pyrene, as
in complexes 1b and 2b (Fig. 2a). The low energy absorption band
in the visible region is assigned to a platinum-to-ligand charge
transfer transition (MLCT) probably mixed with an alkynyl-to-
terpyridine ligand-to-ligand charge transfer character (LLCT).
This band is found near 400 nm in the terpy-substituted alkyne
complexes 1b and 2b, whereas in the pyrene derivatives 1a and 2a
it appears at 492 and 474 nm respectively. This bathochromic
shift is probably induced by the increase of electronic density
imported by the pyrene and the 18 nm shift found in 2a compared
to 1a is due to the donor properties of the tert-butyl fragments.
Interestingly, by complexing the free terpy sites in complexes 2b
and 7 with iron(II) a new intense FeLCT band located around
580–590 nm is seen (Fig. 2b), while the PtLCT band is shifted to
the blue by ca. 40 nm. In the Zn/Pt2 complex 9, no additional
MLCT band was observed but the hypsochromic shift of the
PtLCT band (ca. 40 nm) is retained (Table 1). For complex
In summary, a series of terpyridine–platinum(II) acetylide
complexes bearing various appended moieties (pyrene, 4ꢀ-terpy,
dibutoxyphenylacetylide–terpy) has been successfully synthesized
using a rational protocol. High solubility has been ensured by
the use of tert-butyl substituents on the terpy–Pt center and/or
by linear butyl chains grafted on the central phenoxy moiety. An
X-ray molecular structure determination revealed a coplanar ar-
rangement of the terpy–Pt/ethynylphenoxy/ethynylterpy entities
favourable for conjugation of the different modules. The nature
of the acetylide residues has a strong influence on the absorption
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Fig. 1 TOP: ORTEP view of complex 7 with atom labelling. Probability displacement◦ellipsoids are shown at 50% level. Selected distances and angles:
◦
◦
◦
˚
C24–C25 1.195(10) A, N3–Pt–N2 80.46(19) , N2–Pt–N1 80.7(2) , N3–Pt–N1 161.1(2) , N2–Pt–C16 178.2(2) . Middle: view along the Pt–pyridine N5
axis. Bottom: crystal packing.
3288 | Dalton Trans., 2006, 3285–3290
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In-depth characterization of the photophysical properties of these
new complexes is currently in progress.
Acknowledgements
The Ministe`re de la Recherche et des Nouvelles Technologies is
gratefully acknowledged for financial support of this work in the
form of an Allocation de Recherche for SD and Professor Jack
Harrowfield for careful reading and commenting this manuscript
prior to publication. We also thank one of the reviewers for his
comments concerning the possible electron pairing into a single
terpy orbital.
Notes and references
‡ X-Ray crystal data for 7 at 293 K (C60H60N6O2PtF6P, Mr = 1237.2):
monoclinic, space group P21/c, a = 10.8334(3), b = 33.5765(9), c =
◦
3
−1
˚
˚
16.8022(5) A, b = 104.462(1) , V = 5918.1(3) A , Z = 4, l = 2.462 mm
,
reflections measured: 16893, unique: 9663, R(int) = 0.035, final R1(F) =
0.047, wR2(F2) = 0.1055 for 784 parameters, 25 restraints and 7013
reflections with I > 2r(I) and 1.74 < h < 24.4◦ (corresponding R values
based on all 9658 reflections are 0.0763 and 0.1166, respectively). Largest
difference peak and hole are 1.071 and −0.696. CCDC reference number
600978. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b603603a
Fig. 2 Overlay of absorption spectra recorded for various complexes in
dimethylformamide solution.
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