TTF-annulated phenanthroline and unexpected oxidative cleavage of the C=C bond in its ruthenium(II) complex

Article abstract of DOI:10.1021/ic902230f

Tetrathiafulvalene (TTF) and 1, 10-phenanthroline have been fused together via a simple and efficient synthetic procedure that provides a new bidentate ligand, 40, 50-ethylenedithiotetrathiafulvenyl[4, 5-f]-[1, 10]phenanthroline (EDT-TTF-phen, 1). Its ruthenium(II) complex exhibits a unique packing of TTF subunits in the solid state. In an acetonitrile solution, [Ru(bpy) ₂(1)](PF₆)₂ undergoes facile oxidative cleavage of the C=C double bond, which cannot be observed in the dark or under anaerobic conditions. This points to the photocatalytic role played by the ruthenium(II) chromophore in this conversion.

Full text of DOI:10.1021/ic902230f

Inorg. Chem. 2010, 49, 1307–1309 1307
DOI: 10.1021/ic902230f
TTF-Annulated Phenanthroline and Unexpected Oxidative Cleavage of the CdC
Bond in Its Ruthenium(II) Complex
Lawrence K. Keniley, Jr., Lipika Ray, Kirill Kovnir, Logan A. Dellinger, Jordan M. Hoyt, and Michael Shatruk*
Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306
Received November 11, 2009
Tetrathiafulvalene (TTF) and 1,10-phenanthroline have been fused
together via a simple and efficient synthetic procedure that provides
a new bidentate ligand, 40,50-ethylenedithiotetrathiafulvenyl[4,5-f]-
[1,10]phenanthroline (EDT-TTF-phen, 1). Its ruthenium(II) complex
exhibits a unique packing of TTF subunits in the solid state. In an
acetonitrile solution, [Ru(bpy)2(1)](PF6)2 undergoes facile oxidative
cleavage of the CdC double bond, which cannot be observed in the
dark or under anaerobic conditions. This points to the photocatalytic
role played by the ruthenium(II) chromophore in this conversion.
1,10-Phenanthroline (phen) and 2,20-bipyridine (bpy) are
important chelating ligands in coordination chemistry because
they exhibit high binding affinities toward transition-metal
ions. Complexes such as [Ru(bpy)3](PF6)2 and Fe(phen)2-
(NCS)2 have become “guinea pigs” for the fields of photo-
chemistry and spin crossover, respectively. Until now, however,
examples of TTF molecules derivatized with the phen or bpy
functionality have been rare. Bpy was appended to the TTF
unit through thiomethylene, ethynyl, and amido bridges.6
Decurtins and co-workers fused TTF with dipyridophena-
zines.7 In a somewhat similar approach, a diazafluorene
derivative of TTF also was reported.8
The field of multifunctional materials is expanding
quickly, with molecule-based solids gaining great interest
over the past decade. Among these, a lot of attention has been
devoted to materials incorporating tetrathiafulvalene (TTF)
and its derivatives. This interest is fueled by the large number
of synthetic conductors and semiconductors that are based
on these redox-active molecules. Attempts have been made to
derivatize TTF with functional groups capable of binding to
transition-metal ions. Such binding may enhance the synergy
between the conducting properties of the organic substruc-
ture and optical or magnetic properties of the metal ions.
In particular, thiolate,1 phosphine,2 carboxylate,3 acetylace-
tonate,4 and pyridyl5 derivatives have been used for this
purpose.
Herein, we report the syntheses and properties of TTF-
annulated phen and its ruthenium(II) complex, as well as a
facile, ruthenium-catalyzed photooxidative cleavage of the CdC
double bond of the TTF unit accompanied by recrystallization
of the reactant to the oxidized product under mother liquor.
40,50-Ethylenedithiotetrathiafulvenyl[4,5-f][1,10]phenan-
throline (EDT-TTF-phen, 1) was prepared in good yield via
a synthetic procedure that employs readily available and
inexpensive reactants (Scheme 1). The synthesis starts with
5,6-dibromo-1,10-phenanthroline (2). While the Almeida
group has reported recently that 5,6-bis(benzylthio)-1,10-
phenanthroline (3) is obtained in 71% yield using a Pd2-
(dba)3-catalyzed reaction between 2 and benzylthiol in
the presence of bis[2-(diphenylphosphino)phenyl] ether and
t-BuOK (48 h at 120 °C),9 we discovered that a simple
reaction between 2 and sodium benzylthiolate (generated in
situ from benzylthiol and sodium hydride) provides a
comparable yield of 3 but in just 12 h at room temperature
and without a catalyst.
*To whom correspondence should be addressed. E-mail: shatruk@chem.
fsu.edu.
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2010 American Chemical Society
Published on Web 01/20/2010
pubs.acs.org/IC

1308 Inorganic Chemistry, Vol. 49, No. 4, 2010
Scheme 1. Synthesis of 1a
Keniley et al.
a (i) NaH, C6H5CH2SH, DMF, 12 h; (ii) AlCl3, benzene, 24 h; NaOH,
CS2, reflux, 2 h; (iii) 4,5-ethylenedithio-1,3-dithiol-2-one, P(OEt)3,
reflux, 12 h.
We found the second step in the preparation of 1 to be the
most challenging because attempts to isolate the intermediate
1,10-phenanthroline-5,6-dithiol led to untractable products.
Thus, a one-pot approach was used in which the 1,3-dithiole-
2-thione functionality was formed by reacting the in situ
formed 1,10-phenanthroline-5,6-dithiolate dianion with CS2
under basic conditions. Product 4 was obtained in 95% yield
and recrystallized from acetonitrile.
It must be added that, very recently, an alternative route to
4 was reported in which 5,6-bis(2-cyanoethylsulfanyl)-1,10-
phenanthroline is converted to 4 in a similar manner by
cleaving the thioether groups with t-BuOK and converting
the obtained dithiolate into 1,3-dithiole-2-thione using thio-
phosgene, with the resulting yield of 67%.10
Figure 1. Crystal structures of 1 CHCl3 (a) and [Ru(bpy)2(1)](PF6)2
3
3
3.5CH3CN (b). In the latter, only one out of three crystallographically
independent complexes is shown and the interstitial solvent molecules are
omitted for clarity. (Thermal ellipsoids are at the 50% probability level.)
The latter authors also reported that they were unsuccess-
ful in preparing the corresponding symmetric TTF derivative
by self-coupling of 4 in the presence of triethyl phosphite.
Nevertheless, we successfully converted 4 to the asymmetric
EDT-TTF-phen 1 by cross-coupling with 4,5-ethylenedithio-
1,3-dithiol-2-one.
The crystal structure determination showed that the mole-
cule of 1 is essentially planar (Figure 1a), with the exception
of the ethylenedithio subunit, the C atoms of which are
˚
disordered over two positions at ∼0.4 A above and below
the molecular plane. The molecules are stacked in columns
along the a axis in a head-to-tail fashion (Figure S1 in the
Supporting Information, SI) and exhibit π-π contacts with
Figure 2. Side (a) and top (b) views of the packing of TTF-containing
cations in the crystal structure of [Ru(bpy)2(1)](PF6)2. The H atoms have
been omitted for clarity. Color scheme: Ru, light blue; S, yellow; N, blue;
C, gray.
˚
an interplanar separation of 3.56 A. Large channels are
present along the a axis, which are filled with disordered
CHCl3 molecules.
A cyclic voltammogram of 1 (Figure 3) recorded in a
0.100 M acetonitrile/dichloromethane (3:2, v/v) solution11 of
(TBA)PF6 (TBA = tetrabutylammonium) reveals two re-
versible oxidations at E1/2(1) = 0.17 and E1/2(2) = 0.52 V vs
Fc/Fcþ (Fc = ferrocenium). Both oxidations are shifted to
higher potentials as compared to unsubstituted TTF (-0.10
and 0.37 V),12 as expected from the π-accepting nature of the
phen moiety. [Ru(bpy)2(1)](PF6)2 exhibits three reversible
oxidations and two reversible reductions. A comparison of its
redox behavior to that of 1 indicates that the first two
To show the suitability of 1 for the preparation of metal
complexes, we reacted it with Ru(bpy)2Cl2 in refluxing
ethanol. The subsequent addition of an ethanolic solution
of NH4PF6 resulted in the precipitation of [Ru(bpy)2(1)]-
(PF6)2, which was recrystallized by the vapor diffusion of
diethyl ether into an acetonitrile solution of the complex. A
crystal structure analysis of this compound revealed that
ligand 1 remains nearly planar (Figure 1b). Furthermore, the
complex exhibits a unique packing of TTF moieties, in which
an asymmetric unit includes three [Ru(bpy)2(1)]2þ cations
(Figure 2), with the plane-to-plane separation between TTF
(1)
(2)
oxidations at E1/2 = 0.26 and E1/2 = 0.58 V are TTF-
based. They are shifted to slightly more positive potentials
with respect to those observed for 1. The third redox process
˚
units being 3.44-3.78 A. The identity of the complex was also
confirmed by 1H NMR spectroscopy (Figure S2 in the SI) and
(11) The CH3CN/CH2Cl2 solvent mixture was used to remove absorption
effects manifested as a dramatic increase in the anodic current for the redox
process observed at the most negative potential (Figure S7 in the SI).
(12) Bryce, M. R.; Marshallsay, G. J.; Moore, A. J. J. Org. Chem. 1992,
57, 4859–4862.
electrospray ionization mass spectrometry (Figure S3 in the SI).
(10) Chesneau, B.; Passelande, A.; Hudhomme, P. Org. Lett. 2009, 11,
649–652.

Communication
Inorganic Chemistry, Vol. 49, No. 4, 2010 1309
Figure 3. Cyclic voltammograms of 1 and [Ru(bpy)2(1)](PF6)2 (dashed
and solid lines, respectively) in a 0.100 M solution of (TBA)PF6 in
CH3CN/CH2Cl2 (3:2, v/v) at a sweep rate of 0.100 V/s (vs Fc/Fcþ).
Figure 4. UV-visible absorption and emission spectra (solid and
dashed lines, respectively) of [Ru(bpy)2(1)](PF6)2 in an acetonitrile solu-
tion. (For the full absorption spectrum, see Figure S5b in the SI.)
at E1/2(3) = 1.04 V is due to oxidation of the RuII center. All three
oxidations are similar to those reported earlier for [Ru(bpy)2-
(TTF-dppz)](PF6)2 (0.29, 0.61, and 0.99 V).13 Reversible ligand-
Scheme 2. Photoinduced Oxidation of [Ru(bpy)2(1)](PF6)2 to
[Ru(bpy)2(5)](PF6)2 in an Acetonitrile Solution
(4)
based reductions are also observed at E1/2 = -1.54 and
E1/2(5) = -1.92 V.
Density functional theory calculations of the electronic
structure of 1 reveal that the π-type highest occupied and
lowest unoccupied molecular orbitals are centered on the
EDT-TTF and phen fragments, respectively (Figure S4 in the
SI), similarly to the nature of frontier orbitals in TTF-dppz.7
Therefore, the lowest-energy 402 nm band in the optical
spectrum of 1 (Figure S5a in the SI) is assigned to the charge-
transfer transition between the EDT-TTF and phen subunits.
[Ru(bpy)2(1)](PF6)2 exhibits a broad metal-to-ligand
charge-transfer band with a maximum at 450 nm in an
acetonitrile solution (Figure 4). The emission maximum is
observed at 620 nm and red-shifted with respect to that of
[Ru(bpy)3](PF6)2 (607 nm), while the quantum yield of 2.8%
is somewhat lower than that reported for [Ru(bpy)3](PF6)2
(6.2%),14 which is explained by luminescence quenching due
to the presence of the TTF unit.13 A detailed investigation of
other photophysical properties of [Ru(bpy)2(1)](PF6)2 is
currently underway.
Both 1 and its ruthenium(II) complex are stable in the solid
state and can be stored in air for prolonged periods of time.
However, when [Ru(bpy)2(1)](PF6)2 was crystallized from
MeCN/Et2O and the crystals were allowed to remain under
mother liquor for about 1 week, a complete transformation of
this compound to a new crystalline solid was observed
(Scheme 2). Crystal structure analysis revealed that the
product contains 5,6-dithiocarbonato-1,10-phenanthroline
(5) coordinated to the RuII center in place of 1 (Figure S6 in
the SI). This facile oxidative cleavage of the CdC bond in the
TTF unit does not occur when the [Ru(bpy)2(1)](PF6)2 com-
plex is kept under mother liquor in the dark or under an inert
atmosphere. The role of the ruthenium(II) chromophore in
this conversion is yet to be understood, and it might simply be
to generate singlet oxygen that oxidizes the double bond. We
currently are carrying out a detailed study of this unexpected
conversion and will report our findings in due course.
It must be added that, while ruthenium(II) complexes have
been used widely for such processes as photocatalytic water
splitting and carbon dioxide to methane conversion,15 their
applicability as photocatalysts for organic transformations is
only beginning to receive attention,16 and one can anticipate
that this research area will be rapidly expanding.
In summary, we successfully fused phen and TTF hetero-
cycles using a convenient and inexpensive synthetic procedure.
The obtained molecule 1 can serve as a promising ligand for
the preparation of multifunctional metal complexes. We have
shown that 1 forms a mononuclear ruthenium(II) complex
with a unique arrangement of TTF moieties in the crystalline
state. The ruthenium complex also shows unusual photoreacti-
vity in solution that leads to facile oxidative cleavage of the
CdC double bond in the TTF fragment. Currently, we are
extending the use of ligand 1 to other transition-metal ions.
Acknowledgment. A part of this research is supported
by the National Science Foundatoin under Grant CHE-
0911109. Financial support by the Florida State University
is also gratefully acknowledged. We thank Dr. Andrey
Khoroshutin (Moscow State University) and Dr. Igor
Alabugin (Florida State University) for helpful discussions.
Supporting Information Available: Complete experimental
details, spectroscopic data, and CIF files. This material is
available free of charge via the Internet at http://pubs.acs.org.
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